Human and animals heavily rely on good vision to perform their daily tasks and for survival, and indeed eyesight is undoubtedly the most valuable of our precious senses. The eyes being windows for the brain is apt since 80% of the external information reaching the neural networks comes in from the visual system. Thus, visual impairment in any form has a devastatingly negative impact on most people whose greatest fear is blindness. Sadly, the World Health Organization (WHO, 2019) reports that due to increasing poverty, poor nutrition, pollution, smoking, reduction in natural resources, diminishing supply of affordable basic hygiene and healthcare products at a global level and lack of timely diagnosis, the incidence of poor vision and blindness continues to rise. Unsurprisingly, Africa, Asia and South America represent the nations where this situation continues to worsen. However, ready supply of rich foods and a rising tide of obesity in the developed countries also is burdening healthcare systems and increasingly causing a rise in ocular disorders and diseases. Development of cataracts and other refractive errors such as myopia account for the major causes of blindness on our planet. However, the incidence of glaucoma, an optic neuropathy comprising several different forms, is the second leading cause of blindness worldwide affecting nearly 80 million patients and which is expected to debilitate >112 million by 2040 (Tham et al., 2014; Flaxman et al., 2017). More than 195 million suffer from age-related macular degeneration (AMD; both wet and dry forms) and 145 million from diabetic retinopathy. Contextually, the estimated societal economic burden imposed by visual impairment/blindness just in the United States is >$16 billion/annum when accounting for decreased quality of life, disability, morbidity and lost overall productivity. With an increasingly aging world population, ophthalmic disorders represent a rising healthcare issue of huge proportions worldwide (WHO, 2019).

Before delving into the pathological basis of glaucoma, it is necessary to describe the anatomical structures and the functions of the visual axis. The mammalian eye is exposed to the outside world somewhat unprotected although the eye socket offers some protection. Similarly, the protective thick white fibrous outer layer (sclera) of the eyeball affords the eye its shape while insulating the interior components from possible damage. The transparent cornea at the front of the eye consists of five layers of different cell types and represents a specialized scleral tissue (DelMonte and Kim, 2011). The conjunctiva is an extension of the lateral parts of the cornea and sclera but it is highly vascularized (unlike the cornea) and ends up as the tissue lining the underside of the eyelids (Weingeist, 1973). Sitting inside a capsular “pocket” and suspended by ligaments is the lens inside the eye a few centimeters behind the corneal endothelial cell layer. Suspended in front of the lens is the iris that forms the pupil where the iris sphincter muscle contracts or relaxes to regulate the amount of light passing to the lens. Just lateral to the lens ligaments is the ciliary body (CB; Smith, 1973) composed of the ciliary processes (CP)/ciliary epithelium (CE) and the ciliary muscle (CM) which is attached to the lens to allow accommodation (Figures 1A,B, 2A,B). The space between the lens and the cornea is the anterior chamber of the eye which is filled with a watery solution (aqueous humor [AQH]) which is produced by the CP/CE of the CB (Civan and Macknight, 2004). Together with the sclera and cornea (and the vitreous humor in the posterior chamber behind the lens [see ahead]), the AQH helps maintain the overall shape of the eyeball.

The AQH also nourishes the cells lining the ANC of the eye as it percolates through the ANC to exit via the angular space between cornea and iris via a filtration system, trabecular meshwork (TM), that is connected to the Schlemm’s canal (SC) that allows the AQH to pass into the veinous circulation via series of complex plexi of smaller vessels (Figures 1A,B, 2A,B). Under normal physiological conditions the amount of AQH produced (∼2 µL/hour) by the non-pigmented cells of the CE equals the amount draining from the ANC, and thus homeostasis is achieved. Recent single cell-based transcriptome research has revealed that at least eight different types of cells reside within the TM/SC outflow pathway and as many as seven sub-types within the UVS pathway in human and monkey eyes with mice and pigs showing similar expression patterns (van Zyle et al., 2020). The filtration area was comprised of two-types of TM cells (expressing known marker genes such as MYOC, MGP and PDPN), SC cells and resident macrophages. However, the structural elements of the TM were composed of Schwalbe’s line cells (TM-beam cells; two-types expressing gene markers FABP4 and TMEFFs) in the non-filtering area of the TM around the juxtacanicular region. The juxtacanicular cells differentially expressed various genes including CH13L1, ANGPTL7, RSPO4, FMOD and NELL2. Interestingly, SC cells displayed a profile made up of both lymphatic endothelia and blood macrophages. Lastly, there were differences in the genetic expression profiles of TM cell-types (e.g., MYOC, FOXC1, PITX2, CYP1B1, LOXL1, ANGPT1 and EFEMP1) and SC cells (e.g., CAV1, CAV2, TEK, PRSS23, ANGPT2), with differences extending to genes involving OHT-related high IOP vs controls. Differences were also observed in RGCs genes which were unrelated to elevated IOP. Therefore, the collective studies from several groups highlight the fact IOP generation and regulation are mediated by a complex array of cell-types in the outflow pathways, and that these elements appear to be well conserved across multiple species Liton et al., 2009; Liu et al., 2016; Kizhatil et al., 2014; Stamer et al., 2015; Sathiyanathan et al., 2017; Carnes et al., 2018; Patel et al., 2020; van Zyl et al., 2020).

Additional single-cell transcriptomic investigations of human ANC cells revealed up to twelve distinct cell-types (Patel et al., 2020). Of the TM cells, myoblastic and fibroblastic signatures were obtained yielding a Schwann cell and macrophagic profile of genes. In contrast, the SC cells exhibited a more lymphatic/blood vasculature genetic signature (Patel et al., 2020). These features correlate well with the ability of TM cells to contract/relax (Wiederholt et al., 2000), and their ability to phagocytose cellular debris, ECM and chemical agents (Alvarado et al., 1981; Alvarado et al., 1984; Grierson and Howes, 1987; Matsumoto and Johnson, 1997; Sherwood and Richardson, 1988), and for the SC pathway cells to drain away the filtered AQH to the veinous circulation (Thomson et al., 2014; Bernier-Latmani and Petrova, 2017; Acott et al., 2020).

In contrast, the posterior segment of the eye behind the lens is filled with a jelly-like material (vitreous humor [VH]), and this chamber is substantially larger than the ANC. The VH, composed of many different proteins and water, also helps in shaping the eyeball and also provides much cushioning and protection to the retina that lines the inside of sclera at the back of the eyeball, and it probably is also involved in removing some of the waste products of the retina. Just as the cornea at the front of the eye is composed of many cell layers, the retina is highly complex and contains many specialized cell-types.

Since glaucoma results from structural and functional failures at multiple levels throughout the visual system, it is important to be aware of the composition and organization of the various components. In the rear of the human eyeball, the retina contains many different types of cells that are also essentially transparent such that light reaches the back of the eye and penetrates to the deeper layers of the retina where the photo-sensitive cells (photoreceptors; rods and cones) reside. Whilst cones are concentrated in a central region of the retina called fovea, the rods are mostly located in the peripheral regions of the retina. Whereas the cones are specialized for high acuity tasks like reading and color perception, rods are responsible for night vision and respond best to dim light (Grossniklaus et al., 2015).

The jelly-like VH fills the majority of the posterior segment of the eye. At the back of the eye, the VH is separated from the retinal tissue by the inner limiting membrane. Next comes a multilayered retinal nerve fiber layer (RNFL) composed of the axons of the RGCs that converge at the optic nerve head (ONH)/optic disc area (Herrera et al., 2019), pass through a delicate tissue (lamina cribosa; LC; Daguman and Delfin, 2018; Figures 3A,B) at and behind ONH and are bundled together to form the optic nerve that exits the eyeball. Behind the RGCs (Russo et al., 2016; Detwiler 2018) are several layers of interneurons comprising bipolar, amacrine and horizontal cells followed by the photo-sensitive photoreceptors (rods and cones) and then the retinal pigment epithelial (RPE) cells (Figures 1A,B, 2A,B) (Vecino et al., 2016; Grünert and Martin, 2020). Muller glial cells run the full length of the retina anteriorally from the RPE cells to the RGCs. The RPE cells are seperated from the capillaries of the choroidal circulation by the outer limiting membrane (Bruch’s membrane) (Figures 1A,B, 2A,B). Each optic nerve travels to the optic chiasm and crosses over to reach the contralateral area of the thalamic brain nuclei. The latter process the information and send it on to the visual cortex thereby conveying the visual signal information from the RGCs to the brain.

The retina is a high energy-demand tissue and it receives nutrients and oxygen from the retinal arteries and the choroid (Kiel 2010; Ansari and Nadeem, 2016). The blood supply to the eye originates from the internal carotid artery as the ophthalmic artery whose branches include the central retinal artery, the short and long posterior ciliary arteries, and the anterior ciliary arteries. The central retinal artery (CRA) bends many times before reaching the optic disc and it, together with the short posterior ciliary arteries provide blood supply to the retina. The CRA travels in or beside the optic nerve as it enters the sclera at the back of the eye, and from where it then branches out to supply the layers of the inner retina which are closest to the inner limiting membrane/VH. The central retinal vein and vortex veins collect the venous blood (Kiel 2010; Ansari and Nadeem, 2016). The latter veins merge with the inferior and superior ophthalmic veins that drain into the pterygoid venous plexus and the facial vein. The retinal venules and veins merge into the central retinal vein (CRV) which exits the eye with the optic nerve parallel and counter-current to the CRA (Kiel, 2010; Ansari and Nadeem, 2016) (Figure 4A).

The dura, arachnoid and the pial membrane sheaths encase the optic nerve (Anderson, 1973) which is composed of three zones referenced relative to the LC. While collaterals from the choroid and retinal circulation supply the prelaminar zone (i.e., inside the eye relative to the LC), short posterior ciliary and pial arteries supply the laminar zone. Lastly, the post laminar zone is supplied by the pial arteries. Venous drainage occurs via the CRV and the pial vein (Kiel, 2010; Ansari and Nadeem, 2016) (Figure 4A). For the optic nerve vessels, the laminar zone marks the transition from exposure to the IOP to the cerebral fluid pressure (intracranial fluid pressure; ICFP) within the optic nerve sheath.

Much joy and happiness comes to us through our eyes as we appreciate the beauty around us, especially the display mother nature arranges for us. It is no wonder then that eyesight is so precious.

At a macrolevel, vision occurs when light entering the cornea passes through the pupil and is focused by the lens onto the retinal photoreceptors. Here, many complex biochemical and electrical signals are created and processed as neurotransmitters are released at synapses and they activate receptors/ion-channels to relay the information within 0.1–0.2 milliseconds. Thus, as the photoreceptors absorb the light, Na⁺-channels are closed to allow intracellular levels of Na⁺ ions to build-up and the cell membrane potential becomes positive. Now, the Ca²⁺-channels are closed, and inhibitory transmitter levels are decreased which removes the inhibition and the bipolar cells are stimulated/excited creating an actional potential in the RGCs which are sent down their axons to the thalamic brain nuclei. Defects in the membrane repolarization process and locally produced endothelin impairs axonal transport and deprives RGCs of neurotrophin support from the brain (Stokely et al., 2002; Fischer et al., 2019).

The RGCs are separated from the VH by a thin transparent inner limiting membrane. Neurons in the inner two retinal layers exhibit complex receptive fields that allow them to detect small changes in contrast reflecting shadows or edges of objects. The RGCs collect all this array of information, including color, and transmit it to the brain down their axons, which are bundled together to form the optic nerve. The optic nerves from each eye cross over at the optic chiasm as they traverse to the first brain relay station, the lateral geniculate nucleus (LGN) of the thalamus (Liang et al., 2018; Masri et al., 2020). The optic nerve also provides input to the pretectum (that controls pupillary response to light) and to the superior colliculus (SC) which is responsible for moving the eyes in response to ambulatory signals. The information about the image is passed from the thalamus to the visual cortex for final decoding and visual perception (Kravitz et al., 2013). Signal transmission from the retina to the brain occurs in approximately 0.15 s, and information transfer happens at roughly 10 million bits per second and vision occurs.

As described above, most optic nerve projections from the retina traverse to the LGN which sorts retinal signals into parallel streams containing color, structure, motion and contrast. Since the LGN is also structurally layered, its top four parvocellular layers handle color and fine structure, while the bottom two magnocellular layers process contrast and motion. Not surprisingly, the primary visual cortex is also highly organized to receive LGN inputs to maintain fidelity and complexity of the information being sent to it. The LGN magnocellular and parvocellular layer cells send long axons to the ancient part of the brain comprised of the primary visual cortex (PVC) (Masri et al., 2020). Here in the V1 region of the PVC, the cells are arranged in complex ways which permit the visual system to assign the objects being perceived in a spatially precise manner. Specifically, these V1-PVC cells are organized so that a direct mapping of RGCs is essentially and precisely imprinted in the visual cortex on a point-by-point basis in a columnar pattern of connections alternating between the left and right eyes. This allows the V1 area of PVC to position objects perceptually in the horizontal and vertical axes. Rapid comparison of the signal inputs from the two eyes by V1-PVC cells allows perception of depth of vision thereby rendering the images into three-dimensions. As V1 cells sharpen the lines and edges of images, the cells of V2 region of PVC refine the coloration of the object images. Color and form perception in V3 and V4 regions of PVC, inferior temporal lobe recognition of face and object, and motion and spatial awareness in the parietal lobe of the cortex cover the majority of the permutations of visual perception. The above illustrates how intricate and yet efficient and strong the visual system is in so many animals and humans. Such is the wonderous tale of light entering the eye and the miracle of seeing the outside world around us. It is therefore imperative that we do not take sight for granted and do everything possible to preserve and cherish it. By actively researching into the causes of vision loss and finding suitable treatment modalities, we can all try to give the gift of sight to those unfortunate people in our communities who are afflicted with visual impairment.

Focusing on the ANC of the eye will now permit a detailed description and discussion about ocular hypertension (OHT) and glaucoma. This ocular disease is represented by a group of pathological conditions which all culminate in the death of RGCs and eventual loss of much of their axonal connections to the brain structures mentioned above (Weinreb and Khaw, 2004; Quigley, 2011; Weinreb et al., 2014; Jonas et al., 2017; Sharif, 2017; Bhandari et al., 2019). The disease is most often associated with elevated intraocular pressure (IOP), and results in gradual loss of peripheral vision in the early stages, with eventual impact on central vision before irreversible blindness if left undiagnosed and untreated. Sadly >50% of the patients who eventually learn about their glaucoma were unaware of their ocular condition WHO, 2019). Diagnostic imaging includes stereoscopic photos and optical coherence tomography (OCT) of the optic nerve for characteristic structural changes at the retinal nerve fiber layer (RNFL), while visual field testing helps to detect functional changes in vision (Quigley, 2011; Weinreb et al., 2014; Jonas et al., 2017; Sharif, 2017; WHO, 2019; Burton et al., 2021).

Many different forms of glaucoma are known (Zukerman et al., 2020). The most common form is primary open-angle glaucoma (POAG) where the angle between the cornea and the iris is normal but the drainage of the AQH via the TM/SC outflow pathway is slowed down or is blocked, and where the uveoscleral (UVS) outflow pathway is poorly operational and results in elevated intraocular pressure (IOP). In primary closed-angle glaucoma (PACG; Sun et al., 2017; Chan et al., 2019; Wang et al., 2019), the iris is displaced and it obstructs the AQH egress from the ANC to rapidly raise IOP (Gazzard et al., 2003) (Figures 1A,B). Both these forms of glaucoma cause OHT by elevating the IOP which then damages the retina and the optic nerve at the back of the eye. In normotensive glaucoma (NTG; Mallick et al., 2016), which is very common in Japan, the eye pressure is in the normal range (14–21 mmHg) but the patient continues to lose vision due to other deleterious factors (Collaborative Normal-Tension Glaucoma Study Group, 1998a; Collaborative Normal-Tension Glaucoma Study Group, 1998b). In some newborn children the ANC eye pressure is abnormally elevated and this leads to congenital glaucoma.

Secondary glaucomas, as the name implies, occurs due to an ocular insult or due to another medical problem such as ANC inflammation (causing uveitic glaucoma) or due to steroid-induced accumulation of complexed extracellular matrix proteins in the TM that prevent AQH efflux, for example. Neovascular glaucoma occurs when abnormal growth of new blood vessels obstructs the AQH drainage pathways in the ANC and it is usually caused by diabetes or high blood pressure. Pigment dispersion syndrome (pigmentary glaucoma) results from aberrantly released melanin granules from the iris which then occlude the AQH drainage pathways thereby causing OHT. Myopic young white men are prone to pigment dispersion syndrome than other people. The most well known animal model of pigmentary glaucoma is the DBA/2J mouse and its variants (Williams et al., 2013; Cooper et al., 2016; Harder et al., 2020). Lastly, there is exfoliation glaucoma that occurs in some patients who have exfoliation syndrome (Ritch et al., 2003; Anderson et al., 2018), a disorder that causes detachment of cells and other debris from the ANC to clog the TM/SC which blocks AQH fluid from draining, thereby raising the IOP. Recent research shows that exfoliation glaucoma may have a genetic linkage to disease (Aung et al., 2018).

Inflammatory and immunogenic elements produce cardinal signs and symptoms of uveitis, including in uveitic glaucoma (Kwon et al., 2017). Anterior uveitis includes inflammation of the iris and ciliary body within the ANC of the eye and can account for up to 90% of all uveitic episodes reported by patients and healthcare professionals. This disorder can be episodic or chronic in nature. Intermediate uveitis, also known as pars planitis, consists of vitritis—vitreous cavity cell inflammation due to deposition of inflammatory cells in the vitreous. Regardless, combined uveitic disease is the 3^rd largest cause of blindness worldwide, and glaucoma associated with uveitis is a very serious disease requiring immediate attention since the IOP can rise suddenly and to a high magnitude (Kwon et al., 2017).

The elevated IOP caused by uveitis and the IOP-spikes associated within these acute or chronic episodes of intraocular inflammation can cause rapid damage to the optic nerve unless the high IOP is reduced quickly and then maintained at a relatively low level (Siddique et al., 2013). As mentioned above, since systemic inflammation and abnormal immune responses can greatly contribute to uveitic glaucoma, the patient requires attention by both an ophthalmologist and a rheumotologist. Glaucoma surgery is usually required to rapidly reduce the elevated IOP, coupled with immunosuppression and/or treatment with corticosteroids to prevent visual impairment and eventual vision loss. Non-infectious posterior uveitis, which is rarer than the anterior uveal uveitis, can now be treated using intravitreal injection of sirolimus, an immunosuppresant that inhibits the mammalian target of rapmycin (mTOR) (Nguyen et al., 2011). Such treatment modalities may be useful for treating uveitic glaucoma once the initially high IOP has been reduced somewhat. Likewise, in a form of pigmentary glaucoma model (DBA/2J mouse model), it appears that lengthening of the nodes of Ranvier in the optic nerve and redistribution of Na⁺-channel precede axonal transport deficits and eye-brain signaling (Pease et al., 2000; Ito and Di Polo, 2017; Crish and Calkins, 2011; Crish et al., 2010; Crish et al., 2013), features that parallel changes seen early on in multiple sclerosis (MS) axonopathy (Smith et al., 2018; McGrady et al., 2020). These deficits could be abrogated by a week of systemic immunosuppressant therapy with fingolimod, a sphingosine-1-phosphate receptor agonist, a drug that is used in relapsing-remitting multiple sclerosis disease, thus offering another therapeutic approach to combat optic nerve damage due to pigment accumulation in the ANC of the eye that prevents AQH drainage and results in OHT. Mycophenolate, sustained release corticosteroids (Borkar et al., 2017), other pharmacological agents, trabeculectomy and AQH drainage shunts (Kwon et al., 2017), along with new generation biologics (Thomas et al., 2019) provide much hope for patients who succumb to non-infectious posterior uveitis, and uveitis-associated OHT/glaucoma (Kesav et al., 2020).

If we focus on the two chief forms of glaucoma impacted by direct features and events in the ANC of the eye, we can summarize the major risk factors associated with POAG and PACG. Thus, decades of research have concluded that POAG risks factors include: elevated IOP (ocular hypertension, OHT), low intracranial fluid pressure, low retinal perfusion, advanced age, African-Caribbean-Latin American ancestry, family history of glaucoma, thin corneas, myopia, diabetes, high blood pressure and low diastolic pressure. PACG occurs suddenly when the iris is displaced and blocks the AQH drainage pathway causing a rapid elevation of IOP, perhaps as high as 70 mmHg. Its clinical manifestations include nausea, blurred vision, ocular pain, cloudy corneas and halos around lights. The risk factors linked to PACG include: advanced age, shallow ANC angles, Asian-Eskimo ancestry, family history of PACG, hyperopia and female gender.

Several clinical trials conducted in the late 1900s early 2000s revealed that lowering of the IOP is highly beneficial and directly slows down and can prevent glaucoma progression in most forms of glaucoma (GLT, 1995; Collaborative Normal-Tension Glaucoma Study Group, 1998a; Collaborative Normal-Tension Glaucoma Study Group, 1998b; AGIS, 2001; Lichter et al., 2001; Heijl et al., 2002; Kass et al., 2002; Gordon et al., 2002; Leske et al., 2007). Consequently, clinical medicine has focused on lowering and controlling IOP to help preserve sight of patients with glaucoma and these aspects will be discussed ahead. However, glaucoma etiology is complex and a number of pathophysiological events converge to induce RGC death and RGC axonal loss: elevated IOP (OHT), retinal ischemia, oxidative stress in the TM/SC and retina, neurotrophin and energy deprivation, and toxicity due to locally elevated levels of glutamate, endothelin, cytokines, nitric oxide (and perhaps carbon monoxide), and proteases (Bucolo and Drago, 2011; Fu and Sretavan, 2012; Evangelho et al., 2019) (Figure 4B). These aspects will be discussed below.

Since OHT due to excess AQH accumulation in the ANC is the root cause of POAG, this requires a detailed appraisal. As mentioned earlier, the CE (mainly non-pigmented ciliary epithelial cells [NPCECs]) of the CP within the CB generate the clear AQH fluid by passive diffusion, ultrafiltration and secretion, with the latter being the predominant event (reviewed by Civan and Macknight, 2004). Since the ANC is avascular, the AQH flowing through the ANC has very important functions that provide a nurturing (provision of O₂, glucose and growth factors) and a stable environment (achieved by balancing nutrient provision and removal of waste products) for the cells/tissue lining the ANC (Kaufman, 2020). As shown in Figure 1B, POAG begins most frequently with an imbalance of AQH production by the CE and its drainage from the ANC of the eye via the conventional (TM/SC) and unconventional (uveoscleral, UVS) outflow pathways (Acott et al., 2020; Wang et al., 2020). TM outflow of AQH accounts for 70–90% and UVS outflow for 10–30% of total AQH drainage from the ANC. Research has shown that TM/SC-mediated AQH efflux is pressure-dependent, and that IOP is mainly due to the back-pressure generated due to blockage of this system (Acott et al., 2020; Wang et al., 2020). Since AQH ultimately exits the eye via collector channels of the TM/SC system and empties into the intrascleral venous plexus, deep-scleral plexus, and into episcleral vessels and thus the veinous circulation, the total resistance within this collective group of channels/plexi/blood vessels causes the elevation of IOP (Thomson et al., 2014; Kaufman, 2020). A closer look at the TM/SC structures (Abu-Hassan et al., 2014; Figures 5A,B) illustrates that several different cell-types coexist and that the majority of the resistance to AQH outflow from the conventional system occurs at the innermost uveal region, the middle corneal scleral region and the outermost juxtacanalicular tissue which essentially becomes obstructed during development of OHT/POAG.

Under normal circumstances, especially in young eyes, the TM cells are abundant and actively phagocytose cellular debris and other accumulating substances such as collagen and fibronectin within the extracellular matrix (ECM; De Groef et al., 2016) to keep the AQH flow constant as the TM filters the AQH in a relatively passive manner. The SC endothelial cells appear to actively transport AQH by generating pores across their surfaces, and perhaps via aquaporins, thereby helping AQH exit the ANC (Acott et al., 2020; Wang et al., 2020). During aging and due to other local deleterious events and processes and ECM accumulation, the number of TM cells decreases as does their remaining phagocytic/autophagic activity (Alvarado et al., 1981; Alvarado et al., 1984; Grierson and Howes, 1987; Matsumoto and Johnson, 1997; Sherwood and Richardson, 1988). Thus, in normal eyes, AQH generation is maintained at a constant rate and a stable IOP is achieved by changes in regulation of AQH outflow/alteration of resistance in the outflow pathway. Normal average IOP is 15 mmHg and >90% of human subject have IOPs between 10 and 21 mmHg. However, in POAG, it is the AQH outflow pathway that is compromised (due to ECM/cellular debris accumulation due to increased transforming growth factor-β (TGF-β) (Tripathi et al., 1994; Hasenbach et al., 2016; Rao and Stubbs, 2021), aberrant cross-linking of ECM which stiffens the TM cells (Yemanyi et al., 2020), possible defects in the SC pore/aquaporin system (Wang et al., 2020), increased resistance at the SC due to angiopoietin/Tie-2 pathway defects (Thomson et al., 2014; Thomson et al., 2019; Bernier-Latmani and Petrova, 2017) and the IOP continues to rise due to the constant addition of AQH to the ANC from the CB. It is estimated that the veinous blood vessel component of the AQH drainage system accounts for <25–50% of total outflow resistance whereas the juxtacanalicular region of the TM/SC area is the major contributor providing >50–75% of total outflow resistance in POAG. However, the resistive impact of distal AQH drainage vessels on IOP probably increases due to their constriction under pathological conditions due to bloodborne and/or AQH-borne vasoconstrictor mediators such as reactive O₂ species, thromboxane and endothelin. Even though TM/SC cells, which have endothelial cell morphology and physiology, appear to show an upregulated nitric oxide (NO) synthase and release the vasodilator gaseous transmitter NO under pressure to compensate for the increased resistance (Schneemann et al., 2003), these measures may be insufficient to decrease the IOP and these elements may overall be substantially down-regulated in glaucomatous ANC. Furthermore, there may be a deficiency of MMPs being released locally around the TM/SC resulting in aberrantly diminished tissue remodeling and ECM accumulation (De Groef et al., 2016). Also, as the IOP changes during the day and night, IOP spikes and general fluctuations are more common in glaucomatous eyes than in normotensive eyes (Bengtsson et al., 2007; Caprioli and Coleman, 2008; Siddique et al., 2013; Kim and Caprioli, 2018). It is believed that such irregular eye pressure changes are highly detrimental to the visual system, and since they occur mostly at night, it is imperative that these IOP fluctuations (Jasien et al., 2019) are diminished as much as possible in order to de-stress the vulnerable elements of the retina/optic nerve at the back of the eye in the quest to protect the RGCs and their axons from this glaucomatous optic neuropathy (GON) (Sharif, 2017, 2018a,b) (Figure 6).

As noted above, even though the AQH dynamics imbalance in the anterior segment of the eye is responsible for onset of POAG/PACG, the whole visual system saccumbs to the overall disease. Again, in order to better understand how glaucomatous damage occurs, it is important to be aware of the anatomy of the retina and the optic nerve.

The retina is a highly specialized structure that possesses a diverse population of cells ranging from photoreceptors, interneurons, retinal ganglion cells (RGC; both pigmented and non-pigmented; Cruz-Martín et al., 2014; Sanes and Masland, 2015; Baden et al., 2016; Russo et al., 2016; Detwiler, 2018) and contain three sub-types of glial cells which serve a diverse range of functions. Glial cells were thought to be mainly structural components of the retina (Figures 2A,B). However, cell profiling has revealed distinct physiological and morphological differences amongst them. Indeed, microglia, Muller cells (Bringmann et al., 2006) and astrocytes are not passive bystanders in the etiology of GON. Astrocytes are star-shaped and appear similar to microglia though they are significantly larger than the latter cells. Astrocytes are fairly stable in their quiscient state and are homeostatic in their role, maintaining blood-brain/blood-retina barriers, regulating blood flow, recycling neurotransmitters, and maintaining synaptic connections (Liu and Neufeld, 2003; Hernandez et al., 2008; Williams et al., 2017c). Astrocytes perform these roles through contacts with capillaries, neuronal cell bodies and RGC axonal bundles in the rear of the retina and at the optic nerve level. Sensing trauma or injury as from elevated IOP-induced deformation of the ONH, astrocytes release glial fibrillary acid protein (GFAP) (Inman and Horner, 2007). This results in the formation of a scar that attempts to isolate the site of injury. Additionally, the astrocytes chronically release various cytokines which upregulate different genes and proteins needed in summoning help and repairing the surrounding tissue (Liu and Neufeld, 2003; Hernandez et al., 2008; Williams et al., 2017c) (Figure 6).

Muller cells appear to be the true benefactor cells of the retina and they are exclusive to the retina, being quite populous (Bringmann et al., 2006). Their cell bodies are located in the inner nuclear layer, but they extend their processes throughout the retinal layers to ensure contact with the majority of the resident cell-types they serve (Figure 2B). Under normal circumstances, Muller glia retain their morphology and physiology helping to maintain a stable local cellular environment in the retina in terms of provision of nutrients, maintaining ionic strength and pH (Reichenbach and Bringmann, 2013; Goldman, 2014; Lust and Wittbrodt, 2018). These cells also provide neurotrophins to neurons and phagocytose local debris. They also ingest and recycle unused local neurotransmitters and toxic waste, and they support and maintain rods, cones and RPE cells. As with astrocytes, Muller glia change their morphology and functions in the wake of deleterious conditions within the retina such as hypoxia or inflammation. They then begin releasing GFAP and upregulate Toll-like-receptors (TLRs), and start secreting cytokines and chemokines thereby signaling trauma to the eye surveillance system(s) (Reichenbach and Bringmann, 2013; Goldman, 2014; Lust and Wittbrodt, 2018).

Microglia represent the CNS-related macrophages that are quiet surveyors of the local extracellular environment (Chen et al., 2002) (Figure 4B). Beginning as monocytes, they enter the CNS during early development and differentiate into resident microglia that spread out throughout the brain, spinal cord and the retina/optic nerve. They check on the health of the neighboring cells as they move around. In the visual system, these quiscent microglia become activated when they sense ischemia, pathogenic infection, aberrant cell death in the retina, at the ONH and/or the optic nerve (Neufeld, 1999), and along with disturbances of the synaptic connections within the brain relay nuclei (Lam et al., 2009; Ebneter et al., 2010). The activated microglia (Yuan and Neufeld, 2000; Yuan and Neufeld, 2001) now release nitric oxide (NO) (Neufeld, et al., 1997), cytokines and chemokines which cause local vasodilation and allow monocytes to infiltrate and transform into more reactive glia thereby amplifying the inflammatory/immunological response to injury/trauma in the visual axis (Neufeld, 1999; Chen et al., 2002; Bosco et al., 2008; Bosco et al., 2011; Wei et al., 2019; Rashid et al., 2019) (Figure 4B). TLRs, major histocompatibility complex (MHC) recognition molecules and complement activation also ensues (Borucki et al., 2020).

Oligodendrocytes and Schwann cells represent additional neuroglia relevant to the retinal-brain axis. The function of the Schwann cells is to produce layers of insulating myelin that encases each RGC axon, and provision of nutritients and structural support to the Schwann cells and the axons befalls on the oligodendrocytes (Fitzner et al., 2011). In this manner, each RGC axon becomes myelinated, except in a small area directly adjacent to the back of the eyeball where the axons exit the rear of the eyeball. Bundling of the RGC axons results in the formation of the optic nerve that crosses over at the optic chiasm and travels on to the brain thalamic relay stations. This unmylenated area of the optic nerve, together with the ONH at the LC, and also the spaces in between the thicker myelin segments along the optic nerve (nodes of Ranvier) are the most delicate components within the visual system and contribute to optic neuropathy/optic neuritis (Bastakis et al., 2019). They are susceptible to injury from intrinsic and extrinsic damaging factors and insults, including the stress and strain brought on by the mechanical forces of elevated IOP in POAG/PACG, especially IOP spikes (Siddique et al., 2013; Pan et al., 2014; Tan et al., 2018; Sherman and Cafiero-Chin, 2019). Lastly, other important retinal cells include amacrine and horizontal cells which modulate communication between photoreceptor- and the major neural cell-types, including bipolar-cells and RGCs of which many types exist (Ou et al., 2016; Vecino et al., 2016; Grünert and Martin, 2020). Early-stage OHT alters the electrical transmission of signals from the RGS to the thalamus (Bhandari et al., 2019), and this deficit increases over time (Trivedi et al., 2019).

Globally, the predominant form of glaucoma is POAG (currently ∼54 million suffering) which represents ∼75% of all forms of glaucoma. Primary closed-angle glaucoma (PACG) has the second highest occurrence (∼23 million patients). Unfortunately, Asian and African nations have the highest number of POAG and PCG patients, greatly outnumbering those afflicted with these glaucoma forms in Europe and North America. Close to 5 million patients are projected to have POAG in North America by 2040.

Akin to most chronic diseases, POAG has a age-related occurrence with patients being diagnosed with this disease almost exponentially between age 40 to age 80 in almost all geographical locations. POAG is asymptomatic, the patient feels no pain and is oblivious to the disease development since other warning signs are all absent. This “silent thief of sight” makes its detection and diagnosis difficult, and it is estimated that 50% of future POAG patients remain undiagnosed. Additionally, during early stages of POAG/PCG/NTG, the patient does not notice much change in the vision and the brain compensates (Bham et al., 2020) for losses of RGCs that have already occurred and are happening (Weinreb et al., 2014; Jonas et al., 2017; Sharif, 2017; Sharif, N. A. 2018). However, as the conditions progress, a tipping-point is reached where the patient notices loss of peripheral (lateral) vision and objects observed appear incomplete and/or become blurry and/or infrequent double vision occurs (Crabb, 2016). Sadly, this signals the demise of >400k RGCs and their axons in the affected eye of the patient (Calkins and Horner, 2012). Ophthalmic examination of the POAG/PCG afflicted patient, including optical coherence tomography (OCT) (Quigley et al., 1989) and visual field tests (Harwerth and Quigley, 2006; Sasaoka et al., 2008; Sponsel et al., 2014), reveal that significant damage has occurred at the ONH (Burgoyne et al., 2005) causing increased optic disc cupping (Downs et al., 2011), that the retinal nerve fiber layer (RNFL) has thinned (due to RGC axon loss) (Harwerth and Quigley, 2006; Soto et al., 2011; Yu et al., 2015; Tu et al., 2019); and scotomas (dead zones) developed in the visual field (Harwerth and Quigley, 2006; Sasaoka et al., 2008; Sponsel et al., 2014). The level of IOP increase is well correlated with optic nerve damage (Chauhan and Drance, 1992; Guo et al., 2005; Yohannan and Boland, 2017; Torres and Hatanaka, 2019) and decrease in retinal function (Parisi, 2003; Kasi et al., 2019; Torres and Hatanaka, 2019; Turkey et al., 2019). The patient’s vision loss begins to accelerate and immediate treatment is required to retard further visual impairment (Karaca et al., 2020). Initially the patient is prescribed topical ocular eyedrops containing medicine (usually a prostaglandin analog (Hellberg et al., 2001, 2002) but it may be beta-blocker like timolol, betaxolol or levobetaxolol to lower the IOP in the ANC of the eye (Weinreb et al., 2014). If the patient is unresponsive or the IOP is poorly controlled, the physician will switch patient to another type of IOP-lowering drug and may resort to adjunctive therapies to achieve the desired IOP reduction using fixed-dose combination products (Hollo et al., 2014). Lasering of the TM may be necessary to create holes for the AQH to egress from the blocked TM system (Weinreb et al., 2014; Jonas et al., 2017). Surgery may be necessary to implant an AQH shunt (Sehi et al., 2010; Pahlitzsch et al., 2017; Sadruddin et al., 2019) that drains the AQH from the ANC of the eye. Finally, if all fails the patient will undergo direct physical surgery to literally create a hole and a flap (bleb) to drain the fluid from the ANC (Bhandari et al., 1997; Vinod and Gedde, 2021). These and newer treatment modalities to treat POAG will be discussed later once the etiological aspects of POAG and GON are described in more detail.

So, even though GON eventually results from the demise of the RGCs and the elimination of the RGC-axonal connections to the brain, in most instances the disease really begins in the ANC. How the afore-mentioned cascade of events operates in OHT/POAG/NTG can be described as follows based on our current understanding. The aging processes and cellular dysfunctions brought on by mitochondrial defects (Wentz-Hunter and Ueda, 2002; He et al., 2008; Izzotti et al., 2011) causing energy deficiency within TM cells lead to their senescence. Thus, a reduced TM cell population is left to perform the AQH filtration and cleansing (Alvarado et al., 1981; Alvarado et al., 1984; Grierson and Howes, 1987; Caballero et al., 2004; Babizhayev and Yegorov, 2011). Additionally, the phagocytic (Sherwood and Richardson, 1988; Matsumoto and Johnson, 1997; Zhou et al., 1999) and autophagic activities (Munemasa and Kitaoka, 2015; Porter et al., 2015; Stothert et al., 2016; Hirt and Liton, 2017) of the remaining TM cells is also substantially reduced resulting in a build-up of ECM (Teixeira et al., 2015; Shen et al., 2020) and other cellular debris within the corneoiridial angle of the ANC of the eye (Gabelt et al., 2005). The reduced perfusion of the ANC (Johnson, 1996) causes an accumulation of cellular waste products and oxidized proteins like lipofucin, ceramide and other lipid metabolites (Aljohani et al., 2013), reactive O₂ species (ROS; He et al., 2008), elevated levels of homocysteine and reduced anti-oxidants (You et al., 2018), elevated endothelin (Zhao et al., 2020), IL-6 and TGF-β1 in the AQH (Liton et al., 2009; Perera et al., 2016). Furthermore, miR-29b is down-regulated which increases ECM secretion (Luna et al., 2012), and additional ATP is screted (Li et al., 2012), along with stimulation of activation factor-4 all of which increase endoplasmic reticulum stress in the TM cells (He et al., 2008; Izzotti et al., 2011; Kasetti et al., 2017) causing TM cell death (Kasetti et al., 2020; Zhao et al., 2020; Ying et al., 2021). Additionally, damaged or dying TM cells release numerous deleterious compounds (e.g., nestin, A-kinase anchor protein, actin-related protein 2/3 complex, numerous miRNAs that negatively impact the ANC) which increase cell aging, TM cell apoptosis and ECM deposition (Izzotti et al., 2011) thus exacerbating the issues in the ANC. Additionally, as mentioned earlier, increased resistance at the SC (Stamer et al., 2015; Wang et al., 2020) and distal veinous drainage plexi resulting from upregulation of the angiopoietin/Tie-2 pathway (Thomson et al., 2014; Thomson et al., 2019; Bernier-Latmani and Petrova, 2017) also contributes to the overall OHT and eventual RGC death (Figure 6).

In certain cases, topical ocular dexamethasome treatment for ocular surface or ANC inflammation causes protein complexing (Yemanyi et al., 2020) and formation of cross-linked actin networks (Bermudez et al., 2017) and results in steroid-induced glaucoma. Moreover, there’s evidence that accumulation of mutant myocilin (Kasetti et al., 2016; Kasetti et al., 2021) and amyloid proteins (Orwig et al., 2012), and IL-6-induced release of TGF-β1, which promotes ECM deposition in the ANC (Liton et al., 2009) and causes secretion of endothelin (Von Zee et al., 2012), causes IOP elevation. Glaucomatous mutant myocilin suppresses autophay and activates the IL-1/NFκB inflammatory cascade within TM cells and some of them die (Itakura et al., 2015; Lynch et al., 2018). All these processes coupled with other deleterious events in the TM/SC reduces the flexibility of these structures (Borrás and Comes, 2009; Liu et al., 2016; Borrás, 2017; Wang et al., 2017) thereby reducing their strength and capacity to filter and drain the AQH from the ANC. The compromized mechanoelasticity (Overby et al., 2014; Pang, 2021) of the TM/SC structures causes adverse gene expression (Vittal et al., 2005) and further restricts AQH outflow drainage (Acott et al., 2020). The resultant elevation of IOP and IOP spikes (Asrani et al., 2000), as the eye tries to regain homeostasis, starts to expand the eyeball as the fluidic pressure distorts the ocular structural components throughout the eye, beginning at the cornea and being transmitted to the rear of the eyeball (Gottanka et al., 1997). This mechanical distortion severely stresses the fragile ONH (Hernandez et al., 1989; Burgoyne et al., 2005; Downs et al., 2011; Xu et al., 2014; Park et al., 2015; Quillen et al., 2020) and the LC (Lee et al., 2013; Daguman and Delfin, 2018) at the back of the eye where the optic nerve leaves the eyeball. Scleral stiffining at the level of the ONH/LC (Coudrillier et al., 2016) prevents the latter tissues from being able to absorb the pressure created by increased IOP and local inflammation ensues. As the eye seeks to normalize the ocular physiology, IOP spikes (Asrani et al., 2000) are generated and these are even worse than the initial OHT, causing constriction of the optic nerve axons at the LC (Hollander et al., 1995) and thus more damage to the ONH/LC tissues (Coudrillier et al., 2016). Mechanical pressure-sensitive (MP-S) cell-types such as LC cells, ONH astrocytes and peripapillary scleral cells detect the increased IOP, and perhaps also the lower intracranial fluid pressure behind the eyeball (ICFP; Berdahl et al., 2008; Wostyn et al., 2015; Jóhannesson et al., 2018; Price et al., 2020). Stretch-linked ion-channels (e.g., TRPV-1 and TRPV-4) are also aberrantly activated in these stressed cells (Ryskamp et al., 2016). Either due to presence of amyloid and/or tau-derived pore-forming peptides (Last and Miranker, 2013; Camilleri et al., 2020; Farrugia et al., 2020; Vassallo, 2021), or high levels of TGF-β2, or independent of these, mitochondral dysfunction ensues resulting in oxidative stress and excess intracellular Ca²⁺-accumulation stresses the LC cells (McElnea et al., 2011) and the local microglia are activated (Bosco et al., 2008; Bosco et al., 2011; Gauthier and Liu, 2016). The latter release MMPS, proteases, and other inflammatory cytokines and chemokines (Ha et al., 2015) which then cause remodelling of the tissue thereby weakening the LC structure. These events cause the optic nerve and the associated blood vessels to bend resulting in vasocontriction/ischemia and reducing axonal flow which eventually kills the RGCs (Martin et al., 2006). As mentioned above, similar changes occur in the ANC where TM, and perhaps the juxtacanicular cells adjoined to the SC cells, become oxidatively stressed under pressure and due to accumulated TGF-β2 (Tripathi et al., 1994; Rao and Stubbs, 2021) and release detrimental factors such as reactive oxygen molecules, endothelin, and copious amounts of NO (and perhaps carbon monoxide [CO]; Bucolo and Drago, 2011) that have toxic effects on the surrounding cells, probably damaging them or killing some of them. During such distress the compromised TM/SC cells also release more TGF-β1/2 that remodels the TM and excerbates the situation causing further increases in resistance to AQH outflow, contributing to IOP elevation in the ANC. This vicious cycle continues at the front and back of the eye and any potential regeneration of axons is suppressed by intracellular kinases and transcription factors (e.g., Chintala et al., 2015; Apara et al., 2017).

The pressure-induced TRPV-channel activation within RGCs and their axons at the ONH begins disrupting their ionic balance which rapidly drains their energy and leakage of cellular constituents such as oxidized proteins, ATP, glutamate, and other substances into the extracellular space ensues. The oxidative stress generates and causes accumulation of extracellular lipofusin, an intralysosomal, non-degradable auto-fluorescent macromolecule which reduces autophagy and clearance of the debris (McElnea et al., 2014). ONH microglia become activated and start releasing pro-inflammatory cytokines, matrix metalloproteases (MMPs)/proteases, and vasodilatory and other toxic substances such as NO (Cho et al., 2011). The upregulated cytokines (e.g., interleukin-1 [IL-1], IL-6, IL-8, tumor neurosis factor [TNFα]) and NO/CO at the ONH induce vasodilation and edema at the ONH/LC region and within the optic nerve (Figures 3A,B). Attracted by the cytokines and chemokines, monocytes and leukocytes surge into the retina and optic nerve, further amplifying the inflammatory cascade with resultant damage. Additionally, the LC tissue is weakend by the MMP-induced degradation of the supporting ECM. The weight of the RGC axons and the associated retinal blood vessels bends and further distorts the structures at the ONH/LC which leads to ischemia/hypoxia and oxidative stress (Neufeld et al., 2002; Nguyen et al., 2011), local release of endothelin causing further vasoconstriction thereby reducing retinal perfusion, and the vicious cycle continues unabated (Prassana et al., 2011). Adding insult to injury, the increased tortoucity of the RGC axons causes disruption of the axonal transport of growth factors and mitochondria from the brain back to the RGC bodies (Dengler-Crish et al., 2014; Fahy et al., 2016). The inflammatory and immunologic insults on the optic nerve components induces a dislocation of the RGC axonal terminal connections to the thalamic LGN and pretectal regions of the brain (Dai et al., 2012; Ghaffarieh and Levin, 2012; Fujishiro et al., 2020) with ensuing retrograde atrophy of the RGC axons and neuronal loss in the LGN (Yucel et al., 2000; Yucel et al., 2001; Gupta et al., 2007). Dropout of RGC axons thins the optic nerve and the RNFL at the ONH. This cascade of events negatively impacts the neurones in the brain and the retina. The decline in retinal-LGN-superior collicular connectivity results in C1q/C3, MAC and GFAP accumulation in the thalamus and visual cortex where the neurones also begin to die. RGCs whose axons had been disconnected from the LGN/pretectum spiral into apoptotic demise as their energy and trophic support diminishes and then stops. The dying RGCs empty their intracellular contents (e.g., ATP, glutamate and DAMPs) into the extracellular space. The microglia and astroctyes detect these distress signals and begin the cellular clearance mechanisms using proteases to prune RGC dendrites and activate the phagocytic/autophagic removal of dead retinal neurones. Glia fill the spaces where the RGCs and interneurones resided and patient’s vision in that region is lost. Cerebral neuronal plasticity compensates for such early losses of RGCs (Bham et al., 2020). Even though these events occur over many years, the unrelenting chronic retinal and brain inflammation/immune response cascades rob the OHT/POAG/PACG patient of additional peripheral vision as more and more RGCs die. With only 600k RGCs left of the original million, the pateint now notices significant visual disturbances amounting to diminished visual acuity, contrast sensitivity, occasional double-vision and a narrower visual field. This is a critical point in the patient’s journey living with OHT/POAG/PACG where diagnosis and treatment initiation are paramount in order to preserve eyesight in the affected eye. AQH drainage from the ANC of the eye is now urgently needed at this stage by pharmaceutical and/or surgical intervention.

Acute inflammation signals the need to initiate damage limitation and beginning of the protective remedial process once the body experiences stress, trauma or injury. However, the local and pan-immune system is activated when a chronic microbial infection and/or inflammation sets in. Numerous cell-types mediate the inflammatory and immunogenic response during the initial phase and progression of the infection(s). Historically, bacterial and viral infections led to the development of the mammalian immune system (Rowan and Taylor, 2018; Flemming, 2018; Tang et al., 2020). Subsequent adaptation of this system has permitted the sensing of cellular stress and the clearance of dead or dying cells in order recycle cellular components to conserve energy, aid neurogensis and to epigenetically increase survival. Consequently, the host defense mechanism became responsiveness to structural characteristics found in pathogens known as pathogen-associated molecular patterns (PAMPs) which microbes express (Rowan and Taylor, 2018; Flemming, 2018; Tang et al., 2020). Astrocytes and microglia possess such signal responsive mechanisms, and they also detect damage-associated molecular patterns (DAMPs, e.g., heat-shock proteins [HSPs], uric acid, αβ-crystallin and double-stranded DNA) which stressed and/or injured cells release to alert the surveillance system (Howell et al., 2007; Soto and Howell, 2014). Thus, pattern recognition receptors (PRRs; e.g., TLRs; Luo et al., 2010; Poyomtip, 2019) on microglia and astrocytes recognize PAMPs and DAMPs and initiate and mediate the inflammatory response within the stressed tissue (Zhou et al., 2005; Vernazza et al., 2020; Parsadaniantz et al., 2020) (Figure 4B) Upon activation, TLRs signal transduction initiates recruiting and stimulation of mitogen-activated protein kinase (MAPK) and kinase IκB (Luo et al., 2010; Poyomtip, 2019) which then help elaborate numerous transcription factors (e.g., NK-κ-B, AP-1, interferon-regulator factor). The latter encode and cause the production and release of inflammatory cytokines and chemokines (Yuan and Neufeld, 2000; Tezel et al., 2010; Ha et al., 2015; Wilson et al., 2015). Mannose receptor (activated by C-type lectin), purinergic receptors (e.g., P2XR7; activated by ATP released during RGC stress/lysis) (Resta et al., 2007; Hu et al., 2010; Sakamoto et al., 2015), the multiprotein unit complexes called the inflammasome, and scavenger receptor (that eliminates leaked lipids and oxidized proteins) are other important PRRs (Howell et al., 2007; Soto and Howell, 2014). The latter collectively enhance inflammation via caspase activation (and thus production of inflammatory cytokines after inflammasome activation by ATP/TLR-4/amyloid-derived peptides) (Chi et al., 2014; Yerramothu et al., 2018). These events, along with activation by mannose residues and antigen-antibody complexes, trigger the complement cascade (Duce et al., 2006; Stasi et al., 2006), the most ancient elemental aspect of the innate immunity system. C1q and C3 components of the complement system, together with the deposition of the membrane attack complex (MAC), destroy the vulnerable cells in the injured/traumatized tissue through cell lysis (Stasi et al., 2006; Howell et al., 2011; Howell et al., 2013; Howell et al., 2014; Williams et al., 2016) (Figure 4B). Activated microglial and astrocytic-derived cytokines and chemokines chemotactically recruit monocytes and leukocytes from the dilated vasculature to the site of injury, and the inflammatory response is amplified further (Yuan and Neufeld, 2000; Tezel, 2011; Tezel, 2013; Tezel et al., 2010; Ha et al., 2015; Wilson et al., 2015). Defects in the cellular debris clearance mechanism(s), autophagy, exacerbate the situation and probably cause further cellular demise (Chintala 2006; Deng et al., 2013; Lin and Kuang, 2014; Munemasa and Kitaoka, 2015; Adornetto et al., 2020). Stress and hypoxia-induced glutamate release and newly generated reactive oxygen species contribute further to the neurotoxicity within the retina and ONH area (Ju et al., 2008; Januschowski et al., 2015).

The classic debate of which happens first, failed function or failed structure is a complex one. Regardless, structural changes within the optic nerve and retina precede the functional visual deficits resulting from chronic OHT, POAG/PACG and NTG. Temporally, the lag period between onset of OHT and glaucoma diagnosis contributes heavily to visual deficits, and thus preservation of vision requires the earliest possible diagnosis of these asymptomatic diseases. The strong correlation between structural damage throughout the visual system and eyesight deficits of animals and humans with glaucoma has been confirmed by numerous investigators (Quigley et al., 1989; Burgoyne et al., 2005; Weinreb and Lindsey, 2005; Harwerth and Quigley, 2006; Sasaoka et al., 2008; Downs et al., 2011; Struebing and Geisert, 2015; Torres and Hatanaka, 2019). Similarly, many studies have demonstrated a progessive optic nerve damage due to increasing IOP in OHT animal eyes (e.g., Figure 7A), and indeed several clinical trials have demonstrated that IOP reduction delays glaucoma progression (reviewed in Weinreb et al., 2014) (Figure 7B). The multiplicity of factors and events mediating the pathogenesis of OHT/POAG/PACG/NTG appear to be reproduced to a large extent in animal models of these ocular disorders as described above.

One intriguing possibility that has some experimental support is that GON has an autoimmunogenic component in addition to the IOP-induced component (Wax and Tezel, 2009; Tezel, 2011; Tezel, 2013; Wax 2011; Geyer and Levo, 2020). Thus, significantly altered antibody directed against ocular cells/tissues were detected in serum, AQH and retinal samples of glaucoma patients, observations that were reproduced in IOP-independent animal models of glaucoma. As IgG autoantibodies switch-on the complement system, C1q/C3 and MAC complexes were found in retinas of OHT rodents, OHT primates in glaucoma patients. Similarly, anti-heat-shock-protein (HSPs) antibodies to HSP-27 and HSP-60 were found in sera of glaucoma and NTG patients (Wax, 2011; Grus et al., 2006; Grus et al., 2008). Additionally, high circulating levels of antibodies to glutathione-S-transferase, gamma-enolase and alpha-fodrin were also detected in these patients (Kremmer et al., 2001; Joachim et al., 2005; Joachim et al., 2007; Joachim et al., 2008; Joachim et al., 2012; Joachim et al., 2014; Grus et al., 2006; Grus et al., 2008). Even though the latter autoantibodies are pathogenic, others like beta-crystallin and vimentin are protective, and these appear to be down-regulated in GON. Antibodies to myelin basic protein and IgG-antibodies were found in the retinas of POAG, NTG and pseudoexfoliation glaucoma patients suggesting that a generalized autoimmune response against visual cell/tissue components can occur (Joachim et al., 2007a,b; Grus et al., 2008; Hammam et al., 2008). Clearly, much more research is needed to find and confirm such etiological aspects of the immune response in OHT and various forms of glaucoma (Gramlich et al., 2013; Joachim et al., 2014; Gramlich et al., 2016; Joachim et al., 2013; Skonieczna and Grabska-Liberek, 2014; Beutgen et al., 2019). Nevertheless, it is clear that age-related and/or pathological events occurring at the ANC-level (i.e., OHT) and at the LC/ONH-level cause progressive loss of RGCs and their axons, resulting in thinning of the RNFL and reduced connectivity to the brain, which leads to visual impairment and can cause blindness unless treatment(s) are started for the patient (Figure 8).

Several pioneering clinical trials in the early 2000s and animal models of OHT/glaucoma have provided compelling evidence that elevated IOP/OHT is a principle root cause of several forms of glaucoma. Consequently, a multipronged approach has been adopted to address this aspect of the disease. Thus, reducing IOP with ocular hypotensive drugs, removal of excess AQH from the ANC of the eye with microshunts and/or surgically promoting AQH drainage from the ANC of the eye of OHT/POAG/NTG patients slows down disease progression and preserve sight by reducing the death of RGCs and their axons (reviewed in Weinreb et al., 2014; Zhang et al., 2019).

In order to provide a perspective of the drug discovery processes involved in finding suitable ocular hypotensive drugs a brief outline of the in vitro assays and in vivo animal models used in such endeavors will be provided next.

Even though NTG patients have IOPs in the normal range (16–21 mmHg), they continue to experience vision loss. Also, it is estimated that 30–80% of OHT/glaucoma patients’ disease continues to progress despite adequate IOP reduction. Conversely, many humans have abnormally elevated IOPs but they do not progress to GON and their vision is unaffected by the raised IOP. These observations strongly suggest that either IOP-independent events and pathologies underlie the development of GON in the NTG patients or that their visual system components are much more sensitive to the IOP levels which are considered “normal”. The same may apply to other patients with other forms of glaucoma. Indeed, accumulating evidence indicates that OHT alone is not the sole contributor to glaucomatous damage and the resultant visual impairment as discussed earlier. OHT and other damaging events ongoing simultaneously probably conspire to cause visual impairment in NTG/POAG/PACG patients. Thus, IOP-independent mechanisms involving retinal vascular dysfunctions (Flammer and Orgul, 1998; Broadway and Drance, 1998; Cull et al., 2015; Pasquale, 2016) and the accompanying oxidative stress (Tezel, 2006; Nickells et al., 2012; Peters et al., 2015) and RGC/axonal energy depletion due to mitochondrial defects (Osborne, 2010; Osborne et al., 2016; Li et al., 2015; Chaphalkar et al., 2020) appear to be culprits. As discussed in the earlier section above, local inflammation at the ONH/LC due to release of injurous cytokines/chemokines/proteases (Albalawi et al., 2017) and at the RGC dendritic/soma sites are responsible for destroying the components of the optic nerve and the RGCs and other retinal interneurons. Excessive release of cellular ATP from dying cells leads to further damage to neighboring cells through activation of the purinergic receptors on RGCs and other retinal neurons through the generation of inflammasomes (Romano et al., 2020). Furthermore, recent research has shown that retrograde axonal flow of mitochondria and neurotrophins (Iwabe et al., 2007) from the brain to the RGC somas becomes defective with OHT (Gaasterland et al., 1978; Radius and Anderson, 1981; Pease et al., 2000; Band et al., 2009; Crish et al., 2013; Ito and Di Polo, 2017) and these changes negatively impact the overall RGC and optic nerve health. Additionally, defects in the generation and maintenance of sufficient levels of intracellular ATP by mitochondria and/or deficits in nicotinamide adenine dinucleotide (NAD⁺) or its congeners, and/or abnormal mitochondrial metabolism/catabolism within retinal neurons of patients with NTG/POAG may heavily contribute to their GON and visual deficits (Thomas et al., 2000; Barron et al., 2004; Bessero and Clarke, 2010). Hence, direct protection of RGCs, their axons and the ONH architecture from such IOP-dependent and IOP-independent insults is necessary in addition to further reducing the patients’ IOPs.

Significant progress has been made in the discovery and characterization of preclinically effective neuroprotectants/cytoprotectants at least from in vitro assay systems using a variety of animal and human isolated retinal, LC and ONH cells subjected to various chemical and mechanical stressors (see He et al., 2018; Sharif, 2018a; Sharif, 2018b; Sharif, 2020a; Sharif, 2020b for reviews). Additional supportive data for the neuroprotective activity of numerous classess of drugs and treatment modalities have been gathered using various animal models of GON (Sharif, 2018a; Sharif, 2018b). The burgeoning list of cyto-/neuro-protective agents include anti-oxidants (Aksar et al., 2015), B-vitamin supplements and other nutriceuticals (Luna et al., 2012; Davis et al., 2017; Williams et al., 2017a; Williams et al., 2017b; Saccà et al., 2019; Cammalleri et al., 2020; Chou et al., 2020), antagonists of excitotoxic glutamate receptors (Sharif et al., 2001; Ju et al., 2015; Opere et al., 2018), valproic acid (Kimura et al., 2015a; Kimura et al., 2015b), Ca²⁺-channel blockers, anti-inflammatory agents/glial cell modulators (Cueva Vargas et al., 2016), ATP-sensitive K⁺-channel openers, neurotrophic compounds (Hu et al., 2005; Khatib and Martin, 2017), neurosteroids (Ishikawa et al., 2014), α2-adrenoceptor agonists, anti-epileptics, nicotininc receptor agonist (Iwamoto et al., 2014), mTOR inhibitors, Janus kinase inhibitors, MAP kinase inhibitors, delta opioid receptor agonists, endothelin antagonists, sigma-1 receptor agonists (Ellis DZ. et al., 2017), etc., etc (recently reviewed by He et al., 2018; Sharif, 2018) which are effective ameliorators of cell death of RGCs/LC cells/TM cells, and which also possess in vivo efficacy in various animal models of ischemia/hypoxia/oxidative stress/neurotrophin- and glucose-deprivation/direct excitotoxicity/optic nerve crush or transection emulating what may be happening in the human GON condition in patients with NTG/POAG (see He et al., 2018; Sharif, 2018 for reviews). Delivery of mitochondrial-targeted anti-oxidants (Pang et al., 2015), inactivation of the NFkB system in astroglia (Dvoriantchikova et al., 2009; Dvoriantchikova and Ivanov, 2014), use of inducible NO synthase inhibitors (Schnichels and Joachim, 2021), RGC and/or stem cell transplatation in the retina (Johnson and Martin, 2013; Behtaj et al., 2020; Liu and Lee, 2021), migroglial removal and repopulation (Barnett et al., 2021), delivery of a gene-therapy-based neurotrophin receptor-ligand complex (Khatib et al., 2021), and direct delivery of Schwann cells to the optic nerve also represent promising approaches to treat GON (Guo et al., 2014; Smedowski et al., 2016). Directly addressing mitochondrial defects/deficiencies using mitofusin activators (Rocha et al., 2018; Dang et al., 2020) may support mitochondrail rescue and help in axonal regeneration (Zhou et al., 2016).

Of the drugs that have been clinically evaluated for neuroprotective activity and to retard vision loss or impairment in POAG patients, for example brimonidine and memantine, none have conclusively proven effective thus far (Almasieh and Levin, 2017; Levin, 2017). It is worth noting that a very recent study demonstrated the neuroprotective activity of brimonidine after retinal ischemia (Conti et al., 2021). There is some hope and promise that oral vitamin-B3/nicotinamide may offord some degree of functional vision preservation in POAG patients (Williams et al., 2017c; Hui et al., 2020). Likewise, the potential utility of senolytic drugs such as quercetin, nicotinib, imatinib, toclizumab and dasatinib (El-Nimri et al., 2020) that help remove dead RGCs and neuroprotect may become commonly used therapeutics to treat POAG/NTG in addition to using IOP-lowering drugs. As with many reports of this kind, these results need to tbe confirmed by other researchers in different parts of the world in multiple additional clinical trials. Moreover, given the multiplicity of deleterious factors and conditions that trigger and sustain neuronal/axonal injury and demise (see above), and the gross heterogeniety of RGC-types (Sanes and Masland, 2015; Ou et al., 2016; Della Santina and Ou, 2017; Tran et al., 2019) with varying degrees of susceptibility to or resiliency to damage, it is likely that a combinatorial approach to mitigate and stem the vision damage/loss from GON caused by different forms of glaucoma would be necessary. This poses significant hurdles from a regulatory perspective to conduct suitable clinical trials and to gain approval from health agencies around the world. Nevertheless, a concerted collaborative effort by researchers and health autority personnel could prove productive in order to help save the eyesight of millions of patients afflicted with GON due to POAG/PACG/NTG. Hence, successful clinical translation of the neuroprotection paradigm in the glaucoma patients is eagerly awaited (Boia et al., 2020), which may also involve the use of suitable stem cells (Chamling et al., 2016) and/or their secretome (Harrell et al., 2019), intravitreally injected Schwann cells and potential optic nerve regeneration (Li et al., 2004; Li et al., 2017), chimeric perpheral nerve grafts (Cui et al., 2003).

The above discourse provides strong evidence for involvement of defective anterior chamber physiology in the onset of POAG, PACG and other forms of glaucoma. Specifically, the aging process leads to energy depletion in mitochondria (Kleesattel et al., 2015; Eells, 2019) and cellular debris accumulation in the corneoiridial angle of the ANC. Such TM occlusion coupled with reduced TM cellularity, phagocytotic activity and flexibility in the conventional outflow pathway directly contribute to the increased resistance to AQH outflow causing elevation of IOP. This OHT-induced mechanical pressure leads to a cascade of detrimental events, including inflammation (Slater et al., 2013), at the retinal ONH/LC and optic nerve regions which ultimately causes RGC axonal damage and RGC death resulting in vision loss, especially resulting in peripheral vision defects (Tribble et al., 2020). Several strategies to pharmaceutically and surgically (including use of implanted microshunts) reduce IOP have been developed to treat OHT. However, since effective IOP control does not prevent visual impairment in many patients suffering from glaucoma, especially those with NTG who have normal IOPs (Jampel, 2007), the notion that IOP-independent mechanisms cause GON is now well accepted. Therefore, future treatment modalities for glaucoma will necessitate the use of neuroprotectants and neuroregenerative paradigms whilst still controlling the IOP (Goldberg, 2012; Guymer et al., 2019; Tsai, 2020). Several classes of pharmaceutical agents/nutriceuticals and regenerative compounds have demonstrated efficacy in cell-based assays and in a number of in vivo models of GON as described above (Goldberg, 2012; Guymer et al., 2019; El-Nimiri et al., 2020). However, translation of such efficacy in glaucoma patients needs to be demonstrated in multiple clinical trials on a worldwide basis to help save eyesight for millions of people around our planet. It is also imperative that high priority be assigned to early diagnosis of the glaucomatous conditions so that patients begin their treatments to slwo down their visual impairment. The development of innovative imaging techniques such as those using nucleic acid dyes (Tsuda et al., 2016) and DARC (detecting apoptosizing retinal cells; Cordeiro et al., 2017), adaptive optics (Bower et al., 2021), angiography-coupled optical coherance tomography (Kaizu et al., 2017), and of course magnetic resonance imaging (MRI; Chow and Paley, 2021) should aid in early diagnosis and quantification of the damage to retinal neurons/optic nerve with and without neuroprotective treatment modalities. Likewise, the future appears bright for use of gene therapy and cell-therapy to potentially correct genetic or disease-induced abnormalities in TM/SC and retinal cells (Khatib and Martin, 2017; Ratican et al., 2018). Perhaps replacing or adding new healthy cells to the ANC and retina, and/or operationalizing gain of function of the latter by normalizing metabolic defects at the mitochondrial level of these cells with appropriate food supplementation or direct injection to the vitreous in sustained delivery vehicles may be possible. Direct repair and/or protection of RGC bodies and dendrites, and the myelin sheath and other components of the RGC axons (Fang et al., 2010), regenration of the optic nerve projecting to the brain (Van de Velde et al., 2015), also hold great promise. Normalization of intracranial fluid pressure to help physically support the optic nerve is potentially another treatment option in the future for patients whose fluid dynamics at the ANC and intracranially become defective due to the aging process or induced by disease mechanisms as discussed above. Ability to monitor IOP round the clock using appropriate sensors (De Moraes et al., 2018; e.g., Figure 18) will offer clinicians tools to diagnose POAG earlier, and perhaps patients to self-evaluate their eye health and seek treatment early as well. Similarly, the role of artificial intelligence in drug discovery, dignosis of POAG, and other aspects of ocular disease treatment represents a novel platform to increase awareness/diagnosis of glaucoma (Zheng et al., 2019).