RBPs are a diverse set of proteins that play crucial roles in ribonucleic acid (RNA) splicing, transport, translation, and stability. The presence of these RBPs proteins is necessary to regulate proper cell function, being essential for the maintenance of cell homeostasis (Tesco and Lomoio, 2022). Its main characteristic is to regulate the expression of a great diversity of genes, which confers stability to messenger RNA (mRNA).

These RBPs have different binding domains to RNA and other proteins, which is why they are multifunctional proteins (Šušnjar et al., 2022). The absence or modification of one of these RBPs could cause a loss and/or gain of cellular function and result in a pathological state (Ash et al., 2010; Kraemer et al., 2010).

Some of these modifications occur within the nucleus, for example, the transcription of C9orf72 produces transcripts that can form secondary RNA structures that have the ability to sequester RBPs, resulting in the formation of RNA foci (Wang Z. F. et al., 2019).

In ALS, the subsequent translation of the C9orf72 gene results in the production of dipeptide repeat proteins (DPRs) that not only disrupt the nucleocytoplasmic transport machinery but also cause TDP-43 mislocalization Table 1 (Ryan et al., 2022).

These mislocalizations of RBPs might be triggered during disrupted proteostasis by heat shock, hyperosmolarity, oxidative stress (Chang et al., 2013; Sama et al., 2013; Chen et al., 2016; Hock et al., 2018) or by a mutated transportin, a nuclear import receptor (Dormann et al., 2010; Neumann et al., 2012). Additionally, mutations in the RBP genes and/or aberrant post-translational modifications may cause RBP to precipitate and aggregate (Bosco et al., 2010; Dewey et al., 2011; Figure 1A).

The cell will turn on protective mechanisms against aggregation, like TDP-43 phosphorylation in the C-terminal region (Gruijs da Silva et al., 2022) or control by chaperon proteins like the heat shock proteins HSP70, HSP90, and HSPB1 (Lin et al., 2021; Lu et al., 2022; Carrasco et al., 2023) in order to overcome aggregation but, if these mechanisms are unsuccessful, then the precipitation of aberrant aggregates will progress.

In the case of neurodegenerative diseases, there is growing evidence that shows that an excessive increase in reactive oxygen species (ROS) leads to cellular problems, including cell death (Patel, 2016; Wilson et al., 2023). A significant percentage of patients with ALS were identified with mutated SOD1 which is a very important defense enzyme against oxidative stress (Berdyński et al., 2022). These mutations modify the catalytic speed of SOD1, allowing the generation of ROS such as peroxynitrite, increasing the nitration of proteins (Pansarasa et al., 2018).

These mismanagements of oxidative stress and inefficient protective mechanisms will result in misfolding and mislocalization of nuclear RBPs such as FUS, TDP-43, or co-aggregates with poly (GR) dipeptides Table 1 (Kino et al., 2011; Patel, 2016; Cook et al., 2020; Ratti et al., 2020). TDP-43 actively participates in the generation of cytoplasmic inclusions such as P-Bodies and stress granules (SGs; Li et al., 2013; Guo et al., 2018) and causes damage to different organelles including mitochondria (Islam, 2017).

Some mechanisms controlling gene expression in MS are post-transcriptional RNA modifications such as oxidation. In RNA, the nucleoside that is more susceptible to oxidation is guanosine, resulting in the mutagenic 8-oxoguanosine. This molecule not only binds to its natural pair but also gains the ability to bind to adenosine, leading to defective proteins.

In an MS oxidative microenvironment, mRNAs are not equally affected. This phenomenon may explain why some molecules such as N-acetyl-aspartate transferase 8 (NAT8L) mRNA are expressed at lower levels. Its cognate protein is involved in the catalytic synthesis of N-acetyl aspartic acid (NAA), required for myelin synthesis and its decrease is a pathological characteristic (Sumi et al., 2017; Kharel et al., 2023).

Another consequence of oxidative stress in MS is the oxidation of phosphatidylcholine, one of the end-product markers of oxidative stress. It was found to be associated with injury and neuroinflammation (Qin et al., 2007). The presence of this produces neurodegeneration in the white matter of the spinal cord and demyelination (Dong et al., 2021; Figure 2A).

TDP-43, which is coded by the TARDBP gene, has been identified as a significant contributor to the development of ALS (Gao et al., 2019; Chhangani et al., 2021). Mutations in the TDP-43 gene may favor the aggregation of misfolded TDP-43 proteins, achieving a prion-like configuration (Buratti, 2015). The anomalous accumulations of TDP-43 are commonly known as “TDP-43 proteinopathy” (Gao et al., 2019; Cook et al., 2020; Figure 1B; Table 1).

TDP-43 proteinopathy in ALS interferes with multiple pathways including RNA metabolism (Polymenidou and Cleveland, 2011), protein translation (Freibaum et al., 2010; Tollervey et al., 2011), stress-induced response (Colombrita et al., 2009), autophagy (Xia et al., 2016), endocytosis (Liu G. et al., 2017), ubiquitin-proteasome system and mitochondrial function (Wang et al., 2016).

Even though protein aggregation is not regarded as primary damage in MS pathology, increasing evidence suggests that the clustering of proteins like RBPs may have a crucial part in the progression of the condition, which could lead to both neurodegeneration and inflammation.

This RBP protein dysfunction has been observed in the experimental model of autoimmune encephalomyelitis (EAE) (Salapa H. A.-O. et al., 2020). In the spinal cord tissue, there is mislocalization and SGs formation of TDP-43 and heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1). Interestingly, there is a positive correlation between EAE score and RBPs mislocalization. This correlation has been reported for ALS as well (Brettschneider et al., 2013, 2014; Kim et al., 2013; Philips and Rothstein, 2015; Purice and Taylor, 2018; Figure 2B). Knock-down of hnRNP A1 in vitro reduced neurite outgrowth, suggesting that the loss-of-function of this RBP by cytoplasmic mislocalization disrupts very important biological processes and leads to cell death (Anees et al., 2021).

To this day, there is no information available on whether RBPs can be found pathologically accumulated in the spinal cord gray matter of patients, but spinal cord atrophy is reported (Zeydan et al., 2018; Bischof et al., 2022). RBPs accumulation is reported for OLs; TDP-43 is mislocalized in active lesions in patients with MS. TDP-43 aggregates in OLs have also been described in ALS (Masaki et al., 2020; Valori and Neumann, 2021) but, in this pathology, OLs proliferation is increased, producing defective mature cells (Table 1). Normal physiological TDP-43 is needed for myelination and OLs survival; it was also found that RPB depletion in these cells does not affect MNs or the neuromuscular junction, suggesting that neurodegeneration could be an independent phenomenon that requires its own triggers (Wang et al., 2018).

Staufen1 protein (STAU1) is another RBP that forms SGs and has been recently linked to ALS pathology (Paul et al., 2021). STAU1, a mammalian target of rapamycin (mTOR) and phosphorylated-Ser2448-mTOR was found to be highly abundant in fibroblasts from ALS patients with reported TDP-43 mutations.

Nuclear accumulation of STAU1 has been described for an ALS/FT model in drosophila and it has been proposed as a possible new hallmark for the disease (Kim et al., 2021). It is reported that cell cultures with over-expressed STAU1 increased mTOR levels by mTOR-mRNA interaction which impaired autophagy (Paul et al., 2023).

Cellular metabolism depends on the proper functioning of mitochondria. At a structural level, mitochondria have several compartments to ensure their proper functioning, thus mitochondria are delimited by the external mitochondrial membrane, and inside it have an internal mitochondrial membrane where the oxidative phosphorylation machinery (OXPHOS) is mounted; this is where the greatest function entrusted to the mitochondria is carried out, which is the generation of adenosine triphosphate (ATP; Protasoni and Zeviani, 2021).

When mitochondria are under stress, morphological changes can appear and present vacuoles inside (Zuo et al., 2021). Normally the TDP-43 protein is associated with the inner mitochondrial membrane since it seems to regulate mitochondrial activities. Some binding sites of TDP-43 to the mitochondria are called M1, M3, and M5. In ALS, the presence of TDP-43 proteins with mutations (Berning and Walker, 2019) and in large amounts can result in mitochondrial damage, causing their death through the activation of autophagy (Buratti, 2015) and mitophagy (Gautam et al., 2019). This not only affects mitochondrial functioning but also the metabolism of the cell itself, causing its death (Altanbyek et al., 2016; Wang et al., 2016).

The damage made to the mitochondria by the presence of TDP-43 in ALS causes the membrane potential to fall (m∆ψ), the production of ATP to decrease (Wang et al., 2016), the consumption of oxygen also decreases, and consequently, the amount of Ca²⁺ and NAD⁺ to decrease (Gautam et al., 2022). Apoptosis is activated when the mitochondria have sustained such severe damage (Dafinca et al., 2021; Figure 1C; Table 1).

If the TDP-43 protein colocalizes with the mitochondria (Wang et al., 2016), complex I malfunction and its subsequent disintegration occurs due to the large amount of TDP-43. This may be the direct cause of changes in mitochondrial morphology and dynamics (Wang et al., 2019; Zuo et al., 2021). In ALS, not only apoptosis is activated but also the activation of necrosis induced by TDP-43 has also been observed (Dhuriya and Sharma, 2018).

Correct mitochondrial function is key to the proper functioning of neurons. In MS, dysfunction at the mitochondrial level in the production of ATP causes neuronal activity to decrease and this can cause neuronal degeneration. The stress induced by the generation of ROS in MS exacerbates neuroinflammation causing neuronal death (Barcelos et al., 2019; Blagov et al., 2022; Figure 2C; Table 1).

As the damage in MS disease progresses, defects at the mitochondrial level increase, such as the presence of mutations in mitochondrial DNA (mtDNA), this inevitably leads to a decrease in energy production, which leads to the functional loss of mitochondrial functions, by changes in the expression of mitochondrial genes (Barcelos et al., 2019; Blagov et al., 2022; Table 1).

Different factors can disrupt the cell’s homeostasis and destabilize the proteins in the endoplasmic reticulum (ER). ER is a vital component in maintaining internal balance through the unfolded protein response (UPR) during normal conditions (Li et al., 2022).

However, in the presence of activated UPR, the Integrated Stress Response (ISR) gets triggered, causing the activation of eukaryotic translation initiation factor 2A (eIF2a), which decreases the generation of new proteins. Physiological changes in neurons caused by the stress response alter the long-term synaptic plasticity of cholinergic interneurons (Rittiner et al., 2016), new research indicates that it’s possible to change neuronal excitability in other nuclei as well (Helseth et al., 2021). ER stress-induced eIF2a activation also drives the expression of a cascade of UPR-targeted genes responsible for protein folding, autophagy, and apoptosis, through the activation of transcription factor 4 (ATF4; Dafinca et al., 2021).

Evidence suggesting proteasome malfunction in ALS has been long established. While there is low toxicity with an increase in misfolded proteins in MNs, toxicity increases significantly when the proteasome is inhibited or ceases to function (Kitamura et al., 2014). In ALS, a decrease in the constituent components of the proteasome has been observed in MN cell cultures, and this augments disease progression (Cheroni et al., 2009).

Studies in mice have demonstrated that mimicking ALS symptoms is possible through the dysfunction of the ubiquitin-proteasome system in MNs (Tashiro et al., 2012). Consequently, the functional impairment of the proteasome has been linked to the accumulation of inclusions of the TDP-43 protein, together with poly-GA inclusions (Guo et al., 2018; Riemenschneider et al., 2022; Figure 1D).

In individuals with MS, antibodies targeting the proteasome have been identified in the cerebrospinal fluid (Mayo et al., 2002). This results in a reduction of the proteolytic activity of subunit 20 of the proteasome in both white and gray matter (Zheng and Bizzozero, 2011), leading to a deficiency in proteostasis and increased accumulation of misfolded proteins in the soma. This generates toxic effects that can cause apoptosis, such as in the case of the bassoon protein (Schattling et al., 2019). Conversely, activation of the proteasome can generate a neuronal protective effect by decreasing the accumulation of protein deposits (Schattling et al., 2019; Figure 2D).

When the cell antioxidant defenses are compromised, SGs are formed. The production of free radicals and ROS in the cell causes the agglutination of various cellular components found in the cytosol, some of the structures found in these clusters are translation initiation factors, ribosomal subunits, mRNA, and RBPs (Protter and Parker, 2016). SGs act as a temporary barrier to stress and develop in response to physiological damage to cells.

The conglomerates that give rise to SGs are formed in the cytoplasm where they are completely assembled (Anderson and Kedersha, 2008; Harrison and Shorter, 2017). Proteins with mutations such as some RBPs, including the TDP-43 protein and the hnRNP A1 protein, can be sequestered by these protein conglomerates (Kim et al., 2013; Boeynaems et al., 2016); These SGs can be harmful to the cell when excessive amounts of RBPs are found (Harrison and Shorter, 2017). If these SGs clusters are removed, neurotoxicity can be decreased (Becker et al., 2017; Figure 1E; Table 1).

The lack of ROS clearance and the presence of the SOD1 mutant protein are the first signs of oxidative stress in ALS (Kirkinezos et al., 2005; Berdyński et al., 2022). When large conglomerates of SGs are made, it is when a proteinopathy that is mediated by the amount of TDP-43 occurs (Li et al., 2013; Wang et al., 2020).

Due to elevated ROS, TDP-43 protein clumps in ALS patients (Ratti et al., 2020; Zuo et al., 2021). Furthermore, SGs formation is induced (Ratti et al., 2020) due to the increased accumulation of TDP-43, this increase in SGs impairs nucleocytoplasmic transport, permitting the buildup of nucleocytoplasmic transport factors and RNA/protein complexes in the cytoplasm (Zhang et al., 2018; Chen and Cohen, 2019). TDP-43 mutations are one of the additional causes of SGs in ALS for example Q331K, A315T, or Q343R (Liu-Yesucevitz et al., 2010).

MS and EAE showed pathogenic RBP (hnRNP A1) dysfunction. This RBP rises up in the cytoplasm instead of the nucleus causing cytoplasmic mislocalization and showing accumulation in SGs (Douglas et al., 2016; Salapa et al., 2018). Also, dysfunction of this RBP increases the formation of SGs (Clarke et al., 2021). TDP-43 and hnRNP A1 can interact, and their colocalization in SGs structures further highlights the significance of SGs in MS (Libner et al., 2020; Salapa H. E. et al., 2020; Figure 2E; Table 1).

The change in neuronal excitability is a consequence of different factors such as the increase in cellular stress and ROS by misfolding proteins. At different stages of ALS, patients have shown hyperexcitability in cortical MNs (Table 1; Vucic et al., 2008). These neurons from the patient cortex were used to have induced pluripotent stem cells (iPSCs) and these cells show TARDBP or C9orf72 dysfunction, also their excitability response and synaptic plasticity were increased (Devlin et al., 2015; Perkins et al., 2021). The alterations observed might be attributed to the rise in calcium levels within the cell (Burley et al., 2022), or elevated glutamate receptor expression levels (Shi et al., 2019; Figure 1F).

This increase in calcium levels led to hyperexcitability and then to excitotoxic MN degeneration (Table 1; Ramírez-Jarquín and Tapia, 2018). Although it has been discussed whether the change in the expression of sodium channels on the surface of the MN is also involved, it is the change in the permeability of α-amino- 3-hydroxy- 5-methyl-4-isoxazole propionic acid (AMPA) (overactivation of AMPA receptors) or N-methyl-D-aspartate (NMDA) receptors that leads to Glutamate-mediated excitotoxicity (Netzahualcoyotzi and Tapia, 2015; Santa-Cruz et al., 2016).

These changes in excitability were reproduced in the SOD1^G93A animal model, showing less contribution of persistent γ-Aminobutyric acid (GABA) current in trigeminal MNs and an increase of L-Type Ca²⁺ currents. All these changes contributed to the imbalance in excitation-inhibition during the pathology (Laszlo et al., 2022; Venugopal et al., 2023).

There are several sources of excitability in MNs, astrocytes being one of them. In ALS, astrocyte-mediated neuronal hyperexcitability could happen via connexin 43 (Cx43) gap junctions (Almad et al., 2022). Loss of astrocyte Cx43 increases MN survival and decreases disease progression (LoRusso et al., 2019). An increase in Cx43 expression in astrocytes reduces MNs survival (Almad et al., 2016; Figure 1F).

Hyperexcitability in MS is closely related to the inflammatory state (Table 1). In patients, an increase in several cytokines, including interleukin 1 beta (IL-1β) and tumor necrosis factor (TNF), has been found in the cerebrospinal fluid. This increase is enough to raise the excitability of neurons in corticostriatal synapses (Rossi et al., 2012a, 2014). Also, IL-1β has a dual effect because it inhibits GABA transmission and reduces the spontaneous inhibitory postsynaptic currents (sIPSC; Rossi et al., 2012b), both effects contributing to possible excitotoxic neurodegeneration. In an animal model of MS, research findings suggest that the AMPA/kainate receptors are responsible for the mediation of glutamate excitotoxicity in OLs (Pitt et al., 2000; Figure 2F).

Axonal degeneration, commonly referred to as “axonopathy” can be an early symptom of neurodegeneration. Thus, the first sign of axonal mitochondrial malfunction in response to oxidative stress and neuroinflammation is the dysregulation of mitochondrial transport (Brown and Bal-Price, 2003; Errea et al., 2015). This axon degeneration may be brought on by low ATP levels activating the protease and Ca²⁺ changes (Dutta et al., 2006) which could be specific by neuron subtype.

Recently, it was shown that myelin attenuates calcium transient currents via clusters of mitochondria, specifically on parvalbumin (PV) interneurons. The number of mitochondria is increased in glutamatergic neurons, whereas it is decreased in PV-GABA interneurons, which have lower overall excitability (Ramírez-Jarquín and Tapia, 2018; Kole et al., 2022).

Axonal transport (AT) directs the delivery of organelles from the cell body to synaptic compartments, removes and degrades unusable materials, and preserves the structural and metabolic variability of axonal subdomains (Brady and Morfini, 2017).

In healthy neurons, phosphorylation processes are precisely controlled both spatially and temporally, but these processes are frequently disrupted in ALS. Several of the kinase pathways linked to ALS produce an aberrant pattern of phosphorylation of numerous neuronal protein substrates that influences AT, which in turn promotes synaptic dysfunction and neuritic pathology in affected neurons (Guo et al., 2020). Thus, AT is a crucial cellular process that underlies the formation and maintenance of neuronal architecture and connectivity over the course of a neuron’s lengthy lifespan (Morfini et al., 2009; Brady and Morfini, 2017; Figure 1G).

It is well known that ALS is characterized by the activation of certain kinases, such as P38 MAP, in the MNs of the spinal cords of both sporadic and familial ALS patients as well as animal models (Bendotti et al., 2004; Sama et al., 2017). But also MAP/microtubule affinity-regulating kinases (MARKs), cyclin-dependent protein kinase 5 (cdk5), casein kinase 2 (CK2), and casein kinase I (CK1) are among the other kinases that assist with microtubule stability, within these kinases, glycogen synthase kinase 3 (GSK3) is the kinase activity that is most consistently linked to ALS patients, and its inactivation improves the survival of the neuron (Hu et al., 2003; Gibbs et al., 2018).

Multiple mechanisms can cause axonal damage in MS, which can occur at various stages of the disease. During the initial inflammatory phases, the primary cause of axonal damage is the inflammatory cascade, microglia activation, oxidative stress, and mitochondrial dysfunction. In contrast, during the later chronic stages, the loss of trophic support from myelin leads to an increase in energy demands. This results in the redistribution of sodium channels, an increase in calcium inside axons, and ultimately axonal fragmentation due to insufficient energy supply (Errea et al., 2015).

Astrocytes and/or microglia become “activated” by inflammatory mediators in MS. Microglia apparently release high levels of glutamate via a specific transporter in exchange for cystine, and this release is increased by inflammatory activation of the microglia. The reversible sodium-dependent glutamate transporter may also have increased activity in activated microglia and result in glutamate release in conditions where extracellular potassium is raised. It has been suggested that the mechanisms by which activated glia lead to neuronal death in vitro include the release of nitric oxide, ROS, glutamate and/or cytokines (Brown and Bal-Price, 2003; Figure 2G).

Activated microglia and macrophages are found in MS in close association with damaged axons. The expression of inducible nitric oxide synthase (iNOS) in astrocytes and/or the presence of nitrotyrosine were also found in chronic and acute MS lesions in postmortem tissues of MS patients. Nitric oxide (NO) could play a role in the pathology of demyelination and destruction of the OLs as this type of glial cell is also sensitive to NO induced cell death (Brown and Bal-Price, 2003).

In MS, an abnormal accumulation of glutamate in the synaptic cleft can occur due to an increase in release and/or a deficiency in the reuptake of glutamate by astrocytes. This can lead to overstimulation of glutamate receptors (GluRs) and cause excitotoxic damage to neurons and OLs. The downregulation of metabolizing enzymes, glutamate dehydrogenase, and glutamine synthetase in OLs exacerbates this effect.

In addition, the upregulation of glutaminase, an enzyme responsible for glutamate synthesis, is observed in MS lesions. This creates a permanent increase in proinflammatory cytokine levels, and the increased availability of glutamate can upregulate neuronal GluR expression, worsen synaptic dysfunction, and reinforce the local glutamate excitotoxicity (Sulkowski et al., 2014; Mandolesi et al., 2015).

In contrast to the excess of glutamate, GABA levels are reduced in both the CSF and CNS in patients with MS. Furthermore, the molecular mechanisms responsible for regulating GABA levels appear to be dysregulated in MS patients (Dutta et al., 2006).

Finally, neuroinflammation can affect synaptic transmission at different levels, with a huge impact on synaptic excitability and, therefore, neuronal function. Several characteristics of the spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) recorded from neurons are affected by inflammation (Mandolesi et al., 2015).

ALS is closely linked with the hexanucleotide repeat expansion present in the C9orf72 gene (Renton et al., 2011; Yang et al., 2020). The C9orf72 gene is associated with proteins involved in autophagy initiation, Table 1 (Sullivan et al., 2016). Therefore, a malfunction in the C9orf72 causes autophagy and lysosome defects that increase cell deterioration in ALS. During the early stages of the disease, autophagy can have beneficial effects, however, during the later stages, it can actually exacerbate the pathology (Rudnick et al., 2017).

Also, progressive lysosomal deficits in MNs impair autophagic degradation because autophagosome generation has limited responsiveness to proteotoxic stress (Xie et al., 2015; Evans and Holzbaur, 2019). Another mechanism by which there is a dysfunction in the regulation of lysosomes and autophagy is through TDP-43 which induces the nuclear translocalization of Transcription Factor EB (TFEB), which regulates autophagy and lysosome biogenesis (Xia et al., 2016; Figure 1H; Table 1). In ALS-like models, it was found that RBPs accumulation impairs autophagy via mTOR dysregulation (Paul et al., 2023).

An essential feature of MS is the presence of demyelinating plaques present in the CNS and the spinal cord, leading to long-term axonal loss; these damages are due to chronic inflammation (Monaghan and Wan, 2020).

MS also has features of an autoimmune disorder, as there is the abnormal presence of T helper 1 (Th-1) and Th-17 lymphocytes in the CNS, these lymphocytes secrete proinflammatory cytokines which continue to chronically damage the entire CNS. This chronic neuroinflammation will determine the progression of the disease (Mcginley et al., 2018; Nitsch et al., 2021).

The presence of antibodies to attack myelin causes the loss of the protective covering of the nerve fibers of the neurons, causing deficiencies in the transmission of the nerve impulse by losing its protection (Ransohoff, 2018; Nitsch et al., 2021; Figure 2H).

In ALS, cells are not equally affected by degeneration, even though proteins like TDP-43 or mutant SOD1 are expressed ubiquitously, for example, abnormal TDP-43 can be found in the basal ganglia and the substantia nigra, those areas are not affected in the disease. A wealth of evidence shows that some MNs subpopulations are more vulnerable than others to cell damage but it is unknown why. Spinal and hypoglossal MNs are among the first to degenerate and, oculomotor neurons and Onuf’s nuclei MNs are last to do so, this is why vision, sexual, and bladder function are maintained undisrupted during the course of the disease (Ragagnin et al., 2019).

Extrinsic factors like the type and density of non-neuronal cells or GABA input from interneurons, and intrinsic factors like neural excitability and receptors for inflammatory molecules, determine the susceptibility to neurodegeneration by excitotoxicity in ALS (Nijssen et al., 2017; Ragagnin et al., 2019; Allodi et al., 2021), such as the fast-twitch motor neuron alpha motor neuron (FF-alfa-MN), which primarily degenerates before the cranial nerves III, IV, and VI that control motor ocular movements (Caligari et al., 2013).

However, using the connectome, which indicates trans-synaptic pathways where interneurons are important, the propagation could be accomplished through either continuous or discontinuous spread (Hossaini et al., 2011; Kanouchi et al., 2012; de Carvalho et al., 2014) as in work by You et al. (2022) where the PV positive interneurons and alpha MNs are lost in the model of ALS.

Neurodegeneration is a component of MS, present in all stages of the pathology, even in RRMS (Mey et al., 2023). For many years, neurodegeneration was thought to be a consequence of inflammatory demyelination, but some research suggests that neurodegeneration occurs in all disease stages (Trapp and Nave, 2008; Frischer et al., 2009; Lassmann and van Horssen, 2011). Biopsies from patients with MS show an association between demyelinated plaques and axonal loss in the white matter tracts of the spinal cord (Table 1), suggesting that their mechanism probably contributes to neurodegeneration in the disease (Dutta et al., 2006; Levin et al., 2014; Mandolesi et al., 2015).

MS progression does not seem to follow any pattern like ALS does, that is, there is no susceptibility to damage depending on cellular subtypes. Actually, MS lesions are disseminated in space and time and may include IgG oligoclonal bands in the cerebrospinal fluid (Thompson et al., 2018). Patients may or may not experience inflammatory disease activity and the disease can follow various courses, relapsing–remitting MS being the most common one (Confavreux et al., 2020; Mey et al., 2023).

During the early stages of MS, the brain attempts to compensate for demyelination by redistributing voltage-gated Na⁺ channels that were concentrated in the Ranvier nodes, along the axon; this allows for slow but effective nerve conduction (Dutta et al., 2006). Senol et al. (2022) show that there are alterations in the initial segment of axons that could possibly lead to increased neural excitability, in both active and inactive lesions from patients.

Mitochondrial dysfunction and reorganization of voltage-gated Na⁺ channels might be one of the main drivers of degeneration in advanced stages of MS. Evidence shows that upper MNs firing rates increase, in part due to a lack of inhibition, given that GABAergic neurotransmission is reduced pre and postsynaptically in MS patients. This translates into a higher energetic demand that cannot be met due to mitochondrial damage, as described above.

This increases Na⁺ levels inside the cell, that under non-pathological conditions would be exchanged for extracellular K⁺ by the Na⁺/K⁺ ATPase. It is believed that intracellular Na⁺ can be exchanged by the Na⁺/Ca²⁺ exchanger that is non-ATP dependent, increasing axoplasmic Ca²⁺ and resulting in degeneration (Figure 2F; Dutta et al., 2006). In postmortem studies, neurons from patients with progressive MS show severe mitochondrial dysfunction, given the decreased activity of complexes I and III from the respiratory chain (Nikić et al., 2011; Libner et al., 2020).

Accumulations of phosphorylated Ser409/410 TDP-43 (pTDP-43) were found in intramuscular nerve bundles (INBs) in postmortem biopsies from patients with sporadic ALS (sALS). Also, a study in 114 biopsies from patients with no muscle diseases and/or ALS causing variants was done; patients with abnormal deposits of pTDP-43 in INBs were later diagnosed with ALS, suggesting that these findings could lead to a new diagnostic biomarker for the disease (Kurashige et al., 2022).

Even though TDP-43 accumulation has been reported in MS and EAE, there are currently no studies that show its dysfunctionality in muscular structures.

Another important feature of ALS is the disruption of the neuromuscular junction (NMJ; Bruneteau et al., 2013; Badu-Mensah et al., 2022). To this day, there are no reports of how MS pathology affects the structure of the neuromuscular junction (NMJ).

The generation of animal models for the study of ALS can be difficult, due to the fact that there are two main variants of the disease; sporadic and familial. There are no models for sALS but great progress has been made with the ones that replicate a known mutation from familial ALS (fALS).

The first ever transgenic model for ALS was created by Gurney and colleagues in 1994 by expressing a mutant form of the human SOD gene, this produced paralyzed mice with MN loss from the spinal cord and eventually death. Other transgenic models, targeting different genes have been elaborated, for example, C9orf72 knockdown zebrafish model (Ciura et al., 2013), FVB-C9orf72 bacterial artificial chromosome (BAC) mouse model (Liu et al., 2016), TDP43-Q331K mouse model (Arnold et al., 2013).

Given the importance of TDP-43 aggregation in the pathology, several models were developed, being ALS10 the first one; in 2009 a mouse expressing the disease-associated mutant TDP43-A315T was created (Table 1; Wegorzewska et al., 2009). More modern models express TDP-43 at more endogenous levels, like the single-copy BAC model of TDP43-M337V (Todd and Petrucelli, 2022).

Another approach is to focus on one hallmark of the pathology, and to try to reproduce it in animals like the group of Ricardo Tapia did, by developing a model of MN death induced by excitotoxicity; In this model, a glutamate receptor agonist, AMPA, is administered with a mini osmotic pump (Almoshari, 2022) directly into the ventral horns of the spinal cord, resulting in progressive paralysis of lower limbs and death of MNs (Tovar-Y-Romo et al., 2007).

In vitro models such as ALS-on-a-chip technology can implement optogenetics for the induction of MNs spheroids (Osaki et al., 2018) or TDP-43 opto-aggregation can be induced in cell cultures (Mann et al., 2019).

OptoGranueles is another approach that uses optogenetics for the multimerization of some scaffold proteins necessary for SD assembly (Zhang et al., 2019).

Lastly, in 2020, Asakawa et al. (2020) developed an in vivo model that uses optogenetic TDP-43 (opTDP-43) in zebrafish. Light stimulation induced cytoplasmic opTDP-43 mislocalization in spinal MNs.

MS is a very complex and heterogeneous pathology, with an unknown origin that can affect patients in several ways, thus its study in the laboratory becomes challenging.

There are many animal models that have been developed with the intention to mimic MS as closely as possible and even though models are imperfect, great advances have been made. EAE or experimental autoimmune encephalomyelitis was developed by Rivers in 1933 and it is the most commonly used animal model in MS research (Constantinescu et al., 2011).

Nowadays EAE is induced by administering animals with immune triggering peptides such as myelin OLs glycoprotein (MOG), myelin proteolipid protein (PLP) and Myelin basic protein (MBP) emulsified in Freund’s complete adjuvant (CFA) (Laaker et al., 2021; Huntemann et al., 2022; Takano et al., 2023) and complementing with Pertussis toxin (PTx) to permeate the BBB and increase disease severity (Singh et al., 2021; Zuo et al., 2022).

The type of immune response that will be developed depends on the animal strain and the protein used, among other factors. In C57BL/6 mice, MOG_35-55 results in a T-cell mediated response (Saligrama et al., 2019; Huntemann et al., 2022) whereas, MOG_1-125 produces a B-cell dependent outcome (Stevens et al., 2020). Another popular alternative is the SJL mouse, which responds better to PLP induction and is useful for the study of EAE’s initial wave.

A great number of authors report a higher susceptibility to EAE in females (Rassy et al., 2020) but, some articles report results in mixed genre groups, making no distinction in sex susceptibility (Othy et al., 2020).

EAE is a useful model to understand the pathogenesis of MS, however, there are several differences; in the EAE model, greater damage is seen in the white matter of the spinal cord, while in patients with MS, there is demyelination mainly in the cortex and cerebellum. It seems that the neuroinflammation is of a different origin in the EAE, causing this damage in the spinal cord and not in the cortex and cerebellum as would be expected in MS (Kutzelnigg et al., 2007; Mcginley et al., 2018).

Viral-induced models are another option for the study of such pathology being Theiler’s murine encephalomyelitis virus (TMEV) (DePaula-Silva et al., 2017) and the murine hepatitis virus (MHV) the most common ones. Since some of the viral induced models can cause more severe symptoms than EAE models do, they are harder to work with.

Some of the viruses can also work as seizure generating agents (Löscher and Howe, 2022), making them useful models for epilepsy (Patel et al., 2022). These models can help study mechanisms for myelin loss that might not be immune-mediated by B and/or T cells (Buttigieg et al., 2023).

Lastly, demyelination in mammals can be triggered by toxic reagents such as ethidium bromide (EtBr), lysolecithin (LPC), and cuprizone. Some of them can be selective to a cell type, by various mechanisms, for example, cuprizone induces OLs apoptosis by homeostasis alteration and, lysolecithin produces OLs death by dissolving its cell membrane given its detergent properties. EtBr is non cell specific (Buttigieg et al., 2023).