Since its initial isolation, GDNF has been regarded as a potential therapeutic neurotrophic factor playing a role in the development of Substantia nigra (SN) Dopaminergic (DA) neurons (Lin et al., 1993). In spite of its name, GDNF is not physiologically expressed in glial cells of the murine nervous system, but rather in neurons, particularly in parvalbumin-positive (PV+) interneurons, cholinergic and somatostatin-positive interneurons in the striatum, as demonstrated in transgenic mice with the lacZ cassette at the GDNF locus (Pascual et al., 2008; Gonzalez-Reyes et al., 2012; Hidalgo-Figueroa et al., 2012). The pre-α-pro-GDNF isoform appears to be the predominant isoform in the dopaminergic system, particularly in the striatum and substantia nigra (Airavaara et al., 2011). In healthy adult human brains, GDNF mRNA levels are typically low; however, they have been found to increase under pathological conditions such as Parkinson’s disease, not only in neurons but also in astrocytes, microglia, and macrophages (Nakagawa et al., 2005; Bäckman et al., 2006; Azevedo et al., 2020).

GDNF principal receptors, GFRα1 and RET, mRNA and protein have been found to be expressed in rodent midbrain DA neurons from early embryonic development through to adulthood (Trupp et al., 1997; Golden et al., 1999; Airaksinen and Saarma, 2002). Several studies on rodent models have investigated the role of GDNF, GFRα1, and RET in the midbrain dopaminergic system. In rat and mouse models of Parkinson’s disease, a transient increase followed by a decline in GFRα1 and RET mRNA levels was observed in the substantia nigra after 6-hydroxydopamine (6-OHDA) exposure (Marco et al., 2002). A similar reduction in RET receptor levels was reported in the striatum following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment (Hirata and Kiuchi, 2007), with both studies linking decreased GFRα1 and RET expression to the loss of tyrosine hydroxylase (TH)-positive dopaminergic neurons in the midbrain and their diminished innervation of the striatum (Hirata and Kiuchi, 2007). GDNF injection during the postnatal period has been shown to have a protective effect on the survival of substantia nigra (SN) dopaminergic neurons, while transgenic models overexpressing GDNF exhibit increased dopamine levels and striatal innervation (Burke, 2004; Kumar et al., 2015). However, GDNF and GFRα1 knockout mice die shortly after birth without evident alterations in the dopaminergic system. In contrast, RET deficiency leads to a progressive loss of SN dopaminergic neurons in aging mice (Kramer et al., 2007). Moreover, in a recently published study, the disruption of the gdnf gene in zebrafish embryos, utilizing CRISPR/Cas9 gene editing, resulted in a significant reduction (∼20%) of dopaminergic neurons in specific diencephalic clusters. This decrease was associated with altered expression of key transcription factors, including otpb and lmx1b.1, which are critical for dopaminergic neuron differentiation. Additionally, gdnf-deficient zebrafish exhibited impaired locomotor activity at 7 days post-fertilization and increased susceptibility to neurotoxic insults. These findings suggest that GDNF plays a conserved and essential role in the early development and functional maintenance of dopaminergic neurons (Wong et al., 2021). On the other hand, constitutive RET activation caused by a missense Meth918Thr mutation in the receptor results in an increased number of dopaminergic neurons in the SN and greater resistance to neurotoxins such as MPTP and 6-OHDA (Mijatovic et al., 2011). Moreover, conditional deletion studies of GDNF and RET have produced conflicting results regarding their necessity for neuronal survival in adulthood, suggesting the presence of compensatory mechanisms or alternative ligands (Pascual et al., 2008; Kopra et al., 2017). Several questions remain open, including the identity of the essential ligand for RET in the midbrain and the intracellular signaling pathways involved in neuronal survival.

GDNF and its receptors, GFRα1 and NCAM, are expressed in hippocampal neurons during embryonic and early postnatal development, with GFRa1 localized at both pre- and post-synaptic sites, while NCAM is restricted to presynaptic terminals (Ledda, 2007). In hippocampal neuron cultures, GDNF enhances synapse formation, and the interaction of GFRα1-coated beads with neurons in the presence of soluble GDNF can induce ectopic presynaptic sites, demonstrating an instructive role of GDNF/GFRα1 signaling in synaptogenesis (Ledda, 2007), a process that partially relies on NCAM at presynaptic terminals. In vivo, mutant mice with reduced GDNF levels exhibit impaired presynaptic maturation and a decreased number of presynaptic sites during hippocampal development, further supporting the role of GDNF in synaptic assembly (Bonafina et al., 2019). Furthermore, utilizing crystallography and electron microscopy, it has been identified a decameric assembly comprising two GFRα1 pentamers bridged by five GDNF dimers. This configuration facilitates synaptic adhesion by forming complexes that bridge adjacent cell membranes. Further experiments demonstrated that the presence of the RET receptor and heparan sulfate can inhibit the formation of this adhesion complex by competing for the same binding interfaces (Bonafina et al., 2019). These findings suggest a dual role for GFRα1: promoting synaptic adhesion independently of RET and engaging in RET-mediated trophic signaling. This dual functionality provides insights into the molecular mechanisms underlying neuronal connectivity and the modulation of synaptic structures (Bonafina et al., 2019).

Additionally, GDNF/GFRα1 signaling is critical for the structural and functional integration of adult-born granule cells into preexisting hippocampal circuits. Conditional GFRα1 knockout mice display deficits in behavioural pattern separation, a function linked to adult neurogenesis. Notably, physical activity enhances GDNF expression in the dentate gyrus, promoting GFRα1-dependent CREB activation and dendritic maturation. These findings highlight GDNF/GFRα1 signaling as a key regulator of both developmental and adult hippocampal plasticity, orchestrating synaptogenesis and the incorporation of new neurons into functional circuits (Bonafina et al., 2019). Recently, it has also been demonstrated that elevated levels of GDNF enhance GABAergic inhibitory inputs onto pyramidal neurons in the CA1 region of the hippocampus (Mikroulis et al., 2022). This effect is mediated through the activation of the RET receptor pathway, facilitated by the co-receptor GFRα1. Notably, the other GDNF receptors, namely as NCAM or Syndecan3, are not implicated in this process. The study also demonstrated similar enhancements in inhibitory synaptic transmission in human hippocampal slices obtained from epilepsy patients. These findings suggest that GDNF’s ability to strengthen inhibitory signaling may contribute to its observed seizure-suppressant effects in various epilepsy models (Paolone et al., 2019; Wahlberg et al., 2020; Mikroulis et al., 2022).

Recent research has revealed significant interactions of GDNF with the serotonergic (5-HT) system. Serotonergic neurons of the raphe nuclei express GDNF receptors, particularly GFRα1 and RET, suggesting direct responsiveness to GDNF (Okaty et al., 2015; 2020; Huang et al., 2019; Ren et al., 2019). The effect of GDNF on the brain 5-HT system is controversial, with some studies showing positive effects on brain 5-HT (Pertusa et al., 2008; Naumenko et al., 2013), and others reporting little or no effect (Hudson et al., 1995; Mijatovic et al., 2007). On the other hand, serotonin increases GDNF expression and secretion from C6 rat glioma cells, acting predominantly via 5-HT2A receptors (Hisaoka et al., 2004; Tsuchioka et al., 2008). While moderate increases in GDNF enhance serotonin neuron number, serotonergic gene expression, and 5-HT levels, excessive GDNF leads to a decrease in serotonergic neurons differentiation (Menegola et al., 2004). This nonlinear relationship may explain previous conflicting reports on GDNF’s effects on the serotonergic system.

Human data with antidepressants provide indirect evidence of a connection between GDNF and the serotonergic system. Acute or chronic administration of antidepressants increasing the levels of synaptic serotonin (such as tricyclic antidepressants, tetracyclic antidepressants, and serotonin-selective reuptake inhibitors) is indeed able to increase GDNF expression both in cell culture (Mercier et al., 2004; Hisaoka et al., 2007; Golan et al., 2011; Kajitani et al., 2012; Hisaoka-Nakashima et al., 2015; 2019; Abe et al., 2019) and in serum of patients with depression (Zhang et al., 2008). Preclinical data show that animals exposed to chronic unpredictable stress exhibit depression-like behavior and decreased GDNF expression in the hippocampus, that is reverted by chronic tricyclic antidepressant treatment (Uchida et al., 2011; Liu Q. et al., 2012). In addition, GDNF may be decreased in the peripheral blood of patients with major depressive disorder (Takebayashi et al., 2006; Lin and Tseng, 2015; Sharma et al., 2016).

These findings suggest that the modulation of GDNF production may be a component of the therapeutic effect of antidepressants and that the fine-tuning of the serotonergic system by GDNF could be implied in the mechanisms of development of neuropsychiatric disorders. This nuanced relationship positions GDNF as a key neurotrophic factor beyond its classical dopaminergic role, critically involved in serotonergic system regulation (Popova et al., 2017).

Glial cell line-derived neurotrophic factor (GDNF) plays a multifaceted role in neurogenesis, influencing neural progenitor proliferation, migration, and differentiation across developmental and adult stages. GDNF has been recognized as a chemoattractant and differentiation signal for neuronal precursors, particularly in the subventricular zone and rostral migratory stream (Paratcha et al., 2006), where it modulates key signaling pathways, including RET/GFRα1 and PI3K/Akt, enhancing neurogenic output and synaptic integration (Airaksinen and Saarma, 2002).

In the forebrain, inhibitory GABAergic interneurons originate in the ventral telencephalon and migrate tangentially to reach the developing cortex, hippocampus, and olfactory bulb (Bartolini et al., 2013). The ganglionic eminences serve as temporary neurogenic regions, with the medial and caudal ganglionic eminences (MGE and CGE) generating most cortical GABAergic neurons, whereas the lateral ganglionic eminence (LGE) primarily contributes interneurons to the olfactory bulb. Both GDNF and its co-receptor GFRα1 are expressed in the MGE and along the migratory routes of GABAergic neurons (Pozas and Ibáñez, 2005), where GDNF facilitates differentiation and functions as a chemoattractant (Pozas and Ibáñez, 2005; Paratcha et al., 2006). These effects depend on GFRα1 but not on NCAM or RET, and the addition of soluble GFRα1 to MGE cultures enhances differentiation and migration even in cells lacking endogenous GFRα1 (Perrinjaquet et al., 2011). Moreover, syndecan-3 has been proposed as an alternative GDNF receptor in MGE-derived GABAergic neurons, independently of GFRα1 (Bespalov et al., 2011). Mice deficient in GFRα1 exhibit reduced migration of GABAergic neurons, leading to a lower number of inhibitory neurons in the cortex and hippocampus at birth (Pozas and Ibáñez, 2005), as well as disrupted integration of parvalbumin-expressing neurons and altered social behaviors linked to increased cortical excitability (Canty et al., 2009), a phenomenon consistent with certain autism models (Tabuchi et al., 2007). GFRα1 signaling is also crucial for olfactory system development, as its loss results in deficits in multiple GABAergic interneuron populations in the olfactory bulb, along with impairments in neurogenesis, migration, and sensory axon growth (Marks et al., 2012).

In stroke models, direct GDNF infusion significantly increases neurogenesis in the striatum (Kobayashi et al., 2006), while in the hippocampus, GDNF and its receptor GFRα1 are essential for proper integration of adult-born granule neurons (Bonafina et al., 2019). Following ischemia and traumatic brain injury, expression of various growth factors is increased and modulates neurogenesis, NSPC biology, and striatum connectivity (Christie and Turnley, 2013; Bacigaluppi et al., 2020; Butti et al., 2022). More recently, the neurogenic efficacy of GDNF has been harnessed through biomaterial-based delivery systems that enhance spinal cord repair and remyelination (Liu et al., 2024). Transplanting mesenchymal stem cells engineered to overexpress GDNF significantly improves neuroregenerative outcomes following ischemic or traumatic injury (Salgado et al., 2015; Chiavellini et al., 2022). This therapeutic strategy offers promising avenues in treating neurodegenerative disorders highlighting GDNF not only as a neuroprotective factor but also as a potent pro-neurogenic agent with promising translational potential in regenerative therapies.

Neuroplasticity, the ability of the central nervous system to promote neurogenesis and renew connections, is influenced by different psychological, environmental and physiological factors, often involving the synthesis and secretion of neurotrophins (Kempermann et al., 2018).

For example, several recent studies highlight that engaging in regular physical exercise, whether aerobic or strength-based, can enhance cognitive function and promote neuroplasticity through different mechanisms (De Sousa Fernandes et al., 2020). Aerobic exercise primarily boosts glutamatergic signaling and neurotrophic factors like BDNF and CREB, while resistance training engages pathways involving PKCα and inflammatory cytokines, while it has been shown that both kinds of exercise share as a common outcome the upregulation of GDNF in plantaris myofibers (Gyorkos and Spitsbergen, 2014; Rabelo et al., 2017; Vilela et al., 2017). GDNF likely plays a central role in mediating the cognitive benefits of exercise by promoting hippocampal remodelling and resilience to age-related neural decline (Vilela et al., 2017). In young adult mice, voluntary exercise increases levels of GDNF and BDNF in the dentate gyrus (DG) (Cotman and Berchtold, 2002; Farmer et al., 2004). This upregulation correlates with enhanced dendritic growth and complexity in newly generated granule cells, suggesting a role for these factors in mediating activity-dependent neuronal integration. Notably, the effects of exercise on granule cells maturation seems to be dependent on GFRα1, the co-receptor for GDNF. Mechanistically, GDNF/GFRα1 signaling may act through the activation of the transcription factor CREB, which is known to regulate activity-dependent dendritic development (Jagasia et al., 2009). In vitro stimulation of DG-derived neural stem cell cultures with GDNF lead to phosphorylation of both CREB and Erk1/2, confirming that GDNF directly activates this signaling cascade in differentiating neurons (Bonafina et al., 2019).

A growing body of evidence is pointing at a fundamental role of astrocyte-derived GDNF, which contributes to neuroprotection by modulating synaptic function, reducing oxidative stress, and promoting neuronal survival. Studies on primary astrocytic culture highlighted that inflammatory stimuli distinctly regulate GDNF and Neuregulin-1 (NRG-1) in rodent astrocytes and microglia, with LPS treatment significantly increasing GDNF expression in astrocytes (Bresjanac and Antauer, 2000; Iravani et al., 2012; Kronenberg et al., 2019; Merienne et al., 2019). However, inflammatory stimuli are not the only ones able to impact GDNF expression. In an in vitro model of ischemia, utilizing neuron-glia and astrocyte cortical cultures subjected to oxygen and glucose deprivation, astrocyte-GDNF emerged as one of the released factors that mediated neuroprotection, elicited by high-frequency repetitive magnetic stimulation (Gava-Junior et al., 2023). Cinnamon and its metabolite sodium benzoate (NaB) can upregulate GDNF in human astrocytes. Oral administration of NaB and cinnamon increased astrocytic expression of GDNF in a model of PD in vivo, conferring neuroprotection of TH neurons of the Substantia Nigra Pars Compacta. However, this effect was absent in astrocyte-specific GDNF knockout mice (GDNF∆astro) (Patel et al., 2019). In another PD model, Gemfibrozil, a lipid-lowering drug approved by the FDA, has been shown to stimulate astrocytic GDNF, protecting dopaminergic neurons, through a PPARα-dependent pathway. Interestingly, Gemfibrozil was not effective in GDNF∆astro mice lacking GDNF, specifically in astrocytes (Gottschalk et al., 2021). Reactive astrocytes in Parkinson’s disease models also exhibit neurotrophic functions, with Nestin-positive astrocytes expressing GDNF (Chen et al., 2006). Furthermore, astrocytic GDNF mitigates cognitive decline post-anesthesia by improving hippocampal synaptic plasticity (Lin et al., 2024).

Conversely, exposure to di-(2-ethylhexyl) phthalate (DEHP), an environmental endocrine-disrupting compound used in food packages, medical devices, office supplies, and children’s toys, reduces the secretion of GDNF, interfering with the estrogen pathway, by downregulating the ERK/c-fos signaling in astrocytes (Wang et al., 2021). Excessive GDNF levels have been linked to astrocyte proliferation and potential gliomagenesis via the GFRα1/RET/MAPK/pCREB/LOXL2 axis (Wang et al., 2022). Finally, chronic overexpression of GDNF in brain astrocytes in a transgenic mouse model appears to have a detrimental influence on nigrostriatal dopamine metabolism and neurotransmission (Sotoyama et al., 2017).

GDNF produced by activated microglia and macrophages can aid in the repair of CNS injuries. In a model of spinal cord injury, LPS-induced macrophage activation enhanced GDNF expression at lesion sites, promoting functional recovery of the spinal cord by sustaining a neurotrophic environment, while mitigating oxidative stress (Hashimoto et al., 2005). GDNF mRNA was also upregulated a few hours post-injury, in a model of mechanical injury in the mouse striatum, with brain macrophages as the primary source, establishing a critical link between GDNF expression and dopaminergic neuron protection (Liberatore et al., 1997). Microglia, which persist long after injury, contribute significantly to neurotrophic support, predominantly secreting GDNF and BDNF. This immune-mediated neurotrophic environment facilitates dopaminergic axonal sprouting and tissue repair, particularly in neurodegenerative contexts such as PD (Batchelor et al., 1999; 2002). Notably, sprouting dopaminergic fibers associate closely with neurotrophic factor-expressing microglia, exploiting them as structural support to navigate toward lesion edges (Batchelor et al., 2002). NG2-positive and Iba1-positive cells in the substantia nigra express GDNF and it has been shown that they are localized close to surviving TH-positive neurons in the Substantia Nigra Pars Compacta, suggesting a neuroprotective role for these cells in dopaminergic neuron survival (Kitamura et al., 2010). Moreover, experiments on BV2 cells have shown that GDNF exerts an anti-inflammatory effect by decreasing the release of pro-inflammatory cytokines such as TNF-α, TGF-β, IL-1β, and IL-12β in a model of inflammation induced by amyloid beta. This study highlights the involvement of the Hippo/YAP signaling pathway in this process (Qing et al., 2020). These findings collectively highlight the interplay between immune cells and neurotrophic factors in CNS repair.

GDNF has long been recognized for its pivotal role in promoting axonal growth and neuronal regeneration, particularly within the peripheral nervous system (Lee et al., 2016). However, emerging evidence underscores its potent anti-inflammatory effects, highlighting its critical role in modulating neuroinflammation in several neurological diseases (Bido et al., 2024; Dingledine et al., 2024). Neuroinflammation is a complex process involving immune cell activation, vascular modulation, and alterations of resident cells, namely astrocytes and microglia. It can have both beneficial and detrimental effects on brain pathologies (Simonato et al., 2006; DiSabato et al., 2016).

A key aspect of this response is mediated by pattern recognition receptors (PRRs), which detect pathogen- and damage-associated molecular patterns (PAMPs and DAMPs), leading to immune cell activation (Iravani et al., 2012; Singh et al., 2022). Recent studies have shown that neurotrophic factors, including BDNF, NGF, and GDNF, play a crucial role in regulating neuroinflammatory pathways (Rocha et al., 2012; Guarino et al., 2022). Among these, GDNF is expressed by both astrocytes and microglia in pathological conditions, with astrocyte-secreted GDNF exerting a strong inhibitory effect on microglial activation, thereby mitigating neuroinflammation. GDNF can modulate the activation of microglia after Zymosan A (yeast-derived immune stimulant) treatment. It has been shown that the effect is dependent on GFRα1 and neutralization of GDNF or GFRα1, as well as GDNF silencing in astrocyte cultures, abolishes its regulatory effects, confirming that GDNF binding to the microglial GFRα1 receptor initiates intracellular signaling cascades responsible for suppressing microglial activation (Rocha et al., 2012).

In hippocampal astrocytes, GDNF/GFRα1 signaling contributes to neuroprotection by regulating immune responses. The upregulation of GDNF and its receptor GFRα-1 in hippocampal neurons and astrocytes enhances resilience against thrombin-induced neurotoxicity, known mechanisms to activate microglia, and induce neuronal death (Yun et al., 2020). Recently, it has been highlighted that astrocytic Sterile Alpha and TIR Motif Containing 1 (SARM1), which plays a critical role in axonal degeneration and inflammation in Multiple Sclerosis (MS), promotes neuroinflammation and axonal demyelination by suppressing GDNF expression. Moreover, pharmacological reduction of GDNF worsened disease progression, reinforcing its neuroprotective function and therapeutic potential in MS (Jin et al., 2022). Beyond microglial regulation, GDNF modulates immune responses through GFL receptors, which are also expressed in immune cells. Activation of these receptors leads to the suppression of immune cell activity and the regulation of pro-inflammatory mediator release (Vargas-Leal et al., 2005).