Neurotrophic factors (NTFs) are a family of secreted proteins that play essential roles in the survival, growth, differentiation, and maintenance of neurons in both the developing and adult nervous system [@mufson2019]. In the context of neurodegenerative diseases, NTF signaling is profoundly disrupted — deficiency of specific neurotrophic factors contributes to the selective vulnerability of neuronal populations, while therapeutic delivery of NTFs represents one of the most actively pursued strategies for neuroprotection and neural repair [@allen2013][@chao2014]. Each family signals through distinct receptor systems and supports specific neuronal populations, making their dysfunction particularly relevant to diseases that affect those populations.
Depletion of neurotrophic factors has been mechanistically linked to disease pathology in Alzheimer's disease (NGF, BDNF), Parkinson's disease (GDNF, BDNF), Huntington's disease (BDNF), ALS (BDNF, GDNF, CNTF, VEGF), CDNF (Cerebral Dopamine Neurotrophic Factor), and other neurodegenerative conditions [@chao2014]. NGF signals primarily through the TrkA receptor tyrosine kinase and the p75NTR pan-neurotrophin receptor.
Neurotrophic factors (NTFs) are a family of secreted proteins that play essential roles in the survival, growth, differentiation, and maintenance of neurons in both the developing and adult nervous system [@mufson2019]. In the context of neurodegenerative diseases, NTF signaling is profoundly disrupted — deficiency of specific neurotrophic factors contributes to the selective vulnerability of neuronal populations, while therapeutic delivery of NTFs represents one of the most actively pursued strategies for neuroprotection and neural repair [@allen2013][@chao2014]. Each family signals through distinct receptor systems and supports specific neuronal populations, making their dysfunction particularly relevant to diseases that affect those populations.
Depletion of neurotrophic factors has been mechanistically linked to disease pathology in Alzheimer's disease (NGF, BDNF), Parkinson's disease (GDNF, BDNF), Huntington's disease (BDNF), ALS (BDNF, GDNF, CNTF, VEGF), CDNF (Cerebral Dopamine Neurotrophic Factor), and other neurodegenerative conditions [@chao2014]. NGF signals primarily through the TrkA receptor tyrosine kinase and the p75NTR pan-neurotrophin receptor.
The neurotrophin family includes nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5). These proteins share a common structural fold known as the cystine knot motif and are initially synthesized as precursor proteins (proneurotrophins) that undergo proteolytic cleavage to generate mature, biologically active forms [@mufson2019]. The balance between proneurotrophins and mature neurotrophins, along with the expression of specific receptors, determines whether neurotrophin signaling promotes neuronal survival or death.
Role in normal brain function: NGF is the primary survival and maintenance factor for basal forebrain cholinergic neurons (BFCNs), which project to the hippocampus, cortex, and amygdala and are critical for learning and memory [@mufson2019][@chao2014]. NGF also supports peripheral sensory and sympathetic neurons. The retrograde transport of NGF from target tissues to neuronal cell bodies is a critical process that maintains neuronal viability, and this transport is dependent on dynein-mediated movement along microtubules [@mufson2019].
Role in Alzheimer's Disease: The cholinergic hypothesis of AD is intimately linked to NGF biology. In AD, BFCNs undergo progressive degeneration, contributing to cognitive decline. While NGF production in cortical target areas is largely preserved in AD, retrograde transport of NGF from the cortex to BFCNs is impaired, leading to a "target-derived" trophic deficit [@mufson2019]. This transport failure is associated with amyloid-beta accumulation, endosomal dysfunction, and proNGF (the precursor form of NGF) accumulation. ProNGF, which preferentially binds p75NTR, can paradoxically promote apoptosis rather than survival [@mufson2019]. The accumulation of proNGF in AD brains represents a pathological shift that contributes to cholinergic degeneration through its pro-apoptotic signaling via p75NTR and sortilin co-receptors [@mufson2019].
Therapeutic approaches: Direct intracerebroventricular infusion of NGF in AD patients caused significant side effects (pain, weight loss). Gene therapy approaches using AAV2-NGF (CERE-110) delivered to the nucleus basalis of Meynert showed safety but limited efficacy in Phase 2 trials [@tuszynski2005]. Encapsulated cell biodelivery (ECB) implants secreting NGF are under investigation. Small molecule TrkA agonists and proNGF-blocking antibodies represent emerging strategies [@allen2013]. The challenge of delivering neurotrophic factors across the blood-brain barrier remains a major obstacle, leading to the exploration of intranasal delivery, viral vector-mediated gene therapy, and cell-based delivery systems.
BDNF is the most abundant and widely distributed neurotrophin in the adult brain [@zuccato2009]. It signals through the TrkB receptor and is critical for synaptic plasticity, long-term potentiation, and memory consolidation.
Role in normal brain function: BDNF regulates synaptic transmission at glutamatergic and GABAergic synapses, promotes dendritic arborization and dendritic spine morphogenesis, supports survival of hippocampal and cortical neurons, and is a key mediator of activity-dependent plasticity [@zuccato2009][@chao2014]. BDNF expression is regulated by neuronal activity, exercise, and environmental enrichment, linking it to cognitive reserve. The activity-dependent secretion of BDNF from postsynaptic neurons and subsequent activation of presynaptic TrkB receptors facilitates retrograde signaling that strengthens synaptic connections [@zuccato2009].
BDNF mediates its effects through full-length TrkB (TrkB-FL) and truncated TrkB (TrkB-T1) receptors. While TrkB-FL contains the intracellular tyrosine kinase domain necessary for classical neurotrophin signaling, the truncated isoforms modulate signaling by forming heterodimers with full-length receptors or by acting as dominant-negative regulators [@zuccato2009]. The complexity of TrkB receptor isoforms adds another layer of regulation to BDNF signaling in the healthy and diseased brain.
BDNF Val66Met polymorphism: A common single nucleotide polymorphism (rs6265) results in a valine-to-methionine substitution at codon 66 (Val66Met). The Met allele impairs activity-dependent BDNF secretion by disrupting intracellular trafficking and sorting of BDNF into regulated secretory vesicles [@lim2021]. The Met allele is associated with:
Role in Alzheimer's Disease: BDNF levels are reduced in the hippocampus and cortex of AD patients [@zuccato2009]. Decreased BDNF signaling contributes to synaptic dysfunction, spine loss, and impaired long-term potentiation. Amyloid-beta oligomers downregulate BDNF expression and impair TrkB signaling. Tau pathology disrupts BDNF axonal transport [@zuccato2009]. BDNF deficiency exacerbates neuroinflammation by reducing microglial phagocytic capacity. The reciprocal relationship between BDNF and amyloid-beta is particularly important — while BDNF can protect against amyloid toxicity, amyloid-beta in turn suppresses BDNF expression through multiple mechanisms including transcriptional repression and disrupted signaling cascades [@zuccato2009].
Role in Huntington's Disease: Wild-type huntingtin protein promotes BDNF transcription and vesicular transport from the cortex to the striatum. Mutant huntingtin in Huntington's disease impairs both BDNF transcription (through sequestration of the transcription factor REST/NRSF) and anterograde transport, leading to BDNF deficiency in striatal medium spiny neurons — the population most vulnerable in HD [@zuccato2009]. REST/NRSF dysregulation in HD leads to widespread changes in gene expression, with BDNF being one of the most critically affected neuronal survival genes [@zuccato2009].
Role in Parkinson's Disease: BDNF supports the survival and function of dopaminergic neurons in the substantia nigra (see dopaminergic neurodegeneration). Reduced BDNF levels in the substantia nigra correlate with dopaminergic neuron loss in PD [@zuccato2009]. The corticostriatal BDNF pathway is particularly important for motor learning and habit formation, functions that are impaired in PD. Additionally, alpha-synuclein pathology can directly interfere with BDNF signaling and transport, creating a double hit on dopaminergic neuronal survival [@zuccato2009].
Therapeutic approaches: Direct BDNF delivery is challenging due to its short half-life and poor blood-brain barrier penetrance. Strategies include: TrkB receptor agonists (e.g., 7,8-dihydroxyflavone, LM22A-4); AAV-BDNF gene therapy in preclinical models; exercise-induced BDNF upregulation (aerobic exercise consistently elevates peripheral and central BDNF levels); and BDNF-mimetic peptides [@allen2013].
Emerging approaches include small molecule TrkB agonists that can penetrate the blood-brain barrier and promote BDNF-independent activation of TrkB signaling. The flavonoid 7,8-dihydroxyflavone (7,8-DHF) and its derivatives have shown promise in preclinical models of AD, PD, and HD [@allen2013]. Additionally, gene therapy approaches using adeno-associated viruses (AAV) to deliver BDNF or TrkB directly to brain regions have demonstrated efficacy in animal models, with several programs advancing toward clinical translation [@allen2013].
NT-3 signals through TrkC and supports proprioceptive sensory neurons, certain motor neuron populations, and cerebellar granule neurons [@chao2014]. NT-3 has shown neuroprotective effects in models of Charcot-Marie-Tooth disease and peripheral neuropathies. NT-3 also plays important roles in hippocampal synaptic plasticity and memory formation, with some studies suggesting it may have complementary or overlapping functions with BDNF in certain neuronal populations [@chao2014].
The expression of NT-3 in the brain is more restricted than BDNF, with highest levels in the hippocampus and cerebellum during development. In the adult brain, NT-3 continues to be expressed in regions including the hippocampus, cortex, and basal forebrain, where it contributes to synaptic maintenance and plasticity [@chao2014]. The therapeutic potential of NT-3 in neurodegenerative diseases has been explored primarily in the context of peripheral neuropathies and motor neuron diseases, where its effects on sensory and motor neurons are most relevant.
NT-4/5 signals through TrkB (like BDNF) and supports motor neurons, striatal neurons, and retinal ganglion cells [@chao2014]. NT-4/5 has been investigated as a potential therapeutic for ALS due to its motor neuron trophic activity. Unlike BDNF, NT-4/5 is expressed at lower levels in the brain but shows distinct spatial and temporal expression patterns that suggest specialized functions [@chao2014].
The therapeutic use of NT-4/5 has faced similar challenges as other neurotrophic factors, including delivery problems and limited diffusion in brain tissue. However, its unique receptor profile and potential for supporting specific neuronal populations that are less responsive to BDNF make it an attractive candidate for certain neurodegenerative applications [@chao2014].
GDNF is the prototypical member of the GDNF family ligands (GFLs), which also includes neurturin (NRTN), artemin (ARTN), and persephin (PSPN) [@lindholm2007]. GDNF signals through the GFRα1 co-receptor and the RET receptor tyrosine kinase. The GDNF family represents a structurally distinct class of neurotrophic factors that do not share sequence homology with the neurotrophin family but serve complementary roles in supporting specific neuronal populations.
The GDNF family ligands (GFLs) are synthesized as precursor proteins that undergo proteolytic processing to generate mature, active forms. Unlike neurotrophins, the GFLs are GPI-anchored to the cell surface and can be released through proteolytic cleavage or alternative splicing [@lindholm2007]. This membrane-associated nature of GFLs has implications for their signaling mechanisms, as they can mediate both cell-autonomous and non-cell-autonomous effects.
Role in dopaminergic neuron survival: GDNF is the most potent known survival factor for midbrain dopaminergic neurons [@allen2013][@airavaara2012]. GDNF supports the survival of dopaminergic neurons in the substantia nigra pars compacta (SNc) and promotes neurite outgrowth and dopamine uptake. The mechanism of GDNF-mediated neuroprotection involves activation of multiple downstream pathways including PI3K-AKT, RAS-MAPK-ERK, and PLCγ, ultimately leading to enhanced neuronal survival, protection against oxidative stress, and improved mitochondrial function [@airavaara2012].
GDNF receptor signaling involves a complex interplay between RET (the signaling-competent receptor) and GFRα co-receptors (GFRα1-4). The GFRα1/RET complex is the primary signaling unit for GDNF, but alternative co-receptor combinations can mediate signaling for other GFLs (GFRα2 for neurturin, GFRα3 for artemin, GFRα4 for persephin) [@lindholm2007]. This receptor specificity explains the distinct neuronal populations supported by each GFL.
Clinical trials in PD: GDNF has been the most extensively clinically tested neurotrophic factor for PD:
Neurturin signals through GFRα2/RET and supports both dopaminergic and enteric neurons. AAV2-neurturin (CERE-120) was tested in two large randomized PD trials [@marks2010]. Both failed to meet primary endpoints, potentially because advanced PD patients had too few remaining dopaminergic terminals to respond to trophic support, and because alpha-synuclein pathology may impair retrograde transport of neurturin from striatal injection sites to cell bodies in the substantia nigra. This led to the design of dual-site injection protocols (putamen + substantia nigra) [@chao2014].
The failure of neurturin trials underscores the importance of understanding the underlying pathophysiology when designing neurotrophic factor therapies. In PD, alpha-synuclein pathology may not only impair retrograde transport but may also disrupt receptor signaling at multiple levels, creating barriers to effective trophic support even when adequate protein is delivered to target regions [@marks2010].
CDNF and MANF constitute a unique neurotrophic factor family that functions primarily through modulating endoplasmic reticulum stress and the unfolded protein response [@lindholm2007]. These proteins represent a distinct structural class of neurotrophic factors that lack sequence homology with either neurotrophins or the GDNF family. Their mechanism of action involves binding to the ER chaperone GRP78/BiP and modulating ER stress signaling pathways [@lindholm2007].
CDNF has shown potent neuroprotective effects in PD animal models, and a Phase 1-2 clinical trial (2020) demonstrated safety of intraputamenal CDNF infusion in PD patients [@lindholm2007]. The unique mechanism of CDNF and MANF, involving ER stress modulation, makes them particularly attractive for neurodegenerative diseases characterized by protein misfolding and ER stress, including PD and related disorders.
MANF (also known as ARMET) is constitutively expressed in the brain and is upregulated in response to various cellular stresses. Its role in neuronal survival appears to be mediated through both ER stress-dependent and independent mechanisms. Preclinical studies have shown that MANF can protect dopaminergic neurons in models of PD and may have therapeutic potential in other neurodegenerative conditions [@lindholm2007].
CNTF supports motor neurons, retinal ganglion cells, and oligodendrocytes. A Phase 3 clinical trial of recombinant CNTF in ALS failed due to systemic side effects (weight loss, fever) from peripheral administration [@allen2013]. Modified CNTF variants and targeted delivery approaches are under development. CNTF signals through a tripartite receptor complex comprising CNTFRα, gp130, and LIFR, activating the JAK-STAT and PI3K-AKT signaling pathways [@allen2013].
The failure of the CNTF trial in ALS highlights the challenges of delivering neurotrophic factors systemically, as peripheral administration leads to significant side effects that limit dosing. Alternative approaches including intrathecal delivery, gene therapy, and modified CNTF variants with improved safety profiles are being explored to overcome these limitations [@allen2013].
IGF-1 signals through the IGF-1 receptor and activates PI3K-AKT and RAS-MAPK survival pathways [@chao2014]. IGF-1 supports motor neurons and is implicated in ALS pathogenesis — some studies show reduced IGF-1 signaling in ALS motor neurons. IGF-1 also interacts with brain insulin signaling and insulin resistance pathways relevant to AD [@allen2013].
The role of IGF-1 in neurodegeneration is complex, with both neuroprotective and potentially detrimental effects depending on context. In ALS, reduced IGF-1 signaling in motor neurons contributes to vulnerability, and therapeutic administration of IGF-1 has shown promise in preclinical models. However, the systemic effects of IGF-1 and its potential to promote tumor growth have limited clinical development [@chao2014].
In Alzheimer's disease, the relationship between IGF-1 and neurodegeneration is complicated by the intersection of brain insulin signaling and neurotrophic support. IGF-1 resistance has been proposed as a mechanism linking metabolic dysfunction to cognitive decline in AD, and strategies to improve IGF-1 signaling in the brain are under investigation [@allen2013].
Beyond its angiogenic role, VEGF has direct neurotrophic and neuroprotective effects [@allen2013]. Reduced VEGF is associated with motor neuron degeneration in ALS models, and VEGF gene deletion in mice causes motor neuron degeneration resembling ALS. Intrathecal VEGF delivery and VEGF gene therapy (AAV-VEGF) have shown preclinical promise in ALS models. VEGF also supports blood-brain barrier integrity and neurovascular unit function.
The neurotrophic effects of VEGF are mediated through VEGFR2 (Flk-1) receptor expressed on neurons and glial cells. VEGF can promote neuronal survival, stimulate neurite outgrowth, and modulate synaptic function through both direct receptor signaling and indirect effects on the neurovascular unit [@allen2013]. The dual role of VEGF in both angiogenesis and neuroprotection makes it a unique therapeutic target, particularly in diseases involving both vascular and neuronal dysfunction.
The fibroblast growth factor family, particularly FGF-2 (basic FGF), has neurotrophic and neuroprotective properties in the central nervous system [@allen2013]. FGF-2 supports the survival and proliferation of neural stem cells, promotes neuronal differentiation, and has been investigated in models of stroke and neurodegenerative diseases. FGF signaling through FGFR receptors activates both PI3K-AKT and RAS-MAPK pathways, contributing to neuronal survival and plasticity [@allen2013].
All neurotrophin receptors (TrkA, TrkB, TrkC) are receptor tyrosine kinases that activate three major downstream cascades [@chao2014]:
The pan-neurotrophin receptor p75NTR can promote either survival or death depending on co-receptor expression and ligand form (mature neurotrophin vs. proneurotrophin) [@mufson2019]. In the context of neurodegeneration, increased p75NTR expression and elevated proneurotrophin levels can shift signaling toward apoptosis and necroptosis.
In Alzheimer's disease, multiple components of neurotrophin signaling are disrupted. TrkA and TrkB signaling is impaired through multiple mechanisms including reduced receptor expression, impaired trafficking, and direct interference by pathological proteins. Amyloid-beta can directly bind to p75NTR and promote apoptotic signaling, while tau pathology disrupts axonal transport of neurotrophin-containing vesicles [@mufson2019][@zuccato2009].
In Parkinson's disease, alpha-synuclein oligomers can interfere with retrograde transport of neurotrophic factors and disrupt signaling at multiple levels. GDNF and BDNF signaling through their respective receptors is compromised in PD models, and restoration of these pathways is a key therapeutic goal [@marks2010][@zuccato2009].
In Huntington's disease, mutant huntingtin disrupts BDNF transcription and transport, leading to reduced trophic support for striatal neurons. Additionally, REST/NRSF dysregulation leads to broad changes in gene expression that amplify the trophic deficit [@zuccato2009].
In ALS, multiple neurotrophic factor pathways are affected, including reduced BDNF, GDNF, and CNTF signaling in motor neurons. The failure of CNTF clinical trials and the ongoing challenges with GDNF delivery highlight the complexity of translating neurotrophic factor therapy for motor neuron diseases [@allen2013].
The blood-brain barrier (BBB) remains a major obstacle for neurotrophic factor therapy. These proteins are too large to cross the BBB via passive diffusion, necessitating alternative delivery strategies. Approaches under investigation include:
The development of small molecule receptor agonists that can penetrate the BBB represents an attractive alternative to protein delivery. TrkA agonists for NGF signaling, TrkB agonists for BDNF signaling, and RET agonists for GDNF family signaling are under development [@allen2013]. These compounds offer advantages including oral bioavailability, better tissue distribution, and reduced immunogenicity compared to protein therapeutics.
Given the complex and multifactorial nature of neurodegenerative diseases, combination approaches targeting multiple pathways may be necessary. Potential combinations include:
The development of biomarkers to predict treatment response and monitor therapeutic efficacy is crucial for advancing neurotrophic factor therapies. Potential biomarkers include:
Neurotrophic factors represent a fundamental component of neuronal survival and function, and their dysregulation is a common feature of neurodegenerative diseases. Despite significant challenges in therapeutic delivery and receptor specificity, neurotrophic factor-based therapies remain a promising approach for neuroprotection and neural repair. Advances in delivery technology, receptor-selective small molecule agonists, and gene therapy approaches continue to drive progress in this field, with multiple clinical trials ongoing or planned for Alzheimer's disease, Parkinson's disease, Huntington's disease, and ALS.
The following diagram shows the key molecular relationships involving Neurotrophic Factors in Neurodegeneration discovered through SciDEX knowledge graph analysis: