Adult neurogenesis — the generation of new functional neurons from neural stem cells (NSCs) in the mature brain — was first conclusively demonstrated in humans by Eriksson et al. in 1998[@eriksson1998]. This finding overturned the long-held dogma that the adult mammalian brain is incapable of generating new neurons. Since then, impaired neurogenesis has emerged as a common hallmark across neurodegenerative diseases, contributing to neuronal loss and the failure of endogenous repair mechanisms that might otherwise slow disease progression.
Adult neurogenesis — the generation of new functional neurons from neural stem cells (NSCs) in the mature brain — was first conclusively demonstrated in humans by Eriksson et al. in 1998[@eriksson1998]. This finding overturned the long-held dogma that the adult mammalian brain is incapable of generating new neurons. Since then, impaired neurogenesis has emerged as a common hallmark across neurodegenerative diseases, contributing to neuronal loss and the failure of endogenous repair mechanisms that might otherwise slow disease progression.
| Feature | Alzheimer's Disease | Parkinson's Disease | ALS | Frontotemporal Dementia | Huntington's Disease |
|---------|-------------------|---------------------|-----|----------------------|---------------------|
| Primary abnormality | Decreased SGZ neurogenesis, impaired differentiation | SVZ/SGZ reduction, olfactory dysfunction | NSC niche disruption, motor cortex involvement | Frontal/subventricular zone impairment | Striatal neurogenesis loss, RMS impairment |
| Key pathological trigger | Amyloid-beta, tau hyperphosphorylation | Alpha-synuclein, dopamine loss | TDP-43, SOD1, C9orf72 | TDP-43, progranulin | Mutant huntingtin, CAG repeat |
| Affected niche | Hippocampal SGZ primarily | SVZ and olfactory bulb | SVZ, motor cortex subventricular | Frontal SVZ, subcallosal | Striatal RMS, subventricular |
| Neurotrophic factor decline | BDNF, NGF, IGF-1 | BDNF, GDNF | BDNF, CNTF | BDNF, NGF | BDNF, GDNF |
| Inflammatory contribution | Microglial activation, cytokines impair NSC | Neuroinflammation blocks neurogenesis | TDP-43 triggers neuroinflammation | Progranulin affects microglia | mHTT activates microglia |
| Clinical correlation | Memory deficits, pattern separation loss | Olfactory dysfunction, mood changes | Motor neuron loss, cognitive decline | Executive dysfunction | Motor symptoms, cognitive decline |
| Therapeutic targeting | Exercise, BDNF agonists, NSC transplantation | Exercise, GDNF, olfactory ensheathing cells | BDNF, CNTF, NSC therapy | BDNF, progranulin modulation | BDNF, NSC therapy |
The subgranular zone lies at the inner border of the granule cell layer in the hippocampal dentate gyrus. Radial glia-like Type 1 neural stem cells (expressing [GFAP](/entities/glial-fibrillary-acidic-protein), [SOX2](/proteins/sox2-protein), and Nestin) give rise to transit-amplifying Type 2 progenitors, which differentiate into Type 3 neuroblasts (expressing doublecortin/DCX and PSA-NCAM), ultimately maturing into glutamatergic dentate granule neurons that integrate into existing hippocampal circuits.
Approximately 700 new neurons are estimated to be added to each human hippocampus per day under normal conditions, based on carbon-14 birth-dating studies. These new neurons are critical for pattern separation, spatial memory formation, and contextual fear conditioning — functions central to hippocampal processing and vulnerable in early [Alzheimer's disease](/diseases/alzheimers-disease).
The subventricular zone lines the lateral walls of the lateral ventricles and is the largest neurogenic niche in the adult mammalian brain. It contains Type B neural stem cells and Type C transit-amplifying cells that proliferate and differentiate continuously. In rodents, SVZ-derived neuroblasts migrate along the rostral migratory stream (RMS) to the olfactory bulb, where they differentiate into GABAergic and dopaminergic interneurons.
In humans, SVZ neurogenesis is prominent in early life but declines substantially with age, with the RMS becoming largely vestigial by adulthood. However, SVZ-derived progenitors may retain capacity for reactive neurogenesis following injury.
Adult hippocampal neurogenesis is significantly impaired in [Alzheimer's disease](/diseases/alzheimers-disease), contributing directly to the memory deficits that characterize early-stage disease. Multiple studies have demonstrated that while thousands of immature neurons (DCX-positive) are identifiable in the dentate gyrus of neurologically healthy subjects up to the ninth decade of life, the number and maturation of these neurons progressively decline as AD advances[@sorrells2018][@boldrini2018][@tobin2019][@moreno-jimenez2019].
Amyloid-beta effects: [Amyloid-beta](/proteins/amyloid-beta) disrupts [Wnt/β-catenin signaling](/mechanisms/wnt-beta-catenin-signaling-neurodegeneration), a critical pathway for NSC proliferation and differentiation. Aβ also directly impairs neuronal maturation and reduces dendritic integration of new neurons.
Tau hyperphosphorylation: [Tau](/proteins/tau) hyperphosphorylation impairs cytoskeletal integrity in neural progenitor cells, disrupting their migration and differentiation. Tau pathology in the dentate gyrus correlates with reduced neurogenesis in AD patients[@adneurogenesis2023].
Neuroinflammation: Chronic [microglial activation](/entities/microglia) releases cytokines (IL-1β, IL-6, TNF-α) that suppress NSC proliferation and promote NSC differentiation toward gliogenesis rather than neurogenesis.
Neurotrophic factor decline: BDNF levels are reduced in the AD hippocampus, impairing survival and integration of new neurons. Reduced [IGF-1](/mechanisms/igf-1-signaling-pathway) signaling further compromises neurogenesis.
In [Parkinson's disease](/diseases/parkinsons-disease), neurogenesis impairment affects both the SVZ-olfactory bulb pathway and the hippocampal SGZ. Olfactory dysfunction in PD correlates with SVZ impairment, while hippocampal neurogenesis deficits contribute to non-motor symptoms including depression and cognitive impairment[@pdsynuclein2024].
Alpha-synuclein pathology: [Alpha-synuclein](/proteins/alpha-synuclein) accumulation in the SVZ directly impairs neural stem cell function. Oligomeric forms of α-syn are particularly toxic to NSCs.
Dopamine depletion: Dopaminergic signaling is required for proper neurogenesis in the SVZ. Loss of dopaminergic neurons reduces trophic support for NSCs.
Neuroinflammation: As in AD, microglial activation contributes to reduced neurogenesis through cytokine-mediated inhibition.
Adult neurogenesis is impaired in [ALS](/diseases/amyotrophic-lateral-sclerosis) through multiple mechanisms related to TDP-43 pathology, the most common proteinopathy in ALS[@terreros2021]. Both the SVZ and the motor cortex-associated neurogenic niches are affected.
TDP-43 pathology: TDP-43 inclusions in NSCs impair their function and promote premature differentiation. The loss of TDP-43 nuclear function disrupts splicing of genes essential for NSC maintenance.
SOD1 mutations: Mutations in [SOD1](/genes/sod1) cause direct toxicity to NSCs and alter the neurogenic niche environment.
C9orf72 expansion: The most common genetic cause of ALS also affects NSC function through RNA toxicity and dipeptide repeat proteins.
[Frontotemporal dementia](/diseases/frontotemporal-dementia) involves neurogenesis impairment particularly in the frontal SVZ and subcallosal zone, correlating with the executive dysfunction that characterizes FTD.
TDP-43 pathology: As in ALS, TDP-43 inclusions in NSCs impair their function. The majority of FTD cases (including behavioral variant FTD and semantic variant PPA) involve TDP-43 pathology.
Progranulin deficiency: Progranulin mutations cause FTLD-TDP and directly impair neurogenesis. Progranulin is required for proper NSC proliferation and differentiation.
Frontotemporal niche vulnerability: The frontal lobe SVZ shows particular vulnerability to progranulin-related pathology.
In [Huntington's disease](/diseases/huntingtons), neurogenesis is severely impaired in both the SVZ and the striatal niche. The striatal neurogenic niche, which normally produces interneurons, is particularly affected.
Mutant huntingtin toxicity: [Huntingtin](/genes/htt) protein with expanded CAG repeats directly impairs NSC function. Mutant huntingtin (mHTT) disrupts gene expression programs required for NSC maintenance and differentiation.
Striatal interneuron loss: The normal striatal neurogenesis that produces GABAergic interneurons is disrupted, contributing to the inhibitory/excitatory imbalance characteristic of HD.
BDNF deficiency: mHTT impairs BDNF transcription and transport, reducing trophic support for neurogenesis in both the SVZ and hippocampus.
Across all five diseases, decline in neurotrophic factors — particularly [BDNF](/mechanisms/bdnf-signaling-neurodegeneration), [GDNF](/mechanisms/gdnf-signaling-pathway), [IGF-1](/mechanisms/igf-1-signaling-pathway), and [VEGF](/mechanisms/vegf-angiogenesis-neurodegeneration) — is a common mechanism impairing neurogenesis. These factors are essential for NSC survival, proliferation, and neuronal differentiation.
[Microglial activation](/entities/microglia) and the associated release of pro-inflammatory cytokines (IL-1β, IL-6, TNF-α, IFN-γ) suppresses neurogenesis across all neurodegenerative conditions. Activated microglia shift NSC differentiation from neurogenesis toward gliogenesis.
Baseline adult neurogenesis declines with age in all humans[@boldrini2018]. Neurodegenerative diseases accelerate this decline through the disease-specific mechanisms outlined above, creating a "double hit" of age-related and disease-related impairment.
| Target | Approach | Disease Relevance | Clinical Status |
|--------|----------|-------------------|------------------|
| BDNF | Agonists, gene therapy | All | Preclinical/Phase I |
| VEGF | VEGF gene therapy | AD, PD | Phase I/II |
| GDNF | Protein delivery, gene therapy | PD, HD | Phase I/II |
| Exercise | Aerobic exercise | All | Established |
| NSC transplantation | Cell therapy | All | Phase I/II |
| Anti-inflammatory | Microglial modulators | All | Phase I/II |
| Wnt/β-catenin | Small molecule activators | AD | Preclinical |
| NCT ID | Intervention | Disease | Phase |
|--------|--------------|---------|-------|
| NCT02795052 | Neural stem cell transplant | PSP, CBD | Phase I |
| NCT04801021 | MSC-NTF cell therapy | ALS | Phase I |
| NCT03738392 | NSC transplantation | PD | Phase I/II |
| NCT03296618 | Exercise + cognitive training | AD | Phase II |
| NCT04139547 | BDNF gene therapy | PD | Phase I |
The following diagram shows the key molecular relationships involving Neurogenesis Disease Comparison discovered through SciDEX knowledge graph analysis: