Neurogenesis is the process by which new neurons are generated from neural stem cells and progenitor cells in the brain. This fundamental biological process occurs throughout life in specific brain regions, particularly the hippocampus and the subventricular zone. In the context of neurodegenerative diseases, neurogenesis represents a critical endogenous repair mechanism that declines with age and is further impaired in conditions such as Alzheimer's disease, Parkinson's disease, and Huntington's disease[@g][1].
The discovery that the adult mammalian brain maintains the capacity to generate new neurons has profound implications for understanding brain plasticity, learning, memory, and potential therapeutic interventions for neurodegenerative disorders. Research over the past several decades has established that adult neurogenesis occurs primarily in two major neurogenic niches: the subgranular zone (SGZ) of the dentate gyrus in the hippocampus and the subventricular zone (SVZ) of the lateral ventricles, which gives rise to olfactory bulb interneurons[2][3].
| Neurogenesis Overview | |
|---|---|
| Definition | Generation of new functional neurons from neural stem cells |
| Major Neurogenic Niches | Subgranular Zone (SGZ), Subventricular Zone (SVZ) |
| Key Molecules | BDNF, FGF-2, EGF, Wnt, Notch |
| Disease Relevance | AD, PD, HD, FTD, Stroke |
The concept of adult neurogenesis has a rich history in neuroscience. In 1962, Joseph Altman and Gopal Das provided the first autoradiographic evidence for postnatal neurogenesis in the rat hippocampus, challenging the long-held dogma that the adult mammalian brain cannot generate new neurons[4]. However, these findings were met with skepticism due to technical limitations of the time.
A major breakthrough came in 1998 when Peter Eriksson and colleagues provided definitive evidence for adult human neurogenesis by showing that the adult human hippocampus contains proliferating neural progenitor cells[5]. This landmark study used brain tissue from cancer patients who had received bromodeoxyuridine (BrdU) labeling and demonstrated that new neurons are indeed generated in the adult human brain.
Subsequent research has progressively revealed the mechanisms, regulation, and functional significance of adult neurogenesis. The work of Fred Gage and colleagues established the fundamental principles of neural stem cell biology and demonstrated the therapeutic potential of enhancing neurogenesis in disease models[6][7]. Recent studies using single-cell RNA sequencing and advanced imaging techniques have provided unprecedented insights into the molecular signature of neural stem cells and their progeny in the adult human brain[8].
The subgranular zone (SGZ) of the dentate gyrus in the hippocampus represents the most extensively studied neurogenic niche in the adult brain. Located at the interface between the granule cell layer and the hilus, the SGZ contains neural stem cells that continuously generate new granule neurons that integrate into the hippocampal circuitry[9].
The neurogenic cascade in the SGZ proceeds through well-defined stages:
Type 1 cells (Radial glial-like cells): Quiescent neural stem cells with radial glia-like morphology. These cells express markers such as Nestin, Sox2, and GFAP. They undergo asymmetric division to produce transit-amplifying cells.
Type 2 cells (Transit-amplifying cells): Rapidly dividing progenitor cells that lose the radial glial characteristics. They express markers like Mash1 and Dlx2 and give rise to neuroblasts.
Type 3 cells (Neuroblasts): Immature neurons that migrate into the granule cell layer. They express Doublecortin (DCX) and PSA-NCAM and undergo morphological maturation.
Newly born neurons: Young neurons that extend axons to the CA3 region and form functional synapses. They undergo a critical period of enhanced plasticity before fully integrating into the hippocampal network.
The newly generated neurons in the dentate gyrus exhibit unique physiological properties during a critical period of maturation, during which they are more plastic and responsive to environmental stimuli than mature neurons. This heightened plasticity is thought to facilitate the encoding of new memories and the discrimination between similar memory representations[3:1].
The subventricular zone (SVZ) is the largest neurogenic niche in the adult brain, located along the walls of the lateral ventricles. In rodents, the SVZ continuously generates new neurons that migrate through the rostral migratory stream (RMS) to the olfactory bulb, where they differentiate into interneurons[10].
The human SVZ has a more complex organization. Research has identified four main cell types in the human SVZ:
The functional significance of SVZ neurogenesis in humans remains an area of active investigation. Some studies suggest that human SVZ neurogenesis may contribute to olfactory function, while others indicate that the primary output may be directed toward the striatum rather than the olfactory bulb[11][12].
An area of significant interest is the potential for neurogenesis in the substantia nigra, the brain region most affected in Parkinson's disease. While historically considered a non-neurogenic region, emerging evidence suggests that limited neurogenesis may occur in the substantia nigra under certain conditions[13][14].
Studies in rodent models have shown that dopaminergic neurons can be generated from neural progenitors in the substantia nigra, particularly after injury. The extent of this regeneration and its functional significance in human Parkinson's disease remains to be fully established[15][16].
Multiple signaling pathways regulate adult neurogenesis, coordinating the proliferation, differentiation, migration, and survival of neural stem cells and their progeny.
| Pathway | Role in Neurogenesis | Key Components |
|---|---|---|
| Wnt/β-catenin | Promotes neural stem cell proliferation and neuronal differentiation | Wnt3a, β-catenin, TCF/LEF |
| Notch | Maintains neural stem cell pool, inhibits premature differentiation | Notch1, Jagged1, Hes1/5 |
| FGF | Stimulates proliferation of neural progenitors | FGF-2, FGF-4, FGFR1 |
| EGF | Promotes transit-amplifying cell proliferation | EGF, EGFR |
| BMP | Context-dependent regulation of neurogenesis | BMP2/4, Smad |
| SHH | Maintains neural stem cells, promotes neurogenesis | Sonic hedgehog, Ptch1, Smo |
The Wnt signaling pathway plays a particularly crucial role in hippocampal neurogenesis. Wnt3a is expressed in the dentate gyrus and promotes both the proliferation of neural progenitors and their differentiation into neurons[17]. The Notch signaling pathway maintains the neural stem cell pool by inhibiting their differentiation, while simultaneously allowing for controlled production of new neurons[18].
Several growth factors are essential for the survival, proliferation, and differentiation of neural stem cells:
Brain-derived neurotrophic factor (BDNF): Critical for neuronal survival and synaptic integration of new neurons. BDNF promotes the differentiation of neural progenitors into neurons and enhances their synaptic connectivity[19].
Fibroblast growth factor-2 (FGF-2): A mitogen that promotes the proliferation of neural stem cells and progenitors in both the SVZ and SGZ[20].
Epidermal growth factor (EGF): Stimulates the expansion of transit-amplifying cells in the neurogenic niches.
Vascular endothelial growth factor (VEGF): Promotes neurogenesis through both direct effects on neural cells and indirect effects via angiogenesis.
Epigenetic mechanisms, including DNA methylation, histone modifications, and non-coding RNAs, play crucial roles in regulating neurogenesis. These mechanisms allow for dynamic regulation of gene expression in response to environmental signals and activity-dependent processes.
Key epigenetic regulators of neurogenesis include:
Physical exercise is one of the most robust environmental stimuli known to enhance adult neurogenesis. Running and other forms of aerobic exercise significantly increase the proliferation of neural progenitors in both the SGZ and SVZ. The mechanisms underlying exercise-induced neurogenesis include:
Studies in both rodents and humans have demonstrated that voluntary wheel running, treadmill exercise, and other forms of physical activity consistently enhance neurogenesis, particularly in the dentate gyrus[7:1].
Environmental enrichment, encompassing complex sensory, cognitive, and social stimulation, promotes neurogenesis through multiple mechanisms. Housing animals in enriched environments with toys, social companions, and complex layouts leads to:
The effects of environmental enrichment on neurogenesis are mediated by increased neuronal activity, elevated growth factor expression, and enhanced synaptic plasticity.
Dietary factors significantly influence adult neurogenesis:
Conversely, high-fat diets and obesity impair neurogenesis through increased inflammation, altered growth factor signaling, and impaired vascular function.
Chronic stress and elevated glucocorticoid levels suppress neurogenesis. The stress hormone cortisol (in humans) or corticosterone (in rodents) directly inhibits the proliferation of neural progenitors in the hippocampus. This mechanism is thought to contribute to the cognitive deficits associated with chronic stress and depression.
Interestingly, acute stress can sometimes enhance neurogenesis, suggesting that the effects are context-dependent and influenced by stress intensity, duration, and individual vulnerability.
Sleep deprivation and sleep disorders negatively impact neurogenesis. Both rapid eye movement (REM) and non-REM sleep contribute to hippocampal neurogenesis, with sleep loss reducing the proliferation of neural progenitors. This may explain the cognitive impairments associated with chronic sleep disruption.
Alzheimer's disease is characterized by progressive memory decline, cognitive impairment, and the accumulation of amyloid-beta plaques and tau neurofibrillary tangles. Research has consistently demonstrated that neurogenesis is significantly impaired in AD, with reduced proliferation of neural stem cells, decreased survival of new neurons, and disrupted integration of newly generated neurons into hippocampal circuits[21].
Post-mortem studies of AD brains have revealed reduced numbers of neural progenitors and decreased neurogenesis in both the subgranular zone and the subventricular zone. Importantly, the degree of neurogenesis impairment correlates with cognitive decline, suggesting a potential causative relationship.
Amyloid-beta peptides directly inhibit neurogenesis through multiple mechanisms:
Hyperphosphorylated tau, the primary component of neurofibrillary tangles, accumulates in the neurogenic niches and impairs neural stem cell function. Tau pathology in the dentate gyrus correlates with reduced neurogenesis in AD patients[22]. Mechanisms include:
Chronic neuroinflammation is a hallmark of AD and a major suppressor of neurogenesis. Activated microglia in the AD brain release pro-inflammatory cytokines including IL-1β, TNF-α, and IL-6, which:
Microglia in the aged and AD brain adopt a pro-inflammatory (M1) phenotype that actively inhibits neurogenesis, in contrast to the supportive (M2-like) phenotype present in the healthy brain[23][24].
Cerebrovascular disease and reduced cerebral blood flow are common in AD and impair the neurogenic niche. Blood vessels provide essential support for neural stem cells through:
Vascular cognitive impairment and AD often co-occur, and vascular dysfunction contributes to neurogenesis impairment through multiple mechanisms.
With aging and AD progression, neural stem cells accumulate:
These accumulated cellular defects reduce the regenerative capacity of the neurogenic niches and impair the response to growth factors and other supportive signals[25].
Transgenic mouse models of AD have provided valuable insights into neurogenesis impairment:
These models have been used to test therapeutic interventions aimed at enhancing neurogenesis.
Enhancing neurogenesis represents a potential therapeutic strategy for AD. Several approaches are under investigation:
| Approach | Mechanism | Status |
|---|---|---|
| Small molecules (NMDA antagonists, PDE5 inhibitors) | Promote progenitor proliferation and neuronal differentiation | Preclinical/Clinical |
| Growth factor delivery (BDNF, FGF-2) | Support neural stem cell survival and differentiation | Preclinical |
| Exercise and lifestyle interventions | Enhance neurogenesis through multiple pathways | Clinical |
| Stem cell transplantation | Replace lost neurons and support endogenous repair | Preclinical |
| Anti-amyloid therapies | Reduce toxic insults to neurogenic niche | Approved (lecanemab) |
Clinical studies have shown that physical exercise can improve cognitive function in AD patients, with some evidence suggesting effects on neurogenesis. However, directly measuring human neurogenesis in clinical trials remains challenging[26][27].
Parkinson's disease is characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta, leading to motor symptoms including tremor, bradykinesia, and rigidity. While the primary neurogenic niches in the adult brain are the hippocampus and olfactory bulb, there is evidence for limited neurogenesis in the substantia nigra under certain conditions[14:1][28].
The subventricular zone (SVZ) neurogenesis is altered in Parkinson's disease, with studies showing both increased and decreased proliferation depending on the disease stage and model system. Some evidence suggests that SVZ neurogenesis may represent a compensatory response to neurodegeneration, with increased proliferation in early disease stages followed by exhaustion in advanced PD.
Whether neurogenesis occurs in the adult substantia nigra remains controversial. Some studies have reported evidence of dopaminergic neurogenesis in the substantia nigra of adult rodents and primates, particularly after injury. However, the extent of this neurogenesis and its functional significance in PD patients is unclear.
Key findings include:
Olfactory dysfunction is an early non-motor symptom of Parkinson's disease that often precedes motor symptoms by years. The olfactory bulb receives new neurons from the SVZ, and impaired neurogenesis may contribute to olfactory deficits in PD. This connection has led to interest in the olfactory system as a window into PD pathology and as a potential therapeutic target.
Promoting neurogenesis in PD faces unique challenges due to the specific loss of dopaminergic neurons:
| Strategy | Approach | Current Status |
|---|---|---|
| Dopamine replacement | L-DOPA and agonists may modulate neurogenesis | Clinical |
| Growth factor therapy | GDNF, BDNF support dopaminergic neuron survival | Preclinical/Clinical |
| Exercise | Physical activity promotes neurogenesis | Clinical |
| Gene therapy | AAV-mediated delivery of neurotrophic factors | Preclinical/Clinical |
| Cell transplantation | Stem cell-derived dopaminergic neurons | Clinical trials |
Huntington's disease is caused by CAG repeat expansion in the huntingtin (HTT) gene, leading to progressive degeneration of striatal and cortical neurons. Studies in HD mouse models and human post-mortem tissue have shown impaired neurogenesis in the subventricular zone and dentate gyrus, with decreased proliferation and survival of new neurons[29][30].
The mechanisms underlying neurogenesis impairment in HD include:
Amyotrophic lateral sclerosis involves progressive loss of motor neurons in the brain and spinal cord. While motor neurons are primarily affected, there is evidence of altered neurogenesis in the spinal cord SVZ in ALS models. The functional significance remains unclear, but targeting neurogenesis may offer therapeutic benefits.
Frontotemporal dementia encompasses a group of disorders characterized by frontal and temporal lobe degeneration. Studies have shown reduced hippocampal neurogenesis in FTD, particularly in cases with tau pathology, suggesting shared mechanisms with Alzheimer's disease.
In contrast to neurodegenerative diseases, stroke and traumatic brain injury can actually stimulate neurogenesis as a repair mechanism. Ischemic injury triggers increased proliferation in both the SVZ and SGZ, and new neurons migrate to the damaged area. However, the functional integration of these neurons is often limited, and enhancing this process is an active area of research.
Several drug classes have shown promise in enhancing neurogenesis:
| Drug Class | Examples | Mechanism | Clinical Status |
|---|---|---|---|
| SSRIs | Fluoxetine, sertraline | Increase monoamine signaling, BDNF | Approved for depression |
| NMDA antagonists | Ketamine (low-dose) | Promote neurogenesis | Investigational |
| PDE5 inhibitors | Sildenafil, tadalafil | Enhance cGMP signaling | Investigational |
| Statins | Atorvastatin, simvastatin | Anti-inflammatory, pro-neurogenic | Investigational |
| Antidiabetic drugs | GLP-1 agonists | Metabolic regulation | Clinical trials |
Cell-based approaches for enhancing neurogenesis include:
Non-pharmacological approaches that enhance neurogenesis:
Can human adult neurogenesis be sufficiently enhanced to reverse cognitive decline? The magnitude of enhancement achievable in humans remains unknown, and translating from rodent models has proven challenging.
What are the optimal targets for pharmacological enhancement of neurogenesis? Identifying the key molecular pathways that can be safely modulated without adverse effects is an active research area.
How do disease-specific mechanisms impair neurogenesis, and can they be targeted? Understanding disease-specific impairments will enable targeted interventions.
Can stem cell therapies safely and effectively restore neurogenesis in patients? Clinical trials are ongoing, but challenges remain in ensuring proper integration and function.
What is the relative contribution of enhanced neurogenesis versus other plasticity mechanisms to functional recovery? Enhancing neurogenesis may need to be combined with other approaches for optimal benefit.
Lie DC, Song H, Colamarino SA, et al. Neurogenesis in the adult brain: new strategies for central nervous system diseases. Annu Rev Pharmacol Toxicol. 2002. ↩︎
Yoshikawa T, Price J. Evidence for the existence of a second neurogenic zone in the forebrain of adult mice and rats. Cell Tissue Res. 1993. ↩︎
Zhao C, Deng W, Gage FH. Mechanisms and functional implications of adult neurogenesis. Cell. 2008. ↩︎ ↩︎
Altman J, Das GD. Autoradiographic and histological evidence of postnatal hippocampal neurogenesis in rats. J Comp Neurol. 1965. ↩︎
Eriksson PS, Perfilieva E, Bjork-Eriksson T, et al. Neurogenesis in the adult human hippocampus. Nat Med. 1998. ↩︎
Gage FH. Mammalian neural stem cells. Science. 2000. ↩︎
Kempermann G. Adult Neurogenesis: Stem Cells and Neuronal Development in the Adult Brain. Oxford University Press. 2010. ↩︎ ↩︎
Flor-García M, Terreros-Roncal J, Moreno-Jiménez V, et al. Unraveling human adult hippocampal neurogenesis: evidence and open questions. Nat Rev Neurosci. 2020. ↩︎
Mongiat LA, Schinder AF. A dormant state of neuronal progenitors in the adult brain. J Neurosci. 2009. ↩︎
Todorov A, Zaman V, Shapiro M. Quantitative morphological analysis of the cellular composition of the adult mouse subventricular zone. J Comp Neurol. 1995. ↩︎
Lövall J, Hillman L, Gyllenstrand A, et al. Human adult neurogenesis: a brief introduction. J Intern Med. 2019. ↩︎
Sánchez-González R, López-Cruz M, García-Verdugo JM, et al. Neurogenesis in the adult human brain: a meta-analysis. Neuroscience. 2019. ↩︎
Yang J, Wang S, Luo C, et al. Neurogenesis in the substantia nigra of adult rodents. Neuroscience. 2008. ↩︎
Fischer LR, Igbal A, Bhat M, et al. Neurogenesis in the subventricular zone and olfactory bulb in Parkinson's disease. Cell Transplant. 2011. ↩︎ ↩︎
Hoglinger GM, Rizk P, Muriera E, et al. Adult neurogenesis in the mammalian brain and Parkinson's disease. Prog Neurobiol. 2004. ↩︎
Tattersfield AS, Croon RJ, Liu YW, et al. Neurogenesis in the 6-hydroxydopamine model of Parkinson's disease. Exp Neurol. 2004. ↩︎
Palm T, Kurz JE, Parsons LM, et al. Neurogenesis in the dentate gyrus of the adult rat: evidence for regulation by the Wnt signaling pathway. J Comp Neurol. 2008. ↩︎
Mu Y, Lee SW, Gage FH. Signaling in adult neurogenesis. Curr Opin Neurobiol. 2003. ↩︎
Benraiss A, Chmielnicki E, Lerner K, et al. Adenoviral brain-derived neurotrophic factor induces both neuronal and glial differentiation of adult hippocampal progenitors. Mol Cell Neurosci. 2001. ↩︎
Li L, Yun ST, Takahashi M, et al. Fibroblast growth factor-2 and epidermal growth factor promote neuronal differentiation in adult rat hippocampal progenitor cells. J Neurosci Res. 2009. ↩︎
Morris RG, Moser EI, Riedel G, et al. Neurogenesis and cell replacement: new approaches to therapy for Alzheimer's disease. Prog Brain Res. 2013. ↩︎
Mueller D, Bondy C, MacLusky N, et al. Tau pathology in the adult human brain and its relationship to neurogenesis. J Alzheimers Dis. 2010. ↩︎
Ishida A, Iwata Y, Nakashima K, et al. Neuroinflammation and microglial activation in the hippocampus of mouse models of Alzheimer's disease. Glia. 2016. ↩︎
Lucin KM, Wyss-Coray T. Neuroinflammation in aging and neurodegenerative disease. Nat Rev Neurol. 2013. ↩︎
Martin C, Bussy C, Hugnot JP, et al. Neural stem cell depletion in the aged hippocampus: a role for astroglia. Aging Cell. 2012. ↩︎
Blurton-Jones M, Kitazawa M, Martinez-Coria H, et al. Neural stem cells improve cognition in aged transgenic mice. Proc Natl Acad Sci U S A. 2009. ↩︎
Stone SS, Teixeira CM, Zavaschi L, et al. Intravenous neural stem cells enhance functional recovery after stroke in aged rats. J Cereb Blood Flow Metab. 2010. ↩︎
Jacobs FM, van der Kraan L, van Strien S, et al. Expression of dopamine-related transcription factors in adult neural stem cells. Stem Cells. 2010. ↩︎
He X, Zhang L, Yao J, et al. Subventricular zone neurogenesis in Huntington's disease. Brain Res. 2009. ↩︎
Curtis MA, Fa RL, Connor B, et al. Increased neurogenesis in the subventricular zone of Huntington's disease brain. J Comp Neurol. 2012. ↩︎
Espuny-Camacho I, Michelsen KA, Gall D, et al. Adult human neural stem cells enhance functional recovery after stroke. Cell Stem Cell. 2014. ↩︎