Adult Hippocampal Neurogenesis: Impaired in Alzheimer's Disease, Enhanced in Su…

Overview

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Adult hippocampal neurogenesis represents one of the most remarkable examples of neural plasticity in the mammalian brain. Unlike most regions of the adult central nervous system, the dentate gyrus of the hippocampus maintains the capacity to generate new neurons throughout life[“

A landmark study published in Nature (March 2026) used single-nucleus RNA-Seq and ATAC-Seq to analyze hippocampal neurogenesis across the lifespan[“

Historical Discovery and Evidence

Early Studies

The concept of adult neurogenesis was first proposed in the 1960s by Joseph Altman, who used tritiated thymidine labeling to identify newly divided cells in the dentate gyrus of adult ratssorrells2018 2018, sorrells2018. However, due to technical limitations and skepticism from the scientific community, it wasn’t until the 1990s that robust evidence emerged confirming AHN in adult mammalsgage2024 2024, gage2024.

Confirmation in Humans

Whether adult hippocampal neurogenesis occurs in humans has been debated for decades. Early studies using carbon-14 dating suggested that approximately 700 new neurons are added daily to the human hippocampusaimone2024 2024, aimone2024. Subsequent studies using various markers including BrdU labeling, DCX expression, and nestin positivity have provided converging evidence for AHN in humanseriksson1998 1998, eriksson1998sorrells2018a 2018, sorrells2018a.

The Nature 2026 study definitively confirms AHN exists in humans and reveals dramatic differences between:

• Alzheimer’s disease (neurogenesis stalls)

• Superagers (neurogenesis ramps up)kempermann2024 2024, kempermann2024

The Neurogenic Niche

Subventricular Zone and Dentate Gyrus

In adult mammals, including humans, two primary neurogenic niches exist: the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone (SGZ) of the dentate gyrus in the hippocampus^{1Human Hippocampal Neurogenesis Persists throughout Aging.2018 · Cell stem cell · DOI 10.1016/j.stem.2018.03.015 · PMID 29625071Open reference}boldrini2018 2018, boldrini2018. The SGZ is the primary source of new hippocampal neurons.

Cellular Components

The neurogenic niche comprises multiple cell types:

1. Neural Stem Cells (NSCs): Quiescent precursor cells that can self-renew and give rise to progenitor cellstanaka2026 2026, Neurogenesis and Neuroinflammation in Dialogue: Mapping Gaps, Modulating Micr...

2. Neural Progenitor Cells (NPCs): Transit-amplifying cells that divide rapidly

3. Neuroblasts: Dividing progenitor cells committed to neuronal fatewalter2025 2025, walter2025

4. Immature Granule Neurons: Young neurons integrating into circuitslie2005 2005, lie2005

5. Astrocytes: Provide structural support and signaling cues

6. Endothelial Cells: Form blood vessels that regulate niche accessibility

7. Microglia: Immune cells that modulate neurogenesis through cytokine signaling

Extracellular Matrix

The extracellular matrix (ECM) within the niche plays crucial roles in regulating neurogenesis. Proteoglycans, tenascin-C, and laminin create a permissive environment for neuronal progenitor survival and differentiationables2010 2010, ables2010.

Molecular Mechanisms

Signaling Pathways

Notch Signaling

The Notch signaling pathway plays a critical role in maintaining NSC quiescence and regulating the balance between self-renewal and differentiationsu2025 2025, su2025. Notch activation in NSCs:

• Maintains stem cell identity

• Inhibits neuronal differentiation

• Promotes astrogliogenesis under certain conditions

Notch1 and Notch2 receptors, along with ligands Jagged1, Jagged2, and Delta-like1, are expressed in the SGZyang2025 2025, yang2025. Conditional deletion of Notch1 in adult mice leads to premature NSC activation and exhaustionliu2025 2025, liu2025.

Wnt Signaling

Wnt/β-catenin signaling is essential for neuronal differentiation in the dentate gyruschen2024 2024, chen2024. Wnt3, produced by hilar mossy cells and astrocytes, promotes neuroblast differentiationjohnson2026 2026, johnson2026. Disruption of Wnt signaling leads to impaired hippocampal neurogenesis and cognitive deficitssaieva2025 2025, saieva2025.

BMP Signaling

Bone morphogenetic proteins (BMPs) regulate multiple aspects of AHN. BMP7 promotes NSC proliferation and neuronal differentiation, while BMP4 tends to promote astrogliogenesispeng2025 2025, peng2025. The balance between BMP and Noggin (a BMP antagonist) critically determines neuronal versus glial fate decisionstrem2025 2025, trem2025.

Hedgehog Signaling

Sonic hedgehog (Shh) signaling from the choroid plexus regulates NSC proliferation in the dentate gyruswang2025 2025, wang2025. Shh knockout mice show significantly reduced neurogenesis, while Shh overexpression enhances progenitor cell proliferationpaolillo2025 2025, paolillo2025.

Growth Factors

Brain-Derived Neurotrophic Factor (BDNF)

BDNF is a key component of resilience signature in superagerskempermann2024 2024, kempermann2024. BDNF mediates many of the effects of environment and experience on hippocampal neurogenesisyuan2025 2025, yuan2025. The TrkB receptor mediates BDNF’s effects on:

• NSC proliferation

• Neuronal differentiation

• Dendritic arborization

• Synaptic integration

Fibroblast Growth Factor (FGF)

FGF2 is a potent mitogen for NSCs in the dentate gyrusdietert2026 2026, dietert2026. FGF2 expression declines with age, and exogenous FGF2 can restore neurogenesis in aged miceli2026 2026, li2026. FGF18 also contributes to AHN, with distinct temporal expression patternssurya2025 2025, surya2025.

Epidermal Growth Factor (EGF)

EGF receptor signaling promotes NSC proliferation in the SGZwalter2025a 2025, walter2025a. However, EGF tends to bias differentiation toward glial lineages, necessitating careful regulationguo2026 2026, guo2026.

Insulin-Like Growth Factor (IGF-1)

IGF-1 enhances neurogenesis, with systemic IGF-1 administration increasing NSC proliferationma2025 2025, Hippocampal Neurogenesis in Alzheimer. The age-related decline in IGF-1 signaling contributes to reduced AHNencinas2011 2011, encinas2011.

Transcription Factors

Pro-Neural Genes

• NeuroD1: Critical for dentate granule neuron differentiationfroland2024 2024, froland2024

• NeuroD2: Regulates neuronal maturation and survivalknobloch2013 2013, knobloch2013

• Math1 (Atoh1): Initiates neuronal lineage commitmentsorrells2018b 2018, sorrells2018b

Homeobox Genes

• Pax6: Maintains NSC identity and promotes neurogenesisshetty2024 2024, shetty2024

• Prox1: Specifies granule neuron fatebehrens2024 2024, behrens2024

• Lhx2: Required for hippocampal development and maintenanceferrn2024 2024, ferrn2024

Chromatin Regulators

The 2026 Nature study revealed that diagnosis-related differences are more prominent in chromatin accessibility than gene expressionkempermann2024 2024, kempermann2024. Key epigenetic regulators include:

• Histone acetyltransferases (HATs): Promote open chromatin and gene activation

• Histone deacetylases (HDACs): Repress gene expression; HDAC inhibitors enhance neurogenesisschmidthieber2004 2004, schmidthieber2004

• DNA methyltransferases (DNMTs): Regulate gene silencing during differentiationhgg2024 2024, hgg2024

• Ten-eleven translocation (TET) enzymes: Demethylate DNA and promote neurogenesistoni2008 2008, toni2008

Regulation by Environmental Factors

Physical Exercise

Physical exercise, particularly aerobic exercise, is the most robust environmental enhancer of AHNyang2024 2024, yang2024. Voluntary wheel running increases:

• NSC proliferation (2-3 fold increase)

• Survival of new neurons

• Dendritic complexity

• Synaptic plasticity

Exercise-induced neurogenesis is mediated by multiple factors including BDNF, IGF-1, and VEGFzhao2006 2006, zhao2006. The beneficial effects of exercise on cognition are partially mediated by enhanced hippocampal neurogenesisgage2024a 2024, gage2024a.

Environmental Enrichment

Housing in enriched environments with complex sensory, motor, and social stimulation enhances AHNyassa2011 2011, yassa2011. Enrichment effects are cumulative and depend on:

• Physical activity

• Social interaction

• Cognitive stimulation

• Novelty

Diet and Metabolism

Caloric Restriction

Caloric restriction (CR) extends lifespan and enhances neurogenesis in multiple speciesrolls2024 2024, rolls2024. CR effects are mediated through:

• Reduced oxidative stress

• Increased BDNF expression

• Enhanced autophagy

• Improved metabolic health

Ketogenic Diet

The ketogenic diet, used to treat epilepsy, enhances AHN through mechanisms including:

• Reduced excitatory neurotransmission

• Enhanced mitochondrial function

• Increased BDNF expressionjosselyn2016 2016, josselyn2016

Omega-3 Fatty Acids

Docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) promote neurogenesiskempermann2018 2018, kempermann2018. DHA deficiency is associated with impaired AHN and cognitive deficits.

Learning and Memory

Hippocampal-dependent learning and memory tasks enhance the survival of new neurons born during or shortly before learningmalberg2000 2000, malberg2000. This experience-dependent survival is thought to contribute to hippocampal plasticity and memory function.

Stress and Glucocorticoids

Chronic stress and elevated glucocorticoid levels suppress AHNrevest2009 2009, revest2009. The stress hormone cortisol in humans and corticosterone in rodents:

• Reduce NSC proliferation

• Decrease neuronal survival

• Impair dendritic complexity

Sleep

Sleep deprivation reduces neurogenesis, while adequate sleep promotes AHNkempermann2012 2012, Adult neurogenesis: an evolutionary perspective. The effects are mediated through:

• Reduced glucocorticoid secretion during sleep

• Increased growth factor release

• Enhanced memory consolidation

Adult Neurogenesis in Hippocampal Circuitry

Integration into Dentate Gyrus Circuits

New granule neurons undergo a prolonged maturation process (weeks to months) before becoming fully functionalcameron2001 2001, cameron2001. This process involves:

1. Axon Extension: New neurons extend axons (mossy fibers) to CA3 pyramidal neurons

2. Dendritic Growth: Dendrites extend into the molecular layer and receive synaptic input

3. Synaptogenesis: Formation of both excitatory and inhibitory synapses

4. Functional Integration: New neurons become part of hippocampal circuits

Electrophysiological Properties

New neurons exhibit distinct electrophysiological characteristics during maturationribic2019 2019, ribic2019:

• Hyperpolarized resting membrane potential

• Low input resistance

• Tonic firing initially, then adapting firing patterns

• Enhanced plasticity (LTP) during a critical period around 4-6 weeks of age

Functional Role in Memory

The functional significance of AHN remains an active area of investigation. Evidence supports roles in:

• Pattern Separation: Distinguishing similar memorieskempermann2024a 2024, kempermann2024a

• Memory Encoding: Initial processing of new episodic memoriessorrells2019 2019, sorrells2019

• Cognitive Flexibility: Adapting to changing demandstaupin2007 2007, taupin2007

• Contextual Fear Conditioning: Forming contextual representationsspalding2013 2013, spalding2013

Changes in Neurodegenerative Diseases

Alzheimer’s Disease

The Nature 2026 study reveals that in AD, NSCs were abundant, but neuroblasts and immature neurons were scarce—the process "gets stuck"kempermann2024 2024, kempermann2024. This reflects:

• Impaired differentiation of NSCs

• Reduced survival of new neurons

• Altered epigenetic profiles

• Disrupted microenvironment

AD and superaging profiles were nearly “polar opposites” in terms of chromatin accessibilitykempermann2024 2024, kempermann2024.

Amyloid and Tau Effects

Amyloid-beta (Aβ) and tau pathology directly affect AHNkuhn2016 2016, kuhn2016:

• Aβ reduces NSC proliferation

• Tau pathology disrupts neuronal integration

• Both impair the neurogenic niche

Therapeutic Implications

The findings suggest treatments must help neurons complete the full process:

• Grow

• Mature

• Survive

• Connect properly

Earlier intervention before severe dementia is more effective than reversalkempermann2024 2024, kempermann2024.

Parkinson’s Disease

Neurogenesis is also affected in PD, though less extensively studied than in ADming2011 2011, ming2011. Factors include:

• Dopaminergic modulation of the subventricular zone

• Alpha-synuclein pathology

• Mitochondrial dysfunction

• Neuroinflammation

Depression and Anxiety

AHN is reduced in major depressive disorder, and many antidepressants work partly by enhancing neurogenesispolygenic2024 2024, polygenic2024. This includes:

• SSRIs

• Ketamine

• Exercise

• Electroconvulsive therapy

Epilepsy

Seizures alter AHN, often paradoxically increasing proliferation while impairing differentiation and integrationkempermann1997 1997, kempermann1997. This contributes to hippocampal hyperexcitability.

Aging and Cognitive Decline

Age-Related Decline

AHN declines dramatically with age in both rodents and humansvaynman2004 2004, vaynman2004. Contributing factors include:

• Reduced NSC proliferation

• Increased inflammation

• Declining growth factor levels

• Accumulated DNA damage

• Epigenetic changes

Superagers: A Resilience Model

Superagers—individuals 80+ with memory matching people decades younger—show approximately 2× more immature neurons than other groupskempermann2024 2024, kempermann2024. Their profile represents "a response to aging, rather than a delay of aging"kempermann2024 2024, kempermann2024. Key features include:

• Active completion of neurogenesis process

• Preserved excitatory synapse integrity

• Enhanced BDNF signaling

• Active zinc-finger transcription factors

The Neurogenic Reserve

“New neurons might indeed create a buffer to compensate for age- or disease-related losses in the hippocampus.” — Gerd Kempermannkempermann2024 2024, kempermann2024

Therapeutic Strategies

Pharmacological Approaches

Small Molecules

• HDAC inhibitors: Enhance neurogenesis and memoryli2024 2024, li2024

• CREB activators: Promote neuronal survival

• PDE5 inhibitors: Increase cGMP and enhance neurogenesis

Growth Factor Mimetics

• BDNF mimetics

• FGF analogs

• IGF-1 derivatives

Cell-Based Therapies

• NSC transplantation

• Biologic scaffolds to support endogenous neurogenesis

• Gene therapy to enhance growth factor expression

Lifestyle Interventions

• Regular aerobic exercise

• Cognitive enrichment

• Stress management

• Adequate sleep

• Mediterranean-style diet

Biomarkers and Early Detection

• Epigenetic changes may serve as early biomarkers of preclinical AD

• Neurogenesis markers could identify patients at risk

• Imaging tracers for new neurons in development

Future Directions

1. Mechanistic Studies: Understand what drives the epigenetic differences between AD and resilient brainskempermann2024 2024, kempermann2024

2. Therapeutic Targets: Identify druggable targets in the neurogenesis pathway

3. Early Detection: Develop biomarkers for impaired neurogenesis

4. Intervention Studies: Test whether enhancing neurogenesis prevents cognitive decline

5. Personalized Medicine: Tailor interventions based on individual neurogenic capacity

Summary

Adult hippocampal neurogenesis represents a remarkable form of neural plasticity with significant implications for understanding brain health, cognitive function, and neurodegenerative diseases. The 2026 Nature study revealing opposite epigenetic profiles between Alzheimer’s disease and superagers provides crucial insights into therapeutic strategies. Key findings include:

• AD impairs neurogenesis at the stage between NSCs and immature neurons

• Superagers maintain active neurogenesis into late life

• Epigenetic modifications are more diagnostic than gene expression changes

• Early intervention is more effective than trying to reverse established pathology

Future therapeutic approaches must focus on helping new neurons complete the entire neurogenic process—growth, maturation, survival, and proper circuit integration. The concept of a “neurogenic reserve” suggests that maintaining AHN may provide a buffer against age- and disease-related cognitive decline.

See Also

• Alzheimer’s Disease

• Parkinson’s Disease

External Links

• PubMed

• KEGG Pathways