NAD+ Metabolism in Neurodegeneration

Introduction

Nicotinamide adenine dinucleotide (NAD+) is a fundamental coenzyme present in every living cell, serving as both a critical electron carrier in mitochondrial energy production and an essential substrate for a diverse family of signaling enzymes. NAD+ levels decline significantly with aging—by approximately 50% between youth and old age in multiple tissues—and this decline is accelerated in neurodegenerative diseases including Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington’s disease (HD)^{1(2019). NAD+ in brain aging and neurodegenerative disorders2019 · Cell Metabolism · PMID 31577933Open reference}lautrup2019 2019, lautrup2019.

The therapeutic strategy of boosting NAD+ levels has emerged as one of the most promising approaches to combating age-related neurodegeneration. NAD+ augmentation addresses multiple pathological processes simultaneously—mitochondrial dysfunction, impaired DNA repair, defective autophagy/mitophagy, neuroinflammation, and cellular senescence—making it an attractive disease-modifying strategy rather than a single-target intervention^{2(2025). NAD augmentation as a disease-modifying strategy for neurodegeneration2025 · Trends in Pharmacological Sciences · PMID 40287324Open reference}fang2025 2025, Fang et al. (2025).

Pathway Diagram

flowchart TD
    NAD_["NAD+"]
    style NAD_ fill:#006494,stroke:#4fc3f7,stroke-width:3px,color:#e0e0e0
    Blood_Brain_Barrier_Damage["Blood-Brain Barrier Damage"]
    NAD_ -->|"protects against"| Blood_Brain_Barrier_Damage
    Purinergic_Signaling["Purinergic Signaling"]
    NAD_ -->|"involved in"| Purinergic_Signaling
    Redox_Metabolism["Redox Metabolism"]
    NAD_ -->|"involved in"| Redox_Metabolism
    DNA_Repair["DNA Repair"]
    NAD_ -->|"involved in"| DNA_Repair
    Energy_Metabolism["Energy Metabolism"]
    NAD_ -->|"involved in"| Energy_Metabolism
    Blood_Brain_Barrier["Blood-Brain Barrier"]
    NAD_ -->|"protects against"| Blood_Brain_Barrier
    Aging_Induced_Blood_Brain_Barr["Aging-Induced Blood-Brain Barrier Damage"]
    NAD_ -->|"treats"| Aging_Induced_Blood_Brain_Barr
    Glycolysis["Glycolysis"]
    NAD_ -->|"involved in"| Glycolysis
    NAMPT["NAMPT"]
    NAMPT -->|"modulates"| NAD_
    NMN["NMN"]
    NMN -->|"involved in"| NAD_
    AMPK["AMPK"]
    AMPK -->|"activates"| NAD_
    SIRT["SIRT"]
    SIRT -->|"substrate of"| NAD_
    NAMPT -->|"associated with"| NAD_
    PARP["PARP"]
    PARP -.->|"downregulates"| NAD_
    SIRT -.->|"downregulates"| NAD_
    CD38["CD38"]
    CD38 -.->|"downregulates"| NAD_
    style Blood_Brain_Barrier_Damage fill:#6d3000,stroke:#4fc3f7,color:#e0e0e0
    style Purinergic_Signaling fill:#5d4400,stroke:#4fc3f7,color:#e0e0e0
    style Redox_Metabolism fill:#5d4400,stroke:#4fc3f7,color:#e0e0e0
    style DNA_Repair fill:#5d4400,stroke:#4fc3f7,color:#e0e0e0
    style Energy_Metabolism fill:#5d4400,stroke:#4fc3f7,color:#e0e0e0
    style Blood_Brain_Barrier fill:#455a64,stroke:#4fc3f7,color:#e0e0e0
    style Aging_Induced_Blood_Brain_Barr fill:#ef5350,stroke:#4fc3f7,color:#e0e0e0
    style Glycolysis fill:#5d4400,stroke:#4fc3f7,color:#e0e0e0
    style NAMPT fill:#4a1a6b,stroke:#4fc3f7,color:#e0e0e0
    style NMN fill:#006494,stroke:#4fc3f7,color:#e0e0e0
    style AMPK fill:#4a1a6b,stroke:#4fc3f7,color:#e0e0e0
    style SIRT fill:#4a1a6b,stroke:#4fc3f7,color:#e0e0e0
    style PARP fill:#4a1a6b,stroke:#4fc3f7,color:#e0e0e0
    style CD38 fill:#4a1a6b,stroke:#4fc3f7,color:#e0e0e0

NAD+ Biochemistry and Brain Function

NAD+ as an Energy Currency

NAD+ exists in two forms: oxidized (NAD+) and reduced (NADH). In energy metabolism, NAD+ serves as the primary electron acceptor in glycolysis and the TCA cycle, with NADH subsequently donating electrons to the mitochondrial electron transport chain (ETC) to drive oxidative phosphorylation and ATP synthesis. The brain, despite comprising only ~2% of body mass, consumes approximately 20% of total body glucose and oxygen, making it exquisitely dependent on NAD+-driven energy metabolismxie2016 2016, (2016). NAD+ metabolism: biomedical implications.

The NAD+/NADH ratio is a critical determinant of cellular metabolic state. A low ratio indicates an oxidized cellular environment and supports catabolic processes, while a high ratio promotes anabolic metabolism. In neurodegeneration, the NAD+/NADH ratio is typically reduced due to impaired mitochondrial function, creating a self-reinforcing cycle of declining energy production and increased oxidative stress.

NAD+ as a Signaling Substrate

Beyond energy metabolism, NAD+ serves as a consumed substrate (not merely a cofactor) for three major enzyme familiesimai2014 2014, imai2014:

Sirtuins (SIRT1-7)

The sirtuin family of NAD+-dependent protein deacetylases and deacylases regulate:

• SIRT1: Nuclear; deacetylates transcription factors (FOXO3, NF-κB, p53, PGC-1α), promoting mitochondrial biogenesis, anti-inflammatory signaling, and autophagy

• SIRT2: Primarily cytoplasmic; regulates microtubule dynamics and myelination in oligodendrocytes

• SIRT3: Mitochondrial; master regulator of mitochondrial protein acetylation, oxidative stress defense (activates SOD2), and fatty acid oxidation

• SIRT5: Mitochondrial; regulates succinylation, critical for TCA cycle enzyme function

• SIRT6: Nuclear; essential for DNA repair, telomere maintenance, and glucose homeostasis

Sirtuins function as metabolic sensors, translating cellular NAD+ levels into downstream signaling cascades. When NAD+ declines, sirtuin activity decreases, leading to dysregulated mitochondrial quality control, increased inflammation, and impaired stress resistance.

Poly(ADP-Ribose) Polymerases (PARPs)

PARP1 and PARP2 consume NAD+ to synthesize poly(ADP-ribose) chains on proteins at DNA damage sites. In neurodegeneration, chronic DNA damage leads to PARP hyperactivation, which can deplete cellular NAD+ pools—a phenomenon termed “NAD+ stealing.” PARP1 hyperactivation has been documented in AD and PD brains and contributes to neuronal energy failurefang2021 2021, fang2021.

CD38 NADase

CD38 is the dominant NAD+ consumer in mammalian tissues. This ectoenzyme degrades NAD+ and its precursors (NMN, NR) through its NADase and cyclase activities. CD38 expression increases with aging and neuroinflammation, making it a therapeutic target for NAD+ restorationhogan2019 2019, hogan2019.

NAD+ Biosynthetic Pathways

The brain maintains NAD+ through three interconnected pathways:

1. Salvage pathway (major): Recycling of nicotinamide (NAM) released from NAD+-consuming reactions. NAMPT (nicotinamide phosphoribosyltransferase) catalyzes the rate-limiting conversion of nicotinamide (NAM) to NMN, which is then converted to NAD+ by NMNAT1-3.

2. Preiss-Handler pathway: Converts dietary nicotin acid (vitamin B3) to NAD+ via a three-step enzymatic process.

3. De novo synthesis (kynurenine pathway): Converts tryptophan to quinolinic acid and ultimately NAD+. This pathway intersects with the kynurenine pathway, which is dysregulated in neurodegeneration—inflammatory activation shunts tryptophan metabolism toward neurotoxic quinolinic acid rather than NAD+ productionbraidy2019 2019, braidy2019.

NAD+ Decline in Neurodegenerative Diseases

Alzheimer’s Disease

NAD+ depletion in AD results from the convergence of multiple pathological processeswei2024 2024, (2024). NAD+ depletion in Alzheimer:

• PARP1 hyperactivation: Amyloid-beta oligomers and tau hyperphosphorylation cause DNA damage, triggering PARP1 overactivation that depletes NAD+ pools

• CD38 upregulation: Neuroinflammation drives CD38 expression on activated microglia

• NAMPT downregulation: The rate-limiting NAD+ biosynthetic enzyme is reduced in AD brain, particularly in the hippocampus

• Mitochondrial dysfunction: Reduced ETC function impairs NAD+/NADH cycling, creating a vicious cycle of declining energy metabolism and increased oxidative DNA damage

A landmark 2025 study demonstrated that REV-ERBα regulates brain NAD+ via an NFIL3-CD38 axis, and disruption of this circadian regulatory circuit contributes to NAD+ depletion and tauopathykim2025 2025, kim2025.

Functionally, NAD+ depletion in AD impairs autophagy and mitophagy—clearance pathways essential for removing amyloid plaques, hyperphosphorylated tau, and damaged mitochondria. Restoration of NAD+ levels rescues defective mitophagy in AD model organisms, demonstrating the functional significance of this depletionfang2019 2019, fang2019.

Parkinson’s Disease

In PD, NAD+ deficiency intersects with dopaminergic neurodegenerationschndorf2021 2021, schndorf2021:

• PINK1/Parkin pathway: NAD+ is required for SIRT1-mediated deacetylation of mitophagy regulators. NAD+ depletion impairs the PINK1/Parkin mitophagy pathway, leading to accumulation of damaged mitochondria in dopaminergic neurons

• Alpha-synuclein aggregation: NAD+ depletion reduces SIRT1/3 activity, increasing protein acetylation and promoting alpha-synuclein aggregation

• Oxidative vulnerability: Substantia nigra dopaminergic neurons have high basal oxidative stress from dopamine metabolism, making them especially sensitive to NAD+-dependent antioxidant defense mechanisms (SOD2 activation by SIRT3)

Clinical evidence supports NAD+ therapy: the NADPARK study demonstrated that high-dose NR (1000 mg/day) was safe and increased brain NAD+ levels measured by 31P-MRS in PD patients, with trends toward improved clinical scoresbrakedal2022 2022, (2022). The NADPARK study: a randomized trial of nicotinamide riboside in Par....

Amyotrophic Lateral Sclerosis

NAD+ metabolism is disrupted in ALS through multiple mechanismsharlan2021 2021, (2021). NAD+ metabolism in ALS: implications for sirtuins and PARPs:

• SOD1 mutations: Mutant SOD1 increases oxidative DNA damage, driving PARP1 activation and NAD+ consumption

• TDP-43 pathology: TDP-43 proteinopathy disrupts mitochondrial function and NAD+ homeostasis in motor neurons

• SARM1 activation: The SARM1 NADase drives programmed axon degeneration (Wallerian degeneration) by consuming axonal NAD+. SARM1 inhibition is a promising therapeutic strategy for ALS

Huntington’s Disease

Huntington polyglutamine expansion disrupts NAD+ metabolism:

• Mutant huntingtin impairs PGC-1α transcriptional activity, reducing mitochondrial biogenesis

• NAMPT expression is reduced in striatal medium spiny neurons, the cell population most vulnerable in HD

• NAD+ supplementation rescues mitochondrial function and improves motor phenotypes in HD animal modelschou2021 2021, (2021). NAD+ and Huntington

Therapeutic Strategies

NAD+ Precursor Supplementation

Two NAD+ precursors have been most extensively studiedzhang2023 2023, (2023). NAD+ precursors in neurodegenerative disease: a comparative review:

Nicotinamide Riboside (NR)

NR is converted to NMN by nicotinamide riboside kinases (NRK1/2), then to NAD+ by NMNATs:

• Preclinical data: NR restores mitophagy, reduces amyloid-β proteotoxicity, decreases BACE1

• NADAPT trial: Phase 2 study testing high-dose NR (3000 mg/day) in atypical parkinsonism (PSP, MSA, corticobasal syndrome)

Nicotinamide Mononucleotide (NMN)

NMN is the direct precursor to NAD+, converted by NMNAT enzymes:

• Preclinical data: NMN restores NAD+ profiles and improves mitochondrial stress response through the ATF4-dependent mitochondrial unfolded protein response (UPRmt) in AD models

• Gut-Brain connection: NMN reverses D-galactose-induced neurodegeneration and enhances intestinal barrier function via the SIRT1 pathway

• Clinical translation: Multiple NMN supplementation trials are ongoing, though none have yet demonstrated robust cognitive endpoints in neurodegenerative populations

Nicotinamide (NAM) / Niacinamide

High-dose nicotinamide (vitamin B3) has been tested directly in AD:

• The NACT study (Phase 2) tested NAM 1500 mg twice daily in mild-to-moderate AD. Results showed no significant changes in CSF total tau, phosphorylated tau, or amyloid-β, though a nominally significant slowing of cognitive decline was observed

CD38 Inhibition

Blocking CD38-mediated NAD+ degradation is emerging as a complementary or alternative strategy to precursor supplementationpei2025 2025, pei2025:

• Small molecule CD38 inhibitors (e.g., 78c, apigenin, quercetin): Increase NAD+ levels by blocking its degradation rather than increasing synthesis

• Anti-CD38 antibodies: A 2025 study showed that targeting CD38 restored metabolic fitness and improved cognition in AD mice, with reduced microglial neuroinflammation

• Dual approach: Combining CD38 inhibition with NAD+ precursors may achieve greater NAD+ restoration than either strategy alone

NAMPT Activators

Enhancing the rate-limiting step of NAD+ salvage:

• P7C3 compounds (e.g., P7C3-A20) activate NAMPT and restore NAD+ homeostasis without producing supraphysiologic NAD+ levels. P7C3-A20 has shown remarkable effects in reversing cognitive deficits and neuropathology in advanced AD mouse modelspieper2025 2025, pieper2025

• SBI-797812 is a direct NAMPT activator under preclinical development

SARM1 Inhibition

For axonal protection, particularly relevant to ALS and other motor neuron diseases:

• SARM1 is an NAD+ hydrolase that triggers axon degeneration when activated

• SARM1 inhibitors preserve axonal NAD+ and prevent Wallerian degeneration

• Multiple pharmaceutical programs target SARM1 for ALS and peripheral neuropathygille2024 2024, gille2024

Sirtuin Activators

Since many NAD+ benefits are mediated through sirtuins:

• Resveratrol and SRT2104 activate SIRT1, promoting mitochondrial function and anti-inflammatory signaling

• Honokiol activates SIRT3, protecting mitochondrial function in neurons

Challenges and Controversies

Bioavailability and Brain Penetration

A critical question is whether orally administered NAD+ precursors effectively raise brain NAD+ levelsxie2025 2025, (2025). Brain bioavailability of NAD+ precursors: challenges and opportunities:

• NR and NMN increase blood and peripheral tissue NAD+ in human studies

• Brain NAD+ measurement is challenging; 31P-MRS can estimate brain NAD+ but with limited sensitivity

• The NADPARK trial demonstrated that NR increases brain NAD+ in PD patients, providing important proof-of-concept

Dosing and Duration

Optimal dosing remains uncertain:

• Most trials have used relatively short durations (8-12 weeks) and moderate doses

• Higher doses (3000 mg/day NR in NADAPT) may be needed for neurodegenerative conditions

• Long-term safety data are limited for high-dose regimens

NAD+ and Cancer Risk

A theoretical concern is that NAD+ augmentation could promote cancer cell survival and proliferation. However, long-term supplementation studies have not shown increased cancer incidence, and the neuroprotective doses used in neurodegenerative disease trials are within the range of dietary vitamin B3 supplementation.

Combination vs. Monotherapy

The complexity of NAD+ metabolism suggests that targeting a single node may be insufficient. Combining precursor supplementation with CD38 inhibition addresses both synthesis and degradation, while adding sirtuin activators may amplify downstream signaling benefits.

Current Research and Future Directions

The field is rapidly evolving with several key research directionsyaku2023 2023, (2023). NAD+ metabolism: therapeutic targets and future directions:

• Circadian regulation of NAD+: NAD+ levels oscillate with circadian rhythms, and circadian disruption in neurodegeneration may compound NAD+ depletion. The REV-ERBα/NFIL3/CD38 axis represents a novel therapeutic target

• Cell-type-specific NAD+ metabolism: Single-cell genomics approaches are revealing that NAD+ metabolism differs dramatically between neurons, astrocytes, and microglia

• NAD+ biosensors: Genetically encoded NAD+ sensors enable real-time monitoring of NAD+ dynamics in living neurons

• Gut-Brain Axis: The gut microbiome influences NAD+ metabolism, and NMN effects on the intestinal barrier may contribute to neuroprotection through reduced peripheral inflammation

• Biomarker development: Plasma biomarkers of NAD+ metabolites (NAD+, NMN, NAM, ADPR) could enable treatment monitoring and patient stratification

• Sex differences: NAD+ metabolism and response to supplementation may differ between sexes, potentially relating to hormonal regulation of NAMPT and CD38

Connections to Other Mechanisms

See Also

• Mitochondrial dysfunction

• Autophagy

• Alzheimer’s Disease

• Parkinson’s Disease

• Huntington’s Disease

• SIRT1 Protein

• SIRT3 Protein

• PARP1 Protein

• NAMPT Protein

NAD+ and Neuroinflammation

Neuroinflammation and NAD+ metabolism are deeply interconnected. Activated microglia and astrocytes consume large amounts of NAD+ through CD38 and PARP activation, creating a self-perpetuating cycle where inflammation depletes NAD+, and reduced NAD+ impairs the anti-inflammatory functions of sirtuins.

The sirtuins, particularly SIRT1 and SIRT3, play crucial roles in modulating neuroinflammatory responses. SIRT1 deacetylates NF-κB subunits, reducing the transcription of pro-inflammatory cytokines. In neurodegenerative conditions, declining NAD+ levels lead to reduced SIRT1 activity, resulting in a hyperinflammatory state that further drives neurodegeneration. This creates a feedforward loop where neuroinflammation depletes NAD+ while NAD+ depletion promotes neuroinflammationfang2021 2021, fang2021.

CD38 expression on microglia increases dramatically in response to inflammatory stimuli, making it a major consumer of NAD+ in the inflamed brain. Therapeutic targeting of CD38 not only raises NAD+ levels but also reduces microglial activation, creating a dual benefit. The recent development of brain-penetrant CD38 inhibitors represents a promising approach to break this cyclepei2025 2025, pei2025.

NAD+ and Mitochondrial Quality Control

Mitochondrial function is absolutely dependent on adequate NAD+ levels. The electron transport chain requires NADH as an electron donor, and the NAD+/NADH ratio determines the efficiency of oxidative phosphorylation. When NAD+ declines, mitochondrial ATP production falls, leading to energy failure—a hallmark of all neurodegenerative diseases.

Beyond energy production, NAD+ is essential for mitochondrial quality control through multiple mechanisms. SIRT3 deacetylates and activates manganese superoxide dismutase (SOD2), enabling the neutralization of mitochondrial reactive oxygen species. SIRT1 regulates PGC-1α, the master regulator of mitochondrial biogenesis. The PINK1/Parkin mitophagy pathway requires NAD+-dependent deacetylation events for optimal function.

In Alzheimer’s disease, impaired mitophagy leads to accumulation of damaged mitochondria that generate excessive ROS, further driving oxidative stress and neuronal death. NAD+ replenishment restores mitophagy flux, enabling clearance of dysfunctional mitochondria and improving cellular energeticsfang2019 2019, fang2019.

In Parkinson’s disease, dopaminergic neurons are particularly vulnerable to mitochondrial dysfunction due to their high energy demands and the oxidative stress inherent in dopamine metabolism. The PINK1/Parkin pathway is critical for maintaining mitochondrial quality in these cells, and NAD+ supplementation enhances this pathway through SIRT1 activationschndorf2021 2021, schndorf2021.

NAD+ and DNA Repair

Neurons are post-mitotic cells that must maintain genomic integrity throughout life. DNA damage accumulates with age from oxidative stress, environmental toxins, and normal cellular metabolism. The base excision repair (BER) pathway is the primary mechanism for repairing oxidative DNA lesions in neurons, and this pathway consumes NAD+ through PARP activity.

PARP1 and PARP2 are activated by single-strand DNA breaks, which are constantly occurring in neurons. Under normal conditions, PARP activation is transient and NAD+ levels are quickly restored through the salvage pathway. However, in neurodegeneration, chronic oxidative stress leads to sustained PARP activation, depleting NAD+ pools and impairing cellular energy metabolism.

The “NAD+ stealing” hypothesis proposes that pathological PARP activation in neurodegenerative disease diverts NAD+ away from other essential functions, creating a metabolic crisis. This is particularly relevant in conditions with high oxidative stress, such as Alzheimer’s and Parkinson’s diseases. PARP inhibitors are being explored as a complementary strategy to preserve NAD+ in these conditionsfang2021 2021, fang2021.

NAD+ and Cellular Senescence

Cellular senescence is a state of irreversible cell cycle arrest that contributes to aging and age-related diseases. Senescent cells secrete a pro-inflammatory senescence-associated secretory phenotype (SASP) that drives chronic neuroinflammation. NAD+ levels decline in senescent cells, and this decline is both a cause and consequence of the senescent state.

Sirtuins, particularly SIRT1, are key regulators of cellular senescence. SIRT1 activation extends cellular replicative lifespan and delays the onset of senescence in multiple cell types. NAD+ repletion can reverse some aspects of cellular senescence by restoring sirtuin activity and improving mitochondrial function.

In the brain, senescent astrocytes and microglia contribute to neuroinflammation through their SASP. Targeting NAD+ metabolism in these cells may reduce the inflammatory burden in neurodegenerative diseases and improve the neural environment for neuronal survival.

Sex Differences in NAD+ Metabolism

Emerging evidence suggests significant sex differences in NAD+ metabolism that may influence the response to NAD+-targeted therapies. Women generally have higher baseline NAD+ levels than men, but the decline with aging is more pronounced in females. This may relate to hormonal regulation of NAMPT and CD38 expression.

Postmenopausal women experience a rapid decline in NAD+ levels that correlates with increased neurodegenerative disease risk. Estrogen replacement has been shown to increase NAMPT expression and NAD+ levels in preclinical models, suggesting a hormonal component to NAD+ regulation. However, the role of hormone therapy in NAD+ restoration for neurodegeneration remains to be established.

Understanding these sex-specific differences may enable personalized approaches to NAD+ supplementation based on sex, age, and hormonal status. Future clinical trials should stratify by sex to identify differential responses to NAD+-targeted interventions.

Biomarkers for NAD+ Treatment Response

Developing biomarkers for NAD+ treatment response is critical for clinical translation. Several approaches are being explored:

• Plasma NAD+ metabolites: NMN, NR, NAM, and ADPR can be measured by mass spectrometry to assess NAD+ flux

• 31P-MRS brain imaging: This technique can estimate brain NAD+ levels non-invasively, as demonstrated in the NADPARK study

• Sirtuin activity assays: Measuring downstream sirtuin function may reflect cellular NAD+ status more accurately than NAD+ itself

• Expression of NAD+ consumer genes: CD38, PARP1, and SIRT expression patterns may indicate NAD+ homeostasis status

Combining multiple biomarkers may enable patient selection and treatment monitoring for NAD+-targeted therapies. This personalized approach could improve trial outcomes and enable dose optimization.

Conclusion

NAD+ metabolism represents a central hub in neurodegenerative disease pathogenesis, connecting energy production, signaling, stress responses, and cellular quality control. The decline of NAD+ with aging and in neurodegeneration is not merely a biomarker but a pathogenic mechanism that drives multiple disease processes simultaneously.

The therapeutic potential of NAD+ augmentation has moved from preclinical promise to clinical reality, with multiple trials demonstrating safety and biological activity. Remaining challenges include optimizing brain penetration, identifying optimal dosing regimens, and developing biomarkers for patient selection. The integration of NAD+ targeting with other disease-modifying approaches offers the most promising path forward for treating these devastating disorders.