Brain-derived neurotrophic factor (BDNF) is a protein encoded by the BDNF gene located on chromosome 11, which provides instructions for producing a member of the neurotrophin family of structurally related polypeptide growth factors found primarily in the brain and spinal cord.[1] It is synthesized as a precursor protein (pro-BDNF) in the endoplasmic reticulum and cleaved into its mature form (approximately 13 kDa), which is secreted and acts as a key regulator of neuronal survival, growth, differentiation, and maintenance.[2] BDNF exerts its effects mainly through binding to the high-affinity tyrosine kinase receptor TrkB and the low-affinity p75 neurotrophin receptor (p75NTR), triggering intracellular signaling pathways such as PLC, PI3K, and Ras-MAPK that promote synaptic plasticity, long-term potentiation (LTP), and neurogenesis.[3]Discovered and purified in 1982 as a factor supporting neuronal survival and growth, BDNF is highly expressed in brain regions like the hippocampus, cortex, and basal forebrain, with expression levels increasing postnatally and stabilizing in adulthood.[2] In the central nervous system, it plays a pivotal role in guiding neuronal development, enhancing synaptic transmission, and modulating neurotransmitter release, thereby contributing to processes like learning, memory formation, and adaptive responses to environmental stimuli.[3] Beyond the brain, BDNF influences peripheral systems, including energy metabolism, appetite regulation, and body weight control, with circulating levels averaging around 92.5 pg/mL in human plasma and varying by factors such as age, gender, and physical activity—exercise, for instance, acutely elevates BDNF to support cognitive enhancement.[2][1]Clinically, BDNF has significant implications across neurological, psychiatric, and metabolic disorders due to its involvement in neuroprotection and plasticity.[2] Reduced BDNF levels are associated with neurodegenerative conditions like Alzheimer's disease, Parkinson's disease, and Huntington's disease, where it fails to mitigate neuronal loss and synaptic dysfunction.[2] Genetic variations, such as the Val66Met polymorphism in the BDNF gene, impair protein secretion and increase susceptibility to psychiatric disorders including bipolar disorder, anxiety, schizophrenia, and eating disorders, as well as neurodevelopmental issues like autism spectrum disorder.[1] Additionally, BDNF deletions contribute to severe obesity and intellectual disability in syndromes like WAGR (often denoted as WAGRO), while its dysregulation links to opioid addiction, type 2 diabetes, and cardiovascular risks, positioning it as a potential therapeutic target for interventions like exercise regimens or pharmacological TrkB agonists.[1][2]
Molecular Biology
Gene Structure and Discovery
Brain-derived neurotrophic factor (BDNF) was first identified in 1982 as a survival-promoting activity for sensory neurons from dorsal root ganglia in extracts of pig brain. This neurotrophic factor was purified from mammalian brain tissue by Yves-Alain Barde and colleagues, who isolated approximately 400 μg of the protein and demonstrated its distinct biological activity compared to nerve growth factor (NGF). The discovery built on earlier work with NGF, highlighting BDNF's role in supporting neuronal survival in vitro.The BDNF gene in humans is located on the short arm of chromosome 11 at position 11p14.1 and spans approximately 70 kb of genomic DNA. It features a complex structure with nine 5' non-coding exons (I–IXa), each driven by individual promoters that enable tissue- and activity-specific transcription, spliced alternatively to a single 3' coding exon (IX) that encodes the prepro-BDNF polypeptide. This organization allows for multiple BDNF mRNA transcripts, contributing to regulated expression across different physiological contexts.The cDNA for BDNF was first cloned in 1989 from porcine brain, revealing its sequence and confirming its expression as a 27 kDa precursor protein. Subsequent cloning efforts identified homologous genes in rat and human, establishing BDNF as the second member of the neurotrophin family, alongside NGF, neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4).[4] BDNF exhibits high evolutionary conservation across vertebrates, with the mature protein sequence showing over 90% identity between mammals and even greater similarity in functional domains compared to NGF, underscoring its preserved role in neural development.[5]
Protein Structure and Processing
Brain-derived neurotrophic factor (BDNF) is initially synthesized as a precursor protein known as preproBDNF, consisting of 247 amino acids.[6] This precursor undergoes cleavage of its N-terminal signal peptide (residues 1–18) in the endoplasmic reticulum to form proBDNF, a 229-amino-acid protein (residues 19–247) with an approximate molecular weight of 32 kDa.[7] ProBDNF can then be further processed either intracellularly or extracellularly to yield the mature form of BDNF (mBDNF), a 119-amino-acid polypeptide (~14 kDa) that dimerizes to form a ~27-28 kDa homodimer.[8]The crystal structure of mBDNF, determined through X-ray crystallography of a BDNF/neurotrophin-3 heterodimer at 2.3 Å resolution, reveals a compact fold with a central core composed of antiparallel β-strands forming an extended β-sheet.[9] This core is stabilized by three conserved disulfide bonds (Cys58–Cys108, Cys86–Cys107, and Cys90–Cys94), while variable loop regions extending from the β-sheet structure contribute to receptor specificity and ligand-receptor interactions.[9] The dimeric interface involves hydrophobic contacts and hydrogen bonds, underscoring the structural homology among neurotrophins.Secretion of BDNF isoforms is differentially regulated in neurons. ProBDNF is primarily released via a constitutive secretory pathway from the trans-Golgi network, independent of neuronal activity.[10] In contrast, mBDNF secretion occurs through activity-dependent exocytosis from regulated dense-core vesicles, triggered by depolarization and calcium influx in hippocampal and cortical neurons.[11]Processing of proBDNF to mBDNF involves specific proteases acting at the dibasic cleavage site (Arg128–Ser129). Intracellularly, furin and proprotein convertases in the trans-Golgi network or secretory granules catalyze this cleavage to generate mBDNF prior to secretion.[12] Extracellularly, plasmin and matrix metalloproteinases (such as MMP-3, MMP-7, and MMP-9) further cleave secreted proBDNF to mBDNF, with activity-dependent regulation of these enzymes modulating the proBDNF/mBDNF ratio in the synaptic cleft.[13]
Expression Patterns
Central Nervous System Expression
Brain-derived neurotrophic factor (BDNF) exhibits prominent expression within specific regions of the central nervous system (CNS), particularly in areas associated with learning and memory. In both rodents and humans, high levels of BDNF mRNA and protein are observed in the hippocampus, including the CA1-CA3 pyramidal cell layers and dentate gyrus granule cells, as well as in the cerebral cortex across layers II-VI, the basal forebrain, and the amygdala.[14][15] These patterns reflect BDNF's role in supporting neuronal populations critical for cognitive functions, with expression primarily localized to neurons such as pyramidal and granule cells.[16]During development, BDNF expression in the rodent CNS is low prenatally, with detectable mRNA emerging around embryonic day 15.5 in the hippocampus and later in the cortex. Postnatally, expression rises dramatically, peaking around postnatal days 7-14 in the hippocampus and cortex, a timeline that aligns with periods of intense synaptogenesis and circuit maturation.[17][16] This surge supports the establishment of synaptic connections during early brain development.[18]BDNF expression in the CNS is highly regulated by neuronal activity. Neuronal depolarization, such as that induced by seizures or learning tasks, triggers calcium influx through L-type voltage-sensitive calcium channels, leading to activation of transcription factors like CREB and subsequent upregulation of BDNF transcription, particularly of activity-dependent exons such as exon IV.[19] For instance, limbic seizures in adult rats rapidly increase BDNF mRNA in the forebrain and hippocampus within hours.[20] External stimuli like exercise can further enhance this activity-dependent expression in hippocampal neurons.[21]Regionally, BDNF shows specificity within the CNS, with lower expression in the cerebellum and brainstem compared to the hippocampus and cortex. Under basal conditions, BDNF is largely absent from most glial cells, though it can be induced in astrocytes under certain activity states.[14][16][22]
Peripheral Expression and Regulation
Brain-derived neurotrophic factor (BDNF) is expressed in various components of the peripheral nervous system, including sensory and motor neurons, where its mRNA has been detected in adult human tissues, supporting neuronal survival and regeneration.[23] In the retina, BDNF is produced by retinal ganglion cells and other layers, promoting the survival and maintenance of these neurons during development and in response to injury.[24] Beyond neuronal sites, BDNF expression occurs in non-neuronal peripheral tissues such as skeletal muscle, where it is secreted in response to contractile activity, enhancing glucose uptake and fat oxidation; vascular endothelium, particularly in smooth muscle cells that express both BDNF and its receptors; and platelets, which store high concentrations of BDNF in α-granules and release it upon activation to modulate hemostasis and inflammation.[25][26][27]Peripheral BDNF expression is dynamically regulated by physiological and environmental factors. Physical exercise, particularly aerobic training, upregulates BDNF in peripheral tissues like skeletal muscle and elevates circulating levels, with acute high-intensity sessions increasing serum BDNF by approximately 25% in healthy adults.[28] Hypoxia induces BDNF expression through activation of the hypoxia-inducible factor-1α (HIF-1α) pathway, which stabilizes BDNF mRNA and promotes adaptive responses in peripheral neurons and vascular cells.[29] Similarly, inflammatory signals enhance BDNF via the nuclear factor-κB (NF-κB) pathway, as seen in endothelial and immune cells during acute stress, though chronic inflammation may suppress it.[30]The majority of circulating BDNF in serum and plasma is derived from peripheral sources, particularly platelets, which store high concentrations in α-granules and release it upon activation. Although the brain produces BDNF, its contribution to circulating levels is small due to the blood-brain barrier, with potential modest increases during exercise.[31] These peripheral contributions become more prominent post-exercise or in response to systemic stressors. Vitamin D status also influences circulating BDNF, as deficiency (serum 25(OH)D <30 ng/mL) correlates with lower levels, and high-dose supplementation (≥2000 IU/day for ≥12 weeks) raises serum BDNF by about 7% in deficient individuals, potentially via enhanced gene transcription in peripheral tissues.[32]Recent reviews from 2024 highlight how exercise-induced peripheral BDNF elevations correlate with cognitive improvements in older adults, such as better executive function linked to higher plasma BDNF/irisin ratios following regular aerobic activity.[33] In chronic conditions like multiple sclerosis, 12-week aerobic training programs increase serum BDNF and reduce fatigue severity, suggesting a role in alleviating systemic symptoms through peripheral mechanisms.[34]
Receptors and Mechanisms of Action
TrkB Receptor Activation
TrkB, encoded by the NTRK2 gene, is a receptor tyrosine kinase that serves as the primary high-affinity receptor for brain-derived neurotrophic factor (BDNF).[35] The receptor consists of an extracellular ligand-binding domain, a single transmembrane domain, and an intracellular tyrosine kinase domain essential for signal transduction.[36] Upon binding of BDNF to the extracellular domain, TrkB undergoes dimerization, which is a critical step in its activation.[37]The binding affinity of BDNF to TrkB is exceptionally high, with a dissociation constant (Kd) of approximately 10−11 M, enabling sensitive detection of the ligand even at low concentrations.[38] This interaction induces conformational changes that activate the intracellular kinase domain, leading to autophosphorylation on specific tyrosine residues, including Y490, Y785, and Y816.[39] These phosphorylated tyrosines create docking sites for SH2-domain-containing adaptor proteins, such as Shc and phospholipase C-γ (PLC-γ), which initiate downstream signaling cascades.[40] For instance, phosphorylation at Y490 recruits proteins involved in Ras-MAPK pathway activation, while Y785 and Y816 facilitate PLC-γ binding and subsequent calcium signaling.[39]Mature BDNF (mBDNF), the processed form of BDNF, preferentially binds and activates TrkB to promote pro-survival and neurotrophic effects in neurons, distinguishing it from the precursor proBDNF, which has higher affinity for the p75NTR receptor and mediates opposing functions like apoptosis.[41] This specificity ensures that mBDNF-TrkB signaling supports neuronal maintenance and plasticity.[42]TrkB is expressed in multiple isoforms due to alternative splicing of the NTRK2 transcript. The full-length isoform (TrkB.FL) contains the complete kinase domain and is responsible for canonical signaling upon BDNF binding.[43] In contrast, truncated isoforms such as TrkB.T1 and TrkB.T2 lack the kinase domain but retain the extracellular and transmembrane regions; these act as dominant negatives by heterodimerizing with TrkB.FL, thereby inhibiting its activation and BDNF-mediated responses.[44] Truncated forms are particularly abundant in the adult brain and may modulate the intensity of trophic signaling.[45]
p75NTR (LNGFR) Interactions
The p75 neurotrophin receptor (p75NTR), encoded by the NGFR gene, serves as a low-affinity receptor for all mammalian neurotrophins, including brain-derived neurotrophic factor (BDNF), with a dissociation constant (Kd) of approximately 1–10 nM.[46] This binding occurs through the extracellular cysteine-rich domains of p75NTR, which recognize a conserved region in the neurotrophins. In contrast to the high-affinity binding of mature BDNF (mBDNF) to TrkB receptors, the precursor form proBDNF exhibits preferential affinity for p75NTR, often forming heterocomplexes with co-receptors such as sortilin to initiate signaling.[47] These proBDNF-p75NTR-sortilin complexes promote pro-apoptotic pathways, particularly in contexts requiring neuronal refinement.[42]In neuronal pruning during development, p75NTR activation by proBDNF triggers intracellular cascades involving c-Jun N-terminal kinase (JNK) and nuclear factor kappa B (NF-κB) pathways, culminating in caspase activation and programmed cell death of excess or inappropriate synapses and neurons.[47] This mechanism helps sculpt neural circuits by eliminating competing projections, as evidenced in studies of hippocampal and cortical development where p75NTR-mediated apoptosis balances neurotrophic support.[42] The pro-apoptotic bias of p75NTR signaling contrasts with TrkB's pro-survival effects, ensuring precise wiring in the immature nervous system.As a co-receptor, p75NTR interacts directly with TrkB via both extracellular and intracellular domains, enhancing the specificity of TrkB activation for BDNF over other neurotrophins like NT-3 and NT-4/5, particularly at low ligand concentrations.[48] This association also modulates the retrograde axonal transport of BDNF-TrkB complexes, facilitating their delivery from distal terminals to cell bodies for sustained signaling.[49] Such cooperative functions refine neurotrophin responsiveness without altering overall binding affinity.p75NTR expression is prominent in the developing central nervous system (CNS), including cerebral cortex, hippocampus, basal forebrain, and spinal motoneurons, as well as in peripheral sensory and sympathetic neurons, where it supports circuit maturation.[50] Levels are subsequently downregulated in the adult CNS and periphery, though re-expression occurs in response to injury or pathology.[51] This dynamic pattern aligns with p75NTR's role in developmental plasticity rather than baseline adult maintenance.
Intracellular Signaling Pathways
Upon ligand binding to the TrkB receptor, BDNF initiates three principal intracellular signaling cascades: the Ras-MAPK/ERK pathway, the PI3K-Akt pathway, and the PLCγ-IP3/Ca²⁺ pathway. These pathways mediate diverse neuronal responses, from survival to plasticity, through receptor autophosphorylation at specific tyrosine residues that recruit adaptor proteins and kinases.[52]The Ras-MAPK/ERK pathway drives gene transcription and neuronal differentiation by sequentially activating Ras, Raf, MEK, and ERK kinases, ultimately phosphorylating transcription factors such as CREB and c-Fos to induce expression of genes involved in growth and survival.[52] This cascade is initiated via the adaptor proteins Shc and Grb2, which link TrkB to SOS and Ras, and features a positive feedback mechanism where activated ERK upregulates BDNF transcription, amplifying the signal.[53] Temporal dynamics are notable: ERK phosphorylation occurs rapidly within minutes to support acute synaptic modulation, whereas sustained activation lasting hours facilitates long-term potentiation (LTP) and structural remodeling.[54]The PI3K-Akt pathway promotes anti-apoptotic neuronal survival and local protein synthesis by recruiting PI3K to phosphotyrosines on TrkB, generating PIP3 to activate Akt (also known as PKB), which in turn stimulates mTOR for translational control of dendritically localized mRNAs.[55] A key aspect of cross-talk involves Akt-mediated phosphorylation and inhibition of FOXO transcription factors, suppressing pro-death genes and enhancing cell resilience.[53] This pathway sustains signaling over extended periods, contributing to LTP maintenance and synaptic consolidation.The PLCγ pathway supports synaptic plasticity by phosphorylating PLCγ at TrkB Tyr816, leading to hydrolysis of PIP2 into IP3 and diacylglycerol; IP3 triggers Ca²⁺ release from intracellular stores, activating Ca²⁺-dependent enzymes like CaMKII and promoting CREB phosphorylation for activity-dependent gene expression.[56] This rapid Ca²⁺ influx, occurring within seconds to minutes of stimulation, coordinates with ERK and Akt for integrated effects on dendritic spine dynamics and transmitter release.[53] Cross-talk among the pathways is evident, as ERK and Akt enhance PLCγ outputs while shared upstream adaptors like Shc ensure coordinated activation.[57]
Physiological Functions
Neurotrophic Effects
Brain-derived neurotrophic factor (BDNF) plays a crucial role in promoting the survival of various neuronal populations during development, particularly dopaminergic neurons in the mesencephalon, cholinergic neurons in the basal forebrain, and sensory neurons in peripheral ganglia.[58][59] In cultured mesencephalic neurons, BDNF increases survival rates by supporting differentiation and protecting against degeneration, with effects observed at concentrations as low as 10 ng/ml.[60] Similarly, BDNF enhances cholinergic neuron survival and differentiation in embryonic basal forebrain cultures, stimulating choline acetyltransferase activity and neurite outgrowth.[61] For sensory neurons, BDNF acts as a target-derived trophic factor, rescuing nodose and dorsal root ganglion neurons from programmed cell death during early postnatal stages. These survival-promoting effects are mediated primarily through activation of the TrkB receptor.[62]The necessity of BDNF for neuronal survival is underscored by phenotypes in BDNF knockout mice, where homozygous mutants exhibit perinatal lethality, typically within 2-3 days after birth, due to severe respiratory and feeding difficulties.[63] These mice display profound sensory deficits, including a drastic reduction (up to 60-70%) in the number of neurons in dorsal root ganglia, trigeminal ganglia, and nodose ganglia, leading to impaired coordination, balance, and proprioception.[64] Heterozygous mutants survive to adulthood but show progressive sensorimotor impairments, highlighting BDNF's dose-dependent role in maintaining sensory neuron populations.[63]BDNF also facilitates axon growth and branching, guiding target innervation through concentration gradients in both peripheral and central nervous system targets. In Xenopus laevis models, BDNF gradients direct mandibular trigeminal axons toward peripheral targets like the cement gland, promoting arborization and precise innervation without affecting initial pathfinding.[65] In mammalian cortical cultures, BDNF acts as a chemoattractant, inducing growth cone turning and elongation in a cAMP-dependent manner, with effective gradients spanning 100-200 μm. These actions ensure appropriate topographic mapping, as seen in retinotectal projections where BDNF gradients refine axonal branching in the superior colliculus.[66]Beyond structural support, BDNF provides metabolic sustenance to neurons by enhancing mitochondrial function and energy metabolism via upregulation of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α). In cortical neurons, BDNF treatment increases PGC-1α expression, leading to elevated mitochondrial biogenesis, improved respiratory coupling, and higher ATP production, which buffers against oxidative stress. This pathway involves TrkB-mediated activation of downstream transcription factors, sustaining neuronal energy demands during growth and maintenance.[67]In the adult nervous system, BDNF maintains neuronal integrity post-injury, particularly by preventing atrophy in facial motor neurons. Following facial nerve axotomy in neonatal rats, local BDNF infusion rescues motor neurons from soma shrinkage and degeneration, preserving cell size and cholinergic phenotype for up to two weeks post-injury. Retrograde transport of BDNF from target muscles further supports this maintenance, as demonstrated by its accumulation in axotomized facial nuclei.[68]
Synaptic Plasticity and Transmission
Brain-derived neurotrophic factor (BDNF) plays a pivotal role in modulating synaptic plasticity, the process underlying learning and memory, by influencing both pre- and postsynaptic mechanisms at synapses throughout the brain. BDNF enhances long-term potentiation (LTP), a key form of synaptic strengthening, particularly in the hippocampus, where it promotes presynaptic neurotransmitter vesicle release and facilitates the trafficking of postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors to the synaptic membrane. This dual action increases synaptic efficacy, as demonstrated in rodent hippocampal slices where exogenous BDNF application potentiates LTP induction by strengthening excitatory transmission.In glutamatergic synapses, BDNF signaling via its receptor TrkB elevates N-methyl-D-aspartate (NMDA) receptor surface expression and increases dendritic spine density, thereby enhancing synaptic connectivity and responsiveness to stimuli. For GABAergic synapses, BDNF shifts the balance between inhibition and excitation, refining neural network activity by modulating inhibitory interneuron function and chloride homeostasis, which supports circuit maturation during development and activity-dependent plasticity. These effects collectively enable adaptive synaptic remodeling, with BDNF acting as a retrograde signal to coordinate presynaptic and postsynaptic changes.Long-term BDNF exposure stabilizes mature synapses by upregulating activity-regulated cytoskeleton-associated protein (Arc) and postsynaptic density protein 95 (PSD-95), proteins essential for maintaining synaptic structure and function over extended periods. Acutely, BDNF boosts the frequency of miniature excitatory postsynaptic currents (mEPSCs), reflecting rapid enhancement of spontaneous neurotransmitter release without altering amplitude, thus fine-tuning synaptic transmission dynamics. These mechanisms distinguish BDNF's contributions to established synaptic function from broader neurotrophic support.BDNF-TrkB signaling contributes to homeostatic plasticity in cortical circuits, where it compensates for chronic changes in network activity by scaling synaptic strengths to preserve overall excitability. This homeostatic role ensures circuit stability amid fluctuating inputs, underscoring BDNF's integrative function in both Hebbian and non-Hebbian plasticity forms.[69]
Neurogenesis and Neuronal Maturation
Brain-derived neurotrophic factor (BDNF) plays a pivotal role in adult neurogenesis by enhancing the proliferation and survival of neural progenitor cells in the subgranular zone of the dentate gyrus in the hippocampus. Infusion of BDNF directly into the adult rat hippocampus significantly increases the number of newly generated granule cells, demonstrating its capacity to promote neurogenesis in this neurogenic niche. This effect is mediated through activation of the TrkB receptor, which triggers downstream signaling via the PI3K/Akt pathway to support the differentiation and survival of these progenitors. Conditional knockout of TrkB in the adult hippocampus markedly reduces neurogenesis, underscoring the essential nature of this BDNF-TrkB-Akt axis for maintaining neural progenitor dynamics in the dentate gyrus.During neuronal development, BDNF drives dendritogenesis by promoting dendritic arborization and spine formation in cortical and hippocampal neurons. In layer 4 pyramidal neurons of the developing ferretvisual cortex, endogenous BDNF stimulates dendritic growth, while opposing influences from NT-3 highlight its specific regulatory role in shaping dendritic architecture. In hippocampal neurons, BDNF application enhances proximal dendritic branching through CREB-dependent transcriptional mechanisms, leading to increased complexity and spine density that facilitate connectivity. These effects are activity-dependent, as BDNF's promotion of dendritic elaboration requires synaptic input to fully manifest during early postnatal stages.BDNF also guides synaptogenesis by facilitating the initial formation of synapses between axons and dendrites in embryonic neurons. In early developmental stages of the Xenopus optic tectum, BDNF increases the density of synaptic sites on dendrites of tectal neurons, coordinating presynaptic and postsynaptic assembly. This process involves BDNF's regulation of spontaneous correlated network activity, which synchronizes gene expression and promotes GABAergicsynaptogenesis in embryonic hippocampal cultures. Such mechanisms ensure proper wiring of nascent neural circuits during embryogenesis.In the context of neuronal maturation, BDNF accelerates the functional integration of new neurons into existing circuits, with effects peaking during adolescence when hippocampal neurogenesis is particularly robust. Local translation of BDNF from long 3' UTR mRNAs in dendrites of adult-born granule cells in the dentate gyrus enhances their differentiation and maturation via GABAergicinterneuron signaling. This BDNF-mediated process supports the timely incorporation of these neurons into hippocampal networks, optimizing circuit function during the adolescent period of heightened plasticity.
Genetic Variations
Common SNPs and Their Effects
Single nucleotide polymorphisms (SNPs) in the BDNF gene are common genetic variations that can influence BDNF expression, protein function, and associated phenotypes. One of the most studied is rs6265, also known as Val66Met, which results in a valine-to-methionine substitution at codon 66 in the proBDNF protein; the Met allele frequency is approximately 20-30% in Caucasian populations and 40-50% in East Asian populations.[70] While detailed effects of rs6265 are addressed elsewhere, it exemplifies how BDNF SNPs can broadly impact neurotrophic activity.Another frequent SNP, rs908867, located in the 5' regulatory region near the BDNF promoter, has been linked to variations in gene expression levels and antidepressant response. Heterozygote carriers of rs908867 show improved response to antidepressants compared to homozygotes, suggesting an influence on BDNF-mediated neuroplasticity.[71] This SNP also correlates with neurocognitive performance in major depressive disorder, where certain alleles associate with deficits in executive function and memory.[72] In individuals with mild traumatic brain injury, rs908867 variants are associated with reduced hippocampal volume on MRI imaging, indicating potential effects on brain structure.[73]The intronic SNP rs2030324, situated in intron 4 of the BDNF gene, may affect splicing efficiency and mRNA processing, leading to altered BDNF transcript stability and expression. This polymorphism has been associated with increased risk of Alzheimer's disease, with specific genotypes elevating susceptibility.[74] In schizophrenia patients, rs2030324 influences cognitive domains such as language performance, particularly in drug-naïve first-episode cases.[75] Functional studies suggest that BDNF SNPs like rs2030324 contribute to inter-individual variability in plasma BDNF levels, potentially through changes in transcription or protein secretion.[70]Overall, common BDNF SNPs such as rs908867 and rs2030324 modify BDNF function by altering transcription rates, mRNA splicing, protein secretion, or stability, resulting in heterogeneous BDNF signaling across populations. These variations are implicated in neuroimaging findings, including reduced hippocampal volumes in carriers of risk alleles, as observed in structural MRI studies of healthy and diseased cohorts.[76]
Val66Met Polymorphism
The Val66Met polymorphism (rs6265) is a single nucleotide polymorphism in the BDNF gene, characterized by a guanine-to-adenine (G>A) substitution in exon V. This change results in the replacement of valine (Val) with methionine (Met) at amino acid position 66 within the prodomain of the proBDNF precursor protein.[77] The polymorphism is common in populations of European descent, with the Met allele frequency typically ranging from 20-30%.[77]At the molecular level, the Val66Met substitution disrupts the intracellular trafficking and sorting of proBDNF into regulated secretory vesicles, thereby impairing activity-dependent BDNF release. This defect arises because the Met variant alters the conformation of the prodomain, preventing efficient packaging into dense-core vesicles and reducing depolarization-induced secretion in neurons, particularly in the hippocampus, by approximately 18-30%. Consequently, mature BDNF levels available for synaptic function are diminished under activity-dependent conditions, while constitutive secretion remains largely unaffected.[77]Phenotypically, Met allele carriers exhibit structural and functional alterations in the brain. Meta-analyses have consistently shown that individuals with at least one Met allele have smaller hippocampal volumes compared to Val/Val homozygotes, with effect sizes indicating a modest but significant reduction of about 4-10% in total hippocampal gray matter. This polymorphism is also linked to impaired episodic memory, as Met carriers demonstrate poorer performance on memory tasks involving hippocampal-dependent recall, accompanied by abnormal fMRI activation patterns in the hippocampus during encoding and retrieval.[77] Additionally, the Met allele alters fear extinction processes; human and rodent studies reveal that Met carriers display slower extinction of conditioned fear responses, potentially due to reduced BDNF-mediated plasticity in the amygdala-hippocampal circuit.Recent analyses highlight the Met allele's complex role in stress-related traits. A 2023 meta-analysis on panic disorder cohorts found that the Met/Met genotype confers an increased risk for developing this anxiety disorder, with odds ratios indicating approximately 20-25% elevated susceptibility in Met/Met carriers compared to others.[78] However, in specific high-stress environments, such as early-onset panic disorder, the Met allele may exert a protective effect by attenuating hyperreactive stress responses, as observed in genotype-stress interaction studies.
BDNF Antisense RNA (BDNF-AS)
BDNF antisense RNA (BDNF-AS), also known as BDNFOS, is a long non-coding RNA (lncRNA) that serves as a natural antisense transcript (NAT) to the BDNF gene, transcribed from the opposite strand and overlapping with BDNF exon IV. This genomic arrangement enables BDNF-AS to form double-stranded RNA duplexes with the complementary BDNF mRNA, which is a common mechanism for antisense-mediated regulation. The gene spans approximately 70 kb on chromosome 11p14.1 and produces multiple splice variants through alternative promoters and splicing, mirroring the complex structure of the BDNF locus itself.The primary function of BDNF-AS is to negatively regulate BDNF expression, primarily through epigenetic silencing rather than mRNA stabilization. It recruits the Polycomb Repressive Complex 2 (PRC2), including the EZH2 subunit, to the BDNF promoter, increasing histone H3lysine 27 trimethylation (H3K27me3) marks and thereby repressing transcription. Experimental knockdown of BDNF-AS in neuronal models results in a 2- to 7-fold upregulation of BDNF mRNA and protein levels, confirming its suppressive role and highlighting potential for therapeutic intervention via antisense inhibition. Additionally, BDNF-AS can act as a competing endogenous RNA (ceRNA), sponging microRNAs like miR-9-5p to indirectly modulate BDNF-related pathways.BDNF-AS is co-expressed with BDNF across various brain regions, exhibiting tissue- and region-specific patterns, with notable abundance in the amygdala and cortex. Its expression is dynamically upregulated in response to stress, as observed in early-onset alcohol use disorder models where BDNF-AS levels positively correlate with daily alcohol intake (r = 0.523) and contribute to reduced BDNF in the amygdala, impairing synaptic plasticity. In neurotoxicity contexts, such as hypoxia-ischemia or amyloid-beta exposure, BDNF-AS is elevated, promoting neuronal apoptosis and ferroptosis while suppressing protective BDNF signaling.In Alzheimer's disease, BDNF-AS has been implicated as a contributor to pathology, with significantly higher plasma levels in late-stage patients compared to healthy controls, where it enhances BACE1 expression via miR-9-5p competition, exacerbating amyloid-beta accumulation and BDNF suppression. Silencing BDNF-AS in amyloid-beta-treated PC12 cells reduces oxidative stress, apoptosis, and neurotoxicity while restoring BDNF levels and cell viability, positioning it as a promising therapeutic target for derepressing BDNF to counteract neurodegeneration.
Clinical Relevance
Role in Neurodevelopmental and Neurodegenerative Disorders
Brain-derived neurotrophic factor (BDNF) plays a critical role in neurodevelopmental disorders, where its dysregulation contributes to impaired neuronal maturation. In Rett syndrome, caused by mutations in the MECP2 gene, BDNF transcripts are significantly reduced in affected brains, leading to compromised dendritic arborization and synaptic density. This reduction disrupts activity-dependent transcription of BDNF, which is essential for neurite growth and spine maturation, exacerbating the neurodevelopmental deficits characteristic of the disorder. Similarly, in autism spectrum disorders, altered BDNF signaling is associated with abnormal dendritic spine morphology and reduced dendritogenesis, contributing to connectivity issues in cortical and hippocampal regions.Prenatal and early postnatal BDNF levels have predictive value for cognitive outcomes in neurodevelopment. In infants born to mothers with gestational diabetes, lower serum BDNF levels at 12 months correlate with poorer language composite scores on developmental assessments, indicating that early BDNF deficits may forecast delays in cognitive and linguistic maturation. Higher BDNF concentrations in preterm infants, conversely, are linked to reduced odds of developmental domain failures, underscoring BDNF's protective role in early brain wiring.In neurodegenerative disorders, BDNF levels decline progressively, correlating with disease advancement and neuronal loss. In Alzheimer's disease, oligomeric amyloid-beta peptides downregulate BDNF mRNA, particularly transcripts IV and V, impairing BDNF-TrkB signaling and promoting synaptic dysfunction. This reduction exacerbates tau pathology by upregulating δ-secretase activity, which cleaves tau to form neurofibrillary tangles. In Parkinson's disease, BDNF mRNA expression is diminished by approximately 70% in the substantia nigrapars compacta, largely due to the loss of BDNF-expressing dopaminergic neurons, with surviving neurons showing 20% lower levels and heightened vulnerability to degeneration. Alpha-synuclein aggregation further blocks BDNF-TrkB neurotrophic activities by binding to TrkB, inhibiting its trafficking and signaling, which accelerates dopaminergiccell death. In Huntington's disease, cortical BDNF levels decrease from the early symptomatic stage, correlating with motor dysfunction onset and enkephalinergic neuronal degeneration, thereby worsening disease progression.Genetic variations in BDNF, such as the Val66Met polymorphism, modulate susceptibility to these disorders. These findings emphasize BDNF's mechanistic links to pathology across neurodevelopmental and neurodegenerative conditions.
Implications in Psychiatric Conditions
Brain-derived neurotrophic factor (BDNF) has been implicated in the pathophysiology of major depressive disorder (MDD), with meta-analyses consistently showing that peripheral BDNF levels are significantly lower in individuals with MDD compared to healthy controls.[79] This reduction is observed across serum and plasma measurements, reflecting potential deficits in neurotrophic support for mood-regulating circuits in the hippocampus and prefrontal cortex.[80]Antidepressant treatments, such as selective serotonin reuptake inhibitors (SSRIs), have been shown to elevate BDNF expression through activation of the extracellular signal-regulated kinase (ERK) and cAMP response element-binding protein (CREB) signaling pathways, which may contribute to therapeutic neuroplasticity.[81]In schizophrenia, postmortem analyses of brain tissue reveal reduced BDNF expression in the hippocampus, suggesting impaired neurotrophic maintenance of neuronal integrity and synaptic function in regions critical for cognition and psychosis.[82] The Val66Met polymorphism in the BDNF gene (rs6265) is associated with this disorder, particularly influencing cognitive deficits such as memory and executivefunction impairments, as Met allele carriers exhibit altered BDNF secretion and hippocampal volume reductions.[83] These genetic and molecular alterations underscore BDNF's role in the neurodevelopmental aspects of schizophrenia, linking reduced trophic support to symptom severity.A 2024 network meta-analysis of BDNF levels across psychiatric disorders identified disorder-specific patterns, with decreased peripheral BDNF in bipolar disorder (BD), MDD, obsessive-compulsive disorder (OCD), panic disorder (PD), and schizophrenia (SCZ) relative to controls, while levels were significantly elevated in post-traumatic stress disorder (PTSD).[84] In BD, lower BDNF correlates with mood episode severity during manic and depressive phases, potentially disrupting emotional regulation via impaired synaptic plasticity.[85] Conversely, the increase in PTSD may reflect compensatory mechanisms in response to chronic stress, though it contrasts with reductions seen in other anxiety-related conditions like OCD and PD.In epilepsy, particularly temporal lobe epilepsy (TLE), BDNF overexpression in the hippocampus promotes aberrant mossy fiber sprouting, a pathological reorganization of granule cell axons that contributes to hyperexcitability and seizure propagation.[86] This sprouting, observed in both human TLE tissue and animal models, is driven by seizure-induced BDNF upregulation, which enhances excitatory synaptic transmission and may perpetuate epileptogenic circuits.[87]
BDNF as a Biomarker and Therapeutic Target
Serum levels of brain-derived neurotrophic factor (BDNF) serve as a peripheral proxy for central nervous system changes, reflecting alterations in brain BDNF expression and activity due to their correlation with cerebrospinal fluid levels.[88] In major depressive disorder (MDD) and Alzheimer's disease (AD), serum BDNF concentrations are consistently decreased compared to healthy controls, with meta-analyses reporting significantly lower levels in AD patients (SMD = -0.282).[89][90] Acute and chronic exercise interventions reliably elevate serum BDNF by 20-40%, with high-intensity protocols inducing immediate increases that support neuroplasticity and cognitive benefits.[91]Therapeutic strategies targeting BDNF focus on mimetics and delivery systems to overcome endogenous deficits. Small-molecule TrkB agonists, such as 7,8-dihydroxyflavone (7,8-DHF), mimic BDNF's neurotrophic effects by activating TrkB receptors, demonstrating preclinical efficacy in amyotrophic lateral sclerosis (ALS) models by preserving motor neurons and in AD models by reducing amyloid-beta pathology and synaptic loss.[92] Derivatives like BrAD-R13, an optimized 7,8-DHF analog, have advanced to phase I/II clinical trials for mild-to-moderate AD as of 2024, showing improved cognitive outcomes without significant adverse effects.[93]Gene therapy approaches, including adeno-associated virus (AAV)-mediated BDNF delivery, have restored dopamine neuron survival and function in Parkinson's disease animal models, such as MPTP-lesioned mice, by enhancing dopaminergic neurotransmission and mitigating mitochondrial dysfunction.[94] These vectors target the substantia nigra, achieving sustained BDNF expression for up to 6 months post-administration.[95]A primary challenge in BDNF-based therapies is the blood-brain barrier (BBB), which prevents significant delivery of BDNF across the BBB following systemic administration.[96] This necessitates alternative routes like intranasal or intracerebroventricular administration, though scalability remains limited. Non-pharmacological interventions, such as aerobic exercise, boost endogenous BDNF by up to 30% through mechanisms involving increased hippocampal expression, while vitamin D supplementation enhances BDNF levels by 15-25% in deficient individuals, with combined exercise-vitamin D protocols yielding synergistic neuroprotection in clinical trials.[97][98]In precision medicine, BDNF genotyping, particularly for the Val66Met polymorphism, informs personalized antidepressant selection, as 2025 meta-analyses indicate Met carriers exhibit faster response rates to selective serotonin reuptake inhibitors (SSRIs) in East Asian populations.[99] This approach integrates BDNF levels with genetic data for optimized dosing and monitoring.[100]