Brain-derived neurotrophic factor (BDNF), or abrineurin, [5] is a protein [6] that, in humans, is encoded by the BDNF gene. [7] [8] BDNF is a member of the neurotrophin family of growth factors, which are related to the canonical nerve growth factor (NGF), a family which also includes NT-3 and NT-4/NT-5. Neurotrophic factors are found in the brain and the periphery. BDNF was first isolated from a pig brain in 1982 by Yves-Alain Barde and Hans Thoenen. [9]
BDNF activates the TrkB tyrosine kinase receptor. [10] [11]
BDNF acts on certain neurons of the central nervous system and the peripheral nervous system expressing TrkB, helping to support survival of existing neurons, and encouraging growth and differentiation of new neurons and synapses. [12] [13] In the brain it is active in the hippocampus, cortex, and basal forebrain—areas vital to learning, memory, and higher thinking. [14] BDNF is also expressed in the retina, kidneys, prostate, motor neurons, and skeletal muscle, and is also found in saliva. [15] [16]
BDNF itself is important for long-term memory. [17] Although the vast majority of neurons in the mammalian brain are formed prenatally, parts of the adult brain retain the ability to grow new neurons from neural stem cells in a process known as neurogenesis. Neurotrophins are proteins that help to stimulate and control neurogenesis, BDNF being one of the most active. [18] [19] [20] Mice born without the ability to make BDNF have developmental defects in the brain and sensory nervous system, and usually die soon after birth, suggesting that BDNF plays an important role in normal neural development. [21] Other important neurotrophins structurally related to BDNF include NT-3, NT-4, and NGF.
BDNF is made in the endoplasmic reticulum and secreted from dense-core vesicles. It binds carboxypeptidase E (CPE), and disruption of this binding has been proposed to cause the loss of sorting BDNF into dense-core vesicles. The phenotype for BDNF knockout mice can be severe, including postnatal lethality. Other traits include sensory neuron losses that affect coordination, balance, hearing, taste, and breathing. Knockout mice also exhibit cerebellar abnormalities and an increase in the number of sympathetic neurons. [22]
Certain types of physical exercise have been shown to markedly (threefold) increase BDNF synthesis in the human brain, a phenomenon which is partly responsible for exercise-induced neurogenesis and improvements in cognitive function. [16] [23] [24] [25] [26] Niacin appears to upregulate BDNF and tropomyosin receptor kinase B (TrkB) expression as well. [27]
BDNF binds at least two receptors on the surface of cells that are capable of responding to this growth factor, TrkB (pronounced "Track B") [10] [11] and the LNGFR (for low-affinity nerve growth factor receptor, also known as p75). [28] It may also modulate the activity of various neurotransmitter receptors, including the Alpha-7 nicotinic receptor. [29] BDNF has also been shown to interact with the reelin signaling chain. [30] The expression of reelin by Cajal–Retzius cells goes down during development under the influence of BDNF. [31] The latter also decreases reelin expression in neuronal culture.
The TrkB receptor is encoded by the NTRK2 gene and is member of a receptor family of tyrosine kinases that includes TrkA and TrkC. TrkB autophosphorylation is dependent upon its ligand-specific association with BDNF, [10] [11] a widely expressed activity-dependent neurotrophic factor that regulates plasticity and is dysregulated following hypoxic injury. The activation of the BDNF-TrkB pathway is important in the development of short-term memory and the growth of neurons.[ citation needed ]
The role of the other BDNF receptor, p75, is less clear. While the TrkB receptor interacts with BDNF in a ligand-specific manner, all neurotrophins can interact with the p75 receptor. [32] When the p75 receptor is activated, it leads to activation of NFkB receptor. [32] Thus, neurotrophic signaling may trigger apoptosis rather than survival pathways in cells expressing the p75 receptor in the absence of Trk receptors. Recent studies have revealed a truncated isoform of the TrkB receptor (t-TrkB) may act as a dominant negative to the p75 neurotrophin receptor, inhibiting the activity of p75, and preventing BDNF-mediated cell death. [33]
The BDNF protein is encoded by a gene that is also called BDNF, found in humans on chromosome 11. [7] [8] Structurally, BDNF transcription is controlled by eight different promoters, each leading to different transcripts containing one of eight untranslated 5' exons (I to VIII) spliced to the 3' encoding exon. Promoter IV activity, leading to the translation of exon IV-containing mRNA, is strongly stimulated by calcium and is primarily under the control of a Cre regulatory component, suggesting a putative role for the transcription factor CREB and the source of BDNF's activity-dependent effects . [34] There are multiple mechanisms through neuronal activity that can increase BDNF exon IV specific expression. [34] Stimulus-mediated neuronal excitation can lead to NMDA receptor activation, triggering a calcium influx. Through a protein signaling cascade requiring Erk, CaM KII/IV, PI3K, and PLC, NMDA receptor activation is capable of triggering BDNF exon IV transcription. BDNF exon IV expression also seems capable of further stimulating its own expression through TrkB activation. BDNF is released from the post-synaptic membrane in an activity-dependent manner, allowing it to act on local TrkB receptors and mediate effects that can lead to signaling cascades also involving Erk and CaM KII/IV. [34] [35] Both of these pathways probably involve calcium-mediated phosphorylation of CREB at Ser133, thus allowing it to interact with BDNF's Cre regulatory domain and upregulate transcription. [36] However, NMDA-mediated receptor signaling is probably necessary to trigger the upregulation of BDNF exon IV expression because normally CREB interaction with CRE and the subsequent translation of the BDNF transcript is blocked by of the basic helix–loop–helix transcription factor protein 2 (BHLHB2). [37] NMDA receptor activation triggers the release of the regulatory inhibitor, allowing for BDNF exon IV upregulation to take place in response to the activity-initiated calcium influx. [37] Activation of dopamine receptor D5 also promotes expression of BDNF in prefrontal cortex neurons. [38]
BDNF has several known single nucleotide polymorphisms (SNP), including, but not limited to, rs6265, C270T, rs7103411, rs2030324, rs2203877, rs2049045 and rs7124442. As of 2008, rs6265 is the most investigated SNP of the BDNF gene [39] [40]
A common SNP in the BDNF gene is rs6265. [41] This point mutation in the coding sequence, a guanine to adenine switch at position 196, results in an amino acid switch: valine to methionine exchange at codon 66, Val66Met, which is in the prodomain of BDNF. [41] [40] Val66Met is unique to humans. [41] [40]
The mutation interferes with normal translation and intracellular trafficking of BDNF mRNA, as it destabilizes the mRNA and renders it prone to degradation. [41] The proteins resulting from mRNA that does get translated, are not trafficked and secreted normally, as the amino acid change occurs on the portion of the prodomain where sortilin binds; and sortilin is essential for normal trafficking. [41] [40] [42]
The Val66Met mutation results in a reduction of hippocampal tissue and has since been reported in a high number of individuals with learning and memory disorders, [40] anxiety disorders, [43] major depression, [44] and neurodegenerative diseases such as Alzheimer's and Parkinson's. [45]
A meta-analysis indicates that the BDNF Val66Met variant is not associated with serum BDNF. [46]
Glutamate is the brain's major excitatory neurotransmitter and its release can trigger the depolarization of postsynaptic neurons. AMPA and NMDA receptors are two ionotropic glutamate receptors involved in glutamatergic neurotransmission and essential to learning and memory via long-term potentiation. While AMPA receptor activation leads to depolarization via sodium influx, NMDA receptor activation by rapid successive firing allows calcium influx in addition to sodium. The calcium influx triggered through NMDA receptors can lead to expression of BDNF, as well as other genes thought to be involved in LTP, dendritogenesis, and synaptic stabilization.
NMDA receptor activation is essential to producing the activity-dependent molecular changes involved in the formation of new memories. Following exposure to an enriched environment, BDNF and NR1 phosphorylation levels are upregulated simultaneously, probably because BDNF is capable of phosphorylating NR1 subunits, in addition to its many other effects. [47] [48] One of the primary ways BDNF can modulate NMDA receptor activity is through phosphorylation and activation of the NMDA receptor one subunit, particularly at the PKC Ser-897 site. [47] The mechanism underlying this activity is dependent upon both ERK and PKC signaling pathways, each acting individually, and all NR1 phosphorylation activity is lost if the TrKB receptor is blocked. [47] PI3 kinase and Akt are also essential in BDNF-induced potentiation of NMDA receptor function and inhibition of either molecule eliminated receptor BDNF can also increase NMDA receptor activity through phosphorylation of the NR2B subunit. BDNF signaling leads to the autophosphorylation of the intracellular domain of the TrkB receptor (ICD-TrkB). Upon autophosphorylation, Fyn associates with the pICD-TrkB through its Src homology domain 2 (SH2) and is phosphorylated at its Y416 site. [49] [50] Once activated, Fyn can bind to NR2B through its SH2 domain and mediate phosphorylation of its Tyr-1472 site. [51] Similar studies have suggested Fyn is also capable of activating NR2A although this was not found in the hippocampus. [52] [53] Thus, BDNF can increase NMDA receptor activity through Fyn activation. This has been shown to be important for processes such as spatial memory in the hippocampus, demonstrating the therapeutic and functional relevance of BDNF-mediated NMDA receptor activation. [52]
In addition to mediating transient effects on NMDAR activation to promote memory-related molecular changes, BDNF should also initiate more stable effects that could be maintained in its absence and not depend on its expression for long term synaptic support. [54] It was previously mentioned that AMPA receptor expression is essential to learning and memory formation, as these are the components of the synapse that will communicate regularly and maintain the synapse structure and function long after the initial activation of NMDA channels. BDNF is capable of increasing the mRNA expression of GluR1 and GluR2 through its interaction with the TrkB receptor and promoting the synaptic localization of GluR1 via PKC- and CaMKII-mediated Ser-831 phosphorylation. [55] It also appears that BDNF is able to influence Gl1 activity through its effects on NMDA receptor activity. [56] BDNF significantly enhanced the activation of GluR1 through phosphorylation of tyrosine830, an effect that was abolished in either the presence of a specific NR2B antagonist or a trk receptor tyrosine kinase inhibitor. [56] Thus, it appears BDNF can upregulate the expression and synaptic localization of AMPA receptors, as well as enhance their activity through its postsynaptic interactions with the NR2B subunit. This suggests BDNF is not only capable of initiating synapse formation through its effects on NMDA receptor activity, but it can also support the regular every-day signaling necessary for stable memory function.
One mechanism through which BDNF appears to maintain elevated levels of neuronal excitation is through preventing GABAergic signaling activities. [57] While glutamate is the brain's major excitatory neurotransmitter and phosphorylation normally activates receptors, GABA is the brain's primary inhibitory neurotransmitter and phosphorylation of GABAA receptors tend to reduce their activity.[ clarification needed ] Blockading BDNF signaling with a tyrosine kinase inhibitor or a PKC inhibitor in wild type mice produced significant reductions in spontaneous action potential frequencies that were mediated by an increase in the amplitude of GABAergic inhibitory postsynaptic currents (IPSC). [57] Similar effects could be obtained in BDNF knockout mice, but these effects were reversed by local application of BDNF. [57] This suggests BDNF increases excitatory synaptic signaling partly through the post-synaptic suppression of GABAergic signaling by activating PKC through its association with TrkB. [57] Once activated, PKC can reduce the amplitude of IPSCs through to GABAA receptor phosphorylation and inhibition. [57] In support of this putative mechanism, activation of PKCε leads to phosphorylation of N-ethylmaleimide-sensitive factor (NSF) at serine 460 and threonine 461, increasing its ATPase activity which downregulates GABAA receptor surface expression and subsequently attenuates inhibitory currents. [58]
BDNF also enhances synaptogenesis. Synaptogenesis is dependent upon the assembly of new synapses and the disassembly of old synapses by β-adducin. [59] Adducins are membrane-skeletal proteins that cap the growing ends of actin filaments and promote their association with spectrin, another cytoskeletal protein, to create stable and integrated cytoskeletal networks. [60] Actins have a variety of roles in synaptic functioning. In pre-synaptic neurons, actins are involved in synaptic vesicle recruitment and vesicle recovery following neurotransmitter release. [61] In post-synaptic neurons they can influence dendritic spine formation and retraction as well as AMPA receptor insertion and removal. [61] At their C-terminus, adducins possess a myristoylated alanine-rich C kinase substrate (MARCKS) domain which regulates their capping activity. [60] BDNF can reduce capping activities by upregulating PKC, which can bind to the adducing MRCKS domain, inhibit capping activity, and promote synaptogenesis through dendritic spine growth and disassembly and other activities. [59] [61]
Local interaction of BDNF with the TrkB receptor on a single dendritic segment is able to stimulate an increase in PSD-95 trafficking to other separate dendrites as well as to the synapses of locally stimulated neurons. [62] PSD-95 localizes the actin-remodeling GTPases, Rac and Rho, to synapses through the binding of its PDZ domain to kalirin, increasing the number and size of spines. [63] Thus, BDNF-induced trafficking of PSD-95 to dendrites stimulates actin remodeling and causes dendritic growth in response to BDNF.
Laboratory studies indicate that BDNF may play a role in neurogenesis. BDNF can promote protective pathways and inhibit damaging pathways in the NSCs and NPCs that contribute to the brain's neurogenic response by enhancing cell survival. This becomes especially evident following suppression of TrkB activity. [32] TrkB inhibition results in a 2–3 fold increase in cortical precursors displaying EGFP-positive condensed apoptotic nuclei and a 2–4 fold increase in cortical precursors that stained immunopositive for cleaved caspase-3. [32] BDNF can also promote NSC and NPC proliferation through Akt activation and PTEN inactivation. [64] Some studies suggest that BDNF may promote neuronal differentiation. [32] [65]
Preliminary research has focused on the possible links between BDNF and clinical conditions, such as depression, [66] schizophrenia, [67] and Alzheimer's disease. [68]
Preliminary studies have assessed a possible relationship between schizophrenia and BDNF. [69] It has been shown that BDNF mRNA levels are decreased in cortical layers IV and V of the dorsolateral prefrontal cortex of schizophrenic patients, an area associated with working memory. [70]
The neurotrophic hypothesis of depression states that depression is associated with a decrease in the levels of BDNF. [66]
Levels of both BDNF mRNA and BDNF protein are known to be up-regulated in epilepsy. [71]
Neurotrophins are a family of proteins that induce the survival, development, and function of neurons.
Synaptogenesis is the formation of synapses between neurons in the nervous system. Although it occurs throughout a healthy person's lifespan, an explosion of synapse formation occurs during early brain development, known as exuberant synaptogenesis. Synaptogenesis is particularly important during an individual's critical period, during which there is a certain degree of synaptic pruning due to competition for neural growth factors by neurons and synapses. Processes that are not used, or inhibited during their critical period will fail to develop normally later on in life.
Nerve growth factor (NGF) is a neurotrophic factor and neuropeptide primarily involved in the regulation of growth, maintenance, proliferation, and survival of certain target neurons. It is perhaps the prototypical growth factor, in that it was one of the first to be described. Since it was first isolated by Nobel Laureates Rita Levi-Montalcini and Stanley Cohen in 1956, numerous biological processes involving NGF have been identified, two of them being the survival of pancreatic beta cells and the regulation of the immune system.
Tropomyosin receptor kinase A (TrkA), also known as high affinity nerve growth factor receptor, neurotrophic tyrosine kinase receptor type 1, or TRK1-transforming tyrosine kinase protein is a protein that in humans is encoded by the NTRK1 gene.
Tropomyosin receptor kinase B (TrkB), also known as tyrosine receptor kinase B, or BDNF/NT-3 growth factors receptor or neurotrophic tyrosine kinase, receptor, type 2 is a protein that in humans is encoded by the NTRK2 gene. TrkB is a receptor for brain-derived neurotrophic factor (BDNF). The standard pronunciation for this protein is "track bee".
The p75 neurotrophin receptor (p75NTR) was first identified in 1973 as the low-affinity nerve growth factor receptor (LNGFR) before discovery that p75NTR bound other neurotrophins equally well as nerve growth factor. p75NTR is a neurotrophic factor receptor. Neurotrophic factor receptors bind Neurotrophins including Nerve growth factor, Neurotrophin-3, Brain-derived neurotrophic factor, and Neurotrophin-4. All neurotrophins bind to p75NTR. This also includes the immature pro-neurotrophin forms. Neurotrophic factor receptors, including p75NTR, are responsible for ensuring a proper density to target ratio of developing neurons, refining broader maps in development into precise connections. p75NTR is involved in pathways that promote neuronal survival and neuronal death.
Tropomyosin receptor kinase C (TrkC), also known as NT-3 growth factor receptor, neurotrophic tyrosine kinase receptor type 3, or TrkC tyrosine kinase is a protein that in humans is encoded by the NTRK3 gene.
Neurotrophic factors (NTFs) are a family of biomolecules – nearly all of which are peptides or small proteins – that support the growth, survival, and differentiation of both developing and mature neurons. Most NTFs exert their trophic effects on neurons by signaling through tyrosine kinases, usually a receptor tyrosine kinase. In the mature nervous system, they promote neuronal survival, induce synaptic plasticity, and modulate the formation of long-term memories. Neurotrophic factors also promote the initial growth and development of neurons in the central nervous system and peripheral nervous system, and they are capable of regrowing damaged neurons in test tubes and animal models. Some neurotrophic factors are also released by the target tissue in order to guide the growth of developing axons. Most neurotrophic factors belong to one of three families: (1) neurotrophins, (2) glial cell-line derived neurotrophic factor family ligands (GFLs), and (3) neuropoietic cytokines. Each family has its own distinct cell signaling mechanisms, although the cellular responses elicited often do overlap.
Neurotrophin-3 is a protein that in humans is encoded by the NTF3 gene.
Neurotrophin-4 (NT-4), also known as neurotrophin-5 (NT-5), is a protein that in humans is encoded by the NTF4 gene. It is a neurotrophic factor that signals predominantly through the TrkB receptor tyrosine kinase. NT-4 was first discovered and isolated from xenopus and viper in the year 1991 by Finn Hallbook et.al
Trk receptors are a family of tyrosine kinases that regulates synaptic strength and plasticity in the mammalian nervous system. Trk receptors affect neuronal survival and differentiation through several signaling cascades. However, the activation of these receptors also has significant effects on functional properties of neurons.
Potassium-chloride transporter member 5 is a neuron-specific chloride potassium symporter responsible for establishing the chloride ion gradient in neurons through the maintenance of low intracellular chloride concentrations. It is a critical mediator of synaptic inhibition, cellular protection against excitotoxicity and may also act as a modulator of neuroplasticity. Potassium-chloride transporter member 5 is also known by the names: KCC2 for its ionic substrates, and SLC12A5 for its genetic origin from the SLC12A5 gene in humans.
The glutamate hypothesis of schizophrenia models the subset of pathologic mechanisms of schizophrenia linked to glutamatergic signaling. The hypothesis was initially based on a set of clinical, neuropathological, and, later, genetic findings pointing at a hypofunction of glutamatergic signaling via NMDA receptors. While thought to be more proximal to the root causes of schizophrenia, it does not negate the dopamine hypothesis, and the two may be ultimately brought together by circuit-based models. The development of the hypothesis allowed for the integration of the GABAergic and oscillatory abnormalities into the converging disease model and made it possible to discover the causes of some disruptions.
Neurotrophic factor receptors or neurotrophin receptors are a group of growth factor receptors which specifically bind to neurotrophins.
Activity-regulated cytoskeleton-associated protein is a plasticity protein that in humans is encoded by the ARC gene. The gene is believed to derive from a retrotransposon. The protein is found in the neurons of tetrapods and other animals where it can form virus-like capsids that transport RNA between neurons.
Alcoholism is a chronic disease characterized by trouble controlling the consumption of alcohol, dependence, and withdrawal upon rapid cessation of drinking. According to ARDI reports approximately 88,000 people had alcohol-related deaths in the United States between the years of 2006 and 2010. Furthermore, chronic alcohol use is consistently the third leading cause of death in the United States. In consequence, research has sought to determine the factors responsible for the development and persistence of alcoholism. From this research, several molecular and epigenetic mechanisms have been discovered.
ANA-12 is a selective, small-molecule non-competitive antagonist of TrkB, the main receptor of brain-derived neurotrophic factor (BDNF). ANA-12 was originally discovered and developed by Cazorla M. and colleagues at Université Paris and Inserm in 2011. The compound crosses the blood-brain-barrier and exerts central TrkB blockade, producing effects as early as 30 minutes and as long as 6 hours following intraperitoneal injection in mice. It blocks the neurotrophic actions of BDNF without compromising neuron survival.
Sandra M. Garraway is a Canadian-American neuroscientist and assistant professor of physiology in the Department of Physiology at Emory University School of Medicine in Atlanta, Georgia. Garraway is the director of the Emory Multiplex Immunoassay Core (EMIC) where she assists researchers from both academia and industry to perform, analyze, and interpret their multiplexed immunoassays. Garraway studies the neural mechanisms of spinal nociceptive pain after spinal cord injury and as a postdoctoral researcher she discovered roles for both BDNF and ERK2 in pain sensitization and developed novel siRNA technology to inhibit ERK2 as a treatment for pain.
Neurotrophin mimetics are small molecules or peptide like molecules that can modulate the action of the neurotrophin receptor. One of the main causes of neurodegeneration involves changes in the expression of neurotrophins (NTs) and/or their receptors. Indeed, these imbalances or changes in their activity, lead to neuronal damage resulting in neurological and neurodegenerative conditions. The therapeutic properties of neurotrophins attracted the focus of many researchers during the years, but the poor pharmacokinetic properties, such as reduced bioavailability and low metabolic stability, the hyperalgesia, the inability to penetrate the blood–brain barrier and the short half-lives render the large neurotrophin proteins not suitable to be implemented as drugs.
The neurotrophic hypothesis of depression proposes that major depressive disorder (MDD) is caused, at least partly, by impaired neurotrophic support. Neurotrophic factors are a family of closely related proteins which regulate the survival, development, and function of neurons in both the central and peripheral nervous systems.
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ignored (help); see the chapter "A Tale of Two Genes: Reelin and BDNF"; pp. 237–45