What Is Brain-derived Neurotrophic Factor?

Brain-derived neurotrophic factor, also known as BDNF, is a protein that, in humans, is encoded by the BDNF gene. BDNF is a member of the neurotrophin family of growth factors, which are related to the canonical Nerve Growth Factor. Neurotrophic factors are found in the brain and the periphery.

BDNF acts on certain neurons of the central nervous system and the peripheral nervous system, helping to support the survival of existing neurons, and encourage the growth and differentiation of new neurons and synapses. In the brain, it is active in the hippocampus, cortex, and basal forebrain — areas vital to learning, memory, and higher thinking. It is also expressed in the retina, motor neurons, the kidneys, saliva, and the prostate.

Brain-derived Neurotrophic Factor Function

BDNF itself is important for storage of long-term memory. 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.

Mice born without the ability to make BDNF suffer 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. Other important neurotrophins structurally related to BDNF include NT-3, NT-4, and NGF.

Brain-derived neurotrophic factor is made in the endoplasmic reticulum and secreted from dense-core vesicles. It binds carboxypeptidase E (CPE), and the disruption of this binding has been proposed to cause the loss of sorting of 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.

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. Niacin appears to upregulate BDNF and tropomyosin receptor kinase B (TrkB) expression as well.

Role In Synaptic Transmission

Glutamatergic Signaling

Glutamate is the brain’s major excitatory neurotransmitter and its release can trigger the depolarization of postsynaptic neurons. AMPA and NMDA receptors are two major ionotropic receptors that are especially suspected of being involved in learning and memory. While AMPA receptor activation leads to depolarization via sodium influx, NMDA receptor activation leads to depolarization via calcium and sodium influx.

The calcium influx triggered through NMDA receptors can lead to the activity-dependent expression of many different genes, proteins, and receptors that are thought to be involved in processes involving learning, memory, neurogenesis, and environmental responses. The activity-dependent synaptic responses also lead to rapid insertion of AMPA receptors into the postsynaptic membrane, which will act to maintain ongoing glutamatergic transmission as sustained calcium influx could result in excitotoxicity.

NMDA Receptor Activity

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, brain-derived neurotrophic factor and NR1 phosphorylation levels are upregulated simultaneously, probably because BDNF is capable of phosphorylating NR1 subunits, in addition to its many other effects.

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. 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. PI3 kinase and Akt are also essential in BDNF-induced potentiation of NMDA receptor function and inhibition of either molecule completely eliminated receptor activity.

Brain-derived neurotrophic factor 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.

Once activated, Fyn can bind to NR2B through its SH2 domain and mediate phosphorylation of its Tyr-1472 site. Similar studies have suggested Fyn is also capable of activating NR2A although this was not found in the hippocampus. 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.

Synapse Stability

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 synapse support. 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.

Brain-derived neurotrophic factor 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. It also appears that BDNF is able to influence Gl1 activity through its effects on NMDA receptor activity. 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.

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.

GABAergic Signaling

One mechanism through which BDNF appears to maintain elevated levels of neuronal excitation is through preventing GABAergic signaling activities. While glutamate is the brain’s major excitatory neurotransmitter and phosphorylation normally activates receptors, GABA is the brain’s primary inhibitory neurotransmitter and phoshorylation of GABAA receptors tend to reduce their activity.

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).

Similar effects could be obtained in BDNF knockout mice, but these effects were reversed by local application of BDNF. This suggests BDNF increases excitatory synaptic signaling partly through the post-synaptic suppression of GABAergic of signaling by activating PKC through its association with TrkB. Once activated, PKC can reduce the amplitude of IPSCs through to GABAA receptor phosphorylation and inhibition.

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.

Synaptogenesis

Brain-derived neurotrophic factor also enhances synaptogenesis. Synaptogenesis is dependent upon the assembly of new synapses and the disassembly of old synapses by β-adducin. 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.

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. In post-synaptic neurons they can influence dendritic spine formation and retraction as well as AMPA receptor insertion and removal.

At their C-terminus, adducins possess a myristoylated alanine-rich C kinase substrate (MARCKS) domain which regulates their capping activity. BDNF can reduce capping activities by upregulating PKC, which can bind to the adducing MRCKS domain, inhibit capping activity, and promote synatogenesis through dendritic spine growth and disassembly and other activities.

Neurogenesis

BDNF plays a significant 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.

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. BDNF can also promote NSC and NPC proliferation through Akt activation and PTEN inactivation.

There have been many in vivo studies demonstrating BDNF is a strong promoter of neuronal differentiation. Infusion of BDNF into the lateral ventricles doubled the population of newborn neurons in the adult rat olfactory bulb and viral overexpression of BDNF can similarly enhance SVZ neurogenesis.

Brain-derived neurotrophic factor might also play a role in NSC/NPC migration. By stabilizing p35 (CDK5R1), in utero electroporation studies revealed BDNF was able to promote cortical radial migration by about 2.3-fold in embryonic rats, an effect which was dependent on the activity of the trkB receptor.

Cognitive Function

Enriched housing provides the opportunity for exercise and exposure to multimodal stimuli. The increased visual, physical, and cognitive stimulation all translates into more neuronal activity and synaptic communication, which can produce structural or molecular activity-dependent alterations.

Sensory inputs from environmental stimuli are initially processed by the cortex before being transmitted to the hippocampus along an afferent pathway, suggesting the activity-mediated effects of enrichment can be far-reaching within the brain. Brain-derived neurotrophic factor expression is significantly enhanced by environmental enrichment and appears to be the primary source of environmental enrichments ability to enhance cognitive processes.

Environmental enrichment enhances synaptogenesis, dendridogenesis, and neurogenesis, leading to improved performance on various learning and memory tasks. BDNF mediates more pathways involved in these enrichment-induced processes than any other molecule and is strongly regulated by calcium activity making it incredibly sensitive to neuronal activity.

Pathology

Various studies have shown possible links between BDNF and conditions, such as depression, schizophrenia, obsessive-compulsive disorder, Alzheimer’s disease, Huntington’s disease, Rett syndrome, and dementia, as well as anorexia nervosa and bulimia nervosa. Increased levels of BDNF can induce a change to an opiate-dependent-like reward state when expressed in the ventral tegmental area in rats.

Schizophrenia

A plethora of recent evidence suggests the linkage between schizophrenia and BDNF. Given that BDNF is critical for the survival of central nervous system (CNS) and peripheral nervous system (PNS) neurons and synaptogenesis during and even after development, BDNF alterations may play a role in the pathogenesis of schizophrenia.

BDNF has been found within many areas of the brain and plays an important role is supporting the formation of memories. 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 that is known to be involved with working memory.

Since schizophrenic patients often suffer from impairments in working memory, and BDNF mRNA levels have been shown to be decreased in the DLPFC of schizophrenic patients, it is highly likely that BDNF plays some role in the etiology of this neurodevelopmental disorder of the CNS.

Depression

Exposure to stress and the stress hormone corticosterone has been shown to decrease the expression of BDNF in rats, and, if exposure is persistent, this leads to an eventual atrophy of the hippocampus. Atrophy of the hippocampus and other limbic structures has been shown to take place in humans suffering from chronic depression.

In addition, rats bred to be heterozygous for BDNF, therefore reducing its expression, have been observed to exhibit similar hippocampal atrophy. This suggests that an etiological link between the development of depression and BDNF exists.

Supporting this, the excitatory neurotransmitter glutamate, voluntary exercise, caloric restriction, intellectual stimulation, curcumin and various treatments for depression (such as antidepressants and electroconvulsive therapy) increase expression of BDNF in the brain. In the case of some treatments such as drugs and electroconvulsive therapy this has been shown to protect against or reverse this atrophy.

Aging

Brain-derived neurotrophic factor levels appear to be highly regulated throughout the lifetime both in the early developmental stages and in the later stages of life. For example, BDNF appears to be critical for the morphological development such as dendrite orientation and number along with soma size. This is important as neuron morphology is critical in behavioral processes like learning and motor skills development.

Research has reported that the interaction between BDNF and TrkB (the receptor to BDNF) is highly important in inducing dendritic growth; some have noted that the phosphorylation of TrkB by another molecule, cdk5 is necessary for this interaction to occur. Thus, high BDNF and active TrkB interaction appears to be necessary during a critical developmental period as it is regulatory in neuron morphology.v

Although BNDF is needed in the developmental stages, BDNF levels have been shown to decrease in tissues with aging. Studies using human subjects have found that hippocampal volume decreases with decreasing plasma levels of BDN. Although this does not mean BDNF necessary impacts hippocampal volume, it does suggest there is a relationship that might explain some of the cognitive decline that occurs during aging.

Top Image: Robinson RC, Radziejewski C, Stuart DI, Jones EY (April 1995). “Structure of the brain-derived neurotrophic factor/neurotrophin 3 heterodimer

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