Francis S. Lee

Molecular adaptations underlying susceptibility and resistance to social defeat in brain reward regions.

While stressful life events are an important cause of psychopathology, most individuals exposed to adversity maintain normal psychological functioning. The molecular mechanisms underlying such resilience are poorly understood. Here, we demonstrate that an inbred population of mice subjected to social defeat can be separated into susceptible and unsusceptible subpopulations that differ along several behavioral and physiological domains. By a combination of molecular and electrophysiological techniques, we identify signature adaptations within the mesolimbic dopamine circuit that are uniquely associated with vulnerability or insusceptibility. We show that molecular recapitulations of three prototypical adaptations associated with the unsusceptible phenotype are each sufficient to promote resistant behavior. Our results validate a multidisciplinary approach to examine the neurobiological mechanisms of variations in stress resistance, and illustrate the importance of plasticity within the brains reward circuits in actively maintaining an emotional homeostasis.

Variant Brain-Derived Neurotrophic Factor (BDNF) (Met66) Alters the Intracellular Trafficking and Activity-Dependent Secretion of Wild-Type BDNF in Neurosecretory Cells and Cortical Neurons

Brain-derived neurotrophic factor (BDNF) plays a critical role in nervous system and cardiovascular development and function. Recently, a common single nucleotide polymorphism in the bdnf gene, resulting in a valine to methionine substitution in the prodomain (BDNFMet), has been shown to lead to memory impairment and susceptibility to neuropsychiatric disorders in humans heterozygous for the variant BDNF. When expressed by itself in hippocampal neurons, less BDNFMet is secreted in an activity-dependent manner. The nature of the cellular defect when both BDNFMet and wild-type BDNF (BDNFVal) are present in the same cell is not known. Given that this is the predominant expression profile in humans, we examined the effect of coexpressed BDNFMet on BDNFVal intracellular trafficking and processing. Our data indicate that abnormal trafficking of BDNFMet occurred only in neuronal and neurosecretory cells and that BDNFMet could alter the intracellular distribution and activity-dependent secretion of BDNFVal. We determined that, when coexpressed in the same cell, ∼70% of the variant BDNF forms BDNFVal·BDNFMet heterodimers, which are inefficiently sorted into secretory granules resulting in a quantitative decreased secretion. Finally, we determined the form of BDNF secreted in an activity-dependent manner and observed no differences in the forms of BDNFMet or the BDNFVal·BDNFMet heterodimer compared with BDNFVal. Together, these findings indicate that components of the regulated secretory machinery interacts specifically with a signal in the BDNF prodomain and that perturbations in BDNF trafficking may lead to selective impairment in CNS function.

ProBDNF Induces Neuronal Apoptosis via Activation of a Receptor Complex of p75NTR and Sortilin

Brain-derived neurotrophic factor (BDNF) is best characterized for critical roles in neuronal survival, differentiation, and synaptic modulation mediated by the TrkB receptor tyrosine kinase. Developmentally regulated death signaling by BDNF has also been demonstrated via activation of p75NTR. Because recent studies suggest that proNGF, the precursor form of NGF, is more active than mature NGF in inducing apoptosis after binding to p75NTR and a coreceptor, sortilin, we asked whether the precursor of BDNF (proBDNF) is also a proapoptotic ligand in the nervous system. proBDNF is secreted by cultured neurons, and recombinant proBDNF binds to sortilin. In sympathetic neurons coexpressing sortilin and p75NTR, we found that proBDNF is an apoptotic ligand that induces death at subnanomolar concentrations. In contrast, mature BDNF, but not proBDNF, is effective in inducing TrkB phosphorylation. proBDNF effects are dependent on cellular coexpression of both p75NTR and sortilin, because neurons deficient in p75NTR are resistant to proBDNF-induced apoptosis, and competitive antagonists of sortilin block sympathetic neuron death. Moreover, addition of preformed complexes of soluble sortilin and proBDNF failed to induce apoptosis of cells coexpressing both sortilin and p75NTR, suggesting that interaction of proBDNF with both receptors on the cell surface is required to initiate cell death. Together with our past findings, these data suggest that the neurotrophin family is capable of modulating diverse biological processes via differential processing of the proneurotrophins.

Neurotrophin signalling in health and disease

Neurotrophins are a unique family of polypeptide growth factors that influence the proliferation, differentiation, survival and death of neuronal and non-neuronal cells. They are essential for the health and well-being of the nervous system. NGF (nerve growth factor), BDNF (brain-derived neurotrophic factor), NT-3 (neurotrophin-3) and NT-4 (neurotrophin-4) also mediate additional higher-order activities, such as learning, memory and behaviour, in addition to their established functions for cell survival. The effects of neurotrophins depend upon their levels of availability, their affinity of binding to transmembrane receptors and the downstream signalling cascades that are stimulated after receptor activation. Alterations in neurotrophin levels have been implicated in neurodegenerative disorders, such as Alzheimers disease and Huntingtons disease, as well as psychiatric disorders, including depression and substance abuse. Difficulties in administering trophic factors have led to the consideration of using small molecules, such as GPCR (G-protein-coupled receptor) ligands, which can participate in transactivation events. In this review, we consider the signalling pathways activated by neurotrophins in both health and disease states.

A Genetic Variant BDNF Polymorphism Alters Extinction Learning in Both Mouse and Human

Of Mice and Men Just how closely must mouse models replicate the known features of human disorders to be accepted as useful for mechanistic and therapeutic studies? Soliman et al. (p. 863, published online 14 January) compared mice that vary only in their allelic composition at one position within the gene encoding brain-derived neurotrophic factor (BDNF) with humans exhibiting the same range of allelic variation. Individuals (mice and humans) carrying the allele that codes for a methionine-containing variant of BDNF retained a fearful response to a threatening stimulus even after its removal in comparison to those with the valine variant. Furthermore, in both cases, this linkage was mediated by diminished activity in the ventral-medial region of the prefrontal cortex. This deficit in extinction learning may contribute to differential responses to extinction-based therapies for anxiety disorders. A common genetic variation affecting fear learning and extinction operates through the same pathways in mice and men. Mouse models are useful for studying genes involved in behavior, but whether they are relevant to human behavior is unclear. Here, we identified parallel phenotypes in mice and humans resulting from a common single-nucleotide polymorphism in the brain-derived neurotrophic factor (BDNF) gene, which is involved in anxiety-related behavior. An inbred genetic knock-in mouse strain expressing the variant BDNF recapitulated the phenotypic effects of the human polymorphism. Both were impaired in extinguishing a conditioned fear response, which was paralleled by atypical frontoamygdala activity in humans. Thus, this variant BDNF allele may play a role in anxiety disorders showing impaired learning of cues that signal safety versus threat and in the efficacy of treatments that rely on extinction mechanisms, such as exposure therapy.

Activation of Trk neurotrophin receptors in the absence of neurotrophins

Neurotrophins regulate neuronal cell survival and synaptic plasticity through activation of Trk receptor tyrosine kinases. Binding of neurotrophins to Trk receptors results in receptor autophosphorylation and downstream phosphorylation cascades. Here, we describe an approach to use small molecule agonists to transactivate Trk neurotrophin receptors. Activation of TrkA receptors in PC12 cells and TrkB in hippocampal neurons was observed after treatment with adenosine, a neuromodulator that acts through G protein-coupled receptors. These effects were reproduced by using the adenosine agonist CGS 21680 and were counteracted with the antagonist ZM 241385, indicating that this transactivation event by adenosine involves adenosine 2A receptors. The increase in Trk activity could be inhibited by the use of the Src family-specific inhibitor, PP1, or K252a, an inhibitor of Trk receptors. In contrast to other G protein-coupled receptor transactivation events, adenosine used Trk receptor signaling with a longer time course. Moreover, adenosine activated phosphatidylinositol 3-kinase/Akt through a Trk-dependent mechanism that resulted in increased cell survival after nerve growth factor or brain-derived neurotrophic factor withdrawal. Therefore, adenosine acting through the A2A receptors exerts a trophic effect through the engagement of Trk receptors. These results provide an explanation for neuroprotective actions of adenosine through a unique signaling mechanism and raise the possibility that small molecules may be used to elicit neurotrophic effects for the treatment of neurodegenerative diseases.

Neuronal release of proBDNF

Pro–brain-derived neurotrophic factor (proBDNF) and mature BDNF utilize distinct receptors to mediate divergent neuronal actions. Using new tools to quantitate endogenous BDNF isoforms, we found that mouse neurons secrete both proBDNF and mature BDNF. The highest levels of proBDNF and p75 were observed perinatally and declined, but were still detectable, in adulthood. Thus, BDNF actions are developmentally regulated by secretion of proBDNF or mature BDNF and by local expression of p75 and TrkB.

Sortilin controls intracellular sorting of brain-derived neurotrophic factor to the regulated secretory pathway

Brain-derived neurotrophic factor (BDNF), after activity-dependent secretion from neurons, modulates critical nervous system functions. Recently, a variant in the human bdnf gene, resulting in a valine to methionine substitution in the prodomain, has been shown to lead to defective regulated secretion from neurons and memory impairment. Here, we report a novel function for a Vps10p domain protein, sortilin, in controlling BDNF sorting to the regulated secretory pathway. Sortilin interacts specifically with BDNF in a region encompassing the methionine substitution and colocalizes with BDNF in secretory granules in neurons. A truncated form of sortilin causes BDNF missorting to the constitutive secretory pathway without affecting neurotrophin-4 (NT-4) secretion. In addition, sortilin small interfering RNA introduced into primary neurons also led to BDNF missorting from the regulated to the constitutive secretory pathway. Together, these data suggest a mechanism to understand the defect associated with variant BDNF and provide a framework, based on divergent presynaptic regulation of sorting to secretory pathways, to explain how two ligands for tropomyosin-related kinase B, BDNF and NT-4, can mediate diverse biological responses.

Transactivation of Trk Neurotrophin Receptors by G-Protein-Coupled Receptor Ligands Occurs on Intracellular Membranes

Neurotrophins, such as NGF and BDNF, activate Trk receptor tyrosine kinases through receptor dimerization at the cell surface followed by autophosphorylation and intracellular signaling. It has been shown that activation of Trk receptor tyrosine kinases can also occur via a G-protein-coupled receptor (GPCR) mechanism, without involvement of neurotrophins. Two GPCR ligands, adenosine and pituitary adenylate cyclase-activating polypeptide (PACAP), can activate Trk receptor activity to increase the survival of neural cells through stimulation of Akt activity. To investigate the mechanism of Trk receptor transactivation, we have examined the localization of Trk receptors in PC12 cells and primary neurons after treatment with adenosine agonists and PACAP. In contrast to neurotrophin treatment, Trk receptors were sensitive to transcriptional and translational inhibitors, and they were found predominantly in intracellular locations particularly associated with Golgi membranes. Biotinylation and immunostaining experiments confirm that most of the transactivated Trk receptors are found in intracellular membranes. These results indicate that there are alternative modes of activating Trk receptor tyrosine kinases in the absence of neurotrophin binding at the cell surface and that receptor signaling may occur and persist inside of neuronal cells.

Variant BDNF (Val66Met) impact on brain structure and function.

Neurotrophins, such as brain-derived neurotrophic factor (BDNF), are a unique family of polypeptide growth factors that influence differentiation and survival of neurons in the developing nervous system. In adults, BDNF is important in regulating synaptic plasticity and connectivity in the brain. Recently, a common single-nucleotide polymorphism in the human BDNF gene, resulting in a valine to methionine substitution in the prodomain (Val66Met), has been shown to lead to memory impairment and susceptibility to neuropsychiatric disorders. An understanding of how this naturally occurring polymorphism affects behavior, anatomy, and cognition in adults is an important first step in linking genetic alterations in the neurotrophin system to definable biological outcomes in humans. We review the recent literature linking this BDNF polymorphism to cognitive impairment in the context of in vitro and transgenic animal studies that have established BDNF’s central role in neuronal functioning in the adult brain.