Maribel Rios, Ph.D. - US grants

[unreadable] DESCRIPTION (provided by applicant): Depressive disorders are debilitating conditions that affect millions of individuals and create an enormous burden on society. Close to 100 billion dollars per year are spent treating patients with severe and mild forms of depression in the United States alone. However, the underlying molecular mechanisms that trigger depression remain to be elucidated so that treatment alternatives for patients that are unresponsive to the current forms of therapy can be created. A potential target for the design of novel treatment strategies is brain derived neurotrophic factor (BDNF). Compelling evidence shows that BDNF modulates affective behavior but the specific role and the mechanism of action of this neurotrophin remain elusive. We recently generated conditional mutations of BDNF using the cre recombinase/IoxP system. These mice have a pre or postnatal depletion of BDNF in the central nervous system that does not compromise their viability as the global depletion of BDNF does. These mutants display dramatic changes in behavior including hyperaggression and hypersensitivity to stress, both of which are often symptoms of depression. We propose using these mutants, and others that we are currently generating, as genetic models of depressive disorders to dissect the role of BDNF in the regulation of behavior. Different lines of mutants that through genetic manipulation have depletion or over expression of BDNF in different regions of the brain associated with mood disorders will be tested using standard behavioral models for depression and aggression. [unreadable] [unreadable]

DESCRIPTION (provided by applicant): Close to a third of the population in the United States is obese and over 60% is overweight. This is an alarming trend as obesity significantly increases susceptibility to type 2 diabetes, cardiovascular disease and other medical disorders. Mounting evidence indicates that the brain-derived neurotrophic factor (BDNF)/TrkB pathway plays a critical part in energy balance regulation and is a promising target for novel therapies. Accordingly, reduced BDNF signaling in mice and humans results in hyperphagic behavior and dramatic obesity. It remained unclear whether BDNF, which supports neuronal survival, differentiation and synaptic plasticity, acted as a required satiety factor in the adult brain or as a developmental facilitator of feeding neural circuits. As part of the previously funded project, we showed that BDNF acts in the adult animal to promote satiety and that the ventromedial hypothalamus (VMH) is a critical source of this neurotrophin. Supportive evidence includes the robust effects of energy status on expression of BDNF and TrkB in the VMH and the hyperphagia and obesity elicited by selectively deleting BDNF in the VMH of adult mice. The cellular and molecular mechanisms underlying the anorexigenic effects of BDNF in the VMH remain to be elucidated. Our recent analysis of the transcriptome of cells laser-captured from the VMH of mice with central (BDNF2L/2LCk-cre) or VMH-specific depletion of BDNF revealed decreased expression of a2d-1 and of two other genes associated with obesity susceptibility in a recent large-scale human study. Our preliminary studies show that BDNF's anorexigenic effects are mediated by a2d-1, a high voltage-gated calcium channel subunit that enhances calcium currents and mediates excitatory synaptogenesis. We found that: i) selective a2d-1 inhibition by chronic gabapentin infusion into wild type VMH increased food intake and body weight and ii) viral-mediated a2d-1 delivery to the VMH of BDNF2L/2LCk-cre mutants ameliorated their hyperphagia and body weight gain. This competitive renewal proposal seeks to build on these findings by ascertaining cellular mechanisms underlying the effects of BDNF and a2d-1. Because calcium currents in VMH cells of BDNF mutants are normal, the satiety effects of a2d-1 might be related to its ability to induce excitatory synaptogenesis in a calcium-independent manner. Thus, Aim 1 will investigate the role of BDNF and a2d-1 in dynamic changes in VMH excitatory drive onto anorexigenic POMC neurons induced by nutritional cues using anatomical and electrophysiological approaches. In Aim 2, we will investigate whether BDNF and a2d-1 also regulate excitatory drive onto anorexigenic VMH neurons. In Aim 3, we will test whether the reduced expression of two other gene candidates in the BDNF mutant VMH contributes to the emergence of hyperphagic behavior and obesity and whether they act in common pathways with a2d-1 in the VMH. These studies will provide a mechanistic understanding of anorexigenic actions of BDNF and reveal novel avenues for the treatment of obesity.

? DESCRIPTION (provided by applicant): Obesity is a risk factor for the development of type 2 diabetes, cardiovascular disease and other afflictions. Our previous studies identified a novel and critical role for brain-derived neurotrophic factor (BDNF) in central neural circuits controllig food intake and body weight. In agreement, mice with global central (BDNF2L/2LCk-cre) or selective BDNF depletion in the adult ventromedial hypothalamus (VMH) exhibit excessive feeding, obesity and metabolic disturbances. Diminished BDNF function has also been associated with hyperphagic behavior and severe obesity in humans. These findings have significant clinical implications as the BdnfVal66Met variant, which interferes with BDNF signaling, is highly prevalent among Americans. Here, we propose investigating whether BDNF regulates astrocyte structural plasticity and function to increase the excitatory drive of anorexigenic neurons in the VMH, a satiety center. Among known energy balance centers, BDNF is most abundant in the VMH, where it plays a required satiety role. Supportive evidence includes: i) robust effects of energy status on expression of BDNF and its receptor, TrkB, in the VMH, ii) the hyperphagia and obesity elicited by selectively deleting Bdnf in the VMH of adult mice iii) reduced density of excitatory synapses and decreased frequency of excitatory post synaptic currents in the VMH inBDNF2L/2LCk-cre mice. The cellular and molecular mechanisms underlying the effects of BDNF on neuronal excitability in the VMH remain to be fully elucidated. BDNF is a dynamic regulator of neuronal plasticity and alters morphology and mediates calcium signaling in astrocytes. These are significant effects as changes in glial morphology are associated with synaptic contact remodeling. Moreover, glutamate clearance by perisynaptic astrocytes is important in maintaining glutamate homeostasis and shaping synaptic currents. These effects of BDNF on astrocytes have been identified in several brain regions, but not in feeding circuits. Indeed, the role of astrocyte-neuron interactions in hypothalamic feeding circuits and the regulation of energy balance remains a vastly under studied research area. It warrants examination as dynamic changes in synaptic connectivity of hypothalamic circuits, including those involving the VMH, are thought to contribute to appetite control. As a first step t understand how astrocytes in the VMH influence feeding circuits, we propose examining the effect of energy status and BDNF on this cell population. Studies comprise examination of effects of energy status and BDNF on structural plasticity, glutamate uptake kinetics and dynamic changes in translating mRNAs in VMH astrocytes using cutting edge anatomical, electrophysiological and molecular approaches. The planned studies will elucidate novel mechanisms involving glial-neuron communication in the VMH that regulate activity of feeding circuits and satiety and thereby identify new targets for therapeutic strategies to treat obesity.

Project Summary Brain-derived neurotrophic factor (BDNF) is a chief regulator of excitatory synaptic plasticity, and diminished function of this neurotrophin is associated with obesity and metabolic dysfunction in humans and rodents. We showed that mice with global central BDNF depletion (BDNF2L/2LCk-cre) exhibit obesity, insulin resistance, hyperglycemia and dyslipidaemia. These findings have significant clinical implications as the BdnfVal66Met variant, which interferes with BDNF signaling, is highly prevalent in humans. Previous findings indicate that the ventromedial hypothalamus (VMH), a critical region for energy and glucose balance control, is an important site of BDNF action. The mechanisms underlying the effects of BDNF are poorly defined, especially those acting independently from feeding control. This proposal will investigate the novel role of metabotropic glutamate receptor 5 (mGluR5) mediating effects of BDNF on VMH neuronal activity and glycemic control. It emanates from our discovery that mGluR5 expression is significantly reduced in VMH of BDNF2L/2LCk-cre mutants. mGluR5 is highly expressed in the VMH and present in neurons (including SF1+) and astrocytes in this region and plays paramount roles in excitatory synaptic plasticity in the adult brain. Notably, we found that selective deletion of mGluR5 in VMH SF1+ neurons significantly impaired glycemic control and lipid metabolism without affecting body weight in mutant (mGluR5f/f:SF1-cre) female but not in male mice. Thus, we hypothesize that mGluR5 acts downstream of BDNF to mediate glucose and lipid homeostasis in a weight-independent and sex-specific manner by elevating the excitatory tone of SF1+ neurons. Aim 1 will test the hypothesis that sex-specific effects of mGluR5 in SF-1+ neurons involve functional interactions with estrogen receptors (ER). Aim 2 consists of anatomical, electrophysiological and molecular studies ascertaining the sex-specific role of mGluR5 regulating activity of SF1+ neurons, Aim 3 will define molecular mechanisms in SF1+ neurons governed by ER-mGluR5 interactions to control neuronal activity and glucose and lipid homeostasis and Aim 4 will test whether effects of BDNF on metabolic function are mediated by mGluR5-ER functional interactions in females. In aggregate, these investigations will inform novel mechanisms mediating glucose homeostasis and new avenues to tackle diabetes and its associated medical complications. In particular, it will inform the higher risk of insulin resistance and metabolic disorders reported in post menopausal women.  

Project Summary Alzheimer?s disease (AD) is a progressive neurodegenerative disorder and the most common form of dementia. It afflicts an evergrowing number of individuals with devastating consequences. Key features of AD pathology are amyloid plaques holding pathological forms of Ab and neurofibrillary tangles containing hyperphosphorylated Tau. Human association and animal studies suggest that obesity and the accompanying metabolic syndrome are risk factors for AD. The mechanisms underlying these putative effects of metabolic dysfunction remain poorly understood and warrant examination considering that obesity is a global health problem. Our previous studies identified a critical role for brain-derived neurotrophic factor (BDNF) in central neural circuits controlling energy and glucose balance. The parent grant for this administrative supplement application investigates whether BDNF signaling through the truncated form of the TrkB receptor (TrkB.T1) in astrocytes in the ventromedial hypothalamus (VMH) is one mechanism mediating energy balance and body weight control. The data so far indicate that TrkB.T1 in VMH astrocytes inhibits expression of the astrocytic glutamate transporter GLT-1 and synaptic glutamate clearance. This effect elevates the excitatory tone onto anorexigenic VMH neurons and suppresses appetite. Moreover, we found that chronic intake of a high fat diet in normal mice elevates expression of TrkB.T1 in hippocampus and prefrontal cortex (PFC), two brain regions involved in cognitive function and affected in AD. These findings are relevant to AD because elevated and reduced levels of TrkB.T1 and GLT-1, respectively, have been reported in AD brain. We hypothesize that HFD-induced obesity and Ab accumulation cooperate to increase levels of TrkB.T1 in cortical and hippocampal astrocytes. TrkB.T1, for its part, impedes synaptic glutamate clearance and the consequent accumulation of extracellular glutamate elicits synaptic dysfunction, exitotoxicity, neurodegeneration and ultimately, cognitive decline. To test this idea, we propose a series of studies examining the effects of HFD consumption on TrkB.T1 in astrocytes and on glutamate uptake kinetics in hippocampus and PFC of a mouse model of AD. Findings from these investigations will serve as a foundation for future studies informing the relationship between obesity and the onset of AD.