Shenfeng Qiu, Ph.D. - US grants

DESCRIPTION (provided by applicant): This proposed Pathway to Independence award describes a five-year career development plan leading to independent academic research. The applicant is a committed scientist conducting postdoctoral research work in the field of Neurobiology at Vanderbilt for the past four and half years. In July, 2009, he will move with his mentor, Pat Levitt, to the Zilkha Neurogenetic Institute, Keck School of Medicine of the University of Southern California, where this proposed work will be carried out. The long-term objective of this proposed research aims at understanding the role of the Met receptor tyrosine kinase in neural development and circuitry formation. Met activation by its ligand, hepatocyte growth factor, plays a pleiotropic role in the ontogenesis of multiple organs. In the developing forebrain, Met expression is temporally regulated and peaks during the period of extensive neuronal growth and synapse formation. Recent human genetic studies conducted in our lab and others have identified MET as a risk gene for autism spectrum disorder, a major neurodevelopmental syndrome with disrupted neuronal activity and connectivity. However, the role of Met in synapse function and microcircuit assembly is currently unknown. Three specific aims are proposed: 1) to investigate role of Met signaling in the development and function of the hippocampus. Both morphological and functional alterations in the developing hippocampus will be determined as a result of altered Met signaling (over-expression, knockdown and conditional genetic deletion);2) to determine molecular mechanisms regulating Met-induced neuronal growth and synaptogenisis in developing hippocampal neurons. In particular, the role of the members of Rho family small GTPases will be studied;3) to explore potential alterations in the local prefrontal cortex synaptic circuitry resulted from forebrain conditional Met deletion. These studies are important and highly relevant in that they provide mechanistic insights on Met-mediated signaling in forebrain development at molecular, cellular and system levels. Perspectives gained from this study will help the applicant establish research independence and form the scientific basis for achieving his long-term career goals in translational neuroscience research. PUBLIC HEALTH RELEVANCE: Project Narrative Human genetics studies have established MET as a risk gene for autism spectrum disocrders. This proposed study investigates the mechanistic role of Met-mediated signaling in the brain development. Insights gained from this study could offer better understanding of autism pathophysiology and thus be useful for future novel developmental interventions.

Human genetic studies have established MET as a prominent risk gene for autism spectrum disorder, a highly heritable psychiatric disorder with disrupted ontogeny of neural connectivity. MET protein is a receptor tyrosine kinase that is tightly regulated during early brain development, peaks at a period of rapid neurite growth and synaptogenesis, and is precipitously down-regulated prior to neuronal maturation. The goal of this project is to elucidate the nature of the time-delimited signaling by investigating how it regulates key brain development events, including synaptogenesis, maturation, circuit connectivity and refinement. Preliminary results from the PI's laboratory reveal that disruption of MET signaling in mice results in altered cortical interlaminar excitatory connectivity, aberrant neuronal morphology and maturation of glutamatergic synapses, as well as impaired circuit connectivity indicative of defective synapse pruning and circuit refinement. Using a controllable transgenic mouse model created in the lab of the PI, this research team recently found that MET activation engages the Rho family small GTPases, Cdc42 and Rac1, and leads to inhibition of the actin depolymerizing factor cofilin, processes that are critical for dendritic spine morphogenesis and excitatory synapse development. This has led to the central hypothesis that MET signaling promotes early dendritic spine morphogenesis, while its down-regulation is required for dendritic spine and glutamatergic synapse maturation later in brain development. In this application, the research group brings together an interdisciplinary team and takes an integrated approach combining neuroanatomy, molecular genetics, in vivo two photon imaging, and patch clamp electrophysiology combined with laser scanning photostimulation for circuit mapping to test the following hypotheses: 1) developmental down-regulation of MET expression is required for normal glutamatergic synapse maturation ; 2) persistent MET signaling impairs developmental synapse pruning and refinement cortical circuit connectivity; and 3) disrupted MET signaling and the resulting change in forebrain developmental trajectory alter mouse behavior. Impact: It is anticipated that successful completion of these proposed studies will define an in-depth, mechanistic, and multifaceted role of MET in neural development and establishment of functional connectivity in the developing forebrain. These mechanisms collectively may be unique to MET, and may illuminate novel interventions in autism by targeting the temporal profiles of glutamatergic synapse development in specific brain circuits.

PROJECT SUMMARY / ABSTRACT Depression is a complex, severe debilitating mental disorder that affects about 10% of Americans. While it is well established that environmental factors, such as stress, plays an etiology role, the brain mechanisms, particularly the role of specific neural circuits mediating the pathogenesis of depression, remain to be elucidated. Chronic social defeat stress (cSDS) in mice is a highly relevant, validated model to study brain mechanisms of depression. This behavior paradigm has been shown to induce morphological and functional changes in multiple brain regions including the prefrontal cortex (PFC), which is interconnected with other limbic brain regions such as the nucleus accumbens (NAc), ventral tegmental area, hippocampus and amygdala. It is not clear whether a particular neuronal type in the PFC, defined by its synaptic connectivity, is more relevant to the pathogenesis of depression. Using a recently developed neuronal activity reporter mouse line, termed fosTRAP, the applicant's lab has determined that acute and chronic social defeat stress activate distinct populations of projection neurons in the PFC. This raises the question of whether specific circuit connectivity is pertinent to depression and if future therapeutic strategies could be devised, using a `precision medicine in psychiatry' approach, to target the relevant circuits to combat the core symptoms of depression while alleviating off target effects. In this R21 proposal, the expertise of two junior faculty laboratories (Qiu, neurophysiology and functional circuit mapping; Ferguson, mouse depression behavior and optogenetics), will be merged to test the hypothesis that chronic social defeat-induced, depression-related behaviors are encoded within a specific neural circuit in the PFC. These PIs will employ the fosTRAP:AI14 reporter mouse combined with tamoxifen to gain genetic access to the prefrontal neurons that are activated by cSDS. They will further explore whether disrupted synaptic homeostasis selectively occurs in the proportion of L5 neurons that are activated by the chronic social defeat stress. This will be investigated using whole cell patch clamp electrophysiology and laser scanning photostimulation for functional circuit mapping studies in NAc-projecting L5 prefrontal neurons in fosTRP:AI14 reporter mice following cSDS (Aim 1). They will also test whether disrupted synaptic homeostasis occurs selectively in the susceptible mice populations. In Aim 2, they will use targeted optogenetic manipulation of neural activity in the L5 PFC projection neurons that are selectively activated by cSDS and also selectively express opsins. These investigators will test the novel hypothesis that optogenetic inhibition of this specific neuron ensemble during the acquisition of depressive-like behavior confers resistance, while repeated activation of these neurons leads to susceptibility. This study could reveal a paradigm-shifting practice in circuit-based therapeutics aimed at restoring prefrontal synaptic homeostasis and could establish a specific corticolimbic circuit as a lead target for preventing the development of depression, which is otherwise not possible by previous studies examining an indiscriminate population of PFC neurons.