Identification and Characterization of Novel Mechanisms of Functional and Structural Synapse Remodeling: Focus on Vav Guanine Nucleotide Exchange Factors and MEF2 Transcription Factors

Proper development of synaptic connectivity is a dynamic process requiring formation, elimination, maintenance, and plasticity of synapses. During early postnatal development, excess synapses are formed in most neural circuits, which are subsequently pruned during adolescence in a sensory- and activity-dependent mechanism. The brain also exhibits experience-dependent synaptic modifications that may enhance or weaken functional synapse strength. Investigation of numerous neurodevelopmental and psychiatric disorders reveals dysfunctions in synapse formation and function; however, underlying molecular mechanisms remain poorly understood. In Part One of this study, I identify a novel role for Vav guanine nucleotide exchange factors (GEFs) in brain-derived neurotrophic factor (BDNF)-dependent synapse plasticity. BNDF and its receptor, TrkB, are well-established positive modulators of hippocampal long-term potentiation (LTP), and increasing evidence suggests that BDNF/TrkB facilitates LTP in part through the stimulation of Rho GTPases and subsequent F-actin remodeling and dendritic spine structural dynamics. I report that Vav-family GEFs are activated by BDNF/TrkB signaling, and are required for BDNF-induced Rac-GTP formation. Vav GEFs, which are enriched at hippocampal glutamatergic synapses, are necessary for rapid BDNF-induced dendritic spine growth and CA3-CA1 LTP. Furthermore, Vav2/3-deficient mice have impaired contextual fear conditioning, as well as reduced anxiety. Together, findings support a role for Vav-dependent F-actin dynamics in BDNF-stimulated dendritic spine head enlargement and LTP, and normal hippocampal-dependent learning and memory and anxiety in mice. Part Two of this study reports the identification of common MEF2 and FMRP mRNA targets that are required for MEF2-induced synapse elimination. The activity-dependent transcription factor myocyte enhancer factor 2 (MEF2) is a key negative regulator of excitatory synapse number, promoting synapse removal in neurons through a complex program of gene expression. The RNA binding protein and translational regulator fragile X mental retardation protein (FMRP) was recently identified as an essential downstream component of MEF2-induced synapse elimination, suggesting that these autism-linked proteins coordinate transcriptional and translational control of common transcripts to mediate proper synaptic connectivity. Using high throughput sequencing of RNA isolated by cross-linking immunoprecipitation (HITS-CLIP) of FMRP, I find a large overlap of MEF2-induced transcripts and FMRP-associated mRNAs, consistent with their shared roles in synapse elimination. More specifically, protocadherin 17 (Pcdh17) mRNA is induced by MEF2 and exhibits differential binding to FMRP following MEF2 activation. Reducing Pcdh17 alone does not alter basal synapse number, but reducing Pcdh17 levels blocks MEF2-induced dendritic spine elimination of hippocampal neurons. These data suggest that MEF2-induced synapse elimination requires Pcdh17 – a MEF2 target gene and FMRP-associated transcript.