The roles of postsynaptic density-95 interactors in dendrite development and recovery after traumatic brain injury

Postsynaptic density 95 (PSD-95) is the major scaffolding protein at excitatory synapses, and it plays a major role in synaptic plasticity. Furthermore, PSD-95 and its interactor, cytosolic PSD-95 interactor (cypin), regulate dendrite branching by altering microtubule dynamics. Additionally, other PSD-95 binding proteins, end-binding protein 3 (EB3) and adenomatous polyposis coli (APC), promote microtubule bundling and stabilization. Thus, PSD-95 and binding partners may regulate the dendritic arbor during development and after injury. I first addressed the role of cypin in the brain in vivo. While generating cypin knockout mice, I identified a novel short isoform of cypin, termed cypinS, which also binds to PSD-95 and regulates dendrite branching, although an increase in dendrites occur more distal from the soma when cypinS is overexpressed compared to when cypin is overexpressed. In addition, unlike cypin, cypinS does not have guanine deaminase activity. Overexpression of cypin, but not cypinS, decreases spine density, suggestingiv that the regulation of spine density but not dendrite branching by cypin is dependent on guanine deaminase activity. Furthermore, I have uncovered novel presynaptic roles for both isoforms as overexpression of either isoform leads to increases in miniature excitatory postsynaptic current (mEPSC) frequency. Thus, cypin and cypinS, play distinct roles in neuronal development. I then chose to study the roles of PSD-95 and interactors in recovery after traumatic brain injury (TBI). We previously identified cypin as a novel target for TBI, and thus, here I studied the role of PSD-95 and its interaction with APC or EB3 in after injury induced by in vitro and in vivo models of TBI. I show that our in vitro model of mechanical stretch injury mimics moderate injury induced by controlled cortical impact (CCI) in mice. During the early stage (1-7 days) post-moderate CCI, the interaction of PSD-95 with APC and EB3 increases. Furthermore, downregulation of PSD-95 prevents stretch-injury mediated decreases in secondary dendrite number and total dendrite length, suggesting a required role of PSD-95 in injury-mediated insults to dendrites. Thus, PSD-95 may sequester APC and EB3 from microtubules to cause decreases in dendrite branching after TBI, and PSD-95 can be targeted as a novel approach for the treatment of patients with TBI.Ph.D.Includes bibliographical referencesby Mihir V. Pate