The extent of long-term functional recovery following traumatic brain injury (TBI) is clearly associated with the capacity for adaptive synaptic plasticity within injured circuitry. Recent evidence supports a role for extracellular matrix proteins (ECMs) and their regulatory metalloproteinases (MMPs) in neurite growth and synaptic reorganization after CNS trauma. Given that these molecules are found within brain regions vulnerable to TBI, we have begun to examine members of the gelatinase and stromelysin MMP families during injury-induced synaptic plasticity. We hypothesize that the interaction of MMPs and their ECM substrates during synaptic reorganization determines the success of long-term recovery following TBI. Specifically, we posit that MMPs control ECM dissociation during removal of degenerating terminals, and subsequently regulate distribution of ECM associated molecules involved with synaptogenesis. Our approach will first identify the spatio-temporal pattern of MMP expression and functional activity after unilateral entorhinal lesion (UEC), an insult which induces the well-defined process of reactive synaptogenesis within the hippocampus. The neuronal plasticity induced in this model results in adaptive restoration of synaptic structure and function. With the UEC pattern as a baseline for comparison, we will profile MMP expression and function after brain trauma using the rat TBI model which combines excessive neuroexcitation of percussive injury with targeted hippocampal deafferentation of entorhinal lesion (TBI+BEC insult). We have shown that this model reliably produces a persistent, maladaptive synaptic plasticity and severe long-term cognitive deficits. Initially, we will examine both protein (LM/EM immunohistochemistry, Western blots) and mRNA (RT-PCR, Northern blot and/or in situ hybridization) expression for select MMPs (gelatinases A and B; stromelysin) and their associated ECM substrates (collagenase IV, chondroitin sulfated proteoglycan, enascin) after injury. Additional experiments will determine how effects on protein and mRNA are correlated with MMP enzyme activity (gel zymography and chromogenic enzyme assay). Next, we will establish whether these injury-induced changes in MMPs/ECMs are associated with alterations in electrophysiological measures of synaptic plasticity (LTP, paired-pulse facilitation, current-source-density analysis) and changes in cognitive outcome (Morris Water Maze performance). Finally we will test the association between MMPs and synaptic reorganization following TBI by: 1) applying specific MMP inhibitors and assessing the extent of synaptic plasticity generated, and 2) enhancing injury-induced plasticity with compounds targeting NMDA and dopamine receptors and then assessing MMP expression and functional activity. Together, these studies will establish whether or not MMPs play a role in regenerative processes evoked by TBI and potentially identify novel therapies for brain trauma victims.
