Defense Date

2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Neuroscience

First Advisor

Linda Phillips

Abstract

Traumatic brain injury (TBI) is a worldwide epidemic as the number of victims living with the resulting cognitive and physical impairment continues to rise, principally due to limited treatment options which fail to address its multifaceted sequelae. By approaching TBI therapy from a molecular standpoint, we have the opportunity to develop a better understanding of the mechanisms which prevent effective recovery. With this information, we can move toward the identification of novel therapeutic treatments which target specific molecules to improve patient outcome following TBI. Here, we have focused on the therapeutic potential of osteopontin (OPN), an extracellular matrix (ECM) protein which is a substrate of several matrix metalloproteinases (MMPs), and capable of acting as both a cytokine and modulator of axonal outgrowth during synaptic recovery. The ECM and its components are of particular interest with respect to selecting novel TBI therapeutics since this network has been implicated in neuronal plasticity during both development and following central nervous system (CNS) insult. In this dissertation study, the temporal and spatial profile of OPN expression, its protein and transcript localization within reactive glia (IBA1 positive microglia or GFAP positive astroglia), and its interaction with the cytoarchitectural protein (microtubule associated protein 1B, MAP1B) after injury were each compared under conditions of deafferentation induced synaptogenesis. Two TBI models were employed: one exhibiting adaptive synaptic plasticity (unilateral entorhinal cortex lesion, UEC), and the other generating maladaptive synaptic plasticity (central fluid percussion injury followed by bilateral entorhinal cortex lesions, TBI+BEC), in each case targeting 1, 2, and 7d postinjury intervals. In addition, we examined the potential for converting the adaptive response to one of maladaptive plasticity by attenuating immune reactivity through acute administration of the tricyclic antibiotic minocycline, utilizing a dosing paradigm previously demonstrated to reduce inflammation. To more clearly confirm that OPN has a role in successful synaptic regeneration, we developed a colony of OPN knockout (KO) mice which were used to profile synaptic structure and functional outcome under conditions of UEC-induced synaptogenesis. In Chapter 2, we report that full length OPN responds robustly in the acute (1-2d postinjury) degenerative period following UEC and TBI+BEC. After UEC, time-dependent differences were observed for two alternative, MMP-processed OPN forms, including early increase in a RGD 45 kD, integrin binding fragment (1d), and delayed increase in a C-terminal 32 kD OPN peptide (7d). OPN transcript was also elevated acutely after UEC, a finding which was pronounced in enriched dentate molecular layer (ML) fractions. Parallel immunohistochemistry (IHC) and in situ hybridization localized OPN protein and transcript to reactive glia following UEC. This localization was concentrated within microglia which delineated the border between the intact and deafferented ML, a pattern which was less pronounced in maladaptive TBI+BEC animals. The timing of this glial movement suggests that OPN regulates microglial migration and, potentially, could act as an astrokine to recruit activated astrocytes for influencing subsequent synaptic regeneration. MAP1B staining confirmed dendritic loss during axonal degeneration and dendritic atrophy, with a reemergence during collateral axonal sprouting. However, OPN colocalization with MAP1B was minimal, suggesting a minor role for OPN in reorganization of dendritic/axonal cytoarchitecture in this model of deafferentation. Minocycline reduced acute OPN protein response 2d after UEC, and caused a more random OPN positive glial distribution, similar to that of the maladaptive TBI+BEC. The role of OPN in the inflammation-directed degeneration of terminals is supported by reduced MMP-9 activity, which is temporally correlated with the reduction of MMP-generated OPN lytic fragments (45 kD). Interestingly, this reduction of integrin-binding OPN peptide also matched the impaired removal of presynaptic terminals, evidenced by diminished synapsin 1 clearance in animals which received postinjury minocycline. In Chapter 3, we sought to more precisely evaluate the role of OPN following deafferentation, utilizing wild type (WT) C57BL/6 and OPN KO mice subjected to UEC, comparing the spatio-temporal injury response between WT and KO. To do this we profiled several outcome measures which assessed OPN role in different aspects of recovery: 1) expression of select proteins important in various stages of synaptic recovery, 2) glial response, 3) cognitive recovery, and 4) MMP enzymatic activity. Compared to WT mice, OPN KO mice did not show significant differences in the acute injury-induced alteration of proteins important to cytoarchitectural reorganization (MAP1B) or stabilization of the synaptic junction (N-cadherin). However, both Western blot and IHC analyses showed OPN KO mice had impaired presynaptic terminal clearance, supported by attenuated synapsin 1 breakdown, a result quite similar to that of the minocycline-treated rats with OPN reduction in Chapter 2. This impaired degeneration in OPN KO mice at 2d postinjury correlated with IHC evidence for altered microglial morphology, and hippocampal function assessed by the novel object recognition (NOR) task. Our NOR results confirmed cognitive dysfunction in OPN KO mice during the 4-21d period of synapse reorganization after UEC. In addition, OPN KO decreased MMP-9 activity, an effect associated with reduced MMP-9 bound lipocalin 2 (LCN2), a persistently activated form of that MMP. These latter findings further support the hypothesis that MMP processing of OPN contributes to effective regenerative response after injury. Collectively, the studies presented in the two chapters of this dissertation provide evidence that OPN is a critical element in the acute immune response following injury-induced CNS deafferentation. They suggest that the cytokine can be produced by reactive microglia, may mediate cell migration and acute degenerative clearance, potentially serves as an astrokine to recruit those glia to sites of synaptic repair, and that these processes are disrupted when OPN is either reduced or ablated. Interestingly, this OPN role in synaptogenesis appears to involve ECM interaction with MMP-9, possibly regulated by LCN2. Most importantly, OPN involvement seems to affect the time-dependent progression of synaptic repair, an effect which can be measured by efficacy of functional outcome

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

October 2013

Included in

Neurosciences Commons

Share

COinS