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The Role of Matrix Metalloproteinase 9 and Osteopontin in Synaptogenesis and Reinnervation of the Olfactory Bulb Following Brain Injury

Traumatic brain injury (TBI) is a serious health concern, causing cognitive, motor, and sensory deficits, including olfactory dysfunction. This dissertation explores the effects of TBI on synaptic plasticity within the olfactory system, seeking to define mechanisms guiding postinjury sensory reinnervation. Physical forces induced by TBI can axotomize olfactory receptor neurons (ORNs), which innervate olfactory bulb (OB). These axons regenerate OB projections after injury, a process involving growth through a complex extracellular matrix (ECM). As such, we investigated a potential molecular mechanism capable of modifying local OB ECM to support postinjury synaptogenesis. Since matrix metalloproteinases (MMPs) and their ECM substrates are recognized for TBI therapeutic potential, we explored the role of MMP9 and its substrate osteopontin (OPN) in promoting ORN reinnervation of the OB after mild fluid percussion injury (FPI). First, we confirmed that FPI deafferented the mouse OB. In Chapter 2, we showed concurrent activation of neuroglia, elevated spectrin proteolysis and reduction in ORN-specific olfactory marker protein (OMP). As OMP normalized during regeneration, growth associated protein-43kD (GAP-43) peaked, marking OB entry of ORN growth cones. Ultrastructural analysis revealed ongoing ORN axon shrinkage and degeneration, glial phagocytosis of cellular debris, and a reorganization of synaptic structure. To explore ECM role in mediating postinjury OB reinnervation, we defined the time course of MMP9 activity and several downstream targets. Chapter 3 reports biphasic MMP9 activity increase during acute/subacute degeneration, accompanied by robust generation of 48kD OPN cell signaling peptide. OPN receptor CD44 also increased during the acute/subacute interval, suggesting potential interaction of the two proteins. Finally, we utilized MMP9 knockout (MMP9KO) mice to confirm MMP9 role in OB synaptogenesis. In Chapter 4, MMP9KO reversed FPI-induced lysis of 49kD OPN and altered postinjury expression of ORN axon degeneration marker OMP. Additional ultrastructural analysis verified delayed recovery of OB synaptic features within the injured MMP9KO. Overall, we demonstrated that mild FPI elicits ORN axotomy to induce OB reactive synaptogenesis, and that MMP9 supports reinnervation by processing OPN for activation of local glia, cells which reorganize the ECM for synapse regeneration.

Identiferoai:union.ndltd.org:vcu.edu/oai:scholarscompass.vcu.edu:etd-5702
Date01 January 2016
CreatorsPowell, Melissa A
PublisherVCU Scholars Compass
Source SetsVirginia Commonwealth University
Detected LanguageEnglish
Typetext
Formatapplication/pdf
SourceTheses and Dissertations
Rights© Melissa A. Powell

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