The Role of Matrix Metalloproteinases in Axon Guidance and Neurite Outgrowth

by Dinglasan, Lu Anne

Axons navigating the complex environment of the developing CNS use extracellular guidance cues to help find their correct synaptic target. Matrix metalloproteinases (MMPs), a family of zinc-dependent proteolytic enzymes, have been shown to regulate axon guidance by degrading extracellular matrix (ECM) or by cleaving guidance cues and their receptors. The olfactory system is an excellent model for studying the role of MMPs in axon guidance due to its capacity for continuous nerve regeneration and topographic maintenance during synaptic targeting. I hypothesized that MMPs may play a role in guiding olfactory sensory neurons to their correct glomerular target by sculpting the ECM and influencing axon interactions with the environment. To investigate this, I used RT-PCR to screen 19 members of the MMP family and their four endogenous inhibitors (tissue inhibitors of metalloproteinases, TIMPs) and performed immunohistochemistry to localize candidate MMP and TIMP proteins. Two MMP sub-families, the gelatinases and the membrane-bound MMPs (MT-MMPs) showed distinctive spatio-temporal expression patterns across different stages of olfactory development, which were consistent with their having a role in axon pathway formation. To assess gelatinases in their active form, I performed in situ zymography and found restricted patterns of proteolytic activity within the developing olfactory nerve. Finally, to study the role of MMPs in pathway formation, I applied active recombinant MT-MMPs to common ECM molecules found in the developing olfactory system, such as tenascin and proteoglycans, and examined subsequent changes in neurite outgrowth. The inhibitory effects of these substrata were decreased with enzyme treatment, with MT-MMPs having different substrate specificities and degradation efficiencies that allow for increased neurite outgrowth in culture. Collectively, the data suggest that MMPs are active in the developing olfactory system and have a role in axon guidance and neuronal pathway formation.