The union of form and function is a core tenet of biology. In animals, tissues often adopt their characteristic form early in development and maintain it through periods of tremendous growth, a process that often demands that cells remain largely fixed in place. Tissue shape and cell migration is dictated in large part by the basement membrane, a special type of extracellular matrix that surrounds tissues, bestowing on them structural support and resiliency. Yet, how tissues adopt and retain their form and how cells remain anchored in place during growth and development remain key questions in biology. The Drosophila larval CNS is an ideal system in which to explore the genetic and molecular mechanisms that govern tissue structure and anchor cells in place. The CNS is fully enwrapped by a thick basement membrane that provides structural support to the CNS via its physical properties and interactions with underlying surface glia; the CNS grows rapidly during larval stages, maintaining its form despite tremendous growth, and during this time, neural lineages remain largely fixed in place. The CNS basement membrane must then maintain its structure and that of the CNS while continually remodeling itself and its interactions with glia to allow for its growth and that of the CNS. The genetic and molecular principles that bestow upon the CNS basement membrane, and likely most basement membranes, this power remain cloudy ? their elucidation represents the focus of this proposal. We identify AdamTS-A, an extra-cellular metalloprotease, as a key organizer of CNS structure. Reduction in AdamTS-A function disrupts CNS structure and induces a mass exodus of neurons and cortex glia out of the CNS. Our studies indicate that AdamTS-A acts in surface glia, the outermost cell layer of the CNS that directly underlies the CNS basement membrane, to maintain CNS structure and to anchor the underlying neural cells in place by opposing the actions of collagen IV and integrin, which promote tissue stiffness. Increased tissue stiffness has been shown to promote cell migration. Thus, we hypothesize that reduction of AdamTS-A function in surface glia increases the stiffness of the overlying CNS basement membrane, which in a cell non- autonomous manner then triggers hundreds of CNS neurons and cortex glia to tunnel through the nerves that project from the CNS toward peripheral tissues fully enwrapped the entire time by the membranes of surface glia. In this grant, we leverage the strengths of the fly system to clarify the underlying genetic and molecular mechanisms through which AdamTS-A maintains tissue structure and keeps cells rooted in place in the CNS.
Our specific aims are ? (i) to complete a systematic phenotypic characterization of AdamTS-A in the CNS, (ii) to identify the substrates and interacting proteins of AdamTS-A in the CNS, and (iii) to uncover the genes and pathways activated in the migrating cells in response to reduced AdamTS-A function in surface glia.
The proposed project seeks to uncover the genetic and molecular mechanisms that underlie the observed mass migration of neural cells out of the CNS through nerves toward peripheral tissues in response to loss of function in a conserved extracellular protease in Drosophila. This phenotype resembles perineural invasion, a major mode of a cancer spread where tumor cells migrate along or through nerves to distant sites, and reduced expression of the cognate gene in humans has been linked to metastasis of head/neck cancers. Moreover, from flies to humans, the extracellular protease we study appears to be expressed in the outermost cell layer of the central nervous system, the surface glia in flies and the glia limitans in humans. Thus, our work holds great promise to uncover key clues on the genetic and molecular basis of human disease.
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