This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The Notch signaling pathway is comprised of cell surface proteins that are fundamental to cell-cell communication in development of the nervous system. Notch receptors are activated by binding the Delta protein, which promotes several sequential cleavages in the Notch receptor. We have recently shown that Delta-Notch engagement promotes similar proteolytic events in Delta, raising the question of how proteolysis regulates the function of this fundamental pathway. Cleavage in Notch requires a sequential regulated intramembrane proteolysis (RIP) mechanism involving an ADAM metalloprotease and Presenilin, the aspartyl protease involved in the etiology of Alzheimer's disease. Surrently we are investigating the role of proteolysis of Notch and Delta in a Drosophila (fruit fly) model by examining the proteolytic products of Notch and Delta in both cultured cells and in vivo. Our results have uncovered a mechanism of regulated proteolysis of Delta that is distinct from that of Notch in that it is not sequential and involves a novel thiol-sensitive protease that is distinct from Presenilin. Cell associated Delta cleavage products derived from the Kuzbanian ADAM protease and the novel thiol-sensitive protease predict isoforms that can act at the plasma membrane and in the nucleus, respectively. Furthermore, we have identified a unique pattern of Delta expression in subsets of developing neuronal lineages at the point in which initial neurite contacts are established. Altogether, our results point to a novel activity for Delta in the developing central nervous system and define a model system in which to carry out the investigation. Also we are studying the sensitivity of Notch signaling to ADAM protease activity. In these studies we have identified an activity of organomercurial compounds in promoting ADAM activity. Our recent findings demonstrate a direct effect of organomercurials on Notch activity, which relies on both metalloprotease-dependent and independent activities. These studies give us the tools to refine the mechanism of Notch activation, as well as provide new lines of inquiry into the toxic mechanisms of environmental forms of mercury, notably methylmercury.
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