| The neurotrophins are a family of soluble growth factors that are synthesized as pro-proteins. While the mature neurotrophins promote survival and outgrowth in the central nervous system, the pro-neurotrophins can be secreted and induce apoptosis via binding to a complex of p75NTR and sortilin. Sortilin is a member of a family of proteins that displays high homology to the yeast vacuolar sortilin protein Vps10p in the extracellular domain. In addition to promoting cell death during development and disease, sortilin is required for the regulated secretion of BDNF via a process that is dependent on sorting motifs in the cytoplasmic tail. Furthermore, sortilin has been shown to mediate a number of additional cellular processes via binding to a diverse array of ligands, suggesting a complex system of regulation.;Both sortilin and its coreceptor p75NTR have been shown to undergo ectodomain shedding, a process by which an increasing number of transmembrane molecules are shown to be regulated. My studies were conducted to gain insight into the regulation and biological role of sortilin proteolysis. Using pharmacology as well as loss of function studies, I identified ADAM10 as the major sortilin sheddase and showed that cleavage can occur at the cell surface. Generation of an uncleavable mutant sortilin identified the extracellular juxtamembrane stalk as required for shedding and provided me with a useful tool to investigate the role of shedding in trafficking. Because production of the soluble sortilin ectodomain may also be affected by alternative spliceforms, I analyzed mRNA from rodents and humans and identified a rodent isoform that is present in young animals and contains an additional exon (exon 17a) that elongates the juxtamembrane stalk. In contrast, the putative human exon 17a encodes a stop codon that would result in generation of a soluble sortilin isoform, but my studies failed to identify a corresponding transcript. Finally, I characterized the synthesis and release of soluble sortilin and pro-BDNF from human platelets and showed that like in neurons, sortilin may regulate BDNF release from platelets. |
