Thomas F J Martin

Neural and endocrine cells serve as key physiological integrators that both receive and emit signals in the form of hormones and neurotransmitters. The secretion of peptide hormones and neurotransmitters from endocrine and neural cells occurs via the exocytotic fusion of secretory granules/vesicles with the plasma membrane. An elevation of cytoplasmic calcium, caused by activation of signal transduction pathways, is the principal trigger for granule/vesicle fusion; however, the mechanism of calcium triggering remains to be elucidated. The operation of the regulated secretory pathway also requires ATP; however, the reactions that consume ATP remain to be identified. Our current research is focused on identifying the calcium-activated and ATP-dependent biochemical reactions that are responsible for regulated fusion.

We developed methods for cell permeabilization that enable preservation of secretory mechanisms in semi-intact cells. A key result from early studies was that soluble cytosolic proteins were required for the process.

A two-stage assay was developed in which it was demonstrated that ATP acted at a step prior to calcium, and each step was shown to require distinct cytosolic proteins. Two of three proteins required for ATP-dependent priming (PEP proteins) were identified as phosphatidylinositol transfer protein, phosphatidylinositol 4-monophosphate 5-kinase, leading to the novel concept that the requirement for ATP in the regulated secretory pathway is for the phosphorylation of phospholipids. A single protein was required to reconstitute the calcium-regulated step. Termed CAPS (for Calcium-dependent Activator Protein in Secretion), this protein is a novel calcium-dependent protein that interacts with phosphorylated lipids and with membranes.

Research in progress is focused on the biochemical and molecular biological characterization of the CAPS and PEP proteins. Other studies are directed at discovering new proteins that are required for regulated secretion and determining the role of additional known membrane proteins.

The mechanisms for regulated neurotransmitter and peptide hormone secretion share common features. Research in this area hopes to uncover universal mechanisms that underlie processes as apparently diverse as learning, inflammatory responses and insulin secretion.