Molecular Mechanism of Granule-mediated Apoptosis

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Cytotoxic cell granule-mediated apoptosis is a unique form of cell signaling that entails the intracellular delivery of granule-associated serine proteases (granzymes) by the pore forming protein perforin. The identification of genetic defects in the granule secretion pathway have clearly established that granule- mediated apoptosis is crucial for immune homeostasis. Acquired disruption of cytotoxic cell effector function by viral proteins are likely to trigger lymphoproliferation and autoreactivity as well as diminish host defense responsiveness. A virus-infected or tumor cell may also resist granule mediated apoptosis by inhibiting virtually any aspect of this pathway. Delineating the multiple steps of this poorly understood phenomenon has become increasingly important to comprehend the immunopathogenesis of a number of disease states. To achieve an understanding of granzyme delivery by perforin requires molecular characterization of the most proximal steps - the events that surround the binding, internalization and early trafficking of these granule components. Granzyme B remains bound to the granule-associated proteoglycan, serglycin, after secretion by a cytotoxic cell. Unexpectedly, the granzyme undergoes electrostatic exchange from serglycin to target cell sulfated glycosaminoglycans which then facilitate binding to undefined receptor(s) for optimal internalization. In addition, the granzyme appears to manifest membranolytic activity if a sufficient endosomal concentration is achieved. Entitled the conjoined Perforin-Granzyme delivery hypothesis - both perforin and granzymes are predicted to undergo electrostatic exchange to the cell surface from serglycin and together contribute to endosomolytic delivery process. Extending previous efforts to the glycobiology of granzyme B-cell surface interactions and emphasizing imaging techniques, particularly new methods that directly visualize the interaction of the granzyme and perforin with target cells, the Specific Aims are: 1. Study the transfer of granzyme B from serglycin to the target cell surface 2. Ascertain role of perforin in granzyme delivery - a molecular analysis 3. Assess plasma membrane and endosomal integrity of the target cell during cytotoxic cell-mediated apoptosis