A Role for Serglycin Proteoglycan in Mast Cell Apoptosis Induced by a Secretory Granule-mediated Pathway

Abstract

Mast cell secretory granules (secretory lysosomes) contain large amounts of fully active proteases bound to serglycin proteoglycan. Damage to the granule membrane will thus lead to the release of serglycin and serglycin-bound proteases into the cytosol, which potentially could lead to proteolytic activation of cytosolic pro-apoptotic compounds. We therefore hypothesized that mast cells are susceptible to apoptosis induced by permeabilization of the granule membrane and that this process is serglycin-dependent. Indeed, we show that wild-type mast cells are highly sensitive to apoptosis induced by granule permeabilization, whereas serglycin-deficient cells are largely resistant. The reduced sensitivity of serglycin ؊/؊ cells to apoptosis was accompanied by reduced granule damage, reduced release of proteases into the cytosol, and defective caspase-3 activation. Mechanistically, the apoptosis-promoting effect of serglycin involved serglycin-dependent proteases, as indicated by reduced sensitivity to apoptosis and reduced caspase-3 activation in cells lacking individual mast cell-specific proteases. Together, these findings implicate serglycin proteoglycan as a novel player in mast cell apoptosis.

Active Caspase-3 Is Stored within Secretory Compartments of Viable Mast Cells

“…Unexpectedly, we noted signs of the presence of active caspase-3 even in the absence of cell deathinducing agent (13). Possibly, the activated caspase-3 could have been present in an apoptotic subpopulation of the primary mast cell population used for those studies.…”

“…In a previous study, we showed that mast cells are sensitive to cell death induced by secretory granule-permeabilizing agents, and that mast cell apoptosis in response to such agents is accompanied by caspase-3 activation (13). Unexpectedly, we noted signs of the presence of active caspase-3 even in the absence of cell deathinducing agent (13).…”

“…If so, disruption of the mast cell granule membrane would have the potential to result in release of active caspase-3 into the cytosol. To evaluate this possibility, we subjected mast cells to LLME, a lysosomotropic agent that has mast cell granule-permeabilizing properties (13,21), and analyzed cytosolic extracts for the presence of active caspase-3. As shown in Fig.…”

“…Mice (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18) wk old) were all on C57BL/6J genetic background, and all animal experiments were approved by the local ethical committee. mM NaCl, 5 mM KCl, 1.4 mM CaCl 2 , 1 mM MgCl 2 , 5.6 mM glucose, 10 mM HEPES, and 0.1% BSA [pH 7.4]) and stimulated with TNP-OVA (0.4 mg/ml).…”

Serglycin: A Structural and Functional Chameleon with Wide Impact on Immune Cells

“…2), and these proteases may have the potential to cause apoptosis by proteolytic activation of proapoptotic compounds present in the cytosol. Indeed, MCs were shown to be highly sensitive to apoptosis induced by secretory granule destabilization, and moreover, serglycin 2/2 MCs and MCs that lack individual serglycin-dependent proteases were less sensitive than were WT cells (16). A striking finding was that serglycin 2/2 cells, when eventually undergoing cell death, die preferentially by necrosis rather than by apoptosis (16).…”