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

Fabio Rabelo Melo,Ida Waern,Elin Rönnberg,Magnus Åbrink,David M Lee,Susan M Schlenner,Thorsten B Feyerabend,Hans-Reimer Rodewald,Boris Turk,Sara Wernersson,Gunnar Pejler

doi:10.1074/jbc.m110.176461

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.

Highlights

Serglycin (SG)2 is a proteoglycan expressed by numerous hematopoietic cell types, including mast cells (MCs), cytotoxic T lymphocytes, platelets, neutrophils, and macrophages [1, 2]
It has been shown that the pro-apoptotic BH-3 only proteins, Puma [40] and Bim [41], have key roles in MC apoptosis induced by growth factor deprivation, and it was recently demonstrated that increased sensitivity of IL-15Ϫ/Ϫ MCs to growth factor deprivation-induced or endogenous acid sphingomyelinase-induced apoptosis involved cathepsin D [42]
MC secretory granules are known to contain extraordinarily large amounts of various proteases. These proteases could have the ability to proteolytically activate pro-apoptotic compounds, and we hypothesized that their release from the granule compartment into the cytosol would cause MC apoptosis

Summary

Serglycin in Apoptosis

Proteases such as cysteine and aspartic acid cathepsins (24 – 27) as well as other lysosomal hydrolases, including ␤-hexosaminidase. Damage to the MC secretory granules will lead to a massive release of active proteases into the cytosol. These may proteolytically activate various pro-apoptotic compounds, and we hypothesized that MCs may be prone to apoptosis via agents that induce secretory granule/lysosome permeabilization. Because many of the proteases within MC granules are dependent on SG for proper storage [4, 5], we hypothesized that apoptosis initiated by a granule/lysosome pathway may be SG-dependent. We show here that MCs are highly sensitive to apoptosis induced by permeabilization of the secretory granules and that this pathway of MC apoptosis is strongly dependent on SG. We have identified SG as a novel player in apoptosis

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION