Serglycin proteoglycan is required for secretory granule integrity in mucosal mast cells
Résumé
Serglycin (SG) proteoglycans (PGs) are strongly implicated in the assembly of mast cell (MC) granules. However, this notion is mainly been based on studies of MCs of the connective tissue subtype, whereas the role of SG PG in mucosal MCs has not been explored. In this study we have addressed the latter issue by using mice with an inactivated SG gene. Bone marrow cells were in vitro differentiated into mucosal MC phenotype, expressing the markers mouse MC protease 1 (mMCP-1) and -2. Biosynthetic labeling experiments performed on these cells revealed an ~80% reduction of 35}SO 4} 2-} incorporation into PGs recovered from SG -/-} cells as compared to SG +/+} counterparts, indicating that SG is the dominating cell-associated PG of mucosal MCs. Moreover, the absence of SG led to defective metachromatic staining of mucosal MCs, both in vivo and in the in vitro-derived mucosal MCs. Ultrastructural analysis showed that granules were present in similar numbers in SG +/+} and SG -/-} cells but that their morphology was markedly affected by the absence of SG, e.g. with electron dense core formation only seen in SG{+/+} granules. Analysis of the MC-specific proteases showed that mMCP-1 and mMCP-7 were completely independent on SG for storage, whereas mMCP-2 showed a partial dependence. In contrast, mMCP-4, -6 and carboxypeptidase A were strongly dependent on SG for storage. Together, our data indicate that SG PG is of crucial importance for assembly of mature mucosal MC granules, but that the specific dependence on SG for storage varies between individual granule compounds.
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