Does b-Hexosaminidase Function Only as a Degranulation Indicator in Mast Cells? The Primary Role of b-Hexosaminidase in Mast Cell Granules

b-Hexosaminidase, which is generally present in the lysosome, is essential for glycoprotein metabolism in the maintenance of cell homeostasis. In mast cells (MCs), large amounts of b-hexosaminidase are present in the granules as opposed to the lysosome, and the biological role of MC b-hexosaminidase has yet to be fully elucidated. Therefore, we investigated the biological role of b-hexosaminidase in MC granules. Bone marrow-derived MCs from C57BL/6 (BL/6-BMMC) or b-hexosaminidase gene–deficient (hexb 2/2-BMMC) mice were transplanted into MC-deficient (WBB6F1/J-Kit W /Kit W-v [W/W v ]) mice to generate MC-reconstituted models. In asthma model experiments, no differences were observed in the symptoms of BL/6, W/W v , BL/6-BMMC–reconstituted W/W v , or hexb 2/2-BMMC–reconstituted W/W v mice. In Staphylococcus epidermidis experimental infection model experiments, the severity of symptoms and frequency of death were markedly higher in W/W v and hexb 2/2-BMMC–reconstituted W/W v mice than in BL/6 and BL/6-BMMC–reconstituted W/W v mice. The growth of S. epidermidis in an in vitro study was clearly inhibited by addition of BL/6-BMMC lysate, but not by addition of hexb 2/2-BMMC lysate. Moreover, suppression of bacterial proliferation was completely recovered when bacteria were incubated with hexb 2/2-BMMC lysate plus b-hexosaminidase. Transmission electron microscopy indicated that the cell wall of S. epidermidis was heavily degraded following coincubation of bacteria with BL/6-BMMC lysate, but not following coincubation with hexb 2/2-BMMC lysate. These findings strongly suggest that MC granule b-hexosaminidase is crucial for defense against bacterial invasion, but is not involved in the allergic response. Our results also suggest that the bactericidal mechanism of b-hexosaminidase involves degradation of bacterial cell wall peptidoglycan. T he enzyme b-hexosaminidase (Enzyme Commission No. 3.2.1.52) cleaves the terminal linked N-acetylhexosamine residues in N-acetyl-b-hexosaminides (1, 2). In the ly-sosome, removal of the terminal linked N-acetyl-b-D-galactos-amine from GM2 ganglioside is catalyzed by b-hexosaminidase (1). Three b-hexosaminidase isozymes composed of a combination of a and b subunits have been identified and are designated b-hexosaminidase A (HexA), b-hexosaminidase B (HexB), and b-hexosaminidase S (HexS) (3). HexA is a heterodimer composed of an a and b subunit, whereas HexB and HexS consist of b and a subunit homodimers, respectively (4). In mammalian cells, HexA and HexB are the major forms of b-hexosaminidase, whereas HexS is a minor form and is physiologically labile (5). Mutations in hexb, the gene encoding the b subunit, disrupt HexA and HexB activity, resulting in massive accumulation of GM2 ganglioside, especially in neuronal cells of …