Tissue distribution and subcellular localization of hyaluronan synthase isoenzymes

Hyaluronan synthases (HAS) are unique plasma membrane glycosyltransferases secreting this glycosaminoglycan directly to the extracellular space. The three HAS isoenzymes (HAS1, HAS2, and HAS3) expressed in mammalian cells differ in their enzymatic properties and regulation by external stimuli, but clearly distinct functions have not been established. To overview the expression of different HAS isoenzymes during embryonic development and their subcellular localization, we immunostained mouse embryonic samples and cultured cells with HAS antibodies, correlating their distribution to hyaluronan staining. Their subcellular localization was further studied by GFP-HAS fusion proteins. Intense hyaluronan staining was observed throughout the development in the tissues of mesodermal origin, like heart and cartilages, but also for example during the maturation of kidneys and stratified epithelia. In general, staining for one or several HASs correlated with hyaluronan staining. The staining of HAS2 was most widespread, both spatially and temporally, correlating with hyaluronan staining especially in early mesenchymal tissues and heart. While epithelial cells were mostly negative for HASs, stratified epithelia became HAS positive during differentiation. All HAS isoenzymes showed cytoplasmic immunoreactivity, both in tissue sections and cultured cells, while plasma membrane staining was also detected, often in cellular extensions. HAS1 had brightest signal in Golgi, HAS3 in Golgi and microvillous protrusions, whereas most of the endogenous HAS2 immunoreactivity was localized in the ER. This differential pattern was also observed with transfected GFP-HASs. The large proportion of intracellular HASs suggests that HAS forms a reserve that is transported to the plasma membrane for rapid activation of hyaluronan synthesis.