Regulated Secretion from Hemopoietic Cells

Open Access
Publisher Website
Google Scholar
Abstract
Secretory lysosomes are a mixture of lysosomes and secretory granules on many different levels. Secretory lysosomes contain both the hydrolases and membrane proteins characteristic of lysosomes as well as the specialized secretory products of different cell types. Functionally, they serve both as the lysosome of the cell and as the secretory granule. The acidic pH is optimal for the action of lysosomal hydrolases and contributes to keeping secretory products inactive before release. As is true for lysosomes, proteins can reach secretory lysosomes by both endocytic and biosynthetic routes as demonstrated by HRP uptake and the delivery of newly synthesized proteins. Morphologically, secretory lysosomes are a mixture of the multilamellar structures characteristic of lysosomes and the dense cores characteristic of secretory granules. Studies on secretory lysosome biogenesis in T lymphocytes indicate that the dense core forms and enlarges within a multivesicular structure (Griffiths and Argon 1995). In mature T lymphocytes, there is a high degree of overlap between secretory and lysosomal markers, suggesting that the majority of lysosomes are secretory lysosomes (Stinchcombe, J.C., and G.M. Griffiths, unpublished observation). However, in neutrophils, lysosomal structures lacking VAMP-2 can be identified alongside other VAMP-2–labeled granules (Brumell et al. 1995), suggesting that these cells may possess both true lysosomes and secretory lysosomes (Table 1).