Regulated Restructuring of Mucins During Secretory Granule Maturation In Vivo

Abstract

Mucins are large, highly glycosylated transmembrane and secreted proteins that line and protect epithelial surfaces. However, the details of mucin biosynthesis and packaging in vivo are largely unknown. Here, we demonstrate that multiple distinct mucins undergo intragranular restructuring during secretory granule maturation in vivo, forming unique structures that are spatially segregated within the same granule. We further identify temporally-regulated genes that influence mucin restructuring, including those controlling pH ( Vha16-1 ), Ca 2+ ions ( fwe ) and Cl − ions ( Clic and ClC-c ). Finally, we show that altered mucin glycosylation influences the dimensions of these structures, thereby affecting secretory granule morphology. This study elucidates key steps and factors involved in intragranular, rather than intergranular segregation of mucins through regulated restructuring events during secretory granule maturation. Understanding how multiple distinct mucins are efficiently packaged into and secreted from secretory granules may provide insight into diseases resulting from defects in mucin secretion.

Rab26 controls secretory granule maturation and breakdown in Drosophila

“…Acidification of insulin granules during their maturation before exocytosis appears to be important in pancreatic beta cells [ 15 ]. Acidification also appears to promote restructuring of glue granule contents in Drosophila [ 16 ]. We thus decided to find out whether Rab26 plays a role in secretory granule acidification.…”

“…The mucoprotein content of the granules is gradually reorganized during glue granule maturation, which is clearly visible under the electron microscope. Since this process appears to depend on acidification [ 16 ], we performed ultrastructural analysis of rab26 mutants. Small secretory granules with a typical diameter of 2–3 µm appear in large numbers in control salivary gland cells at the wandering L3 stage (Fig.…”

“…Indeed, our loss-of-function studies revealed that glue granule maturation and acidification are accelerated in rab26 mutants, and its overexpression delays this process. Acidification appears to be important for insulin granule secretion [ 15 ] and glue granule maturation in Drosophila [ 16 ]. Of note, Rab26 does not play a clear positive regulatory role in secretory granule maturation in fruit fly salivary gland cells [ 9 ].…”

“…Our GFP-Lamp1 localization data show that Rab26 knockdown results in earlier fusions with lysosomes or Lamp carrier vesicles. Since secretory granules acidify during their maturation [ 15 , 16 ], we propose that lysosomal transport plays a dual role in the progression of glue granules, being important for proper maturation and release as well as for the crinophagic degradation of residual granules that remain inside the cells after the wave of glue secretion.…”

“…For instance, acidification and chloride ion uptake of the insulin granules were shown to be required for exocytosis in pancreas beta cells [ 15 ]. Recently, the lowering of granular pH accompanied by specific changes in calcium and chloride ion concentration has also been shown to be required for normal secretory granule maturation in Drosophila [ 16 ]. Moreover, uncontrolled secretion of granules (enlarged partly due to fusions with lysosomes) can be observed in the secretory glands of TRPML1 (a lysosomal Ca 2+ -permeable cation channel) mutant mice developing mucolipidosis type IV [ 17 ].…”