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DIFFERENTIAL SUBCELLULAR AND CELLULAR STORAGE OF GLP-1 AND PYY, AND ITS IMPLICATIONS
Intestinal L cells have key roles in the detection of the chemical environment in the gut lumen, to which they react by the release of hormones that influence appetite, proximal gut motility, insulin secretion and mucosal function (Furness et al, Nature Gastroenterology, 10, 729–740, 2013). Important amongst L cell hormones are glucagon-like peptide1 (GLP-1) and peptide tyrosine-tyrosine (PYY), which are products of separate genes. The conventional description of their localisation is that GLP-1 and PYY are in the same storage vesicles in the same cells. However, GLP-1 and PYY have different functions, particularly in relation to insulin secretion and mucosal function. We have used super-resolution (3D-SIM) microscopy and double-labelling immunohistochemistry to investigate the subcellular localisations of the hormones, and digital scanning microscopy to investigate cell populations.
Super-resolution microscopy revealed that GLP-1 and PYY are in separate storage organelles in enteroendocrine cells from mouse, rat, pig and human. The majority of the organelles were 150-170 nm or less in diameter, and are concluded to be secretory vesicles. Only 10-20% of organelles had immunoreactivity for both hormones. Even this may be an overestimate, as touching or very close vesicles may not be effectively resolved, even with super-resolution microscopy. In investigating co-localisation at the cell level, we included glucagon-like insulinotropic peptide (GIP), an incretin of K cells, in the analysis. The work shows that there is a K/L cell gradient in the mouse intestine. From the duodenum to the distal colon, there are populations of cells with GIP alone, GLP alone, PYY alone and all combinations of the three hormones. Greatest numbers of GIP cells were in the doudenum and jejunum, where 30-40% contained only GIP and the remainder also contained GLP-1. A small proportion also contained PYY. Similar patterns of overlap occurred in the proximal and distal ileum, where GLP-1 was the dominant peptide, which was often alone, or co-localised with PYY. In the large intestine the majority of cells contained both GLP-1 and PYY, but cells with only one of these and cells with all three hormones were found.
The findings reveal a structural basis for the separate or preferential control of GLP-1, PYY (and possibly GIP) release. A number of physiological studies imply that there can be differential release of GLP-1 and PYY. This should be investigated further.
Keywords: enteroendocrine cells, incretins, glucagon like peptide, peptideYY