Regulation of Mouse Intestinal L Cell Progenitors Proliferation by the Glucagon Family of Peptides.

Abstract

Glucagon like peptide-1 (GLP-1) and glucagon like peptide-2 (GLP-2) are hormones secreted by intestinal L cells that stimulate glucose-dependent insulin secretion and regulate intestinal growth, respectively. Mice with deletion of the glucagon receptor (Gcgr) have high levels of circulating GLP-1 and GLP-2. In the first part of my study, I sought to determine whether the increased level of the glucagon-like peptides in Gcgr-/- mice is due to L cell hyperplasia. I found, first, that high circulating levels of the glucagon-like peptides increase L cell number but do not affect the numbers of other intestinal epithelial cell types. Second, a large proportion of ileal L cells of Gcgr-/- mice co-expressed glucose-dependent insulinotropic peptide (GIP), a hormone that induces an increase in insulin secretion following a meal. Cells co-expressing GIP and GLP-1 are termed LK cells. Third, the increase in L cell number was due to a higher rate of proliferation of L cell progenitors rather than of mature L cells. Fourth, high circulating levels of the glucagon-like peptides were associated with increased mRNA levels of transcription factors expressed by late but not early enteroendocrine progenitors. Fifth, administration of exendin 9-39, a GLP-1 receptor antagonist, to Gcgr-/- mice decreased the rate of L cell precursor proliferation, indicating that the rate of cell division is regulated by GLP-1. Finally, we determined that L cells do not express the GLP-1 receptor, suggesting that the effect of GLP-1 is mediated by paracrine and/or neuronal signals. These results indicate that GLP-1 plays an important role in the regulation of L cell number via activation of its precursor population. In the second part of my study, I sought to characterize L and K cells during embryonic development. I found that GLP-1+ cells first appear at embryonic (e) day 15 of development and that their number is increased at e-17. Most GLP-1+ cells of ileum of e-17 embryos co-express GIP and, therefore, are LK cells. In addition, a subset of GLP-1+ cells of embryos but not of neonates, co-express glucagon, indicating that the expression of Glu in GLP-1+ cells disappears after birth. Taken together, these observations indicate that the phenotype of L cells changes after birth, presumably, reflecting the initiation of intestinal functional activity. Analogs of the hormones secreted by L cells are currently used in the clinic. My studies suggest a mechanism that will allow to increase the endogenous source of the glucagon-like peptides. This strategy will help to circumvent harmful side effects of pharmacological doses of the drugs. Therefore, the identification of the signals controlling the number of this important cell type will provide crucial information for the regulation of endogenous levels of GLP-1 and the control of glucose homeostasis.