Abstract
The peptide hormone Urocortin 3 (Ucn 3) is abundantly and exclusively expressed in mouse pancreatic beta cells where it regulates insulin secretion. Here we demonstrate that Ucn 3 first appears at embryonic day (E) 17.5 and, from approximately postnatal day (p) 7 and onwards throughout adult life, becomes a unifying and exclusive feature of mouse beta cells. These observations identify Ucn 3 as a potential beta cell maturation marker. To determine whether Ucn 3 is similarly restricted to beta cells in humans, we conducted comprehensive immunohistochemistry and gene expression experiments on macaque and human pancreas and sorted primary human islet cells. This revealed that Ucn 3 is not restricted to the beta cell lineage in primates, but is also expressed in alpha cells. To substantiate these findings, we analyzed human embryonic stem cell (hESC)-derived pancreatic endoderm that differentiates into mature endocrine cells upon engraftment in mice. Ucn 3 expression in hESC-derived grafts increased robustly upon differentiation into mature endocrine cells and localized to both alpha and beta cells. Collectively, these observations confirm that Ucn 3 is expressed in adult beta cells in both mouse and human and appears late in beta cell differentiation. Expression of Pdx1, Nkx6.1 and PC1/3 in hESC-derived Ucn 3+ beta cells supports this. However, the expression of Ucn 3 in primary and hESC-derived alpha cells demonstrates that human Ucn 3 is not exclusive to the beta cell lineage but is a general marker for both the alpha and beta cell lineages. Ucn 3+ hESC-derived alpha cells do not express Nkx6.1, Pdx1 or PC1/3 in agreement with the presence of a separate population of Ucn 3+ alpha cells. Our study highlights important species differences in Ucn 3 expression, which have implications for its utility as a marker to identify mature beta cells in (re)programming strategies.
Highlights
Urocortin 3 (Ucn 3) is a peptide hormone that belongs to the corticotropin-releasing factor (CRF) subfamily of peptide hormone, which includes Ucn 1 and 22 [1,2,3]
Ucn 3 expression was retained in purified mouse primary beta cells obtained by FACS sorting dissociated islets from MIP-GFP reporter mice, further confirming the localization of Ucn 3 expression to beta cells (Fig. 1E), while expression of the alpha cell marker glucagon and the delta cell marker somatostatin is lost
Ucn 3 was co-discovered by our group a decade ago as the fourth member of the corticotropin-releasing factor family of peptide hormones [32,33]
Summary
Urocortin 3 (Ucn 3) is a peptide hormone that belongs to the corticotropin-releasing factor (CRF) subfamily of peptide hormone, which includes Ucn 1 and 22 [1,2,3]. Ucn 3 secretion from the beta cell is glucose-dependent and involves the ATP-sensitive potassium (KATP) channel [5] These islet-autonomous actions of Ucn 3 suggest the local presence of cognate receptors, which we confirmed by demonstrating expression of the alpha isoform of the CRFR2 receptor in MIN6 insulinoma cells and primary rodent and human islets [6]. Our increased understanding of the complex sequence of events that is required to drive beta cell differentiation culminated in detailed in vitro differentiation protocols [7,8,9] While these protocols are effective in driving the differentiation from hESCs to pancreatic endoderm and endocrine progenitor cells in vitro, the resulting cells can only mature into glucose responsive functional beta cells when implanted into rodent recipients [9,10,11]. While these observations identify Ucn 3 expression as a feature of mature beta cells in rodents, they suggest that in humans, Ucn 3 is instead a general maturation marker of alpha and beta cells
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