Developmental Biology of the Psammomys obesus Pancreas: Cloning and Expression of the Neurogenin-3 Gene

Genome sequence of a diabetes-prone rodent reveals a mutation hotspot around the ParaHox gene cluster

The sand rat Psammomys obesus is a gerbil species native to deserts of North Africa and the Middle East, and is constrained in its ecology because high carbohydrate diets induce obesity and type II diabetes that, in extreme cases, can lead to pancreatic failure and death. We report the sequencing of the sand rat genome and discovery of an unusual, extensive, and mutationally biased GC-rich genomic domain. This highly divergent genomic region encompasses several functionally essential genes, and spans the ParaHox cluster which includes the insulin-regulating homeobox gene Pdx1. The sequence of sand rat Pdx1 has been grossly affected by GC-biased mutation, leading to the highest divergence observed for this gene across the Bilateria. In addition to genomic insights into restricted caloric intake in a desert species, the discovery of a localized chromosomal region subject to elevated mutation suggests that mutational heterogeneity within genomes could influence the course of evolution.

The islet ghrelin cell

The islets of Langerhans are key regulators of glucose homeostasis and have been known as a structure for almost one and a half centuries. During the twentieth century several different cell types were described in the islets of different species and at different developmental stages. Six cell types with identified hormonal product have been described so far by the use of histochemical staining methods, transmission electron microscopy, and immunohistochemistry. Thus, glucagon-producing α-cells, insulin-producing β-cells, somatostatin-producing δ-cells, pancreatic polypeptide-producing PP-cells, serotonin-producing enterochromaffin-cells, and gastrin-producing G-cells have all been found in the mammalian pancreas at least at some developmental stage. Species differences are at hand and age-related differences are also to be considered. Eleven years ago a novel cell type, the ghrelin cell, was discovered in the human islets. Subsequent studies have shown the presence of islet ghrelin cells in several animals, including mouse, rat, gerbils, and fish. The developmental regulation of ghrelin cells in the islets of mice has gained a lot of interest and several studies have added important pieces to the puzzle of molecular mechanisms and the genetic regulation that lead to differentiation into mature ghrelin cells. A body of evidence has shown that ghrelin is an insulinostatic hormone, and the potential for blockade of ghrelin signalling as a therapeutic avenue for type 2 diabetes is intriguing. Furthermore, ghrelin-expressing pancreatic tumours have been reported and ghrelin needs to be taken into account when diagnosing pancreatic tumours. In this review article, we summarise the knowledge about islet ghrelin cells obtained so far.

Diet-induced diabetes in the sand rat (Psammomys obesus)

Insulin deficiency is the underlying cause of hyperglycemia in type 2 diabetes. The gerbil Psammomys obesus (P. obesus) is a naturally insulin resistant rodent with tendency to develop diet-induced hyperglycemia associated with obesity. P. obesus does not exhibit hyperglycemia in its natural desert habitat, feeding on low caloric vegetation. However, when fed regular laboratory chow containing higher caloric density, the animals develop moderate obesity and hyperglycemia. Diabetes development and progression is very fast in P. obesus. The animals reach the irreversible hypoinsulinemic stage of the disease, in which a marked reduction of β-cell mass is apparent, within 4-6 weeks of high caloric diet. The present review describes the P. obesus of the Hebrew University colony, with emphasis on its use for the study of β-cell dysfunction in type 2 diabetes.

The morphology of islets of Langerhans is only mildly affected by the lack of Pdx-1 in the pancreas of adult Meriones jirds

The Meriones Jirds belong to the genus of Gerbillinae (Rodentia: Muridae). We and others have previously reported the lack of the pancreatic beta-cell transcription factor, Pdx-1 in the fat sand rat, Psammomys obesus. The aim of the study was to investigate the expression and localization of Pdx-1 in phylogenetically related members of the Gerbillinae subfamily. In addition, we characterized by IHC the expression pattern of islet hormones and additional important pancreatic transcription factors in order to evaluate overall endocrine pancreas appearance. PCR showed that Pdx-1 was easily amplified from a wide range of phylogenetically distant species but not from 13 different gerbilline species. Identical to P. obesus the important beta-cell transcription factor Pdx-1 was absent from all five jirds. However, expression of other critical islet transcription factors and islet hormones was generally normal. Insulin was localized in the center of the islets with glucagon, somatostatin and pancreatic polypeptide (PP) found in the islet mantle. PYY cells were also observed and colocalized with PP cells. The NKX family of transcription factors were localized to the same cell types as seen in other rodents. MafA was nuclear localized in some of the insulin immunoreactive but not in other cell types, while MafB was found not only in the glucagon cells but also in many of the insulin cells. In conclusion, Pdx-1 appears to be lacking in all gerbils and despite the lack of Pdx-1, the Meriones Jirds have islets that are morphologically similar to other rodents and express hormones and transcription factors in the expected pattern except for MafA and MafB.

An immunohistochemical study of the endocrine pancreas of the African ice rat, Otomys sloggetti robertsi

The African ice rat, Otomys sloggetti robertsi, is a member of the subfamily Otomyinae, in the superfamily of Muroidea, to which all rodents belong. Very little is known about this unique family of rodents. The study reported here examines the endocrine pancreas of this species using immunohistochemical techniques. The islets of Langerhans were scattered in the exocrine pancreas and tended to be quite small. Scattered single endocrine cells (mostly immunoreactive for insulin) were found in the exocrine pancreas and were not generally associated with ducts (as marked by pan-cytokeratin labeling). The normal islet architecture of insulin in the center and glucagon, somatostatin (SS) and pancreatic polypeptide (PP) in the rim was observed, but the islets tended to have 2-3 layers of glucagon immunoreactive cells. Examining for rarer endocrine cell types, we found that cocaine amphetamine regulated transcript (CART) immunoreactive cells were co-localized with SS; and peptide YY (PYY) immunoreactive cells could be found that were singly immunoreactive or co-localized with either PP or glucagon. Ghrelin cells were not found. MafA co-localized only with the insulin cells, while MafB, which localizes to the glucagon cells, also showed a low level of immunoreactivity in most insulin immunoreactive cells. The Nkx family of transcription factors (Nkx6.1 and 2.2) and PDX-1 were all detected in the pancreas in a similar manner to that seen in mouse and rat. In conclusion, the endocrine pancreas of the African ice rat is quite similar to that of other studied rodents, but these animals have more glucagon and SS cells than rat (Rattus) or mouse (Mus) species.