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

Abstract

(AEK) S U M M A R Y The desert gerbil Psammomys obesus, an established model of type 2 diabetes (T2D), has previously been shown to lack pancreatic and duodenal homeobox gene 1 (Pdx-1) expression. Pdx-1 deficiency leads to pancreas agenesis in both mice and humans. We have therefore further examined the pancreas of P. obesus during embryonic development. Using Pdx-1 antisera raised against evolutionary conserved epitopes, we failed to detect Pdx-1 immunoreactivity at any time points. However, at E14.5, Nkx6.1 immunoreactivity marks the nuclei of all epithelial cells of the ventral and dorsal pancreatic buds and the only endocrine cell types found at this time point are glucagon and PYY. At E18.5 the pancreas is well branched and both glucagon-and ghrelin-positive cells are scattered or found in clusters, whereas insulin-positive cells are not found. At E22.5, the acini of the exocrine pancreas are starting to mature, and amylase and carboxypeptidase A immunoreactivity is found scattered and not in all acini. Ghrelin-, glucagon-, PYY-, gastrin-, somatostatin (SS)-, pancreatic polypeptide (PP)-, and insulin-immunoreactive cells are found scattered or in small groups within or lining the developing ductal epithelium as marked by cytokeratin 19. Using degenerate PCR, the P. obesus Neurogenin-3 (Ngn-3) gene was cloned. Nucleotide and amino acid sequences show high homology with known Ngn-3 sequences. Using specific antiserum, we can observe that Ngn-3-immunoreactive cells are rare at E14.5 but readily detectable at E18.5 and E22.5. In conclusion, despite the lack of detection of Pdx-1, the P. obesus pancreas develops similarly to Muridae species, and the Ngn-3 sequence and expression pattern is highly conserved in P. obesus. (J Histochem Cytochem 55:97-104, 2007) K E Y W O R D S Psammomys obesus neurogenin-3 type 2 diabetes developmental biology glucagon insulin pancreas sand rat THE GERBIL PSAMMOMYS OBESUS is a promising animal model for the study of type 2 diabetes (T2D) (Hackel et al. 1967;Kalman et al. 2001). The natural habitat of the P. obesus is the arid zone of the Eastern Mediterranean region and North Africa from where it was collected in the 1960s. Our animals are from the colony in Jerusalem that was established in the 1970s when it was collected from the arid shores of the Dead Sea (Shafrir and Ziv 1998). In its native habitat, the P. obesus feeds on succulent leaves of, e.g., the salt bush, which is a low-energy diet (2.98 digestible kcal/g). When transferred to laboratory rodent chow (Purina 5008, 3.31 digestible kcal/g; Charles River Laboratories, Wilmington, MA), which is high in energy to the P. obesus, z80% of our population develops diabetes. Development of diabetes begins with hyperinsulinemia, after which hyperglycemia sets in and finally overt T2D occurs. This phenotypic pattern closely resembles the pattern observed in human populations, making the P. obesus a good model for the study of T2D (Walder et al. 2000). Donev et al. (1989) found that the normoglycemic P. obesus pancreas expres...

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

“…S11), indicating that the coding region is under functional constraint despite extensive mutation. Extreme deviation from the expected sequence explains why antibodies and PCR failed to detect Pdx1 in sand rat, Mongolian jird, and, potentially, other gerbil species (14)(15)(16).…”

“…However, Pdx1 is readily detectable in the closely related spiny mouse, Acomys cahirinus (Fig. 1B), leading to the hypothesis that the gene has been lost within the Gerbillinae subfamily, contributing to the compromised ability to regulate insulin in the sand rat (14)(15)(16). Such a conclusion would raise further questions, because in addition to its adult functions, Pdx1 is also essential for pancreatic development in the embryo.…”

Acomys, the closest relatives to Gerbils, do express Pdx-1 protein and have similar islet morphology to Gerbils

“…What causes the defective insulin secretion in Acomys is not related to lack of Pdx-1 protein as maybe the case in P. obesus. 27,28 Furthermore, based on genetic data, it has been argued that the spiny mouse (Acomys) is more closely related to gerbils (Gerbillinae) than to real mice (Murinae). 15 In addition, we have recently shown that maturity-onset diabetes of the young and type 2 diabetes; absence of the gene leads to complete pancreas agenesis.…”

“…Interestingly, Leibowitz et al 27 and Vedtofte et al 28 have shown the apparent absence of this crucial factor in the sand rat (Psammomys obesus). In the same subfamily (Gerbillinae), we have observed that five species of the genus Meriones demonstrate the same absence of Pdx-1 protein and 13 species lack the gene.…”

The islet ghrelin cell

“…Important pieces to the puzzle have, however, been added recently. Vedtofte et al (2007) reported that the islets of the desert gerbil (Psammomys obesus) also harbour ghrelin cells during development. The desert gerbil is closely related to the mouse and the data are largely in line with previous studies in mice (Prado et al 2004, Heller et al 2005.…”