Abrogation of Protein Convertase 2 Activity Results in Delayed Islet Cell Differentiation and Maturation, Increased α-Cell Proliferation, and Islet Neogenesis

Abstract

To date, the role of pancreatic hormones in pancreatic islet growth and differentiation is poorly understood. To address this issue, we examined mice with a disruption in the gene encoding prohormone convertase 2 (PC2). These mice are unable to process proglucagon, prosomatostatin, and other neuroendocrine precursors into mature hormones. Initiation of insulin (IN) expression during development was delayed in PC2 mutant mice. Cells containing IN were first detected in knockout embryos on d 15 of development, 5 d later than in wild-type littermates. However, the IN(+) cells of d 15 PC2 mutant mice coexpressed glucagon, as did the first appearing beta-cells of controls. In addition, lack of PC2 perturbed the pattern of expression of transcription factors presumed to be involved in the determination of the mature alpha-cell phenotype. Thus, in contrast to controls, alpha-cells of mutant mice had protracted expression of Nkx 6.1 and Pdx-1, but did not express Brn-4. Islets of adult mutant mice also contained cells coexpressing insulin and somatostatin, an immature cell type found only in islets of the wild-type strain during development. In addition to the effects on islet cell differentiation, the absence of PC2 activity resulted in a 3-fold increase in the rate of proliferation of proglucagon cells during the perinatal period. This increase contributed to the development of alpha-cell hyperplasia during postnatal life. Furthermore, the total beta-cell volume was increased 2-fold in adult mutants compared with controls. This increase was due to islet neogenesis, as the number of islets per section was significantly higher in knockout mice compared with wild-type mice, whereas both strains had similar rates of IN cell proliferation. These results indicate that hormones processed by PC2 affected processes that regulate islet cell differentiation and maturation in embryos and adults.

Nestin expression in pancreatic endocrine and exocrine cells of mice lacking glucagon signaling

“…The persistence of nestin expression in differentiated endocrine cells of PC2 Ϫ/Ϫ and Gcgr Ϫ/Ϫ embryos follows the pattern of other embryonal traits that have protracted expression in islet cells of these two mutant lines (Vincent et al, 2003;Vuguin et al, 2006). Thus, while the pancreatic specific transcription factor Pdx-1 is expressed by pancreatic cells of PC2 ϩ/ϩ…”

“…Similar to Gcgr ϩ/ϩ and PC2 ϩ/ϩ embryos, the pancreatic primordia of e-11 Gcgr Ϫ/Ϫ and PC2 Ϫ/Ϫ embryos contained nestin ϩ glucagon ϩ cells (results not shown). However, e-11 Gcgr Ϫ/Ϫ and PC2 Ϫ/Ϫ embryos lacked IN ϩ cells (Vincent et al, 2003;Vuguin et al, 2006). Lack of glucagon signaling also affected the time-table of expression of nestin in islet cells.…”

“…Since our previous results indicated that pancreas of adult PC2 Ϫ/Ϫ and Gcgr Ϫ/Ϫ contain subsets of islet cells expressing traits characteristic of endocrine cells of embryos (Vincent et al, 2003;Vuguin et al, 2006), we sought to determine whether these cells also expressed nestin. Large numbers of nestin cells were found in approximately 10% of the islets in both strains of mutant mice and some of these cells co-expressed insulin (Fig.…”

“…and Gcgr ϩ/ϩ embryos but only by beta cells of adults, non-beta cells of embryos and adult PC2 Ϫ/Ϫ and Gcgr Ϫ/Ϫ mice expressed Pdx-1 (Vincent et al, 2003;Vuguin et al, 2006). Similarly, the glucose transporter 2 (GLUT2), which is transiently expressed by glucagon cells in control mice during development, is expressed by alpha cells of adult Gcgr Ϫ/Ϫ mice (Vuguin et al, 2006).…”

“…In the present study, we sought to determine whether nestin is expressed in vivo by islet cells of two mutant mouse models characterized by protracted expression of several embryonal properties (Vincent et al, 2003;Vuguin et al, 2006), speculating that they may also retain expression of nestin after the initiation of hormone expression. One mutant line comprised mice with a deletion in the gene encoding for prohormone convertase-2 (PC2 Ϫ/Ϫ ), responsible for the conversion of proglucagon into the glucagon (Furuta et al, 1997;Steiner, 1998) while a second line consisted of mice with a global deletion of the glucagon receptor (Gcgr Ϫ/Ϫ ) (Gelling et al, 2003).…”