A transgenic model for studying islet development

Absence of PARP-1 affects Cxcl12 expression by increasing DNA demethylation

Poly [ADP‐ribose] polymerase 1 (PARP‐1) has an inhibitory effect on C‐X‐C motif chemokine 12 gene (Cxcl12) transcription. We examined whether PARP‐1 affects the epigenetic control of Cxcl12 expression by changing its DNA methylation pattern. We observed increased expression of Cxcl12 in PARP‐1 knock‐out mouse embryonic fibroblasts (PARP1−/−) in comparison to wild‐type mouse embryonic fibroblasts (NIH3T3). In the Cxcl12 gene, a CpG island is present in the promoter, the 5′ untranslated region (5′ UTR), the first exon and in the first intron. The methylation state of Cxcl12 in each cell line was investigated by methylation‐specific PCR (MSP) and high resolution melting analysis (HRM). Both methods revealed strong demethylation in PARP1−/− compared to NIH3T3 cells in all four DNA regions. Increased expression of the Ten‐eleven translocation (Tet) genes in PARP1−/− cells indicated that TETs could be important factors in Cxcl12 demethylation in the absence of PARP‐1, accounting for its increased expression. Our results showed that PARP‐1 was a potential upstream player in (de)methylation events that modulated Cxcl12 expression.

The Importance of the CXCL12/CXCR4 Axis in Therapeutic Approaches to Diabetes Mellitus Attenuation

The pleiotropic chemokine (C–X–C motif) ligand 12 (CXCL12) has emerged as a crucial player in several diseases. The role of CXCL12 in diabetes promotion and progression remains elusive due to its multiple functions and the overwhelming complexity of diabetes. Diabetes is a metabolic disorder resulting from a failure in glucose regulation due to β-cell loss and/or dysfunction. In view of its ability to stimulate the regeneration, proliferation, and survival of β-cells, as well as its capacity to sustain local immune-isolation, CXCL12 has been considered in approaches aimed at attenuating type 1 diabetes. However, a note of caution emerges from examinations of the involvement of CXCL12 in the development of diabetes and its complications, as research data indicate that CXCL12 displays effects that range from protective to detrimental. Therefore, as a beneficial effect of CXCL12 in one process could have deleterious consequences in another, a more complete understanding of CXCL12 effects, in particular its functioning in the cellular microenvironment, is essential before CXCL12 can be considered in therapies for diabetes treatment.

The SDF-1α/CXCR4 axis is required for proliferation and maturation of human fetal pancreatic endocrine progenitor cells

The chemokine receptor CXCR4 and ligand SDF-1α are expressed in fetal and adult mouse islets. Neutralization of CXCR4 has previously been shown to diminish ductal cell proliferation and increase apoptosis in the IFNγ transgenic mouse model in which the adult mouse pancreas displays islet regeneration. Here, we demonstrate that CXCR4 and SDF-1α are expressed in the human fetal pancreas and that during early gestation, CXCR4 colocalizes with neurogenin 3 (ngn3), a key transcription factor for endocrine specification in the pancreas. Treatment of islet like clusters (ICCs) derived from human fetal pancreas with SDF-1α resulted in increased proliferation of epithelial cells in ICCs without a concomitant increase in total insulin expression. Exposure of ICCs in vitro to AMD3100, a pharmacological inhibitor of CXCR4, did not alter expression of endocrine hormones insulin and glucagon, or the pancreatic endocrine transcription factors PDX1, Nkx6.1, Ngn3 and PAX4. However, a strong inhibition of β cell genesis was observed when in vitro AMD3100 treatment of ICCs was followed by two weeks of in vivo treatment with AMD3100 after ICC transplantation into mice. Analysis of the grafts for human C-peptide found that inhibition of CXCR4 activity profoundly inhibits islet development. Subsequently, a model pancreatic epithelial cell system (CFPAC-1) was employed to study the signals that regulate proliferation and apoptosis by the SDF-1α/CXCR4 axis. From a selected panel of inhibitors tested, both the PI 3-kinase and MAPK pathways were identified as critical regulators of CFPAC-1 proliferation. SDF-1α stimulated Akt phosphorylation, but failed to increase phosphorylation of Erk above the high basal levels observed. Taken together, these results indicate that SDF-1α/CXCR4 axis plays a critical regulatory role in the genesis of human islets.

Enhanced beta-cell mass without increased proliferation following chronic mild glucose infusion

The physiological mechanisms underlying pancreatic beta-cell mass (BCM) homeostasis are complex and not fully resolved. Here we examined the factors contributing to the increased BCM following a mild glucose infusion (GI) whereby normoglycemia was maintained through 96 h. We used morphometric and immunochemical methods to investigate enhanced beta-cell growth and survival in Sprague-Dawley rats. BCM was elevated >2.5-fold over saline-infused control rats by 48 h and increased modestly thereafter. Unexpectedly, increases in beta-cell proliferation were not observed at any time point through 4 days. Instead, enhanced numbers of insulin(+) cell clusters and small islets (400-12,000 microm(2); approximately 23- to 124-microm diameter), mostly scattered among the acini, were observed in the GI rats by 48 h despite no difference in the numbers of medium to large islets. We previously showed that increased beta-cell growth in rodent models of insulin resistance and pancreatic regeneration involves increased activated Akt/PKB, a key beta-cell signaling intermediate, in both islets and endocrine cell clusters. GI in normal rats also leads to increased Akt activation in islet beta-cells, as well as in insulin(+) and insulin(-) cells in the common duct epithelium and endocrine clusters. This correlated with strong Pdx1 expression in these same cells. These results suggest that mechanisms other than proliferation underlie the rapid beta-cell growth response following a mild GI in the normal rat and involve Akt-regulated enhanced beta-cell survival potential and neogenesis from epithelial precursors.

Identification and expansion of pancreatic stem/progenitor cells

Pancreatic islet transplantation represents an attractive approach for the treatment of diabetes. However, the limited availability of donor islets has largely hampered this approach. In this respect, the use of alternative sources of islets such as the ex vivo expansion and differentiation of functional endocrine cells for treating diabetes has become the major focus of diabetes research. Adult pancreatic stem cells /progenitor cells have yet to be recognized because limited markers exist for their identification. While the pancreas has the capacity to regenerate under certain circumstances, questions where adult pancreatic stem/progenitor cells are localized, how they are regulated, and even if the pancreas harbors a stem cell population need to be resolved. In this article, we review the recent achievements both in the identification as well as in the expansion of pancreatic stem/progenitor cells.

The stromal cell-derived factor-1alpha/CXCR4 ligand-receptor axis is critical for progenitor survival and migration in the pancreas

The SDF-1α/CXCR4 ligand/chemokine receptor pair is required for appropriate patterning during ontogeny and stimulates the growth and differentiation of critical cell types. Here, we demonstrate SDF-1α and CXCR4 expression in fetal pancreas. We have found that SDF-1α and its receptor CXCR4 are expressed in islets, also CXCR4 is expressed in and around the proliferating duct epithelium of the regenerating pancreas of the interferon (IFN) γ–nonobese diabetic mouse. We show that SDF-1α stimulates the phosphorylation of Akt, mitogen-activated protein kinase, and Src in pancreatic duct cells. Furthermore, migration assays indicate a stimulatory effect of SDF-1α on ductal cell migration. Importantly, blocking the SDF-1α/CXCR4 axis in IFNγ-nonobese diabetic mice resulted in diminished proliferation and increased apoptosis in the pancreatic ductal cells. Together, these data indicate that the SDF-1α–CXCR4 ligand receptor axis is an obligatory component in the maintenance of duct cell survival, proliferation, and migration during pancreatic regeneration.