Paired box 6 programs essential exocytotic genes in the regulation of glucose-stimulated insulin secretion and glucose homeostasis

Gene therapy targeting key beta cell regulators as a potential intervention for diabetes

In this Comment, Wing Yan So and Weiping Han discuss the recent achievements and therapeutic potential of gene therapy targeting beta cell regulators for the treatment of diabetes.

YAP/TAZ Drive Agrin-Matrix Metalloproteinase 12-Mediated Diabetic Skin Wound Healing

Macroscopic loss of extracellular matrix can lead to chronic defects in skin wound healing, but supplementation of extracellular matrix holds promise for facilitating wound closure, particularly in diabetic wound healing. We recently showed that the extracellular matrix proteoglycan agrin accelerates cutaneous wound healing by improving mechanoperception of migrating keratinocytes and allowing them to respond to mechanical stresses through matrix metalloproteinase 12 (MMP12). RNA-sequencing analysis revealed that in addition to a disorganized extracellular matrix, agrin-depleted skin cells have impaired YAP/TAZ transcriptional outcomes, leading us to hypothesize that YAP/TAZ, as central mechanosensors, drive the functionality of agrin-MMP12 signaling during cutaneous wound repair. In this study, we demonstrate that agrin activates YAP/TAZ during migration of keratinocytes after wounding in vitro and in vivo. Mechanistically, YAP/TAZ sustain agrin and MMP12 protein expression during migration after wounding through positive feedback. YAP/TAZ silencing abolishes agrin-MMP12-mediated force recognition and geometrical constraints. Importantly, soluble agrin therapy accelerates wound closure in diabetic mouse models by engaging MMP12-YAP. Because patients with diabetic foot ulcers and impaired wound healing have reduced expression of agrin-MMP12 that correlates with YAP/TAZ inactivation, we propose that timely activation of YAP/TAZ by soluble agrin therapy can accentuate mechanobiological microenvironments for efficient wound healing, under normal and diabetic conditions.

Taurine promotes insulin synthesis by enhancing Isl-1 expression through miR-7a/RAF1/ERK1/2 pathway

It has been well established that the circulating taurine affects the insulin synthesis in pancreatic islet β-cells, whereas miR-7a and LIM-homeodomain transcription factor Isl-1 are important intracellular factors regulating insulin transcription and synthesis. However, it still remains unknown whether taurine regulates insulin synthesis by affecting miR-7a and/or Isl-1 expressions in mouse pancreatic islet β-cells. The present study was thus proposed to identify the effects of taurine on the expressions of miR-7a and/or Isl-1 and their relations to insulin synthesis in mouse pancreatic islet β-cells by using miR-7a2 knockout (KO) and taurine transporter (TauT) KO mouse models and the related in vitro experiments. The results demonstrated that taurine supplement significantly decreased the pancreas miR-7a expression, but sharply upregulated the pancreas Isl-1 and insulin expressions, and serum insulin levels. However, the enhanced effects of taurine on Isl-1 expression and insulin synthesis were mitigated in the TauT KO and miR-7a2 KO mice. In addition, our results confirmed that taurine markedly increased pancreas RAF1 and ERK1/2 expressions. Collectively, the present study firstly demonstrates that taurine regulates insulin synthesis through TauT/miR-7a/RAF1/ERK1/2/Isl-1 signaling pathway, which are crucial for our understanding the mechanisms of taurine affecting insulin synthesis, and also potential for establishing the therapeutic strategies for diabetes and the diseases related to metabolism.

Paired box 6 gene delivery preserves beta cells and improves islet transplantation efficacy

Loss of pancreatic beta cells is the central feature of all forms of diabetes. Current therapies fail to halt the declined beta cell mass. Thus, strategies to preserve beta cells are imperatively needed. In this study, we identified paired box 6 (PAX6) as a critical regulator of beta cell survival. Under diabetic conditions, the human beta cell line EndoC-βH1, db/db mouse and human islets displayed dampened insulin and incretin signalings and reduced beta cell survival, which were alleviated by PAX6 overexpression. Adeno-associated virus (AAV)-mediated PAX6 overexpression in beta cells of streptozotocin-induced diabetic mice and db/db mice led to a sustained maintenance of glucose homeostasis. AAV-PAX6 transduction in human islets reduced islet graft loss and improved glycemic control after transplantation into immunodeficient diabetic mice. Our study highlights a previously unappreciated role for PAX6 in beta cell survival and raises the possibility that ex vivo PAX6 gene transfer into islets prior to transplantation might enhance islet graft function and transplantation outcome.

BCAS2 Participates in Insulin Synthesis and Secretion via mRNA Alternative Splicing in Mice

Insulin secreted by pancreatic β cells is essential for maintaining blood glucose levels. Diabetes is caused primarily by a loss of β cells or impairment of β-cell function. A previous whole-transcriptome analysis of islets from a type 2 diabetes group and a control group showed that a splicing disorder occurred in approximately 25% of splicing events. Breast carcinoma amplified sequence 2 (BCAS2) is a spliceosome component whose function in islet β cells is unclear. Here, we report that knockdown of Bcas2 decreased glucose- and KCl-stimulated insulin secretion in the NIT-1 cell line. Pancreas weight, glucose tolerance, and insulin sensitivity were measured in normal chow-fed Bcas2 f/f-βKO mice, and β-cell mass and islet size were analyzed by immunohistochemistry. Glucose intolerance developed in Bcas2 f/f-βKO mice, but there were no significant differences in pancreas weight, insulin sensitivity, β-cell mass, or islet size. Furthermore, observation of glucose-stimulated insulin secretion and insulin secretion granules in normal chow-fed mice revealed that the insulin level in serum and the number of insulin secretion granules were decreased in Bcas2 f/f-βKO mice. These differences were related to abnormal splicing of Syt7 and Tcf7l2 pre-mRNA. Taken together, these results demonstrate that BCAS2 is involved in alternative splicing during insulin synthesis and secretion.

Zhx2 maintains islet β-cell mass and function by transcriptionally regulating Pax6

Emerging evidence shows that pancreatic β-cell function and quality are key determinants in the progression of type 2 diabetes (T2D). The transcription factor zinc finger homeobox 2 (Zhx2) is involved in proliferation and development of multiple cells. However, the exact role of Zhx2 in β-cells and T2D remains completely unknown. Here, we report that Zhx2 orchestrates β-cell mass and function by regulating paired box protein pax-6 (Pax6). We found that β-cell-specific knockout Zhx2 (Zhx2BKO) mice showed a decrease in β-cell proliferation and glucose homeostasis. Under prediabetic and diabetic conditions, we discovered glucose intolerance in both Zhx2BKO-HFD mice and Zhx2BKO-db/db mice, with reduced β-cell mass and insulin secretion. Mechanistically, we demonstrated that Zhx2 targeted the Pax6 promoter region (-1740∼-1563; -862∼-559; -251∼+75), enhanced promoter activity. Overall, Zhx2 maintains β-cell function by transcriptionally regulating Pax6, which provides a therapeutic target for diabetes intervention.

Mutated lncRNA increase the risk of type 2 diabetes by promoting β cell dysfunction and insulin resistance

Islet β cell dysfunction and insulin resistance are the main pathogenesis of type 2 diabetes (T2D), but the mechanism remains unclear. Here we identify a rs3819316 C > T mutation in lncRNA Reg1cp mainly expressed in islets associated with an increased risk of T2D. Analyses in 16,113 Chinese adults reveal that Mut-Reg1cp individuals had higher incidence of T2D and presented impaired insulin secretion as well as increased insulin resistance. Mice with islet β cell specific Mut-Reg1cp knock-in have more severe β cell dysfunction and insulin resistance. Mass spectrometry assay of proteins after RNA pulldown demonstrate that Mut-Reg1cp directly binds to polypyrimidine tract binding protein 1 (PTBP1), further immunofluorescence staining, western blot analysis, qPCR analysis and glucose stimulated insulin secretion test reveal that Mut-Reg1cp disrupts the stabilization of insulin mRNA by inhibiting the phosphorylation of PTBP1 in β cells. Furthermore, islet derived exosomes transfer Mut-Reg1cp into peripheral tissue, which then promote insulin resistance by inhibiting AdipoR1 translation and adiponectin signaling. Our findings identify a novel mutation in lncRNA involved in the pathogenesis of T2D, and reveal a new mechanism for the development of T2D.

Stepwise differentiation of functional pancreatic β cells from human pluripotent stem cells

Pancreatic β cells differentiated from stem cells provide promise for cell replacement therapy of diabetes. Human pluripotent stem cells could be differentiated into definitive endoderm, followed by pancreatic progenitors, and then subjected to endocrinal differentiation and maturation in a stepwise fashion. Many achievements have been made in making pancreatic β cells from human pluripotent stem cells in last two decades, and a couple of phase I/II clinical trials have just been initiated. Here, we overview the major progresses in differentiating pancreatic β cells from human pluripotent stem cells with the focus on recent technical advances in each differentiation stage, and briefly discuss the current limitations as well.

Dietary intervention preserves β cell function in mice through CTCF-mediated transcriptional reprogramming

Pancreatic β cell plasticity is the primary determinant of disease progression and remission of type 2 diabetes (T2D). However, the dynamic nature of β cell adaptation remains elusive. Here, we establish a mouse model exhibiting the compensation-to-decompensation adaptation of β cell function in response to increasing duration of high-fat diet (HFD) feeding. Comprehensive islet functional and transcriptome analyses reveal a dynamic orchestration of transcriptional networks featuring temporal alteration of chromatin remodeling. Interestingly, prediabetic dietary intervention completely rescues β cell dysfunction, accompanied by a remarkable reversal of HFD-induced reprogramming of islet chromatin accessibility and transcriptome. Mechanistically, ATAC-based motif analysis identifies CTCF as the top candidate driving dietary intervention-induced preservation of β cell function. CTCF expression is markedly decreased in β cells from obese and diabetic mice and humans. Both dietary intervention and AAV-mediated restoration of CTCF expression ameliorate β cell dysfunction ex vivo and in vivo, through transducing the lipid toxicity and inflammatory signals to transcriptional reprogramming of genes critical for β cell glucose metabolism and stress response.

A distinct role of STING in regulating glucose homeostasis through insulin sensitivity and insulin secretion

The role of STING in maintaining glucose homeostasis remains unknown. Herein, using global and β-cell–specific STING knockout mouse models, we revealed a distinct role of STING in the regulation of glucose homeostasis through β-cells and peripheral tissues. Specially, while global STING knockout beneficially alleviated insulin resistance and glucose intolerance induced by high-fat diet, it surprisingly impaired islet glucose-stimulated insulin secretion (GSIS). Further analyses revealed that STING deficiency down-regulated expression of β-cell key transcription factor Pax6, impairing Pax6 nuclear localization and binding activity to the promoters of its target genes, including Glut2 and Abcc8, causing impaired GSIS. These data highlight pathophysiological significance of fine-tuned STING signaling in β-cells and insulin target tissues for maintaining glucose homeostasis.

Insulin resistance and β-cell dysfunction are two main molecular bases yet to be further elucidated for type 2 diabetes (T2D). Accumulating evidence indicates that stimulator of interferon genes (STING) plays an important role in regulating insulin sensitivity. However, its function in β-cells remains unknown. Herein, using global STING knockout (STING^−/−) and β-cell–specific STING knockout (STING-βKO) mouse models, we revealed a distinct role of STING in the regulation of glucose homeostasis through peripheral tissues and β-cells. Specially, although STING^−/− beneficially alleviated insulin resistance and glucose intolerance induced by high-fat diet, it surprisingly impaired islet glucose-stimulated insulin secretion (GSIS). Importantly, STING is decreased in islets of db/db mice and patients with T2D, suggesting a possible role of STING in β-cell dysfunction. Indeed, STING-βKO caused glucose intolerance due to impaired GSIS, indicating that STING is required for normal β-cell function. Islet transcriptome analysis showed that STING deficiency decreased expression of β-cell function–related genes, including Glut2, Kcnj11, and Abcc8, contributing to impaired GSIS. Mechanistically, the assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) and cleavage under targets and tagmentation (CUT&Tag) analyses suggested that Pax6 was the transcription factor that might be associated with defective GSIS in STING-βKO mice. Indeed, Pax6 messenger RNA and protein levels were down-regulated and its nuclear localization was lost in STING-βKO β-cells. Together, these data revealed a function of STING in the regulation of insulin secretion and established pathophysiological significance of fine-tuned STING within β-cells and insulin target tissues for maintaining glucose homeostasis.

Genome-Wide Placental Gene Methylations in Gestational Diabetes Mellitus, Fetal Growth and Metabolic Health Biomarkers in Cord Blood

Gestational diabetes mellitus (GDM) "program" an elevated risk of metabolic syndrome in the offspring. Epigenetic alterations are a suspected mechanism. GDM has been associated with placental DNA methylation changes in some epigenome-wide association studies. It remains unclear which genes or pathways are affected, and whether any placental differential gene methylations are correlated to fetal growth or circulating metabolic health biomarkers. In an epigenome-wide association study using the Infinium MethylationEPIC Beadchip, we sought to identify genome-wide placental differentially methylated genes and enriched pathways in GDM, and to assess the correlations with fetal growth and metabolic health biomarkers in cord blood. The study samples were 30 pairs of term placentas in GDM vs. euglycemic pregnancies (controls) matched by infant sex and gestational age at delivery in the Shanghai Birth Cohort. Cord blood metabolic health biomarkers included insulin, C-peptide, proinsulin, IGF-I, IGF-II, leptin and adiponectin. Adjusting for maternal age, pre-pregnancy BMI, parity, mode of delivery and placental cell type heterogeneity, 256 differentially methylated positions (DMPs,130 hypermethylated and 126 hypomethylated) were detected between GDM and control groups accounting for multiple tests with false discovery rate <0.05 and beta-value difference >0.05. WSCD2 was identified as a differentially methylated gene in both site- and region-level analyses. We validated 7 hypermethylated (CYP1A2, GFRA1, HDAC4, LIMS2, NAV3, PAX6, UPK1B) and 10 hypomethylated (DPP10, CPLX1, CSMD2, GPR133, NRXN1, PCSK9, PENK, PRDM16, PTPRN2, TNXB) genes reported in previous epigenome-wide association studies. We did not find any enriched pathway accounting for multiple tests. DMPs in 11 genes (CYP2D7P1, PCDHB15, ERG, SIRPB1, DKK2, RAPGEF5, CACNA2D4, PCSK9, TSNARE1, CADM2, KCNAB2) were correlated with birth weight (z score) accounting for multiple tests. There were no significant correlations between placental gene methylations and cord blood biomarkers. In conclusions, GDM was associated with DNA methylation changes in a number of placental genes, but these placental gene methylations were uncorrelated to the observed metabolic health biomarkers (fetal growth factors, leptin and adiponectin) in cord blood. We validated 17 differentially methylated placental genes in GDM, and identified 11 differentially methylated genes relevant to fetal growth.