Downregulation of Glis3 in INS1 cells exposed to chronically elevated glucose contributes to glucotoxicity-associated β cell dysfunction

Chronically elevated levels of glucose are deleterious to pancreatic β cells and contribute to β cell dysfunction, which is characterized by decreased insulin production and a loss of β cell identity. The Krüppel-like transcription factor, Glis3 has previously been shown to positively regulate insulin transcription and mutations within the Glis3 locus have been associated with the development of several pathologies including type 2 diabetes mellitus. In this report, we show that Glis3 is significantly downregulated at the transcriptional level in INS1 832/13 cells within hours of being subjected to high glucose concentrations and that diminished expression of Glis3 is at least partly attributable to increased oxidative stress. CRISPR/Cas9-mediated knockdown of Glis3 indicated that the transcription factor was required to maintain normal levels of both insulin and MafA expression and reduced Glis3 expression was concomitant with an upregulation of β cell disallowed genes. We provide evidence that Glis3 acts similarly to a pioneer factor at the insulin promoter where it permissively remodels the chromatin to allow access to a transcriptional regulatory complex including Pdx1 and MafA. Finally, evidence is presented that Glis3 can positively regulate MafA transcription through its pancreas-specific promoter and that MafA reciprocally regulates Glis3 expression. Collectively, these results suggest that decreased Glis3 expression in β cells exposed to chronic hyperglycemia may contribute significantly to reduced insulin transcription and a loss of β cell identity.

Whole Mount Immunohistochemistry in Zebrafish Embryos and Larvae

Immunohistochemistry is a widely used technique to explore protein expression and localization during both normal developmental and disease states. Although many immunohistochemistry protocols have been optimized for mammalian tissue and tissue sections, these protocols often require modification and optimization for non-mammalian model organisms. Zebrafish are increasingly used as a model system in basic, biomedical, and translational research to investigate the molecular, genetic, and cell biological mechanisms of developmental processes. Zebrafish offer many advantages as a model system but also require modified techniques for optimal protein detection. Here, we provide our protocol for whole-mount fluorescence immunohistochemistry in zebrafish embryos and larvae. This protocol additionally describes several different mounting strategies that can be employed and an overview of the advantages and disadvantages each strategy provides. We also describe modifications to this protocol to allow detection of chromogenic substrates in whole mount tissue and fluorescence detection in sectioned larval tissue. This protocol is broadly applicable to the study of many developmental stages and embryonic structures.

PIAS-family proteins negatively regulate Glis3 transactivation function through SUMO modification in pancreatic β cells

Gli-similar 3 (Glis3) is Krüppel-like transcription factor associated with the transcriptional regulation of insulin. Mutations within the Glis3 locus have been implicated in a number of pathologies including diabetes mellitus and hypothyroidism. Despite its clinical significance, little is known about the proteins and posttranslational modifications that regulate Glis3 transcriptional activity. In this report, we demonstrate that the SUMO-pathway associated proteins, PIASy and Ubc9 are capable of regulating Glis3 transactivation function through a SUMO-dependent mechanism. We present evidence that SUMOylation of Glis3 by PIAS-family proteins occurs at two conserved lysine residues within the Glis3 N-terminus and modification of Glis3 by SUMO dramatically inhibited insulin transcription. Finally, we provide evidence that Glis3 SUMOylation increases under conditions of chronically elevated glucose and correlates with decreased insulin transcription. Collectively, these results indicate that SUMOylation may serve as a mechanism to regulate Glis3 activity in β cells.

The Krüppel-like protein Gli-similar 3 (Glis3) functions as a key regulator of insulin transcription

Transcriptional regulation of insulin in pancreatic β-cells is mediated primarily through enhancer elements located within the 5' upstream regulatory region of the preproinsulin gene. Recently, the Krüppel-like transcription factor, Gli-similar 3 (Glis3), was shown to bind the insulin (INS) promoter and positively influence insulin transcription. In this report, we examined in detail the synergistic activation of insulin transcription by Glis3 with coregulators, CREB-binding protein (CBP)/p300, pancreatic and duodenal homeobox 1 (Pdx1), neuronal differentiation 1 (NeuroD1), and v-maf musculoaponeurotic fibrosarcoma oncogene homolog A (MafA). Our data show that Glis3 expression, the binding of Glis3 to GlisBS, and its recruitment of CBP are required for optimal activation of the insulin promoter in pancreatic β-cells not only by Glis3, but also by Pdx1, MafA, and NeuroD1. Mutations in the GlisBS or small interfering RNA-directed knockdown of GLIS3 diminished insulin promoter activation by Pdx1, NeuroD1, and MafA, and neither Pdx1 nor MafA was able to stably associate with the insulin promoter when the GlisBS were mutated. In addition, a GlisBS mutation in the INS promoter implicated in the development of neonatal diabetes similarly abated activation by Pdx1, NeuroD1, and MafA that could be reversed by increased expression of exogenous Glis3. We therefore propose that recruitment of CBP/p300 by Glis3 provides a scaffold for the formation of a larger transcriptional regulatory complex that stabilizes the binding of Pdx1, NeuroD1, and MafA complexes to their respective binding sites within the insulin promoter. Taken together, these results indicate that Glis3 plays a pivotal role in the transcriptional regulation of insulin and may serve as an important therapeutic target for the treatment of diabetes.

Modulation of the transactivation function and stability of Krüppel-like zinc finger protein Gli-similar 3 (Glis3) by Suppressor of Fused

Glis3 is a member of the Glis subfamily of Krüppel-like zinc finger transcription factors. Recently, Glis3 has been linked to both type I and type II diabetes and shown to positively regulate insulin gene expression. In this study, we have identified a region within the N terminus of Glis3 that shares high levels of homology with the Cubitus interruptus (Ci)/Gli family of proteins. We demonstrated that Glis3 interacts with Suppressor of Fused (SUFU), which involves a VYGHF motif located within this conserved region. We further showed that SUFU is able to inhibit the activation of the insulin promoter by Glis3 but not the activation by a Glis3 mutant deficient in its ability to bind SUFU, suggesting that the inhibitory effect is dependent on the interaction between the two proteins. Exogenous SUFU did not affect the nuclear localization of Glis3; however, Glis3 promoted the nuclear accumulation of SUFU. Additionally, we demonstrated that SUFU stabilizes Glis3 in part by antagonizing the Glis3 association with a Cullin 3-based E3 ubiquitin ligase that promotes the ubiquitination and degradation of Glis3. This is the first reported instance of Glis3 interacting with SUFU and suggests a novel role for SUFU in the modulation of Glis3 signaling. Given the critical role of Glis3 in pancreatic β-cell generation and maintenance, the elevated Glis3 expression in several cancers, and the established role of SUFU as a tumor suppressor, these data provide further insight into Glis3 regulation and its function in development and disease.