SIX2 and SIX3 coordinately regulate functional maturity and fate of human pancreatic β cells

Abstract

The physiological functions of many vital tissues and organs continue to mature after birth, but the genetic mechanisms governing this postnatal maturation remain an unsolved mystery. Human pancreatic β cells produce and secrete insulin in response to physiological cues like glucose, and these hallmark functions improve in the years after birth. This coincides with expression of the transcription factors SIX2 and SIX3, whose functions in native human β cells remain unknown. Here, we show that shRNA-mediated SIX2 or SIX3 suppression in human pancreatic adult islets impairs insulin secretion. However, transcriptome studies revealed that SIX2 and SIX3 regulate distinct targets. Loss of SIX2 markedly impaired expression of genes governing β-cell insulin processing and output, glucose sensing, and electrophysiology, while SIX3 loss led to inappropriate expression of genes normally expressed in fetal β cells, adult α cells, and other non-β cells. Chromatin accessibility studies identified genes directly regulated by SIX2. Moreover, β cells from diabetic humans with impaired insulin secretion also had reduced SIX2 transcript levels. Revealing how SIX2 and SIX3 govern functional maturation and maintain developmental fate in native human β cells should advance β-cell replacement and other therapeutic strategies for diabetes.

Maturation of beta cells: lessons from in vivo and in vitro models

“…Even the oligomerisation state of the glycolytic enzyme GAPDH (rather than expression level) has been associated with beta cell functional maturation [34]. Some recent findings have also identified the SIX homeobox 2 (SIX2) and SIX homeobox 3 (SIX3) transcription factors as regulators of beta cell functional maturation [121,122]; this has been demonstrated in SC-islet knockdown models of SIX2, wherein GSIS function was strongly impaired [123]. Interestingly, although SIX2 is necessary for SC-islet functional acquisition in vitro, SIX3 expression appears to be important for advanced maturation events and is not detected in SC-islets in vitro or after extended murine engraftment [35,123].…”

Relationships between type 2 diabetes, cell dysfunction, and redox signaling: A meta‐analysis of single‐cell gene expression of human pancreatic α‐ and β‐cells

“…SIX3, which represses Wnt activity and activates the SHH, 35 was identified as top overexpressed gene in β‐cells from T2DM donors, and SIX3 was also one of the top overexpressed genes in senescent vs non‐senescent β‐cells. SIX3 has been identified as a transcription factor that governs functional β‐cell maturation and may be a potential target for β‐cell dysfunction in T2DM 47 …”

“…SIX3 has been identified as a transcription factor that governs functional β-cell maturation and may be a potential target for β-cell dysfunction in T2DM. 47 As outlined in Figure 6, lipotoxicity and glucotoxicity associated with T2DM increase oxidative stress and can increase NFE2L2 activation. As expected, αand β-cells from T2DM donors have increased NFE2L2 activation.…”

Immune Protection of Stem Cell-Derived Islet Cell Therapy for Treating Diabetes

“…Notably, these insulinproducing cells reverse diabetes in vivo after transplantation into rodents (52)(53)(54). Although these pioneering protocols recaptured well-differentiated mono-hormonal b cell features, such as coexpression of PDX1, NKX6-1 and insulin/c-peptide and high glucose-stimulated insulin secretion, cells exhibited slow insulin secretion speeds in response to glucose and aberrant Ca 2+ signaling, which are critical for insulin exocytosis (52,53) (55)(56)(57)(58)(59)(60)(61)(62)(63). MAFA regulates INS and G6PC2 gene expression, which are required for enhancing insulin production, and suppressing insulin secretion at lower glucose concentrations to enhance the amplitude of glucose-stimulated insulin secretion (GSIS), respectively (64)(65)(66).…”