Epigenetic mechanisms regulating pancreatic beta versus alpha cell fate — ASN Events
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Epigenetic mechanisms regulating pancreatic beta versus alpha cell fate (#8)

Rohan K Humphrey 1 , Rudolf T Pillich 1 , Andrew Hinton 1 , Sumati Gonuguntla 1 , Ana D Lopez 1 , Ryan Anderson 2 , Ulupi S Jhala 1
  1. Pediatric Diabetes Research Center, School of Medicine, University of California San Diego, San Diego, California, USA
  2. Herman B Wells Center for Pediatric Research, School of Medicine, Indiana University, Indiana, Indianapolis, USA

The first two authors contributed equally to this study.

At the center of cell based therapies for treatment of diabetes is the generation of cells that stably and persistently express the beta cell phenotype. In eukaryotic cells, stable commitment to a differentiated phenotype is epigenetically regulated via DNA methylation, histone modification and chromatin remodeling. Therefore, reprogramming embryonic stem cells, or non-beta cell types into insulin producing cells requires an understanding of epigenetic events that determine beta cell identity. 

The homeodomain protein PDX1 is a master regulator of pancreas specification. As development proceeds, PDX1 expression gets progressively restricted to the endocrine beta cell, where it regulates key genes required for optimal beta cell function. Conversely, PDX1 silencing in non-beta cells is equally critical, and forced expression of PDX1 in endocrine progenitor cells results in islets devoid of alpha cells.

Consistent with this notion, we found that PDX1 gene promoter was completely methylated in FACS-sorted primary alpha cells, but no methylation was detected in beta cells. Further analysis showed that the aristaless related factor ARX, a crucial inducer of the alpha cell-fate, actively suppresses PDX1 expression by recruiting a repressor complex including histone demethylase KDM1 and deacetylase HDAC1. Knock-down of ARX expression reversed key repressor histone modifications of PDX1 gene in alpha cells.  Accordingly, injecting zebrafish embryos with ARX morpholinos was sufficient to upregulate PDX1 and insulin in alpha cells.  

Finally, we examined the epigenetic status of PDX1 in human ES cells induced to differentiate into insulin producing cells. Current protocols are limited in that they generate mixed-hormone endocrine cells lacking PDX1 expression. We found that poor PDX1 expression was correlated with promoter methylation. Remarkably, small molecule epigenetic modifiers that derepress PDX1 in alpha cells, shifted the mixed endocrine fate into insulin producing cells. These studies underscore the practical biomedical implications of understanding the role of epigenetics in cell fate determination.