Characterization of a Novel Vesicular Protein of the Pancreatic Beta Cell

Abstract

Type I diabetes results from the autoimmune destruction of pancreatic beta cells, leading to chronic hyperglycemia absent lifelong exogenous insulin treatment. Cell replacement therapy from stem cell derived beta cells offers a promising therapeutic avenue, by restoring endogenous insulin signaling, and a virtually unlimited supply of beta cells for basic science. It is unclear to what degree other beta cell proteins are involved in regulating glucose metabolism. We describe here ERseq08, identified in the lab by a novel sequencing technique, endoplasmic reticulum sequencing (ERseq), indicating the presence of its transcript in beta cells and positioned it as likely secreted from the same vesicles as is insulin. We examined the nature of the protein, determining that its expression pattern is limited to beta cells and other rare endocrine populations, identified the short isoform predominantly expressed, and observed increased expression in more functional beta cell populations. At the protein level, ERseq08 is present in insulin vesicles. We observe homozygous lethality in three separate loss of function alleles. Investigation of this phenotype revealed mild metabolic and secretory defects in the pancreas and liver of neonatal mutants. We also developed and explored a beta-cell specific overexpression allele but observed no significant changes to whole body metabolism with this expression system.