Determining the Mechanisms Underlying Islet Dysfunction and Clinical Heterogeneity in Type 1 Diabetes (T1D)

Abstract

Recent observations have challenged the long held concept that all β cells are eventually destroyed in type 1 diabetes (T1D). Indeed, many individuals with T1D produce small amounts of C-peptide long after disease onset and β cells are present within the pancreas even after longstanding disease. In addition, individuals with T1D are unable to mount the appropriate glucagon secretory response to hypoglycemia, contributing to the inaccuracy of exogenous insulin therapy to effectively correct blood glucose levels. The features of these β and α cells are largely unknown due to difficulty of comprehensive investigation of relevant human pancreatic samples. Using a new approach to study the pancreas and isolated islets from 8 individuals with T1D of varying disease duration, we show these remaining β cells maintained their differentiation state (expression of PDX1, NKX6.1, NKX2.2) and had nearly normal insulin secretion (relative to islet insulin content). Unexpectedly, glucagon secretion by T1D α cells was reduced and these cells had alterations in expression of transcription factors constituting α and β cell identity (MAFB, ARX, NKX6.1). In the native pancreas and after placing the T1D islets into a non-autoimmune, normoglycemic in vivo environment, there was no evidence of α-to-β cell conversion however we saw partial recovery of α cell specific markers. Transcriptional profiling of the T1D α cell revealed changes in genes important for α cell identity and function. These results indicate that β cells persisting for many years in T1D maintain features of regulated insulin secretion and suggest a new explanation for the disordered glucagon secretion in the T1D counterregulatory response to hypoglycemia. As part of studies of T1D pancreata, we noted surprising findings in the pancreatic tissue and isolated islets from an additional two donors. DNA sequencing revealed these donors carried variants in genes associated with monogenic forms of diabetes. Integration of clinical information and molecular and cellular analyses in these cases identified what appeared to be T1D was in fact part of a broader spectrum of insulin-deficient diabetes and provides translational insight into incompletely understood forms of diabetes. To further investigate the molecular mechanisms of α cell dysfunction in T1D, we adapted existing methodology to develop a system where human islet dispersion and re-aggregation using a modified hanging droplet method generates human pseudoislets that closely resemble native whole islets in morphology and function. Here, we establish how pseudoislets can be used to study human islet cell physiology and pathophysiology.