Jasmine Pipella

P16INK4A belongs to the family of proteins called cyclin-dependent kinase inhibitors, which arrest the cell cycle by inhibiting cyclin/cyclin-dependent kinase (CDK) complexes. p16INK4A is best know...

Aims/hypothesis Residual pancreatic beta cells in type 1 diabetes show reduced insulin production but the mechanisms remain unclear. Beta cells undergo stress responses during type 1 diabetes, including endoplasmic reticulum (ER) stress and DNA damage-associated senescence, which may affect insulin production. ER stress reduces insulin production but whether senescence disrupts insulin production in human beta cells has not been investigated. Methods DNA damage-mediated senescence was induced using bleomycin in human donor islets. Relative levels of prohormone convertase 1/3 (PC1/3), prohormone convertase 2 (PC2), carboxypeptidase E (CPE) and the endogenous PC1/3 inhibitor, proprotein convertase subtilisin/kexin type 1 inhibitor (proSAAS), were quantified by western blot. Levels of proinsulin and insulin were measured by ELISA. Flow cytometry was used to measure insulin expression in islet cells. FACS was used to sort endogenous senescent beta cells from islets for analysis of insulin content. Proinsulin immunofluorescence staining was quantified in endogenous senescent vs non-senescent beta cells in pancreas tissue from control donors and donors with type 1 diabetes. Publicly available datasets were used to interrogate relationships between senescence effectors, proinsulin-processing genes and insulin content. DNA damage was induced with bleomycin in the non-proliferative female-fetus-derived EndoC-βH5 human beta cell model to study the impact of the DNA damage response on insulin production in clonal cells growth-arrested due to p16^INK4A expression. Results DNA damage-mediated senescence led to increased PC1/3 without changes in levels of PC2, CPE or proSAAS in human islets. Consistent with these changes, no significant differences in proinsulin or insulin content were observed, compared with control islets. Flow cytometry confirmed maintenance of insulin content in DNA damage-mediated senescent beta cells vs control cells and sorted endogenous senescent beta cells had similar insulin content to non-senescent beta cells. Proinsulin staining was similar in endogenous senescent vs non-senescent beta cells from a control donor and donor with type 1 diabetes. Analysis of proteomics datasets from Humanislets.com and single-cell RNA-seq datasets from the Human Pancreas Analysis Program corroborated these findings. In EndoC-βH5 beta cells, which are growth-arrested, DNA damage led to decreased levels of CPE and proSAAS, and reduced levels of insulin. Conclusions/interpretation Our findings suggest that the expression of proinsulin-processing enzymes and the production of insulin are sustained in both chemically induced DNA damage-related senescence and in endogenous senescent adult human beta cells. Collectively, these findings suggest that senescent beta cells may be a source of insulin production among residual beta cells in type 1 diabetes. Graphical Abstract

Beta cell senescence is an emerging contributor to the pathogenesis of type 1 diabetes, but candidate therapeutic targets have not been identified in human beta cells. In this study, we examined the role of a secreted factor, GDF15, and found that although it is not required to maintain viability during senescence, it is required to fine-tune gene expression programs involved in the senescence response during DNA damage in human beta cells.

Type 1 diabetes results from the poorly understood process of islet autoimmunity, which ultimately leads to the loss of functional pancreatic beta cells. Mounting evidence supports the notion that the activation and evolution of islet autoimmunity in genetically susceptible people is contingent upon early life exposures affecting the islets, especially beta cells. Here, we review some of the recent advances and studies that highlight the roles of these changes as well as antigen presentation and stress response pathways in beta cells in the onset and propagation of the autoimmune process in type 1 diabetes. Future progress in this area holds promise for advancing islet- and beta cell-directed therapies that could be implemented in the early stages of the disease and could be combined with immunotherapies. Graphical Abstract