1770-P: Islet ß-Cells Shed DOC2B-Laden EVs via Sorting Signals Present in DOC2B Protein Free

Loss of β-cell function occurs early in type 1 diabetes (T1D). However, C-peptide is insufficient to detect early pre-type 1 diabetes-related β-cell dysfunction. Thus, the discovery of clinically accessible β-cell health biomarkers may enable early therapeutic interventions, mitigate T1D, and prevent complete loss of β-cell mass. The exocytosis regulatory protein double C2-containing protein β (DOC2B) abundance is closely related to β-cell health and function. Our recent publication demonstrated the correlation between the decline in DOC2B abundance in blood-derived platelets and the loss of β-cell function in early-onset T1D and pre-diabetic nonobese diabetic (NOD) mice. Indeed, preliminary data show that DOC2B reduction in plasma precedes C-peptide reduction in prediabetic mice, suggesting that reduced DOC2B levels in plasma may serve as an early biomarker of β-cell dysfunction. However, the mechanisms by which β-cells export DOC2B into the circulation remain unknown. We hypothesize that DOC2B is uniquely sorted into β-cell extracellular vesicles (EVs). Preliminary data show DOC2B is detected in EVs isolated from human plasma, and that DOC2B-laden EVs can stem from β-cells. Because DOC2B is a broadly expressed protein, we questioned whether DOC2B-laden EVs are predominantly released by β-cells, relative to that of other cell types known to express DOC2B protein. Biochemical analyses reveal that DOC2B largely exists as luminal cargo in β-cell-derived EVs and is largely absent from EVs released by liver, brain, and skeletal muscle cells. Mechanistically, the research has delineated that the sufficient structural domain enabling DOC2B sorting into β-cell-derived EVs is that which encompasses the tandem C2 domains at the C-terminus of DOC2B. Taken together, these data indicate that DOC2B in circulation could originate from DOC2B-laden EVs released from β-cells, and that the mechanism by which DOC2B is sorted into those EVs involves one or both C2 domains of the DOC2B protein.