CD59 double knockout mice express a CD59ba hybrid fusion protein that mediates insulin secretion

CD59 is a cell-surface inhibitor of the terminal step in the complement cascade. However, in addition to its complement inhibitory function, a non-canonical role of CD59 in pancreatic beta cells has been identified. Two recently discovered intracellular alternative splice forms of CD59, IRIS-1 and IRIS-2, are involved in insulin exocytosis through interactions with SNARE-complex components. In mice, the CD59 gene has undergone duplication and to further explore the role of CD59 in insulin secretion, blood glucose homeostasis was studied in a CD59 double knockout (CD59abKO) mouse model. However, no phenotypic deviation related to insulin secretion or blood glucose homeostasis was observed for the CD59abKO mice. Instead, a CD59ba hybrid transcript formed as a consequence of the mutation induced to generate the model was identified. This hybrid transcript is expressed in pancreatic islets of the CD59abKO mice and is comprised of the remaining exons of the two CD59 genes spliced together. Similar to canonical CD59, the CD59ba hybrid was found to be glycosylated and present on the cell surface when exogenously expressed in INS-1 832/13 cells. Furthermore, INS-1 832/13 cells over-expressing the mouse CD59ba hybrid retained normal insulin secretion following siRNA-mediated knockdown of canonical CD59. Hence, although the CD59ba hybrid has lost the complement inhibitory function, the intracellular insulin secretory function remains. These results provide further information concerning the structural requirements of CD59 in its intracellular role relative to its role as a complement inhibitor. It also highlights the importance of carefully assessing plausible consequences of induced mutations in research models.