1742-P: Loss of MAFB Compromises a-Cell Identity and Glucagon Secretion in Adult Human Islets Free

Islet-enriched transcription factors (TFs) are critical regulators of α and β cell development and function. One such TF, MAFB, exhibits a distinct expression pattern between species in that it is restricted to α cells postnatally in rodents, but produced in both developing and adult human α and β cells. While MAFB is essential for insulin production and secretion in human embryonic stem cell models, the impact of MAFB loss in mature α and β cells is unknown. Importantly, both T1D and T2D are associated with dramatic loss of islet MAFB expression. To test the hypothesis that MAFB is a critical regulator in adult human islets, dispersed islet cells were infected with lentiviruses expressing control or MAFB knockdown (MAFB^KD) shRNAs, and reaggregated for 6 days to form pseudoislets (PIs; N=6 donors, ages 16-50 years). MAFB^KD PIs exhibited a 56.4±4.2% reduction (P<0.001) in MAFB mRNA compared to control. Likewise, glucagon mRNA and hormone content were reduced by 42.5±6.9 and 28.6±7.6% (P<0.05), respectively, whereas insulin expression and content were unchanged. In static hormone secretion experiments, IBMX- and arginine-stimulated glucagon secretion was blunted in MAFB^KD PIs (~33% lower than control; P<0.05), an observation that was replicated in α cell-enriched MAFB^KD PIs. Insulin secretion, on the other hand, was largely unaffected. Bulk RNA-sequencing analysis of MAFB^KD versus control PIs illustrated downregulation of α cell identity genes, upregulation of β, delta, and gamma cell identity genes, and misexpression of PYY, a gut peptide gene. Upregulated genes were also highly enriched in gene sets associated with epithelial mesenchymal transition and transforming growth factor β signaling. Intriguingly, marker genes in both pathways were shown recently to be upregulated in T2D α cells. Our findings suggest that MAFB is a molecular “gatekeeper” of α cell identity through gene activating and repressive functions, with its loss leading to dysregulation of human glucagon secretion.