Insulin Null β-cells Have a Prohormone Processing Defect That Is Not Reversed by AAV Rescue of Proinsulin Expression

Area of Science:

• Endocrinology
• Molecular Biology
• Gene Therapy

Background:

• Monogenic diabetes, caused by insulin gene mutations, affects up to 6% of diabetes cases.
• Gene therapy presents a potential treatment for these patients.
• Adeno-associated virus (AAV)-mediated gene therapy is being explored for diabetes treatment.

Purpose of the Study:

• To assess the efficacy of AAV-mediated gene therapy delivering human or mouse insulin genes to beta cells.
• To investigate prohormone processing defects in insulin-deficient mouse models.
• To explore the role of endogenous proinsulin production in beta cell function.

Main Methods:

• Utilized mouse models of permanent neonatal and adult inducible diabetes.
• Employed AAV vectors with rat insulin 1 promoter (Ins1) to deliver human insulin (INS) or mouse insulin 1 (Ins1) genes.
• Assessed beta cell infection, insulin expression, prohormone processing, secretion, and subcellular localization of prohormone convertase 1/3 (PC1/3).

Main Results:

• AAV delivery successfully conferred insulin expression but revealed prohormone processing defects in insulin-null beta cells.
• Secretion of proinsulin transiently reversed diabetes, but processing defects persisted.
• Impaired pro-islet amyloid polypeptide (proIAPP) processing and elevated circulating proIAPP were observed in inducible models.
• Defective sorting of PC1/3 to glycogen-containing vacuoles and ER retention were identified as potential mechanisms.
• Persistent endogenous proinsulin production was found necessary for proper beta cell storage and processing.

Conclusions:

• AAV-mediated insulin gene therapy faces challenges due to beta cell prohormone processing defects.
• Endogenous proinsulin production is crucial for maintaining normal beta cell function, including proinsulin storage and processing.
• Further research is needed to overcome processing defects for effective gene therapy in monogenic diabetes.