Mouse Alport podocytes are susceptible to AAV9 transduction in vivo

Introduction: Alport syndrome features a defective glomerular basement membrane (GBM) due to variants in COL4A3, COL4A4, and COL4A5. The most severe forms, which lack the GBM's collagen α3α4α5(IV) network, progress from hematuria in early childhood to proteinuria, chronic kidney disease, and kidney failure by the age of ∼30. As a monogenic disease without specific treatments, and with podocytes being the only glomerular cells that synthesize collagen α3α4α5(IV), the ability to efficiently deliver genes to Alport podocytes could open up new possibilities for treatment. Methods: As a proof-of-concept study, we investigated whether podocytes in X-linked Alport mice were susceptible to transduction by adeno-associated virus (AAV)9. ssAAV9-CAG-tdTomato and scAAV9-CMV-Cre were intravenously injected into Col4a5^-/y and Col4a5^-/y; Ai14 (Cre-activatable tdTomato) Alport mice, respectively. The kidneys were collected two weeks later and subjected to quantification of podocyte transduction by fluorescence assays using synaptopodin as a podocyte marker. Results: ssAAV9-CAG-tdTomato delivered to controls failed to transduce podocytes, but heterozygous female and hemizygous male Alport podocytes showed a range of transduction efficiencies, from 1.8% to 26%, which correlated positively with levels of albuminuria. Similar correlation between podocyte transduction and albuminuria was observed in the more sensitive Ai14 system with scAAV9-CMV-Cre administration. A subset of tubular and mesangial cells could also be transduced, the former in Alport mice and the latter in both Alport and control mice. Conclusions: The Alport GBM becomes leaky to AAV9 as mice mature, allowing viruses to reach and transduce a substantial subset of podocytes. This is promising for someday using AAV9 or other vehicles in gene therapy for patients with Alport syndrome. Interestingly, mesangial cells of control and young Alport mice were moderately susceptible to transduction, demonstrating that gene delivery to mesangial cells in mice is a viable approach for investigating mesangial cell biology in any context.