Retromer Combinatorials for Gene-Therapy Across a Spectrum of Neurological Diseases

Endosomal trafficking is a biological pathway implicated in Alzheimer’s and Parkinson’s disease, and a growing number of other neurological disorders. For this category of diseases, the endosome’s trafficking complex retromer has emerged as a validated therapeutic target. Retromer’s core is a heterotrimeric complex composed of the scaffold protein VPS35 to which VPS26 and VPS29 bind. Unless it is deficient, increasing expression of VPS35 by viral vectors has a limited effect on other trimeric members and on retromer’s overall function. Here we set out to address these constraints and, based on prior insight, hypothesized that co-expressing VPS35 and VPS26 would synergistically interact and elevate retromer’s trimeric expression and function. Neurons, however, are distinct in expressing two VPS26 paralogs, VPS26a and VPS26b, and so to test the hypothesis we generated three novel AAV9 vectors harboring the VPS35, or VPS26a, or VPS26b transgene. First, we optimized their expression in neuroblastoma cell lines, then, in a comprehensive series of neuronal culture experiments, we expressed VPS35, VPS26a, and VPS26b individually and in all possible combinations. Confirming our hypothesis, expressing individual proteins failed to affect the trimer, while VPS35 and VPS26 combinatorials synergized the trimer’s expression. In addition, we illustrate functional synergy by showing that only VPS35 and VPS26 combinatorials significantly increase levels of Sorl1, a key retromer-receptor deficient in Alzheimer’s disease. Collectively, and together with other recent observations, these results suggest a precision-medicine logic when applying retromer gene therapy to a host of neurological disorders, depending on each disorder’s specific retromer-related molecular and anatomical phenotype.