Abstract
Background
Convergent evidence indicates that deficits in the endosomal recycling pathway underlies pathogenesis of Alzheimer’s disease (AD). SORL1 encodes the retromer‐associated receptor SORLA that plays an essential role in recycling of AD‐associated cargos such as the amyloid precursor protein and the glutamatergic AMPA receptor. Importantly, loss of function pathogenic SORL1 variants are associated with AD. Moreover, both SORLA and retromer protein levels are reduced in the most vulnerable regions of AD brains. Thus, restoration of SORLA levels and function via viral transduction is an attractive therapeutic strategy. Unfortunately, the full‐length SORL1 is too large to be packaged into a viral capsid for gene therapy.
Method
We identified and tested endosomal recycling effects of a truncated SORL1 mini‐gene construct after transient transfections in HEK293 cells. We developed a gene therapy lead by packaging SORL1 mini‐gene into AAV9. This lead was tested for expression and activity in neurons differentiated from human induced pluripotent cells (hiPSC) as well as in vivo after brain infusions in wild‐type mice and SORL1 haploinsufficient Göttingen minipigs by biochemistry and immunohistochemistry.
Result
Expression of the SORL1 mini‐gene in HEK293 cells increased endosomal recycling of full‐length SORLA as shown by elevated cell surface expression and shedding of the SORLA mini receptor. In hiPSC neurons, AAV9‐SORL1 treatment resulted in strong endosomal expression of the SORLA mini‐receptor that co‐localized with components of the retromer complex in endosomes. In vivo, we observed dose‐dependent transduction and expression of SORLA mini‐receptor in neurons, which localized to endosome‐like punctate structures. Evaluation of functional effects is underway.
Conclusion
We conclude that the AAV9‐SORL1 mini‐gene expresses SORLA mini‐receptor in neurons which resides in intracellular vesicular endosomal structures together with the retromer complex and has the capacity to boost the endosomal recycling pathway. Thus, the SORL1 mini‐gene is a promising candidate for gene‐therapy for AD patients.