SORL1 and its involvement in Alzheimer's disease pathogenesis and pathophysiology - Project 1: Characterizing the structural biology of SORL1 variants

Columbia University Health Sciences, New York NY

Investigators

Abstract

PROJECT 1 SUMMARY As discussed in the Overview, the PPG will rely on new biological insights into SORL1-retromer’s functions as an endosomal recycling unit to understand how pathogenic variants in the SORL1 gene are linked to both familial and sporadic Alzheimer’s disease. Alzheimer’s disease (AD), the only major cause of death for which no preventive or significant disease-modifying treatment exists, will cost the U.S. a trillion dollars annually by 2050. Only 4 truly causal genes have been identified: APP, PSEN1, PSEN2, and SORL1. Of these, certain alleles of SORL1 cause late onset AD that phenocopies the common sporadic disease. SORL1 encodes a 2214-residue type I transmembrane protein with 24 distinct domains. Its function in neurons is to help traffic membrane proteins such as the glutamate receptor subunit GLUA1, APP, and other cargo to their correct destinations after endocytosis, by forming a specific complex with cargo and with the retromer multiprotein assembly, the master regulator of trafficking out of early endosomes. Many pathogenic mutations seem to cause endosomal traffic jams, ultimately leading to neurodegeneration. Since pathogenic SORL1 variants (>500) are involved in ~3% of all AD cases, it is very important to determine if a given variant is pathogenic, and if the afflicted patient is a candidate for therapy aimed at normalizing protein trafficking out of the endosome. The PPG’s overall approach is for Cores B (genetics) and C (hiPS cells) to first establish and clarify the pathogenicity of the variants so that the Projects can then more deeply probe their biology--- integrating structural (Project 1), cellular (Project 2 and Core D), anatomical (Project 3), and biofluidic (Project 4) biology. The Cores and particularly the Projects are also designed to be innovative in expanding our mechanistic understanding into how SORL1-retromer dysfunction drives and modulates the disease. Project 1 supports the PPG by determining the effects of pathological mutations in the SORL1 gene on the structure, stability and cargo binding of the protein encoded by that variant. Both biochemical (dimerization, binding to the retromer multiprotein complex, and binding of APP) and biophysical (overall and local protein folding, protein stability, dynamical properties of the protein structure) will be measured. In the case of mutations in the VPS10/10CC region of SORL1, the crystal structures of selected mutant proteins will be determined to provide atomic-level details of the effect of the mutation on the local and global conformations of these two domains. In the case of mutations elsewhere in this large protein, we will use a model we have built (employing the deep learning algorithm AlphaFold3) of the ectodomain to aid in structure determination by cryoEM in vivo and in cells, enabling us to predict the likely effects of other mutations on the conformation – and biochemical functions - of SORL1. This information will be incorporated, along with cell biology, histopathology, human genetics and mouse model data from the other PPG Projects and Cores, to provide a comprehensive picture of the relationship between a given variant’s effects on SORL1 structure and function and its pathogenicity in human disease.