Endoplasmic Reticulum Alterations in Living Cells Expressing INF2 Variants Causing Glomerulosclerosis and Peripheral Neuropathy

The endoplasmic reticulum (ER) is a single dynamic, and continuous network, which mediates a variety of biological processes. The ER spreads throughout the cytoplasm as an interconnected network mainly consisting of flat (sheets) or reticular (tubules) networks. Such ER integrity is regulated by actin-microtubule interaction. INF2 is an actin assembly factor that exclusively located in ER and is mutated in hereditary form of glomerulopathy (focal segmental glomerulosclerosis, FSGS) and peripheral neuropathy (Charcot-Marie Tooth, CMT-DIE, MIM 614455). It remains unclear how INF2 variants could affect ER morphology. High-resolution, live-imaging of HeLa cells revealed that pathogenic INF2-CAAX variants disrupt peripheral ER complexity, generating focal clustering of polygonal tubules and preferential sheet-like appearance. G73D (causing CMT+FSGS) induced more remarkable alterations than T161N, N202S and R218W (leading to FSGS). Both actin and microtubule inhibitors shifted the ER balance towards sheet predominance and focal compaction of ER tubules, suggesting the role of cytoskeleton in shaping tubular ER networks. INF2 variants induced mitochondria fragmentation with peripheral mis-distribution. The mitochondrial alterations correlated with the degree of cytoskeletal disorganization, leading to defective respiratory function. Moreover, lysosomal trafficking was restricted by INF2 variants in the cell cortex. These organelle-cytoskeletal interactions were more remarkably impaired by CMT+FSGS variant than in FSGS variants. Our observations underscore that INF2 variants disrupt ER integrity by disorganizing cytoskeletons, which leads to defective mitochondria function and vesicle trafficking in INF2 disorders. INF2 CMT+FSGS variants impair ER-organelles interaction more prominent than FSGS variants, suggesting the existence of specific mediators for CMT+FSGS variants.