Synaptopodin deficiency exacerbates kidney disease in a mouse model of Alport syndrome

Abstract

Synaptopodin (Synpo) is an actin-associated protein in podocyte foot processes. By generating mice that completely lack Synpo, we previously showed that Synpo is dispensable for normal kidney function. However, the lack of Synpo worsened Adriamycin nephropathy, indicating a protective role for Synpo in injured podocytes. Here we investigated whether the lack of Synpo directly impacts a genetic disease, Alport syndrome (AS), because Synpo is reduced in the podocytes of affected humans and mice; whether this is merely an association or pathogenic is unknown. We used Col4a5 mutant mice that model X-linked AS, showing glomerular basement membrane (GBM) abnormalities, eventual foot process effacement, and progression to ESKD. We intercrossed mice carrying mutations in Synpo and Col4a5 to produce doubly mutant mice. Urine and tissue were taken at select time points to evaluate albuminuria, histopathology, and glomerular capillary wall composition and ultrastructure. The lack of Synpo in Col4a5-/Y, Col4a5-/-, or Col4a5+/- Alport mice led to acceleration of disease progression, including more severe proteinuria and glomerulosclerosis. The absence of Synpo attenuated the shift of myosin IIA from the podocyte cell body and major processes to the actin cables near the GBM in the areas of effacement. We speculate that this is mechanistically associated with enhanced loss of podocytes due to easier detachment from the GBM. We conclude that Synpo deletion exacerbates the disease phenotype in Alport mice, revealing the podocyte actin cytoskeleton as a target for therapy in patients with AS.

Synaptopodin stress fiber and contractomere at the epithelial junction

“…Now we know that synaptopodin depleted cells are missing stress fibers and contractomeres. Synaptopodin depletion decreases the rate of wound migration in endothelial cells, epithelial cells, and podocytes ( Anekal et al, 2015 ; Ning et al, 2020 ; Ning et al, 2021 ; Rochman et al, 2017 ), which is consistent with the role of stress fibers in lamella formation in migrating cells ( Burridge and Guilluy, 2016 ). Several studies have also shown that epithelial integrity is disrupted to a greater extent in synaptopodin depleted cells when challenged by chemical insult, indicating a protective role and a survival advantage provided by synaptopodin.…”

“…Synaptopodin is essential for the maintenance of permeability barrier in multiple systems including MDCK cells, T84 intestinal cells, mouse intestine, and kidney podocytes ( Kannan and Tang, 2015 ; Ning et al, 2020 ; Ning et al, 2021 ; Wang et al, 2020 ). However, the mechanism for its protective function is not known.…”

Enhanced podocyte differentiation and changing drug toxicity sensitivity through pressure-controlled mechanical filtration stress on a glomerulus-on-a-chip

“…These results demonstrate that the filtration, the vertical shear stress stimulus against to the cell adhesion surface, promotes the expression of multiple podocyte differentiation markers. Moreover, based on the previous reports that SYNPO, PODXL, CD2AP, VEGFA expression contributes to the maintenance of foot process structure in podocytes [59][60][61][62] , these results suggest that physical filtration stimuli have the potential to induce and maintain the process-like structure in podocytes as shown in Fig. 5 via upregulated SYNPO, PODXL, CD2AP and VEGFA expression.…”

An ex vivo culture model of kidney podocyte injury reveals mechanosensitive, synaptopodin-templating, sarcomere-like structures

“…3E ). Although no specific pathogenic phenotype has been observed in synaptopodin knockout mice at baseline, the loss of synaptopodin exacerbates both drug-induced and genetic kidney injuries ( 10 , 23 ). Our ex vivo system may be useful for further clarifying synaptopodin’s roles in podocyte injury and kidney glomerular disease.…”

“…Studies of glomeruli in vivo have revealed that in mouse models of kidney diseases, podocytes contain the normally absent contractile protein myosin IIA in the basal aspect of the areas of foot process effacement, and the injured podocytes develop sarcomere-like structures (SLSs) ( 22 ). On the basis of these in vivo data, we have speculated that SLSs are associated with responses to mechanobiological cues associated with pathology and possibly associated with podocyte migration and healing ( 10 , 23 ). However, because SLSs are difficult to study in vivo because of their nanoscale size and because they have not been observed in any current in vitro systems, this hypothesis has not been possible to test.…”