Podocyte-Specific Sialylation-Deficient Mice Serve as a Model for Human FSGS
Authors: Niculovic Kristina M. , Blume Linda , Wedekind Henri , Kats Elina , Albers Iris , Groos Stephanie , Abeln Markus , Schmitz Jessica , Beuke Esther , Bräsen Jan H. , Melk Anette , Schiffer Mario , Weinhold Birgit , Münster-Kühnel Anja K. 
Published: April 2019

Significance Statement

Although glycosylation of the endothelial surface layer is known to be crucial for kidney function, the role of podocyte glycosylation is not well understood. The authors demonstrated that podocyte-specific ablation of sialylation in mice caused a phenotype resembling human FSGS. Loss of sialylation resulted in disturbance of podocyte homeostasis and podocyte loss in vivo. In vitro, sialylation-deficient podocytes were able to proliferate and differentiate, but did not grow out of isolated glomeruli; they also showed impaired adhesion to collagen, suggesting a crucial role of sialylation for podocyte interaction with the glomerular basement membrane. These findings strongly implicate sialylation as a factor in FSGS pathogenesis, and it therefore might serve as both a diagnostic marker and a therapeutic target to delay disease progression.

Background

The etiology of steroid-resistant nephrotic syndrome, which manifests as FSGS, is not completely understood. Aberrant glycosylation is an often underestimated factor for pathologic processes, and structural changes in the glomerular endothelial glycocalyx have been correlated with models of nephrotic syndrome. Glycans are frequently capped by sialic acid (Sia), and sialylation’s crucial role for kidney function is well known. Human podocytes are highly sialylated; however, sialylation’s role in podocyte homeostasis remains unclear.

Methods

We generated a podocyte-specific sialylation-deficient mouse model (PCmas−/−
) by targeting CMP-Sia synthetase, and used histologic and ultrastructural analysis to decipher the phenotype. We applied CRISPR/Cas9 technology to generate immortalized sialylation-deficient podocytes (asialo-podocytes) for functional studies.

Results

Progressive loss of sialylation in PCmas−/−
mice resulted in onset of proteinuria around postnatal day 28, accompanied by foot process effacement and loss of slit diaphragms. Podocyte injury led to severe glomerular defects, including expanded capillary lumen, mesangial hypercellularity, synechiae formation, and podocyte loss. In vivo, loss of sialylation resulted in mislocalization of slit diaphragm components, whereas podocalyxin localization was preserved. In vitro, asialo-podocytes were viable, able to proliferate and differentiate, but showed impaired adhesion to collagen IV.

Conclusions

Loss of cell-surface sialylation in mice resulted in disturbance of podocyte homeostasis and FSGS development. Impaired podocyte adhesion to the glomerular basement membrane most likely contributed to disease development. Our data support the notion that loss of sialylation might be part of the complex process causing FSGS. Sialylation, such as through a Sia supplementation therapy, might provide a new therapeutic strategy to cure or delay FSGS and potentially other glomerulopathies.