Deficits in Sialylation Impair Podocyte Maturation

Abstract

The role of sialylation in kidney biology is not fully understood. The synthesis of sialoglycoconjugates, which form the outermost structures of animal cells, requires CMP-sialic acid, which is a product of the nuclear enzyme CMAS. We used a knock-in strategy to create a mouse with point mutations in the canonical nuclear localization signal of CMAS, which relocated the enzyme to the cytoplasm of transfected cells without affecting its activity. Although insufficient to prevent nuclear entry in mice, the mutation led to a drastically reduced concentration of nuclear-expressed enzyme. Mice homozygous for the mutation died from kidney failure within 72 hours after birth. The Cmas nls mouse exhibited podocyte foot process effacement, absence of slit diaphragms, and massive proteinuria, recapitulating features of nephrinknockout mice and of patients with Finnish-type congenital nephrotic syndrome. Although the Cmas nls mouse displayed normal sialylation in all organs including kidney, a critical shortage of CMP-sialic acid prevented sialylation of nephrin and podocalyxin in the maturing podocyte where it is required during the formation of foot processes. Accordingly, the sialylation defects progressed with time and paralleled the morphologic changes. In summary, sialylation is critical during the development of the glomerular filtration barrier and required for the proper function of nephrin. Whether altered sialylation impairs nephrin function in human disease requires further study.

Overexpression of TGF-β Inducible microRNA-143 in Zebrafish Leads to Impairment of the Glomerular Filtration Barrier by Targeting Proteoglycans

“…The direct involvement of podocyte proteoglycans for the proper function of the GFB was further documented by the development of nephrotic syndrome in the absence of these proteins [8,[29][30][31].…”

Systemic Blockade of Sialylation in Mice with a Global Inhibitor of Sialyltransferases

“…These changes are likely a reflection of the loss of sialic acid on podocalyxin, a highly sialylated glycoprotein on glomerular podocytes that forms an electrostatic barrier integral to the filtering capacity of glomeruli (42). Indeed, loss of sialylation on podocalyxin is known to cause kidney dysfunction in patients as well as mice with mutations in glucosamine [UDP-N-acetyl]-2-epimerase/N-acetylmannosamine kinase and cytidine monophosphate N-acetylneuraminic acid synthetase (32)(33)(34). The resialylation of podocalyxin by day 54 as seen by Western blot but continued loss of kidney function may reflect the desialylation on other highly sialylated proteins, such as nephrin, which also serves a critical filtration role in podocytes (34).…”

“…Indeed, loss of sialylation on podocalyxin is known to cause kidney dysfunction in patients as well as mice with mutations in glucosamine [UDP-N-acetyl]-2-epimerase/N-acetylmannosamine kinase and cytidine monophosphate N-acetylneuraminic acid synthetase (32)(33)(34). The resialylation of podocalyxin by day 54 as seen by Western blot but continued loss of kidney function may reflect the desialylation on other highly sialylated proteins, such as nephrin, which also serves a critical filtration role in podocytes (34). To this end, the dramatic loss of sialic acid on podocalyxin that we observed after treatment of mice with 3F-NeuAc strongly correlated with kidney dysfunction and the onset of proteinuria.…”

“…It is noteworthy that although blockade of sialylation with 3F-NeuAc did not alter the viability of cultured cells (29), genetic disruption of sialic acid biosynthesis is embryonically lethal in mice (10). In contrast, transgenic mice with partial disruption of sialic acid biosynthesis (32)(33)(34) or deficiencies in expression of individual sialyltransferases are viable and have very selective and often subtle phenotypes (16 -28), making it difficult to predict the effect of a sialyltransferase inhibitor post-development.…”

Intellectual disability and bleeding diathesis due to deficient CMP–sialic acid transport

“…4,5 Moreover, a number of gene defects in glycosylation pathways are known to cause embryonic lethality. [8][9][10][11][12] Herein, we describe a 22-year-old patient who presented with mental retardation, seizures, and bleeding diathesis, as well as renal and cardiac dysfunction. Golgi-localized sialyltransferases, with differential but partly overlapping acceptor specificities, catalyze the addition of Sia to glycoconjugates.…”