Mutation in the key enzyme of sialic acid biosynthesis causes severe glomerular proteinuria and is rescued by N-acetylmannosamine

Abstract

Mutations in the key enzyme of sialic acid biosynthesis, uridine diphospho-N-acetylglucosamine 2-epimerase/N-acetylmannosamine (ManNAc) kinase (GNE/MNK), result in hereditary inclusion body myopathy (HIBM), an adult-onset, progressive neuromuscular disorder. We created knockin mice harboring the M712T Gne/Mnk mutation. Homozygous mutant (Gne M712T/M712T ) mice did not survive beyond P3. At P2, significantly decreased Gne-epimerase activity was observed in Gne M712T/M712T muscle, but no myopathic features were apparent. Rather, homozygous mutant mice had glomerular hematuria, proteinuria, and podocytopathy. Renal findings included segmental splitting of the glomerular basement membrane, effacement of podocyte foot processes, and reduced sialylation of the major podocyte sialoprotein, podocalyxin. ManNAc administration yielded survival beyond P3 in 43% of the Gne M712T/M712T pups. Survivors exhibited improved renal histology, increased sialylation of podocalyxin, and increased Gne/Mnk protein expression and Gneepimerase activities. These findings establish this Gne M712T/M712T knockin mouse as what we believe to be the first genetic model of podocyte injury and segmental glomerular basement membrane splitting due to hyposialylation. The results also support evaluation of ManNAc as a treatment not only for HIBM but also for renal disorders involving proteinuria and hematuria due to podocytopathy and/or segmental splitting of the glomerular basement membrane.

Deficits in Sialylation Impair Podocyte Maturation

“…This selective phenotypic manifestation clearly differentiates the new mouse model from the earlier described Gne M712T/M712T mouse, 21 in which podocytopathy developed on the background of a constitutively and systemically depressed sialylation (see the defective NCAM polysialylation in Gne M712T/M712T mouse brain). Moreover, Gne M712T/M712T mice are smaller than wt littermates at the day of birth, showing petechial hemorrhages and split GBMs.…”

“…21 Not surprising thus that animal models exhibiting mutations that lead to a general decrease in cellular Sia levels develop nephropathies. 21,30,31 The nls/nls mouse model, however, is the first that unequivocally identified the candidates and processes that crucially depend on sialylation during the construction of the size and charge controlled filtration barrier. Although CMAS mutations in humans presumably result in embryonic lethality and may not be of postnatal clinical relevance, the pathology of acquired sialylation deficiencies caused by neuraminidase producing germs may include targets as described for nls/nls mice.…”

Peracetylated N-Acetylmannosamine, a Synthetic Sugar Molecule, Efficiently Rescues Muscle Phenotype and Biochemical Defects in Mouse Model of Sialic Acid-deficient Myopathy

“…In certain muscle glycoproteins, including ␣-dystroglycan, increased reactivity to peanut agglutinin, a lectin that is reactive to desialylated forms of O-glycans, was observed (30); thus, it was proposed that the stability of such glycoproteins could be influenced by sialylation, and therefore these proteins could be involved in the pathomechanism of DMRV/hIBM. Likewise, it was also hypothesized that hyposialylation of neural cell adhesion molecule and NEP (26,32) may contribute to the symptomatology of disease. In this study, we showed that hyposialylation of transferrin in blood, ␣SG and NEP in skeletal muscle, and podocalyxin in both kidney and skeletal muscles of DMRV/hIBM mouse were almost completely recovered after Ac 4 ManNAc treatment, although until now, the relevance of changes in specific glycoproteins was still being clarified.…”

“…) (32). In the kidney and skeletal muscle of DMRV/hIBM mice, podocalyxin from DMRV/hIBM placebo mice was indeed hyposialylated, migrating more slowly than littermate control (supplemental Fig.…”

Podocalyxin in the Diagnosis and Treatment of Cancer

“…At least some of PC's molecular functions in normal and neoplastic tissues can be explained by viewing PC as a key electrostatic anti-adhesive force between extracellular membrane surfaces (Gelberg et al, 1996;Takeda et al, 2000;Galeano et al, 2007;Strilic et al, 2009). However, since the intracellular domain of PC is phosphorylated on Ser/Thr and Tyr residues and PC assembles signalling complexes in response to cell activation, PC undoubtedly influences the behaviour of tumour cells by regulating intracellular signalling pathways.…”