Mutation in the key enzyme of sialic acid biosynthesis causes severe glomerular proteinuria and is rescued by N-acetylmannosamine
Belinda Galeano, Riko Klootwijk, Irini Manoli, MaoSen Sun, Carla Ciccone, Daniel Darvish, Matthew F. Starost, Patricia M. Zerfas, Victoria J. Hoffmann, Shelley Hoogstraten-Miller, Donna M. Krasnewich, William A. Gahl, Marjan Huizing
Belinda Galeano, Riko Klootwijk, Irini Manoli, MaoSen Sun, Carla Ciccone, Daniel Darvish, Matthew F. Starost, Patricia M. Zerfas, Victoria J. Hoffmann, Shelley Hoogstraten-Miller, Donna M. Krasnewich, William A. Gahl, Marjan Huizing
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Research Article

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 (GneM712T/M712T) mice did not survive beyond P3. At P2, significantly decreased Gne-epimerase activity was observed in GneM712T/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 GneM712T/M712T pups. Survivors exhibited improved renal histology, increased sialylation of podocalyxin, and increased Gne/Mnk protein expression and Gne-epimerase activities. These findings establish this GneM712T/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.

Authors

Belinda Galeano, Riko Klootwijk, Irini Manoli, MaoSen Sun, Carla Ciccone, Daniel Darvish, Matthew F. Starost, Patricia M. Zerfas, Victoria J. Hoffmann, Shelley Hoogstraten-Miller, Donna M. Krasnewich, William A. Gahl, Marjan Huizing

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Figure 1

Intracellular sialic acid metabolism.

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Intracellular sialic acid metabolism. Cytosolic glucose is converted in ...
Cytosolic glucose is converted in several steps into UDP-GlcNAc, which serves as substrate for the bifunctional, rate-limiting, committed enzyme of sialic acid biosynthesis, GNE/MNK. The GNE catalytic activity (EC 5.1.3.14) epimerizes UDP-GlcNAc to ManNAc, followed by the phosphorylation of ManNAc to ManNAc-6-phosphate (MaNAc-6-P) by the MNK kinase catalytic domain (EC 2.7.1.60). ManNAc-6-P is then further converted into Neu5Ac (sialic acid), which is activated into cytidine monophosphate–sialic acid (CMP–sialic acid) in the nucleus. CMP–sialic acid can subsequently be utilized in the Golgi complex as a substrate for the biosynthesis of sialyl-oligosaccharides by sialyltransferases. Cytosolic CMP–sialic acid displays strong feedback inhibition (dotted line) of GNE enzymatic activity by binding to its allosteric site (1), thereby contributing to the tight regulation of intracellular sialic acid biosynthesis. Even more complexity is added to this pathway by the presence of ancillary kinases, such as GlcNAc kinase (NAGK; EC 2.7.1.59) with high intrinsic MNK activity (19), which can also convert ManNAc to ManNAc-6-P. CTP, cytidine triphosphate; PEP, phosphoenolpyruvate; OGS, oligosaccharides.