Gangliosides (sialylated glycolipids) play an essential role in the CNS by regulating recognition and signaling in neurons. Metabolic blocks in processing and catabolism of gangliosides result in the development of severe neurologic disorders, including gangliosidoses manifesting with neurodegeneration and neuroinflammation. We demonstrate that 2 mammalian enzymes, neuraminidases 3 and 4, play important roles in catabolic processing of brain gangliosides by cleaving terminal sialic acid residues in their glycan chains. In neuraminidase 3 and 4 double-knockout mice, G ganglioside is stored in microglia, vascular pericytes, and neurons, causing micro- and astrogliosis, neuroinflammation, accumulation of lipofuscin bodies, and memory loss, whereas their cortical and hippocampal neurons have lower rate of neuritogenesis Double-knockout mice also have reduced levels of G ganglioside and myelin in neuronal axons. Furthermore, neuraminidase 3 deficiency drastically increased storage of G in the brain tissues of an asymptomatic mouse model of Tay-Sachs disease, a severe human gangliosidosis, indicating that this enzyme is responsible for the metabolic bypass of β-hexosaminidase A deficiency. Together, our results provide the first evidence that neuraminidases 3 and 4 have important roles in CNS function by catabolizing gangliosides and preventing their storage in lipofuscin bodies.-Pan, X., De Britto Pará De Aragão, C., Velasco-Martin, J. P., Priestman, D. A., Wu, H. Y., Takahashi, K., Yamaguchi, K., Sturiale, L., Garozzo, D., Platt, F. M., Lamarche-Vane, N., Morales, C. R., Miyagi, T., Pshezhetsky, A. V. Neuraminidases 3 and 4 regulate neuronal function by catabolizing brain gangliosides.

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By regulating recognition, signaling and physiology of neurons, gangliosides are essential to CNS function. The composition of gangliosides in the brain changes during development and aging being regulated by neuraminidases presumably, which cleave their terminal sialic acid residues. Two mammalian neuraminidases, Neu3 and Neu4 are major candidates for processing brain gangliosides, however, the corresponding knockout mice showed no changes in brain ganglioside metabolism or deficits in brain function.To elucidate the physiological functions of Neu4 and Neu3 in the brain, we generated a gene-targeted mouse strain devoid of both enzymes. At the age of 10 months the mice showed deficits in spatial and short-term memory, whereas neuraminidase activity against gangliosides in their brain tissues was below detection levels. GM3 ganglioside was stored in lysosomes of neurons, microglia and vascular pericytes and GM1-ganglioside decreased in the axons. The number of astrocytes and activated microglia was increased, consistent with neuroinflammation. Cultured hippocampal and cortical neurons also showed reduced levels of GM1 ganglioside in the axons and impaired neuritogenesis.Our data provide evidence that Neu3 and Neu4 have unique roles in the regulation of brain ganglioside composition essential for CNS function: Neu4 generates the neuronal GM1 necessary for axon growth whereas Neu3 catabolizes GM3.Ganglioside expression patterns in the brain are cell and organelle-specific, dynamic and are generated and maintained by highly coordinated processes of their biosynthesis, trafficking, processing and catabolism. General ganglioside content and composition of the mammalian brain also constantly changes over an individual lifetime.Early in development (such as E14 in the rat), the mammalian brain is dominated by GM3 and GD3 4,5 . By the time of birth, GM3 and GD3 become minor components of the brain, with GD1a and GT1b taking their place as the major sialoglycans. Finally, as the brain matures further to adulthood, GM1 and GD1b content increases until the four species, GM1a, GD1a, GD1b, and GT1b are present at comparable levels together representing ~97% of brain gangliosides 6 . Until now, it has been assumed that brain ganglioside composition is mainly regulated by changes in the rate of their biosynthesis in the Golgi apparatus by sialyltransferases that catalyze the transfer of Sia from CMP-Sia to an acceptor carbohydrate reviewed in 2 .Genetic deficiencies of individual sialyltransferases in human patients or in gene-targeted mouse models resulted in both absence of individual gangliosides and in severe neurological manifestations reviewed in 2 . However, transcriptional regulation of the sialyltransferase gene network alone cannot explain how the composition of brain gangliosides changes during development. Mice deficient in individual sialyltransferases lack the whole series of gangliosides: GD3 and GM3 in St8sia1 KO mice, GM1a, GD1a/b and GT1b gangliosides in B4galnt1 KO mice or GD1a and GT1b gangliosides in St3gal2...