Investigating the Functions of Endogenous Neuraminidases Neu1 and Neu3 in Blood Cell and Protein Homeostasis

Abstract

Neuraminidases cleave terminal sialic acid linkages on the glycans of glycoproteins and glycolipids. In some cases this exposes underlying galactose linkages that can be recognized by lectins including the endocytic hepatic Ashwell‐Morell receptor that thereby modulate protein function through endocytic clearance. Neuraminidases are widely distributed in vertebrates and microorganisms, although the neuraminidases of mammalian origin seems to differ from the microbial variants in genetic sequences and enzymatic properties. Four mammalian neuraminidases, encoded by NEU1‐4, have been identified and characterized. Mice deficient in the lysosomal resident Neu1 have been reported with molecular features resembling sialidosis, a severe human lysosomal storage disorder, whilst Neu3 deficiency does not result in an obvious physiological phenotype. Neu1 is found in a complex with b‐galactosidase and protective protein/cathepsin A in the lysosome. The subcellular localization of Neu3 has been suggested to be the plasma membrane and endosomal compartments. Interestingly, the membrane attachment mechanism and topology is yet to be fully clarified, however Neu3 is commonly reported to be membrane‐associated. A variety of mammalian cell types express neuraminidases, including endothelial cells as well as hematopoietic lineages spanning platelets, monocytes, and the erythrocyte surface. Both Neu1 and Neu3 have been found in the bloodstream and in extracellular compartments, by one or more secretion mechanisms that are not fully defined. We have previously noted that circulating neuraminidase activity is measurable in normal mammalian blood at physiological pH, and de‐sialylate plasma proteins with increasing molecular age, and this was further demonstrated to result in lectin‐mediated clearance in the determinations of glycoprotein half‐lives. To further investigate the roles of each of these mammalian neuraminidases, we are using multiple approaches in Neu1‐ and Neu3‐deficient mice, including measurements with novel lectins that detect different types of sialic acid linkages and their modifications, among the endothelium and the surfaces of hematopoietic cells in parallel with comparative blood proteomics, flow cytometry, and other molecular identification strategies. These studies will be presented with our current findings that may provide further information regarding mammalian Neu activation and function in glycoprotein homeostasis.Support or Funding InformationNIH grant HL131474.Post Doctoral Fellowship: Swedish Research Council dnr 2017‐00192.