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X. Ma et al.
such as platelet homeostasis, has just begun to be appreci- ated.
of CMP-SA biosynthesis. In the de novo pathway, sialic acids are synthesized in the cytoplasm from the substrate uridine diphosphate (UDP)-N-acetylglucosamine through serial reactions catalyzed by the enzymes UDP-N-acetyl- glucosamine 2-epimerase, N-acetylneuraminic acid syn- thase, and N-acetylneuraminate-9-phosphate-phos- phatase. On the other hand, sialic acid can also be gener- ated during desialylation of sialylated molecules. Sialic acids are attached to CMP catalyzed by CMP-SA synthase in the nucleus, and subsequently CMP-SA is transported into the Golgi lumen by the CMP-sialic acid transporter SLC35A1.
There are more than 20 known genes in mice and humans encoding sialyltransferases. a2,3-linked sialic acids on glycans of megakaryocytes and platelets are gen- erated by six different a2,3-sialyltransferases (ST3Gal-I – VI). Studies with mice lacking ST3Gal-I or ST3Gal-IV have shown the importance of sialylation in platelet biology, especially platelet clearance by hepatocytes and/or Küpffer cells.17 However, due to redundant expression of these sialyltransferases in platelets, such as ST3Gal-I, ST3Gal-IV, and ST6Gal-I,18,19 the overall function of sialy- lation in platelet homeostasis remains to be studied.
The CMP-SA transporter (CST) is encoded by the SLC35A1 gene (Figure 1A). Several patients with SLC35A1 mutations have been reported and are described as having
a2,3-linked sialic acid is the main form of platelet sialy- lation and is commonly linked to the penultimate galac- tose (Gal) or N-acetylgalactosamine (GalNAc) on complex N-glycans and O-glycans.7 The sialylation level on platelets is reduced in conditions such as cold storage of platelets, sepsis, and a subset of immune thrombocytope- nia.10,11 It has been reported that desialylated platelets express terminal galactose and are cleared by hepatocytes through interactions with hepatic asialoglycoprotein receptor.12-14 This mechanism is considered to regulate nor- mal platelet homeostasis and contributes to thrombocy- topenia under pathological conditions. Our recent study revealed that Küpffer cells, rather than hepatocytes, phagocytose desialylated or O-glycan-deficient platelets.7 Glycosylation is also important for megakaryocytopoiesis and platelet production. Mice with constitutive or inducible global loss of O-glycans exhibited thrombocy- topenia due to defects in terminal megakaryocyte differ- entiation and platelet production, demonstrating that O- glycosylation is critical for thrombocytogenesis.15,16
Sialylation occurs in the trans-Golgi by transferring sialic acids from the donor substrate cytidine-5’-monophos- phate-sialic acid (CMP-SA) to acceptor N- and/or O-gly- cans (Figure 1A).8,9 There are de novo and salvage pathways
A
B
CD
Figure 1. Generation of mice with megakaryocyte- and platelet-spe- cific deletion of Slc35a1. (A) The function of Slc35a1-encoded CMP-sialic acid transporter (CST). (B) Targeting strategy for condi- tional deletion of the Slc35a1 gene. Diagram of wild-type (WT)
(Slc35a1), loxP
(Slc35a1f/f), and null (Slc35a1–/–) alleles of Slc35a1. Arrowheads indicate the position of loxP. (C) Genotyping polymerase chain reaction of tail genomic DNA from offspring was used to identify Plt Slc35a1–/– mice. Lane 3 repre- sents the genotype of Plt Slc35a1–/– mice. (D) Slc35a1 gene expression in WT and Plt Slc35a1–/– mice was determined by quantitative reverse transcrip- tase polymerase chain reaction using RNA extracted from bone marrow megakaryocytes (n=3 for each group).
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haematologica | 2021; 106(3)