In vivo evidence for GDP-fucose transport in the absence of transporter SLC35C1 and putative transporter SLC35C2

Slc35c1 encodes an antiporter that transports GDP-fucose into the Golgi and returns GMP to the cytoplasm. The closely related gene Slc35c2 encodes a putative GDP-fucose transporter and promotes Notch fucosylation and Notch signaling in cultured cells. Here, we show that HEK293T cells lacking SLC35C1 transferred reduced amounts of O-fucose to secreted epidermal growth factor-like repeats from NOTCH1 or secreted thrombospondin type I repeats from thrombospondin 1. However, cells lacking SLC35C2 did not exhibit reduced fucosylation of these epidermal growth factor-like repeats or thrombospondin type I repeats. To investigate SLC35C2 functions in vivo, WW6 embryonic stem cells were targeted for Slc35c2. Slc35c2[-/-] mice were viable and fertile and exhibited no evidence of defective Notch signaling during skeletal or T cell development. By contrast, mice with inactivated Slc35c1 exhibited perinatal lethality and marked skeletal defects in late embryogenesis, typical of defective Notch signaling. Compound Slc35c1[-/-]Slc35c2[-/-] mutants were indistinguishable in skeletal phenotype from Slc35c1[-/-] embryos and neonates. Double mutants did not exhibit the exacerbated skeletal defects predicted if SLC35C2 was functionally important for Notch signaling in vivo. In addition, NOTCH1 immunoprecipitated from Slc35c1[-/-]Slc35c2[-/-] neonatal lung carried fucose detected by binding of Aleuria aurantia lectin. Given that the absence of both SLC35C1, a known GDP-fucose transporter, and SLC35C2, a putative GDP-fucose transporter, did not lead to afucosylated NOTCH1 nor to the severe Notch signaling defects and embryonic lethality expected if all GDP-fucose transport were abrogated, at least one more mechanism of GDP-fucose transport into the secretory pathway must exist in mammals.

Ablation of the GDP-fucose transporter suppresses lung cancer cell proliferation and migration by reducing expression of PD-L1

Fucosylation, an important post-translational modification, is closely related to the development of tumors. In the microenvironment of lung cancer, expression of PD-L1 and fucosylation is abnormally upregulated. However, the correlation between PD-L1 expression and its fucosylation in lung adenocarcinoma (LUAD) remains unclear. The GDP-fucose transporter (GFT) is a key molecule in cellular fucosylation. To explore the correlation between fucosylation and PD-L1 expression, we knocked out the GFT-encoding gene SLC35C1 in mouse Lewis lung adenocarcinoma cells and in human H1299 lung adenocarcinoma cells. Loss of SLC35C1 impaired the phosphorylation of EGFR and its downstream target ERK. Moreover, loss of SLC35C1 up-regulated the expression of β-TrCP, a PD-L1 E3 ligase, thereby promoting the ubiquitination of PD-L1 and its subsequent degradation. The down-regulated expression of PD-L1 leads to a decline in lung cancer cell proliferation and migration. These results suggest that fucosylation partially influences LUAD tumorigenesis by regulating PD-L1 expression.

Hematologically important mutations: Leukocyte adhesion deficiency (second update)

Leukocyte adhesion deficiency (LAD) is an immunodeficiency caused by defects in the adhesion of leukocytes (especially neutrophils) to the blood vessel wall. As a result, patients with LAD suffer from severe bacterial infections and impaired wound healing, accompanied by neutrophilia. In LAD-I, characterized directly after birth by delayed separation of the umbilical cord, mutations are found in ITGB2, the gene that encodes the β subunit (CD18) of the β₂ integrins. In the rare LAD-II disease, the fucosylation of selectin ligands is disturbed, caused by mutations in SLC35C1, the gene that encodes a GDP-fucose transporter of the Golgi system. LAD-II patients lack the H and Lewis Le^a and Le^b blood group antigens. Finally, in LAD-III, the conformational activation of the hematopoietically expressed β integrins is disturbed, leading to leukocyte and platelet dysfunction. This last syndrome is caused by mutations in FERMT3, encoding the kindlin-3 protein in all blood cells, involved in the regulation of β integrin conformation. This article contains an update of the mutations that we consider to be relevant for the various forms of LAD.

CHO-gmt5, a novel CHO glycosylation mutant for producing afucosylated and asialylated recombinant antibodies

Engineered zinc-finger nucleases (ZFNs) are powerful tools for creating double-stranded-breaks (DSBs) in genomic DNA in a site-specific manner. These DSBs generated by ZFNs can be repaired by homology-directed repair or nonhomologous end joining, in which the latter can be exploited to generate insertion or deletion mutants. Based on published literature, we designed a pair of zinc-finger nucleases and inactivated the GDP-fucose transporter gene (Slc35c1) in a previously reported CHO mutant that has a dysfunctional CMP-sialic acid transporter gene (Slc35a1). The resulting mutant cell line, CHO-gmt5, lacks functional GDP-fucose transporter and CMP-sialic acid transporter. As a result, these cells can only produce asialylated and afucosylated glycoproteins. It is now widely recognized that removal of the core fucose from the N-glycans attached to Asn(297) of human IgG1 significantly enhances its binding to its receptor, FcγRIIIa, and thereby dramatically improves antibody-dependent cellular cytotoxicity (ADCC). Recent reports showed that removal of sialic acid from IgG1 also enhances ADCC. Therefore, CHO-gmt5 may represent a more advantageous cell line for the production of recombinant antibodies with enhanced ADCC. These cells show comparable growth rate to wild type CHO-K1 cells and uncompromised transfection efficiency, which make them desirable for use as a production line.