N-Glycan Branching Is Required for Development of Mature B Cells

Abstract

Galectins have been implicated in inhibiting BCR signaling in mature B cells but promoting pre-BCR signaling during early development. Galectins bind to branched N-glycans attached to cell surface glycoproteins to control the distribution, clustering, endocytosis, and signaling of surface glycoproteins. During T cell development, N-glycan branching is required for positive selection of thymocytes, inhibiting both death by neglect and negative selection via enhanced surface retention of the CD4/CD8 coreceptors and limiting TCR clustering/signaling, respectively. The role of N-glycan branching in B cell development is unknown. In this study, we report that N-glycan branching is absolutely required for development of mature B cells in mice. Elimination of branched N-glycans in developing B cells via targeted deletion of N-acetylglucosaminyl transferase I (Mgat1) markedly reduced cellularity in the bone marrow and/or spleen and inhibited maturation of pre-, immature, and transitional stage 2 B cells. Branching deficiency markedly reduced surface expression of the pre-BCR/BCR coreceptor CD19 and promoted spontaneous death of pre-B cells and immature B cells in vitro. Death was rescued by low-dose pre-BCR/BCR stimulation but exacerbated by high-dose pre-BCR/BCR stimulation as well as antiapoptotic Bcl xL overexpression in pre-B cells. Branching deficiency also enhanced Nur77 induction, a marker of negative selection. Together, these data suggest that, as in T cells, N-glycan branching promotes positive selection of B cells by augmenting pre-BCR/BCR signaling via CD19 surface retention, whereas limiting negative selection from excessive BCR engagement.

N-Glycan Branching Decouples B Cell Innate and Adaptive Immunity to Control Inflammatory Demyelination

“…However, the dramatic reduction in BCR and CD19 surface expression induced by Mgat1 (but not Mgat2) deficiency is dominant, thereby reducing BCR signaling despite a potential decrease in galectin-9 binding to BCR. Consistent with this, we recently observed that deleting Mgat1 in pro/pre-bone-marrow B cells completely blocks development of mature B cells by reducing CD19/BCR signaling (Mortales et al, 2020).…”

“…Mgat1 deficiency is expected to produce a more severe phenotype than Mgat2 deficiency, as the former eliminates, whereas the latter only reduces branching ( Figure S1 A) ( Mkhikian et al, 2016 ; Zhou et al., 2014 ). Indeed, whereas CD19-driven cre-deletion of Mgat2 had no impact on B cell development, deletion of Mgat1 completely blocked B cell development ( Mortales et al., 2020 ). Therefore, to examine Mgat1 deficiency in mature peripheral B cells, we utilized Mgat1 f/f /tetO-cre/ROSA-rtTA mice ( Zhou et al., 2014 ), where doxycycline consistently induces Mgat1 deficiency in ∼35%–40% of mature ex vivo splenic B cells.…”

Abnormal B cell glycosylation in autoimmunity: A new potential treatment strategy

“…Interestingly, α2-6 sialic acid is absent in pSS B cells and barely present in SLE, whereas α-fucose is reduced only in SLE B cells. In-depth characterisations have revealed that memory B cells are remarkably altered, and these glycosylation changes could explain the positive selection of autoreactive B cells ( 49 , 53 ), and the increased activation of BCR caused by the absence of CD22 sialic acid ligands ( 39 , 52 , 54 ). In contrast to pSS B cells, SLE B cells exhibit Gal-GlcNAc/Gal-GalNAc terminal patterns, suggesting a lack of terminal glycosylation.…”

Glycoproteomic analysis of the changes in protein N-glycosylation during neuronal differentiation in human-induced pluripotent stem cells and derived neuronal cells

“…N-glycosylation is one of the most important post-translational protein modifications 1 . N-glycans are involved in various biological phenomena such as development 1 – 3 , pluripotency 4 – 6 , differentiation 5 – 8 , regeneration 9 , 10 , and carcinogenesis 11 , 12 . In the neural system, N-glycans play crucial roles in neuronal system development 13 , 14 , neuroplasticity 14 , 15 , differentiation 7 , 14 , the regulation of signal transduction 13 , and are involved in the onset of neurodegenerative diseases 16 .…”