Lack of lacto/neolacto-glycolipids enhances the formation of glycolipid-enriched microdomains, facilitating B cell activation

In a previous study, we demonstrated that β1,3-N-acetylglucosaminyltransferase 5 (B3gnt5) is a lactotriaosylceramide (Lc₃Cer) synthase that synthesizes a precursor structure for lacto/neolacto-series glycosphingolipids (GSLs) in in vitro experiments. Here, we generated B3gnt5-deficient (B3gnt5 -/- ) mice to investigate the in vivo biological functions of lacto/neolacto-series GSLs. In biochemical analyses, lacto/neolacto-series GSLs were confirmed to be absent and no Lc₃Cer synthase activity was detected in the tissues of these mice. These results demonstrate that β3GnT5 is the sole enzyme synthesizing Lc₃Cer in vivo. Ganglioside GM1, known as a glycosphingolipid-enriched microdomain (GEM) marker, was found to be up-regulated in B3gnt5 -/- B cells by flow cytometry and fluorescence microscopy. However, no difference in the amount of GM1 was observed by TLC-immunoblotting analysis. The GEM-stained puncta on the surface of B3gnt5 -/- resting B cells were brighter and larger than those of WT cells. These results suggest that structural alteration of GEM occurs in B3gnt5 -/- B cells. We next examined whether BCR signaling-related proteins, such as BCR, CD19, and the signaling molecule Lyn, had moved into or out of the GEM fraction. In B3gnt5 -/- B cells, these molecules were enriched in the GEM fraction or adjacent fraction. Moreover, B3gnt5 -/- B cells were more sensitive to the induction of intracellular phosphorylation signals on BCR stimulation and proliferated more vigorously than WT B cells. Together, these results suggest that lacto/neolacto-series GSLs play an important role in clustering of GEMs and tether-specific proteins, such as BCR, CD19, and related signaling molecules to the GEMs.