3A). The MFG-E8 transcript that included the cryptic exon encoded an MFG-E8 protein that was truncated at the C2 domain (designated as C2del) (Fig. 3A). Studies on mouse and bovine MFG-E8 show that the C1/C2-homologous domains are required for binding to phosphatidylserine 7, 20. To characterize C2del, we prepared human rMFG-E8 using HeLa cell transformants that produced the transgene in a tetracycline-dependent manner. On SDS-PAGE, the purified C2del ran as a smeared band of approximately 50 kDa, which was significantly bigger than the 46-kDa wild-type MFG-E8 (Fig. 3B). This was unexpected considering that C2del had a truncation of 96 amino acids and contained
only one of three N-linked glycosylation sites present in the wild-type protein. The treatment of C2del with PNGase selleck chemicals llc F reduced its molecular weight to 32.6 kDa (Fig. 3C), and a mutation of the remaining N-glycosylation site (Asn238) also reduced its molecular weight (data not shown). Neuraminidase treatment significantly reduced C2del’s molecular weight (Fig. 3D), indicating that it was sialylated. These results suggested that this C-terminal NVP-BEZ235 truncation of human MFG-E8 caused it to be aberrantly glycosylated. We next examined the
ability of C2del to recognize apoptotic cells. As shown in Fig. 3E, C2del dose-dependently bound to phosphatidylserine. The dissociation constants (Kd) determined by Biacore for the wild-type and C2del MFG-E8 Ribose-5-phosphate isomerase were 1.1 and 8.0 nM, respectively. C2del supported phagocytosis with a bell-shaped dosage effect and the same dose dependency as the wild-type molecule (Fig. 3F). However, the ability of C2del to enhance the engulfment at the optimum concentration was consistently lower than that observed with the wild-type MFG-E8. As described above, C2del was aberrantly glycosylated, and in particular, sialylated. The sialylation of proteins is known to prolong their half-life in vivo21, 22. To examine whether this was true for C2del, the wild-type MFG-E8
and C2del proteins were injected into C57BL/6 mice, and their levels in serum were monitored by ELISA. As shown in Fig. 4A, when 12 pmol of the wild-type or mutant MFG-E8 was injected into the tail vein, about 20 pM wild-type MFG-E8 was found in the serum after 60 min, whereas the concentration of C2del was more than 1 nM at the same time point. These results suggested that C2del was sustained longer than the wild-type protein in the blood. We previously showed that excess MFG-E8 prevents the efficient engulfment of apoptotic cells and that some SLE patients carry a significantly increased level of MFG-E8 in their blood 15. Accordingly, the injection of wild-type MFG-E8 into mice induced the development of autoimmune diseases 16. Since C2del lasted longer in vivo than wild-type MFG-E8, we hypothesized that the administration of C2del might cause autoimmune disease in mice at a lower dose than the wild-type molecule. As shown in Fig.