Epitope Mapping of the Anti-Diacylglycerol Kinase Monoclonal Antibody DhMab-4 for Immunohistochemical Analysis

Diacylglycerol kinase (DGK) plays a pivotal role in intracellular signaling pathways in mammals. Activated G protein-coupled receptor activates phospholipase C (PLC) through heterotrimeric G protein, following which PLC hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP₂) into diacylglycerol (DG) and inositol 1,4,5-trisphosphate (IP₃). DGK catalyzes DG phosphorylation to produce phosphatidic acid. DG and phosphatidic acid function as second messengers and their intracellular concentrations are regulated by DGK; therefore, DGK plays an important role in regulating many biological processes. There are ten DGK isozymes, of which DGKη is classified as a type II DGK. Reports have shown that DGKη is associated with several diseases; for example, it is highly expressed in the hippocampus and cerebellum and is a key element in bipolar disorder. Although a DGKη-specific monoclonal antibody (mAb) is necessary to reveal the association between the expression of DGKη and diseases, an anti-DGKη mAb for immunohistochemistry has not yet been established. In this study, we established a specific anti-human DGKη (hDGKη) mAb, DhMab-4 (mouse IgG_2b, kappa). DhMab-4 strongly stained Purkinje cells of human cerebellum in immunohistochemistry analysis. For epitope mapping of DhMab-4, we produced deletion or point mutants of hDGKη and performed western blotting to determine the binding epitope of DhMab-4. DhMab-4 reacted with dN745 mutant but not with dN750 mutant, indicating that the N-terminus of the DhMab-4 epitope is located between amino acids 745 and 750. More detailed analysis using point mutants demonstrated that five mutants, that is, D747A, P748A, F749A, G750A, and T752A, were not detected by DhMab-4. These results indicate that Asp747, Pro748, Phe749, Gly750, and Thr752 are important for DhMab-4 binding to hDGKη.