br DAG kinase activity is confined to specific cell compartm

DAG kinase activity is confined to specific cell compartments A number of reports demonstrating agonist-dependent translocation of DGKs to distinct membrane compartments suggest that DGK activity is restricted to localized DAG pools generated after activation of receptors. Perhaps the best evidence of spatially restricted DAG kinase activity was demonstrated by van der Bend et al. [69]. This group measured DAG kinase activity in cells following receptor activation—which caused physiological DAG production—or after treating the cells with exogenous PLC—which caused global, nonspecific DAG generation. They detected significant DAG kinase activity upon activating a receptor, but found very little DAG kinase activity after treating the cells with exogenous PLC. Their data suggested that DGKs are active only in spatially restricted compartments following physiological generation of DAG. Consistent with this conclusion, Nurrish et al. [12] found in C. elegans that dgk-1, an ortholog of human DGKθ, regulated DAG signalling that was necessary for GSK 4112 release. Their data suggested a model where serotonin signalling—which inhibits locomotion—activated the DGK to reduce DAG accumulation. DGKs appear to be active in a number of cell compartments. For example, Nagaya et al. [21] demonstrated that overexpressed DGKδ partly localized in the endoplasmic reticulum, while Abramovici et al. [70] found endogenous DGKζ at the neuromuscular junction. Several groups have noted DGK activity in the cell fractions containing cytoskeleton components. For example, Tolias et al. [71] noted that DGK activity associated with a complex of proteins including a PIP5K, Rac, Rho, Cdc42, and Rho-GDI, all of which regulate cytoskeleton dynamics. We found that several DGK isotypes co-immunoprecipitated with either Rac, Rho, or Cdc42 when overexpressed in cells (M.K.T. and B.L., unpublished observations), and Houssa et al. [44] showed that active RhoA associated with DGKθ. Additionally, we found that DGKζ interacted with human PIP5K type Iα and increased its activity by generating phosphatidic acid [56]. The physiological significance of these interactions is not entirely clear, but there are data demonstrating that DGKs can modulate cytoskeleton remodeling. For example, DGK inhibitors—which primarily affect type I enzymes—augmented platelet secretion and aggregation [72], and Abramovici et al. [70] recently demonstrated that expression of a DGKζ mutant that localized strongly with the plasma membrane enhanced membrane ruffles and caused the formation of large intracellular vesicles. Consistent with an effect on cytoskeleton dynamics, endogenous DGKζ co-purified with components of the cytoskeleton [70] and it localized at the leading edge of both glioblastoma cells [73] and C2 myoblasts [70]. Together, these data suggest that DGKs have a broad role in regulating the cytoskeleton, but at this point, their specific roles are not clear.