Cellular expression and localization of DGKζ-interacting NAP1-like proteins in the brain and functional implications under hypoxic stress.

Diacylglycerol kinase (DGK) catalyzes conversion of a lipid second messenger diacylglycerol to another messenger molecule phosphatidic acid. Consequently, DGK plays a pivotal role in cellular pathophysiology by regulating the levels of these two messengers. We reported previously that DGKζ translocates from the nucleus to cytoplasm in hippocampal neurons under ischemic/hypoxic stress. In addition, we also identified nucleosome assembly protein 1 (NAP1)-like proteins NAP1L1 and NAP1L4 as novel DGKζ-interacting partners using a proteomic approach and revealed that these NAP1-like proteins induce cytoplasmic translocation of DGKζ in overexpressed cells because NAP1-like proteins associate with the nuclear localization signal of DGKζ and block its nuclear import via importin α. In the present study, we examined whether NAP1-like proteins are expressed in the brain and whether the molecular interaction of DGKζ and NAP1-like proteins would be changed in the brain after hypoxic stress. Immunohistochemistry revealed that NAP1L1 and NAP1L4 are widely expressed in neurons and glial cells in the brain with some differences. After 3 days of transient whole-body hypoxic stress, DGKζ translocated from the nucleus to cytoplasm in hippocampal pyramidal neurons, whereas NAP1-like proteins remained in the cytoplasm. Contrary to our expectations, NAP1-like proteins showed no change in their expression levels. The molecular interaction between DGKζ and NAP1-like proteins was attenuated after hypoxic stress. These results suggest that DGKζ cytoplasmic translocation in neurons under hypoxic stress is regulated by some mechanism which differs from that mediated by NAP1-like proteins.