Inhibition of Lipid Signaling Enzyme Diacylglycerol Kinase ϵ Attenuates Mutant Huntingtin Toxicity

Abstract

Huntington disease (HD) is a dominantly inherited neurodegenerative disease caused by a polyglutamine expansion in the protein huntingtin (Htt). Striatal and cortical neuronal loss are prominent features of this disease. No disease-modifying treatments have been discovered for HD. To identify new therapeutic targets in HD, we screened a kinase inhibitor library for molecules that block mutant Htt cellular toxicity in a mouse HD striatal cell model, Hdh(111Q/111Q) cells. We found that diacylglycerol kinase (DGK) inhibitor II (R59949) decreased caspase-3/7 activity after serum withdrawal in striatal Hdh(111Q/111Q) cells. In addition, R59949 decreased the accumulation of a 513-amino acid N-terminal Htt fragment processed by caspase-3 and blocked alterations in lipid metabolism during serum withdrawal. To identify the diacylglycerol kinase mediating this effect, we knocked down all four DGK isoforms expressed in the brain (β, γ, ε, and ζ) using siRNA. Only the knockdown of the family member, DGKε, blocked striatal Hdh(111Q/111Q)-mediated toxicity. We also investigated the significance of these findings in vivo. First, we found that reduced function of the Drosophila DGKε homolog significantly improves Htt-induced motor dysfunction in a fly model of HD. In addition, we find that the levels of DGKε are increased in the striatum of R6/2 HD transgenic mice when compared with littermate controls. Together, these findings indicate that increased levels of kinase DGKε contribute to HD pathogenesis and suggest that reducing its levels or activity is a potential therapy for HD.