How Huntingtin May Regulate Transcription

Abstract

Polyglutamate expansion in the huntingtin (Htt) protein is responsible for the neurodegenerative disease Huntington's disease. Marcora et al. performed a yeast two-hybrid screen for partners of the neural and endocrine transcription factor NeuroD, which is important for neuronal and pancreatic beta cell survival in mammals, and identified huntingtin-associated protein 1 (HAP1) and mixed lineage kinase 2 (MLK2), both of which were known to interact with Htt. Coimmunoprecipitation experiments in transfected mouse neuroblastoma cells (N2A) showed that when NeuroD was coexpressed with HAP1, Htt also was precipitated. In cells transfected with both NeuroD and MLK2, NeuroD exhibited a shift in mobility consistent with phosphorylation by MLK2. Furthermore, MLK2 phosphorylated NeuroD in vitro; however, only NeuroD immunoprecipitated from cells was an adequate substrate--recombinant NeuroD was not efficiently phosphorylated. Using an ectopic neurogenesis assay in Xenopus embryos, the authors showed that coexpression of NeuroD and MLK2 stimulated ectopic neurogenesis more frequently that did NeuroD alone. Coexpression of a partial Htt (containing the interaction domains for HAP1 and MLK2), NeuroD, HAP1, and MLK2 increased the frequency of ectopic neurogenesis from 15% in response to NeuroD alone to 86% in the quadruple expression condition. A mutant version of Htt also promoted neurogenesis in the assay, which is consistent with the known ability of mutant Htt to support embryonic neurogenesis. Thus, Htt and HAP1 may serve to promote the interaction of NeuroD and MLK2, thereby stimulating NeuroD activity. These results may begin to answer why Htt is selectively toxic to neurons, because they link Htt to the activity of a neuronal specific transcription factor. E. Marcora, K. Gowan, J. E. Lee, Stimulation of NeuroD activity by huntingtin and huntingtin-associated proteins HAP1 and MLK2. Proc. Natl. Acad. Sci. U.S.A. 100 , 9578-9583 (2003). [Abstract] [Full Text]