Elucidating a normal function of huntingtin by functional and microarray analysis of huntingtin-null mouse embryonic fibroblasts

Hua Zhang¹ , Sudipto Das² , Quan-Zhen Li³ , Ioannis Dragatsis⁴ , Joyce Repa¹ , Scott Zeitlin⁵ , György Hajnóczky² and Ilya Bezprozvanny¹

¹Department of Physiology, UT Southwestern Medical Center at Dallas, Dallas, TX 75390, USA

²Department of Pathology and Cell Biology, Thomas Jefferson University, Philadelphia, PA 19107, USA

³Department of Immunology, UT Southwestern Medical Center at Dallas, Dallas, TX 75390, USA

⁴Department of Physiology, The University of Tennessee Health Science Center, Memphis, TN 38163, USA

⁵Department of Neuroscience, University of Virginia School of Medicine, Charlottesville, VA 22908, USA

author email corresponding author email

BMC Neuroscience 2008, 9:38doi:10.1186/1471-2202-9-38


Published:	15 April 2008

Abstract

Background

The polyglutamine expansion in huntingtin (Htt) protein is a cause of Huntington's disease (HD). Htt is an essential gene as deletion of the mouse Htt gene homolog (Hdh) is embryonic lethal in mice. Therefore, in addition to elucidating the mechanisms responsible for polyQ-mediated pathology, it is also important to understand the normal function of Htt protein for both basic biology and for HD.

Results

To systematically search for a mouse Htt function, we took advantage of the Hdh +/- and Hdh-floxed mice and generated four mouse embryonic fibroblast (MEF) cells lines which contain a single copy of the Hdh gene (Hdh-HET) and four MEF lines in which the Hdh gene was deleted (Hdh-KO). The function of Htt in calcium (Ca²⁺) signaling was analyzed in Ca²⁺imaging experiments with generated cell lines. We found that the cytoplasmic Ca²⁺spikes resulting from the activation of inositol 1,4,5-trisphosphate receptor (InsP₃R) and the ensuing mitochondrial Ca²⁺signals were suppressed in the Hdh-KO cells when compared to Hdh-HET cells. Furthermore, in experiments with permeabilized cells we found that the InsP₃-sensitivity of Ca²⁺mobilization from endoplasmic reticulum was reduced in Hdh-KO cells. These results indicated that Htt plays an important role in modulating InsP₃R-mediated Ca²⁺signaling. To further evaluate function of Htt, we performed genome-wide transcription profiling of generated Hdh-HET and Hdh-KO cells by microarray. Our results revealed that 106 unique transcripts were downregulated by more than two-fold with p < 0.05 and 173 unique transcripts were upregulated at least two-fold with p < 0.05 in Hdh-KO cells when compared to Hdh-HET cells. The microarray results were confirmed by quantitative real-time PCR for a number of affected transcripts. Several signaling pathways affected by Hdh gene deletion were identified from annotation of the microarray results.

Conclusion

Functional analysis of generated Htt-null MEF cells revealed that Htt plays a direct role in Ca²⁺signaling by modulating InsP₃R sensitivity to InsP₃. The genome-wide transcriptional profiling of Htt-null cells yielded novel and unique information about the normal function of Htt in cells, which may contribute to our understanding and treatment of HD.