Research Interests
Transcriptional Regulation in Eukaryotes
Figure 1. Transcriptional activation of a gene.
The expression of eukaryotic genes is precisely regulated in order to carry out defined programs of cell growth and differentiation required for development of an organism. Many genes are controlled at the level of transcriptional initiation, a process orchestrated through a network of interactions between proteins bound to DNA, an array of intermediary factors and the general transcription machinery. The elaborate organization of the eukaryotic genome into highly ordered chromatin structures confers an additional level of regulation to the transcription machinery.
Studies in my laboratory have focused on understanding the molecular details that allow accurate initiation of transcription in response to intra- and extra-cellular signals, an area fundamental to essentially all aspects of biology. These studies investigate the mechanisms underlying the communication initiated by gene-specific transcription factors that result in the recruitment of the general transcription machinery to promoter DNA. The ultimate goal is to unravel at the molecular level how the blueprint of genomic information is transcribed into the complex patterns of regulated gene expression required for vertebrate development.
Although many of the proteins involved in transcription have been identified over the past decade, deciphering how they work together as tightly regulated complex machinery remains one of the exciting challenges in biology. Our laboratory has been studying the role of the transcription factor TFIID in mediating RNA polymerase II-dependent transcriptional activation in human cells. Specifically, we have molecularly cloned and characterized human TBP (TATA box binding protein)-associated factor TAFII130 (recently renamed TAF4), an integral subunit of the TFIID complex. TAF4 serves as a co-activator for a subset of sequence-specific transcriptional activators. Our recent studies have uncovered new functions of TAF4 that promise to lead us in new directions including investigations into human disease mechanisms. It has become increasingly clear that chromatin plays a major role in the regulation of gene expression. We have begun to study biochemical and functional properties of the components of the human SWI/SNF chromatin remodeling complex that plays a fundamental role in transcriptional initiation.
Does TAF4 function as a switch between transcriptionally repressed and activated states?
Modulation of chromatin structure plays a fundamental role in gene expression because transcription factors must contend with the nucleosomes, which are generally inhibitory to transcription. A diverse array of post-translational modifications is made to the core histone tails that are thought to bring about distinct events affecting gene expression. We carried out a yeast two-hybrid screen and identified a novel interaction between TAF4 and HP1, a chromatin-associated protein whose function has been implicated in gene silencing. It is possible that TAF4 interacts with HP1 to facilitate the retention of TFIID in a promoter-specific manner, forming repressed pre-loaded TFIID poised for rapid activation. Work is under way to identify target genes of TAF4 and test the exciting possibility that TAF4 may function as a transcriptional switch in vivo.
Does transcriptional deregulation play a role in the pathogenesis of Huntington's disease?
Defining the molecular mechanisms underlying the pathogenesis of a human disease is an ultimate challenge for researchers in the basic sciences. In collaboration with Dimitri Krainc, M.D., Ph.D. (Massachusetts General Hospital), we have made important discoveries that shed new insights into the pathogenesis of Huntington's disease (HD), an inherited neurodegenerative disorder. HD is caused by an expansion of CAG trinucleotide repeats encoding polyglutamines in huntingtin, a 300 kD protein of unknown function. Although the precise mechanisms by which mutant huntingtin causes neuropathologic damage remains to be determined, we have found that the site-specific transcription factor Sp1 and TAF4, a component of the basal transcription factor TFIID, interact with huntingtin leading to transcriptional deregulation of neuronal genes. Strikingly, over-expression of both Sp1 and TAF4 in cultured striatal cells reversed mutant huntingtin-mediated transcriptional inhibition, and protected neurons from huntingtin-induced cellular toxicity. Collectively, these experiments suggest that huntingtin physically associates with glutamine-rich transcription factors, thereby interfering with their ability to activate select target genes. Future experiments will be aimed at defining the mechanisms of huntingtin-induced repression using biochemical and cell biological methods.
How does mammalian SWI/SNF complex control cell growth?
Eukaryotic genomes require significant compaction in order to fit into the nucleus. This extraordinary degree of packing is achieved by wrapping chromosomal DNA around a core of histones to form nucleosomes, the building blocks of chromatin. Modulation of chromatin structure plays a fundamental role in gene expression because nucleosomes generally inhibit transcription. Genetic and biochemical approaches have led to the discovery of multiple protein complexes that activate or repress transcription by targeting histones or nucleosomes. We previously purified the Brahma chromatin remodeling complex to homogeneity from Drosophila cells and demonstrated that Osa, a protein previously shown to be required for photoreceptor differentiation and embryonic segmentation, is an integral component of the Drosophila SWI/SNF complex (in collaboration with Jessica Treisman, NYU School of Medicine). Our lab subsequently isolated cDNA clones encoding two distinct human homologues of Osa and demonstrated their role in transcriptional activation by steroid hormone receptors. Cell lines induced to over-express hOsa protein exhibit growth and cell cycle defects. Similar to the observations made in flies, hOsa proteins may contribute to activation or repression of target genes in a pathway associated with tumor development. Ongoing research focuses on determining how hOsa regulates specific growth and cell cycle regulatory genes in order to gain better understanding of the processes leading to tumorigenesis.