Characterization of molecular mechanisms involving the transcriptional repression functions of mammalian GrouchoTLE proteins

To investigate the molecular mechanisms regulating mammalian neurogenesis, studies were performed that centered on a family of transcriptional co-repressors whose founding member is Drosophila Groucho (Gro). Genetic studies show that Groucho plays fundamental roles in a variety of cell-differentiation events, including neurogenesis. The overall hypothesis of this thesis is that Groucho's mammalian homologues, the t&barbelow;ransducin-l&barbelow;ike E&barbelow;nhancer of split (TLE) proteins, are general transcriptional co-repressors important in regulating a variety of cell differentiation events in mammals. The main goal of these studies was to elucidate the molecular mechanisms underlying TLE function.Groucho binds to Drosophila H&barbelow;airy/E&barbelow;nhancer of split (HES) bHLH proteins to repress the transcription of genes that promote neuronal fate. TLEs and mammalian HES proteins are shown here to also physically interact. Along with the co-expression of TLE and mammalian HES proteins in neural precursor cells undergoing neuronal differentiation, these results suggest that mammalian TLE/HES complexes function in a manner resembling their Drosophila counterparts.The present studies also show that both TLEs and HES proteins interact functionally with members of the Runt-related protein family, termed the Cbfa proteins. Moreover, TLE and HES proteins are shown to antagonistically contribute towards modulating Cbfa's transactivating ability.Further studies show that TLEs oligomerize and contain two separate transcription repression domains. The TLE oligomerization domain and the amino-terminal repression domain overlap, suggesting that multimerization contributes to the repressive abilities of TLEs.Finally, functional similarities between Groucho/TLEs and the yeast general transcriptional co-repressor, TUP1, were investigated. Groucho and the Drosophila Engrailed homeodomain protein are known to directly interact and repress transcription together. Similarity, TUP1 forms a complex with SSN6 and both TUP1 and SSN6 directly interact with the homeodomain factor alpha2 to repress transcription. TLEs are shown here to complex with UTY/X, mammalian proteins related to the yeast protein SSN6. Furthermore, both TLE and UTY/X are shown to interact with mammalian Engrailed proteins.Taken together, these studies strongly suggest that TLEs are general transcriptional co-repressors that perform analogous developmental functions to Drosophila Groucho and exhibit molecular mechanisms of transcriptional repression similar to those of yeast TUPI