Muscle tissue development (myogenesis) involves the formation of specific fibers (myotubes) from muscle cells (myoblasts). For this to occur, the sequential expression of Myogenic Regulatory Factors (MRFs), such as MyoD and myogenin, is required. The expression of these MRFs is regulated posttranscriptionally by the RNA-binding protein HuR, whereby HuR associates with the 3'-untranslated regions of MyoD and myogenin mRNA, leading to a significant increase in their half-lives. Here we show that the cleavage of HuR by caspases at the aspartate (D) 226 residue is one of the main regulators of its pro-myogenic function. This proteolytic activity generates two cleavage products (CPs), HuR-CP1 and HuR-CP2, that differentially affect the myogenic process. Myoblasts overexpressing HuR-CP1 or the non-cleavable mutant of HuR, HuRD226A, are not able to engage myogenesis, while overexpressing HuR-CP2 enhances myotube formation. HuR-CP2 but not -CP1 promotes myogenesis by stabilizing the MyoD and myogenin mRNAs to the same levels as wt-HuR. Conversely, the inhibitory effects of HuR-CP1 and HuRD226A depend on their abilities to associate during myogenesis with the HuR import receptor, Trn2, leading to HuR accumulation in the cytoplasm. Therefore, we propose a model whereby the caspase-mediated cleavage of HuR generates two CPs that collaborate to regulate myogenesis; HuR-CP1 by interfering with the Trn2-mediated import of HuR and HuR-CP2 by participating in the stabilization of mRNAs encoding key MRFs.

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Mechanisms of post-transcriptional control are critical to deploy RNAs and proteins asymmetrically to a discrete region of cytoplasm at the posterior of the Drosophila oocyte and embryo, called the pole plasm and thus allow differentiation of the germline. Research presented in this thesis investigates the post-transcriptional control of Drosophila pole plasm component germ cell-less (gcl ). Maternal gcl activity is required for germ cell specification and gcl RNA and protein accumulate asymmetrically in the pole plasm. gcl RNA, but not Gcl protein, is also detected in somatic regions of the embryo, and ectopic expression of Gcl in the soma causes repression of somatic patterning genes suggesting that gcl RNA is subject to translational control. I find that Gcl is expressed during oogenesis, where its expression is regulated by translational repressor Bruno (Bru). Increased levels of Gcl are observed in the oocyte when Bru is reduced (i.e., in an arrest heterozygote) and Bru overexpression reduces the amount of Gcl. Consistent with this, reduction of the maternal dosage of Bru leads to ectopic Gcl expression in the embryo, which, in turn, causes repression of anterior huckebein RNA expression. Bruno binds directly to the gcl3'UTR in vitro, but surprisingly, this binding is largely independent of a Bruno Response Element (BRE) in the gcl 3'UTR and depends upon a novel site. Furthermore, the gcl BRE-like region is not required to repress Gcl expression during oogenesis or embryogenesis. I concluded that Bru regulates gcl translation in a BRE-independent manner. In addition, I established the role of the gcl 3'UTR in gcl RNA localization and translation using transgenes that replace the endogenous 3'UTR with the alpha-tubulin 3'UTR or place it in tandem to the bicoid 3'UTR. I find that accumulation of gcl RNA in the embryonic pole plasm requires the gcl 3'UTR. Moreover, Gel is restricted to the pole plasm by translational repression mediated by the gcl 3'UTR and a limiting pool of trans-acting translational repressors. The phenotypic consequences of loss of this translational control are relatively mild, suggesting that gcl translation does not require stringent repression in the soma.

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