This project has addressed the role of translation in accelerating the rate of total protein synthesis during load-induced cardiac hypertrophy. Another aspect of hypertrophy is qualitative changes in the expression of a relatively small number of genes, which are required for regulating cardiac growth and remodeling. This renewal will focus on how select proteins are preferentially synthesized in the adult cardiocyte in response to increased load, specifically proteins which function in a regulatory capacity during hypertrophic growth. Individual mRNAs must compete with each other for binding to ribosomes in order to initiate translation. The capability of an individual mRNA molecule to bind ribosomes is determined by intrinsic features such as the amount of secondary structure in the 5'-untranslated region (5'-UTR), and by extrinsic factors such as the activity of translation initiation factors. The majority of mRNAs are considered to be iestrongld with respect to translation because they have minimal secondary structure and/or because they possess features favorable for ribosome binding. Some examples are mRNAs encoding for sarcomeric proteins such as actin and myosin. In contrast, regulatory proteins such as transcription factors, growth factors and cell signalling molecules are derived from mRNAs that tend to have an extensive amount of secondary structure in their respective 5'-UTRs. These mRNAs are considered to be """"""""weak"""""""" with respect to translation. The expression of weak mRNAs is normally low at steady state because ribosomes bind preferentially to the stronger mRNAs. For cardiocyte growth to occur, we posit that translational efficiency of weak mRNAs must be selectively improved relative to strong mRNAs. Therefore, this proposal will test the hypothesis that an increase in load triggers the preferential translation of weaker mRNAs, many of which encode for proteins required for cardiocyte growth.
The specific aims are: 1) to identify mRNAs that are preferentially translated in response to changes in load by measuring the efficiency of candidate mRNAs which have features in the 5'-UTR characteristic of weak (Class I) mRNAs; 2) to examine how structural elements in the 5'-UTR preferentially regulate translational efficiency by generating adenoviral reporter constructs containing 5'-UTRs derived from weak (Class I and Class II) and strong (Class III) mRNAs; 3) to examine the link between structural elements in the 5'-UTR and specific proteins involved in regulating translational efficiency by measuring interactions between eIF-4E and messenger ribonucleoprotein particles (mRNPs). The significance of this proposal is that will examine how gene expression is regulated beyond transcription by addressing how the adult cardiocyte preferentially translates a select set of proteins, which are integral to the growth process.
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