Cells adapt to extrinsic and intrinsic stresses by rapidly adjusting the rates of protein synthesis. An important mechanism for this translational control involves phosphorylation of eIF2, a translation initiator factor that couples with GTP and delivers Met-tRNAi(Met) to ribosomes. For example during nutrient limitation, GCN2 (EIF2AK4) phosphorylation of eIF2 (eIF2-P) blocks the exchange of eIF2-GDP to eIF2-GTP, thus reducing global translation initiation which allows cells to conserve resources and reprogram gene expression. GCN2 is also activated by UV-B irradiation, disruption in protein degradation, and during differentiation of certain cell types. Coincident with repression of global protein synthesis, eIF2-P enhances translation of select mRNAs, such as ATF4, encoding a transcriptional activator of genes subject to the Integrated Stress Response (ISR). Translational control in the ISR involves upstream open reading frames (uORFs) that serve as ?bar codes? for scanning ribosomes to delineate mRNAs that are preferentially translated from those that are repressed by or indifferent to eIF2-P. It is important to emphasize that a majority of mammalian mRNAs contain uORFs, so their presence alone is not sufficient to direct preferential translation. Rather we determined that the sequence and context of uORFs are critical determinants for preferential translation in the ISR. Furthermore, while diverse stresses induce the ISR, our research suggests that the ISR implementation of translational control provides for different programs of gene expression that are best suited for cell adaptation to a given stress condition. Our hypothesis is that GCN2 is activated by diverse cell perturbations, facilitating patterns of gene expression that are tailored to adapt to a specific stress condition. In this proposal we address important gaps in our knowledge of the ISR. Using innovative concepts and technologies that feature biochemistry, molecular and cellular biology, and genomic and structural perspectives, we will address three fundamental ISR questions. 1) What are the mechanisms by which GCN2 recognizes diverse stress conditions and invokes translational control? 2) How does eIF2-P induce different patterns of mRNA translation, whereby some mRNAs are preferentially translated, whereas others are tolerant of or repressed by eIF2-P? Finally, 3) How does translational control invoked by eIF2-P, combined with stress-induced proteolysis, change the proteome and signaling pathways that direct cell survival? Completion of the proposed studies will garner new insights into the mechanisms by which diverse stresses activate GCN2 and the processes by which ribosomes differentially translate mRNAs. These studies will also provide an understanding for how the ISR contributes to the progression of stress-related diseases, including diabetes and related metabolic disorders, neuropathologies, and cancer, with the promise of developing new strategies for diagnosis and treatment.
This proposal is focused on understanding the molecular mechanisms regulating protein synthesis and stress response pathways and how these processes are beneficial or adverse in a given biomedical context. Specifically, we are addressing how a collection of protein kinases that phosphorylate a translation initiation factor, eIF2, recognize nutrient stresses and disruptions in protein folding and implement differential translational control that serves to repair and sustain cells. This research is important for understanding the progression of many diseases, including diabetes, obesity, and Non-alcoholic fatty liver disease (NAFLD), with a long term goal of development of biomarkers and therapeutic treatments.
