Regulation of Lifespan in Drosophila by Modulation of Genes in the TOR Signaling Pathway

In many species, reducing nutrient intake without causing malnutrition extends lifespan 1, 2, 3. Like DR (dietary restriction), modulation of genes in the insulin-signaling pathway, known to alter nutrient sensing, has been shown to extend lifespan in various species 1, 2, 3, 4. In Drosophila, the target of rapamycin (TOR) and the insulin pathways have emerged as major regulators of growth and size. Hence we examined the role of TOR pathway genes in regulating lifespan by using Drosophila. We show that inhibition of TOR signaling pathway by alteration of the expression of genes in this nutrient-sensing pathway, which is conserved from yeast to human, extends lifespan in a manner that may overlap with known effects of dietary restriction on longevity. In Drosophila, TSC1 and TSC2 (tuberous sclerosis complex genes 1 and 2) act together to inhibit TOR (target of rapamycin), which mediates a signaling pathway that couples amino acid availability to S6 kinase, translation initiation, and growth [5]. We find that overexpression of dTsc1, dTsc2, or dominant-negative forms of dTOR or dS6K all cause lifespan extension. Modulation of expression in the fat is sufficient for the lifespan-extension effects. The lifespan extensions are dependent on nutritional condition, suggesting a possible link between the TOR pathway and dietary restriction

4E-BP Extends Lifespan upon Dietary Restriction by Enhancing Mitochondrial Activity in Drosophila

Dietary restriction (DR) extends lifespan in multiple species. To examine the mechanisms of lifespan extension upon DR, we assayed genome-wide translational changes in Drosophila. A number of nuclear encoded mitochondrial genes, including those in Complex I and IV of the electron transport chain, showed increased ribosomal loading and enhanced overall activity upon DR. We found that various mitochondrial genes possessed shorter and less structured 5′UTRs, which were important for their enhanced mRNA translation. The translational repressor 4E-BP, the eukaryotic translation initiation factor 4E binding protein, was upregulated upon DR and mediated DR dependent changes in mitochondrial activity and lifespan extension. Inhibition of individual mitochondrial subunits from Complex I and IV diminished the lifespan extension obtained upon DR, reflecting the importance of enhanced mitochondrial function during DR. Our results imply that translational regulation of nuclear-encoded mitochondrial gene expression by 4E-BP plays an important role in lifespan extension upon DR

Promoter sequences direct cytoplasmic localization and translation of mRNAs during starvation in yeast.

A universal feature of the response to stress and nutrient limitation is transcriptional upregulation of genes that encode proteins important for survival. Under many such conditions, the overall protein synthesis level is reduced, thereby dampening the stress response at the level of protein expression. For example, during glucose starvation in Saccharomyces cerevisiae (yeast), translation is rapidly repressed, yet the transcription of many stress- and glucose-repressed genes is increased. Here we show, using ribosomal profiling and microscopy, that this transcriptionally upregulated gene set consists of two classes: one class produces messenger RNAs that are translated during glucose starvation and are diffusely localized in the cytoplasm, including many heat-shock protein mRNAs; and the other class produces mRNAs that are not efficiently translated during glucose starvation and are concentrated in foci that co-localize with P bodies and stress granules, a class that is enriched for mRNAs involved in glucose metabolism. Surprisingly, the information specifying the differential localization and protein production of these two classes of mRNA is encoded in the promoter sequence: promoter responsiveness to heat-shock factor 1 (Hsf1) specifies diffuse cytoplasmic localization and higher protein production on glucose starvation. Thus, promoter sequences can influence not only the levels of mRNAs but also the subcellular localization of mRNAs and the efficiency with which they are translated, enabling cells to tailor protein production to the environmental conditions

Promoter sequences direct cytoplasmic localization and translation of mRNAs during starvation in yeast.

A universal feature of the response to stress and nutrient limitation is transcriptional upregulation of genes that encode proteins important for survival. Under many such conditions, the overall protein synthesis level is reduced, thereby dampening the stress response at the level of protein expression. For example, during glucose starvation in Saccharomyces cerevisiae (yeast), translation is rapidly repressed, yet the transcription of many stress- and glucose-repressed genes is increased. Here we show, using ribosomal profiling and microscopy, that this transcriptionally upregulated gene set consists of two classes: one class produces messenger RNAs that are translated during glucose starvation and are diffusely localized in the cytoplasm, including many heat-shock protein mRNAs; and the other class produces mRNAs that are not efficiently translated during glucose starvation and are concentrated in foci that co-localize with P bodies and stress granules, a class that is enriched for mRNAs involved in glucose metabolism. Surprisingly, the information specifying the differential localization and protein production of these two classes of mRNA is encoded in the promoter sequence: promoter responsiveness to heat-shock factor 1 (Hsf1) specifies diffuse cytoplasmic localization and higher protein production on glucose starvation. Thus, promoter sequences can influence not only the levels of mRNAs but also the subcellular localization of mRNAs and the efficiency with which they are translated, enabling cells to tailor protein production to the environmental conditions

Post-transcriptional control of mitochondrial protein composition in changing environmental conditions.

In fluctuating environmental conditions, organisms must modulate their bioenergetic production in order to maintain cellular homeostasis for optimal fitness. Mitochondria are hubs for metabolite and energy generation. Mitochondria are also highly dynamic in their function: modulating their composition, size, density, and the network-like architecture in relation to the metabolic demands of the cell. Here, we review the recent research on the post-transcriptional regulation of mitochondrial composition focusing on mRNA localization, mRNA translation, protein import, and the role that dynamic mitochondrial structure may have on these gene expression processes. As mitochondrial structure and function has been shown to be very important for age-related processes, including cancer, metabolic disorders, and neurodegeneration, understanding how mitochondrial composition can be affected in fluctuating conditions can lead to new therapeutic directions to pursue

An integrated approach reveals regulatory controls on bacterial translation elongation.

Ribosomes elongate at a nonuniform rate during translation. Theoretical models and experiments disagree on the in vivo determinants of elongation rate and the mechanism by which elongation rate affects protein levels. To resolve this conflict, we measured transcriptome-wide ribosome occupancy under multiple conditions and used it to formulate a whole-cell model of translation in E. coli. Our model predicts that elongation rates at most codons during nutrient-rich growth are not limited by the intracellular concentrations of aminoacyl-tRNAs. However, elongation pausing during starvation for single amino acids is highly sensitive to the kinetics of tRNA aminoacylation. We further show that translation abortion upon pausing accounts for the observed ribosome occupancy along mRNAs during starvation. Abortion reduces global protein synthesis, but it enhances the translation of a subset of mRNAs. These results suggest a regulatory role for aminoacylation and abortion during stress, and our study provides an experimentally constrained framework for modeling translation

Stress‐induced mRNP granules: Form and function of processing bodies and stress granules

In response to stress, cells must quickly reprogram gene expression to adapt and survive. This is achieved in part by altering levels of mRNAs and their translation into proteins. Recently, the formation of two stress-induced messenger ribonucleoprotein (mRNP) assemblies named stress granules and processing bodies has been postulated to directly impact gene expression during stress. These assemblies sequester and concentrate specific proteins and RNAs away from the larger cytoplasm during stress, thereby providing a layer of posttranscriptional gene regulation with the potential to directly impact mRNA levels, protein translation, and cell survival. The function of these granules has generally been ascribed either by the protein components concentrated into them or, more broadly, by global changes that occur during stress. Recent proteome- and transcriptome-wide studies have provided a more complete view of stress-induced mRNP granule composition in varied cell types and stress conditions. However, direct measurements of the phenotypic and functional consequences of stress granule and processing body formation are lacking. This leaves our understanding of their roles during stress incomplete. Continued study into the function of these granules will be an important part in elucidating how cells respond to and survive stressful environmental changes. This article is categorized under: Translation > Translation Regulation RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Export and Localization > RNA Localization

Stress‐induced mRNP granules: Form and function of processing bodies and stress granules

In response to stress, cells must quickly reprogram gene expression to adapt and survive. This is achieved in part by altering levels of mRNAs and their translation into proteins. Recently, the formation of two stress-induced messenger ribonucleoprotein (mRNP) assemblies named stress granules and processing bodies has been postulated to directly impact gene expression during stress. These assemblies sequester and concentrate specific proteins and RNAs away from the larger cytoplasm during stress, thereby providing a layer of posttranscriptional gene regulation with the potential to directly impact mRNA levels, protein translation, and cell survival. The function of these granules has generally been ascribed either by the protein components concentrated into them or, more broadly, by global changes that occur during stress. Recent proteome- and transcriptome-wide studies have provided a more complete view of stress-induced mRNP granule composition in varied cell types and stress conditions. However, direct measurements of the phenotypic and functional consequences of stress granule and processing body formation are lacking. This leaves our understanding of their roles during stress incomplete. Continued study into the function of these granules will be an important part in elucidating how cells respond to and survive stressful environmental changes. This article is categorized under: Translation > Translation Regulation RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Export and Localization > RNA Localization

An integrated approach reveals regulatory controls on bacterial translation elongation.

Ribosomes elongate at a nonuniform rate during translation. Theoretical models and experiments disagree on the in vivo determinants of elongation rate and the mechanism by which elongation rate affects protein levels. To resolve this conflict, we measured transcriptome-wide ribosome occupancy under multiple conditions and used it to formulate a whole-cell model of translation in E. coli. Our model predicts that elongation rates at most codons during nutrient-rich growth are not limited by the intracellular concentrations of aminoacyl-tRNAs. However, elongation pausing during starvation for single amino acids is highly sensitive to the kinetics of tRNA aminoacylation. We further show that translation abortion upon pausing accounts for the observed ribosome occupancy along mRNAs during starvation. Abortion reduces global protein synthesis, but it enhances the translation of a subset of mRNAs. These results suggest a regulatory role for aminoacylation and abortion during stress, and our study provides an experimentally constrained framework for modeling translation