mTOR signaling in growth control and disease

The mechanistic target of rapamycin (mTOR) signaling pathway senses and integrates a variety of environmental cues to regulate organismal growth and homeostasis. The pathway regulates many major cellular processes and is implicated in an increasing number of pathological conditions, including cancer, obesity, type 2 diabetes, and neurodegeneration. Here, we review recent advances in our understanding of the mTOR pathway and its role in health, disease, and aging. We further discuss pharmacological approaches to treat human pathologies linked to mTOR deregulation.     

New modes of translational control in development, behavior, and disease

Over the last 10 years, the field of translational control has been enriched by atomic resolution structures of ribosomal complexes and factors in different functional states, and increased in sophistication by wedding genetics, reconstituted systems, and structural biology to elucidate basic reactions and mRNA-specific control mechanisms. New regulatory principles have emerged, including repression by micro-RNAs (miRNAs) and mRNA sequestration in cytoplasmic granules, and the field has extended its reach into development, brain function, and human disease. Here we seek to highlight some of the exciting developments of the last decade from the perspectives of our own approaches and expertise; accordingly, many highly noteworthy achievements could not be mentioned and we refer interested readers to the numerous excellent reviews cited below.     

Staurosporine inhibits phosphorylation of translational regulators linked to mTOR

Treatment of Swiss 3T3 cells with staurosporine resulted in dephosphorylation of two proteins which play key roles in regulating mRNA translation. This occurred before the execution of apoptosis, assessed by caspase-3 activity. These translation regulators are p70 S6 kinase, which phosphorylates ribosomal protein S6, and eukaryotic initiation factor (eIF) 4E binding protein 1 (4E-BP1), which both lie downstream of the mammalian target of rapamycin (mTOR). This resulted in decreased p70 S6 kinase activity, dephosphorylation of ribosomal protein S6, increased binding of 4E-BP1 to eIF4E and a concomitant decrease in eIF4F complexes. Our data show that staurosporine impairs mTOR signalling in vivo but that this not due to direct inhibition of mTOR or to inhibition of protein kinase C. It is becoming clear that agents which cause apoptosis inactivate mTOR signalling as a common early response prior to the execution of apoptosis, i.e., before caspase activation.     

Cell-cycle-dependent translational control

Control of translation in eukaryotes occurs mainly at the initiation step. Translation rates in mammals are robust in the G1 phase of the cell cycle but are low during mitosis. These changes correlate with the activity of several canonical translation initiation factors, which is modulated during the cell cycle to regulate translation.     

Cytoplasmic polyadenylation and translational control

Cytoplasmic polyadenylation is the process by which dormant, translationally inactive mRNAs become activated via the elongation of their poly(A) tails in the cytoplasm. This process is regulated by the conserved cytoplasmic polyadenylation element binding (CPEB) protein family. Recent studies have advanced our understanding of the molecular code that dictates the timing of CPEB-mediated poly(A) tail elongation and the extent of translational activation. In addition, evidence for CPEB-independent mechanisms has accumulated, and the breath of biological circumstances in which cytoplasmic polyadenylation plays a role has expanded. These observations underscore the versatility of CPEB as a translational regulator, and highlight the diversity of cytoplasmic polyadenylation mechanisms.     

mTOR signaling in disease

The target of rapamycin (TOR) is a highly conserved serine/threonine kinase and a central controller of cell growth, metabolism and aging. Mammalian TOR (mTOR) is activated in response to nutrients, growth factors and cellular energy. Dysregulated mTOR signaling has been implicated in major disease. Here we review recent findings on the role of mTOR in cancer, metabolic disorders, neurological diseases, and inflammation.     

Transient CPEB dimerization and translational control

During oocyte development, the cytoplasmic polyadenylation element-binding protein (CPEB) nucleates a set of factors on mRNA that controls cytoplasmic polyadenylation and translation. The regulation of polyadenylation is mediated in part through serial phosphorylations of CPEB, which control both the dynamic integrity of the cytoplasmic polyadenylation apparatus and CPEB stability, events necessary for meiotic progression. Because the precise stoichiometry between CPEB and CPE-containing RNA is responsible for the temporal order of mRNA polyadenylation during meiosis, we hypothesized that, if CPEB production exceeded the amount required to bind mRNA, the excess would be sequestered in an inactive form. One attractive possibility for the sequestration is protein dimerization. We demonstrate that not only does CPEB form a dimer, but dimerization requires its RNA-binding domains. Dimer formation prevents CPEB from being UV cross-linked to RNA, which establishes a second pool of CPEB that is inert for polyadenylation and translational control. During oocyte maturation, the dimers are degraded much more rapidly than the CPEB monomers, due to their greater affinity for polo-like kinase 1 (plx1) and the ubiquitin E3 ligase β-TrCP. Because dimeric CPEB also binds cytoplasmic polyadenylation factors with greater affinity than monomeric CPEB, it may act as a hub or reservoir for the polyadenylation machinery. We propose that the balance between CPEB and its target mRNAs is maintained by CPEB dimerization, which inactivates spare proteins and prevents them from inducing polyadenylation of RNAs with low affinity binding sites. In addition, the dimers might serve as molecular hubs that release polyadenylation factors for translational activation upon CPEB dimer destruction.     

Location,location, location: translational control in development and neurobiology

Control of gene expression by regulating mRNA translation is essential for proper growth and development in species ranging from yeast to mammals. Recently, translational regulation has also been implicated in neuronal function, where long-lasting changes in synaptic strength and the formation of memory require new protein synthesis. Crucial to both developmental and neurobiological function, translational control allows for the asymmetric distribution of gene products and a rapid and local response to stimulation--as discussed at a recent Serono Foundation EMBO Workshop held in Mallorca, Spain.     

Mutational analysis of sites in the translational regulator, PHAS-I, that are selectively phosphorylated by mTOR.

Results obtained with PHAS-I proteins having Ser to Ala mutations in the five known phosphorylation sites indicate that mTOR preferentially phosphorylates Thr36 and Thr45. The effects of phosphorylating these sites on eIF4E binding were assessed in a far-Western analysis with a labeled eIF4E probe. Phosphorylation of Thr36 only slightly attenuated binding of PHAS-I to eIF4E, while phosphorylation of Thr45 markedly inhibited binding. Phosphorylation of neither site affected the electrophoretic mobility of the protein, indicating that results of studies that rely solely on a gel-shift assay to assess changes in PHAS-I phosphorylation must be interpreted with caution.     

Neuronal homeostasis through translational control

Translational repression is a key component of the mechanism that establishes segment polarity during early embryonic development in the fruitfly Drosophila melanogaster. Two proteins, Pumilio (Pum) and Nanos, block the translation of hunchback messenger RNA in only the posterior segments, thereby promoting an abdominal fate. More recent studies focusing on postembryonic neuronal function have shown that Pum is also integral to numerous mechanisms that allow neurons to adapt to the changing requirements placed on them in a dynamic nervous system. These mechanisms include those contributing to dendritic structure, synaptic growth, neuronal excitability, and formation of long-term memory. This article describes these new studies and highlights the role of translational repression in regulation of neuronal processes that compensate for change.     

Cytoplasmic nonpolysomal ribonucleoprotein complexes and translational control

Summary In this article, we discuss our attempts to establish the existence in the cytoplasm of regulatory molecules involved in translational control. Our studies have revealed the presence of cAMP independent protein kinase in the free mRNP complex capable of phosphorylating a Mr = 38 000 polypeptide, also part of the same complex. Both the kinase and the acceptor protein were found also as free proteins in the cytoplasmic pool. This kinase has been shown to be distinct from the heme regulated enzyme that phosphorylates the small subunit of eIF-2.Other regulatory molecules include small molecular weight RNAs found as part of an RNP complex. A 4S fraction isolated from this complex inhibited the translation of both capped and uncapped mRNAs in a cell-free protein synthesizing system.The biological role of the protein kinase and the 4S RNA fraction is considered.     

mTOR signaling for biological control and cancer

Mammalian target of rapamycin (mTOR) is a major intersection that connects signals from the extracellular milieu to corresponding changes in intracellular processes. When abnormally regulated, the mTOR signaling pathway is implicated in a wide spectrum of cancers, neurological diseases, and proliferative disorders. Therefore, pharmacological agents that restore the regulatory balance of the mTOR pathway could be beneficial for a great number of diseases. This review summarizes current understanding of mTOR signaling and some unanswered questions in the field. We describe the composition of the mTOR complexes, upstream signals that activate mTOR, and physiological processes that mTOR regulates. We also discuss the role of mTOR and its downstream effectors in cancer, obesity and diabetes, and autism. J. Cell. Physiol. 228: 1658–1664, 2013. © 2013 Wiley Periodicals, Inc.     

Cup-ling oskar RNA localization and translational control

RNA localization and spatially restricted translational control can serve to deploy specific proteins to particular places within a cell. oskar (osk) RNA is a key initiatior of posterior patterning and germ cell specification in Drosophila, and its localization and translation are under elaborate control. In this issue, Wilhelm et al. (2003) show that the protein Cup both promotes osk localization and participates in repressing translation of unlocalized osk.     

mTOR: at the crossroads of aging,chaperones, and Alzheimer's disease

Read the full article ‘Over‐expression of heat shock factor 1 phenocopies the effect of chronic inhibition of TOR by rapamycin and is sufficient to ameliorate Alzheimer's‐like deficits in mice modeling the disease’ on doi: 10.1111/jnc.12080