An investigation of nutrition-dependent mRNA translation in Drosophila larvae

Dietary nutrients promote organismal growth via activation of cellular pathways that are necessary for cellular growth and division. However, upon conditions of nutrient scarcity, organisms ranging from yeast to mammals alter their metabolism to conserve nutrients and energy. One of the key cellular events required for sustained growth downstream of nutrition is mRNA translation. In order to coordinate cellular and organismal growth with nutrient availability, organisms have devised several control checkpoints to limit the energy consuming process of translation under starvation. Work on mammalian cell lines have shown that mRNA translation is inhibited in response to a multitude of stresses including nutritional stress, DNA damage, viral stress, hypoxia and oxidative stress. However, it is unclear as to how an organism in general responds to starvation in terms of mRNA translation. I have used polysome gradient analysis in developing Drosophila larvae to show that starvation led to a rapid decrease in translation, seen as early as 30 minutes after removal of larvae from food. A maximal decrease in translation was observed after 6-18 hours of starvation. Using qRT-PCR I have looked at the translation profile of 18 individual genes and observed that starvation led to a general decrease in translation, irrespective of the starvation-mediated change in total transcript levels. Sugars and amino acids have been shown to be important regulators of translation in animal cells. I have determined that neither sugars nor amino acids were sufficient to maintain translation in larvae removed from food. However, a complex diet such as yeast was sufficient to maintain translation in larvae removed from food. The majority of work in the field of translation suggest three main signaling pathways functioning downstream of nutrition to regulate translation - insulin signaling,TOR signaling and eIF2α signaling. I have shown that TOR signaling is required for translation in fed conditions. On the other hand, genetic activation of TOR and/or insulin signaling was not sufficient to prevent the starvation-mediated inhibition of translation. I have also shown that the eiF2α kinases - GCN2 and PERK were not required for starvation-mediated suppression of translation

An investigation of nutrient-dependent mRNA translation in Drosophila larvae

The larval period of the Drosophila life cycle is characterized by immense growth. In nutrient rich conditions, larvae increase in mass approximately two hundred-fold in five days. However, upon nutrient deprivation, growth is arrested. The prevailing view is that dietary amino acids drive this larval growth by activating the conserved insulin/PI3 kinase and Target of rapamycin (TOR) pathways and promoting anabolic metabolism. One key anabolic process is protein synthesis. However, few studies have attempted to measure mRNA translation during larval development or examine the signaling requirements for nutrient-dependent regulation. Our work addresses this issue. Using polysome analyses, we observed that starvation rapidly (within thirty minutes) decreased larval mRNA translation, with a maximal decrease at 6–18 hours. By analyzing individual genes, we observed that nutrient-deprivation led to a general reduction in mRNA translation, regardless of any starvation-mediated changes (increase or decrease) in total transcript levels. Although sugars and amino acids are key regulators of translation in animal cells and are the major macronutrients in the larval diet, we found that they alone were not sufficient to maintain mRNA translation in larvae. The insulin/PI3 kinase and TOR pathways are widely proposed as the main link between nutrients and mRNA translation in animal cells. However, we found that genetic activation of PI3K and TOR signaling, or regulation of two effectors – 4EBP and S6K – could not prevent the starvation-mediated translation inhibition. Similarly, we showed that the nutrient stress-activated eIF2α kinases, GCN2 and PERK, were not required for starvation-induced inhibition of translation in larvae. These findings indicate that nutrient control of mRNA translation in larvae is more complex than simply amino acid activation of insulin and TOR signaling

Outcomes of ST Segment Elevation Myocardial Infarction without Standard Modifiable Cardiovascular Risk Factors – Newer Insights from a Prospective Registry in India

Objectives: Patients with ST elevation myocardial infarction (STEMI) without standard modifiable cardiovascular risk factors (SMuRFs; dyslipidaemia, hypertension, diabetes mellitus and smoking) are reported to have a worse clinical outcome compared to those with SMuRFs. However, robust prospective data and low-and middle-income country perspective are lacking. We aimed to study the patients with first STEMI and assess the influence of SMuRFs on clinical outcomes by comparing the patients with and without SMuRFs. Methods: We included all consecutive STEMI patients without prior coronary artery disease enrolled in the Madras Medical College STEMI Registry from September 2018 to October 2019. We collected baseline clinical characteristics, revascularisation strategies and clinical outcome. We analysed suboptimal self-reported sleep duration as a 5th extended SMuRF (eSMuRF). Primary outcome was in-hospital mortality. Secondary outcomes included in-hospital complications and one-year all-cause mortality. Results: Among 2,379 patients, 605 patients (25.4%) were SMuRF-less. More women were SMuRF-less than men (27.1% vs 22.1%; P = 0.012). SMuRF-less patients were older (57.44 ± 13.95 vs 55.68 ± 11.74; P < 0.001), more often former tobacco users (10.4% vs 5.0%; P < 0.001), with more anterior wall MI (62.6% vs 52.1%; P = 0.032). The primary outcome [in-hospital mortality (10.7% vs 11.3%; P = 0.72)] and secondary outcomes [in-hospital complications (29.1% vs 31.7%; P = 0.23) and one-year all-cause mortality (22.3% vs 22.7%; P = 0.85)] were similar in both groups. Addition of suboptimal self-reported sleep duration as a 5th eSMuRF yielded similar results. Conclusions: 25% of first STEMI patients were SMuRF-less. Clinical outcomes of patients without SMuRFs were similar to those with SMuRFs. Suboptimal sleep duration did not account for the risk associated with the SMuRF-less status

Control of Drosophila endocycles by E2F and CRL4(CDT2)

Endocycles are variant cell cycles comprised of DNA synthesis (S)- and gap (G)-phases but lacking mitosis1,2. Such cycles facilitate post-mitotic growth in many invertebrate and plant cells, and are so ubiquitous that they may account for up to half the world’s biomass3,4. DNA replication in endocycling Drosophila cells is triggered by cyclin E/cyclin dependent kinase 2 (CYCE/CDK2), but this kinase must be inactivated during each G-phase to allow the assembly of pre-Replication Complexes (preRCs) for the next S-phase5,6. How CYCE/CDK2 is periodically silenced to allow re-replication has not been established. Here, using genetic tests in parallel with computational modelling, we show that the endocycles of Drosophila are driven by a molecular oscillator in which the E2F1 transcription factor promotes CycE expression and S-phase initiation, S-phase then activates the CRL4CDT2 ubiquitin ligase, and this in turn mediates the destruction of E2F1 (ref. 7). We propose that it is the transient loss of E2F1 during S phases that creates the window of low Cdk activity required for preRC formation. In support of this model overexpressed E2F1 accelerated endocycling, whereas a stabilized variant of E2F1 blocked endocycling by deregulating target genes, including CycE, as well as Cdk1 and mitotic cyclins. Moreover, we find that altering cell growth by changing nutrition or target of rapamycin (TOR) signalling impacts E2F1 translation, thereby making endocycle progression growth-dependent. Many of the regulatory interactions essential to this novel cell cycle oscillator are conserved in animals and plants1,2,8, indicating that elements of this mechanism act in most growth-dependent cell cycles