In a recent publication in Cell, Wolin et al.1 describe a massively multiplexed variation of CLIP called SPIDR that allows the simultaneous profiling of targets of arbitrary numbers of RNA-binding proteins. Using SPIDR, they uncover unexpected RNA-protein interactions controlled by the nutrient-regulated mTOR pathway.

mRNA therapeutics offer a potentially universal strategy for the efficient development and delivery of therapeutic proteins. Current mRNA vaccines include chemically modified nucleotides to reduce cellular immunogenicity. Here, we develop an efficient, high-throughput method to measure human translation initiation on therapeutically modified as well as endogenous RNAs. Using systems-level biochemistry, we quantify ribosome recruitment to tens of thousands of human 5' untranslated regions (UTRs) including alternative isoforms and identify sequences that mediate 200-fold effects. We observe widespread effects of coding sequences on translation initiation and identify small regulatory elements of 3-6 nucleotides that are sufficient to potently affect translational output. Incorporation of N1-methylpseudouridine (m1J) selectively enhances translation by specific 5' UTRs that we demonstrate surpass those of current mRNA vaccines. Our approach is broadly applicable to dissecting mechanisms of human translation initiation and engineering more potent therapeutic mRNAs.

PDF file - 719K, Supplemental figure 1. Torin2 on PC-3 wide type cell line, the IB was made after 30 min treatment of drug at indicated concentrations. Supplemental figure 2. Torin1, Torin2, AZD8055 and BEZ235 on HeLa and HCT-116 cell line at different time point with a dose responsive drug treatment. Supplemental figure 3. PET-CT experiment of short term treatment of drugs. Supplemental figure 4. PDG-PET determination of drug treatment response with statistics

Legends for Supplementary Figures S1-S4.

Both, p38/MAPK, and ERK/MEK pathway contribute to MYC and YAP up-regulation in resistant cell lines.

MYC and YAP expression mediate drug resistance towards PI3K/mTOR inhibition.

Structurally diverse PI3K/mTOR inhibitors induce MYC and YAP up-regulation in resistant MCAS-R cells.

Description of additional methods and procedures used in the study.

PI3K/mTOR inhibition leads to p38 activation and DNA damage response, which is suppressed in drug resistant cell lines.

The control of mRNA stability is fundamental to gene regulation, and a deeper understanding of this post-transcriptional regulatory step can provide key insights into gene function. Measuring mRNA half-lives directly, however, is challenging. The most common strategies for evaluating mRNA stability and decay involve blocking general transcription and then measuring the decline in mRNA levels over time. The downside of these approaches, however, is that they severely impact cell function and viability, indirectly perturbing gene expression. Here, we describe Roadblock-qPCR, a simple method for measuring mRNA decay kinetics in living cells that is both economical and quick. Cells are first incubated with the nucleoside analog 4-thiouridine (4sU), which is readily incorporated into nascent mRNAs during transcription. RNA is then extracted and treated with N-ethylmaleimide (NEM), a sulfhydryl alkylating agent that selectively modifies 4sU, before proceeding to cDNA synthesis. Because the NEM-modified 4sU creates a chemical "roadblock" that interferes with reverse transcription, this treatment ultimately results in the depletion of the nascent 4sU-containing transcripts from the cDNA pool. As such, the decay rate of the non-4sU-labeled pre-existing mRNAs can be monitored by quantitative PCR (qPCR). In combination with spike-in standards, this approach can be used to efficiently and accurately measure the half-lives of endogenous mRNAs with a wide range of stabilities, while avoiding the artifacts of transcription shutoff strategies. © 2022 Wiley Periodicals LLC. Basic Protocol: Roadblock-qPCR Support Protocol: Synthesis of spike-in mRNA.

mRNA regulatory sequences control gene expression at multiple levels including translation initiation and mRNA decay. The 5′ terminal sequences of mRNAs have unique regulatory potential because of their proximity to key post-transcriptional regulators. Here we have systematically probed the function of 5′ terminal sequences in gene expression in human cells. Using a library of reporter mRNAs initiating with all possible 7-mer sequences at their 5′ ends, we find an unexpected impact on transcription that underlies 200-fold differences in mRNA expression. Library sequences that promote high levels of transcription mirrored those found in native mRNAs and define two basic classes with similarities to classic Initiator (Inr) and TCT core promoter motifs. By comparing transcription, translation and decay rates, we identify sequences that are optimized for both efficient transcription and growth-regulated translation and stability, including variants of terminal oligopyrimidine (TOP) motifs. We further show that 5′ sequences of endogenous mRNAs are enriched for multi-functional TCT/TOP hybrid sequences. Together, our results reveal how 5′ sequences define two general classes of mRNAs with distinct growth-responsive profiles of expression across synthesis, translation and decay.

The stability of mRNAs is fundamental to determining expression level and dynamics. Nonetheless, current approaches for measuring transcript half-lives (e.g., transcription shutoff) are generally toxic or technically complex. Here we describe an alternative strategy for targeted measurements of endogenous mRNA stability that is simple, inexpensive, and nontoxic. Cells are first metabolically labeled with the nucleoside analog 4-thiouridine (4sU). Extracted mRNA can then be treated with the thiol-reactive compound N-ethylmaleimide. This compound modifies 4sU nucleotides and sterically interferes with reverse transcription of 4sU-containing transcripts, disrupting their conversion into cDNA. The decay rate of non-4sU-containing preexisting mRNA can then be monitored by quantitative PCR (qPCR). Importantly, this approach avoids the biochemical isolation of 4sU-labeled transcripts and/or RNA-seq analysis required for other metabolic-labeling strategies. In summary, our method combines the simplicity of "transcription shutoff" strategies with the accuracy of metabolic-labeling strategies for measurements of mRNA stability across a wide range of half-lives.

The RNA-binding protein LARP1 has generated interest in recent years for its role in the mTOR signalling cascade and its regulation of terminal oligopyrimidine (TOP) mRNA translation. Paradoxically, some scientists have shown that LARP1 represses TOP translation while others that LARP1 activates it. Here, we present opinions from four leading scientists in the field to discuss these and other contradictory findings.

A rationally designed class of mTORC1 inhibitors combines the selectivity of rapamycin with the potency of mTOR kinase inhibitors.

Terminal oligopyrimidine (TOP) motifs are sequences at the 5' ends of mRNAs that link their translation to the mTOR Complex 1 (mTORC1) nutrient-sensing signaling pathway. They are commonly regarded as discrete elements that reside on ∼100 mRNAs that mostly encode translation factors. However, the full spectrum of TOP sequences and their prevalence throughout the transcriptome remain unclear, primarily because of uncertainty over the mechanism that detects them. Here, we globally analyzed translation targets of La-related protein 1 (LARP1), an RNA-binding protein and mTORC1 effector that has been shown to repress TOP mRNA translation in a few specific cases. We establish that LARP1 is the primary translation regulator of mRNAs with classical TOP motifs genome-wide, and also that these motifs are extreme instances of a broader continuum of regulatory sequences. We identify the features of TOP sequences that determine their potency and quantify these as a metric that accurately predicts mTORC1/LARP1 regulation called a TOPscore. Analysis of TOPscores across the transcriptomes of 16 mammalian tissues defines a constitutive "core" set of TOP mRNAs, but also identifies tissue-specific TOP mRNAs produced via alternative transcription initiation sites. These results establish the central role of LARP1 in TOP mRNA regulation on a transcriptome scale and show how it connects mTORC1 to a tunable and dynamic program of gene expression that is tailored to specific biological contexts.

The coordination of metabolic processes to allow increased nutrient uptake and utilization for macromolecular synthesis is central for cell growth. Although studies of bulk cell populations have revealed important metabolic and signaling requirements that impact cell growth on long time scales, whether the same regulation influences short-term cell growth remains an open question. Here we investigate cell growth by monitoring mass accumulation of mammalian cells while rapidly depleting particular nutrients. Within minutes following the depletion of glucose or glutamine, we observe a growth reduction that is larger than the mass accumulation rate of the nutrient. This indicates that if one particular nutrient is depleted, the cell rapidly adjusts the amount that other nutrients are accumulated, which is consistent with cooperative nutrient accumulation. Population measurements of nutrient sensing pathways involving mTOR, AKT, ERK, PKA, MST1, or AMPK, or pro-survival pathways involving autophagy suggest that they do not mediate this growth reduction. Furthermore, the protein synthesis rate does not change proportionally to the mass accumulation rate over these time scales, suggesting that intracellular metabolic pools buffer the growth response. Our findings demonstrate that cell growth can be regulated over much shorter time scales than previously appreciated.

The mammalian target of rapamycin (mTOR) kinase is the catalytic subunit of two functionally distinct complexes, mTORC1 and mTORC2, that coordinately promote cell growth, proliferation, and survival. Rapamycin is a potent allosteric mTORC1 inhibitor with clinical applications as an immunosuppressant and anti-cancer agent. Here we find that Torin1, a highly potent and selective ATP-competitive mTOR inhibitor that directly inhibits both complexes, impairs cell growth and proliferation to a far greater degree than rapamycin. Surprisingly, these effects are independent of mTORC2 inhibition and are instead because of suppression of rapamycin-resistant functions of mTORC1 that are necessary for cap-dependent translation and suppression of autophagy. These effects are at least partly mediated by mTORC1-dependent and rapamycin-resistant phosphorylation of 4E-BP1. Our findings challenge the assumption that rapamycin completely inhibits mTORC1 and indicate that direct inhibitors of mTORC1 kinase activity may be more successful than rapamycin at inhibiting tumors that depend on mTORC1.