Exploring the pan-cancer landscape of posttranscriptional regulation.

Understanding the mechanisms underlying cancer gene expression is critical for precision oncology. Posttranscriptional regulation is a key determinant of protein abundance and cancer cell behavior. However, to what extent posttranscriptional regulatory mechanisms impact protein levels and cancer progression is an ongoing question. Here, we exploit cancer proteogenomics data to systematically compare mRNA-protein correlations across 14 different human cancer types. We identify two clusters of genes with particularly low mRNA-protein correlations across all cancer types, shed light on the role of posttranscriptional regulation of cancer driver genes and drug targets, and unveil a cohort of 55 mutations that alter systems-wide posttranscriptional regulation. Surprisingly, we find that decreased levels of posttranscriptional control in patients correlate with shorter overall survival across multiple cancer types, prompting further mechanistic studies into how posttranscriptional regulation affects patient outcomes. Our findings underscore the importance of a comprehensive understanding of the posttranscriptional regulatory landscape for predicting cancer progression.

Inhibition of AXL receptor tyrosine kinase enhances brown adipose tissue functionality in mice.

The current obesity epidemic and high prevalence of metabolic diseases necessitate efficacious and safe treatments. Brown adipose tissue in this context is a promising target with the potential to increase energy expenditure, however no pharmacological treatments activating brown adipose tissue are currently available. Here, we identify AXL receptor tyrosine kinase as a regulator of adipose function. Pharmacological and genetic inhibition of AXL enhance thermogenic capacity of brown and white adipocytes, in vitro and in vivo. Mechanistically, these effects are mediated through inhibition of PI3K/AKT/PDE signaling pathway, resulting in induction of nuclear FOXO1 localization and increased intracellular cAMP levels via PDE3/4 inhibition and subsequent stimulation of the PKA-ATF2 pathway. In line with this, both constitutive Axl deletion as well as inducible adipocyte-specific Axl deletion protect animals from diet-induced obesity concomitant with increases in energy expenditure. Based on these data, we propose AXL receptor as a target for the treatment of obesity.

Monitoring the 5'UTR landscape reveals isoform switches to drive translational efficiencies in cancer.

Transcriptional and translational control are key determinants of gene expression, however, to what extent these two processes can be collectively coordinated is still poorly understood. Here, we use Nanopore long-read sequencing and cap analysis of gene expression (CAGE-seq) to document the landscape of 5' and 3' untranslated region (UTR) isoforms and transcription start sites of epidermal stem cells, wild-type keratinocytes and squamous cell carcinomas. Focusing on squamous cell carcinomas, we show that a small cohort of genes with alternative 5'UTR isoforms exhibit overall increased translational efficiencies and are enriched in ribosomal proteins and splicing factors. By combining polysome fractionations and CAGE-seq, we further characterize two of these UTR isoform genes with identical coding sequences and demonstrate that the underlying transcription start site heterogeneity frequently results in 5' terminal oligopyrimidine (TOP) and pyrimidine-rich translational element (PRTE) motif switches to drive mTORC1-dependent translation of the mRNA. Genome-wide, we show that highly translated squamous cell carcinoma transcripts switch towards increased use of 5'TOP and PRTE motifs, have generally shorter 5'UTRs and expose decreased RNA secondary structures. Notably, we found that the two 5'TOP motif-containing, but not the TOP-less, RPL21 transcript isoforms strongly correlated with overall survival in human head and neck squamous cell carcinoma patients. Our findings warrant isoform-specific analyses in human cancer datasets and suggest that switching between 5'UTR isoforms is an elegant and simple way to alter protein synthesis rates, set their sensitivity to the mTORC1-dependent nutrient-sensing pathway and direct the translational potential of an mRNA by the precise 5'UTR sequence.

A pan-cancer metabolic atlas of the tumor microenvironment.

Tumors are heterogeneous cellular environments with entwined metabolic dependencies. Here, we use a tumor transcriptome deconvolution approach to profile the metabolic states of cancer and non-cancer (stromal) cells in bulk tumors of 20 solid tumor types. We identify metabolic genes and processes recurrently altered in cancer cells across tumor types, highlighting pan-cancer upregulation of deoxythymidine triphosphate (dTTP) production. In contrast, the tryptophan catabolism rate-limiting enzymes IDO1 and TDO2 are highly overexpressed in stroma, raising the hypothesis that kynurenine-mediated suppression of antitumor immunity may be predominantly constrained by the stroma. Oxidative phosphorylation is the most upregulated metabolic process in cancer cells compared to both stromal cells and a large atlas of cancer cell lines, suggesting that the Warburg effect may be less pronounced in cancer cells in vivo. Overall, our analysis highlights fundamental differences in metabolic states of cancer and stromal cells inside tumors and establishes a pan-cancer resource to interrogate tumor metabolism.

GPR180 is a component of TGFß signalling that promotes thermogenic adipocyte function and mediates the metabolic effects of the adipocyte-secreted factor CTHRC1.

Activation of thermogenic brown and beige adipocytes is considered as a strategy to improve metabolic control. Here, we identify GPR180 as a receptor regulating brown and beige adipocyte function and whole-body glucose homeostasis, whose expression in humans is associated with improved metabolic control. We demonstrate that GPR180 is not a GPCR but a component of the TGFß signalling pathway and regulates the activity of the TGFß receptor complex through SMAD3 phosphorylation. In addition, using genetic and pharmacological tools, we provide evidence that GPR180 is required to manifest Collagen triple helix repeat containing 1 (CTHRC1) action to regulate brown and beige adipocyte activity and glucose homeostasis. In this work, we show that CTHRC1/GPR180 signalling integrates into the TGFß signalling as an alternative axis to fine-tune and achieve low-grade activation of the pathway to prevent pathophysiological response while contributing to control of glucose and energy metabolism.

FGF-2-dependent signaling activated in aged human skeletal muscle promotes intramuscular adipogenesis.

Aged skeletal muscle is markedly affected by fatty muscle infiltration, and strategies to reduce the occurrence of intramuscular adipocytes are urgently needed. Here, we show that fibroblast growth factor-2 (FGF-2) not only stimulates muscle growth but also promotes intramuscular adipogenesis. Using multiple screening assays upstream and downstream of microRNA (miR)-29a signaling, we located the secreted protein and adipogenic inhibitor SPARC to an FGF-2 signaling pathway that is conserved between skeletal muscle cells from mice and humans and that is activated in skeletal muscle of aged mice and humans. FGF-2 induces the miR-29a/SPARC axis through transcriptional activation of FRA-1, which binds and activates an evolutionary conserved AP-1 site element proximal in the miR-29a promoter. Genetic deletions in muscle cells and adeno-associated virus-mediated overexpression of FGF-2 or SPARC in mouse skeletal muscle revealed that this axis regulates differentiation of fibro/adipogenic progenitors in vitro and intramuscular adipose tissue (IMAT) formation in vivo. Skeletal muscle from human donors aged >75 y versus <55 y showed activation of FGF-2-dependent signaling and increased IMAT. Thus, our data highlights a disparate role of FGF-2 in adult skeletal muscle and reveals a pathway to combat fat accumulation in aged human skeletal muscle.

Pan-Cancer Analysis of Ligand-Receptor Cross-talk in the Tumor Microenvironment.

Signaling between cancer and nonmalignant (stromal) cells in the tumor microenvironment (TME) is a key to tumor progression. Here, we deconvoluted bulk tumor transcriptomes to infer cross-talk between ligands and receptors on cancer and stromal cells in the TME of 20 solid tumor types. This approach recovered known transcriptional hallmarks of cancer and stromal cells and was concordant with single-cell, in situ hybridization and IHC data. Inferred autocrine cancer cell interactions varied between tissues but often converged on Ephrin, BMP, and FGFR-signaling pathways. Analysis of immune checkpoints nominated interactions with high levels of cancer-to-immune cross-talk across distinct tumor types. Strikingly, PD-L1 was found to be highly expressed in stromal rather than cancer cells. Overall, our study presents a new resource for hypothesis generation and exploration of cross-talk in the TME. SIGNIFICANCE: This study provides deconvoluted bulk tumor transcriptomes across multiple cancer types to infer cross-talk in the tumor microenvironment.

MicroRNA-7a2 Regulates Prolactin in Developing Lactotrophs and Prolactinoma Cells.

Prolactin production is controlled by a complex and temporally dynamic network of factors. Despite this tightly coordinated system, pathological hyperprolactinemia is a common endocrine disorder that is often not understood, thereby highlighting the need to expand our molecular understanding of lactotroph cell regulation. MicroRNA-7 (miR-7) is the most highly expressed miRNA family in the pituitary gland and the loss of the miR-7 family member, miR-7a2, is sufficient to reduce prolactin gene expression in mice. Here, we used conditional loss-of-function and gain-of-function mouse models to characterize the function of miR-7a2 in lactotroph cells. We found that pituitary miR-7a2 expression undergoes developmental and sex hormone-dependent regulation. Unexpectedly, the loss of mir-7a2 induces a premature increase in prolactin expression and lactotroph abundance during embryonic development, followed by a gradual loss of prolactin into adulthood. On the other hand, lactotroph development is delayed in mice overexpressing miR-7a2. This regulation of lactotroph function by miR-7a2 involves complementary mechanisms in multiple cell populations. In mouse pituitary and rat prolactinoma cells, miR-7a2 represses its target Raf1, which promotes prolactin gene expression. These findings shed light on the complex regulation of prolactin production and may have implications for the physiological and pathological mechanisms underlying hyperprolactinemia.

The Diabetes Gene JAZF1 Is Essential for the Homeostatic Control of Ribosome Biogenesis and Function in Metabolic Stress.

The ability of pancreatic ß-cells to respond to increased demands for insulin during metabolic stress critically depends on proper ribosome homeostasis and function. Excessive and long-lasting stimulation of insulin secretion can elicit endoplasmic reticulum (ER) stress, unfolded protein response, and ß-cell apoptosis. Here we show that the diabetes susceptibility gene JAZF1 is a key transcriptional regulator of ribosome biogenesis, global protein, and insulin translation. JAZF1 is excluded from the nucleus, and its expression levels are reduced upon metabolic stress and in diabetes. Genetic deletion of Jazf1 results in global impairment of protein synthesis that is mediated by defects in ribosomal protein synthesis, ribosomal RNA processing, and aminoacyl-synthetase expression, thereby inducing ER stress and increasing ß-cell susceptibility to apoptosis. Importantly, JAZF1 function and its pleiotropic actions are impaired in islets of murine T2D and in human islets exposed to metabolic stress. Our study identifies JAZF1 as a central mediator of metabolic stress in ß-cells.

Author Correction: Coherent diffractive imaging of microtubules using an X-ray laser.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Coherent diffractive imaging of microtubules using an X-ray laser.

X-ray free electron lasers (XFELs) create new possibilities for structural studies of biological objects that extend beyond what is possible with synchrotron radiation. Serial femtosecond crystallography has allowed high-resolution structures to be determined from micro-meter sized crystals, whereas single particle coherent X-ray imaging requires development to extend the resolution beyond a few tens of nanometers. Here we describe an intermediate approach: the XFEL imaging of biological assemblies with helical symmetry. We collected X-ray scattering images from samples of microtubules injected across an XFEL beam using a liquid microjet, sorted these images into class averages, merged these data into a diffraction pattern extending to 2 nm resolution, and reconstructed these data into a projection image of the microtubule. Details such as the 4 nm tubulin monomer became visible in this reconstruction. These results illustrate the potential of single-molecule X-ray imaging of biological assembles with helical symmetry at room temperature.

Proteomics profiling of arginine methylation defines PRMT5 substrate specificity.

Protein arginine methyltransferases (PRMTs) catalyze arginine methylation on both chromatin-bound and cytoplasmic proteins. Accumulating evidence supports the involvement of PRMT5, the major type II PRMT, in cell survival and differentiation pathways that are important during development and in tumorigenesis. PRMT5 is an attractive drug target in various cancers, and inhibitors are currently in oncological clinical trials. Nonetheless, given the complex biology of PRMT5 and its multiple nonhistone substrates, it is paramount to fully characterize these dynamic changes in methylation and to link them to the observed anticancer effects to fully understand the functions of PRMT5 and the consequences of its inhibition. Here, we used a newly established pipeline coupling stable isotope labeling with amino acids in cell culture (SILAC) with immunoenriched methyl peptides to globally profile arginine monomethylation and symmetric dimethylation after PRMT5 inhibition by a selective inhibitor. We adopted heavy methyl SILAC as an orthogonal validation method to reduce the false discovery rate. Through in vitro methylation assays, we validated a set of PRMT5 targets identified by mass spectrometry and provided previously unknown mechanistic insights into the preference of the enzyme to methylate arginine sandwiched between two neighboring glycines (a Gly-Arg-Gly, or "GRG," sequence). Our analysis led to the identification of previously unknown PRMT5 substrates, thus both providing insight into the global effects of PRMT5 and its inhibition in live cells, beyond chromatin, and refining our knowledge of its substrate specificity.

Structural evidence for the roles of divalent cations in actin polymerization and activation of ATP hydrolysis.

The structure of the actin filament is known at a resolution that has allowed the architecture of protein components to be unambiguously assigned. However, fully understanding the chemistry of the system requires higher resolution to identify the ions and water molecules involved in polymerization and ATP hydrolysis. Here, we find experimental evidence for the association of cations with the surfaces of G-actin in a 2.0-Šresolution X-ray structure of actin bound to a Cordon-Bleu WH2 motif and in previously determined high-resolution X-ray structures. Three of four reoccurring divalent cation sites were stable during molecular dynamics (MD) simulations of the filament, suggesting that these sites may play a functional role in stabilizing the filament. We modeled the water coordination at the ATP-bound Mg2+, which also proved to be stable during the MD simulations. Using this model of the filament with a hydrated ATP-bound Mg2+, we compared the cumulative probability of an activated hydrolytic water molecule approaching the γ-phosphorous of ATP, in comparison with G-actin, in the MD simulations. The cumulative probability increased in F-actin in line with the activation of actin's ATPase activity on polymerization. However, inclusion of the cations in the filament lowered cumulative probability, suggesting the rate of hydrolysis may be linked to filament flexibility. Together, these data extend the possible roles of Mg2+ in polymerization and the mechanism of polymerization-induced activation of actin's ATPase activity.

Advances in Structural Biology and the Application to Biological Filament Systems.

Structural biology has experienced several transformative technological advances in recent years. These include: development of extremely bright X-ray sources (microfocus synchrotron beamlines and free electron lasers) and the use of electrons to extend protein crystallography to ever decreasing crystal sizes; and an increase in the resolution attainable by cryo-electron microscopy. Here we discuss the use of these techniques in general terms and highlight their application for biological filament systems, an area that is severely underrepresented in atomic resolution structures. We assemble a model of a capped tropomyosin-actin minifilament to demonstrate the utility of combining structures determined by different techniques. Finally, we survey the methods that attempt to transform high resolution structural biology into more physiological environments, such as the cell. Together these techniques promise a compelling decade for structural biology and, more importantly, they will provide exciting discoveries in understanding the designs and purposes of biological machines.

Flow-aligned, single-shot fiber diffraction using a femtosecond X-ray free-electron laser.

A major goal for X-ray free-electron laser (XFEL) based science is to elucidate structures of biological molecules without the need for crystals. Filament systems may provide some of the first single macromolecular structures elucidated by XFEL radiation, since they contain one-dimensional translational symmetry and thereby occupy the diffraction intensity region between the extremes of crystals and single molecules. Here, we demonstrate flow alignment of as few as 100 filaments (Escherichia coli pili, F-actin, and amyloid fibrils), which when intersected by femtosecond X-ray pulses result in diffraction patterns similar to those obtained from classical fiber diffraction studies. We also determine that F-actin can be flow-aligned to a disorientation of approximately 5 degrees. Using this XFEL-based technique, we determine that gelsolin amyloids are comprised of stacked ß-strands running perpendicular to the filament axis, and that a range of order from fibrillar to crystalline is discernable for individual α-synuclein amyloids.

Yersinia effector protein (YopO)-mediated phosphorylation of host gelsolin causes calcium-independent activation leading to disruption of actin dynamics.

Pathogenic Yersinia bacteria cause a range of human diseases. To modulate and evade host immune systems, these yersiniae inject effector proteins into host macrophages. One such protein, the serine/threonine kinase YopO (YpkA in Yersinia pestis), uses monomeric actin as bait to recruit and phosphorylate host actin polymerization-regulating proteins, including the actin-severing protein gelsolin, to disrupt actin filaments and thus impair phagocytosis. However, the YopO phosphorylation sites on gelsolin and the consequences of YopO-mediated phosphorylation on actin remodeling have yet to be established. Here we determined the effects of YopO-mediated phosphorylation on gelsolin and identified its phosphorylation sites by mass spectrometry. YopO phosphorylated gelsolin in the linker region between gelsolin homology domains G3 and G4, which, in the absence of calcium, are compacted but adopt an open conformation in the presence of calcium, enabling actin binding and severing. Using phosphomimetic and phosphodeletion gelsolin mutants, we found that YopO-mediated phosphorylation partially mimics calcium-dependent activation of gelsolin, potentially contributing to a reduction in filamentous actin and altered actin dynamics in phagocytic cells. In summary, this work represents the first report of the functional outcome of serine/threonine phosphorylation in gelsolin regulation and provides critical insight into how YopO disrupts normal gelsolin function to alter host actin dynamics and thus cripple phagocytosis.

Novel actin filaments from Bacillus thuringiensis form nanotubules for plasmid DNA segregation.

Here we report the discovery of a bacterial DNA-segregating actin-like protein (BtParM) from Bacillus thuringiensis, which forms novel antiparallel, two-stranded, supercoiled, nonpolar helical filaments, as determined by electron microscopy. The BtParM filament features of supercoiling and forming antiparallel double-strands are unique within the actin fold superfamily, and entirely different to the straight, double-stranded, polar helical filaments of all other known ParMs and of eukaryotic F-actin. The BtParM polymers show dynamic assembly and subsequent disassembly in the presence of ATP. BtParR, the DNA-BtParM linking protein, stimulated ATP hydrolysis/phosphate release by BtParM and paired two supercoiled BtParM filaments to form a cylinder, comprised of four strands with inner and outer diameters of 57 Å and 145 Å, respectively. Thus, in this prokaryote, the actin fold has evolved to produce a filament system with comparable features to the eukaryotic chromosome-segregating microtubule.

Structural complexity of filaments formed from the actin and tubulin folds.

From yeast to man, an evolutionary distance of 1.3 billion years, the F-actin filament structure has been conserved largely in line with the 94% sequence identity. The situation is entirely different in bacteria. In comparison to eukaryotic actins, the bacterial actin-like proteins (ALPs) show medium to low levels of sequence identity. This is extreme in the case of the ParM family of proteins, which often display less than 20% identity. ParMs are plasmid segregation proteins that form the polymerizing motors that propel pairs of plasmids to the extremities of a cell prior to cell division, ensuring faithful inheritance of the plasmid. Recently, exotic ParM filament structures have been elucidated that show ParM filament geometries are not limited to the standard polar pair of strands typified by actin. Four-stranded non-polar ParM filaments existing as open or closed nanotubules are found in Clostridium tetani and Bacillus thuringiensis, respectively. These diverse architectures indicate that the actin fold is capable of forming a large variety of filament morphologies, and that the conception of the "actin" filament has been heavily influenced by its conservation in eukaryotes. Here, we review the history of the structure determination of the eukaryotic actin filament to give a sense of context for the discovery of the new ParM filament structures. We describe the novel ParM geometries and predict that even more complex actin-like filaments may exist in bacteria. Finally, we compare the architectures of filaments arising from the actin and tubulin folds and conclude that the basic units possess similar properties that can each form a range of structures. Thus, the use of the actin fold in microfilaments and the tubulin fold for microtubules likely arose from a wider range of filament possibilities, but became entrenched as those architectures in early eukaryotes.

The evolution of compositionally and functionally distinct actin filaments.

The actin filament is astonishingly well conserved across a diverse set of eukaryotic species. It has essentially remained unchanged in the billion years that separate yeast, Arabidopsis and man. In contrast, bacterial actin-like proteins have diverged to the extreme, and many of them are not readily identified from sequence-based homology searches. Here, we present phylogenetic analyses that point to an evolutionary drive to diversify actin filament composition across kingdoms. Bacteria use a one-filament-one-function system to create distinct filament systems within a single cell. In contrast, eukaryotic actin is a universal force provider in a wide range of processes. In plants, there has been an expansion of the number of closely related actin genes, whereas in fungi and metazoa diversification in tropomyosins has increased the compositional variety in actin filament systems. Both mechanisms dictate the subset of actin-binding proteins that interact with each filament type, leading to specialization in function. In this Hypothesis, we thus propose that different mechanisms were selected in bacteria, plants and metazoa, which achieved actin filament compositional variation leading to the expansion of their functional diversity.