Monophosphothreonyl extracellular signal-regulated kinases 1 and 2 (ERK1/2) are formed endogenously in intact cardiac myocytes and are enzymically active

Concordant and Heterogeneity of Single-Cell Transcriptome in Cardiac Development of Human and Mouse

Normal heart development is vital for maintaining its function, and the development process is involved in complex interactions between different cell lineages. How mammalian hearts develop differently is still not fully understood. In this study, we identified several major types of cardiac cells, including cardiomyocytes (CMs), fibroblasts (FBs), endothelial cells (ECs), ECs/FBs, epicardial cells (EPs), and immune cells (macrophage/monocyte cluster, MACs/MONOs), based on single-cell transcriptome data from embryonic hearts of both human and mouse. Then, species-shared and species-specific marker genes were determined in the same cell type between the two species, and the genes with consistent and different expression patterns were also selected by constructing the developmental trajectories. Through a comparison of the development stage similarity of CMs, FBs, and ECs/FBs between humans and mice, it is revealed that CMs at e9.5 and e10.5 of mice are most similar to those of humans at 7 W and 9 W, respectively. Mouse FBs at e10.5, e13.5, and e14.5 are correspondingly more like the same human cells at 6, 7, and 9 W. Moreover, the e9.5-ECs/FBs of mice are most similar to that of humans at 10W. These results provide a resource for understudying cardiac cell types and the crucial markers able to trace developmental trajectories among the species, which is beneficial for finding suitable mouse models to detect human cardiac physiology and related diseases.

Cardiomyocyte BRAF and type 1 RAF inhibitors promote cardiomyocyte and cardiac hypertrophy in mice in vivo

The extracellular signal-regulated kinase 1/2 (ERK1/2) cascade promotes cardiomyocyte hypertrophy and is cardioprotective, with the three RAF kinases forming a node for signal integration. Our aims were to determine if BRAF is relevant for human heart failure, whether BRAF promotes cardiomyocyte hypertrophy, and if Type 1 RAF inhibitors developed for cancer (that paradoxically activate ERK1/2 at low concentrations: the 'RAF paradox') may have the same effect. BRAF was up-regulated in heart samples from patients with heart failure compared with normal controls. We assessed the effects of activated BRAF in the heart using mice with tamoxifen-activated Cre for cardiomyocyte-specific knock-in of the activating V600E mutation into the endogenous gene. We used echocardiography to measure cardiac dimensions/function. Cardiomyocyte BRAFV600E induced cardiac hypertrophy within 10 d, resulting in increased ejection fraction and fractional shortening over 6 weeks. This was associated with increased cardiomyocyte size without significant fibrosis, consistent with compensated hypertrophy. The experimental Type 1 RAF inhibitor, SB590885, and/or encorafenib (a RAF inhibitor used clinically) increased ERK1/2 phosphorylation in cardiomyocytes, and promoted hypertrophy, consistent with a 'RAF paradox' effect. Both promoted cardiac hypertrophy in mouse hearts in vivo, with increased cardiomyocyte size and no overt fibrosis. In conclusion, BRAF potentially plays an important role in human failing hearts, activation of BRAF is sufficient to induce hypertrophy, and Type 1 RAF inhibitors promote hypertrophy via the 'RAF paradox'. Cardiac hypertrophy resulting from these interventions was not associated with pathological features, suggesting that Type 1 RAF inhibitors may be useful to boost cardiomyocyte function.

A walk-through MAPK structure and functionality with the 30-year-old yeast MAPK Slt2

Mitogen-activated protein kinases (MAPKs) are evolutionarily conserved signaling proteins involved in the regulation of most eukaryotic cellular processes. They are downstream components of essential signal transduction pathways activated by the external stimuli, in which the signal is conveyed through phosphorylation cascades. The excellent genetic and biochemical tractability of simple eukaryotes such as Saccharomyces cerevisiae has significantly contributed to gain fundamental information into the physiology of these key proteins. The budding yeast MAPK Slt2 was identified 30 years ago and was later revealed as a fundamental element of the cell wall integrity (CWI) pathway, one of the five MAPK routes of S. cerevisiae. As occurs with other MAPKs, whereas Slt2 displays the core typical structural traits of eukaryotic protein kinases, it also features conserved domains among MAPKs that allow an exquisite spatio-temporal regulation of their activity and binding to activating kinases, downregulatory phosphatases, or nuclear transcription factors. Additionally, Slt2 bears a regulatory extra C-terminal tail unique among S. cerevisiae MAPKs. Here, we review the structural and functional basis for the signaling role of Slt2 in the context of the molecular architecture of this important family of protein kinases.

Differential Role of Threonine and Tyrosine Phosphorylation in the Activation and Activity of the Yeast MAPK Slt2

The Mitogen-Activated Protein Kinase (MAPK) Slt2 is central to signaling through the yeast Cell Wall Integrity (CWI) pathway. MAPKs are regulated by phosphorylation at both the threonine and tyrosine of the conserved TXY motif within the activation loop (T190/Y192 in Slt2). Since phosphorylation at both sites results in the full activation of MAPKs, signaling through MAPK pathways is monitored with antibodies that detect dually phosphorylated forms. However, most of these antibodies also recognize monophosphorylated species, whose relative abundance and functionality are diverse. By using different phosphospecific antibodies and phosphate-affinity (Phos-tag) analysis on distinct Slt2 mutants, we determined that Y192- and T190-monophosphorylated species coexist with biphosphorylated Slt2, although most of the Slt2 pool remains unphosphorylated following stress. Among the monophosphorylated forms, only T190 exhibited biological activity. Upon stimulation, Slt2 is first phosphorylated at Y192, mainly by the MAPKK Mkk1, and this phosphorylation is important for the subsequent T190 phosphorylation. Similarly, dephosphorylation of Slt2 by the Dual Specificity Phosphatase (DSP) Msg5 is ordered, with dephosphorylation of T190 depending on previous Y192 dephosphorylation. Whereas Y192 phosphorylation enhances the Slt2 catalytic activity, T190 is essential for this activity. The conserved T195 residue is also critical for Slt2 functionality. Mutations that abolish the activity of Slt2 result in a high increase in inactive Y192-monophosphorylated Slt2. The coexistence of different Slt2 phosphoforms with diverse biological significance highlights the importance of the precise detection of the Slt2 phosphorylation status.

Distinct biological activity of threonine monophosphorylated MAPK isoforms during the stress response in fission yeast

Mitogen-activated protein kinases (MAPKs) define a specific group of eukaryotic protein kinases which regulate a number of cellular functions by transducing extracellular signals to intracellular responses. Unlike other protein kinases, catalytic activation of MAPKs by MAPKKs depends on dual phosphorylation at two tyrosine and threonine residues within the conserved TXY motif, and this has been proposed to occur in an ordered fashion, where the initial phosphorylation on tyrosine is followed by phosphorylation at the threonine residue. However, monophosphorylated MAPKs also exist in vivo, and although threonine phosphorylated isoforms retain some catalytic activity, their functional significance remains to be further elucidated. In the fission yeast Schizosaccharomyces pombe MAPKs Sty1 and Pmk1 control multiple aspects of fission yeast life cycle, including morphogenesis, cell cycle, and cellular response to a variety of stressful situations. In this work we show that a trapping mechanism increases MAPKK binding and tyrosine phosphorylation of both Sty1 and Pmk1 when subsequent phosphorylation at threonine is hampered, indicating that a sequential and likely processive mechanism might be responsible for MAPK activation in this simple organism. Whereas threonine-monophosphorylated Sty1 showed a limited biological activity particularly at the transcriptional level, threonine-monophosphorylated Pmk1 was able to execute most of the biological functions of the dually phosphorylated kinase. Thus, threonine monophosphorylated MAPKs might display distinct functional relevance among eukaryotes.

Cardiac protein kinases: the cardiomyocyte kinome and differential kinase expression in human failing hearts

AIMS

Protein kinases are potential therapeutic targets for heart failure, but most studies of cardiac protein kinases derive from other systems, an approach that fails to account for specific kinases expressed in the heart and the contractile cardiomyocytes. We aimed to define the cardiomyocyte kinome (i.e. the protein kinases expressed in cardiomyocytes) and identify kinases with altered expression in human failing hearts.

METHODS AND RESULTS

Expression profiling (Affymetrix microarrays) detected >400 protein kinase mRNAs in rat neonatal ventricular myocytes (NVMs) and/or adult ventricular myocytes (AVMs), 32 and 93 of which were significantly up-regulated or down-regulated (greater than two-fold), respectively, in AVMs. Data for AGC family members were validated by qPCR. Proteomics analysis identified >180 cardiomyocyte protein kinases, with high relative expression of mitogen-activated protein kinase cascades and other known cardiomyocyte kinases (e.g. CAMKs, cAMP-dependent protein kinase). Other kinases are poorly investigated (e.g. Slk, Stk24, Oxsr1). Expression of Akt1/2/3, BRaf, ERK1/2, Map2k1, Map3k8, Map4k4, MST1/3, p38-MAPK, PKCδ, Pkn2, Ripk1/2, Tnni3k, and Zak was confirmed by immunoblotting. Relative to total protein, Map3k8 and Tnni3k were up-regulated in AVMs vs. NVMs. Microarray data for human hearts demonstrated variation in kinome expression that may influence responses to kinase inhibitor therapies. Furthermore, some kinases were up-regulated (e.g. NRK, JAK2, STK38L) or down-regulated (e.g. MAP2K1, IRAK1, STK40) in human failing hearts.

CONCLUSION

This characterization of the spectrum of kinases expressed in cardiomyocytes and the heart (cardiomyocyte and cardiac kinomes) identified novel kinases, some of which are differentially expressed in failing human hearts and could serve as potential therapeutic targets.

Modelling negative feedback networks for activating transcription factor 3 predicts a dominant role for miRNAs in immediate early gene regulation

Activating transcription factor 3 (Atf3) is rapidly and transiently upregulated in numerous systems, and is associated with various disease states. Atf3 is required for negative feedback regulation of other genes, but is itself subject to negative feedback regulation possibly by autorepression. In cardiomyocytes, Atf3 and Egr1 mRNAs are upregulated via ERK1/2 signalling and Atf3 suppresses Egr1 expression. We previously developed a mathematical model for the Atf3-Egr1 system. Here, we adjusted and extended the model to explore mechanisms of Atf3 feedback regulation. Introduction of an autorepressive loop for Atf3 tuned down its expression and inhibition of Egr1 was lost, demonstrating that negative feedback regulation of Atf3 by Atf3 itself is implausible in this context. Experimentally, signals downstream from ERK1/2 suppress Atf3 expression. Mathematical modelling indicated that this cannot occur by phosphorylation of pre-existing inhibitory transcriptional regulators because the time delay is too short. De novo synthesis of an inhibitory transcription factor (ITF) with a high affinity for the Atf3 promoter could suppress Atf3 expression, but (as with the Atf3 autorepression loop) inhibition of Egr1 was lost. Developing the model to include newly-synthesised miRNAs very efficiently terminated Atf3 protein expression and, with a 4-fold increase in the rate of degradation of mRNA from the mRNA/miRNA complex, profiles for Atf3 mRNA, Atf3 protein and Egr1 mRNA approximated to the experimental data. Combining the ITF model with that of the miRNA did not improve the profiles suggesting that miRNAs are likely to play a dominant role in switching off Atf3 expression post-induction.

Activating transcription factor 3 (Atf3) is an important regulatory transcription factor which is associated with inflammation, restraint of the immune response and cancer. In this work, we develop a series of mathematical models to understand how Atf3 may be regulated. Informed with data from the literature and our own experiments, we show that self-regulation of Atf3 does not allow for variation between experimentally observed Atf3 mRNA and Atf3 protein expression profiles. A fast-acting signal via phosphorylated RSK is also shown to be implausible for similar reasons. Extending our mathematical model further, we postulate for the first time, that the observed dynamical variation in Atf3 mRNA and protein can be described by microRNAs downstream of RSKs. The further inclusion of an inhibitory transcription factor for Atf3 expression has little effect on these findings.

SAICAR induces protein kinase activity of PKM2 that is necessary for sustained proliferative signaling of cancer cells

Abnormal metabolism and sustained proliferation are hallmarks of cancer. Pyruvate kinase M2 (PKM2) is a metabolic enzyme that plays important roles in both processes. Recently, PKM2 was shown to have protein kinase activity phosphorylating histone H3 and promoting cancer cell proliferation. However, the mechanism and extent of this protein kinase in cancer cells remain unclear. Here, we report that binding of succinyl-5-aminoimidazole-4-carboxamide-1-ribose-5'-phosphate (SAICAR), a metabolite abundant in proliferating cells, induces PKM2's protein kinase activity in vitro and in cells. Protein microarray experiments revealed that more than 100 human proteins, mostly protein kinases, are phosphorylated by PKM2-SAICAR. In particular, PKM2-SAICAR phosphorylates and activates Erk1/2, which in turn sensitizes PKM2 for SAICAR binding through phosphorylation. Additionally, PKM2-SAICAR was necessary to induce sustained Erk1/2 activation and mitogen-induced cell proliferation. Thus, the ligand-induced protein kinase activity from PKM2 is a mechanism that directly couples cell proliferation with intracellular metabolic status.

p90 ribosomal S6 kinases play a significant role in early gene regulation in the cardiomyocyte response to G(q)-protein-coupled receptor stimuli, endothelin-1 and α(1)-adrenergic receptor agonists

ERK1/2 (extracellular-signal-regulated kinase 1/2) and their substrates RSKs (p90 ribosomal S6 kinases) phosphorylate different transcription factors, contributing differentially to transcriptomic profiles. In cardiomyocytes ERK1/2 are required for >70% of the transcriptomic response to endothelin-1. In the present study we investigated the role of RSKs in the transcriptomic responses to the G_q-protein-coupled receptor agonists endothelin-1, phenylephrine (a generic α₁-adrenergic receptor agonist) and A61603 (α_1A-adrenergic receptor selective). Phospho-ERK1/2 and phospho-RSKs appeared in cardiomyocyte nuclei within 2–3 min of stimulation (endothelin-1>A61603≈phenylephrine). All agonists increased nuclear RSK2, but only endothelin-1 increased the nuclear RSK1 content. PD184352 (inhibits ERK1/2 activation) and BI-D1870 (inhibits RSKs) were used to dissect the contribution of RSKs to the endothelin-1-responsive transcriptome. Of the 213 RNAs up-regulated after 1 h, 51% required RSKs for their up-regulation, whereas 29% required ERK1/2 but not RSKs. The transcriptomic response to phenylephrine overlapped with, but was not identical with, endothelin-1. As with endothelin-1, PD184352 inhibited the up-regulation of most phenylephrine-responsive transcripts, but the greater variation in the effects of BI-D1870 suggests that differential RSK signalling influences global gene expression. A61603 induced similar changes in RNA expression in cardiomyocytes as phenylephrine, indicating that the signal was mediated largely through α_1A-adrenergic receptors. A61603 also increased expression of immediate early genes in perfused adult rat hearts and, as in cardiomyocytes, up-regulation of the majority of genes was inhibited by PD184352. PD184352 or BI-D1870 prevented the increased surface area induced by endothelin-1 in cardiomyocytes. Thus RSKs play a significant role in regulating cardiomyocyte gene expression and hypertrophy in response to G_q-protein-coupled receptor stimulation.

Feedback regulation by Atf3 in the endothelin-1-responsive transcriptome of cardiomyocytes: Egr1 is a principal Atf3 target

Endothelin-1 promotes cardiomyocyte hypertrophy by inducing changes in gene expression. Immediate early genes including Atf3 (activating transcription factor 3), Egr1 (early growth response 1) and Ptgs2 (prostaglandin-endoperoxide synthase 2) are rapi-dly and transiently up-regulated by endothelin-1 in cardiomyocytes. Atf3 regulates the expression of downstream genes and is implicated in negative feedback regulation of other immediate early genes. To identify Atf3-regulated genes, we knocked down Atf3 expression in cardiomyocytes exposed to endothelin-1 and used microarrays to interrogate the transcriptomic effects. The expression of 23 mRNAs (including Egr1 and Ptgs2) was enhanced and the expression of 25 mRNAs was inhibited by Atf3 knockdown. Using quantitative PCR, we determined that knockdown of Atf3 had little effect on up-regulation of Egr1 mRNA over 30 min, but abolished the subsequent decline, causing sustained Egr1 mRNA expression and enhanced protein expression. This resulted from direct binding of Atf3 to the Egr1 promoter. Mathematical modelling established that Atf3 can suffice to suppress Egr1 expression. Given the widespread co-regulation of Atf3 with Egr1, we suggest that the Atf3–Egr1 negative feedback loop is of general significance. Loss of Atf3 caused abnormal cardiomyocyte growth, presumably resulting from the dysregulation of target genes. The results of the present study therefore identify Atf3 as a nexus in cardiomyocyte hypertrophy required to facilitate the full and proper growth response.

A novel non-canonical mechanism of regulation of MST3 (mammalian Sterile20-related kinase 3)

The canonical pathway of regulation of the GCK (germinal centre kinase) III subgroup member, MST3 (mammalian Sterile20-related kinase 3), involves a caspase-mediated cleavage between N-terminal catalytic and C-terminal regulatory domains with possible concurrent autophosphorylation of the activation loop MST3(Thr¹⁷⁸), induction of serine/threonine protein kinase activity and nuclear localization. We identified an alternative ‘non-canonical’ pathway of MST3 activation (regulated primarily through dephosphorylation) which may also be applicable to other GCKIII (and GCKVI) subgroup members. In the basal state, inactive MST3 co-immunoprecipitated with the Golgi protein GOLGA2/gm130 (golgin A2/Golgi matrix protein 130). Activation of MST3 by calyculin A (a protein serine/threonine phosphatase 1/2A inhibitor) stimulated (auto)phosphorylation of MST3(Thr¹⁷⁸) in the catalytic domain with essentially simultaneous cis-autophosphorylation of MST3(Thr³²⁸) in the regulatory domain, an event also requiring the MST3(341–376) sequence which acts as a putative docking domain. MST3(Thr¹⁷⁸) phosphorylation increased MST3 kinase activity, but this activity was independent of MST3(Thr³²⁸) phosphorylation. Interestingly, MST3(Thr³²⁸) lies immediately C-terminal to a STRAD (Sterile20-related adaptor) pseudokinase-like site identified recently as being involved in binding of GCKIII/GCKVI members to MO25 scaffolding proteins. MST3(Thr¹⁷⁸/Thr³²⁸) phosphorylation was concurrent with dissociation of MST3 from GOLGA2/gm130 and association of MST3 with MO25, and MST3(Thr³²⁸) phosphorylation was necessary for formation of the activated MST3–MO25 holocomplex.

Interaction between ionizing radiation and estrogen: what we are missing?

Following complexity as a new approach in science of 21st century biomonitoring of biological effects caused by ionizing radiation received an option of a new dimension. Insight in biological response of mammals to ionizing radiation exposure by integration of genome, non-genome and distant organ bystander effects will significantly change evaluation of health risk and preventive measures. Impact of estrogen on carcinogenesis caused by occupational or accidental exposure to ionizing radiation additionally enables biodosimetry to recognize vulnerable subpopulations according to gender and age. Estrogen, as a potent molecule involved in number of biological pathways during development and adulthood, shows close interaction with pathological processes launched by overexposure to ionizing radiation which should be included in future research and radiation protection.