Mst1 promotes cardiac myocyte apoptosis through phosphorylation and inhibition of Bcl-xL

Hippo Signaling in the Heart - Non-Canonical Pathways Impact Growth, Survival and Function

Initially identified inDrosophila melanogaster, the Hippo signaling pathway regulates organ size through modulation of cell proliferation, survival and differentiation. This pathway is evolutionarily conserved and canonical signaling involves a kinase cascade that phosphorylates and inhibits the downstream effector Yes-associated protein (YAP). Recent research has demonstrated a fundamental role of Hippo signaling in cardiac development, homeostasis, injury and regeneration, and remains the subject of intense investigation. However, 2 prominent members of this pathway, RASSF1A and Mst1, have been shown to influence heart function and stress responses through YAP-independent mechanisms. This review summarizes non-canonical targets of RASSF1A and Mst1 and discusses their role in the context of cardiac hypertrophy, autophagy, apoptosis and function. (Circ J 2016; 80: 1504-1510).

Mst1 shuts off cytosolic antiviral defense through IRF3 phosphorylation

Here, Meng et al. investigated how interferon regulatory factor 3 (IRF3) activation, a key signal mediator/transcriptional factor of the antiviral-sensing pathway, is regulated. They demonstrate that Mst1, a stress response kinase, represses cytosolic antiviral sensing and defense through the repression of RNA virus-induced activation of TBK1 and interference with the IRF3 homodimerization and chromatin binding via direct phosphorylation of IRF3 Thr253 and Thr75 residues.

Cytosolic RNA/DNA sensing elicits primary defense against viral pathogens. Interferon regulatory factor 3 (IRF3), a key signal mediator/transcriptional factor of the antiviral-sensing pathway, is indispensible for interferon production and antiviral defense. However, how the status of IRF3 activation is controlled remains elusive. Through a functional screen of the human kinome, we found that mammalian sterile 20-like kinase 1 (Mst1), but not Mst2, profoundly inhibited cytosolic nucleic acid sensing. Mst1 associated with IRF3 and directly phosphorylated IRF3 at Thr75 and Thr253. This Mst1-mediated phosphorylation abolished activated IRF3 homodimerization, its occupancy on chromatin, and subsequent IRF3-mediated transcriptional responses. In addition, Mst1 also impeded virus-induced activation of TANK-binding kinase 1 (TBK1), further attenuating IRF3 activation. As a result, Mst1 depletion or ablation enabled an enhanced antiviral response and defense in cells and mice. Therefore, the identification of Mst1 as a novel physiological negative regulator of IRF3 activation provides mechanistic insights into innate antiviral defense and potential antiviral prevention strategies.

Mst1-mediated phosphorylation of Bcl-xL is required for myocardial reperfusion injury

Mst1 is a central Ser-Thr kinase in the Hippo pathway, which promotes apoptosis and inhibits cell proliferation. We have shown previously that, in cardiomyocytes, oxidative stress activates Mst1 at mitochondria, where Mst1 phosphorylates Bcl-xL at Ser14, inducing dissociation of Bcl-xL from Bax and thereby promoting apoptosis. However, the functional significance of Ser14 phosphorylation of endogenous Bcl-xL in vivo remains elusive. We generated knockin (KI) mice in which Ser14 of Bcl-xL is replaced with Ala. KI mice were born at the expected Mendelian ratio, and adult KI mice exhibited normal cardiac morphology and function at baseline. However, KI mice were protected from myocardial ischemia/reperfusion (I/R) injury and exhibited reduced cardiomyocyte apoptosis. Although suppression of endogenous Mst1 also reduced I/R injury, there was no additive protective effect when Mst1 was inhibited in KI mice. The development of dilated cardiomyopathy induced by cardiac-specific overexpression of Mst1 was also ameliorated in KI mice. Lats2 and YAP, two other key components of the Hippo pathway, were not affected in KI mice. These results suggest that Ser14 phosphorylation of Bcl-xL plays an essential role in mediating both cardiomyocyte apoptosis and myocardial injury by acting as a key downstream mediator of Mst1 independently of the canonical Hippo pathway.

Phosphoproteome Analysis of Invasion and Metastasis-Related Factors in Pancreatic Cancer Cells

Mechanisms of abnormal protein phosphorylation that regulate cell invasion and metastasis in pancreatic cancer remain obscure. In this study, we used high-throughput phosphorylation array to test two pancreatic cancer cell lines (PC-1 cells with a low, and PC-1.0 cells with a high potential for invasion and metastasis). We noted that a total of 57 proteins revealed a differential expression (fold change ≥ 2.0). Six candidate proteins were further validated by western blot with results found to be accordance with the array. Of 57 proteins, 32 up-regulated proteins (e.g. CaMK1-α and P90RSK) were mainly involved in ErbB and neurotrophin signaling pathways as determined using DAVID software, while 25 down-regulated proteins (e.g. BID and BRCA1) were closely involved in apoptosis and p53 signaling pathways. Moreover, four proteins (AKT1, Chk2, p53 and P70S6K) with different phosphorylation sites were found, not only among up-regulated, but also among down-regulated proteins. Importantly, specific phosphorylation sites can affect cell biological functions. CentiScaPe software calculated topological characteristics of each node in the protein-protein interaction (PPI) network: we found that AKT1 owns the maximum node degrees and betweenness in the up-regulation protein PPI network (26 nodes, average path length: 1.89, node degrees: 6.62±4.18, betweenness: 22.23±35.72), and p53 in the down-regulation protein PPI network (17 nodes, average path length: 2.04, node degrees: 3.65±2.47, betweenness: 16.59±29.58). In conclusion, the identification of abnormal protein phosphorylation related to invasion and metastasis may allow us to identify new biomarkers in an effort to develop novel therapeutic drug targets for pancreatic cancer treatment.

Unique epigenetic influence of H2AX phosphorylation and H3K56 acetylation on normal stem cell radioresponses

Normal stem cells from tissues often exhibiting radiation injury are highly radiosensitive and exhibit a muted DNA damage response, in contrast to differentiated progeny. These radioresponses can be attributed to unique epigenetic regulation in stem cells, identifying potential therapeutic targets for radioprotection.

Normal tissue injury resulting from cancer radiotherapy is often associated with diminished regenerative capacity. We examined the relative radiosensitivity of normal stem cell populations compared with non–stem cells within several radiosensitive tissue niches and culture models. We found that these stem cells are highly radiosensitive, in contrast to their isogenic differentiated progeny. Of interest, they also exhibited a uniquely attenuated DNA damage response (DDR) and muted DNA repair. Whereas stem cells exhibit reduced ATM activation and ionizing radiation–induced foci, they display apoptotic pannuclear H2AX-S139 phosphorylation (γH2AX), indicating unique radioresponses. We also observed persistent phosphorylation of H2AX-Y142 along the DNA breaks in stem cells, which promotes apoptosis while inhibiting DDR signaling. In addition, down-regulation of constitutively elevated histone-3 lysine-56 acetylation (H3K56ac) in stem cells significantly decreased their radiosensitivity, restored DDR function, and increased survival, signifying its role as a key contributor to stem cell radiosensitivity. These results establish that unique epigenetic landscapes affect cellular heterogeneity in radiosensitivity and demonstrate the nonubiquitous nature of radiation responses. We thus elucidate novel epigenetic rheostats that promote ionizing radiation hypersensitivity in various normal stem cell populations, identifying potential molecular targets for pharmacological radioprotection of stem cells and hopefully improving the efficacy of future cancer treatment.

TRAF2 functions as an activator switch in the reactive oxygen species-induced stimulation of MST1

Reactive oxygen species (ROS) have many physiological and pathological effects on diverse cellular events. In particular, excessive ROS causes oxidative stress that leads to cell death. The mammalian STE20-like kinase-1 (MST1), a multifunctional serine-threonine kinase, plays a pivotal role in oxidative stress-induced cellular signaling events. Tumor necrosis factor receptor (TNFR)-associated factor 2 (TRAF2) is also known to be essential for oxidative stress-induced cell death. Here, we showed that H2O2 induced the physical interaction between TRAF2 and MST1, and that this interaction promoted the homodimerization as well as the activation of MST1. Furthermore, TRAF2 was required for MST1 to mediate the H2O2-induced stimulation of c-Jun N-terminal kinase and p38 kinase as well as apoptosis. Taken together, our results suggest that TRAF2 functions as a key activator of MST1 in oxidative stress-induced intracellular signaling processes.

Tocilizumab inhibits neuronal cell apoptosis and activates STAT3 in cerebral infarction rat model

Cerebral infarction is a severe hypoxic ischemic necrosis with accelerated neuronal cell apoptosis in the brain. As a monoclonal antibody against interleukin 6, tocilizumab (TCZ) is widely used in immune diseases, whose function in cerebral infarction has not been studied. This study aims to reveal the role of TCZ in regulating neuronal cell apoptosis in cerebral infarction. The cerebral infarction rat model was constructed by middle cerebral artery occlusion and treated with TCZ. Cell apoptosis in hippocampus and cortex of the brain was examined with TUNEL method. Rat neuronal cells cultured in oxygen-glucose deprivation (OGD) conditions and treated with TCZ were used to compare cell viability and apoptosis. Apoptosis-related factors including B-cell lymphoma extra large (Bcl-xL) and Caspase 3, as well as the phosphorylated signal transducer and activator of transcription 3 (p-STAT3) in brain cortex were analyzed from the protein level. Results indicated that TCZ treatment could significantly prevent the promoted cell apoptosis caused by cerebral infarction or OGD (P < 0.05 or P < 0.01). In brain cortex of the rat model, TCZ up-regulated Bcl-xL and down-regulated Caspase 3, consistent with the inhibited cell apoptosis. It also promoted tyrosine 705 phosphorylation of STAT3, which might be the potential regulatory mechanism of TCZ in neuronal cells. This study provided evidence for the protective role of TCZ against neuronal cell apoptosis in cerebral infarction. Based on these fundamental data, TCZ is a promising option for treating cerebral infarction, but further investigations on related mechanisms are still necessary.

A new mitochondrial pool of cyclin E, regulated by Drp1, is linked to cell-density-dependent cell proliferation

The regulation and function of the crucial cell cycle regulator cyclin E (CycE) remains elusive. Unlike other cyclins, CycE can be uniquely controlled by mitochondrial energetics, the exact mechanism being unclear. Using mammalian cells (in vitro) and Drosophila (in vivo) model systems in parallel, we show that CycE can be directly regulated by mitochondria through its recruitment to the organelle. Active mitochondrial bioenergetics maintains a distinct mitochondrial pool of CycE (mtCycE) lacking a key phosphorylation required for its degradation. Loss of the mitochondrial fission protein dynamin-related protein 1 (Drp1, SwissProt O00429 in humans) augments mitochondrial respiration and elevates the mtCycE pool allowing CycE deregulation, cell cycle alterations and enrichment of stem cell markers. Such CycE deregulation after Drp1 loss attenuates cell proliferation in low-cell-density environments. However, in high-cell-density environments, elevated MEK-ERK signaling in the absence of Drp1 releases mtCycE to support escape of contact inhibition and maintain aberrant cell proliferation. Such Drp1-driven regulation of CycE recruitment to mitochondria might be a mechanism to modulate CycE degradation during normal developmental processes as well as in tumorigenic events.

Kinases Mst1 and Mst2 positively regulate phagocytic induction of reactive oxygen species and bactericidal activity

Mitochondria need to be juxtaposted to phagosomes to synergistically produce ample reactive oxygen species (ROS) in phagocytes for pathogens killing. However, how phagosomes transmit signal to recruit mitochondria remains unclear. Here, we report that the kinases Mst1 and Mst2 function to control ROS production by regulating mitochondrial trafficking and mitochondrion-phagosome juxtaposition. Mst1 and Mst2 activate Rac GTPase to promote Toll-like receptor (TLR)-triggered assembly of the TRAF6-ECSIT complex that is required for mitochondrial recruitment to phagosomes. Inactive forms of Rac, including the human Rac2^D57N mutant, disrupt the TRAF6-ECSIT complex by sequestering TRAF6, and severely dampen ROS production and greatly increase susceptibility to bacterial infection. These findings demonstrate the TLR-Mst1-Mst2-Rac signalling axis to be critical for effective phagosome-mitochondrion function and bactericidal activity.

miR-206 Mediates YAP-Induced Cardiac Hypertrophy and Survival

RATIONALE

In Drosophila, the Hippo signaling pathway negatively regulates organ size by suppressing cell proliferation and survival through the inhibition of Yorkie, a transcriptional cofactor. Yes-associated protein (YAP), the mammalian homolog of Yorkie, promotes cardiomyocyte growth and survival in postnatal hearts. However, the underlying mechanism responsible for the beneficial effect of YAP in cardiomyocytes remains unclear.

OBJECTIVES

We investigated whether miR-206, a microRNA known to promote hypertrophy in skeletal muscle, mediates the effect of YAP on promotion of survival and hypertrophy in cardiomyocytes.

METHODS AND RESULTS

Microarray analysis indicated that YAP increased miR-206 expression in cardiomyocytes. Increased miR-206 expression induced cardiac hypertrophy and inhibited cell death in cultured cardiomyocytes, similar to that of YAP. Downregulation of endogenous miR-206 in cardiomyocytes attenuated YAP-induced cardiac hypertrophy and survival, suggesting that miR-206 plays a critical role in mediating YAP function. Cardiac-specific overexpression of miR-206 in mice induced hypertrophy and protected the heart from ischemia/reperfusion injury, whereas suppression of miR-206 exacerbated ischemia/reperfusion injury and prevented pressure overload-induced cardiac hypertrophy. miR-206 negatively regulates Forkhead box protein P1 expression in cardiomyocytes and overexpression of Forkhead box protein P1 attenuated miR-206-induced cardiac hypertrophy and survival, suggesting that Forkhead box protein P1 is a functional target of miR-206.

CONCLUSIONS

YAP increases the abundance of miR-206, which in turn plays an essential role in mediating hypertrophy and survival by silencing Forkhead box protein P1 in cardiomyocytes.

Executioner Caspase-3 and 7 Deficiency Reduces Myocyte Number in the Developing Mouse Heart

Executioner caspase-3 and -7 are proteases promoting cell death but non-apoptotic roles are being discovered. The heart expresses caspases only during development, suggesting they contribute to the organ maturation process. Therefore, we aimed at identifying novel functions of caspases in heart development. We induced simultaneous deletion of executioner caspase-3 and -7 in the mouse myocardium and studied its effects. Caspase knockout hearts are hypoplastic at birth, reaching normal weight progressively through myocyte hypertrophy. To identify the molecular pathways involved in these effects, we used microarray-based transcriptomics and multiplexed quantitative proteomics to compare wild type and executioner caspase-deficient myocardium at different developmental stages. Transcriptomics showed reduced expression of genes promoting DNA replication and cell cycle progression in the neonatal caspase-deficient heart suggesting reduced myocyte proliferation, and expression of non-cardiac isoforms of structural proteins in the adult null myocardium. Proteomics showed reduced abundance of proteins involved in oxidative phosphorylation accompanied by increased abundance of glycolytic enzymes underscoring retarded metabolic maturation of the caspase-null myocardium. Correlation between mRNA expression and protein abundance of relevant genes was confirmed, but transcriptomics and proteomics indentified complementary molecular pathways influenced by caspases in the developing heart. Forced expression of wild type or proteolytically inactive caspases in cultured cardiomyocytes induced expression of genes promoting cell division. The results reveal that executioner caspases can modulate heart’s cellularity and maturation during development, contributing novel information about caspase biology and heart development.

STK4 (MST1) deficiency in two siblings with autoimmune cytopenias: A novel mutation

Combined immunodeficiencies (CIDs) are heterogeneous group of disorders characterized by abrogated/impaired T cell development and/or functions that resulted from diverse genetic defects. In addition to the susceptibility to infections with various microorganisms, the patients may have lymphoproliferation, autoimmunity, inflammation, allergy and malignancy. Recently, three groups have independently reported patients having mutations in STK4 gene that cause a novel autosomal recessive (AR) CID. We describe here two siblings with a novel STK4 mutation identified during the evaluation of a group of patients with features highly overlapping with those of DOCK-8 deficiency, a form of AR hyperimmunoglobulin E syndrome. The patients' clinical features include autoimmune cytopenias, viral skin (molluscum contagiosum and perioral herpetic infection) and bacterial infections, mild onychomycosis, mild atopic and seborrheic dermatitis, lymphopenia (particularly CD4 lymphopenia), and intermittent mild neutropenia. Determination of the underlying defect and reporting the patients are required for the description of the phenotypic spectrum of each immunodeficiency.

Significance of expression of MST1 in homocysteine-induced hepatocyte apoptosis

AIM: To study the role of the mammalian sterile 20-like kinase 1 (MST1) gene in homocysteine-induced hepatocyte apoptosis.

METHODS: Five-week-old C57BL/6J mice of SPF grade were divided into four groups: a normal control group, an ApoE^-/- group, a high methionine diet group, and an intervention group (n = 12 each). In the normal control group, normal mice were fed a normal diet. In the ApoE^-/- group, male ApoE^-/- mice were fed a normal diet. In the high methionine diet group, male ApoE^-/- mice were fed a high methionine diet. In the intervention group, male ApoE^-/- mice were fed a high methionine diet plus folic acid and vitamin B₁₂. Transmission electron microscopy and DAPI staining were used to determine the level of apoptosis in hepatic tissue. qRT-PCR and Western blot were used to determine the expression of MST1. Hepatocytes were then cultured in the presence or absence of homocysteine (100 μmol/L) alone or 100 μmol/L homocysteine plus folic acid and vitamin B₁₂; flow cytometry was used to determine the level of hepatocytes apoptosis, and the expression of MST1 was detected by qRT-PCR and Western blot.

RESULTS: After the mice were fed for 14 wk, serum homocysteine level in the high methionine diet group was 2.3 and 1.9 times as high as that in the normal control group and the ApoE^-/- group (P < 0.01), respectively. Serum homocysteine level in the intervention group was 28% lower than that in the high methionine diet group (P < 0.01). These findings suggest that the model was successfully established. Electron microscopy showed that in the high methionine diet group, there were chromosome swelling or condensation, mitochondrial swelling, marked endoplasmic reticulum swelling and break, which suggested the trend of cell apoptosis in hepatic tissue. Compared with the normal control group and ApoE^-/- group, hepatic apoptosis level in the high methionine diet group was higher. However, hepatic apoptosis level in the intervention group was lower than that in the high methionine diet group. Compared with the normal control group and ApoE^-/- group, the expression of MST1 mRNA and protein in heapatic tissue in the high methionine diet group was upregulated (P < 0.05 or P < 0.01); however, MST1 expression in the intervention group was significantly lower than that in the high methionine diet group (P < 0.05). In vitro, compared with the normal control group, hepatocytes apoptosis level in the homocysteine alone group was significantly higher (P < 0.01); however, hepatocytes apoptosis level in the intervention group was significantly lower than that in the homocysteine alone group (P < 0.05). Compared with the normal control group, the expression of MST1 mRNA and protein in the homocysteine alone group was upregulated (P < 0.01); however, MST1 expression in the intervention group was significantly lower than that in the homocysteine alone group (P < 0.05).

CONCLUSION: MST1 expression is upregulated in homocysteine-induced hepatocyte apoptosis, and folic acid and vitamin B₁₂ can suppress the up-regulation of MST1.

Regulation of signal transduction by reactive oxygen species in the cardiovascular system

Oxidative stress has long been implicated in cardiovascular disease, but more recently, the role of reactive oxygen species (ROS) in normal physiological signaling has been elucidated. Signaling pathways modulated by ROS are complex and compartmentalized, and we are only beginning to identify the molecular modifications of specific targets. Here, we review the current literature on ROS signaling in the cardiovascular system, focusing on the role of ROS in normal physiology and how dysregulation of signaling circuits contributes to cardiovascular diseases, including atherosclerosis, ischemia-reperfusion injury, cardiomyopathy, and heart failure. In particular, we consider how ROS modulate signaling pathways related to phenotypic modulation, migration and adhesion, contractility, proliferation and hypertrophy, angiogenesis, endoplasmic reticulum stress, apoptosis, and senescence. Understanding the specific targets of ROS may guide the development of the next generation of ROS-modifying therapies to reduce morbidity and mortality associated with oxidative stress.

mTORC2 regulates cardiac response to stress by inhibiting MST1

The mTOR and Hippo pathways have recently emerged as the major signaling transduction cascades regulating organ size and cellular homeostasis. However, direct crosstalk between two pathways is yet to be determined. Here, we demonstrate that mTORC2 is a direct negative regulator of the MST1 kinase, a key component of the Hippo pathway. mTORC2 phosphorylates MST1 at serine 438 in the SARAH domain, thereby reducing its homodimerization and activity. We found that Rictor/mTORC2 preserves cardiac structure and function by restraining the activity of MST1 kinase. Cardiac-specific mTORC2 disruption through Rictor deletion leads to a marked activation of MST1 that, in turn, promotes cardiac dysfunction and dilation, impairing cardiac growth and adaptation in response to pressure overload. In conclusion, our study demonstrates the existence of a direct crosstalk between mTORC2 and MST1 that is critical for cardiac cell survival and growth.

Mst1 inhibition rescues β1-adrenergic cardiomyopathy by reducing myocyte necrosis and non-myocyte apoptosis rather than myocyte apoptosis

It is generally held that inhibition of mammalian sterile 20-like kinase 1 (Mst1) protects the heart through reducing myocyte apoptosis. We determined whether inhibition with a dominant-negative Mst1 (DN-Mst1) would protect against the cardiomyopathy induced by chronic β1-adrenergic receptor (β1-AR) stimulation by preventing myocyte apoptosis. DN-Mst1 mice were mated with β1-AR transgenic (Tg) mice and followed for 20 months. β1-AR Tg mice developed cardiomyopathy as they aged, as reflected by premature mortality and depressed cardiac function, which were rescued in β1-AR × DN-Mst1 bigenic mice. Surprisingly, myocyte apoptosis did not significantly decrease with Mst1 inhibition. Instead, Mst1 inhibition predominantly reduced non-myocyte apoptosis, e.g., fibroblasts, macrophages, neutrophils and endothelial cells. Fibrosis in the hearts with cardiomyopathy increased fivefold and this increase was nearly abolished in the bigenic mice with Mst1 inhibition. Regression analysis showed no correlation between myocyte apoptosis and cardiac function or myocyte number, whereas the latter two correlated significantly, p < 0.05, with fibrosis, which generally results from necrosis. To examine the role of myocyte necrosis, chronic β-AR stimulation with isoproterenol was induced for 24 h and myocyte necrosis was assessed by 1% Evans blue dye. Compared to WT, DN-Mst1 mice showed significant inhibition, p < 0.05, of myocyte necrosis. We confirmed this result in Mst1-knockout mice, which also showed significant protection, p < 0.05, against myocyte necrosis compared to WT. These data indicate that Mst1 inhibition rescued cardiac fibrosis and myocardial dysfunction in β1-AR cardiomyopathy. However, this did not occur through Mst1 inhibition of myocyte apoptosis but rather by inhibition of cardiomyocyte necrosis and non-myocyte apoptosis, features of Mst1 not considered previously.

The hippo signaling pathway: implications for heart regeneration and disease

Control of cell number and organ size is critical for appropriate development and tissue homeostasis. Studies in both Drosophila and mammals have established the Hippo signaling pathway as an important modulator of organ size and tumorigenesis. Upon activation, this kinase cascade modulates gene expression through the phosphorylation and inhibition of transcription co-activators that are involved in cell proliferation, differentiation, growth and apoptosis. Hippo signaling serves to limit organ size and suppress malignancies, and has been implicated in tissue regeneration following injury. These outcomes highlight the important role that Hippo signaling plays in regulating both physiologic and pathologic processes. In this review, an overview of the signaling pathway will be discussed as well as recent work that has investigated its role in cardiac development, regeneration and disease.

miR-133a enhances the protective capacity of cardiac progenitors cells after myocardial infarction

miR-133a and miR-1 are known as muscle-specific microRNAs that are involved in cardiac development and pathophysiology. We have shown that both miR-1 and miR-133a are early and progressively upregulated during in vitro cardiac differentiation of adult cardiac progenitor cells (CPCs), but only miR-133a expression was enhanced under in vitro oxidative stress. miR-1 was demonstrated to favor differentiation of CPCs, whereas miR-133a overexpression protected CPCs against cell death, targeting, among others, the proapoptotic genes Bim and Bmf. miR-133a-CPCs clearly improved cardiac function in a rat myocardial infarction model by reducing fibrosis and hypertrophy and increasing vascularization and cardiomyocyte proliferation. The beneficial effects of miR-133a-CPCs seem to correlate with the upregulated expression of several relevant paracrine factors and the plausible cooperative secretion of miR-133a via exosomal transport. Finally, an in vitro heart muscle model confirmed the antiapoptotic effects of miR-133a-CPCs, favoring the structuration and contractile functionality of the artificial tissue.

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miR-1 and miR-133a have a role in adult cardiac progenitor cells (CPCs)

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miR-133a-CPCs protect cardiac function

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miR-133a-CPCs increase vascularization and protect against hypertrophy

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miR-133a is enriched in CPCs-derived exosomes

The RASSF1A tumor suppressor regulates XPA-mediated DNA repair

RASSF1A may be the most frequently inactivated tumor suppressor identified in human cancer so far. It is a proapoptotic Ras effector and plays an important role in the apoptotic DNA damage response (DDR). We now show that in addition to DDR regulation, RASSF1A also plays a key role in the DNA repair process itself. We show that RASSF1A forms a DNA damage-regulated complex with the key DNA repair protein xeroderma pigmentosum A (XPA). XPA requires RASSF1A to exert full repair activity, and RASSF1A-deficient cells exhibit an impaired ability to repair DNA. Moreover, a cancer-associated RASSF1A single-nucleotide polymorphism (SNP) variant exhibits differential XPA binding and inhibits DNA repair. The interaction of XPA with other components of the repair complex, such as replication protein A (RPA), is controlled in part by a dynamic acetylation/deacetylation cycle. We found that RASSF1A and its SNP variant differentially regulate XPA protein acetylation, and the SNP variant hyperstabilizes the XPA-RPA70 complex. Thus, we identify two novel functions for RASSF1A in the control of DNA repair and protein acetylation. As RASSF1A modulates both apoptotic DDR and DNA repair, it may play an important and unanticipated role in coordinating the balance between repair and death after DNA damage.

The Hippo signal transduction network in skeletal and cardiac muscle

The discovery of the Hippo pathway can be traced back to two areas of research. Genetic screens in fruit flies led to the identification of the Hippo pathway kinases and scaffolding proteins that function together to suppress cell proliferation and tumor growth. Independent research, often in the context of muscle biology, described Tead (TEA domain) transcription factors, which bind CATTCC DNA motifs to regulate gene expression. These two research areas were joined by the finding that the Hippo pathway regulates the activity of Tead transcription factors mainly through phosphorylation of the transcriptional coactivators Yap and Taz, which bind to and activate Teads. Additionally, many other signal transduction proteins crosstalk to members of the Hippo pathway forming a Hippo signal transduction network. We discuss evidence that the Hippo signal transduction network plays important roles in myogenesis, regeneration, muscular dystrophy, and rhabdomyosarcoma in skeletal muscle, as well as in myogenesis, organ size control, and regeneration of the heart. Understanding the role of Hippo kinases in skeletal and heart muscle physiology could have important implications for translational research.