Characterization and function of MYPT2, a target subunit of myosin phosphatase in heart

Myosin Light Chain Phosphatase Plays an Important Role in Cardiac Fibrosis in a Model of Mineralocorticoid Receptor-Associated Hypertension

BACKGROUND

Myosin phosphatase targeting subunit 2 (MYPT2) is an important subunit of cardiac MLC (myosin light chain) phosphatase, which plays a crucial role in regulating the phosphorylation of MLC to phospho-MLC (p-MLC). A recent study demonstrated mineralocorticoid receptor-related hypertension is associated with RhoA/Rho-associated kinase/MYPT1 signaling upregulation in smooth muscle cells. Our purpose is to investigate the effect of MYPT2 on cardiac function and fibrosis in mineralocorticoid receptor-related hypertension.

METHODS AND RESULTS

HL-1 murine cardiomyocytes were incubated with different concentrations or durations of aldosterone. After 24-hour stimulation, aldosterone increased CTGF (connective tissue growth factor) and MYPT2 and decreased p-MLC in a dose-dependent manner. MYPT2 knockdown decreased CTGF. Cardiac-specific MYPT2-knockout (c-MYPT2-/-) mice exhibited decreased type 1 phosphatase catalytic subunit β and increased p-MLC. A disease model of mouse was induced by subcutaneous aldosterone and 8% NaCl food for 4 weeks after uninephrectomy. Blood pressure elevation and left ventricular hypertrophy were observed in both c-MYPT2-/- and MYPT2+/+ mice, with no difference in heart weights or nuclear localization of mineralocorticoid receptor in cardiomyocytes. However, c-MYPT2-/- mice had higher ejection fraction and fractional shortening on echocardiography after aldosterone treatment. Histopathology revealed less fibrosis, reduced CTGF, and increased p-MLC in c-MYPT2-/- mice. Basal global radial strain and global longitudinal strain were higher in c-MYPT2-/- than in MYPT2+/+ mice. After aldosterone treatment, both global radial strain and global longitudinal strain remained higher in c-MYPT2-/- mice compared with MYPT2+/+ mice.

CONCLUSIONS

Cardiac-specific MYPT2 knockout leads to decreased myosin light chain phosphatase and increased p-MLC. MYPT2 deletion prevented cardiac fibrosis and dysfunction in a model of mineralocorticoid receptor-associated hypertension.

The MYPT2-regulated striated muscle-specific myosin light chain phosphatase limits cardiac myosin phosphorylation in vivo

The physiological importance of cardiac myosin regulatory light chain (RLC) phosphorylation by its dedicated cardiac myosin light chain kinase has been established in both humans and mice. Constitutive RLC-phosphorylation, regulated by the balanced activities of cardiac myosin light chain kinase and myosin light chain phosphatase (MLCP), is fundamental to the biochemical and physiological properties of myofilaments. However, limited information is available on cardiac MLCP. In this study, we hypothesized that the striated muscle-specific MLCP regulatory subunit, MYPT2, targets the phosphatase catalytic subunit to cardiac myosin, contributing to the maintenance of cardiac function in vivo through the regulation of RLC-phosphorylation. To test this hypothesis, we generated a floxed-PPP1R12B mouse model crossed with a cardiac-specific Mer-Cre-Mer to conditionally ablate MYPT2 in adult cardiomyocytes. Immunofluorescence microscopy using the gene-ablated tissue as a control confirmed the localization of MYPT2 to regions where it overlaps with a subset of RLC. Biochemical analysis revealed an increase in RLC-phosphorylation in vivo. The loss of MYPT2 demonstrated significant protection against pressure overload-induced hypertrophy, as evidenced by heart weight, qPCR of hypertrophy-associated genes, measurements of myocyte diameters, and expression of β-MHC protein. Furthermore, mantATP chase assays revealed an increased ratio of myosin heads distributed to the interfilament space in MYPT2-ablated heart muscle fibers, confirming that RLC-phosphorylation regulated by MLCP, enhances cardiac performance in vivo. Our findings establish MYPT2 as the regulatory subunit of cardiac MLCP, distinct from the ubiquitously expressed canonical smooth muscle MLCP. Targeting MYPT2 to increase cardiac RLC-phosphorylation in vivo may improve baseline cardiac performance, thereby attenuating pathological hypertrophy.

FAT10 differentially stabilizes MYPT2 isoforms

Myosin phosphatase (MP) is an enzyme complex that regulates muscle contraction and plays important roles in various physiological and pathological conditions. Myosin phosphatase targeting subunit (MYPT) 2, a subunit of MP, interacts with protein phosphatase 1c to regulate its phosphatase activity. MYPT2 exists in various isoforms that differ in the composition of essential motifs that contribute to its function. However, regulatory mechanisms underlying these isoforms are poorly understood. Human leukocyte antigen-F adjacent transcript 10 (FAT10) is a ubiquitin-like modifier that not only targets proteins for proteasomal degradation but also stabilizes its interacting proteins. In this study, we investigated the effect of the interaction between FAT10 and MYPT2 isoform a (the canonical full-length form of MYPT2) or MYPT2 isoform f (the natural truncated form of MYPT2). FAT10 interacted with both MYPT2 isoforms a and f; however, only MYPT2 isoform f was increased by FAT10, whereas MYPT2 isoform a remained unaffected by FAT10. We further confirmed that, in contrast to MYPT2 isoform a, MYPT2 isoform f undergoes rapid degradation via the ubiquitin-proteasome pathway and that FAT10 stabilizes MYPT2 isoform f by inhibiting its ubiquitination. Therefore, our findings suggest that the interaction between FAT10 and MYPT2 isoforms leads to distinct stabilization effects on each isoform, potentially modulating MP activity.

Clinical report and genetic analysis of a neonate with genitourinary and/or brain malformation syndrome caused by a non-coding sequence variant of PPP1R12A

BACKGROUND

Genitourinary and/or brain malformation syndrome (GUBS) is a recently discovered syndrome involving abnormalities of the neurological or urogenital system. PPP1R12A may be the pathological gene causing this syndrome. Currently, to our knowledge, there is only one study related to GUBS in the world. Here, we report a clinical case of a Chinese newborn with congenital micropenis caused by a non-coding sequence pathogenic variant of PPP1R12A, providing additional evidence on genetic causes of genital malformation.

METHODS

The genetic cause of the patient's malformation was detected using trio-whole exome sequencing and Sanger sequencing, and reverse transcription-PCR analysis was performed by constructing the minigene mutant plasmid in vitro.

RESULTS

Genetic testing revealed a novel heterozygous variant, c.2666+3A>G, of the PPP1R12A gene of the patient. The parents at this site were wild-type, indicating that this might be a de novo variant. The minigene experiment showed that the c.2666+3A>G plasmid led to the deletion of 17 bp in exon 20, and a new mRNA product c.2650_2666del (p.Thr884IleTer2) with skipping of exon 20 was produced. This may lead to PPP1R12A haploinsufficiency and cause biological harm.

CONCLUSIONS

To our knowledge, this is the first clinical study on a rare variant of PPP1R12A in the Chinese population. The c.2666+3A>G may lead to external genitalia malformation, such as congenital micropenis in male neonates. The results of this study further verified the correlation between GUBS and PPP1R12A haploinsufficiency and revealed the important role of a non-coding sequence variant in the pathogenesis of the disease.

Pepducin ICL1-9-Mediated β2-Adrenergic Receptor-Dependent Cardiomyocyte Contractility Occurs in a Gi Protein/ROCK/PKD-Sensitive Manner

PURPOSE

β-Adrenergic receptors (βAR) are essential targets for the treatment of heart failure (HF); however, chronic use of βAR agonists as positive inotropes to increase contractility in a Gs protein-dependent manner is associated with increased mortality. Alternatively, we previously reported that allosteric modulation of β2AR with the pepducin intracellular loop (ICL)1-9 increased cardiomyocyte contractility in a β-arrestin (βarr)-dependent manner, and subsequently showed that ICL1-9 activates the Ras homolog family member A (RhoA). Here, we aimed to elucidate both the proximal and downstream signaling mediators involved in the promotion of cardiomyocyte contractility in response to ICL1-9.

METHODS

We measured adult mouse cardiomyocyte contractility in response to ICL1-9 or isoproterenol (ISO, as a positive control) alone or in the presence of inhibitors of various potential components of βarr- or RhoA-dependent signaling. We also assessed the contractile effects of ICL1-9 on cardiomyocytes lacking G protein-coupled receptor (GPCR) kinase 2 (GRK2) or 5 (GRK5).

RESULTS

Consistent with RhoA activation by ICL1-9, both Rho-associated protein kinase (ROCK) and protein kinase D (PKD) inhibition were able to attenuate ICL1-9-mediated contractility, as was inhibition of myosin light chain kinase (MLCK). While neither GRK2 nor GRK5 deletion impacted ICL1-9-mediated contractility, pertussis toxin attenuated the response, suggesting that ICL1-9 promotes downstream RhoA-dependent signaling in a Gi protein-dependent manner.

CONCLUSION

Altogether, our study highlights a novel signaling modality that may offer a new approach to the promotion, or preservation, of cardiac contractility during HF via the allosteric regulation of β2AR to promote Gi protein/βarr-dependent activation of RhoA/ROCK/PKD signaling.

Phosphatase regulatory subunit MYPT2 knock-out partially compensates for the cardiac dysfunction in mice caused by lack of myosin light chain kinase 3

Cardiac contraction is modulated by the phosphorylation state of myosin regulatory light chain 2 (MLC-2v). The level of MLC-2v phosphorylation is dependent on the opposing activities of MLC kinases and phosphatases. The predominant MLC phosphatase found in cardiac myocytes contains Myosin Phosphatase Targeting Subunit 2 (MYPT2). Overexpression of MYPT2 in cardiac myocytes results in a decreased level of MLC phosphorylation, reduced left ventricular contraction and induction of hypertrophy; however, the effect of knocking out MYPT2 on cardiac function is unknown. We obtained heterozygous mice containing a MYPT2 null allele from the Mutant Mouse Resource Center. These mice were produced in a C57BL/6N background which lack MLCK3, the main regulatory light chain kinase in cardiac myocytes. We found that mice null for MYPT2 were viable and had no obvious phenotypic abnormality when compared to wild-type mice. Additionally, we determined that wild-type C57BL/6N mice had a low basal level of MLC-2v phosphorylation which was significantly increased when MYPT2 was absent. At 12-weeks, MYPT2 knock-out mice had smaller hearts and showed down-regulation of genes involved in cardiac remodeling. Using cardiac echo, we found that 24-week-old male MYPT2 knock-out mice had decreased heart size with increased fractional shortening compared to their MYPT2 wild-type littermates. Collectively, these studies highlight the important role that MYPT2 plays in cardiac function in vivo and demonstrate that its deletion can partially compensate for the lack of MLCK3.

Myosin light chain phosphatase catalytic subunit dephosphorylates cardiac myosin via mechanisms dependent and independent of the MYPT regulatory subunits

Cardiac muscle myosin regulatory light chain (RLC) is constitutively phosphorylated at ∼0.4 mol phosphate/mol RLC in normal hearts, and phosphorylation is maintained by balanced activities of dedicated cardiac muscle–specific myosin light chain kinase and myosin light chain phosphatase (MLCP). Previously, the identity of the cardiac-MLCP was biochemically shown to be similar to the smooth muscle MLCP, which is a well-characterized trimeric protein comprising the regulatory subunit (MYPT1), catalytic subunit PP1cβ, and accessory subunit M20. In smooth muscles in vivo, MYPT1 and PP1cβ co-stabilize each other and are both necessary for normal smooth muscle contractions. In the cardiac muscle, MYPT1 and MYPT2 are both expressed, but contributions to physiological regulation of cardiac myosin dephosphorylation are unclear. We hypothesized that the main catalytic subunit for cardiac-MLCP is PP1cβ, and maintenance of RLC phosphorylation in vivo is dependent on regulation by striated muscle–specific MYPT2. Here, we used PP1cβ conditional knockout mice to biochemically define cardiac-MLCP proteins and developed a cardiac myofibrillar phosphatase assay to measure the direct contribution of MYPT-regulated and MYPT-independent phosphatase activities toward phosphorylated cardiac myosin. We report that (1) PP1cβ is the main isoform expressed in the cardiac myocyte, (2) cardiac muscle pathogenesis in PP1cβ knockout animals involve upregulation of total PP1cα in myocytes and non-muscle cells, (3) the stability of cardiac MYPT1 and MYPT2 proteins in vivo is not dependent on the PP1cβ expression, and (4) phosphorylated myofibrillar cardiac myosin is dephosphorylated by both myosin-targeted and soluble MYPT-independent PP1cβ activities. These results contribute to our understanding of the cardiac-MLCP in vivo.

MicroRNA-4463 facilitates the development of colon cancer by suppression of the expression of PPP1R12B

PURPOSE

In the present work, we investigated the expression pattern of miR-4463 in the non-metastasis and metastasis colorectal cancer (CRC) patients and its regulation axis.

METHODS

RT-qPCR assay was performed to assess miR-4463 expression in the serum and tissues of patients with non-metastasis and metastasis, and in the CRC cell lines. MTT assay, colony formation assay, transwell assay, and flow cytometry assay were used to examine the role of miR-4463 in CRC cell viability, proliferation, and migration. Bioinformatic analysis was used to identify the potential target gene of miR-4463, and the targeting relationship between miR-4463 and PPP1R12B was verified in vitro using dual luciferase assay. Western blotting assay was used to determine the protein level of the target gene PPP1R12B in CRC cells under the transfections of miR-4463 mimic, inhibitor and vectors overexpressing PPP1R12B.

RESULTS

miR-4463 was markedly increased in the non-metastasis CRC tissues, and increased even higher in the metastasis CRC tissues, while miR-4463 expression had no significant difference in serum from non-metastasis and metastasis CRC samples. Besides, miR-4463 was upregulated in CRC cell lines. Functionally, miR-4463 promoted CRC cell proliferation, migration, and inhibiting cell apoptosis. Further analysis revealed that the miR-4463/PPP1R12B axis was responsible for the role of this miRNA.

CONCLUSION

We reported the roles of miR-4463 in CRC proliferation and migration, supporting that miR-4463 could be a potential predictive diagnostic marker for colon cancer.

Novel γ-sarcoglycan interactors in murine muscle membranes

BACKGROUND

The sarcoglycan complex (SC) is part of a network that links the striated muscle cytoskeleton to the basal lamina across the sarcolemma. The SC coordinates changes in phosphorylation and Ca⁺⁺-flux during mechanical deformation, and these processes are disrupted with loss-of-function mutations in gamma-sarcoglycan (Sgcg) that cause Limb girdle muscular dystrophy 2C/R5.

METHODS

To gain insight into how the SC mediates mechano-signaling in muscle, we utilized LC-MS/MS proteomics of SC-associated proteins in immunoprecipitates from enriched sarcolemmal fractions. Criteria for inclusion were co-immunoprecipitation with anti-Sgcg from C57BL/6 control muscle and under-representation in parallel experiments with Sgcg-null muscle and with non-specific IgG. Validation of interaction was performed in co-expression experiments in human RH30 rhabdomyosarcoma cells.

RESULTS

We identified 19 candidates as direct or indirect interactors for Sgcg, including the other 3 SC proteins. Novel potential interactors included protein-phosphatase-1-catalytic-subunit-beta (Ppp1cb, PP1b) and Na⁺-K⁺-Cl^--co-transporter NKCC1 (SLC12A2). NKCC1 co-localized with Sgcg after co-expression in human RH30 rhabdomyosarcoma cells, and its cytosolic domains depleted Sgcg from cell lysates upon immunoprecipitation and co-localized with Sgcg after detergent permeabilization. NKCC1 localized in proximity to the dystrophin complex at costameres in vivo. Bumetanide inhibition of NKCC1 cotransporter activity in isolated muscles reduced SC-dependent, strain-induced increases in phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2). In silico analysis suggests that candidate SC interactors may cross-talk with survival signaling pathways, including p53, estrogen receptor, and TRIM25.

CONCLUSIONS

Results support that NKCC1 is a new SC-associated signaling protein. Moreover, the identities of other candidate SC interactors suggest ways by which the SC and NKCC1, along with other Sgcg interactors such as the membrane-cytoskeleton linker archvillin, may regulate kinase- and Ca⁺⁺-mediated survival signaling in skeletal muscle.

Regulation of myosin light-chain phosphorylation and its roles in cardiovascular physiology and pathophysiology

The regulation of muscle contraction is a critical function in the cardiovascular system, and abnormalities may be life-threatening or cause illness. The common basic mechanism in muscle contraction is the interaction between the protein filaments myosin and actin. Although this interaction is primarily regulated by intracellular Ca²⁺, the primary targets and intracellular signaling pathways differ in vascular smooth muscle and cardiac muscle. Phosphorylation of the myosin regulatory light chain (RLC) is a primary molecular switch for smooth muscle contraction. The equilibrium between phosphorylated and unphosphorylated RLC is dynamically achieved through two enzymes, myosin light chain kinase, a Ca²⁺-dependent enzyme, and myosin phosphatase, which modifies the Ca²⁺ sensitivity of contractions. In cardiac muscle, the primary target protein for Ca²⁺ is troponin C on thin filaments; however, RLC phosphorylation also plays a modulatory role in contraction. This review summarizes recent advances in our understanding of the regulation, physiological function, and pathophysiological involvement of RLC phosphorylation in smooth and cardiac muscles.

Complex genetic dependencies among growth and neurological phenotypes in healthy children: Towards deciphering developmental mechanisms

The genetic mechanisms of childhood development in its many facets remain largely undeciphered. In the population of healthy infants studied in the Growing Up in Singapore Towards Healthy Outcomes (GUSTO) program, we have identified a range of dependencies among the observed phenotypes of fetal and early childhood growth, neurological development, and a number of genetic variants. We have quantified these dependencies using our information theory-based methods. The genetic variants show dependencies with single phenotypes as well as pleiotropic effects on more than one phenotype and thereby point to a large number of brain-specific and brain-expressed gene candidates. These dependencies provide a basis for connecting a range of variants with a spectrum of phenotypes (pleiotropy) as well as with each other. A broad survey of known regulatory expression characteristics, and other function-related information from the literature for these sets of candidate genes allowed us to assemble an integrated body of evidence, including a partial regulatory network, that points towards the biological basis of these general dependencies. Notable among the implicated loci are RAB11FIP4 (next to NF1), MTMR7 and PLD5, all highly expressed in the brain; DNMT1 (DNA methyl transferase), highly expressed in the placenta; and PPP1R12B and DMD (dystrophin), known to be important growth and development genes. While we cannot specify and decipher the mechanisms responsible for the phenotypes in this study, a number of connections for further investigation of fetal and early childhood growth and neurological development are indicated. These results and this approach open the door to new explorations of early human development.

Controlling Ser/Thr protein phosphatase PP1 activity and function through interaction with regulatory subunits

Protein phosphatase 1 is a major Ser/Thr protein phosphatase activity in eukaryotic cells. It is composed of a catalytic polypeptide (PP1C), with little substrate specificity, that interacts with a large variety of proteins of diverse structure (regulatory subunits). The diversity of holoenzymes that can be formed explain the multiplicity of cellular functions under the control of this phosphatase. In quite a few cases, regulatory subunits have an inhibitory role, downregulating the activity of the phosphatase. In this chapter we shall introduce PP1C and review the most relevant families of PP1C regulatory subunits, with particular emphasis in describing the structural basis for their interaction.

Designer Approaches for G Protein-Coupled Receptor Modulation for Cardiovascular Disease

The new horizon for cardiac therapy may lie beneath the surface, with the downstream mediators of G protein–coupled receptor (GPCR) activity. Targeted approaches have shown that receptor activation may be biased toward signaling through G proteins or through GPCR kinases (GRKs) and β-arrestins, with divergent functional outcomes. In addition to these canonical roles, numerous noncanonical activities of GRKs and β-arrestins have been demonstrated to modulate GPCR signaling at all levels of receptor activation and regulation. Further, research continues to identify novel GRK/effector and β-arrestin/effector complexes with distinct impacts on cardiac function in the normal heart and the diseased heart. Coupled with the identification of once orphan receptors and endogenous ligands with beneficial cardiovascular effects, this expands the repertoire of GPCR targets. Together, this research highlights the potential for focused therapeutic activation of beneficial pathways, with simultaneous exclusion or inhibition of detrimental signaling, and represents a new wave of therapeutic development.

Thick Filament Protein Network, Functions, and Disease Association

Sarcomeres consist of highly ordered arrays of thick myosin and thin actin filaments along with accessory proteins. Thick filaments occupy the center of sarcomeres where they partially overlap with thin filaments. The sliding of thick filaments past thin filaments is a highly regulated process that occurs in an ATP-dependent manner driving muscle contraction. In addition to myosin that makes up the backbone of the thick filament, four other proteins which are intimately bound to the thick filament, myosin binding protein-C, titin, myomesin, and obscurin play important structural and regulatory roles. Consistent with this, mutations in the respective genes have been associated with idiopathic and congenital forms of skeletal and cardiac myopathies. In this review, we aim to summarize our current knowledge on the molecular structure, subcellular localization, interacting partners, function, modulation via posttranslational modifications, and disease involvement of these five major proteins that comprise the thick filament of striated muscle cells. © 2018 American Physiological Society. Compr Physiol 8:631-709, 2018.

Cyclic stretch-induced the cytoskeleton rearrangement and gene expression of cytoskeletal regulators in human periodontal ligament cells

OBJECTIVE

This study aimed to explore the mechanism of the stretch-induced cell realignment and cytoskeletal rearrangement by identifying several mechanoresponsive genes related to cytoskeletal regulators in human PDL cells.

MATERIAL AND METHODS

After the cells were stretched by 1, 10 and 20% strains for 0.5, 1, 2, 4, 6, 12 or 24 h, the changes of the morphology and content of microfilaments were recorded and calculated. Meanwhile, the expression of 84 key genes encoding cytoskeletal regulators after 6 and 24 h stretches with 20% strain was detected by using real-time PCR array. Western blot was applied to identify the protein expression level of several cytoskeletal regulators encoded by these differentially expressed genes.

RESULTS

The confocal fluorescent staining results confirmed that stretch-induced realignment of cells and rearrangement of microfilaments. Among the 84 genes screened, one gene was up-regulated while two genes were down-regulated after 6 h stretch. Meanwhile, three genes were up-regulated while two genes were down-regulated after 24 h stretch. These genes displaying differential expression included genes regulating polymerization/depolymerization of microfilaments (CDC42EP2, FNBP1L, NCK2, PIKFYVE, WASL), polymerization/depolymerization of microtubules (STMN1), interacting between microfilaments and microtubules (MACF1), as well as a phosphatase (PPP1R12B). Among the proteins encoded by these genes, the protein expression level of Cdc42 effector protein-2 (encoded by CDC42EP2) and Stathmin-1 (encoded by STMN1) was down-regulated, while the protein expression level of N-WASP (encoded by WASL) was up-regulated.

CONCLUSION

The present study confirmed the cyclic stretch-induced cellular realignment and rearrangement of microfilaments in the human PDL cells and indicated several force-sensitive genes with regard to cytoskeletal regulators.

Genome-Wide Temporal Profiling of Transcriptome and Open Chromatin of Early Cardiomyocyte Differentiation Derived From hiPSCs and hESCs

RATIONALE

Recent advances have improved our ability to generate cardiomyocytes from human induced pluripotent stem cells (hiPSCs) and human embryonic stem cells (hESCs). However, our understanding of the transcriptional regulatory networks underlying early stages (ie, from mesoderm to cardiac mesoderm) of cardiomyocyte differentiation remains limited.

OBJECTIVE

To characterize transcriptome and chromatin accessibility during early cardiomyocyte differentiation from hiPSCs and hESCs.

METHODS AND RESULTS

We profiled the temporal changes in transcriptome and chromatin accessibility at genome-wide levels during cardiomyocyte differentiation derived from 2 hiPSC lines and 2 hESC lines at 4 stages: pluripotent stem cells, mesoderm, cardiac mesoderm, and differentiated cardiomyocytes. Overall, RNA sequencing analysis revealed that transcriptomes during early cardiomyocyte differentiation were highly concordant between hiPSCs and hESCs, and clustering of 4 cell lines within each time point demonstrated that changes in genome-wide chromatin accessibility were similar across hiPSC and hESC cell lines. Weighted gene co-expression network analysis (WGCNA) identified several modules that were strongly correlated with different stages of cardiomyocyte differentiation. Several novel genes were identified with high weighted connectivity within modules and exhibited coexpression patterns with other genes, including noncoding RNA LINC01124 and uncharacterized RNA AK127400 in the module related to the mesoderm stage; E-box-binding homeobox 1 (ZEB1) in the module correlated with postcardiac mesoderm. We further demonstrated that ZEB1 is required for early cardiomyocyte differentiation. In addition, based on integrative analysis of both WGCNA and transcription factor motif enrichment analysis, we determined numerous transcription factors likely to play important roles at different stages during cardiomyocyte differentiation, such as T and eomesodermin (EOMES; mesoderm), lymphoid enhancer-binding factor 1 (LEF1) and mesoderm posterior BHLH transcription factor 1 (MESP1; from mesoderm to cardiac mesoderm), meis homeobox 1 (MEIS1) and GATA-binding protein 4 (GATA4) (postcardiac mesoderm), JUN and FOS families, and MEIS2 (cardiomyocyte).

CONCLUSIONS

Both hiPSCs and hESCs share similar transcriptional regulatory mechanisms underlying early cardiac differentiation, and our results have revealed transcriptional regulatory networks and new factors (eg, ZEB1) controlling early stages of cardiomyocyte differentiation.

Altered Expression of Human Smooth Muscle Myosin Phosphatase Targeting (MYPT) Isovariants with Pregnancy and Labor

Background

Myosin light-chain phosphatase is a trimeric protein that hydrolyses phosphorylated myosin II light chains (MYLII) to cause relaxation in smooth muscle cells including those of the uterus. A major component of the phosphatase is the myosin targeting subunit (MYPT), which directs a catalytic subunit to dephosphorylate MYLII. There are 5 main MYPT family members (MYPT1 (PPP1R12A), MYPT2 (PPP1R12B), MYPT3 (PPP1R16A), myosin binding subunit 85 MBS85 (PPP1R12C) and TIMAP (TGF-beta-inhibited membrane-associated protein (PPP1R16B)). Nitric oxide (NO)-mediated smooth muscle relaxation has in part been attributed to activation of the phosphatase by PKG binding to a leucine zipper (LZ) dimerization domain located at the carboxyl-terminus of PPP1R12A. In animal studies, alternative splicing of PPP1R12A can lead to the inclusion of a 31-nucleotide exonic segment that generates a LZ negative (LZ-) isovariant rendering the phosphatase less sensitive to NO vasodilators and alterations in PPP1R12ALZ- and LZ+ expression have been linked to phenotypic changes in smooth muscle function. Moreover, PPP1R12B and PPP1R12C, but not PPP1R16A or PPP1R16B, have the potential for LZ+/LZ- alternative splicing. Yet, by comparison to animal studies, the information on human MYPT genomic sequences/mRNA expressions is scant. As uterine smooth muscle undergoes substantial remodeling during pregnancy we were interested in establishing the patterns of expression of human MYPT isovariants during this process and also following labor onset as this could have important implications for determining successful pregnancy outcome.

Objectives

We used cross-species genome alignment, to infer putative human sequences not available in the public domain, and isovariant-specific quantitative PCR, to analyse the expression of mRNA encoding putative LZ+ and LZ- forms of PPP1R12A, PPP1R12B and PPP1R12C as well as canonical PPP1R16A and PPP1R16B genes in human uterine smooth muscle from non-pregnant, pregnant and in-labor donors.

Results

We found a reduction in the expression of PPP1R12A, PPP1R12BLZ+, PPP1R16A and PPP1R16B mRNA in late pregnancy (not-in-labor) relative to non-pregnancy. PPP1R12ALZ+ and PPP1R12ALZ- mRNA levels were similar in the non-pregnant and pregnant not in labor groups. There was a further reduction in the uterine expression of PPP1R12ALZ+, PPP1R12CLZ+ and PPP1R12ALZ- mRNA with labor relative to the pregnant not-in-labor group. PPP1R12A, PPP1R12BLZ+, PPP1R16A and PPP1R16B mRNA levels were invariant between the not in labor and in-labor groups.

Conclusions

MYPT proteins are crucial determinants of smooth muscle function. Therefore, these alterations in human uterine smooth muscle MYPT isovariant expression during pregnancy and labor may be part of the important molecular physiological transition between uterine quiescence and activation.

Role of myosin light chain phosphatase in cardiac physiology and pathophysiology

Maintenance of contractile performance of the heart is achieved in part by the constitutive 40% phosphorylation of myosin regulatory light chain (RLC) in sarcomeres. The importance of this extent of RLC phosphorylation for optimal cardiac performance becomes apparent when various mouse models and resultant phenotypes are compared. The absence or attenuation of RLC phosphorylation results in poor performance leading to heart failure, whereas increased RLC phosphorylation is associated with cardiac protection from stresses. Although information is limited, RLC phosphorylation appears compromised in human heart failure which is consistent with data from mouse studies. The extent of cardiac RLC phosphorylation is determined by the balanced activities of cardiac myosin light chain kinases and phosphatases, the regulatory mechanisms of which are now emerging. This review thusly focuses on kinases that may participate in phosphorylating RLC to make the substrate for cardiac myosin light chain phosphatases, in addition to providing perspectives on the family of myosin light chain phosphatases and involved signaling mechanisms. Because biochemical and physiological information about cardiac myosin light chain phosphatase is sparse, such studies represent an emerging area of investigation in health and disease.

The neglected messengers: Control of cardiac myofilaments by protein phosphatases

Cardiac myofilaments act as the central contractile apparatus of heart muscle cells. Covalent modification of constituent proteins through phosphorylation is a rapid and powerful mechanism to control myofilament function, and is increasingly seen as a mechanism of disease. While the relationship between protein kinases and cardiac myofilaments has been widely examined, the impact of protein dephosphorylation by protein phosphatases is poorly understood. This review outlines the mechanisms by which the mostly widely expressed protein phosphatases in cardiac myocytes regulate myofilament function, and the emerging role of myofilament-associated protein phosphatases in heart failure. The importance of regulatory subunits and subcellular compartmentalization in determining the functional impact of protein phosphatases on myofilament and myocardial function is also discussed, as are discrepancies about the roles of protein phosphatases in regulating myofilament function. The potential for targeting these molecular messengers in the treatment of heart failure is discussed as a key future direction.

Cellular adhesome screen identifies critical modulators of focal adhesion dynamics, cellular traction forces and cell migration behaviour

Cancer cells migrate from the primary tumour into surrounding tissue in order to form metastasis. Cell migration is a highly complex process, which requires continuous remodelling and re-organization of the cytoskeleton and cell-matrix adhesions. Here, we aimed to identify genes controlling aspects of tumour cell migration, including the dynamic organization of cell-matrix adhesions and cellular traction forces. In a siRNA screen targeting most cell adhesion-related genes we identified 200+ genes that regulate size and/or dynamics of cell-matrix adhesions in MCF7 breast cancer cells. In a subsequent secondary screen, the 64 most effective genes were evaluated for growth factor-induced cell migration and validated by tertiary RNAi pool deconvolution experiments. Four validated hits showed significantly enlarged adhesions accompanied by reduced cell migration upon siRNA-mediated knockdown. Furthermore, loss of PPP1R12B, HIPK3 or RAC2 caused cells to exert higher traction forces, as determined by traction force microscopy with elastomeric micropillar post arrays, and led to considerably reduced force turnover. Altogether, we identified genes that co-regulate cell-matrix adhesion dynamics and traction force turnover, thereby modulating overall motility behaviour.

CaMKII and at least two unidentified kinases phosphorylate regulatory light chain in non-contracting cardiomyocytes

In cardiac tissue, regulatory light chain (RLC, myosin light chain 2) phosphorylation (Ser(15)) leads to modulation of muscle contraction through Ca(2+)-sensitization. To elucidate which kinases that are involved in the basal (diastolic phase) RLC phosphorylation, we studied non-contracting adult rat cardiomyocytes. RLC kinase activities in situ were unmasked by maximally inhibiting myosin light chain phosphatase (MLCP) by calyculin A in the absence and presence of various protein kinase inhibitors. Surprisingly MLCK did not contribute to the phosphorylation of RLC in the non-contracting cardiomyocytes. Two kinase activity groups were revealed by different sensitivities to staurosporine. The fraction with the highest sensitivity to staurosporine was inhibited by KN-93, a selective CaMKII inhibitor, producing a 23% ± 7% reduction in RLC phosphorylation. Calmodulin antagonism (W7) and reduction in Ca(2+) (EGTA) combined with low concentration of staurosporine caused a larger decrease in RLC phosphorylation than staurosporine alone. These data strongly suggest that in addition to CaMKII, there is another Ca(2+)/calmodulin-dependent kinase and a Ca(2+)/calmodulin-independent kinase phosphorylating RLC. Thus the RLC phosphorylation seems to be ensured by redundant kinase activities.

A post-MI power struggle: adaptations in cardiac power occur at the sarcomere level alongside MyBP-C and RLC phosphorylation

Myocardial remodeling in response to chronic myocardial infarction (CMI) progresses through two phases, hypertrophic "compensation" and congestive "decompensation." Nothing is known about the ability of uninfarcted myocardium to produce force, velocity, and power during these clinical phases, even though adaptation in these regions likely drives progression of compensation. We hypothesized that enhanced cross-bridge-level contractility underlies mechanical compensation and is controlled in part by changes in the phosphorylation states of myosin regulatory proteins. We induced CMI in rats by left anterior descending coronary artery ligation. We then measured mechanical performance in permeabilized ventricular trabecula taken distant from the infarct zone and assayed myosin regulatory protein phosphorylation in each individual trabecula. During full activation, the compensated myocardium produced twice as much power and 31% greater isometric force compared with noninfarcted controls. Isometric force during submaximal activations was raised >2.4-fold, while power was 2-fold greater. Electron and confocal microscopy demonstrated that these mechanical changes were not a result of increased density of contractile protein and therefore not an effect of tissue hypertrophy. Hence, sarcomere-level contractile adaptations are key determinants of enhanced trabecular mechanics and of the overall cardiac compensatory response. Phosphorylation of myosin regulatory light chain (RLC) increased and remained elevated post-MI, while phosphorylation of myosin binding protein-C (MyBP-C) was initially depressed but then increased as the hearts became decompensated. These sensitivities to CMI are in accordance with phosphorylation-dependent regulatory roles for RLC and MyBP-C in crossbridge function and with compensatory adaptation in force and power that we observed in post-CMI trabeculae.

Revisiting Frank-Starling: regulatory light chain phosphorylation alters the rate of force redevelopment (ktr ) in a length-dependent fashion

Key points

Regulatory light chain (RLC) phosphorylation has been shown to alter the ability of muscle to produce force and power during shortening and to alter the rate of force redevelopment (k_tr) at submaximal [Ca²⁺].

Increasing RLC phosphorylation ∼50% from the in vivo level in maximally [Ca²⁺]‐activated cardiac trabecula accelerates k_tr.

Decreasing RLC phosphorylation to ∼70% of the in vivo control level slows k_tr and reduces force generation.

k_tr is dependent on sarcomere length in the physiological range 1.85–1.94 μm and RLC phosphorylation modulates this response.

We demonstrate that Frank–Starling is evident at maximal [Ca²⁺] activation and therefore does not necessarily require length‐dependent change in [Ca²⁺]‐sensitivity of thin filament activation.

The stretch response is modulated by changes in RLC phosphorylation, pinpointing RLC phosphorylation as a modulator of the Frank–Starling law in the heart.

These data provide an explanation for slowed systolic function in the intact heart in response to RLC phosphorylation reduction.

Abstract

Force and power in cardiac muscle have a known dependence on phosphorylation of the myosin‐associated regulatory light chain (RLC). We explore the effect of RLC phosphorylation on the ability of cardiac preparations to redevelop force (k_tr) in maximally activating [Ca²⁺]. Activation was achieved by rapidly increasing the temperature (temperature‐jump of 0.5–20ºC) of permeabilized trabeculae over a physiological range of sarcomere lengths (1.85–1.94 μm). The trabeculae were subjected to shortening ramps over a range of velocities and the extent of RLC phosphorylation was varied. The latter was achieved using an RLC‐exchange technique, which avoids changes in the phosphorylation level of other proteins. The results show that increasing RLC phosphorylation by 50% accelerates k_tr by ∼50%, irrespective of the sarcomere length, whereas decreasing phosphorylation by 30% slows k_tr by ∼50%, relative to the k_tr obtained for in vivo phosphorylation. Clearly, phosphorylation affects the magnitude of k_tr following step shortening or ramp shortening. Using a two‐state model, we explore the effect of RLC phosphorylation on the kinetics of force development, which proposes that phosphorylation affects the kinetics of both attachment and detachment of cross‐bridges. In summary, RLC phosphorylation affects the rate and extent of force redevelopment. These findings were obtained in maximally activated muscle at saturating [Ca²⁺] and are not explained by changes in the Ca²⁺‐sensitivity of acto‐myosin interactions. The length‐dependence of the rate of force redevelopment, together with the modulation by the state of RLC phosphorylation, suggests that these effects play a role in the Frank–Starling law of the heart.

Regulatory light chain (RLC) phosphorylation has been shown to alter the ability of muscle to produce force and power during shortening and to alter the rate of force redevelopment (k_tr) at submaximal [Ca²⁺].

Increasing RLC phosphorylation ∼50% from the in vivo level in maximally [Ca²⁺]‐activated cardiac trabecula accelerates k_tr.

Decreasing RLC phosphorylation to ∼70% of the in vivo control level slows k_tr and reduces force generation.

k_tr is dependent on sarcomere length in the physiological range 1.85–1.94 μm and RLC phosphorylation modulates this response.

We demonstrate that Frank–Starling is evident at maximal [Ca²⁺] activation and therefore does not necessarily require length‐dependent change in [Ca²⁺]‐sensitivity of thin filament activation.

The stretch response is modulated by changes in RLC phosphorylation, pinpointing RLC phosphorylation as a modulator of the Frank–Starling law in the heart.

These data provide an explanation for slowed systolic function in the intact heart in response to RLC phosphorylation reduction.

The Function of Rho-Associated Kinases ROCK1 and ROCK2 in the Pathogenesis of Cardiovascular Disease

Rho-associated kinases ROCK1 and ROCK2 are serine/threonine kinases that are downstream targets of the small GTPases RhoA, RhoB, and RhoC. ROCKs are involved in diverse cellular activities including actin cytoskeleton organization, cell adhesion and motility, proliferation and apoptosis, remodeling of the extracellular matrix and smooth muscle cell contraction. The role of ROCK1 and ROCK2 has long been considered to be similar; however, it is now clear that they do not always have the same functions. Moreover, depending on their subcellular localization, activation, and other environmental factors, ROCK signaling can have different effects on cellular function. With respect to the heart, findings in isoform-specific knockout mice argue for a role of ROCK1 and ROCK2 in the pathogenesis of cardiac fibrosis and cardiac hypertrophy, respectively. Increased ROCK activity could play a pivotal role in processes leading to cardiovascular diseases such as hypertension, pulmonary hypertension, angina pectoris, vasospastic angina, heart failure, and stroke, and thus ROCK activity is a potential new biomarker for heart disease. Pharmacological ROCK inhibition reduces the enhanced ROCK activity in patients, accompanied with a measurable improvement in medical condition. In this review, we focus on recent findings regarding ROCK signaling in the pathogenesis of cardiovascular disease, with a special focus on differences between ROCK1 and ROCK2 function.

New Isoform of Cardiac Myosin Light Chain Kinase and the Role of Cardiac Myosin Phosphorylation in α1-Adrenoceptor Mediated Inotropic Response

Background & Aims

Cardiac myosin light chain kinase (cMLCK) plays an obligatory role in maintaining the phosphorylation levels of regulatory myosin light chain (MLC2), which is thought to be crucial for regulation of cardiac function. To test this hypothesis, the role played by ventricular MLC2 (MLC2v) phosphorylation was investigated in the phenylephrine-induced increase in twitch tension using the naturally-occurring mouse strain, C57BL/6N, in which cMLCK is down regulated.

Methods and Results

By Western blot and nanoLC-MS/MS analysis, cMLCKs with molecular mass of 61-kDa (cMLCK-2) and/or 86-kDa were identified in mice heart. Among various mouse strains, C57BL/6N expressed cMLCK-2 alone and the closest relative strain C57BL/6J expressed both cMLCKs. The levels of MLC2v phosphorylation was significantly lower in C57BL/6N than in C57BL/6J. The papillary muscle twitch tension induced by electrical field stimulation was smaller in C57BL/6N than C57BL/6J. Phenylephrine had no effect on MLC2v phosphorylation in either strains but increased the twitch tension more potently in C57BL/6J than in C57BL/6N. Calyculin A increased papillary muscle MLC2v phosphorylation to a similar extent in both strains but increased the phenylephrine-induced inotropic response only in C57BL/6N. There was a significant positive correlation between the phenylephrine-induced inotropic response and the levels of MLC2v phosphorylation within ranges of 15–30%.

Conclusions

We identified a new isoform of cMLCK with a molecular mass of 61kDa(cMLCK-2) in mouse heart. In the C57BL/6N strain, only cMLCK-2 was expressed and the basal MLC2v phosphorylation levels and the phenylephrine-induced inotropic response were both smaller. We suggest that a lower phenylephrine-induced inotropic response may be caused by the lower basal MLC2v phosphorylation levels in this strain.

Myosin regulatory light chain phosphorylation enhances cardiac β-myosin in vitro motility under load

Familial hypertrophic cardiomyopathy (HCM) is characterized by left ventricular hypertrophy and myofibrillar disarray, and often results in sudden cardiac death. Two HCM mutations, N47K and R58Q, are located in the myosin regulatory light chain (RLC). The RLC mechanically stabilizes the myosin lever arm, which is crucial to myosin's ability to transmit contractile force. The N47K and R58Q mutations have previously been shown to reduce actin filament velocity under load, stemming from a more compliant lever arm (Greenberg, 2010). In contrast, RLC phosphorylation was shown to impart stiffness to the myosin lever arm (Greenberg, 2009). We hypothesized that phosphorylation of the mutant HCM-RLC may mitigate distinct mutation-induced structural and functional abnormalities. In vitro motility assays were utilized to investigate the effects of RLC phosphorylation on the HCM-RLC mutant phenotype in the presence of an α-actinin frictional load. Porcine cardiac β-myosin was depleted of its native RLC and reconstituted with mutant or wild-type human RLC in phosphorylated or non-phosphorylated form. Consistent with previous findings, in the presence of load, myosin bearing the HCM mutations reduced actin sliding velocity compared to WT resulting in 31-41% reductions in force production. Myosin containing phosphorylated RLC (WT or mutant) increased sliding velocity and also restored mutant myosin force production to near WT unphosphorylated values. These results point to RLC phosphorylation as a general mechanism to increase force production of the individual myosin motor and as a potential target to ameliorate the HCM-induced phenotype at the molecular level.

Cardiac myosin light chain phosphorylation and inotropic effects of a biased ligand, TRV120023, in a dilated cardiomyopathy model

AIMS

Therapeutic approaches to treat familial dilated cardiomyopathy (DCM), which is characterized by depressed sarcomeric tension and susceptibility to Ca(2+)-related arrhythmias, have been generally unsuccessful. Our objective in the present work was to determine the effect of the angiotensin II type 1 receptor (AT1R) biased ligand, TRV120023, on contractility of hearts of a transgenic mouse model of familial DCM with mutation in tropomyosin at position 54 (TG-E54K). Our rationale is based on previous studies, which have supported the hypothesis that biased G-protein-coupled receptor ligands, signalling via β-arrestin, increase cardiac contractility with no effect on Ca(2+) transients. Our previous work demonstrated that the biased ligand TRV120023 is able to block angiotensin-induced hypertrophy, while promoting an increase in sarcomere Ca(2+) response.

METHODS AND RESULTS

We tested the hypothesis that the depression in cardiac function associated with DCM can be offset by infusion of the AT1R biased ligand, TRV120023. We intravenously infused saline, TRV120023, or the unbiased ligand, losartan, for 15 min in TG-E54K and non-transgenic mice to obtain left ventricular pressure-volume relations. Hearts were analysed for sarcomeric protein phosphorylation. Results showed that the AT1R biased ligand increases cardiac performance in TG-E54K mice in association with increased myosin light chain-2 phosphorylation.

CONCLUSION

Treatment of mice with an AT1R biased ligand, acting via β-arrestin signalling, is able to induce an increase in cardiac contractility associated with an increase in ventricular myosin light chain-2 phosphorylation. AT1R biased ligands may prove to be a novel inotropic approach in familial DCM.

Constitutive phosphorylation of cardiac myosin regulatory light chain in vivo

In beating hearts, phosphorylation of myosin regulatory light chain (RLC) at a single site to 0.45 mol of phosphate/mol by cardiac myosin light chain kinase (cMLCK) increases Ca(2+) sensitivity of myofilament contraction necessary for normal cardiac performance. Reduction of RLC phosphorylation in conditional cMLCK knock-out mice caused cardiac dilation and loss of cardiac performance by 1 week, as shown by increased left ventricular internal diameter at end-diastole and decreased fractional shortening. Decreased RLC phosphorylation by conventional or conditional cMLCK gene ablation did not affect troponin-I or myosin-binding protein-C phosphorylation in vivo. The extent of RLC phosphorylation was not changed by prolonged infusion of dobutamine or treatment with a β-adrenergic antagonist, suggesting that RLC is constitutively phosphorylated to maintain cardiac performance. Biochemical studies with myofilaments showed that RLC phosphorylation up to 90% was a random process. RLC is slowly dephosphorylated in both noncontracting hearts and isolated cardiac myocytes from adult mice. Electrically paced ventricular trabeculae restored RLC phosphorylation, which was increased to 0.91 mol of phosphate/mol of RLC with inhibition of myosin light chain phosphatase (MLCP). The two RLCs in each myosin appear to be readily available for phosphorylation by a soluble cMLCK, but MLCP activity limits the amount of constitutive RLC phosphorylation. MLCP with its regulatory subunit MYPT2 bound tightly to myofilaments was constitutively phosphorylated in beating hearts at a site that inhibits MLCP activity. Thus, the constitutive RLC phosphorylation is limited physiologically by low cMLCK activity in balance with low MLCP activity.

Degradation of cardiac myosin light chain kinase by matrix metalloproteinase-2 contributes to myocardial contractile dysfunction during ischemia/reperfusion

Although ischemia/reperfusion (I/R)-induced myocardial contractile dysfunction is associated with a prominent decrease in myofilament Ca(2+) sensitivity, the underlying mechanisms have not yet been fully clarified. Phosphorylation of ventricular myosin light chain 2 (MLC-2v) facilitates actin-myosin interactions and enhances contractility, however, its level and regulation by cardiac MLC kinase (cMLCK) and cMLC phosphatase (cMLCP) in I/R hearts are debatable. In this study, the levels and/or effects of MLC-2v phosphorylation, cMLCK, cMLCP, and proteases during I/R were determined. Global myocardial I/R-suppressed cardiac performance in isolated rat hearts was concomitant with decreases of MLC-2v phosphorylation, myofibrillar Ca(2+)-stimulated ATPase activity, and cMLCK content, but not cMLCP proteins. Consistently, simulated I/R in isolated cardiomyocytes inhibited cell shortening, Ca(2+) transients, MLC-2v phosphorylation, and myofilament sensitivity to Ca(2+). These observations were reversed by cMLCK overexpression, while the specific cMLCK knockdown by short hairpin RNA (shRNA) had the opposite effect. Moreover, the inhibition of matrix metalloproteinase-2 (MMP-2, a zinc-dependent endopeptidase) reversed IR-decreased cMLCK, MLC-2v phosphorylation, myofibrillar Ca(2+)-stimulated ATPase activity, myocardial contractile function, and myofilament sensitivity to Ca(2+), while the inhibition or knockdown of cMLCK by ML-9 or specific shRNA abolished MMP-2 inhibition-induced cardioprotection. Finally, the co-localization in cardiomyocytes and interaction in vivo of MMP-2 and cMLCK were observed. Purified recombinant rat cMLCK was concentration- and time-dependently degraded by rat MMP-2 in vitro, and this was prevented by the inhibition of MMP-2. These findings reveal that the I/R-activated MMP-2 leads to the degradation of cMLCK, resulting in a reduction of MLC-2v phosphorylation, and myofibrillar Ca(2+)-stimulated ATPase activity, which subsequently suppresses myocardial contractile function through a decrease of myofilament Ca(2+) sensitivity.

Quantitative phosphoproteomics reveals novel phosphorylation events in insulin signaling regulated by protein phosphatase 1 regulatory subunit 12A

Serine/threonine protein phosphatase 1 regulatory subunit 12A (PPP1R12A) modulates the activity and specificity of the catalytic subunit of protein phosphatase 1, regulating various cellular processes via dephosphorylation. Nonetheless, little is known about phosphorylation events controlled by PPP1R12A in skeletal muscle insulin signaling. Here, we used quantitative phosphoproteomics to generate a global picture of phosphorylation events regulated by PPP1R12A in a L6 skeletal muscle cell line, which were engineered for inducible PPP1R12A knockdown. Phosphoproteomics revealed 3876 phosphorylation sites (620 were novel) in these cells. Furthermore, PPP1R12A knockdown resulted in increased overall phosphorylation in L6 cells at the basal condition, and changed phosphorylation levels for 698 sites (assigned to 295 phosphoproteins) at the basal and/or insulin-stimulated conditions. Pathway analysis on the 295 phosphoproteins revealed multiple significantly enriched pathways related to insulin signaling, such as mTOR signaling and RhoA signaling. Moreover, phosphorylation levels for numerous regulatory sites in these pathways were significantly changed due to PPP1R12A knockdown. These results indicate that PPP1R12A indeed plays a role in skeletal muscle insulin signaling, providing novel insights into the biology of insulin action. This new information may facilitate the design of experiments to better understand mechanisms underlying skeletal muscle insulin resistance and type 2 diabetes.

BIOLOGICAL SIGNIFICANCE

These results identify a large number of potential new substrates of serine/threonine protein phosphatase 1 and suggest that serine/threonine protein phosphatase 1 regulatory subunit 12A indeed plays a regulatory role in multiple pathways related to insulin action, providing novel insights into the biology of skeletal muscle insulin signaling. This information may facilitate the design of experiments to better understand the molecular mechanism responsible for skeletal muscle insulin resistance and associated diseases, such as type 2 diabetes and cardiovascular diseases.

Getting the skinny on thick filament regulation in cardiac muscle biology and disease

Thin (actin) filament accessory proteins are thought to be the regulatory force for muscle contraction in cardiac muscle; however, compelling new evidence suggests that thick (myosin) filament regulatory proteins are emerging as having independent and important roles in regulating cardiac muscle contraction. Key to these new findings is a growing body of evidence that point to an influential and, more recently, direct role for ventricular myosin light chain-2 (MLC2v) phosphorylation in regulating cardiac muscle contraction, function, and disease. This includes the discovery and characterization of a cardiac-specific myosin light chain kinase capable of phosphorylating MLC2v as well as a myosin phosphatase that dephosphorylates MLC2v in the heart, which provides added mechanistic insights on MLC2v regulation within cardiac muscle. Here, we review evidence for an emerging and critical role for MLC2v phosphorylation in regulating cardiac myosin cycling kinetics, function, and disease, based on recent studies performed in genetic mouse models and humans. We further provide new perspectives on future avenues for targeting these pathways as therapies in alleviating cardiac disease.

The frequency of single nucleotide polymorphisms and their association with uric acid concentration based on data from genome-wide association studies in the Korean population

We aimed, first, to investigate the minor allele frequencies (MAFs) of single nucleotide polymorphisms (SNPs) associated with serum uric acid (SUA) level in the Korean population and compare these with data from other ethnic groups and, second, to investigate whether the SNPs are associated with altered SUA levels. We examined the frequencies of risk alleles, investigated the MAFs of 40 previously described SNPs associated with SUA level in the Korean population (a total of 1,957 subjects), and compared results with data for other ethnic groups. We also analyzed associations with SUA concentrations based on data from genome-wide association studies in the Korean population (a total of 402 rheumatoid arthritis subjects) and tested whether polymorphism of any of the 40 SNPs associated with SUA identified previously was associated with SUA levels. The MAFs of SNPs associated with SUA level in the Korean population were quite similar to those among Japanese, but not in populations of European descent. SNP rs12734001 (PPP1R12B) proved to have the most probable association with SUA concentrations (P_trend = 2.29 × 10(-9)). We also analyzed 13 SNPs shown previously by meta-analysis to be associated with SUA, and SNP rs3741414 (INHBC) was found to have probable association with SUA level observed in the present study (P_trend = 0.01). The pattern of variants controlling SUA levels in the Korean population is not similar to that in European population. SNP rs12734001 (PPP1R12B) is significantly associated with SUA level among Koreans.

Myosin regulatory light chain (RLC) phosphorylation change as a modulator of cardiac muscle contraction in disease

Understanding how cardiac myosin regulatory light chain (RLC) phosphorylation alters cardiac muscle mechanics is important because it is often altered in cardiac disease. The effect this protein phosphorylation has on muscle mechanics during a physiological range of shortening velocities, during which the heart generates power and performs work, has not been addressed. We have expressed and phosphorylated recombinant Rattus norvegicus left ventricular RLC. In vitro we have phosphorylated these recombinant species with cardiac myosin light chain kinase and zipper-interacting protein kinase. We compare rat permeabilized cardiac trabeculae, which have undergone exchange with differently phosphorylated RLC species. We were able to enrich trabecular RLC phosphorylation by 40% compared with controls and, in a separate series, lower RLC phosphorylation to 60% of control values. Compared with the trabeculae with a low level of RLC phosphorylation, RLC phosphorylation enrichment increased isometric force by more than 3-fold and peak power output by more than 7-fold and approximately doubled both maximum shortening speed and the shortening velocity that generated peak power. We augmented these measurements by observing increased RLC phosphorylation of human and rat HF samples from endocardial left ventricular homogenate. These results demonstrate the importance of increased RLC phosphorylation in the up-regulation of myocardial performance and suggest that reduced RLC phosphorylation is a key aspect of impaired contractile function in the diseased myocardium.

Identification of bovine hibernation-specific protein complex and evidence of its regulation in fasting and aging

Hibernation-specific protein (HP) is a plasma protein that regulates hibernation in chipmunks. The HP complex (HP20c) consists of three homologous proteins, HP20, HP25 and HP27, all produced by liver and belonging to the C1q family. To date, HP20c has not been identified in any mammalian species except chipmunk and ground squirrel hibernators. Here, we report a bovine HP20 gene isolated from liver tissue and aortic endothelial cells. Total homology between bovine and chipmunk variants was 63% at the amino acid level. Gene expression was highest in the liver. Western blot revealed HP20 protein in foetal, newborn, calf and adult serum, with highest concentrations in the adult. Similar proteins were detected in sera of other ruminants but not in humans, bears, mice or rats. Bovine HP20 protein was found mainly in ovaries, stomach, heart, kidneys, lungs, testes and prostate, but not in the skeletal muscle. Native HP20 was purified from bovine adult serum as a complex containing 25 and 27 kDa proteins. Mass spectrometry revealed that these proteins are orthologues of chipmunk HP25 and HP27, respectively. Interestingly, bovine HP20 was highly expressed in cattle serum after fasting. Native bovine HP20c may be a useful tool for investigating HP function.

Distinct distribution and localization of Rho-kinase in mouse epithelial, muscle and neural tissues

The small GTP-binding protein Rho plays a crucial role in a wide variety of cellular functions through various effector proteins. Rho-kinase is a key effector protein of Rho, which is composed of two isoforms, ROCK1 and ROCK2. To clarify the site of action of ROCK1 and ROCK2, we performed immunofluorescence and immunoelectron microscopic analyses using isoform-specific antibodies in mouse tissues. In the large and small intestines, ROCK1 immunoreactivity was predominantly identified in epithelial cells, and ROCK2 immunoreactivity was negligible. In these epithelial cells, ROCK1 immunoreactivity was distributed on plasma membranes, while ROCK1 immunogold signals were localized at cell-cell contacts and cell adhesion sites, especially at the adherens junctions at the ultrastructural level. In the bladder epithelium, however, ROCK1 and ROCK2 signals were identified at intermediate filaments, and ROCK2 signals were also observed in nuclei. In the three types of muscular cells-smooth, cardiac, and skeletal muscle cells-ROCK1 and ROCK2 also showed differential distribution. ROCK1 signals were localized at actin filaments, plasma membranes, and vesicles near plasma membranes in smooth muscle cells; at the lysosomes in skeletal muscle cells; and were undetectable in cardiac muscle cells. ROCK2 signals were localized at actin filaments and centrosomes in smooth muscle cells, at intercalated discs in cardiac muscle cells, and at Z-discs and sarcoplasmic reticulum in skeletal muscle cells. In the brain, ROCK1 immunoreactivity was distributed in glia, whereas ROCK2 immunoreactivity was observed in neurons. These results indicate that the two isoforms of Rho-kinase distribute differentially to accomplish their specific functions.

Insulin-stimulated phosphorylation of protein phosphatase 1 regulatory subunit 12B revealed by HPLC-ESI-MS/MS

BACKGROUND

Protein phosphatase 1 (PP1) is one of the major phosphatases responsible for protein dephosphorylation in eukaryotes. Protein phosphatase 1 regulatory subunit 12B (PPP1R12B), one of the regulatory subunits of PP1, can bind to PP1cδ, one of the catalytic subunits of PP1, and modulate the specificity and activity of PP1cδ against its substrates. Phosphorylation of PPP1R12B on threonine 646 by Rho kinase inhibits the activity of the PP1c-PPP1R12B complex. However, it is not currently known whether PPP1R12B phosphorylation at threonine 646 and other sites is regulated by insulin. We set out to identify phosphorylation sites in PPP1R12B and to quantify the effect of insulin on PPP1R12B phosphorylation by using high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry.

RESULTS

14 PPP1R12B phosphorylation sites were identified, 7 of which were previously unreported. Potential kinases were predicted for these sites. Furthermore, relative quantification of PPP1R12B phosphorylation sites for basal and insulin-treated samples was obtained by using peak area-based label-free mass spectrometry of fragment ions. The results indicate that insulin stimulates the phosphorylation of PPP1R12B significantly at serine 29 (3.02 ± 0.94 fold), serine 504 (11.67 ± 3.33 fold), and serine 645/threonine 646 (2.34 ± 0.58 fold).

CONCLUSION

PPP1R12B was identified as a phosphatase subunit that undergoes insulin-stimulated phosphorylation, suggesting that PPP1R12B might play a role in insulin signaling. This study also identified novel targets for future investigation of the regulation of PPP1R12B not only in insulin signaling in cell models, animal models, and in humans, but also in other signaling pathways.

Protein phosphatase 1 regulatory subunit 12A and catalytic subunit δ, new members in the phosphatidylinositide 3 kinase insulin-signaling pathway

Skeletal muscle insulin resistance is an early abnormality in individuals with metabolic syndrome and type 2 diabetes (T2D). Insulin receptor substrate-1 (IRS1) plays a key role in insulin signaling, the function of which is regulated by both phosphorylation and dephosphorylation of tyrosine and serine/threonine residues. Numerous studies have focused on kinases in IRS1 phosphorylation and insulin resistance; however, the mechanism for serine/threonine phosphatase action in insulin signaling is largely unknown. Recently, we identified protein phosphatase 1 (PP1) regulatory subunit 12A (PPP1R12A) as a novel endogenous insulin-stimulated interaction partner of IRS1 in L6 myotubes. The current study was undertaken to better understand PPP1R12A's role in insulin signaling. Insulin stimulation promoted an interaction between the IRS1/p85 complex and PPP1R12A; however, p85 and PPP1R12A did not interact independent of IRS1. Moreover, kinase inhibition experiments indicated that insulin-induced interaction between IRS1 and PPP1R12A was reduced by treatment with inhibitors of phosphatidylinositide 3 kinase, PDK1, Akt, and mTOR/raptor but not MAPK. Furthermore, a novel insulin-stimulated IRS1 interaction partner, PP1 catalytic subunit (PP1cδ), was identified, and its interaction with IRS1 was also disrupted by inhibitors of Akt and mTOR/raptor. These results indicate that PPP1R12A and PP1cδ are new members of the insulin-stimulated IRS1 signaling complex, and the interaction of PPP1R12A and PP1cδ with IRS1 is dependent on Akt and mTOR/raptor activation. These findings provide evidence for the involvement of a particular PP1 complex, PPP1R12A/PP1cδ, in insulin signaling and may lead to a better understanding of dysregulated IRS1 phosphorylation in insulin resistance and T2D.

Berbamine increases myocardial contractility via a Ca2+-independent mechanism

Berbamine (BM), a natural compound derived from Berberis vulgaris L, has been reported to inhibit cardiac contractile function at higher concentrations. Here, we report that BM had concentration-dependent biphasic effects on myocardial contraction in Langendorff-perfused rat hearts, that is, at lower concentrations (30-100 nM), it displayed positive inotropic and lusitropic effects, whereas at a higher concentration of 1 μM, it caused a negative inotropic effect after an initially weak increase. These effects were further confirmed in cardiomyocytes isolated from the left ventricles of rats. Moreover, the increased cell shortening by BM at concentrations from 0.1 to 100 nM was not associated with an alteration of intracellular Ca transients. Consistently, at 30 nM, BM shifted the cell shortening--Ca transient relationship curve induced by cumulative elevation of extracellular Ca concentration to the left. Furthermore, BM significantly increased membrane-bound but not filament-bound protein kinase C epsilon (PKCε) in the isolated hearts and cardiomyocytes. Such a translocation was inhibited by PKCε-specific inhibitor PKCε V1-2 concomitant with the abolishment of the BM-induced increase in contraction. These findings reveal the positive inotropic effect of BM in the myocardium and demonstrate that BM increases myocardial contractility by increasing myofilament Ca sensitivity via a PKCε-dependent signaling pathway.

Site-specific phosphorylation of protein phosphatase 1 regulatory subunit 12A stimulated or suppressed by insulin

Protein phosphatase 1 (PP1) is one of the major phosphatases responsible for protein dephosphorylation in eukaryotes. So far, only few specific phosphorylation sites of PP1 regulatory subunit 12A (PPP1R12A) have been shown to regulate the PP1 activity. The effect of insulin on PPP1R12A phosphorylation is largely unknown. Utilizing a mass spectrometry based phosphorylation identification and quantification approach, we identified 21 PPP1R12A phosphorylation sites (7 novel sites, including Ser20, Thr22, Thr453, Ser478, Thr671, Ser678, and Ser680) and quantified 16 of them under basal and insulin stimulated conditions in hamster ovary cells overexpressing the insulin receptor (CHO/IR), an insulin sensitive cell model. Insulin stimulated the phosphorylation of PPP1R12A significantly at Ser477, Ser478, Ser507, Ser668, and Ser695, while simultaneously suppressing the phosphorylation of PPP1R12A at Ser509 (more than 2-fold increase or decrease compared to basal). Our data demonstrate that PPP1R12A undergoes insulin stimulated/suppressed phosphorylation, suggesting that PPP1R12A phosphorylation may play a role in insulin signal transduction. The novel PPP1R12A phosphorylation sites as well as the new insulin-responsive phosphorylation sites of PPP1R12A in CHO/IR cells provide targets for investigation of the regulation of PPP1R12A and the PPP1R12A-PP1cδ complex in insulin action and other signaling pathways in other cell models, animal models, and humans.

Myosin light chain kinase and the role of myosin light chain phosphorylation in skeletal muscle

Skeletal muscle myosin light chain kinase (skMLCK) is a dedicated Ca(2+)/calmodulin-dependent serine-threonine protein kinase that phosphorylates the regulatory light chain (RLC) of sarcomeric myosin. It is expressed from the MYLK2 gene specifically in skeletal muscle fibers with most abundance in fast contracting muscles. Biochemically, activation occurs with Ca(2+) binding to calmodulin forming a (Ca(2+))(4)•calmodulin complex sufficient for activation with a diffusion limited, stoichiometric binding and displacement of a regulatory segment from skMLCK catalytic core. The N-terminal sequence of RLC then extends through the exposed catalytic cleft for Ser15 phosphorylation. Removal of Ca(2+) results in the slow dissociation of calmodulin and inactivation of skMLCK. Combined biochemical properties provide unique features for the physiological responsiveness of RLC phosphorylation, including (1) rapid activation of MLCK by Ca(2+)/calmodulin, (2) limiting kinase activity so phosphorylation is slower than contraction, (3) slow MLCK inactivation after relaxation and (4) much greater kinase activity relative to myosin light chain phosphatase (MLCP). SkMLCK phosphorylation of myosin RLC modulates mechanical aspects of vertebrate skeletal muscle function. In permeabilized skeletal muscle fibers, phosphorylation-mediated alterations in myosin structure increase the rate of force-generation by myosin cross bridges to increase Ca(2+)-sensitivity of the contractile apparatus. Stimulation-induced increases in RLC phosphorylation in intact muscle produces isometric and concentric force potentiation to enhance dynamic aspects of muscle work and power in unfatigued or fatigued muscle. Moreover, RLC phosphorylation-mediated enhancements may interact with neural strategies for human skeletal muscle activation to ameliorate either central or peripheral aspects of fatigue.

Evidence for somatic gene conversion and deletion in bipolar disorder, Crohn's disease, coronary artery disease, hypertension, rheumatoid arthritis, type-1 diabetes, and type-2 diabetes

BACKGROUND

During gene conversion, genetic information is transferred unidirectionally between highly homologous but non-allelic regions of DNA. While germ-line gene conversion has been implicated in the pathogenesis of some diseases, somatic gene conversion has remained technically difficult to investigate on a large scale.

METHODS

A novel analysis technique is proposed for detecting the signature of somatic gene conversion from SNP microarray data. The Wellcome Trust Case Control Consortium has gathered SNP microarray data for two control populations and cohorts for bipolar disorder (BD), cardiovascular disease (CAD), Crohn's disease (CD), hypertension (HT), rheumatoid arthritis (RA), type-1 diabetes (T1D) and type-2 diabetes (T2D). Using the new analysis technique, the seven disease cohorts are analyzed to identify cohort-specific SNPs at which conversion is predicted. The quality of the predictions is assessed by identifying known disease associations for genes in the homologous duplicons, and comparing the frequency of such associations with background rates.

RESULTS

Of 28 disease/locus pairs meeting stringent conditions, 22 show various degrees of disease association, compared with only 8 of 70 in a mock study designed to measure the background association rate (P < 10-9). Additional candidate genes are identified using less stringent filtering conditions. In some cases, somatic deletions appear likely. RA has a distinctive pattern of events relative to other diseases. Similarities in patterns are apparent between BD and HT.

CONCLUSIONS

The associations derived represent the first evidence that somatic gene conversion could be a significant causative factor in each of the seven diseases. The specific genes provide potential insights about disease mechanisms, and are strong candidates for further study.

The myosin phosphatase targeting protein (MYPT) family: a regulated mechanism for achieving substrate specificity of the catalytic subunit of protein phosphatase type 1δ

The mammalian MYPT family consists of the products of five genes, denoted MYPT1, MYPT2, MBS85, MYPT3 and TIMAP, which function as targeting and regulatory subunits to confer substrate specificity and subcellular localization on the catalytic subunit of type 1δ protein serine/threonine phosphatase (PP1cδ). Family members share several conserved domains, including an RVxF motif for PP1c binding and several ankyrin repeats that mediate protein-protein interactions. MYPT1, MYPT2 and MBS85 contain C-terminal leucine zipper domains involved in dimerization and protein-protein interaction, whereas MYPT3 and TIMAP are targeted to membranes via a C-terminal prenylation site. All family members are regulated by phosphorylation at multiple sites by various protein kinases; for example, Rho-associated kinase phosphorylates MYPT1, MYPT2 and MBS85, resulting in inhibition of phosphatase activity and Ca(2+) sensitization of smooth muscle contraction. A great deal is known about MYPT1, the myosin targeting subunit of myosin light chain phosphatase, in terms of its role in the regulation of smooth muscle contraction and, to a lesser extent, non-muscle motile processes. MYPT2 appears to be the key myosin targeting subunit of myosin light chain phosphatase in cardiac and skeletal muscles. MBS85 most closely resembles MYPT2, but little is known about its physiological function. Little is also known about the physiological role of MYPT3, although it is likely to target myosin light chain phosphatase to membranes and thereby achieve specificity for substrates involved in regulation of the actin cytoskeleton. MYPT3 is regulated by phosphorylation by cAMP-dependent protein kinase. TIMAP appears to target PP1cδ to the plasma membrane of endothelial cells where it serves to dephosphorylate proteins involved in regulation of the actin cytoskeleton and thereby control endothelial barrier function. With such a wide range of regulatory targets, MYPT family members have been implicated in diverse pathological events, including hypertension, Parkinson's disease and cancer.

Signaling to myosin regulatory light chain in sarcomeres

Myosin regulatory light chain (RLC) phosphorylation in skeletal and cardiac muscles modulates Ca(2+)-dependent troponin regulation of contraction. RLC is phosphorylated by a dedicated Ca(2+)-dependent myosin light chain kinase in fast skeletal muscle, where biochemical properties of RLC kinase and phosphatase converge to provide a biochemical memory for RLC phosphorylation and post-activation potentiation of force development. The recent identification of cardiac-specific myosin light chain kinase necessary for basal RLC phosphorylation and another potential RLC kinase (zipper-interacting protein kinase) provides opportunities for new approaches to study signaling pathways related to the physiological function of RLC phosphorylation and its importance in cardiac muscle disease.

Heart-specific small subunit of myosin light chain phosphatase activates rho-associated kinase and regulates phosphorylation of myosin phosphatase target subunit 1

Phosphorylation of myosin regulatory light chain (MLC) plays a regulatory role in muscle contraction, and the level of MLC phosphorylation is balanced by MLC kinase and MLC phosphatase (MLCP). MLCP consists of a catalytic subunit, a large subunit (MYPT1 or MYPT2), and a small subunit. MLCP activity is regulated by phosphorylation of MYPTs, whereas the role of small subunit in the regulation remains unknown. We previously characterized a human heart-specific small subunit (hHS-M(21)) that increased the sensitivity to Ca(2+) in muscle contraction. In this study, we investigated the role of hHS-M(21) in the regulation of MLCP phosphorylation. Two isoforms of hHS-M(21), hHS-M(21)A and hHS-M(21)B, preferentially bound the C-terminal one-third region of MYPT1 and MYPT2, respectively. Amino acid substitutions at a phosphorylation site of MYPT1, Ser-852, impaired the binding of MYPT1 and hHS-M(21). The hHS-M(21) increased the phosphorylation level of MYPT1 at Thr-696, which was attenuated by Rho-associated kinase (ROCK) inhibitors and small interfering RNAs for ROCK. In addition, hHS-M(21) bound ROCK and enhanced the ROCK activity. These findings suggest that hHS-M(21) is a heart-specific effector of ROCK and plays a regulatory role in the MYPT1 phosphorylation at Thr-696 by ROCK.

Tissue-specific and ubiquitous expression patterns from alternative promoters of human genes

BACKGROUND

Transcriptome diversity provides the key to cellular identity. One important contribution to expression diversity is the use of alternative promoters, which creates mRNA isoforms by expanding the choice of transcription initiation sites of a gene. The proximity of the basal promoter to the transcription initiation site enables prediction of a promoter's location based on the gene annotations. We show that annotation of alternative promoters regulating expression of transcripts with distinct first exons enables a novel methodology to quantify expression levels and tissue specificity of mRNA isoforms.

PRINCIPAL FINDINGS

The use of distinct alternative first exons in 3,296 genes was examined using exon-microarray data from 11 human tissues. Comparing two transcripts from each gene we found that the activity of alternative promoters (i.e., P1 and P2) was not correlated through tissue specificity or level of expression. Furthermore neither P1 nor P2 conferred any bias for tissue-specific or ubiquitous expression. Genes associated with specific diseases produced transcripts whose limited expression patterns were consistent with the tissue affected in disease. Notably, genes that were historically designated as tissue-specific or housekeeping had alternative isoforms that showed differential expression. Furthermore, only a small number of alternative promoters showed expression exclusive to a single tissue indicating that "tissue preference" provides a better description of promoter activity than tissue specificity. When compared to gene expression data in public databases, as few as 22% of the genes had detailed information for more than one isoform, whereas the remainder collapsed the expression patterns from individual transcripts into one profile.

CONCLUSIONS

We describe a computational pipeline that uses microarray data to assess the level of expression and breadth of tissue profiles for transcripts with distinct first exons regulated by alternative promoters. We conclude that alternative promoters provide individualized regulation that is confirmed through expression levels, tissue preference and chromatin modifications. Although the selective use of alternative promoters often goes uncharacterized in gene expression analyses, transcripts produced in this manner make unique contributions to the cell that requires further exploration.

Smoothelin-like 1 protein regulates myosin phosphatase-targeting subunit 1 expression during sexual development and pregnancy

Pregnancy coordinately alters the contractile properties of both vascular and uterine smooth muscles reducing systemic blood pressure and maintaining uterine relaxation. The precise molecular mechanisms underlying these pregnancy-induced adaptations have yet to be fully defined but are likely to involve changes in the expression of proteins regulating myosin phosphorylation. Here we show that smoothelin like protein 1 (SMTNL1) is a key factor governing sexual development and pregnancy induced adaptations in smooth and striated muscle. A primary target gene of SMTNL1 in these muscles is myosin phosphatase-targeting subunit 1 (MYPT1). Deletion of SMTNL1 increases expression of MYPT1 30-40-fold in neonates and during development expression of both SMTNL1 and MYPT1 increases over 20-fold. Pregnancy also regulates SMTNL1 and MYPT1 expression, and deletion SMTNL1 greatly exaggerates expression of MYPT1 in vascular smooth muscle, producing a profound reduction in force development in response to phenylephrine as well as sensitizing the muscle to acetylcholine. We also show that MYPT1 is expressed in Type2a muscle fibers in mice and humans and its expression is regulated during pregnancy, suggesting unrecognized roles in mediating skeletal muscle plasticity in both species. Our findings define a new conserved pathway in which sexual development and pregnancy mediate smooth and striated muscle adaptations through SMTNL1 and MYPT1.

Overexpression of myosin phosphatase reduces Ca(2+) sensitivity of contraction and impairs cardiac function

BACKGROUND

Phosphorylation of the regulatory light chain of myosin (MLC) has roles in cardiac function. In vitro, myosin phosphatase target subunit 2 (MYPT2) is a strongly suspected regulatory subunit of cardiac myosin phosphatase (MP), but there is no in-vivo evidence regarding the functions of MYPT2 in the heart.

METHODS AND RESULTS

Transgenic mice (Tg) overexpressing MYPT2 were generated using the alpha-MHC promoter. Tg hearts showed an increased expression of MYPT2 and concomitant increase of the endogenous catalytic subunit of type 1 phosphatase (PP1cdelta), resulting in an increase of the MP holoenzyme. The level of phosphorylation of ventricular MLC was reduced. The pCa-tension relationship, using beta-escin permeabilized fibers, revealed decreased Ca(2+) sensitization of contraction in the Tg heart. LV enlargement with associated impairment of function was observed in the Tg heart and ultrastructural examination showed cardiomyocyte degeneration.

CONCLUSIONS

Overexpression of MYPT2 and the increase in PP1cdelta resulted in an increase of the MP holoenzyme and a decrease in the level of MLC phosphorylation. The latter induced Ca(2+) desensitization of contraction and decreased LV contractility, resulting in LV enlargement. Thus, MYPT2 is truly the regulatory subunit of cardiac MP in-vivo and plays a significant role in modulating cardiac function. (Circ J 2010; 74: 120 - 128).

Prostanoid F receptors elicit an inotropic effect in rat left ventricle by enhancing myosin light chain phosphorylation

AIMS

The aims of this study were to determine if the prostanoid F receptor (FPR)-mediated inotropic effect in rat ventricle is mediated by increased phosphorylation of myosin light chain-2 (MLC-2) and to elucidate the signalling pathway(s) activated by FPRs to regulate MLC-2 phosphorylation.

METHODS AND RESULTS

Contractility was measured in left ventricular strips from adult male rats. Strips were also snap-frozen, and changes in the phosphorylation level of both MLC-2 and myosin phosphatase targeting subunit-2 (MYPT-2) were quantified. FPR stimulation with fluprostenol increased contractility by approximately 100% above basal and increased phosphorylation of both MLC-2 (by approximately 30%) and MYPT-2 (by approximately 50%). The FPR-mediated inotropic effect and MLC-2 phosphorylation were reduced by a similar magnitude in the presence of the myosin light chain kinase (MLCK) inhibitor ML-7 (approximately 60-70%) and an inhibitor of Ca(2+)/calmodulin, W-7 (approximately 35%). Inhibition of Rho-associated kinase by Y-27632 reduced the FPR-mediated inotropic effect and MLC-2 phosphorylation by approximately 40-45% and MYPT-2 phosphorylation by approximately 70%. ML-7 and Y-27632 together reduced contractility and MLC-2 phosphorylation by approximately 70-80%. The FPR-mediated inotropic effect was only modestly affected by high concentrations of the inositol tris-phosphate (IP(3)) receptor blocker 2-APB, but not by the protein kinase C (PKC) inhibitor bisindolylmaleimide.

CONCLUSION

The FPR-evoked inotropic effect is mediated by increasing the phosphorylation of MLC-2 through regulation of both MLCK and myosin light chain phosphatase activities. The second messenger IP(3) and PKC are unlikely to be involved in the signalling cascade of the FPR-mediated positive inotropic effect. Therefore, FPR signalling mechanism(s) regulating MLC-2 phosphorylation likely extend beyond those classically established for G(q/11)-coupled receptors.

The actin cytoskeleton in cancer cell motility

Cancer cell metastasis is a multi-stage process involving invasion into surrounding tissue, intravasation, transit in the blood or lymph, extravasation, and growth at a new site. Many of these steps require cell motility, which is driven by cycles of actin polymerization, cell adhesion and acto-myosin contraction. These processes have been studied in cancer cells in vitro for many years, often with seemingly contradictory results. The challenge now is to understand how the multitude of in vitro observations relates to the movement of cancer cells in living tumour tissue. In this review we will concentrate on actin protrusion and acto-myosin contraction. We will begin by presenting some general principles summarizing the widely-accepted mechanisms for the co-ordinated regulation of actin polymerization and contraction. We will then discuss more recent studies that investigate how experimental manipulation of actin dynamics affects cancer cell invasion in complex environments and in vivo.