Regulation and tuning of smooth muscle myosin

Paxillin participates in the sphingosylphosphorylcholine-induced abnormal contraction of vascular smooth muscle by regulating Rho-kinase activation

BACKGROUND

The Ca2+-independent contraction of vascular smooth muscle is a leading cause of cardiovascular and cerebrovascular spasms. In the previous study, we demonstrated the involvement of Src family protein tyrosine kinase Fyn and Rho-kinase in the sphingosylphosphorylcholine (SPC)-induced abnormal and Ca2+-independent contraction of vascular smooth muscle, but the specific mechanism has not been completely clarified.

METHODS

Paxillin knockdown human coronary artery smooth muscle cells (CASMCs) and smooth muscle-specific paxillin knockout mice were generated by using paxillin shRNA and the tamoxifen-inducible Cre-LoxP system, respectively. CASMCs contraction was observed by time-lapse recording. The vessel contractility was measured by using a myography assay. Fyn, Rho-kinase, and myosin light chain activation were assessed by immunoprecipitation and western blotting. The paxillin expression and actin stress fibers were visualized by histological analysis and immunofluorescent staining.

RESULTS

The SPC-induced abnormal contraction was inhibited in paxillin knockdown CASMCs and arteries of paxillin knockout mice, indicating that paxillin is involved in this abnormal contraction. Further study showed that paxillin knockdown inhibited the SPC-induced Rho-kinase activation without affecting Fyn activation. In addition, paxillin knockdown significantly inhibited the SPC-induced actin stress fiber formation and myosin light chain phosphorylation. These results suggest that paxillin, as an upstream molecule of Rho-kinase, is involved in the SPC-induced abnormal contraction of vascular smooth muscle.

CONCLUSIONS

The present study demonstrated that paxillin participates in the SPC-induced abnormal vascular smooth muscle contraction by regulating Rho-kinase activation. Video Abstract.

Integrative analysis of noncoding mutations identifies the druggable genome in preterm birth

Preterm birth affects ~10% of pregnancies in the US. Despite familial associations, identifying at-risk genetic loci has been challenging. We built deep learning and graphical models to score mutational effects at base resolution via integrating the pregnant myometrial epigenome and large-scale patient genomes with spontaneous preterm birth (sPTB) from European and African American cohorts. We uncovered previously unidentified sPTB genes that are involved in myometrial muscle relaxation and inflammatory responses and that are regulated by the progesterone receptor near labor onset. We studied genomic variants in these genes in our recruited pregnant women administered progestin prophylaxis. We observed that mutation burden in these genes was predictive of responses to progestin treatment for preterm birth. To advance therapeutic development, we screened ~4000 compounds, identified candidate molecules that affect our identified genes, and experimentally validated their therapeutic effects on regulating labor. Together, our integrative approach revealed the druggable genome in preterm birth and provided a generalizable framework for studying complex diseases.

Filament evanescence of myosin II and smooth muscle function

Smooth muscle is an integral part of hollow organs. Many of them are constantly subjected to mechanical forces that alter organ shape and modify the properties of smooth muscle. To understand the molecular mechanisms underlying smooth muscle function in its dynamic mechanical environment, a new paradigm has emerged that depicts evanescence of myosin filaments as a key mechanism for the muscle’s adaptation to external forces in order to maintain optimal contractility. Unlike the bipolar myosin filaments of striated muscle, the side-polar filaments of smooth muscle appear to be less stable, capable of changing their lengths through polymerization and depolymerization (i.e., evanescence). In this review, we summarize accumulated knowledge on the structure and mechanism of filament formation of myosin II and on the influence of ionic strength, pH, ATP, myosin regulatory light chain phosphorylation, and mechanical perturbation on myosin filament stability. We discuss the scenario of intracellular pools of monomeric and filamentous myosin, length distribution of myosin filaments, and the regulatory mechanisms of filament lability in contraction and relaxation of smooth muscle. Based on recent findings, we suggest that filament evanescence is one of the fundamental mechanisms underlying smooth muscle’s ability to adapt to the external environment and maintain optimal function. Finally, we briefly discuss how increased ROCK protein expression in asthma may lead to altered myosin filament stability, which may explain the lack of deep-inspiration–induced bronchodilation and bronchoprotection in asthma.

EF-hand proteins and the regulation of actin-myosin interaction in the eutardigrade Hypsibius klebelsbergi (tardigrada)

Many tardigrade species resist harsh environmental conditions by entering anhydrobiosis or cryobiosis. Desiccation as well as freeze resistance probably leads to changes of the ionic balance that includes the intracellular calcium concentration. In order to search for protein modifications affecting the calcium homoeostasis, we studied the regulatory system controlling actin-myosin interaction of the eutardigrade Hypsibius klebelsbergi and identified full-length cDNA clones for troponin C (TnC, 824 bp), calmodulin (CaM, 1,407 bp), essential myosin light chain (eMLC, 1,015 bp), and regulatory myosin light chain (rMLC, 984 bp) from a cDNA library. All four proteins belong to the EF-hand superfamily typified by a calcium coordinating helix-loop-helix motif. Further, we cloned and obtained recombinant TnC and both MLCs. CaM and TnC revealed four and two potential calcium-binding domains, respectively. Gel mobility shift assays demonstrated calcium-induced conformational transition of TnC. From both MLCs, only the rMLC showed one potential N-terminal EF-hand domain. Additionally, sequence properties suggest phosphorylation of this myosin light chain. Based on our results, we suggest a dual-regulated system at least in somatic muscles for tardigrades with a calcium-dependent tropomyosin-troponin complex bound to the actin filaments and a phosphorylation of the rMLC turning on and off both actin and myosin. Our results indicate no special modifications of the molecular structure and function of the EF-hand proteins in tardigrades. Phylogenetic trees of 131 TnCs, 96 rMLCs, and 62 eMLCs indicate affinities to Ecdysozoa, but also to some other taxa suggesting that our results reflect the complex evolution of these proteins rather than phylogenetic relationships.

α(1) adrenergic receptor agonist, phenylephrine, actively contracts early rat rib fracture callus ex vivo

Early, soft fracture callus that links fracture ends together is smooth muscle-like in nature. We aimed to determine if early fracture callus could be induced to contract and relax ex vivo by similar pathways to smooth muscle, that is, contraction via α(1) adrenergic receptor (α(1) AR) activation with phenylephrine (PE) and relaxation via β(2) adrenergic receptor (β(2) AR) stimulation with terbutaline. A sensitive force transducer quantified 7 day rat rib fracture callus responses in modified Krebs-Henseliet (KH) solutions. Unfractured ribs along with 7, 14, and 21 day fracture calluses were analyzed for both α(1) AR and β(2) AR gene expression using qPCR, whilst 7 day fracture callus was examined via immunohistochemistry for both α(1) AR and β(2) AR- immunoreactivity. In 7 day callus, PE (10(-6)  M) significantly induced an increase in force that was greater than passive force generated in calcium-free KH (n = 8, mean 51% increase, 95% CI: 26-76%). PE-induced contractions in calluses were attenuated by the α(1) AR antagonist, prazosin (10(-6)  M; n = 7, mean 5% increase, 95% CI: 2-11%). Terbutaline did not relax callus. Gene expression of α(1) ARs was constant throughout fracture healing; however, β(2) AR expression was down-regulated at 7 days compared to unfractured rib (p < 0.01). Furthermore, osteoprogenitor cells of early fibrous callus displayed considerable α(1) AR-like immunoreactivity but not β(2) AR-like immunoreactivity. Here, we demonstrate for the first time that early fracture callus can be pharmacologically induced to contract. We propose that increased concentrations of α(1) AR agonists such as noradrenaline may tonically contract callus in vivo to promote osteogenesis.

The molecular effects of skeletal muscle myosin regulatory light chain phosphorylation

Phosphorylation of the myosin regulatory light chain (RLC) in skeletal muscle has been proposed to act as a molecular memory of recent activation by increasing the rate of force development, ATPase activity, and isometric force at submaximal activation in fibers. It has been proposed that these effects stem from phosphorylation-induced movement of myosin heads away from the thick filament backbone. In this study, we examined the molecular effects of skeletal muscle myosin RLC phosphorylation using in vitro motility assays. We showed that, independently of the thick filament backbone, the velocity of skeletal muscle myosin is decreased upon phosphorylation due to an increase in the myosin duty cycle. Furthermore, we did not observe a phosphorylation-dependent shift in calcium sensitivity in the absence of the myosin thick filament. These data suggest that phosphorylation-induced movement of myosin heads away from the thick filament backbone explains only part of the observed phosphorylation-induced changes in myosin mechanics. Last, we showed that the duty cycle of skeletal muscle myosin is strain dependent, consistent with the notion that strain slows the rate of ADP release in striated muscle.

Affinity for MgADP and force of unbinding from actin of myosin purified from tonic and phasic smooth muscle

Smooth muscle is unique in its ability to maintain force at low MgATP consumption. This property, called the latch state, is more prominent in tonic than phasic smooth muscle. Studies performed at the muscle strip level have suggested that myosin from tonic muscle has a greater affinity for MgADP and therefore remains attached to actin longer than myosin from phasic muscle, allowing for cross-bridge dephosphorylation and latch-bridge formation. An alternative hypothesis is that after dephosphorylation, myosin reattaches to actin and maintains force. We investigated these fundamental properties of smooth muscle at the molecular level. We used an in vitro motility assay to measure actin filament velocity (nu(max)) when propelled by myosin purified from phasic or tonic muscle at increasing [MgADP]. Myosin was 25% thiophosphorylated and 75% unphosphorylated to approximate in vivo conditions. The slope of nu(max) versus [MgADP] was significantly greater for tonic (-0.51+/-0.04) than phasic muscle myosin (-0.15+/-0.04), demonstrating the greater MgADP affinity of myosin from tonic muscle. We then used a laser trap assay to measure the unbinding force from actin of populations of unphosphorylated tonic and phasic muscle myosin. Both myosin types attached to actin, and their unbinding force (0.092+/-0.022 pN for phasic muscle and 0.084+/-0.017 pN for tonic muscle) was not statistically different. We conclude that the greater affinity for MgADP of tonic muscle myosin and the reattachment of dephosphorylated myosin to actin may both contribute to the latch state.

Host defense genes in asthma and sepsis and the role of the environment

PURPOSE OF REVIEW

There is growing evidence that innate immunity genes contribute to asthma pathogenesis. At the core of the innate immune response are ubiquitous, soluble fragments of bacterial lipopolysaccharide or endotoxin, and chronic exposure to domestic endotoxin has been shown to influence asthma severity. Asthmatic and atopic individuals are more sensitive to endotoxin than nonallergic individuals, suggesting a role for genetics in the innate immunity response, and the potential for gene-environment interactions. Variants in genes associated with classic innate immunity-related disorders, such as sepsis, may be unique candidates for asthma susceptibility.

RECENT FINDINGS

Candidate genes for asthma and allergic diseases co-associated with sepsis including innate immunity receptors and related molecules (CD14, TLR4 and AOAH) and novel genes such as MYLK provide good examples of pleitropic effects of innate immunity genes, where variants conferring risk to specific traits (i.e. sepsis) under one set of genetic and environmental circumstances confer a reduced risk in a different (but possibly related) clinical outcome (i.e. allergic asthma), and support the 'common variant/multiple disease' hypothesis.

SUMMARY

Collectively, these observations suggest a greater role for the innate immunity response in allergic asthma than previously assumed, and implicate host defense genes in disease pathology.

Polymorphisms in the myosin light chain kinase gene that confer risk of severe sepsis are associated with a lower risk of asthma

BACKGROUND

Myosin light chain kinase (MYLK) is a multifunctional protein involved in regulation of airway hyperreactivity and other activities relevant to asthma.

OBJECTIVE

To determine the role of MYLK gene variants in asthma among African Caribbean and African American populations.

METHODS

We performed association tests between single nucleotide polymorphisms (SNPs) in the MYLK gene and asthma susceptibility and total serum IgE concentrations in 2 independent, family-based populations of African descent. Previously we identified variants/haplotypes in MYLK that confer risk for sepsis and acute lung injury; we compared findings from our asthma populations to findings in the African American sepsis and acute lung injury groups.

RESULTS

Significant associations between MYLK SNPs and asthma and total serum IgE concentrations were observed in the African Caribbean families: a promoter SNP (rs936170) in the smooth muscle form gave the strongest association (P = .009). A haplotype including rs936170 corresponding to the actin-binding activity of the nonmuscle and smooth muscle forms was negatively associated with asthma (eg, decreased risk) in both the American (P = .005) and Caribbean families (P = .004), and was the same haplotype that conferred risk for severe sepsis (P = .002). RNA expression studies on PBMCs and rs936170 suggested a significant decrease in MYLK expression among patients with asthma with this variant (P = .025).

CONCLUSION

MYLK polymorphisms may function as a common genetic factor in clinically distinct diseases involving bronchial smooth muscle contraction and inflammation.

CLINICAL IMPLICATIONS

Genetic variants in MYLK are significantly associated with both asthma and sepsis in populations of African ancestry.

The essential light chain N-terminal extension alters force and fiber kinetics in mouse cardiac muscle

The functional significance of the actin-binding region at the N terminus of the cardiac myosin essential light chain (ELC) remains elusive. In a previous experiment, the endogenous ventricular ELC was replaced with a protein containing a 10-amino acid deletion at positions 5-14 (ELC1vDelta5-14, referred to as 1vDelta5-14), a region that interacts with actin. 1vDelta5-14 mice showed no discernable mutant phenotype in skinned ventricular strips. However, because the myofilament lattice swells upon skinning, the mutant phenotype may have been concealed by the inability of the ELC to reach the actin-binding site. Using the same mouse model, we repeated earlier measurements and performed additional experiments on skinned strips osmotically compressed to the intact lattice spacing as determined by x-ray diffraction. 1vDelta5-14 mice exhibited decreased maximum isometric tension without a change in calcium sensitivity. The decreased force was most evident in 5-6-month-old mice compared with 13-15-month-old mice and may account for the greater ventricular wall thickness in young 1vDelta5-14 mice compared with age-matched controls. No differences were observed in unloaded shortening velocity at maximum calcium activation. However, 1vDelta5-14 mice exhibited a significant difference in the frequency at which minimum complex modulus amplitude occurred, indicating a change in cross-bridge kinetics. We hypothesize that the ELC N-terminal extension interaction with actin inhibits the reversal of the power stroke, thereby increasing isometric force. Our results strongly suggest that an interaction between residues 5-14 of the ELC N terminus and the C-terminal residues of actin enhances cardiac performance.

Smooth muscle myosin: regulation and properties

The relationship of the biochemical states to the mechanical events in contraction of smooth muscle cross-bridges is reviewed. These studies use direct measurements of the kinetics of Pi and ADP release. The rate of release of Pi from thiophosphorylated cycling cross-bridges held isometric was biphasic with turnovers of 1.8 s-1 and 0.3 s-1, reflecting properties and forces directly acting on cross-bridges through mechanisms such as positive strain and inhibition by high-affinity MgADP binding. Fluorescent transients reporting release of an ADP analogue 3'-deac-edaADP were significantly faster in phasic than in tonic smooth muscles. Thiophosphorylation of myosin regulatory light chains (RLCs) increased and positive strain decreased the release rate around twofold. The rates of ADP release from rigor cross-bridges and the steady-state Pi release from cycling isometric cross-bridges are similar, indicating that the ADP-release step or an isomerization preceding it may limit the ATPase rate. Thus ADP release in phasic and tonic smooth muscles is a regulated step with strain- and dephosphorylation-dependence. High affinity of cross-bridges for ADP and slow ADP release prolong the fraction of the duty cycle occupied by strongly bound AM.ADP state(s) and contribute to the high economy of force that is characteristic of smooth muscle. RLC thiophosphorylation led to structural changes in smooth muscle cross-bridges consistent with our findings that thiophosphorylation and strain modulate product release.

Myosin regulatory light chain phosphorylation and strain modulate adenosine diphosphate release from smooth muscle Myosin

The effects of myosin regulatory light chain (RLC) phosphorylation and strain on adenosine diphosphate (ADP) release from cross-bridges in phasic (rabbit bladder (Rbl)) and tonic (femoral artery (Rfa)) smooth muscle were determined by monitoring fluorescence transients of the novel ADP analog, 3'-deac-eda-ADP (deac-edaADP). Fluorescence transients reporting release of 3'-deac-eda-ADP were significantly faster in phasic (0.57 +/- 0.06 s(-1)) than tonic (0.29 +/- 0.03 s(-1)) smooth muscles. Thiophosphorylation of regulatory light chains increased and strain decreased the release rate approximately twofold. The calculated (k-ADP/k+ADP) dissociation constant, Kd of unstrained, unphosphorylated cross-bridges for ADP was 0.6 microM for rabbit bladder and 0.3 microM for femoral artery. The rates of ADP release from rigor bridges and reported values of Pi release (corresponding to the steady-state adenosine triphosphatase (ATPase) rate of actomyosin (AM)) from cross-bridges during a maintained isometric contraction are similar, indicating that the ADP-release step or an isomerization preceding it may be limiting the adenosine triphosphatase rate. We conclude that the strain- and dephosphorylation-dependent high affinity for and slow ADP release from smooth muscle myosin prolongs the fraction of the duty cycle occupied by strongly bound actomyosin.ADP state(s) and contributes to the high economy of force.

Cross-linking of actin filaments by myosin II is a major contributor to cortical integrity and cell motility in restrictive environments

Cells are frequently required to move in a local environment that physically restricts locomotion, such as during extravasation or metastatic invasion. In order to model these events, we have developed an assay in which vegetative Dictyostelium amoebae undergo chemotaxis under a layer of agarose toward a source of folic acid [Laevsky, G. and Knecht, D. A. (2001). Biotechniques 31, 1140-1149]. As the concentration of agarose is increased from 0.5% to 3% the cells are increasingly inhibited in their ability to move under the agarose. The contribution of myosin II and actin cross-linking proteins to the movement of cells in this restrictive environment has now been examined. Cells lacking myosin II heavy chain (mhcA-) are unable to migrate under agarose overlays of greater than 0.5%, and even at this concentration they move only a short distance from the trough. While attempting to move, the cells become stretched and fragmented due to their inability to retract their uropods. At higher agarose concentrations, the mhcA- cells protrude pseudopods under the agarose, but are unable to pull the cell body underneath. Consistent with a role for myosin II in general cortical stability, GFP-myosin dynamically localizes to the lateral and posterior cortex of cells moving under agarose. Cells lacking the essential light chain of myosin II (mlcE-), have no measurable myosin II motor activity, yet were able to move normally under all agarose concentrations. Mutants lacking either ABP-120 or alpha-actinin were also able to move under agarose at rates similar to wild-type cells. We hypothesize that myosin stabilizes the actin cortex through its cross-linking activity rather than its motor function and this activity is necessary and sufficient for the maintenance of cortical integrity of cells undergoing movement in a restrictive environment. The actin cross-linkers alpha-actinin and ABP-120 do not appear to play as major a role as myosin II in providing this cortical integrity.

Structural model of the regulatory domain of smooth muscle heavy meromyosin

The goal of this study was to provide structural information about the regulatory domains of double-headed smooth muscle heavy meromyosin, including the N terminus of the regulatory light chain, in both the phosphorylated and unphosphorylated states. We extended our previous photo-cross-linking studies (Wu, X., Clack, B. A., Zhi, G., Stull, J. T., and Cremo, C. R. (1999) J. Biol. Chem. 274, 20328-20335) to determine regions of the regulatory light chain that are cross-linked by a cross-linker attached to Cys(108) on the partner regulatory light chain. For this purpose, we have synthesized two new biotinylated sulfhydryl reactive photo-cross-linking reagents, benzophenone, 4-(N-iodoacetamido)-4'-(N-biotinylamido) and benzophenone, 4-(N-maleimido)-4'-(N-biotinylamido). Cross-linked peptides were purified by avidin affinity chromatography and characterized by Edman sequencing and mass spectrometry. Labeled Cys(108) from one regulatory light chain cross-linked to (71)GMMSEAPGPIN(81), a loop in the N-terminal half of the regulatory light chain, and to (4)RAKAKTTKKRPQR(16), a region for which there is no atomic resolution data. Both cross-links were to the partner regulatory light chain and occurred in unphosphorylated but not phosphorylated heavy meromyosin. Using these data, data from our previous study, and atomic coordinates from various myosin isoforms, we have constructed a structural model of the regulatory domain in an unphosphorylated double-headed molecule that predicts the general location of the N terminus. The implications for the structural basis of the phosphorylation-mediated regulatory mechanism are discussed.

Heterotrimeric G protein signaling: role in asthma and allergic inflammation

Asthma and rhinitis are pathophysiologic conditions associated with a prototypical allergic response to inhaled allergens consisting of both neuromechanical and inflammatory components. Heptahelical receptors that bind guanosine triphosphate-binding proteins (G proteins), referred to as G protein-coupled receptors (GPCRs), have been intimately linked with asthma and allergic inflammation for many years. G protein signaling mediates responses throughout the immune, nervous, and muscular systems that might contribute to the pathogenesis of allergic processes and asthma. For example, GPCR agonists or antagonists are used as therapies for asthma either by promoting airway smooth muscle relaxation (beta2 adrenergic receptor agonists) or by inhibiting inflammation in the nasal mucosa and airways (cysteinyl leukotriene receptor antagonists). The focus of this review is to explore how downstream signaling cascades elicited by GPCR activation contribute to the allergic phenotype and the mechanism by which pharmaceuticals alter signaling to generate a therapeutic effect. We also discuss physiologic modulators of G protein signaling, such as regulator of G protein signaling proteins and G protein receptor kinases, inasmuch as they represent potential new therapeutic targets in the treatment of atopy and other inflammatory conditions.

Purification of native myosin filaments from muscle

Analysis of the structure and function of native thick (myosin-containing) filaments of muscle has been hampered in the past by the difficulty of obtaining a pure preparation. We have developed a simple method for purifying native myosin filaments from muscle filament suspensions. The method involves severing thin (actin-containing) filaments into short segments using a Ca(2+)-insensitive fragment of gelsolin, followed by differential centrifugation to purify the thick filaments. By gel electrophoresis, the purified thick filaments show myosin heavy and light chains together with nonmyosin thick filament components. Contamination with actin is below 3.5%. Electron microscopy demonstrates intact thick filaments, with helical cross-bridge order preserved, and essentially complete removal of thin filaments. The method has been developed for striated muscles but can also be used in a modified form to remove contaminating thin filaments from native smooth muscle myofibrils. Such preparations should be useful for thick filament structural and biochemical studies.