'Sarcomeres' of smooth muscle: functional characteristics and ultrastructural evidence

Reconstitution of contractile actomyosin bundles

Contractile actomyosin bundles are critical for numerous aspects of muscle and nonmuscle cell physiology. Due to the varying composition and structure of actomyosin bundles in vivo, the minimal requirements for their contraction remain unclear. Here, we demonstrate that actin filaments and filaments of smooth muscle myosin motors can self-assemble into bundles with contractile elements that efficiently transmit actomyosin forces to cellular length scales. The contractile and force-generating potential of these minimal actomyosin bundles is sharply sensitive to the myosin density. Above a critical myosin density, these bundles are contractile and generate large tensile forces. Below this threshold, insufficient cross-linking of F-actin by myosin thick filaments prevents efficient force transmission and can result in rapid bundle disintegration. For contractile bundles, the rate of contraction decreases as forces build and stalls under loads of ∼0.5 nN. The dependence of contraction speed and stall force on bundle length is consistent with bundle contraction occurring by several contractile elements connected in series. Thus, contraction in reconstituted actomyosin bundles captures essential biophysical characteristics of myofibrils while lacking numerous molecular constituents and structural signatures of sarcomeres. These results provide insight into nonsarcomeric mechanisms of actomyosin contraction found in smooth muscle and nonmuscle cells.

Active tension adaptation at a shortened arterial muscle length: inhibition by cytochalasin-D

Unlike the static length-tension curve of striated muscle, airway and urinary bladder smooth muscles display a dynamic length-tension curve. Much less is known about the plasticity of the length-tension curve of vascular smooth muscle. The present study demonstrates that there were significant increases of ∼15% in the phasic phase and ∼10% in the tonic phase of a third KCl-induced contraction of a rabbit femoral artery ring relative to the first contraction after a 20% decrease in length from an optimal muscle length (L(0)) to 0.8-fold L(0). Typically, three repeated contractions were necessary for full length adaptation to occur. The tonic phase of a third KCl-induced contraction was increased by ∼50% after the release of tissues from 1.25-fold to 0.75-fold L(o). The mechanism for this phenomenon did not appear to lie in thick filament regulation because there was no increase in myosin light chain (MLC) phosphorylation to support the increase in tension nor was length adaptation abolished when Ca(2+) entry was limited by nifedipine and when Rho kinase (ROCK) was blocked by H-1152. However, length adaptation of both the phasic and tonic phases was abolished when actin polymerization was inhibited through blockade of the plus end of actin by cytochalasin-D. Interestingly, inhibition of actin polymerization when G-actin monomers were sequestered by latrunculin-B increased the phasic phase and had no effect on the tonic phase of contraction during length adaptation. These data suggest that for a given level of cytosolic free Ca(2+), active tension in the femoral artery can be sensitized not only by regulation of MLC phosphatase via ROCK and protein kinase C, as has been reported by others, but also by a nonmyosin regulatory mechanism involving actin polymerization. Dysregulation of this form of active tension modulation may provide insight into alterations of large artery stiffness in hypertension.

Emergence of airway smooth muscle functions related to structural malleability

The function of a complex system such as a smooth muscle cell is the result of the active interaction among molecules and molecular aggregates. Emergent macroscopic manifestations of these molecular interactions, such as the length-force relationship and its associated length adaptation, are well documented, but the molecular constituents and organization that give rise to these emergent muscle behaviors remain largely unknown. In this minireview, we describe emergent properties of airway smooth muscle that seem to have originated from inherent fragility of the cellular structures, which has been increasingly recognized as a unique and important smooth muscle attribute. We also describe molecular interactions (based on direct and indirect evidence) that may confer malleability on fragile structural elements that in turn may allow the muscle to adapt to large and frequent changes in cell dimensions. Understanding how smooth muscle works may hinge on how well we can relate molecular events to its emergent macroscopic functions.

Influence of dispersion in myosin filament orientation and anisotropic filament contractions in smooth muscle

A new constitutive model for the biomechanical behaviour of smooth muscle tissue is proposed. The active muscle contraction is accomplished by the relative sliding between actin and myosin filaments, comprising contractile units in the smooth muscle cells. The orientation of the myosin filaments, and thereby the contractile units, are taken to exhibit a statistical dispersion around a preferred direction. The number of activated cross-bridges between the actin and myosin filaments governs the contractile force generated by the muscle and also the contraction speed. A strain-energy function is used to describe the mechanical behaviour of the smooth muscle tissue. Besides the active contractile apparatus, the mechanical model also incorporates a passive elastic part. The constitutive model was compared to histological and isometric tensile test results for smooth muscle tissue from swine carotid artery. In order to be able to predict the active stress at different muscle lengths, a filament dispersion significantly larger than the one observed experimentally was required. Furthermore, a comparison of the predicted active stress for a case of uniaxially oriented myosin filaments and a case of filaments with a dispersion based on the experimental histological data shows that the difference in generated stress is noticeable but limited. Thus, the results suggest that myosin filament dispersion alone cannot explain the increase in active muscle stress with increasing muscle stretch.

Length oscillation mimicking periodic individual deep inspirations during tidal breathing attenuates force recovery and adaptation in airway smooth muscle

Airway smooth muscle (ASM) is able to generate maximal force under static conditions, and this isometric force can be maintained over a large length range due to length adaptation. The increased force at short muscle length could lead to excessive narrowing of the airways. Prolonged exposure of ASM to submaximal stimuli also increases the muscle's ability to generate force in a process called force adaptation. To date, the effects of length and force adaptation have only been demonstrated under static conditions. In the mechanically dynamic environment of the lung, ASM is constantly subjected to periodic stretches by the parenchyma due to tidal breathing and deep inspiration. It is not known whether force recovery due to muscle adaptation to a static environment could occur in a dynamic environment. In this study the effect of length oscillation mimicking tidal breathing and deep inspiration was examined. Force recovery after a length change was attenuated in the presence of length oscillation, except at very short lengths. Force adaptation was abolished by length oscillation. We conclude that in a healthy lung (with intact airway-parenchymal tethering) where airways are not allowed to narrow excessively, large stretches (associated with deep inspiration) may prevent the ability of the muscle to generate maximal force that would occur under static conditions irrespective of changes in mean length; mechanical perturbation on ASM due to tidal breathing and deep inspiration, therefore, is the first line of defense against excessive bronchoconstriction that may result from static length and force adaptation.

Dense-body aggregates as plastic structures supporting tension in smooth muscle cells

The wall of hollow organs of vertebrates is a unique structure able to generate active tension and maintain a nearly constant passive stiffness over a large volume range. These properties are predominantly attributable to the smooth muscle cells that line the organ wall. Although smooth muscle is known to possess plasticity (i.e., the ability to adapt to large changes in cell length through structural remodeling of contractile apparatus and cytoskeleton), the detailed structural basis for the plasticity is largely unknown. Dense bodies, one of the most prominent structures in smooth muscle cells, have been regarded as the anchoring sites for actin filaments, similar to the Z-disks in striated muscle. Here, we show that the dense bodies and intermediate filaments formed cable-like structures inside airway smooth muscle cells and were able to adjust the cable length according to cell length and tension. Stretching the muscle cell bundle in the relaxed state caused the cables to straighten, indicating that these intracellular structures were connected to the extracellular matrix and could support passive tension. These plastic structures may be responsible for the ability of smooth muscle to maintain a nearly constant tensile stiffness over a large length range. The finding suggests that the structural plasticity of hollow organs may originate from the dense-body cables within the smooth muscle cells.

A multiscale, spatially distributed model of asthmatic airway hyper-responsiveness

We present a multiscale, spatially distributed model of lung and airway behaviour with the goal of furthering the understanding of airway hyper-responsiveness and asthma. The model provides an initial computational framework for linking events at the cellular and molecular levels, such as Ca(2+) and crossbridge dynamics, to events at the level of the entire organ. At the organ level, parenchymal tissue is modelled using a continuum approach as a compressible, hyperelastic material in three dimensions, with expansion and recoil of lung tissue due to tidal breathing. The governing equations of finite elasticity deformation are solved using a finite element method. The airway tree is embedded in this tissue, where each airway is modelled with its own airway wall, smooth muscle and surrounding parenchyma. The tissue model is then linked to models of the crossbridge mechanics and their control by Ca(2+) dynamics, thus providing a link to molecular and cellular mechanisms in airway smooth muscle cells. By incorporating and coupling the models at these scales, we obtain a detailed, computational multiscale model incorporating important physiological phenomena associated with asthma.

Abl activation regulates the dissociation of CAS from cytoskeletal vimentin by modulating CAS phosphorylation in smooth muscle

Abl is a nonreceptor tyrosine kinase that is required for smooth muscle contraction. However, the mechanism by which Abl regulates smooth muscle contraction is not completely understood. In the present study, Abl underwent phosphorylation at Tyr412 (an index of Abl activation) in smooth muscle in response to contractile activation. Treatment with a cell-permeable decoy peptide, but not the control peptide, attenuated Abl phosphorylation during contractile stimulation. Treatment with the decoy peptide did not affect the association of Abl with the cytoskeletal protein vinculin and the spatial location of vinculin in smooth muscle. Inhibition of Abl phosphorylation by the decoy peptide attenuated the agonist-induced phosphorylation of Crk-associated substrate (CAS), an adapter protein participating in the signaling processes that regulate force development in smooth muscle. Additionally, previous studies have shown that contractile stimulation triggers the dissociation of CAS from the vimentin network, which is important for cytoskeletal signaling and contraction in smooth muscle. In this report, the decrease in the amount of CAS in cytoskeletal vimentin in response to contractile activation was reversed by the Abl inhibition with the decoy peptide. Moreover, force development and the enhancement of F-actin-to-G-actin ratios (an indication of actin polymerization) upon contractile activation were also attenuated by the Abl inhibition. However, myosin phosphorylation induced by contractile activation was not affected by the inhibition of Abl. These results suggest that Abl regulates the dissociation of CAS from the vimentin network, actin polymerization, and contraction by modulating CAS phosphorylation in smooth muscle.

Increase in passive stiffness at reduced airway smooth muscle length: potential impact on airway responsiveness

The amplitude of strain in airway smooth muscle (ASM) produced by oscillatory perturbations such as tidal breathing or deep inspiration (DI) influences the force loss in the muscle and is therefore a key determinant of the bronchoprotective and bronchodilatory effects of these breathing maneuvers. The stiffness of unstimulated ASM (passive stiffness) directly influences the amplitude of strain. The nature of the passive stiffness is, however, not clear. In this study, we measured the passive stiffness of ovine ASM at different muscle lengths (relative to in situ length, which was used as a reference length, Lref) and states of adaptation to gain insights into the origin of this muscle property. The results showed that the passive stiffness was relatively independent of muscle length, possessing a constant plateau value over a length range from 0.62 to 1.25 Lref. Following a halving of ASM length, passive stiffness decreased substantially (by 71%) but redeveloped over time (∼30 min) at the shorter length to reach 65% of the stiffness value at Lref, provided that the muscle was stimulated to contract at least once over a ∼30-min period. The redevelopment and maintenance of passive stiffness were dependent on the presence of Ca2+ but unaffected by latrunculin B, an inhibitor of actin filament polymerization. The maintenance of passive stiffness was also not affected by blocking myosin cross-bridge cycling using a myosin light chain kinase inhibitor or by blocking the Rho-Rho kinase (RhoK) pathway using a RhoK inhibitor. Our results suggest that the passive stiffness of ASM is labile and capable of redevelopment following length reduction. Redevelopment and maintenance of passive stiffness following muscle shortening could contribute to airway hyperresponsiveness by attenuating the airway wall strain induced by tidal breathing and DI.

Adaptation of the length-active tension relationship in rabbit detrusor

Studies have shown that the length-tension (L-T) relationships in airway and vascular smooth muscles are dynamic and can adapt to length changes over a period of time. Our prior studies have shown that the passive L-T relationship in rabbit detrusor smooth muscle (DSM) is also dynamic and that DSM exhibits adjustable passive stiffness (APS) characterized by a passive L-T curve that can shift along the length axis as a function of strain history and activation history. The present study demonstrates that the active L-T curve for DSM is also dynamic and that the peak active tension produced at a particular muscle length is a function of both strain and activation history. More specifically, this study reveals that the active L-T relationship, or curve, does not have a unique peak tension value with a single ascending and descending limb, but instead reveals that multiple ascending and descending limbs can be exhibited in the same DSM strip. This study also demonstrates that for DSM strips not stretched far enough to reveal a descending limb, the peak active tension produced by a maximal KCl-induced contraction at a short, passively slack muscle length of 3 mm was reduced by 58.6 +/- 4.1% (n = 15) following stretches to and contractions at threefold the original muscle length, 9 mm. Moreover, five subsequent contractions at the short muscle length displayed increasingly greater tension; active tension produced by the sixth contraction was 91.5 +/- 9.1% of that produced by the prestretch contraction at that length. Together, these findings indicate for the first time that DSM exhibits length adaptation, similar to vascular and airway smooth muscles. In addition, our findings demonstrate that preconditioning, APS and adaptation of the active L-T curve can each impact the maximum total tension observed at a particular DSM length.

p130 Crk-associated substrate (CAS) in vascular smooth muscle

Vascular smooth muscle is a key effector in the wall of blood vessels during the pathogenesis of hypertension. Various factors directly elicit smooth muscle cell contraction, migration, growth, and hypertrophy, which lead to the progression of hypertension. Crk-associated substrate (CAS), the first discovered member of the adapter protein CAS family, has recently emerged as a critical cellular component that regulates smooth muscle functions. In this review, the molecular structure and protein interactions of the CAS family members are summarized. Evidence for the role of CAS in the regulation of vascular smooth muscle contractility, cell migration, hypertrophy, and growth is presented. Regulation of CAS by novel tyrosine kinases/phosphatases and unique downstream signaling partners of CAS are also discussed. These new findings establish the important role for CAS in regulating vascular smooth muscle functions. The CAS-associated processes may be new biological targets for the development of new treatment of cardiovascular diseases such as hypertension.

Chronic activation in shortened airway smooth muscle: a synergistic combination underlying airway hyperresponsiveness?

Airway smooth muscle (ASM) in individuals with asthma is continuously stimulated by spasmogens released as part of chronic airway inflammation. This chronic submaximal stimulation of ASM produces "tone," which may or may not narrow airways sufficiently to induce respiratory symptoms. However, when coupled with a bronchoprovocative challenge with a nonspecific contractile agonist, this increased tone could contribute to the manifestation of airway hyperresponsiveness (AHR). In this study, we examined the effect of chronic acetylcholine (ACh) exposure at different muscle lengths to gain insights into the consequence of increased tone on the mechanical properties of ASM. The total force (the ACh-induced tone plus active force induced by a second stimulus-electric field stimulation [EFS]) increased immediately after induction of muscle tone, and increased further over time in the presence of the tone in a process termed "force adaptation." The phenomenon of force adaptation was observed over a wide range of muscle lengths and did not prevent length adaptation when the muscle was adapted to the tone before being subjected to a length change, suggesting that both length and force adaptations can occur sequentially and in an independent fashion in the same tissue. Together, these results suggest that adaptation of ASM to shortened length in the presence of muscle tone produced a condition that favored excessive force generation in response to a second stimulus (herein EFS) at reduced muscle length. In vivo these changes will be translated into excessive airway narrowing in response to naturally occurring and pharmacological bronchoconstricting stimuli.

Stress and strain in the contractile and cytoskeletal filaments of airway smooth muscle

Stress and strain are omnipresent in the lung due to constant lung volume fluctuation associated with respiration, and they modulate the phenotype and function of all cells residing in the airways including the airway smooth muscle (ASM) cell. There is ample evidence that the ASM cell is very sensitive to its physical environment, and can alter its structure and/or function accordingly, resulting in either desired or undesired consequences. The forces that are either conferred to the ASM cell due to external stretching or generated inside the cell must be borne and transmitted inside the cytoskeleton (CSK). Thus, maintaining appropriate levels of stress and strain within the CSK is essential for maintaining normal function. Despite the importance, the mechanisms regulating/dysregulating ASM cytoskeletal filaments in response to stress and strain remained poorly understood until only recently. For example, it is now understood that ASM length and force are dynamically regulated, and both can adapt over a wide range of length, rendering ASM one of the most malleable living tissues. The malleability reflects the CSK's dynamic mechanical properties and plasticity, both of which strongly interact with the loading on the CSK, and all together ultimately determines airway narrowing in pathology. Here we review the latest advances in our understanding of stress and strain in ASM cells, including the organization of contractile and cytoskeletal filaments, range and adaptation of functional length, structural and functional changes of the cell in response to mechanical perturbation, ASM tone as a mediator of strain-induced responses, and the novel glassy dynamic behaviors of the CSK in relation to asthma pathophysiology.

Regulatory mechanism of smooth muscle contraction studied with gelsolin-treated strips of taenia caeci in guinea pig

The potential roles of the regulatory proteins actin, tropomyosin (Tm), and caldesmon (CaD), i.e., the components of the thin filament, in smooth muscle have been extensively studied in several types of smooth muscles. However, controversy remains on the putative physiological significance of these proteins. In this study, we intended to determine the functional roles of Tm and CaD in the regulation of smooth muscle contraction by using a reconstitution system of the thin filaments. At appropriate conditions, the thin (actin) filaments within skinned smooth muscle strips of taenia caeci in guinea pigs could be selectively removed by an actin-severing protein, gelsolin, without irreversible damage to the contractile apparatus, and then the thin filaments were reconstituted with purified components of thin filaments, i.e., actin, Tm, and CaD. We found that the structural remodeling of actin filaments or thin filaments was functionally linked to the Ca2+-induced force development and reduction in muscle cross-sectional area (CSA). That is, after the reconstitution of the gelsolin-treated skinned smooth muscle strips with pure actin, the Ca2+-dependent force development was partially restored, but the Ca2+-induced reduction in CSA occurred once. In contrast, the reconstitution with actin, followed by Tm and CaD, restored not only the force generation but also both its Ca2+sensitivity and the reversible Ca2+-dependent reduction in CSA. We confirmed that both removal of the thin filaments by gelsolin treatment and reconstitution of the actin (thin) filaments with Tm and CaD caused no significant changes in the level of myosin regulatory light chain phosphorylation. We thus conclude that Tm and CaD are necessary for the full regulation of smooth muscle contraction in addition to the other regulatory systems, including the myosin-linked one.

Disrupting actin-myosin-actin connectivity in airway smooth muscle as a treatment for asthma?

Breathing is known to functionally antagonize bronchoconstriction caused by airway muscle contraction. During breathing, tidal lung inflation generates force fluctuations that are transmitted to the contracted airway muscle. In vitro, experimental application of force fluctuations to contracted airway smooth muscle strips causes them to relengthen. Such force fluctuation-induced relengthening (FFIR) likely represents the mechanism by which breathing antagonizes bronchoconstriction. Thus, understanding the mechanisms that regulate FFIR of contracted airway muscle could suggest novel therapeutic interventions to increase FFIR, and so to enhance the beneficial effects of breathing in suppressing bronchoconstriction. Here we propose that the connectivity between actin filaments in contracting airway myocytes is a key determinant of FFIR, and suggest that disrupting actin-myosin-actin connectivity by interfering with actin polymerization or with myosin polymerization merits further evaluation as a potential novel approach for preventing prolonged bronchoconstriction in asthma.

Smooth muscle signalling pathways in health and disease

Smooth muscle contractile activity is a major regulator of function of the vascular system, respiratory system, gastrointestinal system and the genitourinary systems. Malfunction of contractility in these systems leads to a host of clinical disorders, and yet, we still have major gaps in our understanding of the molecular mechanisms by which contractility of the differentiated smooth muscle cell is regulated. This review will summarize recent advances in the molecular understanding of the regulation of smooth muscle myosin activity via phosphorylation/dephosphorylation of myosin, the regulation of the accessibility of actin to myosin via the actin-binding proteins calponin and caldesmon, and the remodelling of the actin cytoskeleton. Understanding of the molecular 'players' should identify target molecules that could point the way to novel drug discovery programs for the treatment of smooth muscle disorders such as cardiovascular disease, asthma, functional bowel disease and pre-term labour.

Smooth muscle signalling pathways in health and disease

Smooth muscle contractile activity is a major regulator of function of the vascular system, respiratory system, gastrointestinal system and the genitourinary systems. Malfunction of contractility in these systems leads to a host of clinical disorders, and yet, we still have major gaps in our understanding of the molecular mechanisms by which contractility of the differentiated smooth muscle cell is regulated. This review will summarize recent advances in the molecular understanding of the regulation of smooth muscle myosin activity via phosphorylation/dephosphorylation of myosin, the regulation of the accessibility of actin to myosin via the actin-binding proteins calponin and caldesmon, and the remodelling of the actin cytoskeleton. Understanding of the molecular 'players' should identify target molecules that could point the way to novel drug discovery programs for the treatment of smooth muscle disorders such as cardiovascular disease, asthma, functional bowel disease and pre-term labour.

Airway mechanics and methods used to visualize smooth muscle dynamics in vitro

Contraction of airway smooth muscle (ASM) is regulated by the physiological, structural and mechanical environment in the lung. We review two in vitro techniques, lung slices and airway segment preparations, that enable in situ ASM contraction and airway narrowing to be visualized. Lung slices and airway segment approaches bridge a gap between cell culture and isolated ASM, and whole animal studies. Imaging techniques enable key upstream events involved in airway narrowing, such as ASM cell signalling and structural and mechanical events impinging on ASM, to be investigated.

Adaptation of airway smooth muscle to basal tone: relevance to airway hyperresponsiveness

Lung inflammation and airway hyperresponsiveness (AHR) are hallmarks of asthma, but their interrelationship is unclear. Excessive shortening of airway smooth muscle (ASM) in response to bronchoconstrictors is likely an important determinant of AHR. Hypercontractility of ASM could stem from a change in the intrinsic properties of the muscle, or it could be due to extrinsic factors such as chronic exposure of the muscle to inflammatory mediators in the airways. The latter could be the link between lung inflammation and AHR. The present study was designed to examine the influence of chronic exposure to a contractile agonist on the force-generating capacity of ASM. Force generation in response to electric field stimulation (EFS) was measured in ovine trachealis with or without a basal tone induced by acetylcholine (ACh). While the tone was maintained, the EFS-induced force decreased transiently but increased over time to reach a plateau in approximately 50 minutes. The total force (ACh tone + EFS force) increased monotonically and in proportion to ACh concentration. The results indicate that the muscle adapted to the basal tone and regained its contractile ability in response to a second stimulus (EFS) over time. Analysis suggests that this is due to a cytoskeletal transformation that allows the cytoskeleton to bear force, thus freeing up actomyosin crossbridges to generate more force. Force adaptation in ASM as a consequence of prolonged exposure to the many spasmogens found in asthmatic airways could be a mechanism contributing to AHR seen in asthma.

Smooth muscle titin Zq domain interaction with the smooth muscle alpha-actinin central rod

Actin-myosin II filament-based contractile structures in striated muscle, smooth muscle, and nonmuscle cells contain the actin filament-cross-linking protein alpha-actinin. In striated muscle Z-disks, alpha-actinin interacts with N-terminal domains of titin to provide a structural linkage crucial for the integrity of the sarcomere. We previously discovered a long titin isoform, originally smitin, hereafter sm-titin, in smooth muscle and demonstrated that native sm-titin interacts with C-terminal EF hand region and central rod R2-R3 spectrin-like repeat region sites in alpha-actinin. Reverse transcription-PCR analysis of RNA from human adult smooth muscles and cultured rat smooth muscle cells and Western blot analysis with a domain-specific antibody presented here revealed that sm-titin contains the titin gene-encoded Zq domain that may bind to the alpha-actinin R2-R3 central rod domain as well as Z-repeat domains that bind to the EF hand region. We investigated whether the sm-titin Zq domain binds to alpha-actinin R2 and R3 spectrin repeat-like domain loops that lie in proximity with two-fold symmetry on the surface of the central rod. Mutations in alpha-actinin R2 and R3 domain loop residues decreased interaction with expressed sm-titin Zq domain in glutathione S-transferase pull-down and solid phase binding assays. Alanine mutation of a region of the Zq domain with high propensity for alpha-helix formation decreased apparent Zq domain dimer formation and decreased Zq interaction with the alpha-actinin R2-R3 region in surface plasmon resonance assays. We present a model in which two sm-titin Zq domains interact with each other and with the two R2-R3 sites in the alpha-actinin central rod.

Physiologic properties and regulation of the actin cytoskeleton in vascular smooth muscle

Vascular smooth muscle tone plays a fundamental role in regulating blood pressure, blood flow, microcirculation, and other cardiovascular functions. The cellular and molecular mechanisms by which vascular smooth muscle contractility is regulated are not completely elucidated. Recent studies show that the actin cytoskeleton in smooth muscle is dynamic, which regulates force development. In this review, evidence for actin polymerization in smooth muscle upon external stimulation is summarized. Protein kinases such as Abelson tyrosine kinase, focal adhesion kinase, Src, and mitogen-activated protein kinase have been documented to coordinate actin polymerization in smooth muscle. Transmembrane integrins have also been reported to link to signaling pathways modulating actin dynamics. The roles of Rho family of the small proteins that bind to guanosine triphosphate (GTP), also known as GTPases, and the actin-regulatory proteins, including Crk-associated substrate, neuronal Wiskott-Aldrich Syndrome protein, the Arp2/3 complex, and profilin, and heat shock proteins in regulating actin assembly are discussed. These new findings promote our understanding on how smooth muscle contraction is regulated at cellular and molecular levels.

A mathematical model of airway and pulmonary arteriole smooth muscle

Airway hyperresponsiveness is a major characteristic of asthma and is believed to result from the excessive contraction of airway smooth muscle cells (SMCs). However, the identification of the mechanisms responsible for airway hyperresponsiveness is hindered by our limited understanding of how calcium (Ca2+), myosin light chain kinase (MLCK), and myosin light chain phosphatase (MLCP) interact to regulate airway SMC contraction. In this work, we present a modified Hai-Murphy cross-bridge model of SMC contraction that incorporates Ca2+ regulation of MLCK and MLCP. A comparative fit of the model simulations to experimental data predicts 1), that airway and arteriole SMC contraction is initiated by fast activation by Ca2+ of MLCK; 2), that airway SMC, but not arteriole SMC, is inhibited by a slower activation by Ca2+ of MLCP; and 3), that the presence of a contractile agonist inhibits MLCP to enhance the Ca2+ sensitivity of airway and arteriole SMCs. The implication of these findings is that murine airway SMCs exploit a Ca2+-dependent mechanism to favor a default state of relaxation. The rate of SMC relaxation is determined principally by the rate of release of the latch-bridge state, which is predicted to be faster in airway than in arteriole. In addition, the model also predicts that oscillations in calcium concentration, commonly observed during agonist-induced smooth muscle contraction, cause a significantly greater contraction than an elevated steady calcium concentration.

Physical integrity of smooth muscle myosin filaments is enhanced by phosphorylation of the regulatory myosin light chain

BACKGROUND AND AIMS

Smooth muscle myosin monomers self-assemble in solution to form filaments. Phosphorylation of the 20-kD regulatory myosin light chain (MLC20) enhances filament formation. It is not known whether the phosphorylated and non-phosphorylated filaments possess the same structural integrity.

METHODS

We purified myosin from bovine trachealis to form filaments, in ATP-containing zero-calcium solution during a slow dialysis that gradually reduced the ionic strength. Sufficient myosin light chain kinase and phosphatase, as well as calmodulin, were retained after the myosin purification and this enabled phosphorylation of MLC20 within 20-40s after addition of calcium to the filament suspension. The phosphorylated and non-phosphorylated filaments were then partially disassembled by ultrasonification. The extent of filament disintegration was visualized and quantified by atomic force microscopy.

RESULTS

MLC20 phosphorylation reduced the diameter of the filaments and rendered the filaments more resistant to ultrasonic agitation. Electron microscopy revealed a similar reduction in filament diameter in intact smooth muscle when the cells were activated.

CONCLUSION

Modification of the structural and physical properties of myosin filaments by MLC20 phosphorylation may be a key regulation step in smooth muscle where formation and dissolution of the filaments are required in the cells' adaptation to different cell length.

Mechanism of partial adaptation in airway smooth muscle after a step change in length

The phenomenon of length adaptation in airway smooth muscle (ASM) is well documented; however, the underlying mechanism is less clear. Evidence to date suggests that the adaptation involves reassembly of contractile filaments, leading to reconfiguration of the actin filament lattice and polymerization or depolymerization of the myosin filaments within the lattice. The time courses for these events are unknown. To gain insights into the adaptation process, we examined ASM mechanical properties and ultrastructural changes during adaptation. Step changes in length were applied to isolated bundles of ASM cells; changes in force, shortening velocity, and myosin filament mass were then quantified. A greater decrease in force was found following an acute decrease in length, compared with that of an acute increase in length. A decrease in myosin filament mass was also found with an acute decrease in length. The shortening velocity measured immediately after the length change was the same as that measured after the muscle had fully adapted to the new length. These observations can be explained by a model in which partial adaptation of the muscle leads to an intermediate state in which reconfiguration of the myofilament lattice occurred rapidly, followed by a relatively slow process of polymerization of myosin filaments within the lattice. The partially adapted intermediate state is perhaps more physiologically relevant than the fully adapted state seen under static conditions, and it simulates a more realistic behavior for ASM in vivo.

Ultrastructural basis of airway smooth muscle contractionThis article is one of a selection of papers published in the Special Issue on Recent Advances in Asthma Research

The sliding filament theory of contraction that was developed for striated muscle is generally believed to be also applicable to smooth muscle. However, the well-organized myofilament lattice (i.e., the sarcomeric structure) found in striated muscle has never been clearly delineated in smooth muscle. There is evidence that the myofilament lattice in some smooth muscles, such as airway smooth muscle, is malleable; it can be reshaped to fit a large range of cell dimensions while the maximal overlap between the contractile filaments is maintained. In this review, some early models of the structurally static contractile apparatus of smooth muscle are described. The focus of the review, however, is on the recent findings supporting a model of structurally dynamic contractile apparatus and cytoskeleton for airway smooth muscle. A list of unanswered questions regarding smooth muscle ultrastructure is also proposed in this review, in the hope that it will provide some guidance for future research.

Airway smooth muscle dynamics: a common pathway of airway obstruction in asthma

Excessive airway obstruction is the cause of symptoms and abnormal lung function in asthma. As airway smooth muscle (ASM) is the effecter controlling airway calibre, it is suspected that dysfunction of ASM contributes to the pathophysiology of asthma. However, the precise role of ASM in the series of events leading to asthmatic symptoms is not clear. It is not certain whether, in asthma, there is a change in the intrinsic properties of ASM, a change in the structure and mechanical properties of the noncontractile components of the airway wall, or a change in the interdependence of the airway wall with the surrounding lung parenchyma. All these potential changes could result from acute or chronic airway inflammation and associated tissue repair and remodelling. Anti-inflammatory therapy, however, does not "cure" asthma, and airway hyperresponsiveness can persist in asthmatics, even in the absence of airway inflammation. This is perhaps because the therapy does not directly address a fundamental abnormality of asthma, that of exaggerated airway narrowing due to excessive shortening of ASM. In the present study, a central role for airway smooth muscle in the pathogenesis of airway hyperresponsiveness in asthma is explored.

The extensive length-force relationship of porcine airway smooth muscle

The full functional length range of trachealis muscle was measured to identify a precise reference length and to assess the length changes that the myofilament lattice can accommodate. The initial reference length ( L10%) was that where rest tension equaled 10% of total force (passive tension plus active force). Total force at this length served as a force reference (Fref = 219 ± 12 kPa, N = 7). Muscles initially adapted at L10% for 30–60 min had no rest tension when shortened to <0.9 L10%. Passive tension rose steeply and linearly with slope 11.2 Fref/ L10% at lengths >1.04 L10%. Rest tension at 1.1 L10% declined by <10% over 1 h. The steep slope and stability of rest tension at long lengths suggest that a parameter of the slope could serve as a precise, reproducible reference length. Active force was nearly constant at lengths 0.33–1.0 L10% and declined steeply at lengths between 0.1 and 0.2 L10%, extrapolating to zero at 0.076 L10%. Muscles visibly reextended during relaxation at lengths <0.25 L10%. At long lengths, force extrapolated to zero at 1.175 L10%. The >15-fold length range (0.076–1.175 L10%) for force generation and nearly constant force over a greater than threefold length range is likely produced by several structural accommodations, including filament sliding, an increased number of sliding filaments in series, and increased length of passive structures in series with the sliding filaments. Visible reextension during relaxation suggests that the lattice does not undergo plastic adaptations at lengths <25% L10% and that lattice plasticity is limited to a three- to fourfold length range.

Adjustable passive length-tension curve in rabbit detrusor smooth muscle

Until the 1990s, the passive and active length-tension (L-T) relationships of smooth muscle were believed to be static, with a single passive force value and a single maximum active force value for each muscle length. However, recent studies have demonstrated that the active L-T relationship in airway smooth muscle is dynamic and adapts to length changes over a period of time. Furthermore, our prior work showed that the passive L-T relationship in rabbit detrusor smooth muscle (DSM) is also dynamic and that in addition to viscoelastic behavior, DSM displays strain-softening behavior characterized by a loss of passive stiffness at shorter lengths following a stretch to a new longer length. This loss of passive stiffness appears to be irreversible when the muscle is not producing active force and during submaximal activation but is reversible on full muscle activation, which indicates that the stiffness component of passive force lost to strain softening is adjustable in DSM. The present study demonstrates that the passive L-T curve for DSM is not static and can shift along the length axis as a function of strain history and activation history. This study also demonstrates that adjustable passive stiffness (APS) can modulate total force (35% increase) for a given muscle length, while active force remains relatively unchanged (4% increase). This finding suggests that the structures responsible for APS act in parallel with the contractile apparatus, and the results are used to further justify the configuration of modeling elements within our previously proposed mechanical model for APS.

Models of contractile units and their assembly in smooth muscle

It is believed that the contractile filaments in smooth muscle are organized into arrays of contractile units (similar to the sarcomeric structure in striated muscle), and that such an organization is crucial for transforming the mechanical activities of actomyosin interaction into cell shortening and force generation. Details of the filament organization, however, are still poorly understood. Several models of contractile filament architecture are discussed here. To account for the linear relationship observed between the force generated by a smooth muscle and the muscle length at the plateau of an isotonic contraction, a model of contractile unit is proposed. The model consists of 2 dense bodies with actin (thin) filaments attached, and a myosin (thick) filament lying between the parallel thin filaments. In addition, the thick filament is assumed to span the whole contractile unit length, from dense body to dense body, so that when the contractile unit shortens, the amount of overlap between the thick and thin filaments (i.e., the distance between the dense bodies) decreases in exact proportion to the amount of shortening. Assembly of the contractile units into functional contractile apparatus is assumed to involve a group of cells that form a mechanical syncytium. The contractile apparatus is assumed malleable in that the number of contractile units in series and in parallel can be altered to accommodate strains on the muscle and to maintain the muscle's optimal mechanical function.

Myosin filament assembly in an ever-changing myofilament lattice of smooth muscle

A major development in smooth muscle research in recent years is the recognition that the myofilament lattice of the muscle is malleable. The malleability appears to stem from plastic rearrangement of contractile and cytoskeletal filaments in response to stress and strain exerted on the muscle cell, and it allows the muscle to adapt to a wide range of cell lengths and maintain optimal contractility. Although much is still poorly understood, we have begun to comprehend some of the basic mechanisms underlying the assembly and disassembly of contractile and cytoskeletal filaments in smooth muscle during the process of adaptation to large changes in cell geometry. One factor that likely facilitates the plastic length adaptation is the ability of myosin filaments to form and dissolve at the right place and the right time within the myofilament lattice. It is proposed herein that formation of myosin filaments in vivo is aided by the various filament-stabilizing proteins, such as caldesmon, and that the thick filament length is determined by the dimension of the actin filament lattice. It is still an open question as to how the dimension of the dynamic filament lattice is regulated. In light of the new perspective of malleable myofilament lattice in smooth muscle, the roles of many smooth muscle proteins could be assigned or reassigned in the context of plastic reorganization of the contractile apparatus and cytoskeleton.

Length adaptation of airway smooth muscle: a stochastic model of cytoskeletal dynamics

To account for cytoskeleton remodeling as well as smooth muscle length adaptation, here we represent the cytoskeleton as a two-dimensional network of links (contractile filaments or stress fibers) that connect nodes (dense plaques or focal adhesions). The network evolves in continuous turnover with probabilities of link formation and dissolution. The probability of link formation increases with the available fraction of contractile units, increases with the degree of network activation, and decreases with increasing distance between nodes, d, as 1/d(s), where s controls the distribution of link lengths. The probability of link dissolution decays with time to mimic progressive cytoskeleton stabilization. We computed network force (F) as the vector summation of link forces exerted at all nodes, unloaded shortening velocity (V) as being proportional to the average link length, and network compliance (C) as the change in network length per change in elastic force. Imposed deformation caused F to decrease transiently and then recover dynamically; recovery ability decreased with increasing time after activation, mimicking observed biological behavior. Isometric contractions showed small sensitivity of F to network length, thus maintaining high force over a wide range of lengths; V and C increased with increasing length. In these behaviors, link length regulation, as described by the parameter s, was found to be crucial. Concerning length adaptation, all phenomena reported thus far in the literature were captured by this extremely simple network model.

The role of airway smooth muscle during an attack of asthma simulated in vitro

Excessive narrowing of airways in response to contractile agonists is a characteristic feature of asthma. We hypothesized that airway smooth muscle (ASM) adaptation to short lengths could contribute to exaggerated airway narrowing during an acute attack of asthma by allowing the muscle to regain its ability to generate maximal force at a shortened length. To test this hypothesis we mimicked, in vitro, the sequence of contractile events that would occur during a spontaneous attack of asthma. Trachealis muscle was challenged with carbachol (300 nM, submaximal dose) and allowed to shorten to approximately half of its original length. After 30 min of adaptation at the shortened length in the presence of carbachol, muscle force, amount and rate of shortening in response to electrical stimulation were compared with corresponding values obtained from control experiments during which the ASM was not adapted to the short length. After adaptation at the shortened length the developed force, amount and rate of shortening increased by 1.93 +/- 0.08-, 1.57 +/- 0.12-, and 1.75 +/- 0.2-fold, respectively. Shortening of ASM in response to contractile agonists can lead to adaptation of the muscle to the shortened length that, in turn, can result in further shortening and the potential for airway closure.