Distribution of alpha-actinin in single isolated smooth muscle cells

Membrane adhesion junctions regulate airway smooth muscle phenotype and function

The local environment surrounding airway smooth muscle (ASM) cells has profound effects on the physiological and phenotypic properties of ASM tissues. ASM is continually subjected to the mechanical forces generated during breathing and to the constituents of its surrounding extracellular milieu. The smooth muscle cells within the airways continually modulate their properties to adapt to these changing environmental influences. Smooth muscle cells connect to the extracellular cell matrix (ECM) at membrane adhesion junctions that provide mechanical coupling between smooth muscle cells within the tissue. Membrane adhesion junctions also sense local environmental signals and transduce them to cytoplasmic and nuclear signaling pathways in the ASM cell. Adhesion junctions are composed of clusters of transmembrane integrin proteins that bind to ECM proteins outside the cell and to large multiprotein complexes in the submembranous cytoplasm. Physiological conditions and stimuli from the surrounding ECM are sensed by integrin proteins and transduced by submembranous adhesion complexes to signaling pathways to the cytoskeleton and nucleus. The transmission of information between the local environment of the cells and intracellular processes enables ASM cells to rapidly adapt their physiological properties to modulating influences in their extracellular environment: mechanical and physical forces that impinge on the cell, ECM constituents, local mediators, and metabolites. The structure and molecular organization of adhesion junction complexes and the actin cytoskeleton are dynamic and constantly changing in response to environmental influences. The ability of ASM to rapidly accommodate to the ever-changing conditions and fluctuating physical forces within its local environment is essential for its normal physiological function.

LPP is a Src substrate required for invadopodia formation and efficient breast cancer lung metastasis

We have previously shown that lipoma preferred partner (LPP) mediates TGFβ-induced breast cancer cell migration and invasion. Herein, we demonstrate that diminished LPP expression reduces circulating tumour cell numbers, impairs cancer cell extravasation and diminishes lung metastasis. LPP localizes to invadopodia, along with Tks5/actin, at sites of matrix degradation and at the tips of extravasating breast cancer cells as revealed by intravital imaging of the chick chorioallantoic membrane (CAM). Invadopodia formation, breast cancer cell extravasation and metastasis require an intact LPP LIM domain and the ability of LPP to interact with α-actinin. Finally, we show that Src-mediated LPP phosphorylation at specific tyrosine residues (Y245/301/302) is critical for invadopodia formation, breast cancer cell invasion and metastasis. Together, these data define a previously unknown function for LPP in the formation of invadopodia and reveal a requirement for LPP in mediating the metastatic ability of breast cancer cells.

Preparation, characterization, in vitro drug release, and cellular interactions of tailored paclitaxel releasing polyethylene oxide films for drug-coated balloons

Drug-coated balloons (DCBs) are used to treat various cardiovascular diseases. Currently available DCBs carry drug on the balloon surface either solely or using different carriers. Several studies have shown that a significant amount of drug is lost in the blood stream during balloon tracking to deliver only a sub-therapeutic level of drug at the treatment site. This research is focused on developing paclitaxel (PAT) loaded polyethylene oxide (PEO) films (PAT-PEO) as a controlled drug delivery carrier for DCBs. An array of PAT-PEO films were developed in this study to provide tailored release of >90% of drug only at specific time intervals, which is the time frame required for carrying out balloon-based therapy. The characterizations of PAT-PEO films using SEM, FTIR, and DSC showed that the films developed were homogenous and the PAT was molecularly dispersed in the PEO matrix. Mechanical tests showed that most PAT-PEO films developed were flexible and ductile, with yield and tensile strengths not affected after PAT incorporation. The viability, proliferation, morphology, and phenotype of smooth muscle cells (SMCs) interacted with control-PEO and PAT-PEO films were investigated. All control-PEO and PAT-PEO films showed a significant inhibitory effect on the growth of SMCs, with the degree of inhibition strongly dependent on the w/v% of the polymer used. The PAT-PEO coating was produced on the balloons. The integrity of PAT-PEO coating was well maintained without any mechanical defects occurring during balloon inflation or deflation. The drug release studies showed that only 15% of the total PAT loaded was released from the balloons within the initial 1min (typical balloon tracking time), whereas 80% of the PAT was released between 1min and 4min (typical balloon treatment time). Thus, this study demonstrated the use of PEO as an alternate drug delivery system for the balloons.

STATEMENT OF SIGNIFICANCE

Atherosclerosis is primarily responsible for cardiovascular diseases (CVDs) in millions of patients every year. Drug-coated balloons (DCBs) are commonly used to treat various CVDs. However, in several currently used DCBs, a significant amount of drug is lost in the blood stream during balloon tracking to deliver only a sub-therapeutic level of drug at the treatment site. In this study, paclitaxel containing polyethylene oxide (PEO) films were developed to provide unique advantages including drug release profiles specifically tailored for balloon-based therapy, homogeneous films with molecularly dispersed drug, flexible and ductile films, and exhibits significant inhibitory effect on smooth muscle cell growth. Thus, this study demonstrated the use of PEO as an alternate drug delivery platform for DCBs to improve its efficacy.

Active patterning and asymmetric transport in a model actomyosin network

Cytoskeletal networks, which are essentially motor-filament assemblies, play a major role in many developmental processes involving structural remodeling and shape changes. These are achieved by nonequilibrium self-organization processes that generate functional patterns and drive intracellular transport. We construct a minimal physical model that incorporates the coupling between nonlinear elastic responses of individual filaments and force-dependent motor action. By performing stochastic simulations we show that the interplay of motor processes, described as driving anti-correlated motion of the network vertices, and the network connectivity, which determines the percolation character of the structure, can indeed capture the dynamical and structural cooperativity which gives rise to diverse patterns observed experimentally. The buckling instability of individual filaments is found to play a key role in localizing collapse events due to local force imbalance. Motor-driven buckling-induced node aggregation provides a dynamic mechanism that stabilizes the two-dimensional patterns below the apparent static percolation limit. Coordinated motor action is also shown to suppress random thermal noise on large time scales, the two-dimensional configuration that the system starts with thus remaining planar during the structural development. By carrying out similar simulations on a three-dimensional anchored network, we find that the myosin-driven isotropic contraction of a well-connected actin network, when combined with mechanical anchoring that confers directionality to the collective motion, may represent a novel mechanism of intracellular transport, as revealed by chromosome translocation in the starfish oocyte.

Vasoconstrictor-induced endocytic recycling regulates focal adhesion protein localization and function in vascular smooth muscle

Turnover of focal adhesions (FAs) is known to be critical for cell migration and adhesion of proliferative vascular smooth muscle (VSM) cells. However, it is often assumed that FAs in nonmigratory, differentiated VSM (dVSM) cells embedded in the wall of healthy blood vessels are stable structures. Recent work has demonstrated agonist-induced actin polymerization and Src-dependent FA phosphorylation in dVSM cells, suggesting that agonist-induced FA remodeling occurs. However, the mechanisms and extent of FA remodeling are largely unknown in dVSM. Here we show, for the first time, that a distinct subpopulation of dVSM FA proteins, but not the entire FA, remodels in response to the α-agonist phenylephrine. Vasodilator-stimulated phosphoprotein and zyxin displayed the largest redistributions, while β-integrin and FA kinase showed undetectable redistribution. Vinculin, metavinculin, Src, Crk-associated substrate, and paxillin displayed intermediate degrees of redistribution. Redistributions into membrane fractions were especially prominent, suggesting endosomal mechanisms. Deconvolution microscopy, quantitative colocalization analysis, and Duolink proximity ligation assays revealed that phenylephrine increases the association of FA proteins with early endosomal markers Rab5 and early endosomal antigen 1. Endosomal disruption with the small-molecule inhibitor primaquine inhibits agonist-induced redistribution of FA proteins, confirming endosomal recycling. FA recycling was also inhibited by cytochalasin D, latrunculin B, and colchicine, indicating that the redistribution is actin- and microtubule-dependent. Furthermore, inhibition of endosomal recycling causes a significant inhibition of the rate of development of agonist-induced dVSM contractions. Thus these studies are consistent with the concept that FAs in dVSM cells, embedded in the wall of the aorta, remodel during the action of a vasoconstrictor.

A multi-scale approach to airway hyperresponsiveness: from molecule to organ

Airway hyperresponsiveness (AHR), a characteristic of asthma that involves an excessive reduction in airway caliber, is a complex mechanism reflecting multiple processes that manifest over a large range of length and time scales. At one extreme, molecular interactions determine the force generated by airway smooth muscle (ASM). At the other, the spatially distributed constriction of the branching airways leads to breathing difficulties. Similarly, asthma therapies act at the molecular scale while clinical outcomes are determined by lung function. These extremes are linked by events operating over intermediate scales of length and time. Thus, AHR is an emergent phenomenon that limits our understanding of asthma and confounds the interpretation of studies that address physiological mechanisms over a limited range of scales. A solution is a modular computational model that integrates experimental and mathematical data from multiple scales. This includes, at the molecular scale, kinetics, and force production of actin-myosin contractile proteins during cross-bridge and latch-state cycling; at the cellular scale, Ca²⁺ signaling mechanisms that regulate ASM force production; at the tissue scale, forces acting between contracting ASM and opposing viscoelastic tissue that determine airway narrowing; at the organ scale, the topographic distribution of ASM contraction dynamics that determine mechanical impedance of the lung. At each scale, models are constructed with iterations between theory and experimentation to identify the parameters that link adjacent scales. This modular model establishes algorithms for modeling over a wide range of scales and provides a framework for the inclusion of other responses such as inflammation or therapeutic regimes. The goal is to develop this lung model so that it can make predictions about bronchoconstriction and identify the pathophysiologic mechanisms having the greatest impact on AHR and its therapy.

Contractile units in disordered actomyosin bundles arise from F-actin buckling

Bundles of filaments and motors are central to contractility in cells. The classic example is striated muscle, where actomyosin contractility is mediated by highly organized sarcomeres which act as fundamental contractile units. However, many contractile bundles in vivo and in vitro lack sarcomeric organization. Here we propose a model for how contractility can arise in bundles without sarcomeric organization and validate its predictions with experiments on a reconstituted system. In the model, internal stresses in frustrated arrangements of motors with diverse velocities cause filaments to buckle, leading to overall shortening. We describe the onset of buckling in the presence of stochastic motor head detachment and predict that buckling-induced contraction occurs in an intermediate range of motor densities. We then calculate the size of the "contractile units" associated with this process. Consistent with these results, our reconstituted actomyosin bundles show contraction at relatively high motor density, and we observe buckling at the predicted length scale.

Requirements for contractility in disordered cytoskeletal bundles

Actomyosin contractility is essential for biological force generation, and is well understood in highly organized structures such as striated muscle. Additionally, actomyosin bundles devoid of this organization are known to contract both in vivo and in vitro, which cannot be described by standard muscle models. To narrow down the search for possible contraction mechanisms in these systems, we investigate their microscopic symmetries. We show that contractile behavior requires non-identical motors that generate large-enough forces to probe the nonlinear elastic behavior of F-actin. This suggests a role for filament buckling in the contraction of these bundles, consistent with recent experimental results on reconstituted actomyosin bundles.

Airways in smooth muscle α-actin null mice experience a compensatory mechanism that modulates their contractile response

We hypothesized that ablation of smooth muscle α-actin (SM α-A), a contractile-cytoskeletal protein expressed in airway smooth muscle (ASM) cells, abolishes ASM shortening capacity and decreases lung stiffness. In both SM α-A knockout and wild-type (WT) mice, airway resistance (Raw) determined by the forced oscillation technique rose in response to intravenous methacholine (Mch). However, the slope of Raw (cmH(2)O·ml(-1)·s) vs. log(2) Mch dose (μg·kg(-1)·min(-1)) was lower (P = 0.007) in mutant (0.54 ± 0.14) than in WT mice (1.23 ± 0.19). RT-PCR analysis performed on lung tissues confirmed that mutant mice lacked SM α-A mRNA and showed that these mice had robust expressions of both SM γ-A mRNA and skeletal muscle (SKM) α-A mRNA, which were not expressed in WT mice, and an enhanced SM22 mRNA expression relative to that in WT mice. Compared with corresponding spontaneously breathing mice, mechanical ventilation-induced lung mechanical strain increased the expression of SM α-A mRNA in WT lungs; in mutant mice, it augmented the expressions of SM γ-A mRNA and SM22 mRNA and did not alter that of SKM α-A mRNA. In mutant mice, the expression of SM γ-A mRNA in the lung during spontaneous breathing and its enhanced expression following mechanical ventilation are consistent with the likely possibility that in the absence of SM α-A, SM γ-A underwent polymerization and interacted with smooth muscle myosin to produce ASM shortening during cholinergic stimulation. Thus our data are consistent with ASM in mutant mice experiencing compensatory mechanisms that modulated its contractile muscle capacity.

Time course of isotonic shortening and the underlying contraction mechanism in airway smooth muscle

Although the structure of the contractile unit in smooth muscle is poorly understood, some of the mechanical properties of the muscle suggest that a sliding-filament mechanism, similar to that in striated muscle, is also operative in smooth muscle. To test the applicability of this mechanism to smooth muscle function, we have constructed a mathematical model based on a hypothetical structure of the smooth muscle contractile unit: a side-polar myosin filament sandwiched by actin filaments, each attached to the equivalent of a Z disk. Model prediction of isotonic shortening as a function of time was compared with data from experiments using ovine tracheal smooth muscle. After equilibration and establishment of in situ length, the muscle was stimulated with ACh (100 μM) until force reached a plateau. The muscle was then allowed to shorten isotonically against various loads. From the experimental records, length-force and force-velocity relationships were obtained. Integration of the hyperbolic force-velocity relationship and the linear length-force relationship yielded an exponential function that approximated the time course of isotonic shortening generated by the modeled sliding-filament mechanism. However, to obtain an accurate fit, it was necessary to incorporate a viscoelastic element in series with the sliding-filament mechanism. The results suggest that a large portion of the shortening is due to filament sliding associated with muscle activation and that a small portion is due to continued deformation associated with an element that shows viscoelastic or power-law creep after a step change in force.

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.

Development of an intestinal cell culture model to obtain smooth muscle cells and myenteric neurones

This paper reports on the development of an entirely new intestinal smooth muscle cell (ISMC) culture model using rat neonates for use in pharmacological research applications. Segments of the duodenum, jejunum and ileum were obtained from Sprague-Dawley rat neonates. The cell extraction technique consisted of ligating both ends of the intestine and incubating (37 degrees C) in 0.25% trypsin for periods of 30-90 min. Isolated cells were suspended in DMEM-HEPES, plated and allowed to proliferate for 7 days. Cell culture quality was assessed via a series of viability tests using the dye exclusion assay. In separate experiments, tissues were exposed to trypsin for varying durations and subsequently histological procedures were applied. Cell purification techniques included differential adhesion technique for minimizing fibroblasts. Selective treatments with neurotoxin scorpion venom (30 microg mL(-1)) and anti-mitotic cytosine arabinoside (6 microm) were also applied to purify respectively ISMC and myenteric neurones selectively. The different cell populations were identified in regard to morphology and growth characteristics via immunocytochemistry using antibodies to smooth muscle alpha-actin, alpha-actinin and serotonin-5HT3 receptors. Based on both viability and cell confluence experiments, results demonstrated that intestinal cells were best obtained from segments of the ileum dissociated in trypsin for 30 min. This provided the optimum parameters to yield highly viable cells and confluent cultures. The finding was further supported by histological studies demonstrating that an optimum incubation time of 30 min is required to isolate viable cells from the muscularis externae layer. When cell cultures were treated with cytosine arabinoside, the non-neuronal cells were abolished, resulting in the proliferation of cell bodies and extended neurites. Conversely, cultures treated with scorpion venom resulted in complete abolition of neurones and proliferation of increasing numbers of ISMC, which were spindle-shaped and uniform throughout the culture. When characterized by immunocytochemistry, neurones were stained with antibody to 5HT3 receptors but not with antibodies to alpha-smooth muscle actin and alpha-actinin. Conversely, ISMC were stained with antibodies to alpha-smooth muscle actin and alpha-actinin but not with antibody to 5HT3 receptors. The present study provides evidence that our method of dissociation and selectively purifying different cell populations will allow for pharmacological investigation of each cell type on different or defined mixtures of different cell types.

Dynamic association between alpha-actinin and beta-integrin regulates contraction of canine tracheal smooth muscle

The adhesion junctions of smooth muscle cells may be dynamically regulated during smooth muscle contraction, and this dynamic regulation may be important for the development of active tension. In the present study, the role of alpha-actinin during smooth muscle contraction was evaluated in tracheal smooth muscle tissues and freshly dissociated cells. Stimulation with acetylcholine (ACh) increased the localization of alpha-actinin at the membrane of freshly dissociated smooth muscle cells, and increased the amount of beta1 integrin that coprecipitated with alpha-actinin from muscle tissue homogenates. GFP-alpha-actinin fusion proteins were expressed in muscle tissues and visualized in live freshly dissociated cells. GFP-alpha-actinin translocated to the membrane within seconds of stimulation of the cells with ACh. Expression of the integrin-binding rod domain of alpha-actinin in smooth muscle tissues depressed active contraction in response to ACh. Expression of the alpha-actinin rod domain also inhibited the translocation of endogenous alpha-actinin to the membrane, and inhibited the association of endogenous alpha-actinin with beta1-integrin in alpha-actinin immunoprecipitates from tissue extracts. However, the expression of alpha-actinin rod domain peptides did not inhibit increases in myosin light chain phosphorylation or actin polymerization in response to stimulation with ACh. Results suggest that contractile stimulation of smooth muscle causes the rapid recruitment of alpha-actinin to beta-integrin complexes at the membrane, and that the recruitment of alpha-actinin to integrin complexes is necessary for active tension development in smooth muscle.

Unloaded shortening velocity in single permeabilized vascular smooth muscle cells is independent of microtubule status

Microtubules may influence smooth muscle contraction either via involvement in signal transduction processes or by serving as an internal load that opposes contraction. To test the latter hypothesis, microtubule distribution and the unloaded shortening velocity were investigated in freshly isolated single vascular smooth muscle cells (VSMCs) treated with microtubule modulating drugs. Immunocytochemical studies showed that microtubules run mainly longitudinally in relaxed VSMCs. They are oriented more obliquely, almost transversely to the long axis of the cells after contraction, suggesting that microtubules are compressed during shortening, and thus might impart an internal passive load. Quantitative immunocytochemical analysis revealed that, relative to the control group, colchicine (15 microM) decreased the microtubule density by 40% while taxol (10 microM) increased the microtubule density by 46%. However, alteration of microtubule polymerization status by these microtubule-modulating drugs did not have a significant effect on unloaded shortening velocity in alpha-toxin permeabilized VSMCs under maximal activating conditions or submaximal activating conditions (about 36% of maximal velocity). These results suggest that microtubules do not present an appreciable internal load to dampen single VSMCs shortening in the present experimental system, and that their influence on smooth muscle contraction is primarily via signal transduction mechanisms.

Tension development during contractile stimulation of smooth muscle requires recruitment of paxillin and vinculin to the membrane

Cytoskeletal reorganization of the smooth muscle cell in response to contractile stimulation may be an important fundamental process in regulation of tension development. We used confocal microscopy to analyze the effects of cholinergic stimulation on localization of the cytoskeletal proteins vinculin, paxillin, talin and focal adhesion kinase (FAK) in freshly dissociated tracheal smooth muscle cells. All four proteins were localized at the membrane and throughout the cytoplasm of unstimulated cells, but their concentration at the membrane was greater in acetylcholine (ACh)-stimulated cells. Antisense oligonucleotides were introduced into tracheal smooth muscle tissues to deplete paxillin protein, which also inhibited contraction in response to ACh. In cells dissociated from paxillin-depleted muscle tissues, redistribution of vinculin to the membrane in response to ACh was prevented, but redistribution of FAK and talin was not inhibited. Muscle tissues were transfected with plasmids encoding a paxillin mutant containing a deletion of the LIM3 domain (paxillin LIM3 dl 444–494), the primary determinant for targeting paxillin to focal adhesions. Expression of paxillin LIM3 dl in muscle tissues also inhibited contractile force and prevented cellular redistribution of paxillin and vinculin to the membrane in response to ACh, but paxillin LIM3 dl did not inhibit increases in intracellular Ca2+or myosin light chain phosphorylation. Our results demonstrate that recruitment of paxillin and vinculin to smooth muscle membrane is necessary for tension development and that recruitment of vinculin to the membrane is regulated by paxillin. Vinculin and paxillin may participate in regulating the formation of linkages between the cytoskeleton and integrin proteins that mediate tension transmission between the contractile apparatus and the extracellular matrix during smooth muscle contraction.

Myosin cross-bridge kinetics in airway smooth muscle: a comparative study of humans, rats, and rabbits

To analyze the kinetics and unitary force of cross bridges (CBs) in airway smooth muscle (ASM), we proposed a new formalism of Huxley's equations adapted to nonsarcomeric muscles (Huxley AF. Prog Biophys Biophys Chem 7: 255-318, 1957). These equations were applied to ASM from rabbits, rats, and humans (n = 12/group). We tested the hypothesis that species differences in whole ASM mechanics were related to differences in CB mechanics. We calculated the total CB number per square millimeter at peak isometric tension (Psi x10(9)), CB unitary force (Pi), and the rate constants for CB attachment (f(1)) and detachment (g(1) and g(2)). Total tension, Psi, and Pi were significantly higher in rabbits than in humans and rats. Values of Pi were 8.6 +/- 0.1 pN in rabbits, 7.6 +/- 0.3 pN in humans, and 7.7 +/- 0.2 pN in rats. Values of Psi were 4.0 +/- 0.5 in rabbits, 1.2 +/- 0.1 in humans, and 1.9 +/- 0.2 in rats; f(1) was lower in humans than in rabbits and rats; g(2) was higher in rabbits than in rats and in rats than in humans. In conclusion, ASM mechanical behavior of different species was characterized by specific CB kinetics and CB unitary force.

Mechanosensitive modulation of receptor-mediated crossbridge activation and cytoskeletal organization in airway smooth muscle

Recent findings indicate that mechanical strain (deformation) exerted by the extracellular matrix modulates activation of airway smooth muscle cells. Furthermore, cytoskeletal organization in airway smooth muscle appears to be dynamic, and subject to modulation by receptor activation and mechanical strain. Mechanosensitive modulation of crossbridge activation and cytoskeletal organization may represent intracellular feedback mechanisms that limit the shortening of airway smooth muscle during bronchoconstriction. Recent findings suggest that receptor-mediated signal transduction is the primary target of mechanosensitive modulation. Mechanical strain appears to regulate the number of functional G-proteins and/or phospholipase C enzymes in the cell membrane possibly by membrane trafficking and/or protein translocation. Dense plaques, membrane structures analogous to focal adhesions, appear to be the primary target of cytoskeletal regulation. Mechanical strain and receptor-binding appear to regulate the assembly and phosphorylation of dense plaque proteins in airway smooth muscle cells. Understanding these mechanisms may reveal new pharmacological targets for controlling airway resistance in airway diseases.

Mechanical strain memory in airway smooth muscle

We investigated the effect of a single rapid stretch on poststretch force and myosin phosphorylation in bovine tracheal smooth muscle. When unstimulated muscle strips were stretched from suboptimal length to optimal length (L(o)), poststretch steady-state force was not significantly different from that of unstretched control at L(o). However, when carbachol-activated muscle strips were stretched from suboptimal length to L(o), poststretch force and myosin phosphorylation were lower than control and significantly correlated with initial length. When poststretch muscle strips were allowed to relax for 1 h and then activated by K(+) depolarization, the developed force remained significantly correlated with initial length. When the same strain was applied in 23 increments to minimize peak stress, poststretch force and myosin phosphorylation increased significantly, approaching the levels expected at L(o). Furthermore, poststretch force development increased after each cycle of contraction and relaxation, approaching the control level after four cycles. These results suggest that activated airway smooth muscle cells can retain relatively precise memory of past strain when they are stretched rapidly with high stress.

Influence of microtubules on vascular smooth muscle contraction

Microtubules are ubiquitous in eukaryotic cells and play key roles in many cellular activities. The purpose of this study was to investigate the influence of microtubules on vascular smooth muscle contraction. Quantitative immunocytochemical analysis of rat aortic tissue revealed that, relative to the control group, colchicine (15 muM, 90 min) and nocodazole (15 muM, 90 min) decreased the microtubule density by 40-50% while taxol (10 muM, 90 min) increased the microtubule density by 33%. Isometric contraction studies demonstrated that both colchicine and nocodazole caused an upward shift in the phenylephrine (10(-8) to 10(-5) M) dose-response curve while taxol caused no significant change when compared to the control group. Potassium chloride (30 mM) induced 55 +/- 5% P0 contraction in DMSO treated vessel rings. The active tension increased to 73 +/- 5% P0 and 71 +/- 6% P0 after pretreatment of the aortic rings with colchicine or nocodazole, respectively. Taxol did not cause a significant change in the active tension (56 +/- 7% P0). These results indicate that microtubule depolymerization enhances isometric contraction of vascular smooth muscle and this enhanced contraction is not receptor dependent. Pretreatment of the aortic rings with an inhibitor of nitric oxide synthase (NOS) (Nomega-nitro-L-arginine) did not change the increased contractile response to phenylephrine due to microtubule depolymerization suggesting that this phenomenon is not mediated by endothelium dependent relaxation.

Integration of intermediate filaments into cellular organelles

The intermediate filaments represent core components of the cytoskeleton and are known to interact with several membranous organelles. Classic examples of this are the attachment of keratin filaments to the desmosomes and the association of the lamin filament meshwork with the inner nuclear membrane. At this point, the molecular mechanisms by which the filaments link to membranes are not clearly understood. However, since a substantial body of information has been amassed, the time is now ripe for comparing notes and formulating working hypotheses. With this objective in mind, we review here pioneering studies on this subject, together with work that has appeared more recently in the literature.

Structure-function relationships in smooth muscle: the missing links

Smooth muscle cells have developed a contractile machinery that allows them to exert tension on the surrounding extracellular matrix over their entire length. This has been achieved by coupling obliquely organized contractile filaments to a more-or-less longitudinal framework of cytoskeletal elements. Earlier structural data suggested that the cytoskeleton was composed primarily of intermediate filaments and played only a passive role. More recent findings highlight the segregation of actin isotypes and of actin-associated proteins between the contractile and cytoskeletal domains and raise the possibility that the cytoskeleton performs a more active function. Current efforts focus on defining the relative contributions of myosin cross-bridge cycling and actin-associated protein interactions to the maintenance of tension in smooth muscle tissue.

Immunocytochemistry of the muscle cell cytoskeleton

The muscle cell cytoskeleton is defined for this review as any structure or protein primarily involved in linking or connecting protein filaments to each other or to anchoring sites. In striated muscle, the M line connects thick filaments at their centers to adjacent thick filaments. Titin forms elastic filaments that extend from the M line to the Z line and may contribute to the resting tension properties of striated muscle. Nebulin forms inextensible filaments in skeletal muscle that are closely associated with thin filaments and that may provide a length template for thin filaments. Z lines anchor thin filaments from adjacent sarcomeres via the actin-binding function of alpha-actinin. Other proteins located at the Z line include Cap Z, Z-nin, Z protein, and zeugmatin. Intermediate filaments connect myofibrils to each other at the level of the Z line and to the sarcolemma at the Z- and possibly the M-line levels. Immunolocalization has identified the adhesion plaque proteins spectrin, vinculin, dystrophin, ankyrin, and talin at subsarcolemmal sites where they may be involved with filament attachment. Smooth muscle cell cytoskeletons are believed to include membrane associated dense bodies (MADBs), intermediate filaments, cytoplasmic dense bodies (CDBs), and perhaps a subset of actin filaments. MADBs contain a menu of attachment plaque proteins and anchor both thin filaments and intermediate filaments to the sarcolemma. CDBs are intracellular analogs of striated muscle Z lines and anchor thin filaments and intermediate filaments.

Intracellular mechanisms involved in the regulation of vascular smooth muscle tone

Vascular smooth muscle contraction is thought to occur by a mechanism similar to that described for striated muscles, i.e., via a cross-bridge cycling--sliding filament mechanism. This symposium focused on Ca2+ signalling and the role of intracellular free Ca2+ concentration, [Ca2+]i, in regulating vascular tone: how contractile stimuli leading to an increase in [Ca2+]i trigger vasoconstriction and how relaxant signals reduce [Ca2+]i causing vasodilation. M.P. Walsh opened the symposium with an overview emphasizing the central role of myosin phosphorylation-dephosphorylation in the regulation of vascular tone and identifying recent developments concerning regulation of [Ca2+]i, Ca2+ sensitization and desensitization of the contractile response, Ca(2+)-independent protein kinase C induced contraction, and direct regulation of cross-bridge cycling by the thin filament associated proteins caldesmon and calponin. The remainder of the symposium focused on three specific areas related to the regulation of vascular tone: Ca2+ signalling in relation to smooth muscle structure, structure-function relations of myosin, and the role of cyclic GMP (cGMP) dependent protein kinase. G.J. Kargacin described how smooth muscle cells are structured and how second messenger signals such as Ca2+ might be modified or influenced by this structure. J. Kendrick-Jones then discussed the results of mutagenesis studies aimed at understanding how the myosin light chains, particularly the phosphorylatable (Ca(2+)-calmodulin dependent) regulatory light chains, control myosin. The vasorelaxant effects of signalling molecules such as beta-adrenergic agents and nitrovasodilators are mediated by cyclic nucleotide dependent protein kinases, leading principally to a reduction in [Ca2+]i. T.M. Lincoln described the roles of cyclic nucleotide dependent protein kinases, in particular cyclic GMP dependent protein kinase, in vasodilation.

A high molecular mass protein isolated from chicken gizzard: its localization at the dense plaques and dense bodies of smooth muscle and the Z-disks of skeletal muscle

We purified a 450 kDa protein from a low-salt alkaline extract of chicken gizzard smooth muscle. This high molecular mass protein could be extracted with the low-salt alkaline solution at 37 degrees C but not at 4 degrees C. The 450 kDa protein was isolated from the extract by ammonium sulfate fractionation and following sequential column chromatography using hydroxylapatite, DEAE-Cellulofine A-800m and phenyl-Sepharose CL-4B resins. The partially purified protein molecule resembled a flexible rod with a globular head and an irregular-shaped tail. Its length was approximately 300 nm. The nucleotide sequence of the partial cDNA encoding this protein was determined and analyzed with a data base. The analysis showed that the protein revealed significant homology with the rod region of chicken filamin (57% homology in amino acid sequence). Immunoblot analysis showed that an affinity-purified antibody reacted exclusively with the 450 kDa protein band of smooth, skeletal and cardiac muscle tissues. By indirect immunofluorescence microscopy, we examined the localization of the 450 kDa protein in smooth and skeletal muscle cells. The affinity-purified antibody against the 450 kDa protein stained the dense plaques and dense bodies of smooth muscle, the peripheral region of Z-disks and the subsarcolemmal region of skeletal muscle. Immunoelectron microscopy confirmed the localization of the 450 kDa protein at the peripheral regions of the actin anchoring structures mentioned above. Judging from its amino acid sequence, molecular size, molecular shape, immunological reactivity and localization in muscle cells, the 450 kDa protein seemed to be a new component associated with the actin-anchoring structures of muscle tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced three-dimensional reconstruction from confocal scanning microscope images. II. Depth discrimination versus signal-to-noise ratio in partially confocal images

The enhanced depth discrimination of a confocal scanning optical microscope is produced by a pinhole aperture placed in front of the detector to reject out-of-focus light. Strictly confocal behavior is impractical because an infinitesimally small aperture would collect very little light and would result in images with a poor signal-to-noise ratio (SNR), while a finite-sized partially confocal aperture provides a better SNR but reduced depth discrimination. Reconstruction algorithms, such as the expectationmaximization algorithm for maximum likelihood, can be applied to partially confocal images in order to achieve better resolution, but because they are sensitive to noise in the data, there is a practical trade-off involved. With a small aperture, fewer iterations of the reconstruction algorithm are necessary to achieve the desired resolution, but the low a priori SNR will result in a noisy reconstruction, at least when no regularization is used. With a larger aperture the a priori SNR is larger but the resolution is lower, and more iterations of the algorithm are necessary to reach the desired resolution; at some point the a posteriori SNR is lower than the a priori value. We present a theoretical analysis of the SNR-toresolution trade-off partially confocal imaging, and we present two studies that use the expectationmaximization algorithm as a postprocessor; these studies show that a for a given task there is an optimum aperture size, departures from which result in a lower a posteriori SNR.

Actin isoform compartments in chicken gizzard smooth muscle cells

Differentiated smooth muscle cells typically contain a mixture of muscle (alpha and gamma) and cytoplasmic (beta and gamma) actin isoforms. Of the cytoplasmic actins the beta-isoform is the more dominant, making up from 10% to 30% of the total actin complement. Employing an antibody raised against the N-terminal peptide specific to beta-actin, which labels only the beta-isoform on two-dimensional gel immunoblots, we have shown that this isoform has a restricted localisation in smooth muscle. Using double-label immunofluorescence and immunoelectron microscopy of ultrathin sections of chicken gizzard, beta-actin was localised in the dense bodies and in longitudinal channels linking consecutive dense bodies that were also occupied by desmin. It was additionally found in the membrane-associated dense plaques, but was excluded from the actomyosin-containing regions of the contractile apparatus. Taken together with earlier results these findings identify a cytoskeletal compartment containing intermediate filaments, cytoplasmic actin and the actin cross-linking protein filamin. Using an antibody specific only for muscle actin, labelling was found generally around the myosin filaments of the contractile apparatus, but was absent from the core of the dense bodies that contained beta-actin. Thus, if dense bodies act as dual-purpose anchorage sites, for the cytoskeletal actin and the contractile actin, the thin filaments of the contractile apparatus must be anchored at the periphery of the dense bodies. A model of the structural organisation of the cell is presented and the possible roles of the cytoskeleton are discussed.

Substructure of cytoplasmic dense bodies and changes in distribution of desmin and alpha-actinin in developing smooth muscle cells

The substructure of assembling cytoplasmic dense bodies (CDBs) and changes in the distribution of desmin and alpha-actinin during development of smooth muscle were studied in gizzard samples from 10- and 16-day embryos and from 1- and 7-day post-hatch chickens. CDBs in these cells lack the density of CDBs in mature or adult smooth muscle cells and, thus, allow observations of the changes inside CDBs. The random filament orientation seen in younger embryonic cells is first modified to include relatively small patches of IFs that are somewhat straighter and are approaching a side-by-side arrangement. As development proceeds, the IFs in these arrays become straighter, are parallel over longer lengths of the IFs and later acquire the density characteristic of mature CDBs. Anti-desmin labeling in embryonic 10- and 16-day cells showed that desmin intermediate filaments (IFs) were located in the myofilament compartment but were concentrated in or near assembling CDBs. Anti-desmin labeling shifted to the perimeter of CDBs after hatching. Cross sections, longitudinal sections, and stereo pairs all show that IF profiles are present inside unlabeled assembling CDBs. Anti-alpha-actinin labeling was directly on CDBs and was often associated with the cross-connecting filaments (CCFs) (average diameter of 2-3nm) inside CDBs. We propose, based on these data, that desmin IFs, alpha-actinin-containing CCFs, and actin filaments are the principal components of the substructure of assembling CDBs. We also present a proposed model for CDB assembly.

Enhancement of axial resolution in fluorescence microscopy by standing-wave excitation

The use of fluorescence microscopy for investigating the three-dimensional structure of cells and tissue is of growing importance in cell biology, biophysics and biomedicine. Three-dimensional data are obtained by recording a series of images of the specimen as it is stepped through the focal plane of the microscope. Whether by direct imaging or by confocal scanning, diffraction effects and noise generally limit axial resolution to about 0.5 microns. Here we describe a fluorescence microscope in which axial resolution is increased to better than 0.05 microns by using the principle of standing-wave excitation of fluorescence. Standing waves formed by interference in laser illumination create an excitation field with closely spaced nodes and antinodes, allowing optical sectioning of the specimen at very high resolution. We use this technique to obtain images of actin fibres and filaments in fixed cells, actin single filaments in vitro and myosin II in a living cell.

Three-dimensional structure of dense bodies in rabbit renal artery smooth muscle

In this report, we present a three-dimensional computer assisted reconstruction study from serial thin sections through a rabbit renal artery smooth muscle cell. In a series of 32 consecutive thin (100-nm) sections, one longitudinally oriented cell was followed and photographed in alternating sections. The profiles of the cell surface and dense bodies were reconstructed from these 16 planes and the distribution, size, shape, and spatial relationships among these components was examined. The reconstructed images showed that the cell decreases in diameter from its widest region in the center to the two ends in a step-wise taper. Within the cell, dense bodies are numerous. Relative to the cell axes, a membrane associated dense body (MDB) can be less than or equal to 3.5 microns long, 0.25 micron wide, and may extend up to 2 microns in depth. While the MDB profile in one section may be aligned with the long axis of the cell, in an adjacent section the same dense body may appear almost circular or wedge shaped. The same is true of cytoplasmic dense bodies (CDBs). Compared with MDBs, CDBs are smaller in all dimensions. Some, but not all, CDBs line up in strings oriented with the long axis of the cell. The continuity of dense bodies over considerable cell depth and their change in shape may have important implications for integration of contractile activity and for transmitting passive tension to the extracellular matrix.

Organization of rat mesenteric artery after removal of cells of extracellular matrix components

Rat mesenteric arteries, perfusion fixed in relaxed or contracted conditions, were digested with acid and elastase, bleach (sodium hypochlorite), or alkali to selectively remove collagen, elastin, or cells. Scanning electron microscopy was used to study the three-dimensional organization of the remaining cells or extracellular components. Smooth muscle cells of the tunica media were elongated and circumferentially oriented. Superior mesenteric artery cells had an irregular surface with numerous projections and some ends were forked. Small mesenteric artery cells were spindle shaped with longitudinal surface ridges, and showed extensive corrugations upon contraction. Elastin was present both as laminae and as an interconnected fibrous meshwork. Collagen was arranged in an irregular network of individual fibrils and small bundles of fibrils that formed nests around the cells in both arteries. This irregular arrangement persisted, with no apparent reordering or loss of order, upon contraction. The lack of an ordered arrangement or specialized organization at the cell ends suggests mechanical coupling of the cells to elastin or collagen throughout the length of the cell, allowing for force transmission in a number of directions. The tunica media is thus a "composite" material consisting of cells, elastin, and collagen. The isotropic network of fibers is well suited for transmitting the shearing forces placed on it by contraction of smooth muscle cells and by pressure-induced loading.

The cytoskeletal and contractile apparatus of smooth muscle: contraction bands and segmentation of the contractile elements

Confocal laser scanning microscopy of isolated and antibody-labeled avian gizzard smooth muscle cells has revealed the global organization of the contractile and cytoskeletal elements. The cytoskeleton, marked by antibodies to desmin and filamin is composed of a mainly longitudinal, meandering and branched system of fibrils that contrasts with the plait-like, interdigitating arrangement of linear fibrils of the contractile apparatus, labeled with antibodies to myosin and tropomyosin. Although desmin and filamin were colocalized in the body of the cell, filamin antibodies labeled additionally the vinculin-containing surface plaques. In confocal optical sections the contractile fibrils showed a continuous label for myosin for at least 5 microns along their length: there was no obvious or regular interruption of label as might be expected for registered myosin filaments. The cytoplasmic dense bodies, labeled with antibodies to alpha-actinin exhibited a regular, diagonal arrangement in both extended cells and in cells shortened in solution to one-fifth of their extended length: after the same shortening, the fibrils of the cytoskeleton that showed colocalization with the dense bodies in extended cells became crumpled and disordered. It is concluded that the dense bodies serve as coupling elements between the cytoskeletal and contractile systems. After extraction with Triton X-100, isolated cells bound so firmly to a glass substrate that they were unable to shorten as a whole when exposed to exogenous Mg ATP. Instead, they contracted internally, producing integral of 10 regularly spaced contraction nodes along their length. On the basis of differences of actin distribution two types of nodes could be distinguished: actin-positive nodes, in which actin straddled the node, and actin-negative nodes, characterized by an actin-free center flanked by actin fringes of 4.5 microns minimum length on either side. Myosin was concentrated in the center of the node in both cases. The differences in node morphology could be correlated with different degrees of coupling of the contractile with the cytoskeletal elements, effected by a preparation-dependent variability of proteolysis of the cells. The nodes were shown to be closely related to the supercontracted cell fragments shown in the accompanying paper (Small et al., 1990) and furnished further evidence for long actin filaments in smooth muscle. Further, the segmentation of the contractile elements pointed to a hierarchial organization of the myofilaments governed by as yet undetected elements.

Enhanced 3-D reconstruction from confocal scanning microscope images. 1: Deterministic and maximum likelihood reconstructions

Significant improvement in the longitudinal (optical axis) resolution of a microscope has previously been obtained either by posterior digital processing of optical sections collected with a conventional microscope, or by collecting sections with a confocal scanning microscope. In this paper we report the feasibility of obtaining longitudinal resolution comparable to the lateral diffraction limit by posterior processing of confocal sections. A confocal through-focus image series was simulated numerically and restored by constrained iterative deconvolution and by maximum likelihood. Several typical imaging situations were simulated. The results support the possibility of achieving equal longitudinal and lateral resolution.

Myosin filaments isolated from skinned amphibian smooth muscle cells are side-polar

The structure of myosin filaments isolated from skinned toad stomach smooth muscle cells has been examined by electron microscopy as a step toward identifying the in vivo structure. When negatively stained following exposure to relaxing conditions, the filaments exhibited a continuous 14-nm axial repeat of crossbridge projections with no central bare zone. The filaments thus differed from the bipolar filaments found in striated muscle and displayed instead features resembling side-polar and mixed-polarity filament models. By rotation of isolated filaments around their longitudinal axes it was found that cross bridges occurred only along two sides of the filament, an arrangement consistent with the side-polar but not the mixed-polarity model. The polarity is thus similar to that proposed for ribbons (Small & Squire, J. molec. Biol. 67, (1972) 17-149) and for synthetic smooth muscle myosin filaments (Craig and Megerman, J. Cell Biol. 75, (1977) 990-996); their appearance in cross-section, however, shows that these structures are filaments (i.e. with two axes of similar dimensions) and not broad ribbons. As the filaments were derived directly from skinned cells which contracted and relaxed in response to physiological levels of MgATP and Ca2+ at rates comparable to those of native, isolated cells, this unusual arrangement of cross bridges appears to be an effective, functional form of myosin in the contractile apparatus. Side-polar filaments therefore merit consideration as plausible candidates for the native organization of myosin in vertebrate smooth muscle cells.

Periodic organization of the contractile apparatus in smooth muscle revealed by the motion of dense bodies in single cells

To study the organization of the contractile apparatus in smooth muscle and its behavior during shortening, the movement of dense bodies in contracting saponin skinned, isolated cells was analyzed from digital images collected at fixed time intervals. These cells were optically lucent so that punctate structures, identified immunocytochemically as dense bodies, were visible in them with the phase contrast microscope. Methods were adapted and developed to track the bodies and to study their relative motion. Analysis of their tracks or trajectories indicated that the bodies did not move passively as cells shortened and that nearby bodies often had similar patterns of motion. Analysis of the relative motion of the bodies indicated that some bodies were structurally linked to one another or constrained so that the distance between them remained relatively constant during contraction. Such bodies tended to fall into laterally oriented, semirigid groups found at approximately 6-microns intervals along the cell axis. Other dense bodies moved rapidly toward one another axially during contraction. Such bodies were often members of separate semirigid groups. This suggests that the semirigid groups of dense bodies in smooth muscle cells may provide a framework for the attachment of the contractile structures to the cytoskeleton and the cell surface and indicates that smooth muscle may be more well-ordered than previously thought. The methods described here for the analysis of the motion of intracellular structures should be directly applicable to the study of motion in other cell types.

Arrangement of desmin intermediate filaments in smooth muscle cells as shown by high-resolution immunocytochemistry

To gain additional information about the arrangement of intermediate filaments (IF) in normal smooth muscle, fresh avian gizzard was processed for immunoelectron microscopy. The protein A-gold immunocytochemical technique was applied for the localization of desmin antigenic sites. Desmin-containing IFs were located in an axial bundle that partially surrounds the nucleus and were associated with numerous mitochondria near the poles of the nucleus. The bundle probably extends the length of the cell. Antibody labeling also showed concentrations of IF around and between cytoplasmic dense bodies (CDB) and also between CDB and membrane-associated dense bodies (MADB). The relationship between the axial bundle and the nucleus and associated mitochondria suggests that the bundle may support and define the position of these organelles in the cell. A fraying or branching of the bundle may integrate the bundle into the remaining cytoskeletal network of the cell.

Corkscrew-like shortening in single smooth muscle cells

The slower and more economical contraction of smooth muscle as compared to that of skeletal muscle may relate to the arrangement of its contractile apparatus. Because the arrangement of the contractile apparatus determines the manner in which a single smooth muscle cell shortens, shortening of a contracting cell was examined by tracking of marker bead movements on the cell surface by means of digital video microscopy. Smooth muscle cells were observed to freely shorten in a unique corkscrew-like fashion with a pitch of 1.4 cell lengths (that is, the length change required for one complete rotation of cell) at a rate of 27 degrees per second. Corkscrew-like shortening was interpreted in terms of a structural model in which the contractile apparatus or cytoskeleton (or both) are helically oriented within the cell. Such an arrangement of these cytoarchitectural elements may help to explain in part the contractile capabilities of smooth muscle.

The mitogenic effects of vasoactive neuropeptides on cultured smooth muscle cell lines

In order to investigate the relationship between the biochemical pathways that characterize contraction and cell growth, we have studied both contraction, mitogenesis and protein synthesis induced by the vasoactive neuropeptides, substance P (SP), calcitonin gene related peptide (CGRP) and vasoactive intestinal polypeptide (VIP) on four different established vascular and nonvascular smooth muscle cell lines. Contraction in vitro was evaluated by light microscopy and recorded photographically. Mitogenesis and protein synthesis were evaluated by [3H]-thymidine incorporation into cells and [3H]-amino acid incorporation into trichloroacetic acid precipitated materials, respectively. SP stimulated mitogenesis of A7r5 cells (embryonic rat aorta), but failed to induce significant contraction of these cells, whereas, SP induced contraction of cultured adult rat vascular smooth muscle cells (VSMC), but failed to stimulate mitogenesis. CGRP and VIP stimulated mitogenesis and protein synthesis, respectively, of DDT1MF-2 cells (hamster vas deferens), but neither induced contraction of this cell line. All three neuropeptides showed no effect on BC3H1 (mouse smooth muscle-like) cells. These results suggest that neuropeptides with vasoactive properties modulate different stages of cellular mitogenic responses which may be regulated by the degree of maturation of smooth muscle cell.