Unique geometry of actin-membrane anchorage sites in avian gizzard smooth muscle cells

Ordering of myosin II filaments driven by mechanical forces: experiments and theory

Myosin II filaments form ordered superstructures in both cross-striated muscle and non-muscle cells. In cross-striated muscle, myosin II (thick) filaments, actin (thin) filaments and elastic titin filaments comprise the stereotypical contractile units of muscles called sarcomeres. Linear chains of sarcomeres, called myofibrils, are aligned laterally in registry to form cross-striated muscle cells. The experimentally observed dependence of the registered organization of myofibrils on extracellular matrix elasticity has been proposed to arise from the interactions of sarcomeric contractile elements (considered as force dipoles) through the matrix. Non-muscle cells form small bipolar filaments built of less than 30 myosin II molecules. These filaments are associated in registry forming superstructures ('stacks') orthogonal to actin filament bundles. Formation of myosin II filament stacks requires the myosin II ATPase activity and function of the actin filament crosslinking, polymerizing and depolymerizing proteins. We propose that the myosin II filaments embedded into elastic, intervening actin network (IVN) function as force dipoles that interact attractively through the IVN. This is in analogy with the theoretical picture developed for myofibrils where the elastic medium is now the actin cytoskeleton itself. Myosin stack formation in non-muscle cells provides a novel mechanism for the self-organization of the actin cytoskeleton at the level of the entire cell.This article is part of the theme issue 'Self-organization in cell biology'.

Anatomy of the cavernous muscles of the kangaroo penis highlights marsupial-placental dichotomy

The mammalian penis is a complex hydraulic organ of cavernous (spongy) tissue supported by both smooth and skeletal muscle structures. In placental mammals, the paired ‎Musculus ischiocavernosi anchor the corpora cavernosa to the pelvis (at the ischium), and the paired M. bulbospongiosi converge as they envelop the base of the corpus spongiosum. Male marsupials have a dramatically different anatomy, however, in which both sets of paired muscles remain separate, have a bulbous, globular shape and do not have any direct connection to the pelvis. Here we provide the first detailed anatomical investigation of the muscles of the penis in the western grey kangaroo (Macropus fuliginosus) incorporating dissection, histology, vascular casting and computed tomography. The M. ischiocavernosus and M. bulbospongiosus form massive, multipennate bodies of skeletal muscle surrounding the paired roots of the corpus cavernosum and corpus spongiosum, respectively. Bilateral vascular supply is via both the artery of the penis and the ventral perineal artery. Histological examination reveals cavernous tissues with substantial smooth muscle supported by fibroelastic trabeculae, surrounded by the thick collagenous tunica albuginea. The M. ischiocavernosus and M. bulbospongiosus are known to function during erection of the penis and ejaculation via muscular contraction increasing blood pressure within cavernous vascular tissues. The thick muscular anatomy of the kangaroo would be well suited to this function. The absence of any connection to the bony pelvis in marsupials suggests the possibility of different mechanisms of action of these muscles with regard to reduction of venous return, eversion from the cloaca, or movements such as penile flips, which have been described in some placental mammals. This highlights a greater diversity in form and function in the evolution of the mammalian penis than has been previously considered.

Aortic aneurysm generation in mice with targeted deletion of integrin-linked kinase in vascular smooth muscle cells

RATIONALE

Integrin-linked kinase (ILK) is located at focal adhesions and links the extracellular matrix (ECM) to the actin cytoskeleton via β1- and β3-integrins. ILK plays a role in the activation of kinases including protein kinase B/Akt and glycogen synthase kinase 3β and regulates cell proliferation, motility, and survival.

OBJECTIVE

To determine the function of ILK in vascular smooth muscle cells (SMCs) in vivo.

METHODS AND RESULTS

SM22Cre(+)Ilk(Fl/Fl) conditional mutant mice were generated in which the Ilk gene was selectively ablated in SMCs. SM22Cre(+)Ilk(Fl/Fl) conditional mutant mice survive to birth but die in the perinatal period exhibiting multiple vascular pathologies including aneurysmal dilatation of the aorta and patent ductus arteriosus (PDA). Defects in morphogenetic development of the aorta were observed as early as E12.5 in SM22Cre(+)Ilk(Fl/Fl) mutant embryos. By late gestation (E16.5 to 18.5), striking expansion of the thoracic aorta was observed in ILK mutant embryos. Histological analyses revealed that the structural organization of the arterial tunica media is severely disrupted with profound derangements in SMC morphology, cell-cell, and cell-matrix relationships, including disruption of the elastic lamellae. ILK deletion in primary aortic SMCs results in alterations of RhoA/cytoskeletal signaling transduced through aberrant localization of myocardin-related transcription factor (MRTF)-A repressing the transcription and expression of SMC genes, which are required for the maintenance of the contractile SMC phenotype.

CONCLUSIONS

These data identify a molecular pathway linking ILK signaling to the contractile SMC gene program. Activation of this pathway is required for morphogenetic development of the aorta and ductus arteriosus during embryonic and postnatal survival.

Actin cytoskeletal dynamics in smooth muscle: a new paradigm for the regulation of smooth muscle contraction

A growing body of data supports a view of the actin cytoskeleton of smooth muscle cells as a dynamic structure that plays an integral role in regulating the development of mechanical tension and the material properties of smooth muscle tissues. The increase in the proportion of filamentous actin that occurs in response to the stimulation of smooth muscle cells and the essential role of stimulus-induced actin polymerization and cytoskeletal dynamics in the generation of mechanical tension has been convincingly documented in many smooth muscle tissues and cells using a wide variety of experimental approaches. Most of the evidence suggests that the functional role of actin polymerization during contraction is distinct and separately regulated from the actomyosin cross-bridge cycling process. The molecular basis for the regulation of actin polymerization and its physiological roles may vary in diverse types of smooth muscle cells and tissues. However, current evidence supports a model for smooth muscle contraction in which contractile stimulation initiates the assembly of cytoskeletal/extracellular matrix adhesion complex proteins at the membrane, and proteins within this complex orchestrate the polymerization and organization of a submembranous network of actin filaments. This cytoskeletal network may serve to strengthen the membrane for the transmission of force generated by the contractile apparatus to the extracellular matrix, and to enable the adaptation of smooth muscle cells to mechanical stresses. Better understanding of the physiological function of these dynamic cytoskeletal processes in smooth muscle may provide important insights into the physiological regulation of smooth muscle tissues.

Characterization of the column and autocellular junctions that define the vasculature of gill lamellae

Novel adhesion junctions have been characterized that are formed at the interface between pillar cells and collagen columns, both of which are essential constituents of the gill lamellae in fish. We termed these junctions the "column junction" and "autocellular junction" and determined their molecular compositions by immunofluorescence microscopy using pufferfish. We visualized collagen columns by concanavalin A staining and found that the components of integrin-mediated cell-matrix adhesion, such as talin, vinculin, paxillin, and fibronectin, were concentrated on plasma membranes surrounding collagen columns (column membranes). This connection is analogous to the focal adhesion of cultured mammalian cells, dense plaque of smooth muscle cells, and myotendinous junction of skeletal muscle cells. We named this connection the "column junction." In the cytoplasm near the column, actin fibers, actinin, and a phosphorylated myosin light chain of 20 kDa are densely located, suggesting the contractile nature of pillar cells. The membrane infoldings surrounding the collagen columns were found to be connected by the autocellular junction, whose components are highly tyrosine-phosphorylated and contain the tight junction protein ZO-1. This study represents the first molecular characterization and fluorescence visualization of the column and autocellular junctions involved in both maintaining structural integrity and the hemodynamics of the branchial lamellae.

Identification of a novel interaction between the Ca2+-binding protein S100A11 and the Ca2+- and phospholipid-binding protein annexin A6

S100A11 is a member of the S100 family of EF-hand Ca2+-binding proteins, which is expressed in smooth muscle and other tissues. Ca2+binding to S100A11 induces a conformational change that exposes a hydrophobic surface for interaction with target proteins. Affinity chromatography with immobilized S100A11 was used to isolate a 70-kDa protein from smooth muscle that bound to S100A11 in a Ca2+-dependent manner and was identified by mass spectrometry as annexin A6. Direct Ca2+-dependent interaction between S100A11 and annexin A6 was confirmed by affinity chromatography of the purified bacterially expressed proteins, by gel overlay of annexin A6 with purified S100A11, by chemical cross-linking, and by coprecipitation of S100A11 with annexin A6 bound to liposomes. The expression of S100A11 and annexin A6 in the same cell type was verified by RT-PCR and immunocytochemistry of isolated vascular smooth muscle cells. The site of binding of S100A11 on annexin A6 was investigated by partial tryptic digestion and deletion mutagenesis. The unique NH2terminal head region of annexin A6 was not required for S100A11 binding, but binding sites were identified in both NH2- and COOH-terminal halves of the molecule. We hypothesize that an agonist-induced increase in cytosolic free [Ca2+] leads to formation of a complex of S100A11 and annexin A6, which forms a physical connection between the plasma membrane and the cytoskeleton, or plays a role in the formation of signaling complexes at the level of the sarcolemma.

Biochemical characterization of detergent-resistant membranes: a systematic approach

Lateral segregation of cholesterol- and sphingomyelin-rich rafts and glycerophospholipid-containing non-raft microdomains has been proposed to play a role in a variety of biological processes. The most compelling evidence for membrane segregation is based on the observation that extraction with non-ionic detergents leads to solubilization of a subset of membrane components only. However, one decade later, a large body of inconsistent detergent-extraction data is threatening the very concept of membrane segregation. We have assessed the validity of the existing paradigms and we show the following. (i) The localization of a membrane component within a particular fraction of a sucrose gradient cannot be taken as a yardstick for its solubility: a variable localization of the DRMs (detergent-resistant membranes) in sucrose gradients is the result of complex associations between the membrane skeleton and the lipid bilayer. (ii) DRMs of variable composition can be generated by using a single detergent, the increasing concentration of which gradually extracts one protein/lipid after another. Therefore any extraction pattern obtained by a single concentration experiment is bound to be 'investigator-specific'. It follows that comparison of DRMs obtained by different detergents in a single concentration experiment is prone to misinterpretations. (iii) Depletion of cholesterol has a graded effect on membrane solubility. (iv) Differences in detergent solubility of the members of the annexin protein family arise from their association with chemically different membrane compartments; however, these cannot be attributed to the 'brick-like' raft-building blocks of fixed size and chemical composition. Our findings demonstrate a need for critical re-evaluation of the accumulated detergent-extraction data.

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.

Fast and slow dynamics of the cytoskeleton

Material moduli of the cytoskeleton (CSK) influence a wide range of cell functions. There is substantial evidence from reconstituted F-actin gels that a regime exists in which the moduli scale with frequency with a universal exponent of 3/4. Such behaviour is entropic in origin and is attributable to fluctuations in semiflexible polymers driven by thermal forces, but it is not obvious a priori that such entropic effects are responsible for the elasticity of the CSK. Here we demonstrate the existence of such a regime in the living cell, but only at high frequencies. Fast events scaled with frequency in a manner comparable to semiflexible-polymer dynamics, but slow events scaled with a non-universal exponent that was systematically smaller than 3/4 and probably more consistent with a soft-glass regime. These findings strongly suggest that at smaller timescales elasticity arises from entropic fluctuations of a semiflexible-filament network, whereas on longer timescales slow (soft-glass-like) dynamics of a different origin prevail. The transition between these two regimes occurred on timescales of the order of 0.01 s, thus setting within the slow glassy regime cellular events such as spreading, crawling, contracting, and invading.

Smooth muscle adherens junctions associated proteins are stable at the cell periphery during relaxation and activation

This study was performed to determine the stability of the adherens junction (AJ)-associated proteins at the smooth muscle cell (SMC) plasma membrane during relaxing and activating conditions. Dog stomach, ileum, colon, and trachea tissues were stored in Ca2+-free PSS or regular PSS or were activated in 10 μM carbachol in PSS before rapid freezing. The tissues were subsequently sectioned and immunoreacted using antibodies for vinculin, talin, fibronectin, and caveolin to determine their cellular distribution in these tissues under these conditions. In all four tissues and under all three conditions, the distribution of these four proteins remained localized to the periphery of the cell. In transverse tissue sections, the AJ-associated proteins formed a distinct punctate pattern around the periphery of the SMCs at the plasma membrane. These domains alternated with the caveolae (as identified by the presence of caveolin). In longitudinal tissue sections, the AJ-associated proteins formed continuous tracks or staves, while the caveolae remained punctate in this dimension as well. Caveolin is not present in the tapered ends of the SMCs, where the AJ-associated proteins appear continuous around the periphery. Densitometry of the fluorophore distribution of these proteins showed no shift in their localization from the SMC periphery when the tissues were relaxed or when they were activated before freezing. These results suggest that under physiologically relaxing and activating conditions, AJ-associated proteins remain stably localized at the plasma membrane.

Domain architecture of the smooth-muscle plasma membrane: regulation by annexins

Individual signalling events are processed in distinct, spatially segregated domains of the plasma membrane. In a smooth muscle, the sarcolemma is divided into domains of focal adhesions alternating with caveolae-rich zones, both harbouring a specific subset of membrane-associated proteins. Recently, we have demonstrated that the sarcolemmal lipids are similarly segregated into domains of cholesterol-rich lipid rafts and glycerophospholipid-rich non-raft regions. In the present study, we provide a detailed structural analysis of the relationship between these proteinaceous and lipid domains. We demonstrate that the segregation of plasmalemmal protein constituents is intimately linked to that of the membrane lipids. Our results imply that lipid segregation is critical for the preservation of membrane protein architecture and essential for directional translocation of proteins to the sarcolemma. We show that the membrane lipid segregation is supported by the annexin protein family in a Ca2+-dependent manner. Eukaryotic cells harbour numerous, tissue-specific subsets of annexins. By examining the significance of this variety in a smooth muscle, we demonstrate that four different annexins target membrane sites of distinct lipid composition and that each annexin requires a different [Ca2+] for its translocation to the sarcolemma. Our results suggest that the interactions of annexins with distinct plasma membrane regions promote membrane segregation and, in combination with their individual Ca2+ sensitivity, might allow a spatially confined, graded response to a multitude of extra- or intracellular stimuli.

Smooth muscle membrane organization in the normal and dysfunctional human urinary bladder: a structural analysis

PURPOSE

The decline in contractile properties is a characteristic feature of the dysfunctional bladder as a result of infravesical outlet obstruction. During clinical progression of the disease, smooth muscle cells undergo structural modifications. Since adaptations to constant changes in length require a high degree of structural organization within the sarcolemma, we have investigated the expression of several proteins, which are involved in smooth muscle membrane organization, in specimens derived from normal and dysfunctional organs.

MATERIALS AND METHODS

Specimen from patients with urodynamically normal/equivocal (n = 4), obstructed (n = 2), and acontractile (n = 2) bladders were analyzed relative to their structural features and sarcolemmal protein profile.

RESULTS

Smooth muscle cells within the normal urinary bladder display a distinct sarcolemmal domain structure, characterized by firm actin-attachment sites, alternating with flexible "hinge" regions. In obstructed bladders, foci of cells displaying degenerative sarcolemmal changes alternate with areas of hypertrophic cells in which the membrane appears unaffected. In acontractile organs, the overall membrane structure remains intact, however annexin 6, a protein belonging to a family of Ca2+-dependent, "membrane-organizers," is downregulated.

CONCLUSION

Degenerative changes in smooth muscle cells, which are chronically working against high resistance, are preferentially located within the actin-attachment sites. In acontractile bladders, the downregulation of annexin 6 might have a bearing on the fine-tuning of the plasma membrane during contraction/relaxation cycles.

Probing the viscoelastic behavior of cultured airway smooth muscle cells with atomic force microscopy: stiffening induced by contractile agonist

Complex rheology of airway smooth muscle cells and its dynamic response during contractile stimulation involves many molecular processes, foremost of which are actomyosin cross-bridge cycling and actin polymerization. With an atomic force microscope, we tracked the spatial and temporal variations of the viscoelastic properties of cultured airway smooth muscle cells. Elasticity mapping identified stiff structural elements of the cytoskeletal network. Using a precisely positioned microscale probe, picoNewton forces and nanometer level indentation modulations were applied to cell surfaces at frequencies ranging from 0.5 to 100 Hz. The resulting elastic storage modulus (G') and dissipative modulus (G'') increased dramatically, with hysteresivity (eta = G''/G') showing a definitive decrease after stimulation with the contractile agonist 5-hydroxytryptamine. Frequency-dependent assays showed weak power-law structural damping behavior and universal scaling in support of the soft-glassy material description of cellular biophysics. Additionally, a high-frequency component of the loss modulus (attributed to cellular Newtonian viscosity) increased fourfold during the contractile process. The complex shear modulus showed a strong sensitivity to the degree of actin polymerization. Inhibitors of myosin light chain kinase activity had little effect on the stiffening response to contractile stimulation. Thus, our measurements appear to be particularly well suited for characterization of dynamic actin rheology during airway smooth muscle contraction.

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.

Membrane segregation and downregulation of raft markers during sarcolemmal differentiation in skeletal muscle cells

Muscle contraction implies flexibility in combination with force resistance and requires a high degree of sarcolemmal organization. Smooth muscle cells differentiate largely from mesenchymal precursor cells and gradually assume a highly periodic sarcolemmal organization. Skeletal muscle undergoes an even more striking differentiation programme, leading to cell fusion and alignment into myofibrils. The lipid bilayer of each cell type is further segregated into raft and non-raft microdomains of distinct lipid composition. Considering the extent of developmental rearrangement in skeletal muscle, we investigated sarcolemmal microdomain organization in skeletal and smooth muscle cells. The rafts in both muscle types are characterized by marker proteins belonging to the annexin family which localize to the inner membrane leaflet, as well as glycosyl-phosphatidyl-inositol (GPI)-anchored enzymes attached to the outer leaflet. We demonstrate that the profound structural rearrangements that occur during skeletal muscle maturation coincide with a striking decrease in membrane lipid segregation, downregulation of annexins 2 and 6, and a significant decrease in raft-associated 5'-nucleotidase activity. The relative paucity of lipid rafts in mature skeletal in contrast to smooth muscle suggests that the organization of sarcolemmal microdomains contributes to the muscle-specific differences in stimulatory responses and contractile properties.

Direct interaction between the actin-binding protein filamin-A and the inwardly rectifying potassium channel, Kir2.1

The role of filamins in actin cross-linking and membrane stabilization is well established, but recently their ability to interact with a variety of transmembrane receptors and signaling proteins has led to speculation of additional roles in scaffolding and signal transduction. Here we report a direct interaction between filamin-A and Kir2.1, an isoform of inwardly rectifying potassium channel expressed in vascular smooth muscle and an important regulator of vascular tone. Yeast two-hybrid screening of a porcine coronary artery cDNA library using the carboxyl terminus of Kir2.1 as bait yielded cDNA encoding a fragment of filamin-A (residues 2481-2647). Interaction between filamin-A and Kir2.1 was confirmed by in vitro overlay assay of membrane-bound Kir2.1 with glutathione S-transferase fusion protein of the isolated filamin clone. Additionally, antibodies directed against Kir2.1 coimmunoprecipitated filamin-A from arterial smooth muscle cell lysates, and immunocytochemical analysis of individual arterial smooth muscle cells showed that Kir2.1 and filamin co-localize in "hotspots" at the cell membrane. Interaction with filamin-A was found to have no effect on Kir2.1 channel behavior but, rather, increased the number of functional channels resident within the membrane. We conclude that filamin-A is potentially an important regulator of Kir2.1 surface expression and location within vascular smooth muscle.

Cytoskeletal remodeling of the airway smooth muscle cell: a mechanism for adaptation to mechanical forces in the lung

Airway smooth muscle is continuously subjected to mechanical forces caused by changes in lung volume during breathing. These mechanical oscillations have profound effects on airway smooth muscle contractility both in vivo and in vitro. Alterations in airway smooth muscle properties in response to mechanical forces may result from adaptive changes in the organization of the actin cytoskeleton. Recent advances suggest that in airway smooth muscle, two cytosolic signaling proteins that associate with focal adhesion complexes, focal adhesion kinase (FAK) and paxillin, are involved in transducing external mechanical signals. FAK and paxillin regulate changes in the organization of the actin cytoskeleton and the activation of contractile proteins. Actin is in a dynamic state in airway smooth muscle and undergoes polymerization and depolymerization during the contraction-relaxation cycle. The organization of the cytoskeletal proteins, vinculin, talin, and alpha-actinin, which mediate linkages between actin filaments and transmembrane integrins, is also regulated by contractile stimulation in airway smooth muscle. The fluidity of the cytoskeletal structure of the airway smooth muscle cell may be fundamental to its ability to adapt and respond to the mechanical forces imposed on it in the lung during breathing.

Calcium-dependent and calcium-independent contractions in smooth muscles

This review examines signal transduction pathways mediating agonist-induced contraction of circular muscle in the body of the esophagus and in the lower esophageal sphincter (LES). In the LES, circular muscle agonists activate a well-defined contractile pathway, involving calcium (Ca(2+))-induced activation of calmodulin and myosin kinase, causing phosphorylation of 20-kDa myosin light chains (MLCs) and contraction. In this pathway, phosphorylation and contraction may be modulated by other factors, resulting, for instance, in inhibition of phosphatase activity, which may potentiate MLC phosphorylation. The agonist-activated contractile pathway of circular muscle from the esophageal body is not as well defined, and it is different from the LES contractile pathway, as it depends on activation of a Ca(2+)-independent protein kinase C (PKC), PKC-epsilon. In this pathway, agonist-induced Ca(2+) influx and/or release activate phospholipases to produce second messengers, such as diacylglycerol and arachidonic acid. The second messengers, however, activate a PKC-epsilon and a contractile pathway, which is Ca(2+) independent. This contractile pathway depends on activation of the mitogen-activated protein (MAP) kinases ERK1 and ERK2 and of p38 MAP kinase. These kinases are, in turn, linked to the small heat-shock protein HSP27, to integrin-linked kinase, and perhaps to other Ca(2+)-independent kinases, such as zipper kinase capable of producing MLC phosphorylation and contraction.

Invited review: focal adhesion and small heat shock proteins in the regulation of actin remodeling and contractility in smooth muscle

Smooth muscle cells are able to adapt rapidly to chemical and mechanical signals impinging on the cell surface. It has been suggested that dynamic changes in the actin cytoskeleton contribute to the processes of contractile activation and mechanical adaptation in smooth muscle. In this review, evidence for functionally important changes in actin polymerization during smooth muscle contraction is summarized. The functions and regulation of proteins associated with "focal adhesion complexes" (membrane-associated dense plaques) in differentiated smooth muscle, including integrins, focal adhesion kinase (FAK), c-Src, paxillin, and the 27-kDa small heat shock protein (HSP27) are described. Integrins in smooth muscles are key elements of mechanotransduction pathways that communicate with and are regulated by focal adhesion proteins that include FAK, c-Src, and paxillin as well as proteins known to mediate cytoskeletal remodeling. Evidence that functions of FAK and c-Src protein kinases are closely intertwined is discussed as well as evidence that focal adhesion proteins mediate key signal transduction events that regulate actin remodeling and contraction. HSP27 is reviewed as a potentially significant effector protein that may regulate actin dynamics and cross-bridge function in response to activation of p21-activated kinase and the p38 mitogen-activated protein kinase signaling pathway by signaling pathways linked to integrin proteins. These signaling pathways are only part of a large number of yet to be defined pathways that mediate acute adaptive responses of the cytoskeleton in smooth muscle to environmental stimuli.

Selected contribution: time course and heterogeneity of contractile responses in cultured human airway smooth muscle cells

We measured the time course and heterogeneity of responses to contractile and relaxing agonists in individual human airway smooth muscle (HASM) cells in culture. To this end, we developed a microrheometer based on magnetic twisting cytometry adapted with a novel optical detection system. Ferromagnetic beads (4.5 microm) coated with Arg-Gly-Asp peptide were bound to integrins on the cell surface. The beads were twisted in a sinusoidally varying magnetic field at 0.75 Hz. Oscillatory bead displacements were recorded using a phase-synchronized video camera. The storage modulus (cell stiffness; G'), loss modulus (friction; G"), and hysteresivity (eta; ratio of G" to G') could be determined with a time resolution of 1.3 s. Within 5 s after addition of histamine (100 microM), G' increased by 2.2-fold, G" increased by 3.0-fold, and eta increased transiently from 0.27 to 0.34. By 20 s, eta decreased to 0.25, whereas G' and G" remained above baseline. Comparable results were obtained with bradykinin (1 microM). These changes in G', G", and eta measured in cells were similar to but smaller than those reported for intact muscle strips. When we ablated baseline tone by adding the relaxing agonist dibutyryl cAMP (1 mM), G' decreased within 5 min by 3.3-fold. With relaxing and contracting agonists, G' could be manipulated through a contractile range of 7.3-fold. Cell populations exhibited a log-normal distribution of baseline stiffness (geometric SD = 2.8) and a heterogeneous response to both contractile and relaxing agonists, partly attributable to variability of baseline tone between cells. The total contractile range of the cells (from maximally relaxed to maximally stimulated), however, was independent of baseline stiffness. We conclude that HASM cells in culture exhibit a clear, although heterogeneous, response to contractile and relaxing agonists and express the essential mechanical features characteristic of the contractile response observed at the tissue level.

Alterations at the intercalated disk associated with the absence of muscle LIM protein

In this study, we investigated cardiomyocyte cytoarchitecture in a mouse model for dilated cardiomyopathy (DCM), the muscle LIM protein (MLP) knockout mouse and substantiated several observations in a second DCM model, the tropomodulin-overexpressing transgenic (TOT) mouse. Freshly isolated cardiomyocytes from both strains are characterized by a more irregular shape compared with wild-type cells. Alterations are observed at the intercalated disks, the specialized areas of mechanical coupling between cardiomyocytes, whereas the subcellular organization of contractile proteins in the sarcomeres of MLP knockout mice appears unchanged. Distinct parts of the intercalated disks are affected differently. Components from the adherens junctions are upregulated, desmosomal proteins are unchanged, and gap junction proteins are downregulated. In addition, the expression of N-RAP, a LIM domain- containing protein located at the intercalated disks, is upregulated in MLP knockout as well as in TOT mice. Detailed analysis of intercalated disk composition during postnatal development reveals that an upregulation of N-RAP expression might serve as an early marker for the development of DCM. Altered expression levels of cytoskeletal proteins (either the lack of MLP or an increased expression of tropomodulin) apparently lead to impaired function of the myofibrillar apparatus and to physiological stress that ultimately results in DCM and is accompanied by an altered appearance and composition of the intercalated disks.

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.

Annexins in cell membrane dynamics. Ca(2+)-regulated association of lipid microdomains

The sarcolemma of smooth muscle cells is composed of alternating stiff actin-binding, and flexible caveolar domains. In addition to these stable macrodomains, the plasma membrane contains dynamic glycosphingolipid- and cholesterol-enriched microdomains, which act as sorting posts for specific proteins and are involved in membrane trafficking and signal transduction. We demonstrate that these lipid rafts are neither periodically organized nor exclusively confined to the actin attachment sites or caveolar regions. Changes in the Ca(2+) concentration that are affected during smooth muscle contraction lead to important structural rearrangements within the sarcolemma, which can be attributed to members of the annexin protein family. We show that the associations of annexins II, V, and VI with smooth muscle microsomal membranes exhibit a high degree of Ca(2+) sensitivity, and that the extraction of annexins II and VI by detergent is prevented by elevated Ca(2+) concentrations. Annexin VI participates in the formation of a reversible, membrane-cytoskeleton complex (Babiychuk, E.B., R.J. Palstra, J. Schaller, U. Kämpfer, and A. Draeger. 1999. J. Biol. Chem. 274:35191-35195). Annexin II promotes the Ca(2+)-dependent association of lipid raft microdomains, whereas annexin V interacts with glycerophospholipid microcompartments. These interactions bring about a new configuration of membrane-bound constituents, with potentially important consequences for signaling events and Ca(2+) flux.

Role of Rho in Ca(2+)-insensitive contraction and paxillin tyrosine phosphorylation in smooth muscle

We investigated whether Rho activation is required for Ca(2+)-insensitive paxillin phosphorylation, myosin light chain (MLC) phosphorylation, and contraction in tracheal muscle. Tyrosine-phosphorylated proteins have been implicated in the Ca(2+)-insensitive contractile activation of smooth muscle tissues. The contractile activation of tracheal smooth muscle increases tyrosine phosphorylation of the cytoskeletal proteins paxillin and focal adhesion kinase. Paxillin is implicated in integrin-mediated signal transduction pathways that regulate cytoskeletal organization and cell motility. In fibroblasts and other nonmuscle cells, paxillin tyrosine phosphorylation depends on the activation of Rho and is inhibited by cytochalasin, an inhibitor of actin polymerization. In permeabilized muscle strips, we found that ACh induced Ca(2+)-insensitive contraction, MLC phosphorylation, and paxillin tyrosine phosphorylation. Ca(2+)-insensitive contraction and MLC phosphorylation induced by ACh were inhibited by C3 transferase, an inhibitor of Rho activation; however, C3 transferase did not inhibit paxillin tyrosine phosphorylation. Ca(2+)-insensitive paxillin tyrosine phosphorylation was also not inhibited by the Rho kinase inhibitor Y-27632, by cytochalasin D, or by the inhibition of MLC phosphorylation. We conclude that, in tracheal smooth muscle, Rho mediates Ca(2+)-insensitive contraction and MLC phosphorylation but that Rho is not required for Ca(2+)-insensitive paxillin tyrosine phosphorylation. Paxillin phosphorylation also does not require actomyosin activation, nor is it inhibited by the actin filament capping agent cytochalasin D.

Annexin VI participates in the formation of a reversible, membrane-cytoskeleton complex in smooth muscle cells

The plasmalemma of smooth muscle cells is periodically banded. This arrangement ensures efficient transmission of contractile activity, via the firm, actin-anchoring regions, while the more elastic caveolae-containing "hinge" regions facilitate rapid cellular adaptation to changes in cell length. Since cellular mechanics are undoubtedly regulated by components of the membrane and cytoskeleton, we have investigated the potential role played by annexins (a family of phospholipid- and actin-binding, Ca(2+)-regulated proteins) in regulating sarcolemmal organization. Stimulation of smooth muscle cells elicited a relocation of annexin VI from the cytoplasm to the plasmalemma. In smooth, but not in striated muscle extracts, annexins II and VI coprecipitated with actomyosin and the caveolar fraction of the sarcolemma at elevated Ca(2+) concentrations. Recombination of actomyosin, annexins, and caveolar lipids in the presence of Ca(2+) led to formation of a structured precipitate. Participation of all 3 components was required, indicating that a Ca(2+)-dependent, cytoskeleton-membrane complex had been generated. This association, which occurred at physiological Ca(2+) concentrations, corroborates our biochemical fractionation and immunohistochemical findings and suggests that annexins play a role in regulating sarcolemmal organization during smooth muscle contraction.

Influence of intercellular tissue connections on airway muscle mechanics

Contraction of smooth muscle in visceral organs is modified by structures external to the muscle. Within muscle tissue itself, connective tissue plays an important role in force transference among the contractile cells. Connections arranged radially can affect contractile mechanics by limiting tissue expansion at short lengths. Previous work suggests that increased stiffness at extreme shortening is due to such radial constraints. Two approaches to further study of these effects are reported. To increase radial constraints, very thin Silastic bands were placed loosely about strips of canine trachealis muscle at rest length. The strips were allowed to shorten under light afterloads, expanding until restrained by the bands. Subsequent removal of the bands allowed increased shortening, with less increase in stiffness at short lengths. Related isometric effects were observed. To reduce constraints, muscle strips were partially digested with collagenase. Compared with control conditions, this treatment permitted further shortening, with less increase in stiffness at short lengths. These results emphasize the role of extracellular structures in determining mechanical function of smooth muscle.

Mechanosensitive tyrosine phosphorylation of paxillin and focal adhesion kinase in tracheal smooth muscle

We investigated the role of the integrin-associated proteins focal adhesion kinase (FAK) and paxillin as mediators of mechanosensitive signal transduction in tracheal smooth muscle. In muscle strips contracted isometrically with ACh, we observed higher levels of tyrosine phosphorylation of FAK and paxillin at the optimal muscle length (Lo) than at shorter muscle lengths of 0.5 or 0.75 Lo. Paxillin phosphorylation was also length sensitive in muscles activated by K+ depolarization and adjusted rapidly to changes in muscle length imposed after contractile activation by either ACh or K+ depolarization. Ca2+ depletion did not affect the length sensitivity of paxillin and FAK phosphorylation in muscles activated with ACh, indicating that the mechanotransduction process can be mediated by a Ca2+-independent pathway. Since Ca2+-depleted muscles do not generate significant active tension, this suggests that the mechanotransduction mechanism is sensitive to muscle length rather than tension. We conclude that FAK and paxillin participate in an integrin-mediated mechanotransduction process in tracheal smooth muscle. We propose that this pathway may initiate alterations in smooth muscle cell structure and contractility via the remodeling of actin filaments and/or via the mechanosensitive regulation of signaling molecules involved in contractile protein activation.

The cytoskeleton of the vertebrate smooth muscle cell

Smooth muscle cells possess a structural lattice composed of two primary parts: the 'cytoskeleton' that pervades the cytoplasm and the 'membrane skeleton' that provides anchorage for the cytoskeleton and contractile apparatus at the cell surface. The cytoskeleton contains two major components: first, a complement of actin filaments that links the cytoplasmic dense bodies at equispaced intervals in longitudinal fibrils; and second, a network of desmin intermediate filaments that co-distributes with the cytoskeletal actin. The actin filaments of the contractile apparatus are presumed to interface with the cytoskeleton at the cytoplasmic dense bodies and with the longitudinal rib-like arrays of dense plaques of the membrane skeleton that couple to the extracellular matrix. The present report focuses attention on the functional role of intermediate filaments and on the molecular domain structure of the protein calponin, which is found both in the cytoskeleton and the contractile apparatus. New information about the role of intermediate filaments in smooth muscle has come from studies of transgenic mice in which desmin expression has been ablated. These have shown that while desmin is dispensable for normal development and viability its absence has significant consequences for the mechanical properties of muscle tissue. Thus, the visceral smooth muscles develop only 40% of the normal contractile force and the maximal shortening velocity is reduced by 25-40%. Intermediate filaments therefore play an active role in force transmission and do not contribute solely to cell shape maintenance, as has hitherto been presumed. Recent studies on calponin have revealed a second actin binding domain at the C-terminus of the molecule and have also pinpointed an N-terminal domain that shares homology with a growing family of actin binding and signalling molecules. How these newly identified features of calponin relate to its function in vivo remains to be established.

Human foetal lung (IMR-90) cells: myofibroblasts with smooth muscle-like contractile properties

Single cells displaying many characteristics in common with smooth muscle are now being identified in many organs. Although their origin remains elusive, they are nonetheless known to play a major role in fibroproliferative diseases and stromal reactions by virtue of their contractile properties. We have investigated the contractile properties and expression of smooth muscle-associated proteins in such a cell line derived from human foetal lung (IMR-90). For almost two decades, these cells have served as model fibroblasts in a wide variety of studies. And yet, IMR-90 cells manifest many features characteristic of differentiated smooth muscle cells: they exhibit the same elongated, slender form and the strikingly regular, longitudinal alignment of their actin-attachment sites, which are indispensable for coordinated contraction. Moreover, these adhaerens junctions also contain metavinculin, the smooth muscle analogue of vinculin, the stable expression of which has not previously been demonstrated in cultured cells. Since sm 22, as well as the smooth muscle-associated alpha-actin isoform and calponin are also expressed, IMR-90 cells must now be classified as myofibroblasts. When cultivated on a rubbery silicon surface, IMR-90 cells respond to stimulation with a rate of contraction which is considerably faster than that exhibited by fibroblasts derived from other established lines. Taken together, the regular longitudinal orientation of the adhaerens junctions, the stable expression of metavinculin, and the rapid speed of shortening in IMR-90 cells suggest, by implication, that the periodicity of actin attachment sites is a fundamental determinant of contractile efficiency in smooth muscle cells; this spacing may be mediated by metavinculin.

Transcriptional regulation of the chicken caldesmon gene. Activation of gizzard-type caldesmon promoter requires a CArG box-like motif

Caldesmon, which plays a vital role in the actomyosin system, is distributed in smooth muscle and non-muscle cells, and its isoformal interconversion between a high M(r) form and low M(r) form is a favorable molecular event for studying phenotypic modulation of smooth muscle cells. Genomic analysis reveals two promoters, of which the gizzard-type promoter displays much higher activity than the brain-type promoter. Here, we have characterized transcriptional regulation of the gizzard-type promoter. Transient transfection assays in chick gizzard smooth muscle cells, chick embryo fibroblasts, mouse skeletal muscle cell line (C2C12), and HeLa cells revealed that the promoter activity was high in smooth muscle cells and fibroblasts, but was extremely low in other cells. Cell type-specific promoter activity depended on an element, CArG1, containing a unique CArG box-like motif (CCAAAAAAGG) at -315, while multiple E boxes were not directly involved in this event. Gel shift assays showed the specific interaction between the CArG1 and nuclear protein factors in smooth muscle cells and fibroblasts. These results suggest that the CArG1 is an essential cis-element for cell type-specific expression of caldesmon and that the function of CArG1 might be controlled under phenotypic modulation of smooth muscle cells.

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.

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)

Calponin is localised in both the contractile apparatus and the cytoskeleton of smooth muscle cells

Calponin and caldesmon are two thin filament-binding proteins found in smooth muscle that have both been attributed a role in modulating the interaction of actin and myosin. Using high-resolution dual-label immunocytochemistry we have determined the distribution of calponin relative to the contractile and cytoskeletal compartments of the smooth muscle cell. We show, using chicken gizzard smooth muscle, that calponin occurs in the cytoskeleton, with beta-cytoplasmic actin, filamin and desmin, as well as in the contractile apparatus, with myosin and caldesmon. According to the observed labelling intensities, calponin was more concentrated in the cytoskeleton and it was additionally localised in the cytoplasmic dense bodies as well as in the adhesion plaques at the cell surface, which both harbour the beta-cytoplasmic isoform of actin. It is probable that these results explain earlier conflicting reports on the composition of smooth muscle thin filaments and suggest that calponin, together with a Ca(2+)-receptor protein, could just as likely serve a role in the cytoskeleton of smooth muscle as in the contractile apparatus.

A novel phosphoglucomutase-related protein is concentrated in adherens junctions of muscle and nonmuscle cells

Using five monoclonal antibodies raised against a human uterine smooth muscle extract, we have identified a novel antigen which runs as a closely spaced doublet in SDS-gels. The proteins (60/63 kDa) co-purify, are present in a 1:1 ratio as judged by Coomassie Blue staining, and are immunologically closely related, if not identical. No N-terminal sequence could be obtained from a mixture of the 60/63 kDa proteins, but the sequence of four polypeptides liberated by V8 protease or cyanogen bromide cleavage showed that the proteins are closely related to the glycolytic enzyme phosphoglucomutase type 1. Affinity-purified polyclonal antibodies and three different monoclonal antibodies to the 60/63 kDa proteins cross-reacted with rabbit skeletal muscle phosphoglucomutase type 1, whilst two additional monoclonal antibodies were specific for the 60/63 kDa proteins. Peptide maps of the 60/63 kDa proteins and phosphoglucomutase 1 are markedly different, and the purified proteins have no detectable phosphoglucomutase activity. Staining of cultured smooth muscle cells and fibroblasts with antibodies to 60/63 kDa proteins showed that the antigen is concentrated in focal contacts at the ends of actin bundles and is also associated with actin filaments. About 60% of the cellular 60/63 kDa proteins were found in the detergent-insoluble fraction, suggesting a physical association with the cytoskeleton. The highest levels of protein immunoreactivity were found in muscles. The antigen is concentrated in muscle adherens junctions, including smooth muscle dense plaques, cardiomyocyte intercalated disks, and striated muscle myotendinous junctions. Among epithelial cells, the 63 kDa isoform of the protein was found only in cultured keratinocytes where immunofluorescent staining was localized in cell-to-cell adherens junctions. Expression of the 60/63 kDa proteins in vascular smooth muscle cells is developmentally regulated and correlates with the differentiated contractile phenotype of these cells.

Immunoblotting of contractile and cytoskeletal proteins of canine basilar artery in vasospasm

Vasospasm was produced in the canine basilar arteries by a two-hemorrhage method, and voltage- and receptor-dependent contractions of the normal canine basilar arteries were induced by local applications of potassium chloride (KCI) and serotonin, respectively, after transclival exposure. Actin, myosin, desmin, filamin, talin, vinculin, and alpha-actinin in the basilar artery were studied by immunoblotting. The immunoblots showed a decrease or loss in immunoreactivity of some native proteins and generation of protein fragments, smaller in size than native proteins, in spastic, KCI, and serotonin groups, indicating a proteolytic degradation. In the spastic group on Day 2, actin, desmin, and filamin were usually degraded slightly; myosin moderately; and talin and alpha-actinin substantially. Vinculin and metavinculin remained intact. In the spastic group on Day 7, actin and desmin were usually decomposed slightly; myosin, filamin, and vinculin substantially; and talin, metavinculin, and alpha-actinin markedly. In the KCI and serotonin groups, slight degradation was usually observed in filamin, often in alpha-actinin, and occasionally in actin, whereas desmin, vinculin, and metavinculin were not degraded. In addition, myosin was usually degraded moderately in the KCI group and slightly in the serotonin group, and talin was generally decomposed slightly in the KCI group and moderately in the serotonin group. The degraded fragments, although variable in number and immunoreactivity, were similar in size in the three groups. We suggest that the intracellular devices responsible for contraction of the basilar arteries are degraded more severely in the spastic group than in the KCI or serotonin group, probably by similar proteolytic mechanism and progressively with the passage of time after subarachnoid hemorrhage in vasospasm.

Probing actin and liposome interaction of talin and talin-vinculin complexes: a kinetic, thermodynamic and lipid labeling study

Talin purified from human platelets and chicken gizzard smooth muscle is an actin and lipid binding protein. Here, we have investigated the effect of vinculin on (a) talin-nucleated actin polymerization and (b) insertion of talin into lipid bilayers. Calorimetric data show ternary complex formation between talin, vinculin, and actin. Actin-talin, actin-vinculin and actin-(talin-vinculin) binding and rate constants as well as actin polymerization rates for all three protein species have been determined by steady state titration, stopped-flow, and fluorescence assay. In contrast to an increase of the polymerization rate by a factor of less than 2 for actin-talin and actin-(talin-vinculin) when lowering the temperature, we measured a decrease in rates for actin alone and actin-vinculin. The overall equilibrium constants (Keq) in the van't Hoff plot proved linear and were of one-step reactions. Thermodynamic data exhibited signs of van der Waal's binding forces. Using the photoactivatable lipid analogue [3H]PTPC/11, which selectively labels membrane-embedded hydrophobic domains of proteins, we also show that talin partially inserts into the hydrophobic bilayer of liposomes. This insertion occurs in a similar manner irrespective of preincubation with vinculin.

Ordered distribution of membrane-associated dense plaques in intact quail gizzard smooth muscle cells revealed by freeze-fracture following treatment with cholesterol probes

The surface distribution of membrane-associated dense plaques in intact quail gizzard smooth muscle cells was investigated by freeze-fracture. Replicas of fractured smooth muscle cell plasma membrane showed caveola-free regions with few intramembrane particles, interspersed with caveola-populated areas with a higher intramembrane particle density. Electron microscopy of thin sections of quail gizzard smooth muscle revealed the regions free of caveolae to be occupied by membrane-associated dense plaques; anchoring sites for the contractile filaments of the cell. Demarcation between the caveola-populated and caveola-free regions on the relicated intramembrane surface was not clear and thus provided little information concerning the distribution of dense plaque sites. However, treatment of the smooth muscle tissue with the cholesterol-binding agents filipin or tomatin prior to freeze-fracture allowed the dense plaque sites to be easily observed as the sites remained free of the membrane deformations characteristic of these agents. The dense plaque sites consist of caveola-free oval areas juxtaposed in regular bands that traverse the long axis of the cell. The dense plaque sites on the freeze-fracture replica were confirmed by electron microscopy of thin sections of filipin-treated quail gizzard smooth muscle cells, which showed the plasma membrane associated with the dense plaques to be unaffected by the actions of filipin, whereas that of the caveola-populated region was severely deformed. The observations presented in this study provide evidence for a highly ordered distribution of dense plaques at the cell surface of intact quail gizzard smooth muscle cells and thus corroborate existing evidence for an organized substructure of smooth muscle cells.