Avian Smooth Muscle Research Articles

Organization of F-actin by Fesselin (avian smooth muscle synaptopodin 2).

Fesselin or avian synaptopodin 2 is a member of the synaptopodin family of actin binding proteins. Fesselin promotes G-actin polymerization and the formation of large actin complexes that can be collected by low-speed centrifugation. Because of the potential role of fesselin in some cancers and its effects on actin, we further investigated the effect of fesselin on actin. Fesselin initiated actin polymerization under a variety of conditions, including the virtual absence of salt. Actin filaments formed at low salt concentrations in the presence of fesselin were similar to filaments polymerized in the presence of 100 mM KCl. In both cases, the filaments were long and straight with a common orientation. Highly ordered actin bundles formed with increasing times of incubation. Blockers of actin growth at the barbed end (cytochalasin D and CapZ) did not prevent fesselin from polymerizing actin. Low concentrations of fesselin increased the critical concentration of actin. Both observations are consistent with preferential growth at the pointed end of actin filaments. These results indicate a role of fesselin in organizing cellular actin. These and other results indicate that fesselin is part of a cellular actin organizing center.

Fibroblast Growth Factor 2 Promotes the Self-Renewal of Bipotent Glial Smooth Muscle Neural Crest Progenitors

The neural crest (NC) is an attractive system for investigating the mechanisms underlying cell lineage diversification in higher vertebrates. The NC contains a mixed population of already defined precursors and multipotent cells that can give rise to a great variety of cell types, including glial cells and neurons of the peripheral nervous system, melanocytes, and smooth muscle cells (SMCs). Microenvironmental factors, such as the fibroblast growth factor 2 (FGF2), found along migratory paths and in target tissues, strongly influence the fate of multipotent NC precursors. We have previously demonstrated that the FGF2 promotes the differentiation of NC cells to glial phenotypes, while the epidermal growth factor induces NC differentiation to neurons and melanocytes. In the present study, we used mass cultures and single-cell culture assays to demonstrate that FGF2 influences NC cell differentiation and increases the proportion of multipotent progenitors. Furthermore, we demonstrate for the first time that avian tripotent glial, melanocyte and smooth muscle NC progenitors, as well as bipotent melanocyte and smooth muscle NC progenitors, are capable of self-renewal. FGF2 significantly stimulated the self-renewal of bipotent progenitor cells with glial cells and SMC potentials. These cells propagated for many generations and behaved as stem cells. These results suggest an important role of FGF2 in maintaining the stemness of avian NC cells.

Organization of F-Actin by Avian Smooth Muscle Synaptopodin 2 (Fesselin)

Fesselin is a member of the synaptopodin family of actin binding proteins and it is rich in smooth muscle tissue. Fesselin accelerates actin polymerization in a calcium-calmodulin dependent fashion and forms insoluble aggregates with actin. The polymerization of actin occurs even in the absence of salts usually required to initiate polymerization. The question is whether the properties of fesselin are indicative of a function in forming structures rich in fesselin such as dense bodies and Z-lines. Electron microscopy of actin filaments shows that fesselin initially produces long actin filaments that become organized into thick bundles. The structure of the bundles depends on the ratio of fesselin to actin. Most frequently there are dense parallel ordered bundles. In some cases, the bundles are frayed and thus display single actin filaments. The filaments within a bundle are in-register and have either common initiation or termination points. Decoration with S1 showed that the filaments of a bundle had the same orientation. Actin filaments were occasionally seen to be radiating from densely stained complexes. Solution studies support the idea that fesselin stimulates pointed end growth of actin filaments. That is, fesselin (a) increased the critical concentration for polymerization and (b) induced actin growth even in the presence of barbed end blockers. When added to actin in the absence of other factors, fesselin can nucleate networks of linear actin filaments in which growth occurs primarily at the pointed ends of actin filaments.

Multiple Isoforms of Fesselin (Avian Synaptopodin 2) are expressed in Smooth, Skeletal and Heart Muscle

The synaptopodin 2 gene can be differently spliced, resulting in three mRNAs with varying 3′ ends coding for different C-termini (De Ganck et al 2008). The calculated molecular weights of these proteins are 117, 119, and 136 kDa. These isoforms require the expression of exons 1–3. However, only a single protein product has been detected in mammalian muscle lacking the product of these first exons. We extracted four fesselin isoforms from avian smooth muscle tissue. These include the first isolated 79 and 103 kDa isoforms (Leinweber et al. 1999). The newly detected isoforms migrated on SDS gels with apparent molecular masses of 140 and 160 kDa. In contrast to our initial assumption that the 79 kDa was a proteolytic product of the 103 kDa protein we now show that they are different spliceforms. The 79 kDa isoform forms the core of synaptopodin 2. The other isoforms have different extensions at either the N- or C-terminal regions. The different isoforms were differentially extracted by different buffers. Extractions were most complete under conditions that depolymerized both actin and intermediate filaments. Surprisingly although fesselin binds to actin and myosin none of the four different isoforms was extracted with the acto-myosin-complex. The extraction data suggest that fesselin functions in actin filament organization rather than in regulation of actin-myosin interactions. In contrast to smooth muscle tissue we detected one isoform of fesselin in skeletal and heart muscle tissue in agreement with the findings in mammalian tissue. In avian skeletal muscle we observed a 79 kDa isoform and in heart muscle a 170 kDa isoform. The reason for the differential expression of fesselin in different muscle types is unknown.

The actin binding protein, fesselin, is a member of the synaptopodin family

Fesselin is a natively unfolded protein that is abundant in avian smooth muscle. Like many natively unfolded proteins, fesselin has multiple binding partners including actin, myosin, calmodulin and α-actinin. Fesselin accelerates actin polymerization and bundles actin. These and other observations suggest that fesselin is a component of the cytoskeleton. We have now cloned fesselin and have determined the cDNA derived amino acid sequence. We verified parts of the sequence by Edman analysis and by mass spectroscopy. Our results confirmed fesselin is homologous to human synaptopodin 2 and belongs to the synaptopodin family of proteins.

PHI-1 interacts with the catalytic subunit of myosin light chain phosphatase to produce a Ca 2+ independent increase in MLC 20 phosphorylation and force in avian smooth muscle

In avian smooth muscles, GTP γS produces a Rho kinase mediated increase in PHI-1 phosphorylation and force, but whether this correlation is causal is unknown. We examined the effect of phosphorylated PHI-1 (P-PHI-1) on force and myosin light chain (MLC 20) phosphorylation at a constant [Ca 2+]. P-PHI-1, but not PHI-1, increased MLC 20 phosphorylation and force, and phosphorylation of PHI-1 increased the interaction of PHI-1 with PP1c. Microcystin induced a dose-dependent reduction in the binding of PHI-1 to PP1c. These results suggest PHI-1 inhibits myosin light chain phosphatase by interacting with the active site of PP1c to produce a Ca 2+ independent increase in MLC 20 phosphorylation and force.

MYPT1 mutants demonstrate the importance of aa 888–928 for the interaction with PKGIα

During nitric oxide signaling, type Ialpha cGMP-dependent protein kinase (PKGIalpha) activates myosin light chain (MLC) phosphatase through an interaction with the 130-kDa myosin targeting subunit (MYPT1), leading to dephosphorylation of 20-kDa MLC and vasodilatation. It has been suggested that the MYPT1-PKGIalpha interaction is mediated by the COOH-terminal leucine zipper (LZ) of MYPT1 and the NH(2)-terminal LZ of PKGIalpha (HK Surks and ME Mendelsohn. Cell Signal 15: 937-944, 2003; HK Surks et al. Science 286: 1583-1587, 1999), but we previously showed that PKGIalpha interacts with LZ-positive (LZ+) and LZ-negative (LZ-) MYPT1 isoforms (13). Interestingly, PKGIalpha is known to preferentially bind to RR and RK motifs (WR Dostmann et al. Proc Natl Acad Sci USA 97: 14772-14777, 2000), and there is an RK motif within the aa 888-928 sequence of MYPT1 in LZ+ and LZ- isoforms. Thus, to localize the domain of MYPT1 important for the MYPT1-PKGIalpha interaction, we designed four MYPT1 fragments that contained both the aa 888-928 sequence and the downstream LZ domain (MYPT1FL), lacked both the aa 888-928 sequence and the LZ domain (MYPT1TR), lacked only the aa 888-928 sequence (MYPT1SO), or lacked only the LZ domain (MYPT1TR2). Using coimmunoprecipitation, we found that only the fragments containing the aa 888-928 sequence (MYPT1FL and MYPT1TR2) were able to form a complex with PKGIalpha in avian smooth muscle tissue lysates. Furthermore, mutations of the RK motif at aa 916-917 (R(916)K(917)) to AA decreased binding of MYPT1 to PKGIalpha in chicken gizzard lysates; these mutations had no effect on binding in chicken aorta lysates. However, mutation of R(916)K(917) to E(916)E(917) eliminated binding, suggesting that one factor important for the PKGIalpha-MYPT1 interaction is the charge at aa 916-917. These results suggest that, during cGMP-mediated signaling, aa 888-928 of MYPT1 mediate the PKGIalpha-MYPT1 interaction.

Dynamic changes in free Ca-calmodulin levels in adult cardiac myocytes

Despite its multifunctional role in cardiac myocyte function, little is known about dynamic changes in activation state of calmodulin (CaM). Thus, the purpose of this study was to develop a tool to measure Ca bound CaM (Ca–CaM) levels in intact cardiac myocytes. For dynamic measurements of Ca–CaM, we generated an adenoviral vector which expresses a cyan and a yellow fluorescent protein linked by a modified version of the Ca–CaM binding domain of avian smooth muscle myosin light chain kinase. Adult rabbit cardiac myocytes were infected with the Ca–CaM sensing probe or simultaneously infected with viruses containing CaM and the Ca–CaM sensing probe for 24–48 h. Myocytes were then field stimulated (1 Hz) and excited at 440 nm with emitted fluorescence measured at 485 and 535 nm. Changes in [Ca–CaM] are expressed as the ratio of 485 nm/535 nm. Small beat-to-beat changes of [Ca–CaM] were detected, but only when CaM was co-expressed with the sensor. However, upon β-adrenergic stimulation with isoproterenol, there was an increase in the amplitude of the signals during each beat (parallel to the shortening, which is an indirect measure of [Ca] i) and also a rise in the diastolic [Ca–CaM]. Total [CaM] measured by both competitive ELISA and semi-quantitative Western blots was 5–6 μM in isolated adult ventricular myocytes. Our results indicate that there are dynamic changes in free Ca–CaM levels (a phasic component tracking [Ca] i) as well as system memory that integrates the [Ca] i signals (a tonic component).

Expression of Smoothelin in the Normal and the Overactive Human Bladder

We established the expression pattern of smoothelin, a marker protein for contractile smooth muscle cells, in the human detrusor and investigated its possible impact on bladder overactivity. Detrusor samples of 13 overactive bladders (sensory urge and detrusor instability) were obtained before botulinum toxin injection and compared to those of 8 normally contractile, nonobstructed bladders obtained during radical cystectomy. Smoothelin mRNA expression patterns were investigated by Northern blot and variant specific reverse transcriptase-polymerase chain reaction as well as by quantitative reverse transcriptase-polymerase chain reaction on laser capture, microdissected smooth muscle. At the protein level smoothelin was investigated by standard and quantitative immunohistochemistry. The bladder muscularis expressed vascular and visceral smoothelin isoforms, and 2 of the known splice variants. In the smooth muscle of patients with detrusor instability and sensory urge a significant 2.4 and 2.2-fold increase, respectively, in smoothelin variant 1 mRNA was observed in comparison to that of normal controls. Analyses at the smoothelin protein level confirmed significant up-regulation in these bladder dysfunctions by a factor of 2.3 and 1.8, respectively. No significant difference in smoothelin expression was observed between detrusor instability and sensory urge. Increased expression of smoothelin in patients with detrusor instability and sensory urge implies that the etiology of these dysfunctions includes changes in myogenic parameters. In addition, our data support the new classification of the International Continence Society for overactive bladder proposing that sensory urge and detrusor instability represent a single clinical entity.

Ca2+−Calmodulin Regulates Fesselin-Induced Actin Polymerization,

Fesselin is a proline-rich actin-binding protein that was isolated from avian smooth muscle. Fesselin bundles actin and accelerates actin polymerization by facilitating nucleation. We now show that this polymerization of actin can be regulated by Ca(2+)-calmodulin. Fesselin was shown to bind to immobilized calmodulin in the presence of Ca(2+). The fesselin-calmodulin interaction was confirmed by a Ca(2+)-dependent increase in 2-(4-maleimidoanilino)naphthalene-6-sulfonic acid (MIANS) fluorescence upon addition of fesselin to MIANS-labeled wheat germ calmodulin. The affinity was estimated to be approximately 10(9) M(-1). The affinity of Ca(2+)-calmodulin to the fesselin F-actin complex was approximately 10(8) M(-1). Calmodulin binding to fesselin appeared to be functionally significant. In the presence of fesselin and calmodulin, the polymerization of actin was Ca(2+)-dependent. Ca(2+)-free calmodulin either had no effect or enhanced the ability of fesselin to accelerate actin polymerization. Ca(2+)-calmodulin not only reversed the stimulatory effect of fesselin but reduced the rate of polymerization below that observed in the absence of fesselin. While Ca(2+)-calmodulin had a large effect on the interaction of fesselin with G-actin, the effect on F-actin was small. Neither the binding of fesselin to F-actin nor the subsequent bundling of F-actin was greatly affected by Ca(2+)-calmodulin. Fesselin may function as an actin-polymerizing factor that is regulated by Ca(2+) levels.

Unzipping the Role of Myosin Light Chain Phosphatase in Smooth Muscle Cell Relaxation

Recently, it has been hypothesized that myosin light chain (MLC) phosphatase is activated by cGMP-dependent protein kinase (PKG) via a leucine zipper-leucine zipper (LZ-LZ) interaction through the C-terminal LZ in the myosin-binding subunit (MBS) of MLC phosphatase and the N-terminal LZ of PKG (Surks, H. K., Mochizuki, N., Kasai, Y., Georgescu, S. P., Tang, K. M., Ito, M., Lincoln, T. M., and Mendelsohn, M. E. (1999) Science 286, 1583-1587). Alternative splicing of a 3'-exon produces a LZ+ or LZ- MBS, and the sensitivity to cGMP-mediated smooth muscle relaxation correlates with the relative expression of LZ+/LZ- MBS isoforms (Khatri, J. J., Joyce, K. M., Brozovich, F. V., and Fisher, S. A. (2001) J. Biol. Chem. 276, 37250 -37257). In the present study, we determined the effect of LZ+/LZ- MBS isoforms on cGMP-induced MLC20 dephosphorylation. Four avian smooth muscle MBS-recombinant adenoviruses were prepared and transfected into cultured embryonic chicken gizzard smooth muscle cells. The expressed exogenous MBS isoforms were shown to replace the endogenous isoform in the MLC phosphatase holoenzyme. The interaction of type I PKG (PKGI) with the MBS did not depend on the presence of cGMP or the MBS LZ. However, direct activation of PKGI by 8-bromo-cGMP produced a dose-dependent decrease in MLC20 phosphorylation (p<0.05) only in smooth muscle cells expressing a LZ+ MBS. These results suggest that the activation of MLC phosphatase by PKGI requires a LZ+ MBS, but the binding of PKGI to the MBS is not mediated by a LZ-LZ interaction. Thus, the relative expression of LZ+/LZ- MBS isoforms could explain differences in tissue sensitivity to NO-mediated vasodilatation.

A novel monoclonal antibody identifies all avian embryonic myogenic cells and adult smooth muscle cells.

A novel monoclonal antibody, designated GIIF3, recognized prospective and differentiated smooth muscle cells in avian species studied - guinea fowl, chicken and quail. The GIIF3 antigen appeared in the myocardial, and the myotomal cells of the embryos at Hamburger-Hamilton stages 10 and 14, respectively. The expression of the GIIF3 molecule in the vascular smooth muscle cells emerged in the ventral wall of the dorsal aorta at Hamburger-Hamilton stage 16. The visceral smooth muscle cells started to produce the GIIF3 molecule from Hamburger-Hamilton stage 28 onwards. In both cardiac and skeletal muscles the GIIF3 expression gradually diminished, and it was lost by the end of the embryonic period, unlike in the differentiated vascular and visceral smooth muscle cells. In the latter cells the GIIF3 immunoreactive product showed a fine granular pattern that accumulated in the central region of the cytoplasm; it also occurred in the nucleus. A heavily stained discontinuous layer was associated with the cell membrane. The immunoblotting of the GIIF3 antibody recognized protein bands at 50 and 42 kDa in lysates of adult avian gizzard. A detailed comparative immunohistochemical study was made by smooth muscle markers, which confirmed the results of the immunoblotting, namely, that the GIIF3 monoclonal antibody recognized an avian myogenic cell specific molecule. During smooth muscle cell differentiation the GIIF3 molecule appeared as early as the alpha-smooth muscle actin, and in adult birds continued to be expressed; therefore the GIIF3 molecule could be regarded as a novel avian smooth muscle specific marker.

Characterisation of muscarinic receptor subtypes in avian smooth muscle

The identity of the muscarinic receptor subtype in the chick ileum was investigated in functional and binding studies. Preliminary studies [Choo, L.-K., Mitchelson, F., Napier, P. 1988. J. Auton. Pharmacol. 8, 259–266] suggested apparent avian and mammalian family differences in the muscarinic receptor profile of ileal smooth muscle. In the current study, further characterisation was undertaken using a greater range of antagonists exhibiting high affinity for specific muscarinic receptor subtypes. Dissociation constants from functional and binding experiments were compared with published values for antagonists at each of the five muscarinic receptor subtypes. Linear regression and correlation analyses revealed the receptor initiating the contractile response was most likely of the muscarinic M 3 receptor subtype as the slope of the linear regression was 1.01±0.14 and the corresponding correlation coefficient ( r) was 0.95. The mammalian muscarinic M 5 receptor subtype also showed a high correlation with the data giving a slope of 0.89±0.27 and r value of 0.76. These findings were in direct contrast to those from binding experiments in which the single binding site detected was of the muscarinic M 2 receptor subtype. The slope of the linear regression was 1.14±0.24 with an r value of 0.87. Thus, these results suggest that there exists a high proportion of the muscarinic M 2 receptor subtype within the tissue that does not contribute to the functional response.

Properties of the Novel Intermediate Filament Protein Synemin and Its Identification in Mammalian Muscle

We examined specific properties of highly purifiedsynemin (230 kDa), recently identified as a novel intermediate filament (IF) protein, from avian smooth muscle. Soluble synemin in 10 mM Tris–HCl, pH 8.5, appears as ∼11-nm-diameter globular structures by negative-stain and low-angle shadow electron microscopy. Chemical crosslinking and SDS–PAGE analysis indicate that soluble synemin molecules contain two 230-kDa subunits. The pH- and ionic strength-dependent solubility properties of synemin are similar to those of the type III IF protein desmin, but under physiological-like conditions in which desmin self-assembles into long ∼10-nm-diameter IFs, synemin self-associates into complex, approx 15- to 25-nm-diameter globular structures. Calpain digestion demonstrated that synemin is extremely proteolytically labile. Western blot analysis, with monospecific polyclonal antibodies against avian synemin, shows the presence of the reactive 230-kDa synemin band in samples of adult avian skeletal, cardiac, and smooth muscle and of two reactive bands at ∼225 kDa (major) and ∼195 kDa in adult porcine skeletal, cardiac, and smooth muscle. Partial purification of synemin from porcine smooth muscle also resulted in fractions highly enriched in the ∼225- and ∼195-kDa polypeptides. Conventional immunofluorescence and immunoconfocal microscopy of isolated myofibrils and of frozen sections also demonstrated, for the first time, that synemin is present in all three adult porcine muscle cell types and is colocalized with desmin in skeletal and cardiac muscle cells at the myofibrillar Z-lines.

Vitronectin regulates smooth muscle contractility via αv and β1 integrin(s)

Previous work from this laboratory has established a method for maintaining physiological contractility of dissociated avian smooth muscle in a defined medium at low density. The present report emphasizes the dramatic potency of serum to alter smooth muscle phenotype and induce a loss of contractility. Vitronectin, a molecule purified from plasma, mimicked these effects of serum via an integrin that is RGD-sensitive. Studies utilizing blocking antibodies against vitronectin demonstrated that the presence of this specific adhesion molecule was necessary for the serum-induced loss of contractility. Based on the actions of function-blocking antibodies and RGD-containing peptides, the integrin alphavbeta1 appears to be the primary receptor involved in vitronectin's ability to induce phenotypic transformation in amniotic smooth muscle. The influence of vitronectin on smooth muscle contractility is particularly relevant, because this molecule is abundant in whole blood and plasma (approx. 400 microg/ml). The results suggest that smooth muscle needs to be continually protected from normal blood constituents in vivo. The implications of these results for smooth muscle-related diseases like atherosclerosis, restenosis and Kaposi's sarcoma are discussed.

Molecular cloning and expression of avian smooth muscle S100A11 (calgizzarin, S100C)

S100A11 (calgizzarin or S100C), a member of the S100 family of Ca(2+)-binding proteins, was first identified in chicken gizzard smooth muscle and subsequently detected in several mammalian species and tissues. We now report the full-length coding sequence of avian smooth muscle S100A11. The cloned nucleotide sequence is 515 bases in length, which includes in-frame start and stop codons and encodes a protein of 101 amino acids. The chicken S100A11 sequence differs from human S100A11 at 25 positions (9 conserved) and is four residues shorter (overall identity 72.4%, similarity 81%). The protein contains two EF hand and conserved hydrophobic residues involved in dimer formation. Cloned avian S100A11 expressed in Escherichia coli and purified by Ca(2+)-dependent hydrophobic interaction chromatography and ion-exchange chromatography was recognized by polyclonal antibodies raised against tissue-purified protein and, like tissue-purified S100A11, bound 45Ca2+ in a gel overlay assay.

CRP1, a LIM domain protein implicated in muscle differentiation, interacts with alpha-actinin.

Members of the cysteine-rich protein (CRP) family are LIM domain proteins that have been implicated in muscle differentiation. One strategy for defining the mechanism by which CRPs potentiate myogenesis is to characterize the repertoire of CRP binding partners. In order to identify proteins that interact with CRP1, a prominent protein in fibroblasts and smooth muscle cells, we subjected an avian smooth muscle extract to affinity chromatography on a CRP1 column. A 100-kD protein bound to the CRP1 column and could be eluted with a high salt buffer; Western immunoblot analysis confirmed that the 100-kD protein is alpha-actinin. We have shown that the CRP1-alpha-actinin interaction is direct, specific, and saturable in both solution and solid-phase binding assays. The Kd for the CRP1-alpha-actinin interaction is 1.8 +/- 0.3 microM. The results of the in vitro protein binding studies are supported by double-label indirect immunofluorescence experiments that demonstrate a colocalization of CRP1 and alpha-actinin along the actin stress fibers of CEF and smooth muscle cells. Moreover, we have shown that alpha-actinin coimmunoprecipitates with CRP1 from a detergent extract of smooth muscle cells. By in vitro domain mapping studies, we have determined that CRP1 associates with the 27-kD actin-binding domain of alpha-actinin. In reciprocal mapping studies, we showed that alpha-actinin interacts with CRP1-LIM1, a deletion fragment that contains the NH2-terminal 107 amino acids (aa) of CRP1. To determine whether the alpha-actinin binding domain of CRP1 would localize to the actin cytoskeleton in living cells, expression constructs encoding epitope-tagged full-length CRP1, CRP1-LIM1(aa 1-107), or CRP1-LIM2 (aa 108-192) were microinjected into cells. By indirect immunofluorescence, we have determined that full-length CRP1 and CRP1-LIM1 localize along the actin stress fibers whereas CRP1-LIM2 fails to associate with the cytoskeleton. Collectively these data demonstrate that the NH2-terminal part of CRP1 that contains the alpha-actinin-binding site is sufficient to localize CRP1 to the actin cytoskeleton. The association of CRP1 with alpha-actinin may be critical for its role in muscle differentiation.

Localization of smoothelin in avian smooth muscle and identification of a vascular-specific isoform

Smoothelin is a smooth muscle-specific protein of minor abundance first identified via a monoclonal antibody obtained using an avian gizzard extract as antigen. Dual labelling of ultrathin sections with antibodies to smoothelin together with antibodies to other smooth muscle proteins showed that smoothelin was co-distributed with filamin and desmin in the cytoskeleton domain of the smooth muscle cell. From the finding that smoothelin, unlike desmin, was readily extracted by Triton X-100 as well as under conditions that solubilized myosin, β-actin and filamin, we conclude that smoothelin is most likely associated with the actin cytoskeleton. Western blot analysis of gizzard smooth muscle tissue revealed an immunoreactive protein band with an apparent molecular weight of 59 kDa that separated into 3–4 isolated variants, while avian vascular muscle showed a polypeptide band of 95 kDa. These results point to the presence of specific isoforms in visceral and vascular smooth muscles. The 59 kDa isoform was shown to be distinct from the 60 kDa filamin-binding protein, described by Maekawa and Sakai (FEBS Lett. 221, 68–72, 1987). As compared to other smooth muscle markers, such as calponin and SM22, smoothelin appeared very late during differentiation in the chick gizzard, on about the 18th embryonic day.

Molecular cloning and expression of avian smooth muscle S100A11 (calgizzarin, S100C)

S100A11 (calgizzarin or S100C), a member of the S100 family of Ca(2+)-binding proteins, was first identified in chicken gizzard smooth muscle and subsequently detected in several mammalian species and tissues. We now report the full-length coding sequence of avian smooth muscle S100A11. The cloned nucleotide sequence is 515 bases in length, which includes in-frame start and stop codons and encodes a protein of 101 amino acids. The chicken S100A11 sequence differs from human S100A11 at 25 positions (9 conserved) and is four residues shorter (overall identity 72.4%, similarity 81%). The protein contains two EF hand and conserved hydrophobic residues involved in dimer formation. Cloned avian S100A11 expressed in Escherichia coli and purified by Ca(2+)-dependent hydrophobic interaction chromatography and ion-exchange chromatography was recognized by polyclonal antibodies raised against tissue-purified protein and, like tissue-purified S100A11, bound 45Ca2+ in a gel overlay assay.

Immunogold localization of β1 integrin in mature avian smooth muscle

Integrins are α β heterodimeric transmembrane glycoproteins. The extracellular aminoterminal portion of the dimer contains the ligand-binding region and provides a linkage to the extracellular matrix and to other cells. The relatively smaller cytoplasmic carboxyterminal region interacts with the cytoskeleton and specifically interacts with α-actinin and with talin. Integrins are, thus, thought to be involved in signal transduction at the surface of many cell types. Smooth muscle cells contain α1β1integrin which has been immunolocalized in fibroblast focal contacts. Because aα-actinin and talin are components of attachment plaques which are sometimes referred to as plasma membrane-bound dense bodies in smooth muscle, we used immunogold labeling and transmission electron microscopy (TEM) to determine the location of β1integrin at the smooth muscle cell surface.Strips of smooth muscle from mature chicken gizzards were isometrically clamped and were fixed in 2% paraformaldehyde in modified Krebs Ringer that contained 2mM Ca2+and 2mM Mg2+for 2½ hrs. at 2° C. Fixed strips were cut into 0.5-0.8mm cubes that were immersed in 0.15 M glycine for 1 hr.