Monoclonal antibodies against caldesmon, a Ca++/calmodulin- and actin-binding protein of smooth muscle and nonmuscle cells

Tropomyosin and caldesmon regulate cytokinesis speed and membrane stability during cell division

The contractile ring and the cell cortex generate force to divide the cell while maintaining symmetrical shape. This requires temporal and spatial regulation of the actin cytoskeleton at these areas. We force-expressed misregulated versions of actin-binding proteins, tropomyosin and caldesmon, into cells and analyzed their effects on cell division. Cells expressing proteins that increase actomyosin ATPase, such as human tropomyosin chimera (hTM5/3), significantly speed up division, whereas cells expressing proteins that inhibit actomyosin, such as caldesmon mutants defective in Ca(2+)/calmodulin binding (CaD39-AB) and in cdk1 phosphorylation sites (CaD39-6F), divide slowly. hTM5 and hTM5/3-expressing cells lift one daughter cell off the substrate and twist. Furthermore, CaD39-AB- and CaD39-6F-expressing cells are sensitive to hypotonic swelling and show severe blebbing during division, whereas hTM5/3-expressing cells are resistant to hypotonic swelling and produce membrane bulges. These results support a model where Ca(2+)/calmodulin and cdk1 dynamically control caldesmon inhibition of tropomyosin-activated actomyosin to regulate division speed and to suppress membrane blebs.

Fascins, and their roles in cell structure and function

The fascins are a structurally unique and evolutionarily conserved group of actin cross-linking proteins. Fascins function in the organisation of two major forms of actin-based structures: dynamic, cortical cell protrusions and cytoplasmic microfilament bundles. The cortical structures, which include filopodia, spikes, lamellipodial ribs, oocyte microvilli and the dendrites of dendritic cells, have roles in cell-matrix adhesion, cell interactions and cell migration, whereas the cytoplasmic actin bundles appear to participate in cell architecture. We discuss the current understanding of the cellular mechanisms that regulate the binding of fascin to actin and how these processes contribute to the organisation or disassembly of cell protrusions. Although the in vivo roles of fascin have been studied principally in Drosophila, several human diseases are associated with inherited or acquired alterations in the expression of fascins. Strategies to modulate fascin-containing protrusions and thereby cell adhesive and migratory behaviour could have potential for therapeutic intervention in these conditions. The supplementary material referred to in this section can be found at http://www.interscience.wiley.com/jpages/0265-9247/suppmat/2002/v24.350.html

The role of myofibroblasts at the tumor border of invasive colorectal adenocarcinomas

BACKGROUND

In order to elucidate the significance of myofibroblasts in invasive growth of colorectal adenocarcinomas, we examined the number of myofibroblasts at the tumor border of colorectal adenocarcinomas.

METHOD

A total of 91 invasive colorectal adenocarcinomas were examined immunohistochemically using anti-alpha-smooth muscle actin (ASMA) and high-molecular-weight caldesmon (h-CD) antibodies; 25 carcinomas confined to the submucosa (sm carcinomas), 40 carcinomas confined to the muscularis propria (mp carcinomas) and 26 carcinomas invading the subserosa or adventitia (ss carcinomas). We considered ASMA-positive and h-CD-negative stromal cells as myofibroblasts.

RESULTS

Twenty-seven (67%) of the 40 mp carcinomas and 25 (96%) of the 26 ss carcinomas had a small number of myofibroblasts at the tumor border facing the muscularis propria.

CONCLUSIONS

Although direct evidence is lacking, there is a possibility that the further immediately vertical and radial invasion of carcinoma cells into the subserosa or adventitia is associated with a smaller number of myofibroblasts at the tumor border facing the muscularis propria in mp carcinomas, resulting in a low incidence of mp and a high incidence of ss carcinomas in the colorectum.

Tropomyosin isoforms in nonmuscle cells

Vertebrate nonmuscle cells, such as human and rat fibroblasts, express multiple isoforms of tropomyosin, which are generated from four different genes and a combination of alternative promoter activities and alternative splicing. The amino acid variability among these isoforms is primarily restricted to three alternatively spliced exon regions; an amino-terminal region, an internal exon, and a carboxyl-terminal exon. Recent evidence reveals that these variable exon regions encode amino acid sequences that may dictate isoform-specific functions. The differential expression of tropomyosin isoforms found in cell transformation and cell differentiation, as well as the differential localization of tropomyosin isoforms in some types of culture cells and developing neurons suggest a differential isoform function in vivo. Tropomyosin in striated muscle works together with the troponin complex to regulate muscle contraction in a Ca(2+)-dependent fashion. Both in vitro and in vivo evidence suggest that multiple isoforms of tropomyosin in nonmuscle cells may be required for regulating actin filament stability, intracellular granule movement, cell shape determination, and cytokinesis. Tropomyosin-binding proteins such as caldesmon, tropomodulin, and other unidentified proteins may be required for some of these functions. Strong evidence for the distinct functions carried out by different tropomyosin isoforms has been generated from genetic analysis of yeast and Drosophila tropomyosin mutants.

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.

Expression and epitopic conservation of calponin in different smooth muscles and during development

Calponin is a thin filament associated protein found in smooth muscle as a potential modulator of contraction. Five mouse monoclonal antibodies (mAbs CP1, CP3, CP4, CP7, and CP8) were prepared against chicken gizzard alpha-calponin. The CP1 epitopic structure is conserved in smooth muscles across vertebrate phyla and is highly sensitive to CNBr cleavage in contrast with the chicken-specific CP4 and the avian-mammalian-specific CP8 epitopes that are resistant to CNBr fragmentation. Using this panel of mAbs against multiple epitopes, only alpha-calponin was detected in adult chicken smooth muscles and throughout development of the gizzard. Western blotting showed that the calponin content varied among different smooth muscle tissues and correlated with that of h-caldesmon. In contrast with the constitutive expression of calponin in phasic smooth muscle of the digestive tract, very low levels of calponin were detected in adult avian tracheas and no calponin expression was detected in embryonic and young chick tracheas. These results provide information on the structural conservation of calponins and suggest a relationship between calponin expression and smooth muscle functional states.

Vascular smooth muscle cells of H-2Kb-tsA58 transgenic mice. Characterization of cell lines with distinct properties

BACKGROUND

The vascular wall is composed of at least two different populations of smooth muscle cells that are distinct in their structure and protein composition. According to the developmental stage of tissue taken for culture, the ratio between cells of epithelioid phenotype and spindle-shaped cells is variable. In particular, the epithelioid cells display characteristic features associated with immaturity. Because their increased appearance can be observed in endothelial denudation, the represent a dedifferentiated, proliferative smooth muscle cell type with a repair function in vascular injury.

METHODS AND RESULTS

To investigate this cellular heterogeneity, we established vascular smooth muscle cell lines from H-2Kb-tsA58 transgenic mice. Due to temperature-sensitive expression of the SV 40 large T-antigen in cells derived from this mouse strain, our smooth muscle lines were conditionally immortalized from the onset of their life in culture. Thus, we were able to clone cell lines representing the two different phenotypes described so far. Epithelioid cells derived from newborn animals are characterized by their expression of cytokeratins and the development of tight junctional complexes. Spindle-shaped cells, which could be isolated from newborn or adult animals, corresponded in phenotype and protein expression to smooth muscle cell lines established previously.

CONCLUSIONS

The special properties of vascular smooth muscle cells of the epithelioid phenotype suggest an endothelial replacement function in the course of injury to the vascular wall.

Forced expression of chimeric human fibroblast tropomyosin mutants affects cytokinesis

Human fibroblasts generate at least eight tropomyosin (TM) isoforms (hTM1, hTM2, hTM3, hTM4, hTM5, hTM5a, hTM5b, and hTMsm alpha) from four distinct genes, and we have previously demonstrated that bacterially produced chimera hTM5/3 exhibits an unusually high affinity for actin filaments and a loss of the salt dependence typical for TM-actin binding (Novy, R.E., J. R. Sellers, L.-F. Liu, and J.J.-C. Lin, 1993. Cell Motil. & Cytoskeleton. 26: 248-261). To examine the functional consequences of expressing this mutant TM isoform in vivo, we have transfected CHO cells with the full-length cDNA for hTM5/3 and compared them to cells transfected with hTM3 and hTM5. Immunofluorescence microscopy reveals that stably transfected CHO cells incorporate force-expressed hTM3 and hTM5 into stress fibers with no significant effect on general cell morphology, microfilament organization or cytokinesis. In stable lines expressing hTM5/3, however, cell division is slow and sometimes incomplete. The doubling time and the incidence of multinucleate cells in the stable hTM5/3 lines roughly parallel expression levels. A closely related chimeric isoform hTM5/2, which differs only in the internal, alternatively spliced exon also produces defects in cytokinesis, suggesting that normal TM function may involve coordination between the amino and carboxy terminal regions. This coordination may be prevented in the chimeric mutants. As bacterially produced hTM5/3 and hTM5/2 can displace hTM3 and hTM5 from actin filaments in vitro, it is likely that CHO-expressed hTM5/3 and hTM5/2 can displace endogenous TMs to act dominantly in vivo. These results support a role for nonmuscle TM isoforms in the fine tuning of microfilament organization during cytokinesis. Additionally, we find that overexpression of TM does not stabilize endogenous microfilaments, rather, the hTM-expressing cells are actually more sensitive to cytochalasin B. This suggests that regulation of microfilament integrity in vivo requires stabilizing factors other than, or in addition to, TM.

Heterogeneity of smooth muscle-associated proteins in mammalian brain microvasculature

In the brain, the microvascular system is composed of endothelial cells surrounded by a layer of pericytes. The lack of smooth muscle cells in this tissue suggests that any contractile function must be performed by one or both of these cell types. The present study was undertaken in order to identify cells in terminal blood vessels that contain smooth muscle-like contractile machinery. Endothelial cells were reactive with antibodies against smooth muscle myosin but showed no other smooth muscle-related features. In contrast, pericytes of intact microvessels showed a pattern of protein expression similar to that of smooth muscle cells. Pericytes also behaved in tissue culture like cultured smooth muscle cells, with regard to the changes in expression of smooth muscle-related proteins. These data confirm the close relationship between smooth muscle cells and pericytes, and point to their contractile function in the brain microvessels.

Overexpression of human fibroblast caldesmon fragment containing actin-, Ca++/calmodulin-, and tropomyosin-binding domains stabilizes endogenous tropomyosin and microfilaments

Fibroblast caldesmon is a protein postulated to participate in the modulation of the actin cytoskeleton and the regulation of actin-based motility. The cDNAs encoding the NH2-terminal (aa.1-243, CaD40) and COOH-terminal (aa.244-538, CaD39) fragments of human caldesmon were subcloned into expression vectors and we previously reported that bacterially produced CaD39 protein retains its actin-binding properties as well as its ability to enhance low M(r) tropomyosin (TM) binding to actin and to inhibit TM-actin-activated HMM ATPase activity in vitro (Novy, R. E., J. R. Sellers, L.-F. Liu, and J. J.-C. Lin. 1993. Cell Motil. Cytoskeleton. 26:248-261). Bacterially produced CaD40 does not bind actin. To study the in vivo effects of CaD39 expression on the stability of actin filaments in CHO cells, we isolated and characterized stable CHO transfectants which express varying amounts of CaD39. We found that expression of CaD39 in CHO cells stabilized microfilament bundles as well as endogenous TM. CaD39-expressing clones displayed an increased resistance to cytochalasin B and Triton X-100 treatments and yielded increased amounts of TM-containing actin filaments in microfilament isolation procedures. In addition, analysis of these clones with immunoblotting and indirect immunofluorescence microscopy with anti-TM antibody revealed that stabilized endogenous TM and enhanced TM-containing microfilament bundles parallel increased amounts of CaD39 expression. The increased TM observed corresponded to a decrease in TM turnover rate and did not appear to be due to increased synthesis of endogenous TM. Additionally, the phenomenon of stabilized TM did not occur in stable CHO clones expressing CaD40. Therefore, it is likely that CaD39 can enhance TM's binding to F-actin in vivo, thus reducing TM's rate of turnover and stabilizing actin microfilament bundles.

Forced expression and assembly of rat cardiac troponin T isoforms in cultured muscle and nonmuscle cells

Cardiac troponin T (cTnT), a tropomyosin (TM)-binding subunit of the troponin complex, undergoes a developmentally regulated isoform switch from embryonic form to adult form in the rat heart. To investigate the in vivo assembly of cTnT isoforms, we transiently transfected cDNA clones of either rat cTnT isoform into nonmuscle CHO cells and chick embryo myogenic (CEM) cells. As determined by Western blotting, both isoforms can be expressed in CHO and CEM cells. The expressed proteins had the same mobility as native rat cTnT proteins on SDS polyacrylamide gels and were recognized by anti-TnT antibodies. Conventional and confocal microscopy of transfected cells, double-labelled with antibodies against cTnT and against TM, revealed that neither isoform appears to associate with the nonmuscle TM in CHO cells, although both are able to colocalize with muscle TM-containing microfilament bundles in the myogenic CEM cells. There was no appreciable cTnT isoform-related difference in association with TM, suggesting that the functional significance of isoform variability in rat cTnT does not correspond to an assembly advantage for the maturing cardiac thin filament. To help determine whether cTnT nonassembly in CHO environment is primarily due to the nonmuscle nature of the endogenous TM, or if it involves the absence of other factors specific to muscle, we have isolated several stably-transfected clones of skeletal beta TM-expressing CHO cells which incorporate this muscle TM onto stress fibres. When either isoform of cTnT was transiently expressed in these beta TM-CHO cells, the strictly filamentous beta TM staining pattern was no longer observed. Instead, beta TM codistributed with cTnT in brightly staining aggregates not associated with the intact stress fibres. This suggests that both isoforms of cTnT are interacting with the beta TM in the nonmuscle environment and that other muscle-specific proteins may indeed be required for stable assembly of cTnT onto microfilaments. It also suggests that the interaction between cTnT and muscle TM is stronger than that between muscle TM and nonmuscle microfilaments.

Electron microscopic images suggest both ends of caldesmon interact with actin filaments

An improved rotary shadowing technique enabled us to visualize chicken gizzard caldesmon (CaD) and its complexes with one or two covalently linked calmodulin (CaM) molecules by electron microscopy. Using a monoclonal antibody against an epitope in the N-terminal region of CaD (anti-N), we can now identify the end of the molecule that is involved in binding to another protein molecule. Thus in the 1:1 complex of CaD and CaM, the CaM molecule was almost always associated with the C-terminus of CaD, indicating preferential CaM-binding to the C-terminal region. We have also studied binding of CaD to filamentous actin (F-actin), using an EM technique that avoids spraying or freeze drying and thereby preserves the structure of F-actin. Only one end of CaD appeared to bind to F-actin, leaving the rest of the molecule projecting away from the filament. While the majority of anti-N bound at the free end of CaD, some antibody molecules were found on F-actin. These findings suggest that either end of CaD can bind to F-actin. Experiments using a monoclonal antibody against the C-terminus of CaD (anti-C) supported this idea. When the native thin filaments that contain endogenous CaD were incubated with anti-N, almost all the bound antibodies were found on the filaments, indicating that the N-terminal regions of CaD interact with actin, and that the binding affinity of the N-terminal region of CaD for actin is higher in vivo than that in vitro, either because the properties of CaD have been altered during purification, or because of the presence of some other component(s) associated with the native filaments.

Expression of wild-type and nuclear localization-deficient human lamin A in chick myogenic cells

Previous analysis of chick embryonic muscle (CEM) differentiation in vivo and in ovo demonstrated that lamin A accumulation to steady-state levels preceded the accumulation of muscle-specific proteins. These observations have suggested the appearance of A-type lamins may be important for differentiation. To test this hypothesis, we have temporally and quantitatively altered the expression of A-type lamins in CEM cells by transient transfection of wild-type (wt; pHLA) or nuclear localization-deficient (NLd; pHLA-del) human lamin A expression plasmids. Transfected CEM cells synthesized the wt and NLd human lamin As to high levels, both of which were resistant to high-salt extraction. The wt human lamin A localized to the nucleus, whereas the NLd protein showed cytoplasmic staining patterns, as well as time-dependent nuclear localization. The presence of endogenous chicken lamins A and B2 in NLd human lamin A cytoplasmic structures suggested the interspecies lamin copolymerization. Thus, this approach may provide a possible method for analysis of lamin-lamin or lamin-lamina component interactions in vivo. With regard to muscle differentiation, CEM cells transfected with either pHLA or pHLA-del demonstrated moderate and transient increased levels of the muscle-specific myosin heavy chain and creatine kinase activity. These increases appeared temporally and quantitatively to reflect the transient accumulation of the human lamin As. In contrast, beta-tubulin and skeletal tropomyosin protein accumulations appeared unaffected. On the basis of these results, we suggest that nuclear lamina content and structure may play a limited, permissive and indirect role in the temporally regulated expression of the myogenic phenotype.

Protein kinase C phosphorylates caldesmon77 and vimentin and enhances albumin permeability across cultured bovine pulmonary artery endothelial cell monolayers

Cytoskeletal protein (CSP) interactions are critical to the contractile response in muscle and non-muscle cells. Current concepts suggest that activation of the contractile apparatus occurs through selective phosphorylation by specific cellular kinase systems. Because the Ca(2+)-phospholipid-dependent protein kinase C (PKC) is involved in the regulation of a number of key endothelial cell responses, the hypothesis that PKC modulates endothelial cell contraction and monolayer permeability was tested. Phorbol myristate acetate (PMA), a direct PKC activator, and alpha-thrombin, a receptor-mediated agonist known to increase endothelial cell permeability, both induced rapid, dose-dependent activation and translocation of PKC in bovine pulmonary artery endothelial cells (BPAEC), as assessed by gamma-[32P]ATP phosphorylation of H1 histone in cellular fractions. This activation was temporally associated with evidence of agonist-mediated endothelial cell contraction as demonstrated by characteristic changes in cellular morphology. Agonist-induced activation of the contractile apparatus was associated with increases in BPAEC monolayer permeability to albumin (approximately 200% increase with 10(-6) MPMA, approximately 400% increase with 10(-8) M alpha-thrombin). To more closely examine the role of PKC in activation of the contractile apparatus, PKC-mediated phosphorylation of two specific CSPs, the actin- and calmodulin-binding protein, caldesmon77, and the intermediate filament protein, vimentin, was assessed. In vitro phosphorylation of both caldesmon and vimentin was demonstrated by addition of exogenous, purified BPAEC PKC to unstimulated BPAEC homogenates, to purified bovine platelet caldesmon77, or to purified smooth muscle caldesmon150. Caldesmon77 and vimentin phosphorylation were observed in intact [32P]-labeled BPAEC monolayers stimulated with either PMA or alpha-thrombin, as detected by immunoprecipitation. In addition, BPAEC pretreatment with the PKC inhibitor, staurosporine, prevented alpha-thrombin- and PMA-induced phosphorylation of both cytoskeletal proteins, attenuated morphologic evidence of contraction, and abolished agonist-induced barrier dysfunction. These results demonstrate that agonist-stimulated PKC activity results in cytoskeletal protein phosphorylation in BPAEC monolayer, an event which occurs in concert with agonist-mediated endothelial cell contraction and resultant barrier dysfunction.

Apolipoprotein A-1 expression is resistant to dimethyl sulfoxide inhibition of myogenic differentiation

Primary cultures of chick embryonic muscle (CEM) were analyzed for the differential expression of a 26-kDa protein during myogenesis. We have identified this 26-kDa protein as apolipoprotein A-1 (Apo A-1), the major protein of serum high density lipoprotein particles. Apo A-1 was expressed in a pattern temporally similar to those of muscle-specific proteins, by myoblasts at very low levels, and by myotubes at high levels. The half-life of Apo A-1 in CEM cell homogenates was 23 min. This fast turnover rate appeared to be due to the secretion of Apo A-1 into the culture medium. To further characterize the relationship of Apo A-1 expression and myogenic differentiation, CEM cultures were treated with dimethyl sulfoxide (DMSO). In the presence of 2% DMSO, myotubes exhibited an atrophied morphology and an inhibition of the synthesis and accumulation of muscle-specific sarcomeric proteins. During recovery from DMSO treatment, the expression and accumulation of muscle-specific proteins returned to high levels. In contrast, the rates of synthesis and secretion of Apo A-1 in control, DMSO-treated, and DMSO-recovered CEM cells were nearly equivalent. These results indicate that the expression of Apo A-1 is not strictly linked to the expression of muscle-specific sarcomeric proteins in skeletal muscle and suggest that independent, or additional regulatory mechanisms exist which modulate Apo A-1 expression during myogenesis.

The molecular anatomy of caldesmon

Images

Immunogold labelling of the intermediate filament-lamina-nuclear matrix system in HeLa and BHK-21 cells

Whole-mount, sequentially extracted cells combined with immunogold electron microscopy were developed to demonstrate the intermediate filaments, lamina, and nuclear matrix (IF-L-NM) and to identify their protein components. The IFs of HeLa cells were reacted both with keratin and vimentin monoclonal antibodies; meanwhile, the IF network of BHK-21 cell was reacted only with vimentin monoclonal antibody. The lamina and nuclear matrices of both HeLa and BHK-21 cell were labelled, respectively, with lamin monoclonal antibody-gold complex and 280 Kd nuclear matrix protein monoclonal antibody-gold complex. The monoclonal antibody to keratin could cross-react with the lamina both of HeLa and BHK-21 cells.

Formation of the perinuclear theca in spermatozoa of diverse mammalian species: relationship of the manchette and multiple band polypeptides

The perinuclear theca is a novel cytoskeletal consisting of a densely layered lamina that surrounds the nucleus of mammalian sperm. Using antibodies specific for the multiple band polypeptides present in the perinuclear theca of bull sperm, we show that a heterogeneous group of immunological related proteins are present in the sperm heads of other mammals with greatly different morphologies, including guinea pig, hamster, rat, and mouse. In none of the species were identical groups of immunoreactive polypeptides found, although immunoreactive proteins of molecular weights 65,000 to 80,000 were present in the sperm heads of all species examined. Immunoreactive proteins less than Mr 55,000 were prominent in rat sperm heads and mouse sperm: guinea pig, hamster, and rat sperm heads and mouse sperm had one band in common at approximately Mr 50,000. Different immunoreactive proteins were present in isolated sperm tails. The perinuclear theca first appeared in the subacrosomal space of round to elongating spermatids. Later, with the caudal movement of the manchette, the postacrosomal segment of the perinuclear theca was deposited in a cephalad to caudal direction along the sperm nucleus. Concomitantly, the cytoplasmic space between the nuclear envelope and the plasma membrane narrowed such that only the theca occupied this portion of the sperm head. Immunoreactivity accompanied the ultrastructural appearance of the subacrosomal layer and the postacrosomal segment. The periods of spermiogenesis, in which sub- and post-acrosomal components of the perinuclear theca are formed and the morphogenesis of sperm organelles with which these elements are associated, suggest that components of this cytoskeletal structure function to join the acrosome and the postacrosomal plasma membrane to the nucleus.

Caldesmon-induced polymerization of actin from profilactin

We have investigated the effect of caldesmon, a Ca2+/calmodulin-regulated actin-binding protein, on the complex between profilin and G-actin (profilactin). We found that smooth muscle caldesmon dissociates this complex rapidly and induces the polymerization of the released actin. Native profilactin (e.g. the complex isolated from calf thymus) proved more resistant to the attack of caldesmon than a heterologous complex reconstituted from calf thymus profilin and skeletal muscle actin. The mode of caldesmon-induced profilactin dissociation was similar to that described for Mg2+, and 2 mM MgCl2 potentiated the caldesmon effect. Since both caldesmon and profilin have been found enriched in ruffling membranes of animal cells, our in vitro findings may be relevant to the regulation of actin filaments in living cells.

Epitope mapping of monoclonal antibodies against caldesmon and their effects on the binding of caldesmon to Ca++/calmodulin and to actin or actin-tropomyosin filaments

The effects of monoclonal anti-caldesmon antibodies, C2, C9, C18, C21, and C23, on the binding of caldesmon to F-actin/F-actin-tropomyosin filaments and to Ca++/calmodulin were examined in an in vitro reconstitution system. In addition, the antibody epitopes were mapped by Western blot analysis of NTCB (2-nitro-5-thiocyanobenzoic acid) and CNBr (cyanogen bromide) fragments of caldesmon. Both C9 and C18 recognize an amino terminal fragment composed of amino acid residues 19 to 153. The C23 epitope lies within a fragment ranging from residues 230 to 386. Included in this region is a 13-residue repeat sequence. Interestingly this repetitive sequence shares sequence similarity with a sequence found in nuclear lamin A, a protein which is also recognized by C23 antibody. Therefore, it is likely that the C23 epitope corresponds to this 13-residue repeat sequence. A carboxyl-terminal 10K fragment contains the epitopes for antibodies C2 and C21. Among these antibodies, only C21 drastically inhibits the binding of caldesmon to F-actin/F-actin-tropomyosin filaments and to Ca++/calmodulin. When the molar ratio of monoclonal antibody C21 to caldesmon reached 1.0, a maximal inhibition (90%) on the binding of caldesmon to F-actin filaments was observed. However, it required double amounts of C21 antibody to exhibit a maximal inhibition of 70% on the binding of caldesmon to F-actin-tropomyosin filaments. These results suggest that the presence of tropomyosin in F-actin enhances caldesmon's binding. Furthermore, C21 antibody also effectively inhibits the caldesmon binding to Ca++/calmodulin. The kinetics of C21 inhibition on caldesmon's binding to Ca++/calmodulin is very similar to the inhibition obtained by preincubation of caldesmon with free Ca++/calmodulin. This result suggests that there is only one Ca++/calmodulin binding domain on caldesmon and this domain appears to be very close to the C21 epitope. Apparently, the Ca++/calmodulin-binding domain and the actin-binding domain are very close to each other and may interfere with each other. In an accompanying paper, we have further demonstrated that microinjection of C21 antibody into living chicken embryo fibroblasts inhibit intracellular granule movement, suggesting an in vivo interference with the functional domains [Hegmann et al., 1991: Cell Motil. Cytoskeleton 20:109-120].

Inhibition of intracellular granule movement by microinjection of monoclonal antibodies against caldesmon

Monoclonal antibodies, C2, C9, C18, and C21, against chicken gizzard caldesmon (called high molecular weight isoform) were shown to crossreact with a low molecular weight isoform of caldesmon in chicken embryo fibroblasts (CEF). These antibodies were used in a microinjection study to investigate the in vivo function of caldesmon in nonmuscle cell motility. Injected cells did not appear to change their morphology significantly; the cells displayed a flat appearance and were able to ruffle and locomote normally. However, in the C21 injected cells, saltatory movements of granules and organelles appeared to be greatly inhibited. This inhibition of granule movement was reversible, so that by 3 hr after injection, granules in injected cells had already recovered to normal speed. The inhibition of granule movement in cells injected with C2, C9, or C18 antibody, or with C21 antibody preabsorbed with caldesmon, were not significantly different from that in uninjected cells. In a previous epitope study, we demonstrated that, of the antibodies used in this study, only C21 antibody was able to compete with the binding of caldesmon to Ca++/calmodulin and to F-actin, although both C21 and C2 antibodies recognized the same carboxyl-terminal 10K fragment of gizzard caldesmon [Lin et al., 1991: Cell Motil. Cytoskeleton 20:95-108]. The caldesmon distribution in C21 injected cells changed from stress-fiber localization to a more diffuse appearance, when the injection was performed at 10-30 mg/ml of C21 antibody. We have previously shown that a monoclonal anti-tropomyosin antibody exhibited motility-dependent recognition of an epitope, and that microinjection of this antibody specifically inhibited intracellular granule movements of CEF cells [Hegmann et al., 1989: J. Cell Biol. 109:1141-1152]. Therefore, it is likely that tropomyosin and caldesmon may both function in intracellular granule movement by regulating the contractile system in response to [Ca++] change inside nonmuscle cells.

Probing the role of nonmuscle tropomyosin isoforms in intracellular granule movement by microinjection of monoclonal antibodies

Chicken embryo fibroblast (CEF) cells were microinjected with several different monoclonal antibodies that recognize certain nonmuscle isoforms of tropomyosin. Immediately after injection, cells were recorded with a time-lapse video imaging system; later analysis of the tapes revealed that particles in cells injected with one of these antibodies (CG1, specific for CEF tropomyosin isoforms 1 and 3) showed a dramatic decrease in instantaneous speed while moving, distance moved per saltation, and proportion of time spent in motion. Injection of Fab fragments of CG1 resulted in similar changes in the pattern of granule movement. This inhibition of granule movement by CG1 antibody was reversible; at 2.5 h after injection, granules in injected cells had already reached three-fourths of normal speed. The speed of granule movement in cells injected either with antibody specific for tropomyosin isoforms not present in CEF cells, or with CG1 antibody preabsorbed with tropomyosin, was not significantly different from the speed of granules in uninjected cells. When cells were injected with CG1 or Fab fragments of CG1, fixed, and counter-stained with rabbit antibodies to reveal the microtubule, microfilament, and intermediate filament systems, no obvious differences from the patterns normally seen in uninjected cells were observed. Examination of the ultrastructure of injected cells by EM confirmed the presence of apparently intact and normal microtubule, actin, and intermediate filament networks. These experiments suggest that tropomyosin may play an important role in the movement of vesicles and organelles in the cell cytoplasm. Also, we have shown previously that the CG1 determinant can undergo a motility-dependent change in reactivity, that may be important for the regulatory function of nonmuscle tropomyosin (Hegmann, T. E., J. L.-C. Lin, and J. J.-C. Lin. 1988. J. Cell Biol. 106:385-393). Therefore, in addition to postulated microtubule-based motors, microfilaments may play a critical role in regulating granule movement in nonmuscle cells.

Expression of nuclear lamin A and muscle-specific proteins in differentiating muscle cells in ovo and in vitro

Primary cultures and tissue samples of chicken embryonic muscle were immunologically probed for the expression of muscle-specific proteins, such as myosin heavy chain and the tropomyosins, as well as for the nuclear lamina protein, lamin A. As determined by quantitative immunoblotting, the expression of lamin A and the muscle-specific proteins were at low levels or absent in predifferentiation myoblasts both in vitro and in ovo. During differentiation, an increase of lamin A expression preceded the induction to high levels of expression of muscle-specific proteins. Immunofluorescence staining of chicken embryonic muscle cells in culture also indicates an accumulation of lamin A before the induction of muscle-specific proteins expression. Furthermore, the accumulation of lamin A reached a plateau before the muscle-specific proteins during muscle development. In two dimensional NEPHGE gel analysis of immunoprecipitated lamin A, no detectable change in the ratio of the acidic/basic isoelectric variants of lamin A was observed during myogenesis. A potential role for lamin A in the mechanisms which underlie the differential and coordinate expression of muscle-specific genes is proposed.