A rapid degradation of calponin 2 is required for cytokinesis.

Calponin 2 is an actin cytoskeleton-associated protein and plays a role in regulating cell motility-related functions such as phagocytosis, migration, and division. We previously reported that overexpression of calponin 2 inhibits the rate of cell proliferation. To investigate the underlying mechanism, our present study found that the levels of endogenous calponin 2 in NIH3T3 and HEK293 cells rapidly decreased before cell division characterized by an absence at the actin contractile ring. In cells lacking endogenous calponin 2, transfective expression of GFP-fusion calponin 2 inhibited cell proliferation similar to that of nonfusion calponin 2. Fluorescent imaging studies of mitotic cells indicated that a proper level of calponin 2 expression and effective degradation during cytokinesis are necessary for normal cell division. Computer-assisted dynamic image analysis of dividing cells revealed that overexpression of calponin 2 significantly affects motility and shape behaviors of cells only on the interval from the start of anaphase to the start of cytokinesis, i.e., the pre-cytokinesis phase, but not on the interval from the start of cytokinesis to 50% completion of cytokinesis. The pre-cytokinesis degradation of calponin 2 was attenuated by MG132 inhibition of the ubiquitin proteasome and inhibitor of protein kinase C (PKC), suggesting that PKC phosphorylation-triggered degradation of calponin 2 could determine the rate of cytokinesis. The novel role of calponin 2 in regulating the rate of cytokinesis may be targeted for therapeutic applications such as in an inhibition of malignant tumor growth.

Localization and function of Xinα in mouse skeletal muscle.

The Xin repeat-containing proteins were originally found in the intercalated discs of cardiac muscle with implicated roles in cardiac development and function. A pair of paralogous genes, Xinα (Xirp1) and Xinß (Xirp2), is present in mammals. Ablation of the mouse Xinα (mXinα) did not affect heart development but caused late-onset adulthood cardiac hypertrophy and cardiomyopathy with conductive defects. Both mXinα and mXinß are also found in the myotendinous junction (MTJ) of skeletal muscle. Here we investigated the structural and functional significance of mXinα in skeletal muscle. In addition to MTJ and the contact sites between muscle and perimysium, mXinα but not mXinß was found in the blood vessel walls, whereas both proteins were absent in neuromuscular junctions and nerve fascicles. Coimmunoprecipitation suggested association of mXinα with talin, vinculin, and filamin, but not ß-catenin, in adult skeletal muscle, consistent with our previous report of colocalization of mXinα with vinculin. Loss of mXinα in mXinα-null mice had subtle effects on the MTJ structure and the levels of several MTJ components. Diaphragm muscle of mXinα-null mice showed hypertrophy. Compared with wild-type controls, mouse extensor digitorum longus (EDL) muscle lacking mXinα exhibited no overt change in contractile and relaxation velocities or maximum force development but better tolerance to fatigue. Loaded fatigue contractions generated stretch injury in wild-type EDL muscle as indicated by a fragmentation of troponin T. This effect was blunted in mXinα-null EDL muscle. The results suggest that mXinα play a role in MTJ conductance of contractile and stretching forces.

Intercalated disc protein Xinß is required for Hippo-YAP signaling in the heart.

Intercalated discs (ICD), specific cell-to-cell contacts that connect adjacent cardiomyocytes, ensure mechanical and electrochemical coupling during contraction of the heart. Mutations in genes encoding ICD components are linked to cardiovascular diseases. Here, we show that loss of Xinß, a newly-identified component of ICDs, results in cardiomyocyte proliferation defects and cardiomyopathy. We uncovered a role for Xinß in signaling via the Hippo-YAP pathway by recruiting NF2 to the ICD to modulate cardiac function. In Xinß mutant hearts levels of phosphorylated NF2 are substantially reduced, suggesting an impairment of Hippo-YAP signaling. Cardiac-specific overexpression of YAP rescues cardiac defects in Xinß knock-out mice-indicating a functional and genetic interaction between Xinß and YAP. Our study reveals a molecular mechanism by which cardiac-expressed intercalated disc protein Xinß modulates Hippo-YAP signaling to control heart development and cardiac function in a tissue specific manner. Consequently, this pathway may represent a therapeutic target for the treatment of cardiovascular diseases.

Sorting of a nonmuscle tropomyosin to a novel cytoskeletal compartment in skeletal muscle results in muscular dystrophy.

Tropomyosin (Tm) is a key component of the actin cytoskeleton and >40 isoforms have been described in mammals. In addition to the isoforms in the sarcomere, we now report the existence of two nonsarcomeric (NS) isoforms in skeletal muscle. These isoforms are excluded from the thin filament of the sarcomere and are localized to a novel Z-line adjacent structure. Immunostained cross sections indicate that one Tm defines a Z-line adjacent structure common to all myofibers, whereas the second Tm defines a spatially distinct structure unique to muscles that undergo chronic or repetitive contractions. When a Tm (Tm3) that is normally absent from muscle was expressed in mice it became associated with the Z-line adjacent structure. These mice display a muscular dystrophy and ragged-red fiber phenotype, suggestive of disruption of the membrane-associated cytoskeletal network. Our findings raise the possibility that mutations in these tropomyosin and these structures may underpin these types of myopathies.

Effect of eradication of Helicobacter pylori on the histology and cellular phenotype of gastric intestinal metaplasia.

BACKGROUND & AIMS: Eradication of Helicobacter pylori appears to reduce gastric cancer incidence. We examined the effect of successful H pylori therapy on histology, phenotype of gastric intestinal metaplasia (GIM) (complete vs incomplete), and expression of several biomarkers related to carcinogenesis. METHODS: Ninety-six H pylori-positive patients from Japan were treated successfully and followed up prospectively over 4 years with yearly endoscopy and were classified into 3 groups: group CG, chronic gastritis without GIM (n = 36); group IM, chronic gastritis with GIM (n = 33); group DYS, and GIM with dysplasia/cancer in a different location of the stomach (n = 27). A total of 288 endoscopic procedures were performed. Histology, mucin-histochemistry, and immunoperoxidase assays using monoclonal antibodies (mAbs) for cell phenotype (monoclonal antibody Das-1/colonic) and for neoplasia (TC22 and p53) were performed. RESULTS: The GIM histologic score was higher in group DYS than in group IM (P < .05) and group CG (P < .0001). The GIM scores did not change in groups IM and DYS over 4 years. mAb Das-1 reactivity was higher in group DYS (63%) than in group IM (39%) and group GC (0%). After eradication of H pylori, mAb Das-1 reactivity disappeared in 40% of patients (P < .0001) despite the unchanged GIM scores, and regression of TC22-4 was noted in the same patients. CONCLUSIONS: H pylori eradication does not reduce the histologic GIM score, but changes the cellular phenotype of GIM. This change of phenotype may be an important factor in the reduction of cancer incidence after eradication of H pylori.

Specific features of neuronal size and shape are regulated by tropomyosin isoforms.

Spatially distinct populations of microfilaments, characterized by different tropomyosin (Tm) isoforms, are present within a neuron. To investigate the impact of altered tropomyosin isoform expression on neuronal morphogenesis, embryonic cortical neurons from transgenic mice expressing the isoforms Tm3 and Tm5NM1, under the control of the beta-actin promoter, were cultured in vitro. Exogenously expressed Tm isoforms sorted to different subcellular compartments with Tm5NM1 enriched in filopodia and growth cones, whereas the Tm3 was more broadly localized. The Tm5NM1 neurons displayed significantly enlarged growth cones accompanied by an increase in the number of dendrites and axonal branching. In contrast, Tm3 neurons displayed inhibition of neurite outgrowth. Recruitment of Tm5a and myosin IIB was observed in the peripheral region of a significant number of Tm5NM1 growth cones. We propose that enrichment of myosin IIB increases filament stability, leading to the enlarged growth cones. Our observations support a role for different tropomyosin isoforms in regulating interactions with myosin and thereby regulating morphology in specific intracellular compartments.

Specification of actin filament function and molecular composition by tropomyosin isoforms.

The specific functions of greater than 40 vertebrate nonmuscle tropomyosins (Tms) are poorly understood. In this article we have tested the ability of two Tm isoforms, TmBr3 and the human homologue of Tm5 (hTM5(NM1)), to regulate actin filament function. We found that these Tms can differentially alter actin filament organization, cell size, and shape. hTm5(NM1) was able to recruit myosin II into stress fibers, which resulted in decreased lamellipodia and cellular migration. In contrast, TmBr3 transfection induced lamellipodial formation, increased cellular migration, and reduced stress fibers. Based on coimmunoprecipitation and colocalization studies, TmBr3 appeared to be associated with actin-depolymerizing factor/cofilin (ADF)-bound actin filaments. Additionally, the Tms can specifically regulate the incorporation of other Tms into actin filaments, suggesting that selective dimerization may also be involved in the control of actin filament organization. We conclude that Tm isoforms can be used to specify the functional properties and molecular composition of actin filaments and that spatial segregation of isoforms may lead to localized specialization of actin filament function.

Autoimmunity against human tropomyosin isoforms in ulcerative colitis: localization of specific human tropomyosin isoforms in the intestine and extraintestinal organs.

BACKGROUND: Tropomyosins (TMs) are microfilament cytoskeletal proteins, and 5 major human TM isoforms (hTM1-5) are described. hTMs, particularly isoform 5 (hTM5), is capable of inducing autoantibodies and T-cell response in ulcerative colitis (UC). However, cellular localization of hTM isoforms in the colon and in extraintestinal organs commonly involved in UC is unknown. METHODS: Using isoform-specific monoclonal antibodies, we localized hTMs through immunoperoxidase assay in normal colon (n = 12), small intestine (n = 14), esophagus (n = 10), skin (n = 19), eye (n = 12), gallbladder (n = 16), liver, including bile duct at the porta hepatis (n = 4), lungs (n = 4), and pancreas (n = 4). RESULTS: There is intense expression of hTM5, but not other isoforms, in the epithelium of the colon, gallbladder, and skin. In the eye, hTM5 is expressed only in the nonpigmented ciliary epithelium. Although extrahepatic and interlobar large ductal biliary epithelium was positive, bile canaliculi at the portal tract are negative. The immunoreactivity in epithelial cells from these organs is diffuse cytoplasmic and along the periphery. In colon epithelium, there is intense expression along basolateral areas and luminal (apical) surface. In the small intestinal epithelium, however, hTM5 expression is weak and distinctly different than in the colon. hTM5 was not detected in the squamous epithelium of the esophagus, although it was strongly positive in the skin. hTM1, hTM2, and hTM3 are localized predominantly in smooth muscle of the intestine and blood vessel wall but not the epithelium. HTM4 is localized in the endothelial cells and basement membrane of the colonic epithelium. CONCLUSIONS: hTM5 is the predominant isoform in the epithelium of colon and extraintestinal organs commonly involved in UC. The unique expression of hTM5 may allow its interaction with effector immune cells involved in the immunopathogenesis of UC and its extraintestinal manifestations.

Discontinuous thoracic venous cardiomyocytes and heart exhibit synchronized developmental switch of troponin isoforms.

Cardiomyocyte-like cells have been reported in thoracic veins of rodents and other mammals, but their differentiation state and relationship to the muscle mass in the heart remain to be characterized. Here we investigated the distribution, ultrastructure, expression and developmental regulation of myofilament proteins of mouse and rat pulmonary and azygos venous cardiomyocytes. Tracing cardiomyocytes in transgenic mouse tissues using a lacZ reporter gene driven by a cloned rat cardiac troponin T promoter demonstrated scattered distribution of cardiomyocytes discontinuous from the atrial sleeves. The longitudinal axis of venous cardiomyocytes is perpendicular to that of the vessel. These cells contain typical sarcomere structures and intercalated discs as shown in electron microscopic images, and express cardiac isoforms of troponin T, troponin I and myosin. The expression of troponin I isoform genes and the alternative splicing of cardiac troponin T in thoracic venous cardiomyocytes are regulated during postnatal development in precise synchrony with that in the heart. However, the patterns of cardiac troponin T splicing in adult rat thoracic venous cardiomyocytes are slightly but clearly distinct from those in the atrial and ventricular muscles. The data indicate that mouse and rat thoracic venous cardiomyocytes residing in extra-cardiac tissue possess a physiologically differentiated state and an intrinsically pre-set developmental clock, which are apparently independent of the very different hemodynamic environments and functional features of the vessels and heart.

Tropomyosin isoforms and reagents.

Tropomyosins are rod-like dimers which form head-to-tail polymers along the length of actin filaments and regulate the access of actin binding proteins to the filaments.1 The diversity of tropomyosin isoforms, over 40 in mammals, and their role in an increasing number of biological processes presents a challenge both to experienced researchers and those new to this field. The increased appreciation that the role of these isoforms expands beyond that of simply stabilizing actin filaments has lead to a surge of reagents and techniques to study their function and mechanisms of action. This report is designed to provide a basic guide to the genes and proteins and the availability of reagents which allow effective study of this family of proteins. We highlight the value of combining multiple techniques to better evaluate the function of different tm isoforms and discuss the limitations of selected reagents. Brief background material is included to demystify some of the unfortunate complexity regarding this multi-gene family of proteins including the unconventional nomenclature of the isoforms and the evolutionary relationships of isoforms between species. Additionally, we present step-by-step detailed experimental protocols used in our laboratory to assist new comers to the field and experts alike.

Changes in protein profiles during course of experimental glomerulonephritis.

Better characterization of the molecular mechanisms underlying glomerular cell proliferation may improve our understanding of the pathogenesis of glomerulonephritis and yield disease-specific markers. We used two-dimensional gel electrophoresis (2DE) and mass spectrometry (MS) to generate expression profiles of glomerular proteins in the course of anti-Thy-1 nephritis. Glomeruli were isolated from Wistar rats by sieving, and proteins were separated by 2DE. In preliminary studies using normal rats, we identified known glomerular proteins from microfilaments [tropomyosin (Tm)] and intermediate filaments (vimentin and lamin A), proteins involved in assembly (alpha-actinin-4, F-actin capping protein) and membrane cytoskeletal linking (ezrin), as well as several enzymes (protein disulfide isomerase, ATP synthase, and aldehyde dehydrogenase). Comparison of glomerular protein abundance between normal rats and rats in the early phase of anti-Thy-1 nephritis yielded 28 differentially expressed protein spots. MS analysis identified 16 differentially expressed proteins including Tm. Altered Tm abundance in the course of anti-Thy-1 nephritis was confirmed, and specific isoforms were characterized by Western blotting. We demonstrated a complex change in Tm isoform abundance in the course of anti-Thy-1 nephritis. The early mesangiolytic phase of the disease was characterized by decreased abundance of low-molecular-weight isoforms Tm5a/5b and increased abundance of high-molecular-weight isoforms Tm6, Tm1, Tm2, and Tm3. The late proliferative phase of the disease was associated with increased abundance of isoforms Tm5a/5b, Tm6, and Tm1 and decreased abundance of Tm3. Isoforms Tm4 and Tm5 remained unchanged in the course of this model of experimental glomerulonephritis. Characterization of Tm isoform abundance in the course of clinical glomerulonephritis may identify disease-specific markers.

Divergent regulation of the sarcomere and the cytoskeleton.

The existence of a feedback mechanism regulating the precise amounts of muscle structural proteins, such as actin and the actin-associated protein tropomyosin (Tm), in the sarcomeres of striated muscles is well established. However, the regulation of nonmuscle or cytoskeletal actin and Tms in nonmuscle cell structures has not been elucidated. Unlike the thin filaments of striated muscles, the actin cytoskeleton in nonmuscle cells is intrinsically dynamic. Given the differing requirements for the structural integrity of the actin thin filaments of the sarcomere compared with the requirement for dynamicity of the actin cytoskeleton in nonmuscle cells, we postulated that different regulatory mechanisms govern the expression of sarcomeric versus cytoskeletal Tms, as key regulators of the properties of the actin cytoskeleton. Comprehensive analyses of tissues from transgenic and knock-out mouse lines that overexpress the cytoskeletal Tms, Tm3 and Tm5NM1, and a comparison with sarcomeric Tms provide evidence for this. Moreover, we show that overexpression of a cytoskeletal Tm drives the amount of filamentous actin.

The intercalated disk protein, mXinalpha, is capable of interacting with beta-catenin and bundling actin filaments [corrected].

Targeted deletion of mXinalpha results in cardiac hypertrophy and cardiomyopathy with conduction defects (Gustafson-Wagner, E., Sinn, H. W., Chen, Y.-L., Wang, D.-Z., Reiter, R. S., Lin, J. L.-C., Yang, B., Williamson, R. A., Chen, J. N., Lin, C.-I., and Lin, J. J.-C. (2007) Am. J. Physiol. 293, H2680-H2692). To understand the underlying mechanisms leading to such cardiac defects, the functional domains of mXinalpha and its interacting proteins were investigated. Interaction studies using co-immunoprecipitation, pull-down, and yeast two-hybrid assays revealed that mXinalpha directly interacts with beta-catenin. The beta-catenin-binding site on mXinalpha was mapped to amino acids 535-636, which overlaps with the known actin-binding domains composed of the Xin repeats. The overlapping nature of these domains provides insight into the molecular mechanism for mXinalpha localization and function. Purified recombinant glutathione S-transferase- or His-tagged mXinalpha proteins are capable of binding and bundling actin filaments, as determined by co-sedimentation and electron microscopic studies. The binding to actin was saturated at an approximate stoichiometry of nine actin monomers to one mXinalpha. A stronger interaction was observed between mXinalpha C-terminal deletion and actin as compared with the interaction between full-length mXinalpha and actin. Furthermore, force expression of green fluorescent protein fused to an mXinalpha C-terminal deletion in cultured cells showed greater stress fiber localization compared with force-expressed GFP-mXinalpha. These results suggest a model whereby the C terminus of mXinalpha may prevent the full-length molecule from binding to actin, until the beta-catenin-binding domain is occupied by beta-catenin. The binding of mXinalpha to beta-catenin at the adherens junction would then facilitate actin binding. In support of this model, we found that the actin binding and bundling activity of mXinalpha was enhanced in the presence of beta-catenin.

Loss of mXinalpha, an intercalated disk protein, results in cardiac hypertrophy and cardiomyopathy with conduction defects.

The intercalated disk protein Xin was originally discovered in chicken striated muscle and implicated in cardiac morphogenesis. In the mouse, there are two homologous genes, mXinalpha and mXinbeta. The human homolog of mXinalpha, Cmya1, maps to chromosomal region 3p21.2-21.3, near a dilated cardiomyopathy with conduction defect-2 locus. Here we report that mXinalpha-null mouse hearts are hypertrophied and exhibit fibrosis, indicative of cardiomyopathy. A significant upregulation of mXinbeta likely provides partial compensation and accounts for the viability of the mXinalpha-null mice. Ultrastructural studies of mXinalpha-null mouse hearts reveal intercalated disk disruption and myofilament disarray. In mXinalpha-null mice, there is a significant decrease in the expression level of p120-catenin, beta-catenin, N-cadherin, and desmoplakin, which could compromise the integrity of the intercalated disks and functionally weaken adhesion, leading to cardiac defects. Additionally, altered localization and decreased expression of connexin 43 are observed in the mXinalpha-null mouse heart, which, together with previously observed abnormal electrophysiological properties of mXinalpha-deficient mouse ventricular myocytes, could potentially lead to conduction defects. Indeed, ECG recordings on isolated, perfused hearts (Langendorff preparations) show a significantly prolonged QT interval in mXinalpha-deficient hearts. Thus mXinalpha functions in regulating the hypertrophic response and maintaining the structural integrity of the intercalated disk in normal mice, likely through its association with adherens junctional components and actin cytoskeleton. The mXinalpha-knockout mouse line provides a novel model of cardiac hypertrophy and cardiomyopathy with conduction defects.

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.

Requirement of reversible caldesmon phosphorylation at P21-activated kinase-responsive sites for lamellipodia extensions during cell migration.

Caldesmon is believed to be one of the key regulators for actin dynamics and thereby cell polarity, membrane extension, and cell motility. We have shown previously that stress fiber formation and cell movement are severely impaired in the cells expressing human fibroblast caldesmon fragment defective in Ca2+/CaM binding sites. Both Ser458 and Ser489, adjacent to the Ca2+/CaM-binding sites, are phosphorylated by p21-activated kinase (PAK) in vitro. Here we report that Ser458 is phosphorylated in response to cell movement. We substituted Ser458 and Ser489 on C-terminal caldesmon (CaD39) with alanine or glutamic acid to mimic under-phosphorylated (CaD39-PAKA) or constitutively phosphorylated (CaD39-PAKE) caldesmon. In vitro, CaD39-PAKE, but not CaD39-PAKA, fails to inhibit myosin ATPase activity and exhibits reduced binding to Ca2+/CaM. When stably expressed in Chinese Hamster Ovary cells, both CaD39-PAKA and CaD39-PAKE incorporate into stress fibers and localize to the leading edge of the migrating cell. Expression of CaD39-PAKE, but not CaD39-PAKA, fails to protect stress fibers from cytochalasin depolymerization. However, both mutations inhibit cell polarization and lead to defects in membrane extension and cell migration. We conclude that phosphorylation of caldesmon by PAK is a dynamic process required to regulate actin dynamics and membrane protrusions in wound-induced cell migration.

h2-Calponin is regulated by mechanical tension and modifies the function of actin cytoskeleton.

Calponin is an extensively studied actin-binding protein, but its function is not well understood. Among three isoforms of calponin, h2-calponin is found in both smooth muscle and non-muscle cells. The present study demonstrates that epidermal keratinocytes and fibroblast cells express significant amounts of h2-calponin. The expression of h2-calponin is cell anchorage-dependent. The levels of h2-calponin decrease when cells are rounded up and remain low when cells are prevented from adherence to a culture dish. h2-calponin expression resumes after the floating cells are allowed to form a monolayer in plastic dish. Cell cultures on polyacrylamide gels of different stiffness demonstrated that h2-calponin expression is affected by the mechanical properties of the culture matrix. When cells are cultured on soft gel that applies less traction force to the cell and, therefore, lower mechanical tension in the cytoskeleton, the level of h2-calponin is significantly lower than that in cells cultured on hard gel or rigid plastic dish. Force-expression of h2-calponin enhanced the resistance of the actin filaments to cytochalasin B treatment. Keratinocyte differentiation is accompanied by a mechanical tension-related up-regulation of h2-calponin. Lowering the tension of actin cytoskeleton by inhibiting non-muscle myosin II ATPase decreased h2-calponin expression. In contrast to the mechanical tension regulation of endogenous h2-calponin, the expression of h2-calponin using a cytomegalovirus promotor was independent of the stiffness of culture matrix. The results suggest that h2-calponin represents a novel manifestation of mechanical tension responsive gene regulation that may modify cytoskeleton function.

Localization of the novel Xin protein to the adherens junction complex in cardiac and skeletal muscle during development.

Previously, we demonstrated that chick embryos treated with antisense oligonucleotides against a striated muscle-specific Xin exhibit abnormal cardiac morphogenesis (Wang et al. [1999] Development 126:1281-1294); therefore, we surmised a role for Xin in cardiac development. Herein, we examine the developmental expression of Xin through immunofluorescent staining of whole-mount mouse embryos and frozen heart sections. Xin expression is first observed within the heart tube of embryonic day 8.0 (E8.0) mice, exhibiting a peripheral localization within the cardiomyocytes. Colocalization of Xin with both beta-catenin and N-cadherin is observed throughout embryogenesis and into adulthood. Additionally, Xin is found associated with beta-catenin within the N-cadherin complex in embryonic chick hearts by coimmunoprecipitation. Xin is detected earlier than vinculin in the developing heart and colocalizes with vinculin at the intercalated disc but not at the sarcolemma within embryonic and postnatal hearts. At E10.0, Xin is also detected in the developing somites and later in the myotendon junction of skeletal muscle but not within the costameric regions of muscle. In cultured C2C12 myotubes, the Xin protein is found in many speckled and filamentous structures, coincident with tropomyosin in the stress fibers. Additionally, Xin is enriched in the regions of cell-cell contacts. These data demonstrate that Xin is one of the components at the adherens junction of cardiac muscle, and its counterpart in skeletal muscle, the myotendon junction. Furthermore, temporal and spatial expressions of Xin in relation to intercalated disc proteins and thin filament proteins suggest roles for Xin in the formation of cell-cell contacts and possibly in myofibrillogenesis.

Role of VEGF family members and receptors in coronary vessel formation.

The specific roles of vascular endothelial growth factor (VEGF) family members and their receptors (VEGFRs) in coronary vessel formation were studied. By using the quail heart explant model, we found that neutralizing antibodies to VEGF-B or VEGF-C inhibited tube formation on the collagen gel more than anti-VEGF-A. Soluble VEGFR-1, a receptor for VEGF-A and -B, inhibited tube formation by 87%, a finding consistent with that of VEGF-B inhibition. In contrast, addition of soluble VEGFR-2, a receptor for VEGF family members A, C, D, and E, inhibited tube formation by only 43%. Acidic FGF-induced tube formation dependency on VEGF was demonstrated by the attenuating effect of a soluble VEGFR-1 and -2 chimera. The localization of VEGF R-2 and R-3 was demonstrated by in situ hybridization of serial sections, which documented marked accumulations of transcripts for both receptors at the base of the truncus arteriosus coinciding with the temporal and spatial formation of the coronary arteries by means of ingrowth of capillary plexuses. This finding suggests that both VEGFR-2 and R-3 may play a role in the formation of the coronary artery roots. In summary, these experiments document a role for multiple members of the VEGF family and their receptors in formation of the coronary vascular bed.

Isolation and sequencing of a novel tropomyosin isoform preferentially associated with colon cancer.

BACKGROUND & AIMS: Nonmuscle human tropomyosin (hTM) isoforms have distinct functions and may play important roles in various disease processes. METHODS: In an attempt to identify colon epithelial tropomyosin isoform, a complementary DNA library prepared from a human colon cancer cell line T84 was screened by an oligonucleotide probe complementary to messages of all known hTM isoforms. A novel clone called TC22 was obtained. The amino acid sequence of TC22 isoform is identical to isoform 5 (hTM5) apart from the C terminal domain, amino acids 222-247 coding the exon 9. RESULTS: Northern blot analysis showed that TC22 message is expressed in transformed epithelial cell lines and tumor tissues but not in normal epithelial cells. We developed a monoclonal antibody specific to TC22 isoform (TC22-4). By Western blot and immunoperoxidase assays, we analyzed 105 colonic specimens (fresh frozen and formalin fixed) from 96 patients with colon polyps (hyperplastic) or adenomas with or without dysplasia and cancer. Twenty-one of 22 (95%) of colon cancer specimens showed the presence of TC22, compared with only 1 of the 17 normal colon specimens and none of the 13 hyperplastic polyps (P < 0.0001). As assayed by immunoperoxidase staining, TC22 expression progressively increased in benign adenomatous polyps (35%) and polyps with mild and severe dysplasia (57% and 100%, respectively). CONCLUSIONS: We cloned and sequenced a novel hTM isoform, TC22, which is strongly associated with colonic neoplasia and carcinoma. TC22 may provide a useful biomarker for surveillance of colon cancer.