Developmental change of protein constituents in chicken gizzards

Transgelin-2: Biochemical and Clinical Implications in Cancer and Asthma

Transgelin-2 has been regarded as an actin-binding protein that induces actin gelation and regulates actin cytoskeleton. However, transgelin-2 has recently been shown to relax the myosin cytoskeleton of the airway smooth muscle cells by acting as a receptor for extracellular metallothionein-2. From a clinical perspective, these results support transgelin-2 as a promising therapeutic target for diseases such as cancer and asthma. The inhibition of transgelin-2 prevents actin gelation and thereby cancer cell proliferation, invasion, and metastasis. Conversely, the activation of transgelin-2 with specific agonists relaxes airway smooth muscles and reduces pulmonary resistance in asthma. Here, we review new studies on the biochemical properties of transgelin-2 and discuss their clinical implications for the treatment of immune, oncogenic, and respiratory disorders.

Development of visceral smooth muscle

The development of the smooth musculature of viscera has attracted the interest of only relatively few investigators, and thus the field appears somewhat underexplored. The major emphasis on histochemical evidence--at the expense of ultrastructural and functional studies--may have limited the progress in this area. Mature tissue is formed through the differentiation of precursors into muscle cells and through the organization of these cells into a complex tissue where distribution and orientation of muscle cells, deployment of abundant extracellular materials and addition of other cellular elements (interstitial cells, fibroblasts, nerves, blood vessels) are characteristic and specific features. The precursor cells are found at sites where a muscle develops, and they derive predominantly from the mesoderm, but also from the neuroectoderm and from the endoderm. The process starts at different times in different organs. The earliest stages of differentiation are characterized by the precursor cells aggregating and becoming elongated; their longitudinal axis lies in a position similar to the one they will have in the mature muscle. Both the cytological and the histochemical differentiation follow distinct patterns in various muscles, with characteristic temporal sequences in the appearance of key features. This process must impart distinct functional properties to a muscle cell at each stage of its development. However, the chronological correspondence between ultrastructural and histochemical development is poorly understood. Histochemical studies have detected gradients of maturation of the muscle cells, for example, across the thickness of the gizzard musculature and along the length of the small intestine; ultrastructural studies have not yet confirmed the existence of these gradients. Muscle growth is accounted for by muscle cell enlargement (without nucleus duplication) and an increase in muscle cell number by mitosis of pre-existing differentiated muscle cells. De-differentiation and division of muscle cells, migration of muscle cells and late development of muscle cell precursors have all also been considered as possible mechanisms for muscle growth. Several authors have described the presence of precursor cells within developing smooth muscles, and they have described late differentiation of some muscle cells or waves of differentiation that would give rise to phenotypic heterogeneity of the mature muscle cell population. In contrast, other studies, mainly by electron microscopy, have suggested that, within large visceral muscles, the muscle cells differentiate synchronously. There are interesting data on the influence of adjacent tissues on the development of a smooth muscle, but the interplay of these and other factors has not been fully investigated. Smooth muscles contract from early in their development, hence mechanical factors are likely to influence development: on the one hand, passive stresses imposed on the muscle by other tissues, such as adjacent muscles or the contents of the viscera and, on the other hand, active forces generated by the muscle itself. The very attraction of visceral smooth muscles in the study of cellular morphogenesis--an attraction that has not yet been highlighted or exploited in scientific studies, either descriptively or experimentally--is that, onto a single type of cell, a large range of factors interact, such as the genetic expression, chemical influences (from other muscles, endocrine glands, nerves, other intramuscular cells) and mechanical factors.

Functional specificity of actin isoforms

Actin, one of the main proteins of muscle and cytoskeleton, exists as a variety of highly conserved isoforms whose distribution in vertebrates is tissue-specific. Synthesis of specific actin isoforms is accompanied by their subcellular compartmentalization, with both processes being regulated by factors of cell proliferation and differentiation. Actin isoforms cannot substitute for each other, and the high-level synthesis of exogenous actins leads to alterations in cell organization and morphology. This indicates that the highly conserved actins are functionally specialized for the tissues in which they predominate. The first goal of this review is to analyze the data on the polymerizability of actin isoforms to show that cytoskeleton isoactins form less stable polymers than skeletal muscle actin. This difference correlates with the dynamics of actin microfilaments versus the stability of myofibrillar systems. The three-dimensional actin structure as well as progress in the analysis of conformational changes in both the actin monomer and the filament allows us to view the data on the structure and polymerization of isoactins in terms of structure-function relationships within the actin molecule. Most of the amino acid substitutions that distinguish actin isoforms are located apart from actin-actin contact sites in the polymer. We suggest that these substitutions can modulate the ability of actin monomers to form more or less stable polymers by long-range (allosteric) regulation of the contact sites.

Phenotype-dependent expression of alpha-smooth muscle actin in visceral smooth muscle cells

Alpha-Smooth muscle actin is one of the molecular markers for a phenotype of vascular smooth muscle cells, because the actin is a major isoform expressed in vascular smooth muscle cells and its expression is upregulated during differentiation. Here, we first demonstrate that the phenotype-dependent expression of this actin in visceral smooth muscles is quite opposite to that in vascular smooth muscles. This actin isoform is not expressed in adult chicken visceral smooth muscles including gizzard, trachea, and intestine except for the inner layer of intestinal muscle layers, whereas its expression is clearly detected in these visceral smooth muscles at early stages of the embryo (10-day-old embryo) and is developmentally downregulated. In cultured gizzard smooth muscle cells maintaining a differentiated phenotype, alpha-smooth muscle actin is not detected while its expression dramatically increases during serum-induced dedifferentiation. Promoter analysis reveals that a sequence (-238 to -219) in the promoter region of this actin gene acts as a novel negative cis-element. In conclusion, the phenotype-dependent expression of alpha-smooth muscle actin would be regulated by the sum of the cooperative contributions of the negative element and well-characterized positive elements, purine-rich motif, and CArG boxes and their respective transacting factors.

cFKBP/SMAP; a novel molecule involved in the regulation of smooth muscle differentiation

During embryogenesis, smooth muscle cells of the gut differentiate from mesenchymal cells derived from splanchnic mesoderm. We have isolated a gene involved in the differentiation of smooth muscle cells in the gut using differential display between the chicken proventriculus in which the smooth muscle layer develops poorly and the gizzard in which smooth muscles develop abundantly. The protein encoded by this gene showed highest similarity to mouse FK506 binding protein, FKBP65, and from the function of this protein it was designated chicken FKBP/smooth muscle activating protein (cFKBP/SMAP). cFKBP/SMAP was first expressed in smooth muscle precursor cells of the gut and, after smooth muscles differentiate, expression was restricted to smooth muscle cells. In organ culture of the gizzard, the differentiation of smooth muscle cells was inhibited by the addition of FK506, the inhibitor of FKBPs. Moreover, overexpression of cFKBP/SMAP in lung and gizzard mesenchymal cells induced smooth muscle differentiation. In addition, cFKBP/SMAP-induced smooth muscle differentiation was inhibited by FK506. We postulate therefore that cFKBP/SMAP plays a crucial role in smooth muscle differentiation in the gut and provides a powerful tool to study smooth muscle differentiation mechanisms, which have been poorly analyzed so far.

The developmentally regulated expression of serum response factor plays a key role in the control of smooth muscle-specific genes

Serum response factor (SRF) is a MADS box transcription factor that has been shown to be important in the regulation of a variety of muscle-specific genes. We have previously shown SRF to be a major component of multiple cis/trans interactions found along the smooth muscle gamma-actin (SMGA) promoter. In the studies reported here, we have further characterized the role of SRF in the regulation of the SMGA gene in the developing gizzard. EMSA analyses, using nuclear extracts derived from gizzards at various stages in development, showed that the SRF-containing complexes were not present early in gizzard smooth muscle development, but appeared as development progressed. We observed an increase in SRF protein and mRNA levels during gizzard development by Western and Northern blot analyses, with a large increase just preceding an increase in SMGA expression. Thus, changes in SRF DNA-binding activity were paralleled with increased SRF gene expression. Immunohistochemical analyses demonstrated a correspondence of SRF and SMGA expression in differentiating visceral smooth muscle cells (SMCs) during gizzard tissue development. This correspondence of SRF and SMGA expression was also observed in cultured smooth muscle mesenchyme induced to express differentiated gene products in vitro. In gene transfer experiments with SMGA promoter-luciferase reporter gene constructs we observed four- to fivefold stronger SMGA promoter activity in differentiated SMCs relative to replicating visceral smooth muscle cells. Further, we demonstrate the ability of a dominant negative SRF mutant protein to specifically inhibit transcription of the SMGA promoter in visceral smooth muscle, directly linking SRF with the control of SMGA gene expression. Taken together, these data suggest that SRF plays a prominent role in the developmental regulation of the SMGA gene.

Morphological changes during ontogeny of the canine proximal colon

The development of the canine proximal colon from the completion of organogenesis through 43 days after birth was studied using light microscopy, immunofluorescence and electron microscopy. During this period the tunica muscularis increased in thickness from 42+/-6 microm in animals midway through the gestation period to 317+/-29 microm in animals 25-30 days old. This increase in thickness resulted from an increase in the number and size of smooth muscle cells in the circular and longitudinal muscle layers. The cross-sectional thickness of the circular muscle layer increased from 10+/-2 smooth muscle cells midway through the gestation period to 92+/-7 cells in animals 25-30 days old. The longitudinal layer increased in thickness from 1.5+/-1 cells in animals midway through the gestation period to 44+/-2 cells in animals 25-30 days old. Smooth muscle cells from both layers also increased in diameter and length. Ultrastructural and immunohistochemical studies suggested that many of the smooth muscle cells were undergoing development throughout the fetal period. Midway through the gestation period, the circular layer was positive for desmin-like immunoreactivity (D-LI), while both the circular and longitudinal layers were positive for vimentin-like immunoreactivity (V-LI). By birth, V-LI was suppressed in the circular and longitudinal layers, and both layers expressed D-LI. The enteric nervous system was already established midway through the gestation period, and submucosal and myenteric ganglia could be identified, although the chemical coding and mature morphology of neurons were incomplete. NADPH-diaphorase-positive neurons, indicating the expression of nitric oxide synthase, developed by the time of birth. Interstitial cells of Cajal (IC) could not clearly be identified midway through gestation, however, potential precursors to ICs were observed. Several classes of ICs were identifiable at birth.

Embryonic chicken gizzard: expression of the smooth muscle regulatory proteins caldesmon and myosin light chain kinase

The patterns of expression of the smooth muscle regulatory proteins caldesmon and myosin light chain kinase were investigated in the developing chicken gizzard. Immunofluorescent studies revealed that both proteins were expressed as early as E5 throughout the mesodermal gizzard anlage, together with actin, alpha-actin-in and a small amount of nonmuscle myosin. These proteins appear to form the scaffold for smooth muscle development, defined by the onset of smooth muscle myosin expression. During E6, a period of extensive cell division, smooth muscle myosin begins to appear in the musculi laterales close to the serosal border and, later, also in the musculi intermedii. Until about E10, myosin reactivity expands into the pre-existing thin filament scaffold. Later in development, the contractile and regulatory proteins co-localize and show a regular uniform staining pattern comparable to that seen in adult tissue. By using immunoblotting techniques, the low-molecular mass form of caldesmon and myosin light chain kinase were detected as early as E5. During further development, the expression of caldesmon switched from the low-molecular mass to the high-molecular mass form; in neonatal and adult tissue, high-molecular mass caldesmon was the only isoform expressed. The level of expression of myosin light chain kinase increased continously during embryonic development, but no embryo-specific isoform with a different molecular mass was detected.

Embryonic chicken gizzard: smooth muscle and non-muscle myosin isoforms

Antibodies to smooth muscle and non-muscle myosin allow the development of smooth muscle and its capillary system in the embryonic chicken gizzard to be followed by immunofluorescent techniques. Although smooth muscle development proceeds in a serosal to luminal direction, angiogenetic cell clusters develop independently at the luminal side close to the epithelial layer, and the presumptive capillaries invade the developing muscle in a luminal to serosal direction. The smooth muscle and non-muscle myosin heavy chains in this avian system cannot be separated by SDS polyacrylamide gel electrophoresis and do not show isoform specificity in immunoblotting, unlike the system found in mammals. Only two myosin heavy chains with M(r) of 200 and 196 kDa were separable and considerable immunological cross-reactivity was found between the denatured myosin isoform heavy chains.

Actin isoform compartments in chicken gizzard smooth muscle cells

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

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

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

Biochemical and molecular characterization of the chicken cysteine-rich protein, a developmentally regulated LIM-domain protein that is associated with the actin cytoskeleton

LIM domains are present in a number of proteins including transcription factors, a proto-oncogene product, and the adhesion plaque protein zyxin. The LIM domain exhibits a characteristic arrangement of cysteine and histidine residues and represents a novel zinc binding sequence (Michelsen et al., 1993). Previously, we reported the identification of a 23-kD protein that interacts with zyxin in vitro (Sadler et al., 1992). In this report, we describe the purification and characterization of this 23-kD zyxin-binding protein from avian smooth muscle. Isolation of a cDNA encoding the 23-kD protein has revealed that it consists of 192 amino acids and exhibits two copies of the LIM motif. The 23-kD protein is 91% identical to the human cysteine-rich protein (hCRP); therefore we refer to it as the chicken cysteine-rich protein (cCRP). Examination of a number of chick embryonic tissues by Western immunoblot analysis reveals that cCRP exhibits tissue-specific expression. cCRP is most prominent in tissues that are enriched in smooth muscle cells, such as gizzard, stomach, and intestine. In primary cell cultures derived from embryonic gizzard, differentiated smooth muscle cells exhibit the most striking staining with anti-cCRP antibodies. We have performed quantitative Western immunoblot analysis of cCRP, zyxin, and alpha-actinin levels during embryogenesis. By this approach, we have demonstrated that the expression of cCRP is developmentally regulated.

Beta-actin specific monoclonal antibody

Using a synthetic peptide mimicking the NH2-terminus of beta-actin we have raised a monoclonal antibody specific for this cytoplasmic actin isoform. Specificity of the antibody was demonstrated by its labelling of the actin polypeptide only in tissues containing the beta isoform, by its exclusive recognition of the synthetic beta-actin peptide amongst those mimicking all six vertebrate isoactins, and by its selective recognition of the beta-actin spot in two-dimensional electrophoresis gels of smooth muscle extracts. The antibody bound to actin filaments in both living and fixed fibroblasts where it labelled the stress fiber bundles and, more predominantly, the peripheral actin rich lamellipodia. The characteristics of the antibody indicate that it should serve as a useful tool for studying isoactin distribution and function.

Molecular cloning and expression of the chicken smooth muscle gamma-actin mRNA

We have investigated the expression of chicken smooth muscle gamma-actin mRNA by isolation and characterization of cDNAs representing this actin isoform and utilizing the cDNA to probe RNA from adult and developing cells. Nucleotide sequence elucidated from an apparent full length smooth muscle gamma-actin cDNA revealed that it contained 94 bp of 5' non-translated sequence, an open reading frame of 1131 bp, and 97 bp of 3' non-translated sequence. Within the 376 amino acid sequence deduced from the chicken cDNA were diagnostic amino acids at the NH2- and COOH-terminal regions which provided unequivocal identification of the gamma-enteric smooth muscle actin isoform. In addition, the chicken gamma-enteric actin deduced from our cDNA clones was found to differ from the sequence reported in earlier protein studies [J. Vandekerckhove and K. Weber, FEBS Lett. 102:219, 1979] by containing a proline rather than a glutamine at position 359 of the protein, indicating that the avian gamma-enteric actin isoform is identical to its mammalian counterpart. Comparison of the 5' and 3' non-translated sequence determined from the chicken cDNA to that elucidated for rat, mouse, and human showed that there is not a high degree of cross-species sequence conservation outside of the coding regions among these mRNAs. Northern hybridization analyses demonstrated that the gamma-enteric actin mRNA is expressed in adult aorta and oviduct tissues but not in adult skeletal muscle, cardiac muscle, liver, brain, and spleen tissues. The gamma-enteric actin mRNA was first observed in measurable quantities in gizzard tissue from 4-5 day embryos and increased in content in developing smooth muscle cells through 16-17 embryonic days. Following this initial increase during embryonic development, the gamma-enteric actin mRNA exhibits a decline in content until approximately 7 days posthatching, after which there is an increase in content to maximal levels found in adult gizzard tissue. In general, the developmental appearance of the gamma-enteric mRNA parallels that observed for this protein in previous studies indicating that the developmental expression of smooth muscle gamma-actin is regulated, in part, by an increased content of mRNA in chicken visceral smooth muscle cells during myogenesis.

Embryonic chicken gizzard: immunolocalization of collagen and smooth muscle myosin

Antibodies to chicken gizzard myosin and to chicken skin collagen type I allow the myofibrillar and connective tissue development in the embryonic chicken gizzard to be followed. Fibroblasts are assumed to synthesize collagen prior to the onset of smooth muscle cell development in the muscle primordium (day 5); they are presumably also responsible for collagen synthesis close to the presumptive lamina propria and in the developing tubular glands (day 14 to 17). From day 6 to 8, myosin and collagen are colocalized intracellularly, and from day 9 onward collagen fibers start to appear extracellularly, eventually forming the trellis-like connective tissue septa that give the rhomboid profile found in the adult muscle. The close association of collagen and myosin in early development suggests that the muscle cells themselves produce and export collagen.

Histological distribution and developmental changes of tropomyosin isoforms in three chicken digestive organs

Histological localization of tropomyosin isoforms in three digestive organs from embryonic and adult chickens was performed by using rabbit antisera against chicken skeletal muscle tropomyosin and against low-Mr-type tropomyosin from chicken small intestine mucosa. The former antiserum (named TM-SH) reacted with alpha, beta, and high-Mr-type isoforms, and the latter (named TM-HL) reacted with alpha, beta, high-Mr-type and low-Mr-type isoforms, alpha and beta Isoforms were detected in muscle cells of the muscular layer and the muscularis mucosa. Low-Mr-type isoforms, however, were detected along the cell membrane and cytoplasm of almost all nonmuscle cells, especially in terminal webs of epithelial cells. Developmental changes of tropomyosin isoforms in digestive organs were studied by two-dimensional gel electrophoresis and image analysis. The relative amounts of alpha and beta isoforms increased in the course of development, but those of low-Mr-type and high-Mr-type isoforms decreased.

Assembly of contractile and cytoskeletal elements in developing smooth muscle cells

Specific developmental changes in smooth muscle were studied in gizzards obtained from 6-, 8-, 10-, 12-, 14-, 16-, 18-, and 20-day chick embryos and from 1- and 7-day posthatch chicks. Myoblasts were actively replicating in tissue from 6-day embryos. Cytoplasmic dense bodies (CDBs) first appeared at Embryonic Day 8 (E8) and were recognized as patches of increased electron density that consisted of actin filaments (AFs), intermediate filaments (IFs), and cross-connecting filaments (CCFs). Although the assembly of CDBs was not synchronized within a cell, the number, size, and electron density of CDBs increased as age increased. Membrane-associated dense bodies (MADBs) also could be recognized at E8. The number and size of MADBs increased as age increased, especially after E16. Filaments with the diameter of thick filaments first appeared at E12. Smooth muscle cells were able to divide as late as E20. The axial intermediate filament bundle (IFB) could first be identified in 1-day posthatch cells and became larger and more prominent in 7-day posthatch cells. Immunogold labeling of 1- and 7-day posthatch cells with anti-desmin showed that the IFB contained desmin IFs. The developmental events during this 23-day period were classified into seven stages, based primarily on the appearance and the growth of contractile and cytoskeletal elements. These stages are myoblast proliferation, dense body appearance, thick filament appearance, dense body growth, muscle cell replication, IFB appearance, and appearance of adult type cells. Smooth muscle cells in each stage express similar developmental characteristics. The mechanism of assembly of myofilaments and cytoskeletal elements in smooth muscle in vivo indicates that myofilaments (AFs and thick filaments) and filament attachment sites (CDBs and MADBs) are assembled before the axial IFB, a major cytoskeletal element.

Expression of stress fibers in bullfrog mesothelial cells in response to tension

The relationship between stress fibers and tension in mesothelial cells of the bullfrog small intestine was examined by fluorescence cytochemistry using en face mesothelial cell preparations. In nontreated controls, actin revealed by rhodamine-phalloidin staining was localized only along the margins of the mesothelial cells. On the other hand, many stress fibers were formed in the mesothelial cells within 5-7 min after stretching of the intestinal wall in a given direction. The orientation of stress fibers within the cells was coincident with the direction of the tension applied. These cytoplasmic fibers disappeared almost completely from the mesothelial cells within 30 min after the release of tension. According to a difference in the intensity of tension necessary for stress fiber expression, the intestinal mesothelial cells were classified into two groups. Furthermore, cells containing stress fibers in each group showed a rapid increase in number once a given value of tension was applied. The present results indicate that the mesothelial cells of bullfrog small intestine may develop stress fibers to counteract tension exerted on the intestinal wall. Such stress fibers may serve to maintain cellular integrity by strengthening the cellular attachment to subepithelial tissue.

Smooth muscle specific expression of calponin

Calponin is an actin-, calmodulin-, and tropomyosin-binding protein that has been isolated from smooth muscle tissue. Using a monoclonal antibody specific for avian calponin, we demonstrate a differentiation-linked increase in calponin expression in embryonic chick gizzard. Cultivation of gizzard smooth muscle cells in vitro resulted in a down-regulation of calponin expression after the first 48 h that was paralleled by a loss of synthesis of metavinculin and the high molecular weight isoform of caldesmon. In early cultures of smooth muscle cells calponin was localised in the actin-containing stress fibres but labelling was restricted to the central parts of the actin cytoskeleton. Calponin expression is suggested as a potentially useful index of smooth muscle differentiation.

Coordinate and discoordinate accumulation of protein constituents in chicken breast muscle

Accumulation of protein constituents in developing chicken breast muscle was examined by two-dimensional gel electrophoresis. Quantitative analysis of the two-dimensional gels showed a moderate coordination in accumulation among contractile proteins (actin, tropomyosin and myosin light chains) during postnatal development in spite of their isoform transition. Creatine kinase was also accumulated coordinately with contractile proteins during development. In contrast, accumulation kinetics of glycolytic enzymes (glyceraldehyde-3-phosphate dehydrogenase, aldolase and enolase) showed discoordination with those of contractile proteins. These findings suggest that there are two distinct phases in muscle maturation: (1) structural maturation and (2) metabolic maturation.

Stress fibers in the mesenteric mesothelial cells of the large intestine of the bullfrog, Rana catesbeiana

Actin-containing cytoplasmic fibers were visualized in the mesenteric mesothelial cells of the large intestine of bullfrog tadpoles by rhodamine-phalloidin staining of en face preparations of mesothelial cells. These fibers were concurrently stained by immunofluorescence using antibodies to myosin or alpha-actinin. Electron microscopy showed the presence of bundles of microfilaments in the basal cytoplasm of the cells. Such fibers in the mesothelial cells may be comparable to the stress fibers present in cultured cells. The mesothelial cells initially formed axially oriented stress fibers when they changed from a rhombic to a slender spindle-like shape. On the other hand, stress fibers disappeared as cells transformed from elongated to polygonal shapes during the period of metamorphic climax. Expression of stress fibers in these cells appears to be related to the degree of tension loaded on the mesentery, which may be generated by mesenteric winding. These stress fibers in the mesothelial cells may serve to regulate cellular transformation. They may also help to maintain cellular integrity by strengthening the cellular attachment to subepithelial tissue against tensile stress exerted on the mesentery.

Development of smooth muscle: ultrastructural study of the chick embryo gizzard

The growth and differentiation of smooth muscle in the chicken gizzard were studied by electron microscopy from the 10th day in ovo to 6 months after hatching; during this period the organ grows 1000-fold in weight. At the earliest stage studied, smooth muscle cells, interstitial cells, and fibroblasts are immature but can already be clearly distinguished. The structural components of muscle cells develop in a characteristic sequence. Mitochondria are more abundant in immature muscle cells (8% in 14 days embryos and 7% in 19 days embryos) than in the adult (5%). Caveolae are virtually absent in the 11 day embryo; they become more common at the end of embryonic life, but continue to increase in relative frequency after hatching. Gap junctions appear around the 16th day in ovo as minute aggregates of connexons, which then grow in size, probably by addition of new connexons. In the earliest stages studied, myofilaments occupy 25% of the cell profile and are assembled into bundles accompanied by dense bodies and surrounded by loosely arranged intermediate filaments. By contrast, membrane-bound dense bands are scarce until the latter part of embryonic life, an observation suggesting that myofilament formation and alignment is not a process initiated near the cell membrane or directed by the cell membrane, and that only late in development bundles of myofilaments become extensively anchored to dense bands over the entire cell surface: at that time myofilaments occupy more than 75% of the cell volume. The muscle cells increase about four-fold in volume over the period studied; the 1000-fold increase in muscle volume is mainly accounted for by an increase in muscle cell number. Mitoses are found in the gizzard musculature at all embryonic ages with a peak at 17-19 days; they occur in muscle cells with a high degree of differentiation. These cells divide at a stage when they are packed with myofilaments and form junctions with neighbouring cells: the mitotic process affects the middle portion of the cell, which takes up an ovoid shape and eventually divides, whereas the remaining portions of the cell do not differ in appearance from the surrounding muscle cells. At all stages of development the population of muscle cells has a uniform appearance (apart from the cells in mitosis), and the growth and differentiation seem to proceed at the same pace in all the cells. There are no undifferentiated cells left behind in the tissue for later development.

Calcitonin gene-related peptide-binding sites of porcine cardiac muscles and coronary arteries: solubilization and characterization

Calcitonin gene-related peptide (CGRP)-binding sites were solubilized, using digitonin, from the porcine spinal cord, atria, and coronary arteries. The specific binding of 125I-human alpha-CGRP to the solubilized binding sites was inhibited by human alpha- and beta-CGRP and by rat alpha-CGRP, but not by angiotensin II or human calcitonin. Scatchard plot analysis of saturation gave the same KD value for CGRP in the crude membrane fractions of the tissues examined. The affinity of CGRP to the binding sites was decreased by solubilization in the atria and coronary arteries, but not in the spinal cord. Affinity labeling followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed distinct molecular sizes of the specific binding sites among the tissues; 70K for the spinal cord, 70K and 90K for the coronary arteries, and 70K and 120K for the atria. These results indicate that the molecular characteristics of the specific binding sites of CGRP in the cardiovascular system are distinct from those in the central nervous system.

Coordinate accumulation of troponin subunits in chicken breast muscle

The accumulation of troponin subunits in developing chicken breast muscle was determined by two-dimensional gel electrophoresis and an image analyzing system. Many troponin T isoforms, including those hidden behind creatine kinase, were detected on the two-dimensional pattern by the addition of 6 M urea in the second dimension. These troponin T isoforms were classified into four types by developmental order, isoelectric point, and molecular weight: leg-muscle type (L), neonatal breast-muscle type (BN), young chicken breast-muscle type (BC), and adult breast-muscle type (BA). The L-, BN-, and BC-type troponin Ts were transiently expressed at specific developmental stages. Quantitative analysis of two-dimensional patterns of troponin subunits including troponin I and troponin C showed moderate coordination in accumulation among the three subunits throughout postnatal development, when the total amount of all isoforms of troponin T was taken into account.

Expression of high and low molecular weight caldesmons during phenotypic modulation of smooth muscle cells

We investigated the expression of two molecular weight forms of caldesmon in a wide range of tissues and cells. The distribution of high molecular weight caldesmon (h-caldesmon, Mr 120,000-150,000) was restricted to smooth muscles where it was found in large quantity. The low molecular weight protein (l-caldesmon, Mr 70,000-80,000) was widely distributed in nonmuscle tissues and cells. Therefore, the expression of h-caldesmon might be much more specific to smooth muscles. We then examined the expressional changes of two caldesmons during phenotypic modulation of smooth muscle cells (SMCs). In developing gizzards, the expression of caldesmons switched from the l- to the h-form. Contrarily, the expression turned from h- to l-caldesmon in association with dedifferentiation of aortic SMCs in primary culture. In agreement with these observations, the levels of those mRNAs that direct the synthesis of both caldesmons were apparently in proportion to the quantities of protein, as determined by use of an in vitro translation system. In addition, h-caldesmon in smooth muscle-like BC3H1 cells increased in its amount with a concomitant reduction of l-caldesmon following serum-depleted and contact-inhibited cytodifferentiation. These results suggest that the expressional changes of two caldesmons are closely correlated with the phenotypic modulation of SMCs.

Induction of polyploidy in cultures of neonatal rat aortic smooth muscle cells

Arterial smooth muscle cells become tetraploid with age and hypertension. To further study this phenomenon, neonatal rat aortic smooth muscle cells were placed in cell culture and studied over time. Numerous cells with tetraploid and even octaploid DNA content appeared beginning in primary cultures. These increases in DNA content per cell were determined by quantitative fluorescence microscopy and flow cytometry, and true polyploidy was confirmed by chromosome counts. In contrast, cells from adult rat aortas failed to produce significant polyploid cells over time in culture. In vitro culture of neonatal aortic cells may therefore be a model system for studying the initiation of polyploidy in arterial smooth muscle.

Change of actin isomers during differentiation of smooth muscle

Changes of actin isomers during development and differentiation of chicken gizzard were investigated by polyacrylamide gel electrophoresis. The two-dimensional gel electrophoresis with SDS-polyacrylamide gels in the presence of urea as the second dimension clearly separated three actin isomers which appear during development of the smooth muscle. The three actin isomers change the relative concentrations during development as follows: gizzard-type gamma-actin begins to be synthesized late on the 7th day of embryogenesis and increases in amount until hatching, nonmuscle-type gamma-actin exists only at earlier stages (before 15 days of embryogenesis), and the amount of beta-actin increases in proportion to the decrease of nonmuscle type gamma-actin, the amount of nonmuscle actin in gizzards then becoming constant. Actin composition of gizzard before 7 days of embryonic age was nonmuscle type and consisted of beta-actin and nonmuscle-type gamma-actin. These observations indicate that developmental process of gizzard smooth muscle cells are classified as three stages: nonmuscle, intermediate and smooth muscle stages.