Cardiac fibronectin: developmental distribution and quantitative comparison of possible sites of synthesis

Fibronectin accumulation within cardiac myocytes in rats with elevated plasma angiotensin II

Elevation in plasma angiotensin II (AngII) is associated with cardiac myocyte necrosis. Myocyte necrosis followed by wound healing and fibrosis represents a structural remodeling of the myocardium thought to contribute to abnormal myocardial function. Fibronectin (FN) is generally considered an early component of the healing process that precedes collagen accumulation. To better understand the time course of this remodeling process involving both cardiac myocytes and extracellular matrix, (i.e., FN and collagen), we used two animal models: (1) endogenous activation of the renin-angiotensin system by surgical induction of renovascular hypertension and (2) exogenous AngII administration (150 ng/min/kg). Animals were killed at different time points within the first two weeks. Both "cellular" (cFN) and "plasma" (pFN) FN immunolabeling were compared with collagen distribution (picrosirius red stain), together with histopathologic (hematoxylin-eosin stain) and ultrastructural examination of cardiac myocytes. In each experimental group, the pattern and time course of FN immunolabeling was coincident with histopathologic evidence of myocyte injury and/or remodeling. We found different patterns of FN labeling of cardiac myocytes: (a) homogenous intracellular distribution in necrotic myocytes, most obvious on days 1 and 2; (b) patchy intracellular distribution in nonnecrotic myocytes starting on day 4; and (c) marking internalized capillaries. Both FNs were codistributed throughout the myocardium of each ventricle; however, cFN was less pronounced and not seen in mature scars. Ultrastructural examination revealed different kinds of intramyocytic inclusions, characterized by vacuoles containing fibrillar/flocculent material, remnants of unknown origin, or internalized capillaries. We conclude that FNs are markers of cardiac myocyte necrosis and early interstitial remodeling and that renovascular hypertension and AngII administration exhibit the same time course and pattern of FN and collagen expression.

Extracellular matrix profiles in the progression to heart failure. European Young Physiologists Symposium Keynote Lecture-Bratislava 2007

The myocardial extracellular matrix (ECM), which preserves the geometry and integrity of the myocardium, is a dynamic structure whose component proteins are maintained by a finely controlled homeostatic balance between deposition and degradation. One of the key targets in cardiology is the elucidation of the molecular mechanisms which mediate pathological remodelling of this matrix causing the transition from compensatory hypertrophy to congestive decompensated heart failure. In response to injury or increased workload, cardiac remodelling including myocyte hypertrophy, develops as the heart attempts to compensate for increased wall stresses. Persistence of these stresses over extended time periods leads to disruption of ECM homeostasis resulting in irreversible maladaptive cardiac remodelling, ventricular dilatation and finally heart failure. ECM remodelling is regulated by the matrix metalloproteinases (MMPs) and their endogenous inhibitors (TIMPs). Clinical studies and experimental models of cardiac disease states have reported alterations in the balance between the MMPs and TIMPs in the failing heart and crucially at intermediate time points in the progression to failure. This article reviews the recent clinical, genetic and experimental approaches employed to compare ECM, MMP and TIMP profiles in healthy, compensated and failing hearts and identifies common themes in the perturbation of ECM homeostasis in the transition to heart failure.

Expression of extracellular matrix components fibronectin and laminin in the human fetal heart

It has been well documented that the extracellular matrix components fibronectin and laminin promote or regulate morphogenesis of the myocardial cells in mammalian heart. However, their chronological change of expression (or localization) in the human heart remains elusive. In this study, fibronectin and laminin in the left ventricle of forty-two human fetuses aged from 8 to 26 weeks gestation and left ventricular tissues obtained from a 2-week old infant and two adults were investigated by Western blot analyses and indirect immunofluorescence technique with monoclonal antibodies. In the fetal heart, fibronectins were present along the endocardium, epicardium, and linings of larger blood vessels. In 14-16 weeks gestation, fibronectin immunofluorescence became stronger but not evenly dispersed in the interstitium. After 24 weeks gestation, they were strongly positive only in the relatively larger blood vessels, as well as those in the infant and adult cardiac tissues. Laminins were strongly positive along the endocardium and basement membrane of the myocardial cells and fibroblasts during fetal life. After birth, laminins formed fine fibrillar network along the basement membrane in association with the transverse tubules of myocardial cell; these morphological characteristics remained in the adult cardiac tissues. These results indicate that fibronectin expression is relatively constant during fetal life but decreases after birth; in contrast, laminin expression is not age-dependent and constant throughout the life.

Fibronectin concentrations correlate with ovarian follicular size and estradiol values in equine follicular fluid

The amounts of total protein, albumin, fibronectin, alpha 2-macroglobulin (alpha 2-M), immunoglobulin G, ceruloplasmin and antithrombin were determined in fluids collected from 53 preovulatory equine follicles and compared with the contents of estradiol-17 beta, progesterone and androstenedione, with follicle size and the amounts of the equivalent proteins in normal equine plasma. The concentration of fibronectin and the fibronectin/albumin ratios increased significantly with follicle size and with follicular estradiol levels. The alpha 2-M levels and alpha 2-M/albumin ratios correlated with follicle size but not with hormone content. Both fibronectin and alpha 2-M were present in lower amounts in follicular fluid compared with plasma while the other proteins were present in similar amounts. Among the proteins evaluated, there was a positive correlation between the amount of the protein in the follicular fluid and the molecular weight of the protein.

Matrix remodelling in dilated cardiomyopathy entails the occurrence of oncofetal fibronectin molecular variants

OBJECTIVES

To investigate whether disturbance of the cellular homoeostasis and integrity of cardiomyocytes in dilated cardiomyopathy (DCM) is accompanied by alterations in cell-matrix relations as indicated by changes in the deposition of fibronectin (FN) isoforms.

DESIGN

Tissue from a case series of patients with DCM was investigated by immunohistochemistry with antibodies against FN (all variants, clone IST4), ED-A+ FN (clone IST9), ED-B+ FN (clone BC1), and oncofetal glycosylated FN (clone 5C10). The sites of de novo synthesis of FN were demonstrated by means of non-radioactive RNA in situ hybridisation (ISH) with biotinylated FN cDNA fragments as the probe.

SETTING

University hospital.

PATIENTS

Samples from 10 patients with clinical criteria and histological diagnosis of DCM and from 3 individuals with normal hearts.

INTERVENTIONS

Samples were obtained by right ventricular endomyocardial biopsy.

MAIN OUTCOME MEASURE

Distribution of oncofetal FN variants in DCM hearts.

RESULTS

Immunostaining of FN (IST4, all variants) showed a coarse interstitial network in normal and diseased myocardium. ED-A+ FN was deposited as fine interstitial spots in normal myocardium and in DCM samples. Immunostaining for oncofetal glycosylated FN and ED-B+ FN was not seen in normal adult myocardium, whereas myocardium from DCM patients showed focal and delicate staining in the interstitium. RNA ISH showed that these deposits resulted from local FN synthesis.

CONCLUSION

The results accord with de novo expression of oncofetal FN variants in hearts from patients with DCM. The oncofetal FN variants may serve as disease markers in myocardium affected by DCM.

Fibronectin expression during physiological and pathological cardiac growth

Fibronectin (FN) is a dimeric glycoprotein found in the extracellular matrix of most tissues that serves as a bridge between cells and the interstitial collagen meshwork and influences diverse processes including cell growth, adhesion, migration, and wound repair. Multiple FN forms arise by the alternative splicing of a primary transcript originating from a single gene. The spatial and temporal alterations in FN expression in the myocardium has been studied in models of cardiac growth in vivo such as fetal development, and hypertrophy secondary to pressure overload. This review focuses on the differential expression of FN isoforms that are observed in different models of cardiac growth. Using a combination of qualitative and quantitative analyses it is shown that in the rat myocardium: (1) the FN phenotype is developmentally regulated, (2) the re-expression of the fetal FN isoforms is observed in different models of cardiac hypertrophy secondary to a sudden or progressive hypertension and (3) the changes in cardiac FN expression affect mostly the coronary artery smooth muscle cells.

Role of extracellular matrix proteins in heart function

The cardiac interstitium is populated by nonmyocyte cell types including transcriptionally active cardiac fibroblasts and endothelial cells. Since these cells are the source of many components of the cardiac extracellular matrix, and because changes in cardiac extracellular matrix are suspected of contributing to the genesis of cardiovascular complications in disease states such as diabetes, hypertension, cardiac hypertrophy and congestive heart failure, interest in the mechanisms of activation of fibroblasts and endothelial cells has led to progress in understanding these processes. Recent work provides evidence for the role of the renin-angiotensin-aldosterone system in the pathogenesis of abnormal deposition of extracellular matrix in the cardiac interstitium during the development of inappropriate cardiac hypertrophy and failure. The cardiac extracellular matrix is also known to change in response to altered cardiac performance associated with post-natal aging, and in response to environmental stimuli including intermittent hypoxia and abnormal nutrition. It is becoming clear that the extracellular matrix mainly consists of molecules of collagen types I and III; they form fibrils and provide most of the connective material for typing together myocytes and other structures in the myocardium and thus is involved in the transmission of developed mechanical force. The data available in the literature support the view that the extracellular matrix is a dynamic entity and alterations in this structure result in the development of heart dysfunction.

The extracellular matrix glycoproteins BM-90 and tenascin are expressed in the mesenchyme at sites of endothelial-mesenchymal conversion in the embryonic mouse heart

BM-90 is a novel glycoprotein initially isolated from the extracellular matrix of a mouse tumor. We here studied the expression of BM-90 during embryonic development of the mouse heart and compared its expression pattern with that of tenascin and laminin. Distribution was studied by immunofluorescence using antibodies specifically raised against mouse BM-90, laminin and tenascin. Some expression of BM-90 was seen in myocardial basement membranes at early developmental stages, but expression abruptly decreased from these sites at day 12 of embryogenesis. Laminin B chains were also found in the muscle basement membranes early but did not decrease with advancing development. The most striking observation was the markedly enriched expression of BM-90 in the endocardial cushion tissue (ECT). The ECT is derived from mesenchymal cells converted from endothelium and they will form the cardiac valves and septa. In the ECT, BM-90 showed considerable co-distribution with tenascin, but tenascin expression was more focal and did not mark all areas of the ECT. Northern blot data show that BM-90 and tenascin were produced by the developing heart. With antibodies detecting A, B1 and B2 chains of mouse laminin, no immunoreactivity was seen in the ECT. Our data thus show clear-cut differences in the molecular composition of the ECT and muscle basement membranes in the developing heart. The focal expression of BM-90 in the ECT suggests that BM-90 could be involved in epithelial-mesenchymal transitions.

Age-related changes in fibronectin expression in spontaneously hypertensive, Wistar-Kyoto, and Wistar rat hearts

The effects of age and blood pressure on fibronectin expression in the rat heart were studied in the normotensive Wistar and Wistar-Kyoto (WKY) strains and in the spontaneously hypertensive rat (SHR). Fibronectin mRNA expression decreased threefold between 10 and 40 weeks of age in Wistar hearts, with changes of similar magnitude occurring between 6 and 24 weeks in WKY rats. In contrast, no decrease in fibronectin mRNA was observed in SHR hearts during this time span. These results are in contrast to changes observed previously in the aorta, where an increase in fibronectin mRNA occurred with age in all three rat strains. Ribonuclease protection analysis showed a small age-specific increase in the relative content of EIIIA+ fibronectin mRNA isoforms in hearts from Wistar rats, whereas no change was found in the relative amount of either EIIIA or EIIIB isoforms in SHR hearts. Changes similar to those observed for fibronectin mRNA, although of different magnitudes, were observed in mRNA levels for collagen alpha 1(III) and beta 1 integrin. In Wistar hearts, collagen alpha 1(III) mRNA levels decreased fivefold to sixfold between 10 and 40 weeks of age, whereas a twofold to threefold decrease in beta 1 integrin was observed in WKY hearts between 6 and 24 weeks of age. Western blot analysis revealed a positive correlation between fibronectin mRNA and protein for age-dependent changes in ventricular tissue but not in the atria, suggesting that the regulation of fibronectin expression during the changes common to both aging and hypertrophy could involve both transcriptional and posttranscriptional mechanisms.

Fibronectin and laminin in transverse tubules of cardiac myocytes studied by laser confocal microscopy and immunocytochemistry

The distribution of two non-collagenous glycoproteins of high molecular weight, fibronectin (FN) and laminin (LMN), was investigated in myocardial cells from the ventricle of rats, and from biopsies collected from the auricle of patients undergoing a coronary bypass operation. In order to elucidate the expression of FN and LMN across cells, non-invasive serial sectioning has been carried out by laser scanning confocal microscopy of frozen, immunostained tissue sections. In addition, immunoelectron microscopy was used to study the distribution of these antigens at higher magnifications. These studies show that FN is part of the basement membrane of the surface sarcolemma of both ventricular and atrial cells, in addition to being an abundant protein of the extracellular matrix (ECM). Along transverse tubular(TT)-membranes, FN was only detected in tubules exceeding 200 nm in diameter. Even here, the intensity of labelling varied greatly and was generally low. By contrast, a heavy investment of LMN was organized in the basal lamina along the surface sarcolemma and along ramifications of the entire TT-system in ventricular heart muscle cells. In this way, the network of TT-membrane systems of working heart muscle cells provides a supply of LMN to all depths of the myocardial fibre. In human atrial muscle cells, a regular TT-system appears to be absent. Instead occasional, deep sarcolemmal invaginations occur with diameters of 300-500 nm, the surfaces of which are also invested with LMN. The significance of the present findings has been discussed, with special reference to LMN as a possible component of a series of proteins involved in transmembrane communication between the ECM and the sarcoplasm.

Rapid expression of fibronectin in the rabbit heart after myocardial infarction with and without reperfusion

The expression of fibronectin in the repair process after myocardial infarction was studied using two protocols of coronary occlusion in the rabbit: a permanent occlusion or 3 h of occlusion followed by reperfusion (too late for salvage). We found a rapid and progressive increase in cardiac fibronectin expression in the infarcted region of the ventricle. Steady-state mRNA levels for fibronectin increased 13- and 16-fold, respectively, in the permanent and reperfused infarcts 1 d postinfarction. Immunological detection of the protein with a polyclonal antibody against plasma fibronectin showed significant increases of the protein fibronectin in the infarcted myocardium by day 3 in the reperfused group and by day 5 in the permanent coronary occlusion group. Ribonuclease protection assays established the induction of EIIIB containing fibronectin mRNA in both models by day 1 and use of a monoclonal antibody showed an increase in the EIIIA isoform 2 d postinfarction. Increases in steady-state mRNA levels for several collagen types were found in both groups, but these changes occurred after those noted for fibronectin. Thus fibronectin mRNA and protein expression increased rapidly postinfarction suggesting a functional role in the repair process.

Developmental modulation of embryonic cardiac myocyte adhesion to cardiac collagens in vitro

The goal of this investigation is to identify molecules that mediate embryonic cardiac myocyte adhesion during chick cardiac morphogenesis. The assay used employs culturing embryonic myocytes on substrata containing embryonic heart proteins separated by molecular weight. This assay shows that embryonic myocytes from 10- to 14-day-old embryos will bind to 140,000 and 128,000 Da proteins present in embryonic hearts and do not require Mg2+ or Ca2+ for adhesion. Myocytes from embryos younger than 10 days or older than 14 days display little or no binding. Embryonic heart fibroblasts collected at these same ages do not bind to these proteins. The 140- and 128-kDa proteins were found to copurify in extraction procedures for procollagens. Amino acid analysis shows that both proteins contain high glycine and hydroxyproline, indicating that they are collagens. However, glycine and imino acid levels are low relative to other known collagens, indicating a nonhelical domain present in each molecule and most closely resembled levels present in procollagens. Immunoblots show that antisera to chick collagen type I recognizes the 128-kDa protein while anti-collagen type III recognizes the 140-kDa protein. Monoclonal antibodies to the amino terminal propeptide of collagen type I recognize the 128-kDa protein in immunoblotting procedures. Embryonic chick myocytes bind to 140/128 kDa proteins present in extracts of sympathetic trunk, although they do not bind to 140/128 kDa proteins in embryonic tendon. The findings thereby indicate that forms of type III and type I collagens in embryonic heart support direct adhesion of embryonic myocytes for a restricted period of cardiac myogenesis and that these proteins differ from collagen types I and III present in other tissues and from fully processed collagen types I and III.

Fibronectin expression in the normal and hypertrophic rat heart

We examined changes in the expression of fibronectin during the induction of cardiac hypertrophy by L-triiodothyronine administration and by mineralocorticoid- and salt-induced experimental hypertension. By use of Northern and Western blotting procedures, fibronectin was localized mainly in the atria of normal rat hearts. Atria contained 10- and 5-fold higher relative concentrations of fibronectin mRNA and protein, respectively, compared with ventricles. During the progression of cardiac hypertrophy induced by L-triiodothyronine over a 10-d period, there was a progressive increase in fibronectin mRNA for the first 6 d followed by a return to control levels. The major change could be accounted for by changes in ventricular mRNA, which increased about four- to sixfold. In contrast, protein levels in ventricles increased progressively over the 10-d treatment period. Ribonuclease protection analysis indicated that the relative amounts of fibronectin isoforms containing exons designated EIIIA and EIIIB increased during the progression of hypertrophy. When cardiac hypertrophy was induced by mineralocorticoid and salt treatment, increases in ventricular fibronectin mRNA and protein and the induction of alternatively spliced forms of fibronectin were also observed. However, the extent and temporal pattern of fibronectin expression differed between the two experimental models.

Extracellular matrix of the developing heart in normal and cardiac lethal mutant axolotls, Ambystoma mexicanum

As part of an ongoing study of heart development in normal and cardiac lethal mutant axolotls (Mexican salamanders) we examined the extracellular matrix (ECM) by microscopical methods. With scanning electron microscopy we are unable to detect ECM on the apical surface of cells of the early cardiogenic mesoderm. During the period of lateral plate migration, which coincides with the period of cardiogenic induction of mesoderm by anterior endoderm, there is little ECM, aside from some microfibrils, on the basal surface of the endoderm or mesoderm of the pharyngeal region. Later, a basal lamina (BL) is found on the endoderm and along portions of the developing endocardial and myocardial tubes. By the time of heartbeat initiation the BLs are complete and invested with striated collagen-like fibrils that are sparsely distributed in the "cardiac jelly" of normal and mutant hearts. Striated fibril deposition, which increases with time, is generally random in orientation, with the exception of some regions where there is a preferred directionality. During the post-hatching period striated fibrils appear in the subepicardial space. In addition, branching fibers that are probably elastin appear in the bulbus arteriosus. In these later stages the density of fibrils in the cardiac lethal mutant heart is much less than normal. Indirect immunofluorescent microscopy reveals laminin and fibronectin in the basal laminae of the endocardial and myocardial tubes of both normal and cardiac lethal mutant hearts. In addition, punctate and fibrillar staining for fibronectin, and punctate staining for laminin are found in the cardiac jelly. These matrix proteins are not abundant at the apical (exterior) surface of the myocardium until the epicardium appears.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanically induced orientation of adult rat cardiac myocytes in vitro

A population of freshly isolated adult rat cardiac myocytes is spatially oriented using a computerized mechanical cell stimulator device for tissue cultured cells. A continuous unidirectional stretch of the substratum at 60 to 400 micron/min for 120 to 30 min, respectively, during the cell attachment period in serum-free medium induces a significant three-fold increase in the number of rod-shaped myocytes oriented parallel to the direction of movement. The myocytes orient less well with unidirectional substratum stretching after their adhesion to the substratum. In contrast, adult myocytes plated onto a substratum undergoing continuous 10% stretch-relaxation cycling show no significant change in myocyte orientation or cytoskeletal organization. Orientation of rod-shaped myocytes is dependent on several factors other than the type of mechanical activity. These include: a) the speed of substratum movement; b) the final stretch amplitude; and c) the timing between initiation of substratum stretching and adhesion of myocytes to the substratum. Oriented adult rod shaped myocytes representing 65 to 70% of the total myocyte population in this model system can now be submitted to different patterns of repetitive mechanical stimulation for the study of stretch-induced alterations in cell growth and gene expression.

Changes in lung tissue fibronectin content and synthesis during postnatal lung growth

We studied changes in lung tissue fibronectin content and synthesis during postnatal lung growth in rats. We reasoned that fibronectin, which is important in cell differentiation, migration, and adhesion, and in the organization of the extracellular matrix, might play a role in the rapid cell proliferation and alveolar septal formation that occurs postnatally in mammalian lungs. Newborn rats were sacrificed at 4, 7, 11, 14, and 21 days after birth. The lungs were perfused and lavaged, tissue fibronectin was extracted using urea and heparin (Bray et al, Science 1981; 214:793) and the extracted fibronectin was measured by enzyme-linked immunoassay. Tissue fibronectin synthesis was measured by the in vivo incorporation of 35S-methionine into fibronectin that was extracted from lung tissue and immunoprecipitated. Lavage fibronectin and albumin content and lung tissue collagen (hydroxyproline) content were also determined. Lung tissue fibronectin content per g dry lung almost doubled between days 4 and 7 after birth, was slightly higher at day 14 than at day 7, and decreased sharply between days 14 and 21. Lung tissue fibronectin synthesis per g dry lung increased steadily between days 4 and 14 to reach a peak value of about 2.5 times the 4-day value; it then decreased sharply between days 14 and 21. The period of increased fibronectin content and synthesis (4 to 14 days) coincided with the period during which lung cell proliferation and secondary alveolar septa formation are known to be the most active, and the sharp decrease in fibronectin content and synthesis (between 14 and 21 days) coincided with the period during which lung growth and remodeling markedly decrease.(ABSTRACT TRUNCATED AT 250 WORDS)

Fibronectin distribution during early chick embryo heart development

The distribution of fibronectin (FN) during early stages of chick embryo heart development has been studied by indirect immunofluorescence methods. The cardiac extracellular matrix (cardiac jelly) was almost devoid of FN-positive material throughout the period studied (stages 8-18). Intensely extracellular fluorescent material was only demonstrated at the heart midline and in the dorsal mesocardium. Fluorescence associated with the basal surface of the myocardium was demonstrated first at the time of fusion of the two heart tubes. While the heart remains attached to the embryonic trunk by the dorsal mesocardium, two different myocardial basal zones can be distinguished according to the intensity of fluorescence: an intensely stained dorsal zone and a much less fluorescent ventral zone. The endocardium did not present a strongly fluorescent basement membrane until stage 13. The intensity of fluorescence of the endocardial basal surface varied according to the rostrocaudal levels of the heart and also to the development stage of the embryo. The levels of fluorescence increased in myocardium and endocardium at the onset of trabeculation but decreased as trabeculation was completed. The quantitative and qualitative variations of FN distribution have been associated with a number of developmental events.