Development of a bioactive tunable hyaluronic-protein bioconjugate hydrogel for tissue regenerative applications

Every year, there are approximately 500 000 peripheral nerve injury (PNI) procedures due to trauma in the US alone. Autologous and acellular nerve grafts are among current clinical repair options; however, they are limited largely by the high costs associated with donor nerve tissue harvesting and implant processing, respectively. Therefore, there is a clinical need for an off-the-shelf nerve graft that can recapitulate the native microenvironment of the nerve. In our previous work, we created a hydrogel scaffold that incorporates mechanical and biological cues that mimic the peripheral nerve microenvironment using chemically modified hyaluronic acid (HA). However, with our previous work, the degradation profile and cell adhesivity was not ideal for tissue regeneration, in particular, peripheral nerve regeneration. To improve our previous hydrogel, HA was conjugated with fibrinogen using Michael-addition to assist in cell adhesion and hydrogel degradability. The addition of the fibrinogen linker was found to contribute to faster scaffold degradation via active enzymatic breakdown, compared to HA alone. Additionally, cell count and metabolic activity was significantly higher on HA conjugated fibrinogen compared previous hydrogel formulations. This manuscript discusses the various techniques deployed to characterize our new modified HA fibrinogen chemistry physically, mechanically, and biologically. This work addresses the aforementioned concerns by incorporating controllable degradability and increased cell adhesivity while maintaining incorporation of hyaluronic acid, paving the pathway for use in a variety of applications as a multi-purpose tissue engineering platform.

The SARS-CoV-2 spike protein induces long-term transcriptional perturbations of mitochondrial metabolic genes, causes cardiac fibrosis, and reduces myocardial contractile in obese mice

BACKGROUND

As the pandemic evolves, post-acute sequelae of CoV-2 (PASC) including cardiovascular manifestations have emerged as a new health threat. This study aims to study whether the Spike protein plus obesity can exacerbate PASC-related cardiomyopathy.

METHODS

A Spike protein-pseudotyped (Spp) virus with the proper surface tropism of SARS-CoV-2 was developed for viral entry assay in vitro and administration into high fat diet (HFD)-fed mice. The systemic viral loads and cardiac transcriptomes were analyzed at 2 and 24 h, 3, 6, and 24 weeks post introducing (wpi) Spp using RNA-seq or real time RT-PCR. Echocardiography was used to monitor cardiac functions.

RESULTS

Low-density lipoprotein cholesterol enhanced viral uptake in endothelial cells, macrophages, and cardiomyocyte-like H9C2 cells. Selective cardiac and adipose viral depositions were observed in HFD mice but not in normal-chow-fed mice. The cardiac transcriptional signatures in HFD mice at 3, 6, and 24 wpi showed systemic suppression of mitochondria respiratory chain genes including ATP synthases and nicotinamide adenine dinucleotide:ubiquinone oxidoreductase gene members, upregulation of stress pathway-related crucial factors such as nuclear factor-erythroid 2-related factor 1 and signal transducer and activator of transcription 5A, and increases in expression of glucose metabolism-associated genes. As compared with the age-matched HFD control mice, cardiac ejection fraction and fractional shortening were significantly decreased, while left ventricular end-systolic diameter and volume were significantly elevated, and cardiac fibrosis was increased in HFD mice at 24 wpi.

CONCLUSION

Our data demonstrated that the Spike protein could induce long-term transcriptional suppression of mitochondria metabolic genes and cause cardiac fibrosis and myocardial contractile impairment in obese mice, providing mechanistic insights to PASC-related cardiomyopathy.

The SARS-CoV-2 Spike protein induces long-term transcriptional perturbations of mitochondrial metabolic genes, causes cardiac fibrosis, and reduces myocardial contractile in obese mice

Background

As the pandemic evolves, post-acute sequelae of CoV-2 (PACS) including cardiovascular manifestations have emerged as a new health threat. This study aims to study whether the Spike protein plus obesity can exacerbate PACS-related cardiomyopathy.

Methods

A Spike protein-pseudotyped (Spp) virus with the proper surface tropism of SARS-CoV-2 was developed for viral entry assay in vitro and administration into high fat diet (HFD)-fed mice. The systemic viral loads and cardiac transcriptomes were analyzed at 2 and 24 hrs, 3, 6, and 24 weeks post introducing (wpi) Spp using RNA-seq or real time RT-PCR. Echocardiography was used to monitor cardiac functions.

Results

Low-density lipoprotein cholesterol enhanced viral uptake in endothelial cells, macrophages, and cardiomyocyte-like H9C2 cells. Selective cardiac and adipose viral depositions were observed in HFD mice but not in normal-chow-fed mice. The cardiac transcriptional signatures in HFD mice at 3, 6, and 24 wpi showed systemic suppression of mitochondria respiratory chain genes including ATP synthases and nicotinamide adenine dinucleotide:ubiquinone oxidoreductase gene members, upregulation of stress pathway-related crucial factors such as nuclear factor-erythroid 2-related factor 1 and signal transducer and activator of transcription 5A, and increases in expression of glucose metabolism-associated genes. As compared with the age-matched HFD control mice, cardiac ejection fraction and fractional shortening were significantly decreased, while left ventricular end-systolic diameter and volume were significantly elevated, and cardiac fibrosis was increased in HFD mice at 24 wpi.

Conclusion

Our data demonstrated that the Spike protein could induce long-term transcriptional suppression of mitochondria metabolic genes and cause cardiac fibrosis and myocardial contractile impairment, providing mechanistic insights to PACS-related cardiomyopathy.

Small-diameter artery decellularization: Effects of anionic detergent concentration and treatment duration on porcine internal thoracic arteries

Engineered replacement materials have tremendous potential for vascular applications where over 400,000 damaged and diseased blood vessels are replaced annually in the United States alone. Unlike large diameter blood vessels, which are effectively replaced by synthetic materials, prosthetic small-diameter vessels are prone to early failure, restenosis, and reintervention surgery. We investigated the differential response of varying 0%-6% sodium dodecyl sulfate and sodium deoxycholate anionic detergent concentrations after 24 and 72 h in the presence of DNase using biochemical, histological, and biaxial mechanical analyses to optimize the decellularization process for xenogeneic vascular tissue sources, specifically the porcine internal thoracic artery (ITA). Detergent concentrations greater than 1% were successful at removing cytoplasmic and cell surface proteins but not DNA content after 24 h. A progressive increase in porosity and decrease in glycosaminoglycan (GAG) content was observed with detergent concentration. Augmented porosity was likely due to the removal of both cells and GAGs and could influence recellularization strategies. The treatment duration on the other hand, significantly improved decellularization by reducing DNA content to trace amounts after 72 h. Prolonged treatment times reduced laminin content and influenced the vessel's mechanical behavior in terms of altered circumferential stress and stretch while further increasing porosity. Collectively, DNase with 1% detergent for 72 h provided an effective and efficient decellularization strategy to be employed in the preparation of porcine ITAs as bypass graft scaffolding materials with minor biomechanical and histological penalties.

Effects of emodin, a plant-derived anthraquinone, on TGF-β1-induced cardiac fibroblast activation and function

Cardiac fibrosis accompanies a number of pathological conditions and results in altered myocardial structure, biomechanical properties and function. The signaling networks leading to fibrosis are complex, contributing to the general lack of progress in identifying effective therapeutic approaches to prevent or reverse this condition. Several studies have shown protective effects of emodin, a plant-derived anthraquinone, in animal models of fibrosis. A number of questions remain regarding the mechanisms whereby emodin impacts fibrosis. Transforming growth factor beta 1 (TGF-β1) is a potent stimulus of fibrosis and fibroblast activation. In the present study, experiments were performed to evaluate the effects of emodin on activation and function of cardiac fibroblasts following treatment with TGF-β1. We demonstrate that emodin attenuates TGF-β1-induced fibroblast activation and collagen accumulation in vitro. Emodin also inhibits activation of several canonical (SMAD2/3) and noncanonical (Erk1/2) TGF-β signaling pathways, while activating the p38 pathway. These results suggest that emodin may provide an effective therapeutic agent for fibrosis that functions via specific TGF-β signaling pathways.

Spike protein of SARS-CoV-2 activates macrophages and contributes to induction of acute lung inflammation in male mice

The spike protein of severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) plays a crucial role in mediating viral entry into host cells. However, whether it contributes to pulmonary hyperinflammation in patients with coronavirus disease 2019 is not well known. In this study, we developed a spike protein–pseudotyped (Spp) lentivirus with the proper tropism of the SARS‐CoV‐2 spike protein on the surface and determined the distribution of the Spp lentivirus in wild‐type C57BL/6J male mice that received an intravenous injection of the virus. Lentiviruses with vesicular stomatitis virus glycoprotein (VSV‐G) or with a deletion of the receptor‐binding domain (RBD) in the spike protein [Spp (∆RBD)] were used as controls. Two hours postinfection (hpi), there were 27‐75 times more viral burden from Spp lentivirus in the lungs than in other organs; there were also about 3‐5 times more viral burden from Spp lentivirus than from VSV‐G lentivirus in the lungs, liver, kidney, and spleen. Deletion of RBD diminished viral loads in the lungs but not in the heart. Acute pneumonia was observed in animals 24 hpi. Spp lentivirus was mainly found in SPC⁺ and LDLR⁺ pneumocytes and macrophages in the lungs. IL6, IL10, CD80, and PPAR‐γ were quickly upregulated in response to infection in the lungs as well as in macrophage‐like RAW264.7 cells. Furthermore, forced expression of the spike protein in RAW264.7 cells significantly increased the mRNA levels of the same panel of inflammatory factors. Our results demonstrated that the spike protein of SARS‐CoV‐2 confers the main point of viral entry into the lungs and can induce cellular pathology. Our data also indicate that an alternative ACE2‐independent viral entry pathway may be recruited in the heart and aorta.

Longitudinal histomechanical heterogeneity of the internal thoracic artery

The internal thoracic artery (ITA) is the principal choice for coronary artery bypass grafting (CABG) due to its mechanical compatibility, histological composition, anti-thrombogenic lumen, and single anastomotic junction. Originating at the subclavian artery, traversing the thoracic cavity, and terminating at the superior epigastric and musculophrenic bifurcation, bilateral ITAs follow a protracted circuitous pathway. The physiological hemodynamics, anatomical configuration, and perivascular changes that occur throughout this length influence the tissue's microstructure and gross mechanical properties. Since histomechanics play a major role in premature graft failure we used inflation-extension testing to quantify the regional material and biaxial mechanical properties at four distinct locations along the left (L) and right (R) ITA and fit the results to a structurally-motivated constitutive model. Our comparative analysis of 44 vessel segments revealed a significant increase in the amount of collagen but not smooth muscle and a significant decrease in elastin and elastic lamellae present with distance from the heart. A subsequent decrease in the total deformation energy and isotropic contribution to the strain energy was present in the LITA but not RITA. Circumferential stress and compliance generally decreased along the length of the LITA while axial stress increased in the RITA. When comparing RITAs to LITAs, some morphological and histological differences were found in proximal sections while distal sections revealed differences predominantly in compliance and axial stress. Overall, this information can be used to better guide graft selection, graft preparation, and xenograft-based tissue-engineering strategies for CABG.

Spike Protein of SARS-CoV-2 Activates Macrophages and Contributes to Induction of Acute Lung Inflammations in Mice

Background

Coronavirus disease 2019 (COVID-19) patients exhibit multiple organ malfunctions with a primary manifestation of acute and diffuse lung injuries. The Spike protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is crucial to mediate viral entry into host cells; however, whether it can be cellularly pathogenic and contribute to pulmonary hyper-inflammations in COVID-19 is not well known.

Methods and Findings

In this study, we developed a Spike protein-pseudotyped (Spp) lentivirus with the proper tropism of SARS-CoV-2 Spike protein on the surface and tracked down the fate of Spp in wild type C57BL/6J mice receiving intravenous injection of the virus. A lentivirus with vesicular stomatitis virus glycoprotein (VSV-G) was used as the control. Two hours post-infection (hpi), Spp showed more than 27-75 times more viral burden in the lungs than other organs; it also exhibited about 3-5 times more viral burden than VSV-G lentivirus in the lungs, liver, kidney and spleen. Acute pneumonia was evident in animals 24 hpi. Spp lentivirus was mainly found in LDLR+ macrophages and pneumocytes in the lungs, but not in MARC1+ macrophages. IL6, IL10, CD80 and PPAR-γ were quickly upregulated in response to infection of Spp lentivirus in the lungs in vivo as well as in macrophage-like RAW264.7 cells in vitro. We further confirmed that forced expression of the Spike protein in RAW264.7 cells could significantly increase the mRNA levels of the same panel of inflammatory factors.

Conclusions

Our results demonstrate that the Spike protein of SARS-CoV-2 alone can induce cellular pathology, e.g. activating macrophages and contributing to induction of acute inflammatory responses.

Roles of miRNAs in spinal cord injury and potential therapeutic interventions

Spinal cord injury (SCI) affects approximately 200,000 individuals per year worldwide. There are more than 27 million people worldwide living with long-term disability due to SCI. Historically, it was thought that the central nervous system (CNS) had little ability for regeneration; however, more recent studies have demonstrated potential for repair within the CNS. Because of this, there exists a renewed interest in the discovery of novel approaches to promote regeneration in the CNS including the spinal cord. It is important to know the roles of the microRNAs (miRNAs) in modulation of pathogenesis in SCI and the potentials of the miRNA-based clinical interventions for controlling post-injury symptoms and improving functional recovery. The miRNAs, which are non-coding RNAs with an average of 22 nucleotides in length, are post-transcriptional gene regulators that cause degradation of the target mRNAs and thus negatively control their translation. This review article focuses on current research related to miRNAs and their roles in modulating SCI symptoms, asserting that miRNAs contribute to critical post-SCI molecular processes including neuroplasticity, functional recovery, astrogliosis, neuropathic pain, inflammation, and apoptosis. In particular, miR-96 provides a promising therapeutic opportunity to improve the outcomes of clinical interventions, including the way SCI injuries are evaluated and treated.

Effects of the isothiocyanate sulforaphane on TGF-β1-induced rat cardiac fibroblast activation and extracellular matrix interactions

An important step in many pathological conditions, particularly tissue and organ fibrosis, is the conversion of relatively quiescent cells into active myofibroblasts. These are highly specialized cells that participate in normal wound healing but also contribute to pathogenesis. These cells possess characteristics of smooth muscle cells and fibroblasts, have enhanced synthetic activity secreting abundant extracellular matrix components, cytokines, and growth factors, and are capable of generating contractile force. As such, these cells have become potential therapeutic targets in a number of disease settings. Transforming growth factor β (TGF-β) is a potent stimulus of fibrosis and myofibroblast formation and likewise is an important therapeutic target in several disease conditions. The plant-derived isothiocyanate sulforaphane has been shown to have protective effects in several pathological models including diabetic cardiomyopathy, carcinogenesis, and fibrosis. These studies suggest that sulforaphane may be an attractive preventive agent against disease progression, particularly in conditions involving alterations of the extracellular matrix and activation of myofibroblasts. However, few studies have evaluated the effects of sulforaphane on cardiac fibroblast activation and their interactions with the extracellular matrix. The present studies were carried out to determine the potential effects of sulforaphane on the conversion of quiescent cardiac fibroblasts to an activated myofibroblast phenotype and associated alterations in signaling, expression of extracellular matrix receptors, and cellular physiology following stimulation with TGF-β1. These studies demonstrate that sulforaphane attenuates TGF-β1-induced myofibroblast formation and contractile activity. Sulforaphane also reduces expression of collagen-binding integrins and inhibits canonical and noncanonical TGF-β signaling pathways.

Type II diabetes promotes a myofibroblast phenotype in cardiac fibroblasts

AIMS

Cardiovascular disease is the leading cause of death for individuals diagnosed with type II diabetes mellitus (DM). Changes in cardiac function, left ventricular wall thickness and fibrosis have all been described in patients and animal models of diabetes; however, the factors mediating increased matrix deposition remain unclear. The goal of this study was to evaluate whether cardiac fibroblast function is altered in a rat model of type II DM.

MAIN METHODS

Cardiac fibroblasts were isolated from 14 week old Zucker diabetic and lean control (LC) adult male rat hearts. Fibroblasts were examined for their ability to remodel 3-dimensional collagen matrices, their adhesion, migration and proliferation on collagen and changes in gene expression associated with collagen remodeling.

KEY FINDINGS

Cardiac fibroblasts from diabetic animals demonstrated significantly greater ability to contract 3-dimensional collagen matrices compared to cardiac fibroblasts from LC animals. The enhanced contractile behavior was associated with an increase in diabetic fibroblast proliferation and elevated expression of α-smooth muscle actin and type I collagen, suggesting the transformation of diabetic fibroblasts into a myofibroblast phenotype.

SIGNIFICANCE

Cardiac fibrosis is a common complication in diabetic cardiomyopathy which may contribute to the observed cardiac dysfunction associated with this disease. Identifying and understanding the changes in fibroblast behavior which contribute to the increased deposition of collagen and other matrix proteins may provide novel therapeutic targets for reducing the devastating effects of diabetes on the heart.

Mechanical loading promotes mast cell degranulation via RGD-integrin dependent pathways

Mast cells are known to respond to a number of stimuli, such as IgE antibody-antigen complexes, pathogens, chemical compounds, and physical stimulation, resulting in the activation of these cells and subsequent release of cytokines, inflammatory mediators and granules which can influence the pathophysiology of neighboring cells. Although different forms of physical stimulation (i.e. shear stress and acupuncture) have been investigated, the effect of cyclic tensile loading on mast cell activation has not. To characterize the response of mast cells to tensile loading, RBL-2H3 cells were embedded in a 3-dimensional fibrin construct and subjected to 24h of cyclic loading at 0%, 5% or 10% peak tensile strain. Mechanical loading significantly increased RBL-2H3 cell secretion of β-hexosaminidase (2.1- to 2.3-fold, respectively) in a load- and time-dependent manner when compared to the controls. Furthermore, no evidence of load-induced cell death or alterations in cell proliferation was observed. To determine if RGD-dependent integrins mediated the degranulation of mast cells during mechanical loading, cell-matrix interactions were inhibited by treating the cells with echistatin, a disintegrin that binds RGD-dependent integrins. Treatment with echistatin significantly attenuated load-induced degranulation without compromising cell viability. These results suggest a novel mechanism through which mechanical loading induces mast cell activation via RGD binding integrins.

Effects of interleukin-33 on cardiac fibroblast gene expression and activity

Interleukin-33 (IL-33) is a recently described member of the interleukin-1 (IL-1) family. It is produced by diverse cell types in response to a variety of stresses including hemorrhage and increased mechanical load. Though only relatively recently discovered, IL-33 has been shown to participate in several pathological processes including promoting type 2 T helper cell-associated autoimmune diseases. In contrast, IL-33 has been also found to have protective effects in cardiovascular diseases. Recent studies have illustrated that IL-33 attenuates cardiac fibrosis induced by increased cardiovascular load in mice (transaortic constriction). Since cardiac fibrosis is largely dependent on increased production of extracellular matrix by cardiac fibroblasts, we hypothesized that IL-33 directly inhibits pro-fibrotic activities of these cells. Experiments have been carried out with isolated rat cardiac fibroblasts to evaluate the effects of IL-33 on the modulation of cardiac fibroblast gene expression and function to test this hypothesis. The expression of the IL-33 receptor, interleukin-1 receptor-like 1 (ST2), was detected at the mRNA and protein levels in isolated adult rat cardiac fibroblasts. Subsequently, the effects of IL-33 treatment (0-100 ng/ml) on the expression of extracellular matrix proteins and pro-inflammatory cytokines/chemokines were examined as well as the effects on rat cardiac fibroblast activities including proliferation, collagen gel contraction and migration. While IL-33 did not directly inhibit collagen I and collagen III production, it yielded a dose-dependent increase in the expression of interleukin-6 and monocyte chemotactic protein-1. Treatment of rat cardiac fibroblasts with IL-33 also impaired the migratory activity of these cells. Further experiments illustrated that IL-33 rapidly activated multiple signaling pathways including extracellular signal-regulated kinases, p38 mitogen-activated protein kinase, c-Jun N-terminal kinases and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) in a dose-dependent manner. Experiments were carried out with pharmacological inhibitors to determine the role of specific signaling pathways in the response of fibroblasts to IL-33. These experiments illustrated that the activation of p38 mitogen-activated protein kinase and extracellular signal-regulated kinases are critical to the increased production of interleukin-6 and monocyte chemotactic protein-1 in response to IL-33. These studies suggest that IL-33 has an important role in the modulation of fibroblast function and gene expression. Surprisingly, IL-33 had no effect on the expression of genes encoding extracellular matrix components or on proliferation, markers typical of fibrosis. The major effects of IL-33 detected in these studies included inhibition of cell migration and activation of cytokine/chemokine expression. The previously reported inhibition of cardiac fibrosis may include more complicated mechanisms that involve other cardiac cell types. Future studies aimed at determining the effects of IL-33 on other cardiac cell types are warranted.

Diabetes-induced alterations in the extracellular matrix and their impact on myocardial function

Diabetes is an increasing public health problem that is expected to escalate in the future due to the growing incidence of obesity in the western world. While this disease is well known for its devastating effects on the kidneys and vascular system, diabetic individuals can develop cardiac dysfunction, termed diabetic cardiomyopathy, in the absence of other cardiovascular risk factors such as hypertension or atherosclerosis. While much effort has gone into understanding the effects of elevated glucose or altered insulin sensitivity on cellular components within the heart, significant changes in the cardiac extracellular matrix (ECM) have also been noted. In this review article we highlight what is currently known regarding the effects diabetes has on both the expression and chemical modification of proteins within the ECM and how the fibrotic response often observed as a consequence of this disease can contribute to reduced cardiac function.

Effects of interleukin-18 on cardiac fibroblast function and gene expression

Fibroblasts are the primary cell type responsible for synthesis and remodeling of the extracellular matrix in the heart. A number of factors including growth factors, hormones and mechanical forces have been identified that modulate the production of extracellular matrix by cardiac fibroblasts. Inflammatory mediators including pro-inflammatory cytokines and chemokines also impact fibrosis of the heart. Recent studies have illustrated that interleukin-18 promotes a pro-fibrotic response in cardiac fibroblasts; however the effects of this cytokine on other aspects of fibroblast function have not been examined. While fibroblasts have long been known for their role in production and remodeling of the extracellular matrix, other functions of these cells are only now beginning to be appreciated. We hypothesize that exposure to interleukin-18 will stimulate other aspects of fibroblast behavior important in myocardial remodeling including proliferation, migration and collagen reorganization. Fibroblasts were isolated from adult male rat hearts and bioassays performed to determine the effects of interleukin-18 on fibroblast function. Treatment of fibroblasts with interleukin-18 (1-100ng/ml) resulted in increased production of extracellular matrix components and remodeling or contraction of three-dimensional collagen scaffolds by these cells. Furthermore, exposure to interleukin-18 stimulated fibroblast migration and proliferation. Treatment of heart fibroblasts with interleukin-18 resulted in the rapid activation of the c-Jun N-terminal kinase (JNK) and phosphoinositide 3-kinase (PI3-kinase) pathways. Studies with pharmacological inhibitors illustrated that activation of these pathways is critical to interleukin-18 mediated alterations in fibroblast function. These studies illustrate that interleukin-18 plays a role in modulation of cardiac fibroblast function and may be an important component of the inflammation-fibrosis cascade during pathological myocardial remodeling.

Temporal alterations in cardiac fibroblast function following induction of pressure overload

Increases in cardiovascular load (pressure overload) are known to elicit ventricular remodeling including cardiomyocyte hypertrophy and interstitial fibrosis. While numerous studies have focused on the mechanisms of myocyte hypertrophy, comparatively little is known regarding the response of the interstitial fibroblasts to increased cardiovascular load. Fibroblasts are the most numerous cell type in the mammalian myocardium and have long been recognized as producing the majority of the myocardial extracellular matrix. It is only now becoming appreciated that other aspects of fibroblast behavior are important to overall cardiac function. The present studies were performed to examine the temporal alterations in fibroblast activity in response to increased cardiovascular load. Rat myocardial fibroblasts were isolated at specific time-points (3, 7, 14, and 28 days) after induction of pressure overload by abdominal aortic constriction. Bioassays were performed to measure specific parameters of fibroblast function including remodeling and contraction of 3-dimensional collagen gels, migration, and proliferation. In addition, the expression of extracellular matrix receptors of the integrin family was examined. Myocardial hypertrophy and fibrosis were evident within 7 days after constriction of the abdominal aorta. Collagen gel contraction, migration, and proliferation were enhanced in fibroblasts from pressure-overloaded animals compared to fibroblasts from sham animals. Differences in fibroblast function and protein expression were evident within 7 days of aortic constriction, concurrent with the onset of hypertrophy and fibrosis of the intact myocardium. These data provide further support for the idea that rapid and dynamic changes in fibroblast phenotype accompany and contribute to the progression of cardiovascular disease.

Fractal and image analysis of morphological changes in the actin cytoskeleton of neonatal cardiac fibroblasts in response to mechanical stretch

Cardiac fibroblasts are the most numerous cells in the heart and are critical in the formation and normal functioning of the organ. Cardiac fibroblasts are firmly attached to and surrounded by extracellular matrix (ECM). Mechanical forces transmitted through interaction with the ECM can result in changes of overall cellular shape, cytoskeletal organization, proliferation, and gene expression of cardiac fibroblasts. These responses may be different in the normally functioning heart, when compared with various pathological conditions, including inflammation or hypertrophy. It is apparent that cellular phenotype and physiology, in turn, are affected by multiple signal transduction pathways modulated directly by the state of polymerization of the actin cytoskeleton. Morphological changes in actin organization resulting from response to adverse conditions in fibroblasts and other cell types are basically descriptive. Some studies have approached quantifying changes in actin cytoskeletal morphology, but these have involved complex and difficult procedures. In this study, we apply image analysis and non-Euclidian geometrical fractal analysis to quantify and describe changes induced in the actin cytoskeleton of cardiac fibroblasts responding to mechanical stress. Characterization of these rapid responses of fibroblasts to mechanical stress may provide insight into the regulation of fibroblasts behavior and gene expression during heart development and disease.

Cardiac fibroblasts: friend or foe?

Cardiac function is determined by the dynamic interaction of various cell types and the extracellular matrix that composes the heart. This interaction varies with the stage of development and the degree and duration of mechanical, chemical, and electrical signals between the various cell types and the ECM. Understanding how these complex signals interact at the molecular, cellular, and organ levels is critical to understanding the function of the heart under a variety of physiological and pathophysiological conditions. Quantitative approaches, both in vivo and in vitro, are essential to understand the dynamic interaction of mechanical, chemical, and electrical stimuli that govern cardiac function. The fibroblast can thus be a friend in normal function or a foe in pathophysiological conditions.

Exposure of cardiac fibroblasts to the herbicide nitrofen causes altered interactions with the extracellular matrix

A large proportion of congenital heart defects result from dysmorphogenesis of valvuloseptal precursors, the endocardial cushions. Intrinsic to formation and maturation of these tissues are developmental changes in cell-cell and cell-extracellular matrix interactions. Interactions between cells and the extracellular matrix play critical roles in modulating cellular processes including proliferation, migration, differentiation and even survival. While significant progress is being made in the elucidation of the cellular events involved in valvuloseptal development, little is known regarding how environmental factors may affect this process. Embryonic exposure to the herbicide nitrofen has been shown to result in congenital heart defects associated with altered endocardial cushion formation or maturation. The present studies were performed to begin to address the cellular mechanisms of these nitrofen-induced effects. Heart fibroblasts were isolated and treated with varying doses of nitrofen in vitro. Experiments were performed to determine the effects of this herbicide on important cellular processes including migration, proliferation and apoptosis. These studies illustrated a dose-dependent decrease in collagen gel contraction and proliferation in response to nitrofen. Assays were also performed to determine the effects of nitrofen on fibroblast gene expression. Increased expression of collagen type I and specific integrins were seen following nitrofen exposure. These studies illustrate that nitrofen has direct effects on cardiac fibroblast proliferation and extracellular matrix remodeling, cellular events important in valvuloseptal development.

Influence of the extracellular matrix on the regulation of cardiac fibroblast behavior by mechanical stretch

Fibroblasts are responsible in large part for production, organization, and turnover of the extracellular matrix (ECM), thereby regulating the fibrotic content of the heart. Excessive fibrosis, which has been associated with certain forms of hemodynamic overload such as hypertension, is thought to result in increased ventricular chamber stiffness, and eventual heart failure. As such, the role of mechanical stretch in regulating fibroblast activity is crucial to our understanding of healthy and diseased hearts. However, little is known about the effects of alterations in the composition of the ECM in regulating mechanotransduction in cardiac fibroblasts. In order to address this question, rat cardiac fibroblasts were cultured on silastic membranes coated with different ECM substrates, and cyclically stretched for various durations. Experiments were designed to assess the activation of signaling pathways, as well as changes in collagen production, cellular proliferation, and morphology. Mitogen activated protein kinase (MAP kinase) was most rapidly activated, and collagen I expression was most abundant, in cells stretched on randomly organized collagen, and uncoated charged membranes. Regardless of the nature of the ECM substrate, stretched cells decreased proliferation, however, this effect was most marked in cells stretched on randomly organized collagen. Finally, cells stretched on all ECM substrates increased their surface area, but this was observed most significantly in cells adherent to aligned collagen, randomly organized collagen, and uncoated, charged membranes. Taken together, these results suggest cardiac fibroblasts may differentially interpret a mechanical stimulus, in terms of both signal transduction, and specific long-term events such as gene transcription, based on the composition and organization of the ECM.

Effects of collagen density on cardiac fibroblast behavior and gene expression

Interactions between cells and the extracellular matrix (ECM) play essential roles in modulating cell behavior during development and disease. The myocardial ECM is composed predominantly of interstitial collagen type I and type III. The composition, organization, and accumulation of these collagens are altered concurrent with cardiovascular development and disease. Changes in these parameters are thought to play significant roles in myocardial function. While a number of studies have examined how changes in the ECM affect myocardial function as a whole, much less is known regarding the response at the cellular level to changes in the collagenous ECM. Experiments were carried out to determine the effects of alterations in collagen density and ECM stiffness on the behavior of isolated heart fibroblasts. In vitro bioassays were performed to measure the effects of changes in collagen concentration (0.75-1.25 mg/ml) on adhesion, migration, spreading, and gene expression by heart fibroblasts. Increased density of collagen in 3-dimensional gels resulted in more efficient adhesion, spreading, and migration by heart fibroblasts. These experiments indicated that the density of the collagen matrix has a significant impact on fibroblast function. These studies begin to elucidate the effects of ECM density at the cellular level in the myocardium.

Integrin shedding as a mechanism of cellular adaptation during cardiac growth

Integrin-mediated cell-extracellular matrix (ECM) interactions are essential for multiple cellular processes; however, little is known regarding integrin turnover during these events. Recent studies have demonstrated shedding of cell surface molecules and suggested this as a potential mechanism for integrin turnover. Confocal microscopy of mouse hearts under different physiological conditions demonstrated the presence of beta(1)-integrin-immunoreactive material in the interstitium. Culture media from neonatal rat cardiac myocytes and fibroblasts contained a 55-kDa fragment of beta(1)-integrin. Attachment to ECM components, response to phorbol 12-myristate 13-acetate stimulation, and matrix metalloproteinase inhibition assays demonstrated that fibroblasts responded differently to the fragment compared with myocytes. The beta(1)-integrin fragment stimulated myocyte attachment to collagen and the fragment itself bound a variety of ECM proteins. These studies indicate that as myocytes and fibroblasts change size and shape, cellular contacts with the ECM are altered, resulting in the liberation of a beta(1)-integrin fragment from the cell surface. Integrin shedding may represent a novel mechanism of rapidly modifying cell-ECM contacts during various cellular processes.

IGF-1 modulation of rat cardiac fibroblast behavior and gene expression is age-dependent

The collagenous extracellular matrix (ECM) forms a stress-tolerant network that is essential for proper function of the vertebrate heart. Profound changes have been detected in the interstitial ECM concurrent with developmental and disease processes of the heart. These alterations in either the organization or accumulation of ECM components markedly affect myocardial function. Studies have shown that a number of biochemical factors, including angiotensin II, transforming growth factor-beta, and insulin-like growth factors, modulate collagen expression by heart fibroblasts, however, few studies have examined the differential effects of these factors on fibroblasts from animals of different physiological backgrounds. The present studies were carried out to determine whether cardiac fibroblasts isolated from different aged animals (fetal, neonatal, and adult) have diverse responses to insulin-like growth factor-1 (IGF-1). Fibroblasts isolated from fetal, neonatal, and adult rat hearts were treated with IGF-1, and several downstream responses were measured, including collagen gel contraction, adhesion to ECM, and expression of interstitial collagen and integrins. IGF-1 affected these parameters to different degrees, depending on the age of the animal from which the fibroblasts were isolated. These experiments indicate that IGF-1 is a potent modulator of fibroblast behavior in general; however, significant differences are apparent in the responsiveness of cells to this growth factor depending on the age of the animal of origin. Future experiments will be directed at determining how the in vivo chemical and biomechanical environment affects the response of heart fibroblasts to growth factors such as IGF-1.

Effects of mast cells on the behavior of isolated heart fibroblasts: modulation of collagen remodeling and gene expression

The extracellular matrix plays a critical role in the development and maintenance of the vertebrate heart. Changes in the accumulation, composition, or organization of the extracellular matrix are known to deleteriously affect heart function. Mast cells are thought to stimulate collagen expression and fibroblast proliferation accompanying fibrosis in some organs; however, the effects of mast cells on the heart interstitium are largely unexplored. The present studies were carried out to determine the effects of mast cells on isolated heart fibroblasts. Several in vitro assays were used including collagen gel contraction to examine the effects of mast cells on the function of isolated fibroblasts. Neonatal heart fibroblasts were cultured either with mast cells, mast cell-conditioned medium, or mast cell extracts, and their ability to contract collagen gels measured. Results from these experiments indicated that mast cells inhibit heart fibroblast migration and contraction of 3-dimensional collagen gels. Further experiments indicated that incubation of neonatal heart fibroblasts with extracts of mast cells altered the expression of collagen, matrix metalloproteases, and matrix receptors of the integrin family. These studies suggest that mast cells play an important role in the regulation of the cardiac interstitial matrix. Further studies are warranted to determine the mechanisms whereby mast cells modulate fibroblast activity.

Modulation of heart fibroblast migration and collagen gel contraction by IGF-I

Dynamic interactions between cells and the extracellular matrix are essential in the regulation of a number of cellular processes including migration, adhesion, proliferation and differentiation. A variety of factors have been identified which modulate these interactions including transforming growth factor-beta, platelet-derived growth factor and others. Insulin-like growth factors have been shown to regulate collagen production by heart fibroblasts; however, the effects of this growth factor on the interactions of heart fibroblasts with the extracellular matrix have not been examined. The present studies were carried out to determine the effects of IGF-I on the ability of fibroblasts to interact with the extracellular matrix and to begin to determine the mechanisms of this response. These experiments illustrate that IGF-I treatment results in increased migration, collagen reorganization and gel contraction by heart fibroblasts. IGF-I has been shown to activate both the mitogen-activated protein kinase and phophatidylinositol-3 kinase pathways in isolated cells. Experiments with pharmacological antagonists of these pathways indicate that the mitogen-activated protein kinase pathway is essential for IGF-I stimulated collagen gel contraction by fibroblasts. These studies illustrate that IGF-I modulates the ability of fibroblasts to interact with the collagen matrix and that activation of multiple signaling pathways by IGF-I may produce distinct downstream responses in these cells.

Identification of Pneumocystis carinii in the lungs of infants dying of sudden infant death syndrome

BACKGROUND

Recently Pneumocystis carinii has been identified in a significant number of infants diagnosed as having died from sudden infant death syndrome (SIDS) in South America and Europe.

METHODS

We examined lung sections of 79 infants who died with a diagnosis of SIDS in Rochester, NY, and Connecticut for the presence of P. carinii.

RESULTS

Organisms with a characteristic silver stain appearance for P. carinii were identified in 14% of the lung sections.

CONCLUSIONS

These data suggest that a possible link between some cases of SIDS and infection with P. carinii should be further evaluated and that infection of young infants may serve as an important reservoir for human P. carinii.

Angiotensin II-stimulated collagen gel contraction by heart fibroblasts: role of the AT1 receptor and tyrosine kinase activity

The accumulation and organization of extracellular matrix (ECM) components play critical roles in development, maintenance, and pathogenesis of most organ systems. These processes are regulated by the precisely orchestrated expression of ECM components, their receptors, and matrix proteases. The collagen gel culture system has been extensively used as a model to examine ECM remodeling similar to that which occurs during development and wound healing. Growth factors, including transforming growth factor-beta, platelet-derived growth factor, insulin-like growth factor, and angiotensin II, have been shown to stimulate collagen gel contraction. The present studies were undertaken to begin to examine the mechanisms through which angiotensin II stimulates collagen remodeling and gel contraction. These studies indicate that angiotensin II stimulates collagen gel contraction by isolated heart fibroblasts in a dose-dependent manner and that this response is inhibited by the AT1 receptor antagonist Losartan. Furthermore, stimulation of collagen gel contraction by angiotensin II is also blocked by the src-related tyrosine kinase inhibitors genistein and herbimycin, indicating that activation of tyrosine kinases plays critical roles in this process. Stimulation of gel contraction by angiotensin II also involves the activation of JAK2, a member of the JAK/STAT pathways of transcriptional activation. Immunoprecipitation of surface-labeled fibroblasts indicate that cell surface levels of collagen-binding integrins also increase in response to angiotensin II treatment. Determining the underlying mechanisms regulating ECM remodeling is essential to understanding the role of ECM organization in development and disease.

Angiotensin II-stimulated collagen gel contraction by heart fibroblasts: role of the AT1 receptor and tyrosine kinase activity

The accumulation and organization of extracellular matrix (ECM) components play critical roles in development, maintenance, and pathogenesis of most organ systems. These processes are regulated by the precisely orchestrated expression of ECM components, their receptors, and matrix proteases. The collagen gel culture system has been extensively used as a model to examine ECM remodeling similar to that which occurs during development and wound healing. Growth factors, including transforming growth factor-beta, platelet-derived growth factor, insulin-like growth factor, and angiotensin II, have been shown to stimulate collagen gel contraction. The present studies were undertaken to begin to examine the mechanisms through which angiotensin II stimulates collagen remodeling and gel contraction. These studies indicate that angiotensin II stimulates collagen gel contraction by isolated heart fibroblasts in a dose-dependent manner and that this response is inhibited by the AT1 receptor antagonist Losartan. Furthermore, stimulation of collagen gel contraction by angiotensin II is also blocked by the src-related tyrosine kinase inhibitors genistein and herbimycin, indicating that activation of tyrosine kinases plays critical roles in this process. Stimulation of gel contraction by angiotensin II also involves the activation of JAK2, a member of the JAK/STAT pathways of transcriptional activation. Immunoprecipitation of surface-labeled fibroblasts indicate that cell surface levels of collagen-binding integrins also increase in response to angiotensin II treatment. Determining the underlying mechanisms regulating ECM remodeling is essential to understanding the role of ECM organization in development and disease.

Abnormal interactions of embryonic mouse trisomy 16 heart fibroblasts with extracellular matrix components in vitro

Trisomy 16 mice have cardiovascular abnormalities thought to arise from altered development and maturation of the cardiac cushions. Cell-cell and cell-extracellular matrix (ECM) interactions play critical roles in heart morphogenesis. To begin to examine the potential involvement of cell-ECM interactions in abnormal trisomy 16 heart development, fibroblasts were isolated from normal and trisomy 16 embryonic mouse hearts. Behavior of these cells was compared in bioassays involving cell-ECM interactions including cell attachment and collagen gel contraction. Significant differences in cell-ECM interactions were found between fibroblasts isolated from normal and trisomy 16 embryonic hearts. Trisomy 16 cells attached poorly to collagen and laminin compared to normal fibroblasts. Trisomy 16 heart fibroblasts also contracted collagen gels less effectively than normal heart fibroblasts. Cell-ECM interactions are largely mediated by ECM receptors of the integrin family. Expression of beta 1 integrins was examined at the mRNA and protein levels in normal and trisomy 16 fibroblasts. Analyses of integrin expression indicated the pattern of integrins produced by normal and trisomy 16 fibroblasts to be similar. These results indicate that fibroblasts isolated from embryonic trisomy 16 mouse hearts interact with several ECM components including collagen and laminin less efficiently than fibroblasts from normal mouse embryos. As cell-ECM interactions play significant roles in cardiac cushion development, abnormal interactions may contribute to defective atrioventricular septal morphogenesis in the trisomy 16 mouse.

The effects of angiotensin II and specific angiotensin receptor blockers on embryonic cardiac development and looping patterns

The role of angiotensin II (Ang II) in the early embryonic development of the heart has not been examined. We have used RT-PCR to identify mRNA for angiotensinogen, angiotensin-converting enzyme, and the Ang II AT1 and AT2 receptors in embryonic day 10.25 Sprague-Dawley rats, and have used confocal microscopy to localize the AT1 receptor to the greater curvature of the developing ventricle in these animals at embryonic days (ED) 9.25 and 10.25. The antibodies used in immunolocalization studies did not distinguish between the AT1a and AT1b receptor subtypes. In whole embryo culture, Ang II added to the culture media resulted in increased ventricular growth and myocyte hypertrophy when treated embryos were compared to cultured littermate controls. Use of Losartan and PD123,319 to block the Ang II AT1 and AT2 receptors resulted in reduced ventricular development and cardiac dilation when compared to control and Ang II-treated embryos. Addition of Ang II and PD123,319 to the culture media also resulted in cardiac loop inversions which may be associated with disruption of normal myofibrillar development. These results clearly indicate an important role for Ang II in the early embryonic development of the heart.

Localization of alpha1(I) collagen mRNA in myocardium from the spontaneously hypertensive rat during the transition from compensated hypertrophy to failure

Spontaneously hypertensive rats (SHR) commonly develop impairment of myocardial function between ages 18-24 months. Isolated muscle studies demonstrate depressed myocardial contractility and increased passive stiffness. Studies of the extracellular matrix in SHR with failure (SHR-F) demonstrate an increased expression of genes encoding extracellular matrix components (ECM), hydroxyproline concentration and fibrosis relative to age-matched non-failing animals. In the present study, tissue sections of hearts from SHR-F, non-failing SHR (SHR-NF) and non-hypertensive Wistar Kyoto rats (WKY) were hybridized with a cDNA probe for alpha1(I) collagen mRNA, which was found by Northern blot analysis to be elevated in SHR-F relative to hearts from control animals. In situ hybridization studies demonstrate increased perivascular and interstitial collagen alpha1(I) gene expression in myocardium from the SHR relative to WKY. In addition, failing hearts from the SHR demonstrate focal alpha1(I) collagen mRNA accumulation in the endocardium and at sites of degenerating single myocardial cells.

Vinculin is an essential component for normal myofibrillar arrangement in fetal mouse cardiac myocytes

Vinculin is a cytoskeletal protein that is believed to be an essential component in the linkage of cytoskeletal actin filaments to the plasma membrane. To investigate the precise function of vinculin in the development of cardiac myofibrils, antisense oligodeoxynucleotides complementary to vinculin mRNA were used to perturb the expression of the protein during myofibril assembly and arrangement in mouse cardiac myocytes. Fetal (day 18-20 post-conception) mouse cardiac myocytes were isolated by collagenase digestion, separated by Percoll density gradient centrifugation, and plated on aligned collagen gels. By 72 h of culture, mouse myocytes displayed an elongated in vivo-like phenotype in parallel with the aligned fibrils of the collagen gels with polarized arrays of myofibrils. Two different antisense oligonucleotides (20-mer) altered the formation of the tissue-like phenotype of myocytes. These antisense oligonucleotides suppressed vinculin protein expression at 43.5+/-26.8% and 48.7+/-20.9% when compared to myocytes that were not treated. Examination of these myocytes by confocal scanning laser and transmission electron microscopy revealed a disruption of the aligned in vivo-like phenotype, assembly of thick and thin filaments, and formulation of Z-bands. Random sequence 20-mer oligonucleotides used as controls had little detectable effect on vinculin protein expression (94.2+/-14.8%), cell shape, normal alignment or assembly of myofibrils. These results indicate that vinculin is a critical cytoskeletal component, that functions in the determination of cell shape and the arrangement and organization of developing myofibrils.

Role of the alpha 1 beta 1 integrin complex in collagen gel contraction in vitro by fibroblasts

Matrix remodeling, critical to embryonic morphogenesis and wound healing, is dependent on the expression of matrix components, their receptors, and matrix proteases. The collagen gel assay has provided an effective model for the examination of the functional role(s) of each of these groups of molecules in matrix remodeling. Previous investigations have indicated that collagen gel contraction involves the beta 1 integrin family of matrix receptors and is stimulated by several growth factors, including TGF-beta, PDGF, and angiotensin II. In particular, collagen gel remodeling by human cells involves the alpha 2 beta 1 and, to a lesser extent, the alpha 1 beta 1 integrin complexes. The present studies were undertaken to determine the role of the alpha 1 integrin chain, a collagen/laminin receptor, in collagen gel contraction by rodent and avian fibroblasts. A high degree of correlation was found between the expression of the alpha 1 beta 1 integrin complex and the relative ability of cells to contract collagen gels. Further studies using antibodies and antisense oligonucleotides against the alpha 1 integrin indicated a significant role for this integrin chain in contraction of collagen gels by rat cardiac fibroblasts. In addition, antibodies to the alpha 1 integrin chain inhibited migration of these fibroblasts on a collagen substratum, suggesting that at least one role of this integrin is in migration of cells in collagen gels. These results indicate that the alpha 1 beta 1 integrin complex plays a significant role in cellular interactions with interstitial collagen that are involved in matrix remodeling such as is seen during morphogenesis and wound healing.

Distribution of beta-1 integrin in the developing rat heart

Cell-cell and cell-matrix interactions play critical roles in various developmental processes including differentiation, proliferation, and migration. Members of the integrin family of cell surface components are important mediators of these cell-extracellular matrix (ECM) contacts or interactions. The ECM provides signals to individual cells essential for development and differentiation and plays essential roles in establishing and maintaining the complex structure of the vertebrate heart. Integrins provide a fundamental link for transduction of developmental signals to cells. Integrin expression by cardiac myocytes is altered during neonatal heart development and disease; however, little is known regarding the spatial and temporal patterns of integrin expression during embryonic and fetal heart development. Essential to understanding the role of integrins in the organization of the heart, the present studies have localized beta-1 integrin protein and mRNA in fetal and neonatal rat hearts. Beta-1 integrin is predominantly found in regions of remodeling (trabeculae) in the early heart (10-13 days of gestation). Later in development (15 days of gestation onward), beta-1 integrin is abundant in regions containing an elaborate ECM, such as the valves. These studies further support the hypothesis that the expressions of integrins and ECM are coordinately regulated in the developing heart.

Expression and accumulation of interstitial collagen in the neonatal rat heart

Significant physiological changes occur in the heart following birth including increased arterial blood pressure and heart rate. Concurrently, biochemical and structural alterations are evident in the neonatal heart in response to these dynamic physiological properties. Prominent among these is the elaborate development of the cardiac extracellular matrix, composed primarily of interstitial collagen. The collagenous fibers, together with other matrix components, form an elastic, stress-tolerant network which functions in the dissipation of force throughout the heart wall. The present studies have used biochemical and molecular techniques to show the temporal and spatial patterns of interstitial collagen accumulation and expression during late fetal and neonatal development of the rat heart. The use of biochemical and particularly molecular methodologies allows the analysis of the expression of matrix components at a resolution previously not attained by structural studies alone. These data show relative increases in interstitial collagen immediately following birth as well as spatial differences in collagen mRNAs within the heart. The data presented provide further evidence for a role of mechanical stimulation in the regulation of collagen gene expression during this period of heart development.

Expression of collagenase and IL-1 alpha in developing rat hearts

During development, extracellular matrix (ECM) molecules are thought to play a major role in regulating the formation of the heart. The change in the heart from a simple tube to a complex, four-chambered organ requires the modification of both the cellular components as well as the surrounding ECM. Matrix metalloproteinases (MMP), which include collagenases, are enzymes present in the ECM that have the potential to modify the existing ECM during the development of the heart. Using both monoclonal and polyclonal antisera against collagenase, specific temporal and spatial patterns have been documented during critical periods of heart development. The cytokine interleukin 1 alpha (IL-1 alpha), a potent inducer of the MMP expression, was also shown to have a similar staining pattern in the developing heart. The monoclonal anti-rat collagenase (Mab) intensely stained the surfaces of the myocytes in the trabeculae and the ventricular and atrial walls of the 11.5 or 12.5 embryonic day (ED) rat hearts. In contrast, the polyclonal anti-human collagenase (Pab) stained not only the cardiomyocytes but also the hypertrophic endocardial cells. Pab appeared to stain the leading edge of the mesenchymal cells that migrate into the cardiac jelly of the 11.5 or 12.5 ED hearts. Immunohistochemical staining showed IL-1 alpha on the endocardial endothelium and the surface of cardiomyocytes near the cardiac jelly just before or coincident with the appearance of migrating cells. IL-1 alpha was detected on the endocardial endothelium, cardiomyocytes in the trabeculae, and the ventricular and atrial walls, as well as in the myocardial basement membrane of the truncal or atrioventricular region. However, no staining could be detected on the migrating cells in the cardiac cushions. These results indicate the presence of collagenase and IL-1 alpha on the surface of cardiomyocytes and mesenchymal cells at times when the heart is undergoing acute remodeling during septation and trabeculation. These data suggest a role for collagenase/cytokine interaction in tissue remodeling during critical stages of cardiac embryogenesis where modification of the ECM is essential to cardiac morphogenesis.

Collagen expression in mechanically stimulated cardiac fibroblasts

The cardiac extracellular matrix, composed predominantly of collagenous fibers, forms a stress-tolerant network that facilitates the distribution of forces generated in the heart and provides for proper alignment of cardiac myocytes. Although considerable information exists regarding the morphological organization of the heart extracellular matrix, little is known about the regulation of the synthesis and accumulation of extracellular matrix components. A potentially significant factor in the cardiovascular system is mechanical stimulation including changes in physical tension and pressure. We recently have developed an in vitro model system to elucidate the effects of mechanical stretch on isolated populations of heart cells. In the present study, we have used biochemical and molecular biological techniques to analyze changes in collagen synthesis by cardiac fibroblasts in response to mechanical stretch. These studies show that the ratio of collagen type III to collagen type I increases in mechanically stretched cells. They also show that type III collagen mRNA levels are increased in response to cyclic mechanical stretch for durations as short as 12 hours. Type I collagen mRNA levels were not found to change under the stretch conditions used in this study. Our results emphasize the potential regulatory role of mechanical stimulation in the expression of specific genes in the heart and support previous studies indicating this to be an intriguing in vitro model of cardiac hypertrophy.

Expression of collagen binding integrins during cardiac development and hypertrophy

The interaction between components of the extracellular matrix and the cell surface of cardiac myocytes appears to be regulated in part by receptors belonging to the integrin superfamily. The expression of the integrins was investigated at different stages of development of the heart as well as during cardiac hypertrophy. The characterization of the membrane proteins showed that a beta 1-integrin and associated alpha-chains were responsible for the interaction with collagen, laminin, and fibronectin. Immunoprecipitation data indicated that the presence of specific alpha-chains varied with development. These data were correlated with the ability of the isolated myocytes to attach to specific components of the extracellular matrix. The expression of the alpha 1-chain was prominently associated with the recognition of interstitial collagens. The presence of the alpha 1-chain was also associated with stages when collagen synthesis was increased, especially during fetal and neonatal growth and cardiac hypertrophy. Immunohistochemical localization with the antiserum against beta 1-integrin demonstrated its specific localization near the Z lines of cardiac myocytes. The localization both in vitro and in vivo indicated that the beta 1-integrin may play a role in myofibrillogenesis during development. The present immunohistochemical, cell adhesion, and biochemical data clearly indicate that integrins play a major role in the regulation of the interaction between cardiac myocytes and the extracellular matrix during development and disease.

Distribution of vinculin in the Z-disk of striated muscle: analysis by laser scanning confocal microscopy

Vinculin is a major cytoskeletal component in striated muscle, where it has been reported to form a rib-like structure between the cell membrane and the Z-disk termed a costamere. This arrangement of vinculin has been purported to be involved in the alignment of the myofibrils. However, the three-dimensional arrangement of vinculin in relation to the Z-disk of the myofibril was not known. In the present study, we examined the distribution of vinculin in striated muscle with monospecific antibodies using immunofluorescence and laser scanning confocal microscopy. Isolated cardiac and skeletal muscle cells from a variety of species, tissue sections, and neonatal myocytes with developing myofibrils were examined. Optical sectioning in the X-Y and X-Z planes demonstrated that vinculin immunoreactivity was heaviest at the periphery of the cell; however, the immunoreactivity was also distributed within the Z-disk although at a relatively reduced level. This distribution is potentially significant in understanding the physiological significance of vinculin in striated muscle function and in myofibrillogenesis.