MR imaging of caveolin gene-specific alterations in right ventricular wall thickness

Magnetic resonance imaging and spectroscopy of the murine cardiovascular system

Magnetic resonance imaging (MRI) has emerged as a powerful and reliable tool to noninvasively study the cardiovascular system in clinical practice. Because transgenic mouse models have assumed a critical role in cardiovascular research, technological advances in MRI have been extended to mice over the last decade. These have provided critical insights into cardiac and vascular morphology, function, and physiology/pathophysiology in many murine models of heart disease. Furthermore, magnetic resonance spectroscopy (MRS) has allowed the nondestructive study of myocardial metabolism in both isolated hearts and in intact mice. This article reviews the current techniques and important pathophysiological insights from the application of MRI/MRS technology to murine models of cardiovascular disease.

Characterization of the molecular architecture of human caveolin-3 and interaction with the skeletal muscle ryanodine receptor

Caveolin-3 (cav-3), an integral membrane protein, is a building block of caveolae as well as a regulator of a number of physiological processes by facilitating the formation of multiprotein signaling complexes. We report that the expression of cav-3 in insect (Sf9) cells induces caveola formation, comparable in size with those observed in native tissue. We have also purified the recombinant cav-3 determining that it forms an oligomer of ∼220 kDa. We present the first three-dimensional structure for cav-3 (using transmission electron microscopy and single particle analysis methods) and show that nine cav-3 monomers assemble to form a complex that is toroidal in shape, ∼16.5 nm in diameter and ∼ 5.5 nm in height. Labeling experiments and reconstitution of the purified cav-3 into liposomes have allowed a proposal for the orientation of the protein with respect to the membrane. We have identified multiple caveolin-binding motifs within the ryanodine receptor (RyR1) sequence employing a bioinformatic analysis. We have then shown experimentally that there is a direct interaction between recombinant cav-3 nonamers and purified RyR1 homotetramers that would imply that at least one of the predicted cav-3-binding sites is exposed within the fully assembled RyR1 structure. The cav-3 three-dimensional model provides new insights as to how a cav-3 oligomer can bind multiple partners in close proximity to form signaling complexes. Furthermore, a direct interaction with RyR1 suggests a possible role for cav-3 as a modifier of muscle excitation-contraction coupling and/or for localization of the receptor to regions of the sarcoplasmic reticulum.

Caveolae, caveolin, and cavins: potential targets for the treatment of cardiac disease

Caveolae are omega-shaped membrane invaginations present in essentially all cell types of the cardiovascular system, including endothelial cells, smooth muscle cells, macrophages, cardiac myocytes, and fibroblasts. Numerous functions have been ascribed to this omega-shaped structure. Caveolae are enriched with different signaling molecules and ion channel regulatory proteins and function both in protein trafficking and signal transduction in these cell types. Caveolins are the structural proteins that are necessary for the formation of caveola membrane domains. Mechanistically, caveolins interact with a variety of downstream signaling molecules, as, for example, Src-family tyrosine kinase, p42/44 mitogen-activated protein (MAP) kinase, and endothelial nitric oxide synthase (eNOS) and hold the signal transducers in the inactive condition until activated with proper stimulus. Caveolae are gradually acquiring increasing attention as cellular organelles contributing to the pathogenesis of several structural and functional processes including cardiac hypertrophy, atherosclerosis, and heart failure. At present, very little is known about the role of caveolae in cardiac function and dysfunction, although recent studies with caveolin knock-out mouse have shown that caveolae and caveolins play a pivotal role in various human pathobiological conditions. This review will discuss the possible role and mechanism of action of caveolae and caveolins in different cardiac diseases.

Caveolins and heart diseases

Caveolins serve as a platform in plasma membrane associated caveolae to orchestrate various signaling molecules to effectively communicate extracellular signals into the interior of cell. All three types of caveolin, Cav-1, Cav-2 and Cav-3 are expressed throughout the cardiovascular system especially by the major cell types involved including endothelial cells, cardiac myocytes, smooth muscle cells and fibroblasts. The functional significance of caveolins in the cardiovascular system is evidenced by the fact that caveolin loss leads to the development of severe cardiac pathology. Caveolin gene mutations are associated with altered expression of caveolin protein and inherited arrhythmias. Altered levels of caveolins and related downstream signaling molecules in cardiomyopathies validate the integral participation of caveolin in normal cardiac physiology. This chapter will provide an overview of the role caveolins play in cardiovascular disease. Furthering our understanding of the role for caveolins in cardiovascular pathophysiology has the potential to lead to the manipulation of caveolins as novel therapeutic targets.

Lipid raft in cardiac health and disease

Lipid rafts are sphingolipid and cholesterol rich micro-domains of the plasma membrane that coordinate and regulate varieties of signaling processes. Lipid rafts are also present in cardiac myocytes and are enriched in signaling molecules and ion channel regulatory proteins. Lipid rafts are receiving increasing attention as cellular organelles contributing to the pathogenesis of several structural and functional processes including cardiac hypertrophy and heart failure. At present, very little is known about the role of lipid rafts in cardiac function and dysfunction. This review will discuss the possible role of lipid rafts in cardiac health and disease.

Caveolae as organizers of pharmacologically relevant signal transduction molecules

Caveolae, a subset of membrane (lipid) rafts, are flask-like invaginations of the plasma membrane that contain caveolin proteins, which serve as organizing centers for cellular signal transduction. Caveolins (-1, -2, and -3) have cytoplasmic N and C termini, palmitolylation sites, and a scaffolding domain that facilitates interaction and organization of signaling molecules so as to help provide coordinated and efficient signal transduction. Such signaling components include upstream entities (e.g., G protein-coupled receptors (GPCRs), receptor tyrosine kinases, and steroid hormone receptors) and downstream components (e.g., heterotrimeric and low-molecular-weight G proteins, effector enzymes, and ion channels). Diseases associated with aberrant signaling may result in altered localization or expression of signaling proteins in caveolae. Caveolin-knockout mice have numerous abnormalities, some of which may reflect the impact of total body knockout throughout the life span. This review provides a general overview of caveolins and caveolae, signaling molecules that localize to caveolae, the role of caveolae/caveolin in cardiac and pulmonary pathophysiology, pharmacologic implications of caveolar localization of signaling molecules, and the possibility that caveolae might serve as a therapeutic target.

Contrast-enhanced MRI of right ventricular abnormalities in Cx43 mutant mouse embryos

Imaging of the mammalian cardiac right ventricle (RV) is particularly challenging, especially when a two-dimensional method such as conventional histology is used to evaluate the morphology of this asymmetric, crescent-shaped chamber. MRI may improve the characterization of mutants with RV phenotypes by allowing analysis of the samples in any plane and by facilitating three-dimensional image reconstruction. MRI was used to examine the conditional knockout Cx43-PCKO mouse line known to have RV malformations. To help delineate the cardiovascular system and facilitate identification of the right ventricular outflow tract (RVOT), embryonic day (E) 17.5 embryos were perfusion fixed through the umbilical vein followed by a gadolinium-based contrast agent mixed in 7% gelatin. Micro-MRI experiments were performed at 7 T and followed by paraffin embedding of specimens, histological sectioning and hematoxylin and eosin (H&E) staining. Imaging of up to four embryos simultaneously allowed for higher throughput than traditional individual imaging techniques, while intravascular contrast afforded excellent signal-to-noise characteristics. All control embryos (n = 4) and heterozygous Cx43 knockout embryos (n = 4) had normal-appearing right ventricular outflow tract contours by MRI. Obvious abnormalities in the RVOT, including abnormal bulging and infiltration of contrast into the wall of the RV, were seen in three out of four Cx43-PCKO mutants with MRI. Furthermore, three-dimensional reconstruction of MR images with orthogonal projections as well as maximum-intensity projection allowed for visualization of the relationship of infundibular bulging segments to the pulmonary trunk in Cx43-PCKO mutant hearts. The addition of MRI to standard histology in the characterization of RV malformations in mutant mouse embryos aids in the assessment and understanding of morphologic abnormalities. Flexibility in the viewing of MR images, which can be retrospectively sectioned in any desired orientation, is particularly useful in the investigation of the RV, an asymmetric chamber that is difficult to analyze with two-dimensional techniques.

MR in mouse models of cardiac disease

Transgenic and knockout mice can be used to study the genes and basic mechanisms involved in heart disease, and have therefore assumed a central role in modern cardiac research. MRI and MRS techniques have recently been developed for mice that enable the quantitative or semi-quantitative in vivo assessment of cardiac anatomy, function, perfusion, infarction, Ca(2+) influx, and metabolism. With these techniques, the normal mouse heart has been shown to be well suited as a model of human cardiac disease. The roles of individual genes in normal cardiac physiology have recently been studied by MR, including the role of neuronal nitric oxide synthase in beta-adrenergic stimulation, the roles of the inducible nitric oxide synthase and myoglobin in function, dilation, and energetics, and the role of cardiac troponin I in contractility. Furthermore, with a mouse model of myocardial infarction, the roles of the angiotensin II type 2 receptor, xanthine oxidase inhibitors, blood coagulation factor XIII, and inducible nitric oxide synthase in post-infarct function and remodeling have been further elucidated. Non-invasive in vivo MRI and MRS in mice provide a unique and powerful means for phenotyping genetically engineered mice and can improve our understanding of the roles of specific genes and proteins in cardiac physiology and pathophysiology.

Role of transcription factor NFAT in glucose and insulin homeostasis

Compromised immunoregulation contributes to obesity and complications in metabolic pathogenesis. Here, we demonstrate that the nuclear factor of activated T cell (NFAT) group of transcription factors contributes to glucose and insulin homeostasis. Expression of two members of the NFAT family (NFATc2 and NFATc4) is induced upon adipogenesis and in obese mice. Mice with the Nfatc2-/- Nfatc4-/- compound disruption exhibit defects in fat accumulation and are lean. Nfatc2-/- Nfatc4-/- mice are also protected from diet-induced obesity. Ablation of NFATc2 and NFATc4 increases insulin sensitivity, in part, by sustained activation of the insulin signaling pathway. Nfatc2-/- Nfatc4-/- mice also exhibit an altered adipokine profile, with reduced resistin and leptin levels. Mechanistically, NFAT is recruited to the transcription loci and regulates resistin gene expression upon insulin stimulation. Together, these results establish a role for NFAT in glucose/insulin homeostasis and expand the repertoire of NFAT function to metabolic pathogenesis and adipokine gene transcription.

Dyskinesis in Chagasic myocardium: centerline analysis of wall motion using cardiac-gated magnetic resonance images of mice

We report on the use of centerline analysis of cardiac-gated magnetic resonance images to measure wall motion abnormalities in mice infected with Trypanosoma cruzi. To our knowledge, this is the first report of segmental wall motion abnormalities in an animal model of Chagas' disease. Chagas' disease patients with severe cardiac involvement exhibit mild hypokinesis in an extensive region of the left ventricle and dyskinesis in the apical region. We observed dyskinetic segments in a similar region of the hearts of infected wild-type mice. Dyskinesis was not observed in infected mice lacking macrophage inflammatory protein-1alpha, a chemokine that may play an important role in the cardiac remodeling that is normally observed in mouse models of Chagas' disease and in human patients. This study aimed to demonstrate the utility of cardiac-gated magnetic resonance imaging and centerline analysis as a straightforward method for monitoring regional left ventricular wall motion in transgenic and/or diseased mice.

Magnetic resonance imaging in experimental Chagas disease: a brief review of the utility of the method for monitoring right ventricular chamber dilatation

Chagas' disease caused by infection with Trypanosoma cruzi leads to a myocardiopathy that evolves from the acute to the chronic phase. Magnetic resonance imaging (MRI) is an important tool for monitoring cardiac morphology and function both in humans and in animals. In the present work, we present a brief review of MRI applications for the study of ventricular hypertrophy and dilatation of the right ventricle in murine models of Chagas' disease. Studies using MRI demonstrate an increase in right ventricular chamber dimension during both phases of infection, indicating that increase of the right ventricle is a marker for experimental chagasic myocardiopathy. Based on previous studies using MRI in these models we propose that this technique is an excellent approach for monitoring heart functionality from the acute through the chronic phase of infection in different parasite-host pairs and for monitoring the efficacy of cardioprotective or immune-therapeutic agents.