An essential role for connexin43 gap junctions in mouse coronary artery development

Congenital Coronary Blood Vessel Anomalies: Animal Models and the Integration of Developmental Mechanisms

Coronary blood vessels are in charge of sustaining cardiac homeostasis. It is thus logical that coronary congenital anomalies (CCA) directly or indirectly associate with multiple cardiac conditions, including sudden death. The coronary vascular system is a sophisticated, highly patterned anatomical entity, and therefore a wide range of congenital malformations of the coronary vasculature have been described. Despite the clinical interest of CCA, very few attempts have been made to relate specific embryonic developmental mechanisms to the congenital anomalies of these blood vessels. This is so because developmental data on the morphogenesis of the coronary vascular system derive from complex studies carried out in animals (mostly transgenic mice), and are not often accessible to the clinician, who, in turn, possesses essential information on the significance of CCA. During the last decade, advances in our understanding of normal embryonic development of coronary blood vessels have provided insight into the cellular and molecular mechanisms underlying coronary arteries anomalies. These findings are the base for our attempt to offer plausible embryological explanations to a variety of CCA as based on the analysis of multiple animal models for the study of cardiac embryogenesis, and present them in an organized manner, offering to the reader developmental mechanistic explanations for the pathogenesis of these anomalies.

Anomalous right coronary artery originating from the aorta: a series of nine pediatric cases

BACKGROUND

To investigate the clinical manifestations, prognosis, and possibly related genes of anomalous right coronary artery originating from the aorta (ARCA-L) in children.

METHODS

This case series study included pediatric patients diagnosed with ARCA-L at the Department of Cardiology in Beijing Children's Hospital affiliated to Capital Medical University, between January 2017 and December 2019.

RESULTS

Nine pediatric patients (aged 3 months to 12 years, 4 boys) were included. Two cases presented with cardiac insufficiency as their primary manifestation, while the remaining seven had post-infection or post-exercise symptoms such as chest pain, chest tightness, long exhalation, lack of strength, and dizziness. Six patients displayed varying degrees of ST-T changes on the electrocardiograph, while two patients had a reduced left ventricular ejection fraction (LVEF) of 20-32% according to echocardiography. Multislice computed tomographic angiography confirmed the presence of ARCA-L in all patients. One patient underwent the unroofing technique. The remaining eight received conservative treatment. After a follow-up of 2-64 months, eight children had a good prognosis and survived. One child experienced sudden death due to aggravated heart failure. Whole exome sequencing revealed that one child tested negative, one had mutations in the RYR2 and LDB3 genes, and the remaining four patients had a mutation in the GDF1, LRP6, MEF2A, and KALRN genes, respectively.

CONCLUSIONS

ARCA-L in children might have a wide variation in clinical manifestations and a risk of sudden death. The occurrence of the disease might be associated with genetic defects.

Desmosomes in Cell Fate Determination: From Cardiogenesis to Cardiomyopathy

Desmosomes play a vital role in providing structural integrity to tissues that experience significant mechanical tension, including the heart. Deficiencies in desmosomal proteins lead to the development of arrhythmogenic cardiomyopathy (AC). The limited availability of preventative measures in clinical settings underscores the pressing need to gain a comprehensive understanding of desmosomal proteins not only in cardiomyocytes but also in non-myocyte residents of the heart, as they actively contribute to the progression of cardiomyopathy. This review focuses specifically on the impact of desmosome deficiency on epi- and endocardial cells. We highlight the intricate cross-talk between desmosomal proteins mutations and signaling pathways involved in the regulation of epicardial cell fate transition. We further emphasize that the consequences of desmosome deficiency differ between the embryonic and adult heart leading to enhanced erythropoiesis during heart development and enhanced fibrogenesis in the mature heart. We suggest that triggering epi-/endocardial cells and fibroblasts that are in different "states" involve the same pathways but lead to different pathological outcomes. Understanding the details of the different responses must be considered when developing interventions and therapeutic strategies.

Cohesin: an emerging master regulator at the heart of cardiac development

Cohesins are ATPase complexes that play central roles in cellular processes such as chromosome division, DNA repair, and gene expression. Cohesinopathies arise from mutations in cohesin proteins or cohesin complex regulators and encompass a family of related developmental disorders that present with a range of severe birth defects, affect many different physiological systems, and often lead to embryonic fatality. Treatments for cohesinopathies are limited, in large part due to the lack of understanding of cohesin biology. Thus, characterizing the signaling networks that lie upstream and downstream of cohesin-dependent pathways remains clinically relevant. Here, we highlight alterations in cohesins and cohesin regulators that result in cohesinopathies, with a focus on cardiac defects. In addition, we suggest a novel and more unifying view regarding the mechanisms through which cohesinopathy-based heart defects may arise.

Connexinplexity: the spatial and temporal expression of connexin genes during vertebrate organogenesis

Animal development requires coordinated communication between cells. The Connexin family of proteins is a major contributor to intercellular communication in vertebrates by forming gap junction channels that facilitate the movement of ions, small molecules, and metabolites between cells. Additionally, individual hemichannels can provide a conduit to the extracellular space for paracrine and autocrine signaling. Connexin-mediated communication is widely used in epithelial, neural, and vascular development and homeostasis, and most tissues likely use this form of communication. In fact, Connexin disruptions are of major clinical significance contributing to disorders developing from all major germ layers. Despite the fact that Connexins serve as an essential mode of cellular communication, the temporal and cell-type-specific expression patterns of connexin genes remain unknown in vertebrates. A major challenge is the large and complex connexin gene family. To overcome this barrier, we determined the expression of all connexins in zebrafish using single-cell RNA-sequencing of entire animals across several stages of organogenesis. Our analysis of expression patterns has revealed that few connexins are broadly expressed, but rather, most are expressed in tissue- or cell-type-specific patterns. Additionally, most tissues possess a unique combinatorial signature of connexin expression with dynamic temporal changes across the organism, tissue, and cell. Our analysis has identified new patterns for well-known connexins and assigned spatial and temporal expression to genes with no-existing information. We provide a field guide relating zebrafish and human connexin genes as a critical step toward understanding how Connexins contribute to cellular communication and development throughout vertebrate organogenesis.

The gap junction protein connexin 43 controls multiple aspects of cranial neural crest cell development

Gap junctions are intercellular channels between cells that facilitate cell-cell communication. Connexin 43 (Cx43; also known as GJA1), the predominant gap junction protein in vertebrates, is expressed in premigratory cranial neural crest cells and is maintained throughout the neural crest cell epithelial-to-mesenchymal transition (EMT), but its function in these cells is unknown. To this end, we used a combination of in vivo and ex vivo experiments to assess gap junction formation, and Cx43 function, in chick cranial neural crest cells. Our results demonstrate that gap junctions exist between premigratory and migratory cranial neural crest cells and depend on Cx43 for their function. In the embryo, Cx43 knockdown just prior to EMT delays the emergence of Cx43-depleted neural crest cells from the neural tube, but these cells eventually successfully emigrate and join the migratory stream. This delay can be rescued by introduction of full-length Cx43 into Cx43-depleted cells. Furthermore, Cx43 depletion reduces the size of the premigratory neural crest cell domain through an early effect on neural crest cell specification. Collectively, these data identify new roles for Cx43 in chick cranial neural crest cell development.

Therapeutic strategies targeting connexins

The connexin family of channel-forming proteins is present in every tissue type in the human anatomy. Connexins are best known for forming clustered intercellular channels, structurally known as gap junctions, where they serve to exchange members of the metabolome between adjacent cells. In their single-membrane hemichannel form, connexins can act as conduits for the passage of small molecules in autocrine and paracrine signalling. Here, we review the roles of connexins in health and disease, focusing on the potential of connexins as therapeutic targets in acquired and inherited diseases as well as wound repair, while highlighting the associated clinical challenges.

Familial clustering of cardiac conditions in patients with anomalous aortic origin of a coronary artery and myocardial bridges

BACKGROUND

Anomalous aortic origin of a coronary artery is the second leading cause of sudden cardiac arrest/death in young athletes in the United States of America. Limited data are available regarding family history in this patient population.

METHODS

Patients were evaluated prospectively from 12/2012 to 02/2017 in the Coronary Anomalies Program at Texas Children's Hospital. Relevant family history included the presence of CHD, sudden cardiac arrest/death, arrhythmia/pacemaker use, cardiomyopathy, and atherosclerotic coronary artery disease before the age of 50 years. The presence of one or more of these in 1st- or 2nd-degree relatives was considered significant.

RESULTS

Of 168 unrelated probands (171 patients total) included, 36 (21%) had significant family history involving 19 (53%) 1st-degree and 17 (47%) 2nd-degree relatives. Positive family history led to cardiology referral in nine (5%) patients and the presence of abnormal tests/symptoms in the remaining patients. Coronary anomalies in probands with positive family history were anomalous right (27), anomalous left (five), single right coronary artery (two), myocardial bridge (one), and anomalous circumflex coronary artery (one). Conditions present in their family members included sudden cardiac arrest/death (15, 42%), atherosclerotic coronary artery disease (14, 39%), cardiomyopathy (12, 33%), CHD (11, 31%), coronary anomalies (3, 8%), myocardial bridge (1, 3%), long-QT syndrome (2, 6%), and Wolff-Parkinson-White (1, 3%).

CONCLUSION

In patients with anomalous aortic origin of a coronary artery and/or myocardial bridges, there appears to be familial clustering of cardiac diseases in approximately 20% of patients, half of these with early occurrence of sudden cardiac arrest/death in the family.

Coronary Artery Development: Origin, Malformations, and Translational Vascular Reparative Therapy

After thickening of the cardiac chamber walls during embryogenesis, oxygen and nutrients can no longer be adequately supplied to cardiac cells via passive diffusion; therefore, a primitive vascular network develops to supply these vital structures. This plexus further matures into coronary arteries and veins, which ensures continued development of the heart. Various models have been proposed to account for the growth of the coronary arteries. However, lineage-tracing studies in the last decade have identified 3 major sources, namely, the proepicardium, the sinus venosus, and endocardium. Although the exact contribution of each source remains unknown, the emerging model depicts alternative pathways and progenitor cells, which ensure successful coronary angiogenesis. We aim to explore the current trends in coronary artery development, the cellular and molecular signals regulating heart vascularization, and its implications for heart disease and vascular regeneration.

Spatiotemporal expression pattern of Connexin 43 during early chick embryogenesis

During embryogenesis, a single cell develops into new tissues and organs that are made up of a number of different cell types. The assembly of the trigeminal ganglion (cranial nerve V), an important component of the peripheral nervous system, typifies this process. The trigeminal ganglia perform key sensory functions, including sensing pain and touch in the face, and arise from cells of two different progenitor populations, the neural crest and the cranial placodes. One question that remains poorly understood is how these two populations of cells interact with each other during development to form a functional ganglion. Gap junctions are intercellular channels that allow for the passage of small solutes between connected cells and could serve as one potential mechanism by which neural crest and placode cells communicate to create the trigeminal ganglia. To this end, we have generated a comprehensive spatiotemporal expression profile for the gap junction protein Connexin 43, a highly expressed member of the Connexin protein family during development. Our results reveal that Connexin 43 is expressed in the neural folds during neural fold fusion and in premigratory neural crest cells prior to the epithelial-to-mesenchymal transition (EMT), during EMT, and in migratory neural crest cells. During trigeminal gangliogenesis, Connexin 43 is expressed in cranial neural crest cells and the mesenchyme but is strikingly absent in the placode-derived neurons. These data underscore the complexity of bringing two distinct cell populations together to form a new tissue during development and suggest that Connexin 43 may play a key role within neural crest cells during EMT, migration, and trigeminal gangliogenesis.

Non-muscle myosin IIB (Myh10) is required for epicardial function and coronary vessel formation during mammalian development

The coronary vasculature is an essential vessel network providing the blood supply to the heart. Disruptions in coronary blood flow contribute to cardiac disease, a major cause of premature death worldwide. The generation of treatments for cardiovascular disease will be aided by a deeper understanding of the developmental processes that underpin coronary vessel formation. From an ENU mutagenesis screen, we have isolated a mouse mutant displaying embryonic hydrocephalus and cardiac defects (EHC). Positional cloning and candidate gene analysis revealed that the EHC phenotype results from a point mutation in a splice donor site of the Myh10 gene, which encodes NMHC IIB. Complementation testing confirmed that the Myh10 mutation causes the EHC phenotype. Characterisation of the EHC cardiac defects revealed abnormalities in myocardial development, consistent with observations from previously generated NMHC IIB null mouse lines. Analysis of the EHC mutant hearts also identified defects in the formation of the coronary vasculature. We attribute the coronary vessel abnormalities to defective epicardial cell function, as the EHC epicardium displays an abnormal cell morphology, reduced capacity to undergo epithelial-mesenchymal transition (EMT), and impaired migration of epicardial-derived cells (EPDCs) into the myocardium. Our studies on the EHC mutant demonstrate a requirement for NMHC IIB in epicardial function and coronary vessel formation, highlighting the importance of this protein in cardiac development and ultimately, embryonic survival.

Connexin43 and Runx2 Interact to Affect Cortical Bone Geometry, Skeletal Development, and Osteoblast and Osteoclast Function

The coupling of osteoblasts and osteocytes by connexin43 (Cx43) gap junctions permits the sharing of second messengers that coordinate bone cell function and cortical bone acquisition. However, details of how Cx43 converts shared second messengers into signals that converge onto essential osteogenic processes are incomplete. Here, we use in vitro and in vivo methods to show that Cx43 and Runx2 functionally interact to regulate osteoblast gene expression and proliferation, ultimately affecting cortical bone properties. Using compound hemizygous mice for the Gja1 (Cx43) and Runx2 genes, we observed a skeletal phenotype not visible in wild-type or singly hemizygous animals. Cortical bone analysis by micro-computed tomography (μCT) revealed that 8-week-old male, compound Gja1^+/- Runx2^+/- mice have a marked increase in cross-sectional area, endosteal and periosteal bone perimeter, and an increase in porosity compared to controls. These compound Gja1^+/- Runx2^+/- mice closely approximate the cortical bone phenotypes seen in osteoblast-specific Gja1-conditional knockout models. Furthermore, μCT analysis of skulls revealed an altered interparietal bone geometry in compound hemizygotes. Consistent with this finding, Alizarin red/Alcian blue staining of 2-day-old Gja1^+/- Runx2^+/- neonates showed a hypomorphic interparietal bone, an exacerbation of the open fontanelles, and a further reduction in the hypoplastic clavicles compared to Runx2^+/- neonates. Expression of osteoblast genes, including osteocalcin, osterix, periostin, and Hsp47, was markedly reduced in tibial RNA extracts from compound hemizygous mice, and osteoblasts from compound hemizygous mice exhibited increased proliferative capacity. Further, the reduced osteocalcin expression and hyperproliferative nature of osteoblasts from Cx43 deficient mice was rescued by Runx2 expression. In summary, these findings provide evidence that Cx43 and Runx2 functionally intersect in vivo to regulate cortical bone properties and affect osteoblast differentiation and proliferation, and likely contributes to aspects of the skeletal phenotype of Cx43 conditional knockout mice. © 2017 American Society for Bone and Mineral Research.

Gap junctional signaling in pattern regulation: Physiological network connectivity instructs growth and form

Gap junctions (GJs) are aqueous channels that allow cells to communicate via physiological signals directly. The role of gap junctional connectivity in determining single-cell functions has long been recognized. However, GJs have another important role: the regulation of large-scale anatomical pattern. GJs are not only versatile computational elements that allow cells to control which small molecule signals they receive and emit, but also establish connectivity patterns within large groups of cells. By dynamically regulating the topology of bioelectric networks in vivo, GJs underlie the ability of many tissues to implement complex morphogenesis. Here, a review of recent data on patterning roles of GJs in growth of the zebrafish fin, the establishment of left-right patterning, the developmental dysregulation known as cancer, and the control of large-scale head-tail polarity, and head shape in planarian regeneration has been reported. A perspective in which GJs are not only molecular features functioning in single cells, but also enable global neural-like dynamics in non-neural somatic tissues has been proposed. This view suggests a rich program of future work which capitalizes on the rapid advances in the biophysics of GJs to exploit GJ-mediated global dynamics for applications in birth defects, regenerative medicine, and morphogenetic bioengineering. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 643-673, 2017.

Cellular and molecular mechanisms of coronary vessel development

Development of coronary vessels is a complex process in developmental biology and it may have clinical implications. Although coronary vessels develop as a form of vasculogenesis followed by angiogenesis, the cells of the entire coronary system do not arise from the developing heart. The key events of the coronary system formation include the generation of primordium and proepicardial organ; formation of epicardium; generation of subepicardial mesenchymal cells, and the formation, remodeling and maturation of the final vascular plexus. These events represent a complex regulation of the cell fate determination, cellular migration, epicardial/mesenchymal transformation, and patterning of vasculatures. Recent studies suggest that several transcription factors, adhesion molecules, growth factors and signaling molecules play essential roles in these events. This article reviews the literature on the development of coronary vessels, and discusses current advances and controversies of molecular and cellular mechanisms, thereby directing future investigations.

Rescue of perfluorooctanesulfonate (PFOS)-mediated Sertoli cell injury by overexpression of gap junction protein connexin 43

Perfluorooctanesulfonate (PFOS) is an environmental toxicant used in developing countries, including China, as a stain repellent for clothing, carpets and draperies, but it has been banned in the U.S. and Canada since the late 2000s. PFOS perturbed the Sertoli cell tight junction (TJ)-permeability barrier, causing disruption of actin microfilaments in cell cytosol, perturbing the localization of cell junction proteins (e.g., occluden-ZO-1, N-cadherin-ß-catenin). These changes destabilized Sertoli cell blood-testis barrier (BTB) integrity. These findings suggest that human exposure to PFOS might induce BTB dysfunction and infertility. Interestingly, PFOS-induced Sertoli cell injury associated with a down-regulation of the gap junction (GJ) protein connexin43 (Cx43). We next investigated if overexpression of Cx43 in Sertoli cells could rescue the PFOS-induced cell injury. Indeed, overexpression of Cx43 in Sertoli cells with an established TJ-barrier blocked the disruption in PFOS-induced GJ-intercellular communication, resulting in the re-organization of actin microfilaments, which rendered them similar to those in control cells. Furthermore, cell adhesion proteins that utilized F-actin for attachment became properly distributed at the cell-cell interface, resealing the disrupted TJ-barrier. In summary, Cx43 is a good target that might be used to manage PFOS-induced reproductive dysfunction.

Tumor-induced loss of mural Connexin 43 gap junction activity promotes endothelial proliferation

Background

Proper functional association between mural cells and endothelial cells (EC) causes EC of blood vessels to become quiescent. Mural cells on tumor vessels exhibit decreased attachment to EC, which allows vessels to be unstable and proliferative. The mechanisms by which tumors prevent proper association between mural cells and EC are not well understood. Since gap junctions (GJ) play an important role in cell-cell contact and communication, we investigated whether loss of GJ plays a role in tumor-induced mural cell dissociation.

Methods

Mural cell regulation of endothelial proliferation was assessed by direct co-culture assays of fluorescently labeled cells quantified by flow cytometry or plate reader. Gap junction function was assessed by parachute assay. Connexin 43 (Cx43) protein in mural cells exposed to conditioned media from cancer cells was assessed by Western and confocal microscopy; mRNA levels were assessed by quantitative real-time PCR. Expression vectors or siRNA were utilized to overexpress or knock down Cx43. Tumor growth and angiogenesis was assessed in mouse hosts deficient for Cx43.

Results

Using parachute dye transfer assay, we demonstrate that media conditioned by MDA-MB-231 breast cancer cells diminishes GJ communication between mural cells (vascular smooth muscle cells, vSMC) and EC. Both protein and mRNA of the GJ component Connexin 43 (Cx43) are downregulated in mural cells by tumor-conditioned media; media from non-tumorigenic MCF10A cells had no effect. Loss of GJ communication by Cx43 siRNA knockdown, treatment with blocking peptide, or exposure to tumor-conditioned media diminishes the ability of mural cells to inhibit EC proliferation in co-culture assays, while overexpression of Cx43 in vSMC restores GJ and endothelial inhibition. Breast tumor cells implanted into mice heterozygous for Cx43 show no changes in tumor growth, but exhibit significantly increased tumor vascularization determined by CD31 staining, along with decreased mural cell support detected by NG2 staining.

Conclusions

Our data indicate that i) functional Cx43 is required for mural cell-induced endothelial quiescence, and ii) downregulation of Cx43 GJ by tumors frees endothelium to respond to angiogenic cues. These data define a novel and important role for maintained Cx43 function in regulation of vessel quiescence, and suggest its loss may contribute to pathological tumor angiogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1420-9) contains supplementary material, which is available to authorized users.

Cellular origin and developmental program of coronary angiogenesis

Coronary artery disease causes acute myocardial infarction and heart failure. Identifying coronary vascular progenitors and their developmental program could inspire novel regenerative treatments for cardiac diseases. The developmental origins of the coronary vessels have been shrouded in mystery and debated for several decades. Recent identification of progenitors for coronary vessels within the endocardium, epicardium, and sinus venosus provides new insights into this question. In addition, significant progress has been achieved in elucidating the cellular and molecular programs that orchestrate coronary artery development. Establishing adequate vascular supply will be an essential component of cardiac regenerative strategies, and these findings raise exciting new strategies for therapeutic cardiac revascularization.

Coronary anomalies in mice with congenital heart defects

BACKGROUND

Coronary anomalies are frequently associated with congenital cardiac defects. Accordingly, we tested the hypothesis that the development of the tunica media of coronary arteries/arterioles is compromised in mice with cardiac defects of the outflow tract (persistent truncus arteriosus, double outlet right ventricle and transposition of the great arteries) by studying hearts of G7-9 generation mice bred from mutagenized mice caused by N-ethyl-N-nitrosourea. Mice were studied at embryonic days E16.5, E17.5, and postnatal days 1 and 11. Data were based on immunohistochemistry of serial sections.

RESULTS

In 21 of 24 mice with outflow tract defects, the development of smooth muscle in arteries and arterioles was retarded; most commonly arterioles had an incomplete layer of smooth muscle or in a few instances, lacked a tunica media. In this model, an absence of a coronary ostium occurred in only 2 mice, indicating that the mechanisms underlying the formation of coronary ostia and the recruitment and differentiation of vascular smooth muscle differ. Coronary fistulas were present in 20% and dilated vessels in 30% of the hearts with cardiac defects.

CONCLUSIONS

The data suggest that vascular smooth muscle recruitment and differentiation are not necessarily linked to other coronary anomalies, such as absence of a main coronary artery or branching patterns.

VEGF-C and aortic cardiomyocytes guide coronary artery stem development

Coronary arteries (CAs) stem from the aorta at 2 highly stereotyped locations, deviations from which can cause myocardial ischemia and death. CA stems form during embryogenesis when peritruncal blood vessels encircle the cardiac outflow tract and invade the aorta, but the underlying patterning mechanisms are poorly understood. Here, using murine models, we demonstrated that VEGF-C-deficient hearts have severely hypoplastic peritruncal vessels, resulting in delayed and abnormally positioned CA stems. We observed that VEGF-C is widely expressed in the outflow tract, while cardiomyocytes develop specifically within the aorta at stem sites where they surround maturing CAs in both mouse and human hearts. Mice heterozygous for islet 1 (Isl1) exhibited decreased aortic cardiomyocytes and abnormally low CA stems. In hearts with outflow tract rotation defects, misplaced stems were associated with shifted aortic cardiomyocytes, and myocardium induced ectopic connections with the pulmonary artery in culture. These data support a model in which CA stem development first requires VEGF-C to stimulate vessel growth around the outflow tract. Then, aortic cardiomyocytes facilitate interactions between peritruncal vessels and the aorta. Derangement of either step can lead to mispatterned CA stems. Studying this niche for cardiomyocyte development, and its relationship with CAs, has the potential to identify methods for stimulating vascular regrowth as a treatment for cardiovascular disease.

Coronary arterial development: a review of normal and congenitally anomalous patterns

Coronary artery development is a delicate, complex, and finely tuned process that includes multiple interactions among many pathways, especially in the pericardium and the developing myocardium. There still exists some controversy on the exact origin of certain cellular components. Nevertheless, an understanding of this extremely important developmental process is paramount in identifying some of the causes of anomalous coronary development. There are different patterns of anomalous coronary arteries, with variable risk of myocardial ischemia, malignant arrhythmias, and sudden cardiac death. These anomalies can be broadly categorized into 2 basic anatomic subsets: those with origin of the anomalous coronary artery from the opposite aortic sinus, and those with origin of the anomalous coronary artery from the pulmonary artery. Diagnosis and management of such patterns continues to be challenging. A good knowledge of the normal and abnormal coronary artery development could potentially help us explore new avenues in the treatment of ischemic heart disease as well as anomalous coronary arteries.

Induction of the Proepicardium

The proepicardium is a transient extracardiac embryonic tissue that gives rise to the epicardium and a number of coronary vascular cell lineages. This important extracardiac tissue develops through multiple steps of inductive events, from specification of multiple cell lineages to morphogenesis. This article will review our current understanding of inductive events involved in patterning of the proepicardium precursor field, specification of cell types within the proepicardium, and their extension and attachment to the heart.

New insights into the developmental mechanisms of coronary vessels and epicardium

During heart development, the epicardium, which originates from the proepicardial organ (PE), is a source of coronary vessels. The PE develops from the posterior visceral mesoderm of the pericardial coelom after stimulation with a combination of weak bone morphogenetic protein and strong fibroblast growth factor (FGF) signaling. PE-derived cells migrate across the heart surface to form the epicardial sheet, which subsequently seeds multipotent subepicardial mesenchymal cells via epithelial-mesenchymal transition, which is regulated by several signaling pathways including retinoic acid, FGF, sonic hedgehog, Wnt, transforming growth factor-β, and platelet-derived growth factor. Subepicardial endothelial progenitors eventually generate the coronary vascular plexus, which acquires an arterial or venous phenotype, connects with the sinus venosus and aortic sinuses, and then matures through the recruitment of vascular smooth muscle cells under the regulation of complex growth factor signaling pathways. These developmental programs might be activated in the adult heart after injury and play a role in the regeneration/repair of the myocardium.

Differential gene expression profiles during embryonic heart development in diabetic mice pregnancy

Congenital heart defects (CHD) are one of the most common defects in offspring of diabetic mothers. There is a clear association between maternal diabetes and CHD; however the underlying molecular mechanism remains unknown. We hypothesized that maternal diabetes affects with the expression of early developmental genes that regulate the essential developmental processes of the heart, thereby resulting in the pathogenesis of CHD. We analyzed genome-wide expression profiling in the developing heart of embryos from diabetic and control mice by using the oligonucleotide microarray. Microarray analysis revealed that a total of 878 genes exhibited more than 1.5 fold changes in expression level in the hearts of experimental embryos in either E13.5 or E15.5 compared with their respective controls. Expression pattern of genes that is differentially expressed in the developing heart was further examined by the real-time reverse transcriptase-polymerase chain reaction. Several genes involved in a number of molecular signaling pathways such as apoptosis, proliferation, migration and differentiation in the developing heart were differentially expressed in embryos of diabetic pregnancy. It is concluded that altered expression of several genes involved in heart development may contribute to CHD in offspring of diabetic mothers.

Evaluating the role of connexin43 in congenital heart disease: Screening for mutations in patients with outflow tract anomalies and the analysis of knock-in mouse models

Background:

GJA1 gene encodes a gap junction protein known as connexin 43 (Cx43). Cx43 is abundantly expressed in the ventricular myocardium and in cardiac neural crest cells. Cx43 is proposed to play an important role in human congenital heart disease, as GJA1 knock-out mice die neonatally from outflow tract obstruction. In addition, patients with visceroatrial heterotaxia or hypoplastic left heart syndrome were reported to have point mutations in GJA1 at residues that affect protein kinase phosphorylation and gating of the gap junction channel. However, as these clinical findings were not replicated in subsequent studies, the question remains about the contribution of GJA1 mutations in human congenital heart disease (CHD).

Materials and Methods:

We analyzed the GJA1 coding sequence in 300 patients with CHD from two clinical centers, focusing on outflow tract anomalies. This included 152 with Tetralogy of Fallot from over 200 patients exhibiting outflow tract anomalies, as well as other structural heart defects including atrioventricular septal defects and other valvar anomalies. Our sequencing analysis revealed only two silent nucleotide substitutions in 8 patients. To further assess the possible role of Cx43 in CHD, we also generated two knock-in mouse models with point mutations at serine residues subject to protein kinase C or casein kinase phosphorylation, sites that are known to regulate gating and trafficking of Cx43, respectively.

Results:

Both heterozygous and homozygous knock-in mice were long term viable and did not exhibit overt CHD.

Conclusion:

The combined clinical and knock-in mouse mutant studies indicate GJA1 mutation is not likely a major contributor to CHD, especially those involving outflow tract anomalies.

Gap junction proteins on the move: connexins, the cytoskeleton and migration

Connexin43 (Cx43) has roles in cell-cell communication as well as channel independent roles in regulating motility and migration. Loss of function approaches to decrease Cx43 protein levels in neural cells result in reduced migration of neurons during cortical development in mice and impaired glioma tumor cell migration. In other cell types, correlations between Cx43 expression and cell morphology, adhesion, motility and migration have been noted. In this review we will discuss the common themes that have been revealed by a detailed comparison of the published results of neuronal cells with that of other cell types. In brief, these comparisons clearly show differences in the stability and directionality of protrusions, polarity of movement, and migration, depending on whether a) residual Cx43 levels remain after siRNA or shRNA knockdown, b) Cx43 protein levels are not detectable as in cells from Cx43(-/-) knockout mice or in cells that normally have no endogenous Cx43 expression, c) gain-of-function approaches are used to express Cx43 in cells that have no endogenous Cx43 and, d) Cx43 is over-expressed in cells that already have low endogenous Cx43 protein levels. What is clear from our comparisons is that Cx43 expression influences the adhesiveness of cells and the directionality of cellular processes. These observations are discussed in light of the ability of cells to rearrange their cytoskeleton and move in an organized manner. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Connexin43 modulates cell polarity and directional cell migration by regulating microtubule dynamics

Knockout mice deficient in the gap junction gene connexin43 exhibit developmental anomalies associated with abnormal neural crest, primordial germ cell, and proepicardial cell migration. These migration defects are due to a loss of directional cell movement, and are associated with abnormal actin stress fiber organization and a loss of polarized cell morphology. To elucidate the mechanism by which Cx43 regulates cell polarity, we used a wound closure assays with mouse embryonic fibroblasts (MEFs) to examine polarized cell morphology and directional cell movement. Studies using embryonic fibroblasts from Cx43 knockout (Cx43KO) mice showed Cx43 deficiency caused cell polarity defects as characterized by a failure of the Golgi apparatus and the microtubule organizing center to reorient with the direction of wound closure. Actin stress fibers at the wound edge also failed to appropriately align, and stabilized microtubule (Glu-tubulin) levels were markedly reduced. Forced expression of Cx43 with deletion of its tubulin-binding domain (Cx43dT) in both wildtype MEFs and neural crest cell explants recapitulated the cell migration defects seen in Cx43KO cells. However, forced expression of Cx43 with point mutation causing gap junction channel closure had no effect on cell motility. TIRF imaging revealed increased microtubule instability in Cx43KO cells, and microtubule targeting of membrane localized Cx43 was reduced with expression of Cx43dT construct in wildtype cells. Together, these findings suggest the essential role of Cx43 gap junctions in development is mediated by regulation of the tubulin cytoskeleton and cell polarity by Cx43 via a nonchannel function.

The cytoplasmic domain of TGFβR3 through its interaction with the scaffolding protein, GIPC, directs epicardial cell behavior

The epicardium is a major contributor of the cells that are required for the formation of coronary vessels. Mice lacking both copies of the gene encoding the Type III Transforming Growth Factor β Receptor (TGFβR3) fail to form the coronary vasculature, but the molecular mechanism by which TGFβR3 signals coronary vessel formation is unknown. We used intact embryos and epicardial cells from E11.5 mouse embryos to reveal the mechanisms by which TGFβR3 signals and regulates epicardial cell behavior. Analysis of E13.5 embryos reveals a lower rate of epicardial cell proliferation and decreased epicardially derived cell invasion in Tgfbr3(-/-) hearts. Tgfbr3(-/-) epicardial cells in vitro show decreased proliferation and decreased invasion in response to TGFβ1 and TGFβ2. Unexpectedly, loss of TGFβR3 also decreases responsiveness to two other important regulators of epicardial cell behavior, FGF2 and HMW-HA. Restoring full length TGFβR3 in Tgfbr3(-/-) cells rescued deficits in invasion in vitro in response TGFβ1 and TGFβ2 as well as FGF2 and HMW-HA. Expression of TGFβR3 missing the 3 C-terminal amino acids that are required to interact with the scaffolding protein GIPC1 did not rescue any of the deficits. Overexpression of GIPC1 alone in Tgfbr3(-/-) cells did not rescue invasion whereas knockdown of GIPC1 in Tgfbr3(+/+) cells decreased invasion in response to TGFβ2, FGF2, and HMW-HA. We conclude that TGFβR3 interaction with GIPC1 is critical for regulating invasion and growth factor responsiveness in epicardial cells and that dysregulation of epicardial cell proliferation and invasion contributes to failed coronary vessel development in Tgfbr3(-/-) mice.

Ectopic origin of coronary arteries from the aorta in Syrian hamsters (Mesocricetus auratus)

An ectopic origin of the coronary artery from the aorta beyond the sinotubular junction, a condition commonly referred to as 'coronary artery high take-off', has been described in man and C57BL/6 mice. The present paper reports this congenital coronary artery anomaly in the Syrian hamster (Mesocricetus auratus). Hearts from 14 individuals, aged 53-350 days, were examined by means of a corrosion-cast technique, scanning electron microscopy or histological and immunohistochemical techniques. In 11 hamsters, the right coronary artery was the ectopic vessel. In the other three animals there was a solitary coronary ostium in the aorta. In all cases, the ectopic coronary artery originated at an acute angle and a valve-like ridge was in front of the coronary artery ostium. The ectopic arteries examined microscopically showed an intramural trajectory within the aortic wall. In the hearts with a solitary ostium in the aorta, the left main coronary artery coursed between the aorta and the pulmonary artery. In man, all of these anomalous conditions place the individual at risk of myocardial ischaemia and sudden death. However, none of the affected hamsters had clinical signs of disease. Intimal thickenings of increasing size with age were present in the intramural coronary artery segment of eight hamsters aged 106 days or older, examined histologically. The present findings fit with the notion that coronary arteries with acute angle take-off and an intramural course are subjected to unusual wear and tear, leading to tissue changes in the vessel wall.

Sox4 mediates Tbx3 transcriptional regulation of the gap junction protein Cx43

Tbx3, a T-box transcription factor, regulates key steps in development of the heart and other organ systems. Here, we identify Sox4 as an interacting partner of Tbx3. Pull-down and nuclear retention assays verify this interaction and in situ hybridization reveals Tbx3 and Sox4 to co-localize extensively in the embryo including the atrioventricular and outflow tract cushion mesenchyme and a small area of interventricular myocardium. Tbx3, SOX4, and SOX2 ChIP data, identify a region in intron 1 of Gja1 bound by all tree proteins and subsequent ChIP experiments verify that this sequence is bound, in vivo, in the developing heart. In a luciferase reporter assay, this element displays a synergistic antagonistic response to co-transfection of Tbx3 and Sox4 and in vivo, in zebrafish, drives expression of a reporter in the heart, confirming its function as a cardiac enhancer. Mechanistically, we postulate that Sox4 is a mediator of Tbx3 transcriptional activity.

Developmental defects of coronary vasculature in rat embryos administered bis-diamine

BACKGROUND

Conotruncal anomalies are often associated with abnormal coronary arteries. Although bis-diamine is known to induce conotruncal defects, its pathological effects on coronary vascular development have not been demonstrated. This study sought to assess the teratogenic effects of bis-diamine on coronary vascular development and the pathogenesis of this anomalous association.

METHODS AND RESULTS

A single 200 mg dose of bis-diamine was administered to pregnant Wistar rats at 10.5 days of gestation. Fifty-two embryos from 10 mother rats underwent morphological analysis of the coronary arteries. Three embryos each were removed from four mothers on embryonic days (ED) 14.5, 15.5, 16.5, and 17.5 and used for immunohistochemical studies using the anti-vascular cell adhesion molecule (VCAM)-1 antibody. Conotruncal anomalies were detected in 48 of 52 embryos, and an aplastic or hypoplastic left coronary artery was found in all of them. In control embryos at ED 16.5, VCAM-1-positive epicardial cells were transformed into mesenchymal cells in vascular plexus, which appeared to differentiate into the endothelial cells of coronary vasculature. In the heart at ED 17.5, coronary vasculature was well developed and connected with coronary ostia near the aorta. However, poor epicardial-mesenchymal transformation and subsequent differentiation was revealed in bis-diamine-treated embryos at EDs 16.5 and 17.5, causing abnormal development of the coronary vasculature and incomplete connections with coronary ostia of the aorta.

CONCLUSIONS

Anomalous coronary arteries in the bis-diamine-treated embryos are induced by the disruption of epicardial-mesenchymal transformation and subsequent poor development of coronary vasculature. Incomplete hatching of the coronary ostium is associated with abnormal truncal division.

Anomalous origin of the left coronary artery from the right pulmonary artery presenting following relief of left heart obstruction: a distinct and predictable clinico-pathological syndrome

INTRODUCTION

Pre-operative recognition of significant abnormalities of the coronary arteries is important in a variety of congenital cardiac conditions. Failure to diagnose anomalous origin of the coronary artery from the pulmonary artery during repair of other anomalies is important because reduction in pulmonary artery pressure will reduce myocardial perfusion pressure.

PATIENTS

We report two cases of the rare association of anomalous origin of the left coronary artery from the right pulmonary artery, aortic coarctation, and mitral stenosis.

CONCLUSIONS

Definitive imaging of coronary artery anatomy by echocardiography or other modalities should form a routine part of diagnostic assessment in all congenital heart disease patients but particularly those with left heart obstruction.

Connexins, cell motility, and the cytoskeleton

Connexins (Cx) comprise a family of transmembrane proteins, which form intercellular channels between plasma membranes of two adjoining cells, commonly known as gap junctions. Recent reports revealed that Cx proteins interact with diverse cellular components to form a multiprotein complex, which has been termed "Nexus". Potential interaction partners include proteins such as cytoskeletal proteins, scaffolding proteins, protein kinases and phosphatases. These interactions allow correct subcellular localization of Cxs and functional regulation of gap junction-mediated intercellular communication. Evidence is accruing that Cxs might have channel-independent functions, which potentially include regulation of cell migration, cell polarization and growth control. In the current review, we summarize recent knowledge on Cx interactions with cytoskeletal proteins and highlight some aspects of their role in cellular motility.

Gap junction heterogeneity as mechanism for electrophysiologically distinct properties across the ventricular wall

Gap junctions are critical to maintaining synchronized impulse propagation and repolarization. Heterogeneous expression of the principal ventricular gap junction protein connexin43 (Cx43) is associated with action potential duration (APD) dispersion across the anterior ventricular wall. Little is known about Cx43 expression patterns and their disparate impact on regional electrophysiology throughout the heart. We aimed to determine whether the anterior and posterior regions of the heart are electrophysiologically distinct. Multisegment, high-resolution optical mapping was performed in canine wedge preparations harvested separately from the anterior left ventricle (aLV; n = 8) and posterior left ventricle (pLV; n = 8). Transmural APD dispersion was significantly greater on the aLV than the pLV (45 +/- 13 vs. 26 +/- 8.0 ms; P < 0.05). Conduction velocity dispersion was also significantly higher (P < 0.05) across the aLV (39 +/- 7%) than the pLV (16 +/- 3%). Carbenoxolone perfusion significantly enhanced APD and conduction velocity dispersion on the aLV (by 1.53-fold and 1.36-fold, respectively), but not the pLV (by 1.27-fold and 1.2-fold, respectively), and produced a 4.2-fold increase in susceptibility to inducible arrhythmias in the aLV. Confocal immunofluorescence microscopy revealed significantly (P < 0.05) greater transmural dispersion of Cx43 expression on the aLV (44 +/- 10%) compared with the pLV wall (8.3 +/- 0.7%), suggesting that regional expression of Cx43 expression patterns may account for regional electrophysiological differences. Computer simulations affirmed that localized uncoupling at the epicardial-midmyocardial interface is sufficient to produce APD gradients observed on the aLV. These data demonstrate that the aLV and pLV differ importantly with respect to their electrophysiological properties and Cx43 expression patterns. Furthermore, local underexpression of Cx43 is closely associated with transmural electrophysiological heterogeneity on the aLV. Therefore, regional and transmural heterogeneous Cx43 expression patterns may be an important mechanism underlying arrhythmia susceptibility, particularly in disease states where gap junction expression is altered.

Genetic fate mapping demonstrates contribution of epicardium-derived cells to the annulus fibrosis of the mammalian heart

The annulus fibrosis electrically insulates the atria and ventricles, allowing the timed sequential beating of these structures that is necessary for efficient heart function. Abnormal development of the annulus fibrosis leads to persistence of accessory electrical pathways from atria to ventricles, providing the anatomical substrate for re-entrant cardiac arrhythmias such as Wolff-Parkinson-White syndrome. To better understand the development of the annulus fibrosis and the etiology of these cardiac arrhythmias, we used Cre-LoxP technology to assess the contribution of epicardium derived cells (EPDCs) to the annulus fibrosis. We found that EPDCs migrated into the region of the forming annulus fibrosis, marked by the protein periostin. These EPDCs also stained positive for procollagen I, suggesting that the EPDCs themselves synthesize proteins of the annulus fibrosis. To further test the hypothesis that EPDCs contribute to cells that synthesize the annulus fibrosis, we purified genetically marked EPDCs from the atrioventricular region and measured gene expression by quantitative PCR. These EPDCs were highly enriched for mRNAs encoding periostin, procollagen I, fibronectin I, vimentin, discoidin domain receptor 2, and tenascin C, markers of fibroblasts and components of the annulus fibrosis. In addition, these EPDCs were highly enriched for Snail, Smad1, Slug, and Twist1, markers for epithelial-to-mesenchymal transition (EMT), and a metalloprotease, Mmp2, that contributes to cellular migration. Our work provides for the first time definitive evidence that epicardium contributes to formation of the mammalian annulus fibrosis through EMT. Abnormalities of this differentiation process may underlie development of some forms of re-entrant atrioventricular tachycardia.

Loss of glypican-3 function causes growth factor-dependent defects in cardiac and coronary vascular development

Glypican-3 (Gpc3) is a heparan sulfate proteoglycan (HSPG) expressed widely during vertebrate development. Loss-of-function mutations cause Simpson-Golabi-Behmel syndrome (SGBS), a rare and complex congenital overgrowth syndrome with a number of associated developmental abnormalities including congenital heart disease. We found that Gpc3-deficient mice display a high incidence of congenital cardiac malformations like ventricular septal defects, common atrioventricular canal and double outlet right ventricle. In addition we observed coronary artery fistulas, which have not been previously reported in SGBS. Coronary artery fistulas are noteworthy because little is known about the molecular basis of this abnormality. Formation of the coronary vascular plexus in Gpc3-deficient embryos was delayed compared to wild-type, and consistent with GPC3 functioning as a co-receptor for fibroblast growth factor-9 (FGF9), we found a reduction in Sonic Hedgehog (Shh) mRNA expression and signaling in embryonic mutant hearts. Interestingly, we found an asymmetric reduction in SHH signaling in cardiac myocytes, as compared with perivascular cells, resulting in excessive coronary artery formation in the Gpc3-deficient animals. We hypothesize that the excessive development of coronary arteries over veins enables the formation of coronary artery fistulas. This work has broad significance to understanding the genetic basis of coronary development and potentially to molecular mechanisms relevant to revascularization following ischemic injury to the heart.

Coronary vessel development and insight towards neovascular therapy

Formation of the coronary arteries consists of a precisely orchestrated series of morphogenetic and molecular events which can be divided into three distinct processes: vasculogenesis, angiogenesis and arteriogenesis (Risau 1997; Carmeliet 2000). Even subtle perturbations in this process may lead to congenital coronary artery anomalies, as occur in 0.2-1.2% of the general population (von Kodolitsch et al. 2004). Contrary to the previously held dogma, the process of vasculogenesis is not limited to prenatal development. Both vasculogenesis and angiogenesis are now known to actively occur within the adult heart. When the need for regeneration arises, for example in the setting of coronary artery disease, a reactivation of embryonic processes ensues, redeploying many of the same molecular regulators. Thus, an understanding of the mechanisms of embryonic coronary vasculogenesis and angiogenesis may prove invaluable in developing novel strategies for cardiovascular regeneration and therapeutic coronary angiogenesis.

Overexpression of the transcription factor Hand1 causes predisposition towards arrhythmia in mice

Elevated levels of the cardiac transcription factor Hand1 have been reported in several adult cardiac diseases but it is unclear whether this change is itself maladaptive with respect to heart function. To test this possibility, we have developed a novel, inducible transgenic system, and used it to overexpress Hand1 in adult mouse hearts. Overexpression of Hand1 in the adult mouse heart leads to mild cardiac hypertrophy and a reduction in life expectancy. Treated mice show no significant fibrosis, myocyte disarray or congestive heart failure, but have a greatly reduced threshold for induced ventricular tachycardia, indicating a predisposition to cardiac arrhythmia. Within 48 h, they show a significant loss of connexin43 protein from cardiac intercalated discs, with increased intercalated disc beta-catenin expression at protein and RNA levels. These changes are sustained during prolonged Hand1 overexpression. We propose that cardiac overexpression of Hand1 offers a useful mouse model of arrhythmogenesis and elevated HAND1 may provide one of the molecular links between the failing heart and arrhythmia.

Histology atlas of the developing mouse heart with emphasis on E11.5 to E18.5

In humans, congenital heart diseases are common. Since the rapid progression of transgenic technologies, the mouse has become the major animal model of defective cardiovascular development. Moreover, genetically modified mice frequently die in utero, commonly due to abnormal cardiovascular development. A variety of publications address specific developmental stages or structures of the mouse heart, but a single reference reviewing and describing the anatomy and histology of cardiac developmental events, stage by stage, has not been available. The aim of this color atlas, which demonstrates embryonic/fetal heart development, is to provide a tool for pathologists and biomedical scientists to use for detailed histological evaluation of hematoxylin and eosin (H&E)-stained sections of the developing mouse heart with emphasis on embryonic days (E) 11.5-18.5. The selected images illustrate the main structures and developmental events at each stage and serve as reference material for the confirmation of the chronological age of the embryo/early fetus and assist in the identification of any abnormalities. An extensive review of the literature covering cardiac development pre-E11.5 is summarized in the introduction. Although the focus of this atlas is on the descriptive anatomic and histological development of the normal mouse heart from E11.5 to E18.5, potential embryonic cardiac lesions are discussed with a list of the most common transgenic pre- and perinatal heart defects. Representative images of hearts at E11.5-15.5 and E18.5 are provided in Figures 2-4, 6, 8, and 9. A complete set of labeled images (Figures E11.5-18.5) is available on the CD enclosed in this issue of Toxicologic Pathology. All digital images can be viewed online at https://niehsimages.epl-inc.com with the username "ToxPath" and the password "embryohearts."

Expression of active Notch1 in avian coronary development

Notch1 is an important regulator of intercellular interactions in cardiovascular development. We show that the nuclear-localized, cleaved and active form of Notch1, the Notch1 intracellular domain (N1ICD), appeared in mesothelial cells of the pro-epicardium during epicardial formation at looped heart stages. N1ICD was also present in mesothelial cells and mesenchymal cells specifically within the epicardium at sulcus regions. N1ICD-positive endothelial cells were detected within the nascent vessel plexus at the atrioventricular junction and within the compact myocardium (Hamburger and Hamilton stage [HH] 25-HH30). The endothelial cells expressing N1ICD were surrounded by N1ICD-positive smooth muscle cells after coronary orifice formation (HH32-HH35), while N1ICD expression was absent in the mesenchymal and mesothelial cells surrounding mature coronary vessels. We propose that differential activation of the hypoxia/HIF1-VEGF-Notch pathway may play a role in epicardial cell interactions that promote epicardial epithelial/mesenchymal transition and coronary progenitor cell differentiation during epicardial development and coronary vasculogenesis in particularly hypoxic sulcus regions.

A role for connexin43 in macrophage phagocytosis and host survival after bacterial peritoneal infection

The pathways that lead to the internalization of pathogens via phagocytosis remain incompletely understood. We now demonstrate a previously unrecognized role for the gap junction protein connexin43 (Cx43) in the regulation of phagocytosis by macrophages and in the host response to bacterial infection of the peritoneal cavity. Primary and cultured macrophages were found to express Cx43, which localized to the phagosome upon the internalization of IgG-opsonized particles. The inhibition of Cx43 using small interfering RNA or by obtaining macrophages from Cx43 heterozygous or knockout mice resulted in significantly impaired phagocytosis, while transfection of Cx43 into Fc-receptor expressing HeLa cells, which do not express endogenous Cx43, conferred the ability of these cells to undergo phagocytosis. Infection of macrophages with adenoviruses expressing wild-type Cx43 restored phagocytic ability in macrophages from Cx43 heterozygous or deficient mice, while infection with viruses that expressed mutant Cx43 had no effect. In understanding the mechanisms involved, Cx43 was required for RhoA-dependent actin cup formation under adherent particles, and transfection with constitutively active RhoA restored a phagocytic phenotype after Cx43 inactivation. Remarkably, mortality was significantly increased in a mouse model of bacterial peritonitis after Cx43 inhibition and in Cx43 heterozygous mice compared with untreated and wild-type counterparts. These findings reveal a novel role for Cx43 in the regulation of phagocytosis and rearrangement of the F-actin cytoskeleton, and they implicate Cx43 in the regulation of the host response to microbial infection.

Endothelial injury induces vascular smooth muscle cell proliferation in highly localized regions of a direct contact co-culture system

Though previous studies have indicated a relationship between the proliferation of endothelial cells and vascular smooth muscle cells (VSMCs) in co-culture, the results have been contradictory and the signaling mechanism poorly understood. In this transmembrane co-culture study, VSMCs and endothelial cells were grown to confluence on opposite sides of a microporous membrane to mimic the intima/media border of vessels. The endothelial layer was injured, and then cultured for 3 days, resulting in partial re-endothelialization. VSMC proliferation across from the injured/partially recovered endothelial region was significantly higher than across from the de-endothelialized region (a sevenfold increase) and the uninjured region (a threefold increase). ELISA indicated that PDGF, which was undetectable in uninjured co-culture and homotypic controls, increased after injury and the addition of a piperazinyl-quinazoline carboxamide PDGF receptor inhibitor blocked VSMC proliferation across from the injured/partially recovered region. We conclude that co-culture signaling initiated by endothelial cell injury locally stimulates VSMC proliferation and that this signaling could be mediated by PDGF-BB.

Prenatal zinc deficiency: influence on heart morphology and distribution of key heart proteins in a rat model

The etiology of congenital heart disease is multifactorial, with genetics and nutritional deficiencies recognized as causative agents. Maternal zinc (Zn) deficiency is associated with an increased risk for fetal heart malformations; however, the contributing mechanisms have yet to be identified. In this study, we fed pregnant rats a Zn-adequate diet (ZnA), a Zn-deficient (ZnD), or a restricted amount of Zn adequate diet (RF) beginning on gestation day (GD) 4.5, to examine whether increased cell death and changes in cardiac neural crest cells (NCC) play a role in Zn deficiency-induced heart defects. Fetuses were collected on GD 13.5, 15.5, and 18.5 and processed for GATA-4, FOG-2, connexin-43 (Cx43), HNK-1, smooth muscle alpha-actin (SMA) and cleaved caspase-3 protein expression. Fetuses from ZnA-fed dams showed normal heart development, whereas fetuses from dams fed with the ZnD diet exhibited a variety of heart anomalies, particularly in the region of the outflow tract. HNK-1 expression was lower than normal in the hearts of GD13.5 and 15.5 ZnD fetuses, particularly in the right atrium and in the distal tip of the interventricular septum. Conversely, Cx43 immunoreactivity was increased throughout the heart in fetuses from ZnD dams compared to fetuses from control dams. The distribution and intensity of expression of SMA, GATA-4, FOG-2, and markers of apoptosis were similar among the three groups. We propose that Zn deficiency induced alterations in the distribution of Cx43 and HNK-1 in fetal hearts contribute to the occurrence of the developmental heart anomalies.

Connexons and cell adhesion: a romantic phase

Recent evidence indicates, that gap junction forming proteins do not only contribute to intercellular communication (Kanno and Saffitz in Cardiovasc Pathol 10:169–177, 2001; Saez et al. in Physiol Rev 83:1359–1400, 2003), ion homeostasis and volume control (Goldberg et al. in J Biol Chem 277:36725–36730, 2002; Saez et al. in Physiol Rev 83:1359–1400, 2003). They also serve biological functions in a mechanical sense, supporting adherent connections between neighbouring cells of epithelial and non-epithelial tissues (Clair et al. in Exp Cell Res 314:1250–1265, 2008; Shaw et al. in Cell 128:547–560, 2007), where they stabilize migratory pathways in the developing central nervous system (Elias et al. in Nature 448:901–907, 2007; Malatesta et al. in Development 127:5253–5263, 2000; Noctor et al. in Nature 409:714–720, 2001; Rakic in Brain Res 33:471–476, 1971; J Comp Neurol 145:61–83 1972; Science 241:170–176, 1988), or mediate polarized movements and directionality of neural crest cells during organogenesis (Kirby and Waldo in Circ Res 77:211–215, 1995; Xu et al. in Development 133:3629–3639, 2006). Since, most data describing adhesive properties of gap junctions delt with connexin 43 (Cx43) (Beardslee et al. in Circ Res 83:629–635, 1998), we will focus our brief review on this isoform.