Biomedical subjects

Daniel Gros

Publications and source records attributed to Daniel Gros.

8 recordsLinked to original sources

PubMed2006

Connexins and cardiac arrhythmias.

During cardiac remodeling, impulse conduction in the heart is altered by changes in excitability, electrical coupling, and tissue architecture. The impairment of normal impulse conduction is one of the factors that increases the propensity for arrhythmias. This chapter focuses on the relationship between electrical coupling between ventricular myocytes and arrhythmogenesis. Mouse models of decreased electrical coupling in the heart have shown that a clinically relevant 50% reduction in gap junctions in the heart has no effect on impulse conduction or arrhythmogenesis. To impair conduction and arrhythmias, coupling has to be reduced to very low levels. Apparently, there is a large conduction reserve, which can preserve normal impulse conduction even when electrical coupling is moderately reduced. However, cardiac remodeling is also associated with reduced excitability and increased levels of collagen deposition (fibrosis). It is therefore presumably the combination of, in itself ineffective, reduction of electrical coupling with other impairments like fibrosis or reduced excitability that causes the limits of conduction reserve to be exceeded, thereby resulting in abnormal impulse conduction and enhanced arrhythmogenesis.

Animals↗

PubMed2005

Discontinuous conduction in mouse bundle branches is caused by bundle-branch architecture.

BACKGROUND: Recordings of the electrical activity of mouse bundle branches (BBs) suggest reduced conduction velocity (CV) in the midseptal compared with the proximal part of the BB. The present study was performed to elucidate the mechanism responsible for this slowing of conduction. METHODS AND RESULTS: Hearts of 16 mice were isolated and Langendorff perfused. After the right and left ventricular free walls were removed, the extracellular activity of the BB was mapped with a 247-point electrode. Premature stimulation was used to estimate CV restitution in the BBs. Expression/distribution of connexin40 (Cx40), Cx43, and Cx45 was determined. Morphology of the conduction system was assessed by whole-mount acetylcholine esterase staining and in Cx40(+/KI-GFP) hearts. Effective CV in the midseptal part of the left and right BBs was reduced by 50% compared with the proximal BB. CV restitution in the proximal and midseptal parts of the BBs was similar. Myocytes labeled positive for Cx40 and Cx45 in the entire BB. Cx43 colocalized with Cx40 and Cx45 only in the very distal BB. Subcellular distribution of gap junctions differed between proximal and distal BBs. Geometry of the midseptal and distal BBs revealed on both sides a profuse network of interlacing fibers, whereas the proximal BB consisted of a single (right BB) or multiple (left BB) parallel fibers. CONCLUSIONS: Comparison of connexin expression/distribution, geometry of the BBs, and CV characteristics suggests that increased path length for activation resulting from BB geometry is responsible for the apparently reduced CV in the midseptal BB of the mouse heart.

Animals↗

PubMed2005

Mouse model of SCN5A-linked hereditary Lenègre's disease: age-related conduction slowing and myocardial fibrosis.

BACKGROUND: We have previously linked hereditary progressive cardiac conduction defect (hereditary Lenègre's disease) to a loss-of-function mutation in the gene encoding the main cardiac Na+ channel, SCN5A. In the present study, we investigated heterozygous Scn5a-knockout mice (Scn5a+/- mice) as a model for hereditary Lenègre's disease. METHODS AND RESULTS: In Scn5a+/- mice, surface ECG recordings showed age-related lengthening of the P-wave and PR- and QRS-interval duration, coinciding with previous observations in patients with Lenègre's disease. Old but not young Scn5a+/- mice showed extensive fibrosis of their ventricular myocardium, a feature not seen in wild-type animals. In old Scn5a+/- mice, fibrosis was accompanied by heterogeneous expression of connexin 43 and upregulation of hypertrophic markers, including beta-MHC and skeletal alpha-actin. Global connexin 43 expression as assessed with Western blots was similar to wild-type mice. Decreased connexin 40 expression was seen in the atria. Using pangenomic microarrays and real-time PCR, we identified in Scn5a+/- mice an age-related upregulation of genes encoding Atf3 and Egr1 transcription factors. Echocardiography and hemodynamic investigations demonstrated conserved cardiac function with aging and lack of ventricular hypertrophy. CONCLUSIONS: We conclude that Scn5a+/- mice convincingly recapitulate the Lenègre's disease phenotype, including progressive impairment with aging of atrial and ventricular conduction associated with myocardial rearrangements and fibrosis. Our work provides the first demonstration that a monogenic ion channel defect can progressively lead to myocardial structural anomalies.

Age Factors↗

PubMed2005

Dysregulation of connexins and inactivation of NFATc1 in the cardiovascular system of Nkx2-5 null mutants.

In humans, mutations of the gene encoding the transcription factor Nkx2-5 result in the heart in electrical conduction defects and morphological abnormalities. In this organ Nkx2-5 is expressed in both the myocardium and the endocardium. Connexins (Cxs) are gap junction channel proteins that have been shown to be involved in both heart development and cardiac electrical conduction, suggesting a possible correlation between expression of Cxs and Nkx2-5. To evaluate this correlation, the expression of Cxs has been investigated in the cardiovascular system of wild-type and Nkx2-5-/- 9.2 days post-conception (dpc) mouse embryos. The disruption of the Nkx2-5 gene results in the loss of Cx43 in the heart, due in part to the poor development of the ventricular trabecular network, as well as specific downregulation of Cx45 gene expression. In addition, the nuclear translocation of NFATc1 in the endocardial endothelial cells is inhibited in the Nkx2-5-/- embryos. These results indicate for the first time that Nkx2-5 is involved in the transcriptional regulation of the Cx45 gene expression. In the mutant embryos the aorta is collapsed, and the vascular endothelial Cxs, Cx40 and Cx37, are no longer expressed in its posterior region. Poor development of the trabeculae and downregulation of Cx45 may contribute both to failure of the myocardial function and to hemodynamic insufficiency. The latter, in turn, may result in the dysregulation of Cx40 and -37 expressions along the whole length of the aorta. Direct or indirect effects of Nkx2-5 inactivation on the Cx45 gene expression could explain the absence of the endocardial cushions in the heart of Nkx2-5-/- embryos.

Animals↗

PubMed2004

Architectural and functional asymmetry of the His-Purkinje system of the murine heart.

OBJECTIVE: The aim of this work was to target a vital reporter gene in the mouse cardiac conduction system (CS) to distinguish this tissue from the surrounding myocardium in the adult heart. METHODS: A transgenic mouse line has been created in which EGFP is expressed under the control of the Cx40 gene. Correlative investigations associating EGFP imaging and electrophysiological techniques were carried out on the adult heart and isolated cardiomyocytes. RESULTS: In the heart of the Cx40(EGFP/+) mice, EGFP signal was seen in the coronary arteries, the atria, the atrioventricular (AV) node and the His-Purkinje system. The latter was found to be structurally and functionally asymmetrical. The anatomical asymmetry was apparent in both the number of strands or fasciculi making up the His bundle branches (BBs) (1 strand on the right, 20 or so on the left), and the density (low on the right, high on the left) of the network of Purkinje fibers (PFs) that extends over the ventricular wall surfaces. The profiles of the electrical activation patterns recorded on the right and left flanks of the septum were also asymmetrical, mirroring the architecture of the branches. EGFP made it easy to identify the Purkinje cells in populations of dissociated cardiomyocytes and they were investigated using the patch-clamp technique. The hyperpolarization-activated current (If) was recorded in all spontaneously active Purkinje cells. CONCLUSIONS: This investigation provides positive evidence of the asymmetry of the His-Purkinje system of the adult mouse, and the first patch-clamp recording data on murine cardiac Purkinje cells. This mouse model opens up new perspectives for investigating the contribution of specific genes to the morphology and function of the His-Purkinje system.

Action Potentials↗

PubMed2004

Genetically modified mice: tools to decode the functions of connexins in the heart-new models for cardiovascular research.

It has long been known that gap junctions are required for the propagation of electrical impulse in the heart. A good deal later, the connexins (Cxs), which are probably exclusive components of the junctional channels that constitute the gap junctions, were identified. More recently, the in vivo functions of cardiac Cxs have been investigated by the analysis of genetically modified mice. These studies have confirmed that Cxs are involved in cardiac impulse conduction, and, unexpectedly, in heart morphogenesis. In addition, cardiac abnormalities described in mice genetically modified for Cx genes, and those observed in certain human cardiac diseases, have been proven to be similar.

Animals↗

PubMed2003

Replacement of connexin40 by connexin45 in the mouse: impact on cardiac electrical conduction.

Gap junction channels, required for the propagation of cardiac impulse, are intercellular structures composed of connexins (Cx). Cx43, Cx40, and Cx45 are synthesized in the cardiomyocytes, and each of them has a unique cardiac expression pattern. Cx40 knock-in Cx45 mice were generated to explore the ability of Cx45 to replace Cx40, and to assess the functional equivalence of these two Cxs that are both expressed in the conduction system. ECGs revealed that the consequences resulting from the biallelic replacement of Cx40 by Cx45 were an increased duration of the P wave, and a prolonged and fractionated QRS complex. Epicardial mapping indicated that the conduction velocities (CV) in the right atrium and the ventricular myocardium, as well as conduction through the AV node, were unaffected. The significant reduction of the CV in the left atrium would be the most likely cause of the P-wave lengthening. In the right ventricle, a changed and prolonged activation in sinus rhythm was found in homozygous mutant mice, which may explain the prolongation and splitting of the QRS complex. Electrical mapping of the His bundle branches revealed that this was due to slow conduction measured in the right branch. The CV in the left branch was unchanged. Therefore, in the absence of Cx40, the upregulation of Cx45 in the heart results in a normal impulse propagation in the right atrium, the AV node, and the left His bundle branch only.

Animals↗

PubMed2003

Genomic organization and alternative transcripts of the human Connexin40 gene.

The human Cx40 gene (NT_004434.5) was sorted out from the GenBank database and as a result of a BLAST homology search, two ESTs (BE784549 from a human lung database, and BE732411 from a human placenta database) overlapping with the coding exon 2 sequence and upstream regions of the gene were identified. These ESTs correspond to two transcripts 1A and 1B, which diverge from each other in their 5' regions. The transcript 1A corresponds to the only transcript previously identified for the mouse and rat Cx40 genes; whereas the transcript 1B is a new transcript. The human Cx40 gene therefore comprises three exons: exon 1A (100 bp), exon 1B (132 bp) and coding exon 2, with the exons 1A and 1B at 14 and 1.3 kb of the exon 2, respectively. The expression of these transcripts is cell-type specific. Transcript 1A is expressed in endothelial cells. Its expression was demonstrated in human umbilical vein endothelial cells (HUVEC). Transcript 1B is expressed in placental cytotrophoblasts. Its expression was demonstrated in malignant trophoblastic cells, BeWo, JAR and JEG-3, and purified cytotrophoblasts from human first trimester placental tissues. Interestingly, both transcripts 1A and 1B are expressed in the right atrial appendages (RAA), although the cell-type expression of the two transcripts in this particular tissue has not yet been determined. Both transcripts were found to be expressed in the various heart regions investigated, where transcript 1B was found to always occur rarely in comparison with transcript 1A. Transcripts 1A and 1B are both more abundant in the atria than in the ventricles. Luciferase reporter gene assays demonstrated that two genomic regions containing the exons 1A and 1B induced a cell-type specific expression. The 1.2 kb sequence, containing the exon 1A, induced an increase of the luciferase activity in HUVEC; whereas the 1.9 kb sequence, containing the exon 1B, induces an increase of expression of the luciferase activity in BeWo cells. The DNA sequence upstream of the exon 1A contains SP1 binding sites, but no TATA- or CAAT-box; whereas the region upstream of the exon 1B is preceded by three CAAT-boxes. Thus, in contrast to the mouse and rat Cx40 genes, the human Cx40 gene organized in three exons and generates two transcripts, which are cell-type specific.

5' Flanking Region↗