Background information. The sensing of head movement in mammals depends upon the vestibular endorgan of the inner ear, a complex structure made up of the semicircular canals and otoliths. Due to the ...similarity between the human and mouse vestibular apparatus, the analysis of mutant mouse is a valuable strategy aiming to identify genes involved in the control of balance and movement.
Results. In the course of a genome‐wide chemical‐mutagenesis programme, we isolated a recessive mutation, named ied (inner ear defect), which induced a severe loss‐of‐balance. A detailed phenotypic analysis of the mutant mice demonstrates that the balance impairment does not affect the motor activity and can be rescued, in part, by training, despite a complete agenesis of otoconia in the utricule and the saccule of the inner ear. Molecular characterization of the ied mutation revealed a transversion that affects the splicing of the second exon of the Otopetrin1 gene located on mouse chromosome 5. The consequence of such a mutation leads to a disruption of the transcription of the gene.
Conclusions. The identification of the ied knock‐down allele strengthens the role of the Otopetrin1 in the sensing of balance. Moreover, the rescue of the ied mutant phenotype in specific behavioural tasks confirmed that other sensory inputs or neural plasticity can compensate, to some extent, for the loss‐of‐balance. In the future, the ied mutant mice might be helpful to study the genetic control of the compensation strategies developed by organisms to counteract balance defects.
Congenital heart disease is a major cause of disability and morbidity. Relatively little is known about the molecular mechanism of cardiac adaptation (hypertrophy) and maladaptation (apoptosis) ...underlying cardiac pathogenesis. Several lines of evidence suggest that serotonin (5-hydroxytryptamine, 5-HT) is a neurohormone that regulates cardiovascular functions. We have recently shown that inactivation of the Gq-coupled 5-HT sub(2B)R gene leads to partial embryonic lethality due to trabeculae defects. Newborn 5-HT sub(2B)R mutant mice exhibit cardiac dilation resulting from contractility deficits and structural deficits at the intercellular junctions between cardiomyocytes. We used cultured cardiomyocytes and 5-HT sub(2B) receptor knockout mice as an animal model of dilated cardiomyopathy to identify the molecular mechanism of cardiac functions triggered by serotonin. These findings identify 5-HT as a novel survival factor targeting mitochondria in cardiomyocytes. Modulation of 5-HT sub(2B) receptor signaling may have applications in the treatment of acute myocardial infarction and congestive heart failure.
The MDM2 proto-oncogene is overexpressed in human tumours and regulates the activities of the tumour suppressors p53 and pRB. We created mice that overexpress MDM2 under the control of the CMV ...promoter. These mice did not display an increased tumour incidence, but rather a specific skin phenotype, characterized by desquamation and hyperkeratosis. Transgenic MDM2 was found to be overexpressed in the epidermis, a tissue that normally expresses high levels of MDM2. The phenotype appeared during the first week after birth and then lessened with age, closely following the level of expression of the transgene. MDM2 overexpression was associated with an increase in proliferation in the basal layer, thickening of the epidermis, altered expression of the differentiation markers cytokeratin CK14, CK10 and CK1, and a decrease in the size and the number of granules that contain products of differentiation. Transgenic mice on a p53 null background displayed similar although not identical changes, showing that the effects of MDM2 are to a certain degree p53 independent. The skin is a major site of MDM2 expression in mice, raising the possibility that MDM2 overexpression perturbs the normal pattern of MDM2 expression and inhibits differentiation of the epidermis.
To investigate the relative contribution of retinoic acid receptor (RAR)β isoforms in conotruncal septation, RARβ1 and β3 were inactivated in the mouse. Mice lacking RARβ1 and β3 appear normal. ...Disruption of these isoforms in RARα or RARγ null genetic backgrounds results in a high postpartum lethality. However, except for ocular defects found in RARβ1-3/RARγ compound mutants, the double null mutants display only abnormalities seen in single null mutants. This probably reflects a functional redundancy with other RARs, most notably with RARβ2 which is five- to sixfold more abundant than RARβ1 and β3 and whose domain of expression is largely overlapping. The conotruncal ridges form normally in retinoid X receptor (RXR)α/RARβ compound mutants but fail to fuse, apparently as a result of excessive apoptosis of mesenchymal cells. Additionally, many cardiomyocytes in the conotruncal wall of these mutants appear necrotic. Although RARβ1 and β3 are expressed specifically in the conotruncal ridges, failure of fusion of these structures is not more frequent in RXRα/RARβ1-3 double null mutants than in RXRα single null mutants. Similarly, the disruption of the sole RARβ2 isoform in a RXRα null genetic background does not result in an increase of the frequency of conotruncal septum agenesis. However, this agenesis is fully penetrant in RXRα/RARβ
+/−mutants, which reflects distinct roles of RXRα:RARβ1 (and β3) and RXRα:RARβ2 heterodimers in promoting the survival of conotruncal mesenchymal cells. Unexpectedly, we discovered that, in wild-type embryos, the conotruncal mesenchyme is a major site of morphogenetic cell death and that conotruncal myocytes are occasionally necrotic. Thus, excessive cell death in the conotruncus is a potential cause of ventricular septal defects in humans.
Background— Identification of factors regulating myocardial structure and function is important to understand the pathogenesis of heart disease. We previously reported that 5-HT 2B receptor ablation ...in mice leads to dilated cardiomyopathy. In this study, we investigated the pathological consequence of overexpressing 5-HT 2B receptors in heart in vivo. Methods and Results— We have generated transgenic mice overexpressing the Gq-coupled 5-HT 2B receptor specifically in heart. We found that overexpression of 5-HT 2B receptor in heart leads to ventricular hypertrophy as the result of increased cell number and size. Increased atrial natriuretic peptide and myosin heavy chain expression demonstrated activation of the molecular program for cardiac hypertrophy. Echocardiographic analysis indicated the presence of thickened ventricular free wall without alteration of the systolic function, showing that transgenic mice have compensated hypertrophy. Electron microscopic analysis revealed structural abnormalities including mitochondrial proliferation, as also manifested by histological staining. Transgenic mouse heart displayed a specific reduction in the expression levels of the adenine nucleotide translocator associated to increase in the succinate dehydrogenase and cytochrome C oxidase mitochondrial activities. Conclusions— Our results constitute the first genetic evidence that overexpression of the 5-HT 2B receptor in the heart leads to compensated hypertrophic cardiomyopathy associated with proliferation of the mitochondria. This observation suggests a role for mitochondria in the hypertrophic signaling that is regulated by serotonin. These transgenic mice provide a new genetic model for hypertrophic heart disease.
Identification of factors regulating cardiomyocyte survival and growth is important to understand the pathogenesis of congenital heart diseases. Little is known about the molecular mechanism of ...cardiac functions triggered by serotonin. The link between signaling circuitry of external stimuli and the mitochondrial apoptotic machinery is of wide interest in cardiac diseases. Using cultured cardiomyocytes and 5-hydroxytryptamine (5-HT)2B-receptor knockout mice as an animal model of dilated cardiomyopathy, for the first time we show that serotonin via the Gq-coupled 5-HT2B-receptor protect cardiomyocytes against serum deprivation-induced apoptosis as manifested by DNA fragmentation, nuclear chromatin condensation, and TUNEL labeling. Serotonin prevents cytochrome c release and caspase-9 and -3 activation after serum deprivation via cross-talks between phosphatidylinositol-3 kinase/Akt and extracellular signal-regulated kinase (ERK) 1/2 signaling pathways. Serotonin binding to 5-HT2B-receptor activates ERK kinases to inhibit Bax expression induced by serum deprivation. Serotonin via phosphatidylinositol-3 kinase/Akt can activate NF-kappaB that is required for the regulation of the mitochondrial adenine nucleotide translocator (ANT-1). Parallel to these observations, ultrastructural analysis in the 5-HT2B-receptor knockout mice heart revealed pronounced mitochondrial defects in addition to altered mitochondrial enzyme activities (cytochrome oxidase and succinate dehydrogenase) and ANT-1 and Bax expressions. These findings identify 5-HT as a novel survival factor targeting mitochondria in cardiomyocytes.
To investigate the relative contribution of retinoic acid receptor (RAR)β isoforms in conotruncal septation, RARβ1 and β3 were inactivated in the mouse. Mice lacking RARβ1 and β3 appear normal. ...Disruption of these isoforms in RARα or RARγ null genetic backgrounds results in a high postpartum lethality. However, except for ocular defects found in RARβ1-3/RARγ compound mutants, the double null mutants display only abnormalities seen in single null mutants. This probably reflects a functional redundancy with other RARs, most notably with RARβ2 which is five- to sixfold more abundant than RARβ1 and β3 and whose domain of expression is largely overlapping. The conotruncal ridges form normally in retinoid X receptor (RXR)α/RARβ compound mutants but fail to fuse, apparently as a result of excessive apoptosis of mesenchymal cells. Additionally, many cardiomyocytes in the conotruncal wall of these mutants appear necrotic. Although RARβ1 and β3 are expressed specifically in the conotruncal ridges, failure of fusion of these structures is not more frequent in RXRα/RARβ1–3 double null mutants than in RXRα single null mutants. Similarly, the disruption of the sole RARβ2 isoform in a RXRα null genetic background does not result in an increase of the frequency of conotruncal septum agenesis. However, this agenesis is fully penetrant in RXRα/RARβ
+/− mutants, which reflects distinct roles of RXRα:RARβ1 (and β3) and RXRα:RARβ2 heterodimers in promoting the survival of conotruncal mesenchymal cells. Unexpectedly, we discovered that, in wild-type embryos, the conotruncal mesenchyme is a major site of morphogenetic cell death and that conotruncal myocytes are occasionally necrotic. Thus, excessive cell death in the conotruncus is a potential cause of ventricular septal defects in humans.
BACKGROUND INFORMATION: The sensing of head movement in mammals depends upon the vestibular endorgan of the inner ear, a complex structure made up of the semicircular canals and otoliths. Due to the ...similarity between the human and mouse vestibular apparatus, the analysis of mutant mouse is a valuable strategy aiming to identify genes involved in the control of balance and movement. RESULTS: In the course of a genome-wide chemical-mutagenesis programme, we isolated a recessive mutation, named ied (inner ear defect), which induced a severe loss-of-balance. A detailed phenotypic analysis of the mutant mice demonstrates that the balance impairment does not affect the motor activity and can be rescued, in part, by training, despite a complete agenesis of otoconia in the utricule and the saccule of the inner ear. Molecular characterization of the ied mutation revealed a transversion that affects the splicing of the second exon of the Otopetrin1 gene located on mouse chromosome 5. The consequence of such a mutation leads to a disruption of the transcription of the gene. CONCLUSIONS: The identification of the ied knock-down allele strengthens the role of the Otopetrin1 in the sensing of balance. Moreover, the rescue of the ied mutant phenotype in specific behavioural tasks confirmed that other sensory inputs or neural plasticity can compensate, to some extent, for the loss-of-balance. In the future, the ied mutant mice might be helpful to study the genetic control of the compensation strategies developed by organisms to counteract balance defects.
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