Functional alterations of the Nppa promoter are linked to cardiac ventricular hypertrophy in WKY/WKHA rat crosses

Molecular Signaling Mechanisms and Function of Natriuretic Peptide Receptor-A in the Pathophysiology of Cardiovascular Homeostasis

The discovery of atrial, brain, and C-type natriuretic peptides (ANP, BNP, and CNP) and their cognate receptors has greatly increased our knowledge of the control of hypertension and cardiovascular homeostasis. ANP and BNP are potent endogenous hypotensive hormones that elicit natriuretic, diuretic, vasorelaxant, antihypertrophic, antiproliferative, and antiinflammatory effects, largely directed toward the reduction of blood pressure (BP) and cardiovascular diseases (CVDs). The principal receptor involved in the regulatory actions of ANP and BNP is guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), which produces the intracellular second messenger cGMP. Cellular, biochemical, molecular, genetic, and clinical studies have facilitated understanding of the functional roles of natriuretic peptides (NPs), as well as the functions of their receptors, and signaling mechanisms in CVDs. Transgenic and gene-targeting (gene-knockout and gene-duplication) strategies have produced genetically altered novel mouse models and have advanced our knowledge of the importance of NPs and their receptors at physiological and pathophysiological levels in both normal and disease states. The current review describes the past and recent research on the cellular, molecular, genetic mechanisms and functional roles of the ANP-BNP/NPRA system in the physiology and pathophysiology of cardiovascular homeostasis as well as clinical and diagnostic markers of cardiac disorders and heart failure. However, the therapeutic potentials of NPs and their receptors for the diagnosis and treatment of cardiovascular diseases, including hypertension, heart failure, and stroke have just begun to be expanded. More in-depth investigations are needed in this field to extend the therapeutic use of NPs and their receptors to treat and prevent CVDs.

Exposure to Oxadiazon-Butachlor causes cardiac toxicity in zebrafish embryos

Oxadiazon-Butachlor (OB) is a widely used herbicide for controlling most annual weeds in rice fields. However, its potential toxicity in aquatic organisms has not been evaluated so far. We used the zebrafish embryo model to assess the toxicity of OB, and found that it affected early cardiac development and caused extensive cardiac damage. Mechanistically, OB significantly increased oxidative stress in the embryos by inhibiting antioxidant enzymes that resulted in excessive production of reactive oxygen species (ROS), eventually leading to cardiomyocyte apoptosis. In addition, OB also inhibited the WNT signaling pathway and downregulated its target genes includinglef1, axin2 and β-catenin. Reactivation of this pathway by the Wnt activator BML-284 and the antioxidant astaxanthin rescued the embryos form the cardiotoxic effects of OB, indicating that oxidative stress, and inhibition of WNT target genes are the mechanistic basis of OB-induced damage in zebrafish. Our study shows that OB exposure causes cardiotoxicity in zebrafish embryos and may be potentially toxic to other aquatic life and even humans.

Genetic Ablation and Guanylyl Cyclase/Natriuretic Peptide Receptor-A: Impact on the Pathophysiology of Cardiovascular Dysfunction

Mice bearing targeted gene mutations that affect the functions of natriuretic peptides (NPs) and natriuretic peptide receptors (NPRs) have contributed important information on the pathogenesis of hypertension, kidney disease, and cardiovascular dysfunction. Studies of mice having both complete gene disruption and tissue-specific gene ablation have contributed to our understanding of hypertension and cardiovascular disorders. These phenomena are consistent with an oligogenic inheritance in which interactions among a few alleles may account for genetic susceptibility to hypertension, renal insufficiency, and congestive heart failure. In addition to gene knockouts conferring increased risks of hypertension, kidney disorders, and cardiovascular dysfunction, studies of gene duplications have identified mutations that protect against high blood pressure and cardiovascular events, thus generating the notion that certain alleles can confer resistance to hypertension and heart disease. This review focuses on the intriguing phenotypes of Npr1 gene disruption and gene duplication in mice, with emphasis on hypertension and cardiovascular events using mouse models carrying Npr1 gene knockout and/or gene duplication. It also describes how Npr1 gene targeting in mice has contributed to our knowledge of the roles of NPs and NPRs in dose-dependently regulating hypertension and cardiovascular events.

Identification of differentially expressed genes and preliminary validations in cardiac pathological remodeling induced by transverse aortic constriction

Cardiac remodeling predisposes to heart failure if the burden is unresolved, and heart failure is an important cause of mortality in humans. The aim of the present study was to identify the key genes involved in cardiac pathological remodeling induced by pressure overload. Gene expression profiles of the GSE5500, GSE18224, GSE36074 and GSE56348 datasets were downloaded from the Gene Expression Omnibus database. Differentially expressed genes (DEGs), defined as |log2FC|>1 (FC, fold change) and an adjusted P‑value of <0.05, were screened using the R software with the limma package. Gene ontology enrichment analysis was performed and a protein‑protein interaction (PPI) network of the DEGs was constructed. A cardiac remodeling model induced by transverse aortic constriction (TAC) was established. Furthermore, consistent DEGs were further validated using reverse transcription‑quantitative polymerase chain reaction (RT‑PCR) analysis, western blotting and immunohistochemistry in the ventricular tissue samples after TAC or sham operation. A total of 24 common DEGs were identified (23 significantly upregulated and 1 downregulated), of which 9 genes had been previously confirmed to be directly involved in cardiac remodeling. Hence, the level of expression of the other 15 genes was detected in subsequent studies via RT‑PCR. Based on the results of the PPI network analysis and RT‑PCR, we further detected the protein levels of Itgbl1 and Asporin, which were consistent with the results of bioinformatics analysis and RT‑PCR. The expression of Itgbl1, Aspn, Fstl1, Mfap5, Col8a1, Ltbp2, Mfap4, Pamr1, Cnksr1, Aqp8, Meox1, Gdf15 and Srpx was found to be upregulated in a mouse model of cardiac remodeling, while that of Retnla was downregulated. Therefore, the present study identified the key genes implicated in cardiac remodeling, aiming to provide new insight into the underlying mechanism.

Rare mutations of ADAM17 from TOFs induce hypertrophy in human embryonic stem cell-derived cardiomyocytes via HB-EGF signaling

Tetralogy of Fallot (TOF) is the most common cyanotic form of congenital heart defects (CHDs). The right ventricular hypertrophy is associated with the survival rate of patients with repaired TOF. However, very little is known concerning its genetic etiology. Based on mouse model studies, a disintergrin and metalloprotease 10/17 (ADAM10 and ADAM17) are the key enzymes for the NOTCH and ErbB pathways, which are critical pathways for heart development. Mutations in these two genes have not been previously reported in human TOF patients. In this study, we sequenced ADAM10 and ADAM17 in a Han Chinese CHD cohort comprised of 80 TOF patients, 286 other CHD patients, and 480 matched healthy controls. Three missense variants of ADAM17 were only identified in 80 TOF patients, two of which (Y42D and L659P) are novel and not found in the Exome Aggregation Consortium (ExAC) database. Point mutation knock-in (KI) and ADAM17 knock-out (KO) human embryonic stem cells (hESCs) were generated by CRISPR/Cas9 and programmed to differentiate into cardiomyocytes (CMs). Y42D or L659P KI cells or complete KO cells all developed hypertrophy with disorganized sarcomeres. RNA-seq results showed that phosphatidylinositide 3-kinases/protein kinase B (PI3K/Akt), which is downstream of epidermal growth factor receptor (EGFR) signaling, was affected in both ADAM17 KO and KI hESC-CMs. In vitro experiments showed that these two mutations are loss-of-function mutations in shedding heparin-binding EGF-like growth factor (HB-EGF) but not NOTCH signaling. Our results revealed that CM hypertrophy in TOF could be the result of mutations in ADAM17 which affects HB-EGF/ErbB signaling.

Rapamycin attenuates pathological hypertrophy caused by an absence of trabecular formation

Cardiac trabeculae are mesh-like muscular structures within ventricular walls. Subtle perturbations in trabeculation are associated with many congenital heart diseases (CHDs), and complete failure to form trabeculae leads to embryonic lethality. Despite the severe consequence of an absence of trabecular formation, the exact function of trabeculae remains unclear. Since ErbB2 signaling plays a direct and essential role in trabecular initiation, in this study, we utilized the erbb2 zebrafish mutant as a model to address the function of trabeculae in the heart. Intriguingly, we found that the trabeculae-deficient erbb2 mutant develops a hypertrophic-like (HL) phenotype that can be suppressed by inhibition of Target of Rapamycin (TOR) signaling in a similar fashion to adult mammalian hearts subjected to mechanical overload. Further, cell transplantation experiments demonstrated that erbb2 mutant cells in an otherwise wildtype heart did not undergo hypertrophy, indicating that erbb2 mutant HL phenotypes are due to a loss of trabeculae. Together, we propose that trabeculae serve to enhance contractility and that defects in this process lead to wall-stress induced hypertrophic remodeling.

Dual Linkage of a Locus to Left Ventricular Mass and a Cardiac Gene Co-Expression Network Driven by a Chromosome Domain

We have previously reported Lvm1 as a quantitative trait locus (QTL) on chromosome 13 that links to cardiac left ventricular mass (LVM) in a panel of AxB/BxA mouse recombinant inbred strains (RIS). When performing a gene expression QTL (eQTL) analysis, we detected 33 cis-eQTLs that correlated with LVM. Among the latter, a group of eight cis-eQTLs clustered in a genomic region smaller than 6 Mb and surrounding the Lvm1 peak on chr13. Co-variant analysis indicated that all eight genes correlated with the phenotype in a causal rather than a reactive fashion, a finding that (despite its functional interest) did not provide grounds to prioritize any of these candidate genes. As a complementary approach, we performed weighted gene co-expression network analysis, which allowed us to detect 49 modules of highly connected genes. The module that correlated best with LVM: (1) showed linkage to a module QTL whose boundaries matched closely those of the phenotypic Lvm1 QTL on chr13; (2) harbored a disproportionately high proportion of genes originating from a small genomic region on chromosome 13 (including the 8 previously detected cis-eQTL genes); (3) contained genes that, beyond their individual level of expression, correlated with LVM as a function of their inter-connectivity; and (4) showed increased abundance of polymorphic insertion–deletion elements in the same region. Taken together, these data suggest that a domain on chromosome 13 constitutes the biologic principle responsible for the organization and linkage of the gene co-expression module, and indicate a mechanism whereby genetic variants within chromosome domains may associate to phenotypic changes via coordinate changes in the expression of several genes. One other possible implication of these findings is that candidate genes to consider as contributors to a particular phenotype should extend further than those that are closest to the QTL peak.

Differential metal content and gene expression in rat left ventricular hypertrophy due to hypertension and hyperactivity

The spontaneously hypertensive rat (SHR) has been studied extensively as a model of left ventricular hypertrophy (LVH) and associated cardiac dysfunction due to hypertension (HT). The SHR also possesses a hyperactive trait (HA). Crossbreeding SHR with Wistar-Kyoto (WKY) control rats, which are nonHT and nonHA, followed by selected inbreeding produced two additional homozygous strains: WKHT and WKHA, in which the traits of HT and HA, respectively, are expressed separately. WKHT, WKHA and SHR all display LVH, but only the SHR exhibits cardiac dysfunction. We hypothesized that cardiac dysfunction in the SHR is uniquely characterized by calcium overload. We measured total cardiac Ca, Cu, Fe, K, Mg and Zn in the four strains. We found elevated Ca and depressed Cu, Mg and Zn with HT, but not unique to SHR. We surmise that HT promotes aberrant regulation of cardiac Ca(2+), Cu(2+), Mg(2+) and Zn(2+), which does not necessarily result in cardiac dysfunction. Interestingly, Cu was elevated in HA strains compared to nonHA counterparts. We then analyzed gene expression as mRNA of Cu-containing proteins, most notably mitochondrial-Cox, Dbh, Lox, Loxl1, Loxl2, Sod1 and Tyr. The gene expression profiles of Lox, Loxl1, Loxl2 and Sod1 were found especially high in the WKHA, which if reflective of protein content could account for the high Cu content in the WKHA. The mRNA of other genes, notably Mb, Fxyd1, Maoa and Maob were also examined. We found that Maoa gene expression and monoamine oxidase-A (MAO-A) protein content were low in the SHR compared to the other strains. The finding that MAO-A protein is low in the SHR and normal in the WKHT and WKHA strains is most consistent with the idea that MAO-A protects against the development of cardiac dysfunction in LVH but not against LVH in these rats.

Mapping and confirmation of a major left ventricular mass QTL on rat chromosome 1 by contrasting SHRSP and F344 rats

An abnormal increase in left ventricular (LV) mass, i.e., LV hypertrophy (LVH), represents an important target organ damage in arterial hypertension and has been associated with poor clinical outcome. Genetic factors are contributing to variation in LV mass in addition to blood pressure and other factors such as dietary salt intake. We set out to map quantitative trait loci (QTL) for LV mass by comparing the spontaneously hypertensive stroke-prone (SHRSP) rat with LVH and normotensive Fischer rats (F344) with contrasting low LV mass. To this end we performed a genome-wide QTL mapping analysis in 232 F2 animals derived from SHRSP and F344 exposed to high-salt (4% in chow) intake for 8 wk. We mapped one major QTL for LV mass on rat chromosome 1 (RNO1) that demonstrated strong linkage (peak logarithm of odds score 8.4) to relative LV weight (RLVW) and accounted for ∼19% of the variance of this phenotype in F2 rats. We therefore generated a consomic SHRSP-1(F344) strain in which RNO1 from F344 was introgressed into the SHRSP background. Consomic and SHRSP animals showed similar blood pressures during conventional intra-arterial measurements, while RLVW was already significantly lower (-17.7%, P<0.05) in SHRSP-1(F344) in response to a normal-salt diet; a similar significant reduction of LV mass was also observed in consomic rats after high-salt intake (P<0.05 vs. SHRSP). Thus, a major QTL on RNO1 was confirmed with significant impact on LV mass in the hypertensive background of SHRSP.

Emerging Roles of Natriuretic Peptides and their Receptors in Pathophysiology of Hypertension and Cardiovascular Regulation

Thus far, three related natriuretic peptides (NPs) and three distinct receptors have been identified, which have advanced our knowledge towards understanding the control of high blood pressure, hypertension, and cardiovascular disorders to a great extent. Biochemical and molecular studies have been advanced to examine receptor function and signaling mechanisms and the role of second messenger cGMP in pathophysiology of hypertension, renal hemodynamics, and cardiovascular functions. The development of gene-knockout and gene-duplication mouse models along with transgenic mice have provided a framework for understanding the importance of the antagonistic actions of natriuretic peptides receptor in cardiovascular events at the molecular level. Now, NPs are considered as circulating markers of congestive heart failure, however, their therapeutic potential for the treatment of cardiovascular diseases such as hypertension, renal insufficiency, cardiac hypertrophy, congestive heart failure, and stroke has just begun to unfold. Indeed, the alternative avenues of investigations in this important are need to be undertaken, as we are at the initial stage of the molecular therapeutic and pharmacogenomic implications.

Guanylyl cyclase / atrial natriuretic peptide receptor-A: role in the pathophysiology of cardiovascular regulation

Atrial natriuretic factor (ANF), also known as atrial natriuretic peptide (ANP), is an endogenous and potent hypotensive hormone that elicits natriuretic, diuretic, vasorelaxant, and anti-proliferative effects, which are important in the control of blood pressure and cardiovascular events. One principal locus involved in the regulatory action of ANP and brain natriuretic peptide (BNP) is guanylyl cyclase / natriuretic peptide receptor-A (GC-A/NPRA). Studies on ANP, BNP, and their receptor, GC-A/NPRA, have greatly increased our knowledge of the control of hypertension and cardiovascular disorders. Cellular, biochemical, and molecular studies have helped to delineate the receptor function and signaling mechanisms of NPRA. Gene-targeted and transgenic mouse models have advanced our understanding of the importance of ANP, BNP, and GC-A/NPRA in disease states at the molecular level. Importantly, ANP and BNP are used as critical markers of cardiac events; however, their therapeutic potentials for the diagnosis and treatment of hypertension, heart failure, and stroke have just begun to be realized. We are now just at the initial stage of molecular therapeutics and pharmacogenomic advancement of the natriuretic peptides. More investigations should be undertaken and ongoing ones be extended in this important field.

The functional genomics of guanylyl cyclase/natriuretic peptide receptor-A: perspectives and paradigms

The cardiac hormones atrial natriuretic peptide and B-type natriuretic peptide (brain natriuretic peptide) activate guanylyl cyclase (GC)-A/natriuretic peptide receptor-A (NPRA) and produce the second messenger cGMP. GC-A/NPRA is a member of the growing family of GC receptors. The recent biochemical, molecular and genomic studies on GC-A/NPRA have provided important insights into the regulation and functional activity of this receptor protein, with a particular emphasis on cardiac and renal protective roles in hypertension and cardiovascular disease states. The progress in this field of research has significantly strengthened and advanced our knowledge about the critical roles of Npr1 (coding for GC-A/NPRA) in the control of fluid volume, blood pressure, cardiac remodeling, and other physiological functions and pathological states. Overall, this review attempts to provide insights and to delineate the current concepts in the field of functional genomics and signaling of GC-A/NPRA in hypertension and cardiovascular disease states at the molecular level.

Evaluation of LEXF/FXLE rat recombinant inbred strains for genetic dissection of complex traits

Recombinant inbred (RI) strains are formed from an outcross between two well-characterized inbred stains followed by at least 20 generations of inbreeding. RI strains can be utilized for the analysis of many complex phenotypic traits. The LEXF/FXLE RI strain set consists of 34 RI strains derived by reciprocal crossing of LE/Stm and F344/Stm. Here we report on genetic dissections of complex traits using this RI set and their parental strains. We have developed strain distribution patterns for 232 informative simple sequence length polymorphism markers. The framework map covers the rat genome except for chromosome Y. Seventy-six phenotype parameters, which included physiological and behavioral traits, were examined for these RI lines. Quantitative trait locus (QTL) analysis of these parameters revealed 27 significant and 91 suggestive QTLs, illustrating the potential of this RI resource for the detection of underlying gene functions for various phenotypes. Although this RI set was originally developed to study susceptibility to chemical-induced tumors, it has been shown to be equally powerful for a wide spectrum of traits. The LEXF/FXLE RI strains have been deposited at the National Bio Resource Project for the Rat in Japan and are maintained under specific pathogen-free conditions. They are available at http://www.anim.med.kyoto-u.ac.jp/nbr .

Cardiac mass and cardiomyocyte size are governed by different genetic loci on either autosomes or chromosome Y in recombinant inbred mice

Left ventricular hypertrophy is one of the main risk factors for cardiovascular mortality and morbidity. It has been proposed that hypertrophic stimuli act in great part by increasing the size of cardiomyocytes, and that the latter characteristic is a necessary condition to differentiate left ventricular hypertrophy from other benign forms of cardiac enlargement. To test whether the same genetic loci control the size of cardiomyocytes and left ventricular mass, we performed whole genome linkage analyses in a panel of 24 recombinant inbred AXB/BXA mouse strains. Whereas one major locus was linked to left ventricular mass in both males and females, loci linked to the size of cardiomyocytes were clearly distinct and showed sex-specific linkage. Moreover, the parental origin of chromosome Y had strong effects on the size of cardiomyocytes in male mice but did not affect left ventricular mass. In addition to showing that genetic loci that increase the size of cardiomyocytes are not necessarily linked to increased left ventricular mass, our findings have important consequences in evaluating cardiac phenotypes when performing genetic manipulations in mice, and in determining the cause of sex-specific differences when using models derived from C57BL/6J mice.

Fine-mapping and comprehensive transcript analysis reveals nonsynonymous variants within a novel 1.17 Mb blood pressure QTL region on rat chromosome 10

The presence of blood pressure (BP) quantitative trait loci (QTL) on rat chromosome 10 has been clearly demonstrated by linkage analysis and substitution mapping. Using congenic strains containing the LEW rat chromosomal segments on the Dahl salt-sensitive (S) rat background, further iterations of congenic substrains were constructed and characterized to fine-map a chromosome 10 region (QTL1) linked to blood pressure. Comparison of seven congenic substrains refined QTL1 to a 1.17 Mb segment flanked by D10Mco88 and D10Mco89, which are located at 71,513,116 and 72,684,774 bp, respectively. The newly defined QTL1, containing 18 genes, is captured in its entirety within a single congenic substrain. A thorough transcript analysis revealed that 3 of these 18 genes, Ccl5, Ddx52, and RGD1559577, had nonsynonymous allelic variations between the S rat and the LEW rat. None of the detected transcripts within the newly defined QTL1 are implicated directly in BP control in humans or model organisms. Therefore, the present work defines a novel blood pressure QTL with three potential quantitative trait nucleotides.

Transcriptional alterations in the left ventricle of three hypertensive rat models

Left ventricular hypertrophy (LVH) is commonly associated with hypertension and represents an independent cardiovascular risk factor. The aim of this study was to test the hypothesis that the cardiac overload related to hypertension is associated to a specific gene expression pattern independently of genetic background. Gene expression levels were obtained with microarrays for 15,866 transcripts from RNA of left ventricles from 12-wk-old rats of three hypertensive models [spontaneously hypertensive rat (SHR), Lyon hypertensive rat (LH), and heterozygous TGR(mRen2)27 rat] and their respective controls. More than 60% of the detected transcripts displayed significant changes between the three groups of normotensive rats, showing large interstrain variability. Expression data were analyzed with respect to hypertension, LVH, and chromosomal distribution. Only four genes had significantly modified expression in the three hypertensive models among which a single gene, coding for sialyltransferase 7A, was consistently overexpressed. Correlation analysis between expression data and left ventricular mass index (LVMI) over all rats identified a larger set of genes whose expression was continuously related with LVMI, including known genes associated with cardiac remodeling. Positioning the detected transcripts along the chromosomes pointed out high-density regions mostly located within blood pressure and cardiac mass quantitative trait loci. Although our study could not detect a unique reprogramming of cardiac cells involving specific genes at early stage of LVH, it allowed the identification of some genes associated with LVH regardless of genetic background. This study thus provides a set of potentially important genes contained within restricted chromosomal regions involved in cardiovascular diseases.

Cardiomyocyte function associated with hyperactivity and/or hypertension in genetic models of LV hypertrophy

We examined cardiomyocyte intracellular calcium ([Ca2+]i) dynamics and sarcomere shortening dynamics in genetic rat models of left ventricular (LV) hypertrophy associated with or without hypertension (HT) and with or without hyperactive (HA) behavior. Previous selective breeding of the spontaneously hypertensive rat (SHR) strain, which is HA and HT, with the Wistar-Kyoto (WKY) rat strain, which is not hyperactive (NA) and not hypertensive (NT), has led to two unique strains: the WKHA strain, selected for HA and NT, and the WKHT strain, selected for NA and HT. Cardiomyocytes were isolated from young adult males and females of each strain, paced at 2, 3, and 4 Hz in 1.2 mM external Ca2+ concentration at 37 degrees C, and cardiomyocyte [Ca2+]i and sarcomere dynamics were recorded simultaneously. Under these conditions, LV cardiomyocyte systolic and diastolic [Ca2+]i dynamics and diastolic sarcomere dynamics in the WKHT were significantly enhanced compared with WKY controls, suggesting an underlying LV hypertrophic response that successfully compensated for HT in the absence of HA. LV cardiomyocyte [Ca2+]i dynamics in the WKHA and SHR were strikingly similar to each other and only slightly reduced compared with WKY. LV cardiomyocyte systolic and diastolic sarcomere dynamics, on the other hand, were significantly reduced in the SHR compare with WKHA and more so in male than in female SHR. We conclude from these data that HT alone is an insufficient descriptor of the cause of LV hypertrophy and diminished LV cardiomyocyte function in the SHR rat. These data further suggest that HA (augmented by male sex) in the SHR may interact with the HT state to initiate impaired cardiomyocyte function and thereby inhibit or undermine an otherwise compensatory response that may occur with HT in the absence of HA.

Distinct QTLs are linked to cardiac left ventricular mass in a sex-specific manner in a normotensive inbred rat inter-cross

Genetic mapping of the progeny of an F(2) inter-cross between WKY and WKHA rats had previously allowed us to detect male-specific linkage between locus Cm 24 and left ventricular mass index (LVMI). By further expanding that analysis, we detected additional loci that were all linked to LVMI in a sex-specific manner despite their autosomal location. In males, we detected one additional locus (Lvm 8) on Chromosome 5 (LOD=3.4), the two loci Lvm 13 (LOD=4.5) and Lvm 9 (LOD=2.8) on Chromosome 17, and locus Lvm 10 (LOD=4.2) on Chromosome 12. The locus Lvm 13 had the same boundaries as locus Cm 26 previously reported by others using a different cross. None of these loci showed linkage to LVM in females. In contrast, we identified in females the novel locus Lvm 11 on Chromosome 15 (LOD=2.8) and locus Lvm 12 (LOD=2.7) that had the same boundaries on Chromosome 3 as locus Cm 25 detected previously by others using a cross of other normotensive strains. In prepubertal males, there were no differences in the width of cardiomyocytes from WKY and WKHA rats, but cardiomyocytes from WKHA became progressively wider than that of WKY as sexual maturation progressed. Altogether, these results provide evidence that distinct genes may influence LVMI of rats in a sex-dependent manner, maybe by involving sex-specific interactions of sex steroids with particular genes involved in the determination of LVMI and/or cardiomyocyte width.

Identification of quantitative trait loci for cardiac hypertrophy in two different strains of the spontaneously hypertensive rat

Cardiac hypertrophy and left ventricular hypertrophy are known to be substantially controlled by genetic factors. As an experimental model, we undertook genome-wide screens for cardiac mass in F2 populations bred from the stroke-prone spontaneously hypertensive rats (SHRSP) and normal spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY) of a Japanese colony. Two F2 cohorts were independently produced: F2(SHRSP x WKY) (110 male and 110 female rats) and F2(SHR x WKY) (151 male rats). The ratio of heart weight to body weight (Hw/Bw) was evaluated at 12 months of age in F2(SHRSP x WKY) after salt-loading for 7 months, and at around 15 weeks of age in F2(SHR x WKY) who had been fed a normal rat chow diet. Subsequent to an initial screen with 251 markers in F2(SHRSP x WKY) male progeny, 170 and 161 markers were selected and characterized in F2(SHRSP x WKY) female progeny and F2(SHR x WKY) male progeny, respectively. Markers from four chromosomal regions showed suggestive or significant linkage to Hw/Bw. The strongest and the most consistent linkage was found in the vicinity of D3Mgh16 on rat chromosome (RNO) 3 (a maximal log of the odds score reached 4.0 to 6.6 across the F2 populations studied). In the other three regions on RNO6, RNO10 and RNO13, the degree of linkage was more prominent in either males or females. These data provide solid evidence for a "principal" RNO3 quantitative trait loci regulating Hw/Bw in SHRSP and SHR, and also suggest the possible presence of sexual dimorphism in regard to genetic susceptibility for cardiac hypertrophy.

Biology of natriuretic peptides and their receptors

Increasing evidence suggests that natriuretic peptides (NPs) play diverse roles in mammals, including renal hemodynamics, neuroendocrine, and cardiovascular functions. Collectively, NPs are classified as hypotensive hormones; the main actions of NPs are implicated in eliciting natriuretic, diuretic, steroidogenic, antiproliferative, and vasorelaxant effects, important factors in the control of body fluid volume and blood pressure homeostasis. One of the principal loci involved in the regulatory actions of NPs is their cognate plasma membrane receptor molecules, which are activated by binding with specific NPs. Interaction of NPs with their receptors plays a central role in physiology and pathophysiology of hypertension and cardiovascular disorders. Gaining insight into the intricacies of NPs-specific receptor signaling pathways is of pivotal importance for understanding both hormone-receptor biology and the disease states arising from abnormal hormone receptor interplay. During the last decade there has been a surge in interest in NP receptors; consequently, a wealth of information has emerged concerning molecular structure and function, signaling mechanisms, and use of transgenics and gene-targeted mouse models. The objective of this present review is to summarize and document the previous findings and recent discoveries in the field of the natriuretic peptide hormone family and receptor systems with emphasis on the structure-function relationship, signaling mechanisms, and the physiological and pathophysiological significance in health and disease.

Strategies for mapping and cloning quantitative trait genes in rodents

Over the past 15 years, more than 2,000 quantitative trait loci (QTLs) have been identified in crosses between inbred strains of mice and rats, but less than 1% have been characterized at a molecular level. However, new resources, such as chromosome substitution strains and the proposed Collaborative Cross, together with new analytical tools, including probabilistic ancestral haplotype reconstruction in outbred mice, Yin-Yang crosses and in silico analysis of sequence variants in many inbred strains, could make QTL cloning tractable. We review the potential of these strategies to identify genes that underlie QTLs in rodents.

Cosegregation analysis of natriuretic peptide genes and blood pressure in the spontaneously hypertensive rat

1. The natriuretic peptide precursor A (Nppa) and B (Nppb) genes are candidate genes for hypertension and cardiac hypertrophy in the spontaneously hypertensive rat (SHR). The purpose of the present study was to determine the role of the Nppa and Nppb genes in the development of hypertension in the SHR. 2. A cohort (n = 162) of F2 segregating intercross animals was established between strains of hypertensive SHR and normotensive Wistar-Kyoto rats. Blood pressure and heart weight were measured in each rat at 12-16 weeks of age. Rats were genotyped using 11 informative microsatellite markers, distributed in the vicinity of the Nppa marker on rat chromosome 5 including an Nppb marker. The phenotype values were compared with genotype using the computer package mapmaker 3.0 (Whitehead Institute, Boston, MA, USA) to determine whether there was a link between the genetic variants of the natriuretic peptide family and blood pressure or cardiac hypertrophy. 3. A strong correlation was observed between the Nppa marker and blood pressure. A quantitative trait locus (QTL) for blood pressure on chromosome 5 was identified between the Nppa locus and the D5Mgh15 marker, less than 2 cM from the Nppa locus. The linkage score for the blood pressure QTL on chromosome 5 was 3.8 and the QTL accounted for 43% of the total variance of systolic blood pressure, 54% of diastolic blood pressure and 59% of mean blood pressure. No association was found between the Nppb gene and blood pressure. This is the first report of linkage between the Nppa marker and blood pressure in the rat. There was no correlation between the Nppa or Nppb genes or other markers in this region and either heart weight or left ventricular weight in F2 rats. 4. These findings suggest the existence of a blood pressure-dependent Nppa marker variant or a gene close to Nppa predisposing to spontaneous hypertension in the rat. It provides a strong foundation for further detailed genetic studies in congenic strains, which may help to narrow down the location of this gene and lead to positional cloning.

Comparison of the effects of omapatrilat and irbesartan/hydrochlorothiazide on endothelial function and cardiac hypertrophy in the stroke-prone spontaneously hypertensive rat

OBJECTIVE

The novel antihypertensive agent, omapatrilat, is both an inhibitor of neutral endopeptidase and angiotensin-converting enzyme. This study investigated the effects of omapatrilat in comparison with an angiotensin I-receptor antagonist/diuretic combination on blood pressure, endothelial function and cardiac hypertrophy in stroke-prone spontaneously hypertensive rats (SHRSP).

METHODS

Male and female SHRSP were treated orally with omapatrilat or irbesartan plus hydrochlorothiazide (I + H) or vehicle for 8 weeks. Systolic blood pressure was measured weekly by tail-cuff. Cardiac hypertrophy was monitored by echocardiography at 8, 12 and 16 weeks of age. Endothelial function [basal nitric oxide (NO) bioavailability and stimulated NO release] was examined in carotid arteries using organ bath pharmacology and in mesenteric resistance arteries using wire myography.

RESULTS

Compared with untreated controls, omapatrilat and I + H significantly attenuated hypertension [male control, 198.3 +/- 6.9 mmHg versus omapatrilat, 149.6 +/- 3.8 mmHg (F = 8.63 P < 0.0001), versus I + H, 145.6 +/- 5.1 mmHg (F = 7.38 P < 0.0001); female control, 170.3 +/-8.3 mmHg versus omapatrilat, 120.0 +/- 4.6 mmHg (F = 8.36, P < 0.0001), versus I + H, 112.2 +/- 2.9 mmHg (F = 9.08, P < 0.0001)] and left ventricular hypertrophy [male + female controls, 3.02 +/- 0.38 mg/g versus omapatrilat, 2.47 +/- 0.26 mg/g (P < 0.0001; 95% confidence interval, 0.27, 0.83), versus I + H, 2.49 +/- 0.21 mg/g (P < 0.0001; 95% confidence interval, 0.25, 0.83)]. Both treatments also significantly increased male carotid artery basal NO bioavailability relative to control [control, 0.62 +/- 0.17 g/g versus omapatrilat, 1.95 +/- 0.17 g/g (P < 0.0001; 95% confidence interval, -1.83, -0.36), versus I + H, 1.57 +/- 0.21 g/g (P < 0.026; 95% confidence interval, -1.31, -0.12)]. However, stimulated NO (EC50) was only improved in omapatrilat-treated males [controls, 0.19 +/- 0.06 micromol/l versus omapatrilat, 0.05 +/- 0.01 micromol/l (P = 0.05; 95% confidence interval, -1.16, -0.03)].

CONCLUSIONS

Omapatrilat treatment significantly reduced left ventricular hypertrophy and improved endothelial function in carotid arteries from male SHRSP by NO-dependent mechanisms. Despite equivalent antihypertensive and antihypertrophic actions, a similar improvement in endothelial function, specifically stimulated NO release, was not observed after treatment with I + H.

Expression of constitutively active guanylate cyclase in cardiomyocytes inhibits the hypertrophic effects of isoproterenol and aortic constriction on mouse hearts

Evidence from several rodent models has suggested that a reduction of either atrial natriuretic peptide or its receptor in the heart affects cardiac remodeling by promoting the onset of cardiac hypertrophy. The atrial natriuretic peptide receptor mediates signaling at least in part via the generation of intracellular cyclic GMP. To directly test whether accumulation of intracellular cyclic GMP conveys protection against cardiac hypertrophy, we engineered transgenic mice that overexpress a catalytic fragment of constitutively active guanylate cyclase domain of the atrial natriuretic peptide receptor in a cardiomyocyte-specific manner. Expression of the transgene increased the intracellular concentration of cyclic GMP specifically within cardiomyocytes and had no detectable effect on cardiac performance under basal conditions. However, expression of the transgene attenuated the effects of the pharmacologic hypertrophic agent isoproterenol on cardiac wall thickness and prevented the onset of the fetal gene expression program normally associated with cardiac hypertrophy. Likewise, expression of the transgene inhibited the hypertrophic effects of abdominal aortic constriction, since it abolished its effects on ventricular wall thickness and greatly attenuated its effects on cardiomyocyte size. Altogether, our results suggest that cyclic GMP is a cardioprotective agent against hypertrophy that acts via a direct local effect on cardiomyocytes.

Distinct gene-sex interactions regulate adult rat cardiomyocyte width and length independently

Wistar-Kyoto (WKY) and WKY-derived hyperactive (WKHA) rats are two genetically-related inbred strains of rats that are both normotensive yet exhibit differences in left ventricular mass (LVM). We had shown previously that cardiomyocytes from male WKHA are wider than that of male WKY, and that there was genetic linkage between LVM and a locus on chromosome 5 (RNO5) in the male progeny of a F2 WKHA/WKY cross. We show here that cardiomyocyte width is linked to the same RNO5 locus in male reciprocal congenic rats derived from WKHA and WKY. Contrary to males, we found no genetic linkage between LVM and the RNO5 locus in female rats. However, ventricular hypertrophy in females might be of a different nature, because cardiomyocytes from female WKHA were shorter than their WKY counterparts (with no difference in width). The RNO5 locus contains that of the natriuretic peptide precursor A (Nppa) gene. In male congenic rats, changes in cardiomyocyte width always correlated with reciprocal changes in the LV concentration of atrial natriuretic factor (ANF, i.e., the peptide product of Nppa). Taken together with other functional data, the small size of the RNO5 locus (approximately 63 cR) increased the likelihood that both cardiomyocyte width and LV ANF concentration could be linked to only one gene (possibly Nppa) in male rats. Moreover, our results support the notion that genes and sex interact to regulate cardiomyocyte width and length independently from one another.

Two closely linked interactive blood pressure QTL on rat chromosome 5 defined using congenic Dahl rats

Previously we reported the construction of a congenic strain, S.LEW, spanning a large region of rat chromosome 5. The Lewis (LEW) strain was the donor, and the Dahl salt-sensitive (S) strain was the recipient. The congenic strain included a blood pressure quantitative trait locus (QTL). In the present work, a series of nine congenic substrains were constructed from S.LEW which defined two closely linked blood pressure QTL in the region previously thought to contain only one. LEW low-blood-pressure alleles at both QTL were required for a major effect on blood pressure. Neither LEW allele alone had a significant effect on blood pressure. The two QTL were localized to regions 6.3 and 4.6 cM, and these were 1.0 cM apart.

Characterization of myocardium, isolated cardiomyocytes, and blood pressure in WKHA and WKY rats

We previously reported that the left ventricular (LV) mass of Wistar-Kyoto (WKY)-derived hyperactive (WKHA) rats was higher than that of WKY rats in the absence of a difference in systolic blood pressure. To extend these earlier observations, we conducted a series of functional and morphological investigations on both strains. Analysis of tissue sections revealed that the surface of ventricular tissue from WKHA rats was higher than that of WKY rats, without any enlargement of the cavity area. Analysis of isolated adult cells showed that cell width (as well as cell volume) of ventricular cardiomyocytes was significantly higher in WKHA than WKY rats. However, LV of WKHA rats contained approximately 33% less cardiomyocytes than those from WKY rats. Mean intracellular free calcium concentration of cardiomyocytes was also higher in WKHA than WKY rats. Hemodynamic measurements revealed that the values of the maximum rates of pressure change (dP/dt) were higher in LV from WKHA rats. However, these differences were reduced (-dP/dt) or abolished (+dP/dt) when the values were normalized for both the number and mean cross-sectional area of ventricular cardiomyocytes. Mean levels of systolic and diastolic blood pressure (corresponding to the 24-h average of measurements obtained continuously in conscious unrestrained animals using radiotelemetric implants) were not different between strains. However, circadian rhythm was more evident in WKY rats, because the difference between morning and night values of systolic and diastolic blood pressure was greater (by 3 mmHg) in WKY rats. Altogether, our data validate the use of WKHA rats as models of predominantly concentric LV hypertrophy developing in the absence of increased mean levels of hemodynamic cardiac load and show that the hypertrophy phenotype is more pronounced in isolated cardiomyocytes than at the level of the whole ventricle.

Natriuretic peptide signalling: molecular and cellular pathways to growth regulation

The natriuretic peptides (NPs) constitute a family of polypeptide hormones that regulate mammalian blood volume and blood pressure. The ability of the NPs to modulate cardiac hypertrophy and cell proliferation as well is now beginning to be recognized. The NPs interact with three membrane-bound receptors, all of which contain a well-characterized extracellular ligand-binding domain. The R1 subclass of NP receptors (NPR-A and NPR-B) contains a C-terminal guanylyl cyclase domain and is responsible for most of the NPs downstream actions through their ability to generate cGMP. The R2 subclass lacks an obvious catalytic domain and functions primarily as a clearance receptor. This review focuses on the signal transduction pathways initiated by ligand binding and other factors that help to determine signalling specificities, including allosteric factors modulating cGMP generation, receptor desensitization, the activation and function of cGMP-dependent protein kinase (PKG), and identification of potential nuclear or cytoplasmic targets such as the mitogen-activated protein kinase signalling (MAPK) cascade. The inhibition of cardiac growth and hypertrophy may be an important but underappreciated action of the NP signalling system.