Naturally occurring variation in cardiovascular traits among inbred mouse strains

Unlocking metabolic insights with mouse genetic diversity

As part of EMBO Journal’s 2024 metabolism methods series, this commentary revisits the impact of genetics on metabolic studies, enabling dissection of novel mechanisms and phenotypes.

Longitudinal changes in blood pressure are preceded by changes in albuminuria and accelerated by increasing dietary sodium intake

BACKGROUND

Dietary sodium is a well-known risk factor for cardiovascular and renal disease; however, direct evidence of the longitudinal changes that occur with aging, and the influence of dietary sodium on the age-associated alterations are scarce.

METHODS

C57BL/6 mice were maintained for 13 months on a low (LS, 0.02 % Na+), normal (NS, 0.3 % Na+) or high (HS, 1.6 % Na+) salt diet. We assessed 1) the longitudinal trajectories for two markers of cardiovascular and renal dysfunction (blood pressure (BP) and albuminuria), as well as hormonal changes, and 2) end-of-study cardiac and renal parameters.

RESULTS

The effect of aging on BP and kidney damage did not reach significance levels in the LS group; however, relative to baseline, there were significant increases in these parameters for animals maintained on NS and HS diets, starting as early as month 7 and month 5, respectively. Furthermore, changes in albuminuria preceded the changes in BP relative to baseline, irrespective of the diet. Circulating aldosterone and plasma renin activity displayed the expected decreasing trends with age and dietary sodium loading. As compared to LS - higher dietary sodium consumption associated with increasing trends in left ventricular mass and volume indices, consistent with an eccentric dilated phenotype. Functional and molecular markers of kidney dysfunction displayed similar trends with increasing long-term sodium levels: higher renovascular resistance, increased glomerular volumes, as well as higher levels of renal angiotensin II type 1 and mineralocorticoid receptors, and lower renal Klotho levels.

CONCLUSION

Our study provides a timeline for the development of cardiorenal dysfunction with aging, and documents that increasing dietary salt accelerates the age-induced phenotypes. In addition, we propose albuminuria as a prognostic biomarker for the future development of hypertension. Last, we identified functional and molecular markers of renal dysfunction that associate with long-term dietary salt loading.

Animal Study Translation: The Other Reproducibility Challenge

Animal research is currently an irreplaceable contributor to our efforts to protect and improve public health. Its relevance, importance, and contributions are represented in historical precedent, regulatory expectations, evidence of our rapidly developing understanding of human health and disease, as well as success in the development of novel therapeutics that are improving quality of life and extending human and animal life expectancy. The rapid and evolving success in responding to the current COVID pandemic significantly supported by animal studies is a clear example of the importance of animal research. But there is growing interest in reducing our dependence on animals and challenges to the effective translation of current animal studies to human applications. There are several potential contributors to gaps in the translatability of animal research to humans, including our approaches to choosing or rationalizing the relevance of a particular animal model, our understanding of their biological variability and how that applies to outcomes, the data we collect from animal studies, and even how we manage the animals. These important contributors to the success of animal research are explored in this issue of the ILAR Journal.

Spontaneous Right Ventricular Pseudoaneurysms and Increased Arrhythmogenicity in a Mouse Model of Marfan Syndrome

Patients with Marfan syndrome (MFS), a connective tissue disorder caused by pathogenic variants in the gene encoding the extracellular matrix protein fibrillin-1, have an increased prevalence of primary cardiomyopathy, arrhythmias, and sudden cardiac death. We have performed an in-depth in vivo and ex vivo study of the cardiac phenotype of Fbn1mgR/mgR mice, an established mouse model of MFS with a severely reduced expression of fibrillin-1. Using ultrasound measurements, we confirmed the presence of aortic dilatation and observed cardiac diastolic dysfunction in male Fbn1mgR/mgR mice. Upon post-mortem examination, we discovered that the mutant mice consistently presented myocardial lesions at the level of the right ventricular free wall, which we characterized as spontaneous pseudoaneurysms. Histological investigation demonstrated a decrease in myocardial compaction in the MFS mouse model. Furthermore, continuous 24 h electrocardiographic analysis showed a decreased heart rate variability and an increased prevalence of extrasystolic arrhythmic events in Fbn1mgR/mgR mice compared to wild-type littermates. Taken together, in this paper we document a previously unreported cardiac phenotype in the Fbn1mgR/mgR MFS mouse model and provide a detailed characterization of the cardiac dysfunction and rhythm disorders which are caused by fibrillin-1 deficiency. These findings highlight the wide spectrum of cardiac manifestations of MFS, which might have implications for patient care.

Genetic variation regulates opioid-induced respiratory depression in mice

In the U.S., opioid prescription for treatment of pain nearly quadrupled from 1999 to 2014. The diversion and misuse of prescription opioids along with increased use of drugs like heroin and fentanyl, has led to an epidemic in addiction and overdose deaths. The most common cause of opioid overdose and death is opioid-induced respiratory depression (OIRD), a life-threatening depression in respiratory rate thought to be caused by stimulation of opioid receptors in the inspiratory-generating regions of the brain. Studies in mice have revealed that variation in opiate lethality is associated with strain differences, suggesting that sensitivity to OIRD is genetically determined. We first tested the hypothesis that genetic variation in inbred strains of mice influences the innate variability in opioid-induced responses in respiratory depression, recovery time and survival time. Using the founders of the advanced, high-diversity mouse population, the Diversity Outbred (DO), we found substantial sex and genetic effects on respiratory sensitivity and opiate lethality. We used DO mice treated with morphine to map quantitative trait loci for respiratory depression, recovery time and survival time. Trait mapping and integrative functional genomic analysis in GeneWeaver has allowed us to implicate Galnt11, an N-acetylgalactosaminyltransferase, as a gene that regulates OIRD.

Commonly used mouse strains have distinct vascular properties

Mice are the most common animal model to investigate human disease and explore physiology. Mice are practical, cost efficient, and easily used for genetic manipulations. Although variability in cardiac structure and function among mouse strains is well noted, the effect of mouse strain on vascular stiffness indices is not known. Here, we compared mouse strain-dependent differences in key vascular stiffness indices among frequently used inbred mouse strains-C57Bl/6J, 129S, and Bl6/129S. In young healthy animals, baseline blood pressure and heart rate were identical in all strains, and independent of gender. However, both active in vivo and passive ex vivo vascular stiffness indices exhibited distinct differences. Specifically, both male and female 129S animals demonstrated the highest tensile stiffness, were least responsive to acetylcholine-induced vasorelaxation, and showed the lowest pulse wave velocity (PWV), an index of in vivo stiffness. C57Bl/6J mice demonstrated the highest PWV, lowest tensile stiffness, and the highest response to acetylcholine-induced vasorelaxation. Interestingly, within each strain, female mice had more compliant aortas. C57Bl/6J mice had thinner vessel walls with fewer layers, whereas 129S mice had the thickest walls with the most layers. Values in the Bl6/129S mixed background mice fell between C57Bl/6J and 129S mice. In conclusion, we show that underlying vascular properties of different inbred wild-type mouse strains are distinct, despite superficial similarities in blood pressure. For each genetic modification, care should be taken to identify proper controls, and conclusions might need to be verified in more than one strain to minimize the risk of false positive studies.

Mylk3 null C57BL/6N mice develop cardiomyopathy, whereas Nnt null C57BL/6J mice do not

The C57BL/6J and C57BL/6N mice have well-documented phenotypic and genotypic differences, including the infamous nicotinamide nucleotide transhydrogenase (Nnt) null mutation in the C57BL/6J substrain, which has been linked to cardiovascular traits in mice and cardiomyopathy in humans. To assess whether Nnt loss alone causes a cardiovascular phenotype, we investigated the C57BL/6N, C57BL/6J mice and a C57BL/6J-BAC transgenic rescuing NNT expression, at 3, 12, and 18 mo. We identified a modest dilated cardiomyopathy in the C57BL/6N mice, absent in the two B6J substrains. Immunofluorescent staining of cardiomyocytes revealed eccentric hypertrophy in these mice, with defects in sarcomere organisation. RNAseq analysis identified differential expression of a number of cardiac remodelling genes commonly associated with cardiac disease segregating with the phenotype. Variant calling from RNAseq data identified a myosin light chain kinase 3 (Mylk3) mutation in C57BL/6N mice, which abolishes MYLK3 protein expression. These results indicate the C57BL/6J Nnt-null mice do not develop cardiomyopathy; however, we identified a null mutation in Mylk3 as a credible cause of the cardiomyopathy phenotype in the C57BL/6N.

Coagulation phenotype of wild-type mice on different genetic backgrounds

Genetically engineered mouse models are used to investigate beneficial treatment in haemophilia by comparison with wild-type mice. It has been recognized that wild-type and haemophilic mice of different genetic backgrounds show different bleeding phenotypes. We assessed ex-vivo coagulation parameters in nine wild-type substrains of 129S1/Sv, BALB/c and C57BL/6 mice applying thromboelastography (TEG), activated partial thromboplastin time (aPTT), prothrombin time (PT) and fibrinogen levels. The comprehensive ex-vivo data are discussed in view of results from a tail-tip bleeding assay. Time to first clot formation (R-time) showed higher within-substrain (CV range: 28–54%) and higher between-substrain (median range: 25.53–42.60 min) variation for BALB/c than for C57BL/6 mice (CV range: 14–31%; median range: 22.45–24.93 min). Median R-time for 129S1/Sv mice was 30.42 min (CV: 33%). No distinct strain differences were observed for maximum amplitude (MA), aPTT, or PT, but males generally showed higher MA and shorter aPTT than females. Males of all substrains had higher fibrinogen levels than females. The heightened in-vivo variability (CV range: 81–171%; median range: 36.00–469.50 mg) in the tail-tip bleeding assay and increased blood loss in wild-type C57BL/6 male mice was not reflected in ex-vivo coagulation parameters. In general, ex-vivo coagulation results appeared consistent within substrains, but showed substrain and sex differences of variable magnitudes. We conclude that alignment of the mouse substrain genetic background to the experimental model is critical to reduce data variability and animal numbers.

Physical activity and impaired left ventricular relaxation in middle aged adults

The aim of this study was to examine the relationship between physical activity level and impaired left ventricular (LV) relaxation in a large sample of apparently healthy men and women. We conducted a cross-sectional study in 57,449 adults who underwent echocardiography as part of a comprehensive health examination between March 2011 and December 2014. Physical activity level was assessed using the Korean version of the International Physical Activity Questionnaire Short Form. The presence of impaired LV relaxation was determined based on echocardiographic findings. Physical activity levels were inversely associated with the prevalence of impaired LV relaxation. The multivariable-adjusted odds ratios (95% confidence interval) for impaired LV relaxation comparing minimally active and health-enhancing physically active groups to the inactive group were 0.84 (0.77–0.91) and 0.64 (0.58–0.72), respectively (P for trend < 0.001). These associations were modified by sex (p for interaction <0.001), with the inverse association observed in men, but not in women. This study demonstrated an inverse linear association between physical activity level and impaired LV relaxation in a large sample of middle-aged Koreans independent of potential confounders. Our findings suggest that increasing physical activity may be independently important in reducing the risk of impaired LV relaxation.

4D cardiac magnetic resonance imaging, 4D and 2D transthoracic echocardiography: a comparison of in-vivo assessment of ventricular function in rats

Preclinical cardiovascular research is the foundation of our understanding and broad knowledge of heart function and cardiovascular disease. Reliable cardiac imaging modalities are the basis for applicable results. Four-dimensional cardiac magnetic resonance (4D CMR) has been set as the gold standard for in-vivo assessment of ventricular function in rodents. However, technical improvements in echocardiography now allow us to image the whole heart, which makes four-dimensional echocardiography (4DE) a possible alternative to 4D CMR. To date, no study has systematically assessed 4DE in comparison with 4D CMR in rats. In total we studied 26 juvenile Sprague-Dawley rats (Crl: CD (SD) IGS). Twenty rats underwent echocardiographic imaging (2D and 4D) and 4D CMR. Five of those rats underwent a ligation of the superior and inferior vena cava to reduce the cardiac inflow as a disease model. Six additional rats were used to assess reproducibility of echocardiography and underwent three echocardiographic examinations. 4D CMR was performed on a 7T scanner; 2D and 4D echocardiography was conducted using a 40 MHz transducer. Correlation between 4D CMR, 4DE and 2DE for left-ventricular ejection fraction (LVEF) was assessed. An excellent correlation was observed between 4DE and 4D CMR ( r = 0.95, p < 0.001). Correlation of 2DE and 4D CMR was weak ( r = 0.57, p < 0.01). 4DE provides results that are equally precise as 4D CMR and highly reproducible with less technical effort than 4D CMR.

Forced Running Endurance Is Influenced by Gene(s) on Mouse Chromosome 10

Phenotypic diversity between laboratory mouse strains provides a model for studying the underlying genetic mechanisms. The A/J strain performs poorly in various endurance exercise models. The aim of the study was to test if endurance capacity and contractility of the fast- and slow-twitch muscles are affected by the genes on mouse chromosome 10. The C57BL/6J (B6) strain and C57BL/6J-Chr 10^A/J/NaJ (B6.A10) consomic strain which carries the A/J chromosome 10 on a B6 strain background were compared. The B6.A10 mice compared to B6 were larger in body weight (p < 0.02): 27.2 ± 1.9 vs. 23.8 ± 2.7 and 23.4 ± 1.9 vs. 22.9 ± 2.3 g, for males and females, respectively, and in male soleus weight (p < 0.02): 9.7 ± 0.4 vs. 8.6 ± 0.9 mg. In the forced running test the B6.A10 mice completed only 64% of the B6 covered distance (p < 0.0001). However, there was no difference in voluntary wheel running (p = 0.6) or in fatigability of isolated soleus (p = 0.24) or extensor digitorum longus (EDL, p = 0.7) muscles. We conclude that chromosome 10 of the A/J strain contributes to reduced endurance performance. We also discuss physiological mechanisms and methodological aspects relevant to interpretation of these findings.

Cardiac Morphology and Function, and Blood Gas Transport in Aquaporin-1 Knockout Mice

We have studied cardiac and respiratory functions of aquaporin-1-deficient mice by the Pressure-Volume-loop technique and by blood gas analysis. In addition, the morphological properties of the animals' hearts were analyzed. In anesthesia under maximal dobutamine stimulation, the mice exhibit a moderately elevated heart rate of < 600 min⁻¹ and an O₂ consumption of ~0.6 ml/min/g, which is about twice the basal rate. In this state, which is similar to the resting state of the conscious animal, all cardiac functions including stroke volume and cardiac output exhibited resting values and were identical between deficient and wildtype animals. Likewise, pulmonary and peripheral exchange of O₂ and CO₂ were normal. In contrast, several morphological parameters of the heart tissue of deficient mice were altered: (1) left ventricular wall thickness was reduced by 12%, (2) left ventricular mass, normalized to tibia length, was reduced by 10–20%, (3) cardiac muscle fiber cross sectional area was decreased by 17%, and (4) capillary density was diminished by 10%. As the P-V-loop technique yielded normal end-diastolic and end-systolic left ventricular volumes, the deficient hearts are characterized by thin ventricular walls in combination with normal intraventricular volumes. The aquaporin-1-deficient heart thus seems to be at a disadvantage compared to the wild-type heart by a reduced left-ventricular wall thickness and an increased diffusion distance between blood capillaries and muscle mitochondria. While under the present quasi-resting conditions these morphological alterations have no consequences for cardiac function, we expect that the deficient hearts will show a reduced maximal cardiac output.

Preclinical Assessment of the Efficacy of Anti-Angiogenic Therapies in Hepatocellular Carcinoma

Diffuse hepatocellular carcinoma (HCC) is a complex affliction in which comorbidities can bias global outcome of cancer therapy. Better methods are thus warranted to directly assess effects of therapy on tumor angiogenesis and growth. As tumor angiogenesis is invariably associated with changes in local blood flow, we assessed the utility of ultrasound imaging in evaluation of the efficacy of anti-angiogenic therapy in a spontaneous transgenic mouse model of HCC. Blood flow velocities were measured monthly in the celiac trunk before and after administration of sorafenib or bevacizumab at doses corresponding to those currently used in clinical practice. Concordant with clinical experience, sorafenib, but not bevacizumab, reduced microvascular density and suppressed tumor growth relative to controls. Evolution of blood flow velocities correlated with microvascular density and with the evolution of tumor size. Ultrasound imaging thus provides a useful non-invasive tool for preclinical evaluation of new anti-angiogenic therapies for HCC.

Heterozygous disruption of activin receptor-like kinase 1 is associated with increased arterial pressure in mice

The activin receptor-like kinase 1 (ALK-1) is a type I cell-surface receptor for the transforming growth factor-β (TGF-β) family of proteins. Hypertension is related to TGF-β1, because increased TGF-β1 expression is correlated with an elevation in arterial pressure (AP) and TGF-β expression is upregulated by the renin-angiotensin-aldosterone system. The purpose of this study was to assess the role of ALK-1 in regulation of AP using Alk1 haploinsufficient mice (Alk1(+/-)). We observed that systolic and diastolic AP were significantly higher in Alk1(+/-) than in Alk1(+/+) mice, and all functional and structural cardiac parameters (echocardiography and electrocardiography) were similar in both groups. Alk1(+/-) mice showed alterations in the circadian rhythm of AP, with higher AP than Alk1(+/+) mice during most of the light period. Higher AP in Alk1(+/-) mice is not a result of a reduction in the NO-dependent vasodilator response or of overactivation of the peripheral renin-angiotensin system. However, intracerebroventricular administration of losartan had a hypotensive effect in Alk1(+/-) and not in Alk1(+/+) mice. Alk1(+/-) mice showed a greater hypotensive response to the β-adrenergic antagonist atenolol and higher concentrations of epinephrine and norepinephrine in plasma than Alk1(+/+) mice. The number of brain cholinergic neurons in the anterior basal forebrain was reduced in Alk1(+/-) mice. Thus, we concluded that the ALK-1 receptor is involved in the control of AP, and the high AP of Alk1(+/-) mice is explained mainly by the sympathetic overactivation shown by these animals, which is probably related to the decreased number of cholinergic neurons.

Loss of Function Mutations in NNT Are Associated With Left Ventricular Noncompaction

BACKGROUND

Left ventricular noncompaction (LVNC) is an autosomal-dominant, genetically heterogeneous cardiomyopathy with variable severity, which may co-occur with cardiac hypertrophy.

METHODS AND RESULTS

Here, we generated whole exome sequence data from multiple members from 5 families with LVNC. In 4 of 5 families, the candidate causative mutation segregates with disease in known LVNC genes MYH7 and TPM1. Subsequent sequencing of MYH7 in a larger LVNC cohort identified 7 novel likely disease causing variants. In the fifth family, we identified a frameshift mutation in NNT, a nuclear-encoded mitochondrial protein, not implicated previously in human cardiomyopathies. Resequencing of NNT in additional LVNC families identified a second likely pathogenic missense allele. Suppression of nnt in zebrafish caused early ventricular malformation and contractility defects, probably driven by altered cardiomyocyte proliferation. In vivo complementation studies showed that mutant human NNT failed to rescue nnt morpholino-induced heart dysfunction, indicating a probable haploinsufficiency mechanism.

CONCLUSIONS

Together, our data expand the genetic spectrum of LVNC and demonstrate how the intersection of whole exome sequence with in vivo functional studies can accelerate the identification of genes that drive human genetic disorders.

Segmental aortic stiffening contributes to experimental abdominal aortic aneurysm development

BACKGROUND

Stiffening of the aortic wall is a phenomenon consistently observed in age and in abdominal aortic aneurysm (AAA). However, its role in AAA pathophysiology is largely undefined.

METHODS AND RESULTS

Using an established murine elastase-induced AAA model, we demonstrate that segmental aortic stiffening precedes aneurysm growth. Finite-element analysis reveals that early stiffening of the aneurysm-prone aortic segment leads to axial (longitudinal) wall stress generated by cyclic (systolic) tethering of adjacent, more compliant wall segments. Interventional stiffening of AAA-adjacent aortic segments (via external application of surgical adhesive) significantly reduces aneurysm growth. These changes correlate with the reduced segmental stiffness of the AAA-prone aorta (attributable to equalized stiffness in adjacent segments), reduced axial wall stress, decreased production of reactive oxygen species, attenuated elastin breakdown, and decreased expression of inflammatory cytokines and macrophage infiltration, and attenuated apoptosis within the aortic wall, as well. Cyclic pressurization of segmentally stiffened aortic segments ex vivo increases the expression of genes related to inflammation and extracellular matrix remodeling. Finally, human ultrasound studies reveal that aging, a significant AAA risk factor, is accompanied by segmental infrarenal aortic stiffening.

CONCLUSIONS

The present study introduces the novel concept of segmental aortic stiffening as an early pathomechanism generating aortic wall stress and triggering aneurysmal growth, thereby delineating potential underlying molecular mechanisms and therapeutic targets. In addition, monitoring segmental aortic stiffening may aid the identification of patients at risk for AAA.

Vital signs monitoring during injectable and inhalant anesthesia in mice

Selecting the appropriate anesthetic protocol for the individual animal is an essential part of laboratory animal experimentation. The present study compared the characteristics of four anesthetic protocols in mice, focusing on the vital signs. Thirty-two male ddY mice were divided into four groups and administered anesthesia as follows: pentobarbital sodium monoanaesthesia; ketamine and xylazine combined (K/X); medetomidine, midazolam, and butorphanol combined (M/M/B); and isoflurane. In each group, rectal temperature, heart rate, respiratory rate, and O₂ saturation (SPO₂) were measured, and the changes over time and instability in these signs were compared. The anesthetic depth was also evaluated in each mouse, and the percentage of mice achieving surgical anesthesia was calculated. K/X anesthesia caused remarkable bradycardia, while the respiratory rate and SPO₂ were higher than with the others, suggesting a relatively strong cardiac influence and less respiratory depression. The M/M/B group showed a relatively lower heart rate and SPO₂, but these abnormalities were rapidly reversed by atipamezole administration. The pentobarbital group showed a lower SPO₂, and 62.5% of mice did not reach a surgical anesthetic depth. The isoflurane group showed a marked decrease in respiratory rate compared with the injectable anesthetic groups. However, it had the most stable SPO₂ among the groups, suggesting a higher tidal volume. The isoflurane group also showed the highest heart rate during anesthesia. In conclusion, the present study showed the cardiorespiratory characteristics of various anesthetic protocols, providing basic information for selecting an appropriate anesthetic for individual animals during experimentation.

Genetic perturbations that impair functional trait interactions lead to reduced bone strength and increased fragility in mice

Functional adaptation may complicate the choice of phenotype used in genetic studies that seek to identify genes contributing to fracture susceptibility. Often, genetic variants affecting one trait are compensated by coordinated changes in other traits. Bone fracture is a prototypic example because mechanical function of long bones (stiffness and strength) depends on how the system coordinately adjusts the amount (cortical area) and quality (tissue-mineral density, TMD) of bone tissue to mechanically offset the natural variation in bone robustness (total area/length). We propose that efforts aimed at identifying genes regulating fracture resistance will benefit from better understanding how functional adaptation contributes to the genotype-phenotype relationship. We analyzed the femurs of C57BL/6J-Chr(A/J)/NaJ Chromosome Substitution Strains (CSSs) to systemically interrogate the mouse genome for chromosomes harboring genes that regulate mechanical function. These CSSs (CSS-i, i=the substituted chromosome) showed changes in mechanical function on the order of -26.6 to +11.5% relative to the B6 reference strain after adjusting for body size. Seven substitutions showed altered robustness, cortical area, or TMD, but no effect on mechanical function (CSS-4, 5, 8, 9, 17, 18, 19); six substitutions showed altered robustness, cortical area, or TMD, and reduced mechanical function (CSS-1, 2, 6, 10, 12, 15); and one substitution also showed reduced mechanical function but exhibited no significant changes in the three physical traits analyzed in this study (CSS-3). A key feature that distinguished CSSs that maintained function from those with reduced function was whether the system adjusted cortical area and TMD to the levels needed to compensate for the natural variation in bone robustness. These results provide a novel biomechanical mechanism linking genotype with phenotype, indicating that genes control function not only by regulating individual traits, but also by regulating how the system coordinately adjusts multiple traits to establish function.

High-throughput phenotypic assessment of cardiac physiology in four commonly used inbred mouse strains

Mice with genetic alterations are used in heart research as model systems of human diseases. In the last decade there was a marked increase in the recognition of genetic diversity within inbred mouse strains. Increasing numbers of inbred mouse strains and substrains and analytical variation of cardiac phenotyping methods require reproducible, high-throughput methods to standardize murine cardiovascular physiology. We describe methods for non-invasive, reliable, easy and fast to perform echocardiography and electrocardiography on awake mice. This method can be used for primary screening of the murine cardiovascular system in large-scale analysis. We provide insights into the physiological divergence of C57BL/6N, C57BL/6J, C3HeB/FeJ and 129P2/OlaHsd mouse hearts and define the expected normal values. Our report highlights that compared to the other three strains tested C57BL/6N hearts reveal features of heart failure such as hypertrophy and reduced contractile function. We found several features of the mouse ECG to be under genetic control and obtained several strain-specific differences in cardiac structure and function.

SHP-2 deletion in postmigratory neural crest cells results in impaired cardiac sympathetic innervation

Autonomic innervation is an essential component of cardiovascular regulation that is first established from the neural crest (NC) lineage in utero and continues developing postnatally. Although in vitro studies have indicated that SH2-containing protein tyrosine phosphatase 2 (SHP-2) is a signaling factor critical for regulating sympathetic neuron differentiation, this has yet to be shown in the complex in vivo environment of cardiac autonomic innervation. Targeting SHP-2 within postmigratory NC lineages resulted in a fully penetrant mouse model of diminished sympathetic cardiac innervation and concomitant bradycardia. Immunohistochemistry of the sympathetic nerve marker tyrosine hydroxylase revealed a progressive loss of adrenergic ganglionic neurons and reduction of cardiac sympathetic axon density in Shp2 cKOs. Molecularly, Shp2 cKOs exhibit lineage-specific suppression of activated phospo-ERK1/2 signaling but not of other downstream targets of SHP-2 such as pAKT. Genetic restoration of the phosphorylated-extracellular signal-regulated kinase (pERK) deficiency via lineage-specific expression of constitutively active MEK1 was sufficient to rescue the sympathetic innervation deficit and its physiological consequences. These data indicate that SHP-2 signaling specifically through pERK in postmigratory NC lineages is essential for development and maintenance of sympathetic cardiac innervation postnatally.

Small and large animal models in cardiac contraction research: advantages and disadvantages

The mammalian heart is responsible for not only pumping blood throughout the body but also adjusting this pumping activity quickly depending upon sudden changes in the metabolic demands of the body. For the most part, the human heart is capable of performing its duties without complications; however, throughout many decades of use, at some point this system encounters problems. Research into the heart's activities during healthy states and during adverse impacts that occur in disease states is necessary in order to strategize novel treatment options to ultimately prolong and improve patients' lives. Animal models are an important aspect of cardiac research where a variety of cardiac processes and therapeutic targets can be studied. However, there are differences between the heart of a human being and an animal and depending on the specific animal, these differences can become more pronounced and in certain cases limiting. There is no ideal animal model available for cardiac research, the use of each animal model is accompanied with its own set of advantages and disadvantages. In this review, we will discuss these advantages and disadvantages of commonly used laboratory animals including mouse, rat, rabbit, canine, swine, and sheep. Since the goal of cardiac research is to enhance our understanding of human health and disease and help improve clinical outcomes, we will also discuss the role of human cardiac tissue in cardiac research. This review will focus on the cardiac ventricular contractile and relaxation kinetics of humans and animal models in order to illustrate these differences.

Effect of chromosome substitution on intrinsic exercise capacity in mice

Previous research identified a locus on Chromosome 14 as an important regulator of endurance exercise capacity in mice. The aim of this study was to investigate the effect of chromosome substitution on intrinsic exercise capacity and identify quantitative trait loci (QTL) associated with exercise capacity in mice. Mice from a chromosome substitution strain (CSS) derived from A/J and C57Bl/6J (B6), denoted as B6.A14, were used to assess the contribution of Chromosome 14 to intrinsic exercise capacity. All mice performed a graded exercise test to exhaustion to determine exercise capacity expressed as time (min) or work (kg·m). Exercise time and work were significantly greater in B6 mice than B6.A14 and A/J mice, indicating the presence of a QTL on Chromosome 14 for exercise capacity. To localize exercise-related QTL, 155 B6.A14 x B6 F ₂ mice were generated for linkage analysis. Suggestive QTL for exercise time (57 cM, 1.75 LOD) and work (57 cM, 2.08 LOD) were identified in the entire B6.A14 x B6 F ₂ cohort. To identify putative sex-specific QTL, male and female F ₂ cohorts were analyzed separately.  In males, a significant QTL for exercise time (55 cM, 2.28 LOD) and a suggestive QTL for work (55 cM, 2.19 LOD) were identified.  In the female cohort, no QTL was identified for time, but a suggestive QTL for work was located at 16 cM (1.8 LOD). These data suggest that one or more QTL on Chromosome 14 regulate exercise capacity. The putative sex-specific QTL further suggest that the genetic architecture underlying exercise capacity is different in males and females.  Overall, the results of this study support the use of CSS as a model for the genetic analysis of exercise capacity. Future studies should incorporate the full panel of CSS using male and female mice to dissect the genetic basis for differences in exercise capacity.

Mouse strain differences in metabolic fluxes and function of ex vivo working hearts

In mice, genetic background is known to influence various parameters, including cardiac function. Its impact on cardiac energy substrate metabolism-a factor known to be closely related to function and contributes to disease development-is, however, unclear. This was examined in this study. In commonly used control mouse substrains SJL/JCrNTac, 129S6/SvEvTac, C57Bl/6J, and C57Bl/6NCrl, we assessed the functional and metabolic phenotypes of 3-mo-old working mouse hearts perfused ex vivo with physiological concentrations of (13)C-labeled carbohydrates (CHO) and a fatty acid (FA). Marked variations in various functional and metabolic flux parameters were observed among all mouse substrains, although the pattern observed differed for these parameters. For example, among all strains, C57Bl/6NCrl hearts had a greater cardiac output (+1.7-fold vs. SJL/JCrNTac and C57Bl/6J; P < 0.05), whereas at the metabolic level, 129S6/SvEvTac hearts stood out by displaying (vs. all 3 strains) a striking shift from exogenous FA (~-3.5-fold) to CHO oxidation as well as increased glycolysis (+1.7-fold) and FA incorporation into triglycerides (+2-fold). Correlation analyses revealed, however, specific linkages between 1) glycolysis, FA oxidation, and pyruvate metabolism and 2) cardiac work, oxygen consumption with heart rate, respectively. This implies that any genetically determined factors affecting a given metabolic flux parameter may impact on the associated functional parameters. Our results emphasize the importance of selecting the appropriate control strain for cardiac metabolic studies using transgenic mice, a factor that has often been neglected. Understanding the molecular mechanisms underlying the diversity of strain-specific cardiac metabolic and functional profiles, particularly the 129S6/SvEvTac, may ultimately disclose new specific metabolic targets for interventions in heart disease.

Non-invasive assessment of cardiac function in a mouse model of renovascular hypertension

Hypertension continues to be a significant cause of morbidity and mortality, underscoring the need to better understand its early effects on the myocardium. The aim of this study is to determine the feasibility of in vivo longitudinal assessment of cardiac function, particularly diastolic function, in a mouse model of renovascular hypertension. Renovascular hypertension (RVH) was induced in 129S1/SvImJ male mice (n=9). To assess left ventricular (LV) systolic and diastolic function, M-mode echocardiography, pulsed-wave Doppler echocardiography and tissue Doppler imaging were performed at baseline, 2 and 4 weeks after the induction of renal artery stenosis. Myocardial tissue was collected to assess cellular morphology, fibrosis, extracellular matrix remodeling and inflammation ex vivo. RVH led to a significant increase in systolic blood pressure after 2 and 4 weeks (baseline: 99.26±1.09 mm Hg; 2 weeks: 140.90±7.64 mm Hg; 4 weeks: 147.52±5.91 mm Hg, P<0.05), resulting in a significant decrease in LV end-diastolic volume, associated with a significant elevation in ejection fraction and preserved cardiac output. Furthermore, the animals developed an abnormal diastolic function profile, with a shortening in the E velocity deceleration time as well as increases in the E/e' and the E/A ratio. The ex vivo analysis revealed a significant increase in myocyte size and deposition of extracellular matrix. Non-invasive high-resolution ultrasonography allowed assessment of the diastolic function profile in a small animal model of renovascular hypertension.

Response to ethanol induced ataxia between C57BL/6J and 129X1/SvJ mouse strains using a treadmill based assay

More sensitive assays of mouse motor ataxia may provide a better understanding of the pathological profile. Treadmill gait analysis using ventral imaging allows for unhindered access to the ambulating mouse. In contrast to genetic mutations or exogenous brain injury, ethanol (EtOH) allows for the detection of dose dependent changes in motor behavior, which can be used to assess an assay's detection sensitivity. EtOH induced ataxia was assessed in C57BL/6J (B6) and 129X1/SvJ (129) mice using the DigiGait imaging system. Gait was analyzed across EtOH dosage (1.75, 2.25 and 2.75 g/kg) in each strain using a linear mixed effects model. Overall, 129 mice displayed greater susceptibility to EtOH ataxia than their B6 counterparts. In both strains, hind paws exhibited greater sensitivity to EtOH dosage than fore paws. Across most variables analyzed, only a modest EtOH-induced change in motor behavior was observed in each strain with the 1.75 g/kg EtOH doses failing to elicit significant change. These data indicate the ability to detect motor differences between strains, yet only moderate ability to detect change across EtOH dosage using the automated treadmill. Rotarod assays, however, were able to detect motor impairment at lower doses of EtOH. The significant, but opposite changes in paw placement with increasing EtOH doses highlight strain-specific differences in biophysical adaptations in response to acute EtOH intoxication.

Genetic modifiers of hypertension in soluble guanylate cyclase α1-deficient mice

Nitric oxide (NO) plays an essential role in regulating hypertension and blood flow by inducing relaxation of vascular smooth muscle. Male mice deficient in a NO receptor component, the α1 subunit of soluble guanylate cyclase (sGCα1), are prone to hypertension in some, but not all, mouse strains, suggesting that additional genetic factors contribute to the onset of hypertension. Using linkage analyses, we discovered a quantitative trait locus (QTL) on chromosome 1 that was linked to mean arterial pressure (MAP) in the context of sGCα1 deficiency. This region is syntenic with previously identified blood pressure-related QTLs in the human and rat genome and contains the genes coding for renin. Hypertension was associated with increased activity of the renin-angiotensin-aldosterone system (RAAS). Further, we found that RAAS inhibition normalized MAP and improved endothelium-dependent vasorelaxation in sGCα1-deficient mice. These data identify the RAAS as a blood pressure-modifying mechanism in a setting of impaired NO/cGMP signaling.

Strategies and methods to study sex differences in cardiovascular structure and function: a guide for basic scientists

Background

Cardiovascular disease remains the primary cause of death worldwide. In the US, deaths due to cardiovascular disease for women exceed those of men. While cultural and psychosocial factors such as education, economic status, marital status and access to healthcare contribute to sex differences in adverse outcomes, physiological and molecular bases of differences between women and men that contribute to development of cardiovascular disease and response to therapy remain underexplored.

Methods

This article describes concepts, methods and procedures to assist in the design of animal and tissue/cell based studies of sex differences in cardiovascular structure, function and models of disease.

Results

To address knowledge gaps, study designs must incorporate appropriate experimental material including species/strain characteristics, sex and hormonal status. Determining whether a sex difference exists in a trait must take into account the reproductive status and history of the animal including those used for tissue (cell) harvest, such as the presence of gonadal steroids at the time of testing, during development or number of pregnancies. When selecting the type of experimental animal, additional consideration should be given to diet requirements (soy or plant based influencing consumption of phytoestrogen), lifespan, frequency of estrous cycle in females, and ability to investigate developmental or environmental components of disease modulation. Stress imposed by disruption of sleep/wake cycles, patterns of social interaction (or degree of social isolation), or handling may influence adrenal hormones that interact with pathways activated by the sex steroid hormones. Care must be given to selection of hormonal treatment and route of administration.

Conclusions

Accounting for sex in the design and interpretation of studies including pharmacological effects of drugs is essential to increase the foundation of basic knowledge upon which to build translational approaches to prevent, diagnose and treat cardiovascular diseases in humans.

Sex hormone control of left ventricular structure/function: mechanistic insights using echocardiography, expression, and DNA methylation analyses in adult mice

Calcium flux into and out of the sarco(endo)plasmic reticulum is vitally important to cardiac function because the cycle of calcium entry and exit controls contraction and relaxation. Putative estrogen and androgen consensus binding sites near to a CpG island are present in the cardiac calsequestrin 2 (CSQ2) promoter. Cardiomyocytes express sex hormone receptors and respond to sex hormones. We hypothesized that sex hormones control CSQ2 expression in cardiomyocytes and so affect cardiac structure/function. Echocardiographic analysis of male and female C57bl6n mice identified thinner walled and lighter hearts in females and significant concentric remodeling after long-term gonadectomy. CSQ2 and sodium-calcium exchanger-1 (NCX1) expression was significantly increased in female compared with male hearts and decreased postovariectomy. NCX1, but not CSQ2, expression was increased postcastration. CSQ2 expression was reduced when H9c2 cells were cultured in hormone-deficient media; increased when estrogen receptor-α (ERα), estrogen receptor-β (ERβ), or androgen agonists were added; and increased in hearts from ERβ-deficient mice. CSQ2 expression was reduced in mice fed a diet low in the methyl donor folic acid and in cells treated with 5-azadeoxycytidine suggesting an involvement of DNA methylation. DNA methylation in CpG in the CSQ2 CpG island was significantly different in males and females and was additionally changed postgonadectomy. Expression of DNA methyltransferases 1, 3a, and 3b was unchanged. These studies strongly link sex hormone-directed changes in CSQ2 expression to DNA methylation with changed expression correlated with altered left ventricular structure and function.

New approaches in small animal echocardiography: imaging the sounds of silence

Systolic and diastolic dysfunction of the left ventricle (LV) is a hallmark of most cardiac diseases. In vivo assessment of heart function in animal models, particularly mice, is essential to refining our understanding of cardiovascular disease processes. Ultrasound echocardiography has emerged as a powerful, noninvasive tool to serially monitor cardiac performance and map the progression of heart dysfunction in murine injury models. This review covers current applications of small animal echocardiography, as well as emerging technologies that improve evaluation of LV function. In particular, we describe speckle-tracking imaging-based regional LV analysis, a recent advancement in murine echocardiography with proven clinical utility. This sensitive measure enables an early detection of subtle myocardial defects before global dysfunction in genetically engineered and rodent surgical injury models. Novel visualization technologies that allow in-depth phenotypic assessment of small animal models, including perfusion imaging and fetal echocardiography, are also discussed. As imaging capabilities continue to improve, murine echocardiography will remain a critical component of the investigator's armamentarium in translating animal data to enhanced clinical treatment of cardiovascular diseases.

A novel role for the fifth component of complement (C5) in cardiac physiology

We have previously demonstrated that C5-deficient A/J and recombinant congenic BcA17 mice suffer from cardiac dysfunction when infected with C. albicans blastospores intravenously. During these studies we had observed that, even in the control un-infected state, BcA17 hearts displayed alterations in gene expression that have been associated with pathological cardiac hypertrophy in comparison to parental C5-sufficient C57Bl/6J (B6) mice. Of note was an increase in the expression of Nppb, a member of the fetal gene program and a decrease in the expression of Rgs2, an inhibitor of the hypertrophic response. We now report that C5-deletion has also affected the expression of other elements of the fetal gene program. Moreover deleting the C5a receptor, C5aR, has essentially the same effect as deleting C5, indicating a key role for C5a-C5aR signaling in the phenotype. Having noted a pathological phenotype in the un-infected state, we investigated the role of C5 in the response to cardiac stress. In previous studies, comparison of the expression profiles of C. albicans-infected BcA17 and similarly infected B6 hearts had revealed a paucity of cardioprotective genes in the C5-deficient heart. To determine whether this was also directly linked to C5-deficiency, we tested the expression of 5 such genes in the C. albicans-infected C5aR(-/-) mice. We found again that deletion of C5aR recapitulated the alterations in stress response of BcA17. To determine whether our observations were relevant to other forms of cardiac injury, we tested the effect of C5-deficiency on the response to isoproterenol-induced hypertrophic stimulation. Consistent with our hypothesis, A/J, BcA17 and C5aR(-/-) mice responded with higher levels of Nppa expression than B6 and BALB/c mice. In conclusion, our results suggest that an absence of functional C5a renders the heart in a state of distress, conferring a predisposition to cardiac dysfunction in the face of additional injury.

Disruption of glycogen synthase kinase-3-beta activity leads to abnormalities in physiological measures in mice

Dysregulation of glycogen synthase kinase-3-beta (GSK-3β) signaling pathways is thought to underlie the pathophysiology of mood disorders. In order to demonstrate that the loss of normal GSK-3β activity results in disturbances of physiological measures, we attempted to determine whether sleep-wake architecture, circadian rhythms of core body temperature and activity were altered in transgenic mice overexpressing GSK-3β activity specifically in the brain. Cortical electroencephalographic activity, body temperature (BT) and body locomotor activity (LMA) were continuously monitored using a biopotential telemetry probe. Normal circadian patterns were maintained for different measurements in both genotypes. No differences were found in total time spent asleep and waking over the 24-h recording session. However, transgenic animals overexpressing GSK-3β showed alteration in sleep continuity characterized by an increases in number of non rapid eye movement (NREM) sleep episodes (GSK-3β, 227.2 ± 1.7 vs. WT, 172.6 ± 1.4, p < 0.05) and decreases in mean episode duration (GSK-3β, 3.0 ± 0.1 vs. WT, 4.4 ± 0.2, p < 0.05). Additionally, transgenic animals exhibited marked enhancement of basal LMA and BT levels during the first part of the dark period, under both light-dark and free running dark-dark circadian cycles. Our findings indicate that transgenic mice overexpressing GSK-3β activity exhibit severe fragmentation of sleep-wake cycle during both the light and dark periods, without showing deviancy in total durations of vigilance states. The results strongly suggest that GSK-3β activity is elemental for the maintenance of circadian motor behavior levels required for proper regulation of BT and sleep-wake organization.

An RF-powered wireless multi-channel implantable bio-sensing microsystem

An RF-powered wireless three-channel implantable bio-sensing microsystem is developed with blood pressure, EKG, and core body temperature sensing capability for untethered genetically engineered laboratory mice real-time monitoring. A flat silicone blood pressure sensing cuff with a MEMS capacitive pressure sensor is employed to form a novel less-invasive blood pressure sensor, which avoids vessel occlusion, bleeding, and blood clotting associated with the conventional catheter-based sensors. The implantable microsystem can be powered by an adaptively controlled external RF energy source at 4 MHz to ensure a stable on-chip power supply. The overall system dissipates 200 microW and achieves a blood pressure sensing resolution of 1 mmHg within 1 kHz bandwidth, an EKG sensing resolution of 7.4 bits, and a temperature sensitivity of 19 mV/°C measured from 22 °C to 43 °C. A prototype packaged sensor exhibits a weight of 495 mg, which is approximately 2% of a laboratory mouse body mass. On-going research effort is devoted to demonstrate in vivo performance in laboratory animals.

Variation in echocardiographic and cardiac hemodynamic effects of PM and ozone inhalation exposure in strains related to Nppa and Npr1 gene knock-out mice

Elevated levels of ambient co-pollutants are associated with adverse cardiovascular outcomes shown by epidemiology studies. The role of particulate matter (PM) and ozone (O3) as co-pollutants in this association is unclear. We hypothesize that cardiac function following PM and O3 exposure is variably affected by genetic determinants (Nppa and Npr1 genes) and age. Heart function was measured before and after 2 days each of the following exposure sequence; (1) 2-h filtered air (FA) and 3-h carbon black (CB; 0.5 microg/m(3)); (2) 2-h O3 (0.6 ppm) and 3-h FA; (3) 5-h FA; and, (4) 2-h O3 and 3-h CB. Two age groups (5 and 18 months old (mo)) were tested in C57Bl/6J (B6) and 129S1/SvImJ (129) mice using echocardiographic (echo) and in vivo hemodynamic (IVH) measurements. With echo, posterior wall thickness was significantly (P < 0.01) greater in 129 relative to B6 mice at baseline. With CB exposure, young B6 and older 129 mice show significant (P < 0.01) reductions in fractional shortening (FS) compared to FA. With O3 exposure, FS was significantly (P < 0.01) diminished in young 129, which was attributable to significant increases in end-systolic left ventricular diameter. With O3 and CB combined, notable (P < 0.01) declines in heart rate and end-systolic posterior wall thickness occurred in young 129 mice. The IVH measurements showed striking (P < 0.05) compromises in cardiac function after CB and O3 exposure; however, strain differences were undetectable. These results suggest that PM and O3 exposures, alone and combined, lead to different cardiac functional changes, and these unique changes are age-specific and dependent on Nppa and Npr1 genes.

Ventilatory patterning in a mouse model of stroke

Cheyne-Stokes respiration (CSR) is a breathing pattern characterized by waxing and waning of breath volume and frequency, and is often recognized following stroke, when causal pathways are often obscure. We used an animal model to address the hypothesis that cerebral infarction is a mechanism for producing breathing instability. Fourteen male A/J mice underwent either stroke (n=7) or sham (n=7) procedure. Ventilation was measured using whole body plethysmography. Respiratory rate (RR), tidal volume (V(T)) and minute ventilation (V(e)) mean values and coefficient of variation were computed for ventilation and oscillatory behaviors. In addition, the ventilatory data were computationally fit to models to quantify autocorrelation, mutual information, sample entropy and a nonlinear complexity index. At the same time post-procedure, stroke when compared to sham animal breathing consisted of a lower RR and autocorrelation, higher coefficient of variation for V(T) and higher coefficient of variation for V(e). Mutual information and the nonlinear complexity index were higher in breathing following stroke which also demonstrated a waxing/waning pattern. The absence of stroke in the sham animals was verified anatomically. We conclude that ventilatory pattern following cerebral infarction demonstrated increased variability with increased nonlinear patterning and a waxing/waning pattern, consistent with CSR.

Influence of genetic background on ex vivo and in vivo cardiac function in several commonly used inbred mouse strains

Inbred mouse strains play a critical role in biomedical research. Genetic homogeneity within inbred strains and their general amenability to genetic manipulation have made them an ideal resource for dissecting the physiological function(s) of individual genes. However, the inbreeding that makes inbred mice so useful also results in genetic divergence between them. This genetic divergence is often unaccounted for but may be a confounding factor when comparing studies that have utilized distinct inbred strains. Here, we compared the cardiac function of C57BL/6J mice to seven other commonly used inbred mouse strains: FVB/NJ, DBA/2J, C3H/HeJ, BALB/cJ, 129X1/SvJ, C57BL/10SnJ, and 129S1/SvImJ. The assays used to compare cardiac function were the ex vivo isolated Langendorff heart preparation and in vivo real-time hemodynamic analysis using conductance micromanometry. We report significant strain-dependent differences in cardiac function between C57BL/6J and other commonly used inbred strains. C57BL/6J maintained better cardiac function than most inbred strains after ex vivo ischemia, particularly compared with 129S1/SvImJ, 129X1/SvJ, and C57BL/10SnJ strains. However, during in vivo acute hypoxia 129X1/SvJ and 129S1/SvImJ maintained relatively normal cardiac function, whereas C57BL/6J animals showed dramatic cardiac decompensation. Additionally, C3H/HeJ showed rapid and marked cardiac decompensation in response to esmolol infusion compared with effects of other strains. These findings demonstrate the complex effects of genetic divergence between inbred strains on cardiac function. These results may help inform analysis of gene ablation or transgenic studies and further demonstrate specific quantitative traits that could be useful in discovery of genetic modifiers relevant to cardiac health and disease.

beta-Arrestin-biased agonism of the angiotensin receptor induced by mechanical stress

beta-Arrestins, which were originally characterized as terminators of heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptor (GPCR) signaling, also act as important signal transducers. An emerging concept in GPCR signaling is beta-arrestin-biased agonism, in which specific ligand-activated GPCR conformational states selectively signal through beta-arrestins, rather than through G proteins. Here, we show that mechanical stretch induced beta-arrestin-biased signaling downstream of angiotensin II type I receptors (AT1Rs) in the absence of ligand or G protein activation. Mechanical stretch triggered an AT1R-mediated conformational change in beta-arrestin similar to that induced by a beta-arrestin-biased ligand to selectively stimulate receptor signaling in the absence of detectable G protein activation. Hearts from mice lacking beta-arrestin or AT1Rs failed to induce responses to mechanical stretch, as shown by blunted extracellular signal-regulated kinase and Akt activation, impaired transactivation of the epidermal growth factor receptor, and enhanced myocyte apoptosis. These data show that the heart responds to acute increases in mechanical stress by activating beta-arrestin-mediated cell survival signals.

Differential effect of wall tension on matrix metalloproteinase promoter activation in the thoracic aorta

BACKGROUND

Vascular remodeling relies upon extracellular matrix restructuring by the matrix metalloproteinases (MMPs). Induction of MMP-2 and MMP-9 by biological signaling molecules has been defined, but whether a mechanical stimulus such as elevated wall tension may generate MMP promoter activation remains unknown. Accordingly, this study examined whether MMP promoter activation would occur as a function of wall tension.

MATERIALS AND METHODS

The MMP-2 or MMP-9 promoter sequences were fused to the reporter gene lacZ and inserted into the mouse genome. Thoracic aortic rings were harvested (6 preparations/construct) and maintained under physiological conditions at predetermined tension values corresponding to 0, 70, 85, and 100 mm Hg for 3 h. Relative gene expression of lacZ, directly reflecting MMP promoter activity, was then quantified by QPCR.

RESULTS

MMP-2 promoter activity decreased to 0.42 +/- 0.11 at 0 mm Hg and increased to 1.57 +/- 0.24-fold at 100 mm Hg (P < 0.05), whereas MMP-9 was unaffected.

CONCLUSIONS

Using unique transgenic constructs with homology to human MMP promoters, this study demonstrated that a physiologically relevant mechanical stimulus was sufficient to differentially induce MMP promoter activation.

Parent-of-origin effects on cardiac response to pressure overload in mice

Left ventricular (LV) hypertrophy (LVH) is an independent risk factor for cardiovascular mortality and is commonly caused by hypertension. In rodents, transverse aortic constriction (TAC) is a model regularly employed in mechanistic studies of the response of the LV to pressure overload. We previously reported that inbred strains of male mice manifest different cardiac responses to TAC, with C57BL/6J (B6) developing LV dilatation and impaired contractility and 129S1/SvImJ (129) males displaying concentric LVH. In the present study, we investigated sex and parent-of-origin effects on the response to TAC by comparing cardiac function, organ weights, expression of cardiac hypertrophy markers, and histology in female B6 and female 129 mice and in F1 progeny of reciprocal crosses between B6 and 129 mice (B6129F1 and 129B6F1). Five weeks after TAC, heart weight increased to the greatest extent in 129B6F1 mice and the least extent in 129 and B6129F1 mice. Female 129B6F1 and B6 mice were relatively protected from the increase in heart weight that occurs in their male counterparts with pressure overload. The response to TAC in 129 consomic mice bearing the B6 Y chromosome resembled that of 129 rather than 129B6F1 mice, indicating that the B6 Y chromosome does not account for the differences in the reciprocal cross. Our results suggest that susceptibility to LVH is more complex than simple Mendelian inheritance and that parental origin effects strongly impact the LV response to TAC in these commonly used inbred strains.

Murine cardiac mtDNA: effects of transgenic manipulation of nucleoside phosphorylation

Mitochondrial toxicity results from pyrimidine nucleoside reverse transcriptase inhibitors (NRTIs) for HIV/AIDS. In the heart, this can deplete mitochondrial (mt) DNA and cause cardiac dysfunction (eg, left ventricle hypertrophy, LVH). Four unique transgenic, cardiac-targeted overexpressors (TGs) were generated to determine their individual impact on native mitochondrial biogenesis and effects of NRTI administration on development of mitochondrial toxicity. TGs included cardiac-specific overexpression of native thymidine kinase 2 (TK2), two pathogenic TK2 mutants (H121N and I212N), and a mutant of mtDNA polymerase, pol-gamma (Y955C). Each was treated with antiretrovirals (AZT-HAART, 3 or 10 weeks, zidovudine (AZT) + lamivudine (3TC) + indinavir, or vehicle control). Parameters included left ventricle (LV) performance (echocardiography), LV mtDNA abundance (real-time PCR), and mitochondrial fine structure (electron microscopy, EM) as a function of duration of treatment and presence of TG. mtDNA abundance significantly decreased in Y955C TG, increased in TK2 native and I212N TGs, and was unchanged in H121N TGs at 10 weeks regardless of treatment. Y955C and I212N TGs exhibited LVH during growth irrespective of treatment. Y955C TGs exhibited cardiomyopathy (CM) at 3 and 10 weeks irrespective of treatment, whereas H121N and I212N TGs exhibited CM only after 10 weeks AZT-HAART. EM features were consistent with cardiac dysfunction. mtDNA abundance and cardiac functional changes were related to TG expression of mitochondrially related genes, mutations thereof, and NRTIs.

Other Transgenic Animal Models Used in Cardiovascular Studies

Previous chapters have described a large number of transgenic animal models used to study specific cardiovascular syndromes. This chapter will fill in some gaps. Many of these transgenic animals were developed to study normal and/or abnormal physiological responses in other organ systems, or to study basic biochemical and molecular reactions or pathways. These models were then discovered to also have effects on the cardiovascular system, some of them unanticipated.

A word of caution, particularly when highly inbred mouse strains are used to develop transgenic models - not all strains of a particular species are created equal. When cardiovascular parameters of age- and sex-matched A/J and C57BL/6J inbred mice were compared the C57BL/6J mice demonstrated eccentric physiologic ventricular hypertrophy, increased ventricular function, lower heart rates, and increased exercise endurance.¹

Ultrasound imaging of renal vaso-occlusive events in transgenic sickle mice exposed to hypoxic stress

One of the major clinical manifestations of sickle cell disease (SCD) is vaso-occlusive crisis in response to hypoxic exposure, leading to acute and chronic organ damages, especially in kidneys. In a SCD transgenic murine model, ultrasound imaging allowed us to characterize the circulatory changes in renal arteries during vaso-occlusive crisis. Cardiac output, heart rate and renal blood flow velocities (BFV) were measured in 10 male transgenic and 10 male wild-type (WT) mice with a conventional echograph (Vivid 7, GE Medical), before and after hypoxic exposure (8%O(2), 18h). To assess entrapment of red cells, histologic study of the kidneys was performed in both groups. Hypoxic exposure decreased heart rates in both groups (-17%, p < 0.001). Cardiac output remained stable in WT, and decreased in transgenic (-26%, p < 0.01). Peak systolic BFV in the renal artery was not modified in both groups. End-diastolic and mean BFV remained stable in WT, but decreased in sickle transgenic (-56%, p < 0.01 and -47%, p < 0.001, respectively). Transgenic mice displayed marked congestion in peritubular capillaries and glomerular abnormalities with trapped sickle red cells, whereas WT did not present any histologic injury. Five hours after hypoxic exposure, blood flow velocities returned to basal values in both groups. Decrease in end-diastolic and mean BFV in absence of peak systolic BFV after hypoxic exposure strongly indicated that the increase in vascular resistance in kidneys related to sickling of red cells. Thus, ultrasound imaging of the renal artery in mouse is a powerful, noninvasive, easy-to-repeat method to evidence circulatory changes in murine models of vascular renal human diseases.

Ultrasonic assessment of cerebral blood flow changes during ischemia-reperfusion in 7-day-old rats

A model of ischemic brain injury in 7-day-old rat pups has been developed to study perinatal ischemia. It combines permanent occlusion of the distal left middle cerebral artery (LMCA) and transient occlusion of homolateral common carotid artery (LCCA). At removal of the clip on LCCA, reflow allowed brain reperfusion through cortical anastomoses. In 10 rat pups, we measured blood flow velocities (BFV) in main cerebral arteries with 12-MHz ultrasound imaging. At basal states, peak systolic BFV in proximal LMCA was 16.0 +/- 3.0 cm.s(-1). Occlusion of LMCA did not yield significant modifications. Occlusion of LCCA involved only a decrease in BFV to 9.5 +/- 2.6 cm.s(-1) (p < 0.001). Indeed, LMCA was then supply by the right internal carotid and the vertebral arteries through the circle of Willis. In three rat pups, release of occlusion of LCCA was followed by restoration of BFV in the left internal carotid artery and in LMCA, in seven pups, by a reversed flow in the LICA and lower BFV in LMCA (11.9 +/- 2.3, p < 0.05). BFV returned to basal values from h5 to h48 in all animals. In addition, ultrasound imaging is a useful, reproducible, non invasive, easy-to-repeat, method to assess and monitor arterial cerebral blood flow supply in small animals. It helps to characterize changes occurring during cerebral ischemia and reperfusion, particularly the depth of the hypoperfusion, as well as the variability of reflow. In preclinical studies, this method could help to identify what can be assigned to a neuroprotective treatment and what depends on changes in cerebral blood flow supply.

The genetic contribution to heart rate and heart rate variability in quiescent mice

Recent studies have suggested a genetic component to heart rate (HR) and HR variability (HRV). However, a systematic examination of the genetic contribution to the variation in HR and HRV has not been performed. This study investigated the genetic contribution to HR and HRV using a wide range of inbred and recombinant inbred (RI) mouse strains. Electrocardiogram data were recorded from 30 strains of inbred mice and 29 RI strains. Significant differences in mean HR and total power (TP) HRV were identified between inbred strains and RI strains. Multiple significant differences within the strain sets in mean low-frequency (LF) and high-frequency (HF) power were also found. No statistically significant concordance was found between strain distribution patterns for HR and HRV phenotypes. Genomewide interval mapping identified a significant quantitative trait locus (QTL) for HR [LOD (likelihood of the odds) score = 3.763] on chromosome 6 [peak at 53.69 megabases (Mb); designated HR 1 (Hr1)]. Suggestive QTLs for TP were found on chromosomes 2, 4, 5, 6, and 14. A suggestive QTL for LF was found on chromosome 16; for HF, we found one significant QTL on chromosome 5 (LOD score = 3.107) [peak at 53.56 Mb; designated HRV-high-frequency 1 (Hrvhf1)] and three suggestive QTLs on chromosomes 2, 11 and 15. In conclusion, the results demonstrate a strong genetic component in the regulation of resting HR and HRV evidenced by the significant differences between strains. A lack of correlation between HR and HRV phenotypes in some inbred strains suggests that different sets of genes control the phenotypes. Furthermore, QTLs were found that will provide important insight to the genetic regulation of HR and HRV at rest.

Feature identification in circadian rhythms of mice strains using in vivo information

The objective of this work was to identify strain-specific characteristics from real-time measurements of circadian rhythms of two inbred mouse strains. In particular, heart rate, temperature, and activity data collected from A/J and C57BL/6J (B6) mice using telemetry are analyzed. The influence of activity on heart rate and temperature is minimized by correlation analysis followed by regression analysis. The correlation analysis is used to determine the length of the activity data filter that results in the best correlation between activity data and heart rate or temperature. After the activity data are filtered, they are used in regression analysis. The temperature and heart rate rhythms obtained as the intercepts of the regression analysis are interpreted as the zero-activity rhythms and consequently are good estimates of the circadian rhythms. The circadian temperature rhythms for the B6 mice follow a smoother cosine-like time waveform, whereas those for the A/J mice follow a more square-wave-like waveform. To quantify the difference between these two temperature rhythms, a feature based on Fourier analysis of the time-series data is used. Detrended fluctuation analysis is used to identify features in the heart rate rhythms. The results of this work show that the features for the circadian temperature and heart rate rhythms can be used as distinguishing characteristics of the A/J and B6 strains. This work provides the foundation for future studies directed at investigating the influence of chromosomal substitutions on the regulation of circadian rhythms in these two strains.

In vivo RF powering for advanced biological research

An optimized remote powering architecture with a miniature and implantable RF power converter for an untethered small laboratory animal inside a cage is proposed. The proposed implantable device exhibits dimensions less than 6 mmx6 mmx1 mm, and a mass of 100 mg including a medical-grade silicon coating. The external system consists of a Class-E power amplifier driving a tuned 15 cmx25 cm external coil placed underneath the cage. The implant device is located in the animal's abdomen in a plane parallel to the external coil and utilizes inductive coupling to receive power from the external system. A half-wave rectifier rectifies the received AC voltage and passes the resulting DC current to a 2.5 kOmega resistor, which represents the loading of an implantable microsystem. An optimal operating point with respect to operating frequency and number of turns in each coil inductor was determined by analyzing the system efficiency. The determined optimal operating condition is based on a 4-turn external coil and a 20-turn internal coil operating at 4 MHz. With the Class-E amplifier consuming a constant power of 25 W, this operating condition is sufficient to supply a desired 3.2 V with 1.3 mA to the load over a cage size of 10 cmx20 cm with an animal tilting angle of up to 60 degrees, which is the worst case considered for the prototype design. A voltage regulator can be designed to regulate the received DC power to a stable supply for the bio-implant microsystem.

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.