Developmental changes in left and right ventricular diastolic filling patterns in mice

Developmental regulation of cardiovascular function is dependent on both genotype and environment

Adverse developmental environments can increase the risk of adult cardiovascular disease, but not all individuals are affected, suggesting the importance of genotype. Genetically distinct mouse strains allow the genetic dissection of complex traits; however, they have not been used to evaluate the developmental origins of adult cardiovascular disease. Our objective was to determine the effect of prenatal nutrient restriction (R) on adult cardiovascular function in A/J (AJ) and C57BL/6J (B6) mice and whether a postnatal high-fat (HF) diet exacerbates these effects. Pregnant AJ and B6 mice underwent a 30% R or ad libitum diet, and their offspring underwent a HF or control diet. Hypertension (+17 mmHg; P < 0.001) was observed in B6R mice at 9 wk, and their arterial pressure tended to remain high at 25 wk (+13 mmHg; not significant). In AJR mice, the normal decrement in arterial pressure over this age range in this strain was abolished. Heart rate prematurely increased in B6R and decreased in AJR (all; P < 0.05) mice from 9 to 25 wk. There was no effect of postnatal HF diet on these relationships. The Tei index (from a 26-wk microultrasound) was increased in both AJR and B6R mice (all; P < 0.05), suggesting an improved global myocardial performance. Neither R nor HF alone changed diastolic (ratio of E wave to A wave) or systolic (%fractional shortening) function in either strain; however, R and HE combined improved diastolic function in B6 ( P < 0.05) but not in AJ mice. Therefore, there are strain-dependent alterations in adult cardiovascular function in response to prenatal nutrient restriction. Unexpectedly, a postnatal HF diet did not exacerbate the effects of prenatal nutrient restriction on postnatal cardiovascular outcomes.

High-frequency ultrasound assessment of the murine heart from embryo through to juvenile

AIM

The aim of this study is to assess the murine heart of normal embryos, neonates, and juveniles using high-frequency ultrasound.

METHODS

Diastolic function was measured with E/A ratio (E wave velocity/A wave velocity) and isovolumetric relaxation time (IRT), systolic function with isovolumetric contraction time (ICT), percentage fractional shortening (FS %), percentage ejection fraction (EF %). Global cardiac performance was quantified using myocardial performance index (MPI).

RESULTS

Isovolumetric relaxation time remained stable from E10.5 to 3 weeks. Systolic function (ICT) improved with gestation and remained stable from E18.5 onward. Myocardial performance index showed improvement in embryonic life (0.82- 0.63) and then stabilized from 1 to 3 week (0.60-0.58). Percentage ejection fraction remained high during gestation (77%-69%) and then decreased from the neonate to juvenile (68%-51%).

CONCLUSION

The ultrasound biomicroscope allows for noninvasive in-depth assessment of cardiac function of embryos and pups. Detailed physiological and functional cardiac function readouts can be obtained, which is invaluable for comparison to mouse models of disease.

A new 15-50 MHz array-based micro-ultrasound scanner for preclinical imaging

Most institutions now have a suite of imaging tools to follow mouse models of human disease. Micro-ultrasound is one of these tools and is second after whole-mouse fluorescence or bioluminescent imaging, in terms of installed systems. We report in this paper the first commercially available array transducer-based ultrasound imaging system that enables micro-ultrasound imaging at center frequencies between 15 and 50 MHz. At the heart of the new scanner is a laser-machined high-frequency 256 element, linear transducer array capable of forming dynamic diffraction limited beams. The power of the linear array approach is embodied in the uniform high resolution maintained over the full field of view. This leads to greatly expanded scope for real-time functional imaging that is demonstrated in this paper. The unprecedented images made with the new imaging system will enable many new applications not previously possible. These include real-time visualization of flow in the mouse placenta, visualization of flow development in the embryo, studies of embryonic to adult cardiac development/disease, and studies of real-time blood flow in mouse models of tumour angiogenesis.

Effects of fixed-dose isosorbide dinitrate/hydralazine on diastolic function and exercise capacity in hypertension-induced diastolic heart failure

Hypertension-induced diastolic heart failure accounts for a large proportion of all heart failure presentations. Hypertension also induces left ventricular (LV) hypertrophy. Fixed-dose isosorbide dinitrate/hydralazine (HISDN) decreased mortality in human systolic heart failure but it is unknown whether it improves maladaptive myocardial remodeling. We sought to test the hypothesis that chronic HISDN modulates LV hypertrophy and myocardial remodeling in hypertension-induced diastolic heart failure. FVB mice underwent either saline (n=18) or aldosterone (n=28) infusion. All underwent uninephrectomy and drank 1% salt water for 4 weeks. Mice were randomized after surgery to regular chow or chow containing HISDN (isosorbide dinitrate: 26 mg/kg per day; hydralazine: 50 mg/kg per day) for 4 weeks. Aldosterone infusion increased tail-cuff blood pressure (161+/-3 mm Hg) versus saline-infused mice (129+/-2 mm Hg). Aldosterone induced LV hypertrophy versus saline-infused mice (LV:body weight ratio: 4.2+/-0.1 versus 3.6+/-0.1 mg/g). HISDN attenuated the aldosterone-induced increased in systolic blood pressure (137+/-5 mm Hg) and also lowered blood pressure in saline-infused mice (114+/-2 mm Hg). However, HISDN did not cause LV hypertrophy regression in aldosterone-infused mice. Aldosterone increased LV end-diastolic dimensions that were not attenuated by HISDN. Similarly, neither aldosterone infusion nor HISDN affected LV end-systolic dimensions. LV ejection fraction and wet:dry lung ratio were not different between aldosterone-untreated and aldosterone-HISDN mice. However, mitral Doppler E/A ratio (a measure of diastolic function), exercise capacity, and plasma soluble vascular cell adhesion molecule 1 levels were improved in aldosterone-HISDN hearts. In conclusion, fixed-dose HISDN improved hypertension, diastolic function, and exercise capacity and reduced soluble vascular cell adhesion molecule 1 levels. There were no reductions in LV hypertrophy, cardiac fibrosis, or pulmonary congestion. These functional improvements are likely related to extracardiac effects, such as effects on the vasculature.

In vivo virtual histology of mouse embryogenesis by ultrasound biomicroscopy and magnetic resonance imaging

Feasibility of magnetic resonance imaging (MRI) and ultrasound biomicroscopy (UBM) for sequential in vivo study of mouse embryo development between Days 6.5 and 13.5 of pregnancy was assessed in a first experiment. A second trial, based on the results of the first, determined the accuracy of UBM for imaging morphogenesis from implantation to the late embryo stage (Days 4.5 to 15.5). MRI allowed imaging of the entire uterus and all gestational sacs and embryos inside whilst the small scanning range of UBM precluded accurate counting of fetuses; however, its high resolution identified the decidual reaction at implantation sites from Day 4.5. At later stages, it was possible to assess key morphogenetic processes such as differentiation of the placenta, the cephalic region, the thoracic and abdominal organs, the skeletal system and the limbs, and dynamic structures such as the cardiovascular system. Thus, both techniques are reliable for in utero imaging of mouse embryo development. MRI may be more appropriate for studying embryo lethality and intrauterine growth retardation, because the entire uterus can be viewed. UBM may be more suitable for studies of cellular components of organs and tissues and assessment of haemodynamic changes in the circulatory system.

High-frequency subharmonic pulsed-wave Doppler and color flow imaging of microbubble contrast agents

A recent study has shown the feasibility of subharmonic (SH) flow imaging at a transmit frequency of 20 MHz. This paper builds on these results by examining the performance of SH flow imaging as a function of transmit pressure. Further, we also investigate the feasibility of SH pulsed-wave Doppler (PWD) imaging. In vitro flow experiments were performed with a 1-mm-diameter wall-less vessel cryogel phantom using the ultrasound contrast agent Definity and an imaging frequency of 20 MHz. The phantom results show that there is an identifiable pressure range where accurate flow velocity and power estimates can be made with SH imaging at 10 MHz (SH10), above which velocity estimates are biased by radiation force effects and unstable bubble behavior, and below which velocity and power estimates are degraded by poor SNR. In vivo validation of SH PWD was performed in an arteriole of a rabbit ear, and blood velocity estimates compared well with fundamental (F20) mode PWD. The ability to suppress tissue signals using SH signals may enable the use of higher frame rates and improve sensitivity to microvascular flow or slow velocities near large vessel walls by reducing or eliminating the need for clutter filters.

Impaired relaxation is the main manifestation in transgenic mice expressing a restrictive cardiomyopathy mutation, R193H, in cardiac TnI

Transgenic mice were generated to express a restrictive cardiomyopathy (RCM) human cardiac troponin I (cTnI) R192H mutation in the heart (cTnI(193His) mice). The objective of this study was to assess cardiac function during the development of diastolic dysfunction and to gain insight into the pathophysiological impact of the RCM cTnI mutation. Cardiac function and pathophysiological changes were monitored in cTnI193His mice and wild-type littermates for a period of 12 mo. It progressed gradually from abnormal relaxation to diastolic dysfunction characterized with high-resolution echocardiography by a reversed E-to-A ratio, increased deceleration time, and prolonged isovolumetric relaxation time. At the age of 12 mo, cardiac output in cTnI(193His) mice was significantly declined, and some transgenic mice showed congestive heart failure. The negative impact of cTnI193His on ventricular contraction and relaxation was further demonstrated in isolated mouse working heart preparations. The main morphological change in cTnI193His myocytes was shortened cell length. Dobutamine stimulation increased heart rate in cTnI193His mice but did not improve CO. The cTnI193His mice had a phenotype similar to that in human RCM patients carrying the cTnI mutation characterized morphologically by enlarged atria and restricted ventricles and functionally by diastolic dysfunction and diastolic heart failure. The results demonstrate a critical role of the COOH-terminal domain of cTnI in the diastolic function of cardiac muscle.

Comparative Study of High-resolution Microimaging with 30-MHz Scanner for Evaluating Cardiac Function in Mice

BACKGROUND

The accurate assessment of cardiac function in mice is challenging because of their small heart size and rapid heart rate.

METHODS

We examined the usefulness of novel high-resolution echocardiography (HRE) with a 30-MHz transducer in evaluating cardiac function in 20 mice compared with conventional echocardiography (CE) with a 13-MHz transducer. The left ventricular (LV) regional wall motion (RWM), LV end-diastolic dimension, fractional shortening, anterior LV wall thickness, E/A, and myocardial performance index were assessed.

RESULTS

RWM analysis was more feasible by HRE than by CE (P < .05). Interobserver agreement in RWM analysis and correlation in LV end-diastolic dimension, fractional shortening, anterior LV wall thickness, E/A, and myocardial performance index were all better with HRE than CE.

CONCLUSIONS

HRE is superior to CE in assessing LV function in mice. HRE is potentially a useful method for accurate assessment of cardiac function in various mice models.

Progressive troponin I loss impairs cardiac relaxation and causes heart failure in mice

Cardiac troponin I (TnI) knockout mice exhibit a phenotype of sudden death at 17–18 days after birth due to a progressive loss of TnI. The objective of this study was to gain insight into the physiological consequences of TnI depletion and the cause of death in these mice. Cardiac function was monitored serially between 12 and 17 days of age by using high-resolution ultrasonic imaging and Doppler echocardiography. Two-dimensional B-mode and anatomical M-mode imaging and Doppler echocardiography were performed using a high-frequency (∼20–45 MHz) ultrasound imaging system on homozygous cardiac TnI mutant mice (cTnI−/−) and wild-type littermates. On day 12, cTnI−/−mice were indistinguishable from wild-type mice in terms of heart rate, atrial and LV (LV) chamber dimensions, LV posterior wall thickness, and body weight. By days 16 through 17, wild-type mice showed up to a 40% increase in chamber dimensions due to normal growth, whereas cTnI−/−mice showed increases in atrial dimensions of up to 97% but decreases in ventricular dimensions of up to 70%. Mitral Doppler analysis revealed prolonged isovolumic relaxation time and pronounced inversion of the mitral E/A ratio (early ventricular filling wave-to-late atrial contraction filling wave) only in cTnI−/−mice indicative of impaired LV relaxation. cTnI−/−mouse hearts showed clear signs of failure on day 17, characterized by >50% declines in cardiac output, ejection fraction, and fractional shortening. B-mode echocardiography showed a profoundly narrowed tube-like LV and enlarged atria at this time. Our data are consistent with TnI deficiency causing impaired LV relaxation, which leads to diastolic heart failure in this model.

Determination of cell types and numbers during cardiac development in the neonatal and adult rat and mouse

Cardiac fibroblasts, myocytes, endothelial cells, and vascular smooth muscle cells are the major cellular constituents of the heart. The aim of this study was to observe alterations in myocardial cell populations during early neonatal development in the adult animal and to observe any variations of the cardiac cell populations in different species, specifically, the rat and mouse. Whole hearts were isolated from either mice or rats during the neonatal and adult stages of development, and single cell suspensions were prepared via sequential collagenase digestion. Heterogeneous cell populations were immunolabeled for specific cell types and analyzed using fluorescence-activated cell sorting (FACS). In addition, the left ventricle, right ventricle, and septa were isolated, fixed, and sectioned for morphometric analyses. These same cardiac regions were also analyzed using FACS. We observed that the adult murine myocardium is composed of approximately 56% myocytes, 27% fibroblasts, 7% endothelial cells, and 10% vascular smooth muscle cells. Moreover, our morphometric and FACS data demonstrated similar percentages in the three regions examined. During murine neonatal cardiac development, we observed a marked increase in numbers of cardiac fibroblasts and a resultant decrease in percentages of myocytes in late neonatal development (day 15). Finally, FACS analyses of the rat heart during development displayed similar results in relation to increases in cardiac fibroblasts during development; however, cell populations in the rat differed markedly from those observed in the mouse. Taken together, these data enabled us to establish a homeostatic model for the myocardium that can be compared with genetic and cardiac disease models.

Fgl2 deficiency causes neonatal death and cardiac dysfunction during embryonic and postnatal development in mice

We hypothesized that cardiac dysfunction was responsible for the high perinatal lethality that we previously reported in fibrinogen-like protein 2 (Fgl2) knockout (KO) mice. We therefore used ultrasound biomicroscopy to assess left ventricular (LV) cardiac structure and function during development in Fgl2 KO and wild-type (WT) mice. The only deaths observed between embryonic day (E)8.5 (onset of heart beating) and postnatal day (P)28 (weaning) were within 3 days after birth, when 33% of Fgl2 KO pups died. Histopathology and Doppler assessments suggested that death was due to acute congestive cardiac failure without evidence of valvular or other obvious cardiac structural abnormalities. Heart rates in Fgl2 KO embryos were significantly reduced at E8.5 and E17.5, and irregular heart rhythms were significantly more common in Fgl2 KO (21/26) than WT (2/21) embryos at E13.5. Indexes of systolic and/or diastolic cardiac function were also abnormal in KO mice at E13.5 and E17.5, in postnatal mice studied at P1, and in KO mice surviving to P28. M-mode analysis showed no difference in LV diastolic chamber dimension, although posterior wall thickness was thinner at P7 and P28 in Fgl2 KO mice. We conclude that Fgl2 deficiency is not associated with obvious structural cardiac defects but is associated with a high incidence of neonatal death as well as contractile dysfunction and rhythm abnormalities during embryonic and postnatal development in mice.

Up-regulation of parvalbumin expression in newborn and adult rat heart

Parvalbumin (PV), a cytoplasmic calcium-binding protein, functions as a relaxing factor and has recently been detected in rat heart. Developmental changes in PV localization and expression were investigated in the heart of Wistar rats at different ages. Ten hearts from newborn, 3-month-old (young), 6-month-old (young adult), and 12-month-old (adult) rats were processed for immunohistochemistry and Western blot assay. PV was detected in hearts of all the age groups of the rats from newborn to 12-month-old by both immunohistochemistry and Western blotting. A variable distribution of PV immunoreactivity was present in newborn cardiac myocytes. In the 3-, 6-, and 12-month-old rat hearts, identical PV immunoreactivity was found in all cardiac myocytes and the intensity of PV immunoreactivity increased with increasing age. By using Western blotting, it was found that the expression of PV was low in the newborn rat heart and increased with increasing age. The presence of PV may correlate with the physiological age, and possibly serves to maintain proper relaxation of the cardiac myocytes to cope with an increasing workload of the heart during body growth.

L’échographie haute résolution de la souris

Small-animal ultrasound imaging has been made possible using high-resolution imaging devices. The spatial resolution is therefore sufficient to accurately measure anatomical parameters in mice. This paper reviews some of the main applications of high-resolution ultrasound imaging of the mouse and highlights what could be the forthcoming advances.

Fetal Mouse Imaging Using Echocardiography: A Review of Current Technology

Advances in genetic research have led to the need for phenotypic analysis of small animal models. However, often these genetic alterations, especially when affecting the cardiovascular system, can result in fetal or perinatal death. Noninvasive ultrasound imaging is an ideal method for detecting and studying such congenital malformations, as it allows early recognition of abnormalities in the living fetus and the progression of disease can be followed in utero with longitudinal studies. Two platforms for fetal mouse echocardiography exist, the clinical systems with 15-MHz phased array transducers and research systems with 20-55-MHz mechanical transducers. The clinical ultrasound system has limited two-dimensional (2D) resolution (axial resolution of 440 microm), but the availability of color and spectral Doppler allows quick interrogations of blood flows, facilitating the detection of structural abnormalities. M-mode imaging further provides important functional data, although, the proper imaging planes are often difficult to obtain. In comparison, the research biomicroscope system has significantly improved 2D resolution (axial resolution of 28 microm). Spectral Doppler imaging is also available, but in the absence of color Doppler, imaging times are increased and the detection of flow abnormalities is more difficult. M-mode imaging is available and equivalent to the clinical ultrasound system. Overall, the research system, given its higher 2D resolution, is best suited for in-depth analysis of mouse fetal cardiovascular structure and function, while the clinical ultrasound systems, equipped with phase array transducers and color Doppler imaging, are ideal for high-throughput fetal cardiovascular screens.

Performance and characterization of new micromachined high-frequency linear arrays

A new approach for fabricating high frequency (> 20 MHz) linear array transducers, based on laser micromachining, has been developed. A 30 MHz, 64-element, 74-microm pitch, linear array design is presented. The performance of the device is demonstrated by comparing electrical and acoustic measurements with analytical, equivalent circuit, and finite-element analysis (FEA) simulations. All FEA results for array performance have been generated using one global set of material parameters. Each fabricated array has been integrated onto a flex circuit for ease of handling, and the flex has been integrated onto a custom printed circuit board test card for ease of testing. For a fully assembled array, with an acoustic lens, the center frequency was 28.7 MHz with a one-way -3 dB and -6 dB bandwidth of 59% and 83%, respectively, and a -20 dB pulse width of -99 ns. The per-element peak acoustic power, for a +/- 30 V single cycle pulse, measured at the 10 mm focal length of the lens was 590 kPa with a -6 dB directivity span of about 30 degrees. The worst-case total cross talk of the combined array and flex assembly is for nearest neighboring elements and was measured to have an average level -40 dB across the -6 dB bandwidth of the device. Any significant deviation from simulation can be explained through limitations in apparatus calibration and in device packaging.

Atrioventricular valve development during late embryonic and postnatal stages involves condensation and extracellular matrix remodeling

Although the signaling molecules regulating the early stages of valvular development have been well described, little is known on the late steps leading to mature fibrous leaflets. We hypothesized that atrioventricular (AV) valve development continues after birth to adjust to the postnatal maturation of the heart. By doing a systematic analysis of the AV valves of mice from embryonic day (E) 15.5 to 8 weeks old, we identified key developmental steps that map the maturation process of embryonic cushion-like leaflets into adult stress-resistant valves. Condensation of the mesenchymal cells occurred between E15.5 and E18.5 and was accompanied by increased cellular proliferation and adhesion. Cellular proliferation also contributed transiently to the concomitant elongation of the leaflets. Patterning of the extracellular matrix (ECM) proteins along the AV axis was achieved 1 week after birth, with the differentiation of two reciprocal structural regions, glycosaminoglycans and versican at the atrial side, and densely packed collagen fibers at the ventricular side. Formation and remodeling of the nodular thickenings at the closure points of the leaflets occurred between N4.5 and N11.5. In conclusion, AV valve development during late embryonic and postnatal stages includes condensation, elongation, formation of nodular thickenings, and remodeling of tension-resistant ECM proteins.

Imaging tools for the developmental biologist: ultrasound biomicroscopy of mouse embryonic development

Progress has been rapid in the elucidation of genes responsible for cardiac development. Strategies to ascertain phenotypes, however, have lagged behind advances in genomics, particularly in the in vivo mouse embryo, considered a model organism for mammalian development, and for human development and disease. Over the past several years, our laboratory and others have pioneered a variety of ultrasound biomicroscopy (UBM)-Doppler approaches to study in vivo development in both normal and mutant mouse embryos. This state-of-the-art review will discuss the development and potential of ultrasound biomicroscopy as a tool for the in vivo imaging and phenotyping of both cardiac and non-cardiac organ systems in the early developing mouse. Broad, long-term research objectives are to define living structure-function relationships during critical periods of mammalian morphogenesis.

Cardiovascular function in mice during normal pregnancy and in the absence of endothelial NO synthase

In humans, the increased cardiovascular demands of pregnancy are met by increases in cardiac output (CO), stroke volume (SV), plasma volume (PV), and cardiac and aortic inner dimensions and a concurrent decrease in arterial pressure that indicates a fall in total peripheral vascular resistance. The mechanisms responsible for these changes are incompletely understood, but NO synthase (NOS) is believed to play a central role. We assessed whether C57Bl/6J (B6) mice show similar changes and whether these changes are altered in mice lacking the gene for endothelial NOS (eNOS). The CO of B6 mice increased 28% by day 9.5 of gestation because of a 25% increase in SV, and increased 48% by day 17.5 because of a 41% increase in SV. The increase in SV at day 17.5 was associated with a 27% increase in PV, a 15% decrease in arterial pressure, and 10% to 15% increases in aortic and left-ventricular inner dimensions. In the absence of eNOS, CO increased 22% by day 9.5 because of increases in SV (14%) and heart rate (9%), but increased no further by day 17.5. SV near term was lower than B6 mice despite similar 26% increases in PV and 14% decreases in arterial pressure in association with blunted left-ventricular chamber enlargement. All reported changes are P<0.05. We conclude that cardiovascular changes during pregnancy are similar in B6 mice and humans. eNOS plays a critical role in increasing stroke volume in late gestation by promoting cardiac remodeling.

Noninvasive localization of nuclear factor of activated T cells c1-/- mouse embryos by ultrasound biomicroscopy-Doppler allows genotype-phenotype correlation

Ultrasound biomicroscopy (UBM)-Doppler allows study of cardiovascular physiology in the in utero mouse embryo from embryonic day (E)8.25 onward. We determined the accuracy of localization of embryos by transabdominal, noninvasive 40-MHz UBM-Doppler imaging. Nuclear factor of activated T cells c1-/- mice lack semilunar valves, exhibit outflow tract regurgitation, and die in utero. In timed pregnant mice generated from heterozygote crosses, an UBM-derived map of the in situ litter was compared with a definitive laparotomy map, and UBM-Doppler cardiac screen attempted for each embryo. All 109 living and dead (nonresorbed) E10.5 to 17.5 embryos were imaged and accurately localized. All 10 embryos with reversed diastolic aortic flow and 7 of 9 dead embryos genotyped were nuclear factor of activated T cells c1-/-. In 30 embryos followed up serially over 1 to 2 days from E12.5 to E16.5, we again achieved 100% accuracy in localizing at follow-up. Noninvasive localization and UBM-Doppler analysis of in situ mouse embryos can provide accurate genotype-phenotype correlation, along with nontraumatic serial imaging of embryos.

Embryonic and Neonatal Phenotyping of Genetically Engineered Mice

Considerable progress has been made in adapting existing and developing new technologies to enable increasingly detailed phenotypic information to be obtained in embryonic and newborn mice. Sophisticated methods for imaging mouse embryos and newborns are available and include ultrasound and magnetic resonance imaging (MRI) for in vivo imaging, and MRI, vascular corrosion casts, micro-computed tomography, and optical projection tomography (OPT) for postmortem imaging. In addition, Doppler and M-mode ultrasound are useful noninvasive tools to monitor cardiac and vascular hemodynamics in vivo in embryos and newborns. The developmental stage of the animals being phenotyped is an important consideration when selecting the appropriate technique for anesthesia or euthanasia and for labeling animals in longitudinal studies. Study design also needs to control for possible differences between inter- and intralitter variability, and for possible long-term developmental effects caused by anesthesia and/or procedures. Noninvasive or minimally invasive intravenous or intracardiac injections or blood sampling, and arterial pressure and electrocardiography (ECG) measurements are feasible in newborns. Whereas microinjection techniques are available for embryos as young as 6.5 days of gestation, further advances are required to enable minimally invasive fluid or tissue samples, or blood pressure or ECG measurements, to be obtained from mouse embryos in utero. The growing repertoire of techniques available for phenotyping mouse embryos and newborns promises to accelerate knowledge gained from studies using genetically engineered mice to understand molecular regulation of morphogenesis and the etiology of congenital diseases.

Comparison of contraction and calcium handling between right and left ventricular myocytes from adult mouse heart: a role for repolarization waveform

In the mammalian heart, the right ventricle (RV) has a distinct structural and electrophysiological profile compared to the left ventricle (LV). However, the possibility that myocytes from the RV and LV have different contractile properties has not been established. In this study, sarcomere shortening, [Ca2+]i transients and Ca2+ and K+ currents in unloaded myocytes isolated from the RV, LV epicardium (LVepi) and LV endocardium (LVendo) of adult mice were evaluated. Maximum sarcomere shortening elicited by field stimulation was graded in the order: LVendo > LVepi > RV. Systolic [Ca2+]i was higher in LVendo myocytes than in RV myocytes. Voltage-clamp experiments in which action potential (AP) waveforms from RV and LVendo were used as the command signal, demonstrated that total Ca2+ influx and myocyte shortening were larger in response to the LVendo AP, independent of myocyte subtypes. Evaluation of possible regional differences in myocyte Ca2+ handling was based on: (i) the current-voltage relation of the Ca2+ current; (ii) sarcoplasmic reticulum Ca2+ uptake; and (iii) mRNA expression of important components of the Ca2+ handling system. None of these were significantly different between RV and LVendo. In contrast, the Ca2+-independent K+ current, which modulates AP repolarization, was significantly different between RV, LVepi and LVendo. These results suggest that these differences in K+ currents can alter AP duration and modulate the [Ca2+]i transient and corresponding contraction. In summary, these findings provide an initial description of regional differences in excitation-contraction coupling in the adult mouse heart [corrected]

Pulsed Doppler Signal Processing for Use in Mice: Applications

We have developed a high-frequency, high-resolution Doppler spectrum analyzer (DSPW) and compared its performance against an adapted clinical Medasonics spectrum analyzer (MSA) and a zero-crossing interval histogram (ZCIH) used previously by us to evaluate cardiovascular physiology in mice. The aortic velocity (means +/- SE: 92.7 +/- 2.5 versus 82.2 +/- 1.8 cm/s) and aortic acceleration (8194 +/- 319 versus 5178 +/- 191 cm/s2) determined by the DSPW were significantly higher compared to those by the MSA. Aortic ejection time was shorter (48.3 +/- 0.9 versus 64.6 +/- 1.8 ms) and the isovolumic relaxation was longer (17.6 +/- 0.6 versus 13.5 +/- 0.6 ms) when determined by the DSPW because it generates shorter temporal widths in the velocity spectra when compared to the MSA. These data indicate that the performance of the DSPW in evaluating cardiovascular physiology was better than that of the MSA. There were no significant differences between the aortic pulse wave velocity determined by using the ZCIH (391 +/- 16 cm/s) and the DSPW (394 +/- 20 cm/s). Besides monitoring cardiac function, we have used the DSPW for studying peripheral vascular physiology in normal, transgenic, and surgical models of mice. Several applications such as the detection of high stenotic jet velocities (> 4 m/s), vortex shedding frequencies (250 Hz), and subtle changes in wave shapes in peripheral vessels which could not obtained with clinical Doppler systems are now made possible with the DSPW.

Murine Echocardiography: A Practical Approach for Phenotyping Genetically Manipulated and Surgically Modeled Mice

There have now been literally hundreds of genetically manipulated mouse models developed during the past decade of cardiac research. Echocardiography is considered an extremely important tool to noninvasively assess and serially follow the phenotype of genetically and surgically altered mice. This review describes in detail the technical considerations, various routinely used methods to assess cardiac function, and some emerging techniques in the assessment of cardiac function in experimental mouse models of cardiac disease.

Molecular and Functional Signature of Heart Hypertrophy During Pregnancy

During pregnancy, the heart develops a reversible physiological hypertrophic growth in response to mechanical stress and increased cardiac output; however, underlying molecular mechanisms remain unknown. Here, we investigated pregnancy-related changes in heart structure, function, and gene expression of known markers of pathological hypertrophy and cell stretching in mice hearts. In late pregnancy, hearts show eccentric hypertrophy, as expected for a response to volume overload, with normal left ventricular diastolic function and a moderate reduction in systolic function. Pregnancy-related physiological heart hypertrophy does not induce expression changes of known markers of pathological hypertrophy like: α- and β-myosin heavy chain, atrial natriuretic factor, phospholamban, and sarcoplasmic reticulum Ca 2+ -ATPase. Instead, it induces the remodeling of Kv4.3 channel and increased c-Src tyrosine kinase activity, a stretch-responsive kinase. Cardiac Kv4.3 channel gene expression was downregulated by ≈3- to 5-fold, both at the mRNA and protein levels, and was paralleled by a reduction in transient outward K + currents, a longer action potential and by prolongation of the QT interval. Downregulation of cardiac Kv4.3 transcripts was mimicked by estrogen treatment in ovariectomized mice, and was prevented by the estrogen receptor antagonist ICI 182,780. c-Src activity increased by ≈2-fold in late pregnancy and after estrogen treatment. We propose that, in addition to mechanical stress, the rise of estrogen toward the end of pregnancy contributes to pregnancy-related heart hypertrophy by increased c-Src activity and that the rise of estrogen is one factor that down regulates cardiac Kv4.3 gene expression providing a molecular correlate for a longer QT interval in pregnancy.

Three-dimensional ultrasound biomicroscopy for xenograft growth analysis

We reported the use of high-frequency ultrasound biomicroscopy (UBM) in the quantitative analysis of early tumor growth in mice bearing melanoma xenografts in a noninvasive longitudinal assay. Initially, measurements of tumor width, depth and length were obtained using on-screen UBM calipers in real time and tumor volume was calculated with the standard ellipsoid formula w d l pi/6. We were able to detect initiating minute tumor nodules, with the lower limit of detection at approximately 0.01 mm(3) in volume. Successive parallel cross-sectional UBM images (33 microm step) encompassing the complete length of these tumors were also obtained and reconstructed into 3-D representations. Subsequent segmentational volumetric analysis provided a measure of tumor volume. Volume measurements using the two techniques were highly correlated when all 33 xenografts were studied (r = 0.9813, p < 0.0001) and a lower degree of correlation was measured with a subset of early small tumors (r = 0.7973, n = 16, p = 0.0004). Further analysis demonstrated that 3-D segmentational volumetric analysis yielded volume estimates that were often smaller than the caliper-and-formula calculation for most early developing xenografts. Thus, 3-D UBM imaging and segmentation is expected to be especially valuable for small tumors that were observed to grow in irregular shapes other than ellipsoids.

Abnormal cardiac inflow patterns during postnatal development in a mouse model of Holt-Oram syndrome

Tbx5(del/+) mice provide a model of human Holt-Oram syndrome. In this study, the cardiac functional phenotypes of this mouse model were investigated with 30-MHz ultrasound by comparing 12 Tbx5(del/+) mice with 12 wild-type littermates at 1, 2, 4, and 8 wk of age. Cardiac dimensions were measured with two-dimensional and M-mode imaging. The flow patterns in the left and right ventricular inflow channels were evaluated with Doppler flow sampling. Compared with wild-type littermates, Tbx5(del/+) mice showed significant changes in the mitral flow pattern, including decreased peak velocity of the left ventricular (LV) early filling wave (E wave), increased peak velocity of the late filling wave (A wave), and decreased or even reversed peak E-to-A ratio. The prolongation of LV isovolumic relaxation time was detected in Tbx5(del/+) neonates as early as 1 wk of age. In Tbx5(del/+) mice, LV wall thickness appeared normal but LV chamber dimension was significantly reduced. LV systolic function did not differ from that in wild-type littermates. In contrast, the Doppler flow spectrum in the enlarged tricuspid orifice of Tbx5(del/+) mice demonstrated increased peak velocities of both E and A waves and increased total time-velocity integral but unchanged peak E/A. In another 13 mice (7 Tbx5(del/+), 6 wild-type) at 2 wk of age, significant correlation was found between Tbx5 gene expression level in ventricular myocardium and LV filling parameters. In conclusion, the LV diastolic function of Tbx5(del/+) mice is significantly deteriorated, whereas the systolic function remains normal.

Development of spontaneous mouth/tongue movement and related neural activity, and their repression in fetal mice lacking glutamate decarboxylase 67

Spontaneous body movement starts at early fetal stage, at embryonic day (E) 12-15 in mice. In the present study, the movement of the head region was studied in E13-14 mice by in utero ultrasound imaging, together with the in vitro recording of underlying neural activities in the hypoglossal nerve and the ventral root of the upper cervical cord of an isolated brainstem-spinal cord preparation. The role of gamma-aminobutyric acid (GABA) in the generation of fetal movement was assessed using mice lacking GABA-synthesizing glutamate decarboxylase 67 (GAD67). At E14, mouth opening and tongue withdrawal were observed independently at frequency of 14/h each. This movement was rarely observed in the GAD67-deficient mouse. The intraventricular administration of picrotoxin or 3-mercaptopropionic acid abolished mouth opening in the wild-type mice. In a brainstem-spinal cord preparation, three types of neural discharge were recorded: mouth/tongue-moving burst, respiratory burst and irregular activity on the basis of their waveform, regularity in occurrence and concomitant muscle activity. In the GAD67-deficient mice, the occurrence of mouth/tongue-moving burst and irregular activity was inhibited to about 15 and 40% of those in the wild-type mice, respectively. Respiratory burst was slightly inhibited but the difference was not significant. Picrotoxin greatly reduced the frequency of mouth/tongue-moving burst. These results indicate that GABA is involved in rhythm generation in movement of the head region and support the hypothesis that cleft palate in the GAD67-deficient mouse is due to the impairment of mouth or tongue movement that assists palate formation.

Comprehensive transthoracic cardiac imaging in mice using ultrasound biomicroscopy with anatomical confirmation by magnetic resonance imaging

High-frequency ultrasound biomicroscopy (UBM) has recently emerged as a high-resolution means of phenotyping genetically altered mice and has great potential to evaluate the cardiac morphology and hemodynamics of mouse mutants. However, there is no standard procedure of in vivo transthoracic cardiac imaging using UBM to comprehensively phenotype the adult mice. In this paper, the characteristic mouse thoracic anatomy is elucidated using magnetic resonance (MR) imaging on fixed mice. Besides the left parasternal and apical windows commonly used for transthoracic ultrasound cardiac imaging, a very useful right parasternal window is found. We present strategies for optimal visualization using UBM of key cardiac structures including: 1) the right atrial inflow channels such as the right superior vena cava; 2) the right ventricular inflow tract via the tricuspid orifice; 3) the right ventricular outflow tract to the main pulmonary artery; 4) the left atrial inflow channel, e.g., pulmonary vein; 5) the left ventricular inflow tract via the mitral orifice; 6) the left ventricular outflow tract to the ascending aorta; 7) the left coronary artery; and 8) the aortic arch and associated branches. Two-dimensional ultrasound images of these cardiac regions are correlated to similar sections in the three-dimensional MR data set to verify anatomical details of the in vivo UBM imaging. Dimensions of the left ventricle and ascending aorta are measured by M-mode. Flow velocities are recorded using Doppler at six representative intracardiac locations: right superior vena cava, tricuspid orifice, main pulmonary artery, pulmonary vein, mitral orifice, and ascending aorta. The methodologies and baseline measurements of inbred mice provide a useful guide for investigators applying the high-frequency ultrasound imaging to mouse cardiac phenotyping.

Developmental Control of Titin Isoform Expression and Passive Stiffness in Fetal and Neonatal Myocardium

Developmental changes in contractile behavior are known to occur during fetal and postnatal heart development. In this study, we examined whether adaptations take place in titin. A range of species was used to evaluate titin isoform expression and altered function during cardiac muscle development. A novel titin exon microarray that allows all 363 titin exons to be monitored simultaneously was used for transcript studies. Results reveal expression of fetal titin isoforms, characterized by additional spring elements both in the tandem Ig and PEVK region of the molecule. At the protein level, the fetal cardiac isoform predominates in fetal and neonatal myocardium and gradually disappears during postnatal development with a time course that varies in different species. Passive myocardium, contrary to previous reports, was found to be less stiff in the neonate than in the adult. This can be explained by the unique spring composition of fetal cardiac titin expressed by the neonate. Changes in titin expression are likely to impact functional transitions and diastolic filling behavior during development of the heart.

Developmental changes in ventricular diastolic function correlate with changes in ventricular myoarchitecture in normal mouse embryos

Both genetic and epigenetic factors, such as abnormal hemodynamics, affect cardiac morphogenesis and the pathogenesis of congenital heart disease. Diastolic function is an important determinant of cardiac function, and tools for evaluating diastolic function in the embryo would be very valuable for assessment of cardiac performance. Using histological measurements of ventricular myoarchitecture, Doppler assessment of ventricular inflow velocities, and direct measurement of ventricular pressure, we investigated developmental changes of ventricular diastolic function in the mouse embryos from embryonic days 9.5 to 19.5. Regression analysis showed that peak velocity of A wave (an index of passive compliance) correlated with the area of trabecular myocardium in right ventricle (RV) (r2=0.92, P<0.0001) and left ventricle (LV) (r2=0.93, P<0.0001). Peak velocity of E wave (an index of active relaxation) exponentially correlated with the area of compact myocardium in RV (r2=0.98, P<0.0001) and LV (r2=0.97, P<0.0001). We used these techniques to analyze FOG-2 null embryos. FOG-2 null embryos had thin compact myocardium, higher EDP and E/A ratio, smaller -dP/dt, and diminished sucking pressure than wild-type littermates, indicating that decreased ventricular diastolic function might be the primary cause of embryonic lethality. In conclusion, during embryogenesis the development of compact myocardium tightly regulates the development of ventricular distensibility. Our study in normal mice forms the basis for future studies of embryonic cardiac function in genetically manipulated mice with abnormalities of the cardiovascular system.