Increasing myocardial contraction and blood pressure in C57BL/6 mice during early postnatal development

Loss of Cardiac PFKFB2 Drives Metabolic, Functional, and Electrophysiological Remodeling in the Heart

BACKGROUND

Phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2) is a critical glycolytic regulator responsible for upregulation of glycolysis in response to insulin and adrenergic signaling. PFKFB2, the cardiac isoform of PFK-2, is degraded in the heart in the absence of insulin signaling, contributing to diabetes-induced cardiac metabolic inflexibility. However, previous studies have not examined how the loss of PFKFB2 affects global cardiac metabolism and function.

METHODS AND RESULTS

To address this, we have generated a mouse model with a cardiomyocyte-specific knockout of PFKFB2 (cKO). Using 9-month-old cKO and control mice, we characterized the impacts of PFKFB2 on cardiac metabolism, function, and electrophysiology. cKO mice have a shortened life span of 9 months. Metabolically, cKO mice are characterized by increased glycolytic enzyme abundance and pyruvate dehydrogenase activity, as well as decreased mitochondrial abundance and beta oxidation, suggesting a shift toward glucose metabolism. This was supported by a decrease in the ratio of palmitoyl carnitine to pyruvate-dependent mitochondrial respiration in cKO relative to control animals. Metabolomic, proteomic, and Western blot data support the activation of ancillary glucose metabolism, including pentose phosphate and hexosamine biosynthesis pathways. Physiologically, cKO animals exhibited impaired systolic function and left ventricular dilation, represented by reduced fractional shortening and increased left ventricular internal diameter, respectively. This was accompanied by electrophysiological alterations including increased QT interval and other metrics of delayed ventricular conduction.

CONCLUSIONS

Loss of PFKFB2 results in metabolic remodeling marked by cardiac ancillary pathway activation. This could delineate an underpinning of pathologic changes to mechanical and electrical function in the heart.

The genetics of cardiomyocyte polyploidy

The regulation of ploidy in cardiomyocytes is a complex and tightly regulated aspect of cardiac development and function. Cardiomyocyte ploidy can range from diploid (2N) to 8N or even 16N, and these states change during key stages of development and disease progression. Polyploidization has been associated with cellular hypertrophy to support normal growth of the heart, increased contractile capacity, and improved stress tolerance in the heart. Conversely, alterations to ploidy also occur during cardiac pathogenesis of diseases, such as ischemic and non-ischemic heart failure and arrhythmia. Therefore, understanding which genes control and modulate cardiomyocyte ploidy may provide mechanistic insight underlying cardiac growth, regeneration, and disease. This chapter summarizes the current knowledge regarding the genes involved in the regulation of cardiomyocyte ploidy. We discuss genes that have been directly tested for their role in cardiomyocyte polyploidization, as well as methodologies used to identify ploidy alterations. These genes encode cell cycle regulators, transcription factors, metabolic proteins, nuclear scaffolding, and components of the sarcomere, among others. The general physiological and pathological phenotypes in the heart associated with the genetic manipulations described, and how they coincide with the respective cardiomyocyte ploidy alterations, are further discussed in this chapter. In addition to being candidates for genetic-based therapies for various cardiac maladies, these genes and their functions provide insightful evidence regarding the purpose of widespread polyploidization in cardiomyocytes.

Loss of cardiac PFKFB2 drives Metabolic, Functional, and Electrophysiological Remodeling in the Heart

Background

Phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2) is a critical glycolytic regulator responsible for upregulation of glycolysis in response to insulin and adrenergic signaling. PFKFB2, the cardiac isoform of PFK-2, is degraded in the heart in the absence of insulin signaling, contributing to diabetes-induced cardiac metabolic inflexibility. However, previous studies have not examined how the loss of PFKFB2 affects global cardiac metabolism and function.

Methods

To address this, we have generated a mouse model with a cardiomyocyte-specific knockout of PFKFB2 (cKO). Using 9-month-old cKO and control (CON) mice, we characterized impacts of PFKFB2 on cardiac metabolism, function, and electrophysiology.

Results

cKO mice have a shortened lifespan of 9 months. Metabolically, cKO mice are characterized by increased glycolytic enzyme abundance and pyruvate dehydrogenase (PDH) activity, as well as decreased mitochondrial abundance and beta oxidation, suggesting a shift toward glucose metabolism. This was supported by a decrease in the ratio of palmitoyl carnitine to pyruvate-dependent mitochondrial respiration in cKO relative to CON animals. Metabolomic, proteomic, and western blot data support the activation of ancillary glucose metabolism, including pentose phosphate and hexosamine biosynthesis pathways. Physiologically, cKO animals exhibited impaired systolic function and left ventricular (LV) dilation, represented by reduced fractional shortening and increased LV internal diameter, respectively. This was accompanied by electrophysiological alterations including increased QT interval and other metrics of delayed ventricular conduction.

Conclusions

Loss of PFKFB2 results in metabolic remodeling marked by cardiac ancillary pathway activation. This could delineate an underpinning of pathologic changes to mechanical and electrical function in the heart.

Clinical Perspective

What is New?: We have generated a novel cardiomyocyte-specific knockout model of PFKFB2, the cardiac isoform of the primary glycolytic regulator Phosphofructokinase-2 (cKO).The cKO model demonstrates that loss of cardiac PFKFB2 drives metabolic reprogramming and shunting of glucose metabolites to ancillary metabolic pathways.The loss of cardiac PFKFB2 promotes electrophysiological and functional remodeling in the cKO heart.What are the Clinical Implications?: PFKFB2 is degraded in the absence of insulin signaling, making its loss particularly relevant to diabetes and the pathophysiology of diabetic cardiomyopathy.Changes which we observe in the cKO model are consistent with those often observed in diabetes and heart failure of other etiologies.Defining PFKFB2 loss as a driver of cardiac pathogenesis identifies it as a target for future investigation and potential therapeutic intervention.

Cardiomyocyte Ploidy, Metabolic Reprogramming and Heart Repair

The adult heart is made up of cardiomyocytes (CMs) that maintain pump function but are unable to divide and form new myocytes in response to myocardial injury. In contrast, the developmental cardiac tissue is made up of proliferative CMs that regenerate injured myocardium. In mammals, CMs during development are diploid and mononucleated. In response to cardiac maturation, CMs undergo polyploidization and binucleation associated with CM functional changes. The transition from mononucleation to binucleation coincides with unique metabolic changes and shift in energy generation. Recent studies provide evidence that metabolic reprogramming promotes CM cell cycle reentry and changes in ploidy and nucleation state in the heart that together enhances cardiac structure and function after injury. This review summarizes current literature regarding changes in CM ploidy and nucleation during development, maturation and in response to cardiac injury. Importantly, how metabolism affects CM fate transition between mononucleation and binucleation and its impact on cell cycle progression, proliferation and ability to regenerate the heart will be discussed.

Deletion of Notch3 Impairs Contractility of Renal Resistance Vessels Due to Deficient Ca2+ Entry

Notch3 plays an important role in the differentiation and development of vascular smooth muscle cells. Mice lacking Notch3 show deficient renal autoregulation. The aim of the study was to investigate the mechanisms involved in the Notch3-mediated control of renal vascular response. To this end, renal resistance vessels (afferent arterioles) were isolated from Notch3^-/- and wild-type littermates (WT) and stimulated with angiotensin II (ANG II). Contractions and intracellular Ca²⁺ concentrations were blunted in Notch3^-/- vessels. ANG II responses in precapillary muscle arterioles were similar between the WT and Notch3^-/- mice, suggesting a focal action of Notch3 in renal vasculature. Abolishing stored Ca²⁺ with thapsigargin reduced Ca²⁺ responses in the renal vessels of the two strains, signifying intact intracellular Ca²⁺ mobilization in Notch3^-/-. EGTA (Ca²⁺ chelating agent), nifedipine (L-type channel-blocker), or mibefradil (T-type channel-blocker) strongly reduced contraction and Ca²⁺ responses in WT mice but had no effect in Notch3^-/- mice, indicating defective Ca²⁺ entry. Notch3^-/- vessels responded normally to KCl-induced depolarization, which activates L-type channels directly. Differential transcriptomic analysis showed a major down-regulation of Cacna1h gene expression, coding for the α_1H subunit of the T-type Ca²⁺ channel, in Notch3^-/- vessels. In conclusion, renal resistance vessels from Notch3^-/- mice display altered vascular reactivity to ANG II due to deficient Ca²⁺-entry. Consequently, Notch3 is essential for proper excitation-contraction coupling and vascular-tone regulation in the kidney.

Sarcomere maturation: function acquisition, molecular mechanism, and interplay with other organelles

During postnatal cardiac development, cardiomyocytes mature and turn into adult ones. Hence, all cellular properties, including morphology, structure, physiology and metabolism, are changed. One of the most important aspects is the contractile apparatus, of which the minimum unit is known as a sarcomere. Sarcomere maturation is evident by enhanced sarcomere alignment, ultrastructural organization and myofibrillar isoform switching. Any maturation process failure may result in cardiomyopathy. Sarcomere function is intricately related to other organelles, and the growing evidence suggests reciprocal regulation of sarcomere and mitochondria on their maturation. Herein, we summarize the molecular mechanism that regulates sarcomere maturation and the interplay between sarcomere and other organelles in cardiomyocyte maturation.

This article is part of the theme issue ‘The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease’.

The focal adhesion protein β-parvin controls cardiomyocyte shape and sarcomere assembly in response to mechanical load

Physiological and pathological cardiac stress induced by exercise and hypertension, respectively, increase the hemodynamic load for the heart and trigger specific hypertrophic signals in cardiomyocytes leading to adaptive or maladaptive cardiac hypertrophy responses involving a mechanosensitive remodeling of the contractile cytoskeleton. Integrins sense load and have been implicated in cardiac hypertrophy, but how they discriminate between the two types of cardiac stress and translate mechanical loads into specific cytoskeletal signaling pathways is not clear. Here, we report that the focal adhesion protein β-parvin is highly expressed in cardiomyocytes and facilitates the formation of cell protrusions, the serial assembly of newly synthesized sarcomeres, and the hypertrophic growth of neonatal rat ventricular cardiomyocytes (NRVCs) in vitro. In addition, physiological mechanical loading of NRVCs by either the application of cyclic, uni-axial stretch, or culture on physiologically stiff substrates promotes NRVC elongation in a β-parvin-dependent manner, which is achieved by binding of β-parvin to α/β-PIX, which in turn activates Rac1. Importantly, loss-of-function studies in mice also revealed that β-parvin is essential for the exercise-induced cardiac hypertrophy response in vivo. Our results identify β-parvin as a novel mechano-responsive signaling hub in hypertrophic cardiomyocytes that drives cell elongation in response to physiological mechanical loads.

Deletion of Orphan G Protein-Coupled Receptor GPR37L1 in Mice Alters Cardiovascular Homeostasis in a Sex-Specific Manner

GPR37L1 is a family A orphan G protein-coupled receptor (GPCR) with a putative role in blood pressure regulation and cardioprotection. In mice, genetic ablation of Gpr37l1 causes sex-dependent effects; female mice lacking Gpr37l1 (GPR37L1^-/-) have a modest but significant elevation in blood pressure, while male GPR37L1^-/- mice are more susceptible to cardiovascular dysfunction following angiotensin II-induced hypertension. Given that this receptor is highly expressed in the brain, we hypothesize that the cardiovascular phenotype of GPR37L1^-/- mice is due to changes in autonomic regulation of blood pressure and heart rate. To investigate this, radiotelemetry was employed to characterize baseline cardiovascular variables in GPR37L1^-/- mice of both sexes compared to wildtype controls, followed by power spectral analysis to quantify short-term fluctuations in blood pressure and heart rate attributable to alterations in autonomic homeostatic mechanisms. Additionally, pharmacological ganglionic blockade was performed to determine vasomotor tone, and environmental stress tests were used to assess whether cardiovascular reactivity was altered in GPR37L1^-/- mice. We observed that mean arterial pressure was significantly lower in female GPR37L1^-/- mice compared to wildtype counterparts, but was unchanged in male GPR37L1^-/- mice. GPR37L1^-/- genotype had a statistically significant positive chronotropic effect on heart rate across both sexes when analyzed by two-way ANOVA. Power spectral analysis of these data revealed a reduction in power in the heart rate spectrum between 0.5 and 3 Hz in female GPR37L1^-/- mice during the diurnal active period, which indicates that GPR37L1^-/- mice may have impaired cardiac vagal drive. GPR37L1^-/- mice of both sexes also exhibited attenuated depressor responses to ganglionic blockade with pentolinium, indicating that GPR37L1 is involved in maintaining sympathetic vasomotor tone. Interestingly, when these mice were subjected to aversive and appetitive behavioral stressors, the female GPR37L1^-/- mice exhibited an attenuation of cardiovascular reactivity to aversive, but not appetitive, environmental stimuli. Together, these results suggest that loss of GPR37L1 affects autonomic maintenance of blood pressure, giving rise to sex-specific cardiovascular changes in GPR37L1^-/- mice.

Changes in Cardiomyocyte Cell Cycle and Hypertrophic Growth During Fetal to Adult in Mammals

The failure of adult cardiomyocytes to reproduce themselves to repair an injury results in the development of severe cardiac disability leading to death in many cases. The quest for an understanding of the inability of cardiac myocytes to repair an injury has been ongoing for decades with the identification of various factors which have a temporary effect on cell‐cycle activity. Fetal cardiac myocytes are continuously replicating until the time that the developing fetus reaches a stage of maturity sufficient for postnatal life around the time of birth. Recent reports of the ability for early neonatal mice and pigs to completely repair after the severe injury has stimulated further study of the regulators of the cardiomyocyte cell cycle to promote replication for the remuscularization of injured heart. In all mammals just before or after birth, single‐nucleated hyperplastically growing cardiomyocytes, 1X2N, undergo ≥1 additional DNA replications not followed by cytokinesis, resulting in cells with ≥2 nuclei or as in primates, multiple DNA replications (polyploidy) of 1 nucleus, 2X2(+)N or 1X4(+)N. All further growth of the heart is attributable to hypertrophy of cardiomyocytes. Animal studies ranging from zebrafish with 100% 1X2N cells in the adult to some strains of mice with up to 98% 2X2N cells in the adult and other species with variable ratios of 1X2N and 2X2N cells are reviewed relative to the time of conversion. Various structural, physiologic, metabolic, genetic, hormonal, oxygenation, and other factors that play a key role in the inability of post‐neonatal and adult myocytes to undergo additional cytokinesis are also reviewed.

Inflammation shapes pathogenesis of murine arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (AC) is an incurable genetic disease, whose pathogenesis is poorly understood. AC is characterized by arrhythmia, fibrosis, and cardiodilation that may lead to sudden cardiac death or heart failure. To elucidate AC pathogenesis and to design possible treatment strategies of AC, multiple murine models have been established. Among them, mice carrying desmoglein 2 mutations are particularly valuable given the identification of desmoglein 2 mutations in human AC and the detection of desmoglein 2 auto-antibodies in AC patients. Using two mouse strains producing either a mutant desmoglein 2 or lacking desmoglein 2 in cardiomyocytes, we test the hypothesis that inflammation is a major component of disease pathogenesis. We show that multifocal cardiomyocyte necrosis initiates a neutrophil-dominated inflammatory response, which also involves macrophages and T cells. Increased expression of Ccl2/Ccr2, Ccl3/Ccr5, and Cxcl5/Cxcr2 mRNA reflects the observed immune cell recruitment. During the ensuing acute disease phase, Mmp12⁺ and Spp1⁺ macrophages and T cells accumulate in scars, which mature from cell- to collagen-rich. The expression of Cx3cl1/Cx3cr1, Ccl2/Ccr2, and Cxcl10/Cxcr3 dominates this disease phase. We furthermore find that during chronic disease progression macrophages and T cells persist within mature scars and are present in expanding interstitial fibrosis. Ccl12 and Cx3cl1 are predominant chemokines in this disease phase. Together, our observations provide strong evidence that specific immune cell populations and chemokine expression profiles modulate inflammatory and repair processes throughout AC progression.

Cardiomyocyte Contractility and Autophagy in a Premature Senescence Model of Cardiac Aging

Globally, cardiovascular diseases are the leading cause of death in the aging population. While the clinical pathology of the aging heart is thoroughly characterized, underlying molecular mechanisms are still insufficiently clarified. The aim of the present study was to establish an in vitro model system of cardiomyocyte premature senescence, culturing heart muscle cells derived from neonatal C57Bl/6J mice for 21 days. Premature senescence of neonatal cardiac myocytes was induced by prolonged culture time in an oxygen-rich postnatal environment. Age-related changes in cellular function were determined by senescence-associated β-galactosidase activity, increasing presence of cell cycle regulators, such as p16, p53, and p21, accumulation of protein aggregates, and restricted proteolysis in terms of decreasing (macro-)autophagy. Furthermore, the culture system was functionally characterized for alterations in cell morphology and contractility. An increase in cellular size associated with induced expression of atrial natriuretic peptides demonstrated a stress-induced hypertrophic phenotype in neonatal cardiomyocytes. Using the recently developed analytical software tool Myocyter, we were able to show a spatiotemporal constraint in spontaneous contraction behavior during cultivation. Within the present study, the 21-day culture of neonatal cardiomyocytes was defined as a functional model system of premature cardiac senescence to study age-related changes in cardiomyocyte contractility and autophagy.

Polyploidy in Cardiomyocytes: Roadblock to Heart Regeneration?

The hallmark of most cardiac diseases is the progressive loss of cardiomyocytes. In the perinatal period, cardiomyocytes still proliferate, and the heart shows the capacity to regenerate upon injury. In the adult heart, however, the actual rate of cardiomyocyte renewal is too low to efficiently counteract substantial cell loss caused by cardiac injury. In mammals, cardiac growth by cell number expansion changes to growth by cardiomyocyte enlargement soon after birth, coinciding with a period in which most cardiomyocytes increase their DNA content by multinucleation and nuclear polyploidization. Although cardiomyocyte hypertrophy is often associated with these processes, whether polyploidy is a prerequisite or a consequence of hypertrophic growth is unclear. Both the benefits of cardiomyocyte enlargement over proliferative growth of the heart and the physiological role of polyploidy in cardiomyocytes are enigmatic. Interestingly, hearts in animal species with substantial cardiac regenerative capacity dominantly comprise diploid cardiomyocytes, raising the hypothesis that cardiomyocyte polyploidy poses a barrier for cardiomyocyte proliferation and subsequent heart regeneration. On the contrary, there is also evidence for self-duplication of multinucleated myocytes, suggesting a more complex picture of polyploidy in heart regeneration. Polyploidy is not restricted to the heart but also occurs in other cell types in the body. In this review, we explore the biological relevance of polyploidy in different species and tissues to acquire insight into its specific role in cardiomyocytes. Furthermore, we speculate about the physiological role of polyploidy in cardiomyocytes and how this might relate to renewal and regeneration.

Molecular mechanisms of heart regeneration

The adult mammalian heart is incapable of clinically relevant regeneration. The regenerative deficit in adult mammalian heart contrasts with the fetal and neonatal heart, which demonstrate substantial regenerative capacity after injury. This deficiency in adult mammals is attributable to the lack of resident stem cells after birth, combined with an inability of pre-existing cardiomyocytes to complete cytokinesis. Studies of neonatal heart regeneration in mammals suggest that latent regenerative potential can be re-activated. Dissecting the cellular and molecular mechanisms that promote cardiomyocyte proliferation is key to stimulating true regeneration in adult humans. Here, we review recent advances in our understanding of cardiomyocyte proliferation that suggest molecular approaches to heart regeneration.

High calories but not fat content of lard-based diet contribute to impaired mitochondrial oxidative phosphorylation in C57BL/6J mice heart

Purpose

High calorie intake leads to obesity, a global socio-economic and health problem, reaching epidemic proportion in children and adolescents. Saturated and monounsaturated fatty acids from animal (lard) fat are major components of the western-pattern diet and its regular consumption leads to obesity, a risk factor for cardiovascular disease. However, no clear evidence exists whether consumption of diet rich in saturated (SFAs) and monounsaturated (MUFAs) fatty acids has detrimental effects on cardiac structure and energetics primarily due to excessive calories. We, therefore, sought to determine the impact of high calories versus fat content in diet on cardiac structure and mitochondrial energetics.

Methods

Six-week-old C57BL/6J mice were fed with high calorie, high lard fat-based diet (60% fat, HFD), high-calorie and low lard fat-based diet (10% fat, LFD), and lower-calorie and fat diet (standard chow, 12% fat, SCD) for 10 weeks.

Results

The HFD- and LFD-fed mice had higher body weight, ventricular mass and thickness of posterior and septal wall with increased cardiomyocytes diameter compared to the SCD-fed mice. These changes were associated with a reduction in the mitochondrial oxidative phosphorylation (OXPHOS) complexes I and III activity compared to the SCD-fed mice without significant differences between the HFD- and LFD-fed animals. The HFD-fed animals had higher level of malondialdehyde (MDA) than LFD and SCD-fed mice.

Conclusions

We assume that changes in cardiac morphology and selective reduction of the OXPHOS complexes activity observed in the HFD- and LFD-fed mice might be related to excessive calories with additional effect of fat content on oxidative stress.

Genetic and epigenetic regulation of cardiomyocytes in development, regeneration and disease

Embryonic and postnatal life depend on the uninterrupted function of cardiac muscle cells. These cells, termed cardiomyocytes, display many fascinating behaviors, including complex morphogenic movements, interactions with other cell types of the heart, persistent contractility and quiescence after birth. Each of these behaviors depends on complex interactions between both cardiac-restricted and widely expressed transcription factors, as well as on epigenetic modifications. Here, we review recent advances in our understanding of the genetic and epigenetic control of cardiomyocyte differentiation and proliferation during heart development, regeneration and disease. We focus on those regulators that are required for both heart development and disease, and highlight the regenerative principles that might be manipulated to restore function to the injured adult heart.

Antiepileptic Drugs Impair Shortening of Isolated Cardiomyocytes

Background

Most antiepileptic drugs (AEDs) inhibit seizure generation by acting on voltage-dependent ion channels. Voltage-dependent sodium and calcium channels are commonly expressed in brain and heart, suggesting that AEDs may have considerable cardiodepressive effects, thereby facilitating sudden cardiac death as a potential cause of sudden unexpected death in epilepsy. Here, we investigated the effects of carbamazepine (CBZ), lamotrigine (LTG), and levetiracetam (LEV) alone and in combination on the shortening properties of isolated ventricular cardiomyocytes of wild-type mice.

Methods

Properties of murine cardiomyocytes were determined by recording the sarcomere shortening with a video imaging system before, during, and after administration of AEDs in different concentrations and combinations. We assessed (i) the number of successful shortenings during continuous electrical stimulation (electromechanical coupling) and (ii) the shortening amplitude as well as other shortening-related properties upon repetitive electrical stimulation at 4 Hz. Data are given as mean ± SEM.

Results

At 100 μM, CBZ (10 cells), LTG (11 cells), and LEV (11 cells) alone had no effect on the electromechanical coupling but reversibly reduced shortening amplitudes by 15 ± 4, 24 ± 3, and 11 ± 3%, respectively. Increasing the LTG concentration to 250 (21 cells) and 500 μM (4 cells) reversibly inhibited the electromechanical coupling in 62 and 100% of the experiments. Importantly, simultaneous application of CBZ, LTG, and LEV at 100 μM also impaired the electromechanical coupling in 8 of 19 cardiomyocytes (42%) and reduced the shortening amplitude by 21 ± 4%.

Conclusion

Our data show that AEDs reversibly impair cardiac excitation and contraction. Importantly, the blocking effect on electromechanical coupling appears to be additive when different AEDs are simultaneously applied. The translational value of these experimental findings into clinical practice is limited. Our results, however, suggest that rationale AED therapy may be important with respect to cardiac side effects and potential facilitation of serious cardiac dysfunction especially when AEDs are used in combination or at very high doses.

Triptolide attenuates pressure overload-induced myocardial remodeling in mice via the inhibition of NLRP3 inflammasome expression

Triptolide is the predominant active component of the Chinese herb Tripterygium wilfordii Hook F (TwHF) that has been widely used to treat several chronic inflammatory diseases due to its immunosuppressive, anti-inflammatory, and anti-proliferative properties. In the present study, we elucidated the cardioprotective effects of triptolide against cardiac dysfunction and myocardial remodeling in chronic pressure-overloaded hearts. Furthermore, the potential mechanisms of triptolide were investigated. For this purpose, C57/BL6 mice were anesthetized and subjected to transverse aortic constriction (TAC) or sham operation. Six weeks after the operation, all mice were randomly divided into 4 groups: sham-operated with vehicle group, TAC with vehicle group, and TAC with triptolide (20 or 100 μg/kg/day intraperitoneal injection) groups. Our data showed that the levels of NLRP3 inflammasome were significantly increased in the TAC group and were associated with increased inflammatory mediators and profibrotic factor production, resulting in myocardial fibrosis, cardiomyocyte hypertrophy, and impaired cardiac function. Triptolide treatment attenuated TAC-induced myocardial remodeling, improved cardiac diastolic and systolic function, inhibited the NLRP3 inflammasome and downstream inflammatory mediators (IL-1β, IL-18, MCP-1, VCAM-1), activated the profibrotic TGF-β1 pathway, and suppressed macrophage infiltration in a dose-dependent manner. Our study demonstrated that the protective effect of triptolide against pressure overload in the heart may act by inhibiting the NLRP3 inflammasome-induced inflammatory response and activating the profibrotic pathway.

Engineered Context-Sensitive Agonism: Tissue-Selective Drug Signaling through a G Protein-Coupled Receptor

Drug discovery strives for selective ligands to achieve targeted modulation of tissue function. Here we introduce engineered context-sensitive agonism as a postreceptor mechanism for tissue-selective drug action through a G protein-coupled receptor. Acetylcholine M₂-receptor activation is known to mediate, among other actions, potentially dangerous slowing of the heart rate. This unwanted side effect is one of the main reasons that limit clinical application of muscarinic agonists. Herein we show that dualsteric (orthosteric/allosteric) agonists induce less cardiac depression ex vivo and in vivo than conventional full agonists. Exploration of the underlying mechanism in living cells employing cellular dynamic mass redistribution identified context-sensitive agonism of these dualsteric agonists. They translate elevation of intracellular cAMP into a switch from full to partial agonism. Designed context-sensitive agonism opens an avenue toward postreceptor pharmacologic selectivity, which even works in target tissues operated by the same subtype of pharmacologic receptor.

Tyrosine hydroxylase haploinsufficiency prevents age-associated arterial pressure elevation and increases half-life in mice

Catecholamines are essential for the maintenance of physiological homeostasis under basal and stress conditions. We aim to determine the impact of deletion of a single allele of the tyrosine hydroxylase (Th) gene might have on aging arterial pressure and life-span. We found that Th haploinsufficiency prevents age-associated increase of arterial pressure (AP) in mature adult mice, and it results in the extension of the half-life of Th-heterozygous (TH-HET) mice respect to their wild-type (WT) littermates. Heart performance was similar in both genotypes. To further investigate the lack of increase in AP with age in TH-HET mice, we measured the AP response to intra-peritoneal administration of substances involved in AP regulation. The response to acetylcholine and the basal sympathetic tone were similar in both genotypes, while norepinephrine had a greater pressor effect in TH-HET mice, which correlated with altered adrenoreceptor expression in blood vessels and the heart. Furthermore, sympatho-adrenomedular response to stress was attenuated in TH-HET mice. Plasma catecholamine levels and urine glucose increased markedly in WT but not in TH-HET mice after stress. Our results showed that TH-HET mice are resistant to age-associated hypertension, present a reduction in the sympathetic response to stress and display an extended half-life.

The toxic effect of R350P mutant desmin in striated muscle of man and mouse

Mutations of the human desmin gene on chromosome 2q35 cause autosomal dominant, autosomal recessive and sporadic forms of protein aggregation myopathies and cardiomyopathies. We generated R349P desmin knock-in mice, which harbor the ortholog of the most frequently occurring human desmin missense mutation R350P. These mice develop age-dependent desmin-positive protein aggregation pathology, skeletal muscle weakness, dilated cardiomyopathy, as well as cardiac arrhythmias and conduction defects. For the first time, we report the expression level and subcellular distribution of mutant versus wild-type desmin in our mouse model as well as in skeletal muscle specimens derived from human R350P desminopathies. Furthermore, we demonstrate that the missense-mutant desmin inflicts changes of the subcellular localization and turnover of desmin itself and of direct desmin-binding partners. Our findings unveil a novel principle of pathogenesis, in which not the presence of protein aggregates, but disruption of the extrasarcomeric intermediate filament network leads to increased mechanical vulnerability of muscle fibers. These structural defects elicited at the myofiber level finally impact the entire organ and subsequently cause myopathy and cardiomyopathy.

Targeted ablation of nesprin 1 and nesprin 2 from murine myocardium results in cardiomyopathy, altered nuclear morphology and inhibition of the biomechanical gene response

Recent interest has focused on the importance of the nucleus and associated nucleoskeleton in regulating changes in cardiac gene expression in response to biomechanical load. Mutations in genes encoding proteins of the inner nuclear membrane and nucleoskeleton, which cause cardiomyopathy, also disrupt expression of a biomechanically responsive gene program. Furthermore, mutations in the outer nuclear membrane protein Nesprin 1 and 2 have been implicated in cardiomyopathy. Here, we identify for the first time a role for the outer nuclear membrane proteins, Nesprin 1 and Nesprin 2, in regulating gene expression in response to biomechanical load. Ablation of both Nesprin 1 and 2 in cardiomyocytes, but neither alone, resulted in early onset cardiomyopathy. Mutant cardiomyocytes exhibited altered nuclear positioning, shape, and chromatin positioning. Loss of Nesprin 1 or 2, or both, led to impairment of gene expression changes in response to biomechanical stimuli. These data suggest a model whereby biomechanical signals are communicated from proteins of the outer nuclear membrane, to the inner nuclear membrane and nucleoskeleton, to result in changes in gene expression required for adaptation of the cardiomyocyte to changes in biomechanical load, and give insights into etiologies underlying cardiomyopathy consequent to mutations in Nesprin 1 and 2.

TLR2 stimulation induces cardiac inflammation but not cardiac depression in vivo

BACKGROUND

Bacteria such as Staphylococcus aureus induce myocardial dysfunction in vivo. To rectify conflicting evidence about the role of TLR2 signaling and cardiac dysfunction, we hypothesized that the specific TLR2 agonist purified lipoteichoic acid (LTA) from S. aureus contributes to cardiac dysfunction in vitro and in vivo.

METHODS

Wildtype (WT-) and TLR2-deficient (TLR2-D) mice were challenged with LTA and in comparison with equivalent doses of lipopolysaccharide (LPS) and CpG-oligodeoxynucleotide (CpG-ODN). TLR2-expression, NFκB as well as cytokine response were determined. Sarcomere shortening of isolated cardiomyocytes was analyzed in vitro and cardiac function in vivo after stimulation with LTA.

RESULTS

LTA induced up-regulation of TLR2 mRNA, activation of NFκB and cytokine expression within 2-6 h in WT-, but not in TLR2-D hearts. Cytokines were also elevated in the serum. LPS and CpG-ODN induced a more severe cardiac inflammation. In vitro incubation of cardiomyocytes with LTA reduced sarcomere shortening via NO at stimulation frequencies ≤ 8 Hz only in WT cells. However, hemodynamic parameters in vivo were not affected by LTA challenge.

CONCLUSIONS

LTA induced cardiac inflammation was relatively weak and sarcomere shortening was reduced only below physiological heart rates. This may explain the apparent contradiction between the in vivo and in vitro LTA effects.

Cardiac assessment in pediatric mice: strain analysis as a diagnostic measurement

Echocardiography is a robust tool for assessing cardiac function in both humans and laboratory animals. Conventional echocardiographic measurements, including chamber dimensions, wall thickness, and ejection fraction are routinely obtained to assess cardiac function in mice. Recently, myocardial strain and strain rate measurements have been added to functional assessments to provide additional details on regional abnormalities that are not evident using conventional measurements. To date, all studies of strain and strain rate in mice or rats have involved adult animals. This study serves to outline methods for acquiring echocardiographic images in pediatric mice and to provide myocardial strain and strain rate values for healthy C57BL/6J mice between 3 and 11 weeks old. Between weeks 3 and 11, left ventricular radial strain ranged from 32 to 43% and longitudinal strain ranged from -15 to -19%, with analysis over time showing no significant changes with aging (radial strain, P = 0.192 and longitudinal strain, P = 0.264; n = 4 for each time point evaluated). In conclusion, myocardial strain analysis in pediatric mice is technically feasible and has potential application in studying the pathophysiology of pediatric cardiovascular disease.

Toll-like receptor 9 promotes cardiac inflammation and heart failure during polymicrobial sepsis

Background. Aim was to elucidate the role of toll-like receptor 9 (TLR9) in cardiac inflammation and septic heart failure in a murine model of polymicrobial sepsis. Methods. Sepsis was induced via colon ascendens stent peritonitis (CASP) in C57BL/6 wild-type (WT) and TLR9-deficient (TLR9-D) mice. Bacterial load in the peritoneal cavity and cardiac expression of inflammatory mediators were determined at 6, 12, 18, 24, and 36 h. Eighteen hours after CASP cardiac function was monitored in vivo. Sarcomere length of isolated cardiomyocytes was measured at 0.5 to 10 Hz after incubation with heat-inactivated bacteria. Results. CASP led to continuous release of bacteria into the peritoneal cavity, an increase of cytokines, and differential regulation of receptors of innate immunity in the heart. Eighteen hours after CASP WT mice developed septic heart failure characterised by reduction of end-systolic pressure, stroke volume, cardiac output, and parameters of contractility. This coincided with reduced cardiomyocyte sarcomere shortening. TLR9 deficiency resulted in significant reduction of cardiac inflammation and a sustained heart function. This was consistent with reduced mortality in TLR9-D compared to WT mice. Conclusions. In polymicrobial sepsis TLR9 signalling is pivotal to cardiac inflammation and septic heart failure.

Transthoracic echocardiography reference values in juvenile and adult 129/Sv mice

Background

In the recent years, the use of Doppler-echocardiography has become a standard non-invasive technique in the analysis of cardiac malformations in genetically modified mice. Therefore, normal values have to be established for the most commonly used inbred strains in whose genetic background those mutations are generated. Here we provide reference values for transthoracic echocardiography measurements in juvenile (3 weeks) and adult (8 weeks) 129/Sv mice.

Methods

Echocardiographic measurements were performed using B-mode, M-mode and Doppler-mode in 15 juvenile (3 weeks) and 15 adult (8 weeks) mice, during isoflurane anesthesia. M-mode measurements variability of left ventricle (LV) was determined.

Results

Several echocardiographic measurements significantly differ between juvenile and adult mice. Most of these measurements are related with cardiac dimensions. All B-mode measurements were different between juveniles and adults (higher in the adults), except for fractional area change (FAC). Ejection fraction (EF) and fractional shortening (FS), calculated from M-mode parameters, do not differ between juvenile and adult mice. Stroke volume (SV) and cardiac output (CO) were significantly different between juvenile and adult mice. SV was 31.93 ± 8.67 μl in juveniles vs 70.61 ± 24.66 μl in adults, ρ < 0.001. CO was 12.06 ± 4.05 ml/min in juveniles vs 29.71 ± 10.13 ml/min in adults, ρ < 0.001. No difference was found in mitral valve (MV) and tricuspid valve (TV) related parameters between juvenile and adult mice. It was demonstrated that variability of M-mode measurements of LV is minimal.

Conclusions

This study suggests that differences in cardiac dimensions, as wells as in pulmonary and aorta outflow parameters, were found between juvenile and adult mice. However, mitral and tricuspid inflow parameters seem to be similar between 3 weeks and 8 weeks mice. The reference values established in this study would contribute as a basis to future studies in post-natal cardiovascular development and diagnosing cardiovascular disorders in genetically modified mouse mutant lines.

Dysfunction in elastic fiber formation in fibulin-5 null mice abrogates the evolution in mechanical response of carotid arteries during maturation

Elastin fragmentation is a common characteristic of vascular diseases, such as abdominal aortic aneurysms, peripheral arterial disease, and aortic dissection. Examining growth and remodeling in the presence of dysfunctional elastic fibers provides insight into the adaptive or maladaptive changes that tissues undergo in compensating for structural deficiencies. This study used the maturation of fibulin-5 knockout (KO) and wild-type mice to study the effects of fragmented elastic fibers on the growth and remodeling of carotid arteries. The microstructural content and organization and the biaxial mechanical behavior of common carotid arteries were measured, and parameter estimation performed from KO and WT mice aged 3, 4, 8, and 13 wk. Gross measurements and biaxial tests revealed significant differences in pressure-diameter behavior, in vivo axial stretch, opening angle, compliance, and wall stresses during maturation of wild-type arteries, but little change in these values in KO mice. Multiphoton microscopy used to image collagen fibers across the vessel wall in pressurized and stretched arteries suggests that there is little variation in fiber angles between different ages. Parameter estimation revealed significant differences in material parameters between genotypes and age groups. This study suggests that neonatal formation and cross-linking of functional elastic fibers, followed by increases in artery size due to growth with little remodeling of the elastic fibers, endow arteries with large distensibility and contribute to the evolution of mechanical behavior of arteries during maturation. Dysfunction in neonatal formation of elastic fibers abrogates many of the changes in mechanical response that take place during the maturation.

Multi-scale biomechanical remodeling in aging and genetic mutant murine mitral valve leaflets: insights into Marfan syndrome

Mitral valve degeneration is a key component of the pathophysiology of Marfan syndrome. The biomechanical consequences of aging and genetic mutation in mitral valves are poorly understood because of limited tools to study this in mouse models. Our aim was to determine the global biomechanical and local cell-matrix deformation relationships in the aging and Marfan related Fbn1 mutated murine mitral valve. To conduct this investigation, a novel stretching apparatus and gripping method was implemented to directly quantify both global tissue biomechanics and local cellular deformation and matrix fiber realignment in murine mitral valves. Excised mitral valve leaflets from wild-type and Fbn1 mutant mice from 2 weeks to 10 months in age were tested in circumferential orientation under continuous laser optical imaging. Mouse mitral valves stiffen with age, correlating with increases in collagen fraction and matrix fiber alignment. Fbn1 mutation resulted in significantly more compliant valves (modulus 1.34 ± 0.12 vs. 2.51 ± 0.31 MPa, respectively, P<.01) at 4 months, corresponding with an increase in proportion of GAGs and decrease in elastin fraction. Local cellular deformation and fiber alignment change linearly with global tissue stretch, and these slopes become more extreme with aging. In comparison, Fbn1 mutated valves have decoupled cellular deformation and fiber alignment with tissue stretch. Taken together, quantitative understanding of multi-scale murine planar tissue biomechanics is essential for establishing consequences of aging and genetic mutations. Decoupling of local cell-matrix deformation kinematics with global tissue stretch may be an important mechanism of normal and pathological biomechanical remodeling in valves.

An improved isolation procedure for adult mouse cardiomyocytes

Isolated adult mouse cardiomyocytes are an important tool in cardiovascular research, but are challenging to prepare. Because the energy supply determines cell function and viability, we compared total creatine ([Cr]) and [ATP] in isolated cardiomyocytes with the intact mouse heart. Isolated myocytes suffered severe losses of Cr (-70%) and ATP (-53%). Myocytes were not able to replete [Cr] during a 5 h incubation period in medium supplemented with 1 mM Cr. In contrast, adding 20 mM Cr to the digestion buffers was sufficient to maintain normal [Cr]. Supplementing buffers with 5 mM of inosine (Ino) and adenosine (Ado) to prevent loss of cellular nucleosides partially protected against loss of ATP. To test whether maintaining [ATP] and [Cr] improves contractile function, myocytes were challenged by varying pacing rate from 0.5 to 10 Hz and by adding isoproterenol (Iso) at 5 and 10 Hz. All groups performed well up to 5 Hz, showing a positive cell shortening-frequency relationship; however, only 16% of myocytes isolated under standard conditions were able to sustain pacing with Iso challenge at 10 Hz. In contrast, 30-50% of the myocytes with normal Cr levels were able to contract and maintain low diastolic [Ca(2+)]. Cell yield also improved in Cr and the Cr/Ino/Ado-treated groups (85-90% vs. 70-75% rod shaped in untreated myocytes). These data suggest that viability and performance of isolated myocytes are improved when they are protected from the severe loss of Cr and ATP during the isolation, making them an even better research tool.

Transforming growth factor β₁ oppositely regulates the hypertrophic and contractile response to β-adrenergic stimulation in the heart

BACKGROUND

Neuroendocrine activation and local mediators such as transforming growth factor-β₁ (TGF-β₁) contribute to the pathobiology of cardiac hypertrophy and failure, but the underlying mechanisms are incompletely understood. We aimed to characterize the functional network involving TGF-β₁, the renin-angiotensin system, and the β-adrenergic system in the heart.

METHODS

Transgenic mice overexpressing TGF-β₁ (TGF-β₁-Tg) were treated with a β-blocker, an AT₁-receptor antagonist, or a TGF-β-antagonist (sTGFβR-Fc), were morphologically characterized. Contractile function was assessed by dobutamine stress echocardiography in vivo and isolated myocytes in vitro. Functional alterations were related to regulators of cardiac energy metabolism.

RESULTS

Compared to wild-type controls, TGF-β₁-Tg mice displayed an increased heart-to-body-weight ratio involving both fibrosis and myocyte hypertrophy. TGF-β₁ overexpression increased the hypertrophic responsiveness to β-adrenergic stimulation. In contrast, the inotropic response to β-adrenergic stimulation was diminished in TGF-β₁-Tg mice, albeit unchanged basal contractility. Treatment with sTGF-βR-Fc completely prevented the cardiac phenotype in transgenic mice. Chronic β-blocker treatment also prevented hypertrophy and ANF induction by isoprenaline, and restored the inotropic response to β-adrenergic stimulation without affecting TGF-β₁ levels, whereas AT₁-receptor blockade had no effect. The impaired contractile reserve in TGF-β₁-Tg mice was accompanied by an upregulation of mitochondrial uncoupling proteins (UCPs) which was reversed by β-adrenoceptor blockade. UCP-inhibition restored the contractile response to β-adrenoceptor stimulation in vitro and in vivo. Finally, cardiac TGF-β₁ and UCP expression were elevated in heart failure in humans, and UCP--but not TGF-β₁--was downregulated by β-blocker treatment.

CONCLUSIONS

Our data support the concept that TGF-β₁ acts downstream of angiotensin II in cardiomyocytes, and furthermore, highlight the critical role of the β-adrenergic system in TGF-β₁-induced cardiac phenotype. Our data indicate for the first time, that TGF-β₁ directly influences mitochondrial energy metabolism by regulating UCP3 expression. β-blockers may act beneficially by normalizing regulatory mechanisms of cellular hypertrophy and energy metabolism.

Cardiomyoplasty Improves Contractile Reserve after Myocardial Injury in Mice: Functional and Morphological Investigations with Reconstructive Three-Dimensional Echocardiography

Cellular cardiomyoplasty (CMP) is a novel therapeutic approach to myocardial injury (MI). Post-MI remodeling of the left ventricle (LV) comprises dilatation and impairment of systolic function and gives rise to progressive hemodynamic deterioration. We aimed to investigate: a) the impact of CMP on global and regional parameters of LV remodeling (LVR) as well as contractile reserve and b) the suitability and validity of different echocardiographic methods in this scenario. Murine ventricular cardiomyocytes (E13.5–E16.5) were transplanted into cryolesioned hearts of male HIM-OF1 mice. Echocardiography was performed at rest 4 and 14 days postoperatively. For quantification of akinetic myocardial mass and contractile reserve 2 weeks postoperatively additionally low-dose dobutamine stress echocardiography was conducted. Reconstructive 3D-echocardiography (r3D-echo) was compared to “plain” echocardiographic investigations and was compared to invasive measurements with conduction catheter. CMP significantly attenuated LV dilatation and reduced LV function decline on day 14, as obtained with all echocardiographic modalities and confirmed with conduction catheter measurements. In contrast to plain echocardiography and invasive testing, r3D-echo allowed noninvasive quantification of scar size and assessment of regional contractile reserve. Cell transplanted hearts demonstrated a significant decrease of akinetic myocardial mass (-CMP: 13 ± 2%; +CMP 7 ± 1%; p < 0.001) and increased regional contractile reserve, an indirect sign of myocardial viability. The present study demonstrates beneficial effects of CMP on global and regional parameters of LVR and contractile reserve after MI. In contrast to “simple” 2D echocardiography, r3D-echo allowed the assessment of regional contractile reserve and quantification of akinetic myocardial mass as additive functional and morphological measures of LVR.

A juvenile murine heart failure model of pressure overload

Persistent pressure overload can cause cardiac hypertrophy and progressive heart failure (HF). The authors developed a pressure-overload HF model of juvenile mice to study the cardiac response to pressure overload that may be applicable to clinical processes in children. Severe thoracic aortic banding (sTAB) was performed using a 28-gauge needle for 40 juvenile (age, 3 weeks) and 47 adult (age, 6 weeks) C57BL/6 male mice. To monitor the structural and functional changes, M-mode echocardiography was performed for conscious mice that had undergone sTAB and sham operation. Cardiac hypertrophy, dilation, and HF occurred in both juvenile and adult mice after sTAB. Compared with adults, juvenile HF is characterized by greater impairment of ventricular contractility and less hypertrophy. In addition, juvenile mice had significantly higher rates of survival than adult mice during the early postoperative weeks. Consistent with clinical HF seen in children, juvenile banded mice demonstrated a lower growth rate than either adult banded mice or juvenile control mice that had sham operations. The authors first developed a juvenile murine model of pressure-overload HF. Learning the unique characteristics of pressure-overload HF in juveniles should aid in understanding age-specific pathologic changes for HF development in children.

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.

Aging induces endothelial dysfunction while sparing arterial thrombosis

OBJECTIVE

To assess the effects of aging on arterial thrombus formation by comparing 2-year-old with 11-week-old C57Bl6 mice.

METHODS AND RESULTS

Aging is a major risk factor for cardiovascular disease. In humans, assessing the direct effects of aging on vascular homeostasis is difficult because it occurs in the presence of other risk factors. Arterial thrombosis is the critical event in cardiovascular diseases; however, it is not known whether aging per se promotes its occurrence. Mice represent an interesting system to address this issue because they age without spontaneously developing other risk factors. Organ chamber experiments confirmed the advanced level of aging of old mice. As previously shown, old mice exhibited endothelial dysfunction; however, arterial thrombosis induced by photochemical injury was unchanged. Arterial tissue factor expression and activity; expressions of tissue factor pathway inhibitor, thrombomodulin, and plasminogen activator inhibitor 1; prothrombin time; partial thromboplastin time; thrombin-antithrombin complex; and platelet activation were comparable in both groups.

CONCLUSIONS

Although these results cannot be directly extrapolated to humans, this study contributes novel important information on the direct effect of aging on arterial thrombosis and underscores the importance of controlling modifiable risk factors in aged individuals.

Feasibility of functional cardiac MR imaging in mice using a clinical 3 Tesla whole body scanner

OBJECTIVES

To test the feasibility of cardiac MR imaging in mice using a clinical 3 Tesla whole body MR system for structural and functional analysis. Standard protocols for bright blood cine imaging were adapted for murine dimensions. To validate measurements of functional parameters the MR data were compared with high-resolution echocardiographic measurements.

MATERIALS AND METHODS

Cardiac imaging was carried out in CD 1 wild-type mice (n = 8). MR imaging studies were performed using a clinical 3 Tesla MR system (Achieva, Philips). All mice received 2 MR scans and 1 echocardiographic evaluation. For optimal MR signal detection a dedicated solenoid receive-only coil was used. Electrocardiogram signal was recorded using a dedicated small animal electrocardiogram monitoring unit. For imaging we used a retrospectively triggered TFE sequence with a repetition time of 12 ms and an echo time of 4 ms. A dedicated software patch allowed for triggering of cardiac frequency of up to 600 BPM. Doppler-echocardiography was performed using a VisualSonics Vevo 770 high-resolution imaging system with a 30 MHz scanhead. Axial/lateral resolution was 40 of 100 microm and temporal resolution was 150 to 300 frames/s (B-mode) and 1000 frames/s (M-mode) depending on the setting.

RESULTS

MR imaging was successfully carried out in all mice with a sufficient temporal resolution and good signal-to-noise ratio and contrast-to-noise ratio levels allowing for identification of all relevant structures. Accordingly, there was a good scan-rescan reproducibility of MR measurements: Interassay coefficients of variance ranged from 4% for ejection fraction to 12% for endsystolic volume (ESV). Magnetic resonance imaging and echocardiography gave comparable results when using the same geometric model (Teichholz method): EDV: 60.2 +/- 6.1 microL/59.1 +/- 12.3 microL, ESV: 20.0 +/- 2.6 microL/20.7 +/- 7.7 microL, EF: 66.7% +/- 4.0%/65.2% +/- 9.9%, CO 19.5 +/- 3.6 mL/17.9 +/- 2.9 mL. Bland-Altman analysis gave acceptable limits of agreement between both methods: EDV (+28.2/-26.1), ESV (+16.3/-17.7), EF (+19.0/-16.1), CO (10.7/-7.5). When applying the Simpson's method MR volume estimates were significantly higher compared with echocardiography resulting in a lower estimate for the ejection fraction (60% +/- 3.9% vs. 66.7% +/- 4.0%).

CONCLUSIONS

Cardiac MR imaging of mice using a clinical 3 Tesla MR system for functional analysis is feasible with sufficient spatial and temporal resolution, good repeatability and reliable results when compared with high-resolution echocardiography.

Interaction between age and obesity on cardiomyocyte contractile function: role of leptin and stress signaling

Objectives

This study was designed to evaluate the interaction between aging and obesity on cardiac contractile and intracellular Ca²⁺ properties.

Methods

Cardiomyocytes from young (4-mo) and aging (12- and 18-mo) male lean and the leptin deficient ob/ob obese mice were treated with leptin (0.5, 1.0 and 50 nM) for 4 hrs in vitro. High fat diet (45% calorie from fat) and the leptin receptor mutant db/db obesity models at young and older age were used for comparison. Cardiomyocyte contractile and intracellular Ca²⁺ properties were evaluated including peak shortening (PS), maximal velocity of shortening/relengthening (± dL/dt), time-to-PS (TPS), time-to-90% relengthening (TR₉₀), intracellular Ca²⁺ levels and decay. O₂⁻ levels were measured by dihydroethidium fluorescence.

Results

Our results revealed reduced survival in ob/ob mice. Aging and obesity reduced PS, ± dL/dt, intracellular Ca²⁺ rise, prolonged TR₉₀ and intracellular Ca²⁺ decay, enhanced O₂⁻ production and p^47phox expression without an additive effect of the two, with the exception of intracellular Ca²⁺ rise. Western blot analysis exhibited reduced Ob-R expression and STAT-3 phosphorylation in both young and aging ob/ob mice, which was restored by leptin. Aging and obesity reduced phosphorylation of Akt, eNOS and p38 while promoting pJNK and pIκB. Low levels of leptin reconciled contractile, intracellular Ca²⁺ and cell signaling defects as well as O₂⁻ production and p^47phox upregulation in young but not aging ob/ob mice. High level of leptin (50 nM) compromised contractile and intracellular Ca²⁺ response as well as O₂⁻ production and stress signaling in all groups. High fat diet-induced and db/db obesity displayed somewhat comparable aging-induced mechanical but not leptin response.

Conclusions

Taken together, our data suggest that aging and obesity compromise cardiac contractile function possibly via phosphorylation of Akt, eNOS and stress signaling-associated O₂⁻ release.

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.

Complete loss of murine Xin results in a mild cardiac phenotype with altered distribution of intercalated discs

AIMS

Xin is a striated muscle-specific F-actin binding protein that has been implicated in cardiomyopathies. In cardiomyocytes, Xin is localized at intercalated discs (IDs). Mice lacking only two of the three Xin isoforms (XinAB(-/-) mice) develop severe cardiac hypertrophy. To further investigate the function of Xin variants in the mammalian heart, we generated XinABC(-/-) mice deficient in all Xin isoforms.

METHODS AND RESULTS

XinABC(-/-) mice showed a very mild phenotype: heart weight, heart weight to tibia length ratios, and cardiac dimensions were not altered. Increased perivascular fibrosis was only observed in hearts of young XinABC(-/-) mice. Striking differences were revealed in isolated cardiomyocytes: XinABC(-/-) cells demonstrated a significantly increased number of non-terminally localized ID-like structures. Furthermore, resting sarcomere length was increased, sarcomere shortening, peak shortening at 0.5-1 Hz, and the duration of shortening were decreased, and shortening and relengthening velocities were accelerated at frequencies above 4 Hz in XinABC(-/-) cardiomyocytes. ECG showed a significantly shorter HV interval and a trend towards shorter QRS interval in XinABC(-/-) mice, suggesting a faster conduction velocity of the ventricular-specific conduction system. In human cardiac tissue, expression of XinC protein was detected solely in samples from patients with cardiac hypertrophy.

CONCLUSION

Total Xin deficiency leads to topographical ID alterations, premature fibrosis and subtle changes in contractile behaviour; this is a milder cardiac phenotype than that observed in XinAB(-/-) mice, which still can express XinC. Together with the finding that XinC is detected solely in cardiomyopathic human tissues, this suggests that its expression is responsible for the stronger dominant phenotype in XinAB(-/-) mice. Furthermore, it indicates that XinC may be involved in the development of human cardiac hypertrophy.

Functional impact of targeted closed-chest transplantation of bone marrow cells in rats with acute myocardial ischemia/reperfusion injury

Intramyocardial transplantation of bone marrow-derived stem cells is a potential therapeutic option after myocardial infarction (MI). Intramyocardial administration is invasive but allows efficient and targeted stem cell delivery. Aims of this study were validation of minimal-invasive, echo-guided closed-chest cell transplantation (CTx) of mononuclear (MNC) or mesenchymal stem cells (MSC) and quantification of systolic left ventricular function and assessment of contractile reserve with high-resolution reconstructive 3D-echocardiography (r3D-echo) 3 weeks after CTx. Female Fischer344 rats received syngeneic male MNC, MSC, or medium after myocardial ischemia and reperfusion via echo-guided percutaneous injection (open-chest for control). Left ventricular systolic function was measured and dysfunctional myocardium was quantified with r3D-echo. For investigation of contractile reserve and myocardial viability r3D-echo was additionally conducted during low-dose dobutamine 3 weeks after CTx. Cell persistence after echo-guided CTx was quantified via real-time PCR; scar size was measured histologically. Echo-guided percutaneous CTx was feasible in all animals (n = 30) without periprocedural complications. After 3 weeks, 1.4 +/- 1.1% of transplanted MNC and 1.9 +/- 1.2% of MSC were detected. These numbers were comparable to those after open-chest intramyocardial injection of MNC (0.8 +/- 1.1%; n = 8, p = 0.3). In r3D-echo no functional benefit was associated with CTx after MI and reperfusion. All groups (MNC, MSC, and controls) revealed a significant decrease of dysfunctional myocardium and similar contractile reserve during inotropic stimulation.In conclusion, percutaneous echo-guided closed-chest CTx promises to be an effective and safe approach for CTx in small-animal research. However, intramyocardial CTx of MNC or MSC had no influence on systolic function and contractile reserve after reperfused MI.

Normal impulse propagation in the atrioventricular conduction system of Cx30.2/Cx40 double deficient mice

Connexin (Cx) 30.2, Cx40 and Cx45 containing gap junctional channels contribute to electrical impulse propagation through the mouse atrioventricular node (AV-node). The cross talk in between these Cxs may be of great importance for AV-nodal conduction. We generated Cx30.2/Cx40 double deficient mice (Cx30.2(LacZ/LacZ)Cx40(-/-)) and analyzed the relative impact of Cx30.2 and Cx40 on cardiac conductive properties in vivo by use of electrophysiological examination. Cx30.2(LacZ/LacZ)Cx40(-/-) mice exhibited neither obvious cardiac malformations nor impaired contractile function. In surface-ECG analyses, Cx30.2(LacZ/LacZ)Cx40(-/-) and Cx40 deficient animals (Cx40(-/-)) showed significantly longer P-wave durations, PQ-intervals and prolonged QRS-complexes relative to wildtype littermates (WT). Cx30.2-deficient mice (Cx30.2(LacZ/LacZ)) developed shorter PQ-intervals as compared to WT, Cx40(-/-) or Cx30.2/Cx40 double deficient mice. Intracardiac evaluation of the atria-His (AH) and His-ventricle (HV) intervals representing supra and infra-Hisian conduction yielded significant acceleration of supra-Hisian conductivity in Cx30.2(LacZ/LacZ) (AH: 28.2+/-4.3 ms) and prolongation of infra-Hisian conduction in Cx40(-/-) mice (HV: 13.7+/-2.6 ms). These parameters were unchanged in the Cx30.2(LacZ/LacZ)Cx40(-/-) mice (AH: 37.3+/-5.5 ms, HV: 11.7+/-2.6 ms), which exhibited AV-nodal and ventricular conduction times similar to WT animals (AH: 35.9+/-4.4 ms, HV: 10.5+/-1.9 ms). We conclude that the remaining Cx45 gap junctional channels are sufficient to maintain electrical coupling and cardiac impulse propagation in the AV-node and proximal ventricular conduction system in mice. We suggest that Cx30.2 and Cx40 act as counterparts in the AV-node and His-bundle, decreasing or increasing, respectively, electrical coupling and conduction velocity in these areas.

Echocardiographic assessment of global left ventricular function in mice

Doppler-echocardiographic assessment of cardiovascular structure and function in murine models has developed into one of the most commonly used non-invasive techniques during the last decades. Recent technical improvements even expanded the possibilities. In this review, we summarize the current options to assess global left ventricular (LV) function in mice using echocardiographic techniques. In detail, standard techniques as structural and functional assessment of the cardiovascular phenotype using one-dimensional M-mode echocardiography, two-dimensional B-mode echocardiography and spectral Doppler signals from mitral inflow respective aortal outflow are presented. Further pros and contras of recently implemented techniques as three-dimensional echocardiography and strain and strain rate measurements are discussed. Deduced measures of LV function as the myocardial performance index according to Tei, estimation of the mean velocity of circumferential fibre shortening, LV wall stress and different algorithms to estimate the LV mass are described in detail. Last but not least, specific features and limitations of murine echocardiography are presented. Future perspectives in respect to new examination techniques like targeted molecular imaging with advanced ultrasound contrast bubbles or improvement of equipment like new generation matrix transducers for murine echocardiography are discussed.

An overview of Notch3 function in vascular smooth muscle cells

Proteins of the Notch family are cell surface receptors that transduce signals between neighbouring cells. The Notch signalling pathway is highly evolutionarily conserved and critical for cell fate determination during embryonic development, including many aspects of vascular development. The interaction of Notch receptors with ligands leads to cleavage of the Notch intracellular domain (NICD) which then translocates to the nucleus and activates the transcription factor CBF1/JBP-Jkappa, regulating downstream gene expression. To date four Notch receptors have been found in mammals. Of these, Notch3 is predominantly expressed in adult arterial smooth muscle cells in human. NOTCH3 gene mutations cause the autosomal dominant condition, cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoecephelopathy (CADASIL), an inherited early stroke syndrome leading to dementia due to systemic vascular degeneration. This suggests that Notch3 plays a critical role in maintaining the phenotypic stability of vascular smooth muscle cells (VSMCs). Recent publications indicate that Notch3 is involved in vascular injury and is a determinant of VSMC survival, but its exact function is unknown. The molecular mechanisms underlying CADASIL pathology are therefore intriguing. Investigation of CADASIL mutant Notch3 shows that the majority of mutations do not change CBF1/JBP-Jkappa mediated classic Notch activation, so the pathological consequences of NOTCH3 mutations in CADASIL patients can not be simply explained by loss- or gain-of-function in the classic Notch signalling pathway. This suggests that a novel Notch3-mediated signalling pathway may be present in VSMCs, or cross-regulation of Notch3 to other signalling pathway(s) may play a critical role on VSMCs survival. Alternatively, the mutant Notch3 may gain a novel or toxic function in VSMCs. This review will focus on recent findings of Notch3 in vascular development and in regulating the VSMC behaviour and phenotype, and will use findings on investigating the molecular pathology of the single gene disorder CADASIL to understand the function of Notch3 in VSMCs.

Cardiovascular characterization of Pkd2+/LacZ mice, an animal model for the autosomal dominant polycystic kidney disease type 2 (ADPKD2)

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in PKD1 or PKD2. Patients with ADPKD have an increased incidence of cardiac valve abnormalities and left ventricular hypertrophy. Systematic analyses of cardiovascular involvement have so far been performed only on genetically unclassified patients or on ADPKD1 patients, but not on genetically defined ADPKD2 patients. Even existing Pkd1 or Pkd2 mouse models were not thoroughly analyzed in this respect. Therefore, the aim of this project was the noninvasive functional cardiovascular characterization of a mouse model for ADPKD2.

METHODS

Pkd2(+/LacZ) mice and wildtype controls were classified into 8 groups with respect to gender, age and genotype. In addition, two subgroups of female mice were analyzed for cardiac function before and during advanced pregnancy. Doppler-echocardiographic as well as histological studies were performed.

RESULTS

Doppler-echocardiography did not reveal significant cardiovascular changes. Heart rate and left ventricular (LV) length, LV mass, LV enddiastolic and LV endsystolic diameters did not differ significantly among the various groups when comparing wildtype and knockout mice. There were no significant differences except for a tendency towards higher maximal early and late flow velocities over the mitral valve in old wildtype mice.

CONCLUSIONS

Non-invasive phenotyping using ultrasound did not reveal significant cardiovascular difference between adult Pkd2(+/LacZ) and WT mice. Due to the lack of an obvious renal phenotype in heterozygous mice, it is likely that in conventional ADPKD knock out mouse models severe cardiac problems appear too late to be identified during the reduced lifespan of the animals.

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.

Replacement of connexin43 by connexin26 in transgenic mice leads to dysfunctional reproductive organs and slowed ventricular conduction in the heart

Background

In order to further distinguish unique from general functions of connexin43, we have generated mice in which the coding region of connexin43 was replaced by that of connexin26.

Results

Heterozygous mothers showed impaired mammary gland development responsible for decreased lactation and early postnatal death of the pups which could be partially rescued by wild type foster mothers. Only about 17% of the homozygous connexin43 knock-in connexin26 mice instead of 25% expected according to Mendelian inheritance, were born and only 6% survived to day 21 post partum and longer. Neonatal and adult connexin43 knock-in connexin26 mice exhibited slowed ventricular conduction in their hearts, i.e. similar but delayed electrophysiological abnormalities as connexin43 deficient mice. Furthermore, connexin43 knock-in connexin26 male and female mice were infertile and exhibited hypotrophic gonads. In testes, tubuli seminiferi were developed and spermatogonia as well as some primary spermatocytes were present, but further differentiated stages of spermatogenesis were absent. Ovaries of female connexin43 knock-in connexin26 mice revealed only few follicles and the maturation of follicles was completely impaired.

Conclusion

The impaired gametogenesis of homozygous males and females can explain their infertility.