Dual roles of myocardial mitochondrial AKT on diabetic cardiomyopathy and whole body metabolism

BACKGROUND

The PI3K/AKT pathway transduces the majority of the metabolic actions of insulin. In addition to cytosolic targets, insulin-stimulated phospho-AKT also translocates to mitochondria in the myocardium. Mouse models of diabetes exhibit impaired mitochondrial AKT signaling but the implications of this on cardiac structure and function is unknown. We hypothesized that loss of mitochondrial AKT signaling is a critical step in cardiomyopathy and reduces cardiac oxidative phosphorylation.

METHODS

To focus our investigation on the pathophysiological consequences of this mitochondrial signaling pathway, we generated transgenic mouse models of cardiac-specific, mitochondria-targeting, dominant negative AKT1 (CAMDAKT) and constitutively active AKT1 expression (CAMCAKT). Myocardial structure and function were examined using echocardiography, histology, and biochemical assays. We further investigated the underlying effects of mitochondrial AKT1 on mitochondrial structure and function, its interaction with ATP synthase, and explored in vivo metabolism beyond the heart.

RESULTS

Upon induction of dominant negative mitochondrial AKT1, CAMDAKT mice developed cardiac fibrosis accompanied by left ventricular hypertrophy and dysfunction. Cardiac mitochondrial oxidative phosphorylation efficiency and ATP content were reduced, mitochondrial cristae structure was lost, and ATP synthase structure was compromised. Conversely, CAMCAKT mice were protected against development of diabetic cardiomyopathy when challenged with a high calorie diet. Activation of mitochondrial AKT1 protected cardiac function and increased fatty acid uptake in myocardium. In addition, total energy expenditure was increased in CAMCAKT mice, accompanied by reduced adiposity and reduced development of fatty liver.

CONCLUSION

CAMDAKT mice modeled the effects of impaired mitochondrial signaling which occurs in the diabetic myocardium. Disruption of this pathway is a key step in the development of cardiomyopathy. Activation of mitochondrial AKT1 in CAMCAKT had a protective role against diabetic cardiomyopathy as well as improved metabolism beyond the heart.

The lipolysis inhibitor acipimox reverses the cardiac phenotype induced by electronic cigarettes

Electronic cigarettes (e-cigarettes) are a prevalent alternative to conventional nicotine cigarettes among smokers and people who have never smoked. Increased concentrations of serum free fatty acids (FFAs) are crucial in generating lipotoxicity. We studied the effects of acipimox, an antilipolytic drug, on e-cigarette-induced cardiac dysfunction. C57BL/6J wild-type mice on high fat diet were treated with saline, e-cigarette with 2.4% nicotine [e-cigarette (2.4%)], and e-cigarette (2.4%) plus acipimox for 12 weeks. Fractional shortening and ejection fraction were diminished in mice exposed to e-cigarettes (2.4%) compared with saline and acipimox-treated mice. Mice exposed to e-cigarette (2.4%) had increased circulating levels of inflammatory cytokines and FFAs, which were diminished by acipimox. Gene Set Enrichment Analysis revealed that e-cigarette (2.4%)-treated mice had gene expression changes in the G2/M DNA damage checkpoint pathway that was normalized by acipimox. Accordingly, we showed that acipimox suppressed the nuclear localization of phospho-p53 induced by e-cigarette (2.4%). Additionally, e-cigarette (2.4%) increased the apurinic/apyrimidinic sites, a marker of oxidative DNA damage which was normalized by acipimox. Mice exposed to e-cigarette (2.4%) had increased cardiac Heme oxygenase 1 protein levels and 4-hydroxynonenal (4-HNE). These markers of oxidative stress were decreased by acipimox. Therefore, inhibiting lipolysis with acipimox normalizes the physiological changes induced by e-cigarettes and the associated increase in inflammatory cytokines, oxidative stress, and DNA damage.

Electronic Cigarette Use and the Risk of Cardiovascular Diseases

Electronic cigarettes or e-cigarettes are the most frequently used tobacco product among adolescents. Despite the widespread use of e-cigarettes and the known detrimental cardiac consequences of nicotine, the effects of e-cigarettes on the cardiovascular system are not well-known. Several in vitro and in vivo studies delineating the mechanisms of the impact of e-cigarettes on the cardiovascular system have been published. These include mechanisms associated with nicotine or other components of the aerosol or thermal degradation products of e-cigarettes. The increased hyperlipidemia, sympathetic dominance, endothelial dysfunction, DNA damage, and macrophage activation are prominent effects of e-cigarettes. Additionally, oxidative stress and inflammation are unifying mechanisms at many levels of the cardiovascular impairment induced by e-cigarette exposure. This review outlines the contribution of e-cigarettes in the development of cardiovascular diseases and their molecular underpinnings.

Treatment of Wound Infections in a Mouse Model Using Zn2+-Releasing Phage Bound to Gold Nanorods

Infections caused by drug-resistant bacteria, particularly Gram-negative organisms, are increasingly difficult to treat using antibiotics. A potential alternative is "phage therapy", in which phages infect and lyse the bacterial host. However, phage therapy poses serious drawbacks and safety concerns, such as the risk of genetic transduction of antibiotic resistance genes, inconsistent pharmacokinetics, and unknown evolutionary potential. In contrast, metallic nanoparticles possess precise, tunable properties, including efficient conversion of electronic excitation into heat. In this work, we demonstrate that engineered phage-nanomaterial conjugates that target the Gram-negative pathogen Pseudomonas aeruginosa are highly effective as a treatment of infected wounds in mice. Photothermal heating, performed as a single treatment (15 min) or as two treatments on consecutive days, rapidly reduced the bacterial load and released Zn²⁺ to promote wound healing. The phage-nanomaterial treatment was significantly more effective than systemic standard-of-care antibiotics, with a >10× greater reduction in bacterial load and ∼3× faster healing as measured by wound size reduction when compared to fluoroquinolone treatment. Notably, the phage-nanomaterial was also effective against a P. aeruginosa strain resistant to polymyxins, a last-line antibiotic therapy. Unlike these antibiotics, the phage-nanomaterial showed no detectable toxicity or systemic effects in mice, consistent with the short duration and localized nature of phage-nanomaterial treatment. Our results demonstrate that phage therapy controlled by inorganic nanomaterials can be a safe and effective antimicrobial strategy in vivo.

Targeted Genetic Reduction of Mutant Huntingtin Lessens Cardiac Pathology in the BACHD Mouse Model of Huntington's Disease

Individuals affected by Huntington's disease (HD) present with progressive degeneration that results in a wide range of symptoms, including cardiovascular (CV) dysfunction. The huntingtin gene (HTT) and its product are ubiquitously expressed, hence, the cardiomyopathy could also be driven by defects caused by its mutated form (mHTT) in the cardiomyocytes themselves. In the present study, we sought to determine the contribution of the mHTT expressed in the cardiomyocytes to CV symptoms. We utilized the BACHD mouse model, which exhibits many of the HD core symptoms, including CV dysfunction. This model allows the targeted genetic reduction of mHTT expression in the cardiomyocytes while maintaining the expression of the mHTT in the rest of the body. The BACHD line was crossed with a line of mice in which the expression of Cre recombinase is driven by the cardiac-specific alpha myosin-heavy chain (Myh6) promoter. The offspring of this cross (BMYO mice) exhibited a dramatic reduction in mHTT in the heart but not in the striatum. The BMYO mice were evaluated at 6 months old, as at this age, the BACHD line displays a strong CV phenotype. Echocardiogram measurements found improvement in the ejection fraction in the BMYO line compared to the BACHD, while hypertrophy was observed in both mutant lines. Next, we examined the expression of genes known to be upregulated during pathological cardiac hypertrophy. As measured by qPCR, the BMYO hearts exhibited significantly less expression of collagen1a as well as Gata4, and brain natriuretic peptide compared to the BACHD. Fibrosis in the hearts assessed by Masson's trichrome stain and the protein levels of fibronectin were reduced in the BMYO hearts compared to BACHD. Finally, we examined the performance of the mice on CV-sensitive motor tasks. Both the overall activity levels and grip strength were improved in the BMYO mice. Therefore, we conclude that the reduction of mHtt expression in the heart benefits CV function in the BACHD model, and suggest that cardiomyopathy should be considered in the treatment strategies for HD.

A small molecule inhibitor of the chloride channel TMEM16A blocks vascular smooth muscle contraction and lowers blood pressure in spontaneously hypertensive rats

Hypertension is a major cause of cardiovascular morbidity and mortality, despite the availability of antihypertensive drugs with different targets and mechanisms of action. Here, we provide evidence that pharmacological inhibition of TMEM16A (ANO1), a calcium-activated chloride channel expressed in vascular smooth muscle cells, blocks calcium-activated chloride currents and contraction in vascular smooth muscle in vitro and decreases blood pressure in spontaneously hypertensive rats. The acylaminocycloalkylthiophene TMinh-23 fully inhibited calcium-activated TMEM16A chloride current with nanomolar potency in Fischer rat thyroid cells expressing TMEM16A, and in primary cultures of rat vascular smooth muscle cells. TMinh-23 reduced vasoconstriction caused by the thromboxane mimetic U46619 in mesenteric resistance arteries of wild-type and spontaneously hypertensive rats, with a greater inhibition in spontaneously hypertensive rats. Blood pressure measurements by tail-cuff and telemetry showed up to a 45-mmHg reduction in systolic blood pressure lasting for four-six hours in spontaneously hypertensive rats after a single dose of TMinh-23. A minimal effect on blood pressure was seen in wild-type rats or mice treated with TMinh-23. Five-day twice daily treatment of spontaneously hypertensive rats with TMinh-23 produced sustained reductions of 20-25 mmHg in daily mean systolic and diastolic blood pressure. TMinh-23 action was reversible, with blood pressure returning to baseline in spontaneously hypertensive rats by three days after treatment discontinuation. Thus, our studies provide validation for TMEM16A as a target for antihypertensive therapy and demonstrate the efficacy of TMinh-23 as an antihypertensive with a novel mechanism of action.

Electronic cigarettes cause alteration in cardiac structure and function in diet-induced obese mice

In spite of the widespread use of electronic cigarettes, also known as e-cigarettes, and the proposed adverse cardiac effects of nicotine, the detrimental effects of e-cigarettes on the heart are not well known. This study examines the detrimental effects of e-cigarettes with nicotine at doses that yield circulating nicotine and cotinine in the ranges similar to the levels found in habitual smokers, and a high fat diet (HFD) on cardiac structure and function in a commonly used model of diet-induced obesity (DIO). C57BL/6J mice on an HFD were exposed to e-cigarette in the presence (2.4% nicotine) or absence (0% nicotine) of nicotine and saline aerosol for 12 weeks. Echocardiographic data demonstrated a decrease in left ventricular (LV) fractional shortening, LV ejection fraction, and velocity of circumferential fiber shortening (VCF) in mice treated with e-cigarette (2.4% nicotine) compared to e-cigarette (0% nicotine) or saline exposed mice. Cardiomyocytes (CMs) of mice treated with e-cigarette (2.4% nicotine) exhibited LV abnormalities, including lipid accumulation (ventricular steatosis), myofibrillar derangement and destruction, and mitochondrial hypertrophy, as revealed by transmission electron microscopy. The detrimental effects of e-cigarettes (2.4% nicotine) on cardiac structure and function was accompanied by increased oxidative stress, plasma free fatty acid levels, CM apoptosis, and inactivation of AMP-activated protein kinase and activation of its downstream target, acetyl-CoA-carboxylase. Our results indicate profound adverse effects of e-cigarettes (2.4% nicotine) on the heart in obese mice and raise questions about the safety of the nicotine e-cigarettes use.

Abstract MP30: Small Molecule Inhibitor Of Chloride Channel Tmem16a Blocks Vascular Smooth Muscle Contraction And Lowers Blood Pressure In Spontaneously Hypertensive Rats

Hypertension is a major cause of cardiovascular morbidity and mortality, despite the availability of antihypertensive drugs with different targets and mechanisms of action. There is an unmet need for antihypertensive drugs with novel mechanisms of action to better control hypertension and reduce cardiovascular morbidity and mortality. Here, we provide evidence that pharmacological inhibition of TMEM16A (transmembrane member 16A or anoctamin-1), a Ca 2+ -activated Cl - channel expressed in vascular smooth muscle cells, reduces in vitro vascular smooth muscle contraction and decreases blood pressure in spontaneously hypertensive rats (SHR). We recently identified by high-throughput screening and subsequent medicinal chemistry, small molecule TMEM16A inhibitor TM inh -23 (2-bromodifluoroacetylamino-5,6,7,8-tetrahydro-4H-cyclohepta[b]thiophene-3-carboxylic acid o-tolylamide) that inhibits TMEM16A current fully, with IC 50 ~ 30 nM. TM inh -23 pretreatment blocked maximum in vitro vascular smooth muscle contractions induced by a thromboxane mimetic (U46619) in rat mesenteric arteries by 90%. Intraperitoneal (ip) administration of TM inh -23 to rodents at 10 mg/kg produced sustained serum concentrations of >10 μM for >4 hours. BP measurements by tail-cuff and telemetry showed a maximum ~45 mmHg reduction in SBP in spontaneously hypertensive rats (SHR) after a single dose (10 mg/kg, ip) TM inh -23 administration compared to vehicle treatment, with BP gradually returning to baseline values within 6-8 hours after TM inh -23 treatment. Minimal effect on BP (less than 10 mmHg decrease in SBP) was seen in wild-type rats and mice with TM inh -23 treatment (10 mg/kg, ip). Chronic 5-day treatment of SHR with TM inh -23 (10 mg/kg, ip, twice daily) caused sustained decreases (~20-25 mmHg) in daily average SBP, DBP and MAP during the treatment period. TM inh -23 action was reversible, with BP returning to baseline (~170/115 mmHg) by 3 days after discontinuation of treatment. These studies provide validation for TMEM16A as a target for antihypertensive therapy, and demonstrate the proof-of-concept for efficacy of TM inh -23 as an antihypertensive with a novel mechanism of action.

Chronic intermittent electronic cigarette exposure induces cardiac dysfunction and atherosclerosis in apolipoprotein-E knockout mice

Electronic cigarettes (e-cigarettes), also known as electronic nicotine delivery systems, are a popular alternative to conventional nicotine cigarettes, both among smokers and those who have never smoked. In spite of the widespread use of e-cigarettes and the proposed detrimental cardiac and atherosclerotic effects of nicotine, the effects of e-cigarettes on these systems are not known. In this study, we investigated the cardiovascular and cardiac effects of e-cigarettes with and without nicotine in apolipoprotein-E knockout (ApoE^-/-) mice. We developed an e-cigarette exposure model that delivers nicotine in a manner similar to that of human e-cigarettes users. Using commercially available e-cigarettes, bluCig PLUS, ApoE^-/- mice were exposed to saline, e-cigarette without nicotine [e-cigarette (0%)], and e-cigarette with 2.4% nicotine [e-cigarette (2.4%)] aerosol for 12 wk. Echocardiographic data show that mice treated with e-cigarette (2.4%) had decreased left ventricular fractional shortening and ejection fraction compared with e-cigarette (0%) and saline. Ventricular transcriptomic analysis revealed changes in genes associated with metabolism, circadian rhythm, and inflammation in e-cigarette (2.4%)-treated ApoE^-/- mice. Transmission electron microscopy revealed that cardiomyocytes of mice treated with e-cigarette (2.4%) exhibited ultrastructural abnormalities indicative of cardiomyopathy. Additionally, we observed increased oxidative stress and mitochondrial DNA mutations in mice treated with e-cigarette (2.4%). ApoE^-/- mice on e-cigarette (2.4%) had also increased atherosclerotic lesions compared with saline aerosol-treated mice. These results demonstrate adverse effects of e-cigarettes on cardiac function in mice.NEW & NOTEWORTHY The present study is the first to show that mice exposed to nicotine electronic cigarettes (e-cigarettes) have decreased cardiac fractional shortening and ejection fraction in comparison with controls. RNA-seq analysis reveals a proinflammatory phenotype induced by e-cigarettes with nicotine. We also found increased atherosclerosis in the aortic root of mice treated with e-cigarettes with nicotine. Our results show that e-cigarettes with nicotine lead to detrimental effects on the heart that should serve as a warning to e-cigarette users and agencies that regulate them.

Neurocardiovascular deficits in the Q175 mouse model of Huntington's disease

Cardiovascular dysautonomia as well as the deterioration of circadian rhythms are among the earliest detectable pathophysiological changes in individuals with Huntington's disease (HD). Preclinical research requires mouse models that recapitulate disease symptoms and the Q175 knock-in model offers a number of advantages but potential autonomic dysfunction has not been explored. In this study, we sought to test the dual hypotheses that cardiovascular dysautonomia can be detected early in disease progression in the Q175 model and that this dysfunction varies with the daily cycle. Using radiotelemetry implants, we observed a significant reduction in the diurnal and circadian activity rhythms in the Q175 mutants at the youngest ages. By middle age, the autonomically driven rhythms in core body temperature were highly compromised, and the Q175 mutants exhibited striking episodes of hypothermia that increased in frequency with mutant huntingtin gene dosage. In addition, Q175 mutants showed higher resting heart rate (HR) during sleep and greatly reduced correlation between activity and HR HR variability was reduced in the mutants in both time and frequency domains, providing more evidence of autonomic dysfunction. Measurement of the baroreceptor reflex revealed that the Q175 mutant could not appropriately increase HR in response to a pharmacologically induced decrease in blood pressure. Echocardiograms showed reduced ventricular mass and ejection fraction in mutant hearts. Finally, cardiac histopathology revealed localized points of fibrosis resembling those caused by myocardial infarction. Thus, the Q175 mouse model of HD exhibits cardiovascular dysautonomia similar to that seen in HD patients with prominent sympathetic dysfunction during the resting phase of the activity rhythm.

Muscleblind-like 3 deficit results in a spectrum of age-associated pathologies observed in myotonic dystrophy

Myotonic dystrophy type I (DM1) exhibits distinctive disease specific phenotypes and the accelerated onset of a spectrum of age-associated pathologies. In DM1, dominant effects of expanded CUG repeats result in part from the inactivation of the muscleblind-like (MBNL) proteins. To test the role of MBNL3, we deleted Mbnl3 exon 2 (Mbnl3(ΔE2)) in mice and examined the onset of age-associated diseases over 4 to 13 months of age. Accelerated onset of glucose intolerance with elevated insulin levels, cardiac systole deficits, left ventricle hypertrophy, a predictor of a later onset of heart failure and the development of subcapsular and cortical cataracts is observed in Mbnl3(ΔE2) mice. Retention of embryonic splice isoforms in adult organs, a prominent defect in DM1, is not observed in multiple RNAs including the Insulin Receptor (Insr), Cardiac Troponin T (Tnnt2), Lim Domain Binding 3 (Ldb3) RNAs in Mbnl3(ΔE2) mice. Although rare DM1-like splice errors underlying the observed phenotypes cannot be excluded, our data in conjunction with the reported absence of alternative splice errors in embryonic muscles of a similar Mbnl3(ΔE2) mouse by RNA-seq studies, suggest that mechanisms distinct from the adult retention of embryonic splice patterns may make important contributions to the onset of age-associated pathologies in DM1.

Abstract 79: Sarcospan Has a Protective Effect During Development of Cardiac Disease

Sarcospan (SSPN) has an important role in stabilizing sarcolemmal dystrophin- and utrophin-glycoprotein adhesion complexes at the cell membrane. Loss of cell adhesion leads to contraction-induced muscle damage, causing muscle dysfunction and cell death. Recently we have shown a specific role for SSPN in modulating cardiac cell adhesion and physiological function. After transthoracic aortic constriction (TAC), SSPN-null mice transitioned toward failure faster than wild-type (WT) mice. Muscle histology revealed large focal areas of collagen deposition in SSPN-null hearts after TAC compared to WT hearts, suggesting that increased membrane fragility affected cardiomyocyte survival. Our laboratory has shown that SSPN loss reduces sarcolemmal dystrophin levels and associated adhesion complexes in the heart. Whereas, the complete loss of dystrophin leads to development of Duchenne muscular dystrophy (DMD), causing cardiac dysfunction and early mortality. Overexpression of SSPN in DMD mice increased cell adhesion and laminin binding in hearts, leading to improvements in tissue histopathology and increased expression of utrophin, a functional homologue of dystrophin. Examining the restorative potential of SSPN in dystrophic cardiac tissue, led us to query whether compensatory upregulation of SSPN occurs in failing hearts of TAC-treated WT mice. In failing non-DMD hearts, we found that SSPN expression is increased. We have evidence of a chaperone role for SSPN, and its increased expression in the failing heart may contribute to the increased localization of dystrophin and associated glycoprotein complexes at the sarcolemma, which we observed in failing WT hearts compared to untreated controls. The upregulation of cell-stabilizing cell adhesion complexes may compensate for increased wall stress and counter pathological processes that culminate in cardiomyocyte demise, and we are exploring whether naturally increased expression or transgenic overexpression of SSPN in the heart may protect against damage. In summary, we have found that SSPN promotes cardiac function by maintaining cell adhesion and promoting cell survival during disease conditions.

Cardiac Dysfunction in the BACHD Mouse Model of Huntington's Disease

While Huntington's disease (HD) is classified as a neurological disorder, HD patients exhibit a high incidence of cardiovascular events leading to heart failure and death. In this study, we sought to better understand the cardiovascular phenotype of HD using the BACHD mouse model. The age-related decline in cardiovascular function was assessed by echocardiograms, electrocardiograms, histological and microarray analysis. We found that structural and functional differences between WT and BACHD hearts start at 3 months of age and continue throughout life. The aged BACHD mice develop cardiac fibrosis and ultimately apoptosis. The BACHD mice exhibited adaptive physiological changes to chronic isoproterenol treatment; however, the medication exacerbated fibrotic lesions in the heart. Gene expression analysis indicated a strong tilt toward apoptosis in the young mutant heart as well as changes in genes involved in cellular metabolism and proliferation. With age, the number of genes with altered expression increased with the large changes occurring in the cardiovascular disease, cellular metabolism, and cellular transport clusters. The BACHD model of HD exhibits a number of changes in cardiovascular function that start early in the disease progress and may provide an explanation for the higher cardiovascular risk in HD.

Sarcospan Regulates Cardiac Isoproterenol Response and Prevents Duchenne Muscular Dystrophy-Associated Cardiomyopathy

Background

Duchenne muscular dystrophy is a fatal cardiac and skeletal muscle disease resulting from mutations in the dystrophin gene. We have previously demonstrated that a dystrophin‐associated protein, sarcospan (SSPN), ameliorated Duchenne muscular dystrophy skeletal muscle degeneration by activating compensatory pathways that regulate muscle cell adhesion (laminin‐binding) to the extracellular matrix. Conversely, loss of SSPN destabilized skeletal muscle adhesion, hampered muscle regeneration, and reduced force properties. Given the importance of SSPN to skeletal muscle, we investigated the consequences of SSPN ablation in cardiac muscle and determined whether overexpression of SSPN into mdx mice ameliorates cardiac disease symptoms associated with Duchenne muscular dystrophy cardiomyopathy.

Methods and Results

SSPN‐null mice exhibited cardiac enlargement, exacerbated cardiomyocyte hypertrophy, and increased fibrosis in response to β‐adrenergic challenge (isoproterenol; 0.8 mg/day per 2 weeks). Biochemical analysis of SSPN‐null cardiac muscle revealed reduced sarcolemma localization of many proteins with a known role in cardiomyopathy pathogenesis: dystrophin, the sarcoglycans (α‐, δ‐, and γ‐subunits), and β1D integrin. Transgenic overexpression of SSPN in Duchenne muscular dystrophy mice (mdx^TG) improved cardiomyofiber cell adhesion, sarcolemma integrity, cardiac functional parameters, as well as increased expression of compensatory transmembrane proteins that mediate attachment to the extracellular matrix.

Conclusions

SSPN regulates sarcolemmal expression of laminin‐binding complexes that are critical to cardiac muscle function and protects against transient and chronic injury, including inherited cardiomyopathy.

Deletion of low molecular weight protein tyrosine phosphatase (Acp1) protects against stress-induced cardiomyopathy

The low molecular weight protein tyrosine phosphatase (LMPTP), encoded by the ACP1 gene, is a ubiquitously expressed phosphatase whose in vivo function in the heart and in cardiac diseases remains unknown. To investigate the in vivo role of LMPTP in cardiac function, we generated mice with genetic inactivation of the Acp1 locus and studied their response to long‐term pressure overload. Acp1^−/− mice develop normally and ageing mice do not show pathology in major tissues under basal conditions. However, Acp1^−/− mice are strikingly resistant to pressure overload hypertrophy and heart failure. Lmptp expression is high in the embryonic mouse heart, decreased in the postnatal stage, and increased in the adult mouse failing heart. We also show that LMPTP expression increases in end‐stage heart failure in humans. Consistent with their protected phenotype, Acp1^−/− mice subjected to pressure overload hypertrophy have attenuated fibrosis and decreased expression of fibrotic genes. Transcriptional profiling and analysis of molecular signalling show that the resistance of Acp1^−/− mice to pathological cardiac stress correlates with marginal re‐expression of fetal cardiac genes, increased insulin receptor beta phosphorylation, as well as PKA and ephrin receptor expression, and inactivation of the CaMKIIδ pathway. Our data show that ablation of Lmptp inhibits pathological cardiac remodelling and suggest that inhibition of LMPTP may be of therapeutic relevance for the treatment of human heart failure. © 2015 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Sleep dysfunction and EEG alterations in mice overexpressing alpha-synuclein

Background:

Sleep disruptions occur early and frequently in Parkinson’s disease (PD). PD patients also show a slowing of resting state activity. Alpha-synuclein is causally linked to PD and accumulates in sleep-related brain regions. While sleep problems occur in over 75% of PD patients and severely impact the quality of life of patients and caregivers, their study is limited by a paucity of adequate animal models.

Objective:

The objective of this study was to determine whether overexpression of wildtype alpha-synuclein could lead to alterations in sleep patterns reminiscent of those observed in PD by measuring sleep/wake activity with rigorous quantitative methods in a well-characterized genetic mouse model.

Methods:

At 10 months of age, mice expressing human wildtype alpha-synuclein under the Thy-1 promoter (Thy1-aSyn) and wildtype littermates underwent the subcutaneous implantation of a telemetry device (Data Sciences International) for the recording of electromyograms (EMG) and electroencephalograms (EEG) in freely moving animals. Surgeries and data collection were performed without knowledge of mouse genotype.

Results:

Thy1-aSyn mice showed increased non-rapid eye movement sleep during their quiescent phase, increased active wake during their active phase, and decreased rapid eye movement sleep over a 24-h period, as well as a shift in the density of their EEG power spectra toward lower frequencies with a significant decrease in gamma power during wakefulness.

Conclusions:

Alpha-synuclein overexpression in mice produces sleep disruptions and altered oscillatory EEG activity reminiscent of PD, and this model provides a novel platform to assess mechanisms and therapeutic strategies for sleep dysfunction in PD.

Loss of muscleblind-like 1 results in cardiac pathology and persistence of embryonic splice isoforms

Cardiac dysfunction is a prominent cause of mortality in myotonic dystrophy I (DM1), a disease where expanded CUG repeats bind and disable the muscleblind-like family of splice regulators. Deletion of muscleblind-like 1 (Mbnl1^ΔE2/ΔE2) in 129 sv mice results in QRS, QTc widening, bundle block and STc narrowing at 2–4 months of age. With time, cardiac function deteriorates further and at 6 months, decreased R wave amplitudes, sinus node dysfunction, cardiac hypertrophy, interstitial fibrosis, multi-focal myocardial fiber death and calcification manifest. Sudden death, where no end point illness is overt, is observed at a median age of 6.5 and 4.8 months in ~67% and ~86% of male and female Mbnl1^ΔE2/ΔE2 mice, respectively. Mbnl1 depletion results in the persistence of embryonic splice isoforms in a network of cardiac RNAs, some of which have been previously implicated in DM1, regulating sodium and calcium currents, Scn5a, Junctin, Junctate, Atp2a1, Atp11a, Cacna1s, Ryr2, intra and inter cellular transport, Clta, Stx2, Tjp1, cell survival, Capn3, Sirt2, Csda, sarcomere and cytoskeleton organization and function, Trim55, Mapt, Pdlim3, Pdlim5, Sorbs1, Sorbs2, Fhod1, Spag9 and structural components of the sarcomere, Myom1, Tnnt2, Zasp. Thus this study supports a key role for Mbnl1 loss in the initiation of DM1 cardiac disease.

INDs for PET molecular imaging probes-approach by an academic institution

We have developed an efficient, streamlined, cost-effective approach to obtain Investigational New Drug (IND) approvals from the Food and Drug Administration (FDA) for positron emission tomography (PET) imaging probes (while the FDA uses the terminology PET drugs, we are using "PET imaging probes," "PET probes," or "probes" as the descriptive terms). The required application and supporting data for the INDs were collected in a collaborative effort involving appropriate scientific disciplines. This path to INDs was successfully used to translate three [(18) F]fluoro-arabinofuranosylcytosine (FAC) analog PET probes to phase 1 clinical trials. In doing this, a mechanism has been established to fulfill the FDA regulatory requirements for translating promising PET imaging probes from preclinical research into human clinical trials in an efficient and cost-effective manner.

Abstract 314: Loss of Sarcospan has a Deleterious Effect on Cardiac Function in Aged Mice

Sarcospan (SSPN) has been shown to have an important role in stabilizing sarcolemmal dystrophin- and utrophin-glycoprotein adhesion complexes. Loss of sarcolemmal integrity leads to immune cell infiltration and inappropriate exchange of cellular contents with the extracellular milieu. Our laboratory has shown SSPN loss destabilizes skeletal muscle adhesion and reduces sarcolemmal dystrophin localization, whereas its complete loss due to mutation underlies development of Duchenne muscular dystrophy (DMD). Loss of dystrophin leads to cardiac dysfunction and early mortality in DMD patients. The role of SSPN in the heart is unknown. We present immunofluorescence data revealing reduction of dystrophin and the sarcoglycans with a coordinate increase of β1D integrin levels at the SSPN-null cardiac sarcolemma relative to WT. Also, SSPN loss decreases cardiac P-Akt levels, disrupting signaling promoting compensatory physiological hypertrophy. These studies suggest a fundamental role for SSPN in cardiac maintenance and function, since left ventricular mass increases with age and upon isoproterenol administration (0.8 mg/day for two wks). Aged SSPN-null mice developed hypertrophy, evidenced as increased heart/body weight ratio and left ventricular wall dimension. The SSPN-null mice lacked the characteristic initial rise in cardiac output, left ventricular ejection fraction (LvEF %), induced by chronic β-adrenergic stimulation. Functionally, aged SSPN-null hearts had an increased E/A ratio indicating restrictive ventricular filling and decreased fractional shortening F/S (%) upon isoproterenol administration. Aged untreated SSPN-null hearts had increased fibrosis compared to aged WT controls, however isoproterenol treated SSPN-null hearts displayed exacerbated fibrotic response compared to WT. To assess whether SSPN-null hearts have altered gene expression profiles during progression of pathogenesis, qRT-PCR will be utilized to measure differences in expression of fetal gene and calcium-handling proteins. In summary, we have found that SSPN has an important role in maintaining cardiac function, its loss exacerbates the hypertrophic response and localization of stabilizing adhesion complexes at the cardiac muscle sarcolemma.

Abstract 13: Nkx2-5-notch Signaling Axis Regulates The Proliferation Of The Atrial Myocytes And Conduction System

Rationale: Tight control of cardiomyocyte proliferation is essential for the formation of four-chambered heart. Although human mutation of NKX2-5 is linked to septal defects and atrioventricular conduction abnormalities, early lethality and hemodynamic alteration in the mutant models have caused controversy as to whether Nkx2-5 regulates cardiomyocyte proliferation.

Objective: In this study, we circumvented these limitations by atrial-restricted deletion of Nkx2-5.

Method and Results: Atrial-specific Nkx2-5 mutants died shortly after birth with hyperplastic working myocytes and conduction system including two nodes and internodal tracts. Multicolor reporter analysis revealed that Nkx2-5-null cardiomyocytes displayed clonal proliferative activity throughout the atria, indicating the suppressive role of Nkx2-5 in the cardiomyocyte proliferation after chamber ballooning stages. Transcriptome analysis revealed that aberrant activation of Notch signaling underlies hyperproliferation of mutant cardiomyocytes, and forced activation of Notch signaling recapitulates hyperproliferation of working myocytes but not conduction system.

Conclusion: Collectively, these data suggest that Nkx2-5 regulates proliferation of atrial working and conduction myocardium in coordination with Notch pathway.

Decreased myocardial injury and improved contractility after administration of a peptide derived against the alpha-interacting domain of the L-type calcium channel

Background

Myocardial infarction remains the leading cause of morbidity and mortality associated with coronary artery disease. The L‐type calcium channel (I_Ca‐L) is critical to excitation and contraction. Activation of the channel also alters mitochondrial function. Here, we investigated whether application of a alpha‐interacting domain/transactivator of transcription (AID‐TAT) peptide, which immobilizes the auxiliary β₂ subunit of the channel and decreases metabolic demand, could alter mitochondrial function and myocardial injury.

Methods and Results

Treatment with AID‐TAT peptide decreased ischemia‐reperfusion injury in guinea‐pig hearts ex vivo (n=11) and in rats in vivo (n=9) assessed with uptake of nitroblue tetrazolium, release of creatine kinase, and lactate dehydrogenase. Contractility (assessed with catheterization of the left ventricle) was improved after application of AID‐TAT peptide in hearts ex vivo (n=6) and in vivo (n=8) up to 12 weeks before sacrifice. In search of the mechanism for the effect, we found that intracellular calcium ([Ca²⁺]_i, Fura‐2), superoxide production (dihydroethidium fluorescence), mitochondrial membrane potential (Ψ_m, JC‐1 fluorescence), reduced nicotinamide adenine dinucleotide production, and flavoprotein oxidation (autofluorescence) are decreased after application of AID‐TAT peptide.

Conclusions

Application of AID‐TAT peptide significantly decreased infarct size and supported contractility up to 12 weeks postcoronary artery occlusion as a result of a decrease in metabolic demand during reperfusion.

Long-term administration of the TNF blocking drug Remicade (cV1q) to mdx mice reduces skeletal and cardiac muscle fibrosis, but negatively impacts cardiac function

Duchenne muscular dystrophy (DMD) is a degenerative skeletal muscle disease caused by mutations in the gene encoding dystrophin (DYS). Tumor necrosis factor (TNF) has been implicated in the pathogenesis since short-term treatment of mdx mice with TNF blocking drugs proved beneficial; however, it is not clear whether long-term treatment will also improve long-term outcomes of fibrosis and cardiac health. In this investigation, short and long-term dosing studies were carried out using the TNF blocking drug Remicade and a variety of outcome measures were assessed. Here we show no demonstrable benefit to muscle strength or morphology with 10mg/kg or 20mg/kg Remicade; however, 3mg/kg produced positive strength benefits. Remicade treatment correlated with reductions in myostatin mRNA in the heart, and concomitant reductions in cardiac and skeletal fibrosis. Surprisingly, although Remicade treated mdx hearts were less fibrotic, reductions in LV mass and ejection fraction were also observed, and these changes coincided with reductions in AKT phosphorylation on threonine 308. Thus, TNF blockade benefits mdx skeletal muscle strength and fibrosis, but negatively impacts AKT activation, leading to deleterious changes to dystrophic heart function. These studies uncover a previously unknown relationship between TNF blockade and alteration of muscle growth signaling pathways.

Nkx2-5 suppresses the proliferation of atrial myocytes and conduction system

RATIONALE

Tight control of cardiomyocyte proliferation is essential for the formation of four-chambered heart. Although human mutation of NKX2-5 is linked to septal defects and atrioventricular conduction abnormalities, early lethality and hemodynamic alteration in the mutant models have caused controversy as to whether Nkx2-5 regulates cardiomyocyte proliferation.

OBJECTIVE

In this study, we circumvented these limitations by atrial-restricted deletion of Nkx2-5.

METHOD AND RESULTS

Atrial-specific Nkx2-5 mutants died shortly after birth with hyperplastic working myocytes and conduction system including two nodes and internodal tracts. Multicolor reporter analysis revealed that Nkx2-5-null cardiomyocytes displayed clonal proliferative activity throughout the atria, indicating the suppressive role of Nkx2-5 in cardiomyocyte proliferation after chamber ballooning stages. Transcriptome analysis revealed that aberrant activation of Notch signaling underlies hyperproliferation of mutant cardiomyocytes, and forced activation of Notch signaling recapitulates hyperproliferation of working myocytes but not the conduction system.

CONCLUSIONS

Collectively, these data suggest that Nkx2-5 regulates the proliferation of atrial working and conduction myocardium in coordination with Notch pathway.

Complete atrial-specific knockout of sodium-calcium exchange eliminates sinoatrial node pacemaker activity

The origin of sinoatrial node (SAN) pacemaker activity in the heart is controversial. The leading candidates are diastolic depolarization by "funny" current (If) through HCN4 channels (the "Membrane Clock" hypothesis), depolarization by cardiac Na-Ca exchange (NCX1) in response to intracellular Ca cycling (the "Calcium Clock" hypothesis), and a combination of the two ("Coupled Clock"). To address this controversy, we used Cre/loxP technology to generate atrial-specific NCX1 KO mice. NCX1 protein was undetectable in KO atrial tissue, including the SAN. Surface ECG and intracardiac electrograms showed no atrial depolarization and a slow junctional escape rhythm in KO that responded appropriately to β-adrenergic and muscarinic stimulation. Although KO atria were quiescent they could be stimulated by external pacing suggesting that electrical coupling between cells remained intact. Despite normal electrophysiological properties of If in isolated patch clamped KO SAN cells, pacemaker activity was absent. Recurring Ca sparks were present in all KO SAN cells, suggesting that Ca cycling persists but is uncoupled from the sarcolemma. We conclude that NCX1 is required for normal pacemaker activity in murine SAN.

Abstract 171: Nkx2-5 Suppresses The Proliferation Of Atrial Cardiomyocytes And Formation Of The Conduction Cells During Cardiac Development

Mutation of human NKX2-5 is linked to atrioventricular conduction abnormalities and atrial septal defects. However, it has been difficult to rigorously examine the primary role of Nkx2-5 during the formation of the atria and cardiac conduction cells due to the concurrent hemodynamic aberration associated with ventricular dysfunction. Here, we examined the role of Nkx2-5 in the formation of the atria and conduction cells. Nkx2-5 was conditionally ablated in the atrial cardiomyocytes using atrial-specific Sln-Cre mouse line. The conditional mutants died shortly after birth, which is earlier and more severe than ventricular-specific knockout of Nkx2-5. While no evidence of ventricular failure was identified, atrial-specific conditional mutants developed congenital heart abnormalities including enlarged foramen ovale and hyperplastic atrial myocardium. Histological analyses revealed that the overproliferation of the atrial myocytes underlies the hyperplastic atrial myocardium prior to the development of the atrial septal enlargement. Moreover, conditional mutants showed massive expansion of SA node, AV node and internodal conduction cells, resulting in atrioventricular blocks that were further exacerbated by catecholamine stimulation. Given that the hemodynamic alteration is minimal during the gestational stages in our model, the misspecification of the conduction cells is not likely due to the alteration of the hemodynamics. Together, these data suggest that Nkx2-5 plays a suppressive role in the proliferation of the atrial cardiomyocytes and the formation of the conduction cells.

Impaired baroreflex function in mice overexpressing alpha-synuclein

Cardiovascular autonomic dysfunction, such as orthostatic hypotension consequent to baroreflex failure and cardiac sympathetic denervation, is frequently observed in the synucleinopathy Parkinson’s disease (PD). In the present study, the baroreceptor reflex was assessed in mice overexpressing human wildtype alpha-synuclein (Thy1-aSyn), a genetic mouse model of synucleinopathy. The beat-to-beat change in heart rate (HR), computed from R–R interval, in relation to blood pressure was measured in anesthetized and conscious mice equipped with arterial blood pressure telemetry transducers during transient bouts of hypertension and hypotension. Compared to wildtype, tachycardia following nitroprusside-induced hypotension was significantly reduced in Thy1-aSyn mice. Thy1-aSyn mice also showed an abnormal cardiovascular response (i.e., diminished tachycardia) to muscarinic blockade with atropine. We conclude that Thy1-aSyn mice have impaired basal and dynamic range of sympathetic and parasympathetic-mediated changes in HR and will be a useful model for long-term study of cardiovascular autonomic dysfunction associated with PD.

Voluntary scheduled exercise alters diurnal rhythms of behaviour, physiology and gene expression in wild-type and vasoactive intestinal peptide-deficient mice

The circadian system co-ordinates the temporal patterning of behaviour and many underlying biological processes. In some cases, the regulated outputs of the circadian system, such as activity, may be able to feed back to alter core clock processes. In our studies, we used four wheel-access conditions (no access; free access; early night; and late night) to manipulate the duration and timing of activity while under the influence of a light-dark cycle. In wild-type mice, scheduled wheel access was able to increase ambulatory activity, inducing a level of exercise driven at various phases of the light-dark cycle. Scheduled exercise also manipulated the magnitude and phasing of the circadian-regulated outputs of heart rate and body temperature. At a molecular level, the phasing and amplitude of PER2::LUCIFERASE (PER2::LUC) expression rhythms in the SCN and peripheral tissues of Per2::Luc knockin mice were altered by scheduled exercise. We then tested whether scheduled wheel access could improve deficits observed in vasointestinal polypeptide-deficient mice under the influence of a light-dark cycle. We found that scheduled wheel access during the late night improved many of the behavioural, physiological and molecular deficits previously described in vasointestinal polypeptide-deficient mice. Our results raise the possibility that scheduled exercise could be used as a tool to modulate daily rhythms and, when applied, may counteract some of the negative impacts of ageing and disease on the circadian system.

Myocardial macronutrient transporter adaptations in the adult pregestational female intrauterine and postnatal growth-restricted offspring

Associations between exponential childhood growth superimposed on low birth weight and adult onset cardiovascular disease with glucose intolerance/type 2 diabetes mellitus exist in epidemiological investigations. To determine the metabolic adaptations that guard against myocardial failure on subsequent exposure to hypoxia, we compared with controls (CON), the effect of intrauterine (IUGR), postnatal (PNGR), and intrauterine and postnatal (IPGR) calorie and growth restriction (n = 6/group) on myocardial macronutrient transporter (fatty acid and glucose) -mediated uptake in pregestational young female adult rat offspring. A higher myocardial FAT/CD36 protein expression in IUGR, PNGR, and IPGR, with higher FATP1 in IUGR, FATP6 in PNGR, FABP-c in PNGR and IPGR, and no change in GLUT4 of all groups was observed. These adaptive macronutrient transporter protein changes were associated with no change in myocardial [(3)H]bromopalmitate accumulation but a diminution in 2-deoxy-[(14)C]glucose uptake. Examination of the sarcolemmal subfraction revealed higher basal concentrations of FAT/CD36 in PNGR and FATP1 and GLUT4 in IUGR, PNGR, and IPGR vs. CON. Exogenous insulin uniformly further enhanced sarcolemmal association of these macronutrient transporter proteins above that of basal, with the exception of insulin resistance of FATP1 and GLUT4 in IUGR and FAT/CD36 in PNGR. The basal sarcolemmal macronutrient transporter adaptations proved protective against subsequent chronic hypoxic exposure (7 days) only in IUGR and PNGR, with notable deterioration in IPGR and CON of the echocardiographic ejection fraction. We conclude that the IUGR and PNGR pregestational adult female offspring displayed a resistance to insulin-induced translocation of FATP1, GLUT4, or FAT/CD36 to the myocardial sarcolemma due to preexistent higher basal concentrations. This basal adaptation of myocardial macronutrient transporters ensured adequate fatty acid uptake, thereby proving protective against chronic hypoxia-induced myocardial compromise.

Hybrid coaxial electrospun nanofibrous scaffolds with limited immunological response created for tissue engineering

Electrospinning using synthetic and natural polymers is a promising technique for the fabrication of scaffolds for tissue engineering. Numerous synthetic polymers are available to maximize durability and mechanical properties (polyurethane) versus degradability and cell adhesion (polycaprolactone). In this study, we explored the feasibility of creating scaffolds made of bicomponent nanofibers from both polymers using a coaxial electrospinning system. We used a core of poly(urethane) and a sheath of a mixture of poly(ε-caprolactone) and gelatin, all dissolved in 1,1,1,3,3,3-hexafluror-2-propanol. These nanofibrous scaffolds were then evaluated to confirm their core-sheath nature and characterize their morphology and mechanical properties under static and dynamic conditions. Furthermore, the antigenicity of the scaffolds was studied to confirm that there is no significant foreign body response to the scaffold itself that would preclude its use in vivo. The results show the advantages of combining both natural and synethic polymers to create a coaxial scaffold capable of withstanding dynamic culture conditions and encourage cellular migration to the interior of the scaffold for tissue-engineering applications. Also, the results show that there is no significant immunoreactivity in vivo to the components of the scaffolds.

Dysfunctions in circadian behavior and physiology in mouse models of Huntington's disease

Many patients with Huntington's disease (HD) exhibit disturbances in their daily cycle of sleep and wake as part of their symptoms. These patients have difficulty sleeping at night and staying awake during the day, which has a profound impact on the quality of life of the patients and their care-givers. In the present study, we examined diurnal and circadian rhythms of four models of HD including the BACHD, CAG 140 knock-in and R6/2 CAG 140 and R6/2 CAG 250 lines of mice. The BACHD and both R6/2 lines showed profound circadian phenotypes as measured by wheel-running activity. Focusing on the BACHD line for further analysis, the amplitude of the rhythms in the BACHD mice declined progressively with age. In addition, the circadian regulation of heart rate and body temperature in freely behaving BACHD mice were also disrupted. Furthermore, the distribution of sleep as well as the autonomic regulation of heart rate was disrupted in this HD model. To better understand the mechanistic underpinnings of the circadian disruption, we used electrophysiological tools to record from neurons within the central clock in the suprachiasmatic nucleus (SCN). The BACHD mice exhibit reduced rhythms in spontaneous electrical activity in SCN neurons. Interestingly, the expression of the clock gene PERIOD2 was not altered in the SCN of the BACHD line. Together, this data is consistent with the hypothesis that the HD mutations interfere with the expression of robust circadian rhythms in behavior and physiology. The data raise the possibility that the electrical activity within the central clock itself may be altered in this disease.

Evidence of a role for insulin-like growth factor binding protein (IGFBP)-3 in metabolic regulation

IGF-binding protein (IGFBP)-3 is a metabolic regulator that has been shown to inhibit insulin-stimulated glucose uptake in murine models. This finding contrasts with epidemiological evidence of decreased serum IGFBP-3 in patients with type 2 diabetes. The purpose of this study was to clarify the role of IGFBP-3 in metabolism. Four-week-old male IGFBP-3(-/-) and control mice were subjected to a high-fat diet (HFD) for 12 wk. IGFBP-3(-/-) mice were heavier before the initiation of HFD and at the end of the study period. Resting metabolic rate was significantly decreased in knockout mice; however, respiratory exchange ratio was not significantly different. Fasting blood glucose and insulin levels were significantly elevated in IGFBP-3(-/-) mice. However, IGFBP-3(-/-) mice had relatively normal glucose tolerance because the relative glucose excursion over time was not different between the groups. During hyperinsulinemic clamps, IGFBP-3(-/-) mice had increased basal hepatic glucose production, but after insulin stimulation, no differences in hepatic glucose production were observed. A second cohort of older IGFBP-3(-/-) mice on HFD displayed unexpected evidence of hepatic steatosis. In summary, glucose tolerance and clamp testing indicate that IGFBP-3(-/-) mice preserve insulin sensitivity despite evidence of increased basal glucose turnover and hepatic steatosis. We provide evidence that genetic deletion of IGFBP-3 modulates hepatic carbohydrate and lipid metabolism.

Circadian regulation of cardiovascular function: a role for vasoactive intestinal peptide

The circadian system, driven by the suprachiasmatic nucleus (SCN), regulates properties of cardiovascular function. The dysfunction of this timing system can result in cardiac pathology. The neuropeptide vasoactive intestinal peptide (VIP) is crucial for circadian rhythms in a number of biological processes including SCN electrical activity and wheel running behavior. Anatomic evidence indicates that SCN neurons expressing VIP are well positioned to drive circadian regulation of cardiac function through interactions with the autonomic centers. In this study, we tested the hypothesis that loss of VIP would result in circadian deficits in heart rate (HR) and clock gene expression in cardiac tissue. We implanted radiotelemetry devices into VIP-deficient mice and wild-type (WT) controls and continuously recorded HR, body temperature, and cage activity in freely moving mice. Under light-dark conditions, VIP-deficient mice displayed weak rhythms in HR, body temperature, and cage activity, with onsets that were advanced in phase compared with WT mice. Similarly, clock gene expression in cardiac tissue was rhythmic but phase advanced in mutant mice. In constant darkness, the normal circadian rhythms in HR were lost in VIP-deficient mice; however, most mutant mice continued to exhibit circadian rhythms of body temperature with shortened free-running period. The loss of VIP altered, but did not abolish, autonomic regulation of HR. Analysis of the echocardiograms did not find any evidence for a loss of cardiac function in VIP-deficient mice, and the size of the hearts did not differ between genotypes. These results demonstrate that VIP is an important regulator of physiological circadian rhythmicity in the heart.

Arginine metabolism by macrophages promotes cardiac and muscle fibrosis in mdx muscular dystrophy

Background

Duchenne muscular dystrophy (DMD) is the most common, lethal disease of childhood. One of 3500 new-born males suffers from this universally-lethal disease. Other than the use of corticosteroids, little is available to affect the relentless progress of the disease, leading many families to use dietary supplements in hopes of reducing the progression or severity of muscle wasting. Arginine is commonly used as a dietary supplement and its use has been reported to have beneficial effects following short-term administration to mdx mice, a genetic model of DMD. However, the long-term effects of arginine supplementation are unknown. This lack of knowledge about the long-term effects of increased arginine metabolism is important because elevated arginine metabolism can increase tissue fibrosis, and increased fibrosis of skeletal muscles and the heart is an important and potentially life-threatening feature of DMD.

Methodology

We use both genetic and nutritional manipulations to test whether changes in arginase metabolism promote fibrosis and increase pathology in mdx mice. Our findings show that fibrotic lesions in mdx muscle are enriched with arginase-2-expressing macrophages and that muscle macrophages stimulated with cytokines that activate the M2 phenotype show elevated arginase activity and expression. We generated a line of arginase-2-null mutant mdx mice and found that the mutation reduced fibrosis in muscles of 18-month-old mdx mice, and reduced kyphosis that is attributable to muscle fibrosis. We also observed that dietary supplementation with arginine for 17-months increased mdx muscle fibrosis. In contrast, arginine-2 mutation did not reduce cardiac fibrosis or affect cardiac function assessed by echocardiography, although 17-months of dietary supplementation with arginine increased cardiac fibrosis. Long-term arginine treatments did not decrease matrix metalloproteinase-2 or -9 or increase the expression of utrophin, which have been reported as beneficial effects of short-term treatments.

Conclusions/Significance

Our findings demonstrate that arginine metabolism by arginase promotes fibrosis of muscle in muscular dystrophy and contributes to kyphosis. Our findings also show that long-term, dietary supplementation with arginine exacerbates fibrosis of dystrophic heart and muscles. Thus, commonly-practiced dietary supplementation with arginine by DMD patients has potential risk for increasing pathology when performed for long periods, despite reports of benefits acquired with short-term supplementation.

Myocardial function with reduced expression of the sodium-calcium exchanger

BACKGROUND

The complete removal of the cardiac sodium-calcium exchanger (NCX1) is associated with embryonic lethality, whereas its overexpression is linked to heart failure. To determine whether or not a reduced expression of NCX1 is compatible with normal heart structure and function, we studied 2 knockout (KO) mouse models with reduced levels of NCX1: a heterozygous global KO (HG-KO) with a 50% level of NCX1 expression in all myocytes, and a ventricular-specific KO (V-KO) with NCX1 expression in only 10% to 20% of the myocytes.

METHODS AND RESULTS

Both groups of mice were evaluated at baseline, after transaortic constriction (TAC), and after acute or chronic beta-adrenergic stimulation. At baseline, the HG-KO mice had smaller hearts and the V-KO mice had larger hearts than their wild-type (WT) controls (P < .05). The HG-KO and their control WT mice had normal responses to TAC and beta-adrenergic stimulation. However, the V-KO group was intolerant to TAC and had a significantly (P < .05) blunted response to beta-adrenergic stimulation as compared with the HG-KO mice and WT controls. Unlike the HG-KO mice, the V-KO mice did not tolerate chronic isoproterenol infusion. Telemetric analysis of the electrocardiogram, body temperature, and activity revealed a normal diurnal rhythm in all groups of mice, but confirmed shorter QT intervals along with increased arrhythmias and reduced R wave to P wave amplitude ratios in the V-KO mice.

CONCLUSIONS

Though NCX1 can be reduced by half in all myocytes without significant functional alterations, it must be expressed in more than 20% of the myocytes to prevent severe remodeling and heart failure in mouse heart.

Nav2 is necessary for cranial nerve development and blood pressure regulation

BACKGROUND

All-trans retinoic acid (atRA) is required for nervous system development, including the developing hindbrain region. Neuron navigator 2 (Nav2) was first identified as an atRA-responsive gene in human neuroblastoma cells (retinoic acid-induced in neuroblastoma 1, Rainb1), and is required for atRA-mediated neurite outgrowth. In this paper, we explore the importance of Nav2 in nervous system development and function in vivo.

RESULTS

Nav2 hypomorphic homozygous mutants show decreased survival starting at birth. Nav2 mutant embryos show an overall reduction in nerve fiber density, as well as specific defects in cranial nerves IX (glossopharyngeal) and X (vagus). Nav2 hypomorphic mutant adult mice also display a blunted baroreceptor response compared to wild-type controls.

CONCLUSIONS

Nav2 functions in mammalian nervous system development, and is required for normal cranial nerve development and blood pressure regulation in the adult.

Dysfunction of ouabain-induced cardiac contractility in mice with heart-specific ablation of Na,K-ATPase beta1-subunit

Na,K-ATPase is composed of two essential alpha- and beta-subunits, both of which have multiple isoforms. Evidence indicates that the Na,K-ATPase enzymatic activity as well as its alpha(1), alpha(3) and beta(1) isoforms are reduced in the failing human heart. The catalytic alpha-subunit is the receptor for cardiac glycosides such as digitalis, used for the treatment of congestive heart failure. The role of the Na,K-ATPase beta(1)-subunit (Na,K-beta(1)) in cardiac function is not known. We used Cre/loxP technology to inactivate the Na,K-beta(1) gene exclusively in the ventricular cardiomyocytes. Animals with homozygous Na,K-beta(1) gene excision were born at the expected Mendelian ratio, grew into adulthood, and appeared to be healthy until 10 months of age. At 13-14 months, these mice had 13% higher heart/body weight ratios, and reduced contractility as revealed by echocardiography compared to their wild-type (WT) littermates. Pressure overload by transverse aortic constriction (TAC) in younger mice, resulted in compensated hypertrophy in WT mice, but decompensation in the Na,K-beta(1) KO mice. The young KO survivors of TAC exhibited decreased contractile function and mimicked the effects of the Na,K-beta(1) KO in older mice. Further, we show that intact hearts of Na,K-beta(1) KO anesthetized mice as well as isolated cardiomyocytes were insensitive to ouabain-induced positive inotropy. This insensitivity was associated with a reduction in NCX1, one of the proteins involved in regulating cardiac contractility. In conclusion, our results demonstrate that Na,K-beta(1) plays an essential role in regulating cardiac contractility and that its loss is associated with significant pathophysiology of the heart.

A cyclin D2-Rb pathway regulates cardiac myocyte size and RNA polymerase III after biomechanical stress in adult myocardium

Normally, cell cycle progression is tightly coupled to the accumulation of cell mass; however, the mechanisms whereby proliferation and cell growth are linked are poorly understood. We have identified cyclin (Cyc)D2, a G(1) cyclin implicated in mediating S phase entry, as a potential regulator of hypertrophic growth in adult post mitotic myocardium. To examine the role of CycD2 and its downstream targets, we subjected CycD2-null mice to mechanical stress. Hypertrophic growth in response to transverse aortic constriction was attenuated in CycD2-null compared with wild-type mice. Blocking the increase in CycD2 in response to hypertrophic agonists prevented phosphorylation of CycD2-target Rb (retinoblastoma gene product) in vitro, and mice deficient for Rb had potentiated hypertrophic growth. Hypertrophic growth requires new protein synthesis and transcription of tRNA genes by RNA polymerase (pol) III, which increases with hypertrophic signals. This load-induced increase in RNA pol III activity is augmented in Rb-deficient hearts. Rb binds and represses Brf-1 and TATA box binding protein (TBP), subunits of RNA pol III-specific transcription factor B, in adult myocardium under basal conditions. However, this association is disrupted in response to transverse aortic constriction. RNA pol III activity is unchanged in CycD2(-/-) myocardium after transverse aortic constriction, and there is no dissociation of TBP from Rb. These investigations identify an essential role for the CycD2-Rb pathway as a governor of cardiac myocyte enlargement in response to biomechanical stress and, more fundamentally, as a regulator of the load-induced activation of RNA pol III.

Autocrine VEGF signaling is required for vascular homeostasis

Vascular endothelial growth factor (VEGF) is essential for developmental and pathological angiogenesis. Here we show that in the absence of any pathological insult, autocrine VEGF is required for the homeostasis of blood vessels in the adult. Genetic deletion of vegf specifically in the endothelial lineage leads to progressive endothelial degeneration and sudden death in 55% of mutant mice by 25 weeks of age. The phenotype is manifested without detectable changes in the total levels of VEGF mRNA or protein, indicating that paracrine VEGF could not compensate for the absence of endothelial VEGF. Furthermore, wild-type, but not VEGF null, endothelial cells showed phosphorylation of VEGFR2 in the absence of exogenous VEGF. Activation of the receptor in wild-type cells was suppressed by small molecule antagonists but not by extracellular blockade of VEGF. These results reveal a cell-autonomous VEGF signaling pathway that holds significance for vascular homeostasis but is dispensable for the angiogenic cascade.

Hypertrophy and heart failure in mice overexpressing the cardiac sodium-calcium exchanger

BACKGROUND

The cardiac sodium-calcium exchanger (NCX1) is a key sarcolemmal protein for the maintenance of calcium homeostasis in the heart. Because heart failure is associated with increased expression of NCX1, heterozygous (HET) and homozygous (HOM) transgenic mice overexpressing NCX1 were developed and evaluated.

METHODS AND RESULTS

The NCX1 transgenic mice display 2.3-fold (HET) and 3.1-fold (HOM) increases in exchanger activity from wild-type (WT) mice. Functional information was obtained by echocardiography and catheterizations before and after hemodynamic stress from pregnancy, treadmill exercise or transaortic constriction (TAC). HET and HOM mice exhibited hypertrophy and blunted responses with beta-adrenergic stimulation. Postpartum mice from all groups were hypertrophied, but only the HOM mice exhibited premature death from heart failure. HOM mice became exercise intolerant after 6 weeks of daily treadmill running. After 21 days TAC, HET, and HOM mice exhibited significant contractile dysfunction and 15% to 40% mortality with clinical evidence of heart failure.

CONCLUSIONS

Hemodynamic stress results in a compensated hypertrophy in WT mice, but NCX1 transgenic mice exhibit decreased contractile function and heart failure in proportion to their level of NCX1 expression. Thus exchanger overexpression in mice leads to abnormal calcium handling and a decompensatory transition to heart failure with stress.

Mechanism of shortened action potential duration in Na+-Ca2+ exchanger knockout mice

In cardiac-specific Na(+)-Ca(2+) exchanger (NCX) knockout (KO) mice, the ventricular action potential (AP) is shortened. The shortening of the AP, as well as a decrease of the L-type Ca(2+) current (I(Ca)), provides a critical mechanism for the maintenance of Ca(2+) homeostasis and contractility in the absence of NCX (Pott C, Philipson KD, Goldhaber JI. Excitation-contraction coupling in Na(+)-Ca(2+) exchanger knockout mice: reduced transsarcolemmal Ca(2+) flux. Circ Res 97: 1288-1295, 2005). To investigate the mechanism that underlies the accelerated AP repolarization, we recorded the transient outward current (I(to)) in patch-clamped myocytes isolated from wild-type (WT) and NCX KO mice. Peak I(to) was increased by 78% and decay kinetics were slowed in KO vs. WT. Consistent with increased I(to), ECGs from KO mice exhibited shortened QT intervals. Expression of the I(to)-generating K(+) channel subunit Kv4.2 and the K(+) channel interacting protein was increased in KO. We used a computer model of the murine AP (Bondarenko VE, Szigeti GP, Bett GC, Kim SJ, and Rasmusson RL. Computer model of action potential of mouse ventricular myocytes. Am J Physiol Heart Circ Physiol 287: 1378-1403, 2004) to determine the relative contributions of increased I(to), reduced I(Ca), and reduced NCX current (I(NCX)) on the shape and kinetics of the AP. Reduction of I(Ca) and elimination of I(NCX) had relatively small effects on the duration of the AP in the computer model. In contrast, AP repolarization was substantially accelerated when I(to) was increased in the computer model. Thus, the increase in I(to), and not the reduction of I(Ca) or I(NCX), is likely to be the major mechanism of AP shortening in KO myocytes. The upregulation of I(to) may comprise an important regulatory mechanism to limit Ca(2+) influx via a reduction of AP duration, thus preventing Ca(2+) overload in situations of reduced myocyte Ca(2+) extrusion capacity.

Hypertrophic growth in cardiac myocytes is mediated by Myc through a Cyclin D2-dependent pathway

c-Myc (Myc) is highly expressed in developing embryos where it regulates body size by controlling proliferation but not cell size. However, Myc is also induced in many postmitotic tissues, including adult myocardium, in response to stress where the predominant form of growth is an increase in cell size (hypertrophy) and not number. The function of Myc induction in this setting is unproven. Therefore, to explore Myc's role in hypertrophic growth, we created mice where Myc can be inducibly inactivated, specifically in adult myocardium. Myc-deficient hearts demonstrated attenuated stress-induced hypertrophic growth, secondary to a reduction in cell growth of individual myocytes. To explore the dependence of Myc-induced cell growth on CycD2, we created bigenic mice where Myc can be selectively activated in CycD2-null adult myocardium. Myc-dependent hypertrophic growth and cell cycle reentry is blocked in CycD2-deficient hearts. However, in contrast to Myc-induced DNA synthesis, hypertrophic growth is independent of CycD2-induced Cdk2 activity. These data suggest that Myc is required for a normal hypertrophic response and that its growth-promoting effects are also mediated through a CycD2-dependent pathway.

Comparing isoflurane with tribromoethanol anesthesia for echocardiographic phenotyping of transgenic mice

Cardiac phenotyping of transgenic mice typically requires anesthesia. Chemical-grade tribromoethanol (TBE) is commonly used for this purpose due to its relatively short duration of action, modest cardiodepressive effects, and its noncontrolled status. In the present study, we used both genders of C57BL/6;C3H-Tg(Slc8a1)hKdp transgenic (TG) mice and C57BL/6;C3H wild-type (WT) mice to evaluate isoflurane (ISF) as a pharmaceutical-grade alternative to TBE for echocardiography and electrocardiography. Baseline target physiologic heart rates (beats per minute) were established by use of telemetry as 544 +/- 10 in WT mice and 580 +/- 21 in TG mice. TG and WT animals were anesthetized with either 0.8% to 1% inhalational ISF or 250 mg/kg intraperitoneal TBE. The following parameters were measured or calculated according to the previously defined physiologic heart rates: end diastolic and systolic dimensions; posterior wall and ventricular septal thicknesses; left ventricular mass, aortic ejection times; left ventricular fractional shortening; velocity of circumferential fiber shortening; and left ventricular ejection fraction. No significant difference between anesthetics was found for any measured cardiac parameters. However, the time required for data acquisition was significantly shorter for ISF (10 min) than for TBE (14 min). This study demonstrates that comparable echocardiographic results can be obtained at higher throughput by use of pharmaceuticalgrade ISF than with chemical-grade TBE.

Cardiac manifestations in the mouse model of mucopolysaccharidosis I

Mucopolysaccharidosis I (MPS I, alpha-l-iduronidase deficiency disease) is a heritable lysosomal storage disorder involving multiple organs, including the heart. Malfunction of the heart is also a major manifestation in the mouse model of MPS I, progressing in severity from 6 to 10 months (of a 1 year life span). In comparisons of MPS I with wild-type mice, the heart was found enlarged, with thickened septal and posterior walls, primarily because of infiltration of the muscle by storage-laden cells. Heart valves were enlarged and misshapen, and contained large numbers of highly vacuolated interstitial cells. The thickened aortic wall contained vacuolated smooth muscle cells and interrupted elastic fibers. Hemodynamic measurements and echocardiography revealed reduced left ventricular function as well as mitral and aortic regurgitation. But despite these abnormalities, free-roaming MPS I mice implanted with radio telemetry devices showed surprisingly normal heart rate and blood pressure, though their electrocardiograms were abnormal. An incidental finding of the telemetry studies was a disturbed circadian rhythm in the MPS I mice. Restoration of enzyme activity in the heart of one mouse, by transplantation of retrovirally modified bone marrow, resulted in normalization of left ventricular function as well as loss of storage vacuoles in myocytes and endothelial cells, though not in valvular interstitial cells. This study demonstrates the usefulness of the mouse model for in-depth studies of the cardiovascular component of MPS I.

Cardiomyopathy in dystrophin-deficient hearts is prevented by expression of a neuronal nitric oxide synthase transgene in the myocardium

Null mutation of dystrophin causes the lethal pathology of Duchenne muscular dystrophy (DMD) in which there is progressive pathology of skeletal and cardiac muscles. A large proportion of DMD patient deaths are attributable to cardiac dysfunction associated with ventricular fibrosis, arrhythmias and conduction abnormalities, although the relationships between the dystrophin mutation and the cardiac defects are unknown. Here, we tested whether cardiac pathology in dystrophin-deficient mdx mice can be corrected by the elevated production of nitric oxide (NO) by the myocardium. Dystrophin-deficient mdx mice were produced in which there was myocardial expression of a neuronal nitric oxide synthase (nNOS) transgene. Expression of the transgene prevented the progressive ventricular fibrosis of mdx mice and greatly reduced myocarditis. Electrocardiographs (ECG) attained by radiotelemetry of freely ambulatory mice showed that mdx mice displayed cardiac abnormalities that are characteristic of DMD patients, including deep Q-waves, diminished S:R ratios, polyphasic R-waves and frequent premature ventricular contractions. All of these ECG abnormalities in mdx mice were improved or corrected by nNOS transgene expression. In addition, defects in mdx cardiac autonomic function, which were reflected by decreased heart rate variability, were significantly reduced by nNOS transgene expression. These findings indicate that increasing NO production by dystrophic hearts may have therapeutic value.

Overlapping roles of pocket proteins in the myocardium are unmasked by germ line deletion of p130 plus heart-specific deletion of Rb

The pocket protein family of tumor suppressors, and Rb specifically, have been implicated as controlling terminal differentiation in many tissues, including the heart. To establish the biological functions of Rb in the heart and overcome the early lethality caused by germ line deletion of Rb, we used a Cre/loxP system to create conditional, heart-specific Rb-deficient mice. Mice that are deficient in Rb exclusively in cardiac myocytes (CRbL/L) are born with the expected Mendelian distribution, and the adult mice displayed no change in heart size, myocyte cell cycle distribution, myocyte apoptosis, or mechanical function. Since both Rb and p130 are expressed in the adult myocardium, we created double-knockout mice (CRbL/L p130-/-) to determine it these proteins have a shared role in regulating cardiac myocyte cell cycle progression. Adult CRbL/L p130-/- mice demonstrated a threefold increase in the heart weight-to-body weight ratio and showed increased numbers of bromodeoxyuridine- and phosphorylated histone H3-positive nuclei, consistent with persistent myocyte cycling. Likewise, the combined deletion of Rb plus p130 up-regulated myocardial expression of Myc, E2F-1, and G1 cyclin-dependent kinase activities, synergistically. Thus, Rb and p130 have overlapping functional roles in vivo to suppress cell cycle activators, including Myc, and maintain quiescence in postnatal cardiac muscle.