Remodeling the cardiac sarcomere using transgenesis

Localization of transcripts, translation, and degradation for spatiotemporal sarcomere maintenance

The mechanisms responsible for maintaining macromolecular protein complexes, with their proper localization and subunit stoichiometry, are incompletely understood. Here we studied the maintenance of the sarcomere, the basic contractile macromolecular complex of cardiomyocytes. We performed single-cell analysis of cardiomyocytes using imaging of mRNA and protein synthesis, and demonstrate that three distinct mechanisms are responsible for the maintenance of the sarcomere: mRNAs encoding for sarcomeric proteins are localized to the sarcomere, ribosomes are localized to the sarcomere with localized sarcomeric protein translation, and finally, a localized E3 ubiquitin ligase allow efficient degradation of excess unincorporated sarcomeric proteins. We show that these mechanisms are distinct, required, and work in unison, to ensure both spatial localization, and to overcome the large variability in transcription. Cardiomyocytes simultaneously maintain all their sarcomeres using localized translation and degradation processes where proteins are continuously and locally synthesized at high rates, and excess proteins are continuously degraded.

Cardiomyocyte-Specific Telomere Shortening is a Distinct Signature of Heart Failure in Humans

Background

Telomere defects are thought to play a role in cardiomyopathies, but the specific cell type affected by the disease in human hearts is not yet identified. The aim of this study was to systematically evaluate the cell type specificity of telomere shortening in patients with heart failure in relation to their cardiac disease, age, and sex.

Methods and Results

We studied cardiac tissues from patients with heart failure by utilizing telomere quantitative fluorescence in situ hybridization, a highly sensitive method with single‐cell resolution. In this study, total of 63 human left ventricular samples, including 37 diseased and 26 nonfailing donor hearts, were stained for telomeres in combination with cardiomyocyte‐ or α‐smooth muscle cell‐specific markers, cardiac troponin T, and smooth muscle actin, respectively, and assessed for telomere length. Patients with heart failure demonstrate shorter cardiomyocyte telomeres compared with nonfailing donors, which is specific only to cardiomyocytes within diseased human hearts and is associated with cardiomyocyte DNA damage. Our data further reveal that hypertrophic hearts with reduced ejection fraction exhibit the shortest telomeres. In contrast to other reported cell types, no difference in cardiomyocyte telomere length is evident with age. However, under the disease state, telomere attrition manifests in both young and older patients with cardiac hypertrophy. Finally, we demonstrate that cardiomyocyte‐telomere length is better sustained in women than men under diseased conditions.

Conclusions

This study provides the first evidence of cardiomyocyte‐specific telomere shortening in heart failure.

Understanding cardiac sarcomere assembly with zebrafish genetics

Mutations in sarcomere genes have been found in many inheritable human diseases, including hypertrophic cardiomyopathy. Elucidating the molecular mechanisms of sarcomere assembly shall facilitate understanding of the pathogenesis of sarcomere-based cardiac disease. Recently, biochemical and genomic studies have identified many new genes encoding proteins that localize to the sarcomere. However, their precise functions in sarcomere assembly and sarcomere-based cardiac disease are unknown. Here, we review zebrafish as an emerging vertebrate model for these studies. We summarize the techniques offered by this animal model to manipulate genes of interest, annotate gene expression, and describe the resulting phenotypes. We survey the sarcomere genes that have been investigated in zebrafish and discuss the potential of applying this in vivo model for larger-scale genetic studies.

Cell biology of sarcomeric protein engineering: disease modeling and therapeutic potential

The cardiac sarcomere is the functional unit for myocyte contraction. Ordered arrays of sarcomeric proteins, held in stoichiometric balance with each other, respond to calcium to coordinate contraction and relaxation of the heart. Altered sarcomeric structure-function underlies the primary basis of disease in multiple acquired and inherited heart disease states. Hypertrophic and restrictive cardiomyopathies are caused by inherited mutations in sarcomeric genes and result in altered contractility. Ischemia-mediated acidosis directly alters sarcomere function resulting in decreased contractility. In this review, we highlight the use of acute genetic engineering of adult cardiac myocytes through stoichiometric replacement of sarcomeric proteins in these disease states with particular focus on cardiac troponin I. Stoichiometric replacement of disease causing mutations has been instrumental in defining the molecular mechanisms of hypertrophic and restrictive cardiomyopathy in a cellular context. In addition, taking advantage of stoichiometric replacement through gene therapy is discussed, highlighting the ischemia-resistant histidine-button, A164H cTnI. Stoichiometric replacement of sarcomeric proteins offers a potential gene therapy avenue to replace mutant proteins, alter sarcomeric responses to pathophysiologic insults, or neutralize altered sarcomeric function in disease.

Animal and in silico models for the study of sarcomeric cardiomyopathies

Over the past decade, our understanding of cardiomyopathies has improved dramatically, due to improvements in screening and detection of gene defects in the human genome as well as a variety of novel animal models (mouse, zebrafish, and drosophila) and in silico computational models. These novel experimental tools have created a platform that is highly complementary to the naturally occurring cardiomyopathies in cats and dogs that had been available for some time. A fully integrative approach, which incorporates all these modalities, is likely required for significant steps forward in understanding the molecular underpinnings and pathogenesis of cardiomyopathies. Finally, novel technologies, including CRISPR/Cas9, which have already been proved to work in zebrafish, are currently being employed to engineer sarcomeric cardiomyopathy in larger animals, including pigs and non-human primates. In the mouse, the increased speed with which these techniques can be employed to engineer precise 'knock-in' models that previously took years to make via multiple rounds of homologous recombination-based gene targeting promises multiple and precise models of human cardiac disease for future study. Such novel genetically engineered animal models recapitulating human sarcomeric protein defects will help bridging the gap to translate therapeutic targets from small animal and in silico models to the human patient with sarcomeric cardiomyopathy.

Sarcomere mutation-specific expression patterns in human hypertrophic cardiomyopathy

BACKGROUND

Heterozygous mutations in sarcomere genes in hypertrophic cardiomyopathy (HCM) are proposed to exert their effect through gain of function for missense mutations or loss of function for truncating mutations. However, allelic expression from individual mutations has not been sufficiently characterized to support this exclusive distinction in human HCM.

METHODS AND RESULTS

Sarcomere transcript and protein levels were analyzed in septal myectomy and transplant specimens from 46 genotyped HCM patients with or without sarcomere gene mutations and 10 control hearts. For truncating mutations in MYBPC3, the average ratio of mutant:wild-type transcripts was ≈1:5, in contrast to ≈1:1 for all sarcomere missense mutations, confirming that nonsense transcripts are uniquely unstable. However, total MYBPC3 mRNA was significantly increased by 9-fold in HCM samples with MYBPC3 mutations compared with control hearts and with HCM samples without sarcomere gene mutations. Full-length MYBPC3 protein content was not different between MYBPC3 mutant HCM and control samples, and no truncated proteins were detected. By absolute quantification of abundance with multiple reaction monitoring, stoichiometric ratios of mutant sarcomere proteins relative to wild type were strikingly variable in a mutation-specific manner, with the fraction of mutant protein ranging from 30% to 84%.

CONCLUSIONS

These results challenge the concept that haploinsufficiency is a unifying mechanism for HCM caused by MYBPC3 truncating mutations. The range of allelic imbalance for several missense sarcomere mutations suggests that certain mutant proteins may be more or less stable or incorporate more or less efficiently into the sarcomere than wild-type proteins. These mutation-specific properties may distinctly influence disease phenotypes.

Cardiac remodeling in Drosophila arises from changes in actin gene expression and from a contribution of lymph gland-like cells to the heart musculature

Understanding the basis of normal heart remodeling can provide insight into the plasticity of the cardiac state, and into the potential for treating diseased tissue. In Drosophila, the adult heart arises during metamorphosis from a series of events, that include the remodeling of an existing cardiac tube, the elaboration of new inflow tracts, and the addition of a layer of longitudinal muscle fibers. We have identified genes active in all these three processes, and studied their expression in order to characterize in greater detail normal cardiac remodeling. Using a Transglutaminase-lacZ transgenic line, that is expressed in the inflow tracts of the larval and adult heart, we confirm the existence of five inflow tracts in the adult structure. In addition, expression of the Actin87E actin gene is initiated in the remodeling cardiac tube, but not in the longitudinal fibers, and we have identified an Act87E promoter fragment that recapitulates this switch in expression. We also establish that the longitudinal fibers are multinucleated, characterizing these cells as specialized skeletal muscles. Furthermore, we have defined the origin of the longitudinal fibers, as a subset of lymph gland cells associated with the larval dorsal vessel. These studies underline the myriad contributors to the formation of the adult Drosophila heart, and provide new molecular insights into the development of this complex organ.

Transgenic cardiac-targeted overexpression of human thymidylate kinase

Thymidylate kinase (TMPK) is a nucleoside monophosphate kinase that catalyzes phosphorylation of thymidine monophosphate to thymidine diphosphate. TMPK also mediates phosphorylation of monophosphates of thymidine nucleoside analog (NA) prodrugs on the pathway to their active triphosphate antiviral or antitumor moieties. Novel transgenic mice (TG) expressing human (h) TMPK were genetically engineered using the alpha-myosin heavy chain promoter to drive its cardiac-targeted overexpression. In '2 by 2' protocols, TMPK TGs and wild-type (WT) littermates were treated with the NA zidovudine (a deoxythymidine analog, 3'-azido-3'deoxythymidine (AZT)) or vehicle for 35 days. Alternatively, TGs and WTs were treated with a deoxycytidine NA (racivir, RCV) or vehicle. Changes in mitochondrial DNA (mtDNA) abundance and mitochondrial ultrastructure were defined quantitatively by real-time PCR and transmission electron microscopy, respectively. Cardiac performance was determined echocardiographically. Results showed TMPK TGs treated with either AZT or RCV exhibited decreased cardiac mtDNA abundance. Cardiac ultrastructural changes were seen only with AZT. AZT-treated TGs exhibited increased left ventricle (LV) mass. In contrast, LV mass in RCV-treated TGs and WTs remained unchanged. In all cohorts, LV end-diastolic dimension remained unchanged. This novel cardiac-targeted overexpression of hTMPK helps define the role of TMPK in mitochondrial toxicity of antiretrovirals.

Myofilament incorporation determines the stoichiometry of troponin I in transgenic expression and the rescue of a null mutation

The highly organized contractile machinery in skeletal and cardiac muscles requires an assembly of myofilament proteins with stringent stoichiometry. To understand the maintenance of myofilament protein stoichiometry under dynamic protein synthesis and catabolism in muscle cells, we investigated the equilibrium of troponin I (TnI) in mouse cardiac muscle during developmental isoform switching and in under- and over-expression models. Compared with the course of developmental TnI isoform switching in normal hearts, the postnatal presence of slow skeletal muscle TnI lasted significantly longer in the hearts of cardiac TnI (cTnI) knockout (cTnI-KO) mice, in which the diminished synthesis was compensated by prolonging the life of myofilamental TnI. Transgenic postnatal expression of an N-terminal truncated cTnI (cTnI-ND) using alpha-myosin heavy chain promoter effectively rescued the lethality of cTnI-KO mice and shortened the postnatal presence of slow TnI in cardiac muscle. cTnI-KO mice rescued with different levels of cTnI-ND over-expression exhibited similar levels of myocardial TnI comparable to that in wild type hearts, demonstrating that excessive synthesis would not increase TnI stoichiometry in the myofilaments. Consistently, haploid under-expression of cTnI in heterozygote cTnI-KO mice was sufficient to sustain the normal level of myocardial cTnI, indicating that cTnI is synthesized in excess in wild type cardiomyocytes. Altogether, these observations suggest that under wide ranges of protein synthesis and turnover, myofilament incorporation determines the stoichiometry of troponin subunits in muscle cells.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Genetic approaches for changing the heart and dissecting complex syndromes

The genetic, biochemical and molecular bases of human cardiac disease have been the focus of extensive research efforts for many years. Early animal models of cardiovascular disease used pharmacologic or surgical interventions, or took advantage of naturally occurring genetic abnormalities and the data obtained were largely correlative. The inability to directly alter an organism's genetic makeup and cellular protein content and accurately measure the results of that manipulation precluded rigorous examination of true cause-effect and structure-function relationships. Directed genetic manipulation in the mouse gave researchers the ability to modify and control the mammalian heart's protein content, resulting in the rational design of models that could provide critical links between the mutated or absent protein and disease. Two techniques that have proven particularly useful are transgenesis, which involves the random insertion of ectopic genetic material of interest into a "host" genome, and gene targeting, which utilizes homologous recombination at a pre-selected locus. Initially, transgenesis and gene targeting were used to examine systemic loss-of-function and gain-of-function, respectively, but further refinements in both techniques have allowed for investigations of organ-specific, cell type-specific, developmental stage-sensitive and dose-dependent effects. Genetically engineered animal models of pediatric and adult cardiac disease have proven that, when used appropriately, these tools have the power to extend mere observation to the establishment of true causative proof. We illustrate the power of the general approach by showing how genetically engineered mouse models can define the precise signaling pathways that are affected by the gain-of-function mutation that underlies Noonan syndrome. Increasingly precise and modifiable animal models of human cardiac disease will allow researchers to determine not only pathogenesis, but also guide treatment and the development of novel therapies.

A novel mutant cardiac troponin C disrupts molecular motions critical for calcium binding affinity and cardiomyocyte contractility

Troponin C (TnC) belongs to the superfamily of EF-hand (helix-loop-helix) Ca(2+)-binding proteins and is an essential component of the regulatory thin filament complex. In a patient diagnosed with idiopathic dilated cardiomyopathy, we identified two novel missense mutations localized in the regulatory Ca(2+)-binding Site II of TnC, TnC((E59D,D75Y)). Expression of recombinant TnC((E59D,D75Y)) in isolated rat cardiomyocytes induced a marked decrease in contractility despite normal intracellular calcium homeostasis in intact cardiomyocytes and resulted in impaired myofilament calcium responsiveness in Triton-permeabilized cardiomyocytes. Expression of the individual mutants in cardiomyocytes showed that TnC(D75Y) was able to recapitulate the TnC((E59D,D75Y)) phenotype, whereas TnC(E59D) was functionally benign. Force-pCa relationships in TnC((E59D,D75Y)) reconstituted rabbit psoas fibers and fluorescence spectroscopy of TnC((E59D,D75Y)) labeled with 2-[(4'-iodoacetamide)-aniline]naphthalene-6-sulfonic acid showed a decrease in myofilament Ca(2+) sensitivity and Ca(2+) binding affinity, respectively. Furthermore, computational analysis of TnC showed the Ca(2+)-binding pocket as an active region of concerted motions, which are decreased markedly by mutation D75Y. We conclude that D75Y interferes with proper concerted motions within the regulatory Ca(2+)-binding pocket of TnC that hinders the relay of the thin filament calcium signal, thereby providing a primary stimulus for impaired cardiomyocyte contractility. This in turn may trigger pathways leading to aberrant ventricular remodeling and ultimately a dilated cardiomyopathy phenotype.

Targeted transgenic overexpression of mitochondrial thymidine kinase (TK2) alters mitochondrial DNA (mtDNA) and mitochondrial polypeptide abundance: transgenic TK2, mtDNA, and antiretrovirals

Mitochondrial toxicity limits nucleoside reverse transcriptase inhibitors (NRTIs) for acquired immune deficiency syndrome. NRTI triphosphates, the active moieties, inhibit human immunodeficiency virus reverse transcriptase and eukaryotic mitochondrial DNA polymerase pol-gamma. NRTI phosphorylation seems to correlate with mitochondrial toxicity, but experimental evidence is lacking. Transgenic mice (TGs) with cardiac overexpression of thymidine kinase isoforms (mitochondrial TK2 and cytoplasmic TK1) were used to study NRTI mitochondrial toxicity. Echocardiography and nuclear magnetic resonance imaging defined cardiac performance and structure. TK gene copy and enzyme activity, mitochondrial (mt) DNA and polypeptide abundance, succinate dehydrogenase and cytochrome oxidase histochemistry, and electron microscopy correlated with transgenesis, mitochondrial structure, and biogenesis. Antiretroviral combinations simulated therapy. Untreated hTK1 or TK2 TGs exhibited normal left ventricle mass. In TK2 TGs, cardiac TK2 gene copy doubled, activity increased 300-fold, and mtDNA abundance doubled. Abundance of the 17-kd subunit of complex I, succinate dehydrogenase histochemical activity, and cristae density increased. NRTIs increased left ventricle mass 20% in TK2 TGs. TK activity increased 3 logs in hTK1 TGs, but no cardiac phenotype resulted. NRTIs abrogated functional effects of transgenically increased TK2 activity but had no effect on TK2 mtDNA abundance. Thus, NRTI mitochondrial phosphorylation by TK2 is integral to clinical NRTI mitochondrial toxicity.

Genetic engineering and therapy for inherited and acquired cardiomyopathies

The cardiac myofilaments consist of a highly ordered assembly of proteins that collectively generate force in a calcium-dependent manner. Defects in myofilament function and its regulation have been implicated in various forms of acquired and inherited human heart disease. For example, during cardiac ischemia, cardiac myocyte contractile performance is dramatically downregulated due in part to a reduced sensitivity of the myofilaments to calcium under acidic pH conditions. Over the last several years, the thin filament regulatory protein, troponin I, has been identified as an important mediator of this response. Mutations in troponin I and other sarcomere genes are also linked to several distinct inherited cardiomyopathic phenotypes, including hypertrophic, dilated, and restrictive cardiomyopathies. With the cardiac sarcomere emerging as a central player for such a diverse array of human heart diseases, genetic-based strategies that target the myofilament will likely have broad therapeutic potential. The development of safe vector systems for efficient gene delivery will be a critical hurdle to overcome before these types of therapies can be successfully applied. Nonetheless, studies focusing on the principles of acute genetic engineering of the sarcomere hold value as they lay the essential foundation on which to build potential gene-based therapies for heart disease.

Decreased mtDNA, oxidative stress, cardiomyopathy, and death from transgenic cardiac targeted human mutant polymerase gamma

POLG is the human gene that encodes the catalytic subunit of DNA polymerase gamma (Pol gamma), the replicase for human mitochondrial DNA (mtDNA). A POLG Y955C point mutation causes human chronic progressive external ophthalmoplegia (CPEO), a mitochondrial disease with eye muscle weakness and mtDNA defects. Y955C POLG was targeted transgenically (TG) to the murine heart. Survival was determined in four TG (+/-) lines and wild-type (WT) littermates (-/-). Left ventricle (LV) performance (echocardiography and MRI), heart rate (electrocardiography), mtDNA abundance (real time PCR), oxidation of mtDNA (8-OHdG), histopathology and electron microscopy defined the phenotype. Cardiac targeted Y955C POLG yielded a molecular signature of CPEO in the heart with cardiomyopathy (CM), mitochondrial oxidative stress, and premature death. Increased LV cavity size and LV mass, bradycardia, decreased mtDNA, increased 8-OHdG, and cardiac histopathological and mitochondrial EM defects supported and defined the phenotype. This study underscores the pathogenetic role of human mutant POLG and its gene product in mtDNA depletion, mitochondrial oxidative stress, and CM as it relates to the genetic defect in CPEO. The transgenic model pathophysiologically links human mutant Pol gamma, mtDNA depletion, and mitochondrial oxidative stress to the mtDNA replication apparatus and to CM.

Expression and function of COOH-terminal myosin heavy chain isoforms in mouse smooth muscle

Isoforms of the smooth muscle myosin motor, SM1 and SM2, differ in length at the carboxy terminal tail region. Their proportion changes with development, hormonal status and disease, but their function is unknown. We developed mice carrying the myosin heavy chain (MyHC) transgenes SM1, cMyc-tagged SM1, SM2, and V5-tagged SM2, and all transgenes corresponded to the SMa NH(2)-terminal isoform. Transgene expression was targeted to smooth muscle by the smooth muscle alpha-actin promoter. Immunoblot analysis showed substantial expression of the cMyc-tagged SM1 and V5-tagged SM2 MyHC protein in aorta and bladder and transgene mRNA was expressed in mice carrying unlabeled SM1 or SM2 transgenes. Despite significant protein expression of tagged MyHCs we found only small changes in the SM1:SM2 protein ratio. Significant changes in functional phenotype were observed in mice carrying unlabeled SM1 or SM2 transgenes. Force in aorta and bladder was increased (72 +/- 14%, 92 +/- 11%) in SM1 and decreased to 57 +/- 1% and 80 +/- 3% in SM2 transgenic mice. SM1 transgenic bladders had faster (1.8 +/- 0.3 s) and SM2 slower (7.1 +/- 0.5 s) rates of force redevelopment following a rapid step shortening. We hypothesize that small changes in the SM1:SM2 ratio could be amplified if they are associated with changes in thick filament assembly and underlie the altered contractility. These data provide evidence indicating an in vivo function for the COOH-terminal isoforms of smooth muscle myosin and suggest that the SM1:SM2 ratio is tightly regulated in smooth muscle tissues.

Blocking cardiac growth in hypertrophic cardiomyopathy induces cardiac dysfunction and decreased survival only in males

Mutations in myosin heavy chain (MyHC) can cause hypertrophic cardiomyopathy (HCM) that is characterized by hypertrophy, histopathology, contractile dysfunction, and sudden death. The signaling pathways involved in the pathology of HCM have not been elucidated, and an unresolved question is whether blocking hypertrophic growth in HCM may be maladaptive or beneficial. To address these questions, a mouse model of HCM was crossed with an antihypertrophic mouse model of constitutive activated glycogen synthase kinase-3beta (caGSK-3beta). Active GSK-3beta blocked cardiac hypertrophy in both male and female HCM mice. However, doubly transgenic males (HCM/GSK-3beta) demonstrated depressed contractile function, reduced sarcoplasmic (endo) reticulum Ca(2+)-ATPase (SERCA) expression, elevated atrial natriuretic factor (ANF) expression, and premature death. In contrast, female HCM/GSK-3beta double transgenic mice exhibited similar cardiac histology, function, and survival to their female HCM littermates. Remarkably, dietary modification from a soy-based diet to a casein-based diet significantly improved survival in HCM/GSK-3beta males. These findings indicate that activation of GSK-3beta is sufficient to limit cardiac growth in this HCM model and the consequence of caGSK-3beta was sexually dimorphic. Furthermore, these results show that blocking hypertrophy by active GSK-3beta in this HCM model is not therapeutic.

Antiretroviral nucleosides, deoxynucleotide carrier and mitochondrial DNA: evidence supporting the DNA pol gamma hypothesis

DESIGN

Nucleoside reverse transcriptase inhibitors (NRTIs) exhibit mitochondrial toxicity. The mitochondrial deoxynucleotide carrier (DNC) transports nucleotide precursors (or phosphorylated NRTIs) into mitochondria for mitochondrial (mt)DNA replication or inhibition of mtDNA replication by NRTIs. Transgenic mice (TG) expressing human DNC targeted to murine myocardium served to define mitochondrial events from NRTIs in vivo and findings were corroborated by biochemical events in vitro.

METHODS

Zidovudine (3'-azido-2',3'-deoxythymidine; ZDV), stavudine (2', 3'-didehydro-2', 3'-deoxythymidine; d4T), or lamivudine ((-)-2'-deoxy-3'-thiacytidine; 3TC) were administered individually to TGs and wild-type (WT) littermates (35 days) at human doses with drug-free vehicle as control. Left ventricle (LV) mass was defined echocardiographically, mitochondrial ultrastructural defects were identified by electron microscopy, the abundance of cardiac mtDNA was quantified by real time polymerase chain reaction, and mtDNA-encoded polypeptides were quantified.

RESULTS

Untreated TGs exhibited normal LV mass with minor mitochondrial damage. NRTI monotherapy (either d4T or ZDV) increased LV mass in TGs and caused significant mitochondrial destruction. Cardiac mtDNA was depleted in ZDV and d4T-treated TG hearts and mtDNA-encoded polypeptides decreased. Changes were absent in 3TC-treated cohorts. In supportive structural observations from molecular modeling, ZDV demonstrated close contacts with K947 and Y951 in the DNA pol gamma active site that were absent in the HIV reverse transcriptase active site.

CONCLUSIONS

NRTIs deplete mtDNA and polypeptides, cause mitochondrial structural and functional defects in vivo, follow inhibition kinetics with DNA pol gamma in vitro, and are corroborated by molecular models. Disrupted pools of nucleotide precursors and inhibition of DNA pol gamma by specific NRTIs are mechanistically important in mitochondrial toxicity.

Histidine button engineered into cardiac troponin I protects the ischemic and failing heart

The myofilament protein troponin I (TnI) has a key isoform-dependent role in the development of contractile failure during acidosis and ischemia. Here we show that cardiac performance in vitro and in vivo is enhanced when a single histidine residue present in the fetal cardiac TnI isoform is substituted into the adult cardiac TnI isoform at codon 164. The most marked effects are observed under the acute challenges of acidosis, hypoxia, ischemia and ischemia-reperfusion, in chronic heart failure in transgenic mice and in myocytes from failing human hearts. In the isolated heart, histidine-modified TnI improves systolic and diastolic function and mitigates reperfusion-associated ventricular arrhythmias. Cardiac performance is markedly enhanced in transgenic hearts during reperfusion despite a high-energy phosphate content similar to that in nontransgenic hearts, providing evidence for greater energetic economy. This pH-sensitive 'histidine button' engineered in TnI produces a titratable molecular switch that 'senses' changes in the intracellular milieu of the cardiac myocyte and responds by preferentially augmenting acute and long-term function under pathophysiological conditions. Myofilament-based inotropy may represent a therapeutic avenue to improve myocardial performance in the ischemic and failing heart.

HIV viral protein R causes atrial cardiomyocyte mitosis, mesenchymal tumor, dysrhythmia, and heart failure

HIV viral protein R (Vpr) affects the immunocyte cell cycle and circulates as free polypeptide in plasma of AIDS patients. Effects of Vpr on cardiomyocytes were explored using transgenic mice (TG) with Vpr targeted to cardiomyocytes by the alpha-myosin heavy-chain promoter. TG and WT littermate hearts were evaluated histopathologically, ultrastructurally, molecularly via RNA microarray analysis and quantitative RT-PCR, and functionally by cardiac magnetic resonance imaging (MRI) and electrocardiograms (ECG). Six hemizygous lines were created (Vpr(a,b,c,d,e,h)). Vpr RNA was expressed exclusively in myocardium and Vpr mRNA expression correlated with phenotypic changes. Vpr(b) exhibited the highest expression and mortality. TGs developed congestive heart failure ( approximately 8 weeks), abnormal cardiomyocyte nuclei and mitoses ( approximately 12 weeks), and became moribund ( approximately 20 weeks) with atrial mesenchymal tumors. MRI revealed four-chamber dilation, defective contraction, and atrial masses. Pathologically, cardiomegaly and atrial mesenchymal tumors occurred ( approximately 16-20 weeks). ECGs showed prolonged R-R, Q-T, and P-R intervals ( approximately 12 weeks). RNA encoding collagen and bone morphogenic protein 4, 6, and 7 were increased. Vpr targeted to cardiomyocytes caused defective contractility and atrial tumors. Since some Vpr cardiomyocytic effects resemble those found in terminally differentiated immunocytes, some pathogenetic mechanisms may be shared at the subcellular level.

Transforming growth factor-beta induces the expression of ANF and hypertrophic growth in cultured cardiomyoblast cells through ZAK

Transforming growth factor-beta (TGF-beta) has been associated with the onset of cardiac cell hypertrophy, but the mechanisms underlying this dissociation are not completely understood. By a previous study, we investigated the involvement of a MAP3K, ZAK, which in cultured H9c2 cardiac cells is a positive mediator of cell hypertrophy. Our results showed that expression of a dominant-negative form of ZAK inhibited the characteristic TGF-beta-induced features of cardiac hypertrophy, including increased cell size, elevated expression of atrial natriuretic factor (ANF), and increased organization of actin fibers. Furthermore, dominant-negative MKK7 effectively blocked both TGF-beta-and ZAK-induced ANF expression. In contrast, a JNK/SAPK specific inhibitor, sp600125, had little effect on TGF-beta- or ZAK-induced ANF expression. Our findings suggest that a ZAK mediates TGF-beta-induced cardiac hypertrophic growth via a novel TGF-beta signaling pathway that can be summarized as TGF-beta>ZAK>MKK7>ANF.

ZAK re-programs atrial natriuretic factor expression and induces hypertrophic growth in H9c2 cardiomyoblast cells

Various intracellular or intercellular stimuli have been associated with the development of cardiac cell hypertrophy. However, the mechanisms underlying this association are not completely understood. In a previous study we determined that ZAK mRNA expression is abundant in heart. ZAK is a mitogen-activated protein kinase kinase kinase (MAP3K) that activates the stress-activated protein kinase/c-jun N-terminal kinase pathway and activates NF-kappaB. We, therefore, investigated the potential involvement of ZAK (which in cultured H9c2 cardiomyoblast cell is a positive mediator of cell hypertrophy). Our results showed that the expression of a wild-type form of ZAK induces the characteristic hypertrophic growth features, including increased cell size, elevated atrial natriuretic factor expression, and increased actin fiber organization.

Use of the transgenic mouse in models of AIDS cardiomyopathy

Heart disease in AIDS, particularly cardiomyopathy (CM), is an increasingly recognized clinical problem with as yet undefined pathogenetic mechanisms. Among the potential etiologies of AIDS CM are HIV-1 infection of cardiac myocytes and subsequent cardiac dysfunction, opportunistic infection, inflammatory reactions, cytokine effects, and cardiotoxicity of prescribed or illicit drugs. It seems probable that multiple factors may impact on the development of CM in AIDS. Transgenic mice (TG) are useful biological tools to explore mechanisms of cardiac function and disease. In AIDS models, TG offer novel ways to elucidate mechanisms of AIDS CM through combined in vivo and in vitro studies. With targeted and non-targeted TG, structural and functional effects of specific HIV-1 gene products on heart tissue may be addressed. The impact of environmental agents including therapeutics or cardiotoxins may also be defined. To address the complexity of AIDS CM using TG, an experimental approach has been employed in our laboratories to model the clinical condition. We utilize AIDS TG with generalized expression of HIV-1 gene products in CM models with combined antiretroviral regimens to define the cardiovascular effects of AIDS and its therapy on the structure and function of the murine heart. We are developing a series of cardiac specific TG bearing selected HIV-1 genes. These TG target the selected HIV-1 genes expressed in cardiac ventricular myocytes. Tissue-specific targeting of this type enables us to define structural and functional effects of specific HIV-1 gene products on the cardiac myocyte.

Myofilament calcium sensitivity and cardiac disease: insights from troponin I isoforms and mutants

The heightened Ca2+ sensitivity of force found with hypertrophic cardiomyopathy (HCM)-associated mutant cardiac troponin I (cTnIR145G; R146G in rodents) has been postulated to be an underlying cause of hypertrophic growth and premature sudden death in humans and in animal models of the disease. Expression of slow skeletal TnI (ssTnI), a TnI isoform naturally expressed in developing heart, also increases myofilament Ca2+ sensitivity, yet its expression in transgenic mouse hearts is not associated with overt cardiac disease. Gene transfer of TnI isoforms or mutants into adult cardiac myocytes is used here to ascertain if expression levels or functional differences between HCM TnI and ssTnI could help explain these divergent organ-level effects. Results showed significantly reduced myofilament incorporation of cTnIR146G compared with ssTnI or wild-type cTnI. Despite differences in myofilament incorporation, ssTnI and cTnIR146G expression each resulted in enhanced myofilament tension in response to submaximal Ca2+ under physiological ionic conditions. Myofilament expression of an analogous HCM mutation in ssTnI (ssTnIR115G) did not further increase myofilament Ca2+ sensitivity of tension compared with ssTnI. In contrast, there was a divergent response under acidic pH conditions, a condition associated with the myocardial ischemia that often accompanies hypertrophic cardiomyopathy. The acidic pH-induced decrease in myofilament Ca2+ sensitivity was significantly greater in myocytes expressing cTnIR146G and ssTnIR115G compared with ssTnI. These results suggest that differences in pH sensitivities between wild-type ssTnI and mutant TnI proteins may be one factor in helping explain the divergent organ and organismal outcomes in TnI HCM- and ssTnI-expressing mice.

Cardiac dysfunction in hypertrophic cardiomyopathy mutant tropomyosin mice is transgene-dependent, hypertrophy-independent, and improved by beta-blockade

Familial hypertrophic cardiomyopathy (FHC) has been linked to mutations in proteins of the cardiac contractile apparatus, including alpha-tropomyosin (Tm). Mice expressing alphaTm in the heart were developed to determine the effects of FHC mutant Tm on cardiac structure and function from single cardiac myocytes to whole organ function in vivo. Expression of E180G mutant Tm did not produce cardiac hypertrophy or detectable changes in cardiac muscle morphology. However, E180G mutant Tm expression increased the Ca2+ sensitivity of force production in single cardiac myocytes in a transgene expression-dependent manner. Contractile dysfunction in single myocytes manifested organ level dysfunction, as conductance-micromanometry showed E180G Tm mice had significantly slowed relaxation (diastolic dysfunction) under physiological conditions. The diastolic dysfunction in E180G Tm mice was no longer evident during beta-blockade because propranolol eliminated the effect of E180G Tm to slow myocardial relaxation. Cellular and organ level dysfunction were evident in E180G Tm mice in the absence of significant cardiac structural abnormalities normally associated with FHC. These findings therefore suggest that diastolic dysfunction in FHC may be a direct consequence of FHC mutant protein expression. In addition, because diastolic dysfunction in E180G Tm mice is dependent on inotropic status, cardiovascular stress may play an important role in FHC pathogenesis.

Targeted myocardial transgenic expression of HIV Tat causes cardiomyopathy and mitochondrial damage

Cardiac effects of human immunodeficiency virus (HIV) transactivator (Tat) are unclear, but Tat decreases liver glutathione (an important mitochondrial antioxidant) when ubiquitously expressed in transgenic mice (TG). With an alpha-myosin heavy chain promoter, Tat was selectively targeted to murine cardiac myocytes. One high-expression hemizygous ((+/-)Tat(high); 12 copies) and two low-expression ((+/-)Tat(lowA,B); 2-5 copies) TG lines were created. Cardiomyopathy was documented with increased left ventricle (LV) mass, ventricular expression of atrial natriuretic factor (ANF) mRNA, mitochondrial ultrastructural defects, and myocardial depletion of glutathione. In (+/-)Tat(high) TGs, normalized LV mass (determined echocardiographically) increased 46% (90 days), 134% (240 days), and 96% (365 days) compared with wild-type littermates (WT). LV fractional shortening was decreased to 28% (90 days), 27% (240 days), and 19% (365 days). (+/-)Tat(low) LV mass was unchanged (<or=365 days). ANF in (+/-)Tat(high) ventricles (180 days) was twofold WT values. Glutathione was selectively decreased in (+/-)Tat(high) hearts (120 days). (+/-)Tat(high) hearts contained damaged mitochondria (>or=210 days); however, profound mitochondrial destruction occurred in homozygous (+/+)Tat(high) hearts (10 days) and the pups died (14 days). Tat caused cardiac dysfunction in this TG and may impact on cardiomyopathy in acquired immunodeficiency syndrome.

Examining the in vivo role of the amino terminus of the essential myosin light chain

The functional significance of the actin binding region at the amino terminus of the cardiac essential myosin light chain (ELC) remains obscure. Previous experiments carried out in vitro indicated that modulation of residues 5-14 could induce an inotropic effect, increasing maximal ATPase activity at submaximal Ca(2+) concentrations (Rarick, H. M., Opgenorth, T. J., von Geldern, T. W., Wu-Wong, J. R., and Solaro, R. J. (1996) J. Biol. Chem. 271, 27039-27043). Using transgenesis, we effected a cardiac-specific replacement of ELC with a protein containing a 10-amino acid deletion at positions 5-14. Both the ventricular (ELC1vDelta5-14) and atrial (ELC1aDelta5-14) isoforms lacking this peptide were stably incorporated into the sarcomere at high efficiencies. Surprisingly when the kinetics of skinned fibers isolated from the ELC1vDelta5-14 or ELC1aDelta5-14 mice were examined, no alterations in either unloaded shortening or maximum shortening velocities were apparent. Myofibrillar Mg(2+)-ATPase activity was also unchanged in these preparations. No significant changes in the fiber kinetics in the cognate compartments were observed when either deletion-containing protein replaced endogenous ELC1v or ELC1a. The data indicate that the previously postulated importance of this region in mediating critical protein interactions between the cardiac ELCs and the carboxyl-terminal residues of actin in vivo should be reassessed.

[Genetically modified animal models in cardiovascular research]

It is a basic tenet of molecular and clinical medicine that specific protein complements underlie cell and organ function. Since cellular and ultimately organ function depend upon the polypeptides that are present, it is not surprising that when function is altered changes in the protein pools occur. In the heart, numerous examples of contractile protein changes correlate with functional alterations, both during normal development and during the development of numerous pathologies. Similarly, different congenital heart diseases are characterized by certain shifts in the motor proteins. To understand these relationships, and to establish models in which the pathogenic processes can be studied longitudinally, it is necessary to direct the heart to stably synthesize, in the absence of other peliotropic changes, the candidate protein. Subsequently, one can determine if the protein's presence causes the effects directly or indirectly with the goal being to define potential therapeutic targets. By affecting the heart's protein complement in a defined manner, one has the means to establish both mechanism and the function of the different mutated proteins of protein isoforms. Gene targeting and transgenesis in the mouse provides a means to modify the mammalian genome and the cardiac motor protein complement. By directing expression of an engineered protein to the heart, one is now able to effectively remodel the cardiac protein profile and study the consequences of a single genetic manipulation at the molecular, biochemical, cytological and physiologic levels, both under normal and stress stimuli.

A genetic model provides evidence that the receptor for atrial natriuretic peptide (guanylyl cyclase-A) inhibits cardiac ventricular myocyte hypertrophy

Guanylyl cyclase-A (NPR-A; GC-A) is the major and possibly the only receptor for atrial natriuretic peptide (ANP) or B-type natriuretic peptide. Although mice deficient in GC-A display an elevated blood pressure, the resultant cardiac hypertrophy is much greater than in other mouse models of hypertension. Here we overproduce GC-A in the cardiac myocytes of wild-type or GC-A null animals. Introduction of the GC-A transgene did not alter blood pressure or heart rate as a function of genotype. Cardiac myocyte size was larger (approximately 20%) in GC-A null than in wild-type animals. However, introduction of the GC-A transgene reduced cardiac myocyte size in both wild-type and null mice. Coincident with the reduction in myocyte size, both ANP mRNA and ANP content were significantly reduced by overexpression of GC-A, and this reduction was independent of genotype. This genetic model, therefore, separates a regulation of cardiac myocyte size by blood pressure from local regulation by a GC-mediated pathway.