In vivo acceleration of heart relaxation performance by parvalbumin gene delivery

Defective cardiac muscle relaxation plays a causal role in heart failure. Shown here is the new in vivo application of parvalbumin, a calcium-binding protein that facilitates ultrafast relaxation of specialized skeletal muscles. Parvalbumin is not naturally expressed in the heart. We show that parvalbumin gene transfer to the heart in vivo produces levels of parvalbumin characteristic of fast skeletal muscles, causes a physiologically relevant acceleration of heart relaxation performance in normal hearts, and enhances relaxation performance in an animal model of slowed cardiac muscle relaxation. Parvalbumin may offer the unique potential to correct defective relaxation in energetically compromised failing hearts because the relaxation-enhancement effect of parvalbumin arises from an ATP-independent mechanism. Additionally, parvalbumin gene transfer may provide a new therapeutic approach to correct cellular disturbances in calcium signaling pathways that cause abnormal growth or damage in the heart or other organs.

Direct, convergent hypersensitivity of calcium-activated force generation produced by hypertrophic cardiomyopathy mutant alpha-tropomyosins in adult cardiac myocytes

Familial hypertrophic cardiomyopathy is a clinically and genetically diverse autosomal dominant disorder characterized by ventricular hypertrophy and myocyte disarray in the absence of known hypertrophic stimuli. It has been linked to many cardiac contractile proteins, including four point mutations in alpha-tropomyosin (Tm). Here we use adenoviral-mediated gene transfer into adult cardiac myocytes in vitro to show that all four hypertrophic cardiomyopathy alpha-Tm proteins can be expressed and incorporated into normal sarcomeric structures in cardiac myocytes at similar levels as normal alpha-Tm proteins after 5-6 days in culture. Isometric force recordings of single cardiac myocytes demonstrated inappropriate increased force output at submaximal calcium concentration with a specific mutant allele hierarchy. These data indicate that the severity of direct calcium-sensitizing effect of mutations in alpha-Tm may predict the clinical severity of mutant alpha-Tm-associated hypertrophic cardiomyopathy.

Effects of myosin heavy chain isoform switching on Ca2+-activated tension development in single adult cardiac myocytes

Cardiac myosin heavy chain (MHC) isoforms are known to play a key role in defining the dynamic contractile behavior of the heart during development. It remains unclear, however, whether cardiac MHC isoforms influence other important features of cardiac contractility, including the Ca2+ sensitivity of isometric tension development. To address this question, adult rats were treated chemically to induce the hypothyroid state and cause a transition in the ventricular cardiac MHC isoform expression pattern from predominantly the alpha-MHC isoform to exclusively the beta-MHC isoform. We found a significant desensitization in the Ca2+ sensitivity of tension development in beta-MHC-expressing ventricular myocytes (pCa50=5. 51+/-0.03, where pCa is -log[Ca2+], and pCa50 is pCa at which tension is one-half maximal) compared with that in predominantly alpha-MHC-expressing myocytes (pCa50=5.68+/-0.05). No differences between the 2 groups were observed in the steepness of the tension-pCa relationship or in the maximum isometric force generated. Instantaneous stiffness measurements were made that provide a relative measure of changes in the numbers of myosin crossbridges attached to actin during Ca2+ activation. Results showed that the relative stiffness-pCa relationship was shifted to the right in beta-MHC-expressing myocytes compared with the alpha-MHC-expressing cardiac myocytes (pCa50=5.47+/-0.05 versus 5.76+/-0.05, respectively). We conclude that MHC isoform switching in adult cardiac myocytes alters the Ca2+ sensitivity of stiffness and tension development. These results suggest that the activation properties of the thin filament are in part MHC isoform dependent in cardiac muscle, indicating an additional role for MHC isoforms in defining cardiac contractile function.

POMT1 mutation results in defective glycosylation and loss of laminin-binding activity in alpha-DG

Walker-Warburg syndrome (WWS) is a congenital muscular dystrophy associated with neuronal migration disorder and structural eye abnormalities. The mutations in the O-mannosyltransferase 1 gene (POMT1) were identified recently in 20% of patients with WWS. The authors report on a patient with WWS and a novel POMT1 mutation. Their patient expressed alpha-dystroglycan (alpha-DG) core protein, but fully glycosylated alpha-DG antibody epitopes were absent, associated with the loss of laminin-binding activity.

Thin filament protein dynamics in fully differentiated adult cardiac myocytes: toward a model of sarcomere maintenance

Sarcomere maintenance, the continual process of replacement of contractile proteins of the myofilament lattice with newly synthesized proteins, in fully differentiated contractile cells is not well understood. Adenoviral-mediated gene transfer of epitope-tagged tropomyosin (Tm) and troponin I (TnI) into adult cardiac myocytes in vitro along with confocal microscopy was used to examine the incorporation of these newly synthesized proteins into myofilaments of a fully differentiated contractile cell. The expression of epitope-tagged TnI resulted in greater replacement of the endogenous TnI than the replacement of the endogenous Tm with the expressed epitope-tagged Tm suggesting that the rates of myofilament replacement are limited by the turnover of the myofilament bound protein. Interestingly, while TnI was first detected in cardiac sarcomeres along the entire length of the thin filament, the epitope-tagged Tm preferentially replaced Tm at the pointed end of the thin filament. These results support a model for sarcomeric maintenance in fully differentiated cardiac myocytes where (a) as myofilament proteins turnover within the cell they are rapidly exchanged with newly synthesized proteins, and (b) the nature of replacement of myofilament proteins (ordered or stochastic) is protein specific, primarily affected by the structural properties of the myofilament proteins, and may have important functional consequences.

Expression of peroxisomal proliferator-activated receptors and retinoid X receptors in the kidney

The discovery that 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) is a ligand for the gamma-isoform of peroxisome proliferator-activated receptor (PPAR) suggests nuclear signaling by prostaglandins. Studies were undertaken to determine the nephron localization of PPAR isoforms and their heterodimer partners, retinoid X receptors (RXR), and to evaluate the function of this system in the kidney. PPARalpha mRNA, determined by RT-PCR, was found predominately in cortex and further localized to proximal convoluted tubule (PCT); PPARgamma was abundant in renal inner medulla, localized to inner medullary collecting duct (IMCD) and renal medullary interstitial cells (RMIC); PPARbeta, the ubiquitous form of PPAR, was abundant in all nephron segments examined. RXRalpha was localized to PCT and IMCD, whereas RXRbeta was expressed in almost all nephron segments examined. mRNA expression of acyl-CoA synthase (ACS), a known PPAR target gene, was stimulated in renal cortex of rats fed with fenofibrate, but the expression was not significantly altered in either cortex or inner medulla of rats fed with troglitazone. In cultured RMIC cells, both troglitazone and 15d-PGJ2 significantly inhibited cell proliferation and dramatically altered cell shape by induction of cell process formation. We conclude that PPAR and RXR isoforms are expressed in a nephron segment-specific manner, suggesting distinct functions, with PPARalpha being involved in energy metabolism through regulating ACS in PCT and with PPARgamma being involved in modulating RMIC growth and differentiation.

Parvalbumin gene transfer corrects diastolic dysfunction in diseased cardiac myocytes

Heart failure frequently involves diastolic dysfunction that is characterized by a prolonged relaxation. This prolonged relaxation is typically the result of a decreased rate of intracellular Ca(2+) sequestration. No effective treatment for this decreased Ca(2+) sequestration rate currently exists. As an approach to possibly correct diastolic dysfunction, we hypothesized that expression of the Ca(2+) binding protein parvalbumin in cardiac myocytes would lead to increased rates of Ca(2+) sequestration and mechanical relaxation. Parvalbumin, which is normally absent in cardiac tissue, is known to act as a soluble relaxing factor in fast skeletal muscle fibers by acting as a delayed Ca(2+) sink. As a test of the hypothesis, gene transfer was used to express parvalbumin in isolated adult cardiac myocytes. We report here that expression of parvalbumin dramatically increases the rate of Ca(2+) sequestration and the relaxation rate in normal cardiac myocytes. Importantly, parvalbumin fully restored the relaxation rate in diseased cardiac myocytes isolated from an animal model of human diastolic dysfunction. These findings indicate that parvalbumin gene transfer offers unique potential as a possible direct treatment for diastolic dysfunction in failing hearts.

A nemaline myopathy mutation in alpha-tropomyosin causes defective regulation of striated muscle force production

Nemaline myopathy (NM) is a rare autosomal dominant skeletal muscle myopathy characterized by severe muscle weakness and the subsequent appearance of nemaline rods within the muscle fibers. Recently, a missense mutation inTPM3, which encodes the slow skeletal alpha-tropomyosin (alphaTm), was linked to NM in a large kindred with an autosomal-dominant, childhood-onset form of the disease. We used adenoviral gene transfer to fully differentiated rat adult myocytes in vitro to determine the effects of NM mutant human alphaTm expression on striated muscle sarcomeric structure and contractile function. The mutant alphaTm was expressed and incorporated correctly into sarcomeres of adult muscle cells. The primary defect caused by expression of the mutant alphaTm was a decrease in the sensitivity of contraction to activating Ca(2+), which could help explain the hypotonia seen in NM. Interestingly, NM mutant alphaTm expression did not directly result in nemaline rod formation, which suggests that rod formation is secondary to contractile dysfunction and that load-dependent processes are likely involved in nemaline rod formation in vivo.

Physiological consequences of tropomyosin mutations associated with cardiac and skeletal myopathies

Mutations have been identified in alpha-tropomyosin (Tm), a key regulatory protein in striated muscle cells, that are associated with a human cardiac myopathy, hypertrophic cardiomyopathy (FHC) and a human skeletal myopathy, nemaline myopathy (NM). In this review, we highlight experiments aimed at identifying the underlying mechanisms by which mutations in alpha-Tm cause inherited diseases of cardiac and skeletal muscle. Gene transfer of normal and mutant alpha-Tm to isolated adult cardiac myocytes was used to study the primary effects of mutant alpha-Tm proteins on the structure and contractile function of fully differentiated striated muscle cells. Both FHC and NM mutant alpha-Tm proteins incorporated normally into the adult muscle sarcomere, similar to normal Tm but exerted differential "dominant-negative" effects on the contractile function of the muscle cell. FHC mutant alpha-Tm proteins produced hypersensitivity of Ca2+-activated force production with a hierarchy that was related to the clinical severity of each mutation. Conversely, the NM mutant alpha-Tm produced a hyposensitivity of Ca2+-activated force production that may underlie, at least in part, the muscle weakness observed in NM. Taken together, the results suggest that the differential changes in the ability of the mutant Tm proteins to regulate muscle contraction in response to changing Ca2+ concentrations underlie the differential clinical presentation of the cardiac and skeletal muscle myopathies associated with mutations in alpha-Tm.

Power spectral analysis of heart rate in subjects over a hundred years old

Altered autonomic regulation of cardiac function may contribute to the onset of cardiovascular disease and provide a substrate for malignant ventricular arrhythmias. This study was designed to assess cardiovascular neuroautonomic status in healthy subjects with short-term power spectral analysis of heart rate variability, including a group over 100 years of age, to identify a neuroautonomic pattern that could help to protect ultra-centenarians against cardiovascular disease. One hundred and twelve subjects (22 men and 90 women, age range 20 to 107 years) were subdivided into five age groups: <40 years (N=26, mean age 30.6+/-0.9); 41 to 60 years (N=27, mean age 51.9+/-1.2); 61 to 80 years (N=37, mean age 70.3+/-1.1); 81 to 100 (N=10, mean age 85.2+/-0.8) and older than 101 years (N=13, mean age: 103.6+/-0.6). Power spectral analysis with autoregressive algorithm provides two indexes of autonomic activity: a low-frequency component oscillating around 0.10 Hz, mainly reflecting sympathetic activity and a high-frequency component around 0.30 Hz, reflecting parasympathetic activity. Subjects 40 years of age or younger had significantly higher spectral high-frequency power values expressed in logarithmic form than the other age groups (P<0.05), the age group from 41 to 100 years had values similar to those of the other groups. However, the age group over 101 years had significantly higher values than the group from 81 to 100 years (P<0.05). Low-frequency spectral density expressed in logarithmic form and in normalized units decreased with age (P<0.0001). These data confirm an age-related decline in sympathetic activity. Compared with elderly subjects from 81 to 100 years of age ultra-centenarians have significantly higher spectral parasympathetic indexes. Parasympathetic predominance may be the neuroautonomic feature that helps to protect ultra-centenarians against cardiovascular disease.

Effects of aging on single cardiac myocyte function in Fischer 344 x Brown Norway rats

The Fischer 344 x Brown Norway (F344xBN) rat has been demonstrated to have a lower incidence of age-related pathology than other rat strains. Therefore, to elucidate the effects of aging on cardiac function, uncomplicated by compensatory effects caused by age-related pathology, cardiac myocytes were isolated from female F344xBN rats at 6 (young) and 32-33 (old) mo of age. Myocytes showed an increase in the relative amount of beta-myosin heavy chain with advanced age and a significant rightward shift in the tension-pCa curve from 5.78 +/- 0.02 pCa units in young adult myocytes to 5.66 +/- 0.03 pCa units. Consistent with a shift to a slower myosin isoform, the time from stimulation to peak sarcomere shortening increased with age from 50.5 +/- 1.3 to 58.9 +/- 1.0 ms. In contrast, no age-related difference was found in either the relengthening parameters or the Ca(2+) transient, indicating that relaxation is not directly altered by aging. This latter finding is at variance with previous studies in rat strains with higher rates of pathology. We conclude, therefore, that the primary effect of aging in isolated cardiac myocytes from the F344xBN rat model is a shift in the myosin heavy chain isoform. Changes in relaxation seen in other rat strains may result from compensatory mechanisms induced by pathological conditions.

Contractile dysfunction in hypertrophic cardiomyopathy: elucidating primary defects of mutant contractile proteins by gene transfer

Hypertrophic cardiomyopathy (HCM) is an inherited disorder of cardiac muscle that has been linked to mutations in the contractile proteins that make up the cardiac muscle sarcomere. Recent advances in cardiovascular molecular biology, including gene targeting and transgenesis in mice, and gene transfer technology to adult cardiac myocytes in primary culture, have provided new insights into how these mutations alter the structure-function of the cardiac muscle pump and the molecular mechanisms of HCM pathogenesis. In this review, we highlight the contributions of the application of gene transfer technology to adult cardiac myocytes in vitro that aim at sorting the primary effects of HCM mutant contractile proteins on the structure and function of cardiac muscle cells from the compensatory and secondary phenomenon that occur during HCM pathogenesis in vivo. The elucidation of the primary molecular mechanisms underlying the development of HCM forms a foundation by which to identify the key targets for disease treatment or prevention.

Vanadates form insoluble complexes with histones

Vanadium oxoanions are known to have a variety of physiological effects including insulin-like activity, inhibition of phosphotyrosine phosphatases, as well as direct interactions with a variety of cellular proteins, such as microtubules. In this study, vanadate was found to form insoluble complexes with histones, as well as other positively charged proteins, in a concentration dependent fashion. This interaction occurred over a 0.5-10 mM range which corresponds to the concentration range required for many of vanadate's known physiological effects. Results from precipitation experiments using vanadate solutions with or without the yellow-orange decavanadate indicated that the decamer form is primarily responsible for this precipitation. Vanadate was able to selectively precipitate histones from soluble chromatin as well as from a soluble bacterial protein extract to which a low concentration of histones had been added. Vanadate was also able to effectively precipitate histone from solutions as low as 0.006 mg/mL histone. Thus, the selective precipitation of histones and other positively charged proteins by vanadate can be utilized as a tool for protein purification. In addition, this interaction may provide insight into the mechanisms for the physiological effects of vanadate.

Histone H4 stimulates glucose uptake through the insulin receptor

Histone H4 stimulates the uptake of glucose in rat adipocytes and muscle cells. However, the mechanism of this unusual activity is not known. Therefore, we have begun to investigate the mechanism by which histone H4 stimulates the glucose uptake in rat adipocytes. We report that histone H4 requires 15-20 min to achieve its maximum effect and its time course is virtually indistinguishable from the time course of insulin itself. Reduction of the concentration of insulin receptors on the surface of adipocytes, either by trypsin digestion of the receptor, or by insulin-induced down regulation of the receptor, reduced the histone H4 effect as well as the insulin effects. Also, quercetin, a bioflavenoid that inhibits the insulin receptor tyrosine kinase activity, inhibits the actions of both histone H4 and insulin. However, histone H4 activity is somewhat more resistant to these interventions than insulin activity. In contrast to the activity of insulin, histone H4 does not appear to be able to down regulate the insulin receptor, since the pretreatment of adipocytes with histone H4 did not affect the subsequent actions of either insulin or histone H4. Finally, Scatchard analysis of the binding of 125I-insulin in the presence and absence of histone H4 increases the specific binding of insulin in a concentration dependent fashion. Histone H2b, a histone that does not have insulin-like activity, does not affect insulin binding. Taken together, these data suggest that the greatest portion of the insulin-like activity of histone H4 is initiated at the insulin receptor. However, the interaction of histone H4 and the insulin receptor is more complex than a simple binding of H4 to the insulin binding site. These studies may provide additional insight into alternate mechanisms for activation of the insulin receptor.