Biomedical subjects

Maria L Valencik

Publications and source records attributed to Maria L Valencik.

6 recordsLinked to original sources

PubMed2006

Integrin activation in the heart: a link between electrical and contractile dysfunction?

Integrins mechanically link the cytoskeleton to the extracellular matrix in cardiac myocytes and are thereby involved in mechanotransduction. Integrins appear to be necessary for cardiac myocyte hypertrophy. To determine the effect of increased integrin ligation and signaling on adult cardiac function, a heart-specific truncated alpha(5) integrin (gain of function) was conditionally expressed in mice. Four days later, we observed an 80% reduction in amplitude of the QRS complex, profound systolic dysfunction, decreased connexin43, loss of gap junctions, and abnormal intercalated discs. Surprisingly, isolated left ventricular myocytes contracted normally and exhibited normal Ca(2+) transients. This suggested that cell/cell electrical and/or mechanical coupling was disrupted. To distinguish electrical from mechanical coupling deficits, we compared the papillary muscle force generated by electrically stimulated versus rapid cooling contractions in which intracellular Ca(2+) is released without electrical depolarization. Both were decreased in the transgenic muscle. However, electrically stimulated contractions were more significantly reduced than rapid cooling contractures. This suggests a component of cell/cell electrical uncoupling. Optical mapping revealed a loss of the normal elliptical isochronal activation pattern implying a loss of preferential conduction through gap junctions. For the first time, we have shown that integrins can regulate both mechanical and electrical coupling in the adult heart, even in the absence of primary hemodynamic alterations. Furthermore, we demonstrated that unregulated integrin activation leads to both contractile dysfunction and arrhythmias.

Animals↗

PubMed2005

Cardiac-specific attenuation of natriuretic peptide A receptor activity accentuates adverse cardiac remodeling and mortality in response to pressure overload.

Atrial (ANP) and brain (BNP) natriuretic peptides are hormones of myocardial cell origin. These hormones bind to the natriuretic peptide A receptor (NPRA) throughout the body, stimulating cGMP production and playing a key role in blood pressure control. Because NPRA receptors are present on cardiomyocytes, we hypothesized that natriuretic peptides may have direct autocrine or paracrine effects on cardiomyocytes or adjacent cardiac cells. Because both natriuretic peptides and NPRA gene expression are upregulated in states of pressure overload, we speculated that the effects of the natriuretic peptides on cardiac structure and function would be most apparent after pressure overload. To attenuate cardiomyocyte NPRA activity, transgenic mice with cardiac specific expression of a dominant-negative (DN-NPRA) mutation (HCAT D 893A) in the NPRA receptor were created. Cardiac structure and function were assessed (avertin anesthesia) in the absence and presence of pressure overload produced by suprarenal aortic banding. In the absence of pressure overload, basal and BNP-stimulated guanylyl cyclase activity assessed in cardiac membrane fractions was reduced. However, systolic blood pressure, myocardial cGMP, log plasma ANP levels, and ventricular structure and function were similar in wild-type (WT-NPRA) and DN-NPRA mice. In the presence of pressure overload, myocardial cGMP levels were reduced, and ventricular hypertrophy, fibrosis, filling pressures, and mortality were increased in DN-NPRA compared with WT-NPRA mice. In addition to their hormonal effects, endogenous natriuretic peptides exert physiologically relevant autocrine and paracrine effects via cardiomyocyte NPRA receptors to modulate cardiac hypertrophy and fibrosis in response to pressure overload.

Animals↗

PubMed2005

Effect of fiber orientation on propagation: electrical mapping of genetically altered mouse hearts.

BACKGROUND: Epicardial potentials reveal the strong effects of fiber anisotropy, rotation, imbrication, and coupling on propagation in the intact heart. From the patterns of the surface potentials, we can obtain information about the local fiber orientation, anisotropy, the transmural fiber rotation, and which direction the wave front is traveling through the wall. In this study, lessons learned from epicardial potential mapping of large hearts were applied to studies conducted in genetically altered mouse hearts. METHODS: An inducible model of the overexpression of a gain-of-function alpha5 integrin (cytoplasmic domain truncation) was created in mouse. After 3 days of administration of doxycycline, the animals exhibited an altered electrical phenotype of markedly reduced amplitude of the QRS complex on the surface electrocardiogram. Epicardial potentials were recorded from Langendorff-perfused mouse hearts with alpha5 integrin gain-of-function mutations and from wild-type (WT) control hearts. A cylindrical electrode array consisting of 184 sites with 1-mm uniform interelectrode spacing was placed around the heart, and unipolar electrograms were recorded during atrial and ventricular stimulation at different basic cycle lengths. RESULTS: The total ventricular activation time for the transgenic animals was greater than that of the WT hearts for atrial and ventricular pacing locations. The isopotential maps from the mutated hearts showed a loss of anisotropy, as revealed by the more rounded and less elliptically shaped wave fronts seen immediately after epicardial point stimulation when compared with WT hearts. The weaker potential maxima in the mutated hearts did not exhibit the normal expansion and rotation associated with an advancing wave front in a normal heart, suggesting abnormalities in myocyte coupling in these hearts. Isopotential maps provided additional information about fiber architecture from the electric field that was not obtained from optical recordings alone. These findings provided a phenotypic characterization and specific insights into the mechanisms of the electrical abnormalities associated with altered integrin signaling in cardiac myocytes.

Action Potentials↗

PubMed2004

Doxycycline inducible expression of SERCA2a improves calcium handling and reverts cardiac dysfunction in pressure overload-induced cardiac hypertrophy.

Delayed cardiac relaxation in failing hearts has been attributed to reduced activity and/or expression of sarco(endo)plasmic reticulum Ca2+-ATPase 2a (SERCA2a). Although constitutive overexpression of SERCA2a has proven effective in preventing cardiac dysfunction, it is unclear whether increasing SERCA2a expression in hearts with preexisting hypertrophy will be therapeutic. To test this hypothesis, we generated a binary transgenic (BTG) system that allows tetracycline-inducible, cardiac-specific SERCA2a expression. In this system (tet-on SERCA2a), a FLAG-tagged SERCA2a transgene is expressed in the presence of doxycycline (Dox) but not in the absence of Dox (2.3-fold more mRNA, 45% more SERCA2a protein). Calcium transients measured in isolated cardiac myocytes from nonbanded Dox-treated BTG mice showed an accelerated calcium decline and an increased systolic Ca2+ peak. Sarcoplasmic reticulum (SR) calcium loading was increased by 45% in BTG mice. In the presence of pressure overload (aortic banding), echocardiographic analysis revealed that expression of SERCA2a-FLAG caused an improvement in fractional shortening. SERCA2a-FLAG expression alleviated the resultant cardiac dysfunction. This was illustrated by an increase in the rate of decline of the calcium transient. Cell shortening and SR calcium loading were also improved in cardiac myocytes isolated from banded BTG mice after SERCA2a overexpression. In conclusion, we generated a novel transgenic mouse that conditionally overexpresses SERCA2a. This model is suitable for both long- and short-term studies of the effects of controlled SERCA2a expression on cardiac function. In addition, inducible overexpression of SERCA2a improved cardiac function and calcium handling in mice with established contractile dysfunction.

Animals↗

PubMed2004

Cardiac-specific induction of the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha promotes mitochondrial biogenesis and reversible cardiomyopathy in a developmental stage-dependent manner.

Recent evidence has identified the peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) as a regulator of cardiac energy metabolism and mitochondrial biogenesis. We describe the development of a transgenic system that permits inducible, cardiac-specific overexpression of PGC-1alpha. Expression of the PGC-1alpha transgene in this system (tet-on PGC-1alpha) is cardiac-specific in the presence of doxycycline (dox) and is not leaky in the absence of dox. Overexpression of PGC-1alpha in tet-on PGC-1alpha mice during the neonatal stages leads to a dramatic increase in cardiac mitochondrial number and size coincident with upregulation of gene markers associated with mitochondrial biogenesis. In contrast, overexpression of PGC-1alpha in the hearts of adult mice leads to a modest increase in mitochondrial number, derangements of mitochondrial ultrastructure, and development of cardiomyopathy. The cardiomyopathy in adult tet-on PGC-1alpha mice is characterized by an increase in ventricular mass and chamber dilatation. Surprisingly, removal of dox and cessation of PGC-1alpha overexpression in adult mice results in complete reversal of cardiac dysfunction within 4 weeks. These results indicate that PGC-1alpha drives mitochondrial biogenesis in a developmental stage-dependent manner permissive during the neonatal period. This unique murine model should prove useful for the study of the molecular regulatory programs governing mitochondrial biogenesis and characterization of the relationship between mitochondrial dysfunction and cardiomyopathy and as a general model of inducible, reversible cardiomyopathy.

Adenosine Triphosphate↗

PubMed2002

A lethal perinatal cardiac phenotype resulting from altered integrin function in cardiomyocytes.

BACKGROUND: Integrins are heterodimeric receptors that couple the extracellular matrix to intracellular signaling pathways and the cyoskeleton. Integrins are strain transducers and candidates for modulators or effectors of cardiac hypertrophy. METHODS: To begin to probe this function, we have transgenically expressed a chimeric protein that alters integrin function in cardiomyocytes. The transgene (Tac-beta(1D)) consists of the biologically inert extracellular and transmembrane domain of the interleukin-2 receptor alpha subunit (Tac) fused to the cytoplasmic tail of the human beta(1D) integrin driven by the cardiac alpha-myosin heavy chain promoter. Transgene expression results in a severe, usually fatal, perinatal cardiac phenotype, characterized by initial electrocardiographic abnormalities followed by extensive myocyte loss, macrophage infiltration, and replacement fibrosis. RESULTS: Expression of Tac-beta(1D) resulted in displacement of endogenous beta(1D) integrin from Z-lines and T-tubules, decreased expression of endogenous beta(1D), and disrupted the fibronectin pericellular matrix. These results are consistent with an essential role for beta(1) integrins in maintenance of cardiomyocyte viability and interaction with extracellular matrix. CONCLUSION: The appearance of conduction abnormalities before morphologic changes suggests that integrins are important in the development or maintenance of the conducting system of the heart.

Animals↗