Measurements of the cross-bridge attachment/detachment process within intact sarcomeres by surface plasmon resonance

Synchronous force and Ca2+ measurements for repeated characterization of excitation-contraction coupling in human myocardium

Dysfunctional Ca2+ signaling affects the myocardial systole and diastole, may trigger arrhythmia and cause transcriptomic and proteomic modifications in heart failure. Thus, synchronous real-time measurement of Ca2+ and force is essential to investigate the relationship between contractility and Ca2+ signaling and the alteration of excitation-contraction coupling (ECC) in human failing myocardium. Here, we present a method for synchronized acquisition of intracellular Ca2+ and contraction force in long-term cultivated slices of human failing myocardium. Synchronous time series of contraction force and intracellular Ca2+ were used to calculate force-calcium loops and to analyze the dynamic alterations of ECC in response to various pacing frequencies, post-pause potentiation, high mechanical preload and pharmacological interventions in human failing myocardium. We provide an approach to simultaneously and repeatedly investigate alterations of contractility and Ca2+ signals in long-term cultured myocardium, which will allow detecting the effects of electrophysiological or pharmacological interventions on human myocardial ECC.

Phosphoregulation of Cardiac Inotropy via Myosin Binding Protein-C During Increased Pacing Frequency or β1-Adrenergic Stimulation

BACKGROUND

Mammalian hearts exhibit positive inotropic responses to β-adrenergic stimulation as a consequence of protein kinase A-mediated phosphorylation or as a result of increased beat frequency (the Bowditch effect). Several membrane and myofibrillar proteins are phosphorylated under these conditions, but the relative contributions of these to increased contractility are not known. Phosphorylation of cardiac myosin-binding protein-C (cMyBP-C) by protein kinase A accelerates the kinetics of force development in permeabilized heart muscle, but its role in vivo is unknown. Such understanding is important because adrenergic responsiveness of the heart and the Bowditch effect are both depressed in heart failure.

METHODS AND RESULTS

The roles of cMyBP-C phosphorylation were studied using mice in which either WT or nonphosphorylatable forms of cMyBP-C [ser273ala, ser282ala, ser302ala: cMyBP-C(t3SA)] were expressed at similar levels on a cMyBP-C null background. Force and [Ca(2+)]in measurements in isolated papillary muscles showed that the increased force and twitch kinetics because increased pacing or β1-adrenergic stimulation were nearly absent in cMyBP-C(t3SA) myocardium, even though [Ca(2+)]in transients under each condition were similar to WT. Biochemical measurements confirmed that protein kinase A phosphorylated ser273, ser282, and ser302 in WT cMyBP-C. In contrast, CaMKIIδ, which is activated by increased pacing, phosphorylated ser302 principally, ser282 to a lesser degree, and ser273 not at all.

CONCLUSIONS

Phosphorylation of cMyBP-C increases the force and kinetics of twitches in living cardiac muscle. Further, cMyBP-C is a principal mediator of increased contractility observed with β-adrenergic stimulation or increased pacing because of protein kinase A and CaMKIIδ phosphorylations of cMyB-C.

Cardiomyocyte contractile status is associated with differences in fibronectin and integrin interactions

Integrins link the extracellular matrix (ECM) with the intracellular cytoskeleton and other cell adhesion-associated signaling proteins to function as mechanotransducers. However, direct quantitative measurements of the cardiomyocyte mechanical state and its relationship to the interactions between specific ECM proteins and integrins are lacking. The purpose of this study was to characterize the interactions between the ECM protein fibronectin (FN) and integrins in cardiomyocytes and to test the hypothesis that these interactions would vary during contraction and relaxation states in cardiomyocytes. Using atomic force microscopy, we quantified the unbinding force (adhesion force) and adhesion probability between integrins and FN and correlated these measurements with the contractile state as indexed by cell stiffness on freshly isolated mouse cardiomyocytes. Experiments were performed in normal physiological (control), high-K(+) (tonically contracted), or low-Ca(2+) (fully relaxed) solutions. Under control conditions, the initial peak of adhesion force between FN and myocyte alpha(3)beta(1)- and/or alpha(5)beta(1)-integrins was 39.6 +/- 1.3 pN. The binding specificity between FN and alpha(3)beta(1)- and alpha(5)beta(1)-integrins was verified by using monoclonal antibodies against alpha(3)-, alpha(5)-, alpha(3) + alpha(5)-, or beta(1)-integrin subunits, which inhibited binding by 48%, 65%, 70%, or 75%, respectively. Cytochalasin D or 2,3-butanedione monoxime (BDM), to disrupt the actin cytoskeleton or block myofilament function, respectively, significantly decreased the cell stiffness; however, the adhesion force and binding probability were not altered. Tonic contraction with high-K(+) solution increased total cell adhesion (1.2-fold) and cell stiffness (27.5-fold) compared with fully relaxed cells with low-Ca(2+) solution. However, it could be partially prevented by high-K(+) bath solution containing BDM, which suppresses contraction by inhibiting the actin-myosin interactions. Thus, our results demonstrate that integrin binding to FN is modulated by the contractile state of cardiac myocytes.

Surface Plasmon Resonance Monitoring of Cell Monolayer Integrity: Implication of Signaling Pathways Involved in Actin-Driven Morphological Remodeling

Morphological changes occurring in individual cells largely influence the physiological functions of various cell layers. The control of barrier function of epithelia and endothelia is a prime example of processes highly dependent on cellular morphology and cell layer integrity. Here, we applied the surface plasmon resonance (SPR) technique to the quantification of cellular activity of an epithelial cell monolayer stimulated by angiotensin II. The analysis of the SPR signal shows reproducible concentration-dependent biphasic responses after cell activation with angiotensin II. Phase-contrast and confocal microscopy imaging was performed to link the SPR signal to molecular and global morphological remodeling. The SPR signal was observed to be in relation with the rapid cell contraction and the subsequent cell spreading observed by phase-contrast microscopy. Additionally, the temporal redistribution of actin, observed by confocal microscopy after angiotensin II stimulation, was also found to be consistent with the SPR signal variation. The modulation of signaling pathways involved in actin-myosin driven cell contraction confirms the direct implication of actin structures in the SPR response. Additionally, we show that the intracellular calcium mobilization associated with angiotensin II stimulation did not produce any significant SPR signal variation. Altogether, our results demonstrate that SPR is a rapid label-free method to study cellular activity and molecular mechanisms implicated in the modulation of the integrity of a cell monolayer in relation to cytoskeleton remodeling with associated cell morphological changes.

Acceleration of crossbridge kinetics by protein kinase A phosphorylation of cardiac myosin binding protein C modulates cardiac function

Normal cardiac function requires dynamic modulation of contraction. beta1-adrenergic-induced protein kinase (PK)A phosphorylation of cardiac myosin binding protein (cMyBP)-C may regulate crossbridge kinetics to modulate contraction. We tested this idea with mechanical measurements and echocardiography in a mouse model lacking 3 PKA sites on cMyBP-C, ie, cMyBP-C(t3SA). We developed the model by transgenic expression of mutant cMyBP-C with Ser-to-Ala mutations on the cMyBP-C knockout background. Western blots, immunofluorescence, and in vitro phosphorylation combined to show that non-PKA-phosphorylatable cMyBP-C expressed at 74% compared to normal wild-type (WT) and was correctly positioned in the sarcomeres. Similar expression of WT cMyBP-C at 72% served as control, ie, cMyBP-C(tWT). Skinned myocardium responded to stretch with an immediate increase in force, followed by a transient relaxation of force and finally a delayed development of force, ie, stretch activation. The rate constants of relaxation, k(rel) (s-1), and delayed force development, k(df) (s-1), in the stretch activation response are indicators of crossbridge cycling kinetics. cMyBP-C(t3SA) myocardium had baseline k(rel) and k(df) similar to WT myocardium, but, unlike WT, k(rel) and k(df) were not accelerated by PKA treatment. Reduced dobutamine augmentation of systolic function in cMyBP-C(t3SA) hearts during echocardiography corroborated the stretch activation findings. Furthermore, cMyBP-C(t3SA) hearts exhibited basal echocardiographic findings of systolic dysfunction, diastolic dysfunction, and hypertrophy. Conversely, cMyBP-C(tWT) hearts performed similar to WT. Thus, PKA phosphorylation of cMyBP-C accelerates crossbridge kinetics and loss of this regulation leads to cardiac dysfunction.

Roles of phosphorylation of myosin binding protein-C and troponin I in mouse cardiac muscle twitch dynamics

A normal heart increases its contractile force with increasing heart rate. Although calcium handling and myofibrillar proteins have been implicated in maintaining this positive force-frequency relationship (FFR), the exact mechanisms by which it occurs have not been addressed. In this study, we have developed an analytical method to define the calcium-force loop data, which characterizes the function of the contractile proteins in response to calcium that is independent of the calcium handling proteins. Results demonstrate that increasing the stimulation frequency causes increased force production per unit calcium concentration and decreased frequency-dependent calcium sensitivity during the relaxation phase. We hypothesize that phosphorylation of myosin binding protein-C (MyBP-C) and troponin I (TnI) acts coordinately to change the rates of force generation and relaxation, respectively. To test this hypothesis, we performed simultaneous calcium and force measurements on stimulated intact mouse papillary bundles before and after inhibition of MyBP-C and TnI phosphorylation using the calcium/calmodulin kinase II (CaMK2) inhibitor autocamtide-2 related inhibitory peptide, or the protein kinase A (PKA) inhibitor 14-22 amide. CaMK2 inhibition reduced both MyBP-C and TnI phosphorylation and decreased active force without changing the magnitude of the [Ca(2+)](i) transient. This reduced the normalized change in force per change in calcium by 19-39%. Data analyses demonstrated that CaMK2 inhibition changed the myofilament characteristics via a crossbridge feedback mechanism. These results strongly suggest that the phosphorylation of MyBP-C and TnI contributes significantly to the rates of force development and relaxation.

Effect of varying electric potential on surface-plasmon resonance sensing

The high sensitivity of surface-plasmon resonance (SPR) sensors allows measurements of small variations in surface potentials to be made. We studied the changes of the SPR angle when an oscillating electric potential was applied to a gold film on which surface plasmons were excited. The shifts of the SPR resonance angle were observed for various aqueous solutions as an adjacent medium. A model that takes into account the redistribution of charges at the double layer near the metal-liquid interface as well as the oxidation of the gold film was developed. It was found that a change in the electronic density at voltages below the oxidation potential and, in addition, the oxidation of the gold surface above this potential are the main mechanisms that account for the observed dependences. It was shown that relatively slow oxidation-reduction processes can explain the observed hysteresis effect. Application of these techniques to studies of dielectric properties and conformational changes of polar biomolecules, such as tubulin, are discussed.

Survey of the year 2001 commercial optical biosensor literature

We have assembled references of 700 articles published in 2001 that describe work performed using commercially available optical biosensors. To illustrate the technology's diversity, the citation list is divided into reviews, methods and specific applications, as well as instrument type. We noted marked improvements in the utilization of biosensors and the presentation of kinetic data over previous years. These advances reflect a maturing of the technology, which has become a standard method for characterizing biomolecular interactions.