Response of rigor cross-bridges to stretch detected by fluorescence lifetime imaging microscopy of myosin essential light chain in skeletal muscle fibers

Phosphorylation of the regulatory light chain of myosin in striated muscle: methodological perspectives

Phosphorylation of the regulatory light chain (RLC) of myosin modulates cellular functions such as muscle contraction, mitosis, and cytokinesis. Phosphorylation defects are implicated in a number of diseases. Here we focus on striated muscle where changes in RLC phosphorylation relate to diseases such as hypertrophic cardiomyopathy and muscular dystrophy, or age-related changes. RLC phosphorylation in smooth muscle and non-muscle cells are covered briefly where relevant. There is much scientific interest in controlling the phosphorylation levels of RLC in vivo and in vitro in order to understand its physiological function in striated muscles. A summary of available and emerging in vivo and in vitro methods is presented. The physiological role of RLC phosphorylation and novel pathways are discussed to highlight the differences between muscle types and to gain insights into disease processes.

Does Interaction between the Motor and Regulatory Domains of the Myosin Head Occur during ATPase Cycle? Evidence from Thermal Unfolding Studies on Myosin Subfragment 1

Myosin head (myosin subfragment 1, S1) consists of two major structural domains, the motor (or catalytic) domain and the regulatory domain. Functioning of the myosin head as a molecular motor is believed to involve a rotation of the regulatory domain (lever arm) relative to the motor domain during the ATPase cycle. According to predictions, this rotation can be accompanied by an interaction between the motor domain and the C-terminus of the essential light chain (ELC) associated with the regulatory domain. To check this assumption, we applied differential scanning calorimetry (DSC) combined with temperature dependences of fluorescence to study changes in thermal unfolding and the domain structure of S1, which occur upon formation of the ternary complexes S1-ADP-AlF4- and S1-ADP-BeFx that mimic S1 ATPase intermediate states S1**-ADP-Pi and S1*-ATP, respectively. To identify the thermal transitions on the DSC profiles (i.e. to assign them to the structural domains of S1), we compared the DSC data with temperature-induced changes in fluorescence of either tryptophan residues, located only in the motor domain, or recombinant ELC mutants (light chain 1 isoform), which were first fluorescently labeled at different positions in their C-terminal half and then introduced into the S1 regulatory domain. We show that formation of the ternary complexes S1-ADP-AlF4- and S1-ADP-BeFx significantly stabilizes not only the motor domain, but also the regulatory domain of the S1 molecule implying interdomain interaction via ELC. This is consistent with the previously proposed concepts and also adds some new interesting details to the molecular mechanism of the myosin ATPase cycle.

Effect of a myosin regulatory light chain mutation K104E on actin-myosin interactions

Familial hypertrophic cardiomyopathy (FHC) is the most common cause of sudden cardiac death in young individuals. Molecular mechanisms underlying this disorder are largely unknown; this study aims at revealing how disruptions in actin-myosin interactions can play a role in this disorder. Cross-bridge (XB) kinetics and the degree of order were examined in contracting myofibrils from the ex vivo left ventricles of transgenic (Tg) mice expressing FHC regulatory light chain (RLC) mutation K104E. Because the degree of order and the kinetics are best studied when an individual XB makes a significant contribution to the overall signal, the number of observed XBs in an ex vivo ventricle was minimized to ∼20. Autofluorescence and photobleaching were minimized by labeling the myosin lever arm with a relatively long-lived red-emitting dye containing a chromophore system encapsulated in a cyclic macromolecule. Mutated XBs were significantly better ordered during steady-state contraction and during rigor, but the mutation had no effect on the degree of order in relaxed myofibrils. The K104E mutation increased the rate of XB binding to thin filaments and the rate of execution of the power stroke. The stopped-flow experiments revealed a significantly faster observed dissociation rate in Tg-K104E vs. Tg-wild-type (WT) myosin and a smaller second-order ATP-binding rate for the K104E compared with WT myosin. Collectively, our data indicate that the mutation-induced changes in the interaction of myosin with actin during the contraction-relaxation cycle may contribute to altered contractility and the development of FHC.

FLIM-FRET for Cancer Applications

Optical imaging assays, especially fluorescence molecular assays, are minimally invasive if not completely noninvasive, and thus an ideal technique to be applied to live specimens. These fluorescence imaging assays are a powerful tool in biomedical sciences as they allow the study of a wide range of molecular and physiological events occurring in biological systems. Furthermore, optical imaging assays bridge the gap between the in vitro cell-based analysis of subcellular processes and in vivo study of disease mechanisms in small animal models. In particular, the application of Förster resonance energy transfer (FRET) and fluorescence lifetime imaging (FLIM), well-known techniques widely used in microscopy, to the optical imaging assay toolbox, will have a significant impact in the molecular study of protein-protein interactions during cancer progression. This review article describes the application of FLIM-FRET to the field of optical imaging and addresses their various applications, both current and potential, to anti-cancer drug delivery and cancer research.

Localizing protein-protein interactions in living cells using fluorescence lifetime imaging microscopy

In the past decade, advances in fluorescence lifetime imaging have extensively applied in the life sciences, from fundamental biological investigations to advanced clinical diagnosis. Fluorescence lifetime imaging microscopy (FLIM) is now routinely used in the biological sciences to monitor dynamic signaling events inside living cells, e.g., Protein-Protein interactions. In this chapter, we describe the calibration of both time-correlated single-photon counting (TCSPC) and frequency domain (FD) FLIM systems and the acquisition and analysis of FLIM-FRET data for investigating Protein-Protein interactions in living cells.

Phosphorylation of myosin regulatory light chain has minimal effect on kinetics and distribution of orientations of cross bridges of rabbit skeletal muscle

Force production in muscle results from ATP-driven cyclic interactions of myosin with actin. A myosin cross bridge consists of a globular head domain, containing actin and ATP-binding sites, and a neck domain with the associated light chain 1 (LC1) and the regulatory light chain (RLC). The actin polymer serves as a "rail" over which myosin translates. Phosphorylation of the RLC is thought to play a significant role in the regulation of muscle relaxation by increasing the degree of skeletal cross-bridge disorder and increasing muscle ATPase activity. The effect of phosphorylation on skeletal cross-bridge kinetics and the distribution of orientations during steady-state contraction of rabbit muscle is investigated here. Because the kinetics and orientation of an assembly of cross bridges (XBs) can only be studied when an individual XB makes a significant contribution to the overall signal, the number of observed XBs was minimized to ∼20 by limiting the detection volume and concentration of fluorescent XBs. The autofluorescence and photobleaching from an ex vivo sample was reduced by choosing a dye that was excited in the red and observed in the far red. The interference from scattering was eliminated by gating the signal. These techniques decrease large uncertainties associated with determination of the effect of phosphorylation on a few molecules ex vivo with millisecond time resolution. In spite of the remaining uncertainties, we conclude that the state of phosphorylation of RLC had no effect on the rate of dissociation of cross bridges from thin filaments, on the rate of myosin head binding to thin filaments, and on the rate of power stroke. On the other hand, phosphorylation slightly increased the degree of disorder of active cross bridges.

Automatic segmentation of fluorescence lifetime microscopy images of cells using multiresolution community detection--a first study

Inspired by a multiresolution community detection based network segmentation method, we suggest an automatic method for segmenting fluorescence lifetime (FLT) imaging microscopy (FLIM) images of cells in a first pilot investigation on two selected images. The image processing problem is framed as identifying segments with respective average FLTs against the background in FLIM images. The proposed method segments a FLIM image for a given resolution of the network defined using image pixels as the nodes and similarity between the FLTs of the pixels as the edges. In the resulting segmentation, low network resolution leads to larger segments, and high network resolution leads to smaller segments. Furthermore, using the proposed method, the mean-square error in estimating the FLT segments in a FLIM image was found to consistently decrease with increasing resolution of the corresponding network. The multiresolution community detection method appeared to perform better than a popular spectral clustering-based method in performing FLIM image segmentation. At high resolution, the spectral segmentation method introduced noisy segments in its output, and it was unable to achieve a consistent decrease in mean-square error with increasing resolution.

Comparison of orientation and rotational motion of skeletal muscle cross-bridges containing phosphorylated and dephosphorylated myosin regulatory light chain

Calcium binding to thin filaments is a major element controlling active force generation in striated muscles. Recent evidence suggests that processes other than Ca(2+) binding, such as phosphorylation of myosin regulatory light chain (RLC) also controls contraction of vertebrate striated muscle (Cooke, R. (2011) Biophys. Rev. 3, 33-45). Electron paramagnetic resonance (EPR) studies using nucleotide analog spin label probes showed that dephosphorylated myosin heads are highly ordered in the relaxed fibers and have very low ATPase activity. This ordered structure of myosin cross-bridges disappears with the phosphorylation of RLC (Stewart, M. (2010) Proc. Natl. Acad. Sci. U.S.A. 107, 430-435). The slower ATPase activity in the dephosporylated moiety has been defined as a new super-relaxed state (SRX). It can be observed in both skeletal and cardiac muscle fibers (Hooijman, P., Stewart, M. A., and Cooke, R. (2011) Biophys. J. 100, 1969-1976). Given the importance of the finding that suggests a novel pathway of regulation of skeletal muscle, we aim to examine the effects of phosphorylation on cross-bridge orientation and rotational motion. We find that: (i) relaxed cross-bridges, but not active ones, are statistically better ordered in muscle where the RLC is dephosporylated compared with phosphorylated RLC; (ii) relaxed phosphorylated and dephosphorylated cross-bridges rotate equally slowly; and (iii) active phosphorylated cross-bridges rotate considerably faster than dephosphorylated ones during isometric contraction but the duty cycle remained the same, suggesting that both phosphorylated and dephosphorylated muscles develop the same isometric tension at full Ca(2+) saturation. A simple theory was developed to account for this fact.

Fluorescence lifetime imaging microscopy using near-infrared contrast agents

Although single-photon fluorescence lifetime imaging microscopy (FLIM) is widely used to image molecular processes using a wide range of excitation wavelengths, the captured emission of this technique is confined to the visible spectrum. Here, we explore the feasibility of utilizing near-infrared (NIR) fluorescent molecular probes with emission >700 nm for FLIM of live cells. The confocal microscope is equipped with a 785 nm laser diode, a red-enhanced photomultiplier tube, and a time-correlated single photon counting card. We demonstrate that our system reports the lifetime distributions of NIR fluorescent dyes, cypate and DTTCI, in cells. In cells labelled separately or jointly with these dyes, NIR FLIM successfully distinguishes their lifetimes, providing a method to sort different cell populations. In addition, lifetime distributions of cells co-incubated with these dyes allow estimate of the dyes' relative concentrations in complex cellular microenvironments. With the heightened interest in fluorescence lifetime-based small animal imaging using NIR fluorophores, this technique further serves as a bridge between in vitro spectroscopic characterization of new fluorophore lifetimes and in vivo tissue imaging.

Mutations of ventricular essential myosin light chain disturb myosin binding and sarcomeric sorting

AIMS

We tested the hypothesis that mutations in the human ventricular essential myosin light chain (hVLC-1) that are associated with hypertrophic cardiomyopathy (HCM) affect protein structure, binding to the IQ1 motif of cardiac myosin heavy chain (MYH) and sarcomeric sorting in neonatal cardiomyocytes.

METHODS AND RESULTS

We employed circular dichroism and surface plasmon resonance spectroscopy to investigate structural properties and protein-protein interactions of a recombinant head-rod fragment of rat cardiac β-MYH (amino acids 664-915) with alanine-mutated IQ2 domain (rβ-MYH(664-915)IQ2(ala4)) and normal or five mutated (M149V, E143K, A57G, E56G, R154H) hVLC-1 forms. Double epitope-tagging competition was used to monitor the intracellular localization of exogenously introduced normal and E56G-mutated (hVLC-1(E56G)) hVLC-1 constructs in neonatal rat cardiomyocytes. Fluorescence lifetime imaging microscopy was applied to map the microenvironment of normal and E56G-mutated hVLC-1 in permeabilized muscle fibres. Affinity of M149V, E143K, A57G, and R154H mutated hVLC-1/rβ-MYH(664-915)IQ2(ala4) complexes was significantly lower compared with the normal hVLC-1/rβ-MYH(664-915)IQ2(ala4) complex interaction. In particular, the E56G mutation induced an ∼30-fold lower MYH affinity. Sorting specificity of E56G-mutated hVLC-1 was negligible compared with normal hVLC-1. Fluorescence lifetime of fibres replaced with hVLC-1(E56G) increased significantly compared with hVLC-1-replaced fibres.

CONCLUSION

Disturbed myosin binding of mutated hVLC-1 may provide a pathomechanism for the development of HCM.