Changes in the crystallographic structures of cardiac myosin filaments detected by polarization-dependent second harmonic generation microscopy

Estimation of crossbridge-state during cardiomyocyte beating using second harmonic generation

Estimation of dynamic change of crossbridge formation in living cardiomyocytes is expected to provide crucial information for elucidating cardiomyopathy mechanisms, efficacy of an intervention, and others. Here, we established an assay system to dynamically measure second harmonic generation (SHG) anisotropy derived from myosin filaments depended on their crossbridge status in pulsating cardiomyocytes. Experiments utilizing an inheritable mutation that induces excessive myosin-actin interactions revealed that the correlation between sarcomere length and SHG anisotropy represents crossbridge formation ratio during pulsation. Furthermore, the present method found that ultraviolet irradiation induced an increased population of attached crossbridges that lost the force-generating ability upon myocardial differentiation. Taking an advantage of infrared two-photon excitation in SHG microscopy, myocardial dysfunction could be intravitally evaluated in a Drosophila disease model. Thus, we successfully demonstrated the applicability and effectiveness of the present method to evaluate the actomyosin activity of a drug or genetic defect on cardiomyocytes. Because genomic inspection alone may not catch the risk of cardiomyopathy in some cases, our study demonstrated herein would be of help in the risk assessment of future heart failure.

Monitoring aging-associated structural alterations in Caenorhabditis elegans striated muscles via polarization-dependent second-harmonic generation measurements

The in-vivo elucidation of the molecular mechanisms underlying muscles dysfunction due to aging via non-invasive label free imaging techniques is an important issue with high biological significance. In this study, polarization-dependent second-harmonic generation (PSHG) was used to evaluate structural alterations in the striated muscles during Caenorhabditis elegans lifespan. Young and old wild-type animals were irradiated. The obtained results showed that it was not feasible to detect differences in the structure of myosin that could be correlated with the aging of the worms, via the implementation of the classical, widely used, cylindrical symmetry model and the calculation of the SHG anisotropy values. A trigonal symmetry model improved the extracted results; however, the best outcome was originated from the application of a general model. Myosin structural modifications were monitored via the estimation of the difference in spectral phases derived from discrete Fourier transform analysis. Age classification of the nematodes was achieved by employing both models, proving the usefulness of the usage of PSHG microscopy as a potential diagnostic tool for the investigation of muscle diseases.

X-ray diffraction and second harmonic imaging reveal new insights into structural alterations caused by pressure-overload in murine hearts

We demonstrate a label-free imaging approach to study cardiac remodeling of fibrotic and hypertrophic hearts, bridging scales from the whole organ down to the molecular level. To this end, we have used mice subjected to transverse aortic constriction and imaged adjacent cardiac tissue sections by microfocus X-ray diffraction and second harmonic generation (SHG) imaging. In this way, the acto-myosin structure was probed in a spatially resolved manner for entire heart sections. From the recorded diffraction data, spatial maps of diffraction intensity, anisotropy and orientation were obtained, and fully automated analysis depicted the acto-myosin filament spacing and direction. X-ray diffraction presented an overview of entire heart sections and revealed that in regions of severe cardiac remodeling the muscle mass is partly replaced by connective tissue and the acto-myosin lattice spacing is increased at these regions. SHG imaging revealed sub-cellular structure of cardiac tissue and complemented the findings from X-ray diffraction by revealing micro-level distortion of myofibrils, immune cell infiltration at regions of cardiac remodeling and the development of fibrosis down to the scale of a single collagen fibril. Overall, our results show that both X-ray diffraction and SHG imaging can be used for label-free and high-resolution visualization of cardiac remodeling and fibrosis progression at different stages in a cardiac pressure-overload mouse model that cannot be achieved by conventional histology.

Internal structure and remodeling in dystrophin-deficient cardiomyocytes using second harmonic generation

Duchenne muscular dystrophy (DMD) is a debilitating disorder related to dystrophin encoding gene mutations, often associated with dilated cardiomyopathy. However, it is still unclear how dystrophin deficiency affects cardiac sarcomere remodeling and contractile dysfunction. We employed second harmonic generation (SHG) microscopy, a nonlinear optical imaging technique that allows studying contractile apparatus organization without histologic fixation and immunostaining. Images were acquired on alive DMD (mdx) and wild type cardiomyocytes at different ages and at various external calcium concentrations. An automated image processing was developed to identify individual myofibrils and extract data about their organization. We observed a structural aging-dependent remodeling in mdx cardiomyocytes affecting sarcomere sinuosity, orientation and length that could not be anticipated from standard optical imaging. These results revealed for the first time the interest of SHG to evaluate the intracellular and sarcomeric remodeling of DMD cardiac tissue in an age-dependent manner that could participate in progressive contractile dysfunction.

Dual- and single-shot susceptibility ratio measurements with circular polarizations in second-harmonic generation microscopy

Polarization-resolved second-harmonic generation (P-SHG) microscopy is a technique capable of characterizing nonlinear optical properties of noncentrosymmetric biomaterials by extracting the nonlinear susceptibility tensor components ratio χzzz2'/χzxx2' , with z-axis parallel and x-axis perpendicular to the C₆ symmetry axis of molecular fiber, such as a myofibril or a collagen fiber. In this paper, we present two P-SHG techniques based on incoming and outgoing circular polarization states for a fast extraction of χzzz2'/χzxx2' : A dual-shot configuration where the SHG circular anisotropy generated using incident right- and left-handed circularly-polarized light is measured; and a single-shot configuration for which the SHG circular anisotropy is measured using only one incident circular polarization state. These techniques are used to extract the χzzz2'/χzxx2' of myosin fibrils in the body wall muscles of Drosophila melanogaster larva. The results are in good agreement with values obtained from the double Stokes-Mueller polarimetry. The dual- and single-shot circular anisotropy measurements can be used for fast imaging that is independent of the in-plane orientation of the sample. They can be used for imaging of contracting muscles, or for high throughput imaging of large sample areas.