Kinetics of a single cross-bridge in familial hypertrophic cardiomyopathy heart muscle measured by reverse Kretschmann fluorescence

Plasmon Coupling Enhanced Micro-Spectroscopy and Imaging for Sensitive Discrimination of Membrane Domains of a Single Cell

Different cell membrane domains play different roles in many cell processes, and the discrimination of these domains is of considerable importance for the elucidation of cellular functions. However, the strategies available for distinguishing these cell membrane domains are limited. A novel technique called plasmon coupling enhanced micro-spectroscopy and imaging to discriminate basal and lateral membrane domains of a single cell combines the application of an additional plasmonic silver film for surface plasmon (SP) excitation to selectively excite and enhance the basal membranes in the near-field with directional enhanced microscopic imaging and spectroscopy. The SP and critical evanescent fields are induced upon excitation through a silver-coated semitransparent coverslip at the surface plasmon resonance and critical angles, respectively. The basal and lateral membrane domains located within the SP and critical evanescent fields can be selectively excited and distinguished by adjusting the incident angle of laser irradiation. Moreover, the brighter images and more intense spectra of membrane-targeting fluorescence-Raman probes under directional excitation than in conventional EPI mode allow clear identification of the membrane domains.

Surface plasmon-coupled emission imaging for biological applications

Fluorescence imaging technology has been extensively applied in chemical and biological research profiting from its high sensitivity and specificity. Much attention has been devoted to breaking the light diffraction-limited spatial resolution. However, it remains a great challenge to improve the axial resolution in a way that is accessible in general laboratories. Surface plasmon-coupled emission (SPCE), generated by the interactions between surface plasmons and excited fluorophores in close vicinity of the thin metal film, offers an opportunity for optical imaging with potential application in analysis of molecular and biological systems. Benefiting from the highly directional and distance-dependent properties, SPCE imaging (SPCEi) has displayed excellent performance in bioimaging with improved sensitivity and axial confinement. Herein, we give a brief overview of the development of SPCEi. We describe the unique optical characteristics and constructions of SPCEi systems and highlight recent advances in the use of SPCEi for biological applications. We hope this review provides readers with both the insights and future prospects of SPCEi as a new promising imaging platform for potentially widespread applications in biological research and medical diagnostics. Graphical abstract.

Live-cell fluorescence imaging with extreme background suppression by plasmonic nanocoatings

Fluorescence microscopy allows specific and selective imaging of biological samples. Unfortunately, unspecific background due to auto-fluorescence, scattering, and non-ideal labeling efficiency often adversely affect imaging. Surface plasmon-coupled emission (SPCE) is known to selectively mediate fluorescence that spatially originates from regions close to the metal interface. However, SPCE combined with fluorescence imaging has not been widely successful so far, most likely due to its limited photon yield, which makes it tedious to identify the exact window of the application. As the strength of SPCE based imaging is its unique sectioning capabilities. We decided to identify its clear beneficial operational regime for biological settings by interrogating samples in the presence of ascending background levels. For fluorescent beads as well as live-cell imaging as examples, we show how to extend the imaging performance in extremely high photon background environments. In a common setup using plasmonic gold-coated coverslips using an objective-based total internal reflection fluorescence microscope (TIRF-M), we theoretically and experimentally characterize our fluoplasmonics (f-Pics) approach by providing general user guidance in choosing f-Pics over TIRF-M or classical wide-field (WF).

High performance dual-mode surface plasmon coupled emission imaging apparatus integrating Kretschmann and reverse Kretschmann configurations for flexible measurements

A Kretschmann (KR) and reverse Kretschmann (RK) dual-mode surface plasmon coupled emission (SPCE) imaging apparatus based on prism coupling was built up. Highly directional and polarized fluorescence images for both RK and KR configurations were obtained. Besides, surface plasmon field-enhanced fluorescence and free space imaging can also be measured conveniently from this apparatus. Combining the high sensitivity of KR mode and the simplicity of RK mode, the multifunctional imaging system is flexible to provide different configurations for imaging applications. Compared to the free space imaging, SPCE imaging provides enhanced fluorescence, especially large enhancement up to about 50 fold in KR configuration. Additionally, the degree of evanescent field enhancement effect was easily estimated experimentally using the apparatus to compare the different imaging configurations. We believed that the dual-mode SPCE imaging apparatus will be useful in fundamental study of plasmon-controlled fluorescence and be a powerful tool for optical imaging, especially for microarray and biological applications.

Mesoscopic analysis of motion and conformation of cross-bridges

The orientation of a cross-bridge is widely used as a parameter in determining the state of muscle. The conventional measurements of orientation, such as that made by wide-field fluorescence microscopy, electron paramagnetic resonance (EPR) or X-ray diffraction or scattering, report the average orientation of 10¹²-10⁹ myosin cross-bridges. Under conditions where all the cross-bridges are immobile and assume the same orientation, for example in normal skeletal muscle in rigor, it is possible to determine the average orientation from such global measurements. But in actively contracting muscle, where a parameter indicating orientation fluctuates in time, the measurements of the average value provide no information about cross-bridge kinetics. To avoid problems associated with averaging information from trillions of cross-bridges, it is necessary to decrease the number of observed cross-bridges to a mesoscopic value (i.e. the value affected by fluctuations around the average). In such mesoscopic regimes, the averaging of the signal is minimal and dynamic behavior can be examined in great detail. Examples of mesoscopic analysis on skeletal and cardiac muscle are provided.

Surface plasmon-coupled emission: what can directional fluorescence bring to the analytical sciences?

Surface plasmon-coupled emission (SPCE) arose from the integration of fluorescence and plasmonics, two rapidly expanding research fields. SPCE is revealing novel phenomena and has potential applications in bioanalysis, medical diagnostics, drug discovery, and genomics. In SPCE, excited fluorophores couple with surface plasmons on a continuous thin metal film; plasmophores radiate into a higher-refractive index medium with a narrow angular distribution. Because of the directional emission, the sensitivity of this technique can be greatly improved with high collection efficiency. This review describes the unique features of SPCE. In particular, we focus on recent advances in SPCE-based analytical platforms and their applications in DNA sensing and the detection of other biomolecules and chemicals.

Myosin regulatory light chain mutation found in hypertrophic cardiomyopathy patients increases isometric force production in transgenic mice

FHC (familial hypertrophic cardiomyopathy) is a heritable form of cardiac hypertrophy caused by mutations in genes encoding sarcomeric proteins. The present study focuses on the A13T mutation in the human ventricular myosin RLC (regulatory light chain) that is associated with a rare FHC variant defined by mid-ventricular obstruction and septal hypertrophy. We generated heart-specific Tg (transgenic) mice with ~10% of human A13T-RLC mutant replacing the endogenous mouse cardiac RLC. Histopathological examinations of longitudinal heart sections from Tg-A13T mice showed enlarged interventricular septa and profound fibrotic lesions compared with Tg-WT (wild-type), expressing the human ventricular RLC, or non-Tg mice. Functional studies revealed an abnormal A13T mutation-induced increase in isometric force production, no change in the force-pCa relationship and a decreased Vmax of the acto-myosin ATPase. In addition, a fluorescence-based assay showed a 3-fold lower binding affinity of the recombinant A13T mutant for the RLC-depleted porcine myosin compared with WT-RLC. These results suggest that the A13T mutation triggers a hypertrophic response through changes in cardiac sarcomere organization and myosin cross-bridge function leading to abnormal remodelling of the heart. The significant functional changes observed, despite a low level of A13T mutant incorporation into myofilaments, suggest a 'poison-peptide' mechanism of disease.

Familial hypertrophic cardiomyopathy can be characterized by a specific pattern of orientation fluctuations of actin molecules

A single-point mutation in the gene encoding the ventricular myosin regulatory light chain (RLC) is sufficient to cause familial hypertrophic cardiomyopathy (FHC). Most likely, the underlying cause of this disease is an inefficient energy utilization by the mutated cardiac muscle. We set out to devise a simple method to characterize two FHC phenotypes caused by the R58Q and D166V mutations in RLC. The method is based on the ability to observe a few molecules of actin in working ex vivo heart myofibril. Actin is labeled with extremely diluted fluorescent dye, and a small volume within the I-band ( approximately 10(-16) L), containing on average three actin molecules, is observed by confocal microscopy. During muscle contraction, myosin cross-bridges deliver cyclic impulses to actin. As a result, actin molecules undergo periodic fluctuations of orientation. We measured these fluctuations by recording the parallel and perpendicular components of fluorescent light emitted by an actin-bound fluorophore. The histograms of fluctuations of fluorescent actin molecules in wild-type (WT) hearts in rigor were represented by perfect Gaussian curves. In contrast, histograms of contracting heart muscle were peaked and asymmetric, suggesting that contraction occurred in at least two steps. Furthermore, the differences between histograms of contracting FHC R58Q and D166V hearts versus corresponding contracting WT hearts were statistically significant. On the basis of our results, we suggest a simple new method of distinguishing between healthy and FHC R58Q and D166V hearts by analyzing the probability distribution of polarized fluorescence intensity fluctuations of sparsely labeled actin molecules.