Second-harmonic generation sensitivity to transmembrane potential in normal and tumor cells

Second-harmonic generation imaging of membrane potential with retinal analogues

Second-harmonic generation (SHG) by membrane-incorporated probes is a nonlinear optical signal that is voltage-sensitive and the basis of a sensitive method for imaging membrane potential. The voltage dependence of SHG by four different probes, three retinoids (all-trans retinal), and two new retinal analogs, 3-methyl-7-(4'-dimethylamino-phenyl)-2,4,6-heptatrienal (AR-3) and 3,7-dimethyl-9-(4'-dimethylamino-phenyl)-2,4,6,8-nonatetraenal (AR-4), and a styryl dye (FM4-64), were compared in HEK-293 cells. Results were analyzed by fitting data with an expression based on an electrooptic mechanism for SHG, which depends on the complex-valued first- and second-order nonlinear electric susceptibilities (χ² and χ³) of the probe. This gave values for the voltage sensitivity at the cell's resting potential, the voltage where the SHG is minimal, and the amplitude of the signal at that voltage for each of the four compounds. These measures show that χ² and χ³ are complex numbers for all compounds except all-trans retinal, consistent with the proximities of excitation and/or emission wavelengths to molecular resonances. Estimates of probe orientation and location in the membrane electric field show that, for the far-from-resonance case, the shot noise-limited signal/noise ratio depends on the location of the probe in the membrane, and on χ³ but not on χ².

Probing myosin structural conformation in vivo by second-harmonic generation microscopy

Understanding of complex biological processes requires knowledge of molecular structures and measurement of their dynamics in vivo. The collective chemomechanical action of myosin molecules (the molecular motors) in the muscle sarcomere represents a paradigmatic example in this respect. Here, we describe a label-free imaging method sensitive to protein conformation in vivo. We employed the order-based contrast enhancement by second-harmonic generation (SHG) for the functional imaging of muscle cells. We found that SHG polarization anisotropy (SPA) measurements report on the structural state of the actomyosin motors, with significant sensitivity to the conformation of myosin. In fact, each physiological/biochemical state we probed (relaxed, rigor, isometric contraction) produced a distinct value of polarization anisotropy. Employing a full reconstruction of the contributing elementary SHG emitters in the actomyosin motor array at atomic scale, we provide a molecular interpretation of the SPA measurements in terms of myosin conformations. We applied this method to the discrimination between attached and detached myosin heads in an isometrically contracting intact fiber. Our observations indicate that isometrically contracting muscle sustains its tetanic force by steady-state commitment of 30% of myosin heads. Applying SPA and molecular structure modeling to the imaging of unstained living tissues provides the basis for a generation of imaging and diagnostic tools capable of probing molecular structures and dynamics in vivo.

Three-dimensional (3D) backward and forward second harmonic generation (SHG) microscopy of biological tissues

In this work we aim to show how it is possible to exploit the second harmonic generation (SHG) signal for producing multimodal microscopic images of biological tissues. SHG microscopy constitutes an important tool for high-resolution, high-contrast, three-dimensional studies of live cell and tissue architectures. The physical origins of SHG within these tissues are addressed and reported in a comprehensive image gallery. Although SHG is a coherent process, the multiple scattering of tissue samples determines the ability to acquire signal in both backward and forward direction. We discuss here some key elements related to the backward and forward SHG signal in terms of acquisition architecture and related microscopic imaging.

Biological second and third harmonic generation microscopy

Multiphoton microscopy has become a standard method for noninvasive imaging of thick specimens with subcellular resolution. Higher harmonic generation microscopy (HHGM), based on nonlinear multiphoton excitation, is a contrast mechanism for the structural and molecular imaging of native samples in cell culture and in fixed and live tissues, for both, three-dimensional and four-dimensional reconstructions. HHGM comprises second and third harmonic generation (SHG, THG) of ordered molecules, can be obtained without exogenous labels, and provides detailed real-time optical reconstruction of fibrillar collagen, myosin, microtubules, and membrane potential, as well as cell depolarization. This unit presents the principles of SHG and THG and the basic setup of a HHGM system, and summarizes current applications in cell biology. Multimodal multiphoton microscopy using HHGM together with two-photon excited fluorescence will develop into a key approach to real-time imaging of cell dynamics in the context of live tissues.

New techniques in linear and non-linear laser optics in muscle research

This review proposes a brief summary of two applications of lasers to muscle research. The first application (laser tweezers), is now a well-established technique in the field, adopted by several laboratories in the world and producing a constant stream of original data, fundamental for our improved understanding of muscle contraction at the level of detail that only single molecule measurements can provide. As an example of the power of this technique, here we focus on some recent results, revealing the performance of the working stroke in at least two distinct steps also in skeletal muscle myosin. A second laser-based technique described here is second-harmonic generation; the application of this technique to muscle research is very recent. We describe the main results obtained thus far in this area and the potentially remarkable impact that this technology may have in muscle research.

Optical recording of fast neuronal membrane potential transients in acute mammalian brain slices by second-harmonic generation microscopy

Although nonlinear microscopy and fast (approximately 1 ms) membrane potential (Vm) recording have proven valuable for neuroscience applications, their potentially powerful combination has not yet been shown for studies of Vm activity deep in intact tissue. We show that laser illumination of neurons in acute rat brain slices intracellularly filled with FM4-64 dye generates an intense second-harmonic generation (SHG) signal from somatic and dendritic plasma membranes with high contrast >125 microm below the slice surface. The SHG signal provides a linear response to DeltaVm of approximately 7.5%/100 mV. By averaging repeated line scans (approximately 50), we show the ability to record action potentials (APs) optically with a signal-to-noise ratio (S/N) of approximately 7-8. We also show recording of fast Vm steps from the dendritic arbor at depths inaccessible with previous methods. The high membrane contrast and linear response of SHG to DeltaVm provides the advantage that signal changes are not degraded by background and can be directly quantified in terms of DeltaVm. Experimental comparison of SHG and two-photon fluorescence Vm recording with the best known probes for each showed that the SHG technique is superior for Vm recording in brain slice applications, with FM4-64 as the best tested SHG Vm probe.