Second harmonic imaging microscopy

Second harmonic generation microscopy: a powerful tool for bio-imaging

Second harmonic generation (SHG) microscopy is an important optical imaging technique in a variety of applications. This article describes the history and physical principles of SHG microscopy and its more advanced variants, as well as their strengths and weaknesses in biomedical applications. It also provides an overview of SHG and advanced SHG imaging in neuroscience and microtubule imaging and how these methods can aid in understanding microtubule formation, structuration, and involvement in neuronal function. Finally, we offer a perspective on the future of these methods and how technological advancements can help make SHG microscopy a more widely adopted imaging technique.

Molecule-Membrane Interactions in Biological Cells Studied with Second Harmonic Light Scattering

The nonlinear optical phenomenon second harmonic light scattering (SHS) can be used for detecting molecules at the membrane surfaces of living biological cells. Over the last decade, SHS has been developed for quantitatively monitoring the adsorption and transport of small and medium size molecules (both neutral and ionic) across membranes in living cells. SHS can be operated with both time and spatial resolution and is even capable of isolating molecule-membrane interactions at specific membrane surfaces in multi-membrane cells, such as bacteria. In this review, we discuss select examples from our lab employing time-resolved SHS to study real-time molecular interactions at the plasma membranes of biological cells. We first demonstrate the utility of this method for determining the transport rates at each membrane/interface in a Gram-negative bacterial cell. Next, we show how SHS can be used to characterize the molecular mechanism of the century old Gram stain protocol for classifying bacteria. Additionally, we examine how membrane structures and molecular charge and polarity affect adsorption and transport, as well as how antimicrobial compounds alter bacteria membrane permeability. Finally, we discuss adaptation of SHS as an imaging modality to quantify molecular adsorption and transport in sub-cellular regions of individual living cells.

Harmonic Generation Microscopy 2.0: New Tricks Empowering Intravital Imaging for Neuroscience

Optical harmonic generation, e.g., second- (SHG) and third-harmonic generation (THG), provides intrinsic contrasts for three-dimensional intravital microscopy. Contrary to two-photon excited fluorescence (TPEF), however, they have found relatively specialized applications, such as imaging collagenous and non-specific tissues, respectively. Here we review recent advances that broaden the capacity of SHG and THG for imaging the central nervous system in particular. The fundamental contrast mechanisms are reviewed as they encode novel information including molecular origin, spectroscopy, functional probes, and image analysis, which lay foundations for promising future applications in neuroscience.

Techniques to assess bone ultrastructure organization: orientation and arrangement of mineralized collagen fibrils

Bone's remarkable mechanical properties are a result of its hierarchical structure. The mineralized collagen fibrils, made up of collagen fibrils and crystal platelets, are bone's building blocks at an ultrastructural level. The organization of bone's ultrastructure with respect to the orientation and arrangement of mineralized collagen fibrils has been the matter of numerous studies based on a variety of imaging techniques in the past decades. These techniques either exploit physical principles, such as polarization, diffraction or scattering to examine bone ultrastructure orientation and arrangement, or directly image the fibrils at the sub-micrometre scale. They make use of diverse probes such as visible light, X-rays and electrons at different scales, from centimetres down to nanometres. They allow imaging of bone sections or surfaces in two dimensions or investigating bone tissue truly in three dimensions, in vivo or ex vivo, and sometimes in combination with in situ mechanical experiments. The purpose of this review is to summarize and discuss this broad range of imaging techniques and the different modalities of their use, in order to discuss their advantages and limitations for the assessment of bone ultrastructure organization with respect to the orientation and arrangement of mineralized collagen fibrils.

Characterization of Protein Nanocrystals Based on the Reversibility of Crystallization

A new approach is described to screen for protein nanocrystals based on the reversibility of crystallization. Methods to characterize nanocrystals are in strong need to facilitate sample preparation for serial femtosecond X-ray nanocrystallography (SFX). SFX enables protein structure determination by collecting X-ray diffraction from nano- and microcrystals using a free electron laser. This technique is especially valuable for challenging proteins as for example membrane proteins and is in general a powerful method to overcome the radiation damage problem and to perform time-resolved structure analysis. Nanocrystal growth cannot be monitored with common methods used in protein crystallography, as the resolution of bright field microscopy is not sufficient. A high-performance method to screen for nanocrystals is second order nonlinear imaging of chiral crystals (SONICC). However, the high cost prevents its use in every laboratory, and some protein nanocrystals may be "invisible" to SONICC. In this work using a crystallization robot and a common imaging system precipitation comprised of nanocrystals and precipitation caused by aggregated protein can be distinguished.

Full determination of single ferroelectric nanocrystal orientation by Pockels electro-optic microscopy

We present a nanoscale electro-optic imaging method allowing access to the phase response, which is not amenable to classical second-harmonic generation microscopy. This approach is used to infer the vectorial orientation of single domain ferroelectric nanocrystals, based on polarization-resolved Pockels microscopy. The electro-optic phase response of KTP nanoparticles yields the full orientation in the laboratory frame of randomly dispersed single nanoparticles, together with their electric polarization dipole. The complete vector determination of the dipole orientation is a prerequisite to important applications including ferroelectric nanodomain orientation, membrane potential imaging, and rotational dynamics of single biomolecules.

Polarization-sensitive sum-frequency generation microscopy of collagen fibers

Point-scanning sum-frequency generation (SFG) microscopy enables the generation of images of collagen I fibers in tissues by tuning into specific vibrational resonances of the polypeptide. It is shown that when collagen-rich tissues are visualized near the 2954 cm(-1) stretching vibration of methylene groups, the SFG image contrast is higher compared to the contrast seen in nonresonant second-harmonic generation (SHG) imaging. Polarization and spectrally resolved analysis of the SFG signal as a function of fiber orientation in the CH-stretching range of the vibrational spectrum enabled a comparative characterization of the achiral tensor elements of collagen's second-order susceptibility. This analysis reveals that selected on-resonance tensor elements are enhanced over other elements, giving rise to a much stronger anisotropy ρ of the signal for SFG (ρ ≈ 15) compared to SHG (ρ ≈ 3). The improved anisotropy of the vibrationally resonant signal contributes to the higher contrast seen in the SFG tissue images.

Pulse-shaping based two-photon FRET stoichiometry

Förster Resonance Energy Transfer (FRET) based measurements that calculate the stoichiometry of intermolecular interactions in living cells have recently been demonstrated, where the technique utilizes selective one-photon excitation of donor and acceptor fluorophores to isolate the pure FRET signal. Here, we present work towards extending this FRET stoichiometry method to employ two-photon excitation using a pulse-shaping methodology. In pulse-shaping, frequency-dependent phases are applied to a broadband femtosecond laser pulse to tailor the two-photon excitation conditions to preferentially excite donor and acceptor fluorophores. We have also generalized the existing stoichiometry theory to account for additional cross-talk terms that are non-vanishing under two-photon excitation conditions. Using the generalized theory we demonstrate two-photon FRET stoichiometry in live COS-7 cells expressing fluorescent proteins mAmetrine as the donor and tdTomato as the acceptor.

Engineering a genetically-encoded SHG chromophore by electrostatic targeting to the membrane

Although second harmonic generation (SHG) microscopy provides unique imaging advantages for voltage imaging and other biological applications, genetically-encoded SHG chromophores remain relatively unexplored. SHG only arises from non-centrosymmetric media, so an anisotropic arrangement of chromophores is essential to provide strong SHG signals. Here, inspired by the mechanism by which K-Ras4B associates with plasma membranes, we sought to achieve asymmetric arrangements of chromophores at the membrane-cytoplasm interface using the fluorescent protein mVenus. After adding a farnesylation motif to the C-terminus of mVenus, nine amino acids composing its β-barrel surface were replaced by lysine, forming an electrostatic patch. This protein (mVe9Knus-CVIM) was efficiently targeted to the plasma membrane in a geometrically defined manner and exhibited SHG in HEK293 cells. In agreement with its design, mVe9Knus-CVIM hyperpolarizability was oriented at a small angle (~7.3°) from the membrane normal. Genetically-encoded SHG chromophores could serve as a molecular platform for imaging membrane potential.

Monitoring membrane potential with second-harmonic generation

This protocol describes the nonlinear optical phenomenon known as second-harmonic generation (SHG) and discusses its special attributes for imaging membrane-potential changes in single cells and multicellular preparations. Undifferentiated N1E-115 mouse neuroblastoma cells are used as a model cellular system for membrane electrophysiology. Styryl and naphthylstyryl dyes, also known as hemicyanines, are a class of electrochromic membrane-staining probes that have been used to monitor membrane potential by fluorescence; they also produce SHG images of cell membranes with SHG intensities that are sensitive to voltage. These experiments allow for the precise characterization of the voltage sensitivity of SHG and identification of the optimal wavelength for the incident laser fundamental light. This protocol presents the steps for the culture, staining, patching, and imaging of cells. The details of the imaging system and the measurements obtained are discussed, as are the prospects of this technology for imaging membrane potential changes in neuronal preparations.

Imaging voltage in neurons

In the last decades, imaging membrane potential has become a fruitful approach to study neural circuits, especially in invertebrate preparations with large, resilient neurons. At the same time, particularly in mammalian preparations, voltage imaging methods suffer from poor signal to noise and secondary side effects, and they fall short of providing single-cell resolution when imaging of the activity of neuronal populations. As an introduction to these techniques, we briefly review different voltage imaging methods (including organic fluorophores, SHG chromophores, genetic indicators, hybrid, nanoparticles, and intrinsic approaches) and illustrate some of their applications to neuronal biophysics and mammalian circuit analysis. We discuss their mechanisms of voltage sensitivity, from reorientation, electrochromic, or electro-optical phenomena to interaction among chromophores or membrane scattering, and highlight their advantages and shortcomings, commenting on the outlook for development of novel voltage imaging methods.

Optical microscopy in photosynthesis

Emerging as well as the most frequently used optical microscopy techniques are reviewed and image contrast generation methods in a microscope are presented, focusing on the nonlinear contrasts such as harmonic generation and multiphoton excitation fluorescence. Nonlinear microscopy presents numerous advantages over linear microscopy techniques including improved deep tissue imaging, optical sectioning, and imaging of live unstained samples. Nonetheless, with the exception of multiphoton excitation fluorescence, nonlinear microscopy is in its infancy, lacking protocols, users and applications; hence, this review focuses on the potential of nonlinear microscopy for studying photosynthetic organisms. Examples of nonlinear microscopic imaging are presented including isolated light-harvesting antenna complexes from higher plants, starch granules, chloroplasts, unicellular alga Chlamydomonas reinhardtii, and cyanobacteria Leptolyngbya sp. and Anabaena sp. While focusing on nonlinear microscopy techniques, second and third harmonic generation and multiphoton excitation fluorescence microscopy, other emerging nonlinear imaging modalities are described and several linear optical microscopy techniques are reviewed in order to clearly describe their capabilities and to highlight the advantages of nonlinear microscopy.

Imaging nervous system activity

This unit describes methods for loading ion- and voltage-sensitive dyes into neurons, with a particular focus on the spinal cord as a model system. In addition, we describe the use of these dyes to visualize neural activity. Although the protocols described here concern spinal networks in culture or an intact in vitro preparation, they can be, and have been, widely used in other parts of the nervous system.

Invited review article: Imaging techniques for harmonic and multiphoton absorption fluorescence microscopy

We review the current state of multiphoton microscopy. In particular, the requirements and limitations associated with high-speed multiphoton imaging are considered. A description of the different scanning technologies such as line scan, multifoci approaches, multidepth microscopy, and novel detection techniques is given. The main nonlinear optical contrast mechanisms employed in microscopy are reviewed, namely, multiphoton excitation fluorescence, second harmonic generation, and third harmonic generation. Techniques for optimizing these nonlinear mechanisms through a careful measurement of the spatial and temporal characteristics of the focal volume are discussed, and a brief summary of photobleaching effects is provided. Finally, we consider three new applications of multiphoton microscopy: nonlinear imaging in microfluidics as applied to chemical analysis and the use of two-photon absorption and self-phase modulation as contrast mechanisms applied to imaging problems in the medical sciences.

Second-harmonic generation imaging of membrane potential with photon counting

Second-harmonic generation (SHG) can be used for imaging membrane potential in neurons, but poor signal-to-noise (S/N) limits accurate measurements of small voltage transients. We use photon counting to improve the S/N of weak SHG signal detection. Photon counting generates shot-noise limited and integrable signals, eliminates pulse-to-pulse variation, and built-in discriminators reduces the background to practically zero. In single trials, by using photon counting, we obtain a more than a twofold S/N increase over analog voltage detection. Trial-to-trial variability is also reduced by 50%. Finally, we show that, using photon counting, the kinetics of fast events such as action potentials can be recorded more accurately.

Laser applications and system considerations in ocular imaging

We review laser applications for primarily in vivo ocular imaging techniques, describing their constraints based on biological tissue properties, safety, and the performance of the imaging system. We discuss the need for cost effective sources with practical wavelength tuning capabilities for spectral studies. Techniques to probe the pathological changes of layers beneath the highly scattering retina and diagnose the onset of various eye diseases are described. The recent development of several optical coherence tomography based systems for functional ocular imaging is reviewed, as well as linear and nonlinear ocular imaging techniques performed with ultrafast lasers, emphasizing recent source developments and methods to enhance imaging contrast.

Second-harmonic generation for studying structural motion of biological molecules in real time and space

SHG and sum-frequency generation (SFG) are surface-selective, nonlinear optical techniques whose ability to measure the average tilt angle of molecules on surfaces is well known in non-biological systems. By labeling molecules with a second-harmonic-active dye probe, SHG detection is extended to any biological molecule. The method has been used in previous work to detect biomolecules at an interface and their ligand-induced conformational changes. Here I demonstrate that SHG can be used to study structural motion quantitatively using a probe placed at a specific site (Cys-77) in adenylate kinase, a protein. The protein is also labeled non-site-specifically via amines. Labeled protein is absorbed to a surface and a baseline SH signal is measured. Upon introducing ATP, AMP or a specific inhibitor, AP(5)A, the baseline signal changes depending on the ligand and the labeling site. In particular, a substantial change in SH intensity is produced upon binding ATP to the amine-labeled protein, consistent with the X-ray crystal structures. In contrast, SHG polarization measurements are used to quantitatively determine that no rotation occurs at site Cys-77, in agreement with the lack of motion observed at this site in the X-ray crystal structures. A method for building a global map of conformational change in real time and space is proposed using a set of probes placed at different sites in a biomolecule. For this purpose, SH-active unnatural amino acids are attractive complements to exogenous labels.

Unique contrast patterns from resonance-enhanced chiral SHG of cell membranes

Chirality can produce novel nonlinear optical effects that may form the basis for new imaging contrast agents. In this paper, we developed a new chiral chromophore 2, which is the dimer of a known voltage sensitive dye, monomer 1, with the chirality originating from the twisted orientation between two subunits. Racemic dimer and monomer 1 were used as the references to study the effect of chirality in SHG microscopy of live cells. All these dyes selectively stain the outer leaflets of cell membranes, producing strong resonance-enhanced SHG images. At the symmetric junction between two adherent cells, monomer or racemic dimer SHG is forbidden due to centrosymmetry, and indeed little SHG was observed (10 +/- 1% relative to nonjunction). When stained with the chiral dimer, the junction is no longer centrosymmetric and much stronger SHG was observed (39 +/- 4% relative to nonjunction). Plane polarized light produces highly polarized images of spherical cells stained with racemic dye, but for the chiral dye, the polarized pattern is largely eliminated by the chiral SHG emanating from the subresolution membrane convolutions.

Nonlinear optical potentiometric dyes optimized for imaging with 1064-nm light

Nonlinear optical phenomena, such as two-photon fluorescence (2PF) and second harmonic generation (SHG), in combination with voltage sensitive dyes, can be used to acquire high-resolution spatio temporal maps of electrical activity in excitable cells and tissue. Developments in 1064-nm fiber laser technology have simplified the generation of high-intensity, long-wavelength, femtosecond light pulses, capable of penetrating deep into tissue. To merge these two advances requires the design and synthesis of new dyes that are optimized for longer wavelengths and that produce fast and sensitive responses to membrane potential changes. In this work, we have systematically screened a series of new dyes with varying chromophores and sidechains that anchor them in cell membranes. We discovered several dyes that could potentially be used for in vivo measurements of cellular electrical activity because of their rapid and sensitive responses to membrane potential. Some of these dyes show optimal activity for SHG; others for 2PF. This regulated approach to dye screening also allows significant insight into the molecular mechanisms behind both SHG and 2PF. In particular, the differing patterns of sensitivity and kinetics for these two nonlinear optical modalities indicate that their voltage sensitivity originates from differing mechanisms.

Second harmonic generation in neurons: electro-optic mechanism of membrane potential sensitivity

Second harmonic generation (SHG) from membrane-bound chromophores can be used to image membrane potential in neurons. We investigate the biophysical mechanism responsible for the SHG voltage sensitivity of the styryl dye FM 4-64 in pyramidal neurons from mouse neocortical slices. SHG signals are exquisitely sensitive to the polarization of the incident laser light. Using this polarization sensitivity in two complementary approaches, we estimate a approximately 36 degrees tilt angle of the chromophore to the membrane normal. Changes in membrane potential do not affect the polarization of the SHG signal. The voltage response of FM 4-64 is faster than 1 ms and does not reverse sign when imaged at either side of its absorption peak. We conclude that FM 4-64 senses membrane potential through an electro-optic mechanism, without significant chromophore membrane reorientation, redistribution, or spectral shift.

The role of backscattering in SHG tissue imaging

We investigate the properties of second-harmonic generation (SHG) tissue imaging for the functional biological unit fascia, skeletal muscle, and tendon. Fascia and Achilles tendon primarily consist of similar collagen type I arrays that can be imaged using SHG microscopy. For muscle, it is the myosin molecules represented within the A bands. For fascia and tendon tissue samples, we observe, in addition to a stronger signal in forward images, vastly different features for the backward versus the forward images. In vivo as well as intact ex vivo thick tissue imaging requires backward detection. The obtained image is a result of the direct backward components plus a certain fraction of the forward components that are redirected (backscattered) toward the objective as they propagate within the tissue block. As the forward and the backward images are significantly different from each other for the imaged collagen type I tissue, it is crucial to determine the fraction of the forward signal that contributes to the overall backward signal. For intact ex vivo SHG imaging of Achilles tendon, we observe a significant contribution of forward features in the resulting image. For fascia, the connective tissue immediately surrounding muscle, we only observe backward features, due to low backscattering in muscle.

Influence of semicrystalline order on the second-harmonic generation efficiency in the anisotropic bands of myocytes

The influence of semicrystalline order on the second-harmonic generation (SHG) efficiency in the anisotropic bands of Drosophila melanogaster sarcomeres from larval and adult muscle has been investigated. Differences in the semicrystalline order were obtained by using wild-type and mutant strains containing different amounts of headless myosin. The reduction in semicrystalline order without altering the chemical composition of myofibrils was achieved by observing highly stretched sarcomeres and by inducing a loss of viability in myocytes. In all cases the reduction of semicrystalline order in anisotropic bands of myocytes resulted in a substantial decrease in SHG. Second-harmonic imaging during periodic contractions of myocytes revealed higher intensities when sarcomeres were in the relaxed state compared with the contracted state. This study demonstrates that an ordered semicrystalline arrangement of anisotropic bands plays a determining role in the efficiency of SHG in myocytes.

Fluorescence microscopy today

Fluorescence microscopy has undergone a renaissance in the last decade. The introduction of green fluorescent protein (GFP) and two-photon microscopy has allowed systematic imaging studies of protein localization in living cells and of the structure and function of living tissues. The impact of these and other new imaging methods in biophysics, neuroscience, and developmental and cell biology has been remarkable. Further advances in fluorophore design, molecular biological tools and nonlinear and hyper-resolution microscopies are poised to profoundly transform many fields of biological research.

Imaging membrane potential in dendritic spines

Dendritic spines mediate most excitatory inputs in the brain. Although it is clear that spines compartmentalize calcium, it is still unknown what role, if any, they play in integrating synaptic inputs. To investigate the electrical function of spines directly, we used second harmonic generation (SHG) imaging of membrane potential in pyramidal neurons from hippocampal cultures and neocortical brain slices. With FM 4-64 as an intracellular SHG chromophore, we imaged membrane potential in the soma, dendritic branches, and spines. The SHG response to voltage was linear and seemed based on an electro-optic mechanism. The SHG sensitivity of the chromophore in spines was similar to that of the parent dendritic shaft and the soma. Backpropagation of somatic action potentials generated SHG signals at spines with similar amplitude and kinetics to somatic ones. Our optical measurements of membrane potential from spines demonstrate directly that backpropagating action potentials invade the spines.

Characterization and application of a new optical probe for membrane lipid domains

In this article, we characterize the fluorescence of an environmentally sensitive probe for lipid membranes, di-4-ANEPPDHQ. In large unilamellar lipid vesicles (LUVs), its emission spectrum shifts up to 30 nm to the blue with increasing cholesterol concentration. Independently, it displays a comparable blue shift in liquid-ordered relative to liquid-disordered phases. The cumulative effect is a 60-nm difference in emission spectra for cholesterol containing LUVs in the liquid-ordered state versus cholesterol-free LUVs in the liquid-disordered phase. Given these optical properties, we use di-4-ANEPPDHQ to image the phase separation in giant unilamellar vesicles with both linear and nonlinear optical microscopy. The dye shows green and red fluorescence in liquid-ordered and -disordered domains, respectively. We propose that this reflects the relative rigidity of the molecular packing around the dye molecules in the two phases. We also observe a sevenfold stronger second harmonic generation signal in the liquid-disordered domains, consistent with a higher concentration of the dye resulting from preferential partitioning into the disordered phase. The efficacy of the dye for reporting lipid domains in cell membranes is demonstrated in polarized migrating neutrophils.

Dendritic spines and linear networks

The function of the cortical microcircuitry is still mysterious. Using a bottom-up analysis based on the biophysics and connectivity of cortical neurons, we propose the hypothesis that the neocortex is essentially a linear integrator of inputs. Dendritic spines would slow the neuron and contribute to linearize input summation. Since excitatory axons are relatively straight, they appeared designed to help disperse information to a large number of recipient neurons, generating a distributed circuit. A linear summation regime will ensure the full benefit of a distributed connectivity matrix. Linear integration could also help the neocortex decode the sensory world and may have additional computational advantages. In this view, spines would be the anatomical signature of linear networks.

Wavelength- and Time-Dependence of Potentiometric Non-linear Optical Signals from Styryl Dyes

Second harmonic generation (SHG) imaging microscopy is an important emerging technique for biological research, complementing existing one- and two-photon fluorescence (2PF) methods. A non-linear phenomenon employing light from mode-locked Ti:sapphire or fiber-based lasers, SHG results in intrinsic optical sectioning without the need for a confocal aperture. Furthermore, as a second-order process SHG is confined to loci lacking a center of symmetry, a constraint that is readily satisfied by lipid membranes with only one leaflet stained by a dye. Of particular interest is "resonance-enhanced" SHG from styryl dyes in cellular membranes and the possibility that SHG is sensitive to transmembrane potential. We have previously confirmed this, using simultaneous voltage-clamping and non-linear imaging of cells to find that SHG is up to four times more sensitive to potential than fluorescence. In this work, we have extended these results in two directions. First, with a range of wavelengths available from a mode-locked Ti:sapphire laser and a fiber-based laser, we have more fully investigated SHG and 2PF voltage-sensitivity from ANEP and ASTAP chromophores, obtaining SHG sensitivity spectra that are consistent with resonance enhancements. Second, we have modified our system to coordinate the application of voltage-clamp steps with non-linear image acquisition to more precisely characterize the time dependence of SHG and 2PF voltage sensitivity, finding that, at least for some dyes, SHG responds more slowly than fluorescence to changes in transmembrane potential.

Characterization of the myosin-based source for second-harmonic generation from muscle sarcomeres

Several biologically important protein structures give rise to strong second-harmonic generation (SHG) in their native context. In addition to high-contrast optical sections of cells and tissues, SHG imaging can provide detailed structural information based on the physical constraints of the optical effect. In this study we characterize, by biochemical and optical analysis, the critical structures underlying SHG from the complex muscle sarcomere. SHG emission arises from domains of the sarcomere containing thick filaments, even within nascent sarcomeres of differentiating myocytes. SHG from isolated myofibrils is abolished by extraction of myosin, but is unaffected by removal or addition of actin filaments. Furthermore, the polarization dependence of sarcomeric SHG is not affected by either the proportion of myosin head domains or the orientation of myosin heads. By fitting SHG polarization anisotropy readings to theoretical response curves, we find an orientation for the elemental harmonophore that corresponds well to the pitch of the myosin rod alpha-helix along the thick filament axis. Taken together, these data indicate that myosin rod domains are the key structures giving SHG from striated muscle. This study should guide the interpretation of SHG contrast in images of cardiac and skeletal muscle tissue for a variety of biomedical applications.

Sensitivity of second harmonic generation from styryl dyes to transmembrane potential

In this article we present results from the simultaneous nonlinear (second harmonic generation and two-photon excitation fluorescence) imaging and voltage clamping of living cells. Specifically, we determine the sensitivity to transmembrane potential of second harmonic generation by ANEP-chromophore styryl dyes as a function of excitation wavelength and dye structure. We have measured second harmonic sensitivities of up to 43% per 100 mV, more than a factor of four better than the nominal voltage sensitivity of the dyes under "one-photon" fluorescence. We find a dependence of voltage sensitivity on excitation wavelength that is consistent with a two-photon resonance, and there is a significant dependence of voltage sensitivity on the structure of the nonchromophore portion of the dyes.

Second harmonic imaging of membrane potential of neurons with retinal

We present a method to optically measure and image the membrane potential of neurons, using the nonlinear optical phenomenon of second harmonic generation (SHG) with a photopigment retinal as the chromophore [second harmonic retinal imaging of membrane potential (SHRIMP)]. We show that all-trans retinal, when adsorbed to the plasma membrane of living cells, can report on the local electric field via its change in SHG. Using a scanning mode-locked Ti-sapphire laser, we collect simultaneous two-photon excited fluorescence (TPEF) and SHG images of retinal-stained kidney cells and cultured pyramidal neurons. Patch clamp experiments on neurons stained with retinal show an increase of 25% in SHG intensity per 100-mV depolarization. Our data are the first demonstration of optical measurements of membrane potential of mammalian neurons with SHG. SHRIMP could have wide applicability in neuroscience and, by modifying rhodopsin, could in principle be subject for developing genetically engineered voltage sensors.

Second-harmonic imaging microscopy for visualizing biomolecular arrays in cells, tissues and organisms

Although the nonlinear optical effect known as second-harmonic generation (SHG) has been recognized since the earliest days of laser physics and was demonstrated through a microscope over 25 years ago, only in the past few years has it begun to emerge as a viable microscope imaging contrast mechanism for visualization of cell and tissue structure and function. Only small modifications are required to equip a standard laser-scanning two-photon microscope for second-harmonic imaging microscopy (SHIM). Recent studies of the three-dimensional in vivo structures of well-ordered protein assemblies, such as collagen, microtubules and muscle myosin, are beginning to establish SHIM as a nondestructive imaging modality that holds promise for both basic research and clinical pathology. Thus far the best signals have been obtained in a transmitted light geometry that precludes in vivo measurements on large living animals. This drawback may be addressed through improvements in the collection of SHG signals via an epi-illumination microscope configuration. In addition, SHG signals from certain membrane-bound dyes have been shown to be highly sensitive to membrane potential. Although this indicates that SHIM may become a valuable tool for probing cell physiology, the small signal size would limit the number of photons that could be collected during the course of a fast action potential. Better dyes and optimized microscope optics could ultimately lead to the imaging of neuronal electrical activity with SHIM.

On the electrical function of dendritic spines

Dendritic spines mediate most excitatory inputs in the brain, yet their function is still unclear. Imaging experiments have demonstrated their role in biochemical compartmentalization at individual synapses, yet theoretical studies have suggested that they could serve an electrical function in transforming synaptic inputs and transmitting dendritic spikes. Recent data indicate that spines possess voltage-dependent conductances and that these channels can be spine-specific. Although direct experimental investigations of the electrical properties of spines have not yet taken place, spines could play a significant electrical role, greatly influencing dendritic integration and the function of neural circuits.