Second- and third-harmonic generation microscopies for the structural imaging of intact tissues

One principal advantage of multiphoton excitation microscopy is that it preserves its three-dimensional micrometer resolution when imaging inside light-scattering samples. For that reason two-photon-excited fluorescence microscopy has become an invaluable tool for cellular imaging in intact tissue, with applications in many fields of physiology. This success has driven increasing interest in other forms of nonlinear microscopy that can provide additional information on cells and tissues, such as second- (SHG) and third- (THG) harmonic generation microscopies. In recent years, significant progress has been made in understanding the contrast mechanisms of these recent methodologies, and high-resolution imaging based on intrinsic sources of signal has been demonstrated in cells and tissues. Harmonic generation exhibits structural rather than chemical specificity and can be obtained from a variety of non-fluorescent samples. SHG is observed specifically in dense, non-centrosymmetric arrangements of polarizable molecules, such as collagen fibrils, myofilaments, and polarized microtubule bundles. SHG imaging is therefore emerging as a novel approach for studying processes such as the physiopathological remodelling of the collagen matrix and myofibrillogenesis in intact tissue. THG does not require a non-centrosymmetric system ; however no signal can be obtained from a homogeneous medium. THG imaging therefore provides maps of sub-micrometer heterogeneities (interfaces, inclusions) in unstained samples, and can be used as a general purpose structural imaging tool. Recent studies showed that this technique can be used to image embryo development in small organisms and to characterize the accumulation of large lipid bodies in specialized cells. SHG and THG microscopy both rely on femtosecond laser technology and are easily combined with two-photon microscopy.     

Second harmonic generation microscopy for quantitative analysis of collagen fibrillar structure

Second-harmonic generation (SHG) microscopy has emerged as a powerful modality for imaging fibrillar collagen in a diverse range of tissues. Because of its underlying physical origin, it is highly sensitive to the collagen fibril/fiber structure, and, importantly, to changes that occur in diseases such as cancer, fibrosis and connective tissue disorders. We discuss how SHG can be used to obtain more structural information on the assembly of collagen in tissues than is possible by other microscopy techniques. We first provide an overview of the state of the art and the physical background of SHG microscopy, and then describe the optical modifications that need to be made to a laser-scanning microscope to enable the measurements. Crucial aspects for biomedical applications are the capabilities and limitations of the different experimental configurations. We estimate that the setup and calibration of the SHG instrument from its component parts will require 2-4 weeks, depending on the level of the user's experience.     

Biological applications of second harmonic imaging

Second Harmonic Generation (SHG) microscopy dates back to 1974, but effective biological use of the technique has a history of barely 10 years. It is now widely used to image collagen in many different applications, and is becoming useful for imaging myosin and some polysaccharides. A separate line on research has focussed on SHG dyes, which can provide high-speed indication of membrane potential and are now in use in neurobiology. This review looks at the progress to date in these different fields.     

Second harmonic generation imaging of endogenous structural proteins

We show that structural protein arrays consisting largely of collagen, myosin, and tubulin, and their associated proteins can be imaged in three dimensions with high contrast and resolution by laser-scanning second harmonic generation (SHG) microscopy. SHG is a nonlinear optical scheme and this form of microscopy shares several common advantages with multiphoton excited fluorescence, namely, intrinsic three-dimensionality and reduced out-of-plane photobleaching and phototoxicity. SHG does not arise from absorption and in-plane photodamage considerations are therefore also greatly reduced. In particular, structural protein arrays that are highly ordered and birefringent produce large SHG signals without the need for any exogenous labels. We demonstrate that thick tissues including muscle and bone can be imaged and sectioned through several hundred micrometers of depth. Combining SHG with two-photon excited green fluorescent protein (GFP) imaging allows inference of the molecular origin of the SHG contrast in Caenorhabditis elegans sarcomeres. Symmetry and organization of microtubule structures in dividing C. elegans embryos are similarly studied by comparing the endogenous tubulin contrast with that of GFP::tubulin fluorescence. It is found that SHG provides molecular level data on radial and lateral symmetries that GFP constructs cannot. The physical basis of SHG is discussed and compared with that of two-photon excitation as well as that of polarization microscopy. Due to the intrinsic sectioning, lack of photobleaching, and availability of molecular level data, SHG is a powerful tool for in vivo imaging.     

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.     

Three-Dimensional High-Resolution Second-Harmonic Generation Imaging of Endogenous Structural Proteins in Biological Tissues

We find that several key endogenous protein structures give rise to intense second-harmonic generation (SHG)—nonabsorptive frequency doubling of an excitation laser line. Second-harmonic imaging microscopy (SHIM) on a laser-scanning system proves, therefore, to be a powerful and unique tool for high-resolution, high-contrast, three-dimensional studies of live cell and tissue architecture. Unlike fluorescence, SHG suffers no inherent photobleaching or toxicity and does not require exogenous labels. Unlike polarization microscopy, SHIM provides intrinsic confocality and deep sectioning in complex tissues. In this study, we demonstrate the clarity of SHIM optical sectioning within unfixed, unstained thick specimens. SHIM and two-photon excited fluorescence (TPEF) were combined in a dual-mode nonlinear microscopy to elucidate the molecular sources of SHG in live cells and tissues. SHG arose not only from coiled-coil complexes within connective tissues and muscle thick filaments, but also from microtubule arrays within interphase and mitotic cells. Both polarization dependence and a local symmetry cancellation effect of SHG allowed the signal from species generating the second harmonic to be decoded, by ratiometric correlation with TPEF, to yield information on local structure below optical resolution. The physical origin of SHG within these tissues is addressed and is attributed to the laser interaction with dipolar protein structures that is enhanced by the intrinsic chirality of the protein helices.     

Surface functionalization of barium titanate SHG nanoprobes for in vivo imaging in zebrafish

To address the need for a bright, photostable labeling tool that allows long-term in vivo imaging in whole organisms, we recently introduced second harmonic generating (SHG) nanoprobes. Here we present a protocol for the preparation and use of a particular SHG nanoprobe label, barium titanate (BT), for in vivo imaging in living zebrafish embryos. Chemical treatment of the BT nanoparticles results in surface coating with amine-terminal groups, which act as a platform for a variety of chemical modifications for biological applications. Here we describe cross-linking of BT to a biotin-linked moiety using click chemistry methods and coating of BT with nonreactive poly(ethylene glycol) (PEG). We also provide details for injecting PEG-coated SHG nanoprobes into zygote-stage zebrafish embryos, and in vivo imaging of SHG nanoprobes during gastrulation and segmentation. Implementing the PROCEDURE requires a basic understanding of laser-scanning microscopy, experience with handling zebrafish embryos and chemistry laboratory experience. Functionalization of the SHG nanoprobes takes ～3 d, whereas zebrafish preparation, injection and imaging setup should take approximately 2-4 h.     

用光学二次谐波成像技术观察皮肤内不同种类的胶原

目的：用光学二次谐波成像的方法比较成熟皮肤与新生皮肤内不同种类胶原的含量,以及正常皮肤与创伤皮肤内胶原种类的变化。方法：用前向及背向二次谐波观察正常及创伤皮肤内的胶原,并与传统的天狼猩红染色法相对照。结果：与传统方法相比,二次谐波可以更快速,更灵敏地检测组织中的胶原。背向二次谐波信号强度随着切片厚度的增加而增强。结论：光学二次谐波成像技术是一种灵敏、简单、快速检测皮肤组织内胶原的新方法,具有很好的应用前景,可应用于活体检测。     

Imaging today's infectious animalcules

The study of pathogens and their interactions with host cells has advanced hand-in-hand with developments in optical microscopy. Whereas microbiology benefits enormously from modern imaging technologies, for example, digital imaging and confocal microscopy, it also presents unique challenges. To overcome these, microbiologists are adept at customising imaging methods, and recently there have been studies using state-of-the-art quantitative imaging methods to probe host-pathogen interactions at the single-cell level. Of particular interest are the studies using combined light and electron microscopy methods, bi-arsenical tetra-cysteine tag labelling and automated image-acquisition and analysis for high-throughput/high-content experimentation. These applications demonstrate how imaging methodologies, adapted for microbiology, continue to open avenues for studies that previously have proven inaccessible.     

Hyperglycemia-Induced Abnormalities in Rat and Human Corneas: The Potential of Second Harmonic Generation Microscopy

Background

Second Harmonic Generation (SHG) microscopy recently appeared as an efficient optical imaging technique to probe unstained collagen-rich tissues like cornea. Moreover, corneal remodeling occurs in many diseases and precise characterization requires overcoming the limitations of conventional techniques. In this work, we focus on diabetes, which affects hundreds of million people worldwide and most often leads to diabetic retinopathy, with no early diagnostic tool. This study then aims to establish the potential of SHG microscopy for in situ detection and characterization of hyperglycemia-induced abnormalities in the Descemet’s membrane, in the posterior cornea.

Methodology/Principal Findings

We studied corneas from age-matched control and Goto-Kakizaki rats, a spontaneous model of type 2 diabetes, and corneas from human donors with type 2 diabetes and without any diabetes. SHG imaging was compared to confocal microscopy, to histology characterization using conventional staining and transmitted light microscopy and to transmission electron microscopy. SHG imaging revealed collagen deposits in the Descemet’s membrane of unstained corneas in a unique way compared to these gold standard techniques in ophthalmology. It provided background-free images of the three-dimensional interwoven distribution of the collagen deposits, with improved contrast compared to confocal microscopy. It also provided structural capability in intact corneas because of its high specificity to fibrillar collagen, with substantially larger field of view than transmission electron microscopy. Moreover, in vivo SHG imaging was demonstrated in Goto-Kakizaki rats.

Conclusions/Significance

Our study shows unambiguously the high potential of SHG microscopy for three-dimensional characterization of structural abnormalities in unstained corneas. Furthermore, our demonstration of in vivo SHG imaging opens the way to long-term dynamical studies. This method should be easily generalized to other structural remodeling of the cornea and SHG microscopy should prove to be invaluable for in vivo corneal pathological studies.     

Label-free 3D visualization of cellular and tissue structures in intact muscle with second and third harmonic generation microscopy

Second and Third Harmonic Generation (SHG and THG) microscopy is based on optical effects which are induced by specific inherent physical properties of a specimen. As a multi-photon laser scanning approach which is not based on fluorescence it combines the advantages of a label-free technique with restriction of signal generation to the focal plane, thus allowing high resolution 3D reconstruction of image volumes without out-of-focus background several hundred micrometers deep into the tissue. While in mammalian soft tissues SHG is mostly restricted to collagen fibers and striated muscle myosin, THG is induced at a large variety of structures, since it is generated at interfaces such as refraction index changes within the focal volume of the excitation laser. Besides, colorants such as hemoglobin can cause resonance enhancement, leading to intense THG signals. We applied SHG and THG microscopy to murine (Mus musculus) muscles, an established model system for physiological research, to investigate their potential for label-free tissue imaging. In addition to collagen fibers and muscle fiber substructure, THG allowed us to visualize blood vessel walls and erythrocytes as well as white blood cells adhering to vessel walls, residing in or moving through the extravascular tissue. Moreover peripheral nerve fibers could be clearly identified. Structure down to the nuclear chromatin distribution was visualized in 3D and with more detail than obtainable by bright field microscopy. To our knowledge, most of these objects have not been visualized previously by THG or any label-free 3D approach. THG allows label-free microscopy with inherent optical sectioning and therefore may offer similar improvements compared to bright field microscopy as does confocal laser scanning microscopy compared to conventional fluorescence microscopy.     

Microtubule plus-end dynamics in Xenopus egg extract spindles

Microtubule dynamics underlie spindle assembly, yet we do not know how the spindle environment affects these dynamics. We developed methods for measuring two key parameters of microtubule plus-end dynamic instability in Xenopus egg extract spindles. To measure plus-end polymerization rates and localize growing plus ends, we used fluorescence confocal imaging of EB1. This revealed plus-end polymerization throughout the spindle at approximately 11 microm/min, similar to astral microtubules, suggesting polymerization velocity is not regionally regulated by the spindle. The ratio of EB1 to microtubule fluorescence revealed an enrichment of polymerizing ends near the spindle middle, indicating enhanced nucleation or rescue there. We measured depolymerization rates by creating a front of synchronized depolymerization in spindles severed with microneedles. This front could be tracked by polarization and fluorescence microscopy as it advanced from each cut edge toward the associated pole. Both imaging modalities revealed rapid depolymerization ( approximately 30 microm/min) superimposed on a subset of microtubules stable to depolymerization. Larger spindle fragments contained a higher percentage of stable microtubules, which we believe were oriented with their minus ends facing the cut. Depolymerization was blocked by the potent microtubule stabilizing agent hexylene glycol, but was unaffected by alpha-MCAK antibody and AMPPNP, which block catastrophe and kinesin motility, respectively. These measurements move us closer to understanding the complete life history of a spindle microtubule.     

Quantitative Imaging of Microtubule Alteration as an Early Marker of?Axonal Degeneration after Ischemia in Neurons

Neuronal death can be preceded by progressive dysfunction of axons. Several pathological conditions such as ischemia can disrupt the neuronal cytoskeleton. Microtubules are basic structural components of the neuronal cytoskeleton that regulate axonal transport and neuronal function. Up-to-date, high-resolution observation of microtubules in living neuronal cells is usually accomplished using fluorescent-based microscopy techniques. However, this needs exogenous fluorescence markers to produce the required contrast. This is an invasive procedure that may interfere with the microtubule dynamics. In this work, we show, for the first time to our knowledge, that by using the endogenous (label-free) contrast provided by second harmonic generation (SHG) microscopy, it is possible to identify early molecular changes occurring in the microtubules of living neurons under ischemic conditions. This is done by measuring the intensity modulation of the SHG signal as a function of the angular rotation of the incident linearly polarized excitation light (technique referred to as PSHG). Our experiments were performed in microtubules from healthy control cultured cortical neurons and were compared to those upon application of several periods of oxygen and glucose deprivation (up to 120 min) causing ischemia. After 120-min oxygen and glucose deprivation, a change in the SHG response to the polarization was measured. Then, by using a three-dimensional PSHG biophysical model, we correlated this finding with the structural changes occurring in the microtubules under oxygen and glucose deprivation. To our knowledge, this is the first study performed in living neuronal cells that is based on direct imaging of axons and that provides the means of identifying the early symptoms of ischemia. Live observation of this process might bring new insights into understanding the dynamics and the mechanisms underlying neuronal degeneration or mechanisms of protection or regeneration.     

Non-Linear Optical Microscopy Sheds Light on Cardiovascular Disease

Many cardiac diseases have been associated with increased fibrosis and changes in the organization of fibrillar collagen. The degree of fibrosis is routinely analyzed with invasive histological and immunohistochemical methods, giving a limited and qualitative understanding of the tissue''s morphological adaptation to disease. Our aim is to quantitatively evaluate the increase in fibrosis by three-dimensional imaging of the collagen network in the myocardium using the non-linear optical microscopy techniques Two-Photon Excitation microscopy (TPE) and Second Harmonic signal Generation (SHG). No sample staining is needed because numerous endogenous fluorophores are excited by a two-photon mechanism and highly non-centrosymmetric structures such as collagen generate strong second harmonic signals. We propose for the first time a 3D quantitative analysis to carefully evaluate the increased fibrosis in tissue from a rat model of heart failure post myocardial infarction. We show how to measure changes in fibrosis from the backward SHG (B_SHG) alone, as only backward-propagating SHG is accessible for true in vivo applications. A 5-fold increase in collagen I fibrosis is detected in the remote surviving myocardium measured 20 weeks after infarction. The spatial distribution is also shown to change markedly, providing insight into the morphology of disease progression.     

Interpreting second-harmonic generation images of collagen I fibrils

Fibrillar collagen, being highly noncentrosymmetric, possesses a tremendous nonlinear susceptibility. As a result, second-harmonic generation (SHG) microscopy of collagen produces extremely bright and robust signals, providing an invaluable tool for imaging tissue structure with submicron resolution. Here we discuss fundamental principles governing SHG phase matching with the tightly focusing optics used in microscopy. Their application to collagen imaging yields several biophysical features characteristic of native collagen structure: SHG radiates from the shell of a collagen fibril, rather than from its bulk. This SHG shell may correspond to the supporting element of the fibril. Physiologically relevant changes in solution ionic strength alter the ratio of forward-to-backward propagating SHG, implying a resulting change in the SHG shell thickness. Fibrillogenesis can be resolved in immature tissue by directly imaging backward-propagating SHG. Such findings are crucial to the design and development of forthcoming diagnostic and research tools.     

Faster STORM using compressed sensing

In super-resolution microscopy methods based on single-molecule switching, the rate of accumulating single-molecule activation events often limits the time resolution. Here we developed a sparse-signal recovery technique using compressed sensing to analyze images with highly overlapping fluorescent spots. This method allows an activated fluorophore density an order of magnitude higher than what conventional single-molecule fitting methods can handle. Using this method, we demonstrated imaging microtubule dynamics in living cells with a time resolution of 3 s.     

Applications of second harmonic generation (SHG)/sum-frequency generation (SFG) imaging for biophysical characterization of the plasma membrane

The plasma membrane is a lipid bilayer of < 10 nm width that separates intra- and extra-cellular environments and serves as the site of cell-cell communication, as well as communication between cells and the extracellular environment. As such, biophysical phenomena at and around the plasma membrane play key roles in determining cellular physiology and pathophysiology. Thus, the selective visualization and characterization of the plasma membrane are crucial aspects of research in wide areas of biology and medicine. However, the specific characterization of the plasma membrane has been a challenge using conventional imaging techniques, which are unable to effectively distinguish between signals arising from the plasma membrane and those from intracellular lipid structures. In this regard, interface-specific second harmonic generation (SHG) and sum-frequency generation (SFG) imaging demonstrate great potential. When combined with exogenous SHG/SFG active dyes, SHG/SFG can specifically highlight the plasma membrane as the most prominent interface associated with cells. Furthermore, SHG/SFG imaging can be readily extended to multimodal multiphoton microscopy with simultaneous occurrence of other multiphoton phenomena, including multiphoton excitation and coherent Raman scattering, which shed light on the biophysical properties of the plasma membrane from different perspectives. Here, we review traditional and current applications, as well as the prospects of long-known but unexplored SHG/SFG imaging techniques in biophysics, with special focus on their use in the biophysical characterization of the plasma membrane.     

Continuous Grading of Early Fibrosis in NAFLD Using Label-Free Imaging: A Proof-of-Concept Study

Background and Aims

Early detection of fibrosis is important in identifying individuals at risk for advanced liver disease in non-alcoholic fatty liver disease (NAFLD). We tested whether second-harmonic generation (SHG) and coherent anti-Stokes Raman scattering (CARS) microscopy, detecting fibrillar collagen and fat in a label-free manner, might allow automated and sensitive quantification of early fibrosis in NAFLD.

Methods

We analyzed 32 surgical biopsies from patients covering histological fibrosis stages 0–4, using multimodal label-free microscopy. Native samples were visualized by SHG and CARS imaging for detecting fibrillar collagen and fat. Furthermore, we developed a method for quantitative assessment of early fibrosis using automated analysis of SHG signals.

Results

We found that the SHG mean signal intensity correlated well with fibrosis stage and the mean CARS signal intensity with liver fat. Little overlap in SHG signal intensities between fibrosis stages 0 and 1 was observed. A specific fibrillar SHG signal was detected in the liver parenchyma outside portal areas in all samples histologically classified as having no fibrosis. This signal correlated with immunohistochemical location of fibrillar collagens I and III.

Conclusions

This study demonstrates that label-free SHG imaging detects fibrillar collagen deposition in NAFLD more sensitively than routine histological staging and enables observer-independent quantification of early fibrosis in NAFLD with continuous grading.