Single-photon counting multicolor multiphoton fluorescence microscope

Measurement of drug-target engagement in live cells by two-photon fluorescence anisotropy imaging

The ability to directly image and quantify drug-target engagement and drug distribution with subcellular resolution in live cells and whole organisms is a prerequisite to establishing accurate models of the kinetics and dynamics of drug action. Such methods would thus have far-reaching applications in drug development and molecular pharmacology. We recently presented one such technique based on fluorescence anisotropy, a spectroscopic method based on polarization light analysis and capable of measuring the binding interaction between molecules. Our technique allows the direct characterization of target engagement of fluorescently labeled drugs, using fluorophores with a fluorescence lifetime larger than the rotational correlation of the bound complex. Here we describe an optimized protocol for simultaneous dual-channel two-photon fluorescence anisotropy microscopy acquisition to perform drug-target measurements. We also provide the necessary software to implement stream processing to visualize images and to calculate quantitative parameters. The assembly and characterization part of the protocol can be implemented in 1 d. Sample preparation, characterization and imaging of drug binding can be completed in 2 d. Although currently adapted to an Olympus FV1000MPE microscope, the protocol can be extended to other commercial or custom-built microscopes.

Near-common-path interferometer for imaging Fourier-transform spectroscopy in wide-field microscopy

Imaging Fourier-transform spectroscopy (IFTS) is a powerful method for biological hyperspectral analysis based on various imaging modalities, such as fluorescence or Raman. Since the measurements are taken in the Fourier space of the spectrum, it can also take advantage of compressed sensing strategies. IFTS has been readily implemented in high-throughput, high-content microscope systems based on wide-field imaging modalities. However, there are limitations in existing wide-field IFTS designs. Non-common-path approaches are less phase-stable. Alternatively, designs based on the common-path Sagnac interferometer are stable, but incompatible with high-throughput imaging. They require exhaustive sequential scanning over large interferometric path delays, making compressive strategic data acquisition impossible. In this paper, we present a novel phase-stable, near-common-path interferometer enabling high-throughput hyperspectral imaging based on strategic data acquisition. Our results suggest that this approach can improve throughput over those of many other wide-field spectral techniques by more than an order of magnitude without compromising phase stability.

Real-time high dynamic range laser scanning microscopy

In conventional confocal/multiphoton fluorescence microscopy, images are typically acquired under ideal settings and after extensive optimization of parameters for a given structure or feature, often resulting in information loss from other image attributes. To overcome the problem of selective data display, we developed a new method that extends the imaging dynamic range in optical microscopy and improves the signal-to-noise ratio. Here we demonstrate how real-time and sequential high dynamic range microscopy facilitates automated three-dimensional neural segmentation. We address reconstruction and segmentation performance on samples with different size, anatomy and complexity. Finally, in vivo real-time high dynamic range imaging is also demonstrated, making the technique particularly relevant for longitudinal imaging in the presence of physiological motion and/or for quantification of in vivo fast tracer kinetics during functional imaging.

Confocal and multiphoton fluorescence microscopy often suffers from low dynamic range. Here the authors develop a high dynamic range, laser scanning fluorescence technique by simultaneously recording different light intensity ranges. The method can be adapted to commercial systems.

Surface biology of collagen scaffold explains blocking of wound contraction and regeneration of skin and peripheral nerves

We review the details of preparation and of the recently elucidated mechanism of biological (regenerative) activity of a collagen scaffold (dermis regeneration template, DRT) that has induced regeneration of skin and peripheral nerves (PN) in a variety of animal models and in the clinic. DRT is a 3D protein network with optimized pore size in the range 20-125 µm, degradation half-life 14 ± 7 d and ligand densities that exceed 200 µM α1β1 or α2β1 ligands. The pore has been optimized to allow migration of contractile cells (myofibroblasts, MFB) into the scaffold and to provide sufficient specific surface for cell-scaffold interaction; the degradation half-life provides the required time window for satisfactory binding interaction of MFB with the scaffold surface; and the ligand density supplies the appropriate ligands for specific binding of MFB on the scaffold surface. A dramatic change in MFB phenotype takes place following MFB-scaffold binding which has been shown to result in blocking of wound contraction. In both skin wounds and PN wounds the evidence has shown clearly that contraction blocking by DRT is followed by induction of regeneration of nearly perfect organs. The biologically active structure of DRT is required for contraction blocking; well-matched collagen scaffold controls of DRT, with structures that varied from that of DRT, have failed to induce regeneration. Careful processing of collagen scaffolds is required for adequate biological activity of the scaffold surface. The newly understood mechanism provides a relatively complete paradigm of regenerative medicine that can be used to prepare scaffolds that may induce regeneration of other organs in future studies.

In Situ Quantification of Surface Chemistry in Porous Collagen Biomaterials

Cells inside a 3D matrix (such as tissue extracellular matrix or biomaterials) sense their insoluble environment through specific binding interactions between their adhesion receptors and ligands present on the matrix surface. Despite the critical role of the insoluble matrix in cell regulation, there exist no widely-applicable methods for quantifying the chemical stimuli provided by a matrix to cells. Here, we describe a general-purpose technique for quantifying in situ the density of ligands for specific cell adhesion receptors of interest on the surface of a 3D matrix. This paper improves significantly the accuracy of the procedure introduced in a previous publication by detailed marker characterization, optimized staining, and improved data interpretation. The optimized methodology is utilized to quantify the ligands of integrins α 1 β 1, α 2 β 1 on two kinds of matched porous collagen scaffolds, which are shown to possess significantly different ligand density, and significantly different ability to induce peripheral nerve regeneration in vivo. Data support the hypothesis that cell adhesion regulates contractile cell phenotypes, recently shown to be inversely related to organ regeneration. The technique provides a standardized way to quantify the surface chemistry of 3D matrices, and a means for introducing matrix effects in quantitative biological models.

A pragmatic guide to multiphoton microscope design

Multiphoton microscopy has emerged as a ubiquitous tool for studying microscopic structure and function across a broad range of disciplines. As such, the intent of this paper is to present a comprehensive resource for the construction and performance evaluation of a multiphoton microscope that will be understandable to the broad range of scientific fields that presently exploit, or wish to begin exploiting, this powerful technology. With this in mind, we have developed a guide to aid in the design of a multiphoton microscope. We discuss source selection, optical management of dispersion, image-relay systems with scan optics, objective-lens selection, single-element light-collection theory, photon-counting detection, image rendering, and finally, an illustrated guide for building an example microscope.

Depth resolved hyperspectral imaging spectrometer based on structured light illumination and Fourier transform interferometry

A depth resolved hyperspectral imaging spectrometer can provide depth resolved imaging both in the spatial and the spectral domain. Images acquired through a standard imaging Fourier transform spectrometer do not have the depth-resolution. By post processing the spectral cubes (x, y, λ) obtained through a Sagnac interferometer under uniform illumination and structured illumination, spectrally resolved images with depth resolution can be recovered using structured light illumination algorithms such as the HiLo method. The proposed scheme is validated with in vitro specimens including fluorescent solution and fluorescent beads with known spectra. The system is further demonstrated in quantifying spectra from 3D resolved features in biological specimens. The system has demonstrated depth resolution of 1.8 μm and spectral resolution of 7 nm respectively.

Differential Multiphoton Laser Scanning Microscopy

Multifocal multiphoton microscopy (MMM) in the biological and medical sciences has become an important tool for obtaining high resolution images at video rates. While current implementations of MMM achieve very high frame rates, they are limited in their applicability to essentially those biological samples that exhibit little or no scattering. In this paper, we report on a method for MMM in which imaging detection is not necessary (single element point detection is implemented), and is therefore fully compatible for use in imaging through scattering media. Further, we demonstrate that this method leads to a new type of MMM wherein it is possible to simultaneously obtain multiple images and view differences in excitation parameters in a single shot.

Quantifying the dynamics of bacterial secondary metabolites by spectral multiphoton microscopy

Phenazines, a group of fluorescent small molecules produced by the bacterium Pseudomonas aeruginosa, play a role in maintaining cellular redox homeostasis. Phenazines have been challenging to study in vivo due to their redox activity, presence both intra- and extracellularly, and their diverse chemical properties. Here, we describe a noninvasive in vivo optical technique to monitor phenazine concentrations within bacterial cells using time-lapsed spectral multiphoton fluorescence microscopy. This technique enables simultaneous monitoring of multiple weakly fluorescent molecules (phenazines, siderophores, NAD(P)H) expressed by bacteria in culture. This work provides the first in vivo measurements of reduced phenazine concentration as well as the first description of the temporal dynamics of the phenazine-NAD(P)H redox system in Pseudomonas aeruginosa, illuminating an unanticipated role for 1-hydroxyphenazine. Similar approaches could be used to study the abundance and redox dynamics of a wide range of small molecules within bacteria, both as single cells and in communities.

Photon counting, censor corrections, and lifetime imaging for improved detection in two-photon microscopy

We present a high-speed photon counter for use with two-photon microscopy. Counting pulses of photocurrent, as opposed to analog integration, maximizes the signal-to-noise ratio so long as the uncertainty in the count does not exceed the gain-noise of the photodetector. Our system extends this improvement through an estimate of the count that corrects for the censored period after detection of an emission event. The same system can be rapidly reconfigured in software for fluorescence lifetime imaging, which we illustrate by distinguishing between two spectrally similar fluorophores in an in vivo model of microstroke.

An optical method to quantify the density of ligands for cell adhesion receptors in three-dimensional matrices

The three-dimensional matrix that surrounds cells is an important insoluble regulator of cell phenotypes. Examples of such insoluble surfaces are the extracellular matrix (ECM), ECM analogues and synthetic polymeric biomaterials. Cell-matrix interactions are mediated by cell adhesion receptors that bind to chemical entities (adhesion ligands) on the surface of the matrix. There are currently no established methods to obtain quantitative estimates of the density of adhesion ligands recognized by specific cell adhesion receptors. This article presents a new optical-based methodology for measuring ligands of adhesion receptors on three-dimensional matrices. The study also provides preliminary quantitative results for the density of adhesion ligands of integrins alpha(1)beta(1) and alpha(2)beta(1) on the surface of collagen-based scaffolds, similar to biomaterials that are used clinically to induce regeneration in injured skin and peripheral nerves. Preliminary estimates of the surface density of the ligands of these two major collagen-binding receptors are 5775 +/- 2064 ligands microm(-2) for ligands of alpha(1)beta(1) and 17 084 +/- 5353 ligands microm(-2) for ligands of alpha(2)beta(1). The proposed methodology can be used to quantify the surface chemistry of insoluble surfaces that possess biological activity, such as native tissue ECM and biomaterials, and therefore can be used in cell biology, biomaterials science and regenerative medical studies for quantitative description of a matrix and its effects on cells.

Quantitative morphometric measurements using site selective image cytometry of intact tissue

Site selective two-photon tissue image cytometry has previously been successfully applied to measure the number of rare cells in three-dimensional tissue specimens up to cubic millimetres in size. However, the extension of this approach for high-throughput quantification of cellular morphological states has not been demonstrated. In this paper, we report the use of site-selective tissue image cytometry for the study of homologous recombination (HR) events during cell division in the pancreas of transgenic mice. Since HRs are rare events, recombinant cells distribute sparsely inside the organ. A detailed measurement throughout the whole tissue is thus not practical. Instead, the site selective two-photon tissue cytometer incorporates a low magnification, wide field, one-photon imaging subsystem that rapidly identifies regions of interest containing recombinant cell clusters. Subsequently, high-resolution three-dimensional assays based on two-photon microscopy can be performed only in these regions of interest. We further show that three-dimensional morphology extraction algorithms can be used to analyse the resultant high-resolution two-photon image stacks providing information not only on the frequency and the distribution of these recombinant cell clusters and their constituent cells, but also on their morphology.

Changes in the biochemical constituents and morphologic appearance of the human cervical stroma during pregnancy

OBJECTIVE

The cervix is the lower portion of the uterus. It is composed of fibrous tissue and its mechanical integrity is crucial for maintaining a healthy gestation. During normal pregnancy, the cervical extracellular matrix (ECM) remodels in preparation for labor. The objective of this study was to investigate the biochemical and morphological changes in cervical stroma associated with physiological remodeling during normal pregnancy.

STUDY DESIGN

Using human cervical tissue obtained from pregnant and non-pregnant patients, the ECM was analyzed for its biochemical constituents and histologic morphology. The ECM was assayed for hydration, collagen concentration, collagen solubility, total sulfated glycosaminoglycan concentration, and individual disaccharide concentration. The ECM morphology was visualized using conventional histological techniques (Masson's trichrome stain, polarized light microscopy) as well as second harmonic generation (SHG) imaging.

RESULTS

When comparing pregnant to non-pregnant tissue, significant increases were measured for total sulfated glycosaminoglycans, hyaluronic acid, and collagen solubility. The microscopy studies confirmed that the collagenous network of the cervical stroma was anisotropic and pregnancy was associated with a discernable decrease in collagen organization.

CONCLUSION

Significant changes were seen in the concentration and organization of cervical ECM constituents during normal pregnancy.

Spectral unmixing: analysis of performance in the olfactory bulb in vivo

Background

The generation of transgenic mice expressing combinations of fluorescent proteins has greatly aided the reporting of activity and identification of specific neuronal populations. Methods capable of separating multiple overlapping fluorescence emission spectra, deep in the living brain, with high sensitivity and temporal resolution are therefore required. Here, we investigate to what extent spectral unmixing addresses these issues.

Methodology/Principal Findings

Using fluorescence resonance energy transfer (FRET)-based reporters, and two-photon laser scanning microscopy with synchronous multichannel detection, we report that spectral unmixing consistently improved FRET signal amplitude, both in vitro and in vivo. Our approach allows us to detect odor-evoked FRET transients 180–250 µm deep in the brain, the first demonstration of in vivo spectral imaging and unmixing of FRET signals at depths greater than a few tens of micrometer. Furthermore, we determine the reporter efficiency threshold for which FRET detection is improved by spectral unmixing.

Conclusions/Significance

Our method allows the detection of small spectral variations in depth in the living brain, which is essential for imaging efficiently transgenic animals expressing combination of multiple fluorescent proteins.

ULTRASHORT PULSE MULTISPECTRAL NONLINEAR OPTICAL MICROSCOPY

Ultrashort pulse, multispectral nonlinear optical microscopy (NLOM) is developed and used to image, simultaneously, a mixed population of cells expressing different fluorescent protein mutants in a 3D tissue model of angiogenesis. Broadband, sub-10-fs pulses are used to excite multiple fluorescent proteins and generate second harmonic in collagen simultaneously. A 16-channel multispectral detector is used to delineate the multiple nonlinear optical signals, pixel by pixel, in NLOM. The ability to image multiple fluorescent protein mutants and collagen, simultaneously, enables serial measurements of cell-cell and cell-matrix interactions in our 3D tissue model and characterization of fundamental processes in angiogenic morphogenesis.

Single-photon-counting detector for increased sensitivity in two-photon laser scanning microscopy

We present the use and characterization of a photon-counting detector for increased sensitivity at low signal levels in fluorescence laser scanning microscopy (LSM). Conventional LSM photomultiplier tube detectors utilize analog current integration and thus suffer from excessive noise at low signal levels, introduced during current measurement. In this Letter we describe the implementation of a fast single-photon-counting (SPC) detector on a conventional two-photon laser scanning microscope and detail its use in imaging low fluorescence intensities. We show that for a low photon flux, the SPC detector is shot-noise limited and thus provides increased detection sensitivity compared with analog current integration.

In vivo nonlinear spectral imaging microscopy of visible and ultraviolet irradiated hairless mouse skin tissues

We demonstrate the capability of nonlinear spectral imaging microscopy (NSIM) in investigating ultraviolet and visible light induced effects on albino Skh:HR-1 hairless mouse skin non-invasively.

Analog single-photon counter for high-speed scanning microscopy

We introduce a novel single-photon sensitive photodetection method of analog single-photon counting (SPC) for the application of high-speed scanning microscopy that requires high measurement speed and wide dynamic range for the photodetector. This scheme is based on analog electronic circuits which can perform proper differentiation and integration operations before and after discrimination of the analog signal from the photomultiplier tube (PMT), respectively. In spite of its simpler implementation, our analog SPC scheme exhibits good sensitivity and operation stability. Related with the dynamic range, the maximum count rate of our analog SPC is significantly improved due to the fast operation of the analog circuitry. This characteristic of the higher counting rate makes this scheme very suitable for high-speed scanning microscopy. It has also been demonstrated that the afterpulsing problem of an analog-mode PMT is the major noise source that degrades the image quality in the application of scanning microscopy, and our SPC scheme successfully neutralizes this kind of impulse noises to obtain a nearly shot-noise-limited imaging performance.

Spectrally resolved fluorescence correlation spectroscopy based on global analysis

Multicolor fluorescence correlation spectroscopy has been recently developed to study chemical interactions of multiple chemical species labeled with spectrally distinct fluorophores. In the presence of spectral overlap, there exists a lower detectability limit for reaction products with multicolor fluorophores. In addition, the ability to separate bound product from reactants allows thermodynamic properties such as dissociation constants to be measured for chemical reactions. In this report, we utilize a spectrally resolved two-photon microscope with single-photon counting sensitivity to acquire spectral and temporal information from multiple chemical species. Further, we have developed a global fitting analysis algorithm that simultaneously analyzes all distinct auto- and cross-correlation functions from 15 independent spectral channels. We have demonstrated that the global analysis approach allows the concentration and diffusion coefficients of fluorescent particles to be resolved despite the presence of overlapping emission spectra.

Three-dimensional tissue cytometer based on high-speed multiphoton microscopy

Image cytometry technology has been extended to 3D based on high-speed multiphoton microscopy. This technique allows in situ study of tissue specimens preserving important cell-cell and cell-extracellular matrix interactions. The imaging system was based on high-speed multiphoton microscopy (HSMPM) for 3D deep tissue imaging with minimal photodamage. Using appropriate fluorescent labels and a specimen translation stage, we could quantify cellular and biochemical states of tissues in a high throughput manner. This approach could assay tissue structures with subcellular resolution down to a few hundred micrometers deep. Its throughput could be quantified by the rate of volume imaging: 1.45 mm(3)/h with high resolution. For a tissue containing tightly packed, stratified cellular layers, this rate corresponded to sampling about 200 cells/s. We characterized the performance of 3D tissue cytometer by quantifying rare cell populations in 2D and 3D specimens in vitro. The measured population ratios, which were obtained by image analysis, agreed well with the expected ratios down to the ratio of 1/10(5). This technology was also applied to the detection of rare skin structures based on endogenous fluorophores. Sebaceous glands and a cell cluster at the base of a hair follicle were identified. Finally, the 3D tissue cytometer was applied to detect rare cells that had undergone homologous mitotic recombination in a novel transgenic mouse model, where recombination events could result in the expression of enhanced yellow fluorescent protein in the cells. 3D tissue cytometry based on HSMPM demonstrated its screening capability with high sensitivity and showed the possibility of studying cellular and biochemical states in tissues in situ. This technique will significantly expand the scope of cytometric studies to the biomedical problems where spatial and chemical relationships between cells and their tissue environments are important.

Multifunctional Imaging of Endogenous Contrast by Simultaneous Nonlinear and Optical Coherence Microscopy of Thick Tissues

A variety of high resolution optical microscopy techniques have been developed in recent years for basic and clinical studies of biological systems. We demonstrate a trimodal microscope combining optical coherence microscopy (OCM) with two forms of nonlinear microscopy, namely two-photon excited fluorescence (2PF) and second harmonic generation (SHG), for imaging turbid media. OCM combines the advantages of confocal detection and coherence gating for structural imaging in highly scattering tissues. Nonlinear microscopy enables the detection of biochemical species, such as elastin, NAD(P)H, and collagen. While 2PF arises from nonlinear excitation of fluorescent species, SHG is a form of nonlinear scattering observed in materials that lack a center of inversion symmetry, such as type I collagen. Characterization of the microscope showed nearly diffraction-limited spatial resolution in all modalities. Images were obtained in fish scales and excised human skin samples. The primary endogenous sources of contrast in the dermis were due to elastin autofluorescence and collagen SHG. Multimodal microscopy allows the simultaneous visualization of structural and functional information of biological systems.

Frequency domain lifetime and spectral imaging microscopy

In the femtoliter observation volume of a two-photon microscope, multiple fluorophores can be present and complex photophysics can take place. Combined detection of the fluorescence emission spectra and lifetimes can provide deeper insight into specimen properties than these two imaging modalities taken separately. Therefore, we have developed a detection scheme based on a frequency-modulated multichannel photomultiplier, which measures simultaneously the spectrum and the lifetime of the emitted fluorescence. Experimentally, the efficiency of the frequency domain lifetime measurement was compared to a time domain set-up. The performance of this spectrally and lifetime-resolved microscope was evaluated on reference specimens and living cells labeled with three different stains targeting the membrane, the mitochondria, and the nucleus.

Hyperspectral confocal microscope

We have developed a new, high performance, hyperspectral microscope for biological and other applications. For each voxel within a three-dimensional specimen, the microscope simultaneously records the emission spectrum from 500 nm to 800 nm, with better than 3 nm spectral resolution. The microscope features a fully confocal design to ensure high spatial resolution and high quality optical sectioning. Optical throughput and detection efficiency are maximized through the use of a custom prism spectrometer and a backside thinned electron multiplying charge coupled device (EMCCD) array. A custom readout mode and synchronization scheme enable 512-point spectra to be recorded at a rate of 8300 spectra per second. In addition, the EMCCD readout mode eliminates curvature and keystone artifacts that often plague spectral imaging systems. The architecture of the new microscope is described in detail, and hyperspectral images from several specimens are presented.

Comparing the quantification of Forster resonance energy transfer measurement accuracies based on intensity, spectral, and lifetime imaging

The measurement of Forster resonance energy transfer (FRET) in microscopes can be realized by different imaging modalities. In the present work, reference FRET constructs are developed to allow the comparison of FRET microscopy measurements using intensity, spectral, and lifetime imaging. Complimentary DNA strands are respectively labeled with Oregon Green 488 (OG488) or tetramethylrhodamine (TMR). The OG488 dye is fixed at the 5(') end of one strand, and the TMR label position is allowed to vary along the complimentary strand. Since OG488 and TMR are FRET pairs, the FRET efficiency can be determined theoretically from the distance separating the two dyes of the double-stranded DNA molecules. Microscopic images are formed by imaging microcapillaries containing various mixtures of oligonucleotides labeled with the FRET fluorophore pair, only the donor, or only acceptor. Traditional two-channel intensity measurements are compared with spectrally resolved imaging and fluorescence lifetime imaging by calculating a FRET index. The latter proves to be the best method to quantify FRET efficiency in the image. More importantly, the intensity fraction of molecules undergoing FRET can be quantitatively measured in each pixel of the image.

Simultaneous imaging of GFP, CFP and collagen in tumors in vivo using multiphoton microscopy

BACKGROUND

The development of multiphoton laser scanning microscopy has greatly facilitated the imaging of living tissues. However, the use of genetically encoded fluorescent proteins to distinguish different cell types in living animals has not been described at single cell resolution using multiphoton microscopy.

RESULTS

Here we describe a method for the simultaneous imaging, by multiphoton microscopy, of Green Fluorescent Protein, Cyan Fluorescent Protein and collagen in vivo in living tumors. This novel method enables: 1) the simultaneous visualization of overall cell shape and sub-cellular structures such as the plasma membrane or proteins of interest in cells inside living animals, 2) direct comparison of the behavior of single cells from different cell lines in the same microenvironment in vivo.

CONCLUSION

Using this multi-fluor, multiphoton technique, we demonstrate that motility and metastatic differences between carcinoma cells of differing metastatic potential can be imaged in the same animal simultaneously at sub-cellular resolution.