Two-photon microscopes and in vivo multiphoton tomographs--powerful diagnostic tools for tissue engineering and drug delivery

Extended-working-distance multiphoton micromanipulation microscope for deep-penetration imaging in live mice and tissue

We constructed a multiphoton (2-P) microscope with space to mount and operate microphysiology hardware, and still acquire high quality 2-P images of tumor cells deep within tissues of live mice. We reconfigured for nondescanned 2-P imaging, a dedicated electrophysiology microscope, the Nikon FN1. This microscope is compact, with retractable objectives, allowing more stage space. The instrument is fitted with long-working-distance objectives (2.5- to 3.5-mm WD) with a narrow bore, high NA, and efficient UV and IR light transmission. The system is driven by a powerful 3.5-W peak power pulsed Ti-sapphire laser with a broad tuning range. This 2-P system images a fluorescent standard to a depth of 750 to 800 microm, acquires images of murine pancreatic tumors in vivo, and also images fluorescently labeled T-cells inside live, externalized mouse lymph nodes. Effective imaging depths range between 100 and 500 microm. This compares favorably with the 100- to 300 microm micron depth attained by many 2-P systems, especially descanned 2-P instruments, and 40-microm-deep imaging with confocal microscopes. The greater depth penetration is attributable to the use of high-NA long-working-distance water-dipping lenses incorporated into a nondescanned instrument with carefully configured laser beam introduction and image-acquisition optics. Thus the new system not only has improved imaging capabilities, but allows micromanipulation and maintenance of tissues and organs.

Sensitivity and specificity of multiphoton laser tomography for in vivo and ex vivo diagnosis of malignant melanoma

The incidence of malignant melanoma has shown a dramatic increase over the past three decades. Patient outcome and curability depend on early diagnosis. In vivo multiphoton laser tomography represents a recently developed diagnostic tool that allows non-invasive tissue imaging. We aim to demonstrate the application of multiphoton laser tomography for the in vivo differentiation and diagnosis of melanoma. Laser radiation in the near infrared spectrum was used to image endogenous fluorophores by multiphoton excitation. Eighty-three melanocytic skin lesions have been investigated. The results showed distinct morphological differences in melanoma compared with melanocytic nevi. In particular, six characteristic features of malignant melanoma were specified and statistically evaluated. Sensitivity values up to 95% (range: 71-95%) and specificity values up to 97% (range: 69-97%) were achieved for diagnostic classification. Logistic regression analysis was performed to identify the most significant diagnostic criteria. We found that architectural disarray of the epidermis, poorly defined keratinocyte cell borders as well as the presence of pleomorphic or dendritic cells were of prime importance. By means of this procedure accuracy values up to 97% were reached. These findings underline the potential applicability of multiphoton laser tomography in melanoma diagnosis of melanocytic skin lesions.

Noninvasive in vitro and in vivo assessment of epidermal hyperkeratosis and dermal fibrosis in atopic dermatitis

Atopic dermatitis (AD) is characterized by hyperkeratosis of epidermis and fibrosis within dermis in chronic skin lesions. Thus far, the histology of skin lesions has been evaluated only by examination of excised specimens. A noninvasive in vivo tool is essential to evaluate the histopathological changes during the clinical course of AD. We used Cr:forsterite laser-based multimodality nonlinear microscopy to analyze the endogenous molecular signals, including third-harmonic generation (THG), second-harmonic generation (SHG), and two-photon fluorescence (TPF) from skin lesions in AD. Significant differences in thickness of epidermis and stratum corneum (SC), and modified degrees of fibrosis in dermis (measured by THG signals and SHG signals, respectively), are clearly demonstrated in in vitro studies. Increased TPF levels are positively associated with the levels of the THG signals from the SC. Our in vitro observations of histological changes are replicated in the in vivo studies. These findings were reproducible in skin lesions from human AD. For the first time, we demonstrate the feasibility of preclinical applications of Cr:forsterite laser-based nonlinear microscopy. Our findings suggest that the optical signatures of THG, TPF, and SHG can be used as molecular markers to assess the pathophysiological process of AD and the effects of local treatment.

Two-photon autofluorescence dynamics imaging reveals sensitivity of intracellular NADH concentration and conformation to cell physiology at the single-cell level

Reduced nicotinamide adenine dinucleotide, NADH, is a major electron donor in the oxidative phosphorylation and glycolytic pathways in cells. As a result, there has been recent resurgence in employing intrinsic NADH fluorescence as a natural probe for a range of cellular processes that include apoptosis, cancer pathology, and enzyme kinetics. Here, we report on two-photon fluorescence lifetime and polarization imaging of intrinsic NADH in breast cancer (Hs578T) and normal (Hs578Bst) cells for quantitative analysis of the concentration and conformation (i.e., free-to-enzyme-bound ratios) of this coenzyme. Two-photon fluorescence lifetime imaging of intracellular NADH indicates sensitivity to both cell pathology and inhibition of the respiratory chain activities using potassium cyanide (KCN). Using a newly developed non-invasive assay, we estimate the average NADH concentration in cancer cells (168+/-49 microM) to be approximately 1.8-fold higher than in breast normal cells (99+/-37 microM). Such analyses indicate changes in energy metabolism and redox reactions in normal breast cells upon inhibition of the respiratory chain activity using KCN. In addition, time-resolved associated anisotropy of cellular autofluorescence indicates population fractions of free (0.18+/-0.08) and enzyme-bound (0.82+/-0.08) conformations of intracellular NADH in normal breast cells. These fractions are statistically different from those in breast cancer cells (free: 0.25+/-0.08; bound: 0.75+/-0.08). Comparative studies on the binding kinetics of NADH with mitochondrial malate dehydrogenase and lactate dehydrogenase in solution mimic our findings in living cells. These quantitative studies demonstrate the potential of intracellular NADH dynamics (rather than intensity) imaging for probing mitochondrial anomalies associated with neurodegenerative diseases, cancer, diabetes, and aging. Our approach is also applicable to other metabolic and signaling pathways in living cells, without the need for cell destruction as in conventional biochemical assays.

In vitro and in vivo imaging of xenobiotic transport in human skin and in the rat liver

Multiphoton tomography was used to examine xenobiotic transport in vivo. We used the photochemical properties of zinc oxide and fluorescein and multiphoton tomography to study their transport in the skin and in the rat liver in vivo. Zinc oxide nanoparticles were visualised in human skin using the photoluminescence properties of zinc oxide and either a selective emission wavelength band pass filter or a filter with fluorescence lifetime imaging (FLIM). Zinc oxide nanoparticles (30 nm) did not penetrate into human skin in vitro and in vivo and this was validated by scanning electron microscopy with X-ray photoelectron spectroscopy. Fluorescein was measured in the liver using FLIM. Fluorescein is rapidly extracted from the blood into the liver cells and then transported into the bile. It is suggested that multiphoton tomography may be of particular use in defining in vivo 4D (in both space and time) pharmacokinetics.

Non-invasive multiphoton imaging of extracellular matrix structures

Multiphoton microscopy has become a powerful method for the artifact-free, nondestructive evaluation of deep-tissue cells and extracellular matrix (ECM) structures in their native environment. By interacting with highly non-centrosymmetric molecular assemblies such as fibrillar collagen, the non-linear process called second harmonic generation (SHG) has also proven to be an important diagnostic tool for the visualization of ECM compartments in situ with submicron resolution without the need for tissue processing. This review reports on applications of multiphoton-induced autofluorescence and SHG microscopy to identify collagen and elastic fiber orientation in native, tissue-engineered and processed, as well as healthy and diseased, tissues and organs. SHG signal profiling was used to quantify ECM damage in various cardiovascular and exocrine tissues, as well as cartilage. These novel imaging modalities open the general possibility of high-resolution in situ and more important in vivo imaging of ECM structures, cells and intracellular organelles in living intact tissues.

Biocompatibility of silica coated NaYF(4) upconversion fluorescent nanocrystals

Here we report the synthesis of uniform silica coated hexagonal-phase NaYF(4) nanocrystals with strong NIR-to-visible upconversion fluorescence and its cytotoxicity and biodistribution in a rat model. The silica coated NaYF(4) nanocrystals were incubated with rat skeletal myoblasts and bone marrow-derived mesenchymal stem cells and cytotoxicity was assessed by using MTS and LDH assay. Healthy rats were injected intravenously with the nanocrystals so as to further investigate their biocompatibility and tissue distribution. The results from this study revealed that the silica coated NaYF(4) upconversion nanocrystals displayed good in vitro and in vivo biocompatibility, demonstrating their potential applications in both cellular and animal imaging systems.

Quantitative assessment of neuronal differentiation in three-dimensional collagen gels using enhanced green fluorescence protein expressing PC12 pheochromocytoma cells

There is a paucity of quantitative methods for evaluating the morphological differentiation of neuronal cells in a three-dimensional (3-D) system to assist in quality control of neural tissue engineering constructs for use in reparative medicine. Neuronal cells tend to aggregate in the 3-D scaffolds, hindering the application of two-dimensional (2-D) morphological methods to quantitate neuronal differentiation. To address this problem, we developed a stable transfectant green fluorescence protein (GFP)-PC12 neuronal cell model, in which the differentiation process in 3-D can be monitored with high sensitivity by fluorescence microscopy. Under 2-D conditions, the green cells showed collagen adherence, round morphology, proliferation properties, expression of the nerve growth factor (NGF) receptors TrkA and p75(NTR), stimulation of extracellular signal-regulated kinase phosphorylation by NGF and were able to differentiate in a dose-dependent manner upon NGF treatment, like wild-type (wt)-PC12 cells. When grown within 3-D collagen gels, upon NGF treatment, the GFP-PC12 cells differentiated, expressing long neurite outgrowths. We describe here a new validated method to measure NGF-induced differentiation in 3-D. Having properties similar to those of wt-PC12 and an ability to grow and differentiate in 3-D structures, these highly visualized GFP-expressing PC12 cells may serve as an ideal model for investigating various aspects of differentiation to serve in neural engineering.

Quantitative second harmonic generation imaging of cartilage damage

Cartilage damage was studied using non-invasive multiphoton-excited autofluorescence and quantitative second harmonic generation (SHG) microscopy. Two cryopreservation techniques based upon freezing and vitrification methods, respectively, were employed to determine whether or not the collagen fiber structure of full thickness porcine articular cartilage was affected by cryopreservation and whether the level of collagen damage could be determined quantitatively in non-processed (non-fixed, non-sliced, non-stained) tissues. Multiphoton-induced autofluorescence imaging revealed the presence of chondrocytes, as well as collagenous structures in all fresh, vitrified and frozen cryopreserved cartilage samples. SHG imaging of the frozen cryopreserved specimens showed a dramatic loss of mean gray value intensities when compared to both fresh and vitrified tissues (P<0.05), indicating structural changes of the extracellular matrix, in particular the deformation and destruction of the collagen fibers in the analyzed articular cartilage. A 0.9974 correlation coefficient was observed between the metabolic cell activity assessed by the alamarBlue technique, and retention of collagen structure between the three experimental groups. These studies suggest that multiphoton-induced autofluorescence imaging combined with quantitative SHG signal profiling may prove to be useful tools for the investigation of extracellular matrix changes in preserved cartilage, giving insights on the structural quality prior to implantation.

Increased degradation of extracellular matrix structures of lacrimal glands implicated in the pathogenesis of Sjögren's syndrome

Lacrimal glands (LGs) of male non-obese diabetic (NOD) mice display many features of human LGs in patients afflicted with the autoimmune disease Sjögren's syndrome (SS), including the loss of secretory functions and a lymphocytic infiltration into the glands by 4 months of age. So far, research has mainly focused on the intracellular events that are involved in initiating LG dysfunction; however, the impact of SS on extracellular matrix (ECM) structures of the diseased LGs has not yet been determined. In this study we identified and compared LG ECM formation and integrity of age-matched male healthy (BALB/c) and diseased (NOD) mice. LG tissues were examined using routine histological, biochemical, immunohistochemical and gene expression analysis. Multiphoton imaging and second-harmonic generation (SHG) microscopy permitted the non-invasive analysis of major LG ECM structures including collagen- and elastin-containing fibers. Biochemical testing demonstrated a significant loss of collagen, glycosaminoglycans and desmosine in NOD LGs when compared to healthy BALB/c LGs. Immunohistochemical staining and gene expression analysis confirmed this disease-related alteration of LG ECM structures. Furthermore, laser-induced autofluorescence and SHG microscopy revealed dramatic changes in the structural organization of most collagenous and elastic fibers of the diseased LG tissues that were more pronounced than those displayed by histological analysis. Our results clearly show an enhanced degradation of ECM proteins accompanied by the severe disorganization and deformation of ECM structures of diseased LG tissues. These new insights into the involvement of ECM degradation in SS may lead to novel therapies for patients suffering from dry eye disease.

Quantitative biomarkers of stem cell differentiation based on intrinsic two-photon excited fluorescence

We present the use of two-photon excited fluorescence (TPEF) as a noninvasive means to monitor differentiation of human mesenchymal stem cells (hMSCs) into an adipogenic pathway relying entirely on endogenous sources of contrast. Specifically, we demonstrate that TPEF can be used to reveal quantitative differences in the biochemical status and the shape of differentiating and nondifferentiating stem cells in two-dimensional (2-D) cultures. We find that even in simple 2-D cultures, not all cells are undergoing differentiation at the same rate. Last, such noninvasive approaches may also ultimately allow for determination of the lineage toward which the cells are differentiating (e.g., fat versus bone). Thus, intrinsic TPEF imaging provides quantitative morphological and biochemical biomarkers associated with stem cell differentiation and could serve as an important enabling technology in tissue engineering applications.

In vivo imaging of elastic fibers using sulforhodamine B

Until now, the imaging of elastic fibers was restricted to tissue sections using the endofluorescence properties of elastin or histological dyes. Methods to study their morphology in vivo and in situ have been lacking. We present and characterize a new application of a fluorescent dye for two-photon microscopy: sulforhodamine B (SRB), which is shown to specifically stain elastic fibers in vivo. SRB staining of elastic fibers is demonstrated to be better than using elastin endofluorescence for two-photon microscopy. Our imaging method of elastic fibers is shown to be suitable for simultaneous imaging with both other fluorescent intravital dyes and second-harmonic generation (SHG). We illustrate these findings with intravital imaging of elastic and collagen fibers in muscle epimysium and endomysium and in blood vessel walls. We expect SRB staining to become a key method to study elastic fibers in vivo.

Intravital imaging and cell invasion

The main cause of cancer treatment failure is the invasion of normal tissues by cancer cells that have migrated from a primary tumor. An important obstacle to understanding cancer invasion has been the inability to acquire detailed, direct observations of the process over time in a living system. Intravital imaging, and the rodent dorsal skinfold window chamber in particular, were developed several decades ago to address this need. However, it is just recently, with the advent of sophisticated new imaging systems such as confocal and multiphoton microscopy together with the development of a wide range of fluorescent cellular and intracellular markers, that intravital methods and the window chamber have acquired powerful new potential for the study of cancer cell invasion. Moreover, the interaction of various cell signaling pathways with the integrin class of cell surface receptors has increasingly been shown to play a key role in cancer invasion. The window chamber in combination with integrin-knockout rodent models, integrin-deficient tumor cell lines, and integrin antagonists, allows real-time observation of integrin-mediated cancer invasion and angiogenesis. The present review outlines the history, uses, and recent methods of the rodent dorsal skinfold window chamber. The introduction of labeled tumor cells into the chamber is described, and imaging of tumors and angiogenic vessels within chambers using standard brightfield, confocal, and multiphoton microscopy is discussed in detail, along with the presentation of sample images.

Progression of bacterial infections studied in real time--novel perspectives provided by multiphoton microscopy

The holy grail of infection biology is to study a pathogen within its natural infectious environment, the living host. Advances in in vivo imaging techniques have begun to introduce the possibility to visualize, in real time, infection progression within a living model. In this review we detail the current advancements and knowledge in multiphoton microscopy and how it can be related to the field of microbial infections. This technology is a new and very valuable tool for in vivo imaging, and using this technique it is possible to begin to study various microbes within their natural infectious environment - the living host.

Optimized preservation of extracellular matrix in cardiac tissues: implications for long-term graft durability

BACKGROUND

Cryopreservation of human tissues, particularly heart valves, is widespread in clinical practice although the effects of this process on underlying tissue structures and its potential impact on valve durability have been poorly studied. Multiphoton imaging and second-harmonic generation (SHG) microscopy permit high-resolution, noninvasive analysis of living tissues at a subcellular level. In the present study we used these novel imaging modalities to compare the effects of vitreous and frozen cryopreservation on the extracellular matrix (ECM) of cardiac tissues.

METHODS

Conventional histology, electron microscopy, and multiphoton imaging to obtain autofluorescence and SHG images were performed on cardiac tissues to characterize the ECM in fresh, vitrified, and frozen cryopreserved tissues.

RESULTS

Autofluorescence and particularly SHG images revealed that conventional frozen cryopreservation of cardiac valves, when compared with fresh or vitrified tissues, leads to the loss of normal ECM structures in valve leaflets. Similar results were found in all other cardiac tissues suggesting that structural deterioration of the ECM is a common consequence of frozen cryopreservation.

CONCLUSIONS

Our results demonstrate that conventional cryopreservation, when compared with fresh or vitrified tissues, causes more destruction of normal ECM structure, which might contribute to eventual graft dysfunction. Whether vitrification preservation will translate into greater durability or less valve failure will need to be determined.