Autofluorescence properties of rat liver under hypermetabolic conditions

Study of laser fluorescence spectroscopy in livers of rats with hypothermic ischemia

PURPOSE

After partial hepatectomy (PH), the remaining liver (RL) undergoes regenerative response proportional to the host. Limited literature exists on hepatic viability after tissue injury during hypothermic preservation. Spectroscopy measures cellular fluorescence and is explored for tissue characterization and parameter investigation. This study aimed to assess fluorescence analysis (spectroscopy) in evaluating liver viability and its relationship with hepatic tissue regeneration 24 hours after PH. Additionally, we analyzed liver regeneration in RL after 70% partial hepatectomy under hypothermic conditions with laser irradiation.

METHODS

Fifty-six Wistar rats were divided into four groups: total non-perfused liver (control), total perfused liver, partial hepatectomy "in situ", and partial hepatectomy "ex situ". Tissue analysis was performed at 0 and 24 hours using spectroscopy with laser devices emitting at 532 (green) and 405 nm (violet).

RESULTS

Spectroscopy identified tissue viability based on consistent results with Ki67 staining. The fluorescence spectra and Ki67 analysis displayed similar patterns, linking proliferative activity and absorption intensity.

CONCLUSIONS

Fluorescence spectroscopy proves to be promising for real-time analysis of cellular activity and viability. Metabolic activity was observed in groups of live animals and hypothermically preserved samples, indicating cellular function even under blood deprivation and hypothermic conditions.

Deconvolution of the fluorescence spectra measured through a needle probe to assess the functional state of the liver

OBJECTIVES

Currently, one of the most pressing issues for surgeons in the treatment of obstructive jaundice is the ability to assess the functional state of the liver and to detect and determine the degree of liver failure in a timely manner with simple and objective techniques. In this regard, the use of fluorescence spectroscopy method can be considered as one of the ways to improve the informativity of existing diagnostic algorithms in clinical practice and to introduce new diagnostic tools. Thus, the aim of the work was to study in vivo the functional state of liver parenchyma by the method of fluorescence spectroscopy implemented through a needle probe and assess the contribution of the main tissue fluorophores to reveal new diagnostic criteria.

MATERIALS AND METHODS

We compared data from 20 patients diagnosed with obstructive jaundice and 11 patients without this syndrome. Measurements were performed using a fluorescence spectroscopy method at excitation wavelengths of 365 and 450 nm. Data were collected using a 1 mm fiber optic needle probe. The analysis was based on the comparison of the results of deconvolution with the combinations of Gaussian curves reflecting the contribution of the pure fluorophores in the liver tissues.

RESULTS

The results showed a statistically significant increase in the contribution of curves reflecting NAD(P)H fluorescence, bilirubin, and flavins in the group of patients with obstructive jaundice. This and the calculated redox ratio values indicated that the energy metabolism of the hepatocytes may have shifted to glycolysis due to hypoxia. An increase in vitamin A fluorescence was also observed. It may also serve as a marker of liver damage, indicating impaired vitamin A mobilization from the liver due to cholestasis.

CONCLUSIONS

The results obtained reflect changes associated with shifts in the content of the main fluorophores characterizing hepatocyte dysfunction caused by accumulation of bilirubin and bile acids and after disturbance of oxygen utilization. The contributions of NAD(P)H, flavins, and bilirubin as well as vitamin A can be used for further studies as promising diagnostic and prognostic markers for the course of liver failure. Further work will include collecting fluorescence spectroscopy data in patients with different clinical effects of obstructive jaundice on postoperative clinical outcome after biliary decompression.

Autofluorescence-based optical biopsy: An effective diagnostic tool in hepatology

Autofluorescence emission of liver tissue depends on the presence of endogenous biomolecules able to fluoresce under suitable light excitation. Overall autofluorescence emission contains much information of diagnostic value because it is the sum of individual autofluorescence contributions from fluorophores involved in metabolism, for example, NAD(P)H, flavins, lipofuscins, retinoids, porphyrins, bilirubin and lipids, or in structural architecture, for example, fibrous proteins, in close relationship with normal, altered or diseased conditions of the liver. Since the 1950s, hepatocytes and liver have been historical models to study NAD(P)H and flavins as in situ, real-time autofluorescence biomarkers of energy metabolism and redox state. Later investigations designed to monitor organ responses to ischaemia/reperfusion were able to predict the risk of dysfunction in surgery and transplantation or support the development of procedures to ameliorate the liver outcome. Subsequently, fluorescent fatty acids, lipofuscin-like lipopigments and collagen were characterized as optical biomarkers of liver steatosis, oxidative stress damage, fibrosis and disease progression. Currently, serum AF is being investigated to improve non-invasive optical diagnosis of liver disease. Validation of endogenous fluorophores and in situ discrimination of cancerous from non-cancerous tissue belong to the few studies on liver in human subjects. These reports along with other optical techniques and the huge work performed on animal models suggest many optically based applications in hepatology. Optical diagnosis is currently offering beneficial outcomes in clinical fields ranging from the respiratory and gastrointestinal tracts, to dermatology and ophthalmology. Accordingly, this review aims to promote an effective bench to bedside transfer in hepatology.

NAD(P)H and Flavin Autofluorescence Correlation with ATP in Rat Livers with Different Metabolic Steady-State Conditions

The monitoring of NAD(P)H and flavin autofluorescence (AF) is at the basis of numerous investigations on energy metabolism. Nevertheless, the ability of these AF biomarkers to accurately represent the energy currency, ATP, is poorly explored. Here, we focused on the AF/ATP correlation in lean and fatty livers with different steady-state metabolic conditions, achieved after organ isolation, preservation and recovery, in a likely dependence on both liver intrinsic metabolic features and externally induced perturbations. Within these eventual, various conditions, a significant correlation was detected between liver NAD(P)H and flavin AF, measured via fiber-optic probe, and biochemical ATP data, strengthening AF as biomarker of energy metabolism in steady-state conditions for wide-ranging experimental and diagnostic applications.

Probing metabolic states of differentiating stem cells using two-photon FLIM

The ability of stem cells to differentiate into specialized cell types presents a number of opportunities for regenerative medicine, stem cell therapy and developmental biology. Because traditional assessments of stem cells are destructive, time consuming, and logistically intensive, the use of a non-invasive, label-free approach to study of cell differentiation provides a powerful tool for rapid, high-content characterization of cell and tissue cultures. Here, we elucidate the metabolic changes in MSCs during adipogenic differentiation, based on the fluorescence of the metabolic co-factors NADH, NADPH, and FAD using the methods of two-photon fluorescence microscopy combined with FLIM. To estimate the contribution of energy metabolism and lipogenesis in the observed changes of the metabolic profile, a separate analysis of NADH and NADPH is required. In our study we demonstrated, for the first time, an increased contribution of protein-bound NADPH in adipocytes that is associated with lipogenesis. The optical redox ratio FAD/NAD(P)H decreased during adipogenic differentiation, and that this was likely to be explained by the intensive biosynthesis of lipids and the enhanced NADPH production associated with this. Based on the data on the fluorescence lifetime contribution of protein-bound NAD(P)H, we registered a metabolic switch from glycolysis to oxidative phosphorylation in adipocytes.

Autofluorescence spectroscopy and imaging: a tool for biomedical research and diagnosis

Native fluorescence, or autofluorescence (AF), consists in the emission of light in the UV-visible, near-IR spectral range when biological substrates are excited with light at suitable wavelength. This is a well-known phenomenon, and the strict relationship of many endogenous fluorophores with morphofunctional properties of the living systems, influencing their AF emission features, offers an extremely powerful resource for directly monitoring the biological substrate condition. Starting from the last century, the technological progresses in microscopy and spectrofluorometry were convoying attention of the scientific community to this phenomenon. In the future, the interest in the autofluorescence will certainly continue. Current instrumentation and analytical procedures will likely be overcome by the unceasing progress in new devices for AF detection and data interpretation, while a progress is expected in the search and characterization of endogenous fluorophores and their roles as intrinsic biomarkers.

Bilirubin: an autofluorescence bile biomarker for liver functionality monitoring

Excitation at 366-465 nm of bilirubin in aqueous solution with solubilizing agents results in emission spectra composed by two main bands. The variation of their relative contributions as shown by changes in the spectral shape are consistent with the bilirubin bichromophore nature. This latter accounts for an exciton-coupling phenomenon, intramolecular interchromophore energy transfer efficiency being affected by microenvironment. Excitation at 366 nm, despite the poor absorption of bilirubin, gives rise to appreciable emission signals from both pure compounds and bile - collected from functionally altered rat livers - favouring the spectral shape response to environment and molecular conformation changes. As compared to the merely bile flow estimation, real-time detection of fluorescence, revealing composition variations, improves near-UV optical-biopsy diagnostic potential in hepatology.

Integrated autofluorescence characterization of a modified-diet liver model with accumulation of lipids and oxidative stress

Oxidative stress in fatty livers is mainly generated by impaired mitochondrial β-oxidation, inducing tissue damages and disease progression. Under suitable excitation, light liver endogenous fluorophores can give rise to autofluorescence (AF) emission, the properties of which depend on the organ morphofunctional state. In this work, we characterized the AF properties of a rat liver model of lipid accumulation and oxidative stress, induced by a 1–9-week hypercaloric methionine-choline deficient (MCD) diet administration. The AF analysis (excitation at 366 nm) was performed in vivo, via fiber optic probe, or ex vivo. The contribution of endogenous fluorophores involved in redox reactions and in tissue organization was estimated through spectral curve fitting analysis, and AF results were validated by means of different histochemical and biochemical assays (lipids, collagen, vitamin A, ROS, peroxidised proteins, and lipid peroxidation -TBARS-, GSH, and ATP). In comparison with the control, AF spectra changes found already at 1 week of MCD diet reflect alterations both in tissue composition and organization (proteins, lipopigments, and vitamin A) and in oxidoreductive pathway engagement (NAD(P)H, flavins), with a subsequent attempt to recover redox homeostasis. These data confirm the AF analysis potential to provide a comprehensive diagnostic information on negative effects of oxidative metabolism alteration.

Autofluorescence of liver tissue and bile: organ functionality monitoring during ischemia and reoxygenation

BACKGROUND AND OBJECTIVE

Autofluorescence (AF) based optical biopsy of liver tissue is a powerful approach for the real-time diagnosis of its functionality. Since increasing attention is given to the bile production and composition to monitor the liver metabolic engagement in surgery and transplantation, we have investigated the bile AF properties as a potential, additional diagnostic parameter.

STUDY DESIGN/MATERIALS AND METHODS

Spectrofluorometric analysis has been performed in real time on a rat liver model of warm ischemia and reperfusion-60 minutes partial portal vein and hepatic artery clamping and subsequent restoration of blood circulation-in comparison with sham operated rats. The AF spectra have been recorded through a single fiber optic probe (366 nm excitation) from both liver tissue and bile, collected from the cannulated bile duct, and analyzed by means of curve fitting procedures. Bile composition has been also analyzed through biochemical assays of bilirubin, total bile acids (TBA) and proteins.

RESULTS

Both liver and bile AF signal amplitude and spectral shape undergo changes during induction of ischemia and subsequent reperfusion. The liver tissue response is mainly ascribable to changes in NAD(P)H and flavins and their redox state, largely dependent on oxygen supply, and to the decrease of both vitamin A and fatty acid AF contributions. During comparable times, sham operated rat livers undergo smaller alterations in AF spectral shape, indicating a continuous, slight increase in the oxidized state. Bile AF emission shows a region in the 510-600 nm range ascribable to bilirubin, and resulting from the contribution of two bands, centered at about 515-523 and 570 nm, consistently with its bichromophore nature. Variations in the balance between these two bands depend on the influence of microenvironment on bilirubin intramolecular interchromophore energy transfer efficiency and are likely indicating alteration in a bile composition. This event is supported also by changes observed in the 400-500 nm emission region, ascribable to other bile components.

CONCLUSIONS

In parallel with the intratissue AF properties, mainly reflecting redox metabolic activities, the bile AF analysis can provide additional information to assess alterations and recovery in the balance of liver metabolic activities.

Optical imaging using endogenous contrast to assess metabolic state

Optical microscopic imaging offers opportunities to perform noninvasive assessments of numerous parameters associated with the biochemistry, morphology, and functional state of biological samples. For example, it is possible to detect the endogenous fluorescence from a small number of important biomolecules, including NADH and FAD, which are two coenzymes involved in key metabolic pathways such as glycolysis, the Krebs cycle, and oxidative phosphorylation. Here, we review different imaging approaches to isolate the fluorescence from these chromophores in two- and three-dimensional samples and discuss the origins and potential interpretation of the observed signals in terms of cell metabolic status. Finally, we discuss the challenges and limitations of these approaches, as well as important research directions that we expect will evolve in the near future.

Efficient enrichment of hepatic cancer stem-like cells from a primary rat HCC model via a density gradient centrifugation-centered method

Background

Because few definitive markers are available for hepatic cancer stem cells (HCSCs), based on physical rather than immunochemical properties, we applied a novel method to enrich HCSCs.

Methodology

After hepatic tumor cells (HTCs) were first isolated from diethylinitrosamine-induced F344 rat HCC model using percoll discontinuous gradient centrifugation (PDGC) and purified via differential trypsinization and differential attachment (DTDA), they were separated into four fractions using percoll continuous gradient centrifugation (PCGC) and sequentially designated as fractions I–IV (FI–IV). Morphological characteristics, mRNA and protein levels of stem cell markers, proliferative abilities, induced differentiation, in vitro migratory capacities, in vitro chemo-resistant capacities, and in vivo malignant capacities were determined for the cells of each fraction.

Findings

As the density of cells increased, 22.18%, 11.62%, 4.73% and 61.47% of primary cultured HTCs were segregated in FI–FIV, respectively. The cells from FIII (density between 1.041 and 1.062 g/ml) displayed a higher nuclear-cytoplasmic ratio and fewer organelles and expressed higher levels of stem cell markers (AFP, EpCAM and CD133) than cells from other fractions (P<0.01). Additionally, in vitro, the cells from FIII showed a greater capacity to self-renew, differentiate into mature HTCs, transit across membranes, close scratches, and carry resistance to chemotherapy than did cells from any other fraction; in vivo, injection of only 1×10⁴ cells from FIII could generate tumors not only in subcutaneous tissue but also in the livers of nude mice.

Conclusions

Through our novel method, HCSC-like cells were successfully enriched in FIII. This study will greatly contribute to two important areas of biological interest: CSC isolation and HCC therapy.

Multi-excitation fluorescence spectroscopy for analysis of non-alcoholic fatty liver disease

BACKGROUND AND OBJECTIVES

The increasing incidence of non-alcoholic fatty liver diseases (NAFLD) and the consequent progression to cirrhosis is expected to become a major cause of liver transplantation. This will exacerbate the organ donor shortage and mean that 'marginal' fatty liver grafts are more frequently used. Autofluorescence spectroscopy is a fast, objective, and non-destructive method to detect change in the endogenous fluorophores distribution and could prove to be a valuable tool for NAFLD diagnosis and transplant graft assessment.

MATERIALS AND METHODS

A system was constructed consisting of a fibre probe with two laser diodes that provided excitation light at 375 and 405 nm, and an imaging spectrograph system. This was used to distinguish fluorescence spectra acquired from the harvested livers from mice with NAFLD of differing severity (healthy, mild steatotic and steatohepatitic). The fluorescence data were entered into a sparse multiclass probabilistic algorithm for disease classification. Histopathology, thiobarbituric acid reactive substances (TBARS) and alanine transaminase (ALT) assays were conducted in addition to the fluorescence measurements

RESULTS

TBARS and ALT assays enabled differentiation of the steatohepatitic group from the mild steatosis and control groups (P ≤ 0.028) but failed to separate the mild steatotic group from the control group. The three groups were all clearly differentiated from each other using fluorescence spectroscopy, and classification accuracy was found to be 95%.

CONCLUSION

Fluorescence spectroscopy appears to be a promising approach for the analysis of diseased liver tissue.

Human liver autofluorescence: an intrinsic tissue parameter discriminating normal and diseased conditions

BACKGROUND AND OBJECTIVE

Autofluorescence (AF) emission is an intrinsic parameter that can provide real-time information on morpho-functional properties of biological tissue, being strictly related with their biochemical composition and structural organization. The diagnostic potentials of AF-based techniques have been investigated on normal, fibrotic, and steatotic liver tissues, in reference to histological features as evidenced by specific histochemical stainings.

MATERIALS AND METHODS

AF emission under excitation at 366 nm has been examined on cryostatic tissue sections obtained from biopsies collected during surgical operation, by means of fluorescence imaging and microspectrofluorometric techniques.

RESULTS

NAD(P)H, collagen, and vitamin A were found to be the endogenous fluorophores characterizing normal, fibrotic, and steatotic liver tissue AF, respectively. The differences of their photo-physical properties, in terms of emission amplitude, spectral shape, and response to irradiation, give rise to modifications of overall AF signal collected from tissues that allow the liver conditions to be distinguished.

CONCLUSION

The study provides a valid premise for a development of AF-based optical biopsy techniques for a real-time discrimination of liver anatomo-pathological patterns.

Two-photon microscopy for non-invasive, quantitative monitoring of stem cell differentiation

Background

The engineering of functional tissues is a complex multi-stage process, the success of which depends on the careful control of culture conditions and ultimately tissue maturation. To enable the efficient optimization of tissue development protocols, techniques suitable for monitoring the effects of added stimuli and induced tissue changes are needed.

Methodology/Principal Findings

Here, we present the quantitative use of two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) as a noninvasive means to monitor the differentiation of human mesenchymal stem cells (hMSCs) using entirely endogenous sources of contrast. We demonstrate that the individual fluorescence contribution from the intrinsic cellular fluorophores NAD(P)H, flavoproteins and lipofuscin can be extracted from TPEF images and monitored dynamically from the same cell population over time. Using the redox ratio, calculated from the contributions of NAD(P)H and flavoproteins, we identify distinct patterns in the evolution of the metabolic activity of hMSCs maintained in either propagation, osteogenic or adipogenic differentiation media. The differentiation of these cells is mirrored by changes in cell morphology apparent in high resolution TPEF images and by the detection of collagen production via SHG imaging. Finally, we find dramatic increases in lipofuscin levels in hMSCs maintained at 20% oxygen vs. those in 5% oxygen, establishing the use of this chromophore as a potential biomarker for oxidative stress.

Conclusions/Significance

In this study we demonstrate that it is possible to monitor the metabolic activity, morphology, ECM production and oxidative stress of hMSCs in a non-invasive manner. This is accomplished using generally available multiphoton microscopy equipment and simple data analysis techniques, such that the method can widely adopted by laboratories with a diversity of comparable equipment. This method therefore represents a powerful tool, which enables researchers to monitor engineered tissues and optimize culture conditions in a near real time manner.

Liver autofluorescence properties in animal model under altered nutritional conditions

Autofluorescence spectroscopy is a promising and powerful approach for an in vivo, real time characterization of liver functional properties. In this work, preliminary results on the dependence of liver autofluorescence parameters on the nutritional status are reported, with particular attention to vitamin A and lipid accumulation in liver tissue. Normally fed and 24 h starving rats were used as animal models. Histochemical and autofluorescence analysis showed that lipids and vitamin A colocalize in the liver parenchyma. Fasting condition results in a parallel increase in both lipids and vitamin A. Autofluorescence imaging and microspectrofluorometric analysis carried out on unfixed, unstained tissue sections under 366 nm excitation, evidenced differences in both spectral shape and response to continuous irradiation between liver biopsies from fed and starving rats. As to photobleaching, in particular, fitting analysis evidenced a reduction of about 85% of the signal attributable solely to vitamin A during the first 10 s of irradiation. The tissue whole emission measured in fed and starving rat livers exhibited reductions of about 35% and 52%, respectively, that are closely related to vitamin A contents. The findings open interesting perspectives for the set up of an in situ, real time diagnostic procedure for the assessment of liver lipid accumulation, exploiting the photophysical properties of vitamin A.