Pretransplantation assessment of renal viability with NADH fluorimetry

Label-Free Metabolic Imaging In Vivo by Two-Photon Fluorescence Lifetime Endomicroscopy

NADH intensity and fluorescence lifetime characteristics have proved valuable intrinsic biomarkers for profiling the cellular metabolic status of living biological tissues. To fully leverage the potential of NADH fluorescence lifetime imaging microscopy (FLIM) in (pre)clinical studies and translational applications, a compact and flexible endomicroscopic embodiment is essential. Herein we present our newly developed two-photon fluorescence (2PF) lifetime imaging endomicroscope (2p-FLeM) that features an about 2 mm diameter, subcellular resolution, and excellent emission photon utilization efficiency and can extract NADH lifetime parameters of living tissues and organs reliably using a safe excitation power (~30 mW) and moderate pixel dwelling time (≤10 μs). In vivo experiments showed that the 2p-FLeM system was capable of tracking NADH lifetime dynamics of cultured cancer cells and subcutaneous mouse tumor models subject to induced apoptosis, and of a functioning mouse kidney undergoing acute ischemia-reperfusion perturbation. The complementary structural and metabolic information afforded by the 2p-FLeM system promises functional histological imaging of label-free internal organs in vivo and in situ for practical clinical diagnosis and therapeutics applications.

Highly sensitive voltammetric determination of NADH based on N-CQDs decorated SnO2/ionic liquid/carbon paste electrode

A highly sensitive and selective modified electrode was successfully developed for the monitoring of nicotinamide adenine dinucleotide (NADH) in the presence of folic acid. In this regard, a carbon paste electrode (CPE) was functionalized by the nitrogen-doped carbon quantum dots/tin oxide (N-CQDs/SnO₂) nanocomposite and 1-butyl-2,3-dimethyl imidazolium hexafluorophosphate ([C₄DMIM][PF₆]) ionic liquid (IL). The structure and surface morphology of the nanocomposite were characterized by various methods, including field emission scanning electron microscopy, energy dispersive spectroscopy (EDS), high-resolution transmission electron microscopy (HR-TEM), and x-ray diffraction (XRD). The modified electrode displayed powerful and long-lasting electron mediating activity, with well-separated NADH and folic acid oxidation peaks. The sensing response of the developed [C₄DMIM][PF₆]/N-CQDs/SnO₂/CPE platform was evaluated by determining NADH via the voltammetric technique under the optimized operating conditions. The current peaks of the square wave voltammograms of NADH and folic acid increased linearly with enhancing its concentrations within the ranges of 0.003-275μM NADH and 0.4-380μM folic acid. The detection limits for NADH and folic acid were obtained at 0.8 nM and 0.1μM, respectively. Interference species such as glucose, urea, tryptophan, glycine, methionine, and vitamin B₁₂had no influence on the ability of the fabricated modified electrode to detect the target species. The low detection limit, high sensitivity, excellent selectivity, superior stability, and cost-effectiveness made it suitable for the quantification of NADH in the real biological samples with the recovery percent values in the range of 97.5%-103%.

Predictive assessment of kidney functional recovery following ischemic injury using optical spectroscopy

Functional changes in rat kidneys during the induced ischemic injury and recovery phases were explored using multimodal autofluorescence and light scattering imaging. The aim is to evaluate the use of noncontact optical signatures for rapid assessment of tissue function and viability. Specifically, autofluorescence images were acquired in vivo under 355, 325, and 266 nm illumination while light scattering images were collected at the excitation wavelengths as well as using relatively narrowband light centered at 500 nm. The images were simultaneously recorded using a multimodal optical imaging system. The signals were analyzed to obtain time constants, which were correlated to kidney dysfunction as determined by a subsequent survival study and histopathological analysis. Analysis of both the light scattering and autofluorescence images suggests that changes in tissue microstructure, fluorophore emission, and blood absorption spectral characteristics, coupled with vascular response, contribute to the behavior of the observed signal, which may be used to obtain tissue functional information and offer the ability to predict posttransplant kidney function.

Dynamic multiphoton microscopy: focusing light on acute kidney injury

Acute kidney injury (AKI) is a major global health problem; much research has been conducted on AKI, and numerous agents have shown benefit in animal studies, but none have translated into treatments. There is, therefore, a pressing unmet need to increase knowledge of the pathophysiology of AKI. Multiphoton microscopy (MPM) provides a tool to non-invasively visualize dynamic events in real time and at high resolution in rodent kidneys, and in this article we review its application to study novel mechanisms and treatments in different forms of AKI.

Activated protein C ameliorates impaired renal microvascular oxygenation and sodium reabsorption in endotoxemic rats

Introduction

We aimed to test whether continuous recombinant human activated protein C (APC) administration would be able to protect renal oxygenation and function during endotoxemia in order to provide more insight into the role of coagulation and inflammation in the development of septic acute kidney injury.

Methods

In anesthetized, mechanically ventilated Wistar rats, endotoxemia was induced by lipopolysaccharide administration (10 mg/kg i.v. over 30 min). One hour later, the rats received fluid resuscitation with 0 (LPS + FR group; n = 8), 10 (APC10 group; n = 8), or 100 (APC100 group; n = 8) μg/kg/h APC for 2 h. Renal microvascular oxygenation in the cortex and medulla were measured using phosphorimetry, and renal creatinine clearance rate and sodium reabsorption were measured as indicators of renal function. Statistical significance of differences between groups was tested using two-way ANOVA with Bonferroni post hoc tests.

Results

APC did not have notable effects on systemic and renal hemodynamic and oxygenation variables or creatinine clearance. The changes in renal microvascular oxygenation in both the cortex (r = 0.66; p < 0.001) and medulla (r = 0.80; p < 0.001) were correlated to renal sodium reabsorption_.

Conclusion

Renal sodium reabsorption is closely correlated to renal microvascular oxygenation during endotoxemia. In this study, fluid resuscitation and APC supplementation were not significantly effective in protecting renal microvascular oxygenation and renal function. The specific mechanisms responsible for these effects of APC warrant further study.

Electronic supplementary material

The online version of this article (doi:10.1186/2197-425X-1-5) contains supplementary material, which is available to authorized users.

In vivo multiphoton imaging of mitochondrial structure and function during acute kidney injury

Mitochondrial dysfunction has been implicated in the pathogenesis of acute kidney injury due to ischemia and toxic drugs. Methods for imaging mitochondrial function in cells using confocal microscopy are well established; more recently, it was shown that these techniques can be utilized in ex vivo kidney tissue using multiphoton microscopy. We extended this approach in vivo and found that kidney mitochondrial structure and function can be imaged in anesthetized rodents using multiphoton excitation of endogenous and exogenous fluorophores. Mitochondrial nicotinamide adenine dinucleotide increased markedly in rat kidneys in response to ischemia. Following intravenous injection, the mitochondrial membrane potential-dependent dye TMRM was taken up by proximal tubules; in response to ischemia, the membrane potential dissipated rapidly and mitochondria became shortened and fragmented in proximal tubules. In contrast, the mitochondrial membrane potential and structure were better maintained in distal tubules. Changes in mitochondrial structure, nicotinamide adenine dinucleotide, and membrane potential were found in the proximal, but not distal, tubules after gentamicin exposure. These changes were sporadic, highly variable among animals, and were preceded by changes in non-mitochondrial structures. Thus, real-time changes in mitochondrial structure and function can be imaged in rodent kidneys in vivo using multiphoton excitation of endogenous and exogenous fluorophores in response to ischemia-reperfusion injury or drug toxicity.

Mitochondrial function and tissue vitality: bench-to-bedside real-time optical monitoring system

BACKGROUND

The involvement of mitochondria in pathological states, such as neurodegenerative diseases, sepsis, stroke, and cancer, are well documented. Monitoring of nicotinamide adenine dinucleotide (NADH) fluorescence in vivo as an intracellular oxygen indicator was established in 1950 to 1970 by Britton Chance and collaborators. We use a multiparametric monitoring system enabling assessment of tissue vitality. In order to use this technology in clinical practice, the commercial developed device, the CritiView (CRV), is tested in animal models as well as in patients.

METHODS AND RESULTS

The new CRV enables the optical monitoring of four different parameters, representing the energy balance of various tissues in vivo. Mitochondrial NADH is measured by surface fluorometry/reflectometry. In addition, tissue microcirculatory blood flow, tissue reflectance and oxygenation are measured as well. The device is tested both in vitro and in vivo in a small animal model and in preliminary clinical trials in patients undergoing vascular or open heart surgery. In patients, the monitoring is started immediately after the insertion of a three-way Foley catheter (urine collection) to the patient and is stopped when the patient is discharged from the operating room. The results show that monitoring the urethral wall vitality provides information in correlation to the surgical procedure performed.

Real-time assessment of renal cortical microvascular perfusion heterogeneities using near-infrared laser speckle imaging

Laser speckle imaging (LSI) is able to provide full-field perfusion maps of the renal cortex and allows quantification of the average LSI perfusion within an arbitrarily set region of interest and the recovery of LSI perfusion histograms within this region. The aim of the present study was to evaluate the use of LSI for mapping renal cortical microvascular perfusion and to demonstrate the capability of LSI to assess renal perfusion heterogeneities. The main findings were that: 1) full-field LSI measurements of renal microvascular perfusion were highly correlated to single-point LDV measurements; 2) LSI is able to detect differences in reperfusion dynamics following different durations of ischemia; and 3) renal microvascular perfusion heterogeneities can be quantitatively assessed by recovering LSI perfusion histograms.

Mitochondrial complex activity in donor renal grafts, cold ischemia time, and recovery of graft function

Indexed mitochondrial complex activities (MCAi) were determined in biopsies obtained from 52 donor kidneys at the end of cold ischemia (8-32 hr) to see if longer anoxia affected MCAi and accounted for the increased risk of delayed graft function (DGF) in recipients of grafts with longer cold ischemia time (CIT) or from non-heart-beating donors (NHBD). CITs were significantly different between those with and without DGF (P=0.02), being shorter in the latter, but MCAi were similar. CIT was correlated (r=0.43, P=0.003) with the time taken for creatinine concentration to fall to half the perioperative value (Crt(1/2)) but not with MCAi. Frequency of DGF, greater in NHBD, was significantly different from that of heart-beating donors (P=0.04), but CIT and MCAi were similar. However, Crt(1/2), was significantly different being longer in NHBD. Thus, the frequency of DGF increased and the speed of recovery diminished with longer CIT, whereas MCAi remained stable suggesting other factors determined tissue recovery.

Evaluation of the contribution of the renal capsule and cortex to kidney autofluorescence intensity under ultraviolet excitation

The use of reduced nicotinamide adenine dinucleotide (NADH) fluorescence to gain metabolic information on kidneys in response to an alteration in oxygen availability has previously been experimentally demonstrated, but signal quantification has not, to date, been addressed. In this work the relative contribution to rat kidney autofluorescence of the capsule versus cortex under ultraviolet excitation is determined from experimental results obtained using autofluorescence microscopy and a suitable mathematical model. The results allow for a quantitative assessment of the relative contribution of the signal originating in the metabolically active cortex as a function of capsule thickness for different wavelengths.

A non-contact method and instrumentation to monitor renal ischemia and reperfusion with optical spectroscopy

The potential of NADH autofluorescence as an in vivo intrinsic optical signature to monitor tissue metabolism is well recognized and supported by experimental results mainly in animal models. In this work, we propose a non-contact implementation of this method using large area excitation and employing a normalization method to account for non-metabolic signal changes. Proof of principle in vivo experiments were carried out using an autofluorescence imaging experimental system and a rat renal ischemia model. A hand-held fiber-optic probe was utilized to test the ability of the signal normalization method to address operational conditions associated with the translation of this method to a clinical setting. Preliminary pre-clinical in vivo test of the probe system was carried out using the same rat model.

Renal hypoxia and dysoxia after reperfusion of the ischemic kidney

Ischemia is the most common cause of acute renal failure. Ischemic-induced renal tissue hypoxia is thought to be a major component in the development of acute renal failure in promoting the initial tubular damage. Renal oxygenation originates from a balance between oxygen supply and consumption. Recent investigations have provided new insights into alterations in oxygenation pathways in the ischemic kidney. These findings have identified a central role of microvascular dysfunction related to an imbalance between vasoconstrictors and vasodilators, endothelial damage and endothelium-leukocyte interactions, leading to decreased renal oxygen supply. Reduced microcirculatory oxygen supply may be associated with altered cellular oxygen consumption (dysoxia), because of mitochondrial dysfunction and activity of alternative oxygen-consuming pathways. Alterations in oxygen utilization and/or supply might therefore contribute to the occurrence of organ dysfunction. This view places oxygen pathways' alterations as a potential central player in the pathogenesis of acute kidney injury. Both in regulation of oxygen supply and consumption, nitric oxide seems to play a pivotal role. Furthermore, recent studies suggest that, following acute ischemic renal injury, persistent tissue hypoxia contributes to the development of chronic renal dysfunction. Adaptative mechanisms to renal hypoxia may be ineffective in more severe cases and lead to the development of chronic renal failure following ischemia-reperfusion. This paper is aimed at reviewing the current insights into oxygen transport pathways, from oxygen supply to oxygen consumption in the kidney and from the adaptation mechanisms to renal hypoxia. Their role in the development of ischemia-induced renal damage and ischemic acute renal failure are discussed.

Renal hypoxia and dysoxia after reperfusion of the ischemic kidney

Ischemia is the most common cause of acute renal failure. Ischemic-induced renal tissue hypoxia is thought to be a major component in the development of acute renal failure in promoting the initial tubular damage. Renal oxygenation originates from a balance between oxygen supply and consumption. Recent investigations have provided new insights into alterations in oxygenation pathways in the ischemic kidney. These findings have identified a central role of microvascular dysfunction related to an imbalance between vasoconstrictors and vasodilators, endothelial damage and endothelium-leukocyte interactions, leading to decreased renal oxygen supply. Reduced microcirculatory oxygen supply may be associated with altered cellular oxygen consumption (dysoxia), because of mitochondrial dysfunction and activity of alternative oxygen-consuming pathways. Alterations in oxygen utilization and/or supply might therefore contribute to the occurrence of organ dysfunction. This view places oxygen pathways' alterations as a potential central player in the pathogenesis of acute kidney injury. Both in regulation of oxygen supply and consumption, nitric oxide seems to play a pivotal role. Furthermore, recent studies suggest that, following acute ischemic renal injury, persistent tissue hypoxia contributes to the development of chronic renal dysfunction. Adaptative mechanisms to renal hypoxia may be ineffective in more severe cases and lead to the development of chronic renal failure following ischemia-reperfusion. This paper is aimed at reviewing the current insights into oxygen transport pathways, from oxygen supply to oxygen consumption in the kidney and from the adaptation mechanisms to renal hypoxia. Their role in the development of ischemia-induced renal damage and ischemic acute renal failure are discussed.

Mitochondrial function in vivo evaluated by NADH fluorescence: from animal models to human studies

Normal mitochondrial function is a critical factor in maintaining cellular homeostasis in various organs of the body. Due to the involvement of mitochondrial dysfunction in many pathological states, the real-time in vivo monitoring of the mitochondrial metabolic state is crucially important. This type of monitoring in animal models as well as in patients provides real-time data that can help interpret experimental results or optimize patient treatment. The goals of the present review are the following: 1) to provide an historical overview of NADH fluorescence monitoring and its physiological significance; 2) to present the solid scientific ground underlying NADH fluorescence measurements based on published materials; 3) to provide the reader with basic information on the methodologies used in the past and the current state of the art fluorometers; and 4) to clarify the various factors affecting monitored signals, including artifacts. The large numbers of publications by different groups testify to the valuable information gathered in various experimental conditions. The monitoring of NADH levels in the tissue provides the most important information on the metabolic state of the mitochondria in terms of energy production and intracellular oxygen levels. Although NADH signals are not calibrated in absolute units, their trend monitoring is important for the interpretation of physiological or pathological situations. To understand tissue function better, the multiparametric approach has been developed where NADH serves as the key parameter. The development of new light sources in UV and visible spectra has led to the development of small compact units applicable in clinical conditions for better diagnosis of patients.

A review of attenuation correction techniques for tissue fluorescence

Fluorescence intensity measurements have the potential to facilitate the diagnoses of many pathological conditions. However, accurate interpretation of the measurements is complicated by the distorting effects of tissue scattering and absorption. Consequently, different techniques have been developed to attempt to compensate for these effects. This paper reviews currently available correction techniques with emphasis on clinical application and consideration given to the intrinsic accuracy and limitations of each technique.

Quantitative analysis of brain NADH in the presence of hemoglobin using microfiber spectrofluorometry: a pre-calibration approach

Dysfunction of mitochondria links a variety of central nervous system disorders and other neurodegenerative diseases. The primary respiratory chain substrate reduced-form nicotinamide adenine dinucleotide (NADH) is an important regulator of respiratory chain function in mitochondria and, because of its fluorescent properties, has been used to assess mitochondrial pathophysiology in cells and tissues. However, assessment of changes in tissue NADH has been limited to qualitative analysis primarily because hemoglobin (Hb) interferes with NADH fluorescence measurements by absorbing both excitation and emission light. This report presents a computer-assisted approach to estimate tissue NADH and Hb concentrations quantitatively at the same time. The method is based on a two-dimensionally interpolated database model that is calibrated by fluorescence emission spectra with known-value standard chemical solutions. Quantitative concentrations for NADH and Hb can be determined by the corresponding known-value spectral data that have the minimum error to the sample spectrum obtained from an experiment. Repeatability and reliability tests are also presented in this report. Results demonstrate that this method can feasibly quantify the NADH content regardless of the Hb background in living hippocampal cells during hypoxia, suggesting that it has the potential to be applied to in vivo experiments in the future.

Spectroscopic imaging for detection of ischemic injury in rat kidneys by use of changes in intrinsic optical properties

It is currently impossible to consistently predict kidney graft viability and function before and after transplantation. We explored optical spectroscopy to assess the degree of ischemic damage in kidney tissue. Tunable UV laser excitation was used to record autofluorescence images, at different spectral ranges, of injured and contralateral control rat kidneys to reveal the excitation conditions that offered optimal contrast. Autofluorescence and near-infrared cross-polarized light-scattering imaging were both used to monitor changes in intensity and spectral characteristics, as a function of exposure time to ischemic injury. These two modalities provided different temporal behaviors, arguably arising from two different mechanisms providing direct correlation of intrinsic optical signatures to ischemic injury time.

Preservation of complex I function during hypoxia-reoxygenation-induced mitochondrial injury in proximal tubules

Inhibition of complex I has been considered to be an important contributor to mitochondrial dysfunction in tissues subjected to ischemia-reperfusion. We have investigated the role of complex I in a severe energetic deficit that develops in kidney proximal tubules subjected to hypoxia-reoxygenation and is strongly ameliorated by supplementation with specific citric acid cycle metabolites, including succinate and the combination of -ketoglutarate plus malate. NADH: ubiquinone reductase activity in the tubules was decreased by only 26% during 60-min hypoxia and did not change further during 60-min reoxygenation. During titration of complex I activity with rotenone, progressive reduction of NAD+ to NADH was detected at >20% complex I inhibition, but substantial decreases in ATP levels and mitochondrial membrane potential did not occur until >70% inhibition. NAD+ was reduced to NADH during hypoxia, but the NADH formed was fully reoxidized during reoxygenation, consistent with the conclusion that complex I function was not limiting for recovery. Extensive degradation of cytosolic and mitochondrial NAD(H) pools occurred during either hypoxia or severe electron transport inhibition by rotenone, with patterns of metabolite accumulation consistent with catabolism by both NAD+ glycohydrolase and pyrophosphatase. This degradation was strongly blocked by alpha-ketoglutarate plus malate. The data demonstrate surprisingly little sensitivity of these cells to inhibition of complex I and high levels of resistance to development of complex I dysfunction during hypoxia-reoxygenation and indicate that events upstream of complex I are important for the energetic deficit. The work provides new insight into fundamental aspects of mitochondrial pathophysiology in proximal tubules during acute renal failure.

Protein binding of NADH on chemical preconditioning

Chemical preconditioning, an emerging neuroprotective strategy described in recent years, results in preserved energy metabolism during hypoxia via yet unknown mechanisms. The hypoxic increase of NADH content is attenuated by preconditioning. The goal of the present study was to investigate whether attenuation of the hypoxic NADH increase is due to a shift between free and protein-bound NADH. NADH in solution has a fluorescence maximum at 469.2 nm. In untreated mouse hippocampal slices, lambda(control onset) is 456.2 +/- 5.3 nm in CA1 (mean +/- SD; p < 0.01 vs. solution) and 454.6 +/- 6.1 nm in CA3 [p < 0.01 vs. solution, not significant (NS) to lambda(control onset) in CA1]. In slices prepared from animals pretreated in vivo with 20 mg/kg 3-nitropropionate, lambda(preconditioning onset) is 439.2 +/- 5.0 nm (p < 0.001 vs. control) in CA1 and 434.2 +/- 6.4 nm in CA3 (p < 0.001 vs. control; NS to lambda(preconditioning onset) in CA1). In controls, the fluorescence maximum shifts to lambda(control hypoxia) 458.2 +/- 1.3 nm in CA1 (NS vs. onset) and 456.0 +/- 3.6 nm in CA3 (NS vs. onset). On preconditioning with 3-nitropropionate, lambda(preconditioning hypoxia) shifts to 446.4 +/- 4.3 nm in CA1 (p < 0.03 vs. onset) and 438.6 +/- 6.9 nm in CA3 (p < 0.03 vs. onset). Posthypoxic decay of free and protein-bound NADH is diminished after preconditioning. We conclude that the free NADH level is reduced on an increase of hypoxic tolerance by chemical preconditioning. Reduction of free NADH content is maintained during hypoxia after preconditioning.