Fumarase activity: an in vivo and in vitro biomarker for acute kidney injury

Enzymatic Reactions Observed with Zero- and Low-Field Nuclear Magnetic Resonance

We demonstrate that enzyme-catalyzed reactions can be observed in zero- and low-field NMR experiments by combining recent advances in parahydrogen-based hyperpolarization methods with state-of-the-art magnetometry. Specifically, we investigated two model biological processes: the conversion of fumarate into malate, which is used in vivo as a marker of cell necrosis, and the conversion of pyruvate into lactate, which is the most widely studied metabolic process in hyperpolarization-enhanced imaging. In addition to this, we constructed a microfluidic zero-field NMR setup to perform experiments on microliter-scale samples of [1-13C]fumarate in a lab-on-a-chip device. Zero- to ultralow-field (ZULF) NMR has two key advantages over high-field NMR: the signals can pass through conductive materials (e.g., metals), and line broadening from sample heterogeneity is negligible. To date, the use of ZULF NMR for process monitoring has been limited to studying hydrogenation reactions. In this work, we demonstrate this emerging analytical technique for more general reaction monitoring and compare zero- vs low-field detection.

Evaluation of Selected Markers in Kidneys of Cynomolgus Monkey (Macaca fascicularis) with Induced Diabetes during Renal Ischemia-reperfusion Injury

We previously reported that induced type 1 diabetes mellitus (DM) increases the susceptibility of acute kidney injury in- duced by ischemia-reperfusion injury (IRI) in cynomolgus monkeys. In this follow-up study, we compared the expression of selected markers in the renal tissues of monkeys subjected to bilateral renal IRI with and without diabetes. All tissues were obtained from the original study. Renal biopsies were obtained before and 24 and 48 h after ischemia and were examined for expression of KI-67 (tubular proliferation), Na+ /K+ ATPase (sodium-potassium pump), TNF-α(tumor necrosis factor-α, inflammation), CD31 (microvessels), CD3 (T-cells), 2 fibrotic markers (fibroblast specific protein-1, FSP-1;α-smooth muscle actin,α -SMA), and cleaved caspase 3 (apoptosis). Generally, the expression of these markers differed in monkeys with and without DM. As compared with non-DM monkeys, DM monkeys had more cells that expressed KI-67 during progression of acute kidney injury (AKI). Na+ /K+ ATPase expression was clearly present at baseline in the basolateral tubular areas only in the non-DM monkeys. At 48 h, its expression in the basolateral area was not visible in DM monkeys, but was still present in intercellular junctions of non-DM monkeys. The expression of TNF-αwas higher in DM before and 48 h after ischemia. Before and 24 h after ischemia, the number of CD31-positive capillaries was not different between 2 groups, although more collapsed vessels were found at in DM at 24 h. At 48 h, the number of capillaries was less in DM compared with those from non-DM animals. DM monkeys had more interstitial CD3-positive cells than did non-DM monkeys at 24 and 48 h after ischemia. Finally, FSP-1-stained cells were more abundant in DM than non-DM at 24 and 48 h. Our results show that DM aggravates the recovery of renal ischemia/reperfusion injury by affecting tubular proliferation, capillary density, T cell infil- tration and by altering protein and mRNA expression of various genes involved in ion channel, inflammation, and fibrotic change. The results from this observational study demonstrate that DM aggravates the recovery of renal ischemia/reperfusion injury by affecting multiple events including tubular necrosis, proliferation, function, inflammation and by inducing capillary rarefaction in cynomolgus monkeys.

Developing a metabolic clearance rate framework as a translational analysis approach for hyperpolarized 13C magnetic resonance imaging

Hyperpolarized carbon-13 magnetic resonance imaging is a promising technique for in vivo metabolic interrogation of alterations between health and disease. This study introduces a formalism for quantifying the metabolic information in hyperpolarized imaging. This study investigated a novel perfusion formalism and metabolic clearance rate (MCR) model in pre-clinical stroke and in the healthy human brain. Simulations showed that the proposed model was robust to perturbations in T₁, transmit B₁, and k_PL. A significant difference in ipsilateral vs contralateral pyruvate derived cerebral blood flow (CBF) was detected in rats (140 ± 2 vs 89 ± 6 mL/100 g/min, p < 0.01, respectively) and pigs (139 ± 12 vs 95 ± 5 mL/100 g/min, p = 0.04, respectively), along with an increase in fractional metabolism (26 ± 5 vs 4 ± 2%, p < 0.01, respectively) in the rodent brain. In addition, a significant increase in ipsilateral vs contralateral MCR (0.034 ± 0.007 vs 0.017 ± 0.02/s, p = 0.03, respectively) and a decrease in mean transit time (31 ± 8 vs 60 ± 2 s, p = 0.04, respectively) was observed in the porcine brain. In conclusion, MCR mapping is a simple and robust approach to the post-processing of hyperpolarized magnetic resonance imaging.

Preclinical models versus clinical renal ischemia reperfusion injury: A systematic review based on metabolic signatures

Despite decennia of research and numerous successful interventions in the preclinical setting, renal ischemia reperfusion (IR) injury remains a major problem in clinical practice, pointing toward a translational gap. Recently, two clinical studies on renal IR injury (manifested either as acute kidney injury or as delayed graft function) identified metabolic derailment as a key driver of renal IR injury. It was reasoned that these unambiguous metabolic findings enable direct alignment of clinical with preclinical data, thereby providing the opportunity to elaborate potential translational hurdles between preclinical research and the clinical context. A systematic review of studies that reported metabolic data in the context of renal IR was performed according to the PRISMA guidelines. The search (December 2020) identified 35 heterogeneous preclinical studies. The applied methodologies were compared, and metabolic outcomes were semi-quantified and aligned with the clinical data. This review identifies profound methodological challenges, such as the definition of IR injury, the follow-up time, and sampling techniques, as well as shortcomings in the reported metabolic information. In light of these findings, recommendations are provided in order to improve the translatability of preclinical models of renal IR injury.

Ratiometric Mass Spectrometry Imaging for Stain-Free Delineation of Ischemic Tissue and Spatial Profiling of Ischemia-Related Molecular Signatures

Mass spectrometry imaging (MSI) serves as an emerging tool for spatial profiling of metabolic dysfunction in ischemic tissue. Prior to MSI data analysis, commonly used staining methods, e.g., triphenyltetrazole chloride (TTC) staining, need to be implemented on the adjacent tissue for delineating lesion area and evaluating infarction, resulting in extra consumption of the tissue sample as well as morphological mismatch. Here, we propose an in situ ratiometric MSI method for simultaneous demarcation of lesion border and spatial annotation of metabolic and enzymatic signatures in ischemic tissue on identical tissue sections. In this method, the ion abundance ratio of a reactant pair in the TCA cycle, e.g., fumarate to malate, is extracted pixel-by-pixel from an ambient MSI dataset of ischemic tissue and used as a surrogate indicator for metabolic activity of mitochondria to delineate lesion area as if the tissue has been chemically stained. This method is shown to be precise and robust in identifying lesions in brain tissues and tissue samples from different ischemic models including heart, liver, and kidney. Furthermore, the proposed method allows screening and predicting metabolic and enzymatic alterations which are related to mitochondrial dysfunction. Being capable of concurrent lesion identification, in situ metabolomics analysis, and screening of enzymatic alterations, the ratiometric MSI method bears great potential to explore ischemic damages at both metabolic and enzymatic levels in biological research.

Hyperpolarized pyruvate to measure the influence of PKM2 activation on glucose metabolism in the healthy kidney

The purpose of the current study was to investigate if hyperpolarized [1-13 C]pyruvate can inform us on the metabolic consequences for the kidney glucose metabolism upon treatment with the pyruvate kinase M2 (PKM2) activator TEPP-46, which has shown promise as a novel therapeutic target for diabetic nephropathy. A healthy male Wistar rat model was employed to study the conversion of [1-13 C]pyruvate to [1-13 C]lactate in the kidney 2 and 4 h after treatment with TEPP-46. All rats were scanned with hyperpolarized [1-13 C]pyruvate kidney MR and vital parameters and blood samples were taken after scanning. The PKM2 activator TEPP-46 increases the glycolytic activity in the kidneys, leading to an increased lactate production, as seen by hyperpolarized pyruvate-to-lactate conversion. The results are supported by an increase in blood lactate, a decreased blood glucose level and an increased pyruvate kinase (PK) activity. The metabolic changes observed in both kidneys following treatment with TEPP-46 are largely independent of renal function and could as such represent a new and extremely sensitive metabolic readout for future drugs targeting PKM2. These results warrant further studies in disease models to evaluate if [1-13 C]pyruvate-to-[1-13 C]lactate conversion can predict treatment outcome.

Non-invasive molecular imaging of kidney diseases

In nephrology, differential diagnosis or assessment of disease activity largely relies on the analysis of glomerular filtration rate, urinary sediment, proteinuria and tissue obtained through invasive kidney biopsies. However, currently available non-invasive functional parameters, and most serum and urine biomarkers, cannot capture intrarenal molecular disease processes specifically. Moreover, although histopathological analyses of kidney biopsy samples enable the visualization of pathological morphological and molecular alterations, they only provide information about a small part of the kidney and do not allow longitudinal monitoring. These limitations not only hinder understanding of the dynamics of specific disease processes in the kidney, but also limit the targeting of treatments to active phases of disease and the development of novel targeted therapies. Molecular imaging enables non-invasive and quantitative assessment of physiological or pathological processes by combining imaging technologies with specific molecular probes. Here, we discuss current preclinical and clinical molecular imaging approaches in nephrology. Non-invasive visualization of the kidneys through molecular imaging can be used to detect and longitudinally monitor disease activity and can therefore provide companion diagnostics to guide clinical trials, as well as the safe and effective use of drugs.

Involvement of Tricarboxylic Acid Cycle Metabolites in Kidney Diseases

Mitochondria are complex organelles that orchestrate several functions in the cell. The primary function recognized is energy production; however, other functions involve the communication with the rest of the cell through reactive oxygen species (ROS), calcium influx, mitochondrial DNA (mtDNA), adenosine triphosphate (ATP) levels, cytochrome c release, and also through tricarboxylic acid (TCA) metabolites. Kidney function highly depends on mitochondria; hence mitochondrial dysfunction is associated with kidney diseases. In addition to oxidative phosphorylation impairment, other mitochondrial abnormalities have been described in kidney diseases, such as induction of mitophagy, intrinsic pathway of apoptosis, and releasing molecules to communicate to the rest of the cell. The TCA cycle is a metabolic pathway whose primary function is to generate electrons to feed the electron transport system (ETS) to drives energy production. However, TCA cycle metabolites can also release from mitochondria or produced in the cytosol to exert different functions and modify cell behavior. Here we review the involvement of some of the functions of TCA metabolites in kidney diseases.

Magnetic resonance hyperpolarization imaging detects early myocardial dysfunction in a porcine model of right ventricular heart failure

Aims

Early detection of heart failure is important for timely treatment. During the development of heart failure, adaptive intracellular metabolic processes that evolve prior to macro-anatomic remodelling, could provide an early signal of impending failure. We hypothesized that metabolic imaging with hyperpolarized magnetic resonance would detect the early development of heart failure before conventional echocardiography could reveal cardiac dysfunction.

Methods and results

Five 8.5 kg piglets were subjected to pulmonary banding and subsequently examined by [1-¹³C]pyruvate hyperpolarization, conventional magnetic resonance imaging, echocardiography, and blood testing, every 4 weeks for 16 weeks. They were compared with a weight matched, healthy control group. Conductance catheter examination at the end of the study showed impaired right ventricular systolic function along with compromised left ventricular diastolic function. After 16 weeks, we saw a significant decrease in the conversion ratio of pyruvate/bicarbonate in the left ventricle from 0.13 (0.04) in controls to 0.07 (0.02) in animals with pulmonary banding, along with a significant increase in the lactate/bicarbonate ratio to 3.47 (1.57) compared with 1.34 (0.81) in controls. N-terminal pro-hormone of brain natriuretic peptide was increased by more than 300%, while cardiac index was reduced to 2.8 (0.95) L/min/m² compared with 3.9 (0.95) in controls. Echocardiography revealed no changes.

Conclusion

Hyperpolarization detected a shift towards anaerobic metabolism in early stages of right ventricular dysfunction, as evident by an increased lactate/bicarbonate ratio. Dysfunction was confirmed with conductance catheter assessment, but could not be detected by echocardiography. Hyperpolarization has a promising future in clinical assessment of heart failure in both acquired and congenital heart disease.

Constant-adiabaticity ultralow magnetic field manipulations of parahydrogen-induced polarization: application to an AA'X spin system

The field of magnetic resonance imaging with hyperpolarized contrast agents is rapidly expanding, and parahydrogen-induced polarization (PHIP) is emerging as an inexpensive and easy-to-implement method for generating the required hyperpolarized biomolecules. Hydrogenative PHIP delivers hyperpolarized proton spin order to a substrate via chemical addition of H₂ in the spin-singlet state, but it is typically necessary to transfer the proton polarization to a heteronucleus (usually ¹³C) which has a longer spin lifetime. Adiabatic ultralow magnetic field manipulations can be used to induce the polarization transfer, but this is necessarily a slow process, which is undesirable since the spins continually relax back to thermal equilibrium. Here we demonstrate two constant-adiabaticity field sweep methods, one in which the field passes through zero, and one in which the field is swept from zero, for optimal polarization transfer on a model AA'X spin system, [1-¹³C]fumarate. We introduce a method for calculating the constant-adiabaticity magnetic field sweeps, and demonstrate that they enable approximately one order of magnitude faster spin-order conversion compared to linear sweeps. The present method can thus be utilized to manipulate nonthermal order in heteronuclear spin systems.

Design of Nuclear Magnetic Resonance Molecular Probes for Hyperpolarized Bioimaging

Nuclear hyperpolarization has emerged as a method to dramatically enhance the sensitivity of NMR spectroscopy. By application of this powerful tool, small molecules with stable isotopes have been used for highly sensitive biomedical molecular imaging. The recent development of molecular probes for hyperpolarized in vivo analysis has demonstrated the ability of this technique to provide unique metabolic and physiological information. This review presents a brief introduction of hyperpolarization technology, approaches to the rational design of molecular probes for hyperpolarized analysis, and examples of molecules that have met with success in vitro or in vivo.

Fumaric acid ester-induced renal Fanconi syndrome: evidence of mitochondrial toxicity

Background

Fumaric acid esters (FAEs) are used to treat chronic plaque psoriasis. Fumarate is a crucial component of the Krebs cycle and mitochondrial function. Proximal tubule cells have high energy demands and rely on aerobic respiration. Proximal tubular dysfunction can cause renal Fanconi syndrome and acute kidney injury. We sought to better understand the mechanism for this in the context of FAE therapy.

Methods

We describe a case series of 10 patients with FAE-associated Fanconi syndrome. Patients were diagnosed and managed at a tertiary renal tubular disorder clinic, with examination of serum and urine biochemistry. Five patients had a renal biopsy with examination of the specimens by electron microscopy.

Results

The median age was 36.5 years [interquartile range (IQR) 32.25–54.25]. The median dose of FAE was 720 mg/day (IQR 390–720). There was low molecular weight proteinuria: the median urinary retinol-binding protein (RBP) at presentation was 8385 μg/mL (IQR 2793–14 600) and the RBP:creatinine ratio was 710 (IQR 390–2415). All patients had hyperphosphaturia [median fractional excretion of phosphate 24.2% (IQR 20.8–26.9), normal range <20%] as well as relative hypophosphataemia, with a median serum phosphate concentration of 0.93 mmol/L (IQR 0.83–0.97). Renal histology showed proximal tubular damage and abnormal mitochondrial morphology. Two patients had a favourable biochemical response to treatment with probenecid.

Conclusions

We document for the first time that FAE-associated renal Fanconi syndrome is associated with mitochondrial damage visible on electron microscopy. This effect may be ameliorated by antagonism of the organic anion transporter with probenecid.

Differential roles of VPS and RAAS in water homeostasis and a risk for kidney dysfunction in rats undergoing rapid fasting/dehydration with regular exercise

Purpose

We examined the effects of rapid restriction of food and fluid intake on the pathways of water homeostasis, the vasopressinergic system (VPS), and the renin–angiotensin–aldosterone system (RAAS), in rats with or without regular exercise.

Methods

Sprague Dawley rats were divided into the following groups: no intervention, rapid restriction, regular exercise, and rapid restriction combined with regular exercise. Rats in the exercise group performed climbing exercise for 4 weeks. All rats consumed food ad libitum, and those in the rapid restriction group fasted for the last 3 days with no water on the last 1 day.

Results

Despite no significant differences in body weight among the groups, the kidney weight was decreased when rapid restriction and regular exercise were combined. Rapid restriction reduced the urine volume and increased the urine osmolality, whereas regular exercise did not. Rapid restriction but not regular exercise increased the levels of circulating aldosterone and the renal expression levels of the ion channel SGK‐1 compared to those without rapid restriction, indicating the stimulation of RAAS. Conversely, VPS showed no significant response to these interventions. Moreover, rapid restriction combined with regular exercise induced the renal expression levels of proinflammatory cytokines and increased the active forms of apoptotic effector caspase‐3 compared with the no intervention group.

Conclusions

Functional significance may differ between VPS and RAAS in water homeostasis in response to rapid restriction. Moreover, the combination of rapid restriction and regular exercise has potentially deleterious effects on the kidney.

Sodium (23Na) MRI of the Kidney: Basic Concept

The handling of sodium by the renal system is a key indicator of renal function. Alterations in the corticomedullary distribution of sodium are considered important indicators of pathology in renal diseases. The derangement of sodium handling can be noninvasively imaged using sodium magnetic resonance imaging (²³Na MRI), with data analysis allowing for the assessment of the corticomedullary sodium gradient. Here we introduce sodium imaging, describe the existing methods, and give an overview of preclinical sodium imaging applications to illustrate the utility and applicability of this technique for measuring renal sodium handling.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction chapter is complemented by two separate chapters describing the experimental procedure and data analysis.

Noninvasive Assessment of Fibrosis Following Ischemia/Reperfusion Injury in Rodents Utilizing Na Magnetic Resonance Imaging

Fibrosis is often heterogeneously distributed, and classical biopsies do not reflect this. Noninvasive methods for renal fibrosis have been developed to follow chronic kidney diseases (CKD) and to monitor anti-fibrotic therapy. In this study, we combined two approaches to assess fibrosis regression following renal ischemia-reperfusion injury (IRI): magnetic resonance imaging (MRI) and noninvasive extracellular matrix (ECM) biomarkers. MRI was used to evaluate fibrosis in bilateral IRI in rats after reperfusion at 7, 14, and 21 days. This was performed with ¹HT₁ and T₂* mapping, dynamic contrast-enhanced (DCE)-MRI, and chemical shift imaging (CSI)-²³Na. The degradation of laminin gamma-1 chain (LG1M) and type III collagen (C3M) was measured in urine and plasma. Fibrosis was analyzed in tissue using fibronectin (FN) and alpha-smooth muscle actin (α-SMA) using quantitative polymerase chain reaction qPCR and western blotting. We found increased fibrosis 7 days after reperfusion, which dropped to sham levels after 21 days. Single kidney glomerular filtration rate (skGFR), perfusion (DCE-MRI), and total ²³Na kidney content correlated positively with fibrotic markers FN and α-SMA as well as noninvasive LG1M and C3M. We showed that novel MRI protocols and ECM markers could track fibrogenic development. This could give rise to a multi-parametric practice to diagnose and assess fibrosis whilst treating kidney disease without using invasive methods.

Hyperpolarized [1,4-13C]fumarate imaging detects microvascular complications and hypoxia mediated cell death in diabetic nephropathy

Today, there is a general lack of prognostic biomarkers for development of renal disease and in particular diabetic nephropathy. Increased glycolytic activity, lactate accumulation and altered mitochondrial oxygen utilization are hallmarks of diabetic kidney disease. Fumarate hydratase activity has been linked to mitochondrial dysfunction as well as activation of the hypoxia inducible factor, induction of apoptosis and necrosis. Here, we investigate fumarate hydratase activity in biofluids in combination with the molecular imaging probe, hyperpolarized [1,4-¹³C₂]fumarate, to identify the early changes associated with hemodynamics and cell death in a streptozotocin rat model of type 1 diabetes. We found a significantly altered hemodynamic signature of [1,4-¹³C₂]fumarate in the diabetic kidneys as well as an systemic increased metabolic conversion of fumarate-to-malate, indicative of increased cell death associated with progression of diabetes, while little to no renal specific conversion was observed. This suggest apoptosis as the main cause of cell death in the diabetic kidney. This is likely resulting from an increased reactive oxygen species production following uncoupling of the electron transport chain at complex II. The mechanism coupling the enzyme leakage and apoptotic phenotype is hypoxia inducible factor independent and seemingly functions as a protective mechanism in the kidney cells.

Metabolic reprogramming associated with progression of renal ischemia reperfusion injury assessed with hyperpolarized [1-13C]pyruvate

Acute kidney injury is a major clinical challenge affecting as many as 1 percent of all hospitalized patients. Currently it is not possible to accurately stratify and predict the outcome of the individual patient. Increasing evidence supports metabolic reprogramming as a potential target for new biomarkers. Hyperpolarized [1-¹³C]pyruvate imaging is a promising new tool for evaluating the metabolic status directly in the kidneys. We here investigate the prognostic potential of hyperpolarized [1-¹³C]pyruvate in the setting of acute kidney injury in a rodent model of ischemia reperfusion. A significant correlation was found between the intra-renal metabolic profile 24 hours after reperfusion and 7 days after injury induction, as well as a correlation with the conventional plasma creatinine biomarker of renal function and markers of renal injury. This leads to a possible outcome prediction of renal function and injury development from a metabolic profile measured in vivo. The results support human translation of this new technology to renal patients as all experiements have been performed using clinical MRI equipment.

Hyperpolarized 13C MRI: A novel approach for probing cerebral metabolism in health and neurological disease

Cerebral metabolism is tightly regulated and fundamental for healthy neurological function. There is increasing evidence that alterations in this metabolism may be a precursor and early biomarker of later stage disease processes. Proton magnetic resonance spectroscopy (1H-MRS) is a powerful tool to non-invasively assess tissue metabolites and has many applications for studying the normal and diseased brain. However, the technique has limitations including low spatial and temporal resolution, difficulties in discriminating overlapping peaks, and challenges in assessing metabolic flux rather than steady-state concentrations. Hyperpolarized carbon-13 magnetic resonance imaging is an emerging clinical technique that may overcome some of these spatial and temporal limitations, providing novel insights into neurometabolism in both health and in pathological processes such as glioma, stroke and multiple sclerosis. This review will explore the growing body of pre-clinical data that demonstrates a potential role for the technique in assessing metabolism in the central nervous system. There are now a number of clinical studies being undertaken in this area and this review will present the emerging clinical data as well as the potential future applications of hyperpolarized 13C magnetic resonance imaging in the brain, in both clinical and pre-clinical studies.

Hyperpolarized [1-13 C] alanine production: A novel imaging biomarker of renal fibrosis

PURPOSE

Renal tubulointerstitial fibrosis is strongly linked to the progressive decline of renal function seen in chronic kidney disease. State-of-the-art noninvasive diagnostic modalities are currently unable to detect the earliest changes associated with the onset of fibrosis. This study was undertaken to evaluate the potential for detecting the earliest alterations in fibrogenesis using a biofluid-based method and metabolic hyperpolarized [1-¹³ C]pyruvate imaging.

METHODS

We evaluated renal fibrosis in a combined ischemia reperfusion-induced and streptozotocin-induced diabetic nephropathy rodent model by hyperpolarized [1-¹³ C]pyruvate MRI and correlated the metabolic MRI parameters with biomarkers of fibrosis measured on renal tissue and plasma/urine.

RESULTS

The hyperglycemic rats experienced maladaptive injury repair after the ischemic insults, as shown by the elevation in the injury markers kidney injury molecule-1 and neutrophil gelatinase-associated lipocalin. Renal function was significantly impaired in the ischemic hyperglycemic kidney, as seen in the reduced perfusion and single-kidney glomerular filtration rate. A deranged energy metabolism was detected in the ischemic hyperglycemic kidney, as seen in the reduced fractional perfusion of lactate. Renal fibrosis biomarkers correlated significantly with the alanine production.

CONCLUSION

Hyperpolarized carbon-13 MRI provides a promising approach to assess renal fibrosis in an animal model of fibrotic chronic kidney disease. In particular, the metabolic supply of amino acids for fibrogenesis (alanine production) correlates well with biomarkers of fibrosis. Thus, [1-¹³ C]pyruvate-to-[1-¹³ C]alanine conversion might be a candidate for noninvasive assessment of renal fibrogenesis.

Acquisition strategies for spatially resolved magnetic resonance detection of hyperpolarized nuclei

Hyperpolarization is an emerging method in magnetic resonance imaging that allows nuclear spin polarization of gases or liquids to be temporarily enhanced by up to five or six orders of magnitude at clinically relevant field strengths and administered at high concentration to a subject at the time of measurement. This transient gain in signal has enabled the non-invasive detection and imaging of gas ventilation and diffusion in the lungs, perfusion in blood vessels and tissues, and metabolic conversion in cells, animals, and patients. The rapid development of this method is based on advances in polarizer technology, the availability of suitable probe isotopes and molecules, improved MRI hardware and pulse sequence development. Acquisition strategies for hyperpolarized nuclei are not yet standardized and are set up individually at most sites depending on the specific requirements of the probe, the object of interest, and the MRI hardware. This review provides a detailed introduction to spatially resolved detection of hyperpolarized nuclei and summarizes novel and previously established acquisition strategies for different key areas of application.

Hyperpolarised 13C-MRI metabolic and functional imaging: an emerging renal MR diagnostic modality

Magnetic resonance imaging (MRI) is a well-established modality for assessing renal morphology and function, as well as changes that occur during disease. However, the significant metabolic changes associated with renal disease are more challenging to assess with MRI. Hyperpolarized carbon-13 MRI is an emerging technique which provides an opportunity to probe metabolic alterations at high sensitivity by providing an increase in the signal-to-noise ratio of 20,000-fold or more. This review will highlight the current status of hyperpolarised 13C-MRI and its translation into the clinic and how it compares to metabolic measurements provided by competing technologies such as positron emission tomography (PET).

Detection of acute kidney injury with hyperpolarized [13 C, 15 N]Urea and multiexponential relaxation modeling

PURPOSE

To assess the utility of Laplacian fitting to describe the differences in hyperpolarized [¹³ C, ¹⁵ N]urea T₂ relaxation in ischemic and healthy rodent kidneys.

METHODS

Six rats with unilateral renal ischemia were investigated. [¹³ C, ¹⁵ N]Urea T₂ mapping was undertaken with a radial fast spin echo method, with subsequent postprocessing performed with regularized Laplacian fitting.

RESULTS

Simulations showed that Laplacian fitting was stable down to a signal-to-noise ratio of 20. In vivo results showed a significant increase in the mono- and decrease in biexponential pools in ischemia reperfusion injury kidneys, in comparison to healthy (14 ± 10% versus 4 ± 2%, 85 ± 10% versus 95 ± 3%; P < .05).

CONCLUSION

We demonstrate, for the first time, the differences in multiexponential behavior of [¹³ C, ¹⁵ N]urea between the healthy and ischemic rodent kidney. The distribution of relaxation pools were found to be both visually and numerically significantly different. The ability to improve the information level in hyperpolarized MR, by using the relaxation contrast mechanisms is an appealing option, that can easily be adopted in large animals and even in clinical studies in the near future.

Hyperpolarized 13C MRI: State of the Art and Future Directions

Hyperpolarized (HP) carbon 13 (¹³C) MRI is an emerging molecular imaging method that allows rapid, noninvasive, and pathway-specific investigation of dynamic metabolic and physiologic processes that were previously inaccessible to imaging. This technique has enabled real-time in vivo investigations of metabolism that are central to a variety of diseases, including cancer, cardiovascular disease, and metabolic diseases of the liver and kidney. This review provides an overview of the methods of hyperpolarization and ¹³C probes investigated to date in preclinical models of disease. The article then discusses the progress that has been made in translating this technology for clinical investigation. In particular, the potential roles and emerging clinical applications of HP [1-¹³C]pyruvate MRI will be highlighted. The future directions to enable the adoption of this technology to advance the basic understanding of metabolism, to improve disease diagnosis, and to accelerate treatment assessment are also detailed.

Renal Energy Metabolism Following Acute Dichloroacetate and 2,4-Dinitrophenol Administration: Assessing the Cumulative Action with Hyperpolarized [1-13C]Pyruvate MRI

Numerous patient groups receive >1 medication and as such represent a potential point of improvement in today's healthcare setup, as the combined or cumulative effects are difficult to monitor in an individual patient. Here we show the ability to monitor the pharmacological effect of 2 classes of medications sequentially, namely, 2,4-dinitrophenol, a mitochondrial uncoupler, and dichloroacetate, a pyruvate dehydrogenase kinase inhibitor, both targeting the oxygen-dependent energy metabolism. We show that although the 2 drugs target 2 different metabolic pathways connected ultimately to oxygen metabolism, we could distinguish the 2 in vivo by using hyperpolarized [1-¹³C]pyruvate magnetic resonance imaging. A statistically significantly different pyruvate dehydrogenase flux was observed by reversing the treatment order of 2,4-dinitrophenol and dichloroacetate. The significance of this study is the demonstration of the ability to monitor the metabolic cumulative effects of 2 distinct therapeutics on an in vivo organ level using hyperpolarized magnetic resonance imaging.

Optimized GC-MS metabolomics for the analysis of kidney tissue metabolites

INTRODUCTION

Metabolomics is a promising approach for discovery of relevant biomarkers in cells, tissues, organs, and biofluids for disease identification and prediction. The field has mostly relied on blood-based biofluids (serum, plasma, urine) as non-invasive sources of samples as surrogates of tissue or organ-specific conditions. However, the tissue specificity of metabolites pose challenges in translating blood metabolic profiles to organ-specific pathophysiological changes, and require further downstream analysis of the metabolites.

OBJECTIVES

As part of this project, we aim to develop and optimize an efficient extraction protocol for the analysis of kidney tissue metabolites representative of key primate metabolic pathways.

METHODS

Kidney cortex and medulla tissues of a baboon were homogenized and extracted using eight different extraction protocols including methanol/water, dichloromethane/methanol, pure methanol, pure water, water/methanol/chloroform, methanol/chloroform, methanol/acetonitrile/water, and acetonitrile/isopropanol/water. The extracts were analyzed by a two-dimensional gas chromatography time-of-flight mass-spectrometer (2D GC-ToF-MS) platform after methoximation and silylation.

RESULTS

Our analysis quantified 110 shared metabolites in kidney cortex and medulla tissues from hundreds of metabolites found among the eight different solvent extractions spanning low to high polarities. The results revealed that medulla is metabolically richer compared to the cortex. Dichloromethane and methanol mixture (3:1) yielded highest number of metabolites across both the tissue types. Depending on the metabolites of interest, tissue type, and the biological question, different solvents can be used to extract specific groups of metabolites.

CONCLUSION

This investigation provides insights into selection of extraction solvents for detection of classes of metabolites in renal cortex and medulla, which is fundamentally important for identification of prognostic and diagnostic metabolic kidney biomarkers for future therapeutic applications.

Hyperpolarized carbon-13 magnetic resonance spectroscopic imaging: a clinical tool for studying tumour metabolism

Glucose metabolism in tumours is reprogrammed away from oxidative metabolism, even in the presence of oxygen. Non-invasive imaging techniques can probe these alterations in cancer metabolism providing tools to detect tumours and their response to therapy. Although Positron Emission Tomography with (18F)2-fluoro-2-deoxy-D-glucose (18F-FDG PET) is an established clinical tool to probe cancer metabolism, it has poor spatial resolution and soft tissue contrast, utilizes ionizing radiation and only probes glucose uptake and phosphorylation and not further downstream metabolism. Magnetic Resonance Spectroscopy (MRS) has the capability to non-invasively detect and distinguish molecules within tissue but has low sensitivity and can only detect selected nuclei. Dynamic Nuclear Polarization (DNP) is a technique which greatly increases the signal-to-noise ratio (SNR) achieved with MR by significantly increasing nuclear spin polarization and this method has now been translated into human imaging. This review provides a brief overview of this process, also termed Hyperpolarized Carbon-13 Magnetic Resonance Spectroscopic Imaging (HP 13C-MRSI), its applications in preclinical imaging, an outline of the current human trials that are ongoing, as well as future potential applications in oncology.

Molecular imaging of tumor photoimmunotherapy: Evidence of photosensitized tumor necrosis and hemodynamic changes

Near-infrared photoimmunotherapy (NIR PIT) employs the photoabsorbing dye IR700 conjugated to antibodies specific for cell surface epidermal growth factor receptor (EGFR). NIR PIT has shown highly selective cytotoxicity in vitro and in vivo. Cell necrosis is thought to be the main mode of cytotoxicity based mainly on in vitro studies. To better understand the acute effects of NIR PIT, molecular imaging studies were performed to assess its cellular and vascular effects. In addition to in vitro studies for cytotoxicity of NIR PIT, the in vivo tumoricidal effects and hemodynamic changes induced by NIR PIT were evaluated by ¹³C MRI using hyperpolarized [1,4-¹³C₂] fumarate, R₂* mapping from T₂*-weighted MRI, and photoacoustic imaging. In vitro studies confirmed that NIR PIT resulted in rapid cell death via membrane damage, with evidence for rapid cell expansion followed by membrane rupture. Following NIR PIT, metabolic MRI using hyperpolarized fumarate showed the production of malate in EGFR-expressing A431 tumor xenografts, providing direct evidence for photosensitized tumor necrosis induced by NIR PIT. R₂* mapping studies showed temporal changes in oxygenation, with an accompanying increase of deoxyhemoglobin at the start of light exposure followed by a sustained decrease after cessation of light exposure. This result suggests a rapid decrease of blood flow in EGFR-expressing A431 tumor xenografts, which is supported by the results of the photoacoustic imaging experiments. Our findings suggest NIR PIT mediates necrosis and hemodynamic changes in tumors by photosensitized oxidation pathways and that these imaging modalities, once translated, may be useful in monitoring clinical treatment response.

Hyperpolarized fumarate via parahydrogen

We produce hyperpolarized [1-¹³C]fumarate in the proton nuclear spin singlet state by pairwise trans-addition of parahydrogen to a molecular precursor using a ruthenium-based catalyst in water.

Simultaneous imaging of hyperpolarized [1,4-13 C2 ]fumarate, [1-13 C]pyruvate and 18 F-FDG in a rat model of necrosis in a clinical PET/MR scanner

A co-polarization scheme for [1,4-¹³ C₂ ]fumarate and [1-¹³ C]pyruvate is presented to simultaneously assess necrosis and metabolism in rats with hyperpolarized ¹³ C magnetic resonance (MR). The co-polarization was performed in a SPINlab polarizer. In addition, the feasibility of simultaneous positron emission tomography (PET) and MR of small animals with a clinical PET/MR scanner is demonstrated. The hyperpolarized metabolic MR and PET was demonstrated in a rat model of necrosis. The polarization and T₁ of the co-polarized [1,4-¹³ C₂ ]fumarate and [1-¹³ C]pyruvate substrates were measured in vitro and compared with those obtained when the substrates were polarized individually. A polarization of 36 ± 4% for fumarate and 37 ± 6% for pyruvate was obtained. We found no significant difference in the polarization and T₁ values between the dual and single substrate polarization. Rats weighing about 400 g were injected intramuscularly in one of the hind legs with 200 μL of turpentine to induce necrosis. Two hours later, ¹³ C metabolic maps were obtained with a chemical shift imaging sequence (16 × 16) with a resolution of 3.1 × 5.0 × 25.0 mm³ . The ¹³ C spectroscopic images were acquired in 12 s, followed by an 8-min ¹⁸ F-2-fluoro-2-deoxy-d-glucose (¹⁸ F-FDG) PET acquisition with a resolution of 3.5 mm. [1,4-¹³ C₂ ]Malate was observed from the tissue injected with turpentine indicating necrosis. Normal [1-¹³ C]pyruvate metabolism and ¹⁸ F-FDG uptake were observed from the same tissue. The proposed co-polarization scheme provides a means to utilize multiple imaging agents simultaneously, and thus to probe various metabolic pathways in a single examination. Moreover, it demonstrates the feasibility of small animal research on a clinical PET/MR scanner for combined PET and hyperpolarized metabolic MR.

Hyperpolarized 13C Magnetic Resonance Imaging Can Detect Metabolic Changes Characteristic of Penumbra in Ischemic Stroke

Magnetic resonance imaging (MRI) is increasingly the method of choice for rapid stroke assessment in patients and for guiding patient selection in clinical trials. The underlying metabolic status during stroke and following treatment is recognized as an important prognostic factor; thus, new methods are required to monitor local biochemistry following cerebral infarction, rapidly and in vivo. Hyperpolarized MRI with the tracer [1-¹³C]pyruvate enables rapid detection of localized [1-¹³C]lactate production, which has recently been shown in patients, supporting its translation to assess clinical stroke. Here we show the ability of hyperpolarized ¹³C MRI to detect the metabolic alterations characteristic of endothelin-1-induced ischemic stroke in rodents. In the region of penumbra, determined via T2-weighted ¹H MRI, both [1-¹³C]pyruvate delivery and [1-¹³C]pyruvate cellular uptake independently increased. Furthermore, we observed a 33% increase in absolute [1-¹³C]lactate signal in the penumbra, and we determined that half of this increase was due to increased intracellular [1-¹³C]pyruvate supply and half was mediated by enhanced lactate dehydrogenase-mediated [1-¹³C]lactate production. Future work to characterize the kinetics of delivery, uptake, and enzymatic conversions of hyperpolarized tracers following ischemic stroke could position hyperpolarized ¹³C MRI as an ideal technology for rapid assessment of the penumbra during the critical time window following ischemic stroke in patients.