Autoradiography of manganese: accumulation and retention in the pancreas

Anatomy, Functionality, and Neuronal Connectivity with Manganese Radiotracers for Positron Emission Tomography

PURPOSE

Manganese ion has been extensively used as a magnetic resonance imaging (MRI) contrast agent in preclinical studies to assess tissue anatomy, function, and neuronal connectivity. Unfortunately, its use in human studies has been limited by cellular toxicity and the need to use a very low dose. The much higher sensitivity of positron emission tomography (PET) over MRI enables the use of lower concentrations of manganese, potentially expanding the methodology to humans.

PROCEDURES

PET tracers manganese-51 (Mn-51, t_1/2 = 46 min) and manganese-52 (Mn-52, t_1/2 = 5.6 days) were used in this study. The biodistribution of manganese in animals in the brain and other tissues was studied as well as the uptake in the pancreas after glucose stimulation as a functional assay. Finally, neuronal connectivity in the olfactory pathway following nasal administration of the divalent radioactive Mn-52 ([⁵²Mn]Mn²⁺) was imaged.

RESULTS

PET imaging with the divalent radioactive Mn-51 ([⁵¹Mn]Mn²⁺) and [⁵²Mn]Mn²⁺ in both rodents and monkeys demonstrates that the accumulation of activity in different organs is similar to that observed in rodent MRI studies following systemic administration. Furthermore, we demonstrated the ability of manganese to enter excitable cells. We followed activity-induced [⁵¹Mn]Mn²⁺ accumulation in the pancreas after glucose stimulation and showed that [⁵²Mn]Mn²⁺ can be used to trace neuronal connections analogous to manganese-enhanced MRI neuronal tracing studies.

CONCLUSIONS

The results were consistent with manganese-enhanced MRI studies, despite the much lower manganese concentration used for PET (100 mM Mn²⁺ for MRI compared to ~ 0.05 mM for PET). This indicates that uptake and transport mechanisms are comparable even at low PET doses. This helps establish the use of manganese-based radiotracers in both preclinical and clinical studies to assess anatomy, function, and connectivity.

Manganese Enhanced MRI for Use in Studying Neurodegenerative Diseases

MRI has been extensively used in neurodegenerative disorders, such as Alzheimer’s disease (AD), frontal-temporal dementia (FTD), mild cognitive impairment (MCI), Parkinson’s disease (PD), Huntington’s disease (HD) and amyotrophic lateral sclerosis (ALS). MRI is important for monitoring the neurodegenerative components in other diseases such as epilepsy, stroke and multiple sclerosis (MS). Manganese enhanced MRI (MEMRI) has been used in many preclinical studies to image anatomy and cytoarchitecture, to obtain functional information in areas of the brain and to study neuronal connections. This is due to Mn²⁺ ability to enter excitable cells through voltage gated calcium channels and be actively transported in an anterograde manner along axons and across synapses. The broad range of information obtained from MEMRI has led to the use of Mn²⁺ in many animal models of neurodegeneration which has supplied important insight into brain degeneration in preclinical studies. Here we provide a brief review of MEMRI use in neurodegenerative diseases and in diseases with neurodegenerative components in animal studies and discuss the potential translation of MEMRI to clinical use in the future.

Manganese-Enhanced Magnetic Resonance Imaging as a Diagnostic and Dispositional Tool after Mild-Moderate Blast Traumatic Brain Injury

Traumatic brain injury (TBI) caused by explosive munitions, known as blast TBI, is the signature injury in recent military conflicts in Iraq and Afghanistan. Diagnostic evaluation of TBI, including blast TBI, is based on clinical history, symptoms, and neuropsychological testing, all of which can result in misdiagnosis or underdiagnosis of this condition, particularly in the case of TBI of mild-to-moderate severity. Prognosis is currently determined by TBI severity, recurrence, and type of pathology, and also may be influenced by promptness of clinical intervention when more effective treatments become available. An important task is prevention of repetitive TBI, particularly when the patient is still symptomatic. For these reasons, the establishment of quantitative biological markers can serve to improve diagnosis and preventative or therapeutic management. In this study, we used a shock-tube model of blast TBI to determine whether manganese-enhanced magnetic resonance imaging (MEMRI) can serve as a tool to accurately and quantitatively diagnose mild-to-moderate blast TBI. Mice were subjected to a 30 psig blast and administered a single dose of MnCl2 intraperitoneally. Longitudinal T1-magnetic resonance imaging (MRI) performed at 6, 24, 48, and 72 h and at 14 and 28 days revealed a marked signal enhancement in the brain of mice exposed to blast, compared with sham controls, at nearly all time-points. Interestingly, when mice were protected with a polycarbonate body shield during blast exposure, the marked increase in contrast was prevented. We conclude that manganese uptake can serve as a quantitative biomarker for TBI and that MEMRI is a minimally-invasive quantitative approach that can aid in the accurate diagnosis and management of blast TBI. In addition, the prevention of the increased uptake of manganese by body protection strongly suggests that the exposure of an individual to blast risk could benefit from the design of improved body armor.

β-Cell subcellular localization of glucose-stimulated Mn uptake by X-ray fluorescence microscopy: implications for pancreatic MRI

Manganese (Mn) is a calcium (Ca) analog that has long been used as a magnetic resonance imaging (MRI) contrast agent for investigating cardiac tissue functionality, for brain mapping and for neuronal tract tracing studies. Recently, we have extended its use to investigate pancreatic β-cells and showed that, in the presence of MnCl(2), glucose-activated pancreatic islets yield significant signal enhancement in T(1)-weigheted MR images. In this study, we exploited for the first time the unique capabilities of X-ray fluorescence microscopy (XFM) to both visualize and quantify the metal in pancreatic β-cells at cellular and subcellular levels. MIN-6 insulinoma cells grown in standard tissue culture conditions had only a trace amount of Mn, 1.14 ± 0.03 × 10(-11)µg/µm(2), homogenously distributed across the cell. Exposure to 2 mM glucose and 50 µM MnCl(2) for 20 min resulted in nonglucose-dependent Mn uptake and the overall cell concentration increased to 8.99 ± 2.69 × 10(-11) µg/µm(2). When cells were activated by incubation in 16 mM glucose in the presence of 50 µM MnCl(2), a significant increase in cytoplasmic Mn was measured, reaching 2.57 ± 1.34 × 10(-10) µg/µm(2). A further rise in intracellular concentration was measured following KCl-induced depolarization, with concentrations totaling 1.25 ± 0.33 × 10(-9) and 4.02 ± 0.71 × 10(-10) µg/µm(2) in the cytoplasm and nuclei, respectively. In both activated conditions Mn was prevalent in the cytoplasm and localized primarily in a perinuclear region, possibly corresponding to the Golgi apparatus and involving the secretory pathway. These data are consistent with our previous MRI findings, confirming that Mn can be used as a functional imaging reporter of pancreatic β-cell activation and also provide a basis for understanding how subcellular localization of Mn will impact MRI contrast.

Tissue-specific MR contrast agents

The purpose of this review is to outline recent trends in contrast agent development for magnetic resonance imaging. Up to now, small molecular weight gadolinium chelates are the workhorse in contrast enhanced MRI. These first generation MR contrast agents distribute into the intravascular and interstitial space, thus allowing the evaluation of physiological parameters, such as the status or existence of the blood-brain-barrier or the renal function. Shortly after the first clinical use of paramagnetic metallochelates in 1983, compounds were suggested for liver imaging and enhancing a cardiac infarct. Meanwhile, liver specific contrast agents based on gadolinium, manganese or iron become reality. Dedicated blood pool agents will be available within the next years. These gadolinium or iron agents will be beneficial for longer lasting MRA procedures, such as cardiac imaging. Contrast enhanced lymphography after interstitial or intravenous injection will be another major step forward in diagnostic imaging. Metastatic involvement will be seen either after the injection of ultrasmall superparamagnetic iron oxides or dedicated gadolinium chelates. The accumulation of both compound classes is triggered by an uptake into macrophages. It is likely that similar agents will augment MRI of atheriosclerotic plaques, a systemic inflammatory disease of the arterial wall. Thrombus-specific agents based on small gadolinium labeled peptides are on the horizon. It is very obvious that the future of cardiovascular MRI will benefit from the development of new paramagnetic and superparamagnetic substances. The expectations for new tumor-, pathology- or receptor-specific agents are high. However, is not likely that such a compound will be available for daily routine MRI within the next decade.

Uptake from water, release and tissue distribution of 54Mn in the Rainbow trout (Oncorhynchus mikiss Walbaum)

As part of a research programme on the transfer of several radionuclides along a pelagic trophic chain, two groups of 12 trout were kept for 8 weeks in water contaminated with 30 Bq ml(-1) of (54)Mn. In order to simulate chronic contamination and limit alterations in the physical and chemical characteristics of the medium, the water was renewed every 2 days. The kinetics of the accumulation and elimination of the radionuclide were monitored in one group of fish. The second group was used to study the contamination of the main organs and tissues at the end of the accumulation phase. The dynamics of contamination can be described by a bi-compartmental model, taking into account the fluctuations in the concentration of (54)Mn in the water, as well as the biological dilution resulting from the growth of the fish. The theoretical value of the steady-state concentration factor for zero growth is 13 (w.w.) and the radionuclide release is characterised by two biological half-lives of 6 and 97 days. At the end of the accumulation phase, the (54)Mn is preferentially fixed in the bone, gills, skin and brain. The data obtained at the end of the depuration phase allow one to classify the organs in two groups with different elimination kinetics. The first group consists of organs of penetration or transit, such as the skin, gills, kidneys, liver, primary and secondary gut and viscera, whereas the second group is made up of the receptor and storage organs and tissues such as the bone, head, fins and muscle.

Evidence for the dissociation of the hepatobiliary MRI contrast agent Mn-DPDP

These experiments assessed and quantitated the release of free manganese Mn++ from the hepatobiliary contrast agent Mn-DPDP (manganese dipyridoxal diphosphate), using several magnetic resonance techniques (EPR spectroscopy, 31P-NMR spectroscopy, and relaxometry) to differentiate between free Mn++ and Mn++ in complexes in various preparations. The presence of calcium and magnesium in physiological concentrations in aqueous solutions induced the release of Mn++ from the complex, as did incubation of the complex in liver homogenates. After intravenous injection of 15 mumol/kg of Mn-DPDP, both EPR and 31P-NMR spectroscopy demonstrated that Mn-DPDP is partly dissociated (approximately 25%) in the liver. By comparing in vitro and ex vivo data from the liver, we concluded that the dissociation of Mn-DPDP occurs primarily in the liver, whereas a minor portion of the dissociated. Mn found in the liver comes from dissociation of the complex in the blood. Most of the dissociated Mn in liver becomes bound to macromolecules and is responsible for the enhancement of relaxivity observed with this agent.

Assessment of myocardial perfusion using contrast-enhanced MR imaging: current status and future developments

Excellent inherent tissue contrast is one of the great promises of clinical magnetic resonance (MR) imaging, but functional information is relatively limited. However, MR imaging complemented by the administration of contrast agents can provide such functional assessment. The perfusion status of the myocardium is one of the most important functional information in cardiovascular imaging. Because the clinical acceptance of a contrast agent is measured by its ability to improve patient outcome and to guide therapy, it is unlikely that detection of myocardial infarction, the final stage of ischemic heart disease, should be the target for contrast media development. It would obviously be better if occult regional myocardial perfusion deficits could be reliably detected. The current article was prepared to help the clinical radiologist to keep pace with new strategies for myocardial enhancement and their potential clinical applicability for detection of early perfusion deficits. Several techniques for noninvasive measurement of myocardial perfusion are currently evolving which have the potential to be introduced into routine MR imaging. Most investigators favor a first-pass analysis of the contrast agent bolus through the myocardium using ultrafast sequences. However, such a technique may require clinical introduction of a blood pool agent. There are good reasons to favor T1-weighted sequences over susceptibility imaging in such first-pass studies. In the future, assessment of myocardial perfusion status using contrast-enhanced MR imaging may be done producing perfusion maps with high spatial resolution (e.g., 256 x 128), with sequences available on most scanners without special hardware requirements (e.g., IR-Turboflash, keyhole imaging), and requiring only a short period of time for examination (approximately 3 min).

Mn-DPDP-enhanced MR imaging of malignant liver lesions: efficacy and safety in 20 patients

Twenty patients with malignant liver lesions underwent magnetic resonance (MR) imaging with manganese (II) DPDP [N,N'-dipyridoxylethylenediamine-N,N'-diacetate 5,5'-bis(phosphate)] to evaluate the safety and efficacy of the contrast agent. In two groups of 10 patients each, 5 mumol/kg Mn-DPDP was administered intravenously (3 mL/min) at a concentration of either 50 or 10 mumol/mL. T1- and T2-weighted images were obtained with a 1.5-T imager. Six patients reported a total of eight instances of side effects (flush, feeling of warmth, metallic taste) of which seven occurred at the 50 mumol/mL concentration. A significant decrease in alkaline phosphatase levels 2 hours after injection was recorded. On T1-weighted images, the 10 mumol/mL formulation yielded significantly greater increases in contrast-to-noise ratio (79.8%-137.5%) than the 50 mumol/mL formulation (46.2%-86.6%). In a blinded reader study of 10 patients with one to five lesions each, no lesion was missed on Mn-DPDP--enhanced T1-weighted images; however, four false-positive foci were identified. The authors conclude that slow administration of 5 mumol/kg Mn-DPDP at a concentration of 10 mumol/mL is safe and efficient enough to proceed to further clinical trials.

Distribution of manganese in rat pancreas and identification of its primary binding protein as pro-carboxypeptidase B

Distribution of manganese (Mn) and its binding to specific proteins were examined in rat pancreas. A MnCl2 solution was injected subcutaneously into Wistar rats daily at a single dose of 15 mg of Mn/kg body weight for 10 days and the animals were killed 1 day after the last injection. The concentration of Mn in the pancreas increased considerably from 1.4 +/- 0.2 (control) to 13.3 +/- 3.7 micrograms/g wet tissue by the repeated injection of Mn. The distribution of Mn in the soluble fraction of the pancreas (170,000 g supernatant) was determined on a gel-filtration column (Asahipak GST-520) using an h.p.l.c.-inductively coupled argon plasma atomic-emission spectrometry (i.c.p.) technique. The metal was eluted as a single peak in the high-molecular-mass protein fraction, where Mn had been observed as a small peak in the control profile, suggesting that the administered Mn was bound to the same Mn-binding component as that in the control. On the basis of enzymic and chemical characterization of the protein, it was identified as a zymogen of carboxypeptidase B (pro-carboxypeptidase B, pro-CPB). The elution profiles of the protein by h.p.l.c.-i.c.p. indicated that Mn and zinc (Zn) were bound to the zymogen with a molar ratio of 1:4 in normal rat pancreas. Mn bound to the zymogen was easily replaced by Zn in vitro, suggesting that Mn was bound to the Zn-binding site and that the binding affinity to Zn was higher than that to Mn. The present results indicate that pro-CPB is the primary Mn-binding protein in the pancreas of control and also Mn-administered rats.

Direct ESR measurement of free radicals in mouse pancreatic lesions

In this experiment, free radicals in the pancreas of endotoxemia and ethionine induced acute pancreatitis in mice were attempted to be detected directly by ESR spectroscopy, using 77 K freeze-trapping and 25 degrees C DMPO spin trapping techniques. In the 77 K freeze-trapping method, Mn (II) ion and R-00. radical were detected in endotoxemia and ethionine induced pancreatic lesions. The heme-NO radical was observed at 6 and 24 h after isolation of the normal pancreas, and signal intensity was increased with time. This finding supports that ESR spectroscopy is a useful method for detecting the tissue degeneration process from ischemia to necrosis. Using the DMPO spin trapping technique (25 degrees C), 6-line was detected at 6 h after intraperitoneal administration of E. coli in the model of endotoxemia, and 3- and 6-lines and a signal suggestive of DMPO-OH adduct were noted at 12 and 24 h in ethionine pancreatitis. These findings suggest that impaired pancreatic tissues exist in a considerably oxidative environment and oxygen derived free radicals may be considered to play an important role in the development of pancreatic lesions.

Comparison of the biodistribution of 153Gd-labeled Gd(DTPA)2-, Gd(DOTA)-, and Gd(acetate)n in mice

The biodistributions of 153Gd-labeled Gd(DTPA)2-, Gd(DOTA)-, and Gd(acetate)n were determined in mice at five residence times. Gd(DTPA)2- and Gd(DOTA)- had similar distributions with greater than 89% renally excreted in 1 h. At 7 days and longer after Gd(DTPA)2- administration, 153Gd bone levels were higher than could be attributed to free 153Gd3+, suggesting that Gd(DTPA)2- dechelates in vivo. Gd(DOTA)- did not appear to dechelate. Gd(acetate)n did not readily clear and 153Gd levels remained high in liver, lungs, and spleen for 28 days.

Manganese: its acquisition by and function in the lactic acid bacteria

The transition metal manganese is considered to be a minor micronutrient in both pro- and eukaryotes, usually being required from the environment at subnanomolar levels. Until recently, Mn was only known to function in cells as a cofactor for a few enzymatic reactions. A notable exception has been reported in many lactic acid bacterial species which require micromolar medium Mn levels for growth and contain up to 35 mM Mn. These high Mn concentrations are accompanied by the near or complete absence of intracellular iron and superoxide dismutase (SOD). Lacking hemes, Lactobacillus plantarum and related species contain a unique Mn-cofactored catalase as well as millimolar Mn(II) in a nonenzymic complex performing the function of the micromolar superoxide dismutase found in most other aerotolerant cells. The high Mn(II) levels are accumulated via an efficient active transport system and are stored intracellularly in a high molecular weight complex. Study of Lactobacillus plantarum has provided an interesting example of the substitution of Mn for Fe in several of the biological roles of Fe, an alternative mechanism of aerotolerance, and a better understanding of the unique biochemistry of the lactic acid bacteria.

Application of whole-body autoradiography in toxicology

Whole-body autoradiography enables the drugs and toxicants to be distributed throughout the animal. Good results are obtained with this technique. However, certain artifacts can occur that could lead to misinterpretation, and these must be known. These artifacts are described. From the metabolic point of view, autoradiography provides data on the distribution kinetics of a compound and the elimination of radioactivity in various organs. These data are a guide for quantitative research into the metabolism of a compound. From the toxicological point of view, it must be admitted that the main purpose of this technique is to reveal the sites of retention of radioactivity. Such specific organ retention could be the consequence of the activation of a minor metabolite into a very reactive compound. If this is so, it is a specific organ effect which could not be studied by other techniques and could lead the way to a more specific organ effect which could not be studied by other techniques and could lead the way to a more appropriate line of research in the study of chronic toxicity. However, it must be recalled that the fact that a compound is retained by a specific organ does not always mean that the compound exerts a toxic effect upon the said organ. With this technique, distribution study can be performed on pregnant animals, and it provides us with more data concerning the transplacental passage of radioactive metabolites. All these aspects of the technique clearly indicate that whole-body autoradiography should be insisted upon during the early stages of development of new molecules. Successive experiments could then lead to selecting the best experimental conditions for metabolic pharmacokinetics and studies in toxicology.