Differentiation and characterization of healthy versus pathological tau using chemical exchange saturation transfer.

Neurofibrillary tangles of tau constitute one of the key biological hallmarks of Alzheimer's disease. Currently, the assessment of regional tau accumulation requires intravenous administration of radioactive tracers for PET imaging. A noninvasive MRI-based solution would have significant clinical implications. Herein, we utilized an MRI technique known as chemical exchange saturation transfer (CEST) to determine the imaging signature of tau in both its monomeric and pathologic fibrillated conformations. Three sets of purified recombinant full-length (4R) tau protein were prepared for collection of CEST spectra using a 9.4 T NMR spectrometer at varying temperatures (25, 37, and 42 °C) and RF intensities (0.7, 1.0, 1.5, and 2.2 µT). Monomeric and fibrillated tau were readily distinguished based on their Z-spectrum profiles. Fibrillated tau demonstrated a less prominent peak at 3.5 ppm with additional peaks near 0.5 and 1.5 ppm. No significant differences were identified between fibrillated tau prepared using heparin versus seed-competent tau. In conclusion, monomeric and fibrillated tau can be readily detected and distinguished based on their CEST-derived Z-spectra, pointing to the potential utility of CEST-MRI as a noninvasive biomarker of regional pathologic tau accumulation in the brain. Further testing and validation in vitro and in vivo will be necessary before this can be applied clinically.

Enhancing r1 Relaxivity in GdDOTA-Monoamide Complexes through Polar Group-Mediated Ordering of Second-Sphere Water Molecules.

This study was designed to test whether the single appended phosphonate group in GdDOTA-1AmP is sufficient for catalyzing the exchange of proton from the single inner-sphere water-exchanging molecule. Unlike the other phosphonate derivatives in this series, GdDOTA-1AmP showed a surprisingly smooth increase in r1 relaxivity from 3.0 to 6.3 mM-1 s-1 at 20 MHz as the pH was lowered from 9 to 2.5. In comparison to the bis-, tris-, and tetrakis-phosphonate analogues, which all show a biphasic dependence of r1 with changes in pH, the unique r1 versus pH characteristics of GdDOTA-1AmP are shown to closely parallel deprotonation of the single appended phosphonate group. Although the tissue biodistribution and clearance rates of GdDOTA-1AmP are more favorable than the other more highly charged phosphonate derivatives, the pH dependency of r1 is substantially reduced at magnetic fields typically used for small animal imaging (7 and 9.4T), so the attractiveness of this new molecule for quantitative imaging of tissue pH is diminished. However, this study provides some new insights into the feasibility of designing pH-responsive MRI contrast agents based upon fundamental acid-base prototropic mechanisms.

Investigations into the Signaling Pathways Involving Glucose-Stimulated Zinc Secretion (GSZS) from Prostate Epithelial Cells In Vitro and In Vivo.

PURPOSE: Recently, we reported that exposure of prostate cells in vitro or the in vivo prostate to high glucose results in release of Zn2+ ions, a process now referred to as glucose-stimulated zinc secretion (GSZS). To our knowledge, the metabolic event(s) that trigger GSZS remain largely unknown. Here, we explore several signaling pathways both in vitro using a prostate epithelial cell line and in vivo from the rat prostate. METHODS: PNT1A cells grown to confluence were washed and tagged with ZIMIR to monitor zinc secretion by optical methods. The expression levels of GLUT1, GLUT4, and Akt in cells cultured in either zinc-rich or zinc-poor media and after exposure to high versus low glucose were determined. Zinc secretion from the rat prostate in vivo as detected by MRI was compared in control animals after injection of glucose, deoxyglucose, or pyruvate to initiate zinc secretion and in animals pre-treated with WZB-117 (a GLUT1 inhibitor) or S961 (a peripheral insulin receptor inhibitor). RESULTS: PNT1A cells exposed to high levels of glucose secrete zinc whereas cells exposed to an equivalent amount of deoxyglucose or pyruvate do not. Expression of Akt was dramatically altered by zinc supplementation of the culture media but not after exposure to glucose while GLUT1 and GLUT4 levels were less affected. Rats pre-treated with WZB-117 prior to imaging showed a reduction in GSZS from the prostate compared to controls whereas rats pre-treated with S961 showed no difference. Interestingly, in comparison to PNT1A cells, pyruvate and deoxyglucose also stimulate zinc secretion in vivo likely through indirect mechanisms. CONCLUSIONS: GSZS requires metabolism of glucose both in vitro (PNT1A cells) and in vivo (rat prostate). Pyruvate also stimulates zinc secretion in vivo but likely via an indirect pathway involving rapid production of glucose via gluconeogenesis. These combined results support the conclusion that glycolytic flux is required to trigger GSZS in vivo.

MRI Methods for Imaging Beta-Cell Function in the Rodent Pancreas.

The role of Zn2+ ions in proper storage of insulin in ß-cell granules is well-established so when insulin is secreted from ß-cells stimulated by an increase in plasma glucose, free Zn2+ ions are also released. This local increase in Zn2+ can be detected in the pancreas of rodents in real time by the use of a zinc-responsive MR contrast agent. This method offers the opportunity to monitor ß-cell function longitudinally in live rodents. The methods used in our lab are fully described in this short report and some MR images of a rat pancreas showing clearly enhanced hot spots in the tail are presented.

Prospects and limitations of paramagnetic chemical exchange saturation transfer agents serving as biological reporters in vivo.

The concept of using paramagnetic metal ion complexes as chemical exchange saturation transfer agents (paraCEST) for molecular imaging of various biological processes first appeared in the literature about 20 years ago. The first paraCEST agent was based on a highly shifted, inner-sphere, slowly exchanging water molecule that could be activated at a frequency far away from bulk water, a substantial advantage for selective activation of the agent alone. Many other paraCEST agent designs followed that were based on activation of exchanging -NH or -OH proton on the chelate itself. Both types of paraCEST designs are attractive for molecular imaging because the rates of water molecule or ligand proton exchange can be designed to be sensitive to a biological or physiological property such as pH, enzyme activity, or redox. Hence, the intensity or frequency of the resulting CEST signal provides a direct readout of that property. Many molecular designs have appeared in the literature over the past 20 years, mostly reported as proof-of-concept designs but, unfortunately, only a few reports have explored the limitations of paraCEST agents for imaging a biological process in vivo. As a community, we now know that the sensitivity of paraCEST agents is lower than one might anticipate based upon simple chemical exchange principles and, in general, it appears the sensitivity of paraCEST agents is even lower in vivo than in vitro. In this short review, we address some of the factors that contribute to the limited sensitivity of paraCEST agents in vivo, offer some thoughts on approaches that could lead to dramatically improved paraCEST sensitivity, and challenge the scientific community to perform more in vivo experiments designed to test these ideas.

Recent progress in analysis of intermediary metabolism by ex vivo 13 C NMR.

Advanced imaging technologies, large-scale metabolomics, and the measurement of gene transcripts or enzyme expression all enable investigations of intermediary metabolism in human patients. Complementary information about fluxes in individual metabolic pathways may be obtained by ex vivo 13 C NMR of blood or tissue biopsies. Simple molecules such as 13 C-labeled glucose are readily administered to patients prior to surgical biopsies, and 13 C-labeled glycerol is easily administered orally to outpatients. Here, we review recent progress in practical applications of 13 C NMR to study cancer biology, the response to oxidative stress, gluconeogenesis, triglyceride synthesis in patients, as well as new insights into compartmentation of metabolism in the cytosol. The technical aspects of obtaining the sample, preparing material for analysis, and acquiring the spectra are relatively simple. This approach enables convenient, valuable, and quantitative insights into intermediary metabolism in patients.

Review and consensus recommendations on clinical APT-weighted imaging approaches at 3T: Application to brain tumors.

Amide proton transfer-weighted (APTw) MR imaging shows promise as a biomarker of brain tumor status. Currently used APTw MRI pulse sequences and protocols vary substantially among different institutes, and there are no agreed-on standards in the imaging community. Therefore, the results acquired from different research centers are difficult to compare, which hampers uniform clinical application and interpretation. This paper reviews current clinical APTw imaging approaches and provides a rationale for optimized APTw brain tumor imaging at 3 T, including specific recommendations for pulse sequences, acquisition protocols, and data processing methods. We expect that these consensus recommendations will become the first broadly accepted guidelines for APTw imaging of brain tumors on 3 T MRI systems from different vendors. This will allow more medical centers to use the same or comparable APTw MRI techniques for the detection, characterization, and monitoring of brain tumors, enabling multi-center trials in larger patient cohorts and, ultimately, routine clinical use.

Dynamic 13 C MR spectroscopy as an alternative to imaging for assessing cerebral metabolism using hyperpolarized pyruvate in humans.

PURPOSE: This study is to investigate time-resolved 13 C MR spectroscopy (MRS) as an alternative to imaging for assessing pyruvate metabolism using hyperpolarized (HP) [1-13 C]pyruvate in the human brain. METHODS: Time-resolved 13 C spectra were acquired from four axial brain slices of healthy human participants (n = 4) after a bolus injection of HP [1-13 C]pyruvate. 13 C MRS with low flip-angle excitations and a multichannel 13 C/1 H dual-frequency radiofrequency (RF) coil were exploited for reliable and unperturbed assessment of HP pyruvate metabolism. Slice-wise areas under the curve (AUCs) of 13 C-metabolites were measured and kinetic analysis was performed to estimate the production rates of lactate and HCO3- . Linear regression analysis between brain volumes and HP signals was performed. Region-focused pyruvate metabolism was estimated using coil-wise 13 C reconstruction. Reproducibility of HP pyruvate exams was presented by performing two consecutive injections with a 45-minutes interval. RESULTS: [1-13 C]Lactate relative to the total 13 C signal (tC) was 0.21-0.24 in all slices. [13 C] HCO3- /tC was 0.065-0.091. Apparent conversion rate constants from pyruvate to lactate and HCO3- were calculated as 0.014-0.018 s-1 and 0.0043-0.0056 s-1 , respectively. Pyruvate/tC and lactate/tC were in moderate linear relationships with fractional gray matter volume within each slice. White matter presented poor linear regression fit with HP signals, and moderate correlations of the fractional cerebrospinal fluid volume with pyruvate/tC and lactate/tC were measured. Measured HP signals were comparable between two consecutive exams with HP [1-13 C]pyruvate. CONCLUSIONS: Dynamic MRS in combination with multichannel RF coils is an affordable and reliable alternative to imaging methods in investigating cerebral metabolism using HP [1-13 C]pyruvate.

Co-Polarized [1-13C]Pyruvate and [1,3-13C2]Acetoacetate Provide a Simultaneous View of Cytosolic and Mitochondrial Redox in a Single Experiment.

Cellular redox is intricately linked to energy production and normal cell function. Although the redox states of mitochondria and cytosol are connected by shuttle mechanisms, the redox state of mitochondria may differ from redox in the cytosol in response to stress. However, detecting these differences in functioning tissues is difficult. Here, we employed 13C magnetic resonance spectroscopy (MRS) and co-polarized [1-13C]pyruvate and [1,3-13C2]acetoacetate ([1,3-13C2]AcAc) to monitor production of hyperpolarized (HP) lactate and ß-hydroxybutyrate as indicators of cytosolic and mitochondrial redox, respectively. Isolated rat hearts were examined under normoxic conditions, during low-flow ischemia, and after pretreatment with either aminooxyacetate (AOA) or rotenone. All interventions were associated with an increase in [Pi]/[ATP] measured by 31P NMR. In well-oxygenated untreated hearts, rapid conversion of HP [1-13C]pyruvate to [1-13C]lactate and [1,3-13C2]AcAc to [1,3-13C2]ß-hydroxybutyrate ([1,3-13C2]ß-HB) was readily detected. A significant increase in HP [1,3-13C2]ß-HB but not [1-13C]lactate was observed in rotenone-treated and ischemic hearts, consistent with an increase in mitochondrial NADH but not cytosolic NADH. AOA treatments did not alter the productions of HP [1-13C]lactate or [1,3-13C2]ß-HB. This study demonstrates that biomarkers of mitochondrial and cytosolic redox may be detected simultaneously in functioning tissues using co-polarized [1-13C]pyruvate and [1,3-13C2]AcAc and 13C MRS and that changes in mitochondrial redox may precede changes in cytosolic redox.

Detection of glucose-derived D- and L-lactate in cancer cells by the use of a chiral NMR shift reagent.

BACKGROUND: Excessive lactate production, a hallmark of cancer, is largely formed by the reduction of pyruvate via lactate dehydrogenase (LDH) to L-lactate. Although D-lactate can also be produced from glucose via the methylglyoxal pathway in small amounts, less is known about the amount of D-lactate produced in cancer cells. Since the stereoisomers of lactate cannot be distinguished by conventional 1H NMR spectroscopy, a chiral NMR shift reagent was used to fully resolve the 1H NMR resonances of D- and L-lactate. METHODS: The production of L-lactate from glucose and D-lactate from methylglyoxal was first demonstrated in freshly isolated red blood cells using the chiral NMR shift reagent, YbDO3A-trisamide. Then, two different cell lines with high GLO1 expression (H1648 and H 1395) were selected from a panel of over 80 well-characterized human NSCLC cell lines, grown to confluence in standard tissue culture media, washed with phosphate-buffered saline, and exposed to glucose in a buffer for 4 h. After 4 h, a small volume of extracellular fluid was collected and mixed with YbDO3A-trisamide for analysis by 1H NMR spectroscopy. RESULTS: A suspension of freshly isolated red blood cells exposed to 5mM glucose produced L-lactate as expected but very little D-lactate. To evaluate the utility of the chiral NMR shift reagent, methylglyoxal was then added to red cells along with glucose to stimulate the production of D-lactate via the glyoxalate pathway. In this case, both D-lactate and L-lactate were produced and their NMR chemical shifts assigned. NSCLC cell lines with different expression levels of GLO1 produced both L- and D-lactate after incubation with glucose and glutamine alone. A GLO1-deleted parental cell line (3553T3) showed no production of D-lactate from glucose while re-expression of GLO1 resulted in higher production of D-lactate. CONCLUSIONS: The shift-reagent-aided NMR technique demonstrates that D-lactate is produced from glucose in NSCLC cells via the methylglyoxal pathway. The biological role of D-lactate is uncertain but a convenient method for monitoring D-lactate production could provide new insights into the biological roles of D- versus L-lactate in cancer metabolism.

Imaging Beta-Cell Function in the Pancreas of Non-Human Primates Using a Zinc-Sensitive MRI Contrast Agent.

Non-invasive beta cell function measurements may provide valuable information for improving diabetes diagnostics and disease management as the integrity and function of pancreatic beta cells have been found to be compromised in Type-1 and Type-2 diabetes. Currently, available diabetes assays either lack functional information or spatial identification of beta cells. In this work, we introduce a method to assess the function of beta cells in the non-human primate pancreas non-invasively with MRI using a Gd-based zinc(II) sensor as a contrast agent, Gd-CP027. Additionally, we highlight the role of zinc(II) ions in the paracrine signaling of the endocrine pancreas via serological measurements of insulin and c-peptide. Non-human primates underwent MRI exams with simultaneous blood sampling during a Graded Glucose Infusion (GGI) with Gd-CP027 or with a non-zinc(II) sensitive contrast agent, gadofosveset. Contrast enhancement of the pancreas resulting from co-release of zinc(II) ion with insulin was observed focally when using the zinc(II)-specific agent, Gd-CP027, whereas little enhancement was detected when using gadofosveset. The contrast enhancement detected by Gd-CP027 increased in parallel with an increased dose of infused glucose. Serological measurements of C-peptide and insulin indicate that Gd-CP027, a high affinity zinc(II) contrast agent, potentiates their secretion only as a function of glucose stimulation. Taken in concert, this assay offers the possibility of detecting beta cell function in vivo non-invasively with MRI and underscores the role of zinc(II) in endocrine glucose metabolism.

31 P-MRS of healthy human brain: Measurement of guanosine diphosphate mannose at 7 T.

Guanosine diphosphate mannose (GDP-Man) is the donor substrate required for mannosylation in the synthesis of glycoproteins, glycolipids and the newly discovered glycoRNA. Normal GDP-Man biosynthesis plays a crucial role in support of a variety of cellular functions, including cell recognition, cell communication and immune responses against viruses. Here, we report the detection of GDP-Man in human brain for the first time, using 31 P MRS at 7 T. The presence of GDP-Man is evidenced by the detection of a weak 31 P doublet at -10.7 ppm that can be assigned to the phosphomannosyl group (Pß) of the GDP-Man molecule. This weak but well-resolved signal lies 0.9 ppm upfield of UDP(G) Pß-multiplet from a mixture of UDP-Glc, UDP-Gal, UDP-GlcNAc and UDP-GalNAc. In reference to ATP (2.8 mM), the concentration of GDP-Man in human brain was estimated to be 0.02 ± 0.01 mM, about 15-fold lower than the total concentration of UDP(G) (0.30 ± 0.04, N = 17) and consistent with previous reports of UDP-Man in cells and brain tissue extracts measured by high-performance liquid chromatography. The reproducibility of the measured GDP-Man between test and 2-week retest was 21% ± 15% compared with 5% ± 4% for UDP(G) (N = 7). The measured concentrations of GDP-Man and UDP(G) are linearly correlated ([UDP(G)] = 4.3 [GDP-Man] + 0.02, with R = 0.66 and p = 0.0043), likely reflecting the effect of shared sugar precursors, which may vary among individuals in response to variation in nutritional intake and consumption. Given that GDP-Man has another set of doublet (Pα) at -8.3 ppm that overlaps with NAD(H) and UDP(G)-Pα signals, the amount of GDP-Man could potentially interfere with the deconvolution of these mixed signals in composition analysis. Importantly, this new finding may be useful in advancing our understanding of glycosylation and its role in the development of cancer, as well as infectious and neurodegenerative diseases.

Magnetic Resonance Imaging Detection of Glucose-Stimulated Zinc Secretion in the Enlarged Dog Prostate as a Potential Method for Differentiating Prostate Cancer From Benign Prostatic Hyperplasia.

OBJECTIVES: In the United States, prostate cancer (PCa) is the most common cancer in men. Multi-parametric magnetic resonance imaging (MRI) is increasingly being relied upon for the diagnosis and characterization of PCa, but differentiating malignancy from benign prostatic hyperplasia (BPH) in the transition zone using MRI can be challenging. The characteristically high levels of zinc in human prostate tissue and a close relationship between malignant proliferation and zinc homeostatic dysregulation create opportunities to visualize PCa with novel contrast media. In mouse models, glucose-stimulated zinc secretion (GSZS) can be preferentially observed in healthy prostate tissue compared with malignant tissue; in vivo, these differences can be captured with MRI by using Gdl1, a gadolinium-based zinc-responsive contrast agent. In this study, we examined whether this technology can be applied in a large animal model by imaging older dogs with clinically diagnosed BPH. MATERIALS AND METHODS: Four intact male dogs 6 years or older with enlarged prostates were imaged (T1-weighted turbo spin-echo, TE/TR, 12/400 milliseconds and T2-weighted, TE/TR, 112/5000 milliseconds) using a 3 T scanner before and at multiple time points after intravenous injection of 0.05 mmol/kg GdL1 plus either (a) 2 mL/kg of 50% dextrose in 1 session or (b) 2 mL/kg normal saline in another session. The two sessions were one week apart, and their order was randomly determined for each dog. During postprocessing, regions of interest were generated in prostate tissue and in paraspinal muscles to evaluate the contrast-to-noise ratio (CNR). The ratio of CNR at any postinjection time point compared with baseline CNR was defined as r-CNR. After the second imaging session, the dogs were euthanized, and their prostates were harvested for histopathological examination. Baseline and postintervention plasma and urine samples were analyzed for total zinc by inductively coupled plasma mass spectrometry. RESULTS: The mean ± SD r-CNR values at 13 minutes postinjection in the dextrose versus saline imaging sessions were 134% ± 10% and 127% ± 7%, respectively (P < 0.01). The histopathologic evaluation of prostate tissues confirmed BPH in all dogs. Interestingly, prostatic intraepithelial neoplasia was detected in 1 animal, and a suspicious mass was found in the same region on T2-weighted scans. The r-CNR of the mass was calculated as 113% ± 4% and 111% ± 6% in the dextrose and saline groups, respectively, with no significant differences between the 2 interventions (P = 0.54), whereas there was a statistically significant difference between the r-CNR of the whole prostate in the dextrose (130% ±11%) and saline (125% ± 9%) interventions (P = 0.03). Inductively coupled plasma mass spectrometry analyses showed a significantly higher urinary zinc in the dextrose versus saline groups, but no differences were found in plasma zinc levels. CONCLUSIONS: T1-weighted MRI of the enlarged canine prostate showed higher r-CNR after injection of GdL1 plus dextrose compared with GdL1 plus saline, consistent with GSZS from BPH tissues. One small region of neoplastic tissue was identified in a single dog on the basis of less GSZS from that region by MRI. These findings suggest a new method for the detection of PCa by MRI that could facilitate the differentiation of BPH from PCa in the transition zone.

In Vivo ZIMIR Imaging of Mouse Pancreatic Islet Cells Shows Oscillatory Insulin Secretion.

Appropriate insulin secretion is essential for maintaining euglycemia, and impairment or loss of insulin release represents a causal event leading to diabetes. There have been extensive efforts of studying insulin secretion and its regulation using a variety of biological preparations, yet it remains challenging to monitor the dynamics of insulin secretion at the cellular level in the intact pancreas of living animals, where islet cells are supplied with physiological blood circulation and oxygenation, nerve innervation, and tissue support of surrounding exocrine cells. Herein we presented our pilot efforts of ZIMIR imaging in pancreatic islet cells in a living mouse. The imaging tracked insulin/Zn2+ release of individual islet ß-cells in the intact pancreas with high spatiotemporal resolution, revealing a rhythmic secretion activity that appeared to be synchronized among islet ß-cells. To facilitate probe delivery to islet cells, we also developed a chemogenetic approach by expressing the HaloTag protein on the cell surface. Finally, we demonstrated the application of a fluorescent granule zinc indicator, ZIGIR, as a selective and efficient islet cell marker in living animals through systemic delivery. We expect future optimization and integration of these approaches would enable longitudinal tracking of beta cell mass and function in vivo by optical imaging.

31 P-MRS of the healthy human brain at 7 T detects multiple hexose derivatives of uridine diphosphate glucose.

Nucleotide sugars are required for the synthesis of glycoproteins and glycolipids, which play crucial roles in many cellular functions such as cell communication and immune responses. Uridine diphosphate-glucose (UDP-Glc) was previously believed to be the only nucleotide sugar detectable in brain by 31 P-MRS. Using spectra of high SNR and high resolution acquired at 7 T, we showed that multiple nucleotide sugars are coexistent in brain and can be measured simultaneously. In addition to UDP-Glc, these also include UDP-galactose (UDP-Gal), -N-acetyl-glucosamine (UDP-GlcNAc) and -N-acetyl-galactosamine (UDP-GalNAc), collectively denoted as UDP(G). Coexistence of these UDP(G) species is evident from a quartet-like multiplet at -9.8 ppm (M-9.8 ), which is a common feature seen across a wide age range (24-64 years). Lineshape fitting of M-9.8 allows an evaluation of all four UDP(G) components, which further aids in analysis of a mixed signal at -8.2 ppm (M-8.2 ) for deconvolution of NAD+ and NADH. For a group of seven young healthy volunteers, the concentrations of UDP(G) species were 0.04 ± 0.01 mM for UDP-Gal, 0.07 ± 0.03 mM for UDP-Glc, 0.06 ± 0.02 mM for UDP-GalNAc and 0.08 ± 0.03 mM for UDP-GlcNA, in reference to ATP (2.8 mM). The combined concentration of all UDP(G) species (average 0.26 ± 0.06 mM) was similar to the pooled concentration of NAD+ and NADH (average 0.27 ± 0.06 mM, with a NAD+ /NADH ratio of 6.7 ± 2.1), but slightly lower than previously found in an older cohort (0.31 mM). The in vivo NMR analysis of UDP-sugar composition is consistent with those from tissue extracts by other modalities in the literature. Given that glycosylation is dependent on the availability of nucleotide sugars, assaying multiple nucleotide sugars may provide valuable insights into potential aberrant glycosylation, which has been implicated in certain diseases such as cancer and Alzheimer's disease.

Manganese(II)-Based Responsive Contrast Agent Detects Glucose-Stimulated Zinc Secretion from the Mouse Pancreas and Prostate by MRI.

A Mn(II)-based zinc-sensitive MRI contrast agent, MnPyC3A-BPEN, was prepared, characterized, and applied in imaging experiments to detect glucose-stimulated zinc secretion (GSZS) from the mouse pancreas and prostate in vivo. Thermodynamic and kinetic stability tests showed that MnPyC3A-BPEN has superior kinetic inertness compared to GdDTPA, is less susceptible to transmetalation in the presence of excess Zn2+ ions, and less susceptible to transchelation by albumin. In comparison with other gadolinium-based zinc sensors bearing a single zinc binding moiety, MnPyC3A-BPEN appears to be a reliable alternative for imaging ß-cell function in the pancreas and glucose-stimulated zinc secretion from the prostate.

Imaging ß-Cell Function Using a Zinc-Responsive MRI Contrast Agent May Identify First Responder Islets.

An imaging method for detecting ß-cell function in real-time in the rodent pancreas could provide new insights into the biological mechanisms involving loss of ß-cell function during development of type 2 diabetes and for testing of new drugs designed to modulate insulin secretion. In this study, we used a zinc-responsive MRI contrast agent and an optimized 2D MRI method to show that glucose stimulated insulin and zinc secretion can be detected as functionally active "hot spots" in the tail of the rat pancreas. A comparison of functional images with histological markers show that insulin and zinc secretion does not occur uniformly among all pancreatic islets but rather that some islets respond rapidly to an increase in glucose while others remain silent. Zinc and insulin secretion was shown to be altered in streptozotocin and exenatide treated rats thereby verifying that this simple MRI technique is responsive to changes in ß-cell function.

13C-Labeled Diethyl Ketoglutarate Derivatives as Hyperpolarized Probes of 2-Ketoglutarate Dehydrogenase Activity.

The TCA cycle is a central metabolic pathway for energy production and biosynthesis. A major control point of metabolic flux through the cycle is the decarboxylation of 2-ketoglutarate by the TCA cycle enzyme 2-ketoglutarate dehydrogenase (2-KGDH). In this project, we developed 13C labeled 2-ketoglutarate derivatives to monitor 2-KGDH activity in vivo. 13C NMR analysis of liver extracts revealed that uniformly 13C labeled 2-ketogutarate, in its cell permeable ester form, was rapidly taken up and hydrolyzed in liver and underwent extensive metabolism to produce labeled glutamate, succinate, lactate and other metabolites. Diethyl [1,2-13C2]-2-ketoglutarate was successfully polarized by dynamic nuclear polarization and within seconds after injection into rats, the probe produced hyperpolarized [13C]bicarbonate in the liver reflecting flux through the TCA cycle. These experiments demonstrate that this tracer offers the possibility of directly monitoring flux through 2-KGDH in vivo.

The Roles of ZnT1 and ZnT4 in Glucose-Stimulated Zinc Secretion in Prostate Epithelial Cells.

PURPOSE: We have previously demonstrated by MRI that high glucose stimulates efflux of zinc ions from the prostate. To our knowledge, this phenomena had not been reported previously and the mechanism remains unknown. Here, we report some initial observations that provide new insights into zinc processing during glucose-stimulated zinc secretion (GSZS) in the immortalized human prostate epithelial cell line, PNT1A. Additionally, we identified the subtypes of zinc-containing cells in human benign prostatic hyperplasia (BPH) tissue to further identify which cell types are likely responsible for zinc release in vivo. PROCEDURE: An intracellular fluorescence marker, FluoZin-1-AM, was used to assess the different roles of ZnT1 and ZnT4 in zinc homeostasis in wild type (WT) and mRNA knockdown PNT1A cell lines. Additionally, Bafilomycin A1 (Baf) was used to disrupt lysosomes and assess the role of lysosomal storage during GSZS. ZIMIR, an extracellular zinc-responsive fluorescent marker, was used to assess dynamic zinc efflux of WT and ZnT1 mRNA knockdown cells exposed to high glucose. Electron microscopy was used to assess intracellular zinc storage in response to high glucose and evaluate how Bafilomycin A1 affects zinc trafficking. BPH cells were harvested from transurtheral prostatectomy tissue and stained with fluorescent zinc granule indicator (ZIGIR), an intracellular zinc-responsive fluorescent marker, before being sorted for cell types using flow cytometry. RESULTS: Fluorescent studies demonstrate that ZnT1 is the major zinc efflux transporter in prostate epithelial cells and that loss of ZnT1 via mRNA knockdown combined with lysosomal storage disruption results in a nearly 4-fold increase in cytosolic zinc. Knockdown of ZnT1 dramatically reduces zinc efflux during GSZS. Electron microscopy (EM) reveals that glucose stimulation significantly increases lysosomal storage of zinc; disruption of lysosomes via Baf or ZnT4 mRNA knockdown increases multi-vesicular body (MVB) formation and cytosolic zinc levels. In human BPH tissue, only the luminal epithelial cells contained significant amounts of zinc storage granules. CONCLUSIONS: Exposure of prostate epithelial cells to high glucose alters zinc homeostasis by inducing efflux of zinc ions via ZnT1 channels and increasing lysosomal storage via ZnT4. Given that prostate cancer cells undergo profound metabolic changes that result in reduced levels of total zinc, understanding the complex interplay between glucose exposure and zinc homeostasis in the prostate may provide new insights into the development of prostate carcinogenesis.

Imaging Tissue Physiology In Vivo by Use of Metal Ion-Responsive MRI Contrast Agents.

Paramagnetic metal ion complexes, mostly based on gadolinium (Gd3+), have been used for over 30 years as magnetic resonance imaging (MRI) contrast agents. Gd3+-based contrast agents have a strong influence on T1 relaxation times and are consequently the most commonly used agents in both the clinical and research environments. Zinc is an essential element involved with over 3000 different cellular proteins, and disturbances in tissue levels of zinc have been linked to a wide range of pathologies, including Alzheimer's disease, prostate cancer, and diabetes mellitus. MR contrast agents that respond to the presence of Zn2+ in vivo offer the possibility of imaging changes in Zn2+ levels in real-time with the superior spatial resolution offered by MRI. Such responsive agents, often referred to as smart agents, are typically composed of a paramagnetic metal ion with a ligand encapsulating it and one or more chelating units that selectively bind with the analyte of interest. Translation of these agents into clinical radiology is the next goal. In this review, we discuss Gd3+-based MR contrast agents that respond to a change in local Zn2+ concentration.