Shape-changing magnetic assemblies as high-sensitivity NMR-readable nanoprobes.

Fluorescent and plasmonic labels and sensors have revolutionized molecular biology, helping visualize cellular and biomolecular processes. Increasingly, such probes are now being designed to respond to wavelengths in the near-infrared region, where reduced tissue autofluorescence and photon attenuation enable subsurface in vivo sensing. But even in the near-infrared region, optical resolution and sensitivity decrease rapidly with increasing depth. Here we present a sensor design that obviates the need for optical addressability by operating in the nuclear magnetic resonance (NMR) radio-frequency spectrum, where signal attenuation and distortion by tissue and biological media are negligible, where background interferences vanish, and where sensors can be spatially located using standard magnetic resonance imaging (MRI) equipment. The radio-frequency-addressable sensor assemblies presented here comprise pairs of magnetic disks spaced by swellable hydrogel material; they reversibly reconfigure in rapid response to chosen stimuli, to give geometry-dependent, dynamic NMR spectral signatures. The sensors can be made from biocompatible materials, are themselves detectable down to low concentrations, and offer potential responsive NMR spectral shifts that are close to a million times greater than those of traditional magnetic resonance spectroscopies. Inherent adaptability should allow such shape-changing systems to measure numerous different environmental and physiological indicators, thus providing broadly generalizable, MRI-compatible, radio-frequency analogues to optically based probes for use in basic chemical, biological, medical and engineering research.

Lambda exonuclease digestion of CGG trinucleotide repeats.

Fragile X syndrome and other trinucleotide diseases are characterized by an elongation of a repeating DNA triplet. The ensemble-averaged lambda exonuclease digestion rate of different substrates, including one with an elongated FMR1 gene containing 120 CGG repeats, was measured using absorption and fluorescence spectroscopy. By use of magnetic tweezers sequence-dependent digestion rates and pausing was measured for individual lambda exonucleases. Within the triplet repeats a lower average and narrower distribution of rates and a higher frequency of pausing was observed.

Manganese-Enhanced MRI in Patients with Multiple Sclerosis.

BACKGROUND AND PURPOSE: Cellular uptake of the manganese ion, when administered as a contrast agent for MR imaging, can noninvasively highlight cellular activity and disease processes in both animals and humans. The purpose of this study was to explore the enhancement profile of manganese in patients with multiple sclerosis. MATERIALS AND METHODS: Mangafodipir is a manganese chelate that was clinically approved for MR imaging of liver lesions. We present a case series of 6 adults with multiple sclerosis who were scanned at baseline with gadolinium, then injected with mangafodipir, and followed at variable time points thereafter. RESULTS: Fourteen new lesions formed during or shortly before the study, of which 10 demonstrated manganese enhancement of varying intensity, timing, and spatial pattern. One gadolinium-enhancing extra-axial mass, presumably a meningioma, also demonstrated enhancement with manganese. Most interesting, manganese enhancement was detected in lesions that formed in the days after mangafodipir injection, and this enhancement persisted for several weeks, consistent with contrast coming from intracellular uptake of manganese. Some lesions demonstrated a diffuse pattern of manganese enhancement in an area larger than that of both gadolinium enhancement and T2-FLAIR signal abnormality. CONCLUSIONS: This work demonstrates the first use of a manganese-based contrast agent to enhance MS lesions on MR imaging. Multiple sclerosis lesions were enhanced with a temporal and spatial profile distinct from that of gadolinium. Further experiments are necessary to uncover the mechanism of manganese contrast enhancement as well as cell-specific uptake.

The fabrication of uniform cylindrical nanoshells and their use as spectrally tunable MRI contrast agents.

A new form of tunable magnetic resonance imaging agent based on precisely dimensioned cylindrical magnetic nanoshells is introduced. Using top-down prepatterned substrates, the nanoshells are fabricated by exploiting what is usually regarded as a detrimental processing side-effect, namely the redeposition of material back-sputtered during ion-milling. The well-resolved nuclear magnetic resonance peaks of the resulting nanostructures attest to the nanoscale fabrication control and the general feasibility of such sputter redeposition for fabrication of a variety of self-supporting, highly monodisperse nanoscale structures.

Manganese-Enhanced MRI of the Brain in Healthy Volunteers.

BACKGROUND AND PURPOSE: The manganese ion is used as an intracellular MR imaging contrast agent to study neuronal function in animal models, but it remains unclear whether manganese-enhanced MR imaging can be similarly useful in humans. Using mangafodipir (Teslascan, a chelated manganese-based contrast agent that is FDA-approved), we evaluated the dynamics of manganese enhancement of the brain and glandular structures in the rostral head and neck in healthy volunteers. MATERIALS AND METHODS: We administered mangafodipir intravenously at a rate of 1 mL/minute for a total dose of 5 µmol/kg body weight. Nine healthy adult volunteers (6 men/3 women; median age, 43 years) completed baseline history and physical examination, 3T MR imaging, and blood work. MR imaging also followed mangafodipir administration at various time points from immediate to 7 days, with delayed scans at 1-3 months. RESULTS: The choroid plexus and anterior pituitary gland enhanced within 10 minutes of infusion, with enhancement persisting up to 7 and 30 days, respectively. Exocrine (parotid, submandibular, sublingual, and lacrimal) glands also enhanced avidly as early as 1 hour postadministration, generally resolving by 1 month; 3 volunteers had residual exocrine gland enhancement, which resolved by 2 months in 1 and by 3 months in the other 2. Mangafodipir did not affect clinical parameters, laboratory values, or T1-weighted signal in the basal ganglia. CONCLUSIONS: Manganese ions released from mangafodipir successfully enable noninvasive visualization of intra- and extracranial structures that lie outside the blood-brain barrier without adverse clinical effects, setting the stage for future neuroradiologic investigation in disease.

2H-nuclear magnetic resonance imaging of tumor blood flow: spatial and temporal heterogeneity in a tissue-isolated mammary adenocarcinoma.

2H-Nuclear magnetic resonance imaging of deuteron accumulation in tissue following an i.v. bolus of deuterium oxide provides a noninvasive means of constructing maps of tissue perfusion. With a measured arterial input function and a simple model for tissue-capillary exchange, these data can provide quantitative estimates of local flow. This technique was tested in rat brain and then applied to the study of spatial heterogeneity and temporal variation of blood flow in the tissue-isolated R3230AC mammary adenocarcinoma. Global flow from the brain averaged 0.96 ml/min.g, in good agreement with results obtained from other methods; the perfusion of brain was relatively homogeneous. Global tumor blood flow averaged 0.32 ml/min.g, ranging from 0.11 to 0.96 ml/min.g. Imaging revealed variations in perfusion both within and between the tumors that far exceeded those expected from brain flow heterogeneity and uncertainty in the flow estimates. By obtaining repeated flow images at 30-min intervals, it was possible to show that the regional blood flow shifted with time in single pixels and in multipixel regions. These experiments show that 2H-nuclear magnetic resonance may be useful in obtaining noninvasive and quantitative measurement of temporal blood flow changes in a solid tumor in vivo.

Changes in pyridine nucleotide levels alter oxygen consumption and extra-mitochondrial phosphates in isolated mitochondria: a 31P-NMR and NAD(P)H fluorescence study.

Isolated rat-liver mitochondria were used to study the relation between mitochondrial NADH levels, oxygen consumption (QO2), and extra-mitochondrial phosphates. Alterations in NADH and QO2 were accomplished by incubating mitochondria with different substrates or varying amounts of exogenous ATPase while monitoring QO2 and NAD(P)H fluorescence. Two sets of conditions were studied: (1) in the presence of excess ADP and inorganic phosphate, an increase in NAD(P)H fluorescence was associated with a linear increase in QO2; (2) when QO2 was driven by the steady-state hydrolysis of ATP by exogenous ATPase, increases in QO2 were associated with proportional decreases in NAD(P)H fluorescence. For all substrates tested this relation was linear; however, the slope was substrate dependent. Different substrates were able to maintain different NAD(P)H levels at the same QO2. To investigate this further, effects of changing substrates at constant QO2 on NAD(P)H and extra-mitochondrial phosphates were determined. Addition of glutamate + malate to mitochondria respiring on citrate caused a 50% increase in NAD(P)H fluorescence, a 41% decrease in ADP, and a 30% decrease in inorganic phosphate. Similar changes for the substrate jump, pyruvate + malate to glutamate + malate were found. Finally, it was determined that a linear relation holds between increases in NAD(P)H fluorescence and increases in QO2 when substrates were varied at constant, physiologic levels of extra-mitochondrial ADP. These results indicate that QO2 depends on NAD(P)H levels as well as on extra-mitochondrial phosphates over a wide range of respiratory rates.

Response of normal and reperfused livers to glucagon stimulation: NMR detection of blood flow and high-energy phosphates.

The effects of glucagon on blood flow and high-energy phosphates in control and in rat livers damaged by ischemia were studied using in vivo nuclear magnetic resonance (NMR) spectroscopy. Normal livers and livers which had been made ischemic for 20, 40, and 60 min followed by 60 min of reperfusion were studied. Ischemia led to a loss in adenosine triphosphate (ATP) within 30 min. Reperfusion after 20 min of ischemia led to complete recovery of ATP. 60 min of reperfusion after 40 or 60 min of ischemia led to only a 76% and 48% recovery of ATP, respectively. Glucagon, at doses up to 2.5 mg/kg body weight, caused no changes in the inorganic phosphate (P(i)) to ATP ratio in normal livers as measured by 31P-NMR spectroscopy. In livers which had been made ischemic for 20, 40, or 60 min, glucagon caused an increase in the P(i)/ATP ratio of 18%, 40%, and 40%, respectively. 19F-NMR detection of the washout of trifluoromethane from liver was used to measure blood flow. Glucagon-stimulated flow in the normal liver in a dose-dependent manner, with 2.5 mg glucagon/kg body weight leading to a 95% increase in flow. Ischemia for 20, 40, and 60 min followed by 60 min of reperfusion led to hepatic blood flows which were 63%, 68%, and 58% lower than control liver. In reperfused livers, blood flow after glucagon-stimulation was reduced to 56%, 43%, and 48% of control glucagon-stimulated flow after 20, 40, and 60 min of ischemia. These results indicate that ischemia followed by reperfusion leads to decreases in hepatic blood flow prior to alterations in ATP and the response of the liver to glucagon is altered in the reperfused liver.

Calibration of the calcium dissociation constant of Rhod(2)in the perfused mouse heart using manganese quenching.

Both theoretical and experimental results are presented for in vivo calibration of the dissociation constant K(Ca)(d)of the calcium-sensitive fluorescent dye Rhod(2)in the perfused mouse heart, using manganese quenching of fluorescence transients. An analytical model is derived, based on the biochemical equilibrium of manganese competition with calcium for Rhod(2)binding. Expressing the differential of the changes between systole and diastole in fluorescence transient (delta Delta F(sys-dia)). delta DeltaF(sys-dia)in a beating heart as a function of the perfusate manganese concentration [Mn(2+)](p)allows correlation of the measured differential transient changes delta Delta F(sys-dia)with the calcium dissociation constant K(Ca)(d)of Rhod(2)and the calcium concentration in the heart. Numerical modeling indicates that the K(Ca)(d)predominantly affects the asymptotic slope of the delta Delta F(sys-dia)versus [Mn(2+)](p)curve at certain manganese concentrations, which suggests that the K(Ca)(d)can be inversely calculated by partially fitting the delta Delta F(sys-dia)distribution as a function of the perfusate manganese concentration. The feasibility of this approach is confirmed by quenching of calcium transients by manganese infusion into isolated perfused beating mouse hearts. The resulting calculated dissociation constant K(Ca)(d)of Rhod(2)is 720nM. Using the same approach, we are able to also estimate intracellular calcium concentrations of 700nM at peak systole and 300nM in diastole. This is in good agreement with values obtained by calibration of fluorescence values with a calcium saturation tetanization procedure in the same perfused mouse heart model.

Changes in the mitochondrial proteome from mouse hearts deficient in creatine kinase.

Creatine kinase (CK) is an abundant enzyme, important for maintenance of high-energy phosphate homeostasis in many tissues including heart. Double-knockout CK (DbKO-CK) mice missing both the muscle (MM) and sarcomeric mitochondrial (ScMit) isoforms of CK have recently been studied. Despite a large change in skeletal muscle function in DbKO-CK mice, there is little functional change in the heart. To investigate whether there are specific changes in cardiac mitochondrial proteins associated with the loss of MM- and ScMit-CK isoforms, we have used difference gel electrophoresis (DIGE) to compare mitochondrial proteins from wild-type and DbKO-CK mice. Mass spectrometry fingerprinting was used to identify 40 spots as known mitochondrial proteins. We have discovered that the loss of MM- and ScMit-CK isoforms did not cause large scale changes in heart mitochondrial proteins. The loss of ScMit-CK was readily detected in the DbKO-CK samples. We have also detected a large decrease in the precursor form of aconitase. Furthermore, two mitochondrial protein differences have been found in the parent mouse strains of the DbKO-CK mice.

Simultaneous glutamate and perfusion fMRI responses to regional brain stimulation.

Functional magnetic resonance imaging (fMRI) rests on the assumption that regional brain activity is closely coupled to regional cerebral blood flow (rCBF) in vivo. To test the degree of coupling, cortical brain activity was locally stimulated in rats by reversed microdialysis infusion of picrotoxinin, alphagamma-aminobutyric acid-A antagonist. Before and during the first 30 minutes of infusion, simultaneous fMRI (rCBF) and neurochemical (interstitial glutamate concentration) measures of brain activity were highly correlated (r = 0.83). After 30 minutes of picrotoxinin-induced stimulation, glutamate levels decreased but rCBF remained elevated, suggesting that additional factors modulate the relationship between neuronal neurotransmitters and hemodynamics at these later stages.

The B isozyme of creatine kinase is active as a fusion protein in Escherichia coli: in vivo detection by 31P NMR.

A cDNA encoding the B isozyme of creatine kinase (CKB) has been expressed in Escherichia coli from a fusion with lacZ carried by lambda gt11. Western blots indicate that a stable polypeptide with the appropriate mobility for the beta-galactosidase-creatine kinase (beta-gal-CKB) fusion protein cross-reacts with both beta-gal and CKB antiserum. No significant CK activity is detected in control E. coli; however, extracts from cells containing the lambda gt11-CKB construct have a CK activity of 1.54 +/- 0.07 mumol/min per mg protein. The fusion protein appears to provide this activity because immunoprecipitation of protein with beta-gal antiserum leads to a loss of CK activity from extracts. That the enzyme is active in vivo was demonstrated by detection of a phosphocreatine (PCr) peak in the 31P NMR spectrum from E. coli grown on medium supplemented with creatine. As in mammalian brain and muscle, the PCr peak detected was sensitive to the energy status of the E. coli.

Respiratory control in the glucose perfused heart. A 31P NMR and NADH fluorescence study.

The phosphate metabolites, adenosine diphosphate (ADP), inorganic phosphate (Pi), and adenosine triphosphate (ATP), are potentially important regulators of mitochondrial respiration in vivo. However, previous studies on the heart in vivo and in vitro have not consistently demonstrated an appropriate correlation between the concentration of these phosphate metabolites and moderate changes in work and respiration. Recently, mitochondrial NAD(P)H levels have been proposed as a potential regulator of cardiac respiration during alterations in work output. In order to understand better the mechanism of respiratory control under these conditions, we investigated the relationship between the phosphate metabolites, the NAD(P)H levels, and oxygen consumption (Q02) in the isovolumic perfused rat heart during alterations in work output with pacing. ATP, creatine phosphate (CrP), Pi and intracellular pH were measured using 31P NMR. Mitochondrial NAD(P)H levels were monitored using spectrofluorometric techniques. Utilizing glucose as the sole substrate, an increase in paced heart rate led to an increase in Q02 from 1.73 +/- 0.09 to 2.29 +/- 0.12 mmol Q2/h per g dry wt. No significant changes in the levels of Pi, PCr, ATP, or the calculated ADP levels were detected. Under identical conditions, an increase in heart rate was associated with a 23 + 3% increase in NAD(P)H fluorescence. Thus, under the conditions of these studies, an increase in Q02 was not associated with an increase in ADP or Pi. In contrast, increases in Q02 were associated with an increase in NAD(P)H. These data are consistent with the notion that increases in the mitochondrial NADH redox state regulate steady-state levels of respiration when myocardial work is increased.

The role of creatine kinase in inhibition of mitochondrial permeability transition.

Cyclosporin A sensitive swelling of mitochondria isolated from control mouse livers and from the livers of transgenic mice expressing human ubiquitous mitochondrial creatine kinase occurred in the presence of both 40 microM calcium and 5 microM atractyloside which was accompanied by a 2.5-fold increase over state 4 respiration rates. Creatine and cyclocreatine inhibited the latter only in transgenic liver mitochondria. Protein complexes isolated from detergent solubilised rat brain extracts, containing octameric mitochondrial creatine kinase, porin and the adenine nucleotide translocator, were reconstituted into malate loaded lipid vesicles. Dimerisation of creatine kinase in the complexes and exposure of the reconstituted complexes to >200 microM calcium induced a cyclosporin A sensitive malate release. No malate release occurred with complexes containing octameric creatine kinase under the same conditions.

The role of magnesium in postischemic cardiac dysfunction.

BACKGROUND: The biochemical basis for postischemic myocardial stunning is not fully elucidated. Magnesium is an important regulator of cellular energetic processes and excitation-contraction coupling. We hypothesized that the decrease in function in the postischemic period may be the result of an alteration in magnesium regulation. METHODS: In a Langendorf perfused rabbit heart model, we used 31P nuclear magnetic resonance spectroscopy to noninvasively determine intracellular Mg2+ and high-energy phosphate levels in the preischemic period and after a 30-minute period of normothermic ischemia. We measured adenosine triphosphate (ATP), phosphocreatine, and the phosphocreatine/inorganic phosphate ratio and calculated the free energy of ATP hydrolysis (delta GATP). On reperfusion, hearts were divided into three groups (n = 7 per group)--those receiving unmodified Krebs-Henseleit (control), 192 ng/ml dobutamine, or 5 mmol/L pyruvate. RESULTS: Function (expressed as the rate-pressure product) was approximately 77% of preischemic values in the control group, whereas in both dobutamine and pyruvate groups it returned to preischemic levels. ATP was decreased similarly in all groups in the postischemic period. Phosphocreatine/inorganic phosphate ratio and delta GATP were higher in the pyruvate group compared with the other groups. Intracellular Mg2+ was elevated significantly in the unmodified control postischemic group compared with preischemic, postischemic dobutamine, and pyruvate groups (1.0 +/- 0.12 vs 0.80 +/- 0.08, 0.64 +/- 0.08, and 0.70 +/- 0.05 mmol/L, respectively; p less than 0.05). CONCLUSIONS: We conclude that (1) postischemic "stunned" hearts have elevated Mg2+ levels in association with impaired contractile function, (2) inotropic agents improve contractile function in association with a decline in Mg2+ to preischemic levels despite differing effects on intracellular energetics, and (3) Mg2+ may play an important regulatory role in the heart after ischemia.

Myofibrillar or mitochondrial creatine kinase deficiency alone does not impair mouse diaphragm isotonic function.

Creatine kinase (CK) provides ATP buffering in skeletal muscle and is expressed as 1) cytosolic myofibrillar CK (M-CK) and 2) sarcomeric mitochondrial CK (ScCKmit) isoforms that differ in their subcellular localization. The diaphragm (Dia) expresses both M-CK and ScCKmit in abundance. We compared the power and work output of 1) control CK-sufficient (Ctl), 2) M-CK-deficient [M-CK(-/-)], 3) ScCKmit-deficient [ScCKmit(-/-)], and 4) combined M-CK/ScCKmit-deficient null mutant [CK(-/-)] Dia during repetitive isotonic activations to determine the effect of CK phenotype on Dia function. Maximum power was obtained at approximately 0.4 tetanic force in all groups. M-CK(-/-) and ScCKmit(-/-) Dia were able to sustain power and work output at Ctl levels during repetitive isotonic activation (75 Hz, 330-ms duration repeated each second at 0.4 tetanic force load), and the duration of sustained Dia shortening was 67 +/- 4 s in M-CK(-/-), 60 +/- 4 s in ScCKmit(-/-), and 62 +/- 5 s in Ctl Dia. In contrast, CK(-/-) Dia power and work declined acutely and failed to sustain shortening altogether by 40 +/- 6 s. We conclude that Dia power and work output are not absolutely dependent on the presence of either M-CK or ScCKmit, whereas the complete absence of CK acutely impairs Dia shortening capacity during repetitive activation.

Combined myofibrillar and mitochondrial creatine kinase deficiency impairs mouse diaphragm isotonic function.

Creatine kinase (CK) is an enzyme central to cellular high-energy phosphate metabolism in muscle. To characterize the physiological role of CK in respiratory muscle during dynamic contractions, we compared the force-velocity relationships, power, and work output characteristics of the diaphragm (Dia) from mice with combined myofibrillar and sarcomeric mitochondrial CK deficiency (CK[-/-]) with CK-sufficient controls (Ctl). Maximum velocity of shortening was significantly lower in CK[-/-] Dia (14.1 +/- 0.9 Lo/s, where Lo is optimal fiber length) compared with Ctl Dia (17.5 +/- 1.1 Lo/s) (P < 0.01). Maximum power was obtained at 0.4-0.5 tetanic force in both groups; absolute maximum power (2,293 +/- 138 W/m2) and work (201 +/- 9 J/m2) were lower in CK[-/-] Dia compared with Ctl Dia (2,744 +/- 146 W/m2 and 284 +/- 26 J/m2, respectively) (P < 0.05). The ability of CK[-/-] Dia to sustain shortening during repetitive isotonic activation (75 Hz, 330-ms duration repeated each second at 0.4 tetanic force load) was markedly impaired, with CK[-/-] Dia power and work declining to zero by 37 +/- 4 s, compared with 61 +/- 5 s in Ctl Dia. We conclude that combined myofibrillar and sarcomeric mitochondrial CK deficiency profoundly impairs Dia power and work output, underscoring the functional importance of CK during dynamic contractions in skeletal muscle.

Absence of myofibrillar creatine kinase and diaphragm isometric function during repetitive activation.

Creatine kinase (CK) provides ATP buffering in skeletal muscle and is expressed as 1) cytosolic myofibrillar CK (M-CK) and 2) sarcomeric mitochondrial CK (ScCKmit) isoforms that differ in their subcellular localization. We compared the isometric contractile and fatigue properties of 1) control CK-sufficient (Ctl), 2) M-CK-deficient (M-CK[-/-]), and 3) combined M-CK/ScCKmit-deficient null mutant (CK[-/-]) diaphragm (Dia) to determine the effect of the absence of M-CK activity on Dia performance in vitro. Baseline contractile properties were comparable across groups except for specific force, which was approximately 16% lower in CK[-/-] Dia compared with M-CK[-/-] and Ctl Dia. During repetitive activation (40 Hz, (1)/(3) duty cycle), force declined in all three groups. This decline was significantly greater in CK[-/-] Dia compared with Ctl and M-CK[-/-] Dia. The pattern of force decline did not differ between M-CK[-/-] and Ctl Dia. We conclude that Dia isometric muscle function is not absolutely dependent on the presence of M-CK, whereas the complete absence of CK acutely impairs isometric force generation during repetitive activation.

Radial echo-planar imaging.

A new ultrafast magnetic resonance imaging pulse sequence named radial echo-planar imaging (rEPI) is introduced. The sequence is based on a modification of the echo-planar imaging (EPI) sequence to scan k-space radially, in an attempt to combine the speed of EPI with the benefits of radial sampling. Like in EPI, all the desired lines in k-space are scanned consecutively in opposite directions. The unique feature of this new sequence, however, is that the orientation of the readout gradient is incrementally rotated, so that all the echoes are refocused through the center of k-space. Therefore, rEPI data are acquired in a polar grid, and image reconstruction can be done either by means of filtered back-projection or by regridding the data to a Cartesian matrix followed by 2D Fourier transform. First results show that rEPI images can be acquired with the same speed and signal-to-noise ratio of EPI images. rEPI images are also shown to be less sensitive to off-resonance effects than EPI images. Further studies are underway to investigate the usefulness of rEPI for spectroscopic imaging and applications affected by motion.