Lactate oxidation in Paracoccus denitrificans

Paracoccus denitrificans has a classical cytochrome-dependent electron transport chain and two alternative oxidases. The classical transport chain is very similar to that in eukaryotic mitochondria. Thus, P. denitrificans can serve as a model of the mammalian mitochondrion that may be more tractable in elucidating mechanisms of regulation of energy production than are mitochondria. In a previous publication we reported detailed studies on respiration in P. denitrificans grown aerobically on glucose or malate. We noted that P. denitrificans has large stores of lactate under various growth conditions. This is surprising because P. denitrificans lacks an NAD+-dependent lactate dehydrogenase. The aim of this study was to investigate the mechanisms of lactate oxidation in P. denitrificans. We found that the bacterium grows well on either d-lactate or l-lactate. Growth on lactate supported a rate of maximum respiration that was equal to that of cells grown on glucose or malate. We report proteomic, metabolomic, and biochemical studies that establish that the metabolism of lactate by P. denitrificans is mediated by two non-NAD+-dependent lactate dehydrogenases. One prefers d-lactate over l-lactate (D-iLDH) and the other prefers l-lactate (L-iLDH). We cloned and produced the D-iLDH and characterized it. The Km for d-lactate was 34 μM, and for l-lactate it was 3.7 mM. Pyruvate was not a substrate, rendering the reaction unidirectional with lactate being converted to pyruvate for entry into the TCA cycle. The intracellular lactate was ∼14 mM such that both isomers could be metabolized by the enzyme. The enzyme has 1 FAD per molecule and utilizes a quinone rather than NAD + as an electron acceptor. D-iLDH provides a direct entry of lactate reducing equivalents into the cytochrome chain, potentially explaining the high respiratory capacity of P. denitrificans in the presence of lactate.

Cardiac nitric oxide scavenging: role of myoglobin and mitochondria

Vascular production of nitric oxide (NO) regulates vascular tone. However, highly permeable NO entering the cardiomyocyte would profoundly impact metabolism and signalling without scavenging mechanisms. The purpose of this study was to establish mechanisms of cardiac NO scavenging. Quantitative optical studies of normoxic working hearts demonstrated that micromolar NO concentrations did not alter mitochondria redox state or respiration despite detecting NO oxidation of oxymyoglobin to metmyoglobin. These data are consistent with proposals that the myoglobin/myoglobin reductase (Mb/MbR) system is the major NO scavenging site. However, kinetic studies in intact hearts reveal a minor role (∼9%) for the Mb/MbR system in NO scavenging. In vitro, oxygenated mitochondria studies confirm that micromolar concentrations of NO bind cytochrome oxidase (COX) and inhibit respiration. Mitochondria had a very high capacity for NO scavenging, importantly, independent of NO binding to COX. NO is also known to quickly react with reactive oxygen species (ROS) in vitro. Stimulation of NO scavenging with antimycin and its inhibition by substrate depletion are consistent with NO interacting with ROS generated in Complex I or III under aerobic conditions. Extrapolating these in vitro data to the intact heart supports the hypothesis that mitochondria are a major site of cardiac NO scavenging. KEY POINTS: Cardiomyocyte scavenging of vascular nitric oxide (NO) is critical in maintaining normal cardiac function. Myoglobin redox cycling via myoglobin reductase has been proposed as a major NO scavenging site in the heart. Non-invasive optical spectroscopy was used to monitor the effect of NO on mitochondria and myoglobin redox state in intact beating heart and isolated mitochondria. These non-invasive studies reveal myoglobin/myoglobin reductase plays a minor role in cardiac NO scavenging. A high capacity for NO scavenging by heart mitochondria was demonstrated, independent of cytochrome oxidase binding but dependent on oxygen and high redox potentials consistent with generation of reactive oxygen species.

Increased mitochondrial free Ca2+ during ischemia is suppressed, but not eliminated by, germline deletion of the mitochondrial Ca2+ uniporter

Mitochondrial Ca2+ overload is proposed to regulate cell death via opening of the mitochondrial permeability transition pore. It is hypothesized that inhibition of the mitochondrial Ca2+ uniporter (MCU) will prevent Ca2+ accumulation during ischemia/reperfusion and thereby reduce cell death. To address this, we evaluate mitochondrial Ca2+ in ex-vivo-perfused hearts from germline MCU-knockout (KO) and wild-type (WT) mice using transmural spectroscopy. Matrix Ca2+ levels are measured with a genetically encoded, red fluorescent Ca2+ indicator (R-GECO1) using an adeno-associated viral vector (AAV9) for delivery. Due to the pH sensitivity of R-GECO1 and the known fall in pH during ischemia, hearts are glycogen depleted to decrease the ischemic fall in pH. At 20 min of ischemia, there is significantly less mitochondrial Ca2+ in MCU-KO hearts compared with MCU-WT controls. However, an increase in mitochondrial Ca2+ is present in MCU-KO hearts, suggesting that mitochondrial Ca2+ overload during ischemia is not solely dependent on MCU.

Spectroscopic identification of the catalytic intermediates of cytochrome c oxidase in respiring heart mitochondria

The catalytic cycle of cytochrome c oxidase (COX) couples the reduction of oxygen to the translocation of protons across the inner mitochondrial membrane and involves several intermediate states of the heme a3-CuB binuclear center with distinct absorbance properties. The absorbance maximum close to 605 nm observed during respiration is commonly assigned to the fully reduced species of hemes a or a3 (R). However, by analyzing the absorbance of isolated enzyme and mitochondria in the Soret (420-450 nm), alpha (560-630 nm) and red (630-700 nm) spectral regions, we demonstrate that the Peroxy (P) and Ferryl (F) intermediates of the binuclear center are observed during respiration, while the R form is only detectable under nearly anoxic conditions in which electrons also accumulate in the higher extinction coefficient low spin a heme. This implies that a large fraction of COX (>50 %) is active, in contrast with assumptions that assign spectral changes only to R and/or reduced heme a. The concentration dependence of the COX chromophores and reduced c-type cytochromes on the transmembrane potential (ΔΨm) was determined in isolated mitochondria during substrate or apyrase titration to hydrolyze ATP. The cytochrome c-type redox levels indicated that soluble cytochrome c is out of equilibrium with respect to both Complex III and COX. Thermodynamic analyses confirmed that reactions involving the chromophores we assign as the P and F species of COX are ΔΨm-dependent, out of equilibrium, and therefore much slower than the ΔΨm-insensitive oxidation of the R intermediate, which is undetectable due to rapid oxygen binding.

Assessment of Lung Structure and Regional Function Using 0.55 T MRI in Patients With Lymphangioleiomyomatosis

OBJECTIVES

Contemporary lower-field magnetic resonance imaging (MRI) may offer advantages for lung imaging by virtue of the improved field homogeneity. The aim of this study was to evaluate the utility of lower-field MRI for combined morphologic imaging and regional lung function assessment. We evaluate low-field MRI in patients with lymphangioleiomyomatosis (LAM), a rare lung disease associated with parenchymal cysts and respiratory failure.

MATERIALS AND METHODS

We performed lung imaging on a prototype low-field (0.55 T) MRI system in 65 patients with LAM. T2-weighted imaging was used for assessment of lung morphology and to derive cyst scores, the percent of lung parenchyma occupied by cysts. Regional lung function was assessed using oxygen-enhanced MRI with breath-held ultrashort echo time imaging and inhaled 100% oxygen as a T1-shortening MR contrast agent. Measurements of percent signal enhancement from oxygen inhalation and percentage of lung with low oxygen enhancement, indicating functional deficits, were correlated with global pulmonary function test measurements taken within 2 days.

RESULTS

We were able to image cystic abnormalities using T2-weighted MRI in this patient population and calculate cyst score with strong correlation to computed tomography measurements (R = 0.86, P < 0.0001). Oxygen-enhancement maps demonstrated regional deficits in lung function of patients with LAM. Heterogeneity of oxygen enhancement between cysts was observed within individual patients. The percent low-enhancement regions showed modest, but significant, correlation with FEV1 (R = -0.37, P = 0.007), FEV1/FVC (R = -0.33, P = 0.02), and cyst score (R = 0.40, P = 0.02). The measured arterial blood ΔT1 between normoxia and hyperoxia, used as a surrogate for dissolved oxygen in blood, correlated with DLCO (R = -0.28, P = 0.03).

CONCLUSIONS

Using high-performance 0.55 T MRI, we were able to perform simultaneous imaging of pulmonary structure and regional function in patients with LAM.

Metabolic design in a mammalian model of extreme metabolism, the North American least shrew (Cryptotis parva)

Mitochondrial adaptations are fundamental to differentiated function and energetic homeostasis in mammalian cells. But the mechanisms that underlie these relationships remain poorly understood. Here, we investigated organ-specific mitochondrial morphology, connectivity and protein composition in a model of extreme mammalian metabolism, the least shrew (Cryptotis parva). This was achieved through a combination of high-resolution 3D focused ion beam electron microscopy imaging and tandem mass tag mass spectrometry proteomics. We demonstrate that liver and kidney mitochondrial content are equivalent to the heart, permitting assessment of mitochondrial adaptations in different organs with similar metabolic demand. Muscle mitochondrial networks (cardiac and skeletal) are extensive, with a high incidence of nanotunnels - which collectively support the metabolism of large muscle cells. Mitochondrial networks were not detected in the liver and kidney as individual mitochondria are localized with sites of ATP consumption. This configuration is not observed in striated muscle, likely due to a homogeneous ATPase distribution and the structural requirements of contraction. These results demonstrate distinct, fundamental mitochondrial structural adaptations for similar metabolic demand that are dependent on the topology of energy utilization process in a mammalian model of extreme metabolism. KEY POINTS: Least shrews were studied to explore the relationship between metabolic function, mitochondrial morphology and protein content in different tissues. Liver and kidney mitochondrial content and enzymatic activity approaches that of the heart, indicating similar metabolic demand among tissues that contribute to basal and maximum metabolism. This allows an examination of mitochondrial structure and composition in tissues with similar maximum metabolic demands. Mitochondrial networks only occur in striated muscle. In contrast, the liver and kidney maintain individual mitochondria with limited reticulation. Muscle mitochondrial reticulation is the result of dense ATPase activity and cell-spanning myofibrils which require networking for adequate metabolic support. In contrast, liver and kidney ATPase activity is localized to the endoplasmic reticulum and basolateral membrane, respectively, generating a locally balanced energy conversion and utilization. Mitochondrial morphology is not driven by maximum metabolic demand, but by the cytosolic distribution of energy-utilizing systems set by the functions of the tissue.

Energy homeostasis is a conserved process: Evidence from Paracoccus denitrificans' response to acute changes in energy demand

Paracoccus denitrificans is a model organism for the study of oxidative phosphorylation. We demonstrate a very high respiratory capacity compared to mitochondria when normalizing to cytochrome aa₃ content even in the absence of alternative terminal oxidases. To gain insight into conserved mechanisms of energy homeostasis, we characterized the metabolic response to K⁺ reintroduction. A rapid 3-4-fold increase in respiration occurred before substantial cellular K⁺ accumulation followed by a sustained increase of up to 6-fold that persisted after net K⁺ uptake stopped. Proton motive force (Δp) was slightly higher upon addition of K⁺ with ΔpH increasing and compensating for membrane potential (ΔΨ) depolarization. Blocking the F₀F₁-ATP synthase (Complex V) with venturicidin revealed that the initial K⁺-dependent respiratory activation was primarily due to K⁺ influx. However, the ability to sustain an increased respiration rate was partially dependent on Complex V activity. The 6-fold stimulation of respiration by K⁺ resulted in a small net reduction of most cytochromes, different from the pattern observed with chemical uncoupling and consistent with balanced input and utilization of reducing equivalents. Metabolomics showed increases in glycolytic and TCA cycle intermediates together with a decrease in basic amino acids, suggesting an increased nitrogen mobilization upon K⁺ replenishment. ATP and GTP concentrations increased after K⁺ addition, indicating a net increase in cellular potential energy. Thus, K⁺ stimulates energy generation and utilization resulting in an almost constant Δp and increased high-energy phosphates during large acute and steady state changes in respiration. The specific energy consuming processes and signaling events associated with this simultaneous activation of work and metabolism in P. denitrificans remain unknown. Nevertheless, this homeostatic behavior is very similar to that observed in mitochondria in tissues when cellular energy requirements increase. We conclude that the regulation of energy generation and utilization to maintain homeostasis is conserved across the prokaryote/eukaryote boundary.

Monitoring mitochondrial calcium and metabolism in the beating MCU-KO heart

Optical methods for measuring intracellular ions including Ca2+ revolutionized our understanding of signal transduction. However, these methods are not extensively applied to intact organs due to issues including inner filter effects, motion, and available probes. Mitochondrial Ca2+ is postulated to regulate cell energetics and death pathways that are best studied in an intact organ. Here, we develop a method to optically measure mitochondrial Ca2+ and demonstrate its validity for mitochondrial Ca2+ and metabolism using hearts from wild-type mice and mice with germline knockout of the mitochondria calcium uniporter (MCU-KO). We previously reported that germline MCU-KO hearts do not show an impaired response to adrenergic stimulation. We find that these MCU-KO hearts do not take up Ca2+, consistent with no alternative Ca2+ uptake mechanisms in the absence of MCU. This approach can address the role of mitochondrial Ca2+ to the myriad of functions attributed to alterations in mitochondrial Ca2+.

Energy metabolism design of the striated muscle cell

The design of the energy metabolism system in striated muscle remains a major area of investigation. Here, we review our current understanding and emerging hypotheses regarding the metabolic support of muscle contraction. Maintenance of ATP free energy, so called energy homeostasis, via mitochondrial oxidative phosphorylation is critical to sustained contractile activity, and this major design criterion is the focus of this review. Cell volume invested in mitochondria reduces the space available for generating contractile force, and this spatial balance between mitochondria acontractile elements to meet the varying sustained power demands across muscle types is another important design criterion. This is accomplished with remarkably similar mass-specific mitochondrial protein composition across muscle types, implying that it is the organization of mitochondria within the muscle cell that is critical to supporting sustained muscle function. Beyond the production of ATP, ubiquitous distribution of ATPases throughout the muscle requires rapid distribution of potential energy across these large cells. Distribution of potential energy has long been thought to occur primarily through facilitated metabolite diffusion, but recent analysis has questioned the importance of this process under normal physiological conditions. Recent structural and functional studies have supported the hypothesis that the mitochondrial reticulum provides a rapid energy distribution system via the conduction of the mitochondrial membrane potential to maintain metabolic homeostasis during contractile activity. We extensively review this aspect of the energy metabolism design contrasting it with metabolite diffusion models and how mitochondrial structure can play a role in the delivery of energy in the striated muscle.

Oxygen-enhanced functional lung imaging using a contemporary 0.55 T MRI system

The purpose of this study was to evaluate oxygen-enhanced pulmonary imaging at 0.55 T with 3D stack-of-spirals ultrashort-T_E (UTE) acquisition. Oxygen-enhanced pulmonary MRI offers the measurement of regional lung ventilation and perfusion using inhaled oxygen as a contrast agent. Low-field MRI systems equipped with contemporary hardware can provide high-quality structural lung imaging by virtue of the prolonged T₂ *. Fortuitously, the T₁ relaxivity of oxygen increases at lower field strengths, which is expected to improve the sensitivity of oxygen-enhanced lung MRI. We implemented a breath-held T₁ -weighted 3D stack-of-spirals UTE acquisition with a 7 ms spiral-out readout. Measurement repeatability was assessed using five repetitions of oxygen-enhanced lung imaging in healthy volunteers (n = 7). The signal intensity at both normoxia and hyperoxia was strongly dependent on lung tissue density modulated by breath-hold volume during the five repetitions. A voxel-wise correction for lung tissue density improved the repeatability of percent signal enhancement maps (coefficient of variation = 34 ± 16%). Percent signal enhancement maps were compared in 15 healthy volunteers and 10 patients with lymphangioleiomyomatosis (LAM), a rare cystic disease known to reduce pulmonary function. We measured a mean percent signal enhancement of 9.0 ± 3.5% at 0.55 T in healthy volunteers, and reduced signal enhancement in patients with LAM (5.4 ± 4.8%, p = 0.02). The heterogeneity, estimated by the percent of lung volume exhibiting low enhancement, was significantly increased in patients with LAM compared with healthy volunteers (11.1 ± 6.0% versus 30.5 ± 13.1%, p = 0.01), illustrating the capability to measure regional functional deficits.

Mitochondrial DNA segregation and replication restrict the transmission of detrimental mutation

Although mitochondrial DNA (mtDNA) is prone to accumulate mutations and lacks conventional DNA repair mechanisms, deleterious mutations are exceedingly rare. How the transmission of detrimental mtDNA mutations is restricted through the maternal lineage is debated. Here, we demonstrate that mitochondrial fission, together with the lack of mtDNA replication, segregate mtDNA into individual organelles in the Drosophila early germarium. After mtDNA segregation, mtDNA transcription begins, which activates respiration. Mitochondria harboring wild-type genomes have functional electron transport chains and propagate more vigorously than mitochondria containing deleterious mutations in hetreoplasmic cells. Therefore, mtDNA expression acts as a stress test for the integrity of mitochondrial genomes and sets the stage for replication competition. Our observations support selective inheritance at the organelle level through a series of developmentally orchestrated mitochondrial processes. We also show that the Balbiani body has a minor role in mtDNA selective inheritance by supplying healthy mitochondria to the pole plasm. These two mechanisms may act synergistically to secure the transmission of functional mtDNA through Drosophila oogenesis.

Abstract 409: Investigating the Role of the Mitochondrial Calcium Uniporter (mcu) in Calcium Regulation of Cardiac Mitochondria Using Transmural Spectroscopy and an Integrating Sphere

Cardiac mitochondria uptake calcium through the mitochondrial calcium uniporter (MCU). To better understand the role of MCU and mitochondrial calcium in regulating heart physiology and pathophysiology, we developed a method to measure mitochondrial matrix calcium in beating, perfused hearts. Langendorff perfusing hearts are loaded with 4.5 uM rhod-2-AM at 30°C and then perfused at 37°C to washout uncleaved dye. We determined that rhod-2 localized primarily to the mitochondria under our loading conditions, as shown by loading in a heart expressing a GFP-tagged mitochondrial outer membrane protein and analyzing heart slices under super resolution microscopy. Further, addition of Mn 2+ , which quenches cytosolic rhod-2, has little effect on rhod-2 signal. We insert an optical catheter with both white light and 532nm laser into the left ventricle and interleaved the collection of spectra from both light sources. The perfused heart is center mounted in an integrating sphere for spectra collection. The light passes through the ventricle and reflects off the integrating sphere resulting in a near uniform sampling of the transmitted light. Spectral properties from both light sources are determined using a rapid scanning spectrophotometer. Myoglobin oxygenation, cytochrome redox state, and rhod-2 loading are determined by white light absorbance. Ca 2+ bound rhod-2 emission is determined by removing background tissue effects from laser emission spectra and normalizing to tissue absorbance. Using this method we are able to measure changes in Ca 2+ and cytochromes during treatments such as isoproterenol and ischemia-reperfusion. The use of an integrating sphere transmural spectroscopy provides us an unique method to study mitochondrial Ca 2+ signaling in perfused mouse heart loaded with Rhod-2.

Perfused murine heart optical transmission spectroscopy using optical catheter and integrating sphere: Effects of ischemia/reperfusion

Tissue transmission optical absorption spectroscopy provides dynamic information on metabolism and function. Murine genetic malleability makes it a major model for heart research. The diminutive size of the mouse heart makes optical transmission studies challenging. Using a perfused murine heart center mounted in an integrating sphere for light collection with a ventricular cavity optical catheter as an internal light source provided an effective method of optical data collection in this model. This approach provided high signal to noise optical spectra which when fit with model spectra provided information on tissue oxygenation and redox state. This technique was applied to the study of cardiac ischemia and ischemia reperfusion which generates extreme heart motion, especially during the ischemic contracture. The integrating sphere reduced motion artifacts associated with a fixed optical pickup and methods were developed to compensate for changes in tissue thickness. During ischemia, rapid decreases in myoglobin oxygenation occurred along with increases in cytochrome reduction levels. Surprisingly, when ischemic contracture occurred, myoglobin remained fully deoxygenated, while the cytochromes became more reduced consistent with a further, and critical, reduction of mitochondrial oxygen tension during ischemic contraction. This optical arrangement is an effective method of monitoring murine heart metabolism.

Opportunities in Interventional and Diagnostic Imaging by Using High-Performance Low-Field-Strength MRI

Background Commercial low-field-strength MRI systems are generally not equipped with state-of-the-art MRI hardware, and are not suitable for demanding imaging techniques. An MRI system was developed that combines low field strength (0.55 T) with high-performance imaging technology. Purpose To evaluate applications of a high-performance low-field-strength MRI system, specifically MRI-guided cardiovascular catheterizations with metallic devices, diagnostic imaging in high-susceptibility regions, and efficient image acquisition strategies. Materials and Methods A commercial 1.5-T MRI system was modified to operate at 0.55 T while maintaining high-performance hardware, shielded gradients (45 mT/m; 200 T/m/sec), and advanced imaging methods. MRI was performed between January 2018 and April 2019. T1, T2, and T2* were measured at 0.55 T; relaxivity of exogenous contrast agents was measured; and clinical applications advantageous at low field were evaluated. Results There were 83 0.55-T MRI examinations performed in study participants (45 women; mean age, 34 years ± 13). On average, T1 was 32% shorter, T2 was 26% longer, and T2* was 40% longer at 0.55 T compared with 1.5 T. Nine metallic interventional devices were found to be intrinsically safe at 0.55 T (<1°C heating) and MRI-guided right heart catheterization was performed in seven study participants with commercial metallic guidewires. Compared with 1.5 T, reduced image distortion was shown in lungs, upper airway, cranial sinuses, and intestines because of improved field homogeneity. Oxygen inhalation generated lung signal enhancement of 19% ± 11 (standard deviation) at 0.55 T compared with 7.6% ± 6.3 at 1.5 T (P = .02; five participants) because of the increased T1 relaxivity of oxygen (4.7e-4 mmHg-1sec-1). Efficient spiral image acquisitions were amenable to low field strength and generated increased signal-to-noise ratio compared with Cartesian acquisitions (P < .02). Representative imaging of the brain, spine, abdomen, and heart generated good image quality with this system. Conclusion This initial study suggests that high-performance low-field-strength MRI offers advantages for MRI-guided catheterizations with metal devices, MRI in high-susceptibility regions, and efficient imaging. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Grist in this issue.

Intra-cardiac Side-Firing Light Catheter for Monitoring Cellular Metabolism using Transmural Absorbance Spectroscopy of Perfused Mammalian Hearts

Absorbance spectroscopy of cardiac muscle provides non-destructive assessment of cytosolic and mitochondrial oxygenation via myoglobin and cytochrome absorbance respectively. In addition, numerous aspects of the mitochondrial metabolic status such as membrane potential and substrate entry can also be estimated. To perform cardiac wall transmission optical spectroscopy, a commercially available side-firing optical fiber catheter is placed in the left ventricle of the isolated perfused heart as a light source. Light passing through the heart wall is collected with an external optical fiber to perform optical spectroscopy of the heart in near real- time. The transmission approach avoids numerous surface scattering interference occurring in widely used reflection approaches. Changes in transmural absorbance spectra were deconvolved using a library of chromophore reference spectra, providing quantitative measures of all the known cardiac chromophores simultaneously. This spectral deconvolution approach eliminated intrinsic errors that may result from using common dual wavelength methods applied to overlapping absorbance spectra, as well as provided a quantitative evaluation of the goodness of fit. A custom program was designed for data acquisition and analysis, which permitted the investigator to monitor the metabolic state of the preparation during the experiment. These relatively simple additions to the standard heart perfusion system provide a unique insight into the metabolic state of the heart wall in addition to conventional measures of contraction, perfusion, and substrate/oxygen extraction.

Paradoxical arteriole constriction compromises cytosolic and mitochondrial oxygen delivery in the isolated saline-perfused heart

The isolated saline-perfused heart is used extensively to study cardiac physiology. Previous isolated heart studies have demonstrated lower tissue oxygenation compared with in vivo hearts based on myoglobin oxygenation and the mitochondrial redox state. These data, consistent with small anoxic regions, suggest that the homeostatic balance between work and oxygen delivery is impaired. We hypothesized that these anoxic regions are caused by inadequate local perfusion due to a paradoxical arteriole constriction generated by a disrupted vasoregulatory network. We tested this hypothesis by applying two exogenous vasodilatory agents, adenosine and cromakalim, to relax vascular tone in an isolated, saline-perfused, working rabbit heart. Oxygenation was monitored using differential optical transmission spectroscopy and full spectral fitting. Increases in coronary flow over control with adenosine (27 ± 4 ml/min) or cromakalim (44 ± 4 ml/min) were associated with proportional spectral changes indicative of myoglobin oxygenation and cytochrome oxidase (COX) oxidation, consistent with a decrease in tissue anoxia. Quantitatively, adenosine decreased deoxymyoglobin optical density (OD) across the wall by 0.053 ± 0.008 OD, whereas the reduced form of COX was decreased by 0.039 ± 0.005 OD. Cromakalim was more potent, decreasing deoxymyoglobin and reducing the level of COX by 0.070 ± 0.019 OD and 0.062 ± 0.019 OD, respectively. These effects were not species specific, as Langendorff-perfused mouse hearts treated with adenosine demonstrated similar changes. These data are consistent with paradoxical arteriole constriction as a major source of regional anoxia during saline heart perfusion. We suggest that the vasoregulatory network is disrupted by the washout of interstitial vasoactive metabolites in vitro. NEW & NOTEWORTHY Regional tissue anoxia is a common finding in the ubiquitous saline-perfused heart but is not found in vivo. Noninvasive optical techniques confirmed the presence of regional anoxia under control conditions and demonstrated that anoxia is diminished using exogenous vasodilators. These data are consistent with active arteriole constriction, occurring despite regional anoxia, generated by a disrupted vasoregulatory network. Washout of interstitial vasoactive metabolites may contribute to the disruption of normal vasoregulatory processes in vitro.

The interactome of intact mitochondria by cross-linking mass spectrometry provides evidence for coexisting respiratory supercomplexes

Mitochondria exert an immense amount of cytophysiological functions, but the structural basis of most of these processes is still poorly understood. Here we use cross-linking mass spectrometry to probe the organization of proteins in native mouse heart mitochondria. Our approach provides the largest survey of mitochondrial protein interactions reported so far. In total, we identify 3,322 unique residue-to-residue contacts involving half of the mitochondrial proteome detected by bottom-up proteomics. The obtained mitochondrial protein interactome gives insights in the architecture and submitochondrial localization of defined protein assemblies, and reveals the mitochondrial localization of four proteins not yet included in the MitoCarta database. As one of the highlights, we show that the oxidative phosphorylation complexes I-V exist in close spatial proximity, providing direct evidence for supercomplex assembly in intact mitochondria. The specificity of these contacts is demonstrated by comparative analysis of mitochondria after high salt treatment, which disrupts the native supercomplexes and substantially changes the mitochondrial interactome.

Cardiac performance is limited by oxygen delivery to the mitochondria in the crystalloid-perfused working heart

The left ventricular working, crystalloid-perfused heart is used extensively to evaluate basic cardiac function, pathophysiology, and pharmacology. Crystalloid-perfused hearts may be limited by oxygen delivery, as adding oxygen carriers increases myoglobin oxygenation and improves myocardial function. However, whether decreased myoglobin oxygen saturation impacts oxidative phosphorylation (OxPhos) is unresolved, since myoglobin has a much lower affinity for oxygen than cytochrome c oxidase (COX). In the present study, a laboratory-based synthesis of an affordable perfluorocarbon (PFC) emulsion was developed to increase perfusate oxygen carrying capacity without impeding optical absorbance assessments. In left ventricular working hearts, along with conventional measurements of cardiac function and metabolic rate, myoglobin oxygenation and cytochrome redox state were monitored using a novel transmural illumination approach. Hearts were perfused with Krebs-Henseleit (KH) or KH supplemented with PFC, increasing perfusate oxygen carrying capacity by 3.6-fold. In KH-perfused hearts, myoglobin was deoxygenated, consistent with cytoplasmic hypoxia, and the mitochondrial cytochromes, including COX, exhibited a high reduction state, consistent with OxPhos hypoxia. PFC perfusate increased aortic output from 76 ± 6 to 142 ± 4 ml/min and increased oxygen consumption while also increasing myoglobin oxygenation and oxidizing the mitochondrial cytochromes. These results are consistent with limited delivery of oxygen to OxPhos resulting in an adapted lower cardiac performance with KH. Consistent with this, PFCs increased myocardial oxygenation, and cardiac work was higher over a wider range of perfusate Po₂. In summary, heart mitochondria are limited by oxygen delivery with KH; supplementation of KH with PFC reverses mitochondrial hypoxia and improves cardiac performance, creating a more physiological tissue oxygen delivery. NEW & NOTEWORTHY Optical absorbance spectroscopy of intrinsic chromophores reveals that the commonly used crystalloid-perfused working heart is oxygen limited for oxidative phosphorylation and associated cardiac work. Oxygen-carrying perfluorocarbons increase myocardial oxygen delivery and improve cardiac function, providing a more physiological mitochondrial redox state and emphasizing cardiac work is modulated by myocardial oxygen delivery.

Intracardiac light catheter for rapid scanning transmural absorbance spectroscopy of perfused myocardium: measurement of myoglobin oxygenation and mitochondria redox state

Absorbance spectroscopy of intrinsic cardiac chromophores provides nondestructive assessment of cytosolic oxygenation and mitochondria redox state. Isolated perfused heart spectroscopy is usually conducted by collecting reflected light from the heart surface, which represents a combination of surface scattering events and light that traversed portions of the myocardium. Reflectance spectroscopy with complex surface scattering effects in the beating heart leads to difficulty in quantitating chromophore absorbance. In this study, surface scattering was minimized and transmural path length optimized by placing a light source within the left ventricular chamber while monitoring transmurally transmitted light at the epicardial surface. The custom-designed intrachamber light catheter was a flexible coaxial cable (2.42-Fr) terminated with an encapsulated side-firing LED of 1.8 × 0.8 mm, altogether similar in size to a Millar pressure catheter. The LED catheter had minimal impact on aortic flow and heart rate in Langendorff perfusion and did not impact stability of the left ventricule of the working heart. Changes in transmural absorbance spectra were deconvoluted using a library of chromophore reference spectra to quantify the relative contribution of specific chromophores to the changes in measured absorbance. This broad-band spectral deconvolution approach eliminated errors that may result from simple dual-wavelength absorbance intensity. The myoglobin oxygenation level was only 82.2 ± 3.0%, whereas cytochrome c and cytochrome a + a₃ were 13.3 ± 1.4% and 12.6 ± 2.2% reduced, respectively, in the Langendorff-perfused heart. The intracardiac illumination strategy permits transmural optical absorbance spectroscopy in perfused hearts, which provides a noninvasive real-time monitor of cytosolic oxygenation and mitochondria redox state.NEW & NOTEWORTHY Here, a novel nondestructive real-time approach for monitoring intrinsic indicators of cardiac metabolism and oxygenation is described using a catheter-based transillumination of the left ventricular free wall together with complete spectral analysis of transmitted light. This approach is a significant improvement in the quality of cardiac optical absorbance spectroscopic metabolic analyses.

Robust fluorescent labelling of micropipettes for use in fluorescence microscopy: application to the observation of a mosquito borne parasite infection

The ability to monitor micropipette injections with a high-resolution fluorescent microscope has utility for a variety of applications. Herein, different approaches were tested for creating broad-band fluorescently labelled glass micropipettes including: UV cured glass glues, baked glass enamel containing fluorescent dyes as well as nanodiamonds attached during pipette formation in the microforge. The most robust and simplest approach was to use labelled baked enamel on the exterior of the pipette. This approach was tested using pipettes designed to mimic a mosquito proboscis for the injection of the malaria parasite, Plasmodium spp., into the dermis of a living mouse ear. The pipette (∼30 micron diameter) was easily detected in the microscopy field of view and tolerated multiple insertions through the skin. This simple inexpensive approach to fluorescently labelling micropipettes will aid in the development of procedures under the fluorescent microscope.

Assessment of Myocardial Microstructural Dynamics by In Vivo Diffusion Tensor Cardiac Magnetic Resonance

BACKGROUND

Cardiomyocytes are organized in microstructures termed sheetlets that reorientate during left ventricular thickening. Diffusion tensor cardiac magnetic resonance (DT-CMR) may enable noninvasive interrogation of in vivo cardiac microstructural dynamics. Dilated cardiomyopathy (DCM) is a condition of abnormal myocardium with unknown sheetlet function.

OBJECTIVES

This study sought to validate in vivo DT-CMR measures of cardiac microstructure against histology, characterize microstructural dynamics during left ventricular wall thickening, and apply the technique in hypertrophic cardiomyopathy (HCM) and DCM.

METHODS

In vivo DT-CMR was acquired throughout the cardiac cycle in healthy swine, followed by in situ and ex vivo DT-CMR, then validated against histology. In vivo DT-CMR was performed in 19 control subjects, 19 DCM, and 13 HCM patients.

RESULTS

In swine, a DT-CMR index of sheetlet reorientation (E2A) changed substantially (E2A mobility ~46°). E2A changes correlated with wall thickness changes (in vivo r² = 0.75; in situ r² = 0.89), were consistently observed under all experimental conditions, and accorded closely with histological analyses in both relaxed and contracted states. The potential contribution of cyclical strain effects to in vivo E2A was ~17%. In healthy human control subjects, E2A increased from diastole (18°) to systole (65°; p < 0.001; E2A mobility = 45°). HCM patients showed significantly greater E2A in diastole than control subjects did (48 ; p < 0.001) with impaired E2A mobility (23°; p < 0.001). In DCM, E2A was similar to control subjects in diastole, but systolic values were markedly lower (40° ; p < 0.001) with impaired E2A mobility (20°; p < 0.001).

CONCLUSIONS

Myocardial microstructure dynamics can be characterized by in vivo DT-CMR. Sheetlet function was abnormal in DCM with altered systolic conformation and reduced mobility, contrasting with HCM, which showed reduced mobility with altered diastolic conformation. These novel insights significantly improve understanding of contractile dysfunction at a level of noninvasive interrogation not previously available in humans. (J Am Coll Cardiol 2017;69:661–76) Published by Elsevier on behalf of the American College of Cardiology Foundation.

Power Grid Protection of the Muscle Mitochondrial Reticulum

Mitochondrial network connectivity enables rapid communication and distribution of potential energy throughout the cell. However, this connectivity puts the energy conversion system at risk, because damaged elements could jeopardize the entire network. Here, we demonstrate the mechanisms for mitochondrial network protection in heart and skeletal muscle (SKM). We find that the cardiac mitochondrial reticulum is segmented into subnetworks comprising many mitochondria linked through abundant contact sites at highly specific intermitochondrial junctions (IMJs). In both cardiac and SKM subnetworks, a rapid electrical and physical separation of malfunctioning mitochondria occurs, consistent with detachment of IMJs and retraction of elongated mitochondria into condensed structures. Regional mitochondrial subnetworks limit the cellular impact of local dysfunction while the dynamic disconnection of damaged mitochondria allows the remaining mitochondria to resume normal function within seconds. Thus, mitochondrial network security is comprised of both proactive and reactive mechanisms in striated muscle cells.

Mitochondrial Function, Biology, and Role in Disease: A Scientific Statement From the American Heart Association

Cardiovascular disease is a major leading cause of morbidity and mortality in the United States and elsewhere. Alterations in mitochondrial function are increasingly being recognized as a contributing factor in myocardial infarction and in patients presenting with cardiomyopathy. Recent understanding of the complex interaction of the mitochondria in regulating metabolism and cell death can provide novel insight and therapeutic targets. The purpose of this statement is to better define the potential role of mitochondria in the genesis of cardiovascular disease such as ischemia and heart failure. To accomplish this, we will define the key mitochondrial processes that play a role in cardiovascular disease that are potential targets for novel therapeutic interventions. This is an exciting time in mitochondrial research. The past decade has provided novel insight into the role of mitochondria function and their importance in complex diseases. This statement will define the key roles that mitochondria play in cardiovascular physiology and disease and provide insight into how mitochondrial defects can contribute to cardiovascular disease; it will also discuss potential biomarkers of mitochondrial disease and suggest potential novel therapeutic approaches.

Improving the physiological realism of experimental models

The Virtual Physiological Human (VPH) project aims to develop integrative, explanatory and predictive computational models (C-Models) as numerical investigational tools to study disease, identify and design effective therapies and provide an in silico platform for drug screening. Ultimately, these models rely on the analysis and integration of experimental data. As such, the success of VPH depends on the availability of physiologically realistic experimental models (E-Models) of human organ function that can be parametrized to test the numerical models. Here, the current state of suitable E-models, ranging from in vitro non-human cell organelles to in vivo human organ systems, is discussed. Specifically, challenges and recent progress in improving the physiological realism of E-models that may benefit the VPH project are highlighted and discussed using examples from the field of research on cardiovascular disease, musculoskeletal disorders, diabetes and Parkinson's disease.

The electrochemical transmission in I-Band segments of the mitochondrial reticulum

Within the mitochondrial reticulum of skeletal muscle, the I-Band segments (IBS) traverse the cell and form a contiguous matrix with the mitochondrial segments at the periphery (PS) of the cell. A tight electrical coupling via the matrix between the PS and IBS has been demonstrated. In addition, oxidative phosphorylation complexes that generate the proton motive force (PMF) are preferentially located in the PS, while Complex V, which utilizes the PMF, is primarily located along the IBS. This has led to the hypothesis that PS can support the production of ATP in the IBS by maintaining the potential energy available to produce ATP deep in the muscle cell via conduction of the PMF down the IBS. However, the mechanism of transmitting the PMF down the IBS is poorly understood. This theoretical study was undertaken to establish the physical limits governing IBS conduction as well as potential mechanisms for balancing the protons entering the matrix along the IBS with the ejection of protons in the PS. The IBS was modeled as a 300 nm diameter, water-filled tube, with an insulated circumferential wall. Two mechanisms were considered to drive ion transport along the IBS: the electrical potential and/or concentration gradients between the PS to the end of the IBS. The magnitude of the flux was estimated from the maximum ATP production rate for skeletal muscle. The major transport ions in consideration were H(+), Na(+), and K(+) using diffusion coefficients from the literature. The simulations were run using COMSOL Multiphysics simulator. These simulations suggest conduction along the IBS via H(+) alone is unlikely requiring un-physiological gradients, while Na(+) or K(+) could carry the current with minor gradients in concentration or electrical potential along the IBS. The majority of conduction down the IBS is likely dependent on these abundant ions; however, this presents a question as to how H(+) is recycled from the matrix of the IBS to the PS for active extrusion. We propose that the abundant cation-proton antiporter in skeletal muscle mitochondria operates in opposite directions in the IBS and PS to permit local recycling of H(+) at each site driven by cooperative gradients in H(+) and Na(+)/K(+) which favor H(+) entry in the PS and H(+) efflux in the IBS. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016,' edited by Prof. Paolo Bernardi.

In vivo microscopy reveals extensive embedding of capillaries within the sarcolemma of skeletal muscle fibers

OBJECTIVE

To provide insight into mitochondrial function in vivo, we evaluated the 3D spatial relationship between capillaries, mitochondria, and muscle fibers in live mice.

METHODS

3D volumes of in vivo murine TA muscles were imaged by MPM. Muscle fiber type, mitochondrial distribution, number of capillaries, and capillary-to-fiber contact were assessed. The role of Mb-facilitated diffusion was examined in Mb KO mice. Distribution of GLUT4 was also evaluated in the context of the capillary and mitochondrial network.

RESULTS

MPM revealed that 43.6 ± 3.3% of oxidative fiber capillaries had ≥50% of their circumference embedded in a groove in the sarcolemma, in vivo. Embedded capillaries were tightly associated with dense mitochondrial populations lateral to capillary grooves and nearly absent below the groove. Mitochondrial distribution, number of embedded capillaries, and capillary-to-fiber contact were proportional to fiber oxidative capacity and unaffected by Mb KO. GLUT4 did not preferentially localize to embedded capillaries.

CONCLUSIONS

Embedding capillaries in the sarcolemma may provide a regulatory mechanism to optimize delivery of oxygen to heterogeneous groups of muscle fibers. We hypothesize that mitochondria locate to PV regions due to myofibril voids created by embedded capillaries, not to enhance the delivery of oxygen to the mitochondria.

Parallel transmit excitation at 1.5 T based on the minimization of a driving function for device heating

PURPOSE

To provide a rapid method to reduce the radiofrequency (RF) E-field coupling and consequent heating in long conductors in an interventional MRI (iMRI) setup.

METHODS

A driving function for device heating (W) was defined as the integration of the E-field along the direction of the wire and calculated through a quasistatic approximation. Based on this function, the phases of four independently controlled transmit channels were dynamically changed in a 1.5 T MRI scanner. During the different excitation configurations, the RF induced heating in a nitinol wire immersed in a saline phantom was measured by fiber-optic temperature sensing. Additionally, a minimization of W as a function of phase and amplitude values of the different channels and constrained by the homogeneity of the RF excitation field (B1) over a region of interest was proposed and its results tested on the benchtop. To analyze the validity of the proposed method, using a model of the array and phantom setup tested in the scanner, RF fields and SAR maps were calculated through finite-difference time-domain (FDTD) simulations. In addition to phantom experiments, RF induced heating of an active guidewire inserted in a swine was also evaluated.

RESULTS

In the phantom experiment, heating at the tip of the device was reduced by 92% when replacing the body coil by an optimized parallel transmit excitation with same nominal flip angle. In the benchtop, up to 90% heating reduction was measured when implementing the constrained minimization algorithm with the additional degree of freedom given by independent amplitude control. The computation of the optimum phase and amplitude values was executed in just 12 s using a standard CPU. The results of the FDTD simulations showed similar trend of the local SAR at the tip of the wire and measured temperature as well as to a quadratic function of W, confirming the validity of the quasistatic approach for the presented problem at 64 MHz. Imaging and heating reduction of the guidewire were successfully performed in vivo with the proposed hardware and phase control.

CONCLUSIONS

Phantom and in vivo data demonstrated that additional degrees of freedom in a parallel transmission system can be used to control RF induced heating in long conductors. A novel constrained optimization approach to reduce device heating was also presented that can be run in just few seconds and therefore could be added to an iMRI protocol to improve RF safety.

Mitochondrial reticulum for cellular energy distribution in muscle

Intracellular energy distribution has attracted much interest and has been proposed to occur in skeletal muscle via metabolite-facilitated diffusion; however, genetic evidence suggests that facilitated diffusion is not critical for normal function. We hypothesized that mitochondrial structure minimizes metabolite diffusion distances in skeletal muscle. Here we demonstrate a mitochondrial reticulum providing a conductive pathway for energy distribution, in the form of the proton-motive force, throughout the mouse skeletal muscle cell. Within this reticulum, we find proteins associated with mitochondrial proton-motive force production preferentially in the cell periphery and proteins that use the proton-motive force for ATP production in the cell interior near contractile and transport ATPases. Furthermore, we show a rapid, coordinated depolarization of the membrane potential component of the proton-motive force throughout the cell in response to spatially controlled uncoupling of the cell interior. We propose that membrane potential conduction via the mitochondrial reticulum is the dominant pathway for skeletal muscle energy distribution.

A Model-based approach for microvasculature structure distortion correction in two-photon fluorescence microscopy images

This paper investigates a postprocessing approach to correct spatial distortion in two-photon fluorescence microscopy images for vascular network reconstruction. It is aimed at in vivo imaging of large field-of-view, deep-tissue studies of vascular structures. Based on simple geometric modelling of the object-of-interest, a distortion function is directly estimated from the image volume by deconvolution analysis. Such distortion function is then applied to subvolumes of the image stack to adaptively adjust for spatially varying distortion and reduce the image blurring through blind deconvolution. The proposed technique was first evaluated in phantom imaging of fluorescent microspheres that are comparable in size to the underlying capillary vascular structures. The effectiveness of restoring three-dimensional (3D) spherical geometry of the microspheres using the estimated distortion function was compared with empirically measured point-spread function. Next, the proposed approach was applied to in vivo vascular imaging of mouse skeletal muscle to reduce the image distortion of the capillary structures. We show that the proposed method effectively improve the image quality and reduce spatially varying distortion that occurs in large field-of-view deep-tissue vascular dataset. The proposed method will help in qualitative interpretation and quantitative analysis of vascular structures from fluorescence microscopy images.

Stimulation of oxidative phosphorylation by calcium in cardiac mitochondria is not influenced by cAMP and PKA activity

Cardiac oxidative ATP generation is finely tuned to match several-fold increases in energy demand. Calcium has been proposed to play a role in the activation of ATP production via PKA phosphorylation in response to intramitochondrial cAMP generation. We evaluated the effect of cAMP, its membrane permeable analogs (dibutyryl-cAMP, 8-bromo-cAMP), and the PKA inhibitor H89 on respiration of isolated pig heart mitochondria. cAMP analogs did not stimulate State 3 respiration of Ca2 +-depleted mitochondria (82.2 ± 3.6% of control), in contrast to the 2-fold activation induced by 0.95 μM free Ca2 +, which was unaffected by H89. Using fluorescence and integrating sphere spectroscopy, we determined that Ca2 + increased the reduction of NADH (8%), and of cytochromes bH (3%), c1 (3%), c (4%), and a (2%), together with a doubling of conductances for Complex I + III and Complex IV. None of these changes were induced by cAMP analogs nor abolished by H89. In Ca2 +-undepleted mitochondria, we observed only slight changes in State 3 respiration rates upon addition of 50 μM cAMP (85 ± 9.9%), dibutyryl-cAMP (80.1 ± 5.2%), 8-bromo-cAMP (88.6 ± 3.3%), or 1 μM H89 (89.7 ± 19.9%) with respect to controls. Similar results were obtained when measuring respiration in heart homogenates. Addition of exogenous PKA with dibutyryl-cAMP or the constitutively active catalytic subunit of PKA to isolated mitochondria decreased State 3 respiration by only 5–15%. These functional studies suggest that alterations in mitochondrial cAMP and PKA activity do not contribute significantly to the acute Ca2 + stimulation of oxidative phosphorylation

Study of the development of the mouse thoracic aorta three-dimensional macromolecular structure using two-photon microscopy

Using the intrinsic optical properties of collagen and elastin, two-photon microscopy was applied to evaluate the three-dimensional (3D) macromolecular structural development of the mouse thoracic aorta from birth to 60 days old. Baseline development was established in the Scavenger Receptor Class B Type I-Deficient, Hypomorphic Apolipoprotein ER61 (SR-BI KO/ApoeR61(h/h)) mouse in preparation for modeling atherosclerosis. Precise dissection enabled direct observation of the artery wall in situ. En-face, optical sectioning of the aorta provided a novel assessment of the macromolecular structural development. During aortic development, the undulating lamellar elastin layers compressed consistent with the increases in mean aortic pressure with age. In parallel, a net increase in overall wall thickness (p<0.05, in day 60 compared with day 1 mice) occurred with age whereas the ratio of the tunicas adventitia and media to full aortic thickness remained nearly constant across age groups (~1:2.6, respectively). Histochemical analyses by brightfield microscopy and ultrastructure validated structural proteins and lipid deposition findings derived from two-photon microscopy. Development was associated with decreased decorin but not biglycan proteoglycan expression. This non-destructive 3D in situ approach revealed the aortic wall microstructure development. Coupling this approach with the intrinsic optical properties of the macromolecules may provide unique vascular wall 3D structure in many pathological conditions, including aortic atherosclerosis, dissections and aneurysms.

Motion compensation for in vivo subcellular optical microscopy

In this review, we focus on the impact of tissue motion on attempting to conduct subcellular resolution optical microscopy, in vivo. Our position is that tissue motion is one of the major barriers in conducting these studies along with light induced damage, optical probe loading as well as absorbing and scattering effects on the excitation point spread function and collection of emitted light. Recent developments in the speed of image acquisition have reached the limit, in most cases, where the signal from a subcellular voxel limits the speed and not the scanning rate of the microscope. Different schemes for compensating for tissue displacements due to rigid body and deformation are presented from tissue restriction, gating, adaptive gating and active tissue tracking. We argue that methods that minimally impact the natural physiological motion of the tissue are desirable because the major reason to perform in vivo studies is to evaluate normal physiological functions. Towards this goal, active tracking using the optical imaging data itself to monitor tissue displacement and either prospectively or retrospectively correct for the motion without affecting physiological processes is desirable. Critical for this development was the implementation of near real time image processing in conjunction with the control of the microscope imaging parameters. Clearly, the continuing development of methods of motion compensation as well as significant technological solutions to the other barriers to tissue subcellular optical imaging in vivo, including optical aberrations and overall signal-to-noise ratio, will make major contributions to the understanding of cell biology within the body.

Use of independent component analysis to improve signal-to-noise ratio in multi-probe fluorescence microscopy

In conventional multi-probe fluorescence microscopy, narrow bandwidth filters on detectors are used to avoid bleed-through artefacts between probes. The limited bandwidth reduces the signal-to-noise ratio of the detection, often severely compromising one or more channels. Herein, we describe a process of using independent component analysis to discriminate the position of different probes using only a dichroic mirror to differentiate the signals directed to the detectors. Independent component analysis was particularly effective in samples where the spatial overlap between the probes is minimal, a very common case in cellular microscopy. This imaging scheme collects nearly all of the emitted light, significantly improving the image signal-to-noise ratio. In this study, we focused on the detection of two fluorescence probes used in vivo, NAD(P)H and ANEPPS. The optimal dichroic mirror cutoff frequency was determined with simulations using the probes spectral emissions. A quality factor, defined as the cross-channel contrast-to-noise ratio, was optimized to maximize signals while maintaining spatial discrimination between the probes after independent component analysis post-processing. Simulations indicate that a ∼3 fold increase in signal-to-noise ratio using the independent component analysis approach can be achieved over the conventional narrow-band filtering approach without loss of spatial discrimination. We confirmed this predicted performance from experimental imaging of NAD(P)H and ANEPPS in mouse skeletal muscle, in vivo. For many multi-probe studies, the increased sensitivity of this 'full bandwidth' approach will lead to improved image quality and/or reduced excitation power requirements.

Influence of the renal artery ostium flow diverter on hemodynamics and atherogenesis

The structure and function of the renal artery ostium flow diverter on the caudal side of the renal branch point were previously reported; in this study, we further evaluate the diverter׳s possible functions. The protrusion of this structure into the abdominal aorta suggests that the diverter may preferentially direct blood flow to the renal arteries, and that it may also influence flow patterns and recirculation known to be involved in atherogenesis. Three-dimensional computational fluid dynamics (CFD) simulations of steady and pulsatile blood flow are performed to investigate the influence of diverter size and position, and vascular geometry, on the flow patterns and fluid mechanical forces in the neighborhood of the diverter. CFD results show that the flow diverter does affect the blood distribution; depending on the diverter׳s position, the flow to the renal arteries may be increased or reduced. Calculated results also demonstrate the diverter׳s effect on the wall shear stress (WSS) distribution, and suggest that the diverter contributes to an atherogenic environment in the abdominal aorta, while being atheroprotective in the renal arteries themselves. These results support previous clinical findings, and suggest directions for further clinical study. The results of this work have direct implications in understanding the physiological significance of the diverter, and its potential role in the pathophysiological development of atherosclerosis.

Decorin and biglycan retain LDL in disease-prone valvular and aortic subendothelial intimal matrix

OBJECTIVE

Subendothelial LDL retention by intimal matrix proteoglycans is an initial step in atherosclerosis and calcific aortic valve disease. Herein, we identify decorin and biglycan as the proteoglycans that preferentially retain LDL in intimal matrix at disease-prone sites in normal valve and vessel wall.

METHODS

The porcine aortic valve and renal artery ostial diverter, initiation sites of calcific valve disease and renal atherosclerosis, respectively, from normal non-diseased animals were used as models in these studies.

RESULTS

Fluorescent human LDL was selectively retained on the lesion-prone collagen/proteoglycan-enriched aortic surface of the valve, where the elastic lamina is depleted, as previously observed in lesion-prone sites in the renal ostium. iTRAQ mass spectrometry of valve and diverter protein extracts identified decorin and biglycan as the major subendothelial intimal matrix proteoglycans electrostatically retained on human LDL affinity columns. Decorin levels correlated with LDL binding in lesion-prone sites in both tissues. Collagen binding to LDL was shown to be proteoglycan-mediated. All known basement membrane proteoglycans bound LDL suggesting they may modulate LDL uptake into the subendothelial matrix. The association of purified decorin with human LDL in an in vitro microassay was blocked by serum albumin and heparin suggesting anti-atherogenic roles for these proteins in vivo.

CONCLUSIONS

LDL electrostatic interactions with decorin and biglycan in the valve leaflets and vascular wall is a major source of LDL retention. The complementary electrostatic sites on LDL or these proteoglycans may provide a novel therapeutic target for preventing one of the earliest events in these cardiovascular diseases.

Compact non-contact total emission detection for in vivo multiphoton excitation microscopy

We describe a compact, non-contact design for a total emission detection (c-TED) system for intra-vital multiphoton imaging. To conform to a standard upright two-photon microscope design, this system uses a parabolic mirror surrounding a standard microscope objective in concert with an optical path that does not interfere with normal microscope operation. The non-contact design of this device allows for maximal light collection without disrupting the physiology of the specimen being examined. Tests were conducted on exposed tissues in live animals to examine the emission collection enhancement of the c-TED device compared to heavily optimized objective-based emission collection. The best light collection enhancement was seen from murine fat (5×-2× gains as a function of depth), whereas murine skeletal muscle and rat kidney showed gains of over two and just under twofold near the surface, respectively. Gains decreased with imaging depth (particularly in the kidney). Zebrafish imaging on a reflective substrate showed close to a twofold gain throughout the entire volume of an intact embryo (approximately 150 μm deep). Direct measurement of bleaching rates confirmed that the lower laser powers, enabled by greater light collection efficiency, yielded reduced photobleaching in vivo. The potential benefits of increased light collection in terms of speed of imaging and reduced photo-damage, as well as the applicability of this device to other multiphoton imaging methods is discussed.

Optical spectroscopy in turbid media using an integrating sphere: mitochondrial chromophore analysis during metabolic transitions

Recent evidence suggests that the activity of mitochondrial oxidative phosphorylation complexes (MOPCs) is modulated at multiple sites. Here, a method of optically monitoring electron distribution within and between MOPCs is described using a center-mounted sample in an integrating sphere (to minimize scattering effects) with a rapid-scanning spectrometer. The redox-sensitive MOPC absorbances (∼465-630 nm) were modeled using linear least squares analysis with individual chromophore spectra. Classical mitochondrial activity transitions (e.g., ADP-induced increase in oxygen consumption) were used to characterize this approach. Most notable in these studies was the observation that intermediates of the catalytic cycle of cytochrome oxidase are dynamically modulated with metabolic state. The MOPC redox state, along with measurements of oxygen consumption and mitochondrial membrane potential, was used to evaluate the conductances of different sections of the electron transport chain. This analysis then was applied to mitochondria isolated from rabbit hearts subjected to ischemia/reperfusion (I/R). Surprisingly, I/R resulted in an inhibition of all measured MOPC conductances, suggesting a coordinated down-regulation of mitochondrial activity with this well-established cardiac perturbation.

Effect of calcium on the oxidative phosphorylation cascade in skeletal muscle mitochondria

Calcium is believed to regulate mitochondrial oxidative phosphorylation, thereby contributing to the maintenance of cellular energy homeostasis. Skeletal muscle, with an energy conversion dynamic range of up to 100-fold, is an extreme case for evaluating the cellular balance of ATP production and consumption. This study examined the role of Ca(2+) in the entire oxidative phosphorylation reaction network in isolated skeletal muscle mitochondria and attempted to extrapolate these results back to the muscle, in vivo. Kinetic analysis was conducted to evaluate the dose-response effect of Ca(2+) on the maximal velocity of oxidative phosphorylation (V(maxO)) and the ADP affinity. Force-flow analysis evaluated the interplay between energetic driving forces and flux to determine the conductance, or effective activity, of individual steps within oxidative phosphorylation. Measured driving forces [extramitochondrial phosphorylation potential (ΔG(ATP)), membrane potential, and redox states of NADH and cytochromes b(H), b(L), c(1), c, and a,a(3)] were compared with flux (oxygen consumption) at 37 °C; 840 nM Ca(2+) generated an ~2-fold increase in V(maxO) with no change in ADP affinity (~43 μM). Force-flow analysis revealed that Ca(2+) activation of V(maxO) was distributed throughout the oxidative phosphorylation reaction sequence. Specifically, Ca(2+) increased the conductance of Complex IV (2.3-fold), Complexes I and III (2.2-fold), ATP production/transport (2.4-fold), and fuel transport/dehydrogenases (1.7-fold). These data support the notion that Ca(2+) activates the entire muscle oxidative phosphorylation cascade, while extrapolation of these data to the exercising muscle predicts a significant role of Ca(2+) in maintaining cellular energy homeostasis.

Increased oxidative metabolism in the Li-Fraumeni syndrome

There is growing evidence that alterations in metabolism may contribute to tumorigenesis. Here, we report on members of families with the Li-Fraumeni syndrome who carry germline mutations in TP53, the gene encoding the tumor-suppressor protein p53. As compared with family members who are not carriers and with healthy volunteers, family members with these mutations have increased oxidative phosphorylation of skeletal muscle. Basic experimental studies of tissue samples from patients with the Li-Fraumeni syndrome and a mouse model of the syndrome support this in vivo finding of increased mitochondrial function. These results suggest that p53 regulates bioenergetic homeostasis in humans. (Funded by the National Heart, Lung, and Blood Institute and the National Institutes of Health; ClinicalTrials.gov number, NCT00406445.).

Living without creatine: unchanged exercise capacity and response to chronic myocardial infarction in creatine-deficient mice

RATIONALE

Creatine is thought to be involved in the spatial and temporal buffering of ATP in energetic organs such as heart and skeletal muscle. Creatine depletion affects force generation during maximal stimulation, while reduced levels of myocardial creatine are a hallmark of the failing heart, leading to the widely held view that creatine is important at high workloads and under conditions of pathological stress.

OBJECTIVE

We therefore hypothesised that the consequences of creatine-deficiency in mice would be impaired running capacity, and exacerbation of heart failure following myocardial infarction.

METHODS AND RESULTS

Surprisingly, mice with whole-body creatine deficiency due to knockout of the biosynthetic enzyme (guanidinoacetate N-methyltransferase [GAMT]) voluntarily ran just as fast and as far as controls (>10 km/night) and performed the same level of work when tested to exhaustion on a treadmill. Furthermore, survival following myocardial infarction was not altered, nor was subsequent left ventricular (LV) remodelling and development of chronic heart failure exacerbated, as measured by 3D-echocardiography and invasive hemodynamics. These findings could not be accounted for by compensatory adaptations, with no differences detected between WT and GAMT(-/-) proteomes. Alternative phosphotransfer mechanisms were explored; adenylate kinase activity was unaltered, and although GAMT(-/-) hearts accumulated the creatine precursor guanidinoacetate, this had negligible energy-transfer activity, while mitochondria retained near normal function.

CONCLUSIONS

Creatine-deficient mice show unaltered maximal exercise capacity and response to chronic myocardial infarction, and no obvious metabolic adaptations. Our results question the paradigm that creatine is essential for high workload and chronic stress responses in heart and skeletal muscle.

Continuous monitoring of enzymatic activity within native electrophoresis gels: application to mitochondrial oxidative phosphorylation complexes

Native gel electrophoresis allows the separation of very small amounts of protein complexes while retaining aspects of their activity. In-gel enzymatic assays are usually performed by using reaction-dependent deposition of chromophores or light-scattering precipitates quantified at fixed time points after gel removal and fixation, limiting the ability to analyze the enzyme reaction kinetics. Herein, we describe a custom reaction chamber with reaction medium recirculation and filtering and an imaging system that permits the continuous monitoring of in-gel enzymatic activity even in the presence of turbidity. Images were continuously collected using time-lapse high-resolution digital imaging, and processing routines were developed to obtain kinetic traces of the in-gel activities and analyze reaction time courses. This system also permitted the evaluation of enzymatic activity topology within the protein bands of the gel. This approach was used to analyze the reaction kinetics of two mitochondrial complexes in native gels. Complex IV kinetics showed a short initial linear phase in which catalytic rates could be calculated, whereas Complex V activity revealed a significant lag phase followed by two linear phases. The utility of monitoring the entire kinetic behavior of these reactions in native gels, as well as the general application of this approach, is discussed.

Three-dimensional motion tracking for high-resolution optical microscopy, in vivo

When conducting optical imaging experiments, in vivo, the signal to noise ratio and effective spatial and temporal resolution is fundamentally limited by physiological motion of the tissue. A three-dimensional (3D) motion tracking scheme, using a multiphoton excitation microscope with a resonant galvanometer, (512 × 512 pixels at 33 frames s(-1)) is described to overcome physiological motion, in vivo. The use of commercially available graphical processing units permitted the rapid 3D cross-correlation of sequential volumes to detect displacements and adjust tissue position to track motions in near real-time. Motion phantom tests maintained micron resolution with displacement velocities of up to 200 μm min(-1), well within the drift observed in many biological tissues under physiologically relevant conditions. In vivo experiments on mouse skeletal muscle using the capillary vasculature with luminal dye as a displacement reference revealed an effective and robust method of tracking tissue motion to enable (1) signal averaging over time without compromising resolution, and (2) tracking of cellular regions during a physiological perturbation.