Assessing Myocardial Microstructure With Biophysical Models of Diffusion MRI.

Biophysical models are a promising means for interpreting diffusion weighted magnetic resonance imaging (DW-MRI) data, as they can provide estimates of physiologically relevant parameters of microstructure including cell size, volume fraction, or dispersion. However, their application in cardiac microstructure mapping (CMM) has been limited. This study proposes seven new two-compartment models with combination of restricted cylinder models and a diffusion tensor to represent intra- and extracellular spaces, respectively. Three extended versions of the cylinder model are studied here: cylinder with elliptical cross section (ECS), cylinder with Gamma distributed radii (GDR), and cylinder with Bingham distributed axes (BDA). The proposed models were applied to data in two fixed mouse hearts, acquired with multiple diffusion times, q-shells and diffusion encoding directions. The cylinderGDR-pancake model provided the best performance in terms of root mean squared error (RMSE) reducing it by 25% compared to diffusion tensor imaging (DTI). The cylinderBDA-pancake model represented anatomical findings closest as it also allows for modelling dispersion. High-resolution 3D synchrotron X-ray imaging (SRI) data from the same specimen was utilized to evaluate the biophysical models. A novel tensor-based registration method is proposed to align SRI structure tensors to the MR diffusion tensors. The consistency between SRI and DW-MRI parameters demonstrates the potential of compartment models in assessing physiologically relevant parameters.

Subtle Role for Adenylate Kinase 1 in Maintaining Normal Basal Contractile Function and Metabolism in the Murine Heart.

AIMS: Adenylate kinase 1 (AK1) catalyses the reaction 2ADP ↔ ATP + AMP, extracting extra energy under metabolic stress and promoting energetic homeostasis. We hypothesised that increased AK1 activity would have negligible effects at rest, but protect against ischaemia/reperfusion (I/R) injury. METHODS AND RESULTS: Cardiac-specific AK1 overexpressing mice (AK1-OE) had 31% higher AK1 activity (P = 0.009), with unchanged total creatine kinase and citrate synthase activities. Male AK1-OE exhibited mild in vivo dysfunction at baseline with lower LV pressure, impaired relaxation, and contractile reserve. LV weight was 19% higher in AK1-OE males due to higher tissue water content in the absence of hypertrophy or fibrosis. AK1-OE hearts had significantly raised creatine, unaltered total adenine nucleotides, and 20% higher AMP levels (P = 0.05), but AMP-activated protein kinase was not activated (P = 0.85). 1H-NMR revealed significant differences in LV metabolite levels compared to wild-type, with aspartate, tyrosine, sphingomyelin, cholesterol all elevated, whereas taurine and triglycerides were significantly lower. Ex vivo global no-flow I/R, caused four-of-seven AK1-OE hearts to develop terminal arrhythmia (cf. zero WT), yet surviving AK1-OE hearts had improved functional recovery. However, AK1-OE did not influence infarct size in vivo and arrhythmias were only observed ex vivo, probably as an artefact of adenine nucleotide loss during cannulation. CONCLUSION: Modest elevation of AK1 may improve functional recovery following I/R, but has unexpected impact on LV weight, function and metabolite levels under basal resting conditions, suggesting a more nuanced role for AK1 underpinning myocardial energy homeostasis and not just as a response to stress.

Adaptation to HIF1α Deletion in Hypoxic Cancer Cells by Upregulation of GLUT14 and Creatine Metabolism.

Hypoxia-inducible factor 1α is a key regulator of the hypoxia response in normal and cancer tissues. It is well recognized to regulate glycolysis and is a target for therapy. However, how tumor cells adapt to grow in the absence of HIF1α is poorly understood and an important concept to understand for developing targeted therapies is the flexibility of the metabolic response to hypoxia via alternative pathways. We analyzed pathways that allow cells to survive hypoxic stress in the absence of HIF1α, using the HCT116 colon cancer cell line with deleted HIF1α versus control. Spheroids were used to provide a 3D model of metabolic gradients. We conducted a metabolomic, transcriptomic, and proteomic analysis and integrated the results. These showed surprisingly that in three-dimensional growth, a key regulatory step of glycolysis is Aldolase A rather than phosphofructokinase. Furthermore, glucose uptake could be maintained in hypoxia through upregulation of GLUT14, not previously recognized in this role. Finally, there was a marked adaptation and change of phosphocreatine energy pathways, which made the cells susceptible to inhibition of creatine metabolism in hypoxic conditions. Overall, our studies show a complex adaptation to hypoxia that can bypass HIF1α, but it is targetable and it provides new insight into the key metabolic pathways involved in cancer growth. IMPLICATIONS: Under hypoxia and HIF1 blockade, cancer cells adapt their energy metabolism via upregulation of the GLUT14 glucose transporter and creatine metabolism providing new avenues for drug targeting.

Over-expression of mitochondrial creatine kinase in the murine heart improves functional recovery and protects against injury following ischaemia-reperfusion.

Aims: Mitochondrial creatine kinase (MtCK) couples ATP production via oxidative phosphorylation to phosphocreatine in the cytosol, which acts as a mobile energy store available for regeneration of ATP at times of high demand. We hypothesized that elevating MtCK would be beneficial in ischaemia-reperfusion (I/R) injury. Methods and results: Mice were created over-expressing the sarcomeric MtCK gene with αMHC promoter at the Rosa26 locus (MtCK-OE) and compared with wild-type (WT) littermates. MtCK activity was 27% higher than WT, with no change in other CK isoenzymes or creatine levels. Electron microscopy confirmed normal mitochondrial cell density and mitochondrial localization of transgenic protein. Respiration in isolated mitochondria was unaltered and metabolomic analysis by 1 H-NMR suggests that cellular metabolism was not grossly affected by transgene expression. There were no significant differences in cardiac structure or function under baseline conditions by cine-MRI or LV haemodynamics. In Langendorff-perfused hearts subjected to 20 min ischaemia and 30 min reperfusion, MtCK-OE exhibited less ischaemic contracture, and improved functional recovery (Rate pressure product 58% above WT; P < 0.001). These hearts had reduced myocardial infarct size, which was confirmed in vivo: 55 ± 4% in WT vs. 29 ± 4% in MtCK-OE; P < 0.0001). Isolated cardiomyocytes from MtCK-OE hearts exhibited delayed opening of the mitochondrial permeability transition pore (mPTP) compared to WT, which was confirmed by reduced mitochondrial swelling in response to calcium. There was no detectable change in the structural integrity of the mitochondrial membrane. Conclusions: Modest elevation of MtCK activity in the heart does not adversely affect cellular metabolism, mitochondrial or in vivo cardiac function, but modifies mPTP opening to protect against I/R injury and improve functional recovery. Our findings support MtCK as a prime therapeutic target in myocardial ischaemia.

Impaired cardiac contractile function in arginine:glycine amidinotransferase knockout mice devoid of creatine is rescued by homoarginine but not creatine.

AIMS: Creatine buffers cellular adenosine triphosphate (ATP) via the creatine kinase reaction. Creatine levels are reduced in heart failure, but their contribution to pathophysiology is unclear. Arginine:glycine amidinotransferase (AGAT) in the kidney catalyses both the first step in creatine biosynthesis as well as homoarginine (HA) synthesis. AGAT-/- mice fed a creatine-free diet have a whole body creatine-deficiency. We hypothesized that AGAT-/- mice would develop cardiac dysfunction and rescue by dietary creatine would imply causality. METHODS AND RESULTS: Withdrawal of dietary creatine in AGAT-/- mice provided an estimate of myocardial creatine efflux of ∼2.7%/day; however, in vivo cardiac function was maintained despite low levels of myocardial creatine. Using AGAT-/- mice naïve to dietary creatine we confirmed absence of phosphocreatine in the heart, but crucially, ATP levels were unchanged. Potential compensatory adaptations were absent, AMPK was not activated and respiration in isolated mitochondria was normal. AGAT-/- mice had rescuable changes in body water and organ weights suggesting a role for creatine as a compatible osmolyte. Creatine-naïve AGAT-/- mice had haemodynamic impairment with low LV systolic pressure and reduced inotropy, lusitropy, and contractile reserve. Creatine supplementation only corrected systolic pressure despite normalization of myocardial creatine. AGAT-/- mice had low plasma HA and supplementation completely rescued all other haemodynamic parameters. Contractile dysfunction in AGAT-/- was confirmed in Langendorff perfused hearts and in creatine-replete isolated cardiomyocytes, indicating that HA is necessary for normal cardiac function. CONCLUSIONS: Our findings argue against low myocardial creatine per se as a major contributor to cardiac dysfunction. Conversely, we show that HA deficiency can impair cardiac function, which may explain why low HA is an independent risk factor for multiple cardiovascular diseases.

Increasing creatine kinase activity protects against hypoxia / reoxygenation injury but not against anthracycline toxicity in vitro.

The creatine kinase (CK) phosphagen system is fundamental to cellular energy homeostasis. Cardiomyocytes express three CK isoforms, namely the mitochondrial sarcomeric CKMT2 and the cytoplasmic CKM and CKB. We hypothesized that augmenting CK in vitro would preserve cell viability and function and sought to determine efficacy of the various isoforms. The open reading frame of each isoform was cloned into pcDNA3.1, followed by transfection and stable selection in human embryonic kidney cells (HEK293). CKMT2- CKM- and CKB-HEK293 cells had increased protein and total CK activity compared to non-transfected cells. Overexpressing any of the three CK isoforms reduced cell death in response to 18h hypoxia at 1% O2 followed by 2h re-oxygenation as assayed using propidium iodide: by 33% in CKMT2, 47% in CKM and 58% in CKB compared to non-transfected cells (P<0.05). Loading cells with creatine did not modify cell survival. Transient expression of CK isoforms in HL-1 cardiac cells elevated isoenzyme activity, but only CKMT2 over-expression protected against hypoxia (0.1% for 24h) and reoxygenation demonstrating 25% less cell death compared to non-transfected control (P<0.01). The same cells were not protected from doxorubicin toxicity (250nM for 48h), in contrast to the positive control. These findings support increased CK activity as protection against ischaemia-reperfusion injury, in particular, protection via CKMT2 in a cardiac-relevant cell line, which merits further investigation in vivo.

Validation of diffusion tensor MRI measurements of cardiac microstructure with structure tensor synchrotron radiation imaging.

BACKGROUND: Diffusion tensor imaging (DTI) is widely used to assess tissue microstructure non-invasively. Cardiac DTI enables inference of cell and sheetlet orientations, which are altered under pathological conditions. However, DTI is affected by many factors, therefore robust validation is critical. Existing histological validation is intrinsically flawed, since it requires further tissue processing leading to sample distortion, is routinely limited in field-of-view and requires reconstruction of three-dimensional volumes from two-dimensional images. In contrast, synchrotron radiation imaging (SRI) data enables imaging of the heart in 3D without further preparation following DTI. The objective of the study was to validate DTI measurements based on structure tensor analysis of SRI data. METHODS: One isolated, fixed rat heart was imaged ex vivo with DTI and X-ray phase contrast SRI, and reconstructed at 100 µm and 3.6 µm isotropic resolution respectively. Structure tensors were determined from the SRI data and registered to the DTI data. RESULTS: Excellent agreement in helix angles (HA) and transverse angles (TA) was observed between the DTI and structure tensor synchrotron radiation imaging (STSRI) data, where HADTI-STSRI = -1.4° ± 23.2° and TADTI-STSRI = -1.4° ± 35.0° (mean ± 1.96 standard deviation across all voxels in the left ventricle). STSRI confirmed that the primary eigenvector of the diffusion tensor corresponds with the cardiomyocyte long-axis across the whole myocardium. CONCLUSIONS: We have used STSRI as a novel and high-resolution gold standard for the validation of DTI, allowing like-with-like comparison of three-dimensional tissue structures in the same intact heart free of distortion. This represents a critical step forward in independently verifying the structural basis and informing the interpretation of cardiac DTI data, thereby supporting the further development and adoption of DTI in structure-based electro-mechanical modelling and routine clinical applications.

Evaluation of non-Gaussian diffusion in cardiac MRI.

PURPOSE: The diffusion tensor model assumes Gaussian diffusion and is widely applied in cardiac diffusion MRI. However, diffusion in biological tissue deviates from a Gaussian profile as a result of hindrance and restriction from cell and tissue microstructure, and may be quantified better by non-Gaussian modeling. The aim of this study was to investigate non-Gaussian diffusion in healthy and hypertrophic hearts. METHODS: Thirteen rat hearts (five healthy, four sham, four hypertrophic) were imaged ex vivo. Diffusion-weighted images were acquired at b-values up to 10,000 s/mm2 . Models of diffusion were fit to the data and ranked based on the Akaike information criterion. RESULTS: The diffusion tensor was ranked best at b-values up to 2000 s/mm2 but reflected the signal poorly in the high b-value regime, in which the best model was a non-Gaussian "beta distribution" model. Although there was considerable overlap in apparent diffusivities between the healthy, sham, and hypertrophic hearts, diffusion kurtosis and skewness in the hypertrophic hearts were more than 20% higher in the sheetlet and sheetlet-normal directions. CONCLUSION: Non-Gaussian diffusion models have a higher sensitivity for the detection of hypertrophy compared with the Gaussian model. In particular, diffusion kurtosis may serve as a useful biomarker for characterization of disease and remodeling in the heart. Magn Reson Med 78:1174-1186, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Resolving Fine Cardiac Structures in Rats with High-Resolution Diffusion Tensor Imaging.

Cardiac architecture is fundamental to cardiac function and can be assessed non-invasively with diffusion tensor imaging (DTI). Here, we aimed to overcome technical challenges in ex vivo DTI in order to extract fine anatomical details and to provide novel insights in the 3D structure of the heart. An integrated set of methods was implemented in ex vivo rat hearts, including dynamic receiver gain adjustment, gradient system scaling calibration, prospective adjustment of diffusion gradients, and interleaving of diffusion-weighted and non-diffusion-weighted scans. Together, these methods enhanced SNR and spatial resolution, minimised orientation bias in diffusion-weighting, and reduced temperature variation, enabling detection of tissue structures such as cell alignment in atria, valves and vessels at an unprecedented level of detail. Improved confidence in eigenvector reproducibility enabled tracking of myolaminar structures as a basis for segmentation of functional groups of cardiomyocytes. Ex vivo DTI facilitates acquisition of high quality structural data that complements readily available in vivo cardiac functional and anatomical MRI. The improvements presented here will facilitate next generation virtual models integrating micro-structural and electro-mechanical properties of the heart.

Protective Effect of Creatine Elevation against Ischaemia Reperfusion Injury Is Retained in the Presence of Co-Morbidities and during Cardioplegia.

AIMS: Ischaemic heart disease is most prevalent in the ageing population and often exists with other comorbidities; however the majority of laboratory research uses young, healthy animal models. Several recent workshops and focus meetings have highlighted the importance of using clinically relevant models to help aid translation to realistic patient populations. We have previously shown that mice over-expressing the creatine transporter (CrT-OE) have elevated intracellular creatine levels and are protected against ischaemia-reperfusion injury. Here we test whether elevating intracellular creatine levels retains a cardioprotective effect in the presence of common comorbidities and whether it is additive to protection afforded by hypothermic cardioplegia. METHODS AND RESULTS: CrT-OE mice and wild-type controls were subjected to transverse aortic constriction for two weeks to induce compensated left ventricular hypertrophy (LVH). Hearts were retrogradely perfused in Langendorff mode for 15 minutes, followed by 20 minutes ischaemia and 30 minutes reperfusion. CrT-OE hearts exhibited significantly improved functional recovery (Rate pressure product) during reperfusion compared to WT littermates (76% of baseline vs. 59%, respectively, P = 0.02). Aged CrT-OE mouse hearts (78±5 weeks) also had enhanced recovery following 15 minutes ischaemia (104% of baseline vs. 67%, P = 0.0007). The cardioprotective effect of hypothermic high K+ cardioplegic arrest, as used during cardiac surgery and donor heart transplant, was further enhanced in prolonged ischaemia (90 minutes) in CrT-OE Langendorff perfused mouse hearts (76% of baseline vs. 55% of baseline as seen in WT hearts, P = 0.02). CONCLUSIONS: These observations in clinically relevant models further support the development of modulators of intracellular creatine content as a translatable strategy for cardiac protection against ischaemia-reperfusion injury.

Prospective acceleration of diffusion tensor imaging with compressed sensing using adaptive dictionaries.

PURPOSE: Diffusion MRI requires acquisition of multiple diffusion-weighted images, resulting in long scan times. Here, we investigate combining compressed sensing and a fast imaging sequence to dramatically reduce acquisition times in cardiac diffusion MRI. METHODS: Fully sampled and prospectively undersampled diffusion tensor imaging data were acquired in five rat hearts at acceleration factors of between two and six using a fast spin echo (FSE) sequence. Images were reconstructed using a compressed sensing framework, enforcing sparsity by means of decomposition by adaptive dictionaries. A tensor was fit to the reconstructed images and fiber tractography was performed. RESULTS: Acceleration factors of up to six were achieved, with a modest increase in root mean square error of mean apparent diffusion coefficient (ADC), fractional anisotropy (FA), and helix angle. At an acceleration factor of six, mean values of ADC and FA were within 2.5% and 5% of the ground truth, respectively. Marginal differences were observed in the fiber tracts. CONCLUSION: We developed a new k-space sampling strategy for acquiring prospectively undersampled diffusion-weighted data, and validated a novel compressed sensing reconstruction algorithm based on adaptive dictionaries. The k-space undersampling and FSE acquisition each reduced acquisition times by up to 6× and 8×, respectively, as compared to fully sampled spin echo imaging. Magn Reson Med 76:248-258, 2016. © 2015 Wiley Periodicals, Inc.

Augmentation of Creatine in the Heart.

Creatine is a principle component of the creatine kinase (CK) phosphagen system common to all vertebrates. It is found in excitable cells, such as cardiomyocytes, where it plays an important role in the buffering and transport of chemical energy to ensure that supply meets the dynamic demands of the heart. Multiple components of the CK system, including intracellular creatine levels, are reduced in heart failure, while ischaemia and hypoxia represent acute crises of energy provision. Elevation of myocardial creatine levels has therefore been suggested as potentially beneficial, however, achieving this goal is not trivial. This mini-review outlines the evidence in support of creatine elevation and critically examines the pharmacological approaches that are currently available. In particular, dietary creatine-supplementation does not sufficiently elevate creatine levels in the heart due to subsequent down-regulation of the plasma membrane creatine transporter (CrT). Attempts to increase passive diffusion and bypass the CrT, e.g. via creatine esters, have yet to be tested in the heart. However, studies in mice with genetic overexpression of the CrT demonstrate proof-of-principle that elevated creatine protects the heart from ischaemia-reperfusion injury. This suggests activation of the CrT as a major unmet pharmacological target. However, translation of this finding to the clinic will require a greater understanding of CrT regulation in health and disease and the development of small molecule activators.

Dipeptidyl peptidase-4 inhibitors and GLP-1 reduce myocardial infarct size in a glucose-dependent manner.

BACKGROUND: The dipeptidyl peptidase-4 (DPP-4) inhibitors Sitagliptin and Vildagliptin lower blood glucose by augmenting endogenous levels of glucagon-like peptide-1 (GLP-1), an incretin which also confers cardioprotection. As such, we hypothesized that treatment with DPP-4 inhibitors are also cardioprotective. METHODS: In ex vivo experiments: Male Sprague-Dawley rats were randomized to receive by oral gavage either Vildagliptin (20 mg/kg/day), Sitagliptin (100 mg/kg/day), or water for 2 weeks. Excised hearts were Langendorff-perfused with buffer containing either 5 mmol/L or 11 mmol/L glucose and subjected to 35 minutes ischaemia/120 minutes reperfusion. In in vivo experiments: Male young Wistar and Sprague-Dawley rats, middle aged Wistar and Goto-Kakizaki diabetic rats were randomized to receive by oral gavage either Sitagliptin (100 mg/kg/day), or water for 2 weeks. Rats were then subjected to 30 minutes ischaemia/120 minutes reperfusion and infarct size ascertained. RESULTS: Two weeks pre-treatment with either Vildagliptin or Sitagliptin reduced ex vivo myocardial infarction (MI) size in hearts perfused with buffer containing 11 mmol/L glucose but not 5 mmol/L glucose. This effect was abolished by Exendin 9-39 (GLP-1 receptor antagonist) and H-89 (PKA antagonist). Treatment of perfused hearts with native GLP-1 was also glucose-sensitive, reducing MI size, at glucose concentrations 7, 9, and 11 mmol/L but not at 5 mmol/L. Finally, Sitagliptin reduced in vivo MI size in middle aged Wistar (7-8 mmol/L glucose) and Goto-Kakizaki (9-10 mmol/L glucose) rats where blood glucose was elevated, but not in young Wistar (5 mmol/L glucose) or Sprague-Dawley (5 mmol/L glucose) rats, where blood glucose was normal. CONCLUSIONS: We find that chronic treatment with DPP-4 inhibitors reduced MI size, via the GLP-1 receptor-PKA pathway, in a glucose-dependent manner. Glucose-sensitive cardioprotection of endogenous GLP-1 in diabetic patients may in part explain why intensive control of serum glucose levels has been associated with increased cardiovascular risk.

Cardioprotection in the aging, diabetic heart: the loss of protective Akt signalling.

AIMS: Old age and diabetes are risk factors that often coexist increasing the vulnerability of the heart to the lethal effects of ischaemia-reperfusion injury (IRI). However, to our knowledge, no investigations have examined IRI and cardioprotective signalling in animal models bearing these co-morbidities concomitantly. The ability of the heart to recover following IRI is greatly dependent on its innate cardioprotective potential, in which a central role is played by Akt. We aimed to investigate in an aging diabetic rat model, the susceptibility of the heart to IRI, the achievability of ischaemic preconditioning (IPC) against this lethal event, and the changes in Akt signalling, as the main prosurvival intracellular pathway. METHODS AND RESULTS: Our data showed that the isolated hearts of aged, diabetic Goto-Kakizaki rats were more susceptible to sub-lethal injury and not amenable to cardioprotection via IPC, compared with younger diabetic rat hearts. Western blot analysis of the heart tissue suggested a chronic up-regulation of Akt phosphorylation, and reduced expression of the mitochondrial regulator PGC-1α and of the anti-oxidant enzyme catalase, potentially due to the Akt up-regulation. Moreover, no further activation of Akt could be achieved following IPC. CONCLUSION: An increased susceptibility to IRI in the aged, diabetic heart could be a consequence of impaired Akt signalling due to chronic Akt phosphorylation. Additional Akt phosphorylation required for IPC protection may therefore not be possible in the aged, diabetic rat heart and may explain why this cardioprotective manoeuvre cannot be achieved in these hearts.

Chronic metformin associated cardioprotection against infarction: not just a glucose lowering phenomenon.

PURPOSE: Clinical and experimental investigations demonstrated that metformin, a widely used anti-diabetic drug, exhibits cardioprotective properties against myocardial infarction. Interestingly, metformin was previously shown to increase the expression of PGC-1α a key controller of energy metabolism in skeletal muscle, which is down-regulated in diabetic conditions. We hypothesized that chronic treatment with metformin could protect the aged, diabetic heart against ischemia-reperfusion injury (IRI) by up-regulating PGC-1α and improving the impaired functionality of diabetic mitochondria. METHODS: Following 4 weeks of metformin (300 mg/kg) administered in the drinking water, 12 month-old diabetic Goto Kakizaki and non-diabetic Wistar rat hearts were assigned for infarct measurement following 35 min ischemia and 60 min reperfusion or for electron microscopy (EM) and Western blotting (WB) investigations. RESULTS: Metformin elicited a cardioprotective effect in both non-diabetic and diabetic hearts. In contrast with the diabetic non-treated hearts, the diabetic hearts treated with metformin showed more organized and elongated mitochondria and demonstrated a significant increase in phosphorylated AMPK and in PGC-1α expression. CONCLUSIONS: In summary these results show for the first time that chronic metformin treatment augments myocardial resistance to ischemia-reperfusion injury, by an alternative mechanism in addition to the lowering of blood glucose. This consisted of a positive effect on mitochondrial structure possibly via a pathway involving AMPK activation and PGC-1α. Thus, metformin prescribed chronically to patients may lead to a basal state of cardioprotection thereby potentially limiting the occurrence of myocardial damage by cardiovascular events.

The diabetic heart: too sweet for its own good?

Diabetes mellitus is a major risk factor for ischemic heart disease (IHD). Patients with diabetes and IHD experience worse clinical outcomes, suggesting that the diabetic heart may be more susceptible to ischemia-reperfusion injury (IRI). In contrast, the animal data suggests that the diabetic heart may be either more, equally, or even less susceptible to IRI. The conflicting animal data may be due to the choice of diabetic and/or IRI animal model. Ischemic conditioning, a phenomenon in which the heart is protected against IRI by one or more brief nonlethal periods of ischemia and reperfusion, may provide a novel cardioprotective strategy for the diabetic heart. Whether the diabetic heart is amenable to ischemic conditioning remains to be determined using relevant animal models of IRI and/or diabetes. In this paper, we review the limitations of the current experimental models used to investigate IRI and cardioprotection in the diabetic heart.

Slow binding inhibition and mechanism of resistance of non-nucleoside polymerase inhibitors of hepatitis C virus.

The binding affinity of four palm and thumb site representative non-nucleoside inhibitors (NNIs) of HCV polymerase NS5B to wild-type and resistant NS5B polymerase proteins was determined, and the influence of RNA binding on NNI binding affinity was investigated. NNIs with high binding affinity potently inhibited HCV RNA polymerase activity and replicon replication. Among the compounds tested, HCV-796 showed slow binding kinetics to NS5B. The binding affinity of HCV-796 to NS5B increased 27-fold over a 3-h incubation period with an equilibrium Kd of 71 +/- 2 nm. Slow binding kinetics of HCV-796 was driven by slow dissociation from NS5B with a k(off) of 4.9 +/- 0.5 x 10(-4) s(-1). NS5B bound a long, 378-nucleotide HCV RNA oligonucleotide with high affinity (Kd = 6.9 +/- 0.3 nm), whereas the binding affinity was significantly lower for a short, 21-nucleotide RNA (Kd = 155.1 +/- 16.2 nm). The formation of the NS5B-HCV RNA complex did not affect the slow binding kinetics profile and only slightly reduced NS5B binding affinity of HCV-796. The magnitude of reduction of NNI binding affinity for the NS5B proteins with various resistance mutations in the palm and thumb binding sites correlated well with resistance -fold shifts in NS5B polymerase activity and replicon assays. Co-crystal structures of NS5B-Con1 and NS5B-BK with HCV-796 revealed a deep hydrophobic binding pocket at the palm region of NS5B. HCV-796 interaction with the induced binding pocket on NS5B is consistent with slow binding kinetics and loss of binding affinity with mutations at amino acid position 316.