Sympathetic nerve damage and restoration after ischemia-reperfusion injury as assessed by (11)C-hydroxyephedrine

Metformin confers longitudinal cardiac protection by preserving mitochondrial homeostasis following myocardial ischemia/reperfusion injury

PURPOSE

Myocardial ischemia-reperfusion (I/R) injury is associated with systemic oxidative stress, cardiac mitochondrial homeostasis, and cardiomyocyte apoptosis. Metformin has been recognized to attenuate cardiomyocyte apoptosis. However, the longitudinal effects and pathomechanism of metformin on the regulation of myocardial mitohormesis following I/R treatment remain unclear. This study aimed to investigate the longitudinal effects and mechanism of metformin in regulating cardiac mitochondrial homeostasis by serial imaging with the 18-kDa translocator protein (TSPO)-targeted positron emission tomography (PET) tracer ¹⁸F-FDPA.

METHODS

Myocardial I/R injury was established in Sprague-Dawley rats, which were treated with or without metformin (150 mg/kg per day). Serial gated ¹⁸F-FDG and ¹⁸F-FDPA PET imaging were performed at 1, 4, and 8 weeks after surgery, followed by analysis of ventricular remodelling and cardiac mitochondrial homeostasis. The correlation between Hsp60 and ¹⁸F-FDPA uptake was analyzed. After PET imaging, the activity of antioxidant enzymes, immunostaining, and western blot analysis were performed to analyze the spatio-temporal effects and pathomechanism of metformin for cardiac protection after myocardial I/R injury.

RESULTS

Oxidative stress and apoptosis increased 1 week after myocardial I/R injury (before significant progression of ventricular remodelling). TSPO expression was correlated with Hsp60 expression and was co-localized with inflammatory CD68⁺ macrophages in the infarct area, and TSPO uptake was associated with an upregulation of AMPK-p/AMPK and a downregulation of Bcl-2/Bax. However, these effects were reversed with metformin treatment. Eight weeks after myocardial I/R injury (representing the advanced stage of heart failure), ¹⁸F-FDPA uptake in myocardial cells in the distal non-infarct area increased without CD68⁺ expression, whereas the activity decreased with metformin treatment.

CONCLUSION

Taken together, these results show that a prolonged metformin treatment has pleiotropic protective effects against myocardial I/R injury associated with a regional and temporal dynamic balance between mitochondrial homeostasis and cardiac outcome, which were assessed by TSPO-targeted imaging during cardiac remodelling.

Cardiac innervation in acute myocardial ischaemia/reperfusion injury and cardioprotection

Acute myocardial infarction (AMI) and the heart failure (HF) that often complicates this condition, are among the leading causes of death and disability worldwide. To reduce myocardial infarct (MI) size and prevent heart failure, novel therapies are required to protect the heart against the detrimental effects of acute ischaemia/reperfusion injury (IRI). In this regard, targeting cardiac innervation may provide a novel therapeutic strategy for cardioprotection. A number of cardiac neural pathways mediate the beneficial effects of cardioprotective strategies such as ischaemic preconditioning and remote ischaemic conditioning, and nerve stimulation may therefore provide a novel therapeutic strategy for cardioprotection. In this article, we provide an overview of cardiac innervation and its impact on acute myocardial IRI, the role of extrinsic and intrinsic cardiac neural pathways in cardioprotection, and highlight peripheral and central nerve stimulation as a cardioprotective strategy with therapeutic potential for reducing MI size and preventing HF following AMI. This article is part of a Cardiovascular Research Spotlight Issue entitled 'Cardioprotection Beyond the Cardiomyocyte', and emerged as part of the discussions of the European Union (EU)-CARDIOPROTECTION Cooperation in Science and Technology (COST) Action, CA16225.

Recent advances in radiotracers targeting norepinephrine transporter: structural development and radiolabeling improvements

The norepinephrine transporter (NET) is a major target for the evaluation of the cardiac sympathetic nerve system in patients with heart failure and Parkinson's disease. It is also used in the therapeutic applications against certain types of neuroendocrine tumors, as exemplified by the clinically used ^123/131I-MIBG as theranostic single-photon emission computed tomography (SPECT) agent. With the development of more advanced positron emission tomography (PET) technology, more radiotracers targeting NET have been reported, with superior temporal and spatial resolutions, along with the possibility of functional and kinetic analysis. More recently, fluorine-18-labelled NET tracers have drawn increasing attentions from researchers, due to their longer radiological half-life relative to carbon-11 (110 min vs. 20 min), reduced dependence on on-site cyclotrons, and flexibility in the design of novel tracer structures. In the heart, certain NET tracers provide integral diagnostic information on sympathetic innervation and the nerve status. In the central nervous system, such radiotracers can reveal NET distribution and density in pathological conditions. Most radiotracers targeting cardiac NET-function for the cardiac application consistent of derivatives of either norepinephrine or MIBG with its benzylguanidine core structure, e.g. ¹¹C-HED and ¹⁸F-LMI1195. In contrast, all NET tracers used in central nervous system applications are derived from clinically used antidepressants. Lastly, possible applications of NET as selective tracers over organic cation transporters (OCTs) in the kidneys and other organs controlled by sympathetic nervous system will also be discussed.

Renal denervation restrains the inflammatory response in myocardial ischemia-reperfusion injury

Myocardial ischemia-reperfusion (I/R) injury leads to intensive sympathetic nervous system (SNS) activation and inflammatory reactions. Whether renal sympathetic denervation (RDN) could be a new therapeutic strategy to modulate I/R inflammation and reduce infarct size after myocardial I/R injury needs to be explored. First, we investigated the correlation between plasma norepinephrine concentrations and circulating myeloid cell numbers in patients with acute myocardial infarction. And then, C57BL/6 mice underwent a "two-hit" operation, with 10% phenol applied to bilateral renal nerves to abrogate sympathoexcitation, and a 45-min ligation of the left coronary artery to induce myocardial I/R injury. The effects of RDN on the mobilization of immune cells in mice following myocardial I/R injury were explored. We observed a strong association between SNS overactivation and myeloid cell excessive accumulation in patients. In animal experiments, there was a significant reduction in infarct size per area at risk in the denervated-I/R group when compared to that of the innervated-I/R group (39.2% versus 49.8%; p < 0.005), and RDN also improved the left ventricular ejection fraction by 20% after 1 week. Furthermore, the denervated-I/R group showed a decrease in the number of neutrophils and macrophages in the blood and the myocardium as reflected by immunohistochemical staining and flow cytometry analysis (p < 0.05); the decrease was associated with a significant reduction in the circulating production of IL-1, IL-6 and TNF-α (p < 0.05). In summary, our study reveals a novel link between the SNS activity and inflammatory response undergoing myocardium I/R injury and identifies RDN as a potential therapeutic strategy against myocardium I/R injury via preserving the spleen immune cells mobilization.

Ventricular Distribution Pattern of the Novel Sympathetic Nerve PET Radiotracer 18F-LMI1195 in Rabbit Hearts

We aimed to determine a detailed regional ventricular distribution pattern of the novel cardiac nerve PET radiotracer ¹⁸F-LMI1195 in healthy rabbits. Ex-vivo high resolution autoradiographic imaging was conducted to identify accurate ventricular distribution of ¹⁸F-LMI1195. In healthy rabbits, ¹⁸F-LMI1195 was administered followed by the reference perfusion marker ²⁰¹Tl for a dual-radiotracer analysis. After 20 min of ¹⁸F-LMI1195 distribution time, the rabbits were euthanized, the hearts were extracted, frozen, and cut into 20-μm short axis slices. Subsequently, the short axis sections were exposed to a phosphor imaging plate to determine ¹⁸F-LMI1195 distribution (exposure for 3 h). After complete ¹⁸F decay, sections were re-exposed to determine ²⁰¹Tl distribution (exposure for 7 days). For quantitative analysis, segmental regions of Interest (ROIs) were divided into four left ventricular (LV) and a right ventricular (RV) segment on mid-ventricular short axis sections. Subendocardial, mid-portion, and subepicardial ROIs were placed on the LV lateral wall. ¹⁸F-LMI1195 distribution was almost homogeneous throughout the LV wall without any significant differences in all four LV ROIs (anterior, posterior, septal and lateral wall, 99 ± 2, 94 ± 5, 94 ± 4 and 97 ± 3%LV, respectively, n.s.). Subepicardial ²⁰¹Tl uptake was significantly lower compared to the subendocardial portion (subendocardial, mid-portion, and subepicardial activity: 90 ± 3, 96 ± 2 and *80 ± 5%LV, respectively, *p < 0.01 vs. mid-portion). This was in contradistinction to the transmural wall profile of ¹⁸F-LMI1195 (90 ± 4, 96 ± 5 and 84 ± 4%LV, n.s.). A slight but significant discrepant transmural radiotracer distribution pattern of ²⁰¹Tl in comparison to ¹⁸F-LMI1195 may be a reflection of physiological sympathetic innervation and perfusion in rabbit hearts.

Moving into the next era of PET myocardial perfusion imaging: introduction of novel 18F-labeled tracers

The heart failure epidemic continues to rise with coronary artery disease as one of its main causes. Novel concepts for risk stratification to guide the referring cardiologist towards revascularization procedures are of significant value. Myocardial perfusion imaging using single-photon emission computed tomography (SPECT) agents has demonstrated high accuracy for the detection of clinically relevant stenoses. With positron emission tomography (PET) becoming more widely available, mainly due to its diagnostic performance in oncology, perfusion imaging with that modality is more practical than in the past and overcomes existing limitations of SPECT MPI. Advantages of PET include more reliable quantification of absolute myocardial blood flow, the routine use of computed tomography for attenuation correction, a higher spatiotemporal resolution and a higher count sensitivity. Current PET radiotracers such as rubidium-82 (half-life, 76 s), oxygen-15 water (2 min) or nitrogen-13 ammonia (10 min) are labeled with radionuclides with very short half-lives, necessitating that stress imaging is performed under pharmacological vasodilator stress instead of exercise testing. However, with the introduction of novel ¹⁸F-labeled MPI PET radiotracers (half-life, 110 min), the intrinsic advantages of PET can be combined with exercise testing. Additional advantages of those radiotracers include, but are not limited to: potentially improved cost-effectiveness due to the use of pre-existing delivery systems and superior imaging qualities, mainly due to the shortest positron range among available PET MPI probes. In the present review, widely used PET MPI radiotracers will be reviewed and potential novel ¹⁸F-labeled perfusion radiotracers will be discussed.

Regional interaction between myocardial sympathetic denervation, contractile dysfunction, and fibrosis in heart failure with preserved ejection fraction: 11C-hydroxyephedrine PET study

PURPOSE

This investigation aimed to identify significant predictors of regional sympathetic denervation quantified by ¹¹C-hydroxyephedrine (HED) positron emission tomography (PET) in patients with heart failure with preserved left ventricular ejection fraction (HFpEF).

METHODS

Included in the study were 34 patients (age 63 ± 15 years, 23 men) with HFpEF (left ventricular ejection fraction ≥40%) and 11 age-matched volunteers without heart failure. Cardiac magnetic resonance imaging was performed to measure left ventricular size and function, and the extent of myocardial late gadolinium enhancement (LGE). ¹¹C-HED PET was performed to quantify myocardial sympathetic innervation that was expressed as a ¹¹C-HED retention index (RI, %/min). To identify predictors of regional ¹¹C-HED RI in HFpEF patients, we propose a multivariate mixed-effects model for repeated measures over segments with an unstructured covariance matrix.

RESULTS

Global ¹¹C-HED RI was significantly lower and more heterogeneous in HFpEF patients than in volunteers (P < 0.01 for all). Regional ¹¹C-HED RI was correlated positively with systolic wall thickening (r = 0.42, P < 0.001) and negatively with the extent of LGE (r = -0.43, P < 0.001). Segments in HFpEF patients with a large extent of LGE had the lowest regional ¹¹C-HED RI among all segments (P < 0.001 in post hoc tests). Multivariate analysis demonstrated that systolic wall thickening and the extent of LGE were significant predictors of regional ¹¹C-HED RI in HFpEF patients (both P ≤ 0.001).

CONCLUSION

Regional sympathetic denervation was associated with contractile dysfunction and fibrotic burden in HFpEF patients, suggesting that regional sympathetic denervation may provide an integrated measure of myocardial damage in HFpEF.

11C-Methionine PET of Myocardial Inflammation in a Rat Model of Experimental Autoimmune Myocarditis

Myocarditis represents a major cause of dilated cardiomyopathy and sudden cardiac death in younger adults. Currently, definitive diagnosis of myocarditis requires endomyocardial biopsy, which is highly invasive and has the drawback of variable sensitivity due to inherent sampling error. Therefore, reliable noninvasive methods to detect and monitor cardiac inflammation are clinically relevant. In this study, we explored the potential of radiolabeled methionine to assess myocardial inflammatory activity in a rat model of experimental autoimmune myocarditis (EAM).

METHODS

Autoimmune myocarditis was induced by immunizing Lewis rats twice with porcine cardiac myosin and Freund complete adjuvant. Control animals were treated with adjuvant alone. Dual-tracer autoradiography was performed to assess ¹⁴C-methionine uptake and to compare the distributions of ¹⁴C-methionine versus ¹⁸F-FDG. Hematoxylin and eosin staining and anti-CD68 macrophage staining were performed for histologic analysis. Additionally, cardiac ¹¹C-methionine PET was performed to evaluate the feasibility of in vivo imaging. ¹⁸F-FDG PET was also conducted to compare the in vivo uptake of ¹¹C-methionine and ¹⁸F-FDG.

RESULTS

Multiple focal cardiac inflammatory lesions were histologically identified in myosin-immunized rats, whereas no cardiac lesions were observed in the controls. Autoradiographic images clearly showed a high-density accumulation of ¹⁴C-methionine in inflammatory lesions of EAM rats, whereas no significant uptake was observed in the control animals. ¹⁴C-methionine uptake was significantly higher in inflammatory lesions than in remote noninflammatory areas and control rat hearts. The distribution of ¹⁴C-methionine correlated well with that of ¹⁸F-FDG and with macrophage density. The contrast between inflammatory and noninflammatory areas was higher for ¹⁸F-FDG than for ¹⁴C-methionine (3.45 ± 0.68 vs. 2.07 ± 0.21, respectively; P < 0.05). In the PET imaging study, the regional ¹¹C-methionine uptake (percentage injected dose per cubic centimeter) observed in EAM rats was significantly higher than the values obtained for control animals (0.64 ± 0.09 vs. 0.28 ± 0.02, respectively; P < 0.001). A good positive correlation between ¹¹C-methionine and ¹⁸F-FDG uptake was found.

CONCLUSION

In a rat model of autoimmune myocarditis, we demonstrated the colocalization of radiolabeled methionine accumulation with ¹⁸F-FDG uptake in histologically proven inflammatory lesions. These data suggest that ¹¹C-methionine might represent a promising candidate for the noninvasive detection and monitoring of myocarditis.