Inhibition of mitochondrial translocator protein prevents atrial fibrillation

Radiotracers for Imaging of Inflammatory Biomarkers TSPO and COX-2 in the Brain and in the Periphery

Inflammation involves the activation of innate immune cells and is believed to play an important role in the development and progression of both infectious and non-infectious diseases such as neurodegeneration, autoimmune diseases, pulmonary and cancer. Inflammation in the brain is marked by the upregulation of translocator protein (TSPO) in microglia. High TSPO levels are also found, for example, in macrophages in cases of rheumatoid arthritis and in malignant tumor cells compared to their relatively low physiological expression. The same applies for cyclooxgenase-2 (COX-2), which is constitutively expressed in the kidney, brain, thymus and gastrointestinal tract, but induced in microglia, macrophages and synoviocytes during inflammation. This puts TSPO and COX-2 in the spotlight as important targets for the diagnosis of inflammation. Imaging modalities, such as positron emission tomography and single-photon emission tomography, can be used to localize inflammatory processes and to track their progression over time. They could also enable the monitoring of the efficacy of therapy and predict its outcome. This review focuses on the current development of PET and SPECT tracers, not only for the detection of neuroinflammation, but also for emerging diagnostic measures in infectious and other non-infectious diseases such as rheumatic arthritis, cancer, cardiac inflammation and in lung diseases.

Translocator protein in the rise and fall of central nervous system neurons

Translocator protein (TSPO), a 18 kDa protein found in the outer mitochondrial membrane, has historically been associated with the transport of cholesterol in highly steroidogenic tissues though it is found in all cells throughout the mammalian body. TSPO has also been associated with molecular transport, oxidative stress, apoptosis, and energy metabolism. TSPO levels are typically low in the central nervous system (CNS), but a significant upregulation is observed in activated microglia during neuroinflammation. However, there are also a few specific regions that have been reported to have higher TSPO levels than the rest of the brain under normal conditions. These include the dentate gyrus of the hippocampus, the olfactory bulb, the subventricular zone, the choroid plexus, and the cerebellum. These areas are also all associated with adult neurogenesis, yet there is no explanation of TSPO's function in these cells. Current studies have investigated the role of TSPO in microglia during neuron degeneration, but TSPO's role in the rest of the neuron lifecycle remains to be elucidated. This review aims to discuss the known functions of TSPO and its potential role in the lifecycle of neurons within the CNS.

Development of a novel low-order model for atrial function and a study of atrial mechano-electric feedback

Numerical models of the cardiovascular system have largely focused on the function of the ventricles, with atrial function often neglected. Furthermore, the time-varying elastance method that prescribes the pressure-volume relationship rather than calculating it consistently is frequently used for the ventricles and atrium. This method has yet to be validated however, so its applicability for cardiac modelling is frequently questioned. To overcome this challenge, we propose a synergistic model of left atrium (LA) and left ventricle (LV) by self-consistently integrating various feedback mechanisms among the electro-mechanical and chemical functions of the micro-scale myofiber, the macro-scale dynamics of the LA and LV, the atrioventricular node (AV), and circulation. The model is tested and shown to reproduce the essential features of the atrium cycling, such as the characteristic figure of eight pressure-volume loops. Our model is further developed to investigate the effect of dysfunctions of the mechanical-electric feedback (MEF) in the atrium. Our model not only successfully reproduces key experimental MEF observations such as prolonged action-potential and increases in action-potential magnitude induced by atrial stretch but also shows how MEF and arrhythmia of the atrium lead to a degradation of cardiac output and pumping power with significant consequences. In particular, MEF reproduces arrhythmia such as ectopic and erratic cycling, missed heart beats and restricted function.

Automated synthesis and preliminary evaluation of [18F]FDPA for cardiac inflammation imaging in rats after myocardial infarction

A translocator protein 18 kDa targeted radiotracer, N,N-diethyl-2-(2-(4-[¹⁸F]fluorophenyl)-5,7-dimethylpyrazolo[1,5-a] pyrimidin-3-yl) acetamide ([¹⁸F]FDPA), was automated synthetized and evaluated for cardiac inflammation imaging. Various reaction conditions for an automated synthesis were systematically optimized. MicroPET/CT imaging were performed on normal rats and rats with myocardial infarction (MI). Normalized SUV ratios of [¹⁸F]FDPA to [¹³N]NH₃ (NSRs) in different regions were calculated to normalize the uptake of [¹⁸F]FDPA to perfusion. The amount of TBAOMs and the volume/proportion of water were crucial for synthesis. After optimization, the total synthesis time was 68 min. The non-decay corrected radiochemical yields (RCYs) and molar activities were 19.9 ± 1.7% and 169.7 ± 46.5 GBq/μmol, respectively. In normal rats, [¹⁸F]FDPA showed a high and stable cardiac uptake and fast clearance from other organs. In MI rats, NSRs in the peri-infarct and infarct regions, which were infiltrated with massive inflammatory cells revealed by pathology, were higher than that in the remote region (1.20 ± 0.01 and 1.08 ± 0.10 vs. 0.89 ± 0.05, respectively). [¹⁸F]FDPA was automated synthesized with high RCYs and molar activities. It showed a high uptake in inflammation regions and offered a wide time window for cardiac imaging, indicating it could be a potential cardiac inflammation imaging agent.

Cardiac Mechano-Electric Coupling: Acute Effects of Mechanical Stimulation on Heart Rate and Rhythm

The heart is vital for biological function in almost all chordates, including humans. It beats continually throughout our life, supplying the body with oxygen and nutrients while removing waste products. If it stops, so does life. The heartbeat involves precise coordination of the activity of billions of individual cells, as well as their swift and well-coordinated adaption to changes in physiological demand. Much of the vital control of cardiac function occurs at the level of individual cardiac muscle cells, including acute beat-by-beat feedback from the local mechanical environment to electrical activity (as opposed to longer term changes in gene expression and functional or structural remodeling). This process is known as mechano-electric coupling (MEC). In the current review, we present evidence for, and implications of, MEC in health and disease in human; summarize our understanding of MEC effects gained from whole animal, organ, tissue, and cell studies; identify potential molecular mediators of MEC responses; and demonstrate the power of computational modeling in developing a more comprehensive understanding of ‟what makes the heart tick.ˮ.

TSPO: An Evolutionarily Conserved Protein with Elusive Functions

TSPO (18 kDa translocator protein) was identified decades ago in a search for peripheral tissue binding sites for benzodiazepines, and was formerly called the peripheral benzodiazepine receptor. TSPO is a conserved protein throughout evolution and it is implicated in the regulation of many cellular processes, including inflammatory responses, oxidative stress, and mitochondrial homeostasis. TSPO, apart from its broad expression in peripheral tissues, is highly expressed in neuroinflammatory cells, such as activated microglia. In addition, emerging studies employing the ligands of TSPO suggest that TSPO plays an important role in neuropathological settings as a biomarker and therapeutic target. However, the precise molecular function of this protein in normal physiology and neuropathology remains enigmatic. This review provides an overview of recent advances in our understanding of this multifaceted molecule and identifies the knowledge gap in the field for future functional studies.

Rabbit models of cardiac mechano-electric and mechano-mechanical coupling

Cardiac auto-regulation involves integrated regulatory loops linking electrics and mechanics in the heart. Whereas mechanical activity is usually seen as 'the endpoint' of cardiac auto-regulation, it is important to appreciate that the heart would not function without feed-back from the mechanical environment to cardiac electrical (mechano-electric coupling, MEC) and mechanical (mechano-mechanical coupling, MMC) activity. MEC and MMC contribute to beat-by-beat adaption of cardiac output to physiological demand, and they are involved in various pathological settings, potentially aggravating cardiac dysfunction. Experimental and computational studies using rabbit as a model species have been integral to the development of our current understanding of MEC and MMC. In this paper we review this work, focusing on physiological and pathological implications for cardiac function.

Translocator protein (18 kDa): a promising therapeutic target and diagnostic tool for cardiovascular diseases

The translocator protein (18 kDa) (TSPO) is a five transmembrane domain protein in mitochondria, abundantly expressed in a variety of organs and tissues. TSPO contributes to a wide range of biological processes, including cholesterol transportation, mitochondrial membrane potential and respiratory chain regulation, apoptosis, and oxidative stress. Recent studies have demonstrated that TSPO might also be involved in the physiological regulation of cardiac chronotropy and inotropy. Accordingly, TSPO ligands play significant roles in protecting the cardiovascular systems under pathological conditions through cardiac electrical activity retention, intracellular calcium maintenance, mitochondrial energy provision, mitochondrial membrane potential equilibrium, and reactive oxygen species inhibition. This paper focuses on the physiological and pathological characteristics of TSPO in the cardiovascular systems and also summarizes the properties of TSPO ligands. TSPO represents a potential therapeutic target and diagnostic tool for cardiovascular diseases including arrhythmia, myocardial infarction, cardiac hypertrophy, atherosclerosis, myocarditis, and large vessel vasculitis.