Complementary roles of gasotransmitters CO and H2S in sleep apnea

Intelligent Metal Carbonyl Metal-Organic Framework Nanocomplex for Fluorescent Traceable H2 O2 -Triggered CO Delivery

The recognized therapeutic benefits from carbon monoxide (CO) have caused booming attention to develop a CO therapy for various major diseases, such as cancer. However, the controlled release of CO gas and the monitoring of the CO release are vitally important to the on-demand CO administration for a safe and efficient therapy, but greatly challenging. In this work, a new CO-releasing nanocomplex was constructed by the adsorption and coordination of manganese carbonyl ([MnBr(CO)₅ ], abbreviated as MnCO) with a Ti-based metal-organic framework (Ti-MOF) to realize an intratumoral H₂ O₂ -triggered CO release and real-time CO release monitoring by fluorescence imaging. A high CO prodrug loading capacity (0.532 g MnCO per gram Ti-MOF) is achieved due to the high surface area of Ti-MOF, and the intracellular H₂ O₂ -triggered CO release from the MnCO@Ti-MOF is realized to enable the nanocomplex selectively release CO in tumor cells and kill tumor cells rather than normal cells. Particularly significant is that the real-time fluorescence imaging monitoring of the CO release is realized based on an annihilation effect of the fluorescence after MnCO loading into Ti-MOF and an activation effect of the fluorescence after CO release from Ti-MOF. The quantitative relationship between the fluorescence intensity and the released CO amount is established in great favor of guiding on-demand CO administration. The results demonstrate the advantage of versatile MOFs for high efficient CO delivery and monitoring, which is critical for the improvement of the effectiveness of future therapeutic application.

The Dynamic Basis of Respiratory Rhythm Generation: One Breath at a Time

Rhythmicity is a universal timing mechanism in the brain, and the rhythmogenic mechanisms are generally dynamic. This is illustrated for the neuronal control of breathing, a behavior that occurs as a one-, two-, or three-phase rhythm. Each breath is assembled stochastically, and increasing evidence suggests that each phase can be generated independently by a dedicated excitatory microcircuit. Within each microcircuit, rhythmicity emerges through three entangled mechanisms: ( a) glutamatergic transmission, which is amplified by ( b) intrinsic bursting and opposed by ( c) concurrent inhibition. This rhythmogenic triangle is dynamically tuned by neuromodulators and other network interactions. The ability of coupled oscillators to reconfigure and recombine may allow breathing to remain robust yet plastic enough to conform to nonventilatory behaviors such as vocalization, swallowing, and coughing. Lessons learned from the respiratory network may translate to other highly dynamic and integrated rhythmic systems, if approached one breath at a time.

The role of hypoxia-inducible factors in carotid body (patho) physiology

Hypoxia-inducible factors mediate adaptive responses to reduced O₂ availability. In patients with obstructive sleep apnoea, repeated episodes of hypoxaemia and reoxygenation (intermittent hypoxia) are sensed by the carotid body (CB). The ensuing CB chemosensory reflex activates the sympathetic nervous system and increased secretion of catecholamines by the adrenal medulla, resulting in hypertension and breathing abnormalities. In the CB, intermittent hypoxia induces the formation of reactive oxygen species (ROS) and increased intracellular Ca²⁺ levels, which drive increased expression of hypoxia-inducible factor (HIF) 1α and a decrease in the levels of HIF-2α.  Intermittent hypoxia increases HIF-1α-dependent expression of Nox2, encoding the pro-oxidant enzyme NADPH oxidase 2, and decreased HIF-2α-dependent expression of Sod2, encoding the anti-oxidant enzyme superoxide dismutase 2. These changes in gene expression drive persistently elevated ROS levels in the CB, brainstem, and adrenal medulla that are required for the development of hypertension and breathing abnormalities. The ROS generated by dysregulated HIF activity in the CB results in oxidation and inhibition of haem oxygenase 2, and the resulting reduction in the levels of carbon monoxide leads to increased hydrogen sulfide production, triggering glomus cell depolarization. Thus, the pathophysiology of obstructive sleep apnoea involves the dysregulation of O₂ -regulated transcription factors, gasotransmitters, and sympathetic outflow that affects blood pressure and breathing.

An H2S-sensing/CO-releasing Flavonol that Operates via Logic Gates

Signaling molecules, including H2S and CO, are involved in a complex interplay within cells to maintain cellular homeostasis. In order to investigate the intracellular interplay of different gasotransmitters, new molecular tools are needed that combine sensing and release capabilities of different small molecules in a single, multifunctional, and highly-regulated molecular platform. This report gives the first example of a combined H2S sensor/CO-releasing molecule. This flavonol-based molecular tool operates intracellularly via a highly regulated AND logic gated input-output sequence and provides fluorescent feedback for the H2S detection and CO release steps. This linear sequence can be combined with a fluorescent molecular sensor for CO detection via a third distinct emission. Overall, this is the first molecular framework that can combine the sensing of H2S with the controlled release of CO, two gaseous molecules that are known to exhibit reciprocal regulation and have overlapping targets in cellular environments.

KLF2 mediates enhanced chemoreflex sensitivity, disordered breathing and autonomic dysregulation in heart failure

KEY POINTS

Enhanced carotid body chemoreflex activity contributes to development of disordered breathing patterns, autonomic dysregulation and increases in incidence of arrhythmia in animal models of reduced ejection fraction heart failure. Chronic reductions in carotid artery blood flow are associated with increased carotid body chemoreceptor activity. Krüppel-like Factor 2 (KLF2) is a shear stress-sensitive transcription factor that regulates the expression of enzymes which have previously been shown to play a role in increased chemoreflex sensitivity. We investigated the impact of restoring carotid body KLF2 expression on chemoreflex control of ventilation, sympathetic nerve activity, cardiac sympatho-vagal balance and arrhythmia incidence in an animal model of heart failure. The results indicate that restoring carotid body KLF2 in chronic heart failure reduces sympathetic nerve activity and arrhythmia incidence, and improves cardiac sympatho-vagal balance and breathing stability. Therapeutic approaches that increase KLF2 in the carotid bodies may be efficacious in the treatment of respiratory and autonomic dysfunction in heart failure.

ABSTRACT

Oscillatory breathing and increased sympathetic nerve activity (SNA) are associated with increased arrhythmia incidence and contribute to mortality in chronic heart failure (CHF). Increased carotid body chemoreflex (CBC) sensitivity plays a role in this process and can be precipitated by chronic blood flow reduction. We hypothesized that downregulation of a shear stress-sensitive transcription factor, Krüppel-like Factor 2 (KLF2), mediates increased CBC sensitivity in CHF and contributes to associated autonomic, respiratory and cardiac sequelae. Ventilation (Ve), renal SNA (RSNA) and ECG were measured at rest and during CBC activation in sham and CHF rabbits. Oscillatory breathing was quantified as the apnoea-hypopnoea index (AHI) and respiratory rate variability index (RRVI). AHI (control 6 ± 1/h, CHF 25 ± 1/h), RRVI (control 9 ± 3/h, CHF 29 ± 3/h), RSNA (control 22 ± 2% max, CHF 43 ± 5% max) and arrhythmia incidence (control 50 ± 10/h, CHF 300 ± 100/h) were increased in CHF at rest ( FIO2 21%), as were CBC responses (Ve, RSNA) to 10% FIO2 (all P < 0.05 vs. control). In vivo adenoviral transfection of KLF2 to the carotid bodies in CHF rabbits restored KLF2 expression, and reduced AHI (7 ± 2/h), RSNA (18 ± 2% max) and arrhythmia incidence (46 ± 13/h) as well as CBC responses to hypoxia (all P < 0.05 vs. CHF empty virus). Conversely, lentiviral KLF2 siRNA in the carotid body decreased KLF2 expression, increased chemoreflex sensitivity, and increased AHI (6 ± 2/h vs. 14 ± 3/h), RRVI (5 ± 3/h vs. 20 ± 3/h) and RSNA (24 ± 4% max vs. 34 ± 5% max) relative to scrambled-siRNA rabbits. In conclusion, down-regulation of KLF2 in the carotid body increases CBC sensitivity, oscillatory breathing, RSNA and arrhythmia incidence during CHF.

International Union of Basic and Clinical Pharmacology. CII: Pharmacological Modulation of H2S Levels: H2S Donors and H2S Biosynthesis Inhibitors

Over the last decade, hydrogen sulfide (H2S) has emerged as an important endogenous gasotransmitter in mammalian cells and tissues. Similar to the previously characterized gasotransmitters nitric oxide and carbon monoxide, H2S is produced by various enzymatic reactions and regulates a host of physiologic and pathophysiological processes in various cells and tissues. H2S levels are decreased in a number of conditions (e.g., diabetes mellitus, ischemia, and aging) and are increased in other states (e.g., inflammation, critical illness, and cancer). Over the last decades, multiple approaches have been identified for the therapeutic exploitation of H2S, either based on H2S donation or inhibition of H2S biosynthesis. H2S donation can be achieved through the inhalation of H2S gas and/or the parenteral or enteral administration of so-called fast-releasing H2S donors (salts of H2S such as NaHS and Na2S) or slow-releasing H2S donors (GYY4137 being the prototypical compound used in hundreds of studies in vitro and in vivo). Recent work also identifies various donors with regulated H2S release profiles, including oxidant-triggered donors, pH-dependent donors, esterase-activated donors, and organelle-targeted (e.g., mitochondrial) compounds. There are also approaches where existing, clinically approved drugs of various classes (e.g., nonsteroidal anti-inflammatories) are coupled with H2S-donating groups (the most advanced compound in clinical trials is ATB-346, an H2S-donating derivative of the non-steroidal anti-inflammatory compound naproxen). For pharmacological inhibition of H2S synthesis, there are now several small molecule compounds targeting each of the three H2S-producing enzymes cystathionine-β-synthase (CBS), cystathionine-γ-lyase, and 3-mercaptopyruvate sulfurtransferase. Although many of these compounds have their limitations (potency, selectivity), these molecules, especially in combination with genetic approaches, can be instrumental for the delineation of the biologic processes involving endogenous H2S production. Moreover, some of these compounds (e.g., cell-permeable prodrugs of the CBS inhibitor aminooxyacetate, or benserazide, a potentially repurposable CBS inhibitor) may serve as starting points for future clinical translation. The present article overviews the currently known H2S donors and H2S biosynthesis inhibitors, delineates their mode of action, and offers examples for their biologic effects and potential therapeutic utility.