Biology subjects

Munoz, M. F.

Publications and source records attributed to Munoz, M. F..

4 recordsLinked to original sources

bioRxiv2026

Endogenous subthreshold noise and PIP2 tune diastolic coherence across the intact sinoatrial node through stochastic resonance

The heartbeat originates in the sinoatrial (SA) node, where spontaneously firing pacemaker myocytes interact to generate a coherent rhythm. The classical entrainment account is deterministic: the fastest cells entrain the rest, and variability is an error term that coupling suppresses. We proposed previously that the node instead exploits its own noise through stochastic resonance, with rhythm quality peaking at intermediate fluctuation amplitude, and since mapped a metabolic gradient capable of supplying it. Two-photon imaging of the voltage sensor ASAP5 in the intact mouse node resolved the subthreshold regime that tissue-scale mapping averages away. Diastolic coherence followed an inverted-U against endogenous noise amplitude in both poles, with optima 3.5-fold apart, and each pole operated near its own. {beta}-adrenergic stimulation raised noise in both poles to a common level. The inferior pole, which began below its optimum, gained coherence, while the superior pole, already at its optimum, lost interval regularity. Blocking HCN-mediated If with ivabradine lowered superior coherence without changing superior noise, and lowered inferior noise without lowering inferior coherence, dissociating the two coordinates. Thus, coherence is a surface defined by noise and coupled-clock drive. Phosphoinositide 4,5-bisphosphate (PIP2), an ATP-dependent lipid cofactor for HCN channels, increased firing rate and noise in both poles but improved coherence only in the energy-poor inferior node. Prior If block abolished that rescue. Pacemaking is therefore graded by a bioenergetic supply chain that sets the noise, not by entrainment alone. Noise, long treated as a nuisance to be averaged away, is an active ingredient of the pacemaker.

physiology↗

bioRxiv2025

Beat-locked ATP microdomains in the sinoatrial node map a calcium-timed energetic hierarchy and regional pacemaker roles

Pacemaker myocytes of the sinoatrial (SA) node initiate each heartbeat through coupled voltage and Ca2+ oscillators, but whether ATP supply is regulated beat-by-beat in these cells remains unclear. Using genetically encoded sensors targeted to the cytosol and mitochondria, we tracked beat-resolved ATP dynamics in intact mouse SA node and isolated myocytes. Cytosolic ATP rose transiently with each Ca2+ transient and segregated into high- and low-gain phenotypes defined by the Ca2+-ATP coupling slope. Mitochondrial ATP flux adopted two stereotyped waveforms--Mode 1 "gains" and Mode 2 "dips." Within Mode 1 cells, ATP gains mirrored the cytosolic high/low-gain dichotomy; Mode 2 dips scaled linearly with Ca2+ load and predominated in slower-firing cells. High-gain/Mode 1 phenotypes localized to superior regions and low-gain/Mode 2 to inferior regions, paralleling gradients in rate, mitochondrial volume, and capillary density. Mechanistic dissection placed sarcoplasmic reticulum (SR) Ca2+ release upstream of ATP production, showing that Ca2+ triggers metabolic transients while membrane voltage primarily modulates their frequency. Inhibiting mitochondrial Ca2+ uptake and adenine nucleotide exchange eliminated beat-locked mito- and cyto-ATP signals, indicating that the mitochondrial Ca2+ uniporter (MCU)-adenine nucleotide translocase (ANT) machinery couples Ca2+ release to ATP fluctuations. Mode 2 recovery kinetics indicate slower ATP replenishment, which would favor low-frequency, fluctuation-rich firing in a subset of cells. Together, these findings reveal beat-locked metabolic microdomains in which Ca2+ transients time oxidative phosphorylation under a local O2 ceiling, unifying vascular architecture, mitochondrial organization, and Ca2+ signaling to match energy supply to excitability. This energetic hierarchy helps explain why some pacemaking myocytes are more likely to set the rate, whereas others may widen the bandwidth. SummaryBeat-locked cytosolic and mitochondrial ATP transients in SA-node myocytes sort into high-gain, low-gain, or consumption-dominant modes aligned with superior-inferior vascular-mitochondrial gradients. This energetic hierarchy lets high-gain cells set fast rates while low-gain/dip cells stabilize slow rhythms, broadening operating range but capping maximal bandwidth.

physiology↗

bioRxiv2024

Impaired S-nitrosylation of Cx43 prevents arrhythmogenicity and myocardial injury upon cardiac stress in Duchenne Muscular Dystrophy

Connexin-43 (Cx43) plays a critical role in the propagation of action potentials and cardiac contractility. In healthy cardiomyocytes, Cx43 is mainly located at the intercalated disk; however, Cx43 remodeling is observed in cardiac pathologies and is linked with arrhythmogenesis and sudden cardiac death. Using a mouse model of Duchenne muscular dystrophy (DMD), we previously demonstrated that Cx43 localizes to the lateral side of dystrophic cardiomyocytes, forming undocked hemichannels. {beta}-adrenergic signaling-induced cardiac stress promotes S-nitrosylation and the opening of undocked Cx43 hemichannels leading to disrupted cardiac membrane excitability and deadly arrhythmogenic behaviors. To establish the direct role of S-nitrosylated Cx43 in DMD cardiomyopathy, we generated knockin DMDmdx mice with reduced levels of S-nitrosylated Cx43, by replacing cysteine 271 with a serine in one Cx43 of the unique site for S-nitrosylation of Cx43 (DMDmdx:C271S+/-). Immunofluorescence analysis revealed that cardiac Cx43 lateralization in DMDmdx:C271S+/- mice was similar to DMDmdx mice, indicating that the genetic modification did not prevent Cx43 remodeling. Upon isoproterenol treatment, DMDmdx mice displayed a higher incidence of arrhythmogenic events when compared to DMDmdx:C271S+/- mice, which more closely resemble wild-type mice. Optical mapping imaging in isolated hearts showed that DMDmdx mice displayed aberrant Ca2+ signaling and prolonged action potentials, which is restored in DMDmdx:C271S+/- mice. Isoproterenol treatment evoked severe myocardial injury in DMDmdx mice, which was significantly attenuated in DMDmdx:C271S+/- mice. Notably, DMDmdx mice treated with Gap19, a Cx43 hemichannel blocker, exhibited cardioprotection against myocardial injury. We concluded that S-nitrosylation of Cx43 proteins is a fundamental NO-mediated mechanism involved in arrhythmias and myocardial injury in DMDmdx, occurring through the opening of hemichannels following {beta}-adrenergic stress.

physiology↗

bioRxiv2022

Remodeled Connexin 43 hemichannels alter cardiac excitability and promote arrhythmias

Connexin-43 (Cx43) is the most abundant protein forming gap junction channels (GJCs) in cardiac ventricles. In multiple cardiac pathologies, including hypertrophy and heart failure, Cx43 is found remodeled at the lateral side of the intercalated discs of ventricular cardiomyocytes. Remodeling of Cx43 has been long linked to spontaneous ventricular arrhythmia, yet the mechanisms by which arrhythmias develop are still debated. Using a model of a dystrophic cardiomyopathy, we previously showed that remodeled Cx43 function as aberrant hemichannels (non-forming GJCs) that alter cardiomyocyte excitability and, consequently, promote arrhythmias. Here, we aim to evaluate if opening of remodeled Cx43 can serve as a general mechanism to alter cardiac excitability independent of cellular dysfunction associated with a particular cardiomyopathy. To address this issue, we used a genetically modified Cx43 knock-in mouse (S3A) that promotes cardiac remodeling of Cx43 protein without apparent cardiac dysfunction. Importantly, when S3A mice were subjected to cardiac stress using the {beta}-adrenergic agonist isoproterenol (Iso), they displayed acute and severe arrhythmias, which were not observed in WT mice. Pre-treatment of S3A mice with the Cx43 hemichannel blocker, Gap19, prevented Iso-induced abnormal electrocardiographic behavior. At the cellular level, when compared with WT, Iso-treated S3A cardiomyocytes showed increased membrane permeability and greater plasma membrane depolarization, which subsequently leads to triggered activity. These cellular dysfunctions were also prevented by Cx43 hemichannel blockers. Our results support the notion that opening of remodeled Cx43 hemichannels, regardless of the type of cardiomyopathy, is sufficient to mediate cardiac stress-induced arrhythmogenicity.

physiology↗