Wei, E. Q. | Search bioRxiv

bioRxiv2025

FGF13 regulates cardiomyocyte impulse propagation via Cx43 trafficking independent of voltage-gated sodium channels

BackgroundFibroblast growth factor homologous factor (FHF) variants associate with arrhythmias. Although FHFs are best characterized as regulators of voltage gated sodium channel (VGSC) gating, recent studies suggest broader, non-VGSC-related functions, including regulation of Cx43 gap junctions and/or hemichannels, mechanisms that have generally been understudied or disregarded. MethodsWe assessed cardiac conduction and cardiomyocyte action potentials in mice with constitutive cardiac-specific Fgf13 ablation (cFgf13KO) while targeting Cx43 gap junctions and hemichannels pharmacologically. Using immunostaining and biochemistry, we characterized FGF13 regulation of Cx43 abundance and subcellular distribution. With proximity labeling proteomics, we investigated novel candidate mechanisms underlying FGF13 regulation of Cx43. ResultsFGF13 ablation prolonged the QRS and QT intervals on the surface electrocardiogram. Carbenoxolone, a Cx43 gap junction uncoupler, markedly prolonged the QRS duration leading to conduction system block in cFgf13KO but not in WT mice. Optical mapping revealed markedly decreased conduction velocity (CV) during ventricular pacing. Microscopy revealed markedly perturbed trafficking of Cx43, reduced localization in the intercalated disc, and suggested decreased membrane Cx43 but increased Cx43 hemichannels in cardiomyocytes from cFgf13KOmice. Resting membrane potential (RMP) was depolarized and APD50 was prolonged in cFgf13KOcardiomyocytes. Both were restored towards wildtype (WT) values with Gap19 (a Cx43 hemichannel inhibitor), expression of FGF13, or expression of a mutant FGF13 incapable of binding to VGSCs, emphasizing VGSC-independent regulation by FGF13. To assess the functional impact of RMP depolarization, hearts were subjected to hypokalemia, which had no effect in WT hearts but fully rescued CV in cFgf13KO hearts. Proteomic analyses revealed candidate roles for FGF13 in the regulation of vesicular-mediated transport. Biochemistry and immunocytochemistry showed that FGF13 ablation destabilized microtubules and reduced the expression of tubulins and MAP4, the major cardiac microtubule regulator. ConclusionsFGF13 regulates microtubule-dependent trafficking and targeting of Cx43, thereby impacting cardiac impulse propagation via VGSC-independent mechanisms.

physiology↗

bioRxiv2023

Multiple beta cell-independent mechanisms drive hypoglycemia in Timothy syndrome

The canonical G406R gain of function mutation that reduces inactivation and increases Ca2+ influx through the CACNA1C-encoded CaV1.2 voltage gated Ca2+ channel underlies the multisystem disorder Timothy syndrome (TS), characterized by invariant Long QT syndrome and consequent life-threatening arrhythmias. Severe episodic hypoglycemia, which exacerbates arrhythmia risk, is among the myriad non-cardiac TS pathologies that are poorly characterized. While hypoglycemia is thought to result from increased Ca2+ influx through CaV1.2 channels in pancreatic beta cells and consequent hyperinsulinism, this mechanism has never been demonstrated due to a lack of informative animal models, thus hampering development of preventive strategies. We generated a CaV1.2 G406R knockin mouse model that recapitulates key TS features including hypoglycemia. Unexpectedly, these mice did not show hyperactive beta cells or hyperinsulinism in the setting of normal intrinsic beta cell function, suggesting dysregulated glucose homeostasis. We discovered multiple alternative contributors to hypoglycemia, including perturbed counterregulatory hormone responses with defects in glucagon secretion and abnormal hypothalamic glucose sensing. Together, these data provide new insights into physiological contributions of the broadly expressed CaV1.2 channel and reveal integrated consequences of the mutant channel that underlie the life-threatening events in TS. Brief SummaryGain of function mutant CaV1.2 channels drive hypoglycemia through adverse effects on counterregulatory hormones and central nervous system glucose sensing

physiology↗