ABSTRACT
BACKGROUND: The dual Na+ and cardiac Ca2+-release channel inhibitor, Flecainide (FLEC) is effective in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT), a disease caused by mutations in cardiac Ca2+-release channels (RyR2), calsequestrin (Casq2), or calmodulin. FLEC suppresses spontaneous Ca2+ waves in Casq2-knockout (Casq2-/-) cardiomyocytes, a CPVT model. However, a report failed to find FLEC efficacy against Ca2+ waves in another CPVT model, RyR2-R4496C heterozygous mice (RyR2R4496C+/-), raising the possibility that FLEC efficacy may be mutation dependent. OBJECTIVE: To address this controversy, we compared FLEC in Casq2-/- and RyR2R4496C+/- cardiomyocytes and mice under identical conditions. METHODS: After 30 min exposure to FLEC (6 µM) or vehicle (VEH), spontaneous Ca2+ waves were quantified during a 40 s pause after 1 Hz pacing train in the presence of isoproterenol (ISO, 1 µM). FLEC efficacy was also tested in vivo using a low dose (LOW: 3 mg/kg ISO + 60 mg/kg caffeine) or a high dose catecholamine challenge (HIGH: 3 mg/kg ISO + 120 mg/kg caffeine). RESULTS: In cardiomyocytes, FLEC efficacy was dependent on extracellular [Ca2+]. At 2 mM [Ca2+], only Casq2-/- myocytes exhibited Ca2+ waves, which were strongly suppressed by FLEC. At 3 mM [Ca2+] both groups exhibited Ca2+ waves that were suppressed by FLEC. At 4 mM [Ca2+], FLEC no longer suppressed Ca2+ waves in both groups. Analogous to the results in myocytes, RyR2R4496C+/- mice (n = 12) had significantly lower arrhythmia scores than Casq2-/- mice (n = 9), but the pattern of FLEC efficacy was similar in both groups (i.e., reduced FLEC efficacy after HIGH dose catecholamine challenge). CONCLUSION: FLEC inhibits Ca2+ waves in RyR2R4496C+/- cardiomyocytes, indicating that RyR2 channel block by FLEC is not mutation-specific. However, FLEC efficacy is reduced by Ca2+ overload in vitro or by high dose catecholamine challenge in vivo, which could explain conflicting literature reports.
ABSTRACT
STAT6 participates in classical IL-4/IL-13 signaling and stimulator of interferon genes-mediated antiviral innate immune responses. Aberrations in STAT6-mediated signaling are linked to development of asthma and diseases of the immune system. In addition, STAT6 remains constitutively active in multiple types of cancer. Therefore, targeting STAT6 is an attractive proposition for treating related diseases. Although a lot is known about the role of STAT6 in transcriptional regulation, molecular details on how STAT6 recognizes and binds specific segments of DNA to exert its function are not clearly understood. Here, we report the crystal structures of a homodimer of phosphorylated STAT6 core fragment (STAT6CF) alone and bound with the N3 and N4 DNA binding site. Analysis of the structures reveals that STAT6 undergoes a dramatic conformational change on DNA binding, which was further validated by performing molecular dynamics simulation studies and small angle X-ray scattering analysis. Our data show that a larger angle at the intersection where the two protomers of STAT meet and the presence of a unique residue, H415, in the DNA-binding domain play important roles in discrimination of the N4 site DNA from the N3 site by STAT6. H415N mutation of STAT6CF decreased affinity of the protein for the N4 site DNA, but increased its affinity for N3 site DNA, both in vitro and in vivo. Results of our structure-function studies on STAT6 shed light on mechanism of DNA recognition by STATs in general and explain the reasons underlying STAT6's preference for N4 site DNA over N3.
