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Articles by X Qi
Total Records ( 2 ) for X Qi
  X Qi , Y. H Yeh , D Chartier , L Xiao , Y Tsuji , B. J.J.M Brundel , I Kodama and S. Nattel

Background— Sustained bradycardia is associated with long-QT syndrome in human beings and causes spontaneous torsades de pointes in rabbits with chronic atrioventricular block (CAVB), at least partly by downregulating delayed-rectifier K+-current to cause action potential (AP) prolongation. We addressed the importance of altered Ca2+ handling, studying underlying mechanisms and consequences.

Methods and Results— We measured ventricular cardiomyocyte [Ca2+]i (Indo1-AM), L-type Ca2+-current (ICaL) and APs (whole-cell perforated-patch), and Ca2+-handling protein expression (immunoblot). CAVB increased AP duration, cell shortening, systolic [Ca2+]i transients, and caffeine-induced [Ca2+]i release, and CAVB cells showed spontaneous early afterdepolarizations (EADs). ICaL density was unaffected by CAVB, but inactivation was shifted to more positive voltages, increasing the activation-inactivation overlap zone for ICaL window current. Ca2+-calmodulin–dependent protein kinase-II (CaMKII) autophosphorylation was enhanced in CAVB, indicating CaMKII activation. CAVB also enhanced CaMKII-dependent phospholamban-phosphorylation and accelerated [Ca2+]i-transient decay, consistent with phosphorylation-induced reductions in phospholamban inhibition of sarcoplasmic reticulum (SR) Ca2+-ATPase as a contributor to enhanced SR Ca2+ loading. The CaMKII-inhibitor KN93 reversed CAVB-induced changes in caffeine-releasable [Ca2+]i and ICaL inactivation voltage and suppressed CAVB-induced EADs. Similarly, the calmodulin inhibitor W7 suppressed CAVB-induced ICaL inactivation voltage shifts and EADs, and a specific CaMKII inhibitory peptide prevented ICaL inactivation voltage shifts. The SR Ca2+-uptake inhibitor thapsigargin and the SR Ca2+ release inhibitor ryanodine also suppressed CAVB-induced EADs, consistent with an important role for SR Ca2+ loading and release in arrhythmogenesis. AP-duration changes reached a maximum after 1 week of bradypacing, but peak alterations in CaMKII and [Ca2+]i required 2 weeks, paralleling the EAD time course.

Conclusions— CAVB-induced remodeling enhances [Ca2+]i load and activates the Ca2+-calmodulin-CaMKII system, producing [Ca2+]i-handling abnormalities that contribute importantly to CAVB-induced arrhythmogenic afterdepolarizations.

  M Xin , E. M Small , L. B Sutherland , X Qi , J McAnally , C. F Plato , J. A Richardson , R Bassel Duby and E. N. Olson

Vascular injury triggers dedifferentiation and cytoskeletal remodeling of smooth muscle cells (SMCs), culminating in vessel occlusion. Serum response factor (SRF) and its coactivator, myocardin, play a central role in the control of smooth muscle phenotypes by regulating the expression of cytoskeletal genes. We show that SRF and myocardin regulate a cardiovascular-specific microRNA (miRNA) cluster encoding miR-143 and miR-145. To assess the functions of these miRNAs in vivo, we systematically deleted them singly and in combination in mice. Mice lacking both miR-143 and miR-145 are viable and do not display overt abnormalities in smooth muscle differentiation, although they show a significant reduction in blood pressure due to reduced vascular tone. Remarkably, however, neointima formation in response to vascular injury is profoundly impeded in mice lacking these miRNAs, due to disarray of actin stress fibers and diminished migratory activity of SMCs. These abnormalities reflect the regulation of a cadre of modulators of SRF activity and actin dynamics by miR-143 and miR-145. Thus, miR-143 and miR-145 act as integral components of the regulatory network whereby SRF controls cytoskeletal remodeling and phenotypic switching of SMCs during vascular disease.

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