AIM:To explore the expression of calcium binding and coiled-coil domain 2(CALCOCO2)in ani-mal models of atrial fibrillation(AF)and its role and mechanism in reversing atrial remodeling in AF mice.METHODS:Twelve rats and 12 mice were randomly divided into the following 2 groups(n=6 each):saline control group(saline group)and angiotensin Ⅱ(Ang Ⅱ)-induced AF group(Ang Ⅱ group).Western blot and immunohistochemistry(IHC)were used to detect CALCOCO2 expression in rat and mouse atrial muscle tissues.Another 24 mice were randomly divided into 4 groups(n=6 each):saline-oeNC,Ang Ⅱ-oeNC,saline-oeCALCOCO2,and Ang Ⅱ-oeCALCOCO2.An adeno-asso-ciated virus was used to induce CALCOCO2 overexpression in mouse atrial myocardium.Subsequently,transthoracic echocardiography and intracardiac electrophysiological testing were used to compare mouse cardiac function among the 4 groups.Western blot and TUNEL staining were also used to evaluate the effect of CALCOCO2 on apoptosis of cardiomyo-cytes in AF models.Additionally,IHC was used to assess the effect of CALCOCO2 on the levels of oxidative stress-related proteins[NADPH oxidase 2(NOX2)and NOX4]and fibrosis-related proteins[collagen type Ⅰ(Col Ⅰ),connexin 40(Cx40)and α-smooth muscle actin(α-SMA)]in AF atrial myocardium.RESULTS:The CALCOCO2 protein level in the atrial myocardium of rats and mice was significantly decreased in Ang Ⅱ group compared with saline group,as detected by Western blot and IHC(0.19±0.01 vs 0.32±0.03 for rats,0.37±0.10 vs 1.00±0.10 for mice,P<0.01).Compared with Ang Ⅱ-oeNC group,the mice in Ang Ⅱ-oeCALCOCO2 group exhibited decreased left atrial inner diameter,shorter AF duration,and increased ejection fraction(P<0.05).Semi-quantitative analysis of TUNEL staining revealed a signifi-cantly decreased apoptosis rate of mouse atrial myocytes in Ang Ⅱ-oeCALCOCO2 group compared with Ang Ⅱ-oeNC group(0.30±0.06 vs 0.61±0.03,P<0.01),which was consistent with the Western blot trend of apoptosis-related proteins such as BAX(1.94±0.34 vs 3.14±0.34,P<0.05)and cleaved caspase-3(2.19±0.41 vs 3.52±0.55,P<0.05).The semi-quantitative results of IHC and immunofluorescence revealed significantly increased levels of oxidative stress-related pro-teins(NOX2 and NOX4)and fibrosis-related proteins(Col Ⅰ and α-SMA),as well as decreased Cx40 levels,in Ang Ⅱ-oeNC group compared with saline-oeNC group.However,the expression levels of these proteins were significantly re-versed after CALCOCO2 overexpression(all P<0.05).CONCLUSION:Overexpression of CALCOCO2 reverses AF-in-duced electrical and structural remodeling by inhibiting the oxidative stress,apoptosis and fibrosis in mouse atrial tissues.