Targeted disruption of the Akt gene in mice induces a

Targeted disruption of the Akt1 gene in mice induces a growth retardation phenotype [4,19]. Akt2 KO mice reveal mild growth retardation and insulin resistance [4,20,21]. Akt1/2 double KO (DKO) mice display severe growth deficiency and die shortly after birth. These mice exhibit impaired bone and skin development and severe skeletal muscle atrophy because of a marked decrease in individual muscle cell size, and impeded adipogenesis [9]. However, the effect of cardiac-specific Akt1/2 DKO on cardiac function and structure remains to be systematically investigated. The present study revealed an association between cardiac dysfunction and 5'-Iodoresiniferatoxin of gap junction proteins, Cx43 and ZO-1, which are regulated by Akt1/2. In vertebrates, gap junctions are present in most tissues and play important roles in growth regulation, development, cell-to-cell communication, and tissue homeostasis [22]. They are organized from hexameric connexin hemichannels, encoded by 21 or 20 distinct genes in humans or rodents respectively [23]. The major cardiac connexin proteins are Cx40, Cx43, and Cx45, having obvious expression patterns and essential roles in heart development, metabolic coupling, propagation of action potentials, and tissue homeostasis [[24], [25], [26], [27], [28], [29]]. The tight junction protein ZO-1 is a member of the membrane-associated guanylate kinase (MAGUK) family of proteins [30]. A previous study showed that ZO-1 was found at 96% of the intercalated discs in non-failing human hearts, where it colocalized with Cx43. In contrast, ZO-1 immunostaining was observed in only 5% of intercalated discs in failing hearts, coincident with a reduction in Cx43 staining. Immunoblotting analysis showed that there was a 95% reduction in ZO-1 expression in human heart failure. Loss of ZO-1 at intercalated discs in heart failure might play a critical role in remodeling Cx43 gap junctions [30]. Divergent interactions between connexins and the tight junction proteins ZO-1, ZO-2, and ZO-3 in regulating connexins in gap junction transition have been described and vary depending on the connexin protein [[31], [32], [33], [34]]. Increased interaction between ZO-1 and Cx43 has been implicated in Cx43 downregulation and reduced Cx43 gap junction size in congestive heart failure [35]. The novel finding of the present study is that Akt1−/−/iAkt2 KO mice demonstrate abnormal gap junction structure and downregulation of Cx43 and ZO-1, prior to the onset of contractile dysfunction, which was evident as early as three days after the first tamoxifen injection. These results suggested that the Akt pathway is important to conserve gap junction proteins, which might play an important role in maintaining cardiac contractility. Another interesting finding of the present study was the early onset of heart failure in the absence of structural remodeling developing as early as four days after the first tamoxifen injection. In another study using a cardiac-specific KO mice model, IRS1/2 DKO mice exhibited heart failure with cell death, fibrosis, and decreased ventricular mass at the age of 5 weeks [36]. In contrast, the present study using the Akt1−/−/iAkt2 KO model induced heart failure at the age of 4 days. These two different DKO models support the view that insulin signaling defects can induce heart failure, despite clear differences in the timing and characteristics of heart failure. Akt1−/−/iAkt2 KO mice developed heart failure at 4 days after the loss of Akt1/2, with a very rapid onset, without typical characteristics of heart failure, such as cell death and fibrosis. These results imply that distinct mechanisms induce the early-onset heart failure in Akt1−/−/iAkt2 KO mice in the absence of gross pathological changes that characterize other models with long-term reduction of upstream activators of Akt such as IRS proteins [37]. Taken together, we speculate that reduction of Cx43 and ZO-1 expression by reduced Akt1 and Akt2 signaling represents an early defect that may contribute to cardiac dysfunction.