Cardiac hypertrophy induced by overexpression of IP3-released inositol 1, 4, 5-trisphosphate receptor-binding protein...

IRBIT, also known as Ahcyl1, is an IP3 receptor (IP3R)-binding protein released with IP3 and was first described as a competitive inhibitor of the mentioned receptor. Studies have shown that overexpression of IP3Rs is associated with cardiac hypertrophy in both human and animal models. Given that IP3Rs play a role in cardiac hypertrophy, IRBIT may also be involved in this condition.

Although IRBIT heart expression has been reported, its function in cardiac tissues remains unclear. Thus, we aimed to study the cardiac outcomes of up-and downregulation of IRBIT to establish its pathophysiological role.

We found that IRBIT is expressed in mouse ventricles and atria, fibroblasts and cardiomyocytes isolated from neonatal mice, and in the myoblast cell line H9c2. Mice with transverse aortic constriction showed a significant increase in both the mRNA and protein expression of IRBIT. Furthermore, we described the differential expression of IRBIT in human myocardial samples of dilated and ischemic cardiomyopathy. IRBIT cardiac overexpression in mice using an adenoassociated virus (AAV9) at two different time points (neonatal mice, day 4 and adult mice, 3 months) resulted in the development of cardiac hypertrophy with impaired systolic function by four months of age. A decrease in the mRNA levels of the IP3 receptor was also observed in both models. Isolated myocytes from the IRBIT-overexpressing neonatal model showed a significantly decreased Ca²⁺ transient amplitude and slower rise of the global Ca²⁺ transient, without changes in sarcoplasmic reticulum (SR) Ca²⁺ content or spontaneous Ca²⁺ wave frequency. However, the velocity of Ca²⁺ wave propagation was reduced. Moreover, we found that the dyssynchrony index (DI) is significantly increased under IRBIT overexpression. Nuclear Ca²⁺ dynamics were assessed, showing no significant changes, but IRBIT overexpression reduced the number of nuclear envelope invaginations. In addition, reducing IRBIT expression using AAV9-shRNA did not result in any changes in the heart morphometric parameters.

Our study describes for the first time that IRBIT plays a critical role in the pathophysiology of the heart. Our findings demonstrate that IRBIT overexpression disrupts Ca²⁺ signaling, contributing to hypertrophic remodeling and impaired cardiac function. The altered wave propagation, the increase in DI and the decrease of the rate of the Ca²⁺ transient suggests that IRBIT influences Ca²⁺ - induced Ca²⁺ release. This study provides the first evidence linking IRBIT to pathological cardiac remodeling and Ca²⁺ handling dysregulation. Although significant progress has been made, further research is required to better understand the cardiovascular function of IRBIT and its mechanisms.