Abstract
Background
In 2010, we developed cardiac direct reprogramming, in which fibroblasts can be directly converted into cardiomyocytes (induced cardiomyocyte like cells; iCMs) by overexpression of myocardial specific transcription factors (Ieda et al., Cell 2010). Furthermore, we succeeded in the cardiac regeneration of myocardial infarction model mice (Inagawa et al., Circ Res 2012). However, in order to actualize cardiac regenerative medicine, it is necessary to improve reprogramming efficiency and clarify the molecular mechanism. Recently, it was reported that cell differentiation was controlled by the stiffness of the scaffold, which suggested that mechanical stress like stiffness works on gene regulation (mechanotransduction). However, mechanotransduction in cell reprogramming has not yet been elucidated. In this study, we aimed at revealing the mechanotransduction in cardiac direct reprograming and improving the induction efficiency of iCMs.
Methods and Results
Previous studies suggested that the efficiency of cardiac direct reprogramming in vivo was better than in vitro (Ieda et al., Cell 2010, Inagawa et al., Circ Res 2012). So, we reproduced the stiffness of myocardial tissue by using a hydrogel, because the stiffness of myocardial tissue (10 kPa) is far softer than cell culture dishes (10^6 kPa). As a result of cardiac direct reprogramming on the hydrogel, the number of beating iCMs (functional iCMs) increased three-fold compared with that of cell culture dishes. To investigate the molecular mechanism of this improvement, we carried out microarray test and found that the hard scaffold upregulated YAP/TAZ-related genes. YAP/TAZ is known to shuttle from the cytoplasm to the nucleus by the hard scaffold, which increases cell proliferation and migration. So, it can be thought that YAP/TAZ negatively regulated cardiac direct reprogramming. Then, we knocked down YAP/TAZ during cardiac direct reprograming, and found that the number of beating iCMs was increased by suppression of YAP/TAZ. These results suggested that YAP/TAZ promoted the trait of fibroblasts, and suppressed the conversion into iCMs. Conversely, suppressing YAP/TAZ promotes cardiac direct reprogramming.
Conclusions
We succeeded in improving the efficiency of cardiac direct reprogramming by mechanotransduction, and found that YAP/TAZ negatively regulates cardiac direct reprogramming. This is the first study that revealed mecanotransduction in cell reprogramming.
