Expression of sumoylation deficient Nkx2.5 mutant in Nkx2.5 haploinsufficient mice leads to congenital heart defects

Nkx2.5 is a cardiac specific homeobox gene critical for normal heart development. We previously identified Nkx2.5 as a target of sumoylation, a posttranslational modification implicated in a variety of cellular activities. Sumoylation enhanced Nkx2.5 activity via covalent attachment to the lysine residue 51, the primary SUMO acceptor site. However, how sumoylation regulates the activity of Nkx2.5 in vivo remains unknown. We generated transgenic mice overexpressing sumoylation deficient mutant K51R (conversion of lysine 51 to arginine) specifically in mouse hearts under the control of cardiac α-myosin heavy chain (α-MHC) promoter (K51R-Tg). Expression of the Nkx2.5 mutant transgene in the wild type murine hearts did not result in any overt cardiac phenotype. However, in the presence of Nkx2.5 haploinsufficiency, cardiomyocyte-specific expression of the Nkx2.5 K51R mutant led to congenital heart diseases (CHDs), accompanied with decreased cardiomyocyte proliferation. Also, a number of human CHDs-associated Nkx2.5 mutants exhibited aberrant sumoylation. Our work demonstrates that altered sumoylation status may underlie the development of human CHDs associated with Nkx2.5 mutants.     

PIASx结构与功能的研究进展

PIAS（protein inhibitor of activated STAT）蛋白家族能够与许多蛋白质发生相互作用,其中大部分为转录因子。PIASx是4个组成成员中的一种,其包括两个亚型。PIASx通过与不同种类的蛋白相互作用,影响它们的活性和功能。PIASx蛋白的调控机制主要有两种：一种是通过其自身所具有的SUMO（smallubiquitin-related modifiers）E3连接酶活性,促进对一些转录因子、转录辅因子的SUMO化修饰,从而调控它们的转录活性;另一种是作为构架蛋白,通过与雄激素受体的作用参与雄激素介导的基因转录调节。PIAS蛋白的上述两种作用机制并不是完全互相排斥的,这体现了PIASx蛋白功能的特异性和复杂性。     

Myocardin sumoylation transactivates cardiogenic genes in pluripotent 10T1/2 fibroblasts

Myocardin, a serum response factor (SRF)-dependent cofactor, is a potent activator of smooth muscle gene activity but a poor activator of cardiogenic genes in pluripotent 10T1/2 fibroblasts. Posttranslational modification of GATA4, another myocardin cofactor, by sumoylation strongly activated cardiogenic gene activity. Here, we found that myocardin's activity was strongly enhanced by SUMO-1 via modification of a lysine residue primarily located at position 445 and that the conversion of this residue to arginine (K445R) impaired myocardin transactivation. PIAS1 was involved in governing myocardin activity via its E3 ligase activity that stimulated myocardin sumoylation on an atypical sumoylation site(s) and by its physical association with myocardin. Myocardin initiated the expression of cardiac muscle-specified genes, such as those encoding cardiac alpha-actin and alpha-myosin heavy chain, in an SRF-dependent manner in 10T1/2 fibroblasts, but only in the presence of coexpressed SUMO-1/PIAS1. Thus, SUMO modification acted as a molecular switch to promote myocardin's role in cardiogenic gene expression.