1 | Introduction
Aortic aneurysm is a fatal vascular disease, lack of effective drug treatment, surgery is the only choice. Most aortic aneurysms are asymptomatic before catastrophic complications such as dissection or rupture occur. Although surgery can stabilize large aneurysms, the recurrence rate and mortality rate are still high. Therefore, it is urgent to clarify the molecular mechanism of the occurrence and development of aortic aneurysm and determine the target of intervention.
Smooth muscle cells (SMCs) located in the media of aorta are key to the vascular remodeling of aortic aneurysm. Most SMCs are in the state of differentiation under physiological conditions to maintain the elasticity and tensile strength of the vascular wall. However, SMCs have significant phenotypic plasticity and can quickly adapt to environmental fluctuation signals. The transition of SMCs from a differentiated state to a dedifferentiated state is called phenotypic modulation, characterized by enhanced proliferation, migration, and expression of fibrotic and inflammatory proteins, which is the key cellular basis of aortic aneurysm (Ailawadi et al., 2009). Myocardin (MYOCD) is a differentiation marker of SMCs, and its deletion in SMCs causes severe aortic aneurysm (Tang et al., 2008). Matrix cytokine osteopontin (OPN) is a marker of dedifferentiation of SMCs, and its serum level predicts the progression of aortic aneurysm (Golledge et al., 2007). Collagen type I and type III (Col I , Col III ) correlate well with the size of aortic aneurysm, and they are also SMC dedifferentiation markers (Meng et al., 2014; Wang et al., 2012a) .MMP2 degrades elastin and collagen (mainly Col IV in basal laminae), destroys vascular integrity, critical in angiotensin II-induced aortic aneurysm (Wang et al., 2012b). NF-κB is a transcriptional factor and its activation favors its nuclear translocation to up-regulate the gene expression of MMP2, vascular cell adhesion molecule 1 (VCAM1) and intercellular adhesion molecule 1 (ICAM1) , promotes SMC proliferation, migration, and inflammation (de Winther et al., 2005; Monaco et al., 2004; Tsai et al., 2017). NF-κB is up-regulated in the aortic aneurysm wall, and inhibition of NF-κB decreases the size of aortic aneurysm (Miyake et al., 2007).
Sarco/endoplasmic reticulum calcium ATPase (SERCA) is a key enzyme to maintain calcium homeostasis by taking up Ca2+ from cytosol to sarcoplasmic reticulum and endoplasmic reticulum. SERCA2 is the main subtype of SERCA in the vasculature, and the S -glutathiolation of the amino acid residue Cys674 (C674) is key to increase the activity of SERCA2 under physiological conditions (Adachi et al., 2004; Tong et al., 2008), but this post-translational protein modification is prevented by the irreversible oxidation of C674 thiol in pathological situations characterised by high levels of ROS, such as aging, diabetes mellitus, atherosclerosis, and hypertension (Adachi et al., 2004; Liu et al., 2020; Qin et al., 2013; Ying et al., 2008), which are risk factors for aortic aneurysm. We have reported that the irreversible oxidation of C674 occurred broadly in mouse and human aortic aneurysms (Que et al., 2020). We used heterozygous SERCA2 C674S knock-in (SKI) mice, where half of C674 was substituted by serine, to represent partial irreversible oxidative inactivation of C674 under pathological conditions. The inactivation of C674 by causing the accumulation of intracellular Ca2+ to activate Ca2+-dependent calcineurin that promoted the nuclear translocation of nuclear factor of activated T-lymphocytes (NFAT) and NF-κB, therefore down-regulated the expression of MYOCD, while up-regulated the expression of OPN, MMP2, Col I, Col III, and VCAM1, accelerated SMC proliferation, migration and macrophage adhesion to SMCs, and exacerbated angiotensin II-induced aortic aneurysm (Que et al., 2020). We hypothesize that inactivation of SERCA2 C674 not only activates NFAT/NF-κB, but also inhibits the protective factors that restrict the phenotypic modulation of SMCs, thus accelerating the occurrence of aortic aneurysm. This study was designed to elucidate the downstream targets of C674 in regulating SMC phenotypic modulation and aortic aneurysm.