A BAV is the most common congenital cardiac malformation, which carries an extremely high risk for thoracic aortic aneurysm development. Other conditions which also carry an increased susceptibility for thoracic aortopathy are related to specific genetic conditions, such as Marfan syndrome, Loeys-Dietz syndrome, Ehlers-Danlos syndrome, and familial thoracic aortic aneurysms and dissections. Thoracic aortopathy is however most commonly seen in patients with a TAV, in the form of degenerative thoracic aortopathy. Many studies have recently compared the ascending aortic wall pathology in abovementioned conditions and discovered specific histopathological features in the aortic wall make-up. The disease-of-interest, thoracic aortopathy, is a very heterogeneous disorder with multiple distinct underlying genetic mutations but common clinical phenotypes and histopathologic and molecular findings [
27,
53,
60,
61]. As a thoracic aortic aneurysm does not have any preceding symptoms, the first manifestation often is chest pain due to a ruptured aorta. Management of thoracic aortopathy therefore remains a challenge in elective as well as emergency cases. The identification of common pathological mechanisms would significantly improve the individual cardiovascular risk stratification and thus will result in a major improvement in the field of personalized medicine. The aim of the present review is to discuss the role of TGF-β signaling in both the development of the vascular wall and on how this complex signaling pathway may be involved in thoracic aortic aneurysm formation in TAV and BAV patients.
The TGF-β signaling pathway not only plays a crucial role in vascular development, but also in degenerative thoracic aortic aneurysm formation in the TAV population. Thoracic aortopathy develops as a result of maladaptive remodeling of the vascular extracellular matrix. In normal conditions, a balance exists in vascular remodeling with matrix deposition and degradation resulting in maintenance of the structural integrity of the vascular wall. Recent evidence highlights that a dysregulation of TGF-β signaling disrupts the balance in favor of enhanced proteolysis resulting in pathological remodeling of the vascular extracellular matrix [
62]. A dysregulation of the TGF-β canonical signaling pathway has further been shown to lead to a fragmentation of the elastic lamellae [
63] causing weakening of the aortic architecture and increased susceptibility for aortic dilatation and dissection [
64]. In the bicuspid research field, recent focus has shifted towards a common embryonic origin of both valvular and vascular development. Genetic defects in early development can account for both a deformed aortic valve and defective composition of the ascending aortic wall. Progenitor cells are responsible for region-specific vascular SMCs in the aortic wall. Therefore, defects in the neural crest and second heart field signaling in the BAV patients lead to BAV-related aortopathy in the proximal thoracic aorta that is rarely found in the descending thoracic aorta [
27]. In comparison in Marfan patients, defects are seen in the neural crest, second heart field, and paraxial mesoderm which cause aortopathy in both the ascending and descending aorta. In this review, we discussed the characteristic features of the bicuspid aortic wall, including a thin intimal layer, a phenotypical switch defect resulting in less differentiated SMCs, excessive mucoid extracellular matrix accumulation, and lack of atherosclerosis. Previous studies have shown that the ascending aortic wall in Marfan syndrome and a dissected aortic wall share many features in common with the BAV, in form of a thin intimal layer, immature vascular SMCs, and lack of atherosclerosis [
4,
65] (Table
1). Considering the functions of TGF-β in the development of the intimal layer and normal vascular remodeling and phenotypical switch of vascular SMCs, it is plausible that a defect of this signaling pathway is responsible for the aortic pathology encountered in patients with a BAV. Expression of TGF-β and downstream signalers has been studied in several ways. Our group investigated the expression of TGF-β and pSMAD2 in the ascending aortic wall in the non- and dilated BAV and TAV patients. We found that the intima lacked TGF-β and pSMAD expression and the medial expression was significantly lower in the BAV as compared to the TAV dilated specimen. Our findings are in line with many other studies which found a decreased expression of TGF-β in the BAV aorta as compared to the TAV [
63,
66], also investigating the canonical SMAD-mediated pathway. In murine aneurysmal studies, an increased TGF-β signal through the non-canonical pathway has been described leading to extracellular matrix degradation [
67,
68]. Unfortunately, special attention to the intimal layer has not been given by any other study. Besides differences in expression between valve morphologies, we were able to identify non-dilated BAVs with increased susceptibility for future complications on basis of the medial expression of TGF-β, pSMAD2, and MMP9 [
25]. Ikonomidis et al. also suggested in their study that a unique profile of plasma MMPs, tissue inhibitors of MMPs, and micro ribonucleic acids (microRNAs) could possibly predict the increased risk for thoracic aortopathy in BAV and TAV using a plasma multianalyte regression strategy [
24]. Expression of TGF-β and markers of the non- and canonical pathway should therefore be further investigated as a marker to distinguish BAV patients with an increased risk for future aortic complications. Differences in the amount of TGF-β in the aortic wall have further been reported to correlate well with the levels of shear stress on the wall [
69]; therefore, in future studies, the role of shear stress on the aortic wall should also be considered. It is interesting to note that a specific group of animals like reptiles normally present with bicuspid semilunar valves. Evidently, these have a different physiology, being “cold-blooded” as well as anatomy, as they present two aortas [
70]. Nevertheless, here the bicuspid valves function efficiently, even until advanced ages without signs of thoracic aortopathy. It would therefore be interesting to study the TGF-β expression in reptiles where BAV prevails under normal conditions.