Thoracic aortic aneurysms (TAA) are permanent and localized dilations of the aorta that predispose patients to a life-threatening risk of aortic dissection or rupture. The identification of pathogenic variants that cause hereditary forms of TAA has delineated fundamental molecular processes required to maintain aortic homeostasis. Vascular smooth muscle cells (VSMCs) elaborate and remodel the extracellular matrix (ECM) in response to mechanical and biochemical cues from their environment. Causal variants for hereditary forms of aneurysm compromise the function of gene products involved in the transmission or interpretation of these signals, initiating processes that eventually lead to degeneration and mechanical failure of the vessel. These include mutations that interfere with transduction of stimuli from the matrix to the actin–myosin cytoskeleton through integrins, and those that impair signaling pathways activated by transforming growth factor-β (TGF-β). In this review, we summarize the features of the healthy aortic wall, the major pathways involved in the modulation of VSMC phenotypes, and the basic molecular functions impaired by TAA-associated mutations. We also discuss how the heterogeneity and balance of adaptive and maladaptive responses to the initial genetic insult might contribute to disease.

Heterozygous, loss of function mutations in positive regulators of the Transforming Growth Factor-β (TGF-β) pathway cause hereditary forms of thoracic aortic aneurysm. It is unclear whether and how the initial signaling deficiency triggers secondary signaling upregulation in the remaining functional branches of the pathway, and if this contributes to maladaptive vascular remodeling. To examine this process in a mouse model in which time-controlled, partial interference with postnatal TGF-β signaling in vascular smooth muscle cells (VSMCs) could be assessed, we used a VSMC-specific tamoxifen-inducible system, and a conditional allele, to inactivate Smad3 at 6 weeks of age, after completion of perinatal aortic development. This intervention induced dilation and histological abnormalities in the aortic root, with minor involvement of the ascending aorta. To analyze early and late events associated with disease progression, we performed a comparative single cell transcriptomic analysis at 10- and 18-weeks post-deletion, when aortic dilation is undetectable and moderate, respectively. At the early time-point, Smad3-inactivation resulted in a broad reduction in the expression of extracellular matrix components and critical components of focal adhesions, including integrins and anchoring proteins, which was reflected histologically by loss of connections between VSMCs and elastic lamellae. At the later time point, however, expression of several transcripts belonging to the same functional categories was normalized or even upregulated; this occurred in association with upregulation of transcripts coding for TGF-β ligands, and persistent downregulation of negative regulators of the pathway. To interrogate how VSMC heterogeneity may influence this transition, we examined transcriptional changes in each of the four VSMC subclusters identified, regardless of genotype, as partly reflecting the proximal-to-distal anatomic location based on in situ RNA hybridization. The response to Smad3-deficiency varied depending on subset, and VSMC subsets over-represented in the aortic root, the site most vulnerable to dilation, most prominently upregulated TGF-β ligands and pro-pathogenic factors such as thrombospondin-1, angiotensin converting enzyme, and pro-inflammatory mediators. These data suggest that Smad3 is required for maintenance of focal adhesions, and that loss of contacts with the extracellular matrix has consequences specific to each VSMC subset, possibly contributing to the regional susceptibility to dilation in the aorta.

Angiotensin II (Ang II) type 1 receptor (AT1R) signaling controls both physiological and pathogenetic responses in the vasculature. In mouse models of Loeys-Dietz syndrome (LDS), a hereditary disorder characterized by aggressive aortic aneurysms, treatment with angiotensin receptor blockers (ARBs) prevents aortic root dilation and associated histological alterations. In this study we use germline and conditional genetic inactivation of Agtr1a (coding for the AT1a receptor) to assess the effect of systemic and localized AT1R signaling attenuation on aortic disease in a mouse model of LDS (Tgfbr1M318R/+). Aortic diameters and histological features were examined in control and Tgfbr1M318R/+ mice with either germline or Mef2CSHF-Cre mediated genetic inactivation of Agtr1a, the latter resulting in deletion in second heart field (SHF)-derived lineages in the aortic root and proximal aorta. Both systemic and regional AT1R signaling attenuation resulted in reduction of diameters and improvement of tissue morphology in the aortic root of LDS mice; these outcomes were associated with reduced levels of Smad2/3 and ERK phosphorylation, signaling events previously linked to aortic disease in LDS. However, regional AT1a inactivation in SHF-derived lineages resulted in a more modest reduction in aortic diameters relative to the more complete effect of germline Agtr1a deletion, which was also associated with lower blood pressure. Our findings suggest that the therapeutic effects of AT1R antagonisms in preclinical models of aortic disease depend on both regional and systemic factors and suggest that combinatorial approaches targeting both processes may prove beneficial for aneurysm mitigation.