Comparison of 10 murine models reveals a distinct biomechanical phenotype in thoracic aortic aneurysms

Abstract

Thoracic aortic aneurysms are life-threatening lesions that afflict young and old individuals alike. They frequently associate with genetic mutations and are characterized by reduced elastic fibre integrity, dysfunctional smooth muscle cells, improperly remodelled collagen and pooled mucoid material. There is a pressing need to understand better the compromised structural integrity of the aorta that results from these genetic mutations and renders the wall vulnerable to dilatation, dissection or rupture. In this paper, we compare the biaxial mechanical properties of the ascending aorta from 10 murine models: wild-type controls, acute elastase-treated, and eight models with genetic mutations affecting extracellular matrix proteins, transmembrane receptors, cytoskeletal proteins, or intracellular signalling molecules. Collectively, our data for these diverse mouse models suggest that reduced mechanical functionality, as indicated by a decreased elastic energy storage capability or reduced distensibility, does not predispose to aneurysms. Rather, despite normal or lower than normal circumferential and axial wall stresses, it appears that intramural cells in the ascending aorta of mice prone to aneurysms are unable to maintain or restore the intrinsic circumferential material stiffness, which may render the wall biomechanically vulnerable to continued dilatation and possible rupture. This finding is consistent with an underlying dysfunctional mechanosensing or mechanoregulation of the extracellular matrix, which normally endows the wall with both appropriate compliance and sufficient strength.

Keywords: 5; actomyosin; angiotensin II; fibrillin-1; fibulin-4; transforming growth factor-β; tuberous sclerosis complex-1.

Figure 1.

Biaxial mechanical data (a–d), gross anatomy (e–n) and VVG-stained cross sections (o–x) from control, elastase-exposed controls and mutant/treated ascending aortas suggest a correlation between the degree and location of elastic fibre loss (star symbols, w,x) or fragmentation (white arrows, r,t,u,v), the decrease in circumferential distension (a,c) and axial extension (b,d) and the likelihood of aneurysm development/progression (e–n). White arrowheads in (m,n) indicate marked aneurysms. The lower-case letter h in (k) shows an intramural haematoma. Notice that elastase-exposed vessels do not mimic per se any in vivo condition, but they reveal the effect of a complete loss of competent elastic fibres (p) without concomitant matrix remodelling. As such, control and elastase-exposed control vessels define bounds for the stress–stretch responses in the circumferential direction (c). Note: elastase-exposed control, Fbn1^mgR/mgR and Fbln4^SMKO ascending aortas were perfusion-fixed (luminal pressure <100 mm Hg for the elastase-treated vessels, <20 mm Hg for the aneurysmal vessels) at their in vivo axial length to avoid collapse due to the large diameter. Other vessels were fixed in the unloaded state.

Figure 2.

Mechanical metrics of stretch (a,b), wall stress (c,d), stiffness (e,f), energy (g) and distensibility (h) are plotted as a function of D_norm (i.e. the outer diameter at systole normalized to the average diameter at systole for the control group) for all specimens. Note that an aneurysm is defined by D_norm > 1.5. The circumferential (a) and axial (b) stretch at systole progressively decrease as the ascending aorta dilates. Whereas material circumferential stiffness (e) increases with dilatation, the distensibility, a common metric of structural stiffness, does not correlate with the propensity towards aneurysm formation (h). Overall, vessels from mutant mice reach normal or lower-than-normal values of both circumferential (c) and axial (d) stress as well as axial stiffness (f). Finally, all of the genetic mutations and pharmacological treatments considered in this study decrease the elastic energy stored at systole, thus impairing the ability of the aorta to perform a key mechanical function (g). Notice that data from elastase-exposed controls (thick white diamonds embedded within the grey shaded area) serve as good comparisons since there is no associated cell-mediated remodelling following the structural and material insult; these data were not included in the regressions, however.