The high blood pressure associated with the Eln+/–
genotype is consistent with systemic hypertension that is a frequent complication of SVAS and WS, diseases of elastin haploinsufficiency in humans (24
). MAP is 30–40% higher in Eln+/–
mice than control animals, a trait that shows complete penetrance in the Eln+/–
genotype. In humans, the incidence of hypertension is less than the complete penetrance we observe in mice. In WS, for example, approximately 80% of affected individuals have clinically apparent SVAS (27
) and 40–60% have documented arterial hypertension (24
). For reasons not yet understood, the incidence of hypertension in isolated SVAS is much lower. Of the two forms of SVAS, the hemizygosity of the elastin gene in WS is genetically most like the Eln+/–
The association of hypertension with elastin haploinsufficiency strongly suggests that vessel wall proteins, particularly elastin, should be considered as causal genes for essential hypertension. Any factor that reduces elastin protein concentration or alters vessel compliance during a critical window of vessel wall formation could have a modifying effect on the progression of, or susceptibility to, hypertension or other vascular diseases. This could include mutations within any of the other genes that participate in elastic fiber assembly or perhaps polymorphisms within the elastin gene itself. Secondary factors could also impact elastin deposition and vascular function. There is evidence, for example, that people who had low birth weight tend to have higher blood pressure in later life. The authors of these studies argue that fetuses whose growth is impaired synthesize less elastin in the aorta and large arteries and that this deficiency leads to changes that could predispose an individual to higher blood pressure (29
). There is also evidence that maternal and postnatal vitamin D ingestion lowers aortic elastin content (vitamin D is known to downregulate elastin production in cultured cells) and alters vascular compliance similar to that which is seen in our mice (31
). Taken together, these studies suggest that environmental or nutritional factors could impact directly vascular development or might act as modifiers on phenotypes involving elastin gene mutations or polymorphisms.
In a previous study, we noted thinner elastic lamellae and decreased elastin mRNA levels in the arterial wall of Eln+/–
). Quantitation of desmosine levels as an index of elastin protein confirmed what was predicted by these earlier findings, namely, that elastin protein is significantly reduced in the Eln+/–
animals. When normalized to total protein, our findings show that elastin levels are approximately 35% lower in the ascending, abdominal, and carotid arteries from Eln+/–
mice. The ratio of collagen to total protein was identical in both genotypes, indicating that relative collagen synthesis and accumulation is unchanged.
The decreased elastin to collagen ratio suggests that arteries in the Eln+/–
mouse should be stiffer than their wild-type counterparts. This was confirmed through mechanical studies that documented a difference in vessel distensibility between the two genotypes (Figures and ). In our initial studies, we reported that the aorta from Eln+/+
mice had similar extensibilities at a presumed physiologic pressure of 100 mmHg (15
). The dramatic difference in blood pressure between the two genotypes, however, alters our interpretation of the earlier physiological findings. At their higher physiological pressure (and above), Eln+/–
vessels are stiffer and have a higher circumferential wall stress, circumferential wall strain, and incremental elasticity modulus than vessels from Eln+/+
animals. At lower pressures, however, Eln+/–
vessels are more elastic and show greater dilation than do arteries from wild-type mice. Similar changes in vessel compliance have been reported in humans with WS. Using noninvasive ultrasound, Salaymeh and Banerjee found that children with WS have a stiffer aorta and a less-compliant systemic arterial bed (28
). Interestingly, a similar study found that the compliance of the carotid artery is not modified in WS, even though increased intima-media thickness and lower arterial stiffness were consistent features (32
). Despite the structural alterations in the Eln+/–
vessel wall, no significant change in the functional potential of the vascular cells was detected. Vessels from Eln+/–
mice responded appropriately and to the same extent as Eln+/+
vessels to both vasodilators and vasoconstrictors.
A major difference between the human pathology associated with SVAS and that seen in Eln+/– mice is that humans, but not mice, develop severe localized aortic occlusion due to subendothelial SMC proliferation. A possible explanation for this difference may relate to the higher vascular wall stress in humans compared with mice, due to their larger size. Higher circumferential wall stress could make vessels more prone to pressure-related damage, leading to stenosis. It is important to note that at their higher physiological pressures, Eln+/– vessels are working close to their maximum strain, suggesting that these animals may be more prone to develop hypertensive cardiovascular pathologies when stressed, since their vessels have a lower potential for distension if the blood pressure increases.
The smaller ID of the large elastic arteries coupled with increased arterial stiffness and elevated cardiac output is predicted to be disadvantageous to cardiac function. Under normal circumstances, this should lead to cardiac hypertrophy, circulatory dysfunction, and possibly death. When we analyzed the hearts of 6-month-old Eln+/–
animals, we observed that the total heart weight as well as the LV weight was increased by 15% and 13%, respectively, over wild-type controls. It is therefore somewhat of a paradox that the Eln+/–
animals have a normal life span, exhibit no overt signs of degenerative cardiovascular disease, and show none of the adverse effects observed with other animal models of induced or spontaneous hypertension (33
). One explanation for the normal characteristics of the Eln+/–
mouse is that the elevated blood pressure is an important adaptive response for maintaining cardiovascular function. Figure shows that at the higher systemic physiological pressure of the Eln+/–
mouse, the effective working ID of the ascending aorta, is comparable to that of the wild-type animal. Because the functional demands of the organism require that normal perfusion be preserved, the adjustment of vessel ID through an increase in blood pressure could be a necessary adaptation to maintain vessel patency appropriate to accommodate cardiac output and perfusion.
Figure 5 Vascular IDs are similar at physiological blood pressures of Eln+/– and Eln+/+ genotypes. Comparison of ascending aorta IDs of Eln+/+ and Eln+/– mice at their respective physiological pressures (more ...)
While we cannot completely exclude structural alterations in the microvasculature as being responsible for hypertension in the Eln+/– mice, pharmacological studies with vasoactive agents suggested that vascular dysfunction secondary to hypertrophy of the resistance vasculature is not the main cause of the hypertensive phenotype. Compared with wild-type controls, Eln+/– mice displayed an equivalent increase in blood pressure in response to a maximal dose of Ang II and an equivalent decrease in blood pressure after infusion of candesartan (Figure ). In both cases, however, the difference in MAP between wild-type and Eln+/– animals persisted at each end point.
The change in blood pressure seen in the Eln+/–
but not Eln+/+
animals in response to saralasin was of particular interest because in both human and animal studies, saralasin infusion reliably identifies an Ang pressor response associated with high renin forms of hypertension (35
). Our finding of elevated renin activity in plasma of Eln+/–
mice is consistent with high renin levels predicted by saralasin inhibition. The equivalent effects of saralasin, a nonselective Ang II receptor antagonist, and candesartan, an antagonist specific for the Ang I receptor, indicates that blood pressure elevation is occurring through the Ang I receptor. Taken together, the Ang II receptor inhibitor studies and high plasma renin levels suggest a role for the kidney and the renin-angiotensin system (RAS) in maintaining high blood pressure in Eln+/–
mice. Increased cardiac stroke volume and cardiac output in Eln+/–
animals are also consistent with the activation of the RAS and indicate expansion of the intravascular volume and increased contractility of the myocardium. It is interesting that aldosterone levels are equivalent in the two genotypes, confirming that the mechanism underlying the hypertension in the Eln+/–
mouse is not permanently mediated by actions of this important hormone.
In a previous study we speculated that the increased number of lamellar units in the arterial wall of SVAS individuals and in Eln+/–
mice arose during vascular development in response to altered wall stress (15
). A role for hemodynamics in vessel wall development (38
) and in modulating elastin production (40
) has been suggested from numerous studies of vascular remodeling in response to altered pressure and flow. In the developing chick coronary artery, for example, SMC recruitment from undifferentiated mesenchyme does not occur until the connection to the aorta is made and actual blood flow through these vessels has begun (42
). When the vessel wall is forming, SMC differentiation, lamellar number, and elastin content coordinately increase with the gradual rise in blood pressure until the proper number of lamellar units are organized (43
). The relatively constant tension per lamellar unit and their uniformity of composition, regardless of species, indicate that the proportion of collagen, elastin, and SMCs in the media is optimal for the stresses to which the aorta is subjected (1
). This is why the increased number of elastic lamellae in the arterial wall of Eln+/–
mice is unique.
Many studies in mature organisms have shown that the response of fully developed blood vessels to hemodynamic stress is clearly different from what we have documented in Eln+/–
mice. In spontaneous or essential hypertension in humans and in experimental hypertension in animals (33
), vessel walls become thickened through cellular maturation and increased matrix deposition, but there is no change in lamellar number (38
). The reason that fetal and mature vascular wall cells respond differently to hemodynamic stress may reflect the effects of the extensive matrix found in older vessels. Because there is more elastin in the mature vessel wall, the ECM plays a greater role in accommodating wall stress than in earlier developmental stages. Hence, the most efficient adaptive mechanism for a mature vessel to use to deal with changes in pressure is that of altering the amount of the load-bearing ECM. In elastin insufficiency, however, SMCs cannot make sufficient elastin, and the increased number of smooth muscle layers (i.e., lamellar units) may be an attempt by the cells to normalize wall stress. We know that the increase in lamellar number is established at the time of birth (15
) and that elevated blood pressure can be documented in neonatal Eln+/–
mice as early as pressure measurements can be obtained, suggesting that alterations in Eln+/–
vessel wall structure and hemodynamics occur early in formation of the arterial wall. The changes found in the Eln+/–
arterial wall suggest that presumptive vascular SMCs are capable of altering vessel wall structure by sensing and responding to wall stress and that mechanical forces play an important role in determining lamellar number. Elucidation of cellular mechanisms for sensing mechanical signals will have important implications for understanding vascular development generally as well as furthering our understanding of vascular pathology in elastin-related human genetic diseases such as SVAS, WS, and hypertension in general.