Arterial stiffening is a hallmark of aging and cardiovascular disease. While it is well established that vascular smooth muscle cells (SMCs) contribute to arterial stiffness by synthesizing and remodeling the arterial extracellular matrix, the direct contributions of SMC contractility and mechanosensors to arterial stiffness, and particularly the arterial response to pressure, remain less well understood despite being a long-standing question of biomedical importance. Here, we have examined this issue by combining the use of pressure myography of intact carotid arteries, pharmacologic inhibition of contractility, and genetic deletion of SMC focal adhesion kinase (FAK). Biaxial inflation-extension tests performed at physiological pressures showed that acute inhibition of cell contractility with blebbistatin or EGTA altered vessel geometry and preferentially reduced circumferential, as opposed to axial, arterial stiffness in wild-type mice. Similarly, genetic deletion of SMC FAK, which attenuated arterial contraction to KCl, reduced vessel wall thickness and circumferential arterial stiffness in response to pressure while having minimal effect on axial mechanics. Moreover, these effects of FAK deletion were lost by treating arteries with blebbistatin or by inhibiting myosin light-chain kinase. The expression of arterial fibrillar collagens, the integrity of arterial elastin, or markers of SMC differentiation were not affected by the deletion of SMC FAK. Our results connect cell contractility and SMC FAK to the regulation of arterial wall thickness and directionally specific arterial stiffening.
Emilia Roberts, Tina Xu, and Richard K Assoian
Ryan von Kleeck, Paola Castagnino, and Richard K Assoian
Hutchinson–Guilford Progeria syndrome (HGPS) is a rare genetic disease of premature aging and early death due to cardiovascular disease. The arteries of HGPS children and mice are pathologically stiff, and HGPS mice also display reduced arterial contractility. We recently showed that reduced contractility is an early event in HGPS and linked to an aberrantly low expression of smooth muscle myosin heavy chain (smMHC). Here, we have explored the basis for reduced smMHC abundance and asked whether it is a direct effect of progerin expression or a longer-term adaptive response. Myh11, the gene encoding for smMHC, is regulated by myocardin-related transcription factors (MRTFs), and we show that HGPS aortas have a reduced MRTF signature. Additionally, smooth muscle cells (SMCs) isolated from HGPS mice display reduced MRTF nuclear localization. Acute progerin expression in WT SMCs phenocopied both the decrease in MRTF nuclear localization and expression of Myh11 seen in HGPS. Interestingly, RNA-mediated depletion of MRTF-A in WT SMCs reproduced the preferential inhibitory effect of progerin on Myh11 mRNA relative to Acta2 mRNA. Our results show that progerin expression acutely disrupts MRTF localization to the nucleus and suggest that the consequent decrease in nuclear coactivator activity can help to explain the reduction in smMHC abundance and SMC contractility seen in HGPS.