Role of Vascular Smooth Muscle Bcl11b in Arterial Stiffness

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Arterial stiffening (AS), the progressive loss of compliance in elastic arteries, increases the risk of developing cardiovascular diseases (CVDs). However, cellular and molecular mechanisms of AS are poorly understood. Determining these mechanisms may lead to innovative strategies that can slow or reverse AS and thus decrease the risk of developing CVD. A gene desert locus on chromosome 14, downstream of the gene Bcl11b, has recently been shown to harbor single nucleotide polymorphisms (SNPs) with a highly significant association with AS. We were the first to show knocking out Bcl11b in mice (BSMKO), both globally and specifically in vascular smooth muscle (VSM), caused elevated AS, and increased the incidence of angiotensin II-induced aortic aneurysms. In addition, we showed Bcl11b expression is downregulated in aortas of two models of AS (mice fed high fat, high sucrose (HFHS) diet and aged mice), and Bcl11b transcriptionally regulates contractile protein expression, including smooth muscle myosin (MYH11) and smooth muscle α-actin (α-SMA). Taken together, we hypothesize that SNP variants in the 3'-Bcl11b gene desert region regulate and suppress Bcl11b expression, playing a causative role in the pathogenesis of AS. We hypothesize that stiff aortas have decreased Bcl11b expression, stimulating alterations in VSM contractile phenotype and/or extracellular matrix (ECM) remodeling, or a combination of these factors, thereby impairing structural and functional integrity of the aorta. In Aim 1, we will use biaxial mechanical testing on wild type and BSMKO aortas together with a microstructurally-motivated constitutive model to dissect the contribution of smooth muscle cells, elastin and collagen to aortic wall stiffness, at baseline and after contractile agonist stimulation. We will then correlate the results of biaxial tests to molecular expressions of VSM contractile, actin polymerization, and focal adhesion proteins known to contribute to VSM tone and stiffness. In Aim 2, we will use chromatin immunoprecipitation (ChIP)-sequencing on VSM homogenates to identify Bcl11b VSM-specific DNA binding sites. We will also test the hypothesis that Bcl11b epigenetically regulates MYH11 and α-SMA gene expression by recruiting the histone deacetylase sirtuin-1 at G/C motifs in their gene promoters. In Aim 3, we will overexpress Bcl11b using transgenic mice or, for a more translational approach, by administering a AAV2/5 vector, to determine if increasing Bcl11b rescues impaired VSM-specific molecular mechanisms of VSM contraction and/or VSM cell-extracellular matrix interaction (e.g., contractile proteins, focal adhesion complexes, actin polymerization) in obese and aged mice. Definitive decreases in VSM stiffness and pulse wave velocity, the in vivo index of AS, would establish that targeting Bcl11b is a viable strategy for ameliorating AS and thus preventing CVD.