Fibulin-4 deficiency differentially affects cytoskeleton structure and dynamics as well as TGFβ signaling.

Fibulin-4 is an extracellular matrix (ECM) protein essential for elastogenesis and mutations in this protein lead to aneurysm formation. In this study, we isolated vascular smooth muscle cells (VSMCs) from mice with reduced fibulin-4 protein expression (Fibulin-4^R/R) and from mice with a smooth muscle cell specific deletion of the Fibulin-4 gene (Fibulin-4^f/-/SM22Cre⁺). We subsequently analyzed and compared the molecular consequences of reduced Fibulin-4 expression versus total ablation of Fibulin-4 expression with regard to effects on the SMC specific contractile machinery, cellular migration and TGFβ signaling. Analysis of the cytoskeleton showed that while Fibulin-4^f/-/SM22Cre⁺ VSMCs lack smooth muscle actin (SMA) fibers, Fibulin-4^R/R VSMCs were able to form SMA fibers. Furthermore, Fibulin-4^f/-/SM22Cre⁺ VSMCs showed a decreased pCofilin to Cofilin ratio, suggesting increased actin depolymerization, while Fibulin-4^R/R VSMCs did not display this decrease. Yet, both Fibulin-4 mutant VSMCs showed decreased migration. We found increased activation of TGFβ signaling in Fibulin-4^R/R VSMCs. However, TGFβ signaling was not increased in Fibulin-4^f/-/SM22Cre⁺ VSMCs. From these results we conclude that both reduction and absence of Fibulin-4 leads to structural and functional impairment of the SMA cytoskeleton. However, while reduced levels of Fibulin-4 result in increased TGFβ activation, complete absence of Fibulin-4 does not result in increased TGFβ activation. Since both mouse models show thoracic aortic aneurysm formation, we conclude that not only hampered TGFβ signaling, but also SMA cytoskeleton dynamics play an important role in aortic aneurysmal disease.

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