Filamin-A as a Balance between Erk/Smad Activities During Cardiac Valve Development

Mitral valve prolapse (MVP) affects 2.4% of the population and has poorly understood etiology. Recent genetic studies have begun to unravel the complexities of MVP and through these efforts, mutations in the FLNA (Filamin-A) gene were identified as disease causing. Our in vivo and in vitro studies have validated these genetic findings and have revealed FLNA as a central regulator of valve morphogenesis. The mechanisms by which FLNA mutations result in myxomatous mitral valve disease are currently unknown, but may involve proteins previously associated with mutated regions of the FLNA protein, such as the small GTPase signaling protein, R-Ras. Herein, we report that Filamin-A is required for R-Ras expression and activation of the Ras–Mek–Erk pathway. Loss of the Ras/Erk pathway correlated with hyperactivation of pSmad2/3, increased extracellular matrix (ECM) production and enlarged mitral valves. Analyses of integrin receptors in the mitral valve revealed that Filamin-A was required for β1-integrin expression and provided a potential mechanism for impaired ECM compaction and valve enlargement. Our data support Filamin-A as a protein that regulates the balance between Erk and Smad activation and an inability of Filamin-A deficient valve interstitial cells to effectively remodel the increased ECM production through a β1-integrin mechanism. As a consequence, loss of Filamin-A function results in increased ECM production and generation of a myxomatous phenotype characterized by improperly compacted mitral valve tissue. Anat Rec, 302:117–124, 2019. © 2018 Wiley Periodicals, Inc.