Role of Periostin in Cardiac Valve Development.

Although periostin plays a significant role in adult cardiac remodeling diseases, the focus of this review is on periostin as a valvulogenic gene. Periostin is expressed throughout valvular development, initially being expressed in endocardial endothelial cells that have been activated to transform into prevalvular mesenchyme termed "cushion tissues" that sustain expression of periostin throughout their morphogenesis into mature (compacted) valve leaflets. The phenotype of periostin null indicates that periostin is not required for endocardial transformation nor the proliferation of its mesenchymal progeny but rather promotes cellular behaviors that promote migration, survival (anti-apoptotic), differentiation into fibroblastic lineages, collagen secretion and postnatal remodeling/maturation. These morphogenetic activities are promoted or coordinated by periostin signaling through integrin receptors activating downstream kinases in cushion cells that activate hyaluronan synthetase II (Akt/PI3K), collagen synthesis (Erk/MapK) and changes in cytoskeletal organization (Pak1) which regulate postnatal remodeling of cells and associated collagenous matrix into a trilaminar (zonal) histoarchitecture. Pak1 binding to filamin A is proposed as one mechanism by which periostin supports remodeling. The failure to properly remodel cushions sets up a trajectory of degenerative (myxomatous-like) changes that over time reduce biomechanical properties and increase chances for prolapse, regurgitation or calcification of the leaflets. Included in the review are considerations of lineage diversity and the role of periostin as a determinant of mesenchymal cell fate.