The molecular pathways regulating valve development are only partially understood. Recent studies indicate that dysregulation of
mitogen-activated protein kinase (MAPK) signaling might play a major role in the pathogenesis of congenital valvular malformations, and, in this study, we explored the role of
extracellular signal-regulated kinase (ERK) 1/2 activation in valve primordia expressing the
Noonan syndrome mutation Q79R-Shp2.
Noonan syndrome is an autosomal dominant disease characterized by dysmorphic features and cardiac abnormalities, with frequent
pulmonic stenosis. The Q79R mutation of PTPN11 previously identified in
Noonan syndrome families results in a gain-of-function of the encoded
protein tyrosine phosphatase Shp2. We compared the effects of wild-type Shp2 and Q79R-Shp2 on endocardial cushion development. Atrioventricular and outflow tract endocardial cushions were excised from chick embryos, infected with wild-type Shp2 or Q79R-Shp2 adenovirus and embedded in a gel matrix. Q79R-Shp2, but not wild-type-Shp2, expression resulted in increased outgrowth of cells into the gel. The dependence of the Q79R-Shp2 effect on ERK1/2 and
p38 MAPK signaling was then determined. The
MAPK/ERK kinase (MEK)-1 inhibitor
U0126, but not the p38-MAPK pathway inhibitor
SB203580, abolished the effect of Q79R-Shp2 on cushion outgrowth.
Coinfection with Q79R-Shp2 and dominant negative MEK-1 prevented enhanced endocardial cushion outgrowth, whereas expression of constitutively active MEK-1 mimicked the effect of Q79R-Shp2. Furthermore, dissociated cushion cells displayed increased
5-bromodeoxyuridine incorporation when infected with Q79R-Shp2 but not with wild-type Shp2. This promitotic effect was eliminated by
U0126. Our results demonstrate that ERK1/2 activation is both necessary and sufficient to mediate the hyperproliferative effect of a gain-of-function mutation of Shp2 on mesenchymal cells in valve primordia.