title: The ras-GTPase activity of neurofibromin restrains ERK-dependent FGFR signaling during endochondral bone formation.
abstract: The severe defects in growth plate development caused by chondrocyte extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) gain or loss-of-function suggest that tight spatial and temporal regulation of mitogen-activated protein kinase signaling is necessary to achieve harmonious growth plate elongation and structure. We provide here evidence that neurofibromin, via its Ras guanosine triphosphatase -activating activity, controls ERK1/2-dependent fibroblast growth factor receptor (FGFR) signaling in chondrocytes. We show first that neurofibromin is expressed in FGFR-positive prehypertrophic and hypertrophic chondrocytes during growth plate endochondral ossification. Using mice lacking neurofibromin 1 (Nf1) in type II collagen-expressing cells, (Nf1col2(-/-) mutant mice), we then show that lack of neurofibromin in post-mitotic chondrocytes triggers a number of phenotypes reminiscent of the ones observed in mice characterized by FGFR gain-of-function mutations. Those include dwarfism, constitutive ERK1/2 activation, strongly reduced Ihh expression and decreased chondrocyte proliferation and maturation, increased chondrocytic expression of Rankl, matrix metalloproteinase 9 (Mmp9) and Mmp13 and enhanced growth plate osteoclastogenesis, as well as increased sensitivity to caspase-9 mediated apoptosis. Using wildtype (WT) and Nf1(-/-) chondrocyte cultures in vitro, we show that FGF2 pulse-stimulation triggers rapid ERK1/2 phosphorylation in both genotypes, but that return to the basal level is delayed in Nf1(-/-) chondrocytes. Importantly, in vivo ERK1/2 inhibition by daily injection of a recombinant form of C-type natriuretic peptide to post-natal pups for 18 days was able to correct the short stature of Nf1col2(-/-) mice. Together, these results underscore the requirement of neurofibromin and ERK1/2 for normal endochondral bone formation and support the notion that neurofibromin, by restraining RAS-ERK1/2 signaling, is a negative regulator of FGFR signaling in differentiating chondrocytes.
authors.affiliation: Department of Medicine, Vanderbilt University Medical Center, Nashville, TN 37232, USA.
authors.forename: Koichiro
authors.initials: K
authors.lastname: Ono
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Karolak
authors.forename: Jean de la Croix
authors.initials: Jde L
authors.lastname: Ndong
authors.forename: Weixi
authors.initials: W
authors.lastname: Wang
authors.forename: Xiangli
authors.initials: X
authors.lastname: Yang
authors.forename: Florent
authors.initials: F
authors.lastname: Elefteriou
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000235:genetics; D049109:Cell Proliferation; D019902:Chondrocytes / Q000378:metabolism; D020219:Chondrogenesis / Q000502:physiology*; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism*; D015870:Gene Expression; D006132:Growth Plate / Q000378:metabolism; D018345:Mice, Knockout; D025542:Neurofibromin 1 / Q000235:genetics / Q000378:metabolism*; D010012:Osteogenesis / Q000502:physiology*; D017468:Receptors, Fibroblast Growth Factor / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: D025542:Neurofibromin 1; D017468:Receptors, Fibroblast Growth Factor; D048049:Extracellular Signal-Regulated MAP Kinases