Are ERK and Wnt Fate d to Reinforce Calvarial Osteoblast Identity?

Abstract

Wnt signaling regulates cell fate decisions in diverse contexts during development and disease. In the mouse cranial mesenchyme (CM) Wnt signaling pathway components and reporters are spatially distributed during calvarial osteoblast fate selection. Within 24 hours, loss of Wnt signaling in the mouse embryonic CM results in a robust and binary cell fate switch, from calvarial bone to ectopic cartilage. The mechanism by which Wnt signaling regulates this cell fate switch is not clear. Extracellular signal-regulated protein kinase 1 and 2 (ERK1/2) activation is required for bone-cartilage cell fate decisions in long bone perichondrium, and we have demonstrated that ERK1/2 activation is present in calvarial osteoblasts. Here, we test the hypothesis that ERK signaling is a mediator of binary Wnt-dependent bone-cartilage cell fate decisions. First, we used three distinct Wnt signaling loss-of-function mouse models to demonstrate that Wnt signaling is required for activation of ERK signaling in calvarial bone progenitors. Second, we showed that loss of ERK signaling precedes formation of ectopic cartilage in CM-Wnt signaling mutants and ERK activation is highly sensitive to levels of Wnt signaling within the cranial mesenchyme. Third, loss of Erk1/2 in the CM results in elevated levels of the master cartilage determinant, SOX9, by E13.5 and loss of calvarial bone by E16.5. These results demonstrate a link between the Wnt and ERK signaling pathways in regulating calvarial bone cell fate decisions in vivo. We propose a new model whereby reciprocal regulation of both canonical and non-canonical Wnt pathways in the CM generates a gradient of Wnt signaling and utilizes ERKs to reinforce binary cell fate decisions in vivo. This offers a new opportunity for therapeutic targeting of Wnt signaling in craniofacial skeletal defects and disease.