Global directional cues in establishing planar cell polarity during skeletal morphogenesis

Abstract

The longitudinal growth of long bone requires proximal-distal elongation of the cartilaginous anlage. However, the mechanism underlying directional elongation of the long bone cartilage is unclear. Planar Cell Polarity (PCP), which controls uniformly polarized cellular behaviors in a field of cells, is critically required for directional morphogenesis in many fundamentally important developmental processes in both invertebrates and vertebrates. Our previous studies have shown that forming chondrocytes are polarized along the proximal-distal (P-D) axis in mouse developing limbs through a process that requires PCP. PCP is established in response to a global directional cue. However, the nature and functional mechanisms of such cue(s) have not been revealed. Morphogen gradients that determine the body axes can act as global cues, and Wnt ligands have been shown to regulate PCP. Nevertheless, it is not defined yet whether Wnt signaling plays permissively or instructively to convey directional information. Here we show with multiple genetic approaches that Wnt5a, which is expressed in a graded fashion along the P-D axis in limb bud, provides a directional cue that plays an instructive role to polarize limb mesenchymal cells when they are differentiating to chondrocytes. In addition, we have also identified a previously unappreciated role of Fgf signaling in PCP establishment through modulating Wnt5a signaling. Our results suggest that the initiation of the planar polarization is coordinately regulated by multiple morphogen gradients during skeletal morphogenesis.