Reelin/DAB-1 signaling in the embryonic limb regulates the chondrogenic differentiation of digit mesodermal progenitors.

Reelin is a bioactive component of some extracellular matrices. Most studies on this signaling glycoprotein have been performed in the developing nervous system, where Reelin binds to the very-low-density lipoprotein receptor (VLDLR) and apolipoprotein E receptor 2 (ApoER2) of target cells. This induces phosphorylation of the intracellular adaptor protein Disabled-1 (Dab-1), which subsequently activates downstream effectors to regulate important aspects of neuroblast biology. Here, we show that the components of the Reelin signaling pathway exhibit a dynamic expression pattern during the development of the digits in chick and mouse embryonic limbs. Reelin and Dab-1 are highly expressed in the differentiating digit cartilages and tendinous blastemas. Immunolabeling of phospho-Dab-1 indicates that the pattern of gene expression correlates with zones of active signaling. Intense signaling is also present in the early stages of cartilage differentiation in micromass cultures of digit mesodermal progenitors. In this in vitro assay, disruption of the Reelin signaling pathway by gene silencing causes cystoskeletal and cell shape modifications accompanied by reduced chondrogenesis and down-regulation of specific cartilage molecular markers. Of note, Scleraxis and Six2, which are master genes of tendinous blastemas, become up-regulated in these experiments. We further show that the receptors ApoER2 and VLDLR are differentially expressed in cartilage and tendons and that these receptors show temporal expression differences in the micromass cultures. Sox9 and other chondrogenic markers were downregulated in micromass cultures after ApoER2 gene silencing, while gene silencing of VLDLR up-regulates Scleraxis. In summary, our findings provide evidence of a role for Reelin signaling in skeletogenesis that promotes chondrogenesis through ApoER2 and inhibits tenogenic differentiation through VLDLR.