Expression of Trk receptors during cartilage differentiation.

The products of the tyrosine kinase trk family of proto-oncogenes bind neurotrophins (NTFs) and are components of their high-affinity receptors. The trk gene encodes for the TrkA transmembrane glycoprotein, which functions as a receptor for nerve growth factor (NGF). Brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4) are the preferred ligands for TrkB. and neurotrophin-3 (NT-3) for TrkC (reviewed in reference 1). The presence of Trk receptors in nonneuronal cells of developing tissues and organs indicates that these molecules may play a role in embryonic development that would be distinct from a neurotrophic function. Cartilage development involves mesoderm-derived cells (chondrocytes) that contribute to appositional growth of the limb skeleton. Chondrocytes synthesize macromolecules characteristic of cartilage such as collagen type I1 and sulfated proteoglycans. The majority of studies dealing with cartilage cell differentiation have been carried out on mesenchymal cells isolated from limb buds.* However, the molecular mechanisms regulating cartilage formation are not filly understood. In the present study we analyzed the expression of Trk receptors during mouse limb development in vivo and in micromass cultures in vitro. Because little is known about their regulation, we studied the effects of various growth factors on expression of Trk receptors in vitro. F 1 (CBA x NMRI) and Balb C mouse embryos (E 10-El 8) were used. Immunohistochemistry was performed according to Mitsiadis et aL3 Fivepm sections were incubated with TrkA (B. B. Rudkin), TrkB (Santa Cruz Biotechnology, USA), and TrkC (D. Martin-Zanca) affinity purified antibodies against the extracellular domain of the receptors, and cTrk (Santa Cruz Biotechnology, USA) an-