The tyrosine kinase receptors for the neurotrophins (Trk) are a family of transmembrane receptors that regulate the differentiation and survival of different neuronal populations. Neurotrophin binding to Trk leads to the activation of several signalling pathways including a rapid, but moderate, increase in intracellular calcium levels. We have previously described the role of calcium and its sensor protein, calmodulin, in Trk-activated intracellular pathways. Here we demonstrate that calmodulin is able to precipitate TrkA from PC12 cell lysates. Using recombinant GST-fusion proteins containing the complete intracellular domain of TrkA, or fragments of this region, we show that calmodulin binds directly to the C-terminal domain of TrkA in a Ca2+-dependent manner. We have also co-immunoprecipitated endogenous Trk and calmodulin in primary cultures of cortical neurones. Moreover, we provide evidence that calmodulin is involved in the regulation of TrkA processing in PC12 cells. Calmodulin inhibition results in the generation of a TrkA-derived p41 fragment from the cytosolic portion of the protein. This fragment is autophosphorylated in tyrosines and can recruit PLCγ and Shc adaptor proteins. These results suggest that calmodulin binding to Trk may be important for the regulation of Trk intracellular localization and cleavage.

This work was funded by the European Union RTD program (QLG3-CT1999-00602, to JXC), the Ministerio de Ciencia y Tecnología (SAF 2000–0164-C02-01, to JXC and PM99-0118, to DMZ), a FIS program from Ministerio de Sanidad y Consumo (PI021357, to MLL) and Telemarato´ de TV3 (edició 1997: Malalties Degeneratives Hereditàries, to JXC). MLL is supported by a postdoctoral contract from ‘Generalitat de Catalunya’ and YdeP is the recipient of a predoctoral fellowship from ‘Ministerio de Ciencia y Tecnología’, Spanish Government. JXC holds both an award ‘Distinció a la Recerca per a Joves Investigadors’ and the program ‘Suport a Grups de Recerca’ from the ‘Generalitat de Catalunya’.

Peer Reviewed