Summary

Neuronal differentiation may be induced by different mechanisms. In PC12 cells, differentiation can be achieved after stimulation by nerve growth factor through the sustained activation and nuclear translocation of MAPKs. A peptide covering the homeodomain of Drosophila Antennapedia translocates through the cell membrane in primary neurons in culture and reaches their nuclei. This process accelerates neurite elongation. We have examined whether the capacity for neuronal induction is a general characteristic of homeodomains, and whether differentiation proceeds through the same pathway as that induced by growth factors or represents a distinct cellular response. We show here that Engrailed homeodomain is internalized by UR61 cells, a PC12 cell derivative, and that it promotes and sustains neurite outgrowth. This event appears to proceed independently of MAPKs activation, suggesting that either parallel signal transduction pathways are under the control of homeoproteins or that they act downstream of MAPKs. The Fushi tarazu homeodomain also causes neurite outgrowth in UR61 cells and the neurotrophic activities of Engrailed and Fushi tarazu homeodomains correlate with their DNA binding specificities. However, neurite outgrowth is not promoted by Bicoid homeodomain, which recognizes a different DNA sequence. Therefore, the neurotrophic activity of the homeodomains depends not only on DNA-binding ability but also on the specificity of this binding.