NEURONAL DIFFERENTIATION OF PC12 CELLS INDUCED BY ENGRAILED HOMEODOMAIN IS DNA-BINDING SPECIFIC AND INDEPENDENT OF MAP KINASES

Neuronal differentiation may be induced by different mechanisms. In PC 12 cells, differentiation can be achieved after stimulation by nerve g rowth factor through the sustained activation and nuclear translocatio n of MAPKs. A peptide covering the homeodomain of Drosophila Antennape dia translocates through the cell membrane in primary neurons in cultu re and reaches their nuclei. This process accelerates neurite elongati on. We have examined whether the capacity for neuronal induction is a general characteristic of homeodomains, and whether differentiation pr oceeds through the same pathway as that induced by growth factors or r epresents a distinct cellular response. We show here that Engrailed ho meodomain is internalized by UR61 cells, a PC12 cell derivative, and t hat it promotes and sustains neurite outgrowth. This event appears to proceed independently of MAPKs activation, suggesting that either para llel signal transduction pathways are under the control of homeoprotei ns or that they act downstream of MAPKs, The Fushi tarazu homeodomain also causes neurite outgrowth in UR61 cells and the neurotrophic activ ities 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. Ther efore, the neurotrophic activity of the homeodomains depends not only on DNA-hinding ability but also on the specificity of this binding.