Homeoproteins and the molecular basis of neuronal morphological plasticity

The co‐culturing of embryonic neurons and astrocytes derived from distinct brain regions has demonstrated that neuronal morphology and polarity can be influenced through region‐specific neuro‐astroglial interactions. In an attempt to search for region‐specific factors capable of modulating neuronal morphologies a new strategy has been developed, aimed at antagonizing homeoprotein activity which demonstrated that homeoproteins control the morphological traits of post‐mitotic neurons. In the course of the latter experiments we have observed that homeodomains and homeoproteins translocate across biological membranes and gain access to the cytoplasm and nucleus of live cells where they can interfere with the transcriptional activity of endogeneous homeoproteins. We identified the third helix of the homeodomain as the shortest structure necessary and sufficient for internalization and used it to address pharmacological substances inside the cells, in particular peptidic and phosphopeptidic antagonists of distinct signalling pathways. The internalization of full‐length homeoproteins led to the investigation of the possibility that homeoproteins are secreted. It was shown that Engrailed gains access to a compartment compatible with secretion in vivo and in vitro and is secreted in vitro, thus possibly acting as a putative polypeptidic messenger. Finally, to further understand the mode of action of homeoproteins, an original gene trap strategy was developed to identify genes in the genetic pathway of homeogenes and we have identified Bullous Pemphigoid Antigen 1 as a candidate Engrailed target. Copyright © 1999 John Wiley & Sons, Ltd.