Characterization of Platyhelminth POU domain genes: ubiquitous and specific anterior nerve cell expression of different epitopes of GtPOU-1

POU domain proteins are a large family of transcription factors that have been identified in a variety of metazoans, from freshwater sponges, planarians and nematodes to arthropods, echinoderms and vertebrates. Many of these proteins are implicated in the development and establishment of the nervous system. In this paper we describe the identification of the planarian genes GtPOU-1, GtPOU-3 and GtPOU-4, which belong to the subclasses III and IV of POU-domain genes. Their similarity with other members of the POU family is restricted to the POU and homeo domains, plus some peptide sequences scattered in the linker and flanking regions. As with other subclass III POU genes, GtPOU-1 is devoid of introns. Axial transcript distribution by RT–PCR and immunohistochemical assays, performed with a polyclonal antibody raised against the GtPOU-1 fusion protein, indicate that both the GtPOU-1 transcript and protein are continuously expressed along the antero-posterior axis. A monoclonal antibody raised against the same fusion protein indicates that a GtPOU-1-specific epitope, probably obtained by post-translational modification, is present in neural cells from both the central and peripheral nerve systems of the adult planarian's anterior third. Moreover, the GtPOU-1-specific epitope shows a dynamic expression pattern during regeneration, always marking the most anterior region of the planarian nervous system. Both the rapid and general GtPOU-1-specific epitope modification, during posterior regeneration, indicate that regeneration is a global process involving all planarian regions, including those that are far from the wound, by a combination of morphallactic and epimorphic mechanisms.