Misexpression of Polycomb-group proteins in Xenopus alters anterior neural development and represses neural target...

In Drosophila, the Polycomb-group constitutes a set of structurally diverse proteins that act together to silence target genes. Many mammalian Polycomb-group proteins have also been identified and show functional similarities with their invertebrate counterparts. To begin to analyze the function of Polycomb-group proteins in Xenopus development, we have cloned a Xenopus homolog of Drosophila Polycomblike, XPcl1. XPcl1 mRNA is present both maternally and zygotically, with prominent zygotic expression in the anterior central nervous system. Misexpression of Pcl1 by RNA injection into embryos produces defects in the anterior central nervous system. The forebrain and midbrain contain excess neural tissue at the expense of the ventricle and include greatly thickened floor and roof plates. The eye fields are present but Rx2A, an eye-specific marker, is completely repressed. Overexpression of Pcl1 in Xenopus embryos alters two hindbrain markers, repressing En-2 and shifting it and Krox-20 in a posterior direction. Similar neural phenotypes and effects on the En-2 expression pattern were produced by overexpression of three other structurally unrelated Polycomb-group proteins: M33, XBmi-1, and mPh2. These observations indicate an important role for the Polycomb-group in regulating gene expression in the developing anterior central nervous system.