Role of Bmi-1 in Epigenetic Regulation during Early Neural Crest Development

Abstract

The neural crest is a transient, multipotent cell population in the developing vertebrate embryo that migrates extensively and contributes to a staggering diversity of cell lineages. Neural crest progenitors are specified at the neural plate border during gastrulation; however, commitment to the neural crest lineage does not occur for some time. I find that the chick neural plate border is characterized by co-expression of many neural crest specifier genes, previously considered “late” signals, which often overlap with “early” neural plate border genes. This suggested that continuously expressed members of the neural crest gene regulatory network may be modulated or repressed for proper maintenance of the multipotent state. Consistent with this possibility, several members of the Polycomb Group of epigenetic repressors are expressed at these early stages. For example, the stem cell factor Bmi-1 is expressed at the neural plate border, dorsal neural folds, and migrating neural crest, but is extinguished in differentiated derivatives. Morpholino-mediated knock-down of Bmi-1 causes early upregulation of Msx1, FoxD3, and Sox9 in the chick neurula without affecting cell proliferation. Conversely, Bmi-1 over-expression causes a downregulation of Msx1, suggesting that it negatively regulates neural crest network genes. I find that several alternative splice variants of Bmi-1 are expressed in the developing chick and that an N-terminal variant, V4, acts as a dominant-negative regulator of the full-length version, up-regulating Msx1 expression. Taken together, these results suggest that neural crest progenitors are exposed to numerous specification signals during gastrulation, some of which are regulated by Polycomb Group factors such as Bmi-1. The activity of Bmi-1, in turn, is modulated by alternatively spliced variants, demonstrating an additional level of regulatory complexity acting during early neural crest development.