A mechanism regulating the onset of Sox2 expression in the embryonic neural plate

In vertebrate embryos, the earliest definitive marker for the neural plate, which will give rise to the entire central nervous system, is the transcription factor Sox2. Although some of the extracellular signals that regulate neural plate fate have been identified, we know very little about the mechanisms controlling Sox2 expression and thus neural plate identity. Here, we use electroporation for gain- and loss-of-function in the chick embryo, in combination with bimolecular fluorescence complementation, two-hybrid screens, chromatin immunoprecipitation, and reporter assays to study protein interactions that regulate expression of N2, the earliest enhancer of Sox2 to be activated and which directs expression to the largest part of the neural plate. We show that interactions between three coiled-coil domain proteins (ERNI, Geminin, and BERT), the heterochromatin proteins HP1α and HP1γ acting as repressors, and the chromatin-remodeling enzyme Brm acting as activator control the N2 enhancer. We propose that this mechanism regulates the timing of Sox2 expression as part of the process of establishing neural plate identity.
Author Summary During early development, when the embryo has three layers of cells (ectoderm, mesoderm, and endoderm), a region of the ectoderm called the neural plate becomes specified to generate the entire nervous system. One of the earliest molecular markers for the neural plate is the transcription factor Sox2, which is critical for cells to acquire their neural fates and also defines neural progenitor character. We know very little about the intracellular mechanisms by which the neural plate cells acquire these fates. Here, we show that recruitment of transcriptional repressors to chromatin-remodeling complexes regulate the onset of Sox2 expression. Competitive interactions between three proteins, ERNI, BERT, and Geminin, modulate the choice of repressors and regulate Sox2 expression. During gastrulation, when the three embryonic cell layers form, ERNI recruits the repressor HP1γ to prevent Geminin from activating Sox2 prematurely. By the end of gastrulation, this repression is counteracted by competitive binding of BERT to ERNI and Geminin, causing activation of Sox2. We propose that this mechanism regulates the timing of Sox2 activation in the very early neural plate and thus helps to define the domain that will give rise to the nervous system.
The vertebrate nervous system develops from the neural plate, defined by the transcription factor Sox2. This paper uncovers a mechanism that regulates the timing of Sox2 expression, involving interactions between several proteins and chromatin remodeling.