Regulation and functional aspects of <i>Xenopus</i> Brachyury.

Formation of the mesoderm is one of the earliest patterning events in the development of the embryo. Brachyury is restricted to the mesoderm and has become one of the most widely used markers for mesoderm formation. The Xenopus homologue Xbra has been shown to be both necessary and sufficient for formation of mesoderm. Employing a nuclear transplantation technique to create transgenic Xenopus embryos I studied spatial and temporal regulation of a reporter driven by Xbra genomic sequences. The proximal 5’ flanking region of the Xbra promoter was sufficient to confine expression of a reporter to the mesoderm during gastrulation. Deletion analysis and point mutations in putative transcription factor binding sites identified two repressor modules, which are necessary to restrict expression to the marginal zone during gastrulation. This part of the project suggests that the Xbra gene is not confined to the mesoderm by specific activation, but rather by repression in tissues where its activity is not required. To learn more about the functional aspects of the Brachyury protein I created a series of constructs encoding Xbra protein with N-terminal and C-terminal deletions. Nuclear localisation of the protein requires two regions not related to known nuclear localisation sequences. Experiments investigating the dimérisation properties of the protein confirmed that the full-length Xbra protein, binds a palindromic consensus sequence as dimer. However, there was no evidence for Xbra dimers forming without binding to DNA. Finally, I investigated the possibility that the MAP kinase pathway regulates Xbra on a post-translational level. The experiments showed that Xbra is phosphoiylated by MAP kinase in vitro in the linker domain, but not in the DNA binding domain. These results will form the basis for a more detailed study investigating the post-translational regulation of Xbra. The possibilities for future experiments will be discussed in this context

Response variability in balanced cortical networks

We study the spike statistics of neurons in a network with dynamically balanced excitation and inhibition. Our model, intended to represent a generic cortical column, comprises randomly connected excitatory and inhibitory leaky integrate-and-fire neurons, driven by excitatory input from an external population. The high connectivity permits a mean-field description in which synaptic currents can be treated as Gaussian noise, the mean and autocorrelation function of which are calculated self-consistently from the firing statistics of single model neurons. Within this description, we find that the irregularity of spike trains is controlled mainly by the strength of the synapses relative to the difference between the firing threshold and the post-firing reset level of the membrane potential. For moderately strong synapses we find spike statistics very similar to those observed in primary visual cortex.Comment: 22 pages, 7 figures, submitted to Neural Computatio