Bimodal control of Hoxd gene transcription in the spinal cord defines two regulatory subclusters.

The importance of Hox genes in the specification of neuronal fates in the spinal cord has long been recognized. However, the transcriptional controls underlying their collinear expression domains remain largely unknown. Here we show in mice that the correspondence between the physical order of Hoxd genes and their rostral expression boundaries, although respecting spatial collinearity, does not display a fully progressive distribution. Instead, two major anteroposterior boundaries are detected, coinciding with the functional subdivision of the spinal cord. Tiling array analyses reveal two distinct blocks of transcription, regulated independently from one another, that define the observed expression boundaries. Targeted deletions in vivo that remove the genomic fragments separating the two blocks induce ectopic expression of posterior genes. We further evaluate the independent regulatory potential and transcription profile of each gene locus by a tiling array approach using a contiguous series of transgenes combined with locus-specific deletions. Our work uncovers a bimodal type of HoxD spatial collinearity in the developing spinal cord that relies on two separate 'enhancer mini-hubs' to ensure correct Hoxd gene expression levels while maintaining their appropriate anteroposterior boundaries.

Exploring the effects of mechanical feedback on epithelial topology.

Apical cell surfaces in metazoan epithelia, such as the wing disc of Drosophila, resemble polygons with different numbers of neighboring cells. The distribution of these polygon numbers has been shown to be conserved. Revealing the mechanisms that lead to this topology might yield insights into how the structural integrity of epithelial tissues is maintained. It has previously been proposed that cell division alone, or cell division in combination with cell rearrangements, is sufficient to explain the observed epithelial topology. Here, we extend this work by including an analysis of the clustering and the polygon distribution of mitotic cells. In addition, we study possible effects of cellular growth regulation by mechanical forces, as such regulation has been proposed to be involved in wing disc size regulation. We formulated several theoretical scenarios that differ with respect to whether cell rearrangements are allowed and whether cellular growth rates are dependent on mechanical stress. We then compared these scenarios with experimental data on the polygon distribution of the entire cell population, that of mitotic cells, as well as with data on mitotic clustering. Surprisingly, we observed considerably less clustering in our experiments than has been reported previously. Only scenarios that include mechanical-stress-dependent growth rates are in agreement with the experimental data. Interestingly, simulations of these scenarios showed a large decrease in rearrangements and elimination of cells. Thus, a possible growth regulation by mechanical force could have a function in releasing the mechanical stress that evolves when all cells have similar growth rates.