Gene regulation: coming to grips with cactus

The presence of nuclear cactus in the early Drosophila embryo may extend the dynamic range of the dorsal gradient

In a developing embryo, the spatial distribution of a signaling molecule, or a morphogen gradient, has been hypothesized to carry positional information to pattern tissues. Recent measurements of morphogen distribution have allowed us to subject this hypothesis to rigorous physical testing. In the early Drosophila embryo, measurements of the morphogen Dorsal, which is a transcription factor responsible for initiating the earliest zygotic patterns along the dorsal-ventral axis, have revealed a gradient that is too narrow to pattern the entire axis. In this study, we use a mathematical model of Dorsal dynamics, fit to experimental data, to determine the ability of the Dorsal gradient to regulate gene expression across the entire dorsal-ventral axis. We found that two assumptions are required for the model to match experimental data in both Dorsal distribution and gene expression patterns. First, we assume that Cactus, an inhibitor that binds to Dorsal and prevents it from entering the nuclei, must itself be present in the nuclei. And second, we assume that fluorescence measurements of Dorsal reflect both free Dorsal and Cactus-bound Dorsal. Our model explains the dynamic behavior of the Dorsal gradient at lateral and dorsal positions of the embryo, the ability of Dorsal to regulate gene expression across the entire dorsal-ventral axis, and the robustness of gene expression to stochastic effects. Our results have a general implication for interpreting fluorescence-based measurements of signaling molecules.

A computational statistics approach for estimating the spatial range of morphogen gradients

A crucial issue in studies of morphogen gradients relates to their range: the distance over which they can act as direct regulators of cell signaling, gene expression and cell differentiation. To address this, we present a straightforward statistical framework that can be used in multiple developmental systems. We illustrate the developed approach by providing a point estimate and confidence interval for the spatial range of the graded distribution of nuclear Dorsal, a transcription factor that controls the dorsoventral pattern of the Drosophila embryo.

A microfluidic array for large-scale ordering and orientation of embryos

Quantitative studies of embryogenesis require the ability to monitor pattern formation and morphogenesis in large numbers of embryos, at multiple time points and in diverse genetic backgrounds. We describe a simple approach that greatly facilitates these tasks for Drosophila melanogaster embryos, one of the most advanced models of developmental genetics. Based on passive hydrodynamics, we developed a microfluidic embryo-trap array that can be used to rapidly order and vertically orient hundreds of embryos. We describe the physical principles of the design and used this platform to quantitatively analyze multiple morphogen gradients in the dorsoventral patterning system. Our approach can also be used for live imaging and, with slight modifications, could be adapted for studies of pattern formation and morphogenesis in other model organisms.

Developmental effects of over-expression of normal and mutated forms of a Xenopus NF-kappa B homologue

High level over-expression of XrelA1, a homologue of the p65 sub-unit of NF-kappa B and of Drosophila dorsal, arrests Xenopus development at the gastrula stage, producing a reduction in the levels of expression of various genes of developmental interest without general reduction in transcription or cessation of cell division. There is little Goosecoid expression, even though a dorsal lip forms. At lower levels XrelA1 mRNA primarily produces disruption of the mid-dorsal axis. A dominant interference gene product, delta 222, produces mainly posterior, but also anterior abnormalities. On the basis of these results we postulate that the role of XrelA1 in the vertebrate embryo is unlikely to be in dorsoventral development, but more likely in the formation of the termini.