Mid-Embryo Patterning and Precision in Drosophila Segmentation: Krüppel Dual Regulation of hunchback

Abstract

In early development, genes are expressed in spatial patterns which later define cellular identities and tissue locations. The mechanisms of such pattern formation have been studied extensively in early Drosophila (fruit fly) embryos. The gap gene hunchback (hb) is one of the earliest genes to be expressed in anterior-posterior (AP) body segmentation. As a transcriptional regulator for a number of downstream genes, the spatial precision of hb expression can have significant effects in the development of the body plan. To investigate the factors contributing to hb precision, we used fine spatial and temporal resolution data to develop a quantitative model for the regulation of hb expression in the mid-embryo. In particular, modelling hb pattern refinement in mid nuclear cleavage cycle 14 (NC14) reveals some of the regulatory contributions of simultaneously-expressed gap genes. Matching the model to recent data from wild-type (WT) embryos and mutants of the gap gene Krüppel (Kr) indicates that a mid-embryo Hb concentration peak important in thoracic development (at parasegment 4, PS4) is regulated in a dual manner by Kr, with low Kr concentration activating hb and high Kr concentration repressing hb. The processes of gene expression (transcription, translation, transport) are intrinsically random. We used stochastic simulations to characterize the noise generated in hb expression. We find that Kr regulation can limit the positional variability of the Hb mid-embryo border. This has been recently corroborated in experimental comparisons of WT and Kr- mutant embryos. Further, Kr regulation can decrease uncertainty in mid-embryo hb expression (i.e. contribute to a smooth Hb boundary) and decrease between-copy transcriptional variability within nuclei. Since many tissue boundaries are first established by interactions between neighbouring gene expression domains, these properties of Hb-Kr dynamics to diminish the effects of intrinsic expression noise may represent a general mechanism contributing to robustness in early development.

Sequential construction of a model for modular gene expression control, applied to spatial patterning of theDrosophilagenehunchback

“…5(b)]. This corroborates our¯nding of dual regulation in an earlier project, 55 which used a non-GA approach to¯t a BS-level \¯ne-grained" model of hb regulation. Dual regulation is supported by a number of in vitro studies on Hb-Kr interactions, and also accounts for loss of Hb PS4 in Kr À mutants.…”

“…Dual regulation has been shown to have the capacity to reduce expression hb noise in the PS4 region. 55 Equation 4 WT solutions also included a Kni repression term to limit hb production on the posterior side of the Kr peak; modeling kni À mutants required repression by an additional term in the Kni peak region (in the \extended" È 1 Þ, such as the Nub TF (see Fig. 3(b); TFs at this position include Nub (also called pdm-1), pdm-2, and BLIMP-1 [55][56][57] ).…”

“…The Hb co-activation of any Kr activity is also capable of suppressing any hb peaks on the posterior of the Kr domain. 55 In Eq. (5) scenario [ Fig.…”

“…55 Spatial patterning of Drosophila gene hb using modular gene expression control model Cooperative Bcd activation, for example, with at least second-order Hill equations…”

Experimental and Modeling Approaches for Understanding the Effect of Gene Expression Noise in Biological Development

“…See also Sanchez et al and Monteoliva et al [40,45] on the effect of the number of BSs on transcriptional noise. For later patterning, we modeled the gap-gap interactions producing the mid-embryo Hb concentration peak necessary for the future thorax [46]. Stochastic simulations indicated that hb-Kr interactions reduce expression noise and contribute to the reliability of mid-embryo development, predicting that FISH dot-dot correlation should decrease in Kr − mutants.…”