Robustness of the Dorsal morphogen gradient with respect to morphogen dosage

Abstract

In multicellular organisms, the timing and placement of gene expression in a developing tissue assigns the fate of each cell in the embryo in order for a uniform field of cells to differentiate into a reproducible pattern of organs and tissues. This positional information is often achieved through the action of spatial gradients of morphogens. Spatial patterns of gene expression are paradoxically robust to variations in morphogen dosage, given that, by definition, gene expression must be sensitive to morphogen concentration. In this work we investigate the robustness of the Dorsal/NF-κB signaling module with respect to perturbations to the dosage of maternally-expressed dorsal mRNA. The Dorsal morphogen gradient patterns the dorsal-ventral axis of the early Drosophila embryo, and we found that an empirical description of the Dorsal gradient is highly sensitive to maternal dorsal dosage. In contrast, we found experimentally that gene expression patterns are highly robust. Although the components of this signaling module have been characterized in detail, how their function is integrated to produce robust gene expression patterns to variations in the dorsal maternal dosage is still unclear. Therefore, we analyzed a mechanistic model of the Dorsal signaling module and found that Cactus, a cytoplasmic inhibitor for Dorsal, must be present in the nucleus for the system to be robust. Furthermore, active Toll, the receptor that dissociates Cactus from Dorsal, must be saturated. Finally, the vast majority of robust descriptions of the system require facilitated diffusion of Dorsal by Cactus. Each of these three recently-discovered mechanisms of the Dorsal module are critical for robustness. These mechanisms synergistically contribute to changing the amplitude and shape of the active Dorsal gradient, which is required for robust gene expression. Our work highlights the need for quantitative understanding of biophysical mechanisms of morphogen gradients in order to understand emergent phenotypes, such as robustness.

Dynamic patterning by morphogens illuminated by cis-regulatory studies

“…Cactus facilitates diffusion, or shuttling, of Dl within the syncytial blastoderm by forming a Dl/ Cactus complex (Carrell et al, 2017). This mechanism also helps to ensure that patterning is robust to changes in Dl morphogen concentration (Al Asafen et al, 2020). Activation of the Toll receptor leads to a downstream intracellular cascade that ultimately results in phosphorylation and subsequent degradation of Cactus.…”

A reaction-diffusion network model predicts a dual role of Cactus/IκB to regulate Dorsal/NFκB nuclear translocation in Drosophila

“…On the other hand, even though the reduction in Dl also decreases the amount of Dl that enters the nucleus by direct flow, the consequent Cact decrease leaves more Dorsal dimers free for nuclear translocation. Al Asafen et al ., 2020 have successfully modeled the pattern of the nDl gradient in the middle of the embryo, focusing on their behavior in response to variations in dl expression [ 33 ]. They find that by including Toll receptor saturation, presence of nuclear DlC and shuttling of Cactus and Dorsal along the DV axis, their simulations provide a good fit for nDl in lateral regions of the embryo, as well as dorsal and ventral nuclei.…”

“…All published models include the recruitment of DlC to signaling complexes, which is dependent on Toll activation (herein referred to as reactions k7, k8), as well as downstream steps to Toll-dependent Dl-C dissociation, Dl nuclear translocation and degradation of Cact through the proteasome. The direct flow of Dl in and out of the nucleus (k3, k4) was previously included in other models, either independently or as part of a complex with Cact [ 11 , 33 , 42 ].…”

“…For instance, our model does not incorporate Toll receptor saturation at lateral positions of the embryo, nor does it include the shuttling of DlC complexes towards the ventral side of the embryo as in [ 11 ]. It has been suggested that shuttling by Cact is required to concentrate high levels of Dl in ventral regions, and responsible for the widening of the nDl gradient observed in dl /+ [ 11 , 33 , 42 ]. In our assays, by quantifying the Dl gradient at the anterior tip of the embryo, we do observe a subtle widening of the nDl gradient that is not reproduced in our simulations.…”

“…dimers free for nuclear translocation. Al Asafen et al, 2020 have successfully modeled the pattern of the nDl gradient in the middle of the embryo, focusing on their behavior in response to variations in dl expression [33]. They find that by including Toll receptor saturation, presence of nuclear DlC and shuttling of Cactus and Dorsal along the DV axis, their simulations provide…”