Arnold tongue entrainment reveals dynamical principles of the embryonic segmentation clock

Living systems exhibit an unmatched complexity, due to countless and entangled interactions across scales. Here we aim to understand and gain control of a complex system, such as the segmentation timing of a developing mouse embryo, without a reference to these detailed interactions. To this end, we develop a coarse-grained approach in which theory guides the experimental identification of the system-level responses to entrainment, in the context of a network of coupled cellular oscillators that constitute the embryonic somite segmentation clock. We demonstrate period- and phase-locking of the embryonic system across a wide range of entrainment parameters, including higher-order coupling. These experimental quantifications allow to derive the phase response curve (PRC) and Arnold tongues of the system, revealing the essential dynamical properties of the embryonic segmentation clock. Our results indicate that at the macro-scale, the somite segmentation clock has characteristics of a highly non-linear oscillator close to a saddle-node on invariant cycle (SNIC) bifurcation and suggests the presence of long-term feedbacks. Combined, this coarse-grained theoretical-experimental approach reveals how we can derive simple, essential features of a highly complex dynamical system and hereby provides precise experimental control over the pace and rhythm of the somite segmentation clock.

Histone deacetylase (Rpd3) regulates Drosophila early brain development via regulation of Tailless

Tailless is a committed transcriptional repressor and principal regulator of the brain and eye development in Drosophila . Rpd3, the histone deacetylase, is an established repressor that interacts with co-repressors like Sin3a, Prospero, Brakeless and Atrophin. This study aims at deciphering the role of Rpd3 in embryonic segmentation and larval brain development in Drosophila . It delineates the mechanism of Tailless regulation by Rpd3 , along with its interacting partners. There was a significant reduction in Tailless in Rpd3 heteroallelic mutant embryos, substantiating that Rpd3 is indispensable for the normal Tailless expression. The expression of the primary readout, Tailless was correlative to the expression of the neural cell adhesion molecule homologue, Fascilin2 (Fas2). Rpd3 also aids in the proper development of the mushroom body. Both Tailless and Fas2 expression are reported to be antagonistic to the epidermal growth factor receptor (EGFR) expression. The decrease in Tailless and Fas2 expression highlights that EGFR is upregulated in the larval mutants, hindering brain development. This study outlines the axis comprising Rpd3, dEGFR, Tailless and Fas2, which interact to fine-tune the early segmentation and larval brain development. Therefore, Rpd3 along with Tailless has immense significance in early embryogenesis and development of the larval brain.

A SoxB gene acts as an anterior gap gene and regulates posterior segment addition in the spider Parasteatoda tepidariorum

SummaryThe Sox gene family encode a set of highly conserved HMG domain transcription factors that regulate many key processes during metazoan embryogenesis. In insects, the SoxB gene Dichaete is the only Sox gene known to be involved in embryonic segmentation. To determine if similar mechanisms are used in other arthropods, we investigated the role of Sox genes during segmentation in the spider Parasteatoda tepidariorum. While Dichaete does not appear to be involved in spider segmentation, RNAi knockdown of the closely related Sox21b-1 gene results in a gap like phenotype in the developing prosoma and also perturbs the sequential addition of opisthosomal segments. We show that this is in part due to a role for Sox21b-1 in regulating the expression of Wnt8 and influencing Delta-Notch signalling during the formation of the segment addition zone. Thus, we have found that two different mechanisms for segmentation in a non-mandibulate arthropod are regulated by a Group B Sox gene. Our work provides new insights into the function of an important and conserved gene family across arthropods, and the evolution of the regulation of segmentation in these animals.

dAP-2 and defective proventriculus regulate Serrate and Delta expression in the tarsus of Drosophila melanogaster

The segmentation of the proximal–distal axis of the Drosophila melanogaster leg depends on the localized activation of the Notch receptor. The expression of the Notch ligand genes Serrate and Delta in concentric, segmental rings results in the localized activation of Notch, which induces joint formation and is required for the growth of leg segments. We report here that the expression of Serrate and Delta in the leg is regulated by the transcription factor genes dAP-2 and defective proventriculus. Previous studies have shown that Notch activation induces dAP-2 in cells distal and adjacent to the Serrate/Delta domain of expression. We find that Serrate and Delta are ectopically expressed in dAP-2 mutant legs and that Serrate and Delta are repressed by ectopic expression of dAP-2. Furthermore, Serrate is induced cell-autonomously in dAP-2 mutant clones in many regions of the leg. We also find that the expression of a defective proventriculus reporter overlaps with dAP-2 expression and is complementary to Serrate expression in the tarsal segments. Ectopic expression of defective proventriculus is sufficient to block joint formation and Serrate and Delta expression. Loss of defective proventriculus results in localized, ectopic Serrate expression and the formation of ectopic joints with reversed polarity. Thus, in tarsal segments, dAP-2 and defective proventriculus are necessary for the correct proximal and distal boundaries of Serrate expression and repression of Serrate by defective proventriculus contributes to tarsal segment asymmetry. The repression of the Notch ligand genes Serrate and Delta by the Notch target gene dAP-2 may be a pattern-refining mechanism similar to those acting in embryonic segmentation and compartment boundary formation.

The Wingless homolog WNT5A and its receptor Frizzled-5 regulate inflammatory responses of human mononuclear cells induced by microbial stimulation

Microarray - assisted gene - expression screens of human macrophages revealed WNT5A, a homolog of Wingless, a key regulator of Drosophila melanogaster embryonic segmentation and patterning, to be consistently up-regulated following stimulation with different mycobacterial species and conserved bacterial structures. The expression of WNT5A required Toll-like receptor signaling and NF-κB activation, which identifies a novel induction pathway for a Wingless homolog. We show that human peripheral-blood mononuclear cells express the WNT5A receptor Frizzled-5 (FZD5). Both WNT5A and FZD5 also were detected in granulomatous lesions in the lungs of Mycobacterium tuberculosis–infected patients. Functional studies showed that WNT5A and FZD5 regulate the microbially induced interleukin-12 response of antigen-presenting cells and interferon-γ production by mycobacterial antigenstimulated T cells. Our findings implicate the evolutionarily conserved WNT/Frizzled signaling system in bridging innate and adaptive immunity to infections.