Pathway to a phenocopy: Heat stress effects in early embryogenesis

Abstract

Background Heat shocks applied at the onset of gastrulation in early Drosophila embryos frequently lead to phenocopies of U-shaped mutants – having characteristic failures in the late morphogenetic processes of germband retraction and dorsal closure. The pathway from non-specific heat stress to phenocopied abnormalities is unknown. Results Drosophila embryos subjected to 30-min, 38-°C heat shocks at gastrulation appear to recover and restart morphogenesis. Post-heat-shock development appears normal, albeit slower, until a large fraction of embryos develop amnioserosa holes (diameters > 100 μm). These holes are positively correlated with terminal U-shaped phenocopies. They initiate between amnioserosa cells and open over tens of minutes by evading normal wound healing responses. They are not caused by tissue-wide increases in mechanical stress or decreases in cell-cell adhesion, but instead appear to initiate from isolated apoptosis of amnioserosa cells. Conclusions The pathway from heat shock to U-shaped phenocopies involves the opening of one or more large holes in the amnioserosa that compromise its structural integrity and lead to failures in morphogenetic processes that rely on amnioserosa-generated tensile forces. The proposed mechanism by which heat shock leads to hole initiation and expansion is heterochonicity – i.e., disruption of morphogenetic coordination between embryonic and extra-embryonic cell types.

Cofilin-Mediated Actin Stress Response Is Maladaptive in Heat-Stressed Embryos

“…Embryos were injected with G-actin Red at 18°C and then imaged at either 18°C or under heat stress at 32°C ( Table S1 ). This is a mild heat stress, given that standard Culturing conditions are 18°C–29°C, and heat shock experiments in Drosophila typically use ≥ 37°C (e.g., Bergh and Arking, 1984 ; Crews et al, 2016 ). In injected embryos, single plane, confocal surface views captured furrow tip F-actin, as well as cross sections through nuclei.…”

“…In humans, prenatal exposure to hypoxia, drugs, pathogens, or high temperature is associated with increased risk of fetal death or physical malformations and/or defects following birth ( Edwards, 2006 ; Dixon et al, 2011 ; Hamdoun and Epel, 2007 ). Environmental stress disrupts cellular function in embryos by generating reactive oxygen species, altering gene expression, inducing apoptosis and heterochronicity, and disrupting signaling networks ( Parman et al, 1999 ; Puscheck et al, 2015 ; Salilew-Wondim et al, 2014 ; Crews et al, 2016 ). Whether these stress-induced disruptions impinge on the actin cytoskeleton, the ultimate architectural driver of embryonic morphogenesis, remains unknown.…”

The Common Developmental Origin of Eyespots and Parafocal Elements and a New Model Mechanism for Color Pattern Formation

“…1.3). A possible reason for this effect is that temperature shock freezes the progression of pattern determination, possibly by activating heat shock or stress proteins that stop biosynthetic or transcriptional activity (Mitchell and Lipps 1978;Crews et al 2016;Welte et al 1995). The grassfire model shows that a single pattern element can split into two and that both ocelli and parafocal elements can be produced from a common source.…”

Amnioserosa development and function in Drosophila embryogenesis: Critical mechanical roles for an extraembryonic tissue

“…Finally, retraction defects are observed when epithelial integrity of the amnioserosa is disrupted by laser ablation (Lynch et al, ) or by nonspecific environmental perturbations, like hyperthermia (Eberlein, ). Interestingly, hyperthermia leads to retraction defects when the heat shock is applied early in embryogenesis (at gastrulation) and holes open in the amnioserosa several hours later (Crews et al, ). These examples demonstrate that the mechanisms of germ band retraction rely on the germ band and amnioserosa as two contiguous, coordinated epithelia.…”