Cortical movement of Bicoid in early Drosophila embryos is actin- and microtubule-dependent and disagrees with the SDD diffusion model

Abstract

The Bicoid (Bcd) protein gradient in Drosophila serves as a paradigm for gradient formation in textbooks. The SDD model (synthesis, diffusion, degradation) was proposed to explain the formation of the gradient. The SDD model states that the bcd mRNA is located at the anterior pole of the embryo at all times and serves a source for translation of the Bicoid protein, coupled with diffusion and uniform degradation throughout the embryo. Recently, the ARTS model (active RNA transport, synthesis) challenged the SDD model. In this model, the mRNA is transported at the cortex along microtubules to form a mRNA gradient which serves as template for the production of Bcd, hence little Bcd movement is involved. To test the validity of the SDD model, we developed a sensitive assay to monitor the movement of Bcd during early nuclear cycles. We observed that Bcd moved along the cortex and not in a broad front towards the posterior as the SDD model would have predicted. We subjected embryos to hypoxia where the mRNA remained strictly located at the tip at all times, while the protein was allowed to move freely, thus conforming to an ideal experimental setup to test the SDD model. Unexpectedly, Bcd still moved along the cortex. Moreover, cortical Bcd movement was sparse, even under longer hypoxic conditions. Hypoxic embryos treated with drugs compromising microtubule and actin function affected Bcd cortical movement and stability. Vinblastine treatment allowed the simulation of an ideal SDD model whereby the protein moved throughout the embryo in a broad front. In unfertilized embryos, the Bcd protein followed the mRNA which itself was transported into the interior of the embryo utilizing a hitherto undiscovered microtubular network. Our data suggest that the Bcd gradient formation is probably more complex than previously anticipated.

Primary Cilia and Calcium Signaling Interactions

“…The sensory role of the primary cilium is highly specialized depending upon the tissue or organ system it is localized in. Well-known examples of their specialized roles include mechanosensitive cilia found in blood vessels, kidney, and bone; chemosensitive cilia in the nose; and recently, new evidence has suggested an electrosensitive role of primary cilia in the nervous system [ 21 , 22 , 23 ]. To effectively translate such a diverse range of extracellular signals, ciliary protein and receptor composition, as well as cilia structure, all come together to contribute to context-appropriate signaling [ 24 ].…”

“…This could be explained by either cytoplasmic Ca 2+ being needed to precede cilia sensory function, enabling the cilia to sense the electrical stimuli, or a separate Ca 2+ microdomain forming in the cilia that is distinct from cytosolic Ca 2+ . Together, this suggests that primary cilia have a critical role in translating Ca 2+ -induced cellular responses into electrical stimulation [ 21 ].…”

Seeing is believing: the Bicoid protein reveals its path

“…To precisely monitor the path of Bcd movement during early development, a sensitive approach was developed that allowed for the study of the spatial Bcd movement during the early nuclear cycles using single confocal sections [ 20 ]. This study revealed, for the first time, that Bcd moved at the cortex of the egg but never entered the inner portion filled with yolk (Fig.…”

“…During this sleeping phase, Bcd still moved slowly at the cortex to the posterior. Again, the speed and location of Bcd movement was not compatible with the diffusion properties claimed by the SDD model [ 20 ], nor with that from the nucleocytoplasmic shuttle model [ 22 ]. Notably, the inner part of the egg, i. e. the yolk still acted as a non-permissive territory (Fig.…”

“…Notably, the inner part of the egg, i. e. the yolk still acted as a non-permissive territory (Fig. 2c , d ; [ 20 ]. This finding was unexpected, based on the previous knowledge of the diffusion of fluorescent dextrane particles which easily entered the yolk [ 12 ].…”