Question

If Xenopus laevis embryos were collected within 10mins of fertlization and treated with latrunculine, an actin destabilizing agent, how would that impact their early development? include that aspect of early development would be affected and the expected phenothpes

Answer 1

Actin is intimately involved in several of the events on which development depends. Some of these events are known to occur in embryos of many widely divergent species. The amphibian embryo has been widely used as a model system for the exploration of cell biological events that contribute to development, in part because it is a vertebrate system that can be fertilized in vitro, and is easily manipulated during the early stages of embryogenesis.

The actin cytoskeleton is a dynamic network of filaments made up of a monomeric 43 kDa protein named globular actin (G-actin), which self-assembles into polymers of 8 nm diameter that are also called microfilaments or filamentous actin (F-actin). In cells, approximately half of the actin is kept in monomeric form, and the polymerization of actin is a dynamic process. Generally speaking, F-actin networks are continuously reorganized in cells that rapidly change their shape and in fast migrating cells that swiftly change the direction of movement. Continuous polymerization and depolymerization of actin molecules in cell-surface protrusions have been well investigated and defined; in fact conversion of these two states of actin existence, which is the foremost point of actin functional performance, is very essential for cell survival. The function of the actin cytoskeleton in cells relies on the intrinsic capacity of the actin monomers to reversibly assemble into protein polymers. Actin is an asymmetric molecule, assembling into polar filaments with structurally and functionally distinct ends, characterized by ATP-actin monomer addition at the plus-end (or barbed-end) and loss of ADP-actin monomers at the minus-end (or pointed-end).

During fertilization of most species, the sperm entry into the egg cytoplasm, the exocytosis of cortical granules, and the meeting of sperm and egg pronuclei all appear to require actin filaments. In Xenopus, the pigmented animal hemisphere contracts isometrically within 5–6 min of sperm entry. The contractile cortex of the zygote is 0.5–3 µm thick, excludes yolk platelets, and contains cortical granules, pigment granules, and actin filaments. Within 20–30 min the center of gravity is shifted to the vegetal pole, so that all fertilized eggs are oriented with the animal side up, while unfertilized eggs orient randomly, often on their sides. The first contraction relaxes and microtubule networks form in the vegetal hemisphere that are necessary for the subsequent rotation, which sets up the gray crescent and specifies the embryonic axes. A second contraction of the pigmented cortex occurs approximately 70 min after fertilization, relaxing as the first cleavage furrow forms.

Cytotoxins Latrunculin A and B, isolated from the Red Sea sponge Latrunculia magnifica, are potent inhibitors of actin filament formation. LATs specifically sequester monomeric actin, mimicking proteins such as β-thymosins. They inhibit polymerization of G-actin, promote depolymerization of F-actin most likely by an allosteric mechanism, and form ternary complexes with profilin or thymosin β4-actin in vitro. They are the only known toxins that interact with the ATP-binding cleft of the actin monomer. LAT A in the nanomolar concentration range disrupts the actin cytoskeleton and causes cell rounding.