Turnover of the actomyosin complex in zebrafish embryos directs geometric remodelling and the recruitment of lipid droplets

Abstract

Lipid droplets (LDs), reservoirs of cholesterols and fats, are organelles that hydrolyse lipids in the cell. In zebrafish embryos, the actomyosin complex and filamentous microtubules control the periodic regulation of the LD geometry. Contrary to the existing hypothesis that LD transport involves the kinesin-microtubule system, we find that their recruitment to the blastodisc depends on the actomyosin turnover and is independent of the microtubules. For the first time we report the existence of two distinct states of LDs, an inactive and an active state, that occur periodically, coupled weakly to the cleavage cycles. LDs are bigger, more circular and more stable in the inactive state in which the geometry of the LDs is maintained by actomyosin as well as microtubules. The active state has smaller and irregularly shaped LDs that show shape fluctuations that are linked to actin depolymerization. Because most functions of LDs employ surface interactions, our findings on the LD geometry and its regulation bring new insights to the mechanisms associated with specific functions of LDs, such as their storage capacity for fats or proteins, lipolysis etc.

Myosin-1 inhibition by PClP affects membrane shape, cortical actin distribution and lipid droplet dynamics in early Zebrafish embryos

“…This movement of LDs is associated with its active and inactive states [11]. In case of Myo1 inhibited embryos, we did not observe such in and out motion of LDs.…”

“…All Zebrafish handling and breeding techniques wereused in identical ways as done in recent published work [11]. All live embryos were collected and maintained in E3 media (50 mM NaCl, 0.17 mM KCl, 0.33 mM CaCl2, 0.33 mM MgSO4) for all experimental purpose.…”

“…They also contain numerous dynamic lipid droplets (LDs) in the cortical region of the blastodisc [11]. These LDs have a neutral lipid-sterol ester core surrounded by a phospholipid monolayer [12].…”

Lipid droplet functions beyond energy storage

“…In a number of cases, LD distribution is mediated by the actin-myosin system. In budding yeast, for example, the actin-based motor Myo6 promotes vectorial transport of LDs from the mother cell into the developing bud [207]; during fission yeast spore formation, actin polymerization promotes, directly or indirectly, LD clustering around nuclei; and in early zebrafish embryos, drugs that disrupt or promote actin polymerization or inhibit non-muscle myosin interfere with the developmental redistribution of LDs [211]. In most systems studied to date, however, LD motility is largely or exclusively due to transport along microtubules [203], for examples in various cultured mammalian cells [209, 212] or in Drosophila embryos [154].…”

Role for formin-like 1-dependent acto-myosin assembly in lipid droplet dynamics and lipid storage

“…In murine macrophages, pharmacological disruption of the actin cytoskeleton reduces LD size and lipid storage10 and in 3T3-L1 cells, inhibition of actin dynamics through cofilin-1 depletion disrupts adipogenesis and lipid storage13. At the organismal level, pharmacological perturbation of acto-myosin turnover in zebrafish affects the recruitment of LDs from the yolk-blastodisc to the animal pole14. However, whether LDs associate with specific actin filament structures and what functional role they may have, has not been defined.…”