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.
In zebrafish embryos, the maternally supplied pool of ATP is insufficient to power even the earliest of developmental events (0–3 hpf) such as oocyte-to-embryo transition (OET). The embryos generate an additional pulse (2.5 h long) of ATP (1.25–4 hpf) to achieve the embryonic ATP homeostasis. We demonstrate that the additional pulse of ATP is needed for successful execution of OET. The maternally supplied yolk lipids play a crucial role in maintaining the embryonic ATP homeostasis. In this paper, we identify the source and trafficking routes of free fatty acids (FFAs) that feed the mitochondria for synthesis of ATP. Interestingly, neither the maternally supplied pool of yolk-FFA nor the yolk-FACoA (fatty acyl coenzyme A) is used for ATP homeostasis during 0–5 hpf in zebrafish embryos. With the help of lipidomics, we explore the link between lipid droplet (LD)-mediated lipolysis and ATP homeostasis in zebrafish embryos. Until 5 hpf, the embryonic LDs undergo extensive lipolysis that generates FFAs. We demonstrate that these newly synthesized FFAs from LDs are involved in the maintenance of embryonic ATP homeostasis, rather than the FFAs/FACoA present in the yolk. Thus, the LDs are vital embryonic organelles that maintain the ATP homeostasis during early developmental stages (0–5 hpf) in zebrafish embryos. Our study highlights the important roles carried on by the LDs during the early development of the zebrafish embryos.
Microbes present survival challenge to pre-immune embryos. Our study provides 7 evidence for antimicrobial-secretion-based strategy of zebrafish embryos against microbes 8 during pre-immune stages. Chorion prevents physical contact between embryos and microbes, 9 157 280 Oviparous animals such as zebrafish fertilize their eggs externally. The fertilized eggs (embryos) 281 face two types of survival challenges presented by their environment: 1) starvation and 2) 282 pathogenic microbes. Relatively late onset of adaptive immunity in zebrafish embryos suggests 283 their vulnerability during 0-9 hpf. Therefore, we investigated: (a) if the embryos are vulnerable 284 to common microbes such as E.coli, and if yes, then (b) how do the embryos respond to 285 pathogenic challenges during 0-9 hpf in the absence of any speciallized immune cells? Our 286 observation that every clutch of embryos obtained from a given female fish has unique density of 287 LDs (LDD) in them (Fig. 1), allows us to explore the link between LDs and survival of the 288 embryos challenged with starvation or pathogen. LDs are synthesized on the Endoplasmic 289Reticulum (ER) surface. We speculate that the positive correlation in Fig. 1C arises probably 290 from uniquely inherited ER content by the embryos. Therefore, both the LDD 30 (amount of LD 291 at 30 mpf) and the rate of LD synthesis are expected to be proportional to the ER surface area. 292By mimicking the pathogenic challenges present in the aquatic habitat (using the most ubiquitous 293 microbe E.coli) we also show that the LDs help the embryos survive under bacteiral exposure 294
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