A Flexible Network of Lipid Droplet Associated Proteins Support Embryonic Integrity of C. elegans

Abstract

In addition to coordinating the storage and mobilization of neutral fat, lipid droplets (LDs) are conserved organelles that can accommodate additional cargos in order to support animal development. However, it is unclear if each type of cargo is matched with a specific subset of LDs. Here, we report that SEIP-1/seipin defines a subset of oocyte LDs that are required for proper eggshell formation in C. elegans. Using a photoconvertible fluorescent protein-based imaging assay, we found that SEIP-1 positive LDs were selectively depleted after fertilization, coincident of the formation of a lipid-rich permeability barrier of the eggshell. Loss of SEIP-1 function caused impenetrant embryonic arrest, which could be worsened by FAT-3/fatty acyl-CoA desaturase deficiency or suppressed by PLIN-1/Perilipin deficiency. The embryonic development of seip-1; plin-1 mutant in turn depended on the recruitment of RAB-18/Rab18 to LDs, which was not observed in wild type embryos. We propose that SEIP-1 dependent and independent mechanisms act in parallel to ensure the packaging and export of lipid-rich permeability barrier constituents, which involve LDs. The identity of these LDs, as defined by their associated proteins, exhibits unexpected plasticity that ultimately ensures the survival of embryos ex utero.

Nematode eggshells: A new anatomical and terminological framework, with a critical review of relevant literature and suggested guidelines for the interpretation and reporting of eggshell imagery

“…This error was not caught by Stein and Golden (2018) [103] in their otherwise thorough review of the Olson team's claims and that error is now being propagated by other laboratories participating in the C. elegans Research Community. Indeed, some of them are now referring to Layer #5 (the Permeability Barrier Membrane) as the lipid layer and even as the "lipid-rich layer" [9,26,35,45], thus confounding the already confused situation by conflating their reference to the Permeability Barrier Membrane with the decades of literature referring to Layer #3 as the Lipid-rich Layer.…”

“…The question of how Layer #5 is formed has not been visually established in any imagery we could find. Olson et al (2012) [83] speculated that it might come from a second round of cargos from a different population of cortical granules in the oocyte, and Cao et al (2022) [26] provided evidence that it is formed from lipid droplets containing ascarosides produced by the endoplasmic reticulum of the oocyte and segregated into functionally different populations by some sort of labeling process that depends upon the protein seipin. Layer #5 is first seen soon after anaphase II in C. elegans and constrains the 2nd polar body to within the Spatio Internum [83] The contents of the innermost compartment (the Spatio Internum) appear to be the most fluid-like of all "layers," usually with relatively few inclusions discernible by TEM.…”