Loss of the seipin gene perturbs eggshell formation inC. elegans

Abstract

SEIPIN, an evolutionary conserved protein, plays pivotal roles during lipid droplet (LD) biogenesis and is associated with various human diseases with unclear mechanisms. Here, we analyzed C. elegans mutants deleted of the sole SEIPIN gene, seip-1. Homozygous seip-1 mutants displayed penetrant embryonic lethality, which is caused by the disruption of the lipid-rich permeability barrier, the innermost layer of the C. elegans embryonic eggshell. In C. elegans oocytes and embryos, SEIP-1 is associated with LDs and crucial for controlling LD size and lipid homeostasis. The seip-1 deletion mutants reduced the ratio of polyunsaturated fatty acids (PUFAs) in their embryonic fatty acid pool. Interestingly, dietary supplementation of selected n-6 PUFAs rescued the embryonic lethality and defective permeability barrier. Accordingly, we propose that SEIP-1 may maternally regulate LD biogenesis and lipid homeostasis to orchestrate the formation of the permeability barrier for eggshell synthesis during embryogenesis. A lipodystrophy allele of seip-1 resulted in embryonic lethality as well and could be rescued by PUFA supplementation; these experiments support a great potential of using C. elegans to model SEIPIN-associated human diseases.

C. elegans Apical Extracellular Matrices Shape Epithelia

“…Loss of this permeability layer allows entry of water and small molecules through the eggshell and perturbs development of the peri-embryonic layer and the embryo [ 32 , 34 ]. Supplementing the animals’ diet with specific poly-unsaturated fatty acids (PUFAs) rescues embryo viability and defects in the permeability barrier and peri-embryonic layer in mutants with defects in fatty acid transport [ 35 ], indicating that PUFAs may be the main components of this layer.…”

Nuclear lipid droplets and nuclear damage in Caenorhabditis elegans

“…Studies in other systems have shown that seipin proteins can have important roles in nLD formation in some cell types, but not others [ 67 ]. We examined previously described seip-1 /seipin null mutants [ 68 , 69 ], and found that most of the mutant gonads had no obvious nLDs, although rare nLDs could be slightly larger than wild type ( S9 Fig ). We conclude that nLDs are a common feature of C .…”

“…We examined germ cell nLDs in each of these mutants, and in representative classes of additional Drop mutants (lipid droplet abnormal) whose molecular identities have not been reported (S9 Fig) . The sizes and numbers of germ cell nLDs in these mutants appeared similar to wild type; some mutants had slightly fewer and/or smaller nLDs than wild type, and others had a few nLDs that were slightly larger than typical wild-type nLDs (S9 Fig) . Studies in other systems have shown that seipin proteins can have important roles in nLD formation in some cell types, but not others [67]. We examined previously described seip-1/seipin null mutants [68,69], and found that most of the mutant gonads had no obvious nLDs, although rare nLDs could be slightly larger than wild type (S9 Fig) . We conclude that nLDs are a common feature of C. elegans germ cells, but that several known mutations that affect cLDs have no major effects on germ cell nLDs.…”

The C. elegans PTCHD homolog PTR-4 is required for proper organization of the pre-cuticular apical extracellular matrix

“…A role for PTR-4 in aECM export would be analogous to the roles of MmpL (mycobacterial membrane protein large) proteins, which export various complex glycolipids across the inner cell membrane and into the periplasmic space for eventual incorporation into the outer cell membrane or other layers of the mycobacterial cell envelope (Melly and Purdy, 2019). Although the lipid content of the pre-cuticle aECM is not known, many aECMs, including the C. elegans eggshell and cuticle, do have significant lipid content, which is thought to confer their permeability barrier functions (Bai et al, 2020; Blaxter, 1993; Cohen et al, 2020b; Olson et al, 2012; Schultz and Gumienny, 2012). When viewed by TEM, the pre-cuticle aECM has an outer, electron-dense layer that could serve as a lipid-rich covering to corral developing membrane-proximal aECM layers and keep them separate from the more dynamic matrix in the central part of tube lumens (Cohen et al, 2020b; Gill et al, 2016; Mancuso et al, 2012).…”

“…Prior to the generation of each new cuticle, a molecularly distinct aECM, termed the sheath or pre-cuticle, coats external epithelia (Cohen et al, 2020b;Cohen and Sundaram, 2020;Priess and Hirsh, 1986;Vuong-Brender et al, 2017). Similar to some mammalian aECMs such as lung surfactant (Pérez-Gil, 2008) or eye tear film (Dartt, 2011), all of these C. elegans aECMs appear to contain layers with significant lipid content (Bai et al, 2020;Blaxter, 1993;Dartt, 2011;Forman-Rubinsky et al, 2017;Pu et al, 2017;Schultz and Gumienny, 2012). We were therefore intrigued by the possibility that PTR proteins might transport lipids or other hydrophobic cargos into one or more of these aECMs.…”