A Lipid Transfer Protein Ensures Nematode Cuticular Impermeability

Abstract

The nematode cuticle is a thick and resistant hydroskeleton that acts as a barrier controlling exchanges between the external environment and the internal medium. The cuticle of C. elegans- a free-living nematode - is impermeable to chemicals, toxins and pathogens. Increased permeability is a desirable phenotype because it facilitates chemical uptake, leading to increased sensitivity to agents and reduced risks of false negatives during small molecule screening. Permeability defects can arise from deficiencies in a subset of collagens, surface coat glycosylation or lipid biogenesis. These genes involved in permeability defects are putative targets to fight pathogenic nematodes. Here, we identify the lipid transfer protein GMAP-1 as a critical element setting the permeability of C. elegans cuticle. A deletion mutant for gmap-1 increases cuticular permeability to a range of molecules: sodium azide, levamisole, Hoechst and DiI. GMAP-1 expression in the hypodermis is sufficient to rescue the gmap-1 permeability phenotype. GMAP-1 protein is secreted from organelles in the hypodermis to the aqueous fluid filling the space between collagen fibres of the cuticle. The production or the retention of this fluid within the cuticle is reduced in gmap-1 mutants. GMAP-1 protein binds phosphoglycerides in vitro. In vivo, the lipid composition of the cuticle is modified in gmap-1. Although surface lipids of the cuticle are known to contribute to the permeability barrier of C. elegans, which lipids constitute this barrier and how these lipids are secreted and shuttled to the cuticle surface is unknown. We observe reduced Triacyglycerols, alteration of phosphoglycerides composition and abnormal organisation of the lipids in the cuticle of gmap-1. Extracellular lipid layers, such as stratum corneum or surfactant require lipid transfer proteins to establish. We suggest GMAP-1 is required to form the permeability barrier of C. elegans by setting its lipid content and organisation.

Keywords: C. elegans, lipid transfer protein, cuticle, hypodermis, permeability, epicuticle, surface lipid