Enzymology of the nematode cuticle: A potential drug target?

Abstract

All nematodes possess an external structure known as the cuticle, which is crucial for their development and survival. This structure is composed primarily of collagen, which is secreted from the underlying hypodermal cells. Extensive studies using the free-living nematode Caenorhabditis elegans demonstrate that formation of the cuticle requires the activity of an extensive range of enzymes. Enzymes are required both pre-secretion, for synthesis of component proteins such as collagen, and post-secretion, for removal of the previous developmental stage cuticle, in a process known as moulting or exsheathment. The excretion/secretion products of numerous parasitic nematodes contain metallo-, serine and cysteine proteases, and these proteases are conserved across the nematode phylum and many are involved in the moulting/exsheathment process. This review highlights the enzymes required for cuticle formation, with a focus on the post-secretion moulting events. Where orthologues of the C. elegans enzymes have been identified in parasitic nematodes these may represent novel candidate targets for future drug/vaccine development.

Keywords: C. elegans; Collagen; Cuticle; Ecdysis; Moulting; Nematode; Protease.

Graphical abstract

Fig. 1A

The organization and structure of the C. elegans cuticle. Left image is a transmission electron micrograph (TEM) depicting a longitudinal cross-section of the adult cuticle. Right panel is a cartoon depiction highlighting the distinct structural layers and their composition. With the exception of the epicuticle and surface coat, collagens are present in all major layers. Cuticlins are restricted to the cortical layer. The epicuticle contains lipids and is covered by a glycoprotein-rich coat (scale bar 1 μm).

Fig. 1B

The cuticle collagen biogenesis pathway in C. elegans. Steps within the endoplasmic reticulum (ER) (blue) include, proline hydroxylation by DPY-18, disulphide bond formation by PDI, and proline isomerisation, by proline isomerases (PPIases). Export from the ER requires a functional COPII-pathway (sec-23). All cuticle collagens are predicted to be N-terminally processed by the subtilisin-like protease BLI-4, whereas a subset of collagens are C-terminally processed by the BMP-like protease DPY-31. The final step in collagen maturation involves tyrosine cross-link formation and is catalysed by peroxidase enzymes including BLI-3 and MLT-7. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Collagen prolyl 4-hydroxylase (C-P4H) is essential in C. elegans and B. malayi. (A) Wild type (N2) adult C. elegans showing normal body morphology. (B) Mutation in C. elegans dpy-18 C-P4H α subunit resulting in a dumpy (short fat) phenotype. (C) C. elegans phy-2 RNAi in a dpy-18 mutant background causing larval lethality from loss of both C-P4H α subunits. (D) Wild type morphology of control (C. elegans phy-3) RNAi on B. malayi microfilaria following treatment of cultured adult females (Winter et al., 2013). (E) Mutant microfilaria body morphology phenotype following double C-P4H α subunit RNAi treatment of cultured adult females (Winter et al., 2013). Scale bars 100 μm.

Fig. 3

Moulting defects due to mutations in C. elegans metalloprotease NAS-37. (A) Mid-body constriction due to unshed cuticle and incomplete ecdysis due to mutation in nas-37. (B) SEM image of mid-body cuticle constriction due to nas-37 mutation. Scale bars 100 μm.