Summary

Ecdysozoan animals, including nematodes and arthropods, develop through periodic larval stage molts when the exoskeleton is shed and synthesized anew. Although molting is the hallmark of the most abundant and diverse group of animals on the planet, including a wide variety of human pests and pathogens, the endocrine circuits that regulate molting in response to environmental and physiologic cues are not well understood. Moreover, little is known about the molecular mechanisms of release and de novo production of the exoskeleton.

Endocrine and neuroendocrine pathways regulate molting in arthropods, and likely operate in nematodes as well. In insects, pulses of the steroid hormone ecdysone trigger molting and metamorphosis. The neuropeptide PTTH (prothoracicotropic hormone)stimulates synthesis of ecdysone in the prothoracic glands. Near the very end of each larval stage, the neuropeptide eclosion hormone (EH), combined with a decline in the titer of ecdysone, prompts release of the peptide ecdysis-triggering hormone (ETH) from glands lining the trachea. ETH promotes behaviors essential for escaping the old exoskeleton, and also stimulates neurons to secrete more EH, creating a positive feedback loop that culminates in a hormonal surge decisive for ecdysis. Environmental cues, including photoperiod, temperature, and humidity, as well as physiologic factors including size, stage, and the nutritional status of the organism, modulate secretion of PTTH in various arthropods, suggesting extensive sensory input to the neuroendocrine secretions that govern molting. However, little is known about circuits that initiate, terminate, or set the pace of the molting cycle in any Ecdysozoan.

Although an endocrine trigger for nematode molting has yet to be identified, several lines of evidence implicate steroid hormones in C. elegans molting. Molting of C. elegans requires cholesterol, the biosynthetic precursor of all steroid hormones, as well as the LDL receptor-like protein LRP-1, which is thought to endocytose sterols from the growth medium. A sterol-modifying enzyme synthesized in the intestine, LET-767, is also essential for ecdysis, consistent with the production or modification of a hormone derived from steroids. The best evidence of a hormonal cue for molting of C. elegans is the requirement for two nuclear hormone receptors, NHR-23 and NHR-25, orthologous, respectively, to the ecdysone-responsive gene products DHR3 and Ftz-F1 of Drosophila melanogaster. Ecdysone itself, however, is unlikely to serve as a molting hormone in nematodes because ecdysteroids have not been detected in any free-living nematode, and because orthologs of the ecdysone receptor components EcR and USP (ultraspiracle) have not been identified in the complete genome of C. elegans.

The exoskeleton of nematodes, called the cuticle, is a collagenous extracellular matrix secreted by underlying epithelial cells, known as the hypodermis and seam, and by specialized interfacial cells that line openings of the body, including the buccal cavity, pharynx, vulva, and, anus. Molting involves the synthesis and secretion of a new cuticle underneath the old one, separation of the old cuticle from the epidermis (apolysis), and shedding of the pre-molt cuticle (ecdysis}. Near the very end of each stage, larvae become inactive for a brief period of time, known as lethargus, coinciding with d etachment of the old cuticle from the hypodermis. Stereotypical behaviours then promote ecdysis; larvae flip on their long axis to loosen the body cuticle, regurgitate the anterior half of the pharyngeal cuticle, and then escape the old exoskeleton via forward thrust s. Although e lasticity of the cuticle permits growth during each larval stage, particular structures, such as the buccal cavity, grow saltationally at molts. The distinction between collagen in the nematode exoskeleton and chitin in the insect exoskeleton suggests that the enzymatic cascades that mediate release of the exoskeleton in nematodes may be distinct from those that release the exoskeleton in arthropods. Although two collagenases essential for molting have been identified in C. elegans, the full ensemble of membrane signaling proteins and extracellular matrix enzymes required to remodel the exoskeleton has yet to be illuminated.

Click here for the full summary of our work on the functional analysis of C. elegans molting.