Invertebrates found a way to adapt without the two typical cannabinoid receptors, CB1 and CB2. Emerging evidence suggests insects, such as fruit flies, contain endocannabinoids. But they evolved with two unique receptors alongside a set of sensory channels. C. Elegans is a nematode, a worm species, with an unusually broad genotype. Recent research discovered that the nematodes depend on an exotic endocannabinoid system (ECS) to utilize certain sensory and metabolic functions. (1)
Cannabinoid receptors in insects
Anandamide and 2-AG act as endogenous keys that unlock CB1 receptors in humans and other vertebrates. 2-AG separately binds to human CB2 receptors. Insects, conversely, do not contain CB1 or CB2 receptors, according to earlier research by John McPartland. (2) Following studies have, however, found special receptors in insects that respond to classic endocannabinoids.
Insects contain the neuropeptide receptor NPR-19 for endocannabinoid binding — rather than CB1 or CB2 receptors. NPR-19 has a closely matching amino acid conformation. It is, therefore, known as a functional orthologue of human cannabinoid receptors. And NPR-32 is a function orthologue of additional receptors in the endocannabinoidome, including GRP35 and GPR55. (3, 4)
Cancelling out NPRs
Neuropeptide receptors respond to endocannabinoid ligands, including 2-AG in nematodes. Dopamine and serotonin also release upon NPR-19 activation. But the receptors are likely not the system that 2-AG targets to regulate sterol distribution.
The team assessed NPR-19 and NPR-32 for a role in cholesterol storage. Genes that delete neuropeptide receptors failed to stop 2-AG-driven cholesterol storage, though. This means insects do not require classic cannabinoid receptor orthologues to store or use cholesterol. The researchers did, however, eventually find genes in the insulin pathway allow 2-AG to mediate sterols.
2-AG drives cholesterol storage in worm species
Caenorhabditis Elegans cannot internally synthesize cholesterol and must obtain it from the diet. The worm depends on 2-AG to store cholesterol, a process previously identified in 2016. (5) Subsequently, PLOS One published a new study on nematode endocannabinology. (1)
TRP channels are calcium ion transporters. A gene enables 2-AG’s ability to mobilize cholesterol, known as ocr-2. And the gene allows animals to produce TRPV channels, a family of calcium ion transporters.
Beyond gene mutations, a compound that blocks calcium channels inhibited 2-AG. This further indicates that 2-AG depends on the sensory channel to drive a metabolic complex. Conclusively, C. Elegans sense endocannabinoids internally with TRP channels. Yet a larger system is involved in nematode cholesterol storage downstream of 2-AG.
Metabolic pathways in insect ECS
Elegans critically require 2-AG to activate a signalling chain through an insulin-specific pathway to store cholesterol from their diet. UNC-31 is a protein that helps secrete calcium-dependent neurotransmitters that assist with G-protein activation. Insulin peptides released by 2-AG depend on UNC-31 and require a second protein, HID-1.
2-AG’s second task involves cholesterol removal from storehouses, and its hydrogen bonds allow it to bind to the sterol. Nematodes with defective genes that encode a receptor related to the sterol regulatory element (SREBP) family, sbp-1, also failed to mobilize cholesterol with 2-AG.
DAF-2, a gene that codes for insulin growth factor 1 (IF1), might be responsible for a metabolic shift. Further in the path, dafachronic acids (DA), a steroid hormone, deactivates a nematode defence state when it binds to the hormone-sensing receptor coded by DEF-12 genes. AKT-1, a signalling protein involved in energy homeostasis, inhibits DAF-16 at the end of the signalling path.
How insects recycle 2-AG
An enzyme known as monoglycerol metabolizes 2-AG in humans. However, other enzymes are involved in endocannabinoid degradation in humans but especially in insects. FAAH normally metabolizes anandamide, but a specific type, known as FAAH-4, conversely breaks down 2-AG in C. Elegans.
While metabolism is unique, the synthesis of endocannabinoids is similar in insects and mammals. Nape-1 and nape-2 genes function as endocannabinoid precursors in C. Elegans, which compares to anandamide’s mammalian precursor — NAPE-phospholipase-D.
Conclusion
Nematodes evolved a unique endocannabinoid system with typical sensory features that assist with growth under cholesterol scarcity.
Sources
Hernandez-Cravero, B., Gallino, S., Florman, J., Vranych, C., Diaz, P., Elgoyhen, A. B., Alkema, M. J., & de Mendoza, D. (2022). Cannabinoids activate the insulin pathway to modulate mobilization of cholesterol in C. elegans. PLoS genetics, 18(11), e1010346. Advance online publication. https://doi.org/10.1371/journal.pgen.1010346
McPartland, John & Agraval, J & Gleeson, Dianne & Heasman, Kevin & Glass, Michelle. (2006). Cannabinoid receptors in invertebrates. Journal of evolutionary biology. 19. 366-73. 10.1111/j.1420-9101.2005.01028.x.
Pastuhov, S. I., Matsumoto, K., & Hisamoto, N. (2016). Endocannabinoid signaling regulates regenerative axon navigation in Caenorhabditis elegans via the GPCRs NPR-19 and NPR-32. Genes to cells : devoted to molecular & cellular mechanisms, 21(7), 696–705. https://doi.org/10.1111/gtc.12377
Clarke TL, Johnson RL, Simone JJ, Carlone RL. The Endocannabinoid System and Invertebrate Neurodevelopment and Regeneration. Int J Mol Sci. 2021;22(4):2103. Published 2021 Feb 20. doi:10.3390/ijms22042103
Corsi, Ann & Wightman, Bruce & Chalfie, Martin. (2015). A Transparent Window into Biology: A Primer on Caenorhabditis elegans. WormBook : the online review of C. elegans biology. 200. 1-31. 10.1895/wormbook.1.177.1.
Galles, C., Prez, G.M., Penkov, S. et al. Endocannabinoids in Caenorhabditis elegans are essential for the mobilization of cholesterol from internal reserves. Sci Rep 8, 6398 (2018).