Upstream eventIncrease, CHS-1 inhibition
Decrease, Cuticular chitin content
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|S-adenosylmethionine depletion leading to population decline (1)||adjacent|
|Chitin synthase 1 inhibition leading to mortality||adjacent||Moderate||Low|
Life Stage Applicability
Key Event Relationship Description
Chitin in the arthropod cuticle is synthesized by the chitin synthase isoform 1 (CHS-1) which spans the plasma membrane on the apical plasma membrane of epithelial cells (Locke and Huie 1979; Binnington 1985; Merzendorfer and Zimoch 2003; Merzendorfer 2006). Since CHS-1 is the enzyme to polymerize chitin from UDP-N-Acetylglucosamine (UDP-GlcNAc) (Merzendorfer 2006), it is solely responsible for the content of chitin in the exoskeleton. Consequently, the inhibition of CHS-1 leads to a decrease in chitin content in the arthropod cuticle.
Evidence Supporting this KER
The process of chitin synthesis in arthropods is well characterized. Although the exact mechanism of the polymerization reaction remains elusive, CHS-1 is known to be the key enzyme in the biosynthesis of chitin and therefore, responsible for the cuticular chitin content (Merzendorfer and Zimoch 2003; Merzendorfer 2006). Therefore, the biological plausibility of this KER can be regarded as high.
Empirical evidence for the occurrence of both KEs, the inhibition of CHS-1 and the decrease in cuticular chitin content exist. For example, the occurrence of chitin synthase inhibition was characterized using cell free crude enzyme preparations in vitro from coleopteran, lepidopteran and dipteran insect species upon treatment with polyoxin B, polyoxin D and nikkomycin Z (Cohen and Casida 1982; Kuwano and Cohen 1984; Cohen and Casida 1990; Zhang and Yan Zhu 2013). The cuticular chitin content was characterized in vivo in Artemia salina or using cultured integumental tissue from lepidopteran and dipteran species after exposure to polyoxin D and nikkomycin Z as well as the phthalimides captan, captafol, and folpet (Gijswijt et al. 1979; Turnbull and Howells 1982; Calcott and Fatig 1984; Gelman and Borkovec 1986; Zhuo et al. 2014). However, studies assessing both endpoints and therefore linking both KEs are lacking.
Uncertainties and Inconsistencies
The major uncertainty in this KER is the absence of studies which assess both, the inhibition of the chitin synthase and the decrease in cuticular chitin content.
Quantitative Understanding of the Linkage
Due to the lack of studies linking the inhibition of CHS-1 to the decrease in cuticular chitin content, it is not possible to describe the nature of the response-response relationship.
Due to the lack of studies assessing the inhibition of CHS-1 and the decrease in cuticular chitin content, it is not possible to make a statement on the timescale of the relationship. However, the expression of CHS-1 peaks at the time of ecdysis (Ampasala et al. 2011; Wang et al. 2012), indicating the highest rate of chitin synthesis at this timepoint. Hence it can be assumed that a decrease in chitin content in the newly synthesized cuticle should become apparent shortly after.
Known modulating factors
CHS is dependent on bivalent ions as cofactor such as Mg2+ or Mn2+ (Merzendorfer 2006). Both low and high levels of Mg2+ inhibited CHS activity in vitro (Zhang and Yan Zhu 2013).
Known Feedforward/Feedback loops influencing this KER
Upon knockdown of CHS-1 in the salmon louse Lepeophtheirus salmonis, upregulation of the UDP-GlcNAc pyrophosphorylase (UAP), which catalyzes the conversion of GlcNAc to UDP-GlcNAc, was observed (Braden et al. 2020). The knockdown of UAP also led to upregulation of CHS-1 demonstrating a clear dependence of the two enzymes. Most likely, the upregulation of UAP is a compensatory mechanism with the goal to restore homeostasis in absence of CHS-1. The exact regulation of the feedback, however, remains to be investigated.
Domain of Applicability
Taxonomic: Likely, this KER is applicable to the whole phylum of arthropods as they all depend on the synthesis of chitin.
Life stage: This KER is applicable for organisms synthesizing chitin in order to grow and develop, namely larval stages of insects and all life stages of crustaceans and arachnids.
Sex: This KER is applicable to all sexes.
Chemical: Substances inducing both, the inhibition of CHS-1 and the decrease in cuticular chitin content are of the family of pyrimidine nucleosides (e.g. polyoxin D, polyoxin B and nikkomycin Z) (Gijswijt et al. 1979; Cohen and Casida 1982; Turnbull and Howells 1982; Calcott and Fatig 1984; Kuwano and Cohen 1984; Cohen and Casida 1990; Zhang and Yan Zhu 2013; Zhuo et al. 2014; Osada 2019). The phthalimide captan was also shown to induce CHS-1 inhibition and a decrease in cuticular chitin content (Cohen and Casida 1982; Gelman and Borkovec 1986). However, studies assessing both endpoints in sequence are lacking.
Ampasala DR, Zheng S, Zhang D, Ladd T, Doucet D, Krell PJ, Retnakaran A, Feng Q. 2011. An epidermis-specific chitin synthase cDNA in Choristoneura fumiferana: Cloning, characterization, developmental and hormonal-regulated expression. Arch Insect Biochem Physiol. 76(2):83–96. doi:10.1002/arch.20404.
Binnington KC. 1985. Ultrastructural changes in the cuticle of the sheep blowfly, Lucilia, induced by certain insecticides and biological inhibitors. Tissue Cell. 17(1):131–140. doi:10.1016/0040-8166(85)90021-7.
Braden L, Michaud D, Igboeli OO, Dondrup M, Hamre L, Dalvin S, Purcell SL, Kongshaug H, Eichner C, Nilsen F, et al. 2020. Identification of critical enzymes in the salmon louse chitin synthesis pathway as revealed by RNA interference-mediated abrogation of infectivity. Int J Parasitol. 50(10–11):873–889. doi:10.1016/j.ijpara.2020.06.007. https://doi.org/10.1016/j.ijpara.2020.06.007.
Calcott PH, Fatig RO. 1984. Inhibition of Chitin metabolism by Avermectin in susceptible Organisms. J Antibiot (Tokyo). 37(3):253–259. doi:10.7164/antibiotics.37.253.
Cohen E, Casida JE. 1982. Properties and inhibition of insect integumental chitin synthetase. Pestic Biochem Physiol. 17(3):301–306. doi:10.1016/0048-3575(82)90141-9.
Cohen E, Casida JE. 1990. Insect and Fungal Chitin Synthetase Activity: Specificity of Lectins as Enhancers and Nucleoside Peptides as Inhibitors. Pestic Biochem Physiol. 37(3):249–253. doi:10.1016/0048-3575(90)90131-K.
Gelman DB, Borkovec AB. 1986. The pharate adult clasper as a tool for measuring chitin synthesis and for identifying new chitin synthesis inhibitors. Comp Biochem Physiol Part C, Comp. 85(1):193–197. doi:10.1016/0742-8413(86)90073-3.
Gijswijt MJ, Deul DH, de Jong BJ. 1979. Inhibition of chitin synthesis by benzoyl-phenylurea insecticides, III. Similarity in action in Pieris brassicae (L.) with Polyoxin D. Pestic Biochem Physiol. 12(1):87–94. doi:10.1016/0048-3575(79)90098-1.
Kuwano E, Cohen E. 1984. The use of a Tribolium chitin synthetase assay in studying the effects of benzimidazoles with a terpene moiety and related compounds. Agric Biol Chem. 48(6):1617–1620. doi:10.1080/00021369.1984.10866362.
Locke M, Huie P. 1979. Apolysis and the Turnover of Plasmamembrane Plaques during Cuticle formation in an Insect. Tissue Cell. 11(2):277–291. doi:10.1016/0040-8166(79)90042-9.
Merzendorfer H. 2006. Insect chitin synthases: A review. J Comp Physiol B Biochem Syst Environ Physiol. doi:10.1007/s00360-005-0005-3.
Merzendorfer H, Zimoch L. 2003. Chitin metabolism in insects: structure, function and regulation of chitin synthases and chitinases. J Exp Biol. 206(24):4393 LP – 4412. doi:10.1242/jeb.00709. http://jeb.biologists.org/content/206/24/4393.abstract.
Osada H. 2019. Discovery and applications of nucleoside antibiotics beyond polyoxin. J Antibiot (Tokyo). 72(12):855–864. doi:10.1038/s41429-019-0237-1. http://dx.doi.org/10.1038/s41429-019-0237-1.
Turnbull IF, Howells AJ. 1982. Effects of several larvicidal compounds on chitin biosynthesis by isolated larval integuments of the sheep blowfly Lucilia cuprina. Aust J Biol Sci. 35(5):491–504. doi:10.1071/BI9820491.
Wang Y, Fan HW, Huang HJ, Xue J, Wu WJ, Bao YY, Xu HJ, Zhu ZR, Cheng JA, Zhang CX. 2012. Chitin synthase 1 gene and its two alternative splicing variants from two sap-sucking insects, Nilaparvata lugens and Laodelphax striatellus (Hemiptera: Delphacidae). Insect Biochem Mol Biol. 42(9):637–646. doi:10.1016/j.ibmb.2012.04.009. http://dx.doi.org/10.1016/j.ibmb.2012.04.009.
Zhang X, Yan Zhu K. 2013. Biochemical characterization of chitin synthase activity and inhibition in the African malaria mosquito, Anopheles gambiae. Insect Sci. 20(2):158–166. doi:10.1111/j.1744-7917.2012.01568.x.
Zhuo W, Fang Y, Kong L, Li X, Sima Y, Xu S. 2014. Chitin synthase A: A novel epidermal development regulation gene in the larvae of Bombyx mori. Mol Biol Rep. 41(7):4177–4186. doi:10.1007/s11033-014-3288-1.