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Relationship: 1742

Title

The title of the KER should clearly define the two KEs being considered and the sequential relationship between them (i.e., which is upstream and which is downstream). Consequently all KER titles take the form “upstream KE leads to downstream KE”.  More help

Inhibition, CHS-1 leads to Decrease, Cuticular chitin content

Upstream event
Upstream event in the Key Event Relationship. On the KER page, clicking on the Event name under Upstream Relationship will bring the user to that individual KE page. More help
Downstream event
Downstream event in the Key Event Relationship. On the KER page, clicking on the Event name under Upstream Relationship will bring the user to that individual KE page. More help

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes. Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

This table is automatically generated upon addition of a KER to an AOP. All of the AOPs that are linked to this KER will automatically be listed in this subsection. Clicking on the name of the AOP in the table will bring you to the individual page for that AOP. More help
AOP Name Adjacency Weight of Evidence Quantitative Understanding Point of Contact Author Status OECD Status
S-adenosylmethionine depletion leading to population decline (1) adjacent Agnes Aggy (send email) Under development: Not open for comment. Do not cite
Chitin synthase 1 inhibition leading to mortality adjacent Moderate Low Brendan Ferreri-Hanberry (send email) Open for comment. Do not cite

Taxonomic Applicability

Select one or more structured terms that help to define the biological applicability domain of the KER. In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER. Authors can indicate the relevant taxa for this KER in this subsection. The process is similar to what is described for KEs (see pages 30-31 and 37-38 of User Handbook) More help
Term Scientific Term Evidence Link
crustaceans Daphnia magna Moderate NCBI
insects insects Moderate NCBI

Sex Applicability

Authors can indicate the relevant sex for this KER in this subsection. The process is similar to what is described for KEs (see pages 31-32 of the User Handbook). More help
Sex Evidence
Unspecific Moderate

Life Stage Applicability

Authors can indicate the relevant life stage for this KER in this subsection. The process is similar to what is described for KEs (see pages 31-32 of User Handbook). More help
Term Evidence
larvae High
Juvenile High
Adult Moderate

Key Event Relationship Description

Provide a brief, descriptive summation of the KER. While the title itself is fairly descriptive, this section can provide details that aren’t inherent in the description of the KEs themselves (see page 39 of the User Handbook). This description section can be viewed as providing the increased specificity in the nature of upstream perturbation (KEupstream) that leads to a particular downstream perturbation (KEdownstream), while allowing the KE descriptions to remain generalised so they can be linked to different AOPs. The description is also intended to provide a concise overview for readers who may want a brief summation, without needing to read through the detailed support for the relationship (covered below). Careful attention should be taken to avoid reference to other KEs that are not part of this KER, other KERs or other AOPs. This will ensure that the KER is modular and can be used by other AOPs. More help

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

Assembly and description of the scientific evidence supporting KERs in an AOP is an important step in the AOP development process that sets the stage for overall assessment of the AOP (see pages 49-56 of the User Handbook). To do this, biological plausibility, empirical support, and the current quantitative understanding of the KER are evaluated with regard to the predictive relationships/associations between defined pairs of KEs as a basis for considering WoE (page 55 of User Handbook). In addition, uncertainties and inconsistencies are considered. More help
Biological Plausibility
Define, in free text, the biological rationale for a connection between KEupstream and KEdownstream. What are the structural or functional relationships between the KEs? For example, there is a functional relationship between an enzyme’s activity and the product of a reaction it catalyses. Supporting references should be included. However, it is recognised that there may be cases where the biological relationship between two KEs is very well established, to the extent that it is widely accepted and consistently supported by so much literature that it is unnecessary and impractical to cite the relevant primary literature. Citation of review articles or other secondary sources, like text books, may be reasonable in such cases. The primary intent is to provide scientifically credible support for the structural and/or functional relationship between the pair of KEs if one is known. The description of biological plausibility can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured (see page 40 of the User Handbook for further information).   More help

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.

Uncertainties and Inconsistencies
In addition to outlining the evidence supporting a particular linkage, it is also important to identify inconsistencies or uncertainties in the relationship. Additionally, while there are expected patterns of concordance that support a causal linkage between the KEs in the pair, it is also helpful to identify experimental details that may explain apparent deviations from the expected patterns of concordance. Identification of uncertainties and inconsistencies contribute to evaluation of the overall WoE supporting the AOPs that contain a given KER and to the identification of research gaps that warrant investigation (seep pages 41-42 of the User Handbook).Given that AOPs are intended to support regulatory applications, AOP developers should focus on those inconsistencies or gaps that would have a direct bearing or impact on the confidence in the KER and its use as a basis for inference or extrapolation in a regulatory setting. Uncertainties that may be of academic interest but would have little impact on regulatory application don’t need to be described. In general, this section details evidence that may raise questions regarding the overall validity and predictive utility of the KER (including consideration of both biological plausibility and empirical support). It also contributes along with several other elements to the overall evaluation of the WoE for the KER (see Section 4 of the User Handbook).  More help

The major uncertainty in this KER is the absence of studies which assess both endpoints, the inhibition of the chitin synthase and the decrease in cuticular chitin content after exposure to specific stressors.

Response-response Relationship
This subsection should be used to define sources of data that define the response-response relationships between the KEs. In particular, information regarding the general form of the relationship (e.g., linear, exponential, sigmoidal, threshold, etc.) should be captured if possible. If there are specific mathematical functions or computational models relevant to the KER in question that have been defined, those should also be cited and/or described where possible, along with information concerning the approximate range of certainty with which the state of the KEdownstream can be predicted based on the measured state of the KEupstream (i.e., can it be predicted within a factor of two, or within three orders of magnitude?). For example, a regression equation may reasonably describe the response-response relationship between the two KERs, but that relationship may have only been validated/tested in a single species under steady state exposure conditions. Those types of details would be useful to capture.  More help

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.

Time-scale
This sub-section should be used to provide information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). This can be useful information both in terms of modelling the KER, as well as for analyzing the critical or dominant paths through an AOP network (e.g., identification of an AO that could kill an organism in a matter of hours will generally be of higher priority than other potential AOs that take weeks or months to develop). Identification of time-scale can also aid the assessment of temporal concordance. For example, for a KER that operates on a time-scale of days, measurement of both KEs after just hours of exposure in a short-term experiment could lead to incorrect conclusions regarding dose-response or temporal concordance if the time-scale of the upstream to downstream transition was not considered. More help

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. In studies where CHS-1 was knocked down, chitin contents were assessed after 3 and 7 days and found to be decreased (Arakane et al. 2005, Li et al. 2017, Zhang X. et al. 2010).

Known modulating factors
This sub-section presents information regarding modulating factors/variables known to alter the shape of the response-response function that describes the quantitative relationship between the two KEs (for example, an iodine deficient diet causes a significant increase in the slope of the relationship; a particular genotype doubles the sensitivity of KEdownstream to changes in KEupstream). Information on these known modulating factors should be listed in this subsection, along with relevant information regarding the manner in which the modulating factor can be expected to alter the relationship (if known). Note, this section should focus on those modulating factors for which solid evidence supported by relevant data and literature is available. It should NOT list all possible/plausible modulating factors. In this regard, it is useful to bear in mind that many risk assessments conducted through conventional apical guideline testing-based approaches generally consider few if any modulating factors. More help

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
This subsection should define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits? In some cases where feedback processes are measurable and causally linked to the outcome, they should be represented as KEs. However, in most cases these features are expected to predominantly influence the shape of the response-response, time-course, behaviours between selected KEs. For example, if a feedback loop acts as compensatory mechanism that aims to restore homeostasis following initial perturbation of a KE, the feedback loop will directly shape the response-response relationship between the KERs. Given interest in formally identifying these positive or negative feedback, it is recommended that a graphical annotation (page 44) indicating a positive or negative feedback loop is involved in a particular upstream to downstream KE transition (KER) be added to the graphical representation, and that details be provided in this subsection of the KER description (see pages 44-45 of the User Handbook).  More help

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

As for the KEs, there is also a free-text section of the KER description that the developer can use to explain his/her rationale for the structured terms selected with regard to taxonomic, life stage, or sex applicability, or provide a more generalizable or nuanced description of the applicability domain than may be feasible using standardized terms. More help

Taxonomic: Likely, this KER is likely 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.

References

List of the literature that was cited for this KER description using the appropriate format. Ideally, the list of references should conform, to the extent possible, with the OECD Style Guide (OECD, 2015). More help

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.

Arakane, Y.; Muthukrishnan, S.; Kramer, K. J.; Specht, C. A.; Tomoyasu, Y.; Lorenzen, M. D.; Kanost, M.; Beeman, R. W. The Tribolium Chitin Synthase Genes TcCHS1 and TcCHS2 Are Specialized for Synthesis of Epidermal Cuticle and Midgut Peritrophic Matrix. Insect Mol. Biol. 2005, 14 (5), 453–463. https://doi.org/10.1111/j.1365-2583.2005.00576.x.

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.

Li, T.; Chen, J.; Fan, X.; Chen, W.; Zhang, W. MicroRNA and DsRNA Targeting Chitin Synthase A Reveal a Great Potential for Pest Management of the Hemipteran Insect Nilaparvata Lugens. Pest Manag. Sci. 2017, 73 (7), 1529–1537. https://doi.org/10.1002/ps.4492.

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.; Zhang, J.; Zhu, K. Y. Chitosan/Double-Stranded RNA Nanoparticle-Mediated RNA Interference to Silence Chitin Synthase Genes through Larval Feeding in the African Malaria Mosquito (Anopheles Gambiae). Insect Mol. Biol. 2010, 19 (5), 683–693. https://doi.org/10.1111/j.1365-2583.2010.01029.x.

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.