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Event: 1522

Key Event Title

The KE title should describe a discrete biological change that can be measured. It should generally define the biological object or process being measured and whether it is increased, decreased, or otherwise definably altered relative to a control state. For example “enzyme activity, decreased”, “hormone concentration, increased”, or “growth rate, decreased”, where the specific enzyme or hormone being measured is defined. More help

Inhibition, Chitin synthase 1

Short name
The KE short name should be a reasonable abbreviation of the KE title and is used in labelling this object throughout the AOP-Wiki. The short name should be less than 80 characters in length. More help
Inhibition, CHS-1

Biological Context

Structured terms, selected from a drop-down menu, are used to identify the level of biological organization for each KE. Note, KEs should be defined within a particular level of biological organization. Only KERs should be used to transition from one level of organization to another. Selection of the level of biological organization defines which structured terms will be available to select when defining the Event Components (below). More help
Level of Biological Organization

Cell term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help
Cell term
cuticle secreting cell

Organ term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help
Organ term

Key Event Components

Further information on Event Components and Biological Context may be viewed on the attached pdf.Because one of the aims of the AOP-KB is to facilitate de facto construction of AOP networks through the use of shared KE and KER elements, authors are also asked to define their KEs using a set of structured ontology terms (Event Components). In the absence of structured terms, the same KE can readily be defined using a number of synonymous titles (read by a computer as character strings). In order to make these synonymous KEs more machine-readable, KEs should also be defined by one or more “event components” consisting of a biological process, object, and action with each term originating from one of 22 biological ontologies (Ives, et al., 2017; See List). Biological process describes dynamics of the underlying biological system (e.g., receptor signalling). The biological object is the subject of the perturbation (e.g., a specific biological receptor that is activated or inhibited). Action represents the direction of perturbation of this system (generally increased or decreased; e.g., ‘decreased’ in the case of a receptor that is inhibited to indicate a decrease in the signalling by that receptor).Note that when editing Event Components, clicking an existing Event Component from the Suggestions menu will autopopulate these fields, along with their source ID and description. To clear any fields before submitting the event component, use the 'Clear process,' 'Clear object,' or 'Clear action' buttons. If a desired term does not exist, a new term request may be made via Term Requests. Event components may not be edited; to edit an event component, remove the existing event component and create a new one using the terms that you wish to add. More help
Process Object Action
chitin synthase activity decreased

Key Event Overview

AOPs Including This Key Event

All of the AOPs that are linked to this KE will automatically be listed in this subsection. This table can be particularly useful for derivation of AOP networks including the KE. Clicking on the name of the AOP will bring you to the individual page for that AOP. More help
AOP Name Role of event in AOP Point of Contact Author Status OECD Status
SAM depletion leading to population decline (1) MolecularInitiatingEvent You Song (send email) Under development: Not open for comment. Do not cite
CHS-1 inhibition leading to mortality MolecularInitiatingEvent Simon Schmid (send email) Open for comment. Do not cite


This is a structured field used to identify specific agents (generally chemicals) that can trigger the KE. Stressors identified in this field will be linked to the KE in a machine-readable manner, such that, for example, a stressor search would identify this as an event the stressor can trigger. NOTE: intermediate or downstream KEs in one AOP may function as MIEs in other AOPs, meaning that stressor information may be added to the KE description, even if it is a downstream KE in the pathway currently under development.Information concerning the stressors that may trigger an MIE can be defined using a combination of structured and unstructured (free-text) fields. For example, structured fields may be used to indicate specific chemicals for which there is evidence of an interaction relevant to this MIE. By linking the KE description to a structured chemical name, it will be increasingly possible to link the MIE to other sources of chemical data and information, enhancing searchability and inter-operability among different data-sources and knowledgebases. The free-text section “Evidence for perturbation of this MIE by stressor” can be used both to identify the supporting evidence for specific stressors triggering the MIE as well as to define broad chemical categories or other properties that classify the stressors able to trigger the MIE for which specific structured terms may not exist. More help

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected from an ontology. In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. More help
Term Scientific Term Evidence Link
Anopheles gambiae Anopheles gambiae High NCBI
Tribolium castaneum Tribolium castaneum High NCBI
Trichoplusia ni Trichoplusia ni High NCBI
Hyalophora cecropia Hyalophora cecropia High NCBI
Bradysia hygida Bradysia hygida Moderate NCBI
Mamestra brassicae Mamestra brassicae Moderate NCBI
Chilo suppressalis Chilo suppressalis Moderate NCBI
Locusta migratoria Locusta migratoria Moderate NCBI
Nilaparvata lugens Nilaparvata lugens Moderate NCBI
Aphis glycines Aphis glycines Moderate NCBI
Lepeophtheirus salmonis Lepeophtheirus salmonis Moderate NCBI
Panonychus citri Panonychus citri Moderate NCBI
Grapholita molesta Grapholita molesta Moderate NCBI
Ectropis obliqua Ectropis obliqua Moderate NCBI
Tigriopus japonicus Tigriopus japonicus Moderate NCBI

Life Stages

The structured ontology terms for life-stage are more comprehensive than those for taxa, but may still require further description/development and explanation in the free text section. More help
Life stage Evidence
larvae High
Juvenile High
Adult Moderate

Sex Applicability

No help message More help
Term Evidence
Unspecific Moderate

Key Event Description

A description of the biological state being observed or measured, the biological compartment in which it is measured, and its general role in the biology should be provided. For example, the biological state being measured could be the activity of an enzyme, the expression of a gene or abundance of an mRNA transcript, the concentration of a hormone or protein, neuronal activity, heart rate, etc. The biological compartment may be a particular cell type, tissue, organ, fluid (e.g., plasma, cerebrospinal fluid), etc. The role in the biology could describe the reaction that an enzyme catalyses and the role of that reaction within a given metabolic pathway; the protein that a gene or mRNA transcript codes for and the function of that protein; the function of a hormone in a given target tissue, physiological function of an organ, etc. Careful attention should be taken to avoid reference to other KEs, KERs or AOPs. Only describe this KE as a single isolated measurable event/state. This will ensure that the KE is modular and can be used by other AOPs, thereby facilitating construction of AOP networks. More help

Chitin synthases are essential enzymes for all organisms synthesizing chitin, for example arthropods and fungi (Latgé 2007; Merzendorfer 2011). Chitin synthases polymerize chitin and subsequently translocate chitin through the cell membrane (Merzendorfer 2006; Merzendorfer 2011). In arthropods, two isoforms of the chitin synthase are known, CHS1, which is responsible for the synthesis of cuticular chitin, and chitin synthase isoform 2, which synthesizes chitin in the midgut (Arakane et al. 2005). In this MIE, inhibition of CHS-1 is characterized. The biological state being measured is the activity of the enzyme. CHS-1 has an essential role in the cuticle biology, as it constitutes the last and most critical step in the chitin biosynthetic pathway by catalyzing the polymerization of UDP-GlcNAc to chitin (Merzendorfer and Zimoch 2003; Merzendorfer 2006).

How It Is Measured or Detected

One of the primary considerations in evaluating AOPs is the relevance and reliability of the methods with which the KEs can be measured. The aim of this section of the KE description is not to provide detailed protocols, but rather to capture, in a sentence or two, per method, the type(s) of measurements that can be employed to evaluate the KE and the relative level of scientific confidence in those measurements. Methods that can be used to detect or measure the biological state represented in the KE should be briefly described and/or cited. These can range from citation of specific validated test guidelines, citation of specific methods published in the peer reviewed literature, or outlines of a general protocol or approach (e.g., a protein may be measured by ELISA).Key considerations regarding scientific confidence in the measurement approach include whether the assay is fit for purpose, whether it provides a direct or indirect measure of the biological state in question, whether it is repeatable and reproducible, and the extent to which it is accepted in the scientific and/or regulatory community. Information can be obtained from the OECD Test Guidelines website and the EURL ECVAM Database Service on Alternative Methods to Animal Experimentation (DB-ALM). ?

Since the purification or even recombinant production of CHS-1 has not been achieved yet, the most common way is to use crude enzyme preparations for CHS-1 activity assays. It is noteworthy that in crude enzyme preparations of whole organisms both CHS isoforms, CHS-1 and CHS-2, are present. However, the expression of CHS-1 was shown to be much higher than CHS-2 in Anopheles gambiae (Zhang et al. 2012), therefore the effect of CHS-2 may be regarded as negligible. Alternatively, the digestive tract of the respective organism could be removed before producing the enzyme preparation. Different ways exist to detect the activity of the enzyme. One can incubate the enzyme preparation with radioactively labelled chitin precursors (e.g. 14C-UDP-GlcNAc) and measure radioactivity in the formed chitin chains by scintillation counting (Cohen 1982; Cohen and Casida 1990). Chitin synthase activity can also be measured in a non-radioactive way after the addition of precursors to a crude enzyme extract. There, the detection of CHS-1 activity involves the binding of chitin chains to wheat germ agglutinin (WGA) which possesses specific chitin binding properties (Lucero et al. 2002; Zhang and Yan Zhu 2013). The assay builds on the principle of a sandwich-ELISA, where chitin binds to a layer of WGA. A second layer of WGA which is conjugated to horseradish peroxidase (HRP) is then added and subsequently incubated with a HRP substrate. The cleavage of the HRP substrate leads to color formation and the amount of chitin synthesized can be determined colorimetrically.

Domain of Applicability

This free text section should be used to elaborate on the scientific basis for the indicated domains of applicability and the WoE calls (if provided). While structured terms may be selected to define the taxonomic, life stage and sex applicability (see structured applicability terms, above) of the KE, the structured terms may not adequately reflect or capture the overall biological applicability domain (particularly with regard to taxa). Likewise, the structured terms do not provide an explanation or rationale for the selection. The free-text section on evidence for taxonomic, life stage, and sex applicability can be used to elaborate on why the specific structured terms were selected, and provide supporting references and background information.  More help

Taxonomic: Effect data for the occurrence of CHS1 inhibition exist from Dipteran, Lepidopteran and Coleopteran insect species. Sequence alignment of CHS1 protein sequences using the Sequence Alignment to Predict Across Species Susceptibility (SeqAPASS, tool, yielded susceptibility predictions for various insect species, arachnids and crustacean taxa such as branchiopods, hexanauplia, malocostraca and merostomata. However, most of the protein sequences were not identified as CHS1. The alignment of amino acid residues believed to be critical for ligand binding were therefore carried out with sequences identified as CHS1. Evidence was rated as high for species with a susceptibility prediction and effect data. Evidence was rated as moderate when only alignment data were available. Although most of the sequences are not annotated as CHS1, all arthropods rely on the synthesis of cuticular chitin therefore it is extremely likely that this MIE is applicable to the whole phylum of arthropods.

Life stage: This MIE is applicable for organisms undergoing continuous molt cycles. Namely larval stages of insects and all life stages of crustaceans and arachnids.

Sex: The MIE is applicable to all sexes.

Chemical: Substances known to trigger inhibit CHS-1 are of the family of pyrimidine nucleosides (e.g. polyoxin D, polyoxin B and nikkomycin Z) (Cohen and Casida 1982; Kuwano and Cohen 1984; Cohen and Casida 1990; Zhang and Yan Zhu 2013; Osada 2019). There also exists evidence for the phthalimide captan to inhibit CHS-1 activity in vitro (Cohen and Casida 1982). However, as phthalimides are known to covalently bind to thiol groups in proteins (Lukens and Sisler 1958), it is not clear if the inhibition is due to specific CHS-1 inhibition or due to unspecific protein binding.

Evidence for Perturbation by Stressor

Overview for Molecular Initiating Event

When a specific MIE can be defined (i.e., the molecular target and nature of interaction is known), in addition to describing the biological state associated with the MIE, how it can be measured, and its taxonomic, life stage, and sex applicability, it is useful to list stressors known to trigger the MIE and provide evidence supporting that initiation. This will often be a list of prototypical compounds demonstrated to interact with the target molecule in the manner detailed in the MIE description to initiate a given pathway (e.g., 2,3,7,8-TCDD as a prototypical AhR agonist; 17α-ethynyl estradiol as a prototypical ER agonist). Depending on the information available, this could also refer to chemical categories (i.e., groups of chemicals with defined structural features known to trigger the MIE). Known stressors should be included in the MIE description, but it is not expected to include a comprehensive list. Rather initially, stressors identified will be exemplary and the stressor list will be expanded over time. For more information on MIE, please see pages 32-33 in the User Handbook.

Stressors known to competitively inhibit CHS1 are polyoxin B, polyoxin D and Nikkomycin Z (Cohen and Casida 1982; Cohen and Casida 1990; Zhang and Yan Zhu 2013). There may also be stressors that inhibit CHS-1 in a non-competitive manner which may become apparent in further characterization efforts of this MIE. There is also a study that reports inhibition of CHS-1 by the phthalimide fungicide captan (Cohen and Casida 1982). However, it remains elusive if the observed inhibition is due to specific interaction with the enzyme or due to unspecific protein binding which is the predominant mode of action of phthalimides (Lukens and Sisler 1958).


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

Arakane Y, Muthukrishnan S, Kramer KJ, Specht CA, Tomoyasu Y, Lorenzen MD, Kanost M, Beeman RW. 2005. The Tribolium  chitin synthase genes TcCHS1 and TcCHS2 are specialized for synthesis of epidermal cuticle and midgut peritrophic matrix. Insect Mol Biol. 14(5):453–463. doi:10.1111/j.1365-2583.2005.00576.x.

Cohen E. 1982. In vitro chitin synthesis in an insect: formation and structure of microfibrils. Eur J Cell Biol. 26(2):289–294.

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.

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.

Latgé JP. 2007. The cell wall: A carbohydrate armour for the fungal cell. Mol Microbiol. 66(2):279–290. doi:10.1111/j.1365-2958.2007.05872.x.

Lucero HA, Kuranda MJ, Bulik DA. 2002. A nonradioactive, high throughput assay for chitin synthase activity. Anal Biochem. 305(1):97–105. doi:10.1006/abio.2002.5594.

Lukens RJ, Sisler HD. 1958. 2-Thiazolidinethione-4-carboxylic acid from the reaction of captan with cysteine. Science (80- ). 127(3299):650. doi:10.1126/science.127.3299.650.

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. 2011. The cellular basis of chitin synthesis in fungi and insects: Common principles and differences. Eur J Cell Biol. 90(9):759–769. doi:10.1016/j.ejcb.2011.04.014.

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.

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.

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.

Zhang X, Zhang J, Park Y, Zhu KY. 2012. Identification and characterization of two chitin synthase genes in African malaria mosquito, Anopheles gambiae. Insect Biochem Mol Biol. 42(9):674–682. doi:10.1016/j.ibmb.2012.05.005.