Fig. 2

Network of adverse outcome pathways (AOPs). Individual linear AOPs were combined into a derived network based on commonly shared KEs and KERs. The network was then visualized and interpreted using a software called Cytoscape 3.7.2 (https://cytoscape.org/), an open-source platform for construction, analysis and visualization of biological networks [145]. In Cytoscape, KEs were represented as nodes (or vertices) in a network and KERs as edges connecting nodes. The network was directionally analyzed (meaning the directional relationship between source and target nodes is preserved) using the built-in plugin NetworkAnalyzer 2.7, which computes the topological parameters of the network (such as edge count, and centrality measures) in a directional, or non-directional manner . Six AOPs were used to build the derived AOP network for inhalation toxicity induced by nanomaterials. For visualization purposes, the KERs (depicted as arrowed edges) of each AOP were color coded to clearly highlight which pathway they are associated with. KE node positions (i.e. final placement of the KE node in the graphic, but not connectivity) were manually set to maximize readability and space. This does not modify the results of the topological network analysis. Adjacent KERs are depicted as solid arrows. Associative KERs are highlighted as dashed arrows in the network, but were otherwise treated normally for the directed network analysis. Non-adjacent KEs are depicted using contiguous arrows. The most hyperconnected KEs were determined to be the nodes with a definite in-degree/out-degree ratio > 0 (indicating there are inputs and outputs from the node making it a KE in an AOP) and the highest edge count (number of connecting KERs) following network analysis. Convergent KEs were defined as having a larger in degree than out degree parameter (more relationships feed in than leave this KE). Similarly, divergent KEs were defined as having smaller in degree than out degree values (More relationships branch out from this KE than feed into it). Green: molecular initiating events; Orange: cellular level key events; Purple: tissue level key events; red: adverse outcomes. Arrows are colored based on corresponding AOP. Solid arrows represent adjacent key events. Dashed arrows represent associative events. Contiguous arrows represent non-adjacent KEs. The most highly connected KE is bordered in blue