Bone loss, commonly seen in osteoporosis, is a condition that entails a progressive decline of bone mineral density and microarchitecture, often seen in post-menopausal women. Bone loss has been widely reported in astronauts exposed to a plethora of stressors and in patients with osteoporosis following radiotherapy for cancer. Studies on mechanisms are well documented but the causal connectivity of events to bone loss development remains incompletely understood. Herein, the adverse outcome pathway (AOP) framework was used to organize data and develop a qualitative AOP beginning from deposition of energy (the molecular initiating event) to bone loss (the adverse outcome). A literature review was conducted to compile and evaluate the state of knowledge based on the modified Bradford Hill criteria. Following review of 1865 studies, an empirically supported AOP was developed, showing the progression to bone loss through many factors affecting the activities of bone-forming osteoblasts and bone-resorbing osteoclasts. The structural, functional, and quantitative basis of each proposed relationship was defined, for inference of causal changes between key events. Current knowledge and its gaps relating to dose-, time- and incidence-concordance across the key events were identified, as well as modulating factors that influence linkages. The new priorities for research informed by the AOP highlight areas for improvement to enable development of a quantitative AOP used to support risk assessment strategies for space travel or cancer radiotherapy.

The understanding of radiation-induced non-cancer effects on the central nervous system (CNS) is essential for the medical setting (e.g., radiotherapy), and occupational exposures, such as nuclear workers or astronauts. Herein, the adverse outcome pathway (AOP) approach was used to consolidate relevant studies in the area of cognitive decline for identification of research gaps, countermeasure development, and for eventual use in risk assessments. AOPs are an analytical construct describing critical events to an adverse outcome (AO) in a simplified form beginning with a molecular initiating event (MIE). An AOP was constructed utilizing mechanistic information to build empirical support for the key event relationships (KERs) between the MIE of deposition of energy to the AO of learning and memory impairment through multiple key events (KEs). The evidence for the AOP was developed through a scoping review of the literature. In this AOP, the MIE is connected to the AO via six KEs of increased oxidative stress, increased deoxyribonucleic acid (DNA) strand breaks, altered signaling pathways, tissue resident cell activation, increased pro-inflammatory mediators and neural remodeling. Deposition of energy directly leads to oxidative stress, increased DNA strand breaks, an increase of pro-inflammatory mediators and tissue resident cell activation. These KEs, which are themselves interconnected, converge through increased DNA strand breaks, altered signaling pathways and pro-inflammatory routes and directly lead to neural remodeling. Broadly, it is envisioned that the outcome of these efforts could be applied to other cognitive disorders and support ongoing work by international authorities to review the system of radiological protection.

Cataracts are one of the leading causes of blindness, with estimated 95 million people affected worldwide. A hallmark of cataract development is lens opacification, typically associated not only with aging, but also radiation exposure as encountered by interventional radiologists and astronauts during the long-term space mission. To better understand radiation-induced cataracts, the adverse outcome pathway (AOP) framework was used to structure and evaluate knowledge across biological levels of organization. AOPs identify a sequence of key events (KEs) connected by key event relationships (KERs) beginning with a molecular initiating event (MIE) to an adverse outcome (AO) of relevance to regulatory decision-making. To construct the cataract AOP, a scoping review methodology was used to filter, screen, and review studies based on the modified Bradford Hill criteria. Eight KEs were moderately supported by empirical evidence across the adjacent (directly-linked) relationships using well-established endpoints. Over half of the evidence collected was informed by biological plausibility. Early KEs of oxidative stress and protein modifications could be the focus of countermeasures. Several identified knowledge gaps and inconsistencies in the AOP can be the basis of future research, most notably directed to experiments at low or moderate doses and dose-rates, relevant to radiation workers and other occupational exposures.