Towards learning and memory risk assessment with human brain organoids: barriers and opportunities

Neurodevelopmental conditions, including autism spectrum disorder, intellectual disability, and learning disabilities, as well as neurodegenerative disorders, affect millions of people in the United States alone. Both genetic and environmental factors contribute to their onset, yet traditional neurotoxicity testing often fails to identify specific risks or mechanisms underlying cognitive impairment. Human brain organoids (hBOs), also called neural organoids or brain microphysiological systems, are three-dimensional (3D) stem cell-derived models that recapitulate key features of human brain development and offer greater physiological relevance than traditional 2D in vitro or animal models. The emerging field of "organoid intelligence" integrates these systems with advanced bioengineering and artificial intelligence to model higher-order neural functions and assess learning and memory-relevant endpoints that were previously less explored in vitro. Despite this promise, we have identified four key barriers that hinder the application of hBOs for the hazard identification phase of functional neurotoxic risk assessments: [1] limited maturity and regional complexity, [2] lack of high-throughput defined procedures for assessing cognitive development and function in vitro, [3] limited standardization for reproducibility, and [4] challenges in translating in vitro results to human health outcomes. Here, we outline current efforts to overcome these challenges, i.e., scientific, technical, and regulatory advances. We also illustrate how hBO-based assays can be applied to advance both mechanistic understanding and the regulatory evaluation of environmental (developmental) neurotoxicants, using heavy metals as a model.