Region‐to‐Region Unidirectional Connection In Vitro Brain Model for Studying Directional Propagation of Neuropathologies

ABSTRACT In the human brain, neurons establish long‐range, unidirectional connections between distinct regions, allowing directional transmission of neuronal signals and the transport of neurotoxic proteins. In this study, a region‐to‐region unidirectional connection in an in vitro brain model that enables the one‐way transfer of signals and molecules is reconstructed. By embedding one brain organoid in hydrogel earlier than the other and promoting neurite outgrowth, it is induced preferential axonal extension toward the later‐seeded organoid, where synaptic connections are formed. This structurally defined unidirectional linkage demonstrated functional unidirectionality, as verified by direction‐specific propagation of electrical activity using dual neural probe monitoring. The model also reproduced directional neuroinflammatory spread following region‐specific introduction of activated microglia. Furthermore, it permitted unidirectional transfer of neurotoxic proteins, including amyloid beta (Aβ), Tau, and alpha‐synuclein (α‐synuclein). Owing to its ability to recapitulate in vivo‐like neuronal functionality, this in vitro brain model provides a valuable experimental platform for elucidating the directional propagation of neuropathological processes.