Bridging the gap: Canine in vitro models of the central nervous system as tools to study pathogenetic mechanisms in neurodegenerative disease and neuroinfectiology.

The increasing implementation of the 3R principles of reduce, replace, and refine, as well as costs and ethical considerations associated with animal experiments, will lead to an incremental reduction in in vivo experimentation. To fill the gap between in vitro generated data and findings obtained following in vivo animal experiments, there is an urgent need for reliable and robust in vitro models with high comparability to the respective in vivo situation. This applies in particular for models of the central nervous system (CNS) due its unique anatomical and physiological characteristics with the resulting limited accessibility for diagnostic and therapeutic approaches and, simultaneously, the devastating consequences of CNS diseases. Sophisticated in vitro canine models will contribute to progression in veterinary medicine and are uniquely suited to study a broad range of human diseases, including monogenic hereditary illnesses, cancers, degenerative and inflammatory diseases, and epilepsy. In addition, notable neurotropic or pantropic pathogens such as rabies virus, canine distemper virus, canine herpesvirus, Toxoplasma sp., and tick-borne encephalitis, as well as previously unknown emerging pathogens, can be studied directly, providing much needed information about cell tropism, pathogen spread, and underlying mechanisms. This review summarizes established canine 2-dimensional and 3-dimensional in vitro models of the CNS including traditional monoculture and coculture systems as well as more advanced systems such as neurospheres, organoids, and organotypic brain slice cultures. The aim is to assess their suitability, current state, and potential applications, as well as challenges and limitations with special focus on neurodegenerative and neuroinfectious diseases.