Configurable Models of the Neurovascular Unit

Abstract

The blood-brain barrier (BBB) restricts the paracellular diffusion of compounds into and out of the brain and is extremely important for maintaining brain homeostasis and proper neuronal function. The BBB is composed of multiple cell types arranged in a complex 3D structure that enables sophisticated interplay between endothelial cells, pericytes, glial cells, and neurons. This complex is known as the “neurovascular unit” (NVU). Several neuropathological conditions, including dementia and stroke which are two of the top ten causes of death worldwide, are associated with BBB dysfunction, but there is still often a lack of understanding of the underlying mechanisms and causal relationships. Representative, translatable preclinical models of the NVU are needed to facilitate a better mechanistic understanding of the relationship between BBB dysfunction and neurological diseases, which is critical for developing new treatments. They have the potential to be used to test the efficacy, toxicology, and delivery of drugs. A plethora of factors such as cell origin, co-culture, shear, substrate stiffness, substrate biochemical composition, 3D structure, etc. greatly affect BBB permeability and thus NVU integrity. Advancements in materials science, microfluidics, and fabrication techniques, for example, soft lithography and 3D bio-printing, have enabled increased control of pertinent factors leading to countless different configurations. Hence, the field has seen a gradual movement away from static models, where non-human cells are cultured in 2D in relatively rigid semi-permeable membranes made of PET or polycarbonate, toward dynamic organ-on-a-chip systems where multiple human NVU cell types are co-cultured under physiological shear in 3D tubular structures that recreate in vivo architecture using biomaterials found in native tissue with representative biomechanical properties and biochemical composition. Furthermore, models have been significantly enhanced by the incorporation of analytical technologies, including live cell imaging and TEER measurement, with further improvement possible using bioelectronics. However, several challenges still remain and applications in fields such as neuropsychiatry are still scarce. This chapter aims to discuss the importance and evolution of in vitro NVU models, key considerations, various configurations that have been developed, their main features, and the variety of fabrication methods that have been used to create them.