Organ-On-A-Chip Models of the Blood-Brain Barrier: Recent Advances and Future Prospects

Abstract

The human brain and central nervous system (CNS) present unique challenges in drug development for neurological diseases. One major obstacle is the blood-brain barrier (BBB), which hampers the effective delivery of therapeutic molecules into the brain while protecting it from blood-born neurotoxic substances and maintaining CNS homeostasis. For BBB research, traditional in vitro models rely upon Petri dishes or Transwell systems. However, these static models lack essential microenvironmental factors such as shear stress and proper cell-cell interactions. To this end, organ-on-a-chip (OoC) technology has emerged as a new in vitro modeling approach to better recapitulate the highly dynamic in vivo human brain microenvironment so-called the neural vascular unit (NVU). Such BBB-on-a-chip models have made substantial progress over the last decade, and concurrently there has been increasing interest in modeling various neurological diseases such as Alzheimer's disease and Parkinson's disease using OoC technology. In addition, with recent advances in other scientific technologies, several new opportunities to improve the BBB-on-a-chip platform via multidisciplinary approaches are available. In this review, an overview of the NVU and OoC technology is provided, recent progress and applications of BBB-on-a-chip for personalized medicine and drug discovery are discussed, and current challenges and future directions are delineated.

Keywords: blood-brain barriers; disease modeling; drug discovery; organ-on-a-chips; personalized medicine.

Figure 1.

Schematic representation of the brain microenvironment. ECs form the luminal side of the BBB by creating a monolayer with TJ proteins to segregate blood from the CNS. The abluminal layer is formed by pericytes and astrocytes. Astrocytes form linkages between ECs and neurons at their end feet. Neurons and microglial cells constitute the rest of the parenchymal cell population. The BBB includes brain ECs, astrocytes, and pericytes whereas the NVU refers to the BBB with neurons and microglia.^[27] The inset shows a cross-sectional image of a brain microvessel.

Figure 2.

TJs and AJs at the BBB. TJs and AJs are formed in between adjacent BMECs. ^{[73, 74]} The four major TJ proteins include JAMs, occludin, claudin, and ZO whereas VE-cadherin is a major AJ protein and binds catenins in the cytoplasm. Both TJ and AJ proteins contribute to the structural integrity of ECs through their interactions with actin filaments.

Figure 3.

Six major pathways that regulate the transport of molecules across the BBB.^{[73, 76]} (a) RMT is mediated by receptors on the cell surface to transport target molecules. (b) Adsorptive transcytosis occurs when positively charged molecules come in close proximity to the negatively charged membrane. (c) Efflux transporters such as the ABC function through ATP hydrolysis or ATP binding (e.g. P-gp). (d) In carrier-mediated transport, molecules are loaded onto carriers with high specificity at the cell membrane and transported across the EC layer. (e) Passive diffusion is defined as the non-specific transport of small molecules. (f) Unlike the transcellular pathways, paracellular transport involves hydrophilic molecules passing through the space in between adjacent ECs.

Figure 4.

BBB-on-a-chip designs. (a) Evolution of the in vitro culture platform from a 2D Petri dish model to microfluidic OoC. Examples of recent BBB-on-a-chip models. (b) Sandwiched-channel design (Reproduced with permission.^[77] Copyright 2019, Springer Nature). (c) Parallel-channel design (Reproduced with permission.^[177] Copyright 2021, Springer Nature). (d) Interlinked BBB and brain-on-a-chips with sandwiched design (Reproduced with permission.^[81] Copyright 2018, Springer Nature). (e) 3D printed BBB-on-a-chip with sandwiched-channel design (Reproduced with permission.^[113] Copyright 2021, Wiley Online Library). (f) BBB-on-a-chip with a bioprinted brain construct (Reproduced with permission.^[178] Copyright 2021, Wiley Online Library).

Figure 5.

TEER sensor integration. (a-b) An OoC with four deposited electrodes. Reproduced with permission.^[119] Copyright 2017, Royal Society of Chemistry. (c-d) TEER-MEA. Reproduced with permission.^[201] Copyright 2017, Royal Society of Chemistry. (e-f) An OoC model with Pt-wire electrodes. Reproduced with permission.^[203] Copyright 2021, Royal Society of Chemistry.

Figure 6.

(a) SEM image of the PtTPTBPF palladium base oxygen sensing dye integrated microparticles and the oxygen sensing integrated into the BBB chip. Reproduced with permission.^[207] Copyright 2019, American Chemical Society. (b) Schematic representation of the BBB-on-a-chip integrated with the three-dimensional cell culturing setup, the micro solid-phase extractor, and the final electrospray ion mass spectrometry set-up. Reproduced with permission.^[208] Copyright 2016, Elsevier.

Figure 7.

Alzheimer’s disease (AD)-on-a-chip. (a) Schematic of the AD-on-a-chip device. (b) Experimental protocol used for the study. (c) Comparisons between wildtype (WT) and AD groups of permeability to tracer molecules of different molecular weights. (d) Claudin-5 expression patterns in WT and AD models. Reproduced with permission.^[170] Copyright 2021, Wiley.

Figure 8.

Parkinson’s disease (PD) on a chip. (a) Study protocol used for the development of the PD-on-a-chip. (b) Schematic representation of the PD-on-a-chip platform. (c) Expression of phosphorylated αSyn after treatment with αSyn fibrils versus αSyn monomers. (d) Effects of αSyn fibrils on ZO-1 expression and presence of pSer129-αSyn. (e) Permeability of the BBB after treatment with αSyn fibrils or monomers. Reproduced with permission.^[229] Copyright 2021, Springer Nature.

Figure 9.

Stroke-on-a-chip. (a) (i) Schematic diagram of the stroke-on-a-chip device. (ii) Experimental protocol used in the study. (b) Effects of ischemia on cellular morphology and expression of markers by neurons and endothelial cells. (c) Permeability of the modeled BBB in ischemia versus normoxia group. (d) Tracking of hiPSC-derived neural progenitor cells (hNPCs) via (i) GFP, (ii) stem cell markers, and (iii) differentiation markers. Reproduced with permission.^[177] Copyright 2021, Springer Nature.

Figure 10.

Glioblastoma (GBM)-on-a-chip. (a) Schematic depiction of the GBM-on-a-chip. (b) Experimental protocol followed in the study. (c) Confocal images showing interactions between CD8+ T cells, GBM cells, and EC cells. (d) Proportions of occurrence of apoptosis in GBM cells treated with activated CD8+ T cells in different GBM niches. Reproduced with permission. ^[228] Copyright 2020, eLife.

Figure 11.

Ongoing efforts and proposed future directions to enhance the utility of the current BBB-on-a-chip platform. (a) Potential areas of development. (b) An example process flow for the implementation of a potential next-generation BBB-on-a-chip platform.