Chipping Away at Blood-Brain-Barrier Modeling

In this issue, Vatine et al., 2019Vatine G.D. Barrile R. Workman M.J. Sances S. Barriga B.K. Rahnama M. Barthakur S. Kasendra M. Lucchesi C. Wen N. et al.Human iPSC-derived blood brain barrier-chips enable disease modeling and personalized medicine applications.Cell Stem Cell. 2019; 24Abstract Full Text Full Text PDF PubMed Scopus (236) Google Scholar present a fully human blood-brain barrier chip that accurately predicts drug permeability and can be perfused with whole blood. Utilizing patient-derived tissue, they recapitulate disease-specific defects and establish a platform to advance drug screening and disease modeling. In this issue, Vatine et al., 2019Vatine G.D. Barrile R. Workman M.J. Sances S. Barriga B.K. Rahnama M. Barthakur S. Kasendra M. Lucchesi C. Wen N. et al.Human iPSC-derived blood brain barrier-chips enable disease modeling and personalized medicine applications.Cell Stem Cell. 2019; 24Abstract Full Text Full Text PDF PubMed Scopus (236) Google Scholar present a fully human blood-brain barrier chip that accurately predicts drug permeability and can be perfused with whole blood. Utilizing patient-derived tissue, they recapitulate disease-specific defects and establish a platform to advance drug screening and disease modeling. Neurons in the brain are dependent on a near-constant supply of energy, provided by the bloodstream in the form of glucose and oxygen. However, many other components of blood are neurotoxic. Thus, brain microvessels have evolved a set of structural and functional characteristics that tightly regulate transport at this interface, and this is termed the blood-brain barrier (BBB). Induction and regulation of BBB function is not well understood, and disruption of one or more features of the BBB has been observed during the progression of several neurological diseases (Zhao et al., 2015Zhao Z. Nelson A.R. Betsholtz C. Zlokovic B.V. Establishment and dysfunction of the blood-brain barrier.Cell. 2015; 163: 1064-1078Abstract Full Text Full Text PDF PubMed Scopus (833) Google Scholar), suggesting that an improved understanding of BBB function in health and disease could lead to novel therapeutics. The use of animal models in BBB studies is limited by cross-species differences in the expression and activity of BBB transporters because these might lead to differences in drug uptake and unpredictable clinical results. In addition, some human neurological diseases do not have analogs in animals, thus an in vitro model of the human BBB could help address these knowledge gaps. Microfluidic devices have emerged as high-throughput tissue culture systems that recapitulate various aspects of the vasculature and enable organ-chip systems. These devices are particularly powerful when combined with derivatives of human induced pluripotent stem cells (hiPSCs). For instance, a microfluidic device was recently used to elucidate the role of primary cilia in regulating the response to shear stress in hiPSC-derived endothelial cells (Smith et al., 2018Smith Q. Macklin B. Chan X.Y. Jones H. Trempel M. Yoder M.C. Gerecht S. Differential HDAC6 activity modulates ciliogenesis and subsequent mechanosensing of endothelial cells derived from pluripotent stem cells.Cell Rep. 2018; 24: 895-908.e6Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar). In vitro models of human brain vasculature range in complexity and should be designed to recapitulate key features of the BBB essential for the model's target application. These 'benchmarks' include parameters related to structure, microenvironment, barrier function, and interactions between different cell types (reviewed in DeStefano et al., 2018DeStefano J.G. Jamieson J.J. Linville R.M. Searson P.C. Benchmarking in vitro tissue-engineered blood-brain barrier models.Fluids Barriers CNS. 2018; 15: 32Crossref PubMed Scopus (81) Google Scholar). In this issue, Vatine et al. fuse hiPSC-derived cells with organ-chip technology to provide a fully human BBB-chip for studying disease (Figure 1; (Vatine et al., 2019Vatine G.D. Barrile R. Workman M.J. Sances S. Barriga B.K. Rahnama M. Barthakur S. Kasendra M. Lucchesi C. Wen N. et al.Human iPSC-derived blood brain barrier-chips enable disease modeling and personalized medicine applications.Cell Stem Cell. 2019; 24Abstract Full Text Full Text PDF PubMed Scopus (236) Google Scholar). The authors differentiated patient-derived hiPSCs into brain microvascular endothelial cells (iBMECs), astrocytes, and neural cells and cultured these derivatives in a microfluidic device with separate channels representing the brain or blood sides of brain vasculature. The authors applied this model to investigate BBB dysfunction in Huntington's disease (HD) and Allen-Herndon-Dudley syndrome (AHDS) and to test potential therapies. Vatine et al. began by examining the response of iBMECs to shear stress. Here the authors observed changes, including upregulations in occludin and CD31, in endothelial gene expression, suggesting an advantage over static BBB models. To investigate iBMEC identity, they compared iBMECs with multiple endothelial and epithelial sources and found that iBMECs grouped most closely with endothelial cells. Intriguingly, choroid plexus epithelium grouped closest to primary BMECs, suggesting additional studies are needed to distinguish endothelial and epithelial cells across different tissues. With the addition of primary human pericytes and astrocytes, Vatine et al. observed a 3-fold decrease in the permeability of 3 kDa dextran. Conversely, they noted an increase in permeability in response to the cytokines TNF-α, IL-1β, and IL-8, in agreement with the established effects of these factors in vivo. They also matched brain:blood ratios of IgG, albumin, and transferrin, further illustrating model applicability. In their isogenic co-culture chip, which used a neural cell blend derived from hiPSCs, the authors noted a similar decrease in permeability from co-culture. By isolating and sequencing RNA from iBMECs subjected to each co-culture, the authors found distinct effects, with the neural cell blend markedly boosting iBMEC proliferation. They also demonstrated on-chip measurement of transendothelial electrical resistance (TEER) and reported values that correspond well with Transwell measurements. The authors then examined the use of diseased hiPSC lines, extending observations from 2D Transwells that revealed impaired BBB functionalities in iBMECs derived from patients with HD (Lim et al., 2017Lim R.G. Quan C. Reyes-Ortiz A.M. Lutz S.E. Kedaigle A.J. Gipson T.A. Wu J. Vatine G.D. Stocksdale J. Casale M.S. et al.Huntington's disease iPSC-derived brain microvascular endothelial cells reveal WNT-mediated angiogenic and blood-brain barrier deficits.Cell Rep. 2017; 19: 1365-1377Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar) or AHDS (Vatine et al., 2017Vatine G.D. Al-Ahmad A. Barriga B.K. Svendsen S. Salim A. Garcia L. Garcia V.J. Ho R. Yucer N. Qian T. et al.Modeling psychomotor retardation using iPSCs from MCT8-deficient patients indicates a prominent role for the blood-brain barrier.Cell Stem Cell. 2017; 20: 831-843.e5Abstract Full Text Full Text PDF PubMed Scopus (141) Google Scholar). Using the HD-BBB-Chip, permeability was increased across all dextran sizes examined, suggesting significant disruption to BBB function. AHDS is caused by a deficiency in MCT8, a lipid transporter required to transport thyroid hormone (triiodothyronine, T3) into the brain. Indeed, MCT8-deficient-BBB-Chips demonstrated reduced T3 transport from the blood side to the brain side. Importantly, for these investigations, the authors used three healthy cell lines as a control, demonstrating that inter-individual variability was insignificant compared to differences observed between healthy and diseased lines. The authors then tested the permeabilities of several approved drugs and demonstrated that the BBB-Chip reflected their expected behaviors, suggesting the utility of the BBB-Chip for testing potential therapies. Finally, Vatine et al. demonstrated an additional capability of the BBB-Chip—the ability for the iBMECs to protect the neural cells from blood-induced cytotoxicity. Whereas in the control (no iBMECs), blood components leaked into the brain compartment and reduced neural cell viability, the presence of iBMECs provided a barrier that shielded neural cells from blood. Thus, the platform successfully recapitulated one of the most essential features of the BBB. While most BBB models use cell culture media for convenience, blood has a different viscosity and its complex composition may affect the transport of some molecules typically bound in blood (such as T3), thus the ability to perform studies using whole blood may further increase the model's physiological relevance. It is a challenging task to engineer functional coordination among different cells of the neurovascular unit, and as such, this study also highlights areas deserving of additional exploration. Brain arterioles, capillaries, and venules are unique with regard to structural elements (vessel diameter, perivascular phenotype) and function (regulation of blood flow, immune cell trafficking). The present device, consisting of a single wide channel divided into blood and brain compartments, is limited in its ability to capture differences in zonation along the brain vasculature. It also uses a PDMS scaffold, which does not recapitulate properties of the brain extracellular matrix (ECM). Potential adaptations of the model could incorporate capillary networks formed by self-organization (Campisi et al., 2018Campisi M. Shin Y. Osaki T. Hajal C. Chiono V. Kamm R.D. 3D self-organized microvascular model of the human blood-brain barrier with endothelial cells, pericytes and astrocytes.Biomaterials. 2018; 180: 117-129Crossref PubMed Scopus (328) Google Scholar), microvessels templated in the ECM (Linville et al., 2019Linville R.M. DeStefano J.G. Sklar M.B. Xu Z. Farrell A.M. Bogorad M.I. Chu C. Walczak P. Cheng L. Mahairaki V. et al.Human iPSC-derived blood-brain barrier microvessels: Validation of barrier function and endothelial cell behavior.Biomaterials. 2019; 190-191: 24-37Crossref PubMed Scopus (97) Google Scholar), or feature a hybrid fabrication strategy employing both techniques. In addition, the culture of neural cells on the brain side could be adapted from 2D to 3D to further recapitulate the CNS environment. Although the authors have observed partial coverage of astrocyte endfeet, the coverage of in vivo brain vasculature is more extensive, suggesting a need for further study of the formation of astrocyte endfeet. Lastly, despite their importance in vivo, pericytes were found to be dispensable with regard to barrier function in the present work. Additional studies are ongoing to understand their role in supporting cerebral vasculature (Jamieson et al., 2019Jamieson J.J. Linville R. Ding Y. Gerecht S. Searson P.C. Role of iPSC-derived pericytes on barrier function of iPSC-derived brain microvascular endothelial cells in 2D and 3D.Fluids Barriers CNS. 2019; https://doi.org/10.1186/s12987-019-0136-7Crossref PubMed Scopus (62) Google Scholar, Stebbins et al., 2019Stebbins M.J. Gastfriend B.D. Canfield S.G. Lee M.S. Richards D. Faubion M.G. Li W.J. Daneman R. Palecek S.P. Shusta E.V. Human pluripotent stem cell-derived brain pericyte-like cells induce blood-brain barrier properties.Sci Adv. 2019; 5: eaau7375https://www.ncbi.nlm.nih.gov/pubmed/30891496Crossref PubMed Scopus (92) Google Scholar). Building toward these advances in engineering vasculature and forming heterogeneous cell-cell contacts may help recapitulate and study complex processes such as neurovascular coupling and model BBB dysfunction observed in aging and disease. Human iPSC-Derived Blood-Brain Barrier Chips Enable Disease Modeling and Personalized Medicine ApplicationsVatine et al.Cell Stem CellJune 06, 2019In BriefThe blood-brain barrier (BBB) is a multicellular neurovascular unit that tightly regulates brain homeostasis and is disturbed in several neurological diseases. Combining patient-specific stem cells and microfluidic technologies, Vatine et al. have generated a personalized human BBB-Chip, which recapitulates the human BBB and can predict variability between individuals. Full-Text PDF Open Archive