BMECs generated from iPSC provide new experimental models for studying blood brain barrier function and dysfunction. We have used this protocol to examine whether BMECs retain their barrier properties after expansion and cryopreservation. This technique enables the generation of disease-specific BMECs from patients with disorders in which blood-brain barrier disruption plays a role in the pathophysiology of disease.
To induce BMEC differentiation from human iPSCs, wash the confluent iPSC culture with DPBS one time before incubating the cells with the appropriate volume of enzymatic EDTA for approximately five minutes at 37 degrees Celsius. When a single cell suspension has been obtained collect the cells by centrifugation and resuspend the pellet in an appropriate volume of stem cell medium supplemented with 10 micromolar ROCK inhibitor for counting. Dilute the cells to 1.56 times 10 to the fourth cells per cubic centimeter concentration and seed two milliliters of cells into individual wells of a six-well flat bottom.
After 24 hours in the cell culture incubator replace the supernatant in each well with E6 medium and return the plate to the incubator for four more days. To purify the iPSC-derived BMECs, coat the subculturing plates with the appropriate volume of a freshly prepared collagen IV and fibronectin solution per well and incubate the plates for a minimum of two hours at 37 degrees Celsius. On day six of differentiation collect the cells by centrifugation and resuspend the pellets in the appropriate volume of fresh human endothelial serum-free medium supplemented with B27, basic fibroblasts growth factor, and retinoic acid.
For TEER analysis, plate the differentiated cells into each well of a collagen and fibronectin coated 12-transwell filtered plate. For efflux transporter activity analysis, plate the cells into each well of a collagen and fibronectin coated 24-well flat bottom plate. After 24 hours of subculture, replace the supernatant in each well with fresh human endothelial serum-free medium supplemented only with B27 supplement.
To measure the blood brain barrier properties of the BMECs using TEER, charge the TEER instrument overnight and lightly wipe the instrument and chopstick electrodes with 70%ethanol before placing them into the safety hood. Switch on the power and calibrate the ohm meter according to the manufacturer's specifications. Plug in the electrodes and rinse them one time with 70%ethanol and one time with DPBS.
Place the shorter end of the electrode into the atypical chamber of the insert and the longer end into the basolateral chamber of a blank well. After measuring the negative control response, quickly rinse the electrodes as demonstrated and measure the first experimental well. After all of the measurements have been recorded rinse chopstick electrodes again, gently wipe the electrodes and let them air dry in the safety hood.
To assess the efflux transporter activity of the cells, on day eight of culturing treat the appropriate number of wells with a 10 micromolar efflux transporter inhibitor solution and place in an incubator for one hour at 37 degrees Celsius. At the end of the incubation, treat the cells with a 10 micromolar efflux transporter substrate solution for one hour in the incubator. At the end of the incubation, wash each well twice with 500 microliters of DPBS per well before lysing the cells with 500 microliters DPBS supplemented with 5%Triton-X per well.
After five minutes, use a microplate reader to measure the fluorescence of the lysed cells at a 485 nanometer excitation and a 530 nanometer emission. For wells not used in the transporter assay, wash the cells twice with DPBS before fixing with 4%paraformaldehyde per cell nuclei quantification. After one day of culture in E6 medium, the cell morphology is similar to that of iPSCs.
By day four, the cells are visibly distinct from iPSCs and cover most of the well. Two days of culture in human endothelial serum-free medium induces an elongated and cobblestone appearance in the cells. At day eight, individual cells form a mostly large cobblestone pattern.
A sprouting assay can be performed to demonstrate the angiogenic potential of iPSC-derived BMECs. This leads to the development of tube-like structures after three days of growth factor treatment. The iPSC-derived BMECs generated using this protocol co-expressed tight junction proteins expressed in endothelial cells in the brain, lung, liver, and kidney, as well as vascular endothelial markers expressed at the blood brain barrier.
As observed, 48 hours after subculture and medium change iPSC-derived BMECs demonstrate TEER values within the range described for iPSC-derived BMECs that were co-cultured with rat primary astrocytes. Efflux transporter activity was also observed at this stage. After cryopreservation, TEER measurements are lower compared to freshly derived BMECs.
Western blot analysis reveals reduced levels of tight junction markers, such as occludin, while immunocytochemistry shows freckled and or frayed patterns of tight junction proteins. A too high or too low iPSC density will affect the differentiation efficiency and impact the development of appropriate blood brain barrier properties. This method enables the investigation of barrier properties cellular signaling and transporter activity using individualized BMECs to model the physiology and pathophysiology of the blood-brain barrier function.
