Fig. 4. A potential model for C6-conditioned media protection against hypoxia-induced changes in paracellular permeability. (A) Cells grown in MEM/F12 and subjected to a 24-hour hypoxic stress undergo a breakdown of the TJ, with a resulting increase in paracellular permeability as measured by [¹⁴C]-sucrose flux. This breakdown of the TJ is probably caused by some dissociation of the component proteins, with resulting formation of actin stress fibers and removal of occludin and ZO-1 from their normal membrane-associated subcellular locations (Mark and Davis, 2002). We hypothesize that under C6-CM co-culture conditions (B), secreted factors in C6-CM trigger the activation of signal transduction mechanisms, linked to NFB or other as yet unidentified pathways. This treatment allows for an adaptive response in the BBMEC when they are exposed to 24 hours hypoxic stress, such that they respond by increasing their expression of claudin-1 and actin. These increases in claudin-1 and actin enable the BBMEC to build additional TJ, thereby preventing the increase in paracellular permeability seen under MEM/F12 conditions. However, C6-CM treatment may also protect via the maintenance of already existing TJ; the exact mechanisms remain to be elucidated.