The blood-brain barrier (BBB) is a physical and metabolic barrier that separates the central nervous system from the peripheral circulation. Central nervous system drug delivery across the BBB is challenging, primarily because of the physical restriction of paracellular diffusion between the endothelial cells that comprise the microvessels of the BBB and the activity of efflux transporters that quickly expel back into the capillary lumen a wide variety of xenobiotics. Therapeutic manipulation of protein trafficking is emerging as a novel means of modulating protein function, and in this minireview, the targeting of the trafficking of 2 key BBB proteins, P-glycoprotein and occludin, is presented as a novel, reversible means of optimizing central nervous system drug delivery.
The blood-brain barrier (BBB) is critical to the health of the central nervous system. The BBB is formed primarily by the presence of tight junctions (TJ) between cerebral microvessel endothelial cells. In light of the known effects of nicotine on endothelial cell biology, the specific effects of nicotine on the in vivo BBB were examined. Using in situ brain perfusion, it was found that continuous administration of nicotine (4.5 mg free base x kg(-1) x day(-1)) for 1 and 7 days led to increased permeability of the BBB to [14C]-sucrose without significant changes in its initial volume of distribution. The expression and distribution of the TJ-associated proteins actin, occludin, claudin-1, -3, and -5, and ZO-1 and -2 were analyzed by Western blot and immunofluorescence microscopy. Though no changes in total protein expression were observed, nicotine treatment was associated with altered cellular distribution of ZO-1 and diminished junctional immunoreactivity of claudin-3. It is proposed that nicotine leads to changes in BBB permeability via the modulation of TJ proteins.
Peptide-based drug development is a rapidly growing field within pharmaceutical research. Nevertheless, peptides have found limited clinical use due to several physiological and pathological factors. Pluronic block copolymers represent a growing technology with the potential to enhance efficacy of peptide therapeutics. This investigation assesses Pluronic P85 (P85) and its potential to enhance opioid peptide analgesia. Two opioid peptides, [D-Pen(2),D-Pen(5)]-enkephalin (DPDPE) and biphalin, were examined as to the benefits of P85 coadministration, above (1.0%) and below (0.01%) the critical micelle concentration, with morphine as a nonpeptide control. P85 was examined in vitro to assess blood-brain barrier uptake in association with P-glycoprotein effect, DPDPE and morphine being P-glycoprotein substrates. P85 coadministration with DPDPE and biphalin showed increased (p 0.01) analgesia with both 0.01 and 1.0% P85. Morphine showed increased (p 0.01) analgesia with 0.01% P85 only. This increase in analgesia is due to both an increase in peak effect, as well as a prolongation of effect. P85 increased cellular uptake of (125)I-DPDPE and [(3)H]morphine at 0.01% (p 0.01) and 1.0% (p 0.01 and p 0.05, respectively). Cyclosporin-A coadministration with (125)I-DPDPE and [(3)H]morphine increased cellular uptake (p 0.01 and p 0.05, respectively). (125)I-DPDPE and [(3)H]morphine coadministered with 0.01% P85 and cyclosporin-A increased cellular uptake compared with control (p 0.01) and compared with cyclosporin-A coadministration without P85 (p 0.01 and p 0.05, respectively). This indicates that, in addition to P-gp inhibition, 0.01% P85 increased (125)I-DPDPE and [(3)H]morphine uptake. In our examination, we determined that P85 enhanced the analgesic profile of biphalin, DPDPE, and morphine, both above and below the critical micelle concentration.
Cerebral microvessel endothelial cells that form the blood-brain barrier (BBB) have tight junctions (TJ) that are critical for maintaining brain homeostasis and low permeability. Both integral (claudin-1 and occludin) and membrane-associated zonula occluden-1 and -2 (ZO-1 and ZO-2) proteins combine to form these TJ complexes that are anchored to the cytoskeletal architecture (actin). Disruptions of the BBB have been attributed to hypoxic conditions that occur with ischemic stroke, pathologies of decreased perfusion, and high-altitude exposure. The effects of hypoxia and posthypoxic reoxygenation in cerebral microvasculature and corresponding cellular mechanisms involved in disrupting the BBB remain unclear. This study examined hypoxia and posthypoxic reoxygenation effects on paracellular permeability and changes in actin and TJ proteins using primary bovine brain microvessel endothelial cells (BBMEC). Hypoxia induced a 2.6-fold increase in [(14)C]sucrose, a marker of paracellular permeability. This effect was significantly reduced (~58%) with posthypoxic reoxygenation. After hypoxia and posthypoxic reoxygenation, actin expression was increased (1.4- and 2.3-fold, respectively). Whereas little change was observed in TJ protein expression immediately after hypoxia, a twofold increase in expression was seen with posthypoxic reoxygenation. Furthermore, immunofluorescence studies showed alterations in occludin, ZO-1, and ZO-2 protein localization during hypoxia and posthypoxic reoxygenation that correlate with the observed changes in BBMEC permeability. The results of this study show hypoxia-induced changes in paracellular permeability may be due to perturbation of TJ complexes and that posthypoxic reoxygenation reverses these effects.
The blood-brain barrier (BBB) is the regulated interface between the peripheral circulation and the central nervous system (CNS). Although originally observed by Paul Ehrlich in 1885, the nature of the BBB was debated well into the 20th century. The anatomical substrate of the BBB is the cerebral microvascular endothelium, which, together with astrocytes, pericytes, neurons, and the extracellular matrix, constitute a "neurovascular unit" that is essential for the health and function of the CNS. Tight junctions (TJ) between endothelial cells of the BBB restrict paracellular diffusion of water-soluble substances from blood to brain. The TJ is an intricate complex of transmembrane (junctional adhesion molecule-1, occludin, and claudins) and cytoplasmic (zonula occludens-1 and -2, cingulin, AF-6, and 7H6) proteins linked to the actin cytoskeleton. The expression and subcellular localization of TJ proteins are modulated by several intrinsic signaling pathways, including those involving calcium, phosphorylation, and G-proteins. Disruption of BBB TJ by disease or drugs can lead to impaired BBB function and thus compromise the CNS. Therefore, understanding how BBB TJ might be affected by various factors holds significant promise for the prevention and treatment of neurological diseases.
