Molecular biology of the blood-brain and the blood-cerebrospinal fluid barriers: similarities and differences

Efficient processing of information by the central nervous system (CNS) represents an important evolutionary advantage. Thus, homeostatic mechanisms have developed that provide appropriate circumstances for neuronal signaling, including a highly controlled and stable microenvironment. To provide such a milieu for neurons, extracellular fluids of the CNS are separated from the changeable environment of blood at three major interfaces: at the brain capillaries by the blood-brain barrier (BBB), which is localized at the level of the endothelial cells and separates brain interstitial fluid (ISF) from blood; at the epithelial layer of four choroid plexuses, the blood-cerebrospinal fluid (CSF) barrier (BCSFB), which separates CSF from the CP ISF, and at the arachnoid barrier. The two barriers that represent the largest interface between blood and brain extracellular fluids, the BBB and the BCSFB, prevent the free paracellular diffusion of polar molecules by complex morphological features, including tight junctions (TJs) that interconnect the endothelial and epithelial cells, respectively. The first part of this review focuses on the molecular biology of TJs and adherens junctions in the brain capillary endothelial cells and in the CP epithelial cells. However, normal function of the CNS depends on a constant supply of essential molecules, like glucose and amino acids from the blood, exchange of electrolytes between brain extracellular fluids and blood, as well as on efficient removal of metabolic waste products and excess neurotransmitters from the brain ISF. Therefore, a number of specific transport proteins are expressed in brain capillary endothelial cells and CP epithelial cells that provide transport of nutrients and ions into the CNS and removal of waste products and ions from the CSF. The second part of this review concentrates on the molecular biology of various solute carrier (SLC) transport proteins at those two barriers and underlines differences in their expression between the two barriers. Also, many blood-borne molecules and xenobiotics can diffuse into brain ISF and then into neuronal membranes due to their physicochemical properties. Entry of these compounds could be detrimental for neural transmission and signalling. Thus, BBB and BCSFB express transport proteins that actively restrict entry of lipophilic and amphipathic substances from blood and/or remove those molecules from the brain extracellular fluids. The third part of this review concentrates on the molecular biology of ATP-binding cassette (ABC)-transporters and those SLC transporters that are involved in efflux transport of xenobiotics, their expression at the BBB and BCSFB and differences in expression in the two major blood-brain interfaces. In addition, transport and diffusion of ions by the BBB and CP epithelium are involved in the formation of fluid, the ISF and CSF, respectively, so the last part of this review discusses molecular biology of ion transporters/exchangers and ion channels in the brain endothelial and CP epithelial cells

The poorly membrane permeable antipsychotic drugs amisulpride and sulpiride are substrates of the organic cation transporters from the SLC22 family.

Variations in influx transport at the blood-brain barrier might affect the concentration of psychotropic drugs at their site of action and as a consequence might alter therapy response. Furthermore, influx transporters in organs such as the gut, liver and kidney may influence absorption, distribution, and elimination. Here, we analyzed 30 commonly used psychotropic drugs using a parallel artificial membrane permeability assay. Amisulpride and sulpiride showed the lowest membrane permeability (P e -6 cm/s) and will require influx transport to penetrate the blood-brain barrier and other physiological barriers. We then studied the uptake of amisulpride and sulpiride by the organic cation transporters of the SLC22 family OCT1, OCT2, OCT3, OCTN1, and OCTN2 Amisulpride was found to be transported by all five transporters studied. In contrast, sulpiride was only transported by OCT1 and OCT2. OCT1 showed the highest transport ability both for amisulpride (CLint = 1.9 ml/min/mg protein) and sulpiride (CLint = 4.2 ml/min/mg protein) and polymorphisms in OCT1 significantly reduced the uptake of both drugs. Furthermore, we observed carrier-mediated uptake that was inhibitable by known OCT inhibitors in the immortalized human brain microvascular endothelial cell line hCMEC/D3. In conclusion, this study demonstrates that amisulpride and sulpiride are substrates of organic cation transporters of the SLC22 family. SLC22 transporters may play an important role in the distribution of amisulpride and sulpiride, including their ability to penetrate the blood-brain barrier

The involvement of a Na⁺- and Cl⁻-dependent transporter in the brain uptake of amantadine and rimantadine

Despite their structural similarity, the two anti-influenza adamantane compounds amantadine (AMA) and rimantadine (RIM) exhibit strikingly different rates of blood-brain barrier (BBB) transport. However, the molecular mechanisms facilitating the higher rate of in situ BBB transport of RIM, relative to AMA, remain unclear. The aim of this study, therefore, was to determine whether differences in the extent of brain uptake between these two adamantanes also occurred in vivo, and elucidate the potential carrier protein facilitating their BBB transport using immortalized human brain endothelial cells (hCMEC/D3). Following oral administration to Swiss Outbred mice, RIM exhibited 2.4-3.0-fold higher brain-to-plasma exposure compared to AMA, which was not attributable to differences in the degree of plasma protein binding. At concentrations representative of those obtained in vivo, the hCMEC/D3 cell uptake of RIM was 4.5-15.7-fold higher than that of AMA, with Michaelis-Menten constants 6.3 and 238.4 μM, respectively. The hCMEC/D3 cellular uptake of both AMA and RIM was inhibited by various cationic transporter inhibitors (cimetidine, choline, quinine, and tetraethylammonium) and was dependent on extracellular pH, membrane depolarization and Na⁺ and Cl⁻ ions. Such findings indicated the involvement of the neutral and cationic amino acid transporter B⁰,⁺ (ATB⁰,⁺) in the uptake of AMA and RIM, which was demonstrated to be expressed (at the protein level) in the hCMEC/D3 cells. Indeed, AMA and RIM appeared to interact with this transporter, as shown by a 53-70% reduction in the hCMEC/D3 uptake of the specific ATB⁰,⁺ substrate ³H-glycine in their presence. These studies suggest the involvement of ATB⁰,⁺ in the disposition of these cationic drugs across the BBB, a transporter with the potential to be exploited for targeted drug delivery to the brain

Xenobiotic, Bile Acid, and Cholesterol Transporters: Function and Regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting β polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) α and β] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology