CoChat
Sign Up
Cited by 144Barrier Structure and Function StudiesDrug Transport and Resistance MechanismsCerebrospinal fluid and hydrocephalus

The DOI matches its Crossref record; 129 of the 129 references it cites were found in a registry or an index.

The biological significance of brain barrier mechanisms: help or hindrance in drug delivery to the central nervous system?

Open access
Saunders, Norman R., Habgood, Mark D., Møllgård, Kjeld, Dziegielewska, Katarzyna M.·2016·F1000Research
DOI 10.12688/f1000research.7378.1PMC PMC4786902
Open this paper in CoChat — free accountOpen PDF

Abstract

Barrier mechanisms in the brain are important for its normal functioning and development. Stability of the brain’s internal environment, particularly with respect to its ionic composition, is a prerequisite for the fundamental basis of its function, namely transmission of nerve impulses. In addition, the appropriate and controlled supply of a wide range of nutrients such as glucose, amino acids, monocarboxylates, and vitamins is also essential for normal development and function. These are all cellular functions across the interfaces that separate the brain from the rest of the internal environment of the body. An essential morphological component of all but one of the barriers is the presence of specialized intercellular tight junctions between the cells comprising the interface: endothelial cells in the blood-brain barrier itself, cells of the arachnoid membrane, choroid plexus epithelial cells, and tanycytes (specialized glial cells) in the circumventricular organs. In the ependyma lining the cerebral ventricles in the adult brain, the cells are joined by gap junctions, which are not restrictive for intercellular movement of molecules. But in the developing brain, the forerunners of these cells form the neuroepithelium, which restricts exchange of all but the smallest molecules between cerebrospinal fluid and brain interstitial fluid because of the presence of strap junctions between the cells. The intercellular junctions in all these interfaces are the physical basis for their barrier properties. In the blood-brain barrier proper, this is combined with a paucity of vesicular transport that is a characteristic of other vascular beds. Without such a diffusional restrain, the cellular transport mechanisms in the barrier interfaces would be ineffective. Superimposed on these physical structures are physiological mechanisms as the cells of the interfaces contain various metabolic transporters and efflux pumps, often ATP-binding cassette (ABC) transporters, that provide an important component of the barrier functions by either preventing entry of or expelling numerous molecules including toxins, drugs, and other xenobiotics. In this review, we summarize these influx and efflux mechanisms in normal developing and adult brain, as well as indicating their likely involvement in a wide range of neuropathologies. There have been extensive attempts to overcome the barrier mechanisms that prevent the entry of many drugs of therapeutic potential into the brain. We outline those that have been tried and discuss why they may so far have been largely unsuccessful. Currently, a promising approach appears to be focal, reversible disruption of the blood-brain barrier using focused ultrasound, but more work is required to evaluate the method before it can be tried in patients. Overall, our view is that much more fundamental knowledge of barrier mechanisms and development of new experimental methods will be required before drug targeting to the brain is likely to be a successful endeavor. In addition, such studies, if applied to brain pathologies such as stroke, trauma, or multiple sclerosis, will aid in defining the contribution of brain barrier pathology to these conditions, either causative or secondary.

Overview

The claims and statistics of this paper haven’t been checked yet.

Run the check — free, no account needed. It reads the open-access text and checks the sources it cites.

A few free checks a day · papers already verified open instantly · results are public

Integrity signals

Registry

Crossref · 10.12688/f1000research.7378.1

Cites

129 of 129 references verified0 citation errors · 0 not checkable

Statistics

Figures

Figure 1. Schematic diagram (center left) of the five main barrier interfaces (a–e) in the brain and an additional one in the embryo (f). The barrier-forming cellular layers at each interface are colored green. ( a) The meningeal barrier is structurally the most complex of all the brain barriers. Barrier-forming cells are the outer layer of the arachnoid membrane (the arachnoid barrier cells [ABC]); these have tight junctions (arrowheads) between adjacent cells forming a barrier between the outer cerebrospinal fluid (o-CSF) in the subarachnoid space (SAS) and more superficial dural layers (dural border cells [DBC] and the dura mater). Blood vessels (BV) in the SAS have tight junctions with similar barrier characteristics as cerebral blood vessels without surrounding pericytes and astrocytic end-feet 11– 13. Blood vessels within the dura mater are fenestrated (f-BV); bm = basement membrane, gl = glia limitans. ( b) The blood-brain barrier is situated at the level of cerebral blood vessels (BV). Tight junctions (tj, arrowhead) are present between the endothelial cells (EC) restricting the paracellular cleft ( 11 and Text Box); bm = basement membrane, PC = pericytes, AE = end feet from astroglial cells. ( c) The blood-CSF barrier is situated in the choroid plexus within each brain ventricle. Barrier-forming cells are the epithelial cells (CPE), which have tight junctions 11 at their apical side (CSF facing, arrowheads). Blood vessels (BV) are fenestrated and do not form a barrier (arrows); apical microvilli increase exchange surface of epithelial cells to the internal CSF (i-CSF). ( d) Circumventricular organs (including median eminence, pineal gland, area postrema, subfornical organ). Blood vessels have permeability characteristics similar to elsewhere in the body and have the functional property of allowing feedback penetration of peptide hormones controlled by the hypothalamic-pituitary axis. These peptides and other molecules are prevented from entering the CSF by tanycytes (TC), the specialized ependymal cells of these brain areas, connected by tight junctions between their apices (arrowhead); entry into the rest of the brain is prevented by tight junctions between astroglial cells (GC 19, 20). Away from the tanycyte layer, ependymal cells lining the ventricular system are linked by gap junctions that do not hinder free exchange between the CSF and brain interstitial fluid (broken arrow). ( e) Ependyma in adult brain. Apart from areas where there are specialized tanycytes, ependymal cells are linked by gap junctions that do not restrict exchange of even large molecules, such as proteins, between CSF and interstitial space of brain (solid arrows). ( f) The embryonic CSF-brain barrier. In early brain development, strap junctions (open arrowheads) are present between adjacent neuroepithelial cells (NE); these form a barrier restricting the movement of larger molecules, such as proteins, but not smaller molecules.
Figure 1. Schematic diagram (center left) of the five main barrier interfaces (a–e) in the brain and an additional one in the embryo (f). The barrier-forming cellular layers at each interface are colored green. ( a) The meningeal barrier is structurally the most complex of all the brain barriers. Barrier-forming cells are the outer layer of the arachnoid membrane (the arachnoid barrier cells [ABC]); these have tight junctions (arrowheads) between adjacent cells forming a barrier between the outer cerebrospinal fluid (o-CSF) in the subarachnoid space (SAS) and more superficial dural layers (dural border cells [DBC] and the dura mater). Blood vessels (BV) in the SAS have tight junctions with similar barrier characteristics as cerebral blood vessels without surrounding pericytes and astrocytic end-feet 11– 13. Blood vessels within the dura mater are fenestrated (f-BV); bm = basement membrane, gl = glia limitans. ( b) The blood-brain barrier is situated at the level of cerebral blood vessels (BV). Tight junctions (tj, arrowhead) are present between the endothelial cells (EC) restricting the paracellular cleft ( 11 and Text Box); bm = basement membrane, PC = pericytes, AE = end feet from astroglial cells. ( c) The blood-CSF barrier is situated in the choroid plexus within each brain ventricle. Barrier-forming cells are the epithelial cells (CPE), which have tight junctions 11 at their apical side (CSF facing, arrowheads). Blood vessels (BV) are fenestrated and do not form a barrier (arrows); apical microvilli increase exchange surface of epithelial cells to the internal CSF (i-CSF). ( d) Circumventricular organs (including median eminence, pineal gland, area postrema, subfornical organ). Blood vessels have permeability characteristics similar to elsewhere in the body and have the functional property of allowing feedback penetration of peptide hormones controlled by the hypothalamic-pituitary axis. These peptides and other molecules are prevented from entering the CSF by tanycytes (TC), the specialized ependymal cells of these brain areas, connected by tight junctions between their apices (arrowhead); entry into the rest of the brain is prevented by tight junctions between astroglial cells (GC 19, 20). Away from the tanycyte layer, ependymal cells lining the ventricular system are linked by gap junctions that do not hinder free exchange between the CSF and brain interstitial fluid (broken arrow). ( e) Ependyma in adult brain. Apart from areas where there are specialized tanycytes, ependymal cells are linked by gap junctions that do not restrict exchange of even large molecules, such as proteins, between CSF and interstitial space of brain (solid arrows). ( f) The embryonic CSF-brain barrier. In early brain development, strap junctions (open arrowheads) are present between adjacent neuroepithelial cells (NE); these form a barrier restricting the movement of larger molecules, such as proteins, but not smaller molecules.CC BY · PubMed Central
Figure 2. Transporters and ion channels in choroid plexus epithelial cells. Data for the localization of transporters and ion channels are from Damkier et al. 25 and Brown et al. 96. CSF secretion results from coordinated transport of ions and water from basolateral membrane to cytoplasm, then sequentially across apical membrane into ventricles 25. The genes for many of these transporters and ion channels are differentially expressed in the embryo compared to the adult. This is represented in the three panels. It is emphasized that this represents differential expression, not the absence of a gene at one age (details are in 35). On the plasma-facing membrane is parallel Cl -/HCO 3 - exchange (AE2 [ Slc4a2] > in adult and AEI [ Slc4a1], AE3 [ Slc4a3] > in embryo) and Na +/HCO 3 - co-transport (NBC1 [ Slc4a4] > in embryo) with net function bringing Cl - into cells in exchange for HCO 3 - 97. Also basolaterally located is an Na-dependent Cl -/HCO 3 - exchange (NCBE [ Slc4a10] > in adult) that modulates pH and perhaps CSF formation 98. Apical Na + efflux by NHE5 ( Slc9a5 > in embryo) and ATB1 ( Atb1b1 [Na +/K +-ATPase] > in adult) maintains a low cell Na + that sets up a favorable basolateral gradient to drive Na + uptake 99. Na + is extruded into CSF mainly via the Na +/K +-ATPase pump (ATB1 [ Atb1b1]) and, under some conditions, the Na +/K +-Cl - co-transporter NKCC1, Slc12a2 (see 100 for review). Aquaporin (AQP1/3/4) channels on CSF-facing membrane mediate water flux into ventricles 101. Polarized distribution of carbonic anhydrase (CAR) and Na +/K +-ATPases, and aquaporins, enable net ion and water translocation to CSF (see 100 and 102 for reviews). CLCKA (CLCK1) is an inwardly rectifying chloride channel; its gene ( Clcnka) in embryonic choroid plexus is expressed many orders of magnitude higher than in the adult. Clcnkb is expressed at a higher level in the adult. CAR2 has an intracellular distribution and is functionally important for catalyzing the equilibrium that generates H + and HCO 3 -, which is an important part of the mechanism secreting CSF. There are many more channels that show age-related differential expression in choroid plexus, the functions of which are unclear 26.
Figure 2. Transporters and ion channels in choroid plexus epithelial cells. Data for the localization of transporters and ion channels are from Damkier et al. 25 and Brown et al. 96. CSF secretion results from coordinated transport of ions and water from basolateral membrane to cytoplasm, then sequentially across apical membrane into ventricles 25. The genes for many of these transporters and ion channels are differentially expressed in the embryo compared to the adult. This is represented in the three panels. It is emphasized that this represents differential expression, not the absence of a gene at one age (details are in 35). On the plasma-facing membrane is parallel Cl -/HCO 3 - exchange (AE2 [ Slc4a2] > in adult and AEI [ Slc4a1], AE3 [ Slc4a3] > in embryo) and Na +/HCO 3 - co-transport (NBC1 [ Slc4a4] > in embryo) with net function bringing Cl - into cells in exchange for HCO 3 - 97. Also basolaterally located is an Na-dependent Cl -/HCO 3 - exchange (NCBE [ Slc4a10] > in adult) that modulates pH and perhaps CSF formation 98. Apical Na + efflux by NHE5 ( Slc9a5 > in embryo) and ATB1 ( Atb1b1 [Na +/K +-ATPase] > in adult) maintains a low cell Na + that sets up a favorable basolateral gradient to drive Na + uptake 99. Na + is extruded into CSF mainly via the Na +/K +-ATPase pump (ATB1 [ Atb1b1]) and, under some conditions, the Na +/K +-Cl - co-transporter NKCC1, Slc12a2 (see 100 for review). Aquaporin (AQP1/3/4) channels on CSF-facing membrane mediate water flux into ventricles 101. Polarized distribution of carbonic anhydrase (CAR) and Na +/K +-ATPases, and aquaporins, enable net ion and water translocation to CSF (see 100 and 102 for reviews). CLCKA (CLCK1) is an inwardly rectifying chloride channel; its gene ( Clcnka) in embryonic choroid plexus is expressed many orders of magnitude higher than in the adult. Clcnkb is expressed at a higher level in the adult. CAR2 has an intracellular distribution and is functionally important for catalyzing the equilibrium that generates H + and HCO 3 -, which is an important part of the mechanism secreting CSF. There are many more channels that show age-related differential expression in choroid plexus, the functions of which are unclear 26.CC BY · PubMed Central
Figure 3. Ion gradients between CSF and plasma in developing and adult rat brain. A characteristic of CSF is its stable ionic composition that differs from that of plasma to an extent that cannot be explained by ultrafiltration, as was once thought 24. Data for CSF and plasma (m-equiv/L water) are from 103 and for intracellular ions (mmol/L water) from Figure 8 in 104. The gradients are the consequence of the complex interactions between enzymes (notably carbonic anhydrase) ion transporters and ion channels, as illustrated in Figure 2. The CSF secretion rate in the embryo and newborn is much lower than in the adult 105– 107, which is perhaps explained by the much lower expression of carbonic anhydrase and ATPases in the developing choroid plexus.
Figure 3. Ion gradients between CSF and plasma in developing and adult rat brain. A characteristic of CSF is its stable ionic composition that differs from that of plasma to an extent that cannot be explained by ultrafiltration, as was once thought 24. Data for CSF and plasma (m-equiv/L water) are from 103 and for intracellular ions (mmol/L water) from Figure 8 in 104. The gradients are the consequence of the complex interactions between enzymes (notably carbonic anhydrase) ion transporters and ion channels, as illustrated in Figure 2. The CSF secretion rate in the embryo and newborn is much lower than in the adult 105– 107, which is perhaps explained by the much lower expression of carbonic anhydrase and ATPases in the developing choroid plexus.CC BY · PubMed Central
Figure 4. Summary of inward transporter mechanisms in cerebral endothelial cells. Individual transporters shown are ones identified in human material 29. Tight junctions (tj) between adjacent cells prevent the paracellular passage of hydrophilic compounds. R-M = receptor-mediated, GLUT = glucose transporters, NTs = nucleoside transporters, AAs = amino acid transporters (includes LAT), OATP = organic anion transporting polypeptides, OAT = organic anion transporters, OCT = organic cation transporters, MCT = monocarboxylate transporters, FATP = fatty acid transport protein. Many of these transporters are solute linked carriers (SLCs). Both SLC designations and the original abbreviations are included here. SLC1A2/EAAT2, SLC1A3/EAAT1 high-affinity glutamate, SLC1A4/ASCT1 glutamate/neutral amino acids, SLC2A1/GLUT1, SLC2A3,14/GLUT3 glucose, SLC3A2/4F2hc amino acid transporter heavy chain, SLC6A12/BGT1 neurotransmitter, SLC7A1/CAT1 cationic amino acid, y+ system, SLC7A5/LAT1 amino acid light chain, L system, SLC10A1/NTCP sodium/bile acid cotransporter. SLC16A1/MCT1, SLC16A2/MCT8 monocarboxylates, SLC19A1/RFC folate, SLC22A1/OCT1 organic cations, SLC22A3/OCTN3 organic cations, SLC22A5/OCTN2 organic cation/carnitine, SLC27A1/FATP1 fatty acid, SLC29A1/ENT1 equilibrative nucleosides, SLCO2B1/OATP2B1 organic anions, SLCO1B1/OATP1B1 organic anions. Examples of receptor-mediated transporters are insulin receptor (INSR), transferrin receptor (TFR1), leptin receptor (LEPR), low-density lipoprotein receptor (LDLR), and insulin-like growth factor receptor (IGFR).
Figure 4. Summary of inward transporter mechanisms in cerebral endothelial cells. Individual transporters shown are ones identified in human material 29. Tight junctions (tj) between adjacent cells prevent the paracellular passage of hydrophilic compounds. R-M = receptor-mediated, GLUT = glucose transporters, NTs = nucleoside transporters, AAs = amino acid transporters (includes LAT), OATP = organic anion transporting polypeptides, OAT = organic anion transporters, OCT = organic cation transporters, MCT = monocarboxylate transporters, FATP = fatty acid transport protein. Many of these transporters are solute linked carriers (SLCs). Both SLC designations and the original abbreviations are included here. SLC1A2/EAAT2, SLC1A3/EAAT1 high-affinity glutamate, SLC1A4/ASCT1 glutamate/neutral amino acids, SLC2A1/GLUT1, SLC2A3,14/GLUT3 glucose, SLC3A2/4F2hc amino acid transporter heavy chain, SLC6A12/BGT1 neurotransmitter, SLC7A1/CAT1 cationic amino acid, y+ system, SLC7A5/LAT1 amino acid light chain, L system, SLC10A1/NTCP sodium/bile acid cotransporter. SLC16A1/MCT1, SLC16A2/MCT8 monocarboxylates, SLC19A1/RFC folate, SLC22A1/OCT1 organic cations, SLC22A3/OCTN3 organic cations, SLC22A5/OCTN2 organic cation/carnitine, SLC27A1/FATP1 fatty acid, SLC29A1/ENT1 equilibrative nucleosides, SLCO2B1/OATP2B1 organic anions, SLCO1B1/OATP1B1 organic anions. Examples of receptor-mediated transporters are insulin receptor (INSR), transferrin receptor (TFR1), leptin receptor (LEPR), low-density lipoprotein receptor (LDLR), and insulin-like growth factor receptor (IGFR).CC BY · PubMed Central
Figure 5. Efflux pathways in barrier-forming cells. The main efflux transporters at the blood-brain and blood-CSF interfaces are P-glycoprotein (PGP, MDR1, ABCB1), breast cancer resistance protein (BCRP, ABCG2), and several members of the multidrug resistance protein subfamily (MRP1 ABCC1, MRP2 ABCC2, MRP4 ABCC4, MRP5 ABCC5). In cerebral capillary endothelial cells (blood-brain barrier), PGP 41– 43, BCRP 44, 45, MRP2 46, MRP4 46, and MRP5 47 are localized to the luminal membrane where they export compounds into the blood. In choroid plexus epithelial cells (blood-CSF barrier), MRP1, MRP4, and BCRP are localized to the basolateral membranes where they export compounds into the stroma of the plexus 40, 48, 49. The subcellular localization of PGP in choroid plexus is not clear. Some studies report staining too low to be able to determine localization 40, 48 or positive staining, but localization was not able to be determined 50, 51. Other studies report cytosolic 52 or subapical localization 42. One study has reported apical membrane localization in cultured choroid plexus epithelial cells 53. A common feature of these outwardly directed efflux transporters is a broad substrate specificity and considerable overlap between transporters (see 54). PGP is unusual in that it intercepts lipid-soluble compounds (red symbols) as they pass through the internal leaflet of the plasma membrane and returns them to the extracellular fluid 55, whereas BCRP and the MRPs bind their substrates from within the cell cytoplasm. Compounds can be exported from the cell by one or more of these efflux pathways. For example, a lipid-soluble compound that manages to avoid interception by PGP as it passes into the cell may then be metabolized by phase I enzymes (e.g. cytochrome P450 oxidases), conjugated by phase II enzymes (sulfotransferases [SULTs], uridine-diphospho-glucuronosyltransferase [UGT], or glutathione S-transferase [GST]), and exported by BCRP and/or MRP.
Figure 5. Efflux pathways in barrier-forming cells. The main efflux transporters at the blood-brain and blood-CSF interfaces are P-glycoprotein (PGP, MDR1, ABCB1), breast cancer resistance protein (BCRP, ABCG2), and several members of the multidrug resistance protein subfamily (MRP1 ABCC1, MRP2 ABCC2, MRP4 ABCC4, MRP5 ABCC5). In cerebral capillary endothelial cells (blood-brain barrier), PGP 41– 43, BCRP 44, 45, MRP2 46, MRP4 46, and MRP5 47 are localized to the luminal membrane where they export compounds into the blood. In choroid plexus epithelial cells (blood-CSF barrier), MRP1, MRP4, and BCRP are localized to the basolateral membranes where they export compounds into the stroma of the plexus 40, 48, 49. The subcellular localization of PGP in choroid plexus is not clear. Some studies report staining too low to be able to determine localization 40, 48 or positive staining, but localization was not able to be determined 50, 51. Other studies report cytosolic 52 or subapical localization 42. One study has reported apical membrane localization in cultured choroid plexus epithelial cells 53. A common feature of these outwardly directed efflux transporters is a broad substrate specificity and considerable overlap between transporters (see 54). PGP is unusual in that it intercepts lipid-soluble compounds (red symbols) as they pass through the internal leaflet of the plasma membrane and returns them to the extracellular fluid 55, whereas BCRP and the MRPs bind their substrates from within the cell cytoplasm. Compounds can be exported from the cell by one or more of these efflux pathways. For example, a lipid-soluble compound that manages to avoid interception by PGP as it passes into the cell may then be metabolized by phase I enzymes (e.g. cytochrome P450 oxidases), conjugated by phase II enzymes (sulfotransferases [SULTs], uridine-diphospho-glucuronosyltransferase [UGT], or glutathione S-transferase [GST]), and exported by BCRP and/or MRP.CC BY · PubMed Central

Verification

Claims the paper makes

The claims and statistics of this paper haven’t been checked yet.

Run the check — free, no account needed. It reads the open-access text and checks the sources it cites.

A few free checks a day · papers already verified open instantly · results are public

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

The self-check needs the paper’s full text.

Citations

Cites

Show all 129 references

Research works printed but not linked: 2

  1. M Lewandowsky, 1900, Zur Lehre von der Cerebrospinalflüssgkeit., Z Clin Med., 40, 480-494
  2. G Cotzias, 1968, Experimental treatment of parkinsonism with L-Dopa., Neurology., 18, 276-277

Cited by

Sorted by citations

Related

See all

Metadata

Authors
Saunders, N. R., Habgood, M. D., Møllgård, K., & Dziegielewska, K. M.
Affiliations
  • The University of Melbourne (AU)
  • University of Copenhagen (DK)
Corresponding author
Saunders, Norman R.
ORCID
Venue
F1000Research
Publisher
F1000 Research Ltd
Published
2016-03-10
Volume
5
Pages
313
ISSN
2046-1402 (ISSN-L)
DOI
10.12688/f1000research.7378.1
PMC
PMC4786902
OpenAlex
W2337076198
Cited by
143
References
129
Citations by year
2026: 9 · 2025: 10 · 2024: 12 · 2023: 13 · 2022: 15 · 2021: 12 · 2020: 17 · 2019: 19 · 2018: 12 · 2017: 13 · 2016: 10
FWCI
1.58
Type
journal article
Language
en
Open access
Licence
Locations
  • F1000Research (publishedVersion, cc-by)
  • Europe PMC (PubMed Central) (publishedVersion, cc-by)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • Research at the University of Copenhagen (University of Copenhagen) (publishedVersion, cc-by)
  • Minerva Access (University of Melbourne) (submittedVersion, cc-by)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion)
Topics
  • Life Sciences › Neuroscience › Neurology › Barrier Structure and Function Studies
  • Health Sciences › Medicine › Oncology › Drug Transport and Resistance Mechanisms
  • Health Sciences › Medicine › Clinical Neurology › Cerebrospinal fluid and hydrocephalus
Keywords
blood-brain barrier, brain drug delivery, tight junctions, ABC transporters, efflux pumps, circumventricular organs, choroid plexus epithelium, arachnoid membrane, tanycytes, neuroepithelium, gap junctions, strap junction, focused ultrasound, blood-brain barrier disruption

About these checks

Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.

Statistics and arithmetic are recomputed, not judged.

Confidence bands are shown only for fields calibrated on our annotated test set. None is calibrated yet, so no band is shown.

What counts as a claim ›How we measure confidence ›How we check this ›

Are you an author, or do you see a problem with this page? Request a review