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The blood-brain barrier (BBB) is a separation of circulating blood andcerebrospinal fluid (CSF) in the central nervous system (CNS). It occurs along all capillaries and consists of tight junctions around the capillaries that do not exist in normal circulation. Endothelial cells restrict the diffusion of microscopic objects (e.g. bacteria) and large or hydrophilicmolecules into the CSF, while allowing the diffusion of smallhydrophobic molecules (O₂, hormones, CO₂). Cells of the barrier actively transport metabolic products such as glucose across the barrier with specific proteins.

History

Paul Ehrlich was a bacteriologist studying staining, a procedure that is used in many microscopic studies to make fine biological structures visible using chemical dyes. When Ehrlich injected some of these dyes (notably the aniline dyesthat were then widely-used), the dye would stain all of the organs of some kinds of animals except for their brains. At that time, Ehrlich attributed this lack of staining to the brain's simply not picking up as much of the dye.

However, in a later experiment in 1913, Edwin Goldmann (one of Ehrlich's students) injected the dye into the cerebro-spinal fluids of animals' brains directly. He found that in this case the brains did become dyed, but the rest of the body did not. This clearly demonstrated the existence of some sort of compartmentalization between the two. At that time, it was thought that the blood vessels themselves were responsible for the barrier, since no obvious membrane could be found. The concept of the blood-brain barrier (then termed hematoencephalic barrier) was proposed by Lina Stern in 1921.^[1] It was not until the introduction of the scanning electron microscope to the medical research fields in the 1960s that the actual membrane could be observed and proven to exist.

Physiology

This "barrier" results from the selectivity of the tight junctions between endothelial cells in CNS vessels that restricts the passage of solutes. At the interface between blood and the brain, endothelial cells are stitched together by these tight junctions, which are composed of smaller subunits, frequently biochemical dimers, that are transmembrane proteins such as occludin, claudins, junctional adhesion molecule (JAM), or ESAM, for example. Each of these transmembrane proteins is anchored into the endothelial cells by another protein complex that includes zo-1 and associated proteins.

The blood-brain barrier is composed of high-density cells restricting passage of substances from the bloodstream much more than endothelial cells in capillaries elsewhere in the body. Astrocyte cell projections called astrocytic feet (also known as "glia limitans") surround the endothelial cells of the BBB, providing biochemical support to those cells. The BBB is distinct from the quite similar blood-cerebrospinal fluid barrier, which is a function of the choroidal cells of thechoroid plexus, and from the blood-retinal barrier, which can be considered a part of the whole realm of such barriers.^[2]

Several areas of the human brain are not "behind" the BBB. These include the circumventricular organs. One example of this is the pineal gland, which secretes the hormone melatonin "directly into the systemic circulation"^[3] as this hormone can pass through the blood-brain barrier.^[4]

Environment

Originally, experiments in the 1920s showed that the blood-brain-barrier (BBB) is still immature in newborns. The reason for this fallacy was a mistake in methodology (the osmotic pressure was too high and the delicate embryonal capillary vessels were partially damaged). It was later shown in experiments with a reduced volume of the injected liquids that the markers under investigation could not pass the BBB. It was reported that those natural substances such as albumin, α-1-fetoprotein or transferrin with elevated plasma concentration in the newborn could not be detected extracellular in the brain. The efflux-transporter P-glycoprotein exists already in the embryonal endothelium.

The measurement of brain uptake of acetamide, antipyrine, benzyl alcohol, butanol, caffeine, cytosine, diphenyl hydantoin, ethanol, ethylene glycol, heroin, mannitol, methanol, phenobarbital, propylene glycol, thiourea, and urea in ether-anesthetized newborn vs. adult rabbits shows that newborn rabbit and adult rat brain endothelia are functionally similar with respect to lipid-mediated permeability. These data confirmed no differences in permeability could be detected between newborn and adult BBB capillaries. No difference in brain uptake of glucose, amino acids, organic acids, purines, nucleosides, or choline was observed between adult and newborn rabbits. These experiments indicate that the newborn BBB has restrictive properties similar to the adult. In contrast to suggestions of an immature barrier in young animals, these studies indicate that a sophisticated, selective BBB is operative at birth.

Pathophysiology

The blood-brain barrier acts very effectively to protect the brain from many common bacterial infections. Thus, infections of the brain are very rare. However, since antibodies and antibiotics are too large to cross the blood-brain barrier, infections of the brain that do occur are often very serious and difficult to treat. However, the blood-brain barrier becomes more permeable during inflammation, meaning that some antibiotics can get across. Viruses easily bypass the blood-brain barrier by attaching themselves to circulating immune cells.

An exception to the bacterial exclusion are the diseases caused by spirochetes, such as Borrelia, which causes Lyme disease, and Treponema pallidum, which causes syphilis. These harmful bacteria seem to breach the blood-barrier by physically tunneling through the blood vessel walls.

There are also some biochemical poisons that are made up of large molecules that are too big to pass through the blood-brain barrier. This was especially important in primitive or medieval times when people often ate contaminated food. Neurotoxins such as Botulinum in the food might affect peripheral nerves, but the blood-brain barrier can often prevent such toxins from reaching the central nervous system, where they could cause serious or fatal damage.

Drugs targeting the brain

Overcoming the difficulty of delivering therapeutic agents to specific regions of the brain presents a major challenge to treatment of most brain disorders. In its neuroprotective role, the blood-brain barrier functions to hinder the delivery of many potentially important diagnostic and therapeutic agents to the brain. Therapeutic molecules and genes that might otherwise be effective in diagnosis and therapy do not cross the BBB in adequate amounts.

Mechanisms for drug targeting in the brain involve going either "through" or "behind" the BBB. Modalities for drug delivery through the BBB entail its disruption by osmotic means; biochemically by the use of vasoactive substances such as bradykinin; or even by localized exposure to high-intensity focused ultrasound (HIFU). Other methods used to get through the BBB may entail the use of endogenous transport systems, including carrier-mediated transporters such as glucose and amino acid carriers; receptor-mediated transcytosis for insulin or transferrin; and the blocking of active efflux transporters such as p-glycoprotein. Methods for drug delivery behind the BBB include intracerebral implantation (such as with needles) and convection-enhanced distribution. Mannitol can be used in bypassing the BBB.

Nanoparticles

Nanotechnology may also help in the transfer of drugs across the BBB.^[5] Recently, researchers have been trying to build liposomes loaded with nanoparticles to gain access through the BBB. More research is needed to determine which strategies will be most effective and how they can be improved for patients with brain tumors. The potential for using BBB opening to target specific agents to brain tumors has just begun to be explored.

Delivering drugs across the blood-brain barrier is one of the most promising applications of nanotechnology in clinical neuroscience. Nanoparticles could potentially carry out multiple tasks in a predefined sequence, which is very important in the delivery of drugs across the blood-brain barrier.

A significant amount of research in this area has been spent exploring methods of nanoparticle-mediated delivery ofantineoplastic drugs to tumors in the central nervous system. For example, radiolabeled polyethylene glycol coated hexadecylcyanoacrylate nanospheres targeted and accumulated in a rat gliosarcoma.^[6] However, this method is not yet ready for clinical trials, due to the accumulation of the nanospheres in surrounding healthy tissue.

It should be noted that vascular endothelial cells and associated pericytes are often abnormal in tumors and that the blood-brain barrier may not always be intact in brain tumors. Also, the basement membrane is sometimes incomplete. Other factors, such as astrocytes, may contribute to the resistance of brain tumors to therapy.^[7][8]

Diseases involving the blood-brain barrier

Meningitis

Meningitis is an inflammation of the membranes that surround the brain and spinal cord (these membranes are known as meninges). Meningitis is most commonly caused by infections with various pathogens, examples of which areStreptococcus pneumoniae and Haemophilus influenzae. When the meninges are inflamed, the blood-brain barrier may be disrupted. This disruption may increase the penetration of various substances (including either toxins or antibiotics) into the brain. Antibiotics used to treat meningitis may aggravate the inflammatory response of the central nervous system by releasing neurotoxins from the cell walls of bacteria-like lipopolysaccharide (LPS) ^[9] Treatment with third-generation or fourth-generation cephalosporin is usually preferred.

Epilepsy

Epilepsy is a common neurological disease that is characterized by recurrent and sometimes untreatable seizures. Several clinical and experimental data have implicated the failure of blood-brain barrier function in triggering chronic or acute seizures,^[10][11] some studies implicate the interactions between a common blood protein - albumin andastrocytes.^[12] These findings have shown that acute seizures are a predictable consequence of disruption of the BBB by either artificial or inflammatory mechanisms. In addition, expression of drug resistance molecules and transporters at the BBB are a significant mechanism of resistance to commonly used anti-epileptic drugs.^[13]

Multiple sclerosis (MS)

Multiple sclerosis (MS) is considered to be an auto-immune and neurodegenerative disorder in which the immune system attacks the myelin that protects and electrically insulates the neurons of the central and peripheral nervous systems. Normally, a person's nervous system would be inaccessible to the white blood cells due to the blood-brain barrier. However, it has been shown using Magnetic Resonance Imaging, that when a person is undergoing an MS "attack," the blood-brain barrier has broken down in a section of the brain or spinal cord, allowing white blood cellscalled T lymphocytes to cross over and attack the myelin. It has sometimes been suggested that, rather than being a disease of the immune system, MS is a disease of the blood-brain barrier.^[14] A recent study suggests that the weakening of the blood-brain barrier is a result of a disturbance in the endothelial cells on the inside of the blood vessel, due to which the production of the protein P-glycoprotein is not working well.

There are currently active investigations into treatments for a compromised blood-brain barrier. It is believed thatoxidative stress plays an important role into the breakdown of the barrier. Anti-oxidants such as lipoic acid may be able to stabilize a weakening blood-brain barrier.^[15]

Neuromyelitis optica

Neuromyelitis optica, also known as Devic's disease, is similar to and is often confused with multiple sclerosis. Among other differences from MS, a different target of the autoimmune response has been identified. Patients with neuromyelitis optica have high levels of antibodies against a protein called aquaporin 4 (a component of the astrocytic foot processes in the blood-brain barrier).^[16]

Late-stage neurological trypanosomiasis (Sleeping sickness)

Late-stage neurological trypanosomiasis, or sleeping sickness, is a condition in which trypanosoma protozoa are found in brain tissue. It is not yet known how the parasites infect the brain from the blood, but it is suspected that they cross through the choroid plexus, a circumventricular organ.

Progressive multifocal leukoencephalopathy (PML)

Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease of the central nervous system that is caused by reactivation of a latent papovavirus (the JC polyomavirus) infection, that can cross the BBB. It affects immune-compromised patients and it is usually seen with patients suffering from AIDS.

De Vivo disease

De Vivo disease (also known as GLUT1 deficiency syndrome) is a rare condition caused by inadequate transportation of the sugar, glucose, across the blood-brain barrier, resulting in mental retardation and other neurological problems. Genetic defects in glucose transporter type 1 (GLUT1) appears to be the primary cause of De Vivo disease.^[17][18]

Alzheimer's Disease

Some new evidence indicates ^[19] that disruption of the blood-brain barrier in Alzheimer's Disease patients allows blood plasma containing amyloid beta (Aβ) to enter the brain where the Aβ adheres preferentially to the surface of astrocytes. These findings have led to the hypotheses that (1) breakdown of the blood-brain barrier allows access of neuron-binding autoantibodies and soluble exogenous Aβ42 to brain neurons and (2) binding of these auto-antibodies to neurons triggers and/or facilitates the internalization and accumulation of cell surface-bound Aβ42 in vulnerable neurons through their natural tendency to clear surface-bound autoantibodies via endocytosis. Eventually the astrocyte is overwhelmed, dies, ruptures, and disintegrates, leaving behind the insoluble Aβ42 plaque. Thus, in some patients, Alzheimer’s disease may be caused (or more likely, aggravated) by a breakdown in the blood-brain barrier. [1]

The herpes virus produces the amyloid beta (Aβ), and this virus has been found to be the pathogen responsible for being a major cause of the disease. [2]

HIV Encephalitis

It is believed ^[20] that latent HIV can cross the blood-brain barrier inside circulating monocytes in the bloodstream ("Trojan horse theory") within the first 14 days of infection. Once inside, these monocytes become activated and are transformed into macrophages. Activated macrophages release virions into the brain tissue proximate to brain microvessels. These viral particles likely attract the attention of sentinel brain microglia and perivascular macrophages initiating an inflammatory cascade that may cause a series of intracellular signaling in brain microvascular endothelial cells and damage the functional and structural integrity of the BBB. This inflammation is HIV encephalitis (HIVE). Instances of HIVE probably occur throughout the course of AIDS and are a precursor for HIV-associated dementia(HAD). The premier model for studying HIV and HIVE is the simian model.

References

1. ^ Lina Stern: Science and fate by A.A. Vein. Department of Neurology, Leiden University Medical Centre, Leiden, The Netherlands
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3. ^ Pritchard, Thomas C.; Alloway, Kevin Douglas (1999) (Google books preview). Medical Neuroscience. Hayes Barton Press. pp. 76–77. ISBN 1889325295. Retrieved 2009-02-08.
4. ^ Gilgun-Sherki, Yossi; Melamed, Eldad and Offen, Daniel.(2001). Neuropharmacology. Volume 40, Issue 8, June 2001, Pages 959-975. doi:10.1016/S0028-3908(01)00019-3
5. ^ Silva, GA (December 2008). "Nanotechnology approaches to crossing the blood-brain barrier and drug delivery to the CNS". BMC Neuroscience 9 (Suppl. 3): S4. doi:10.1186/1471-2202-9-S3-S4. PMID 19091001. PMC 2604882.
6. ^ Brigger I, Morizet J, Aubert G, et al. (December 2002). "Poly(ethylene glycol)-coated hexadecylcyanoacrylate nanospheres display a combined effect for brain tumor targeting". J. Pharmacol. Exp. Ther. 303 (3): 928–36.doi:10.1124/jpet.102.039669. PMID 12438511.
7. ^ Hashizume, H; Baluk P, Morikawa S, McLean JW, Thurston G, Roberge S, Jain RK, McDonald DM (April 2000)."Openings between defective endothelial cells explain tumor vessel leakiness". American Journal of Pathology 156(4): 1363–1380. PMID 10751361.
8. ^ Schneider, SW; Ludwig T, Tatenhorst L, Braune S, Oberleithner H, Senner V, Paulus W (March 2004). "Glioblastoma cells release factors that disrupt blood-brain barrier features". Acta Neuropathologica 107 (3): 272–276.doi:10.1007/s00401-003-0810-2. PMID 14730455.
9. ^ Beam, TR Jr.; Allen, JC (December 1977). "Blood, brain, and cerebrospinal fluid concentrations of several antibiotics in rabbits with intact and inflamed meninges". Antimicrobial agents and chemotherapy 12 (6): 710–6. PMID 931369.
10. ^ E. Oby and D. Janigro, The Blood-brain barrier and epilepsy. Epilepsia. 2006 Nov;47(11):1761-74
11. ^ Marchi,N. et al. Seizure-Promoting Effect of Blood-Brain Barrier Disruption. Epilepsia 48(4), 732-742 (2007). Seiffert,E. et al. Lasting blood-brain barrier disruption induces epileptic focus in the rat somatosensory cortex. J. Neurosci. 24, 7829-7836 (2004). Uva,L. et al. Acute induction of epileptiform discharges by pilocarpine in the in vitro isolated guinea-pig brain requires enhancement of blood-brain barrier permeability. Neuroscience (2007). van Vliet,E.A. et al. Blood-brain barrier leakage may lead to progression of temporal lobe epilepsy. Brain 130, 521-534 (2007).
12. ^ Ivens S, Kaufer D, Flores LP, Bechmann I, Zumsteg D, Tomkins O et al. (2007). "TGF-beta receptor-mediated albumin uptake into astrocytes is involved in neocortical epileptogenesis.". Brain 130 (Pt 2): 535–47. doi:10.1093/brain/awl317.PMID 17121744.
13. ^ Awasthi,S. et al. RLIP76, a non-ABC transporter, and drug resistance in epilepsy. BMC. Neurosci. 6, 61 (2005). Loscher,W. & Potschka,H. Drug resistance in brain diseases and the role of drug efflux transporters. Nat. Rev. Neurosci. 6, 591-602 (2005).
14. ^ Waubant E (2006). "Biomarkers indicative of blood-brain barrier disruption in multiple sclerosis". Disease Markers22 (4): 235–44. PMID 17124345.
15. ^ Schreibelt G, Musters RJ, Reijerkerk A, et al. (August 2006). "Lipoic acid affects cellular migration into the central nervous system and stabilizes blood-brain barrier integrity". J. Immunol. 177 (4): 2630–7. PMID 16888025.
16. ^ Lennon VA, Kryzer TJ, Pittock SJ, Verkman AS, Hinson SR (August 2005). "IgG marker of optic-spinal multiple sclerosis binds to the aquaporin-4 water channel". J. Exp. Med. 202 (4): 473–7. doi:10.1084/jem.20050304.PMID 16087714.
17. ^ Pascual, JM; Wang D, Lecumberri B, Yang H, Mao X, Yang R, De Vivo DC (May 2004). "GLUT1 deficiency and other glucose transporter diseases". European journal of endocrinology 150 (5): 627–33. doi:10.1530/eje.0.1500627.PMID 15132717.
18. ^ Klepper, J; Voit T (June 2002). "Facilitated glucose transporter protein type 1 (GLUT1) deficiency syndrome: impaired glucose transport into brain-- a review". European journal of pediatrics 161 (6): 295–304. doi:10.1007/s00431-002-0939-3. PMID 12029447.
19. ^ Microvascular injury and blood–brain barrier leakage in Alzheimer’s disease - Zipser et al 2006
20. ^ http://arjournals.annualreviews.org/doi/pdf/10.1146/annurev.neuro.25.112701.142822?cookieSet=1

Categories: Neurology | Animal physiology | Pharmacokinetics