8 Blood Brain Barrier Flashcards
Functions of the BBB
- Protects the brain from “foreign substances” in the blood that may injure the brain;
- Protects the brain from hormones and neurotransmitters in the rest of the body;
- Transport nutrients into the brain;
- Move out toxins of the brain;
- Maintains a constant environment for the brain;
- Essential for brain normal functions
Drug Delivery and the BBB
- the BBB is permeable to small and lipophilic (fat-loving) molecules (up to 400 Da);
- but larger molecules are not transported across unless there is an active transport system available;
- an additional problem is the very effective drug efflux systems (P-gly-coprotein – P-gp ), which pump the drug back out of cells
What is BBB
BBB is a membrane that controls the passage of substances from the blood into the CNS
Strategies for Drug Delivery to the Brain
GO THROUGH
The use of endogenous system
Disturbing the BBB e.g. osmotic means
Prodrugs
First, is to use endogenous transport systems, including carrier-mediated transporters such as glucose and amino acid carriers; receptor-mediated transcytosis for insulin or transferrin; and blocking of active efflux transporters such as p-glycoprotein
Second strategy is transient BBB disruption. Injection of hypertonic solution, such as mannitol, causes endothelial cell shrinkage and opening of BBB tight junctions for a period of a few hours, and this permits drug delivery to the brain. However, it has risk of entry of toxins from the blood.
Strategies for Drug Delivery to the Brain
GO BEHIND
Intraventricular /intrathecal route
Olfactory pathway
Interstitial delivery - can yield high CNS drug concentration.
One strategy for bypassing the BBB is the intralumbar injection or intraventricular infusion of drugs directly into the CSF. Because the CSF freely exchanges molecules with extracellular fluid of the brain parenchyma, delivering drugs into the CSF could result in therapeutic CNS drug conc.
An alternative CNS drugs delivery strategy is the intranasal route. Drugs delivered intranasally are transported along olfactory sensory neurons to yield significant concentration in the CSF.
How is BBB formed
Brain microvascular endothelial cells (BMEC) in brain capillaries are fused together, forming the barrier between blood and the brain
Where is BBB based and located
BBB is based on the vasculature in the brain and located in the brain capillaries.
How does blood enter the brain
Blood enters brain through large arteries, small arteries (arterioles) and capillaries.
What does the brain uptake from the blood and what does the blood uptake from the brain
Brain uptakes nutrients and oxygen from the blood whilst the blood takes away waste and carbon dioxide from the brain.
How does blood leave the brain
After the exchange blood leaves brain through small veins (venules) and large veins.
How are tight junctions formed
Endocytes of capillary forms tight junction which regulates the influx of most compounds from blood to brain
What are pericytes
Pericytes are contractile cells that have smooth muscle like properties and wrap around the endothelial cells of capillaries. Pericytes stabilise and monitor the maturation of endothelial cells by means of direct communication between the cell membrane as well as through cell signalling. Deficiency of pericytes in CNS can cause the BBB to break down.
What are astrocytes
Astrocytes are star shaped glial cells. They provide biochemical support for brain microvascular endothelial cells, influence BBB morphogenesis and organisation. Factors released by astrocytes are involved in postnatal maturation of BBB; direct contact between endothelial cella and astrocytes necessary to generate BBB.
Brain endothelial resistance in tight junction and peripheral organs
Brain endothelial tight junction resistance is between 1000-2000 ohm/cm2, while in peripheral organs it is between 5-10 ohm/cm2.
Which junction form BBB
BBB is formed by 2 junctions: tight junction and adherens junction, between capillary endothelial cells. Both attach to actin (cytoskeletal protein)
What are claudins
Claudins make up the backbone of TJ strands by forming dimers on adjacent cells to produce the primary seal of the TJ.
What are occludin
Occludin functions as a dynamic regulatory protein, whose presence in the membrane is correlated with increased electrical resistance across the membrane and decreased paracellular permeability.
What are junction adhesion molecules involved in
Junctional adhesion molecule (JAM) is also involved in cell-to-cell adhesion and monocyte transmigration through BBB
Zonula occludens proteins
The zonula occludens proteins (ZO1, ZO2 and ZO3) serve as recognition proteins for tight junctional placement and as a support structure for signal transduction proteins. Cingulin binds preferentially to ZO proteins and to other cingulin molecules.
Which other barrier is present?
Also a smaller less direct interface between blood and cerebrospinal fluid (CSF) - is the blood-CSF barrier which is selectively permeable.
The choroid plexus and the arachnoid membrane..
The choroid plexus and the arachnoid membrane act together at the barriers between the blood and CSF.
How is arachnoid membrane formed
On the external surface of the brain the ependymal cells fold over onto themselves to form a double layered structure, which lies between the dura and pia, this is called the arachnoid membrane.
The arachnoid membrane is generally impermeable to hydrophilic substances, thus forming the Blood-CSF barrier.
Where is the subarachnoid space
Within the double layer is the subarachnoid space. Inside the space, there is CSF.
What are choroid plexuses
The choroid plexuses (CPs) are leaf-like highly vascular structures laying in the ventricles. The adjacent choroidal epithelial cells form tight junctions, forming the blood CSF barrier and prevent most macromolecules from effectively passing into the CSF from the blood. However, these epithelial-like cells have shown a low resistance as compared the cerebral endothelial cells, approximately 200 Wcm2, between blood and CSF, compared to 1000-2000 ohm / cm2 in the BBB.
Chloroid plexus also forms..
Besides forming the blood CSF barrier, the choroid plexus forms the CSF & regulates concentration of molecules in the CSF. CSF is mostly made at choroid plexus at a rate of 0.4ml/min or 500-600 ml/day in adult humans. While CSF in human adult brain is about 140ml, so turn-over is about 4x per day, to clean out metabolites and toxins like beta amyloid.
How much does the brain weigh
Brain is a metabolic highly demanding organ. It weighs only 2% of body weight but demands over 20% of the body’s oxygen and 25% of its glucose.
There are a number of pathways across the BBB..
The BBB actively involves substance exchange between brain and system. There are a number of pathways across the BBB:
(a) BBB provides a large surface area of the lipid membranes of the endothelium, which offers an effective diffusive route for lipid-soluble agents.
(b) Active efflux carriers (ABC transporters) may intercept some of these passively penetrating solutes and pump them out of the endothelial cell.
(c) The endothelium contains transport proteins (carriers) for glucose, amino acids, purine bases, nucleosides, choline and other substances
(d) Certain proteins, such as insulin and transferrin, are taken up by specific receptor-mediated endocytosis and transcytosis; Native plasma proteins such as albumin are poorly transported, but cationization can increase their uptake by adsorptive-mediated transcytosis.
(e) Leukocytes cross the BBB through the interaction with the junction-associated molecules (JAMs) in BBB.
most CNS drugs enter via route a, the diffusion.
What is the most common pathway and give examples of molecules that cross it
Alcohol, nicotine, oxygen, Co2 cross the BBB through diffusion which is the commonest pathway for drug entering the brain
In BBB, the diffusion occurs from..
In BBB, the diffusion occurs from aqueous phase → lipid phase, BBB is free for Lipophilic small molecules smaller than 300Da
In BCSFB, the diffusion occur from..
In BCSFB, the diffusion occur from aqueous phase → aqueous phase, BCSFB is free for Hydrophilic small molecules, smaller than 400Da.
Carrier-mediated transport and examples
There are two forms of carrier-mediated transport, active transport and facilitated diffusion.
Examples of the carriers include: ion Channels/Pores; Glucose transporter, Neurotransmitter transporters; P-glycoprotein, Ion transporters. The carrier mediated transport is characterised by
(1) being saturable;
(2) like substances being able to compete for the services of the carrier so that competitive inhibition occurs.
Transcytosis includes..
Transcytosis includes endocytosis and exocytosis, and is used for transporting various macromolecules across the BBB. Macromolecules are captured in vesicles on one side of the cell, drawn across the cell, and ejected on the other side. There are two types of transcytosis: 1. Receptor mediated transcytosis e.g. transferrin, insulin, EPO; 2. Adsoptive mediated transcytosis, e.g. albumin.
What are efflux systems
Efflux systems function via an energy-dependent mechanism to pump out unwanted toxic substances through specific efflux pumps. Some efflux systems are drug-specific, P-gp is a well-characterized ATP-binding cassette (ABC)-transporter of the multidrug resistance (MDR) subfamily. P-glycoprotein binds a substrate and ATP molecule simultaneously. ATP hydrolysis shifts the substrate into a position to be excreted from the cell.
Summary of BBB
BBB prevents large molecules from entering the brain easily, and does not allow Low lipid soluble molecules to penetrate into the brain. However, lipid soluble molecules rapidly cross the BBB into the brain. Molecules that have a high electrical charge are slowed.
Summary of BBB
The BBB is permeable to small and lipophilic molecules. Small lipophilic drugs may enter by passive diffusion. These drugs must be Lipophilic, and smaller than 400Da, and contains less than 9 hydrogen bonds. Some drugs enter brain via facilitated transport or transcytosis / endocytosis. But, a large proportion of drugs don’t cross BBB, since 98 percent of them are heavier than 500 daltons. Up to 95% of CNS drugs being developed cannot be used in the clinic due to their inability of crossing the BBB.
How uptake of drugs be improved in the brain
Brain uptake of drugs can be improved via prodrug formation.
What is a prodrug
A prodrug is a precursor (forerunner) of a drug. Prodrug itself is not pharmacologically active, and it must undergo chemical conversion by metabolic processes before becoming an active pharmacological agent.
