Blood Brain Barrier Flashcards
When was the BBB discovered and by who
The presence of a “mechanical membrane” that separates blood from brain
was suggested at the beginning of the XX century by Lewandowsky and
Goldman.
How did they discovered the BBB?
When dye is added into the blood stream, only blood was stained.
When dye was added into the CSF –> dyes the brain AND CSF
The brain receives __ % of cardiac output
20%
What is the neuron to vessel ratio
About 1:1. Almost every neuron is perfused by its own vessel
Role of diffusion in distributing nutirents, water, drugs etc. into brain
Minimal role
BBB is __ largest discrete area for solute exchange
3rd after lung and intestine
If blood flow to brain stops–how long does it take to see damage
A stop of seconds, will cause neuronal damage within minutes
The BBB is a ____ and ___ barrier
physical and biochemical
Role of BBB
to control and regulate the access of molecules from blood to brain to maintain homeostasis of the brain microenvironment
The BBB is made of 3 types of cells…
Endothelial cells
Pericytes
Astrocytes
Barrier function is primarily mediated by which cell type(s)
Endothelial cells
Development and maintenance of BBB is primnarily mediated by which cell type(s)
Pericytes and astrocytes
Other cell types that influence the BBB
perivascular inter-neurons and microglia–also contribute to BBB regulation
Regions w/o BBB
Area postrema (of medulla)
Circumventricular organs
Regions of the hypothalamus
Structural reason(s) certain brain regions don’t have BBB
they have fenestrated capilaries–w/o pericytes and astrocyte ensheathment therefore allowing the passage of hydrophobic molecules, peptides and proteins into the brain
Physiological reason(s) certain brain regions don’t have BBB
they need to be able to sample the general circulation (chemoreceptors in area postrema) or release of hormones (hypothalamus)
Feats of BBB
Endothelial cells have _________
No fenestrations
Feats of BBB
Transcellular transport through ____ is ____
pinocytosis is minimal
Feats of BBB
What system removes small molecules from the endothelial cells before they reach the brain parenchima
an efflux transport system ( made up of P-glycoprotein and others)
Feats of BBB
What is the Secondary system consituiting an additional barrier to the BBB
the use of enzymatic systems (C450, peptidases, nucleotidases) to inatcivate neuroactive or toxic compounds that enter the brain
Feats of BBB
What ‘seals’ the paracellular pathway
Tight junctions
What is a tight junction
an adhesion complex found between adjacent endothelial cells that seals the paracellualr pathway-restricting movement b/t cells
Because of tight junctions ____ transport is negligible at the BBB
paracelluar transport
Additional role of tight junctions in BBB
Maintaining highly polarized state of BBB endothelial cells by segregating the apical (luminal) and basal (abluminal) domains of the endothelial cell memb
What structural transmembrane proteins responsible for tight junction regulation?
Occludin
What structural transmembrane proteins responsible for tight junction “tightness”?
Claudins
What structural transmembrane proteins responsible for tight junction stability and maintenance?
Junctional adhesion molecules (JAMs)
What cytoplasmic accessory proteins responsible for tight junction’s ability to anchor transmembrane proteins to the cytoskeleton and modulate junction “tightness”?
Zonula occludens proteins 1 and 2 (ZO-1 and ZO-2)
and cingulin
have a regulatory function in determining tightness of the TJs
How do tight junctions effect transendothelial electrical resistance (TEER)
Tight junction restricts the movement of water and ions (Na+ and Cl-) therefore increasing TEER
TEER (transendothelial electrical reistance)
Measure of the resistance to an electrical current passed across the endothelial monolayer as a measure of ionic permeability
How peripheral TEER compares to cerebral TEER
TEER 2-20 ohms/cm2 peripheral microvessels
>1000 ohms/cm2 in micro microvessels
High TEER due to tight junctions
TEER and tight junction
restrict ion and water flow –> increase TEER
BBB is present in…
all vertebrates
How BBB differs across species
species differences in postnatal murations leads to differences in amount of drugs being able to reach the brain during development
In humans BBB developement is completed by
6 months of age (postnatal)
In humans BBB development starts
During the first trimester of fetal life (completed by 6 months postnatally)
Permeability and drug effects on neonates altered by:
differently than to adults due to incomplete BBB–should take into consideration higher permeability of fetal and neonate BBB
Role of neural stem cells in BBB development
neural stem cells produce Wnt molecules that stimulate endothelial cells of the brain to express BBB-related molecules
Pericytes and astrocytes contribute to
BBB development, maturation and MAINTENANCE
WNT
released by neural stem cells
stimulate production BBB
Astrocyte location
astrocyte end-feet completely surround microvessels
Astrocyte roles
Transport of nutrients to brain
regulation of BBB props
regulation of local bloodflow
Neurovascular coupling
synchronizes neuronal metabolic demands to local cerebral blood flow regulation (increased activity, increased blood flow)
done by astrocytes
How astrocytes participate in neurovascular coupling
detect glutamate-depending synaptic activity and respond by producing prosteoids (bioactive lipids) that cause vasodlation (which increases blood flow to the area of higher activity)
Effects of Astrocyte-produced cytokines and growth factorss
contribute to tighter TJ + induction of transported expression (p-glycoproteon and GLUT-1)
for the maintenance of mature barrier props
In-vitro experiments of endothelial plus astrocytes vs without astrocytes–differneces
TEER increased when co-cultured w/ astrocytes
Endothelial cells alone–some barrier resistance (suggest endothelial cells themselves already have some)
Endothelial cells w/ astrocytes – increased barrier resistance
Where are pericytes
juxtaposed to the endothelial cells and together with them form a matrix of extracellular proteins (basal lamina) that surrounds the vessel
How do pericytes get recruited
Endothelial cells secrete platelet-dervied growth factor B (PDGF-B) recruits them closer to the enodthelial cells and leads to vascular maturation
platelet-dervied growth factor B (PDGF-B)
Secreted by endothelial cells to recruit pericytes
Pericytes produce ____ to induce barrier functions
Angiotensin 1 (ANG-1) and TGF-beta
The actions of pericytes on BBB props is mediated by their ability to _____ and _____
Polarize astrocyte endfeet and limit endothelial transcytosis
Pericyte to endothelium in brain vs muscle
1:3 versus 1:100
Pericytes roles (simplified)
Provide mechanical stability release Ang1, TGF-beta to induce barrier function
Also have contractile function (control blood flow with contraction/relaxation)
How pericytes control blood flow
Via their contractile function (control blood flow with contraction/relaxation)
In-vitro models best reproduce true BBB properties if they have
both astrocytes and pericytes
will have high TEER (as seen in vivo)
BBB transport: no ____ transport, exhange of molecules may occur through _____ or _______
No paracellular transport and minimal pinocytosis
Mainly through: lipid-mediated diffusion or catalysed transport
What undergoes Lipid-mediated free diffusion
lipophilic compounds with a molecular mass <400-500 Da
i.e. free diffusion of small lipophillic molecules
Catalysed transport–3 types
facilitative transport
active-efflux transport
receptor-mediated transcytosis
The permeability of the BBB is directly proportional to (in most cases)…
the lipid solubility of the substance (measured by the oil-water partition coefficient)
More high-philic, more permeable
Exception– higher than expected permeability
Some hydrophilic molecules (ex. D-glucose) are readily taken up by selective endothelial transport–higher than predicted due to selective transport
Exception-lower than expected permeability
Drugs with very high oil-water parition coefficients need to be bound to albumin in blood–> decrease delivery to brain
(Oil-water) Partition coefficient
ratio of concentrations of a compund in the two phases of a mixture of two immiscle solvents, usually water and octanol, at equilibrium
BBB transport of MACROmolecules
either: receptor-mediated transcytosis OR
adsorptive-mediated transcytosis
BOTH forms of trancytosis
Receptor-mediated transcytosis: mechanism
receptor and bound ligand are internalized and move through the cytoplasm to the abluminal side where the ligand is exocytosized
Receptor-mediated transcytosis: ligands
transferrin IgG insulin leptin TNF-alpha, EGF
Adsorptive-mediated transcytosis: mechanism
cationised albumin interacts with negative cell membrane allows passage through endothelial cells
The ability of the sorting endosome to route away from _____ is a special feature of the BBB
LYSOSOMES
in contrast–in the periphery contents of primary endosomes are routed to the acidic lysosome for enzymatic degradation
P-Glycoprotein (aka MDR1 and ABCB1)–structure
transmembrane protein charcteriszed by 2 homologous halves, each with six transmembrane domains and a typical ATP-binding domain
P-Glycoprotein role
hydrolizes ATP to transport molecules across the cell membrane (export bad things from the brain)
expressed in many normal tissues, where together with xenobiotic metabolizing enzymes, it acts to protect the body from potentially toxic xenobiotics
________ is one of the most important transporters for drug disposition in humans
P-glycoprotein
Moves drugs out of tissues including out of the brain
P-glycoprotein: props of good substrates
hydrophillic or antipathic
BROAD range
Trouble with P-glycoprotein
Pumps drugs out of the brain–good if target is the periphery (won’t have neural effects)
BAD if brain is the target –> trouble with neuropharmacology
P-glycoprotein mediated efflux
1) Substrate partitions the bilayer from outside the cell to the inner leaflet and enters the internal drug-binding pocket through an open portal.
2) The AA residues in the drug binding site stabilize the protein-substrate interaction.
3) Upon ATP binding to the nucleotide-binding domain –> P-gp changes conformation and forces the substrate out
P-gp affcets drug absorption and tissue distribiton through
1) limited drug absorption (limits drug entry in gut)
2) Active drug elimination (once in circulation promotes elimination into urine)
3) Limited drug distribution into tissues (limits entry into sensitive tissues–ex. brain or testes)
Effects drug efficacy by decreasing its concentration
MDR-2 gene polymorphism’s role in CNS disease
these polymorphisms affect protein expression/activity incfluencing individual response to treatments and disease progression
Examples of MDR-2 gene polymorphism’s role in CNS disease
pesticide-induced PD associated with certain MDR-1 polymorphisms (pesticides that are usually removed are not –> cause PD);
Drug-resistant epilepsy (drugs removed from the brain –> non effective)
Dysfunction of BBB contributes to development and progression of
Alzheimers PD HIV-related dementia ALS MS
Disruption of the BBB occurs due to these pathologies
Stoke and brain injuries
inflammation
brain tumours
Brain regions w/o BBB
Area postrema
subfornical organ
pineal gland
neurohypophysis
2 roles of tight junctions
- restricting paracellular transport
- maintaining the polarity of endothelial cells (by segregating the apical and basal domains of te endothelial cell memb)
BBB Transport systems
1) facilitative transporters
2) active Na+-dependent transporters
3) facilitative transporters on the luminal membrane
4) ATP-binding cassette (ABC) transporters
Facilitative transporters can be on…
both membranes or only on the luminal membrane
Facilitative transporters on both membranes examples:
GLUT-1, L system MCT-1
Facilitative transporters on the luminal membranes examples:
Xg-transporter
N-transporter
Glu and Gln transporters
follows conc gradient but extrude aa’s accumulated in the endothelial cells
Facilitative transporters mechanism
move molecules following their concentration gradient (high to low)
Active Na+-dependent transporters–mechanism
on the abluminal membrane transport molcules against their graident via co-transport with Na that is then pumped out by the Na+/K+ pump
Active Na+-dependent transporters–examples
glutamate and glycine aa’s pumped out
ATP-binding cassette (ABC) transporters–examples
P-glycoprotein, MRP
Active Na+-dependent transporters–purpose
efflux metabolites, drugs and xenobiotics out of the brain and into the blood
P-GP expression is high in
enterocytes (gut) , kidney cells, heptocytes, placenta and BBB
affects how drugs abosrp and distribute in the body
