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Blood brain barrier
BBB
• The blood-brain barrier (BBB) is a
membrane that controls the passage of
substances from the blood into the central
nervous system.
• It is a physical barrier between the local
blood vessels and most parts of the
central nervous system itself, and stops
many substances from travelling across it.
• The BBB is permeable to alcohol, and
some heavy metals can cross the bloodbrain barrier as well.
BBB
• History
• The existence of such a barrier was - by Paul
Ehrlich in the late-19th century.
• the aniline dyes stain all of the organs of an
animal except the brain
• in 1913, Edwin Goldmann (one of Ehrlich's
students) injected the dye into the spine directly.
• He found that in this case the
brain would
become dyed, but the rest of the body remained
dye-free.
• This demonstrated the existence- of barrier
between the two sections of the body.
• the scanning electron microscope in the 1960s
demonstrated the barrier membrane.
BBB
Anatomy of BBB
• The brain microvessels are composed of:
1. endothelium with 3 structural features
a. tight intercellular junctions without fenestrations
b. paucity pinocytic intracellular vesicles
relative lack of vesicular transport
c. abundant mitochondria
Basement membrane -surrounds endothelium
-40-50 nm thick
contains-proteoglycans, heparin sulfate, lamnin,
entactin & type IV collagen
2. Pericytes with smooth muscle- like properties that reside adjacent to
capillaries
3. Astroglial process –ensheath >95% of the abluminal vessel surface
Anatomy of BBB--
BBB cont
BBB
BBB vs capillaries of peripheral nerve + organs
Characteristic or structure
BBB
capillaries
Tight junction resistance
5-10 ohm/sq.cm
2000 ohm/ sq.cm
Fenestration/clefts
no
yes
Vesicular transport
deficient
abundant
Pinocytic vesicles
rare
abundant
Mitochondria
abundant
rare
Tight junction
++++++
+
Selective transport
++++++
-
Astrocyte foot process
++++++
-
BBB
• Physiology
• In the brain, endothelial cells are packed much
tighter together, due to the existence of zonula
occludens (tight junctions) between them,
blocking the passage of most molecules.
• The blood-brain barrier blocks all molecules
except those that cross cell membranes by means
of lipid solubility (such as oxygen, carbon dioxide,
ethanol, and steroid hormones) and those that are
allowed in by specific transport systems (such as
sugars and some amino acids).
• Substances with a molecular weight higher than
500 daltons generally cannot cross
Selectivity of BBB--• Entry to the CSF is in three ways
1. by diffusion of lipid-soluble substance
- endothelial membrane surface area =180
sq.cm/g of gray matter.
- Efficient exchange of lipid-soluble gases as
oxygen and carbondioxide-- limited by surface
area & blood flow
2. by facilitative and energy- dependent receptormediated transport of specific water soluble
substances
3. by ion channels
Blood brain transport properties
• In normal BBB mechanisms controlling passage of
substances across cell wall are
I. diffusion –lipid soluble substances
II. specific carrier system:
a. metabolic substrates = glucose transporter GLUT-1
-GLUT-1 has 492 amino acids
-non-energy dependent, facilitative, saturable &
stereospecific transporter functioning both at the luminal &
abluminal membrane
- the gene coding for GLUT-1 is on chromosome 1.
-absence of GLUT-1=incompatible with life
-deficient GLUT-1 expression=rare syndrome with mental
retardation, epilepsy & hypoglycorhachia
Blood brain transport properties---
b. amino acids carrier system:- are three
1. the L system- for large neutral amino acids with branched or ringed side
chains such as leucine & valine.
It is a sodium dependent, facilitative transporter & located at both luminal and
abluminal endothelial membrane
-inhibited by 2-aminobicyclohptane-2-carboxilic acid (BCH)
- down concentration gradient-transport
- L-DOPA & dopamine precursors for Rx of Parkinson‘s disease
==transported via
- competitive inhibition of the transport of large neutral amino acids by high
phenylalanine == the neurotoxicity of phenylketonuria
2. the A system- transports neutral amino acids with short linear or polar side
chains, such as alanine or serine
-energy dependent, sodium dependent,& inhibited by methylaminoisobutyric acid
-located exclusively abluminal
-limits accumulation of glycine in spinal cord and glutamate in the brain== the
ATP & sodium required are supplied by sodium potassium ATPase –at abluminal
side
3. ASC-mediated- for alanine, serine & cysteine
-energy dependent, sodium dependent
-located exclusively abluminal
Blood brain transport properties-
c. transmembranous transporters
-Most abundant
-described initially for multiple drug resistance (MDR) to tumor cells
-remove a broad range of natural and synthetic hydrophobic toxins
- MDR-transporter=p-glycoprotein-influence delivery of many cancer therapy drugs eg vinca
alkaloids, actinomycin D
-expressed by vessels of BBB not by others
III. Ion channel & exchangers:
-mediate electrolyte movement across BBB
-non-selective luminal ion channnel inhibited by amiloride & atrial
natriuretic peptide
- Na+/K+ ATPase
Transport across BBB
Figure:Transport mechanism across BBB---
1 = paracellular diffusion (sucrose)
2 = transcellular diffusion (ethanol)
3 = ion channel (K+ gated)
4 = ion-symport channel (Na+/K+/Cl-cotransporter)
5 = ion-antiport channel (Na+/H+ exchange)
6 = facilitated diffusion (Glucose via GLUT-1)
7 = active efflux pump (P-glycoprotein)
8 = active-antiport transport (Na+/K+ ATPase)
9 = receptor mediated endocytosis (transferrin &
insulin)
Metabolic BBB
• DOPA decarboxylase & monoamine oxidase
- in endothelial cells of the barrier rapidly metabolize LDOPA to 3,4-dihydroxyphenylacetic acid-inhibiting the
entry of DOPA to the brain=inhibitor needed
BBB physiology
• Many drugs are unable to pass the barrier, since
98 percent of them are heavier than 500 daltons.
• hormones generally do not penetrate the brain
from the blood, except at the 'circumventricular
organs
• The blood-brain barrier is an effective way to
protect the brain from common infections.
• as antibodies are too large to cross the bloodbrain barrier, infections of the brain when they
do occur can be very serious and difficult to
treat.
Circumventricular Organs
Areas of the brain where the BBB is weak, & allows substances to cross into the
brain somewhat freely are known as "circumventricular organs. These leaky
regions are isolated from the rest of the brain by specialized ependymal cells
called tanycytes located along the ventricular surface close to the midline
• Pineal body
– Secretes melatonin and neuroactive peptides. Associated with circadian
rhythms.
•
Neurohypophysis (posterior pituitary)
– Releases neurohormones like oxytocin and vasopressin
•
Area postrema
– "Vomiting center": when a toxic substance enters the bloodstream it will
get to the area postrema and may cause the animal to throw up
Subfornical organ-chemoreceptive area monitoring blood angiotensin II level
– Important for the regulation of body fluids.
•
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Vascular organ of the lamina terminalis
– A chemosensory area that detects peptides and other molecules.
•
Median eminence
– Regulates anterior pituitary through release of neurohormones.
BBB break
• The BBB can be broken down by:
• Hypertension (high blood pressure): high blood
pressure opens the BBB
• Development: the BBB is not fully formed at birth.
• Hyperosmolitity: a high concentration of a substance
in the blood can open the BBB.
• Microwaves: exposure to microwaves can open the
BBB.
• Radiation: exposure to radiation can open the BBB.
• Infection: exposure to infectious agents can open
the BBB.
• Trauma, Ischemia, Inflammation, Pressure: injury to
the brain can open the BBB.
Diseases affecting the blood-brain barrier
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Meningitis
inflamed meninges disrupt blood-brain barrier
This increase penetration of various substances (including antibiotics) into
the brain
third generation cephalosporin or fourth generation cephalosporin is
preferred.
Multiple sclerosis (MS)
Normally, nervous system is inaccessible for WBC due to BBB.
using MRI in an MS "attack," BBB has broken down in brain or spinal
cord, T lymphocytes crossing over and destroying the myelin.
MS is a disease of the blood-brain barrier.
The yellow ingredient turmeric, found in curry, has been shown to
strengthen the blood-brain barrier to resist attacks.
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Late-stage neurological trypanosomiasis (Sleeping sickness)
Late-stage neurological trypanosomiasis, or sleeping sickness, is a
condition in which trypanosoma protozoa have crossed the blood-brain
barrier.
•
Progressive multifocal leukoencephalopathy (PML)
Blood brain barrier in chemotherapy of glioblastoma
Blood brain barrier in HIV
cont
• Blood-borne immune cells such as lymphocytes,
monocytes and neutrophils cannot penetrate this barrier
• A thin basement membrane, provides mechanical support
and a barrier function
• BBB prevents infiltration of pathogens and restricting
antibody-mediated immune responses in the central
nervous system, as well as for preventing disorganisation
of the fragile neural network
• This, together with a generally muted immune
environment within the brain itself, protects the fragile
neuronal network from the risk of damage that could
ensue from a full-blown immune response
• On occasions, pathogens (e.g. viruses, fungi and prions)
and autoreactive T cells breach the endothelial barrier and
enter the brain
• A local innate immune response is mounted and
pathogens are destroyed and cell debris is removed
Drugs targeting the brain
• the BBB hinders the delivery of diagnostic and therapeutic agents
• Mechanisms for drug targeting in the brain involve going either
"through" or "behind" the BBB.
• Modalities for drug delivery through the BBB entail disruption of the
BBB by osmotic means, biochemically by the use of vasoactive
substances such as bradykinin, or even by localized exposure to
ultrasound.
• strategies to go 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 blocking of active efflux transporters such
as p-glycoprotein.
• Strategies for drug delivery behind the BBB include intracerebral
implantation and convection-enhanced distribution.
• Nanotechnology could also help in the transfer of drugs across the
BBB.
• Recently researchers have been trying to build nanoparticles
loaded with liposomes to gain access through the BBB.
Drug Delivery and the Blood-Brain Barrier
-The BBB is permeable to small and lipophilic
(fat-loving) molecules (up to 800 atomic mass
units), but larger molecules are not transported
across unless there is an active transport system
available
• Thus this is one of the stumbling blocks for drug
delivery
• An additional problem is the very effective drug
efflux systems (P-gly-coprotein – P-gp ), which
pump the drug back out of cells.
drug delivery across BBB
Transport across BBB
cont
• There are three main methods of transport across the BBB, none of
which is perfect .
1. Direct physical injection into site of interest.
2. Permeabilisation of tight junctions using either osmotic disruption
or biochemical opening (RMP-7 Alkermes, vasoactive compounds –
histamine).
3. Enhance transcytosis across the endothelial cells – (transcytosis)
to the underlying brain cells can be achieved by increasing
endocytosis (i.e. internalisation of small extracellular molecules) by
using liposomes or nanoparticles loaded with the drug of interest.
The uptake can be further enhanced by specifically targeting the
delivery system to receptors on the brain endothelium surface that
are capable of receptor mediated endocytosis.
This method is more selective than the tight junction disruption,
especially if brain specific targeting technology is used, but tends to
be less efficient.
It also requires the discovery and development of receptor specific
ligands, which can be attached directly to the drug of interest or the
drug delivery system itself.
Blood CSF barrier
cont
• While the largest interface between blood and brain is the BBB, this
is also smaller less direct interface between blood and cerebrospinal
fluid (CSF)
• Goldmann first demonstrated the existence of the blood-CSF barrier
in 1913. Through the use of dyes with different properties it was
found that the blood-CSF barrier was selectively permeable, rather
than absolute
• The choroid plexus and the arachnoid membrane act together at the
barriers between the blood and CSF
• 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.
Within the double layer is the subarachnoid space, which
participates in CSF drainage
• Passage of substances from the blood through the arachnoid
membrane is prevented by tight junctions
• The arachnoid membrane is generally impermeable to hydrophilic
substances, and its role is forming the Blood-CSF barrier is largely
passive.
cont
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choroid plexus forms the CSF & regulates concentration of molecules in
the CSF
The choroid plexus consist of highly vascularized, "cauliflower-like"
masses of pia mater tissue that dip into pockets formed by ependymal
cells.
choroid plexus is distributed throughout the fourth ventricle near the base
of the brain and in the lateral ventricles inside the right and left cerebral
hemispheres.
The cells of the choroidal are modified and have epithelial characteristics.
These ependymal cells have microvilli on the CSF side, basolateral
interdigitations, and abundant mitochondria
The ependymal cells, which line the ventricles, form a continuous sheet
around the choroid plexus.
While the capillaries of the choroid plexus are fenestrated, non-continuous
and have gaps between the capillary endothelial cells allowing the freemovement of small molecules, the adjacent choroidal epithelial cells form
tight junctions preventing 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 W ·cm2,
between blood and CSF .
cont
cont
• While the brain & CSF are separated by the
somewhat permeable pia mater, the bloodcerebrospinal fluid barrier and the blood-brain
barrier (BBB) represent substantial protection for
the brain against undesirable blood substances.
These barriers are very permeable to water,
oxygen, carbon dioxide and small lipid-soluble
substances.
• They are also somewhat permeable to small
electrolytes -- and special transport systems exist
for some other specific molecules such as essential
amino acids.
• The barriers are the result of endothelial cells which
line capillary walls -- and glial cells called
astrocytes which wrap the capillaries with fibers.