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 blood-brain 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 collagen2. Pericytes with smooth muscle- like properties that reside adjacent to

capillaries3. 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 receptor-mediated 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 L-DOPA 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 blood-brain 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.

• 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

• 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.

• 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

• 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 free-movement 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 blood-cerebrospinal 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.