BBB L1 - 2 Flashcards
Why is drug delivery to the brain an issue for the pharma industry?
- failure in treating neurodegenerative diseases e.g. AD, PD, dementia, and brain tumours e.g. Gliomas
- <5% drugs in trials show clinical benefit - don’t cross BBB
- Cost of failed trials = puts of pharma trying
- don’t give pt false hope
- complex to develop e.g. animal models different metabolic systems
Why does a barrier exist between the brain and the blood?
What is the need for homeostasis of brain ECF?
- separate blood from brain ECF
- shield from composition of blood e.g. glutamic acid = toxic and causes apoptosis
What does Brain ECF include?
1) Brain tissues interstitial fluid (ISF) = between neurones and capillaries
2) Ventricular cerebrospinal fluid (CSF)
What experiment shows evidence for the existence of the BBB?
- Goldmann Trypan Blue
- IV = only body stained
- IT (Intrathecal) = only CNS
1) Why is there need for a barrier - 5 reasons
1) Ion regulation - need optimal Na and K conc for optimal neuronal signalling
2) Molecular traffic: Keep toxins out = low cell death
3) Low protein - proteins = cell division and brain is surrounded by fixed skull
4) Preserve neurotransmitter signalling = separate CNS:PNS
5) ‘Silent’ immune surveillance. Keep leukocytes out to limit inflammation -brain has its own immune cells - microglia
What are three key Clinical Significance points for homeostasis of brain ECF?
1) K+ equilibrium potential = approx. -100 mV –> reduce seizures, action potentials specific, acidity = faster repolarisation
2) Low protein = reduce water retention and brain oedema
3) Low Ca and glutamate = reduce potential excitotoxicity and subsequent neurodegeneration
Sites of brain barriers
BBB = capillaries
Blood-CSF- barrier = choroid plexuses
Stats about brain energy usage and size of anatomy
•Brain has high energy requirement:
•uses ̴ 15% of glucose and O2 but is only 2% of body weight.
•Rich capillary network needed for constant supply of nutrients - similar to exercising muscle- to keep neurones active and Na/K pump operational.
- The brain used 7x more glucose and oxygen than non exercising muscle = need for dense capillaries.
- Human brain – 1500 g
- Capillary total length – 600km
- Surface area – 20 m2
- Capillary volume -17 mL of blood
- Inter-capillary distance - 40um
- Capillary lumen diameter – 7 um – only fits one RBC
- Length per neurone – 10 um
What are the three components that form the BBB?
1) Physical
2) Transport
3) Metabolic
1) Physical Barrier
- what cells overlap and what protein junction forms between them?
- what type of brain cell secrete the factors secrete factors to make the said junctions?
- Endothelial cells
- Tight Junctions
- Astrocyte secrete factors that induce tight junction formation
What is the difference between adherens and tight junctions?
Adherens = formed with cadherin, and are in the body
Tight Junctions = complex protein structure in the brain - made up of occludin, claudio, zonnula occludens (ZO)
What unit is used to measure tightness?
Ohms cm^2
What is the different resistances between muscle capillary, choroid plexus, and brain capillary?
- Muscle capillary = 40 ohms cm^2
- Choroid plexus = 200 ohms cm^2
- Brain capillary = 8000 ohms cm^2 (hence a lot tighter than the rest of the body)
What type of molecules can pass through the BBB?
N.B what certain characteristics do they need to have>
- <400 Da
- log P >1
- small
- lipid soluble
oxygen, nicotine, caffeine, barbiturates, spice, opiates, anaesthetics, alcohol
B) Transport Barrier
- What are the THREE examples of efflux transporters
1) P-glycoprotein
2) BCRP (Breast Cancer Resistant Protein)
3) MRP (Multi-drug Resistance Protein)
What are examples of molecules that the transport barrier for the BBB aims to keep out?
- lipid soluble toxins
- drugs e.g. chemotherapy, antivirals, antibiotics
- pesticides
- plasma amyloid peptide
- glutamic acid
C) Metabolic Barrier
- What enzymes are known to exist within the metabolic barrier for the BBB?
- CYP450 of neurones
- CYP1B1, CYP2U1 - in endothelia
- CYP2J2, CYP2U1 - astrocyte end-foot
- CYP2E1, CYP1A1, CYP2C9= rest of astrocyte cell
What type of metabolism to CYP enzymes carry out?
- metabolize vast array of compounds
- drugs, neurotoxins, neurotransmitters, neurosteroids
3) How to things get into the brain?
Uptake Transporters - on apical blood-facing membrane
How is a cell described that only allows uni-directional flow?
Polarised
- movement in one direction from the blood-facing membrane
What are the key examples and their respective substrates for transporters at the BBB?
GLUT1 - Glucose
Mct1 - Lactate (Monocarboxylate)
Lat1 - Leucine (neutral aa)
TfR - Transferrin - important for iron homeostasis
N.B THESE TRANSPORTERS ARE SPECIFIC - ALLOWS TIGHT CONTROL OF MOLECULES CROSSING THE BBB
What drug takes advantage of carrier-mediated uptake of aa?
L-dopa (Treatment for PD)
What are the two types of vesicular mediated transport?
RMT - receptor mediated transcytosis - where the system is dependent on the molecule/substrate binding
AMT - aDsorptive mediated transcytosis - where the system is dependent on the substrate acquiring a positive charge
4) The Neurovascular unit
- What are the main four cells that make up the neuromuscular unit?
- endothelia
- neuron
- astrocyte
- pericyte
What is the action of the astrocyte at the neurones, and relate this to the subsequent effect this has on pericytes - and the end result of this pathway i.e. how does the pericyte act alike smooth muscle and what is the purpose for this?
- astrocyte acts at neurone = has the same receptor as post-synaptic neuronal membrane
- when Ca2+ released more = increased firing = astrocyte detects this = releases vasoactive compounds to deliver more blood to working neuron
- astrocyte feedback to pericyte = has myosin and actin = alike smooth muscle by being able to relax and cause vasodilation of capillaries - allowing more blood to be delivered to the neurones
- if the pericyte contracts i.e. when the astrocyte signals the neurone is less active - the pericyte can contract and restrict blood supply bu causing vasoconstriction.
Why is the pericyte control of BBB capillary network of clinical significance in treating neurodegenerative diseases?
As neurodegenerative diseases progress - the neurones become less active - hence the pericyte causes vasoconstriction - hence decreased blood supply to these areas.
This is bad as the further the disease progresses - the harder it is to deliver drugs to these sites.
How can the blood-flow in the brain be measured to identify neurodegenerative disease?
MRI - monitor brain neuronal activity.
Reduced blood flow = disease
5) CSF - Cerebrospinal Fluid
- What is the pharmacological significance of the CSF?
Removal of Drugs
want to control removal of CSF - i.e. if want to retain drug in the brain
What is the name of the anatomy that produces CSF?
- Choroid plexuses of the lateral, third, and fourth ventricles
What forms the Blood-CSF-Barrier, and How does it form its barrier?
Choroid plexus has capillary surrounded by epithelium - leaky BUT has tight junctions - this is the barrier
N.B the capillaries here are alike normal body capillaries i.e. not BBB - hence can allow molecules in and out freely but it is the tight junctions of the epithelia that form the barrier here.
What is the function of these epithelia in the B-CSF barrier?
Remove toxins, large molecules e.g. proteins, from the CSF
‘Kidney-like function’
What are the similarities between BBB and BCSFC?
- physical barrier (BUT not as tight junctions as BBB)
- Transport barrier
- Ion homeostasis = needed for optimal signalling (especially K, Ca2+, Mg2+)
What are the 5 ‘extra’ functions of the B-CSF-B?
1) Produce CSF
2) Buoyancy (1400g –> 50g)
3) Nutrition (Vit.C, aa, glucose)
4) Excretion/drainage - i.e. acts alike lymphatic system
5) Hormonal secretion e.g. IGF (insulin-like growth factor), Leptin
How does the CSF by choroid plexuses occur? What membrane protein complex is influential in this?
Na/K ATPase sets up gradient on apical membrane
Causes Na and Cl to move from blood side to CSF side = water follows down osmotic gradient
Water also moves via aquaporins
What are the figures relating to:
1) Secretion volume
2) Pressure
3) Volume
4) Composition
1) 0.5 mL/min
2) 10 mmHg (ISF = 3 mmHg)
3) 150 mL (Ventricles = 35 mL) - replenished 4 x daily = 600 mL daily.
4) similar to plasma BUT lower protein, glucose, aa and also small but significant differences in K and Ca i.e. less of these
Describe
1) The flow pathway, and
2) The drainage of the CSF
1) Flow from ventricles, where some to spinal cord, but then makes its way back up and resides mainly over the SUB-ARACHNOID SPACE - the space between brain and the arachnoid membrane
CSF flows between this
2) CSF drains into VENOUS SINUSES - large vein down the centre of the brain - major route for clearance of metabolites/waste/toxins from the whole brain
How does the venous sinus allow one-way flow? How does this allow adaptation to increased CSF flow?
Has a valve system = allows push of fluid, hence one-way valve allows management of increased CSF flow by pushing the fluid straight into the blood, not allowing any back-pressure
What is the perivascular route?
2 ‘spaces’
- Virchow-Robin space (VRS)
- Peri-arterial space (PAS) in human brain
How does the CSF move between the perivascular route?
Flows back and forth from VRS and PAS
Only flows small-distance
The CSF can transverse the brain tissue by the perivascular route, true or false?
FALSE
it only travels a small distance in and out of the VRS and PAS spaces
What function can the perivascular route play?
- Washing motion - washing out these areas for removal of drugs and,
to aid mobility of drugs across membranes e.g. if high conc. build-up in PAS - the flowing action can spread the drug depot over larger area = aids mobility across the membrane
What does the description of a ‘pulsatile movement’ mean with regards to the flow of CSF via the perivascular route
it flows back and forth
What does the direction of CSF flow via the perivascular route rely on?
Fluid pressure i.e. how much CSF and ISF there is
Drug/metabolite concentration
What would the interruption of CSF flow cause?
What is non-communicating and communicating?
Hydrocephalus: dilated ventricles, swelling of brain, increased intra-cranial pressure = squeezing of brain against the cranium cause damage.
Non-communicating = usually due to blockage of the aqueduct of Sylviu results from closure before birth (congenital), of from brain tumour at any age.
Communicating = blockage of CSF flow in sub-arachnoid space or villi
Treatment: shunt of excess fluid in ventricles e.g. to abdominal cavity
Why is the flow pulsatile?
Aids clearance varies with cardiac cycle, causes movement of CSF back and forth, midnight 42mL/h due to increased BP and minimal at midday 8mL/h. So best to give drug in the day =cleared less
How can clearance from the CSF be decreased by pulsatile movement?
larger sized drug molecule, but issue with cross BBB initially, can bind to glia cell so is retained for longer
Can you break the BBB barrier, When would you do this?
This occurs in neurological conditions eg alzheimers where there is break down of BBB this lets in toxins eg glutamate - therefore this technique should only be used to prolong life =FOR END OF LIFE
what is a cause of BBB breakdown
Traumatic brain injury, stroke, MS, bacterial meningitis’s, epilepsy
what is a consequence of BBB
alzheimers, epilepsy, HIV related dementia, cerebral malaria, depression, schizo, age-related, parkinson’s disease
