BBB Flashcards
why is delivering drugs to brain a problem for industry and whats the way forward?
failure in treating neurodegeneration (Alzhiemer’s, PD) + brain tumours
- because majority of drugs cannot cross the BBB
WAY FORWARD:
- not only develop treatments for disease
- also develop ways to cross the BBB
what is the early evidence for a BBB?
Goldmann’s 1st and 2nd experiments - demonstrated there was some form of selective barrier b/w bloodstream and brain - because when certain dyes were injected into the blood - it stained most tissues but NOT the brain & Goldmann found that substances like trypan blue dye & methylene blue dye could penetrate most tissues but not the brain.
In mice - trypan blue dye was injected IV - body was stained but CNS was unstained
- conversely when dye was injected intrathecally (brain & spinal cord) - CNS stained but body unstained
- 1st indication - that there is separation b/w fluid environments of CNS and fluid environments of rest of body
Why have a barrier?
1) keep low protein - proteins stimulate cell division due to growth factors - so limits cell proliferation - as theres no room for expansion and would crush other cells and neurones - gradually losing neuronal function
- neurones are post mitotic - don’t divide
2) ion regulation - regulation of Na, K, Ca etc. - proper ion balance is crucial for optimal neural signalling inc. APs and synaptic transmission - disrupting ion balance could lead to neuronal hyperexcitability or inhibition
- brain has constant ion concentration unlike blood which fluctuates in sodium
3) molecular traffic - to keep toxins out - low risk of cell death - maintains brain health
4) separates NTs found in blood and brain e.g. adrenaline & glutamate - prevents excessive levels which could lead to overstimulation or excitotoxicity
5) keeps WBCs out of brain - keep leukocytes out to prevent inflammation - prevents swelling and oedema which would crush neurones in small brain space
- instead brain has its own immune cells called microglia which do the same job as WBCs but don’t divide fast and cause inflammatory action
compare K+ conc. in the blood and brain and why this difference is important
Blood K+ conc. = 3.6-5.0 mmol/L
brain K+ conc. = 2.9 mmol/L
- much lower in brain
potentially primary reason for evolutionary adaptation of BBB
- to keep potassium out
- allows RMP of neurones to be low - HYPERPOLARISED - around -100mv (more neg) whereas in the rest of the body it is around -60/70 mv
this reduces the chances of unintended excitation as neurones have a tendency to spontaneously produce APs due to leaking out of K+
- to prevent spontaneous random firing of APs, even if there is a leak - RMP is still hyperpolarised
- if brain tumour that disrupts barrier - could lead to random firing of APs and seizures
compare H+ ion conc. in brain and blood
pH of blood - 7.4
pH of brain - 7.3
one of the rare cases where theres a higher conc. in brain than blood
- increased H+ ions allows faster repolarisation after APs using Na+/K+ ATPase pump
compare albumin, calcium and glutamate, glucose and cholesterol conc. in brain and blood
albumin blood - 6 g/dL, brain - 0.02 g/dL
very low conc. in brain as low protein reduces water retention and oedema and limits cell proliferation
calcium blood 2.4mmol/L, brain 1.2 mmol/L
glutumate blood 40 um, brain 0.05 um
- low calcium and glutumate reduces chance of excitotoxicity and neurodegeneration
glucose blood 100mg/dl, brain 64mg/dl
cholesterol blood 175mg/dl, brain, 0.2 mg/dl
how do we know brain has a high energy requirement and why?
uses 15% of glucose and oxygen but is only 2% of body weight
rich capillary network needed for constant supply of nutrients
needs a lot of energy to power all the Na/K ATPase pumps and ion channels
BBB statistics:
capillary total length - 600 km
surface area - 20 m2
capillary volume - 17 ml of blood
intercapillary distance - 40 um
capillary lumen diameter - 7 um
neuron length - 10 um
How is the barrier formed?
3 parts to BBB:
physical barrier
efflux transporter barrier
metabolic barrier
describe the physical barrier
physical barrier - most imp
- specialised tight junctions b/w capillary endothelial cells
- TJs form when astrocytes are present
- astrocyte end feet secrete factors which induce TJ formation
- normal capillary - no TJ + fenestrations and pores - molecules can pass eaily
- brain capillary - TJs + no pores
TJ structure:
- TJs contain transmembrane proteins - occludins, claudins and junctional adhesion molecules which interact with actin cytoskeleton
- interaction of transmembrane proteins and actin stabilises TJ structure + maintains integrity
TJ function:
- barrier - physical barrier controls movement of ions
- selective permeability - only lipophilic log P > -1 and small molecules < 400 Da
- protection - keep harmful toxins out
- cell to cell communication
compare capillary tightness of brain capillary and general capillary
capillary tightness measured in ohms
- electrical resistance to Na for example
- if Na ions flowing - low resistance
- if no Na ions flowing - high resistance
normal capillary - 40 ohms cm2
brain capillary - 8000 ohms cm2
what molecules can get through BBB and whats the clinical significance
small + lipophilic molecules and gases
- oxygen
- alcohol
- caffeine
- nicotine
- barbiturates
- opiates
- anaesthetic
- 98% of small molecules cannot cross BBB - severely limits drug treatment for CNS diseases
describe the efflux transporter barrier
efflux transporters use ATP to keep lipid soluble molecules out that have started to enter endothelial cells
3 transporters:
PGP - P-glycoprotein
BCRP - breast cancer resistance protein
MRP - multi-drug resistance protein
clinical significance
- keep harmful toxins out but also keeps therapeutic drugs out
- leads to CNS drug resistance - brain becomes resistant to drugs
e.g. chemotherapeutics, antibiotics, antivirals, vitamins
describe the metabolic barrier
if drug gets through efflux transporters into endothelial cells - it will get metabolised and removed
if it gets through endothelial cells metabolised by p450 enzymes from astrocytes
- drugs can be metabolised in endothelial cells and astrocytes
phase 1 - p450, glutathione S-transferase
phase 2 - methyltransferase, COMT, HNMT
- metabolise drugs before they get to neurones
endothelia - CYP1B1, CYP2U1
astrocyte end foot - CYP2J2, CYP2U1
CYP2J2 mutation linked to dementia
