BBB 1

How suggested a mechanical membrane that separates the brain and blood?

Lewandowsky and Goldman

Who coined blood-brain-barrier

Lewandowsky

What can’t cross the BBB

>95% small molecules

100% biologics

How much of the cardiovascular output does the brain receive, capillary length and surface.

20%

400 miles, 20m²

Distance between capillaries

40um

Perfusion and diffusion

Almost every neurons is perfused by it’s own blood vessel

Diffusion plays a minimal role in the distribution of nutrients, water, solutes in brain

What is the BBB

Physical and biochemical barrier formed at the level of brain micro vessels

What type of cells form the BBB

Endothelial cells, pericytes, astrocytes

What cells mediate the main function

Endothelial

Other to are for development and maintenance of these properties

Neurons that contribute

Perivascular inter-neurons and microglia

What areas lack a BBB

• Area postrema

• Circumventricular organs

• Hypothalamus

What properties do regions without a BBB have

• Fenestrated capillaries 

• Lack pericytes and astrocyte enhancement

What can pass through regions without a BBB

Hydrophilic molecules, proteins, peptides

Purpose of areas without a BBB

Direct adaptive response

eg. area postrema has chemoreceptors for vomiting response to toxins

What are the three formation patterns of endothelial cells

• Continuous (basement membrane)

• Fenestrated

• Sinusoid (incomplete basement membrane)

Endothelial cells of BBB

• Continuous

• Endothelial layers= tunica intima

• Tight intercellular cleft

Three types of transport through cells

• Phagocytosis

• Pinocytosis (minimal in BBB)

• Receptor-mediated endocytosis (option for brain)

Two biochemical bariers

• Efflux system (p-glycoprotein)

• Enzymatic systems (cat.p450, peptidase, nucleotidases)

Three physical barriers

• No fenestrations

• Transcellular Pinocytosis is minimal 

• Tight junctions seal paracellular pathway

Tight-junctions

• Adhesion complexes

• Induce polarity=compartementalization

Three structural transmembrane proteins

• Occludin- TJ regulation

• Claudins- Tightness

• Junctional adhesion molecules- stability and maintenance

Cytoplasmic accessory proteins

• Zonula occludens 1&2

• Cingulin

• Anchor transmembrane proteins to cytoskeleton- tightness

Transendothelial electrical resistance (TEER)

• Tight junctions limit water and ion= increased TEER

• Measures resistance to an electrical current

• Measure of ionic permeability

TEER in peripheral vs brain micro vessels

2-20 ohm-cm²

>1000 ohm.cm²

What species is the BBB in

All vertebrates

When are variations between species formed

Postnatal maturation

BBB development

Gradually starts in first trimester

Completed by 6 months of age (high permeability as neonates)

Cells and proteins in development

• Early= neural stem cells producing Wnt molecules

• Want stimulate endothelial cells to express BBB-related

• Angiogenic program : Wet/B-catenin drives vessel formation in the CNS

Astrocytic end feet

• Completely surround microvessels

• Transport of nutrients

• Regulation of BBB properties

• Regulation of local blood flow

What coupling do astrocytes contribute to

Neurovascular

Synchronizes metabolic demands to local cerebral blood flow regulation

Products of astrocytes

Cytokines, grow factors

Tighter TJ and expression of transporters (P-GP, GLUT-1)= maintenance of mature properties

Astrocytes in in vitro systems

Promote formation of mature BBB

Brain compartment/glial cells show GFAP

GFAP (glial fibrillary acidic protein- astrocyte marker)

Bovine aortic endothelial cells vs bovine brain capillary cells

Increased TEER from BCEC, most from BCEC+astrocytes

Pericytes

Juxtaposed to the endothelial

With the latter form a matrix of extracellular proteins )basal) that surrounds the vessels

IB4 and NG2-DsRed

Recruitment of pericytes

Endothelial cells secrete platelet derived growth factor that recruits pericytes

Leads to vascular maturation

3 functions of pericytes

• Produce factors that induce barrier functions in the endothelial cells

• Help polarize astrocyte end feet and. to limit endothelial transcytosis

• Contractile function and contribute to regulate brain capillary blood flow

3 step of improved model of BBB

• Transwell-based model of BBB with 3 cell types

• 3D cultures of endothelial cells, astrocytes, pericytes spontaneously organize into tube-like

• Dynamic microfluidic models incorporating shear forces on endothelial cells model physiological blood flow