Zlokovic 2008 BBB

Question 1

What molecules can cross the BBB without assistance?

Answer 1

Lipid-soluble molecules via diffusion

Question 2

What comprises the BBB?

Answer 2

Endothelial cells with tight and adherens junctions

Question 3

Briefly describe the mechanism of how tight junctions are maintained.

Answer 3

Maintained by claudins and occludins
They are transmembrane proteins that bind with each other to restrict paracellular flow of molecules

Question 4

How do MMPs relate to the BBB?

Answer 4

It can degrade claudins and occludins and thus open the BBB
These TJ proteins accumulate in astrocytes and neurons, possibly due to autophagy after BBB breakdown

Question 5

How is occludin related to diseases like AD and MS?

Answer 5

In AD, there is an accumulation of occludin in non-ECs.
In MS, occludin dephosphorylation as well as ZO-1 loss precedes BBB opening

Question 6

What junctional proteins could be involved in hypertension?

Answer 6

Increased JAM-A can promote hypertension and seen in rodents

Question 7

What are cytoskeletal proteins involved in maintaining BBB?

Answer 7

ZO-1, Caveolin-1, and Actin

Question 8

How does ZO-1 stabilize the BBB?

Answer 8

Anchors TJ proteins to cytoskeletons

Question 9

How is Caveoline-1 involved in BBB maintenance?

Answer 9

Regulates expression of tight and adherens junction proteins like ZO-1 and occludin and VE-cadherin

Question 10

How is actin involved in BBB maintenance?

Answer 10

Proper actin filament arrangement is crucial for correct localization of junctional proteins

Question 11

What are the functional difference between tight and adherens junctions?

Answer 11

Tight = Restricts paracellular flow of macromolecules
Adherens = cell-cell adhesion

Question 12

Briefly describe the molecular mechanism of how adherens junctions are maintained.

Answer 12

By VE-cadherin and PECAM-1
VE-cadherin is linked to the cytoskeleton via catenins

Question 13

How may VE-cadherin be involved in angiogenesis and vascular repair?

Answer 13

Forms complex with VEGFR, which is involved in angiogenesis and repair

Question 14

What is the main role of PECAM-1 and how does it relate to diseases?

Answer 14

Main role is for leukocyte migration across BBB
Promotes leukocyte migration into the brain in EAE (MS model) and also in Abeta mediated migration in AD

Question 15

What are 5 different mechanisms that mediate BBB transport?

Answer 15

• Carrier-mediate transport
• ion transport
• active efflux transport
• receptor-mediated transport
• caveolae-mediated transport

Question 16

What is carrier-mediated transport and what is responsible for?

Answer 16

Responsible for transporting nutrients to the brain like glucose and vitamins
Different from receptor-mediated transport (for proteins)

Question 17

How are glucose, ketones, amino acids and vitamins transported into the brain?

Answer 17

Carrier-mediated transport
Glut1, MCT1, L1 and y+ AA transporter, sodium-dep multivitamin transporter, respectively

Question 18

How does the brain try to compensate for hypoxia?

Answer 18

Through HIF-1 (Hypoxia inducible factor), increases Glut1 transcription

Question 19

State how Glut-1 relates to AD

Answer 19

Glut-1 protein levels are decreased at the BBB in AD and precedes the conversion to AD
This is not due to brain atrophy

Question 20

What does MCT-1 do?

Answer 20

Transports ketone and lactate for metabolic use in the brain

Question 21

What are the types of amino acid transporter and how do they differ?

Answer 21

L1 and y+ transporters
L1 is for neutral essential AAs
y+ for cationic essential and nonessential (eg. L-arginine)

Question 22

What is endothelial EAATs potentially for?

Answer 22

It may provide net removal of glutamate which prevents excitotoxicity, implicated in MS, ALS, AD

Question 23

What ion transporters are important and why?

Answer 23

Na+/K+-ATPase uses ATP to maintain high Na+ gradient for Na+-dependent transport

Also sodium-hydrogen exchanger to maintain pH

Question 24

Provide an example of active efflux.

Answer 24

ABC transporter such as P-gp is important for the removal of toxins and drugs
P-gp also regulates LRP1 expression and thus Abeta clearance

Question 25

Provide examples of receptor-mediated transport.
What is its implication for drugs?

Answer 25

LDL, insulin, transferrin
Drugs can be conjugated to antibodies against receptors for these to allow brain entry

Question 26

How is caveolae involved in transport? What else?

Answer 26

It underlies endocytosis and transcytosis by expressing receptors for things like insulin, LDL, transferrin, albumin etc.
It also involves eNOS and calcium influx proteins

Question 27

How are pericyte-endothelial contacts maintained?

Answer 27

Processes that encircle endothelial cells
Contacts through TJ, AJ, and Gap junctions too

Question 28

What are some of the roles of pericytes on ECs?

Answer 28

Provide physical stability by coverage
Secrete signals that promote quiescence

Question 29

What are examples of pericyte-EC signaling and the consequences of disruption?

Answer 29

PDGFRbeta(pericyte) and PDGFbeta (EC)
TGFbeta and receptors
These promote stabilization of BBB -> disrupt then get microhemorrhages

Question 30

How do pericytes influence angiogenesis?

Answer 30

During development, pericyte migration guides ECs for angiogenesis and later for BBB integrity

Question 31

What changes are seen in microglial through activation?

Answer 31

At resting state they have thin long processes
When activated becomes ameboid and then to phagocytic form

Question 32

What are some examples of microvascular pathologies in AD? (3)

Answer 32

String vessels, basement membrane thickening, changes in vessel diameter

Question 33

What BBB changes are seen in AD and how does it relate to Abeta pathology?

Answer 33

Increased RAGE expression increases Abeta influx, and thus proinflammatory cytokines (TNFalpha, IL6) and ICAM1, generation of ET-1

Blood is also a major source of Abeta

Question 34

Describe one way we clear Abeta from the brain and body. How is it altered in AD?

Answer 34

LRP1 binds Abeta to clear it to blood and afterwards at the liver for systemic clearance
Some reports on reduced vascular LRP1 in aging and AD brain
Abeta can promote degradation of LRP1

Neuronal LRP1 contributes to Abeta retention though

Question 35

How may hypercontraction occur in AD?

Answer 35

Via Abeta-mediated contraction due to ET-1 for example.
Alternatively, expression of VSMC transcription factor like MYOCD that induce hypercontractile phenotype

Question 36

What are vascular risk factors of AD?

Answer 36

Atherosclerosis, age, stroke, hypertension, ischemia, and ApoE4

Question 37

Describe 1 pathway in which hypoxia/hypoperfusion can affect AD.

Answer 37

Hypoxia downregulates MEOX-2 in BEC -> stabilize expression of MYOCD -> hypercontractile SMCs

Question 38

Describe a model of how Abeta and BEC changes interplay with AD pathogenesis.

Answer 38

Changes in key receptors like decreased LRP1 and increased RAGE can promote accumulation of Abeta
Abeta can promote vasoconstrictor secretion (ET1) and expression of hypercontractile SMCs or promote inflammatory (TNFalpha, IL6, IL1) conditions through cytokines and ICAMs, and activated microglia. These cooperate to decrease BBB integrity and thus lead to neuronal damage and decreased activity (due to metabolic insufficiency)
Abeta can also recruit microglia and proinflammatory environment