Bioc L11 Brain

Question 1

Define the Blood-Brain Barrier (BBB) and summarize the 7 components.

What can make it through the barrier w/out a selctive transporter?

Answer 1

The BBB restricts movement of molecules from blood to the extracellular fluid within the central nervous system

1. Tight Junctions between endothelial cells prevents paracellular transport.

2-6. Selective Transport: channels/carriers on the luminal (blood) and abluminal (brain) sides of endothelial cells selectively filter ion/molecules during transcellular transport.

7. Efflux (from brain) is also limited but luminal endothelial membrane usually more permeable/less selective (Except for glucose)

Without a selective transporter molecules must be small, fat soluble and or gaseous to pass across BBB (alcohol, O₂, CO₂, etc)

Question 2

What are the three barriers in the BBB?

Answer 2

1st Barrier: Endothelial cells in capillaries, (tight junctions, selective transporters)

2nd Barrier: Basement membrane

3rd Barrier: Astroglial (astrocyte) endfeet, covers of 90% of abluminal (brain side) endothelial surface. Take up and transfer nutrients, modulate neuronal signaling.

Question 3

Why is the barrier incomplete in some areas?

Answer 3

To allow sensing of toxins, fuel molecules, etc, to mediate the brain’s contribution to the relevant physiology or behavior.

Example: vomiting in response to drinking poison.

Question 4

Describe the BBB selective transport of monosaccharides, fatty acids, amino acids and ketone bodies.

Answer 4

Monosaccharides: mosty glucose by GLUT1 and GLUT3 (passive-mediated transport; transport down gradient only). brain must always have glucose.

Fatty Acids: limited transport, mainly the essential w-3 and w-6 fatty acids. FAs are synthed in brain but mostly for structure, not energy.

Amino Acids: selective transport system, some aa’s heavily restricted

Ketone Bodies: use monocarboxylate transporters (MCTs)

Question 5

What’s the distribution of GLUTs in the BBB?

Answer 5

GLUT1 (45K): mostly on astrocytes.

GLUT1 (55K): on endothelial cells, mostly abluminal side; ensures glucose flux into brain

GLUT3: mostly on Neurons, lower K_m helps maintain glucose flux.

Glucose never leaves the brain! Higher GLUT concentrations at each membrane on way to neurons helps maintain flux into brain.

Question 6

Describe BBB Fatty Acid Transport

Answer 6

Two FA uptake mechanisms:
1. flip-flop mechanism for short/medium chain fatty acids 2. Facilitated Transport via Specific Transporters.

w-3 and w-6 are critical for brain development and function, must come from diet (crosses bloodbrain barrier).

Unsaturated FA’s cross BBB more easily than Saturated FA’s

Question 7

Describe Amino Acid transport

Answer 7

Essential amino acids (11 in brain) are transported by NA⁺-independent facilitated transport (e.g., L-system) with a 2:1 luminal:abluminal ratio of # of transporters (opposite of GLUTs where more are abluminal)

Non-essential amino acids is much more limited.

Na⁺-dependent transports amino acids out of brain
against steep gradient into endothelial cells, followed by facilitated diffusion into blood.

Question 8

Describe Glutamate/Glutamine homeostasis and why it’s important.

Answer 8

EAAT Family“most powerful” of Na⁺ symporters transport glutamate from brain into endothelial cells which transport glutamate to blood via facilitated diffusion.

Glutamine concentration in brain is similar to its concentration in blood (slowing facilitated transport to blood) so endothelial cells convert glutamine to glutamate (glutaminase enzyme) which can enter the blood more easily.

It’s important to clear Glutamate out of brain because it’s a primary neurotransmitter.

EAT family of transporters also transports glutamate into neurons (fewer transporters) and astrocytes which convert glutamate to glutamine via glutamine synthetase.

Question 9

How can hyperammonemia disrupt glutamate/glutamine homeostasis?

Answer 9

NH₃ easily crosses BBB into brain. The NH₃ is converted to NH₄ and 99% of that gets used by Glutamine Synthetase (GS) to convert glutamate to glutamine. Increased GS activity increases astroglial glutamine levels which can lead to astroglial swelling (brain edema).

Also, increased NH₄ concentrations blocks K⁺ buffering which can cause seizures.

Question 10

What are Monocarboxylate Transporters (MCTs) used for?

Answer 10

MCT’s are a family of H⁺-coupled symporters that trasport molecules with COO^- group including:

1. Ketone Bodies (MCT1 is rate limiting enzyme of ketone catabolism)
2. Pyruvate
3. Lactate
4. deaminated BCAAs

Question 11

What is the hypothesis for the Astrocyte-Neuron energy relationship?

Answer 11

** Astrocyte-Neuron Lactate Shuttle: When astrocytes increase glycolysis, they produce lactate which is shuttled to Neurons for energy (astrocytes feed neurons)**.

There is a controversial hypothesis that neurons prefer lactate over glucose.

Question 12

How does the brain produce most of its ATP and what is most of that ATP used for?

What is enriched near synapses?

Answer 12

Nervouse tissue mainly uses glucose (glycolysis, TCA cycle, ETC) to produce ATP. The majority of ATP is used to reset Na and K concentrations across synaptic membranes by Na/ATPase.

Mitochondria and MTCs are increased around synapses to supply local energy.

note: MTC fuels and other sugars can provide energy but Glucose is still absolute requirement and it isn’t known why.

Question 13

How does the brain use ketone bodies?

How do ketones amplify the glucose-sparing affect?

Answer 13

Ketone bodies enter the TCA cycle as acetyl CoA. Ketone use depends on the concentration of ketones in blood (MTC1 is rate limiting), if ketones are present the brain will use them.

Ketones can amplify the glucose-sparing effect by inhibiting glycolysis (e.g. from increasing cysosolic citrate).

Question 14

Where in the brain is glycogen stored and how is it used?

Answer 14

Astrocytes have a small amount of glycogen that is used for local needs (independent of insulin/glucagon) similar to muscle glycogen.

Question 15

What is the Tripartite Synapse?

What is the Glu-Gln cycle and what are the steps?

Answer 15

Tripartite refers to the presynaptic and postsynaptic neurons and the surrounding astrocytes. The astrocytes help “cleanup” glutamate from synapse.

The Glu-Gln cycle refers to the movement and metabolism of glutamate between neruons and astrocytes. Steps:

1. Glutamte released to synaptic cleft
2. Glutamate binds to postsynaptic receptors, signal is terminated by astrocyte EAAT Glu uptake or taken up by presynaptic neuron (also EAAT but less uptake than astrocytes) and recycled.
3. Astrocytes convert Glu to Gln using GS (unique to astrocytes)
4. Neutralized Gln is transported out of astrocyte
5. Neurons convert Gln back to Glu using Glutaminase (also found in vascular endothelial cells)

Question 16

Why is the Glu-Gln cycle costly?

How is astrocytic glycolysis affected?

Answer 16

The astrocyte EAAT is Na-coupled symporter and increases intracellular Na. This activates the Na/K-ATPase to restore ionic balance (uses ATP). Conversion of Glu to Gln in astrocytes requires ATP.

Reduced ATP and increased AMP stimulates PFK-1, increasing the rate of astrocytic glycolysis and thus glucose uptake.

Question 17

What are two in vivo brain imaging techniques and what do they measure?

Answer 17

Positron Emission Tomography (PET) uses radiolabeled tracers to measure glucose and O₂ metabolism.

Functional Magnetic Resonance Imaging (fMRI) uses blood oxygenation and flow to measure metabolism.

Both techniques measure metabolism as a representation of neuronal signaling.

Question 18

What are some examples of clinical applications for brain imaging?

Answer 18

FDG PET used as early diagnosis for Alzheimer’s.

fMRI can map the location of language functions before neurosurgery (to prevent speech impairement)

FDG PET is also used to detect and monitor treatment of cancers in the brain. The disrupted metabolism of tumors is referred to as the “Warburg effect”.