Transcript Lipid Rafts - Chemistry Department

1
THREE DECADES OF QUESTIONS
ON LIPID RAFTS
Whitney Stutts
Outline
2

What are lipid rafts?

Why do they form?

What methods are used to study lipid rafts?

What effects do they have on eukaryotes?

Why all the controversy?
What are lipid rafts?
3

“Lipid rafts are small (10-200nm),
heterogeneous, highly dynamic, sterol- and
sphingolipid-enriched domains that
compartmentalize cellular processes.”
~ 2006 Keystone Symposium
What are lipid rafts?
4

Cholesterol and sphingolipid-enriched membrane
microdomains or platforms
Cholesterol levels double
 Sphingomyelin levels elevated by 50%




Concentrate and segregate proteins within the plane of
the bilayer
More ordered and tightly packed than surrounding
bilayer
Float freely in Lc bilayer
Video
5
Lipid Rafts
http://multimedia.mcb.harvard.edu/anim_innerlife.ht
ml

Two types of lipid rafts
6




Caveolae: small, flaskshaped invaginations of the
plasma membrane enriched
in caveolin
Planar lipid rafts: found in
neurons and enriched in
flotillin
Caleolin and flotillin recruit
signaling proteins
Signaling can be promoted
or dampened
Raft Proteins
7

“True resident proteins”
 GPI-anchored
proteins-prion protein (PrPc)
 Caveolin
 Flotillin

Signaling proteins
 G-protein,

non-receptor tyrosine kinases
Cytoskeletal/Adhesion proteins
 actin,
myosin, vinculin, cofilin, cadherin, ezrin
Why do they form?
8
Why do they form? Cholesterol
9

Cholesterol is the dynamic
“glue” that holds the raft
together




Saturation
Hydroxyl H-bonding with
amide
Up to 25% of cholesterol
is found in the
brain…CNS?
When removed, most
proteins dissociate from
rafts
Misconception
10



Rafts contain only phospholipids with fully saturated
acyl chains….FALSE!
Most glycerophospholipids in membrane rafts
contain at least one monounsaturated acyl chain
Sphygomyelin- usually saturated chains but when
unsaturated the DB is located at C15
Why do rafts form?
11




Driving force- line tension: energy required to create a
boundary between the raft and the surrounding
membrane
Raft thickness
Hydrophobic mismatch
contrast in thickness
line tension
Results in larger, more circular rafts which reduce line
tension and energetic cost of the raft interface length
What methods are used to study lipid
rafts?
12





DRM Isolation - lipid rafts are insoluble in cold non-ionic detergents
(Triton X-100)
Electon microscopy - determines location of raft components and can
detect clustering of proteins
FRET – used to determine whether two raft components are spatially
close
FRAP - probe for the association of proteins to lipid rafts and study
diffusion of proteins in lipid rafts
Manipulation of cholesterol



Sequestration
Depletion or removal
Inhibition of cholesterol synthesis
What effects do rafts have on
eukaryotes?
13

Organizing centers-assembly of signaling molecules


Signaling can be promoted or dampened
Effects membrane fluidity
Acyl chain fluidity
 Lateral diffusion


Involved in trafficking of membrane proteins

Regulation of neurotransmission and receptor trafficking

Neurotrophin receptors embedded in rafts
Hijacking Viruses
14

HIV virus
 Budding

may occur from lipid rafts
Influenza virus
 Raft-associated
glycoproteins in envelope
Disorders & Diseases
15

Mood disorders
 Therapeutic

Alzheimer’s disease
 Platforms

efficacy of antidepressants
for production of amyloid-β (neurotoxic protein)
Prion disorder
 Normal
prion protein (PrPc) is converted to abnormal
proteins (PrPsc) in lipid rafts
(GPI anchor required)
Why all the controversy?
16

Problems with biomembranes
 Lipid
rafts are too small to be resolved by light
microscopy
 Difficult
 Not
to study lipid rafts in intact cells
in thermodynamic equilibrium
Why all the controversy?
17

Problems with synthetic membranes
 Lower
concentration of proteins
 Difficult
 Lack
to model membrane-cytoskeletal interactions
natural lipid asymmetry
 Studied
under equilibrium conditions
More Questions
18

What are the effects of membrane protein levels?

What is the physiological function of lipid rafts?



What effect does flux of membrane lipids have on raft
formation?
What effect do diet and drugs have on lipid rafts?
What effect do proteins located at raft boundaries
have on lipid rafts?
Works Cited
19












Allen, John A. "Lipid raft microdomains and neurotransmitter signaling." Nature 8 (2007): 128-40.
Benarroch, Eduardo E. "Lipid rafts, protein scaffolds, and neurologic disease." Neurology 69 (2007): 1635639.
Hamasaki, Dr. Toshikazu. "Tutorial 2, Plasma Membrane." UCLA. 22 Feb. 2009.
Jacobson, Ken. "Lipid rafts: at a crossroad between cell biology and physics." Nature Cell Biology 9 (2007):
7-13.
Jacques Fantini, Nicolas Garmy, Radhia Mahfoud and Nouara Yahi (2002) Lipid rafts: structure, function
and role in HIV, Alzheimer’s and prion diseases. Exp. Rev. Mol. Med. 20 December,
http://www.expertreviews.org/02005392h.htm
Korade, Zeljka. "Lipid rafts, cholesterol, and the brain." Neuropharmacology 55 (2008): 1265-273.
Luckey, Mary. Membrane Structural Biology : With Biochemical and Biophysical Foundations. New York:
Cambridge UP, 2008.
Pike, Linda J. "The Challenge of Lipid Rafts." Journal of Lipid Research Oct (2008): 1-17.
Simons, Kai, and Ehehalt, R. "Cholesterol, lipid rafts, and disease." The Journal of Clinical Invesigation 110
(2002): 597-603.
Simons, Kai. "Lipid Rafts and Signal Transduction." Nature Reviews 1 (2000): 31-41.
Simons, Kai. "Model Systems, Lipid Rafts, and Cell Membranes." Annu. Rev. Biophys. Biomol. Struct. 33
(2004): 269-95.
Video: Viel, A., Lue R.A., “Inner life of the cell.” The president and Fellows of Harvard College (2007)
http://multimedia.mcb.harvard.edu/anim_innerlife.html