This document summarizes lipid rafts, including what they are, their functions, and their roles in neurological diseases. Lipid rafts are cholesterol-enriched membrane microdomains that compartmentalize cellular processes. They influence protein sorting, signaling, trafficking, and more. Dysfunctions in lipid rafts and their associated proteins have been linked to neurological disorders like Alzheimer's, Huntington's, Parkinson's, and ALS by disrupting processes like amyloid protein processing, vesicle trafficking, and receptor signaling.

1. LIPID RAFTS IN PHYSIOLOGY
AND DISEASES
Presented by-
Moitrayee Majumder
M. PhilNeuroscience 1st year

2. i. What are lipid rafts?
ii. Functionality of Rafts
iii. Lipid rafts in neurological
diseases.

3. BIOLOGICAL MEMBRANES
Cellular lipidomes may contain upto 7000 distinct lipid
species.
Scientific American, Bretscher 1985

4. Structural complexity of membrane Phospholipids
Glycerol
Phosphate
Head
Groups

5. Cholesterol depletion studies using Methyl beta Cyclodextrin

6. Cellular Lipidomics
 Cellular architecture
 Lipid Signaling
 Regulating membrane proteins
 Membrane trafficking
 Creating sub-specific compartments in
membranes

7. MEMBRANE RAFT Models
Simons K, Ikonen E 1997

8. Mechanism of lipid shell formation
Richard G. W.
Anderson, and Ken
Jacobson Science
2002

9. Courtesy: Angue Maguy, 2005
Mosaic Microdomain model
F. Maxfield 2002

10. Features of the microdomains
 Cholesterol and spingolipid enriched domains
 Detergent resistant membranes (DRM)
 Cholesterol-sensitive functional membrane complexes
 Include or exclude proteins to a variable extent.
• Proteins with raft affinity include
glycosylphosphatidylinositol (GPI anchored proteins), doubly
acylated proteins such as Src-family kinases or the α-
subunits of heterotrimeric G proteins, Cholesterol-linked
and palmitoylated proteins such as Hedgehog and
transmembrane proteins particularly Palmitoylated ones.

11. Lipid Raft Hypothesis 1
Epithelial cells have distinct lipid and
proteins composition at their apical and
basolateral surfaces.
Glycerospingolipids and Spingomyelin
are concentrated at the apical surface.
Simons and Ikonen , 1997

12. Lipid Raft Hypothesis 1
BASAL
Viruses budding from apical and basolateral surfaces have distinct spingolipid
composition
Haemagluttinin
viruses
VSV like
Rabies
Simons and Van Meyer, 1988

13. Lipid rafts are hypothesized to be involved in
diverse biological phenomena
 Nervous System
 Immune function
 Nutrient uptake
 Cell Cycle
 Virus budding and entry
 Pathogen biology
 Cell motility

14. GPI-anchored proteins
 GPI anchored proteins consequetively present in
rafts showed apical targetting in Epithelial cells.
 Associated with DRM.

15. Signal Transduction by GPI-anchored proteins
 GPI-anchored proteins are cleaved by Phospholipase C, Phospholipase D,
Notum (in Wnt signaling) cleaves extracellularly. Cleavage of GPI anchors helps in
activation or inactivation of various signalling pathways for growth and
differentiation of cells.
 Antibody cross-linking of GPI-anchored proteins has been known to evoke
signalling responses, including rises in intracellular Ca2+ or tyrosine
phosphorylation.
 The variable surface glycoproteins from the sleeping sickness
protozoan Trypanosoma brucei are attached to the plasma membrane via a GPI
anchor.
( Taylor and Hooper)

16. Models for Signal Initiation in Rafts
Proteins crosslink within
a raft
Altering Partition
dynamics of the proteins

17. Several rafts in the membrane differ in protein composition. Clustering
coalesce rafts containing mixture of proteins viz crosslinkers and enzymes.
Clustering can occur extracellularly or cytosolic as primary co-stimulatory
response.
Simons and Toomre, NR 2000

18. Photonic Force Microscopy
Comparision of Viscous Drag of:
 Raft Vs Non Raft proteins
 Transmembrane and GPI anchored proteins
 Untreated Cells Vs Cholesterol depleted cells
Simons and Horber, J Cell Biol. 2000

19. Local Viscous Drag Measurements
Nanodomain assembly state and Raft Proteins diffuse as 25-
10nm radius

20. Caveolae
 First identified on the basis of
morphology. Palade and Yamada,
1950.
 Caveolins polymerization in rafts.
Have Palmitoylated hairpin like
structure and binds tightly with
Cholesterol.
 Implicated in endocytosis and
Transcytosis of albumin and other
proteins across endothelial monolayer.
 In developing myocytes, ribbons of
many caveole organize into T-tubules
required for calcium regulation of
muscle contraction.
Andrew F.G. Quest Biochem. Cell Biol. 2004

21. Rafts in physiological Signal transduction
Immunological synapse
building up by adaptors and
scaffolding proteins
Lck activation leads to
raft clustering.

22. Rafts in physiological Signal transduction Contd…
GDNF (Glial derived Neurotrophic Factor) Signalling:
GDNF binds to GPI-linked
GDNF receptors α (GFRα)
and transmembrane
tyrosine kinase RET.
Extracellular GDNF
binding with GFRα
triggers RET movement
inside rafts.

23. Rafts in physiological Signal transduction Contd…
Hedgehog Signalling:
Hh is post-translationally
modified to introduce a
Cholesterol moiety at C terminus
and Palmitate moiety at N
terminus.
Cholesterol-modified hh is
membrane bound and has been
shown to associate with lipid
rafts in Drosophila embryos.
Cholesterol modification
restricts signalling range of hh,
making it short range morphogen.
Dispached, is a sterol sensing
protein required for release of
hh.
Simons and Toomre, NR 2000

24. Glycospingolipids in Nervous Tissue
Glycospingolipids
Courtesy: Google images

25. Function of GSL in Nervous Tissue
 Their association with lipid rafts is uncertain.
 The oligosaccharide groups on gangliosides extend well
beyond the surfaces of the cell membranes, and act as
distinguishing surface markers that can serve as specific
determinants in cellular recognition and cell-to-cell
communication.
 These carbohydrate head groups also act as specific
receptors for certain pituitary hormones and certain bacterial
protein toxins such as cholera toxin.
Lehninger Principles of Biochemistry, 4th
edition

26. Lipid Raft associated Neurological
disorders

27. Raft lipid alteration in Alzheimer’s Disease
 ApoE4 gene associated
disrupted lipid distribution.
 Abnormal lipid composition
association with APP proteins in
rafts disrupt normal APP
dependent signaling.
 Promotes cleavage of APP by
secretases in lipid rafts.
 formation of insoluble amyloid
fibrils and the release of toxic
soluble Aβ aggregates.
 Aβ aggregates interact with
lipid raft-associated PrP to
exert their detrimental effect
on neurons.Sonnino et al., 2014

28. Abnormal synaptic traffiking in Huntington’s Disease
 Huntington’s disease (HD), characterized by neurodegeneration of the striatum
and less so of the cerebral cortex.
 Mutant htt associated with rafts accompanied by a significant increase in
raft-associated glycogen synthase kinase 3- which coincides with apoptotic stress.
(Valencia et al, J. Neurosci. 2009)
Gene array analyses indicated that mutant htt inhibited expression of several genes
encoding enzymes needed for cholesterol synthesis as well as expression of
genes involved in vesicle trafficking and synaptic vesicle formation. (S. Sipione, Hum.
Mol. Genet.2002)
 Decrease in Ganglioside content which is implicated in Ca++ transport and
signaling is decreased in HD . Desplats (2007)

29. Parkinson’s Disease
 Parkin and α-synuclein have also been shown to associate
with lipid raft.
 Parkin, found in lipid rafts on the synaptic plasma membrane
and on post-synaptic densities where it has been implicated in
N-methyl-D-aspartate trafficking.
 Targetting ganglioside GM1 induced α-helical structure of
synuclein and prevented formation of α-synuclin fibrils.
Z. Martinez, Biochemistry (2007)

30. Amyotrophic Lateral Sclerosis
 ALS is a neurodegenerative disease characterized by the
progressive loss of function of motor neurons in the brain and spinal
cord resulting in paralysis of voluntary muscles.
 Embryonic muscle cells from rat pups induced with ALS showed
overexpression of BDNF activating TrkB signaling at the level of
excitotoxicity. TrkB which is coupled with GPCR and Src-family
kinases are experimentally observed to be associated with rafts.
 Disruption of rafts with MβCD resulted in neuroprotection in
these cells and the non-raft TrkB signaling compensated for the
loss.
(J. Mojsilovic-Petrovic, J. Neurosci. 2006)

31. Thank you…