Lecture IX Flashcards
What is a mechanism that leads to neuropathies?
unfolded protein response (UPR)
What are hereditary neuropathies?
diseases that are inherited and affect the nervous system
What is myelin?
it is a layer that wraps around the axon and insulates nerves to allow for faster conduction of nerve impulses
it is a protrusion of schwann cells
*Nodes of Ranvier are an exception
What are schwann cells?
a type of glial cell in the PNS that help separate and insulate nerve cells
How many times do schwann cells wrap around the axon?
more than 100
What are the 2 types of schwann cells?
myelinating and non-myelinating
What happens to schwann cells if there is nerve injury?
they can de-differentiate and go back to immature schwann cell types through the activation of negative regulators of myelination (c-Jun)
What is a difference in the PNS that cannot be found in the CNS?
schwann cells are in the PNS and very plasting, which means they are able to differentiate when there is an injury in the myelin or spinal cord and then can remyelinate
What happens when there are problems in the system that gives rise to myelination?
there is a hereditary kind of neuropathy (Charcot-Marie-Tooth (CMT)
What is one of the most common neurodegenerative diseases in the PNS?
Charcot-Marie Tooth
What can Charcot-Marie-Tooth be divided into?
CMT1: demyelinating
CMT2: axonal type based on electrophysiology and morphology
Describe how the nerves of the patients might look:
normal nerve: myelin surrounds axon
case of CMT1A: loss of fiber and in some axons, the onion bulb structure is visible
Dejerine-Sottas Syndrome (DSS): severe in childrena nd their nerve and muscle impulses never develop properly and they have huge nerve onion bulbs
What is the most present protein in the myelin of peripheral nerves?
myelin Protein 0 (P0)
What is myelin Protein 0?
trans-membrane protein which has an extracellular domain and folds like an immunoglobulin domain
Where are there more than 200 different mutations that cause CMT?
P0
Why does P0 form tetramers?
to allow for the packing of the myelin
How is the intracellular space of the schwann cell completely squished and held?
via P0
What happens if there is an absence of P0?
axons are surrounded by schwann cells, but myelin is not formed
What did the deletion of serin 63 (s63del) cause?
classical CMT1 demyelinating disease
What happened with the substitution of S63 with S63C?
caused DSS (more severe phenotype)
Review the 3 main pathways of UPR:
What does the retainment of proteins in the ER usually trigger first?
PERK pathway
What does it mean that the UPR is adaptive?
on one side, there is misfolded protein in the ER
eIF2๐ผ is activated to reduce protein translation
other genes are also activated to try to fold protein or degrade it
What happens in UPR if stress is chronic?
it becomes maladaptive:
through ATF4, there is activation of another transcription factor called CHOP, which leads to the activation of genes (GADD34) which are involved in apoptosis
How can it be that a protein that is only active in myelin is retained in the ER and causing demyelinating neuropathy?
GADD34 is allowing translation to proceed and the removal results in translation to be attenutated and the cells producing less protein (P0 in schwann cells)
*as a result, if there is less protein in the ER, there should be a reduction in stress because the cells produce less protein
What does stress deriving from mutant protein lead to?
eIF2๐ผ being phosphorylated and translation attenuated
What is eIF2๐ผ phosphorylation important for?
protect cells from having a disrupted phenotype
What levels are decreased in the mutant without eIF2๐ผ?
ATF4 and CHOP
*this was expected as they are downstream the phosphorylation of eIF2๐ผ
What was the lack of phosphorylation of eIF2๐ผ doing?
telling the cell to stop myelinating
What is a gene that is a strong regulator of myelination?
c-Jun
What does c-un act through?
phosphor-ERK and phosphor-ATK pathways
How is c-Jun activated?
through p-ERK that is largely increased in the mutant mouse without eIF2๐ผ
What happens when c-Jun is activated?
myelination in the nerves is blocked
What relationship is present in schwann cells?
there is a relationship between the phosphorylation of eIF2๐ผ and the differentiation mechanism through p-ERK
What happens in schwann cells?
they do not react like ฮฒ-cells
instead: p-ERK pathway is activated and increases c-Jun activation, which blocks schwann cell differentiation and myelination
What was determined to be a very potent inhibitor of GADD34 but without the defects of Guanabenz?
Sephinq, which is a technique that increases phosphorylation of eIF2๐ผ because GADD34 is inhibited
What did the treatment of the cultures with Sephin1 improve?
myelination and a reduction of the stress response genes
Can Sephin1 enter the BBB?
yes and this is important as therapies for neurodegenerative diseases need to enter the nervous system which is protected by the BBB
Where does Sephin1 accumulate?
sciatic nerve, but it is able to clear out quickly
What was seen when mice were treated withSephin1?
they were almost completely cured
SUMMARY:
after ER stress and activation of UPR, GADD34 is activated
GADD34 dephosphorylates eIF2๐ผ to restore translation
blockage of GADD34 can be done genetically or with Sephin1, which leads to an increase in eIF2๐ผ, and causes a decrease in the amount of mutant proteins made
proteostatic reset is generated, so cells myelinate better and mice are healthier
What happened when R98C mice were treated with P32-P180 from 1-6 moths?
an improvement in myelation was observed (in particular the thickness of the myelin)
*this led to nerve conduction also being improved slightly
We have ___ different mutations of P0 in the ER.
2
What happens when the 2 different P0 mutation in the Er are treated?
the mice improve
What is the most common CMT in the owrld?
CMT1A
What kind of neuropathy do 50% of CMT1 cases present?
Charcot-m\Marie-Tooth (CMT) neuropathy
In CMT neuropathy, what does the rearrangement cause?
duplication of a 1.4 Mb segment on chromosome 17 that contains the peripheral myelin protein 22 (PMP22) gene and causes duplication of 1 allele
What does the duplication of 1 allele caused by the rearrangement in CMT cause?
the production of 2 copies of the protein, which leads to the major character of the disease
OVERALL:
Describe what happens if only PMP22 is overexpressed.
neuropathy arises, so the duplication on the 1.4 Mb segment on chromosome segment is the major reason for the disease
What happens if someone only has 1 copy of PMP22?
they also have a neuropathy called Herditary neuropathy with pressure palsies (HNPP)
*usually this does not occur unless there is pressure to the nerve
What does a point mutation of PMP22 cause?
neuropathy called CMT1E from a missense mutation in PMP22
What happens in both CMT1A and CMT1E?
activation of stress response mechanism
both cases have an unfolded protein, so the mechanism is similar to what was seen in P0
What are the stress responses?
BiP
CHOP
eIF2๐ผ
What is a key protective step in the pathology of CMT?
phosphorylation of eIF2
What could be a good pharmacological treatment of patients?
using a molecule that inhibits Gadd34
What happens in the IRE1/Xbp1 pathway?
IRE1 dimerizes and leads to the splicing of Xbp1, whihc becomes a potent TF that enters the nucleus and induces the transcription of genes involved in chaperones, lipid syntthesis, and ER-associated degradation
What is the role of Xbp1 in neuropathy?
schwan cells are not affected
B cells no longer produce antibodies
What happened when Xbp1 was removed from S63del mice shwann cells?
phenotype worsened, which indicated Xbp1 is needed to protect the cells at least partially from the accumulation of the mutant protein and toxicity
What kind of myelin does the R98C model have?
thin myelin
What happens when Xbp1 is removed?
stress level is increased
What is the mechanism with which cells dislocate proteins that need to be degraded?
ERAD
Why are ERAD genes increased in the S63del mouse?
they are increased to remove the mutant protein
What happens if ERAD is blocked?
S63del will accumulate suggesting that this is the way it is degraded
What causes protein QC to decrease?
aging
What do the S63del mice that overexpress Xbp1 have?
they have a more efficient ER-associated degredation system, so they are able to degrade the mutant protein faster and allow the WT to go to myelin and start demyelination
In the mutant, the mutant protein accumulates in the ER and leads to what?
stress increasing and UPR being activated
Why is the activation od UPR a good thing for the cell?
it is the mechanism by which the cell tries to degrade the protein in the ER
protein=problem
UPR=solution
What is the problem if UPR tries to degrade the protein Xbp1, which is the protein that drives genes to remove the protein from the ER?
the accumulation of the protein in the ER will increase
stress levels will increase
phenotype will worsen
What happens if Xbp1 is increased genetically or pharmacologically?
ER-associated degredation will increase, which could lead to the reduction in the accumulation of the mutant protein in the ER
stress is reduced
phenotype is ameliorated
SUMMARY:
Xbp1 plays a protective role
IRE1/Xbp1 pathway could be a target for therapy for conformational changes
What 2 therapies could be used for patients?
Sephin1
therapy that target the IRE1/Xbp1 pathway
In the ATF6 pathway, what happens when STF6 is removed?
phenotype worsens, nerve conduction velocity decreases, and F-wave increases
What is a target of ATF6?
BiP
If ATF6 is removed, what happens to BiP?
it is no longer activated and the phenotype is worsened
What role does the activation of the UPR pathways have?
protective role in CMT1B, shown in both mouse models
What are molecules that are able to activate the UPR pathways or protein QC?
ERAD, potentially the proteasome aor additional treatment to Sephin1
TO SUMMARIZE ALL:
in a normal schwann cell, P0 goes to myelin
in mutants, the protein gets stuck in ER and UPR is activated, which is good because it will activate c-Jun, Sox2, or Id2โlimit the amount of myelin which limits the entry of the mutant protein in the ER and keeps the cell in equilibrium
*if Sox2 or Id2 are removed, cells will continue to produce P0 and stress will continue to increase and UPR will not be able to handle it and become maladaptive leading to cell death
**demyelination occurs because the cell is completely stresses
