Lecture neurodegenerative disorders 1: general mechanisms Flashcards
How are protein aggregates formed?
An unfolded native monomer that won’t fold can form β-sheets in the nucleus. These sheets are very sticky and will stick together to form protein aggregates.
Protein aggregation leads to two things. What two things?
- Loss of function
- Gain of toxic function
Answer the following questions:
- Where in the cell are proteins synthesized?
- What needs to happen after proteins have been synthesized?
- In the cytosol
- Synthesized proteins are still in their native unfolded form and need to be folded in a functional conformation. The proteins are therefore translocated to the endoplasmatic reticulum (ER).
⅓ of proteins still misfold in the ER. What solution is there to this?
Protein quality control system by chaperones (e.g. heat-shock proteins). These proteins recognize misfolded proteins and will try to fold the protein in the correct way by keeping proteins in a folding-competent state.
Some heat-shock proteins (HSPs) are ATP dependent and some are not. Name three ATP dependent Hsps and one ATP independent Hsp.
- ATP dependent → Hsp90, Hsp70, Hsp60
- ATP independent → Small Hsp
How does Hsp70 refold misfolded proteins?
Hsp70 has a closed and open conformation. In open conformation, ATP is bound and the substrate-binding domain is exposed so that Hsp70 can bind misfolded proteins. So in its active form, Hsp70 binds a misfolded protein. Next, ATP is hydrolyzed to ADP and the conformation changes to a closed one. The misfolded protein is now ‘trapped’ in the Hsp70 conformation and the protein is refolded. At last, ADP is exchanged for ATP, where the conformation opens and the refolded protein is released.
Thus, binding and release of the unfolded protein is dependent on ATP and a co-chaperone.
What happens when refolding of a misfolded protein fails?
The protein is tagged for degradation by an ubiquitin tag.
Describe the Ubiquitin-Proteasome System (UPS).
This system is meant for monomeric (misfolded) proteins, where these proteins are tagged by a polyubiquitin chain. This chain is recognized by the proteasome for degradation. The proteasome has a cilindric shape with on the inside catalytic activity. The ubiqtuitin tagged protein is moved through the proteasome and gets degraded.
So what happens when you inhibit the proteasome?
It induces accumulation of polyubiquitin-tagged proteins, as can also be seen in the picture.
Another protein-degradation system is autophagy. Describe autophagy.
Autophagy is protein degradation for protein aggregates or other large molecules, like organelles. Here, a double membrane is formed around the molecule targetted for degradation and lysosomes fuse with it, forming an autolysosome. Lysosomes have proteolytic activity that cause the molecule to be broken down.
There are 4 different types of protein conformations that can be targetted for degradation: native, misfolded monomer, oligomer or fibril.
- Describe for each if they can be degraded by proteasome, autophagy or inclusion body.
- Native conformation, can be degraded by the proteasome
- Misfolded monomer, can be degraded by the proteasome or autophagy.
- Oligomer, can be degraded by autophagy or inclusion body.
- Fibril, can be degraded by inclusion body.
Name the three causes (aetiology) of neurodegenartive diseases.
- Sporadic
- Acquired
- Genetic
Huntington’s disease
- What is the aetiology?
- What protein aggregates are causative of the disease?
- What brain region is affected?
- What is a typical symptom for Huntington’s?
- Aetiology: genetic
- Aggregates: Huntingtin protein
- Brain region: basal ganglia
- Symptom: chorea (unpredictable and/or uncontrollable movement)
How does Huntington’s disease occur?
The Huntingtin gene is responsible for the disease. The Huntingtin gene is normally composed of a CAG-triplet with a repeat expansion of 10-26 times. In Huntington’s disease, this repeat expansion is about 37-80 repeats long. This results in protein aggregation of the Huntingtin protein.
Is fibril formation and disease connected?
Yes, definitely. Only quantitatively not → there can be many deposits but little or no disease and there can be little or nog deposits but severe disease.
What are inclusion bodies?
Aggregates of specific protein types that are found in (neuronal) cells.
Are inclusion bodies toxic or protective?
Inclusion bodies are still protein aggregates that could interfere with normal cell function and thus can be toxic. In non-neuronal cells, there’s mitotic clearance of these aggregates. In this situation, inclusion bodies are protective and prevent that a whole part of the cell is lost during mitotic clearance. Only, neuronal cells are post-mitotic cells and so don’t have the ability to get rid of these inclusion bodies via mitotic clearance. So, in neuronal cells inclusion bodies are toxic, while in non-neuronal cells they are protective.
Describe the heat-shock response.
Heat-shock proteins (Hsps) are always present, but are upregulated by the heat shock factor 1 (HSF1) transcription factor when the cell is under stress (e.g. misfolded proteins).
- When a cell is under stress, (inactive) monomeric HSF1 will trimerize, which causes activation of HSF1.
- The trimer translocates to the nucleus and will bind to HSP genes which causes transcription and translation of the Hsps.
- The newly synthesized Hsps (mostly Hsp70 and -90) will try to refold the misfolded proteins. They also bind HSF1 monomer to inhibit trimerization and activation of HSF1.
The sensors of the Unfolded Protein Response all have an individual role in reducing cellular stress caused by the accumulation of unfolded proteins in the E.R. Describe for each sensor what activation leads to.
- ATF6 → binds to stress element promotors upstream of genes that are upregulasted in the UPR.
- PERK → transcription/translation of redox enzymes and proteins important for cell death.
- IRE1 → transcription/translation of chaperones, lipid synthesis and ERAD proteins (Endoplasmic-Reticulum-Associated Protein Degradation)
Reactive astrocytes are different to activated astrocytes and reactive astrogliosis occurs in many neurological diseases. What’s different in reactive astrocytes?
- GFAP is upregulated, an important protein for astrocyte development.
- Hypertrophy of cellular processes
Describe mild to moderate astrogliosis.
Astrocytes undergo hypertrophy and molecular and functional changes. Since the neuroinflammation is mild (to moderate) in this context, there’s still potential for resolution of the reactive astrocytes.
Describe severe astrogliosis.
When there’s a severe insult/damage, astrocytes undergo hypertrophy and other molecular and functional changes that persist. More astrocytes will proliferate, together with other cell types like microglia. These microglia, together with the reactive astrocytes, border along the regions of damaged tissue and create a persisting mature glial scar.
Just study this picture as it supports the latter questions.
Ok
What immune cells are typically activated in Alzheimer’s disease?
Microglia
What is microglial priming?
It’s an exaggerated microglial response, which makes microglia more sensitive to minor stimuli. There’s a change in microglial proliferation, morphology, physiology and biochemical markers (phenotype). This results in increased production of cytokine and inflammatory mediators.
What induces microglial priming?
Aging, microglia get more reactive due to priming as age increases.
