The UPS in NDDs

Question 1

Maintaining proteostasis in neurons is especially important due to

Answer 1

– their complex architecture (ex. long axons)
– long lifespan
– inability to dilute aggregate load through cell division (no cell division therefore get stuck with issues)

Question 2

The UPS is critical for functioning of neuronal synapses:

Answer 2

– Synaptic protein turnover
– Plasticity
– Long-term memory formation

Question 3

Dysfunction of the UPS in NDDs can occur at…

Answer 3

• Impaired ubiquitination
• Loss of proteasome
• Disease-associated misfolded proteins inhibit the proteasome directly
• Secondary impairment of proteasome due to mitochondrial dysfunction
• aggravated by age-related decline of proteasomal catalytic activity

Question 4

Impaired ubiquitination due to

Answer 4

– depletion or reduced activity of E1, E2, or E3 enzymes

– Ubb+1 (decreased recognition by UPS)

Question 5

Result of loss of proteosome activity

Answer 5

results in accumulation of misfolded proteins, which can further impair catalytic activity

Question 6

Secondary impariment of protesome due to mit damage

Answer 6

Mit damage/dysfunction leads to ATP depletion and ROS production –> increase number of abnormal proteins and inhibit proteosome

Question 7

With age…

Answer 7

proteosomal catalytic activity is decreased

Question 8

Misfolded proteins lead to a ___ in proteosome function

Answer 8

decrease/inhibition

Question 9

How is cytotoxic mHTT removed

Answer 9

UNCLEAR
– soluble mHtt is not efficiently targeted by the 26S proteasome
– lack of efficient ubiquitination for proteasomal degradation leads to intracellular aggregation driven by the intrinsic disordered structure of mHtt

Question 10

The data on proteasomal impairment in HD are contradictory

Answer 10

• mHtt impairs the UPS in cultured cells

- Proteasomes are not impaired in many animal models of HD

Question 11

Impairment of UPS in HD has been attributed to:

Answer 11

– inefficient targeting of soluble mHtt for proteasomal degradation
– inhibition of 26S proteasome gate opening by soluble oligomeric mHtt species

Question 12

mHTT inclusions (IBs)in the brains of HD patients and HD mice are ______ (deficient/enriched) in UPS components

Answer 12

enriched
– mHTT species can be initially tagged with Ub but are poor substrates for the proteasome
– suggest direct (mHtt protein) or indirect (proteins associated to mHtt) sequestration of the proteasome into the IBs

Question 13

T/f: proteasome sequestration plays a major role in UPS dysfunction in HD

Answer 13

FALSE

as a large proportion of protesomes remain free (and still active)

Question 14

More recent proposal of UPS and HD

Answer 14

global proteostasis network dysfunction: diffuse mHtt might inhibit the proteasome indirectly by saturating the protein folding machinery and diverting an excess of
other proteins to the UPS that then becomes impaired

Question 15

Collapse of Proteostasis in HD

Answer 15

1) Chronic production of misfolded mHtt overwhelms chaperones
2) Chaperone exhaustion acts synergistically with aging
• decrease chaperone availability, leads to the misfolding and accumulation of other chaperone client protein
3) Misfolded proteins are diverted to the proteasome exceeding its capacity, and causing UPS impairment
3b) Mitochondria dysfunction limits ATP production and impairs energy-dependent protein clearance pathways (Autophagy)
4)Proteostasis collapse increases the levels of diffuse mHtt

Question 16

mHTT can

Answer 16

• trigger apoptosis

* promote aggregation into inclusions bodies (IBs)

Question 17

Inclusion bodies–short term vs long term role

Answer 17

Intially: IBs can extend survival by sequestering mHtt
Long term: IBs can recruit proteostasis components, in a vicious cycle that gradually disrupts cellular homeostasis that eventually culminates in necrosis

Question 18

AD is characterized by

Answer 18

– presence of Ub in senile plaques and neurofibrillary tangles
– downregulation of E1 and E2 enzymes
– changes in E3 enzymes (e.g. reduction of Parkin)
– increased oxidation of DUB
– decreased proteasomal activity

Question 19

Mutations in UPS in AD

Answer 19

A mutation in the Ub gene UBB+1 causes

neurodegeneration and is linked to AD

Question 20

Proteasome and AD

Answer 20

• Proteasomes indirectly influence Ab production (as The UPS processes and degrades presenilin-1, presenilin-2 and b-secretase)
• Proteasome activation also clears tau
• Age-dependent decrease in proteasomal activity may cause both Ab and tau to accumulate
• Once Ab and tau aggregates are formed, they decrease proteasome activity –> creates a vicious cycle that increases Ab and tau accumulation

Question 21

UBB+1

Answer 21

A frameshift form of Ub generated by
molecular misreading
- Ubb+1cannot be used for ubiquitynation
- seen in both AD and HD

Question 22

T/F: Ubb+1 is only presented in demented individuals

Answer 22

FALSE
Ubb+1 mRNA is present in equal levels in non demented control individuals compared with AD patients
BUT it accumulates in AD patients’ neurons

Question 23

Ubb+1 in AD patients

Answer 23

• Ubb+1 accumulates in neurons of all AD patients AND
colocalizes with dystrophic neurites of amyloid plaques and neurofibrillary
tangles

Question 24

Ubb+ in healthy vs aging brain

Answer 24

In Healthy brain: Ubb+1 is continuously
degraded by UPS
In Aging or presence of UPS inhibitors:
Ubb+1 degradation is affected and
protein starts to accumulate

Question 25

Aging and Ubb+

Answer 25

In aging or UPS inhibition Ubb+1 degradation is affected and Ubb+1 starts to accumulate which serves as a endogenous reporter for UPS dysfunction
–Causes Ubb+1 to shift from UPS substrate to inhibitor
– At high levels of Ubb+1 UPS fails to degrade Ubb+1 and Ubb+1 accumulation further induces functional impairment of UPS –> Proteasome inhibition causes neuronal
death

Question 26

Parkin

Answer 26

E3 ligase that mediates multiple forms of ubiquitination, including K48- and K63-linked polyubiquitination

Question 27

PD is caused by

Answer 27

Loss of parkin-catalyzed ubiquitylation is a cause of PD

Homozygous Parkin mutations: autosomal recessive juvenile- onset PD

Question 28

Early studies into parkin show that

Answer 28

– Parkin targets ER-derived proteins and polyQ proteins for proteasomal degradation
– Parkin defect in the UPS-mediated protein quality control may be responsible for PD pathology

Question 29

RECENT studies show

Answer 29

autophagy- related processes are pivotal to the pathology caused by Parkin mutations

Question 30

When parkin is normal (WT) it localizes to

Answer 30

– Lewy bodies in sporadic PD patients

– aggresomes induced by proteasomal impairment

Question 31

When parkin is mutated (ex. parkin-associated PD)

Answer 31

• devoid of Lewy bodies suggesting that parkin-mediated ubiquitination may be directly involved in the formation of Lewy bodies
• inability to form these protective inclusion bodies may underlie the rapid disease onset and progression (in familial PD with disorder parkin–i.e. juvenile onset PD)