Proteotoxicity and ageing 02/04

Question 1

What do ALE and AGE stand for?

Answer 1

Advanced lipoxidation and glycosylation end products.

Question 2

Define proteostasis and it’s five components.

Answer 2

It is the maintenance of proteome homeostasis, which occurs through:

1. Translation
2. Post-translational processing
3. Folding
4. Assembly and disassembly into macro-molecular complexes
5. Stability and clearance.

The main protein systems involved are

• Molecular chaperones preventing the misfolding and aggregation: Heat shock proteins (HSP)
• The ubiquitin/proteasome system, autophagy and lysosomal sequestering

Question 3

Define proteotoxicity

Answer 3

The adverse effects of damaged or misfolded proteins

Question 4

Why are altered proteins deleterious?

Answer 4

1. They interact with normal proteins to form cross-links via reactive protein carbonyl groups. (C=O)
2. They are prone to aggregation
3. They cannot be broken down
4. They can surpass the systems capacity to remove them
5. Cross-links can inhibit proteosome activity, causing further cross-link protein formation

Question 5

What are some of the pathological effects of AGEs?

Answer 5

Advanced glycosylation end products form cross-links with proteins such as collagen in arteries - Increased arterial stiffness

Increased vascular permeability.

Oxidizing LDL - atherosclerosis

Enhanced oxidative stress.

Increased amyloidogenic processing via RAGE/NFkB/BACE-1 pathway

Question 6

The main 3 deleterious protein modifications?

Answer 6

– Protein carbonylation
– Lipid peroxydation
– Protein glycation

Question 7

3 examples of age related alterations in proteins?

Answer 7

• lipofuscin (skin pigment): age spots on skin, organs
• α-crystallin: cataracts (crystallin aggregates in eye lens)
• cartilage protein, collagen: As a result of AGEs, protein strands randomly cross-linked making them stiff and irregular. Occuring in hip joints, arteries

Question 8

Disease related altered proteins?

Answer 8

Protein aggregates in age-related neuropathies: α-synuclein (Lewy bodies in PD), amyloid plaques and tau tangles in AD, Prions in Creuztfeld-Jacob disease…

Question 9

Four causes of formation of altered proteins?

Answer 9

• Mutation and biosynthetic errors
• Post-synthetic damage by reactive oxygen and nitrogen species, reactive aldehydes and glycating agents
• Protein mis-folding
• Incomplete proteolysis

Question 10

How are altered proteins related to ageing?

Answer 10

1. Proteosome and autophagic system can be damaged by ROS and glycation agents.
2. Damage can also affect the chaperone proteins which
   normally assist in correct protein folding.

• through advancing age, proteins tend to start accumulating. Another random damage theory of ageing

Question 11

What are common diabetic complications?

Answer 11

Diabetic complications
resemble premature ageing:
Cataract
Vascular disease
Retinopathy
Neuropathy
Skin changes
Kidney failure

These also occur in normal ageing, but occur earlier in diabetes

Question 12

What are the effects of Advanced lipoxidation and glycosylation end products.

Answer 12

Like ROS and RNS they involve a change of charge and a loss of function. However they also form cross links and induce ROS

Question 13

What are protein carbonyls and how are they introduced?

Answer 13

Protein carbonylation is an irreversible, non-enzymatic
modification of proteins.

It begins with ROS species and can be induced directly:

• By direct oxidation of side chains on proteins, catlaysed by metals
• Oxidative cleavage of protein backbone

or Indirectly by conjugation of reactive species:

• Lipid peroxidation creating reactive aldehydes
• Reaction of reducing sugars creating reactive carbonyls

Question 14

Exogenous sources of AGEs?

Answer 14

Cooked food (maillard reaction)
Duck skin 
Cake 
Donuts 
Cereal 

Drinks: 
Orange juice
Tea
Coffee 
Coke 

It is not clear how they enter the body, but there is lots of evidence. It is also shown that RAGE is expressed in the gut

Question 15

What are the effects of AGEs and calorie restriction?

Answer 15

Calorie restriction increases survival rate significantly, with effects increasing with older age. However, CR combined with AGEs reduces survival rate.

human with high level of plasma AGEs show reduced longevity compared to low level of plasma AGEs

Question 16

How are AGEs and ALEs formed?

Answer 16

ALEs: a class of covalent adducts which are generated by the nonenzymatic reaction of reactive carbonyl compounds (RCCs), produced by lipid peroxidation and lipid metabolism, with the free amino groups of cellular and tissue proteins.

AGEs: a class of covalently modified proteins (adducts) generated through a nonenzymatic reaction between reducing sugars and free amino groups of cellular and tissue proteins.

Question 17

How is glucose metabolism linked to AGE formation?

Answer 17

Highly reactive glycolytic intermediates form methylglyoxal, an RCC.

Question 18

How is methylglyoxal metabolised?

Answer 18

glyoxalase

Over-expression of glyoxalase in C. elegans:
reduces MG-mediated protein modification
reduces mito dysfunction
reduces ROS production
increase lifespan

Activity decreases with age :-(

Question 19

What are the two main consequences of AGE accumulation?

Answer 19

1. Proteins aggregate, become dysfunctional,
   damage cells, overwhelm/inhibit proteolytic apparatus.
2. AGEs may activate intracellular signals through several receptor (MSR and AGE receptors, RAGE receptors) - and non-receptor-mediated mechanisms, leading to increased ROS and Inflammatory cytokines .

Question 20

What do macrophage scavenge receptor (MSR) and AGE receptors do?

Answer 20

MSR = AGE endocytosis and degradation, which leads to AGE detoxification.  
RAGE = cytokine increase, inflammation

Question 21

What are the downstream effects of RAGE activation?

Answer 21

NFKB pathway activation :

• inflammation
• ROS production
• BACE-1 activation

Question 22

Can AGES be modulated?

Answer 22

• calorie restriction reduced AGE formation
• Deoxyglucose (non-metabolisable): decreasing glucose levels (Wan 2016)
• Carnosine (β-alanyl-L-histidine): readily glycated by
  RCCs rather than proteins being glycated.
• AGE cross link breaker

Question 23

What is unusual about the naked mole rat?

Answer 23

Similar size to mouse, it has much higher lipid peroxidation, protein carbonylation and DNA damage even though it lives about 30 yrs (mouse 3 yrs). However, the levels of this remain constant instead of increasing throughout lifetime like the mice.
Proteosome activity seems to increase with age.