A new pathway to improve longevity?

New study reveals that “age-related changes in targeted degradation of structural and regulatory proteins across tissues determine longevity”.

Aging comes with damage to our organism. Damage accumulation can lead to cell loss. Ubiquitin – a regulatory protein found in most tissues of eukaryotes – plays a major role in cell function regulation. Ubiquitination – the addition of ubiquitin – tags structural and regulatory proteins for degradation by the proteasome – a protein complex responsible for the proteolysis of damaged or unneeded proteins [1]. While widely investigated, the effect of ubiquitination on aging is quite unclear, but new research from the Cologne Faculty of Medicine, CECAD and CMMC could change that.

Longevity.Technology: The ubiquitin-proteasome system is central to the regulation of biological processes, while its disruption can lead to many diseases [2]. Proteasome deficiency is a hallmark of senescent cells under stress conditions. Its function can be inhibited by intracellular junks like lipofuscin, commonly found in aging cells. While the ubiquitin-proteasome system is a precipitating factor in neurodegenerative diseases, it could also be part of the solution [3].

In the recent study, the link between ubiquitination and aging was assessed by quantifying ubiquitin modifications (peptides and proteins levels) across the proteome – the entire set of proteins expressed in a cell, tissue or organ – in the worm model Caenorhabditis elegans (C. elegans).

A comparison of ubiquitin-modified proteome at days 1, 5, 10 and 15 was performed between three models: wild-type (WT) and two long-lived genetic models – of dietary restriction (eat-2) and of reduced insulin signalling (daf-2).

David Vilchez
Top: Representative image of muscle actin cytoskeleton in young animals. Middle: Representative image of muscle actin cytoskeleton in old animals. The images show destabilization of muscle cytoskeleton during aging. Bottom: Preventing destabilization of muscle cytoskeleton in old animals by lowering the age-dysregulated high levels of EPS-8, a regulator of actin cytoskeleton.

Aging is associated with a loss of ubiquitination

The levels of modified ubiquitin-peptides (Ub-peptides) increased with age in both WT and genetic models. In the WT model, this change was due to an increase of downregulated ubiquitination levels. The daf-2 model however showed an increase in upregulated Ub-peptides [4]. The global downregulated ubiquitination observed in the WT model supports the idea that aging is associated with a loss of ubiquitination. The surge in upregulated Ub-peptides with age in the longevity models indicates a prevention of age-related changes in ubiquitination.

Ub-protein levels didn’t follow quite the same trajectories. The WT model exhibited a decrease in Ub-protein amounts, while the longevity models either showed no change with age, or an increase for the daf-2 model. As for the specific expression of ubiquitin-encoding genes, aged WT worms expressed similar or higher levels compared to younger counterparts or age-matched longevity models, while the protein’s amount was not affected [4].

Deubiquitinase activity triggers loss of ubiquitination

Ubiquitin ligases and deubiquitinating enzymes (DUBs) work in balance to modulate ubiquitination levels [5]. In the WT model, age changed the expression of 12 out of the 170 ubiquitin ligases, and 15 out of the 45 DUBs [4]. The knockdown of age-dysregulated DUBs improved the age-associated loss of ubiquitination. Treatment with a DUB inhibitor rescues low ubiquitination levels and extends the lifespan of WT worms [4].

Targeted degradation can improve lifespan

Proteins with decreased ubiquitination that aggregate with age represent age-dysregulated proteasomal targets. Their deubiquitination reduces their recognition and degradation by the proteasome. EPS-8 and IFB-2 – essential factors for normal development in C. elegans – are two examples of age-dysregulated proteasomal targets [6,7].

The upregulation of EPS-8 and IFB-2 protein levels in older worms shortens their lifespan; however, the knockdown of EPS-8 and ifb-2 in aged worms extends their longevity [4].

Tissue-specific modulation

Downregulated Ub-peptides and decreased amounts of Ub-proteins with age were found in various tissues such as germline, muscle, intestine, epidermis or neurons, indicating an age-dependent loss of ubiquitination in the entire organism [4]. This deubiquitination across tissues is responsible for distinct hallmarks of aging.

IFB-2 is intestine-specific. Its accumulation triggers a loss of intestinal integrity, resulting in bacterial colonisation – a characteristic of organismal aging. This protein aggregation is accelerated by a loss of ubiquitination but salvaged with a knockdown of ifb-2 in older worms, which also results in a diminished bacterial colonisation [4].

While EPS-8 is found in all the organism’s tissues, it regulates lifespan through its activity in the brain and muscles. EPS-8 aggregation reduces longevity and excessively alters the actin cytoskeleton. Knockdown of EPS-8 in adult worms prevents the destabilisation of muscle actin cytoskeleton and myosin filaments, ameliorating defects in motility typically associated with age; it also reduces the aggregation of actin in muscle and brain [4].

Thus, the global deubiquitination that disrupts the proteasomal degradation of specific lifespan regulators determines definite hallmarks of aging.

As lead author David Vilchez summarised in a Tweetorial: “Aging causes a global loss of ubiquitination triggered by elevated deubiquitinase activity. Inhibiting elevated deubiquitinase in old worms rescues ubiquitination levels and extends lifespan … lowering the levels of age-dysregulated proteasome targets prolongs longevity, whereas preventing their degradation shortens lifespan … our results indicate that age-related changes in targeted degradation of structural and regulatory proteins across tissues determine longevity [8].

Where next?

The ubiquitin-proteasome system has already been mentioned as a potential target for therapeutic interventions [9]. While the present results are promising, it is noteworthy that the experiments were performed on an animal model and that their translation to human studies is not yet underway.

Nevertheless, the current results can be pushed for further investigations on the potential link between longevity and another regulatory pathway: autophagy.
[1] https://pubmed.ncbi.nlm.nih.gov/19489727/
[2] https://www.sciencedirect.com/science/article/pii/S0167488904002344
[3] https://www.frontiersin.org/articles/10.3389/fnmol.2014.00070/full
[4] https://www.nature.com/articles/s41586-021-03781-z
[5] https://www.sciencedirect.com/science/article/pii/S2213231714000226
[6] https://www.nature.com/articles/ncb1198
[7] https://pubmed.ncbi.nlm.nih.gov/15540462/
[8] https://twitter.com/TheVilchezLab/status/1420413363859955724
[9] https://www.frontiersin.org/articles/10.3389/fnmol.2016.00004/full

Images courtesy of the David Vilchez Lab