Menu

Close

Akt/Foxo pathway activation switches apoptosis to senescence in short telomere zebrafish

Mounir El-Maï, Marta Marzullo, Inês Pimenta de Castro, Miguel Godinho Ferreira

Preprint posted on January 10, 2020 https://www.biorxiv.org/content/10.1101/2020.01.10.901603v1

Pushed into senescence: El-Maï and colleagues elucidate an age-related apoptosis to senescence switch in telomerase deficient zebrafish

Selected by Lorenzo Lafranchi

Categories: cell biology

 

Background

Telomeres are nucleoprotein structures present at the end of linear chromosomes. Telomeres protect chromosome termini from being recognized as DNA lesions and, together with telomerase, counterbalance the incomplete replication of terminal DNA. Since telomerase is mainly inactive in human somatic cells, telomeres shorten with aging. When telomeres reach a critical length, they trigger the DNA damage response (DDR) that, through stabilization of the p53 protein, can lead to apoptosis or senescence – the two principal outcomes of an irreversible cell damage. Apoptotic cells are removed from tissues, whereas senescent cells tend to accumulate over time and are associated with aging phenotypes. Although the mechanisms leading to a terminal cell fate are mainly understood, what determines the choice between apoptosis or senescence remains unclarified.

 

Key findings

In this preprint El-Maï and colleagues investigate the cellular outcome of accelerated telomere shortening in zebrafish. It has been previously shown that telomerase deficient (tert-/-) zebrafish have shorter telomeres than wild-typeanimals. tert-/- fish have a shorter lifespan and develop several degenerative conditions, mainly affecting highly proliferative tissues, such as the testis and the gut. While differences in telomere length can already be appreciated in 3-months-old animals, macroscopic and histological differences are only visible at 9 months of age. The authors open this manuscript showing that tert-/- tissues, despite being macroscopically similar to their wild-type counterparts, contain a large number of apoptotic cells. Further analysis confirms that in absence of telomerase, the DNA damage response is already strongly activated at 3 months of age, resulting in p53 stabilization and extensive apoptosis. Interestingly, at this stage tert-/- tissues do not show any sign of cellular senescence. When the authors turn to tissues coming from 9-month-old fish, they face a completely different scenario. In this case, p53 levels are comparable between tert-/- and wt fish and only a modest number of apoptotic cells can be observed in tert-/- tissues. In contrast, at this stage tert-/- tissues entail a large number of senescent cells. At the molecular level, this age-dependent switch from apoptosis to senescence is underscored by an increase in the levels of p15, the homologue of the CDK inhibitor p16. Since p15 induction can be a consequence of the accumulation of reactive oxygen species (ROS) due to dysfunctional mitochondria, the authors investigate if there is an age-related decrease in mitochondrial functionality between tert-/- and wt fish. At the age of 3 months, ROS levels are comparable between mutant and wt, nevertheless ROS significantly accumulates with age in tert-/- fish. This increase is accompanied by a decrease in ATP levels, supporting the hypothesis that the loss of mitochondrial fitness is the cause of the age-dependent switch in cell-fate decision.

 

Figure 1 Immunofluorescence images comparing gut tissues coming from 3- or 9-month-old zebrafish. Telomere attrition trigger apoptosis (visualized by TUNEL assay) in young animals and senescence (indicated by p15/p16 or SA-ß-GAL) in later stages. The figure is adapted from figure 1 of the preprint with permission of the authors.

 

It has been shown in mammalian cells that expression of one of the major ROS scavengers, mitochondrial manganese superoxide dismutase (SOD2), decreases with age. SOD2 expression is controlled by FoxO proteins, a family of transcription factors responsible for a wide range of cellular processes, including the DNA damage response and ROS detoxification. In 9-month-old fish, western blot analysis shows that SOD2 levels are reduced in tert-/- tissues, compared to wt. SOD2 downregulation is accompanied by the inactivation of members of the FoxO family and the activation of their upstream inhibitory kinase AKT. AKT is a central regulator of growth-promoting signals and can directly be activated by the mTOR complex 2. Activation of a pro-proliferative pathway in an organism exhibiting growth defects seems contradictory, but the authors hypothesize that this activation could be a consequence of the high level of apoptosis observed in tissues at 3 months of age. In fact, it has been shown that dying cells of proliferative tissues try to compensate for the empty space, by secreting mitogenic signals to promote proliferation of neighbouring cells. To test their hypothesis, El-Maï and co-workers turn to tert-/- p53-/- fish, in which absence of p53 abrogates the massive cell death observed in absence of telomerase at 3 months of age. Interestingly, all the phenotypes observed in old tert-/- fish were relieved in absence of p53. This data supports the hypothesis that apoptosis and the thereby induced compensatory proliferation are the causes of tissue damages and cellular senescence observed in old fish lacking telomerase. Finally, to directly test the role of the AKT/FoxO pathway in promoting cellular senescence, the authors crossed the tert-/- fish with a strain haploinsufficient for zTOR, the zebrafish homologue of mTOR. In this fish, AKT activation is reduced, resulting in a decreased expression of p15 in old fish. Nevertheless, possibly due to the partial inhibition of zTOR, dampened expression of p15 is not sufficient to restore tissue morphology in old animals.

 

What I like about this work and future directions

The final outcome of activation of the DNA damage response can be highly variable and we currently have a limited understanding of the factors determining the fate of a cell following DNA damage. El-Maï and colleagues start from the observation that there is an age-related transition from apoptosis to senescence following telomere attrition in zebrafish and nicely define the molecular mechanisms responsible for this switch. Now, it remains to be clarified whether this switch in the response to telomere shortening is conserved in other organisms.

 

Questions to the authors

Do wild-type fish experience later in life the same morphological defects you observed in tert-/- mutants? Do you reckon the apoptosis to senescence switch observed in tert-/- mutants also occurs in wild-type fish?

Did you score for senescent cells in tert-/- ztor+/- animals at 11 months of age? And in the G2 tert-/- fish after treatment with the AKT inhibitor?

 

Tags: akt/foxo, apoptosis, dna damage response, senescence, telomerase, telomeres

Posted on: 21st January 2020 , updated on: 22nd January 2020

doi: https://doi.org/10.1242/prelights.16400

Read preprint (No Ratings Yet)




Author's response

Mounir El-Maï, Marta Marzullo and Miguel Godinho Ferreira shared

Do wild-type fish experience later in life the same morphological defects you observed in tert-/- mutants? Do you reckon the apoptosis to senescence switch observed in tert-/- mutants also occurs in wild-type fish?

Our previous studies showed that WT fish exhibit the same defects observed in tert-/- mutants (Carneiro et al.; Plos Genetics 2016), including testis and gut alterations described in this work. In WT fish, tissue degeneration can be observed from 18-24 month of age. Progressive loss of tissue integrity is associated to telomere shortening. Telomere length in old fish is comparable to the one in tert-/- mutants when similar defects are observed, suggesting that short telomeres may initiate the age-associated phenotypes.

In WT zebrafish, our previous work reported a analogous transition from apoptosis to senescence, as in tert-/- fish. Apoptosis indeed appears at early stages of aging while senescence becomes predominant in later ages of WT zebrafish. Like in tert-/- fish, tissue defects are associated with accumulation of senescent cells. We anticipate that the degenerative phenotypes are outcomes of a sequential activation of apoptosis and senescence in aging tissues.

In this study, we demonstrated that the Akt pro-proliferative pathway is involved in the switch between apoptosis and senescence. Our work emphasizes the importance of studying telomere-dependent cell fates in an in vivo setting. Indicating, with other results from our studies, that the same cells may respond differently depending on the surrounding environment.

 

Did you score for senescent cells in tert-/- ztor+/- animals at 11 months of age? And in the G2 tert-/- fish after treatment with the AKT inhibitor?

Thanks for giving us the opportunity to clarify these points.

We performed SA-Beta-Gal assays on tert-/- ztor+/- animals. However, similar to our observations on tissue morphology, we did not see a clear reduction of senescence staining. ztor haploinsufficiency may not be sufficient to fully rescue senescence upon telomere shortening. Potentially, a small reduction of senescent cells may not be visible using this technique. In contrast, RT-qPCR being a quantitative and highly sensitive technique, allowed us to detect a reduction in p15/16 expression in the double tert-/- ztor+/-, indicating that ztor activity is required for the full induction of this senescence marker.

As for the second question, we did not perform this experiment yet. However, it is worth noticing that, in line with a rescue of senescence after Akt inhibition, G2 tert-/- larvae exhibit a reduced p15/p16 protein and mRNA levels compared to untreated larvae.

Considering the reduced regenerative capacity seen in zebrafish in tert-/- and during aging, we are keenly interested in understanding the involvement of Akt and its interplay with p53 in this phenomenon.

Have your say

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Sign up to customise the site to your preferences and to receive alerts

Register here

preLists in the cell biology category:

FENS 2020

A collection of preprints presented during the virtual meeting of the Federation of European Neuroscience Societies (FENS) in 2020

 



List by Ana Dorrego-Rivas

Planar Cell Polarity – PCP

This preList contains preprints about the latest findings on Planar Cell Polarity (PCP) in various model organisms at the molecular, cellular and tissue levels.

 



List by Ana Dorrego-Rivas

BioMalPar XVI: Biology and Pathology of the Malaria Parasite

[under construction] Preprints presented at the (fully virtual) EMBL BioMalPar XVI, 17-18 May 2020 #emblmalaria

 



List by Gautam Dey, Samantha Seah

1

Cell Polarity

Recent research from the field of cell polarity is summarized in this list of preprints. It comprises of studies focusing on various forms of cell polarity ranging from epithelial polarity, planar cell polarity to front-to-rear polarity.

 



List by Yamini Ravichandran

TAGC 2020

Preprints recently presented at the virtual Allied Genetics Conference, April 22-26, 2020. #TAGC20

 



List by Maiko Kitaoka, Madhuja Samaddar, Miguel V. Almeida, Sejal Davla, Jennifer Ann Black, Gautam Dey

3D Gastruloids

A curated list of preprints related to Gastruloids (in vitro models of early development obtained by 3D aggregation of embryonic cells). Preprint missing? Don't hesitate to let us know.

 



List by Paul Gerald L. Sanchez and Stefano Vianello

ECFG15 – Fungal biology

Preprints presented at 15th European Conference on Fungal Genetics 17-20 February 2020 Rome

 



List by Hiral Shah

ASCB EMBO Annual Meeting 2019

A collection of preprints presented at the 2019 ASCB EMBO Meeting in Washington, DC (December 7-11)

 



List by Madhuja Samaddar, Ramona Jühlen, Amanda Haage, Laura McCormick, Maiko Kitaoka

EMBL Seeing is Believing – Imaging the Molecular Processes of Life

Preprints discussed at the 2019 edition of Seeing is Believing, at EMBL Heidelberg from the 9th-12th October 2019

 



List by Gautam Dey

Autophagy

Preprints on autophagy and lysosomal degradation and its role in neurodegeneration and disease. Includes molecular mechanisms, upstream signalling and regulation as well as studies on pharmaceutical interventions to upregulate the process.

 



List by Sandra Malmgren Hill

Lung Disease and Regeneration

This preprint list compiles highlights from the field of lung biology.

 



List by Rob Hynds

Cellular metabolism

A curated list of preprints related to cellular metabolism at Biorxiv by Pablo Ranea Robles from the Prelights community. Special interest on lipid metabolism, peroxisomes and mitochondria.

 



List by Pablo Ranea Robles

BSCB/BSDB Annual Meeting 2019

Preprints presented at the BSCB/BSDB Annual Meeting 2019

 



List by Gautam Dey

MitoList

This list of preprints is focused on work expanding our knowledge on mitochondria in any organism, tissue or cell type, from the normal biology to the pathology.

 



List by Sandra Franco Iborra

Biophysical Society Annual Meeting 2019

Few of the preprints that were discussed in the recent BPS annual meeting at Baltimore, USA

 



List by Joseph Jose Thottacherry

ASCB/EMBO Annual Meeting 2018

This list relates to preprints that were discussed at the recent ASCB conference.

 



List by Gautam Dey, Amanda Haage
Close