Mitochondria in Cell Senescence:Is Mitophagy the Weakest Link?

Cell senescence is increasingly recognized as a major contributor to the loss of health and fitness associated with aging. Senescent cells accumulate dysfunctional mitochondria; oxidative phosphorylation efficiency is decreased and reactive oxygen species production is increased. In this review we will discuss how the turnover of mitochondria (a term referred to as mitophagy) is perturbed in senescence contributing to mitochondrial accumulation and Senescence-Associated Mitochondrial Dysfunction (SAMD). We will further explore the subsequent cellular consequences; in particular SAMD appears to be necessary for at least part of the specific Senescence-Associated Secretory Phenotype (SASP) and may be responsible for tissue-level metabolic dysfunction that is associated with aging and obesity. Understanding the complex interplay between these major senescence-associated phenotypes will help to select and improve interventions that prolong healthy life in humans. Search strategy and selection criteria: Data for this review were identified by searches of MEDLINE, PubMed, and references from relevant articles using the search terms “mitochondria AND senescence”, “(autophagy OR mitophagy) AND senescence”, “mitophagy AND aging” and related terms. Additionally, searches were performed based on investigator names. Abstracts and reports from meetings were excluded. Articles published in English between 1995 and 2017 were included. Articles were selected according to their relevance to the topic as perceived by the authors

Age-related mitochondrial DNA depletion and the impact on pancreatic beta cell function

Type 2 diabetes is characterised by an age-related decline in insulin secretion. We previously identified a 50% age-related decline in mitochondrial DNA (mtDNA) copy number in isolated human islets. The purpose of this study was to mimic this degree of mtDNA depletion in MIN6 cells to determine whether there is a direct impact on insulin secretion. Transcriptional silencing of mitochondrial transcription factor A, TFAM, decreased mtDNA levels by 40% in MIN6 cells. This level of mtDNA depletion significantly decreased mtDNA gene transcription and translation, resulting in reduced mitochondrial respiratory capacity and ATP production. Glucose-stimulated insulin secretion was impaired following partial mtDNA depletion, but was normalised following treatment with glibenclamide. This confirms that the deficit in the insulin secretory pathway precedes K+ channel closure, indicating that the impact of mtDNA depletion is at the level of mitochondrial respiration. In conclusion, partial mtDNA depletion to a degree comparable to that seen in aged human islets impaired mitochondrial function and directly decreased insulin secretion. Using our model of partial mtDNA depletion following targeted gene silencing of TFAM, we have managed to mimic the degree of mtDNA depletion observed in aged human islets, and have shown how this correlates with impaired insulin secretion. We therefore predict that the age-related mtDNA depletion in human islets is not simply a biomarker of the aging process, but will contribute to the age-related risk of type 2 diabetes

Biomarkers of aging in Drosophila

Low environmental temperature and dietary restriction (DR) extend lifespan in diverse organisms. In the fruit fly Drosophila, switching flies between temperatures alters the rate at which mortality subsequently increases with age but does not reverse mortality rate. In contrast, DR acts acutely to lower mortality risk; flies switched between control feeding and DR show a rapid reversal of mortality rate. Dietary restriction thus does not slow accumulation of aging-related damage. Molecular species that track the effects of temperatures on mortality but are unaltered with switches in diet are therefore potential biomarkers of aging-related damage. However, molecular species that switch upon instigation or withdrawal of DR are thus potential biomarkers of mechanisms underlying risk of mortality, but not of aging-related damage. Using this approach, we assessed several commonly used biomarkers of aging-related damage. Accumulation of fluorescent advanced glycation end products (AGEs) correlated strongly with mortality rate of flies at different temperatures but was independent of diet. Hence, fluorescent AGEs are biomarkers of aging-related damage in flies. In contrast, five oxidized and glycated protein adducts accumulated with age, but were reversible with both temperature and diet, and are therefore not markers either of acute risk of dying or of aging-related damage. Our approach provides a powerful method for identification of biomarkers of aging.This work was supported by the Wellcome Trust and in part by I+D grants from the Spanish Ministry of Education and Science (BFU2006-14495 ⁄ BFI), the Spanish Ministry of Health (ISCIII, Red de Envejecimiento y Fragilidad, RD06 ⁄ 0013 ⁄ 0012), and the Generalitat of Catalunya (2005SGR00101) to R.P; the Spanish Ministry of Health (FIS PI081843), Spanish Ministry of Education and Science (AGL2006-12433), and ‘‘La Caixa’’ Foundation to M.P.O. Also supported by the Max Planck Society (J.J. and L.P), COST B-35 Action; Research into Ageing (A.J.L.) and the Medical Research Council and National Institutes of Health (P01 AG025901, PL1 AG032118 and P30 AG025708) (M.D.B.)

Postmitotic neurons develop a p21-dependent senescence-like phenotype driven by a DNA damage response

In senescent cells, a DNA damage response drives not only irreversible loss of replicative capacity but also production and secretion of reactive oxygen species (ROS) and bioactive peptides including pro-inflammatory cytokines. This makes senescent cells a potential cause of tissue functional decline in aging. To our knowledge, we show here for the first time evidence suggesting that DNA damage induces a senescence like state in mature postmitotic neurons in vivo. About 40 80% of Purkinje neurons and 20–40% of cortical, hippocampal and peripheral neurons in the myenteric plexus from old C57Bl ⁄ 6 mice showed severe DNA damage, activated p38MAPkinase, high ROS production and oxidative damage, interleukin IL-6 production, heterochromatinization and senescence-associated b-galactosidase activity. Frequencies of these senescence-like neurons increased with age. Short term caloric restriction tended to decrease frequencies of positive cells. The phenotype was aggravated in brains of late-generation TERC) ⁄ ) mice with dysfunctional telomeres. It was fully rescued by loss of p21(CDKN1A) function in late- generation TERC) ⁄ )CDKN1A) ⁄ ) mice, indicating p21 as the necessary signal transducer between DNA damage response and senescence-like phenotype in neurons, as in senescing fibroblasts and other proliferation competent cells. We conclude that a senescence-like phenotype is possibly not restricted to proliferation-competent cells. Rather, dysfunctional telomeres and ⁄ or accumulated DNA damage can induce a DNA damage response leading to a phenotype in postmitotic neurons that resembles cell senescence in multiple features. Senescence-like neurons might be a source of oxidative and inflammatory stress and a contributor to brain aging

Functional Sperm of the Yellowtail (Seriola quinqueradiata) Were Produced in the Small-Bodied Surrogate, Jack Mackerel (Trachurus japonicus).

Production of xenogeneic gametes from large-bodied, commercially important marine species in closely related smaller surrogates with short generation times may enable rapid domestication of the targeted species. In this study we aimed to produce gametes of Japanese yellowtail (Seriola quinqueradiata) using jack mackerel (Trachurus japonicus) as a surrogate with a smaller body size and shorter maturation period. Donor spermatogonia were collected from the testes of yellowtail males and transferred into the peritoneal cavity of 10- and 12-day-old jack mackerel larvae. Twenty days later, 59.5% of the recipients survived of which 88.2% had donor-derived germ cells in their gonads. One year later, genomic DNA templates were prepared from the semen of 96 male recipients and subjected to PCR analyses using primers specific for the yellowtail vasa sequence, resulting in the detection of positive signals in semen from two recipients. The milt collected from the recipients was used for fertilization with yellowtail eggs. Of eight hatchlings obtained from the crosses, two were confirmed to be derived from donor yellowtail by DNA markers, although the others were gynogenetic diploids. These findings indicate that it is possible to produce donor-derived sperm in xenogeneic recipients with smaller body size and shorter generation time by transplanting spermatogonia. Thus, the xenogeneic transplantation of spermatogonia might be a potential tool to produce gametes of large-bodied, commercially important fish although the efficiency of the method requires further improvement. This is the first report demonstrating that donor-derived sperm could be produced in xenogeneic recipient via spermatogonial transplantation in carangid fishes

Targeting the autophagy-NAD axis protects against cell death in Niemann-Pick type C1 disease models

Impairment of autophagy leads to an accumulation of misfolded proteins and damaged organelles and has been implicated in plethora of human diseases. Loss of autophagy in actively respiring cells has also been shown to trigger metabolic collapse mediated by the depletion of nicotinamide adenine dinucleotide (NAD) pools, resulting in cell death. Here we found that the deficit in the autophagy-NAD axis underpins the loss of viability in cell models of a neurodegenerative lysosomal storage disorder, Niemann-Pick type C1 (NPC1) disease. Defective autophagic flux in NPC1 cells resulted in mitochondrial dysfunction due to impairment of mitophagy, leading to the depletion of both the reduced and oxidised forms of NAD as identified via metabolic profiling. Consequently, exhaustion of the NAD pools triggered mitochondrial depolarisation and apoptotic cell death. Our chemical screening identified two FDA-approved drugs, celecoxib and memantine, as autophagy activators which effectively restored autophagic flux, NAD levels, and cell viability of NPC1 cells. Of biomedical relevance, either pharmacological rescue of the autophagy deficiency or NAD precursor supplementation restored NAD levels and improved the viability of NPC1 patient fibroblasts and induced pluripotent stem cell (iPSC)-derived cortical neurons. Together, our findings identify the autophagy-NAD axis as a mechanism of cell death and a target for therapeutic interventions in NPC1 disease, with a potential relevance to other neurodegenerative disorders

Feedback between p21 and reactive oxygen production is necessary for cell senescence

The sustained activation of CDKN1A (p21/Waf1/Cip1) by a DNA damage response induces mitochondrial dysfunction and reactive oxygen species (ROS) production via signalling through CDKN1A-GADD45A-MAPK14- GRB2-TGFBR2-TGFbeta in senescing primary human and mouse cells in vitro and in vivo.Enhanced ROS production in senescing cells generates additional DNA damage. Although this damage is repairable and transient, it elevates the average levels of DNA damage response permanently, thus forming a positive feedback loop.This loop is necessary and sufficient to maintain the stability of growth arrest until a ‘point of no return' is reached during establishment of senescence

Suppressed basal mitophagy drives cellular aging phenotypes that can be reversed by a p62-targeting small molecule.

Selective degradation of damaged mitochondria by autophagy (mitophagy) is proposed to play an important role in cellular homeostasis. However, the molecular mechanisms and the requirement of mitochondrial quality control by mitophagy for cellular physiology are poorly understood. Here, we demonstrated that primary human cells maintain highly active basal mitophagy initiated by mitochondrial superoxide signaling. Mitophagy was found to be mediated by PINK1/Parkin-dependent pathway involving p62 as a selective autophagy receptor (SAR). Importantly, this pathway was suppressed upon the induction of cellular senescence and in naturally aged cells, leading to a robust shutdown of mitophagy. Inhibition of mitophagy in proliferating cells was sufficient to trigger the senescence program, while reactivation of mitophagy was necessary for the anti-senescence effects of NAD precursors or rapamycin. Furthermore, reactivation of mitophagy by a p62-targeting small molecule rescued markers of cellular aging, which establishes mitochondrial quality control as a promising target for anti-aging interventions. [Abstract copyright: Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.

Mitochondria are required for pro-ageing features of the senescent phenotype

Cell senescence is an important tumour suppressor mechanism and driver of ageing. Both functions are dependent on the development of the senescent phenotype, which involves an overproduction of pro‐inflammatory and pro‐oxidant signals. However, the exact mechanisms regulating these phenotypes remain poorly understood. Here, we show the critical role of mitochondria in cellular senescence. In multiple models of senescence, absence of mitochondria reduced a spectrum of senescence effectors and phenotypes while preserving ATP production via enhanced glycolysis. Global transcriptomic analysis by RNA sequencing revealed that a vast number of senescent‐associated changes are dependent on mitochondria, particularly the pro‐inflammatory phenotype. Mechanistically, we show that the ATM, Akt and mTORC1 phosphorylation cascade integrates signals from the DNA damage response (DDR) towards PGC‐1β‐dependent mitochondrial biogenesis, contributing to a ROS‐mediated activation of the DDR and cell cycle arrest. Finally, we demonstrate that the reduction in mitochondrial content in vivo, by either mTORC1 inhibition or PGC‐1β deletion, prevents senescence in the ageing mouse liver. Our results suggest that mitochondria are a candidate target for interventions to reduce the deleterious impact of senescence in ageing tissues