Portal de Periódicos da CAPES

Sleep: DNA Repair Function for Better Neuronal Aging?

2019; Elsevier BV; Volume: 29; Issue: 12 Linguagem: Inglês

10.1016/j.cub.2019.05.018

1879-0445

Philippe Mourrain, Gordon Wang,

Sleep and related disorders

A novel potential role of sleep is neuronal DNA repair. Live imaging of chromosome dynamics in zebrafish neurons has uncovered how sleep can repair DNA breaks accumulated during wake to maintain genome integrity and likely slow down neuronal aging. A novel potential role of sleep is neuronal DNA repair. Live imaging of chromosome dynamics in zebrafish neurons has uncovered how sleep can repair DNA breaks accumulated during wake to maintain genome integrity and likely slow down neuronal aging. While most of our body cells are renewed during the course of our lives, we die with much of the neuronal cells we are born with. Thus, in contrast to a skin, blood or liver cells, which live from days to months, a neuron may need to preserve its integrity while maintaining its capacity to connect to other neurons in an ever-changing environment across decades. While it is unclear how neuronal tissues achieve such a feat, a recurring period of our lives may be critical for the survival and maintenance of our brain cells, including their genome — sleep. A recent study from Zada et al. [1Zada D. Bronshtein I. Lerer-Goldshtein T. Garini Y. Appelbaum L. Sleep increases chromosome dynamics to enable reduction of accumulating DNA damage in single neurons.Nat. Commun. 2019; 10: 895Crossref Scopus (67) Google Scholar] shows at the single cell level that sleep increases chromosome dynamics in neuronal nuclei to repair DNA double-strand breaks (DSBs) accumulated in the genome during wake [2Suberbielle E. Sanchez P.E. Kravitz A.V. Wang X. Ho K. Eilertson K. Devidze N. Kreitzer A.C. Mucke L. Physiologic brain activity causes DNA double-strand breaks in neurons, with exacerbation by amyloid-β.Nat. Neurosci. 2013; 16: 613-621Crossref PubMed Scopus (317) Google Scholar, 3Bellesi M. Bushey D. Chini M. Tononi G. Cirelli C. Contribution of sleep to the repair of neuronal DNA double-strand breaks: evidence from flies and mice.Sci. Rep. 2016; 6: 36804Crossref Scopus (45) Google Scholar]. Although the functions of sleep have not been fully uncovered, over the past few years an overarching theme has emerged that sleep is critical to mend imbalances and damage generated by the waking state [4Wang G. Grone B. Colas D. Appelbaum L. Mourrain P. Synaptic plasticity in sleep: Learning, homeostasis and disease.Trends Neurosci. 2011; 34: 452-463Abstract Full Text Full Text PDF PubMed Scopus (110) Google Scholar]. The intuitive idea of sleep being a restorative phase should of course be strange to no one, but the accumulation of tangible molecular or subcellular evidence scientifically illustrating such a role is actually fairly recent. The neuronal activity of the brain during wake comes at a cost. As one explores and learns, new connections are created and synaptic strength increases in many regions of the brain, a dynamic that cannot be sustained indefinitely. Thus, one of the first hypothesized functions for sleep is to ensure synaptic homeostasis [5Tononi G. Cirelli C. Sleep and synaptic homeostasis: a hypothesis.Brain Res. Bull. 2003; 62: 143-150Crossref PubMed Scopus (810) Google Scholar, 6Appelbaum L. Wang G. Yokogawa T. Skariah G.M. Smith S.J.S.J. Mourrain P. Mignot E. Circadian and homeostatic regulation of structural synaptic plasticity in hypocretin neurons.Neuron. 2010; 68: 87-98Abstract Full Text Full Text PDF PubMed Scopus (131) Google Scholar]. Correlated to this increased synaptic activity during wake is the production and accumulation of waste and proteins such as beta amyloid (Aβ), and sleep is likely an important period during which major toxic metabolites are cleared from the brain including Aβ [7Xie L. Kang H. Xu Q. Chen M.J. Liao Y. Thiyagarajan M. O'Donnell J. Christensen D.J. Nicholson C. Iliff J.J. et al.Sleep drives metabolite clearance from the adult brain.Science. 2013; 342: 373-377Crossref PubMed Scopus (2487) Google Scholar]. More recently, an additional role for sleep in neuronal genome maintenance has emerged [1Zada D. Bronshtein I. Lerer-Goldshtein T. Garini Y. Appelbaum L. Sleep increases chromosome dynamics to enable reduction of accumulating DNA damage in single neurons.Nat. Commun. 2019; 10: 895Crossref Scopus (67) Google Scholar, 3Bellesi M. Bushey D. Chini M. Tononi G. Cirelli C. Contribution of sleep to the repair of neuronal DNA double-strand breaks: evidence from flies and mice.Sci. Rep. 2016; 6: 36804Crossref Scopus (45) Google Scholar]. This novel DNA repair function of sleep stems from earlier observations showing that wake with exploration causes DSBs in neurons in the mouse brain [2Suberbielle E. Sanchez P.E. Kravitz A.V. Wang X. Ho K. Eilertson K. Devidze N. Kreitzer A.C. Mucke L. Physiologic brain activity causes DNA double-strand breaks in neurons, with exacerbation by amyloid-β.Nat. Neurosci. 2013; 16: 613-621Crossref PubMed Scopus (317) Google Scholar] (Figure 1). Increasing neuronal activity by visual or optogenetic stimulations was sufficient to increase neuronal DSBs, suggesting a causal link between neuronal activity and DNA damage. These DNA breaks were found most abundant in circuits involved in learning and memory in the hippocampus, but surprisingly were repaired within 24 hours [2Suberbielle E. Sanchez P.E. Kravitz A.V. Wang X. Ho K. Eilertson K. Devidze N. Kreitzer A.C. Mucke L. Physiologic brain activity causes DNA double-strand breaks in neurons, with exacerbation by amyloid-β.Nat. Neurosci. 2013; 16: 613-621Crossref PubMed Scopus (317) Google Scholar], suggesting a sleep component. Conversely, Aβ, which is elevated by sleep deprivation [8Shokri-Kojori E. Wang G.-J. Wiers C.E. Demiral S.B. Guo M. Kim S.W. Lindgren E. Ramirez V. Zehra A. Freeman C. et al.β-Amyloid accumulation in the human brain after one night of sleep deprivation.Proc. Natl. Acad. Sci. USA. 2018; 115: 4483-4488Crossref PubMed Scopus (351) Google Scholar], exacerbates DNA damage by eliciting aberrant synaptic activity, prompting further investigation of the potential role of sleep (Figure 1). A subsequent study [3Bellesi M. Bushey D. Chini M. Tononi G. Cirelli C. Contribution of sleep to the repair of neuronal DNA double-strand breaks: evidence from flies and mice.Sci. Rep. 2016; 6: 36804Crossref Scopus (45) Google Scholar] also showed that not mere wake but wake with exploration of an enriched environment induces DSBs in both the fly and mouse brain. Interestingly, DSBs are necessary for the transcription [9Bunch H. Lawney B.P. Lin Y.-F. Asaithamby A. Murshid A. Wang Y.E. Chen B.P.C. Calderwood S.K. Transcriptional elongation requires DNA break-induced signalling.Nat. Commun. 2015; 6: 10191Crossref PubMed Scopus (121) Google Scholar] and expression of neuronal early-response genes [10Madabhushi R. Gao F. Pfenning A.R. Pan L. Yamakawa S. Seo J. Rueda R. Phan T.X. Yamakawa H. Pao P.-C. et al.Activity-induced DNA breaks govern the expression of neuronal early-response genes.Cell. 2015; 161: 1592-1605Abstract Full Text Full Text PDF PubMed Scopus (408) Google Scholar], many of which have been implicated in synaptic plasticity (Figure 1). DSBs could hence potentially be needed for the transcriptional activation of genes critical for learning and memory during wake. Consistent with the repair of DNA damage in less than 24 h observed in [2Suberbielle E. Sanchez P.E. Kravitz A.V. Wang X. Ho K. Eilertson K. Devidze N. Kreitzer A.C. Mucke L. Physiologic brain activity causes DNA double-strand breaks in neurons, with exacerbation by amyloid-β.Nat. Neurosci. 2013; 16: 613-621Crossref PubMed Scopus (317) Google Scholar], Bellesi et al. found that sleep facilitates DSB repair in both mouse and fly brains and DNA repair genes are upregulated during sleep [3Bellesi M. Bushey D. Chini M. Tononi G. Cirelli C. Contribution of sleep to the repair of neuronal DNA double-strand breaks: evidence from flies and mice.Sci. Rep. 2016; 6: 36804Crossref Scopus (45) Google Scholar]. How sleep may promote DNA repair was still an open question though. As brain neuronal activity is lower overall during sleep, Zada et al. [1Zada D. Bronshtein I. Lerer-Goldshtein T. Garini Y. Appelbaum L. Sleep increases chromosome dynamics to enable reduction of accumulating DNA damage in single neurons.Nat. Commun. 2019; 10: 895Crossref Scopus (67) Google Scholar] hypothesized that sleep has evolved in order to regulate functionally linked single neurons to synchronize their silencing or reduce their activity, thus enabling coordinated DNA repair in the absence of neuronal activity, which the previous work suggests would undermine this. Sleep could have a beneficial role at the chromosomal level, either autonomously in single neurons or non-autonomously in synchronized neuronal networks. They also suspected that chromosome dynamics may change during sleep as biophysical studies over the past twenty years had implicated chromatin movement in mechanisms of gene expression as well as DNA repair (for review, see [11Seeber A. Hauer M.H. Gasser S.M. Chromosome dynamics in response to DNA damage.Annu. Rev. Genet. 2018; 52: 295-319Crossref PubMed Scopus (44) Google Scholar]). Chromatin motion may play an active role during the homology search step that must occur in recombination-mediated DSB repair. Chromatin movement is also needed to allow the recruitment of damaged DNA to favored sites of repair [11Seeber A. Hauer M.H. Gasser S.M. Chromosome dynamics in response to DNA damage.Annu. Rev. Genet. 2018; 52: 295-319Crossref PubMed Scopus (44) Google Scholar]. For example, DSBs located in heterochromatin must be moved to another compartment in the nucleus so they can load Rad51, a recombination/repair factor, onto the resected DNA end prior to repair [11Seeber A. Hauer M.H. Gasser S.M. Chromosome dynamics in response to DNA damage.Annu. Rev. Genet. 2018; 52: 295-319Crossref PubMed Scopus (44) Google Scholar]. The authors hence used the transparent zebrafish as a vertebrate model to image chromosome dynamics, DSB occurrence and repair during wake and sleep periods. Zada et al. imaged the dynamics of single chromosomes in live larvae by using telomere and centromere markers fused to enhanced green fluorescent protein. They focused on the fish cerebrum and brainstem. Remarkably, the time-lapse imaging showed that chromosome dynamics increased by approximately two-fold during nighttime sleep in both brain regions. To differentiate between sleep and circadian effects, they repeated their experiments in free running conditions as well as after sleep deprivation and sleep rebound. They found that sleep increases neuronal chromosome dynamics in a homeostatic-dependent manner but independently of the circadian clock, suggesting a sleep-dependent process. To test whether these changes are also present in other cell types, they monitored chromosome dynamics in peripheral endothelial and Schwann cells. Interestingly, chromosome dynamics did not differ between day and night in either cell type, supporting a specific function for sleep in neuron nuclei. Zada et al. next speculated that increased chromosome dynamics could enhance the efficiency of DSB elimination during sleep. Similar to Suberbielle et al. [2Suberbielle E. Sanchez P.E. Kravitz A.V. Wang X. Ho K. Eilertson K. Devidze N. Kreitzer A.C. Mucke L. Physiologic brain activity causes DNA double-strand breaks in neurons, with exacerbation by amyloid-β.Nat. Neurosci. 2013; 16: 613-621Crossref PubMed Scopus (317) Google Scholar] and Bellesi et al. [3Bellesi M. Bushey D. Chini M. Tononi G. Cirelli C. Contribution of sleep to the repair of neuronal DNA double-strand breaks: evidence from flies and mice.Sci. Rep. 2016; 6: 36804Crossref Scopus (45) Google Scholar], they used the γH2AX marker, which is activated as part of the DNA damage response system, to quantify DSBs during the day/night cycle. During the day, the number of DSBs consistently increased, and peaked one hour before darkness. During the night, the number of DSBs dramatically decreased, reached minimum levels in the middle of the 10-hour night, and remained low until the beginning of the day. In parallel, during the day, chromosome dynamics remained low but then, following 1 h of darkness, increased by two-fold and stayed high during the entire night. Chromosome dynamics thus correlated with efficient reduction of DSBs during the night. This correlation was substantiated by the observation that sleep deprivation prevented DSB reduction in neuron chromosomes while sleep rebound allowed it. In contrast to the day/night changes observed in neurons, the number of DSBs were constantly low during day and night in endothelial and Schwann cells, suggesting again that sleep has a specific influence on chromosome dynamics and DSB repair in neurons. Zada et al. then showed that sleep-dependent chromosome dynamics are necessary to reduce DSBs by manipulating chromosome binding to the nuclear lamina and blocking chromosome dynamics [1Zada D. Bronshtein I. Lerer-Goldshtein T. Garini Y. Appelbaum L. Sleep increases chromosome dynamics to enable reduction of accumulating DNA damage in single neurons.Nat. Commun. 2019; 10: 895Crossref Scopus (67) Google Scholar]. As often with sleep function(s) it may be hard to distinguish between a primary function of sleep and an opportunistic function taking advantage of these long periods of body rest. To start answering this question, Zada et al. tested whether DSBs could actually cause changes in sleep. The authors used etoposide (ETO), which induces DSBs [12Smart D.J. Halicka H.D. Schmuck G. Traganos F. Darzynkiewicz Z. Williams G.M. Assessment of DNA double-strand breaks and γH2AX induced by the topoisomerase II poisons etoposide and mitoxantrone.Mutat. Res. Mol. Mech. Mutagen. 2008; 641: 43-47Crossref PubMed Scopus (100) Google Scholar], and the number of γH2AX foci, chromosome dynamics, and sleep time were monitored. 1 h following ETO withdrawal, sleep time increased in ETO-treated larvae. After 2 h of recovery from the ETO treatment, sleep time remained high, and chromosome dynamics increased by approximately two-fold accompanied by a reduction of the number of DSBs. These results suggest that while chromosome dynamics are low during the formation of DSBs, the accumulation of DSBs during wakefulness may trigger sleep, which increases chromosome dynamics and eventually reduces the number of DSBs. The emerging image suggested by this very recent work is that during wake neurons accumulate DNA damage due to synaptic activity, a finding consistent with the fact that in flies, mammals and zebrafish, neuronal activity promotes the formation of DSBs [1Zada D. Bronshtein I. Lerer-Goldshtein T. Garini Y. Appelbaum L. Sleep increases chromosome dynamics to enable reduction of accumulating DNA damage in single neurons.Nat. Commun. 2019; 10: 895Crossref Scopus (67) Google Scholar, 2Suberbielle E. Sanchez P.E. Kravitz A.V. Wang X. Ho K. Eilertson K. Devidze N. Kreitzer A.C. Mucke L. Physiologic brain activity causes DNA double-strand breaks in neurons, with exacerbation by amyloid-β.Nat. Neurosci. 2013; 16: 613-621Crossref PubMed Scopus (317) Google Scholar, 10Madabhushi R. Gao F. Pfenning A.R. Pan L. Yamakawa S. Seo J. Rueda R. Phan T.X. Yamakawa H. Pao P.-C. et al.Activity-induced DNA breaks govern the expression of neuronal early-response genes.Cell. 2015; 161: 1592-1605Abstract Full Text Full Text PDF PubMed Scopus (408) Google Scholar]. These DSBs can be repaired by sleep-dependent, chromosome-level mechanisms requiring chromosome movement (Figure 1). Moreover, these DSBs appear to be associated with the activation of the synaptic glutamate sensor NR2B [2Suberbielle E. Sanchez P.E. Kravitz A.V. Wang X. Ho K. Eilertson K. Devidze N. Kreitzer A.C. Mucke L. Physiologic brain activity causes DNA double-strand breaks in neurons, with exacerbation by amyloid-β.Nat. Neurosci. 2013; 16: 613-621Crossref PubMed Scopus (317) Google Scholar], a glutamate-activated ion channel that is permeable to Ca2+. This synaptic Ca2+ has the potential to initiate DSBs as Ca2+ has been demonstrated to induce activation and nuclear transport of cytosolic topoisomerases via protease cleavage by the Ca2+-sensitive protease calpain [13Chou S.-M. Huang T.-H. Chen H.-C. Li T.-K. Calcium-induced cleavage of DNA topoisomerase I involves the cytoplasmic-nuclear shuttling of calpain 2.Cell. Mol. Life Sci. 2011; 68: 2769-2784Crossref Scopus (9) Google Scholar]. Synaptic activity could also lead to the firing of an action potential, which will open intracellular stores of Ca2+ and temporarily increase nuclear Ca2+. This nuclear elevation of Ca2+ could facilitate further DSBs via topoisomerase 2 (TOP2) as Ca2+ traps TOP2 in the active form of the enzyme [14Osheroff N. Zechiedrich E.L. Calcium-promoted DNA cleavage by eukaryotic topoisomerase II: trapping the covalent enzyme-DNA complex in an active form.Biochemistry. 1987; 26: 4303-4309Crossref PubMed Scopus (119) Google Scholar] (Figure 1). To date it is unknown whether DSBs are linked to synaptic Ca2+ or intracellular Ca2+ via action potentials. Ca2+ could also play an important part in the recovery and repair of DSBs. Burst firing, which is especially prevalent during sleep [15Navarro-Lobato I. Genzel L. The up and down of sleep: From molecules to electrophysiology.Neurobiol. Learn. Mem. 2018; 160: 3-10Crossref Scopus (24) Google Scholar], is extremely efficient in elevating intracellular Ca2+. Intracellular Ca2+ is a potent secondary messenger and is known to regulate enzymes such as poly(ADP-ribose) polymerase-1 (PARP-1), which acts as a DNA damage sensor that responds to both single- and/or double-strand DNA breaks facilitating DNA repair and cell survival [16Bentle M.S. Reinicke K.E. Bey E.A. Spitz D.R. Boothman D.A. Calcium-dependent modulation of poly(ADP-ribose) polymerase-1 alters cellular metabolism and DNA repair.J. Biol. Chem. 2006; 281: 33684-33696Crossref PubMed Scopus (100) Google Scholar]. The PARP-1 gene is also known to be upregulated during sleep [3Bellesi M. Bushey D. Chini M. Tononi G. Cirelli C. Contribution of sleep to the repair of neuronal DNA double-strand breaks: evidence from flies and mice.Sci. Rep. 2016; 6: 36804Crossref Scopus (45) Google Scholar]. Perhaps sleep is a confluence of activities that together facilitates the overall stability of the genome and the health of neurons. During sleep, especially slow wave sleep, there is an overall decrease in neuronal activity [15Navarro-Lobato I. Genzel L. The up and down of sleep: From molecules to electrophysiology.Neurobiol. Learn. Mem. 2018; 160: 3-10Crossref Scopus (24) Google Scholar]. This decrease in activity reduces stress on chromosomal structure; moreover, the synchronized state of neuronal activity during sleep makes neurons fire in bursts, instead of stochastic patterns, which increases intracellular Ca2+ levels and facilitates DNA break detection mechanisms, such as PARP-1 activity. This increased activity of a DSB sensing enzyme could result in increased chromosomal movement and DSB repair, as described in the study by Zada et al. (Figure 1). Reflecting on this study, another interesting aspect is the role of sleep-mediated DNA repair in the course of normal or pathological aging. Neurons are not renewed and DNA repair is not perfect. Thus, over a lifetime, mutations must build up. While the exact numbers of genomic mutations in aging neurons are unknown, in mitochondrial DNA it is known that point mutations increase with age and the number of point mutations is 2–3 times higher in Alzheimer's disease than during normal aging [17Chang S.-W. Zhang D. Chung H.D. Zassenhaus H.P. The frequency of point mutations in mitochondrial DNA is elevated in the Alzheimer's brain. Biochem.Biophys. Res. Commun. 2000; 273: 203-208Crossref PubMed Scopus (50) Google Scholar, 18Coskun P.E. Beal M.F. Wallace D.C. Alzheimer's brains harbor somatic mtDNA control-region mutations that suppress mitochondrial transcription and replication.Proc. Natl. Acad. Sci. USA. 2004; 101: 10726-10731Crossref PubMed Scopus (452) Google Scholar]. Knowing that Aβ exacerbates DNA damage, and that Aβ continually builds over wake and is cleared from the brain during sleep via the glymphatic system [7Xie L. Kang H. Xu Q. Chen M.J. Liao Y. Thiyagarajan M. O'Donnell J. Christensen D.J. Nicholson C. Iliff J.J. et al.Sleep drives metabolite clearance from the adult brain.Science. 2013; 342: 373-377Crossref PubMed Scopus (2487) Google Scholar], suggests that disrupted sleep might act as a nexus of unfortunate events for an aging or degenerating brain. It is well documented that sleep quality and duration is significantly reduced by normal aging and even more significantly in Alzheimer's disease [19Scullin M.K. Bliwise D.L. Sleep, cognition, and normal aging: integrating a half century of multidisciplinary research.Perspect. Psychol. Sci. 2015; 10: 97-137Crossref PubMed Scopus (271) Google Scholar]. This in conjunction with reduction of glymphatic flow during aging and the accumulation of DNA damage could result in a confluence of factors, such as abnormal neuronal firing physiology, disruption of normal neuronal metabolism, even cell death, that slowly grind the nervous system to a halt. One could hypothesize that chronic sleep-dependent DNA repair defects in combination with Aβ pathology could generate over years abnormal or defective cognition and neuronal degeneration eventually. Altogether, the common observations of wake-dependent DSBs and sleep-dependent DSB repair in flies, mice and zebrafish point towards an exciting core function of sleep that could be critical in understanding a diverse range of biology from cellular aging to neural degeneration. For too long, the sleep field has been dominated by system-level descriptions of phenomenology that does not reflect the full biological underpinnings of this critical state. The discovery of sleep functions at the subcellular level finally focuses the need to understand and define sleep at the cellular level.