Postmitotic Tissues Research Articles

D03 Mitochondrial Biogenesis, And Respiratory Chain Assembly And Function, In Skeletal Muscle Of The R6/2 Mouse Model And Human Huntington's Disease

Background Huntington’s disease (HD) primarily affects the brain. However, Huntingtin (HTT) is expressed in peripheral tissues as well. In skeletal muscle, like brain a post-mitotic tissue, multiple mitochondrial DNA deletions as well as variable deficits in complex I of the mitochondrial respiratory chain (MRC), abnormal calcium handling, and reduced expression of PGC-1α have been described. In addition, myoblast cell cultures from pre-manifest and manifest HD subjects showed impaired cell differentiation and HTT inclusions similar to those in brain. Aims To assess mitochondrial biogenesis, MRC assembly and MRC function in R6/2 and human skeletal muscle. Methods/techniques We used quadriceps muscle tissue from 12-week old R6/2 HD transgenic mice, and near to motor onset pre-manifest HD (n = 20), early onset HD patients (n = 20) and sex and age matched healthy controls (n = 20), as part of the Multi-Tissue Molecular signatures in HD project (MTM-HD). Results We report on the expression of PGC1α and one of its downstream targets, the mitochondrial transcription factor TFAM. Since TFAM regulates mitochondrial DNA transcription we determined the amount of mitochondrial DNA copies. Mitochondrial DNA encodes subunits of the respiratory chain complexes I, III and IV. We therefore assessed the assembly and the levels of the MRC complexes and measured their activities.

A21 Muscle Phenotype in Human HD

<h3>Background</h3> Huntington’s disease (HD) primarily affects the brain. However, the HTT CAG repeat expansion mutation is also expressed in peripheral tissues. In skeletal muscle, like brain a post-mitotic tissue, multiple mitochondrial DNA deletions as well as variable deficits in complex I of the mitochondrial respiratory chain (MRC), abnormal calcium handling, and reduced expression of PGC-1α have been described. In addition, myoblast cell cultures from pre-manifest and manifest HD subjects showed impaired cell differentiation and HTT inclusions similar to those in brain. <h3>Aims</h3> To highlight the similarities, and differences, between neurones and muscle. To investigate histological phenotype, mitochondrial biogenesis, MRC assembly and MRC function in quadriceps muscle from patients with HD. <h3>Material and Methods</h3> We investigated tissue from near to motor onset pre-manifest HD (n = 20), early onset HD patients (n = 20) and sex and age matched healthy controls (n = 20), as part of the Multi-Tissue Molecular signatures in HD project (MTM-HD). <h3>Results/outcome</h3> Report on the histopathology; in addition, I will show data on the expression of PGC-1α and one of its downstream targets, the mitochondrial transcription factor TFAM. Since TFAM regulates mitochondrial DNA transcription we determined the amount of mitochondrial DNA copies. Mitochondrial DNA encodes subunits of the respiratory chain complexes I, III and IV. We therefore assessed the assembly and the levels of the MRC complexes and measured their activities. <h3>Conclusions</h3> a) the muscle phenotype itself with potential repercussions for the pathophysiology of the brain; b) how could muscle tissue be used in treatment trials.

Indispensable pre-mitotic endocycles promote aneuploidy in the Drosophila rectum.

The endocycle is a modified cell cycle that lacks M phase. Endocycles are well known for enabling continued growth of post-mitotic tissues. By contrast, we discovered pre-mitotic endocycles in precursors of Drosophila rectal papillae (papillar cells). Unlike all known proliferative Drosophila adult precursors, papillar cells endocycle before dividing. Furthermore, unlike diploid mitotic divisions, these polyploid papillar divisions are frequently error prone, suggesting papillar structures may accumulate long-term aneuploidy. Here, we demonstrate an indispensable requirement for pre-mitotic endocycles during papillar development and also demonstrate that such cycles seed papillar aneuploidy. We find blocking pre-mitotic endocycles disrupts papillar morphogenesis and causes organismal lethality under high-salt dietary stress. We further show that pre-mitotic endocycles differ from post-mitotic endocycles, as we find only the M-phase-capable polyploid cells of the papillae and female germline can retain centrioles. In papillae, this centriole retention contributes to aneuploidy, as centrioles amplify during papillar endocycles, causing multipolar anaphase. Such aneuploidy is well tolerated in papillae, as it does not significantly impair cell viability, organ formation or organ function. Together, our results demonstrate that pre-mitotic endocycles can enable specific organ construction and are a mechanism that promotes highly tolerated aneuploidy.

Killing the Killer: PARC/CUL9 promotes cell survival by destroying cytochrome C.

Balanced amounts of apoptotic cell death are essential for health; its deregulation plays key roles in neurodegeneration, autoimmunity, and cancer. Mitochondria orchestrate apoptosis through a process called mitochondrial outer-membrane permeabilization (MOMP). After MOMP, mitochondrial cytochrome c is released into the cytoplasm, where it binds the adaptor molecule APAF1, triggering caspase protease activation and cell death. In this issue of Science Signaling, Deshmukh and colleagues define a new survival mechanism downstream of mitochondrial permeabilization. Specifically, they identify proteasomal degradation of cytochrome c as a major determinant of cell survival. In an unbiased approach, PARC (also known as CUL9) was found to be the ubiquitin ligase responsible for the ubiquitination and proteasomal degradation of cytochrome c. The consequences of this survival process may be double-edged because both cancer cells and postmitotic cells use PARC/CUL9-mediated cytochrome c degradation to ensure cell survival. Ultimately, differential targeting of this process may promote survival of postmitotic tissue or enhance tumor-specific killing.

MtDNA Segregation in Heteroplasmic Tissues Is Common In Vivo and Modulated by Haplotype Differences and Developmental Stage

The dynamics by which mitochondrial DNA (mtDNA) evolves within organisms are still poorly understood, despite the fact that inheritance and proliferation of mutated mtDNA cause fatal and incurable diseases. When two mtDNA haplotypes are present in a cell, it is usually assumed that segregation (the proliferation of one haplotype over another) is negligible. We challenge this assumption by showing that segregation depends on the genetic distance between haplotypes. We provide evidence by creating four mouse models containing mtDNA haplotype pairs of varying diversity. We find tissue-specific segregation in all models over a wide range of tissues. Key findings are segregation in postmitotic tissues (important for disease models) and segregation covering all developmental stages from prenatal to old age. We identify four dynamic regimes of mtDNA segregation. Our findings suggest potential complications for therapies in human populations: we propose "haplotype matching" as an approach to avoid these issues.

Adeno-associated virus vectors as therapeutic and investigational tools in the cardiovascular system.

The use of vectors based on the small parvovirus adeno-associated virus has gained significant momentum during the past decade. Their high efficiency of transduction of postmitotic tissues in vivo, such as heart, brain, and retina, renders these vectors extremely attractive for several gene therapy applications affecting these organs. Besides functional correction of different monogenic diseases, the possibility to drive efficient and persistent transgene expression in the heart offers the possibility to develop innovative therapies for prevalent conditions, such as ischemic cardiomyopathy and heart failure. Therapeutic genes are not only restricted to protein-coding complementary DNAs but also include short hairpin RNAs and microRNA genes, thus broadening the spectrum of possible applications. In addition, several spontaneous or engineered variants in the virus capsid have recently improved vector efficiency and expanded their tropism. Apart from their therapeutic potential, adeno-associated virus vectors also represent outstanding investigational tools to explore the function of individual genes or gene combinations in vivo, thus providing information that is conceptually similar to that obtained from genetically modified animals. Finally, their single-stranded DNA genome can drive homology-directed gene repair at high efficiency. Here, we review the main molecular characteristics of adeno-associated virus vectors, with a particular view to their applications in the cardiovascular field.

Epigenetic control of cell identity and plasticity

The DNA centered dogma for genetic information and cell identity is now evolving into a much more complex and flexible dimension provided by the discovery of the Epigenome. This comprises those chromosome structural and topological components that complement DNA information and contribute to genome functional organization. Current concept is that the Epigenome constitutes the dynamic molecular interface allowing the Genome to interact with the Environment. Exploring how the genome interacts with the environment is a key to fully understand cellular and complex organism mechanisms of adaptation and plasticity. Our work focuses on the role of an essential, specialized group or chromatin associated proteins named Polycomb (PcG) that control maintenance of transcription programs during development and in adult life. In particular PcG proteins exert epigenetic “memory” function by modifying chromosome structures at various levels to maintain gene silencing in particular through cell division. While in the past decade substantial progress was made in understanding PcG mechanisms acting in development and partially during cell cycle, very little is known about their role in adult post-mitotic tissues and more in general the role of the epigenome in adaptation. To this, we studied the role of PcG in the context of mammalian skeletal muscle cell differentiation. We previously reported specific dynamics of PRC2 proteins in myoblasts and myotubes, in particular the dynamics of PcG Histone H3 K27 Methyl Transferases (HMT), EZH2 and EZH1, the latter apparently replacing for EZH2 in differentiated myotubes. Ezh1 protein, although almost identical to Ezh2, shows a weak H3K27 HMT activity and its primary function remains elusive. Recent ChIPseq studies performed in differentiating muscle cells revealed that Ezh1 associates with active and not repressed regulatory regions to control RNA pol II elongation. Since H3K27 tri-methylation levels are virtually steady in non-cycling myotubes we asked which could be PRC2-EZH1 function in this context. We postulated that the presence of PRC2-EZH1 complex on active promoters, and not on repressed loci, could be explained by a novel role for PcG to control prompt response to specific to stress conditions and silencing of tissue specific active genes in a global coordinated manner. Data in vitro and in vivo will be presented in support of our hypothesis showing that in terminally differentiated myotubes PRC2-Ezh1 complex H3K27m3 methyltransferase activity is dynamically regulated in response to specific environmental stimuli and that EZH1 function is required in particular to prevent muscle atrophy. Further, we discovered that this dynamics involves a novel pathway controlling PRC2 complex formation, with potential general implications for mechanisms controlling the ability of somatic cells to respond to adverse environmental conditions via chromatin plasticity.

Inadequate mito-biogenesis in primary dermal fibroblasts from old humans is associated with impairment of PGC1A-independent stimulation

Extrinsic skin ageing converges on the dermis, a post-mitotic tissue compartment consisting of extracellular matrix and long-lived fibroblasts prone to damage accumulation and maladaptation. Aged human fibroblasts exhibit mitochondrial and nuclear dysfunctions, which may be a cause or consequence of ageing. We report on a systematic study of human dermal fibroblasts retrieved from female donors aged 20–67years and analysed ex vivo at low population doubling precluding replicative senescence. According to gene set enrichment analysis of genome wide array data, the most prominent age-associated change of the transcriptome was decreased expression of mitochondrial genes. Consistent with that, mitochondrial content and cell proliferation declined with donor age. This was associated with upregulation of AMP-dependent protein kinase (AMPK), increased mRNA levels of PPARγ-coactivator 1α (PGC1A) and decreased levels of NAD+-dependent deacetylase sirtuin 1. In the old cells the PGC1A-mediated mito-biogenetic response to direct AMPK-stimulation by AICAR was undiminished, while the PGC1A-independent mito-biogenetic response to starvation was attenuated and accompanied by increased ROS-production. In summary, these observations suggest an age-associated decline in PGC1A-independent mito-biogenesis, which is insufficiently compensated by upregulation of the AMPK/PGC1A-axis leading under baseline conditions to decreased mitochondrial content and reductive overload of residual respiratory capacity.

Muscle physiology: move to translation

Since the discovery of the striation pattern of muscle by Antoni van Leeuwenhoek in the Netherlands, research in muscle physiology has provided an immensely detailed picture of skeletal and cardiac muscle structure and function. Particularly in recent years, molecular techniques, gene expression and imaging techniques have provided a wealth of information about the basic function of muscle and its dysfunction in chronic diseases and congenital myopathies. In this special issue, based on the work presented at the 42nd European Muscle Conference (2013) held in Amsterdam (The Netherlands), an overview of the state-of-the-art in skeletal and cardiac muscle research is presented. Since basic and clinical muscle physiology are two sides of the same coin, combining both perspectives is key to fully profit from advances made in both research fields. The force-generating capacity of muscle is largely determined by the quantity and quality of the contractile apparatus. As striated muscle is generally considered as post-mitotic tissue, hypertrophy relies on a net positive balance between the rate of protein synthesis and degradation. In skeletal muscle including the diaphragm, muscle stem cells, residing between the myofiber basal lamina and sarcolemma (satellite cell, MuSC) have the ability to proliferate, differentiate and fuse with the host myofiber. Whether the pool of myonuclei (i.e. amount of DNA) within a myofiber is limiting protein synthesis, and as such myofibrillar hypertrophy, is still subject of debate. In this issue, Blaauw and Reggiani (2014) discuss the current theories on the role of MuSC in muscle hypertrophy. Is the activation of MuSC and accretion of myonuclei within the myofiber requisite for its hypertrophy? Recent reports suggest that MuSC activation may not be required. Blaauw and Reggiani (2014) argue that local inflammatory responses (elevated cytokines) may attract and activate other muscle resident stem cells, such as mesangioblasts which can differentiate into muscle precursors and possibly contribute to the pool of myonuclei within the myofiber. A key alternative mechanism to increase the rate of protein synthesis is the activation of the mammalian target of rapamycin (mTOR), which is activated by mechanical loading, leucine or growth factors in muscle. Activation of mTOR is generally regarded as a prime stimulator of mRNA translation. In this issue of the journal, Jacobs et al. (2013) reviewed the current literature on how mechanical and biochemical stimuli activate mTOR signaling. Recent evidence from Hornberger’s laboratory suggests that the late endosomal lysosome could serve as a prime regulatory center for controlling the mechanical loading induced activation of mTOR in skeletal muscle. These novel insights suggest that the late endosome/lysosome system plays a critical role in protein synthesis, protein trafficking as well as protein degradation. In addition to muscle fiber size, also muscle quality matters. Both skeletal and cardiac muscle function can be negatively affected by lifestyle and ageing, particularly through an increased oxidative stress. Canton et al. (2014) discuss the role of secondary mechanisms, particularly mediated by oxidative stress, in muscular dystrophies, C. A. C. Ottenheijm R. C. I. Wust J. van der Velden (&) Department of Physiology, Institute for Cardiovascular Research (ICaR-VU), VU University Medical Center, van der Boechorststraat 7, 1081 BT Amsterdam, The Netherlands e-mail: j.vandervelden@vumc.nl

NAD+ Deficiency in Age-Related Mitochondrial Dysfunction

Age-related mitochondrial dysfunction is thought to contribute to mammalian aging, particularly in postmitotic tissues that rely heavily on oxidative phosphorylation. A new study (Gomes etal., 2013) shows that reduced levels of nicotinamide adenine dinucleotide (NAD(+)) contribute to the mitochondrial decay associated with skeletal muscle aging and that sirtuin 1 (SIRT1) modulates this process.

Aging genomes: A necessary evil in the logic of life

Genomes are inherently unstable because of the need for DNA sequence variation as a substrate for evolution through natural selection. However, most multicellular organisms have postmitotic tissues, with limited opportunity for selective removal of cells harboring persistent damage and deleterious mutations, which can therefore contribute to functional decline, disease, and death. Key in this process is the role of genome maintenance, the network of protein products that repair DNA damage and signal DNA damage response pathways. Genome maintenance is beneficial early in life by swiftly eliminating DNA damage or damaged cells, facilitating rapid cell proliferation. However, at later ages accumulation of unrepaired damage and mutations, as well as ongoing cell depletion, promotes cancer, atrophy, and other deleterious effects associated with aging. As such, genome maintenance and its phenotypic sequelae provide yet another example of antagonistic pleiotropy in aging and longevity.

Oxidative Stress in Aging: Advances in Proteomic Approaches

Aging is a gradual, complex process in which cells, tissues, organs, and the whole organism itself deteriorate in a progressive and irreversible manner that, in the majority of cases, implies pathological conditions that affect the individual's Quality of Life (QOL). Although extensive research efforts in recent years have been made, the anticipation of aging and prophylactic or treatment strategies continue to experience major limitations. In this review, the focus is essentially on the compilation of the advances generated by cellular expression profile analysis through proteomics studies (two-dimensional [2D] electrophoresis and mass spectrometry [MS]), which are currently used as an integral approach to study the aging process. Additionally, the relevance of the oxidative stress factors is discussed. Emphasis is placed on postmitotic tissues, such as neuronal, muscular, and red blood cells, which appear to be those most frequently studied with respect to aging. Additionally, models for the study of aging are discussed in a number of organisms, such as Caenorhabditis elegans, senescence-accelerated probe-8 mice (SAMP8), naked mole-rat (Heterocephalus glaber), and the beagle canine. Proteomic studies in specific tissues and organisms have revealed the extensive involvement of reactive oxygen species (ROS) and oxidative stress in aging.

Triple Trans-Splicing Adeno-Associated Virus Vectors Capable of Transferring the Coding Sequence for Full-Length Dystrophin Protein into Dystrophic Mice

Recombinant adeno-associated virus (rAAV) vectors have been shown to permit very efficient widespread transgene expression in skeletal muscle after systemic delivery, making these increasingly attractive as vectors for Duchenne muscular dystrophy (DMD) gene therapy. DMD is a severe muscle-wasting disorder caused by DMD gene mutations leading to complete loss of dystrophin protein. One of the major issues associated with delivery of the DMD gene, as a therapeutic approach for DMD, is its large open reading frame (ORF; 11.1 kb). A series of truncated microdystrophin cDNAs (delivered via a single AAV) and minidystrophin cDNAs (delivered via dual-AAV trans-spliced/overlapping reconstitution) have thus been extensively tested in DMD animal models. However, critical rod and hinge domains of dystrophin required for interaction with components of the dystrophin-associated protein complex, such as neuronal nitric oxide synthase, syntrophin, and dystrobrevin, are missing; these dystrophin domains may still need to be incorporated to increase dystrophin functionality and stabilize membrane rigidity. Full-length DMD gene delivery using AAV vectors remains elusive because of the limited single-AAV packaging capacity (4.7 kb). Here we developed a novel method for the delivery of the full-length DMD coding sequence to skeletal muscles in dystrophic mdx mice using a triple-AAV trans-splicing vector system. We report for the first time that three independent AAV vectors carrying "in tandem" sequential exonic parts of the human DMD coding sequence enable the expression of the full-length protein as a result of trans-splicing events cojoining three vectors via their inverted terminal repeat sequences. This method of triple-AAV-mediated trans-splicing could be applicable to the delivery of any large therapeutic gene (≥11 kb ORF) into postmitotic tissues (muscles or neurons) for the treatment of various inherited metabolic and genetic diseases.

Misregulation of autophagy and protein degradation systems in myopathies and muscular dystrophies

A number of recent studies have highlighted the importance of autophagy and the ubiquitin-proteasome in the pathogenesis of muscle wasting in different types of inherited muscle disorders. Autophagy is crucial for the removal of dysfunctional organelles and protein aggregates, whereas the ubiquitin-proteasome is important for the quality control of proteins. Post-mitotic tissues, such as skeletal muscle, are particularly susceptible to aged or dysfunctional organelles and aggregation-prone proteins. Therefore, these degradation systems need to be carefully regulated in muscles. Indeed, excessive or defective activity of the autophagy lysosome or ubiquitin-proteasome leads to detrimental effects on muscle homeostasis. A growing number of studies link abnormalities in the regulation of these two pathways to myofiber degeneration and muscle weakness. Understanding the pathogenic role of these degradative systems in each inherited muscle disorder might provide novel therapeutic targets to counteract muscle wasting. In this Commentary, we will discuss the current view on the role of autophagy lysosome and ubiquitin-proteasome in the pathogenesis of myopathies and muscular dystrophies, and how alteration of these degradative systems contribute to muscle wasting in inherited muscle disorders. We will also discuss how modulating autophagy and proteasome might represent a promising strategy for counteracting muscle loss in different diseases.

Cisplatin induces a mitochondrial-ROS response that contributes to cytotoxicity depending on mitochondrial redox status and bioenergetic functions.

Cisplatin is one of the most effective and widely used anticancer agents for the treatment of several types of tumors. The cytotoxic effect of cisplatin is thought to be mediated primarily by the generation of nuclear DNA adducts, which, if not repaired, cause cell death as a consequence of DNA replication and transcription blockage. However, the ability of cisplatin to induce nuclear DNA (nDNA) damage per se is not sufficient to explain its high degree of effectiveness nor the toxic effects exerted on normal, post-mitotic tissues. Oxidative damage has been observed in vivo following exposure to cisplatin in several tissues, suggesting a role for oxidative stress in the pathogenesis of cisplatin-induced dose-limiting toxicities. However, the mechanism of cisplatin-induced generation of ROS and their contribution to cisplatin cytotoxicity in normal and cancer cells is still poorly understood. By employing a panel of normal and cancer cell lines and the budding yeast Saccharomyces cerevisiae as model system, we show that exposure to cisplatin induces a mitochondrial-dependent ROS response that significantly enhances the cytotoxic effect caused by nDNA damage. ROS generation is independent of the amount of cisplatin-induced nDNA damage and occurs in mitochondria as a consequence of protein synthesis impairment. The contribution of cisplatin-induced mitochondrial dysfunction in determining its cytotoxic effect varies among cells and depends on mitochondrial redox status, mitochondrial DNA integrity and bioenergetic function. Thus, by manipulating these cellular parameters, we were able to enhance cisplatin cytotoxicity in cancer cells. This study provides a new mechanistic insight into cisplatin-induced cell killing and may lead to the design of novel therapeutic strategies to improve anticancer drug efficacy.

Compensatory cellular hypertrophy: the other strategy for tissue homeostasis

Metazoan tissues have the ability to maintain tissue size and morphology while eliminating aberrant or damaged cells. In the tissue homeostasis system, cell division is the primary strategy cells use not only to increase tissue size during development but also to compensate for cell loss in tissue repair. Recent studies in Drosophila, however, have shown that cells in postmitotic tissues undergo hypertrophic growth without division, contributing to tissue repair as well as organ development. Indeed, similar compensatory cellular hypertrophy (CCH) can be observed in different contexts such as mammalian hepatocytes or corneal endothelial cells. Here we highlight these findings and discuss the underlying mechanisms of CCH, which is likely an evolutionarily conserved strategy for homeostatic tissue growth in metazoans.

Gene regulation and priming by topoisomerase IIα in embryonic stem cells

Topoisomerases resolve torsional stress, while their function in gene regulation, especially during cellular differentiation, remains unknown. Here we find that the expression of topo II isoforms, topoisomerase IIα and topoisomerase IIβ, is the characteristic of dividing and postmitotic tissues, respectively. In embryonic stem cells, topoisomerase IIα preferentially occupies active gene promoters. Topoisomerase IIα inhibition compromises genomic integrity, which results in epigenetic changes, altered kinetics of RNA Pol II at target promoters and misregulated gene expression. Common targets of topoisomerase IIα and topoisomerase IIβ are housekeeping genes, while unique targets are involved in proliferation/pluripotency and neurogenesis, respectively. Topoisomerase IIα targets exhibiting bivalent chromatin resolve upon differentiation, concomitant with their activation and occupancy by topoisomerase IIβ, features further observed for long genes. These long silent genes display accessible chromatin in embryonic stem cells that relies on topoisomerase IIα activity. These findings suggest that topoisomerase IIα not only contributes to stem-cell transcriptome regulation but also primes developmental genes for subsequent activation upon differentiation.

High Preservation of CpG Cytosine Methylation Patterns at Imprinted Gene Loci in Liver and Brain of Aged Mice

A gradual loss of the correct patterning of 5-methyl cytosine marks in gene promoter regions has been implicated in aging and age-related diseases, most notably cancer. While a number of studies have examined DNA methylation in aging, there is no consensus on the magnitude of the effects, particularly at imprinted loci. Imprinted genes are likely candidate to undergo age-related changes because of their demonstrated plasticity in utero, for example, in response to environmental cues. Here we quantitatively analyzed a total of 100 individual CpG sites in promoter regions of 11 imprinted and non-imprinted genes in liver and cerebral cortex of young and old mice using mass spectrometry. The results indicate a remarkably high preservation of methylation marks during the aging process in both organs. To test if increased genotoxic stress associated with premature aging would destabilize DNA methylation we analyzed two DNA repair defective mouse models showing a host of premature aging symptoms in liver and brain. However, also in these animals, at the end of their life span, we found a similarly high preservation of DNA methylation marks. We conclude that patterns of DNA methylation in gene promoters of imprinted genes are surprisingly stable over time in normal, postmitotic tissues and that the multiple documented changes with age are likely to involve exceptions to this pattern, possibly associated with specific cellular responses to age-related changes other than genotoxic stress.

DNA damage leads to progressive replicative decline but extends the life span of long-lived mutant animals

Human-nucleotide-excision repair (NER) deficiency leads to different developmental and segmental progeroid symptoms of which the pathogenesis is only partially understood. To understand the biological impact of accumulating spontaneous DNA damage, we studied the phenotypic consequences of DNA-repair deficiency in Caenorhabditis elegans. We find that DNA damage accumulation does not decrease the adult life span of post-mitotic tissue. Surprisingly, loss of functional ERCC-1/XPF even further extends the life span of long-lived daf-2 mutants, likely through an adaptive activation of stress signaling. Contrariwise, NER deficiency leads to a striking transgenerational decline in replicative capacity and viability of proliferating cells. DNA damage accumulation induces severe, stochastic impairment of development and growth, which is most pronounced in NER mutants that are also impaired in their response to ionizing radiation and inter-strand crosslinks. These results suggest that multiple DNA-repair pathways can protect against replicative decline and indicate that there might be a direct link between the severity of symptoms and the level of DNA-repair deficiency in patients.

‘From Death, Lead Me to Immortality’ – Mantra of Ageing Skeletal Muscle

Skeletal muscle is a post-mitotic tissue maintained by repair and regeneration through a population of stem cell-like satellite cells. Following muscle injury, satellite cell proliferation is mediated by local signals ensuring sufficient progeny for tissue repair. Age–related changes in satellite cells as well as to the local and systemic environment potentially impact on the capacity of satellite cells to generate sufficient progeny in an ageing organism resulting in diminished regeneration. ‘Rejuvenation’ of satellite cell progeny and regenerative capacity by environmental stimuli effectors suggest that a subset of age-dependent satellite cell changes may be reversible. Epigenetic regulation of satellite stem cells that include DNA methylation and histone modifications which regulate gene expression are potential mechanisms for such reversible changes and have been shown to control organismal longevity. The area of health and ageing that is likely to benefit soonest from advances in the biology of adult stem cells is the emerging field of regenerative medicine. Further studies are needed to elucidate the mechanisms by which epigenetic modifications regulate satellite stem cell function and will require an increased understanding of stem-cell biology, the environment of the aged tissue and the interaction between the two.