Progeroid Phenotypes Research Articles

P44, the ‘longevity‐assurance’ isoform of P53, regulates tau phosphorylation and is activated in an age‐dependent fashion

p44 is a short isoform of p53 with ‘longevity-assurance’ activity. Overexpression of p44 in the mouse (p44+/+ transgenic mice) causes a progeroid phenotype that mimics an accelerated form of aging. The phenotype includes abnormal phosphorylation of the microtubule-binding protein tau, synaptic deficits, and cognitive decline. Genetic engineering demonstrated that the phosphorylation status of tau acts upstream of the synaptic deficits. Here, we provide evidence that p44 promotes the phosphorylation of tau in the mouse. Specifically, we show that p44 binds to the promoter of tau kinases Dyrk1A, GSK3β, Cdk5, p35, and p39 and activates their transcription. The upregulation of the above kinases is followed by increased phosphorylation of tau. Finally, we show that p44 is preferentially found in the nucleus and that its levels increase with age in the mouse brain. Taken together, these results suggest that an imbalance in the p53:p44 ratio might be involved with the altered tau metabolism that characterizes aging.

Understanding epigenetic changes in aging stem cells – a computational model approach

During aging, a decline in stem cell function is observed in many tissues. This decline is accompanied by complex changes of the chromatin structure among them changes in histone modifications and DNA methylation which both affect transcription of a tissue-specific subset of genes. A mechanistic understanding of these age-associated processes, their interrelations and environmental dependence is currently lacking. Here, we discuss related questions on the molecular, cellular, and population level. We combine an individual cell-based model of stem cell populations with a model of epigenetic regulation of transcription. The novel model enables to simulate age-related changes of trimethylation of lysine 4 at histone H3 and of DNA methylation. These changes entail expression changes of genes that induce age-related phenotypes (ARPs) of cells. We compare age-related changes of regulatory states in quiescent stem cells occupying a niche with those observed in proliferating cells. Moreover, we analyze the impact of the activity of the involved epigenetic modifiers on these changes. We find that epigenetic aging strongly affects stem cell heterogeneity and that homing at stem cell niches retards epigenetic aging. Our model provides a mechanistic explanation how increased stem cell proliferation can lead to progeroid phenotypes. Adapting our model to properties observed for aged hematopoietic stem cell (HSC) clones, we predict that the hematopoietic ARP activates young HSCs and thereby retards aging of the entire HSC population. In addition, our model suggests that the experimentally observed high interindividual variance in HSC numbers originates in a variance of histone methyltransferase activity.

Structural and functional basis of accelerated involution of the thymus in OXYS rats

Involution of the thymus is one of the most expressed indices of aging of the immune system. The mechanisms of involution of the thymus are relatively well studied, whereas the reasons and mechanisms of accelerated involution of the thymus are less known. We have previously reported that premature aging in OXYS rats is associated with accelerated involution of the thymus. The aim of the present study was to examine morphofunctional conditions of epithelial cells in the thymus of OXYS rats. Immunohistochemical study revealed that in 3.5-month-old OXYS rats, i.e. in the initial period of age-related involution, the network of epithelial cells in the thymus was reduced, and the volume and area of the surface of epithelial cells in the cortical substance was lower compared to control Wistar rats. Electron microscopy found strong changes in the ultrastructure of epithelial cells, such as shrinkage of the cytoplasm volume, a clear decrease in the size and number of secretory vacuoles, and multiple autophagosomes and phagolysosomes. Our data show that a possible mechanism of the reduction of the epithelial network in the thymus of OXYS rats is an enhancement of autophagy, probably related to specific mitochondria dysfunction in these rats. The fact of age-related retardation of autophagy in some tissues is well known. In spite to this, based on the data from OXYS rats, we suppose that long-term deviation of intensity of this process from the physiological level may be directed not only to its attenuation but also to its activation and thus results in degenerative modifications of organs and the formation of the progeroid phenotype of the body.

Chapter Five - Mouse Models and Aging: Longevity and Progeria

Aging is a complex, multifactorial process that is likely influenced by the activities of a range of biological pathways. Genetic approaches to identify genes modulating longevity have been highly successful and recent efforts have extended these studies to mammalian aging. A variety of genetic models have been reported to have enhanced lifespan and, similarly, many genetic interventions lead to progeroid phenotypes. Here, we detail and evaluate both sets of models, focusing on the insights they provide about the molecular processes modulating aging and the extent to which mutations conferring progeroid pathologies really phenocopy accelerated aging.

Decreased cell proliferation and higher oxidative stress in fibroblasts from Down Syndrome fetuses. Preliminary study

Down Syndrome is the most common chromosomal disease and is also known for its decreased incidence of solid tumors and its progeroid phenotype. Cellular and systemic oxidative stress has been considered as one of the Down Syndrome phenotype causes. We correlated, in a preliminary study, the fibroblast proliferation rate and different cell proliferation key regulators, like Rcan1 and the telomere length from Down Syndrome fetuses, with their oxidative stress profile and the Ribonucleic acid and protein expression of the main antioxidant enzymes together with their activity. Increased oxidized glutathione/glutathione ratio and high peroxide production were found in our cell model. These results correlated with a distorted antioxidant shield. The messenger RNA (SOD1) and protein levels of copper/zinc superoxide dismutase were increased together with a decreased mRNA expression and protein levels of glutathione peroxidase (GPx). As a consequence the [Cu/ZnSOD/(catalase+GPx)] activity ratio increases which explains the oxidative stress generated in the cell model. In addition, the expression of thioredoxin 1 and glutaredoxin 1 is decreased. The results obtained show a decreased antioxidant phenotype that correlates with increased levels of Regulator of calcineurin 1 and attrition of telomeres, both related to oxidative stress and cell cycle impairment. Our preliminary results may explain the proneness to a progeroid phenotype.

Cdkn1b overexpression in adult mice alters the balance between genome and tissue ageing

Insufficient cell proliferation has been suggested as a potential cause of age related tissue dysgenesis in mammals. However, genetic manipulation of cell cycle regulators in the germ lines of mice results in changes in animal size but not progeroid phenotypes. Here we increase levels of the cyclin dependent kinase inhibitor Cdkn1b (p27kip1) in adult mice through doxycycline inducible expression and show this results in reduced cell proliferation in multiple tissues. The mice undergo changes resembling aging even in the absence of an elevated DNA damage response or evidence of senescent cells suggesting an altered balance between genetic and tissue aging. In contrast, suppressing cell proliferation by doxycycline treatment of neonates retards growth, but the onset of degenerative changes is delayed during the period of reduced body mass. These results support the hypothesis that many of the most recognizable features of mammalian aging can result from an imbalance between cell production and the mass of tissue that must be maintained.

The many sides of CIN

Surveillance mechanisms, such as the spindle assembly checkpoint (SAC) have evolved to ensure chromosome segregation fidelity. When these quality control mechanisms fail, aneuploidy ensues. This condition of an abnormal chromosome number has long been associated with death, disease and developmental abnormalities. Many spontaneous abortions are found to harbor single chromosome aneuploidies, indicating that aneuploidy compromises viability when present throughout the organism. However, most solid cancers also exhibit multiple aneuploidies. As cancer is defined by unrestrained cell proliferation, this implies that aneuploidy can confer fitness advantages at the cellular level. Clearly, the effects of aneuploidy on cells and organisms are complex and we must develop cell and animal models to fully understand their impact. To directly characterize the consequences of chromosome mis-segregation and aneuploidy, collectively called chromosome instability (CIN), Baker and colleagues induced chromosome missegregation in mice by disrupting the spindle assembly checkpoint. By generating mice homozygous for a hypomorphic mutation in the checkpoint component BubR1, Baker and colleagues obtained viable animals in which chromosomes were continuously missegregated. As a result, approximately one-third of cells harbored one or more whole-chromosome aneuploidies. One may have expected some of these cells to have a proliferative advantage and for the mice to develop tumors. However, quite the opposite occurred. Cells senesced prematurely and mice exhibited a progeroid phenotype. BubR1 hypomorphs developed cataracts, skeletal abnormalities, and tissue atrophy at a young age and the median lifespan was reduced to six months. Correspondingly, the same group subsequently found that reducing chromosome mis-segregation by overexpressing BubR1 was sufficient to delay signs of aging and extend lifespan. A paper is interesting when it changes your perspective—when you put it down (or press apple-quit) and say, “Hmm, did not see that coming.” This paper did this for us. The studies by Baker and colleagues implicate CIN and aneuploidy in yet another phenomenon—aging. They suggest that the majority of aneuploid cells can survive in somatic tissues, but they become senescent rather than precipitate transformation. We must now understand how CIN and aneuploidy can be associated with the disparate phenotypes of limitless proliferation and accelerated senescence. Further investigation into the genetic and cellular context in which aneuploidy occurs should help to resolve this paradox.

The Exonuclease Activity of the Yeast Mitochondrial DNA Polymerase γ Suppresses Mitochondrial DNA Deletions Between Short Direct Repeats in Saccharomyces cerevisiae

The importance of mitochondrial DNA (mtDNA) deletions in the progeroid phenotype of exonuclease-deficient DNA polymerase γ mice has been intensely debated. We show that disruption of Mip1 exonuclease activity increases mtDNA deletions 160-fold, whereas disease-associated polymerase variants were mostly unaffected, suggesting that exonuclease activity is vital to avoid deletions during mtDNA replication.

P21 Both Attenuates and Drives Senescence and Aging in BubR1 Progeroid Mice

BubR1 insufficiency occurs with natural aging and induces progeroid phenotypes in both mice and children with mosaic variegated aneuploidy syndrome. In response to BubR1 insufficiency, skeletal muscle, fat, and lens tissue engage p19(Arf) to attenuate senescence and age-related deterioration. Here, we address how p19(Arf) exerts this caretaker role using BubR1 progeroid mice lacking p53 or its transcriptional target p21. We show that p53 delays functional decline of skeletal muscle and fat in a p21-dependent fashion by inhibiting p16(Ink4a)-mediated senescence of progenitor cells. Strikingly, p53 also attenuates the formation of cataractous lenses, but here its antiaging effect is p21 independent, as we found p21 to promote senescence of lens epithelial cells and cataract formation. Together, these results demonstrate that p53 counteracts tissue destruction in response to BubR1 insufficiency through diverse mechanisms and uncover a causal link between senescence of the progenitor cell compartment and age-related dysfunction.

The role of mitochondria in aging

Over the last decade, accumulating evidence has suggested a causative link between mitochondrial dysfunction and major phenotypes associated with aging. Somatic mitochondrial DNA (mtDNA) mutations and respiratory chain dysfunction accompany normal aging, but the first direct experimental evidence that increased mtDNA mutation levels contribute to progeroid phenotypes came from the mtDNA mutator mouse. Recent evidence suggests that increases in aging-associated mtDNA mutations are not caused by damage accumulation, but rather are due to clonal expansion of mtDNA replication errors that occur during development. Here we discuss the caveats of the traditional mitochondrial free radical theory of aging and highlight other possible mechanisms, including insulin/IGF-1 signaling (IIS) and the target of rapamycin pathways, that underlie the central role of mitochondria in the aging process.

The epidemiology of premature aging and associated comorbidities

Hutchinson–Gilford Progeria Syndrome and Werner syndrome, also known as childhood- and adulthood-progeria, respectively, represent two of the best characterized human progeroid diseases with clinical features mimicking physiological aging at an early age. The discovery of their genetic basis has led to the identification of several gene mutations leading to a spectrum of progeroid phenotypes ranging from moderate and mild–severe to very aggressive forms. In parallel, the creation of disease registers and databases provided available data for the design of relatively large-scale epidemiological studies, thereby allowing a better understanding of the nature and frequency of the premature aging-associated signs and symptoms. The aim of this article is to review the most recent findings concerning the epidemiology of premature aging disorders, their genetic basis, and the most recent reports on the frequency of associated diseases.

Resveratrol Rescues SIRT1-Dependent Adult Stem Cell Decline and Alleviates Progeroid Features in Laminopathy-Based Progeria

Abnormal splicing of LMNA gene or aberrant processing of prelamin A results in progeroid syndrome. Here we show that lamin A interacts with and activates SIRT1. SIRT1 exhibits reduced association with nuclear matrix (NM) and decreased deacetylase activity in the presence of progerin or prelamin A,leading to rapid depletion of adult stem cells (ASCs) in Zmpste24(-/-) mice. Resveratrol enhances the binding between SIRT1 and A-type lamins to increases its deacetylase activity. Resveratrol treatment rescues ASC decline, slows down body weight loss, improves trabecular bone structure and mineral density, and significantly extends the life span in Zmpste24(-/-) mice. Our data demonstrate lamin A as an activator of SIRT1 and provide a mechanistic explanation for the activation of SIRT1 by resveratrol. The link between conserved SIRT1 longevity pathway and progeria suggests a stem cell-based and SIRT1 pathway-dependent therapeutic strategy for progeria.

NF-κB in premature aging

NF-κB in premature aging

A filtering strategy identifies FOXQ1 as a potential effector of lamin A dysfunction.

Small increases in the expression of wild-type prelamin A are sufficient to recapitulate the reduced cell proliferation and altered nuclear membrane morphology observed in cells expressing progerin, the mutant lamin A associated with progeria. We hypothesized that the manifestation of these phenotypes in cells expressing elevated levels of wild-type prelamin A or progerin is caused by the same molecular effectors, which play a central role in the onset of the progeroid phenotype. To experimentally test this hypothesis, we compared the transcriptomes of isogenic diploid fibroblasts expressing progerin or elevated levels of wild-type prelamin A with that of wild-type fibroblasts. We subsequently used the reversion towards normal of two phenotypes, reduced cell growth and dismorphic nuclei, by treatment with farnesyltransferase inhibitor (FTI) or overexpression of ZMPSTE24, as a filtering strategy to identify genes linked to the onset of these two phenotypes. Through this analysis we identified the gene encoding for the transcription factor FOXQ1, as a gene whose expression is induced in both cells expressing progerin and elevated levels of wild-type prelamin A, and subsequently reduced in both cell types upon conditions that ameliorate the phenotypes. We overexpressed FOXQ1 in normal fibroblasts and demonstrated that increased levels of this factor lead to the development of both features that were used in the filtering strategy. These findings suggest a potential link between this transcription factor and cell dysfunction induced by altered prelamin A metabolism.

Abstract 366: Sirtuin 6 Reduces Osteogenic Signaling in Aortic Valve Interstitial Cells and Vascular Smooth Muscle Cells

Cardiovascular calcification is often an active process involving re-differentiation of cells to an osteoblast-like phenotype, and is strongly associated with aging. Sirtuins are histone and protein deacetylases that play a critical role in a number of age-related diseases, and SIRT6-deficient mice have the most profound progeroid phenotype. The role of SIRT6 in the regulation of cardiovascular calcification, however, is poorly understood. We hypothesized that age-related reductions in sirtuins promote osteogenic differentiation in cells from aortic valve and aorta. We used aortic valve interstitial cells isolated from SIRT6-deficient mice, pigs, or humans, and vascular smooth muscle cells from mouse aorta to determine whether osteogenic differentiation of cardiovascular cells is regulated by SIRT6 levels. In line with our hypothesis, we found increased p-SMAD1/5/8 levels in cells from SIRT6-deficient aortic valve and aorta. Second, global SIRT inhibition with NAD+-depletion increased histone acetylation and protein levels of the osteogenic proteins Sp7 and Runx2. Third, inhibition of SIRTs enhanced the transcriptional responses of Sp7 and Runx2 when cells were treated with BMP2. Fourth, knockdown of SIRT6 in mouse and human aorta and aortic valve cells by RNA interference increased Sp7 expression. Finally, chronic pharmacological inhibition of SIRTs promoted calcified nodule formation and calcium deposition in cultured mouse/swine aortic valve cells. Collectively, our data suggest that reductions in SIRT6 levels favor amplification of pro-osteogenic signaling and osteogenic differentiation of cardiovascular cells and promote the formation of calcified nodules in vitro . These data also suggest that sirtuin re-activation may be a useful therapy to slow initiation and/or progression of cardiovascular calcification in vivo.

Bmi1 Is Down-Regulated in the Aging Brain and Displays Antioxidant and Protective Activities in Neurons

Aging increases the risk to develop several neurodegenerative diseases, although the underlying mechanisms are poorly understood. Inactivation of the Polycomb group gene Bmi1 in mice results in growth retardation, cerebellar degeneration, and development of a premature aging-like phenotype. This progeroid phenotype is characterized by formation of lens cataracts, apoptosis of cortical neurons, and increase of reactive oxygen species (ROS) concentrations, owing to p53-mediated repression of antioxidant response (AOR) genes. Herein we report that Bmi1 expression progressively declines in the neurons of aging mouse and human brains. In old brains, p53 accumulates at the promoter of AOR genes, correlating with a repressed chromatin state, down-regulation of AOR genes, and increased oxidative damages to lipids and DNA. Comparative gene expression analysis further revealed that aging brains display an up-regulation of the senescence-associated genes IL-6, p19Arf and p16Ink4a, along with the pro-apoptotic gene Noxa, as seen in Bmi1-null mice. Increasing Bmi1 expression in cortical neurons conferred robust protection against DNA damage-induced cell death or mitochondrial poisoning, and resulted in suppression of ROS through activation of AOR genes. These observations unveil that Bmi1 genetic deficiency recapitulates aspects of physiological brain aging and that Bmi1 over-expression is a potential therapeutic modality against neurodegeneration.

Somatic Progenitor Cell Vulnerability to Mitochondrial DNA Mutagenesis Underlies Progeroid Phenotypes in Polg Mutator Mice

Somatic stem cell (SSC) dysfunction is typical for different progeroid phenotypes in mice with genomic DNA repair defects. MtDNA mutagenesis in mice with defective Polg exonuclease activity also leads to progeroid symptoms, by an unknown mechanism. We found that Polg-Mutator mice had neural (NSC) and hematopoietic progenitor (HPC) dysfunction already from embryogenesis. NSC self-renewal was decreased in vitro, and quiescent NSC amounts were reduced in vivo. HPCs showed abnormal lineage differentiation leading to anemia and lymphopenia. N-acetyl-L-cysteine treatment rescued both NSC and HPC abnormalities, suggesting that subtle ROS/redox changes, induced by mtDNA mutagenesis, modulate SSC function. Our results show that mtDNA mutagenesis affected SSC function early but manifested as respiratory chain deficiency in nondividing tissues in old age. Deletor mice, having mtDNA deletions in postmitotic cells and no progeria, had normal SSCs. We propose that SSC compartment is sensitive to mtDNA mutagenesis, and that mitochondrial dysfunction in SSCs can underlie progeroid manifestations.

Effects of SIRT6 silencing on collagen metabolism in human dermal fibroblasts

SIRT6 (sirtuin 6) has been identified as an anti-aging protein, and SIRT6-deficient mice display numerous progeroid and aging-like phenotypes. However, there are no reports correlating skin aging with SIRT6. To explore how SIRT6 affects collagen metabolism and determine its role in skin aging, the levels of COL1A1 (type I collagen), COL3A1 (type III collagen) and TGFβ1 (transforming growth factor β1) mRNA expression were detected by RT-Q-PCR (real-time quantitative PCR), and MMP1 (matrix metalloproteinase 1) concentration in HDF (human dermal fibroblast) supernatants was determined by ELISA after blocking SIRT6 using siRNA (small interefering RNA). Compared with the control group, blocking SIRT6 significantly decreased the HC (hydroxyproline content). SIRT6 knockdown in HDFs inhibited the transcription of COL1A1 and prompted MMP1 secretion, but had no effect on COL3A1 and TGFβ1. Furthermore, NF-κB p65 (nuclear factor κB p65) protein in the nucleus was increased after blocking SIRT6. The results suggest that SIRT6 knockdown in HDFs influence the synthesis and degradation of collagen by hyperactive NF-κB signalling, which leads to a decrease in dermal collagen fibrils. SIRT6 may therefore play an important role in the process of skin anti-aging.

Post-natal myogenic and adipogenic developmental

A-type lamins are a major component of the nuclear lamina. Mutations in the LMNA gene, which encodes the A-type lamins A and C, cause a set of phenotypically diverse diseases collectively called laminopathies. While adult LMNA null mice show various symptoms typically associated with laminopathies, the effect of loss of lamin A/C on early post-natal development is poorly understood. Here we developed a novel LMNA null mouse (LMNAGT-/-) based on genetrap technology and analyzed its early post-natal development. We detect LMNA transcripts in heart, the outflow tract, dorsal aorta, liver and somites during early embryonic development. Loss of A-type lamins results in severe growth retardation and developmental defects of the heart, including impaired myocyte hypertrophy, skeletal muscle hypotrophy, decreased amounts of subcutaneous adipose tissue and impaired ex vivo adipogenic differentiation. These defects cause death at 2 to 3 weeks post partum associated with muscle weakness and metabolic complications, but without the occurrence of dilated cardiomyopathy or an obvious progeroid phenotype. Our results indicate that defective early post-natal development critically contributes to the disease phenotypes in adult laminopathies.

Broad segmental progeroid changes in short-lived Ercc1 −/Δ7 mice

Genome maintenance is considered a prime longevity assurance mechanism as apparent from many progeroid human syndromes that are caused by genome maintenance defects. The ERCC1 protein is involved in three genome maintenance systems: nucleotide excision repair, interstrand cross-link repair, and homologous recombination. Here we describe in-life and post-mortem observations for a hypomorphic Ercc1 variant, Ercc1−/Δ7, which is hemizygous for a single truncated Ercc1 allele, encoding a protein lacking the last seven amino acids. Ercc1−/Δ7 mice were much smaller and median life span was markedly reduced compared to wild-type siblings: 20 and 118 weeks, respectively. Multiple signs and symptoms of aging were found to occur at an accelerated rate in the Ercc1−/Δ7 mice as compared to wild-type controls, including a decline in weight of both whole body and various organs, numerous histopathological lesions, and immune parameters. Together they define a segmental progeroid phenotype of the Ercc1−/Δ7 mouse model.