Identification of cultivation-independent markers of human endothelial cell senescence in vitro

Inner mitochondrial membrane structure and fusion dynamics are altered in senescent human iPSC-derived and primary rat cardiomyocytes

Dysfunction of the aging heart is a major cause of death in the human population. Amongst other tasks, mitochondria are pivotal to supply the working heart with ATP. The mitochondrial inner membrane (IMM) ultrastructure is tailored to meet these demands and to provide nano-compartments for specific tasks. Thus, function and morphology are closely coupled. Senescent cardiomyocytes from the mouse heart display alterations of the inner mitochondrial membrane. To study the relation between inner mitochondrial membrane architecture, dynamics and function is hardly possible in living organisms. Here, we present two cardiomyocyte senescence cell models that allow in cellular studies of mitochondrial performance. We show that doxorubicin treatment transforms human iPSC-derived cardiomyocytes and rat neonatal cardiomyocytes in an aged phenotype. The treated cardiomyocytes display double-strand breaks in the nDNA, have β-galactosidase activity, possess enlarged nuclei, and show p21 upregulation. Most importantly, they also display a compromised inner mitochondrial structure. This prompted us to test whether the dynamics of the inner membrane was also altered. We found that the exchange of IMM components after organelle fusion was faster in doxorubicin-treated cells than in control cells, with no change in mitochondrial fusion dynamics at the meso-scale. Such altered IMM morphology and dynamics may have important implications for local OXPHOS protein organization, exchange of damaged components, and eventually the mitochondrial bioenergetics function of the aged cardiomyocyte.

Mechanical Fingerprint of Senescence in Endothelial Cells

Endothelial senescence entails alterations of the healthy cell phenotype, which accumulate over time and contribute to cardiovascular disease. Mechanical aspects regulating cell adhesion, force generation, and the response to flow contribute to the senescence-associated drift; however, they remain largely unexplored. Here, we exploit force microscopy to resolve variations of the cell anchoring to the substrate and the tractions generated upon aging in the nanonewton (nN) range. Senescent endothelial cells display a multifold increase in the levels of basal adhesion and force generation supported by mature and strong focal adhesions. The enhanced mechanical interaction with the substrate yields static endothelial monolayers that polarize in response to flow but fail the process of coordinated cell shape remodeling and reorientation. The emerging picture indicates that senescence reinforces the local cell interaction with the substrate and may therefore prevent endothelial denudation; however, it compromises the ability to functionally adapt to the local hemodynamic conditions.

Do senescence markers correlate in vitro and in situ within individual human donors?

Little is known on how well senescence markers in vitro and in situ correlate within individual donors. We studied correlations between the same and different in vitro markers. Furthermore, we tested correlations between in vitro markers with in situ p16INK4a positivity.

From 100 donors (20-91 years), cultured dermal fibroblasts were assessed for reactive oxygen species (ROS), telomere-associated foci (TAF), p16INK4a and senescence-associated β-gal (SAβ-gal), with/ without 0.6 µM rotenone for 3 days (short-term). In fibroblasts from 40 donors, telomere shortening, ROS and SAβ-gal were additionally assessed, with/ without 20 nM rotenone for 7 weeks (long-term). In skin from 52 donors, the number of p16INK4a positive dermal cells was assessed in situ.

More than half of the correlations of the same senescence markers in vitro between duplicate experiments and between short-term versus long-term experiments were significant. Half of the different senescence marker correlations were significant within the short-term and within the long-term experiments. The different senescence markers in vitro were not significantly correlated intra-individually with in situ p16INK4a positivity.
In conclusion, the same and different senescence markers are frequently correlated significantly within and between in vitro experiments, but in vitro senescence markers are not correlated with p16INK4a positivity in situ.

Age-related dysfunction of the autophago-lysosomal pathway in human endothelial cells

Senescent cells, which are cells in a post-proliferative state, show an increased number of dysfunctional mitochondria and oxidatively damaged and aggregated proteins. The mitochondrial-lysosomal axis theory of aging proposes that the autophago-lysosomal system is unable to cope with the rising amount of damaged organelles and proteins. We used human umbilical vein endothelial cells (HUVEC) as in vitro model system to determine which part/s of the autophago-lysosomal pathway become deficient by aging. Senescent HUVEC contained a much larger population of autophagosomes and lysosomes compared to young cells. Transcriptome analysis comparing young and old cells demonstrated several age-related changes of autophagy gene expression. One reason for the observed increase of autophagosomes was an impairment of the autophagic flux in senescent cells due to reduced V-ATPase activity required for acidification of the lysosomes and thus functionality of lysosomal hydrolases. The hypothesis that reduced mitochondrial ATP production underlies low V-ATPase activity was supported by addition of exogenous ATP. This procedure rescued the lysosomal acidification and restored the autophagic flux. Thus, we propose impaired lysosomal acidification due to ATP shortage which may result from mitochondrial dysfunction as a mechanism underlying the accumulation of dysfunctional cellular constituents during aging.

Exosomes from hyperglycemia-stimulated vascular endothelial cells contain versican that regulate calcification/senescence in vascular smooth muscle cells

Background

To determine whether and how exosomes from human umbilical vein endothelial cells (HUVEC-Exos) regulates vascular smooth muscle cells (VSMCs) calcification/senescence in high glucose condition.

Methods

HUVEC-Exos were isolated from normal glucose (NG) and high glucose (HG) stimulated HUVECs (NG/HG-HUVEC-Exos) by super speed centrifugation. HUVEC-Exos were identified by transmission electron microscopy and Western blot of CD63. Protein profile in HUVEC-Exos was examined to screen the candidate molecules that mediate HUVEC-Exos function. VSMCs were incubated with HUVEC-Exos. A series of functional assays in vitro were performed to assess the effects of HUVEC-Exos on the calcification/senescence of VSMCs. The role of the candidate protein in HUVEC-Exos-induced VSMCs dysfunction was assessed.

Results

Exosomes isolated from HG-HUVEC-Exos induced calcification/senescence in VSMCs as assessed by Alizarin Red Staining, senescence-associated β-galactosidase (SA-β-gal) staining, and the expression of ALP and p21. HG-HUVEC-Exos significantly increased LDH activity, as well as the product of lipid peroxidation (MDA content), and decreased oxidative stress marker activity, as compared with NG-HUVEC-Exos. Moreover, mechanism studies showed that mitochondrial membrane potential and the expression levels of mitochondrial function related protein HADHA and Cox-4 were significantly decreased in HG-HUVEC-Exos compared to controls. Proteomic analysis showed that HG-HUVEC-Exos consisted of higher level of versican (VCAN), as compared with NG-HUVEC-Exos. Observation under laser confocal microscopy revealed that most green fluorescence of VCAN could overlap with the red fluorescence came from mitochondria, indicating VCAN is mainly localized to the mitochondria of VSMCs. Knockdown of VCAN with siRNA in HUVECs, inhibited HG-HUVEC-Exos-induced mitochondrial dysfunction and calcification/senescence of VSMCs.

Conclusions

Our data indicate an intracellular role for VCAN in VSMCs. VCAN participates in hyperglycemia-induced calcification/senescence via modulation of mitochondrial function in VSMCs.

Electronic supplementary material

The online version of this article (10.1186/s13578-018-0263-x) contains supplementary material, which is available to authorized users.

AMPK/TSC2/mTOR pathway regulates replicative senescence of human vascular smooth muscle cells

Age-associated diseases, including vascular diseases, are on the rise with the increase in the aging population. However, the mechanisms of aging and age-associated vascular dysfunction remain to be fully elucidated. Replicative senescence of vascular smooth muscle cells (VSMCs) contributes to aging as well as age-associated vascular diseases. Rapamycin may delay aging-associated diseases via inhibition of the mammalian target of rapamycin (mTOR), but its role in VSMC aging has remained elusive. The present study investigated the involvement of mTOR signaling in replicative senescence of VSMCs. Replicative senescence was induced in human VSMCs by extended passages and identified by assessing the cell morphology, senescence-associated β-galactosidase activity, and p53 and p21 protein expression. Protein expression and phosphorylation were determined by western blot analysis. Significant senescence of VSMCs was observed in cells subjected to extended passaging (until passage 15). Significant decreases in adenosine monophosphate-activated protein kinase (AMPK)/tuberous sclerosis complex 2 (TSC2) phosphorylation, but significant increases in mTOR/ribosomal protein S6 kinase 1 (S6K1) phosphorylation, were observed in cells with replicative senescence compared with those in young cells. Pre-treatment of VSMCs with AMPK activator and mTOR inhibitor delayed replicative senescence and reversed changes in AMPKα, TSC2, mTOR and S6K1 phosphorylation in senescent VSMCs. The AMPK/TSC2/mTOR/S6K1 signaling axis was found to have an important role in regulating replicative senescence of human VSMCs.

Featured Article: Deterioration of visual function mediated by senescence-associated endoplasmic reticulum stress in inflammatory tie2-TNF mice

Stress-associated premature senescence plays a major role in retinal diseases. In this study, we investigated the relationship between endothelial dysfunction, endoplasmic reticulum (ER) stress, and cellular senescence in the development of retinal dysfunction. We tested the hypothesis that constant endothelial activation by transmembrane tumor necrosis factor-α (tmTNF-α) exacerbates age-induced visual deficits via senescence-mediated ER stress in this model. To address this, we employed a mouse model of chronic vascular activation using endothelial-specific TNF-α-expressing (tie2-TNF) mice at 5 and 10 months of age. Visual deficits were exhibited by tie2-TNF mice at both 5 months and 10 months of age, with the older mice showing statistically significant loss of visual acuity compared with tie2-TNF mice at age 5 months. The neural defects, as measured by electroretinogram (ERG), also followed a similar trend in an age-dependent fashion, with 10-month-old tie2-TNF mice showing the greatest decrease in "b" wave amplitude at 25 cd.s.m² compared with age-matched wildtype (WT) mice and five-month-old tie2-TNF mice. While gene and protein expression from the whole retinal extracts demonstrated increased inflammatory (Icam1, Ccl2), stress-associated premature senescence (p16, p21, p53), and ER stress (Grp78, p-Ire1α, Chop) markers in five-month-old tie2-TNF mice compared with five-month-old WT mice, a further increase was seen in 10-month-old tie2-TNF mice. Our data demonstrate that tie2-TNF mice exhibit age-associated increases in visual deficits, and these data suggest that inflammatory endothelial activation is at least partly at play. Given the correlation of increased premature senescence and ER stress in an age-dependent fashion, with the loss of visual functions and increased endothelial activation, our data suggest a possible self-enhanced loop of unfolded protein response pathways and senescence in propagating neurovascular defects in this model. Impact statement Vision loss in most retinal diseases affects the quality of life of working age adults. Using a novel animal model that displays constant endothelial activation by tmTNF-α, our results demonstrate exacerbated age-induced visual deficits via premature senescence-mediated ER stress. We have compared mice of 5 and 10 months of age, with highly relevant human equivalencies of approximately 35- and 50-year-old patients, representing mature adult and middle-aged subjects, respectively. Our studies suggest a possible role for a self-enhanced loop of ER stress pathways and senescence in the propagation of retinal neurovascular defects, under conditions of constant endothelial activation induced by tmTNF-α signaling.

Differentiation of Human Pluripotent Stem Cells into Functional Endothelial Cells in Scalable Suspension Culture

Endothelial cells (ECs) are involved in a variety of cellular responses. As multifunctional components of vascular structures, endothelial (progenitor) cells have been utilized in cellular therapies and are required as an important cellular component of engineered tissue constructs and in vitro disease models. Although primary ECs from different sources are readily isolated and expanded, cell quantity and quality in terms of functionality and karyotype stability is limited. ECs derived from human induced pluripotent stem cells (hiPSCs) represent an alternative and potentially superior cell source, but traditional culture approaches and 2D differentiation protocols hardly allow for production of large cell numbers. Aiming at the production of ECs, we have developed a robust approach for efficient endothelial differentiation of hiPSCs in scalable suspension culture. The established protocol results in relevant numbers of ECs for regenerative approaches and industrial applications that show in vitro proliferation capacity and a high degree of chromosomal stability.

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Efficient generation of hiPSC-derived ECs in scalable suspension culture

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High degree of chromosomal stability of hiPSC-ECs after in vitro expansion

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Generation of relevant numbers of hiPSC-ECs for regenerative approaches

Nox4 regulates the eNOS uncoupling process in aging endothelial cells

ROS and its associated signaling contribute to vascular aging-associated endothelial disturbance. Since the non-effective endothelial nitric oxide synthase (eNOS) coupling status is related to vascular aging-related phenotypes, eNOS coupled/uncoupled system signaling was studied in human umbilical vein endothelial cells (HUVEC). Nitric oxide (NO) and eNOS Ser1177 were significantly decreased, whereas O₂^- (superoxide anion radical) increased with passage number. In aging cells, NADPH oxidase 4 (Nox4), one of the main superoxide generating enzymes, and its associated protein disulfide isomerase (PDI) chaperone were highly activated, and the resultant ER redox imbalance leads to disturbance of protein folding capability, namely endoplasmic reticulum (ER) stress, ultimately inducing dissociation between HSP90 and IRE-1α or PERK, decreasing HSP90 stability and dissociating the binding of eNOS from the HSP90 and leading to eNOS uncoupling. Through chemical and Nox4 siRNA approaches, Nox4 and its linked ER stress were shown to mainly contribute to eNOS uncoupling and its associated signaling, suggesting that Nox4 and its related ER stress signaling are key signals of the aging process in endothelial cells.

Mitochondrial elongation in the macular RPE of aging monkeys, evidence of metabolic stress

PURPOSE

This study was conducted to determine whether mitochondria of the macular retinal pigment epithelium (RPE) change with age in rhesus monkeys (Macaca mulatta). Mitochondria are the main instigators of oxidative stress, which has often been considered to play a role in the pathogenesis of age-related macular degeneration (AMD). Any pathological changes in the mitochondria of aging macular RPE, the main target of AMD, would be a clue to the pathogenesis of this common retinal degeneration afflicting both monkey and man.

METHODS

Transmission electron microscopy was used to identify mitochondria and to determine their appearance, their density per unit area of RPE cytoplasm and their length. The eyes of seven monkeys, 1, 2, 6.5, 23, 26, 27 and 35 years of age, were studied. Measurements were kept separate for the basal, middle and apical third of each cell. The basal third of the macular RPE had many more mitochondria than the middle third, and the apical third was almost devoid of mitochondria.

RESULTS

Mitochondrial number decreased and length increased with age. The increase in length was associated with an unusual clustering of mitochondria into parallel arrays of elongated mitochondria, with their long axis orthogonal to the basal membrane of the cell, structures not described before in RPE.

CONCLUSIONS

Mitochondrial elongation is associated with metabolic and/or oxidative stress, which implies that age produces stress in macular RPE. The increased clustering of very elongated mitochondria suggests that pathological changes occur in mitochondrial organization with age. These changes support the hypothesis that age-related mitochondrial dysfunction plays a role in the pathogenesis of AMD.

Cellular responses to egg-oil (charismon©)

Egg-oil (Charismon©) is known for its beneficial action in wound healing and other skin irritancies and its antibacterial activity. The physiological basis for these actions has been investigated using cells in culture: HaCaT-cells (immortalized human keratinocytes), human endothelial cells in culture (HUVEC), peripheral blood mononuclear lymphocytes (PBML) and a full thickness human skin model (FTSM). Emphasis was on the influence of egg-oil on cell migration and IL-8 production in HaCaT cells, respiration, mitochondrial membrane potential, reactive oxygen (ROS) production and proliferation in HUVEC and HaCaT cells, cytokine and interleukin production in PBML and UV-light induced damage of FTSM. IL-8 production by HaCaT cells is stimulated by egg-oil whilst in phythemagglutin in-activated PBMLs production of the interleukins IL-2, IL-6, IL-10 and IFN-γ and TFN-α is reduced. ROS-production after H(2)O(2) stimulation first is enhanced but later on reduced. Respiration becomes activated due to partial uncoupling of the mitochondrial respiratory chain and proliferation of HaCaT and HUVEC is reduced. Recovery of human epidermis cells in FTSM after UV-irradiation is strongly supported by egg-oil. These results support the view that egg-oil acts through reduction of inflammatory processes and ROS production. Both these processes are equally important in cellular aging as in healing of chronic wounds.

Endothelial cell senescence and age-related vascular diseases

Advanced age is an independent risk factor for ageing-related complex diseases, such as coronary artery disease, stroke, and hypertension, which are common but life threatening and related to the ageing-associated vascular dysfunction. On the other hand, patients with progeria syndromes suffer from serious atherosclerosis, suggesting that the impaired vascular functions may be critical to organismal ageing, or vice versa. However, it remains largely unknown how vascular cells, particularly endothelial cell, become senescent and how the senescence impairs the vascular functions and contributes to the age-related vascular diseases over time. Here, we review the recent progress on the characteristics of vascular ageing and endothelial cell senescence in vitro and in vivo, evaluate how genetic and environmental factors as well as autophagy and stem cell influence endothelial cell senescence and how the senescence contributes to the age-related vascular phenotypes, such as atherosclerosis and increased vascular stiffness, and explore the possibility whether we can delay the age-related vascular diseases through the control of vascular ageing.

Lypopolysaccharide downregulates the expression of selected phospholipase C genes in cultured endothelial cells

The signaling system of phosphoinositides (PI) is involved in a variety of cell and tissue functions, including membrane trafficking, ion channel activity, cell cycle, apoptosis, differentiation, and cell and tissue polarity. Recently, PI and related molecules, such as the phosphoinositide-specific phospholipases C (PI-PLCs), main players in PI signaling were supposed to be involved in inflammation. Besides the control of calcium levels, PI-PLCs contribute to the regulation of phosphatydil-inositol bisphosphate metabolism, crucial in cytoskeletal organization. The expression of PI-PLCs is strictly tissue specific and evidences suggest that it varies under different conditions, such as tumor progression or cell activation. In a previous study, we obtained a complete panel of expression of PI-PLC isoforms in human umbilical vein endothelial cells (HUVEC), a widely used experimental model for endothelial cells. In the present study, we analyzed the mRNA concentration of PI-PLCs in lipopolysaccharide (LPS)-treated HUVEC by using the multiliquid bioanalyzer methodology after 3, 6, 24, 48, and 72 h from LPS administration. Marked differences in the expression of most PI-PLC codifying genes were evident.

Do we age because we have mitochondria?

The process of aging remains a great riddle. Production of reactive oxygen species (ROS) by mitochondria is an inevitable by-product of respiration, which has led to a hypothesis proposing the oxidative impairment of mitochondrial components (e.g., mtDNA, proteins, lipids) that initiates a vicious cycle of dysfunctional respiratory complexes producing more ROS, which again impairs function. This does not exclude other processes acting in parallel or targets for ROS action in other organelles than mitochondria. Given that aging is defined as the process leading to death, the role of mitochondria-based impairments in those organ systems responsible for human death (e.g., the cardiovascular system, cerebral dysfunction, and cancer) is described within the context of "garbage" accumulation and increasing insulin resistance, type 2 diabetes, and glycation of proteins. Mitochondrial mass, fusion, and fission are important factors in coping with impaired function. Both biogenesis of mitochondria and their degradation are important regulatory mechanisms stimulated by physical exercise and contribute to healthy aging. The hypothesis of mitochondria-related aging should be revised to account for the limitations of the degradative capacity of the lysosomal system. The processes involved in mitochondria-based impairments are very similar across a large range of organisms. Therefore, studies on model organisms from yeast, fungi, nematodes, flies to vertebrates, and from cells to organisms also add considerably to the understanding of human aging.

MiR-146a as marker of senescence-associated pro-inflammatory status in cells involved in vascular remodelling

In order to identify new markers of vascular cell senescence with potential in vivo implications, primary cultured endothelial cells, including human umbilical vein endothelial cells (HUVECs), human aortic endothelial cells (HAECs), human coronary artery endothelial cells (HCAECs) and ex vivo circulating angiogenic cells (CACs), were analysed for microRNA (miR) expression. Among the 367 profiled miRs in HUVECs, miR-146a, miR-9, miR-204 and miR-367 showed the highest up-regulation in senescent cells. Their predicted target genes belong to nine common pathways, including Toll-like receptor signalling (TLR) that plays a pivotal role in inflammatory response, a key feature of senescence (inflammaging). MiR-146a was the most up-regulated miR in the validation analysis (>10-fold). Mimic and antagomir transfection confirmed TLR's IL-1 receptor-associated kinase (IRAK1) protein modulation in both young and senescent cells. Significant correlations were observed among miR-146a expression and β-galactosidase expression, telomere length and telomerase activity. MiR-146a hyper-expression was also validated in senescent HAECs (>4-fold) and HCAECs (>30-fold). We recently showed that CACs from patients with chronic heart failure (CHF) presented a distinguishing feature of senescence. Therefore, we also included miR-146a expression determination in CACs from 37 CHF patients and 35 healthy control subjects (CTR) for this study. Interestingly, a 1,000-fold increased expression of miR-146a was observed in CACs of CHF patients compared to CTR, along with decreased expression of IRAK1 protein. Moreover, significant correlations among miR-146a expression, telomere length and telomerase activity were observed. Overall, our findings indicate that miR-146a is a marker of a senescence-associated pro-inflammatory status in vascular remodelling cells.

Endothelial cell senescence is associated with disrupted cell-cell junctions and increased monolayer permeability

Background

Cellular senescence is associated with cellular dysfunction and has been shown to occur in vivo in age-related cardiovascular diseases such as atherosclerosis. Atherogenesis is accompanied by intimal accumulation of LDL and increased extravasation of monocytes towards accumulated and oxidized LDL, suggesting an affected barrier function of vascular endothelial cells. Our objective was to study the effect of cellular senescence on the barrier function of non-senescent endothelial cells.

Methods

Human umbilical vein endothelial cells were cultured until senescence. Senescent cells were compared with non-senescent cells and with co-cultures of non-senescent and senescent cells. Adherens junctions and tight junctions were studied. To assess the barrier function of various monolayers, assays to measure permeability for Lucifer Yellow (LY) and horseradish peroxidase (PO) were performed.

Results

The barrier function of monolayers comprising of senescent cells was compromised and coincided with a change in the distribution of junction proteins and a down-regulation of occludin and claudin-5 expression. Furthermore, a decreased expression of occludin and claudin-5 was observed in co-cultures of non-senescent and senescent cells, not only between senescent cells but also along the entire periphery of non-senescent cells lining a senescent cell.

Conclusions

Our findings show that the presence of senescent endothelial cells in a non-senescent monolayer disrupts tight junction morphology of surrounding young cells and increases the permeability of the monolayer for LY and PO.

Autophagy proteins LC3B, ATG5 and ATG12 participate in quality control after mitochondrial damage and influence lifespan

Mitochondrial health is maintained by the quality control mechanisms of mitochondrial dynamics (fission and fusion) and mitophagy. Decline of these processes is thought to contribute to aging and neurodegenerative diseases. To investigate the role of mitochondrial quality control in aging on the cellular level, human umbilical vein endothelial cells (HUVEC) were subjected to mitochondria-targeted damage by combining staining of mitochondria and irradiation. This treatment induced a short boost of reactive oxygen species, which resulted in transient fragmentation of mitochondria followed by mitophagy, while mitochondrial dynamics were impaired. Furthermore, targeted mitochondrial damage upregulated autophagy factors LC3B, ATG5 and ATG12. Consequently these proteins were overexpressed in HUVEC as an in vitro aging model, which significantly enhanced the replicative life span up to 150% and the number of population doublings up to 200%, whereas overexpression of LAMP-1 did not alter the life span. Overexpression of LC3B, ATG5 and ATG12 resulted in an improved mitochondrial membrane potential, enhanced ATP production and generated anti-apoptotic effects, while ROS levels remained unchanged and the amount of oxidized proteins increased. Taken together, these data relate LC3B, ATG5 and ATG12 to mitochondrial quality control after oxidative damage, and to cellular longevity.

Mitochondrial dysfunction in long-term neuronal cultures mimics changes with aging

Background

Aging is a highly complex process that affects various tissues and systems in the body. Senescent changes are relatively more prevalent and severe in the postmitotic cells. Mitochondria play an important role in the aging process. Recently, cell cultures have been widely used as an in vitro model to study aging. The present study was designed to investigate mitochondrial dysfunction associated with aging in a long-term cell culture system.

Material/Methods

Rat hippocampal neurons were maintained in culture in serum-free medium for 30 days in vitro (DIV). The morphology and development of hippocampal neurons was observed by phase contrast microscope. The levels of cellular senescence were evaluated by cytochemical staining of senescence-associated β-galactosidase (SA-β-Gal) at DIV 5, 10, 15, 20, 25 and 30. In addition, we investigated the changes in mitochondrial membrane potential (Δψm) and intracellular reactive oxygen species (ROS) generation of hippocampal neurons by flow cytometry at different ages.

Results

The proportion of the senescent cells steadily increased with age in neuron cultures. Δψm decreased gradually with age in long-term culture, while ROS generation increased.

Conclusions

This study indicates an age-related decrease in mitochondrial function in long-term hippocampal neuronal culture and suggests that DIV 25 neurons could possibly serve as a platform for the future study of anti-aging from the perspective of mitochondrial function.

L-arginine attenuates high glucose-accelerated senescence in human umbilical vein endothelial cells

OBJECTIVE

Endothelial dysfunction is a key event in the onset and progression of atherosclerosis associated with diabetes. Increasing cell senescence may lead to endothelial dysfunction and contribute to vascular complications. Therefore, we aimed to elucidate the possible role and mechanism of L-arginine in preventing cell senescence induced by high glucose.

METHODS

HUVECs were respectively cultured under normal control glucose (5.5mM), high glucose (33mM), co-incubation with L-arginine (800microM)and high glucose, and senescence was identified by beta-galactosidase staining, change of cell cycle and telomerase activity. Akt and eNOS activity was analyzed by western blot.

RESULTS

High glucose significantly increased number of beta-galactosidase-positive stained cells, inhibited telomerase activity, increased proportion of cells in the G(0)/G(1) phase and reduced proportion in the S phase, and decreased NO synthesis. L-arginine significantly attenuated these senescent alterations. Furthermore, high glucose induced a decrease in Akt and eNOS activity, and L-arginine prevented the decrease in activity. The PI3K inhibitor LY294002 or eNOS inhibitor L-NAME attenuated anti-senescence effect of L-arginine.

CONCLUSION

L-arginine may have an anti-senescence effect via the PI3K/Akt pathway in HUVECs exposed to high glucose and it might be a therapeutic agent for diabetic vascular complications.

Do UCP2 and mild uncoupling improve longevity?

Mild uncoupling of mitochondrial respiration is considered to prolong life span of organisms by reducing the production of reactive oxygen species (ROS). Experimental evidence against this hypothesis has been brought forward by premature senescence in cell cultures treated with uncouplers. Exposing HUVEC to a mixture of nutritionally important fatty acids (oil extract of chicken yolk) mild uncoupling with "naturally acting substances" was performed. This treatment also resulted in premature senescence although ROS production did not increase. Fatty acids activate uncoupling proteins (UCP) in the inner mitochondrial membrane. UCP2 expression proved to be sensitive to the presence of fatty acids but remains unchanged during the ageing process. UCP3 expression in senescent HUVEC and avUCP expression in senescent CEF were considerably less than in young cultures. No indication for protonophoric reduction of mitochondrial membrane potential was found in UCP2 overexpressing HeLa cells and only little in HUVEC. ROS levels increased instead of being reduced in these cells. Stable transfection with UCP2-GFP was possible only in chick embryo fibroblasts and HeLa cells and resulted in decreased proliferation. Stable transfection of HUVEC with UCP2-GFP resulted in death of cultures within one or two weeks. The reason for this behaviour most probably is apoptosis preceded by mitochondrial fragmentation and loss of membrane potential.

Decreased expression of Drp1 and Fis1 mediates mitochondrial elongation in senescent cells and enhances resistance to oxidative stress through PINK1

Mitochondria display different morphologies, depending on cell type and physiological situation. In many senescent cell types, an extensive elongation of mitochondria occurs, implying that the increase of mitochondrial length in senescence could have a functional role. To test this hypothesis, human endothelial cells (HUVECs) were aged in vitro. Young HUVECs had tubular mitochondria, whereas senescent cells were characterized by long interconnected mitochondria. The change in mitochondrial morphology was caused by downregulation of the expression of Fis1 and Drp1, two proteins regulating mitochondrial fission. Targeted photodamage of mitochondria induced the formation of reactive oxygen species (ROS), which triggered mitochondrial fragmentation and loss of membrane potential in young cells, whereas senescent cells proved to be resistant. Alterations of the Fis1 and Drp1 expression levels also influenced the expression of the putative serine-threonine kinase PINK1, which is associated with the PARK6 variant of Parkinson's disease. Downregulation of PINK1 or overexpression of a PINK1 mutant (G309D) increased the sensitivity against ROS in young cells. These results indicate that there is a Drp1- and Fis1-induced, and PINK1-mediated protection mechanism in senescent cells, which, when compromised, could contribute to the age-related progression of Parkinson's disease and arteriosclerosis.

Mitochondrial dynamics

Mitochondrial dynamics is a key feature for the interaction of mitochondria with other organelles within a cell and also for the maintenance of their own integrity. Four types of mitochondrial dynamics are discussed: Movement within a cell and interactions with the cytoskeleton, fusion and fission events which establish coherence within the chondriome, the dynamic behavior of cristae and their components, and finally, formation and disintegration of mitochondria (mitophagy). Due to these essential functions, disturbed mitochondrial dynamics are inevitably connected to a variety of diseases. Localized ATP gradients, local control of calcium-based messaging, production of reactive oxygen species, and involvement of other metabolic chains, that is, lipid and steroid synthesis, underline that physiology not only results from biochemical reactions but, in addition, resides on the appropriate morphology and topography. These events and their molecular basis have been established recently and are the topic of this review.

Aging of different avian cultured cells: lack of ROS-induced damage and quality control mechanisms

Elevated reactive oxygen species (ROS) levels have been observed in mammals during aging, implying an important role of ROS in the aging process. Most bird species are known to live longer and to contain lower ROS levels than mammals of the same body weight. The influence of ROS on the aging process of birds has been investigated using pigeon embryonic fibroblasts (PEF) and chicken embryonic fibroblasts (CEF). ROS levels in young avian cells were much lower than in human cells. When cultivated till replicative senescence, PEF proliferated about one-third longer compared to CEF. However, both senescent avian cell populations showed no increased ROS levels or accumulation of ROS-induced damage on the mtDNA or protein level. The investigation for quality control (QC) mechanisms revealed that the autophagosomal/lysosomal pathway was not downregulated in old avian cells and stable overexpression of the autophagy protein ATG5 improved mitochondrial fitness, enhanced the resistance against oxidative stress and prolonged the life span of CEF. Oxidative stress-mediated apoptosis induced a dose-dependent cell proliferation in CEF as well as in PEF. Taken together, our data indicate that autophagy and compensatory proliferation act as QC mechanisms, while ROS did not influence the aging process in avian cells.

Polyploidy impairs human aortic endothelial cell function and is prevented by nicotinamide phosphoribosyltransferase

Polyploid endothelial cells are found in aged and atherosclerotic arteries. However, whether increased chromosome content has an impact on endothelial cell function is unknown. We show here that human aortic endothelial cells become tetraploid as they approach replicative senescence. Furthermore, accumulation of tetraploid endothelial cells was accelerated during growth in high glucose. Interestingly, induction of polyploidy was completely prevented by modest overexpression of the NAD+ regenerating enzyme, nicotinamide phosphoribosyltransferase (Nampt). To determine the impact of polyploidy on endothelial cell function, independent of replicative senescence, we induced tetraploidy using the spindle poison, nocodazole. Global gene expression analyses of tetraploid endothelial cells revealed a dysfunctional phenotype characterized by a cell cycle arrest profile (decreased CCNE2/A2, RBL1, BUB1B; increased CDKN1A) and increased expression of genes involved in inflammation (IL32, TNFRSF21/10C, PTGS1) and extracellular matrix remodeling (COL5A1, FN1, MMP10/14). The protection from polyploidy conferred by Nampt was not associated with enhanced poly(ADP-ribose) polymerase-1 or sirtuin (SIRT) 2 activity, but with increased SIRT1 activity, which reduced cellular reactive oxygen species and the associated oxidative stress stimulus for the induction of polyploidy. We conclude that human aortic endothelial cells are prone to chromosome duplication that, in and of itself, can induce characteristics of endothelial dysfunction. Moreover, the emergence of polyploid endothelial cells during replicative aging and glucose overload can be prevented by optimizing the Nampt-SIRT1 axis.

The NADPH oxidase Nox4 restricts the replicative lifespan of human endothelial cells

The free radical theory of aging proposes that ROS (reactive oxygen species) are major driving forces of aging, and are also critically involved in cellular senescence. Besides the mitochondrial respiratory chain, alternative sources of ROS have been described that might contribute to cellular senescence. Noxs (NADPH oxidases) are well-known sources of superoxide, which contribute to the antimicrobial capabilities of macrophages, a process involving the prototypical member of the family referred to as Nox2. However, in recent years non-phagocytic homologues of Nox2 have been identified that are involved in processes other than the host defence. Superoxide anions produced by these enzymes are believed to play a major role in signalling by MAPKs (mitogen-activated protein kinases) and stress-activated kinases, but could also contribute to cellular senescence, which is known to involve oxygen radicals. In HUVECs (human umbilical vein endothelial cells), Nox4 is predominantly expressed, but its role in replicative senescence of HUVECs remains to be elucidated. Using shRNA (small-hairpin RNA)-mediated knockdown of Nox4, implicating lentiviral vectors, we addressed the question of whether lifelong depletion of Nox4 in HUVECs would influence the senescent phenotype. We found a significant extension of the replicative lifespan of HUVECs upon knockdown of Nox4. Surprisingly, mean telomere length was significantly reduced in Nox4-depleted cells. Nox4 depletion had no discernable influence on the activity of MAPKs and stress-activated kinases, but reduced the degree of oxidative DNA damage. These results suggest that Nox4 activity increases oxidative damage in HUVECs, leading to loss of replicative potential, which is at least partly independent of telomere attrition.

Nicotinamide phosphoribosyltransferase imparts human endothelial cells with extended replicative lifespan and enhanced angiogenic capacity in a high glucose environment

Endothelial dysfunction is a characteristic of aging-related vascular disease and is worsened during diabetes. High glucose can impair endothelial cell (EC) function through cellular accumulation of reactive oxygen species, an insult that can also limit replicative lifespan. Nicotinamide phosphoribosyltransferase (Nampt), also known as PBEF and visfatin, is rate-limiting for NAD+ salvage from nicotinamide and confers resistance to oxidative stress via SIRT1. We therefore sought to determine if Nampt expression could resist the detrimental effects of high glucose and confer a survival advantage to human vascular EC in this pathologic environment. Human aortic EC were infected with retrovirus encoding eGFP or eGFP-Nampt, and FACS-selected to yield populations with similar, modest transgene expression. Using a chronic glucose exposure model we tracked EC populations to senescence, assessed cellular metabolism, and determined in vitro angiogenic function. Overexpression of Nampt increased proliferation and extended replicative lifespan, and did so preferentially during glucose overload. Nampt expression delayed markers of senescence and limited reactive oxygen species accumulation in high glucose through a modest increase in aerobic glycolysis. Furthermore, tube networks formed by Nampt-overexpressing EC were more extensive and glucose-resistant, in accordance with SIRT1-mediated repression of the anti-angiogenic transcription factor, FoxO1. We conclude that Nampt enables proliferating human EC to resist the oxidative stress of aging and of high glucose, and to productively use excess glucose to support replicative longevity and angiogenic activity. Enhancing endothelial Nampt activity may thus be beneficial in scenarios requiring EC-based vascular repair and regeneration during aging and hyperglycemia, such as atherosclerosis and diabetes-related vascular disease.

Insulin-like growth factor-induced signals activate mitochondrial respiration

From experiments with lower eukaryotes it is known that the metabolic rate and also the rate of aging are tightly controlled by the insulin-like growth factor (IGF)/insulin signal transduction pathway. The mitochondrial theory of aging implies that an increased metabolic rate leads to increased mitochondrial activity; increased production of reactive oxygen species due to these alterations would speed up the aging process. To address the question if mitochondrial activity is influenced by insulin/IGF signaling, we have established an experimental system to determine the influence of IGF-I-dependent signaling on mitochondrial function. We used DU145 prostate cancer cells, known for the intact IGF signal transduction pathway, to address the influence of IGF receptor activation on mitochondrial function by high-resolution respirometry. These experiments revealed that indeed mitochondrial function is regulated by IGF signaling, and up-regulation of respiration seems to require phosphoinositide 3-kinase/AKT signaling, but is independent of IGF effects on cell cycle progression. Collectively these data establish a regulatory cross-talk between insulin/IGF signal transduction and mitochondrial function, two major pathways implicated in controlling the rate of aging.

Short- and long-term alterations of mitochondrial morphology, dynamics and mtDNA after transient oxidative stress

Cells are exposed during their life span to fluctuating levels of reactive oxygen species (ROS). To investigate the effects of a single ROS boost in vitro, human endothelial cells (HUVEC) were treated with one short-term dose of hydrogen peroxide. This treatment resulted in a short, dose-dependent ROS peak that caused transient changes in the mitochondrial morphology and fine structure, in the frequency of mitochondrial fission and fusion and in the mRNA levels of distinct fission and fusion factors. Treatment with a higher dose induced prolonged mtDNA damage; these cells exhibited a significantly shortened replicative lifespan, indicating dose-dependent effects of oxidative stress on mitochondria.

Insights into post-translational processing of beta-galactosidase in an animal model resembling late infantile human G-gangliosidosis

G_M1-gangliosidosis is a lysosomal storage disorder caused by a deficiency of ß-galactosidase activity. Human GM1-gangliosidosis has been classified into three forms according to the age of clinical onset and specific biochemical parameters. In the present study, a canine model for type II late infantile human GM1-gangliosidosis was investigated ‘in vitro’ in detail. For a better understanding of the molecular pathogenesis underlying G_M1-gangliosidosis the study focused on the analysis of the molecular events and subsequent intracellular protein trafficking of β-galactosidase. In the canine model the genetic defect results in exclusion or inclusion of exon 15 in the mRNA transcripts and to translation of two mutant precursor proteins. Intracellular localization, processing and enzymatic activity of these mutant proteins were investigated. The obtained results suggested that the β-galactosidase C-terminus encoded by exons 15 and 16 is necessary for correct C-terminal proteolytic processing and enzyme activity but does not affect the correct routing to the lysosomes. Both mutant protein precursors are enzymatically inactive, but are transported to the lysosomes clearly indicating that the amino acid sequences encoded by exons 15 and 16 are necessary for correct folding and association with protective protein/cathepsin A, whereas the routing to the lysosomes is not influenced. Thus, the investigated canine model is an appropriate animal model for the human late infantile form and represents a versatile system to test gene therapeutic approaches for human and canine G_M1-gangliosidosis.