Charakterisierung des Abbaus von Mitochondrien in primären humanen Zellen

Autophagie ist ein evolutionär stark konservierter Degradationsmechanismus für geschädigte Proteine bis hin zu ganzen Organellen eukaryotischer Zellen. Dabei umhüllt eine Doppelmembran, bisher unbekannten Ursprungs, das zu degradierende Material und bildet das Autophagosom. Dies fusioniert später mit Lysosomen, wodurch dessen Inhalt proteolytisch zersetzt und die Bestandteile der Zelle wieder zur Verfügung gestellt werden kann. In dieser Abeit wurde der Fokus auf den mitochondrialen Abbau über Autophagie (Mitophagie) und dessen Funktion als ein mitochondrialer Qualitätsmechanismus gesetzt. Als Zellmodell wurden primäre humane Endothelzellen der Nabelschnurvene (HUVEC) verwendet. Diese zeichenen sich durch einen Übergang von einer mitotischen, jungen in eine lange postmitotische, seneszente Phase während der Kultiverungszeit aus. Dabei durchlaufen sie einer zelluläre und mitochondriale Morphologieänderung. , wodurch sich die Möglichkeit bot , die Autophagie unter verschiedenen Parametern zu betrachten. So wird generell eine Abnahme des autophagosomalen / lysosomalen Weges mit dem Alter beschrieben und die Abhängigkeit der Mitophagie von der mitochondrialen Länge. Mitophagie ist unter normalen Kultivierungsbedingungen ein mikroskopisch selten zu beobachtender Vorgang. Daher wurde ein mitochondriales Schädigungsystem etabliert, welches die photosensibiliesierende Wirkung des Farbstoffs MitoTracker Red Cmx Ros (MTR) nutzt, um Mitochondrien gezielt oxidativ zu schädigen und die Mitophagie zu aktivieren. Mitotische HUVEC zeigten 2 h – 8 h nach oxidativer Schädigung eine mitochondriale Fragmentierung größtenteils begleitet von einem Verlust des Membranpotentials. Über einen Zeitraum von 72h-120h kam es zur Regeneration des mitochondrialen Netzwerks durch Neusynthese mitochondrialer Biomoleküle. Entgegen der rescue Hypothese konnten oxidativ geschädigte Mitochondrien nicht durch eine Fusion mit funktional intakten Mitochondrien gerettet werden und wurden über den autophagosomalen / lysosomalen Weg abgebaut, gekennzeichnet durch die Ubiquitin-Ligase Parkin vermittelte Markierung und finaler Kolokalisation mit den autophagosomalen und lysosomalen Markerproteinen LC3B und LAMP-2A. Auf mRNA- und Proteinebene zeigte sich in diesem Zeitraum eine erhöhte Expression autophagie-relevanter Gene (ATGs) ATG5, ATG12 und LC3B. Der Vergleich von mitotischen mit postmitotischen HUVEC nach oxidativer Schädigung wies zwei grundlegende Unterschiede auf. Zum einem behielten, in Gegensatz zu jungen Zellen, die Mitochondrien alter HUVEC ihre Morphologie und ihr Membranpotential bei. Diese erhöhte Widerstandfähigkeit gegenüber oxidativem Stress konnte auf die erhöhte Expression der mitochondrial lokalisierten Serin / Threonin Kinase PINK1 zurückgeführt werden, ein Schlüsselgen in Parkinson. Die PINK1-Transkription stand invers zu der Expression der mitochondrialen Teilungsfaktoren Fis1- und Drp1, welche in postmitotischen HUVEC stark vermindert war. Andererseits wiesen alte Zellen eine verminderte Degradationsfähigkeit geschädigter Mitochondrien auf. Dieser Umstand war durch eine verminderte lysosomale Azidität bedingt. Eine externe ATP-Zugabe förderte die Azidität der Lysosomen alter Zellen und die Fusion mit Autophagosomen, wodurch Mitochondrien und ihre geringere ATP-Produktion im Alter als ein Faktor der Autophagie ermittelt weden konnte. Die Autophagierate steht in Verbindung mit der Lebensspanne von Zellen bis hin zu ganzen Organismen. Durch die Überexpression autophagie-relevanter GFP-Fusions-Proteine ATG5, ATG12 und LC3B, welche nach oxidativer Schädigung in ihrer Expression verstärkt wurden, förderten die Mitophagie und wurden stabil in junge HUVEC exprimiert. Diese Überexpressionen bewirkten eine verbesserte mitochondriale Qualität, veranschaulicht durch ein erhöhtes Membranpotential und die ATP-Bereitstellung, einer besseren mtDNA Integrität und sie verlängerten die Lebensspanne signifikant, wobei die Produktion von reaktiven Sauerstoffspezien (ROS), entgegen der von Harman aufgestellten Alterungstheorie, keine Verminderung zeigte. Dennoch wiesen sie einen erhöhten Gehalt oxidativ modifizierter Proteine auf, welche letztendlich auf die erhöhten Autophagosomenanzahl zurückgeführt werden konnte, in denen höchstwahrscheinlich das oxidativ geschädigte Material gelagert wird. In dieser Arbeit kann gezeigt werden, dass Mitochondrien nach oxidativer Schädigung eine Teilung vollziehen und geschädigte Mitochondrien selektiv über Autophagie abgebaut werden. Dabei fungiert Mitophagie als ein mitochondrialer Qualitätmechanismus und steht unmittelbar mit der Lebensspanne in Verbindung

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

A New Link to Mitochondrial Impairment in Tauopathies

Tauopathies like the “frontotemporal dementia with Parkinsonism linked to chromosome 17” (FTDP-17) are characterized by an aberrant accumulation of intracellular neurofibrillary tangles composed of hyperphosphorylated tau. For FTDP-17, a pathogenic tau mutation P301L was identified. Impaired mitochondrial function including disturbed dynamics such as fission and fusion are most likely major pathomechanisms of most neurodegenerative diseases. However, very little is known if tau itself affects mitochondrial function and dynamics. We addressed this question using SY5Y cells stably overexpressing wild-type (wt) and P301L mutant tau. P301L overexpression resulted in a substantial complex I deficit accompanied by decreased ATP levels and increased susceptibility to oxidative stress. This was paralleled by pronounced changes in mitochondrial morphology, decreased fusion and fission rates accompanied by reduced expression of several fission and fusion factors like OPA-1 or DRP-1. In contrast, overexpression of wt tau exhibits protective effects on mitochondrial function and dynamics including enhanced complex I activity. Our findings clearly link tau bidirectional to mitochondrial function and dynamics, identifying a novel aspect of the physiological role of tau and the pathomechanism of tauopathies

Loss of PINK1 impairs stress-induced autophagy and cell survival

The mitochondrial kinase PINK1 and the ubiquitin ligase Parkin are participating in quality control after CCCP- or ROSinduced mitochondrial damage, and their dysfunction is associated with the development and progression of Parkinson’s disease. Furthermore, PINK1 expression is also induced by starvation indicating an additional role for PINK1 in stress response. Therefore, the effects of PINK1 deficiency on the autophago-lysosomal pathway during stress were investigated. Under trophic deprivation SH-SY5Y cells with stable PINK1 knockdown showed downregulation of key autophagic genes, including Beclin, LC3 and LAMP-2. In good agreement, protein levels of LC3-II and LAMP-2 but not of LAMP-1 were reduced in different cell model systems with PINK1 knockdown or knockout after addition of different stressors. This downregulation of autophagic factors caused increased apoptosis, which could be rescued by overexpression of LC3 or PINK1. Taken together, the PINK1-mediated reduction of autophagic key factors during stress resulted in increased cell death, thus defining an additional pathway that could contribute to the progression of Parkinson’s disease in patients with PINK1 mutations

Use of HuH6 and other humanderived hepatoma lines for the detection of genotoxins: a new hope for laboratory animals?

Cell lines which are currently used in genotoxicity tests lack enzymes which activate/detoxify mutagens. Therefore, rodent-derived liver preparations are used which reflect their metabolism in humans only partly; as a consequence misleading results are often obtained. Previous findings suggest that certain liver cell lines express phase I/II enzymes and detect promutagens without activation; however, their use is hampered by different shortcomings. The aim of this study was the identification of a suitable cell line. The sensitivity of twelve hepatic cell lines was investigated in single cell gel electrophoresis assays. Furthermore, characteristics of these lines were studied which are relevant for their use in genotoxicity assays (mitotic activity, p53 status, chromosome number, and stability). Three lines (HuH6, HCC1.2, and HepG2) detected representatives of five classes of promutagens, namely, IQ and PhIP (HAAs), B(a)P (PAH), NDMA (nitrosamine), and AFB1 (aflatoxin), and were sensitive towards reactive oxygen species (ROS). In contrast, the commercially available line HepaRG, postulated to be a surrogate for hepatocytes and an ideal tool for mutagenicity tests, did not detect IQ and was relatively insensitive towards ROS. All other lines failed to detect two or more compounds. HCC1.2 cells have a high and unstable chromosome number and mutated p53, these features distract from its use in routine screening. HepG2 was frequently employed in earlier studies, but pronounced inter-laboratory variations were observed. HuH6 was never used in genotoxicity experiments and is highly promising, it has a stable karyotype and we demonstrated that the results of genotoxicity experiments are reproducible.(VLID)473572

Stable PINK1 knockdown in SH-SY5Y cells.

<p>A) SH-SY5Y neuroblastoma cells were either cultivated in RPMI+10% FCS or starved in HBSS with or without addition of LY294002 or rapamycin. After 16 h PINK1 mRNA expression was determined by RT-qPCR and the PINK1 mRNA content of cells kept in RPMI+10% FCS was set as 1. Induction of autophagy by starvation or rapamycin resulted in an induction of PINK1 expression, comparable to the positive control LY294002; n = 4; * expression changes compared to the PINK1 expression in RPMI+10% FCS: LY294002: p<0.01; rapamycin: p<1×10⁻⁴; HBSS: p<0.005; HBSS+LY294002: p<5×10⁻⁶; HBSS+ rapamycin: p<0.0005; # expression changes compared to the PINK1 expression in RPMI+10% FCS+LY294002: p<0.05; <b>+</b> expression changes compared to the PINK1 expression in RPMI+10% FCS+rapamycin: p<0.0005. B) SH-SY5Y cells were either stably transduced with a control (nt) shRNA or a shRNA directed against PINK1 and cultivated in RPMI medium containing 5% or 10% FCS. Their PINK1 mRNA content was determined by RT-qPCR and PINK1 mRNA content of nt cells kept in medium with 10% FCS was set as 1. Serum reduction increased PINK1 mRNA in nt cells in accordance with the data shown in Fig. 1A, while stable PINK1 knockdown (kd) reduced PINK1 content under both conditions; n = 3; * expression changes compared to the PINK1 expression in RPMI+10% FCS: PINK1 kd 10% FCS p<0.0005; PINK1 induction by 5% FCS: p<0.05; # expression changes compared to the PINK1 expression in RPMI+5% FCS: PINK1 kd 5% FCS: p<0.005. C) SH-SY5Y cells without (nt) or with stable PINK1 knockdown (kd) were kept in medium with 5% FCS and either untreated or treated for 2 h with CCCP to stabilize PINK1. Afterwards the PINK1 63 kDa protein (arrowhead) and actin protein levels were determined by western blotting (see representative gel on the right). The quantification revealed a reduction of PINK1 protein under both conditions in PINK1 kd cells.</p

Increased apoptosis in cells with PINK1 knockdown.

<p>5A) nt and PINK1 knockdown (kd) SH-SY5Y cells were either kept in RPMI medium with 5% FCS or starved with HBSS for 12 h. DEVD cleavage as parameter for caspase-3 activity was analyzed and normalized to the protein content. Cells with PINK1 knockdown demonstrated a significantly increased caspase-3 activity under both conditions; n = 4; RPMI+5% FCS: p<0.05; HBSS: p<0.01. 5B) nt and PINK1 knockdown (kd) SH-SY5Y cells were either kept in RPMI medium with 5% FCS for 48 h or starved with HBSS for the indicated time points. In the representative western blot the appearance of the PARP 89 kDa cleaved product is depicted as well as GAPDH for normalizing. 5C) nt and PINK1 knockdown (kd) SH-SY5Y cells were either kept in RPMI medium with 5% FCS for 48 h or starved with HBSS for the indicated time points. For quantification the cleaved PARP 89 kDa product was normalized to the full length 113 kDa PARP and the resulting value normalized to GAPDH. Cells with PINK1 knockdown demonstrated increased PARP cleavage that was significant after 24 h starvation; n = 4; 24 h: p<0.005. 5D) HeLa cells were transfected with scrambled siRNA or PINK1 siRNA and GFP or GFP-Parkin or PINK1-GFP or PINK1G309D-GFP. After transfection cells were starved for additional 24 h with HBSS and afterwards the amount of adherent cells was determined. The number of adherent cells transfected with scrambled siRNA and the indicated plasmid was set as 1. Starvation-mediated increased cell loss after PINK1 knockdown could be rescued by PINK1 or PINK1G309D-GFP but not by GFP or Parkin; GFP: n = 5, p<0.005; Parkin: n = 3, p<0.05; PINK1: n = 5, ns; PINK1G309D-GFP: n = 3, ns. 5E) HeLa cells and HeLa cells stably overexpressing LC3 were transfected with scrambled siRNA or PINK1 siRNA. After 48 h cells were starved in HBSS for additional 24 h and afterwards DEVD cleavage as parameter for caspase-3 activity was analyzed and normalized to 1,000,000 cells. PINK1 knockdown increased caspase-3 activity significantly, which was prevented by LC3 overexpression; n = 3; p<0.05. 5F) HeLa cells and HeLa cells stably expressing LC3 were transfected with scrambled or PINK1 siRNA. 48 h post transfection cells were either left untreated (+ medium) or starved (+HBSS) for additional 24 h and afterwards the amount of adherent cells was determined. The amount of non-starved cells was set as 1. PINK1 knockdown resulted in elevated cell loss, which was prevented by LC3 overexpression; HeLa: n = 6, p<0.05; HeLa LC3: n = 5.</p

Reduced autophagy after PINK1 knockdown.

<p>2A) SH-SY5Y cells were starved for 2 h in HBSS with Bafilomycin (+Baf) or left untreated in RPMI+5% FCS (- Baf). The LC3-II and actin content were determined by western blotting. A representative blot is shown on the right, showing only LC3-II bands in the Bafilomycin-treated samples. The LC3-II bands of Bafilomycin treated samples were normalized to actin and the relative LC3-II content of nt cells was set as 1. Cells with stable PINK1 knockdown exhibited a reduced LC3-II/actin ratio compared to the control (nt) cells; n = 4, p<0.005. 2B) Cortical neurons from 3 different isolations (10-20 DIV) of WT and PINK1 KO mice were either non-starved or starved for 2 h in HBSS and the LC3-II and actin content was determined by western blotting. A representative blot is shown on the right. The relative LC3-II content of WT and PINK1 KO mice, respectively, was set as 1. Primary neurons showed a strong upregulation of autophagy in reaction to starvation but cells derived from PINK1 KO mice exhibited a reduced LC3-II/actin ratio compared to the WT cells; n = 5, p<0.001. 2C) HeLa cells were transiently transfected with scrambled siRNA or PINK1 siRNA. After 48 h the LC3-II and actin content was determined by western blotting. A representative blot is shown on the right. The relative LC3-II content of cells transfected with scrambled siRNA was set as 1. Transient PINK1 knockdown resulted in a reduced LC3-II/actin ratio compared to the cells transfected with scrambled siRNA; n = 6; p<0.005. 2D) HeLa cells were transiently transfected with PINK1-GFP, PINK1G309D-GFP or GFP. After 24 h cells were starved for 2 h in HBSS. Afterwards the LC3-II and actin content were determined by western blotting. A representative blot is shown on the right. The relative LC3-II content of cells transfected with GFP was set as 1. Transient PINK1 or PINK1G309D overexpression resulted in an increased LC3-II/actin ratio compared to cells transfected with GFP; n = 3, PINK1: p<0.01; PINK1G309D: p<0.05.</p