Mitokondriell fusion mättes genom att spåra utjämning av photoconverted matrix målinriktat GFP över mitokondriella nätverket över tid. Så här långt, kan endast en cell skall utsättas för en timmes kinetisk analys vid en tidpunkt. Vi presenterar en metod som samtidigt mäter flera celler, vilket snabbar upp insamlingen av uppgifter.
Mitochondrial fusion plays an essential role in mitochondrial calcium homeostasis, bioenergetics, autophagy and quality control. Fusion is quantified in living cells by photo-conversion of matrix targeted photoactivatable GFP (mtPAGFP) in a subset of mitochondria. The rate at which the photoconverted molecules equilibrate across the entire mitochondrial population is used as a measure of fusion activity. Thus far measurements were performed using a single cell time lapse approach, quantifying the equilibration in one cell over an hour. Here, we scale up and automate a previously published live cell method based on using mtPAGFP and a low concentration of TMRE (15 nm). This method involves photoactivating a small portion of the mitochondrial network, collecting highly resolved stacks of confocal sections every 15 min for 1 hour, and quantifying the change in signal intensity. Depending on several factors such as ease of finding PAGFP expressing cells, and the signal of the photoactivated regions, it is possible to collect around 10 cells within the 15 min intervals. This provides a significant improvement in the time efficiency of this assay while maintaining the highly resolved subcellular quantification as well as the kinetic parameters necessary to capture the detail of mitochondrial behavior in its native cytoarchitectural environment.
Mitochondrial dynamics play a role in many cellular processes including respiration, calcium regulation, and apoptosis1,2,3,13. The structure of the mitochondrial network affects the function of mitochondria, and the way they interact with the rest of the cell. Undergoing constant division and fusion, mitochondrial networks attain various shapes ranging from highly fused networks, to being more fragmented. Interestingly, Alzheimer’s disease, Parkinson’s disease, Charcot Marie Tooth 2A, and dominant optic atrophy have been correlated with altered mitochondrial morphology, namely fragmented networks4,10,13. Often times, upon fragmentation, mitochondria become depolarized, and upon accumulation this leads to impaired cell function18. Mitochondrial fission has been shown to signal a cell to progress toward apoptosis. It can also provide a mechanism by which to separate depolarized and inactive mitochondria to keep the bulk of the network robust14. Fusion of mitochondria, on the other hand, leads to sharing of matrix proteins, solutes, mtDNA and the electrochemical gradient, and also seems to prevent progression to apoptosis9. How fission and fusion of mitochondria affects cell homeostasis and ultimately the functioning of the organism needs further understanding, and therefore the continuous development and optimization of how to gather information on these phenomena is necessary.
Existing mitochondrial fusion assays have revealed various insights into mitochondrial physiology, each having its own advantages. The hybrid PEG fusion assay7, mixes two populations of differently labeled cells (mtRFP and mtYFP), and analyzes the amount of mixing and colocalization of fluorophores in fused, multinucleated, cells. Although this method has yielded valuable information, not all cell types can fuse, and the conditions under which fusion is stimulated involves the use of toxic drugs that likely affect the normal fusion process. More recently, a cell free technique has been devised, using isolated mitochondria to observe fusion events based on a luciferase assay1,5. Two human cell lines are targeted with either the amino or a carboxy terminal part of Renilla luciferase along with a leucine zipper to ensure dimerization upon mixing. Mitochondria are isolated from each cell line, and fused. The fusion reaction can occur without the cytosol under physiological conditions in the presence of energy, appropriate temperature and inner mitochondrial membrane potential. Interestingly, the cytosol was found to modulate the extent of fusion, demonstrating that cell signaling regulates the fusion process 4,5. This assay will be very useful for high throughput screening to identify components of the fusion machinery and also pharmacological compounds that may affect mitochondrial dynamics. However, more detailed whole cell mitochondrial assays will be needed to complement this in vitro assay to observe these events within a cellular environment.
A technique for monitoring whole-cell mitochondrial dynamics has been in use for some time and is based on a mitochondrially-targeted photoactivatable GFP (mtPAGFP)6,11. Upon expression of the mtPAGFP, a small portion of the mitochondrial network is photoactivated (10-20%), and the spread of the signal to the rest of the mitochondrial network is recorded every 15 minutes for 1 hour using time lapse confocal imaging. Each fusion event leads to a dilution of signal intensity, enabling quantification of the fusion rate. Although fusion and fission are continuously occurring in cells, this technique only monitors fusion as fission does not lead to a dilution of the PAGFP signal6. Co-labeling with low levels of TMRE (7-15 nM in INS1 cells) allows quantification of the membrane potential of mitochondria. When mitochondria are hyperpolarized they uptake more TMRE, and when they depolarize they lose the TMRE dye. Mitochondria that depolarize no longer have a sufficient membrane potential and tend not to fuse as efficiently if at all. Therefore, active fusing mitochondria can be tracked with these low levels of TMRE9,15. Accumulation of depolarized mitochondria that lack a TMRE signal may be a sign of phototoxicity or cell death. Higher concentrations of TMRE render mitochondria very sensitive to laser light, and therefore great care must be taken to avoid overlabeling with TMRE. If the effect of depolarization of mitochondria is the topic of interest, a technique using slightly higher levels of TMRE and more intense laser light can be used to depolarize mitochondria in a controlled fashion (Mitra and Lippincott-Schwartz, 2010). To ensure that toxicity due to TMRE is not an issue, we suggest exposing loaded cells (3-15 nM TMRE) to the imaging parameters that will be used in the assay (perhaps 7 stacks of 6 optical sections in a row), and assessing cell health after 2 hours. If the mitochondria appear too fragmented and cells are dying, other mitochondrial markers, such as dsRED or Mitotracker red could be used instead of TMRE.
The mtPAGFP method has revealed details about mitochondrial network behavior that could not be visualized using other methods. For example, we now know that mitochondrial fusion can be full or transient, where matrix content can mix without changing the overall network morphology. Additionally, we know that the probability of fusion is independent of contact duration and organelle dimension, is influenced by organelle motility, membrane potential and history of previous fusion activity8,15,16,17.
In this manuscript, we describe a methodology for scaling up the previously published protocol using mtPAGFP and 15nM TMRE8 in order to examine multiple cells at a time and improve the time efficiency of data collection without sacrificing the subcellular resolution. This has been made possible by the use of an automated microscope stage, and programmable image acquisition software. Zen software from Zeiss allows the user to mark and track several designated cells expressing mtPAGFP. Each of these cells can be photoactivated in a particular region of interest, and stacks of confocal slices can be monitored for mtPAGFP signal as well as TMRE at specified intervals. Other confocal systems could be used to perform this protocol provided there is an automated stage that is programmable, an incubator with CO2, and a means by which to photoactivate the PAGFP; either a multiphoton laser, or a 405 nm diode laser.
Denna metod gör det möjligt att avbilda cirka 10 celler vid en tid, om förvärvet sker var 15: e minuter efter fotoaktivering. Det exakta antalet celler kommer att bero på hur snabbt man kan lokalisera och markera mtPAGFP uttrycker cellerna i kulturen skålen och hur snabbt man kan ställa in alla program parametrar. För att göra automation smidigt ett jämnt lager av celler bör användas eftersom de angivna z-stack marginaler kommer att gälla för alla celler.
Storleken på denna inledande fotoaktivering området kommer att styra jämvikt tiden. För att kunna mäta mitokondrie fusion, är det viktigt att photoactivate endast 10-20% av nätet, så att spridningen av signal till resten av nätverket kan följas över tid. Om alltför mycket av nätet fotoaktiveras, är det möjligt att en fullständig fusion kommer att ske för snabbt, och händelsen kommer inte att fångas.
Extrem försiktighet måste vidtas för attjustera lasereffekten i två-foton-laser samt TMRE koncentration för att undvika fototoxicitet som leder till mitokondriellt depolarisation. Säkerställa att mtPAGFP signalen colocalizes med TMRE signalen kan hjälpa till att bedöma fototoxicitet och allmänna cell hälsa 8,15. Belysning med ljus epifluoerescent bör undvikas. Samtidigt söka efter celler som uttrycker mtPAGFP bör pinhole vara maximalt öppen när du skannar med låg effekt 488 nm excitation. Justering av två-foton-lasereffekten att photoactivate tillräckligt PA-GFP för att mäta signalen över 1 timme, men inte till oversaturate någon av cellerna kan vara svårt 8. Bör dock tiden spenderas i denna optimering steg eftersom när automatiserat program startas, är det jobbigt att stoppa det, att välja fler celler och återuppta.
För kvalitetskontroll, förvärv av en differential interferens kontrast (DIC) bild (eller överföras ljus) för att övervaka fokus på cellerna kan vara myckethjälpsamma och också ett bra sätt att upptäcka bubblor som bildas i immersionsolja vid skanning, vilket händer ibland från rörelser på scenen.
Även med denna mtPAGFP metod samlar uppgifter om enkelriktade flödet av mitokondriens matrix proteiner från fotoaktiveras mitokondrier till dem som inte är märkta, är det tänkbart att utnyttja denna teknik för att studera andra processer. Till exempel kan specifika fluorokromer fästas membranproteiner för att observera deras specifika rörelser under fusionshändelser, vilket har visats för ABC-mig, fusion av vilka inträffar på en annan tidsskala från blandningen av lösliga matrisproteiner 15.
The authors have nothing to disclose.
Vi tackar Tufts Center for Neuroscience Research, P30 NS047243 (Jackson), mitokondrierna Affinity Research Collaborative (mtARC) som stöds av Evans Centrum för tvärvetenskaplig biomedicinsk forskning vid Boston University Medical Campus, Länk medicin Corporation, och Zeiss för att stödja detta arbete.
Name of the reagent | Company | Catalogue number |
COXIII-adenoviral PA-GFP | Dr. Lippincott-Schwartz | |
TMRE | Invitrogen | T669 |
Zeiss LSM 710 confocal | Zeiss |