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Method Article
Analysis of the mitochondrial structure-function relationship is required for a thorough understanding of the regulatory mechanisms of mitochondrial functionality. Specific methods for studying mitochondrial structure and function in live and fixed Drosophila ovaries are described and demonstrated in this paper.
Analysis of the mitochondrial structure-function relationship is required for a thorough understanding of the regulatory mechanisms of mitochondrial functionality. Fluorescence microscopy is an indispensable tool for the direct assessment of mitochondrial structure and function in live cells and for studying the mitochondrial structure-function relationship, which is primarily modulated by the molecules governing fission and fusion events between mitochondria. This paper describes and demonstrates specific methods for studying mitochondrial structure and function in live as well as in fixed tissue in the model organism Drosophila melanogaster. The tissue of choice here is the Drosophila ovary, which can be isolated and made amenable for ex vivo live confocal microscopy. Furthermore, the paper describes how to genetically manipulate the mitochondrial fission protein, Drp1, in Drosophila ovaries to study the involvement of Drp1-driven mitochondrial fission in modulating the mitochondrial structure-function relationship. The broad use of such methods is demonstrated in already-published as well as in novel data. The described methods can be further extended towards understanding the direct impact of nutrients and/or growth factors on the mitochondrial properties ex vivo. Given that mitochondrial dysregulation underlies the etiology of various diseases, the described innovative methods developed in a genetically tractable model organism, Drosophila, are anticipated to contribute significantly to the understanding of the mechanistic details of the mitochondrial structure-function relationship and to the development of mitochondria-directed therapeutic strategies.
Mitochondria are classically described as the cellular powerhouse, since they are the main seats of energy production in differentiated cells. Moreover, mitochondria play a critical role in metabolism, heat generation, lipid modification, calcium and redox homeostasis, the orchestration of cell signaling processes, etc1. Mitochondria also play an active role in the induction of cell death2, as well as in cell cycle regulation3. Such multi-functionality raises the following fundamental questions: a) how do mitochondria perform all these functions simultaneously and b) are there specific mitochondrial pools or subzones that are specialized for distinct functions? In this context, it is important to note that the multifunctional mitochondria are dynamic in their shape, size, and structure within individual cells and that the steady-state shape of mitochondria can vary between cell types. Decades of research from various laboratories suggest that the alteration of mitochondrial shape, size, and structure, collectively called mitochondrial dynamics, is crucial for maintaining various mitochondrial functions4,5,6. These findings raise the possibility that mitochondria may accomplish their multi-functionality by virtue of their structural dynamism.
Extensive efforts are underway to understand the mitochondrial structure-function relationship. The dynamism of mitochondrial structure is primarily maintained by their ability to undergo fission and fusion events with each other. Fission of large mitochondria converts them into smaller mitochondrial elements, while fusion between two smaller mitochondria merges them into a larger mitochondrial element7. Moreover, transient fusion of two mitochondria may occur to allow the mixing of their contents. The fission and fusion events of the inner and outer mitochondrial membranes are carefully governed by specific sets of proteins. The core fission machinery is composed of dynamin-related protein 1 (Drp1), which is recruited from the cytosol to the mitochondria by its interaction with certain bona fide mitochondrial proteins (e.g., Fis1 or Mff1), while Drp1 function can also be regulated by other proteins on the mitochondrial surface4. Although Drp1 operates on the outer membrane, its fission abilities impact the inner membrane as well. The orchestration of the fission of outer and inner mitochondrial membranes is not well understood. On the other hand, fusion of the inner membrane is governed at the core by the activities of Opa1, while mitofusins govern the fusion of the outer-membrane5. The balance of the counteracting fission and fusion events of mitochondria dictate the steady-state mitochondrial shape in a cell. For example, repression of mitochondrial fission would result in complete and unopposed fusion, while the over-activity of mitochondrial fission would result in fragmentation of mitochondria3.
The study of the mitochondrial structure-function relationship primarily involves two complimentary approaches: a) analyses of the cellular and organismal phenotypes after genetic manipulation of mitochondrial fission/fusion proteins and b) direct assessments of mitochondrial structure and function. It is noteworthy that genetic analyses may not always reveal the direct functionality of the molecule at hand (in this case, mitochondrial fission/fusion proteins), as the phenotypes may arise due to secondary effects. Therefore, it is of the utmost importance to develop and use tools to study mitochondrial structure and function directly. Any assessment of mitochondrial structure involves various microscopy tools. Use of fluorescence microscopy of live cells has greatly advanced the studies of mitochondrial dynamics, since mitochondrial dynamism can be monitored both qualitatively and quantitatively using the appropriate fluorescence microscopy tools and techniques8. Fluorescence microscopy-based tools have been developed to study mitochondrial structure and function in live and fixed Drosophila melanogaster tissues, elucidating the significance of mitochondrial dynamism in vivo9. These and related methods are described here, with the goal of studying mitochondrial structure and function in the Drosophila ovary.
The Drosophila ovary consists of germline and somatic lineages, which arise from their respective adult stem cells that reside in the germarium10,11. Sixteen syncytial germ cells (GCs) get encapsulated by somatic follicle cells (FCs) to form individual egg chambers that emerge out of the germarium (Figure 1). One of the 16 GCs get committed to become an oocyte, and the remaining 15 GCs develop into nurse cells that support the growth of the oocyte chamber, facilitating the maturation of the egg before it is laid. The majority of the FCs undergo 9 rounds of mitotic divisions before they exit the mitotic cell cycle to terminally differentiate into a patterned epithelial cell layer consisting of anterior follicle cells (AFCs), posterior follicle cells (PFCs), and main body cells (MBCs). The consecutive egg chambers are connected by stalk cells, which are differentiated cells that are also derived from the FCs early in development. Mitochondrial shape regulated by the mitochondrial fission protein Drp1 is actively involved in the process of differentiation during the normal development of the Drosophila ovarian FC layer9,12. The methods used in these studies to identify the involvement of Drp1 in Drosophila follicle cell layer development are described here.
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1. Preparation of Drosophila (the tools required are depicted in Figure 2A)
2. Dissection of Drosophila Ovaries (the tools required are depicted in Figure 2A)
3. Preparation for Live-tissue Microscopy
NOTE: The tools required are depicted in Figure 2A.
4. Fluorescence Loss In Photobleaching (FLIP) Assay to Assess Mitochondrial Matrix Continuity
NOTE: Mitochondrial matrix continuity in a fused mitochondrial structure is established after the complete fusion of the mitochondrial inner and outer membranes following a progression through the intermediate steps. Fission of mitochondria may follow the same steps but in the reverse direction (Figure 3A). FLIP is a time-lapse microscopy-based semi-quantitative method that can be used to assess mitochondrial matrix continuity in the final fused state of ex vivo mitochondria (steps 3 and 4 in Figure 3A) in live Drosophila ovaries9. The FLIP assay is performed as a small region of interest (ROI) of the mitochondria expressing a fluorescent molecule in the mitochondrial matrix that is photobleached at regular intervals (FLIP ROI in Figure 3A). As a result, any surrounding mitochondrial region that is continuous with the FLIP ROI (experimental ROI in Figure 3A) will lose signal due to the exchange of molecules in the continuous mitochondrial matrix. The FLIP experiments demonstrated here are performed on transgenic Drosophila expressing mitoYFP, which contains the mitochondrial targeting sequence of the human cytochrome oxidase VIII subunit tagged with YFP to target it to the mitochondrial matrix in a freely diffusible form. A similar experiment can also be performed with the mito pUASP-mito-GFP transgene, as reported previously9. A similar FLIP protocol may be used with a probe targeted to the mitochondrial inter-membrane space to be able to detect the continuity resulting from the fusion of the outer but not the inner mitochondrial membranes (step 2 in Figure 3A).
5. Live Staining with Fluorescent Mitochondrial Dyes
NOTE: Steady-state mitochondrial structure and potential can be assessed using dyes that specifically incorporate into mitochondria in live cells and tissues. Live Drosophila ovaries can be stained ex vivo with fluorescent mitochondrial stains to visualize the mitochondria, to assess mitochondrial reactive oxygen species (mito-ROS) production, and to assess mitochondrial potential per unit mass. This can be accomplished by co-staining with the mitochondrial potentiometric dye tetramethylrhodamine ethyl ester (TMRE) and a compatible live mitochondrial stain representing the mitochondrial mass (see Materials Table for the specific dyes).
6. Generation of Drp1 Null Mosaics
NOTE: The clonal strategy used here introduces green fluorescent protein (GFP)-negative Drp1 null clones in the background of a GFP-positive, phenotypically wild-type background that is genotypically heterozygous for the Drp1 null mutation9. Heat shock-induced flippase-flippase recognition target (FLP-FRT)-mediated site-specific mitotic recombination creates homozygous clones of the functionally null drpKG03815 allele. The genotype of Drosophila carrying the Drp1 mutant is drpKG03815 FRT40A/CYO, whereas the genotype carrying the heat shock-induced FLP (hsFLP) and UbiGFP clonal marker is hsflp; ubiquitin nls-GFP (UbiGFP) FRT 40A/CyO. The genotype of the selected offspring of the cross between the above genotypes is hsFLP/+; drpKG03815FRT40A/UbiGFPFRT40A.
7. Co-immunostaining for Cyclin E and Mitochondria
NOTE: To detect Drosophila Cyclin E (dCyclinE), we have used a commercially obtained antibody raised specifically against dCyclinE9 (see Materials Table). As a mitochondrial marker, we used an antibody against ATP-B (a subunit of the mitochondrial ATP synthase complex)9.
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The described methods can be used to study mitochondrial structure and function in live and fixed Drosophila ovaries (Figure 2B). Provided are some examples of anticipated results obtained with the described methods.
Dissection of the Drosophila ovary: When dissected further, the severed abdomens (Figure 3B) from the whole Drosophila (Figure 3A)...
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Critical Steps within the Protocol
Photobleaching: Preventing undue photobleaching of fluorescent samples is absolutely necessary to performing efficient confocal microscopy. Therefore, the time used to locate samples through the eyepiece or to set image acquisition parameters through the live scanning mode should be minimized to minimize photobleaching.
Tissue damage: Since mitochondria are considered to be the sensors of cellular health, it...
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The authors have no competing financial interests.
We acknowledge Leena Patel and Diamond Woodard for helping in the Drosophila medium preparation and Dr. Igor Chesnokov for providing access to the camera-attached stereomicroscope.
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Name | Company | Catalog Number | Comments |
Grace's Media (Insect Dissecting Medium) | Fisher Scientific | 30611031-2 | |
41 Paraformaldehyde AQ | Electronic Microscopy Sciences | 50-259-99 | |
Mitotracker Green (overall mitochondrial stain) | Life Technologies | m7514 | Reconstitute and Aliquot |
Tetramethylrhodamine ethyl ester perchlorate | Sigma Aldrich | 87917-25MG | Reconstitute and Aliquot |
MitoSox (Mito-Ros stain) | Life Technologies | m36008 | Reconstitute and Aliquot |
PolyLysine | MP Biomedicals | ICN15017625 | |
Fly Vials | Fisher Scientific | AS-515 | |
Fly Conicals | Fisher Scientific | AS-355 | |
Fly Vial Flugs | Fisher Scientific | AS273 | |
Fly Conical Flugs | Fisher Scientific | AS 277 | |
Jazzmix Drosophila food (Drosophila food) | Fisher Scientific | AS153 | |
Bovine Serum Albumin | Sigma Aldrich | A9647-50G | |
Cyclin E Antibody (d-300) | Santa Cruz | sc- 33748 | |
ATPB antibody [3D5] - Mitochondrial Marker | AbCam | ab14730 | |
Cy3 AffiniPure Goat Anti-Mouse IgG (H+L) | Jackson ImmunoResearch | 115-165-146 | |
Cy5 AffiniPure Goat Anti-Rabbit IgG (H+L) | Jackson ImmunoResearch | 111-175-144 | |
Hoechst | Fisher Scientific | H3570 | |
VectaShield | Fisher Scientific | H100 | |
Azer Scientific EverMark Select Microscope Slides | Fisher Scientific | 22-026-252 | |
Microscope Cover Glass | Fisher Scientific | 12-542-B | |
Mat Tek Corp Glass Bottom Mircrowell Dish | Fisher Scientific | P35G-0-14-C | |
Active Dried Yeast | Fisher Scientific | ICN10140001 | |
Confocal Microscope | Carl Zeiss | LSM 700 | |
Dumont #5 Forceps | Fine Science Technologies | 11251-20 | |
Moria Nickel Plated Pin Holder | Fine Science Technologies | 26016-12 | |
Minutien Pins | Fine Science Technologies | 26002-15 | |
MYFP ( w[*]; P{w[+mC]=sqh-EYFP-Mito}3 ) | Bloomington Stock Center | 7194 | |
Fly Pad | Fly stuff | 59-118 | |
Blowgun | Fly stuff | 54-104 | |
Blowgun needle | Flystuff | 54-119 | |
Dissecting Microscope | Carl Zeiss | Stemi 2000 | |
Analyses software | Carl Zeiss | Zen | |
Analyses software | Open source | Image J | |
Research Macro Zoom Microscope | Olympus | MVX10 | |
QICAM Fast 1394 Cooled Digital Camera, 12-bit, Mono | QImaging | QIC-F-M-12-C | |
QCapture Pro 5.1 | QImaging |
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