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This protocol describes a mechanism for using correlative light and electron microscopy to visualize the interaction of mitochondria and lysosomes labeled with mEosEM and APEX2, respectively.
Cellular organelles, such as mitochondria and lysosomes, display dynamic structures. Despite the higher resolution of transmission electron microscopy for structural analysis, light microscopy is essential for the visualization of dynamic organelles by target-specific labeling. The following protocol describes a method that combines dual-color correlative light and electron microscopy (CLEM) to observe the interactions between mitochondria and lysosomes. In this study, mitochondria were labeled with mEosEM (Mito-mEosEM) and lysosomes with TMEM192-V5-APEX2. The results obtained from CLEM images enable us to observe the changes in the interactions between mitochondria and lysosomes under external stress conditions. Treatment with bafilomycin (BFA), which inhibits lysosomal function, resulted in an increase in contact between mitochondria and lysosomes, leading to the formation of fragmented mitochondria trapped inside lysosomes. Conversely, treatment with U18666A, which inhibits cholesterol export from lysosomes, caused lysosomes to be surrounded by mitochondria, indicating a distinct form of interaction. This study presents an effective method for observing the interactions between mitochondria and lysosomes in fixed cells. Furthermore, CLEM imaging with dual-color probes offers a powerful tool for future investigations of organelle dynamics and their implications for cell function and pathology.
Mitochondria and lysosomes are the principal membrane-bound organelles that are essential for the maintenance of cellular homeostasis. Mitochondria are highly dynamic cellular organelles modulated by fission and fusion events1. In general, mitochondria are referred to as the powerhouse of the cell and are known for their important role in oxidative phosphorylation, ATP production, and metabolite storage such as calcium2. In addition to their role in energy metabolism, mitochondria are involved in a number of other cellular signaling processes, including apoptosis3,4, calcium signaling5,6, and lipid metabolism7,8. Similarly, lysosomes are highly dynamic cellular organelles that play multiple roles as cellular recycling centers to maintain cellular cleanliness through protein degradation, autophagy, endocytosis, and phagocytosis9. Apart from their role in degradation, lysosomes also play roles in cellular physiology such as nutrient sensing, homeostasis10, lipid metabolism11, and cellular dedifferentiation12.
Despite the existence of numerous studies on the functions of individual cellular organelles, our recent research has concentrated on the less-understood crosstalk between mitochondria and lysosomes, which has been demonstrated to influence both cellular physiology and pathology. Contacts between mitochondria and lysosomes have been observed in various cell types, including cancer cells, neurons, and induced pluripotent stem cell (iPSC)-derived cells13, and they are mainly observed under cellular stress or in neurodegenerative diseases such as Parkinson's disease13,14 and Charcot-Marie-Tooth disease15. The contact between mitochondria and lysosomes plays an important role in maintaining metabolic homeostasis through the exchange of ions (e.g., Ca2+)16, cholesterol17, and iron18 between the two organelles. Furthermore, the contact between mitochondria and lysosomes plays a crucial role in the quality control of cellular organelles. For example, damaged mitochondria can be targeted for degradation through the process of mitophagy, in which they are engulfed by autophagosomes and delivered to lysosomes for degradation. Consequently, the removal of dysfunctional mitochondria can be facilitated before they cause further damage to the cell or to normal mitochondria. This interaction between mitochondria and lysosomes is crucial for maintaining cellular homeostasis and preventing the accumulation of dysfunctional organelles. Thus, only damaged mitochondria are selectively removed, allowing healthy mitochondria to function optimally within the cell.
Correlative light and electron microscopy (CLEM) is an imaging technique that has recently received attention as a research method by combining the advantages of light microscopy (LM) and electron microscopy (EM) to provide detailed structural information at different scales. In CLEM, a sample is first imaged using LM to identify specific regions of interest, followed by EM sample preparation steps such as fixation, dehydration, and embedding. The same sample is then observed under an electron microscope. Although live-cell CLEM methods have been reported recently19,20,21,22, it remains challenging to accurately combine optical images of highly dynamic biological samples with electron microscopy images of fixed cells into a single plane. For highly dynamic organelles like mitochondria and lysosomes, their precise localization can change as they are observed live under the light microscope, and they continue to move even during the short delay time required for fixation.
Therefore, in this protocol, two different molecular tags that remain active even within fixative solutions were employed to visualize the interaction between mitochondria and lysosomes, which play an important role in maintaining metabolic homeostasis through the exchange of ions (e.g., Ca2+)16, cholesterol17, and iron18 between these organelles. The constructs TMEM192-V5-APEX2 and Mito-mEosEM were used to observe the interactions of mitochondria and lysosomes in cultured cells at both the EM and LM levels. For mitochondria, the mEosEM tag was selected for its ability to preserve the fluorescence signal during EM sample preparation in a fixative solution and OsO423. For lysosomes, the APEX2 tag, a protein of approximately 28 kDa, was used to study the localization of a specific protein at both EM and LM levels24. In the presence of hydrogen peroxide, the APEX2's substrate, Amplex-Red, forms an insoluble polymer that can be observed by confocal microscopy at the tag-targeted location. In this protocol, dual-color CLEM was used to visualize the structure of the interactions between lysosomes and mitochondria in fixed cells.
The step-by-step workflow for sample preparation is shown in Figure 1.
1. Cell culture and transfection
2. Confocal microscopic imaging
3. Sample preparation for EM block
4. Sectioning and TEM imaging
5. Image stitching using imageJ Fiji.
6. Resizing the fluorescence image
7. Landmark-based image registration with the Big Warp plugin
8. Overlay of CLEM images
To visualize the interactions between mitochondria and lysosomes using this dual-color CLEM protocol, we employed Mito-mEosEM and TMEM192-V5-APEX2. Fixed cells were first imaged using a light microscope, followed by EM sampling and imaging. The results of the correlated images are shown in Figure 3A,B and Figure 4A. Following the protocol described above, we checked whether we could observe the changes in the interaction between mitochondria and...
This protocol describes dual-color CLEM to observe the interaction of mitochondria and lysosomes in fixed cells using genetically encoded mEosEM and APEX2 tags. Over the past few decades, advances in microscopy have greatly improved the ability to observe changes in organelle networks32,33, and the study of organelle interactions has become increasingly interesting. Super-resolution microscopy has been used to observe changes in highly dynamic contacts between mi...
The authors have no conflicts of interest to disclose.
This research was supported by the KBRI basic research program through Korea Brain Research Institute funded by Ministry of Science and ICT (24-BR-01-03). TMEM192-V5-APEX2 plasmids were kindly provided by Hyun-Woo Rhee (Seoul National University). TEM data were acquired at Brain Research Core Facilities in KBRI.
Name | Company | Catalog Number | Comments |
Chemicals and reagents | |||
25% Glutaraldehyde | Electron Microscopy Sciences | 16200 | Aldehyde fumes are extremely toxic. Use only in fume hood. |
30% Hydrogen peroxide solution | Merck | 107210 | |
4% Paraformaldehyde solution | Biosesang | PC2031-100-00 | ldehyde fumes are extremely toxic. Use only in fume hood. |
Amplex Red | ThermoFisher | A12222 | |
Epon 812 | Electron Microscopy Sciences | 14120 | |
Ethanol | Merck | 100983 | |
Fugene HD | Promega | E2311 | Plasmid transfection |
Glycine | SIGMA | G8898 | |
Lead Citrate 3% | Electron Microscopy Sciences | 22410 | |
Osmium tetroxide 4 % aqueous solution | Electron Microscopy Sciences | 16320 | Very toxic. Use only in fume hood. |
Potassium hexacyanoferrate(II) trihydrate | SIGMA | P3289 | |
Sodium cacodylate trihydrate | Sigma-Aldrich | C0250-500G | |
Uranyl acetate | Electron Microscopy Sciences | 22400 | |
UranyLess EM stain | Electron Microscopy Sciences | 22409 | |
Plasmid construction | |||
Mito-mEosEM | addgene | 132706 | DOI: 10.1016/j.chembiol.2024.02.007 |
TMEM192-V5-APEX2 | Sharma N. et al. | N/A | DOI: 10.1016/j.chembiol.2024.02.007 |
Tools | |||
0.22 um syringe filter | Sartorius | 16534 | |
35 mm Gridded coverslip dish | Mattek | P35G-1.5-14-CGRD | |
BEEM Embedding Capsules | Ted Pella | 130 | |
Formvar Supported Single slot copper grid | Ted Pella | 01705 | |
Diamond knife | DiATOME | DU4530 | |
Ultra-microtome | Leica | ARTOS 3D | |
Microscopes | |||
Inverted confocal microscopy | Nikon | A1 Rsi/Ti-E | |
Transmission electron microscopy | FEI | Tecnai G2 | |
Software and Algorithms | |||
Fiji/Image J | NIH / open source | https://imagej.nih.gov/ij/ | |
ImageJ BigWarp software package | NIH / open source | https://imagej.net/plugins/bigwarp | |
Photozoom Pro 8 | BenVista | https://www.benvista.com/photozoompro Alternative software Image Resizer : https://imageresizer.com/ Gigapixel AI : https://www.topazlabs.com/gigapixel ON1 resize : https://www.on1.com/products/resize-ai/ |
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