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We here introduce a procedure to measure protein oligomers and aggregation in cell lysate and live cells using fluorescence correlation spectroscopy.
Protein aggregation is a hallmark of neurodegenerative disorders such as amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and so on. To detect and analyze soluble or diffuse protein oligomers or aggregates, fluorescence correlation spectroscopy (FCS), which can detect the diffusion speed and brightness of a single particle with a single molecule sensitivity, has been used. However, the proper procedure and know-how for protein aggregation detection have not been widely shared. Here, we show a standard procedure of FCS measurement for diffusion properties of aggregation-prone proteins in cell lysate and live cells: ALS-associated 25 kDa carboxyl-terminal fragment of TAR DNA/RNA-binding protein 43 kDa (TDP25) and superoxide dismutase 1 (SOD1). The representative results show that a part of aggregates of green fluorescent protein (GFP)-tagged TDP25 was slightly included in the soluble fraction of murine neuroblastoma Neuro2a cell lysate. Moreover, GFP-tagged SOD1 carrying ALS-associated mutation shows a slower diffusion in live cells. Accordingly, we here introduce the procedure to detect the protein aggregation via its diffusion property using FCS.
Protein aggregations involving neurodegenerative disorders such as amyotrophic lateral sclerosis (ALS), Alzheimer's disease, Parkinson's disease, Huntington's disease, and so on1 are known to be toxic and would disturb protein homeostasis (proteostasis) in the cells and organs, that could then lead to aging2. The clearance of protein aggregation is expected as a therapeutic strategy; however, chemicals that prevent protein aggregation formation and degrade protein aggregates (e.g., small molecules or drugs) have not been established yet. Moreover, how protein aggregation exerts toxicity remains elusive. Therefore, to promote research projects related to protein aggregation, it is important to introduce high throughput procedures to simply detect protein aggregation. Protein aggregation detection using antibodies recognizing the conformation of protein aggregation and aggregation-specific fluorescent dye has been widely used3. However, it is difficult to detect the aggregation, especially in live cells using such classical procedures.
Förster resonance energy transfer (FRET) is a procedure to detect protein aggregation and structural change. However, FRET is unable to analyze protein dynamics (e.g., diffusion and oligomerization of protein in live cells)3. Therefore, we introduce here a simple protocol to detect protein aggregation in solution (e.g., cell lysate) and live cells using fluorescence correlation spectroscopy (FCS), which measures the diffusion property and brightness of fluorescent molecules with single molecule sensitivity4. FCS is a photon-counting method by using a laser scan confocal microscope (LSM). Using a highly sensitive photon-detector and calculation of autocorrelation function (ACF) of photon arrival time, the pass through time and brightness of the fluorescent molecules in the detection volume is measured. The diffusion slows with an increase of molecular weight; thus, intermolecular interaction can be estimated using FCS. Even more powerfully, an increase in the brightness of the fluorescent molecule indicates homo-oligomerization of the molecules. Therefore, FCS is a powerful tool to detect such protein aggregation.
1. Materials and reagents
2. Cell culture and transfection
3. Cell lysis & medium exchange
4. Fluorescence correlation spectroscopy (FCS) calibration
5. FCS measurement in cell lysate
6. FCS measurement in live cells
We performed FCS measurement of GFP-TDP25 in cell lysate and SOD1-G85R-GFP in live cells. In both cases, a positive amplitude and smooth ACFs were able to be acquired. We have shown that a portion of GFP-TDP25 expressed in Neuro2a cells was recovered in the soluble fraction under the indicated condition6. In the soluble fraction of the cell lysate, extremely bright fluorescence molecules were detected in the photon count rate record using FCS (Figure 2A, top, arrow). ...
Regarding system calibration before measurements, the same glasswares as the one used to measure the sample should be used (e.g., the 8-wells cover glass chamber for cell lysate and the 35-mm glass base dish for live cells). Because of the adsorption of Rh6G on the glass, its effective concentration may sometimes decrease. If so, a highly concentrated Rh6G solution such as 1 μM should be used just for the pinhole adjustment. Extremely high photon count rates must be avoided to protect the detector (e.g., more than 1...
These authors have no conflicts of interest.
A.K. was supported by a Japan Society for Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (C) (#18K06201), by a grant-in-aid from the Nakatani Foundation for Countermeasures against novel coronavirus infections, by a grant from Hokkaido University Office for Developing Future Research Leaders (L-Station), and a grant-in-aid from Hoansha Foundation. M. K. was partially supported by a JSPS Grant-in-Aid for Scientific Research on Innovative Areas "Chemistry for Multimolecular Crowding Biosystems" (#20H04686), and a JSPS Grant-in-Aid for Scientific Research on Innovative Areas "Information physics of living matters" (#20H05522).
Name | Company | Catalog Number | Comments |
0.25% (w/v) Trypsin-1 mmol/L EDTA·4Na Solution with Phenol Red (Trypsin-EDTA) | Fujifilm Wako Pure Chemical Corp. | 201-16945 | |
100-mm plastic dishes | CORNING | 430167 | |
35-mm glass base dish | IWAKI | 3910-035 | For live cell measurement |
35-mm plastic dishes | Thermo Fisher Scientific | 150460 | |
Aluminum plate | Bio-Bik | AB-TC1 | |
C-Apochromat 40x/1.2NA Korr. UV-VIS-IR M27 | Carl Zeiss | Objective | |
Cell scraper | Sumitomo Bakelite Co., Ltd. | MS-93100 | |
Cellulose acetate filter membrane (0.22 mm) | Advantech Toyo | 25CS020AS | |
Cover glass chamber 8-wells | IWAKI | 5232-008 | For solution measurement |
Dulbecco's Modified Eagle's Medium (DMEM) | Sigma-Aldrich | D5796 | basal medium |
Fetal bovine serum (FBS) | biosera | Lot check required | |
Lipofectamine 2000 | Thermo Fisher Scientific | 11668019 | |
LSM510 META + ConfoCor3 | Carl Zeiss | FCS system | |
Murine neuroblastoma Neuro2a cells | ATCC | CCL-131 | Cell line |
Opti-MEM I | Thermo Fisher Scientific | 31985070 | |
pCAGGS | RIKEN | RDB08938 | Plasmid DNA for the transfection carrier |
Penicillin-Streptomycin Solution (×100 ) | Fujifilm Wako Pure Chemical Corp. | 168-23191 | |
pmeGFP-C1-TDP25 | Plasmid DNA for TDP25 tagged with monomeric eGFP | ||
pmeGFP-N1 | Plasmid DNA for eGFP monomer expression | ||
pmeGFP-N1-SOD1-G85R | Plasmid DNA for ALS-linked G85R mutant of SOD1 tagged with monomeric eGFP | ||
Protease inhibitor cocktail | Sigma-Aldrich | P8304 |
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