Combining Mitotic Cell Synchronization and High Resolution Confocal Microscopy to Study the Role of Multifunctional Cell Cycle Proteins During Mitosis

We present a protocol for double thymidine synchronization of HeLa cells followed by analysis using high resolution confocal microscopy. This method is key to obtaining large number of cells that proceed synchronously from S phase to mitosis, enabling studies on mitotic roles of multifunctional proteins which also possess interphase functions.

The overall goal of this procedure is to use double thymidine synchronization and high-resolution confocal microscopy to study the mitotic roles of multifunctional proteins that may possess critical interphase functions. This method can help answer key questions in the cell biology field, about how to delineate the normal functions of proteins involved in cell cycle and mitosis. The main advantages of this technique are that, the cells can maintain their normal physiological behavior and that this method is also very easy to perform.

Demonstrating this procedure will be Dr.Mohammed Amin, a post-doc from my laboratory, who is an expert in the use of this method. For fixed cell imaging of mitotic cell progression, begin by seeding approximately two times 10 to the fifth HeLa cells into each well of a six-well plate, containing a 70%ethanol and UV sterilized cover slip and two milliliters of DMEM medium. After 24 hours in a cell culture incubator, block the cells with two milliliters of freshly diluted thymidine and fresh medium per well, and return the plate to the incubator for another 18 hours.

The next day, wash the cells two times with two milliliters of PBS, and one time with fresh 37 degree Celsius medium. Return the cells to the incubator for nine hours to release the cells from the block, followed by the addition of another two milliliters of thymidine-supplemented medium per well. After the second blocking incubation, wash the cells two times with two milliliters of PBS, and one time with two milliliters of fresh 37 degree Celsius medium, and add 100 nanomolar of freshly prepared siRNA to the appropriate wells.

After nine to 10 hours, aspirate the supernatant and fix the cells onto the cover slips with four percent paraformaldehyde for 20 minutes at room temperature. After washing with PBS, permeabilize the fixed cells with 0.5%detergent for 10 minutes at room temperature, followed by two five-minute washes in PBS. Block the cells with one percent BSA in PBS for one hour at room temperature.

Then label the cells with 50 microliters of the primary antibodies of interest for one hour at 37 degrees Celsius. At the end of the incubation, wash the cells three times in PBS, and label them with 50 microliters of the appropriate secondary antibodies of interest. After one hour at room temperature, wash the cells two times in PBS for five minutes each wash, and label the cells with DAPI for five minutes at room temperature.

Follow the DAPI staining with two five-minute washes in PBS and use the appropriate mounting medium to mount the cover slips on individual clear microscope slides. Then, using 60 or 100 X 1.4 numerical aperture plan apochromatic DIC oil immersion objectives mounted on an inverted high resolution confocal microscope equipped with an appropriate camera, acquire images of the immunostained proteins in Z stacks of 0.2 micrometer thickness. For live cell imaging of mitotic cell progression, seed approximately 0.5 to one times 10 to the fifth HeLa cells stably expressing mCherry H2B and GFP alpha-tubulin into 35-milliliter glass-bottom dishes containing 1.5 milliliters of DMEM medium.

Grow the cells in a cell culture incubator for 24 hours. Then, thymidine block the cells two times as just demonstrated, this time washing the cells two times with two milliliters of PBS and one time with one milliliter of pre-warmed DMEM medium at the end of each thymidine block and treating the cells with siRNA after the first block during the first thymidine washout. Grow the cells in fresh pre-warmed Leibovitz's medium, supplemented with 10%FBS and 20 millimolar HEPES for eight hours to release the cells from the second block.

Then place the first plate in the temperature-controlled chamber of a high-resolution confocal microscope, and use the 60 X objective and bright field imaging to bring the cells into focus. When the cells are visible, manually adjust the plate position to the region of choice and use the image acquisition software to set the laser power and exposure, image acquisition parameters, and experiment duration period. Select the appropriate GFP and mCherry filters and acquire pulsed transmitted light and fluorescence images every 10 minutes for up to 16 hours, to obtain time-lapse images of the mitosis process.

Nine hours after release from the double thymidine block, acquire the images at 12 one-micrometer separated z-planes. Then analyze the images by tracking individual mitotic cells and use the appropriate source software to assemble a corresponding movie. Although most of the control and Cdt1 siRNA cells are at the metaphase stage at nine hours post-release from double thymidine block, fixation at 10 hours reveals that most of the Cdt1 siRNA-treated cells are still arrested in late prometaphase, while the control cells enter anaphase and segregate their chromosomes as expected.

Cold treatment nine hours after release from double thymidine block facilitates the retention of stable kinetochore microtubules which are relatively less robust in Cdt1-depleted cells compared to those within control-depleted cells. DNA replication origin licensing is not perturbed in Cdt1-or control-depleted cells during the G2-M phase, as these cells were not found to induce the accumulation of phosphoralated gamma H2AX, a marker for DNA damage, during the subsequent G2 phase. Cdt1 siRNA transfection of asynchronous cultures, however, induces the accumulation of phospho gamma H2AX positive foci possibly due to the DNA damage induced by improper DNA replication licensing.

After nuclear envelope breakdown, normal cells enter mitosis and undergo anaphase onset to exit from mitosis within about 30 to 60 minutes. RNAi-mediated knockdown of Hec1, a key kinetochore protein required for robust kinetochore microtubule attachment formation, however, delays the normal mitotic progression to anaphase onset or exit from mitosis even after several hours of mitosis delay. Once mastered, this technique can be completed in three to four days if it is performed properly.

While attempting the procedure, it's important to remember to dissolve the thymidine completely after you thaw it out from the freezer. Following this procedure, other methods like western blotting can be performed to answer additional questions like, what are the expression patterns of proteins of interest during mitosis compared to interphase? After its development, this technique paved the way for researchers in the field of cell biology to explore cancer biogenesis in human cell culture models, and to use high-resolution confocal microscopy to identify the cell cycle related functions of proteins of interest.

Don't forget that working with reagents like paraformaldehyde and DAPI can be extremely hazardous and that precautions such as wearing gloves and a lab coat, should always be taken while performing this procedure.