A LIQUID NITROGEN-COOLED LIGHT MICROSCOPE STAGE TO SCREEN SAMPLES FOR CRYOEM

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Rapidly frozen biological samples show near-to-native preservation of structure, but this virtue comes at a price; contrast in frozen-hydrated specimens is modest, and the samples are very sensitive to the electron beam, so it is difficult to use electron microscopy to identify areas of interest in such specimens. We are therefore trying to pre-screen frozen-samples by light microscopy, mapping areas of interest on a TEM-finder grid. Our approach has been designed to make use of tools that are commonly available in cell biology, e.g., GFP-labeled proteins of interest and a conventional fluorescence light microscope. To maintain sample quality during screening, however, one must maintain sample temperature less than or equal to -130 degrees C. With the help of a biophysics laboratory in Moscow we have designed and built a cryostage for our Zeiss fluorescence microscope (¿light-microscope cryo-stage¿, or LM-CS) that can hold a TEM-grid for study and recording. This instrument is made in three parts: the LM-CS itself, a liquid nitrogen evaporation system with Dewar, and a temperature controller unit. The cryostage is equipped with a clamp to hold a TEM-grid, which is held in a cylindrical hole into which a dry objective can be lowered. This recess can be covered with a styrofoam-ring to minimize condensation. The nitrogen evaporation system in the Dewar is attached to the cryotable via thermo-isolated tubing; the liquid nitrogen in the Dewar is heated at a controlled rate, so gas phase nitrogen with temperature of about ¿150 degrees C is boiled up to be fed over the grid. The grid temperature can be maintained with high precision (plus or minus 0.5 degrees C) in the range from room temperature down to ¿146 degrees C. Also connected to the table is a temperature sensor that communicates with the temperature controller; sample temperature is then regulated by the rate of nitrogen evaporation. The LM-CS is now assembled and will be tested with different frozen-hydrated specimen.