[Deep-freeze link for transport of water-containing biological objects to the electron microscope]

Native immunogold labeling of cell surface proteins and viral glycoproteins for cryo-electron microscopy and cryo-electron tomography applications

Numerous methods have been developed for immunogold labeling of thick, cryo-preserved biological specimens. However, most of the methods are permutations of chemical fixation and sample sectioning, which select and isolate the immunolabeled region of interest. We describe a method for combining immunogold labeling with cryo-electron microscopy (cryo-EM) and cryo-electron tomography (cryo-ET) of the surface proteins of intact mammalian cells or the surface glycoproteins of assembling and budding viruses in the context of virus-infected mammalian cells cultured on EM grids. In this method, the cells were maintained in culture media at physiologically relevant temperatures while sequentially incubated with the primary and secondary antibodies. Subsequently, the immunogold-labeled specimens were vitrified and observed under cryo-conditions in the transmission electron microscope. Cryo-EM and cryo-ET examination of the immunogold-labeled cells revealed the association of immunogold particles with the target antigens. Additionally, the cellular structure was unaltered by pre-immunolabeling chemical fixation and retained well-preserved plasma membranes, cytoskeletal elements, and macromolecular complexes. We think this technique will be of interest to cell biologists for cryo-EM and conventional studies of native cells and pathogen-infected cells.

Future developments in instrumentation for electron crystallography

Advances in instrumentation have proceeded at an impressive rate since the invention of the electron microscope. These advances have produced a continuous expansion of the capabilities and range of application of electron microscopy. In order to provide some insights on how continuing advances may enhance cryo-electron microscopy and electron crystallography, we review some of the active areas of instrumentation development. There is strong momentum in areas including detectors, phase contrast devices, and aberration correctors that may have substantial impact on the productivity and expectations of electron crystallographers.

Retrospective on the early development of cryoelectron microscopy of macromolecules and a prospective on opportunities for the future

Methods for preserving specimen hydration in protein crystals were pursued in the early 1970s as a prerequisite for protein crystallography using an electron microscope. Three laboratories approached this question from very different directions. One built a differentially pumped hydration chamber that could maintain the crystal in a liquid water environment, a second maintained hydration by rapidly freezing the protein crystal and examining it in a cold stage, and the third replaced the water of hydration by using glucose in the same way as one had previously used "negative stains". Each of these early efforts succeeded in preserving the structures of protein crystals at high resolution within the vacuum of the electron microscope, as demonstrated by electron diffraction patterns. The next breakthrough came in the early 1980s when a technique was devised to preserve noncrystalline specimens by freezing them within vitreous ice. Since then, with the development of high stability cold stages and transfer mechanisms compatible with many instrument platforms, and by using commercially provided low dose imaging techniques to avoiding radiation damage, there has been an explosion of applications. These now include single particles, helical filaments, 2-D arrays and even whole cells, where the most exciting recent applications involve cryoelectron tomography. These achievements and possibilities generate a new set of research opportunities associated with increasing the reliability and throughput with which specimens can be studied by cryoEM.

Adding the third dimension to virus life cycles: three-dimensional reconstruction of icosahedral viruses from cryo-electron micrographs

Viruses are cellular parasites. The linkage between viral and host functions makes the study of a viral life cycle an important key to cellular functions. A deeper understanding of many aspects of viral life cycles has emerged from coordinated molecular and structural studies carried out with a wide range of viral pathogens. Structural studies of viruses by means of cryo-electron microscopy and three-dimensional image reconstruction methods have grown explosively in the last decade. Here we review the use of cryo-electron microscopy for the determination of the structures of a number of icosahedral viruses. These studies span more than 20 virus families. Representative examples illustrate the use of moderate- to low-resolution (7- to 35-A) structural analyses to illuminate functional aspects of viral life cycles including host recognition, viral attachment, entry, genome release, viral transcription, translation, proassembly, maturation, release, and transmission, as well as mechanisms of host defense. The success of cryo-electron microscopy in combination with three-dimensional image reconstruction for icosahedral viruses provides a firm foundation for future explorations of more-complex viral pathogens, including the vast number that are nonspherical or nonsymmetrical.

Cryo-electron microscopy of vitrified specimens

Cryo-electron microscopy of vitrified specimens was just emerging as a practical method when Richard Henderson proposed that we should teach an EMBO course on the new technique. The request seemed to come too early because at that moment the method looked more like a laboratory game than a useful tool. However, during the months which ellapsed before the start of the course, several of the major difficulties associated with electron microscopy of vitrified specimens found surprisingly elegant solutions or simply became non-existent. The course could therefore take place under favourable circumstances in the summer of 1983. It was repeated the following years and cryo-electron microscopy spread rapidly. Since that time, water, which was once the arch enemy of all electronmicroscopists, became what it always was in nature – an integral part of biological matter and a beautiful substance.

X-ray microanalysis of freeze-dried and frozen-hydrated cryosections

The elemental composition and the ultrastructure of biological cells were studied by scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray microanalysis. The preparation technique involves cryofixation, cryoultramicrotomy, cryotransfer, and freeze-drying of samples. Freeze-dried cryosections 100-nm thick appeared to be appropriate for measuring the distribution of diffusible elements and water in different compartments of the cells. The lateral analytical resolution was less than 50 nm, depending on ice crystal damage and section thickness. The detection limit was in the range of 10 mmol/kg dry weight for all elements with an atomic number higher than 12; for sodium and magnesium the detection limits were about 30 and 20 mmol/kg dry weight, respectively. The darkfield intensity in STEM is linearly related to the mass thickness. Thus, it becomes possible to measure the water content in intracellular compartments by using the darkfield signal of the dry mass remaining after freeze-drying. By combining the X-ray microanalytical data expressed as dry weight concentrations with the measurements of the water content, physiologically more meaningful wet weight concentrations of elements were determined. In comparison to freeze-dried cryosections frozen-hydrated sections showed poor contrast and were very sensitive against radiation damage, resulting in mass loss. The high electron exposure required for recording X-ray spectra made reproducible microanalysis of ultrathin (about 100-nm thick) frozen-hydrated sections impossible. The mass loss could be reduced by carbon coating; however, the improvement achieved thus far is still insufficient for applications in X-ray microanalysis. Therefore, at present only bulk specimens or at least 1-micron thick sections can be used for X-ray microanalysis of frozen-hydrated biological samples.

Ultrastructural visualization of unfixed and unstained whole mounts by high-voltage electron microscopy at low temperatures

A physical procedure for the visualization of cellular fine structures is described as an alternative to chemical preparative techniques. It consists of fixation by fast freezing followed by controlled etching in the cryostage of a million-volt transmission electron microscope. Whole mounts were thus observed under stable conditions with no use of chemical fixatives, solvents, or stains, with no exposure to the atmosphere, and with the improved penetration and resolution in thick specimens that characterize high-voltage electron microscopy. The preservation, contrast, and resolution exhibited by images of preparations obtained by this procedure are discussed.

Preparation of frozen-hydrated specimens for high resolution electron microscopy

A method is presented for preserving the high resolution structure of biological membranes in a frozen-hydrated environment for electron microscopy. The technique consists of sandwiching a specimen between two carbon films and then waiting while some of the solvent evaporates. When the solvent layer is judged to be of an appropriate thickness, the specimen is then frozen in liquid nitrogen. The specimen can then be inserted into the precooled stage of an electron microscope. Electron diffraction studies of the purple membrane of Halobacterium halobium recorded at -120 degrees C have shown that the structure can be preserved to a resolution of 3.5 A. The main advantage of this method over previous techniques is that the hydrating conditions can be accurately controlled.

Electron microscopy of frozen hydrated sections of vitreous ice and vitrified biological samples

The preparation and high resolution observation of frozen hydrated thin sections has been studied by transmission electron microscopy (TEM and STEM) on model systems, including pure water, protein solutions, catalase crystals, myelin sheath and various tissues. The state of the ice is determined by electron diffraction. Mass measurement in the electron microscope is used to determine section thickness and control hydration. An adequate depth of vitrified material for sectioning can be obtained from many biological suspensions or untreated tissues. Frozen hydrated sections around 100 nm thick can be produced under optimal conditions from vitreous ice or from vitrified biological samples. Sectioning, transfer and observation in the electron microscope is feasible without alteration of the sample hydration or its initial vitrification. Biological structures can be preserved and observed down to 10 nm. Under favourable working conditions, specimen compression during sectioning and electron beam damage are the factors limiting high resolution observations.

Imaging vesicular dispersions with cold-stage electron microscopy

A fast-freeze, cold-stage transmission electron microscopy technique which can incorporate in situ freeze-drying of the sample is described. Its use in elucidating structure in unstained and stained, hydrated and freeze-dried, aqueous vesicular dispersions of biological and chemical interest is demonstrated with vesicles of L-alpha-phosphatidylcholine (bovine phosphatidylcholine) and of the synthetic surfactant sodium 4-(1'-heptylnonyl)benzenesulfonate (SHBS). The contrast features observed in transmission electron microscope images of frozen, hydrated samples are identified and explained with the dynamical theory of electron diffraction. Radiolysis by the electron beam is shown to increase contrast in vesicle images and to change their structure and size. Micrographs illustrate the freeze-drying of a dispersion in the microscope; the process causes vesicles to shrink and collapse.

Preparation of biological cryosections for analytical electron microscopy

A cooling chain for studies of ultrastructure and elemental composition of cryofixed biological cells and tissues is described. The technique is demonstrated for yeast cells. The preparation steps such as cryofixation, cryosectioning, transfer into the electron microscope as well as imaging and X-ray microanalysis in the STEM are discussed with respect to the present possibilities and problems. Whereas freeze-dried cryosections can be studied routinely, imaging of the ultrastructure and measurement of the elemental distribution in frozen-hydrated sections turn out to be limited.

A cooling chain for studies of cryofixed biological specimens by scanning transmission electron microscopy and X-ray microanalysis

A cooling chain is described which enables the transfer of frozen hydrated biological specimens (ultrathin cryosections as well as about 1 micrometer thick cultured cells) from a cryoultramicrotome into a scanning transmission electron microscope with a field emission gun. Transfer is done at 118 K, specimen temperature in the microscope is 165 K. Sublimation processes are controlled visually and by mass spectrometry. Electron micrographs and X-ray microanalytical spectra of cryofixed unstained tissue culture cells and rat liver tissue sections are described and discussed. Contamination of the specimen is much reduced by use of the cold stage.

Structure determination of frozen, hydrated, crystalline biological specimens

Low temperature transmission electron microscopy can be used to study the structure of biological materials in the hydrated state. Spatial averaging techniques are necessary to overcome the radiation damage problems, and for this reason the techniques described are most applicable to crystalline objects. However, with thin, crystalline biological specimens there are no difficulties with preserving periodicity during the freezing process. Improved specimen preparation methods are described which achieve the production of very thin aqueous films with the hydrated specimen embedded in them. Defocused bright field images of frozen, hydrated protein crystals possess a surprisingly high contrast, presumably due to the difference in density of the protein and the aqueous phase. The techniques described have been used to study the crystal structure of hydrated catalase and the outermost cell wall of Spirillum serpens.

Quantitative X-ray microanalysis of diffusible ions in the skeletal muscle bulk specimen

X-ray microanalysis in the scanning electron microscope (SEM) is used to obtain information about the distribution and mobility of electrolyte ions in the skeletal muscle tissue. In order to localize the diffusible ions in their physiological state, so far as possible, any chemical fixative is avoided. A special 'cooling chain' preparation method is applied which enables studies on cryofractured bulk specimens at low temperatures. Quantitation of the obtained X-ray spectra is done by the use of glycerol-treated tissue specimens which are incubated with electrolyte solutions of defined ion concentration.

Instrumentation for direct observation of frozen hydrated specimens in the electron microscope

Instrumentation and techniques are described for the transfer and observation of frozen hydrated specimens in the transmission electron microscope. The transfer is accomplished without the complexity of a vacuum transfer device but also without significant sublimation of specimen ice or frosting. Examples are given of the transfer and observation of thin sections of rapidly frozen muscle and of rapidly frozen thin film preparations of isolated cells.