Electron cytochemical stains based on metal chelation

Localization of Mitochondrial Nucleoids by Transmission Electron Microscopy Using the Transgenic Expression of the Mitochondrial Helicase Twinkle and APEX2

Reminiscent of their evolutionary origin, mitochondria contain their own genome (mtDNA) compacted into the mitochondrial chromosome or nucleoid (mt-nucleoid). Many mitochondrial disorders are characterized by disruption of mt-nucleoids, either by direct mutation of genes involved in mtDNA organization or by interfering with other vital proteins for mitochondrial function. Thus, changes in mt-nucleoid morphology, distribution, and structure are a common feature in many human diseases and can be exploited as an indicator of cellular fitness. Electron microscopy provides the highest possible resolution that can be achieved, delivering spatial and structural information about all cellular structures. Recently, the ascorbate peroxidase APEX2 has been used to increase transmission electron microscopy (TEM) contrast by inducing diaminobenzidine (DAB) precipitation. DAB has the ability to accumulate osmium during classical EM sample preparation and, due to its high electron density, provides strong contrast for TEM. Among the nucleoid proteins, the mitochondrial helicase Twinkle fused with APEX2 has been successfully used to target mt-nucleoids, providing a tool to visualize these subcellular structures with high contrast and with the resolution of an electron microscope. In the presence of H₂O₂, APEX2 catalyzes the polymerization of DAB, generating a brown precipitate that can be visualized in specific regions of the mitochondrial matrix. Here, we provide a detailed protocol to generate murine cell lines expressing a transgenic variant of Twinkle, suitable to target and visualize mt-nucleoids. We also describe all the necessary steps to validate the cell lines prior to electron microscopy imaging and offer examples of anticipated results.

An ultrastructural study of histochemical staining of complex carbohydrates in the mouse posterior vitreous body

The vitreous body contains complex carbohydrates that can be demonstrated morphologically. Vitreous hyaluronic acid is very soluble but it can be precipitated by cetylpyridinium chloride (CPC) while being cross-linked by glutaraldehyde. Oligosaccharide chains of vitreous glycoproteins are fixed with glutaraldehyde alone. Mouse eyes were fixed with glutaraldehyde or glutaraldehyde and CPC and the complex carbohydrates of the posterior vitreous cortex were studied by electron microscopy. Cationic dyes were used in the fixative or for block-staining on most fixed tissue blocks to allow detailed observations of complex carbohydrates. Most blocks were postfixed with OsO4. The hyaluronic-acid domain on vitreous collagen fibrils sequentially contracted and expanded in size with various histochemical manipulations. Contraction of the domain of hyaluronic acid generally indicates an increased charge density. OsO4 contributes considerable charge density upon forming osmate esters, but tissue postfixed with OsO4 contained large globular forms of hyaluronic acid rather than the small globules observed in non-osmicated preparations. A model is proposed to explain the seemingly paradoxical findings by reference to suggested mechanisms of polysaccharide-ligand-OsO4 interactions.