Dual effect of tannic acid on the preservation and ultrastructure of phosphatidyl choline vesicles

CryoAPEX - an electron tomography tool for subcellular localization of membrane proteins

We describe a method, termed cryoAPEX, which couples chemical fixation and high-pressure freezing of cells with peroxidase tagging (APEX) to allow precise localization of membrane proteins in the context of a well-preserved subcellular membrane architecture. Further, cryoAPEX is compatible with electron tomography. As an example, we apply cryoAPEX to obtain a high-resolution three-dimensional contextual map of the human FIC (filamentation induced by cAMP) protein, HYPE (also known as FICD). HYPE is a single-pass membrane protein that localizes to the endoplasmic reticulum (ER) lumen and regulates the unfolded protein response. Alternate cellular locations for HYPE have been suggested. CryoAPEX analysis shows that, under normal and/or resting conditions, HYPE localizes robustly within the subdomains of the ER and is not detected in the secretory pathway or other organelles. CryoAPEX is broadly applicable for assessing both lumenal and cytosol-facing membrane proteins.

pH-dependent multilamellar structures in fetal mouse bone: possible involvement of fatty acids in bone mineralization

pH-dependent multilamellar structures in fetal mouse bone: possible involvement of fatty acids in bone mineralization. Am. J. Physiol. 266 (Cell Physiol. 35): C590-C600, 1994.--Multilamellar structures (MLS) were found inside and outside osteoblasts in cultured and uncultured fetal mouse parietal bone fixed at pH 7.3 with glutaraldehyde solution containing tannic acid (TA). Electron-lucent areas (up to 1.5 microns in diameter) surrounded by thin lamellar structures were found in place of MLS in bone and bone cell cultures fixed at pH 6.0 in the presence of TA. Large lipid droplets were found, in place of electron-lucent areas and MLS, in specimens fixed at pH 7.4 in the absence of TA and dehydrated with a procedure that did not extract neutral lipid. Freeze-fracture studies showed phosphatidylcholine formed MLS at both pH 8.1 and pH 6.0, whereas oleic acid formed MLS at pH 8.1 and lipid droplets at pH 6.0. Thus fatty acids probably formed the pH-dependent MLS found in bone. The data suggest that osteoblasts synthesize and secrete fatty acids, as droplets, into the extracellular space. The close association of MLS with calcifying osteoid in specimens processed with TA suggests that fatty acids are directly involved in bone mineralization.

Freeze-substitution of chemically stabilized samples for biological field emission scanning electron microscopy

The high resolution imaging capabilities of modern field emission scanning electron microscopes require adequately improved tissue preparation procedures to prevent the collapse of macromolecular structures and the extraction of molecules. A routine cryo-stabilization technique is described which utilizes chemical crosslinking and cryo-dehydration for mechanical and chemical stabilization of protein and lipid structures and increase of electrical conductivity of the sample. Thiocarbohydrazide (TCH) serves as a general mordant for osmium tetroxide crosslinking. However, extensive washing after all impregnation steps is necessary to dissolve unspecific osmium black precipitations at the sample surface. Collagen I aggregates showed increased stability against collapse after TCH osmification alone, whereas pulmonary surfactant liposomes require additional freeze-substitution in methanol and Freon 113 for stabilization during critical point drying. Environmental scanning electron microscopy (at water vapor pressures of 5-10 torr within the specimen chamber) was used to control, in the wet phase, the stabilization procedure at the level of chemical crosslinkage. It could be confirmed that tannic acid, often used to stabilize lipids, leads to artificial rearrangement of bilayered liposomes into compact presumable multilayered bodies, whereas the TCH osmification preserved liposome structures and their aggregates. The increase of electrical conductivity of sliced tissue was demonstrated on kidney. Support technologies for the cryo-stabilization procedures are described in detail, as well as simple routines for first stabilization trials with new samples. On pulmonary tissue, the excellent preservation of alveolar shape and fine structures of intermediate forms of surfactant are described.