Sodium Cholate-Based Active Delipidation for Rapid and Efficient Clearing and Immunostaining of Deep Biological Samples

Recent surges of optical clearing provided anatomical maps to understand structure-function relationships at organ scale. Detergent-mediated lipid removal enhances optical clearing and allows efficient penetration of antibodies inside tissues, and sodium dodecyl sulfate (SDS) is the most common choice for this purpose. SDS, however, forms large micelles and has a low critical micelle concentration (CMC). Theoretically, detergents that form smaller micelles and higher CMC should perform better but these have remained mostly unexplored. Here, SCARF, a sodium cholate (SC)-based active delipidation method, is developed for better clearing and immunolabeling of thick tissues or whole organs. It is found that SC has superior properties to SDS as a detergent but has serious problems; precipitation and browning. These limitations are overcome by using the ion-conductive film to confine SC while enabling high conductivity. SCARF renders orders of magnitude faster tissue transparency than the SDS-based method, while excellently preserving the endogenous fluorescence, and enables much efficient penetration of a range of antibodies, thus revealing structural details of various organs including sturdy post-mortem human brain tissues at the cellular resolution. Thus, SCARF represents a robust and superior alternative to the SDS-based clearing methods and is expected to facilitate the 3D morphological mapping of various organs.

Application of Zwitterionic Polymer Hydrogels to Optical Tissue Clearing for 3D Fluorescence Imaging

Zwitterionic polymers have both anion and cation groups in the side chain and have been used in various biomedical applications because of the unique properties. In this study, zwitterionic polymer hydrogels are applied to optical tissue clearing for 3D fluorescence imaging. Polyacrylamide hydrogels have been employed in Clear Lipid-exchanged Acrylamide-hybridized Rigid Imaging/Immunostaining/In situ-hybridization-compatible Tissue-hYdrogel method. Zwitterionic polymer hydrogels are produced using zwitterionic monomers, such as 3-[(3-acrylamidopropyl)dimethylammonio]propane-1-sulfonate (DAPS) and 2-methacryloyloxyethyl phosphorylcholine (MPC), and crosslinkers. The hydrogels made from poly(DAPS-co-acrylamide) and MPC homopolymers afford the most transparent tumor tissues. However, the tissues cleared using DAPS copolymers-containing hydrogels became turbid in a refractive index-matching solution, which are unable to obtain clear 3D fluorescence images. In contrast, the 3D fluorescence imaging is achieved in the MPC polymer-treated 2-mm-thick brain slices after immunostaining. The 3D fluorescence imaging of lung metastasis that is cleared by the MPC hydrogel to demonstrate the possible application to cancer diagnosis is performed. The results indicate the increased potentials of zwitterionic polymer hydrogels, especially MPC polymer hydrogels, in biomedical applications.

Whole-Tissue Immunolabeling and 3D Fluorescence Imaging to Visualize Axon Degeneration in the Intact, Unsectioned Mouse Tissues

Axon degeneration destructs functional connectivity of neural circuits and is one of the common, key pathological features of different neurodegenerative diseases. However, conventional histochemistry methods, which largely rely on tissue sections, have intrinsic limitations in examining the 3D distribution of axonal structures on the whole-tissue level. This technical shortcoming has continuously impeded our in-depth understanding of pathological axon degeneration in many scenarios. To overcome such drawback encountered in the research field, we describe here a general protocol of whole-tissue immunolabeling and 3D fluorescence imaging technique to visualize axon degeneration in the intact, unsectioned mouse tissues. In particular, experimental steps of tissue harvesting, whole-tissue immunolabeling, tissue optical clearing, and 3D fluorescence imaging have been systematically optimized, which makes the protocol effective for assessing integrity of the axonal structures in a variety of tissues. Notably, it has enabled the 3D fluorescence imaging of chemotherapy- or traumatic injury-induced axon degeneration within the bones (e.g., femurs) or bone-containing tissues (e.g., hindpaws), which had previously been inaccessible to conventional histochemistry methods. This protocol is therefore readily compatible with many areas of the research on axon degeneration and is poised to serve the field in future investigations.

Development of 3D imaging technique of reconstructed human epidermis with immortalized human epidermal cell line

The epidermis, the outermost layer of the skin, retains moisture and functions as a physical barrier against the external environment. Epidermal cells are continuously replaced by turnover, and thus to understand in detail the dynamic cellular events in the epidermis, techniques to observe live tissues in 3D are required. Here, we established a live 3D imaging technique for epidermis models. We first obtained immortalized human epidermal cell lines which have a normal differentiation capacity and fluorescence-labelled cytoplasm or nuclei. The reconstituted 3D epidermis was prepared with these lines. Using this culture system, we were able to observe the structure of the reconstituted epidermis live in 3D, which was similar to an in vivo epidermis, and evaluate the effect of a skin irritant. This technique may be useful for dermatological science and drug development.