The Japan Aerospace Exploration Agency (JAXA) started a high‐quality protein crystal growth project, now called JAXA PCG, on the International Space Station (ISS) in 2002. Using the counter‐diffusion technique, 14 sessions of experiments have been performed as of 2012 with 580 proteins crystallized in total. Over the course of these experiments, a user‐friendly interface framework for high accessibility has been constructed and crystallization techniques improved; devices to maximize the use of the microgravity environment have been designed, resulting in some high‐resolution crystal growth. If crystallization conditions were carefully fixed in ground‐based experiments, high‐quality protein crystals grew in microgravity in many experiments on the ISS, especially when a highly homogeneous protein sample and a viscous crystallization solution were employed. In this article, the current status of JAXA PCG is discussed, and a rational approach to high‐quality protein crystal growth in microgravity based on numerical analyses is explained.
It is said that the microgravity environment positively affects the quality of protein crystal growth. The formation of a protein depletion zone and an impurity depletion zone due to the suppression of convection flow were thought to be the major reasons. In microgravity, the incorporation of molecules into a crystal largely depends on diffusive transport, so the incorporated molecules will be allocated in an orderly manner and the impurity uptake will be suppressed, resulting in highly ordered crystals. Previously, these effects were numerically studied in a steady state using a simplified model and it was determined that the combination of the diffusion coefficient of the protein molecule (D) and the kinetic constant for the protein molecule () could be used as an index of the extent of these depletion zones. In this report, numerical analysis of these depletion zones around a growing crystal in a non-steady (i.e. transient) state is introduced, suggesting that this model may be used for the quantitative analysis of these depletion zones in the microgravity environment.
Epithelioid trophoblastic tumor (ETT) is a rare type of gestational trophoblastic disease and only 25 cases have been reported so far. It was first proposed by Mazur and Kurman in 1994 as an unusual type of trophoblastic tumor that is distinct from placental site trophoblastic tumor and choriocarcinoma and has features resembling carcinoma. A case of ETT of the lung in a 38-year-old Japanese woman is reported. The patient had suffered from a hydatidiform mole at the age of 27 years, and had four normal deliveries at the ages of 24, 31, 35 and 37 years. Because no tumor lesions were detected in the uterus, the patient was suspected of having metastatic choriocarcinoma with multiple lesions in the lung accompanied by an elevated level of human chorionic gonadotropin (hCG). In order to make an exact diagnosis, a partial resection of metastatic foci in the lung was performed. Microscopically, the tumor showed hemorrhagic necrotic foci and was composed of mainly mononuclear tumor cells and some giant tumor cells resembling trophoblastic cells. Immunohistochemical examination showed that a few large cells were stained positively for hCG, and that other cells were positive for human placental lactogen, pregnancy-specific beta1-glycoprotein, cytokeratin 7 and inhibin-alpha. In the ultrastructure, the tumor cells contained large nuclei and rich organella with desmosomes and well-formed filaments. The diagnosis of ETT was confirmed from the findings as described above.
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