Three-dimensional architecture and assembly mechanism of the egg-shaped shell in testate amoeba Paulinella micropora

Mami Nomura; Keisuke Ohta; Yukinori Nishigami; Takuro Nakayama; Kei-Ichiro Nakamura; Kenjiro Tadakuma; Josephine Galipon

doi:10.3389/fcell.2023.1232685

Three-dimensional architecture and assembly mechanism of the egg-shaped shell in testate amoeba Paulinella micropora

Front Cell Dev Biol. 2023 Sep 4:11:1232685. doi: 10.3389/fcell.2023.1232685. eCollection 2023.

Authors

Mami Nomura¹, Keisuke Ohta^{2

3}, Yukinori Nishigami^{4

5}, Takuro Nakayama⁶, Kei-Ichiro Nakamura^{2

3}, Kenjiro Tadakuma^{7

8}, Josephine Galipon^{7

9

10

11}

Affiliations

¹ Faculty of Science, Yamagata University, Yamagata, Japan.
² Division of Microscopic and Developmental Anatomy, Department of Anatomy, Kurume University School of Medicine, Kurume, Japan.
³ Advanced Imaging Research Center, Kurume University School of Medicine, Kurume, Japan.
⁴ Graduate School of Life Science, Hokkaido University, Sapporo, Japan.
⁵ Research Institute for Electronic Science, Hokkaido University, Sapporo, Japan.
⁶ Center for Computational Sciences, University of Tsukuba, Tsukuba, Japan.
⁷ Graduate School of Information Sciences, Tohoku University, Sendai, Japan.
⁸ Tough Cyberphysical AI Research Center, Tohoku University, Sendai, Japan.
⁹ Institute for Advanced Sciences, Keio University, Tsuruoka, Japan.
¹⁰ Graduate School of Media and Governance, Keio University, Fujisawa, Japan.
¹¹ Graduate School of Science and Engineering, Yamagata University, Yonezawa, Japan.

Abstract

Unicellular euglyphid testate amoeba Paulinella micropora with filose pseudopodia secrete approximately 50 siliceous scales into the extracellular template-free space to construct a shell isomorphic to that of its mother cell. This shell-constructing behavior is analogous to building a house with bricks, and a complex mechanism is expected to be involved for a single-celled amoeba to achieve such a phenomenon; however, the three-dimensional (3D) structure of the shell and its assembly in P. micropora are still unknown. In this study, we aimed to clarify the positional relationship between the cytoplasmic and extracellular scales and the structure of the egg-shaped shell in P. micropora during shell construction using focused ion beam scanning electron microscopy (FIB-SEM). 3D reconstruction revealed an extensive invasion of the electron-dense cytoplasm between the long sides of the positioned and stacked scales, which was predicted to be mediated by actin filament extension. To investigate the architecture of the shell of P. micropora, each scale was individually segmented, and the position of its centroid was plotted. The scales were arranged in a left-handed, single-circular ellipse in a twisted arrangement. In addition, we 3D printed individual scales and assembled them, revealing new features of the shell assembly mechanism of P. micropora. Our results indicate that the shell of P. micropora forms an egg shape by the regular stacking of precisely designed scales, and that the cytoskeleton is involved in the construction process.

Keywords: 3D printer; 3D reconstruction; FIB-SEM; Paulinella; shell formation; testate amoeba.

Grants and funding

This research was supported by JSPS KAKENHI Grant Numbers, 19H05367 (MN), 21H00359 (MN), 21K15131 (MN), 22H05670 (MN), 21H05308 (YN), and JP16H06280; Grant-in-Aid for Scientific Research on Innovative Areas—Platforms for Advanced Technologies and Research Resources “Advanced Bioimaging Support.” A part of this study was supported by the Institute for Fermentation, Osaka (MN), and the Cooperative Research Program of the Network Joint Research Center for Materials and Devices (MN). The study was also supported in part by funds from Yamagata Prefecture and Tsuruoka City.