Biophysical mechanism of ultrafast helical twisting contraction in the giant unicellular ciliate Spirostomum ambiguum

Abstract

The biophysical mechanism of cytoskeletal structures has been fundamental to understanding of cellular dynamics. Here, we present a mechanism for the ultrafast contraction exhibited by the unicellular ciliate Spirostomum ambiguum. Powered by a Ca 2+ binding myoneme mesh architecture, Spirostomum is able to twist its two ends in the same direction and fully contract to 75% of its body length within five milliseconds, followed by a slow elongation mechanism driven by the uncoiling of the microtubules. To elucidate the principles of this rapid contraction and slow elongation cycle, we used high-speed imaging to examine the same-direction coiling of the two ends of the cell and immunofluorescence techniques to visualize and quantify the structural changes in the myoneme mesh, microtubule arrays, and the cell membrane. Lastly, we provide support for our hypotheses using a simple physical model that captures key features of Spirostomum's ultrafast twisting contraction. SIGNIFICANCEUltrafast movements are ubiquitous in nature, and some of the most fascinating ultrafast biophysical systems are found on the cellular level. Quantitative studies and models are key to understand the biophysics of these fast movements. In this work, we study Spirostomum's ultrafast contraction by using high-speed imaging, labeling relevant cytoskeletal structures, and building a physical model to provide a biophysical mechanism especially of the helical same direction twisting of this extremely large single cell organism. Deeper understanding of how single cells can execute extreme shape changes hold potential for advancing basic cell biophysics and also inspire new cellular inspired actuators for engineering applications.

A unified model for the dynamics of ATP-independent ultrafast contraction

“…We obtained Spirostomum organisms from Carolina Biological and cultured them at room temperature. We prepared culturing medium by mixing one part boiled hay medium solution (Ward's Science, 470177-390), four parts boiled spring water, and two boiled wheat grains (Carolina, Item #132425) as described previously 32 . We inoculated organisms into fresh media approximately twice per week.…”

“…Among several other presumably important physical features of the organisms, we have neglected possible volume or area conservation of the cells, any spatial variation in the drag coefficient along the cell's length 38,45 , any twisting dynamics or three-dimensional geometry of the cell bodies 32,45 , any braking dynamics contributed by the entanglement of the internal organelles 75 , and any intermediate Reynolds number corrections to the viscous drag 38,45 . Our main justification for these omissions is that we are interested here in the one-dimensional compression dynamics of the myoneme mesh, neglecting for example the twist dynamics and anisotropic mesh contraction which have been observed in Spirostomum 32 . By neglecting the twist dynamics we do not allow for coupling between twisting and stretching, which is a feature of some biological materials like DNA.…”

“…ATP is subsequently consumed to pump Ca 2+ away from the myoneme fibers and reset the process 11,29 . While several experimental studies have been conducted on myoneme-based contraction a) Electronic mail: mary.elting@ncsu.edu b) Electronic mail: svaikunt@uchicago.edu c) Electronic mail: dinner@uchicago.edu d) Electronic mail: saadb@chbe.gatech.edu dynamics [7][8][9][10][11][30][31][32][33][34][35][36][37][38] , it is difficult to draw general conclusions without a theoretical framework to interpret the observations. Whereas actomyosin contractility has been extensively studied theoretically and computationally [39][40][41][42][43][44] , Ca 2+ -powered myoneme-based contractility is relatively unexplored from a modeling point of view.…”

Giant proteins in a giant cell: Molecular basis of ultrafast Ca 2+ -dependent cell contraction

“…The giant heterotrich ciliate Spirostomum is characterized by its rod-like cell that is usually one to several millimeters in length. Upon contraction, the cell of Spirostomum typically twists and shortens its length up to 75% within 5 ms (7,8). In past decades, several studies have been carried out to elucidate the subcellular structures and biophysical features of this fascinating system (7,(9)(10)(11)(12)(13).…”

A Tunable, Simplified Model for Biological Latch Mediated Spring Actuated Systems

“…Models have been developed to understand the extreme biomechanics of latch-mediated spring actuated organisms. Organism-specific models, including both continuum mechanics-based models (2)(3)(4)(5)(6)(7)(8)(9)(10) and physical modeling with biomimetic devices (2,(10)(11)(12)(13)(14), have been used to test hypotheses about the mechanisms of movement in specific organisms (Table 1 summarizes examples of recent work).…”