Predicting nucleation sites in chemotaxing Dictyostelium discoideum

Abstract

Blebs, pressure driven protrusions of the plasma membrane, facilitate the movement of cell such as the soil amoeba Dictyostelium discoideum in a three dimensional environment. The goal of the article is to develop a means to predict nucleation sites.We accomplish this through an energy functional that includes the influence of cell membrane geometry (membrane curvature and tension), membrane-cortex linking protein lengths as well as local pressure differentials. We apply the resulting functional to the parameterized microscopy images of chemotaxing Dictyostelium cells. By restricting the functional to the cell boundary influenced by the cyclic AMP (cAMP) chemo-attractant (the cell anterior), we find that the next nucleation site ranks high in the top 10 energy values. More specifically, if we look only at the boundary segment defined by the extent of the expected bleb, then 96.8% of the highest energy sites identify the nucleation. February 27, 20191/20This work concerns the prediction of nucleation sites in the soil amoeba-like 1 Dictyostelium discoideum. We define a real valued functional combining input from 2 cortex and membrane geometry such as membrane curvature and tension, cortex to 3 membrane separation and local pressure differences. We show that the functional may 4 be used to predict the location of bleb nucleation. In the region influenced by the 5 cAMP gradient (the cell anterior), the next blebbing site lies in the ten highest energy 6 functional values 70% of the time. The correctness increases to 96.8% provided we 7 restrict attention to the segment in the general location of the next bleb. We verify 8 these claims through the observation of microscopy images. The images are sequential 9 at 1.66 and 0.8 seconds per image. We first identify the earliest sign of the bleb. We 10 then use several observational factors to identify the nucleation site and estimate the 11 corresponding location in the prior image.12 21 D. discoideum is a species of soil-living amoeba-like organism. It is a eukaryotic 22 organism whose motility and shape is controlled predominantly by intricate actin based 23 structures in the cytoplasm. In turn, the cell cytoplasm is encased by a plasma 24 membrane, a 4-5nm thick semi-permeable lipid bilayer [38]. Beneath this membrane is 25 the cell cortex, an assembly of thin cross-linked actin filaments held together firmly by 26 cross linking proteins such as Filamin, α-Actinin, Fimbrin and Fascin. The cortex 27 thickness is several hundred nanometers [1]. The cortex plays a major role in 28 February 27, 2019 2/20 maintaining cell shape and the formation of motility structures. Its contractile 29 capabilities are due to the presence of Myosin II in the network. The membrane is 30 attached to the cortex by trans membrane proteins such as Talin [9]. In addition to the 31 cortex, the cytoplasm includes distinct actin-based structures referred to as the 32 cytoskeleton. The fluid part of the cytoplasm (cytosol) contains water, ions and 33 dissolved molecules. 34 Blebs and blebbing have ...