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. Author summary This work concerns the prediction of nucleation sites in the soil amoeba-like Dictyostelium discoideum . We define a real valued functional combining input from cortex and membrane geometry such as membrane curvature and tension, cortex to membrane separation and local pressure differences. We show that the functional may be used to predict the location of bleb nucleation. In the region influenced by the cAMP gradient (the cell anterior), the next blebbing site lies in the ten highest energy functional values 70% of the time. The correctness increases to 96.8% provided we restrict attention to the segment in the general location of the next bleb. We verify these claims through the observation of microscopy images. The images are sequential at 1.66 and 0.8 seconds per image. We first identify the earliest sign of the bleb. We then use several observational factors to identify the nucleation site and estimate the corresponding location in the prior image.