Why does a cleavage plane develop parallel to the spindle axis in conical sand dollar eggs? A key question for clarifying the mechanism of contractile ring positioning

Classical and Emerging Regulatory Mechanisms of Cytokinesis in Animal Cells

The primary goal of cytokinesis is to produce two daughter cells, each having a full set of chromosomes. To achieve this, cells assemble a dynamic structure between segregated sister chromatids called the contractile ring, which is made up of filamentous actin, myosin-II, and other regulatory proteins. Constriction of the actomyosin ring generates a cleavage furrow that divides the cytoplasm to produce two daughter cells. Decades of research have identified key regulators and underlying molecular mechanisms; however, many fundamental questions remain unanswered and are still being actively investigated. This review summarizes the key findings, computational modeling, and recent advances in understanding of the molecular mechanisms that control the formation of the cleavage furrow and cytokinesis.

Agent-based modeling: case study in cleavage furrow models

The number of studies in cell biology in which quantitative models accompany experiments has been growing steadily. Roughly, mathematical and computational techniques of these models can be classified as "differential equation based" (DE) or "agent based" (AB). Recently AB models have started to outnumber DE models, but understanding of AB philosophy and methodology is much less widespread than familiarity with DE techniques. Here we use the history of modeling a fundamental biological problem-positioning of the cleavage furrow in dividing cells-to explain how and why DE and AB models are used. We discuss differences, advantages, and shortcomings of these two approaches.

How to be at the right place at the right time: the importance of spindle positioning in embryos

Spindle positioning is an imperative cellular process that regulates a number of different developmental events throughout embryogenesis. The spindle must be properly positioned in embryos not only for the segregation of chromosomes, but also to segregate developmental determinants into different daughter blastomeres. In this review, the role of spindle positioning is explored in several different developmental model systems, which have revealed the diversity of factors that regulate spindle positioning. The C. elegans embryo, the Drosophila neuroblast, and ascidian embryos have all been utilized for the study of polarity-dependent spindle positioning, and exploration of the proteins that are required for asymmetric cell division. Work in the sea urchin embryo has examined the influence of cell shape and factors that affect secondary furrow formation. The issue of size scaling in extremely large cells, as well as the requirement for spindle positioning in developmental fate decisions in vertebrates, has been addressed by work in the Xenopus embryo. Further work in mouse oocytes has examined the roles of actin and myosin in spindle positioning. The data generated from these model organisms have made unique contributions to our knowledge of spindle positioning. Future work will address how all of these different factors work together to regulate the position of the spindle.

Theoretically predicted effects of Gaussian curvature on lateral diffusion of membrane molecules

Lateral diffusion on curved biological membranes has been studied theoretically and experimentally. However, how membrane geometries influence the diffusion process remains unclear. Here we show the significance of Gaussian curvature by numerically solving the diffusion equation in a geodesic polar coordinate system with regard to several types of surfaces including elliptic and hyperbolic paraboloids. On surfaces where Gaussian curvature has positive and negative values, diffusion is slower and faster than on the plane, respectively. The deviation from the normal diffusion on the plane tends to get larger as the absolute value of Gaussian curvature increases. Diffusion is anisotropic at a surface region where the normal curvature is anisotropic and Gaussian curvature has nonzero values. The anisotropy can be classified into several types according to whether diffusion is the fastest or the slowest in the principal directions. In the case of diffusion on spheroids, the limited area of a closed surface reduces the diffusion rate so greatly that the slowdown effects of positive values of Gaussian curvature are concealed. Analysis of the diffusion equation suggests that Gaussian curvature causes slowed or accelerated diffusion and anisotropic diffusion in any type of surface. Furthermore, it is discussed the degree to which Gaussian curvature influences diffusive phenomena taking place in real membranes through such effects. These results provide a different image of biological membranes that lateral diffusion of membrane molecules is usually anisotropic and the diffusion rate kaleidoscopically changes according to place.

New roles for Galpha and RGS proteins: communication continues despite pulling sisters apart

Large G protein alpha subunits and their attendant regulators of G-protein signaling (RGS) proteins control both intercellular signaling and asymmetric cell divisions by distinct pathways. The classical pathway, found throughout higher eukaryotic organisms, mediates intercellular communication via hormone binding to G-protein-coupled receptors (GPCRs). Recent studies have led to the discovery of GPCR-independent activation of Galpha subunits by the guanine nucleotide exchange factor RIC-8 in both asymmetric cell division and synaptic vesicle priming in metazoan organisms. Protein-protein interactions and protein function in each pathway are driven through the cycle of GTP binding and hydrolysis by the Galpha subunit. This review builds a conceptual framework for understanding RIC-8-mediated pathways by comparison with the mechanism of classical G-protein activation and inhibition in GPCR signaling.