Biology subjects

Bement, W.

Publications and source records attributed to Bement, W..

4 recordsLinked to original sources

bioRxiv2024

Energy partitioning in the cell cortex

Living systems are driven far from thermodynamic equilibrium through the continuous consumption of ambient energy1. In the cell cortex, this energy is invested in the formation of diverse patterns in chemical and mechanical activities, whose unique spatial and temporal dynamics determine cell phenotypes and behaviors2-6. However, how cells partition internal energy between chemical and mechanical work is unknown7-9. Here we measured the entropy production rate (EPR) of both the chemical and mechanical subsystems of the cell cortex across a broad range of periodic patterns as the system is driven further from equilibrium via manipulation of the Rho GTPase pathway, which controls cortical actin filaments (F-actin) and myosin-II. We find that at lower levels of Rho GAP (GTPase activating protein) expression, which produce pulses or "choppy" Rho and F-actin waves, energy is comparably partitioned between the chemical and mechanical subsystems and is subject to the constraint of Onsager reciprocity. Within the range of reciprocity, the EPR is maximized in choppy waves that resemble the waves associated with cell division3,10. However, as the cortex is driven even further from equilibrium into elaborate labyrinthine or spiral traveling wave trains via increased GAP expression, reciprocity is broken, marking an increasingly differential partitioning of energy and an uncoupling of chemical and mechanical activities. We further demonstrate that energy partitioning and reciprocity are determined by the competition between the timescales of chemical reaction and mechanical relaxation. These results indicate that even within coupled cellular subsystems, both the relative proportions of energy partitioned to each subsystem and the ultimate phenotypic outcome vary dramatically as a function of the overall energy investment.

biophysics↗

bioRxiv2022

A versatile pattern-forming cortical circuit based on Rho, F-actin, Ect2 and RGA-3/4

Many cells can generate complementary traveling waves of actin filaments (F-actin) and cytoskeletal regulators. This phenomenon, termed cortical excitability, results from coupled positive and negative feedback loops of cytoskeletal regulators. The nature of these feedback loops, however, remains poorly understood. We assessed the role of the Rho GAP RGA-3/4 in the cortical excitability that accompanies cytokinesis in both frog and starfish. RGA-3/4 localizes to the cytokinetic apparatus, "chases" Rho waves in an F-actin-dependent manner and, when co-expressed with the Rho GEF Ect2, is sufficient to convert the normally quiescent, immature Xenopus oocyte cortex into a dramatically excited state. Experiments and modeling show that changing the ratio of RGA-3/4 to Ect2 produces a range of cortical behaviors from pulses to complex waves of Rho activity. We conclude that RGA-3/4, Ect2, Rho and F-actin form the core of a circuit that drives a diverse range of cortical behaviors, and demonstrate that the immature oocyte is a powerful model for characterizing these dynamics. SummaryMichaud et al. identify Ect2 and RGA-3/4 as core components of the cortical excitability circuit associated with cytokinesis. Additionally, they demonstrate that the immature Xenopus oocyte is a powerful model for characterizing excitable dynamics.

cell biology↗

bioRxiv2019

Extraction of active RhoGTPases by RhoGDI regulates spatiotemporal patterning of RhoGTPases

The RhoGTPases are characterized as membrane-associated molecular switches cycling between active, GTP-bound and inactive, GDP-bound states. However, 90-95% of RhoGTPases are maintained in a soluble form by RhoGDI, which is generally viewed as a passive shuttle for inactive RhoGTPases. Our current understanding of RhoGTPase:RhoGDI dynamics has been limited by two experimental challenges: direct visualization of the RhoGTPases in vivo and reconstitution of the cycle in vitro. We developed methods to directly image vertebrate RhoGTPases in vivo or on lipid bilayers in vitro. Using these tools, we identified pools of active and inactive RhoGTPase associated with the membrane, showed that RhoGDI can actively extract both inactive and active RhoGTPases, and that the extraction of active RhoGTPase contributes to their spatial regulation around wounds. In contrast to the textbook model of the RhoGTPase cycle, these results indicate that RhoGDI actively contributes to spatiotemporal patterning by removing active RhoGTPases from the plasma membrane.

cell biology↗

bioRxiv2018

Calcium nanodomains in spindles

The role of calcium signaling in specific events of animal cell meiosis or mitosis (M-phase) is a subject of enduring controversy. Early efforts suggested that increases in intracellular free calcium ([Ca2+]i) promote spindle disassembly 1, 2 while subsequent work suggested that global [Ca2+]i increases trigger nuclear envelope breakdown, spindle assembly, the metaphase-anaphase transition, and cytokinesis 3-6. However, further studies led to the conclusion that elevation of [Ca2+]i either has no role in these events, plays a permissive role in these events, or functions as an auxiliary signaling pathway that supplements other mechanisms 7. One potential explanation of the controversy is that specific M-phase events might depend on highly localized increases in [Ca2+]i, variously referred to as microdomains 8 or nanodomains 9, as proposed recently 10. Such domains are hypothesized to arise from rapid shuttling of calcium between closely positioned sources and sinks, rendering them potentially difficult to detect with traditional dyes and largely insensitive to slow chelators such as EGTA 9. Here a novel microtubule-binding calcium sensor--TubeCamp--was used to test the hypothesis 10 that spindles are associated with calcium nanodomains. TubeCamp imaging revealed that spindles in Xenopus eggs, Xenopus embryos, and HeLa cells were all associated with calcium nanodomains at the spindle poles. Calcium nanodomains also formed in spindles assembled in cell extracts and at the center of monopolar spindles, suggesting that they are a basic feature of spindle self-assembly. Disruption of calcium nanodomains via perturbation of inositol-1,4,5-trisphosphate signaling or rapid chelation of [Ca2+]i resulted in spindle disassembly in vivo and vitro. The results demonstrate the existence of spindle-associated calcium nanodomains and indicate that such domains are an essential and common feature of spindles in vertebrates.

cell biology↗