Energy partitioning in the cell cortex

Abstract

Living systems are driven far from thermodynamic equilibrium through the continuous consumption of ambient energy (1). 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 behaviors (2-6). However, how cells partition internal energy between chemical and mechanical work is unknown (7-9). 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.