Rho-GTPases are central regulators within a complex signaling network that controls the cytoskeletal organization and cell movement. This network includes multiple GTPases, such as the most studied Rac1, Cdc42, and RhoA, and their numerous effectors that provide mutual regulation and feedback loops. Here we investigate the temporal and spatial relationship between Rac1 and Cdc42 during membrane ruffling using a simulation model which couples GTPase signaling with cell morphodynamics to capture the GTPase behavior observed with FRET-based biosensors. We show that membrane velocity is regulated by the kinetic rate of GTPase activation rather than the concentration of active GTPase. Our model captures both uniform and polarized ruffling. We also show that cell-type specific time delays between Rac1 and Cdc42 activation can be reproduced with a single signaling motif, in which the delay is controlled by feedback from Cdc42 to Rac1. The resolution of our simulation output matches those of the time-lapsed recordings of cell dynamics and GTPase activity. This approach allows us to validate simulation results with quantitative precision using the same pipeline for the analysis of simulated and experimental data.

/pdf/spatiotemporal-coordination-of-rac1-and-cdc42-at-the-whole-16jbxrk4.pdf

Spatiotemporal Coordination of Rac1 and Cdc42 at the Whole Cell Level during Cell Ruffling

T cells migrate in nearly every healthy, inflamed or diseased tissue. Such 'all-terrain' motility is achieved by a dynamic mechanobiological balance between amoeboid and mesenchymal-like migration modes. Here, we report that septin proteins function as a key regulator of migratory balance in T cells. We show that active septins compartmentalize the lymphocyte's cortex into a peristaltically treadmilling 'tube' during the avoidance response to mechanically crowding hindrances. Cortical peristaltism along segmented T cell mechanically channels nucleus and cytoplasm translocation between mechanically crowding 3D collagen fibers. Septins' inactivation abruptly shifts T cell motility balance towards mesenchymal-like mode, characterized by distinct contact guidance and MAP4-, SEPT9-, HDAC6- mediated enhancement of microtubules and microtubule-associated dynein contractility. The non-stretchable microtubular cables secure structurally coherent cell passage through confining spaces and long-distance transmission of dynein-generated forces, which effectively replace diminished actomyosin contractility. Thus, septins provide T cells with a structural and signaling molecular switch between actomyosin-driven amoeboid and dynein-driven mesenchymal-like migration.

Septin-Mediated Mechanobiological Reprogramming of T Cell Transmigration and 3D Motility

The all-terrain motility of lymphocytes in tissues and tissue-like gels is best described as amoeboid motility. For amoeboid motility, lymphocytes do not require specific biochemical or structural modifications to the surrounding extracellular matrix. Instead, they rely on changing shape and steric interactions with the microenvironment. However, the exact mechanism of amoeboid motility remains elusive. Here we report that septins shape T cells for amoeboid motility. Specifically, septins form F-actin and alpha-actinin-rich cortical rings at the sites of cortex-indenting collisions of T cells with the extracellular matrix. Cortical rings compartmentalize cells into chains of spherical segments that are spatially conformed to the available lumens, forming transient ‘hourglass’-shaped steric locks onto the surrounding collagen fibers. The steric lock facilitates pressure-driven peristaltic propulsion of cytosolic content by individually contracting cell segments. Our results demonstrate that septins provide microenvironment-guided partitioning of actomyosin contractility and steric pivots for amoeboid motility of T cells in tissue-like microenvironments. GLOSSARY Steric interactions - interactions by the means of their spatial collision dependent on objects’ shapes. Steric guidance - cell navigation within crowded 3D environments, determined by the available passages around and between steric hindrances. Peristaltic treadmilling - locomotion mode by the means of a repeated sequence of polarized cell cortex extension, stabilization, and retraction, accompanied by translocation of nucleus and cytoplasm via circumferential cortex contractility. Significance Statement T cells can be highly motile, searching for cognate antigens or better yet targets in chimeric antigen receptor therapy settings. However, mechanisms of motility remain elusive for T cells migrating in structurally and biochemically diverse tissues. Here we address one pivotal question of basic and clinical immunology - How T cells achieve the ‘all-terrain’ motility? Here we decipher and report septin-based T cell motility in a 3D tissue-like environment. Specifically, we show that septins facilitate cell morphological responsiveness to the steric obstacles, i.e., collagen fiber-wise partitioning of actomyosin cortex contractility and cell-obstacle steric interactions. These responses coordinate peristaltic propulsion of the lymphocyte’s cytosolic content along its individually contracting cell segments, forming the obstacle-avoiding motility, i.e., circumnavigation, shared across various tested lymphocytes.

Septins Provide Microenvironment Sensing and Cortical Actomyosin Partitioning in Motile Amoeboid T Lymphocytes

Rho-GTPases are central regulators within a complex signaling network that controls cytoskeletal organization and cell movement. The network includes multiple GTPases, such as the most studied Rac1, Cdc42, and RhoA, along with their numerous effectors that provide mutual regulation through feedback loops. Here we investigate the temporal and spatial relationship between Rac1 and Cdc42 during membrane ruffling, using a simulation model that couples GTPase signaling with cell morphodynamics and captures the GTPase behavior observed with FRET-based biosensors. We show that membrane velocity is regulated by the kinetic rate of GTPase activation rather than the concentration of active GTPase. Our model captures both uniform and polarized ruffling. We also show that cell-type specific time delays between Rac1 and Cdc42 activation can be reproduced with a single signaling motif, in which the delay is controlled by feedback from Cdc42 to Rac1. The resolution of our simulation output matches those of time-lapsed recordings of cell dynamics and GTPase activity. Our data-driven modeling approach allows us to validate simulation results with quantitative precision using the same pipeline for the analysis of simulated and experimental data.

https://www.mdpi.com/2073-4409/12/12/1638/pdf?version=1686823129

Spatiotemporal Coordination of Rac1 and Cdc42 at the Whole Cell Level during Cell Ruffling.

The Rho family GTPases are molecular switches that regulate cytoskeletal dynamics and cell movement through a complex spatiotemporal organization of their activity. In Patiria miniata (starfish) oocytes under in vitro experimental conditions (with overexpressed Ect2, induced expression of Δ90 cyclin B, and roscovitine treatment), such activity generates multiple co-existing regions of coherent propagation of actin waves. Here we use computational modeling to investigate the development and properties of such wave domains. The model reveals that the formation of wave domains requires a balance between the activation and inhibition in the Rho signaling motif. Intriguingly, the development of the wave domains is preceded by a stage of low-activity quasi-static patterns, which may not be readily observed in experiments. Spatiotemporal patterns of this stage and the different paths of their destabilization define the behavior of the system in the later high-activity (observable) stage. Accounting for a strong intrinsic noise allowed us to achieve good quantitative agreement between simulated dynamics in different parameter regimes of the model and different wave dynamics in Patiria miniata and wild type Xenopus laevis (frog) data. For quantitative comparison of simulated and experimental results, we developed an automated method of wave domain detection, which revealed a sharp reversal in the process of pattern formation in starfish oocytes. Overall, our findings provide an insight into spatiotemporal regulation of complex and diverse but still computationally reproducible cell-level actin dynamics.

/pdf/spatiotemporal-development-of-coexisting-wave-domains-of-rho-1ca5sipbhv.pdf

Mary Kho

Papers

Spatiotemporal development of coexisting wave domains of Rho activity in the cell cortex