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1. Probing coordinated co-culture cancer related motility through differential micro-compartmentalized elastic substrates

   https://doi.org/10.1038/s41598-020-74575-y  

   Chou, Szu-Yuan; Lin, Chang-You; Cassino, Theresa; Wan, Li; LeDuc, Philip R. (December 2020, Scientific Reports)

   Abstract Cell development and behavior are driven by internal genetic programming, but the external microenvironment is increasingly recognized as a significant factor in cell differentiation, migration, and in the case of cancer, metastatic progression. Yet it remains unclear how the microenvironment influences cell processes, especially when examining cell motility. One factor that affects cell motility is cell mechanics, which is known to be related to substrate stiffness. Examining how cells interact with each other in response to mechanically differential substrates would allow an increased understanding of their coordinated cell motility. In order to probe the effect of substrate stiffness on tumor related cells in greater detail, we created hard–soft–hard (HSH) polydimethylsiloxane (PDMS) substrates with alternating regions of different stiffness (200 and 800 kPa). We then cultured WI-38 fibroblasts and A549 epithelial cells to probe their motile response to the substrates. We found that when the 2 cell types were exposed simultaneously to the same substrate, fibroblasts moved at an increased speed over epithelial cells. Furthermore, the HSH substrate allowed us to physically guide and separate the different cell types based on their relative motile speed. We believe that this method and results will be important in a diversity of areas including mechanical microenvironment, cell motility, and cancer biology. 

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2. Collective States of Active Particles With Elastic Dipolar Interactions

   https://doi.org/10.3389/fphy.2022.876126  

   Bose, Subhaya; Noerr, Patrick S.; Gopinathan, Ajay; Gopinath, Arvind; Dasbiswas, Kinjal (May 2022, Frontiers in Physics)

   Many types of animal cells exert active, contractile forces and mechanically deform their elastic substrate, to accomplish biological functions such as migration. These substrate deformations provide a mechanism in principle by which cells may sense other cells, leading to long-range mechanical inter–cell interactions and possible self-organization. Here, inspired by cell mechanobiology, we propose an active matter model comprising self-propelling particles that interact at a distance through their mutual deformations of an elastic substrate. By combining a minimal model for the motility of individual particles with a linear elastic model that accounts for substrate-mediated, inter–particle interactions, we examine emergent collective states that result from the interplay of motility and long-range elastic dipolar interactions. In particular, we show that particles self-assemble into flexible, motile chains which can cluster to form diverse larger-scale compact structures with polar order. By computing key structural and dynamical metrics, we distinguish between the collective states at weak and strong elastic interaction strength, as well as at low and high motility. We also show how these states are affected by confinement within a channel geometry–an important characteristic of the complex mechanical micro-environment inhabited by cells. Our model predictions may be generally applicable to active matter with dipolar interactions ranging from biological cells to synthetic colloids endowed with electric or magnetic dipole moments. 

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3. Abstract 1315: Biophysics of polyploidal cancer cells in an aging stroma

   https://doi.org/10.1158/1538-7445  

   Dawson, Michelle; Xuan, Botai; Ghosh, Deepraj (April 2018, Cancer research)

   Senescence is a potent tumor-suppressive mechanism that irreversibly arrests the growth of damaged cells. However, senescent cells that accumulate in tissues eventually develop a senescence-associated secretory phenotype (SASP) that alters the microenvironment to promote cancer. Paracrine factors in the SASP may also contribute to the formation of rare giant polyploidal cancer cells (GPCCs). A single-cell mechanical approach was used to profile cytoskeletal and nuclear mechanics, morphology, motility, and adhesion for breast cancer cells treated with conditioned media from senescent fibroblasts. Our study showed that a small but significant population of MDA-MB-231 breast cancer cells (less than 5%) treated with conditioned media from senescent LF-1 fibroblasts develop an enlarged morphology, chromosomal instability, and polyploidy, a phenotype associated with GPCCs. Although GPCCs are highly invasive and chemoresistant, little is known about their biophysical properties. First, we developed a method for identifying the small subpopulation of GPCCs in a heterogeneous population of cancer cells based on increased nuclear area and confirmed that GPCCs are more resistant to paclitaxel than normal-size MDA-MB-231 cells (NCCs). We then compared critical biophysical properties of NCCs and GPCCs, including cytoskeletal and nuclear mechanics, cell and nuclear morphology, motility, and adhesion. Cells were stained for cytoskeletal proteins actin, tubulin, and vinculin. Cytoskeletal organization was dramatically altered in GPCCs compared to NCCs. GPCCs displayed more disorganized microtubule structure, dense actin stress fibers, and mature focal adhesions. Intracellular particle tracking microrheology was used to measure cytoskeletal and nuclear mechanics. These studies demonstrated that although GPCCs are thought to be highly invasive cancer cells, they are inherently stiffer than NCCs, in terms of both their cytoskeletal and nuclear mechanics. This was surprising since more invasive cancer cells are often more compliant than less invasive cancer cells. This result may be in part to the ability for GPCCs to behave like activated stromal cells that stiffen in the tumor; we confirmed that GPCCs display similar adhesive behavior as activated stromal cells. To determine how mechanics correlates with cell migration, we used time-lapse nuclear tracking to measure cell motility. The average cell speed was higher for NCCs than for GPCCs; however, GPCCs moved longer distances over time because their motion was more directional. These findings highlight the unusual biophysical behavior of GPCCs. To develop pharmacologic tools that target GPCCs, it is imperative to understand their biophysical properties. 

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4. Matrix Stiffness Modulates Mechanical Interactions and Promotes Contact between Motile Cells

   https://doi.org/10.3390/biomedicines9040428  

   Bose, Subhaya; Dasbiswas, Kinjal; Gopinath, Arvind (April 2021, Biomedicines)

   null (Ed.)

   The mechanical micro-environment of cells and tissues influences key aspects of cell structure and function, including cell motility. For proper tissue development, cells need to migrate, interact, and form contacts. Cells are known to exert contractile forces on underlying soft substrates and sense deformations in them. Here, we propose and analyze a minimal biophysical model for cell migration and long-range cell–cell interactions through mutual mechanical deformations of the substrate. We compute key metrics of cell motile behavior, such as the number of cell-cell contacts over a given time, the dispersion of cell trajectories, and the probability of permanent cell contact, and analyze how these depend on a cell motility parameter and substrate stiffness. Our results elucidate how cells may sense each other mechanically and generate coordinated movements and provide an extensible framework to further address both mechanical and short-range biophysical interactions. 

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5. A negative feedback loop between small GTPase Rap1 and mammalian tumor suppressor homologue KrsB regulates cell-substrate adhesion in Dictyostelium

   https://doi.org/10.1091/mbc.E24-11-0507  

   Artemenko, Yulia; Niu, Gengle; Arnold, Megan E; Roberts, Kelsey E; Fernandez, Bianca N; Flores, Tiffany; McClave, Harper D; Paestella, Michael; Borleis, Jane; Devreotes, Peter N (April 2025, Molecular Biology of the Cell)

   Welch, Matthew (Ed.)

   Cell adhesion to the substrate influences a variety of cell behaviors and its proper regulation is essential for migration, although details of the molecular pathways regulating cell adhesion during migration are lacking. Rap1 is a small GTPase that regulates adhesion in mammalian cells, as well as in Dictyostelium discoideum social amoeba, which is an established model for studying directed cell migration. In Dictyostelium, Rap1 controls adhesion via its effects on adhesion mediator talin and Ser/Thr kinase Phg2, which inhibits myosin II function. Kinase responsive to stress B (KrsB), a homologue of mammalian tumor suppressor MST1/2 and Drosophila Hippo, also regulates cell adhesion and migration, although the molecular mechanism of KrsB action is not understood. Because KrsB has been shown to interact with active Rap1 by mass spectroscopy, we investigated the genetic interaction between Rap1 and KrsB. Cells lacking KrsB have increased adhesion to the substrate, which leads to reduced movement. Expression of constitutively active Rap1 G12V increased cell spreading and adhesion even in the absence of KrsB, suggesting that Rap1 does not require KrsB to mediate cell adhesion. In contrast, KrsB activation requires Rap1 since dominant-negative Rap1 S17N impaired KrsB phosphorylation, which has been previously shown to be necessary for KrsB activity and its function in adhesion. Even though Rap1 did not require KrsB for its function in adhesion, KrsB negatively regulates Rap1 function as seen by increased cortical localization of active Rap1 in KrsB-null cells. Consistently, Rap1 S17N completely reversed the overadhesive phenotype of KrsB-null cells. Furthermore, chemoattractant-induced activation of downstream effectors of Rap1, TalB and Phg2, was increased in the absence of KrsB. Taken together, these findings suggest that Rap1 leads to activation of KrsB, which inhibits Rap1 and its downstream targets, shutting off adhesion. The existence of a negative feedback loop between Rap1 and KrsB may contribute to the dynamic regulation of cell adhesion that is necessary for rapid amoeboid-type migration. 

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