A label-free optical system with a nanohole array biosensor for discriminating live single cancer cells from normal cells

Developing a simple, fast, and label-free method for discrimination between live cancer cells and normal cells in biological samples still remains a challenge. Here, a system is described that fulfills these features to analyze individual living cells. The system consists of a gold nanohole array biosensor plus a microscope optical design to isolate the spectral response of a single cell. It is demonstrated that differences in the spectral behavior between tumor (colorectal cancer cell lines and primary cells from colorectal cancer tissue) and non-tumor cells (peripheral blood mononuclear cells, skin fibroblasts and colon epithelial cells) are influenced by the actin cortex, which lies within the short penetration depth of the surface plasmon electromagnetic field. The efficacy of this system was assessed by the analysis of about one thousand single cells showing the highest discrimination capacity between normal colon epithelial cells and colorectal cancer cells from surgical specimens, with values of sensitivity and specificity ranging 80–100% and 87–100%, respectively. It is also demonstrated that cell discrimination capacity of the system is highly reduced by disrupting the formation of actin cortex. This plasmonic system may find wide applications in biomedicine and to study key cellular processes that involve the actin cortex, including proliferation, differentiation, and migration.

3D Computational Modeling of Bleb Initiation Dynamics

Blebbing occurs in cells under high cortical tension when the membrane locally detaches from the actin cortex, resulting in pressure-driven flow of the cytosol and membrane expansion. Some cells use blebs as leading edge protrusions during cell migration, particularly in 3D environments such as a collagen matrix. Blebs can be initiated through either a localized loss of membrane-cortex adhesion or ablation of the cortex in a region. Bleb morphologies resulting from different initiation mechanisms have not been studied in detail, either experimentally or with theoretical models. Additionally, material properties of the cytoplasm, such as elasticity, have been shown to be important for limiting bleb size. A 3D dynamic computational model of the cell is presented that includes mechanics and the interactions of the cytoplasm, the actin cortex, the cell membrane, and the cytoskeleton. The model is used to quantify bleb expansion dynamics and shapes that result from simulations using different initiation mechanisms. The cytoplasm is modeled as a both viscous fluid and as a poroelastic material. Results from model simulations with a viscous fluid cytoplasm model show much broader blebs that expand faster when they are initiated via cortical ablation than when they are initiated by removing only membrane-cortex adhesion. Simulation results using the poroelastic model of the cytoplasm provide qualitatively similar bleb morphologies regardless of the initiation mechanism. Parameter studies on bleb expansion time, cytoplasmic stiffness, and permeability reveal different scaling properties, namely a smaller power-law exponent, in 3D simulations compared to 2D ones.

Passive coupling of membrane tension and cell volume during active response of cells to osmosis

During osmotic changes of their environment, cells actively regulate their volume and plasma membrane tension that can passively change through osmosis. How tension and volume are coupled during osmotic adaptation remains unknown, as their quantitative characterization is lacking. Here, we performed dynamic membrane tension and cell volume measurements during osmotic shocks. During the first few seconds following the shock, cell volume varied to equilibrate osmotic pressures inside and outside the cell, and membrane tension dynamically followed these changes. A theoretical model based on the passive, reversible unfolding of the membrane as it detaches from the actin cortex during volume increase quantitatively describes our data. After the initial response, tension and volume recovered from hypoosmotic shocks but not from hyperosmotic shocks. Using a fluorescent membrane tension probe (fluorescent lipid tension reporter [Flipper-TR]), we investigated the coupling between tension and volume during these asymmetric recoveries. Caveolae depletion and pharmacological inhibition of ion transporters and channels, mTORCs, and the cytoskeleton all affected tension and volume responses. Treatments targeting mTORC2 and specific downstream effectors caused identical changes to both tension and volume responses, their coupling remaining the same. This supports that the coupling of tension and volume responses to osmotic shocks is primarily regulated by mTORC2.

A novel mechanism of bulk cytoplasmic transport by cortical dynein in Drosophila ovary

Cytoplasmic dynein, a major minus-end directed microtubule motor, plays essential roles in eukaryotic cells. Drosophila oocyte growth is mainly dependent on the contribution of cytoplasmic contents from the interconnected sister cells, nurse cells. We have previously shown that cytoplasmic dynein is required for Drosophila oocyte growth, and assumed that it transports cargoes along microtubule tracks from nurse cells to the oocyte. Here we report that instead transporting cargoes along microtubules into the oocyte, cortical dynein actively moves microtubules in nurse cells and from nurse cells to the oocyte via the cytoplasmic bridges, the ring canals. We demonstrate this microtubule movement is sufficient to drag even inert cytoplasmic particles through the ring canals to the oocyte. Furthermore, replacing dynein with a minus-end directed plant kinesin linked to the actin cortex is sufficient for transporting organelles and cytoplasm to the oocyte and driving its growth. These experiments show that cortical dynein can perform bulk cytoplasmic transport by gliding microtubules along the cell cortex and through the ring canals to the oocyte. We propose that the dynein-driven microtubule flow could serve as a novel mode of cargo transport for fast cytoplasmic transfer to support rapid oocyte growth.

Partitioning of ribonucleoprotein complexes from the cellular actin cortex

The cell cortex plays a crucial role in cell mechanics, signaling, and development. However, little is known about the influence of the cortical meshwork on the spatial distribution of cytoplasmic biomolecules. Here, we describe a new fluorescence microscopy method to infer the intracellular distribution of labeled biomolecules with sub-resolution accuracy. Unexpectedly, we find that RNA-binding proteins are partially excluded from the cytoplasmic volume adjacent to the plasma membrane that corresponds to the actin cortex. Complementary diffusion measurements of RNA-protein complexes suggest that a rudimentary model based on excluded volume interactions can explain this partitioning effect. Our results suggest the actin cortex meshwork may play a role in regulating the biomolecular content of the volume immediately adjacent to the plasma membrane.

Decoding natural astrocyte rhythms: dynamic actin waves result from environmental sensing by primary rodent astrocytes

Astrocytes are key regulators of brain homeostasis, which is essential for proper cognitive function. The role of cytoskeletal dynamics in this critical regulatory process is unknown. Here we find that actin is dynamic in certain subcellular regions, especially near the cell boundary. Our results further indicate that actin dynamics concentrates into “hotspot” regions that selectively respond to certain chemophysical stimuli, specifically the homeostatic challenges of ion or water concentration increases. Substrate topography makes actin dynamics more frequent yet weaker, and it also alters actin network structure. Superresolution images analyzed with a filament extraction algorithm demonstrate that surface topography is associated with a predominant perpendicular alignment of actin filaments near the cell boundary whereas flat substrates result in an actin cortex mainly parallel to the cell boundary. Thus, actin structure and dynamics together integrate information from different aspects of the environment that might steer the operation of neural cell networks.TeaserAstrocytes display dynamic actin that is modulated by combinations of chemophysical stimuli and environmental topographies.

The role of actin and myosin II in the cell cortex of adhered and suspended cells

Adhesion induces dramatic morphological and mechanical changes to cells, which are reflected by changes to the actin cortex. Among the many different proteins involved in this sub-membranous layer, motor proteins (e.g., nonmuscle myosin II [NMII]) and actin nucleators (e.g., Arp2/3, formins) are known to have significant influences on its dynamics and structure. The different roles of NMII, Arp2/3, and formins in the dynamics, structure, and mechanics of the actin cortex depend on the adhesion state of the cell. In this study, we unravel the interplay between the dynamics, structure, and mechanics of the actin cortex in adhered cells and in cells in suspension. We show that treatments with extrinsic cellular perturbants lead to alterations of all three properties that are correlated. However, intrinsic actin cortex variations between different cell adhesion states lead to unexpected correlations. Surprisingly, we find that NMII minifilaments have a minor influence on the actin cortex. Using new microscopy techniques, we show that NMII minifilaments are not localized within the actin cortex, as previously thought, but concentrated in a layer beneath it. Our treatments affecting Arp2/3 and formin reveal correlations between the actin cortex characteristics. Our data build towards a comprehensive understanding of the actin cortex. This understanding allows the prediction and control of cortical changes, which is essential for the study of general cellular processes, such as cell migration, metastasis, and differentiation.

Cortical actin flow activates an alpha-catenin clutch to assemble adherens junctions

Adherens junctions (AJs) fundamentally mediate cell-cell adhesion, yet the mechanisms that determine where or when AJs assemble are not understood. Here we reveal a mechanosensitive clutch that initiates AJ assembly. Before cell-cell contact, alpha-catenin couples surface E-cadherin complexes to retrograde flow of the actin cortex. Cortical flows with opposed orientations persist after contact, applying tension to alpha-catenin within trans-ligated cadherin complexes. Tension unfolds the alpha-catenin actin-binding domain (ABD), which is expected to mediate a catch bond with F-actin. However, catch bond behaviour is not sufficient for AJ assembly in a molecular clutch model. Instead, it is also necessary for the activated ABD to promote cis-clustering of E-cadherin molecules by bundling F-actin. Thus, this alpha-catenin clutch transduces the mechanical signal of cortical flow to assemble AJs.