Cortical rigidity of round cells in mitotic phase and suspended state

Dynamics of cell rounding during detachment

Animal cells undergo repeated shape changes, for example by rounding up and respreading as they divide. Cell rounding can be also observed in interphase cells, for example when cancer cells switch from a mesenchymal to an ameboid mode of cell migration. Nevertheless, it remains unclear how interphase cells round up. In this article, we demonstrate that a partial loss of substrate adhesion triggers actomyosin-dependent cortical remodeling and ERM activation, which facilitates further adhesion loss causing cells to round. Although the path of rounding in this case superficially resembles mitotic rounding in involving ERM phosphorylation, retraction fiber formation, and cortical remodeling downstream of ROCK, it does not require Ect2. This work provides insights into the way partial loss of adhesion actives cortical remodeling to drive cell detachment from the substrate. This is important to consider when studying the mechanics of cells in suspension, for example using methods like real-time deformability cytometry (RT-DC).

iASPP contributes to cell cortex rigidity, mitotic cell rounding, and spindle positioning

iASPP is a protein mostly known as an inhibitor of p53 pro-apoptotic activity and a predicted regulatory subunit of the PP1 phosphatase, which is often overexpressed in tumors. We report that iASPP associates with the microtubule plus-end binding protein EB1, a central regulator of microtubule dynamics, via an SxIP motif. iASPP silencing or mutation of the SxIP motif led to defective microtubule capture at the cortex of mitotic cells, leading to abnormal positioning of the mitotic spindle. These effects were recapitulated by the knockdown of the membrane-to-cortex linker Myosin-Ic (Myo1c), which we identified as a novel partner of iASPP. Moreover, iASPP or Myo1c knockdown cells failed to round up upon mitosis because of defective cortical stiffness. We propose that by increasing cortical rigidity, iASPP helps cancer cells maintain a spherical geometry suitable for proper mitotic spindle positioning and chromosome partitioning.

Alpha-mangostin dephosphorylates ERM to induce adhesion and decrease surface stiffness in KG-1 cells

Surface stiffness is a unique indicator of various cellular states and events and needs to be tightly controlled. α-Mangostin, a natural compound with numerous bioactivities, reduces the mechanical stiffness of various cells; however, the mechanism by which it affects the actin cytoskeleton remains unclear. We aimed to elucidate the mechanism underlying α-mangostin activity on the surface stiffness of leukocytes. We treated spherical non-adherent myelomonocytic KG-1 cells with α-mangostin; it clearly reduced their surface stiffness and disrupted their microvilli. The α-mangostin-induced reduction in surface stiffness was inhibited by calyculin A, a protein phosphatase inhibitor. α-Mangostin also induced KG-1 cell adhesion to a fibronectin-coated surface. In KG-1 cells, a decrease in surface stiffness and the induction of cell adhesion are largely attributed to the dephosphorylation of ezrin/radixin/moesin proteins (ERMs); α-mangostin reduced the levels of phosphorylated ERMs. It further increased protein kinase C (PKC) activity. α-Mangostin-induced KG-1 cell adhesion and cell surface softness were inhibited by the PKC inhibitor GF109203X. The results of the present study suggest that α-mangostin decreases stiffness and induces adhesion of KG-1 cells via PKC activation and ERM dephosphorylation.

Mechanical stiffness softening and cell adhesion are coordinately regulated by ERM dephosphorylation in KG-1 cells

Mechanical stiffness is closely related to cell adhesion and rounding in some cells. In leukocytes, dephosphorylation of ezrin/radixin/moesin (ERM) proteins is linked to cell adhesion events. To elucidate the relationship between surface stiffness, cell adhesion, and ERM dephosphorylation in leukocytes, we examined the relationship in the myelogenous leukemia line, KG-1, by treatment with modulation drugs. KG-1 cells have ring-shaped cortical actin with microvilli as the only F-actin cytoskeleton, and the actin structure constructs the mechanical stiffness of the cells. Phorbol 12-myristate 13-acetate and staurosporine, which induced cell adhesion to fibronectin surface and ERM dephosphorylation, caused a decrease in surface stiffness in KG-1 cells. Calyculin A, which inhibited ERM dephosphorylation and had no effect on cell adhesion, did not affect surface stiffness. To clarify whether decreasing cell surface stiffness and inducing cell adhesion are equivalent, we examined KG-1 cell adhesion by treatment with actin-attenuated cell softening reagents. Cytochalasin D clearly diminished cell adhesion, and high concentrations of Y27632 slightly induced cell adhesion. Only Y27632 slightly decreased ERM phosphorylation in KG-1 cells. Thus, decreasing cell surface stiffness and inducing cell adhesion are not equivalent, but these phenomena are coordinately regulated by ERM dephosphorylation in KG-1 cells.

Alpha-mangostin reduces mechanical stiffness of various cells

Alpha-mangostin (α-mangostin) has been identified as a naturally occurring compound with potential anticancer properties. It can induce apoptosis and inhibit the growth and metastasis of cancer cells. Moreover, α-mangostin reduces the mechanical stiffness of lung cancer cells. The objective of this study was to determine the effect of α-mangostin on the mechanical stiffness of various cells, as well as cell viability. The following cell types were examined: human fibroblast TIG-1 cells, human cancerous HeLa cells, human embryonic kidney HEK293 cells, mouse macrophage RAW 264.7 cells, and human myeloblasts KG-1 cells. Cells were treated with α-mangostin, and then examined for cell viability, actin cytoskeletal structures, and surface mechanical stiffness using atomic force microscopy. α-Mangostin demonstrated cytotoxicity against TIG-1, HeLa, HEK293, and KG-1 cells, but not against RAW 264.7 cells. The cytotoxic effect of α-mangostin varies according to cell type. On the other hand, α-mangostin reduced the mechanical stiffness of all cell types, including RAW 264.7 cells. Upon treatment with α-mangostin, F-actin was slightly reduced but the actin cytoskeletal structures were little altered in these cells. Thus, reducing mechanical stiffness of animal cells is an inherent effect of α-mangostin. Our results show that α-mangostin is a naturally occurring compound with potential to change the actin cytoskeletal micro-structures and reduce the surface stiffness of various cells.

Activation of PKC induces leukocyte adhesion by the dephosphorylation of ERM

Although circulating leukocytes are non-adherent cells, they also undergo adhesion in response to external stimuli. To elucidate this switch mechanism, we investigated PMA-induced cell adhesion in myelomonocytic KG-1 cells. PMA induced microvillius collapse, decrease of cell surface rigidity and exclusion of sialomucin from adhesion sites. All these adhesion-contributing events are linked to dephosphorylation of Ezrin/Radixin/Moesin (ERM) proteins. Indeed, PMA-treatment induced quick decrease of phosphorylated ERM proteins, while expression of Moesin-T558D, a phospho-mimetic mutant, inhibited PMA-induced cell adhesion. PMA-induced cell adhesion and ERM-dephophorylation were inhibited by PKC inhibitors or by a phosphatase inhibitor, indicating the involvement of PKC and protein phophatase in these processes. In peripheral T lymphocytes, ERM-dephosphorylation by adhesion-inducing stimuli was inhibited by a PKC inhibitor. Combined, these findings strongly suggest that external stimuli induce ERM-dephosphorylation via the activation of PKC in leukocytes and that ERM-dephosphorylation leads to leukocytes' adhesion.

Sialomucin and phosphorylated-ERM are inhibitors for cadherin-mediated aggregate formation

Cell adhesion is mediated by adhesion molecules, but also regulated by adhesion inhibitory molecules. Molecules such as leukocyte sialomucin and phosphorylated-Ezrin/Radixin/Moesin (ERM) inhibit cell-substratum adhesion. Here we show that these adhesion inhibitory molecules also inhibit aggregate formation of adherent cells in suspension culture. Expression of sialomucin, CD43 or CD34, inhibited formation of packed aggregates in HEK293T cells. Deletion mutant analysis and enzymatic cleavage indicated the significance of the extracellular sialomucin domain for this inhibition. Meanwhile, phosphorylated-ERM were decreased coincidently with aggregate formation. Combined with the inhibition of aggregate formation by the expression of phospho-mimetic Moesin mutant (Moesin-T558D), phosphorylated-ERM are inhibitors for aggregate formation. Increase of phosphorylated-ERM by CD43 and sialomucin-dependence of Moesin-T558D's inhibition indicate that sialomucin and phosphorylated-ERM collaborate to inhibit aggregate formation. Because aggregate formation of HEK293T cells is mediated by N-cadherin, sialomucin and phosphorylated-ERM inhibit cadherin-mediated cell-cell adhesion. Thus, sialomucin and phosphorylated-ERM are inhibitors for both cell-cell adhesion and cell-substratum adhesion, and regulation of these inhibitory molecules is essential for cell adhesion.

Alpha-mangostin inhibits the migration and invasion of A549 lung cancer cells

Several studies have indicated that α-mangostin exerts anti-metastasis and anti-subsistence effects on several types of cancer cells. Especially, the anti-metastatic effect of α-mangostin on cancer cells is a prospective function in cancer treatment. However, the metastasis process is complicated, and includes migration, invasion, intravasation, and extravasation; thus, the main target of anti-metastatic effect of α-mangostin is not known. In this study, we investigated the effects of α-mangostin on the invasion, subsistence, and migration of lung cancer cells under co-culture conditions with normal cells and regular mono-culture conditions. We found that α-mangostin killed the lung cancer and normal cells in a dose-dependent manner. Furthermore, the alteration in the surface mechanical properties of cells was examined by using atomic force microscopy. Although the α-mangostin concentrations of 5 and 10 µM did not affect the short-term cell viability, they considerably decreased the Young's modulus of lung cancer cells implying a decline in cell surface actin cytoskeletal properties. Additionally, these concentrations of α-mangostin inhibited the migration of lung cancer cells. In co-culture conditions (cancer cells with normal cells), the invasive activities of cancer cells on normal cells were discernibly observed, and was inhibited after treatment with 5 and 10 µM of α-mangostin. Taken together, α-mangostin suppressed the subsistence of lung cancer cells and displayed anti-metastatic activities by inhibiting the migration and invasion, and reducing the actin cytoskeleton of cancer cells. Our findings suggest that α-mangostin could be a potential therapeutic agent for cancer treatment.

Different TCR-induced T lymphocyte responses are potentiated by stiffness with variable sensitivity

T cells are mechanosensitive but the effect of stiffness on their functions is still debated. We characterize herein how human primary CD4⁺ T cell functions are affected by stiffness within the physiological Young’s modulus range of 0.5 kPa to 100 kPa. Stiffness modulates T lymphocyte migration and morphological changes induced by TCR/CD3 triggering. Stiffness also increases TCR-induced immune system, metabolism and cell-cycle-related genes. Yet, upon TCR/CD3 stimulation, while cytokine production increases within a wide range of stiffness, from hundreds of Pa to hundreds of kPa, T cell metabolic properties and cell cycle progression are only increased by the highest stiffness tested (100 kPa). Finally, mechanical properties of adherent antigen-presenting cells modulate cytokine production by T cells. Together, these results reveal that T cells discriminate between the wide range of stiffness values found in the body and adapt their responses accordingly.

DOI:http://dx.doi.org/10.7554/eLife.23190.001

Our immune system contains many cells that play various roles in defending the body against infection, cancer and other threats. For example, T cells constantly patrol the body ready to detect and respond to dangers. They do so by gathering cues from their surroundings, which can be specific chemical signals or physical properties such as the stiffness of tissues. Once the T cells are active they respond in several different ways including releasing hormones and dividing to produce more T cells.

Tissue stiffness varies considerably between different organs. Furthermore, disease can lead to changes in tissue stiffness. For example, tissues become more rigid when they are inflamed. The stiffness and other physical properties of the surfaces that T cells interact with affect how the cells respond when they detect a threat, but few details are known about exactly how these cues tune T cell responses. Saitakis et al. studied how human T cells respond to artificial surfaces of varying stiffness that mimic the range found in the body.

The experiments show that T cells that interact with stiff surfaces become more active than T cells that interact with softer surfaces. However, some responses are more sensitive to the stiffness of the surface than others. For example, the ability of the T cells to release hormones was affected by the whole range of stiffnesses tested in the experiments, whereas only very stiff surfaces stimulated the T cells to divide.

These findings show that T cells can detect the stiffness of surfaces in the body and use this to adapt how they respond to threats. Future challenges will be to find out how T cells sense the physical properties of their surroundings and investigate whether cell and tissue stiffness affects immune responses in the body. This will help us to understand how T cells fight infections and other threats, and could be used to develop new ways of boosting these cells to fight cancer and other diseases.

DOI:http://dx.doi.org/10.7554/eLife.23190.002

Functional interaction of PkcA and PldB regulate aggregation and development in Dictyostelium discoideum

Multicellular development in Dictyostelium discoideum involves tightly regulated signaling events controlling the entry into development, initiation of aggregation and chemotaxis, and cellular differentiation. Here we show that PkcA, a Dictyostelium discoideum Protein Kinase C-orthologue, is involved in quorum sensing and the initiation of development, as well as cAMP sensing during chemotaxis. Additionally, by epistasis analysis we provide evidence that PkcA and PldB (a Phospholipase D-orthologue) functionally interact to regulate aggregation, differentiation, and cell-cell adhesion during development. Finally, we show that PkcA acts as a positive regulator of intracellular PLD-activity during development. Taken together, our results suggest that PkcA act through PldB, by regulating PLD-activity, in order to control events during development.

Distinct mechanical behavior of HEK293 cells in adherent and suspended states

The mechanical features of individual animal cells have been regarded as indicators of cell type and state. Previously, we investigated the surface mechanics of cancer and normal stromal cells in adherent and suspended states using atomic force microscopy. Cancer cells possessed specific mechanical and actin cytoskeleton features that were distinct from normal stromal cells in adherent and suspended states. In this paper, we report the unique mechanical and actin cytoskeletal features of human embryonic kidney HEK293 cells. Unlike normal stromal and cancer cells, the surface stiffness of adherent HEK293 cells was very low, but increased after cell detachment from the culture surface. Induced actin filament depolymerization revealed that the actin cytoskeleton was the underlying source of the stiffness in suspended HEK293 cells. The exclusive mechanical response of HEK293 cells to perturbation of the actin cytoskeleton resembled that of adherent cancer cells and suspended normal stromal cells. Thus, with respect to their special cell-surface mechanical features, HEK293 cells could be categorized into a new class distinct from normal stromal and cancer cells.

Actin-based biomechanical features of suspended normal and cancer cells

The mechanical features of individual cells have been regarded as unique indicators of their states, which could constantly change in accordance with cellular events and diseases. Particularly, cancer progression was characterized by the disruption and/or reorganization of actin filaments causing mechanical changes. Thus, mechanical characterization of cells could become an effective cytotechnological approach for early detection of cancer. To develop mechanical cytotechnology, it would be necessary to clarify the mechanical properties in various cell adhesion states. In this study, we investigated the surface mechanical behavior of cancer and normal cells in the adherent and suspended states using atomic force microscopy. Adherent normal stromal cells showed high surface stiffness due to developed actin cap structures on their apical surface, whereas cancer cells did not have developed filamentous actin structures, and their surface stiffness was low. Upon cell detachment from the substrate, filamentous actin structures of adherent normal stromal cells reorganized to the cortical region and their surface stiffness decreased consequently however, the stiffness of suspended normal cells remained higher than that of cancer cells. These suspended state actin structures were similar, regardless of the cell type. Furthermore, the mechanical responses of the cancer and normal stromal cells to perturbation of the actin cytoskeleton were different, suggesting distinct regulatory mechanisms for actin cytoskeleton in cancer and normal cells in both adherent and suspended states. Therefore, cancer cells possess specific mechanical and actin cytoskeleton features different from normal stromal cells.