Nanosecond pulsed electric field suppresses growth and reduces multi-drug resistance effect in pancreatic cancer

Nanosecond pulsed electric fields (nsPEF) have been shown to exert anticancer effects; however, little is known about the mechanisms triggered in cancer cells by nanosecond-length pulses, especially when low, sub-permeabilization voltage is used. In this study, three human pancreatic cancer cell lines were treated with nsPEF and molecular changes at the cellular level were analyzed. Further, we assessed the efficacy of paclitaxel chemotherapy following nsPEF treatment and correlated that with the changes in the expression of multi-drug resistance (MDR) proteins. Finally, we examined the influence of nsPEF on the adhesive properties of cancer cells as well as the formation and growth of pancreatic cancer spheroids. Cell line response differed with the application of a 200 ns, 100 pulses, 8 kV/cm, 10 kHz PEF treatment. PEF treatment led to (1) the release of microvesicles (MV) in EPP85-181RDB cells, (2) electropermeabilization in EPP85-181RNOV cells and (3) cell shrinkage in EPP85-181P cells. The release of MV's in EPP85-181RDB cells reduced the membrane content of P-gp and LRP, leading to a transient increase in vulnerability of the cells towards paclitaxel. In all cell lines we observed an initial reduction in size of the cancer spheroids after the nsPEF treatment. Cell line EPP85-181RNOV exhibited a permanent reduction in the spheroid size after nsPEF. We propose a mechanism in which the surface tension of the membrane, regulated by the organization of actin fibers, modulates the response of cancer cells towards nsPEF. When a membrane's surface tension remains low, we observed some cells form protrusions and release MVs containing MDR proteins. In contrast, when cell surface tension remains high, the cell membrane is being electroporated. The latter effect may be responsible for the reduced tumor growth following nsPEF treatment.

Adherens junctions stimulate and spatially guide integrin activation and extracellular matrix deposition

Cadherins and integrins are intrinsically linked through the actin cytoskeleton and are largely responsible for the mechanical integrity and organization of tissues. We show that cadherin clustering stimulates and spatially guides integrin activation. Adherens junction (AJ)-associated integrin activation depends on locally generated tension and does not require extracellular matrix ligands. It leads to the creation of primed integrin clusters, which spatially determine where focal adhesions will form if ligands are present and where ligands will be deposited. AJs that display integrin activation are targeted by microtubules facilitating their disassembly via caveolin-based endocytosis, showing that integrin activation impacts the stability of the core cadherin complex. Thus, the interplay between cadherins and integrins is more intimate than what was once believed and is rooted in the capacity of active integrins to be stabilized via AJ-generated tension. Altogether, our data establish a mechanism of cross-regulation between cadherins and integrins.

Junctional ER Organization Affects Mechanotransduction at Cadherin-Mediated Adhesions

Cadherin-mediated adhesions (also known as adherens junctions) are adhesive complexes that connect neighboring cells in a tissue. While the role of the actin cytoskeleton in withstanding tension at these sites of contact is well documented, little is known about the involvement of microtubules and the associated endoplasmic reticulum (ER) network in cadherin mechanotransduction. Therefore, we investigated how the organization of ER extensions in close proximity of cadherin-mediated adhesions can affect such complexes, and vice versa. Here, we show that the extension of the ER to cadherin-mediated adhesions is tension dependent and appears to be cadherin-type specific. Furthermore, the different structural organization of the ER/microtubule network seems to affect the localization of ER-bound PTP1B at cadherin-mediated adhesions. This phosphatase is involved in the modulation of vinculin, a molecular clutch which enables differential engagement of the cadherin-catenin layer with the actomyosin cytoskeleton in response to tension. This suggests a link between structural organization of the ER/microtubule network around cadherin-specific adhesions, to control the mechanotransduction of adherens junctions by modulation of vinculin conformational state.

Cell-Cell Adhesion-Driven Contact Guidance and Its Effect on Human Mesenchymal Stem Cell Differentiation

Contact guidance has been extensively explored using patterned adhesion functionalities that predominantly mimic cell-matrix interactions. Whether contact guidance can also be driven by other types of interactions, such as cell-cell adhesion, still remains a question. Herein, this query is addressed by engineering a set of microstrip patterns of (i) cell-cell adhesion ligands and (ii) segregated cell-cell and cell-matrix ligands as a simple yet versatile set of platforms for the guidance of spreading, adhesion, and differentiation of mesenchymal stem cells. It was unprecedently found that micropatterns of cell-cell adhesion ligands can induce contact guidance. Surprisingly, it was found that patterns of alternating cell-matrix and cell-cell strips also induce contact guidance despite providing a spatial continuum for cell adhesion. This guidance is believed to be due to the difference between the potencies of the two adhesions. Furthermore, patterns that combine the two segregated adhesion functionalities were shown to induce more human mesenchymal stem cell osteogenic differentiation than monofunctional patterns. This work provides new insight into the functional crosstalk between cell-cell and cell-matrix adhesions and, overall, further highlights the ubiquitous impact of the biochemical anisotropy of the extracellular environment on cell function.

Microtubules: Evolving roles and critical cellular interactions

Microtubules are cytoskeletal elements known as drivers of directed cell migration, vesicle and organelle trafficking, and mitosis. In this review, we discuss new research in the lens that has shed light into further roles for stable microtubules in the process of development and morphogenesis. In the lens, as well as other systems, distinct roles for characteristically dynamic microtubules and stabilized populations are coming to light. Understanding the mechanisms of microtubule stabilization and the associated microtubule post-translational modifications is an evolving field of study. Appropriate cellular homeostasis relies on not only one cytoskeletal element, but also rather an interaction between cytoskeletal proteins as well as other cellular regulators. Microtubules are key integrators with actin and intermediate filaments, as well as cell–cell junctional proteins and other cellular regulators including myosin and RhoGTPases to maintain this balance.

Impact statement

The role of microtubules in cellular functioning is constantly expanding. In this review, we examine new and exciting fields of discovery for microtubule’s involvement in morphogenesis, highlight our evolving understanding of differential roles for stabilized versus dynamic subpopulations, and further understanding of microtubules as a cellular integrator.

Enhanced cell-cell contact stability and decreased N-cadherin-mediated migration upon fibroblast growth factor receptor-N-cadherin cross talk

N-cadherin adhesion has been reported to enhance cancer and neuronal cell migration either by mediating actomyosin-based force transduction or initiating fibroblast growth factor receptor (FGFR)-dependent biochemical signalling. Here we show that FGFR1 reduces N-cadherin-mediated cell migration. Both proteins are co-stabilised at cell-cell contacts through direct interaction. As a consequence, cell adhesion is strengthened, limiting the migration of cells on N-cadherin. Both the inhibition of migration and the stabilisation of cell adhesions require the FGFR activity stimulated by N-cadherin engagement. FGFR1 stabilises N-cadherin at the cell membrane through a pathway involving Src and p120. Moreover, FGFR1 stimulates the anchoring of N-cadherin to actin. We found that the migratory behaviour of cells depends on an optimum balance between FGFR-regulated N-cadherin adhesion and actin dynamics. Based on these findings we propose a positive feed-back loop between N-cadherin and FGFR at adhesion sites limiting N-cadherin-based single-cell migration.

Recent Advances in Engineering the Stem Cell Microniche in 3D

Conventional 2D cell culture techniques have provided fundamental insights into key biochemical and biophysical mechanisms responsible for various cellular behaviors, such as cell adhesion, spreading, division, proliferation, and differentiation. However, 2D culture in vitro does not fully capture the physical and chemical properties of the native microenvironment. There is a growing body of research that suggests that cells cultured on 2D substrates differ greatly from those grown in vivo. This article focuses on recent progress in using bioinspired 3D matrices that recapitulate as many aspects of the natural extracellular matrix as possible. A range of techniques for the engineering of 3D microenvironment with precisely controlled biophysical and chemical properties, and the impact of these environments on cellular behavior, is reviewed. Finally, an outlook on future challenges for engineering the 3D microenvironment and how such approaches would further our understanding of the influence of the microenvironment on cell function is provided.

β-tubulin carboxy-terminal tails exhibit isotype-specific effects on microtubule dynamics in human gene-edited cells

This study used human gene-edited cell models and image analysis to reveal that the tubulin C-terminal tails specifically regulate the dynamics of individual microtubules and coordinate microtubule behavior across the cell.

Microtubules are highly dynamic structures that play an integral role in fundamental cellular functions. Different α- and β-tubulin isotypes are thought to confer unique dynamic properties to microtubules. The tubulin isotypes have highly conserved structures, differing mainly in their carboxy-terminal (C-terminal) tail sequences. However, little is known about the importance of the C-terminal tail in regulating and coordinating microtubule dynamics. We developed syngeneic human cell models using gene editing to precisely modify the β-tubulin C-terminal tail region while preserving the endogenous microtubule network. Fluorescent microscopy of live cells, coupled with advanced image analysis, revealed that the β-tubulin C-terminal tails differentially coordinate the collective and individual dynamic behavior of microtubules by affecting microtubule growth rates and explorative microtubule assembly in an isotype-specific manner. Furthermore, βI- and βIII-tubulin C-terminal tails differentially regulate the sensitivity of microtubules to tubulin-binding agents and the microtubule depolymerizing protein mitotic centromere-associated kinesin. The sequence of the β-tubulin tail encodes regulatory information that instructs and coordinates microtubule dynamics, thereby fine-tuning microtubule dynamics to support cellular functions.

Functional role for stable microtubules in lens fiber cell elongation

The process of tissue morphogenesis, especially for tissues reliant on the establishment of a specific cytoarchitecture for their functionality, depends a balanced interplay between cytoskeletal elements and their interactions with cell adhesion molecules. The microtubule cytoskeleton, which has many roles in the cell, is a determinant of directional cell migration, a process that underlies many aspects of development. We investigated the role of microtubules in development of the lens, a tissue where cell elongation underlies morphogenesis. Our studies with the microtubule depolymerizing agent nocodazole revealed an essential function for the acetylated population of stable microtubules in the elongation of lens fiber cells, which was linked to their regulation of the activation state of myosin. Suppressing myosin activation with the inhibitor blebbistatin could attenuate the loss of acetylated microtubules by nocodazole and rescue the effect of this microtubule depolymerization agent on both fiber cell elongation and lens integrity. Our results also suggest that acetylated microtubules impact lens morphogenesis through their interaction with N-cadherin junctions, with which they specifically associate in the region where lens fiber cell elongate. Disruption of the stable microtubule network increased N-cadherin junctional organization along lateral borders of differentiating lens fiber cells, which was prevented by suppression of myosin activity. These results reveal a role for the stable microtubule population in lens fiber cell elongation, acting in tandem with N-cadherin cell-cell junctions and the actomyosin network, giving insight into the cooperative role these systems play in tissue morphogenesis.

Inter-dependent apical microtubule and actin dynamics orchestrate centrosome retention and neuronal delamination

Detachment of newborn neurons from the neuroepithelium is required for correct neuronal architecture and functional circuitry. This process, also known as delamination, involves adherens-junction disassembly and acto-myosin-mediated abscission, during which the centrosome is retained while apical/ciliary membranes are shed. Cell-biological mechanisms mediating delamination are, however, poorly understood. Using live-tissue and super-resolution imaging, we uncover a centrosome-nucleated wheel-like microtubule configuration, aligned with the apical actin cable and adherens-junctions within chick and mouse neuroepithelial cells. These microtubules maintain adherens-junctions while actin maintains microtubules, adherens-junctions and apical end-foot dimensions. During neuronal delamination, acto-myosin constriction generates a tunnel-like actin-microtubule configuration through which the centrosome translocates. This movement requires inter-dependent actin and microtubule activity, and we identify drebrin as a potential coordinator of these cytoskeletal dynamics. Furthermore, centrosome compromise revealed that this organelle is required for delamination. These findings identify new cytoskeletal configurations and regulatory relationships that orchestrate neuronal delamination and may inform mechanisms underlying pathological epithelial cell detachment.

Single and collective cell migration: the mechanics of adhesions

Chemical and physical properties of the environment control cell proliferation, differentiation, or apoptosis in the long term. However, to be able to move and migrate through a complex three-dimensional environment, cells must quickly adapt in the short term to the physical properties of their surroundings. Interactions with the extracellular matrix (ECM) occur through focal adhesions or hemidesmosomes via the engagement of integrins with fibrillar ECM proteins. Cells also interact with their neighbors, and this involves various types of intercellular adhesive structures such as tight junctions, cadherin-based adherens junctions, and desmosomes. Mechanobiology studies have shown that cell-ECM and cell-cell adhesions participate in mechanosensing to transduce mechanical cues into biochemical signals and conversely are responsible for the transmission of intracellular forces to the extracellular environment. As they migrate, cells use these adhesive structures to probe their surroundings, adapt their mechanical properties, and exert the appropriate forces required for their movements. The focus of this review is to give an overview of recent developments showing the bidirectional relationship between the physical properties of the environment and the cell mechanical responses during single and collective cell migration.

Multiple roles of filopodial dynamics in particle capture and phagocytosis and phenotypes of Cdc42 and Myo10 deletion

Macrophage filopodia, finger-like membrane protrusions, were first implicated in phagocytosis more than 100 years ago, but little is still known about the involvement of these actin-dependent structures in particle clearance. Using spinning disk confocal microscopy to image filopodial dynamics in mouse resident Lifeact-EGFP macrophages, we show that filopodia, or filopodia-like structures, support pathogen clearance by multiple means. Filopodia supported the phagocytic uptake of bacterial (Escherichia coli) particles by (i) capturing along the filopodial shaft and surfing toward the cell body, the most common mode of capture; (ii) capturing via the tip followed by retraction; (iii) combinations of surfing and retraction; or (iv) sweeping actions. In addition, filopodia supported the uptake of zymosan (Saccharomyces cerevisiae) particles by (i) providing fixation, (ii) capturing at the tip and filopodia-guided actin anterograde flow with phagocytic cup formation, and (iii) the rapid growth of new protrusions. To explore the role of filopodia-inducing Cdc42, we generated myeloid-restricted Cdc42 knock-out mice. Cdc42-deficient macrophages exhibited rapid phagocytic cup kinetics, but reduced particle clearance, which could be explained by the marked rounded-up morphology of these cells. Macrophages lacking Myo10, thought to act downstream of Cdc42, had normal morphology, motility, and phagocytic cup formation, but displayed markedly reduced filopodia formation. In conclusion, live-cell imaging revealed multiple mechanisms involving macrophage filopodia in particle capture and engulfment. Cdc42 is not critical for filopodia or phagocytic cup formation, but plays a key role in driving macrophage lamellipodial spreading.

How cells respond to environmental cues - insights from bio-functionalized substrates

Biomimetic materials have long been the (he)art of bioengineering. They usually aim at mimicking in vivo conditions to allow in vitro culture, differentiation and expansion of cells. The past decade has witnessed a considerable amount of progress in soft lithography, bio-inspired micro-fabrication and biochemistry, allowing the design of sophisticated and physiologically relevant micro- and nano-environments. These systems now provide an exquisite toolbox with which we can control a large set of physicochemical environmental parameters that determine cell behavior. Bio-functionalized surfaces have evolved from simple protein-coated solid surfaces or cellular extracts into nano-textured 3D surfaces with controlled rheological and topographical properties. The mechanobiological molecular processes by which cells interact and sense their environment can now be unambiguously understood down to the single-molecule level. This Commentary highlights recent successful examples where bio-functionalized substrates have contributed in raising and answering new questions in the area of extracellular matrix sensing by cells, cell-cell adhesion and cell migration. The use, the availability, the impact and the challenges of such approaches in the field of biology are discussed.

The formation of ordered nanoclusters controls cadherin anchoring to actin and cell-cell contact fluidity

Visualization of single cadherins within cell membrane at nanometric resolution shows that E-cadherins arrange in ordered clusters and that these clusters control the anchoring of cadherin to actin and cell–cell contact fluidity.

Oligomerization of cadherins could provide the stability to ensure tissue cohesion. Cadherins mediate cell–cell adhesion by forming trans-interactions. They form cis-interactions whose role could be essential to stabilize intercellular junctions by shifting cadherin clusters from a fluid to an ordered phase. However, no evidence has been provided so far for cadherin oligomerization in cellulo and for its impact on cell–cell contact stability. Visualizing single cadherins within cell membrane at a nanometric resolution, we show that E-cadherins arrange in ordered clusters, providing the first demonstration of the existence of oligomeric cadherins at cell–cell contacts. Studying the consequences of the disruption of the cis-interface, we show that it is not essential for adherens junction formation. Its disruption, however, increased the mobility of junctional E-cadherin. This destabilization strongly affected E-cadherin anchoring to actin and cell–cell rearrangement during collective cell migration, indicating that the formation of oligomeric clusters controls the anchoring of cadherin to actin and cell–cell contact fluidity.

Single cell rigidity sensing: A complex relationship between focal adhesion dynamics and large-scale actin cytoskeleton remodeling

Many physiological and pathological processes involve tissue cells sensing the rigidity of their environment. In general, tissue cells have been shown to react to the stiffness of their environment by regulating their level of contractility, and in turn applying traction forces on their environment to probe it. This mechanosensitive process can direct early cell adhesion, cell migration and even cell differentiation. These processes require the integration of signals over time and multiple length scales. Multiple strategies have been developed to understand force- and rigidity-sensing mechanisms and much effort has been concentrated on the study of cell adhesion complexes, such as focal adhesions, and cell cytoskeletons. Here, we review the major biophysical methods used for measuring cell-traction forces as well as the mechanosensitive processes that drive cellular responses to matrix rigidity on 2-dimensional substrates.

Front-Rear Polarization by Mechanical Cues: From Single Cells to Tissues

Directed cell migration is a complex process that involves front-rear polarization, characterized by cell adhesion and cytoskeleton-based protrusion, retraction, and contraction of either a single cell or a cell collective. Single cell polarization depends on a variety of mechanochemical signals including external adhesive cues, substrate stiffness, and confinement. In cell ensembles, coordinated polarization of migrating tissues results not only from the application of traction forces on the extracellular matrix but also from the transmission of mechanical stress through intercellular junctions. We focus here on the impact of mechanical cues on the establishment and maintenance of front-rear polarization from single cell to collective cell behaviors through local or large-scale mechanisms.

Adaptive rheology and ordering of cell cytoskeleton govern matrix rigidity sensing

Matrix rigidity sensing regulates a large variety of cellular processes and has important implications for tissue development and disease. However, how cells probe matrix rigidity, and hence respond to it, remains unclear. Here, we show that rigidity sensing and adaptation emerge naturally from actin cytoskeleton remodeling. Our in vitro experiments and theoretical modeling demonstrate a bi-phasic rheology of the actin cytoskeleton, which transitions from fluid on soft substrates to solid on stiffer ones. Furthermore, we find that increasing substrate stiffness correlates with the emergence of an orientational order in actin stress fibers, which exhibit an isotropic to nematic transition that we characterize quantitatively in the framework of active matter theory. These findings imply mechanisms mediated by a large-scale reinforcement of actin structures under stress, which could be the mechanical drivers of substrate stiffness dependent cell shape changes and cell polarity.

Staurosporine induces formation of two types of extra-long cell protrusions: actin-based filaments and microtubule-based shafts

Staurosporine (STS) has been known as a classic protein kinase C inhibitor and is a broad-spectrum inhibitor targeting over 250 protein kinases. In this study, we observed that STS treatment induced drastic morphologic changes, such as elongation of a very large number of nonbranched, actin-based long cell protrusions that reached up to 30 µm in an hour without caspase activation or PARP cleavage in fibroblasts and epithelial cells. These cell protrusions were elongated not only from the free cell edge but also from the cell-cell junctions. The elongation of STS-dependent protrusions was required for ATP hydrolysis and was dependent on myosin-X and fascin but independent of Cdc42 and VASP. Interestingly, in the presence of an actin polymerization inhibitor, namely, cytochalasin D, latrunculin A, or jasplakinolide, STS treatment induced excess tubulin polymerization, which resulted in the formation of many extra-long microtubule (MT)-based protrusions toward the outside of the cell. The unique MT-based protrusions were thick and linear compared with the STS-induced filaments or stationary filopodia. These protrusions, which were composed of microtubules, have been scarcely observed in cultured non-neuronal cells. Taken together, our findings revealed that STS-sensitive kinases are essential for the maintenance of normal cell morphology, and a common unidentified molecular mechanism is involved in the formation of the following two different types of protrusions: actin-based filaments and MT-based shafts.