Collective Migration: Spatial Tension Relief

Wound repair: Two distinct Rap1 pathways close the gap

Groups of cells often coordinate their movements during normal development, cancer invasion, and wound repair. These coordinated migrations require dynamic cytoskeleton and cell-junction remodeling. Two distinct Rap1 pathways are required to regulate this dynamic remodeling for rapid wound closure.

Disentangling cadherin-mediated cell-cell interactions in collective cancer cell migration

Cell dispersion from a confined area is fundamental in a number of biological processes, including cancer metastasis. To date, a quantitative understanding of the interplay of single-cell motility, cell proliferation, and intercellular contacts remains elusive. In particular, the role of E- and N-cadherin junctions, central components of intercellular contacts, is still controversial. Combining theoretical modeling with in vitro observations, we investigate the collective spreading behavior of colonies of human cancer cells (T24). The spreading of these colonies is driven by stochastic single-cell migration with frequent transient cell-cell contacts. We find that inhibition of E- and N-cadherin junctions decreases colony spreading and average spreading velocities, without affecting the strength of correlations in spreading velocities of neighboring cells. Based on a biophysical simulation model for cell migration, we show that the behavioral changes upon disruption of these junctions can be explained by reduced repulsive excluded volume interactions between cells. This suggests that in cancer cell migration, cadherin-based intercellular contacts sharpen cell boundaries leading to repulsive rather than cohesive interactions between cells, thereby promoting efficient cell spreading during collective migration.

Assembly of the Non-Canonical Myo9a-RhoGAP and RhoA·GDP Transition State Complex in the Presence of MgF3. Protein J 2021;40:842-848. [PMID: 34709522 DOI: 10.1007/s10930-021-10027-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Myo9a is an actin-based molecular motor with a RhoGAP domain in its C-terminal tail. It plays a role in a variety of biological processes, such as in regulating the immune response, neuron development, and cancer progression, and its deregulation can lead to the development of disease conditions. Myo9a acts mainly via its RhoGAP domain. In the current study, we used a pET32a vector with an N-terminal Trx-His6 tag to express Myo9a-RhoGAP in a soluble form. High-purity Myo9a-RhoGAP protein was obtained after two rounds of Ni²⁺ affinity and size-exclusion chromatography. We mixed Myo9a-RhoGAP and RhoA in equimolar ratios in the presence of 5 mM MgCl₂ and 20 mM NaF to achieve a stable RhoA GTP hydrolysis transition state complex. Analytical gel filtration and SDS-PAGE were used to verify complex formation. ITC and GAP assays suggested that Myo9a-RhoGAP could bind to RhoA and accelerate RhoA GTP hydrolysis in vitro. We purified the soluble Myo9a-RhoGAP protein with GAP activity and achieved the Myo9a-RhoGAP/RhoA·GDP/MgF₃^- complex assembly in vitro for the first time. The data may provide novel insights into Myo9a structure and function.

Self-organization of actin networks by a monomeric myosin

The organization of actomyosin networks lies at the center of many types of cellular motility, including cell polarization and collective cell migration during development and morphogenesis. Myosin-IXa is critically involved in these processes. Using total internal reflection fluorescence microscopy, we resolved actin bundles assembled by myosin-IXa. Electron microscopic data revealed that the bundles consisted of highly ordered lattices with parallel actin polarity. The myosin-IXa motor domains aligned across the network, forming cross-links at a repeat distance of precisely 36 nm, matching the helical repeat of actin. Single-particle image processing resolved three distinct conformations of myosin-IXa in the absence of nucleotide. Using cross-correlation of a modeled actomyosin crystal structure, we identified sites of additional mass, which can only be accounted for by the large insert in loop 2 exclusively found in the motor domain of class IX myosins. We show that the large insert in loop 2 binds calmodulin and creates two coordinated actin-binding sites that constrain the actomyosin interactions generating the actin lattices. The actin lattices introduce orientated tracks at specific sites in the cell, which might install platforms allowing Rho-GTPase-activating protein (RhoGAP) activity to be focused at a definite locus. In addition, the lattices might introduce a myosin-related, force-sensing mechanism into the cytoskeleton in cell polarization and collective cell migration.

Circulating tumour cells: the evolving concept and the inadequacy of their enrichment by EpCAM-based methodology for basic and clinical cancer research

Increasing evidence suggests that circulating tumour cells (CTCs) are responsible for metastatic relapse and this has fuelled interest in their detection and quantification. Although numerous methods have been developed for the enrichment and detection of CTCs, none has yet reached the 'gold' standard. Since epithelial cell adhesion molecule (EpCAM)-based enrichment of CTCs offers several advantages, it is one of the most commonly used and has been adapted for high-throughput technology. However, emerging evidence suggests that CTCs are highly heterogeneous: they consist of epithelial tumour cells, epithelial-to-mesenchymal transition (EMT) cells, hybrid (epithelial/EMT(+)) tumour cells, irreversible EMT(+) tumour cells, and circulating tumour stem cells (CTSCs). The EpCAM-based approach does not detect CTCs expressing low levels of EpCAM and non-epithelial phenotypes such as CTSCs and those that have undergone EMT and no longer express EpCAM. Thus, the approach may lead to underestimation of the significance of CTCs, in general, and CTSCs and EMT(+) tumour cells, in particular, in cancer dissemination. Here, we provide a critical review of research literature on the evolving concept of CTCs and the inadequacy of their enrichment by EpCAM-based technology for basic and clinical cancer research. The review also outlines future perspectives in the field.