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1. Mechanosensitive mTORC2 independently coordinates leading and trailing edge polarity programs during neutrophil migration

   https://doi.org/10.1091/mbc.E22-05-0191  

   Saha, Suvrajit; Town, Jason P.; Weiner, Orion D. (May 2023, Molecular Biology of the Cell)

   Huttenlocher, Anna (Ed.)

   By acting both upstream and downstream of biochemical organizers of the cytoskeleton, physical forces function as central integrators of cell shape and movement. Here we use a combination of genetic, pharmacological, and optogenetic perturbations to probe the role of the conserved mechanosensitive mTORC2 programs in neutrophil polarity and motility. We find that the tension-based inhibition of leading edge signals (Rac, F-actin) that underlies protrusion competition is gated by the kinase-independent role of the complex, whereas the regulation of RhoA and Myosin II-based contractility at the trailing edge depend on mTORC2 kinase activity. mTORC2 is essential for spatial and temporal coordination of the front and back polarity programs for persistent migration under confinement. This mechanosensory pathway integrates multiple upstream signals, and we find that membrane stretch synergizes with biochemical co-input PIP3 to robustly amplify mTORC2 activation. Our results suggest that different signalling arms of mTORC2 regulate spatially and molecularly divergent cytoskeletal programs for efficient coordination of neutrophil shape and movement. [Media: see text] [Media: see text] 

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2. Transcription inhibition suppresses nuclear blebbing and rupture independent of nuclear rigidity

   https://doi.org/10.1242/jcs.261547  

   Berg, Isabel K.; Currey, Marilena L.; Gupta, Sarthak; Berrada, Yasmin; Nguyen, Bao V.; Pho, Mai; Patteson, Alison E.; Schwarz, J. M.; Banigan, Edward J.; Stephens, Andrew D. (September 2023, Journal of Cell Science)

   Chromatin is an essential component of nuclear mechanical response and shape that maintains nuclear compartmentalization and function. However, major genomic functions, such as transcription activity, might also impact cell nuclear shape via blebbing and rupture through their effects on chromatin structure and dynamics. To test this idea, we inhibited transcription with several RNA polymerase II inhibitors in wild type cells and perturbed cells that present increased nuclear blebbing. Transcription inhibition suppresses nuclear blebbing for several cell types, nuclear perturbations, and transcription inhibitors. Furthermore, transcription inhibition suppresses nuclear bleb formation, bleb stabilization, and bleb-based nuclear ruptures. Interestingly, transcription inhibition does not alter either H3K9 histone modification state, nuclear rigidity, or actin compression and contraction, which typically control nuclear blebbing. Polymer simulations suggest that RNA pol II motor activity within chromatin could drive chromatin motions that deform the nuclear periphery. Our data provide evidence that transcription inhibition suppresses nuclear blebbing and rupture, separate and distinct from chromatin rigidity. 

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3. Atypical peripheral actin band formation via overactivation of RhoA and nonmuscle myosin II in mitofusin 2-deficient cells

   https://doi.org/10.7554/eLife.88828  

   Wang, Yueyang; Troughton, Lee D; Xu, Fan; Chatterjee, Aritra; Ding, Chang; Zhao, Han; Cifuentes, Laura P; Wagner, Ryan B; Wang, Tianqi; Tan, Shelly; et al (September 2023, eLife)

   Cell spreading and migration play central roles in many physiological and pathophysiological processes. We have previously shown that MFN2 regulates the migration of human neutrophil-like cells via suppressing Rac activation. Here, we show that in mouse embryonic fibroblasts, MFN2 suppresses RhoA activation and supports cell polarization. After initial spreading, the wild-type cells polarize and migrate, whereas theMfn2^-/-cells maintain a circular shape. Increased cytosolic Ca²⁺resulting from the loss of Mfn2 is directly responsible for this phenotype, which can be rescued by expressing an artificial tether to bring mitochondria and endoplasmic reticulum to close vicinity. Elevated cytosolic Ca²⁺activates Ca²⁺/calmodulin-dependent protein kinase II, RhoA, and myosin light-chain kinase, causing an overactivation of nonmuscle myosin II, leading to a formation of a prominent F-actin ring at the cell periphery and increased cell contractility. The peripheral actin band alters cell physics and is dependent on substrate rigidity. Our results provide a novel molecular basis to understand how MFN2 regulates distinct signaling pathways in different cells and tissue environments, which is instrumental in understanding and treating MFN2-related diseases. 

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4. Steroid hormone signaling synchronizes cell migration machinery, adhesion and polarity to direct collective movement

   https://doi.org/10.1242/jcs.261164  

   Bhattacharya, Mallika; Starz-Gaiano, Michelle (March 2024, Journal of Cell Science)

   ABSTRACT Migratory cells – either individually or in cohesive groups – are critical for spatiotemporally regulated processes such as embryonic development and wound healing. Their dysregulation is the underlying cause of formidable health problems such as congenital abnormalities and metastatic cancers. Border cell behavior during Drosophila oogenesis provides an effective model to study temporally regulated, collective cell migration in vivo. Developmental timing in flies is primarily controlled by the steroid hormone ecdysone, which acts through a well-conserved, nuclear hormone receptor complex. Ecdysone signaling determines the timing of border cell migration, but the molecular mechanisms governing this remain obscure. We found that border cell clusters expressing a dominant-negative form of ecdysone receptor extended ineffective protrusions. Additionally, these clusters had aberrant spatial distributions of E-cadherin (E-cad), apical domain markers and activated myosin that did not overlap. Remediating their expression or activity individually in clusters mutant for ecdysone signaling did not restore proper migration. We propose that ecdysone signaling synchronizes the functional distribution of E-cadherin, atypical protein kinase C (aPKC), Discs large (Dlg1) and activated myosin post-transcriptionally to coordinate adhesion, polarity and contractility and temporally control collective cell migration. 

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5. Actin Turnover Required for Adhesion-Independent Bleb Migration

   https://doi.org/10.3390/fluids7050173  

   Copos, Calina; Strychalski, Wanda (May 2022, Fluids)

   Cell migration is critical for many vital processes, such as wound healing, as well as harmful processes, such as cancer metastasis. Experiments have highlighted the diversity in migration strategies employed by cells in physiologically relevant environments. In 3D fibrous matrices and confinement between two surfaces, some cells migrate using round membrane protrusions, called blebs. In bleb-based migration, the role of substrate adhesion is thought to be minimal, and it remains unclear if a cell can migrate without any adhesion complexes. We present a 2D computational fluid-structure model of a cell using cycles of bleb expansion and retraction in a channel with several geometries. The cell model consists of a plasma membrane, an underlying actin cortex, and viscous cytoplasm. Cellular structures are immersed in viscous fluid which permeates them, and the fluid equations are solved using the method of regularized Stokeslets. Simulations show that the cell cannot effectively migrate when the actin cortex is modeled as a purely elastic material. We find that cells do migrate in rigid channels if actin turnover is included with a viscoelastic description for the cortex. Our study highlights the non-trivial relationship between cell rheology and its external environment during migration with cytoplasmic streaming. 

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