Biology subjects

Groisman, A.

Publications and source records attributed to Groisman, A..

10 recordsLinked to original sources

bioRxiv2020

Modeling cell turning by mechanics at the cell rear

In this study, we explore a simulation of a mechanical model of the keratocyte lamellipodium as previously tested and calibrated for straight steady-state motility [1] and for the process of polarization and motility initiation [2]. In brief, this model uses the balance of three essential forces (myosin contraction, adhesive drag and actin network viscosity) to determine the cells mechanical behavior. Cell shape is set by the balance between the actin polymerization-driven protrusion at the cell boundary and myosin-driven retraction of the actin-myosin network. In the model, myosin acts to generate contractile stress applied to a viscous actin network with viscous resistance to actin flow created by adhesion to the substrate. Previous study [3] demonstrated that similar simple model with uniform constant adhesion predicts a rotating behavior of the cell; however, this behavior is idealized, and does not mimic observed features of the keratocytes turning behavior. Our goal is to explore what are the consequences of introducing mechanosensitive adhesions to the model.

biophysics↗

bioRxiv2019

Environmental sensing in dynamic quorum responses

Cell communication and coordinated cell behavior are hallmarks of multicellular behavior of living systems. However, in many cases, including the ancient and archetypal example of bacterial quorum sensing, the meaning of the communicated information remains a subject of debate. It is commonly assumed that quorum sensing encodes the information on the current state of the colony, including cell density and physical colony confinement. Here, we show that quorum sensing can also be exquisitely sensitive to dynamic changes in the environment, including fluctuations of the prevailing nutrient source. We propose a new signaling mechanism accounting for this sensory capability. This mechanism combines regulation by the commonly studied lux operon-encoded network with the environmentally determined balance of protein synthesis and dilution rates, dependent on the rate of cell proliferation. This regulatory mechanism accounts for observed complex spatial distribution of quorum responses, and emergence of sophisticated processing of dynamic inputs, including temporal thresholds and persistent partial induction following a transient change in the environmental conditions. We further show that, in this context, cell communication in quorum sensing acquires a new meaning: education of cells within a population about the past history of transient exposure to adverse conditions by a subset of induced cells. In combination, these signaling and communication features may endow a cell population with increased fitness in diverse fluctuating environments.

systems biology↗

bioRxiv2019

Formation of talin-vinculin pre-complexes dictates maturation of nascent adhesions by accelerated force transmission and vinculin recruitment

Talin, vinculin, and paxillin are mechanosensitive proteins that are recruited early to integrin-based nascent adhesions (NAs). Using machine learning, traction microscopy, single-particle-tracking, and fluorescence fluctuation analysis, we find that talin, vinculin, and paxillin are recruited in near-synchrony to NAs maturing to focal adhesions. After initial recruitment of all three proteins under minimal load, vinculin accumulates in these NAs at a ~5 fold higher rate than in non-maturing NAs and with faster growth in traction. We identify a domain in talin, R8, which exposes a vinculin-binding-site (VBS) without requiring load. Stabilizing this domain via mutation lowers load-free vinculin binding to talin, impairs maturation of NAs, and reduces the rate of additional vinculin recruitment. Taken together, our data show that talins concurrent localization with vinculin, before engagement with integrins, is essential for NA maturation, which entails traction-mediated unfolding of talin and exposure of additional VBSs triggering further vinculin binding.

cell biology↗

bioRxiv2019

A many-to-one sensory circuit encodes oxygen levels and drives respiratory behaviour in Danio rerio

Zebrafish larvae respond to hypoxia by increasing a number of ventilatory behaviors. During development, these animals switch from skin-resident to gill-resident neuroendocrine cells around 7 days post fertilization (d.p.f.) to detect hypoxia and drive adaptive behaviors. Here, we probe the neural pathways that receive inputs from skin-resident neuroendocrine cells and alter pectoral fin movements. We first show that a 5 d.p.f. larva increases its pectoral fin movements and heart activity upon hypoxia exposure. Next, we map the downstream neural circuitry and show that individual vagal sensory neurons receive inputs from multiple oxygen-sensing neuroendocrine cells. We then use calcium imaging to show that neurons in the second, but not third, vagal sensory ganglia show increases in the magnitude of their hypoxia-evoked responses. Finally, we link purinergic signaling between neuroendocrine cells and second vagal sensory neurons to increases in pectoral fin movements. Collectively, we suggest that vagal sensory neurons transform hypoxic stimuli into respiratory behaviors.

neuroscience↗

bioRxiv2019

Modulation of adult-born neuron excitability by coupling to GABAergic networks

The dentate gyrus of the hippocampus is dominated by a strong GABAergic tone that maintains sparse levels of activity. Adult neurogenesis disrupts this balance through the continuous addition of new granule cells (GCs) that display high excitability while develop and connect within the preexisting host circuit. The dynamics of the connectivity map for developing GCs in the local inhibitory networks remains unknown. We used optogenetics to study afferent and efferent synaptogenesis between new GCs and GABAergic interneurons expressing parvalbumin (PV-INs) and somatostatin (SST-INs). Inputs from PV-INs targeted the soma and remained immature until they grew abruptly in >4-week-old GCs. This transition was accelerated by exposure to enriched environment. Inputs from SST-INs were dendritic and developed slowly until reaching maturity by 8 weeks. Synaptic outputs from GCs onto PV-INs matured faster than those onto SST-INs, but also required several weeks. In the mature dentate network, PV-INs exerted an efficient control of GC spiking and were involved in both feedforward and feedback loops, a mechanism that would favor lateral inhibition and sparse coding. Our results reveal a long-lasting transition where adult-born neurons remain poorly coupled to inhibition, which might enable a parallel streaming channel from the entorhinal cortex to CA3 pyramidal cells.

neuroscience↗

bioRxiv2018

Cell Mechanics at the Rear Act To Steer the Direction of Cell Migration

Motile cells navigate complex environments by changing their direction of travel, generating left-right asymmetries in their mechanical subsystems to physically turn. Currently little is known about how external directional cues are propagated along the length scale of the whole cell and integrated with its force-generating apparatus to steer migration mechanically. We examine the mechanics of spontaneous cell turning in fish epidermal keratocytes and find that the mechanical asymmetries responsible for turning behavior predominate at the rear of the cell, where there is asymmetric centripetal actin flow. Using experimental perturbations we identify two linked feedback loops connecting myosin II contractility, adhesion strength and actin network flow in turning cells that are sufficient to recreate observed cell shapes and trajectories in a computational model. Surprisingly, asymmetries in actin polymerization at the cell leading edge play only a minor role in the mechanics of cell turning - that is, cells steer from the rear.\n\nHighlightsO_LIFish keratocytes can migrate with persistent angular velocity, straight or in circles.\nC_LIO_LIAsymmetry in protrusion at the leading edge is not sufficient to generate persistent turning.\nC_LIO_LIAsymmetries in myosin II contraction, actin flow and adhesion at the cell rear cause turns.\nC_LIO_LIOur new computational model of migration predicts observed cell trajectories.\nC_LI

cell biology↗

bioRxiv2018

Hyperactive Rac1 drives MAPK-independent proliferation in melanoma by assembly of a mechanosensitive dendritic actin network

AbstractCancer cells use a variety of mechanisms to subvert growth regulation and overcome environmental challenges. Often, these same mechanisms enable cancer cells to also develop resistance to targeted therapies. Here, we describe how a hyperactivating mutation of the Rac1 GTPase (Rac1P29S) harnesses Rac1s role as a regulator of actin polymer assembly to sustain cell cycle progression in growth limiting conditions. This proliferative advantage supports metastatic colonization of melanoma cells and confers insensitivity to inhibitors of the mitogen-activated protein kinase (MAPK) pathway, a frequent target for melanoma treatment. Rac1P29S bypasses the MAPK axis through a mechanism that necessitates cell-matrix attachment, however, does not depend on integrin-mediated focal adhesion assembly and focal adhesion kinase signaling. Even without involvement of canonical adhesion signaling, cells carrying the Rac1P29S mutation show elevated traction upon drug treatment and require mechanical resistance from their surrounding matrix to gain a proliferative advantage. We describe an alternative arm for cell mechanosensing, whereby actin polymerization against a matrix of minimal rigidity organizes biochemical cues to drive proliferative signals. Hyperactivation of Rac1 by the P29S mutation channels this pathway in melanoma through Arp 2/3-dependent formation of a constrained actin brush network that results in the inactivation of tumor suppressor NF2/Merlin. These data suggest an alternative mechanism for mechanosensitive growth regulation that can be hijacked by cancer cells to circumvent the adverse conditions of foreign microenvironments or drug treatment.

cell biology↗

bioRxiv2018

Organized spatial patterns of activated β2 integrins in arresting neutrophils

The transition from leukocyte rolling to firm adhesion is called arrest. {beta}2 integrins are required for neutrophil arrest1. Chemokines can trigger neutrophil arrest in vivo2 and in vitro3. Resting integrins4 exist in a \"bent-closed\" conformation, i.e., not extended (E-) and not high affinity (H-), unable to bind ligand. Electron microscopic images of isolated {beta}2 integrins in \"open\" and \"closed\" conformations5 inspired the switchblade model of integrin activation from E-H- to E+H- to E+H+67. Recently8, we discovered an alternative pathway of integrin activation from E-H- to E-H+ to E+H+. Spatial patterning of activated integrins is thought to be required for effective arrest, but so far only diffraction-limited localization maps of activated integrins exist8. Here, we combine superresolution microscopy with molecular modeling to identify the molecular patterns of H+E-, H-E+, and H+E+ activated integrins on primary human neutrophils. At the time of neutrophil arrest, E+H+ integrins form oriented (non-random) nanoclusters that contain a total of 4,625{+/-}369 E+H+ {beta}2 integrin molecules.

immunology↗

bioRxiv2017

CYK-4 functions independently of its centralspindlin partner ZEN-4 to cellularize oocytes in germline syncytia

Throughout metazoans, germ cells undergo incomplete cytokinesis to form syncytia connected by intercellular bridges. Formation of gametes ultimately requires bridge closure. Here, we investigate the contribution of the conserved bridge component centralspindlin to oocyte production in C. elegans. Centralspindlin is composed of the Rho family GTPase-activating protein (GAP) CYK-4/MgcRacGAP and the microtubule motor ZEN-4/kinesin-6, which are both essential for cytokinesis. In contrast, we show that oocyte production by the syncytial germline requires CYK-4 but not ZEN-4. Longitudinal imaging after conditional CYK-4 inactivation revealed a role in oocyte cellularization, rather than in generation of syncytial compartments. CYK-4s lipid-binding C1 domain and the GTPase-binding interface of its GAP domain were individually important for oocyte cellularization and for targeting CYK-4 to bridges, where it contributes to enrichment of active RhoA. These results identify a C1-GAP module in CYK-4 that recruits it to bridges in the germline and directs their closure to produce oocytes.\n\nIMPACT STATEMENTThe CYK-4 subunit of centralspindlin, a broadly conserved component of intercellular bridges across metazoa, is required for the cytokinesis-like closure of intercellular bridges that cellularizes oocytes to separate them from germline syncytia.\n\nMAJOR SUBJECT AREASCell Biology, Developmental Biology & Stem Cells

cell biology↗

bioRxiv2016

Blood flow directs arterial-venous remodeling through Notch activation and endothelial cell migration

Arteries and veins are formed independently by different types of endothelial cells (ECs). In vascular remodeling, arteries and veins become connected and some arteries become veins. It is unclear how ECs in transforming vessels change their type and how fates of individual vessels are determined. In embryonic trunk, vascular remodeling transforms arterial intersegmental vessels (ISVs) into a functional network of arteries and veins. We found that, once an ISV is connected to venous circulation, venous blood flow promotes upstream migration of ECs that results in displacement of arterial ECs by venous ECs, completing the transformation of this ISV into a vein without trans-differentiation of ECs. Arterial blood flow initiated in two neighboring ISVs prevents their transformation into veins by activating Notch signaling in ECs. Together, different responses of ECs to arterial and venous blood flow lead to the formation of a balanced network with equal numbers of arteries and veins.

developmental biology↗