Local actin nucleation tunes centrosomal microtubule nucleation during passage through mitosis.

Cells going through mitosis undergo precisely timed changes in cell shape and organisation, which serve to ensure the fair partitioning of cellular components into the two daughter cells. These structural changes are driven by changes in actin filament and microtubule dynamics and organisation. While most evidence suggests that the two cytoskeletal systems are remodelled in parallel during mitosis, recent work in interphase cells has implicated the centrosome in both microtubule and actin nucleation, suggesting the potential for regulatory crosstalk between the two systems. Here, by using both in vitro and in vivo assays to study centrosomal actin nucleation as cells pass through mitosis, we show that mitotic exit is accompanied by a burst in cytoplasmic actin filament formation that depends on WASH and the Arp2/3 complex. This leads to the accumulation of actin around centrosomes as cells enter anaphase and to a corresponding reduction in the density of centrosomal microtubules. Taken together, these data suggest that the mitotic regulation of centrosomal WASH and the Arp2/3 complex controls local actin nucleation, which may function to tune the levels of centrosomal microtubules during passage through mitosis.

Rab5 and rac team up in cell motility.

Cell motility requires actin assembly mediated by Rac, a Rho family GTPase. In this issue, Palamidessi et al. (2008) show that Rab5-directed trafficking of Rac to the cell membrane is required for Rac-mediated actin assembly. Trafficking of Rac may influence the mode of cell migration during morphogenesis and cancer metastasis.

Effect of Bromide on the Surfactant Stabilization Layer Density of Gold Nanorods.

Some studies have speculated that the concentration of bromide ions plays a crucial role in the surfactant density surrounding gold nanorods (AuNR). Small-angle X-ray and neutron scattering (SAXS and SANS) experiments were conducted to analyze any influence the bromide ions might have on the stabilization layer and the aggregation behavior of the ligand CTAB molecules in general. The AuNR were immersed in solutions containing a fixed CTA+ concentration of 2 mM and varying bromide ion concentrations from 0 to 22 mM. A patchy AuNR stabilization shell at low bromide ion concentrations was found, contrary to previously published SANS studies on the AuNR stabilization shell. However, with increasing bromide ion concentration, the density of the stabilization shell increases asymptotically toward a closed/collapsed bilayer configuration. AuNR grown under similar conditions show higher anisotropy with larger bromide ion concentrations. Both results indicate that anisotropic growth strongly depends on a sufficiently dense stabilization layer established by high bromide ion concentrations.

Connecting the dots: combined control of endocytic recycling and degradation.

Endocytosis is an essential process where proteins and lipids are internalised from the plasma membrane in membrane-bound carriers, such as clathrin-coated vesicles. Once internalised into the cell these vesicles fuse with the endocytic network where their contents are sorted towards degradation in the lysosome or recycling to their origin. Initially, it was thought that cargo recycling is a passive process, but in recent years the identification and characterisation of specialised recycling complexes has established a hitherto unthought-of level of complexity that actively opposes degradation. This review will summarise recent developments regarding the composition and regulation of the recycling machineries and their relationship with the degradative pathways of the endosome.

Internal Structure of Nanometer-Sized Droplets Prepared by Antisolvent Precipitation.

Antisolvent precipitation (AP) is a low-cost and less-invasive preparation alternative for organic nanoparticles compared to top-down methods such as high-pressure homogenization or milling. Here we report on particularly small organic nanoparticles (NPs) prepared by AP. It has been found for various materials that these NPs in their liquid state exhibit a significant degree of molecular order at their interface toward the dispersion medium including ubiquinones (coenzyme Q10), triglycerides (trimyristin, tripalmitin), and alkanes (tetracosane). This finding is independent of the use of a stabilizer in the formulation. While this is obviously a quite general interfacial structuring effect, the respective structural details of specific NPs systems might differ. Here, a detailed structural characterization of very small liquid coenzyme Q10 (Q10) NPs is presented as a particular example for this phenomenon. The Q10 NPs have been prepared by AP in the presence of two different stabilizers, sodium dodecyl sulfate (SDS) and pentaethylene glycol monododecyl ether (C12E5), respectively, and without any stabilizer. The NPs' size is initially analyzed by photon correlation spectroscopy (PCS). The SDS-stabilized Q10 NPs have been studied further by differential scanning calorimetry (DSC), small-angle X-ray and neutron scattering (SAXS, SANS), wide-angle X-ray scattering (WAXS), and cryogenic transmission electron microscopy (CryoTEM). A simultaneous analysis of SAXS and contrast variation SANS studies revealed the molecular arrangement within the interface between the NPs and the dispersion medium. The Q10 NPs stabilized by SDS and C12E5, respectively, are small (down to 19.9 nm) and stable (for at least 16 months) even when no stabilizer is used. The SDS-stabilized Q10 NPs reported here, are therewith, to the best of our knowledge, the smallest organic NPs which have been reported to be prepared by AP so far. In particular, these NPs exhibit a core-shell structure consisting of an amorphous Q10 core and a surrounding shell, which is mainly composed of oriented Q10 molecules and aligned SDS molecules. This structure suggests a significant amphiphilic behavior and a rather unexpected stabilizing role of Q10 molecules.

PIKfyve, MTMR3 and their product PtdIns5P regulate cancer cell migration and invasion through activation of Rac1.

Previously, we have shown that the phosphoinositide metabolizing enzymes PIKfyve (phosphoinositide 5-kinase, FYVE finger containing) and MTMR3 (myotubularin-related protein 3), together with their lipid product PtdIns5P, are important for migration of normal human fibroblasts. As these proteins are a kinase and a phosphatase respectively, and thereby considered druggable, we wanted to test their involvement in cancer cell migration and invasion. First, we showed that PIKfyve and MTMR3 are expressed in most cancer cells. Next, we demonstrated that depletion of PIKfyve or MTMR3 resulted in decreased velocity in three different cancer cell lines by using new software for cell tracking. Inhibition of the enzymatic activity of PIKfyve by the inhibitor YM201636 also led to a strong reduction in cell velocity. Mechanistically, we show that PIKfyve and MTMR3 regulate the activation of the Rho family GTPase Rac1. Further experiments also implicated PtdIns5P in the activation of Rac1. The results suggest a model for the activation of Rac1 in cell migration where PIKfyve and MTMR3 produce PtdIns5P on cellular membranes which may then serve to recruit effectors to activate Rac1. Finally, in an invasion assay, we demonstrate that both PIKfyve and MTMR3 are implicated in invasive behaviour of cancer cells. Thus PIKfyve and MTMR3 could represent novel therapeutic targets in metastatic cancer.

Gadkin negatively regulates cell spreading and motility via sequestration of the actin-nucleating ARP2/3 complex.

Regulation of actin dynamics is key to many cell physiological processes, ranging from protrusion formation and control of cell shape to cellular motility, endocytosis, and vesicle movement. The actin-related protein (ARP)2/3 complex is a major actin nucleator organizing branched filament networks in lamellipodial protrusions and during cell migration downstream of nucleation-promoting factors (NPFs). Although many NPFs have been characterized in detail, only few ARP2/3 inhibitors are known. Here, we identify the trans-Golgi network (TGN)/endosomally localized adaptor protein (AP)-1-associated adaptor protein Gadkin as a negative regulator of ARP2/3 function. Loss of Gadkin is associated with a partial redistribution of ARP2/3 to the plasma membrane and with increased cell spreading and migration, phenotypes that depend on the presence of a functional ARP2/3 complex. Gadkin directly binds to ARP2/3 via a conserved tryptophan-based acidic cluster motif reminiscent of ARP2/3-binding sequences of NPFs but fails to facilitate ARP2/3-mediated actin assembly. Consistent with an inhibitory role of Gadkin on ARP2/3 function, ARP2/3 is found on motile Gadkin-containing endosomal vesicles under migration-inhibiting conditions from where it relocalizes to the plasma membrane following activation of NPFs. Together with the observation that Gadkin-mediated inhibition of cell spreading requires its binding to ARP2/3, these data indicate that Gadkin is a negative regulator of ARP2/3 function present on intracellular membranes.

Accumulation of raft lipids in T-cell plasma membrane domains engaged in TCR signalling.

Activating stimuli for T lymphocytes are transmitted through plasma membrane domains that form at T-cell antigen receptor (TCR) signalling foci. Here, we determined the molecular lipid composition of immunoisolated TCR activation domains. We observed that they accumulate cholesterol, sphingomyelin and saturated phosphatidylcholine species as compared with control plasma membrane fragments. This provides, for the first time, direct evidence that TCR activation domains comprise a distinct molecular lipid composition reminiscent of liquid-ordered raft phases in model membranes. Interestingly, TCR activation domains were also enriched in plasmenyl phosphatidylethanolamine and phosphatidylserine. Modulating the T-cell lipidome with polyunsaturated fatty acids impaired the plasma membrane condensation at TCR signalling foci and resulted in a perturbed molecular lipid composition. These results correlate the accumulation of specific molecular lipid species with the specific plasma membrane condensation at sites of TCR activation and with early TCR activation responses.

The Arp2/3 activator WASH regulates α5ß1-integrin-mediated invasive migration.

The actin cytoskeleton provides scaffolding and physical force to effect fundamental processes such as motility, cytokinesis and vesicle trafficking. The Arp2/3 complex nucleates actin structures and contributes to endocytic vesicle invagination and trafficking away from the plasma membrane. Internalisation and directed recycling of integrins are major driving forces for invasive cell motility and potentially for cancer metastasis. Here, we describe a direct requirement for WASH and Arp2/3-mediated actin polymerisation on the endosomal membrane system for α5ß1 integrin recycling. WASH regulates the trafficking of endosomal α5ß1 integrin to the plasma membrane and is fundamental for integrin-driven cell morphology changes and integrin-mediated cancer cell invasion. Thus, we implicate WASH and Arp2/3-driven actin nucleation in receptor recycling leading to invasive motility.

3D matrix adhesion feedback controls nuclear force coupling to drive invasive cell migration.

Cell invasion is a multi-step process, initiated by the acquisition of a migratory phenotype and the ability to move through complex 3D extracellular environments. We determine the composition of cell-matrix adhesion complexes of invasive breast cancer cells in 3D matrices and identify an interaction complex required for invasive migration. ßPix and myosin18A (Myo18A) drive polarized recruitment of non-muscle myosin 2A (NM2A) to adhesion complexes at the tips of protrusions. Actomyosin force engagement then displaces the Git1-ßPix complex from paxillin, establishing a feedback loop for adhesion maturation. We observe active force transmission to the nucleus during invasive migration that is needed to pull the nucleus forward. The recruitment of NM2A to adhesions creates a non-muscle myosin isoform gradient, which extends from the protrusion to the nucleus. We postulate that this gradient facilitates coupling of cell-matrix interactions at the protrusive cell front with nuclear movement, enabling effective invasive migration and front-rear cell polarity.

Proteomic characterization of plasma membrane-proximal T cell activation responses.

Early downstream responses of T lymphocytes following T cell antigen receptor (TCR) activation are mediated by protein complexes that assemble in domains of the plasma membrane. Using stable isotope labeling with amino acids in cell culture and mass spectrometry, we quantitatively related the proteome of αCD3 immunoisolated native TCR signaling plasma membrane domains to that of control plasma membrane fragments not engaged in TCR signaling. Proteins were sorted according to their relative enrichment in isolated TCR signaling plasma membrane domains, identifying a complex protein network that is anchored in the vicinity of activated TCR. These networks harbor widespread mediators of plasma membrane-proximal T cell activities, including propagation, balancing, and attenuation of TCR signaling, immune synapse formation, as well as cytoskeletal arrangements relative to TCR activation clusters. These results highlight the unique potential of systematic characterizations of plasma membrane-proximal T cell activation proteome in the context of its native lipid bilayer platform.

Laminin N-terminus α31 is upregulated in invasive ductal breast cancer and changes the mode of tumour invasion.

Laminin N-terminus α31 (LaNt α31) is an alternative splice isoform derived from the laminin α3 gene. The LaNt α31 protein is enriched around the terminal duct lobular units in normal breast tissue. In the skin and cornea the protein influences epithelial cell migration and tissue remodelling. However, LaNt α31 has never been investigated in a tumour environment. Here we analysed LaNt α31 in invasive ductal carcinoma and determined its contribution to breast carcinoma invasion. LaNt α31 expression and distribution were analysed by immunohistochemistry in human breast tissue biopsy sections and tissue microarrays covering 232 breast cancer samples. This analysis revealed LaNt α31 to be upregulated in 56% of invasive ductal carcinoma specimens compared with matched normal tissue, and further increased in nodal metastasis compared with the tumour mass in 45% of samples. 65.8% of triple negative cases displayed medium to high LaNt α31 expression. To study LaNt α31 function, an adenoviral system was used to induce expression in MCF-7 and MDA-MB-231 cells. 2D cell migration and invasion into collagen hydrogels were not significantly different between LaNt α31 overexpressing cells and control treated cells. However, LaNt α31 overexpression reduced the proliferation rate of MCF-7 and MDA-MB-231 cells. Moreover, LaNt α31 overexpressing MDA-MB-231 cells displayed a striking change in their mode of invasion into laminin-containing Matrigel; changing from multicellular streaming to individual cellular-invasion. In agreement with these results, 66.7% of the tumours with the highest LaNt α31 expression were non-cohesive. Together these findings indicate that breast cancer-associated changes in LaNt α31 expression could contribute to the processes involved in tumour invasion and may represent a new therapeutic target.

Radiolysis-Driven Evolution of Gold Nanostructures - Model Verification by Scale Bridging In Situ Liquid-Phase Transmission Electron Microscopy and X-Ray Diffraction.

Utilizing ionizing radiation for in situ studies in liquid media enables unique insights into nanostructure formation dynamics. As radiolysis interferes with observations, kinetic simulations are employed to understand and exploit beam-liquid interactions. By introducing an intuitive tool to simulate arbitrary kinetic models for radiation chemistry, it is demonstrated that these models provide a holistic understanding of reaction mechanisms. This is shown for irradiated HAuCl4 solutions allowing for quantitative prediction and tailoring of redox processes in liquid-phase transmission electron microscopy (LP-TEM). Moreover, it is demonstrated that kinetic modeling of radiation chemistry is applicable to investigations utilizing X-rays such as X-ray diffraction (XRD). This emphasizes that beam-sample interactions must be considered during XRD in liquid media and shows that reaction kinetics do not provide a threshold dose rate for gold nucleation relevant to LP-TEM and XRD. Furthermore, it is unveiled that oxidative etching of gold nanoparticles depends on both, precursor concentration, and dose rate. This dependency is exploited to probe the electron beam-induced shift in Gibbs free energy landscape by analyzing critical radii of gold nanoparticles.

Cells in Slow Motion: Apparent Undercooling Increases Glassy Behavior at Physiological Temperatures.

Solvent conditions are unexpectedly sufficient to drastically and reversibly slow down cells. In vitro on the molecular level, protein-solvent interactions drastically change in the presence of heavy water (D2 O) and its stronger hydrogen bonds. Adding D2 O to the cell medium of living cells increases the molecular intracellular viscosity. While cell morphology and phenotype remain unchanged, cellular dynamics transform into slow motion in a changeable manner. This is exemplified in the slowdown of cell proliferation and migration, which is caused by a reversible gelation of the cytoplasm. In analogy to the time-temperature superposition principle, where temperature is replaced by D2 O, an increase in viscosity slows down the effective time. Actin networks, crucial structures in the cytoplasm, switch from a power-law-like viscoelastic to a more rubber-like elastic behavior. The resulting intracellular resistance and dissipation impair cell movement. Since cells are highly adaptive non-equilibrium systems, they usually respond irreversibly from a thermodynamic perspective. D2 O induced changes, however, are fully reversible and their effects are independent of signaling as well as expression. The stronger hydrogen bonds lead to glass-like, drawn-out intramolecular dynamics, which may facilitate longer storage times of biological matter, for instance, during transport of organ transplants.

Inhibition of Protein N-Glycosylation Blocks SARS-CoV-2 Infection.

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) extensively N-glycosylates its spike proteins, which are necessary for host cell invasion and the target of both vaccines and immunotherapies. These N-glycans are predicted to modulate spike binding to the host receptor by stabilizing its open conformation and host immunity evasion. Here, we investigated the essentiality of both the host N-glycosylation pathway and SARS-CoV-2 N-glycans for infection. Ablation of host N-glycosylation using RNA interference or inhibitors, including FDA-approved drugs, reduced the spread of the infection, including that of variants B.1.1.7 (Alpha), B.1.351 (Beta), P.1 (Gamma) and B.1.617.2 (Delta). Under these conditions, cells produced fewer virions and some completely lost their infectivity. Furthermore, partial enzymatic deglycosylation of intact virions showed that surface-exposed N-glycans are critical for cell invasion. Altogether, we propose protein N-glycosylation as a targetable pathway with clinical potential for treatment of COVID-19. IMPORTANCE The coronavirus SARS-CoV-2 uses its spike surface proteins to infect human cells. Spike proteins are heavily modified with several N-glycans, which are predicted to modulate their function. In this work, we show that interfering with either the synthesis or attachment of spike N-glycans significantly reduces the spread of SARS-CoV-2 infection in vitro, including that of several variants. As new SARS-CoV-2 variants, with various degrees of resistance against current vaccines, are likely to continue appearing, halting virus glycosylation using repurposed human drugs could result in a complementary strategy to reducing the spread of COVID-19 worldwide.

Plasma membrane segregation during T cell activation: probing the order of domains.

T cell activation leads to a segregation of plasma membrane domains to form TCR signalling clusters and eventually immunological synapses. At these T cell activation sites signalling protein networks reside in plasma membrane regions which adopt a highly ordered physical state. Studies of reconstituted model membranes suggest that aggregation of lipid raft-favouring membrane components may trigger this lipid ordering and condensation of membrane domains in T cells. Activation-induced protein-protein interactions such as anchorage to the cytoskeleton drive this condensation of the plasma membrane. Elucidating the functional role and specific molecular mechanisms of lipid ordering at these domains in the T cell activation cascade will be an essential element in understanding the transmission of outside signals into intracellular responses.

A novel experimental approach for nanostructure analysis: simultaneous small-angle X-ray and neutron scattering.

Exploiting small-angle X-ray and neutron scattering (SAXS/SANS) on the same sample volume at the same time provides complementary nanoscale structural information in two different contrast situations. Unlike an independent experimental approach, the truly combined SAXS/SANS experimental approach ensures the exactness of the probed samples, particularly for in situ studies. Here, an advanced portable SAXS system that is dimensionally suitable for installation in the D22 zone of ILL is introduced. The SAXS apparatus is based on a Rigaku switchable copper/molybdenum microfocus rotating-anode X-ray generator and a DECTRIS detector with a changeable sample-to-detector distance of up to 1.6 m in a vacuum chamber. A case study is presented to demonstrate the uniqueness of the newly established method. Temporal structural rearrangements of both the organic stabilizing agent and organically capped gold colloidal particles during gold nanoparticle growth are simultaneously probed, enabling the immediate acquisition of correlated structural information. The new nano-analytical method will open the way for real-time investigations of a wide range of innovative nanomaterials and will enable comprehensive in situ studies on biological systems. The potential development of a fully automated SAXS/SANS system with a common control environment and additional sample environments, permitting a continual and efficient operation of the system by ILL users, is also introduced.

Membrane Tension Orchestrates Rear Retraction in Matrix-Directed Cell Migration.

In development, wound healing, and cancer metastasis, vertebrate cells move through 3D interstitial matrix, responding to chemical and physical guidance cues. Protrusion at the cell front has been extensively studied, but the retraction phase of the migration cycle is not well understood. Here, we show that fast-moving cells guided by matrix cues establish positive feedback control of rear retraction by sensing membrane tension. We reveal a mechanism of rear retraction in 3D matrix and durotaxis controlled by caveolae, which form in response to low membrane tension at the cell rear. Caveolae activate RhoA-ROCK1/PKN2 signaling via the RhoA guanidine nucleotide exchange factor (GEF) Ect2 to control local F-actin organization and contractility in this subcellular region and promote translocation of the cell rear. A positive feedback loop between cytoskeletal signaling and membrane tension leads to rapid retraction to complete the migration cycle in fast-moving cells, providing directional memory to drive persistent cell migration in complex matrices.

Actin-Based Cell Protrusion in a 3D Matrix.

Cell migration controls developmental processes (gastrulation and tissue patterning), tissue homeostasis (wound repair and inflammatory responses), and the pathobiology of diseases (cancer metastasis and inflammation). Understanding how cells move in physiologically relevant environments is of major importance, and the molecular machinery behind cell movement has been well studied on 2D substrates, beginning over half a century ago. Studies over the past decade have begun to reveal the mechanisms that control cell motility within 3D microenvironments - some similar to, and some highly divergent from those found in 2D. In this review we focus on migration and invasion of cells powered by actin, including formation of actin-rich protrusions at the leading edge, and the mechanisms that control nuclear movement in cells moving in a 3D matrix.

STEF/TIAM2-mediated Rac1 activity at the nuclear envelope regulates the perinuclear actin cap.

The perinuclear actin cap is an important cytoskeletal structure that regulates nuclear morphology and re-orientation during front-rear polarisation. The mechanisms regulating the actin cap are currently poorly understood. Here, we demonstrate that STEF/TIAM2, a Rac1 selective guanine nucleotide exchange factor, localises at the nuclear envelope, co-localising with the key perinuclear proteins Nesprin-2G and Non-muscle myosin IIB (NMMIIB), where it regulates perinuclear Rac1 activity. We show that STEF depletion reduces apical perinuclear actin cables (a phenotype rescued by targeting active Rac1 to the nuclear envelope), increases nuclear height and impairs nuclear re-orientation. STEF down-regulation also reduces perinuclear pMLC and decreases myosin-generated tension at the nuclear envelope, suggesting that STEF-mediated Rac1 activity regulates NMMIIB activity to promote stabilisation of the perinuclear actin cap. Finally, STEF depletion decreases nuclear stiffness and reduces expression of TAZ-regulated genes, indicating an alteration in mechanosensing pathways as a consequence of disruption of the actin cap.