Cortical tension drug screen links mitotic spindle integrity to Rho pathway.

Mechanical force generation plays an essential role in many cellular functions, including mitosis. Actomyosin contractile forces mediate changes in cell shape in mitosis and are implicated in mitotic spindle integrity via cortical tension. An unbiased screen of 150 small molecules that impact actin organization and 32 anti-mitotic drugs identified two molecular targets, Rho kinase (ROCK) and tropomyosin 3.1/2 (Tpm3.1/2), whose inhibition has the greatest impact on mitotic cortical tension. The converse was found for compounds that depolymerize microtubules. Tpm3.1/2 forms a co-polymer with mitotic cortical actin filaments, and its inhibition prevents rescue of multipolar spindles induced by anti-microtubule chemotherapeutics. We examined the role of mitotic cortical tension in this rescue mechanism. Inhibition of ROCK and Tpm3.1/2 and knockdown (KD) of cortical nonmuscle myosin 2A (NM2A), all of which reduce cortical tension, inhibited rescue of multipolar mitotic spindles, further implicating cortical tension in the rescue mechanism. GEF-H1 released from microtubules by depolymerization increased cortical tension through the RhoA pathway, and its KD also inhibited rescue of multipolar mitotic spindles. We conclude that microtubule depolymerization by anti-cancer drugs induces cortical-tension-based rescue to ensure integrity of the mitotic bipolar spindle mediated via the RhoA pathway. Central to this mechanism is the dependence of NM2A on Tpm3.1/2 to produce the functional engagement of actin filaments responsible for cortical tension.

Type 2 diabetes influences intraepithelial corneal nerve parameters and corneal stromal-epithelial nerve penetration sites.

INTRODUCTION: The quantification of intraepithelial corneal basal nerve parameters by in vivo confocal microscopy represents a promising modality to identify the earliest manifestations of diabetic peripheral neuropathy. However, its diagnostic accuracy is hampered by its dependence on neuron length, with minimal consideration for other parameters, including the origin of these nerves, the corneal stromal-epithelial nerve penetration sites. This study sought to utilize high-resolution images of murine corneal nerves to analyze comprehensively the morphological changes associated with type 2 diabetes progression. MATERIALS AND METHODS: ßIII-Tubulin immunostained corneas from prediabetic and type 2 diabetic mice and their respective controls were imaged by scanning confocal microscopy and analyzed automatically for nerve parameters. Additionally, the number and distribution of penetration sites was manually ascertained and the average length of the axons exiting them was computed. RESULTS: The earliest detectable changes included a significant increase in nerve density (6.06 ± 0.41% vs 8.98 ± 1.99%, P = 0.03) and branching (2867.8 ± 271.3/mm2 vs 4912.1 ± 1475.3/mm2 , P = 0.03), and in the number of penetration sites (258.80 ± 20.87 vs 422.60 ± 63.76, P = 0.0002) at 8 weeks of age. At 16 weeks, corneal innervation decreased, most notably in the periphery. The number of penetration sites remained significantly elevated relative to controls throughout the monitoring period. Similarly, prediabetic mice exhibited an increased number of penetration sites (242.2 ± 13.55 vs 305.6 ± 30.96, P = 0.003) without significant changes to the nerves. CONCLUSIONS: Our data suggest that diabetic peripheral neuropathy may be preceded by a phase of neuron growth rather than regression, and that the peripheral cornea is more sensitive than the center for detecting changes in innervation.

Molecular Dynamics and Theratyping in Airway and Gut Organoids Reveal R352Q-CFTR Conductance Defect.

A significant challenge to making targeted cystic fibrosis transmembrane conductance regulator (CFTR) modulator therapies accessible to all individuals with cystic fibrosis (CF) are many mutations in the CFTR gene that can cause CF, most of which remain uncharacterized. Here, we characterized the structural and functional defects of the rare CFTR mutation R352Q, with a potential role contributing to intrapore chloride ion permeation, in patient-derived cell models of the airway and gut. CFTR function in differentiated nasal epithelial cultures and matched intestinal organoids was assessed using an ion transport assay and forskolin-induced swelling assay, respectively. CFTR potentiators (VX-770, GLPG1837, and VX-445) and correctors (VX-809, VX-445, with or without VX-661) were tested. Data from R352Q-CFTR were compared with data of 20 participants with mutations with known impact on CFTR function. R352Q-CFTR has residual CFTR function that was restored to functional CFTR activity by CFTR potentiators but not the corrector. Molecular dynamics simulations of R352Q-CFTR were carried out, which indicated the presence of a chloride conductance defect, with little evidence supporting a gating defect. The combination approach of in vitro patient-derived cell models and in silico molecular dynamics simulations to characterize rare CFTR mutations can improve the specificity and sensitivity of modulator response predictions and aid in their translational use for CF precision medicine.

Molecular dynamics and functional characterization of I37R-CFTR lasso mutation provide insights into channel gating activity.

Characterization of I37R, a mutation located in the lasso motif of the CFTR chloride channel, was conducted by theratyping several CFTR modulators from both potentiator and corrector classes. Intestinal current measurements in rectal biopsies, forskolin-induced swelling (FIS) in intestinal organoids, and short circuit current measurements in organoid-derived monolayers from an individual with I37R/F508del CFTR genotype demonstrated that the I37R-CFTR results in a residual function defect amenable to treatment with potentiators and type III, but not type I, correctors. Molecular dynamics of I37R using an extended model of the phosphorylated, ATP-bound human CFTR identified an altered lasso motif conformation which results in an unfavorable strengthening of the interactions between the lasso motif, the regulatory (R) domain, and the transmembrane domain 2 (TMD2). Structural and functional characterization of the I37R-CFTR mutation increases understanding of CFTR channel regulation and provides a potential pathway to expand drug access to CF patients with ultra-rare genotypes.

Visualization of endogenous p27 and Ki67 reveals the importance of a c-Myc-driven metabolic switch in promoting survival of quiescent cancer cells.

Rationale: Recurrent and metastatic cancers often undergo a period of dormancy, which is closely associated with cellular quiescence, a state whereby cells exit the cell cycle and are reversibly arrested in G0 phase. Curative cancer treatment thus requires therapies that either sustain the dormant state of quiescent cancer cells, or preferentially, eliminate them. However, the mechanisms responsible for the survival of quiescent cancer cells remain obscure. Methods: Dual genome-editing was carried out using a CRISPR/Cas9-based system to label endogenous p27 and Ki67 with the green and red fluorescent proteins EGFP and mCherry, respectively, in melanoma cells. Analysis of transcriptomes of isolated EGFP-p27highmCherry-Ki67low quiescent cells was conducted at bulk and single cell levels using RNA-sequencing. The extracellular acidification rate and oxygen consumption rate were measured to define metabolic phenotypes. SiRNA and inducible shRNA knockdown, chromatin immunoprecipitation and luciferase reporter assays were employed to elucidate mechanisms of the metabolic switch in quiescent cells. Results: Dual labelling of endogenous p27 and Ki67 with differentiable fluorescent probes allowed for visualization, isolation, and analysis of viable p27highKi67low quiescent cells. Paradoxically, the proto-oncoprotein c-Myc, which commonly drives malignant cell cycle progression, was expressed at relatively high levels in p27highKi67low quiescent cells and supported their survival through promoting mitochondrial oxidative phosphorylation (OXPHOS). In this context, c-Myc selectively transactivated genes encoding OXPHOS enzymes, including subunits of isocitric dehydrogenase 3 (IDH3), whereas its binding to cell cycle progression gene promoters was decreased in quiescent cells. Silencing of c-Myc or the catalytic subunit of IDH3, IDH3α, preferentially killed quiescent cells, recapitulating the effect of treatment with OXPHOS inhibitors. Conclusion: These results establish a rigorous experimental system for investigating cellular quiescence, uncover the high selectivity of c-Myc in activating OXPHOS genes in quiescent cells, and propose OXPHOS targeting as a potential therapeutic avenue to counter cancer cells in quiescence.

Neuronal-epithelial cell alignment: A determinant of health and disease status of the cornea.

PURPOSE: How sensory neurons and epithelial cells interact with one another, and whether this association can be considered an indicator of health or disease is yet to be elucidated. METHODS: Herein, we used the cornea, Confetti mice, a novel image segmentation algorithm for intraepithelial corneal nerves which was compared to and validated against several other analytical platforms, and three mouse models to delineate this paradigm. For aging, eyes were collected from 2 to 52 week-old normal C57BL/6 mice (n ≥ 4/time-point). For wound-healing and limbal stem cell deficiency, 7 week-old mice received a limbal-sparing or limbal-to-limbal epithelial debridement to their right cornea, respectively. Eyes were collected 2-16 weeks post-injury (n=4/group/time-point), corneas procured, immunolabelled with ßIII-tubulin, flat-mounted, imaged by scanning confocal microscopy and analyzed for nerve and epithelial-specific parameters. RESULTS: Our data indicate that nerve features are dynamic during aging and their curvilinear arrangement align with corneal epithelial migratory tracks. Moderate corneal injury prompted axonal regeneration and recovery of nerve fiber features. Limbal stem cell deficient corneas displayed abnormal nerve morphology, and fibers no longer aligned with corneal epithelial migratory tracks. Mechanistically, we discovered that nerve pattern restoration relies on the number and distribution of stromal-epithelial nerve penetration sites. CONCLUSIONS: Microstructural changes to innervation may explain corneal complications related to aging and/or disease and facilitate development of new assays for diagnosis and/or classification of ocular and systemic diseases.

Label-Free Isolation and Single Cell Biophysical Phenotyping Analysis of Primary Cardiomyocytes Using Inertial Microfluidics.

To advance the understanding of cardiomyocyte (CM) identity and function, appropriate tools to isolate pure primary CMs are needed. A label-free method to purify viable CMs from mouse neonatal hearts is developed using a simple particle size-based inertial microfluidics biochip achieving purities of over 90%. Purified CMs are viable and retained their identity and function as depicted by the expression of cardiac-specific markers and contractility. The physico-mechanical properties of sorted cells are evaluated using downstream real-time deformability cytometry. CMs exhibited different physico-mechanical properties when compared with non-CMs. Taken together, this CM isolation and phenotyping method could serve as a valuable tool to progress the understanding of CM identity and function, and ultimately benefit cell therapy and diagnostic applications.

Drug Targeting the Actin Cytoskeleton Potentiates the Cytotoxicity of Low Dose Vincristine by Abrogating Actin-Mediated Repair of Spindle Defects.

Antimicrotubule vinca alkaloids are widely used in the clinic but their toxicity is often dose limiting. Strategies that enhance their effectiveness at lower doses are needed. We show that combining vinca alkaloids with compounds that target a specific population of actin filaments containing the cancer-associated tropomyosin Tpm3.1 result in synergy against a broad range of tumor cell types. We discovered that low concentrations of vincristine alone induce supernumerary microtubule asters that form transient multi-polar spindles in early mitosis. Over time these asters can be reconstructed into functional bipolar spindles resulting in cell division and survival. These microtubule asters are organized by the nuclear mitotic apparatus protein (NuMA)-dynein-dynactin complex without involvement of centrosomes. However, anti-Tpm3.1 compounds at nontoxic concentrations inhibit this rescue mechanism resulting in delayed onset of anaphase, formation of multi-polar spindles, and apoptosis during mitosis. These findings indicate that drug targeting actin filaments containing Tpm3.1 potentiates the anticancer activity of low-dose vincristine treatment. IMPLICATIONS: Simultaneously inhibiting Tpm3.1-containing actin filaments and microtubules is a promising strategy to potentiate the anticancer activity of low-dose vincristine.

Flotillins promote T cell receptor sorting through a fast Rab5-Rab11 endocytic recycling axis.

The targeted endocytic recycling of the T cell receptor (TCR) to the immunological synapse is essential for T cell activation. Despite this, the mechanisms that underlie the sorting of internalised receptors into recycling endosomes remain poorly understood. To build a comprehensive picture of TCR recycling during T cell activation, we developed a suite of new imaging and quantification tools centred on photoactivation of fluorescent proteins. We show that the membrane-organising proteins, flotillin-1 and -2, are required for TCR to reach Rab5-positive endosomes immediately after endocytosis and for transfer from Rab5- to Rab11a-positive compartments. We further observe that after sorting into in Rab11a-positive vesicles, TCR recycles to the plasma membrane independent of flotillin expression. Our data suggest a mechanism whereby flotillins delineate a fast Rab5-Rab11a endocytic recycling axis and functionally contribute to regulate the spatial organisation of these endosomes.

A mobile endocytic network connects clathrin-independent receptor endocytosis to recycling and promotes T cell activation.

Endocytosis of surface receptors and their polarized recycling back to the plasma membrane are central to many cellular processes, such as cell migration, cytokinesis, basolateral polarity of epithelial cells and T cell activation. Little is known about the mechanisms that control the organization of recycling endosomes and how they connect to receptor endocytosis. Here, we follow the endocytic journey of the T cell receptor (TCR), from internalization at the plasma membrane to recycling back to the immunological synapse. We show that TCR triggering leads to its rapid uptake through a clathrin-independent pathway. Immediately after internalization, TCR is incorporated into a mobile and long-lived endocytic network demarked by the membrane-organizing proteins flotillins. Although flotillins are not required for TCR internalization, they are necessary for its recycling to the immunological synapse. We further show that flotillins are essential for T cell activation, supporting TCR nanoscale organization and signaling.

PDGF-AB and 5-Azacytidine induce conversion of somatic cells into tissue-regenerative multipotent stem cells.

Current approaches in tissue engineering are geared toward generating tissue-specific stem cells. Given the complexity and heterogeneity of tissues, this approach has its limitations. An alternate approach is to induce terminally differentiated cells to dedifferentiate into multipotent proliferative cells with the capacity to regenerate all components of a damaged tissue, a phenomenon used by salamanders to regenerate limbs. 5-Azacytidine (AZA) is a nucleoside analog that is used to treat preleukemic and leukemic blood disorders. AZA is also known to induce cell plasticity. We hypothesized that AZA-induced cell plasticity occurs via a transient multipotent cell state and that concomitant exposure to a receptive growth factor might result in the expansion of a plastic and proliferative population of cells. To this end, we treated lineage-committed cells with AZA and screened a number of different growth factors with known activity in mesenchyme-derived tissues. Here, we report that transient treatment with AZA in combination with platelet-derived growth factor-AB converts primary somatic cells into tissue-regenerative multipotent stem (iMS) cells. iMS cells possess a distinct transcriptome, are immunosuppressive, and demonstrate long-term self-renewal, serial clonogenicity, and multigerm layer differentiation potential. Importantly, unlike mesenchymal stem cells, iMS cells contribute directly to in vivo tissue regeneration in a context-dependent manner and, unlike embryonic or pluripotent stem cells, do not form teratomas. Taken together, this vector-free method of generating iMS cells from primary terminally differentiated cells has significant scope for application in tissue regeneration.

SiaA/D Interconnects c-di-GMP and RsmA Signaling to Coordinate Cellular Aggregation of Pseudomonas aeruginosa in Response to Environmental Conditions.

Pseudomonas aeruginosa has emerged as an important opportunistic human pathogen that is often highly resistant to eradication strategies, mediated in part by the formation of multicellular aggregates. Cellular aggregates may occur attached to a surface (biofilm), at the air-liquid interface (pellicle), or as suspended aggregates. Compared to surface attached communities, knowledge about the regulatory processes involved in the formation of suspended cell aggregates is still limited. We have recently described the SiaA/D signal transduction module that regulates macroscopic cell aggregation during growth with, or in the presence of the surfactant SDS. Targets for SiaA/D mediated regulation include the Psl polysaccharide, the CdrAB two-partner secretion system and the CupA fimbriae. While the global regulators c-di-GMP and RsmA are known to inversely coordinate cell aggregation and regulate the expression of several adhesins, their potential impact on the expression of the cupA operon remains unknown. Here, we investigated the function of SiaA (a putative ser/thr phosphatase) and SiaD (a di-guanylate cyclase) in cupA1 expression using transcriptional reporter fusions and qRT-PCR. These studies revealed a novel interaction between the RsmA posttranscriptional regulatory system and SiaA/D mediated macroscopic aggregation. The RsmA/rsmY/Z system was found to affect macroscopic aggregate formation in the presence of surfactant by impacting the stability of the cupA1 mRNA transcript and we reveal that RsmA directly binds to the cupA1 leader sequence in vitro. We further identified that transcription of the RsmA antagonist rsmZ is controlled in a SiaA/D dependent manner during growth with SDS. Finally, we found that the siaD transcript is also under regulatory control of RsmA and that overproduction of RsmA or the deletion of siaD results in decreased cellular cyclic di-guanosine monophosphate (c-di-GMP) levels quantified by a transcriptional reporter, demonstrating that SiaA/D connects c-di-GMP and RsmA/rsmY/Z signaling to reciprocally regulate cell aggregation in response to environmental conditions.

γ-Actin plays a key role in endothelial cell motility and neovessel maintenance.

BACKGROUND: Angiogenesis plays a crucial role in development, wound healing as well as tumour growth and metastasis. Although the general implication of the cytoskeleton in angiogenesis has been partially unravelled, little is known about the specific role of actin isoforms in this process. Herein, we aimed at deciphering the function of γ-actin in angiogenesis. METHODS: Localization of ß- and γ-actin in vascular endothelial cells was investigated by co-immunofluorescence staining using monoclonal antibodies, followed by the functional analysis of γ-actin using siRNA. The impact of γ-actin knockdown on the random motility and morphological differentiation of endothelial cells into vascular networks was investigated by timelapse videomicroscopy while the effect on chemotaxis was assessed using modified Boyden chambers. The implication of VE-cadherin, VEGFR-2 and ROCK signalling was then examined by Western blotting and using pharmacological inhibitors. RESULTS: The two main cytoplasmic isoforms of actin strongly co-localized in vascular endothelial cells, albeit with some degree of spatial preference. While ß-actin knockdown was not achievable without major cytotoxicity, γ-actin knockdown did not alter the viability of endothelial cells. Timelapse videomicroscopy experiments revealed that γ-actin knockdown cells were able to initiate morphological differentiation into capillary-like tubes but were unable to maintain these structures, which rapidly regressed. This vascular regression was associated with altered regulation of VE-cadherin expression. Interestingly, knocking down γ-actin expression had no effect on endothelial cell adhesion to various substrates but significantly decreased their motility and migration. This anti-migratory effect was associated with an accumulation of thick actin stress fibres, large focal adhesions and increased phosphorylation of myosin regulatory light chain, suggesting activation of the ROCK signalling pathway. Incubation with ROCK inhibitors, H-1152 and Y-27632, completely rescued the motility phenotype induced by γ-actin knockdown but only partially restored the angiogenic potential of endothelial cells. CONCLUSIONS: Our study thus demonstrates for the first time that ß-actin is essential for endothelial cell survival and γ-actin plays a crucial role in angiogenesis, through both ROCK-dependent and -independent mechanisms. This provides new insights into the role of the actin cytoskeleton in angiogenesis and may open new therapeutic avenues for the treatment of angiogenesis-related disorders.

Fluorescence recovery after photobleaching (FRAP): acquisition, analysis, and applications.

A significant number of biological processes occur at, or involve cellular membranes, including; cell adhesion, migration, endocytosis, signal transduction, and many biochemical reactions involving membrane anchored scaffolds. Each process involves a complex arrangement of interacting molecules whose location in space and time influence the outcome of the event. In this protocol we discuss the application of fluorescence recovery after photobleaching (FRAP) to study the dynamics of membrane associated molecules. We discuss the principles, acquisition and the analysis of FRAP data and address issues surrounding its interpretation.

Cyclical action of the WASH complex: FAM21 and capping protein drive WASH recycling, not initial recruitment.

WASH causes actin to polymerize on vesicles involved in retrograde traffic and exocytosis. It is found within a regulatory complex, but the physiological roles of the other four members are unknown. Here we present genetic analysis of the subunits' individual functions in Dictyostelium. Mutants in each subunit are completely blocked in exocytosis. All subunits except FAM21 are required to drive actin assembly on lysosomes. Without actin, lysosomes never recycle vacuolar-type H(+)-adenosine triphosphatase (V-ATPase) or neutralize to form postlysosomes. However, in FAM21 knockout lysosomes, WASH generates excessive, dynamic streams of actin. These successfully remove V-ATPase, neutralize, and form huge postlysosomes. The distinction between WASH and FAM21 phenotypes is conserved in human cells. Thus, FAM21 and WASH act at different steps of a cyclical pathway in which FAM21 mediates recycling of the complex back to acidic lysosomes. Recycling is driven by FAM21's interaction with capping protein, which couples the WASH complex to dynamic actin on vesicles.

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.

Actin polymerization driven by WASH causes V-ATPase retrieval and vesicle neutralization before exocytosis.

WASP and SCAR homologue (WASH) is a recently identified and evolutionarily conserved regulator of actin polymerization. In this paper, we show that WASH coats mature Dictyostelium discoideum lysosomes and is essential for exocytosis of indigestible material. A related process, the expulsion of the lethal endosomal pathogen Cryptococcus neoformans from mammalian macrophages, also uses WASH-coated vesicles, and cells expressing dominant negative WASH mutants inefficiently expel C. neoformans. D. discoideum WASH causes filamentous actin (F-actin) patches to form on lysosomes, leading to the removal of vacuolar adenosine triphosphatase (V-ATPase) and the neutralization of lysosomes to form postlysosomes. Without WASH, no patches or coats are formed, neutral postlysosomes are not seen, and indigestible material such as dextran is not exocytosed. Similar results occur when actin polymerization is blocked with latrunculin. V-ATPases are known to bind avidly to F-actin. Our data imply a new mechanism, actin-mediated sorting, in which WASH and the Arp2/3 complex polymerize actin on vesicles to drive the separation and recycling of proteins such as the V-ATPase.

Actin on disease--studying the pathobiology of cell motility using Dictyostelium discoideum.

The actin cytoskeleton in eukaryotic cells provides cell structure and organisation, and allows cells to generate forces against membranes. As such it is a central component of a variety of cellular structures involved in cell motility, cytokinesis and vesicle trafficking. In multicellular organisms these processes contribute towards embryonic development and effective functioning of cells of all types, most obviously rapidly moving cells like lymphocytes. Actin also defines and maintains the architecture of complex structures such as neuronal synapses and stereocillia, and is required for basic housekeeping tasks within the cell. It is therefore not surprising that misregulation of the actin cytoskeleton can cause a variety of disease pathologies, including compromised immunity, neurodegeneration, and cancer spread. Dictyostelium discoideum has long been used as a tool for dissecting the mechanisms by which eukaryotic cells migrate and chemotax, and recently it has gained precedence as a model organism for studying the roles of conserved pathways in disease processes. Dictyostelium's unusual lifestyle, positioned between unicellular and multicellular organisms, combined with ease of handling and strong conservation of actin regulatory machinery with higher animals, make it ideally suited for studying actin-related diseases. Here we address how research in Dictyostelium has contributed to our understanding of immune deficiencies and neurological defects in humans, and briefly discuss its future prospects for furthering our understanding of neurodegenerative disorders.