Reconsidering an active role for G-actin in cytoskeletal regulation

Coordination of actin plus-end dynamics by IQGAP1, formin, and capping protein

Cell processes require precise regulation of actin polymerization that is mediated by plus-end regulatory proteins. Detailed mechanisms that explain plus-end dynamics involve regulators with opposing roles, including factors that enhance assembly, e.g., the formin mDia1, and others that stop growth (capping protein, CP). We explore IQGAP1's roles in regulating actin filament plus-ends and the consequences of perturbing its activity in cells. We confirm that IQGAP1 pauses elongation and interacts with plus ends through two residues (C756 and C781). We directly visualize the dynamic interplay between IQGAP1 and mDia1, revealing that IQGAP1 displaces the formin to influence actin assembly. Using four-color TIRF, we show that IQGAP1's displacement activity extends to formin-CP "decision complexes," promoting end-binding protein turnover at plus-ends. Loss of IQGAP1 or its plus-end activities disrupts morphology and migration, emphasizing its essential role. These results reveal a new role for IQGAP1 in promoting protein turnover on filament ends and provide new insights into how plus-end actin assembly is regulated in cells.

The actin binding protein profilin 1 localizes inside mitochondria and is critical for their function

The monomer-binding protein profilin 1 (PFN1) plays a crucial role in actin polymerization. However, mutations in PFN1 are also linked to hereditary amyotrophic lateral sclerosis, resulting in a broad range of cellular pathologies which cannot be explained by its primary function as a cytosolic actin assembly factor. This implies that there are important, undiscovered roles for PFN1 in cellular physiology. Here we screened knockout cells for novel phenotypes associated with PFN1 loss of function and discovered that mitophagy was significantly upregulated. Indeed, despite successful autophagosome formation, fusion with the lysosome, and activation of additional mitochondrial quality control pathways, PFN1 knockout cells accumulate depolarized, dysmorphic mitochondria with altered metabolic properties. Surprisingly, we also discovered that PFN1 is present inside mitochondria and provide evidence that mitochondrial defects associated with PFN1 loss are not caused by reduced actin polymerization in the cytosol. These findings suggest a previously unrecognized role for PFN1 in maintaining mitochondrial integrity and highlight new pathogenic mechanisms that can result from PFN1 dysregulation.

Actin network evolution as a key driver of eukaryotic diversification

Eukaryotic cells have been evolving for billions of years, giving rise to wildly diverse cell forms and functions. Despite their variability, all eukaryotic cells share key hallmarks, including membrane-bound organelles, heavily regulated cytoskeletal networks and complex signaling cascades. Because the actin cytoskeleton interfaces with each of these features, understanding how it evolved and diversified across eukaryotic phyla is essential to understanding the evolution and diversification of eukaryotic cells themselves. Here, we discuss what we know about the origin and diversity of actin networks in terms of their compositions, structures and regulation, and how actin evolution contributes to the diversity of eukaryotic form and function.

Zonal Patterning of Extracellular Matrix and Stromal Cell Populations Along a Perfusable Cellular Microchannel

The spatial organization of biophysical and biochemical cues in the extracellular matrix (ECM) in concert with reciprocal cell-cell signaling is vital to tissue patterning during development. However, elucidating the role an individual microenvironmental factor plays using existing in vivo models is difficult due to their inherent complexity. In this work, we have developed a microphysiological system to spatially pattern the biochemical, biophysical, and stromal cell composition of the ECM along an epithelialized 3D microchannel. This technique is adaptable to multiple hydrogel compositions and scalable to the number of zones patterned. We confirmed that the methodology to create distinct zones resulted in a continuous, annealed hydrogel with regional interfaces that did not hinder the transport of soluble molecules. Further, the interface between hydrogel regions did not disrupt microchannel structure, epithelial lumen formation, or media perfusion through an acellular or cellularized microchannel. Finally, we demonstrated spatially patterned tubulogenic sprouting of a continuous epithelial tube into the surrounding hydrogel confined to local regions with stromal cell populations, illustrating spatial control of cell-cell interactions and signaling gradients. This easy-to-use system has wide utility for modeling three-dimensional epithelial and endothelial tissue interactions with heterogeneous hydrogel compositions and/or stromal cell populations to investigate their mechanistic roles during development, homeostasis, or disease.

Prolonged depletion of profilin 1 or F-actin causes an adaptive response in microtubules

In addition to its well-established role in actin assembly, profilin 1 (PFN1) has been shown to bind to tubulin and alter microtubule growth. However, whether PFN1's predominant control over microtubules in cells occurs through direct regulation of tubulin or indirectly through the polymerization of actin has yet to be determined. Here, we manipulated PFN1 expression, actin filament assembly, and actomyosin contractility and showed that reducing any of these parameters for extended periods of time caused an adaptive response in the microtubule cytoskeleton, with the effect being significantly more pronounced in neuronal processes. All the observed changes to microtubules were reversible if actomyosin was restored, arguing that PFN1's regulation of microtubules occurs principally through actin. Moreover, the cytoskeletal modifications resulting from PFN1 depletion in neuronal processes affected microtubule-based transport and mimicked phenotypes that are linked to neurodegenerative disease. This demonstrates how defects in actin can cause compensatory responses in other cytoskeleton components, which in turn significantly alter cellular function.

Cdc42 activation is necessary for heterosynaptic cooperation and competition

Synapses change their weights in response to neuronal activity and in turn, neuronal networks alter their response properties and ultimately allow the brain to store information as memories. As for memories, not all events are maintained over time. Maintenance of synaptic plasticity depends on the interplay between functional changes at synapses and the synthesis of plasticity-related proteins that are involved in stabilizing the initial functional changes. Different forms of synaptic plasticity coexist in time and across the neuronal dendritic area. Thus, homosynaptic plasticity refers to activity-dependent synaptic modifications that are input-specific, whereas heterosynaptic plasticity relates to changes in non-activated synapses. Heterosynaptic forms of plasticity, such as synaptic cooperation and competition allow neurons to integrate events that occur separated by relatively large time windows, up to one hour. Here, we show that activation of Cdc42, a Rho GTPase that regulates actin cytoskeleton dynamics, is necessary for the maintenance of long-term potentiation (LTP) in a time-dependent manner. Inhibiting Cdc42 activation does not alter the time-course of LTP induction and its initial expression but blocks its late maintenance. We show that Cdc42 activation is involved in the phosphorylation of cofilin, a protein involved in modulating actin filaments and that weak and strong synaptic activation leads to similar levels on cofilin phosphorylation, despite different levels of LTP expression. We show that Cdc42 activation is required for synapses to interact by cooperation or competition, supporting the hypothesis that modulation of the actin cytoskeleton provides an activity-dependent and time-restricted permissive state of synapses allowing synaptic plasticity to occur. We found that under competition, the sequence in which synapses are activated determines the degree of LTP destabilization, demonstrating that competition is an active destabilization process. Taken together, we show that modulation of actin cytoskeleton by Cdc42 activation is necessary for the expression of homosynaptic and heterosynaptic forms of plasticity. Determining the temporal and spatial rules that determine whether synapses cooperate or compete will allow us to understand how memories are associated.

Reconstitution of Arp2/3-Nucleated Actin Assembly with CP, V-1 and CARMIL

Actin polymerization is often associated with membrane proteins containing capping-protein-interacting (CPI) motifs, such as CARMIL, CD2AP, and WASHCAP/Fam21. CPI motifs bind directly to actin capping protein (CP), and this interaction weakens the binding of CP to barbed ends of actin filaments, lessening the ability of CP to functionally cap those ends. The protein V-1 / myotrophin binds to the F-actin binding site on CP and sterically blocks CP from binding barbed ends. CPI-motif proteins also weaken the binding between V-1 and CP, which decreases the inhibitory effects of V-1, thereby freeing CP to cap barbed ends. Here, we address the question of whether CPI-motif proteins on a surface analogous to a membrane lead to net activation or inhibition of actin assembly nucleated by Arp2/3 complex. Using reconstitution with purified components, we discovered that CARMIL at the surface promotes and enhances actin assembly, countering the inhibitory effects of V-1 and thus activating CP. The reconstitution involves the presence of an Arp2/3 activator on the surface, along with Arp2/3 complex, V-1, CP, profilin and actin monomers in solution, recreating key features of cell physiology.

Cytoskeletal activation of NHE1 regulates mechanosensitive cell volume adaptation and proliferation

Mammalian cells can rapidly respond to osmotic and hydrostatic pressure imbalances during an environmental change, generating large fluxes of water and ions that alter cell volume within minutes. While the role of ion pump and leak in cell volume regulation has been well-established, the potential contribution of the actomyosin cytoskeleton and its interplay with ion transporters is unclear. We discovered a cell volume regulation system that is controlled by cytoskeletal activation of ion transporters. After a hypotonic shock, normal-like cells (NIH-3T3, MCF-10A, and others) display a slow secondary volume increase (SVI) following the immediate regulatory volume decrease. We show that SVI is initiated by hypotonic stress induced Ca 2+ influx through stretch activated channel Piezo1, which subsequently triggers actomyosin remodeling. The actomyosin network further activates NHE1 through their synergistic linker ezrin, inducing SVI after the initial volume recovery. We find that SVI is absent in cancer cell lines such as HT1080 and MDA-MB-231, where volume regulation is dominated by intrinsic response of ion transporters. A similar cytoskeletal activation of NHE1 can also be achieved by mechanical stretching. On compliant substrates where cytoskeletal contractility is attenuated, SVI generation is abolished. Moreover, cytoskeletal activation of NHE1 during SVI triggers nuclear deformation, leading to a significant, immediate transcriptomic change in 3T3 cells, a phenomenon that is again absent in HT1080 cells. While hypotonic shock hinders ERK-dependent cell growth, cells deficient in SVI are unresponsive to such inhibitory effects. Overall, our findings reveal the critical role of Ca 2+ and actomyosin-mediated mechanosensation in the regulation of ion transport, cell volume, transcriptomics, and cell proliferation.

Optical Control of G-Actin with a Photoswitchable Latrunculin

Actin is one of the most abundant proteins in eukaryotic cells and is a key component of the cytoskeleton. A range of small molecules has emerged that interfere with actin dynamics by either binding to polymeric F-actin or monomeric G-actin to stabilize or destabilize filaments or prevent their formation and growth, respectively. Among these, the latrunculins, which bind to G-actin and affect polymerization, are widely used as tools to investigate actin-dependent cellular processes. Here, we report a photoswitchable version of latrunculin, termed opto-latrunculin (OptoLat), which binds to G-actin in a light-dependent fashion and affords optical control over actin polymerization. OptoLat can be activated with 390-490 nm pulsed light and rapidly relaxes to its inactive form in the dark. Light activated OptoLat induced depolymerization of F-actin networks in oligodendrocytes and budding yeast, as shown by fluorescence microscopy. Subcellular control of actin dynamics in human cancer cell lines was demonstrated via live cell imaging. Light-activated OptoLat also reduced microglia surveillance in organotypic mouse brain slices while ramification was not affected. Incubation in the dark did not alter the structural and functional integrity of the microglia. Together, our data demonstrate that OptoLat is a useful tool for the elucidation of G-actin dependent dynamic processes in cells and tissues.

Coordination of actin plus-end dynamics by IQGAP1, formin, and capping protein

Cell processes require precise regulation of actin polymerization that is mediated by plus-end regulatory proteins. Detailed mechanisms that explain plus-end dynamics involve regulators with opposing roles, including factors that enhance assembly, e.g., the formin mDia1, and others that stop growth (Capping Protein, CPz). We explore IQGAP1's roles regulating actin filament plus-ends and the consequences of perturbing its activity in cells. We confirm that IQGAP1 pauses elongation and interacts with plus ends through two residues (C756 and C781). We directly visualize the dynamic interplay between IQGAP1 and mDia1, revealing that IQGAP1 displaces the formin to influence actin assembly. Using four-color TIRF we show that IQGAP1's displacement activity extends to formin-CPz 'decision complexes', promoting end-binding protein turnover at plus-ends. Loss of IQGAP1 or its plus-end activities disrupts morphology and migration, emphasizing its essential role. These results reveal a new role for IQGAP1 in promoting protein turnover on filament ends and provide new insights into how plus-end actin assembly is regulated in cells.

Emerging roles of deubiquitinating enzymes in actin cytoskeleton and tumor metastasis

BACKGROUND

Metastasis accounts for the majority of cancer-related deaths. Actin dynamics and actin-based cell migration and invasion are important factors in cancer metastasis. Metastasis is characterized by actin polymerization and depolymerization, which are precisely regulated by molecular changes involving a plethora of actin regulators, including actin-binding proteins (ABPs) and signalling pathways, that enable cancer cell dissemination from the primary tumour. Research on deubiquitinating enzymes (DUBs) has revealed their vital roles in actin dynamics and actin-based migration and invasion during cancer metastasis.

CONCLUSION

Here, we review how DUBs drive tumour metastasis by participating in actin rearrangement and actin-based migration and invasion. We summarize the well-characterized and essential actin cytoskeleton signalling molecules related to DUBs, including Rho GTPases, Src kinases, and ABPs such as cofilin and cortactin. Other DUBs that modulate actin-based migration signalling pathways are also discussed. Finally, we discuss and address therapeutic opportunities and ongoing challenges related to DUBs with respect to actin dynamics.

The actin binding protein profilin 1 is critical for mitochondria function

Profilin 1 (PFN1) is an actin binding protein that is vital for the polymerization of monomeric actin into filaments. Here we screened knockout cells for novel functions of PFN1 and discovered that mitophagy, a type of selective autophagy that removes defective or damaged mitochondria from the cell, was significantly upregulated in the absence of PFN1. Despite successful autophagosome formation and fusion with the lysosome, and activation of additional mitochondrial quality control pathways, PFN1 knockout cells still accumulate damaged, dysfunctional mitochondria. Subsequent imaging and functional assays showed that loss of PFN1 significantly affects mitochondria morphology, dynamics, and respiration. Further experiments revealed that PFN1 is located to the mitochondria matrix and is likely regulating mitochondria function from within rather than through polymerizing actin at the mitochondria surface. Finally, PFN1 mutants associated with amyotrophic lateral sclerosis (ALS) fail to rescue PFN1 knockout mitochondrial phenotypes and form aggregates within mitochondria, further perturbing them. Together, these results suggest a novel function for PFN1 in regulating mitochondria and identify a potential pathogenic mechanism of ALS-linked PFN1 variants.

Cytoskeletal adaptation following long-term dysregulation of actomyosin in neuronal processes

Microtubules, intermediate filaments, and actin are cytoskeletal polymer networks found within the cell. While each has unique functions, all the cytoskeletal elements must work together for cellular mechanics to be fully operative. This is achieved through crosstalk mechanisms whereby the different networks influence each other through signaling pathways and direct interactions. Because crosstalk can be complex, it is possible for perturbations in one cytoskeletal element to affect the others in ways that are difficult to predict. Here we investigated how long-term changes to the actin cytoskeleton affect microtubules and intermediate filaments. Reducing F-actin or actomyosin contractility increased acetylated microtubules and intermediate filament expression, with the effect being significantly more pronounced in neuronal processes. Changes to microtubules were completely reversible if F-actin and myosin activity is restored. Moreover, the altered microtubules in neuronal processes resulting from F-actin depletion caused significant changes to microtubule-based transport, mimicking phenotypes that are linked to neurodegenerative disease. Thus, defects in actin dynamics cause a compensatory response in other cytoskeleton components which profoundly alters cellular function.

Actin polymerization and depolymerization in developing vertebrates

Development is a complex process that occurs throughout the life cycle. F-actin, a major component of the cytoskeleton, is essential for the morphogenesis of tissues and organs during development. F-actin is formed by the polymerization of G-actin, and the dynamic balance of polymerization and depolymerization ensures proper cellular function. Disruption of this balance results in various abnormalities and defects or even embryonic lethality. Here, we reviewed recent findings on the structure of G-actin and F-actin and the polymerization of G-actin to F-actin. We also focused on the functions of actin isoforms and the underlying mechanisms of actin polymerization/depolymerization in cellular and organic morphogenesis during development. This information will extend our understanding of the role of actin polymerization in the physiologic or pathologic processes during development and may open new avenues for developing therapeutics for embryonic developmental abnormalities or tissue regeneration.

How neurons maintain their axons long-term: an integrated view of axon biology and pathology

Axons are processes of neurons, up to a metre long, that form the essential biological cables wiring nervous systems. They must survive, often far away from their cell bodies and up to a century in humans. This requires self-sufficient cell biology including structural proteins, organelles, and membrane trafficking, metabolic, signalling, translational, chaperone, and degradation machinery-all maintaining the homeostasis of energy, lipids, proteins, and signalling networks including reactive oxygen species and calcium. Axon maintenance also involves specialised cytoskeleton including the cortical actin-spectrin corset, and bundles of microtubules that provide the highways for motor-driven transport of components and organelles for virtually all the above-mentioned processes. Here, we aim to provide a conceptual overview of key aspects of axon biology and physiology, and the homeostatic networks they form. This homeostasis can be derailed, causing axonopathies through processes of ageing, trauma, poisoning, inflammation or genetic mutations. To illustrate which malfunctions of organelles or cell biological processes can lead to axonopathies, we focus on axonopathy-linked subcellular defects caused by genetic mutations. Based on these descriptions and backed up by our comprehensive data mining of genes linked to neural disorders, we describe the 'dependency cycle of local axon homeostasis' as an integrative model to explain why very different causes can trigger very similar axonopathies, providing new ideas that can drive the quest for strategies able to battle these devastating diseases.

Optical Control of G-Actin with a Photoswitchable Latrunculin

Actin is one of the most abundant proteins in eukaryotic cells and a key component of the cytoskeleton. A range of small molecules have emerged that interfere with actin dynamics by either binding to polymeric F-actin or monomeric G-actin to stabilize or destabilize filaments or prevent their formation and growth, respectively. Amongst these, the latrunculins, which bind to G-actin and affect polymerization, are widely used as tools to investigate actin-dependent cellular processes. Here, we report a photoswitchable version of latrunculin, termed opto-latrunculin (OptoLat), which binds to G-actin in a light-dependent fashion and affords optical control over actin polymerization. OptoLat can be activated with 390 - 490 nm pulsed light and rapidly relaxes to the inactive form in the dark. Light activated OptoLat induced depolymerization of F-actin networks in oligodendrocytes and budding yeast, as shown by fluorescence microscopy. Subcellular control of actin dynamics in human cancer cell lines was demonstrated by live cell imaging. Light-activated OptoLat also reduced microglia surveillance in organotypic mouse brain slices while ramification was not affected. Incubation in the dark did not alter the structural and functional integrity of microglia. Together, our data demonstrate that OptoLat is a useful tool for the elucidation of G-actin dependent dynamic processes in cells and tissues.

Thymosin β4 and prothymosin α promote cardiac regeneration post-ischaemic injury in mice

AIMS

The adult mammalian heart is a post-mitotic organ. Even in response to necrotic injuries, where regeneration would be essential to reinstate cardiac structure and function, only a minor percentage of cardiomyocytes undergo cytokinesis. The gene programme that promotes cell division within this population of cardiomyocytes is not fully understood. In this study, we aimed to determine the gene expression profile of proliferating adult cardiomyocytes in the mammalian heart after myocardial ischaemia, to identify factors to can promote cardiac regeneration.

METHODS AND RESULTS

Here, we demonstrate increased 5-ethynyl-2'deoxyuridine incorporation in cardiomyocytes 3 days post-myocardial infarction in mice. By applying multi-colour lineage tracing, we show that this is paralleled by clonal expansion of cardiomyocytes in the borderzone of the infarcted tissue. Bioinformatic analysis of single-cell RNA sequencing data from cardiomyocytes at 3 days post ischaemic injury revealed a distinct transcriptional profile in cardiomyocytes expressing cell cycle markers. Combinatorial overexpression of the enriched genes within this population in neonatal rat cardiomyocytes and mice at postnatal day 12 (P12) unveiled key genes that promoted increased cardiomyocyte proliferation. Therapeutic delivery of these gene cocktails into the myocardial wall after ischaemic injury demonstrated that a combination of thymosin beta 4 (TMSB4) and prothymosin alpha (PTMA) provide a permissive environment for cardiomyocyte proliferation and thereby attenuated cardiac dysfunction.

CONCLUSION

This study reveals the transcriptional profile of proliferating cardiomyocytes in the ischaemic heart and shows that overexpression of the two identified factors, TMSB4 and PTMA, can promote cardiac regeneration. This work indicates that in addition to activating cardiomyocyte proliferation, a supportive environment is a key for regeneration to occur.

Purification of human β- and γ-actin from budding yeast

Biochemical studies of human actin and its binding partners rely heavily on abundant and easily purified α-actin from skeletal muscle. Therefore, muscle actin has been used to evaluate and determine the activities of most actin regulatory proteins but there is an underlying concern that these proteins perform differently from actin present in non-muscle cells. To provide easily accessible and relatively abundant sources of human β- or γ-actin (i.e. cytoplasmic actins), we developed Saccharomyces cerevisiae strains that express each as their sole source of actin. Both β- or γ-actin purified in this system polymerize and interact with various binding partners, including profilin, mDia1 (formin), fascin and thymosin-β4 (Tβ4). Notably, Tβ4 and profilin bind to β- or γ-actin with higher affinity than to α-actin, emphasizing the value of testing actin ligands with specific actin isoforms. These reagents will make specific isoforms of actin more accessible for future studies on actin regulation.

Proteomic characterisation of prostate cancer intercellular communication reveals cell type-selective signalling and TMSB4X-dependent fibroblast reprogramming

BACKGROUND

In prostate cancer, the tumour microenvironment (TME) represents an important regulator of disease progression and response to treatment. In the TME, cancer-associated fibroblasts (CAFs) play a key role in tumour progression, however the mechanisms underpinning fibroblast-cancer cell interactions are incompletely resolved. Here, we address this by applying cell type-specific labelling with amino acid precursors (CTAP) and mass spectrometry (MS)-based (phospho)proteomics to prostate cancer for the first time.

METHODS

Reciprocal interactions between PC3 prostate cancer cells co-cultured with WPMY-1 prostatic fibroblasts were characterised using CTAP-MS. Signalling network changes were determined using Metascape and Enrichr and visualised using Cytoscape. Thymosin β4 (TMSB4X) overexpression was achieved via retroviral transduction and assayed by ELISA. Cell motility was determined using Transwell and random cell migration assays and expression of CAF markers by indirect immunofluorescence.

RESULTS

WPMY-1 cells co-cultured with PC3s demonstrated a CAF-like phenotype, characterised by enhanced PDGFRB expression and alterations in signalling pathways regulating epithelial-mesenchymal transition, cytoskeletal organisation and cell polarisation. In contrast, co-cultured PC3 cells exhibited more modest network changes, with alterations in mTORC1 signalling and regulation of the actin cytoskeleton. The expression of the actin binding protein TMSB4X was significantly decreased in co-cultured WPMY-1 fibroblasts, and overexpression of TMSB4X in fibroblasts decreased migration of co-cultured PC3 cells, reduced fibroblast motility, and protected the fibroblasts from being educated to a CAF-like phenotype by prostate cancer cells.

CONCLUSIONS

This study highlights the potential of CTAP-MS to characterise intercellular communication within the prostate TME and identify regulators of cellular crosstalk such as TMSB4X.

Cytoskeletal and Cytoskeleton-Associated Proteins: Key Regulators of Cancer Stem Cell Properties

Cancer stem cells (CSCs) are a subpopulation of cancer cells possessing stemness characteristics that are closely associated with tumor proliferation, recurrence and resistance to therapy. Recent studies have shown that different cytoskeletal components and remodeling processes have a profound impact on the behavior of CSCs. In this review, we outline the different cytoskeletal components regulating the properties of CSCs and discuss current and ongoing therapeutic strategies targeting the cytoskeleton. Given the many challenges currently faced in targeted cancer therapy, a deeper comprehension of the molecular events involved in the interaction of the cytoskeleton and CSCs will help us identify more effective therapeutic strategies to eliminate CSCs and ultimately improve patient survival.

Multiple roles for the cytoskeleton in ALS

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease caused by more than sixty genes identified through classic linkage analysis and new sequencing methods. Yet no clear mechanism of onset, cure, or effective treatment is known. Popular discourse classifies the proteins encoded from ALS-related genes into four disrupted processes: proteostasis, mitochondrial function and ROS, nucleic acid regulation, and cytoskeletal dynamics. Surprisingly, the mechanisms detailing the contribution of the neuronal cytoskeletal in ALS are the least explored, despite involvement in these cell processes. Eight genes directly regulate properties of cytoskeleton function and are essential for the health and survival of motor neurons, including: TUBA4A, SPAST, KIF5A, DCTN1, NF, PRPH, ALS2, and PFN1. Here we review the properties and studies exploring the contribution of each of these genes to ALS.

Rapid assembly of a polar network architecture drives efficient actomyosin contractility

Actin network architecture and dynamics play a central role in cell contractility and tissue morphogenesis. RhoA-driven pulsed contractions are a generic mode of actomyosin contractility, but the mechanisms underlying how their specific architecture emerges and how this architecture supports the contractile function of the network remain unclear. Here we show that, during pulsed contractions, the actin network is assembled by two subpopulations of formins: a functionally inactive population (recruited) and formins actively participating in actin filament elongation (elongating). We then show that elongating formins assemble a polar actin network, with barbed ends pointing out of the pulse. Numerical simulations demonstrate that this geometry favors rapid network contraction. Our results show that formins convert a local RhoA activity gradient into a polar network architecture, causing efficient network contractility, underlying the key function of kinetic controls in the assembly and mechanics of cortical network architectures.

RhoA-driven actomyosin contractility plays a key role in driving cell and tissue contractility during morphogenesis. Tracking individual formins, Costache et al. show that the network assembled downstream of RhoA displays a polar architecture, barbed ends pointing outward, a feature that supports efficient contractility and force transmission during pulsed contractions.

Effect of GNE Mutations on Cytoskeletal Network Proteins: Potential Gateway to Understand Pathomechanism of GNEM

GNE myopathy is an inherited neuromuscular disorder caused by mutations in GNE (UDP-N-acetylglucosamine 2-epimerase/N-acetyl mannosamine kinase) gene catalyzing the sialic acid biosynthesis pathway. The characteristic features include muscle weakness in upper and lower extremities, skeletal muscle wasting, and rimmed vacuole formation. More than 200 GNE mutations in either epimerase or kinase domain have been reported worldwide. In Indian subcontinent, several GNE mutations have been recently identified with unknown functional correlation. Alternate role of GNE in various cellular processes such as cell adhesion, migration, apoptosis, protein aggregation, and cytoskeletal organization have been proposed in recent studies. We aim to understand and compare the effect of various GNE mutations from Indian origin on regulation of the cytoskeletal network. In particular, F-actin dynamics was determined quantitatively by determining F/G-actin ratios in immunoblots for specific proteins. The extent of F-actin polymerization was visualized by immunostaining with Phalloidin using confocal microscopy. The proteins regulating F-actin dynamics such as RhoA, cofilin, Arp2, and alpha-actinin were studied in various GNE mutants. The altered level of cytoskeletal organization network proteins affected cell migration of GNE mutant proteins as measured by wound healing assay. The functional comparison of GNE mutations will help in better understanding of the genotypic severity of the disease in the Indian population. Our study offers a potential for identification of therapeutic molecules regulating actin dynamics in GNE specific mutations.

Contractile apparatus in CNS capillary pericytes

Significance: Whether or not capillary pericytes contribute to blood flow regulation in the brain and retina has long been debated. This was partly caused by failure of detecting the contractile protein α -smooth muscle actin ( α -SMA) in capillary pericytes. Aim: The aim of this review is to summarize recent developments in detecting α -SMA and contractility in capillary pericytes and the relevant literature on the biology of actin filaments. Results: Evidence suggests that for visualization of the small amounts of α -SMA in downstream mid-capillary pericytes, actin depolymerization must be prevented during tissue processing. Actin filaments turnover is mainly based on de/re-polymerization rather than transcription of the monomeric form, hence, small amounts of α -SMA mRNA may evade detection by transcriptomic studies. Similarly, transgenic mice expressing fluorescent reporters under the α -SMA promoter may yield low fluorescence due to limited transcriptional activity in mid-capillary pericytes. Recent studies show that pericytes including mid-capillary ones express several actin isoforms and myosin heavy chain type 11, the partner of α -SMA in mediating contraction. Emerging evidence also suggests that actin polymerization in pericytes may have a role in regulating the tone of downstream capillaries. Conclusions: With guidance of actin biology, innovative labeling and imaging techniques can reveal the molecular machinery of contraction in pericytes.

Biochemical characterization of actin assembly mechanisms with ALS-associated profilin variants

Eight separate mutations in the actin-binding protein profilin-1 have been identified as a rare cause of amyotrophic lateral sclerosis (ALS). Profilin is essential for many neuronal cell processes through its regulation of lipids, nuclear signals, and cytoskeletal dynamics, including actin filament assembly. Direct interactions between profilin and actin monomers inhibit actin filament polymerization. In contrast, profilin can also stimulate polymerization by simultaneously binding actin monomers and proline-rich tracts found in other proteins. Whether the ALS-associated mutations in profilin compromise these actin assembly functions is unclear. We performed a quantitative biochemical comparison of the direct and formin mediated impact for the eight ALS-associated profilin variants on actin assembly using classic protein-binding and single-filament microscopy assays. We determined that the binding constant of each profilin for actin monomers generally correlates with the actin nucleation strength associated with each ALS-related profilin. In the presence of formin, the A20T, R136W, Q139L, and C71G variants failed to activate the elongation phase of actin assembly. This diverse range of formin-activities is not fully explained through profilin-poly-L-proline (PLP) interactions, as all ALS-associated variants bind a formin-derived PLP peptide with similar affinities. However, chemical denaturation experiments suggest that the folding stability of these profilins impact some of these effects on actin assembly. Thus, changes in profilin protein stability and alterations in actin filament polymerization may both contribute to the profilin-mediated actin disruptions in ALS.

TDAExplore: Quantitative analysis of fluorescence microscopy images through topology-based machine learning

Recent advances in machine learning have greatly enhanced automatic methods to extract information from fluorescence microscopy data. However, current machine-learning-based models can require hundreds to thousands of images to train, and the most readily accessible models classify images without describing which parts of an image contributed to classification. Here, we introduce TDAExplore, a machine learning image analysis pipeline based on topological data analysis. It can classify different types of cellular perturbations after training with only 20–30 high-resolution images and performs robustly on images from multiple subjects and microscopy modes. Using only images and whole-image labels for training, TDAExplore provides quantitative, spatial information, characterizing which image regions contribute to classification. Computational requirements to train TDAExplore models are modest and a standard PC can perform training with minimal user input. TDAExplore is therefore an accessible, powerful option for obtaining quantitative information about imaging data in a wide variety of applications.

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TDAExplore combines topological data analysis with machine learning classification

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As few as 20–30 high-resolution images can be used to train TDAExplore models

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TDAExplore is robust to different microscopy modes, dataset size, image features

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TDAExplore quantifies where and how much each image resembles the training data

Traditional intensity-based measurements of fluorescent microscopy data limit its potential to reveal new information about its sample. Here, we present an image analysis pipeline called TDAExplore, which is based on topological data analysis and machine learning classification. In addition to being highly accurate in assigning images to their correct group, TDAExplore quantifies how much images resemble the training data and identifies which parts are different, an improvement over other machine learning models that do not permit insight into how classification tasks were made. The next steps for TDAExplore will be to expand its capabilities into three-dimensional, multivariate, and time series datasets. This work represents progress into a future where machine learning identifies and describes nuanced image features in ways that allow researchers to answer important biological questions and generate new hypotheses for future studies.

Polymerization/depolymerization of actin cooperates with the morphology and stability of cell-sized droplets generated in a polymer solution under a depletion effect

Intercellular fluids in living organisms contain high concentrations of macromolecules such as nucleic acid and protein. Over the past few decades, several studies have examined membraneless organelles in terms of liquid-liquid phase separation. These studies have investigated aggregation/attraction among a rich variety of biomolecules. Here, we studied the association between the polymerization/depolymerization of actin, interconversion between monomeric (G-actin) and filamentous states (F-actin), and water/water phase separation in a binary polymer solution using polyethylene glycol (PEG) and dextran (DEX). We found that actin, which is a representative cytoskeleton, changes its distribution in a PEG/DEX binary solution depending on its polymerization state: monomeric G-actin is distributed homogeneously throughout the solution, whereas polymerized F-actin is localized only within the DEX-rich phase. We extended our study by using fragmin, which is a representative actin-severing and -depolymerizing factor. It took hours to restore a homogeneous actin distribution from localization within the DEX-rich phase, even with the addition of fragmin in an amount that causes complete depolymerization. In contrast, when actin that had been depolymerized by fragmin in advance was added to a solution with microphase-separation, F-actin was found in DEX-rich phase droplets. The micro-droplets tended to deform into a non-spherical morphology under conditions where they contained F-actin. These findings suggest that microphase-separation is associated with the dynamics of polymerization and localization of the actin cytoskeleton. We discuss our observations by taking into consideration the polymer depletion effect.

F-Actin Cytoskeleton Network Self-Organization Through Competition and Cooperation

Many fundamental cellular processes such as division, polarization, endocytosis, and motility require the assembly, maintenance, and disassembly of filamentous actin (F-actin) networks at specific locations and times within the cell. The particular function of each network is governed by F-actin organization, size, and density as well as by its dynamics. The distinct characteristics of different F-actin networks are determined through the coordinated actions of specific sets of actin-binding proteins (ABPs). Furthermore, a cell typically assembles and uses multiple F-actin networks simultaneously within a common cytoplasm, so these networks must self-organize from a common pool of shared globular actin (G-actin) monomers and overlapping sets of ABPs. Recent advances in multicolor imaging and analysis of ABPs and their associated F-actin networks in cells, as well as the development of sophisticated in vitro reconstitutions of networks with ensembles of ABPs, have allowed the field to start uncovering the underlying principles by which cells self-organize diverse F-actin networks to execute basic cellular functions.

Thymosin β4 is essential for thrombus formation by controlling the G-actin/F-actin equilibrium in platelets

Coordinated rearrangements of the actin cytoskeleton are pivotal for platelet biogenesis from megakaryocytes but also orchestrate key functions of peripheral platelets in hemostasis and thrombosis, such as granule release, the formation of filopodia and lamellipodia, or clot retraction. Along with profilin (Pfn) 1, thymosin β4 (encoded by Tmsb4x) is one of the two main G-actin-sequestering proteins within cells of higher eukaryotes, and its intracellular concentration is particularly high in cells that rapidly respond to external signals by increased motility, such as platelets. Here, we analyzed constitutive Tmsb4x knockout (KO) mice to investigate the functional role of the protein in platelet production and function. Thymosin β4 deficiency resulted in a macrothrombocytopenia with only mildly increased platelet volume and an unaltered platelet life span. Megakaryocyte numbers in the bone marrow and spleen were unaltered, however, Tmsb4x KO megakaryocytes showed defective proplatelet formation in vitro and in vivo. Thymosin β4-deficient platelets displayed markedly decreased G-actin levels and concomitantly increased F-actin levels resulting in accelerated spreading on fibrinogen and clot retraction. Moreover, Tmsb4x KO platelets showed activation defects and an impaired immunoreceptor tyrosine-based activation motif (ITAM) signaling downstream of the activating collagen receptor glycoprotein VI. These defects translated into impaired aggregate formation under flow, protection from occlusive arterial thrombus formation in vivo and increased tail bleeding times. In summary, these findings point to a critical role of thymosin β4 for actin dynamics during platelet biogenesis, platelet activation downstream of glycoprotein VI and thrombus stability.

Different translation dynamics of β- and γ-actin regulates cell migration

β- and γ-cytoplasmic actins are ubiquitously expressed in every cell type and are nearly identical at the amino acid level but play vastly different roles in vivo. Their essential roles in embryogenesis and mesenchymal cell migration critically depend on the nucleotide sequences of their genes, rather than their amino acid sequences; however, it is unclear which gene elements underlie this effect. Here we address the specific role of the coding sequence in β- and γ-cytoplasmic actins’ intracellular functions, using stable polyclonal populations of immortalized mouse embryonic fibroblasts with exogenously expressed actin isoforms and their ‘codon-switched’ variants. When targeted to the cell periphery using β-actin 3′UTR; β-actin and γ-actin have differential effects on cell migration. These effects directly depend on the coding sequence. Single-molecule measurements of actin isoform translation, combined with fluorescence recovery after photobleaching, demonstrate a pronounced difference in β- and γ-actins’ translation elongation rates in cells, leading to changes in their dynamics at focal adhesions, impairments in actin bundle formation, and reduced cell anchoring to the substrate during migration. Our results demonstrate that coding sequence-mediated differences in actin translation play a key role in cell migration.

Most mammalian cells make both β- and γ-actin, two proteins which shape the cell’s internal skeleton and its ability to migrate. The molecules share over 99% of their sequence, yet they play distinct roles. In fact, deleting the β-actin gene in mice causes death in the womb, while the animals can survive with comparatively milder issues without their γ-actin gene. How two similar proteins can have such different biological roles is a long-standing mystery.

A closer look could hold some clues: β- and γ-actin may contain the same blocks (or amino acids), but the genetic sequences that encode these proteins differ by about 13%. This is because different units of genetic information – known as synonymous codons – can encode the same amino acid. These ‘silent substitutions’ have no effect on the sequence of the proteins, yet a cell reads synonymous codons (and therefore produces proteins) at different speeds.

To find out the impact of silent substitutions, Vedula et al. swapped the codons for the two proteins, forcing mouse cells to produce β-actin using γ-actin codons, and vice versa. Cells with non-manipulated γ-actin and those with β-actin made using γ-actin codons could move much faster than cells with β-actin. This suggested that silent substitutions were indeed affecting the role of the protein.

Vedula et al. found that cells read γ-codons – and therefore made γ-actin – much more slowly than β-codons: this also affected how quickly the protein could be dispatched where it was needed in the cell. Slower production meant that bundles of γ-actin were shorter, which allowed cells to move faster by providing a weaker anchoring system. Overall, this work provides new links between silent substitutions and protein behavior, a relatively new research area which is likely to shed light on other protein families.

In Vitro-Evolved Peptides Bind Monomeric Actin and Mimic Actin-Binding Protein Thymosin-β4

Actin is the most abundant protein in eukaryotic cells and is key to many cellular functions. The filamentous form of actin (F-actin) can be studied with help of natural products that specifically recognize it, as for example fluorophore-labeled probes of the bicyclic peptide phalloidin, but no synthetic probes exist for the monomeric form of actin (G-actin). Herein, we have panned a phage display library consisting of more than 10 billion bicyclic peptides against G-actin and isolated binders with low nanomolar affinity and greater than 1000-fold selectivity over F-actin. Sequence analysis revealed a strong similarity to a region of thymosin-β4, a protein that weakly binds G-actin, and competition binding experiments confirmed a common binding region at the cleft between actin subdomains 1 and 3. Together with F-actin-specific peptides that we also isolated, we evaluated the G-actin peptides as probes in pull-down, imaging, and competition binding experiments. While the F-actin peptides were applied successfully for capturing actin in cell lysates and for imaging, the G-actin peptides did not bind in the cellular context, most likely due to competition with thymosin-β4 or related endogenous proteins for the same binding site.

Significance of nuclear LOXL2 inhibition in fibroblasts and myofibroblasts in the fibrotic process of acute respiratory distress syndrome

Fibrotic scarring is an important prognostic factor of acute respiratory distress syndrome (ARDS). There are currently no antifibrotic drugs or other therapeutic agents for ARDS. Lysyl oxidase-like 2 (LOXL2), an amine oxidase, contributes to fibrotic scarring by facilitating collagen cross-linking. Recent clinical trials revealed that a monoclonal inhibitory antibody against LOXL2 failed to show benefit over placebo in patients with fibrotic disorders involving the lungs. These clinical results raise the possibility that targeting the extracellular enzymic activity of LOXL2 is not in itself sufficient to prevent fibrotic scarring. We investigated the role of LOXL2 in the pathogenesis of ARDS in vivo, in vitro, and in samples from patients with ARDS. After lung injury, LOXL2 was unevenly expressed in the nuclei of lung fibroblasts and myofibroblasts in the fibrotic phase. Nuclear LOXL2 expression was upregulated in lung fibroblasts after transforming growth factor-beta1 (TGF-β1)-treatment. LOXL2 silencing abrogated the TGF-β1-induced expression of a myofibrogenic-progenitor marker, the appearance of proto-myofibroblasts, and the evolution of differentiated myofibroblasts in lung fibroblasts. Nuclear upregulation of Snail was evident in myofibroblasts during the fibrotic phase after lung injury. We detected high levels of LOXL2 protein in the lungs of ARDS patients, specifically during the proliferative and fibrotic phases. Our results highlight nuclear LOXL2 in fibroblasts as a primary causative driver of cell-fate decision toward myofibroblasts and of the progression of fibrotic scarring. A nuclear-LOXL2-targeted agent could be a promising therapeutic strategy against fibrotic disorders including ARDS.

Actin Remodeling Defects Leading to Autoinflammation and Immune Dysregulation

A growing number of monogenic immune-mediated diseases have been related to genes involved in pathways of actin cytoskeleton remodeling. Increasing evidences associate cytoskeleton defects to autoinflammatory diseases and primary immunodeficiencies. We reviewed the pathways of actin cytoskeleton remodeling in order to identify inflammatory and immunological manifestations associated to pathological variants. We list more than twenty monogenic diseases, ranging from pure autoinflammatory conditions as familial Mediterranean fever, mevalonate kinase deficiency and PAPA syndrome, to classic and novel primary immunodeficiencies as Wiskott-Aldrich syndrome and DOCK8 deficiency, characterized by the presence of concomitant inflammatory and autoimmune manifestations, such as vasculitis and cytopenia, to severe and recurrent infections. We classify these disorders according to the role of the mutant gene in actin cytoskeleton remodeling, and in particular as disorders of transcription, elongation, branching and activation of actin. This expanding field of rare immune disorders offers a new perspective to all immunologists to better understand the physiological and pathological role of actin cytoskeleton in cells of innate and adaptive immunity.

Actin Remodeling Defects Leading to Autoinflammation and Immune Dysregulation

A growing number of monogenic immune-mediated diseases have been related to genes involved in pathways of actin cytoskeleton remodeling. Increasing evidences associate cytoskeleton defects to autoinflammatory diseases and primary immunodeficiencies. We reviewed the pathways of actin cytoskeleton remodeling in order to identify inflammatory and immunological manifestations associated to pathological variants. We list more than twenty monogenic diseases, ranging from pure autoinflammatory conditions as familial Mediterranean fever, mevalonate kinase deficiency and PAPA syndrome, to classic and novel primary immunodeficiencies as Wiskott-Aldrich syndrome and DOCK8 deficiency, characterized by the presence of concomitant inflammatory and autoimmune manifestations, such as vasculitis and cytopenia, to severe and recurrent infections. We classify these disorders according to the role of the mutant gene in actin cytoskeleton remodeling, and in particular as disorders of transcription, elongation, branching and activation of actin. This expanding field of rare immune disorders offers a new perspective to all immunologists to better understand the physiological and pathological role of actin cytoskeleton in cells of innate and adaptive immunity.

Twinfilin bypasses assembly conditions and actin filament aging to drive barbed end depolymerization

Cellular actin networks grow by ATP-actin addition at filament barbed ends and have long been presumed to depolymerize at their pointed ends, primarily after filaments undergo "aging" (ATP hydrolysis and Pi release). The cytosol contains high levels of actin monomers, which favors assembly over disassembly, and barbed ends are enriched in ADP-Pi actin. For these reasons, the potential for a barbed end depolymerization mechanism in cells has received little attention. Here, using microfluidics-assisted TIRF microscopy, we show that mouse twinfilin, a member of the ADF-homology family, induces depolymerization of ADP-Pi barbed ends even under assembly-promoting conditions. Indeed, we observe in single reactions containing micromolar concentrations of actin monomers the simultaneous rapid elongation of formin-bound barbed ends and twinfilin-induced depolymerization of free barbed ends. The data show that twinfilin catalyzes dissociation of subunits from ADP-Pi barbed ends and thereby bypasses filament aging prerequisites to disassemble newly polymerized actin filaments.

Assessing the Multivariate Relationship between the Human Infant Intestinal Exfoliated Cell Transcriptome (Exfoliome) and Microbiome in Response to Diet

Gut microbiota and the host exist in a mutualistic relationship, with the functional composition of the microbiota strongly influencing the health and well-being of the host. In addition to the standard differential expression analysis of host genes to assess the complex cross-talk between environment (diet), microbiome, and host intestinal physiology, data-driven integrative approaches are needed to identify potential biomarkers of both host genes and microbial communities that characterize these interactions. Our findings demonstrate that the complementary application of univariate differential gene expression analysis and multivariate approaches such as sparse Canonical Correlation Analysis (sCCA) and sparse Principal Components Analysis (sPCA) can be used to integrate data from both the healthy infant gut microbial community and host transcriptome (exfoliome) using stool derived exfoliated cells shed from the gut. These approaches reveal host genes and microbial functional categories related to the feeding phenotype of the infants. Our findings also confirm that combinatorial noninvasive -omic approaches provide an integrative genomics-based perspective of neonatal host-gut microbiome interactions.

The Function of Oncogene B-Cell Lymphoma 6 in the Regulation of the Migration and Invasion of Trophoblastic Cells

Human placentation is a highly invasive process. Deficiency in the invasiveness of trophoblasts is associated with a spectrum of gestational diseases, such as preeclampsia (PE). The oncogene B-cell lymphoma 6 (BCL6) is involved in the migration and invasion of various malignant cells. Intriguingly, its expression is deregulated in preeclamptic placentas. We have reported that BCL6 is required for the proliferation, survival, fusion, and syncytialization of trophoblasts. In the present work, we show that the inhibition of BCL6, either by its gene silencing or by using specific small molecule inhibitors, impairs the migration and invasion of trophoblastic cells, by reducing cell adhesion and compromising the dynamics of the actin cytoskeleton. Moreover, the suppression of BCL6 weakens the signals of the phosphorylated focal adhesion kinase, Akt/protein kinase B, and extracellular regulated kinase 1/2, accompanied by more stationary, but less migratory, cells. Interestingly, transcriptomic analyses reveal that a small interfering RNA-induced reduction of BCL6 decreases the levels of numerous genes, such as p21 activated kinase 1, myosin light chain kinase, and gamma actin related to cell adhesion, actin dynamics, and cell migration. These data suggest BCL6 as a crucial player in the migration and invasion of trophoblasts in the early stages of placental development through the regulation of various genes associated with the migratory machinery.

Cytoskeletal Remodeling in Cancer

Simple Summary

Cell migration is an essential process from embryogenesis to cell death. This is tightly regulated by numerous proteins that help in proper functioning of the cell. In diseases like cancer, this process is deregulated and helps in the dissemination of tumor cells from the primary site to secondary sites initiating the process of metastasis. For metastasis to be efficient, cytoskeletal components like actin, myosin, and intermediate filaments and their associated proteins should co-ordinate in an orderly fashion leading to the formation of many cellular protrusions-like lamellipodia and filopodia and invadopodia. Knowledge of this process is the key to control metastasis of cancer cells that leads to death in 90% of the patients. The focus of this review is giving an overall understanding of these process, concentrating on the changes in protein association and regulation and how the tumor cells use it to their advantage. Since the expression of cytoskeletal proteins can be directly related to the degree of malignancy, knowledge about these proteins will provide powerful tools to improve both cancer prognosis and treatment.

Abstract

Successful metastasis depends on cell invasion, migration, host immune escape, extravasation, and angiogenesis. The process of cell invasion and migration relies on the dynamic changes taking place in the cytoskeletal components; actin, tubulin and intermediate filaments. This is possible due to the plasticity of the cytoskeleton and coordinated action of all the three, is crucial for the process of metastasis from the primary site. Changes in cellular architecture by internal clues will affect the cell functions leading to the formation of different protrusions like lamellipodia, filopodia, and invadopodia that help in cell migration eventually leading to metastasis, which is life threatening than the formation of neoplasms. Understanding the signaling mechanisms involved, will give a better insight of the changes during metastasis, which will eventually help targeting proteins for treatment resulting in reduced mortality and longer survival.

The Huntingtin-interacting protein SETD2/HYPB is an actin lysine methyltransferase

The methyltransferase SET domain-containing 2 (SETD2) was originally identified as Huntingtin (HTT) yeast partner B. However, a SETD2 function associated with the HTT scaffolding protein has not been elucidated, and no linkage between HTT and methylation has yet been uncovered. Here, we show that SETD2 is an actin methyltransferase that trimethylates lysine-68 (ActK68me3) in cells via its interaction with HTT and the actin-binding adapter HIP1R. ActK68me3 localizes primarily to the insoluble F-actin cytoskeleton in cells and regulates actin polymerization/depolymerization dynamics. Disruption of the SETD2-HTT-HIP1R axis inhibits actin methylation, causes defects in actin polymerization, and impairs cell migration. Together, these data identify SETD2 as a previously unknown HTT effector regulating methylation and polymerization of actin filaments and provide new avenues for understanding how defects in SETD2 and HTT drive disease via aberrant cytoskeletal methylation.

The LINC00961 transcript and its encoded micropeptide, small regulatory polypeptide of amino acid response, regulate endothelial cell function

AIMS

Long non-coding RNAs (lncRNAs) play functional roles in physiology and disease, yet understanding of their contribution to endothelial cell (EC) function is incomplete. We identified lncRNAs regulated during EC differentiation and investigated the role of LINC00961 and its encoded micropeptide, small regulatory polypeptide of amino acid response (SPAAR), in EC function.

METHODS AND RESULTS

Deep sequencing of human embryonic stem cell differentiation to ECs was combined with Encyclopedia of DNA Elements (ENCODE) RNA-seq data from vascular cells, identifying 278 endothelial enriched genes, including 6 lncRNAs. Expression of LINC00961, first annotated as an lncRNA but reassigned as a protein-coding gene for the SPAAR micropeptide, was increased during the differentiation and was EC enriched. LINC00961 transcript depletion significantly reduced EC adhesion, tube formation, migration, proliferation, and barrier integrity in primary ECs. Overexpression of the SPAAR open reading frame increased tubule formation; however, overexpression of the full-length transcript did not, despite production of SPAAR. Furthermore, overexpression of an ATG mutant of the full-length transcript reduced network formation, suggesting a bona fide non-coding RNA function of the transcript with opposing effects to SPAAR. As the LINC00961 locus is conserved in mouse, we generated an LINC00961 locus knockout (KO) mouse that underwent hind limb ischaemia (HLI) to investigate the angiogenic role of this locus in vivo. In agreement with in vitro data, KO animals had a reduced capillary density in the ischaemic adductor muscle after 7 days. Finally, to characterize LINC00961 and SPAAR independent functions in ECs, we performed pull-downs of both molecules and identified protein-binding partners. LINC00961 RNA binds the G-actin sequestering protein thymosin beta-4x (Tβ4) and Tβ4 depletion phenocopied the overexpression of the ATG mutant. SPAAR binding partners included the actin-binding protein, SYNE1.

CONCLUSION

The LINC00961 locus regulates EC function in vitro and in vivo. The gene produces two molecules with opposing effects on angiogenesis: SPAAR and LINC00961.

Actin is associated with tissue injury in trauma patients and produces a hypercoagulable profile in vitro

BACKGROUND

While tissue injury provokes fibrinolysis shutdown in trauma, the mechanism remains elusive. Cellular death causes release of structural proteins, including actin and myosin, which may interact with clot formation and structure. We hypothesized that tissue injury is associated with high circulating actin and that actin produces a hypercoagulable profile with decreased fibrinolysis in vitro.

METHODS

Blood was collected from trauma activation patients at a single Level I trauma center for thrombelastography and proteomics. Proteomic analyses were performed through targeted liquid chromatography coupled with mass spectrometry using isotope-labeled standards for quantification of actin and its endogenous inhibitor gelsolin. Based on the results, we added physiologic concentrations of cytoskeletal G-actin to whole blood from healthy volunteers and analyzed changes in thrombelastography, as well as to plasma and examined clot architecture using confocal microscopy of fluorescently labeled fibrinogen.

RESULTS

Overall, 108 trauma patients were included: majority (71%) men, median age of 32.7 years, 66% blunt mechanism, median New Injury Severity Score (NISS) of 41. Compared with patients without severe tissue injury (NISS < 15, n = 10), patients with severe tissue injury (NISS > 15, n = 98) had higher levels of circulating actin (0.0428 vs. 0.0301, p = 0.02). Further, there was a trend toward lower gelsolin levels in patients with fibrinolysis shutdown (0.1844 vs. 0.2052, p = 0.17) and tissue plasminogen activator resistance (0.1676 vs. 0.2188, p = 0.06).Ten healthy volunteers were included in the in vitro experiments (50% male; median age, 31.3 years). Actin significantly increased angle (40.0° to 52.9°, p = 0.002) and decreased fibrinolysis (percent clot lysis 30 minutes after reaching maximum amplitude, 4.0% to 1.6%; p = 0.002), provoking fibrinolytic shutdown in three patients. The addition of actin to control plasma decreased fiber resolvability of fibrin clots, monitored by microscopy, and decreased plasmin-mediated fibrinolysis.

CONCLUSION

Actin increases clot propagation and provokes fibrinolysis shutdown in vitro, through a mechanism of plasmin inhibition. High circulating levels of actin are present in trauma patients with severe tissue injury, suggesting actin contributes to fibrinolysis shutdown in the setting of tissue injury.

Profilin choreographs actin and microtubules in cells and cancer

Actin and microtubules play essential roles in aberrant cell processes that define and converge in cancer including: signaling, morphology, motility, and division. Actin and microtubules do not directly interact, however shared regulators coordinate these polymers. While many of the individual proteins important for regulating and choreographing actin and microtubule behaviors have been identified, the way these molecules collaborate or fail in normal or disease contexts is not fully understood. Decades of research focus on Profilin as a signaling molecule, lipid-binding protein, and canonical regulator of actin assembly. Recent reports demonstrate that Profilin also regulates microtubule dynamics and polymerization. Thus, Profilin can coordinate both actin and microtubule polymer systems. Here we reconsider the biochemical and cellular roles for Profilin with a focus on the essential cytoskeletal-based cell processes that go awry in cancer. We also explore how the use of model organisms has helped to elucidate mechanisms that underlie the regulatory essence of Profilin in vivo and in the context of disease.

Profilin: many facets of a small protein

Profilin is a ubiquitously expressed protein well known as a key regulator of actin polymerisation. The actin cytoskeleton is involved in almost all cellular processes including motility, endocytosis, metabolism, signal transduction and gene transcription. Hence, profilin's role in the cell goes beyond its direct and essential function in regulating actin dynamics. This review will focus on the interactions of Profilin 1 and its ligands at the plasma membrane, in the cytoplasm and the nucleus of the cells and the regulation of profilin activity within those cell compartments. We will discuss the interactions of profilin in cell signalling pathways and highlight the importance of the cell context in the multiple functions that this small essential protein has in conjunction with its role in cytoskeletal organisation and dynamics. We will review some of the mechanisms that control profilin expression and the implications of changed expression of profilin in the light of cancer biology and other pathologies.

MicroRNA Regulatory Pathways in the Control of the Actin-Myosin Cytoskeleton

MicroRNAs (miRNAs) are key modulators of post-transcriptional gene regulation in a plethora of processes, including actin-myosin cytoskeleton dynamics. Recent evidence points to the widespread effects of miRNAs on actin-myosin cytoskeleton dynamics, either directly on the expression of actin and myosin genes or indirectly on the diverse signaling cascades modulating cytoskeletal arrangement. Furthermore, studies from various human models indicate that miRNAs contribute to the development of various human disorders. The potentially huge impact of miRNA-based mechanisms on cytoskeletal elements is just starting to be recognized. In this review, we summarize recent knowledge about the importance of microRNA modulation of the actin-myosin cytoskeleton affecting physiological processes, including cardiovascular function, hematopoiesis, podocyte physiology, and osteogenesis.

Genetic suppression of defective profilin by attenuated Myosin II reveals a potential role for Myosin II in actin dynamics in vivo in fission yeast

The actin cytoskeleton plays a variety of roles in eukaryotic cell physiology, ranging from cell polarity and migration to cytokinesis. Key to the function of the actin cytoskeleton is the mechanisms that control its assembly, stability, and turnover. Through genetic analyses in Schizosaccharomyces pombe, we found that myo2-S1 (myo2-G515D), a Myosin II mutant allele, was capable of rescuing lethality caused by partial defects in actin nucleation/stability caused, for example, through compromised function of the actin-binding protein Cdc3-profilin. The mutation in myo2-S1 affects the activation loop of Myosin II, which is involved in physical interaction with subdomain 1 of actin and in stimulating the ATPase activity of Myosin. Consistently, actomyosin rings in myo2-S1 cell ghosts were unstable and severely compromised in contraction on ATP addition. These studies strongly suggest a role for Myo2 in actin cytoskeletal disassembly and turnover in vivo, and that compromise of this activity leads to genetic suppression of mutants defective in actin filament assembly/stability at the division site.

Actin reorganization at the centrosomal area and the immune synapse regulates polarized secretory traffic of multivesicular bodies in T lymphocytes

T-cell receptor stimulation induces the convergence of multivesicular bodies towards the microtubule-organizing centre (MTOC) and the polarization of the MTOC to the immune synapse (IS). These events lead to exosome secretion at the IS. We describe here that upon IS formation centrosomal area F-actin decreased concomitantly with MTOC polarization to the IS. PKCδ-interfered T cell clones showed a sustained level of centrosomal area F-actin associated with defective MTOC polarization. We analysed the contribution of two actin cytoskeleton-regulatory proteins, FMNL1 and paxillin, to the regulation of cortical and centrosomal F-actin networks. FMNL1β phosphorylation and F-actin reorganization at the IS were inhibited in PKCδ-interfered clones. F-actin depletion at the central region of the IS, a requirement for MTOC polarization, was associated with FMNL1β phosphorylation at its C-terminal, autoregulatory region. Interfering all FMNL1 isoforms prevented MTOC polarization; nonetheless, FMNL1β re-expression restored MTOC polarization in a centrosomal area F-actin reorganization-independent manner. Moreover, PKCδ-interfered clones exhibited decreased paxillin phosphorylation at the MTOC, which suggests an alternative actin cytoskeleton regulatory pathway. Our results infer that PKCδ regulates MTOC polarization and secretory traffic leading to exosome secretion in a coordinated manner by means of two distinct pathways, one involving FMNL1β regulation and controlling F-actin reorganization at the IS, and the other, comprising paxillin phosphorylation potentially controlling centrosomal area F-actin reorganization.

Abbreviations

Ab, antibody; AICD, activation-induced cell death; AIP, average intensity projection; APC, antigen-presenting cell; BCR, B-cell receptor for antigen; ^C, centre of mass; cent2, centrin 2; cIS, central region of the immune synapse; CMAC, CellTracker™ Blue (7-amino-4-chloromethylcoumarin); cSMAC, central supramolecular activation cluster; CTL, cytotoxic T lymphocytes; DAG, diacylglycerol; DGKα, diacylglycerol kinase α; Dia1, Diaphanous-1; dSMAC, distal supramolecular activation cluster; ECL, enhanced chemiluminescence; ESCRT, endosomal sorting complex required for traffic; F-actin, filamentous actin; Fact-low cIS, F-actin-low region at the centre of the immune synapse; FasL, Fas ligand; FMNL1, formin-like 1; fps, frames per second; GFP, green fluorescent protein; HBSS, Hank’s balanced salt solution; HRP, horseradish peroxidase; ILV, intraluminal vesicles; IS, immune synapse; MFI, mean fluorescence intensity; MHC, major histocompatibility complex; MIP, maximal intensity projection; MVB, multivesicular bodies; MTOC, microtubule-organizing centre; NS, not significant; PBL, peripheral blood lymphocytes; PKC, protein kinase C; PKCδ, protein kinase C δ isoform; PLC, phospholipase C; PMA, phorbol myristate acetate; Pol. Index, polarization index; pSMAC, peripheral supramolecular activation cluster; PSF, point spread function; ROI, region of interest; SD, standard deviation; shRNA, short hairpin RNA; SEE, Staphylococcus enterotoxin E; SMAC, supramolecular activation cluster; TCR, T-cell receptor for antigen; T-helper (Th); TRANS, transmittance; WB, Western blot.

Arp2/3 and Mena/VASP Require Profilin 1 for Actin Network Assembly at the Leading Edge

Cells have many types of actin structures, which must assemble from a common monomer pool. Yet, it remains poorly understood how monomers are distributed to and shared between different filament networks. Simplified model systems suggest that monomers are limited and heterogeneous, which alters actin network assembly through biased polymerization and internetwork competition. However, less is known about how monomers influence complex actin structures, where different networks competing for monomers overlap and are functionally interdependent. One example is the leading edge of migrating cells, which contains filament networks generated by multiple assembly factors. The leading edge dynamically switches between the formation of different actin structures, such as lamellipodia or filopodia, by altering the balance of these assembly factors' activities. Here, we sought to determine how the monomer-binding protein profilin 1 (PFN1) controls the assembly and organization of actin in mammalian cells. Actin polymerization in PFN1 knockout cells was severely disrupted, particularly at the leading edge, where both Arp2/3 and Mena/VASP-based filament assembly was inhibited. Further studies showed that in the absence of PFN1, Arp2/3 no longer localizes to the leading edge and Mena/VASP is non-functional. Additionally, we discovered that discrete stages of internetwork competition and collaboration between Arp2/3 and Mena/VASP networks exist at different PFN1 concentrations. Low levels of PFN1 caused filopodia to form exclusively at the leading edge, while higher concentrations inhibited filopodia and favored lamellipodia and pre-filopodia bundles. These results demonstrate that dramatic changes to actin architecture can be made simply by modifying PFN1 availability.

Actin dynamics in cell migration

Cell migration is an essential process, both in unicellular organisms such as amoeba and as individual or collective motility in highly developed multicellular organisms like mammals. It is controlled by a variety of activities combining protrusive and contractile forces, normally generated by actin filaments. Here, we summarize actin filament assembly and turnover processes, and how respective biochemical activities translate into different protrusion types engaged in migration. These actin-based plasma membrane protrusions include actin-related protein 2/3 complex-dependent structures such as lamellipodia and membrane ruffles, filopodia as well as plasma membrane blebs. We also address observed antagonisms between these protrusion types, and propose a model - also inspired by previous literature - in which a complex balance between specific Rho GTPase signaling pathways dictates the protrusion mechanism employed by cells. Furthermore, we revisit published work regarding the fascinating antagonism between Rac and Rho GTPases, and how this intricate signaling network can define cell behavior and modes of migration. Finally, we discuss how the assembly of actin filament networks can feed back onto their regulators, as exemplified for the lamellipodial factor WAVE regulatory complex, tightly controlling accumulation of this complex at specific subcellular locations as well as its turnover.