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

Structural and functional mechanisms of actin isoforms

Actin is a highly conserved and fundamental protein in eukaryotes and participates in a broad spectrum of cellular functions. Cells maintain a conserved ratio of actin isoforms, with muscle and non-muscle actins representing the main actin isoforms in muscle and non-muscle cells, respectively. Actin isoforms have specific and redundant functional roles and display different biochemistries, cellular localization, and interactions with myosins and actin-binding proteins. Understanding the specific roles of actin isoforms from the structural and functional perspective is crucial for elucidating the intricacies of cytoskeletal dynamics and regulation and their implications in health and disease. Here, we review how the structure contributes to the functional mechanisms of actin isoforms with a special emphasis on the questions of how post-translational modifications and disease-linked mutations affect actin isoforms biochemistry, function, and interaction with actin-binding proteins and myosin motors.

IntAct: A nondisruptive internal tagging strategy to study the organization and function of actin isoforms

Mammals have 6 highly conserved actin isoforms with nonredundant biological functions. The molecular basis of isoform specificity, however, remains elusive due to a lack of tools. Here, we describe the development of IntAct, an internal tagging strategy to study actin isoforms in fixed and living cells. We identified a residue pair in β-actin that permits tag integration and used knock-in cell lines to demonstrate that IntAct β-actin expression and filament incorporation is indistinguishable from wild type. Furthermore, IntAct β-actin remains associated with common actin-binding proteins (ABPs) and can be targeted in living cells. We demonstrate the usability of IntAct for actin isoform investigations by showing that actin isoform-specific distribution is maintained in human cells. Lastly, we observed a variant-dependent incorporation of tagged actin variants into yeast actin patches, cables, and cytokinetic rings demonstrating cross species applicability. Together, our data indicate that IntAct is a versatile tool to study actin isoform localization, dynamics, and molecular interactions.

Cytosolic actin isoforms form networks with different rheological properties that indicate specific biological function

The implications of the existence of different actins expressed in epithelial cells for network mechanics and dynamics is investigated by microrheology and confocal imaging. γ-actin predominately found in the apical cortex forms stiffer networks compared to β-actin, which is preferentially organized in stress fibers. We attribute this to selective interactions with Mg2+-ions interconnecting the filaments' N-termini. Bundling propensity of the isoforms is different in the presence of Mg2+-ions, while crosslinkers such as α-actinin, fascin, and heavy meromyosin alter the mechanical response independent of the isoform. In the presence of myosin, β-actin networks show a large number of small contraction foci, while γ-actin displays larger but fewer foci indicative of a stronger interaction with myosin motors. We infer that subtle changes in the amino acid sequence of actin isoforms lead to alterations of the mechanical properties on the network level with potential implications for specific biological functions.

Colocalization Analysis of Cytoplasmic Actin Isoforms Distribution in Endothelial Cells

Actin cytoskeleton is an essential component of living cells and plays a decisive role in many cellular processes. In mammals, β- and γ-actin are cytoplasmic actin isoforms in non-muscle cells. Despite minor differences in the amino acid sequence, β- and γ-actin localize in different cell structures and perform different functions. While cytoplasmic β-actin is involved in many intracellular processes including cell contraction, γ-actin is responsible for cell mobility and promotes tumor transformation. Numerous studies demonstrate that β- and γ-actin are spatially separated in the cytoplasm of fibroblasts and epithelial cells; this separation is functionally determined. The spatial location of β/γ-actin in endothelial cells is still a subject for discussion. Using super-resolution microscopy, we investigated the β/γ-actin colocalization in endotheliocytes and showed that the β/γ-actin colocalization degree varies widely between different parts of the marginal regions and near the cell nucleus. In the basal cytoplasm, β-actin predominates, while the ratio of isoforms evens out as it moves to the apical cytoplasm. Thus, our colocalization analysis suggests that β- and γ-actin are segregated in the endotheliocyte cytoplasm. The segregation is greatly enhanced during cell lamella activation in the nocodazole-induced endothelial barrier dysfunction, reflecting a different functional role of cytoplasmic actin isoforms in endothelial cells.

High Actin Expression in Thrombus of Acute Ischemic Stroke Can Be a Biomarker of Atherothrombotic Origin Stroke

Background

As the treatment target, the imaging information and histologic characteristics of the thrombus may differ according to the stroke subtype. This study aimed to provide the correlative study of stroke etiology with the non-contrast CT, and histological composition of retrieved clots in acute ischemic stroke (AIS).

Materials and Methods

A total of 94 patients with AIS who underwent the endovascular treatment with successfully retrieved clots from January 2017 to October 2020 were enrolled in the present study. Histological analysis was performed using hematoxylin and eosin (H&E) staining and immunostaining with CD3, CD20, CD105, and actin antibodies. CT obtained at the patients' admission was to measure the attenuation and volume of all thrombus.

Results

A total of 94 subjects were included in this study. Fifty-six patients were classified as cardioembolic (CE), and 38 were classified with large-artery atherosclerosis (LAA). The subjects with LAA tend to exhibit higher actin and CD105 levels, and lower Hounsfield Unit (HU) values than subjects with CE. After adjusting for confounders, the actin was positively correlated with CD105 but not with HU values. Logistics regression shows actin was valuable for the prediction of LAA (OR, 1.148; 95% CI, 1.075–1.227; p < 0.001), even adjusted for age, sex, and intervention type (OR, 1.129; 95% CI, 1.048–1.216; p = 0.001), CT density and CD105 (OR, 1.161; 95% CI, 1.056–1.277; p = 0.002). Actin levels have a strong accuracy in differentiating LAA from CE, especially combined with CT density and CD105, which yielded a sensitivity of 63.2%, a specificity of 89.3%, with the area under the curve (AUC) at 0.821 (95% CI, 0.731–0.912).

Conclusion

Our findings suggest that actin's level was a major factor differentiating atherothrombotic origin strokes from the cardioembolic stroke.

Clinical Trial Registration

ChiCTR2100051173.

Effects of Low and High Aneurysmal Wall Shear Stress on Endothelial Cell Behavior: Differences and Similarities

Background: Intracranial aneurysms (IAs) result from abnormal enlargement of the arterial lumen. IAs are mostly quiescent and asymptomatic, but their rupture leads to severe brain damage or death. As the evolution of IAs is hard to predict and intricates medical decision, it is essential to improve our understanding of their pathophysiology. Wall shear stress (WSS) is proposed to influence IA growth and rupture. In this study, we investigated the effects of low and supra-high aneurysmal WSS on endothelial cells (ECs).

Methods: Porcine arterial ECs were exposed for 48 h to defined levels of shear stress (2, 30, or 80 dyne/cm²) using an Ibidi flow apparatus. Immunostaining for CD31 or γ-cytoplasmic actin was performed to outline cell borders or to determine cell architecture. Geometry measurements (cell orientation, area, circularity and aspect ratio) were performed on confocal microscopy images. mRNA was extracted for RNAseq analysis.

Results: ECs exposed to low or supra-high aneurysmal WSS were more circular and had a lower aspect ratio than cells exposed to physiological flow. Furthermore, they lost the alignment in the direction of flow observed under physiological conditions. The effects of low WSS on differential gene expression were stronger than those of supra-high WSS. Gene set enrichment analysis highlighted that extracellular matrix proteins, cytoskeletal proteins and more particularly the actin protein family were among the protein classes the most affected by shear stress. Interestingly, most genes showed an opposite regulation under both types of aneurysmal WSS. Immunostainings for γ-cytoplasmic actin suggested a different organization of this cytoskeletal protein between ECs exposed to physiological and both types of aneurysmal WSS.

Conclusion: Under both aneurysmal low and supra-high WSS the typical arterial EC morphology molds to a more spherical shape. Whereas low WSS down-regulates the expression of cytoskeletal-related proteins and up-regulates extracellular matrix proteins, supra-high WSS induces opposite changes in gene expression of these protein classes. The differential regulation in EC gene expression observed under various WSS translate into a different organization of the ECs’ architecture. This adaptation of ECs to different aneurysmal WSS conditions may affect vascular remodeling in IAs.

The Cytoplasmic Actins in the Regulation of Endothelial Cell Function

The primary function of the endothelial cells (EC) lining the inner surface of all vessels is to regulate permeability of vascular walls and to control exchange between circulating blood and tissue fluids of organs. The EC actin cytoskeleton plays a crucial role in maintaining endothelial barrier function. Actin cytoskeleton reorganization result in EC contraction and provides a structural basis for the increase in vascular permeability, which is typical for many diseases. Actin cytoskeleton in non-muscle cells presented two actin isoforms: non-muscle β-cytoplasmic and γ-cytoplasmic actins (β-actins and γ-actins), which are encoded by ACTB and ACTG1 genes, respectively. They are ubiquitously expressed in the different cells in vivo and in vitro and the β/γ-actin ratio depends on the cell type. Both cytoplasmic actins are essential for cell survival, but they perform various functions in the interphase and cell division and play different roles in neoplastic transformation. In this review, we briefly summarize the research results of recent years and consider the features of the cytoplasmic actins: The spatial organization in close connection with their functional activity in different cell types by focusing on endothelial cells.

Knockout of ACTB and ACTG1 with CRISPR/Cas9(D10A) Technique Shows that Non-Muscle β and γ Actin Are Not Equal in Relation to Human Melanoma Cells' Motility and Focal Adhesion Formation

Non-muscle actins have been studied for many decades; however, the reason for the existence of both isoforms is still unclear. Here we show, for the first time, a successful inactivation of the ACTB (CRISPR clones with inactivated ACTB, CR-ACTB) and ACTG1 (CRISPR clones with inactivated ACTG1, CR-ACTG1) genes in human melanoma cells (A375) via the RNA-guided D10A mutated Cas9 nuclease gene editing [CRISPR/Cas9(D10A)] technique. This approach allowed us to evaluate how melanoma cell motility was impacted by the lack of either β actin coded by ACTB or γ actin coded by ACTG1. First, we observed different distributions of β and γ actin in the cells, and the absence of one actin isoform was compensated for via increased expression of the other isoform. Moreover, we noted that γ actin knockout had more severe consequences on cell migration and invasion than β actin knockout. Next, we observed that the formation rate of bundled stress fibers in CR-ACTG1 cells was increased, but lamellipodial activity in these cells was impaired, compared to controls. Finally, we discovered that the formation rate of focal adhesions (FAs) and, subsequently, FA-dependent signaling were altered in both the CR-ACTB and CR-ACTG1 clones; however, a more detrimental effect was observed for γ actin-deficient cells. Our research shows that both non-muscle actins play distinctive roles in melanoma cells’ FA formation and motility.

Mechanical Roles of F-Actin in the Differentiation of Stem Cells: A Review

In the development and differentiation of stem cells, mechanical forces associated with filamentous actin (F-actin) play a crucial role. The present review aims to reveal the relationship among the chemical components, microscopic structures, mechanical properties, and biological functions of F-actin. Particular attention is given to the functions of the cytoplasmic and nuclear microfilament cytoskeleton and their regulation mechanisms in the differentiation of stem cells. The distributions of different types of actin monomers in mammal cells and the functions of actin-binding proteins are summarized. We discuss how the fate of stem cells is regulated by intra/extracellular mechanical and chemical cues associated with microfilament-related proteins, intercellular adhesion molecules, etc. In addition, we also address the differentiation-induced variation in the stiffness of stem cells and the correlation between the fate and geometric shape change of stem cells. This review not only deepens our understanding of the biophysical mechanisms underlying the fates of stem cells under different culture conditions but also provides inspirations for the tissue engineering of stem cells.

Tuning the properties of a cyclic RGD-containing tetrapeptide through backbone fluorination

Stereoselective fluorination is investigated as a method for modulating the properties of a cyclic RGD-containing tetrapeptide. Three key outcomes of fluorination are assessed: (i) the effect on peptide cyclisation efficiency; (ii) the ability to fine-tune the molecular conformation; and (iii) the effect on the cyclic peptides' biological activity. Fluorination is found to exert pronounced effects against all three criteria.

Reconsidering an active role for G-actin in cytoskeletal regulation

Globular (G)-actin, the actin monomer, assembles into polarized filaments that form networks that can provide structural support, generate force and organize the cell. Many of these structures are highly dynamic and to maintain them, the cell relies on a large reserve of monomers. Classically, the G-actin pool has been thought of as homogenous. However, recent work has shown that actin monomers can exist in distinct groups that can be targeted to specific networks, where they drive and modify filament assembly in ways that can have profound effects on cellular behavior. This Review focuses on the potential factors that could create functionally distinct pools of actin monomers in the cell, including differences between the actin isoforms and the regulation of G-actin by monomer binding proteins, such as profilin and thymosin β4. Owing to difficulties in studying and visualizing G-actin, our knowledge over the precise role that specific actin monomer pools play in regulating cellular actin dynamics remains incomplete. Here, we discuss some of these unanswered questions and also provide a summary of the methodologies currently available for the imaging of G-actin.

Cysteine-rich buccal gland protein suppressed the proliferation, migration and invasion of hela cells through akt pathway

Cysteine-rich buccal gland protein (CRBGP) as a member of cysteine-rich secretory proteins (CRISPs) superfamily was isolated from the buccal glands of Lampetra japonica, the blood suckers in the marine. Previous studies showed CRBGP could suppress angiogenesis probably due to its ion channel blocking activity. Whether CRBGP could also affect the activity of tumor cells has not been reported yet. In this study, CRBGP suppressed the proliferation of Hela cells with an IC₅₀ of 6.7 μM by inducing apoptosis. Both microscopic observation and Western blot indicated that CRBGP was able to induce the nuclei shrinking, downregulate the protein level of BCL2 and caspase 3 as well as upregulate the level of BAX in Hela cells, suggested that CRBGP might induce apoptosis of Hela cells in a mitochondrial-dependent pathway. Furthermore, CRBGP could disturb F-actin organization, which would finally cause the Hela cells to lose their shape and to lessen their abilities on adhesion, migration and invasion. Finally, CRBGP was shown to reduce the phosphorylation level of Akt, which indicated that CRBGP might inhibit the proliferation and metastasis of Hela cells through Akt pathway. CRBGP, as a voltage-gated sodium channel blocker, also possesses the anti-tumor abilities which provided information on the effects and action manner of the other CRISPs. © 2017 IUBMB Life, 69(11):856-866, 2017.

Involvement of β- and γ-actin isoforms in actin cytoskeleton organization and migration abilities of bleb-forming human colon cancer cells

Amoeboid movement is characteristic for rounded cells, which do not form strong adhesion contacts with the ECM and use blebs as migratory protrusions. It is well known that actin is the main component of mature forms of these structures, but the exact role fulfilled by non-muscle actin isoforms β- and γ- in bleb formation and migration of these cells is still not fully understood. The aim of this study was to establish the role of β- and γ-actin in migration of bleb-forming cancer cells using isoform-specific antibodies and expression of fluorescently tagged actin isoforms. We observed, after staining with monoclonal antibodies, that both actins are present in these cells in the form of a cortical ring as well as in the area of blebs. Additionally, using simultaneous expression of differentially tagged β- and γ-actin in cells, we observed that the actin isoforms are present together in a single bleb. They were involved during bleb expansion as well as retraction. Also present in the area of these protrusions formed by both isoforms were the bleb markers–ezrin and myosin II. The overexpression of β- or γ-actin led to actin cytoskeletal rearrangement followed by the growth of migration and invasion abilities of examined human colon cancer cells, LS174T line. In summary these data prove that both actin isoforms have an impact on motility of bleb-forming cancer cells. Moreover, we conclude that monoclonal antibodies directed against actin isoforms in combination with the tagged actins are good tools to study their role in important biological processes.

Divergent roles of β- and γ-actin isoforms during spread of vaccinia virus

Actin is a major component of the cytoskeleton and is present as two isoforms in non‐muscle cells: β‐ and γ‐cytoplasmic actin. These isoforms are strikingly conserved, differing by only four N‐terminal amino acids. During spread from infected cells, vaccinia virus (VACV) particles induce localized actin nucleation that propel virus to surrounding cells and facilitate cell‐to‐cell spread of infection. Here we show that virus‐tipped actin comets are composed of β‐ and γ‐actin. We employed isoform‐specific siRNA knockdown to examine the role of the two isoforms in VACV‐induced actin comets. Despite the high level of similarity between the actin isoforms, and their colocalization, VACV‐induced actin nucleation was dependent exclusively on β‐actin. Knockdown of β‐actin led to a reduction in the release of virus from infected cells, a phenotype dependent on virus‐induced Arp2/3 complex activity. We suggest that local concentrations of actin isoforms may regulate the activity of cellular actin nucleator complexes.

Calreticulin attenuated microwave radiation-induced human microvascular endothelial cell injury through promoting actin acetylation and polymerization

Recent work reveals that actin acetylation modification has been linked to different normal and disease processes and the effects associated with metabolic and environmental stressors. Herein, we highlight the effects of calreticulin on actin acetylation and cell injury induced by microwave radiation in human microvascular endothelial cell (HMEC). HMEC injury was induced by high-power microwave of different power density (10, 30, 60, 100 mW/cm2, for 6 min) with or without exogenous recombinant calreticulin. The cell injury was assessed by lactate dehydrogenase (LDH) activity and Cell Counting Kit-8 in culture medium, migration ability, intercellular junction, and cytoskeleton staining in HMEC. Western blotting analysis was used to detected calreticulin expression in cytosol and nucleus and acetylation of globular actin (G-actin). We found that HMEC injury was induced by microwave radiation in a dose-dependent manner. Pretreatment HMEC with calreticulin suppressed microwave radiation-induced LDH leakage and increased cell viability and improved microwave radiation-induced decrease in migration, intercellular junction, and cytoskeleton. Meanwhile, pretreatment HMEC with exogenous calreticulin upregulated the histone acetyltransferase activity and the acetylation level of G-actin and increased the fibrous actin (F-actin)/G-actin ratio. We conclude that exogenous calreticulin protects HMEC against microwave radiation-induced injury through promoting actin acetylation and polymerization.

Gamma-actin is involved in regulating centrosome function and mitotic progression in cancer cells

Reorganization of the actin cytoskeleton during mitosis is crucial for regulating cell division. A functional role for γ-actin in mitotic arrest induced by the microtubule-targeted agent, paclitaxel, has recently been demonstrated. We hypothesized that γ-actin plays a role in mitosis. Herein, we investigated the effect of γ-actin in mitosis and demonstrated that γ-actin is important in the distribution of β-actin and formation of actin-rich retraction fibers during mitosis. The reduced ability of paclitaxel to induce mitotic arrest as a result of γ-actin depletion was replicated with a range of mitotic inhibitors, suggesting that γ-actin loss reduces the ability of broad classes of anti-mitotic agents to induce mitotic arrest. In addition, partial depletion of γ-actin enhanced centrosome amplification in cancer cells and caused a significant delay in prometaphase/metaphase. This prolonged prometaphase/metaphase arrest was due to mitotic defects such as uncongressed and missegregated chromosomes, and correlated with an increased presence of mitotic spindle abnormalities in the γ-actin depleted cells. Collectively, these results demonstrate a previously unknown role for γ-actin in regulating centrosome function, chromosome alignment and maintenance of mitotic spindle integrity.

ERK Signaling Pathway Is Involved in HPV-16 E6 but not E7 Oncoprotein-Induced HIF-1α Protein Accumulation in NSCLC Cells

Extracellular signal-regulated kinase (ERK)1/2 signaling pathway plays a critical role in regulating tumor angiogenesis. Our previous studies have demonstrated that HPV-16 oncoproteins enhanced hypoxia-inducible factor-1α (HIF-1α) protein accumulation and vascular endothelial growth factor (VEGF) and interleukin-8 (IL-8) expression in non-small cell lung cancer (NSCLC) cells, thus contributing to angiogenesis. In this study, we further investigated the role of ERK1/2 signaling pathway in HPV-16 oncoprotein-induced HIF-1α, VEGF, and IL-8 expression and in vitro angiogenesis in NSCLC cells. Our results showed that HPV-16 E6 and HPV-16 E7 oncoproteins promoted the activation of ERK1/2 signaling pathway in A549 and NCI-H460 cells. Moreover, PD98059, a specific inhibitor of ERK1/2, blocked in vitro angiogenesis stimulated by HPV-16 E6 but not E7 oncoprotein. Additionally, HIF-1α protein accumulation and VEGF and IL-8 expression in NSCLC cells induced by HPV-16 E6 but not E7 oncoprotein were significantly inhibited by PD98059. Taken together, our results suggest that ERK1/2 signaling pathway is involved in HPV-16 E6 but not E7 oncoprotein-induced HIF-1α, VEGF, and IL-8 expression in NSCLC cells, leading to the enhanced angiogenesis in vitro.

Mammalian Actins: Isoform-Specific Functions and Diseases

Actin is the central building block of the actin cytoskeleton, a highly regulated filamentous network enabling dynamic processes of cells and simultaneously providing structure. Mammals have six actin isoforms that are very conserved and thus share common functions. Tissue-specific expression in part underlies their differential roles, but actin isoforms also coexist in various cell types and tissues, suggesting specific functions and preferential interaction partners. Gene deletion models, antibody-based staining patterns, gene silencing effects, and the occurrence of isoform-specific mutations in certain diseases have provided clues for specificity on the subcellular level and its consequences on the organism level. Yet, the differential actin isoform functions are still far from understood in detail. Biochemical studies on the different isoforms in pure form are just emerging, and investigations in cells have to deal with a complex and regulated system, including compensatory actin isoform expression.

Anti-angiogenic activities of CRBGP from buccal glands of lampreys (Lampetra japonica)

Cysteine-rich secretory proteins (CRISPs), characterized by 16 conserved cysteines, are distributed in a wide range of organisms, such as secernenteas, amphibians, reptiles and mammals. In the previous studies, a novel CRISP family member (cysteine-rich buccal gland protein, CRBGP) was separated from the buccal gland of lampreys (Lampetra japonica, L. japonica). Lamprey CRBGP could not only suppress depolarization-induced contraction of rat tail arterial smooth muscle, but also block voltage-gated sodium channels (VGSCs). In the present study, the anti-angiogenic activities of lamprey CRBGP were investigated using endothelial cells and chick chorioallantoic membrane (CAM) models. In vitro assays, lamprey CRBGP is able to induce human umbilical vein endothelial cells (HUVECs) apoptosis by disturbing the calcium homeostasis and mitochondria functions. In addition, lamprey CRBGP could inhibit proliferation, adhesion, migration, invasion and tube formation of HUVECs by affecting the organization of F-actin and expression level of matrix metallo-proteinase 2 (MMP-2), matrix metallo-proteinase 9 (MMP-9) and vascular endothelial growth factor A (VEGFA) which are related to angiogenesis. In vivo assays, lamprey CRBGP could suppress the blood vessel formation in CAM models. Therefore, lamprey CRBGP is an important protein present in the buccal gland of lampreys and might help lampreys suppress the contraction of blood vessels, nociceptive responses and wound healing of host fishes during their feeding time. In addition, lamprey CRBGP might have the potential to act as an effective anti-angiogenic factor for the treatment of abnormal angiogenesis induced diseases.