Beta and gamma-cytoplasmic actins display distinct distribution and functional diversity

Urinary Reference Values and First Insight into the Urinary Proteome of Captive Giraffes

Urinalysis is widely recognized to be a useful tool in routine health investigations, since it can diagnose numerous pathologies. Considering the paucity of knowledge concerning giraffes, urine from 44 giraffes (Giraffa camelopardalis) (18 males and 26 females, from 3 months of age to 21 years of age) underwent routine urinalysis, 1D-electrophoresis, and protein identification using mass spectrometry, with the aim of identifying the urinary reference values and the urine proteome. The urine specific gravity (USG), urine total proteins (uTP), urine creatinine (uCr), and urine protein:creatinine ratio (UPC) reference values, reported as the median, and lower limit (LL) and upper limit (UL), were 1.030 (1006-1.049), 17.58 (4.54-35.31) mg/dL, 154.62 (39.59-357.95) mg/dL, and 0.11 (0.07-0.16), respectively. Mass spectrometry, together with electrophoresis, revealed a pattern of common urinary proteins; albumin, lysozyme C, and ubiquitin were the most represented proteins in the giraffe urine. It has been hypothesized that these proteins could act as a defense against microbes. Moreover, in giraffes, urinalysis could be a valid tool for gauging renal function and physiological status changes.

Diversity from similarity: cellular strategies for assigning particular identities to actin filaments and networks

The actin cytoskeleton has the particularity of being assembled into many functionally distinct filamentous networks from a common reservoir of monomeric actin. Each of these networks has its own geometrical, dynamical and mechanical properties, because they are capable of recruiting specific families of actin-binding proteins (ABPs), while excluding the others. This review discusses our current understanding of the underlying molecular mechanisms that cells have developed over the course of evolution to segregate ABPs to appropriate actin networks. Segregation of ABPs requires the ability to distinguish actin networks as different substrates for ABPs, which is regulated in three different ways: (1) by the geometrical organization of actin filaments within networks, which promotes or inhibits the accumulation of ABPs; (2) by the identity of the networks' filaments, which results from the decoration of actin filaments with additional proteins such as tropomyosin, from the use of different actin isoforms or from covalent modifications of actin; (3) by the existence of collaborative or competitive binding to actin filaments between two or multiple ABPs. This review highlights that all these effects need to be taken into account to understand the proper localization of ABPs in cells, and discusses what remains to be understood in this field of research.

When Actin is Not Actin' Like It Should: A New Category of Distinct Primary Immunodeficiency Disorders

An increasing number of primary immunodeficiencies (PIDs) have been identified over the last decade, which are caused by deleterious mutations in genes encoding for proteins involved in actin cytoskeleton regulation. These mutations primarily affect hematopoietic cells and lead to defective function of immune cells, such as impaired motility, signaling, proliferative capacity, and defective antimicrobial host defense. Here, we review several of these immunological "actinopathies" and cover both clinical aspects, as well as cellular mechanisms of these PIDs. We focus in particular on the effect of these mutations on human neutrophil function.

Involvement of SASH1 in the Maintenance of Stable Cell-Cell Adhesion

SASH1 is an adapter and signaling protein that contains SH3 and SAM domains responsible for protein-protein interactions. SASH1 downregulation has been observed in many tumors. We examined localization of SASH1 in cultures of normal IAR-20 epithelial cells and HT-29 colorectal cancer cells using immunofluorescence staining and confocal microscopy. IAR-20 normal epithelial cells and HT-29 cells with epithelial phenotype formed stable linear adherens junctions (AJs) associated with circumferential actin bundles. In both IAR-20 and HT-29 cells, SASH1 was co-localized with zones of circumferential actin bundles and linear AJs. SASH1 was not detected in lamellipodia. IAR-20 and HT-29 cells treated with Epidermal Growth Factor underwent epithelial-mesenchymal transition (EMT). We observed significant differences in the course of EMT between IAR-20 and HT-29 cultures. IAR-20 cells underwent partial EMT acquiring migratory phenotype but retaining E-cadherin in unstable radial AJs. SASH1 was present in these contacts. Disappearance of AJs was observed in HT-29 cell undergoing a complete EMT, which also resulted in disruption of stable cell-cell adhesion. SASH1 was lost from the zones of cell-cell interaction. SASH1 depletion by means of RNA interference in IAR-20 cells led to destruction of stable linear AJs and acquisition of mesenchymal phenotype by some of the cells. These data indicate involvement of SASH1 in the maintenance of stable cell-cell adhesion.

Prenatal presentation and diagnosis of Baraitser-Winter syndrome using exome sequencing

Baraitser-Winter cerebrofrontofacial syndrome (BWCFF) is a rare autosomal dominant developmental disorder associated with missense mutations in the genes ACTB or ACTG1. The classic presentation of BWCFF is discerned by the combination of unique craniofacial characteristics including ocular coloboma, intellectual disability, and hypertelorism. Congenital contractures and organ malformations are often present, including structural defects in the brain, heart, renal, and musculoskeletal system. However, there is limited documentation regarding its prenatal presentation that may encourage healthcare providers to be aware of this disorder when presented throughout pregnancy. Herein we describe a case of a pregnancy with large cystic hygroma and omphalocele. Whole exome sequencing (WES) was performed and a de novo, heterozygous, likely pathogenic mutation in ACTB was detected, c.1004G>A (p.Arg335His), conferring a diagnosis of BWCFF.

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.

Regulation of actin isoforms in cellular and developmental processes

Actin is one of the most abundant and essential intracellular proteins that mediates nearly every form of cellular movement and underlies such key processes as embryogenesis, tissue integrity, cell division and contractility of all types of muscle and non-muscle cells. In mammals, actin is represented by six isoforms, which are encoded by different genes but produce proteins that are 95-99 % identical to each other. The six actin genes have vastly different functions in vivo, and the small amino acid differences between the proteins they encode are rigorously maintained through evolution, but the underlying differences behind this distinction, as well as the importance of specific amino acid sequences for each actin isoform, are not well understood. This review summarizes different levels of actin isoform-specific regulation in cellular and developmental processes, starting with the nuclear actin's role in transcription, and covering the gene-level, mRNA-level, and protein-level regulation, with a special focus on mammalian actins in non-muscle cells.

Quantification of intracellular N-terminal β-actin arginylation

Actin is a ubiquitous, essential, and highly abundant protein in all eukaryotic cells that performs key roles in contractility, adhesion, migration, and leading edge dynamics. The two non-muscle actins, β- and γ-, are ubiquitously present in every cell type and are nearly identical to each other at the amino acid level, but play distinct intracellular roles. The mechanisms regulating this distinction have been the focus of recent interest in the field. Work from our lab has previously shown that β-, but not γ-, actin undergoes N-terminal arginylation on Asp3. While functional evidence suggest that this arginylation may be important to actin's function, progress in these studies so far has been hindered by difficulties in arginylated actin detection, precluding estimations of the abundance of arginylated actin in cells, and its occurrence in different tissues and cell types. The present study represents the first antibody-based quantification of the percentage of arginylated actin in migratory non-muscle cells under different physiological conditions, as well as in different cells and tissues. We find that while the steady-state level of arginylated actin is relatively low, it is consistently present in vivo, and is somewhat more prominent in migratory cells. Inhibition of N-terminal actin acetylation dramatically increases the intracellular actin arginylation level, suggesting that these two modifications may directly compete in vivo. These findings constitute an essential step in our understanding of actin regulation by arginylation, and in uncovering the dynamic interplay of actin's N-terminal modifications in vivo.

Antigen Sampling CSF1R-Expressing Epithelial Cells Are the Functional Equivalents of Mammalian M Cells in the Avian Follicle-Associated Epithelium

The follicle-associated epithelium (FAE) is a specialized structure that samples luminal antigens and transports them into mucosa-associated lymphoid tissues (MALT). In mammals, transcytosis of antigens across the gut epithelium is performed by a subset of FAE cells known as M cells. Here we show that colony-stimulating factor 1 receptor (CSF1R) is expressed by a subset of cells in the avian bursa of Fabricius FAE. Expression was initially detected using a CSF1R-reporter transgene that also label subsets of bursal macrophages. Immunohistochemical detection using a specific monoclonal antibody confirmed abundant expression of CSF1R on the basolateral membrane of FAE cells. CSF1R-transgene expressing bursal FAE cells were enriched for expression of markers previously reported as putative M cell markers, including annexin A10 and CD44. They were further distinguished from a population of CSF1R-transgene negative epithelial cells within FAE by high apical F-actin expression and differential staining with the lectins jacalin, PHA-L and SNA. Bursal FAE cells that express the CSF1R-reporter transgene were responsible for the bulk of FAE transcytosis of labeled microparticles in the size range 0.02-0.1 μm. Unlike mammalian M cells, they did not readily take up larger bacterial sized microparticles (0.5 μm). Their role in uptake of bacteria was tested using Salmonella, which can enter via M cells in mammals. Labeled Salmonella enterica serovar Typhimurium entered bursal tissue via the FAE. Entry was partially dependent upon Type III secretion system-1. However, the majority of invading bacteria were localized to CSF1R-negative FAE cells and in resident phagocytes that express the phosphatidylserine receptor TIM4. CSF1R-expressing FAE cells in infected follicles showed evidence of cell death and shedding into the bursal lumen. In mammals, CSF1R expression in the gut is restricted to macrophages which only indirectly control M cell differentiation. The novel expression of CSF1R in birds suggests that these functional equivalents to mammalian M cells may have different ontological origins and their development and function are likely to be regulated by different growth factors.

Actin-microtubule interplay coordinates spindle assembly in human oocytes

Mammalian oocytes assemble a bipolar acentriolar microtubule spindle to segregate chromosomes during asymmetric division. There is increasing evidence that actin in the spindle interior not only participates in spindle migration and positioning but also protects oocytes from chromosome segregation errors leading to aneuploidy. Here we show that actin is an integral component of the meiotic machinery that closely interacts with microtubules during all major events of human oocyte maturation from the time point of spindle assembly till polar body extrusion and metaphase arrest. With the aid of drugs selectively affecting cytoskeleton dynamics and transiently disturbing the integrity of the two cytoskeleton systems, we identify interdependent structural rearrangements indicative of a close communication between actin and microtubules as fundamental feature of human oocytes. Our data support a model of actin-microtubule interplay that is essential for bipolar spindle assembly and correct partitioning of the nuclear genome in human oocyte meiosis.

Structural Features of Actin Cytoskeleton Required for Endotheliocyte Barrier Function

Cytoplasmic actin structures are essential components of the eukaryotic cytoskeleton. According to the classic concepts, actin structures perform contractile and motor functions, ensuring the possibility of cell shape changes during cell spreading, polarization, and movement both in vitro and in vivo, from the early embryogenesis stages and throughout the life of a multicellular organism. Intracellular organization of actin structures, their biochemical composition, and dynamic properties play a key role in the realization of specific cellular and tissue functions and vary in different cell types. This paper is a review of recent studies on the organization and properties of actin structures in endotheliocytes, interaction of these structures with other cytoskeletal components and elements involved in cell adhesion, as well as their role in the functional activity of endothelial cells.

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.

Mechanical regulation of myofibroblast phenoconversion and collagen contraction

Activated fibroblasts promote physiological wound repair following tissue injury. However, dysregulation of fibroblast activation contributes to the development of fibrosis by enhanced production and contraction of collagen-rich extracellular matrix. At the peak of their activities, fibroblasts undergo phenotypic conversion into highly contractile myofibroblasts by developing muscle-like features, including formation of contractile actin-myosin bundles. The phenotype and function of fibroblasts and myofibroblasts are mechanically regulated by matrix stiffness using a feedback control system that is integrated with the progress of tissue remodelling. The actomyosin contraction machinery and cell-matrix adhesion receptors are critical elements that are needed for mechanosensing by fibroblasts and the translation of mechanical signals into biological responses. Here, we focus on mechanical and chemical regulation of collagen contraction by fibroblasts and the involvement of these factors in their phenotypic conversion to myofibroblasts.

Divergent impact of actin isoforms on cell cycle regulation

We have shown that cytoplasmic actin isoforms play different roles in neoplastic cell transformation. β-Cytoplasmic actin acts as a tumor suppressor, affecting epithelial differentiation, cell growth, cell invasion and tumor growth of colon and lung carcinoma cells. In contrast, γ-cytoplasmic actin enhances malignant features of tumor cells whose actin network regulation is carried out via the γ-actin isoform. The goal of this study was to describe the role of cytoplasmic actins in cell cycle regulation of breast cancer cell lines MCF-7 and MDA-MB-231. The distinct roles of each cytoplasmic actin in the cell cycle driving were observed. β-Actin as well as γ-actin down-regulation inhibited proliferation of breast cancer cells, but only down-regulation of β-actin induced a significant decrease in diploid cell population and accumulation of tetraploid cells. Down-regulation of β-actin stimulated cyclin A2, B1 and D3 expression, whereas down-regulation of γ-actin reduced expression of these cyclins in both cell lines. Moreover, cyclin B1 and γ-actin were co-localized in mitotic control and β-actin-deficient cells. In mitotic MCF-7 cells down-regulation of β-actin caused an enrichment of prophase/metaphase population compared with control. γ-Actin down-regulation induced telophase enrichment.

ERK1/2 and γ-actin co-localization and possible selective binding were revealed in MCF7 cells. β-Actin down-regulation induced ERK1/2 activation, while γ-actin down-regulation led to reduction of p-ERK1/2. A direct interaction of ERK1/2 with γ-actin and cyclin A2 in the same protein complex was also discovered. We suggest that γ-actin down-regulation leads to decrease of cyclin A2 level, inhibits ERK1/2 signaling and deceleration of breast cancer cells proliferation.

Posttranscriptional and Posttranslational Regulation of Actin

Actin is one of the most abundant intracellular proteins, essential in every eukaryotic cell type. Actin plays key roles in tissue morphogenesis, cell adhesion, muscle contraction, and developmental reprogramming. Most actin studies have focused on its regulation at the protein level, either directly or through differential interactions with over a hundred intracellular binding partners. However, numerous studies emerging in recent years demonstrate specific types of nucleotide-level regulation that strongly affect non-muscle actins during cell migration and adhesion and are potentially applicable to other members of the actin family. This regulation involves zipcode-mediated actin mRNA targeting to the cell periphery, proposed to mediate local synthesis of actin at the cell leading edge, as well as the recently discovered N-terminal arginylation that specifically targets non-muscle β-actin via a nucleotide-dependent mechanism. Moreover, a study published this year suggests that actin's essential roles at the organismal level may be entirely nucleotide-dependent. This review summarizes the emerging data on actin's nucleotide-level regulation. Anat Rec, 301:1991-1998, 2018. © 2018 Wiley Periodicals, Inc.

Variants in exons 5 and 6 of ACTB cause syndromic thrombocytopenia

Germline mutations in the ubiquitously expressed ACTB, which encodes β-cytoplasmic actin (CYA), are almost exclusively associated with Baraitser-Winter Cerebrofrontofacial syndrome (BWCFF). Here, we report six patients with previously undescribed heterozygous variants clustered in the 3′-coding region of ACTB. Patients present with clinical features distinct from BWCFF, including mild developmental disability, microcephaly, and thrombocytopenia with platelet anisotropy. Using patient-derived fibroblasts, we demonstrate cohort specific changes to β-CYA filament populations, which include the enhanced recruitment of thrombocytopenia-associated actin binding proteins (ABPs). These perturbed interactions are supported by in silico modeling and are validated in disease-relevant thrombocytes. Co-examination of actin and microtubule cytoskeleton constituents in patient-derived megakaryocytes and thrombocytes indicates that these β-CYA mutations inhibit the final stages of platelet maturation by compromising microtubule organization. Our results define an ACTB-associated clinical syndrome with a distinct genotype-phenotype correlation and delineate molecular mechanisms underlying thrombocytopenia in this patient cohort.

Genetic variants in ACTB and ACTG1 have been associated with Baraitser-Winter Cerebrofrontofacial syndrome. Here, the authors report of a syndromic thrombocytopenia caused by variants in ACTB exons 5 or 6 that compromise the organization and coupling of the cytoskeleton, leading to impaired platelet maturation.

Vascular Smooth Muscle Contractile Function Declines With Age in Skeletal Muscle Feed Arteries

Aging induces a progressive decline in vasoconstrictor responses in central and peripheral arteries. This study investigated the hypothesis that vascular smooth muscle (VSM) contractile function declines with age in soleus muscle feed arteries (SFA). Contractile function of cannulated SFA isolated from young (4 months) and old (24 months) Fischer 344 rats was assessed by measuring constrictor responses of denuded (endothelium removed) SFA to norepinephrine (NE), phenylephrine (PE), and angiotensin II (Ang II). In addition, we investigated the role of RhoA signaling in modulation of VSM contractile function. Structural and functional characteristics of VSM cells were evaluated by fluorescence imaging and atomic force microscopy (AFM). Results indicated that constrictor responses to PE and Ang II were significantly impaired in old SFA, whereas constrictor responses to NE were preserved. In the presence of a Rho-kinase inhibitor (Y27632), constrictor responses to NE, Ang II, and PE were significantly reduced in young and old SFA. In addition, the age-group difference in constrictor responses to Ang II was eliminated. ROCK1 and ROCK2 content was similar in young and old VSM cells, whereas pROCK1 and pROCK2 were significantly elevated in old VSM cells. Aging was associated with a reduction in smooth muscle α-actin stress fibers and recruitment of proteins to cell-matrix adhesions. Old VSM cells presented an increase in integrin adhesion to the matrix and smooth muscle γ-actin fibers that was associated with increased cell stiffness. In conclusion, our results indicate that VSM contractile function declined with age in SFA. The decrement in contractile function was mediated in part by RhoA/ROCK signaling. Upregulation of pROCK in old VSM cells was not able to rescue contractility in old SFA. Collectively, these results indicate that changes at the VSM cell level play a central role in the reduced contractile function of aged SFA.

Mitochondria-targeted antioxidant SkQ1 suppresses fibrosarcoma and rhabdomyosarcoma tumour cell growth

Mitochondria are important regulators of tumour growth and progression due to their specific role in cancer metabolism and modulation of apoptotic pathways. In this paper we describe that mitochondria-targeted antioxidant SkQ1 designed as a conjugate of decyl-triphenylphosphonium cation (TPP⁺) with plastoquinone, suppressed the growth of fibrosarcoma HT1080 and rhabdomyosarcoma RD tumour cells in culture and tumour growth of RD in xenograft nude mouse model. Under the same conditions, no detrimental effect of SkQ1 on cell growth of primary human subcutaneous fibroblasts was observed. The tumour growth suppression was shown to be a result of the antioxidant action of low nanomolar concentrations of SkQ1. We have revealed significant prolongation of mitosis induced by SkQ1 in both tumour cell cultures. Prolonged mitosis and apoptosis could be responsible for growth suppression after SkQ1 treatment in RD cells. Growth suppression in HT1080 cells was accompanied by the delay of telophase and cytokinesis, followed by multinuclear cells formation. The effects of SkQ1 on the cell cycle were proved to be at least partially mediated by inactivation of Aurora family kinases.

ABBREVIATIONS

TPP⁺: Triphenylphosphonium cation; ROS: Reactive oxygen species; mtROS: Mitochondrial reactive oxygen species; NAC: N-acetyl-L-cysteine; DCFH-DA: Dichlorodihydrofluorescein diacetate; APC: Anaphase promoting complex; ABPs: Actin-binding proteins; DMEM: Dulbecco's modified Eagle media; SDS: sodium dodecyl sulfate; HEPES: 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid.

β-Actin shows limited mobility and is required only for supraphysiological insulin-stimulated glucose transport in young adult soleus muscle

Studies in skeletal muscle cell cultures suggest that the cortical actin cytoskeleton is a major requirement for insulin-stimulated glucose transport, implicating the β-actin isoform, which in many cell types is the main actin isoform. However, it is not clear that β-actin plays such a role in mature skeletal muscle. Neither dependency of glucose transport on β-actin nor actin reorganization upon glucose transport have been tested in mature muscle. To investigate the role of β-actin in fully differentiated muscle, we performed a detailed characterization of wild type and muscle-specific β-actin knockout (KO) mice. The effects of the β-actin KO were subtle; however, we confirmed the previously reported decline in running performance of β-actin KO mice compared with wild type during repeated maximal running tests. We also found insulin-stimulated glucose transport into incubated muscles reduced in soleus but not in extensor digitorum longus muscle of young adult mice. Contraction-stimulated glucose transport trended toward the same pattern, but the glucose transport phenotype disappeared in soleus muscles from mature adult mice. No genotype-related differences were found in body composition or glucose tolerance or by indirect calorimetry measurements. To evaluate β-actin mobility in mature muscle, we electroporated green fluorescent protein (GFP)-β-actin into flexor digitorum brevis muscle fibers and measured fluorescence recovery after photobleaching. GFP-β-actin showed limited unstimulated mobility and no changes after insulin stimulation. In conclusion, β-actin is not required for glucose transport regulation in mature mouse muscle under the majority of the tested conditions. Thus, our work reveals fundamental differences in the role of the cortical β-actin cytoskeleton in mature muscle compared with cell culture.

Actin-Based Cell Protrusion in a 3D Matrix

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

Cell migration has been well studied in 2D, but how this relates to movement in physiological 3D tissues and matrix is not clear, particularly in vertebrate interstitial matrix.

In 3D matrix cells actin polymerisation directly contributes to the formation of lamellipodia to facilitate migration and invasion (mesenchymal movement), analogous to 2D migration; actomyosin contractility promotes bleb formation to indirectly promote protrusion (amoeboid movement).

Mesenchymal migration can be characterised by polymerisation of actin to form filopodial protrusions, in the absence of lamellipodia.

Translocation of the nucleus is emerging as a critical step due to the constrictive environment of 3D matrices, and the mechanisms that transmit force to the nucleus and allow movement are beginning to be uncovered.

The makings of the 'actin code': regulation of actin's biological function at the amino acid and nucleotide level

The actin cytoskeleton plays key roles in every eukaryotic cell and is essential for cell adhesion, migration, mechanosensing, and contractility in muscle and non-muscle tissues. In higher vertebrates, from birds through to mammals, actin is represented by a family of six conserved genes. Although these genes have evolved independently for more than 100 million years, they encode proteins with ≥94% sequence identity, which are differentially expressed in different tissues, and tightly regulated throughout embryogenesis and adulthood. It has been previously suggested that the existence of such similar actin genes is a fail-safe mechanism to preserve the essential function of actin through redundancy. However, knockout studies in mice and other organisms demonstrate that the different actins have distinct biological roles. The mechanisms maintaining this distinction have been debated in the literature for decades. This Review summarizes data on the functional regulation of different actin isoforms, and the mechanisms that lead to their different biological roles in vivo We focus here on recent studies demonstrating that at least some actin functions are regulated beyond the amino acid level at the level of the actin nucleotide sequence.

[A change in the expression of membrane-associated proteins and cytoplasmic actin isoforms in the progression of human colon tumors]

Tumor progression is a complex process that also involves the restructuring of the actin cytoskeleton and the weakening of intercellular adhesive contacts due to the tumor cells that pass through the epithelial-mesenchymal transition (EMT).

AIM

Тo identify correlations between clinical features, risk of progression and/or recurrence of human colon adenocarcinomas (CAC), and EMT-related tumor markers.

MATERIAL AND METHODS

Descending colon and sigmoid colon adenocarcinoma samples were examined immunohistochemically. Formalin-fixed paraffin-embedded tissue sections were incubated with antigen-specific antibodies, then secondary antibodies labeled with fluorochromes, and the fluorescence intensity of microscopy images was analyzed.

RESULTS

The cells of a tumor compared to those of intact colon tissue showed a weak staining of E-cadherin in the cell-cell contact areas. The reduced membrane staining and nuclear localization of β-catenin were detected in moderately (G2) and poorly (G3) differentiated tumors. There were substantially decreased β-actin levels in almost all tumor samples and increased γ-actin ones, mainly in the samples belonging to stage IV disease.

CONCLUSION

A correlation was found between stage, tumor differentiation grade, risk for relapse or progression of disease, and the impaired expression of different EMT markers: total or partial loss of E-cadherin expression, β-catenin reorganization in cell-cell contacts, and a change in the ratio of cytoplasmic actin isoforms in the late stages of CAC development. We believe that these molecular markers may have a prognostic potential.

Depletion of mitochondrial reactive oxygen species downregulates epithelial-to-mesenchymal transition in cervical cancer cells

In the course of cancer progression, epithelial cells often acquire morphological and functional characteristics of mesenchymal cells, a process known as epithelial-to-mesenchymal transition (EMT). EMT provides epithelial cells with migratory, invasive, and stem cell capabilities. Reactive oxygen species produced by mitochondria (mtROS) could be of special importance for pro-tumorigenic signaling and EMT.

In our study, we used mitochondria-targeted antioxidant SkQ1 to lower the mtROS level and analyze their role in the regulation of the actin cytoskeleton, adhesion junctions, and signaling pathways critical for tumorigenesis of cervical carcinomas. A decrease in mtROS was found to induce formation of β-cytoplasmic actin stress fibers and circumferential rings in cervical cancer SiHa and Ca-Ski cells. It was accompanied by an upregulation of E-cadherin in SiHa cells and a downregulation of N-cadherin in Ca-Ski cells. In SiHa cells, an increase in E-cadherin expression was accompanied by a reduction of Snail, E-cadherin negative regulator. A stimulation of mtROS by epidermal growth factor (EGF) caused a Snail upregulation in SiHa cells that could be downregulated by SkQ1. SkQ1 caused a decrease in activation of extracellular-signal-regulated kinases 1 and 2 (ERK1/2) in SiHa and Ca-Ski. EGF produced an opposite effect. Incubation with SkQ1 suppressed EGF-induced p-ERK1/2 upregulation in SiHa, but not in Ca-Ski cells. Thus, we showed that scavenging of mtROS by SkQ1 initiated reversal of EMT and suppressed proliferation of cervical cancer cells.

Lymphocyte-specific protein 1 regulates mechanosensory oscillation of podosomes and actin isoform-based actomyosin symmetry breaking

Subcellular fine-tuning of the actomyosin cytoskeleton is a prerequisite for polarized cell migration. We identify LSP (lymphocyte-specific protein) 1 as a critical regulator of actomyosin contractility in primary macrophages. LSP1 regulates adhesion and migration, including the parameters cell area and speed, and also podosome turnover, oscillation and protrusive force. LSP1 recruits myosin IIA and its regulators, including myosin light chain kinase and calmodulin, and competes with supervillin, a myosin hyperactivator, for myosin regulators, and for actin isoforms, notably β-actin. Actin isoforms are anisotropically distributed in myosin IIA-expressing macrophages, and contribute to the differential recruitment of LSP1 and supervillin, thus enabling an actomyosin symmetry break, analogous to the situation in cells expressing two myosin II isoforms. Collectively, these results show that the cellular pattern of actin isoforms builds the basis for the differential distribution of two actomyosin machineries with distinct properties, leading to the establishment of discrete zones of actomyosin contractility.

The actomyosin cytoskeleton plays an important role in polarised cell migration. Here the authors identify lymphocyte-specific protein (LSP)-1 as a regulator of actomyosin contractility in macrophages, by competing with supervillin for myosin IIA activators acting specifically on the β-actin isoform.

Fundamental differences

The differences between β- and γ-actin are deeper than those between the amino acid sequences of these two proteins.

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.

Diverse functions of homologous actin isoforms are defined by their nucleotide, rather than their amino acid sequence

β‐ and γ‐cytoplasmic actin are nearly indistinguishable in their amino acid sequence, but are encoded by different genes that play non‐redundant biological roles. The key determinants that drive their functional distinction are unknown. Here, we tested the hypothesis that β- and γ-actin functions are defined by their nucleotide, rather than their amino acid sequence, using targeted editing of the mouse genome. Although previous studies have shown that disruption of β-actin gene critically impacts cell migration and mouse embryogenesis, we demonstrate here that generation of a mouse lacking β-actin protein by editing β-actin gene to encode γ-actin protein, and vice versa, does not affect cell migration and/or organism survival. Our data suggest that the essential in vivo function of β-actin is provided by the gene sequence independent of the encoded protein isoform. We propose that this regulation constitutes a global ‘silent code’ mechanism that controls the functional diversity of protein isoforms.

Mammalian cells, including human cells, contain high levels of a protein called actin. This protein is essential for many of the processes that organisms use to develop and survive. For example, filaments of actin maintain the shape of cells, and help generate the forces needed for cells to move and divide.

As in many other animals, every cell in the human body contains two related actin proteins – known as β-actin and γ-actin. These proteins are made from almost identical amino acid building blocks. Yet the genes that encode these two proteins vary much more. The two actin proteins also play different roles: disrupting the gene for β-actin causes mouse embryos to die, but mice without the gene for γ-actin develop almost like normal. It was not fully understood how these almost identical proteins could perform such different roles.

Earlier studies exploring the mechanisms that underlie the unique roles of β- and γ-actin focused on the differences in their amino acid sequences. Now, Vedula, Kurosaka et al. test the hypothesis that the differing roles of these two actin proteins are due to the pronounced differences in the DNA sequences of their genes.

A gene-editing technique called CRISPR/Cas9 was used to make small changes to the mouse gene for β-actin so that it coded for the γ-actin protein. As a consequence, these mice did not make any β-actin protein and instead made the γ-actin protein from a mostly intact gene for β-actin. These mice lacking the β-actin protein survived as normal and were fertile. The shape of their organs and the movement of their cells – two other major processes that need β-actin – were also unaffected. Hence, the γ-actin protein can substitute for β-actin when the β-actin gene is intact.

These observations imply that it is the DNA sequence of the gene rather than the amino acid sequence of the protein that determines the essential role of β-actin in cell migration and the organism’s survival. The next step will be to see if other proteins work in a similar way. If so, this mechanism might allow scientists to discover new ways to fine-tune how proteins behave in healthy and diseased human cells.

Cytokinesis requires localized β-actin filament production by an actin isoform specific nucleator

Cytokinesis is initiated by the localized assembly of the contractile ring, a dynamic actomyosin structure that generates a membrane furrow between the segregating chromosomal masses to divide a cell into two. Here we show that the stabilization and organization of the cytokinetic furrow is specifically dependent on localized β-actin filament assembly at the site of cytokinesis. β-actin filaments are assembled directly at the furrow by an anillin-dependent pathway that enhances RhoA-dependent activation of the formin DIAPH3, an actin nucleator. DIAPH3 specifically generates homopolymeric filaments of β-actin in vitro. By employing enhancers and activators, cells can achieve acute spatio-temporal control over isoform-specific actin arrays that are required for distinct cellular functions.

Cytokinesis is initiated by the localized assembly of the contractile ring. Here the authors show that the stabilization and organization of the cytokinetic furrow requires localized β-actin filament assembly at the site of cytokinesis by an actin isoform specific nucleator.

Reversible differentiation of immortalized human bladder smooth muscle cells accompanied by actin bundle reorganization

Previous studies have shown that phenotypic modulation of smooth muscle cells (SMCs) plays a pivotal role in human diseases. However, the molecular mechanisms underlying the reversible differentiation of SMCs remain elusive particularly because cultured SMCs that reproducibly exhibit bidirectional phenotypic modulation have not been established. Here we established an immortalized human bladder SMC line designated as hBS11. Under differentiation-inducing conditions, hBS11 cells underwent smooth muscle differentiation accompanied by the robust expression of smooth muscle differentiation markers and isoform-dependent reorganization of actin bundles. The cholinergic receptor agonist carbachol increased intracellular calcium in differentiated hBS11 cells in an acetylcholine muscarinic receptor-dependent manner. Differentiated hBS11 cells displayed contractile properties depending on the elevation in the levels of intracellular calcium. Depolarization of membrane potential triggered inward sodium current in differentiated hBS11 cells. However, differentiated hBS11 cells lost the differentiated phenotype and resumed mitosis when re-fed with growth medium. Our study provides direct evidence pertaining to the human bladder SMCs being able to retain the capacity of reversible differentiation and that the reorganization of actin bundles is involved in the reinstatement of contractility. Moreover, we have established a human SMC line retaining high proliferating potential without compromising differentiation potential.

Myofibril contraction and crosslinking drive nuclear movement to the periphery of skeletal muscle

Nuclear movements are important for multiple cellular functions, and are driven by polarized forces generated by motor proteins and the cytoskeleton. During skeletal myofibre formation or regeneration, nuclei move from the centre to the periphery of the myofibre for proper muscle function. Centrally located nuclei are also found in different muscle disorders. Using theoretical and experimental approaches, we demonstrate that nuclear movement to the periphery of myofibres is mediated by centripetal forces around the nucleus. These forces arise from myofibril contraction and crosslinking that 'zip' around the nucleus in combination with tight regulation of nuclear stiffness by lamin A/C. In addition, an Arp2/3 complex containing Arpc5L together with γ-actin is required to organize desmin to crosslink myofibrils for nuclear movement. Our work reveals that centripetal forces exerted by myofibrils squeeze the nucleus to the periphery of myofibres.

The Centrosome as the Main Integrator of Endothelial Cell Functional Activity

The centrosome is an intracellular structure of the animal cell responsible for organization of cytoplasmic microtubules. According to modern concepts, the centrosome is a very important integral element of the living cell whose functions are not limited to its ability to polymerize microtubules. The centrosome localization in the geometric center of the interphase cell, the high concentration of various regulatory proteins in this area, the centrosome-organized radial system of microtubules for intracellular transport by motor proteins, the centrosome involvement in the perception of external signals and their transmission - all these features make this cellular structure a unique regulation and distribution center managing dynamic morphology of the animal cell. In conjunction with the tissue-specific features of the centrosome structure, this suggests the direct involvement of the centrosome in execution of cell functions. This review discusses the involvement of the centrosome in the vital activity of endothelial cells, as well as its possible participation in the implementation of barrier function, the major function of endothelium.

Myosin Va plays essential roles in maintaining normal mitosis, enhancing tumor cell motility and viability

Myosin Va, a member of Class V myosin, functions in organelle motility, spindle formation, nuclear morphogenesis and cell motility. The purpose of this study is to explore the expression and localization of myosin Va in testicular cancer and prostate cancer, and its specific roles in tumor progression including cell division, migration and proliferation. We detected myosin Va in testicular and prostate tumor tissues using sqRT-PCR, western blot, and immunofluorescence. Tumor samples showed an increased expression of myosin Va, abnormal actin and myosin Va distribution. Immunofluorescence images during the cell cycle showed that myosin Va tended to gather at cytoplasm during anaphase but co-localized with nucleus during other phases, suggesting the roles of myosin Va in disassembly of spindle microtubule, movement of chromosomes and normal cytokinesis. In addition, multi-nucleation and aberrant nuclear morphology were observed in myosin Va-knockdown cells. Wounding assay and CCK-8-based cell counting were conducted to explore myosin Va roles in cell migration, viability and proliferation. Our results suggest that myosin Va plays essential roles in maintaining normal mitosis, enhancing tumor cell motility and viability, and these properties are the hallmark of tumor progression and metastasis development. Therefore, an increased understanding of myosin Va expression and function will assist in the development of future oncodiagnosis and -therapy.

Actin stress fiber organization promotes cell stiffening and proliferation of pre-invasive breast cancer cells

Studies of the role of actin in tumour progression have highlighted its key contribution in cell softening associated with cell invasion. Here, using a human breast cell line with conditional Src induction, we demonstrate that cells undergo a stiffening state prior to acquiring malignant features. This state is characterized by the transient accumulation of stress fibres and upregulation of Ena/VASP-like (EVL). EVL, in turn, organizes stress fibres leading to transient cell stiffening, ERK-dependent cell proliferation, as well as enhancement of Src activation and progression towards a fully transformed state. Accordingly, EVL accumulates predominantly in premalignant breast lesions and is required for Src-induced epithelial overgrowth in Drosophila. While cell softening allows for cancer cell invasion, our work reveals that stress fibre-mediated cell stiffening could drive tumour growth during premalignant stages. A careful consideration of the mechanical properties of tumour cells could therefore offer new avenues of exploration when designing cancer-targeting therapies.

When cells acquire a malignant phenotype they become less stiff and this helps migration and invasion favouring metastasis. Here the authors show that Src-driven cell transformation and transition to a less stiff state follows an event of membrane stiffening due to stress fibres accumulation.

The Clinical Manifestations and Genetic Implications of Baraitser-Winter Syndrome Type 2

An 18-year-old Caucasian male was born by cesarean section weighing 2.6 kg (5 lb 14 oz) at birth after an uncomplicated pregnancy with no perinatal complications. Around 4 to 5 months of age, the patient's mother initially became concerned as he was experiencing signs of developmental delay and a mild floppy tone, in addition to facial features that resembled some form of mental retardation. The patient's older brother also experienced similar developmental symptoms and facial features that presented around the same age period as our patient. It was initially thought to be Down syndrome; however, both the patient and his brother tested negative for Down syndrome on chromosomal analyses. There was also a question of whether the patient had some form of autism spectrum disorder, but doctors were unable to specifically confirm this. Now at the age of 18 years, the patient has no understandable speech with distinctive facial features such as a broad nasal bridge and prominent epicanthic folds, lissencephaly, smaller than average head size, intellectual disability, and hearing loss. It was discovered, through trio-based exome sequencing, that the patient had a de novo missense mutation (p.Ser155Phe) in the ACTG1 gene, which has been linked to the rare syndrome known as Baraister-Winter syndrome type 2. Baraitser-Winter syndrome 2 is a unique variant that is clinically similar to Baraitser-Winter syndrome type 1; however, only seven previous cases have been reported.

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.

Differential roles of α-, β-, and γ-actin in axon growth and collateral branch formation in motoneurons

α-, β-, and γ-actin differentially regulate cytoskeletal dynamics and stability in axons of motoneurons. Locally translated α-actin contributes to stable actin filaments in axonal branches, whereas β- and γ-actin give rise to highly dynamic filaments that modulate growth cone dynamics.

Axonal branching and terminal arborization are fundamental events during the establishment of synaptic connectivity. They are triggered by assembly of actin filaments along axon shafts giving rise to filopodia. The specific contribution of the three actin isoforms, Actα, Actβ, and Actγ, to filopodia stability and dynamics during this process is not well understood. Here, we report that Actα, Actβ, and Actγ isoforms are expressed in primary mouse motoneurons and their transcripts are translocated into axons. shRNA-mediated depletion of Actα reduces axonal filopodia dynamics and disturbs collateral branch formation. Knockdown of Actβ reduces dynamic movements of growth cone filopodia and impairs presynaptic differentiation. Ablation of Actβ or Actγ leads to compensatory up-regulation of the two other isoforms, which allows maintenance of total actin levels and preserves F-actin polymerization. Collectively, our data provide evidence for specific roles of different actin isoforms in spatial regulation of actin dynamics and stability in axons of developing motoneurons.

Relative importance of βcyto- and γcyto-actin in primary mouse embryonic fibroblasts

The highly homologous β (β_cyto) and γ (γ_cyto) cytoplasmic actins are hypothesized to carry out both redundant and unique essential functions, but studies using targeted gene knockout and siRNA-mediated transcript knockdown to examine β_cyto- and γ_cyto-isoform--specific functions in various cell types have yielded conflicting data. Here we quantitatively characterized actin transcript and protein levels, as well as cellular phenotypes, in both gene- and transcript-targeted primary mouse embryonic fibroblasts. We found that the smooth muscle α_sm-actin isoform was the dominantly expressed actin isoform in WT primary fibroblasts and was also the most dramatically up-regulated in primary β_cyto- or β/γ_cyto-actin double-knockout fibroblasts. Gene targeting of β_cyto-actin, but not γ_cyto-actin, led to greatly decreased cell proliferation, decreased levels of cellular ATP, and increased serum response factor signaling in primary fibroblasts, whereas immortalization induced by SV40 large T antigen supported fibroblast proliferation in the absence of β_cyto-actin. Consistent with in vivo gene-targeting studies in mice, both gene- and transcript-targeting approaches demonstrate that the loss of β_cyto-actin protein is more disruptive to primary fibroblast function than is the loss of γ_cyto-actin.

Baraitser-Winter cerebrofrontofacial syndrome

Baraitser-Winter cerebrofrontofacial syndrome (BWCFF) (BRWS; MIM #243310, 614583) is a rare developmental disorder affecting multiple organ systems. It is characterised by intellectual disability (mild to severe) and distinctive facial appearance (metopic ridging/trigonocephaly, bilateral ptosis, hypertelorism). The additional presence of cortical malformations (pachygyria/lissencephaly) and ocular colobomata are also suggestive of this syndrome. Other features include moderate short stature, contractures, congenital cardiac disease and genitourinary malformations. BWCFF is caused by missense mutations in the cytoplasmic beta- and gamma-actin genes ACTB and ACTG1. We provide an overview of the clinical characteristics (including some novel findings in four recently diagnosed patients), diagnosis, management, mutation spectrum and genetic counselling.

Interaction of microtubules with the actin cytoskeleton via cross-talk of EB1-containing +TIPs and γ-actin in epithelial cells

Actin microfilaments and microtubules are both highly dynamic cytoskeleton components implicated in a wide range of intracellular processes as well as cell-cell and cell-substrate interactions. The interactions of actin filaments with the microtubule system play an important role in the assembly and maintenance of 3D cell structure. Here we demonstrate that cytoplasmic actins are differentially distributed in relation to the microtubule system. LSM, 3D-SIM, proximity ligation assay (PLA) and co-immunoprecipitation methods applied in combination with selective depletion of β- or γ-cytoplasmic actins revealed a selective interaction between microtubules and γ-, but not β-cytoplasmic actin via the microtubule +TIPs protein EB1. EB1-positive comet distribution analysis and quantification have shown more effective microtubule growth in the absence of β-actin. Our data represent the first demonstration that microtubule +TIPs protein EB1 interacts mainly with γ-cytoplasmic actin in epithelial cells.

Tumour-suppressor microRNAs regulate ovarian cancer cell physical properties and invasive behaviour

The activities of pathways that regulate malignant transformation can be influenced by microRNAs (miRs). Recently, we showed that increased expression of five tumour-suppressor miRs, miR-508-3p, miR-508-5p, miR-509-3p, miR-509-5p and miR-130b-3p, correlate with improved clinical outcomes in human ovarian cancer patients, and that miR-509-3p attenuates invasion of ovarian cancer cell lines. Here, we investigate the mechanism underlying this reduced invasive potential by assessing the impact of these five miRs on the physical properties of cells. Human ovarian cancer cells (HEYA8, OVCAR8) that are transfected with miR mimics representing these five miRs exhibit decreased invasion through collagen matrices, increased cell size and reduced deformability as measured by microfiltration and microfluidic assays. To understand the molecular basis of altered invasion and deformability induced by these miRs, we use predicted and validated mRNA targets that encode structural and signalling proteins that regulate cell mechanical properties. Combined with analysis of gene transcripts by real-time PCR and image analysis of F-actin in single cells, our results suggest that these tumour-suppressor miRs may alter cell physical properties by regulating the actin cytoskeleton. Our findings provide biophysical insights into how tumour-suppressor miRs can regulate the invasive behaviour of ovarian cancer cells, and identify potential therapeutic targets that may be implicated in ovarian cancer progression.

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.

Altered redox mitochondrial biology in the neurodegenerative disorder fragile X-tremor/ataxia syndrome: use of antioxidants in precision medicine

A 55-200 expansion of the CGG nucleotide repeat in the 5'-UTR of the fragile X mental retardation 1 gene (FMR1) is the hallmark of the triplet nucleotide disease known as the "premutation" as opposed to those with >200 repeats, known as the full mutation or fragile X syndrome. Originally, premutation carriers were thought to be free of phenotypic traits; however, some are diagnosed with emotional and neurocognitive issues and, later in life, with the neurodegenerative disease fragile X-associated tremor/ataxia syndrome (FXTAS). Considering that mitochondrial dysfunction has been observed in fibroblasts and post-mortem brain samples from carriers of the premutation, we hypothesized that mitochondrial dysfunction-derived ROS may result in cumulative oxidative-nitrative damage. Fibroblasts from premutation carriers (n=31, all FXTAS-free except 8), compared to age- and sex-matched controls (n=25), showed increased mitochondrial ROS production, impaired Complex I activity, lower expression of MIA40 (rate-limiting step of the redox-regulated mitochondrial-disulfide-relay-system), increased mtDNA deletions, and increased biomarkers of lipid and protein oxidative-nitrative damage. Most of the outcomes were more pronounced in FXTAS-affected individuals. Significant recovery of mitochondrial mass and/or function was obtained with superoxide or hydroxyl radicals' scavengers, a glutathione peroxidase analog, or by overexpressing MIA40. The effects of ethanol (a hydroxyl radical scavenger) were deleterious, while others (by N-acetyl-cysteine, quercetin and epigallocatechin-3-gallate) were outcome- and/or carrier-specifics. The use of antioxidants in the context of precision medicine is discussed with the goal of improving mitochondrial function in carriers with the potential of decreasing the morbidity and/or delaying FXTAS onset.

Tumor promotion by γ and suppression by β non-muscle actin isoforms

Here we have shown that β-cytoplasmic actin acts as a tumor suppressor, inhibiting cell growth and invasion in vitro and tumor growth in vivo. In contrast, γ-cytoplasmic actin increases the oncogenic potential via ERK1/2, p34-Arc, WAVE2, cofilin1, PP1 and other regulatory proteins. There is a positive feedback loop between γ-actin expression and ERK1/2 activation. We conclude that non-muscle actin isoforms should not be considered as merely housekeeping proteins and the β/γ-actins ratio can be used as an oncogenic marker at least for lung and colon carcinomas. Agents that increase β- and/or decrease γ-actin expression may be useful for anticancer therapy.

Localizations of γ-Actins in Skin, Hair, Vibrissa, Arrector Pili Muscle and Other Hair Appendages of Developing Rats

Six isoforms of actins encoded by different genes have been identified in mammals including α-cardiac, α-skeletal, α-smooth muscle (α-SMA), β-cytoplasmic, γ-smooth muscle (γ-SMA), and γ-cytoplasmic actins (γ-CYA). In a previous study we showed the localization of α-SMA and other cytoskeletal proteins in the hairs and their appendages of developing rats (Morioka K., et al. (2011) Acta Histochem. Cytochem. 44, 141–153), and herein we determined the localization of γ type actins in the same tissues and organs by immunohistochemical staining. Our results indicate that the expression of γ-SMA and γ-CYA is suggested to be poor in actively proliferating tissues such as the basal layer of the epidermis and the hair matrix in the hair bulb, and as well as in highly keratinized tissues such as the hair cortex and hair cuticle. In contrast, the expression of γ-actins were high in the spinous layer, granular layer, hair shaft, and inner root sheath, during their active differentiations. In particular, the localization of γ-SMA was very similar to that of α-SMA. It was located not only in the arrector pili muscles and muscles in the dermis, but also in the dermal sheath and in a limited area of the outer root sheath in both the hair and vibrissal follicles. The γ-CYA was suggested to be co-localized with γ-SMA in the dermal sheath, outer root sheath, and arrector pili muscles. Sparsely distributed dermal cells expressed both types of γ-actin. The expression of γ-actins is suggested to undergo dynamic changes according to the proliferation and differentiation of the skin and hair-related cells.

Premutation in the Fragile X Mental Retardation 1 (FMR1) Gene Affects Maternal Zn-milk and Perinatal Brain Bioenergetics and Scaffolding

Fragile X premutation alleles have 55–200 CGG repeats in the 5′ UTR of the FMR1 gene. Altered zinc (Zn) homeostasis has been reported in fibroblasts from >60 years old premutation carriers, in which Zn supplementation significantly restored Zn-dependent mitochondrial protein import/processing and function. Given that mitochondria play a critical role in synaptic transmission, brain function, and cognition, we tested FMRP protein expression, brain bioenergetics, and expression of the Zn-dependent synaptic scaffolding protein SH3 and multiple ankyrin repeat domains 3 (Shank3) in a knock-in (KI) premutation mouse model with 180 CGG repeats. Mitochondrial outcomes correlated with FMRP protein expression (but not FMR1 gene expression) in KI mice and human fibroblasts from carriers of the pre- and full-mutation. Significant deficits in brain bioenergetics, Zn levels, and Shank3 protein expression were observed in the Zn-rich regions KI hippocampus and cerebellum at PND21, with some of these effects lasting into adulthood (PND210). A strong genotype × age interaction was observed for most of the outcomes tested in hippocampus and cerebellum, whereas in cortex, age played a major role. Given that the most significant effects were observed at the end of the lactation period, we hypothesized that KI milk might have a role at compounding the deleterious effects on the FMR1 genetic background. A higher gene expression of ZnT4 and ZnT6, Zn transporters abundant in brain and lactating mammary glands, was observed in the latter tissue of KI dams. A cross-fostering experiment allowed improving cortex bioenergetics in KI pups nursing on WT milk. Conversely, WT pups nursing on KI milk showed deficits in hippocampus and cerebellum bioenergetics. A highly significant milk type × genotype interaction was observed for all three-brain regions, being cortex the most influenced. Finally, lower milk-Zn levels were recorded in milk from lactating women carrying the premutation as well as other Zn-related outcomes (Zn-dependent alkaline phosphatase activity and lactose biosynthesis—whose limiting step is the Zn-dependent β-1,4-galactosyltransferase). In premutation carriers, altered Zn homeostasis, brain bioenergetics and Shank3 levels could be compounded by Zn-deficient milk, increasing the risk of developing emotional and neurological/cognitive problems and/or FXTAS later in life.