At least six different actins are expressed in a higher mammal: an analysis based on the amino acid sequence of the amino-terminal tryptic peptide

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.

Bioinformatics analysis of proteins interacting with different actin isoforms

Actin is one of the most widespread and most conserved proteins. At the same time, six actin isoforms are known, encoded by different genes. These isoforms differ slightly in amino acid sequence and have similar structures, but differ in localization and functioning. During functioning, actin interacts with a large number of proteins, which are combined according to this feature into a pool of so-called actin-binding proteins. The question arises whether and how the proteins interacting with different actin isoforms differ. Since the pool of actin-binding proteins includes hundreds of proteins, it was logical to use bioinformatics analysis to solve the questions. In this work, it is shown that the functionality of the α-, β-, and γ-actin interactomes differ significantly, but their structural characteristics are close.

Extreme Phenotypic Variability of ACTG1-Related Disorders in Hearing Loss

Hearing loss is the most common sensory defect in humans, affecting normal communication. In most cases, hearing loss is a multifactorial disorder caused by both genetic and environmental factors, but single-gene mutations can lead to syndromic or non-syndromic hearing loss. Monoallelic variants in ACTG1, coding for gamma (γ)-actin, are associated with classical Baraitser-Winter Syndrome type 2 (BRWS2, nonsyndromic deafness, and a variety of clinical presentations not fitting the original BRWS2 description or nonsyndromic deafness. Here two unrelated patients with ACTG1 variants are reported, having severe hearing loss as a common phenotype but with different clinical presentations, supporting the extreme variability of ACTG1-related disorders.

Insights into Actin Isoform-Specific Interactions with Myosin via Computational Analysis

Actin, which plays a crucial role in cellular structure and function, interacts with various binding proteins, notably myosin. In mammals, actin is composed of six isoforms that exhibit high levels of sequence conservation and structural similarity overall. As a result, the selection of actin isoforms was considered unimportant in structural studies of their binding with myosin. However, recent high-resolution structural research discovered subtle structural differences in the N-terminus of actin isoforms, suggesting the possibility that each actin isoform may engage in specific interactions with myosin isoforms. In this study, we aimed to explore this possibility, particularly by understanding the influence of different actin isoforms on the interaction with myosin 7A. First, we compared the reported actomyosin structures utilizing the same type of actin isoforms as the high-resolution filamentous skeletal α-actin (3.5 Å) structure elucidated using cryo-EM. Through this comparison, we confirmed that the diversity of myosin isoforms leads to differences in interaction with the actin N-terminus, and that loop 2 of the myosin actin-binding sites directly interacts with the actin N-terminus. Subsequently, with the aid of multiple sequence alignment, we observed significant variations in the length of loop 2 across different myosin isoforms. We predicted that these length differences in loop 2 would likely result in structural variations that would affect the interaction with the actin N-terminus. For myosin 7A, loop 2 was found to be very short, and protein complex predictions using skeletal α-actin confirmed an interaction between loop 2 and the actin N-terminus. The prediction indicated that the positively charged residues present in loop 2 electrostatically interact with the acidic patch residues D24 and D25 of actin subdomain 1, whereas interaction with the actin N-terminus beyond this was not observed. Additionally, analyses of the actomyosin-7A prediction models generated using various actin isoforms consistently yielded the same results regardless of the type of actin isoform employed. The results of this study suggest that the subtle structural differences in the N-terminus of actin isoforms are unlikely to influence the binding structure with short loop 2 myosin 7A. Our findings are expected to provide a deeper understanding for future high-resolution structural binding studies of actin and myosin.

The predictive value of serum F-actin on the severity and early neurological deterioration of acute ischemic stroke: Predictive value of F-actin in stroke

BACKGROUND

F-actin is involved in the progression of ischemic stroke and is associated with the disruption of the blood-brain barrier. In this article, we evaluated serum F-actin as a biomarker in stroke severity and early neurological deterioration (END) in acute ischemic stroke.

METHODS

In this study, serum F-actin was measured in consecutively collected 140 AIS patients and 144 healthy controls matched in gender and age by ELISA. Early neurological deterioration (END) was defined as the deterioration of neurological dysfunction within 72 hours of admission, with an increase of ≥ 4 points in the NIHSS score. Severe stroke was defined as a NIHSS score>8 at admission.

RESULTS

The serum F-actin level in AIS was significantly higher than healthy controls (p = 0.041). In large-artery atherosclerosis stroke and cardioembolic stroke, serum F-actin were significantly higher than that in small artery occlusion stroke (padjust = 0.019, padjust < 0.001, respectively).F-actin level above the critical value (>1.37 µg/L) was significantly associated with severe stroke (OR, 3.015; 95 %CI, 1.014-8.963; p = 0.047) . In addition, elevated level of F-actin was significantly associated with END (OR, 1.323; 95 % CI, 1.001-1.747, p = 0.049). When the level of F-actin was above the critical value (>2.17 µg/L), its association with END remained significant (OR, 6.303; 95 %CI, 2.160-18.394; p < 0.001) .

CONCLUSION

F-actin is an important blood biomarker in the early stage of AIS, and high levels of F-actin are valuable in determining the severity of stroke and predicting early neurological deterioration.

Purification of modified mammalian actin isoforms for in vitro reconstitution assays

In vitro reconstitution assays using purified actin have greatly improved our understanding of cytoskeletal dynamics and their regulation by actin-binding proteins. However, early purification methods consisted of harsh conditions to obtain pure actin and often did not include correct maturation and obligate modification of the isolated actin monomers. Novel insights into the folding requirements and N-terminal processing of actin as well as a better understanding of the interaction of actin with monomer sequestering proteins such as DNaseI, profilin and gelsolin, led to the development of more gentle approaches to obtain pure recombinant actin isoforms with known obligate modifications. This review summarizes the approaches that can be employed to isolate natively folded endogenous and recombinant actin from tissues and cells. We further emphasize the use and limitations of each method and describe how these methods can be implemented to study actin PTMs, disease-related actin mutations and novel actin-like proteins.

Actin Polymerization Defects Induce Mitochondrial Dysfunction in Cellular Models of Nemaline Myopathies

Nemaline myopathy (NM) is one of the most common forms of congenital myopathy and it is identified by the presence of "nemaline bodies" (rods) in muscle fibers by histopathological examination. The most common forms of NM are caused by mutations in the Actin Alpha 1 (ACTA1) and Nebulin (NEB) genes. Clinical features include hypotonia and muscle weakness. Unfortunately, there is no curative treatment and the pathogenetic mechanisms remain unclear. In this manuscript, we examined the pathophysiological alterations in NM using dermal fibroblasts derived from patients with mutations in ACTA1 and NEB genes. Patients' fibroblasts were stained with rhodamine-phalloidin to analyze the polymerization of actin filaments by fluorescence microscopy. We found that patients' fibroblasts showed incorrect actin filament polymerization compared to control fibroblasts. Actin filament polymerization defects were associated with mitochondrial dysfunction. Furthermore, we identified two mitochondrial-boosting compounds, linoleic acid (LA) and L-carnitine (LCAR), that improved the formation of actin filaments in mutant fibroblasts and corrected mitochondrial bioenergetics. Our results indicate that cellular models can be useful to study the pathophysiological mechanisms involved in NM and to find new potential therapies. Furthermore, targeting mitochondrial dysfunction with LA and LCAR can revert the pathological alterations in NM cellular models.

The actin cytoskeleton in hair bundle development and hearing loss

Inner ear hair cells assemble mechanosensitive hair bundles on their apical surface that transduce sounds and accelerations. Each hair bundle is comprised of ∼ 100 individual stereocilia that are arranged into rows of increasing height and width; their specific and precise architecture being necessary for mechanoelectrical transduction (MET). The actin cytoskeleton is fundamental to establishing this architecture, not only by forming the structural scaffold shaping each stereocilium, but also by composing rootlets and the cuticular plate that together provide a stable foundation supporting each stereocilium. In concert with the actin cytoskeleton, a large assortment of actin-binding proteins (ABPs) function to cross-link actin filaments into specific topologies, as well as control actin filament growth, severing, and capping. These processes are individually critical for sensory transduction and are all disrupted in hereditary forms of human hearing loss. In this review, we provide an overview of actin-based structures in the hair bundle and the molecules contributing to their assembly and functional properties. We also highlight recent advances in mechanisms driving stereocilia elongation and how these processes are tuned by MET.

A defective mechanosensing pathway affects fibroblast-to-myofibroblast transition in the old male mouse heart

The cardiac fibroblast interacts with an extracellular matrix (ECM), enabling myofibroblast maturation via a process called mechanosensing. Although in the aging male heart, ECM is stiffer than in the young mouse, myofibroblast development is impaired, as demonstrated in 2-D and 3-D experiments. In old male cardiac fibroblasts, we found a decrease in actin polymerization, α-smooth muscle actin (α-SMA), and Kindlin-2 expressions, the latter an effector of the mechanosensing. When Kindlin-2 levels were manipulated via siRNA interference, young fibroblasts developed an old-like fibroblast phenotype, whereas Kindlin-2 overexpression in old fibroblasts reversed the defective phenotype. Finally, inhibition of overactivated extracellular regulated kinases 1 and 2 (ERK1/2) in the old male fibroblasts rescued actin polymerization and α-SMA expression. Pathological ERK1/2 overactivation was also attenuated by Kindlin-2 overexpression. In contrast, old female cardiac fibroblasts retained an operant mechanosensing pathway. In conclusion, we identified defective components of the Kindlin/ERK/actin/α-SMA mechanosensing axis in aged male fibroblasts.

Rupture of Splenic Artery Aneurysm in Patient with ACTN2 Mutation

Here, we report a case of splenic artery aneurysm rupture in a patient with known heterozygosity mutation of the ACTN2 gene (variant c.971G > A p.Arg324Gln). The patient came to our emergency department with epigastric pain radiating to the lumbar area, with an absence of peritonism signs. An abdominal computed tomography angiography showed a ruptured huge (5 cm) splenic artery aneurysm. Therefore, the patient underwent emergency endovascular coil embolization with complete aneurysm exclusion. The postoperative course was uneventful, until postoperative day five when the patient developed a symptomatic supraventricular tachycardia in the absence of echocardiographic alterations. The signs and symptoms disappeared after three days of medical management. The patient was discharged on the 14th postoperative day in good clinical condition under verapamil and anti-platelet therapy. Although ACTN2 mutation was associated with cardiac and peripheral vascular disease occurrence, to the best of our knowledge, the present case is the first report of a visceral (splenic) aneurysm directly linked with this rare mutation.

What about the Cytoskeletal and Related Proteins of Tapeworms in the Host's Immune Response? An Integrative Overview

Recent advances have increased our understanding of the molecular machinery in the cytoskeleton of mammalian cells, in contrast to the case of tapeworm parasites, where cytoskeleton remains poorly characterized. The pertinence of a better knowledge of the tapeworm cytoskeleton is linked to the medical importance of these parasitic diseases in humans and animal stock. Moreover, its study could offer new possibilities for the development of more effective anti-parasitic drugs, as well as better strategies for their surveillance, prevention, and control. In the present review, we compile the results of recent experiments on the cytoskeleton of these parasites and analyze how these novel findings might trigger the development of new drugs or the redesign of those currently used in addition to supporting their use as biomarkers in cutting-edge diagnostic tests.

Fibroblasts-Warriors at the Intersection of Wound Healing and Disrepair

Wound healing is triggered by inflammation elicited after tissue injury. Mesenchymal cells, specifically fibroblasts, accumulate in the injured tissues, where they engage in tissue repair through the expression and assembly of extracellular matrices that provide a scaffold for cell adhesion, the re-epithelialization of tissues, the production of soluble bioactive mediators that promote cellular recruitment and differentiation, and the regulation of immune responses. If appropriately deployed, these processes promote adaptive repair, resulting in the preservation of the tissue structure and function. Conversely, the dysregulation of these processes leads to maladaptive repair or disrepair, which causes tissue destruction and a loss of organ function. Thus, fibroblasts not only serve as structural cells that maintain tissue integrity, but are key effector cells in the process of wound healing. The review will discuss the general concepts about the origins and heterogeneity of this cell population and highlight the specific fibroblast functions disrupted in human disease. Finally, the review will explore the role of fibroblasts in tissue disrepair, with special attention to the lung, the role of aging, and how alterations in the fibroblast phenotype underpin disorders characterized by pulmonary fibrosis.

The Role of non-muscle actin paralogs in cell cycle progression and proliferation

Uncontrolled cell proliferation leads to several pathologies, including cancer. Thus, this process must be tightly regulated. The cell cycle accounts for cell proliferation, and its progression is coordinated with changes in cell shape, for which cytoskeleton reorganization is responsible. Rearrangement of the cytoskeleton allows for its participation in the precise division of genetic material and cytokinesis. One of the main cytoskeletal components is filamentous actin-based structures. Mammalian cells have at least six actin paralogs, four of which are muscle-specific, while two, named β- and γ-actin, are abundantly present in all types of cells. This review summarizes the findings that establish the role of non-muscle actin paralogs in regulating cell cycle progression and proliferation. We discuss studies showing that the level of a given non-muscle actin paralog in a cell influences the cell's ability to progress through the cell cycle and, thus, proliferation. Moreover, we elaborate on the non-muscle actins' role in regulating gene transcription, interactions of actin paralogs with proteins involved in controlling cell proliferation, and the contribution of non-muscle actins to different structures in a dividing cell. The data cited in this review show that non-muscle actins regulate the cell cycle and proliferation through varying mechanisms. We point to the need for further studies addressing these mechanisms.

Single-cell atlas of epithelial and stromal cell heterogeneity by lobe and strain in the mouse prostate

BACKGROUND

Evaluating the complex interplay of cell types in the tissue microenvironment is critical to understanding the origin and progression of diseases in the prostate and potential opportunities for intervention. Mouse models are an essential tool to investigate the molecular and cell-type-specific contributions of prostate disease at an organismal level. While there are well-documented differences in the extent, timing, and nature of disease development in various genetically engineered and exposure-based mouse models in different mouse strains and prostate lobes within each mouse strain, the underlying molecular phenotypic differences in cell types across mouse strains and prostate lobes are incompletely understood.

METHODS

In this study, we used single-cell RNA-sequencing (scRNA-seq) methods to assess the single-cell transcriptomes of 6-month-old mouse prostates from two commonly used mouse strains, friend virus B/NIH jackson (FVB/NJ) (N = 2) and C57BL/6J (N = 3). For each mouse, the lobes of the prostate were dissected (anterior, dorsal, lateral, and ventral), and individual scRNA-seq libraries were generated. In situ and pathological analyses were used to explore the spatial and anatomical distributions of novel cell types and molecular markers defining these cell types.

RESULTS

Data dimensionality reduction and clustering analysis of scRNA-seq data revealed that basal and luminal cells possessed strain-specific transcriptomic differences, with luminal cells also displaying marked lobe-specific differences. Gene set enrichment analysis comparing luminal cells by strain showed enrichment of proto-Oncogene targets in FVB/NJ mice. Additionally, three rare populations of epithelial cells clustered independently of strain and lobe: one population of luminal cells expressing Foxi1 and components of the vacuolar ATPase proton pump (Atp6v0d2 and Atp6v1g3), another population expressing Psca and other stem cell-associated genes (Ly6a/Sca-1, Tacstd2/Trop-2), and a neuroendocrine population expressing Chga, Chgb, and Syp. In contrast, stromal cell clusters, including fibroblasts, smooth muscle cells, endothelial cells, pericytes, and immune cell types, were conserved across strain and lobe, clustering largely by cell type and not by strain or lobe. One notable exception to this was the identification of two distinct fibroblast populations that we term subglandular fibroblasts and interstitial fibroblasts based on their strikingly distinct spatial distribution in the mouse prostate.

CONCLUSIONS

Altogether, these data provide a practical reference of the transcriptional profiles of mouse prostate from two commonly used mouse strains and across all four prostate lobes.

Structural and functional analysis of actin point mutations leading to nemaline myopathy to elucidate their role in actin function

In this work, we analyzed 78 mutations in the actin protein that cause the disease nemaline myopathy. We analyzed how these mutations are distributed in important regions of the actin molecule (folding nucleus, core of the filament, amyloidogenic regions, disordered regions, regions involved in interaction with other proteins). It was found that 54 mutations (43 residues) fall into the folding nucleus (Ф ≥ 0.5), 11 mutations (10 residues) into the filament core, 14 mutations into the amyloidogenic regions (11 residues), 14 mutations (9 residues) in the unstructured regions, and 24 mutations (22 residues) in regions involved in interaction with other proteins. It was also found that the occurrence of single mutations G44V, V45F, T68I, P72R, K338I and S350L leads to the appearance of new amyloidogenic regions that are not present in native actin. The largest number of mutations (54 out of 78) occurs in the folding nucleus; these mutations are important for folding and therefore can affect the protein folding rate. We have shown that almost all of the considered mutations are associated with the structural characteristics of the actin molecule, and some of the residues we have considered have several important characteristics.

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.

The actin cytoskeleton: Morphological changes in pre- and fully developed lung cancer

Actin, a primary component of the cell cytoskeleton can have multiple isoforms, each of which can have specific properties uniquely suited for their purpose. These monomers are then bound together to form polymeric filaments utilizing adenosine triphosphate hydrolysis as a source of energy. Proteins, such as Arp2/3, VASP, formin, profilin, and cofilin, serve important roles in the polymerization process. These filaments can further be linked to form stress fibers by proteins called actin-binding proteins, such as α-actinin, myosin, fascin, filamin, zyxin, and epsin. These stress fibers are responsible for mechanotransduction, maintaining cell shape, cell motility, and intracellular cargo transport. Cancer metastasis, specifically epithelial mesenchymal transition (EMT), which is one of the key steps of the process, is accompanied by the formation of thick stress fibers through the Rho-associated protein kinase, MAPK/ERK, and Wnt pathways. Recently, with the advent of "field cancerization," pre-malignant cells have also been demonstrated to possess stress fibers and related cytoskeletal features. Analytical methods ranging from western blot and RNA-sequencing to cryo-EM and fluorescent imaging have been employed to understand the structure and dynamics of actin and related proteins including polymerization/depolymerization. More recent methods involve quantifying properties of the actin cytoskeleton from fluorescent images and utilizing them to study biological processes, such as EMT. These image analysis approaches exploit the fact that filaments have a unique structure (curvilinear) compared to the noise or other artifacts to separate them. Line segments are extracted from these filament images that have assigned lengths and orientations. Coupling such methods with statistical analysis has resulted in development of a new reporter for EMT in lung cancer cells as well as their drug responses.

Differential N-terminal processing of beta and gamma actin

Cytoplasmic beta- and gamma-actin are ubiquitously expressed in every eukaryotic cell. They are encoded by different genes, but their amino acid sequences differ only by four conservative substitutions at the N-termini, making it difficult to dissect their individual regulation. Here, we analyzed actin from cultured cells and tissues by mass spectrometry and found that beta, unlike gamma actin, undergoes sequential removal of N-terminal Asp residues, leading to truncated actin species found in both F- and G-actin preparations. This processing affects up to ∼3% of beta actin in different cell types. We used CRISPR/Cas-9 in cultured cells to delete two candidate enzymes capable of mediating this type of processing. This deletion abolishes most of the beta actin N-terminal processing and results in changes in F-actin levels, cell spreading, filopodia formation, and cell migration. Our results demonstrate previously unknown isoform-specific actin regulation that can potentially affect actin functions in cells.

Unique and redundant functions of cytoplasmic actins and nonmuscle myosin II isoforms at epithelial junctions

The integrity and functions of epithelial barriers depend on the formation of adherens junctions (AJs) and tight junctions (TJs). A characteristic feature of AJs and TJs is their association with the cortical cytoskeleton composed of actin filaments and nonmuscle myosin II (NM-II) motors. Mechanical forces generated by the actomyosin cytoskeleton are essential for junctional assembly, stability, and remodeling. Epithelial cells express two different actin proteins and three NM-II isoforms, all known to be associated with AJs and TJs. Despite their structural similarity, different actin and NM-II isoforms have distinct biochemical properties, cellular distribution, and functions. The diversity of epithelial actins and myosin motors could be essential for the regulation of different steps of junctional formation, maturation, and disassembly. This review focuses on the roles of actin and NM-II isoforms in controlling the integrity and barrier properties of various epithelia. We discuss the effects of the depletion of individual actin isoforms and NM-II motors on the assembly and barrier function of AJs and TJs in model epithelial monolayers in vitro. We also describe the functional consequences of either total or tissue-specific gene knockout of different actins and NM-II motors, with a focus on the development and integrity of different epithelia in vivo.

Cytoplasmic Beta and Gamma Actin Isoforms Reorganization and Regulation in Tumor Cells in Culture and Tissue

The cytoplasmic actin isoforms (β- and γ-actins) contribute greatly to cellular processes such as cel-cell and cell-matrix interactions, as well as cell polarization, motility and division. Distinct isoforms modulations are linked to serious pathologies, so investigations of underlying mechanisms would be of major relevance not only for fundamental research but also for clinical applications. Therefore, the study of the relevant mechanisms of change in the isoform’s balance is important for basic research and for clinical studies. The disruption of actin cytoskeleton and intercellular adhesions contribute to the neoplastic transformation, as it is important for the tumor growth, invasiveness and metastasis. Cytoplasmic actins display the functional diversity: β-actin is responsible for contractility, whereas γ-actin participates in the submembrane flexible cortex organization and direction cell motility. The involvement of β- and γ-actin in cell architecture, motility, division, and adhesion junctions in normal cells is not equivalent, and the major question was following: whether isoform ratio and the distribution in the cell corresponds to pathological function. Significant data were obtained in the study of tumor and normal cells in culture, as well as on clinical material of human tissues, and via selective regulation of β- and γ-actin’s expression. Investigation of the actins’ diversity and function in cancers may help to choose the benefit treatment strategies, and to design new therapies.

Rodlet cells in kidney of goldfish (Carassius auratus, Linnaeus 1758): A light and confocal microscopy study

Rodlet cells (RCs) have always been an enigma for scientists. RCs have been given a variety of activities over the years, including ion transport, osmoregulation, and sensory function. These cells, presumably as members of the granulocyte line, are present only in teleosts and play a role in the innate immune response. RCs are migratory cells found in a variety of organs, including skin, vascular, digestive, uropoietic, reproductive, and respiratory systems, and present distinct physical properties that make them easily recognizable in tissues and organs. The development of RCs can be divided into four stages: granular, transitional, mature, and ruptured, having different morphological characteristics. Our study aims to characterize the different stages of these cells by histomorphological and histochemical techniques. Furthermore, we characterized these cells at all stages with peroxidase and fluorescence immunohistochemical techniques using different antibodies: S100, tubulin, α-SMA, piscidin, and for the first time TLR-2. From our results, the immunoreactivity of these cells to the antibodies performed may confirm that RCs play a role in fish defense mechanisms, helping to expand the state of the art on immunology and immune cells of teleosts.

The Final Maturation State of β-actin Involves N-terminal Acetylation by NAA80, not N-terminal Arginylation by ATE1

Actin is a hallmark protein of the cytoskeleton in eukaryotic cells, affecting a range of cellular functions. Actin dynamics is regulated through a myriad of actin-binding proteins and post-translational modifications. The mammalian actin family consists of six different isoforms, which vary slightly in their N-terminal (Nt) sequences. During and after synthesis, actins undergo an intricate Nt-processing that yields mature actin isoforms. The ubiquitously expressed cytoplasmic β-actin is Nt-acetylated by N-alpha acetyltransferase 80 (NAA80) yielding the Nt-sequence Ac-DDDI-. In addition, β-actin was also reported to be Nt-arginylated by arginyltransferase 1 (ATE1) after further peptidase-mediated processing, yielding RDDI-. To characterize in detail the Nt-processing of actin, we used state-of-the-art proteomics. To estimate the relative cellular levels of Nt-modified proteoforms of actin, we employed NAA80-lacking cells, in which actin was not Nt-acetylated. We found that targeted proteomics is superior to a commercially available antibody previously used to analyze Nt-arginylation of β-actin. Significantly, despite the use of sensitive mass spectrometry-based techniques, we could not confirm the existence of the previously claimed Nt-arginylated β-actin (RDDI-) in either wildtype or NAA80-lacking cells. A very minor level of Nt-arginylation of the initially cleaved β-actin (DDDI-) could be identified, but only in NAA80-lacking cells, not in wildtype cells. We also identified small fractions of cleaved and unmodified β-actin (DDI-) as well as cleaved and Nt-acetylated β-actin (Ac-DDI-). In sum, we show that the multi-step Nt-maturation of β-actin is terminated by NAA80, which Nt-acetylates the exposed Nt-Asp residues, in the virtual absence of previously claimed Nt-arginylation.

Age-Related Changes in the Microvascular Density of the Human Meniscus

BACKGROUND

The microvascular anatomy of the meniscus of the human knee is regarded as a crucial factor in the injury response. Previous studies have investigated the zone-dependent distribution pattern, but no quantitative data exist on vascular density and its age-related changes.

HYPOTHESIS/PURPOSE

The aim of the present study was to histologically analyze the vascular anatomy of the meniscus as a function of age. It was hypothesized that vascular density would decrease with increasing age.

STUDY DESIGN

Descriptive laboratory study.

METHODS

Human menisci were retrieved from patients who underwent tumor resection or who received total knee replacement because of osteoarthritis. A total of 51 menisci were collected from 28 patients over 9 years (mean age, 25.6 ± 20.4 years; range 3-79 years). Immunohistological staining (alpha-smooth muscle actin) in combination with serial sections and standardized software-based contrast detection were used for the quantitative analysis. Data were analyzed using multiple t tests and the analysis of variance for trends, with a statistical significance level of P < .05.

RESULTS

The overall vascular density in the meniscus was lower in the 61- to 80-year age group than in the age groups of 0 to 10, 11 to 20, and 21 to 30 years (P < .01). A negative linear trend was detected with increasing age (slope, -0.007; P = .016). Within the red-white (RW) zone, a low vessel density was detected for the age groups of 0 to 10 and 11 to 20 years. Beyond these age groups, no vasculature was found in the RW zone. For the white-white (WW) zone, no vessel formations were noted in any age group. Almost 95% of the vessels in the meniscus were located in the capsule.

CONCLUSION

This study reports quantitative histological data for microvascular anatomy as a function of age in a broad cohort of human knee menisci. The overall vascular density decreased with increasing age. No vessel formations were detected in the RW and WW zones after adolescence. Additionally, the capsule is far more densely vascularized than any other part of the meniscus.

CLINICAL RELEVANCE

Vascular density might be an additional factor to consider, along with tear location and patient age, for future treatment options.

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.

Posttranslational modifications of the cytoskeleton

The cytoskeleton plays important roles in many essential processes at the cellular and organismal levels, including cell migration and motility, cell division, and the establishment and maintenance of cell and tissue architecture. In order to facilitate these varied functions, the main cytoskeletal components-microtubules, actin filaments, and intermediate filaments-must form highly diverse intracellular arrays in different subcellular areas and cell types. The question of how this diversity is conferred has been the focus of research for decades. One key mechanism is the addition of posttranslational modifications (PTMs) to the major cytoskeletal proteins. This posttranslational addition of various chemical groups dramatically increases the complexity of the cytoskeletal proteome and helps facilitate major global and local cytoskeletal functions. Cytoskeletal proteins undergo many PTMs, most of which are not well understood. Recent technological advances in proteomics and cell biology have allowed for the in-depth study of individual PTMs and their functions in the cytoskeleton. Here, we provide an overview of the major PTMs that occur on the main structural components of the three cytoskeletal systems-tubulin, actin, and intermediate filament proteins-and highlight the cellular function of these modifications.

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

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

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

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

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

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

The remodelling of actin composition as a hallmark of cancer

Actin is a key structural protein that makes up the cytoskeleton of cells, and plays a role in functions such as division, migration, and vesicle trafficking. It comprises six different cell-type specific isoforms: ACTA1, ACTA2, ACTB, ACTC1, ACTG1, and ACTG2. Abnormal actin isoform expression has been reported in many cancers, which led us to hypothesize that it may serve as an early biomarker of cancer. We show an overview of the different actin isoforms and highlight mechanisms by which they may contribute to tumorigenicity. Furthermore, we suggest how the aberrant expression of actin subunits can confer cells with greater proliferation ability, increased migratory capability, and chemoresistance through incorporation into the normal cellular F-actin network and altered actin binding protein interaction. Studying this fundamental change that takes place within cancer cells can further our understanding of neoplastic transformation in multiple tissue types, which can ultimately aid in the early-detection, diagnosis and treatment of cancer.

Cancer type-specific alterations in actin genes: Worth a closer look?

Actins form a strongly conserved family of proteins that are central to the functioning of the actin cytoskeleton partaking in natural processes such as cell division, adhesion, contraction and migration. These processes, however, also occur during the various phases of cancer progression. Yet, surprisingly, alterations in the six human actin genes in cancer studies have received little attention and the focus was mostly on deregulated expression levels of actins and even more so of actin-binding or regulatory proteins. Starting from the early mutation work in the 1980s, we propose based on reviewing literature and data from patient cancer genomes that alterations in actin genes are different in distinct cancer subtypes, suggesting some specificity. These actin gene alterations include (missense) mutations, gene fusions and copy number alterations (deletions and amplifications) and we illustrate their occurrence for a limited number of examples including actin mutations in lymphoid cancers and nonmelanoma skin cancer and actin gene copy number alterations for breast, prostate and liver cancers. A challenge in the future will be to further sort out the specificity per actin gene, alteration type and cancer subtype. Even more challenging is (experimentally) distinguishing between cause and consequence: which alterations are passengers and which are involved in tumor progression of particular cancer subtypes?

Articular Chondrocyte Phenotype Regulation through the Cytoskeleton and the Signaling Processes That Originate from or Converge on the Cytoskeleton: Towards a Novel Understanding of the Intersection between Actin Dynamics and Chondrogenic Function

Numerous studies have assembled a complex picture, in which extracellular stimuli and intracellular signaling pathways modulate the chondrocyte phenotype. Because many diseases are mechanobiology-related, this review asked to what extent phenotype regulators control chondrocyte function through the cytoskeleton and cytoskeleton-regulating signaling processes. Such information would generate leverage for advanced articular cartilage repair. Serial passaging, pro-inflammatory cytokine signaling (TNF-α, IL-1α, IL-1β, IL-6, and IL-8), growth factors (TGF-α), and osteoarthritis not only induce dedifferentiation but also converge on RhoA/ROCK/Rac1/mDia1/mDia2/Cdc42 to promote actin polymerization/crosslinking for stress fiber (SF) formation. SF formation takes center stage in phenotype control, as both SF formation and SOX9 phosphorylation for COL2 expression are ROCK activity-dependent. Explaining how it is molecularly possible that dedifferentiation induces low COL2 expression but high SF formation, this review theorized that, in chondrocyte SOX9, phosphorylation by ROCK might effectively be sidelined in favor of other SF-promoting ROCK substrates, based on a differential ROCK affinity. In turn, actin depolymerization for redifferentiation would “free-up” ROCK to increase COL2 expression. Moreover, the actin cytoskeleton regulates COL1 expression, modulates COL2/aggrecan fragment generation, and mediates a fibrogenic/catabolic expression profile, highlighting that actin dynamics-regulating processes decisively control the chondrocyte phenotype. This suggests modulating the balance between actin polymerization/depolymerization for therapeutically controlling the chondrocyte phenotype.

The origin of the expressed retrotransposed gene ACTBL2 and its influence on human melanoma cells' motility and focal adhesion formation

We have recently found that β-actin-like protein 2 (actbl2) forms complexes with gelsolin in human melanoma cells and can polymerize. Phylogenetic and bioinformatic analyses showed that actbl2 has a common origin with two non-muscle actins, which share a separate history from the muscle actins. The actin groups' divergence started at the beginning of vertebrate evolution, and actbl2 actins are characterized by the largest number of non-conserved amino acid substitutions of all actins. We also discovered that ACTBL2 is expressed at a very low level in several melanoma cell lines, but a small subset of cells exhibited a high ACTBL2 expression. We found that clones with knocked-out ACTBL2 (CR-ACTBL2) or overexpressing actbl2 (OE-ACTBL2) differ from control cells in the invasion, focal adhesion formation, and actin polymerization ratio, as well as in the formation of lamellipodia and stress fibers. Thus, we postulate that actbl2 is the seventh actin isoform and is essential for cell motility.

Bioinformatics Analysis of Actin Molecules: Why Quantity Does Not Translate Into Quality?

It is time to review all the available data and find the distinctive characteristics of actin that make it such an important cell molecule. The presented double-stranded organization of filamentous actin cannot explain the strong polymorphism of actin fibrils. In this work, we performed bioinformatics analysis of a set of 296 amino acid actin sequences from representatives of different classes of the Chordate type. Based on the results of the analysis, the degree of conservatism of the primary structure of this protein in representatives of the Chordate type was determined. In addition, 155 structures of rabbit actin obtained using X-ray diffraction analysis and electron microscopy have been analyzed over the past 30 years. From pairwise alignments and the calculation of root-mean-square deviations (RMSDs) for these structures, it follows that they are very similar to each other without correlation with the structure resolution and the reconstruction method: the RMSDs for 11,781 pairs did not exceed 3 Å. It turned out that in rabbit actin most of the charged amino acid residues are located inside the protein, which is not typical for the protein structure. We found that two of six exon regions correspond to structural subdomains. To test the double-stranded organization of the actin structure, it is necessary to use new approaches and new techniques, taking into account our new data obtained from the structural analysis of actin.

Loss of β-Cytoplasmic Actin in the Intestinal Epithelium Increases Gut Barrier Permeability in vivo and Exaggerates the Severity of Experimental Colitis

Intestinal epithelial barrier is critical for the maintenance of normal gut homeostasis and disruption of this barrier may trigger or exaggerate mucosal inflammation. The actin cytoskeleton is a key regulator of barrier structure and function, controlling the assembly and permeability of epithelial adherens and tight junctions. Epithelial cells express two actin isoforms: a β-cytoplasmic actin and γ-cytoplasmic actin. Our previous in vitro studies demonstrated that these actin isoforms play distinctive roles in establishing the intestinal epithelial barrier, by controlling the organization of different junctional complexes. It remains unknown, whether β-actin and γ-actin have unique or redundant functions in regulating the gut barrier in vivo. To address this question, we selectively knocked out β-actin expression in mouse intestinal epithelium. Mice with intestinal epithelial knockout of β-actin do not display gastrointestinal abnormalities or gross alterations of colonic mucosal architecture. This could be due to compensatory upregulation of γ-actin expression. Despite such compensation, β-actin knockout mice demonstrate increased intestinal permeability. Furthermore, these animals show more severe clinical symptoms during dextran sodium sulfate induced colitis, compared to control littermates. Such exaggerated colitis is associated with the higher expression of inflammatory cytokines, increased macrophage infiltration in the gut, and accelerated mucosal cell death. Consistently, intestinal organoids generated from β-actin knockout mice are more sensitive to tumor necrosis factor induced cell death, ex vivo. Overall, our data suggests that β-actin functions as an essential regulator of gut barrier integrity in vivo, and plays a tissue protective role during mucosal injury and inflammation.

The actin cytoskeleton orchestra in Entamoeba histolytica

Years of evolution have kept actin conserved throughout various clades of life. It is an essential protein starring in many cellular processes. In a primitive eukaryote named Entamoeba histolytica, actin directs the process of phagocytosis. A finely tuned coordination between various actin-binding proteins (ABPs) choreographs this process and forms one of the virulence factors for this protist pathogen. The ever-expanding world of ABPs always has space to accommodate new and varied types of proteins to the earlier existing repertoire. In this article, we report the identification of 390 ABPs from Entamoeba histolytica. These proteins are part of diverse families that have been known to regulate actin dynamics. Most of the proteins are primarily uncharacterized in this organism; however, this study aims to annotate the ABPs based on their domain arrangements. A unique characteristic about some of the ABPs found is the combination of domains present in them unlike any other reported till date. Calponin domain-containing proteins formed the largest group among all types with 38 proteins, followed by 29 proteins with the infamous BAR domain in them, and 23 proteins belonging to actin-related proteins. The other protein families had a lesser number of members. Presence of exclusive domain arrangements in these proteins could guide us to yet unknown actin regulatory mechanisms prevalent in nature. This article is the first step to unraveling them.

Actin-binding compounds, previously discovered by FRET-based high-throughput screening, differentially affect skeletal and cardiac muscle

Actin's interactions with myosin and other actin-binding proteins are essential for cellular viability in numerous cell types, including muscle. In a previous high-throughput time-resolved FRET (TR-FRET) screen, we identified a class of compounds that bind to actin and affect actomyosin structure and function. For clinical utility, it is highly desirable to identify compounds that affect skeletal and cardiac muscle differently. Because actin is more highly conserved than myosin and most other muscle proteins, most such efforts have not targeted actin. Nevertheless, in the current study, we tested the specificity of the previously discovered actin-binding compounds for effects on skeletal and cardiac α-actins as well as on skeletal and cardiac myofibrils. We found that a majority of these compounds affected the transition of monomeric G-actin to filamentous F-actin, and that several of these effects were different for skeletal and cardiac actin isoforms. We also found that several of these compounds affected ATPase activity differently in skeletal and cardiac myofibrils. We conclude that these structural and biochemical assays can be used to identify actin-binding compounds that differentially affect skeletal and cardiac muscles. The results of this study set the stage for screening of large chemical libraries for discovery of novel compounds that act therapeutically and specifically on cardiac or skeletal muscle.

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.

Generation of an enteric smooth muscle cell line from the pig ileum

Smooth muscle cells (SMCs) play an important role in physiology and production in farm animals such as pigs. Here, we report the generation of a pig SMC line. Our original objective was to establish an enteroendocrine cell line from the pig ileum epithelium through lentiviral transduction of the Simian Virus (SV) 40 large T antigen. However, an initial expression analysis of marker genes in nine cell clones revealed that none of them were enteroendocrine cells or absorptive enterocytes, goblet cells, or Paneth cells, some of the major cell types existing in the ileum epithelium. A more detailed characterization of one clone named PIC7 by RNA-seq showed that these cells expressed many of the known smooth muscle-specific or -enriched genes, including smooth muscle actin alpha 2, calponin 1, calponin 3, myosin heavy chain 11, myosin light chain kinase, smoothelin, tenascin C, transgelin, tropomyosin 1, and tropomyosin 2. Both quantitative PCR and RNA-seq analyses showed that the PIC7 cells had a high expression of mRNA for smooth muscle actin gamma 2, also known as enteric smooth muscle actin. A Western blot analysis confirmed the expression of SV40 T antigen in the PIC7 cells. An immunohistochemical analysis demonstrated the expression of smooth muscle actin alpha 2 filaments in the PIC7 cells. A collagen gel contraction assay showed that the PIC7 cells were capable of both spontaneous contraction and contraction in response to serotonin stimulation. We conclude that the PIC7 cells are derived from an enteric SMC from the pig ileum. These cells may be a useful model for studying the cellular and molecular physiology of pig enteric SMCs. Because pigs are similar to humans in anatomy and physiology, the PIC7 cells may be also used as a model for human intestinal SMCs.

Yeast as a Model to Understand Actin-Mediated Cellular Functions in Mammals-Illustrated with Four Actin Cytoskeleton Proteins

The budding yeast Saccharomyces cerevisiae has an actin cytoskeleton that comprises a set of protein components analogous to those found in the actin cytoskeletons of higher eukaryotes. Furthermore, the actin cytoskeletons of S. cerevisiae and of higher eukaryotes have some similar physiological roles. The genetic tractability of budding yeast and the availability of a stable haploid cell type facilitates the application of molecular genetic approaches to assign functions to the various actin cytoskeleton components. This has provided information that is in general complementary to that provided by studies of the equivalent proteins of higher eukaryotes and hence has enabled a more complete view of the role of these proteins. Several human functional homologues of yeast actin effectors are implicated in diseases. A better understanding of the molecular mechanisms underpinning the functions of these proteins is critical to develop improved therapeutic strategies. In this article we chose as examples four evolutionarily conserved proteins that associate with the actin cytoskeleton: (1) yeast Hof1p/mammalian PSTPIP1, (2) yeast Rvs167p/mammalian BIN1, (3) yeast eEF1A/eEF1A1 and eEF1A2 and (4) yeast Yih1p/mammalian IMPACT. We compare the knowledge on the functions of these actin cytoskeleton-associated proteins that has arisen from studies of their homologues in yeast with information that has been obtained from in vivo studies using live animals or in vitro studies using cultured animal cell lines.

Neutrophil-like HL-60 cells expressing only GFP-tagged β-actin exhibit nearly normal motility

Observations of actin dynamics in living cells using fluorescence microscopy have been foundational in the exploration of the mechanisms underlying cell migration. We used CRISPR/Cas9 gene editing to generate neutrophil‐like HL‐60 cell lines expressing GFP‐β‐actin from the endogenous locus (ACTB). In light of many previous reports outlining functional deficiencies of labeled actin, we anticipated that HL‐60 cells would only tolerate a monoallelic edit, as biallelic edited cells would produce no normal β‐actin. Surprisingly, we recovered viable monoallelic GFP‐β‐actin cells as well as biallelic edited GFP‐β‐actin cells, in which one copy of the ACTB gene is silenced and the other contains the GFP tag. Furthermore, the edited cells migrate with similar speeds and persistence as unmodified cells in a variety of motility assays, and have nearly normal cell shapes. These results might partially be explained by our observation that GFP‐β‐actin incorporates into the F‐actin network in biallelic edited cells at similar efficiencies as normal β‐actin in unedited cells. Additionally, the edited cells significantly upregulate γ‐actin, perhaps helping to compensate for the loss of normal β‐actin. Interestingly, biallelic edited cells have only modest changes in global gene expression relative to the monoallelic line, as measured by RNA sequencing. While monoallelic edited cells downregulate expression of the tagged allele and are thus only weakly fluorescent, biallelic edited cells are quite bright and well‐suited for live cell microscopy. The nondisruptive phenotype and direct interpretability of this fluorescent tagging approach make it a promising tool for studying actin dynamics in these rapidly migrating and highly phagocytic cells.

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.

Stereocilia Rootlets: Actin-Based Structures That Are Essential for Structural Stability of the Hair Bundle

Sensory hair cells of the inner ear rely on the hair bundle, a cluster of actin-filled stereocilia, to transduce auditory and vestibular stimuli into electrical impulses. Because they are long and thin projections, stereocilia are most prone to damage at the point where they insert into the hair cell’s soma. Moreover, this is the site of stereocilia pivoting, the mechanical movement that induces transduction, which additionally weakens this area mechanically. To bolster this fragile area, hair cells construct a dense core called the rootlet at the base of each stereocilium, which extends down into the actin meshwork of the cuticular plate and firmly anchors the stereocilium. Rootlets are constructed with tightly packed actin filaments that extend from stereocilia actin filaments which are wrapped with TRIOBP; in addition, many other proteins contribute to the rootlet and its associated structures. Rootlets allow stereocilia to sustain innumerable deflections over their lifetimes and exemplify the unique manner in which sensory hair cells exploit actin and its associated proteins to carry out the function of mechanotransduction.

Impact of the actin cytoskeleton on cell development and function mediated via tropomyosin isoforms

The physiological function of actin filaments is challenging to dissect because of the pleiotropic impact of global disruption of the actin cytoskeleton. Tropomyosin isoforms have provided a unique opportunity to address this issue. A substantial fraction of actin filaments in animal cells consist of co-polymers of actin with specific tropomyosin isoforms which determine the functional capacity of the filament. Genetic manipulation of the tropomyosins has revealed isoform specific roles and identified the physiological function of the different actin filament types based on their tropomyosin isoform composition. Surprisingly, there is remarkably little redundancy between the tropomyosins resulting in highly penetrant impacts of both ectopic overexpression and knockout of isoforms. The physiological roles of the tropomyosins cover a broad range from development and morphogenesis to cell migration and specialised tissue function and human diseases.

Extracellular Matrix in Regulation of Contractile System in Cardiomyocytes

The contractile apparatus of cardiomyocytes is considered to be a stable system. However, it undergoes strong rearrangements during heart development as cells progress from their non-muscle precursors. Long-term culturing of mature cardiomyocytes is also accompanied by the reorganization of their contractile apparatus with the conversion of typical myofibrils into structures of non-muscle type. Processes of heart development as well as cell adaptation to culture conditions in cardiomyocytes both involve extracellular matrix changes, which appear to be crucial for the maturation of contractile apparatus. The aim of this review is to analyze the role of extracellular matrix in the regulation of contractile system dynamics in cardiomyocytes. Here, the remodeling of actin contractile structures and the expression of actin isoforms in cardiomyocytes during differentiation and adaptation to the culture system are described along with the extracellular matrix alterations. The data supporting the regulation of actin dynamics by extracellular matrix are highlighted and the possible mechanisms of such regulation are discussed.

The Role of Stroma in Ovarian Cancer

Background: Ovarian cancer is one of the gynecological malignancies responsible for thousands of deaths in women worldwide. Malignant solid tumors are formed by malignant cells and stroma that influence each other, where different types of cells in the stromal environment can be recruited by malignant cells to promote tumor growth and facilitate metastasis. The chronic inflammatory response is increasingly accepted in its relation to the pathophysiology of the onset and development of tumors, sustained cell proliferation in an environment rich in inflammatory cells, growth factors, activated stroma and DNA damage agents may increase the risk to develop a neoplasm.Methods: A search for the following keywords was performed in the PubMed database; "Ovarian cancer", "stroma", "tumor-associated macrophages", "cancer-associated fibroblasts", "cytokines", "angiogenesis", "epithelial-mesenchymal transition", and "extracellular matrix".Results: The articles identified were published in English between 1971 and 2018. A total of 154 articles were selected for further analysis. Conclusion: We consider ovarian cancer as a heterogeneous disease, not only in the sense that different histological or molecular subtypes may be behind the same clinical result, but also that multiple cell types besides cancer cells, like other non-cellular components, need to be mobilized and coordinated to support tumor survival, growth, invasion and progression.

Involvement of Actin in Autophagy and Autophagy-Dependent Multidrug Resistance in Cancer

Currently, autophagy in the context of cancer progression arouses a lot of controversy. It is connected with the possibility of switching the nature of this process from cytotoxic to cytoprotective and vice versa depending on the treatment. At the same time, autophagy of cytoprotective character may be one of the factors determining multidrug resistance, as intensification of the process is observed in patients with poorer prognosis. The exact mechanism of this relationship is not yet fully understood; however, it is suggested that one of the elements of the puzzle may be a cytoskeleton. In the latest literature reports, more and more attention is paid to the involvement of actin in the autophagy. The role of this protein is linked to the formation of autophagosomes, which are necessary element of the process. However, based on the proven effectiveness of manipulation of the actin pool, it seems to be an attractive alternative in breaking autophagy-dependent multidrug resistance in cancer.

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.