Spectrin, alpha-actinin, and dystrophin

UTRN as a potential biomarker in breast cancer: a comprehensive bioinformatics and in vitro study

Utrophin (UTRN), known as a tumor suppressor, potentially regulates tumor development and the immune microenvironment. However, its impact on breast cancer's development and treatment remains unstudied. We conducted a thorough examination of UTRN using both bioinformatic and in vitro experiments in this study. We discovered UTRN expression decreased in breast cancer compared to standard samples. High UTRN expression correlated with better prognosis. Drug sensitivity tests and RT-qPCR assays revealed UTRN's pivotal role in tamoxifen resistance. Furthermore, the Kruskal-Wallis rank test indicated UTRN's potential as a valuable diagnostic biomarker for breast cancer and its utility in detecting T stage of breast cancer. Additionally, our results demonstrated UTRN's close association with immune cells, inhibitors, stimulators, receptors, and chemokines in breast cancer (BRCA). This research provides a novel perspective on UTRN's role in breast cancer's prognostic and therapeutic value. Low UTRN expression may contribute to tamoxifen resistance and a poor prognosis. Specifically, UTRN can improve clinical decision-making and raise the diagnosis accuracy of breast cancer.

Role of a Pdlim5:PalmD complex in directing dendrite morphology

Neuronal connectivity is regulated during normal brain development with the arrangement of spines and synapses being dependent on the morphology of dendrites. Further, in multiple neurodevelopmental and aging disorders, disruptions of dendrite formation or shaping is associated with atypical neuronal connectivity. We showed previously that Pdlim5 binds delta-catenin and promotes dendrite branching. We report here that Pdlim5 interacts with PalmD, a protein previously suggested by others to interact with the cytoskeleton (e.g., via adducin/spectrin) and to regulate membrane shaping. Functionally, the knockdown of PalmD or Pdlim5 in rat primary hippocampal neurons dramatically reduces branching and conversely, PalmD exogenous expression promotes dendrite branching as does Pdlim5. Further, we show that each proteins' effects are dependent on the presence of the other. In summary, using primary rat hippocampal neurons we reveal the contributions of a novel Pdlim5:PalmD protein complex, composed of functionally inter-dependent components responsible for shaping neuronal dendrites.

Unveiling Vacuolar H+-ATPase Subunit a as the Primary Target of the Pyridinylmethyl-Benzamide Fungicide, Fluopicolide

An estimated 240 fungicides are presently in use, but the direct targets for the majority remain elusive, constraining fungicide development and efficient resistance monitoring. In this study, we found that Pcα-actinin knockout did not influence the sensitivity of Phytophthora capsici to fluopicolide, which is a notable oomycete inhibitor. Using a combination of Bulk Segregant Analysis Sequencing and Drug Affinity Responsive Target Stability (DARTS) assays, the vacuolar H+-ATPase subunit a (PcVHA-a) was pinpointed as the target protein of fluopicolide. We also confirmed four distinct point mutations in PcVHA-a responsible for fluopicolide resistance in P. capsici through site-directed mutagenesis. Molecular docking, ATPase activity assays, and a DARTS assay suggested a fluopicolide-PcVHA-a interaction. Sequence analysis and further molecular docking validated the specificity of fluopicolide for oomycetes or fish. These findings support the claim that PcVHA-a is the target of fluopicolide, proposing vacuolar H+-ATPase as a promising target for novel fungicide development.

SPTBN1 Mediates the Cytoplasmic Constraint of PTTG1, Impairing Its Oncogenic Activity in Human Seminoma

Seminoma is the most common testicular cancer. Pituitary tumor-transforming gene 1 (PTTG1) is a securin showing oncogenic activity in several tumors. We previously demonstrated that nuclear PTTG1 promotes seminoma tumor invasion through its transcriptional activity on matrix metalloproteinase 2 (MMP-2) and E-cadherin (CDH1). We wondered if specific interactors could affect its subcellular distribution. To this aim, we investigated the PTTG1 interactome in seminoma cell lines showing different PTTG1 nuclear levels correlated with invasive properties. A proteomic approach upon PTTG1 immunoprecipitation uncovered new specific securin interactors. Western blot, confocal microscopy, cytoplasmic/nuclear fractionation, sphere-forming assay, and Atlas database interrogation were performed to validate the proteomic results and to investigate the interplay between PTTG1 and newly uncovered partners. We observed that spectrin beta-chain (SPTBN1) and PTTG1 were cofactors, with SPTBN1 anchoring the securin in the cytoplasm. SPTBN1 downregulation determined PTTG1 nuclear translocation, promoting its invasive capability. Moreover, a PTTG1 deletion mutant lacking SPTBN1 binding was strongly localized in the nucleus. The Atlas database revealed that seminomas that contained higher nuclear PTTG1 levels showed significantly lower SPTBN1 levels in comparison to non-seminomas. In human seminoma specimens, we found a strong PTTG1/SPTBN1 colocalization that decreases in areas with nuclear PTTG1 distribution. Overall, these results suggest that SPTBN1, along with PTTG1, is a potential prognostic factor useful in the clinical management of seminoma.

Role of a Pdlim5:PalmD complex in directing dendrite morphology

Neuronal connectivity is regulated during normal brain development with the arrangement of spines and synapses being dependent on the morphology of dendrites. Further, in multiple neurodevelopmental and aging disorders, disruptions of dendrite formation or shaping is associated with atypical neuronal connectivity. We showed previously that Pdlim5 binds delta-catenin and promotes dendrite branching (Baumert et al., J Cell Biol 2020). We report here that Pdlim5 interacts with PalmD, a protein previously suggested by others to interact with the cytoskeleton (e.g., via adducin/ spectrin) and to regulate membrane shaping. Functionally, the knockdown of PalmD or Pdlim5 in rat primary hippocampal neurons dramatically reduces branching and conversely, PalmD exogenous expression promotes dendrite branching as does Pdlim5. Further, we show that effects of each protein are dependent on the presence of the other. In summary, using primary rat hippocampal neurons we reveal the contributions of a novel Pdlim5:PalmD protein complex, composed of functionally inter-dependent components responsible for shaping neuronal dendrites.

Actin Bundles Dynamics and Architecture

Cells use the actin cytoskeleton for many of their functions, including their division, adhesion, mechanosensing, endo- and phagocytosis, migration, and invasion. Actin bundles are the main constituent of actin-rich structures involved in these processes. An ever-increasing number of proteins that crosslink actin into bundles or regulate their morphology is being identified in cells. With recent advances in high-resolution microscopy and imaging techniques, the complex process of bundles formation and the multiple forms of physiological bundles are beginning to be better understood. Here, we review the physiochemical and biological properties of four families of highly conserved and abundant actin-bundling proteins, namely, α-actinin, fimbrin/plastin, fascin, and espin. We describe the similarities and differences between these proteins, their role in the formation of physiological actin bundles, and their properties-both related and unrelated to their bundling abilities. We also review some aspects of the general mechanism of actin bundles formation, which are known from the available information on the activity of the key actin partners involved in this process.

ACTN2 Mutant Causes Proteopathy in Human iPSC-Derived Cardiomyocytes

Genetic variants in α-actinin-2 (ACTN2) are associated with several forms of (cardio)myopathy. We previously reported a heterozygous missense (c.740C>T) ACTN2 gene variant, associated with hypertrophic cardiomyopathy, and characterized by an electro-mechanical phenotype in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). Here, we created with CRISPR/Cas9 genetic tools two heterozygous functional knock-out hiPSC lines with a second wild-type (ACTN2wt) and missense ACTN2 (ACTN2mut) allele, respectively. We evaluated their impact on cardiomyocyte structure and function, using a combination of different technologies, including immunofluorescence and live cell imaging, RNA-seq, and mass spectrometry. This study showed that ACTN2mut presents a higher percentage of multinucleation, protein aggregation, hypertrophy, myofibrillar disarray, and activation of both the ubiquitin-proteasome system and the autophagy-lysosomal pathway as compared to ACTN2wt in 2D-cultured hiPSC-CMs. Furthermore, the expression of ACTN2mut was associated with a marked reduction of sarcomere-associated protein levels in 2D-cultured hiPSC-CMs and force impairment in engineered heart tissues. In conclusion, our study highlights the activation of proteolytic systems in ACTN2mut hiPSC-CMs likely to cope with ACTN2 aggregation and therefore directs towards proteopathy as an additional cellular pathology caused by this ACTN2 variant, which may contribute to human ACTN2-associated cardiomyopathies.

Role of Spectrin in Endocytosis

Cytoskeletal spectrin is found in (non)erythroid cells. Eukaryotic endocytosis takes place for internalizing cargos from extracellular milieu. The role of spectrin in endocytosis still remains poorly understood. Here, I summarize current knowledge of spectrin function, spectrin-based cytoskeleton and endocytosis of erythrocytes, and highlight how spectrin contributes to endocytosis and working models in different types of cells. From an evolutionary viewpoint, I discuss spectrin and endocytosis in a range of organisms, particularly in plants and yeast where spectrin is absent. Together, the role of spectrin in endocytosis is related to its post-translational modification, movement/rearrangement, elimination (by proteases) and meshwork fencing.

Ancient Origins of Cytoskeletal Crosstalk: Spectraplakin-like Proteins Precede the Emergence of Cortical Microtubule Stabilization Complexes as Crosslinkers

Adhesion between cells and the extracellular matrix (ECM) is one of the prerequisites for multicellularity, motility, and tissue specialization. Focal adhesions (FAs) are defined as protein complexes that mediate signals from the ECM to major components of the cytoskeleton (microtubules, actin, and intermediate filaments), and their mutual communication determines a variety of cellular processes. In this study, human cytoskeletal crosstalk proteins were identified by comparing datasets with experimentally determined cytoskeletal proteins. The spectraplakin dystonin was the only protein found in all datasets. Other proteins (FAK, RAC1, septin 9, MISP, and ezrin) were detected at the intersections of FAs, microtubules, and actin cytoskeleton. Homology searches for human crosstalk proteins as queries were performed against a predefined dataset of proteomes. This analysis highlighted the importance of FA communication with the actin and microtubule cytoskeleton, as these crosstalk proteins exhibit the highest degree of evolutionary conservation. Finally, phylogenetic analyses elucidated the early evolutionary history of spectraplakins and cortical microtubule stabilization complexes (CMSCs) as model representatives of the human cytoskeletal crosstalk. While spectraplakins probably arose at the onset of opisthokont evolution, the crosstalk between FAs and microtubules is associated with the emergence of metazoans. The multiprotein complexes contributing to cytoskeletal crosstalk in animals gradually gained in complexity from the onset of metazoan evolution.

Spectrins and human diseases

Spectrin, as one of the major components of a plasma membrane-associated cytoskeleton, is a cytoskeletal protein composed of the modular structure of α and β subunits. The spectrin-based skeleton is essential for preserving the integrity and mechanical characteristics of the cell membrane. Moreover, spectrin regulates a variety of cell processes including cell apoptosis, cell adhesion, cell spreading, and cell cycle. Dysfunction of spectrins is implicated in various human diseases including hemolytic anemia, neurodegenerative diseases, ataxia, heart diseases, and cancers. Here, we briefly discuss spectrins function as well as the clinical manifestations and currently known molecular mechanisms of human diseases related to spectrins, highlighting that strategies for targeting regulation of spectrins function may provide new avenues for therapeutic intervention for these diseases.

Plasma membrane disruption (PMD) formation and repair in mechanosensitive tissues

Mammalian cells employ an array of biological mechanisms to detect and respond to mechanical loading in their environment. One such mechanism is the formation of plasma membrane disruptions (PMD), which foster a molecular flux across cell membranes that promotes tissue adaptation. Repair of PMD through an orchestrated activity of molecular machinery is critical for cell survival, and the rate of PMD repair can affect downstream cellular signaling. PMD have been observed to influence the mechanical behavior of skin, alveolar, and gut epithelial cells, aortic endothelial cells, corneal keratocytes and epithelial cells, cardiac and skeletal muscle myocytes, neurons, and most recently, bone cells including osteoblasts, periodontal ligament cells, and osteocytes. PMD are therefore positioned to affect the physiological behavior of a wide range of vertebrate organ systems including skeletal and cardiac muscle, skin, eyes, the gastrointestinal tract, the vasculature, the respiratory system, and the skeleton. The purpose of this review is to describe the processes of PMD formation and repair across these mechanosensitive tissues, with a particular emphasis on comparing and contrasting repair mechanisms and downstream signaling to better understand the role of PMD in skeletal mechanobiology. The implications of PMD-related mechanisms for disease and potential therapeutic applications are also explored.

Structural and functional characterization of a plant alpha-actinin

The Australian tree malletwood (Rhodamnia argentea) is unique. The genome of malletwood is the only known plant genome that contains a gene coding for an α‐actinin‐like protein. Several organisms predating the animal‐plant bifurcation express an α‐actinin or α‐actinin‐like protein. Therefore, it appears that plants in general, but not malletwood, have lost the α‐actinin or α‐actinin‐like gene during evolution. In order to characterize its structure and function, we synthesized the gene and expressed the recombinant R. argentea protein. The results clearly show that this protein has all properties of genuine α‐actinin. The N‐terminal actin‐binding domain (ABD), with two calponin homology motifs, is very similar to the ABD of any α‐actinin. The C‐terminal calmodulin‐like domain, as well as the intervening rod domain, are also similar to the corresponding regions in other α‐actinins. The R. argentea α‐actinin‐like protein dimerises in solution and thereby can cross‐link actin filaments. Based on these results, we believe the R. argentea protein represents a genuine α‐actinin, making R. argentea unique in the plant world.

Cytoskeletal Transport, Reorganization, and Fusion Regulation in Mast Cell-Stimulus Secretion Coupling

Mast cells are well known for their role in allergies and many chronic inflammatory diseases. They release upon stimulation, e.g., via the IgE receptor, numerous bioactive compounds from cytoplasmic secretory granules. The regulation of granule secretion and its interaction with the cytoskeleton and transport mechanisms has only recently begun to be understood. These studies have provided new insight into the interaction between the secretory machinery and cytoskeletal elements in the regulation of the degranulation process. They suggest a tight coupling of these two systems, implying a series of specific signaling effectors and adaptor molecules. Here we review recent knowledge describing the signaling events regulating cytoskeletal reorganization and secretory granule transport machinery in conjunction with the membrane fusion machinery that occur during mast cell degranulation. The new insight into MC biology offers novel strategies to treat human allergic and inflammatory diseases targeting the late steps that affect harmful release from granular stores leaving regulatory cytokine secretion intact.

UTRN inhibits melanoma growth by suppressing p38 and JNK/c-Jun signaling pathways

Background

Utrophin (UTRN), as a tumor suppressor gene, is involved in various cancer progression. The function of UTRN in the melanoma process and the related molecular mechanisms are still unclear. Herein, we studied the function of UTRN in melanoma growth and the relevant molecular mechanisms.

Methods

Using the GEO database and UCSC Xena project, we compared the expression of UTRN in non-cancerous and melanoma tissues. Immunohistochemistry (IHC) staining, qRT-PCR and Western Blot (WB) were performed to evaluate UTRN expression in clinical samples. A total of 447 cases with UTRN expression data, patient characteristics and survival data were extracted from TCGA database and analyzed. After stable transduction and single cell cloning, the proliferation ability of A375 human melanoma cells was analyzed by Cell Counting Kit‑8 (CCK) and 5‑ethynyl‑2′‑deoxyuridine (EdU) incorporation assays. GSEA was performed to predict the mechanism by which UTRN regulated melanoma growth. Then WB analysis was used to assess the protein expression levels of pathway signaling in overexpression (EXP) melanoma cells. Epac activator 8-pCPT-2′-O-Me-cAMP was then used to evaluate the proliferation ability by activation of p38 and JNK/c-Jun signaling pathways.

Results

Data from GEO and UCSC Xena project indicated that UTRN expression was decreased in melanoma. Experiment on clinical samples further confirmed our finding. TCGA results showed that a reduced expression of UTRN in 447 melanoma samples was associated with advanced clinical characteristics (T stage, Clark level, ulceration), shorter survival time and poorer prognosis. In addition, up-regulated UTRN expression inhibited melanoma cell proliferation when compared to control group. MAPK signaling pathway was presented in both KEGG and BioCarta databases by using GSEA tool. WB results confirmed the down-regulated expression of p38, JNK1 and c-Jun in EXP group when compared to control group. Epac activator 8-pCPT-2′-O-Me-cAMP treatment could partially rescue proliferation of tumor cells.

Conclusion

We have demonstrated that reduced UTRN predicted poorer prognosis and UTRN inhibited melanoma growth via p38 and JNK1/c-Jun pathways. Therefore, UTRN could serve as a tumor suppressor and novel prognostic biomarker for melanoma patients.

βII spectrin (SPTBN1): biological function and clinical potential in cancer and other diseases

βII spectrin, the most common isoform of non-erythrocyte spectrin, is a cytoskeleton protein present in all nucleated cells. Interestingly, βII spectrin is essential for the development of various organs such as nerve, epithelium, inner ear, liver and heart. The functions of βII spectrin include not only establishing and maintaining the cell structure but also regulating a variety of cellular functions, such as cell apoptosis, cell adhesion, cell spreading and cell cycle regulation. Notably, βII spectrin dysfunction is associated with embryonic lethality and the DNA damage response. More recently, the detection of altered βII spectrin expression in tumors indicated that βII spectrin might be involved in the development and progression of cancer. Its mutations and disorders could result in developmental disabilities and various diseases. The versatile roles of βII spectrin in disease have been examined in an increasing number of studies; nonetheless, the exact mechanisms of βII spectrin are still poorly understood. Thus, we summarize the structural features and biological roles of βII spectrin and discuss its molecular mechanisms and functions in development, homeostasis, regeneration and differentiation. This review highlight the potential effects of βII spectrin dysfunction in cancer and other diseases, outstanding questions for the future investigation of therapeutic targets. The investigation of the regulatory mechanism of βII spectrin signal inactivation and recovery may bring hope for future therapy of related diseases.

N-Terminus of the Third PDZ Domain of PSD-95 Orchestrates Allosteric Communication for Selective Ligand Binding

PDZ domains constitute common models to study single-domain allostery without significant structural changes. The third PDZ domain of PSD-95 (PDZ3) is known to have selective structural features that confer unique modulatory roles to this unit. In this model system, two residues, H372 directly connected to the binding site and G330 holding an off-binding-site position, were designated to assess the effect of mutations on binding selectivity. It has been observed that the H372A and G330T-H372A mutations change ligand preferences from class I (T/S amino acid at position -2 of the ligand) to class II (hydrophobic amino acid at the same position). Alternatively, the G330T single mutation leads to the recognition of both ligand classes. We have performed a series of molecular dynamics (MD) simulations for wild-type, H372A, and G330T single mutants and a double mutant of PDZ3 in the absence and presence of both types of ligands. With the combination of free-energy difference calculations and a detailed analysis of MD trajectories, "class switching" and "class bridging" behavior of PDZ3 mutants, as well as their effects on ligand selection and binding affinities are explained. We show that the dynamics of the charged N-terminus plays a fundamental role in determining the binding preferences in PDZ3 by altering the electrostatic energy. These findings are corroborated by simulations on N-terminus-truncated versions of these systems. The dynamical allostery orchestrated by the N-terminus offers a fresh perspective to the study of communication pathways in proteins.

The intracellular bacterium Rickettsia rickettsii exerts an inhibitory effect on the apoptosis of tick cells

BACKGROUND

Rickettsia rickettsii is a tick-borne obligate intracellular bacterium that causes Rocky Mountain spotted fever, a life-threatening illness. To obtain an insight into the vector-pathogen interactions, we assessed the effects of infection with R. rickettsii on the proteome cells of the tick embryonic cell line BME26.

METHODS

The proteome of BME26 cells was determined by label-free high-performance liquid chromatography coupled with tandem mass spectrometry analysis. Also evaluated were the effects of infection on the activity of caspase-3, assessed by the hydrolysis of a synthetic fluorogenic substrate in enzymatic assays, and on the exposition of phosphatidyserine, evaluated by live-cell fluorescence microscopy after labeling with annexin-V. Finally, the effects of activation or inhibition of caspase-3 activity on the growth of R. rickettsii in BME26 cells was determined.

RESULTS

Tick proteins of different functional classes were modulated in a time-dependent manner by R. rickettsii infection. Regarding proteins involved in apoptosis, certain negative regulators were downregulated at the initial phase of the infection (6 h) but upregulated in the middle of the exponential phase of the bacterial growth (48 h). Microorganisms are known to be able to inhibit apoptosis of the host cell to ensure their survival and proliferation. We therefore evaluated the effects of infection on classic features of apoptotic cells and observed DNA fragmentation exclusively in noninfected cells. Moreover, both caspase-3 activity and phosphatidylserine exposition were lower in infected than in noninfected cells. Importantly, while the activation of caspase-3 exerted a detrimental effect on rickettsial proliferation, its inhibition increased bacterial growth.

CONCLUSIONS

Taken together, these results show that R. rickettsii modulates the proteome and exerts an inhibitory effect on apoptosis in tick cellsthat seems to be important to ensure cell colonization.

A Fresh Look at the Structure, Regulation, and Functions of Fodrin

Fodrin and its erythroid cell-specific isoform spectrin are actin-associated fibrous proteins that play crucial roles in the maintenance of structural integrity in mammalian cells, which is necessary for proper cell function. Normal cell morphology is altered in diseases such as various cancers and certain neuronal disorders. Fodrin and spectrin are two-chain (αβ) molecules that are encoded by paralogous genes and share many features but also demonstrate certain differences. Fodrin (in humans, typically a heterodimer of the products of the SPTAN1 and SPTBN1 genes) is expressed in nearly all cell types and is especially abundant in neuronal tissues, whereas spectrin (in humans, a heterodimer of the products of the SPTA1 and SPTB1 genes) is expressed almost exclusively in erythrocytes. To fulfill a role in such a variety of different cell types, it was anticipated that fodrin would need to be a more versatile scaffold than spectrin. Indeed, as summarized here, domains unique to fodrin and its regulation by Ca²⁺, calmodulin, and a variety of posttranslational modifications (PTMs) endow fodrin with additional specific functions. However, how fodrin structural variations and misregulated PTMs may contribute to the etiology of various cancers and neurodegenerative diseases needs to be further investigated.

Nonsense mutations in alpha-II spectrin in three families with juvenile onset hereditary motor neuropathy

Distal hereditary motor neuropathies are a rare subgroup of inherited peripheral neuropathies hallmarked by a length-dependent axonal degeneration of lower motor neurons without significant involvement of sensory neurons. We identified patients with heterozygous nonsense mutations in the αII-spectrin gene, SPTAN1, in three separate dominant hereditary motor neuropathy families via next-generation sequencing. Variable penetrance was noted for these mutations in two of three families, and phenotype severity differs greatly between patients. The mutant mRNA containing nonsense mutations is broken down by nonsense-mediated decay and leads to reduced protein levels in patient cells. Previously, dominant-negative αII-spectrin gene mutations were described as causal in a spectrum of epilepsy phenotypes.

Mechanical Unfolding of Spectrin Repeats Induces Water-Molecule Ordering

Mechanical processes are involved at many stages of the development of living cells, and often external forces applied to a biomolecule result in its unfolding. Although our knowledge of the unfolding mechanisms and the magnitude of the forces involved has evolved, the role that water molecules play in the mechanical unfolding of biomolecules has not yet been fully elucidated. To this end, we investigated with steered molecular dynamics simulations the mechanical unfolding of dystrophin's spectrin repeat 1 and related the changes in the protein's structure to the ordering of the surrounding water molecules. Our results indicate that upon mechanically induced unfolding of the protein, the solvent molecules become more ordered and increase their average number of hydrogen bonds. In addition, the unfolded structures originating from mechanical pulling expose an increasing amount of the hydrophobic residues to the solvent molecules, and the uncoiled regions adapt a convex surface with a small radius of curvature. As a result, the solvent molecules reorganize around the protein's small protrusions in structurally ordered waters that are characteristic of the so-called "small-molecule regime," which allows water to maintain a high hydrogen bond count at the expense of an increased structural order. We also determined that the response of water to structural changes in the protein is localized to the specific regions of the protein that undergo unfolding. These results indicate that water plays an important role in the mechanically induced unfolding of biomolecules. Our findings may prove relevant to the ever-growing interest in understanding macromolecular crowding in living cells and their effects on protein folding, and suggest that the hydration layer may be exploited as a means for short-range allosteric communication.

Crystal structure of the N-terminal domain of the major virulence factor BB0323 from the Lyme disease agent Borrelia burgdorferi

Lyme disease is an infection caused by the spirochete Borrelia burgdorferi after it is transmitted to a mammalian organism during a tick blood meal. B. burgdorferi encodes at least 140 lipoproteins located on the outer or inner membrane, thus facing the surroundings or the periplasmic space, respectively. However, most of the predicted lipoproteins are of unknown function, and only a few proteins are known to be essential for the persistence and virulence of the pathogen. One such protein is the periplasmic BB0323, which is indispensable for B. burgdorferi to cause Lyme disease and the function of which is associated with cell fission and outer membrane integrity. After expression and transport to the periplasm, BB0323 is cleaved into C-terminal and N-terminal domains by the periplasmic serine protease BB0104. The resulting N-terminal domain is sufficient to ensure the survival of B. burgdorferi throughout the mouse-tick infection cycle. The crystal structure of the N-terminal domain of BB0323 was determined at 2.35 Å resolution. The overall fold of the protein belongs to the spectrin superfamily, with the characteristic interconnected triple-helical bundles known as spectrin repeats that function as linkers between different cell components in other organisms. Overall, the reported three-dimensional structure of the N-terminal domain of BB0323 not only reveals the molecular details of a protein that is essential for B. burgdorferi membrane integrity, cell fission and infectivity, but also suggests that spectrin repeats in bacteria are not limited to the EzrA proteins.

Proteins with calmodulin-like domains: structures and functional roles

The appearance of modular proteins is a widespread phenomenon during the evolution of proteins. The combinatorial arrangement of different functional and/or structural domains within a single polypeptide chain yields a wide variety of activities and regulatory properties to the modular proteins. In this review, we will discuss proteins, that in addition to their catalytic, transport, structure, localization or adaptor functions, also have segments resembling the helix-loop-helix EF-hand motifs found in Ca2+-binding proteins, such as calmodulin (CaM). These segments are denoted CaM-like domains (CaM-LDs) and play a regulatory role, making these CaM-like proteins sensitive to Ca2+ transients within the cell, and hence are able to transduce the Ca2+ signal leading to specific cellular responses. Importantly, this arrangement allows to this group of proteins direct regulation independent of other Ca2+-sensitive sensor/transducer proteins, such as CaM. In addition, this review also covers CaM-binding proteins, in which their CaM-binding site (CBS), in the absence of CaM, is proposed to interact with other segments of the same protein denoted CaM-like binding site (CLBS). CLBS are important regulatory motifs, acting either by keeping these CaM-binding proteins inactive in the absence of CaM, enhancing the stability of protein complexes and/or facilitating their dimerization via CBS/CLBS interaction. The existence of proteins containing CaM-LDs or CLBSs substantially adds to the enormous versatility and complexity of Ca2+/CaM signaling.

Systemic AAV Micro-dystrophin Gene Therapy for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a lethal muscle disease caused by dystrophin gene mutation. Conceptually, replacing the mutated gene with a normal one would cure the disease. However, this task has encountered significant challenges due to the enormous size of the gene and the distribution of muscle throughout the body. The former creates a hurdle for viral vector packaging and the latter begs for whole-body therapy. To address these obstacles, investigators have invented the highly abbreviated micro-dystrophin gene and developed body-wide systemic gene transfer with adeno-associated virus (AAV). Numerous microgene configurations and various AAV serotypes have been explored in animal models in many laboratories. Preclinical data suggests that intravascular AAV micro-dystrophin delivery can significantly ameliorate muscle pathology, enhance muscle force, and attenuate dystrophic cardiomyopathy in animals. Against this backdrop, several clinical trials have been initiated to test the safety and tolerability of this promising therapy in DMD patients. While these trials are not powered to reach a conclusion on clinical efficacy, findings will inform the field on the prospects of body-wide DMD therapy with a synthetic micro-dystrophin AAV vector. This review discusses the history, current status, and future directions of systemic AAV micro-dystrophin therapy.

Biochemical and Functional Interplay Between Ion Channels and the Components of the Dystrophin-Associated Glycoprotein Complex

Dystrophin is a cytoskeleton-linked membrane protein that binds to a larger multiprotein assembly called the dystrophin-associated glycoprotein complex (DGC). The deficiency of dystrophin or the components of the DGC results in the loss of connection between the cytoskeleton and the extracellular matrix with significant pathophysiological implications in skeletal and cardiac muscle as well as in the nervous system. Although the DGC plays an important role in maintaining membrane stability, it can also be considered as a versatile and flexible molecular complex that contribute to the cellular organization and dynamics of a variety of proteins at specific locations in the plasma membrane. This review deals with the role of the DGC in transmembrane signaling by forming supramolecular assemblies for regulating ion channel localization and activity. These interactions are relevant for cell homeostasis, and its alterations may play a significant role in the etiology and pathogenesis of various disorders affecting muscle and nerve function.

α-Helix Unwinding as Force Buffer in Spectrins

Spectrins are cytoskeletal proteins located at the inner face of the plasma membrane, making connections between membrane anchors and the actin cortex, and between actin filaments. Spectrins share a common structure forming a bundle of 3 α-helices and play a major role during cell deformation. Here, we used high-speed force spectroscopy and steered molecular dynamics simulations to understand the mechanical stability of spectrin, revealing a molecular force buffering function. We find that spectrin acts as a soft spring at short extensions (70-100 Å). Under continuous external stretching, its α-helices unwind, leading to a viscous mechanical response over larger extensions (100-300 Å), represented by a constant-force plateau in force/extension curves. This viscous force buffering emerges from a quasi-equilibrium competition between disruption and re-formation of α-helical hydrogen bonds. Our results suggest that, in contrast to β-sheet proteins, which unfold in a catastrophic event, α-helical spectrins dominantly unwind, providing a viscous force buffer over extensions about 5 times their folded length.

Alpha-actinin of the chlorarchiniophyte Bigelowiella natans

The genome of the chlorarchiniophyte Bigelowiella natans codes for a protein annotated as an α-actinin-like protein. Analysis of the primary sequence indicate that this protein has the same domain structure as other α-actinins, a N-terminal actin-binding domain and a C-terminal calmodulin-like domain. These two domains are connected by a short rod domain, albeit long enough to form a single spectrin repeat. To analyse the functional properties of this protein, the full-length protein as well as the separate domains were cloned and isolated. Characerisation showed that the protein is capable of cross-linking actin filaments into dense bundles, probably due to dimer formation. Similar to human α-actinin, calcium-binding occurs to the most N-terminal EF-hand motif in the calmodulin-like C-terminal domain. The results indicate that this Bigelowiella protein is a proper α-actinin, with all common characteristics of a typical α-actinin.

Spectraplakin family proteins - cytoskeletal crosslinkers with versatile roles

The different cytoskeletal networks in a cell are responsible for many fundamental cellular processes. Current studies have shown that spectraplakins, cytoskeletal crosslinkers that combine features of both the spectrin and plakin families of crosslinkers, have a critical role in integrating these different cytoskeletal networks. Spectraplakin genes give rise to a variety of isoforms that have distinct functions. Importantly, all spectraplakin isoforms are uniquely able to associate with all three elements of the cytoskeleton, namely, F-actin, microtubules and intermediate filaments. In this Review, we will highlight recent studies that have unraveled their function in a wide range of different processes, from regulating cell adhesion in skin keratinocytes to neuronal cell migration. Taken together, this work has revealed a diverse and indispensable role for orchestrating the function of different cytoskeletal elements in vivo.

BPAG1, a distinctive role in skin and neurological diseases

Spectraplakins are multifunctional cytoskeletal linker proteins that act as important communicators, connecting cytoskeletal components with each other and to cellular junctions. Bullous pemphigoid antigen 1 (BPAG1)/dystonin is a member of spectraplakin family and expressed in various tissues. Alternative splicing of BPAG1 gene produces various isoforms with unique structure and domains. BPAG1 plays crucial roles in numerous biological processes, such as cytoskeleton organization, cell polarization, cell adhesion, and cell migration as well as signaling transduction. Genetic mutation of BPAG1 isoforms is the miscreant of epidermolysis bullosa and multifarious, destructive neurological diseases. In this review, we summarize the recent advances of BPAG1's role in various biological processes and in skin and neurological diseases.

Tropomodulin 1 Regulation of Actin Is Required for the Formation of Large Paddle Protrusions Between Mature Lens Fiber Cells

Purpose

To elucidate the proteins required for specialized small interlocking protrusions and large paddle domains at lens fiber cell tricellular junctions (vertices), we developed a novel method to immunostain single lens fibers and studied changes in cell morphology due to loss of tropomodulin 1 (Tmod1), an F-actin pointed end–capping protein.

Methods

We investigated F-actin and F-actin–binding protein localization in interdigitations of Tmod1^+/+ and Tmod1^−/− single mature lens fibers.

Results

F-actin–rich small protrusions and large paddles were present along cell vertices of Tmod1^+/+ mature fibers. In contrast, Tmod1^−/− mature fiber cells lack normal paddle domains, while small protrusions were unaffected. In Tmod1^+/+ mature fibers, Tmod1, β2-spectrin, and α-actinin are localized in large puncta in valleys between paddles; but in Tmod1^−/− mature fibers, β2-spectrin was dispersed while α-actinin was redistributed at the base of small protrusions and rudimentary paddles. Fimbrin and Arp3 (actin-related protein 3) were located in puncta at the base of small protrusions, while N-cadherin and ezrin outlined the cell membrane in both Tmod1^+/+ and Tmod1^−/− mature fibers.

Conclusions

These results suggest that distinct F-actin organizations are present in small protrusions versus large paddles. Formation and/or maintenance of large paddle domains depends on a β2-spectrin–actin network stabilized by Tmod1. α-Actinin–crosslinked F-actin bundles are enhanced in absence of Tmod1, indicating altered cytoskeleton organization. Formation of small protrusions is likely facilitated by Arp3-branched and fimbrin-bundled F-actin networks, which do not depend on Tmod1. This is the first work to reveal the F-actin–associated proteins required for the formation of paddles between lens fibers.

Regulation of bacterial cell wall growth

During growth and propagation, a bacterial cell enlarges and subsequently divides its peptidoglycan (PG) sacculus, a continuous mesh-like layer that encases the cell membrane to confer mechanical strength and morphological robustness. The mechanism of sacculus growth, how it is regulated and how it is coordinated with other cellular processes is poorly understood. In this article, we will discuss briefly the current knowledge of how cell wall synthesis is regulated, on multiple levels, from both sides of the cytoplasmic membrane. According to the current knowledge, cytosolic scaffolding proteins connect PG synthases with cytoskeletal elements, and protein phosphorylation regulates cell wall growth in Gram-positive species. PG-active enzymes engage in multiple protein-protein interactions within PG synthesis multienzyme complexes, and some of the interactions modulate activities. PG synthesis is also regulated by central metabolism, and by PG maturation through the action of PG hydrolytic enzymes. Only now are we beginning to appreciate how these multiple levels of regulating PG synthesis enable the cell to propagate robustly with a defined cell shape under different and variable growth conditions.

Structural implications of Ca2+-dependent actin-bundling function of human EFhd2/Swiprosin-1

EFhd2/Swiprosin-1 is a cytoskeletal Ca²⁺-binding protein implicated in Ca²⁺-dependent cell spreading and migration in epithelial cells. EFhd2 domain architecture includes an N-terminal disordered region, a PxxP motif, two EF-hands, a ligand mimic helix and a C-terminal coiled-coil domain. We reported previously that EFhd2 displays F-actin bundling activity in the presence of Ca²⁺ and this activity depends on the coiled-coil domain and direct interaction of the EFhd2 core region. However, the molecular mechanism for the regulation of F-actin binding and bundling by EFhd2 is unknown. Here, the Ca²⁺-bound crystal structure of the EFhd2 core region is presented and structures of mutants defective for Ca²⁺-binding are also described. These structures and biochemical analyses reveal that the F-actin bundling activity of EFhd2 depends on the structural rigidity of F-actin binding sites conferred by binding of the EF-hands to Ca²⁺. In the absence of Ca²⁺, the EFhd2 core region exhibits local conformational flexibility around the EF-hand domain and C-terminal linker, which retains F-actin binding activity but loses the ability to bundle F-actin. In addition, we establish that dimerisation of EFhd2 via the C-terminal coiled-coil domain, which is necessary for F-actin bundling, occurs through the parallel coiled-coil interaction.

Cytoskeletal Integrators: The Spectrin Superfamily

This review discusses the spectrin superfamily of proteins that function to connect cytoskeletal elements to each other, the cell membrane, and the nucleus. The signature domain is the spectrin repeat, a 106-122-amino-acid segment comprising three α-helices. α-actinin is considered to be the ancestral protein and functions to cross-link actin filaments. It then evolved to generate spectrin and dystrophin that function to link the actin cytoskeleton to the cell membrane, as well as the spectraplakins and plakins that link cytoskeletal elements to each other and to junctional complexes. A final class comprises the nesprins, which are able to bind to the nuclear membrane. This review discusses the domain organization of the various spectrin family members, their roles in protein-protein interactions, and their roles in disease, as determined from mutations, and it also describes the functional roles of the family members as determined from null phenotypes.

The F-actin bundler α-actinin Ain1 is tailored for ring assembly and constriction during cytokinesis in fission yeast

The actomyosin contractile ring is a network of cross-linked actin filaments that facilitates cytokinesis in dividing cells. Contractile ring formation has been well characterized in Schizosaccharomyces pombe, in which the cross-linking protein α-actinin SpAin1 bundles the actin filament network. However, the specific biochemical properties of SpAin1 and whether they are tailored for cytokinesis are not known. Therefore we purified SpAin1 and quantified its ability to dynamically bind and bundle actin filaments in vitro using a combination of bulk sedimentation assays and direct visualization by two-color total internal reflection fluorescence microscopy. We found that, while SpAin1 bundles actin filaments of mixed polarity like other α-actinins, SpAin1 has lower bundling activity and is more dynamic than human α-actinin HsACTN4. To determine whether dynamic bundling is important for cytokinesis in fission yeast, we created the less dynamic bundling mutant SpAin1(R216E). We found that dynamic bundling is critical for cytokinesis, as cells expressing SpAin1(R216E) display disorganized ring material and delays in both ring formation and constriction. Furthermore, computer simulations of initial actin filament elongation and alignment revealed that an intermediate level of cross-linking best facilitates filament alignment. Together our results demonstrate that dynamic bundling by SpAin1 is important for proper contractile ring formation and constriction.

The Dystrophin Complex: Structure, Function, and Implications for Therapy

The dystrophin complex stabilizes the plasma membrane of striated muscle cells. Loss of function mutations in the genes encoding dystrophin, or the associated proteins, trigger instability of the plasma membrane, and myofiber loss. Mutations in dystrophin have been extensively cataloged, providing remarkable structure-function correlation between predicted protein structure and clinical outcomes. These data have highlighted dystrophin regions necessary for in vivo function and fueled the design of viral vectors and now, exon skipping approaches for use in dystrophin restoration therapies. However, dystrophin restoration is likely more complex, owing to the role of the dystrophin complex as a broad cytoskeletal integrator. This review will focus on dystrophin restoration, with emphasis on the regions of dystrophin essential for interacting with its associated proteins and discuss the structural implications of these approaches.

Characterisation of Schizosaccharomyces pombe α-actinin

The actin cytoskeleton plays a fundamental role in eukaryotic cells. Its reorganization is regulated by a plethora of actin-modulating proteins, such as a-actinin. In higher organisms, α-actinin is characterized by the presence of three distinct structural domains: an N-terminal actin-binding domain and a C-terminal region with EF-hand motif separated by a central rod domain with four spectrin repeats. Sequence analysis has revealed that the central rod domain of α-actinin from the fission yeast Schizosaccharomyces pombe consists of only two spectrin repeats. To obtain a firmer understanding of the structure and function of this unconventional α-actinin, we have cloned and characterized each structural domain. Our results show that this a-actinin isoform is capable of forming dimers and that the rod domain is required for this. However, its actin-binding and cross-linking activity appears less efficient compared to conventional α-actinins. The solved crystal structure of the actin-binding domain indicates that the closed state is stabilised by hydrogen bonds and a salt bridge not present in other α-actinins, which may reduce the affinity for actin.

Solution structure of the calmodulin-like C-terminal domain of Entamoeba α-actinin2

Cell motility is dependent on a dynamic meshwork of actin filaments that is remodelled continuously. A large number of associated proteins that are severs, cross-links, or caps the filament ends have been identified and the actin cross-linker α-actinin has been implied in several important cellular processes. In Entamoeba histolytica, the etiological agent of human amoebiasis, α-actinin is believed to be required for infection. To better understand the role of α-actinin in the infectious process we have determined the solution structure of the C-terminal calmodulin-like domain using NMR. The final structure ensemble of the apo form shows two lobes, that both resemble other pairs of calcium-binding EF-hand motifs, connected with a mobile linker.

Actin: Structure, Function, Dynamics, and Interactions with Bacterial Toxins

Actin is one of the most abundant proteins in any eukaryotic cell and an indispensable component of the cytoskeleton. In mammalian organisms, six highly conserved actin isoforms can be distinguished, which differ by only a few amino acids. In non-muscle cells, actin polymerizes into actin filaments that form actin structures essential for cell shape stabilization, and participates in a number of motile activities like intracellular vesicle transport, cytokinesis, and also cell locomotion. Here, we describe the structure of monomeric and polymeric actin, the polymerization kinetics, and its regulation by actin-binding proteins. Probably due to its conserved nature and abundance, actin and its regulating factors have emerged as prefered targets of bacterial toxins and effectors, which subvert the host actin cytoskeleton to serve bacterial needs.

Human artificial chromosomes for Duchenne muscular dystrophy and beyond: challenges and hopes

Safe and efficacious vectors able to carry large or several transgenes are of key importance for gene therapy. Human artificial chromosomes can fulfil this essential requirement; moreover, they do not integrate into the host genome. However, drawbacks such as the low efficiency of chromosome transfer and their relatively complex engineering still limit their widespread use. In this article, I summarise the key steps that brought human artificial chromosomes into preclinical research for Duchenne muscular dystrophy, an X-linked, monogenic disorder. I will also review possible future pre-clinical and clinical perspectives for this technology.

The biochemical and mass spectrometric profiling of the dystrophin complexome from skeletal muscle

The development of advanced mass spectrometric methodology has decisively enhanced the analytical capabilities for studies into the composition and dynamics of multi-subunit protein complexes and their associated components. Large-scale complexome profiling is an approach that combines the systematic isolation and enrichment of protein assemblies with sophisticated mass spectrometry-based identification methods. In skeletal muscles, the membrane cytoskeletal protein dystrophin of 427 kDa forms tight interactions with a variety of sarcolemmal, cytosolic and extracellular proteins, which in turn associate with key components of the extracellular matrix and the intracellular cytoskeleton. A major function of this enormous assembly of proteins, including dystroglycans, sarcoglycans, syntrophins, dystrobrevins, sarcospan, laminin and cortical actin, is postulated to stabilize muscle fibres during the physical tensions of continuous excitation-contraction-relaxation cycles. This article reviews the evidence from recent proteomic studies that have focused on the characterization of the dystrophin-glycoprotein complex and its central role in the establishment of the cytoskeleton-sarcolemma-matrisome axis. Proteomic findings suggest a close linkage of the core dystrophin complex with a variety of protein species, including tubulin, vimentin, desmin, annexin, proteoglycans and collagens. Since the almost complete absence of dystrophin is the underlying cause for X-linked muscular dystrophy, a more detailed understanding of the composition, structure and plasticity of the dystrophin complexome may have considerable biomedical implications.

Structure of the 34 kDa F-actin-bundling protein ABP34 from Dictyostelium discoideum

The crystal structure of the 34 kDa F-actin-bundling protein ABP34 from Dictyostelium discoideum was solved by Ca(2+)/S-SAD phasing and refined at 1.89 Å resolution. ABP34 is a calcium-regulated actin-binding protein that cross-links actin filaments into bundles. Its in vitro F-actin-binding and F-actin-bundling activities were confirmed by a co-sedimentation assay and transmission electron microscopy. The co-localization of ABP34 with actin in cells was also verified. ABP34 adopts a two-domain structure with an EF-hand-containing N-domain and an actin-binding C-domain, but has no reported overall structural homologues. The EF-hand is occupied by a calcium ion with a pentagonal bipyramidal coordination as in the canonical EF-hand. The C-domain structure resembles a three-helical bundle and superposes well onto the rod-shaped helical structures of some cytoskeletal proteins. Residues 216-244 in the C-domain form part of the strongest actin-binding sites (193-254) and exhibit a conserved sequence with the actin-binding region of α-actinin and ABP120. Furthermore, the second helical region of the C-domain is kinked by a proline break, offering a convex surface towards the solvent area which is implicated in actin binding. The F-actin-binding model suggests that ABP34 binds to the side of the actin filament and residues 216-244 fit into a pocket between actin subdomains -1 and -2 through hydrophobic interactions. These studies provide insights into the calcium coordination in the EF-hand and F-actin-binding site in the C-domain of ABP34, which are associated through interdomain interactions.

DMD Mutations in 576 Dystrophinopathy Families: A Step Forward in Genotype-Phenotype Correlations

Recent advances in molecular therapies for Duchenne muscular dystrophy (DMD) require precise genetic diagnosis because most therapeutic strategies are mutation-specific. To understand more about the genotype-phenotype correlations of the DMD gene we performed a comprehensive analysis of the DMD mutational spectrum in a large series of families. Here we provide the clinical, pathological and genetic features of 576 dystrophinopathy patients. DMD gene analysis was performed using the MLPA technique and whole gene sequencing in blood DNA and muscle cDNA. The impact of the DNA variants on mRNA splicing and protein functionality was evaluated by in silico analysis using computational algorithms. DMD mutations were detected in 576 unrelated dystrophinopathy families by combining the analysis of exonic copies and the analysis of small mutations. We found that 471 of these mutations were large intragenic rearrangements. Of these, 406 (70.5%) were exonic deletions, 64 (11.1%) were exonic duplications, and one was a deletion/duplication complex rearrangement (0.2%). Small mutations were identified in 105 cases (18.2%), most being nonsense/frameshift types (75.2%). Mutations in splice sites, however, were relatively frequent (20%). In total, 276 mutations were identified, 85 of which have not been previously described. The diagnostic algorithm used proved to be accurate for the molecular diagnosis of dystrophinopathies. The reading frame rule was fulfilled in 90.4% of DMD patients and in 82.4% of Becker muscular dystrophy patients (BMD), with significant differences between the mutation types. We found that 58% of DMD patients would be included in single exon-exon skipping trials, 63% from strategies directed against multiexon-skipping exons 45 to 55, and 14% from PTC therapy. A detailed analysis of missense mutations provided valuable information about their impact on the protein structure.

Functional and Genetic Analysis of Spectraplakins in Drosophila

The cytoskeleton is a dynamic network of filamentous protein polymers required for virtually all cellular processes. It consists of three major classes, filamentous actin (F-actin), intermediate filaments, and microtubules, all displaying characteristic structural properties, functions, cellular distributions, and sets of interacting regulatory proteins. One unique class of proteins, the spectraplakins, bind, regulate, and integrate the functions of all three classes of cytoskeleton proteins. Spectraplakins are giant, evolutionary conserved multidomain proteins (spanning up to 9000 aa) that are true members of the plakin, spectrin, and Gas2-like protein families. They have OMIM-listed disease links to epidermolysis bullosa and hereditary sensory and autonomic neuropathy. Their role in disease is likely underrepresented since studies in model animal systems have revealed critical roles in polarity, morphogenesis, differentiation and maintenance, migration, signaling, and intracellular trafficking in a variety of tissues. This enormous diversity of spectraplakin function is consistent with the numerous isoforms produced from single genomic loci that combine different sets of functional domains in distinct cellular contexts. To study the broad range of functions and complexity of these proteins, Drosophila is a powerful model. Thus, the fly spectraplakin Short stop (Shot) acts as an actin-microtubule linker and plays important roles in many developmental processes, which provide experimentally amenable and relevant contexts in which to study spectraplakin functions. For these studies, a versatile range of relevant experimental resources that facilitate genetics and transgenic approaches, highly refined genomics tools, and an impressive set of spectraplakin-specific genetic and molecular tools are readily available. Here, we use the example of Shot to illustrate how the various tools and strategies available for Drosophila can be employed to decipher and dissect cellular roles and molecular mechanisms of spectraplakins.

Regulation of structure and function of sarcomeric actin filaments in striated muscle of the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has been used as a valuable system to study structure and function of striated muscle. The body wall muscle of C. elegans is obliquely striated muscle with highly organized sarcomeric assembly of actin, myosin, and other accessory proteins. Genetic and molecular biological studies in C. elegans have identified a number of genes encoding structural and regulatory components for the muscle contractile apparatuses, and many of them have counterparts in mammalian cardiac and skeletal muscles or striated muscles in other invertebrates. Applicability of genetics, cell biology, and biochemistry has made C. elegans an excellent system to study mechanisms of muscle contractility and assembly and maintenance of myofibrils. This review focuses on the regulatory mechanisms of structure and function of actin filaments in the C. elegans body wall muscle. Sarcomeric actin filaments in C. elegans muscle are associated with the troponin-tropomyosin system that regulates the actin-myosin interaction. Proteins that bind to the side and ends of actin filaments support ordered assembly of thin filaments. Furthermore, regulators of actin dynamics play important roles in initial assembly, growth, and maintenance of sarcomeres. The knowledge acquired in C. elegans can serve as bases to understand the basic mechanisms of muscle structure and function.

Forskolin Regulates L-Type Calcium Channel through Interaction between Actinin 4 and β3 Subunit in Osteoblasts

Voltage-dependent L-type calcium channels that permit cellular calcium influx are essential in calcium-mediated modulation of cellular signaling. Although the regulation of voltage-dependent L-type calcium channels is linked to many factors including cAMP-dependent protein kinase A (PKA) activity and actin cytoskeleton, little is known about the detailed mechanisms underlying the regulation in osteoblasts. Our present study investigated the modulation of L-type calcium channel activities through the effects of forskolin on actin reorganization and on its functional interaction with actin binding protein actinin 4. The results showed that forskolin did not significantly affect the trafficking of pore forming α_1c subunit and its interaction with actin binding protein actinin 4, whereas it significantly increased the expression of β₃ subunit and its interaction with actinin 4 in osteoblast cells as assessed by co-immunoprecipitation, pull-down assay, and immunostaining. Further mapping showed that the ABD and EF domains of actinin 4 were interaction sites. This interaction is independent of PKA phosphorylation. Knockdown of actinin 4 significantly decreased the activities of L-type calcium channels. Our study revealed a new aspect of the mechanisms by which the forskolin activation of adenylyl cyclase - cAMP cascade regulates the L-type calcium channel in osteoblast cells, besides the PKA mediated phosphorylation of the channel subunits. These data provide insight into the important role of interconnection among adenylyl cyclase, cAMP, PKA, the actin cytoskeleton, and the channel proteins in the regulation of voltage-dependent L-type calcium channels in osteoblast cells.

Toxicogenomics profiling of bone marrow from rats treated with topotecan in combination with oxaliplatin: a mechanistic strategy to inform combination toxicity

Combinations of anticancer agents may have synergistic anti-tumor effects, but enhanced hematological toxicity often limit their clinical use. We examined whether "microarray profiles" could be used to compare early molecular responses following a single dose of agents administered individually with that of the agents administered in a combination. We compared the mRNA responses within bone marrow of Sprague-Dawley rats after a single 30 min treatment with topotecan at 4.7 mg/kg or oxaliplatin at 15 mg/kg alone to that of sequentially administered combination therapy or vehicle control for 1, 6, and 24 h. We also examined the histopathology of the bone marrow following all treatments. Drug-related histopathological lesions were limited to bone marrow hypocellularity for animals dosed with either agent alone or in combination. Lesions had an earlier onset and higher incidence for animals given topotecan alone or in combination with oxaliplatin. Severity increased from mild to moderate when topotecan was administered prior to oxaliplatin compared with administering oxaliplatin first. Notably, six patterns of co-expressed genes were detected at the 1 h time point that indicate regulatory expression of genes that are dependent on the order of the administration. These results suggest alterations in histone biology, chromatin remodeling, DNA repair, bone regeneration, and respiratory and oxidative phosphorylation are among the prominent pathways modulated in bone marrow from animals treated with an oxaliplatin/topotecan combination. These data also demonstrate the potential for early mRNA patterns derived from target organs of toxicity to inform toxicological risk and molecular mechanisms for agents given in combination.

Erythrocyte stiffness during morphological remodeling induced by carbon ion radiation

The adverse effect induced by carbon ion radiation (CIR) is still an unavoidable hazard to the treatment object. Thus, evaluation of its adverse effects on the body is a critical problem with respect to radiation therapy. We aimed to investigate the change between the configuration and mechanical properties of erythrocytes induced by radiation and found differences in both the configuration and the mechanical properties with involving in morphological remodeling process. Syrian hamsters were subjected to whole-body irradiation with carbon ion beams (1, 2, 4, and 6 Gy) or X-rays (2, 4, 6, and 12 Gy) for 3, 14 and 28 days. Erythrocytes in peripheral blood and bone marrow were collected for cytomorphological analysis. The mechanical properties of the erythrocytes were determined using atomic force microscopy, and the expression of the cytoskeletal protein spectrin-α1 was analyzed via western blotting. The results showed that dynamic changes were evident in erythrocytes exposed to different doses of carbon ion beams compared with X-rays and the control (0 Gy). The magnitude of impairment of the cell number and cellular morphology manifested the subtle variation according to the irradiation dose. In particular, the differences in the size, shape and mechanical properties of the erythrocytes were well exhibited. Furthermore, immunoblot data showed that the expression of the cytoskeletal protein spectrin-α1 was changed after irradiation, and there was a common pattern among its substantive characteristics in the irradiated group. Based on these findings, the present study concluded that CIR could induce a change in mechanical properties during morphological remodeling of erythrocytes. According to the unique characteristics of the biomechanical categories, we deduce that changes in cytomorphology and mechanical properties can be measured to evaluate the adverse effects generated by tumor radiotherapy. Additionally, for the first time, the current study provides a new strategy for enhancing the assessment of the curative effects and safety of clinical radiotherapy, as well as reducing adverse effects.

Neuronal porosome lipidome

Cup-shaped lipoprotein structures called porosomes are the universal secretory portals at the cell plasma membrane, where secretory vesicles transiently dock and fuse to release intravesicular contents. In neurons, porosomes measure ∼15 nm and are comprised of nearly 40 proteins, among them SNAREs, ion channels, the G_αo G-protein and several structural proteins. Earlier studies report the interaction of specific lipids and their influence on SNAREs, ion channels and G-protein function. Our own studies demonstrate the requirement of cholesterol for the maintenance of neuronal porosome integrity, and the influence of lipids on SNARE complex assembly. In this study, to further understand the role of lipids on porosome structure-function, the lipid composition of isolated neuronal porosome was determined using mass spectrometry. Using lipid-binding assays, the affinity of porosome-associated syntaxin-1A to various lipids was determined. Our mass spectrometry results demonstrate the presence of phosphatidylinositol phosphates (PIP's) and phosphatidic acid (PA) among other lipids, and the enriched presence of ceramide (Cer), lysophosphatidylinositol phosphates (LPIP) and diacylglycerol (DAG). Lipid binding assays demonstrate the binding of neuronal porosome to cardiolipin, and confirm its association with PIP's and PA. The ability of exogenous PA to alter protein–protein interaction and neurotransmitter release is further demonstrated from the study.

α-Actinin-2 mediates spine morphology and assembly of the post-synaptic density in hippocampal neurons

Dendritic spines are micron-sized protrusions that constitute the primary post-synaptic sites of excitatory neurotransmission in the brain. Spines mature from a filopodia-like protrusion into a mushroom-shaped morphology with a post-synaptic density (PSD) at its tip. Modulation of the actin cytoskeleton drives these morphological changes as well as the spine dynamics that underlie learning and memory. Several PSD molecules respond to glutamate receptor activation and relay signals to the underlying actin cytoskeleton to regulate the structural changes in spine and PSD morphology. α-Actinin-2 is an actin filament cross-linker, which localizes to dendritic spines, enriched within the post-synaptic density, and implicated in actin organization. We show that loss of α-actinin-2 in rat hippocampal neurons creates an increased density of immature, filopodia-like protrusions that fail to mature into a mushroom-shaped spine during development. α-Actinin-2 knockdown also prevents the recruitment and stabilization of the PSD in the spine, resulting in failure of synapse formation, and an inability to structurally respond to chemical stimulation of the N-methyl-D-aspartate (NMDA)-type glutamate receptor. The Ca2+-insensitive EF-hand motif in α-actinin-2 is necessary for the molecule's function in regulating spine morphology and PSD assembly, since exchanging it for the similar but Ca2+-sensitive domain from α-actinin-4, another α-actinin isoform, inhibits its function. Furthermore, when the Ca2+-insensitive domain from α-actinin-2 is inserted into α-actinin-4 and expressed in neurons, it creates mature spines. These observations support a model whereby α-actinin-2, partially through its Ca2+-insensitive EF-hand motif, nucleates PSD formation via F-actin organization and modulates spine maturation to mediate synaptogenesis.