Mechanism of actin filament pointed-end capping by tropomodulin

Myosin-dependent short actin filaments contribute to peripheral widening in developing stereocilia

In the auditory and vestibular systems, stereocilia are actin-based protrusions that convert mechanical stimuli into electrical signals. During development, stereocilia elongate and widen by adding filamentous actin (F-actin), attaining their mature shape necessary for mechanosensitive function. Myosin motors, including MYO3A/B and MYO15A, are required for normal stereocilia growth, but the regulation of actin and the impact of myosins on actin assembly remain unclear. We focused on stereocilia widening, which requires the addition of new filaments to the bundle of linear F-actin comprising the initial stereocilia core. Our findings revealed that newly expressed actin incorporates at the stereocilia tip first, then along the shaft to promote stereocilia widening. The newly incorporated F-actin surrounded the existing F-actin core, suggesting that the core is stable once formed, with additional actin adding only to the periphery. To better understand the nature of incorporating actin, we used several probes to detect globular (G-) actin, F-actin barbed ends, and F-actin pointed ends. While F-actin core filaments are parallel and thought to present only barbed ends at stereocilia tips, we also detected F-actin pointed ends, indicating a previously undetected population of short actin filaments. Overexpression of actin resulted in abundant F-actin pointed and barbed ends along the periphery of the stereocilia shaft, suggesting that short actin filaments contribute to stereocilia widening. Short actin filament levels correlated with the levels of MYO3A/B and MYO15A at stereocilia tips, suggesting these myosins generate or stabilize short actin filaments essential for stereocilia widening and elongation.

Myosin-I synergizes with Arp2/3 complex to enhance the pushing forces of branched actin networks

Class I myosins (myosin-Is) colocalize with Arp2/3 complex-nucleated actin networks at sites of membrane protrusion and invagination, but the mechanisms by which myosin-I motor activity coordinates with branched actin assembly to generate force are unknown. We mimicked the interplay of these proteins using the "comet tail" bead motility assay, where branched actin networks are nucleated by the Arp2/3 complex on the surface of beads coated with myosin-I and nucleation-promoting factor. We observed that myosin-I increased bead movement efficiency by thinning actin networks without affecting growth rates. Myosin-I triggered symmetry breaking and comet tail formation in dense networks resistant to spontaneous fracturing. Even with arrested actin assembly, myosin-I alone could break the network. Computational modeling recapitulated these observations, suggesting myosin-I acts as a repulsive force shaping the network's architecture and boosting its force-generating capacity. We propose that myosin-I leverages its power stroke to amplify the forces generated by Arp2/3 complex-nucleated actin networks.

Molecular mechanisms linking missense ACTG2 mutations to visceral myopathy

Visceral myopathy is a life-threatening disease characterized by muscle weakness in the bowel, bladder, and uterus. Mutations in smooth muscle γ-actin (ACTG2) are the most common cause of the disease, but the mechanisms by which the mutations alter muscle function are unknown. Here, we examined four prevalent ACTG2 mutations (R40C, R148C, R178C, and R257C) that cause different disease severity and are spread throughout the actin fold. R178C displayed premature degradation, R148C disrupted interactions with actin-binding proteins, R40C inhibited polymerization, and R257C destabilized filaments. Because these mutations are heterozygous, we also analyzed 50/50 mixtures with wild-type (WT) ACTG2. The WT/R40C mixture impaired filament nucleation by leiomodin 1, and WT/R257C produced filaments that were easily fragmented by smooth muscle myosin. Smooth muscle tropomyosin isoform Tpm1.4 partially rescued the defects of R40C and R257C. Cryo-electron microscopy structures of filaments formed by R40C and R257C revealed disrupted intersubunit contacts. The biochemical and structural properties of the mutants correlate with their genotype-specific disease severity.

Myosin-I Synergizes with Arp2/3 Complex to Enhance Pushing Forces of Branched Actin Networks

Myosin-Is colocalize with Arp2/3 complex-nucleated actin networks at sites of membrane protrusion and invagination, but the mechanisms by which myosin-I motor activity coordinates with branched actin assembly to generate force are unknown. We mimicked the interplay of these proteins using the "comet tail" bead motility assay, where branched actin networks are nucleated by Arp2/3 complex on the surface of beads coated with myosin-I and the WCA domain of N-WASP. We observed that myosin-I increased bead movement efficiency by thinning actin networks without affecting growth rates. Remarkably, myosin-I triggered symmetry breaking and comet-tail formation in dense networks resistant to spontaneous fracturing. Even with arrested actin assembly, myosin-I alone could break the network. Computational modeling recapitulated these observations suggesting myosin-I acts as a repulsive force shaping the network's architecture and boosting its force-generating capacity. We propose that myosin-I leverages its power stroke to amplify the forces generated by Arp2/3 complex-nucleated actin networks.

A structural vista of phosducin-like PhLP2A-chaperonin TRiC cooperation during the ATP-driven folding cycle

Proper cellular proteostasis, essential for viability, requires a network of chaperones and cochaperones. ATP-dependent chaperonin TRiC/CCT partners with cochaperones prefoldin (PFD) and phosducin-like proteins (PhLPs) to facilitate folding of essential eukaryotic proteins. Using cryoEM and biochemical analyses, we determine the ATP-driven cycle of TRiC-PFD-PhLP2A interaction. PhLP2A binds to open apo-TRiC through polyvalent domain-specific contacts with its chamber's equatorial and apical regions. PhLP2A N-terminal H3-domain binding to subunits CCT3/4 apical domains displace PFD from TRiC. ATP-induced TRiC closure rearranges the contacts of PhLP2A domains within the closed chamber. In the presence of substrate, actin and PhLP2A segregate into opposing chambers, each binding to positively charged inner surface residues from CCT1/3/6/8. Notably, actin induces a conformational change in PhLP2A, causing its N-terminal helices to extend across the inter-ring interface to directly contact a hydrophobic groove in actin. Our findings reveal an ATP-driven PhLP2A structural rearrangement cycle within the TRiC chamber to facilitate folding.

Recessive TMOD1 mutation causes childhood cardiomyopathy

Familial cardiomyopathy in pediatric stages is a poorly understood presentation of heart disease in children that is attributed to pathogenic mutations. Through exome sequencing, we report a homozygous variant in tropomodulin 1 (TMOD1; c.565C>T, p.R189W) in three individuals from two unrelated families with childhood-onset dilated and restrictive cardiomyopathy. To decipher the mechanism of pathogenicity of the R189W mutation in TMOD1, we utilized a wide array of methods, including protein analyses, biochemistry and cultured cardiomyocytes. Structural modeling revealed potential defects in the local folding of TMOD1R189W and its affinity for actin. Cardiomyocytes expressing GFP-TMOD1R189W demonstrated longer thin filaments than GFP-TMOD1wt-expressing cells, resulting in compromised filament length regulation. Furthermore, TMOD1R189W showed weakened activity in capping actin filament pointed ends, providing direct evidence for the variant's effect on actin filament length regulation. Our data indicate that the p.R189W variant in TMOD1 has altered biochemical properties and reveals a unique mechanism for childhood-onset cardiomyopathy.

Fascin - F-actin interaction studied by molecular dynamics simulation and protein network analysis

Actin bundles are an important component of cellular cytoskeleton and participate in the movement of cells. The formation of actin bundles requires the participation of many actin binding proteins (ABPs). Fascin is a member of ABPs, which plays a key role in bundling filamentous actin (F-actin) to bundles. However, the detailed interactions between fascin and F-actin are unclear. In this study, we construct an atomic-level structure of fascin - F-actin complex based on a rather poor cryo-EM data with resolution of 20 nm. We first optimized the geometries of the complex by molecular dynamics (MD) simulation and analyzed the binding site and pose of fascin which bundles two F-actin chains. Next, binding free energy of fascin was calculated by MM/GBSA method. Finally, protein structure network analysis (PSNs) was performed to analyze the key residues for fascin binding. Our results show that residues of K22, E27, E29, K41, K43, R110, R149, K358, R408 and K471 on fascin are important for its bundling, which are in good agreement with the experimental data. On the other hand, the consistent results indicate that the atomic-level model of fascin - F-actin complex is reliable. In short, this model can be used to understand the detailed interactions between fascin and F-actin, and to develop novel potential drugs targeting fascin.Communicated by Ramaswamy H. Sarma.

Mechanisms of actin disassembly and turnover

Cellular actin networks exhibit a wide range of sizes, shapes, and architectures tailored to their biological roles. Once assembled, these filamentous networks are either maintained in a state of polarized turnover or induced to undergo net disassembly. Further, the rates at which the networks are turned over and/or dismantled can vary greatly, from seconds to minutes to hours or even days. Here, we review the molecular machinery and mechanisms employed in cells to drive the disassembly and turnover of actin networks. In particular, we highlight recent discoveries showing that specific combinations of conserved actin disassembly-promoting proteins (cofilin, GMF, twinfilin, Srv2/CAP, coronin, AIP1, capping protein, and profilin) work in concert to debranch, sever, cap, and depolymerize actin filaments, and to recharge actin monomers for new rounds of assembly.

Structures of the free and capped ends of the actin filament

The barbed and pointed ends of the actin filament (F-actin) are the sites of growth and shrinkage and the targets of capping proteins that block subunit exchange, including CapZ at the barbed end and tropomodulin at the pointed end. We describe cryo-electron microscopy structures of the free and capped ends of F-actin. Terminal subunits at the free barbed end adopt a "flat" F-actin conformation. CapZ binds with minor changes to the barbed end but with major changes to itself. By contrast, subunits at the free pointed end adopt a "twisted" monomeric actin (G-actin) conformation. Tropomodulin binding forces the second subunit into an F-actin conformation. The structures reveal how the ends differ from the middle in F-actin and how these differences control subunit addition, dissociation, capping, and interactions with end-binding proteins.

Structural basis of membrane skeleton organization in red blood cells

The spectrin-based membrane skeleton is a ubiquitous membrane-associated two-dimensional cytoskeleton underneath the lipid membrane of metazoan cells. Mutations of skeleton proteins impair the mechanical strength and functions of the membrane, leading to several different types of human diseases. Here, we report the cryo-EM structures of the native spectrin-actin junctional complex (from porcine erythrocytes), which is a specialized short F-actin acting as the central organizational unit of the membrane skeleton. While an α-/β-adducin hetero-tetramer binds to the barbed end of F-actin as a flexible cap, tropomodulin and SH3BGRL2 together create an absolute cap at the pointed end. The junctional complex is strengthened by ring-like structures of dematin in the middle actin layers and by patterned periodic interactions with tropomyosin over its entire length. This work serves as a structural framework for understanding the assembly and dynamics of membrane skeleton and offers insights into mechanisms of various ubiquitous F-actin-binding factors in other F-actin systems.

A structural vista of phosducin-like PhLP2A-chaperonin TRiC cooperation during the ATP-driven folding cycle

Proper cellular proteostasis, essential for viability, requires a network of chaperones and cochaperones. ATP-dependent chaperonin TRiC/CCT partners with cochaperones prefoldin (PFD) and phosducin-like proteins (PhLPs) to facilitate the folding of essential eukaryotic proteins. Using cryoEM and biochemical analyses, we determine the ATP-driven cycle of TRiC-PFD-PhLP2A interaction. In the open TRiC state, PhLP2A binds to the chamber's equator while its N-terminal H3-domain binds to the apical domains of CCT3/4, thereby displacing PFD from TRiC. ATP-induced TRiC closure rearranges the contacts of PhLP2A domains within the closed chamber. In the presence of substrate, actin and PhLP2A segregate into opposing chambers, each binding to the positively charged inner surfaces formed by CCT1/3/6/8. Notably, actin induces a conformational change in PhLP2A, causing its N-terminal helices to extend across the inter-ring interface to directly contact a hydrophobic groove in actin. Our findings reveal an ATP-driven PhLP2A structural rearrangement cycle within the TRiC chamber to facilitate folding.

Drosophila Tropomodulin is required for multiple actin-dependent processes within developing myofibers

Proper muscle contraction requires the assembly and maintenance of sarcomeres and myofibrils. Although the protein components of myofibrils are generally known, less is known about the mechanisms by which they individually function and together synergize for myofibril assembly and maintenance. For example, it is unclear how the disruption of actin filament (F-actin) regulatory proteins leads to the muscle weakness observed in myopathies. Here, we show that knockdown of Drosophila Tropomodulin (Tmod), results in several myopathy-related phenotypes, including reduction of muscle cell (myofiber) size, increased sarcomere length, disorganization and misorientation of myofibrils, ectopic F-actin accumulation, loss of tension-mediating proteins at the myotendinous junction, and misshaped and internalized nuclei. Our findings support and extend the tension-driven self-organizing myofibrillogenesis model. We show that, like its mammalian counterpart, Drosophila Tmod caps F-actin pointed-ends, and we propose that this activity is crucial for cellular processes in different locations within the myofiber that directly and indirectly contribute to the maintenance of muscle function. Our findings provide significant insights to the role of Tmod in muscle development, maintenance and disease.

Alterations in the Proteome of Developing Neocortical Synaptosomes in the Absence of MET Signaling Revealed by Comparative Proteomics

Alterations in the expression of genes encoding proteins involved in synapse formation, maturation, and function are a hallmark of many neurodevelopmental and psychiatric disorders. For example, there is reduced neocortical expression of the MET receptor tyrosine kinase (MET) transcript and protein in Autism Spectrum Disorder (ASD) and Rett syndrome. Preclinical in vivo and in vitro models manipulating MET signaling reveal that the receptor modulates excitatory synapse development and maturation in select forebrain circuits. The molecular adaptations underlying the altered synaptic development remain unknown. We performed a comparative mass spectrometry analysis of synaptosomes generated from the neocortex of wild type and Met null mice during the peak of synaptogenesis (postnatal day 14; data are available from ProteomeXchange with identifier PXD033204). The analyses revealed broad disruption of the developing synaptic proteome in the absence of MET, consistent with the localization of MET protein in pre- and postsynaptic compartments, including proteins associated with the neocortical synaptic MET interactome and those encoded by syndromic and ASD risk genes. In addition to an overrepresentation of altered proteins associated with the SNARE complex, multiple proteins in the ubiquitin-proteasome system and associated with the synaptic vesicle, as well as proteins that regulate actin filament organization and synaptic vesicle exocytosis/endocytosis, were disrupted. Taken together, the proteomic changes are consistent with structural and functional changes observed following alterations in MET signaling. We hypothesize that the molecular adaptations following Met deletion may reflect a general mechanism that produces circuit-specific molecular changes due to loss or reduction of synaptic signaling proteins.

Nuclear Magnetic Resonance-Guided Structural Analysis of Moderate-Affinity Protein Complexes with Intrinsically Disordered Polypeptides

Binding affinity of an individual binding site of an intrinsically disordered protein for its folded partner may be moderate. In such cases, a straightforward determination of the structure of the binding interface is difficult. We offer a hybrid protocol combining NMR chemical shift information, NMR spectral data on amino acid residue sequence substitution effects, residual dipolar coupling, and molecular dynamics simulation that allowed us to determine the structure of a complex between the intrinsically disordered tropomyosin-binding site of leiomodin and a coiled-coil peptide modeling the N-terminal fragment of tropomyosin. The protocol can be used for other moderate-affinity complexes composed of an intrinsically disordered peptide bound to a structured protein partner.

Rapid transcriptomic and physiological changes in the freshwater pennate diatom Mayamaea pseudoterrestris in response to copper exposure

Diatoms function as major primary producers, accumulating large amounts of biomass in most aquatic environments. Given their rapid responses to changes in environmental conditions, diatoms are used for the biological monitoring of water quality and for performing ecotoxicological tests in aquatic ecosystems. However, the molecular basis for their toxicity to chemical compounds remains largely unknown. Here, we sequenced the genome of a freshwater diatom, Mayamaea pseudoterrestris NIES-4280, which has been proposed as an alternative strain of Navicula pelliculosa UTEX 664 for performing the Organisation for Economic Co-operation and Development ecotoxicological test. This study shows that M. pseudoterrestris has a small genome and carries the lowest number of genes among freshwater diatoms. The gene content of M. pseudoterrestris is similar to that of the model marine diatom, Phaeodactylum tricornutum. Genes related to cell motility, polysaccharide metabolism, oxidative stress alleviation, intracellular calcium signalling, and reactive compound detoxification showed rapid changes in their expression patterns in response to copper exposure. Active gliding motility was observed in response to copper addition, and copper exposure decreased intracellular calcium concentration. These findings enhance our understanding of the environmental adaptation of diatoms, and elucidate the molecular basis of toxicity of chemical compounds in algae.

Cleavage of tropomodulin-3 by asparagine endopeptidase promotes cancer malignancy by actin remodeling and SND1/RhoA signaling

Background

Abnormal proliferation and migration of cells are hallmarks of cancer initiation and malignancy. Asparagine endopeptidase (AEP) has specific substrate cleavage ability and plays a pro-cancer role in a variety of cancers. However, the underlying mechanism of AEP in cancer proliferation and migration still remains unclear.

Methods

Co-immunoprecipitation and following mass spectrometry were used to identify the substrate of AEP. Western blotting was applied to measure the expression of proteins. Single cell/nuclear-sequences were done to detect the heterogeneous expression of Tmod3 in tumor tissues. CCK-8 assay, flow cytometry assays, colony formation assay, Transwell assay and scratch wound-healing assay were performed as cellular functional experiments. Mouse intracranial xenograft tumors were studied in in vivo experiments.

Results

Here we showed that AEP cleaved a ubiquitous cytoskeleton regulatory protein, tropomodulin-3 (Tmod3) at asparagine 157 (N157) and produced two functional truncations (tTmod3-N and tTmod3-C). Truncated Tmod3 was detected in diverse tumors and was found to be associated with poor prognosis of high-grade glioma. Functional studies showed that tTmod3-N and tTmod3-C enhanced cancer cell migration and proliferation, respectively. Animal models further revealed the tumor-promoting effects of AEP truncated Tmod3 in vivo. Mechanistically, tTmod3-N was enriched in the cell cortex and competitively inhibited the pointed-end capping effect of wild-type Tmod3 on filamentous actin (F-actin), leading to actin remodeling. tTmod3-C translocated to the nucleus, where it interacted with Staphylococcal Nuclease And Tudor Domain Containing 1 (SND1), facilitating the transcription of Ras Homolog Family Member A/Cyclin Dependent Kinases (RhoA/CDKs).

Conclusion

The newly identified AEP-Tmod3 protease signaling axis is a novel “dual-regulation” mechanism of tumor cell proliferation and migration. Our work provides new clues to the underlying mechanisms of cancer proliferation and invasive progression and evidence for targeting AEP or Tmod3 for therapy.

Pointed-end processive elongation of actin filaments by Vibrio effectors VopF and VopL

According to the cellular actin dynamics paradigm, filaments grow at their barbed ends and depolymerize predominantly from their pointed ends to form polar structures and do productive work. We show that actin can elongate at the pointed end when assisted by Vibrio VopF/L toxins, which act as processive polymerases. In cells, processively moving VopF/L speckles are inhibited by factors blocking the pointed but not barbed ends. Multispectral single-molecule imaging confirmed that VopF molecules associate with the pointed end, actively promoting its elongation even in the presence of profilin. Consequently, VopF/L can break the actin cytoskeleton's polarity by compromising actin-based cellular processes. Therefore, actin filament design allows processive growth at both ends, which suggests unforeseen possibilities for cellular actin organization, particularly in specialized cells and compartments.

The role of leiomodin in actin dynamics: a new road or a secret gate

Leiomodin is an important emerging regulator of thin filaments. As novel molecular, cellular, animal model, and human data accumulate, the mechanisms of its action become clearer. Structural studies played a significant part in understanding the functional significance of leiomodin's interacting partners and functional domains. In this review, we present the current state of knowledge on the structural and cellular properties of leiomodin which has led to two proposed mechanisms of its function. Although it is known that leiomodin is essential for life, numerous domains within leiomodin remain unstudied and as such, we outline future directions for investigations that we predict will provide evidence that leiomodin is a multifunctional protein.

Thin filament cardiomyopathies: A review of genetics, disease mechanisms, and emerging therapeutics

All muscle contraction occurs due to the cyclical interaction between sarcomeric thin and thick filament proteins within the myocyte. The thin filament consists of the proteins actin, tropomyosin, Troponin C, Troponin I, and Troponin T. Mutations in these proteins can result in various forms of cardiomyopathy, including hypertrophic, restrictive, and dilated phenotypes and account for as many as 30% of all cases of inherited cardiomyopathy. There is significant evidence that thin filament mutations contribute to dysregulation of Ca2+ within the sarcomere and may have a distinct pathomechanism of disease from cardiomyopathy associated with thick filament mutations. A number of distinct clinical findings appear to be correlated with thin-filament mutations: greater degrees of restrictive cardiomyopathy and relatively less left ventricular (LV) hypertrophy and LV outflow tract obstruction than that seen with thick filament mutations, increased morbidity associated with heart failure, increased arrhythmia burden and potentially higher mortality. Most therapies that improve outcomes in heart failure blunt the neurohormonal pathways involved in cardiac remodeling, while most therapies for hypertrophic cardiomyopathy involve use of negative inotropes to reduce LV hypertrophy or septal reduction therapies to reduce LV outflow tract obstruction. None of these therapies directly address the underlying sarcomeric dysfunction associated with thin-filament mutations. With mounting evidence that thin filament cardiomyopathies occur through a distinct mechanism, there is need for therapies targeting the unique, underlying mechanisms tailored for each patient depending on a given mutation.

Ca2+ attenuates nucleation activity of leiomodin

A transient increase in Ca2+ concentration in sarcomeres is essential for their proper function. Ca2+ drives striated muscle contraction via binding to the troponin complex of the thin filament to activate its interaction with the myosin thick filament. In addition to the troponin complex, the myosin essential light chain and myosin-binding protein C were also found to be Ca2+ sensitive. However, the effects of Ca2+ on the function of the tropomodulin family proteins involved in regulating thin filament formation have not yet been studied. Leiomodin, a member of the tropomodulin family, is an actin nucleator and thin filament elongator. Using pyrene-actin polymerization assay and transmission electron microscopy, we show that the actin nucleation activity of leiomodin is attenuated by Ca2+ . Using circular dichroism and nuclear magnetic resonance spectroscopy, we demonstrate that the mostly disordered, negatively charged region of leiomodin located between its first two actin-binding sites binds Ca2+ . We propose that Ca2+ binding to leiomodin results in the attenuation of its nucleation activity. Our data provide further evidence regarding the role of Ca2+ as an ultimate regulator of the ensemble of sarcomeric proteins essential for muscle function. SUMMARY STATEMENT: Ca2+ fluctuations in striated muscle sarcomeres modulate contractile activity via binding to several distinct families of sarcomeric proteins. The effects of Ca2+ on the activity of leiomodin-an actin nucleator and thin filament length regulator-have remained unknown. In this study, we demonstrate that Ca2+ binds directly to leiomodin and attenuates its actin nucleating activity. Our data emphasizes the ultimate role of Ca2+ in the regulation of the sarcomeric protein interactions.

The Mechanisms of Thin Filament Assembly and Length Regulation in Muscles

The actin containing tropomyosin and troponin decorated thin filaments form one of the crucial components of the contractile apparatus in muscles. The thin filaments are organized into densely packed lattices interdigitated with myosin-based thick filaments. The crossbridge interactions between these myofilaments drive muscle contraction, and the degree of myofilament overlap is a key factor of contractile force determination. As such, the optimal length of the thin filaments is critical for efficient activity, therefore, this parameter is precisely controlled according to the workload of a given muscle. Thin filament length is thought to be regulated by two major, but only partially understood mechanisms: it is set by (i) factors that mediate the assembly of filaments from monomers and catalyze their elongation, and (ii) by factors that specify their length and uniformity. Mutations affecting these factors can alter the length of thin filaments, and in human cases, many of them are linked to debilitating diseases such as nemaline myopathy and dilated cardiomyopathy.

Heterozygous Tropomodulin 3 mice have improved lung vascularization after chronic hypoxia

The molecular mechanisms leading to high altitude pulmonary hypertension (HAPH) remains poorly understood. We previously analyzed the whole genome sequence of Kyrgyz highland population and identified eight genomic intervals having a potential role in HAPH. Tropomodulin 3 gene (TMOD3) which encodes a protein that binds and caps the pointed ends of actin filaments and inhibits cell migration, was one of the top candidates. Here we systematically sought additional evidence to validate the functional role of TMOD3. In-silico analysis reveals that some of the SNPs in HAPH associated genomic intervals were positioned in a regulatory region that could result in alternative splicing of TMOD3. In order to functionally validate the role of TMOD3 in HAPH, we exposed Tmod3-/+ mice to 4 weeks of constant hypoxia, i.e. 10% O2 and analyzed both functional (hemodynamic measurements) and structural (angiography) parameters related to HAPH. The hemodynamic measurements, such as right ventricular systolic pressure, a surrogate measure for pulmonary arterial systolic pressure, and right ventricular contractility (RV- ± dP/dt), increases with hypoxia did not separate between Tmod3-/+ and control mice. Remarkably, there was a significant increase in the number of lung vascular branches and total length of pulmonary vascular branches (p < 0.001) in Tmod3-/+ after 4 weeks of constant hypoxia as compared to controls. Notably, the Tmod3-/+ endothelial cells migration was also significantly higher than that from the wild-type littermates. Our results indicate that, under chronic hypoxia, lower levels of Tmod3 play an important role in the maintenance or neo-vascularization of pulmonary arteries.

New insights into the function of Fascin in actin bundling: A combined theoretical and experimental study

Fascin, one of actin bundling proteins, plays an important role in the cross-linking of actin filaments (F-actin). Phosphorylation of Fascin is an important posttranslational modification to affect its structure and function. For example, a phosphomimetic mutation of Fascin-S39D decrease its bundling ability with F-actin significantly. In this paper, we studied the actin-bundling activity of Fascin by using molecular dynamics (MD) simulations and biochemical methods. All single-site mutations from serine/threonine to aspartic acid were mimicked by MD simulations. For five mutants (S146D, S156D, S218D, T239D and S259D), the mutated residues in domain 2 of Fascin were found to form salt-bridge interactions with an adjacent residue, indicating that mutations of these residues could potentially reduce actin-bundling activity. Further, F-actin-bundling assays and immunofluorescence technique showed S146D and T239D to have a strong effect on Fascin bundling with F-actin. Finally, we show that single-site mutations do not change the general shape of Fascin, but local structures near the mutated residues in Fascin-S146D and T239D become unstable, thereby affecting the ability of Fascin to bind with F-actin. These findings suggest that targeting domain 2 of Fascin would be very useful for the drug design. In addition, our study indicates that MD simulation is a useful method to screening which residues on Fascin are important.

Unraveling Subcellular and Ultrastructural Changes During Vitrification of Human Spermatozoa: Effect of a Mitochondria-Targeted Antioxidant and a Permeable Cryoprotectant

Mitochondria-targeted antioxidants have great potential to counterbalance the generated reactive oxygen species (ROS) because they cross the inner membrane of the mitochondria. Still, their use was not reported in vitrified human spermatozoa. Our laboratory has successfully vitrified spermatozoa without the use of permeable cryoprotectants, but subcellular-level evidence was missing. Therefore, this study aimed to improve spermatozoa vitrification using a mitochondria-targeted antioxidant (mitoquinone, MitoQ), reveal ultrastructural changes in the spermatozoa due to the use of a permeable cryoprotectant, and report alterations of functional proteins during the spermatozoa vitrification process. For this, each of 20 swim-up-prepared ejaculates was divided into seven aliquots and diluted with a vitrification medium supplemented with varying concentrations of MitoQ (0.02 and 0.2 μM), glycerol (1, 4, and 6%), and a combination of MitoQ and glycerol. All aliquots were vitrified by the aseptic capillary method developed in our laboratory. The spermatozoa function assays revealed that the addition of either MitoQ (0.02 μM), glycerol (1%), or a combination of MitoQ (0.02 μM) and glycerol (1%) in the vitrification medium results in better or equivalent spermatozoa quality relative to the control. Transmission electron microscopy revealed that MitoQ protects the spermatozoa from undergoing ultrastructural alterations, but glycerol induced ultrastructural alterations during the vitrification process. Next, we performed label-free quantitative proteomics and identified 1,759 proteins, of which 69, 60, 90, and 81 were altered in the basal medium, 0.02 μM MitoQ, 1% glycerol, and Mito-glycerol groups, respectively. Actin, tubulins, and outer dense fiber proteins were not affected during the vitrification process. Some of the identified ubiquitinating enzymes were affected during spermatozoa vitrification. Only a few proteins responsible for phosphorylation were altered during vitrification. Similarly, several proteins involved in spermatozoa–egg fusion and fertilization (IZUMO1 and Tektin) were not affected during the vitrification process. In conclusion, MitoQ attenuates the vitrification-induced ultrastructural changes and alterations in the key proteins involved in spermatozoa functions and fertilization.

Actin-Binding Proteins as Potential Biomarkers for Chronic Inflammation-Induced Cancer Diagnosis and Therapy

Actin-binding proteins (ABPs), by interacting with actin, regulate the polymerization, depolymerization, bundling, and cross-linking of actin filaments, directly or indirectly, thereby mediating the maintenance of cell morphology, cell movement, and many other biological functions. Consequently, these functions of ABPs help regulate cancer cell invasion and metastasis when cancer occurs. In recent years, a variety of ABPs have been found to be abnormally expressed in various cancers, indicating that the detection and interventions of unusual ABP expression to alter this are available for the treatment of cancer. The early stages of most cancer development involve long-term chronic inflammation or repeated stimulation. This is the case for breast cancer, gastric cancer, lung cancer, prostate cancer, liver cancer, esophageal cancer, pancreatic cancer, melanoma, and colorectal cancer. This article discusses the relationship between chronic inflammation and the above-mentioned cancers, emphatically introduces relevant research on the abnormal expression of ABPs in chronic inflammatory diseases, and reviews research on the expression of different ABPs in the above-mentioned cancers. Furthermore, there is a close relationship between ABP-induced inflammation and cancer. In simple terms, abnormal expression of ABPs contributes to the chronic inflammation developing into cancer. Finally, we provide our viewpoint regarding these unusual ABPs serving as potential biomarkers for chronic inflammation-induced cancer diagnosis and therapy, and interventions to reverse the abnormal expression of ABPs represent a potential approach to preventing or treating the corresponding cancers.

Actin binding to galectin-13/placental protein-13 occurs independently of the galectin canonical ligand binding site

The gene for galectin-13 (Gal-13, placental protein 13) is only present in primates, and its low expression level in maternal serum may promote pre-eclampsia. In the present study, we used pull-down experiments and biolayer interferometry to assess the interaction between Gal-13 and actin. These studies uncovered that human Gal-13 (hGal-13) and Saimiri boliviensis boliviensis (sGal-13) strongly bind to α- and β/γ-actin, with Ca2+ and ATP significantly enhancing interactions. This in turn suggests that h/sGal-13 may inhibit myosin-induced contraction when vascular smooth muscle cells undergo polarization. Here, we solved the crystal structure of sGal-13 bound to lactose and found that it exists as a monomer in contrast to hGal-13 that is a dimer. The distribution of sGal-13 in HeLa cells is similar to that of hGal-13, indicating that monomeric Gal-13 is the primary form in cells. Even though sGal-13 also binds to actin, hGal-13 ligand binding site mutants do not influence hGal-13/actin binding, whereas the monomeric mutant C136S/C138S binds to actin more strongly than wild type hGal-13. Overall, our study demonstrates that monomeric Gal-13 binds to actin, an interaction that is independent of the galectin canonical ligand binding site.

CAP2 is a regulator of actin pointed end dynamics and myofibrillogenesis in cardiac muscle

The precise assembly of actin-based thin filaments is crucial for muscle contraction. Dysregulation of actin dynamics at thin filament pointed ends results in skeletal and cardiac myopathies. Here, we discovered adenylyl cyclase-associated protein 2 (CAP2) as a unique component of thin filament pointed ends in cardiac muscle. CAP2 has critical functions in cardiomyocytes as it depolymerizes and inhibits actin incorporation into thin filaments. Strikingly distinct from other pointed-end proteins, CAP2's function is not enhanced but inhibited by tropomyosin and it does not directly control thin filament lengths. Furthermore, CAP2 plays an essential role in cardiomyocyte maturation by modulating pre-sarcomeric actin assembly and regulating α-actin composition in mature thin filaments. Identification of CAP2's multifunctional roles provides missing links in our understanding of how thin filament architecture is regulated in striated muscle and it reveals there are additional factors, beyond Tmod1 and Lmod2, that modulate actin dynamics at thin filament pointed ends.

Novel insights into sarcomere regulatory systems control of cardiac thin filament activation

Our review focuses on sarcomere regulatory mechanisms with a discussion of cardiac-specific modifications to the three-state model of thin filament activation from a blocked to closed to open state. We discuss modulation of these thin filament transitions by Ca²⁺, by crossbridge interactions, and by thick filament–associated proteins, cardiac myosin–binding protein C (cMyBP-C), cardiac regulatory light chain (cRLC), and titin. Emerging evidence supports the idea that the cooperative activation of the thin filaments despite a single Ca²⁺ triggering regulatory site on troponin C (cTnC) cannot be considered in isolation of other functional domains of the sarcomere. We discuss long- and short-range interactions among these domains with the regulatory units of thin filaments, including proteins at the barbed end at the Z-disc and the pointed end near the M-band. Important to these discussions is the ever-increasing understanding of the role of cMyBP-C, cRLC, and titin filaments. Detailed knowledge of these control processes is critical to the understanding of mechanisms sustaining physiological cardiac state with varying hemodynamic load, to better defining genetic and acquired cardiac disorders, and to developing targets for therapies at the level of the sarcomeres.

A crucial residue in the hydrophobic core of the solenoid structure of leucine rich repeats

Leucine rich repeats (LRRs) with 20-30 residues form a super helix arrangement. Individual LRRs are separated into a highly conserved segment with a highly conserved (HCS) and a variable segment (VS). In LRRs short β-strands in HCS stack in parallel, while VS adopts various secondary structures. Among eleven classes recognized, only RI-like, Cysteine-containing (CC), and GALA classes adopt an α-helix. However, the repeat unit lengths are usually different from each other. We performed some analyses based on the atomic coordinates in the known LRR structures. In the VS consensuses of the three classes, position 8 in the VS part is, in common, occupied by conserved aliphatic residue adopting an α-helix. This aliphatic residue is near to the two conserved hydrophobic residues at position 4 (in the center of β-strands) in two adjacent HCS parts. The conserved aliphatic residue plays a crucial role to preserve two parallel β-strands.

Twinfilin uncaps filament barbed ends to promote turnover of lamellipodial actin networks

Coordinated polymerization of actin filaments provides force for cell migration, morphogenesis and endocytosis. Capping protein (CP) is a central regulator of actin dynamics in all eukaryotes. It binds to actin filament (F-actin) barbed ends with high affinity and slow dissociation kinetics to prevent filament polymerization and depolymerization. However, in cells, CP displays remarkably rapid dynamics within F-actin networks, but the underlying mechanism remains unclear. Here, we report that the conserved cytoskeletal regulator twinfilin is responsible for CP's rapid dynamics and specific localization in cells. Depletion of twinfilin led to stable association between CP and cellular F-actin arrays, as well as to its retrograde movement throughout leading-edge lamellipodia. These were accompanied by diminished F-actin turnover rates. In vitro single-filament imaging approaches revealed that twinfilin directly promotes dissociation of CP from filament barbed ends, while enabling subsequent filament depolymerization. These results uncover a bipartite mechanism that controls how actin cytoskeleton-mediated forces are generated in cells.

Structural insights into the tropomodulin assembly at the pointed ends of actin filaments

Tropomodulins are a family of important regulators of actin dynamics at the pointed ends of actin filaments. Four isoforms of tropomodulin, Tmod1-Tmod4, are expressed in vertebrates. Binding of tropomodulin to the pointed end is dependent on tropomyosin, an actin binding protein that itself is represented in mammals by up to 40 isoforms. The understanding of the regulatory role of the tropomodulin/tropomyosin molecular diversity has been limited due to the lack of a three-dimensional structure of the tropomodulin/tropomyosin complex. In this study, we mapped tropomyosin residues interacting with two tropomyosin-binding sites of tropomodulin and generated a three-dimensional model of the tropomodulin/tropomyosin complex for each of these sites. The models were refined by molecular dynamics simulations and validated via building a self-consistent three-dimensional model of tropomodulin assembly at the pointed end. The model of the pointed-end Tmod assembly offers new insights in how Tmod binding ensures tight control over the pointed end dynamics.

Lysine acetylation of cytoskeletal proteins: Emergence of an actin code

Reversible lysine acetylation of nuclear proteins such as histones is a long-established important regulatory mechanism for chromatin remodeling and transcription. In the cytoplasm, acetylation of a number of cytoskeletal proteins, including tubulin, cortactin, and the formin mDia2, regulates both cytoskeletal assembly and stability. More recently, acetylation of actin itself was revealed to regulate cytoplasmic actin polymerization through the formin INF2, with downstream effects on ER-to-mitochondrial calcium transfer, mitochondrial fission, and vesicle transport. This finding raises the possibility that actin acetylation, along with other post-translational modifications to actin, might constitute an "actin code," similar to the "histone code" or "tubulin code," controlling functional shifts to these central cellular proteins. Given the multiple roles of actin in nuclear functions, its modifications might also have important roles in gene expression.

Enzymatic Noncovalent Synthesis

Enzymatic reactions and noncovalent (i.e., supramolecular) interactions are two fundamental nongenetic attributes of life. Enzymatic noncovalent synthesis (ENS) refers to a process where enzymatic reactions control intermolecular noncovalent interactions for spatial organization of higher-order molecular assemblies that exhibit emergent properties and functions. Like enzymatic covalent synthesis (ECS), in which an enzyme catalyzes the formation of covalent bonds to generate individual molecules, ENS is a unifying theme for understanding the functions, morphologies, and locations of molecular ensembles in cellular environments. This review intends to provide a summary of the works of ENS within the past decade and emphasize ENS for functions. After comparing ECS and ENS, we describe a few representative examples where nature uses ENS, as a rule of life, to create the ensembles of biomacromolecules for emergent properties/functions in a myriad of cellular processes. Then, we focus on ENS of man-made (synthetic) molecules in cell-free conditions, classified by the types of enzymes. After that, we introduce the exploration of ENS of man-made molecules in the context of cells by discussing intercellular, peri/intracellular, and subcellular ENS for cell morphogenesis, molecular imaging, cancer therapy, and other applications. Finally, we provide a perspective on the promises of ENS for developing molecular assemblies/processes for functions. This review aims to be an updated introduction for researchers who are interested in exploring noncovalent synthesis for developing molecular science and technologies to address societal needs.

Leiomodin creates a leaky cap at the pointed end of actin-thin filaments

Improper lengths of actin-thin filaments are associated with altered contractile activity and lethal myopathies. Leiomodin, a member of the tropomodulin family of proteins, is critical in thin filament assembly and maintenance; however, its role is under dispute. Using nuclear magnetic resonance data and molecular dynamics simulations, we generated the first atomic structural model of the binding interface between the tropomyosin-binding site of cardiac leiomodin and the N-terminus of striated muscle tropomyosin. Our structural data indicate that the leiomodin/tropomyosin complex only forms at the pointed end of thin filaments, where the tropomyosin N-terminus is not blocked by an adjacent tropomyosin protomer. This discovery provides evidence supporting the debated mechanism where leiomodin and tropomodulin regulate thin filament lengths by competing for thin filament binding. Data from experiments performed in cardiomyocytes provide additional support for the competition model; specifically, expression of a leiomodin mutant that is unable to interact with tropomyosin fails to displace tropomodulin at thin filament pointed ends and fails to elongate thin filaments. Together with previous structural and biochemical data, we now propose a molecular mechanism of actin polymerization at the pointed end in the presence of bound leiomodin. In the proposed model, the N-terminal actin-binding site of leiomodin can act as a "swinging gate" allowing limited actin polymerization, thus making leiomodin a leaky pointed-end cap. Results presented in this work answer long-standing questions about the role of leiomodin in thin filament length regulation and maintenance.

Nanoscopy reveals the layered organization of the sarcomeric H-zone and I-band complexes

Sarcomeres are extremely highly ordered macromolecular assemblies where structural organization is intimately linked to their functionality as contractile units. Although the structural basis of actin and Myosin interaction is revealed at a quasiatomic resolution, much less is known about the molecular organization of the I-band and H-zone. We report the development of a powerful nanoscopic approach, combined with a structure-averaging algorithm, that allowed us to determine the position of 27 sarcomeric proteins in Drosophila melanogaster flight muscles with a quasimolecular, ∼5- to 10-nm localization precision. With this protein localization atlas and template-based protein structure modeling, we have assembled refined I-band and H-zone models with unparalleled scope and resolution. In addition, we found that actin regulatory proteins of the H-zone are organized into two distinct layers, suggesting that the major place of thin filament assembly is an M-line-centered narrow domain where short actin oligomers can form and subsequently anneal to the pointed end.

Actin Mutations and Their Role in Disease

Actin is a widely expressed protein found in almost all eukaryotic cells. In humans, there are six different genes, which encode specific actin isoforms. Disease-causing mutations have been described for each of these, most of which are missense. Analysis of the position of the resulting mutated residues in the protein reveals mutational hotspots. Many of these occur in regions important for actin polymerization. We briefly discuss the challenges in characterizing the effects of these actin mutations, with a focus on cardiac actin mutations.

Identification of significant protein markers by mass spectrometry after co-treatment of cells with different drugs: An in vitro survey platform

RATIONALE

Understanding drug-drug interactions and predicting the side effects induced by polypharmacy are difficult because there are few suitable platforms that can predict drug-drug interactions and possible side effects. Hence, developing a platform to identify significant protein markers of drug-drug interactions and their associated side effects is necessary to avoid adverse effects.

METHODS

Human liver cells were treated with ethosuximide in combination with cimetidine, ketotifen, metformin, metronidazole, or phenytoin. After sample preparation and extraction, mitochondrial proteins from liver cells were isolated and digested with trypsin. Then, peptide solutions were detected using a nano ultra-performance liquid chromatographic system combined with tandem mass spectrometry. The Ingenuity Pathway Analysis tool was used to simulate drug-drug interactions and identify protein markers associated with drug-induced adverse effects.

RESULTS

Several protein markers were identified by the proposed method after liver cells were co-treated with ethosuximide and other drugs. Several of these protein markers have previously been reported in the literature, indicating that the proposed platform is workable.

CONCLUSIONS

Using the proposed in vitro platform, significant protein markers of drug-drug interactions could be identified by mass spectrometry. This workflow can then help predict indicators of drug-drug interactions and associated adverse effects for increased safety in clinical prescriptions.

Tropomodulins Control the Balance between Protrusive and Contractile Structures by Stabilizing Actin-Tropomyosin Filaments

Eukaryotic cells have diverse protrusive and contractile actin filament structures, which compete with one another for a limited pool of actin monomers. Numerous actin-binding proteins regulate the dynamics of actin structures, including tropomodulins (Tmods), which cap the pointed end of actin filaments. In striated muscles, Tmods prevent actin filaments from overgrowing, whereas in non-muscle cells, their function has remained elusive. Here, we identify two Tmod isoforms, Tmod1 and Tmod3, as key components of contractile stress fibers in non-muscle cells. Individually, Tmod1 and Tmod3 can compensate for one another, but their simultaneous depletion results in disassembly of actin-tropomyosin filaments, loss of force-generating stress fibers, and severe defects in cell morphology. Knockout-rescue experiments reveal that Tmod's interaction with tropomyosin is essential for its role in the stabilization of actin-tropomyosin filaments in cells. Thus, in contrast to their role in muscle myofibrils, in non-muscle cells, Tmods bind actin-tropomyosin filaments to protect them from depolymerizing, not elongating. Furthermore, loss of Tmods shifts the balance from linear actin-tropomyosin filaments to Arp2/3 complex-nucleated branched networks, and this phenotype can be partially rescued by inhibiting the Arp2/3 complex. Collectively, the data reveal that Tmods are essential for the maintenance of contractile actomyosin bundles and that Tmod-dependent capping of actin-tropomyosin filaments is critical for the regulation of actin homeostasis in non-muscle cells.

Capping proteins regulate fungal development, DON-toxisome formation and virulence in Fusarium graminearum

Deoxynivalenol (DON) is an important trichothecene mycotoxin produced by the cereal pathogen Fusarium graminearum. DON is synthesized in organized endoplasmic reticulum structures called toxisomes. However, the mechanism for toxisome formation and the components of toxisomes are not yet fully understood. In a previous study, we found that myosin I (FgMyo1)-actin cytoskeleton participated in toxisome formation. In the current study, we identified two new components of toxisomes, the actin capping proteins (CAPs) FgCapA and FgCapB. These two CAPs form a heterodimer in F. graminearum, and physically interact with FgMyo1 and Tri1. The deletion mutants ΔFgcapA and ΔFgcapB and the double deletion mutant ΔΔFgcapA/B dramatically reduced hyphal growth, asexual and sexual reproduction and endocytosis. More importantly, the deletion mutants markedly disrupted toxisome formation and DON production, and attenuated virulence in planta. Collectively, these results suggest that the actin CAPs are associated with toxisome formation and contribute to the virulence and development of F. graminearum.

Temporal Profile of Brain Gene Expression After Prey Catching Conditioning in an Anuran Amphibian

A key goal in modern neurobiology is to understand the mechanisms underlying learning and memory. To that end, it is essential to identify the patterns of gene expression and the temporal sequence of molecular events associated with learning and memory processes. It is also important to ascertain if and how these molecular events vary between organisms. In vertebrates, learning and memory processes are characterized by distinct phases of molecular activity involving gene transcription, structural change, and long-term maintenance of such structural change in the nervous system. Utilizing next generation sequencing techniques, we profiled the temporal expression patterns of genes in the brain of the fire-bellied toad Bombina orientalis after prey catching conditioning. The fire-bellied toad is a basal tetrapod whose neural architecture and molecular pathways may help us understand the ancestral state of learning and memory mechanisms in tetrapods. Differential gene expression following conditioning revealed activity in molecular pathways related to immediate early genes (IEG), cytoskeletal modification, axon guidance activity, and apoptotic processes. Conditioning induced early IEG activity coinciding with transcriptional activity and neuron structural modification, followed by axon guidance and cell adhesion activity, and late neuronal pruning. While some of these gene expression patterns are similar to those found in mammals submitted to conditioning, some interesting divergent expression profiles were seen, and differential expression of some well-known learning-related mammalian genes is missing altogether. These results highlight the importance of using a comparative approach in the study of the mechanisms of leaning and memory and provide molecular resources for a novel vertebrate model in the relatively poorly studied Amphibia.

Towards a structural understanding of the remodeling of the actin cytoskeleton

Actin filaments (F-actin) are a key component of eukaryotic cells. Whether serving as a scaffold for myosin or using their polymerization to push onto cellular components, their function is always related to force generation. To control and fine-tune force production, cells have a large array of actin-binding proteins (ABPs) dedicated to control every aspect of actin polymerization, filament localization, and their overall mechanical properties. Although great advances have been made in our biochemical understanding of the remodeling of the actin cytoskeleton, the structural basis of this process is still being deciphered. In this review, we summarize our current understanding of this process. We outline how ABPs control the nucleation and disassembly, and how these processes are affected by the nucleotide state of the filaments. In addition, we highlight recent advances in the understanding of actomyosin force generation, and describe recent advances brought forward by the developments of electron cryomicroscopy.

Key Genes And Pathways Controlled By E2F1 In Human Castration-Resistant Prostate Cancer Cells

Background

Treatment of castration-resistant prostate cancer (CRPC) is an enormous challenge. As E2F transcription factor 1 (E2F1) is an essential factor in CRPC, this study investigated the genes and pathways controlled by E2F1 and their effects on cellular behavior in CRPC.

Methods

In vitro assays were used to evaluate cellular proliferation, apoptosis, and behavior. Cellular expression was quantified by RNA sequencing (RNA-seq). Gene co-expression was assessed using the GeneMANIA database, and correlations were analyzed with the GEPIA server. Altered pathways of differentially expressed genes (DEGs) were revealed by functional annotation. Module analysis was performed using the STRING database and hub genes were filtered with the Cytoscape software. Some DEGs were validated by real-time quantitative PCR (RT-qPCR).

Results

Knockdown of E2F1 significantly inhibited proliferation and accelerated apoptosis in PC3 cells but not in DU145 cells. Invasion and migration were reduced for both cell lines. A total of 1811 DEGs were identified in PC3 cells and 27 DEGs in DU145 cells exhibiting E2F1 knockdown. Ten overlapping DEGs, including TMOD2 and AIF1L, were identified in both knockdown cell lines and were significantly enriched for association with actin filament organization pathways. TMOD2 and KREMEN2 were genes co-expressed with E2F1; six overlapping DEGs were positively correlated with transcription factor E2F1. DEGs of the PC3 and DU145 groups were associated with multiple pathways. Five DEGs that overlapped between the two cell lines and three hub DEGs from PC3 cells were validated by RT-qPCR.

Conclusion

The results of this study suggest that E2F1 has a critical role in regulating actin filaments, as indicated by the change in expression level of several genes, including TMOD2 and AIF1L, in CRPC. This extends our understanding of the cellular responses affected by E2F1 in CRPC.

Mechanism of synergistic actin filament pointed end depolymerization by cyclase-associated protein and cofilin

The ability of cells to generate forces through actin filament turnover was an early adaptation in evolution. While much is known about how actin filaments grow, mechanisms of their disassembly are incompletely understood. The best-characterized actin disassembly factors are the cofilin family proteins, which increase cytoskeletal dynamics by severing actin filaments. However, the mechanism by which severed actin filaments are recycled back to monomeric form has remained enigmatic. We report that cyclase-associated-protein (CAP) works in synergy with cofilin to accelerate actin filament depolymerization by nearly 100-fold. Structural work uncovers the molecular mechanism by which CAP interacts with actin filament pointed end to destabilize the interface between terminal actin subunits, and subsequently recycles the newly-depolymerized actin monomer for the next round of filament assembly. These findings establish CAP as a molecular machine promoting rapid actin filament depolymerization and monomer recycling, and explain why CAP is critical for actin-dependent processes in all eukaryotes.

The cofilin family proteins are actin disassembly factors but the disassembly mechanism is poorly understood. Here authors show that cyclase-associated-protein (CAP) works in synergy with cofilin to accelerate actin filament depolymerization by nearly 100-fold and reveal how CAP destabilizes the interface between terminal actin subunits.

Transforming growth factor β1 alters the 3'-UTR of mRNA to promote lung fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic disease characterized by the pathological remodeling of air sacs as a result of excessive accumulation of extracellular matrix (ECM) proteins, but the mechanism governing the robust protein expression is poorly understood. Our recent findings demonstrate that alternative polyadenylation (APA) caused by NUDT21 reduction is important for the increased expression of fibrotic mediators and ECM proteins in lung fibroblasts by shortening the 3'-untranslated regions (3'-UTRs) of mRNAs and stabilizing their transcripts, therefore activating pathological signaling pathways. Despite the importance of NUDT21 reduction in the regulation of fibrosis, the underlying mechanisms for the depletion are unknown. We demonstrate here that NUDT21 is depleted by TGFβ1. We found that miR203, which is increased in IPF, was induced by TGFβ1 to target the NUDT21 3'-UTR, thus depleting NUDT21 in human and mouse lung fibroblasts. TGFβ1-mediated NUDT21 reduction was attenuated by the miR203 inhibitor antagomiR203 in fibroblasts. TGFβ1 transgenic mice revealed that TGFβ1 down-regulates NUDT21 in fibroblasts in vivo Furthermore, TGFβ1 promoted differential APA of fibrotic genes, including FGF14, RICTOR, TMOD2, and UCP5, in association with increased protein expression. This unique differential APA signature was also observed in IPF fibroblasts. Altogether, our results identified TGFβ1 as an APA regulator through NUDT21 depletion amplifying pulmonary fibrosis.

Tropomodulin 3 modulates EGFR-PI3K-AKT signaling to drive hepatocellular carcinoma metastasis

The mechanism of hepatocellular carcinoma (HCC) metastasis remains poorly understood. Tropomodulin 3 (TMOD3) is a member of the pointed end capping protein family that contributes to invasion and metastasis in several types of malignancies. It has been found to be crucial for the membranous skeleton and embryonic development, although, its role in HCC progression remains largely unclear. We observed increased levels of Tmod3 in HCCs, especially in extrahepatic metastasis. High Tmod3 expression correlated with aggressive carcinoma and poor patient with HCC survival. Loss-of-function studies conducted by us determined Tmod3 as an oncogene that promoted HCC growth and metastasis. Mechanistically, Tmod3 increases transcription of matrix metalloproteinase-2, -7, and -9 which required PI3K-AKT. Interaction between Tmod3 and epidermal growth factor receptor (EGFR) that supports the activation of EGFR phosphorylation, is essential for signaling activation of PI3K-AKT viral oncogene homolog. These findings reveal that Tmod3 enhances aggressive behavior of HCC both in vitro and in vivo by interacting with EFGR and by activating the PI3K-AKT signaling pathway.

Thin filament dysfunctions caused by mutations in tropomyosin Tpm3.12 and Tpm1.1

Tropomyosin is the major regulator of the thin filament. In striated muscle its function is to bind troponin complex and control the access of myosin heads to actin in a Ca²⁺-dependent manner. It also participates in the maintenance of thin filament length by regulation of tropomodulin and leiomodin, the pointed end-binding proteins. Because the size of the overlap between actin and myosin filaments affects the number of myosin heads which interact with actin, the filament length is one of the determinants of force development. Numerous point mutations in genes encoding tropomyosin lead to single amino acid substitutions along the entire length of the coiled coil that are associated with various types of cardiomyopathy and skeletal muscle disease. Specific regions of tropomyosin interact with different binding partners; therefore, the mutations affect diverse tropomyosin functions. In this review, results of studies on mutations in the genes TPM1 and TPM3, encoding Tpm1.1 and Tpm3.12, are described. The paper is particularly focused on mutation-dependent alterations in the mechanisms of actin-myosin interactions and dynamics of the thin filament at the pointed end.

Role of intrinsic disorder in muscle sarcomeres

The role and utility of intrinsically disordered regions (IDRs) is reviewed for two groups of sarcomeric proteins, such as members of tropomodulin/leiomodin (Tmod/Lmod) protein homology group and myosin binding protein C (MyBP-C). These two types of sarcomeric proteins represent very different but strongly interdependent functions, being responsible for maintaining structure and operation of the muscle sarcomere. The role of IDRs in the formation of complexes between thin filaments and Tmods/Lmods is discussed within the framework of current understanding of the thin filament length regulation. For MyBP-C, the function of IDRs is discussed in the context of MYBP-C-dependent sarcomere contraction and actomyosin activation.

The Chinese Medicinal Formulation Guzhi Zengsheng Zhitongwan Modulates Chondrocyte Structure, Dynamics, and Metabolism by Controlling Multiple Functional Proteins

Traditional Chinese medicine is one of the oldest medical systems in the world and has its unique principles and theories in the prevention and treatment of human diseases, which are achieved through the interactions of different types of materia medica in the form of Chinese medicinal formulations. GZZSZTW, a classical and effective Chinese medicinal formulation, was designed and created by professor Bailing Liu who is the only national medical master professor in the clinical research field of traditional Chinese medicine and skeletal diseases. GZZSZTW has been widely used in clinical settings for several decades for the treatment of joint diseases. However, the underlying molecular mechanisms are still largely unknown. In the present study, we performed quantitative proteomic analysis to investigate the effects of GZZSZTW on mouse primary chondrocytes using state-of-the-art iTRAQ technology. We demonstrated that the Chinese medicinal formulation GZZSZTW modulates chondrocyte structure, dynamics, and metabolism by controlling multiple functional proteins that are involved in the cellular processes of DNA replication and transcription, protein synthesis and degradation, cytoskeleton dynamics, and signal transduction. Thus, this study has expanded the current knowledge of the molecular mechanism of GZZSZTW treatment on chondrocytes. It has also shed new light on possible strategies to further prevent and treat cartilage-related diseases using traditional Chinese medicinal formulations.