Tropomodulin1 exacerbates inflammatory response in macrophages by negatively regulating LPS-induced TLR4 endocytosis.

The excessive inflammation caused by the prolonged activation of Toll-like receptor 4 (TLR4) and its downstream signaling pathways leads to sepsis. CD14-mediated endocytosis of TLR4 is the key step to control the amount of TLR4 on cell membrane and the activity of downstream pathways. The actin cytoskeleton is necessary for receptor-mediated endocytosis, but its role in TLR4 endocytosis remains elusive. Here we show that Tropomodulin 1 (Tmod1), an actin capping protein, inhibited lipopolysaccharide (LPS)-induced TLR4 endocytosis and intracellular trafficking in macrophages. Thus it resulted in increased surface TLR4 and the upregulation of myeloid differentiation factor 88 (MyD88)-dependent pathway and the downregulation of TIR domain-containing adaptor-inducing interferon-ß (TRIF)-dependent pathway, leading to the enhanced secretion of inflammatory cytokines, such as TNF-α and IL-6, and the reduced secretion of cytokines, such as IFN-ß. Macrophages deficient with Tmod1 relieved the inflammatory response in LPS-induced acute lung injury mouse model. Mechanistically, Tmod1 negatively regulated LPS-induced TLR4 endocytosis and inflammatory response through modulating the activity of CD14/Syk/PLCγ2/IP3/Ca2+ signaling pathway, the reorganization of actin cytoskeleton, and the membrane tension. Therefore, Tmod1 is a key regulator of inflammatory response and immune functions in macrophages and may be a potential target for the treatment of excessive inflammation and sepsis.

Tropomodulin 3 Overexpression as a Marker for Platinum Resistance and Immune Infiltration in Ovarian Cancer.

The cytoskeleton plays an important role in platinum resistance in ovarian cancer. Tropomodulin 3 (TMOD3) is critical in the development of many tumors, but its role in the drug resistance of ovarian cancer remains unexplored. By analyzing data from the Gene Expression Omnibus (GEO), The Cancer Genome Atlas (TCGA), and Clinical Proteomic Tumor Analysis Consortium (CPTAC) databases, this study compared TMOD3 expression in ovarian cancer and normal tissues, and examined the expression of TMOD3 after platinum treatment in platinum-sensitive and platinum-resistant ovarian cancers. The Kaplan-Meier method was used to assess the effect of TMOD3 on overall survival (OS) and progression-free survival (PFS) in ovarian cancer patients. microRNAs (miRNAs) targeting TMOD3 were predicted using TargetScan and analyzed using the TCGA database. Tumor Immune Estimation Resource (TIMER) and an integrated repository portal for tumor-immune system interactions (TISIDB) were used to determine the relationship between TMOD3 expression and immune infiltration. TMOD3 coexpression networks in ovarian cancer were explored using LinkedOmics, the Search Tool for the Retrieval of Interacting Genes/Proteins (STRING), and The Database for Annotation, Visualization, and Integrated Discovery (DAVID) Bioinformatics. The results showed that TMOD3 was highly expressed in ovarian cancer and was associated with the grading, staging, and metastasis of ovarian cancer. TMOD3 expression was significantly reduced in platinum-treated ovarian cancer cells and patients. However, TMOD3 expression was higher in platinum-resistant ovarian cancer cells and tissues compared to platinum-sensitive ones. Higher TMOD3 expression was significantly associated with lower OS and PFS in ovarian cancer patients treated with platinum-based chemotherapy. miRNA-mediated post-transcriptional regulation is likely responsible for high TMOD3 expression in ovarian cancer and platinum-resistant ovarian tissues. The expression of TMOD3 mRNA was associated with immune infiltration in ovarian cancer. These findings indicate that TMOD3 is highly expressed in ovarian cancer and is closely associated with platinum resistance and immune infiltration.

Silencing of tropomodulin 1 inhibits acute myeloid leukemia cell proliferation and tumor growth by elevating karyopherin alpha 2-mediated autophagy.

Evidence shows that tropomodulin 1 (TMOD1) is a powerful diagnostic marker in the progression of several cancer types. However, the regulatory mechanism of TMOD1 in tumor progression is still unclear. Here, we showed that TMOD1 was highly expressed in acute myeloid leukemia (AML) specimens, and TMOD1-silencing inhibited cell proliferation by inducing autophagy in AML THP-1 and MOLM-13 cells. Mechanistically, the C-terminal region of TMOD1 directly bound to KPNA2, and TMOD1-overexpression promoted KPNA2 ubiquitylation and reduced KPNA2 levels. In contrast, TMOD1-silencing increased KPNA2 levels and facilitated the nuclear transfer of KPNA2, then subsequently induced autophagy and inhibited cell proliferation by increasing the nucleocytoplasmic transport of p53 and AMPK activation. KPNA2/p53 inhibitors attenuated autophagy induced by silencing TMOD1 in AML cells. Silencing TMOD1 also inhibited tumor growth by elevating KPNA2-mediated autophagy in nude mice bearing MOLM-13 xenografts. Collectively, our data demonstrated that TMOD1 could be a novel therapeutic target for AML treatment.

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.

A nemaline myopathy-linked mutation inhibits the actin-regulatory functions of tropomodulin and leiomodin.

Actin is a highly expressed protein in eukaryotic cells and is essential for numerous cellular processes. In particular, efficient striated muscle contraction is dependent upon the precise regulation of actin-based thin filament structure and function. Alterations in the lengths of actin-thin filaments can lead to the development of myopathies. Leiomodins and tropomodulins are members of an actin-binding protein family that fine-tune thin filament lengths, and their dysfunction is implicated in muscle diseases. An Lmod3 mutation [G326R] was previously identified in patients with nemaline myopathy (NM), a severe skeletal muscle disorder; this residue is conserved among Lmod and Tmod isoforms and resides within their homologous leucine-rich repeat (LRR) domain. We mutated this glycine to arginine in Lmod and Tmod to determine the physiological function of this residue and domain. This G-to-R substitution disrupts Lmod and Tmod's LRR domain structure, altering their binding interface with actin and destroying their abilities to regulate thin filament lengths. Additionally, this mutation renders Lmod3 nonfunctional in vivo. We found that one single amino acid is essential for folding of Lmod and Tmod LRR domains, and thus is essential for the opposing actin-regulatory functions of Lmod (filament elongation) and Tmod (filament shortening), revealing a mechanism underlying the development of NM.

Novel secreted STPKLRR from Vibrio splendidus AJ01 promotes pathogen internalization via mediating tropomodulin phosphorylation dependent cytoskeleton rearrangement.

We previously demonstrated that the flagellin of intracellular Vibrio splendidus AJ01 could be specifically identified by tropomodulin (Tmod) and further mediate p53-dependent coelomocyte apoptosis in the sea cucumber Apostichopus japonicus. In higher animals, Tmod serves as a regulator in stabilizing the actin cytoskeleton. However, the mechanism on how AJ01 breaks the AjTmod-stabilized cytoskeleton for internalization remains unclear. Here, we identified a novel AJ01 Type III secretion system (T3SS) effector of leucine-rich repeat-containing serine/threonine-protein kinase (STPKLRR) with five LRR domains and a serine/threonine kinase (STYKc) domain, which could specifically interact with tropomodulin domain of AjTmod. Furthermore, we found that STPKLRR directly phosphorylated AjTmod at serine 52 (S52) to reduce the binding stability between AjTmod and actin. After AjTmod dissociated from actin, the F-actin/G-actin ratio decreased to induce cytoskeletal rearrangement, which in turn promoted the internalization of AJ01. The STPKLRR knocked out strain could not phosphorylated AjTmod and displayed lower internalization capacity and pathogenic effect compared to AJ01. Overall, we demonstrated for the first time that the T3SS effector STPKLRR with kinase activity was a novel virulence factor in Vibrio and mediated self-internalization by targeting host AjTmod phosphorylation dependent cytoskeleton rearrangement, which provided a candidate target to control AJ01 infection in practice.

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.

Suggestive evidence of the genetic association of TMOD1 and PTCSC2 polymorphisms with thyroid carcinoma in the Chinese Han population.

BACKGROUND: The purpose of this study was to survey the associations of six single nucleotide polymorphisms (SNPs) in the TMOD1 and PTCSC2 genes with thyroid carcinoma (TC). METHOD: Peripheral blood samples were obtained from 510 patients with TC and 509 normal controls. Six SNPs were genotyped by the Agena MassARRAY platform. Logistic regression was used to evaluate the association between SNPs and TC susceptibility by calculating odds ratios (ORs) and 95% confidence intervals (CIs). SNP-SNP interactions were analyzed by multifactor dimensionality reduction (MDR). RESULTS: Our study showed that rs925489 (OR = 1.45, p = 0.011) and rs965513 (OR = 1.40, p = 0.021) were significantly associated with an increased risk of TC. Rs10982622 decreased TC risk (OR = 0.74, p = 0.025). Further stratification analysis showed that rs10982622 reduced the susceptibility to TC in patients aged ≤ 45 years (OR = 0.69, p = 0.019) and in females (OR = 0.61, p = 0.014). Rs925489 increased TC risk in people aged > 45 years (OR = 1.54, p = 0.044) and in males (OR = 2.34, p = 0.003). In addition, rs965513 was related to an increased risk of TC in males (OR = 2.14, p = 0.007). Additionally, haplotypes in the block (rs925489|rs965513) significantly increased TC risk (p < 0.05). The best predictive model for TC was the combination of rs1052270, rs10982622, rs1475545, rs16924016, and rs925489. CONCLUSION: TMOD1 and PTCSC2 polymorphisms were separately correlated with a remarkable decrease and increase in TC risk based on the analysis.

Mutual dependence between tropomodulin and tropomyosin in the regulation of sarcomeric actin assembly in Caenorhabditis elegans striated muscle.

Tropomodulin and tropomyosin are important components of sarcomeric thin filaments in striated muscles. Tropomyosin decorates the side of actin filaments and enhances tropomodulin capping at the pointed ends of the filaments. Their functional relationship has been extensively characterized in vitro, but in vivo and cellular studies in mammals are often complicated by the presence of functionally redundant isoforms. Here, we used the nematode Caenorhabditis elegans, which has a relatively simple composition of tropomodulin and tropomyosin genes, and demonstrated that tropomodulin (unc-94) and tropomyosin (lev-11) are mutually dependent on each other in their sarcomere localization and regulation of sarcomeric actin assembly. Mutation of tropomodulin caused sarcomere disorganization with formation of actin aggregates. However, the actin aggregation was suppressed when tropomyosin was depleted in the tropomodulin mutant. Tropomyosin was mislocalized to the actin aggregates in the tropomodulin mutants, while sarcomere localization of tropomodulin was lost when tropomyosin was depleted. These results indicate that tropomodulin and tropomyosin are interdependent in the regulation of organized sarcomeric assembly of actin filaments in vivo.

Erythroid differentiation in mouse erythroleukemia cells depends on Tmod3-mediated regulation of actin filament assembly into the erythroblast membrane skeleton.

Erythroid differentiation (ED) is a complex cellular process entailing morphologically distinct maturation stages of erythroblasts during terminal differentiation. Studies of actin filament (F-actin) assembly and organization during terminal ED have revealed essential roles for the F-actin pointed-end capping proteins, tropomodulins (Tmod1 and Tmod3). Tmods bind tropomyosins (Tpms), which enhance Tmod capping and F-actin stabilization. Tmods can also nucleate F-actin assembly, independent of Tpms. Tmod1 is present in the red blood cell (RBC) membrane skeleton, and deletion of Tmod1 in mice leads to a mild compensated anemia due to mis-regulated F-actin lengths and membrane instability. Tmod3 is not present in RBCs, and global deletion of Tmod3 leads to embryonic lethality in mice with impaired ED. To further decipher Tmod3's function during ED, we generated a Tmod3 knockout in a mouse erythroleukemia cell line (Mel ds19). Tmod3 knockout cells appeared normal prior to ED, but showed defects during progression of ED, characterized by a marked failure to reduce cell and nuclear size, reduced viability, and increased apoptosis. Tmod3 does not assemble with Tmod1 and Tpms into the Triton X-100 insoluble membrane skeleton during ED, and loss of Tmod3 had no effect on α1,ß1-spectrin and protein 4.1R assembly into the membrane skeleton. However, F-actin, Tmod1 and Tpms failed to assemble into the membrane skeleton during ED in absence of Tmod3. We propose that Tmod3 nucleation of F-actin assembly promotes incorporation of Tmod1 and Tpms into membrane skeleton F-actin, and that this is integral to morphological maturation and cell survival during erythroid terminal differentiation.

Genetic dissection of novel myopathy models reveals a role of CapZα and Leiomodin 3 during myofibril elongation.

Myofibrils within skeletal muscle are composed of sarcomeres that generate force by contraction when their myosin-rich thick filaments slide past actin-based thin filaments. Although mutations in components of the sarcomere are a major cause of human disease, the highly complex process of sarcomere assembly is not fully understood. Current models of thin filament assembly highlight a central role for filament capping proteins, which can be divided into three protein families, each ascribed with separate roles in thin filament assembly. CapZ proteins have been shown to bind the Z-disc protein α-actinin to form an anchoring complex for thin filaments and actin polymerisation. Subsequent thin filaments extension dynamics are thought to be facilitated by Leiomodins (Lmods) and thin filament assembly is concluded by Tropomodulins (Tmods) that specifically cap the pointed end of thin filaments. To study thin filament assembly in vivo, single and compound loss-of-function zebrafish mutants within distinct classes of capping proteins were analysed. The generated lmod3- and capza1b-deficient zebrafish exhibited aspects of the pathology caused by variations in their human orthologs. Although loss of the analysed main capping proteins of the skeletal muscle, capza1b, capza1a, lmod3 and tmod4, resulted in sarcomere defects, residual organised sarcomeres were formed within the assessed mutants, indicating that these proteins are not essential for the initial myofibril assembly. Furthermore, detected similarity and location of myofibril defects, apparent at the peripheral ends of myofibres of both Lmod3- and CapZα-deficient mutants, suggest a function in longitudinal myofibril growth for both proteins, which is molecularly distinct to the function of Tmod4.

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 with 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.

Down-regulated in renal cell carcinoma 1 (DRR1) regulates axon outgrowth during hippocampal neuron development.

Down-regulated in renal cell carcinoma 1 (DRR1), a unique stress-induced protein, is highly expressed in the nervous system. This study investigated the roles of DRR1 in the brain by examining its expression pattern at different developmental stages of a rat brain and in cultured primary hippocampal neurons. High expression of DRR1 was observed in all developmental stages of a rat brain and cultured primary hippocampal neurons. We then focused on the role of DRR1 in promoting neurite outgrowth during the early stage of hippocampal neuron development. Results showed that down-regulation of DRR1 suppressed axon outgrowth. Mass spectrometry analysis revealed that tropomodulin-2 (Tmod2) is a novel binding partner of DRR1. Our results showed that both DRR1 and Tmod2 mediate axon formation during the early stage of hippocampal neuron development. Suppression of TMOD2 expression rescued the abnormal axon outgrowth induced by DRR1 knockdown during the early stage of hippocampal neuron development.

Targeted regulation of miR-17-5p on TMOD1 promotes the development of cardia cancer.

OBJECTIVE: Cardia cancer is a common type of gastric cancer. Most clinical prevention and prognosis focus on surgical resection, but the efficacy is not satisfactory. Studying the molecular mechanism of pathogenesis of cardia cancer helps us intervene in prognosis and treatment. MATERIALS AND METHODS: a total of 134 normal cases related to cardia cancer and 62 cases of cardia cancer samples from the Gene Expression Omnibus (GEO) database were collected. A series of bioinformatics analyses, including differential gene analysis, co-expression analysis, enrichment analysis, regulator prediction, and (Protein-protein interaction) PPI analysis validation were performed. RESULTS: Differential analysis highlighted 10882 differential genes (p<0.05). Weighted gene co-expression network analysis indicated 6 functional disorder modules. TMOD1, JAM2, SPARC, ST18, NOS1 were key genes of each module. Enrichment analysis showed the dysfunctional module genes were mainly related to the proteinaceous extracellular matrix and neuroactive ligand-receptor interaction. Pivotal analysis of ncRNA demonstrated miR-17-5p significantly regulates modular genes including m1, m3, and m5. Target genes were backtracked according to the key regulators. Then, the Module_target gene_ncRNA interaction network diagram was constructed. The network shows m1 has the strongest regulation effect in the network. PPI showed that the core gene TMOD1 (Tropomodulin1) of m1 was at TOP10 in the algorithm. In other words, PPI indicated the importance of TMOD1 in the interaction network. CONCLUSIONS: We believe that targeted regulation of miR-17-5p on TMOD1 gene affects the neuroactive ligand-receptor interaction pathway, and it promotes proliferation and apoptosis of cardia cancer cells.

Quantitative proteomics reveals TMOD1-related proteins associated with water balance regulation.

The distal tubule and collecting duct in kidney regulate water homeostasis. TMOD1 is an actin capping protein that plays an important role in controlling the organization of actin filaments. In this study, we found TMOD1 was specifically expressed in distal tubules and collecting ducts. To investigate the role of TMOD1, we created Tmod1flox/flox mice and bred them with Ksp-Cre mice to generate tubule-specific Tmod1 knockout mice, Tmod1flox/flox/Ksp-Cre+ (designated as TFK). As compared with control mice, TFK mice showed oliguria, hyperosmolality urine, and high blood pressure. To determine the mechanisms underlying this phenotype, we performed label-free quantitative proteomics on kidneys of TFK and control mice. Total of 83 proteins were found differentially expressed. Bioinformatic analysis indicated that biological processes, including protein phosphorylation and metabolic process, were involved in TMOD1 regulatory network. Gene set enrichment analysis showed that multiple pathways, such as phosphatidylinositol signaling system and GnRH signaling pathway, were strongly associated with Tmod1 knockout. Western blot validated the down-regulation of three proteins, TGFBR2, SLC25A11, and MTFP1, in kidneys of TFK mice. Our study provides valuable information on the molecular functions and the regulatory network of Tmod1 gene in kidney, as well as the new mechanisms for the regulation of water balance.

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.

Tropomodulin 3 promotes liver cancer progression by activating the MAPK/ERK signaling pathway.

Tropomodulin 3 (TMOD3) is a member of the pointed­end capping protein family that contributes to invasion and metastasis in several types of malignancies. TMOD3 has been found to be crucial for membranous skeleton and embryonic development; however, little is known regarding the role of TMOD3 in liver cancer progression. In addition, to the best of our knowledge, no previous studies have investigated the mechanism underlying the TMOD3­regulated promotion of liver cancer. The aim of the present study was to determine whether TMOD3 is associated with liver cancer progression. TMOD3 expression was found to be elevated in liver cancer cells and tissues. In the in vitro experiments, liver cancer cell proliferation, invasion and migration were inhibited by TMOD3 knockdown and promoted by ectopic expression of TMOD3. Furthermore, mechanistic analysis indicated that TMOD3 overexpression activated mitogen­activated protein kinase (MAPK)/extracellular signal­regulated kinase (ERK) signaling and increased the levels of other targets of this pathway, including matrix metalloproteinase (MMP)2, MMP9 and cyclin D1. TMOD3 overexpression was associated with changes in liver cancer cell morphology and altered expression of epithelial and mesenchymal markers. High TMOD3 expression was hypothesized to promote epithelial­to­mesenchymal transition in liver cancer cells. In conclusion, TMOD3 was shown to promote liver cancer cell growth, invasion and migration through the MAPK/ERK signaling pathway, and it may serve as a candidate biomarker and therapeutic target in liver cancer.

Force-history dependence and cyclic mechanical reinforcement of actin filaments at the single molecular level.

The actin cytoskeleton is subjected to dynamic mechanical forces over time and the history of force loading may serve as mechanical preconditioning. While the actin cytoskeleton is known to be mechanosensitive, the mechanisms underlying force regulation of actin dynamics still need to be elucidated. Here, we investigated actin depolymerization under a range of dynamic tensile forces using atomic force microscopy. Mechanical loading by cyclic tensile forces induced significantly enhanced bond lifetimes and different force-loading histories resulted in different dissociation kinetics in G-actin-G-actin and G-actin-F-actin interactions. Actin subunits at the two ends of filaments formed bonds with distinct kinetics under dynamic force, with cyclic mechanical reinforcement more effective at the pointed end compared to that at the barbed end. Our data demonstrate force-history dependent reinforcement in actin-actin bonds and polarity of the actin depolymerization kinetics under cyclic tensile forces. These properties of actin may be important clues to understanding regulatory mechanisms underlying actin-dependent mechanotransduction and mechanosensitive cytoskeletal dynamics.This article has an associated First Person interview with the first author of the paper.

Novel insight into the genetic basis of high-altitude pulmonary hypertension in Kyrgyz highlanders.

The Central Asian Kyrgyz highland population provides a unique opportunity to address genetic diversity and understand the genetic mechanisms underlying high-altitude pulmonary hypertension (HAPH). Although a significant fraction of the population is unaffected, there are susceptible individuals who display HAPH in the absence of any lung, cardiac or hematologic disease. We report herein the analysis of the whole-genome sequencing of healthy individuals compared with HAPH patients and other controls (total n = 33). Genome scans reveal selection signals in various regions, encompassing multiple genes from the first whole-genome sequences focusing on HAPH. We show here evidence of three candidate genes MTMR4, TMOD3 and VCAM1 that are functionally associated with well-known molecular and pathophysiological processes and which likely lead to HAPH in this population. These processes are (a) dysfunctional BMP signaling, (b) disrupted tissue repair processes and (c) abnormal endothelial cell function. Whole-genome sequence of well-characterized patients and controls and using multiple statistical tools uncovered novel candidate genes that belong to pathways central to the pathogenesis of HAPH. These studies on high-altitude human populations are pertinent to the understanding of sea level diseases involving hypoxia as a main element of their pathophysiology.

The cardiomyopathy-associated K15N mutation in tropomyosin alters actin filament pointed end dynamics.

Correct assembly of thin filaments composed of actin and actin-binding proteins is of crucial importance for properly functioning muscle cells. Tropomyosin (Tpm) mediates the binding of tropomodulin (Tmod) and leiomodin (Lmod) at the slow-growing, or pointed, ends of the thin filaments. Together these proteins regulate thin filament lengths and actin dynamics in cardiac muscle. The K15N mutation in the TPM1 gene is associated with familial dilated cardiomyopathy (DCM) but the effect of this mutation on Tpm's function is unknown. In this study, we introduced the K15N mutation in striated muscle α-Tpm (Tpm1.1) and investigated its interaction with actin, Tmod and Lmod. The mutation caused a ∼3-fold decrease in the affinity of Tpm1.1 for actin. The binding of Lmod and Tmod to Tpm1.1-covered actin filaments also decreased in the presence of the K15N mutation. Furthermore, the K15N mutation in Tpm1.1 disrupted the inhibition of actin polymerization and affected the competition between Tmod1 and Lmod2 for binding at the pointed ends. Our data demonstrate that the K15N mutation alters pointed end dynamics by affecting molecular interactions between Tpm1.1, Lmod2 and Tmod1.