Sequencing, expression analysis, and mapping of three unique human tropomodulin genes and their mouse orthologs

Intra-amniotic delivery of tropomodulin 3 rescues cell apoptosis induced by miR-200b-3p upregulation via non-canonical nuclear factor kappa B pathways in ethylene thiourea induced anorectal malformations fetal rat

Ethylene thiourea (ETU), a metabolite of the fungicide ethylene bisdithiocarbamate (EBDC), has received great concern because of its harmful effects. ETU-induced anorectal malformations (ARMs) in rat models have been reported and widely used in the study of ARMs embryogenesis. Dysplasia of the lumbosacral spinal cord (LSSC), pelvic floor muscles (PFMs), and hindgut (HG) during intrauterine life affects postoperative defecation in patients with ARMs. However, the underlying toxic effects of ETU and pathological mechanisms in the three defecation-related tissues of fetuses with ARMs have not been reported. Thus, this study aimed to elucidate the molecular mechanisms involved in ARMs, with a focus on the dysregulation of miR-200b-3p and its downstream target tropomodulin 3 (TMOD3). The mRNA and protein levels of miR-200b-3p and TMOD3 in LSSC, PFMs, and HG of fetal rats with ARMs were evaluated by reverse transcription quantitative polymerase chain reaction and Western blotting (WB) on embryonic day 17 (E17). Further, a dual-luciferase reporter assay confirmed their targeting relationship. Gene silencing and overexpression of miR-200b-3p and TMOD3 were performed to verify their functions in HEK-293 T cells. Fetal rats with ARMs also received intra-amniotic microinjection of Ad-TMOD3 on E15, and key molecules in nuclear factor kappa (NF-κB) signaling and apoptosis were evaluated by WB on E21. Abnormally high levels of miR-200b-3p inhibited TMOD3 expression by binding with its 3'-untranslated region, leading to the activation of the non-canonical NF-κB signaling pathway, which is critical in the maldevelopment of LSSC, PFMs, and HG in ARMs rats. Furthermore, miR-200b-3p triggered apoptosis by directly targeting TMOD3. Notably, intra-amniotic Ad-TMOD3 microinjection revealed that the upregulation of TMOD3 expression mitigates the effects of miR-200b-3p on the activation of non-canonical NF-κB signaling and apoptosis in fetal rat model of ARMs. A novel miR-200b-3p/TMOD3/non-canonical NF-κB signaling axis triggered the massive apoptosis in LSSC, PFMs, and HG of ARMs, which was restored by the intra-amniotic injection of Ad-TMOD3 during embryogenesis. Our results indicate the potential of TMOD3 as a treatment target to restore defecation.

Role of Actin-Binding Proteins in Skeletal Myogenesis

Maintenance of skeletal muscle quantity and quality is essential to ensure various vital functions of the body. Muscle homeostasis is regulated by multiple cytoskeletal proteins and myogenic transcriptional programs responding to endogenous and exogenous signals influencing cell structure and function. Since actin is an essential component in cytoskeleton dynamics, actin-binding proteins (ABPs) have been recognized as crucial players in skeletal muscle health and diseases. Hence, dysregulation of ABPs leads to muscle atrophy characterized by loss of mass, strength, quality, and capacity for regeneration. This comprehensive review summarizes the recent studies that have unveiled the role of ABPs in actin cytoskeletal dynamics, with a particular focus on skeletal myogenesis and diseases. This provides insight into the molecular mechanisms that regulate skeletal myogenesis via ABPs as well as research avenues to identify potential therapeutic targets. Moreover, this review explores the implications of non-coding RNAs (ncRNAs) targeting ABPs in skeletal myogenesis and disorders based on recent achievements in ncRNA research. The studies presented here will enhance our understanding of the functional significance of ABPs and mechanotransduction-derived myogenic regulatory mechanisms. Furthermore, revealing how ncRNAs regulate ABPs will allow diverse therapeutic approaches for skeletal muscle disorders to be developed.

Whole-exome identifies germline variants in families with obstructive sleep apnea syndrome

Background: Obstructive sleep apnea syndrome (OSAS) (OMIM #107650) is characterized by complete or partial obstruction of the upper airways, resulting in periods of sleep associated apnea. OSAS increases morbidity and mortality risk from cardiovascular and cerebrovascular diseases. While heritability of OSAS is estimated at ∼40%, the precise underlying genes remain elusive. Brazilian families with OSAS that follows as seemingly autosomal dominant inheritance pattern were recruited. Methods: The study included nine individuals from two Brazilian families displaying a seemingly autosomal dominant inheritance pattern of OSAS. Whole exome sequencing of germline DNA were analyzed using Mendel, MD software. Variants selected were analyzed using Varstation^® with subsequent analyses that included validation by Sanger sequencing, pathogenic score assessment by ACMG criteria, co-segregation analyses (when possible) allele frequency, tissue expression patterns, pathway analyses, effect on protein folding modeling using Swiss-Model and RaptorX. Results: Two families (six affected patients and three unaffected controls) were analyzed. A comprehensive multistep analysis yielded variants in COX20 (rs946982087) (family A), PTPDC1 (rs61743388) and TMOD4 (rs141507115) (family B) that seemed to be strong candidate genes for being OSAS associated genes in these families. Conclusion: Sequence variants in COX20, PTPDC1 and TMOD4 seemingly are associated with OSAS phenotype in these families. Further studies in more, ethnically diverse families and non-familial OSAS cases are needed to better define the role of these variants as contributors to OSAS phenotype.

DNA methylome and transcriptome identified Key genes and pathways involved in Speckled Eggshell formation in aged laying hens

BACKGROUND

The quality of poultry eggshells is closely related to the profitability of egg production. Eggshell speckles reflect an important quality trait that influences egg appearance and customer preference. However, the mechanism of speckle formation remains poorly understood. In this study, we systematically compared serum immune and antioxidant indices of hens laying speckled and normal eggs. Transcriptome and methylome analyses were used to elucidate the mechanism of eggshell speckle formation.

RESULTS

The results showed that seven differentially expressed genes (DEGs) were identified between the normal and speckle groups. Gene set enrichment analysis (GSEA) revealed that the expressed genes were mainly enriched in the calcium signaling pathway, focal adhesion, and MAPK signaling pathway. Additionally, 282 differentially methylated genes (DMGs) were detected, of which 15 genes were associated with aging, including ARNTL, CAV1, and GCLC. Pathway analysis showed that the DMGs were associated with T cell-mediated immunity, response to oxidative stress, and cellular response to DNA damage stimulus. Integrative analysis of transcriptome and DNA methylation data identified BFSP2 as the only overlapping gene, which was expressed at low levels and hypomethylated in the speckle group.

CONCLUSIONS

Overall, these results indicate that aging- and immune-related genes and pathways play a crucial role in the formation of speckled eggshells, providing useful information for improving eggshell quality.

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.

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.

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.

Tropomodulin1 Expression Increases Upon Maturation in Dendritic Cells and Promotes Their Maturation and Immune Functions

Dendritic cells (DCs) are the most potent antigen-presenting cells. Upon maturation, DCs express costimulatory molecules and migrate to the lymph nodes to present antigens to T cells. The actin cytoskeleton plays key roles in multiple aspects of DC functions. However, little is known about the mechanisms and identities of actin-binding proteins that control DC maturation and maturation-associated functional changes. Tropomodulin1 (Tmod1), an actin-capping protein, controls actin depolymerization and nucleation. We found that Tmod1 was expressed in bone marrow-derived immature DCs and was significantly upregulated upon lipopolysaccharide (LPS)-induced DC maturation. By characterizing LPS-induced mature DCs (mDCs) from Tmod1 knockout mice, we found that compared with Tmod1+/+ mDCs, Tmod1-deficient mDCs exhibited lower surface expression of costimulatory molecules and chemokine receptors and reduced secretion of inflammatory cytokines, suggesting that Tmod1 deficiency retarded DC maturation. Tmod1-deficient mDCs also showed impaired random and chemotactic migration, deteriorated T-cell stimulatory ability, and reduced F-actin content and cell stiffness. Furthermore, Tmod1-deficient mDCs secreted high levels of IFN-β and IL-10 and induced immune tolerance in an experimental autoimmune encephalomyelitis (EAE) mouse model. Mechanistically, Tmod1 deficiency affected TLR4 signaling transduction, resulting in the decreased activity of MyD88-dependent NFκB and MAPK pathways but the increased activity of the TRIF/IRF3 pathway. Rescue with exogenous Tmod1 reversed the effect of Tmod1 deficiency on TLR4 signaling. Therefore, Tmod1 is critical in regulating DC maturation and immune functions by regulating TLR4 signaling and the actin cytoskeleton. Tmod1 may be a potential target for modulating DC functions, a strategy that would be beneficial for immunotherapy for several diseases.

An epigenome-wide association study of ambient pyrethroid pesticide exposures in California's central valley

BACKGROUND

Pyrethroid pesticide use is increasing worldwide, although the full extent of associated health effects is unknown. An epigenome-wide association study (EWAS) with exploratory pathway analysis may help identify potential pyrethroid-related health effects.

METHODS

We performed an exploratory EWAS of chronic ambient pyrethroid exposure using control participants' blood in the Parkinson's Environment and Genes Study in the Central Valley of California (N = 237). We estimated associations of living and working near agricultural pyrethroid pesticide applications in the past 5 years (binary) with site-specific differential methylation, and used a false discovery rate (FDR) cut off of 0.05 for significance. We controlled for age, sex, education, cell count, and an ancestral marker for Hispanic ethnicity. We normalized methylation values for Type I/II probe bias using Beta-Mixture Quantile (BMIQ) normalization, filtered out cross-reactive probes, and evaluated for remaining bias with Surrogate Variable Analysis (SVA). We also evaluated the effects of controlling for cell count and normalizing for Type I/II probe bias by comparing changes in effect estimates and p-values for the top hits across BMIQ and GenomeStudio normalization methods, and controlling for cell count. To facilitate broader interpretation, we annotated genes to the CpG sites and performed gene set overrepresentation analysis, using genes annotated to CpG sites that were associated with pyrethroids at a raw p < 0.05, and controlling for background representation of CpG sites on the chip. We did this for both a biological process context (Gene Ontology terms) using missMethyl, and a disease set context using WebGestalt. For these gene set overrepresentation analyses we also used an FDR cut off of 0.05 for significance of gene sets.

RESULTS

After controlling for cell count and applying BMIQ normalization, 4 CpG sites were differentially methylated in relation to pyrethroid exposures. When using GenomeStudio's Illumina normalization, 415 CpG sites were differentially methylated, including all four identified with the BMIQ method. In the gene set overrepresentation analyses, we identified 6 GO terms using BMIQ normalization, and 76 using Illumina normalization, including the 6 identified by BMIQ. For disease sets, we identified signals for Alzheimer's disease, leukemia and several other cancers, diabetes, birth defects, and other diseases, for both normalization methods. We identified minimal changes in effect estimates after controlling for cell count, and controlling for cell count generally weakened p-values. BMIQ normalization, however, resulted in different beta coefficients and weakened p-values.

CONCLUSIONS

Chronic ambient pyrethroid exposure is associated with differential methylation at CpG sites that annotate to a wide variety of disease states and biological mechanisms that align with prior research. However, this EWAS also implicates several novel diseases for future investigation, and highlights the relative importance of different background normalization methods in identifying associations.

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.

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.

Multifunctional roles of tropomodulin-3 in regulating actin dynamics

Tropomodulins (Tmods) are proteins that cap the slow-growing (pointed) ends of actin filaments (F-actin). The basis for our current understanding of Tmod function comes from studies in cells with relatively stable and highly organized F-actin networks, leading to the view that Tmod capping functions principally to preserve F-actin stability. However, not only is Tmod capping dynamic, but it also can play major roles in regulating diverse cellular processes involving F-actin remodeling. Here, we highlight the multifunctional roles of Tmod with a focus on Tmod3. Like other Tmods, Tmod3 binds tropomyosin (Tpm) and actin, capping pure F-actin at submicromolar and Tpm-coated F-actin at nanomolar concentrations. Unlike other Tmods, Tmod3 can also bind actin monomers and its ability to bind actin is inhibited by phosphorylation of Tmod3 by Akt2. Tmod3 is ubiquitously expressed and is present in a diverse array of cytoskeletal structures, including contractile structures such as sarcomere-like units of actomyosin stress fibers and in the F-actin network encompassing adherens junctions. Tmod3 participates in F-actin network remodeling in lamellipodia during cell migration and in the assembly of specialized F-actin networks during exocytosis. Furthermore, Tmod3 is required for development, regulating F-actin mesh formation during meiosis I of mouse oocytes, erythroblast enucleation in definitive erythropoiesis, and megakaryocyte morphogenesis in the mouse fetal liver. Thus, Tmod3 plays vital roles in dynamic and stable F-actin networks in cell physiology and development, with further research required to delineate the mechanistic details of Tmod3 regulation in the aforementioned processes, or in other yet to be discovered processes.

Tropomodulin's Actin-Binding Abilities Are Required to Modulate Dendrite Development

There are many unanswered questions about the roles of the actin pointed end capping and actin nucleation by tropomodulins (Tmod) in regulating neural morphology. Previous studies indicate that Tmod1 and Tmod2 regulate morphology of the dendritic arbor and spines. Tmod3, which is expressed in the brain, had only a minor influence on morphology. Although these studies established a defined role of Tmod in regulating dendritic and synaptic morphology, the mechanisms by which Tmods exert these effects are unknown. Here, we overexpressed a series of mutated forms of Tmod1 and Tmod2 with disrupted actin-binding sites in hippocampal neurons and found that Tmod1 and Tmod2 require both of their actin-binding sites to regulate dendritic morphology and dendritic spine shape. Proximity ligation assays (PLAs) indicate that these mutations impact the interaction of Tmod1 and Tmod2 with tropomyosins Tpm3.1 and Tpm3.2. This impact on Tmod/Tpm interaction may contribute to the morphological changes observed. Finally, we use molecular dynamics simulations (MDS) to characterize the structural changes, caused by mutations in the C-terminal helix of the leucine-rich repeat (LRR) domain of Tmod1 and Tmod2 alone and when bound onto actin monomers. Our results expand our understanding of how neurons utilize the different Tmod isoforms in development.

Favorable prognostic role of tropomodulins in neuroblastoma

Neuroblastoma is a pediatric tumor of the sympatoadrenal lineage of the neural crest characterized by high molecular and clinical heterogeneity, which are the main causes of the poor response to standard multimodal therapy. The identification of new and selective biomarkers is important to improve our knowledge on the mechanisms of neuroblastoma progression and to find the targets for innovative cancer therapies. This study identifies a positive correlation among tropomodulins (TMODs) proteins expression and neuroblastoma progression. TMODs bind the pointed end of actin filaments, regulate polymerization and depolymerization processes modifying actin cytoskeletal dynamic and influencing neuronal development processes. Expression levels of TMODs genes were analyzed in 17 datasets comprising different types of tumors, including neuroblastoma, and it was demonstrated that high levels of tropomodulin1 (TMOD1) and tropomodulin 2 (TMOD2) correlate positively with high survival probability and with favorable clinical and molecular characteristics. Functional studies on neuroblastoma cell lines, showed that TMOD1 knockin induced cell cycle arrest, cell proliferation arrest and a mature functional differentiation. TMOD1 overexpression was responsible for particular cell morphology and biochemical changes which directed cells towards a neuronal favorable differentiation profile. TMOD1 downregulation also induced cell proliferation arrest but caused the loss of mature cell differentiation and promoted the development of neuroendocrine cellular characteristics, delineating an aggressive and unfavorable tumor behavior. Overall, these data indicated that TMODs are favorable prognostic biomarkers in neuroblastoma and we believe that they could contribute to unravel a new pathophysiological mechanism of neuroblastoma resistance contributing to the design of personalized therapeutics opportunities.

Characterizing interaction forces between actin and proteins of the tropomodulin family reveals the presence of the N-terminal actin-binding site in leiomodin

Tropomodulin family of proteins includes several isoforms of tropomodulins (Tmod) and leiomodins (Lmod). These proteins can sequester actin monomers or nucleate actin polymerization. Although it is known that their actin-binding properties are isoform-dependent, knowledge on how they vary in strengths of interactions with G-actin is missing. While it is confirmed in many studies that Tmods have two actin-binding sites, information on number and location of actin-binding sites in Lmod2 is controversial. We used atomic force microscopy to study interactions between G-actin and proteins of the tropomodulin family. Unbinding forces between G-actin and Tmod1, Tmod2, Tmod3, or Lmod2 were quantified. Our results indicated that Tmod1 and Tmod3 had unimodal force distributions, Tmod2 had a bimodal distribution and Lmod2 had a trimodal distribution. The number of force distributions correlates with the proteins' abilities to sequester actin or to nucleate actin polymerization. We assigned specific unbinding forces to the individual actin-binding sites of Tmod2 and Lmod2 using mutations that destroy actin-binding sites of Tmod2 and truncated Lmod2. Our results confirm the existence of the N-terminal actin-binding site in Lmod2. Altogether, our data demonstrate how the differences between the number and the strength of actin-binding sites of Tmod or Lmod translate to their functional abilities.

Tropomodulins and Leiomodins: Actin Pointed End Caps and Nucleators in Muscles

Cytoskeletal structures characterized by actin filaments with uniform lengths, including the thin filaments of striated muscles and the spectrin-based membrane skeleton, use barbed and pointed-end capping proteins to control subunit addition/dissociation at filament ends. While several proteins cap the barbed end, tropomodulins (Tmods), a family of four closely related isoforms in vertebrates, are the only proteins known to specifically cap the pointed end. Tmods are ∼350 amino acids in length, and comprise alternating tropomyosin- and actin-binding sites (TMBS1, ABS1, TMBS2, and ABS2). Leiomodins (Lmods) are related in sequence to Tmods, but display important differences, including most notably the lack of TMBS2 and the presence of a C-terminal extension featuring a proline-rich domain and an actin-binding WASP-Homology 2 domain. The Lmod subfamily comprises three somewhat divergent isoforms expressed predominantly in muscle cells. Biochemically, Lmods differ from Tmods, acting as powerful nucleators of actin polymerization, not capping proteins. Structurally, Lmods and Tmods display crucial differences that correlate well with their different biochemical activities. Physiologically, loss of Lmods in striated muscle results in cardiomyopathy or nemaline myopathy, whereas complete loss of Tmods leads to failure of myofibril assembly and developmental defects. Yet, interpretation of some of the in vivo data has led to the idea that Tmods and Lmods are interchangeable or, at best, different variants of two subfamilies of pointed-end capping proteins. Here, we review and contrast the existing literature on Tmods and Lmods, and propose a model of Lmod function that attempts to reconcile the in vitro and in vivo data, whereby Lmods nucleate actin filaments that are subsequently capped by Tmods during sarcomere assembly, turnover, and repair.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

Actin regulation by tropomodulin and tropomyosin in neuronal morphogenesis and function

Actin is a profoundly influential protein; it impacts, among other processes, membrane morphology, cellular motility, and vesicle transport. Actin can polymerize into long filaments that push on membranes and provide support for intracellular transport. Actin filaments have polar ends: the fast-growing (barbed) end and the slow-growing (pointed) end. Depolymerization from the pointed end supplies monomers for further polymerization at the barbed end. Tropomodulins (Tmods) cap pointed ends by binding onto actin and tropomyosins (Tpms). Tmods and Tpms have been shown to regulate many cellular processes; however, very few studies have investigated their joint role in the nervous system. Recent data directly indicate that they can modulate neuronal morphology. Additional studies suggest that Tmod and Tpm impact molecular processes influential in synaptic signaling. To facilitate future research regarding their joint role in actin regulation in the nervous system, we will comprehensively discuss Tpm and Tmod and their known functions within molecular systems that influence neuronal development.

Tropomodulin-3 is essential in asymmetric division during mouse oocyte maturation

The dynamic polymerization and depolymerization of actin filaments is essential for various cellular processes such as cell migration, rotation, cytokinesis, and mammalian oocyte maturation. Tropomodulin 3 (Tmod3) binds to the slow-growing (pointed) ends of the actin filament, thereby protecting the filament from depolymerization. However, the roles of Tmod3 in mammalian oocyte maturation remain elusive. Tmod3 mRNA and protein is present at all stages of mouse oocyte maturation. Tmod3 protein is mainly localized in the cytoplasm and appears enriched near the chromosome during maturation. By knocking down or ectopically overexpressing Tmod3, we confirmed that Tmod3 regulate the level of the intracytoplasmic actin mesh and asymmetric spindle migration. Expression of N-terminal Tmod3 (correspond to 1–155 amino acids), which contains the tropomyosin-binding site, results in decreased density of the actin mesh, thereby demonstrating the importance of the interaction between tropomyosin and tropomodulin for the maintenance of the actin mesh. Taken together, these findings indicate that Tmod3 plays crucial roles in oocyte maturation, presumably by protecting the actin filament from depolymerization and thereby controlling the density of the cytoplasmic actin mesh.

Tropomodulin isoforms utilize specific binding functions to modulate dendrite development

Tropomodulins (Tmods) cap F-actin pointed ends and have altered expression in the brain in neurological diseases. The function of Tmods in neurons has been poorly studied and their role in neurological diseases is entirely unknown. In this article, we show that Tmod1 and Tmod2, but not Tmod3, are positive regulators of dendritic complexity and dendritic spine morphology. Tmod1 increases dendritic branching distal from the cell body and the number of filopodia/thin spines. Tmod2 increases dendritic branching proximal to the cell body and the number of mature dendritic spines. Tmods utilize two actin-binding sites and two tropomyosin (Tpm)-binding sites to cap F-actin. Overexpression of Tmods with disrupted Tpm-binding sites indicates that Tmod1 and Tmod2 differentially utilize their Tpm- and actin-binding sites to affect morphology. Disruption of Tmod1's Tpm-binding sites abolished the overexpression phenotype. In contrast, overexpression of the mutated Tmod2 caused the same phenotype as wild type overexpression. Proximity ligation assays indicate that the mutated Tmods are shuttled similarly to wild type Tmods. Our data begins to uncover the roles of Tmods in neural development and the mechanism by which Tmods alter neural morphology. These observations in combination with altered Tmod expression found in several neurological diseases also suggest that dysregulation of Tmod expression may be involved in the pathology of these diseases. © 2016 Wiley Periodicals, Inc.

Tropomyosin-binding properties modulate competition between tropomodulin isoforms

The formation and fine-tuning of cytoskeleton in cells are governed by proteins that influence actin filament dynamics. Tropomodulin (Tmod) regulates the length of actin filaments by capping the pointed ends in a tropomyosin (TM)-dependent manner. Tmod1, Tmod2 and Tmod3 are associated with the cytoskeleton of non-muscle cells and their expression has distinct consequences on cell morphology. To understand the molecular basis of differences in the function and localization of Tmod isoforms in a cell, we compared the actin filament-binding abilities of Tmod1, Tmod2 and Tmod3 in the presence of Tpm3.1, a non-muscle TM isoform. Tmod3 displayed preferential binding to actin filaments when competing with other isoforms. Mutating the second or both TM-binding sites of Tmod3 destroyed its preferential binding. Our findings clarify how Tmod1, Tmod2 and Tmod3 compete for binding actin filaments. Different binding mechanisms and strengths of Tmod isoforms for Tpm3.1 contribute to their divergent functional capabilities.

Tropomodulin3 as the link between insulin-activated AKT2 and cortical actin remodeling in preparation of GLUT4 exocytosis

It is well established that insulin-induced remodeling of actin filaments into a cortical mesh is required for insulin-stimulated GLUT4 exocytosis. Akt2 and its downstream effectors play a pivotal role in mediating the translocation and membrane fusion of GLUT4-storage vesicle (GSV). However, the direct downstream effector underlying the event of cortical actin reorganization has not been elucidated. In a recent study in Nature Communications, (1) Lim et al identify Tropomodulin3 (Tmod3) as a downstream target of the Akt2 kinase and describe the role of this pointed-end actin-capping protein in regulating insulin-dependent exocytosis of GSVs in adipocytes through the remodeling of the cortical actin network. Phosphorylation of Tmod3 by Akt2 on Ser71 modulates insulin-induced actin remodeling, a key step for GSV fusion with the plasma membrane (PM). Furthermore, the authors establish Tm5NM1 (Tpm3.1 in new nomenclature) (2) as the cognate tropomyosin partner of Tmod3, and an essential role of Tmod3-Tm5NM1 interaction for GSV exocytosis and glucose uptake. This study elucidates a novel effector of Akt2 that provides a direct mechanistic link between Akt2 signaling and actin reorganization essential for vesicle fusion, and suggests that a subset of actin filaments with specific molecular compositions may be dedicated for the process of vesicle fusion.

Tropomodulin3 is a novel Akt2 effector regulating insulin-stimulated GLUT4 exocytosis through cortical actin remodeling

Akt2 and its downstream effectors mediate insulin-stimulated GLUT4-storage vesicle (GSV) translocation and fusion with the plasma membrane (PM). Using mass spectrometry, we identify actin-capping protein Tropomodulin 3 (Tmod3) as an Akt2-interacting partner in 3T3-L1 adipocytes. We demonstrate that Tmod3 is phosphorylated at Ser71 on insulin-stimulated Akt2 activation, and Ser71 phosphorylation is required for insulin-stimulated GLUT4 PM insertion and glucose uptake. Phosphorylated Tmod3 regulates insulin-induced actin remodelling, an essential step for GSV fusion with the PM. Furthermore, the interaction of Tmod3 with its cognate tropomyosin partner, Tm5NM1 is necessary for GSV exocytosis and glucose uptake. Together these results establish Tmod3 as a novel Akt2 effector that mediates insulin-induced cortical actin remodelling and subsequent GLUT4 membrane insertion. Our findings suggest that defects in cytoskeletal remodelling may contribute to impaired GLUT4 exocytosis and glucose uptake.

Phosphorylation of tropomodulin1 contributes to the regulation of actin filament architecture in cardiac muscle

Tropomodulin1 (Tmod1) is an actin-capping protein that plays an important role in actin filament pointed-end dynamics and length in striated muscle. No mechanisms have been identified to explain how Tmod1's functional properties are regulated. The purpose of this investigation was to explore the functional significance of the phosphorylation of Tmod1 at previously identified Thr54. Rat cardiomyocytes were assessed for phosphorylation of Tmod1 using Pro-Q Diamond staining and (32)P labeling. Green fluorescent protein-tagged phosphorylation-mimic (T54E) and phosphorylation-deficient (T54A) versions of Tmod1 were expressed in cultured cardiomyocytes, and the ability of these mutants to assemble and restrict actin lengths was observed. We report for the first time that Tmod1 is phosphorylated endogenously in cardiomyocytes, and phosphorylation at Thr54 causes a significant reduction in the ability of Tmod1 to assemble to the pointed end compared with that of the wild type (WT; 48 vs. 78%, respectively). In addition, overexpression of Tmod1-T54E restricts actin filament lengths by only ∼3%, whereas Tmod1-WT restricts the lengths significantly by ∼8%. Finally, Tmod1-T54E altered the actin filament-capping activity in polymerization assays. Taken together, our data suggest that pointed-end assembly and Tmod1's thin filament length regulatory function are regulated by its phosphorylation state.

Deregulated expression of cytoskeleton related genes in the spinal cord and sciatic nerve of presymptomatic SOD1(G93A) Amyotrophic Lateral Sclerosis mouse model

Early molecular events related to cytoskeleton are poorly described in Amyotrophic Lateral Sclerosis (ALS), especially in the Schwann cell (SC), which offers strong trophic support to motor neurons. Database for Annotation, Visualization and Integrated Discovery (DAVID) tool identified cytoskeleton-related genes by employing the Cellular Component Ontology (CCO) in a large gene profiling of lumbar spinal cord and sciatic nerve of presymptomatic SOD1(G93A) mice. One and five CCO terms related to cytoskeleton were described from the spinal cord deregulated genes of 40 days (actin cytoskeleton) and 80 days (microtubule cytoskeleton, cytoskeleton part, actin cytoskeleton, neurofilament cytoskeleton, and cytoskeleton) old transgene mice, respectively. Also, four terms were depicted from the deregulated genes of sciatic nerve of 60 days old transgenes (actin cytoskeleton, cytoskeleton part, microtubule cytoskeleton and cytoskeleton). Kif1b was the unique deregulated gene in more than one studied region or presymptomatic age. The expression of Kif1b [quantitative polymerase chain reaction (qPCR)] elevated in the lumbar spinal cord (40 days old) and decreased in the sciatic nerve (60 days old) of presymptomatic ALS mice, results that were in line to microarray findings. Upregulation (24.8 fold) of Kif1b was seen in laser microdissected enriched immunolabeled motor neurons from the spinal cord of 40 days old presymptomatic SOD1(G93A) mice. Furthermore, Kif1b was dowregulated in the sciatic nerve Schwann cells of presymptomatic ALS mice (60 days old) that were enriched by means of cell microdissection (6.35 fold), cell sorting (3.53 fold), and primary culture (2.70 fold) technologies. The gene regulation of cytoskeleton molecules is an important occurrence in motor neurons and Schwann cells in presymptomatic stages of ALS and may be relevant in the dying back mechanisms of neuronal death. Furthermore, a differential regulation of Kif1b in the spinal cord and sciatic nerve cells emerged as key event in ALS.

Role of tropomodulin's leucine rich repeat domain in the formation of neurite-like processes

Actin dynamics is fundamental for neurite development; monomer depolymerization from pointed ends is rate-limiting in actin treadmilling. Tropomodulins (Tmod) make up a family of actin pointed end-capping proteins. Of the four known isoforms, Tmod1-Tmod3 are expressed in brain cells. We investigated the role of Tmod's C-terminal (LRR) domain in the formation of neurite-like processes by overexpressing Tmod1 and Tmod2 with deleted or mutated LRR domains in PC12 cells, a model system used to study neuritogenesis. Tmod1 overexpression results in a normal quantity and a normal length of processes, while Tmod2 overexpression reduces both measures. The Tmod2 overexpression phenotype is mimicked by overexpression of Tmod1 with the LRR domain removed or with three point mutations in the LRR domain that disrupt exposed clusters of conserved residues. Removal of Tmod2's LRR domain does not significantly alter the outgrowth of neurite-like processes compared to that of Tmod2. Overexpression of chimeras with the N-terminal and C-terminal domains switched between Tmod1 and Tmod2 reinforces the idea that Tmod1's LRR domain counteracts the reductive effect of the Tmod N-terminal domain upon formation of processes while Tmod2's LRR domain does not. We suggest that the TM-dependent actin capping ability of both Tmods inhibits the formation of processes, but in Tmod1, this inhibition can be controlled via its LRR domain. Circular dichroism, limited proteolysis, and molecular dynamics demonstrate structural differences in the C-terminal region of the LRR domains of Tmod1, Tmod2, and the Tmod1 mutant.

The minus-end actin capping protein, UNC-94/tropomodulin, regulates development of the Caenorhabditis elegans intestine

BACKGROUND

Tropomodulins are actin-capping proteins that regulate the stability of the slow-growing, minus-ends of actin filaments. The C. elegans tropomodulin homolog, UNC-94, has sequence and functional similarity to vertebrate tropomodulins. We investigated the role of UNC-94 in C. elegans intestinal morphogenesis.

RESULTS

In the embryonic C. elegans intestine, UNC-94 localizes to the terminal web, an actin- and intermediate filament-rich structure that underlies the apical membrane. Loss of UNC-94 function results in areas of flattened intestinal lumen. In worms homozygous for the strong loss-of-function allele, unc-94(tm724), the terminal web is thinner and the amount of F-actin is reduced, pointing to a role for UNC-94 in regulating the structure of the terminal web. The non-muscle myosin, NMY-1, also localizes to the terminal web, and we present evidence that increasing actomyosin contractility by depleting the myosin phosphatase regulatory subunit, mel-11, can rescue the flattened lumen phenotype of unc-94 mutants.

CONCLUSIONS

The data support a model in which minus-end actin capping by UNC-94 promotes proper F-actin structure and contraction in the terminal web, yielding proper shape of the intestinal lumen. This establishes a new role for a tropomodulin in regulating lumen shape during tubulogenesis.

The switch role of the Tmod4 in the regulation of balanced development between myogenesis and adipogenesis

Tmod4 (Tropomodulin 4) is a member of Tmod family that plays important role in thin filament length regulation and myofibril assembly. We found that the expression levels of Tmod4 were higher in skeletal muscle and adipose tissues. However, the function and regulation of the Tmod4 gene in the myogenesis and adipogenesis remains unclear. In this study, we found that the expression of Tmod4 was decreased in myogenesis while increased in adipogenesis. Then, the transcriptional regulation analysis of Tmod4 promoter showed that Tmod4 could be regulated directly by myogenic factors and adipogenic factors. Furthermore, the roles of Tmod4 in the myogenesis and adipogenesis were confirmed by its over-expression in C2C12 cells and 3T3 cells, which suggested that Tmod4 could promote adipogenesis by up-regulating the adipogenic factors but moderately delay the myogenesis. These results indicated that the Tmod4 gene may play as a switch between myogenesis and adipogenesis, which resulted in the balanced development between skeletal muscle and adipose tissue. Therefore, the model for switch role of the Tmod4 in the balanced regulation between myogenesis and adipogenesis was proposed. It is showed that the expression of Tmod4 was activated in adipogenesis by adipogenic factors while inhibited in myogenesis by myogenic factors. Moreover, Tmod4 could promote adipogenesis by up-regulating the expression of adipogenic factors while moderately delaying the myogenesis. Our study provides an important basis for further understanding the regulation and function of porcine Tmod4 in muscle and fat development.

Tropomodulins and tropomyosins: working as a team

Actin filaments are major components of the cytoskeleton in eukaryotic cells and are involved in vital cellular functions such as cell motility and muscle contraction. Tmod and TM are crucial constituents of the actin filament network, making their presence indispensable in living cells. Tropomyosin (TM) is an alpha-helical, coiled coil protein that covers the grooves of actin filaments and stabilizes them. Actin filament length is optimized by tropomodulin (Tmod), which caps the slow growing (pointed end) of thin filaments to inhibit polymerization or depolymerization. Tmod consists of two structurally distinct regions: the N-terminal and the C-terminal domains. The N-terminal domain contains two TM-binding sites and one TM-dependent actin-binding site, whereas the C-terminal domain contains a TM-independent actin-binding site. Tmod binds to two TM molecules and at least one actin molecule during capping. The interaction of Tmod with TM is a key regulatory factor for actin filament organization. The binding efficacy of Tmod to TM is isoform-dependent. The affinities of Tmod/TM binding influence the proper localization and capping efficiency of Tmod at the pointed end of actin filaments in cells. Here we describe how a small difference in the sequence of the TM-binding sites of Tmod may result in dramatic change in localization of Tmod in muscle cells or morphology of non-muscle cells. We also suggest most promising directions to study and elucidate the role of Tmod-TM interaction in formation and maintenance of sarcomeric and cytoskeletal structure.

Mutations changing tropomodulin affinity for tropomyosin alter neurite formation and extension

Assembly of the actin cytoskeleton is an important part of formation of neurites in developing neurons. Tropomodulin, a tropomyosin-dependent capping protein for the pointed end of the actin filament, is one of the key players in this process. Tropomodulin binds tropomyosin in two binding sites. Tmod1 and Tmod2, tropomodulin isoforms found in neurons, were overexpressed in PC12 cells, a model system for neuronal differentiation. Tmod1 did not affect neuronal differentiation; while cells expressing Tmod2 showed a significant reduction in the number and the length of neurites. Both tropomodulins bind short α-, γ- and δ-tropomyosin isoforms. Mutations in one of the tropomyosin-binding sites of Tmod1, which increased its affinity to short γ- and δ-tropomyosin isoforms, caused a decrease in binding short α-tropomyosin isoforms along with a 2-fold decrease in the length of neurites. Our data demonstrate that Tmod1 is involved in neuronal differentiation for proper neurite formation and outgrowth, and that Tmod2 inhibits these processes. The mutations in the tropomyosin-binding site of Tmod1 impair neurite outgrowth, suggesting that the integrity of this binding site is critical for the proper function of Tmod1 during neuronal differentiation.

Tropomodulins: pointed-end capping proteins that regulate actin filament architecture in diverse cell types

Tropomodulins are a family of four proteins (Tmods 1-4) that cap the pointed ends of actin filaments in actin cytoskeletal structures in a developmentally regulated and tissue-specific manner. Unique among capping proteins, Tmods also bind tropomyosins (TMs), which greatly enhance the actin filament pointed-end capping activity of Tmods. Tmods are defined by a TM-regulated/Pointed-End Actin Capping (TM-Cap) domain in their unstructured N-terminal portion, followed by a compact, folded Leucine-Rich Repeat/Pointed-End Actin Capping (LRR-Cap) domain. By inhibiting actin monomer association and dissociation from pointed ends, Tmods regulate actin dynamics and turnover, stabilizing actin filament lengths and cytoskeletal architecture. In this review, we summarize the genes, structural features, molecular and biochemical properties, actin regulatory mechanisms, expression patterns, and cell and tissue functions of Tmods. By understanding Tmods' functions in the context of their molecular structure, actin regulation, binding partners, and related variants (leiomodins 1-3), we can draw broad conclusions that can explain the diverse morphological and functional phenotypes that arise from Tmod perturbation experiments in vitro and in vivo. Tmod-based stabilization and organization of intracellular actin filament networks provide key insights into how the emergent properties of the actin cytoskeleton drive tissue morphogenesis and physiology.

Functional diversity of actin cytoskeleton in neurons and its regulation by tropomyosin

Neurons comprise functionally, molecularly, and spatially distinct subcellular compartments which include the soma, dendrites, axon, branches, dendritic spines, and growth cones. In this chapter, we detail the remarkable ability of the neuronal cytoskeleton to exquisitely regulate all these cytoplasmic distinct partitions, with particular emphasis on the microfilament system and its plethora of associated proteins. Importance will be given to the family of actin-associated proteins, tropomyosin, in defining distinct actin filament populations. The ability of tropomyosin isoforms to regulate the access of actin-binding proteins to the filaments is believed to define the structural diversity and dynamics of actin filaments and ultimately be responsible for the functional outcome of these filaments.

Tropomodulin capping of actin filaments in striated muscle development and physiology

Efficient striated muscle contraction requires precise assembly and regulation of diverse actin filament systems, most notably the sarcomeric thin filaments of the contractile apparatus. By capping the pointed ends of actin filaments, tropomodulins (Tmods) regulate actin filament assembly, lengths, and stability. Here, we explore the current understanding of the expression patterns, localizations, and functions of Tmods in both cardiac and skeletal muscle. We first describe the mechanisms by which Tmods regulate myofibril assembly and thin filament lengths, as well as the roles of closely related Tmod family variants, the leiomodins (Lmods), in these processes. We also discuss emerging functions for Tmods in the sarcoplasmic reticulum. This paper provides abundant evidence that Tmods are key structural regulators of striated muscle cytoarchitecture and physiology.

Cytoplasmic gamma-actin and tropomodulin isoforms link to the sarcoplasmic reticulum in skeletal muscle fibers

Tropomodulins, cytoplasmic γ-actin, and small ankyrin 1.5 mechanically stabilize the sarcoplasmic reticulum and maintain myofibril alignment in skeletal muscle fibers.

The sarcoplasmic reticulum (SR) serves as the Ca²⁺ reservoir for muscle contraction. Tropomodulins (Tmods) cap filamentous actin (F-actin) pointed ends, bind tropomyosins (Tms), and regulate F-actin organization. In this paper, we use a genetic targeting approach to examine the effect of Tmod1 deletion on the organization of cytoplasmic γ-actin (γ_cyto-actin) in the SR of skeletal muscle. In wild-type muscle fibers, γ_cyto-actin and Tmod3 defined an SR microdomain that was distinct from another Z line–flanking SR microdomain containing Tmod1 and Tmod4. The γ_cyto-actin/Tmod3 microdomain contained an M line complex composed of small ankyrin 1.5 (sAnk1.5), γ_cyto-actin, Tmod3, Tm4, and Tm5NM1. Tmod1 deletion caused Tmod3 to leave its SR compartment, leading to mislocalization and destabilization of the Tmod3–γ_cyto-actin–sAnk1.5 complex. This was accompanied by SR morphological defects, impaired Ca²⁺ release, and an age-dependent increase in sarcomere misalignment. Thus, Tmod3 regulates SR-associated γ_cyto-actin architecture, mechanically stabilizes the SR via a novel cytoskeletal linkage to sAnk1.5, and maintains the alignment of adjacent myofibrils.

Tropomodulins are negative regulators of neurite outgrowth

Regulation of the actin cytoskeleton is critical for neurite formation. Tropomodulins (Tmods) regulate polymerization at actin filament pointed ends. Previous experiments using a mouse model deficient for the neuron specific isoform Tmod2 suggested a role for Tmods in neuronal function by impacting processes underlying learning and memory. However, the role of Tmods in neuronal function on the cellular level remains unknown. Immunofluorescence localization of the neuronal isoforms Tmod1 and Tmod2 in cultured rat primary hippocampal neurons revealed that Tmod1 is enriched along the proximal part of F-actin bundles in lamellipodia of spreading cells and in growth cones of extending neurites, while Tmod2 appears largely cytoplasmic. Functional analysis of these Tmod isoforms in a mouse neuroblastoma N2a cell line showed that knockdown of Tmod2 resulted in a significant increase in the number of neurite-forming cells and in neurite length. While N2a cells compensated for Tmod2 knockdown by increasing Tmod1 levels, over-expression of exogenous Tmod1 had no effect on neurite outgrowth. Moreover, knockdown of Tmod1 increased the number of neurites formed per cell, without effect on the number of neurite-forming cells or neurite length. Taken together, these results indicate that Tmod1 and Tmod2 have mechanistically distinct inhibitory roles in neurite formation, likely mediated via different effects on F-actin dynamics and via differential localizations during early neuritogenesis.

Mammalian tropomodulins nucleate actin polymerization via their actin monomer binding and filament pointed end-capping activities

Many actin-binding proteins have been shown to possess multiple activities to regulate filament dynamics. Tropomodulins (Tmod1-4) are a conserved family of actin filament pointed end-capping proteins. Our previous work has demonstrated that Tmod3 binds to monomeric actin in addition to capping pointed ends. Here, we show a novel actin-nucleating activity in mammalian Tmods. Comparison of Tmod isoforms revealed that Tmod1-3 but not Tmod4 nucleate actin filament assembly. All Tmods bind to monomeric actin, and Tmod3 forms a 1:1 complex with actin. By truncation and mutagenesis studies, we demonstrated that the second α-helix in the N-terminal domain of Tmod3 is essential for actin monomer binding. Chemical cross-linking and LC-MS/MS further indicated that residues in this second α-helix interact with actin subdomain 2, whereas Tmod3 N-terminal domain peptides distal to this α-helix interact with actin subdomain 1. Mutagenesis of Leu-73 to Asp, which disrupts the second α-helix of Tmod3, decreases both its actin monomer-binding and -nucleating activities. On the other hand, point mutations of residues in the C-terminal leucine-rich repeat domain of Tmod3 (Lys-317 in the fifth leucine-rich repeat β-sheet and Lys-344 or Arg-345/Arg-346 in the C-terminal α6-helix) significantly reduced pointed end-capping and nucleation without altering actin monomer binding. Taken together, our data indicate that Tmod3 binds actin monomers over an extended interface and that nucleating activity depends on actin monomer binding and pointed end-capping activities, contributed by N- and C-terminal domains of Tmod3, respectively. Tmod3 nucleation of actin assembly may regulate the cytoskeleton in dynamic cellular contexts.

Tropomodulin isoforms regulate thin filament pointed-end capping and skeletal muscle physiology

In skeletal muscle fibers, tropomodulin 1 (Tmod1) can be compensated for, structurally but not functionally, by Tmod3 and -4.

During myofibril assembly, thin filament lengths are precisely specified to optimize skeletal muscle function. Tropomodulins (Tmods) are capping proteins that specify thin filament lengths by controlling actin dynamics at pointed ends. In this study, we use a genetic targeting approach to explore the effects of deleting Tmod1 from skeletal muscle. Myofibril assembly, skeletal muscle structure, and thin filament lengths are normal in the absence of Tmod1. Tmod4 localizes to thin filament pointed ends in Tmod1-null embryonic muscle, whereas both Tmod3 and -4 localize to pointed ends in Tmod1-null adult muscle. Substitution by Tmod3 and -4 occurs despite their weaker interactions with striated muscle tropomyosins. However, the absence of Tmod1 results in depressed isometric stress production during muscle contraction, systemic locomotor deficits, and a shift to a faster fiber type distribution. Thus, Tmod3 and -4 compensate for the absence of Tmod1 structurally but not functionally. We conclude that Tmod1 is a novel regulator of skeletal muscle physiology.

Mouse erythrocyte tropomodulin in the brain reported by lacZ knocked-in downstream from the E1 promoter

Erythrocyte tropomodulin (E-Tmod, Tmod1) is a tropomyosin-binding protein that caps the slow-growing end of actin filaments. In erythrocytes, it may favor the formation of short actin protofilaments needed for elastic cell deformation. Previously we created a knockout mouse model in which lacZ was knocked-in downstream of the E1 promoter to report the expression of full length E-Tmod. Here we utilize E-Tmod(+/lacZ) mice to study E-Tmod expression patterns in the CNS. X-gal staining and in situ hybridization of adults revealed its restricted expression in the olfactory bulb, hippocampus, cerebral cortex, basal ganglia, nuclei of brain stem and cerebellum. In neonates, signals in the cortex and caudate putamen increased from days 15 to 40. Immunohistochemistry also revealed that signals for beta-galactosidase coincided with that of NeuN, a post-mitotic nuclear marker for neurons, but not that for GFAP+ astrocytes or APC+ oligodendrocytes, suggesting E-Tmod/lacZ-positive cells in the CNS were neurons. Large neurons, e.g., mitral cells in olfactory bulb and mossy cells in hilus of the dentate gyrus are among those that expressed very high levels of E-Tmod in the CNS.

Tropomodulin binds two tropomyosins: a novel model for actin filament capping

Tropomodulin, a tropomyosin-binding protein, caps the slow-growing (pointed) end of the actin filament regulating its dynamics. Tropomodulin, therefore, is important for determining cell morphology, cell movement, and muscle contraction. For the first time we show that one tropomodulin molecule simultaneously binds two tropomyosin molecules in a cooperative manner. On the basis of the tropomodulin solution structure and predicted secondary structure, we introduced a series of point mutations in regions important for tropomyosin binding and actin capping. Capping activity of these mutants was assayed by measuring actin polymerization using pyrene fluorescence. Using direct methods (circular dichroism and native gel electrophoresis) for detecting tropomodulin/tropomyosin binding, we localized the second tropomyosin-binding site to residues 109-144. Despite previous reports that the second binding site is for erythrocyte tropomyosin only, we found that both short nonmuscle and long muscle alpha-tropomyosins bind there as well, though with different affinities. We propose a model for actin capping where one tropomodulin molecule can bind to two tropomyosin molecules at the pointed end.

Tropomodulin 3 binds to actin monomers

Regulation of the actin cytoskeleton by filament capping proteins is critical to myriad dynamic cellular functions. The ability of these proteins to bind both filaments as well as monomers is often central to their cellular functions. The ubiquitous pointed end capping protein Tmod3 (tropomodulin 3) acts as a negative regulator of cell migration, yet mechanisms behind its cellular functions are not understood. Analysis of Tmod3 effects on kinetics of actin polymerization and steady state monomer levels revealed that Tmod3, unlike previously characterized tropomodulins, sequesters actin monomers with an affinity similar to its affinity for capping pointed ends. Furthermore, Tmod3 is found bound to actin in high speed supernatant cytosolic extracts, suggesting that Tmod3 can bind to monomers in the context of other cytosolic monomer binding proteins. The Tmod3-actin complex can be efficiently cross-linked with 1-ethyl-3-(dimethylaminopropyl)carbodiimide/N-hydroxylsulfosuccinimide in a 1:1 complex. Subsequent tryptic digestion and liquid chromatography/tandem mass spectrometry revealed two binding interfaces on actin, one distinct from other actin monomer binding proteins, and two potential binding sites in Tmod3, which are independent of the previously characterized leucine-rich repeat structure involved in pointed end capping. These data suggest that the Tmod3 isoform may regulate actin dynamics differently in cells than the previously described tropomodulin isoforms.

Leucine 135 of Tropomodulin-1 Regulates Its Association with Tropomyosin, Its Cellular Localization, and the Integrity of Sarcomeres

Tropomodulin-1 (Tmod-1) is a well defined actin-capping protein that interacts with tropomyosin (TM) at the pointed end of actin filaments. Previous studies by others have mapped its TM-binding domain to the amino terminus from amino acid 39 to 138. In this study, we have identified several amino acid residues on Tmod-1 that are important for its interaction with TM5 (a nonmuscle TM isoform). Glutathione S-transferase affinity chromatography and immunoprecipitation assays reveal that Tmod sense mutations of either amino acid 134, 135, or 136 causes various degrees of loss of function of Tmod TM-binding ability. The reduction of TM-binding ability was relatively mild (reduced approximately 20-40%) from the G136A Tmod mutant but more substantially (reduced approximately 50-100%) from the I134D, L135E, and L135V Tmod mutants. In addition, mutation at any of these three sites dramatically alters the subcellular location of Tmod-1 when introduced into mammalian cells. Further analysis of these three mutants uncovered a previously unknown nuclear trafficking function of Tmod-1, and residues 134, 135, and 136 are located within a nuclear export signal motif. As a result, mutation on either residue 134 or residue 135 not only will cause a significant reduction of the Tmod-1 ability to bind to TM5 but also lead to predominant nuclear localization of Tmod-1 by crippling its nuclear export mechanism. The failure of the Tmod mutations to fully associate with TM5 when introduced into neonatal rat cardiomyocytes was also associated with an accelerated and severe fragmentation of sarcomeric structures compared with overexpression of wild type Tmod-1. The multiple losses of function of Tmod engendered by these missense mutations are most severe with the single substitution of residue 135.

Regulation of aromatase expression in estrogen-responsive breast and uterine disease: from bench to treatment

A single gene encodes the key enzyme for estrogen biosynthesis termed aromatase, inhibition of which effectively eliminates estrogen production. Aromatase inhibitors successfully treat breast cancer and endometriosis, whereas their roles in endometrial cancer, uterine fibroids, and aromatase excess syndrome are less clear. Ovary, testis, adipose tissue, skin, hypothalamus, and placenta express aromatase normally, whereas breast and endometrial cancers, endometriosis, and uterine fibroids overexpress aromatase and produce local estrogen that exerts paracrine and intracrine effects. Tissue-specific promoters distributed over a 93-kilobase regulatory region upstream of a common coding region alternatively control aromatase expression. A distinct set of transcription factors regulates each promoter in a signaling pathway- and tissue-specific manner. Three mechanisms are responsible for aromatase overexpression in a pathologic tissue versus its normal counterpart. First, cellular composition is altered to increase aromatase-expressing cell types that use distinct promoters (breast cancer). Second, molecular alterations in stromal cells favor binding of transcriptional enhancers versus inhibitors to a normally quiescent aromatase promoter and initiate transcription (breast/endometrial cancer, endometriosis, and uterine fibroids). Third, heterozygous mutations, which cause the aromatase coding region to lie adjacent to constitutively active cryptic promoters that normally transcribe other genes, result in excessive estrogen formation owing to the overexpression of aromatase in many tissues.

Disruption in the tropomodulin1 (Tmod1) gene compromises cardiomyocyte development in murine embryonic stem cells by arresting myofibril maturation

Tropomodulins (Tmods) comprise a family of capping proteins for actin filament pointed ends. To decipher the significance of Tmod1 functions during de novo myofibrillogenesis, we generated Tmod1 null embryonic stem (ES) cells and studied their differentiation into cardiomyocytes. Strikingly, in vitro cardiomyocyte differentiation of wild type (WT) ES cells faithfully recapitulates in vivo cardiomyocyte differentiation, allowing us to evaluate the phenotypes of Tmod1 knockout (KO) myofibrils irrespective of embryonic lethality of Tmod1 KO mice. Immunofluorescence and electron microscopy studies revealed that Tmod1 null cardiac myocytes were round, morphologically immature, and contained underdeveloped myofibrils that were shorter, narrower, and had fewer thin filaments than those in WT cells. Unexpectedly, clear gaps in the staining pattern for F-actin at the H-zone were detected in most KO cells, indicating the presence of filaments at uniform lengths. This indicates that additional mechanisms other than capping proteins are responsible for thin filament length maintenance in cardiac myocytes. Also unexpectedly, approximately 40% of the KO cardiac myocytes exhibited contractile activity. Our data indicate that differentiating ES cells are a powerful system to investigate the functional properties of contractile proteins and that Tmod1 functions are critical for late stages of myofibrillogenesis, and for the maturation of myofibrils.

Tal1/SCL binding to pericentromeric DNA represses transcription

Tal1/SCL is a basic helix-loop-helix transcription factor critical for normal hematopoiesis. To understand the mechanisms underlying transcriptional regulation by Tal1/SCL, we combined an in vitro DNA binding strategy and an in vivo chromatin immunoprecipitation analysis to search for Tal1/SCL target regions in K562 erythroleukemia cells. A 0.4-kb genomic DNA clone containing two Tal1/SCL binding E-boxes and GATA- and SATB1-binding motifs (EEGS) was identified that localized to the pericentromeric region with high homology to satellite 2 DNA. Pericentric DNA is related to heterochromatin and gene inactivation. We found that Tal1/SCL could complex with the histone H3 lysine 9 (H3K9)-specific methyltransferase Suv39H1. Binding of Tal1/SCL to EEGS chromatin correlated with hypermethylation of H3K9 and the association of heterochromatin protein HP1 to this region. In Rep4 reporter gene assays, EEGS affected repression in a manner dependent on the expression level of Tal1/SCL that was accompanied by increased H3K9 methylation in chromatin associated with EEGS and a linked promoter. A specific histone deacetylase inhibitor, trichostatin A, relieved Tal1/SCL-mediated repression by EEGS. In addition, SATB1 bound EEGS chromatin and promoted Tal1/SCL EEGS-dependent repression. We expand the list of potential interacting partners for Tal1/SCL by demonstrating direct associations of Tal1/SCL with SATB1 and with Suv39H1. These results reveal a novel mechanism of action for Tal1/SCL and implicate heterochromatin-like silencing via a cis-acting binding motif for transcriptional repression.

Structure and tropomyosin binding properties of the N-terminal capping domain of tropomodulin 1

Two families of actin regulatory proteins are the tropomodulins and tropomyosins. Tropomodulin binds to tropomyosin (TM) and to the pointed end of actin filaments and "caps" the pointed end (i.e., inhibits its polymerization and depolymerization). Tropomodulin 1 has two distinct actin-capping regions: a folded C-terminal domain (residues 160-359), which does not bind to TM, and a conserved, N-terminal region, within residues 1-92 that binds TM and requires TM for capping activity. NMR and circular dichroism were used to determine the structure of a peptide containing residues 1-92 of tropomodulin (Tmod1(1-92)) and to define its TM binding site. Tmod1(1-92) is mainly disordered with only one helical region, residues 24-35. This helix forms part of the TM binding domain, residues 1-35, which become more ordered upon binding a peptide containing the N-terminus of an alpha-TM. Mutation of L27 to E or G in the Tmod helix reduces TM affinity. Residues 49-92 are required for capping but do not bind TM. Of these, residues 67-75 have the sequence of an amphipathic helix, but are not helical. Residues 55-62 and 76-92 display negative 1H-15N heteronuclear Overhauser enhancements showing they are flexible. The conformational dynamics of these residues may be important for actin capping activity.

Cytoplasmic Nuclear Transfer of the Actin-capping Protein Tropomodulin

Tropomodulin (Tmod) is a cytoskeletal actin-capping protein that interacts with tropomyosin at the pointed end of actin filaments. E-Tmod is an isoform that expresses predominantly in cardiac cells and slow skeletal muscle fibers. We unexpectedly discovered significant levels of Tmod in nuclei and then defined peptide domains in Tmod responsible for nuclear import and export. These domains resemble, and function as, a nuclear export signal (NES) and a pattern 4 nuclear localization signal (NLS). Both motifs are conserved in other Tmod isoforms and across species. Comparisons of wild-type Tmod and Tmod carrying mutations in these peptide domains revealed that Tmod normally traffics through the nucleus. These observations logically presuppose that Tmod functions may include a nuclear role. Indeed, increasing Tmod in the nucleus severely hampered myogenic differentiation and selectively suppressed muscle-specific gene expression (endogenous p21, myosin heavy chain, myogenin, and Tmod) but did not affect endogenous glyceraldehyde-3-phosphate dehydrogenase or expression from a transfected E-GFP vector. These results suggest that, at least in myogenic cells, nuclear Tmod may be involved in the differentiation process.

Myofibrillogenesis in the developing chicken heart: Role of actin isoforms and of the pointed end actin capping protein tropomodulin during thin filament assembly

Recently, important differences between myofibrillogenesis in cultured cardiomyocytes vs. the three-dimensional setting in situ could be determined. We investigated thin filament assembly in situ by confocal microscopy of whole-mount preparations of immunostained embryonic chicken hearts. Of interest, a distinct localisation of different actin isoforms was observed in immature thin filaments. Cardiac alpha-actin is restricted to filaments with a length comparable to mature thin filaments as soon as the first contractions occur, while vascular alpha-actin makes up filaments that extend toward the M-band. The pointed-end actin filament capping protein tropomodulin can be found initially in close association with the plasma membrane, but attains its mature localisation pattern at the ends of the thin filaments only comparatively late during myofibrillogenesis. Thus tropomodulin acts as a length stabilising element of actin filaments also in developing cardiomyocytes in situ, but plays an additional role together with membrane-associated actin filaments in the earliest steps of myofibril assembly.

Tropomodulin contains two actin filament pointed end-capping domains

Tropomodulin 1 (Tmod1) is a approximately 40-kDa tropomyosin binding and actin filament pointed end-capping protein that regulates pointed end dynamics and controls thin filament length in striated muscle. In vitro, the capping affinity of Tmod1 for tropomyosin-actin filaments (Kd approximately 50 pm) is several thousand-fold greater than for capping of pure actin filaments (Kd approximately 0.1 microM). The tropomyosin-binding region of Tmod1 has been localized to the amino-terminal portion between residues 1 and 130, but the location of the actin-capping domain is not known. We have now identified two distinct actin-capping regions on Tmod1 by testing a series of recombinant Tmod1 fragments for their ability to inhibit actin elongation from gelsolin-actin seeds using pyrene-actin polymerization assays. The carboxyl-terminal portion of Tmod1 (residues 160-359) contains the principal actin-capping activity (Kd approximately 0.4 microM), requiring residues between 323 and 359 for full activity, whereas the amino-terminal portion of Tmod1 (residues 1-130) contains a second, weaker actin-capping activity (Kd approximately 1.8 microM). Interestingly, 160-359 but not 1-130 enhances spontaneous actin nucleation, suggesting that the carboxyl-terminal domain may bind to two actin subunits across the actin helix at the pointed end, whereas the amino-terminal domain may bind to only one actin subunit. On the other hand, the actin-capping activity of the amino-terminal but not the carboxyl-terminal portion of Tmod1 is enhanced several thousand-fold in the presence of skeletal muscle tropomyosin. We conclude that the carboxyl-terminal capping domain of Tmod1 contains a TM-independent actin pointed end-capping activity, whereas the amino-terminal domain contains a TM-regulated pointed end actin-capping activity.