Tropomyosin localization reveals distinct populations of microfilaments in neurites and growth cones

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.

Targeting the cytoskeleton against metastatic dissemination

Cancer is a pathology characterized by a loss or a perturbation of a number of typical features of normal cell behaviour. Indeed, the acquisition of an inappropriate migratory and invasive phenotype has been reported to be one of the hallmarks of cancer. The cytoskeleton is a complex dynamic network of highly ordered interlinking filaments playing a key role in the control of fundamental cellular processes, like cell shape maintenance, motility, division and intracellular transport. Moreover, deregulation of this complex machinery contributes to cancer progression and malignancy, enabling cells to acquire an invasive and metastatic phenotype. Metastasis accounts for 90% of death from patients affected by solid tumours, while an efficient prevention and suppression of metastatic disease still remains elusive. This results in the lack of effective therapeutic options currently available for patients with advanced disease. In this context, the cytoskeleton with its regulatory and structural proteins emerges as a novel and highly effective target to be exploited for a substantial therapeutic effort toward the development of specific anti-metastatic drugs. Here we provide an overview of the role of cytoskeleton components and interacting proteins in cancer metastasis with a special focus on small molecule compounds interfering with the actin cytoskeleton organization and function. The emerging involvement of microtubules and intermediate filaments in cancer metastasis is also reviewed.

SOD1-interacting proteins: Roles of aggregation cores and protein degradation systems

Cu/Zn superoxide dismutase (SOD1) mutations are associated with amyotrophic lateral sclerosis (ALS). SOD1-positive aggregates in motor neurons, as well as proteins that interact with the aggregates are presumably involved in ALS neurotoxicity. We used a proteomics approach to compare differences in protein expression in spinal cord homogenates from non-transgenic (NTG) and ALS model mice. Using the homogenates, we identified proteins that interacted with SOD1 seeds in vitro. We assessed differences in SOD1-interacting proteins in cell cultures treated with proteasome or autophagy inhibitor. In the first experiment, intermediate filamentous and small heat shock proteins were upregulated in glial cells. We identified 26 protein types that interacted with aggregation cores in ALS model homogenates, and unexpectedly, 40 proteins in were detected in NTG mice. In cell cultures treated with proteasome and autophagy inhibitors, we identified 16 and 11 SOD1-interacting proteins, respectively, and seven proteins in untreated cells. These SOD1-interacting proteins were involved in multiple cellular functions such as protein quality control, cytoskeletal organization, and pathways involved in growth factor signaling and their downstream cascades. The complex interactions between pathways could cause further dysregulation, ultimately leading to fatal cellular dysfunction in ALS.

Tropomyosin isoforms regulate cofilin 1 activity by modulating actin filament conformation

Tropomyosin and cofilin are involved in the regulation of actin filament dynamic polymerization and depolymerization. Binding of cofilin changes actin filaments structure, leading to their severing and depolymerization. Non-muscle tropomyosin isoforms were shown before to differentially regulate the activity of cofilin 1; products of TPM1 gene stabilized actin filaments, but products of TPM3 gene promoted cofilin-dependent severing and depolymerization. Here, conformational changes at the longitudinal and lateral interface between actin subunits resulting from tropomyosin and cofilin 1 binding were studied using skeletal actin and yeast wild type and mutant Q41C and S265C actins. Cross-linking of F-actin and fluorescence changes in F-actin labeled with acrylodan at Cys41 (in D-loop) or Cys265 (in H-loop) showed that tropomyosin isoforms differentially regulated cofilin-induced conformational rearrangements at longitudinal and lateral filament interfaces. Tryptic digestion of F-Mg-actin confirmed the differences between tropomyosin isoforms in their regulation of cofilin-dependent changes at actin-actin interfaces. Changes in the fluorescence of AEDANS attached to C-terminal Cys of actin, as well as FRET between Trp residues in actin subdomain 1 and AEDANS, did not show differences in the conformation of the C-terminal segment of F-actin in the presence of different tropomyosins ± cofilin 1. Therefore, actin's D- and H-loop are the sites involved in regulation of cofilin activity by tropomyosin isoforms.

Actin-tropomyosin distribution in non-muscle cells

The interactions of cytoskeletal actin filaments with myosin family motors are essential for the integrity and function of eukaryotic cells. They support a wide range of force-dependent functions. These include mechano-transduction, directed transcellular transport processes, barrier functions, cytokinesis, and cell migration. Despite the indispensable role of tropomyosins in the generation and maintenance of discrete actomyosin-based structures, the contribution of individual cytoskeletal tropomyosin isoforms to the structural and functional diversification of the actin cytoskeleton remains a work in progress. Here, we review processes that contribute to the dynamic sorting and targeted distribution of tropomyosin isoforms in the formation of discrete actomyosin-based structures in animal cells and their effects on actin-based motility and contractility.

Tropomyosin concentration but not formin nucleators mDia1 and mDia3 determines the level of tropomyosin incorporation into actin filaments

The majority of actin filaments in human cells exist as a co-polymer with tropomyosin, which determines the functionality of actin filaments in an isoform dependent manner. Tropomyosin isoforms are sorted to different actin filament populations and in yeast this process is determined by formins, however it remains unclear what process determines tropomyosin isoform sorting in mammalian cells. We have tested the roles of two major formin nucleators, mDia1 and mDia3, in the recruitment of specific tropomyosin isoforms in mammals. Despite observing poorer cell-cell attachments in mDia1 and mDia3 KD cells and an actin bundle organisation defect with mDia1 knock down; depletion of mDia1 and mDia3 individually and concurrently did not result in any significant impact on tropomyosin recruitment to actin filaments, as observed via immunofluorescence and measured via biochemical assays. Conversely, in the presence of excess Tpm3.1, the absolute amount of Tpm3.1-containing actin filaments is not fixed by actin filament nucleators but rather depends on the cell concentration of Tpm3.1. We conclude that mDia1 and mDia3 are not essential for tropomyosin recruitment and that tropomyosin incorporation into actin filaments is concentration dependent.

Tropomyosins Regulate the Severing Activity of Gelsolin in Isoform-Dependent and Independent Manners

The actin cytoskeleton fulfills numerous key cellular functions, which are tightly regulated in activity, localization, and temporal patterning by actin binding proteins. Tropomyosins and gelsolin are two such filament-regulating proteins. Here, we investigate how the effects of tropomyosins are coupled to the binding and activity of gelsolin. We show that the three investigated tropomyosin isoforms (Tpm1.1, Tpm1.12, and Tpm3.1) bind to gelsolin with micromolar or submicromolar affinities. Tropomyosin binding enhances the activity of gelsolin in actin polymerization and depolymerization assays. However, the effects of the three tropomyosin isoforms varied. The tropomyosin isoforms studied also differed in their ability to protect pre-existing actin filaments from severing by gelsolin. Based on the observed specificity of the interactions between tropomyosins, actin filaments, and gelsolin, we propose that tropomyosin isoforms specify which populations of actin filaments should be targeted by, or protected from, gelsolin-mediated depolymerization in living cells.

On-target action of anti-tropomyosin drugs regulates glucose metabolism

The development of novel small molecule inhibitors of the cancer-associated tropomyosin 3.1 (Tpm3.1) provides the ability to examine the metabolic function of specific actin filament populations. We have determined the ability of these anti-Tpm (ATM) compounds to regulate glucose metabolism in mice. Acute treatment (1 h) of wild-type (WT) mice with the compounds (TR100 and ATM1001) led to a decrease in glucose clearance due mainly to suppression of glucose-stimulated insulin secretion (GSIS) from the pancreatic islets. The impact of the drugs on GSIS was significantly less in Tpm3.1 knock out (KO) mice indicating that the drug action is on-target. Experiments in MIN6 β-cells indicated that the inhibition of GSIS by the drugs was due to disruption to the cortical actin cytoskeleton. The impact of the drugs on insulin-stimulated glucose uptake (ISGU) was also examined in skeletal muscle ex vivo. In the absence of drug, ISGU was decreased in KO compared to WT muscle, confirming a role of Tpm3.1 in glucose uptake. Both compounds suppressed ISGU in WT muscle, but in the KO muscle there was little impact of the drugs. Collectively, this data indicates that the ATM drugs affect glucose metabolism in vivo by inhibiting Tpm3.1's function with few off-target effects.

Functional characterisation of filamentous actin probe expression in neuronal cells

Genetically encoded filamentous actin probes, Lifeact, Utrophin and F-tractin, are used as tools to label the actin cytoskeleton. Recent evidence in several different cell types indicates that these probes can cause changes in filamentous actin dynamics, altering cell morphology and function. Although these probes are commonly used to visualise actin dynamics in neurons, their effects on axonal and dendritic morphology has not been systematically characterised. In this study, we quantitatively analysed the effect of Lifeact, Utrophin and F-tractin on neuronal morphogenesis in primary hippocampal neurons. Our data show that the expression of actin-tracking probes significantly impacts on axonal and dendrite growth these neurons. Lifeact-GFP expression, under the control of a pBABE promoter, caused a significant decrease in total axon length, while another Lifeact-GFP expression, under the control of a CAG promoter, decreased the length and complexity of dendritic trees. Utr261-EGFP resulted in increased dendritic branching but Utr230-EGFP only accumulated in cell soma, without labelling any neurites. Lifeact-7-mEGFP and F-tractin-EGFP in a pEGFP-C1 vector, under the control of a CMV promoter, caused only minor changes in neuronal morphology as detected by Sholl analysis. The results of this study demonstrate the effects that filamentous actin tracking probes can have on the axonal and dendritic compartments of neuronal cells and emphasise the care that must be taken when interpreting data from experiments using these probes.

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.

Smooth Muscle-specific α Tropomyosin Is a Marker of Fully Differentiated Smooth Muscle in Lung

Tropomyosin (Tm) is one of the major components of smooth muscle. Currently it is impossible to easily distinguish the two major smooth muscle (sm) forms of Tm at a protein level by immunohistochemistry due to lack of specific antibodies. α-sm Tm contains a unique 2a exon not found in any other Tm. We have produced a polyclonal antibody to this exon that specifically detects α-sm Tm. We demonstrate here the utility of this antibody for the study of smooth muscle. The tissue distribution of α-sm Tm was shown to be highly specific to smooth muscle. α-sm Tm showed an identical profile and tissue colocalization with α-sm actin both by Western blotting and immunohistochemistry. Using lung as a model organ system, we examined the developmental appearance of α-sm Tm in comparison to α-sm actin in both the mouse and human. α-sm Tm is a late-onset protein, appearing much later than actin in both species. There were some differences in onset of appearance in vascular and airway smooth muscle with airway appearing earlier. α-sm Tm can therefore be used as a good marker of mature differentiated smooth muscle cells. Along with α-sm actin and sm-myosin antibodies, α-sm Tm is a valuable tool for the study of smooth muscle.

Tissue-specific Tropomyosin Isoform Composition

Four distinct genes encode tropomyosin (Tm) proteins, integral components of the actin microfilament system. In non-muscle cells, over 40 Tm isoforms are derived using alternative splicing. Distinct populations of actin filaments characterized by the composition of these Tm isoforms are found differentially sorted within cells ( Gunning et al. 1998b ). We hypothesized that these distinct intracellular compartments defined by the association of Tm isoforms may allow for independent regulation of microfilament function. Consequently, to understand the molecular mechanisms that give rise to these different microfilaments and their regulation, a cohort of fully characterized isoform-specific Tm antibodies was required. The characterization protocol initially involved testing the specificity of the antibodies on bacterially produced Tm proteins. We then confirmed that these Tm antibodies can be used to probe the expression and subcellular localization of different Tm isoforms by Western blot analysis, immunofluorescence staining of cells in culture, and immunohistochemistry of paraffin wax-embedded mouse tissues. These Tm antibodies, therefore, have the capacity to monitor specific actin filament populations in a range of experimental systems.

Tropomyosins in the healthy and diseased nervous system

Regulation of the actin cytoskeleton is dependent on a plethora of actin-associated proteins in all eukaryotic cells. The family of tropomyosins plays a key role in controlling the function of several of these actin-associated proteins and their access to actin filaments. In order to understand the regulation of the actin cytoskeleton in highly dynamic subcellular compartments of neurons such as growth cones of developing neurons and the synaptic compartment of mature neurons, it is pivotal to decipher the functional role of tropomyosins in the nervous system. In this review, we will discuss the current understanding and recent findings on the regulation of the actin cytoskeleton by tropomyosins and potential implication that this has for the dysregulation of the actin cytoskeleton in neurological diseases.

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.

Building Blocks of Functioning Brain: Cytoskeletal Dynamics in Neuronal Development

Neural connectivity requires proper polarization of neurons, guidance to appropriate target locations, and establishment of synaptic connections. From when neurons are born to when they finally reach their synaptic partners, neurons undergo constant rearrangment of the cytoskeleton to achieve appropriate shape and polarity. Of particular importance to neuronal guidance to target locations is the growth cone at the tip of the axon. Growth-cone steering is also dictated by the underlying cytoskeleton. All these changes require spatiotemporal control of the cytoskeletal machinery. This review summarizes the proteins that are involved in modulating the actin and microtubule cytoskeleton during the various stages of neuronal development.

Tropomyosin - master regulator of actin filament function in the cytoskeleton

Tropomyosin (Tpm) isoforms are the master regulators of the functions of individual actin filaments in fungi and metazoans. Tpms are coiled-coil parallel dimers that form a head-to-tail polymer along the length of actin filaments. Yeast only has two Tpm isoforms, whereas mammals have over 40. Each cytoskeletal actin filament contains a homopolymer of Tpm homodimers, resulting in a filament of uniform Tpm composition along its length. Evidence for this 'master regulator' role is based on four core sets of observation. First, spatially and functionally distinct actin filaments contain different Tpm isoforms, and recent data suggest that members of the formin family of actin filament nucleators can specify which Tpm isoform is added to the growing actin filament. Second, Tpms regulate whole-organism physiology in terms of morphogenesis, cell proliferation, vesicle trafficking, biomechanics, glucose metabolism and organ size in an isoform-specific manner. Third, Tpms achieve these functional outputs by regulating the interaction of actin filaments with myosin motors and actin-binding proteins in an isoform-specific manner. Last, the assembly of complex structures, such as stress fibers and podosomes involves the collaboration of multiple types of actin filament specified by their Tpm composition. This allows the cell to specify actin filament function in time and space by simply specifying their Tpm isoform composition.

An actin filament population defined by the tropomyosin Tpm3.1 regulates glucose uptake

Actin has an ill-defined role in the trafficking of GLUT4 glucose transporter vesicles to the plasma membrane (PM). We have identified novel actin filaments defined by the tropomyosin Tpm3.1 at glucose uptake sites in white adipose tissue (WAT) and skeletal muscle. In Tpm 3.1-overexpressing mice, insulin-stimulated glucose uptake was increased; while Tpm3.1-null mice they were more sensitive to the impact of high-fat diet on glucose uptake. Inhibition of Tpm3.1 function in 3T3-L1 adipocytes abrogates insulin-stimulated GLUT4 translocation and glucose uptake. In WAT, the amount of filamentous actin is determined by Tpm3.1 levels and is paralleled by changes in exocyst component (sec8) and Myo1c levels. In adipocytes, Tpm3.1 localizes with MyoIIA, but not Myo1c, and it inhibits Myo1c binding to actin. We propose that Tpm3.1 determines the amount of cortical actin that can engage MyoIIA and generate contractile force, and in parallel limits the interaction of Myo1c with actin filaments. The balance between these actin filament populations may determine the efficiency of movement and/or fusion of GLUT4 vesicles with the PM.

Differences in brain transcriptomes of closely related Baikal coregonid species

The aim of this work was to get deeper insight into genetic factors involved in the adaptive divergence of closely related species, specifically two representatives of Baikal coregonids—Baikal whitefish (Coregonus baicalensis Dybowski) and Baikal omul (Coregonus migratorius Georgi)—that diverged from a common ancestor as recently as 10–20 thousand years ago. Using the Serial Analysis of Gene Expression method, we obtained libraries of short representative cDNA sequences (tags) from the brains of Baikal whitefish and omul. A comparative analysis of the libraries revealed quantitative differences among ~4% tags of the fishes under study. Based on the similarity of these tags with cDNA of known organisms, we identified candidate genes taking part in adaptive divergence. The most important candidate genes related to the adaptation of Baikal whitefish and Baikal omul, identified in this work, belong to the genes of cell metabolism, nervous and immune systems, protein synthesis, and regulatory genes as well as to DTSsa4 Tc1-like transposons which are widespread among fishes.

Purification of tropomyosin Br-3 and 5NM1 and characterization of their interactions with actin

Tropomyosins were first identified in neuronal systems in 1973. Although numerous isoforms were found and described since then, many aspects of their function and interactions remained unknown. Tropomyosin isoforms show different sorting pattern in neurogenesis. As one example, TM5NM1/2 is present in developing axons, but it is replaced by TMBr-3 in mature neurons, suggesting that these tropomyosin isoforms contribute differently to the establishment of the functional features of the neuronal actin networks. We developed a method for the efficient purification of TMBr-3 and TM5NM1 as recombinant proteins using bacterial expression system and investigated their interactions with actin. We found that both isoforms bind actin filaments, however, the binding of TM5NM1 was much stronger than that of TMBr-3. TMBr-3 and TM5NM1 modestly affected actin assembly kinetics, in an opposite manner. Consistently with the higher affinity of TM5NM1 it inhibited actin filament disassembly more efficiently than TMBr-3. Similarly to other previously studied tropomyosins TM5NM1 inhibited the Arp2/3 complex-mediated actin assembly. Notably, TMBr-3 did not influence the Arp2/3 complex-mediated polymerization. This is a unique feature of TMBr-3, since so far it is the only known tropomyosin supporting the activity of the Arp2/3 complex, indicating that TMBr-3 may colocalize and work simultaneously with Arp2/3 complex in neuronal cells.

Cytoskeletal tropomyosins: choreographers of actin filament functional diversity

The actin cytoskeleton plays a central role in many essential cellular processes. Its involvement requires actin filaments to form multiple populations with different structural and therefore functional properties in specific subcellular locations. This diversity is facilitated through the interaction between actin and a number of actin binding proteins. One family of proteins, the tropomyosins, are absolutely essential in regulating actin's ability to form such diverse structures. In this review we integrate studies from different organisms and cell types in an attempt to provide a unifying view of tropomyosin dependent regulation of the actin cytoskeleton.

Neural activity imaging with genetically encoded calcium indicators

Genetically encoded calcium indicators (GECIs), together with modern microscopy, allow repeated activity measurement, in real time and with cellular resolution, of defined cellular populations. Recent efforts in protein engineering have yielded several high-quality GECIs that facilitate new applications in neuroscience. Here, we summarize recent progress in GECI design, optimization, and characterization, and provide guidelines for selecting the appropriate GECI for a given biological application. We focus on the unique challenges associated with imaging in behaving animals.

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.

New approaches to targeting the actin cytoskeleton for chemotherapy

The actin cytoskeleton is indispensable for normal cellular function. In particular, several actin-based structures coordinate cellular motility, a process hijacked by tumor cells in order to facilitate their propagation to distant sites. The actin cytoskeleton, therefore, represents a point for chemotherapeutic intervention. The challenge in disrupting the actin cytoskeleton is in preserving actin-driven contraction of cardiac and skeletal muscle. By targeting actin-binding proteins with altered expression in malignancy, it may be possible to achieve tumor-specific toxicity. A number of actin-binding proteins act cooperatively and synergistically to regulate actin structures required for motility. The actin cytoskeleton is characterized by a significant degree of plasticity. Targeting specific actin-binding proteins for chemotherapy will only be successful if no other compensatory mechanisms exist.

Tropomyosin variants describe distinct functional subcellular domains in differentiated vascular smooth muscle cells

Tropomyosin (Tm) is known to be an important gatekeeper of actin function. Tm isoforms are encoded by four genes, and each gene produces several variants by alternative splicing, which have been proposed to play roles in motility, proliferation, and apoptosis. Smooth muscle studies have focused on gizzard smooth muscle, where a heterodimer of Tm from the α-gene (Tmsm-α) and from the β-gene (Tmsm-β) is associated with contractile filaments. In this study we examined Tm in differentiated mammalian vascular smooth muscle (dVSM). Liquid chromatography-tandem mass spectrometry (LC MS/MS) analysis and Western blot screening with variant-specific antibodies revealed that at least five different Tm proteins are expressed in this tissue: Tm6 (Tmsm-α) and Tm2 from the α-gene, Tm1 (Tmsm-β) from the β-gene, Tm5NM1 from the γ-gene, and Tm4 from the δ-gene. Tm6 is by far most abundant in dVSM followed by Tm1, Tm2, Tm5NM1, and Tm4. Coimmunoprecipitation and coimmunofluorescence studies demonstrate that Tm1 and Tm6 coassociate with different actin isoforms and display different intracellular localizations. Using an antibody specific for cytoplasmic γ-actin, we report here the presence of a γ-actin cortical cytoskeleton in dVSM cells. Tm1 colocalizes with cortical cytoplasmic γ-actin and coprecipitates with γ-actin. Tm6, on the other hand, is located on contractile bundles. These data indicate that Tm1 and Tm6 do not form a classical heterodimer in dVSM but rather describe different functional cellular compartments.

Polymorphisms in the tropomyosin TPM1 short isoform promoter alter gene expression and are associated with increased risk of metabolic syndrome

BACKGROUND

Inflammation contributes to the development of atherosclerotic lesions in the metabolic syndrome. Tropomyosin isoform expression is altered in this disease and has a role in inflammatory cell plasticity, motility, and insulin sensitivity. We determined the frequency of haplotype carriage of three single-nucleotide polymorphisms (SNPs) in the short isoform promoter of the TPM1 gene in 300 normal controls and 500 metabolic syndrome patients. The effect of each haplotype on tropomyosin gene expression was assessed.

METHODS

PCR-restriction fragment length polymorphism assays were developed for each polymorphism. Promoter activity was measured using luciferase assays in the insulin-sensitive human embryonic kidney (HEK) 293 and the monocyte THP-1 lines.

RESULTS

The SNPs -111(T/C), -426(T/C), and -491(A/G), relative to the TPM1 short isoform transcription start site, occurred in haplotypes ATT, GCT, GTT, and GTC, and were in strong linkage disequilibrium. ATT had a frequency of 66%. The presence of -491G, which conforms to a predicted binding site for transcription factor AML-1, caused a decrease in gene expression of 24% in the HEK 293 cells. In the THP-1 cells, haplotypes GTC and GTT gave 24% lower expression, whereas haplotype GCT gave expression at wild-type levels. The carriage of a -491G allele gave an odds ratio of 1.4 (95% CI 1.02-1.8) for the metabolic syndrome (P < 0.03).

CONCLUSIONS

A polymorphism in the TPM1 short isoform promoter region is predicted to alter transcription factor binding, alters gene expression and is associated with the metabolic syndrome. This could affect inflammatory cells and cytoskeleton-mediated insulin signaling.

Internal and external paralogy in the evolution of tropomyosin genes in metazoans

Nature contains a tremendous diversity of forms both at the organismal and genomic levels. This diversity motivates the twin central questions of molecular evolution: what are the molecular mechanisms of adaptation, and what are the functional consequences of genomic diversity. We report a 22-species comparative analysis of tropomyosin (PPM) genes, which exist in a variety of forms and are implicated in the emergence of a wealth of cellular functions, including the novel muscle functions integral to the functional diversification of bilateral animals. TPM genes encode either or both of long-form [284 amino acid (aa)] and short-form (approximately 248 aa) proteins. Consistent with a role of TPM diversification in the origins and radiation of bilaterians, we find evidence that the muscle-specific long-form protein arose in proximal bilaterian ancestors (the bilaterian 'stem'). Duplication of the 5' end of the gene led to alternative promoters encoding long- and short-form transcripts with distinct functions. This dual-function gene then underwent strikingly parallel evolution in different bilaterian lineages. In each case, recurrent tandem exon duplication and mutually exclusive alternative splicing of the duplicates, with further association between these alternatively spliced exons along the gene, led to long- and short-form-specific exons, allowing for gradual emergence of alternative "internal paralogs" within the same gene. We term these Mutually exclusively Alternatively spliced Tandemly duplicated Exon sets "MATEs". This emergence of internal paralogs in various bilaterians has employed every single TPM exon in at least one lineage and reaches striking levels of divergence with up to 77% of long- and short-form transcripts being transcribed from different genomic regions. Interestingly, in some lineages, these internal alternatively spliced paralogs have subsequently been "externalized" by full gene duplication and reciprocal retention/loss of the two transcript isoforms, a particularly clear case of evolution by subfunctionalization. This parallel evolution of TPM genes in diverse metazoans attests to common selective forces driving divergence of different gene transcripts and represents a striking case of emergence of evolutionary novelty by alternative splicing.

Alternatively spliced N-terminal exons in tropomyosin isoforms do not act as autonomous targeting signals

Tropomyosin (Tm) polymerises head-to-tail to form a continuous polymer located in the major groove of the actin filament. Multiple Tm isoforms are generated by alternative splicing of four genes, and individual isoforms show specific localisation patterns in many cell types, and can have differing effects on the actin cytoskeleton. Fluorescently-tagged Tm isoforms and mutants were expressed in C2C12 cells to investigate the mechanisms of alternative localisation of high molecular weight (HMW) and low molecular weight (LMW) Tms. Fluorescently-tagged Tm constructs show similar localisation to endogenous Tms as observed by antibodies, with the HMW Tm3 relatively diminished at the periphery of cells compared to LMW isoforms Tm5b or Tm5NM1. Tm3 and Tm5b only differ in their N-terminal exons, but these N-terminal exons do not independently direct localisation within the cell, as chimeric mutants Tm3-Tm5NM1 and Tm5b-Tm5NM1 show an increased peripheral localisation similar to Tm5NM1. The lower abundance of Tm3 at the periphery of the cell is not a result of different protein dynamics, as Tm3 and Tm5b show similar recovery after photobleaching. The relative exclusion of Tm3 from the periphery of cells does, however, require interaction with the actin filament, as mutants with truncations at either the N-terminus or the C-terminus are unable to localise to actin stress fibres, and are present in the most peripheral regions of the cell. We conclude that it is the entire Tm molecule which is the unit of sorting, and that the alternatively spliced N-terminal exons do not act as autonomous targeting signals.

Gestational ethanol exposure alters the behavioral response to ethanol odor and the expression of neurotransmission genes in the olfactory bulb of adolescent rats

Fetal exposure to ethanol is highly predictive of the propensity to ingest ethanol during adolescence and in utero chemosensory plasticity has been implicated as a contributing factor in this process. Recent rodent studies have shown that fetal ethanol exposure results in a tuned unconditioned sniffing and neurophysiological olfactory response to ethanol odor in infant animals. Importantly, a significant proportion of increased ethanol avidity at this age can be attributed to the tuned behavioral response to ethanol odor. These effects are absent in adults. Using behavioral methods and comprehensive gene expression profiling to screen for robust transcriptional differences induced in the olfactory bulb, we examined whether ethanol exposure via maternal diet results in an altered responsiveness to ethanol odor that persists into late adolescence and, if so, the molecular mechanisms that may be associated with such effects. Compared to controls, fetal exposure altered: the adolescent sniffing response to ethanol odor consistent with the previously observed changes in infant animals; and the expression of genes involved in synaptic transmission and plasticity as well as neuronal development (both cell fate and axon/neurite outgrowth). These data provide evidence for a persistence of olfactory-mediated responsiveness to ethanol into the period of adolescence. Further, they provide insight into an important relationship between fetal exposure to ethanol, adolescent odor responsiveness to the drug and potential underlying molecular mechanisms for the odor-guided behavioral response.

Tropomyosin isoform expression regulates the transition of adhesions to determine cell speed and direction

The balance of transition between distinct adhesion types contributes to the regulation of mesenchymal cell migration, and the characteristic association of adhesions with actin filaments led us to question the role of actin filament-associating proteins in the transition between adhesive states. Tropomyosin isoform association with actin filaments imparts distinct filament structures, and we have thus investigated the role for tropomyosins in determining the formation of distinct adhesion structures. Using combinations of overexpression, knockdown, and knockout approaches, we establish that Tm5NM1 preferentially stabilizes focal adhesions and drives the transition to fibrillar adhesions via stabilization of actin filaments. Moreover, our data suggest that the expression of Tm5NM1 is a critical determinant of paxillin phosphorylation, a signaling event that is necessary for focal adhesion disassembly. Thus, we propose that Tm5NM1 can regulate the feedback loop between focal adhesion disassembly and focal complex formation at the leading edge that is required for productive and directed cell movement.

Cytoskeletal tropomyosin Tm5NM1 is required for normal excitation-contraction coupling in skeletal muscle

The functional diversity of the actin microfilaments relies in part on the actin binding protein tropomyosin (Tm). The muscle-specific Tms regulate actin-myosin interactions and hence contraction. However, there is less known about the roles of the numerous cytoskeletal isoforms. We have shown previously that a cytoskeletal Tm, Tm5NM1, defines a Z-line adjacent cytoskeleton in skeletal muscle. Recently, we identified a second cytoskeletal Tm in this region, Tm4. Here we show that Tm4 and Tm5NM1 define separate actin filaments; the former associated with the terminal sarcoplasmic reticulum (SR) and other tubulovesicular structures. In skeletal muscles of Tm5NM1 knockout (KO) mice, Tm4 localization was unchanged, demonstrating the specificity of the membrane association. Tm5NM1 KO muscles exhibit potentiation of T-system depolarization and decreased force rundown with repeated T-tubule depolarizations consistent with altered T-tubule function. These results indicate that a Tm5NM1-defined actin cytoskeleton is required for the normal excitation-contraction coupling in skeletal muscle.

The phenomics and expression quantitative trait locus mapping of brain transcriptomes regulating adaptive divergence in lake whitefish species pairs (Coregonus sp.)

We used microarrays and a previously established linkage map to localize the genetic determinants of brain gene expression for a backcross family of lake whitefish species pairs (Coregonus sp.). Our goals were to elucidate the genomic distribution and sex specificity of brain expression QTL (eQTL) and to determine the extent to which genes controlling transcriptional variation may underlie adaptive divergence in the recently evolved dwarf (limnetic) and normal (benthic) whitefish. We observed a sex bias in transcriptional genetic architecture, with more eQTL observed in males, as well as divergence in genome location of eQTL between the sexes. Hotspots of nonrandom aggregations of up to 32 eQTL in one location were observed. We identified candidate genes for species pair divergence involved with energetic metabolism, protein synthesis, and neural development on the basis of colocalization of eQTL for these genes with eight previously identified adaptive phenotypic QTL and four previously identified outlier loci from a genome scan in natural populations. Eighty-eight percent of eQTL-phenotypic QTL colocalization involved growth rate and condition factor QTL, two traits central to adaptive divergence between whitefish species pairs. Hotspots colocalized with phenotypic QTL in several cases, revealing possible locations where master regulatory genes, such as a zinc-finger protein in one case, control gene expression directly related to adaptive phenotypic divergence. We observed little evidence of colocalization of brain eQTL with behavioral QTL, which provides insight into the genes identified by behavioral QTL studies. These results extend to the transcriptome level previous work illustrating that selection has shaped recent parallel divergence between dwarf and normal lake whitefish species pairs and that metabolic, more than morphological, differences appear to play a key role in this divergence.

Tetratricopeptide repeat domain 9A is an interacting protein for tropomyosin Tm5NM-1

Background

Tetratricopeptide repeat domain 9A (TTC9A) protein is a recently identified protein which contains three tetratricopeptide repeats (TPRs) on its C-terminus. In our previous studies, we have shown that TTC9A was a hormonally-regulated gene in breast cancer cells. In this study, we found that TTC9A was over-expressed in breast cancer tissues compared with the adjacent controls (P < 0.00001), suggesting it might be involved in the breast cancer development process. The aim of the current study was to further elucidate the function of TTC9A.

Methods

Breast samples from 25 patients including the malignant breast tissues and the adjacent normal tissues were processed for Southern blot analysis. Yeast-two-hybrid assay, GST pull-down assay and co-immunoprecipitation were used to identify and verify the interaction between TTC9A and other proteins.

Results

Tropomyosin Tm5NM-1 was identified as one of the TTC9A partner proteins. The interaction between TTC9A and Tm5NM-1 was further confirmed by GST pull-down assay and co-immunoprecipitation in mammalian cells. TTC9A domains required for the interaction were also characterized in this study. The results suggested that the first TPR domain and the linker fragment between the first two TPR domains of TTC9A were important for the interaction with Tm5NM-1 and the second and the third TPR might play an inhibitory role.

Conclusion

Since the primary function of tropomyosin is to stabilize actin filament, its interaction with TTC9A may play a role in cell shape and motility. In our previous results, we have found that progesterone-induced TTC9A expression was associated with increased cell motility and cell spreading. We speculate that TTC9A acts as a chaperone protein to facilitate the function of tropomyosins in stabilizing microfilament and it may play a role in cancer cell invasion and metastasis.

Tropomyosin-based regulation of the actin cytoskeleton in time and space

Tropomyosins are rodlike coiled coil dimers that form continuous polymers along the major groove of most actin filaments. In striated muscle, tropomyosin regulates the actin-myosin interaction and, hence, contraction of muscle. Tropomyosin also contributes to most, if not all, functions of the actin cytoskeleton, and its role is essential for the viability of a wide range of organisms. The ability of tropomyosin to contribute to the many functions of the actin cytoskeleton is related to the temporal and spatial regulation of expression of tropomyosin isoforms. Qualitative and quantitative changes in tropomyosin isoform expression accompany morphogenesis in a range of cell types. The isoforms are segregated to different intracellular pools of actin filaments and confer different properties to these filaments. Mutations in tropomyosins are directly involved in cardiac and skeletal muscle diseases. Alterations in tropomyosin expression directly contribute to the growth and spread of cancer. The functional specificity of tropomyosins is related to the collaborative interactions of the isoforms with different actin binding proteins such as cofilin, gelsolin, Arp 2/3, myosin, caldesmon, and tropomodulin. It is proposed that local changes in signaling activity may be sufficient to drive the assembly of isoform-specific complexes at different intracellular sites.

Isoform sorting of tropomyosins

Cytoskeletal tropomyosin (Tm) isoforms show extensive intracellular sorting, resulting in spatially distinct actin-filament populations. Sorting of Tm isoforms has been observed in a number of cell types, including fibroblasts, epithelial cells, osteoclasts, neurons and muscle cells. Different Tm isoforms have differential impact on the activity of a number of actin-binding proteins and can therefore differentially regulate actin filament function. Functionally distinct sub-populations of actin filaments can therefore be defined on the basis of the Tm isoforms associated with the filaments. The mechanisms that underlie Tm sorting are not yet well understood, but it is clear that Tm sorting is a very fluid and dynamic process, with changes in sorting occurring throughout development and cell differentiation. For this reason, it is unlikely that Tm localization is determined by an intrinsic sorting signal that directs particular isoforms to a single geographical location. Rather, a molecular sink model where isoforms accumulate in actin-based structures where they have the highest affinity, is most consistent with current data. This model would predict Tm sorting to be influenced by changes to actin filament dynamics and organization and collaboration with other actin-binding proteins.

Tropomyosins as discriminators of myosin function

Vertebrate nonmuscle cells express multiple tropomyosin isoforms that are sorted to subcellular compartments that have distinct morphological and dynamic properties. The creation of these compartments has a role in controlling cell morphology, cell migration and polarization of cellular components. There is increasing evidence that nonmuscle myosins are regulated by tropomyosin in these compartments via the regulation of actin attachment, ATPase kinetics, or by stabilization of cytoskeletal tracks for myosin-based transport. In this chapter, I review the literature describing the regulation of various myosins by tropomyosins and consider the mechanisms for this regulation.

Human tropomyosin isoforms in the regulation of cytoskeleton functions

Over the past two decades, extensive molecular studies have identified multiple tropomyosin isoforms existing in all mammalian cells and tissues. In humans, tropomyosins are encoded by TPM1 (alpha-Tm, 15q22.1), TPM2 (beta-Tm, 9p13.2-p13.1), TPM3 (gamma-Tm, 1q21.2) and TPM4 (delta-Tm, 19p13.1) genes. Through the use of different promoters, alternatively spliced exons and different sites of poly(A) addition signals, at least 22 different tropomyosin cDNAs with full-length open reading frame have been cloned. Compelling evidence suggests that these isoforms play important determinants for actin cytoskeleton functions, such as intracellular vesicle movement, cell migration, cytokinesis, cell proliferation and apoptosis. In vitro biochemical studies and in vivo localization studies suggest that different tropomyosin isoforms have differences in their actin-binding properties and their effects on other actin-binding protein functions and thus, in their specification ofactin microfilaments. In this chapter, we will review what has been learned from experimental studies on human tropomyosin isoforms about the mechanisms for differential localization and functions of tropomyosin. First, we summarize current information concerning human tropomyosin isoforms and relate this to the functions of structural homologues in rodents. We will discuss general strategies for differential localization oftropomyosin isoforms, particularly focusing on differential protein turnover and differential isoform effects on other actin binding protein functions. We will then review tropomyosin functions in regulating cell motility and in modulating the anti-angiogenic activity of cleaved high molecular weight kininogen (HKa) and discuss future directions in this area.

Tropomyosin and ADF/cofilin as collaborators and competitors

Dynamics of actin filaments is pivotal to many fundamental cellular processes such as Dcytokinesis, motility, morphology, vesicle and organelle transport, gene transcription and senescence. In vivo kinetics of actin filament dynamics is far from the equilibrium in vitro and these profound differences are attributed to large number of regulatory proteins. In particular, proteins of the ADF/cofilin family greatly increase actin filament dynamics by severing filaments and enhancing depolymerization of ADP-actin monomers from their pointed ends. Cofilin binds cooperatively to a minor conformer of F-actin in which the subunits are slightly under rotated along the filament helical axis. At high stoichiometry of cofilin to actin subunits, cofilin actually stabilizes actin filaments. Many isoforms oftropomyosin appear to compete with ADF/cofilin proteins for binding to actin filaments. Tropomyosin isoforms studied to date prefer binding to the "untwisted" conformer of F-actin and through their protection and stabilization of F-actin, recruit myosin II and assemble different actin superstructures from the cofilin-actin filaments. However, some tropomyosin isoforms may synergize with ADF/cofilin to enhance filament dynamics, suggesting that the different isoforms of tropomyosins, many of which show developmental or tissue specific expression profiles, play major roles in the assembly and turnover of actin superstructures. Different actin superstructures can overlap both spatially and temporally within a cell, but can be differentiated from each other based upon their kinetic and kinematic properties. Furthermore, local regulation of ADF/cofilin activity through signal transduction pathways could be one mechanism to alter the dynamic balance in F-actin-binding of certain tropomyosin isoforms in subcellular domains.

Divergent regulation of the sarcomere and the cytoskeleton

The existence of a feedback mechanism regulating the precise amounts of muscle structural proteins, such as actin and the actin-associated protein tropomyosin (Tm), in the sarcomeres of striated muscles is well established. However, the regulation of nonmuscle or cytoskeletal actin and Tms in nonmuscle cell structures has not been elucidated. Unlike the thin filaments of striated muscles, the actin cytoskeleton in nonmuscle cells is intrinsically dynamic. Given the differing requirements for the structural integrity of the actin thin filaments of the sarcomere compared with the requirement for dynamicity of the actin cytoskeleton in nonmuscle cells, we postulated that different regulatory mechanisms govern the expression of sarcomeric versus cytoskeletal Tms, as key regulators of the properties of the actin cytoskeleton. Comprehensive analyses of tissues from transgenic and knock-out mouse lines that overexpress the cytoskeletal Tms, Tm3 and Tm5NM1, and a comparison with sarcomeric Tms provide evidence for this. Moreover, we show that overexpression of a cytoskeletal Tm drives the amount of filamentous actin.

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.

Tropomyosin isoforms localize to distinct microfilament populations in osteoclasts

Osteoclasts resorb bone through transient rearrangement of their cytoskeletons to create a polarized phenotype in which an apical ruffled membrane is surrounded by a ring of F-actin that creates a tight seal against bone substrate. This process, coupled with the capacity for rapid motility, necessitates the presence of a dynamic, multi-functional actin cytoskeleton. Tropomyosins are a large class of actin-binding proteins that can regulate microfilament stability and organization by recruiting other regulatory proteins to actin, or alternately, by inhibiting their binding. Tropomyosins are expressed from four distinct genes (alpha, beta, gamma, and delta) that are alternately spliced to produce over forty isoforms. In recent years, it has become clear that nonmuscle isoforms of tropomyosin may be differentially distributed among intracellular pools of F-actin possessing different functions. Here we have used Western analysis and immunocytochemistry coupled with confocal microscopy to identify the isoforms of tropomyosin expressed by osteoclasts, as well as their distributions within cells. Osteoclasts express at least seven isoforms with markedly different distributions. The products of the alpha gene (Tm-2, -3, and -5a/5b) are up-regulated during osteoclastogenesis, indicating potential cell-specific functions. Some isoforms (Tm-5a/5b, Tm-4) are specifically enriched within and around osteoclast attachment structures, the sealing zone and podosomes, whereas others are more abundant in internal regions of the cell. This compartmentalization of tropomyosins to specific actin structures within osteoclasts is likely to play a critical role in determining the dynamic properties of the actin cytoskeleton and thus osteoclast activity.

Tropomyosin isoforms: divining rods for actin cytoskeleton function

Actin filament functional diversity is paralleled by variation in the composition of isoforms of tropomyosin in these filaments. Although the role of tropomyosin is well understood in skeletal muscle, where it regulates the actin-myosin interaction, its role in the cytoskeleton has been obscure. The intracellular sorting of tropomyosin isoforms indicated a role in spatial specialization of actin filament function. Genetic manipulation and protein chemistry studies have confirmed that these isoforms are functionally distinct. Tropomyosins differ in their recruitment of myosin motors and their interaction with actin filament regulators such as ADF-cofilin. Tropomyosin isoforms have therefore provided a powerful mechanism to diversify actin filament function in different intracellular compartments.

Accumulation of tropomyosin isoform 5 at the infection sites of host cells during Cryptosporidium invasion

The actin cytoskeleton of host cells has been implicated in Cryptosporidium invasion. However, the underlying mechanism of how actin filaments and associated proteins modulate this process remains unclear. In this study, we use in vitro cultured cell lines, human ileocecal adenocarcinoma HCT-8 and Chinese hamster ovary (CHO), and an in vivo mouse model to investigate the roles of tropomyosin isoforms in Cryptosporidium invasion. Using isoform-specific monoclonal antibodies, we found that the major human tropomyosin (hTM) isoforms expressed in HCT-8 cells are hTM4 and hTM5. HCT-8 cells also express hTM1 at low levels but not hTM2 and hTM3. During Cryptosporidium parvum infection, hTM5 colocalized to the infection sites with a novel parasite membrane protein, CP2. Neither hTM1 nor hTM4 accumulated at infection sites. Similarly, a high level of TM5 and varying amounts of TM4 accumulated at the C. parvum infection sites in CHO cells. CHO cells overexpressing hTM5 exhibit a significantly higher percent of mature meronts early in the infection process relative to CHO cells or CHO cells overexpressing a tropomyosin mutant, chimeric isoform hTM5/3. These results suggest that functional TM5 enhances Cryptosporidium invasion of host cells. In C. parvum-infected mice, accumulation and rearrangement of TM5 and TM4 were detected throughout the infected ileum. Similarly, in the Cryptosporidium muris-infected mice, TM5 accumulated in discrete regions of the epithelial cells of gastric glands and in the oocyst-laden stomach gland lumen. Cryptosporidium infection appears to rearrange and recruit host TM isoforms in both culture cells and in the mouse. Localized accumulation of tropomyosin at the infection sites may facilitate parasite invasion.

Differential transport and local translation of cytoskeletal, injury-response, and neurodegeneration protein mRNAs in axons

Recent studies have begun to focus on the signals that regulate axonal protein synthesis and the functional significance of localized protein synthesis. However, identification of proteins that are synthesized in mammalian axons has been mainly based on predictions. Here, we used axons purified from cultures of injury-conditioned adult dorsal root ganglion (DRG) neurons and proteomics methodology to identify axonally synthesized proteins. Reverse transcription (RT)-PCR from axonal preparations was used to confirm that the mRNA for each identified protein extended into the DRG axons. Proteins and the encoding mRNAs for the cytoskeletal proteins beta-actin, peripherin, vimentin, gamma-tropomyosin 3, and cofilin 1 were present in the axonal preparations. In addition to the cytoskeletal elements, several heat shock proteins (HSP27, HSP60, HSP70, grp75, alphaB crystallin), resident endoplasmic reticulum (ER) proteins (calreticulin, grp78/BiP, ERp29), proteins associated with neurodegenerative diseases (ubiquitin C-terminal hydrolase L1, rat ortholog of human DJ-1/Park7, gamma-synuclein, superoxide dismutase 1), anti-oxidant proteins (peroxiredoxins 1 and 6), and metabolic proteins (e.g., phosphoglycerate kinase 1 (PGK 1), alpha enolase, aldolase C/Zebrin II) were included among the axonally synthesized proteins. Detection of the mRNAs encoding each of the axonally synthesized proteins identified by mass spectrometry in the axonal compartment indicates that the DRG axons have the potential to synthesize a complex population of proteins. Local treatment of the DRG axons with NGF or BDNF increased levels of cytoskeletal mRNAs into the axonal compartment by twofold to fivefold but had no effect on levels of the other axonal mRNAs studied. Neurotrophins selectively increased transport of beta-actin, peripherin, and vimentin mRNAs from the cell body into the axons rather than changing transcription or mRNA survival in the axonal compartment.

Specific features of neuronal size and shape are regulated by tropomyosin isoforms

Spatially distinct populations of microfilaments, characterized by different tropomyosin (Tm) isoforms, are present within a neuron. To investigate the impact of altered tropomyosin isoform expression on neuronal morphogenesis, embryonic cortical neurons from transgenic mice expressing the isoforms Tm3 and Tm5NM1, under the control of the beta-actin promoter, were cultured in vitro. Exogenously expressed Tm isoforms sorted to different subcellular compartments with Tm5NM1 enriched in filopodia and growth cones, whereas the Tm3 was more broadly localized. The Tm5NM1 neurons displayed significantly enlarged growth cones accompanied by an increase in the number of dendrites and axonal branching. In contrast, Tm3 neurons displayed inhibition of neurite outgrowth. Recruitment of Tm5a and myosin IIB was observed in the peripheral region of a significant number of Tm5NM1 growth cones. We propose that enrichment of myosin IIB increases filament stability, leading to the enlarged growth cones. Our observations support a role for different tropomyosin isoforms in regulating interactions with myosin and thereby regulating morphology in specific intracellular compartments.

Sorting of a nonmuscle tropomyosin to a novel cytoskeletal compartment in skeletal muscle results in muscular dystrophy

Tropomyosin (Tm) is a key component of the actin cytoskeleton and >40 isoforms have been described in mammals. In addition to the isoforms in the sarcomere, we now report the existence of two nonsarcomeric (NS) isoforms in skeletal muscle. These isoforms are excluded from the thin filament of the sarcomere and are localized to a novel Z-line adjacent structure. Immunostained cross sections indicate that one Tm defines a Z-line adjacent structure common to all myofibers, whereas the second Tm defines a spatially distinct structure unique to muscles that undergo chronic or repetitive contractions. When a Tm (Tm3) that is normally absent from muscle was expressed in mice it became associated with the Z-line adjacent structure. These mice display a muscular dystrophy and ragged-red fiber phenotype, suggestive of disruption of the membrane-associated cytoskeletal network. Our findings raise the possibility that mutations in these tropomyosin and these structures may underpin these types of myopathies.

Modification of the tropomyosin isoform composition of actin filaments in the brain by deletion of an alternatively spliced exon

Tropomyosin (Tm) in non-muscle cells is involved in stabilisation of the actin cytoskeleton. Some of the 40 isoforms described are found in the brain and exhibit spatial and developmental regulation. Non-muscle isoforms from the gamma Tm gene can be subdivided into three subsets of isoforms differing at the C-terminus, all of which are found throughout the brain and some of which are implicated in different aspects of neuronal function. We have approached the role of different gamma isoforms in neuronal function by knocking out a subset of isoforms. We show here that we can successfully knock out all isoforms containing the brain-specific 9c C-terminus. Brains from these mice did not show any gross abnormalities. Western analysis of adult brains showed that 9c isoforms are reduced in +/- and absent in -/- mice but that a compensation by 9a-containing isoforms resulted in total levels of gamma products remaining the same. No other Tm isoforms were altered. We have therefore specifically altered the Tm composition in these neurons which allows us to study the effects of these changes on the cytoskeleton of neurons during growth, differentiation and maturation and give us insights into the normal roles of these isoforms.