Design, synthesis, and biological evaluation of aminopyridine derivatives as novel tropomyosin receptor kinase inhibitors.

Tropomyosin receptor kinase (TRK) is a successful target for the treatment of various cancers caused by NTRK gene fusions. Herein, based on a rational drug design strategy, we designed and synthesized 35 aminopyrimidine derivatives that were shown to be TRKA inhibitors in the enzyme assay, among which compounds C3, C4, and C6 showed potent inhibitory activities against TRKA with IC50 values of 6.5, 5.0, and 7.0 nM, respectively. In vitro antiproliferative activity study showed that compound C3 significantly inhibited the proliferation of KM-12 cells but had weak inhibitory effect on MCF-7 cells and HUVEC cells. The preliminary druggability evaluation showed that compound C3 exhibited favorable liver microsomal and plasma stabilities and had weak or no inhibitory activity against cytochrome P450 isoforms at 10 µM. Compounds C3, C4, and C6 were also selected for ADME (absorption, distribution, metabolism, and elimination) properties prediction and molecular docking studies. Inhibition experiments showed that compound C3 was not selective for TRK subtypes. All results indicated that compound C3 was a useful candidate for the development of TRK inhibitors.

Design, synthesis and anti-tumor efficacy of novel phenyl thiazole/triazole derivatives as selective TrkA inhibitors.

Aiming to develop novel tropomyosin receptor kinase A (TrkA) inhibitors, a scaffold hopping strategy was utilized by transforming the fused indazole of Entrectinib to phenyl triazole/thiazole skeleton to obtain compounds 7a-7 h and 13a-13 h. In the light of MTT assay, phenyl triazole derivatives 7a-7 h exhibited moderate anti-proliferative activities against KM-12 cells with the IC50 values of 1.78-17.51 µM, while phenyl thiazole derivatives 13a-13 h showed the weaker efficacy. Further structure-guided optimizations by combining the phenyl triazole skeleton with 3,5­difluorophenyl and 3-carbamoyl-4-piperazinylaniline moiety led to compounds 19a-19d and 20. Eventually, 19c bearing (2-(4-methylpiperazin-1-yl)phenyl)(morpholino)methanone moiety exhibited excellent anti-proliferative activity on TrkA-positive KM-12 cells with IC50 value of 0.17 µM. Meanwhile, compound 19c showed the inhibitory potency on TrkA with IC50 value of 1.6 nM, and displayed higher selectivity on TrkA over TrkB (IC50 = 12.3 nM) and TrkC (IC50 = 18.4 nM). The dedicated wound healing and colony formation assay indicated that the optimal compound 19c could suppress migration and significantly inhibit KM-12 cell colony formation in a dose-dependent manner. In addition, 19c could weakly induce apoptosis of KM-12 cell in immunofluorescent staining analysis. Taken together, the above results suggest 19c as a novel TrkA inhibitor worthy of further profiling.

Influenza A virus activates cellular Tropomyosin receptor kinase A (TrkA) signaling to promote viral replication and lung inflammation.

Influenza A virus (IAV) infection causes acute respiratory disease with potential severe and deadly complications. Viral pathogenesis is not only due to the direct cytopathic effect of viral infections but also to the exacerbated host inflammatory responses. Influenza viral infection can activate various host signaling pathways that function to activate or inhibit viral replication. Our previous studies have shown that a receptor tyrosine kinase TrkA plays an important role in the replication of influenza viruses in vitro, but its biological roles and functional mechanisms in influenza viral infection have not been characterized. Here we show that IAV infection strongly activates TrkA in vitro and in vivo. Using a chemical-genetic approach to specifically control TrkA kinase activity through a small molecule compound 1NMPP1 in a TrkA knock-in (TrkA KI) mouse model, we show that 1NMPP1-mediated TrkA inhibition completely protected mice from a lethal IAV infection by significantly reducing viral loads and lung inflammation. Using primary lung cells isolated from the TrkA KI mice, we show that specific TrkA inhibition reduced IAV viral RNA synthesis in airway epithelial cells (AECs) but not in alveolar macrophages (AMs). Transcriptomic analysis confirmed the cell-type-specific role of TrkA in viral RNA synthesis, and identified distinct gene expression patterns under the TrkA regulation in IAV-infected AECs and AMs. Among the TrkA-activated targets are various proinflammatory cytokines and chemokines such as IL6, IL-1ß, IFNs, CCL-5, and CXCL9, supporting the role of TrkA in mediating lung inflammation. Indeed, while TrkA inhibitor 1NMPP1 administered after the peak of IAV replication had no effect on viral load, it was able to decrease lung inflammation and provided partial protection in mice. Taken together, our results have demonstrated for the first time an important biological role of TrkA signaling in IAV infection, identified its cell-type-specific contribution to viral replication, and revealed its functional mechanism in virus-induced lung inflammation. This study suggests TrkA as a novel host target for therapeutic development against influenza viral disease.

Hydrazide-hydrazones as Small Molecule Tropomyosin Receptor Kina se A (TRKA) Inhibitors: Synthesis, Anticancer Activities, In silico ADME and Molecular Docking Studies.

AIM: The aim of the study was to search for new anticancer agents as TRKA inhibitors. BACKGROUND: A series of new salicylic acid hydrazide hydrazones were synthesized and evaluated for their in vitro anticancer activities against lung (A549), ovarian (SK-OV-3), skin (SK-MEL-2), and colon (HCT15) cancer cell lines, and tropomyosin receptor kinase A (TRKA) inhibitory activities. OBJECTIVE: In this study, we focused on the synthesis and anticancer properties evaluation of salicylic acid hydrazide hydrazones as TRKA inhibitors. The in vitro anticancer activities of hydrazone analogs were measured against four cancer cell lines, and the TRKA inhibitory properties were investigated using an enzyme assay to determine their modes of action. In silico molecular docking was conducted using the crystal structure of the TRKA receptor to study the interactions and modes of binding at its active site, and ligand-based target predictions were used to identify putative secondary enzymatic targets of the synthesized compounds. Additionally, pharmacokinetic properties, toxicity effects, and drug scores of the studied molecules were also assessed. METHODS: A series of hydrazide hydrazones were prepared by means of a facile and straight-forward two-step reaction under soft reflux conditions from a methyl ester of substituted aromatic acids and hydrazine hydrate followed by the condensation with substituted aldehydes. In vitro cytotoxic properties of the synthesized compounds were screened against four human cancer cells using the SRB (sulforhodamine-B) colorimetric method. The TRKA inhibitory activity was measured by enzymatic assay. In silico ADME, drug score properties, docking studies, and ligand-based target prediction analyses were performed using Osiris Cheminformatics and AutoDock Vina, and SwissTargetPrediction bioinformatics software. RESULTS: In vitro bioassays revealed that compound 6 exhibited the most potent broad-spectrum anticancer activities with IC50 values of 0.144, <0.001, 0.019, and 0.022 µM against A549, SK-OV-3, SK-MEL-2, and HCT15 cancer cells, respectively, followed by compounds 11, 3a, and 9. In TRKA inhibitory assays, compounds 3e and 11 demonstrated the highest potency with IC50 values of 111 and 614 nM, respectively. The results of docking studies on 3e and 11 with the active site of the TRKA receptor revealed that both compounds interacted as previously reported TRKA inhibitors with high docking scores. CONCLUSION: New salicylic acid hydrazide hydrazones were synthesized, and the most active compounds exhibited significant anticancer properties against A549, SK-OV-3, SK-MEL-2, and HCT15 cancer cells, suggesting to be good candidates for in vivo studies. The results obtained in the present study would help in the design and preparation of new hydrazidehydrazone analogs as potential TRKA inhibitors for cancer treatment.

LncRNA NORAD facilitates oral squamous cell carcinoma progression by sponging miR-577 to enhance TPM4.

BACKGROUND: Long non-coding RNAs (lncRNAs) have been reported to be vital factors to affect the expression of genes and proteins. Also, it has been proved that the abnormal expression or mutation of lncRNAs stands as a signal of metastasis and proliferation of cancer. Nevertheless, the majority of lncRNAs still need to be explored in abundant cancers especially in oral squamous cell carcinoma (OSCC). METHODS: RT-qPCR assays were applied to test the expression of RNAs. Mechanism assays were performed to verify the combination among NORAD, TPM4 and miR-577. Also, functional assays were conducted to verify the function of RNAs on OSCC cells. RESULTS: LncRNA NORAD was highly expressed in OSCC tissues and cells. NORAD silencing repressed the biological behaviors of OSCC cells. MiR-577 was found in OSCC with low expression, and RIP assays illustrated that NORAD, miR-577 and TPM4 coexisted in RNA-induced silencing complexes. Rescue assays proved that the overexpression of TPM4 could recover the effect of NORAD silencing on OSCC progression. CONCLUSIONS: It was revealed that NORAD functioned as a tumor promoter to sponge miR-577 thus elevating TPM4 in OSCC, which indicated that NORAD was worthy to be studied as a target for the treatment of OSCC.

Down-regulation of miR-29c promotes the progression of oral submucous fibrosis through targeting tropomyosin-1.

BACKGROUND/PURPOSE: Various microRNAs (miRs) have been found to be associated with the development of the precancerous condition of the oral cavity, oral submucous fibrosis (OSF). The expression of miR-29c is dysregulated in oral cancer, but its role in OSF has not been investigated. The purpose of the study is to investigate the functional role of miR-29c and its target in OSF. METHODS: The expression levels of miR-29c in OSF tissues and fibrotic buccal mucosal fibroblasts (fBMFs) were assessed using next-generation sequencing and real-time Polymerase Chain Reaction (PCR) analysis. MiR-29c mimic and inhibitors were employed to examine its functional role of myofibroblast transdifferentiation. In addition, several myofibroblast phenotypes, such as collagen gel contraction and migration were tested, and a luciferase reporter assay was conducted to confirm the relationship between miR-29c and its predicted target, tropomyosin-1 (TPM1). RESULTS: We observed that miR-29c expression was downregulated in fBMFs. fBMFs transfected with miR-29c mimics exhibited reduced migration ability and collagen gel contractility, whereas inhibition of miR-29c in normal BMFs induced the myofibroblast phenotypes. Results from the luciferase reporter assay showed that TPM1 was a direct target of miR-29c and the expression of TPM1 was suppressed in the fBMFs transfected with miR-29c mimics. Besides, we confirmed that the expression of miR-29c was indeed downregulated in OSF specimens. CONCLUSION: MiR-29c seems to exert an inhibitory effect on myofibroblast activation, such as collagen gel contractility and migration ability, via suppressing TPM1. These results suggested that approaches to upregulate miR-29c may be able to ameliorate the progression of OSF.

Spotlight on the treatment of infantile fibrosarcoma in the era of neurotrophic tropomyosin receptor kinase inhibitors: International consensus and remaining controversies.

Targeted neurotrophic tropomyosin receptor kinase (TRK) inhibitors offer a highly specific therapeutic option for patients with infantile fibrosarcoma (IFS) carrying the NTRK gene translocation. International recommendations are needed to define the role of TRK inhibitors (TRKI) for infants with IFS. We analysed retrospective data for all published patients with IFS in the European Paediatric Soft tissue sarcoma Study Group and Cooperative Weichteilsarkomstudiengruppe (CWS) experience and developed a consensus strategy with the Children's Oncology Group. Therapies consisted of tumour resection and/or perioperative chemotherapy for extensive tumours. Among the 172 European patients treated, 162 were alive at the end of the follow-up. Sixty-five patients (40% of all survivors) were treated with surgery alone and 64 patients (39%) with surgery combined with chemotherapy. Radiotherapy was delivered to 3% of survivors (five patients). In addition, 28 survivors (17%) exclusively received chemotherapy. Among the 129 patients treated with surgery, 91% had conservative surgery (118 cases). Overall, nine patients died of disease, one from toxicity (6%) and 20 patients (12%) survived with major functional deficits or had mutilating surgery. Overall, conventional conservative strategies before the era of targeted therapy, even in the case of extensive tumours, demonstrate efficacy in IFS, but are associated with acute and some chronic side effects. TRKI have demonstrated very rapid responses in the vast majority of children with IFS with limited acute toxicity. With the current state of our knowledge, both conventional chemotherapy and TRKI should be regarded as options for patients with localised disease at the physician's and parent's discretion. TRKI should be considered in patients with metastatic disease, and before mutilating surgery when conventional chemotherapy fails. Outside a clinical trial, additional data are needed to resolve the lack of consensus about front-line use of conventional chemotherapy versus TRKI in patients with localised disease.

A tropomyosin-like Meretrix meretrix Linnaeus polypeptide inhibits the proliferation and metastasis of glioma cells via microtubule polymerization and FAK/Akt/MMPs signaling.

Glioblastoma (GBM) represents the most common, aggressive and deadliest primary tumors with poor prognosis as available therapeutic approaches fail to control its aberrant proliferation and high invasiveness. Thus, the therapeutic agents targeting these two characteristics will be more effective. In present study, a novel polypeptide (MM15), which was originally purified from Meretrix meretrix Linnaeus and has been proven to possess potent antitumor activity by our laboratory, was recombinant expressed and identified as a tropomyosin homologous protein. The recombinant polypeptide (re-MM15) could induce the U87 cell cycle arrest in G2/M phase and cell apoptosis by inducing tubulin polymerization. Additionally, re-MM15 displayed the significant inhibition to the migration and invasion of U87 cells through downregulating FAK/Akt/MMPs signaling. Furthermore, the in vivo analysis suggested that re-MM15 significantly blocked tumor growth in U87 xenograft model. Collectively, our results indicated that re-MM15, with anti-GBM properties in vitro and in vivo, has promising potential as a new anticancer candidate for GBM.

Binding of Zn(II) to Tropomyosin Receptor Kinase A in Complex with Its Cognate Nerve Growth Factor: Insights from Molecular Simulation and in Vitro Essays.

The binding of the human nerve growth factor (NGF) protein to tropomyosin receptor kinase A (TrkA) is associated with Alzhemeir's development. Owing to the large presence of zinc(II) ions in the synaptic compartments, the zinc ions might be bound to the complex in vivo. Here, we have identified a putative zinc binding site using a combination of computations and experiments. First, we have predicted structural features of the NGF/TrkA complex in an aqueous solution by molecular simulation. Metadynamics free energy calculations suggest that these are very similar to those in the X-ray structure. Here, the "crab" structure of the NGF shape binds tightly to two TrkA "pincers". Transient conformations of the complex include both more extended and more closed conformations. Interestingly, the latter features facial histidines (His60 and His61) among the N-terminal D1-D3 domains, each of which is a potential binding region for biometals. This suggests the presence of a four-His Zn binding site connecting the two chains. To address this issue, we investigated the binding of a D1-D3 domains' peptide mimic by stability constant and nuclear magnetic resonance measurements, complemented by density functional theory-based calculations. Taken together, these establish unambiguously a four-His coordination of the metal ion in the model systems, supporting the presence of our postulated binding site in the NGF/TrkA complex.

Cooperative effects of tropomyosin on the dynamics of the actin filament.

Tropomyosin (Tpm) plays an important role in regulating the organisation and functions of the actin cytoskeleton. Here, we describe a new approach to analyse the effects of Tpm on actin dynamics. Using F-actin proteolytically modified within the DNase-binding loop (ECP-actin), we show that Tpm binding almost completely suppresses the increased subunit exchange intrinsic for this F-actin. The effect is both concentration-dependent and cooperative, with half-maximal inhibition observed at about a 1 : 50 Tpm : actin ratio. Tpm decreases not only the number concentration of ECP-actin filaments, but also the rate of the filament subunit exchange. Our data suggest that Tpm regulates the dynamics of actin filaments by an allosteric strengthening of intermonomer contacts in the actin filament, and that this mechanism may be involved in the modulation of cytoskeletal dynamics.

Effect of malonaldehyde cross-linking on the ability of shrimp tropomyosin to elicit the release of inflammatory mediators and cytokines from activated RBL-2H3 cells.

BACKGROUND: Malonaldehyde, the primary by-product of lipid peroxidation in food, modifies the structural and functional properties of proteins by cross-linking. The aim of this study was to investigate the effect of malonaldehyde on the allergenicity of shrimp tropomyosin. RESULTS: RBL-2H3 cells, a model of type I allergic reactions, were sensitised with sera from patients allergic to shrimp, and were stimulated with native and cross-linked tropomyosin. Release of inflammatory mediators such as ß-hexosaminidase, histamine, tryptase, cysteinyl leukotriene, and prostaglandin D2 was clearly suppressed in a manner that depended on the extent of tropomyosin cross-linking. Release of interleukin-4 (IL-4) and IL-13 was similarly decreased. Notably, cells sensitised with one patient's serum released IL-4 at comparable levels in response to native and cross-linked tropomyosin. CONCLUSION: Cross-linking strongly modulates the ability of shrimp tropomyosin to induce release of inflammatory cytokines and mediators from activated RBL-2H3 cells. © 2016 Society of Chemical Industry.

Coupling of adjacent tropomyosins enhances cross-bridge-mediated cooperative activation in a markov model of the cardiac thin filament.

We developed a Markov model of cardiac thin filament activation that accounts for interactions among nearest-neighbor regulatory units (RUs) in a spatially explicit manner. Interactions were assumed to arise from structural coupling of adjacent tropomyosins (Tms), such that Tm shifting within each RU was influenced by the Tm status of its neighbors. Simulations using the model demonstrate that this coupling is sufficient to produce observed cooperativity in both steady-state and dynamic force-Ca(2+) relationships. The model was further validated by comparison with reported responses under various conditions including inhibition of myosin binding and the addition of strong-binding, non-force-producing myosin fragments. The model also reproduced the effects of 2.5 mM added P(i) on Ca(2+)-activated force and the rate of force redevelopment measured in skinned rat myocardial preparations. Model analysis suggests that Tm-Tm coupling potentiates the activating effects of strongly-bound cross-bridges and contributes to force-Ca(2+) dynamics of intact cardiac muscle. The model further predicts that activation at low Ca(2+) concentrations is cooperatively inhibited by nearest neighbors, requiring Ca(2+) binding to >25% of RUs to produce appreciable levels of force. Without excluding other putative cooperative mechanisms, these findings suggest that structural coupling of adjacent Tm molecules contributes to several properties of cardiac myofilament activation.

Tropomyosin-induced arthritis in rats.

OBJECTIVE: Immunization of rats with alpha-tropomyosin (TPM) led to arthritis, uveitis and dermatitis, typical features of Behçet's disease (BD). The present study characterizes the arthritic features of this animal model, not previously described. METHODS: Lewis rats were immunized with bovine alpha-TPM and another group of rats was treated with neutralizing anti- tumor necrosis factor-alpha (TNF-alpha) antibodies. RESULTS: Clinically more than 90% of the immunized rats developed severe acute arthritis 12 days after vaccination. Rats that were followed-up for 6 months had persistent inflammation of the leg joints. Histologic studies demonstrated predominant mononuclear infiltrations in the acute phase of arthritis; the chronic arthritic process resulted in cartilage and bone damage and abundant fibrosis which led to joint deformations. Male and female rats had a similar clinical course. Analysis of the splenocyte cytokine profile kinetics revealed a persistently high level of interferon-gamma (INF-gamma) and an increase in TNF-alpha secretion during the acute phase. Increasing levels of interleukin (IL)-10 heralded the decline in clinical arthritis. No IL-4 was detected. No arthritis was detected in the rats treated with anti-TNF-alpha antibodies. CONCLUSION: The data indicates that alpha-TPM serves as an autoantigen to induce acute and chronic destructive arthritis in rats. This model is a TNF-alpha dependent autoimmune disease, with a Th1 cytokine profile.

Myosin surface loop 4 modulates inhibition of actomyosin 1b ATPase activity by tropomyosin.

Structural studies of the class I myosin, MyoE, led to the predictions that loop 4, a surface loop near the actin-binding region that is longer in class I myosins than in other myosin subclasses, might limit binding of myosins I to actin when actin-binding proteins, like tropomyosin, are present, and might account for the exclusion of myosin I from stress fibers. To test these hypotheses, mutant molecules of the related mammalian class I myosin, Myo1b, in which loop 4 was truncated (from an amino acid sequence of RMNGLDES to NGLD) or replaced with the shorter and distinct loop 4 found in Dictyostelium myosin II (GAGEGA), were expressed in vitro and their interaction with actin and with actin-tropomyosin was tested. Saturating amounts of expressed fibroblast tropomyosin-2 resulted in a decrease in the maximum actin-activated Mg2+-ATPase activity of wild-type Myo1b but had little or no effect on the actin-activated Mg2+-ATPase activity of the two mutants. In motility assays, few actin filaments bound tightly to Myo1b-WT-coated cover slips when tropomyosin-2 was present, whereas actin filaments both bound and were translocated by Myo1b-NGLD or Myo1b-GAGEGA in both the presence and absence of tropomyosin-2. When expressed in mammalian cells, like the wild type, the mutant myosins were largely excluded from tropomyosin-containing actin filaments, indicating that in the cell additional factors besides loop 4 determine targeting of myosins I to specific subpopulations of actin filaments.

A proteomic approach to the identification of early molecular targets changed by L-ascorbic acid in NB4 human leukemia cells.

The pro-oxidant effect of L-ascorbic acid (LAA) is toxic to leukemia cells. LAA induces the oxidation of glutathione to its oxidized form (GSSG) and this is followed by a concentration-dependent H(2)O(2) accumulation, which occurs in parallel to the induction of apoptosis. To identify early protein targets of LAA in leukemia cells, we used a differential proteomics approach in NB4 human leukemia cells treated with 0.5 mM of LAA for 30 min. This exposure was determined to efficiently block cellular proliferation and to activate oxidative stress-inducible apoptosis. We identified nine proteins that sensitively reacted to LAA treatment by using two-dimensional (2-D) gel electrophoresis and matrix-assisted laser desorption ionization time-of-flight-MS. A subunit of protein-disulfide isomerase (a thiol/disulfide exchange catalyst) and immunoglobulin-heavy-chain binding protein (BiP, identical to Hsp70 chaperone) showed quantitative expression profile differences. A myeloid leukemia associated antigen protein (a tropomyosin isoform) showed changes in pI as a result of phosphorylation. Our studies demonstrate for the first time that the addition of LAA to cells results in an immediate change in the intracellular thiol/disulfide condition and that this includes an increase in the GSH oxidation with changes in the superfamily of thiol/disulfide exchange catalysts. These results suggest that LAA oxidizes intracellular reduced glutathione and modulates disulfide bond formation in proteins.

Conformational dynamics of loop 262-274 in G- and F-actin.

According to the original Holmes model of F-actin structure, the hydrophobic loop 262-274 stabilizes the actin filament by inserting into a pocket formed at the interface between two protomers on the opposing strand. Using a yeast actin triple mutant, L180C/L269C/C374A [(LC)(2)CA], we showed previously that locking the hydrophobic loop to the G-actin surface by a disulfide bridge prevents filament formation. We report here that the hydrophobic loop is mobile in F- as well as in G-actin, fluctuating between the extended and parked conformations. Copper-catalyzed, brief air oxidation of (LC)(2)CA F-actin on electron microscopy grids resulted in the severing of thin filaments and their conversion to amorphous aggregates. Disulfide, bis(methanethiosulfonate) (MTS), and dibromobimane (DBB) cross-linking reactions proceeded in solution at a faster rate with G- than with F-actin. Cross-linking of C180 to C269 by DBB (4.4 A) in either G- or F-actin resulted in shorter and less stable filaments. The cross-linking with a longer MTS-6 reagent (9.6 A) did not impair actin polymerization or filament structure. Myosin subfragment 1 (S1) and tropomyosin inhibited the disulfide cross-linking of phalloidin-stabilized F-actin. Electron paramagnetic resonance measurements with nitroxide spin-labeled actin revealed strong spin-spin coupling and a similar mean interspin distance ( approximately 10 A) in G- and in F-actin, with a broader distance distribution in G-actin. These results show loop 262-274 fluctuations in G- and F-actin and correlate loop dynamics with actin filament formation and stability.

Effects of cardiomyopathic mutations on the biochemical and biophysical properties of the human alpha-tropomyosin.

Mutations in the protein alpha-tropomyosin (Tm) can cause a disease known as familial hypertrophic cardiomyopathy. In order to understand how such mutations lead to protein dysfunction, three point mutations were introduced into cDNA encoding the human skeletal tropomyosin, and the recombinant Tms were produced at high levels in the yeast Pichia pastoris. Two mutations (A63V and K70T) were located in the N-terminal region of Tm and one (E180G) was located close to the calcium-dependent troponin T binding domain. The functional and structural properties of the mutant Tms were compared to those of the wild type protein. None of the mutations altered the head-to-tail polymerization, although slightly higher actin binding was observed in the mutant Tm K70T, as demonstrated in a cosedimentation assay. The mutations also did not change the cooperativity of the thin filament activation by increasing the concentrations of Ca2+. However, in the absence of troponin, all mutant Tms were less effective than the wild type in regulating the actomyosin subfragment 1 Mg2+ ATPase activity. Circular dichroism spectroscopy revealed no differences in the secondary structure of the Tms. However, the thermally induced unfolding, as monitored by circular dichroism or differential scanning calorimetry, demonstrated that the mutants were less stable than the wild type. These results indicate that the main effect of the mutations is related to the overall stability of Tm as a whole, and that the mutations have only minor effects on the cooperative interactions among proteins that constitute the thin filament.

Protein levels of alpha1-tubulin, protein disulfide isomerase, tropomyosins and vimentin are regulated by the tuberous sclerosis gene products.

Tuberous sclerosis (TSC) is an autosomal dominant tumour suppressor gene syndrome affecting about 1 in 6000 individuals. It is characterized by mental retardation and epilepsy. A variety of tumours characteristically occur in different organs of TSC patients. Typically, highly epileptogenic dysplastic lesions (tubers) composed of abnormal shaped neurones can be detected in the cerebral cortex. Two tumour suppressor genes have been shown to be responsible for this disease: TSC1, encoding hamartin, and TSC2, encoding tuberin. In this study we performed a proteomic approach of two-dimensional gel electrophoresis with subsequent mass spectrometrical identification of protein spots after ectopic overexpression of human TSC1 or TSC2. We found the protein levels of alpha1-tubulin, protein disulfide isomerase, tropomyosin 3 and 5 and vimentin to be regulated by the two tuberous sclerosis gene products. The here presented findings suggest that deregulation of the control of these target proteins might contribute to the development of tumours in tuberous sclerosis patients. These data provide important new insights into the molecular development of this disease especially since alpha1-tubulin, protein disulfide isomerase and certain tropomyosins have also been implicated in the regulation of neuronal differentiation.

Behavior of caldesmon upon interaction of thin filaments with myosin subfragment 1 in ghost fibers.

Caldesmon is a component of smooth muscle thin filaments which inhibits their interaction with myosin. We have used polarized fluorescence technique to study the behavior of caldesmon during the interaction of myosin subfragment 1 (S1) with thin filaments reconstituted in rabbit skeletal muscle ghost fibers by incorporation of smooth muscle tropomyosin and caldesmon labeled with acrylodan at cysteine residue located in the C-terminal region. Significant changes in acrylodan fluorescence intensity upon addition of skeletal muscle S1 reflected substantial displacement of caldesmon from thin filaments, while alterations in the calculated fluorescence parameters indicated the simultaneous rearrangement of the remaining caldesmon fraction. The orientation of caldesmon in the S1-thin filament complex relative to the fiber axis changes by approximately 7 degrees and the mobility of the fluorescent probe by about 9%. The alterations in caldesmon orientation were proportional to the strength of S1 binding and diminished respectively upon addition of ADP and ADP-V(i). The changes in orientation of acrylodan-caldesmon evoked by the interaction of S1 with thin filaments were more pronounced than that in AEDANS-F-actin which suggests that the spatial arrangement of caldesmon in the complex is governed not only by F-actin but also by S1. The results may indicate that the changes in spatial arrangement of caldesmon are adjusted to the conformation of F-actin and S1 characteristic for particular steps of the ATP hydrolysis cycle.

Effects of caldesmon, calponin, and tropomyosin on the Mg2+-ATPase activities of smooth muscle myosin.

OBJECTIVE: To test whether in the absence of actin, actin-binding proteins such as caldesmon, calponin, and tropomyosin interact with the myosin of unphosphorylation, Ca2+-dependent phosphorylation (CDP), and Ca2+-independent phosphorylation (CIP) and stimulate myosin Mg2+-ATPase activities. METHODS: Mg2+-ATPase activities were measured to evaluate the effects of caldesmon, calponin, and tropomyosin on the myosin in unphosphorylation, CDP by myosin light chain kinase (MLCK), and CIP by MLCK. RESULTS: (1) At different incubation-time, i.e., 5, 10, 20, 40, and 60 minutes, the highest Mg2+-ATPase activity was observed when myosin was in the state of CDP, the medium was CIP of myosin, and the lowest was the unphosphorylated myosin. (2) In the absence of caldesmon, calponin, and tropomyosin, the Mg2+-ATPase activities from high to low were in the following order: CDP, CIP, and unphosphorylated myosin. However, in the presence of caldesmon, calponin, and tropomyosin, the order of relative value of Mg2+-ATPase activities from high to low was unphosphorylated, CIP, and CDP of myosin respectively compared to the corresponding controls. CONCLUSIONS: The results propose that caldesmon, calponin, and tropomyosin are capable of stimulating Mg2+-ATPase activity of smooth muscle myosin in Ca2+-independent manner, since Ca2+ is not obligating for the stimulating effects of the three proteins. The common characteristic of the three proteins is that when myosin activities are low, their activations are relatively strong and this property might be involved in smooth muscle tension keeping.