Laser-induced transient grating analysis of dynamics of interaction between sensory rhodopsin II D75N and the HtrII transducer.

The interaction between sensory rhodopsin II (SRII) and its transducer HtrII was studied by the time-resolved laser-induced transient grating method using the D75N mutant of SRII, which exhibits minimal visible light absorption changes during its photocycle, but mediates normal phototaxis responses. Flash-induced transient absorption spectra of transducer-free D75N and D75N joined to 120 amino-acid residues of the N-terminal part of the SRII transducer protein HtrII (DeltaHtrII) showed only one spectrally distinct K-like intermediate in their photocycles, but the transient grating method resolved four intermediates (K(1)-K(4)) distinct in their volumes. D75N bound to HtrII exhibited one additional slower kinetic species, which persists after complete recovery of the initial state as assessed by absorption changes in the UV-visible region. The kinetics indicate a conformationally changed form of the transducer portion (designated Tr*), which persists after the photoreceptor returns to the unphotolyzed state. The largest conformational change in the DeltaHtrII portion was found to cause a DeltaHtrII-dependent increase in volume rising in 8 micros in the K(4) state and a drastic decrease in the diffusion coefficient (D) of K(4) relatively to those of the unphotolyzed state and Tr*. The magnitude of the decrease in D indicates a large structural change, presumably in the solvent-exposed HAMP domain of DeltaHtrII, where rearrangement of interacting molecules in the solvent would substantially change friction between the protein and the solvent.

Protein-protein interaction between phototaxis receptor sensory Rhodopsin I and its transducer HtrI

The molecular complex containing the seven transmembrane helix photoreceptor S&barbelow;ensory R&barbelow;hodopsin I&barbelow; (SRI) and transducer protein HtrI (H&barbelow;alobacterial Transducer for SRI&barbelow;) mediates color-sensitive phototaxis responses in the archaeon Halobacterium salinarum. Orange light causes an attractant response by a one-photon reaction and white light (orange + UV light) a repellent response by a two-photon reaction. Three aspects of SRI-HtrI structure/function and the signal transduction pathway were explored. First, the coupling of HtrI to the photoactive site of SRI was analyzed by mutagenesis and kinetic spectroscopy. Second, SRI-HtrI mutations and suppressors were selected and characterized to elucidate the color-sensing mechanism. Third, the signal relay through the transducer-bound histidine kinase was analyzed using an in vitro reconstitution system with known and newly identified taxis components. ^ Twenty-one mutations on HtrI were introduced by site-directed mutagenesis. Several replacements of charged residues perturbed the photochemical kinetics of SRI which led to the finding of a cluster of residues at the membrane/cytoplasm interface in HtrI electrostatically coupled to the photoactive site of SRI. We found by laser-flash kinetic spectroscopy that the transducer and these residues have specific effects on the light-induced proton transfer between the retinal chromophore and the protein. ^ One of the mutations showed an unusual mutant phenotype we called “inverted” signaling, in which the cell produces a repellent response to normally attractant light. Therefore, this mutant (E56Q of HtrI) had lost the color-discrimination by the SRI-HtrI complex. We used suppressor analysis to better understand the phenotype. Certain suppressors resulted in return of attractant responses to orange light but with inversion of the normally repellent response to white light to an attractant response. To explain this and other results, we formulated the Conformational Shuttling model in which the HtrI-SRI complex is poised in a metastable equilibrium of two conformations shifted in opposite directions by orange and white light. We tested this model by behavioral analysis (computerized cell tracking and motion study) of double mutants of inverting and suppressing mutations and the results confirmed the equilibrium-shift explanation. ^ We developed an in vitro system for measuring the effect of purified transducer on the histidine-kinase CheAH that controls the flagellar motor switch. The rate of kinase autophosphorylation was stimulated >2 fold in the reconstitution of the complete signal transduction system from purified components from H. salinarum. The in vitro assay also showed that the kinase activity was reduced in the absence and in the presence of high levels of linker protein CheWH. (Abstract shortened by UMI.) ^

Role of transducer proteins in photoaxis signaling of Halobacterium salinarum

In Halobacterium salinarum phototaxis is mediated by the visual pigment-like photoreceptors sensory rhodopsin I (SRI) and II (SRII). SRI is a receptor for attractant orange and repellent UV-blue light, and SRII is a receptor for repellent blue-green light, and transmit signals through the membrane-bound transducer proteins HtrI and HtrII, respectively. ^ The primary sequences of HtrI and HtrII predict 2 transmembrane helices (TM1 and TM2) followed by a hydrophilic cytoplasmic domain. HtrII shows an additional large periplasmic domain for chemotactic ligand binding. The cytoplasmic regions are homologous to the adaptation and signaling domains of eubacterial chemotaxis receptors and, like their eubacterial homologs, modulate the transfer of phosphate groups from the histidine protein kinase CheA to the response regulator CheY that in turn controls flagellar motor rotation and the cell's swimming behavior. HtrII and Htrl are dimeric proteins which were predicted to contain carboxylmethylation sites in a 4-helix bundle in their cytoplasmic regions, like eubacterial chemotaxis receptors. ^ The phototaxis transducers of H. salinarum have provided a model for studying receptor/tranducer interaction, adaptation in sensory systems, and the role of membrane molecular complexes in signal transduction. ^ Interaction between the transducer HtrI and the photoreceptor SRI was explored by creating six deletion constructs of HtrI, with progressively shorter cytoplasmic domains. This study confirmed a putative chaperone-like function of HtrI, facilitating membrane insertion or stability of the SRI protein, a phenomenon previously observed in the laboratory, and identified the smallest HtrI fragment containing interaction sites for both the chaperone-like function and SRI photocycle control. The active fragment consisted of the N-terminal 147 residues of the 536-residue HtrI protein, a portion of the molecule predicted to contain the two transmembrane helices and the first ∼20% of the cytoplasmic portion of the protein. ^ Phototaxis and chemotaxis sensory systems adapt to stimuli, thereby signaling only in response to changes in environmental conditions. Observations made in our and in other laboratories and homologies between the halobacterial transducers with the chemoreceptors of enteric bacteria anticipated a role for methylation in adaptation to chemo- and photostimuli. By site directed mutagenesis we identified the methylation sites to be the glutamate pairs E265–E266 in HtrI and E513–E514 in HtrII. Cells containing the unmethylatable transducers are still able to perform phototaxis and adapt to light stimuli. By pulse-chase analysis we found that methanol production from carboxylmethyl group hydrolysis occurs upon specific photo stimulation of unmethylatable HtrI and HtrII and is due to turnover of methyl groups on other transducers. We demonstrated that the turnover in wild-type H. salinarum cells that follows a positive stimulus is CheY-dependent. The CheY-feedback pathway does not require the stimulated transducer to be methylatable and operates globally on other transducers present in the cell. ^ Assembly of signaling molecules into architecturally defined complexes is considered essential in transmission of the signals. The spectroscopic characteristics of SRI were exploited to study the stoichiometric composition in the phototaxis complex SRI-HtrI. A molar ratio of 2.1 HtrI: 1 SRI was obtained, suggesting that only 1 SRI binding site is occupied on the HtrI homodimer. We used gold-immunoelectron microscopy and light fluorescence microscopy to investigate the structural organization and the distribution of other halobacterial transducers. We detected clusters of transducers, usually near the cell's poles, providing a ultrastructural basis for the global effects and intertransducer communication we observe. ^

Vessel orientation-dependent sensitivity of optoacoustic imaging using a linear array transducer

For clinical optoacoustic imaging, linear probes are preferably used because they allow versatile imaging of the human body with real-time display and free-hand probe guidance. The two-dimensional (2-D) optoacoustic image obtained with this type of probe is generally interpreted as a 2-D cross-section of the tissue just as is common in echo ultrasound. We demonstrate in three-dimensional simulations, phantom experiments, and in vivo mouse experiments that for vascular imaging this interpretation is often inaccurate. The cylindrical blood vessels emit anisotropic acoustic transients, which can be sensitively detected only if the direction of acoustic radiation coincides with the probe aperture. Our results reveal for this reason that the signal amplitude of different blood vessels may differ even if the vessels have the same diameter and initial pressure distribution but different orientation relative to the imaging plane. This has important implications for the image interpretation, for the probe guidance technique, and especially in cases when a quantitative reconstruction of the optical tissue properties is required.

Helicobacter pylori Inhibits Dendritic Cell Maturation via Interleukin-10-Mediated Activation of the Signal Transducer and Activator of Transcription 3 Pathway.

Helicobacter pylori infects the human gastric mucosa causing a chronic infection that is the primary risk factor for gastric cancer development. Recent studies demonstrate that H. pylori promotes tolerogenic dendritic cell (DC) development indicating that this bacterium evades the host immune response. However, the signaling pathways involved in modulating DC activation during infection remain unclear. Here, we report that H. pylori infection activated the signal transducer and activator of transcription 3 (STAT3) pathway in murine bone marrow-derived DCs (BMDCs) and splenic DCs isolated ex vivo. Isogenic cagA-, cagE-, vacA- and urease-mutants exhibited levels of phosphoSTAT3 that were comparable to in the wild-type (WT) parent strain. H. pylori-infected BMDCs produced increased immunosuppressive IL-10, which activated STAT3 in an autocrine/paracrine fashion. Neutralization of IL-10 prevented H. pylori-mediated STAT3 activation in both BMDCs and splenic DCs. In addition, anti-IL-10 treatment of infected H. pylori-BMDCs was associated with increased CD86 and MHC II expression and enhanced proinflammatory IL-1β cytokine secretion. Finally, increased CD86 and MHC II expression was detected in H. pylori-infected STAT3 knockout DCs when compared to WT controls. Together, these results demonstrate that H. pylori infection induces IL-10 secretion in DCs, which activates STAT3, thereby modulating DC maturation and reducing IL-1β secretion. These findings identify a host molecular mechanism by which H. pylori can manipulate the innate immune response to potentially favor chronic infection and promote carcinogenesis. © 2014 S. Karger AG, Basel.

Molecular mechanisms of signal transducer and activator of transcription (STAT) protein function in hematopoiesis

Hematopoietic growth factors play important roles in regulating blood cell growth and development in vivo. In this work, we investigated the signaling mechanisms of two growth factors with clinical significance, erythropoietin (Epo) and granulocyte colony-stimulating factor (G-CSF). Epo is essential for the survival, proliferation and differentiation of red blood cell progenitors, while G-CSF plays an important role in controlling mature neutrophil production. To identify which amino acid(s) and/or motif in EpoR is responsible for cell survival, wild type or mutant EpoR isoforms were transfected into the growth factor-dependent 32D cell line. Proliferation and apoptosis assays demonstrated that an EpoR isoform that lacks intracellular tyrosine residues and is truncated after 321 amino acids in the cytoplasmic tail (EpoR 1-321) mediates Epo-dependent cell survival. Furthermore, in absence of fetal calf serum (FCS), Epo signaling through wild type or mutant receptors supported anti-apoptosis, but not proliferation during 72 hours in response to Epo. To investigate the signaling pathway by which EpoR regulates cell survival, a dominant negative Stat5b (dnStat5b) isoform was generated and coexpressed with EpoR in stable cell lines. Expression of dnStat5b causes a significant induction of apoptosis in the presence of Epo in cells expressing EpoR 1-321, indicating that Stat5 is essential for survival signaling through tyrosine independent sequences in the EpoR. In a second project to investigate G-CSF signaling, we studied mechanisms by which G-CSF regulates the expression of PU.1, an important transcription factor in myeloid and B cell development. We demonstrated, by immunoblot and real time RT-PCR, that PU.1 is induced by G-CSF ex vivo as well as in vivo. To test whether G-CSF signaling through Stat3 is required for PU.1 regulation, the upstream region of the PU.1 gene was analyzed for potential Stat3 binding motifs. Four potential sites were identified; chromatin immunoprecipitations demonstrated that G-CSF activated Stat3 binds to 3 of the 4 binding motifs. In addition, PU.1 induction by G-CSF was completely abrogated in bone marrow from hematopoietic conditional Stat3 knockout mice. These results indicate an important role for Stat3 in G-CSF-dependent PU.1 gene regulation. Collectively, our works demonstrate that Stat protein play important and diverse roles in hematopoietic growth factor signaling. ^

Significance of signal transducer and activator of transcription 3 during multistage mouse skin carcinogenesis

Signal transducer and activator of transcription 3 (Stat3) is a signaling molecule that transduces signal from cell surface receptors, itself translocates into the nucleus, binds to consensus promoter sequences and activates gene transcription. Here, we showed that Stat3 is constitutively activated in both premalignant tumors (papillomas) and squamous cell carcinomas of mouse skin that is induced by topical treatment with an initiator 7,12-dimethylbenz[a]anthracene (DMBA) followed by a tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Additional data demonstrated that epidermal growth factor signaling contributes to the activation of Stat3 in this model. Using mice where Stat3 function is abrogated in keratinocytes via the Cre-LoxP system (K5Cre.Stat3 flox/flox), we demonstrated that Stat3 is required for de novo carcinogenesis since Stat3 deficiency leads to a complete abrogation of skin tumor development induced by DMBA and TPA. We subsequently showed that Stat3 plays a role in both the initiation and promotion stages of carcinogenesis. During initiation, Stat3 functions as an anti-apoptotic molecule for maintaining the survival of DNA-damaged keratinocyte stem cells. During promotion, Stat3 functions as a critical regulator for G1 to S phase cell cycle progression to confer selective clonal expansion of initiated cells into papillomas. On the other hand, using transgenic mice over-expressing a constitutively dimerized form of Stat3 (Stat3C) in keratinocytes (K5.Stat3C), we revealed a role for Stat3 in tumor progression. After treatment with DMBA and TPA, K5.Stat3C transgenic mice developed skin tumors with a shorter latency when 100% bypassed the premalignant stage and became carcinoma in situ. Histological and immunohistochemical analysis revealed these tumors as highly vascularized and poorly differentiated. More strikingly, these tumors exhibited invasion into surrounding mesenchymal tissue, some of which metastasized into lung. The tumor-mesenchymal front was characterized by partial loss of E-cadherin and elevation of vimentin, markers characterizing epithelial-mesenchymal transition. On the other hand, inhibition of Stat3 via a decoy oligonucleotide led to a significant reduction of tumor size in approximately 50% of all papillomas tested. In conclusion, we demonstrated that Stat3 plays a critical in all three stages (initiation, promotion and progression) of skin carcinogenesis, and it may potentially become a good target for cancer prevention and anti-cancer therapy. ^

Signal relay between two integral membrane proteins: The sensory rhodopsin I/HtrI transducer molecular complex

The molecular complex of sensory rhodopsin I (SRI) and its transducer HtrI mediate color-sensitive phototaxis in the archaeon Halobacterium salinarum. Orange light causes an attractant response by a one-photon reaction and white light causes a repellent response by a two-photon reaction. Three aspects of this molecular complex were explored: (i) We determined the stoichiometry of SRI and HtrI to be 2:2 by gene fusion analysis. A SRI-HtrI fusion protein was expressed in H. salinarum and shown to mediate 1-photon and 2-photon phototaxis responses comparable to wild-type complex. Disulfide crosslinking demonstrated that the fusion protein is a homodimer in the membrane. Measurement of photochemical reaction kinetics and pH titration of absorption spectra established that both SRI domains are complexed to HtrI in the fusion protein, and therefore the stoichiometry is 2:2. (ii) Cytoplasmic channel closure of SRI by HtrI, an important aspect of their interaction, was investigated by incremental HtrI truncation. We found that binding of the membrane-embedded portion of HtrI is insufficient for channel closure, whereas cytoplasmic extension of the second HtrI transmembrane helix by 13 residues blocks proton conduction through the channel as well as full-length HtrI. The closure activity is localized to 5 specific residues, each of which incrementally contributes to reduction of proton conductivity. Moreover, these same residues in the dark incrementally and proportionally increase the pKa of the Asp76 counterion to the protonated Schiff base chromophore. We conclude that this critical region of HtrI alters the dark conformation of SRI as well as light-induced channel opening. (iii) We developed a procedure for reconstituting HtrI-free SRI and the SRI/HtrI complex into liposomes, which exhibit photocycles with opened and closed cytoplasmic channels, respectively, as in the membrane. This opens the way for study of the light-induced conformational change and the interaction in vitro by fluorescence and spin-labeling. Single-cysteine mutations were introduced into helix F of SRI, labeled with a nitroxide spin probe and a fluorescence probe, reconstituted into proteoliposomes, and light-induced conformational changes detected in the complex. The probe signals can now be used as the readout of signaling to analyze mutants and the kinetics of signal relay. ^

Molecular interactions and signal transfer between sensory rhodopsin-I and its cognate transducer

SRI is unique among known photoreceptors in that it produces opposite signals depending on the color of light stimuli. Absorption of orange light (587 nm) triggers an attractant response by the cell, whereas absorption of orange light followed by near-UV light (373 run) triggers a repellent response. Using behavioral mutants that exhibit aberrant color-sensing ability, we tested a two-conformation equilibrium model, using FRET and EPR spectroscopy. The essence of the model applied to SRI-HtrI is that the complex exists in a metastable two-conformer equilibrium which is shifted in one direction by orange light absorption (producing an attractant signal) and in the opposite direction by a second UV-violet photon (producing a repellent signal). First, by FRET we found that the E-F cytoplasmic loop of SRI moves toward the RAMP domain of the HtrI transducer during the formation of the orange-light activated signaling state of the complex. This is the first localization of a change in the physical relationship between the receptor and transducer subunits of the complex and provides a structural property of the two proposed conformers that we can monitor. Second, EPR spectra of a spin label probe at this cytoplasmic position showed shifts in the dark in the mutants toward shorter or longer EF loop-RAMP distances, explaining their behavior in terms of their mutations causing pre-stimulus shifts into one or the other conformer. ^ Next, we applied a novel electrophysiological method for monitoring the directionality of proton movement during photoactivation of SRI, to investigate the process of proton transfer in the photoactive site from the chromophore to proton acceptors on both the wildtype and aberrant color-response mutants. We observed an unexpected and critical difference in the two signaling conformations of the SRI-HtrI complex. The finding is that the vectoriality (i.e. movement away or toward the cytoplasm) of the light-induced proton transfer from the chromophore to the protein is opposite in formation of the two conformations. Retinylidene proton transfer is a common critical process in rhodopsins and these results are the first to show differences in vectoriality in a rhodopsin receptor, and to demonstrate functional importance of the direction of proton transfer. ^

A Novel Mechanism of Skin Tumor Promotion Involving Interferon-gamma (IFNγ)/Signal Transducer and Activator of Transcription-1 (Stat1) Signaling in Epidermis

The JAK-STAT pathway is a major signaling pathway involved in many biological processes including proliferation, apoptosis, and differentiation. Aberrant expression of STATs has been reported in multiple human cancers and murine mouse models of tumorigenesis. Previous studies from our lab and others have established a critical role for Stat3 in epithelial tumorigenesis, but the role of Stat1 is largely unknown. The current study was designed to explore the role of Stat1 during multistage skin carcinogenesis. Topical treatment with both TPA and the anthrone derivative chrysarobin (CHRY) led to rapid phosphorylation of Stat1 on both tyrosine (Tyr701) and serine (Ser727) residues in epidermis. CHRY treatment also led to upregulation of unphosphorylated Stat1 (uStat1) at later time points. In addition, CHRY treatment also led to upregulation of IRF-1 mRNA and protein which was dependent on Stat1. Further analyses demonstrated that topical treatment with CHRY but not TPA upregulated interferon-gamma (IFNg) mRNA in the epidermis and that the induction of both IRF-1 and uStat1 was dependent on IFNg signaling. Stat1 deficient (Stat1-/-) mice were highly resistant to skin tumor promotion by CHRY. In contrast, the tumor response (in terms of both papillomas and squamous cell carcinomas) was similar in Stat1-/- mice and wild-type littermates with TPA as the promoter. Histological evaluation of the proliferative response confirmed the data obtained from the tumor study for both TPA and CHRY. In addition, maximal induction of both cyclooxygenase-2 and inducible nitric oxide synthase in epidermis following treatment with CHRY was also dependent on the presence of functional Stat1. Following CHRY treatment, Stat1-/- mice exhibited reduced macrophage infiltration and reduced production of many immune cell derived chemokines/cytokines. These studies define a novel mechanism associated with skin tumor promotion by the anthrone class of tumor promoters involving upregulation of IFNg signaling in the epidermis and downstream signaling through activated (phosphorylated) Stat1 and subsequent upregulation of IRF-1 and uStat1.

Role of janus tyrosine kinase -3 (JAK3) and signal transducer and activator of transcription (STAT5) pathway in T lymphocyte activation

T cell activation and expansion is essential for immune response against foreign antigens. However, uncontrolled T cell activity can be manifested as a number of lymphoid derived diseases such as autoimmunity, graft versus host disease, and lymphoma. The purpose of this research was to test the central hypothesis that the Jak3/Stat5 pathway is critical for T cell function. To accomplish this objective, two novel Jak3 inhibitors, AG490 and PNU156804, were identified and their effects characterized on Jak3/Stat5 activation and T cell growth. Inhibition of Jak3 selectively disrupted primary human T lymphocyte growth in response to Interleukin-2 (IL-2), as well as other γ c cytokine family members including IL-4, IL-7, IL-9, and IL-15. Inhibition of Jak3 ablated IL-2 induced Stat5 but not TNF-α mediated NF-κβ DNA binding. Loss of Jak3 activity did not affect T cell receptor mediated signals including activation of p56Lck and Zap70, or IL-2 receptor a chain expression. To examine the effects of Jak3/Stat5 inhibition within a mature immune system, we employed a rat heart allograft model of Lewis (RT1 1) to ACI (RT1a). Heart allograft survival was significantly prolonged following Jak3/Stat5 inhibition when rats were treated with AG490 (20mg/kg) or PNU156804 (80mg/kg) compared to non-treated control animals. This effect was synergistically potentiated when Jak3 inhibitors were used in combination with a signal 1/2 disrupter, cyclosporine, but only additively potentiated with another signal 3 inhibitor, rapamycin. This suggested that sequential inhibition of T cell function is more effective. To specifically address the role of Stat5 in maintaining T cell activity, novel Stat5 antisense oligonucleotides were synthesized and characterized in vitro. Primary human T cells and T-cell tumor lines treated with Stat5 antisense oligonucleotide (7.5 μM) rapidly underwent apoptosis, while no changes in cell cycle were observed as measured by FACS analysis utilizing Annexin-V-Fluorescein and Propidium iodide staining. Evidence is provided to suggest that caspase 8 and 9 pathways mediate this event. Thus, Stat5 may act rather as a negative regulator of apoptotic signals and not as a positive regulator of cell cycle as previously proposed. We conclude that the Jak3/Stat5 pathway is critical for γc cytokine mediated gene expression necessary for T cell expansion and normal immune function and represents an therapeutically relevant effector pathway to combat T cell derived disease. ^

Effect of the Wake of the Transducer Supports on the Ultrasonic Anemometer Measurements

The wake produced by the structural supports of the ultrasonic anemometers (UAs)causes distortions in the velocity field in the vicinity of the sonic path. These distortions are measured by the UA, inducing errors in the determination of the mean velocity, turbulence intensity, spectrum, etc.; basic parameters to determine the effect of wind on structures. Additionally, these distortions can lead to indefinition in the calibration function of the sensors (Cuerva et al., 2004). Several wind tunnel tests have been dedicated to obtaining experimental data, from which have been developed fit models to describe and to correct these distortions (Kaimal, 1978 and Wyngaard, 1985). This work explores the effect of a vortex wake generated by the supports of an UA, on the measurement of wind speed done by this instrument. To do this, the Von Karman¿s vortex street potential model is combined with the mathematical model of the measuring process carried out by UAs developed by Franchini et al. (2007). The obtained results are the correction functions of the measured wind velocity, which depends on the geometry of the sonic anemometer and aerodynamic conditions. These results have been validated with the ones obtained in a wind tunnel test done on a single path UA, especially developed for research. The supports of this UA have been modified in order to reproduce the conditions of the theoretical model. Good agreements between experimental and theoretical results have been found.

Integrated phased array transducer for on-board structural health monitoring

Permanently bonded onto a structure, an integrated Phased Array (PhA II) transducer that can provide reliable electromechanical connection with corresponding sophisticated miniaturized ?all in one? SHM electronic device installed directly above it, without need for any interface cabling, during all aerospace structure lifecycle phases and for a huge variety of real harsh service environments of structures to be monitored is presented. This integrated PhA II transducer [1], as a key component of the PAMELA SHM? (Phased Array Monitoring for Enhanced Life Assessment) system, has two principal tasks at the same time, reliably transceive elastic waves in real aerospace service environments and serves as a reliable sole carrier or support for associated integrated on-board SHM electronic device attached above. The PhA II transducer successfully accomplished both required task throughout extensive test campaigns which included low to high temperature tests, temperature cycling, mechanical loading, combined thermo- mechanical loading and vibration resistance, etc. both with and without SHM device attached above due to RTCA DO-160F.

Simulation, construction and characterization of a piezoelectric transducer using rexolite as acoustic matching

This article describes the simulation and characterization of an ultrasonic transducer using a new material called Rexolite to be used as a matching element. This transducer was simulated using a commercial piezoelectric ceramic PIC255 at 8 MHz. Rexolite, the new material, presents an excellent acoustic matching, specially in terms of the acoustic impedance of water. Finite elements simulations were used in this work. Rexolite was considered as a suitable material in the construction of the transducer due to its malleability and acoustic properties, to validate the simulations a prototype transducer was constructed. Experimental measurements were used to determine the resonance frequency of the prototype transducer. Simulated and experimental results were very similar showing that Rexolite may be an excellent matching, particularly for medical applications.

Compact duplexing for a 680-GHz radar using a waveguide orthomode transducer

A compact 680-GHz waveguide orthomode transducer (OMT) and circular horn combination has been designed, tested, and characterized in a radar transceiver's duplexer. The duplexing capability is implemented by a hybrid waveguide quasi optical solution, combining a linear polarization OMT and an external grating polarizer. Isolation between the OMT's orthogonal ports' flanges was measured with a vector network analyzer to exceed 33 dB over a >10% bandwidth between 630 and 710 GHz. Calibrated Y-factor measurements using a mixer attached to the OMT ports reveal losses through the transmit and receive paths that sum to an average of 4.7 dB of two-way loss over 660-690 GHz. This is consistent with radar sensitivity measurements comparing the new OMT/horn with a quasi-optical wire grid beam splitter. Moreover, the radar performance assessment validates the OMT as a suitable compact substitute of the wire grid for the JPL's short-range 680-GHz imaging radar.