Pair Creation with Strong Laser Fields, Compton Scale X Rays, and Heavy Nuclei.

Electron-positron pair creation is considered when intense laser pulses collide head-on with <1 MeV x-ray photons in the presence of stationary Coulomb charges Z(-e). The analysis employs Coulomb-corrected Volkov states and is not limited to Born's approximation in Z. The cross section and the yield increase dramatically with increasing Z, potentially enabling (i) measurable yields with petawatt lasers and (ii) sensitive tests of strong-field QED.

Lensing properties of rotational gas flow: erratum.

This erratum includes additional references relevant to rotational gas flow negative lenses that were omitted in Appl. Opt.57, 9392 (2018)APOPAI0003-693510.1364/AO.57.009392.

Lensing properties of rotational gas flow.

A negative lens comprising a gas in steady axisymmetric flow is demonstrated experimentally and analyzed. The lens has potential applications in high-intensity laser optics and presents the possibility of adjusting the focusing properties on a submillisecond time scale. It can be operated in environments where conventional optical elements are vulnerable.

First Benchmark of Relativistic Photoionization Theories against 3D ab initio Simulation.

Photoelectron spectra and ionization rates encompassing relativistic intensities and hydrogenlike ions with relativistic binding energies are obtained using a quasiclassical S-matrix approach. These results, along with those based on the imaginary time method, are compared with three-dimensional, ½-period ab initio simulations for a wide range of ionization potentials and electric field amplitudes. Significant differences between the three results are demonstrated. Time-dependent simulations indicate that the peak ionization current can occur before the peak of the electric field.

Laser-Accelerated Ions from a Shock-Compressed Gas Foil.

We present results of energetic laser-ion acceleration from a tailored, near solid density gas target. Colliding hydrodynamic shocks compress a pure hydrogen gas jet into a 70 µm thick target prior to the arrival of the ultraintense laser pulse. A density scan reveals the transition from a regime characterized by a wide angle, low-energy beam (target normal sheath acceleration) to one of a more focused beam with a high-energy halo (magnetic vortex acceleration). In the latter case, three-dimensional simulations show the formation of a Z pinch driven by the axial current resulting from laser wakefield accelerated electrons. Ions at the rear of the target are then accelerated by a combination of space charge fields from accelerated electrons and Coulombic repulsion as the pinch dissipates.

Simulation of free-space optical guiding structure based on colliding gas flows: erratum.

A typographical error in Kaganovich et al. [Appl. Opt.54, F144 (2015)10.1364/AO.54.00F144APOPAI0003-6935] is corrected here.

Stimulated Raman scattering and nonlinear focusing of high-power laser beams propagating in water.

The physical processes associated with propagation of a high-power (power > critical power for self-focusing) laser beam in water include nonlinear focusing, stimulated Raman scattering (SRS), optical breakdown, and plasma formation. The interplay between nonlinear focusing and SRS is analyzed for cases where a significant portion of the pump power is channeled into the Stokes wave. Propagation simulations and an analytical model demonstrate that the Stokes wave can re-focus the pump wave after the power in the latter falls below the critical power. It is shown that this novel focusing mechanism is distinct from cross-phase focusing. The phenomenon of gain-focusing discussed here for propagation in water is expected to be of general occurrence applicable to any medium supporting nonlinear focusing and stimulated Raman scattering.

Simulation of free-space optical guiding structure based on colliding gas flows.

Preformed plasma channels with parabolic radial density profiles enable the extended and stable optical guiding of high-intensity laser pulses. High-voltage discharge capillaries, commonly used for channel formation, have limited guiding length and opaque walls, complicating the diagnosis of the plasma within. This paper proposes a free-space gas channel produced by the collision of several gas flows. The collision of the gas flows forms an on-axis density depression surrounded by higher density walls. By offsetting the flows, we demonstrated the creation of what we believe is a novel vortex structure that exhibits a long-lived parabolic density profile. Once ionized, the resulting plasma density profile has a near-parabolic dependence appropriate for guiding. We then performed detailed two-dimensional (2D) fluid dynamics simulations to examine the properties and stability of the guiding structure.

Origin and control of the subpicosecond pedestal in femtosecond laser systems.

The picosecond time scale pedestal of a multiterawatt femtosecond laser pulse is investigated experimentally and analytically. The origin of the pedestal is related to the finite bandwidth of the laser system. By deliberately introducing a modulated spectrum with minima that match this limited bandwidth, the pedestal can be reduced, with no deleterious effect on the main pulse. Using this technique, we experimentally demonstrate a subpicosecond scale order of magnitude enhancement of contrast ratio while preserving the energy in the main pulse.

Laser heating of uncoated optics in a convective medium.

Powerful, long-pulse lasers have a variety of applications. In many applications, optical elements are employed to direct, focus, or collimate the beam. Typically the optic is suspended in a gaseous environment (e.g., air) and can cool by convection. The variation of the optic temperature with time is obtained by combining the effects of laser heating, thermal conduction, and convective loss. Characteristics of the solutions in terms of the properties of the optic material, laser beam parameters, and the environment are discussed and compared with measurements at the Naval Research Laboratory, employing kW-class, 1 µm wavelength, continuous wave lasers and optical elements made of fused silica or BK7 glass. The calculated results are in good agreement with the measurements, given the approximations in the analysis and the expected variation in the absorption coefficients of the glasses used in the experiments.

Measurement of electro-optic shock and electron acceleration in a strongly cavitated laser wakefield accelerator.

Conically emitted second harmonic radiation was observed when a relativistically intense, ultrashort laser pulse was focused into a jet of gas. This second harmonic electro-optic shock is the result of frequency mixing within the sheath of electrons surrounding a highly cavitated plasma region created by the ponderomotive force of the laser. Strong correlation between the second harmonic characteristics and electron acceleration has been observed.

Nonlinear conversion of photon spin to photon orbital angular momentum.

A relativistically intense, ultrashort laser pulse with purely spin angular momentum produces second-harmonic radiation with equal parts of both spin and orbital angular momentum when focused into a plasma. The orbital contribution is due to an azimuthal phase variation that arises in the nonlinear current density. This phase variation is associated with the radial nonuniformity driven by ponderomotive blowout.

T47DCO cells, genetically unstable and containing estrogen receptor mutations, are a model for the progression of breast cancers to hormone resistance.

We postulate that one mechanism for the progression of breast cancers to hormone resistance involves the acquisition of mutant estrogen receptors (ER)4 by genetically unstable cell subpopulations. The T47D human breast cancer cell line may be a model for such progression, having sublines that are ER positive and estrogen responsive, ER positive and estrogen resistant, or ER negative. Also, T47D cells can be either hyperdiploid (HD) or hypertetraploid (HT) or persistently alternate between these states. T47DCO cells are a HD and ER-positive, but estrogen-resistant, subline of T47D cells that undergoes spontaneous tetraploidization. Such a stable variant, designated T47Dv, is 85% HT (Cancer Res., 49: 3943, 1989). We now show that single-cell clones derived from the mixed HD/HT T47Dv can be either HD or HT, and can be either estrogen responsive or estrogen resistant, for growth and for progesterone receptor regulation. To begin the study of ER in this model system of T47DCO and their derivatives, we have generated complementary DNA libraries from the parental HD T47DCO cells and have isolated three ER complementary DNA mutants. These include two frame-shift/termination mutants that would encode ERs truncated in the DNA-binding domain and in the hormone-binding domain and a third mutant with a large in-frame deletion spanning the hinge region and a part of the hormone-binding domain. If expressed, these mutant ERs would lack hormone-binding capacity and would be undetected by the anti-ER antibodies currently in clinical use. Genetic instability, when associated with mutant ERs in subpopulations of breast tumor cells, may provide the selective pressure leading to hormone resistance. T47DCO cells and their clonal derivatives provide a model for the systematic study of ER mutations and other mechanisms of hormone resistance in Stage IV breast cancer.

Expression of the beta-subunit gene of murine thyrotropin results in multiple messenger ribonucleic acid species which are generated by alternative exon splicing.

The current studies reveal that expression of the mouse TSH beta-subunit gene in the pituitary gland and TtT97 murine thyrotropic tumor results in multiple mRNAs which arise by alternative splicing. This was deduced by 1) sequencing the extended products of TSH beta mRNA primed with an oligonucleotide complementary to the protein coding region and 2) demonstrating primer extension of oligonucleotides complementary to the optional exon junctions. These results, when correlated with the sequence of the TSH beta gene, revealed a structure comprising five exons and four introns. Exon 1, which is common to all of the mRNAs, and optional exons 2 and 3 code only for 5'-untranslated (UT) sequences, whereas exons 4 (includes one nucleotide of 5'-UT) and 5 encode the protein coding and 3'-UT regions. Multiple splicing events generate mRNAs with 5'-UT regions of 28, 69, 75, and 116 nucleotides which include exon 1, exons 1 and 3, exons 1 and 2, and exons 1, 2, and 3, respectively. In contrast, in the hypothyroid rat pituitary messages possessing optional exons were not detected, and thus the TSH beta mRNA with a 5'-UT region of 28 nucleotides predominated. We identified an additional transcriptional start site located 40 nucleotides upstream in both the mouse and rat TSH beta gene. Optional exon splicing was not detectable in transcripts initiated from the upstream start site of the murine gene. In the hypothyroid state, transcripts initiated at the upstream site were considerably less abundant than those initiated at the downstream site. Transcription from both start sites was inhibited by thyroid hormone in both mice and rats.

The combination of Pit-1 and Pit-1T have a synergistic stimulatory effect on the thyrotropin beta-subunit promoter but not the growth hormone or prolactin promoters.

Pit-1 is a pituitary-specific transcription factor with protein expression limited to thyrotrope, somatotrope, and lactotrope cells of the anterior pituitary gland. We have recently described a thyrotrope-specific variant isoform of Pit-1, called Pit-1T, which contains an additional 14 amino acids in the activation domain generated by an alternate 3'-splicing choice. Pit-1T, in the presence of Pit-1, stimulates the thyrotropin beta-subunit (TSH beta) promoter in a thyrotrope-derived cell that lacks all Pit-1 isoform proteins. Three laboratories have identified another Pit-1 splice variant, called Pit-1 beta, which contains an additional 26 amino acids in the activation domain that is generated by a similar 3'-alternate splice choice. Pit-1 beta has been shown to stimulate the GH promoter, but not the PRL or TSH beta promoters. In this report, we evaluate the effect of the three Pit-1 isoforms (Pit-1, Pit-1T, and Pit-1 beta) on the GH, PRL, and TSH beta promoters when introduced into different cell types. The combination of Pit-1 and Pit-1T had a synergistic stimulatory effect on the TSH beta promoter, but not on the PRL or GH promoters in a thyrotrope-derived cell line that lacks all Pit-1 protein isoforms (alpha TSH cells). When added to GH3 cells, which lack only the Pit-1T isoform, Pit-1T selectively stimulated the TSH beta promoter and not the GH or PRL promoters, suggesting that the thyrotrope-specific Pit-1T exhibits a promoter-specific effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and characterization of mouse complementary DNAs encoding alpha and beta thyroid hormone receptors from thyrotrope cells: the mouse pituitary-specific beta 2 isoform differs at the amino terminus from the corresponding species from rat pituitary tumor cells.

Thyroid hormones (T3) and their receptors (TR) play a critical role in the function of the pituitary gland, particularly in thyrotropes, where they regulate expression of the alpha- and beta-subunits of TSH. Since the pituitary gland is composed of several cell types, we undertook a characterization of TR subtypes in a murine thyrotropic tumor (TtT-97), an excellent model in which to study thyroid hormone action in thyrotropes. We screened a thyrotrope cDNA library with rat TR alpha 1 and TR beta 1 cDNA probes and isolated cDNAs encoding the mouse TR alpha 1 and TR beta 1 isoforms as well as a partial clone corresponding to the non-T3 binding carboxy-terminal alpha 2 variant. The polymerase chain reaction was used to amplify additional cDNAs for the specific 5' domains of the mouse TR beta 1 and the pituitary-specific TR beta 2 amino-terminal variant. Using hybridization probes that discriminate between the alpha and beta isoforms and their variants, we demonstrated that thyrotropes contain TR alpha 1 and alpha 2 mRNAs as well as transcripts encoding Rev-erbA, which arise by transcription from the opposite strand of the TR alpha gene. In thyrotropes, the ratio of alpha 2 to TR alpha 1 mRNA levels more closely resembled the distribution in mouse brain than that in heart, where the mRNA levels of TR alpha 1 and alpha 2 are comparable. TR beta 1 and TR beta 2 mRNAs were detected in thyrotropes and were of similar size (approximately 6.4 kilobases). Despite the almost complete conservation between the rat and mouse TR beta 1 sequences at the protein level, the mouse and rat TR beta 2-specific N-terminal domains were less conserved, and the mouse protein was shorter by 39 amino acids at the N-terminus. Of the receptor species, only the mRNA encoding the TR beta 2 isoform, which was restricted to thyrotropes, was decreased by T3 treatment, although the mRNA for the alpha 2 variant was also reduced by T3 in thyrotropes and heart tissue. Levels of TR beta 1 mRNA were not changed in liver, but were increased in thyrotropic tumors and also somewhat in brain, an organ that is not responsive to T3 by classical criteria.

The thyrotrope-restricted isoform of the retinoid-X receptor-gamma1 mediates 9-cis-retinoic acid suppression of thyrotropin-beta promoter activity.

TSHbeta is a subunit of TSH that is uniquely expressed and regulated in the thyrotrope cells of the anterior pituitary gland. Thyroid hormone receptors (TR) are known to mediate T3 suppression of TSHbeta gene expression at the level of promoter activity. The role of other nuclear receptors in regulation of this gene is less clearly defined. Retinoid X receptors (RXR) are a family of nuclear transcription factors that function both as 9-cis-retinoic acid (RA) ligand-dependent receptors and heterodimeric partners with TR and other nuclear receptors. Recently, the RXR isoform, RXRgamma, has been identified in the anterior pituitary gland and found to be restricted to thyrotrope cells within the pitutiary. In this report, we have further characterized the distribution of RXRgamma1, the thyrotrope-restricted isoform of RXRgamma, in murine tissues and different cell types. We have found that RXRgamma1 mRNA and protein are expressed in the TtT-97 thyrotropic tumor, but not the thyrotrope-variant alphaTSH cells or somatotrope-derived GH3 cells. Furthermore, we have studied the effects of RXRgamma1 on TSHbeta promoter activity and hormone regulation in these pituitary-derived cell types. Both T3 and 9-cis-RA independently suppressed promoter activity in the TtT-97 thyrotropes. Interestingly, the combination of ligands suppressed promoter activity more than either alone, indicating that these hormones may act cooperatively to regulate TSHbeta gene expression in thyrotropes. The RXRgamma1 isoform was necessary for the 9-cis-RA-mediated suppression of TSHbeta promoter activity in alphaTSH and GH3 cells, both of which lack this isoform. RXRbeta, a more widely distributed isoform, did not mediate these effects. Finally, we showed that the murine TSHbeta promoter region between -200 and -149 mediated a majority of the 9-cis-RA suppression of promoter activity in thyrotropes. This region is distinct from the T3-mediated response region near the transcription start site. These data suggest that retinoids can mediate TSHbeta gene regulation in thyrotropes and the thyrotrope-restricted isoform, RXRgamma1, is required for this effect.

Msx1 is present in thyrotropic cells and binds to a consensus site on the glycoprotein hormone alpha-subunit promoter.

Our studies are aimed at identifying the transcription factors that activate the glycoprotein hormone alpha-subunit promoter. Therefore, we performed a Southwestern screening of a thyrotropic (alphaTSH) cDNA expression library, using the region of the promoter from -490 to -310 that contains sequences critical for expression in thyrotrope cells. A clone was isolated corresponding to part of the coding sequence of Msx1, which is a homeodomain-containing transcription factor that has been found to play an important role in the development of limb buds and craniofacial structures. Northern blot analysis, using the cloned Msx1 cDNA fragment as a probe, demonstrated that alpha-subunit-expressing thyrotrope cells (alphaTSH cells and TtT97 tumors) contained Msx1 RNA transcripts of 2.2 kb, while somatomammotrope (GH3) cells that do not produce the alpha-subunit had barely detectable levels. The presence of Msx1 protein was demonstrated by Western blot analysis in alphaTSH cells. We also demonstrated that transcripts encoding the closely related Msx2 factor were not detectable by Northern blot analysis in either thyrotrope or somatomammotrope-derived cells. Subfragments of the region from -490 to -310 of the alpha-subunit promoter were used in a Southwestern blot assay using bacterially produced Msx1 and demonstrated that binding was localized specifically to the region from -449 to -421. Deoxyribonuclease I protection analysis, using purified Msx1 homeodomain, demonstrated structurally induced differences in DNA digestion patterns between -436 and -413, and sequence analysis of this region revealed a direct repeat of the sequence GXAATTG, which is similar to the Msx1 consensus-binding site. When nucleotides at both sites were mutated, Msx1 binding was dramatically reduced, and the activity of an alpha-subunit promoter construct decreased by approximately 50% in transfected thyrotrope (alphaTSH) cells. These studies suggest that Msx1 may play a role in the expression of the alpha-subunit gene in thyrotrope cells.

Structural and functional characterization of the genomic locus encoding the murine beta 2 thyroid hormone receptor.

beta 1 and beta 2 are functional thyroid hormone receptors (TRs) that are generated from the same genomic locus by splicing of a different amino terminus onto a common carboxyl region containing the DNA and hormone binding domains. TR beta 1 is widely expressed whereas TR beta 2 is found primarily in the pituitary gland although low levels of expression have been described in other tissues. To gain insight into the mechanisms governing expression of this complex transcriptional unit, we cloned mouse genomic fragments containing the common carboxyl terminus as well as the unique TR beta 2 amino-terminal sequence that was located at least 25 kilobases upstream. The DNA and ligand binding exons are identical in size and location of their boundaries to those of the human TR beta 1 gene. To determine whether the region 5' of the TR beta 2 amino terminus represented the promoter region, we examined it for sites of transcriptional initiation and for its ability to function as a promoter in TR beta 2-expressing thyrotrope cells. Multiple transcriptional start sites extending over 400 base pairs (bp) were identified with those more proximal showing inhibition by T3. Transcription was not detected more than 400 bp upstream from the putative AUG codon, although initiation downstream of this AUG was demonstrated indicating alternative AUG usage. A fragment containing 500 bp of the TR beta 2 5'-region exhibited preferential promoter activity when transfected into thyrotrope cells that express endogenous TR beta 2. Deletion studies demonstrated that removal of consensus binding sites for the transcription factor Pit-1 resulted in loss of this cell specificity. We therefore conclude that the promoter region responsible for expression of the TR beta 2 isoform in pituitary thyrotropes is distinct from that described for TR beta 1 and is located many kilobases upstream from their common exons.

Identification of cis-acting promoter elements important for expression of the mouse glycoprotein hormone alpha-subunit gene in thyrotropes.

The glycoprotein hormone alpha-subunit gene is expressed in a cell-specific manner in the anterior pituitary and placenta. Previous studies have shown that the region between -178 to -111 is indispensable for placental-specific expression of the human alpha-subunit gene. Using gene transfer techniques with chimeric luciferase plasmids, this report identifies regions of the mouse alpha-subunit promoter that are important for transcriptional activation in primary thyrotropic cells. Transient expression of a series of 5' flanking DNA deletions resulted in stepwise reductions of basal promoter activity between -480 to -417 (4-fold), -254 to -177 (5-fold), and -177 to -120 (3.5-fold). DNase-I protection analysis with nuclear extracts from thyrotropic tumor cells revealed specific protein-DNA interactions within each of these functionally defined regions. These were mapped to positions -474 to -452, -447 to -419, -213 to -170, and -158 to -101 within the 5' flanking region. In contrast, in mouse fibroblast L-cells no significant difference in alpha-subunit promoter activity was found by deleting the region from -480 to -177. However, a 3-fold decrease, similar to that found in primary thyrotropes, was found by deleting the region from -177 to -120. Further, a smaller region between -138 and -122 was the only area detected by the DNase-I protection assay using L-cell nuclear extracts. Thus, several cis-acting promoter elements located up-stream of position -177 are important for expression in thyrotropes. These elements also bind nuclear factors present in thyrotropes but not in nonpituitary fibroblasts and, therefore, differ from those mediating expression of the human alpha-subunit gene in the placenta.