Observation of Laser Power Amplification in a Self-Injecting Laser Wakefield Accelerator

We report on the depletion and power amplification of the driving laser pulse in a strongly driven laser wakefield accelerator. Simultaneous measurement of the transmitted pulse energy and temporal shape indicate an increase in peak power from 187±11  TW to a maximum of 318±12  TW after 13 mm of propagation in a plasma density of 0.9×10^{18}  cm^{-3}. The power amplification is correlated with the injection and acceleration of electrons in the nonlinear wakefield. This process is modeled by including a localized redshift and subsequent group delay dispersion at the laser pulse front.

Laser-wakefield accelerators as hard x-ray sources for 3D medical imaging of human bone

A bright μm-sized source of hard synchrotron x-rays (critical energy Ecrit > 30 keV) based on the betatron oscillations of laser wakefield accelerated electrons has been developed. The potential of this source for medical imaging was demonstrated by performing micro-computed tomography of a human femoral trabecular bone sample, allowing full 3D reconstruction to a resolution below 50 μm. The use of a 1 cm long wakefield accelerator means that the length of the beamline (excluding the laser) is dominated by the x-ray imaging distances rather than the electron acceleration distances. The source possesses high peak brightness, which allows each image to be recorded with a single exposure and reduces the time required for a full tomographic scan. These properties make this an interesting laboratory source for many tomographic imaging applications.

Compact laser accelerators for X-ray phase-contrast imaging

Advances in X-ray imaging techniques have been driven by advances in novel X-ray sources. The latest fourth-generation X-ray sources can boast large photon fluxes at unprecedented brightness. However, the large size of these facilities means that these sources are not available for everyday applications. With advances in laser plasma acceleration, electron beams can now be generated at energies comparable to those used in light sources, but in university-sized laboratories. By making use of the strong transverse focusing of plasma accelerators, bright sources of betatron radiation have been produced. Here, we demonstrate phase-contrast imaging of a biological sample for the first time by radiation generated by GeV electron beams produced by a laser accelerator. The work was performed using a greater than 300 TW laser, which allowed the energy of the synchrotron source to be extended to the 10-100 keV range.

Rayleigh-Taylor instability of an ultrathin foil accelerated by the radiation pressure of an intense laser

We report experimental evidence for a Rayleigh-Taylor-like instability driven by radiation pressure of an ultraintense (10(21) W/cm(2)) laser pulse. The instability is witnessed by the highly modulated profile of the accelerated proton beam produced when the laser irradiates a 5 nm diamondlike carbon (90% C, 10% H) target. Clear anticorrelation between bubblelike modulations of the proton beam and transmitted laser profile further demonstrate the role of the radiation pressure in modulating the foil. Measurements of the modulation wavelength, and of the acceleration from Doppler-broadening of back-reflected light, agree quantitatively with particle-in-cell simulations performed for our experimental parameters and which confirm the existence of this instability.

Current filamentation instability in laser wakefield accelerators

Experiments using an electron beam produced by laser-wakefield acceleration have shown that varying the overall beam-plasma interaction length results in current filamentation at lengths that exceed the laser depletion length in the plasma. Three-dimensional simulations show this to be a combination of hosing, beam erosion, and filamentation of the decelerated beam. This work suggests the ability to perform scaled experiments of astrophysical instabilities. Additionally, understanding the processes involved with electron beam propagation is essential to the development of wakefield accelerator applications.

Complete temporal characterization of asymmetric pulse compression in a laser wakefield

We present complete experimental characterization of the temporal shape of an intense ultrashort 200-TW laser pulse driving a laser wakefield. The phase of the pulse was uniquely measured by using (second-order) frequency-resolved optical gating. The pulses are asymmetrically compressed and exhibit a positive chirp consistent with the expected asymmetric self-phase-modulation due to photon acceleration or deceleration in a relativistic plasma wave. The measured pulse duration decreases linearly with increasing length and density of the plasma, in quantitative agreement with the intensity-dependent group velocity variation in the plasma wave.

Near-GeV acceleration of electrons by a nonlinear plasma wave driven by a self-guided laser pulse

The acceleration of electrons to approximately 0.8 GeV has been observed in a self-injecting laser wakefield accelerator driven at a plasma density of 5.5x10(18) cm(-3) by a 10 J, 55 fs, 800 nm laser pulse in the blowout regime. The laser pulse is found to be self-guided for 1 cm (>10zR), by measurement of a single filament containing >30% of the initial laser energy at this distance. Three-dimensional particle in cell simulations show that the intensity within the guided filament is amplified beyond its initial focused value to a normalized vector potential of a0>6, thus driving a highly nonlinear plasma wave.

Characterization of high-intensity laser propagation in the relativistic transparent regime through measurements of energetic proton beams

Experiments were performed to investigate the propagation of a high intensity (I approximately 10(21) W cm(-2)) laser in foam targets with densities ranging from 0.9n(c) to 30n(c). Proton acceleration was used to diagnose the interaction. An improvement in proton beam energy and efficiency is observed for the lowest density foam (n(e)=0.9n(c)), compared to higher density foams. Simulations show that the laser beam penetrates deeper into the target due to its relativistic propagation and results in greater collimation of the ensuing hot electrons. This results in the rear surface accelerating electric field being larger, increasing the efficiency of the acceleration. Enhanced collimation of the ions is seen to be due to the self-generated azimuthal magnetic and electric fields at the rear of the target.

Dynamic control of laser-produced proton beams

The emission characteristics of intense laser driven protons are controlled using ultrastrong (of the order of 10(9) V/m) electrostatic fields varying on a few ps time scale. The field structures are achieved by exploiting the high potential of the target (reaching multi-MV during the laser interaction). Suitably shaped targets result in a reduction in the proton beam divergence, and hence an increase in proton flux while preserving the high beam quality. The peak focusing power and its temporal variation are shown to depend on the target characteristics, allowing for the collimation of the inherently highly divergent beam and the design of achromatic electrostatic lenses.

Laser-driven acceleration of electrons in a partially ionized plasma channel

The generation of quasimonoenergetic electron beams, with energies up to 200 MeV, by a laser-plasma accelerator driven in a hydrogen-filled capillary discharge waveguide is investigated. Injection and acceleration of electrons is found to depend sensitively on the delay between the onset of the discharge current and the arrival of the laser pulse. A comparison of spectroscopic and interferometric measurements suggests that injection is assisted by laser ionization of atoms or ions within the channel.

Observation of synchrotron radiation from electrons accelerated in a petawatt-laser-generated plasma cavity

The dynamics of plasma electrons in the focus of a petawatt laser beam are studied via measurements of their x-ray synchrotron radiation. With increasing laser intensity, a forward directed beam of x rays extending to 50 keV is observed. The measured x rays are well described in the synchrotron asymptotic limit of electrons oscillating in a plasma channel. The critical energy of the measured synchrotron spectrum is found to scale as the Maxwellian temperature of the simultaneously measured electron spectra. At low laser intensity transverse oscillations are negligible as the electrons are predominantly accelerated axially by the laser generated wakefield. At high laser intensity, electrons are directly accelerated by the laser and enter a highly radiative regime with up to 5% of their energy converted into x rays.

Bright multi-keV harmonic generation from relativistically oscillating plasma surfaces

The first evidence of x-ray harmonic radiation extending to 3.3 A, 3.8 keV (order n>3200) from petawatt class laser-solid interactions is presented, exhibiting relativistic limit efficiency scaling (eta approximately n{-2.5}-n{-3}) at multi-keV energies. This scaling holds up to a maximum order, n{RO} approximately 8{1/2}gamma;{3}, where gamma is the relativistic Lorentz factor, above which the first evidence of an intensity dependent efficiency rollover is observed. The coherent nature of the generated harmonics is demonstrated by the highly directional beamed emission, which for photon energy hnu>1 keV is found to be into a cone angle approximately 4 degrees , significantly less than that of the incident laser cone (20 degrees ).

Control of strong-laser-field coupling to electrons in solid targets with wavelength-scale spheres

Irradiation of a planar solid by an intense laser pulse leads to fast electron acceleration and hard x-ray production. We have investigated whether this high field production of fast electrons can be controlled by introducing dielectric spheres of well-defined size on the target surface. We find that the presence of spheres with a diameter slightly larger than half the laser wavelength leads to Mie enhancements of the laser field which, accompanied by multipass stochastic heating of the electrons, leads to significantly enhanced hard x-ray yield and temperature.

Polymorphisms of the human UDP-glucuronosyltransferase (UGT) 1A7 gene in colorectal cancer

BACKGROUND

Genetic polymorphisms in the human UDP-glucuronosyltransferase-1A7 (UGT1A7) gene are detected and significantly correlated with sporadic colorectal carcinoma. UGT1A7, which has recently been demonstrated to glucuronidate environmental carcinogens, is now implicated as a cancer risk gene. A silent mutation at codon 11 and missense mutations at codons 129, 131, and 208 lead to the description of three polymorphic alleles designated UGT1A7*2, UGT1A7*3, and UGT1A7*4.

METHODS

UGT1A7 polymorphisms were analysed by polymerase chain reaction amplification and sequencing, as well as temperature gradient gel electrophoresis in 210 healthy blood donors and 78 subjects with colorectal cancer.

RESULTS

Homozygous wild-type UGT1A7 alleles were present in 20% of normal controls but were only detected in 9% of patients with colorectal carcinoma (odds ratio (OR) 0.39 (95% confidence interval (CI) 0.17-0.92); p=0.03). Analysis of individual polymorphic alleles identified a highly significant association between the presence of UGT1A7*3 alleles and colorectal cancer (OR 2.75 (95% CI 1.6 - 4.71); p<0.001). Recombinant expression of UGT1A7 polymorphic cDNA in eukaryotic cell culture showed reduced carcinogen glucuronidation activity in comparison with wild-type UGT1A7. UGT1A7 may therefore represent a modifier gene in colorectal carcinogenesis.

CONCLUSION

We have identified a potential novel risk factor in sporadic colorectal cancer which may contribute to the identification of risk groups and to the elucidation of factors involved in colon carcinogenesis.

Developmental aspects of human hepatic drug glucuronidation in young children and adults

BACKGROUND AND AIMS

The liver represents one of the major sites of human glucuronidation. Many therapeutic drugs are substrates for UDP-glucuronosyltransferases (UGT) leading to the formation of usually inactive glucuronides. Hepatic glucuronidation undergoes significant changes during fetal and neonatal development requiring age adapted drug therapy. Regulation of individual UGT genes during hepatic development has not been defined.

SUBJECTS AND METHODS

Expression of 13 UGT genes and glucuronidation activities were analysed in 16 paediatric liver samples (aged 7-24 months), two fetal samples, and 12 adult liver samples (aged 25-75 years) using duplex reverse transcription-polymerase chain reaction, western blot, and specific catalytic UGT activity assays.

RESULTS

No UGT transcripts were detected in fetal liver at 20 weeks' gestation. In contrast, UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, UGT2B4, UGT2B7, UGT2B10, and UGT2B15 transcripts were present without variation in all 28 hepatic samples after six months of age. Significantly lower expression of UGT1A9 and UGT2B4 mRNA was identified in paediatric liver. Hepatic glucuronidation activity in children aged 13-24 months was found to be lower than in adults for ibuprofen (24-fold), amitriptyline (16-fold), 4-tert-butylphenol (40-fold), estrone (15-fold), and buprenorphine (12-fold).

CONCLUSIONS

An early phase characterised by the appearance of UGT gene transcripts and a later phase characterised by upregulation of UGT expression is demonstrated during human hepatic development. The differential regulation of UGT1A9 and UGT2B4 expression extends beyond two years of age and is capable of influencing hepatic glucuronidation of common therapeutic drugs in children. The development of hepatic UGT activities is significant for paediatric drug therapy and the prevention of adverse drug effects.

Genetic link of hepatocellular carcinoma with polymorphisms of the UDP-glucuronosyltransferase UGT1A7 gene

BACKGROUND & AIMS

Hepatocellular carcinoma is associated with risk factors including hepatitis C, hepatitis B, cirrhosis, genetic liver diseases, and environmental carcinogens. Uridine 5'-diphosphate-glucuronosyltransferases are a superfamily of detoxifying enzymes capable of tobacco-borne carcinogen detoxification and cellular protection. This study examines the association of UGT1A7 and UGT1A9 gene polymorphisms with hepatocellular carcinoma.

METHODS

Genomic DNA from the blood of 59 patients with hepatocellular carcinoma and 70 control subjects without evidence of cancer was analyzed by UGT1A7- and UGT1A9-specific PCR, sequencing analysis, and temperature gradient gel electrophoresis.

RESULTS

Three UGT1A7 missense mutations were detected defining the UGT1A7*2, UGT1A7*3, and UGT1A7*4 alleles. Wild-type UGT1A7 alleles were present in 41.4% of controls but only in 6.8% of cancer patients (P < 0.001; odds ratio [OR], 9.73; 95% confidence interval [CI], 3.17-29.83). UGT1A7 polymorphisms were present in 93.2% of hepatocellular cancer patients, 74.5% carried the UGT1A7*3 allele (P < 0.001; OR, 10.76; 95% CI, 4.75-24.38), which combines the W208R, N129K, and R131K mutations and encodes a protein with low carcinogen detoxification activity. No UGT1A9 polymorphisms were detected.

CONCLUSIONS

The significant association of hepatocellular carcinoma with the UGT1A7*3 allele encoding a low detoxification activity protein is identified and implicates UGT1A7 as a risk gene of hepatocarcinogenesis in addition to a role as potential marker for cancer risk assessment in chronic liver disease.

Polymorphic gene regulation and interindividual variation of UDP-glucuronosyltransferase activity in human small intestine

UDP-glucuronosyltransferases (UGTs) convert dietary constituents, drugs, and environmental mutagens to inactive hydrophilic glucuronides. Recent studies have shown that the expression of the UGT1 and UGT2 gene families is regulated in a tissue-specific fashion. Human small intestine represents a major site of resorption of dietary constituents and orally administered drugs and plays an important role in extrahepatic UGT directed metabolism. Expression of 13 UGT1A and UGT2B genes coupled with functional and catalytic analyses were studied using 18 small intestinal and 16 hepatic human tissue samples. Hepatic expression of UGT gene transcripts was without interindividual variation. In contrast, a polymorphic expression pattern of all the UGT genes was demonstrated in duodenal, jejunal, and ileal mucosa, with the exception of UGT1A10. To complement these studies, interindividual expression of UGT proteins and catalytic activities were also demonstrated. Hyodeoxycholic acid glucuronidation, catalyzed primarily by UGT2B4 and UGT2B7, showed a 7-fold interindividual variation in small intestinal duodenal samples, in contrast to limited variation in the presence of 4-methylumbelliferone, a substrate glucuronidated by most UGT1A and UGT2B gene products. Linkage of RNA expression patterns to protein abundance were also made with several mono-specific antibodies to the UGTs. These results are in contrast to a total absence of polymorphic variation in gene expression, protein abundance, and catalytic activity in liver. In addition, the small intestine exhibits considerable catalytic activity toward most of the different classes of substrates accepted for glucuronidation by the UGTs, which is supported by immunofluorescence analysis of UGT1A protein in the mucosal cell layer of the small intestine. Thus, tissue-specific and interindividual polymorphic regulation of UGT1A and UGT2B genes in small intestine is identified and implicated as molecular biological determinant contributing to interindividual prehepatic drug and xenobiotic metabolism in humans.

Two evolutionarily closely related ABC transporters mediate the uptake of choline for synthesis of the osmoprotectant glycine betaine in Bacillus subtilis

Biosynthesis of the compatible solute glycine betaine in Bacillus subtilis confers a considerable degree of osmotic tolerance and proceeds via a two-step oxidation process of choline, with glycine betaine aldehyde as the intermediate. We have exploited the sensitivity of B. subtilis strains defective in glycine betaine production against glycine betaine aldehyde to select for mutants resistant to this toxic intermediate. These strains were also defective in choline uptake, and genetic analysis proved that two mutations affecting different genetic loci (opuB and opuC) were required for these phenotypes. Molecular analysis allowed us to demonstrate that the opuB and opuC operons each encode a binding protein-dependent ABC transport system that consists of four components. The presumed binding proteins of both ABC transporters were shown to be lipoproteins. Kinetic analysis of [14C]-choline uptake via OpuB (K(m) = 1 microM; Vmax = 21 nmol min-1 mg-1 protein) and OpuC (K(m) = 38 microM; Vmax = 75 nmol min-1 mg-1 protein) revealed that each of these ABC transporters exhibits high affinity and substantial transport capacity. Western blotting experiments with a polyclonal antiserum cross-reacting with the presumed substrate-binding proteins from both the OpuB and OpuC transporter suggested that the expression of the opuB and opuC operons is regulated in response to increasing osmolality of the growth medium. Primer extension analysis confirmed the osmotic control of opuB and allowed the identification of the promoter of this operon. The opuB and opuC operons are located close to each other on the B. subtilis chromosome, and their high sequence identity strongly suggests that these systems have evolved from a duplication event of a primordial gene cluster. Despite the close relatedness of OpuB and OpuC, these systems exhibit a striking difference in substrate specificity for osmoprotectants that would not have been predicted readily for such closely related ABC transporters.

Glycine betaine aldehyde dehydrogenase from Bacillus subtilis: characterization of an enzyme required for the synthesis of the osmoprotectant glycine betaine

Production of the compatible solute glycine betaine from its precursors choline or glycine betaine aldehyde confers a considerable level of tolerance against high osmolarity stress to the soil bacterium Bacillus subtilis. The glycine betaine aldehyde dehydrogenase GbsA is an integral part of the osmoregulatory glycine betaine synthesis pathway. We strongly overproduced this enzyme in an Escherichia coli strain that expressed a plasmid-encoded gbsA gene under T7&phi;10 control. The recombinant GbsA protein was purified 23-fold to apparent homogeneity by fractionated ammonium sulfate precipitation, ion-exchange chromatography on Q-Sepharose, and subsequent hydrophobic interaction chromatography on phenyl-Sepharose. Molecular sieving through Superose 12 and sedimentation centrifugation through a glycerol gradient suggested that the native enzyme is a homodimer with 53.7-kDa subunits. The enzyme was specific for glycine betaine aldehyde and could use both NAD+ and NADP+ as cofactors, but NAD+ was strongly preferred. A kinetic analysis of the GbsA-mediated oxidation of glycine betaine aldehyde to glycine betaine revealed Km values of 125 microM and 143 microM for its substrates glycine betaine aldehyde and NAD+, respectively. Low concentrations of salts stimulated the GbsA activity, and the enzyme was highly tolerant of high ionic conditions. Even in the presence of 2.4 M KCl, 88% of the initial enzymatic activity was maintained. B. subtilis synthesizes high levels of proline when grown at high osmolarity, and the presence of this amino acid strongly stimulated the GbsA activity in vitro. The enzyme was stimulated by moderate concentrations of glycine betaine, and its activity was highly tolerant against molar concentrations of this osmolyte. The high salt tolerance and its resistance to its own reaction product are essential features of the GbsA enzyme and ensure that B. subtilis can produce high levels of the compatible solute glycine betaine under conditions of high osmolarity stress.

The aquaporin-Z water channel gene of Escherichia coli: structure, organization and phylogeny

Aquaporin water channel proteins are found throughout the plant and animal kingdoms, but the first prokaryotic water channel gene, aqpZ, was only recently identified in wild type Escherichia coli (Calamita G et al (1995) J Biol Chem 270, 29063-29066). Here we define the organization of aqpZ in E coli, produce the AqpZ protein and compare the AqpZ phylogeny to that of some known bacterial homologs. Physical mapping and sequence analyses confirmed the location of aqpZ at minute 19.7 on the E coli chromosome where it is transcribed counterclockwise. The monocistronic nature of aqpZ was clearly indicated by the structural organization of its surrounding genes, ybjD and ybjE' and by the presence of a typical Rho-independent transcriptional terminator following the aqpZ stop codon. Computer sequence analysis indicated the -35/-10 region located 72 bases upstream of the aqpZ start codon as the most likely aqpZ promoter. A series of potential cis-regulatory elements were found in the 400 bp region preceding the aqpZ ORF. The AqpZ protein, produced under T7 phi 10 control, showed a size of about 20 kDa by SDS-PAGE. Striking similarities were found between the E coli aqpZ and a gene included in the genome of the cyanobacterium Synechocystis sp PCC6803, a species permanently living a fresh water. These results may represent a fundamental step to characterize the regulation and the physiological features of the AqpZ water channel in prokaryotes.

pOSEX: vectors for osmotically controlled and finely tuned gene expression in Escherichia coli

Expression of the proU operon of Escherichia coli is directly proportional to the osmolarity of the growth medium. The basal level of proU transcription is very low, but a large increase is triggered by a sudden rise in the external osmolarity. This increased expression is maintained for as long as the osmotic stimulus persists. We have capitalized upon these regulatory features of the proU operon and have constructed a series of expression vectors (pOSEX) permitting osmotically controlled expression of heterologous genes governed by regulatory signals of proU. The pOSEX vectors carry the proU promoter, an upstream region required for high-level expression, and part of the first structural gene (proV), which acts as a silencer and is necessary to maintain low-level expression in low osmolarity media. An extended multiple cloning site (MCS) positioned at the 3' end of proV' permits the cloning of heterologous genes into the pOSEX plasmids, and efficient transcription terminators derived from the rrnB operon prevent deleterious read-through transcription into the vector portion. The properties of the pOSEX expression vectors were tested by positioning a promoterless lacZ (encoding beta-galactosidase) gene from E. coli and the gcdA (encoding carboxytransferase) gene from the Gram+ bacterium Acidaminococcus fermentans under the control of the proU regulatory region. Efficient, osmo-regulated and finely tuned expression of both lacZ and gcdA was achieved, and the amount of beta-galactosidase and carboxytransferase synthesized were simply controlled by adjusting the osmolarity of the growth medium with various concentrations of NaCl.

The nucleoid-associated DNA-binding protein H-NS is required for the efficient adaptation of Escherichia coli K-12 to a cold environment

The hns gene is a member of the cold-shock regulon, indicating that the nucleoid-associated, DNA-binding protein H-NS plays an important role in the adaptation of Escherichia coli to low temperatures. We show here that the ability to cope efficiently with a cold environment (12 degrees C and 25 degrees C) is strongly impaired in E. coli strains carrying hns mutations. Growth inhibition is much more pronounced in strains carrying the hns-206 allele (an ampicillin resistance cassette inserted after codon 37) than in those carrying the hns-205 mutation (a Tn10 insertion located in codon 93). A protein fragment (H-NS*) is synthesized in strains carrying the hns-205::Tn10 mutation, suggesting that this truncated polypeptide is partially functional in the cold adaptation process. Analysis of the growth properties of strains harbouring four different low-copy-number plasmid-encoded hns' genes that result in the production of C-terminally truncated H-NS proteins supports this proposal. H-NS* proteins composed of 133, 117 or 94 amino-terminal amino acids partially complemented the severe cold-sensitive growth phenotype of the hns-206 mutant. In contrast, synthesis of a truncated H-NS protein with only 75 amino-terminal amino acids was insufficient to restore growth at low temperature.

Direct regulation of bile secretion by prostaglandins in perfused rat liver

Prostaglandins have been postulated to act as mediators of intercellular communication between liver cell populations in the regulation of liver carbohydrate metabolism. Their role in the regulation of bile secretion is rather unclear. The action of prostaglandins on bile flow and bile acid secretion was studied in the in situ perfused rat liver. Infusion of prostaglandin F2 alpha resulted in reduction of bile flow and bile acid secretion. In addition, portal flow was diminished and glucose output was increased. The parameters were altered in a dose-dependent manner. The effects of prostaglandins D2 and E2 were similar but less potent than those of prostaglandin F2 alpha. To separate the effects on portal flow and bile flow, we used nitroprusside and nifedipine to suppress the hemodynamic changes induced by prostaglandins. In the presence of these substances, the alterations of bile flow and bile acid secretion were even more pronounced. Infusion of inhibitors of prostaglandin synthesis increased bile flow. In conclusion, prostaglandins F2 alpha, D2 and E2 reduce bile flow and bile acid secretion. These effects are independent of hemodynamic changes. Suppression of synthesis of endogenous prostaglandins by inhibitors of prostaglandin synthesis reversed these effects. Prostaglandins are involved in the regulation of bile flow and bile acid secretion.