Achievement of Target Gain Larger than Unity in an Inertial Fusion Experiment

On December 5, 2022, an indirect drive fusion implosion on the National Ignition Facility (NIF) achieved a target gain G_{target} of 1.5. This is the first laboratory demonstration of exceeding "scientific breakeven" (or G_{target}>1) where 2.05 MJ of 351 nm laser light produced 3.1 MJ of total fusion yield, a result which significantly exceeds the Lawson criterion for fusion ignition as reported in a previous NIF implosion [H. Abu-Shawareb et al. (Indirect Drive ICF Collaboration), Phys. Rev. Lett. 129, 075001 (2022)PRLTAO0031-900710.1103/PhysRevLett.129.075001]. This achievement is the culmination of more than five decades of research and gives proof that laboratory fusion, based on fundamental physics principles, is possible. This Letter reports on the target, laser, design, and experimental advancements that led to this result.

Lawson Criterion for Ignition Exceeded in an Inertial Fusion Experiment

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin "burn propagation" into surrounding cold fuel, enabling the possibility of high energy gain. While "scientific breakeven" (i.e., unity target gain) has not yet been achieved (here target gain is 0.72, 1.37 MJ of fusion for 1.92 MJ of laser energy), this Letter reports the first controlled fusion experiment, using laser indirect drive, on the National Ignition Facility to produce capsule gain (here 5.8) and reach ignition by nine different formulations of the Lawson criterion.

Radiographic diagnoses in 80 cats before and 73 cats after unobstructing the urethra

OBJECTIVE

The aim of this study was to investigate causes for feline urethral obstruction and determine whether the frequency of radiographic diagnoses differs between cats radiographed before or after unobstruction of the urethra.

MATERIALS AND METHODS

A retrospective cross-sectional study of cats with naturally occurring urethral obstruction was performed. Only cats presenting for their first urethral obstruction in which radiography was integrated in the initial evaluation were included. The diagnosis frequency (overall and for each disease type) was compared between cats radiographed before or after unobstruction of the urethra.

RESULTS

Eighty cats (52%) had radiographs obtained before unobstructing the urethra and 73 cats (48%) had radiographs taken after unobstructing the urethra. Cats radiographed before unobstruction had a greater frequency of radiographic diagnoses than those radiographed after unobstruction (61% versus 45%). This difference was largely due to a greater frequency of urethral plugs detected before unobstruction versus after unobstruction (45% versus 5.5%).

CLINICAL SIGNIFICANCE

Radiographs obtained before unobstructing the urethra provided a diagnostic advantage for detecting a cause for urethral obstruction compared to radiographs obtained after unobstructing the urethra. Urethral plugs were the most common diagnosis.

Solid Cherenkov detector for studying nucleosynthesis in inertial confinement fusion

Measuring gamma rays emitted from nuclear reactions gives insight into their nuclear structure. Notably, there are several nuclear reactions that produce gamma rays at ∼1 MeV-3 MeV energies such as T(⁴He, γ)⁷Li, ⁴He(³He, γ)⁷Be, and ¹²C(p, γ)¹³N, which may solve questions lingering about big-bang nucleosynthesis and stellar nucleosynthesis. To observe 1 MeV-3 MeV gamma rays in an inertial confinement fusion system, a new style of the Cherenkov detector was developed using aerogel and fused silica as a Cherenkov medium. Utilizing the OMEGA laser facility, both aerogel and fused silica media were compared with the existing gas-medium Cherenkov detector to validate the concept. Gamma ray measurements from high yield inertial confinement fusion implosions (deuterium-tritium and deuterium-³He) demonstrated that aerogel and fused silica were viable Cherenkov media, paving the way for a potential optimized detector to make these cross section measurements on OMEGA or the National Ignition Facility.

Progress on next generation gamma-ray Cherenkov detectors for the National Ignition Facility

Fusion reaction history and ablator areal density measurements for Inertial Confinement Fusion experiments at the National Ignition Facility are currently conducted using the Gamma Reaction History diagnostic (GRH_6m). Future Gas Cherenkov Detectors (GCDs) will ultimately provide ∼100x more sensitivity, reduce the effective temporal response from ∼100 to ∼10 ps, and lower the energy threshold from 2.9 to 1.8 MeV, relative to GRH_6m. The first phase toward next generation GCDs consisted of inserting the existing coaxial GCD-3 detector into a reentrant well which puts it within 4 m of the implosion. Reaction history and ablator gamma measurement results from this Phase I are discussed here. These results demonstrate viability for the follow-on Phases of (II) the use of a revolutionary new pulse-dilation photomultiplier tube to improve the effective measurement bandwidth by >10x relative to current PMT technology; and (III) the design of a NIF-specific "Super" GCD which will be informed by the assessment of the radiation background environment within the well described here.

Aerogel Cherenkov detector for characterizing the intense flash x-ray source, Cygnus, spectrum

An aerogel Cherenkov detector is proposed to measure the X-ray energy spectrum from the Cygnus-intense flash X-ray source operated at the Nevada National Security Site. An array of aerogels set at a variety of thresholds between 1 and 3 MeV will be adequate to map out the bremsstrahlung X-ray production of the Cygnus, where the maximum energy of the spectrum is normally around 2.5 MeV. In addition to the Cherenkov radiation from aerogels, one possible competing light-production mechanism is optical transition radiation (OTR), which may be significant in aerogels due to the large number of transitions from SiO₂ clusters to vacuum voids. To examine whether OTR is a problem, four aerogel samples were tested using a mono-energetic electron beam (varied in the range of 1-3 MeV) at NSTec Los Alamos Operations. It was demonstrated that aerogels can be used as a Cherenkov medium, where the rate of the light production is about two orders magnitude higher when the electron beam energy is above threshold.

Next generation gamma-ray Cherenkov detectors for the National Ignition Facility

The newest generation of Gas Cherenkov Detector (GCD-3) employed in Inertial Confinement Fusion experiments at the Omega Laser Facility has provided improved performance over previous generations. Comparison of reaction histories measured using two different deuterium-tritium fusion products, namely gamma rays using GCD and neutrons using Neutron Temporal Diagnostic (NTD), have provided added credibility to both techniques. GCD-3 is now being brought to the National Ignition Facility (NIF) to supplement the existing Gamma Reaction History (GRH-6m) located 6 m from target chamber center (TCC). Initially it will be located in a reentrant well located 3.9 m from TCC. Data from GCD-3 will inform the design of a heavily-shielded "Super" GCD to be located as close as 20 cm from TCC. It will also provide a test-bed for faster optical detectors, potentially lowering the temporal resolution from the current ∼100 ps state-of-the-art photomultiplier tubes (PMT) to ∼10 ps Pulse Dilation PMT technology currently under development.

Extended performance gas Cherenkov detector for gamma-ray detection in high-energy density experiments

A new Gas Cherenkov Detector (GCD) with low-energy threshold and high sensitivity, currently known as Super GCD (or GCD-3 at OMEGA), is being developed for use at the OMEGA Laser Facility and the National Ignition Facility (NIF). Super GCD is designed to be pressurized to ≤400 psi (absolute) and uses all metal seals to allow the use of fluorinated gases inside the target chamber. This will allow the gamma energy threshold to be run as low at 1.8 MeV with 400 psi (absolute) of C2F6, opening up a new portion of the gamma ray spectrum. Super GCD operating at 20 cm from TCC will be ∼400 × more efficient at detecting DT fusion gammas at 16.7 MeV than the Gamma Reaction History diagnostic at NIF (GRH-6m) when operated at their minimum thresholds.

Conceptual design of the gamma-to-electron magnetic spectrometer for the National Ignition Facility

The Gamma-to-Electron Magnetic Spectrometer (GEMS) diagnostic is designed to measure the prompt γ-ray energy spectrum during high yield deuterium-tritium (DT) implosions at the National Ignition Facility (NIF). The prompt γ-ray spectrum will provide "burn-averaged" observables, including total DT fusion yield, total areal density (ρR), ablator ρR, and fuel ρR. These burn-averaged observables are unique because they are essentially averaged over 4π, providing a global reference for the line-of-sight-specific measurements typical of x-ray and neutron diagnostics. The GEMS conceptual design meets the physics-based requirements: ΔE/E = 3%-5% can be achieved in the range of 2-25 MeV γ-ray energy. Minimum DT neutron yields required for 15% measurement uncertainty at low-resolution mode are: 5 × 10(14) DT-n for ablator ρR (at 0.2 g/cm(2)); 2 × 10(15) DT-n for total DT yield (at 4.2 × 10(-5) γ/n); and 1 × 10(16) DT-n for fuel ρR (at 1 g/cm(2)).

Gamma-to-electron magnetic spectrometer (GEMS): an energy-resolved γ-ray diagnostic for the National Ignition Facility

The gamma-to-electron magnetic spectrometer, having better than 5% energy resolution, is proposed to resolve γ-rays in the range of E(o) ± 20% in single shot, where E(o) is the central energy and is tunable from 2 to 25 MeV. Gamma-rays from inertial confinement fusion implosions interact with a thin Compton converter (e.g., beryllium) located at approximately 300 cm from the target chamber center (TCC). Scattered electrons out of the Compton converter enter an electromagnet placed outside the NIF chamber (approximately 600 cm from TCC) where energy selection takes place. The electromagnet provides tunable E(o) over a broad range in a compact manner. Energy resolved electrons are measured by an array of quartz Cherenkov converters coupled to photomultipliers. Given 100 detectable electrons in the energy bins of interest, 3 × 10(14) minimum deuterium/tritium (DT) neutrons will be required to measure the 4.44 MeV (12)C γ-rays assuming 200 mg/cm(2) plastic ablator areal density and 3 × 10(15) minimum DT neutrons to measure the 16.75 MeV DT γ-ray line.

Mach-Zehnder recording systems for pulsed power diagnostics

Fiber-optic transmission and recording systems, based on Mach-Zehnder modulators, have been developed and installed at the National Ignition Facility (NIF), and are being developed for other pulsed-power facilities such as the Z accelerator at Sandia, with different requirements. We present the design and performance characteristics for the mature analog links, based on the system developed for the Gamma Reaction History diagnostic at the OMEGA laser and at NIF. For a single detector channel, two Mach-Zehnders are used to provide high dynamic range at the full recording bandwidth with no gaps in the coverage. We present laboratory and shot data to estimate upper limits on the radiation effects as they impact recorded data quality. Finally, we will assess the technology readiness level for mature and developing implementations of Mach-Zehnder links for these environments.

Diagnosing inertial confinement fusion gamma ray physics (invited)

The gamma reaction history (GRH) diagnostic is a multichannel, time-resolved, energy-thresholded γ-ray spectrometer that provides a high-bandwidth, direct-measurement of fusion reaction history in inertial confinement fusion implosion experiments. 16.75 MeV deuterium+tritium (DT) fusion γ-rays, with a branching ratio of the order of 10(-5)γ/(14 MeV n), are detected to determine fundamental burn parameters, such as nuclear bang time and burn width, critical to achieving ignition at the National Ignition Facility. During the tritium/hydrogen/deuterium ignition tuning campaign, an additional γ-ray line at 19.8 MeV, produced by hydrogen+tritium fusion with a branching ratio of unity, will increase the available γ-ray signal and may allow measurement of reacting fuel composition or ion temperature. Ablator areal density measurements with the GRH are also made possible by detection of 4.43 MeV γ-rays produced by inelastic scatter of DT fusion neutrons on (12)C nuclei in the ablating plastic capsule material.

Using gamma-ray emission to measure areal density of inertial confinement fusion capsules

Fusion neutrons streaming from a burning inertial confinement fusion capsule generate gamma rays via inelastic nuclear scattering in the ablator of the capsule. The intensity of gamma-ray emission is proportional to the product of the ablator areal density (ρR) and the yield of fusion neutrons, so by detecting the gamma rays we can infer the ablator areal density, provided we also have a measurement of the capsule's total neutron yield. In plastic-shell capsules, for example, (12)C nuclei emit gamma rays at 4.44 MeV after excitation by 14.1 MeV neutrons from D+T fusion. These gamma rays can be measured by a new gamma-ray detector under development. Analysis of predicted signals is in progress, with results to date indicating that the method promises to be useful for diagnosing imploded capsules.

Gamma bang time analysis at OMEGA

Absolute bang time measurements with the gas Cherenkov detector (GCD) and gamma reaction history (GRH) diagnostic have been performed to high precision at the OMEGA laser facility at the University of Rochester with bang time values for the two diagnostics agreeing to within 5 ps on average. X-ray timing measurements of laser-target coupling were used to calibrate a facility-generated laser timing fiducial with rms spreads in the measured coupling times of 9 ps for both GCD and GRH. Increased fusion yields at the National Ignition Facility (NIF) will allow for improved measurement precision with the GRH easily exceeding NIF system design requirements.

GEANT4 simulations of Cherenkov reaction history diagnostics

This paper compares the results from a GEANT4 simulation of the gas Cherenkov detector 1 (GCD1) with previous simulations and experimental data from the Omega laser facility. The GCD1 collects gammas emitted during a deuterium-tritium capsule implosion and converts them, through several processes, to Cherenkov light. Photon signals are recorded using subnanosecond photomultiplier tubes, producing burn reaction histories. The GEANT4 GCD1 simulation is first benchmarked against ACCEPT, an integrated tiger series code, with good agreement. The simulation is subsequently compared with data from the Omega laser facility, where experiments have been performed to measure the effects of Hohlraum materials on reaction history signals, in preparation for experiments at the National Ignition Facility.

Development and characterization of sub-100 ps photomultiplier tubes

We describe the evaluation of a microchannel plate (MCP) photomultiplier tube (PMT), incorporating a 3 μm pore MCP and constant voltage anode and cathode gaps. The use of the small pore size results in PMTs with response functions of the order of 85 ps full-width-half-maximum, while the constant electric field across the anode and cathode gaps produces a uniform response function over the entire operating range of the device. The PMT was characterized on a number of facilities and employed on gas Cherenkov detectors fielded on various deuterium tritium fuel (DT) implosions on the Omega Laser Facility at the University of Rochester. The Cherenkov detectors are part of diagnostic development to measure Gamma ray reaction history for DT implosions on the National Ignition Facility.

Release, biological potency, and biochemical integrity of recombinant human platelet-derived growth factor-BB (rhPDGF-BB) combined with Augment(TM) Bone Graft or GEM 21S beta-tricalcium phosphate (beta-TCP)

Over 10 million surgical procedures are performed annually in the United States to treat musculoskeletal injuries, and a significant portion of these involve orthopedic bone grafting. The goals of the study were to evaluate the in vitro and in vivo release kinetics, biological potency and biochemical integrity of rhPDGF-BB combined with large (1000-2000 microm) and small (250-1000 microm) beta-TCP particles. Recombinant human platelet-derived growth factor B homodimer (rhPDGF-BB) is a protein growth factor under development as a therapeutic for accelerating bone healing. Release of the protein was monitored in vitro by ELISA, and in vivo by measurement of radioactive rhPDGF-BB implanted in rat calvarial defects. Biological activity was measured using a cell-based bioassay, and biochemical integrity was determined by SDS-PAGE and high pressure size exclusion chromatography (HPSEC). Release of rhPDGF-BB occurred rapidly from beta-TCP both in vitro and in vivo. Almost 100% of the rhPDGF-BB was recovered from large and small beta-TCP after 90 min in vitro. Approximately 90% of the rhPDGF-BB was depleted from calvarial defect sites within 72 h of implantation. RhPDGF-BB retained 100% of its biological potency compared to reference standard rhPDGF-BB, manifested as a single band at ~30 kDa by SDS-PAGE and a single peak eluted after 13 min by HPSEC following release from beta-TCP. RhPDGF-BB is rapidly released from large and small beta-TCP particles and is biochemically unaltered following release.

Gamma bang time/reaction history diagnostics for the National Ignition Facility using 90 degrees off-axis parabolic mirrors

Gas Cherenkov detectors (GCDs) have been used to convert fusion gamma into photons to achieve gamma bang time and reaction history measurements. The GCDs designed for OMEGA used Cassegrain reflector optics in order to fit inside a 10 in. manipulator. A novel design for the National Ignition Facility using 90 degrees off-axis parabolic mirrors will increase light collection efficiency from fusion gammas and achieve minimum time dispersion. The broadband Cherenkov light (from 200 to 800 nm) is relayed into a high-speed detector using three parabolic mirrors. Because light is collected from many source planes throughout the CO(2) gas volume, the detector is positioned at the stop position rather than at an image position. The stop diameter and its position are independent of the light-generation location along the gas cell. The current design collects light from a 100 mm diameter by 500 mm long gas volume. Optical ray tracings demonstrate how light can be collected from different angled trajectories of the Compton electrons as they fly through the CO(2) gas volume. A cluster of four channels will allow for increased dynamic range as well as for different gamma energy threshold sensitivities.

Developmental analysis and computer modelling of bioengineered teeth

Here we present the developmental progression of bioengineered pig teeth from 1 to 25 weeks of development. We demonstrate that 2-25 week implants contained embryonic tooth bud- and cap-stage tooth structures consisting of dental epithelium expressing the sonic hedgehog gene and condensed dental mesenchyme. Implants harvested at 18-25 weeks also contained tooth bud-like structures, as well as mature tooth structures containing enamel, dentin and pulp tissues. Immunohistochemical analyses confirmed the expression of dentin- and enamel-specific proteins in differentiated bioengineered tooth tissues. Three-dimensional computer modelling further demonstrated a spatial organization of enamel, dentin and pulp tissues resembling that of natural teeth. We conclude that bioengineered teeth commonly exhibit morphological stages characteristic of naturally forming teeth. Furthermore, the presence of immature tooth buds at all times assayed and increased numbers of bioengineered tooth structures over time suggests that porcine dental progenitor cells maintain the ability to form teeth for at least 25 weeks.

Bioengineered teeth from cultured rat tooth bud cells

The recent bioengineering of complex tooth structures from pig tooth bud tissues suggests the potential for the regeneration of mammalian dental tissues. We have improved tooth bioengineering methods by comparing the utility of cultured rat tooth bud cells obtained from three- to seven-day post-natal (dpn) rats for tooth-tissue-engineering applications. Cell-seeded biodegradable scaffolds were grown in the omenta of adult rat hosts for 12 wks, then harvested. Analyses of 12-week implant tissues demonstrated that dissociated 4-dpn rat tooth bud cells seeded for 1 hr onto PGA or PLGA scaffolds generated bioengineered tooth tissues most reliably. We conclude that tooth-tissue-engineering methods can be used to generate both pig and rat tooth tissues. Furthermore, our ability to bioengineer tooth structures from cultured tooth bud cells suggests that dental epithelial and mesenchymal stem cells can be maintained in vitro for at least 6 days.

Beta-catenin/Tcf activation partially mimics the transforming activity of Wnt-1 in Rat-1 fibroblasts

We have previously demonstrated that Wnt-1 induction of cytosolic beta-catenin and Tcf/Lef transcriptional activation correlate with enhanced proliferation, survival, and post-confluent growth in Rat-1 fibroblasts. To examine whether beta-catenin mediates the biological responses to Wnt-1 in this context, we characterized Rat-1 clonal cell lines expressing different levels of a mutant, stabilized beta-catenin (beta-cateninS37A). Clonal lines exhibit elevated cytosolic and nuclear beta-cateninS37A and Tcf transcriptional activation, comparable to that elicited by Wnt-1 expression. However, expression of beta-cateninS37A does not promote growth of Rat-1 cells in serum-free conditions and only partially promotes growth post-confluence, when compared to that induced by Wnt-1 expression. As ectopic expression of beta-cateninS37A only partially mimics Wnt-1 effects on Rat-1 cells, we conclude that Wnt-1 signaling elicits biochemical events that act in addition to beta-catenin/Tcf signaling to promote cell growth.

Tissue engineering of complex tooth structures on biodegradable polymer scaffolds

Tooth loss due to periodontal disease, dental caries, trauma, or a variety of genetic disorders continues to affect most adults adversely at some time in their lives. A biological tooth substitute that could replace lost teeth would provide a vital alternative to currently available clinical treatments. To pursue this goal, we dissociated porcine third molar tooth buds into single-cell suspensions and seeded them onto biodegradable polymers. After growing in rat hosts for 20 to 30 weeks, recognizable tooth structures formed that contained dentin, odontoblasts, a well-defined pulp chamber, putative Hertwig's root sheath epithelia, putative cementoblasts, and a morphologically correct enamel organ containing fully formed enamel. Our results demonstrate the first successful generation of tooth crowns from dissociated tooth tissues that contain both dentin and enamel, and suggest the presence of epithelial and mesenchymal dental stem cells in porcine third molar tissues.

Hepatic ischemia/reperfusion injury in P-selectin and intercellular adhesion molecule-1 double-mutant mice

Neutrophil adhesion and recruitment represents one of the early cellular events that occur during hepatic ischemia/reperfusion (IR) injury and plays a critical role in determining the extent of tissue damage. The adhesion molecules, such as selectins and intercellular adhesion molecules (ICAM), are important in mediating neutrophil-endothelial cell interactions and neutrophil emigration. The goal of this study was to evaluate the role of P-selectin and ICAM-1 in hepatic IR injury. Male wild-type and P-selectin/ICAM-1-deficient (P/I null) mice underwent 90 minutes of partial hepatic ischemia followed by reperfusion at various time points (0, 1.5, 3, and 6 hours). Reperfusion caused a time-dependent hepatocellular injury in both wild-type and P/I null mice as judged by plasma alanine aminotransferase (ALT) levels and liver histopathology examination. Although ALT levels were slightly lower in the P/I null mice compared with the wild-type mice the differences were not statistically significant. Neutrophil infiltration to the ischemic liver was observed in both mouse groups after 6 hours of reperfusion; however, the infiltration to the midzonal region of the ischemic liver was more pronounced in the wild-type group. This study suggests that hepatocellular injury induced after hepatic IR was independent of P-selectin and ICAM-1 in this model of acute inflammatory tissue injury.

Ionizing radiation modulates vascular endothelial growth factor (VEGF) expression through multiple mitogen activated protein kinase dependent pathways

We investigated the role of radiation-induced mitogen activated protein kinase (MAPK) pathway activity in the regulation of proliferation, cell survival and vascular endothelial growth factor (VEGF) production in primary astrocytes and in T9 and RT2 glioblastoma cells derived from Fisher 344 rats. In these cells, ionizing radiation (2 Gy) caused activation of the MAPK pathway which was blocked by specific inhibitor drugs. Blunting of radiation-induced MAPK activity weakly enhanced radiation-induced apoptosis 24 h after exposure in RT2 cells. Furthermore, blunting of MAPK activation weakly enhanced the ability of radiation to reduce RT2 cell growth in clonogenic growth assays. These findings argue that inhibition of MAPK signaling reduces proliferation and enhances cell killing by ionizing radiation in transformed astrocytes. Proliferation and survival of cancer cells has been linked in vivo to enhanced expression of angiogenic growth factors. Recently we demonstrated that the gene product of a novel rodent radiation-responsive gene, progression elevated gene 3 (PEG-3), could enhance vascular endothelial growth factor (VEGF) promoter activity in rodent fibroblasts, leading to increased VEGF protein levels and tumorigenic behavior in vivo. Thus PEG-3 and VEGF expression could be expected to directly correlate with the oncogenic potential of transformed cells. RT2 cells expressed more PEG-3 and VEGF protein than T9 cells, and were more tumorigenic in vivo than T9 cells. Radiation activated the PEG-3 promoter via MAPK signaling and ectopic over-expression of PEG-3 enhanced both basal MAPK activity and basal VEGF promoter activity. Basal MAPK activity partially correlated with basal VEGF promoter activity and VEGF protein levels in primary astrocytes, T9 and RT2 cells. Radiation increased the activity of the VEGF promoter and VEGF protein levels in primary astrocytes, T9 and RT2 cells which were dependent upon MAPK function. Furthermore, inhibition of AP-1 transcription factor signaling by dominant negative c-Jun (TAM67) also significantly reduced basal, and to a lesser extent radiation-induced, VEGF promoter function in RT2 cells. Collectively, our data demonstrate that radiation-induced MAPK signaling can both protect cells from radiation-induced cell death as well as enhance protein levels of pro-angiogenic factors such as VEGF. Enhanced VEGF expression in RT2 cells may be mediated via MAPK and JNK pathway signaling which converges upon the AP-1 transcription factor complex.

High-pressure and stark hole-burning studies of chlorosome antennas from Chlorobium tepidum

Results from high-pressure and Stark hole-burning experiments on isolated chlorosomes from the green sulfur bacterium Chlorobium tepidum are presented, as well as Stark hole-burning data for bacteriochlorophyll c (BChl c) monomers in a poly(vinyl butyral) copolymer film. Large linear pressure shift rates of -0.44 and -0.54 cm(-1)/MPa were observed for the chlorosome BChl c Q(y)-band at 100 K and the lowest Q(y)-exciton level at 12 K, respectively. It is argued that approximately half of the latter shift rate is due to electron exchange coupling between BChl c molecules. The similarity between the above shift rates and those observed for the B875 and B850 BChl a rings of the light-harvesting complexes of purple bacteria is emphasized. For BChl c monomer, fDeltamu++ = 0.35 D, where Deltamu+ is the dipole moment change for the Q(y) transition and f is the local field correction factor. The data establish that Deltamu+ is dominated by the matrix-induced contribution. The change in polarizability (Deltaalpha) for the Q(y) transition of the BChl c monomer is estimated at 19 A(3), which is essentially identical to that of the Chl a monomer. Interestingly, no Stark effects were observed for the lowest exciton level of the chlorosomes (maximum Stark field of 10(5) V/cm). Possible explanations for this are given, and these include consideration of structural models for the chlorosome BChl c aggregates.

Creation and repair of specific DNA double-strand breaks in vivo following infection with adenovirus vectors expressing Saccharomyces cerevisiae HO endonuclease

To study DNA double-strand break (DSB) repair in mammalian cells, the Saccharomyces cerevisiae HO endonuclease gene, or its recognition site, was cloned into the adenovirus E3 or E1 regions. Analysis of DNA from human A549 cells coinfected with the E3::HO gene and site viruses showed that HO endonuclease was active and that broken viral genomes were detectable 12 h postinfection, increasing with time up to approximately 30% of the available HO site genomes. Leftward fragments of approximately 30 kbp, which contain the packaging signal, but not rightward fragments of approximately 6 kbp, were incorporated into virions, suggesting that broken genomes were not held together tightly after cleavage. There was no evidence for DSB repair in E3::HO virus coinfections. In contrast, such evidence was obtained in E1::HO virus coinfections of nonpermissive cells, suggesting that adenovirus proteins expressed in the permissive E3::HO coinfection can inhibit mammalian DSB repair. To test the inhibitory role of E4 proteins, known to suppress genome concatemer formation late in infection (Weiden and Ginsberg, 1994), A549 cells were coinfected with E3::HO viruses lacking the E4 region. The results strongly suggest that the E4 protein(s) inhibits DSB repair.

PEG-3, a nontransforming cancer progression gene, is a positive regulator of cancer aggressiveness and angiogenesis

Cancer is a progressive disease culminating in acquisition of metastatic potential by a subset of evolving tumor cells. Generation of an adequate blood supply in tumors by production of new blood vessels, angiogenesis, is a defining element in this process. Although extensively investigated, the precise molecular events underlying tumor development, cancer progression, and angiogenesis remain unclear. Subtraction hybridization identified a genetic element, progression elevated gene-3 (PEG-3), whose expression directly correlates with cancer progression and acquisition of oncogenic potential by transformed rodent cells. We presently demonstrate that forced expression of PEG-3 in tumorigenic rodent cells, and in human cancer cells, increases their oncogenic potential in nude mice as reflected by a shorter tumor latency time and the production of larger tumors with increased vascularization. Moreover, inhibiting endogenous PEG-3 expression in progressed rodent cancer cells by stable expression of an antisense expression vector extinguishes the progressed cancer phenotype. Cancer aggressiveness of PEG-3 expressing rodent cells correlates directly with increased RNA transcription, elevated mRNA levels, and augmented secretion of vascular endothelial growth factor (VEGF). Furthermore, transient ectopic expression of PEG-3 transcriptionally activates VEGF in transformed rodent and human cancer cells. Taken together these data demonstrate that PEG-3 is a positive regulator of cancer aggressiveness, a process regulated by augmented VEGF production. These studies also support an association between expression of a single nontransforming cancer progression-inducing gene, PEG-3, and the processes of cancer aggressiveness and angiogenesis. In these contexts, PEG-3 may represent an important target molecule for developing cancer therapeutics and inhibitors of angiogenesis.

The role of the L4 33K gene in adenovirus infection

The late phase of adenovirus infection is characterized not only by the synthesis of late proteins and the assembly of new virions, but also by the inhibition of early gene expression and host cell translation. Previous work has demonstrated that both of these inhibitory effects depend upon expression from the major late transcription unit (MLTU), controlled by the major late promoter (MLP). Furthermore, the repression of early gene expression has been shown to be mediated in trans, suggesting a role for one or more MLTU-encoded soluble factor(s). A possible candidate for such a factor is the L4-encoded 33K gene product, a protein conserved throughout the Mastadenoviridae, but of no known function. To test the role of this protein in viral infection, a stop codon was placed at the 20th position of the 33K ORF. Viable virus with genomes containing the mutation were recovered in an overlap recombination assay. Phenotypic analysis revealed that the mutant virus had a significant deficiency in both kinetics of replication and final yield, as compared to the wild-type virus. Detailed analysis of infected cells showed that there was no detectable change in the regulation of expression of several early genes and the pIX gene. This suggests either that 33K is not involved in this late phase phenomenon or that this function is replaceable by another late protein(s). Late protein synthesis and accumulation were similar to those in wild-type-infected cells. However, the reduced yield of infectious mutant virus could be accounted for by a marked deficiency in the accumulation of intermediate particles and completed capsids, suggesting a role for 33K in the process of assembly. In addition there was a small but reproducible deficiency in the shutoff of host cell translation. These results show that the 33K protein plays an important, although apparently not essential, function in the late phase of virus infection.

The NH2-terminal region of apolipoprotein B is sufficient for lipoprotein association with glycosaminoglycans

An initial event in atherosclerosis is the retention of lipoproteins within the intima of the vessel wall. The co-localization of apolipoprotein (apo) B and proteoglycans within lesions has suggested that retention is due to lipoprotein interaction with these highly electronegative glycoconjugates. Both apoB100- and apoB48-containing lipoproteins, i.e. low density lipoproteins (LDLs) and chylomicron remnants, are atherogenic. This suggests that retention is due to determinants in the initial 48% of apoB. To test this, the interaction of an apoB fragment (apoB17), and apoB48- and apoB100- containing lipoproteins with heparin, subendothelial matrix, and artery wall purified proteoglycans was studied. ApoB100-containing LDL from humans and human apoB transgenic mice and apoB48-containing LDLs from apoE knockout mice were used. Despite the lack of the carboxyl-terminal 52% of apoB, the apoB48-LDL bound to heparin-affinity gel as well as did apoB100-LDL. An NH2-terminal fragment containing 17% of full-length apoB was made using a recombinant adenovirus; apoB17 bound to heparin as well as did LDL. Monoclonal antibodies against the NH2-terminal region of apoB decreased apoB100 LDL binding to heparin, whereas antibodies against the LDL receptor-binding region did not alter LDL-heparin interaction. The role of the NH2-terminal region of apoB in LDL interaction with matrix molecules was also assessed. Media containing apoB17 decreased LDL binding to subendothelial matrix by 42%. Moreover, removal of the apoB17 by immunoprecipitation abrogated the inhibitory effect of these media. Antibodies to the NH2-terminal region decreased LDL binding to matrix and dermatan sulfate proteoglycans. Purified apoB17 effectively competed for binding of LDL to artery derived decorin and to subendothelial matrix. Thus, despite the presence of multiple basic amino acids near the LDL receptor-binding domain of LDL, the NH2-terminal region of apoB is sufficient for the interaction of lipoproteins with glycoconjugates produced by endothelial and smooth muscle cells. The presence of a proteoglycan-binding site in the NH2-terminal region of apoB may explain why apoB48- and apoB100-containing lipoproteins are equally atherogenic.

The cancer growth suppressor gene mda-7 selectively induces apoptosis in human breast cancer cells and inhibits tumor growth in nude mice

A differentiation induction subtraction hybridization strategy is being used to identify and clone genes involved in growth control and terminal differentiation in human cancer cells. This scheme identified melanoma differentiation associated gene-7 (mda-7), whose expression is up-regulated as a consequence of terminal differentiation in human melanoma cells. Forced expression of mda-7 is growth inhibitory toward diverse human tumor cells. The present studies elucidate the mechanism by which mda-7 selectively suppresses the growth of human breast cancer cells and the consequence of ectopic expression of mda-7 on human breast tumor formation in vivo in nude mice. Infection of wild-type, mutant, and null p53 human breast cancer cells with a recombinant type 5 adenovirus expressing mda-7, Ad.mda-7 S, inhibited growth and induced programmed cell death (apoptosis). Induction of apoptosis correlated with an increase in BAX protein, an established inducer of programmed cell death, and an increase in the ratio of BAX to BCL-2, an established inhibitor of apoptosis. Infection of breast carcinoma cells with Ad.mda-7 S before injection into nude mice inhibited tumor development. In contrast, ectopic expression of mda-7 did not significantly alter cell cycle kinetics, growth rate, or survival in normal human mammary epithelial cells. These data suggest that mda-7 induces its selective anticancer properties in human breast carcinoma cells by promoting apoptosis that occurs independent of p53 status. On the basis of its selective anticancer inhibitory activity and its direct antitumor effects, mda-7 may represent a new class of cancer suppressor genes that could prove useful for the targeted therapy of human cancer.

Topological model of the Rhodobacter capsulatus light-harvesting complex I assembly protein LhaA (previously known as ORF1696)

A theoretical topology of the Rhodobacter capsulatus membrane protein LhaA was formulated and evaluated by gene fusion experiments. The apparent topological locations of fusion enzymes were compared with the theoretically derived structure, and a model of LhaA is suggested that consists of 12 transmembrane segments, with the N and C termini residing in the cytoplasm.

Control of adenovirus early gene expression during the late phase of infection

The adenovirus gene regulatory program occurs in two distinct phases, as defined by the onset of DNA replication. During the early phase, the E1A, E1B, E2, E3, and E4 genes are maximally expressed, while the major late promoter (MLP) is minimally expressed and transcription is attenuated. After the onset of DNA replication, the IVa2 and pIX genes are expressed at high levels, transcription from the MLP is unattenuated and fully activated, and early gene expression is repressed. Although the cis elements and trans-acting factors responsible for the late-phase activation of the MLP have been identified and characterized and the role of DNA replication in activation has been established, the mechanism(s) underlying the commensurate decrease in early gene expression has yet to be elucidated. The results of this study demonstrate that this decrease depends on a fully functional MLP. Specifically, virus mutants with severely deficient transcription from the MLP exhibit a marked increase in expression of the E1A, E1B, and E2 early genes. These increases were observed at the level of transcription initiation, mRNA accumulation, and protein production. In addition, expression from the late gene pIX, which is not contained within the major late transcription unit (MLTU), is also markedly increased. To begin the analysis of the mechanisms underlying these late-phase effects, mixed-infection experiments with mutant and wild-type viruses were performed. The results show that the effects on early gene expression, as measured both at the protein and RNA levels, are mediated in trans and not in cis. These observations are consistent either with a model in which one or more late protein products encoded by the MLTU acts as a repressor of early gene expression or with one in which the wild-type MLP competes with early promoters for limiting transcription factors.

Wnt-1 induces growth, cytosolic beta-catenin, and Tcf/Lef transcriptional activation in Rat-1 fibroblasts

Genetic evidence suggests that regulation of beta-catenin and regulation of Tcf/Lef family transcription factors are downstream events of the Wnt signal transduction pathway. However, a direct link between Wnt activity and Tcf/Lef transcriptional activation has yet to be established. In this study, we show that Wnt-1 induces a growth response in a cultured mammalian cell line, Rat-1 fibroblasts. Wnt-1 induces serum-independent cellular proliferation of Rat-1 fibroblasts and changes in morphology. Rat-1 cells stably expressing Wnt-1 (Rat-1/Wnt-1) show a constitutive up-regulation of cytosolic beta-catenin, while membrane-associated beta-catenin remains unaffected. Induction of cytosolic beta-catenin in Rat-1/Wnt-1 cells is correlated with activation of a Tcf-responsive transcriptional element. We thus provide evidence that Wnt-1 induces Tcf/Lef transcriptional activation in a mammalian system. Expression of a mutant beta-catenin (beta-CatS37A) in Rat-1 cells does not result in a proliferative response or a detectable change in the cytosolic beta-catenin protein level. However, beta-CatS37A expression in Rat-1 cells results in strong Tcf/Lef transcriptional activation, comparable to that seen in Wnt-1-expressing cells. These results suggest that Wnt-1 induction of cytosolic beta-catenin may have functions in addition to Tcf/Lef transcriptional activation.

Functional analysis of the CAAT box in the major late promoter of the subgroup C human adenoviruses

Comparisons among sequences predicted to encode the major late promoter (MLP) of adenoviruses from a wide variety of host species show that an inverted CAAT box is among the most highly conserved transcription elements found in the putative MLPs. The high degree of conservation suggests that the CAAT box plays an important role in the function of the MLP in vivo, an idea supported by a previous mutational analysis of the core CCAAT sequence. To address the importance of the CAAT box, in terms both of quantitative levels of transcription and of specificity, a further set of mutations was created and examined in the context of the viral genome. One mutation, CAAT5, contains individual changes at five positions, four of which correspond to invariant residues in a CAAT box consensus derived either by computer analysis or empirically. The CAAT5 mutation had no discernible phenotype by itself but when coupled with the previously described USF0 mutation, which disrupts binding of the upstream stimulating factor (USF) but is otherwise phenotypically silent, gave rise to virus with a severe replication deficiency. Nuclear run-on assays showed that transcription initiation at the mutant MLP was significantly reduced compared with that of the wild type or the virus containing CAAT5 alone. Replication of the double mutant was lower than that of the previously described USF0::CCCAT virus, suggesting that the additional mutations in the CAAT box had further lowered the binding of transcription factor CP1 (also called CBF, NF-Y). Replacement of the CAAT box by an ATF binding site or an OCT1 binding site had no phenotypic effect in an otherwise wild-type background, but replacement in a USF0::CCCAT background led to only partial restoration of the wild-type phenotype. The failure to restore the functional redundancy normally exhibited by the CAAT box and the proximal upstream activating element is consistent with the idea that in the adenovirus MLP the CAAT box is preferred over others as the distal transcriptional element.

Genetic complementation and kinetic analyses of Rhodobacter capsulatus ORF1696 mutants indicate that the ORF1696 protein enhances assembly of the light-harvesting I complex

Rhodobacter capsulatus ORF1696 mutant strains were created by insertion of antibiotic resistance cartridges at different sites within the ORF1696 gene in a strain that lacks the light-harvesting II (LHII) complex. Steady-state absorption spectroscopy profiles and the kinetics of the light-harvesting I (LHI) complex assembly and decay were used to evaluate the function of the ORF1696 protein in various strains. All of the mutant strains were found to be deficient in the LHI complex, including one (deltaNae) with a disruption located 13 codons before the 3' end of the gene. A 5'-proximal disruption after the 31st codon of ORF1696 resulted in a mutant strain (deltaMun) with a novel absorption spectrum. The two strains with more 3' disruptions (deltaStu and deltaNae) were restored nearly to the parental strain phenotype when trans complemented with a plasmid expressing the ORF1696 gene, but deltaMun was not. The absorption spectrum of deltaMun resembled that of a strain which had a polar mutation in ORF1696. We suggest that a rho-dependent transcription termination site exists between the MunI and proximal StuI sites of ORF1696. A comparison of LHI complex assembly kinetics showed that assembly occurred 2.6-fold faster in the parental strain than in strain deltaStu. In contrast, LHI complex decay occurred 1.7-fold faster in the ORF1696 parental strain than in deltaStu. These results indicate that the ORF1696 protein has a major effect on LHI complex assembly, and models of ORF1696 function are proposed.

Functional characterization of the major late promoter of mouse adenovirus type 1

During the late phase of adenovirus infection, the major late promoter (MLP) controls the regulated expression of the genes that encode most viral structural proteins. Recently, the region of the genome of mouse adenovirus type 1 (MAV-1), predicted to contain the MLP, was sequenced and compared to that of the human virus MLP. The general organization of the transcriptional elements of the putative MAV-1 MLP is similar to that of the human virus counterpart, with some interesting differences. We wished to investigate the function of the predicted MLP of MAV-1 and to determine the significance of the differences found in the MAV-1 MLP. To test the activity of the predicted MLP of MAV-1, both Northern blot and primer extension analyses were performed on intracellular RNA isolated from cells infected with MAV-1. The results show that late RNA can be detected 48 hr postinfection and increases up to 6 days p.i. Primer extension analysis revealed that the major start sites of transcription are 28 and 31 nt downstream of the first T residue of the predicted TATA box. To analyze the functional significance of the predicted transcriptional elements, a transient transfection system, using the firefly luciferase gene controlled by the MAV-1 MLP sequence, was established. The predicted MLP sequence was capable of directing luciferase gene expression, to a level some 60% of that of the human virus MLP. Mutations were created in the inverted CAAT box, the SP1 site, and the TATA box, either singly or in combination. Each single-element mutation causes a marked reduction in luciferase gene expression, with the SP1 mutation showing the greatest effect. Double mutations were even more deficient, suggesting a level of functional redundancy among the various transcriptional elements. Finally, the putative SP1-binding site was examined by gel mobility shift assay and shown to interact with purified SP1 protein specifically, supporting the functional significance of this transcriptional element. These findings contribute to a better understanding of gene expression in MAV-1 and to its development as an appropriate model for the study of the molecular basis of pathogenesis in a natural host animal.

The initiator element of the adenovirus major late promoter has an important role in transcription initiation in vivo

Previous results showed that the structure and function of the adenovirus major late promoter (MLP) can be analyzed genetically in its correct location, despite its essential role in the viral life cycle. This genetic approach was extended to investigate the in vivo role of the initiator (INR), a transcriptional element that surrounds the start site of transcription. The analysis was designed to investigate if the INR is an alternative basal element to the canonical TATA box of the MLP, its relative importance in the functioning of the promoter, and if its function was affected by upstream activating elements. Accordingly, two different mutations in the INR were created and tested in the genome, either by themselves or together with mutations in the TATA box or one of the two upstream activating elements, the upstream promoter element (UPE) and the inverted CAAT box. The mutant viruses were examined first in one-step growth experiments, and then levels of late mRNA accumulation were measured by primer extension, transcription initiation was assayed in isolated nuclei, and viral DNA accumulation was determined by Southern hybridization. Neither mutation in the INR alone had any discernible phenotypic effects but when coupled to a phenotypically silent mutation in the TATA box gave rise to viruses with growth defects that were attributable to a significantly lowered rate of transcription initiation from the MLP. These results suggest that the INR plays a role in vivo and can act as an alternative basal element in the absence of a functioning TATA box. A virus with mutations in both the INR and the UPE, although viable, likewise had a severe deficiency in transcription, suggesting that the function of the INR is affected by that of the UPE. This contrasts with the previous report that a TATA box-UPE double mutation is not recoverable in virus. In addition, the virus with mutations in both the INR and the inverted CAAT box was phenotypically wild type, unlike the previously described TATA box-CAAT box double mutant, which had a severe transcription deficiency. Taken together, the present and previous genetic results can be interpreted as evidence that in the MLP, the TATA box and the UPE are the more important of the two basal and activating elements, respectively, but that the INR and CAAT can function in transcription initiation. We consider the role of the INR in the formation of the preinitiation complex and speculate on possible protein-protein interactions.

Prospective evaluation of fluorine-18 fluorodeoxyclucose positron emission tomography in breast cancer for staging of the axilla related to surgery and immunocytochemistry

The noninvasive staging of axillary lymph nodes for metastases is investigated in patients with breast cancer prior to surgery by positron emission tomography (PET) with fluorine-18-fluoro-2-deoxy-d-glucose (18F-FDG). In 124 patients with newly diagnosed breast cancer, whole-body PET was performed to determine the average differential uptake ratio (DUR) of 18F-FDG in the axillary lymph nodes. Results were correlated with the number of the dissected lymph nodes, size of the primary tumor, tumor type, tumor grade, estrogen and progesterone receptors, DNA ploidy, and the proportion of cells in the synthetic phase of the cell cycle (S-phase). In this prospective study of 124 patients with breast carcinoma, PET correctly categorized all 44 tumor-positive axillary lymph nodes, a sensitivity of 100%. Sixty tumor-negative axillary lymph nodes were negative by PET and 20 tumor-negative axillary lymph nodes were positive by PET. No false-negative PET findings were encountered. A weak correlation was found between DUR and tumor size as well as between DUR and the S-phase of the tumor. In patients with breast carcinoma, 18F-FDG PET can be of value in evaluating axillary lymph nodes for metastatic involvement prior to surgery. It is of particular importance that no false-negative PET findings were encountered, and axillary lymph node dissection might not be necessary in patients without axillary uptake by PET. The DUR of the positive axillary lymph nodes seems to bear a relationship with some of the purported prognostic parameters of the primary tumor.

The melanoma differentiation associated gene mda-7 suppresses cancer cell growth

Cancer is a disease characterized by defects in growth control, and tumor cells often display abnormal patterns of cellular differentiation. The combination of recombinant human fibroblast interferon and the antileukemic agent mezerein corrects these abnormalities in cultured human melanoma cells resulting in irreversible growth arrest and terminal differentiation. Subtraction hybridization identifies a melanoma differentiation associated gene (mda-7) with elevated expression in growth arrested and terminally differentiated human melanoma cells. Colony formation decreases when mda-7 is transfected into human tumor cells of diverse origin and with multiple genetic defects. In contrast, the effects of mda-7 on growth and colony formation in transient transfection assays with normal cells, including human mammary epithelial, human skin fibroblast, and rat embryo fibroblast, is quantitatively less than that found with cancer cells. Tumor cells expressing elevated mda-7 display suppression in monolayer growth and anchorage independence. Infection with a recombinant type 5 adenovirus expressing antisense mda-7 eliminates mda-7 suppression of the in vitro growth and transformed phenotype. The ability of mda-7 to suppress growth in cancer cells not expressing or containing defects in both the retinoblastoma (RB) and p53 genes indicates a lack of involvement of these critical tumor suppressor elements in mediating mda-7-induced growth inhibition. The lack of protein homology of mda-7 with previously described growth suppressing genes and the differential effect of this gene on normal versus cancer cells suggests that mda-7 may represent a new class of cancer growth suppressing genes with antitumor activity.

Conservation of DNA sequence in the predicted major late promoter regions of selected mastadenoviruses

The major late promoter (MLP) of the subgroup C human adenoviruses is a preeminent model for the study of the mechanisms of basal and activated transcription, both in vivo and in vitro. However, while the structure and function of the human virus MLP has been the subject of extensive investigation, the conservation of the various promoter elements among the adenoviruses from different species has not been examined. Conservation of specific elements would strongly suggest the importance and universality of their function. To address this issue, sequences were obtained from cloned DNAs of several representative Mastadenoviridae, mouse adenovirus type 1 (MAV-1), Tupaia adenovirus type 1 (TAV-1), and two bovine adenoviruses of two distinct subgroups, BAV-3 and BAV-7. The results of the sequencing studies showed that the TATA box and an upstream inverted CAAT box are conserved in all species and that the binding site for transcription factor USF is present in all except MAV-1, in which a sequence similar to an Sp1-binding site is present at a similar position. The initiator element (INR) sequence is not well conserved, and only one or other of the two downstream activating elements, DE1 and DE2, is predicted to be present in the nonprimate virus MLP regions. Ribonuclease protection assays on RNA isolated from MAV-1-infected cells late in infection indicated that the predicted MLP is functional, and transcription initiation and splice donor sites were identified. The human virus MLP is embedded in the essential DNA polymerase sequence on the opposite DNA strand. The primary amino acid sequences of the C-terminal regions of the predicted DNA polymerases show strong conservation of sequence motifs observed in replicative polymerases ranging from prokaryotes to mammals, and additional regions of strong conservation among the adenovirus polymerases. Pairwise comparisons between the newly sequenced regions of the polymerases and previously published sequences show that BAV-7 is most dissimilar to all others, while TAV-1 has a greater similarity to the primate sequences than to the others. The sequence data from both strands were also used to construct phylogenetic trees, based on BAV-7 as the outgroup. The trees constructed from the two sets of sequences are broadly similar, showing close relationships between primate viruses, but differing in the order of divergence of TAV-1 and MAV-1 branches.

Increased regional cerebral glucose metabolism and semantic memory performance in Alzheimer's disease: a pilot double blind transdermal nicotine positron emission tomography study

Nicotinic receptor dysfunction and impaired semantic memory occur early in Alzheimer's disease patients (AD). Previous research implied that nicotine's ability to enhance alertness, arousal, and cognition in a number of nonclinical populations was a function of its ability to stimulate CNS nicotinic cholinergic receptors. In this study it was hypothesized that transdermal administration of nicotine would increase both regional cerebral glucose metabolism (rCMRglc) and semantic memory (as assessed by verbal fluency). Two mild AD and two elderly controls underwent positron emission tomography scanning during a double blind nicotinic agonist verbal fluency challenge procedure. rCMRglc increases occurred in both AD patients, but not controls. In the two AD patients, verbal fluency scores increased by an average of 17%. One elderly control's verbal fluency increased, and the other decreased. These findings suggest that nicotine's effect on metabolism and verbal fluency is due to its ability to stimulate the cholinergic system.

Implementation of multiple isocentre treatment for dynamic radiosurgery

Radiosurgery using the dynamic rotation technique with a single isocentre was introduced at the Toronto-Bayview Regional Cancer Centre (T-BRCC) in 1988. Since then, over 100 patients have been treated. It was soon recognized that 25-30% of patients were referred with either non-spherical lesions or multiple lesions located sufficiently close together that consideration had to be given to the overlapping dose distributions throughout the treated volume. To treat these more complex targets a multiple isocentre technique was developed which also took account of these effects and the resulting normalization problem. This technique was implemented in September 1992. Comparisons between calculated doses and actual doses delivered have been undertaken using a spherical phantom containing radiochromic film. Measured dose distributions agreed with the planned distributions to within +/- 1 mm. The effect of multiple isocentres on the penumbra of dose distributions has been examined. The methods adopted for the normalization of treatment plans and clinical examples illustrating the application of the multiple isocentre technique are presented.

A modified single-strand annealing model best explains the joining of DNA double-strand breaks mammalian cells and cell extracts

The joining of DNA double-strand breaks in vivo is frequently accompanied by the loss of a few nucleotides at the junction between the interacting partners. In vitro systems mimic this loss and, on detailed analysis, have suggested two models for the mechanism of end-joining. One invokes the use of extensive homologous side-by-side alignment of the partners prior to joining, while the other proposes the use of small regions of homology located at or near the terminus of the interacting molecules. to discriminate between these two models, assays were conducted both in vitro and in vivo with specially designed substrates. In vitro, molecules with limited terminal homology were capable of joining, but analysis of the junctions suggested that the mechanism employed the limited homology available. In vivo, the substrates with no extensive homology end-joined with equal efficiency to those with extensive homology in two different topological arrangements. Taken together, these results suggest that extensive homology is not a prerequisite for efficient end-joining, but that small homologies close to the terminus are used preferentially, as predicted by the modified single-strand annealing model.

Sequence of the mouse adenovirus serotype-1 DNA encoding the precursor to capsid protein VI

The nucleotide sequence predicted to encode the precursor to virion structural protein VI (preVI) of mouse adenovirus (Ad) serotype-1 (MAV-1) was determined. The 237-amino-acid sequence has 45% identity and 66% similarity to the human Ad serotype-2 preVI sequence. There is a marked conservation at the C terminus, the last eleven residues of which may be necessary for activating the Ad endoproteinase, and at the N terminus, including the consensus endoproteinase cleavage site.

Frameless stereotaxy for radiosurgical planning and follow-up

In our centre, 111 patients have been treated with linear accelerator stereotactic radiosurgery. Angiographic, CT and MRI images are generated and the target coordinates calculated in 3 dimensions. For CT scanning, cross sections of perpendicular and oblique fiducial markers are seen. For follow-up CT scans done without the frame, a virtual frame is generated by means of a computer program that places fiducial markers on each CT scan cut, as if the patient had been wearing the OBT frame and the scan produced with the gantry parallel to the base of the frame. The position of the oblique marker may be calculated by knowing the thickness and position of each CT cut. Various natural fiducial markers (bony landmarks) are identified by coordinates in the scan with the patient wearing the real frame and in the scan with the virtual frame applied. A transformation matrix is utilized to establish the equivalence between the original CT scan with the real frame applied and subsequent scans without the real frame but with the virtual frame applied. In effect, the virtual frame is re-applied in exactly the same position as the real frame. Lesion measurements may then be duplicated and growth or regression accurately established. The uncertainty in this system of re-application resides in possible patient movement, CT scan slice thickness and inter-observer error in the identification of natural fiducial markers.