D.P. Shepherd

GSK-3beta inhibition reverses axonal transport defects and behavioural phenotypes in Drosophila

The tauopathies are a group of disorders characterised by aggregation of the microtubule-associated protein tau and include Alzheimers disease (AD) and the fronto-temporal dementias (FTD). We have used Drosophila to analyse how tau abnormalities cause neurodegeneration. By selectively co-expressing wild-type human tau (0N3R isoform) and a GFP vesicle marker in motorneurons, we examined the consequences of tau overexpression on axonal transport in vivo. The results show that overexpression of tau disrupts axonal transport causing vesicle aggregation and this is associated with loss of locomotor function. All these effects occur without neuron death. Co-expression of constitutively active glycogen-synthase kinase-3β (GSK-3β) enhances and two GSK-3β inhibitors, lithium and AR-A014418, reverse both the axon transport and locomotor phenotypes, suggesting that the pathological effects of tau are phosphorylation dependent. These data show that tau abnormalities significantly disrupt neuronal function, in a phosphorylation-dependent manner, before the classical pathological hallmarks are evident and also suggest that the inhibition of GSK-3β might have potential therapeutic benefits in tauopathies.

Compact diode-pumped passively Q-switched tunable Er-Yb double-clad fiber laser

Efficient repetitive passive Q switching of a cladding-pumped Er-Yb fiber laser has been demonstrated by use of an external-cavity configuration containing a Co(2+): ZnS crystal as a saturable absorber. Energies of as much as 60muJ in pulses of durations as short as 3.5 ns (FWHM), corresponding to a peak power of >10kW, have been generated, and the maximum slope efficiency with respect to the absorbed pump power was 13%. Using a bulk diffraction grating in the Littrow configuration to provide wavelength-selective feedback, we tuned the passively Q -switched fiber laser over 31 nm from 1532 to 1563 nm. The prospects for further improvement in performance are discussed.

Developmental architecture of adult-specific lineages in the ventral CNS of Drosophila

In Drosophila most thoracic neuroblasts have two neurogenic periods: an initial brief period during embryogenesis and a second prolonged phase during larval growth. This study focuses on the adult-specific neurons that are born primarily during the second phase of neurogenesis. The fasciculated neurites arising from each cluster of adult-specific neurons express the cell-adhesion protein Neurotactin and they make a complex scaffold of neurite bundles within the thoracic neuropils. Using MARCM clones, we identified the 24 lineages that make up the scaffold of a thoracic hemineuromere. Unlike the early-born neurons that are strikingly diverse in both form and function, the adult specific cells in a given lineage are remarkably similar and typically project to only one or two initial targets, which appear to be the bundled neurites from other lineages. Correlated changes in the contacts between the lineages in different segments suggest that these initial contacts have functional significance in terms of future synaptic partners. This paper provides an overall view of the initial connections that eventually lead to the complex connectivity of the bulk of the thoracic neurons.

Tau and tau reporters disrupt central projections of sensory neurons in Drosophila.

In this paper, the authors report that the expression of tau‐based reporter genes causes severe defects in the morphology of sensory neurons in adult Drosophila. Targeted expression of tau‐green fluorescent protein (tau‐GFP) in sensory neurons, using the galactosidase‐4 (GAL4) system, produced a range of characteristic defects in expressing neurons. The defects observed included loss of axons, abnormal axon bundling, reduced sensory arborisations, and axonal swellings (beads). Blind comparisons of adult sensory neurons labelled with tau‐GFP or CD8‐GFP showed that tau‐GFP neurons exhibited many more defects than CD8‐GFP‐expressing neurons. CD8‐GFP was found to induce no significant defects on sensory neuron morphology. Expression of tau‐lacZ and human tau in sensory neurons produced defects comparable to those seen with tau‐GFP. A developmental study showed that tau‐expressing axons grow normally and innervate the correct regions of the neuropil. The absence of these axons later in development suggests that tau‐expressing axons are lost after initial ingrowth. Examination of silver‐stained sections suggests that the absence of axons is due to axon loss rather than failure of the expression system to label the neurons. The results suggest that the expression of tau‐based reporter constructs causes severe defects in sensory neurons, resulting in degeneration. The results also indicate that Drosophila may provide a useful model system for examining the role of tau in neurodegenerative disorders. J. Comp. Neurol. 428:630–640, 2000.

High-inversion densities in Nd:YAG-upconversion and bleaching

We report on the investigation of upconversion in Nd:YAG and its implications for intensely pumped devices. Analysis of lifetime measurements and the performance of a 1 at.% Nd-doped YAG amplifier give an Auger upconversion rate of 7/spl times/10/sup 3/ s/sup -1/. This is significantly smaller than previously reported, but modeling of the performance of Nd:YAG devices with high-inversion densities shows that even this rate can still seriously degrade the small-signal gain and significantly increase the thermal load. The variation of cross-relaxation and upconversion rates with doping level is also described. Finally, it is found that the effect of bleaching of the Nd:YAG absorption can lead to a reduced spatial overlap between the signal and inversion profiles and thus can also significantly reduce the gain.

Over-expression of tau results in defective synaptic transmission in Drosophila neuromuscular junctions.

We have shown that over-expression of human tau (0N3R) in Drosophila larval motor neurons causes significant morphological and functional disruption to the neuromuscular junctions (NMJs). Tau-expressing NMJs are reduced in size with irregular and abnormal bouton structure. Immunocytochemical analysis shows that the abnormal NMJs still retain synaptotagmin expression and form active zones. Functionally, the NMJs exhibit abnormal endo/exocytosis as revealed by incorporation of the styryl dye FM1-43. Electrophysiological studies showed that with low frequency stimulation (1 Hz), evoked synaptic potentials produced from tau over-expressing motor neurons were indistinguishable from wild type, however, following high frequency stimulation (50 Hz), evoked synaptic potentials were significantly decreased. Analysis of the number and distribution of mitochondria showed that motor neurons over-expressing tau had a significant reduction in functional mitochondria in the presynaptic terminal. Collapsing the mitochondrial membrane potential in wild type larvae phenocopied the effects of tau over-expression on synaptic transmission. Our results demonstrate that tau over-expression in vivo cause a synaptic dysfunction, which may be caused by a reduced complement of functional mitochondria.

Growth and low-threshold laser oscillation of an epitaxially grown Nd:YAG waveguide

We report 1.064-microm laser operation of an epitaxially grown Nd: YAG planar waveguide with thresholds as low as ~0.7 mW when high-reflectivity mirrors are used. The output is single mode and, when a 83% reflectivity output coupler is used, has a diode pumped slope efficiency of ~40%. Output powers in excess of 60 mW have been obtained when pumping with a Rhodamine 6G dye laser.

Soluble hyper-phosphorylated tau causes microtubule breakdown and functionally compromises normal tau in vivo

It has been hypothesised that tau protein, when hyper-phosphorylated as in Alzheimer’s disease (AD), does not bind effectively to microtubules and is no longer able to stabilise them; thus microtubules break down, and axonal transport can no longer proceed efficiently in affected brain regions in AD and related tauopathies (tau-microtubule hypothesis). We have used Drosophila models of tauopathy to test all components of this hypothesis in vivo. We have previously shown that upon expression of human 0N3R tau in Drosophila motor neurons it becomes highly phosphorylated, resulting in disruptions to both axonal transport and synaptic function which culminate in behavioural phenotypes. We now show that the mechanism by which the human tau mediates these effects is twofold: first, as predicted by the tau-microtubule hypothesis, the highly phosphorylated tau exhibits significantly reduced binding to microtubules; and second, it participates in a pathogenic interaction with the endogenous normal Drosophila tau and sequesters it away from microtubules. This causes disruption of the microtubular cytoskeleton as evidenced by a reduction in the numbers of intact correctly-aligned microtubules and the appearance of microtubules that are not correctly oriented within the axon. These deleterious effects of human tau are phosphorylation dependent because treatment with LiCl to suppress tau phosphorylation increases microtubule binding of both human and Drosophila tau and restores cytoskeletal integrity. Notably, all these phospho-tau-mediated phenotypes occur in the absence of tau filament/neurofibrillary tangle formation or neuronal death, and may thus constitute the mechanism by which hyper-phosphorylated tau disrupts neuronal function and contributes to cognitive impairment prior to neuronal death in the early stages of tauopathies.

High-power planar dielectric waveguide lasers

The advantages and potential hazards of using a planar waveguide as the host in a high-power diode-pumped laser system are described. The techniques discussed include the use of proximity-coupled diodes, double-clad waveguides, unstable resonators, tapers, and integrated passive Q switches. Laser devices are described based on Yb3+-, Nd3+-, and Tm3+-doped YAG, and monolithic and highly compact waveguide lasers with outputs greater than 10 W are demonstrated. The prospects for scaling to the 100 W level and for further integration of devices for added functionality in a monolithic laser system are discussed.

Laser operation of an Nd:Gd3Ga5O12 thin‐film optical waveguide fabricated by pulsed laser deposition

We report the laser operation of a thin-film waveguide structure grown by the pulsed laser deposition technique. A 2.7-µm-thick crystalline film of neodymium doped Gd3Ga5O12 (Nd:GGG) lases at a wavelength centered at 1.06µm when pumped by a Ti:sapphire laser at 808 nm.