T. Barber

Depletion of PtdIns(4,5)P2 underlies retinal degeneration in Drosophila trp mutants

Summary The prototypical transient receptor potential (TRP) channel is the major light-sensitive, and Ca2+-permeable channel in the microvillar photoreceptors of Drosophila. TRP channels are activated following hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] by the key effector enzyme phospholipase C (PLC). Mutants lacking TRP channels undergo light-dependent retinal degeneration, as a consequence of the reduced Ca2+ influx. It has been proposed that degeneration is caused by defects in the Ca2+-dependent visual pigment cycle, which result in accumulation of toxic phosphorylated metarhodopsin–arrestin complexes (MPP–Arr2). Here we show that two interventions, which prevent accumulation of MPP–Arr2, namely rearing under red light or eliminating the C-terminal rhodopsin phosphorylation sites, failed to rescue degeneration in trp mutants. Instead, degeneration in trp mutants reared under red light was rescued by mutation of PLC. Degeneration correlated closely with the light-induced depletion of PtdIns(4,5)P2 that occurs in trp mutants due to failure of Ca2+-dependent inhibition of PLC. Severe retinal degeneration was also induced in the dark in otherwise wild-type flies by overexpression of a bacterial PtdInsPn phosphatase (SigD) to deplete PtdIns(4,5)P2. In degenerating trp photoreceptors, phosphorylated Moesin, a PtdIns(4,5)P2-regulated membrane–cytoskeleton linker essential for normal microvillar morphology, was found to delocalize from the rhabdomere and there was extensive microvillar actin depolymerisation. The results suggest that compromised light-induced Ca2+ influx, due to loss of TRP channels, leads to PtdIns(4,5)P2 depletion, resulting in dephosphorylation of Moesin, actin depolymerisation and disintegration of photoreceptor structure.

A charge collection study with dedicated RD50 charge multiplication sensors

We investigate the charge collection efficiency of specially designed charge multiplication silicon strip detectors produced by MICRON in Liverpool under the framework of the CERN RD50 collaboration. Charge collection measurements are performed before and after proton and neutron irradiation to fluences of 1×1015 and 5×1015 1 MeV neq/cm2. Charge multiplication structures on these devices include varying diffusion times and energies for the implantation process, different sensor thicknesses, and several different strip width and pitch geometries. The charge collection for the charge multiplication devices is compared to standard silicon strip sensors with no charge multiplication properties.

The data acquisition and calibration system for the ATLAS Semiconductor Tracker

The Semiconductor Tracker (SCT) data acquisition (DAQ) system has been built to calibrate, configure, and control the approximately six million front-end channels of the ATLAS silicon strip detector