Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, PC221
Understanding the mechanism of retinal degeneration in Drosophila lacking Transient Receptor Potential (TRP) channels
S. Sengupta1, T. R. Barber1, R. C. Hardie1
1. Physiology, Development & Neuroscience, University of Cambridge, Cambridge, United Kingdom.
Drosophila lacking the transient receptor potential (TRP) channels show light dependent retinal degeneration. Using optical neutralization technique (Franceschini and Kirschfeld, 1971) and transmission electron microscopy (TEM), we observed that the retinal degeneration in trp mutants under continuous red light is completely rescued by mutation in phospholipase C (PLC), suggesting an event downstream of PLC activity to be responsible. Severe PI(4,5)P2 depletion has been reported in trp mutants under bright illumination due to lack of Ca2+ dependent inhibition of PLC (Hardie et al., 2001). Thus, we hypothesize that depletion of PI(4,5)P2 could be responsible for the trp retinal degeneration. In agreement with this hypothesis, optical neutralization assay shows that reducing the gene dosage of phosphoinositide (PI) resynthesising enzymes, CDP-DAG synthase (cds) and PI-transfer protein (rdgB), enhanced the extent of trp retinal degeneration observed under 12 hr red light and dark cycle (n>10 flies for each genotype). TEM of the retina of trp mutant flies exposed to red light for different time periods showed complete disruption of the rhabdomere structure. Drosophila moesin (dMoesin), which is required for the maintenance of photoreceptor structure (Karagiosis and Ready, 2004), shows light induced movement from rhabdomere to cystosol (Chorna-Ornan et al., 2005). This movement is dependent on the levels of PI(4,5)P2. The protein in its active phosphorylated state remains associated with the rhabdomere membrane. Upon illumination, which leads to PI(4,5)P2 hydrolysis, it gets dephosphorylated and moves to the cytosol of cell body. Mutations in moesin that impairs phosphorylation of the protein, and thus abolishes its light induced movement, results in light-dependent degeneration of the photoreceptors (Chorna-Ornan et al., 2005). The retina of trp mutants, upon prolonged illumination, showed absence of phophorylated Moesin (p-Moe) from the rhabdomere structure when probed with anti p-Moe antibody (n=3). This effect was rescued by mutating PLC. This finding suggests that, PI(4,5)P2 depletion in the retina of trp mutants exposed to light results in depletion of p-Moe, and possibly, leads to the disruption of the photoreceptor structure causing cell death. These findings support the view that sustained hydrolysis of PI(4,5)P2 by PLC, resulting from lack of Ca2+ influx to the photoreceptor cell, may underlie the observed retinal degeneration in trp flies.
Where applicable, experiments conform with Society ethical requirements