Time course of retinal degeneration associated with the absence of 1, 4, 5-inositol trisphosphate receptor in Drosophila melanogaster

Abstract

The absence of the inositol trisphosphate receptor is associated with a gradual retinal degeneration in Drosophila melanogaster. To characterize the time-course profile of this process, mosaic flies expressing a null allele of the itp gene in the eye were studied by electroretinograms and electronic microscopy. Membrane contour alterations, disrupted mitochondria, altered morphology and even loss of photoreceptors were increased progressively starting 5 d after hatching, were more evident during days 10–15 and promoted highly disorganized structures thereafter. The synaptic transmission and membrane potential of retinal cells were also significantly distorted, showing reduced ON and OFF transients as well as membrane potential from day 10 of hatching, and the functional defects became progressively more severe. Unexpectedly, these alterations were detected not only in the non-pigmented mutant ommatidia, but also in the pigmented ommatidia, including heterozygous and twin clones expressing 1, 4, 5-inositol trisphosphate receptor (IP3R). To explore the mechanism underlying this degenerative process, the progression of pro-oxidant and apoptotic reactions was characterized by immunohistochemical techniques. Mutant ommatidia showed intermittent episodes of increased pro-oxidant reactions (detected as adducts of 4-hydroxy-nonenal) throughout the fly's life. Similarly, several episodes of active caspase 3, an apoptotic effector, were evident with the same time pattern. Episodes of enhanced lipid peroxidation and apoptosis were also observed in the pigmented ommatidia of the mosaic eyes. The results indicate that photoreceptors lacking IP3R suffer episodes of increased lipid peroxidation, which eventually perturb the retinal subcellular organization and disrupt the phototransduction process and cell viability. Pigmented ommatidia also showed a similar pattern of damage, indicating that the degenerative process is non-autonomous and is so intense that it propagated to the non-mutant retinal cells in the mosaic eyes. In conclusion, ommatidia with a null mutation of IP3R degenerate by a process associated with intermittent lipid peroxidation and apoptotic activities.