Microphotometric, ultrastructural, and electrophysiological analyses of light-dependent processes on visual receptors in white-eyed wild-type and norpA (noreceptor potential) mutant Drosophila

Abstract

We examined a white-eyed strain of the norpA mutant (norpA;cn bw) and white (w)norpA+ controls using microspectrophotometry (MSP), electron microscopy (EM), and electroretinography (ERG). These studies revealed that light mediates receptor demise in norpA even though norpA lacks phototransduction. Rhodopsin and the rhabdomere which houses it decrease with increasing age in norpA but not in w with rearing on a 12 h light/12-h dark cycle or in constant light. At higher temperature in norpA;cn bw and w reared in constant light, visual pigment decreases, rhabdomeres diminish, and cells die. Importantly, dark rearing blocked visual pigment loss in norpA;cn bw; the M-potential, an ERG reflection of visual pigment level, corroborated this finding. MSP showed that norpA's visual pigment loss was not due to acute loss of metarhodopsin, rhodopsin's photoproduct. NorpA blocks certain processes expected to be light elicited. The alteration of visual pigment as a function of time of day, present in w controls, is absent in white-eyed norpA, suggesting that light-induced depolarization may be necessary to entrain the rhythm. Microspectrofluorometry using the fluorescent dye, Lucifer yellow, suggested that norpA lacks a light-induced uptake mechanism; using control flies, we determined the stimulus parameters required for uptake in vivo. An attempt to “cure” norpA;cn bw by replacement “therapy” using phospholipase C, missing in norpA's phototransduction cascade, was largely unsuccessful.

Degeneration of photoreceptors in rhodopsin mutants of Drosophila

“…Figure 7( A) shows, in addition, that the crosssectional area of wild-type R1-6 rhabdomeres increases considerably (by about 38% in this sample) between 0 day and 1 week posteclosion. This finding may be the structural basis of the persistent finding in this laboratory (N. Scavarda, J. O'Tousa, L. L. Randall, unpublished data) and elsewhere (Zinkl et al, 1990) that the amount of R 1-6 rhodopsin increases substantially during the first week after eclosion in wild-type flies. The observations thus suggest that R 1-6 rhabdomeres continue to develop during this period.…”

Accumulation of calcium in degenerating photoreceptors of severalDrosophilamutants

“…135 mmole/1 calcium, on the average, in light-raised rdgC retinae), which is about two orders of magnitude above the calcium content usually found in normal cells, reflects an active process of calcium accumulation. This is consistent with other observations indicating viability of the degenerating cells: (1) In a recent study when 10-day-old norpA flies were raised at 28°C under light-dark cycle regime, Stark and colleagues (Zinkl et al, 1990) concluded that "rhabdomeres are reduced, but cells survive and show some aspects of normal membrane turnover." (2) In 7-day-old light-raised rdgB flies, calcium spikes are recorded (Rubinstein et al, l9S9a,b;Minke et al, 1990;Sahly et al, 1992) indicating the existence of functional plasma membrane.…”

“…Although the defective gene products have been identified, it is not clear in any of these mutants how the defective proteins lead to retinal degeneration. In the retinal degeneration C mutant (rdgC, Steele & OTousa, 1990;Steele et al, 1992), neither inactivation nor afterpotential C mutant (ninaC, Pak, 1979;Matsumoto et al, 1987;, Porter et al, 1992Porter & Montell, 1993) and the no receptor potential A (norpA) mutant (Bloomquist et al, 1988;Ostroy, 1978;Meyertholen et al, 1987;Zinkl et al, 1990) the photoreceptors show normal (or close to normal e.g. norpA, Pak, 1975) morphology in young, dark-raised mutants while a few days of intense illumination induce degeneration (Steele & Reprint requests to: Dr. Baruch Minke, Department of Physiology, The Hebrew University-Hadassah Medical School, Jerusalem 91120, Israel.…”

Mutation of a TADR Protein Leads to Rhodopsin and Gq-Dependent Retinal Degeneration inDrosophila

“…Similar to those in humans, fly mutations in phototransduction molecules including rhodopsin (Leonard et al, 1992; Kurada and O'Tousa, 1995; Iakhine et al, 2004), PLC (Meyertholen et al, 1987; Zinkl et al, 1990; Alloway et al, 2000), TRP (Hong et al, 2002; Wang et al, 2005) and arrestins (Dolph et al, 1993; Satoh and Ready, 2005) all cause age-dependent photoreceptor degenerations, which are generally characterized by diminished rhabdomeres. Several other visual proteins such as a diacylgrycerol kinase RDGA and a rhodopsin phosphatase RDGC are also essential for photoreceptor protection (Masai et al, 1993; Kiselev et al, 2000).…”