Birefringent periodicities in amphibian rod outer segments

An inducible expression system to measure rhodopsin transport in transgenic Xenopus rod outer segments

We developed an inducible transgene expression system in Xenopus rod photoreceptors. Using a transgene containing mCherry fused to the carboxyl terminus of rhodopsin (Rho-mCherry), we characterized the displacement of rhodopsin (Rho) from the base to the tip of rod outer segment (OS) membranes. Quantitative confocal imaging of live rods showed very tight regulation of Rho-mCherry expression, with undetectable expression in the absence of dexamethasone (Dex) and an average of 16.5 µM of Rho-mCherry peak concentration after induction for several days (equivalent to >150-fold increase). Using repetitive inductions, we found the axial rate of disk displacement to be 1.0 µm/day for tadpoles at 20 °C in a 12 h dark /12 h light lighting cycle. The average distance to peak following Dex addition was 3.2 µm, which is equivalent to ~3 days. Rods treated for longer times showed more variable expression patterns, with most showing a reduction in Rho-mCherry concentration after 3 days. Using a simple model, we find that stochastic variation in transgene expression can account for the shape of the induction response.

Arrestin migrates in photoreceptors in response to light: a study of arrestin localization using an arrestin-GFP fusion protein in transgenic frogs

Subcellular translocation of phototransduction proteins in response to light has previously been detected by immunocytochemistry. This movement is consistent with the hypothesis that migration is part of a basic cellular mechanism regulating photoreceptor sensitivity. In order to monitor the putative migration of arrestin in response to light, we expressed a functional fusion between the signal transduction protein arrestin and green fluorescent protein (GFP) in rod photoreceptors of transgenic Xenopus laevis. In addition to confirming reports that arrestin is translocated, this alternative approach generated unique observations, raising new questions regarding the nature and time scale of migration. Confocal fluorescence microscopy was performed on fixed frozen retinal sections from tadpoles exposed to three different lighting conditions. A consistent pattern of localization emerged in each case. During early light exposure, arrestin-GFP levels diminished in the inner segments (ISs) and simultaneously increased in the outer segments (OSs), initially at the base and eventually at the distal tips as time progressed. Arrestin-GFP reached the distal tips of the photoreceptors by 45-75 min at which time the ratio of arrestin-GFP fluorescence in the OSs compared to the ISs was maximal. When dark-adaptation was initiated after 45 min of light exposure, arrestin-GFP rapidly re-localized to the ISs and axoneme within 30 min. Curiously, prolonged periods of light exposure also resulted in re-localization of arrestin-GFP. Between 150 and 240 min of light adaptation the arrestin-GFP in the ROS gradually declined until the pattern of arrestin-GFP localization was indistinguishable from that of dark-adapted photoreceptors. This distribution pattern was observed over a wide range of lighting intensity (25-2700 lux). Immunocytochemical analysis of arrestin in wild-type Xenopus retinas gave similar results.

Three-dimensional membrane crystals in amphibian cone outer segments: 2. Crystal type associated with the saddle point regions of cone disks

In light-adapted, perfusion fixed retinas of the Congo eel salamander, Amphiuma, we have observed distinctive 3-D crystalline domains within the axial array of cone outer segment disks. These crystalline domains, each involving 2-12 disks, have been observed in the distal half of cone outer segment, and are associated with saddle point regions and immediately adjacent segments of the cone disk perimeter. In longitudinal sections, the crystals typically display an axially oriented array of cytoplasmic filaments with lateral spacings in the range of 12-13 nm. The width of the intradiskal compartment is expanded to 8-10 nm within the crystal, and approximates the width of the cytoplasmic compartment. In some sections, the cytoplasmic filaments are axially aligned with intradiskal filaments of similar length and width. In transverse sections, the projected lattice appears to be approximately rectangular, with unit cell dimensions of approximately 12 nm x 12.5 nm. In shape, orientation, dimension, location, projection symmetry and associated membrane spacing relationships, the cone outer segment crystal filaments share a strong resemblance with lattice filaments located along the perimeters and preincisures of rod disks. These similarities suggest that the cone crystal filaments may be related to the rim protein of rod disks. Lastly, the preferential association of these crystals with saddle point regions indirectly supports the hypothesis that reductions in COS disk area with apical displacement are accomplished by resorption of disk membrane components through the saddle points.

Thermal control of rod outer segment length and shedding in a fish, Fundulus zebrinus

The effects of temperature on rod outer segment (ROS) length and membrane shedding were studied in a cyprinodont fish, Fundulus zebrinus. After 30 days in 14L/10D cyclic light and 17 degrees C, ROS length averaged 41.2 microns. Fish were then exposed to 7, 17 or 27 degrees C for 10 and 25 days before being sampled 5 hr before and 1-4 hr after light onset. In 7 degrees C ROS shortened to 83.5% of initial controls within 10 days, then only 4.1% further, to 79.4% by day 25 (34.4, 32.7 microns). ROS length did not change significantly in fish remaining at 17 degrees C (39.7 and 40.7 microns at day 10 and 25) or in fish moved to 27 degrees C (41.7 and 41.6 microns). Phagosomes were most numerous in 7 degrees C and least numerous in 17 degrees C, but varied in overall size among the largest phagosomes being more common after light onset. After light onset at day 25, the estimated volume per phagosome was 1.14, 4.73 and 5.75 microns 3 in 7, 17 and 27 degrees C. Total phagosome volume per 100 microns RPE at 27 degrees C was generally double that at 17 degrees C. Apparently, in F. zebrinus, the number of disks shed from ROS is adjusted during thermal acclimation to stabilize ROS length.

Temperature-dependent birefringence patterns in Xenopus rod outer segments

Results reported here show that the birefringence of disk membranes in Xenopus rod photoreceptors depends upon the temperature at which the disks were assembled. Portions of the outer segments produced by the assembly of new disks while frogs are exposed to constant darkness and an ambient temperature of 25 degrees C have a higher birefringence than portions assembled at 18 degrees C. The intensity of birefringence reflects molecular-level structure and macromolecular-level organization of the disk membranes. It has been shown previously that increasing the ambient temperature enhances the rate of assembly of disk membranes in Xenopus rod outer segments. It also has been shown that disk membranes are assembled most frequently in the hours following light onset, and that disks assembled in the light have a lower birefringence than disks assembled in the dark. Therefore, although light and increased temperature both enhance the rate of disk membrane assembly, they have opposite effects upon the birefringence of Xenopus rod outer segments.