Disk membrane initiation and insertion are not required for axial disk displacement in Xenopus laevis rod outer segments

Abstract

PURPOSE. Mechanisms that maintain the close coupling between the formation of photoreceptor disk membranes and the displacement of disk membranes toward the pigment epithelium are poorly understood. This study was designed to determine whether the axial displacement of disk membranes requires the assembly and insertion of new disk lamellae. METHODS. Retinal detachment and treatment with cytochalasin D were employed to interrupt the normal formation of disk membranes in cultured Xenopus laevis retinas. The effect of disrupting disk initiation and assembly upon disk displacement was documented and quantified. RESULTS. Isolating retinas from the retinal pigment epithelium prevented the normal morphogenesis of disks, but previously formed disks moved distally at a rate that is greater than or equal to the rate in attached retinas or in vivo. Treatment of attached retinas in eyecups with cytochalasin D blocked initiation of new disks and resulted in the formation of ectopic, disk-like membranes, but it did not stop axial displacement of previously formed disks. Rod cells in retinas that were cultured while slightly elevated from the retinal pigment epithelium sometimes formed disks of a smaller diameter than normal, even though the rate of initiation and displacement of disks was the same as in vivo. CONCLUSIONS. Observations on detached retinas and or retinas treated with cytochalasin D suggest that disk displacement does not depend upon normal disk formation and that the motive mechanism does not involve filamentous actin. The formation of small diameter disks in elevated retinas suggests that disk initiation and displacement is independent of the completion of normal diameter disks.