Light-Induced Length Shrinkage of Rod Photoreceptor Outer Segments

Abstract

PurposeThis study was designed to verify light-induced outer segment (OS) length shrinkage of rod photoreceptors and to characterize its anatomic source at disc-level resolution.MethodsFrog (Rana pipiens) retinas were used for this study. Time-lapse light microscopy of freshly isolated OSs was employed to test transient rod OS changes at 10 ms temporal resolution. Histological light microscopy of dark- and light-adapted retinas was used to confirm light-induced rod OS length changes; and transmission electron microscopy (TEM) was used to quantify light-driven structural perturbation of rod OSs at disc level resolution.ResultsTime-lapse light microscopy images verified transient length shrinking responses in freshly isolated rod OSs. Histological light microscopy images confirmed reduced rod OS lengths in light-adapted retinas, compared to that of dark-adapted retinas. TEM images disclosed shortened inter-disc distances in light-adapted retinas compared to dark-adapted retinas.ConclusionsLight-induced rod OS length shrinkage was confirmed using time-lapse light microscopy of isolated rod OSs and histological light microscopy of dark- and light-adapted retinas. TEM revealed that the rod OS length shrinkage was correlated to the light-driven decrease of the space between individual discs, not the disc thickness itself.Translational RelevanceLight-induced transient rod response promises a noninvasive biomarker for early diagnosis of age-related macular degeneration and retinitis pigmentosa, in which the rod photoreceptors are known to be more vulnerable than cone photoreceptors.

Mechanosensitivity is an essential component of phototransduction in vertebrate rods

“…In agreement with previous observations [4,24], we confirm that bright flashes of light elicit a transient shortening of the rod OS. These flash-induced movements begin with a delay around 50 ms for a flash of about 2,500 R � /rod (Fig 3).…”

“…Given that Xenopus rods respond to mechanical stimuli, we decided to test whether they also exhibited changes in OS length upon illumination, such as the shrinkage of about 0.4-0.6 μm reported by Lu and colleagues [4,24]. We chose to use OTs because of their high sensitivity, on the order of 1-10 nm, and rapid temporal resolution, in the millisecond range [20]; see also Methods and S4 Fig.…”

“…Photoreceptors are thought to be specialized cells devoted to the transduction of the incoming visual signals. Following strong illumination, rod outer segments (OSs) from mice [1] and fly photoreceptors [2,3] have been reported to increase their length and contract respectively, but rod OSs from frogs shrink their length by about 0.4-0.6 μm [4]. In addition, rod photoreceptors are known to shed old disks from their tip and to synthesize new disks at the base of the OS.…”

Fast intrinsic optical signal correlates with activation phase of phototransduction in retinal photoreceptors

“…63,64 A recent phase-sensitive OCT study has revealed transient reduction of OPL of photoreceptor OSs, 61 which is consistent with the vertical shrinkage of photoreceptor OS. 44 In addition, both the fast photoreceptor-IOS and OS shrinkage occur immediately following the visible light stimulation, [31][32][33] which is much faster than ERG a-wave generation. 45,53,54 These facts appear to provide evidence that the fast IOS and Figure 5, substitution of low-sodium medium did not substantially affect either the time course or the peak amplitude of the fast OS change.…”

On mechanisms of light-induced deformations in photoreceptors

“…Driving this tension change with the known time course of the ERP (Figure 1, [2]), we can calculate the area expansion over time for this quasi-static model. To account for the slow swelling attributed to osmotic changes during phototransduction, as well as any tension change from the late receptor potential, we also include a term linearly increasing with time [12], [25]. As shown in Supplementary Figure 1, this quasi-static model provides a poor fit to OPL changes measured in the living human eye.…”