Cone–like Morphological, Molecular and Electrophysiological Features of Photoreceptors of the Nrl Knockout Mouse

Abstract

Abstract: : Purpose:To test the hypothesis that Nrl ^–/– photoreceptors are cones, by comparing them with WT rods and cones using morphological, histochemical, molecular and electrophysiological criteria. Methods: The photoreceptor layer of fixed retinal tissue of 4–6 wk old mice was examined in plastic sections by electron microscopy, and by confocal microscopy in frozen sections immunolabeled for the mouse UV–cone pigment, and colabeled with PNA. Single– and paired–flash methods were used to extract the spectral sensitivity, kinetics and amplification of the a–wave of the ERG. Results: The outer segments of Nrl ^–/– photoreceptors (∼ 7 µm) were shorter than those of WT rods (∼ 25 µm) and cones (∼ 15 µm), but like WT cones had 25 or more basal discs open to the extracellular space, extracellular matrix sheaths stained by PNA, chromatin "clumping" in their nuclei, and 2–fold shorter mitochondria than rods. Nrl ^–/– photoreceptors expressed the mouse UV cone pigment, cone transducin and cone arrestin in amounts expected given the relative size and density of cones in the two retinas. The ERG a–wave was used to assay the properties of the photocurrent response. The sensitivity of the Nrl ^–/– a–wave was maximal at 360 with a secondary mode at 510 nm having 1/10th the maximal sensitivity, corresponding to the max’s of the two mouse cone pigments. The time to peak of the dim–flash photocurrent response, extracted with paired–flash ERGs, was ∼ 50 ms, more than 2 times faster than that of rods. Conclusions: Many morphological, molecular and electrophysiological features of Nrl ^–/– are cone–like, and strongly distinguish these cells from rods. This cone–dominant mouse retina provides a model for the investigation of cone function and cone–specific genetic disease.

Keywords: photoreceptors • color pigments and opsins • extracellular matrix