Neuropsin-expressing cells in the retina affect melanopsin expression and response to light

Abstract

Purpose : Neuropsin (OPN5) is a non-visual opsin associated with various functions in the retina including regulating the postnatal hyaloid vessel regression and the entrainment of the retinal clock by light. Regulation of these functions was thought to be due to the non-visual opsin melanopsin (OPN4) expressed in mammals’ retinal ganglion cells (RGC). The OPN5 cells and their interaction with the rest of the photoreception system have not been fully characterized. This study aims at characterizing OPN5 cells and evaluating how their absence affects OPN4-expressing cells’ response to light.

Methods : We used the recently generated OPN5-Cre mouse strain and Cre-dependent viruses to express electron and light microscopy reporters, respectively the peroxidase APEX2 and the fluorescent protein tdTomato, to specifically label the retinal cells expressing OPN5 in adult retinas. We then used serial blockface electron microscopy to reconstruct OPN5-expressing cells and light microscopy to estimate the number of cells expressing OPN5 and OPN4.
In parallel, we recorded on a multielectrode array the retinal electrical response to light (405 and 465 nm at various light intensities from 10¹⁰ to 10¹⁵ photons/cm^2/s) of mice deficient in either OPN4 or OPN5 to establish the specific response of both opsins and the consequences of their absence.

Results : We have been able to fully reconstruct OPN5-cells in 3D showing 3 different patterns of dendritic arborization. Their dendrites extended to either the ON sublayer of the inner plexiform layer, the OFF sublayer or both ON and OFF sublayers. In addition, dendrites of OPN5 RGCs cover an average of 42% of the retina surface. These cells do not co-express OPN5 and OPN4 and have distinct dendritic arborization. We however observed a reduced number of OPN4-expressing cells in the retina of mice deficient for OPN5.
The absence of OPN5 also leads to a reduction of the maximal discharge rate compared to WT retinas in response to light stimulations but does not affect the total duration of the response. We did not detect an electrical response to UV light specific to OPN5-expressing cells, even at high irradiance.

Conclusions : OPN5 is not expressed in a specific subtype of RGC but in a small number of various subtypes without overlapping OPN4-expressing subtypes. However, the absence of OPN5 leads to a decreased number of OPN4-expressing cells and a reduced electrical response to light.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.