Extra Photoreceptors In PVmd2-ngn3 Transgenic Mice

Abstract

Purpose: : This study investigates an unconventional approach to generate differentiating photoreceptors - reprogramming RPE cells with a pro-photoreceptor gene to channel RPE’s capabilities of proliferation and plasticity towards photoreceptor production.

Methods: : The underlying theme of the approach is to use Ngn3, a transcription factor in a bHLH hierarchy regulating early retinal development, to initiate photoreceptor differentiation program in the RPE. Transgenic mice, in which Ngn3 is under the control of Betrophin-1 promoter (P_VMD2) were created. The eyes of transgenic mice were analyzed with histology and immunohistology.

Results: : Transgenic mouse eyes contained extra outer nuclear layer (E-ONL). Photoreceptor cells in the E-ONL and in the regular outer nuclear layer (ONL) were identified with anti-recoverin immunostaining. The E-ONL was localized between the RPE and the ONL, and was often lined by dark pigments (presumably of RPE origin). RPE and choroid were maintained at places where E-ONL was present. The E-ONL appeared noncontiguous, possible reflecting the different sites of origin of the extra photoreceptor cells. E-ONL was observed with 3 independent transgenic lines. Cells in the E-ONL appeared similar to those in the ONL. They developed outer segments, which were discernible with bright-field microscopy and were decorated by anti-red opsin and anti-rhodopsin immunostaining. The thickness of the E-ONL varied, from comparable to the regular ONL to consisting only a single layer of cells. The orientation of cells in the E-ONL also varied. Cells in the E-ONL could correctly orient themselves, in respect to the RPE, with their apices (outer segments) point towards the RPE, as do photoreceptor cells in the ONL. In others, red opsin⁺ photoreceptors and rhodopsin⁺ photoreceptors in the E-ONL displayed orientations different from their counterparts in the ONL, including head-to-head, opposing orientation. Sometimes, extra photoreceptor cells were found in the optic nerve.

Conclusions: : Extra photoreceptor cells, often organized as extra ONL, were present in P_VMD2-ngn3 transgenic mice. Further analyses are needed to demonstrate the cellular source(s) of the extra photoreceptor cells, despite that, according the experiments design, transgene ngn3 expression is predominantly in RPE cells.

Keywords: photoreceptors • regeneration • retinal pigment epithelium