Glial cell activation and migration during photoreceptor death in two rat models of inherited retinal degeneration.

Abstract

Purpose : There are many different animal models of retinitis pigmentosa, each bearing a genetic defect responsible for the initiation and progression of the disease. We have examined in control animals and in two animal models with genetic defects in the retinal pigmented epithelium (RPE) or the photoreceptors (PR), the course of retinal degeneration and the changes occurring in retinal glial cells (astrocytes, Müller and microglial cells). We have also analyzed whether the increased microglial cell numbers are due to microglial proliferation.

Methods : Groups (n=4) of female rats of four different strains: homozygous albino P23H-1, pigmented Royal College of Surgeons (RCS), Sprague-Dawley (SD) and pigmented Piebald Virol Glaxo (PVG) rats of different ages (P 10,15, 21, 28, 33, 45 and 60) were processed and their retinas cryostat sectioned, immunoreacted with antibodies against different opsins (S and L/M), rhodopsin, anti-Iba 1 and 4, anti-GFAP and anti-PCNA and stained with DAPI. Automatic photographic reconstructions of 3 complete retinal sections per animal were made (20x) and the number of nuclei rows in the outer nuclear layer (ONL) and of microglial cells in each retinal layer were quantified manually in eight standard areas of each section.

Results : Photoreceptor degeneration starts earlier in P23H-1 rats but proceeds faster in the RCS. Rods degenerate preferentially in P23H-1 rats but remain almost unchanged in the RCS rat at these ages. Increased expression of GFAP starts earlier in P23H-1 rats (P21) than in RCS rats (P45). Both models show increased microglial cell numbers in the retina as a result of increased cell numbers in the ONL, and this happens at younger ages in P23H-1 than in RCS rats. On the contrary, the numbers of microglial cells decrease with age in the retinal ganglion cell layer in both models. Cellular proliferation was only detected in blood vessels but not in microglial cells.

Conclusions : In young P23H-1 and RCS rats, photoreceptor degeneration causes microglial and macroglial activation and microglial migration to the ONL. However, the increased numbers of microglial cells in the ONL cannot be explained only by migration from other retinal layers or microglial proliferation and thus may be result of cellular migration from the subretinal space. Further studies are needed to document this fact.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.