Myo/Nog Cells Migrate to Areas of Cell Death and are Phagocytic in a Murine Model of Retinitis Pigmentosa
Authors: Helm Courtney T. , Souza Rachel , Serpico Scott , Martin Mark , Sugarman Eric , Font Carlos , Crowley Diana , Brahmbhatt Rushil , Tsai E‐Jine , Coughlan Sarah , Woodruff Mary , Lecker Paul , Gorski Grzegorz , Benalcazar John , Gugerty Lindsay , Gerhart Jacqueline , George‐Weinstein Mindy , Bravo Nuevo Arturo 
Published: May 2022

Introduction

Retinitis Pigmentosa (RP) is a set of diseases that leads to progressive visual impairment, affecting over one million people worldwide. Despite the heterogeneity of the disease course and mechanism, the various forms of RP all involve progressive loss of retinal rod and cone cells. Myo/Nog cells have been shown to play a crucial role in ocular development. They have been implicated in neuroprotection in response to acute injuries and stress. In this study, we examined the behavior of Myo/Nog cells in a murine model of congenital retinitis pigmentosa (RP), determined the anatomical location of these cells as the degeneration progresses, and assessed their possible role of clearing out the debris caused by the degeneration.

Methods

C3H/Hej (C3H) mice, which are homozygous for the cGMP phosphodiesterase 6B (PDE6) mutation (also known as the rd1 mutation), were used as the animal model of RP in this study. C3H and C57Bl/6J (C57) control mice were assessed at weeks 2.5, 3, 4, 5, and 6 using scotopic electroretinographs (ERG), ocular computer topography (OCT), and immunofluorescence microscopy. Enucleated eyes were fixed in paraformaldehyde and cryo‐sectioned. Sections were incubated in TUNEL reagent to detect and quantify cell death. Cryosections were labeled with a Myo/Nog specific monoclonal antibody to brain‐specific angiogenesis inhibitor (BAI1). Myo/Nog cell counts were performed by identifying the total number of BAI1+ cells, TUNEL+ cells, and the number of cells that were positive for both BAI1 and TUNEL. Myo/Nog cell counts, photoreceptor counts, outer nuclear layer (ONL), and inner nuclear layer (INL) thicknesses were all obtained using NIS elements software on a FluorScope microscope.

Results

ERG showed decreased amplitudes of the A and B waves in C3H mice when compared to the C57 group. OCTs demonstrated a significant difference in retinal thickness between the two groups. Progressive thinning of the retina in the C3H mice was confirmed by microscopy. We observed early degeneration of the retina of C3H mice in the central and mid‐peripheral areas before week 2.5, which then progressed more significantly in the edge of the retina in later weeks. Myo/Nog cells were significantly more numerous in C3H mice compared to C57 mice at all time points. Most Myo/Nog cells of C3H mice were found in the outer nuclear layer (ONL) and within the neighboring choroid. Data obtained from ERG and histological demonstrated progressive photoreceptor loss over time, and a rapid decline in ONL/INL thickness between weeks 2.5‐4 in C3H mice. Myo/Nog cells appeared to have phagocytosed cell corpses as evidenced by the overlap in Myo/Nog cell labeling with the labeling of cellular debris (the depleted photoreceptors).

Conclusions

These findings are consistent with the behaviors of Myo/Nog cells in other forms of retinal stress and injury. Future studies will examine the importance of Myo/Nog cells in clearance in retinopathies.