Alterations to retinal architecture prior to photoreceptor loss in a mouse model of retinitis pigmentosa

Abstract

Mouse models of retinitis pigmentosa (RP) are essential tools in the pursuit to understand fully what cell types and processes underlie the degeneration observed in RP. Knowledge of these processes is required if we are to develop successful therapies to treat this currently incurable disease. We have used the rd10 mouse model of RP to study retinal morphology prior to photoreceptor loss, using immunohistochemistry and confocal microscopy on cryosections, since little is known about how the mutation affects the retina during this period. We report novel findings that the mutation in the rd10 mouse results in retinal abnormalities earlier than was previously thought. Defects in rod and cone outer segments, bipolar cells, amacrine cells and photoreceptor synapses were apparent in the retina during early stages of postnatal retinal development and prior to the loss of photoreceptors. Additionally, we observed a dramatic response of glial cells during this period. Microglia responded as early as postnatal day (P) 5; ~13 days before any photoreceptor loss is detected with Müller glia and astrocytes exhibiting changes from P10 and P15 respectively. Overall, these findings present pathological aspects to the postnatal development of the rd10 retina, contributing significantly to our understanding of disease onset and progression in the rd10 mouse and provide a valuable resource for the study of retinal dystrophies.

Progesterone Attenuates Microglial-Driven Retinal Degeneration and Stimulates Protective Fractalkine-CX3CR1 Signaling

“…Norgestrel-supplemented rd10’s demonstrated lower levels of TUNEL positivity in the central outer nuclear layer (ONL) at P15 and P20 compared to control (Fig 1A), coinciding with significant preservation of photoreceptors. As expected, control rd10 mice revealed a substantial loss of photoreceptors between P15 and P25 (Fig 1 & S1 Fig) [9]. At P20, ONL thickness in the Norgestrel-fed rd10 mice was comparable to age-matched C57 retina (S1 Fig).…”

“…We sought to evaluate the protection offered by Norgestrel when using a less invasive method of diet-supplementation. We also recognized a need to administer Norgestrel at an earlier age based on recent findings that the rd10 retina displays significant abnormalities prior to photoreceptor cell loss [9]. Dams of newborn rd10s were given a Norgestrel-supplemented diet when their pups reached post-natal day 10 (P10), and all rd10s continued to receive a Norgestrel-supplemented diet post-weaning.…”

“…P16 was chosen as the time-point to harvest primary cells. Microglia respond and migrate to the mutated photoreceptors becoming activated by P15 in the rd10 retina compared to C57 control [9]. This time-point therefore represents a time when microglia have responded to the mutated photoreceptors but photoreceptor cell death has not yet taken place.…”

“…The rd10 mouse model of RP possesses a mutation in the phosphodiesterase-6b ( pde6b) gene [2–4]. A suitable model for studying cell death in RP, the course of photoreceptor cell loss and subsequent retinal degeneration in this mouse closely resembles disease progression in humans [5–9]. In 2011, our group showed that the synthetic progestin ‘Norgestrel’ works as a neuroprotective agent in the retina, thus identifying it as a potential treatment for RP [10].…”

“…Previous studies have highlighted a detrimental role for microglia as drivers of retinal cell degeneration [18–21]. In the rd10 mouse, microglia respond to the mutation as early as P5 and are found in close association with the photoreceptors during the period of initial cell loss [9]. Other studies have shown that when microglial cells are either genetically ablated, their phagocytic ability inhibited [18], or their pro-inflammatory activities dampened by anti-inflammatory drug administration [19], disease progression is attenuated in the rd10 retina.…”