Photoreceptor degeneration, second order neuron remodeling and glia reactivity in an in vivo and in vitro model of retinal neurodegeneration

Abstract: Photoreceptors have the ability to last during the entire lifespan of an individual. Being the first line of neurons in the visual transduction pathway, their health and maintenance is eminent for proper retinal function. However, photoreceptors are susceptible to neurodegenerative retinal dystrophies. A number of retinal pathologies such as retinitis pigmentosa, age-related amacular degeneration and diabetic retinopathy have been linked to photoreceptor death. Moreover, photoreceptor degeneration has been shown to affect downstream inner nuclear layer cells as well as induce reactive responses from Müller cells and microglia. Since current treatments are ineffective in preventing the degeneration of these neurons, intense research is still underway to discover novel treatment modalities. In this thesis, photoreceptor degeneration was assessed in an in vivo and in vitro model of neurodegeneration. Moreover, a possible mode of preserving these neurons by the use of human neural progenitor cells (hNPCs) was investigated. The in vivo pdgf-bret/ret (platelet derived growth factor-b retention motif knockout) mouse model, which shows severe vascular pathology as a result of detachment of pericytes from the vascular endothelium, was studied during the first postnatal month. In a short time span, i.e. between postnatal day (P)10 and P15, retinopathic features were observed. Photoreceptor degeneration related to cell death, cone outer segment (OS) shortening and synapse disassembly in the outer plexiform layer (OPL) was seen. The second order rod bipolar cells underwent remodeling and the Müller cells became gliotic with increased expression of GFAP (glial fibrillary acidic protein). Microglial cells were also observed to convert to their reactive amoeboid-like phenotype. These features seemed to become more severe in the older P28 mutants. In the in vitro porcine retinal explant model, photoreceptor death significantly increased by 3 days in vitro (div). This was associated with loss of cone OSs, opsin mislocalization and loss of synaptic integrity in the OPL. Horizontal cell death and remodeling was also observed together with a severe gliotic response from the Müller cells. Human neural progenitor cell cocultured explants for 3 div had the ability to preserve photoreceptor survival by means of OS conservation, better opsin trafficking and maintaining synaptic integrity. However, Müller cell gliosis was only mitigated by a decreased density of GFAP immunoreactive Müller cells. In conclusion, both the in vivo and in vitro model of neurodegeneration demonstrate the vulnerability of photoreceptors to various mechanisms of retinal injury. Interestingly, hNPC derived neurotrophic factors had neuroprotective qualities in 3 div porcine retinal explants.