Assessment of neuroprotective treatment strategies to promote photoreceptor survival in a murine model of cone-rod dystrophy and a murine model of retinitis pigmentosa

Abstract

Retinal dystrophies, such as cone and cone-rod-dystrophies (CODs/CORDs) or retinitis pigmentosa (RP) are characterised by the progressive loss of photoreceptors and their functionality. Patients with severely progressed retinal dystrophies display, depending on the disorder, symptoms such as photophobia, night blindness, nystagmus, central scotomas, and reduced or absent colour vision. They often exhibit abnormal electroretinograms (ERGs), indicating the loss of rod or cone functionality. These disorders often progress slowly, leading to visual deterioration and eventually blindness. Only few treatment options are currently examined for the treatment of retinal dystrophies, e.g. utilisation recombinant proteins, encapsulated cell therapy (ECT) or injections of viral particles. Fewer still are approved by drug agencies for use in patients. One of the most regarded forms of treatment is the introduction of cytokines or other small proteins into the vitreous cavity of the eye, often in form of direct intravitreal injections of recombinant proteins. However, due to the chronic and often progressive nature of retinal dystrophies, and fast turn-over of molecules in the vitreous humour, the application of such therapeutics has to be repeated regularly, which can be a burden on both patient and the attending physician. Hence, a sustained approach for the treatment of retinal dystrophies is highly desirable. The present thesis analysed the neuroprotective effect of a sustained delivery of different neurotrophic factors (NTFs), such as ciliary neurotrophic factor (CNTF), glial cell line-derived neurotrophic factor (GDNF), leukemia inhibitory factor (LIF), progranulin (PGRN) and the designer cytokine hyper-interleukin-6 (hIL6) on degenerating photoreceptors of two animal models of retinal dystrophy. A sustained delivery of the different NTFs was achieved either by application of lentivirally modified neural stem cells (NSCs) or by injection of adeno-associated viruses (AAVs) (hIL6 only). Experiments were conducted in the Atp1b2Atp1b1 knock-in (ki) mouse, a novel model of a cone-rod dystrophy, and in the rd10 mouse, a well-established model of autosomal-recessive retinitis pigmentosa. Modified NSCs were examined in vitro and in vivo to characterise their potential for adequate performance in the animals. Cells expressed the desired NTFs in sufficient amounts and could survive for a prolonged period of time, differentiating into astrocytes in vivo, still expressing the respective protein. The neuroprotective effect was examined in immunohistochemically treated retinal sections analysing different parameters, such as inflammation, retina thickness, and the number of both rod and cone photoreceptors at various time points after treatment. In Atp1b2Atp1b1 ki mice all applied neurotrophic factors, except for GDNF, were suitable to attenuate loss of photoreceptor cells for several months after application of the modified NSCs. In the rd10 mouse model, which mimics both genotype and phenotype of patients with an autosomal-recessive form of retinitis pigmentosa, sustained delivery of hIL6, either by NSCs or AAVs, can protect from rod photoreceptor degeneration for up to 2 months. The collected data suggests a not inconsiderable impact of neuroinflammation on the magnitude of neuroprotection, since better effects have been observed in retinas with a higher degree of inflammation. The present results indicate that the sustained delivery of neurotrophic factors, such as CNTF, LIF, PGRN and hIL6, by means intravitreal application of lentivirally modified NSCs or AAVs, is suitable to attenuate photoreceptor loss in the retina of Atp1b2Atp1b1 ki mice and rd10 mice. The activation of pro-survival pathways such as the JAK/STAT or the RAS/MAPK pathways as well as the degree of inflammation seem to be key factors in conveying the neuroprotective effects observed in both animal models.