Long-term consequences of developmental vascular defects on retinal vessel homeostasis and function in a mouse model of Norrie disease

Abstract

International audienceLoss of Norrin signalling due to mutations in the Norrie disease pseudoglioma gene causes severe vascular defects in the retina, leading to visual impairment and ultimately blindness. While the emphasis of experimental work so far was on the developmental period, we focus here on disease mechanisms that induce progression into severe adult disease. The goal of this study was the comprehensive analysis of the long-term effects of the absence of Norrin on vascular homeostasis and retinal function. In a mouse model of Norrie disease retinal vascular morphology and integrity were studied by means of in vivo angiography; the vascular constituents were assessed in detailed histological analyses using quantitative retinal morphometry. Finally, electroretinographic analyses were performed to assess the retinal function in adult Norrin deficient animals. We could show that the primary developmental defects not only persisted but developed into further vascular abnormalities and microangio-pathies. In particular, the overall vessel homeostasis, the vascular integrity, and also the cellular constituents of the vascular wall were affected in the adult Norrin deficient retina. Moreover, functional analyses indicated to persistent hypoxia in the neural retina which was suggested as one of the major driving forces of disease progression. In summary, our data provide evidence that the key to adult Norrie disease are ongoing vascular modifications, driven by the persistent hypoxic conditions, which are ineffective to compensate for the primary Norrin-dependent defects

Hypoxia tolerance in the Norrin-deficient retina and the chronically hypoxic brain studied at single-cell resolution

“…Despite the absence of intraretinal capillaries in murine retinas lacking Norrin/Frizzled4 signaling and the resulting hypoxia of the INL, the overall structure and cellularity of the mutant retina appear to be largely preserved, with minimal loss of INL cells over the first several months of life (20). It is likely that INL hypoxia in these retinas is substantially mitigated by loss of the blood-retina barrier (BRB), which allows small molecules, such as glucose, to diffuse into the INL (19).…”

Cystoid edema, neovascularization and inflammatory processes in the murine Norrin-deficient retina

“…As shown previously, at two months of age the vessels of the Norrin deficient retina were still confined to the ganglion cell layer (GCL) and inner plexiform layer (IPL), deeper retinal capillaries were not present leaving the retinal layers completely avascular and thus resulting in tissue hypoxia 9 . In our previous work we also observed vascular remodelling until 2 months of age 9 . To analyse if these modifications further progressed, we made use of in vivo SLO imaging that allows for the long-term monitoring of the same individual 18 .…”

“…In adult Ndph y/− mice, however, the entire vasculature was covered by microangiopathies (Fig. 4C ) which have already been analysed in detail recently 9 . Examination of retinal layer morphology with in vivo imaging revealed that these microaneurysm-like angiopathies were also detectable by OCT analyses (Fig.…”

“…4D , arrows). Since Norrin dependent defects are characterized by the complete lack of deeper retinal capillaries, still at 2 months of age 9 , these hyperreflective alterations can not correspond to vascular structures. Noticeably, these alterations were also evenly distributed throughout the entire retina.…”

“…The respective mouse model, generated in 1996 by homologous recombination in embryonic stem cells (Ndph y/− mouse) 3 , develops symptoms similar to those observed in Norrie disease patients 4 – 6 . During development, the defects in sprouting angiogenesis lead to abnormal vessel growth with a complete lack of deeper retinal capillaries along with a delayed hyaloid vessel regression that result in persistent vitreoretinal membranes 7 – 9 . Recently, by investigating the long-term consequences of Norrin deficiency we observed a close interplay between the primary developmental defects and secondary alterations that affected the constituents of the retinal vasculature and resulted in the development of microaneurysm-like lesions with extensive vascular fenestration 9 .…”