Spp1 drives retinal neuron resiliency in glaucomatous neuropathy but not axotomy

Abstract

Purpose : Chronic ocular hypertension and acute injuries lead to neuron losses as shared outcomes via potentially different mechanisms. Comparing retinal ganglion cell (RGC) responses between glaucomatous conditions and optic nerve crush (ONC) offers potential mechanisms for RGC subtypes and distinct molecular responses.

Methods : Chronic intraocular pressure (IOP) elevation was achieved using the silicone oil-induced ocular hypertension (SO) model. This was directly compared with the ONC model. AAV-mediated Secreted Phosphoprotein 1 (Spp1) knockdown or overexpression in RGCs occurred at 8 weeks of age. Mice were sacrificed at 1 and 4 weeks post-surgery for immunohistochemistry (IHC) analysis. IHC also assessed spp1 expression in post-mortem prenatal and adult human retinae and ELISA to quantify Spp1 in the aqueous humor (AH) from patients with glaucoma.

Results : Using distinct RGC subclass marking lines and immunohistological analysis and in vivo imaging, we showed that αRGCs are uniquely resilient to chronic ocular hypertension (SO model). Our past transcriptomics studies revealed that αRGCs express high levels of Spp1 (Duan et al., 2015), and now we demonstrate that the level of Spp1 is increased in the setting of prolonged IOP elevation in mice. Our loss-of-function and gain-of-function studies in different RGC subsets suggested that Spp1 drives neuronal resiliency in glaucomatous neuropathy. These data indicate that Spp1 is expressed in RGCs and increased Spp1 expression in chronic glaucomatous injury results in RGC resilience. A similar mechanism may also explain the relative resiliency of Melanopsin-positive RGCs, as Spp1 was increased after exposure to the SO model. Correlating the findings onto human RGCs, we found that SPP1 marks adult RGC subsets with relatively large somata, distinct from other RGC subsets. Finally, we sought to determine if SPP1 expression was relevant to humans with glaucomatous neuropathy. Human aqueous humor revealed that more severe glaucoma was associated with higher levels of SPP1 expression.

Conclusions : Our study revealed Spp1 as a key molecular player that drives αRGC-specific neuroprotection in the context of glaucomatous injury. Spp1’s unique role in mediating neuronal resiliency in glaucomatous optic neuropathy that was not present in the optic nerve crush setting offers unique molecular insights into the contribution of each neuron and neuron type in disease etiology in vivo.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.