Glutaminase-driven glutamine catabolism supports cone photoreceptor metabolism, function, and structure

Abstract

Purpose : Despite their similar basic functions of phototransduction, neurotransmission and biosynthesis of outer segments, rods and cones have unique metabolic demands. Prior studies have suggested that cone photoreceptors (PRs) may depend less on glycolysis and as a result, may utilize other anaplerotic substrates as fuels. It has been shown that PRs have the flexibility to utilize fuel sources other than glucose. Glutamine (Gln) is a critical amino acid in many cancer cells, and common metabolic relationships exist between cancer cells and cone PRs. Gln is first converted to glutamate for anaplerosis via glutaminase (GLS), a critical enzyme in many cancer cells. This study examines the role of GLS-driven Gln catabolism in cone PR metabolism, function, and survival.

Methods : Mice carrying a floxed Gls allele were crossed to mice harboring a Cre recombinase under the control of the Opn1mw promoter to create animals with conditional deletion of Gls from cone PRs, specifically (Gls cKO). ERG and immunohistochemistry assessed cone PR function and structure. Cone PR-specific gene expression changes between Gls cKO and wild-type (WT) mice were evaluated with scRNA sequencing. Neural retinas from Gls cKO and WT mice were subjected to targeted and stable isotope tracing metabolomics strategies.

Results : Immunofluorescence confirmed cone PR-specific knockdown of GLS as well as early loss of cone PRs. In accordance with this loss of cones, decreases in the photopic and flicker ERG amplitudes were observed at postnatal day 35 (P35). At P15, scRNA sequencing of cones from the cKO retina revealed the downregulation of Gls as well as genes involved in phototransduction and metabolism, including mitochondrial function. Targeted metabolomics at P14 demonstrated an increase in the Gln to glutamate ratio in the Gls cKO retina and a decline in hexosamine biosynthetic pathway metabolites. Stable isotope tracing with uniformly labeled ¹³C-glutamine is underway to interrogate in vivo Gln utilization in cKO and WT retinas.

Conclusions : This study demonstrates the critical link between GLS-initiated Gln catabolism and cone PR metabolism, function, and viability. An improved understanding of the metabolic pathways that support cone function and survival will provide a framework for developing novel neuroprotective agents that preserve central visual acuity.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.