NMR spectroscopy-based metabolomics of organotypic retinal explants

The retina consumes massive amounts of energy, yet its metabolism and
substrate exploitation remain poorly understood. Here, we used a murine
explant model to manipulate retinal energy metabolism under entirely
controlled conditions and utilized 1H-NMR spectroscopy-based metabolomics,
in situenzyme detection, and cell viability readouts to uncover the
pathways of retinal energy production. Our experimental manipulations
resulted in varying degrees of photoreceptor degeneration, while the inner
retina and retinal pigment epithelium were essentially unaffected. This
selective vulnerability of photoreceptors suggested very specific
adaptations in their energy metabolism. Rod photoreceptors were found to
rely strongly on oxidative phosphorylation, but only mildly on glycolysis.
Conversely, cone photoreceptors were dependent on glycolysis but
insensitive to electron transport chain decoupling. Importantly,
photoreceptors appeared to uncouple glycolytic and Krebs-cycle metabolism
via three different pathways: 1) the mini-Krebs-cycle, fueled by glutamine
and branched-chain amino acids, generating N-acetylaspartate; 2) the
alanine-generating Cahill-cycle; 3) the lactate-releasing Cori-cycle.
Moreover, the metabolomic data indicated a shuttling of taurine and
hypotaurine between the retinal pigment epithelium and photoreceptors,
likely resulting in an additional net transfer of reducing power to
photoreceptors. These findings expand our understanding of retinal
physiology and pathology and shed new light on neuronal energy homeostasis
and the pathogenesis of neurodegenerative diseases.