Phagocytosis-dependent ketogenesis in retinal pigment epithelium

Abstract

Daily, the retinal pigment epithelium (RPE) ingests a bolus of lipid and protein in the form of phagocytized photoreceptor outer segments (OS). The RPE, like the liver, expresses enzymes required for fatty acid oxidation and ketogenesis. This suggests that these pathways play a role in the disposal of lipids from ingested OS, as well as providing a mechanism for recycling metabolic intermediates back to the outer retina. In this study, we examined whether OS phagocytosis was linked to ketogenesis. We found increased levels of β-hydroxybutyrate (β-HB) in the apical medium following ingestion of OS by human fetal RPE and ARPE19 cells cultured on Transwell inserts. No increase in ketogenesis was observed following ingestion of oxidized OS or latex beads. Our studies further defined the connection between OS phagocytosis and ketogenesis in wild-type mice and mice with defects in phagosome maturation using a mouse RPE explant model. In explant studies, the levels of β-HB released were temporally correlated with OS phagocytic burst after light onset. In the mouse where phagosome maturation is delayed, there was a temporal shift in the release of β-HB. An even more pronounced shift in maximal β-HB production was observed in the RPE, in which loss of the ATP-binding cassette A4 transporter results in defective phagosome processing and accumulation of lipid debris. These studies suggest that FAO and ketogenesis are key to supporting the metabolism of the RPE and preventing the accumulation of lipids that lead to oxidative stress and mitochondrial dysfunction.

Succinate can shuttle reducing power from the hypoxic retina to the O2-rich pigment epithelium

“…These observations about retinal metabolism prompted us to explore the role of succinate in the overall metabolic ecosystem of the eye. The RPE relies on its mitochondria to oxidize diverse fuels including lactate, fatty acids, glutamine and proline, and some of these fuels can be supplied to the RPE by the retina (Adijanto et al, 2014;Du et al, 2016;Kanow et al, 2017;Reyes-Reveles et al, 2017;Yam et al, 2019). In this report we show that the RPE/choroid complex has an extraordinary capacity to oxidize succinate.…”

“…The RPE consumes mitochondrial fuels, and RPE cells are in direct contact with the retina and metabolites exchange between these tissues within an eye (Adijanto et al, 2014;Du et al, 2016;Kanow et al, 2017;Nickla and Wallman, 2010;Reyes-Reveles et al, 2017;Swarup et al, 2019;Yam et al, 2019;Young and Bok, 1969). For these reasons, we tested the capacity of RPE cells to consume succinate.…”

Symmetric arrangement of mitochondria:plasma membrane contacts between adjacent photoreceptor cells regulated by Opa1

“…In addition to examining images through the depth of the retina close to the IS-ONL junction by SBFSEM, the projections do not originate from photoreceptors and we unequivocally show they are Müller glial cell derived. It has been proposed in retina there is an metabolic ecosystem, and in addition to the well-established metabolic transport between the RPE and photoreceptors (39)(40)(41)(42), Müller glial cells are involved in shuttling of lactate as well as other metabolites (27,43,44). Due to the positioning of the Müller glial processes close to the IS mitochondria, they may exist to assist in transport of resources towards or away from the IS mitochondria.…”

Phagocytosed photoreceptor outer segments activate mTORC1 in the retinal pigment epithelium

“…In contrast to autophagy, however, POS trafficking and degradation are not merely catabolic processes. RPE phagocytosis stimulates β-oxidation of fatty acids and ketogenesis (46). High mTORC1 activity supports aerobic glycolysis (47) and suppresses ketogenesis (48), which may serve as an off signal after the cells have used free fatty acids derived from POS as metabolic substrates.…”