Enhancing Retinal Metabolism Protects Cone Vision

A new study from Constance Cepko’s lab at Harvard Medical School presents a promising step toward therapies for Retinitis Pigmentosa, a degenerative eye disease affecting about 1 in 4000 people worldwide. Mutations in more than a hundred genes can cause Retinitis Pigmentosa. These mutations affect the physiology of rod photoreceptors, leading to their death, so that the disease most often first manifests as impaired night vision from rod loss. In later stages, however, cone photoreceptors, which are crucial for daylight and color vision, also degenerate. Unlike rods, many of the cone-related changes are thought to stem from metabolic dysregulation rather than direct genetic defects.

The Cepko lab investigates whether supporting retinal metabolism in retinitis pigmentosa can slow cone death and preserve vision, regardless of the specific mutation that causes rod loss. The success of such an approach could help many Retinitis Pigmentosa patients, not only those with a specific mutation, as would be the case in traditional gene therapy approaches. In this study, Cepko and colleagues tested whether restoring retinal pigment epithelium (RPE) metabolism could delay cone degeneration. Using an AAV vector to enhance expression of the lactate transporter MCT2 in the RPE, they aimed to boost lactate uptake from the choroid vasculature which, as a consequence, frees up more glucose to sustain cone photoreceptors. Their results not only showed improved cone survival and function in animal models, but also introduced a new way of directly visualizing glucose and lactate levels in RPE cells.

The Central Role of RPE Metabolism

Retinitis Pigmentosa begins with the death of rod photoreceptors, but cone loss often follows, raising the question of why cones eventually die, even though they do not directly express a mutated gene in most Retinitis Pigmentosa cases. One explanation lies in their unique metabolic dependence. Because the photoreceptor layer is avascular, rods and cones rely on the underlying RPE to supply nutrients. Photoreceptors themselves consume large amounts of glucose, which they metabolize through aerobic glycolysis, producing lactate as a byproduct. This lactate is normally taken up by the RPE through MCT1/2 transporters. Inside the RPE, part of the lactate is metabolized, while the rest feeds back into circulation. Importantly, this lactate influx suppresses RPE glycolysis, ensuring that glucose remains available for photoreceptors.

When rods die, this delicate feedback loop collapses. The RPE receives less lactate, reactivates glycolysis to meet its own energy needs, and consumes glucose that would otherwise nourish cones. The result is cone starvation and progressive vision loss—a metabolic downward spiral. Cepko’s team hypothesized that by artificially boosting lactate uptake in RPE cells, they could restore the balance and protect cones.

AAV-Mediated Delivery of MCT2

To test this idea, the researchers engineered an AAV vector carrying the high-affinity monocarboxylate transporter MCT2 under an RPE-specific promoter. Because MCT2 has a greater capacity for lactate import than MCT1 or MCT3, its overexpression was expected to compensate for the loss of rod-derived lactate. The vector was delivered via subretinal injection in both rat and mouse models of RP, enabling direct comparison of cone survival and function in treated versus untreated tissue.

Cone Survival in Animal Models

In rats carrying a disease-causing mutation, neonatal injection of the MCT2 vector led to measurable benefits. Six months later, cone counts revealed that treated animals retained more cones compared to untransduced ones, both across whole retinas and within vector-transduced regions. While cone rescue was incomplete and did not fully match healthy animals, it clearly outperformed controls.

In two different mouse strains (FVB and P23H), which are known to display characteristic “cone craters” of degeneration, MCT2 treatment again improved outcomes. Cones were more numerous, and the craters were absent in treated eyes. To better track transduced regions, the team used an additional vector expressing GFP in cone nuclei, which confirmed widespread survival benefits. As in rats, however, cone rescue was partial rather than complete.

Functional Preservation of Cone Vision

Morphological survival is encouraging, but the real question is whether rescued cones continue to support vision. To address this, the team used Striatech’s OptoDrum to assess the optomotor reflex in P23H mice. Treated animals showed better visual acuity than controls, though results varied due to differences in injection efficiency and natural inter-animal variability. Importantly, the functional gains declined with age, and by postnatal day 53 the difference between treated and untreated mice was no longer statistically significant. These results suggest that MCT2 transduction can prolong, but not permanently preserve, cone function.

Direct Metabolic Measurements with FLIM Sensors

To confirm that improved cone outcomes were linked to metabolic changes, the researchers employed an innovative approach: fluorescence lifetime imaging (FLIM) biosensors. These genetically encoded sensors, delivered by AAV, undergo conformational changes when binding metabolites such as lactate (LiLac sensor) or glucose (GlucoSnFR-TS), altering their fluorescence lifetime. Combined with two-photon microscopy, this allowed direct, single-cell–level readouts of metabolite concentrations in living RPE tissue ex vivo.

In MCT2-transduced eyecups, lactate sensors reported higher baseline intracellular lactate levels and greater uptake across physiological ranges compared to controls. Glucose sensors, in turn, showed increased intracellular glucose in treated tissue under higher medium concentrations, consistent with reduced glycolytic consumption by the RPE. These findings directly validated the team’s central metabolic hypothesis.

Promise and Limitations of Metabolic Rescue in Retinitis Pigmentosa

Taken together, these experiments show that enhancing MCT2-mediated lactate uptake in the RPE can partially rescue cone survival and prolong visual function in Retinitis Pigmentosa models. The introduction of FLIM biosensors adds a powerful tool for studying retinal metabolism, providing single-cell resolution that circumvents many limitations of older methods.

Still, challenges remain. Even with MCT2 overexpression, cones eventually degenerated, indicating that additional factors—such as inflammation or oxidative stress—contribute to disease progression. Moreover, the ex vivo metabolic assays, while elegant, may not perfectly mirror in vivo conditions. Future work could refine these methods, including live in vivo FLIM imaging, and explore combinatorial strategies that target multiple pathological processes.

Ultimately, this study strengthens the case for gene-agnostic therapies that address metabolic failure as a common pathway of cone degeneration in Retinitis Pigmentosa. By prolonging central vision regardless of the underlying mutation, such approaches could offer broad-spectrum benefits to patients facing this currently incurable disease.

Original source: L.C. Chandler, A. Gardner, & C.L. Cepko, RPE-specific MCT2 expression promotes cone survival in models of retinitis pigmentosa, Proc. Natl. Acad. Sci. U.S.A. 122 (14) e2421978122, https://doi.org/10.1073/pnas.2421978122 (2025).

Blog author: Emilia Kawecka, Technical University of Munich, Student Assistant at Striatech