Aqp9 Gene Deletion Enhances Retinal Ganglion Cell (RGC) Death and Dysfunction Induced by Optic Nerve Crush: Evidence that Aquaporin 9 Acts as an Astrocyte-to-Neuron Lactate Shuttle in Concert with Monocarboxylate Transporters To Support RGC Function and Survival

Abstract

Aquaporin 9 (AQP9) is an aquaglyceroporin that can transport lactate. Accumulating evidence suggests that astrocyte-to-neuron lactate shuttle (ANLS) plays a critical role in energy metabolism in neurons, including retinal ganglion cells (RGCs). To test the hypothesis that AQP9, in concert with monocarboxylate transporters (MCTs), participates in ANLS to maintain function and survival of RGCs, Aqp9-null mice and wild-type (WT) littermates were subjected to optic nerve crush (ONC) with or without intravitreal injection of an MCT2 inhibitor. RGC density was similar between the Aqp9-null mice and WT mice without ONC, while ONC resulted in significantly more RGC density reduction in the Aqp9-null mice than in the WT mice at day 7. Positive scotopic threshold response (pSTR) amplitude values were similar between the two groups without ONC, but were significantly more reduced in the Aqp9-null mice than in the WT mice 7days after ONC. MCT2 inhibitor injection accelerated RGC death and pSTR amplitude reduction only in the WT mice with ONC. Immunolabeling revealed that both RGCs and astrocytes expressed AQP9, that ONC predominantly reduced astrocytic AQP9 expression, and that MCTs 1, 2, and 4 were co-localized with AQP9 at the ganglion cell layer. These retinal MCTs were also co-immunoprecipitated with AQP9 in the WT mice. ONC decreased the co-immunoprecipitation of MCTs 1 and 4, but did not impact co-immunoprecipitation of MCT2. Retinal glucose transporter 1 expression was increased in Aqp9-null mice. Aqp9 gene deletion reduced and increased the intraretinal l-lactate and d-glucose concentrations, respectively. Results suggest that AQP9 acts as the ANLS to maintain function and survival of RGCs.

Changes in Gene Expression Profiling and Phenotype in Aged Multidrug Resistance Protein 4-Deficient Mouse Retinas

“…Electroretinography (ERG) recording was conducted as reported previously [ 29 , 30 ], with some modifications. All the animals were dark-adapted overnight before ERG recordings, and all the procedures were performed under dim red light.…”

Dimethyl Fumarate Promotes the Survival of Retinal Ganglion Cells after Optic Nerve Injury, Possibly through the Nrf2/HO-1 Pathway

“…To visualize RGCs, immunolabeling with an TUBB3 antibody (Biolegend, San Diego, CA, USA) was performed, as reported previously [ 26 , 27 ]. On day 7 after ONC, the mice were sacrificed and perfused using 4% paraformaldehyde (PFA).…”

“…The mice were dark-adapted overnight before testing, and all procedures were performed under dim red light, as reported previously, with a modification [ 26 , 28 ]. Briefly, on the day of the experiment, the mice were anesthetized with an intraperitoneal injection of ketamine (100 mg/kg) and xylazine (10 mg/kg).…”

AQP-9, KCNJ11 and ABCC8 Gene Variants in Open Angle Glaucoma: A Hypothesis

“…The lack of sufficient ATP in the aged TM results in elevated IOP and cell swelling. However, for the neural retina, which depends on lactate for its energy needs [9,21,22], there are grave consequences since aquaporin-9 is essential to transport lactate from astrocytes to neurons [27][28][29] in concert with monocarboxylate transporters [27], which is converted to pyruvate to produce ATP [24].…”

“…In the brain and retina lactate is shuttled from astrocytes to neurons by a family of lactate/pyruvate monocarboxylate transporters (MCTs) [26,27] with the cooperation of aquaporin-9 [27]. Even though the specific role of aquaporin-9 in the retina is not known, it is clear that aquaporin-9 is essential for the survival and function of RGCs [28][29][30].…”