Citation: Shrestha, A.P.; Stiles, M.; Grambergs, R.C.; Boff, J.M.; Madireddy, S.; Mondal, K.; Rajmanna, R.; Porter, H.; Sherry, D.M.; Proia, R.L.; et al. The Role of Sphingosine-1-Phosphate Receptor 2 in Mouse Retina Light Responses. Biomolecules 2023, 13, 1691. https:// doi.org/10.3390/biom13121691 Academic Editors: Viswanathan Natarajan and Robert V. Stahelin Received: 10 October 2023 Revised: 9 November 2023 Accepted: 19 November 2023 Published: 23 November 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). biomolecules Article The Role of Sphingosine-1-Phosphate Receptor 2 in Mouse Retina Light Responses Abhishek P. Shrestha 1,† , Megan Stiles 2,† , Richard C. Grambergs 3,† , Johane M. Boff 1 , Saivikram Madireddy 4 , Koushik Mondal 3 , Rhea Rajmanna 1 , Hunter Porter 5 , David M. Sherry 2 , Richard L. Proia 6 , Thirumalini Vaithianathan 1, * and Nawajes Mandal 3,7, * 1 Department of Pharmacology, Addiction Science, and Toxicology, College of Medicine, University of Tennessee Health Science Center, Memphis, TN 38163, USA 2 Departments of Cell Biology, Neurosurgery, and Pharmacological Sciences, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA 3 Department of Ophthalmology, Hamilton Eye Institute, University of Tennessee Health Science Center, Memphis, TN 38163, USA 4 College of Medicine, The University of Tennessee Health Science Center, Memphis, TN 38163, USA 5 Department of Ophthalmology, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA 6 Genetics of Development and Disease Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892, USA 7 Department of Anatomy and Neurobiology, University of Tennessee Health Science Center, Memphis, TN 38163, USA * Correspondence: tvaithia@uthsc.edu (T.V.); nmandal@uthsc.edu (N.M.); Tel.: +1-901-448-2786 (T.V.); +1-901-448-7740 (N.M.) † These authors contributed equally to this work. Abstract: The bioactive sphingolipid sphingosine-1-phosphate (S1P) acts as a ligand for a family of G protein-coupled S1P receptors (S1PR1-5) to participate in a variety of signaling pathways. However, their specific roles in the neural retina remain unclear. We previously showed that S1P receptor subtype 2 (S1PR2) is expressed in murine retinas, primarily in photoreceptors and bipolar cells, and its expression is altered by retinal stress. This study aims to elucidate the role of S1PR2 in the mouse retina. We examined light responses by electroretinography (ERG), structural differences by optical coherence tomography (OCT), and protein levels by immunohistochemistry (IHC) in wild-type (WT) and S1PR2 knockout (KO) mice at various ages between 3 and 6 months. We found that a- and b-wave responses significantly increased at flash intensities between 400~2000 and 4~2000 cd.s/m 2 , respectively, in S1PR2 KO mice relative to those of WT controls at baseline. S1PR2 KO mice also exhibited significantly increased retinal nerve fiber layer (RNFL) and outer plexiform layer (OPL) thickness by OCT relative to the WT. Finally, in S1PR2 KO mice, we observed differential labeling of synaptic markers by immunohistochemistry (IHC) and quantitative reverse transcription polymerase chain reaction (RT-qPCR). These results suggest a specific involvement of S1PR2 in the structure and synaptic organization of the retina and a potential role in light-mediated functioning of the retina. Keywords: sphingolipid; retina; light response; electroretinography; synaptic proteins; retinal synapses 1. Introduction Neuronal membranes are enriched in sphingolipids [1], which serve pivotal roles in structure and signaling. The bioactive lipid sphingosine-1-phosphate (S1P) is a sphingosine metabolite produced by two sphingosine kinase isoenzymes [2]. S1P plays important roles in synaptic transmission, modulation of neurotransmitter release, membrane excitability, and synaptic strength in the central nervous system. Although sphingolipids are possible mediators of retinal diseases, including diabetic retinopathy [3], age-related macular degen- eration [4], and retinal degenerative processes in general [5], their roles in retinal function are unclear. Biomolecules 2023, 13, 1691. https://doi.org/10.3390/biom13121691 https://www.mdpi.com/journal/biomolecules