Dysregulation of lipid metabolism is strongly associated with age-dependent retinal diseases including age-related macular degeneration (AMD) based on genetic and epidemiological studies. However, the roles of dysregulated lipid metabolism in the development and progression of age-dependent retinal diseases remain largely unknown. Mouse models showing impairment of lipid metabolism and cellular homeostasis in the retina provide excellent tools to study how dysregulated lipid metabolism impacts retinal health and lead to age- dependent retinal diseases. We identified one such mouse strain harboring a mutation in transmembrane protein 135 (Tmem135) that displays signs of accelerated aging in the retina as well as pathologies observed in AMD including retinal pigment epithelium (RPE) cell abnormalities, inflammation and photoreceptor cell degeneration. We found that mitochondrial dynamics are dysregulated in Tmem135 mutant mice. In the previous funding period, we further identified the role of TMEM135 in lipid metabolism in the retina. Our lipidomics data showed reduced DHA levels in Tmem135 mice and indicated that TMEM135 has a role in DHA export from peroxisomes. We also found that TMEM135 is critical for the regulation of peroxisomal number and lipid metabolism in the retina. The number of peroxisomes is correlated with mitochondrial morphology and function in Tmem135 mutant mice and mice overexpressing Tmem135 (Tmgm135 TG), suggesting close interaction between these organelles. Moreover, the Tmem135 mutation changes expression of genes associated with lipid metabolism in mouse eyecups, which are similar to genes differentially expressed in RPE/choroid of AMD patients. Based on these findings, we hypothesize that “Regulation of peroxisomal functions through TMEM135 is essential to maintaining the normal function and integrity of RPE and photoreceptor cells, dysregulation of which leads to age-dependent retinal disease phenotypes.” In this renewal proposal, we will investigate how TMEM135 regulates lipid metabolism, mitochondrial function and retinal function through its role in peroxisomes and DHA synthesis. Specifically, we will 1) test the hypothesis that decreased DHA levels in Tmem135 mutant mice are responsible for the retinal abnormalities, 2) determine the contributions of fatty acid synthesis pathways on Tmem135 mutant retinal pathologies, and 3) test the role of peroxisomes in mitochondrial homeostasis in the RPE. Successful completion of this project will reveal the previously unknown role of Tmem135 in age-dependent changes of retinal cells, and identify factors involved in those processes, which may lead to novel supplementation or treatment options for aging and age-related diseases in the retina.