Characterizing the effects of light on the early onset photoreceptor function loss in Mucolipidosis type IV

Retinal degenerative diseases are a diverse group of retinal disorders thatcause visual impairment. While RDD prevalence is high, little is known about themolecular mechanisms underlying the pathogenesis within some of these disorders. Herewe are trying to elucidate the molecular mechanisms that drive early onset photoreceptorfunction loss in the RDD, Mucolipidosis type IV (MLIV). MLIV is a lysosomal storagedisorder that results from loss of function mutations in the MCOLN1 gene and leads to adisruption of autophagy. The MCOLN1 gene encodes a lysosomal cation channel calledTrpml1. Based on previous data from our lab, we hypothesized that a loss of Trpml1expression in photoreceptors causes a disruption in protein trafficking and intracellularsignaling, which results in decreased light activation, and is exacerbated in light stressconditions. To test this hypothesis, we measured retinal function in 6-week-old controland Mcoln1 KO mice under normal light and light stress conditions. In these experimentswe find that the Mcoln1 KO mice have a significantly decreased photoresponse whencompared to control at all light conditions. We also used immunohistochemistry andwestern analysis to reveal reduction in expression and observed mislocalization ofphototransduction proteins such as rhodopsin and cyclic nucleotide gated (CNG)channels. We saw changes in protein expression and localization that were consistentwith the decrease in Mcoln1 KO mouse photoresponses. These observations inphysiological response and protein expression lead us to test for changes in geneexpression within the Mcoln1 KO mice. To do this we used RNA sequencing on bothcontrol and Mcoln1 KO mice that were raised under normal and stressed light conditions.We used differential expression analysis to identify changes in gene expression across all light conditions. In this data, we saw subtle decreases in rod photoreceptor proteins ofMcoln1 KO mice under all light conditions that were consistent with functional andexpression changes measured through ERG and western analysis. In addition, we lookedat differentially expressed genes that were significant in fold change difference and wefound 4 commonly upregulated across all light conditions, which all function in thecellular immune response within the Mcoln1 KO retina. This data suggests that in theMcoln1 KO retina, there is chronic inflammation that may be inducing increased cell stresson photoreceptors therefore contributing to the decreased photoreceptor responses. Thesefindings provide novel information about the molecular changes that occur during theonset of photoreceptor function loss and the impact of light-stress in the RDD,Mucolipidosis type IV.