RPS23RG1 inhibits SORT1-mediated lysosomal degradation of MDGA2 to protect against autism

<b>Rationale:</b> Mutations in the synaptic protein MAM domain containing glycosylphosphatidylinositol anchor 2 (MDGA2) have been associated with autism spectrum disorder (ASD). Therefore, elucidating the regulatory mechanisms of MDGA2 can help develop effective treatments for ASD. <b>Methods:</b> Liquid chromatography-tandem mass spectrometry was carried out to identify proteins interacting with the extracellular domain of RPS23RG1 and with MDGA2, followed by co-immunoprecipitation assays to confirm protein-protein interactions. RPS23RG1 and SORT1 levels were downregulated by siRNAs to study their effects on MDGA2 degradation, with additional applications of immunoblotting and immunostaining assays. Lysosome isolation was performed to determine the lysosomal degradation of MDGA2 further. <i>Rps23rg1</i> knockout mice and <i>Mdga2</i> ^+/- mice were subjected to various behavioral tests to study their ASD-like phenotypes. AAVs expressing MDGA2 were delivered in <i>Rps23rg1</i> knockout mice, and RPS23RG1-derived peptide was delivered in <i>Mdga2</i> ^+/- mice to study their rescuing effects. <b>Results:</b> We found that both RPS23RG1 and SORT1 interacted with MDGA2. MDGA2 was primarily degraded through the SORT1-mediated lysosomal degradation pathway. RPS23RG1 competed with SORT1 for MDGA2 binding to inhibit MDGA2 degradation. Furthermore, we showed that <i>Rps23rg1</i> knockout mice exhibited decreased MDGA2 levels and ASD-like behaviors, whereas restoration of MDGA2 levels attenuated social defects in <i>Rps23rg1</i> KO mice. Moreover, we identified a crucial region of RPS23RG1 for MDGA2 interaction and found that a peptide derived from this region not only bound MDGA2 and promoted MDGA2 levels, but also rescued social defects in <i>Mdga2</i> ^+/- mice. <b>Conclusion:</b> Our findings highlight a crucial role of RPS23RG1 in antagonizing SORT1-mediated lysosomal degradation of MDGA2 and suggest a potential for targeting the RPS23RG1-MDGA2 axis to treat ASD with MDGA2 deficiency.