Ryosuke Midorikawa

Amyloid β (Aβ) is a central contributor to neuronal damage and cognitive impairment in Alzheimer's disease (AD). Aβ disrupts AMPA receptor-mediated synaptic plasticity, a key factor in early AD progression. Numerous studies propose that Aβ oligomers hinder synaptic plasticity, particularly long-term potentiation (LTP), by disrupting GluA1 (encoded by GRIA1) function, although the precise mechanism remains unclear. In this study, we demonstrate that Aβ mediates the accumulation of GM1 ganglioside in lipid raft domains of cultured cells, and GluA1 exhibits preferential localization in lipid rafts via direct binding to GM1. Aβ enhances the raft localization of GluA1 by increasing GM1 in these areas. Additionally, chemical LTP stimulation induces lipid raft-dependent GluA1 internalization in Aβ-treated neurons, resulting in reduced cell surface and postsynaptic expression of GluA1. Consistent with this, disrupting lipid rafts and GluA1 localization in rafts rescues Aβ-mediated suppression of hippocampal LTP. These findings unveil a novel functional deficit in GluA1 trafficking induced by Aβ, providing new insights into the mechanism underlying AD-associated cognitive dysfunction.

The AMPA‐type glutamate receptor (AMPA‐R) plays a primary role in principal excitatory synaptic transmission and many neuronal functions including synaptic plasticity that underlie learning and memory. N‐glycosylation is one of the major post‐translational modifications of membrane proteins, but its specific roles in neurons remain largely unknown. AMPA‐R subunits are N‐glycosylated at their extracellular domains during their biosynthesis in the lumen of the endoplasmic reticulum and Golgi system. Six N‐glycosylation sites are presumed to exist in the extracellular domain of GluA1, which is a member of the AMPA‐R subunits. We observed that the intracellular trafficking and cell surface expression were strongly suppressed in the GluA1 mutants lacking N‐glycans at N63/N363 in HEK293T cells. Multimer analysis using Blue Native‐PAGE displayed the impaired tetramer formation in the glycosylation mutants (N63S and N363S), indicating that the mis‐transport was caused by impaired tetramer formation. N63S and N363S mutants were primarily degraded via the lysosomal pathway. Flag‐tagged N363S GluA1, but not N63S GluA1, expressed in primary cortical neuron cultures prepared from GluA1 knockout mice was observed to localize at the cell surface. Co‐expression of GluA2 partially rescued tetramer formation and the cell surface expression of N363S GluA1 but not N63S GluA1, in HEK293T cells. Electrophysiological analysis also demonstrated functional heteromers of N363S GluA1 with GluA2. These data suggest that site‐specific N‐glycans on GluA1 subunit regulates tetramer formation, intracellular trafficking, and cell surface expression of AMPA‐R.Open science badges: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/. image

Recent studies have demonstrated protective roles for autophagy in various neurodegenerative disorders, including the polyglutamine diseases; however, the role of autophagy in retinal degeneration has remained unclear. Accumulation of activated rhodopsin in someDrosophilamutants leads to retinal degeneration, and although it is known that activated rhodopsin is degraded in endosomal pathways in normal photoreceptor cells, the contribution of autophagy to rhodopsin regulation has remained elusive. This study reveals that activated rhodopsin is degraded by autophagy in collaboration with endosomal pathways to prevent retinal degeneration. Light-dependent retinal degeneration in theDrosophilavisual system is caused by the knockdown or mutation of autophagy-essential components, such as autophagy-related protein 7 and 8 (atg-7/atg-8), or genes essential for PE (phosphatidylethanolamine) biogenesis and autophagosome formation, including Phosphatidylserine decarboxylase (Psd) and CDP-ethanolamine:diacylglycerol ethanolaminephosphotransferase (Ept). The knockdown ofatg-7/8orPsd/Eptproduced an increase in the amount of rhodopsin localized to Rab7-positive late endosomes. This rhodopsin accumulation, followed by retinal degeneration, was suppressed by overexpression of Rab7, which accelerated the endosomal degradation pathway. These results indicate a degree of cross talk between the autophagic and endosomal/lysosomal pathways. Importantly, a reduction in rhodopsin levels rescuedPsdknockdown-induced retinal degeneration. Additionally, thePsdknockdown-induced retinal degeneration phenotype was enhanced byPpt1inactivation, which causes infantile neuronal ceroid lipofuscinosis, implying that autophagy plays a significant role in its pathogenesis. Collectively, the current data reveal that autophagy suppresses light-dependent retinal degeneration in collaboration with the endosomal degradation pathway and that rhodopsin is a key substrate for autophagic degradation in this context.