上田 洋司

Transient memories are converted to persistent memories at the synapse and circuit/systems levels. The synapse-level consolidation parallels electrophysiological transition from early- to late-phase long-term potentiation of synaptic transmission (E-/L-LTP). While glutamate signaling upregulations coupled with dendritic spine enlargement are common underpinnings of E-LTP and L-LTP, synaptic mechanisms conferring persistence on L-LTP remain unclear. Here, we show that L-LTP induced at the perforant path-hippocampal dentate gyrus (DG) synapses accompanies cytoskeletal remodeling that involves actin and the septin subunit SEPT3. L-LTP in DG neurons causes fast spine enlargement, followed by SEPT3-dependent smooth endoplasmic reticulum (sER) extension into enlarged spines. Spines containing sER show greater Ca2+ responses upon synaptic input and local synaptic activity. Consistently, Sept3 knockout in mice (Sept3-/-) impairs memory consolidation and causes a scarcity of sER-containing spines. These findings indicate a concept that sER extension into active spines serves as a synaptic basis of memory consolidation.

Exosomes are small extracellular vesicles (sEVs) secreted via multivesicular bodies (MVBs)/late endosomes and mediators of cell-cell communication. We previously reported a novel post-translational modification by ubiquitin-like 3 (UBL3). UBL3 is localized in MVBs and the plasma membrane and released outside as sEVs, including exosomes. Approximately 60% of proteins sorted in sEVs are affected by UBL3 and localized in various organelles, the plasma membrane, and the cytosol, suggesting that its dynamic movement in the cell before entering the MVBs. To examine the intracellular dynamics of UBL3, we constructed a sophisticated visualization system via fusing fluorescent timers that changed from blue to red form over time with UBL3 and by its expression under Tet-on regulation. Intriguingly, we found that after synthesis, UBL3 was initially distributed within the cytosol. Subsequently, UBL3 was localized to MVBs and the plasma membrane and finally showed predominant accumulation in MVBs. Furthermore, by super-resolution microscopy analysis, UBL3 was found to be associated with one of its substrates, α-tubulin, in the cytosol, and the complex was subsequently transported to MVBs. This spatiotemporal visualization system for UBL3 will form a basis for further studies to elucidate when and where UBL3 associates with its substrates/binding proteins before localization in MVBs.

Discovery of novel post-translational modifications provides new insights into changes in protein function, localization, and stability. They are also key elements in understanding disease mechanisms and developing therapeutic strategies. We have previously reported that ubiquitin-like 3 (UBL3) serves as a novel post-translational modifier that is highly expressed in the cerebral cortex and hippocampus, in addition to various other organs, and that 60% of proteins contained in small extracellular vesicles (sEVs), including exosomes, are influenced by UBL3. In this study, we generated transgenic mice expressing biotinylated UBL3 in the forebrain under control of the alpha-CaMKII promoter (Ubl3Tg/+). Western blot analysis revealed that the expression of UBL3 in the cerebral cortex and hippocampus was 6- to 7-fold higher than that in the cerebellum. Therefore, we performed immunoprecipitation of protein extracts from the cerebral cortex of Ubl3+/+ and Ubl3Tg/+ mice using avidin beads to comprehensively discover UBL3 interacting proteins, identifying 35 new UBL3 interacting proteins. Nine proteins were annotated as extracellular exosomes. Gene Ontology (GO) analysis suggested a new relationship between sEVs and RNA metabolism in neurodegenerative diseases. We confirmed the association of endogenous UBL3 with the RNA-binding proteins FUS and HPRT1-both listed in the Neurodegenerative Diseases Variation Database (NDDVD)-and with LYPLA1, which is involved in Huntington's disease, using immunoprecipitation (IP)-western blotting analysis. These UBL3 interacting proteins will accelerate the continued elucidation of sEV research about proteins regulated by novel post-translational modifications by UBL3 in the brain.

エクソソームを介した疾患増悪蛋白質の伝播は、様々な疾患に関与している。エクソソームに輸送される一連の蛋白質も、リン酸化やアセチル化、ユビキチン化、脂質修飾などの様々な翻訳後修飾が付加される。ユビキチンと相同な配列を持つ蛋白質もまた翻訳後修飾因子として作用し、標的分子の輸送、機能活性調節、分解などに関与している。ユビキチン様蛋白質であるSUMO、ISG15、UBL3によるエクソソームへの制御機構について、近年の知見を含めて紹介した。

Promoter-associated long non-coding RNAs (lncRNAs) regulate the expression of adjacent genes; however, precise roles of these lncRNAs in skeletal muscle remain largely unknown. Here, we characterize a promoter-associated lncRNA, Myoparr, in myogenic differentiation and muscle disorders. Myoparr is expressed from the promoter region of the mouse and human myogenin gene, one of the key myogenic transcription factors. We show that Myoparr is essential both for the specification of myoblasts by activating neighboring myogenin expression and for myoblast cell cycle withdrawal by activating myogenic microRNA expression. Mechanistically, Myoparr interacts with Ddx17, a transcriptional coactivator of MyoD, and regulates the association between Ddx17 and the histone acetyltransferase PCAF Myoparr also promotes skeletal muscle atrophy caused by denervation, and knockdown of Myoparr rescues muscle wasting in mice. Our findings demonstrate that Myoparr is a novel key regulator of muscle development and suggest that Myoparr is a potential therapeutic target for neurogenic atrophy in humans.

Exosomes, a type of small extracellular vesicles (sEVs), derived from multivesicular bodies (MVBs), mediate cell-to-cell communication by transporting proteins, mRNAs, and miRNAs. However, the molecular mechanism by which proteins are sorted to sEVs is not fully understood. Here, we report that ubiquitin-like 3 (UBL3)/membrane-anchored Ub-fold protein (MUB) acts as a posttranslational modification (PTM) factor that regulates protein sorting to sEVs. We find that UBL3 modification is indispensable for sorting of UBL3 to MVBs and sEVs. We also observe a 60% reduction of total protein levels in sEVs purified from Ubl3-knockout mice compared with those from wild-type mice. By performing proteomics analysis, we find 1241 UBL3-interacting proteins, including Ras. We also show that UBL3 directly modifies Ras and oncogenic RasG12V mutant, and that UBL3 expression enhances sorting of RasG12V to sEVs via UBL3 modification. Collectively, these results indicate that PTM by UBL3 influences the sorting of proteins to sEVs.