Ubiquitination-directed proteasomal degradation of synaptic proteins, presumably mediated by lysine 48 (K48) of ubiquitin, is a key mechanism in synapse and neural circuit remodeling. However, more than half of polyubiquitin (polyUb) species in the mammalian brain are estimated to be non-K48; among them, the most abundant is Lys 63 (K63)-linked polyUb chains that do not tag substrates for degradation but rather modify their properties and activity. Virtually nothing is known about the role of these nonproteolytic polyUb chains at the synapse. Here we report that K63-polyUb chains play a significant role in postsynaptic protein scaffolding and synaptic strength and plasticity. We found that the postsynaptic scaffold PSD-95 (postsynaptic density protein 95) undergoes K63 polyubiquitination, which markedly modifies PSD-95’s scaffolding potentials, enables its synaptic targeting, and promotes synapse maturation and efficacy. TNF receptor-associated factor 6 (TRAF6) is identified as a direct E3 ligase for PSD-95, which, together with the E2 complex Ubc13/Uev1a, assembles K63-chains on PSD-95. In contrast, CYLD (cylindromatosis tumor-suppressor protein), a K63-specific deubiquitinase enriched in postsynaptic densities, cleaves K63-chains from PSD-95. We found that neuronal activity exerts potent control of global and synaptic K63-polyUb levels and, through NMDA receptors, drives rapid, CYLD-mediated PSD-95 deubiquitination, mobilizing and depleting PSD-95 from synapses. Silencing CYLD in hippocampal neurons abolishes NMDA-induced chemical long-term depression. Our results unveil a previously unsuspected role for nonproteolytic polyUb chains in the synapse and illustrate a mechanism by which a PSD-associated K63-linkage–specific ubiquitin machinery acts on a major postsynaptic scaffold to regulate synapse organization, function, and plasticity.

中文翻译：

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不依赖蛋白酶体的聚泛素连接调节突触支架，功效和可塑性

可能由泛素的赖氨酸48（K48）介导，由泛素化控制的突触蛋白的蛋白酶体降解是突触和神经回路重塑的关键机制。但是，哺乳动物大脑中的多聚泛素（polyUb）种类估计超过一半是非K48的；其中，最丰富的是与Lys 63（K63）连接的polyUb链，这些链不会标记要降解的底物，而是修饰其特性和活性。关于这些非蛋白水解的polyUb链在突触中的作用，实际上几乎一无所知。在这里我们报告，K63 polyUb链在突触后蛋白支架和突触强度和可塑性中发挥重要作用。我们发现，突触后支架PSD-95（突触后密度蛋白95）经历K63多泛素化作用，这明显修饰了PSD-95的支架潜力，使其突触靶向，并促进突触成熟和功效。TNF受体相关因子6（TRAF6）被确定为PSD-95的直接E3连接酶，它与E2复合物Ubc13 / Uev1a一起在PSD-95上组装K63链。相反，CYLD（圆柱状瘤病肿瘤抑制蛋白），一种富含K63特异性去泛素酶，富含突触后密度，可从PSD-95切割K63链。我们发现神经元活动对全局和突触的K63-polyUb水平发挥有效的控制作用，并通过NMDA受体，驱动快速的CYLD介导的PSD-95去泛素化，从突触中动员和消耗PSD-95。沉默海马神经元CYLD消除了NMDA诱导的化学长期抑郁症。