Department of Neurology, University of Michigan, Ann Arbor, MI, 48109-2200, USA.
Neuroscience Graduate Program, Northwestern University Feinberg School of Medicine, Chicago, IL, 60611, USA.
Sci Rep. 2021 Jan 11;11(1):287. doi: 10.1038/s41598-020-78775-4.
The brain-expressed ubiquilins, UBQLNs 1, 2 and 4, are highly homologous proteins that participate in multiple aspects of protein homeostasis and are implicated in neurodegenerative diseases. Studies have established that UBQLN2 forms liquid-like condensates and accumulates in pathogenic aggregates, much like other proteins linked to neurodegenerative diseases. However, the relative condensate and aggregate formation of the three brain-expressed ubiquilins is unknown. Here we report that the three ubiquilins differ in aggregation propensity, revealed by in-vitro experiments, cellular models, and analysis of human brain tissue. UBQLN4 displays heightened aggregation propensity over the other ubiquilins and, like amyloids, UBQLN4 forms ThioflavinT-positive fibrils in vitro. Measuring fluorescence recovery after photobleaching (FRAP) of puncta in cells, we report that all three ubiquilins undergo liquid-liquid phase transition. UBQLN2 and 4 exhibit slower recovery than UBQLN1, suggesting the condensates formed by these brain-expressed ubiquilins have different compositions and undergo distinct internal rearrangements. We conclude that while all brain-expressed ubiquilins exhibit self-association behavior manifesting as condensates, they follow distinct courses of phase-separation and aggregation. We suggest that this variability among ubiquilins along the continuum from liquid-like to solid informs both the normal ubiquitin-linked functions of ubiquilins and their accumulation and potential contribution to toxicity in neurodegenerative diseases.
脑表达的泛素结合蛋白 UBQLN1、2 和 4 是高度同源的蛋白质，参与蛋白质动态平衡的多个方面，并与神经退行性疾病有关。研究已经证实，UBQLN2 形成液态凝聚物并在致病聚集体中积累，就像其他与神经退行性疾病相关的蛋白质一样。然而，三种脑表达的泛素结合蛋白的相对凝聚物和聚集体形成尚不清楚。在这里，我们报告说，这三种泛素结合蛋白在体外实验、细胞模型和人脑组织分析中显示出不同的聚集倾向。UBQLN4 比其他泛素结合蛋白表现出更高的聚集倾向，并且与淀粉样蛋白一样，UBQLN4 在体外形成硫黄素 T 阳性纤维。通过测量细胞中斑点的荧光恢复后漂白（FRAP），我们报告说，所有三种泛素结合蛋白都经历了液-液相转变。UBQLN2 和 4 的恢复速度比 UBQLN1 慢，这表明这些脑表达的泛素结合蛋白形成的凝聚物具有不同的组成，并经历了不同的内部重排。我们得出结论，虽然所有脑表达的泛素结合蛋白都表现出自我缔合行为，表现为凝聚物，但它们沿着从液态到固态的连续体经历不同的相分离和聚集过程。我们认为，在从液态到固态的连续体中，泛素结合蛋白的这种变异性既影响了泛素结合蛋白的正常泛素连接功能，也影响了它们的积累及其在神经退行性疾病中的潜在毒性贡献。
