Hexanucleotide repeat expansions of variable size in C9orf72 are the most prevalent genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. Sense and antisense transcripts of the expansions are translated by repeat-associated non-AUG translation into five dipeptide repeat proteins (DPRs). Of these, the polyGR, polyPR and, to a lesser extent, polyGA DPRs are neurotoxic, with polyGA the most abundantly detected DPR in patient tissue. Trans-cellular transmission of protein aggregates has recently emerged as a major driver of toxicity in various neurodegenerative diseases. In vitro evidence suggests that the C9 DPRs can spread. However, whether this phenomenon occurs under more complex in vivo conditions remains unexplored. Here, we used the adult fly brain to investigate whether the C9 DPRs can spread in vivo upon expression in a subset of neurons. We found that only polyGA can progressively spread throughout the brain, which accumulates in the shape of aggregate-like puncta inside recipient cells. Interestingly, GA transmission occurred as early as 3 days after expression induction. By comparing the spread of 36, 100 and 200 polyGA repeats, we found that polyGA spread is enhanced upon expression of longer GA DPRs. Transmission of polyGA is greater in older flies, indicating that age-associated factors exacerbate the spread. These data highlight a unique propensity of polyGA to spread throughout the brain, which could contribute to the greater abundance of polyGA in patient tissue. In addition, we present a model of early GA transmission that is suitable for genetic screens to identify mechanisms of spread and its consequences in vivo.

中文翻译：

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甘氨酸丙氨酸二肽重复在飞脑中以重复的长度和年龄依赖性方式迅速扩散。

C9orf72中可变大小的六核苷酸重复扩增是肌萎缩性侧索硬化和额颞痴呆的最普遍遗传原因。通过重复相关的非AUG翻译将扩增的有义和反义转录物翻译成五个二肽重复蛋白（DPR）。其中，polyGR，polyPR和polyGA DPR（在较小程度上）具有神经毒性，其中polyGA是患者组织中检测到最多的DPR。蛋白质聚集体的跨细胞传播最近已成为各种神经退行性疾病中毒性的主要驱动因素。体外证据表明，C9 DPR可以扩散。然而，这种现象是否在更复杂的体内条件下发生尚待探讨。这里，我们使用了成年苍蝇的大脑来研究C9 DPR在神经元子集中表达后是否可以在体内传播。我们发现只有polyGA才能逐渐扩散到整个大脑，并在受体细胞内以聚集体状的点状聚集。有趣的是，GA传播最早在表达诱导后3天发生。通过比较36、100和200个polyGA重复序列的扩散，我们发现当表达更长的GA DPR时，polyGA扩散得到增强。在较老的果蝇中，polyGA的传播更大，这表明与年龄相关的因素加剧了传播。这些数据突显了polyGA在整个大脑中传播的独特倾向，这可能有助于患者组织中polyGA的丰度更高。此外，