ER stress signaling is linked to the pathophysiological and clinical disease manifestations in amyotrophic lateral sclerosis (ALS). Here, we have investigated ER stress-induced adaptive mechanisms in C9ORF72-ALS/FTD, focusing on uncovering early endogenous neuroprotective mechanisms and the crosstalk between pathological and adaptive responses in disease onset and progression. We provide evidence for the early onset of ER stress-mediated adaptive response in C9ORF72 patient-derived motoneurons (MNs), reflected by the elevated increase in GRP75 expression. These transiently increased GRP75 levels enhance ER–mitochondrial association, boosting mitochondrial function and sustaining cellular bioenergetics during the initial stage of disease, thereby counteracting early mitochondrial deficits. In C9orf72 rodent neurons, an abrupt reduction in GRP75 expression coincided with the onset of UPR, mitochondrial dysfunction and the emergence of PolyGA aggregates, which co-localize with GRP75. Similarly, the overexpression of PolyGA in WT cortical neurons or C9ORF72 patient-derived MNs led to the sequestration of GRP75 within PolyGA inclusions, resulting in mitochondrial calcium (Ca²⁺) uptake impairments. Corroborating these findings, we found that PolyGA aggregate-bearing human post-mortem C9ORF72 hippocampal dentate gyrus neurons not only display reduced expression of GRP75 but also exhibit GRP75 sequestration within inclusions. Sustaining high GRP75 expression in spinal C9orf72 rodent MNs specifically prevented ER stress, normalized mitochondrial function, abrogated PolyGA accumulation in spinal MNs, and ameliorated ALS-associated behavioral phenotype. Taken together, our results are in line with the notion that neurons in C9ORF72-ALS/FTD are particularly susceptible to ER–mitochondrial dysfunction and that GRP75 serves as a critical endogenous neuroprotective factor. This neuroprotective pathway, is eventually targeted by PolyGA, leading to GRP75 sequestration, and its subsequent loss of function at the MAM, compromising mitochondrial function and promoting disease onset.

ER 应激信号与肌萎缩侧索硬化 (ALS) 的病理生理学和临床疾病表现有关。在这里，我们研究了C9ORF72 -ALS/FTD中 ER 应激诱导的适应性机制，重点是揭示早期内源性神经保护机制以及疾病发作和进展过程中病理和适应性反应之间的串扰。我们为C9ORF72中 ER 应激介导的适应性反应的早期发作提供了证据患者来源的运动神经元 (MN)，这反映在 GRP75 表达的增加上。这些瞬时增加的 GRP75 水平增强了 ER-线粒体结合，增强了线粒体功能并在疾病初期维持细胞生物能量学，从而抵消了早期线粒体缺陷。在C9orf72啮齿动物神经元中，GRP75 表达的突然减少与 UPR、线粒体功能障碍和与 GRP75 共定位的 PolyGA 聚集体的出现同时发生。类似地，PolyGA 在WT皮层神经元或C9ORF72患者衍生的 MN 中的过表达导致 GRP75 在 PolyGA 包涵体中的隔离，导致线粒体钙 (Ca ²⁺) 摄取障碍。为证实这些发现，我们发现携带 PolyGA 聚集体的人类死后C9ORF72海马齿状回神经元不仅表现出 GRP75 表达减少，而且表现出包裹体内的 GRP75 隔离。在脊髓C9orf72啮齿动物 MN中维持高 GRP75 表达可特异性预防 ER 应激、使线粒体功能正常化、消除脊髓 MN 中的 PolyGA 积累并改善 ALS 相关行为表型。合在一起，我们的结果符合C9ORF72中的神经元的概念-ALS/FTD 特别容易受到 ER 线粒体功能障碍的影响，并且 GRP75 是一种关键的内源性神经保护因子。这种神经保护途径最终被 PolyGA 靶向，导致 GRP75 隔离，随后在 MAM 处丧失功能，损害线粒体功能并促进疾病发作。