Department of Neuropathology, Charité - Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin and Humboldt-Universität zu Berlin, Berlin, Germany.
Department of Neurology with Institute for Translational Neurology, University Hospital Münster, Münster, Germany.
Brain Pathol. 2022 Nov;32(6):e13084. doi: 10.1111/bpa.13084. Epub 2022 Jun 15.
Patients suffering from immune-mediated necrotizing myopathies (IMNM) harbor, the pathognomonic myositis-specific auto-antibodies anti-SRP54 or -HMGCR, while about one third of them do not. Activation of chaperone-assisted autophagy was described as being part of the molecular etiology of IMNM. Endoplasmic reticulum (ER)/sarcoplasmic reticulum (SR)-stress accompanied by activation of the unfolded protein response (UPR) often precedes activation of the protein clearance machinery and represents a cellular defense mechanism toward restoration of proteostasis. Here, we show that ER/SR-stress may be part of the molecular etiology of IMNM. To address this assumption, ER/SR-stress related key players covering the three known branches (PERK-mediated, IRE1-mediated, and ATF6-mediated) were investigated on both, the transcript and the protein levels utilizing 39 muscle biopsy specimens derived from IMNM-patients. Our results demonstrate an activation of all three UPR-branches in IMNM, which most likely precedes the activation of the protein clearance machinery. In detail, we identified increased phosphorylation of PERK and eIF2a along with increased expression and protein abundance of ATF4, all well-documented characteristics for the activation of the UPR. Further, we identified increased general XBP1-level, and elevated XBP1 protein levels. Additionally, our transcript studies revealed an increased ATF6-expression, which was confirmed by immunostaining studies indicating a myonuclear translocation of the cleaved ATF6-form toward the forced transcription of UPR-related chaperones. In accordance with that, our data demonstrate an increase of downstream factors including ER/SR co-chaperones and chaperones (e.g., SIL1) indicating an UPR-activation on a broader level with no significant differences between seropositive and seronegative patients. Taken together, one might assume that UPR-activation within muscle fibers might not only serve to restore protein homeostasis, but also enhance sarcolemmal presentation of proteins crucial for attracting immune cells. Since modulation of ER-stress and UPR via application of chemical chaperones became a promising therapeutic treatment approach, our findings might represent the starting point for new interventional concepts.
患有免疫介导的坏死性肌病 (IMNM) 的患者携带肌炎特异性自身抗体抗 SRP54 或 -HMGCR，而其中约三分之一的患者没有。伴侣蛋白辅助自噬的激活被描述为 IMNM 分子发病机制的一部分。内质网 (ER)/肌浆网 (SR)-应激伴随着未折叠蛋白反应 (UPR) 的激活，通常先于蛋白质清除机制的激活，代表细胞对蛋白质稳态恢复的防御机制。在这里，我们表明 ER/SR-应激可能是 IMNM 分子发病机制的一部分。为了解决这个假设，我们利用源自 IMNM 患者的 39 个肌肉活检标本，在转录和蛋白质水平上研究了涵盖三个已知分支（PERK 介导、IRE1 介导和 ATF6 介导）的 ER/SR-应激相关关键因子。我们的结果表明，IMNM 中所有三个 UPR 分支均被激活，这很可能先于蛋白质清除机制的激活。具体而言，我们发现 PERK 和 eIF2a 的磷酸化增加，以及 ATF4 的表达和蛋白丰度增加，所有这些都是 UPR 激活的特征。此外，我们还发现了普遍的 XBP1 水平增加和 XBP1 蛋白水平升高。此外，我们的转录研究表明 ATF6 表达增加，免疫染色研究证实了 cleaved ATF6 形式向 UPR 相关伴侣蛋白的强制转录的核内易位，证实了这一点。相应地，我们的数据表明包括 ER/SR 共伴侣和伴侣蛋白（例如 SIL1）在内的下游因子增加，表明 UPR 在更广泛的水平上激活，而在血清阳性和血清阴性患者之间没有显著差异。总之，人们可能会认为肌纤维内的 UPR 激活不仅有助于恢复蛋白质稳态，还增强了吸引免疫细胞的蛋白质的肌膜表达。由于通过应用化学伴侣物来调节 ER 应激和 UPR 已成为一种有前途的治疗方法，我们的发现可能代表新的干预概念的起点。
