Data_Sheet_1_TrkB-ICD Fragment, Originating From BDNF Receptor Cleavage, Is Translocated to Cell Nucleus and Phosphorylates Nuclear and Axonal Proteins.pdf

The signaling of brain-derived neurotrophic factor (BDNF) has been suggested to be impaired in Alzheimer’s disease (AD), which may compromise the function of BDNF upon neuronal activity and survival. Accordingly, decreased levels of BDNF and its tropomyosin-receptor kinase B-full-length (TrkB-FL) have been detected in human brain samples of AD patients. We have previously found that neuronal exposure to amyloid-β (Aβ) peptide, a hallmark of AD, leads to calpain overactivation and subsequent TrkB-FL cleavage leading to decreased levels of TrkB-FL and the generation of two new fragments: a membrane-bound truncated receptor (TrkB-T′) and an intracellular fragment (TrkB-ICD). Importantly, we identified this TrkB-FL cleavage and TrkB-ICD presence in human brain samples, which indicates that this molecular mechanism contributes to the loss of BDNF signaling in humans. The exact role of this TrkB-ICD fragment is, however, unknown. Here, we used a human neuroglioma cell line and rat cortical primary neuronal cultures to track TrkB-ICD intracellularly. Our data show that TrkB-ICD is a relatively stable fragment that accumulates in the nucleus over time, through a phosphorylation-dependent process. We also found that TrkB-ICD has tyrosine kinase activity, inducing the phosphorylation of nuclear and axonal proteins. These findings suggest that TrkB-ICD may lead to a dysregulation of the activity of several proteins, including proteins in the nucleus, to where TrkB-ICD migrates. Since TrkB-ICD is formed by Aβ peptide-induced cleavage of TrkB-FL, the present data highlights a new mechanism that may have a role in AD pathophysiology.

脑源性神经营养因子（brain-derived neurotrophic factor, BDNF）的信号转导通路在阿尔茨海默病（Alzheimer’s disease, AD）中被证实存在受损情况，这可能会削弱BDNF对神经元活动与存活的调控功能。相应地，研究人员已在AD患者的人类脑组织样本中检测到BDNF及其原肌球蛋白受体激酶B全长型（tropomyosin-receptor kinase B-full-length, TrkB-FL）的表达水平显著下降。我们此前的研究发现，神经元暴露于淀粉样β（amyloid-β, Aβ）肽——AD的核心病理标志性物质——会引发钙蛋白酶过度激活，继而介导TrkB-FL发生蛋白切割，导致TrkB-FL水平降低，并同时产生两种全新的蛋白片段：一种是锚定在细胞膜上的截短型受体（TrkB-T′），另一种是细胞内片段（TrkB-ICD）。尤为关键的是，我们在人类AD患者脑组织样本中也检测到了该TrkB-FL切割现象与TrkB-ICD的存在，这表明该分子机制确实参与了人类体内BDNF信号转导的缺失过程。然而，目前对于TrkB-ICD片段的确切生物学功能仍尚不明确。本研究中，我们借助人类神经胶质瘤细胞系与大鼠皮层原代神经元培养体系，对TrkB-ICD的细胞内分布与动态变化进行了追踪分析。实验数据显示，TrkB-ICD是一种相对稳定的蛋白片段，可通过磷酸化依赖的信号通路随时间推移在细胞核内不断积累。此外，我们还证实TrkB-ICD具备酪氨酸激酶活性，能够诱导细胞核与轴突内多种蛋白质发生磷酸化修饰。上述研究结果表明，TrkB-ICD可能会引发多种蛋白质的活性失调，其中就包括TrkB-ICD迁移靶向的细胞核内蛋白质。鉴于TrkB-ICD是由淀粉样β肽诱导切割TrkB-FL所产生的，本研究的数据揭示了一种可能参与AD病理生理进程的全新分子机制。