Chu Jin, Li Jian-Guo, Hoffman Nicholas E, Madesh Muniswamy, Praticò Domenico
Department of Pharmacology, Center for Translational Medicine, Temple University, Philadelphia, Pennsylvania, USA.
J Neurochem. 2015 May;133(3):432-9. doi: 10.1111/jnc.13011. Epub 2015 Feb 13.
A major hallmark feature of Alzheimer's disease is the accumulation of amyloid β (Aβ), whose formation is regulated by the γ-secretase complex and its activating protein (also known as γ-secretase activating protein, or GSAP). Because GSAP interacts with the γ-secretase without affecting the cleavage of Notch, it is an ideal target for a viable anti-Aβ therapy. GSAP derives from a C-terminal fragment of a larger precursor protein of 98 kDa via a caspase 3-mediated cleavage. However, the mechanism(s) involved in its degradation remain unknown. In this study, we show that GSAP has a short half-life of approximately 5 h. Neuronal cells treated with proteasome inhibitors markedly prevented GSAP protein degradation, which was associated with a significant increment in Aβ levels and γ-secretase cleavage products. In contrast, treatment with calpain blocker and lysosome inhibitors had no effect. In addition, we provide experimental evidence that GSAP is ubiquitinated. Taken together, our findings reveal that GSAP is degraded through the ubiquitin-proteasome system. Modulation of the GSAP degradation pathway may be implemented as a viable target for a safer anti-Aβ therapeutic approach in Alzheimer's disease. The GSAP derives from a precursor via a caspase 3-mediated cleavage, is up-regulated in Alzheimer's disease brains and facilitates Aβ production by interacting directly with the γ-secretase complex. Here, we demonstrate that GSAP is ubiquitinated and then selectively degraded via the proteasome system but not the calpains or lysosome pathways. These findings provide further evidence for the involvement of the proteasome system in the regulation of amyloid beta (Aβ) precursor protein metabolism and Aβ formation. AICD, APP intracellular domain; APP, amyloid precursor protein; ATP, adenosine triphosphate; CTF-α, alpha-C-terminal fragment; CTF-β, beta-C-terminal fragment; GSAP, γ-secretase activating protein; Ub, ubiquitin.
阿尔茨海默病的一个主要标志性特征是β淀粉样蛋白（Aβ）的积累，其形成受γ-分泌酶复合物及其激活蛋白（也称为γ-分泌酶激活蛋白，或GSAP）调控。由于GSAP与γ-分泌酶相互作用而不影响Notch的切割，因此它是可行的抗Aβ治疗的理想靶点。GSAP通过半胱天冬酶3介导的切割从一个98 kDa的较大前体蛋白的C末端片段衍生而来。然而，其降解所涉及的机制仍然未知。在本研究中，我们表明GSAP的半衰期约为5小时。用蛋白酶体抑制剂处理的神经元细胞显著阻止了GSAP蛋白的降解，这与Aβ水平和γ-分泌酶切割产物的显著增加有关。相反，用钙蛋白酶阻滞剂和溶酶体抑制剂处理没有效果。此外，我们提供了GSAP被泛素化的实验证据。综上所述，我们的研究结果表明GSAP通过泛素-蛋白酶体系统降解。GSAP降解途径的调节可能作为阿尔茨海默病中更安全的抗Aβ治疗方法的可行靶点。GSAP通过半胱天冬酶3介导的切割从前体衍生而来，在阿尔茨海默病大脑中上调，并通过直接与γ-分泌酶复合物相互作用促进Aβ的产生。在这里，我们证明GSAP被泛素化，然后通过蛋白酶体系统而非钙蛋白酶或溶酶体途径被选择性降解。这些发现为蛋白酶体系统参与淀粉样β（Aβ）前体蛋白代谢和Aβ形成的调节提供了进一步的证据。AICD，APP细胞内结构域；APP，淀粉样前体蛋白；ATP，三磷酸腺苷；CTF-α，α-C末端片段；CTF-β，β-C末端片段；GSAP，γ-分泌酶激活蛋白；Ub，泛素。
