Brijbassi Sonya, Amtul Zareen, Newbigging Susan, Westaway David, St George-Hyslop Peter, Rozmahel Richard F
Department of Biochemistry, University of Western Ontario, London, Ontario, Canada.
Neurobiol Dis. 2007 Feb;25(2):291-6. doi: 10.1016/j.nbd.2006.09.013. Epub 2006 Oct 27.
Nicastrin is an integral member of PS-complexes that perform gamma-secretase cleavage of numerous type I membrane proteins including amyloid precursor protein that underlies Alzheimer's disease; thus, diminishing gamma-secretase activity by reducing levels of functional PS-complexes is suggested as a possible preventative/therapeutic avenue for the disease. One means of reducing PS-complex activity entails decreasing the levels of one or more of its components, such as nicastrin, which is fundamental to its assembly. Two previous studies detailing the effects of decreased nicastrin on gamma-secretase cleavage of APP in nicastrin heterozygous mouse fibroblast, which express relatively low levels of endogenous nicastrin compared to neurons, were contradictory. One report documented a 50% reduction in gamma-secretase cleavage of APP while the second showed markedly higher levels of this activity. Here we report that brains of heterozygous nicastrin mice show no difference in levels of APP gamma-secretase cleavage, APP C-terminal fragments or beta-amyloid peptides, compared to wild-type. This result is explained by the levels of nicastrin protein and functional presenilin complexes being similar between the heterozygous and wild-type brains, though nicastrin mRNA levels were diminished appropriately in the former. These in vivo results indicate that nicastrin mRNA and its immature protein are likely in overabundance in neurons and not limiting for assembly of PS-complexes, and that a 50% reduction of its mRNA or protein production would not affect APP processing, in contrast to fibroblast. Thus, partial reduction (maintaining a level above 50% of normal) of brain nicastrin would likely not be efficacious in reducing functional PS-complexes and gamma-secretase activity as a therapeutic strategy for Alzheimer's disease.
尼卡斯特林是PS复合物的一个组成成员，PS复合物对包括阿尔茨海默病所涉及的淀粉样前体蛋白在内的众多I型膜蛋白进行γ-分泌酶切割；因此，通过降低功能性PS复合物的水平来减少γ-分泌酶活性被认为是该疾病一种可能的预防/治疗途径。降低PS复合物活性的一种方法是降低其一种或多种组分的水平，例如对其组装至关重要的尼卡斯特林。之前有两项研究详细阐述了尼卡斯特林减少对尼卡斯特林杂合小鼠成纤维细胞中APP的γ-分泌酶切割的影响，与神经元相比，这些细胞表达的内源性尼卡斯特林水平相对较低，这两项研究结果相互矛盾。一份报告记录了APP的γ-分泌酶切割减少了50%，而另一份报告则显示该活性水平明显更高。我们在此报告，与野生型相比，杂合尼卡斯特林小鼠的大脑在APP的γ-分泌酶切割水平、APP C末端片段或β-淀粉样肽水平上没有差异。这一结果的解释是，杂合型和野生型大脑中尼卡斯特林蛋白和功能性早老素复合物的水平相似，尽管前者的尼卡斯特林mRNA水平适当降低。这些体内研究结果表明，尼卡斯特林mRNA及其未成熟蛋白在神经元中可能过量存在，并非PS复合物组装的限制因素，并且与成纤维细胞相反，其mRNA或蛋白产量降低50%不会影响APP的加工。因此，作为阿尔茨海默病的治疗策略，部分降低（维持在正常水平的50%以上）大脑中的尼卡斯特林可能无法有效降低功能性PS复合物和γ-分泌酶活性。
