A unique subpopulation of wild-type neurons recapitulating FAD phenotypes

摘要 晚发性散发性阿尔茨海默病(SAD)患者表现出与早发性家族性AD(FAD)患者总体相同的临床和病理特征。然而，这两种遗传上不同的阿尔茨海默病的临床病理共同性背后的机制(S)尚不清楚。我们的总体假设是，大脑中的一个野生型神经元亚群惊人地概括了表达FAD突变体早老素(PSEN)的神经元的表型(可能是通过野生型PSEN1的翻译后修饰)，这种选择性细胞群在SAD神经退行性变中发挥作用。有几个证据支持这一假设。首先，我们证明了SAD大脑中的一部分神经元中的野生型PSEN1显示了类似于FAD突变体PSEN1的构象(Wahlster等人。《神经病理学报》2013)。其次，我们发现PKA介导的PSEN1在Ser310的磷酸化在SAD脑中显著上调，并且这个翻译后的修饰，与另外两个位点的磷酸化一起，引导野生型PSEN1向FAD突变体PSEN1的构象(Maesako等人)。ELife 2017)。最后，我们最近开发了新型的基于基因编码的FRET生物传感器，它首次能够在逐个细胞的基础上定量记录活神经元中随时间推移的伽马分泌酶活性(Maesako等人)。IScience 2020，Houser等人。Sensors 2020，Houser等人。Biosensors 2021，Maesako等人。J Neurosci 2022)。令人惊讶的是，这些生物传感器使我们能够发现一种独特的野生型神经元亚群，显示出内源性伽马分泌酶活性减弱。更重要的是，我们强大的初步数据表明，这个细胞群概括了在表达FAD突变体PSEN的神经元中发现的几个关键特征；这些特征包括伽马分泌酶“加工性”受损，从而主要产生Long Aβ、内溶酶体异常，以及对毒性侮辱的易损性表型。因此，这项建议将进一步使用多种模型系统和互补分析来建立支持我们假设的分子基础和生理相关性。目的1阐明神经元内源性γ-分泌酶活性异质性的分子机制(S)及其后果。目的2将进一步验证功能失调的伽马分泌酶、内切酶异常和神经元易损性之间的因果关系。更重要的是，我们将探索美国FDA批准的化合物的治疗潜力，这些化合物可能作为伽马分泌酶调节剂(GSM)或伽马分泌酶激活剂(GSA)发挥作用。目的3将确定在内源性表达野生型PSEN的“AD”小鼠模型中是否存在独特的FAD样神经元群体，正如我们的初步结果所表明的那样，在IPSCs来源的人类神经元和SAD病例的死后脑中。鉴于促进神经元弹性可能是AD的一种新的治疗策略，更好地了解新发现的选择性脆弱细胞群背后的分子基础将为开发新的治疗机会开辟一条新的途径。

Abstract People with late-onset sporadic Alzheimer’s disease (SAD) display overall the same clinical and pathological features as those with early-onset familial AD (FAD). However, the mechanism(s) underlying the clinicopathologic commonality between these two genetically distinct AD forms is unclear. Our overall hypothesis is that a subpopulation of wild-type neurons in the brain strikingly recapitulates the phenotypes of neurons expressing FAD mutant Presenilin (PSEN) (perhaps via post-translational modification of wild-type PSEN1), and this selective cell population plays a role in SAD neurodegeneration. Several pieces of evidence support this hypothesis. First, we showed that wild-type PSEN1 in a subset of neurons within the SAD brain displays a conformation similar to FAD mutant PSEN1 (Wahlster et al. Acta Neuropathol 2013). Second, we uncovered that PKA-mediated PSEN1 phosphorylation at Ser310 is significantly upregulated in SAD brains, and this post translational modification, together with phosphorylation of two other sites, steers wild-type PSEN1 conformation towards that of FAD mutant PSEN1 (Maesako et al. eLife 2017). Lastly, we have recently developed novel genetically encoded FRET-based biosensors that for the first time allow quantitative recording of the gamma-secretase activity over time, on a cell-by-cell basis, in live neurons (Maesako et al. iScience 2020, Houser et al. Sensors 2020, Houser et al. Biosensors 2021, Maesako et al. J Neurosci 2022). Surprisingly, these biosensors have enabled us to discover a unique subpopulation of wild-type neurons displaying diminished endogenous gamma-secretase activity. More importantly, our strong preliminary data show that this cell population recapitulates several key characteristics that have been identified in neurons expressing FAD mutant PSEN; these include impaired gamma-secretase “processivity” and thus predominant production of long Aβ, endo-lysosomal abnormalities, and vulnerability phenotypes in response to toxic insults. Therefore, this proposal will further employ multiple model systems and complementary assays to establish the molecular basis and physiological relevance that support our hypothesis. Aim 1 will elucidate the molecular mechanism(s) underlying the heterogeneity in endogenous gamma-secretase activity and its consequences in neurons. Aim 2 will further verify the cause-and-effect relationship between dysfunctional gamma-secretase, endo-lysosomal abnormalities, and neuronal vulnerability. More importantly, we will explore the therapeutic potential of the US FDA-approved compounds that could potentially function as gamma-secretase modulators (GSMs) or gamma-secretase activators (GSAs). Aim 3 will determine if the unique FAD-like neuronal population exists in “AD” mouse models endogenously expressing wild-type PSEN, as our preliminary results indicate, in iPSCs derived human neurons and post-mortem brains from SAD cases. Given that promoting neuronal resilience could be a new therapeutic strategy for AD, a better understanding of the molecular basis behind the newly discovered selectively vulnerable cell population will open a new path for developing novel therapeutic opportunities.