Proteostasis dysregulation and the development of Alzheimer's-like neurodegeneration and dementia in Down syndrome

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Down syndrome (DS) is a congenital condition resulting from partial or complete triplication of human chromosome 21. Virtually all the subjects with DS develop widespread neuropathology including amyloid- neuritic plaques, synaptic dysfunction and neurodegeneration, reminiscent of Alzheimer's disease. While the extra copy of amyloid- precursor protein (APP) on chromosome 21 is thought to play a major role in the development of this type of pathologic phenotype in DS, the underlying mechanisms responsible for these changes and their contribution to neurodegeneration are still elusive. Compelling evidence supports the hypothesis that physiological cellular proteostasis is of critical importance for neuronal health and that the mammalian target of rapamycin (mTOR) is a master regulator of this vital cellular function. However, while we know that the system is altered in DS we do not know whether it plays a functional role in the pathogenesis of DS and the onset of neuropathology. In our preliminary data we found that compared with healthy matched controls mTOR is hyperactive in selected brain regions of DS patients. Importantly, we observed that in the same subjects the dysfunction directly correlates with the pathology. Moreover, we show that mTOR is altered in the brain of a well- established mouse model of DS, the Ts65Dn mice, at an early stage of the phenotype, and associates with biochemical evidence of cell loss, suggesting a link between mTOR, neuropathology and neurodegeneration in DS subjects. Taking into consideration the scientific rigor of the previously published literature together with our recent findings we now propose a novel working hypothesis: alteration of mTOR signaling pathway is responsible for the onset of the neuropathologic DS phenotype and represents a novel and viable therapeutic target against it in DS subjects. In this proposal, we will assess the temporal relationship between dysregulation of mTOR in the brain of DS patients and the development of the neuropathology. We will then focus on investigating early events responsible for this dysregulation in the same DS subjects. Next, to prove its direct role in the pathogenesis of the syndrome, we will study the effect that modulation of mTOR activity and expression levels has on behavior impairments and neuropathology using in vivo models of DS. The results of our proposed studies will elucidate early changes and the functional consequences of altered proteostasis secondary to dysregulated mTOR in the development of the neuropathologic phenotype in individuals with DS. Importantly, our findings have the potential to identify new therapeutic opportunities for delaying its onset and /or halting its progression.