Emerging studies implicate Tau as an essential mediator of neuronal atrophy and cognitive impairment in Alzheimer's disease (AD), yet the factors that precipitate Tau dysfunction in AD are poorly understood. Chronic environmental stress and elevated glucocorticoids (GC), the major stress hormones, are associated with increased risk of AD and have been shown to trigger intracellular Tau accumulation and downstream Tau-dependent neuronal dysfunction. However, the mechanisms through which stress and GC disrupt Tau clearance and degradation in neurons remain unclear. Here, we demonstrate that Tau undergoes degradation via endolysosomal sorting in a pathway requiring the small GTPase Rab35 and the endosomal sorting complex required for transport (ESCRT) machinery. Furthermore, we find that GC impair Tau degradation by decreasing Rab35 levels, and that AAV-mediated expression of Rab35 in the hippocampus rescues GC-induced Tau accumulation and related neurostructural deficits. These studies indicate that the Rab35/ESCRT pathway is essential for Tau clearance and part of the mechanism through which GC precipitate brain pathology.

227 words 23 Article Body: 3771 words (excluding M&M, legends, abstract and references) 24 Fig.: 8 25 Tables: 0 26 Supplementary Information: 1 27 28 2 Abstract 29 Imbalance of neuronal proteostasis associated with misfolding and aggregation of Tau protein is a 30 common neurodegenerative feature in Alzheimer's disease (AD) and other Tauopathies. Consistent 31with suggestions that lifetime stress may be an important AD precipitating factor, we previously 32 reported that environmental stress and high glucocorticoid (GC) levels induce accumulation of 33 aggregated Tau; however, the molecular mechanisms for such process remain unclear. Herein, we 34 monitor a novel interplay between RNA-binding proteins (RBPs) and autophagic machinery in the 35 underlying mechanisms through which chronic stress and high GC levels impact on Tau proteostasis 36 precipitating Tau aggregation. Using molecular, pharmacological and behavioral analysis, we 37 demonstrate that chronic stress and high GC trigger a mTOR-dependent inhibition of autophagy, 38 leading to accumulation of Tau aggregates and cell death in P301L-Tau expressing mice and cells. 39In parallel, we found that environmental stress and GC disturb cellular homeostasis and trigger the 40 insoluble accumulation of different RBPs, such as PABP, G3BP1, TIA-1, and FUS, shown to form 41 Stress granules(SGs) and Tau aggregation. Interestingly, an mTOR-driven pharmacological 42 stimulation of autophagy attenuates the GC-driven accumulation of Tau and SG-related proteins as 43 well as the related cell death, suggesting a critical interface between autophagy and the response of 44 the SG-related protein in the neurodegenerative potential of chronic stress and GC. These studies 45 provide novel insights into the RNA-protein intracellular signaling regulating the precipitating role of 46 environmental stress and GC on Tau-driven brain pathology. 47 48 49 50 51 Silva et al 3 Alzheimer's disease (AD) is a multifactorial neurodegenerative disorder with a complex 53 pathophysiology and still undefined initiators. Several risk factors have been associated with AD 54 pathology, with recent evidence supporting a detrimental role of lifetime stress 1-3 . Clinical studies 55 relate distress, high cortisol levels and dysfunction of hypothalamus-pituitary-adrenal (HPA) axis with 56 poor memory scores and earlier disease onset in AD patients highlighting the potential implication of 57 chronic stress and glucocorticoids (GC) in the pathogenesis and/or progression of the disorder 4-6 . In 58 line with the above clinical evidence, experimental studies have shown that chronic stress and 59 exposure to high GC levels trigger Tau hyperphosphorylation and malfunction leading to its 60 accumulation, formation of neurotoxic Tau aggregates and AD pathology 1,7,8 . Despite our little 61knowledge about the molecular mechanisms that underpin stress-driven pathology, experimental 62 evidence suggests that stress/GC reduces Tau turnover 9 , suggesting that stress/GC impact on the 63 chaperones and proteases th...

Tau protein in dendrites and synapses has been recently implicated in synaptic degeneration and neuronal malfunction. Chronic stress, a well-known inducer of neuronal/synaptic atrophy, triggers hyperphosphorylation of Tau protein and cognitive deficits. However, the cause-effect relationship between these events remains to be established. To test the involvement of Tau in stress-induced impairments of cognition, we investigated the impact of stress on cognitive behavior, neuronal structure, and the synaptic proteome in the prefrontal cortex (PFC) of Tau knock-out (Tau-KO) and wild-type (WT) mice. Whereas exposure to chronic stress resulted in atrophy of apical dendrites and spine loss in PFC neurons as well as significant impairments in working memory in WT mice, such changes were absent in Tau-KO animals. Quantitative proteomic analysis of PFC synaptosomal fractions, combined with transmission electron microscopy analysis, suggested a prominent role for mitochondria in the regulation of the effects of stress. Specifically, chronically stressed animals exhibit Tau-dependent alterations in the levels of proteins involved in mitochondrial transport and oxidative phosphorylation as well as in the synaptic localization of mitochondria in PFC. These findings provide evidence for a causal role of Tau in mediating stress-elicited neuronal atrophy and cognitive impairment and indicate that Tau may exert its effects through synaptic mitochondria.

Stress, a well-known sculptor of brain plasticity, is shown to suppress hippocampal neurogenesis in the adult brain; yet, the underlying cellular mechanisms are poorly investigated. Previous studies have shown that chronic stress triggers hyperphosphorylation and accumulation of the cytoskeletal protein Tau, a process that may impair the cytoskeleton-regulating role(s) of this protein with impact on neuronal function. Here, we analyzed the role of Tau on stress-driven suppression of neurogenesis in the adult dentate gyrus (DG) using animals lacking Tau (Tau-knockout; Tau-KO) and wild-type (WT) littermates. Unlike WTs, Tau-KO animals exposed to chronic stress did not exhibit reduction in DG proliferating cells, neuroblasts and newborn neurons; however, newborn astrocytes were similarly decreased in both Tau-KO and WT mice. In addition, chronic stress reduced phosphoinositide 3-kinase (PI3K)/mammalian target of rapamycin (mTOR)/glycogen synthase kinase-3β (GSK3β)/β-catenin signaling, known to regulate cell survival and proliferation, in the DG of WT, but not Tau-KO, animals. These data establish Tau as a critical regulator of the cellular cascades underlying stress deficits on hippocampal neurogenesis in the adult brain.

Chronic stress, a suggested precipitant of brain pathologies, such as depression and Alzheimer’s disease, is known to impact on brain plasticity by causing neuronal remodeling as well as neurogenesis suppression in the adult hippocampus. Although many studies show that stressful conditions reduce the number of newborn neurons in the adult dentate gyrus (DG), little is known about whether and how stress impacts on dendritic development and structural maturation of these newborn neurons. We, herein, demonstrate that chronic stress impacts differentially on doublecortin (DCX)-positive immature neurons in distinct phases of maturation. Specifically, the density of the DCX-positive immature neurons whose dendritic tree reaches the inner molecular layer (IML) of DG is reduced in stressed animals, whereas their dendritic complexity is increased. On the contrary, no change on the density of DCX-positive neurons whose dendritic tree extends to the medial/outer molecular layer (M/OML) of the DG is found under stress conditions, whereas the dendritic complexity of these cells is diminished. In addition, DCX+ cells displayed a more complex and longer arbor in the dendritic compartments located in the granular cell layer of the DG under stress conditions; on the contrary, their dendritic segments localized into the M/OML were shorter and less complex. These findings suggest that the neuroplastic effects of chronic stress on dendritic maturation and complexity of DCX+ immature neurons vary based on the different maturation stage of DCX-positive cells and the different DG sublayer, highlighting the complex and dynamic stress-driven neuroplasticity of immature neurons in the adult hippocampus.