Summary Prion-like propagation of tau aggregation may underlie the stereotyped progression of neurodegenerative tauopathies. True prions stably maintain unique conformations (“strains”) in vivo that link structure to patterns of pathology. We now find that tau meets this criterion. Stably expressed tau repeat domain indefinitely propagates distinct amyloid conformations in a clonal fashion in culture. Reintroduction of tau from these lines into naïve cells re-establishes identical clones. We produced two strains in vitro that induce distinct pathologies in vivo as determined by successive inoculations into three generations of transgenic mice. Immunopurified tau from these mice re-creates the original strains in culture. We used the cell system to isolate tau strains from 29 patients with 5 different tauopathies, finding that different diseases are associated with different sets of strains. Tau thus demonstrates essential characteristics of a prion. This may explain the phenotypic diversity of tauopathies and could enable more effective diagnosis and therapy.

Highlights d Auditory gamma entrainment using sensory stimuli (GENUS) boosts hippocampal function d GENUS affects microglia, astrocytes, and vasculature in auditory cortex and hippocampus d Auditory plus visual GENUS induces microglia clustering around plaques d Auditory plus visual GENUS reduces amyloid pathology throughout neocortex

Increased p25, a proteolytic fragment of the regulatory subunit p35, is known to induce aberrant activity of cyclin-dependent kinase 5 (Cdk5), which is associated with neurodegenerative disorders, including Alzheimer's disease. Previously, we showed that replacing endogenous p35 with the noncleavable mutant p35 (Δp35) attenuated amyloidosis and improved cognitive function in a familial Alzheimer's disease mouse model. Here, to address the role of p25/Cdk5 in tauopathy, we generated double-transgenic mice by crossing mice overexpressing mutant human tau (P301S) with Δ mice. We observed significant reduction of phosphorylated tau and its seeding activity in the brain of double transgenic mice compared with the P301S mice. Furthermore, synaptic loss and impaired LTP at hippocampal CA3 region of mice were attenuated by blocking p25 generation. To further validate the role of p25/Cdk5 in tauopathy, we used frontotemporal dementia patient-derived induced pluripotent stem cells (iPSCs) carrying the Tau P301L mutation and generated P301L:Δp35KI isogenic iPSC lines using CRISPR/Cas9 genome editing. We created cerebral organoids from the isogenic iPSCs and found that blockade of p25 generation reduced levels of phosphorylated tau and increased expression of synaptophysin. Together, these data demonstrate a crucial role for p25/Cdk5 in mediating tau-associated pathology and suggest that inhibition of this kinase can remedy neurodegenerative processes in the presence of pathogenic tau mutation. Accumulation of p25 results in aberrant Cdk5 activation and induction of numerous pathological phenotypes, such as neuroinflammation, synaptic loss, Aβ accumulation, and tau hyperphosphorylation. However, it was not clear whether p25/Cdk5 activity is necessary for the progression of these pathological changes. We recently developed the Δ transgenic mouse that is deficient in p25 generation and Cdk5 hyperactivation. In this study, we used this mouse model to elucidate the role of p25/Cdk5 in FTD mutant tau-mediated pathology. We also used a frontotemporal dementia patient-derived induced pluripotent stem cell carrying the Tau P301L mutation and generated isogenic lines in which p35 is replaced with noncleavable mutant Δp35. Our data suggest that p25/Cdk5 plays an important role in tauopathy in both mouse and human model systems.

Mef2 is induced after enrichment in mice, reduces disease-associated hyperexcitability, and is a feature of cognitive resilience in patients with AD.

Acknowledgements 31We thank E. McNamara and M. Taylor for mouse colony maintenance and members 32 of Tsai lab for discussion and valuable comments on manuscript. We also thank 33Harvard's Center for Macromolecular Interactions for access to instrumentation for 34 Biolayer Interferometry (BLI) and Microscale Thermophoresis. S. Abstract 43Aberrant activity of cyclin-dependent kinase (Cdk5) has been implicated in various 44 neurodegenerative diseases. This effect is mediated by pathological cleavage of the 45 Cdk5 activator p35 to produce the truncated product p25, exhibiting increased 46 stability and altered substrate specificity. The benefit of blocking p25 production has 47 selectively inhibit Cdk5/p25 kinase activity. Using cellular assays, mouse 54 neurodegeneration models and human cerebral organoids generated from patient-55 derived iPSCs, we demonstrate beneficial effects of the Cdk5i peptide on various 56 pathological phenotypes including gliosis, DNA damage, and Tau 57 hyperphosphorylation. 58 59 60 61 62 63 64 65 Hall et al., 2016). It is worth noting that the length of a peptide is the limiting factor in 128 such assays; shorter peptide lengths increased assay efficiency. 129 130 Our earlier efforts determined the structural basis of the Cdk5/p25 interaction 131 (Tarricone et al., 2001) and provided insight for developing a novel inhibitory peptide 132for Cdk5/p25 derived from the Cdk5 sequence which we report here. This 12-amino-133 acid long Cdk5 inhibitory peptide (hereafter referred to as 'Cdk5i peptide') has higher 134