Synaptic distributions of pS214-tau in rhesus monkey prefrontal cortex are associated with spine density, but not with cognitive decline

Comparative neuropathology in aging primates: A perspective

While humans exhibit a significant degree of neuropathological changes associated with deficits in cognitive and memory functions during aging, non-human primates (NHP) present with more variable expressions of pathological alterations among individuals and species. As such, NHP with long life expectancy in captivity offer an opportunity to study brain senescence in the absence of the typical cellular pathology caused by age-related neurodegenerative illnesses commonly seen in humans. Age-related changes at neuronal population, single cell, and synaptic levels have been well documented in macaques and marmosets, while age-related and Alzheimer's disease-like neuropathology has been characterized in additional species including lemurs as well as great apes. We present a comparative overview of existing neuropathologic observations across the primate order, including classic age-related changes such as cell loss, amyloid deposition, amyloid angiopathy, and tau accumulation. We also review existing cellular and ultrastructural data on neuronal changes, such as dendritic attrition and spine alterations, synaptic loss and pathology, and axonal and myelin pathology, and discuss their repercussions on cellular and systems function and cognition.

SIRT1 Regulates Tau Expression and Tau Synaptic Pathology

BACKGROUND

Amyloid plaques and neurofibrillary tangles are two pathological hallmarks of Alzheimer's disease (AD). However, synaptic deficits occur much earlier and correlate stronger with cognitive decline than amyloid plaques and neurofibrillary tangles. Mislocalization of tau is an early hallmark of neurodegeneration and precedes aggregations. Sirtuin type 1 (SIRT1) is a deacetylase which acts on proteins including transcriptional factors and associates closely with AD.

OBJECTIVE

The present study investigated the association between SIRT1 and tau expression/tau localization in cells and in mice brains.

METHODS

Western blot was performed to detected tau, SIRT1, C/EBPα, and GAPDH protein levels. Immunological fluorescence assay was used to assess tau localization in primary cortical neuronal cells. Golgi staining was performed to evaluated dendritic spine morphology in mice brains.

RESULTS

In the present study, we found that SIRT1 negatively regulates expression of tau at the transcriptional level through transcriptional factor C/EBPα. Inhibition of the activity of SIRT1 limits the distribution of tau to the neurites. In the meantime, the alteration of dendritic spine morphology is also observed in the brains of SIRT1+/- mice.

CONCLUSION

SIRT1 may be a potential drug target for early intervention in AD.

Alzheimer's-like pathology in aging rhesus macaques: Unique opportunity to study the etiology and treatment of Alzheimer's disease

Although mouse models of Alzheimer's disease (AD) have provided tremendous breakthroughs, the etiology of later onset AD remains unknown. In particular, tau pathology in the association cortex is poorly replicated in mouse models. Aging rhesus monkeys naturally develop cognitive deficits, amyloid plaques, and the same qualitative pattern and sequence of tau pathology as humans, with tangles in the oldest animals. Thus, aging rhesus monkeys can play a key role in AD research. For example, aging monkeys can help reveal how synapses in the prefrontal association cortex are uniquely regulated compared to the primary sensory cortex in ways that render them vulnerable to calcium dysregulation and tau phosphorylation, resulting in the selective localization of tau pathology observed in AD. The ability to assay early tau phosphorylation states and perform high-quality immunoelectron microscopy in monkeys is a great advantage, as one can capture early-stage degeneration as it naturally occurs in situ. Our immunoelectron microscopy studies show that phosphorylated tau can induce an "endosomal traffic jam" that drives amyloid precursor protein cleavage to amyloid-β in endosomes. As amyloid-β increases tau phosphorylation, this creates a vicious cycle where varied precipitating factors all lead to a similar phenotype. These data may help explain why circuits with aggressive tau pathology (e.g., entorhinal cortex) may degenerate prior to producing significant amyloid pathology. Aging monkeys therefore can play an important role in identifying and testing potential therapeutics to protect the association cortex, including preventive therapies that are challenging to test in humans.