No abstract

The dentate gyrus (DG) of the human hippocampus is a complex and dynamic structure harboring mature and immature granular neurons in diverse proliferative states. While most mammals show persistent neurogenesis through adulthood, human neurogenesis is still under debate. We found nuclear alterations in granular cells in autopsied human brains, detected by immunohistochemistry. These alterations differ from those reported in pyramidal neurons of the hippocampal circuit. Aging and early AD chromatin were clearly differentiated by the increased epigenetic markers H3K9me3 (heterochromatin suppressive mark) and H3K4me3 (transcriptional euchromatin mark). At early AD stages, lamin B2 was redistributed to the nucleoplasm, indicating cell-cycle reactivation, probably induced by hippocampal nuclear pathology. At intermediate and late AD stages, higher lamin B2 immunopositivity in the perinucleus suggests fewer immature neurons, less neurogenesis, and fewer adaptation resources to environmental factors. In addition, senile samples showed increased nuclear Tau interacting with aged chromatin, likely favoring DNA repair and maintaining genomic stability. However, at late AD stages, the progressive disappearance of phosphorylated Tau forms in the nucleus, increased chromatin disorganization, and increased nuclear autophagy support a model of biphasic neurogenesis in AD. Therefore, designing therapies to alleviate the neuronal nuclear pathology might be the only pathway to a true rejuvenation of brain circuits.

Dementia is a public health problem with an extraordinary increase in recent years. Alzheimer's disease (AD) is the most common cause of dementia. This disease has been considered a consequence of cytoplasmic and extracellular accumulations of Tau protein and β-amyloid, respectively. Nevertheless, a nuclear origin of AD has recently emerged. Both Tau protein and the nuclear lamin protect the nuclear and chromatin organization for proper gene expression throughout neuronal life. Accumulation of DNA damage, mainly as a result of aging, drives post-mitotic neurons to initiate DNA repair by entering the cell cycle. The complexity of the nucleus-cytoskeleton prevents neurons from dividing and condemns them to a state of hyperdiploidy ending in neuronal death, after transiently prolonging their life. In AD, hippocampal neurons survive their fatal fate by triggering an aberrant structural and functional transformation of the nucleus. Lamin A expression and Tau protein transfer to the cytoplasm results in loss of the protector role of nuclear Tau and the subsequent global chromatin disorganization. Therefore, the cytoplasmic Tau protein accumulations that characterize AD are consequence of the attempted neuronal repair. Alzheimer's neurons with unstructured nuclei and aberrant cytoskeletons manage to survive by sacrificing their plasticity, which inexorably leads to dementia.