A mouse model of early sporadic tau pathology induces neurogenic plasticity in the hippocampus
Alzheimers disease is characterized by widespread neurodegeneration and the accumulation of insoluble amyloid plaques and neurofibrillary tangles. However, at the earliest stages of the disease cell death and pathological tau are localized to the entorhinal cortex. In particular, the lateral entorhinal cortex, and its functions in object-related memory, are among the most vulnerable in aging. Notably, the entorhinal cortex projects directly to the dentate gyrus subregion of the hippocampus, where neurogenesis proceeds throughout adult life. Immature, adult-born neurons provide a robust source of plasticity to the entorhinal-dentate pathway and they may be uniquely responsive, or vulnerable, to early entorhinal tau pathology. To test this, we injected a human tau-expressing recombinant adeno-associated virus into the lateral entorhinal cortex and used AsclCreER mice to birthdate downstream dentate neurons born in early postnatal development or adulthood. Consistent with known roles in neurodegeneration, lateral entorhinal tau expression caused a loss of mushroom spines in downstream dentate neurons and reduced dendritic complexity of adult-born neurons in male mice. Presynaptic tau also increased neurogenesis levels and increased the density of thin spines on adult-born neurons. Despite these morphological effects, tau did not alter synaptic strength or the magnitude of long-term potentiation at entorhinal synapses onto adult- or developmentally-born neurons. Thus, in a novel model of early sporadic tau pathology, there are morphological changes consistent with neurodegeneration but also compensatory neuroplastic changes, caused by neurogenesis. Since immature neurons have also been identified in the human dentate gyrus, a similar neurogenic plasticity may help maintain entorhinal-hippocampal formation in pathological aging.