Arc in aging and disease
Although no diseases are known to be caused by mutations directly related to Arc expression, the loss of Arc in KO mice leads to hyperexcitability and increased susceptibility to seizures [16]. Interestingly, increased levels of Alzheimer’s-related human amyloid precursor protein (hAPP)-derived amyloid b in transgenic mice expressing hAPP [77] and in cultured cortical neurons [78] impaired Arc expression and also led to hyperexcitable networks and seizures [79]. Indeed, the extent to which Arc was depleted in granule cells from hAPP mice assessed histologically post-hoc was one of the best predictors of behavioral deficits in individual animals [77]. Moreover, similar to Alzheimer’s disease (AD) mouse models, Arc KO mice have reduced calbindin and increased neuropeptide Y(NPY) protein levels in the hippocampus [16], which may indicate adaptive changes that occur to prevent severe epilepsy. Because Arc is required for normal memory formation, this evidence suggests that decreased Arc expression may underlie some of the memory disturbances seen in AD (Table 1). The mechanism by which Arc is reduced in AD is not yet clear. However, adaptive changes in NPY expression in interneurons, which may mitigate hyperexcitability driven by amyloid b, might also interfere with the production of Arc in response to patterns of activity that would normally stimulate plasticity and enduring memories.