Role of hippocalcin in mediating Aβ toxicity

Abstract

Alzheimer’s disease (AD) is the most common cause of dementia. There is no cure available and the current treatments provide only limited symptomatic relief. Histopathological hallmarks of the AD brain are the amyloid-beta (Aβ) plaques and the tau-containing neurofibrillary tangles. These Aβ and tau lesions do not occur at random, rather, the neurodegenerative process is stereotyped in that it is initiated in the entorhinal cortex and hippocampal formation. Interestingly, it is the latter brain area where the calcium-sensing enzyme hippocalcin is highly expressed. As calcium deregulation in the form of excitotoxicity is a well-established pathomechanism in AD, we aimed to address the putative role of hippocalcin in human AD brain and transgenic mouse models. We found that levels of hippocalcin are higher in human AD brains and in Aβ plaque-forming APP23 transgenic mice compared to controls. In APP23 mice there was an inverse relationship between Aβ and hippocalcin staining as those cells with the highest intracellular Aβ had very low levels of hippocalcin. To determine the role of hippocalcin in Aβ toxicity, we treated primary cultures derived from mice lacking hippocalcin with Aβ and found that they were more susceptible to Aβ toxicity than the controls. Likewise, treatment with thapsigargin and ionomycin, respectively, both of which are known to deregulate intracellular calcium levels, caused an increased toxicity in hippocampal neurons from hippocalcin knock-out mice compared to wild-type controls. Our findings suggest that hippocalcin has a neuroprotective role in AD. The data further suggest hippocalcin as a putative biomarker for AD.