[Mitochondrial dysfunction and neuronal apoptosis: new molecular approach to prevent Alzheimer's disease]

Aberrant expression of RCAN1 in Alzheimer's pathogenesis: a new molecular mechanism and a novel drug target

AD, a devastating neurodegenerative disorder, is the most common cause of dementia in the elderly. Patients with AD are characterized by three hallmarks of neuropathology including neuritic plaque deposition, neurofibrillary tangle formation, and neuronal loss. Growing evidences indicate that dysregulation of regulator of calcineurin 1 (RCAN1) plays an important role in the pathogenesis of AD. Aberrant RCAN1 expression facilitates neuronal apoptosis and Tau hyperphosphorylation, leading to neuronal loss and neurofibrillary tangle formation. This review aims to describe the recent advances of the regulation of RCAN1 expression and its physiological functions. Moreover, the AD risk factors-induced RCAN1 dysregulation and its role in promoting neuronal loss, synaptic impairments and neurofibrillary tangle formation are summarized. Furthermore, we provide an outlook into the effects of RCAN1 dysregulation on APP processing, Aβ generation and neuritic plaque formation, and the possible underlying mechanisms, as well as the potential of targeting RCAN1 as a new therapeutic approach.

Regulation of RCAN1 translation and its role in oxidative stress-induced apoptosis

Abnormal expression of regulator of calcineurin 1 (RCAN1) has been implicated in Alzheimer's disease (AD) and Down's syndrome (DS). There are two major isoforms of RCAN1, isoforms 1 and 4. RCAN1 isoform 1 is predominantly expressed in the brain, particularly in neurons. In this report, we showed that there are two translation start codons in RCAN1 exon 1 serving as a functional translation initiation site to generate a longer 41-kDa isoform 1 (RCAN1.1L) and a shorter 31-kDa isoform 1 (RCAN1.1S). The first translation initiation site has higher translation efficiency than the downstream second one, and the translation initiation of two AUG sites is by a Cap-dependent mechanism. Short-term expression of RCAN1.1L protected SH-SY5Y cells from oxidative stress-induced apoptosis by inhibiting caspase-3 activation. However, long-term accumulation of RCAN1.1L in SH-SY5Y cells promoted oxidative stress-induced apoptosis via caspase-3 activation, and terminal deoxynucleotidyl transferase dUTP nick end labeling assay showed that the apoptosis ratio was increased to 499.03 ± 47.56% in SH-1.1L cells compared with 283.93 ± 28.66% in control cells. Furthermore, we found that RCAN1.1L is significantly elevated in the AD brains and patients with DS. RCAN1.1S is expressed at a low level in both human cells and brain tissues. Our results defined the regulatory mechanism underlying RCAN1 expression and the roles of RCAN1.1 in oxidative stress-induced neurodegeneration in AD and DS pathogenesis.