PKC[epsilon] Promotes HuD-Mediated Neprilysin mRNA Stability and Enhances Neprilysin-Induced A[beta] Degradation in Brain Neurons

Amyloid-beta (A[beta]) peptide accumulation in the brain is a pathological hallmark of all forms of Alzheimer's disease. An imbalance between A[beta] production and clearance from the brain may contribute to accumulation of neurotoxic A[beta] and subsequent synaptic loss, which is the strongest correlate of the extent of memory loss in AD. The activity of neprilysin (NEP), a potent A[beta]-degrading enzyme, is decreased in the AD brain. Expression of HuD, an mRNA-binding protein important for synaptogenesis and neuronal plasticity, is also decreased in the AD brain. HuD is regulated by protein kinase C[epsilon] (PKC[epsilon]), and we previously demonstrated that PKC[epsilon] activation decreases A[beta] levels. We hypothesized that PKC[epsilon] acts through HuD to stabilize NEP mRNA, modulate its localization, and support NEP activity. Conversely, loss of PKC[epsilon]-activated HuD in AD leads to decreased NEP activity and accumulation of A[beta]. Here we show that HuD is associated with NEP mRNA in cultures of human SK-N-SH cells. Treatment with bryostatin, a PKC[epsilon]-selective activator, enhanced NEP association with HuD and increased NEP mRNA stability. Activation of PKC[epsilon] also increased NEP protein levels, increased NEP phosphorylation, and induced cell surface expression. In addition, specific PKC[epsilon] activation directly stimulated NEP activity, leading to degradation of a monomeric form of A[beta] peptide and decreased A[beta] neuronal toxicity, as measured by cell viability. Bryostatin treatment also rescued A[beta]-mediated inhibition of HuD-NEP mRNA binding, NEP protein expression, and NEP cell membrane translocation. These results suggest that PKC[epsilon] activation reduces A[beta] by up-regulating, via the mRNA-binding protein HuD, A[beta]-degrading enzymes such as NEP. Thus, PKC[epsilon] activation may have therapeutic efficacy for AD by reducing neurotoxic A[beta] accumulation as well as having direct anti-apoptotic and synaptogenic effects.