Somatostatin-evoked Aß catabolism in the brain: Mechanistic involvement of α-endosulfine-KATP channel pathway.

Alzheimer's disease (AD) is characterized by the deposition of amyloid ß peptide (Aß) in the brain. The neuropeptide somatostatin (SST) regulates Aß catabolism by enhancing neprilysin (NEP)-catalyzed proteolytic degradation. However, the mechanism by which SST regulates NEP activity remains unclear. Here, we identified α-endosulfine (ENSA), an endogenous ligand of the ATP-sensitive potassium (KATP) channel, as a negative regulator of NEP downstream of SST signaling. The expression of ENSA is significantly increased in AD mouse models and in patients with AD. In addition, NEP directly contributes to the degradation of ENSA, suggesting a substrate-dependent feedback loop regulating NEP activity. We also discovered the specific KATP channel subtype that modulates NEP activity, resulting in the Aß levels altered in the brain. Pharmacological intervention targeting the particular KATP channel attenuated Aß deposition, with impaired memory function rescued via the NEP activation in our AD mouse model. Our findings provide a mechanism explaining the molecular link between KATP channel and NEP activation, and give new insights into alternative strategies to prevent AD.

The dopaminergic system promotes neprilysin-mediated degradation of amyloid-ß in the brain.

Deposition of amyloid-ß (Aß) in the brain can impair neuronal function and contribute to cognitive decline in Alzheimer's disease (AD). Here, we found that dopamine and the dopamine precursor levodopa (also called l-DOPA) induced Aß degradation in the brain. Chemogenetic approaches in mice revealed that the activation of dopamine release from ventral tegmental area (VTA) neurons increased the abundance and activity of the Aß-degrading enzyme neprilysin and reduced the amount of Aß deposits in the prefrontal cortex in a neprilysin-dependent manner. Aged mice had less dopamine and neprilysin in the anterior cortex, a decrease that was accentuated in AD model mice. Treating AD model mice with levodopa reduced Aß deposition and improved cognitive function. These observations demonstrate that dopamine promotes brain region-specific, neprilysin-dependent degradation of Aß, suggesting that dopamine-associated strategies have the potential to treat this aspect of AD pathology.

Cell surface expression of the major amyloid-ß peptide (Aß)-degrading enzyme, neprilysin, depends on phosphorylation by mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK) and dephosphorylation by protein phosphatase 1a.

Neprilysin is one of the major amyloid-ß peptide (Aß)-degrading enzymes, the expression of which declines in the brain during aging. The decrease in neprilysin leads to a metabolic Aß imbalance, which can induce the amyloidosis underlying Alzheimer disease. Pharmacological activation of neprilysin during aging therefore represents a potential strategy to prevent the development of Alzheimer disease. However, the regulatory mechanisms mediating neprilysin activity in the brain remain unclear. To address this issue, we screened for pharmacological regulators of neprilysin activity and found that the neurotrophic factors brain-derived neurotrophic factor, nerve growth factor, and neurotrophins 3 and 4 reduce cell surface neprilysin activity. This decrease was mediated by MEK/ERK signaling, which enhanced phosphorylation at serine 6 in the neprilysin intracellular domain (S6-NEP-ICD). Increased phosphorylation of S6-NEP-ICD in primary neurons reduced the levels of cell surface neprilysin and led to a subsequent increase in extracellular Aß levels. Furthermore, a specific inhibitor of protein phosphatase-1a, tautomycetin, induced extensive phosphorylation of the S6-NEP-ICD, resulting in reduced cell surface neprilysin activity. In contrast, activation of protein phosphatase-1a increased cell surface neprilysin activity and lowered Aß levels. Taken together, these results indicate that the phosphorylation status of S6-NEP-ICD influences the localization of neprilysin and affects extracellular Aß levels. Therefore, maintaining S6-NEP-ICD in a dephosphorylated state, either by inhibition of protein kinases involved in its phosphorylation or by activation of phosphatases catalyzing its dephosphorylation, may represent a new approach to prevent reduction of cell surface neprilysin activity during aging and to maintain physiological levels of Aß in the brain.

AMP-activated protein kinase counteracts brain-derived neurotrophic factor-induced mammalian target of rapamycin complex 1 signaling in neurons.

Growth factors and nutrients, such as amino acids and glucose, regulate mammalian target of rapamycin complex 1 (mTORC1) signaling and subsequent translational control in a coordinated manner. Brain-derived neurotrophic factor (BDNF), the most prominent neurotrophic factor in the brain, activates mTORC1 and induces phosphorylation of its target, p70S6 kinase (p70S6K), at Thr389 in neurons. BDNF also increases mammalian target of rapamycin-dependent novel protein synthesis in neurons. Here, we report that BDNF-induced p70S6K activation is dependent on glucose, but not amino acids, sufficiency in cultured cortical neurons. AMP-activated protein kinase (AMPK) is the molecular background to this specific nutrient dependency. Activation of AMPK, which is induced by glucose deprivation, treatment with pharmacological agents such as 2-deoxy-D-glucose, metformin, and 5-aminoimidazole-4-carboxamide ribonucleoside or forced expression of a constitutively active AMPKα subunit, counteracts BDNF-induced phosphorylation of p70S6K and enhanced protein synthesis in cortical neurons. These results indicate that AMPK inhibits the effects of BDNF on mTORC1-mediated translation in neurons.

Brain-derived neurotrophic factor enhances the basal rate of protein synthesis by increasing active eukaryotic elongation factor 2 levels and promoting translation elongation in cortical neurons.

The constitutive and activity-dependent components of protein synthesis are both critical for neural function. Although the mechanisms controlling extracellularly induced protein synthesis are becoming clear, less is understood about the molecular networks that regulate the basal translation rate. Here we describe the effects of chronic treatment with various neurotrophic factors and cytokines on the basal rate of protein synthesis in primary cortical neurons. Among the examined factors, brain-derived neurotrophic factor (BDNF) showed the strongest effect. The rate of protein synthesis increased in the cortical tissues of BDNF transgenic mice, whereas it decreased in BDNF knock-out mice. BDNF specifically increased the level of the active, unphosphorylated form of eukaryotic elongation factor 2 (eEF2). The levels of active eEF2 increased and decreased in BDNF transgenic and BDNF knock-out mice, respectively. BDNF decreased kinase activity and increased phosphatase activity against eEF2 in vitro. Additionally, BDNF shortened the ribosomal transit time, an index of translation elongation. In agreement with these results, overexpression of eEF2 enhanced protein synthesis. Taken together, our results demonstrate that the increased level of active eEF2 induced by chronic BDNF stimulation enhances translational elongation processes and increases the total rate of protein synthesis in neurons.

Leucine induces phosphorylation and activation of p70S6K in cortical neurons via the system L amino acid transporter.

p70S6 kinase is a multipotent kinase that phosphorylates substrates in response to extracellular stimuli. This kinase activity inhibits apoptosis, regulates cell size and controls translation. In the CNS, p70S6K also participates in synaptic plasticity. In this study, we report that leucine, a branched-chain amino acid, induces phosphorylation and activation of p70S6 kinase in cortical neurons. Leucine also induces phosphorylation of S6 protein, a substrate of p70S6K. These effects of leucine are completely inhibited by rapamycin, consistent with mammalian target of rapamycin mediating p70S6 phosphorylation. Finally, we demonstrate that the action of leucine on cortical neurons is mediated by the system L amino acid transporter. Neurons express components of system L amino acid transporter LAT1, LAT2, and CD98. Leucine uptake and its effect on p70S6 kinase are both inhibited by a specific inhibitor of system L amino acid transporter. We propose that leucine plays important roles in regulating signaling by p70S6 kinase by acting as an intercellular communicator in the CNS.

The extreme C-terminus of GluRdelta2 is essential for induction of long-term depression in cerebellar slices.

Long-term depression (LTD) of parallel fibre (PF)-Purkinje cell synapses in the cerebellum is recognized as a cellular substrate of motor learning. Although the delta2 glutamate receptor (GluRdelta2) has been shown to be crucial for LTD, the mechanisms by which GluRdelta2 functions remain elusive. In this study, we developed a virus vector-based gene transfer approach to rescue impaired LTD in GluRdelta2-null Purkinje cells in cerebellar slice preparations. We demonstrated that LTD was restored in GluRdelta2-null Purkinje cells transduced with wild-type but not with mutant GluRdelta2, which lacked the PDZ-ligand domain in the C-terminus. Immunohistochemical analysis revealed no difference in expression levels or spine localization patterns between virally introduced wild-type and mutant GluRdelta2 proteins. Similarly, LTD was abrogated in Purkinje cells that had been acutely perfused with peptides, hampering the interaction of GluRdelta2 with PDZ proteins such as PSD-93, PTPMEG and S-SCAM but not with delphilin. Together, these results indicate that PDZ proteins that bind to the C-terminus of GluRdelta2 are not essential for localizing GluRdelta2 at synapses but are crucial for conveying signals necessary for the induction of LTD.