Intracellular amyloid-β peptide (Aβ) has been implicated in neuronal death associated with Alzheimer's disease. Although Aβ is predominantly secreted into the extracellular space, mechanisms of Aβ transport at the level of the neuronal cell membrane remain to be fully elucidated. We demonstrate that receptor for advanced glycation end products (RAGE) contributes to transport of Aβ from the cell surface to the intracellular space. Mouse cortical neurons exposed to extracellular human Aβ subsequently showed detectable peptide intracellularly in the cytosol and mitochondria by confocal microscope and immunogold electron microscopy. Pretreatment of cultured neurons from wild-type mice with neutralizing antibody to RAGE, and neurons from RAGE knockout mice displayed decreased uptake of Aβ and protection from Aβ-mediated mitochondrial dysfunction. Aβ activated p38 MAPK, but not SAPK/JNK, and then stimulated intracellular uptake of Aβ-RAGE complex. Similar intraneuronal co-localization of Aβ and RAGE was observed in the hippocampus of transgenic mice overexpressing mutant amyloid precursor protein. These findings indicate that RAGE contributes to mechanisms involved in the translocation of Aβ from the extracellular to the intracellular space, thereby enhancing Aβ cytotoxicity.

Several lines of evidence suggest that extracellular signal-regulated kinase1/2 (ERK1/2) and dopaminergic system is involved in learning and memory. However, it remains to be determined if the dopaminergic system and ERK1/2 pathway contribute to cognitive function in the prefrontal cortex (PFC). The amount of phosphorylated ERK1/2 was increased in the PFC immediately after exposure to novel objects in the training session of the novel object recognition test. An inhibitor of ERK kinase impaired long-term recognition memory 24 h after the training although short-term memory tested 1 h after the training was not affected by the treatment. The dopamine D1 receptor agonist increased ERK1/2 phosphorylation in the PFC in vivo as well as in cortical neurons in vitro. Microinjection of the dopamine D1 receptor antagonist into the PFC impaired long-term recognition memory whereas the D2 receptor antagonist had no effect. Immunohistochemistry revealed that exposure to novel objects resulted in an increase in c-Fos expression in the PFC. Microinjection of the protein synthesis inhibitor anisomycin into the PFC impaired the long-term recognition memory. These results suggest that the activation of ERK1/2 following the stimulation of dopamine D1 receptors is necessary for the protein synthesis-dependent long-term retention of recognition memory in the PFC.

Background: Clinically used antidepressants suffer from various side effects. Therefore, we searched for a safe antidepressant with minimal side effects among food ingredients that are distributed to the brain. Here, we focused on ERGO (ergothioneine), which is a hydrophilic antioxidant and contained at high levels in edible golden oyster mushrooms. ERGO is a typical substrate of carnitine/organic cation transporter OCTN1/SLC22A4, which is expressed in the brain and neuronal stem cells, although little is known about its permeation through the BBB (blood–brain barrier) or its neurological activity.Methods: To clarify the exposure of ERGO to brain and the possible antidepressant‐like effect after oral ingestion, ERGO or GOME (golden oyster mushroom extract) which contains 1.2% (w/w) ERGO was mixed with feed and provided to mice for 2 weeks, and then ERGO concentration and antidepressant‐like effect were evaluated by LC‐MS/MS and FST (forced swimming test) or TST (tail suspension test), respectively.Results: Diet containing ERGO or GOME greatly increased the ERGO concentrations in plasma and brain, and significantly decreased the immobility time in both FST and TST. The required amount of GOME (~37 mg/day) to show the antidepressant‐like effect corresponds to at most 8 g/day in humans. In mice receiving GOME‐containing diet, doublecortin‐positive cells showed a significant increase from the basal level, suggesting promotion of neuronal differentiation.Conclusion: Thus, orally ingested ERGO is transported across the BBB into the brain, where it may promote neuronal differentiation and alleviate symptoms of depression at plausibly achieved level of daily ingestion.

Carnitine/organic cation transporter (OCTN1/SLC22A4) accepts various therapeutic agents as substrates in vitro and is expressed ubiquitously, although its function in most organs has not yet been examined. The purpose of the present study was to evaluate functional expression of OCTN1 in small intestine and liver, using octn1 gene knockout [octn1(−/−)] mice. After oral administration of [³H]ergothioneine ([³H]ERGO), a typical substrate of OCTN1, the amount of [³H]ERGO remaining in the small intestinal lumen was much higher in octn1(−/−) mice than in wild-type mice. In addition, uptake of [³H]ERGO by human embryonic kidney 293 cells heterologously expressing OCTN1 gene product and uptake of [³H]ERGO at the apical surface of intestinal everted sacs from wild-type mice were inhibited by OCTN1 substrates, tetraethylammonium and verapamil. Immunohistochemical analysis revealed that OCTN1 is localized on the apical surface of small intestine in mice and humans. These results suggest that OCTN1 is responsible for small intestinal absorption of [³H]ERGO. However, the plasma concentration of [³H]ERGO after oral administration was higher in octn1(−/−) mice than in wild-type mice, despite the lower absorption in octn1(−/−) mice. This was probably because of efficient hepatic uptake of [³H]ERGO, as revealed by integration plot analysis; the uptake clearance was close to the hepatic plasma flow rate. The uptake of [³H]ERGO by isolated hepatocytes was minimal, whereas [³H]ERGO uptake was observed in isolated nonparenchymal cells. This finding is consistent with immunostaining of OCTN1 in liver sinusoids. Thus, our results indicate that OCTN1 is functionally expressed in nonparenchymal liver cells.