title: Molecular mechanism of histamine clearance by primary human astrocytes.
abstract: Histamine clearance is an essential process for avoiding excessive histaminergic neuronal activity. Previous studies using rodents revealed the predominant role of astrocytes in brain histamine clearance. However, the molecular mechanism of histamine clearance has remained unclear. We detected histamine N-methyltransferase (HNMT), a histamine-metabolizing enzyme, in primary human astrocytes and the astrocytes of human brain specimens. Immunocytochemical analysis and subcellular fractionation assays revealed that active HNMT localized to the cytosol, suggesting that histamine transport into the cytosol is crucial for histamine inactivation. We showed that primary human astrocytes transported histamine in a time-dependent manner. Kinetics analysis showed that two low-affinity transporters were involved in histamine transport. Histamine uptake by primary human astrocytes was not dependent on the extracellular Na(+) /Cl(-) concentration. Histamine is reported to be a substrate for three low-affinity and Na(+) /Cl(-) -independent transporters: organic cation transporter 2 (OCT2), OCT3, and plasma membrane monoamine transporter (PMAT). RT-PCR analysis revealed that OCT3 and PMAT were expressed in primary human astrocytes. Immunohistochemistry confirmed OCT3 and PMAT expression in the astrocytes of human brain specimens. Drug inhibition assays and gene knockdown assays revealed the major contribution of PMAT and the minor contribution of OCT3 to histamine transport. The present study demonstrates for the first time that the molecular mechanism of histamine clearance is by primary human astrocytes. These findings might indicate that PMAT, OCT3 and HNMT in human astrocytes play a role in the regulation of extraneuronal histamine concentration and the activities of histaminergic neurons.
authors.affiliation: Department of Pharmacology, Tohoku University Graduate School of Medicine, Aoba-ku, Sendai, Miyagi, Japan. tyoshikawa@med.tohoku.ac.jp
authors.forename: Takeo
authors.initials: T
authors.lastname: Yoshikawa
authors.forename: Fumito
authors.initials: F
authors.lastname: Naganuma
authors.forename: Tomomitsu
authors.initials: T
authors.lastname: Iida
authors.forename: Tadaho
authors.initials: T
authors.lastname: Nakamura
authors.forename: Ryuichi
authors.initials: R
authors.lastname: Harada
authors.forename: Attayeb S
authors.initials: AS
authors.lastname: Mohsen
authors.forename: Atsuko
authors.initials: A
authors.lastname: Kasajima
authors.forename: Hironobu
authors.initials: H
authors.lastname: Sasano
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Yanai
mesh_terms: D001253:Astrocytes / Q000378:metabolism*; D001692:Biological Transport; D001921:Brain / Q000378:metabolism*; D045744:Cell Line, Tumor; D033705:Equilibrative Nucleoside Transport Proteins / Q000235:genetics / Q000378:metabolism; D006632:Histamine / Q000378:metabolism*; D006637:Histamine N-Methyltransferase / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D009474:Neurons / Q000378:metabolism; D027701:Organic Cation Transport Proteins / Q000235:genetics / Q000378:metabolism; D000075102:Organic Cation Transporter 2
chemical_list: D033705:Equilibrative Nucleoside Transport Proteins; D027701:Organic Cation Transport Proteins; D000075102:Organic Cation Transporter 2; C491013:SLC22A2 protein, human; C495988:SLC29A4 protein, human; D006632:Histamine; D006637:Histamine N-Methyltransferase