title: Albumin is taken up by hippocampal NG2 cells and astrocytes and decreases gap junction coupling.
abstract: OBJECTIVE
  Dysfunction of the blood-brain barrier (BBB) and albumin extravasation have been suggested to play a role in the etiology of human epilepsy. In this context, dysfunction of glial cells attracts increasing attention. Our study was aimed to analyze in the hippocampus (1) which cell types internalize albumin injected into the lateral ventricle in vivo, (2) whether internalization into astrocytes impacts their coupling and expression of connexin 43 (Cx43), and (3) whether expression of Kir4.1, the predominating astrocytic K(+) channel subunit, is altered by albumin.
  
  
  METHODS
  The patch-clamp method was combined with single cell tracer filling to investigate electrophysiologic properties and gap junction coupling (GJC). For cell identification, mice with cell type-specific expression of a fluorescent protein (NG2kiEYFP mice) and immunohistochemistry were employed. Semiquantitative real time polymerase chain reaction (RT-PCR) allowed analysis of Kir4.1 and Cx43 transcript levels.
  
  
  RESULTS
  We show that fluorescently labeled albumin is taken up by astrocytes, NG2 cells, and neurons, with NG2 cells standing out in terms of the quantity of uptake. Within 5 days postinjection (dpi), intracellular albumin accumulation was largely reduced suggesting rapid degradation. Electrophysiologic analysis of astrocytes and NG2 cells revealed no changes in their membrane properties at either time point. However, astrocytic GJC was significantly decreased at 1 dpi but returned to control levels within 5 dpi. We found no changes in hippocampal Cx43 transcript expression, suggesting that other mechanisms account for the observed changes in coupling. Kir4.1 mRNA was regulated oppositely in the CA1 stratum radiatum, with a strong increase at 1 dpi followed by a decrease at 5 dpi.
  
  
  CONCLUSIONS
  The present study demonstrates that extravasal albumin in the hippocampus induces rapid changes of astrocyte function, which can be expected to impair ion and transmitter homeostasis and contribute to hyperactivity and epileptogenesis. Therefore, astrocytes may represent alternative targets for antiepileptic therapeutic approaches.
authors.affiliation: Institute of Cellular Neurosciences, University of Bonn, Sigmund Freud Strasse 25, Bonn, Germany.
authors.forename: Oliver
authors.initials: O
authors.lastname: Braganza
authors.forename: Peter
authors.initials: P
authors.lastname: Bedner
authors.forename: Kerstin
authors.initials: K
authors.lastname: Hüttmann
authors.forename: Elena
authors.initials: E
authors.lastname: von Staden
authors.forename: Alon
authors.initials: A
authors.lastname: Friedman
authors.forename: Gerald
authors.initials: G
authors.lastname: Seifert
authors.forename: Christian
authors.initials: C
authors.lastname: Steinhäuser
mesh_terms: D000818:Animals; D001253:Astrocytes / Q000378:metabolism* / Q000473:pathology; D005456:Fluorescent Dyes / Q000008:administration & dosage / Q000378:metabolism; D017629:Gap Junctions / Q000378:metabolism* / Q000473:pathology; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D007276:Injections, Intraventricular; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D012709:Serum Albumin / Q000008:administration & dosage / Q000502:physiology*
chemical_list: D005456:Fluorescent Dyes; D012709:Serum Albumin