title: HIF-1alpha and HIF-2alpha are differentially activated in distinct cell populations in retinal ischaemia.
abstract: BACKGROUND
  Hypoxia plays a key role in ischaemic and neovascular disorders of the retina. Cellular responses to oxygen are mediated by hypoxia-inducible transcription factors (HIFs) that are stabilised in hypoxia and induce the expression of a diverse range of genes. The purpose of this study was to define the cellular specificities of HIF-1alpha and HIF-2alpha in retinal ischaemia, and to determine their correlation with the pattern of retinal hypoxia and the expression profiles of induced molecular mediators.
  
  
  RESULTS
  We investigated the tissue distribution of retinal hypoxia during oxygen-induced retinopathy (OIR) in mice using the bio-reductive drug pimonidazole. We measured the levels of HIF-1alpha and HIF-2alpha proteins by Western blotting and determined their cellular distribution by immunohistochemistry during the development of OIR. We measured the temporal expression profiles of two downstream mediators, vascular endothelial growth factor (VEGF) and erythropoietin (Epo) by ELISA. Pimonidazole labelling was evident specifically in the inner retina. Labelling peaked at 2 hours after the onset of hypoxia and gradually declined thereafter. Marked binding to Müller glia was evident during the early hypoxic stages of OIR. Both HIF-1alpha and HIF-2alpha protein levels were significantly increased during retinal hypoxia but were evident in distinct cellular distributions; HIF-1alpha stabilisation was evident in neuronal cells throughout the inner retinal layers whereas HIF-2alpha was restricted to Müller glia and astrocytes. Hypoxia and HIF-alpha stabilisation in the retina were closely followed by upregulated expression of the downstream mediators VEGF and EPO.
  
  
  CONCLUSIONS
  Both HIF-1alpha and HIF-2alpha are activated in close correlation with retinal hypoxia but have contrasting cell specificities, consistent with differential roles in retinal ischaemia. Our findings suggest that HIF-2alpha activation plays a key role in regulating the response of Müller glia to hypoxia.
authors.affiliation: Department of Genetics, University College London Institute of Ophthalmology, London, United Kingdom.
authors.forename: Freya M
authors.initials: FM
authors.lastname: Mowat
authors.forename: Ulrich F O
authors.initials: UF
authors.lastname: Luhmann
authors.forename: Alexander J
authors.initials: AJ
authors.lastname: Smith
authors.forename: Clemens
authors.initials: C
authors.lastname: Lange
authors.forename: Yanai
authors.initials: Y
authors.lastname: Duran
authors.forename: Sarah
authors.initials: S
authors.lastname: Harten
authors.forename: Deepa
authors.initials: D
authors.lastname: Shukla
authors.forename: Patrick H
authors.initials: PH
authors.lastname: Maxwell
authors.forename: Robin R
authors.initials: RR
authors.lastname: Ali
authors.forename: James W B
authors.initials: JW
authors.lastname: Bainbridge
mesh_terms: D000818:Animals; D051792:Basic Helix-Loop-Helix Proteins / Q000378:metabolism*; D015153:Blotting, Western; D004797:Enzyme-Linked Immunosorbent Assay; D004921:Erythropoietin / Q000378:metabolism; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000378:metabolism*; D007150:Immunohistochemistry; D007511:Ischemia / Q000378:metabolism*; D051379:Mice; D012171:Retinal Vessels / Q000378:metabolism*; D042461:Vascular Endothelial Growth Factor A / Q000378:metabolism; D000099162:Endothelial PAS Domain-Containing Protein 1
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; D004921:Erythropoietin; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D042461:Vascular Endothelial Growth Factor A; D000099162:Endothelial PAS Domain-Containing Protein 1; C497443:Hif1a protein, mouse