title: Retinal expression of the serine protease matriptase-2 (Tmprss6) and its role in retinal iron homeostasis.
abstract: OBJECTIVE
  Matriptase-2 (also known as TMPRSS6) is a critical regulator of the iron-regulatory hormone hepcidin in the liver; matriptase-2 cleaves membrane-bound hemojuvelin and consequently alters bone morphogenetic protein (BMP) signaling. Hemojuvelin and hepcidin are expressed in the retina and play a critical role in retinal iron homeostasis. However, no information on the expression and function of matriptase-2 in the retina is available. The purpose of the present study was to examine the retinal expression of matriptase-2 and its role in retinal iron homeostasis.
  
  
  METHODS
  RT-PCR, quantitative PCR (qPCR), and immunofluorescence were used to analyze the expression of matriptase-2 and other iron-regulatory proteins in the mouse retina. Polarized localization of matriptase-2 in the RPE was evaluated using markers for the apical and basolateral membranes. Morphometric analysis of retinas from wild-type and matriptase-2 knockout (Tmprss6(msk/msk) ) mice was also performed. Retinal iron status in Tmprss6(msk/msk) mice was evaluated by comparing the expression levels of ferritin and transferrin receptor 1 between wild-type and knockout mice. BMP signaling was monitored by the phosphorylation status of Smads1/5/8 and expression levels of Id1 while interleukin-6 signaling was monitored by the phosphorylation status of STAT3.
  
  
  RESULTS
  Matriptase-2 is expressed in the mouse retina with expression detectable in all retinal cell types. Expression of matriptase-2 is restricted to the apical membrane in the RPE where hemojuvelin, the substrate for matriptase-2, is also present. There is no marked difference in retinal morphology between wild-type mice and Tmprss6(msk/msk) mice, except minor differences in specific retinal layers. The knockout mouse retina is iron-deficient, demonstrable by downregulation of the iron-storage protein ferritin and upregulation of transferrin receptor 1 involved in iron uptake. Hepcidin is upregulated in Tmprss6(msk/msk) mouse retinas, particularly in the neural retina. BMP signaling is downregulated while interleukin-6 signaling is upregulated in Tmprss6(msk/msk) mouse retinas, suggesting that the upregulaton of hepcidin in knockout mouse retinas occurs through interleukin-6 signaling and not through BMP signaling.
  
  
  CONCLUSIONS
  The iron-regulatory serine protease matriptase-2 is expressed in the retina, and absence of this enzyme leads to iron deficiency and increased expression of hemojuvelin and hepcidin in the retina. The upregulation of hepcidin expression in Tmprss6(msk/msk) mouse retinas does not occur via BMP signaling but likely via the proinflammatory cytokine interleukin-6. We conclude that matriptase-2 is a critical participant in retinal iron homeostasis.
authors.affiliation: Department of Biochemistry and Molecular Biology, Medical College of Georgia, Georgia Regents University, Augusta, GA.
authors.forename: Jaya P
authors.initials: JP
authors.lastname: Gnana-Prakasam
authors.affiliation: Department of Biochemistry and Molecular Biology, Medical College of Georgia, Georgia Regents University, Augusta, GA.
authors.forename: Renee B
authors.initials: RB
authors.lastname: Baldowski
authors.affiliation: Department of Biochemistry and Molecular Biology, Medical College of Georgia, Georgia Regents University, Augusta, GA.
authors.forename: Sudha
authors.initials: S
authors.lastname: Ananth
authors.affiliation: Department of Biochemistry and Molecular Biology, Medical College of Georgia, Georgia Regents University, Augusta, GA.
authors.forename: Pamela M
authors.initials: PM
authors.lastname: Martin
authors.affiliation: Department of Cellular Biology and Anatomy, Medical College of Georgia, Georgia Regents University, Augusta, GA.
authors.forename: Sylvia B
authors.initials: SB
authors.lastname: Smith
authors.affiliation: Department of Biochemistry and Molecular Biology, Medical College of Georgia, Georgia Regents University, Augusta, GA.
authors.forename: Vadivel
authors.initials: V
authors.lastname: Ganapathy
mesh_terms: D000818:Animals; D019485:Bone Morphogenetic Proteins / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D005293:Ferritins / Q000378:metabolism; D058851:GPI-Linked Proteins; D000071020:Hemochromatosis Protein; D064451:Hepcidins / Q000378:metabolism; D006706:Homeostasis; D015850:Interleukin-6 / Q000378:metabolism; D007501:Iron / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D021381:Protein Transport; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011990:Receptors, Transferrin / Q000378:metabolism; D012160:Retina / Q000201:enzymology* / Q000473:pathology; D055213:Retinal Pigment Epithelium / Q000378:metabolism; D012697:Serine Endopeptidases / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D015854:Up-Regulation
chemical_list: D019485:Bone Morphogenetic Proteins; D058851:GPI-Linked Proteins; C504414:HJV protein, mouse; D000071020:Hemochromatosis Protein; D064451:Hepcidins; D015850:Interleukin-6; D008565:Membrane Proteins; D012333:RNA, Messenger; D011990:Receptors, Transferrin; D005293:Ferritins; D007501:Iron; D012697:Serine Endopeptidases; C467801:matriptase 2