title: A developmental switch in the expression of aquaporin-4 and Kir4.1 from horizontal to Müller cells in mouse retina.
abstract: OBJECTIVE
  In adult retina, aquaporin-4 (AQP4) and inwardly rectifying K(+) (Kir4.1) channels localize to astrocyte and Müller cell membranes facing vascular and vitreous compartments, optimizing clearance of extracellular K(+) and water from the synaptic layers. However, it is unknown whether these channels are expressed at early developmental stages, before gliogenesis or angiogenesis take place in the neural retina. This study was conducted to determine the presence of AQP4 and Kir4.1 proteins in the developing mouse retina.
  
  
  METHODS
  Simultaneous AQP4 and Kir4.1 immunodetection was performed in postnatal mice 1, 9, 15, and 30 days of age. Confocal microscopy was used to identify the cellular distribution of AQP4 and Kir4.1 proteins, as well as their coexpression with the cell-selective immunomarkers Prox-1, calbindin, and neurofilament.
  
  
  RESULTS
  AQP4 and Kir4.1 proteins were coexpressed in calbindin- and Prox1-expressing retinal neurons at birth. These neurons were identified as horizontal cells based on their position and morphology. By P15, when vision starts, AQP4 and Kir4.1 localization coordinately switched from horizontal cells to Müller glial cells.
  
  
  CONCLUSIONS
  The findings showed that AQP4 and Kir4.1 protein expression is confined to differentiating horizontal cells before its expression in Müller cells. The finding of AQP4 in neurons is novel, since AQP4 expression within the central nervous system is restricted to glia. Also, the results demonstrated that AQP4 is a horizontal cell-specific immunomarker in neonatal retina. The transitory coexpression of AQP4 and Kir4.1 proteins by differentiating horizontal interneurons suggests that these cells mediate K(+) and water transcellular uptake until the initiation of phototransduction, when glial cells assume these functions.
authors.affiliation: Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461, USA. abosco@aecom.yu.edu
authors.forename: Alejandra
authors.initials: A
authors.lastname: Bosco
authors.forename: Karen
authors.initials: K
authors.lastname: Cusato
authors.forename: Grazia P
authors.initials: GP
authors.lastname: Nicchia
authors.forename: Antonio
authors.initials: A
authors.lastname: Frigeri
authors.forename: David C
authors.initials: DC
authors.lastname: Spray
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D051401:Aquaporin 4 / Q000378:metabolism*; D064026:Calbindins; D005136:Eye Proteins / Q000378:metabolism*; D019084:Fluorescent Antibody Technique, Indirect; D018398:Homeodomain Proteins / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D018613:Microscopy, Confocal; D016900:Neurofilament Proteins / Q000378:metabolism; D009457:Neuroglia / Q000378:metabolism*; D024661:Potassium Channels, Inwardly Rectifying / Q000378:metabolism*; D012160:Retina / Q000166:cytology / Q000254:growth & development* / Q000378:metabolism; D064030:S100 Calcium Binding Protein G / Q000378:metabolism; D025521:Tumor Suppressor Proteins; D014882:Water-Electrolyte Balance; D000098551:Prospero-Related Homeobox 1 Protein; D000098854:Kcnj10 Channel
chemical_list: C496031:Aqp4 protein, mouse; D051401:Aquaporin 4; D064026:Calbindins; D005136:Eye Proteins; D018398:Homeodomain Proteins; D000098854:Kcnj10 Channel; D016900:Neurofilament Proteins; D024661:Potassium Channels, Inwardly Rectifying; D064030:S100 Calcium Binding Protein G; D025521:Tumor Suppressor Proteins; D000098551:Prospero-Related Homeobox 1 Protein