Rod photoreceptor loss in Rho-/- mice reduces retinal hypoxia and hypoxia-regulated gene expression.
OBJECTIVE
This study was conducted to evaluate whether regions of the retinal neuropile become hypoxic during periods of high oxygen consumption and whether depletion of the outer retina reduces hypoxia and related changes in gene expression.


METHODS
Retinas from rhodopsin knockout (Rho-/-) mice were evaluated along with those of wild-type (WT) control animals. Retinas were also examined at the end of 12-hour dark or light periods, and a separate group was treated with l-cis-diltiazem at the beginning of a 12-hour dark period. Hypoxia was assessed by deposition of hypoxyprobe (HP) and HP-protein adducts were localized by immunohistochemistry and quantified using ELISA. Also, hypoxia-regulated gene expression and transcriptional activity were assessed alongside vascular density.


RESULTS
Hypoxia was observed in the inner nuclear and ganglion cell layers in WT retina and was significantly reduced in Rho-/- mice (P < 0.05). Retinal hypoxia was significantly increased during dark adaptation in WT mice (P < 0.05), whereas no change was observed in Rho-/- or with l-cis-diltiazem-treated WT mice. Hypoxia-inducible factor (HIF)-1alpha DNA-binding and VEGF mRNA expression in Rho-/- retina was significantly reduced in unison with outer retinal depletion (P < 0.05). Retina from the Rho-/- mice displayed an extensive intraretinal vascular network after 6 months, although there was evidence that capillary density was depleted in comparison with that in WT retinas.


CONCLUSIONS
Relative hypoxia occurs in the inner retina especially during dark adaptation. Photoreceptor loss reduces retinal oxygen usage and hypoxia which corresponds with attenuation of the retinal microvasculature. These studies suggest that in normal physiological conditions and diurnal cycles the adult retina exists in a state of borderline hypoxia, making this tissue particularly susceptible to even subtle reductions in perfusion.
de Gooyer
Tanyth E
TE
Centre for Vision Science, Queen's University Belfast, Royal Victoria Hospital, Belfast, Northern Ireland, United Kingdom.
Stevenson
Kathryn A
KA
Humphries
Pete
P
Simpson
David A C
DA
Curtis
Timothy M
TM
Gardiner
Tom A
TA
Stitt
Alan W
AW
D000818:Animals; D001081:Apyrase / Q000378:metabolism; D054707:Cyclic Nucleotide Phosphodiesterases, Type 6; D003623:Dark Adaptation; D004110:Diltiazem / Q000494:pharmacology; D004797:Enzyme-Linked Immunosorbent Assay; D000860:Hypoxia / Q000378:metabolism* / Q000473:pathology; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000235:genetics* / Q000378:metabolism; D008027:Light; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009593:Nitroimidazoles / Q000494:pharmacology; D010101:Oxygen Consumption; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D012162:Retinal Degeneration / Q000378:metabolism* / Q000473:pathology; D017948:Retinal Rod Photoreceptor Cells / Q000378:metabolism*; D012171:Retinal Vessels / Q000201:enzymology / Q000473:pathology; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012243:Rhodopsin / Q000502:physiology*; D015533:Transcriptional Activation / Q000502:physiology*; D042461:Vascular Endothelial Growth Factor A / Q000235:genetics / Q000378:metabolism
C497443:Hif1a protein, mouse; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D009593:Nitroimidazoles; D012333:RNA, Messenger; D042461:Vascular Endothelial Growth Factor A; C467480:vascular endothelial growth factor A, mouse; C033815:pimonidazole; D012243:Rhodopsin; D010727:Phosphoric Diester Hydrolases; D054707:Cyclic Nucleotide Phosphodiesterases, Type 6; C517438:Pde6b protein, mouse; D001081:Apyrase; D004110:Diltiazem