Light-Induced photoreceptor degeneration in the mouse involves activation of the small GTPase Rac1.
OBJECTIVE
Rho GTPases play a central role in actin-based cytoskeleton reorganization, and they participate in signaling pathways that regulate gene transcription, cell cycle entry, and cell survival. This study verifies the role of Rac1 during light-induced retinal degeneration.


METHODS
BALB/c mice were exposed to degenerative light stimulus, and their eyes were enucleated immediately or after the mice were kept in the dark for 6, 24, and 48 hours. Retinas were fixed and processed for immunohistochemical analysis. The distribution of Rac1 and its effectors-p21-activated kinases (PAKs) 1, 2, and 3-was studied by immunohistochemistry, whereas the expression of PAKs 3, 4, and 5 mRNA was analyzed by real-time PCR. Rac1 activity was measured using a pull-down assay.


RESULTS
In control retinas, Rac1 was mostly observed in photoreceptors, plexiform layers, and Müller glial cells. In light-damaged retinas, some TUNEL-positive photoreceptors upregulated Rac1 expression. Conversely, most of the Rac1-positive cells were TUNEL-positive, mainly in early stages of retinal degeneration. The increase in Rac1 expression was preceded by enhanced Rac1 activity, detectable at the end of the light stimulus and still present 48 hours later. The distribution patterns of PAK1, PAK2, and PAK3 did not change in light-damaged retinas. However, there was a marked increase in PAK3 and PAK4 gene expression, whereas that of PAK5 mRNA remained the same.


CONCLUSIONS
Rac1 may play a role in the apoptosis of light-damaged photoreceptors. The increased expression of PAK4 after light stimulus possibly functions as a protective mechanism against apoptosis.
Belmonte
Mônica A
MA
Department of Cell and Developmental Biology, Biomedical Sciences Institute, University of São Paulo, São Paulo, Brazil.
Santos
Marinilce F
MF
Kihara
Alexandre H
AH
Yan
Chao Y I
CY
Hamassaki
Dânia E
DE
D000818:Animals; D017209:Apoptosis; D004789:Enzyme Activation; D019084:Fluorescent Antibody Technique, Indirect; D015971:Gene Expression Regulation, Enzymologic / Q000502:physiology; D020287:In Situ Nick-End Labeling; D008027:Light; D051379:Mice; D008807:Mice, Inbred BALB C; D009479:Neuropeptides / Q000378:metabolism*; D020419:Photoreceptor Cells, Vertebrate / Q000201:enzymology / Q000473:pathology / Q000528:radiation effects*; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D011833:Radiation Injuries, Experimental / Q000201:enzymology* / Q000209:etiology / Q000473:pathology; D012162:Retinal Degeneration / Q000201:enzymology* / Q000209:etiology / Q000473:pathology; D020133:Reverse Transcriptase Polymerase Chain Reaction; D020744:rac GTP-Binding Proteins / Q000378:metabolism*; D020830:rac1 GTP-Binding Protein
D009479:Neuropeptides; D012333:RNA, Messenger; C484602:Rac1 protein, mouse; D017346:Protein Serine-Threonine Kinases; D020744:rac GTP-Binding Proteins; D020830:rac1 GTP-Binding Protein