Mutations associated with retinopathies alter mitogen-activated protein kinase-induced phosphorylation of neural retina leucine-zipper.
OBJECTIVE
Neural retina leucine-zipper (NRL), a member of the basic motif leucine zipper family of transcription factors, is preferentially expressed in rod photoreceptors of the mammalian retina. Mutations in NRL are associated with retinopathies; many of these are suggested to change phosphorylation status and alter NRL-mediated transactivation of rhodopsin promoter. The purpose of this study was to identify potential kinases responsible for the phosphorylation of NRL and determine if such kinase-dependent phosphorylation is altered in disease-associated NRL mutations.


METHODS
Metabolic labeling with 33P-orthophosphate was used to study phosphorylation of NRL in transfected COS-1 cells. NRL or NRL mutants were expressed as glutathione S-transferase (GST)-fusion proteins and used as substrate to screen various kinases by in vitro phosphorylation assays. CV-1 cells were co-transfected with rhodopsin promoter-reporter construct and expression plasmids, with or without specific mitogen-activated protein kinase (MAPK) inhibitors, to examine their effect on NRL-mediated transactivation. Expression of activated MAPKs in postnatal mice retina was determined by immunoblot analysis.


RESULTS
Metabolic labeling of NRL produces multiple phosphorylated protein bands in transfected COS-1 cells. Fewer but more intense radiolabeled bands are observed for NRL-S50T, -S50A, and -P51L mutants compared to wild-type NRL. We show that MAPK2 and p38 induce specific phosphorylation of NRL, but this pattern is altered in NRL mutants. Immunoblot analysis of extracts from developing mouse retina reveals enhanced expression of activated MAPK2 at postnatal day 0-3, concordant with the reported phosphorylation pattern of NRL in vivo. Inhibition of MAPK signaling pathways decreases NRL and CRX-mediated synergistic activation of rhodopsin promoter in transfected CV-1 cells.


CONCLUSIONS
Our results suggest that multiple MAPKs can phosphorylate NRL and this phosphorylation pattern is altered by disease-associated NRL mutations. As inhibition of MAPK signaling pathways decreases NRL-mediated transactivation of rhodopsin promoter, we propose that phosphorylation changes associated with NRL mutations perturb gene expression in rods, leading to photoreceptor degeneration in retinopathies.
Swain
Prabodha
P
Department of Cellular and Molecular Neuroscience, National Brain Research Center, Manesar, Haryana, India. prabodh@nbrc.ac.in
Kumar
Sandeep
S
Patel
Dharmesh
D
Richong
Sushmita
S
Oberoi
Pranav
P
Ghosh
Madhumita
M
Swaroop
Anand
A
D000818:Animals; D000831:Animals, Newborn; D019556:COS Cells; D002522:Chlorocebus aethiops; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005799:Genes, Dominant; D016350:Leucine Zippers / Q000235:genetics / Q000502:physiology*; D020935:MAP Kinase Signaling System / Q000187:drug effects; D051379:Mice; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism; D020928:Mitogen-Activated Protein Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D009154:Mutation / Q000502:physiology*; D010766:Phosphorylation; D011401:Promoter Regions, Genetic / Q000502:physiology; D012160:Retina / Q000201:enzymology / Q000378:metabolism*; D012164:Retinal Diseases / Q000235:genetics*; D012174:Retinitis Pigmentosa / Q000235:genetics / Q000378:metabolism; D012243:Rhodopsin / Q000235:genetics; D015533:Transcriptional Activation / Q000187:drug effects; D014162:Transfection
D004791:Enzyme Inhibitors; D012243:Rhodopsin; D019950:Mitogen-Activated Protein Kinase 1; D020928:Mitogen-Activated Protein Kinases