Mislocalized opsin and cAMP signaling: a mechanism for sprouting by rod cells in retinal degeneration.
OBJECTIVE
In human retinal degeneration, rod photoreceptors reactively sprout neurites. The mechanism is unknown in part because of the paucity of animal models displaying this feature of human pathology. We tested the role of cAMP and opsin in sprouting by tiger salamander rod cells, photoreceptors that can produce reactive growth.


METHODS
In vitro systems of isolated photoreceptor cells and intact neural retina were used. cAMP signaling was manipulated with nucleotide analogues, enzyme stimulators, agonists for adenosine and dopamine receptors, and the opsin agonist, β-ionone. Levels of cAMP were determined by radioimmunoassay, and protein levels by Western blot and quantitative immunocytochemistry. Neuritic growth was assayed by image analysis and conventional and confocal microscopy.


RESULTS
cAMP analogues and stimulation of adenylyl cyclase (AC) directly or through G-protein-coupled receptors resulted in significant increases in neuritic growth of isolated rod, but not cone, cells. The signaling pathway included protein kinase A (PKA) and phosphorylation of the transcription factor cAMP response element-binding protein (pCREB). Opsin, a G-linked receptor, is present throughout the plasmalemma of isolated cells; its activation also induced sprouting. In neural retina, rod sprouting was significantly increased by β-ionone with concomitant increases in cAMP, pCREB, and synaptic proteins. Notably, opsin stimulated sprouting only when mislocalized to the plasmalemma of the rod cell body.


CONCLUSIONS
cAMP causes neuritic sprouting in rod, but not cone, cells through the AC-PKA-CREB pathway known to be associated with synaptic plasticity. We propose that in retinal disease, mislocalized rod opsin gains access to cAMP signaling, which leads to neuritic sprouting.
Department of Neurology and Neuroscience, New Jersey Medical School–University of Medicine and Dentistry of New Jersey, Newark, New Jersey 07103, USA.
Jianfeng
J
Wang
Nan
N
Zhang
Annie
A
Beuve
Ellen
E
Townes-Anderson
D000262:Adenylyl Cyclases / Q000378:metabolism; D000557:Ambystoma; D000818:Animals; D049109:Cell Proliferation; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism*; D017362:Cyclic AMP Response Element-Binding Protein / Q000378:metabolism; D006801:Humans; D016501:Neurites / Q000187:drug effects / Q000502:physiology; D045792:Norisoprenoids / Q000494:pharmacology; D010766:Phosphorylation / Q000187:drug effects / Q000502:physiology; D011554:Pseudopodia / Q000187:drug effects / Q000502:physiology; D011954:Receptors, Dopamine / Q000378:metabolism; D018047:Receptors, Purinergic P1 / Q000378:metabolism; D012162:Retinal Degeneration / Q000378:metabolism* / Q000473:pathology; D017948:Retinal Rod Photoreceptor Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D017299:Rod Opsins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*
D017362:Cyclic AMP Response Element-Binding Protein; D045792:Norisoprenoids; D011954:Receptors, Dopamine; D018047:Receptors, Purinergic P1; D017299:Rod Opsins; D005576:Colforsin; C008157:beta-ionone; D000242:Cyclic AMP; D000262:Adenylyl Cyclases