Functional role of hCngb3 in regulation of human cone cng channel: effect of rod monochromacy-associated mutations in hCNGB3 on channel function.
OBJECTIVE
The human cone photoreceptor cyclic nucleotide-gated (CNG) channel comprises alpha- and beta-subunits, which are respectively encoded by hCNGA3 and hCNGB3. The purpose was to examine the functional role of hCNGB3 in modulation of human cone CNG channels and to characterize functional consequences of rod monochromacy-associated mutations in hCNGB3 (S435F and D633G).


METHODS
Macroscopic patch currents were recorded from human embryonic kidney (HEK) 293 cells expressing homomeric (hCNGA3 and hCNGB3) and heteromeric (hCNGA3/hCNGB3, hCNGA3/hCNGB3-S435F, and hCNGA3/hCNGB3-D633G) channels using inside-out patch-clamp technique.


RESULTS
Both hCNGA3 homomeric and hCNGA3/hCNGB3 heteromeric channels were activated by cGMP, with half-maximally activating concentration (K(1/2)) of 11.1 +/- 1.0 and 26.2 +/- 1.9 micro M, respectively. The hCNGA3 channels appeared to be more sensitive to inhibition by extracellular Ca(2+) compared with hCNGA3/hCNGB3 channels, when assessed by the degree of outward rectification. Coexpression of either of rod monochromacy-associated mutants of hCNGB3 with hCNGA3 significantly reduced K(1/2) value for cGMP but little affected the sensitivity to extracellular Ca(2+), compared with wild-type heteromeric channels. The selectivity of hCNGA3, hCNGA3/hCNGB3, hCNGA3/hCNGB3-S435F, and hCNGA3/hCNGB3-D633G channels for monovalent cations were largely similar. Immunoprecipitation experiments showed association of hCNGA3 subunit with both of wild-type and mutant hCNGB3 subunits.


CONCLUSIONS
The hCNGB3 plays an important modulatory role in the function of human cone CNG channels with respect to cGMP and extracellular Ca(2+) sensitivities. The rod monochromacy-associated S435F and D633G mutations in hCNGB3 evokes a significant increase in the apparent affinity for cGMP, which should alter cone function and thereby contribute at least partly to pathogenesis of the disease.
Okada
Akira
A
Department of Ophthalmology, Shiga University of Medical Science, Seta, Otsu, Japan.
Ueyama
Hisao
H
Toyoda
Futoshi
F
Oda
Sanae
S
Ding
Wei-Guang
WG
Tanabe
Shoko
S
Yamade
Shinichi
S
Matsuura
Hiroshi
H
Ohkubo
Iwao
I
Kani
Kazutaka
K
D002118:Calcium / Q000494:pharmacology; D018929:Cell Culture Techniques; D003001:Cloning, Molecular; D006152:Cyclic GMP / Q000378:metabolism; D054815:Cyclic Nucleotide-Gated Cation Channels; D015870:Gene Expression; D006801:Humans; D007473:Ion Channels / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D007668:Kidney / Q000187:drug effects / Q000196:embryology; D008564:Membrane Potentials; D009154:Mutation; D018408:Patch-Clamp Techniques; D011233:Precipitin Tests; D017949:Retinal Cone Photoreceptor Cells / Q000378:metabolism*; D012162:Retinal Degeneration / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D017948:Retinal Rod Photoreceptor Cells / Q000473:pathology*; D014162:Transfection
C415904:CNGB3 protein, human; D054815:Cyclic Nucleotide-Gated Cation Channels; D007473:Ion Channels; D006152:Cyclic GMP; D002118:Calcium