Rod cGMP-specific 3',5'-cyclic phosphodiesterase subunit beta is the beta subunit of the protein complex PDE6 that is encoded by the PDE6Bgene.[1][2] PDE6 is crucial in transmission and amplification of visual signal. The existence of this beta subunit is essential for normal PDE6 functioning. Mutations in this subunit are responsible for retinal degeneration such as retinitis pigmentosa[3][4] or congenital stationary night blindness.[5]
File:PDE6B.jpgStructure of Rod Phosphodiesterase 6 complex showing the GAF (GAF1 & GAF2) and catalytic (CAT) domains
PDE6 is a protein complex located on the photoreceptor's outer segment, and plays an important role in the phototransduction cascade.[6] There are two types of photoreceptors: cones and rods. The rod and cone PDE6 complexes have different structures. PDE6β together with PDE6α and two identical inhibitory subunits, PDE6γ, form the rod PDE6 holoenzyme while the cone PDE6 complex only consists of two identical PDE6α' catalytic subunits.[7] PDE6β, one of the catalytic units in rod PDE6, is composed of three domains: two N-terminalGAF domains and one C-terminal catalytic domain.The non-catalytic GAF domains are responsible for cGMP binding. The C-terminal interacts with cell membrane by isoprenylation and S-carboxylmethylation.[7]
PDE6 is a highly concentrated protein in retinal photoreceptors. With the presence of the GAF domain, PDE6 can actively bind to the cGMP. The inactive PDE6 in the dark allows cGMP to bind to cGMP gated ion channels. The channel remains open as long as cGMP is binding to it, which allows constant electron flow in to the photoreceptor cell through the plasma membrane. Light causes the visual pigment, rhodopsin, to activate. This process leads to the release of subunit PDE6γ from PDE6αβ, activating PDE6 which leads to the hydrolysis of cGMP. Without the cGMP binding, the ion channel closes, leading to the hyperpolarization.[7] After hyperpolarization the presnaptic transimitter is reduced. Next, the enzyme guanylate cyclase restores cGMP, which reopens the membrane channels. This process is called light adaptation.
Mutation of the PDE6b gene leads to the dysfunction of PDE, which results in failure of hydrolysis of cGMP. The rd1 mouse is a well-characterized animal model of retinitis pigmentosa caused by the mutation of Pde6b gene.[10] The phenotype was first discovered in rodless mice in the 1920s by Keeler.[11] An insertion of Murine leukemia provirus is present near the first exon combined with a point mutation, which introduces a stop codon in exon 7. In addition to the rd1 mouse, a missense mutation (R560C) in exon 13 of the Pde6b gene is the character of another animal model of recessive retinal degeneration.
In rd1 animals, the retinal rod photoreceptor cells begin degenerating at about postnatal day 10, and by 3 weeks no rod photoreceptors remain. Degeneration is preceded by accumulation of cGMP in the retina and is correlated with deficient activity of the rod photoreceptor cGMP-phosphodiesterase.[10][12] Cone photoreceptors undergo a slower degeneration over the course of a year, which causes the mutants to completely go blind.[13] The possibility of altering the course of retinal degeneration through subretinal injection of recombinant replication defective adenovirus that contained the murine cDNA for wildtype PDE6β was tested in rd1 mice.[14] Subretinal injection of rd1 mice was carried out 4 days after birth, before the onset of rod photoreceptor degeneration. Following therapy, Pde6β transcripts and enzyme activity were detected, and histologic studies revealed that photoreceptor cell death was significantly retarded.[2]
The albino FVB mouse laboratory strain become blind by weaning age due to a mutant allele of the PDE6b gene. There are pigmented derivative strains of FVB that lack this trait.
rcd1 dog
Similar to rd1 in mice, Rod-cone dysplasia type 1 (rcd1-PRA) is a form of progressive retinal atrophy (PRA), with early onset of the disease. The Irish Setter is a characterized animal model of rcd1. The mutation is caused by a nonsense mutation in pde6b gene. Photoreceptors start degeneration at postnatal day 13 until a year after the dog is totally blind.[15]
References
↑Bateman JB, Klisak I, Kojis T, Mohandas T, Sparkes RS, Li TS, Applebury ML, Bowes C, Farber DB (Mar 1992). "Assignment of the beta-subunit of rod photoreceptor cGMP phosphodiesterase gene PDEB (homolog of the mouse rd gene) to human chromosome 4p16". Genomics. 12 (3): 601–3. doi:10.1016/0888-7543(92)90454-Z. PMID1313787.
↑Wang Q, Chen Q, Zhao K, Wang L, Wang L, Traboulsi EI (2001). "Update on the molecular genetics of retinitis pigmentosa". Ophthalmic Genetics. 22 (3): 133–54. doi:10.1076/opge.22.3.133.2224. PMID11559856.
↑Danciger, M; Blaney, J; Gao, YQ; Zhao, DY; Heckenlively, JR; Jacobson, SG; Farber, DB (1 November 1995). "Mutations in the PDE6B gene in autosomal recessive retinitis pigmentosa". Genomics. 30 (1): 1–7. doi:10.1006/geno.1995.0001. PMID8595886.
↑ 7.07.17.27.3Cote RH (Jun 2004). "Characteristics of photoreceptor PDE (PDE6): similarities and differences to PDE5". International Journal of Impotence Research. 16 Suppl 1: S28–33. doi:10.1038/sj.ijir.3901212. PMID15224133.
↑Khramtsov NV, Feshchenko EA, Suslova VA, Shmukler BE, Terpugov BE, Rakitina TV, Atabekova NV, Lipkin VM (Aug 1993). "The human rod photoreceptor cGMP phosphodiesterase beta-subunit. Structural studies of its cDNA and gene". FEBS Letters. 327 (3): 275–8. doi:10.1016/0014-5793(93)81003-I. PMID8394243.
↑Anant JS, Ong OC, Xie HY, Clarke S, O'Brien PJ, Fung BK (Jan 1992). "In vivo differential prenylation of retinal cyclic GMP phosphodiesterase catalytic subunits". The Journal of Biological Chemistry. 267 (2): 687–90. PMID1309771.
↑ 10.010.1Farber DB, Lolley RN (1976). "Enzymic basis for cyclic GMP accumulation in degenerative photoreceptor cells of mouse retina". Journal of Cyclic Nucleotide Research. 2 (3): 139–48. PMID6493.
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