PDE6B

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.[12] After hyperpolarization the presnaptic transmitter is reduced. Next, the enzyme guanylate cyclase restores cGMP, which reopens the membrane channels. This process is called light adaptation.

Pde6 ribbonmodel

PDE6β is the only protein that undergoes the two types of post-translational modification, prenylation and carboxymethylation.[15] The geranylgeranyl group of PDE6B is the result of these modifications, which are responsible for the rod PDE6's interaction with membrane.

The figure at left shows the PDE6 aalpha/beta dimer in blue and purple, with the gamma ubunits in green and orange.

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.[16] The phenotype was first discovered in rodless mice in the 1920s by Keeler.[17] 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.[16][18] Cone photoreceptors undergo a slower degeneration over the course of a year, which causes the mutants to completely go blind.[19] 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.[20] 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.[6]

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.

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.[21]