PDE6 in Lamprey Petromyzon marinus:  Implications for the Evolution of the Visual Effector in Vertebrates^,

Abstract

Photoreceptor rod and cone phosphodiesterases comprise the sixth family of cyclic nucleotide phosphodiesterases (PDE6). PDE6s have uniquely evolved as effector enzymes in the vertebrate phototransduction cascade. To understand the evolution of the PDE6 family, we have examined PDE6 in lamprey, an ancient vertebrate group. A single PDE6 catalytic subunit transcript was found in the sea lamprey Petromyzon marinus cDNA library. The lamprey PDE6 sequence showed a high degree of homology with mammalian PDE6 and equally distant relationships with the rod and cone enzymes. In contrast, two different PDE6 inhibitory Pgamma subunits, a cone-type Pgamma1 and a mixed cone/rod-type Pgamma2, have been identified in the lamprey retina. Immunofluorescence analysis demonstrated that Pgamma1 and Pgamma2 are expressed in the long and short photoreceptors of sea lamprey, respectively. The catalytic PDE6 subunit was present in the photoreceptors of both types and colocalized with the Pgamma subunits. Recombinant Pgamma1 and Pgamma2 potently inhibited trypsin-activated lamprey and bovine PDE6 enzymes. Our results point to a high degree of conservation of PDE6 genes during the vertebrate evolution. The apparent duplication of the Pgamma gene in the stem of vertebrate lineage may have been an essential component of the evolution of scotopic vision in early vertebrates.

Single-Photon Sensitivity of Lamprey Rods with Cone-like Outer Segments

“…The “short” photoreceptors respond to brief flashes of light with high sensitivity, and their responses decay slowly as for other vertebrate rods; whereas the “long” photoreceptors have a sensitivity and time course of response nearly indistinguishable from the cones of other vertebrates [see 24]. Our experiments confirm previous evidence of two distinct lamprey photoreceptor classes from differences in the expression of membrane proteins [8] and different classes of transducin [25] and phosphodiesterase gamma [26]. Moreover extracellular recordings from lamprey retina previously indicated that there are two kinds of photoreceptors with differing sensitivities [6].…”

Rod phosphodiesterase-6 PDE6A and PDE6B Subunits Are Enzymatically Equivalent

“…The human cone and bovine rod P␥ subunits were subcloned into the pET15b vector, expressed in E. coli, and purified using His⅐Bind resin and reverse-phase HPLC as described previously (19,26). The K i values for PDE6 inhibition by P␥ were calculated by fitting data to the following equation:…”

Characterization of Human Cone Phosphodiesterase-6 Ectopically Expressed in Xenopus laevis Rods

“…The resulting PCR product was cloned into the pET15b vector using the NdeI/BamHI sites. The cone and rod P␥-subunits were expressed and purified as described previously (24).…”