Phosphorylation of bovine rod photoreceptor cyclic GMP phosphodiesterase

Differential expression of PKCα and -β in the zebrafish retina

The retina is a complex neural circuit, which processes and transmits visual information from light perceiving photoreceptors to projecting retinal ganglion cells. Much of the computational power of the retina rests on signal integrating interneurons, such as bipolar cells. Commercially available antibodies against bovine and human conventional protein kinase C (PKC) α and -β are frequently used as markers for retinal ON-bipolar cells in different species, despite the fact that it is not known which bipolar cell subtype(s) they actually label. In zebrafish (Danio rerio) five prkc genes (coding for PKC proteins) have been identified. Their expression has not been systematically determined. While prkcg is not expressed in retinal tissue, the other four prkc (prkcaa, prkcab, prkcba, prkcbb) transcripts were found in different parts of the inner nuclear layer and some as well in the retinal ganglion cell layer. Immunohistochemical analysis in adult zebrafish retina using fluorescent in situ hybridization and PKC antibodies showed an overlapping immunolabeling of ON-bipolar cells that are most likely of the BON s6 and BON s6L or RRod type. However, comparison of transcript expression with immunolabeling, implies that these antibodies are not specific for one single zebrafish conventional PKC, but rather detect a combination of PKC -α and -β variants.

Phosphorylation of GRK1 and GRK7 by cAMP-dependent protein kinase attenuates their enzymatic activities

Phosphorylation of G protein-coupled receptors is a critical step in the rapid termination of G protein signaling. In rod cells of the vertebrate retina, phosphorylation of rhodopsin is mediated by GRK1. In cone cells, either GRK1, GRK7, or both, depending on the species, are speculated to initiate signal termination by phosphorylating the cone opsins. To compare the biochemical properties of GRK1 and GRK7, we measured the K(m) and V(max) of these kinases for ATP and rhodopsin, a model substrate. The results demonstrated that these kinases share similar kinetic properties. We also determined that cAMP-dependent protein kinase (PKA) phosphorylates GRK1 at Ser(21) and GRK7 at Ser(23) and Ser(36) in vitro. These sites are also phosphorylated when FLAG-tagged GRK1 and GRK7 are expressed in HEK-293 cells treated with forskolin to stimulate the endogenous production of cAMP and activation of PKA. Rod outer segments isolated from bovine retina phosphorylated the FLAG-tagged GRKs in the presence of dibutyryl-cAMP, suggesting that GRK1 and GRK7 are physiologically relevant substrates. Although both GRKs also contain putative phosphorylation sites for PKC and Ca(2+)/calmodulin-dependent protein kinase II, neither kinase phosphorylated GRK1 or GRK7. Phosphorylation of GRK1 and GRK7 by PKA reduces the ability of GRK1 and GRK7 to phosphorylate rhodopsin in vitro. Since exposure to light causes a decrease in cAMP levels in rod cells, we propose that phosphorylation of GRK1 and GRK7 by PKA occurs in the dark, when cAMP levels in photoreceptor cells are elevated, and represents a novel mechanism for regulating the activities of these kinases.

Activity-dependent expression of acyl-coenzyme a-binding protein in retinal muller glial cells evoked by optokinetic stimulation

Long-term horizontal optokinetic stimulation (HOKS) decreases the gain of the horizontal optokinetic reflex and evokes the second phase of optokinetic afternystagmus (OKAN-II). We investigated the possible molecular constituents of this adaptation. We used a differential display reverse transcriptase-PCR screen for mRNAs isolated from retinas of rabbits that received HOKS. In each rabbit, we compared mRNAs from the retina stimulated in the posterior-->anterior (preferred) direction with mRNAs from the retina stimulated in the anterior-->posterior (null) direction. Acyl-CoA-binding protein (ACBP) mRNA was one of four mRNAs selected by this screen, the proteins of which interact with GABA receptors. HOKS in the preferred direction increased ACBP mRNA transcription and ACBP protein expression. ACBP was localized to Muller glial cells by hybridization histochemistry and by immunohistochemistry. ACBP interacts with the alpha1-subunit of the GABA(A) receptor, as determined by a yeast two-hybrid technique. This interaction was confirmed by coimmunoprecipitation of ACBP and the alpha1-subunit of the GABA(A) receptor using an antibody to GABA(A)alpha1. The interaction was also confirmed by a "pull-down" assay in which histidine-tagged ACBP was used to pull down the GABA(A)alpha1. ACBP does not cross the blood-brain barrier. However, smaller truncated proteolytic fragments of ACBP do, increasing the excitability of central cortical neurons. Muller cells may secrete ACBP in the inner plexiform layer, thereby decreasing the sensitivity of GABA(A) receptors expressed on the surface of ganglion cell dendrites. Because retinal directional sensitivity is linked to GABAergic transmission, HOKS-induced expression of ACBP could provide a molecular basis for adaptation to HOKS and for the genesis of OKAN-II.

The time course of light adaptation in vertebrate retinal rods

The photoresponse of a rod wanes over time in steady illumination, as light loses its efficacy in generating the response. Such desensitization is adaptive because it extends the range of ambient light levels over which the rod signals changes in light intensity by several orders of magnitude. Adaptation begins to unfold rapidly after the onset of light with a time constant of approximately 1 s, causing the rod's sensitivity to steady light to decrease by nearly two log units. Thereafter, a much slower phase of adaptation evolves with a time constant of 9 s. During this phase the rod's sensitivity decreases by an additional log unit. Both phases are dependent upon the light-induced fall in intracellular Ca2+. The fast phase of light adaptation can be attributed to Ca2+ feedback processes regulating the lifetime ofphotoactivated rhodopsin, cGMP synthesis and sensitivity of the cGMP-gated channel to cGMP. Although the mechanism(s) of the slow phase is not yet known, it appears to include further regulation of the lifetime of photoactivated rhodopsin.

Regulation of photoreceptor phosphodiesterase (PDE6) by phosphorylation of its inhibitory gamma subunit re-evaluated

Phosphorylation of the inhibitory gamma subunit (Pgamma) of rod cGMP phosphodiesterase (PDE6) has been reported to turn off visual excitation without the requirement for inactivation of the photoreceptor G-protein transducin. We evaluated the significance of Pgamma phosphorylation for PDE6 regulation by preparing Pgamma stoichiometrically phosphorylated at Thr(22) or at Thr(35). Phosphorylation of Pgamma at either residue caused a minor decrease--not the previously reported increase--in the ability of Pgamma to inhibit catalysis at the active site of purified PDE6 catalytic dimers. Likewise, Pgamma phosphorylation had little effect on its potency to inhibit transducin-activated PDE6 depleted of its endogenous Pgamma subunits. The strength of Pgamma interaction with the regulatory GAF domain of PDE6 was reduced severalfold upon Pgamma phosphorylation at Thr(22) (but not Thr(35)), as judged by allosteric changes in cGMP binding to these noncatalytic sites on the enzyme (Mou, H., and Cote, R. H. (2001) J. Biol. Chem. 276, 27527-27534). In contrast, the effects of Pgamma phosphorylation on its interactions with activated transducin were much more pronounced. Phosphorylation of Pgamma at either Thr(22) or Thr(35) greatly diminished its ability to bind activated transducin, consistent with earlier work. In situ phosphorylation of Pgamma by endogenous rod outer segment kinases was enhanced severalfold upon light activation, but only approximately 10% of the endogenous Pgamma was phosphorylated. This is attributed to Pgamma being a poor substrate for protein kinases when associated with the PDE6 holoenzyme. We conclude that, contrary to previous reports, Pgamma phosphorylation at either Thr(22) or Thr(35) modestly weakens its direct interactions with PDE6. However, Pgamma phosphorylation subsequent to its dissociation from PDE6 is likely to abolish its binding to activated transducin and may serve to make phosphorylated Pgamma available to regulate other signal transduction pathways (e.g. mitogen-activated protein kinase; Wan, K. F., Sambi, B. S., Frame, M., Tate, R., and Pyne, N. J. (2001) J. Biol. Chem. 276, 37802-37808) in photoreceptor cells.

Light-mediated activation of diacylglycerol kinase in rat and bovine rod outer segments

The hydrolysis of phosphatidylinositol 4,5-bisphosphate is regulated by light in retinal rod outer segment (ROS) membranes. We recently reported that the activities of phosphatidylinositol synthetase and phosphatidylinositol 3-kinase are also higher in bleached (light-exposed) ROS (B-ROS). In this study, we investigated the effect of bleaching on diacylglycerol (DAG) kinase (DAG-kinase) activity in bovine and rat ROS membranes prepared from dark-adapted (D-ROS) or bleached (B-ROS) retinas. In bovine ROS, DAG-kinase activity toward endogenous DAG substrate was higher in B-ROS than in D-ROS. Quantification of DAG in both sets of membranes showed that the levels were the same, eliminating the possibility that the greater DAG-kinase activity was due to higher levels of endogenous substrate in B-ROS. DAG-kinase activity was also higher in B-ROS against an exogenous, water-soluable substrate (1, 2-didecanoyl-rac-glycerol), which competed with endogenous DAG substrate and saturated at approximately 2 mM. Immunoblot analysis with an anti-DAG-kinase gamma polyclonal antibody demonstrated that the gamma isoform was present in isolated bovine ROS. Immunocytochemistry of frozen bovine retinal sections confirmed the presence of DAG-kinase gamma immunoreactivity in ROS, as well as other retinal cells. Quantification of the immunoreactive products on western blots showed that more DAG-kinase gamma was present in B-ROS than in D-ROS. In an in vivo experiment, ROS prepared from rats exposed to 30 min of room light had greater DAG-kinase activity than ROS prepared from dark-adapted animals. Taken together, these data suggest that light exposure leads to the translocation of DAG-kinase from the cytosol to ROS membranes and that the greater DAG-kinase activity in B-ROS is due to the presence of more protein associated with ROS membranes.

Phosphorylation of rod outer segment proteins modulates phosphatidylethanolamine N-methyltransferase and phospholipase A2 activities in photoreceptor membranes

The activities of enzymes involved in lipid metabolism--phospholipase A2 (PLA2) and phosphatidylethanolamine N-methyltransferase (PE N-MTase)--were found to be differently affected by pre-incubation of rod outer segments (ROS) under protein phosphorylating or dephosphorylating conditions. Exposure to cAMP-dependent protein kinase (PKA), under dark or light conditions, produced a significant increase in PE N-MTase activity, whereas PLA2 activity decreased. Under standard protein kinase C (PKC) phosphorylating conditions in light, PE N-MTase activity was stimulated and PLA2 activity was not affected. When the assays were performed in the dark, both enzymatic activities were unaffected when compared to the corresponding controls. Incubation of ROS membranes in light in the presence of PKC activators phorbol 12,13-dibutyrate (PDBu) and dioctanoylglycerol (DOG) resulted in the same pattern of changes in enzyme activities as described for standard PKC phosphorylating condition. Pre-incubation of membranes with the PKC inhibitor H-7 reduced the stimulation of PDBu on PE N-MTase activity, and had no effect on PLA2 activity in ROS membranes incubated with the phorbol ester. Pre-treatment of isolated ROS with alkaline phosphatase resulted in decreased PE N-MTase activity and produced a significant stimulation of PLA2 activity under dark as well as under light conditions when compared to the corresponding controls. These findings suggest that ROS protein phosphorylation and dephosphorylation modulates PE N-MTase and PLA2 activities in isolated ROS, and that these activities are independently and specifically modulated by particular kinases. Furthermore, dephosphorylation of ROS proteins has the opposite effect to that produced by protein phosphorylation on the enzymes studied.

Protein kinase C activity and light sensitivity of single amphibian rods

Biochemical experiments by others have indicated that protein kinase C activity is present in the rod outer segment, with potential or demonstrated targets including rhodopsin, transducin, cGMP-phosphodiesterase (PDE), guanylate cyclase, and arrestin, all of which are components of the phototransduction cascade. In particular, PKC phosphorylations of rhodopsin and the inhibitory subunit of PDE (PDE ) have been studied in some detail, and suggested to have roles in downregulating the sensitivity of rod photoreceptors to light during illumination. We have examined this question under physiological conditions by recording from a single, dissociated salamander rod with a suction pipette while exposing its outer segment to the PKC activators phorbol-12-myristate,13-acetate (PMA) or phorbol-12,13-dibutyrate (PDBu), or to the PKC-inhibitor GF109203X. No significant effect of any of these agents on rod sensitivity was detected, whether in the absence or presence of a background light, or after a low bleach. These results suggest that PKC probably does not produce any acute downregulation of rod sensitivity as a mechanism of light adaptation, at least for isolated amphibian rods.

Identification of components of a phosphoinositide signaling pathway in retinal rod outer segments

Phototransduction in retinal rods involves a G protein-coupled signaling cascade that leads to cGMP hydrolysis and the closure of cGMP-gated cation channels that are open in darkness, producing a membrane hyperpolarization as the light response. For many years there have also been reports of the presence of a phosphoinositide pathway in the rod outer segment, though its functions and the molecular identities of its components are still unclear. Using immunocytochemistry with antibodies against various phosphoinositide-specific phospholipase C (PLC) isozymes (beta1-4, gamma1-2, and delta1-2), we have found PLCbeta4-like immunoreactivity in rod outer segments. Similar experiments with antibodies against the alpha-subunits of the G(q) family of G proteins, which are known to activate PLCbeta4, have also demonstrated G(alpha11)-like immunoreactivity in this location. Immunoblots of total proteins from whole retina or partially purified rod outer segments with anti-PLCbeta4 and anti-G(alpha11) antibodies gave, respectively, a single protein band of the expected molecular mass, suggesting specific labelings. The retinal locations of the two proteins were also supported by in situ hybridization experiments on mouse retina with probes specific for the corresponding mouse genes. These two proteins, or immunologically identical isoforms, therefore likely mediate the phosphoinositide signaling pathway in the rod outer segment. At present, G(alpha11) or a G(alpha11)-like protein represents the only G protein besides transducin (which mediates phototransduction) identified so far in the rod outer segment. Although absent in the outer segment layer, other PLC isoforms as well as G(alpha q) (another G(q) family member), are present elsewhere in the retina.

Protein kinase C potentiation of the tyrosine kinase inhibitor-stimulated cyclic GMP production in rat pinealocytes

Inhibition of tyrosine kinase activities elevates cyclic GMP (cGMP) levels in rat pinealocytes. Since protein kinase C (PKC) and intracellular Ca2+ both interact with the agonist-stimulated cGMP accumulation, in this study their interactions with the tyrosine kinase inhibitor-mediated cGMP response were investigated. Two tyrosine kinase inhibitors, genistein and tyrphostin B42, increased basal cGMP accumulation concentration dose-dependently. This increase in cGMP accumulation was potentiated by 4 beta-phorbol 12-myristate 13-acetate (PMA), an activator of PKC, and blocked by calphostin C, a specific PKC inhibitor. The tyrosine kinase inhibitors had no effect on the in vitro or PMA-mediated translocation of PKC activity. However, when the phosphodiesterase was inhibited by isobutylmethylxanthine (IBMX), neither the tyrosine kinase inhibitors alone nor in combination with PMA had an effect on cGMP accumulation, suggesting that phosphodiesterase is a probable site of action of the inhibitors. In comparison, elevation of intracellular Ca2+ by BayK 8644, ionomycin, or KCl inhibited the genistein- or tyrphostin B42-mediated increase in cGMP accumulation. This inhibition persisted in the presence of IBMX and was partly reversed by a Ca2+/calmodulin inhibitor. These results suggest that PKC modulates the rate of cGMP degradation through signalling pathways involving tyrosine phosphorylation. However, the inhibitory effect of the Ca(2+)-elevating agents on the tyrosine kinase inhibitor-stimulated cGMP accumulation appears to be independent of phosphodiesterase inhibition.

Phosphorylation and activation of a cAMP-specific phosphodiesterase by the cAMP-dependent protein kinase. Involvement of serine 54 in the enzyme activation

A cAMP-specific phosphodiesterase (PDE4D3) is activated in rat thyroid cells by TSH through a cAMP-dependent phosphorylation (Sette, C., Iona, S., and Conti, M.(1994) J. Biol. Chem. 269, 9245-9252). This short term activation may be involved in the termination of the hormonal stimulation and/or in the induction of desensitization. Here, we have further characterized the protein kinase A (PKA)-dependent phosphorylation of this PDE4D3 variant and identified the phosphorylation site involved in the PDE activation. The PKA-dependent incorporation of phosphate in the partially purified, recombinant rat PDE4D3 followed a time course similar to that of activation. Half-maximal activation of the enzyme was obtained with 0.6 microM ATP and 30 nM of the catalytic subunit of PKA. Phosphorylation altered the Vmax of the PDE without affecting the Km for cAMP. Phosphorylation also modified the Mg2+ requirements and the pattern of inhibition by rolipram. Cyanogen bromide cleavage of the 32P-labeled rat PDE4D3 yielded two or three major phosphopeptide bands, providing a first indication that the enzyme may be phosphorylated at multiple sites in a cell-free system. Site-directed mutagenesis was performed on the serine residues present at the amino terminus of this PDE in the context of preferred motifs for PKA phosphorylation. The PKA-dependent incorporation of 32P was reduced to the largest extent in mutants with both Ser13 --> Ala and Ser54 --> Ala substitutions, confirming the presence of more than one phosphorylation site in rat PDE4D3. While substitution of serine 13 with alanine did not affect the activation by PKA, substitution of Ser54 completely suppressed the kinase activation. Similar conclusions were reached with wild type and mutated PDE4D3 proteins expressed in MA-10 cells, where the endogenous PKA was activated by dibutyryl cAMP. Again, the PDE with the Ser54 --> Ala substitution could not be activated by the endogenous PKA in the intact cell. These findings support the hypothesis that the PDE4D3 variant contains a regulatory domain target for phosphorylation at the amino terminus of the protein and that Ser54 in this domain plays a crucial role in activation.

Protein kinase C in rod outer segments: effects of phosphorylation of the phosphodiesterase inhibitory subunit

The inhibitory subunit (PDE gamma) of the cGMP phosphodiesterase (PDE alpha beta gamma 2) in rod outer segments (ROS) realizes its regulatory role in phototransduction by inhibition of PDE alpha beta catalytic activity. The photoreceptor G-protein, transducin, serves as a transducer from the receptor (rhodopsin) to the effector (PDE) and eliminates the inhibitory effect of PDE gamma by direct interaction with PDE gamma. Our previous study [Udovichenko, Cunnick, Gonzalez and Takemoto (1994) J: Biol. Chem. 269, 9850-9856] has shown that PDE gamma is a substrate for protein kinase C (PKC) from ROS and that phosphorylation by PKC increases the ability of PDE gamma to inhibit PDE alpha beta catalytic activity. Here we report that transducin is less effective in activation of PDE alpha beta (gamma p)2 (a complex of PDE alpha beta with phosphorylated PDE gamma, PDE gamma p) than PDE alpha beta gamma 2. PDE gamma p also increases the rate constant of GTP hydrolysis of transducin (from 0.16 S-1 for non-phosphorylated PDE gamma to 0.21 s-1 for PDE gamma p). These data suggest that phosphorylation of the inhibitory subunit of PDE by PKC may regulate the visual transduction cascade by decreasing the photoresponse.

Regulation of sensitivity in vertebrate rod photoreceptors by calcium

Over the past decade and a half, there have been great advances in our understanding of how light is transduced into electrical signals by the retinal rod and cone photoreceptors in vertebrates. One essential feature of these sensory neurons is their ability to adapt to background illumination, which allows them to function over a broad range of light intensities. This adaptation appears to arise mostly from negative feedback on phototransduction that is mediated by calcium ions. Recent work has suggested that this feedback is fairly complex, and involves several pathways directed at different components of phototransduction. From direct measurements of these feedback pathways in rods, it is possible to evaluate their relative contributions to the overall sensitivity of the cell. At the same time, these feedback mechanisms, as currently known, appear to be sufficient for explaining the change in sensitivity of rods during adaptation to light.

Light adaptation of bovine retinas in situ stimulates phosphatidylinositol synthesis in rod outer segments in vitro

Light-stimulated phosphoinositide turnover has been reported in both vertebrate retina and isolated rod outer segments (ROS). In the current investigation, we examined the incorporation of [3H]-inositol in vitro in bovine ROS isolated from dark adapted (DROS) or bleached (BROS) retinas. Incorporation of [3H]-inositol into phosphoinositides in BROS was 3-5 fold higher than in DROS. The majority (approximately 90%) of [3H]-inositol was found in phosphatidylinositol (PI), whereas phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) accounted for 7-8% of the label. The enhanced labelling of PI was only observed when bovine retinas were light-adapted prior to ROS preparation, suggesting the requirement for an intact photoreceptor for the observed effect. Our data strongly suggest that bleaching of bovine retina in situ stimulates PI synthesis in isolated ROS in vitro.