Mechanism of photoreception in vertebrate vision

Vitamin A and Vision

Visual systems detect light by monitoring the effect of photoisomerization of a chromophore on the release of a neurotransmitter from sensory neurons, known as rod and cone photoreceptor cells in vertebrate retina. In all known visual systems, the chromophore is 11-cis-retinal complexed with a protein, called opsin, and photoisomerization produces all-trans-retinal. In mammals, regeneration of 11-cis-retinal following photoisomerization occurs by a thermally driven isomerization reaction. Additional reactions are required during regeneration to protect cells from the toxicity of aldehyde forms of vitamin A that are essential to the visual process. Photochemical and phototransduction reactions in rods and cones are identical; however, reactions of the rod and cone visual pigment regeneration cycles differ, and perplexingly, rod and cone regeneration cycles appear to use different mechanisms to overcome the energy barrier involved in converting all-trans- to 11-cis-retinoid. Abnormal processing of all-trans-retinal in the rod regeneration cycle leads to retinal degeneration, suggesting that excessive amounts of the retinoid itself or its derivatives are toxic. This line of reasoning led to the development of various approaches to modifying the activity of the rod visual cycle as a possible therapeutic approach to delay or prevent retinal degeneration in inherited retinal diseases and perhaps in the dry form of macular degeneration (geographic atrophy). In spite of great progress in understanding the functioning of rod and cone regeneration cycles at a molecular level, resolution of a number of remaining puzzling issues will offer insight into the amelioration of several blinding retinal diseases.

Cyclic nucleotide analogs as biochemical tools and prospective drugs

Cyclic AMP (cAMP) and cyclic GMP (cGMP) are key second messengers involved in a multitude of cellular events. From the wealth of synthetic analogs of cAMP and cGMP, only a few have been explored with regard to their therapeutic potential. Some of the first-generation cyclic nucleotide analogs were promising enough to be tested as drugs, for instance N(6),O(2)'-dibutyryl-cAMP and 8-chloro-cAMP (currently in clinical Phase II trials as an anticancer agent). Moreover, 8-bromo and dibutyryl analogs of cAMP and cGMP have become standard tools for investigations of biochemical and physiological signal transduction pathways. The discovery of the Rp-diastereomers of adenosine 3',5'-cyclic monophosphorothioate and guanosine 3',5'-cyclic monophosphorothioate as competitive inhibitors of cAMP- and cGMP-dependent protein kinases, as well as subsequent development of related analogs, has proven very useful for studying the molecular basis of signal transduction. These analogs exhibit a higher membrane permeability, increased resistance against degradation, and improved target specificity. Furthermore, better understanding of signaling pathways and ligand/protein interactions has led to new therapeutic strategies. For instance, Rp-8-bromo-adenosine 3',5'-cyclic monophosphorothioate is employed against diseases of the immune system. This review will focus mainly on recent developments in cyclic nucleotide-related biochemical and pharmacological research, but also highlights some historical findings in the field.

Phospholipase C rescues visual defect in norpA mutant of Drosophila melanogaster

Mutations in the norpA gene of Drosophila melanogaster severely affect the light-evoked photoreceptor potential with strong mutations rendering the fly blind. The norpA gene has been proposed to encode phosphatidylinositol-specific phospholipase C (PLC), which enzymes play a pivotal role in one of the largest classes of signaling pathways known. A chimeric norpA minigene was constructed by placing the norpA cDNA behind an R1-6 photoreceptor cell-specific rhodopsin promoter. This minigene was transferred into norpAP24 mutant by P-element-mediated germline transformation to determine whether it could rescue the phototransduction defect concomitant with restoring PLC activity. Western blots of head homogenates stained with norpA antiserum show that norpA protein is restored in heads of transformed mutants. Moreover, transformants exhibit a large amount of measurable PLC activity in heads, whereas heads of norpAP24 mutant exhibit very little to none. Immunohistochemical staining of tissue sections using norpA antiserum confirm that expression of norpA protein in transformants localizes in the retina, more specifically in rhabdomeres of R1-6 photoreceptor cells, but not R7 or R8 photoreceptor cells. Furthermore, electrophysiological analyses reveal that transformants exhibit a restoration of light-evoked photoreceptor responses in R1-6 photoreceptor cells, but not in R7 or R8 photoreceptor cells. This is the strongest evidence thus far supporting the hypothesis that the norpA gene encodes phospholipase C that is utilized in phototransduction.

The rpa (receptor potential absent) visual mutant of the blowfly (Calliphora erythrocephala) is deficient in phospholipase C in the eye

The rpa (receptor potential absent) mutation of the blowfly, Calliphora erythrocephala, reduces the light-evoked responses of photoreceptor cells and renders the fly blind. This phenotype is similar to the phenotype caused by norpA mutations in Drosophila which have been shown to occur within a gene encoding phospholipase C. In Western blots, norpA antiserum stains a protein in homogenates of wild-type Calliphora eye and head that is similar in molecular weight to the norpA protein. Very little staining of this protein is observed in similar homogenates of rpa mutant. Moreover, norpA antiserum strongly stains retina in immunohistochemical assays of wild-type adult head, but not in rpa mutant. Furthermore, eyes of rpa mutant have a reduced amount of phospholipase C activity compared to eye of wild-type Calliphora. These data suggest that the rpa mutation occurs in a phospholipase C gene of the blowfly that is homologous to the norpA gene of Drosophila.

Inhibition of cGMP-dependent protein kinase by (Rp)-guanosine 3',5'-monophosphorothioates

The activation of the cGMP-dependent protein kinase and cAMP-dependent protein kinase by the diastereomers of guanosine 3',5'-monophosphorothioate, (Sp)-cGMPS and (Rp)-cGMPS, and 8-chloroguanosine 3',5'-monophosphorothioate, (Sp)-8-Cl-cGMPS and (Rp)-8-Cl-cGMPS, was investigated using the peptide Kemptide as substrate. The (Sp)-diastereomers, which have an axial exocyclic sulfur atom, bound to the cGMP-dependent protein kinase and stimulated its phosphotransferase activity. In contrast, the (Rp)-isomers, which have an equatorial exocyclic sulfur atom, bound to the enzyme without stimulation of its activity. (Rp)-cGMPS and (Rp)-8-Cl-cGMPS antagonized the activation of the cGMP-dependent protein kinase with a Ki of 20 microM and 1.5 microM, respectively. (Rp)-cGMPS also antagonized the activation of cAMP-dependent protein kinase with a Ki of 20 microM. In contrast, (Rp)-8-cGMPS ws a weak inhibitor of the cAMP-dependent protein kinase with a Ki of 100 microM. (Rp)-8-Cl-cGMPS appears to be a rather selective inhibitor of the cGMP-dependent protein kinase and may be a useful tool for studying the role of cGMP in broken and intact cell systems.

cGMP immunocytochemistry in aorta, kidney, retina and brain tissues of the rat after perfusion with nitroprusside

The distribution of cyclic guanosine 3',5' monophosphate (cGMP) producing cells in various organs of the rat were studied immunocytochemically using antibodies raised against formaldehyde-fixed cGMP. Sodium nitroprusside (SNP), a direct activator of guanylate cyclase and vasodilator, was used to enhance cGMP levels. In order to reach all organs optimally, whole body perfusion was performed using a modified Krebs-Ringer buffer at 37 degrees C, aerated with 5% CO2/95% O2, also containing isobutyl methyl xanthine (IBMX); a phosphodiesterase inhibitor. After 15-min pre-perfusion, SNP was added to the perfusate, followed by fast fixation with ice-cold 4% paraformaldehyde-phosphate buffer. After vehicle perfusion, only the retina showed cGMP immunoreactivity in the photoreceptor and ganglion layer, while other organs lacked cGMP immunoreactivity. After 15-min perfusion with SNP (10 microM), enhanced cGMP immunostaining was seen in smooth muscles of the aorta, amacrine-like cells in the retina, glomeruli of the kidney cortex, blood vessels in the dura mater, as well as cells in the pineal and in the median eminence. The results indicate that the distribution and the reactivity of cGMP producing cells, situated outside the blood brain barrier, can be studied by immunocytochemistry after pharmacological manipulations of the intact tissue with a nitrovasodilator using whole body perfusion.

Molecular cloning and predicted full-length amino acid sequence of the type I beta isozyme of cGMP-dependent protein kinase from human placenta. Tissue distribution and developmental changes in rat

In this study we report the isolation and characterization of three overlapping cDNA clones for the type I beta isozyme of cGMP-dependent protein kinase (cGK) from human placenta libraries. The composite sequence was 3740 nucleotides long and contained 58 nucleotides from the 5'-noncoding region, an open reading frame of 2061 bases including the stop codon, and a 3'-noncoding region of 1621 nucleotides. The predicted full-length human type I beta cGK protein contained 686 amino acids including the initiator methionine, and had an estimated molecular mass of 77,803 Da. On comparison to the published amino acid sequence of bovine lung I alpha, human placenta I beta cGK differed by only two amino acids in the carboxyl-terminal region (amino acids 105-686). In contrast, the amino-terminal region of the two proteins was markedly different (only 36% similarity), and human I beta cGK was 16 amino acids longer. In a specific region in the amino-terminus (amino acids 63-75), 12 out of 13 amino acids of the human I beta cGK were identical to the partial amino acid sequence recently published for a new I beta isoform of cGK from bovine aorta. Northern blot analysis demonstrated a human I beta cGK mRNA, 7 kb in size, in human uterus and weakly in placenta. An mRNA of 7 kb was also observed in rat cerebellum, cerebrum, lung, kidney, and adrenal, whereas an mRNA doublet of 7.5 and 6.5 kb were observed in rat heart. Comparison of Northern and Western blot analyses demonstrated that the mRNA and protein for cerebellar cGK increased during the development of rats from 5 to 30 days old, whereas the 6.5 kb mRNA in rat heart declined.

The effect of ATP, GTP and cAMP on the cGMP-dependent conductance of the fragments from frog rod plasma membrane

Using a 'patch-clamp' method in the 'inside-out' configuration, ATP, ADP, AMP-PCP and AMP-PNP have been shown to increase the cGMP-dependent component of the rod plasma membrane conductance 2-4-fold and GTP, GDP but not GMP or nonhydrolyzable GTP analogs GMP-PNP and GTP-gamma-S to abolish the ATP action. The ATP and GTP effects were observed at [EDTA] = 1 mM when magnesium and calcium ions were absent. In about half of the experiments the cGMP-dependent conductance was shown to be increased by cAMP in the micromolar concentration range by 10-50%, the cAMP action did not depend on the presence of nucleoside triphosphates. In vivo ATP, GTP and cAMP are assumed to modulate the sensitivity of the photoreceptor plasma membrane to cGMP.

The evolutionary origin of eukaryotic transmembrane signal transduction

1. A comparison was made of transmembrane signal transduction mechanisms in different eukaryotes and prokaryotes. 2. Much attention was given to eukaryotic microbes and their signal transduction mechanisms, since these organisms are intermediate in complexity between animals, plants and bacteria. 3. Signal transduction mechanisms in eukaryotic microbes, however, do not appear to be intermediate between those in animals, plants and bacteria, but show features characteristic of the higher eukaryotes. 4. These similarities include the regulation of receptor function, adenylate cyclase activity, the presence of a phosphatidylinositol cycle and of GTP-binding regulatory proteins. 5. It is proposed that the signal transduction systems known to operate in present-day eukaryotes evolved in the earliest eukaryotic cells.

Resolution of transducin subunits by chromatography on blue sepharose

The retinal guanine nucleotide-binding protein, transducin (TD), was subjected to chromatography on Blue Sepharose (BLS). A simple two-step protocol was developed, allowing the resolution of the alpha-subunit and the beta gamma-complex of the protein extracted from bovine retina by the use of a poorly hydrolysable GTP analogue. If TD was applied to BLS in a divalent cation-containing buffer, the beta gamma-complex did not bind to the resin, whereas the alpha-subunit was retained; elution of the latter was achieved by removing the divalent cation from the buffer. Binding of the alpha-subunit to BLS was not affected by nucleotides or by ADP ribosylation catalysed by bacterial toxins. However, adsorption of the alpha-subunit by BLS or by a strong cation exchanger (Mono S) depended strictly on divalent cations. In contrast to previous reports, the data suggest the formation of a complex between a sulphonyl residue of Cibacron Blue, a divalent metal ion, and the alpha-subunit as the relevant binding mechanism causing adsorption of the alpha-subunit to BLS.

Phospholipidic second messengers and calcium

A number of signal molecules bind to surface receptors of target cells and generate intracellular messengers from inositol-containing phospholipids. The phosphatidyl inositol (4, 5) bisphosphate is hydrolyzed into inositol (1, 4, 5) trisphosphate and diacylglycerol. These messengers, via changes in the concentrations of cytosolic Ca2+ and H+ and/or protein phosphorylations, couple the signal to a variety of responses including activation of metabolism, secretion, aggregation, phototransduction, cell proliferation and possibly contraction.

The GTP-binding protein, Go, regulates neuronal calcium channels

In neuronal cells, opioid peptides and opiates inhibit neurotransmitter release, which is a calcium-dependent process. They also inhibit adenylyl cyclase, presumably via the membrane signal-transducing component, Gi, a guanine nucleotide-binding protein (G-protein). No causal relationship between these two events has yet been demonstrated. Besides Gi, membranes of neuronal tissues contain large amounts of Go, a G-protein with unknown function. Both G-proteins are heterotrimers consisting of alpha-, beta- and gamma-subunits; the alpha-subunits can be ADP-ribosylated by an exotoxin from Bordetella pertussis (PT), which modification inhibits receptor-mediated activation of the G-protein. It was recently shown that noradrenaline, dopamine and gamma-aminobutyric acid (GABA) inhibit the voltage-dependent calcium channels in dorsal root and sympathetic ganglia; this inhibition is mimicked by intracellular application of guanine nucleotides and blocked by PT, suggesting the involvement of a G-protein. Here we report an inhibitory effect of the opioid D-Ala2, D-Leu5-enkephalin (DADLE) on the calcium current (ICa) in neuroblastoma X glioma hybrid cells (N X G cells). Pretreatment with PT almost completely abolishes the DADLE effect. The effect is restored by intracellular application of Gi and Go. As the alpha-subunit of Go (with or without beta-gamma complex) is 10 times more potent than Gi, we propose that Go is involved in the functional coupling of opiate receptors to neuronal voltage-dependent calcium channels.