Inositol-1,4,5-trisphosphate receptors in the vertebrate retina

Signaling roles of phosphoinositides in the retina

The field of phosphoinositide signaling has expanded significantly in recent years. Phosphoinositides (also known as phosphatidylinositol phosphates or PIPs) are universal signaling molecules that directly interact with membrane proteins or with cytosolic proteins containing domains that directly bind phosphoinositides and are recruited to cell membranes. Through the activities of phosphoinositide kinases and phosphoinositide phosphatases, seven distinct phosphoinositide lipid molecules are formed from the parent molecule, phosphatidylinositol. PIP signals regulate a wide range of cellular functions, including cytoskeletal assembly, membrane budding and fusion, ciliogenesis, vesicular transport, and signal transduction. Given the many excellent reviews on phosphoinositide kinases, phosphoinositide phosphatases, and PIPs in general, in this review, we discuss recent studies and advances in PIP lipid signaling in the retina. We specifically focus on PIP lipids from vertebrate (e.g., bovine, rat, mouse, toad, and zebrafish) and invertebrate (e.g., Drosophila, horseshoe crab, and squid) retinas. We also discuss the importance of PIPs revealed from animal models and human diseases, and methods to study PIP levels both in vitro and in vivo. We propose that future studies should investigate the function and mechanism of activation of PIP-modifying enzymes/phosphatases and further unravel PIP regulation and function in the different cell types of the retina.

Retinoschisin is linked to retinal Na/K-ATPase signaling and localization

Retinoschisin binds to the extracellular domain of Na/K-ATPase subunit β2. Retinoschisin inhibits Na/K-ATPase–associated signaling cascades and affects Na/K-ATPase localization. The retinoschisin-Na/K-ATPase complex overlaps with signaling mediators. Defective Na/K-ATPase signaling by retinoschisin deficiency may promote retinal dystrophy.

Mutations in the RS1 gene cause X-linked juvenile retinoschisis (XLRS), a hereditary retinal dystrophy. We recently showed that retinoschisin, the protein encoded by RS1, regulates ERK signaling and apoptosis in retinal cells. In this study, we explored an influence of retinoschisin on the functionality of the Na/K-ATPase, its interaction partner at retinal plasma membranes. We show that retinoschisin binding requires the β2-subunit of the Na/K-ATPase, whereas the α-subunit is exchangeable. Our investigations revealed no effect of retinoschisin on Na/K-ATPase–mediated ATP hydrolysis and ion transport. However, we identified an influence of retinoschisin on Na/K-ATPase–regulated signaling cascades and Na/K-ATPase localization. In addition to the known ERK deactivation, retinoschisin treatment of retinoschisin-deficient (Rs1h^-/Y) murine retinal explants decreased activation of Src, an initial transmitter in Na/K-ATPase signal transduction, and of Ca²⁺ signaling marker Camk2. Immunohistochemistry on murine retinae revealed an overlap of the retinoschisin–Na/K-ATPase complex with proteins involved in Na/K-ATPase signaling, such as caveolin, phospholipase C, Src, and the IP3 receptor. Finally, retinoschisin treatment altered Na/K-ATPase localization in photoreceptors of Rs1h^-/Y retinae. Taken together, our results suggest a regulatory effect of retinoschisin on Na/K-ATPase signaling and localization, whereas Na/K-ATPase-dysregulation caused by retinoschisin deficiency could represent an initial step in XLRS pathogenesis.

Calcium stores in vertebrate photoreceptors

This review lays out the emerging evidence for the fundamental role of Ca(2+) stores and store-operated channels in the Ca(2+) homeostasis of rods and cones. Calcium-induced calcium release (CICR) is a major contributor to steady-state and light-evoked photoreceptor Ca(2+) homeostasis in the darkness whereas store-operated Ca(2+) channels play a more significant role under sustained illumination conditions. The homeostatic response includes dynamic interactions between the plasma membrane, endoplasmic reticulum (ER), mitochondria and/or outer segment disk organelles which dynamically sequester, accumulate and release Ca(2+). Coordinated activation of SERCA transporters, ryanodine receptors (RyR), inositol triphosphate receptors (IP3Rs) and TRPC channels amplifies cytosolic voltage-operated signals but also provides a memory trace of previous exposures to light. Store-operated channels, activated by the STIM1 sensor, prevent pathological decrease in [Ca(2+)]i mediated by excessive activation of PMCA transporters in saturating light. CICR and SOCE may also modulate the transmission of afferent and efferent signals in the outer retina. Thus, Ca(2+) stores provide additional complexity, adaptability, tuneability and speed to photoreceptor signaling.

Phosphoinositides and photoreceptors

The importance of phosphoinositides (phosphorylated phosphatidyl inositol derivatives, PIs) for normal cellular function cannot be overstated. Although they represent a small fraction of the total phospholipid within the cell, they are essential regulators of many cellular functions. They direct membrane trafficking by functioning as recruitment factors for vesicular trafficking components, they can modulate ion channel activity through direct binding within cellular membranes, and their hydrolysis generates second messenger signaling molecules. Despite an explosion of information regarding the importance of these lipids in cellular biology, their precise roles in vertebrate retinal photoreceptors has not been established. This review summarizes the literature on potential roles for different phosphoinositides and their regulators in vertebrate rods and cones. A brief description of the importance of PI signaling in other photosensitive cells is also presented. The highly specialized functions of the vertebrate photoreceptor, combined with the established importance of phosphoinositides, promise significant future discoveries in this field.

Progesterone potentiates IP(3)-mediated calcium signaling through Akt/PKB

The activity of cells critically depends on the control of their cytosolic free calcium ion (Ca(2+)) concentration. The objective of the present study was to identify mechanisms of action underlying the control of the gain of intracellular Ca(2+) release by circulating gonadal steroid hormones. Acute stimulation of isolated neurons with progesterone led to IP(3)R-mediated Ca(2+) transients that depend on the activation of the PI3 kinase/Akt/PKB signaling pathway. These results were confirmed at the molecular level and phosphorylation of IP(3)R type 1 by Akt/PKB was identified as the mechanism of action. Hence, it is likely that circulating gonadal steroid hormones control neuronal activity including phosporylation status through receptor- and kinase-mediated signaling. With a direct control of the gain of the Ca(2+) second messenger system as a signaling gatekeeper for neuronal activity the present study identifies a novel pathway for interaction of the endocrine and central nervous system.

Calcium From Internal Stores Triggers GABA Release From Retinal Amacrine Cells

The Ca2+ that promotes transmitter release is generally thought to enter presynaptic terminals through voltage-gated Ca2+channels. Using electrophysiology and Ca2+ imaging, we show that, in amacrine cell dendrites, at least some of the Ca2+ that triggers transmitter release comes from endoplasmic reticulum Ca2+ stores. We show that both inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs) are present in these dendrites and both participate in the elevation of cytoplasmic [Ca2+] during the brief depolarization of a dendrite. Only the Ca2+ released through IP3Rs, however, seems to promote the release of transmitter. Antagonists for the IP3R reduced transmitter release, whereas RyR blockers had no effect. Application of an agonist for metabotropic glutamate receptor, known to liberate Ca2+ from internal stores, enhanced both spontaneous and evoked transmitter release.

Serca isoform expression in the mammalian retina

The sarcoplasmic-endoplasmic reticulum calcium ATPase (SERCA) is a key intracellular calcium transporter, which regulates cellular calcium concentration [Ca2+] by transporting Ca2+ ions from the cytosol into the endoplasmic reticulum. SERCA-mediated Ca2+ sequestration controls proper folding of newly synthesized proteins within the ER as well as the timing and spatial patterning of depolarization-evoked Ca2+ responses in the cytoplasm. To understand the spatial and temporal properties of Ca2+ homeostasis in retinal neurons better, I studied expression and distribution of all three SERCA isoforms in the mouse retina using isoform-specific antibodies. No immunostaining was observed with the SERCA1 antibody. SERCA2 was expressed in photoreceptor inner segments, amacrine and ganglion cells of the mouse retina. Similar SERCA2 localization was observed in adult rat, macaque and ground squirrel retinas. Analysis of distribution of SERCA2 immunofluorescence in the developing mouse retina revealed prominent SERCA2 signals throughout postnatal development. The N89 antibodys used to identify the SERCA3 isoforms labelled cone outer segments, inner segments of photoreceptors and cell processes in the inner nuclear layer of the mouse retina. These results imply that the SERCA2 isoform controls Ca2+ sequestration into the endoplasmic reticulum in most classes of retinal neuron. A potential role for SERCA3 in cone function is suggested.

Circadian rhythm and photic control of cAMP level in chick retinal cell cultures: a mechanism for coupling the circadian oscillator to the melatonin-synthesizing enzyme, arylalkylamine N-acetyltransferase, in photoreceptor cells

Arylalkylamine N-acetyltransferase (AANAT) is the penultimate and key regulatory enzyme in the melatonin biosynthetic pathway. In chicken retina in vivo, AANAT is expressed in a circadian fashion, primarily in photoreceptor cells. AANAT activity is high at night in darkness, low during the daytime, and suppressed by light exposure at night. In the present study, we investigated the circadian and photic regulation of adenosine 3',5'-monophosphate (cAMP) in cultured retinal cells entrained to a daily light-dark (LD) cycle, as well as the role of Ca(2+) and cAMP in the regulation of AANAT activity. Similar to AANAT activity, cAMP levels fluctuate in a daily fashion, with high levels at night in darkness and low levels during the day in light. This daily fluctuation continued with reduced amplitude in constant (24 h/day) darkness (DD). These changes in cAMP appear to be causally related to control of AANAT activity. Adenylyl cyclase and protein kinase A inhibitors suppress the nocturnal increase of AANAT in DD, while 8Br-cAMP augments it. The nocturnal increase of AANAT activity also involves Ca(2+) influx, as it is inhibited by nitrendipine, an inhibitor of L-type voltage-gated channels, and augmented by Bay K 8644, a Ca(2+) channel agonist. The effect of Bay K 8644 was antagonized by the adenylyl cyclase inhibitor MDL 12330A, suggesting a link between Ca(2+) influx, cAMP formation, and AANAT activity in retinal cells. Light exposure at night, which rapidly suppresses AANAT activity, also suppressed cAMP levels. The effect of light on AANAT activity was reversed by Bay K 8644, 8Br-cAMP, and the proteasome inhibitor lactacystin. These results indicate a dynamic interplay of circadian oscillators and light in the regulation of cAMP levels and AANAT activity in photoreceptor cells.

Calcium regulation in photoreceptors

In this review we describe some of the remarkable and intricate mechanisms through which the calcium ion (Ca2+) contributes to detection, transduction and synaptic transfer of light stimuli in rod and cone photoreceptors. The function of Ca2+ is highly compartmentalized. In the outer segment, Ca2+ controls photoreceptor light adaptation by independently adjusting the gain of phototransduction at several stages in the transduction chain. In the inner segment and synaptic terminal, Ca2+ regulates cells' metabolism, glutamate release, cytoskeletal dynamics, gene expression and cell death. We discuss the mechanisms of Ca2+ entry, buffering, sequestration, release from internal stores and Ca2+ extrusion from both outer and inner segments, showing that these two compartments have little in common with respect to Ca2+ homeostasis. We also investigate the various roles played by Ca2+ as an integrator of intracellular signaling pathways, and emphasize the central role played by Ca2+ as a second messenger in neuromodulation of photoreceptor signaling by extracellular ligands such as dopamine, adenosine and somatostatin. Finally, we review the intimate link between dysfunction in photoreceptor Ca2+ homeostasis and pathologies leading to retinal dysfunction and blindness.

A light-dependent increase in free Ca2+ concentration in the salamander rod outer segment

1. The Ca(2+) indicator dye fluo-5F was excited by an argon ion laser to measure changes in free Ca(2+) concentration ([Ca2+]i) in the outer segments of isolated salamander rods rapidly exposed to a 0 Ca(2+), 0 Na(+) solution designed to minimise surface membrane Ca(2+) fluxes. Over 30-60 s of laser illumination, the fluorescence first increased rapidly and then declined at a rate that was much slower than in Ringer solution and consistent with previous physiological evidence that 0 Ca(2+), 0 Na(+) solution greatly retards light-induced changes in [Ca(2+)]i. 2. The initial increase in fluorescence was investigated with a sequence of 100 ms laser flashes presented at 5 s intervals. The fluorescence evoked by the second laser flash was on average 30 % larger than the first, and subsequent responses exhibited a slow decline like that measured with continuous laser exposures. The initial increase in fluorescence did not depend upon the timing of exposure to 0 Ca(2+), 0 Na(+) solution but appeared to be evoked by exposure to the laser light. 3. Both the increase and subsequent decline in fluorescence measured with brief laser flashes could be reduced by incorporation of the Ca(2+) chelator BAPTA. This and other results indicate that the fluorescence increase was unlikely to have been caused by a change in the affinity of fluo-5F for Ca(2+) or an increase in the quantity of incorporated dye available to bind Ca(2+) but reflects an actual release of intracellular Ca(2+) within the outer segment. 4. The pool of Ca(2+) available to be released could be decreased if, before the first laser flash, the rod was exposed to light bright enough to bleach a substantial fraction of the photopigment. The releasable pool could also be depleted by exposure to saturating light of much lower intensity if delivered in Ringer solution but not if delivered in 0 Ca(2+), 0 Na(+) solution. We conclude that Ca(2+) can be released within the outer segment both by the bleaching of rhodopsin and by the reduction in [Ca(2+)]i which normally accompanies illumination in Ringer solution. 5. The activation of rhodopsin appears somehow to induce the release of Ca(2+) from a binding site or store within the outer segment. Substantial release, however, required stimulating light of an intensity sufficient to bleach a considerable fraction of the visual pigment. It therefore seems unlikely that such release contributes to the normal Ca(2+)-mediated modulation of transduction during light adaptation. The mechanism and physiological function of light-induced Ca(2+) release are unknown.

Adaptation in vertebrate photoreceptors

When light is absorbed within the outer segment of a vertebrate photoreceptor, the conformation of the photopigment rhodopsin is altered to produce an activated photoproduct called metarhodopsin II or Rh(*). Rh(*) initiates a transduction cascade similar to that for metabotropic synaptic receptors and many hormones; the Rh(*) activates a heterotrimeric G protein, which in turn stimulates an effector enzyme, a cyclic nucleotide phosphodiesterase. The phosphodiesterase then hydrolyzes cGMP, and the decrease in the concentration of free cGMP reduces the probability of opening of channels in the outer segment plasma membrane, producing the electrical response of the cell. Photoreceptor transduction can be modulated by changes in the mean light level. This process, called light adaptation (or background adaptation), maintains the working range of the transduction cascade within a physiologically useful region of light intensities. There is increasing evidence that the second messenger responsible for the modulation of the transduction cascade during background adaptation is primarily, if not exclusively, Ca(2+), whose intracellular free concentration is decreased by illumination. The change in free Ca(2+) is believed to have a variety of effects on the transduction mechanism, including modulation of the rate of the guanylyl cyclase and rhodopsin kinase, alteration of the gain of the transduction cascade, and regulation of the affinity of the outer segment channels for cGMP. The sensitivity of the photoreceptor is also reduced by previous exposure to light bright enough to bleach a substantial fraction of the photopigment in the outer segment. This form of desensitization, called bleaching adaptation (the recovery from which is known as dark adaptation), seems largely to be due to an activation of the transduction cascade by some form of bleached pigment. The bleached pigment appears to activate the G protein transducin directly, although with a gain less than Rh(*). The resulting decrease in intracellular Ca(2+) then modulates the transduction cascade, by a mechanism very similar to the one responsible for altering sensitivity during background adaptation.

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.

Localization of type I inositol 1,4,5-triphosphate receptor in the outer segments of mammalian cones

Calcium enters the outer segment of a vertebrate photoreceptor through a cGMP-gated channel and is extruded via a Na/Ca, K exchanger. We have identified another element in mammalian cones that might help to control cytoplasmic calcium. Reverse transcription-PCR performed on isolated photoreceptors identified mRNA for the SII- splice variant of the type I receptor for inositol 1,4,5-triphosphate (IP3), and Western blots showed that the protein also is expressed in outer segments. Immunocytochemistry showed type I IP3 receptor to be abundant in red-sensitive and green-sensitive cones of the trichromatic monkey retina, but it was negative or weakly expressed in blue-sensitive cones and rods. Similarly, the green-sensitive cones expressed the receptor in dichromatic retina (cat, rabbit, and rat), but the blue-sensitive cones did not. Immunostain was localized to disk and plasma membranes on the cytoplasmic face. To restore sensitivity after a light flash, cytoplasmic cGMP must rise to its basal level, and this requires cytoplasmic calcium to fall. Cessation of calcium release via the IP3 receptor might accelerate this fall and thus explain why the cone recovers much faster than the rod. Furthermore, because its own activity of the IP3 receptor depends partly on cytoplasmic calcium, the receptor might control the set point of cytoplasmic calcium and thus affect cone sensitivity.

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.

Localization of a 42-kDa inositol 1,3,4,5-tetrakisphosphate receptor protein in retina and change in expression after optic nerve injury

The mRNA and protein expression of a 42-kDa inositol 1,3,4,5-tetrakisphosphate receptor (InsP4R) was investigated in cryostat and paraffin sections from rat, porcine and bovine retina. InsP4R mRNA was localized by in situ hybridization in the ganglion cell layer, the inner nuclear cell layer and the outermost part of the outer nuclear cell layer. For immunocytochemistry, we used an antibody raised against a 19-amino-acid peptide (peptide-3) derived from previous microsequencing of proteolytic fragments of the porcine InsP4R (Stricker et al., FEBS Lett., 370 (1995) 236). The distribution of immunoreactivity was similar in all species investigated. Two cell types, most likely wide-field amacrine and retinal ganglion cells, were intensely stained. Prominent immunoreactivity in the on/off sublaminae of the inner plexiform layer and in the optic nerve layer indicates a pre- and/or post-synaptic localization of the protein. Moreover, significant InsP4R protein expression in the inner segment of photoreceptors points to a putative role of the second messenger InsP4 in signaling processes related to phototransduction. However, also the endfeet of Müller glia cells in the optic nerve layer were intensely stained. Optic nerve crush caused only minor changes in retinal InsP4R mRNA levels whereas InsP4R immunoreactivity was attenuated for more than 4 weeks in the photoreceptor inner segments, wide-field amacrine cells, and in retinal ganglion cells. The immunopositive sublaminae of the inner plexiform layer appeared to have shrunken. However, the signal intensity gradually recovered after 10 weeks. Since in parallel sections stained with a monoclonal antibody directed against the vesicular protein synaptophysin no changes were found, the alterations in InsP4R immunoreactivity induced by nerve injury are not due to a general decline in the expression of pre-synaptic proteins. We, therefore, hypothesize that the InsP4R might be linked to altered intracellular Ca2+ signaling after neuronal injury.

Gene cloning and characterization of CDP-diacylglycerol synthase from rat brain

A cDNA encoded a 462-amino acid protein, which showed CDP-diacylglycerol synthase (CDS) activity was cloned for the first time as the vertebrate enzyme molecule from rat brain cDNA library. The deduced molecular mass of this rat CDS was 53 kDa, and putative primary structure included several possible membrane- spanning regions. At the amino acid sequence level, rat CDS shared 55.5%, 31. 7%, and 20.9% identity with already known Drosophila, Saccharomyces cerevisiae, and Escherichia coli CDS, respectively. This rat CDS preferred 1-stearoyl-2-arachidonoyl phosphatidic acid as a substrate, and its activity was strongly inhibited by phosphatidylglycerol 4, 5-bisphosphate. By immunoblotting analysis of COS cells overexpressed with the epitope-tagged for rat CDS, a 60-kDa band was detected. By epitope-tag immunocytochemistry, the CDS protein was mainly localized in close association with the membrane of the endoplasmic reticulum of the transfected cells. The intense mRNA expression of CDS was localized in the cerebellar Purkinje cells, the pineal body, and the inner segment of photoreceptor cells. Additionally, very intense expression was detected in postmitotic spermatocytes and spermatids.

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.

Pharmacological characterization of inositol-1,4,5,-trisphosphate binding to membranes from retina and retinal cultures

Light and excitatory amino acids (EAA) stimulate the phosphoinositide cycle in the vertebrate retina. The regulation of Ca2+ release from intracellular stores by inositol-1,4, 5-trisphosphate (IP3) involves an interaction of this compound with specific receptors. By means of [3H]IP3-specific binding, we studied the kinetic and pharmacological properties of IP3 receptors in the chick retina as well as in primary cultures of neurons and glia from this tissue. The equilibrium time for the binding reaction was 15 min and was optimal at alkaline pH (8.3). IP3 receptor displayed high affinity (K(B) approximately 40 nM) and selectivity for D-IP3, compared to D-IP4 > L-IP3 > D-IP2 > D-IP1. These characteristics were the same in subcellular fractions from outer (P1) and thinner (P2) plexiform layers, binding sites being more abundant in P2 (2.65 pmol/mg protein). IP3 receptors were present in both neuronal and glial cultures, but were concentrated in neuronal cultures. Binding was not affected by ryanodine, or caffeine, related to calcium-induced calcium release (CICR) channels, nor by the endoplasmic reticulum Ca2+ ATPase inhibitor thapsigargin, while heparin affectively inhibited IP3 binding. GSSG and thimerosal increased the affinity of [3H]IP3 binding from IC50 approximately 80 nM to IC50 approximately 40 nM; this effect was reversed by DTT. Binding in zero Ca2+ was decreased by low concentrations of Ca2+ (350 nM). These results suggest that actions of IP3 in the retina are regulated by physiological changes in intracellular pH and Ca2+ concentrations, as well as by the oxidation state of the receptor. Additionally, the presence of IP3 receptors in Müller glia opens the possibility of IP3 participation in nonsynaptic signalling through Ca2+ waves in glial cells.

Gi proteins and the response to 5-hydroxytryptamine in porcine cultured endothelial cells with impaired release of EDRF

1. The receptor-mediated release of endothelium-derived relaxing factor(s) (EDRF) requires the presence of different functional G proteins in endothelial cells. Release of EDRF in response to 5-hydroxytryptamine (5-HT), which involves activation of pertussis toxin-sensitive Gi proteins, is impaired in both regenerated endothelium of the coronary artery following balloon catheterization and in porcine cultured endothelial cells. This study used porcine cultured endothelial cells as a model of regenerated endothelium to determine if the abnormal release of EDRF in response to 5-HT may be associated with the loss of functional pertussis toxin-sensitive Gi proteins. 2. Binding studies on porcine cultured endothelial cells demonstrated specific binding sites for [3H]-5-HT. Scatchard analyses revealed a single binding site for [3H]-5-HT with Kd of 7.2 +/- 3.5 nM and maximal binding (Bmax) of 121.4 +/- 51.3 fmol mg-1 protein. Binding of [3H]-5-HT was displaced by methiothepin (5-HT1 and 5-HT2 antagonist; Ki = 6.2 +/- 1.2 nM), but not by ketanserin (preferential 5-HT2 antagonist). 3. Gi alpha 1 protein was expressed in cultured but not in native endothelial cells. Gi alpha 2 and Gi alpha 3 proteins were expressed to significant levels in porcine native and cultured endothelial cells, as detected by Northern and Western blot analysis. 4. In membranes from cultured endothelial cells, two bands of 40 and 41 kDa, which corresponded to the Gi alpha 2 and the combination of Gi alpha 3-Gi alpha 1 proteins, respectively, were ADP-ribosylated by pertussis toxin. The labelling intensity was Gi alpha 2>Gi alpha 3-Gi alpha l and the amount of ADP-ribosylation was not different between porcine native and cultured endothelial cells. Stimulation of the cultured cells with 5-HT (3 x 10-6 M; 4 min) decreased significantly further ADP-ribosylation of Gi alpha 2 by pertussis toxin, but not that of Gi alpha 3 and/or Gi alpha l.5. The present results suggest that porcine endothelial cell culture may lead to the abnormal expression of Gi alpha l protein and that the dysfunctional release of EDRF from cultured porcine endothelial cells in response to 5-HT is not associated with the loss of Gi alpha proteins or the absence of 5-HT binding sites.

Two types of mitochondria are evidenced by protein kinase C immunoreactivity in the Müller cells of the carp retina

The localization of protein kinase C (PKC) was studied immunocytochemically in the Müller cells of the carp retina. Electron microscope immunocytochemistry (using a monoclonal antibody to the alpha and beta isoenzymes of PKC) showed PKC-immunoreactivity mainly inside some mitochondria, especially along the mitochondrial cristae whereas other mitochondria in the same Müller cells showed no staining. Despite a detailed analysis we did not find any significant morphological difference between labeled and unlabeled mitochondria. These results demonstrate, for the first time, the presence of PKC immunoreactivity inside mitochondria and suggest that individual mitochondria may differ in signal transduction pathway.