Adenosine A(2a) receptor-mediated inhibition of rod opsin mRNA expression in tiger salamander

Circadian clock organization in the retina: From clock components to rod and cone pathways and visual function

Circadian (24-h) clocks are cell-autonomous biological oscillators that orchestrate many aspects of our physiology on a daily basis. Numerous circadian rhythms in mammalian and non-mammalian retinas have been observed and the presence of an endogenous circadian clock has been demonstrated. However, how the clock and associated rhythms assemble into pathways that support and control retina function remains largely unknown. Our goal here is to review the current status of our knowledge and evaluate recent advances. We describe many previously-observed retinal rhythms, including circadian rhythms of morphology, biochemistry, physiology, and gene expression. We evaluate evidence concerning the location and molecular machinery of the retinal circadian clock, as well as consider findings that suggest the presence of multiple clocks. Our primary focus though is to describe in depth circadian rhythms in the light responses of retinal neurons with an emphasis on clock control of rod and cone pathways. We examine evidence that specific biochemical mechanisms produce these daily light response changes. We also discuss evidence for the presence of multiple circadian retinal pathways involving rhythms in neurotransmitter activity, transmitter receptors, metabolism, and pH. We focus on distinct actions of two dopamine receptor systems in the outer retina, a dopamine D4 receptor system that mediates circadian control of rod/cone gap junction coupling and a dopamine D1 receptor system that mediates non-circadian, light/dark adaptive regulation of gap junction coupling between horizontal cells. Finally, we evaluate the role of circadian rhythmicity in retinal degeneration and suggest future directions for the field of retinal circadian biology.

The adenosine A2A receptor antagonist KW6002 distinctly regulates retinal ganglion cell morphology during postnatal development and neonatal inflammation

Adenosine A_2A receptors (A_2ARs) appear early in the retina during postnatal development, but the roles of the A_2ARs in the morphogenesis of distinct types of retinal ganglion cells (RGCs) during postnatal development and neonatal inflammatory response remain undetermined. As the RGCs are rather heterogeneous in morphology and functions in the retina, here we resorted to the Thy1-YFPH transgenic mice and three-dimensional (3D) neuron reconstruction to investigate how A_2ARs regulate the morphogenesis of three morphologically distinct types of RGCs (namely Type I, II, III) during postnatal development and neonatal inflammation. We found that the A_2AR antagonist KW6002 did not change the proportion of the three RGC types during retinal development, but exerted a bidirectional effect on dendritic complexity of Type I and III RGCs and cell type-specifically altered their morphologies with decreased dendrite density of Type I, decreased the dendritic field area of Type II and III, increased dendrite density of Type III RGCs. Moreover, under neonatal inflammation condition, KW6002 specifically increased the proportion of Type I RGCs with enhanced the dendrite surface area and volume and the proportion of Type II RGCs with enlarged the soma area and perimeter. Thus, A_2ARs exert distinct control of RGC morphologies to cell type-specifically fine-tune the RGC dendrites during normal development but to mainly suppress RGC soma and dendrite volume under neonatal inflammation.

A Circadian Clock in the Retina Regulates Rod-Cone Gap Junction Coupling and Neuronal Light Responses via Activation of Adenosine A2A Receptors

Adenosine, a major neuromodulator in the central nervous system (CNS), is involved in a variety of regulatory functions such as the sleep/wake cycle. Because exogenous adenosine displays dark- and night-mimicking effects in the vertebrate retina, we tested the hypothesis that a circadian (24 h) clock in the retina uses adenosine to control neuronal light responses and information processing. Using a variety of techniques in the intact goldfish retina including measurements of adenosine overflow and content, tracer labeling, and electrical recording of the light responses of cone photoreceptor cells and cone horizontal cells (cHCs), which are post-synaptic to cones, we demonstrate that a circadian clock in the retina itself-but not activation of melatonin or dopamine receptors-controls extracellular and intracellular adenosine levels so that they are highest during the subjective night. Moreover, the results show that the clock increases extracellular adenosine at night by enhancing adenosine content so that inward adenosine transport ceases. Also, we report that circadian clock control of endogenous cone adenosine A_2A receptor activation increases rod-cone gap junction coupling and rod input to cones and cHCs at night. These results demonstrate that adenosine and A_2A receptor activity are controlled by a circadian clock in the retina, and are used by the clock to modulate rod-cone electrical synapses and the sensitivity of cones and cHCs to very dim light stimuli. Moreover, the adenosine system represents a separate circadian-controlled pathway in the retina that is independent of the melatonin/dopamine pathway but which nevertheless acts in concert to enhance the day/night difference in rod-cone coupling.

Adenosine receptor expression in the adult zebrafish retina

Adenosine is an endogenous nucleoside in the central nervous system that acts on adenosine receptors. These are G protein-coupled receptors that have four known subtypes: A1, A2A, A2B, and A3 receptors. In the present study, we aimed to map the location of the adenosine receptor subtypes in adult wild-type zebrafish retina using in situ hybridization and immunohistochemistry. A1R, A2AR, and A2BR mRNA were detected in the ganglion cell layer (GCL), the inner nuclear layer (INL), the outer nuclear layer (ONL), and the outer segment (OS). A3R mRNA was detected in the GCL, ONL, and OS. A1R-immunoreactivity was expressed as puncta in the INL and in the outer plexiform layer (OPL). A1Rs were located within the cone pedicle and contiguous to horizontal cell tips in the OPL. A2AR-immunoreactivity was expressed as puncta in the GCL, inner plexiform layer (IPL), INL, and outer retina. A2AR puncta in the outer retina were situated around the ellipsoids and nuclei of cones, and weakly around the rod nuclei. A1Rs and A2ARs were clustered around ON cone bipolar cell terminals and present in the OFF lamina of the INL but were not expressed on mixed rod/cone response bipolar cell terminals. A2BR-immunoreactivity was mainly localized to the Müller cells, while A3Rs were found to be expressed in retinal ganglion cells of the GCL, INL, ONL, and OS. In summary, all four adenosine receptor subtypes were localized in the zebrafish retina and are in agreement with expression patterns shown in retinas from other species.

Adenosine receptor distribution in Rhesus monkey ocular tissue

Adenosine receptor (ADOR) antagonists, such as 7-methylxanthine (7-MX), have been shown to slow myopia progression in humans and animal models. Adenosine receptors are found throughout the body, and regulate the release of neurotransmitters such as dopamine and glutamate. However, the role of adenosine in eye growth is unclear. Evidence suggests that 7-MX increases scleral collagen fibril diameter, hence preventing axial elongation. This study used immunohistochemistry (IHC) and reverse-transcription quantitative polymerase chain reaction (RT-qPCR) to examine the distribution of the four ADORs in the normal monkey eye to help elucidate potential mechanisms of action. Eyes were enucleated from six Rhesus monkeys. Anterior segments and eyecups were separated into components and flash-frozen for RNA extraction or fixed in 4% paraformaldehyde and processed for immunohistochemistry against ADORA1, ADORA2a, ADORA2b, and ADORA3. RNA was reverse-transcribed, and qPCR was performed using custom primers. Relative gene expression was calculated using the ΔΔCt method normalizing to liver expression, and statistical analysis was performed using Relative Expression Software Tool. ADORA1 immunostaining was highest in the iris sphincter muscle, trabecular meshwork, ciliary epithelium, and retinal nerve fiber layer. ADORA2a immunostaining was highest in the corneal epithelium, trabecular meshwork, ciliary epithelium, retinal nerve fiber layer, and scleral fibroblasts. ADORA2b immunostaining was highest in corneal basal epithelium, limbal stem cells, iris sphincter, ciliary muscle, ciliary epithelium, choroid, isolated retinal ganglion cells and scattered scleral fibroblasts. ADORA3 immunostaining was highest in the iris sphincter, ciliary muscle, ciliary epithelium, choroid, isolated retinal ganglion cells, and scleral fibroblasts. Compared to liver mRNA, ADORA1 mRNA was significantly higher in the brain, retina and choroid, and significantly lower in the iris/ciliary body. ADORA2a expression was higher in brain and retina, ADORA2b expression was higher in retina, and ADORA3 was higher in the choroid. In conclusion, immunohistochemistry and RT-qPCR indicated differential patterns of expression of the four adenosine receptors in the ocular tissues of the normal non-human primate. The presence of ADORs in scleral fibroblasts and the choroid may support mechanisms by which ADOR antagonists prevent myopia. The potential effects of ADOR inhibition on both anterior and posterior ocular structures warrant investigation.

Caffeine exposure alters adenosine system and neurochemical markers during retinal development

Evidence points to beneficial properties of caffeine in the adult central nervous system, but teratogenic effects have also been reported. Caffeine exerts most of its effects by antagonizing adenosine receptors, especially A1 and A2A subtypes. In this study, we evaluated the role of caffeine on the expression of components of the adenosinergic system in the developing avian retina and the impact of caffeine exposure upon specific markers for classical neurotransmitter systems. Caffeine exposure (5-30 mg/kg by in ovo injection) to 14-day-old chick embryos increased the expression of A1 receptors and concomitantly decreased A2A adenosine receptors expression after 48 h. Accordingly, caffeine (30 mg/kg) increased [(3) H]-8-cyclopentyl-1,3-dipropylxanthine (A1 antagonist) binding and reduced [(3) H]-ZM241385 (A2A antagonist) binding. The caffeine time-response curve demonstrated a reduction in A1 receptors 6 h after injection, but an increase after 18 and 24 h. In contrast, caffeine exposure increased the expression of A2A receptors from 18 and 24 h. Kinetic assays of [(3) H]-S-(4-nitrobenzyl)-6-thioinosine binding to the equilibrative adenosine transporter ENT1 revealed an increase in Bmax with no changes in Kd , an effect accompanied by an increase in adenosine uptake. Immunohistochemical analysis showed a decrease in retinal content of tyrosine hydroxylase, calbindin and choline acetyltransferase, but not Brn3a, after 48 h of caffeine injection. Furthermore, retinas exposed to caffeine had increased levels of phosphorylated extracellular signal-regulated kinase and cAMP-response element binding protein. Overall, we show an in vivo regulation of the adenosine system, extracellular signal-regulated kinase and cAMP-response element binding protein function and protein expression of specific neurotransmitter systems by caffeine in the developing retina. The beneficial or maleficent effects of caffeine have been demonstrated by the work of different studies. It is known that during animal development, caffeine can exert harmful effects, impairing the correct formation of CNS structures. In this study, we demonstrated cellular and tissue effects of caffeine's administration on developing chick embryo retinas. Those effects include modulation of adenosine receptors (A1 , A2 ) content, increasing in cAMP response element-binding protein (pCREB) and extracellular signal-regulated kinase phosphorylation (pERK), augment of adenosine equilibrative transporter content/activity, and a reduction of some specific cell subpopulations. ENT1, Equilibrative nucleoside transporter 1.

Adenosine transporters and receptors: key elements for retinal function and neuroprotection

Adenosine is an important neuroactive substance in the central nervous system, including in the retina where subclasses of adenosine receptors and transporters are expressed since early stages of development. Here, we review some evidence showing that adenosine plays important functions in the mature as well as in the developing tissue. Adenosine transporters are divided into equilibrative and concentrative, and the major transporter subtype present in the retina is the ENT1. This transporter is responsible for a bidirectional transport of adenosine and the uptake or release of this nucleoside appears to be regulated by different signaling pathways that are also controlled by activation of adenosine receptors. Adenosine receptors are also key players in retina physiology regulating a variety of functions in the mature and developing tissue. Regulation of excitatory neurotransmitter release and neuroprotection are the main functions played be adenosine in the mature tissue, while regulation of cell survival and neurogenesis are some of the functions played by adenosine in developing retina. Since adenosine is neuroprotective against excitotoxic and metabolic dysfunctions observed in neurological and ocular diseases, the search for adenosine-related drugs regulating adenosine transporters and receptors can be important for advancement of therapeutic strategies against these diseases.

Mislocalized opsin and cAMP signaling: a mechanism for sprouting by rod cells in retinal degeneration

PURPOSE

In human retinal degeneration, rod photoreceptors reactively sprout neurites. The mechanism is unknown in part because of the paucity of animal models displaying this feature of human pathology. We tested the role of cAMP and opsin in sprouting by tiger salamander rod cells, photoreceptors that can produce reactive growth.

METHODS

In vitro systems of isolated photoreceptor cells and intact neural retina were used. cAMP signaling was manipulated with nucleotide analogues, enzyme stimulators, agonists for adenosine and dopamine receptors, and the opsin agonist, β-ionone. Levels of cAMP were determined by radioimmunoassay, and protein levels by Western blot and quantitative immunocytochemistry. Neuritic growth was assayed by image analysis and conventional and confocal microscopy.

RESULTS

cAMP analogues and stimulation of adenylyl cyclase (AC) directly or through G-protein-coupled receptors resulted in significant increases in neuritic growth of isolated rod, but not cone, cells. The signaling pathway included protein kinase A (PKA) and phosphorylation of the transcription factor cAMP response element-binding protein (pCREB). Opsin, a G-linked receptor, is present throughout the plasmalemma of isolated cells; its activation also induced sprouting. In neural retina, rod sprouting was significantly increased by β-ionone with concomitant increases in cAMP, pCREB, and synaptic proteins. Notably, opsin stimulated sprouting only when mislocalized to the plasmalemma of the rod cell body.

CONCLUSIONS

cAMP causes neuritic sprouting in rod, but not cone, cells through the AC-PKA-CREB pathway known to be associated with synaptic plasticity. We propose that in retinal disease, mislocalized rod opsin gains access to cAMP signaling, which leads to neuritic sprouting.

Expression of A1 adenosine receptors in the developing avian retina: in vivo modulation by A(2A) receptors and endogenous adenosine

Little is known about the mechanisms that regulate the expression of adenosine receptors during CNS development. We demonstrate here that retinas from chick embryos injected in ovo with selective adenosine receptor ligands show changes in A1 receptor expression after 48 h. Exposure to A1 agonist N⁶-cyclohexyladenosine (CHA) or antagonist 8-Cyclopentyl-1, 3-dipropylxanthine (DPCPX) reduced or increased, respectively, A1 receptor protein and [³H]DPCPX binding, but together, CHA+DPCPX had no effect. Interestingly, treatment with A(2A) agonist 3-[4-[2-[[6-amino-9-[(2R,3R,4S,5S)-5-(ethylcarbamoyl)-3,4-dihydroxy-oxolan-2-yl]purin-2-yl]amino] ethyl]phenyl] propanoic acid (CGS21680) increased A1 receptor protein and [³H]DPCPX binding, and reduced A(2A) receptors. The A(2A) antagonists 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-trizolo[1,5-c] pyrimidine (SCH58261) and 4-(2-[7-amino-2-[2-furyl][1,2,4]triazolo[2,3-a][1,3,5]triazo-5-yl-amino]ethyl)phenol (ZM241385) had opposite effects on A1 receptor expression. Exposure to CGS21680 + CHA did not change A1 receptor levels, whereas CHA + ZM241385 or CGS21680 + DPCPX had no synergic effect. The blockade of adenosine transporter with S-(4-nitrobenzyl)-6-thioinosine (NBMPR) also reduced [³H]DPCPX binding, an effect blocked by DPCPX, but not enhanced by ZM241385. [³H]DPCPX binding kinetics showed that treatment with CHA reduced and CGS21680 increased the Bmax, but did not affect Kd values. CHA, DPCPX, CGS21680, and ZM241385 had no effect on A1 receptor mRNA. These data demonstrated an in vivo regulation of A1 receptor expression by endogenous adenosine or long-term treatment with A1 and A(2A) receptors modulators.

Neuronal regulation of photo-induced pineal photoreceptor proteins in carp Catla catla

In the present in vitro study on the pineal in carp Catla catla, specific agonist and antagonists of receptors for different neuronal signals and regulators of intra-cellular Ca(++) and cAMP were used to gather basic information on the neuronal signal transduction cascade mechanisms in the photo-induced expression of rod-like opsin and alpha-transducin-like proteins in any fish pineal. Western-blot analysis followed by quantitative analysis of respective immunoblot data for both the proteins revealed that photo-induced expression of each protein was stimulated by cholinergic (both nicotinic and muscarinic) agonists and a dopaminergic antagonist, inhibited by both cholinergic antagonists and a dopaminergic agonist, but not affected by any agonists or antagonists of adrenergic (alpha(1), alpha(2) and beta(1)) receptors. Moreover, expression of each protein was stimulated by voltage gated L type calcium channel blocker, adenylate cyclase inhibitor and phosphodiesterase activator; but suppressed by the activators of both calcium channel and adenylate cyclase, and by phosphodiesterase inhibitor. Collectively, we report for the first time that both cholinergic and dopaminergic signals play an important, though antagonistic, role in the photo-induced expression of photoreceptor proteins in the fish pineal through activation of a signal transduction pathway in which both calcium and cAMP may act as the intracellular messengers.

Cyclic AMP-dependent activation of rhodopsin gene transcription in cultured retinal precursor cells of chicken embryo

The present study describes a robust 50-fold increase in rhodopsin gene transcription by cAMP in cultured retinal precursor cells of chicken embryo. Retinal cells isolated at embryonic day 8 (E8) and cultured for 3 days in serum-supplemented medium differentiated mostly into red-sensitive cones and to a lesser degree into green-sensitive cones, as indicated by real-time RT-PCR quantification of each specific opsin mRNA. In contrast, both rhodopsin mRNA concentration and rhodopsin gene promoter activity required the presence of cAMP-increasing agents [forskolin and 3-isobutyl-1-methylxanthine (IBMX)] to reach significant levels. This response was rod-specific and was sufficient to activate rhodopsin gene transcription in serum-free medium. The increase in rhodopsin mRNA levels evoked by a series of cAMP analogs suggested the response was mediated by protein kinase A, not by EPAC. Membrane depolarization by high KCl concentration also increased rhodopsin mRNA levels and this response was strongly potentiated by IBMX. The rhodopsin gene response to cAMP-increasing agents was developmentally gated between E6 and E7. Rod-specific transducin alpha subunit mRNA levels also increased up to 50-fold in response to forskolin and IBMX, while rod-specific phosphodiesterase-VI and rod arrestin transcripts increased 3- to 10-fold. These results suggest a cAMP-mediated signaling pathway may play a role in rod differentiation.

Purinergic signalling in the subretinal space: a role in the communication between the retina and the RPE

The retinal pigment epithelium (RPE) is separated from the photoreceptor outer segments by the subretinal space. While the actual volume of this space is minimal, the communication that occurs across this microenvironment is important to the visual process, and accumulating evidence suggests the purines ATP and adenosine contribute to this communication. P1 and P2 receptors are localized to membranes on both the photoreceptor outer segments and on the apical membrane of the RPE which border subretinal space. ATP is released across the apical membrane of the RPE into this space in response to various triggers including glutamate and chemical ischemia. This ATP is dephosphorylated into adenosine by a series of ectoenzymes on the RPE apical membrane. Regulation of release and ectoenzyme activity in response to light-sensitive signals can alter the balance of purines in subretinal space, and thus coordinate communication across subretinal space with the visual process.