Association of the AMPA receptor-related postsynaptic density proteins GRIP and ABP with subsets of glutamate-sensitive neurons in the rat retina

The biophysical, molecular, and anatomical landscape of pigeon CRY4: A candidate light-based quantal magnetosensor

The biophysical and molecular mechanisms that enable animals to detect magnetic fields are unknown. It has been proposed that birds have a light-dependent magnetic compass that relies on the formation of radical pairs within cryptochrome molecules. Using spectroscopic methods, we show that pigeon cryptochrome clCRY4 is photoreduced efficiently and forms long-lived spin-correlated radical pairs via a tetrad of tryptophan residues. We report that clCRY4 is broadly and stably expressed within the retina but enriched at synapses in the outer plexiform layer in a repetitive manner. A proteomic survey for retinal-specific clCRY4 interactors identified molecules that are involved in receptor signaling, including glutamate receptor-interacting protein 2, which colocalizes with clCRY4. Our data support a model whereby clCRY4 acts as an ultraviolet-blue photoreceptor and/or a light-dependent magnetosensor by modulating glutamatergic synapses between horizontal cells and cones.

Somatic and neuritic spines on tyrosine hydroxylase-immunopositive cells of rat retina

Dopamine- and tyrosine hydroxylase-immunopositive cells (TH cells) modulate visually driven signals as they flow through retinal photoreceptor, bipolar, and ganglion cells. Previous studies suggested that TH cells release dopamine from varicose axons arborizing in the inner and outer plexiform layers after glutamatergic synapses depolarize TH cell dendrites in the inner plexiform layer and these depolarizations propagate to the varicosities. Although it has been proposed that these excitatory synapses are formed onto appendages resembling dendritic spines, spines have not been found on TH cells of most species examined to date or on TH cell somata that release dopamine when exposed to glutamate receptor agonists. By use of protocols that preserve proximal retinal neuron morphology, we have examined the shape, distribution, and synapse-related immunoreactivity of adult rat TH cells. We report here that TH cell somata, tapering and varicose inner plexiform layer neurites, and varicose outer plexiform layer neurites all bear spines, that some of these spines are immunopositive for glutamate receptor and postsynaptic density proteins (viz., GluR1, GluR4, NR1, PSD-95, and PSD-93), that TH cell somata and tapering neurites are also immunopositive for a γ-aminobutyric acid (GABA) receptor subunit (GABA_A R_α1 ), and that a synaptic ribbon-specific protein (RIBEYE) is found adjacent to some colocalizations of GluR1 and TH in the inner plexiform layer. These results identify previously undescribed sites at which glutamatergic and GABAergic inputs may stimulate and inhibit dopamine release, especially at somata and along varicose neurites that emerge from these somata and arborize in various levels of the retina. J. Comp. Neurol. 525:1707-1730, 2017. © 2016 Wiley Periodicals, Inc.

Rapid glutamate receptor 2 trafficking during retinal degeneration

Background

Retinal degenerations, such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP), are characterized by photoreceptor loss and anomalous remodeling of the surviving retina that corrupts visual processing and poses a barrier to late-stage therapeutic interventions in particular. However, the molecular events associated with retinal remodeling remain largely unknown. Given our prior evidence of ionotropic glutamate receptor (iGluR) reprogramming in retinal degenerations, we hypothesized that the edited glutamate receptor 2 (GluR2) subunit and its trafficking may be modulated in retinal degenerations.

Results

Adult albino Balb/C mice were exposed to intense light for 24 h to induce light-induced retinal degeneration (LIRD). We found that prior to the onset of photoreceptor loss, protein levels of GluR2 and related trafficking proteins, including glutamate receptor-interacting protein 1 (GRIP1) and postsynaptic density protein 95 (PSD-95), were rapidly increased. LIRD triggered neuritogenesis in photoreceptor survival regions, where GluR2 and its trafficking proteins were expressed in the anomalous dendrites. Immunoprecipitation analysis showed interaction between KIF3A and GRIP1 as well as PSD-95, suggesting that KIF3A may mediate transport of GluR2 and its trafficking proteins to the novel dendrites. However, in areas of photoreceptor loss, GluR2 along with its trafficking proteins nearly vanished in retracted retinal neurites.

Conclusions

All together, LIRD rapidly triggers GluR2 plasticity, which is a potential mechanism behind functionally phenotypic revisions of retinal neurons and neuritogenesis during retinal degenerations.

Changes in neuronal response to ischemia in retinas with genetic alterations of somatostatin receptor expression

Ischemia is a primary cause of neuronal death in retinal diseases. The repertoire of expressed transmitter receptors would determine the neurons' responses to ischemic damage, and peptidergic receptors may be involved. With a new in vitro model of the ischemic mouse retina, we investigated whether an altered expression of somatostatin receptors could modulate retinal responses to ischemia. We used retinas of somatostatin receptor 1 (sst(1)) knock out (KO) mice, where sst(2) are over-expressed and over-functional, and of sst(2) KO mice. TUNEL analysis of ischemic retinas showed a marked reduction of cell death in sst(1) KO retinas, while there were no differences between wild-type (WT) and sst(2) KO retinas. In addition, caspase-3 mRNA expression was also reduced in sst(1) KO as compared to WT retinas. An immunohistochemical analysis demonstrated that different cell populations responded differently to the ischemic insult, and that the persistence of some immunohistochemical markers was greater in sst(1) KO than in WT or in sst(2) KO retinas. In particular, rod bipolar cell survival was markedly improved in sst(1) KO retinas, while it was dramatically decreased in sst(2) KO retinas. Furthermore, consistent with a role of glutamate excitotoxicity in ischemia-induced neuronal death, retinal glutamate release was observed to increase under ischemic conditions, but this increase was significantly reduced in sst(1) KO retinas. These observations demonstrate that an increased presence of functional sst(2) protects against retinal ischemia, thus implementing the background for the use of sst(2) analogs in therapies of retinal diseases such as glaucoma or diabetic retinopathy.

State-dependent AMPA receptor trafficking in the mammalian retina

The rapid cycling of AMPA receptors (AMPARs) at the membrane maintains synaptic transmission at a number of CNS synapses and may play a role in several forms of synaptic plasticity. It is unclear, however, how prevalent the trafficking of AMPARs is in the CNS, particularly at synapses not known to exhibit activity-dependent plasticity. Because trafficking is regulated by basal synaptic activity, a question also remains as to how receptor trafficking is modulated at synapses subject to different patterns of synaptic activation. We have investigated whether trafficking of AMPARs occurs in retinal neurons, which are subject to tonic glutamate release. We find two distinct states of AMPAR trafficking in ON ganglion cells. Light adaptation serves to stabilize AMPARs in a noncycling mode. However, dark adaptation for as little as 8 h triggers a switch to a second state of trafficking characterized by rapid cycling. We provide evidence that the activation of AMPARs is critical for switching between cycling and noncycling states. The induction of cycling further appears to be modulated by changes in the function of glutamate receptor 2/3-interacting proteins. Our results suggest that there is a strong link between synaptic activity and AMPAR trafficking in retinal neurons. These results further suggest the existence of a previously unknown form of activity-dependent plasticity in the retina that may be regulated in the course of a normal light/dark cycle.

Membrane localization of membrane type 5 matrix metalloproteinase by AMPA receptor binding protein and cleavage of cadherins

Matrix metalloproteinases (MMPs) have been proposed to remodel the extracellular environment of neurons. Here, we report that the metalloproteinase membrane-type 5 MMP (MT5-MMP) binds to AMPA receptor binding protein (ABP) and GRIP (glutamate receptor interaction protein), two related postsynaptic density (PSD) PDZ (postsynaptic density-95/Discs large/zona occludens-1) domain proteins that target AMPA receptors to synapses. The MT5-MMP C terminus binds ABP PDZ5 and the two proteins coimmunoprecipitated and colocalized in heterologous cells and neurons. MT5-MMP localized in filopodia at the tips of growth cones in young [2-5 d in vitro (DIV)] cultured embryonic hippocampal neurons, and at synapses in mature (21 DIV) neurons. Its enrichment in synaptosomes also indicated a synaptic localization in the mature brain. Deletion of the PDZ binding site impaired membrane trafficking of MT5-MMP, whereas exogenous ABP splice forms that are associated either with the plasma membrane or with the cytosol, respectively, colocalized with MT5-MMP in synaptic spines or recruited MT5-MMP to intracellular compartments. We show that endogenous MT5-MMP is found in cultured neurons and brain lysates in a proenzyme form that is activated by furin and degraded by auto-proteolysis. We also identify cadherins as MT5-MMP substrates. These results suggest that ABP directs MT5-MMP proteolytic activity to growth cones and synaptic sites in neurons, where it may regulate axon pathfinding or synapse remodeling through proteolysis of cadherins or other ECM or cell adhesion molecules.

Somatostatin modulates dopamine release in rat retina

The aim of the present study was to determine the possible role of somatostatin as a modulator of dopamine release in rat retina. Basal release of dopamine, and how this is influenced by somatostatin receptor (sst) selective ligands, was examined ex vivo in rat retinal explants. Dopamine levels were quantified by high-pressure liquid chromatography (HPLC) with electrochemical detection. Basal levels of dopamine were measured over 120 min of tissue incubation and found to be 1.17+/-0.35 ng/ml. Somatostatin (10(-6), 10(-5), 10(-4)M) increased dopamine levels in a concentration-dependent manner, while the sst(2) antagonist CYN154806 (10(-4)M) reversed its actions. BIM23014 (sst(2) agonist) increased dopamine levels in a statistically significant manner only at the concentration of 10(-5)M. The sst(1) agonist L797.591 (10(-5), 10(-4)M) also increased dopamine levels, while activation of the sst(3) receptor (sst(3) agonist, L796.778, 10(-4)M) had no effect. These data substantiate a neuromodulatory role for sst(1) and sst(2) somatostatin receptors in the retina and show for the first time somatostatin's influence on dopamine release.

The molecular pharmacology and cell biology of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) are of fundamental importance in the brain. They are responsible for the majority of fast excitatory synaptic transmission, and their overactivation is potently excitotoxic. Recent findings have implicated AMPARs in synapse formation and stabilization, and regulation of functional AMPARs is the principal mechanism underlying synaptic plasticity. Changes in AMPAR activity have been described in the pathology of numerous diseases, such as Alzheimer's disease, stroke, and epilepsy. Unsurprisingly, the developmental and activity-dependent changes in the functional synaptic expression of these receptors are under tight cellular regulation. The molecular and cellular mechanisms that control the postsynaptic insertion, arrangement, and lifetime of surface-expressed AMPARs are the subject of intense and widespread investigation. For example, there has been an explosion of information about proteins that interact with AMPAR subunits, and these interactors are beginning to provide real insight into the molecular and cellular mechanisms underlying the cell biology of AMPARs. As a result, there has been considerable progress in this field, and the aim of this review is to provide an account of the current state of knowledge.

The postsynaptic scaffold proteins ProSAP1/Shank2 and Homer1 are associated with glutamate receptor complexes at rat retinal synapses

The postsynaptic density (PSD) at glutamatergic synapses is a macromolecular complex of various molecules that organize the different glutamate receptors spatially and link them to their appropriate downstream signaling pathways and to the cytoskeleton. Recently, a new family of multidomain proteins called Shanks or ProSAPs (proline-rich synapse-associated proteins) has been identified. They are suggested to be central adaptor proteins of the PSD of glutamatergic synapses, bridging different types of glutamate receptor complexes. With immunocytochemistry and light and electron microscopy, we examined the cellular, synaptic, and postnatal developmental expression of ProSAP1/Shank2 at the synapses of rat retina. With double-labeling experiments and confocal microscopy, we analyzed the association of ProSAP1/Shank2 with proteins specific for glutamatergic, glycinergic, and gamma-aminobutyric acid (GABA)ergic synapses and with proteins known to be involved in the structural and functional organization of PSDs containing N-methyl-D-aspartate receptors [95-kDa postsynaptic density protein (PSD-95)], group I metabotropic glutamate receptors (Homer1), and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors [glutamate receptor-interacting protein (GRIP)]. ProSAP1/Shank2 was present postsynaptically at the glutamatergic ribbon synapses of photoreceptor and bipolar cells, and it was absent from glycinergic and GABAergic amacrine cell synapses. The double-labeling experiments revealed a high rate of colocalization of ProSAP1/Shank2 with Homer1 and PSD-95, and little colocalization with GRIP. These data suggest that ProSAP1/Shank2 acts as an organizer at PSDs of different glutamatergic retinal synapses.

AMPA receptors mediate acetylcholine release from starburst amacrine cells in the rabbit retina

The light response of starburst amacrine cells is initiated by glutamate released from bipolar cells. To identify the receptors that mediate this response, we used a combination of anatomical and physiological techniques. An in vivo, rabbit eyecup was preloaded with [(3)H]-choline, and the [(3)H]-acetylcholine (ACh) released into the superfusate was monitored. A photopic, 3 Hz flashing light increased ACh release, and the selective AMPA receptor antagonist, GYKI 53655, blocked this light-evoked response. Nonselective AMPA/kainate agonists increased the release of ACh, but the specific kainate receptor agonist, SYM 2081, did not increase ACh release. Selective AMPA receptor antagonists, GYKI 53655 or GYKI 52466, also blocked the responses to agonists. We conclude that the predominant excitatory input to starburst amacrine cells is mediated by AMPA receptors. We also labeled lightly fixed rabbit retinas with antisera to choline acetyltransferase (ChAT), AMPA receptor subunits GluR1, GluR2/3, or GluR4, and kainate receptor subunits GluR6/7 and KA2. Labeled puncta were observed in the inner plexiform layer with each of these antisera to glutamate receptors, but only GluR2/3-IR puncta and GluR4-IR puncta were found on the ChAT-IR processes. The same was true of starburst cells injected intracellularly with Neurobiotin, and these AMPA receptor subunits were localized to two populations of puncta. The AMPA receptors are expected to desensitize rapidly, enhancing the sensitivity of starburst amacrine cells to moving or other rapidly changing stimuli.

Gene expression of AMPA-type glutamate receptor subunits in rod-type ON bipolar cells of rat retina

The retinal rod bipolar cell type is involved in the sign-inverting depolarizing ON-type response to light. This response is mediated by the metabotropic glutamate receptor type 6 (mGluR6) expressed on the rod bipolar dendrites. In a previous immunocytochemical study, an unexpected colocalization was reported [W. Kamphuis et al. (2003) J. Comp. Neurol., 455, 172-186] of mGluR6 with the ionotropic AMPA-type glutamate receptor subunit GluR2 in rod bipolar cells of rat retina. The aim of the present study was to investigate whether expression of both genes could be found at the single-cell level. Two approaches were followed. (i). Retinal cells were isolated by enzymatic and mechanical treatment. Single cells with a bipolar morphology were harvested, subjected to multiplex PCR with protein kinase C (PKC)-, mGluR6- and GluR1-4-specific primers, followed by a real-time quantitative PCR assay. Of 23 studied cells, 74% expressed PKC and 87% expressed mGluR6. Using the presence of both transcripts as the criterion for a rod bipolar cell signature (n = 15), 73% of these cells expressed GluR2, with a minor contribution of GluR1 (20%), GluR3 (7%), and GluR4 (20%). Quantification of the transcript levels demonstrated that mGluR6 and GluR2 genes are expressed at similar levels in rod ON-type bipolar cells. (ii). Rod bipolar cells were identified in retinal sections by immunolabelling with a protein kinase C antibody and isolated using laser pressure catapulting (LPC). Quantitative PCR was employed to assess gene expression levels of reference genes, PKCalpha, mGluR6 and the GluR subunits. However, in samples from PKCalpha-immunopositive somata no significant enrichment of PKCalpha transcript levels was observed when compared with control samples from immunonegative somata. We conclude that this approach lacks sufficient spatial specificity. In conclusion, the results show coexpression of mGluR6 and GluR2 in rod bipolar cells; this is in good agreement with the results of previous immunocytochemical studies. The functional implications of AMPA-type glutamate receptors for ON-type rod bipolar-mediated signal transduction remains to be elucidated.