The light response of ON bipolar neurons requires G[alpha]o

Metabotropic glutamate receptor 6 signaling enhances TRPM1 calcium channel function and increases melanin content in human melanocytes

Mutations in TRPM1, a calcium channel expressed in retinal bipolar cells and epidermal melanocytes, cause complete congenital stationary night blindness with no discernible skin phenotype. In the retina, TRPM1 activity is negatively coupled to metabotropic glutamate receptor 6 (mGluR6) signaling through Gαo and TRPM1 mutations result in the loss of responsiveness of TRPM1 to mGluR6 signaling. Here, we show that human melanocytes express mGluR6, and treatment of melanocytes with L-AP4, a type III mGluR-selective agonist, enhances Ca(2+) uptake. Knockdown of TRPM1 or mGluR6 by shRNA abolished L-AP4-induced Ca(2+) influx and TRPM1 currents, showing that TRPM1 activity in melanocytes is positively coupled to mGluR6 signaling. Gαo protein is absent in melanocytes. However, forced expression of Gαo restored negative coupling of TRPM1 to mGluR6 signaling, but treatment with pertussis toxin, an inhibitor of Gi /Go proteins, did not affect basal or mGluR6-induced Ca(2+) uptake. Additionally, chronic stimulation of mGluR6 altered melanocyte morphology and increased melanin content. These data suggest differences in coupling of TRPM1 function to mGluR6 signaling explain different cellular responses to glutamate in the retina and the skin.

Kir2.4 surface expression and basal current are affected by heterotrimeric G-proteins

Kir2.4, a strongly rectifying potassium channel that is localized to neurons and is especially abundant in retina, was fished with yeast two-hybrid screen using a constitutively active Gαo1. Here, we wished to determine whether and how Gαo affects this channel. Using transfected HEK 293 cells and retinal tissue, we showed that Kir2.4 interacts with Gαo, and this interaction is stronger with the GDP-bound form of Gαo. Using two-electrode voltage clamp, we recorded from oocytes that were injected with Kir2.4 mRNA and a combination of G-protein subunit mRNAs. We found that the wild type and the inactive mutant of Gαo reduce the Kir2.4 basal current, whereas the active mutant has little effect. Other pertussis-sensitive Gα subunits also reduce this current, whereas Gαs increases it. Gβγ increases the current, whereas m-phosducin, which binds Gβγ without affecting the state of Gα, reduces it. We then tested the effect of G-protein subunits on the surface expression of the channel fused to cerulean by imaging the plasma membranes of the oocytes. We found that the surface expression is affected, with effects paralleling those seen with the basal current. This suggests that the observed effects on the current are mainly indirect and are due to surface expression. Similar results were obtained in transfected HEK cells. Moreover, we show that in retinal ON bipolar cells lacking Gβ3, localization of Kir2.4 in the dendritic tips is reduced. We conclude that Gβγ targets Kir2.4 to the plasma membrane, and Gαo slows this down by binding Gβγ.

Neuronal programmed cell death-1 ligand expression regulates retinal ganglion cell number in neonatal and adult mice

OBJECTIVES

During mouse retina maturation, the final number of retinal ganglion cells (RGCs) is determined by highly regulated programmed cell death. Previous studies demonstrated that the immunoregulatory receptor programmed cell death-1 (PD-1) promotes developmental RGC death. To identify the functional signaling partner(s) for PD-1, we identified retinal expression of PD-1 ligands and examined the effect of PD-1 ligand expression on RGC number. We also explored the hypothesis that PD-1 signaling promotes the development of functional visual circuitry.

METHODS

Characterization of retinal and brain programmed cell death-1 ligand 1 (PD-L1) expression were examined by immunofluorescence on tissue sections. The contribution of PD-ligands, PD-L1, and programmed cell death-1 ligand 2 (PD-L2) to RGC number was examined in PD-ligand knockout mice lacking 1 or both ligands. Retinal architecture was assessed by spectral-domain optical coherence tomography, and retinal function was analyzed by electroretinography in wild-type and PD-L1/L2 double-deficient mice.

RESULTS

PD-L1 expression is found throughout the neonatal retina and persists in adult RGCs, bipolar interneurons, and Müller glia. In the absence of both PD-ligands, there is a significant numerical increase in RGCs (34% at postnatal day 2 [P2] and 18% in adult), as compared to wild type, and PD-ligands have redundant function in this process. Despite the increased RGC number, adult PD-L1/L2 double-knockout mice have normal retinal architecture and outer retina function.

CONCLUSION

This study demonstrates that PD-L1 and PD-L2 together impact the final number of RGCs in adult mice and supports a novel role for active promotion of neuronal cell death through PD-1 receptor-ligand engagement.

Melatonin receptors are anatomically organized to modulate transmission specifically to cone pathways in the retina of Xenopus laevis

Melatonin receptors have been identified in several retinal cell types, including photoreceptors, horizontal cells, amacrine cells, and ganglion cells. Recent reports suggest that melatonin potentiates signaling from rods to inner retinal neurons. However, the organization of the melatonin receptors mediating this action in the outer plexiform layer (OPL) is not clear. To assess melatonin receptor localization in the OPL, double-label confocal immunohistochemistry for Mel1a or Mel1b melatonin receptors was performed in combination with markers for cone photoreceptors (calbindin, XAP-1) and ON bipolar cells (guanine nucleotide binding protein alpha, Goα) on the retina of Xenopus laevis. Both Mel1a and Mel1b receptors were specifically associated with processes contacting the pedicles of cones, but localized to processes from different sets of second-order neurons. Mel1a receptors localized to the large axonal processes of horizontal cells, while Mel1b receptors localized to the dendrites of OFF bipolar cells. Both receptors also localized to third-order amacrine and ganglion cells and their processes in the inner plexiform layer. This study indicates that Mel1a and Mel1b melatonin receptors are expressed specifically in the Xenopus OPL to modulate transmission from cones to horizontal cells and OFF bipolar cells, respectively; they are second-order neurons that predominantly contact ribbon synapses and display OFF responses to light. When combined with results from recent physiological studies, the current results suggest a conserved function for melatonin in enhancing transmission from rods to second-order neurons across species, although the precise mechanisms by which melatonin enhances this transmission are likely to vary in a species-dependent manner.

"mGlu Receptors in the Retina" - WIREs Membrane Transport and Signaling

Glutamate, a key neurotransmitter in the vertebrate retina, acts via ionotropic and metabotropic receptors. Retina expresses mRNA for all metabotropic glutamate receptors and proteins for all but mGluR3. Every retinal cell class expresses one or more of these receptors. In general, these receptors are present presynaptically and serve to modulate synaptic transmission. While mGluRs on the photoreceptor terminal act as autoreceptors to titer glutamate levels, those on horizontal cell processes seem to shape the light response. Similarly, autoreceptors on bipolar axon terminals modulate glutamate release and the receptors on amacrine and ganglion cells modulate feedforward signals by modulating K+ or Ca2+ current to fine tune light responses. Since most of the mGluR sub-types are present in amacrine and ganglion cells that belong to many cell types, the pathways downstream of mGluRs are highly diverse with primarily modulatory effects. An exception to most mGluRs which have modulatory function is mGluR6 because it plays a key role in the feedforward transmission from photoreceptors to ON bipolar cells and is also required for the correct localization of the synaptic proteins in the dendritic tips. In humans, mutations in the gene encoding mGluR6 cause autosomal recessive night blindness. In addition, mGluRs appear to play a trophic role in development and after retinal damage, suggesting potential future therapeutic implications.

G-protein-mediated inhibition of the Trp channel TRPM1 requires the Gβγ dimer

ON bipolar cells are critical for the function of the ON pathway in the visual system. They express a metabotropic glutamate receptor (mGluR6) that, when activated, couples to the G(o) class of G protein. The channel that is primarily responsible for the synaptic response has been recently identified as the transient receptor potential cation channel subfamily M member 1 (TRPM1); TRPM1 is negatively coupled to the mGluR6/Go cascade such that activation of the cascade results in closure of the channel. Light indirectly opens TRPM1 by reducing transmitter release from presynaptic photoreceptors, resulting in a decrease in mGluR6 activation. Conversely, in the dark, binding of synaptic glutamate to mGluR6 inhibits TRPM1 current. Closure of TRPM1 by G-protein activation in the dark is a critical step in the process of ON bipolar cell signal transduction, but the precise pathway linking these two events is not understood. To address this question, we measured TRPM1 activity in retinal bipolar cells, in human ependymal melanocytes (HEMs) that endogenously express TRPM1, and in HEK293 cells transfected with TRPM1. Dialysis of the Gβγ subunit dimer, but not Gα(o), closed TRPM1 channels in every cell type that we tested. In addition, activation of an endogenous G-protein-coupled receptor pathway in HEK293 cells that releases Gβγ without activating Go protein also closed TRPM1 channels. These results suggest a model in which the Gβγ dimer that is released as a result of the dissociation from Gα(o) upon activation of mGluR6 closes the TRPM1 channel, perhaps via a direct interaction.

Regulators of G protein signaling RGS7 and RGS11 determine the onset of the light response in ON bipolar neurons

The time course of signaling via heterotrimeric G proteins is controlled through their activation by G-protein coupled receptors and deactivation through the action of GTPase accelerating proteins (GAPs). Here we identify RGS7 and RGS11 as the key GAPs in the mGluR6 pathway of retinal rod ON bipolar cells that set the sensitivity and time course of light-evoked responses. We showed using electroretinography and single cell recordings that the elimination of RGS7 did not influence dark-adapted light-evoked responses, but the concurrent elimination of RGS11 severely reduced their magnitude and dramatically slowed the onset of the response. In RGS7/RGS11 double-knockout mice, light-evoked responses in rod ON bipolar cells were only observed during persistent activation of rod photoreceptors that saturate rods. These observations are consistent with persistently high G-protein activity in rod ON bipolar cell dendrites caused by the absence of the dominant GAP, biasing TRPM1 channels to the closed state.

Novel expression patterns of metabotropic glutamate receptor 6 in the zebrafish nervous system

The metabotropic glutamate receptor 6 (mGluR6 or GRM6) belongs to the class III of the metabotropic glutamate receptor family. It is the only known mGluR that mediates direct synaptic transmission in the nervous system and is thought to mediate the ON-response in the ON-pathway of the vertebrate retina. Phylogenetic and gene structure analysis indicated that the zebrafish genome harbours two mglur6 paralogs, mglur6a and mglur6b. Besides expression in the inner nuclear layer and distinct regions in the brain, both mglur6 paralogs are expressed in ganglion cells of the retina, an expression pattern which can also be observed in the downstream effector molecules gnaoa and gnaob. This unexpected expression pattern is consistent with immunohistological labeling using a peptide antibody specific for the mGluR6b paralog. These expression patterns contradict the existing view that mGluR6 is solely located on ON-bipolar cells where it functions in signal transmission. Consistent with expression in ON-bipolar cells, we report a decreased b-wave amplitude in the electroretinogram after morpholino-based downregulation of mGluR6b, showing a function in the ON response. Our data suggest more widespread functions of mGluR6 mediated signaling in the central nervous system, possibly including sign reversing synapses in the inner retina.

Retinoblastoma (Rb) regulates laminar dendritic arbor reorganization in retinal horizontal neurons

Neuronal differentiation with respect to the acquisition of synaptic competence needs to be regulated precisely during neurogenesis to ensure proper formation of circuits at the right place and time in development. This regulation is particularly important for synaptic triads among photoreceptors, horizontal cells (HCs), and bipolar cells in the retina, because HCs are among the first cell types produced during development, and bipolar cells are among the last. HCs undergo a dramatic transition from vertically oriented neurites that form columnar arbors to overlapping laminar dendritic arbors with differentiation. However, how this process is regulated and coordinated with differentiation of photoreceptors and bipolar cells remains unknown. Previous studies have suggested that the retinoblastoma (Rb) tumor suppressor gene may play a role in horizontal cell differentiation and synaptogenesis. By combining genetic mosaic analysis of individual synaptic triads with neuroanatomic analyses and multiphoton live imaging of developing HCs, we found that Rb plays a cell-autonomous role in the reorganization of horizontal cell neurites as they differentiate. Aberrant vertical processes in Rb-deficient HCs form ectopic synapses with rods in the outer nuclear layer but lack bipolar dendrites. Although previous reports indicate that photoreceptor abnormalities can trigger formation of ectopic synapses, our studies now demonstrate that defects in a postsynaptic partner contribute to the formation of ectopic photoreceptor synapses in the mammalian retina.

mGluR6 deletion renders the TRPM1 channel in retina inactive

In darkness, glutamate released from photoreceptors activates the metabotropic glutamate receptor 6 (mGluR6) on retinal ON bipolar cells. This activates the G protein G(o), which then closes transient receptor potential melastatin 1 (TRPM1) channels, leading to cells' hyperpolarization. It has been generally assumed that deleting mGluR6 would render the cascade inactive and the ON bipolar cells constitutively depolarized. Here we show that the rod bipolar cells in mGluR6-null mice were hyperpolarized. The slope conductance of the current-voltage curves and the current noise were smaller than in wild type. Furthermore, while in wild-type rod bipolar cells, TRPM1 could be activated by local application of capsaicin; in null cells, it did not. These results suggest that the TRPM1 channel in mGluR6-null rod bipolar cells is inactive. To explore the reason for this lack of activity, we tested if mGluR6 deletion affected expression of cascade components. Immunostaining for G protein subunit candidates Gα(o), Gβ(3), and Gγ(13) showed no significant changes in their expression or distribution. Immunostaining for TRPM1 in the dendritic tips was greatly reduced, but the channel was still present in the soma and primary dendrites of mGluR6-null bipolar cells, where a certain fraction of TRPM1 appears to localize to the plasma membrane. Consequently, the lack of TRPM1 activity in the null retina is unlikely to be due to failure of the channels to localize to the plasma membrane. We speculate that, to be constitutively active, TRPM1 channels in ON bipolar cells have to be in a complex, or perhaps require an unidentified factor.

Characterization of Trpm1 desensitization in ON bipolar cells and its role in downstream signalling

ON bipolar cells invert the sign of light responses from hyperpolarizing to depolarizing before passing them on to ganglion cells. Light responses are generated when a cation channel, recently identified as Trpm1, opens. The amplitude of the light response rapidly decays due to desensitization of Trpm1 current. The role of Trpm1 desensitization in shaping light responses both in bipolar and downstream ganglion cells has not been well characterized. Here we show that two parameters, the amount and the rate of recovery from desensitization, depend on the strength of the presynaptic stimulus. Stimuli that activate less than 20% of the maximum Trpm1 current did not promote any detectable desensitization, even for prolonged periods. Beyond this threshold there was a linear relationship between the amount of desensitization and the fractional Trpm1 current. In response to stimuli that open all available channels, desensitization reduced the response to approximately 40% of the peak, with a time constant of 1 s, and recovery was slow, with a time constant of more than 20 s. In dye-filled bipolar cells classified as transient or sustained using morphological criteria, there were no significant differences in Trpm1 desensitization parameters. Trpm1 activation evoked robust EPSCs in ganglion cells, and removal of Trpm1 desensitization strongly augmented a sustained component of the ganglion cell EPSC irrespective of whether ganglion cells were of the ON or ON/OFF type. We conclude that Trpm1 desensitization impacts the kinetics of ganglion cell EPSCs, but does not underlie the sustained/transient dichotomy of neurons in the ON pathway.

TRPM1: the endpoint of the mGluR6 signal transduction cascade in retinal ON-bipolar cells

For almost 30 years the ion channel that initiates the ON visual pathway in vertebrate vision has remained elusive. Recent findings now indicate that the pathway, which begins with unbinding of glutamate from the metabotropic glutamate receptor 6 (mGluR6), ends with the opening of the transient receptor potential (TRP)M1 cation channel. As a component of the mGluR6 signal transduction pathway, mutations in TRPM1 would be expected to cause congenital stationary night blindness (CSNB), and several such mutations have already been identified in CSNB families. Furthermore, expression of TRPM1 in both the retina and skin raises the possibility that a genetic link exists between certain types of visual and skin disorders.

Augmented glucose-induced insulin release in mice lacking G(o2), but not G(o1) or G(i) proteins

Insulin secretion by pancreatic β cells is a complex and highly regulated process. Disruption of this process can lead to diabetes mellitus. One of the various pathways involved in the regulation of insulin secretion is the activation of heterotrimeric G proteins. Bordetella pertussis toxin (PTX) promotes insulin secretion, suggesting the involvement of one or more of three G(i) and/or two G(o) proteins as suppressors of insulin secretion from β cells. However, neither the mechanism of this inhibitory modulation of insulin secretion nor the identity of the G(i/o) proteins involved has been elucidated. Here we show that one of the two splice variants of G(o), G(o2), is a key player in the control of glucose-induced insulin secretion by β cells. Mice lacking G(o2)α, but not those lacking α subunits of either G(o1) or any G(i) proteins, handle glucose loads more efficiently than wild-type (WT) mice, and do so by increased glucose-induced insulin secretion. We thus provide unique genetic evidence that the G(o2) protein is a transducer in an inhibitory pathway that prevents damaging oversecretion of insulin.

Allosteric modulation of metabotropic glutamate receptors

The development of receptor subtype-selective ligands by targeting allosteric sites of G protein-coupled receptors (GPCRs) has proven highly successful in recent years. One GPCR family that has greatly benefited from this approach is the metabotropic glutamate receptors (mGlus). These family C GPCRs participate in the neuromodulatory actions of glutamate throughout the CNS, where they play a number of key roles in regulating synaptic transmission and neuronal excitability. A large number of mGlu subtype-selective allosteric modulators have been identified, the majority of which are thought to bind within the transmembrane regions of the receptor. These modulators can either enhance or inhibit mGlu functional responses and, together with mGlu knockout mice, have furthered the establishment of the physiologic roles of many mGlu subtypes. Numerous pharmacological and receptor mutagenesis studies have been aimed at providing a greater mechanistic understanding of the interaction of mGlu allosteric modulators with the receptor, which have revealed evidence for common allosteric binding sites across multiple mGlu subtypes and the presence for multiple allosteric sites within a single mGlu subtype. Recent data have also revealed that mGlu allosteric modulators can display functional selectivity toward particular signal transduction cascades downstream of an individual mGlu subtype. Studies continue to validate the therapeutic utility of mGlu allosteric modulators as a potential therapeutic approach for a number of disorders including anxiety, schizophrenia, Parkinson's disease, and Fragile X syndrome.

Nonlinearity and noise at the rod-rod bipolar cell synapse

In the retina, rod bipolar (RBP) cells synapse with many rods, and suppression of rod outer segment and synaptic noise is necessary for their detection of rod single-photon responses (SPRs). Depending on the rods' signal-to-noise ratio (SNR), the suppression mechanism will likely eliminate some SPRs as well, resulting in decreased quantum efficiency. We examined this synapse in rabbit, where 100 rods converge onto each RBP. Suction electrode recordings showed that rabbit rod SPRs were difficult to distinguish from noise (independent SNR estimates were 2.3 and 2.8). Nonlinear transmission from rods to RBPs improved response detection (SNR = 8.7), but a large portion of the rod SPRs was discarded. For the dimmest flashes, the loss approached 90%. Despite the high rejection ratio, noise of two distinct types was apparent in the RBP traces: low-amplitude rumblings and discrete events that resembled the SPR. The SPR-like event frequency suggests that they result from thermal isomerizations of rhodopsin, which occurred at the rate 0.033/s/rod. The presence of low-amplitude noise is explained by a sigmoidal input-output relationship at the rod-RBP synapse and the input of noisy rods. The rabbit rod SNR and RBP quantum efficiency are the lowest yet reported, suggesting that the quantum efficiency of the rod-RBP synapse may depend on the SNR in rods. These results point to the possibility that fewer photoisomerizations are discarded for species such as primate, which has a higher rod SNR.

TRPM1: a vertebrate TRP channel responsible for retinal ON bipolar function

The transient receptor potential (TRP) channels affect essential functions widely in sensory systems of various species, both invertebrates and vertebrates. The channel protein encoded by the trp gene, the first identified TRP superfamily molecule, is known to mediate the Drosophila light response. A vertebrate TRP channel playing a crucial role in the visual system has not yet been discovered, although numerous studies have revealed primal functions of TRP superfamily molecules in various sensory systems other than vision. In the retina, which is the entry tissue in the vertebrate visual pathway, the transduction cation channel in ON bipolar cells has been elusive, despite intensive investigation by many researchers over a long period of time. Recent studies finally revealed that TRPM1, the first member of the melanoma-related transient receptor potential (TRPM) subfamily to be discovered, is a visual transduction channel in retinal ON bipolar cells. This review covers the significant discoveries on the physiological function and regulatory mechanism of the TRPM1 channel in retinal ON bipolar cells and the association of human TRPM1 mutations with congenital stationary night blindness.

Coordinated control of sensitivity by two splice variants of Gα(o) in retinal ON bipolar cells

The high sensitivity of scotopic vision depends on the efficient retinal processing of single photon responses generated by individual rod photoreceptors. At the first synapse in the mammalian retina, rod outputs are pooled by a rod “ON” bipolar cell, which uses a G-protein signaling cascade to enhance the fidelity of the single photon response under conditions where few rods absorb light. Here we show in mouse rod bipolar cells that both splice variants of the G_o α subunit, Gα_o1 and Gα_o2, mediate light responses under the control of mGluR6 receptors, and their coordinated action is critical for maximizing sensitivity. We found that the light response of rod bipolar cells was primarily mediated by Gα_o1, but the loss of Gα_o2 caused a reduction in the light sensitivity. This reduced sensitivity was not attributable to the reduction in the total number of G_o α subunits, or the altered balance of expression levels between the two splice variants. These results indicate that Gα_o1 and Gα_o2 both mediate a depolarizing light response in rod bipolar cells without occluding each other’s actions, suggesting they might act independently on a common effector. Thus, Gα_o2 plays a role in improving the sensitivity of rod bipolar cells through its action with Gα_o1. The coordinated action of two splice variants of a single Gα may represent a novel mechanism for the fine control of G-protein activity.

Immunocytochemical evidence that monkey rod bipolar cells use GABA

Certain bipolar cells in most species immunostain for GABA or its synthesizing enzyme glutamic acid decarboxylase. However, it is unknown whether they actually release GABA and, if so, from which cellular compartment and by what release mechanism. We investigated these questions in monkey retina where rod bipolar cells immunostain for GABA. We found that rod bipolar cells immunostain for one isoform of GAD (GAD65) in their somas, dendrites and axon terminals. Near the fovea, the somatic stain of rod bipolar cells is weaker than that of horizontal cells but, at the periphery, it is stronger. Staining for the vesicular GABA transporter in monkey rod bipolar cells is negative. However, staining for the GABA transporter GAT3 is positive in the soma and primary dendrites (but not in the axon terminals). Staining for GAT3 is also positive in horizontal cells. Double staining of rod bipolar cells and the alpha subunit of the GABAA receptor reveals scarce GABAA puncta that appose rod bipolar dendrites. We conclude that monkey rod bipolar cells use GABA and discuss the possibility that they tonically release GABA from their dendrites using a reverse action of GAT3.

Ret-PCP2 colocalizes with protein kinase C in a subset of primate ON cone bipolar cells

Purkinje cell protein 2 (PCP2), a member of the family of guanine dissociation inhibitors and a strong interactor with the G-protein subunit G alpha(o), localizes to retinal ON bipolar cells. The retina-specific splice variant of PCP2, Ret-PCP2, accelerates the light response of rod bipolar cells by modulating the mGluR6 transduction cascade. All ON cone bipolar cells express mGluR6 and G alpha(o), but only a subset expresses Ret-PCP2. Here we test the hypothesis that Ret-PCP2 contributes to shaping the various temporal bandwidths of ON cone bipolar cells in monkey retina. We found that the retinal splice variants in monkey and mouse are similar and longer than the cerebellar variants. Ret-PCP2 is strongly expressed by diffuse cone bipolar type 4 cells (DB4; marked with anti-PKCalpha) and weakly expressed by midget bipolar dendrites (labeled by antibodies against G alpha(o), G gamma 13, or mGluR6). Ret-PCP2 is absent from diffuse cone bipolar type 6 (DB6; marked with anti-CD15) and blue cone bipolar cells (marked with anti-CCK precursor). Thus, cone bipolar cells that terminate in stratum 3 of the inner plexiform layer (DB4) express more Ret-PCP2 than those that terminate in strata 3 + 4 (midget bipolar cells), and these in turn express more than those that terminate in stratum 5 (DB6 and blue cone bipolar cells). This expression pattern approximates the arborization of ganglion cells (GC) with different temporal bandwidths: parasol GCs stratifying near stratum 3 are faster than midget GCs stratifying in strata 3 + 4, and these are probably faster than the sluggish GCs that arborize in stratum 5.

Metabotropic glutamate receptors: physiology, pharmacology, and disease

The metabotropic glutamate receptors (mGluRs) are family C G-protein-coupled receptors that participate in the modulation of synaptic transmission and neuronal excitability throughout the central nervous system. The mGluRs bind glutamate within a large extracellular domain and transmit signals through the receptor protein to intracellular signaling partners. A great deal of progress has been made in determining the mechanisms by which mGluRs are activated, proteins with which they interact, and orthosteric and allosteric ligands that can modulate receptor activity. The widespread expression of mGluRs makes these receptors particularly attractive drug targets, and recent studies continue to validate the therapeutic utility of mGluR ligands in neurological and psychiatric disorders such as Alzheimer's disease, Parkinson's disease, anxiety, depression, and schizophrenia.

TRPM1 mutations are associated with the complete form of congenital stationary night blindness

PURPOSE

To identify human transient receptor potential cation channel, subfamily M, member 1 (TRPM1) gene mutations in patients with congenital stationary night blindness (CSNB).

METHODS

We analyzed four different Japanese patients with complete CSNB in whom previous molecular examination revealed no mutation in either nyctalopin (NYX) or glutamate receptor, metabotropic 6 (GRM6). The ophthalmologic examination included best-corrected visual acuity, refraction, biomicroscopy, ophthalmoscopy, fundus photography, Goldmann kinetic perimetry, color vision tests, and electroretinography (ERG). Exons 2 through 27 and the exon-intron junction regions of human TRPM1 were sequenced.

RESULTS

Five different mutations in human TRPM1 were identified. Mutations were present in three unrelated patients with complete CSNB. All three patients were compound heterozygotes. Fundus examination revealed no abnormalities other than myopic changes, and the single bright-flash, mixed rod-cone ERG showed a "negative-type" configuration with a reduced normal a-wave and a significantly reduced b-wave amplitude. Our biochemical and cell biologic analyses suggest that the two identified IVS mutations lead to abnormal TRPM1 protein production, and imply that the two identified missense mutations lead to the mislocalization of the TRPM1 protein in bipolar cells (BCs).

CONCLUSIONS

Human TRPM1 mutations are associated with the complete form of CSNB in Japanese patients, suggesting that TRPM1 plays an essential role in mediating the photoresponse in ON BCs in humans as well as in mice.

R9AP stabilizes RGS11-G beta5 and accelerates the early light response of ON-bipolar cells

The rate-limiting step in the recovery of the photoreceptor light response is the hydrolysis of GTP by transducin, a reaction that is accelerated by the RGS9-Gbeta5 complex, and its membrane anchor, R9AP. Similar complexes, including RGS7, RGS11, and Gbeta5, are found in retinal ON-bipolar cell dendrites. Here, we present evidence that R9AP is also expressed in the dendritic tips of ON-bipolar cells. Immunofluorescent staining for R9AP revealed a punctate pattern of labeling in the outer plexiform layer, where it colocalized with mGluR6. In photoreceptors, R9AP is required for proteolytic stability of the entire regulator of G protein signaling complex, and we found that genetic deletion of R9AP also results in a marked reduction in the levels of RGS11 and Gbeta5 in the bipolar cell dendrites; the level of RGS7 was unaffected, suggesting the presence of another interaction partner to stabilize RGS7. To determine the effect of R9AP deletion on the response kinetics of ON-bipolar cells, we compared the electroretinogram (ERG) between wild-type and R9AP-deficient mice. The ERG b-wave, reflecting ON-bipolar cell activity, was delayed and larger in the R9AP-deficient mice. Our data indicate that R9AP is required for stable expression of RGS11-Gbeta5 in ON-bipolar cell dendrites. Furthermore, they suggest that the RGS11-Gbeta5-R9AP complex accelerates the initial ON-bipolar cell response to light.

Ectopic synaptic ribbons in dendrites of mouse retinal ON- and OFF-bipolar cells

The ectopic distribution of synaptic ribbons in dendrites of mouse retinal bipolar cells was examined by using genetic ablation of metabotropic glutamate receptor subtype 6 (mGluR6), electron microscopy, and immunocytochemistry. Ectopic ribbons were observed in dendrites of rod and ON-cone bipolar cells in the mGluR6-deficient mouse but not in those of wild-type mice. The number of rod spherules facing the ectopic ribbons in mGluR6-deficient rod bipolar dendrites increased gradually during early growth and reached a plateau level of about 20% at 12 weeks. These ectopic ribbons were immunopositive for RIBEYE, a ribbon-specific protein, but the associated vesicles were immunonegative for synaptophysin, a synaptic-vesicle-specific protein. The presence of ectopic ribbons was correlated with an increase in the roundness of the invaginating dendrites of the rod bipolar cells. We further confirmed ectopic ribbons in dendrites of OFF-cone bipolar cells in wild-type retinas. Of the four types of OFF-cone bipolar cells (T1-T4), only the T2-type, which had a greater number of synaptic ribbons at the axon terminal and a thicker axon cylinder than the other types, had ectopic ribbons. Light-adapted experiments revealed that, in wild-type mice under enhanced-light adaptation (considered similar to the mGluR6-deficient state), the roundness in the invaginating dendrites and axon terminals of rod bipolar cells increased, but no ectopic ribbons were detected. Based on these findings and known mechanisms for neurotransmitter release and protein trafficking, the possible mechanisms underlying the ectopic ribbons are discussed on the basis of intracellular transport for the replenishment of synaptic proteins.

TRPM1 is mutated in patients with autosomal-recessive complete congenital stationary night blindness

Night vision requires signaling from rod photoreceptors to adjacent bipolar cells in the retina. Mutations in the genes NYX and GRM6, expressed in ON bipolar cells, lead to a disruption of the ON bipolar cell response. This dysfunction is present in patients with complete X-linked and autosomal-recessive congenital stationary night blindness (CSNB) and can be assessed by standard full-field electroretinography (ERG), showing severely reduced rod b-wave amplitude and slightly altered cone responses. Although many cases of complete CSNB (cCSNB) are caused by mutations in NYX and GRM6, in approximately 60% of the patients the gene defect remains unknown. Animal models of human diseases are a good source for candidate genes, and we noted that a cCSNB phenotype present in homozygous Appaloosa horses is associated with downregulation of TRPM1. TRPM1, belonging to the family of transient receptor potential channels, is expressed in ON bipolar cells and therefore qualifies as an excellent candidate. Indeed, mutation analysis of 38 patients with CSNB identified ten unrelated cCSNB patients with 14 different mutations in this gene. The mutation spectrum comprises missense, splice-site, deletion, and nonsense mutations. We propose that the cCSNB phenotype in these patients is due to the absence of functional TRPM1 in retinal ON bipolar cells.

Altered G-protein coupling in an mGluR6 point mutant associated with congenital stationary night blindness

The highly specialized metabotropic glutamate receptor type 6 (mGluR6) is postsynaptically localized and expressed only in the dendrites of ON bipolar cells. Upon activation of mGluR6 by glutamate released from photoreceptors, a nonselective cation channel is inhibited, causing these cells to hyperpolarize. Mutations in this gene have been implicated in the development of congenital stationary night blindness type 1 (CSNB1). We investigated five known mGluR6 point mutants that lead to CSNB1 to determine the molecular mechanism of each phenotype. In agreement with other studies, four mutants demonstrated trafficking impairment. However, mGluR6 E775K (E781K in humans) suggested no trafficking or signaling deficiencies measured by our initial assays. Most importantly, our results indicate a switch in G-protein coupling, in which E775K loses G(o) coupling but retains coupling to G(i), which may explain the phenotype.

Naturally occurring animal models with outer retina phenotypes

Naturally occurring and laboratory generated animal models serve as powerful tools with which to investigate the etiology of human retinal degenerations, especially retinitis pigmentosa (RP), Leber congenital amaurosis (LCA), cone dystrophies (CD) and macular degeneration (MD). Much progress has been made in elucidating gene defects underlying disease, in understanding mechanisms leading to disease, and in designing molecules for translational research and gene-based therapy to interfere with the progression of disease. Key to this progress has been study of naturally occurring murine and canine retinal degeneration mutants. This article will review the history, phenotypes and gene defects of select animal models with outer retina (photoreceptor and retinal pigment epithelium) degeneration phenotypes.

Neuronal and glial cell expression of angiotensin II type 1 (AT1) and type 2 (AT2) receptors in the rat retina

The bio-active peptide, angiotensin II (Ang II), has been suggested to exert a neuromodulatory effect on inner retinal neurons. In this study, we examined the distribution of angiotensin receptors (ATRs) in the developing and mature rat retina and optic nerve using immunofluorescence immunocytochemistry. Double-labeling experiments were performed with established markers to identify different retinal cell populations. In adult retinae, ATRs were observed on neurons involved in "ON" pathways of neurotransmission. Angiotensin II type 1 receptors (AT(1)Rs) were expressed by a sub-population of "ON" cone bipolar cells that also labeled for G alpha(0) and islet-1. Extra-neuronal expression of AT(1)Rs was evident on retinal astrocytes, Müller cells and blood vessels. Immunoreactivity for the angiotensin II type 2 receptor (AT(2)R) was observed on conventional and displaced GABAergic amacrine cells. Co-localization studies showed that AT(2)R-expressing amacrine cells constituted at least two separate sub-populations. Cell counts revealed that all wide-field amacrine cells expressing protein kinase C-alpha were also AT(2)R-positive; a further subset of amacrine cells expressing AT(2)Rs and stratifying in sublamina "b" of the inner plexiform layer (IPL) was identified. Developmental expression of AT(1)Rs was dynamic, involving multiple inner neuronal classes. At postnatal day 8 (P8), AT(1)R immunoreactivity was observed on putative ganglion cells. The characteristic bipolar cell labeling observed in adults was not evident until P13. In contrast, AT(2)Rs were detected as early as P2 and localized specifically to amacrine cells from this age onward. These data provide further evidence for the potential role of angiotensin II in the modulation of retinal neurons and glia. The differential pattern of expression of these receptors across these cell types is similar to that observed in the brain and suggests that a similar functional role for Ang II may also exist within the retina.

Isolation of ON bipolar cell genes via hrGFP-coupled cell enrichment using the mGluR6 promoter

mGluR6 expression is a characteristic property of retinal ON bipolar cells. mGluR6 is also the causal gene for a form of congenital night blindness. To elucidate physiological and pathological functions of ON bipolar cells and mGluR6, we thought it important to identify genes specifically expressed in them. We thus made transgenic mouse lines expressing humanized Renilla reniformis green fluorescent protein (hrGFP), under the control of the mGluR6 promoter. From their retina, we isolated hrGFP-positive cells by cell sorting, and analysed the gene-expression profile of these cells by using DNA microarray. Further analysis revealed that about half of the initially selected ON bipolar cell genes were expressed in the expected retinal layer. We confirmed previously ambiguous retinal localization of regulator of G-protein signalling 11 (RGS11) and transient receptor potential cation channel, subfamily M, member 1 (TRPM1). In addition, we showed the expression of calcium channel, voltage-dependent, alpha2/delta subunit 3 (Cacna2d3) in ON bipolar cells for the first time. Although we could not completely exclude the possibility that a small population of hrGFP-positive cells might not be ON bipolar cells, these mice as well as our strategy would be highly valuable for the further analysis of ON bipolar cells.

Analysis of protein expression and compartmentalization in retinal neurons using serial tangential sectioning of the retina

The progress in understanding visual signal transduction in vertebrate photoreceptors, arguably the best studied G protein-mediated signal transduction cascade in modern biology, was facilitated by the unique anatomy of rod photoreceptors. Held only by thin connected cilia, rod outer segments can be readily separated from the rest of the retina simply by shaking, and then purified by gradient centrifugation. The availability of such an efficient procedure of rod outer segment purification not only previously facilitated the identification of many principal visual signaling proteins located in this cellular compartment, but it is also currently being exploited in proteomics studies. In this paper, we describe a simple and inexpensive technique that allows for the quantitative analysis of protein expression within different subcellular compartments of photoreceptors, and could also be used for studying protein expression in the secondary retinal neurons. This technique is based on the Western blot analysis of the protein content of serial sections obtained by tangential sectioning of flat-mounted frozen retinas from mouse and rat, and it could serve as a way to validate proteomic data, similar to the way the quantitative RT-PCR technique is used for validation of gene-microarray data. To demonstrate the utility of this technique, we have determined the expression profiles in normal mouse retina of several signaling, energy-producing, and chaperone proteins, which were recently identified in bovine rod photoreceptors by mass spectrometry.

Probing neurochemical structure and function of retinal ON bipolar cells with a transgenic mouse

Retinal ON bipolar cells make up about 70% of all bipolar cells. Glutamate hyperpolarizes these cells by binding to the metabotropic glutamate receptor mGluR6, activating the G-protein G(o1), and closing an unidentified cation channel. To facilitate investigation of ON bipolar cells, we here report on the production of a transgenic mouse (Grm6-GFP) in which enhanced green fluorescent protein (EGFP), under control of mGluR6 promoter, was expressed in all and only ON bipolar cells. We used the mouse to determine density of ON bipolar cells, which in central retina was 29,600 cells/mm(2). We further sorted the fluorescent cells and created a pure ON bipolar cDNA library that was negative for photoreceptor unique genes. With this library, we determined expression of 27 genes of interest. We obtained positive transcripts for G(o) interactors: regulators of G-protein signaling (RGS), Ret-RGS1 (a variant of RGS20), RGS16, RGS7, purkinje cell protein 2 (PCP2, also called L7 or GPSM4), synembryn (RIC-8), LGN (GPSM2), RAP1GAP, and Gbeta5; cGMP modulators: guanylyl cyclase (GC) 1alpha1, GC1beta1, phosphodiesterase (PDE) 1C, and PDE9A; and channels: inwardly rectifying potassium channel Kir2.4, transient receptor potential TRPC2, and sperm-specific cation channels CatSper 2-4. The following transcripts were not found in our library: AGS3 (GPSM1), RGS10, RGS19 (GAIP), calbindin, GC1alpha2, GC1beta2, PDE5, PDE2A, amiloride-sensitive sodium channel ACCN4, and CatSper1. We then localized Kir2.4 to several cell types and showed that, in ON bipolar cells, the channel concentrates in their dendritic tips. The channels and modulators found in ON bipolar cells likely shape their light response. Additional uses of the Grm6-GFP mouse are also discussed.

Allelic variance between GRM6 mutants, Grm6nob3 and Grm6nob4 results in differences in retinal ganglion cell visual responses

An electroretinogram (ERG) screen identified a mouse with a normal a-wave but lacking a b-wave, and as such it was designated no b-wave3 (nob3). The nob3 phenotype mapped to chromosome 11 in a region containing the metabotropic glutamate receptor 6 gene (Grm6). Sequence analyses of cDNA identified a splicing error in Grm6, introducing an insertion and an early stop codon into the mRNA of affected mice (designated Grm6(nob3)). Immunohistochemistry of the Grm6(nob3) retina showed that GRM6 was absent. The ERG and visual behaviour abnormalities of Grm6(nob3) mice are similar to Grm6(nob4) animals, and similar deficits were seen in compound heterozygotes (Grm6(nob4/nob3)), indicating that Grm6(nob3) is allelic to Grm6(nob4). Visual responses of Grm6(nob3) retinal ganglion cells (RGCs) to light onset were abnormal. Grm6(nob3) ON RGCs were rarely recorded, but when they were, had ill-defined receptive field (RF) centres and delayed onset latencies. When Grm6(nob3) OFF-centre RGC responses were evoked by full-field stimulation, significantly fewer converted that response to OFF/ON compared to Grm6(nob4) RGCs. Grm6(nob4/nob3) RGC responses verified the conclusion that the two mutants are allelic. We propose that Grm6(nob3) is a new model of human autosomal recessive congenital stationary night blindness. However, an allelic difference between Grm6(nob3) and Grm6(nob4) creates a disparity in inner retinal processing. Because the localization of GRM6 is limited to bipolar cells in the On pathway, the observed difference between RGCs in these mutants is likely to arise from differences in their inputs.

Comparisons of structural and functional abnormalities in mouse b-wave mutants

In the most simplistic view, the retinal circuit can be divided into vertical excitatory pathways that use glutamate as their neurotransmitter and lateral inhibitory pathways in the outer and inner synaptic layers that modulate excitation via glycine and GABA. Within the vertical excitatory pathways, the visual signal is initiated in the rod, cone or both photoreceptors, depending on the adaptation state of the retina. This signal is transmitted to the rest of the retina through the bipolar cells, which can be subdivided based on: the photoreceptor that provides their input, their dendritic and axonal morphology, and the polarity of their response evoked by a luminance increment, e.g. depolarizing or hyperpolarizing responses. The polarity of this response is controlled by the type of glutamatergic postsynaptic receptor that is expressed on their dendritic terminals. Hyperpolarizing bipolar cells express AMPA/kainate receptors, whereas depolarizing bipolar cells (DBCs) express the metabotropic glutamate receptor 6 (Grm6). The electroretinogram (ERG) is a non-invasive method used to assess overall retinal function. The initiation of the visual signal in the photoreceptors is reflected in the ERG a-wave and the ensuing depolarization of DBCs in the b-wave. When there is failure of signal transmission from photoreceptors to DBCs or signalling within DBCs, the ERG a-wave is present, while the b-wave is absent or significantly reduced. This ERG phenotype has been found in the human population and is referred to as congenital stationary night blindness. Until recently, it had been assumed that the absence of a b-wave was indicative of a lack of signalling through the On pathway, leaving the Off pathway unaffected. Here we review recent findings that demonstrate that many mouse mutants share a no b-wave ERG phenotype but their retinal morphology and RGC responses differ significantly, suggesting very different effects of the underlying mutations on output from the DBCs to the rest of the retinal circuit.

Deletion of Go2alpha abolishes cocaine-induced behavioral sensitization by disturbing the striatal dopamine system

The alpha-subunits of the trimeric Go class of GTPases, comprising the splice variants Go1alpha and Go2alpha, are abundantly expressed in brain and reside on both plasma membrane and synaptic vesicles. Go2alpha is involved in the vesicular storage of monoamines but its physiological relevance is still obscure. We now show that genetic depletion of Go2alpha reduces motor activity induced by dopamine-enhancing drugs like cocaine, as repeated injections of cocaine fail to provoke behavioral sensitization in Go2alpha(-/-) mice. In Go2alpha(-/-) mice, D1 receptor signaling in the striatum is attenuated due to a reduced expression of Golf alpha and Gs alpha. Following cocaine treatment, Go2alpha(-/-) mice have lower D1 and higher D2 receptor amounts compared to wild-type mice. The lack of behavioral sensitization correlates with reduced dopamine levels in the striatum and decreased expression of tyrosine hydroxylase. One reason for the neurochemical changes may be a reduced uptake of monoamines by synaptic vesicles from Go2alpha(-/-) mice as a consequence of a lowered set point for filling. We conclude that Go2alpha optimizes vesicular filling which is instrumental for normal dopamine functioning and for the development of drug-induced behavioral sensitization.

Treatment of epilepsy: the GABA-transaminase inhibitor, vigabatrin, induces neuronal plasticity in the mouse retina

Vigabatrin was a major drug in the treatment of epilepsy until the discovery that it was associated with an irreversible constriction of the visual field. Nevertheless, the drug is still prescribed for infantile spasms and refractory epilepsy. Disorganization of the photoreceptor nuclear layer and cone photoreceptor damage have been described in albino rats. To investigate the vigabatrin-elicited retinal toxicity further, we examined the retinal tissue of albino mice treated with two vigabatrin doses. The higher dose did not always cause the photoreceptor layer disorganization after 1 month of treatment. However, it triggered a massive synaptic plasticity in retinal areas showing a normal layering of the retina. This plasticity was shown by the withdrawal of rod but not cone photoreceptor terminals from the outer plexiform layers towards their cell bodies. Furthermore, both rod bipolar cells and horizontal cells exhibited dendritic sprouting into the photoreceptor nuclear layer. Withdrawing rod photoreceptors appeared to form ectopic contacts with growing postsynaptic dendrites. Indeed, contacts between rods and bipolar cells, and between bipolar cells and horizontal cells were observed deep inside the outer nuclear layer. This neuronal plasticity is highly suggestive of an impaired glutamate release by photoreceptors because similar observations have been reported in different genetically modified mice with deficient synaptic transmission. Such a synaptic deficit is consistent with the decrease in glutamate concentration induced by vigabatrin. This description of the neuronal plasticity associated with vigabatrin provides new insights into its retinal toxicity in epileptic patients.

Photopic electroretinograms of mGluR6-deficient mice

PURPOSE

To study the properties of the photopic electroretinograms (ERGs) of the metabotropic glutamate receptor subtype 6 (mGluR6)-deficient mice and to investigate the contribution of cone ON-and OFF-pathways to the mouse photopic ERGs.

METHODS

Photopic ERGs were recorded from mGluR6-deficient and wild-type mice. Photopic ERGs were also recorded after an intravitreous injection of cis-2,3 piperidine dicarboxylic acid (PDA) to block the transmission of signals from the photoreceptors to the OFF-bipolar cells, horizontal cells, and other inner retinal neurons.

RESULTS

The amplitude of the b-wave of the photopic ERG was severely reduced in mGluR6-deficient mice, but a small, slow, positive component was seen after the a-wave. Intravitreous injection of PDA eliminated this positive component.

CONCLUSIONS

The mGluR6-deficient mouse is a useful animal model to study the contribution of the ON-and OFF-pathways to the mouse ERG.

Regulation of ON bipolar cell activity

Synaptic transmission from photoreceptors to all types of ON bipolar cells is primarily mediated by the mGluR6 receptor. This receptor, which is apparently expressed uniquely in the nervous system by ON bipolar cells, couples negatively to a nonselective cation channel. This arrangement results in a sign reversal at photoreceptor/ON bipolar cell synapse, which is necessary in order to establish parallel ON and OFF pathways in the retina. The synapse is an important target for second messenger molecules that are known to modulate synaptic transmission elsewhere in the nervous system, second messengers that act on a time scale ranging from milliseconds to minutes. This review focuses on two of these molecules, Ca2+ and cGMP, summarizing our current knowledge of how they modulate gain at the photoreceptor/ON bipolar cell synapse, as well as their proposed sites of action within the mGluR6 cascade. The implications of plasticity at this synapse for retinal function will also be examined.

Effects of the protein tyrosine kinase inhibitor genistein and taurine on retinal function in isolated superfused retina

BACKGROUND

Genistein has the potential to act as an intraocular antiangiogenic agent. Its therapeutical use, however, is limited by toxic side effects on the retina. This study was designed to evaluate the simultaneous use of taurine as a neuroprotective drug.

METHODS

Bovine retinas were isolated and perfused with an oxygen-preincubated nutrient solution. The electroretinogram (ERG) was recorded as a transretinal electrical potential using Ag/AgCl electrodes. At stable ERG amplitudes, genistein at concentrations of 11, 37, and 150 microM was added to the nutrient solution for 45 min, in the absence or presence of taurine (3 mM). Thereafter, the retina was reperfused with the nutrient solution for another 100 min. The percentage of b-wave reduction during genistein and genistein/taurine application was calculated.

RESULTS

The b-wave amplitude was reduced by a smaller amount during the application of genistein (11 and 37 microM) in the presence of taurine compared with genistein alone. For both, genistein/taurine and genistein alone the b-wave recovered completely during the wash-out of the drugs. However, during the application of the highest tested concentration of genistein (150 microM), taurine did not protect completely, leading to an irreversible b-wave reduction.

CONCLUSIONS

The adjuvant use of taurine reduces the genistein-induced retinal toxicity to a certain degree. However, the protective effect of taurine is limited and there is only a narrow therapeutic index for a combined intravitreal administration of genistein in coapplication with taurine to inhibit pathological ocular neovascularization.

Gbeta5-RGS complexes co-localize with mGluR6 in retinal ON-bipolar cells

The time course of G-protein-coupled responses is largely determined by the kinetics of GTP hydrolysis by the G protein alpha subunit, which is accelerated by interaction with regulator of G-protein signaling (RGS) proteins. Light responses of ON-bipolar cells of the vertebrate retina require rapid inactivation of the G protein Galphao, which is activated in the dark by metabotropic glutamate receptor, mGluR6, in their dendritic tips. It is not yet known, however, which RGS protein(s) might be responsible for rapid inactivation kinetics. By immunofluorescence and co-immunoprecipitation, we have identified complexes of the Galphao-selective RGS proteins RGS7 and RGS11, with their obligate binding partner, Gbeta5, that are localized to the dendritic tips of murine rod and cone ON-bipolar cells, along with mGluR6. Experiments using pre- and post-synaptic markers, and a dissociated bipolar cell preparation, clearly identified the location of these complexes as the ON-bipolar cell dendritic tips and not the adjacent photoreceptor terminals or horizontal cell dendrites. In mice lacking mGluR6, the distribution of RGS11, RGS7 and Gbeta5 shifts away from the dendritic tips, implying a functional relationship with mGluR6. The precise co-localization of Gbeta5-RGS7 and Gbeta5-RGS11 with mGluR6, and the dependence of localization on the presence of mGluR6, suggests that Gbeta5-RGS7 and Gbeta5-RGS11 function specifically in the mGluR6 signal transduction pathway, where they may stimulate the GTPase activity of Galphao, thus accelerating the ON-bipolar cell light response, in a manner analogous to the acceleration of photoreceptor light responses by the Gbeta5-RGS9-1 complex.

Functional roles of Otx2 transcription factor in postnatal mouse retinal development

We previously reported that Otx2 is essential for photoreceptor cell fate determination; however, the functional role of Otx2 in postnatal retinal development is still unclear although it has been reported to be expressed in retinal bipolar cells and photoreceptors at postnatal stages. In this study, we first examined the roles of Otx2 in the terminal differentiation of photoreceptors by analyzing Otx2; Crx double-knockout mice. In Otx2+/-; Crx-/- retinas, photoreceptor degeneration and downregulation of photoreceptor-specific genes were much more prominent than in Crx-/- retinas, suggesting that Otx2 has a role in the terminal differentiation of the photoreceptors. Moreover, bipolar cells decreased in the Otx2+/-; Crx-/- retina, suggesting that Otx2 is also involved in retinal bipolar-cell development. To further investigate the role of Otx2 in bipolar-cell development, we generated a postnatal bipolar-cell-specific Otx2 conditional-knockout mouse line. Immunohistochemical analysis of this line showed that the expression of protein kinase C, a marker of mature bipolar cells, was significantly downregulated in the retina. Electroretinograms revealed that the electrophysiological function of retinal bipolar cells was impaired as a result of Otx2 ablation. These data suggest that Otx2 plays a functional role in the maturation of retinal photoreceptor and bipolar cells.

Nyctalopin expression in retinal bipolar cells restores visual function in a mouse model of complete X-linked congenital stationary night blindness

Mutations in the NYX gene that encodes the protein nyctalopin cause congenital stationary night blindness type 1. In no b-wave (nob) mice, a mutation in Nyx results in a functional phenotype that includes the absence of the electroretinogram b-wave and abnormal spontaneous and light-evoked activity in retinal ganglion cells (RGCs). In contrast, there is no morphological abnormality in the retina at either the light or electron microscopic levels. These functional deficits suggest that nyctalopin is required for normal synaptic transmission between retinal photoreceptors and depolarizing bipolar cells (DBCs). However, the synaptic etiology and, specifically, the exact location and function of nyctalopin, remain uncertain. We show that nob DBCs fail to respond to exogenous application of the photoreceptor neurotransmitter, glutamate, thus demonstrating a postsynaptic deficit in photoreceptor to bipolar cell communication. To determine if postsynaptic expression of nyctalopin is necessary and sufficient to rescue the nob phenotype, we constructed transgenic mice that expressed an EYFP-nyctalopin fusion protein on the dendritic tips of the DBCs. Immunohistochemical and ultrastructural studies verified that fusion protein expression was limited to the DBC dendritic tips. Fusion gene expression in nob mice restored normal outer and inner visual function as determined by the electroretinogram and RGC spontaneous and evoked responses. Together, our data show that nyctalopin expression on DBC dendrites is required for normal function of the murine retina.

Regulation of vesicular monoamine and glutamate transporters by vesicle-associated trimeric G proteins: new jobs for long-known signal transduction molecules

Neurotransmitters of neurons and neuroendocrine cells are concentrated first in the cytosol and then in either small synaptic vesicles ofpresynaptic terminals or in secretory vesicles by the activity of specific transporters of the plasma and the vesicular membrane, respectively. In the central nervous system the postsynaptic response depends--amongst other parameters-on the amount of neurotransmitter stored in a given vesicle. Neurotransmitter packets (quanta) vary over a wide range which may be also due to a regulation of vesicular neurotransmitter filling. Vesicular filling is regulated by the availability of transmitter molecules in the cytoplasm, the amount of transporter molecules and an electrochemical proton-mediated gradient over the vesicular membrane. In addition, it is modulated by vesicle-associated heterotrimeric G proteins, Galphao2 and Galphaq. Galphao2 and Galphaq regulate vesicular monoamine transporter (VMAT) activities in brain and platelets, respectively. Galphao2 also regulates vesicular glutamate transporter (VGLUT) activity by changing its chloride dependence. It appears that the vesicular content activates the G protein, suggesting a signal transduction from the luminal site which might be mediated by a vesicular G protein-coupled receptor or as an alternative possibility by the transporter itself. Thus, G proteins control transmitter storage and thereby probablylink the regulation of the vesicular content to intracellular signal cascades.

Mpp4 is required for proper localization of plasma membrane calcium ATPases and maintenance of calcium homeostasis at the rod photoreceptor synaptic terminals

Membrane palmitoylated protein 4 (Mpp4) is a member of the membrane-associated guanylate kinase family. We show that Mpp4 localizes specifically to the plasma membrane of photoreceptor synaptic terminals. Plasma membrane Ca(2+) ATPases (PMCAs), the Ca(2+) extrusion pumps, interact with an Mpp4-dependent presynaptic membrane protein complex that includes Veli3 and PSD95. In mice lacking Mpp4, PMCAs were lost from rod photoreceptor presynaptic membranes. Synaptic ribbons were enlarged, a phenomenon known to correlate with higher Ca(2+). SERCA2 (sarcoplasmic-endoplasmic reticulum Ca(2+) ATPase, type 2), which pumps cytosolic Ca(2+) into intracellular Ca(2+) stores and localizes next to the ribbons, was increased. The distribution of IP(3)RII (InsP(3) receptor, type 2), which releases Ca(2+) from the stores, was shifted away from the synaptic terminals. Synaptic transmission to second-order neurons was maintained but was reduced in amplitude. These data suggest that loss of Mpp4 disrupts a Ca(2+) extrusion mechanism at the presynaptic membranes, with ensuing adaptive responses by the photoreceptor to restore Ca(2+) homeostasis. We propose that Mpp4 organizes a presynaptic protein complex that includes PMCAs and has a role in modulating Ca(2+) homeostasis and synaptic transmission in rod photoreceptors.

Retinal organization in the retinal degeneration 10 (rd10) mutant mouse: a morphological and ERG study

Retinal degeneration 10 (rd10) mice are a model of autosomal recessive retinitis pigmentosa (RP), identified by Chang et al. in 2002 (Vision Res. 42:517-525). These mice carry a spontaneous mutation of the rod-phosphodiesterase (PDE) gene, leading to a rod degeneration that starts around P18. Later, cones are also lost. Because photoreceptor degeneration does not overlap with retinal development, and light responses can be recorded for about a month after birth, rd10 mice mimic typical human RP more closely than the well-known rd1 mutants. The aim of this study is to provide a comprehensive analysis of the morphology and function of the rd10 mouse retina during the period of maximum photoreceptor degeneration, thus contributing useful data for exploiting this novel model to study RP. We analyzed the morphology and survival of retinal cells in rd10 mice of various ages with quantitative immunocytochemistry and confocal microscopy; we also studied retinal function with the electroretinogram (ERG), recorded between P18 and P30. We found that photoreceptor death (peaking around P25) is accompanied and followed by dendritic retraction in bipolar and horizontal cells, which eventually undergo secondary degeneration. ERG reveals alterations in the physiology of the inner retina as early as P18 (before any obvious morphological change of inner neurons) and yet consistently with a reduced band amplification by bipolar cells. Thus, changes in the rd10 retina are very similar to what was previously found in rd1 mutants. However, an overall slower decay of retinal structure and function predicts that rd10 mice might become excellent models for rescue approaches.

Reduced synaptic vesicle density and aberrant synaptic localization caused by a splice site mutation in the Rs1h gene

X-linked retinoschisis (XLRS) is a common form of inherited macular degeneration caused by mutations in the RS1 gene. Whereas the role of RS1 has been implicated in the synaptic structure as well as layer organization in the retina, the pathological effect of a defective RS1 gene on the synaptic interaction between photoreceptor cells and second-order neurons has not been thoroughly investigated. In this study, we perform a detailed characterization of the retinal synaptic phenotypes caused by a splice site mutation in the murine RS1 homolog (Rs1htmgc1). Electron microscopic analysis showed that presynaptic terminals of photoreceptor cells contain a lower areal density of synaptic vesicles in the Rs1htmgc1 retina. Examination of the synaptic interactions in the outer plexiform layer also revealed ectopic localization of photoreceptor cell presynaptic markers and elongation of neurites from postsynaptic neurons (bipolar and horizontal cells), which are observed in other mouse models with defective photoreceptor cell molecules. Consistent with these synaptic abnormalities, ERG analysis of young Rs1htmgc1 mice revealed attenuation of the b-wave with preservation of the a-wave. These results demonstrate that RS1H has functional significance in the morphology and function of the synapse between photoreceptors and second-order neurons. A developmental study from postnatal day (P) 15 through P19 showed that synaptic interactions form normally, and structural abnormalities occur after completion of synaptic formation suggesting that RS1H is important for the maintenance of this synaptic interaction. Thus, Rs1htmgc1 mice may serve as a new genetic model for human XLRS and other synaptic disorders.

G protein coupling profile of mGluR6 and expression of Gα proteins in retinal ON bipolar cells

Metabotropic glutamate receptor 6 (mGluR6) is a group III, pertussis toxin (PTX)-sensitive G protein coupled mGluR that plays a specialized role in the retina. Retinal ON bipolar cells, which receive direct glutamatergic input from photoreceptor cells, express mGluR6 as their primary postsynaptic glutamate receptor. Activation of mGluR6 in these cells initiates an intracellular signaling cascade ultimately leading to inhibition of a cation channel and cell hyperpolarization. The primary mediator of this pathwayin vivois Gαo, but the potential roles of other G proteins from the Gαi/ofamily in the regulation of this or other signaling pathways in ON bipolar cells are unclear. To determine which specific G proteins from the Gαi/ofamily are able to couple to mGluR6, a Gα reconstitution system was employed using PTX-insensitive Gα mutants expressed with mGluR6 in PTX-treated sympathetic neurons from the rat superior cervical ganglion (SCG). The efficiency of coupling to mGluR6 was Goa> Gob, Gi1> Gi2, Gi3, whereas no coupling was observed with Gαz, nor with the retinal Gα proteins, rod (GNAT2) or cone (GNAT1) transducin (GαTr-R, GαTr-C). Finally, the expression of Gα proteins determined to couple with mGluR6 was examined in rat ON bipolar cells using single cell RT-PCR. Co-expression of mGluR6 message was used to distinguish ON from OFF bipolar cells. Expression of Gαowas detected in every ON bipolar cell examined. Message for Gαi1, which coupled moderately to mGluR6, was not detected in ON bipolar cells, nor was Gαi3, which coupled to mGluR6 in only a few cells but on average did not exhibit statistically significant coupling. Finally, though Gαi2was detectable in ON bipolar cells, its coupling to mGluR6 in the SCG system was not significant. Together, these data indicate that signaling through mGluR6 in mammalian ON bipolar cells is highly focused, apparently acting through a single Gα protein subtype.

Rod bipolar cells and horizontal cells form displaced synaptic contacts with rods in the outer nuclear layer of the nob2 retina

The nob2 mouse carries a null mutation in the Cacna1f gene, which encodes the pore-forming subunit of the L-type calcium channel, Ca(v)1.4. The loss of the electroretinogram b-wave in these mice suggests a severe reduction in transmission between photoreceptors and second-order neurons in the retina and supports a central role for the Ca(v)1.4 calcium channel at photoreceptor ribbon synapses, to which it has been localized. Here we show that the loss of Ca(v)1.4 leads to the aberrant outgrowth of rod bipolar cell dendrites and horizontal cell processes into the outer nuclear layer (ONL) of the nob2 retina and to the formation of ectopic synaptic contacts with rod photoreceptors in the ONL. Ectopic contacts are predominantly between rods and rod bipolar cells, with horizontal cell processes also present at some sites. Ectopic contacts contain apposed pre- and postsynaptic specializations, albeit with malformed synaptic ribbons. Cone photoreceptor terminals do not participate in ectopic contacts in the ONL. During retinal development, ectopic contacts appear in the days after eye opening, appearing progressively farther into the ONL at later postnatal stages. Ectopic contacts develop at the tips of rod bipolar cell dendrites and are less frequently associated with the tips of horizontal cell processes, consistent with the adult phenotype. The relative occurrence of pre- and postsynaptic markers in the ONL during development suggests a mechanism for the formation of ectopic synaptic contacts that is driven by the retraction of rod photoreceptor terminals and neurite outgrowth by rod bipolar cell dendrites.