Metabotropic glutamate receptors are differentially regulated under elevated intraocular pressure

Group II metabotropic glutamate receptor agonist promotes retinal ganglion cell survival by reducing neuronal excitotoxicity in a rat chronic ocular hypertension model

Glaucoma, the second leading cause of irreversible blindness worldwide, is characterized by the selective death of retinal ganglion cells (RGCs). The group II metabotropic glutamate receptor (mGluR II) activation has been linked to RGC survival, however, the mechanism by which it promotes neuronal survival remains poorly defined. In the present work, we show that extracellular application of LY341495, an mGluR II antagonist could increase the RGC firing frequency, suggesting that activation of mGluR II by endogenously released glutamate could modulate RGC excitability. LY354740, an mGluR II agonist, significantly decreased RGC excitability and the reduced presynaptic excitatory inputs and post-synaptic Ca2+-permeable currents mediated the LY354740-induced effects. By using a well-characterized in vivo male Sprague-Dawley rat glaucoma model, we further demonstrate that in the early stage of experimental glaucoma, the expression of mGluR II dimer-formed protein was significantly reduced, and pre-activation of mGluR II by intravitreal injection of LY354740 before establishment of the glaucoma model could effectively reduce excitatory inputs, thereby reversing hyperexcitability induced by elevated intraocular pressure. Furthermore, LY354740 could increase the expression level of brain-derived neurotrophic factor in the glaucomatous retinas, further protecting RGCs. Our study indicates that the abnormal expression of mGluR II may accelerate RGC apoptosis in glaucoma, and demonstrates that mGluR II agonist LY354740 can be used as a novel method to counter RGC apoptosis in glaucoma.

Rac1 Modulates Excitatory Synaptic Transmission in Mouse Retinal Ganglion Cells

Ras-related C3 botulinum toxin substrate 1 (Rac1), a member of the Rho GTPase family which plays important roles in dendritic spine morphology and plasticity, is a key regulator of cytoskeletal reorganization in dendrites and spines. Here, we investigated whether and how Rac1 modulates synaptic transmission in mouse retinal ganglion cells (RGCs) using selective conditional knockout of Rac1 (Rac1-cKO). Rac1-cKO significantly reduced the frequency of AMPA receptor-mediated miniature excitatory postsynaptic currents, while glycine/GABA_A receptor-mediated miniature inhibitory postsynaptic currents were not affected. Although the total GluA1 protein level was increased in Rac1-cKO mice, its expression in the membrane component was unchanged. Rac1-cKO did not affect spine-like branch density in single dendrites, but significantly reduced the dendritic complexity, which resulted in a decrease in the total number of dendritic spine-like branches. These results suggest that Rac1 selectively affects excitatory synaptic transmission in RGCs by modulating dendritic complexity.

Involvement of mGluR I in EphB/ephrinB reverse signaling activation induced retinal ganglion cell apoptosis in a rat chronic hypertension model

EphB/ephrinB reverse signaling is involved in retinal ganglion cell (RGC) apoptosis in experimental glaucoma. Here, we further investigated the mechanisms underlying EphB/ephrinB reverse signaling activation induced RGC apoptosis in a rat chronic ocular hypertension (COH) model, using patch-clamp techniques in retinal slices. In COH retinas, RGCs showed higher spontaneous firing frequency and much more depolarized membrane potential as compared to control, which was mimicked by intravitreally injection of EphB2-Fc, an activator of ephrinB2. The changes in RGC spontaneous firing and membrane potential could be reversed by the tyrosine kinase inhibitor PP2, suggesting that EphB/ephrinB reverse signaling activation induced RGC hyperexcitability. Intravitreal pre-injection of either LY367385 or MPEP, selective mGluR1 and mGluR5 antagonists, also blocked the changes in RGC spontaneous firing and membrane potential. Co-immunoprecipitation experiments showed an interaction between ephrinB2 and group I metabotropic glutamate receptor (mGluR I) (mGluR1/mGluR5). Furthermore, intravitreal pre-injection of the mixture of L-NAME (an NO synthase inhibitor) and XPro1595 (a selective inhibitor of soluble TNF-α) could reduce the EphB2-Fc injection induced increase in RGC firing, suggesting that Müller cells might be involved in EphB/ephrinB reverse signaling activation induced change in RGC hyperexcitability. In addition, LY367385/MPEP reduced the numbers of TUNEL-positive RGCs both in EphB2-Fc injected and COH retinas. All results suggest that activation of EphB/ephrinB reverse signaling induces RGC hyperexcitability and apoptosis by interacting with mGluR I in COH rats. Appropriate reduction of EphB/ephrinB reverse signaling could alleviate the loss of RGCs in glaucoma.

Glaucoma: Current and Developing Concepts for Inflammation, Pathogenesis and Treatment

Glaucoma is a prevalent neurodegenerative disorder of the eye. However, the mechanism leading to the disease is still unclear. Increased intraocular pressure (IOP) and subsequent retinal ganglion cell (RGC) death leading to the loss of visual field characterizes the pathology of primary open angle glaucoma, which is the most common form. Possible factors leading to glaucoma include glutamate-induced neurotoxicity, nitric oxide (NO) based damage, disruption of neurotrophic factor transport and immune-induced neurodestruction. Current treatment options primarily aim at decreasing IOP by utilizing pharmacological agents, laser therapy and surgery. Developing treatments target neuroprotection with vaccines, the inhibition of NO synthesis and apoptosis. Gaining a better understanding of the pathogenesis can aid in the development of new treatment options and, perhaps, even a cure for glaucoma.

Proteomics Analysis of Molecular Risk Factors in the Ocular Hypertensive Human Retina

PURPOSE

To better understand ocular hypertension-induced early molecular alterations that may determine the initiation of neurodegeneration in human glaucoma, this study analyzed retinal proteomic alterations in the ocular hypertensive human retina.

METHODS

Retina samples were obtained from six human donors with ocular hypertension (without glaucomatous injury) and six age- and sex-matched normotensive controls. Retinal proteins were analyzed by two-dimensional LC-MS/MS (liquid chromatography and linear ion trap mass spectrometry) using oxygen isotope labeling for relative quantification of protein expression. Proteomics data were validated by Western blot and immunohistochemical analyses of selected proteins.

RESULTS

Out of over 2000 retinal proteins quantified, hundreds exhibited over 2-fold increased or decreased expression in ocular hypertensive samples relative to normotensive controls. Bioinformatics linked the proteomics datasets to various pathways important for maintenance of cellular homeostasis in the ocular hypertensive retina. Upregulated proteins included various heat shock proteins, ubiquitin proteasome pathway components, antioxidants, and DNA repair enzymes, while many proteins involved in mitochondrial oxidative phosphorylation exhibited downregulation in the ocular hypertensive retina. Despite the altered protein expression reflecting intrinsic adaptive/protective responses against mitochondrial energy failure, oxidative stress, and unfolded proteins, no alterations suggestive of an ongoing cell death process or neuroinflammation were detectable.

CONCLUSIONS

This study provides information about ocular hypertension-related molecular risk factors for glaucoma development. Molecular alterations detected in the ocular hypertensive human retina as opposed to previously detected alterations in human donor retinas with clinically manifest glaucoma suggest that proteome alterations determine the individual threshold to tolerate the ocular hypertension-induced tissue stress or convert to glaucomatous neurodegeneration when intrinsic adaptive/protective responses are overwhelmed.

Altered expression of metabotropic glutamate receptor 1 alpha after acute diffuse brain injury: Effect of the competitive antagonist 1-aminoindan-1, 5-dicarboxylic acid

The diffuse brain injury model was conducted in Sprague-Dawley rats, according to Marmarou's free-fall attack. The water content in brain tissue, expression of metabotropic glutamate receptor 1α mRNA and protein were significantly increased after injury, reached a peak at 24 hours, and then gradually decreased. After treatment with the competitive antagonist of metabotropic glutamate receptor 1α, (RS)-1-aminoindan-1, 5-dicarboxylic acid, the water content of brain tissues decreased between 12–72 hours after injury, and neurological behaviors improved at 2 weeks. These experimental findings suggest that the 1-aminoindan-1, 5-dicarboxylic acid may result in marked neuroprotection against diffuse brain injury.

Expression of Glutamate and GABA during the Process of Rat Retinal Synaptic Plasticity Induced by Acute High Intraocular Pressure

Acute high intraocular pressure (HIOP) can induce plastic changes of retinal synapses during which the expression of the presynaptic marker synaptophysin (SYN) has a distinct spatiotemporal pattern from the inner plexiform layer to the outer plexiform layer. We identified the types of neurotransmitters in the retina that participated in this process and determined the response of these neurotransmitters to HIOP induction. The model of acute HIOP was established by injecting normal saline into the anterior chamber of the rat eye. We found that the number of glutamate-positive cells increased successively from the inner part to the outer part of the retina (from the ganglion cell layer to the inner nuclear layer to the outer nuclear layer) after HIOP, which was similar to the spatiotemporal pattern of SYN expression (internally to externally) following HIOP. However, the distribution and intensity of GABA immunoreactivity in the retina did not change significantly at different survival time post injury and had no direct correlation with SYN expression. Our results suggested that the excitatory neurotransmitter glutamate might participate in the plastic process of retinal synapses following acute HIOP, but no evidence was found for the role of the inhibitory neurotransmitter GABA.

Rod photoreceptor ribbon synapses in DBA/2J mice show progressive age-related structural changes

The DBA/2J mouse is a commonly used animal model in glaucoma research. The eyes of DBA/2J mice show severe age-related changes that finally lead to the degeneration of retinal ganglion cells and the optic nerve. Recent electroretinogram studies identified functional deficits, which suggest that also photoreceptor cells are involved in the pathological processes occurring in the DBA/2J mouse retina. In a comparative study, we examined anatomical and molecular changes in the retinae of DBA/2J and C57BL/6 control mice with light and electron microscopy and with PCR analyses. In the retina of the DBA/2J mouse, we found a thinning of the outer plexiform layer, the first synaptic layer in the transfer of visual signals, and age-dependent and progressive degenerative structural changes at rod photoreceptor ribbon synapses. The structural ribbon changes represent a photoreceptor synaptic phenotype that has not yet been described in this animal model of secondary angle-closure glaucoma. Furthermore, genes of the classical complement cascade were upregulated in the photoreceptor cells of aging DBA/2J mice, suggesting a putative link between ribbon synapse degradation and the innate immune system.

"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.

Distribution of nidogen in the murine eye and ocular phenotype of the nidogen-1 knockout mouse

Distribution and lack of nidogen-1, part of numerous basement membranes, were studied in the mouse eye. For that purpose, eyes of C57BL/6 and nidogen-1 knockout mice were stained immunohistochemically for nidogen-1, and intraocular pressure measurements and light- and electron microscopy were used to study the nidogen-1 knockout animals. In normal mice, nidogen-1 was present in many basement membranes, but showed irregularities underneath the corneal epithelium, in Bruch's membrane and in the iris. Homozygous knockout of nidogen-1 in the mouse showed only mild pathological changes. In the anterior eye segment, small interruptions were noted in the nonpigmented ciliary epithelium without further consequences. In the posterior eye segment, interruptions of the inner limiting membrane led to small retinal ectopias and subsequent changes in the optic nerve. In summary, the knockout of nidogen-1 showed mild but significant morphological changes pointing to the importance of this protein which can in part, but not completely; be replaced by nidogen-2.

Changes in the modulation of retinocollicular transmission through group III mGluRs long after an increase in intraocular pressure in a rat model of glaucoma

Metabotropic glutamate receptors (mGluRs) have been shown to be involved in the modulation of retinocollicular neurotransmission. In glaucoma, retinal ganglion cells (RGCs) degenerate, which may have an implication on this transmission as the superior colliculus is their major central target in the much-used rodent models of the disease. We have investigated this using an in vitro slice preparation of the superior colliculus by eliciting field excitatory postsynaptic potentials (fEPSPs) through optic tract stimulation in a rat ocular hypertension model of glaucoma. Application of the group III mGluR agonist L-AP4 reduced the peak amplitude of the fEPSP in superior colliculus slices through presynaptic mechanisms as previously shown in our lab. At 3 and 16 weeks after surgery, there were no significant differences in the effect of L-AP4 on fEPSP peak amplitude in the superior colliculus slices receiving input from the glaucomatous eyes [elevated intraocular pressure (IOP)] compared to those with input from the unoperated eyes (normal IOP). However, at 32 weeks, the fEPSP peak amplitude was reduced to a significantly greater degree during L-AP4 application in the elevated IOP slices compared to normal IOP slices. At all time points, there were no significant changes in the baseline amplitudes of fEPSPs or the stimulus intensities required to evoke fEPSPs. These results suggest that the modulation of synaptic transmission through group III mGluRs on RGC terminals to the superior colliculus is changed at later stages due to RGC degeneration through IOP elevation. These changes may be compensatory changes possibly through plasticity in the RGC terminals of surviving cells, which may be due to increases in the numbers of group III mGluRs. This result may have implications on further treatment studies carried out using these models of glaucoma as changes in the central visual system may need to be considered along with the retinal changes that occur.

Retinal gene expression changes related to IOP exposure and axonal loss in DBA/2J mice

PURPOSE

To determine the effects of cumulative IOP exposure and axonal damage on retinal gene expression in DBA/2 mice.

METHODS

DBA/2J, DBA/2J(pe) (pearl), and C57BL/6 mice from 3 to 12 months of age were used. IOP was measured with a rebound tonometer, and optic nerve (ON) damage was determined by grading of ON sections. Retinal RNA was subjected to microarray analysis. Comparisons explored the effects of cumulative IOP exposure (cIOPx) as well as ON damage (ONd) in the DBA/2J animals compared with that in the C57BL/6 and pearl mice. RT-PCR was performed to confirm some of the genes and bioinformatic analysis to identify affected gene networks.

RESULTS

Microarrays revealed that an increasing number of genes were up- or downregulated in 9- and 12-month DBA/2J mice with various degrees of ONd. A smaller number of genes were expressed differentially between eyes with different cIOPx at the same age, from 6 months on. Expression of 1385 and 1133 genes differed between DBA/2J animals and C57BL/6 or pearl mice, respectively, and some them were confirmed by RT-PCR. Bioinformatics analysis identified functional gene networks, including members of the complement system, that appeared to be related to cIOPx, ONd, or both.

CONCLUSIONS

Gene expression changes occur in retinas of DBA/2 mice with various amounts of cIOPx as well as ONd. Genes involved, code for proteins with diverse cellular functions and include among others the complement system. cIOPx and ONd affect common as well as unique gene sets.

Visual responses in mice lacking critical components of all known retinal phototransduction cascades

The mammalian visual system relies upon light detection by outer-retinal rod/cone photoreceptors and melanopsin-expressing retinal ganglion cells. Gnat1^−/−;Cnga3^−/−;Opn4^−/− mice lack critical elements of each of these photoreceptive mechanisms via targeted disruption of genes encoding rod α transducin (Gnat1); the cone-specific α3 cyclic nucleotide gated channel subunit (Cnga3); and melanopsin (Opn4). Although assumed blind, we show here that these mice retain sufficiently widespread retinal photoreception to drive a reproducible flash electroretinogram (ERG). The threshold sensitivity of this ERG is similar to that of cone-based responses, however it is lost under light adapted conditions. Its spectral efficiency is consistent with that of rod opsin, but not cone opsins or melanopsin, indicating that it originates with light absorption by the rod pigment. The TKO light response survives intravitreal injection of U73122 (a phospholipase C antagonist), but is inhibited by a missense mutation of cone α transducin (Gnat2^cpfl3), suggesting Gnat2-dependence. Visual responses in TKO mice extend beyond the retina to encompass the lateral margins of the lateral geniculate nucleus and components of the visual cortex. Our data thus suggest that a Gnat1-independent phototransduction mechanism downstream of rod opsin can support relatively widespread responses in the mammalian visual system. This anomalous rod opsin-based vision should be considered in experiments relying upon Gnat1 knockout to silence rod phototransduction.

A Thy1-CFP DBA/2J mouse line with cyan fluorescent protein expression in retinal ganglion cells

A DBA/2J (D2) transgenic mouse line with cyan fluorescent protein (CFP) reporter expression in ganglion cells was developed for the analysis of ganglion cells during progressive glaucoma. The Thy1-CFP D2 (CFP-D2) line was created by congenically breeding the D2 line, which develops pigmentary glaucoma, and the Thy1-CFP line, which expresses CFP in ganglion cells. Microsatellite marker analysis of CFP-D2 progeny verified the genetic inclusion of the D2 isa and ipd loci. Specific mutations within these loci lead to dysfunctional melanosomal proteins and glaucomatous phenotype in D2 mice. Polymerase chain reaction analysis confirmed the inclusion of the Thy1-CFP transgene. CFP-fluorescent ganglion cells, 6-20 microm in diameter, were distributed in all retinal regions, CFP processes were throughout the inner plexiform layer, and CFP-fluorescent axons were in the fiber layer and optic nerve head. Immunohistochemistry with antibodies to ganglion cell markers NF-L, NeuN, Brn3a, and SMI32 was used to confirm CFP expression in ganglion cells. Immunohistochemistry with antibodies to amacrine cell markers HPC-1 and ChAT was used to confirm weak CFP expression in cholinergic amacrine cells. CFP-D2 mice developed a glaucomatous phenotype, including iris disease, ganglion cell loss, attrition of the fiber layer, and elevated intraocular pressure. A CFP-D2 transgenic line with CFP-expressing ganglion cells was developed, which has (1) a predominantly D2 genetic background, (2) CFP-expressing ganglion cells, and (3) age-related progressive glaucoma. This line will be of value for experimental studies investigating ganglion cells and their axons in vivo and in vitro during the progressive development of glaucoma.

The effects of acute intraocular pressure elevation on rat retinal glutamate transport

PURPOSE

To investigate the relationship between intraocular pressure (IOP), retinal glutamate transport and retinal hypoxia during acute IOP elevations of varying magnitude.

METHODS

Female Dark Agouti rats were anaesthetized by ketamine/xylazine/acepromazine (10/5/0.5 mg/kg i.p.). The anterior chamber was cannulated with a 30-gauge needle attached to a saline reservoir. The target IOP (20-120 mmHg, in 10 mmHg increments) was obtained by adjusting the reservoir height. After 10 mins of IOP stabilization, 2 microl of the non-endogenous glutamate transporter substrate, D-aspartate, was injected into the vitreous (final concentration 50 microm), and the elevated IOP maintained for a further 60 mins (total duration of IOP elevation was 70 mins). Glutamate transporter function was assessed by the immunohistochemical localization of D-aspartate. Retinal sections were examined for histological integrity. The experiment was repeated substituting the D-aspartate with the cellular hypoxia marker, Hypoxyprobe-1.

RESULTS

Under control conditions, D-aspartate was preferentially taken up into the glial Müller cells by glutamate/aspartate transporter (GLAST). This function was maintained at pressures < or = 70 mmHg, whereafter perturbation of function was evidenced by decreased accumulation of D-aspartate by Müller cells. Failure of GLAST activity was coincident with the appearance of Hypoxyprobe-labelled cells in the inner retina and histological damage.

CONCLUSIONS

Glutamate transport does not appear to change linearly with increased IOP. A pressure threshold exists, above which Müller cell GLAST function is compromised. Moreover, ganglion cell glutamate uptake is only apparent at pressures above those that cause GLAST inhibition. The association between IOP, hypoxia, glutamate transporter dysfunction and subsequent retinal cell death may have important implications for the pathogenesis of IOP/ischaemia-related neuropathy and neuroprotective strategies.

The trick of the tail: protein-protein interactions of metabotropic glutamate receptors

It was initially believed that G-protein-coupled receptors, such as metabotropic glutamate receptors, could simply be described as individual proteins that are associated with intracellular signal cascades via G-proteins. This view is no longer tenable. Today we know that metabotropic glutamate receptors (mGluRs) can dimerize and bind to a variety of proteins in addition to trimeric G-proteins. These newly identified protein interactions led to the discovery of new regulatory mechanisms that are independent of and sometimes synergistic with the classical G-protein-coupled second messenger pathways. Notably, several of these mechanisms connect mGluR-mediated signaling to other receptor classes, thereby creating a network of different receptor types and associated signal cascades. The intracellular C-termini of mGluRs play a key role in the regulation of these networks, and various new protein interactions of these domains were described recently. Because mGluRs are involved in a variety of physiological and pathophysiological processes, some of the proteins interacting with this receptor class have potential as valuable pharmaceutical targets. This review will give a comprehensive overview of proteins interacting with mGluR C-termini, highlight new evolving regulatory mechanisms for glutamatergic signal transduction and discuss possibilities for future drug development.

Alterations of amino acids and glutamate transport in the DBA/2J mouse retina; possible clues to degeneration

BACKGROUND

The DBA/2J mouse spontaneously develops ocular hypertension and time-dependent progressive retinal ganglion cell (RGC) loss. This study examines changes in amino acid levels in the vitreous, and changes in the expression of retinal glutamate transporters and receptors that occur during the progression of this pathology.

METHODS

Retinas were obtained from DBA/2J mice at ages 3, 6 and 11 months. C57BL/6 mice were used as age-matched controls. Vitreal amino acid content was measured with HPLC. Western blotting and immunohistochemistry were performed using specific antibodies against the glutamate transporters (GLAST, GLT-1v, EAAC-1) and glutamate receptors, particularly NMDA (NR1, NR2A, NR2B) and AMPA (GluR1, GluR2/3, GluR4) receptors.

RESULTS

HPLC showed retinal concentrations of glutamate, glutamine, glycine, alanine, lysine, serine, and arginine to be significantly higher in DBA/2J mice at 11 months of age compared to age-matched controls. Western Blots revealed a moderate decrease of GLAST and GLT-1v expression in DBA/2J mice at 6 and 11 months as compared to age-matched controls while there was no change in EAAC1. Immunohistochemically, no changes in expression of NMDA and AMPA receptors were seen.

CONCLUSION

Alterations of amino acid content and enhanced glutamate neurotransmission might be involved in the pathogenesis of retinal neurodegeneration in the DBA/ 2J mouse model of ocular hypertension. Moreover, these mice provide an animal model for studying excitotoxic retinal damage.

Tax1-binding protein 1 is expressed in the retina and interacts with the GABA(C) receptor rho1 subunit

Macromolecular signalling complexes that link neurotransmitter receptors to functionally and structurally associated proteins play an important role in the regulation of neurotransmission. Thus the identification of proteins binding to neurotransmitter receptors describes molecular mechanisms of synaptic signal transduction. To identify interacting proteins of GABA(C) (where GABA is gamma-aminobutyric acid) receptors in the retina, we used antibodies specific for GABA(C) receptor rho1-3 subunits. Analysis of immunoprecipitated proteins by MALDI-TOF MS (matrix-assisted laser-desorption ionization-time-of-flight MS) identified the liver regeneration-related protein 2 that is identical with amino acids 253-813 of the Tax1BP1 (Tax1-binding protein 1). A C-terminal region of Tax1BP1 bound to an intracellular domain of the rho1 subunit, but not to other subunits of GABA(C), GABA(A) or glycine receptors. Confocal laser-scanning microscopy demonstrated co-localization of Tax1BP1 and rho1 in clusters at the cell membrane of transfected cells. Furthermore, Tax1BP1 and GABA(C) receptors were co-expressed in both synaptic layers of the retina, indicating that Tax1BP1 is a component of GABA(C) receptor-containing signal complexes.

Immunolocalization of metabotropic glutamate receptors 1 and 5 in the synaptic layers of the chicken retina

We have examined the distribution of metabotropic glutamate receptors (mGluRs) 1 and 5 in the adult chicken retina using preembedding immuno-electronmicroscopy. Immunoreactivity for mGluRs 1 and 5 was found in both the outer plexiform layer (OPL) and the inner plexiform layer (IPL). For mGluR1, OPL labeling was observed at cone pedicles and horizontal and bipolar cell processes. In the IPL, mGluR1 labeling could be found on bipolar cell terminals, as well as postsynaptic processes, including amacrine cell processes. Neither presynaptic nor postsynaptic elements were labeled at rod synapses. For mGluR5, OPL labeling was associated with cone pedicles as well as bipolar and horizontal cell processes. As for mGluR1, rod synapses were unlabeled. In the IPL, labeling for mGluR5 was found on bipolar cell terminals and amacrine cell processes. The presynaptic expression of these receptors in the OPL was confirmed at the light level by double-labeling experiments with SV2. The distributions of mGluRs 1 and 5 indicate that they have the potential to regulate function in both synaptic layers. Furthermore, the similar expression patterns for these two receptors indicate that they might be co-expressed and thus have the potential to interact functionally.

Assessment of neuroprotective effects of glutamate modulation on glaucoma-related retinal ganglion cell apoptosis in vivo

PURPOSE

To assess the neuroprotective effects of different glutamate modulation strategies, with a nonselective (MK801) and a selective (ifenprodil) NMDA receptor antagonist and a metabotropic glutamate receptor agonist (mGluR Group II, LY354740), in glaucoma-related in vivo rat models of retinal ganglion cell (RGC) apoptosis.

METHODS

RGC apoptosis was induced in Dark Agouti (DA) rats by staurosporine (SSP) treatment. Single agents MK801, ifenprodil, or LY354740, or MK801 and LY354740 combined, were administrated intravitreally at different doses. Eyes were imaged in vivo using a recently established technique and the results confirmed histologically. The most effective combined therapy regimen of MK801 and LY354740 was then assessed in a chronic ocular hypertension (OHT) rat model with application at 0, 1, and 2 weeks after OHT surgery and the effects assessed as described before.

RESULTS

All strategies of glutamate modulation reduced SSP-induced-RGC apoptosis compared with the control, in a dose-dependent manner: MK801 (R2= 0.8863), ifenprodil (R2= 0.4587), and LY354740 (R2= 0.9094), with EC50s of 0.074, 0.0138, and 19 nanomoles, respectively. The most effective combination dose of MK801 and LY354740 was 0.06 and 20 nanomoles (P < 0.05), respectively, and the optimal timing of the therapy was 0 weeks after OHT surgery (P < 0.05).

CONCLUSIONS

This novel SSP model was validated as a useful tool for screening neuroprotective strategies in vivo. Group II mGluR modulation may be a useful treatment for RGC death. Combination therapy optimized to limit neurotoxic effects of MK801 may be an effective neuroprotective approach in retinal degenerative disease. Furthermore, treatments that minimize secondary RGC degeneration may be most useful in glaucoma.

Optic nerve degeneration in the DBA/2NNia mouse: is the lamina cribrosa important in the development of glaucomatous optic neuropathy?

To study optic nerve (ON) degeneration in the DBA/2NNia (DBA) mouse, a species lacking a lamina cribrosa and a model for secondary angle-closure glaucoma, serial semi- and ultra-thin sectioning of the myelinated ON and of the ON head was performed and sections evaluated qualitatively and quantitatively by light and electron microscopy. Immunohistochemistry was performed using antibodies against collagen type I, III, VI, laminin, and connexin43. The major finding on the myelinated ON was a significant decrease in cross section area during ON degeneration which was paralleled by a loss of axons and an increase in microglia. The number of astrocytes and blood vessels did not change. The major findings on the ON papilla were that ON heads with only mild degeneration showed a pronounced focal degeneration around the central retinal artery. In more severely degenerated ON, newly formed bundles of collagen type VI were located between astrocyte processes within the ON head. In a species that has no lamina cribrosa, DBA mice can develop typical signs of glaucomatous optic neuropathy. The entrance of the central retinal vessels into the ONH seems to be a preferentially vulnerable region for axon loss in this mouse model. In addition, astrocytes in the ON head form extracellular material similar to that found in human glaucomatous eyes.

Assessment of glutamate loss from the ganglion cell layer of young DBA/2J mice with glaucoma

OBJECTIVE

To determine whether glutamate contents are decreased in the ganglion cell layer (GCL) of retinas of DBA/2J mice with glaucoma, compared with unaffected control mice.

SAMPLE POPULATION

20 eyes from DBA/2J mice (9-week-old mice [n = 8] and 4- [4], 6- [4], and 12-month-old [4] mice) and 17 eyes from control CD-1 (7) and C57/BL6 (10) mice of similar age.

PROCEDURE

After euthanasia, the eyes were rapidly dissected and fixed. Serial 0.5-microm sections were prepared from eyecups and stained with toluidine blue (to identify damaged cells) or immunogold (to localize glutamate). Microscopic images were captured digitally for comparison; immunostaining densities were assessed via special software.

RESULTS

In the GCL of control mice, few cells appeared damaged; large amounts of glutamate were detected in 83 +/- 8.3% of cells. In DBA/2J mice > or = 9 weeks of age, damaged neurons were observed in retinal sections; the level of glutamate immunoreactivity was high in a few cells near areas of damage (13 +/- 3.2%) and in many cells in less-damaged regions of the same sections (82 +/- 4.2%). Many neurons with low amounts of glutamate in damaged regions did not appear damaged histologically.

CONCLUSIONS AND CLINICAL RELEVANCE

In retinas of young DBA/2J mice, damaged and undamaged GCL cells had decreased levels of immunostaining for glutamate, compared with less-damaged adjacent regions or retinas from control mice. The loss of neuronal glutamate in damaged retinal regions suggests that glutamate is contributing to early retinal damage prior to changes in intraocular pressure.

Transgenic studies on the role of optineurin in the mouse eye

Mutations in the OPTN gene encoding for optineurin have been associated with primary open-angle glaucoma. The functional role(s) of optineurin in the normal and glaucomatous eye are unclear. As optineurin interferes with TNF-alpha mediated cell death in vitro, an involvement of optineurin in the regulatory pathways leading to apoptosis of retinal ganglion cells has been suggested. The goal of the present study was to study the molecular properties of optineurin and its capabilities to prevent apoptosis in vivo in the eyes of transgenic mice. The chicken betaB1-crystallin promoter was used to overexpress ectopic optineurin in the lenses of transgenic mice. The expression of transgenic mRNA was monitored by northern blot analysis. The localization of transgenic optineurin was investigated by one- and two-dimensional western blot analysis and by immunohistochemistry, and compared with that of endogenous optineurin. To assess effects of transgenic optineurin on apoptosis, betaB1-crystallin-OPTN mice were crossbred with betaB1-crystallin-TGFbeta1 mice that undergo substantial TGF-beta1-induced apoptotic cell death in the lens. Two independent betaB1-crystallin-OPTN transgenic lines were established, in which transgenic optineurin was expressed strictly lens-specific as assessed by Northern and Western blotting, and by immunohistochemistry. In contrast, endogenous optineurin was preferentially expressed in the retina, where retinal ganglion cells showed strong labeling. Immunostaining for endogenous optineurin in the anterior eye was considerably weaker than in the posterior eye and was seen in iris, ciliary epithelium, cells of corneal stroma and endothelium, and in the trabecular meshwork. Neither transgenic nor endogenous optineurin was found in the aqueous humor. Transgenic overexpression of optineurin did not have measurable effects on TGFbeta1-induced apoptosis in mixed betaB1-crystallin-OPTN/betaB1-crystallin-TGFbeta1 transgenic mice. Our results show that optineurin is a cytoplasmatic rather than a secretory protein that is preferentially expressed in retinal ganglion cells, and argue against a major role of optineurin for the modulation of apoptosis in vivo.

Inhibition of metabotropic glutamate receptor signaling by the huntingtin-binding protein optineurin

Huntington disease is caused by a polyglutamine expansion in the huntingtin protein (Htt) and is associated with excitotoxic death of striatal neurons. Group I metabotropic glutamate receptors (mGluRs) that are coupled to inositol 1,4,5-triphosphate formation and the release of intracellular Ca(2+) stores play an important role in regulating neuronal function. We show here that mGluRs interact with the Htt-binding protein optineurin that is also linked to normal pressure open angled glaucoma and, when expressed in HEK 293 cells, optineurin functions to antagonize agonist-stimulated mGluR1a signaling. We find that Htt is co-precipitated with mGluR1a and that mutant Htt functions to facilitate optineurin-mediated attenuation of mGluR1a signaling. In striatal cell lines derived from Htt(Q111/Q111) mutant knock-in mice mGluR5-stimulated inositol phosphate formation is also severely impaired when compared with striatal cells derived from Htt(Q7/Q7) knock-in mice. In addition, we show that a missense single nucleotide polymorphism optineurin H486R variant previously identified to be associated with glaucoma is selectively impaired in mutant Htt binding. Although optineurin H486R retains the capacity to bind to mGluR1a, optineurin H486R-dependent attenuation of mGluR1a signaling is not enhanced by the expression of mutant Htt. Because G protein-coupled receptor kinase 2 (GRK2) protein expression is relatively low in striatal tissue, we propose that optineurin may substitute for GRK2 in the striatum to mediate mGluR desensitization. Taken together, these studies identify a novel mechanism for mGluR desensitization and an additional biochemical link between altered glutamate receptor signaling and Huntington disease.

Voltage-Sensitive and Ligand-Gated Channels in Differentiating Neural Stem-Like Cells Derived from the Nonhematopoietic Fraction of Human Umbilical Cord Blood

Fetal cells with the characteristics of neural stem cells (NSCs) can be derived from the nonhematopoietic fraction of human umbilical cord blood (HUCB), expanded as a nonimmortalized cell line (HUCB-NSC), and further differentiated into neuron-like cells (HUCB-NSCD); however, the functional and neuronal properties of these cells are poorly understood. To address this issue, we used whole-cell patch-clamp recordings, gene microarrays, and immunocytochemistry to identify voltage-gated channels and ligand-gated receptors on HUCB-NSCs and HUCB-NSCDs. Gene microarray analysis identified genes for voltage-dependent potassium and sodium channels and the neurotransmitter receptors acetylcholine (ACh), gamma-aminobutyric acid (GABA), glutamate, glycine, 5-hydroxytryptamine (5-HT), and dopamine (DA). Several of these genes (GABA-A, glycine and glutamate receptors, voltage-gated potassium channels, and voltage-gated sodium type XII alpha channels) were not expressed in the HUCB mono-nuclear fraction (HUCB-MC), which served as a starting cell population for HUCB-NSC. HUCB-NSCD acquired neuronal phenotypes and displayed an inward rectifying potassium current (Kir) and an outward rectifying potassium current (I(K+)). Kir was present on most HUCB-NSCs and HUCB-NSCDs, whereas I(K+) was present only on HUCB-NSCDs. Many HUCB-NSCDs were immunopositive for glutamate, glycine, nicotinic ACh, DA, 5-HT, and GABA receptors. Kainic acid (KA), a non-N-methyl-D-asparate (NMDA) glutamate-receptor agonist, induced an inward current in some HUCB-NSCDs. KA, glycine, DA, ACh, GABA, and 5-HT partially blocked Kir through their respective receptors. These results suggest that HUCB-NSCs differentiate toward neuron-like cells, with functional voltage- and ligand-gated channels identified in other neuronal systems.

Subunits of the epithelial sodium channel family are differentially expressed in the retina of mice with ocular hypertension

Glaucoma is a prevalent cause of blindness, resulting in the apoptotic death of retinal ganglion cells and optic nerve degeneration. The disease is often associated with elevated intraocular pressure, however, molecular mechanisms involved in ganglion cell death are poorly understood. To identify proteins contributing to this pathological process, we analysed the retinal gene expression of DBA/2J mice that develop an elevated intraocular pressure by the age of 6 months with subsequent ganglion cell loss. In this study, we identified subunits of the epithelial sodium channel (ENaC) family that are specifically expressed under elevated intraocular pressure. Using reverse transcriptase polymerase chain reaction we observed a significant increase of alpha-ENaC in the neuronal retina of DBA/2J mice when compared with control animals, while beta-ENaC and gamma-ENaC were not detectable in this tissue. Specific immune sera to ENaC subunits showed up-regulation of alpha-ENaC in synaptic and nuclear layers of the retina, and in the retinal pigment epithelium. Consistent with our polymerase chain reaction data, beta-ENaC was not detected by specific antibodies in the retina, while gamma-ENaC was only present in the retinal pigment epithelium under ocular hypertension. Finally, the increase of alpha-ENaC gene expression in the neuronal retina and the retinal pigment epithelium was not observed in other tissues of DBA/2J mice. Since the intraocular pressure is regulated by the transport of aqueous humour across epithelial structures of the eye that in turn is associated with ion flux, the specific up-regulation of ENaC proteins could serve as a protecting mechanism against elevated intraocular pressure.