Neurotoxic effects of L-alpha-aminoadipic acid on the carp retina: a long term observation

Astroglia-Microglia Cross Talk during Neurodegeneration in the Rat Hippocampus

Brain injury triggers a progressive inflammatory response supported by a dynamic astroglia-microglia interplay. We investigated the progressive chronic features of the astroglia-microglia cross talk in the perspective of neuronal effects in a rat model of hippocampal excitotoxic injury. N-Methyl-D-aspartate (NMDA) injection triggered a process characterized within 38 days by atrophy, neuronal loss, and fast astroglia-mediated S100B increase. Microglia reaction varied with the lesion progression. It presented a peak of tumor necrosis factor-α (TNF-α) secretion at one day after the lesion, and a transient YM1 secretion within the first three days. Microglial glucocorticoid receptor expression increased up to day 5, before returning progressively to sham values. To further investigate the astroglia role in the microglia reaction, we performed concomitant transient astroglia ablation with L-α-aminoadipate and NMDA-induced lesion. We observed a striking maintenance of neuronal death associated with enhanced microglial reaction and proliferation, increased YM1 concentration, and decreased TNF-α secretion and glucocorticoid receptor expression. S100B reactivity only increased after astroglia recovery. Our results argue for an initial neuroprotective microglial reaction, with a direct astroglial control of the microglial cytotoxic response. We propose the recovery of the astroglia-microglia cross talk as a tissue priority conducted to ensure a proper cellular coordination that retails brain damage.

Pharmacological disruption of the outer limiting membrane leads to increased retinal integration of transplanted photoreceptor precursors

Retinal degeneration is the leading cause of untreatable blindness in the developed world. Cell transplantation strategies provide a novel therapeutic approach to repair the retina and restore sight. Previously, we have shown that photoreceptor precursor cells can integrate and form functional photoreceptors after transplantation into the subretinal space of the adult mouse. In a clinical setting, however, it is likely that far greater numbers of integrated photoreceptors would be required to restore visual function. We therefore sought to assess whether the outer limiting membrane (OLM), a natural barrier between the subretinal space and the outer nuclear layer (ONL), could be reversibly disrupted and if disruption of this barrier could lead to enhanced numbers of transplanted photoreceptors integrating into the ONL. Transient chemical disruption of the OLM was induced in adult mice using the glial toxin, dl-alpha-aminoadipic acid (AAA). Dissociated early post-natal neural retinal cells were transplanted via subretinal injection at various time-points after AAA administration. At 3 weeks post-injection, the number of integrated, differentiated photoreceptor cells was assessed and compared with those found in the PBS-treated contralateral eye. We demonstrate for the first time that the OLM can be reversibly disrupted in adult mice, using a specific dose of AAA administered by intravitreal injection. In this model, OLM disruption is maximal at 72 h, and recovers by 2 weeks. When combined with cell transplantation, disruption of the OLM leads to a significant increase in the number of photoreceptors integrated within the ONL compared with PBS-treated controls. This effect was only seen in animals in which AAA had been administered 72 h prior to transplantation, i.e. when precursor cells were delivered into the subretinal space at a time coincident with maximal OLM disruption. These findings suggest that the OLM presents a physical barrier to photoreceptor integration following transplantation into the subretinal space in the adult mouse. Reversible disruption of the OLM may provide a strategy for increasing cell integration in future therapeutic applications.

Heterogeneity between hippocampal and septal astroglia as a contributing factor to differential in vivo AMPA excitotoxicity

Astroglial participation in the regional differences of vulnerability to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-induced neurodegeneration was investigated in the rat hippocampus and medial septum using L-alpha-aminoadipate (alpha-AA) as a specific astroglial toxin. alpha-AA was microinjected in the hippocampus and the medial septum and a time-course study was carried out between 2 hr and 3 days. When compared to controls, microinjection of alpha-AA in the hippocampus induced within 3 days a reversible loss of glial fibrillary acidic protein (GFAP) immunostaining and a microglial reaction without any neuronal loss, whereas in the medial septum it caused no effects on astroglial, microglial, or neuronal populations. Differences in hippocampus and medial septum vulnerability were also evidenced when alpha-AA was co-injected with AMPA and neurodegeneration was assessed in terms of neuronal loss, glial reactions, calcification, and atrophy of the area. In the hippocampus, alpha-AA increased AMPA excitotoxicity with marked disorganization of all hippocampal subfields, increased neuronal loss, a more important astroglial reaction, a larger area of microgliosis, and a greater abundance of calcium deposits. By contrast, in the medial septum alpha-AA did not modify any parameter of the AMPA-induced lesion. In conclusion, the presence of different astroglial populations in hippocampus and medial septum results in a different participation to AMPA excitotoxicity that may determine, at least in part, the specific regional vulnerability to neurodegeneration.

Determination of alpha-aminoadipic acid in brain, peripheral tissues, and body fluids using GC/MS with negative chemical ionization

alpha-Aminoadipic acid (alphaAA) is a structural homolog of the excitatory amino acid glutamate and a natural product of lysine metabolism in mammalian cells. Under experimental conditions, alphaAA can influence various elements of glutamatergic neurotransmission. Moreover, as a selective inhibitor of kynurenine aminotransferase II, alphaAA is capable of decreasing the levels of the neuroinhibitory metabolite kynurenic acid in the brain. We now describe the identification of this potential endogenous neuromodulator in tissues and body fluids by gas chromatography/mass spectrometry (GC/MS) analysis of its pentafluorobenzyl (PFB) derivative. alphaAA was recovered from the GC column with a retention time of approximately 7 min. Subsequent MS analysis using electron capture with negative ionization revealed two separate ions for alphaAA (m/z 520, approximately 45% and m/z 322, approximately 55%). Both of these ions were positively identified with two different GC methodologies. In the rat, alphaAA levels ranged from 5 to 30 microM in various brain areas and from 8 to 40 microM in peripheral organs, whereas serum and urine contained only 1-2 microM alphaAA. Levels in the human brain were 18.7+/-2.4 microM (cortex) and 18.0+/-1.7 microM (striatum) alphaAA (n=9 each), and the mouse forebrain contained 8.3+/-1.9 microM alphaAA (n=6). Neuronal depletion, caused in rats by an intrastriatal injection of NMDA (300 nmol/2.5 microl), did not alter the striatal content of alphaAA, indicating that brain alphaAA resides at least in part in glial cells. alphaAA may therefore function as a glia-derived modulator of excitatory neurotransmission.

Targeted disruption of Müller cell metabolism induces photoreceptor dysmorphogenesis

Within the retina, the Müller cells and photoreceptors are in close physical proximity and are metabolically coupled. It is unknown, however, whether Müller cells affect photoreceptor differentiation and outer segment membrane assembly. The objective of this study was to determine whether targeted disruption of Müller cell metabolism would induce photoreceptor dysmorphogenesis. Intact isolated Xenopus laevis embryonic eyes were cultured in medium with or without Müller cell-specific inhibitors (i.e., alpha-aminoadipic acid and fluorocitrate). To assess Müller cell injury, the gross retinal morphology was examined along with immunocytochemical assessment of Müller cell-specific protein expression patterns. The steady-state levels of opsin were quantified to determine whether the Müller cell inhibitors negatively affected photoreceptor protein synthesis. Müller and photoreceptor cell ultrastructure was scrutinized and the organization of the outer segment membranes was graded. In control retinas, there was no swelling of Müller cell cytoplasm. Glial fibrillary acidic protein (GFAP) was undetectable, whereas glutamine synthetase was abundant. The steady-state level of opsin was high and photoreceptors elaborated properly folded outer segments. Exposure to both Müller cell-specific inhibitors induced swelling of Müller cell endfeet, cytoplasmic paling and alterations of Müller cell-specific protein expression patterns. The steady-state level of opsin in retinas exposed to alpha-aminoadipic acid was unchanged compared with control eyes, whereas, in eyes exposed to fluorocitrate, opsin levels were slightly reduced. The most significant finding was that targeted disruption of Müller cell metabolism adversely affected photoreceptor outer segment membrane assembly, causing dysmorphogenesis of nascent outer segments. These results suggest that the termination signal(s) necessary for proper outer segment folding were disrupted by targeted inhibition of Müller cells and support the hypothesis that Müller cells interact with photoreceptors through mechanisms that may regulate, at least in part, the assembly of photoreceptor outer segment membranes.

Slow recovery of goldfish retinal ganglion cells' soma size during regeneration

The goldfish optic nerve regenerates after sectioning. Recently both short-term (30 days) and long-term (4 months) recovery of various goldfish behaviors were observed after optic nerve section. Using intracellular injection of Lucifer Yellow (LY) the morphology of regenerating ganglion cells in goldfish retina after optic nerve section over a 4 month period have been investigated. In normal retinas, most cells (96-98%) were 7-10 microm in soma diameter which increased with increasing distance from the optic disc. Only two or three short, thin processes could be traced with LY. The remaining cells (2-4%) were 13-16 microm in soma diameter and all of the long dendritic trees could be traced with LY. The most conspicuous morphological change observed was cellular hypertrophy, which occurred for 20-90 days after axotomy. Neuronal processes were also hypertrophic in this period. The percentage increase in hypertrophy of the central ganglion cells tended to be slightly higher compared to cells from other regions. These morphological changes peaked at 60 days after axotomy and fully disappeared by 120 days after axotomy. The slow recovery of ganglion cells' soma size may reflect the slow return to the normal number of optic axon terminals in the tectum during regeneration.

Fast and slow recovery phases of goldfish behavior after transection of the optic nerve revealed by a computer image processing system

As the goldfish is a common experimental animal for vision research, including psychophysical behavior, it is very important to quantitatively score fish behavior. We have previously developed a computer image processing system which can acquire the positional coordinates of goldfish moving freely in an aquarium and determine turning directions (go straight, right or left turn). In the present study, an algorithm to determine tilting angles of moving goldfish was constructed. We also made histograms for quantifying the interaction between pairs of goldfish (two-point distance). By using these histograms, we estimated the time-course of behavioral regeneration after optic nerve transection in goldfish. Control goldfish showed an equal percentage of right or left turns and maintained an upright position in a dorsoventral axis. When the optic nerve of a goldfish was unilaterally sectioned, the goldfish showed predominant turning and slight tilting toward the intact eye. The abnormal turning and tilting behaviors lasted for 10-14 days and then gradually decreased, returning to control behaviors by one month after the unilateral transection. When the optic nerve of a single goldfish was bilaterally sectioned, it did not show any preferential turning and tilting behavior, which is similar to what was observed in control goldfish. However, the trace maps showed that, after bilateral sectioning, fish preferred to cross the center of the tank, which was unlike control fish. In control pairs, one goldfish chased the other with a fixed small range of two-point distances. However, in pairs of goldfish with bilateral transection of the optic nerve, the blind goldfish behaved independently of each other, with a long two-point distance. The long two-point distance of the blind goldfish lasted for at least two months and then slowly returned to control two-point distance by four months after bilateral transection. Such fast and slow recovery in goldfish behaviors evoked after unilateral and bilateral transection of the optic nerve is discussed with respect to reconnection of regenerating optic nerves in the fish central nervous system. This computer image processing system is a useful tool with which we can quickly and easily quantify fish behavior.

Characterization of L-alpha-aminoadipic acid transport in cultured rat astrocytes

The mechanism of the selective gliotoxicity of L-alpha-aminoadipate (L-alpha AA) is thought to involve its entry into glia as a substrate for glutamate transporters or, alternatively, its ability to inhibit glial glutamate transport. To clarify the properties of L-alpha AA as a transport substrate, we explored the ionic dependence, kinetics and pharmacology of L-[3H] alpha AA uptake in rat cortical astrocytes. We observed two components of saturable L-alpha AA uptake, one Na(+)-dependent and the other Na(+)-independent. These components exhibited the characteristics of system X-AG, the widespread family of Na(+)-cotransporters of aspartate and glutamate, and system x-c, a Cl(-)-dependent glutamate/cystine exchanger, respectively. The K(m) value of Na(+)-dependent L-alpha AA uptake was 629 +/- 42 microM, and Vmax was 62 +/- 4 nmol.min-1.mg-1 protein, which was more than twice the capacity of Na(+)-dependent glutamate uptake. The kinetic parameters of Na(+)-dependent L-alpha AA uptake (K(m) of 20 +/- 2 microM, Vmax of 1.7 +/- 0.4 nmol.min-1.mg-1 protein did not differ from the values for Na(+)-independent glutamate uptake, indicating that L-alpha AA and glutamate are equally good substrates for system x-c.

Selective ablation of astrocytes by intracerebral injections of alpha-aminoadipate

The efficacy and the specificity of the putative astrotoxin, alpha-aminoadipate, were examined in this study. The integrity of astrocytes was evaluated at several time points following a single injection of alpha-aminoadipate into amygdala of adult rats using immunohistochemistry. The density and the morphological appearance of neurons and the response of microglia were also examined. The injection of alpha-aminoadipate disrupted the astrocytic network in that region. There was a profound loss of glial fibrillary acidic protein-positive and S100 beta-positive astrocytes, normally present in the region, while vimentin immunohistochemistry revealed the presence of deformed cell processes, presumably astrocytic. The presence of reactive microglia at the injection site was suggestive of an active degenerative process, while the normal neuronal density and appearance, as compared to controls, suggested that the damage was confined to astrocytes. The confirmed effectiveness and cellular specificity of alpha-aminoadipate in vivo makes it a potentially important experimental tool for attempting to decipher the functional significance of astrocytes.

Müller glia stabilizes cell columns during retinal development: lateral cell migration but not neuropil growth is inhibited in mixed chick-quail retinospheroids

Radial columnar organization of cell clones is a characteristic feature of vertebrate retinae that is structurally not understood. Here we provide in vitro evidence that Müller glia processes stabilize cells within columns. Dissociated embryonic chick retinal plus pigmented cells regenerate in vitro into fully laminated stratospheroids. After reaggregating chick and quail cells, quail-derived spheroid areas are detected as isolated sectors, as shown by a quail-specific antibody. Each sector contains one or multiple cell columns. The radial borders separating chick and quail sectors are fully congruent with the extension of 3A7-labelled Müller glia processes. While cell somata do not show any lateral interspecies mixing, quail-derived neuropil extends within the inner plexiform areas far into chick sectors. After selective damage of Müller cells by the gliotoxin DL-alpha-aminoadipic acid, the columnar organization is destabilized, as evidenced by a decrease in vimentin expression and by the migration of individual neurons out of their cell column. These data demonstrate that Müller cells actively stabilize cells within their columns, while neuritic growth is not hindered.

Effects of Müller cell disruption on mouse photoreceptor cell development

Müller cells have been proposed to play an important role in photoreceptor cell development during the final stages of retinal maturation. The effect of disrupting Müller cells during mouse retinal development was investigated using the specific glial cell toxin, DL-alpha-aminoadipic acid (AAA). By giving multiple systemic injections over several days, impairment of Müller cell function was maintained during the period of photoreceptor migration and differentiation. Following three consecutive days of AAA treatment [commencing on post-natal (P) day 3, 5, 7 or 9, and examined at P8-P14], clumps of photoreceptor nuclei were displaced through the inner segments, lying immediately beneath the retinal pigment epithelium (RPE). Apart from the scalloped appearance of the outer retina, the overall lamination pattern of the retina was relatively well preserved. Even when AAA treatment commenced as early as P3, several days prior to the formation of the outer nuclear layer, the majority of photoreceptors migrated to their correct position and formed inner and outer segments. Therefore, the signals for photoreceptor migration are either provided by the Müller cells prior to P3, or, alternatively, are derived from different intrinsic or extrinsic cues. Disruption of Müller cell function was evidenced by decreased glutamine synthetase activity as well as by increased glial fibrillary acidic protein (GFAP) and decreased cellular retinaldehyde-binding protein (CRALBP) immunoreactivity. Immunocytochemistry with an antibody to CD44, which labels the microvilli of Müller cells at the outer limiting membrane, coupled with electron microscopic analysis, demonstrated that the zonulae adherentes between Müller cells and photoreceptors were either irregular or absent in areas adjacent to displaced clumps of photoreceptors. Thus AAA treatment of early post-natal mice results in localized disruption of the contacts between Müller cells and photoreceptors. These pathologic changes persist into adulthood since at P28, while short stretches of photoreceptors appeared relatively normal with fully developed outer segments, periodic clumps of displaced photoreceptor nuclei were still present adjacent to the RPE. In conclusion, Müller cell processes at the outer limiting membrane appear to play a critical role in providing a barrier to aberrant photoreceptor migration into the subretinal space.

Cystine/glutamate antiporter expression in retinal Müller glial cells: implications for DL-alpha-aminoadipate toxicity

A cytotoxicity of glutamate or related amino acids (10 mM) mediated by a cystine/glutamate antiporter (system Xc) has recently been demonstrated in N18 neuroblastoma-rat retina hybrid (N18RE105) cells and C6 glioma cells. The antiporter usually transports glutamate outside and cystine inside, thereby maintaining cellular concentrations of glutathione. High concentrations of glutamate inhibit cystine uptake and lead to depletion of cellular levels of glutathione. Among related amino acids, DL-alpha-aminoadipic acid (DL-alpha-AAA), which is well known as a selective gliotoxin in the retina, is also toxic to these cells. However, this does not explain why DL-alpha-AAA acts gliospecifically on the retina. To answer this question we first examined the effects of DL-alpha-AAA on the [35S]cystine uptake with parental N18 neuroblastoma cells and rat retina of the hybrid cells. DL-alpha-AAA showed a competitive inhibition of [35S]cystine uptake in the rat retina but not in the N18 cells. Such a competitive inhibition of cystine uptake by DL-alpha-AAA could also be seen in the carp retina. The cystine uptake with carp retina was mainly Na(+)-independent and Cl(-)-dependent as already described as a characteristic ion dependency of the Xc antiporter. We next examined the effects of exogenous cystine on the glutamate release from the retina. Cystine (1 mM) actually induced a glutamate release approximately twice that of the control. Furthermore, the glutamate release induced by cystine was also Na(+)-independent and Cl(-)-dependent, and was blocked by DL-alpha-AAA. An autoradiogram of [35S]cystine uptake in the carp retina showed typical radial glial Müller cells. A large incorporation of [35S]cystine into retinal glutathione fraction was detected by a high pressure liquid chromatography method during a 1-4-h incubation. A significant or large decrease of retinal levels of glutathione was observed one day ater an intravitreal injection of 8 mumol DL-alpha-AAA or L-alpha-AAA, respectively. Buthionine sulfoximine (2.5 mumol), a specific inhibitor of glutathione synthesis, induced a large decrease of retinal levels of glutathione and a loss of electroretinographic b-wave 20-30 h after treatment. Taken together, our present data with rat and carp retinas strongly indicate that the expression of cystine/glutamate antiporter is enriched in the retina, particularly in the glial Müller cells which have a rapid turnover pool for glutathione. The gliotoxin DL-alpha-AAA inhibits cystine uptake through this antiporter on the glial cells and elicits reduction of cellular levels of glutathione.(ABSTRACT TRUNCATED AT 400 WORDS)

Gliotoxic effects of alpha-aminoadipic acid isomers on the carp retina: a long term observation

The glutamate analogue, alpha-aminoadipic acid was intravitreally administered in the D-, DL- and L-forms to carp (Cyprinus carpio) retina in vivo. To make a quantitative assessment of its gliotoxic action, the activity of glutamine synthetase, whose localization was confirmed in glial Müller cells by an immunohistochemical technique, was examined at various intervals over one month. Intravitreal injection of 8 mumol alpha-aminoadipic acids reduced the glutamine synthetase activity within 4 h and maximally by 24 h. The maximum reduction evoked by L-, DL- and D-forms was about 65, 45 and 28% in reduction, and their minimum effective dose was 0.8, 1.5 and 2.0 mumol, respectively. At three to four days after alpha-aminoadipic acids injection, sodium dodecyl sulphate gel electrophoresis suggested that some retinal proteins including glutamine synthetase were significantly reduced, whilst others were increased. These biochemical changes were fully reversed one to two weeks after administration of the D- or DL-forms, but not until one month with the L-form. The electroretinographic b-wave, reflecting glial activity, was completely blocked by 8 mumol alpha-aminoadipic acids within 4 h. The electroretinographic b-wave was recovered first in the case of D- and then of DL-form at two to three weeks after injection, but only 50% recovery was seen in the case of L-form even two months later. A high dose of DL-alpha-aminoadipic acid (16 mumol) induced as long lasting a suppression in the glutamine synthetase and electroretinographic b-wave activities as 8 mumol L-alpha-aminoadipic acid. Therefore, the gliotoxic efficacy of L-alpha-aminoadipic acid at micromol orders was two-fold higher than that of DL-alpha-aminoadipic acid. Differences in the time-course of recovery of the suppression of glutamate synthetase and electroretinographic b-wave activities induced by alpha-aminoadipic acids are discussed in terms of its gliotoxicity.