Stereospecific effects of the alpha-aminoadipic acid on the retina: a morphological and electrophysiological study

The use of time-lapse optical coherence tomography to image the effects of microapplied toxins on the retina

PURPOSE

We developed a novel technique for accelerated drug screening and retinotoxin characterization using time-lapse optical coherence tomography (OCT) and a drug microapplication device.

METHODS

Using an ex vivo rabbit eyecup preparation, we studied retinotoxin effects in real-time by microperfusing small retinal areas under a transparent fluoropolymer tube. Known retinotoxic agents were applied to the retina for 5-minute periods, while changes in retinal structure, thickness, and reflectance were monitored with OCT. The OCT images of two agents with dissimilar mechanisms, cyanide and kainic acid, were compared to their structural changes seen histologically.

RESULTS

We found the actions of retinotoxic agents tested could be classified broadly into two distinct types: (1) agents that induce neuronal depolarization, such as kainic acid, causing increases in OCT reflectivity or thickness of the inner plexiform and nuclear layers, and decreased reflectivity of the outer retina; and (2) agents that disrupt mitochondrial function, such as cyanide, causing outer retinal structural changes as evidenced by a reduction in the OCT reflectivity of the photoreceptor outer segment and pigment epithelium layers.

CONCLUSIONS

Retinotoxin-induced changes in retinal layer reflectivity and thickness under the microperfusion tube in OCT images closely matched the histological evidence of retinal injury. Time-lapse OCT imaging of the microperfused local retina has the potential to accelerate drug retinotoxicological screening and expand the use of OCT as an evaluation tool for preclinical animal testing.

A T-DNA insertion knockout of the bifunctional lysine-ketoglutarate reductase/saccharopine dehydrogenase gene elevates lysine levels in Arabidopsis seeds

Plants possess both anabolic and catabolic pathways for the essential amino acid lysine (Lys). However, although the biosynthetic pathway was clearly shown to regulate Lys accumulation in plants, the functional significance of Lys catabolism has not been experimentally elucidated. To address this issue, we have isolated an Arabidopsis knockout mutant with a T-DNA inserted into exon 13 of the gene encoding Lys ketoglutarate reductase/saccharopine dehydrogenase. This bifunctional enzyme controls the first two steps of Lys catabolism. The phenotype of the LKR/SDH knockout was indistinguishable from wild-type plants under normal growth conditions, suggesting that Lys catabolism is not an essential pathway under standard growth conditions. However, mature seeds of the knockout mutant over-accumulated Lys compared with wild-type plants. This report provides the first direct evidence for the functional significance of Lys catabolism in regulating Lys accumulation in seeds. Such a knockout mutant may also provide new perspectives to improve the level of the essential amino acid Lys in plant seeds.

Evaluation of methanol-induced retinotoxicity using oscillatory potential analysis

Methanol is an ocular toxicant which causes visual dysfunction often leading to blindness after acute exposure. While the manifestation of the toxicity has been widely studied, the mechanism by which the injury is produced is still uncertain. A major unanswered question pertains to the site of action, i.e. direct retinotoxicity versus primary optic nerve toxicity with secondary retinotoxicity. In the present study, the effect of methanol on the oscillatory potentials (OPs) of the electroretinogram (ERG) were evaluated in acutely treated folate sufficient (FS) and folate reduced (FR) Long-Evans rats. The OP amplitudes of the acutely dosed FR rats displayed non-selective decreases in all OP amplitudes and non-selective increases in all OP latencies at methanol doses ranging from 1.5 to 3.0 g/kg. Comparing decreases of op2 and ERG b-wave amplitudes with blood formate concentration demonstrates that the b-wave is more sensitive than op2 in a blood formate concentration range of 6-14 mM, suggesting that retinal ischemia is not involved in methanol-induced visual system toxicity.

Rapid increase of intracellular Ca2+ concentration caused by aminoadipic acid enantiomers in retinal Müller cells and neurons in vitro

The ability of the gliotoxic compounds D,L-, D- or L-2-aminoadipic acid (AAA) to increase selectively the intracellular concentration of free calcium ion ([Ca2+]i) was examined in Müller cells cultured with or without retinal neurons. The monitoring of [Ca2+]i following exposure to 0.06 to 6 mM AAA was performed by a microfluorometry using a fluorescent Ca2+ indicator, Fura-2 acetoxymethyl ester. A rapid increase of [Ca2+]i occurred in the Müller cells following exposure to a relatively low concentration of the L-isomer. This is compatible with the known strong gliotoxicity of this isomer. The D,L- and D-forms of AAA activated neurons at low concentrations and activated the Müller cells at higher concentrations. The D-isomer appears to act selectively on retinal neurons and may be an agonist of an excitatory amino acid receptor. These results indicate that the ability of AAA to elevate cytosolic [Ca2+]i depends on the stereospecificity of the AAA and on cell type.

Pharmacological modification of eye growth in normally reared and visually deprived chicks

Regular injections of the gliotoxins D-alpha-aminoadipic acid and L-alpha-aminoadipic acid were made into the vitreous chamber of the eyes of newly-hatched chicks reared either in a normal visual environment or under conditions of monocular occlusion. Striking differences in the growth rates of the eyes from the different groups were observed. Injection of D-alpha-aminoadipic acid, which causes the Müller glial cells to swell and diminishes the retinal OFF-response, resulted in a marked increase in the rate of axial growth of the eye compared with normal eyes. However, injection of D-alpha-aminoadipic acid into occluded eyes caused a lesser growth rate than was observed in occluded control eyes. By contrast, injection of L-alpha-aminoadipic acid, which also causes severe glial swelling and abolishes the retinal ON-response, caused reduced eye growth in non-occluded eyes. However, injection of L-alpha-aminoadipic acid into occluded eyes caused eye growth in excess of that recorded in the occluded controls. We concluded that the different growth rates observed is more likely a result of the disruption of the neural ON and OFF mechanisms than of the indisposition of the Müller cells.