Glutamine synthetase (GS) activity and spatial and temporal patterns of GS expression in the developing chick retina: Relationship with synaptogenesis in the outer plexiform layer

Abstract

The profile of glutamine synthetase (GS) activity in the neural retina of chicken embryos and adults was studied alongside the in vivo spatio-temporal patterns of generation and morphogenesis of Müller cell and of retinal synaptogenesis. The rise of GS activity during development is not related to Müller cell differentiation but to synaptogenesis in the outer plexiform layer (opl). GS expression was investigated by immunoreaction with GS-specific antiserum. Three spatial gradients of decreasing labeling were observed between embryonic (E)15 and E18, from central to peripheral retina, dorsal to ventral, and temporal to nasal, which are in spatio-temporal relationship with synaptogenesis in the opl. GS is localized in Müller cells and apparently also in a population of astrocyte-like glial cells, located in the ganglion cell layer throughout the retina. Precocious induction by hydrocortisone, in ovo, at E10, does not show the spatial pattern of GS immunoreactivity observed in control retinas at the time of natural induction (E15). We also show that dissociated (non-cultured) Müller cells of E18-20 retinas, to which only photoreceptors or photoreceptors and neurons remain joined, maintain an immunodetectable level of GS, while those in isolated state lose GS immunoreactivity rapidly. Our results suggest that the induction of GS expression might be mediated by Müller cell-neuron interactions at the opl and also perhaps at the outer nuclear layer (onl). An analysis of our results and those of previous authors suggests that the level of GS in differentiated Müller cells could be determined by conjoint cell interactions at the onl, opl, and inner plexiform layer.

Ability of retinal M�ller glial cells to protect neurons against excitotoxicity in vitro depends upon maturation and neuron-glial interactions

“…In our experiments Glu-induced increases in GS activity occurred only in mixed neuronal-glial cultures (young or mature), correlating with previous studies indicating that neuron-glial cell contacts are required in the regulation of GS (Vardimon et al, 1988;Mearow et al, 1990). Moreover, Germer et al (1997) demonstrated that GS expression in mammalian Mü ller cells was dependent on environmental factors, and recently Prada et al (1998) suggested that GS expression in the chick retina is mediated by Mü ller cell-neuron interactions. Immunolabelling of purified glia using anti-GS antibody did not change after Glu exposure, indicating that EAA by themselves do not upregulate glial GS expression.…”

Morphological characterization of pecteneal hyalocytes in the developing quail retina

“…Glutamine synthetase (GS) is a characteristic marker of glial Müller cells in avian retina (Prada et al 1998).…”

Onset of synaptogenesis in the plexiform layers of the chick retina: A transmission electron microscopic study

“…An array of different endogenously released neurotransmitters (ACh, dopamine/L-Dopa, glycine, GABA, glutamate) were reported to trigger and to modulate calcium waves, which occur in the chick retina from E7/8 on. GABAergic, glutamatergic, muscarinic, nicotinic acetylcholine, and AMPA receptors are present shortly before or during the period when spontaneous activity occurs, spread across the retinal layers, and from central to peripheral retina (Catsicas et al, 1998Hering and Kröger, 1996;Kubrusly et al, 2003;Pearson et al, 2002;Prada et al, 1998;Silveira dos Santos Bredariol and Hamassaki-Britto, 2001;Vailati et al, 2003;Yamashita and Fukuda, 1993).…”

“…Presynaptic proteins are present concurrent with the forming IPL and they spread following the spatial course of the maturating retina (Bergmann et al, 1999(Bergmann et al, , 2000Hering and Kröger, 1996). Glutamine synthetase activity suggests the onset of synaptogenesis in the OPL on E9 (Prada et al, 1998). Consequently, one may assume that synaptogenesis starts much earlier than presently acknowledged and that this process begins in the central retina.…”