Ординатура / Офтальмология / Английские материалы / The Retinal Muller Cell Structure and Function_Sarthy, Ripps_2001

lacking, it is generally assumed that the intracellular retinoid-binding proteins are involved in cytoplasmic transport and routing of retinoids, but they may serve also as substrate carriers for enzymatic reactions (Saari and Bredberg, 1982; Ong et al., 1994; Napoli, 1991).

More definitive clues to the functions of retinoid-binding proteins have come from studies that have examined the phenotypes of knockout mice. In the case of CRBP I, the protein appears to be indispensable for efficient retinyl ester synthesis and storage, and its absence results in 6-fold faster turnover of retinol (Ghyselinck et al., 1999). The mice appear normal, which could mean CRBP is not involved in regulating retinoic acid synthesis. The RBP-knockout mice have a markedly impaired ERG during the first few months of life but develop a normal response when fed a vitamin A-suffi- cient diet (Quandro et al., 1999). They are otherwise normal in appearance. Liver retinyl ester stores accumulate but cannot be mobilized. Thus, the animals are dependent on a regular vitamin A intake. The results suggest that an alternate pathway of delivery of retinol to tissues (including RPE) exists but it is inefficient in utero.

In CRALBP-knockout mice, visual pigment synthesis is delayed by about 15-fold (J. Saari, personal communication). HPLC analysis shows that retinyl esters accumulate during the delay, suggesting that the delay occurs at the isomerohydrolase step. Finally, the kinetics of visual pigment renewal

Figure 3.21. CRALBP is localized in Müller cells. Antibodies to CRALBP stain radial fibers of Müller cells in bovine (A) and monkey (B) retinas. P, Pigment epithelium; m, Müller cell; V, vitreous; C, choroid (Bunt-Milam and Saari, 1983). (Copyright 1983 The Rockefeller University Press, reprinted with permission.)

well as for 11-cis retinol (Saari et al., 1978,1982). Since it had been generally assumed that Müller cells are not involved in any aspect of vitamin A metabolism associated with the visual cycle, the observation was totally unexpected.

Subsequent immunocytochemical studies showed that CRBP, which binds all-trans retinol to form a substrate for esterification, is also localized to Müller cells (Bok et al., 1984; Eisenfeld et al., 1985; De Leeuw et al., 1990). Thus, binding proteins for two of the key retinoids that participate in the visual cycle are found in Müller cells of all species examined (Fig. 3.22). Even more intriguing, two additional retinoid-binding proteins are found in Müller cells. In the primate retina, CRABP has been localized to Müller cells. In addition, a seven-transmembrane, microsomal protein with all-trans retinaldehyde-binding activity has been localized to Müller cells, where it probably acts as a substrate carrier in the oxidation of retinaldehyde to retinoic acid (Pandey et al., 1994).

3.5.2.Müller Cells and the Visual Cycle

Although retinoid-binding proteins have been found in Müller cells, no studies, to our knowledge, have directly addressed the question of

Figure 3.23. Recovery of sensitivity after bleaching in salamander cone photoreceptors. Solid symbols represent thresholds for three cones, before and after bleaching and during recovery with 11-cis-retinol with IRBP as carrier. Open symbols represent controls with bleached cones exposed to IRBP without retinoid (Jones et al., 1989). (Copyright 1989 G. J. Jones, reprinted with permission.)

Interactions between neurons and glial cells occur throughout the nervous system during development, in synapse formation, and in many other functions under both normal and pathological conditions (Barres, 1991; Murphy et al., 1993; Nedergaard, 1994; Pfrieger and Barres, 1996; McCall et al., 1996; Pfrieger and Barres, 1997). Although there is no histological or physiological evidence of gap junctional pathways for direct communication between neurons and glial cells, there is a surprising array of intercellular interactions mediated by ions, metabolites, and various signaling molecules. This chapter describes some of the neuron–glia interactions mediated by neurotransmitters, calcium, and nitric oxide.

In addition to their conventional roles at nerve and muscle synapses, neurotransmitters provide a chemical pathway linking the activities of neurons and glial cells. In the vertebrate retina and throughout the CNS, neurotransmitter amino acids constitute one of the principal classes of molecule mediating intercellular communication. Because precise regulation of the magnitude and duration of transmitter activity is required, there is a need to inactivate or remove the transmitter in an efficient and timely manner from synaptic sites. Unlike the mechanism whereby acetylcholine is inactivated, degradative enzymes for amino acids and other neuroactive compounds are not present extracellularly, and termination of transmitter action depends largely on reuptake into neurons and/or transport into glial cells (cf. Kanner, 1994). After uptake, the transmitter may be degraded, transported into synaptic vesicles for reuse (Kanner and Sharon, 1978; Kish et al., 1989), or converted to a precursor for subsequent transmitter synthesis (cf. Paulsen and Fonnum, 1990). The importance of membrane transporters for the viability and functional integrity of neural tissue cannot be overemphasized (cf. Müller et al., 1998); even cholinergic neurons re quire an active transport process to recapture choline following acetylcholine hydrolysis (Kuhar and Murrin, 1978).

Glial cells possess high-affinity, carrier-mediated transport systems, and

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it is now firmly established that glia are major players in the active removal of neuroactive substances (Henn and Hamberger, 1971; Henn et al., 1974; Erecinska, 1987; Hansson and Rönnbäck, 1991). Furthermore, their biochemical properties suggest that glial cells may be involved in transmitter degradation and recycling, and the electrophysiological properties of their transporters indicate that they are able to discharge neurotransmitter into the extracellular space (cf. Nicholls and Attwell, 1990; Martin, 1992). Indeed, the glutamate released from glial cells has been shown to provide a signaling pathway by which neuronal NMDA receptors can be activated (Parpura et al. 1994, 1995). The transport systems of Müller cells exhibit many of the same features as other forms of glia, and it is likely these cells are engaged in similar activities in the retina (cf. Linser et al., 1984; Sarthy, 1983; Brew and Attwell, 1987; Szatkowski et al., 1990).

Glial cell membranes also contain voltage-gated (Chapter 5) and ligand-gated channels that behave very much like their neuronal counterparts (MacVicar et al., 1989; Wyllie et al., 1991; Clark and Mobbs, 1992). Here, too, the Muller cell has proven a good model for study. The findings have provided new insights into the functional and pharmacological properties of glial receptors and amino acid transporters (Brew and Attwell, 1987; Nicholls andAttwell, 1990; Malchowetal., 1989; Qian et al., 1993,1996; Biedermann et al., 1995; Billups et al., 1996). Although the retina uses a multitude of neurotransmitters (cf. Adler, 1983; Pourcho et al., 1984; Biedermann et al., 1995), this chapter will focus primarily on neuron–glia interactions mediated by GABA and glutamate, the main inhibitory and excitatory amino acids, respectively, in retina and CNS. In addition, there is clear evidence that fluctuations in intracellular calcium, induced by activation of transmitter-gated ion channels, provide an important mechanism for neuronglia signaling. Lastly, the observation that nitric oxide (NO) can serve as an intercellular messenger in neural tissues suggests yet another pathway for communication between neurons and glial cells (Garthwaite, 1991). Both cell types have the capacity to make NO, but whether glial cell NO affects neuronal function—and vice versa—is an issue that remains difficult to resolve. However, the emergence of NO as a major signaling molecule warrants consideration when discussing possible pathways for neuron–glia interactions.

4.1. MEMBRANE TRANSPORT

The rapid removal of amino acid transmitters is accomplished by transport proteins located in the plasma membranes of neurons and glial cells. The active transport process is typically Na+-dependent, and driven by the

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In teleost, avian, and amphibian retinas, 3H-labeled GABA appears to accumulate exclusively in neurons of the inner nuclear and ganglion cell layers (Lam and Steinman, 1971; Marshall and Voaden, 1974a; Voaden et al., 1974; Pourcho et al., 1984). On the other hand, 3H-GABA is taken up primarily by the Müller cells in elasmobranch (Lam, 1975; Ripps and Witkovsky, 1985) and most mammalian retinas (Fig. 4.1). However, in rabbit and

Figure 4.1. Light microscope autoradiographs of 3H-GABA uptake in the retinas of several vertebrate species. Activity appears confined to the Müller cell fibers in rat (B), guinea pig (C), and skate (D) retinas. In rabbit (A), amacrine cells as well as Müller cells are labeled (Marshall and Voaden, 1974b, 1975; Ripps and Witkovsky 1985). ([B] copyright 1974Academic Press, Inc., reprinted with permission. [A], [C] copyright 1975 Elsevier Science, [D] copyright 1985 Elsevier Science, reprinted with permission.)