Aquaporin-4 Water Channel Protein in the Rat Retina and Optic Nerve: Polarized Expression in Müller Cells and Fibrous Astrocytes

Abstract

The water permeability of cell membranes differs by orders of magnitude, and most of this variability reflects the differential expression of aquaporin water channels. We have recently found that the CNS contains a member of the aquaporin family, aquaporin-4 (AQP4). As a prerequisite for understanding the cellular handling of water during neuronal activity, we have investigated the cellular and subcellular expression of AQP4 in the retina and optic nerve where activity-dependent ion fluxes have been studied in detail. In situ hybridization with digoxigenin-labeled riboprobes and immunogold labeling by a sensitive postembedding procedure demonstrated that AQP4 and AQP4 mRNA were restricted to glial cells, including Mü ller cells in the retina and fibrous astrocytes in the optic nerve. A quantitative immunogold analysis of the Mü ller cells showed that these cells exhibited three distinct membrane compartments with regard to AQP4 expression. End feet membranes (facing the vitreous body or blood vessels) were 10-15 times more intensely labeled than non-end feet membranes, whereas microvilli were devoid of AQP4. These data suggest that Mü ller cells play a prominent role in the water handling in the retina and that they direct osmotically driven water flux to the vitreous body and vessels rather than to the subretinal space. Fibrous astrocytes in the optic nerve similarly displayed a differential compartmentation of AQP4. The highest expression of AQP4 occurred in end feet membranes, whereas the membrane domain facing the nodal axolemma was associated with a lower level of immunoreactivity than the rest of the membrane. This arrangement may allow transcellular water redistribution to occur without inducing inappropriate volume changes in the perinodal extracellular space.

Functions of aquaporins in the eye

“…Our lab first localized AQP1 in ciliary epithelium (Hasegawa et al, 1993), and later AQP3 in conjunctiva and AQP4 in ciliary epithelium and retina (Frigeri et al, 1995). These findings were extended by several laboratories in different mammalian species, including humans, and several groups found AQP5 expression in corneal and lacrimal gland epithelia (Nielsen et al, 1993;Hasegawa et al, 1994;Stamer et al, 1994;Patil et al, 1997;Ishida et al, 1997;Hamann et al, 1998;Funaki et al, 1998;Nagelhus et al, 1998;Kenney et al, 2004;Song et al, 2004;Sasaki et al, 2007). In addition to the AQP expression pattern shown in Fig.…”

Organization of Ionotropic Glutamate Receptors at Dendrodendritic Synapses in the Rat Olfactory Bulb

“…Adult (3 months old) Wistar rats were deeply anesthetized by an intraperitoneal injection of sodium pentobarbital (50 mg / kg) and perf used through the left ventricle with 2% ice-cold dextran (molecular weight 70,000) in 0.1 M sodium phosphate buffer (PB), pH 7.4, for 15 sec, and then by 4% formaldehyde (freshly depolymerized from paraformaldehyde) in 0.2 M acetate buffer, pH 6.0, followed by 4% formaldehyde in 0.2 M sodium carbonate buffer, pH 10.5, room temperature (50 ml /min for 20 min). This "pH-shift" protocol has been found previously to provide optimal detection of the antigenic epitopes combined with good ultrastructural preservation (Nagelhus et al, 1998). Specimens from the main olfactory bulb were rinsed in PB with 4% glucose (4°C, overnight), cryoprotected in increasing concentrations of glycerol (10,20,and 30% in PB), and rapidly frozen in liquid propane in a cryofixation unit (K F80; Reichert, Vienna, Austria).…”

Radial migration of developing microglial cells in quail retina: A confocal microscopy study

“…This apparent contradiction can be readily explained by the functional polarization of Mü ller cells, with one domain located at the vitrealmost part of the retina (endfeet) and another extending over the rest of the cell. These domains express distinct molecules (Nagelhus et al, 1998) and exert different influences on retinal cells (Bauch et al, 1998;Steinbach and Schlosshauer, 2000). Therefore, it is possible that the vitreal domain does not stimulate microglial ramification, whereas the nonvitreal domain does.…”