Orthogonal arrays of particles in plasma membranes of Müller cells in the guinea pig retina

Comparative electrophysiology of retinal Müller glial cells-A survey on vertebrate species

Müller cells are the dominant macroglial cells in the retina of all vertebrates. They fulfill a variety of functions important for retinal physiology, among them spatial buffering of K⁺ ions and uptake of glutamate and other neurotransmitters. To this end, Müller cells express inwardly rectifying K⁺ channels and electrogenic glutamate transporters. Moreover, a lot of voltage- and ligand-gated ion channels, aquaporin water channels, and electrogenic transporters are expressed in Müller cells, some of them in a species-specific manner. For example, voltage-dependent Na⁺ channels are found exclusively in some but not all mammalian species. Whereas a lot of data exist from amphibians and mammals, the results from other vertebrates are sparse. It is the aim of this review to present a survey on Müller cell electrophysiology covering all classes of vertebrates. The focus is on functional studies, mainly performed using the whole-cell patch-clamp technique. However, data about the expression of membrane channels and transporters from immunohistochemistry are also included. Possible functional roles of membrane channels and transporters are discussed. Obviously, electrophysiological properties involved in the main functions of Müller cells developed early in vertebrate evolution. GLIA 2017;65:533-568.

Aquaporins: Multiple Roles in the Central Nervous System

Aquaporins (AQPs) represent a diverse family of membrane proteins found in prokaryotes and eukaryotes. The primary aquaporins expressed in the mammalian brain are AQP1, which is densely packed in choroid plexus cells lining the ventricles, and AQP4, which is abundant in astrocytes and concentrated especially in the end-feet structures that surround capillaries throughout the brain and are present in glia limitans structures, notably in osmosensory areas such the supraoptic nucleus. Water movement in brain tissues is carefully regulated from the micro- to macroscopic levels, with aquaporins serving key roles as multifunctional elements of complex signaling assemblies. Intriguing possibilities suggest links for AQP1 in Alzheimer's disease, AQP4 as a target for therapy in brain edema, and a possible contribution of AQP9 in Parkinson's disease. For all the aquaporins, new contributions to physiological functions are likely to continue to be discovered with ongoing work in this rapidly expanding field of research. NEUROSCIENTIST 13(5):470—485, 2007.

A novel look at astrocytes: aquaporins, ionic homeostasis, and the role of the microenvironment for regeneration in the CNS

Aquaporin-4 (AQP4) water channels are located at the basolateral membrane domain of many epithelial cells involved in ion transport and secretion. These epithelial cells separate fluid compartments by forming apical tight junctions. In the brain, AQP4 is located on astrocytes in a polarized distribution: At the border to blood vessels or the pial surface, its density is very high. During ontogeny and phylogeny, astroglial cells go through a stage of expressing tight junctions, separating fluid compartments differently than in adult mammals. In adult mammals, this barrier is formed by arachnoid, choroid plexus, and endothelial cells. The ontogenetic and phylogenetic barrier transition from glial to endothelial cells correlates with the regenerative capacity of neuronal structures: Glial cells forming tight junctions, and expressing no or unpolarized AQP4 are found in the fish optic nerve and the olfactory nerve in mammals both known for their regenerative ability. It is hypothesized that highly polarized AQP4 expression and the lack of tight junctions on astrocytes increase ionic homeostasis, thus improving neuronal performance possibly at the expense of restraining neurogenesis and regeneration.

Structure and functions of aquaporin-4-based orthogonal arrays of particles

Orthogonal arrays or assemblies of intramembranous particles (OAPs) are structures in the membrane of diverse cells which were initially discovered by means of the freeze-fracturing technique. This technique, developed in the 1960s, was important for the acceptance of the fluid mosaic model of the biological membrane. OAPs were first described in liver cells, and then in parietal cells of the stomach, and most importantly, in the astrocytes of the brain. Since the discovery of the structure of OAPs and the identification of OAPs as the morphological equivalent of the water channel protein aquaporin-4 (AQP4) in the 1990s, a plethora of morphological work on OAPs in different cells was published. Now, we feel a need to balance new and old data on OAPs and AQP4 to elucidate the interrelationship of both structures and molecules. In this review, the identity of OAPs as AQP4-based structures in a diversity of cells will be described. At the same time, arguments are offered that under pathological or experimental circumstances, AQP4 can also be expressed in a non-OAP form. Thus, we attempt to project classical work on OAPs onto the molecular biology of AQP4. In particular, astrocytes and glioma cells will play the major part in this review, not only due to our own work but also due to the fact that most studies on structure and function of AQP4 were done in the nervous system.

Immunogold evidence suggests that coupling of K+ siphoning and water transport in rat retinal Müller cells is mediated by a coenrichment of Kir4.1 and AQP4 in specific membrane domains

Postembedding immunogold labeling was used to examine the subcellular distribution of the inwardly rectifying K+ channel Kir4.1 in rat retinal Müller cells and to compare this with the distribution of the water channel aquaporin-4 (AQP4). The quantitative analysis suggested that both molecules are enriched in those plasma membrane domains that face the vitreous body and blood vessels. In addition, Kir4. 1, but not AQP4, was concentrated in the basal approximately 300-400 nm of the Müller cell microvilli. These data indicate that AQP4 may mediate the water flux known to be associated with K+ siphoning in the retina. By its highly differentiated distribution of AQP4, the Müller cell may be able to direct the water flux to select extracellular compartments while protecting others (the subretinal space) from inappropriate volume changes. The identification of specialized membrane domains with high Kir4.1 expression provides a morphological correlate for the heterogeneous K+ conductance along the Müller cell surface.

Direct immunogold labeling of aquaporin-4 in square arrays of astrocyte and ependymocyte plasma membranes in rat brain and spinal cord

Aquaporin (AQP) water channels are abundant in the brain and spinal cord, where AQP1 and AQP4 are believed to play major roles in water metabolism and osmoregulation. Immunocytochemical analysis of the brain recently revealed that AQP4 has a highly polarized distribution, with marked expression in astrocyte end-feet that surround capillaries and form the glia limitans; however, the structural organization of AQP4 has remained unknown. In freeze-fracture replicas, astrocyte end-feet contain abundant square arrays of intramembrane particles that parallel the distribution of AQP4. To determine whether astrocyte and ependymocyte square arrays contain AQP4, we employed immunogold labeling of SDS-washed freeze-fracture replicas and stereoscopic confirmation of tissue binding. Antibodies to AQP4 directly labeled approximately 33% of square arrays in astrocyte and ependymocyte plasma membranes in rat brain and spinal cord. Overall, 84% of labels were present beneath square arrays; 11% were beneath particle clusters that resembled square arrays that had been altered during fixation or cleaving; and 5% were beneath the much larger areas of glial plasma membrane that were devoid of square arrays. Based on this evidence that AQP4 is concentrated in glial square arrays, freeze-fracture methods may now provide biophysical insights regarding neuropathological states in which abnormal fluid shifts are accompanied by alterations in the aggregation state or the molecular architecture of square arrays.

Aquaporin-4 water channel protein in the rat retina and optic nerve: polarized expression in Müller cells and fibrous astrocytes

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 MHller cells in the retina and fibrous astrocytes in the optic nerve. A quantitative immunogold analysis of the MHller 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 MHller 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.

A novel junction-like membrane complex in the optic nerve astrocyte of the Japanese macaque with a possible relation to a potassium ion channel

BACKGROUND

A new type of junction-like membrane complex (JMC) was detected between adjacent astrocytes in the optic nerve of Japanese macaque (macaca fuscata). This membrane complex morphologically resembled a cell junction, but a possible role for potassium ion channels could not be denied based on freeze-fracture replica observation. We attempted to determine the chemical nature and function of the novel JMC.

METHODS

Using an electron microscope, we observed JMCs in the optic nerve astrocyte. In addition, we observed them using a freeze-fracture replica and immunohistochemistry with connexin 43, a gap junction specific protein. Furthermore, immunolocalization of an inwardly rectifying potassium ion channel, K(AB)-2 (Kir4.1), was studied with a confocal laser-scanning microscope, and an electron microscope using a newly developed pre-embedding method.

RESULTS

These JMCs were abundant around the blood vessel in the area just behind the lamina cribrosa. At JMCs the inner leaflet was thicker than the outer leaflet and electron-dense materials were packed in the intercellular space. Freeze-fracture replica observation revealed orthogonal arrays of particles, probably at the place of JMCs, that have been considered a potassium ion channel. No connexin 43 immunoreactivity was detected in JMCs, while K(AB)-2 was mostly localized on either side of the opposing cell membranes of JMC.

CONCLUSIONS

These JMCs do not seem to be a simple junction, but relate to a potassium ion channel. The area just behind the lamina cribrosa may be important in terms of conductance of the optic nerve impulse.

Functional properties of retinal Müller cells following transplantation to the anterior eye chamber

Two types of glial cells occur in the retina, Müller cells and astrocytes. These cells share several structural features such as extending endfeet onto blood vessels of the retina. Retinal vessels express a tight blood-retinal barrier which is comparable to the blood-brain barrier (BBB) of the CNS. While astrocytes have been implicated in the induction of the BBB, the role of Müller cells in the blood-retinal barrier is unknown. To determine if Müller cells are capable of influencing vascular permeability, we have prepared Müller cells that are free of astrocytes and transplanted them to a peripheral target, the anterior eye chamber. Müller cells were identified 2 weeks to 3 months after injection and were predominantly localized within the connective tissue of the ciliary body. The Müller cells occurred as dense clusters of cells closely associated with ciliary blood vessels. The ciliary vessels adjacent to Müller cells were freely permeable to circulating horseradish peroxidase (HRP), suggesting that Müller cells did not induce tight barrier properties from these leaky peripheral vessels. In contrast, cortical astrocytes injected into the anterior eye chamber preferentially formed a monolayer on the anterior surface of the iris, a region known to contain blood vessels that are impermeable to circulating tracers (e.g., Raviola, Exp Eye Res [Suppl] 25:27, 1977). Müller cells were rarely associated with the iris and the few cells that were present were located deep within the iris stroma rather than on the surface. The behaviour of guinea pig Müller cells transplanted to the anterior eye chamber contrasts sharply with that of cortical astrocytes in terms of: 1) the ocular compartment to which Müller cells migrate; 2) the tissue invasiveness of the cells; and 3) the degree of permeability of blood vessels adjacent to transplanted cells. The results of this study emphasize the functional distinctness of the two types of retinal glia and suggest that Müller cells from guinea pig retina may not be active in modifying the permeability properties of peripheral blood vessels, a function that has been suggested for astrocytes.

Müller (glial) cells in the retina of urodeles and anurans reveal different morphology by means of freeze-fracturing

Müller (glial) cells of the retina of various species of amphibia (urodeles and anurans) were investigated by means of the freeze-fracture technique. This was done because Müller cells in anamniotes were believed to differ from those in mammals in that they should lack the so-called orthogonal arrays of particles (OAP) which are a characteristic feature of Müller cells in mammalian retina. However, as we could demonstrate previously (Berg-von der Emde and Wolburg, Glia, 2 (1989) 458), fish retinal Müller cells also contain OAP in their membranes suggesting that OAP are a general marker of Müller cells in all vertebrates. As demonstrated in this study, Müller cells of urodeles (Batrachoseps attenuatus and Pleurodeles waltlii) are OAP-positive, whereas two anurans (Rana esculenta and Xenopus laevis) do not reveal any OAP in their Müller cell membranes. Under phylogenetic aspects, it appears very interesting that frogs are as yet the only vertebrate group that deviates from all other vertebrates in terms of Müller cell membrane morphology.

Orthogonal arrays of particles in astroglial cells: quantitative analysis of their density, size, and correlation with intramembranous particles

Astroglial cells were investigated by means of freeze-fracture in normal rat and mouse brain, cell culture and human gliomas. Membranes of these cells were quantitatively analyzed for their intramembranous particles (IMPs) and orthogonal arrays of particles (OAPs). Measurement of the size of OAPs and IMPs has permitted the search for a correlation between the 7-nm IMPs, which are distributed randomly in the membrane, and the subunits of OAPs (OAP-Su, also 7 nm in diameter). Using cultured astroglial cells treated with basic fibroblast growth factor (bFGF), arginine vasopressin, or sorbitol, good evidence for a relationship between the density of 7-nm IMPs and the size of OAPs can be demonstrated. These findings led to a hypothetical model of OAP modulation. A preliminary report has been published elsewhere (Neuhaus and Wolburg, 1989).

Developmental alterations in membrane organization of rat subpial astrocytes

Subpial astrocytic processes were examined in developing rats, mainly with complementary replicas, to see how orthogonal arrays of particles (OAs) are formed and become numerous in membranes covered by basal lamina. Only a few (4.2%) endfeet in the membranes contacting the basal lamina (subpial membranes) had acquired OAs by the 19-day foetal stage. The number of endfeet provided with OAs increased drastically in the prenatal period, continued to increase at birth (P0), and somewhat more slowly in the early postnatal period (P0-P3), reaching 100% at P10. There were neuronal processes as well abutting on the basal lamina at the pial surface but they were easy to distinguish from astrocytic endfeet because of their larger intramembrane particles (IMPs), which are sparsely distributed and in patch-like aggregations. The distribution density of OAs in differentiated astrocytic endfeet also increased very gradually with age until P0, a little faster in the early postnatal period, and drastically from P10 to adult. Ordinary globular IMPs increased in number with age and continued to increase in the lateral membrane where OAs were still very few, though less rapidly in the subpial membrane as OAs became numerous. With maturation, larger IMPs became conspicuous in the lateral membrane but not in the subpial, suggesting that larger IMPs were predominantly used to form OAs. We have proposed the idea that relatively large IMPs line up to form single linear arrays (SLs), appearing as grooves on the E face, and that occasionally some SLs line up in multiple rows [multiple linear arrays (MLs)] and that SLs or MLs fuse with one another to become rod-like strands, then divide into squares to become OAs. SLs and MLs appeared ontogenetically earlier than OAs, and continued to appear in membranes provided with OAs. In areas where membranes were bent, transition of these three structures was observable and the proportion of OAs increased with age. Further, in such areas, alignment of OAs was different according to membrane curvature: concentric in and around protrusions, perpendicular to the edge of invaginations. This unique association of OA alignment with membrane curvature suggests that OAs contribute to some membrane stability in the area covered by the basal lamina and provide the membrane with special resistance to bending.