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

Host-Cell Type Dependent Features of Recombinant Human Aquaporin-4 Orthogonal Arrays of Particles-New Insights for Structural and Functional Studies

The CNS plasma-membrane water channel aquaporin-4 (AQP4) is expressed as two major isoforms able to aggregate into supramolecular assemblies known as ‘orthogonal arrays of particles’ (OAPs). OAP subnanometric features are largely unknown mainly because a method for the expression, isolation, and crystallization of integral human OAPs has not been developed. Here, the human OAP-forming isoform M23-AQP4 was expressed in insect and mammalian cell lines and AQP4 and OAP features evaluated. Native size exclusion chromatography was employed to isolate and analyze authentically folded OAPs, and neuromyelitis optica (NMO)-specific sandwich ELISA was developed to test OAP-integrity. The results demonstrate that in insect cells most AQP4 remains intracellular and unfolded and that OAPs are largely disassembled after the detergent extraction step. In mammalian cells, AQP4 showed regular plasma membrane targeting and OAPs exhibited strong post-extraction stability. Starting from the mammalian cell expression system, we isolated authentically folded OAPs. Together these data suggest a new strategy for expressing and isolating integral recombinant human OAPs and providing new insights into the cell-type dependent OAP-assembly and post-extraction stability, potentially useful to design new approaches for structural and functional studies of OAP and for other plasma membrane proteins organized into supramolecular structures.

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.

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.

Müller glial cells in anuran retina

Whereas in the brain, the activity of the neurons is supported by several types of glial cells such as astrocytes, oligodendrocytes, and ependymal cells, the retina (evolving from the brain during ontogenesis) contains only one type of macroglial cell, the Müller (radial glial) cells, in most vertebrates including the anurans. These cells span the entire thickness of the tissue, and thereby contact and ensheath virtually every type of neuronal cell body and process. This intimate topographical relationship is reflected by a multitude of functional interactions between retinal neurons and Müller glial cells. Müller cells are the principal stores of retinal glycogen, and are thought to fuel retinal neurons with substrate (lactate/pyruvate) for their oxidative metabolism. Furthermore, Müller cells are involved in the control and homeostasis of many constituents of the extracellular space, such as potassium and perhaps other ions, signaling molecules, and of the extracellular pH. They also seem to play important roles in recycling mechanisms of photopigment molecules and neurotransmitter molecules such as glutamate and GABA. By containing the main retinal stores of glutathione, Müller cells may protect retinal neurons against free radicals. Moreover, Müller cells express receptors for many neuroactive substances, and may also release such substances to their neighbouring neurons. Thus, Müller cells exert many functions crucial for signal processing in the normal retina. Moreover, Müller cells change their properties in cases of retinal disease and injury, and may either support the survival of neuronal cells or accelerate the progress of neuronal degeneration.

Müller glial cells of the tree shrew retina

The tree shrew is one of the few mammalian species whose retinae are strongly cone dominated, which is usually the case in reptilian and avian retinae. Müller cells of the tree shrew (Tupaia belangeri) retina were studied by transmission electron microscopy of tissue sections and freeze-fracture replicas, by immunolabeling of the intermediate filament protein vimentin in radial paraffin sections and in whole retinae, as well as by intracellular dye injection in slices of retinae. In addition, enzymatically isolated cells were stained by Pappenheim's panoptic staining method. The cells showed an ultrastructure that is similar to other mammalian Müller cells with two exceptions: Due to the extensive lateral fins of cone inner segments, the apical microvilli of Müller cells are arranged in peculiar palisades, and the basket-like Müller cell sheaths around neuronal somata in both nuclear layers consist of unusual multilayered membrane lamellae. Unlike Müller cells in other mammalian species studied thus far, but similar to reptilian and avian Müller cells, those of tree shrews commonly have two or more vitread processes rather than one main trunk. Müller cell densities range between some 13,000 mm-2 in the periphery and about 20,000 mm-2 in the retinal center. Neuron:(Müller)glial cell ratios were estimated to be 7.9:1 in the center and 6.2:1 in the periphery. For each Müller cell, about 1.5 (cone) photoreceptor cells, four or five interneurons of the inner nuclear layer, and about one cell of the ganglion cell layer were counted. This is a much lower number of neurons per Müller cell than in most other mammals studied.

Astrocytes in the nonmyelinated lamina cribrosa of the rat are less polarized than in the optic nerve proper: a freeze-fracture study

Astrocytes in the lamina cribrosa of the adult rat optic nerve which is devoid of myelination were investigated by means of quantitative freeze-fracturing. The orthogonal arrays of particles (OAPs) were found to be less concentrated in the membranes of subpial endfeet when compared to the OAP concentration in endfeet of the optic nerve proper. This result corresponds to that found previously in the myelin deficient rat (Rohlmann et al: Glia 5:259, 1992) suggesting that lack of myelination generally correlates with altered astrocytes.