Disturbance of plasmalemmal astrocytic assemblies in focal and selective cerebral ischemia

Dynamic regulation of aquaporin-4 water channels in neurological disorders

Aquaporin-4 water channels play a central role in brain water regulation in neurological disorders. Aquaporin-4 is abundantly expressed at the astroglial endfeet facing the cerebral vasculature and the pial membrane, and both its expression level and subcellular localization significantly influence brain water transport. However, measurements of aquaporin-4 levels in animal models of brain injury often report opposite trends of change at the injury core and the penumbra. Furthermore, aquaporin-4 channels play a beneficial role in brain water clearance in vasogenic edema, but a detrimental role in cytotoxic edema and exacerbate cell swelling. In light of current evidence, we still do not have a complete understanding of the role of aquaporin-4 in brain water transport. In this review, we propose that the regulatory mechanisms of aquaporin-4 at the transcriptional, translational, and post-translational levels jointly regulate water permeability in the short and long time scale after injury. Furthermore, in order to understand why aquaporin-4 channels play opposing roles in cytotoxic and vasogenic edema, we discuss experimental evidence on the dynamically changing osmotic gradients between blood, extracellular space, and the cytosol during the formation of cytotoxic and vasogenic edema. We conclude with an emerging picture of the distinct osmotic environments in cytotoxic and vasogenic edema, and propose that the directions of aquaporin-4-mediated water clearance in these two types of edema are distinct. The difference in water clearance pathways may provide an explanation for the conflicting observations of the roles of aquaporin-4 in edema resolution.

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.

Differential water permeability and regulation of three aquaporin 4 isoforms

Aquaporin 4 (AQP4) is expressed in the perivascular glial endfeet and is an important pathway for water during formation and resolution of brain edema. In this study, we examined the functional properties and relative unit water permeability of three functional isoforms of AQP4 expressed in the brain (M1, M23, Mz). The M23 isoform gave rise to square arrays when expressed in Xenopus laevis oocytes. The relative unit water permeability differed significantly between the isoforms in the order of M1 > Mz > M23. None of the three isoforms were permeable to small osmolytes nor were they affected by changes in external K(+) concentration. Upon protein kinase C (PKC) activation, oocytes expressing the three isoforms demonstrated rapid reduction of water permeability, which correlated with AQP4 internalization. The M23 isoform was more sensitive to PKC regulation than the longer isoforms and was internalized significantly faster. Our results suggest a specific role for square array formation.

New insights into water transport and edema in the central nervous system from phenotype analysis of aquaporin-4 null mice

Aquaporin-4 (AQP4) is the major water channel in the CNS. Its expression at fluid-tissue barriers (blood-brain and brain-cerebrospinal fluid barriers) throughout the brain and spinal cord suggests a role in water transport under normal and pathological conditions. Phenotype studies of transgenic mice lacking AQP4 have provided evidence for a role of AQP4 in cerebral water balance and neural signal transduction. Primary cultures of astrocytes from AQP4-null mice have greatly reduced osmotic water permeability compared with wild-type astrocytes, indicating that AQP4 is the principal water channel in these cells. AQP4-null mice have reduced brain swelling and improved neurological outcome following water intoxication and focal cerebral ischemia, establishing a role of AQP4 in the development of cytotoxic (cellular) cerebral edema. In contrast, brain swelling and clinical outcome are worse in AQP4-null mice in models of vasogenic (fluid leak) edema caused by freeze-injury and brain tumor, probably due to impaired AQP4-dependent brain water clearance. AQP4-null mice also have markedly reduced acoustic brainstem response potentials and significantly increased seizure threshold in response to chemical convulsants, implicating AQP4 in modulation of neural signal transduction. Pharmacological modulation of AQP4 function may thus provide a novel therapeutic strategy for the treatment of stroke, tumor-associated edema, epilepsy, traumatic brain injury, and other disorders of the CNS associated with altered brain water balance.

Correspondence of AQP4 expression and hypoxic-ischaemic brain oedema monitored by magnetic resonance imaging in the immature and juvenile rat

Whether the water channel protein AQP4 is involved in the very early cell swelling and brain oedema observed with cerebral hypoxia-ischaemia (HI) and whether this response depends on the maturity of brain were investigated by comparing regional changes in AQP4 protein expression and signal intensity on magnetic resonance (MR) images in immature and juvenile brains. Maps of T2 and the apparent diffusion coefficient (ADC) of water were acquired in 1- and 4-week-old rats at times prior to HI, within the last 5 min of HI and 1 h or 24 h afterwards. AQP4 expression assessed with Western blotting was not significantly reduced until 24 h post-HI irrespective of age. However, AQP4 immunostaining was decreased at the end of HI and at 1 h or 24 h after HI in the hemisphere ipsilateral to the occlusion with changes being similar in both age groups and coinciding well with regional reductions in ADC. IgG immunostaining to assess blood-brain barrier integrity and T2 were unchanged at early time points in 4-week old rats despite decreases in AQP4 immunostaining. Thus, at early time points there were decreases in AQP4 detected with immunostaining but not Western blotting methods. However, the good correlation between alterations in ADC and AQP4 immunostaining suggests that changes in the AQP4 are involved in some of the early changes in brain water distribution observed in hypoxia-ischemia, and supports the speculation that AQP4 is involved in the transport of water across the perivascular membranes into the vascular lumen.

Aquaporin-4 square array assembly: opposing actions of M1 and M23 isoforms

Osmotic homeostasis in the brain involves movement of water through aquaporin-4 (AQP4) membrane channels. Perivascular astrocyte end-feet contain distinctive orthogonal lattices (square arrays) assembled from 4- to 6-nm intramembrane particles (IMPs) corresponding to individual AQP4 tetramers. Two isoforms of AQP4 result from translation initiation at methionine residues M1 and M23, but no functional differences are known. In this study, Chinese hamster ovary cells were transfected with M1, M23, or M1+M23 isoforms, and AQP4 expression was confirmed by immunoblotting, immunocytochemistry, and immunogold labeling. Square array organization was examined by freeze-fracture electron microscopy. In astrocyte end-feet, >90% of 4- to 6-nm IMPs were found in square arrays, with 65% in arrays of 13-30 IMPs. In cells transfected with M23, 95% of 4- to 6-nm IMPs were in large assemblies (rafts), 85% of which contained >100 IMPs. However, in M1 cells, >95% of 4- to 6-nm IMPs were present as singlets, with <5% in incipient arrays of 2-12 IMPs. In M1+M23 cells, 4- to 6-nm IMPs were in arrays of intermediate sizes, resembling square arrays in astrocytes. Structural cross-bridges of 1 x 2 nm linked >90% of IMPs in M23 arrays ( approximately 1,000 cross-bridges per microm2) but were rarely seen in M1 cells. These studies show that M23 and M1 isoforms have opposing effects on intramembrane organization of AQP4: M23 forms large square arrays with abundant cross-bridges; M1 restricts square array assembly.

Decreased hemispheric Aquaporin-4 is linked to evolving brain edema following controlled cortical impact injury in rats

The cerebral Aquaporin-4 (AQP4) water channel is suggested to be involved in brain edema formation aggravated by reduced cerebral blood flow early after traumatic brain injury (TBI). Therefore, the temporal profile of brain edema formation, AQP4 expression, and cortical perfusion were investigated following focal TBI in rats. Brain edema was maximal by 24 h. Concurrently, AQP4 protein expression was decreased in both hemispheres, being more pronounced in the traumatized hemisphere (-50%) 48 h after trauma. Cortical perfusion was only decreased in the ipsilateral cortex (-40%) between 4 and 8 h after trauma, reaching baseline values at 24 h. Globally reduced AQP4 expression following induction of a focal contusion coincides with edema development and seems to be independent of changes in cortical perfusion.

The elasmobranch renal corpuscle: fine structure of Bowman's capsule and the glomerular capillary wall

The fine structure of the renal corpuscle of the marine elasmobranch fish, the little skate (Raja erinacea), and two species of dogfish sharks, the spiny dogfish (Squalus acanthias) and the smooth dogfish (Mustelus canis), was studied by light microscopy and by transmission (thin sections, freeze-fracture replicas) and scanning electron microscopy. Bowman's capsule was lined by ciliated cells, similar to those of the first part of the tubule, at the urinary pole and squamous cells in the zone between urinary and vascular poles. At the vascular pole the visceral epithelial cells had some closely apposed cuboidal cell bodies with a few processes inserted along the basement membrane, but foot processes were absent. These cuboidal cells were continuous with podocytes, which had primary, secondary and tertiary processes from which the pedicels arose. An inconsistently present slit membrane bridged the pedicels at varying distance from the urinary space. Small maculae and large fasciae occludentes joined the podocytes and/or their processes among which gap junctions were observed. In the skate kidney, the podocyte plasma membrane facing the basement membrane contained orthogonal arrays of particles. The epithelial basement membrane of the glomeruli in all elasmobranchs was consistently thick. The mesangial cells were numerous and partially enveloped in a basement membrane; their long processes almost completely circumscribed the capillary walls. The mesangial matrix was abundant and consisted of loosely arranged collagen fibrils, microfibrils and occasional anchoring fibrils. The endothelial cells had irregularly distributed fenestrations of various sizes and lay on their discontinuous basement membrane which was separate from that under the epithelial cells. The filtration apparatus resembles that of immature mammals and lower vertebrates.