Brain mitochondria contain aquaporin water channels: evidence for the expression of a short AQP9 isoform in the inner mitochondrial membrane

Two isoforms of aquaporin 2 responsive to hypertonic stress in bottlenose dolphin

This study investigated the expression of aquaporin 2 (AQP2) and its newly found alternatively spliced isoform (alternative AQP2) and the functions of these AQP2 isoforms in the cellular hyperosmotic tolerance in the bottlenose dolphin Tursiops truncatus. mRNA sequencing revealed that alternative AQP2 lacks the fourth exon and instead has a longer third exon that includes a part of the original third intron. The portion of the third intron, now part of the coding region of alternative AQP2, is highly conserved among many species of the order Cetacea but not among terrestrial mammals. Semi-quantitative polymerase chain reaction revealed that AQP2 was expressed only in the kidney, similar to terrestrial mammals. In contrast, alternative AQP2 was expressed in all organs examined, with strong expression in the kidney. In cultured renal cells, expression of both AQP2 isoforms was upregulated by the addition to the medium of NaCl but not by the addition of mannitol, indicating that the expression of both isoforms is induced by hypersalimity but not hypertonicity conditions. Treatment with small interfering RNA for both isoforms, resulted in a decrease in cell viability in hypertonic medium (500 mOsm/kg) when compared to controls. These findings indicate that the expression of alternatively spliced AQP2 is ubiquitous in cetacean species and it may be one of the molecules important for cellular osmotic tolerance throughout the body.

The central role of aquaporins in the pathophysiology of ischemic stroke

Stroke is a complex and devastating neurological condition with limited treatment options. Brain edema is a serious complication of stroke. Early edema formation can significantly contribute to infarct formation and thus represents a promising target. Aquaporin (AQP) water channels contribute to water homeostasis by regulating water transport and are implicated in several disease pathways. At least 7 AQP subtypes have been identified in the rodent brain and the use of transgenic mice has greatly aided our understanding of their functions. AQP4, the most abundant channel in the brain, is up-regulated around the peri-infarct border in transient cerebral ischemia and AQP4 knockout mice demonstrate significantly reduced cerebral edema and improved neurological outcome. In models of vasogenic edema, brain swelling is more pronounced in AQP4-null mice than wild-type providing strong evidence of the dual role of AQP4 in the formation and resolution of both vasogenic and cytotoxic edema. AQP4 is co-localized with inwardly rectifying K(+)-channels (Kir4.1) and glial K(+) uptake is attenuated in AQP4 knockout mice compared to wild-type, indicating some form of functional interaction. AQP4-null mice also exhibit a reduction in calcium signaling, suggesting that this channel may also be involved in triggering pathological downstream signaling events. Associations with the gap junction protein Cx43 possibly recapitulate its role in edema dissipation within the astroglial syncytium. Other roles ascribed to AQP4 include facilitation of astrocyte migration, glial scar formation, modulation of inflammation and signaling functions. Treatment of ischemic cerebral edema is based on the various mechanisms in which fluid content in different brain compartments can be modified. The identification of modulators and inhibitors of AQP4 offer new therapeutic avenues in the hope of reducing the extent of morbidity and mortality in stroke.

Aquaporin 9 expression is required for l-lactate to maintain retinal neuronal survival

Aquaporin 9 (AQP9), an aquaglyceroporin, is not only permeable to water but also to non-charged solutes, such as lactate. Lactate can be an energy source for retinal neurons. This study aimed to evaluate the effect of the downregulation of AQP9 expression on the survival rates and reactive oxygen species accumulation in RGC-5 cells cultured in a medium containing lactate. The Live/Dead assay revealed that the cell death rate of RGC-5 cells transfected with the control siRNA (siControl) was 3.65%±0.75% in the 5-mM glucose medium. The death rate was significantly increased by five-fold in the no glucose and 10-mM d-lactate media but not in the 10-mM l-lactate medium. In comparison, the death rate of cells transfected with siRNA targeting AQP9 (siAQP9) was 8.07%±1.01% in the 5-mM glucose medium, which was significantly increased by two-fold in the other medium conditions, indicating that the downregulation of AQP9 expression eliminated the prosurvival effect of l-lactate. Few RGC-5 cells transfected with siControl showed dichlorofluorescein (DCF) fluorescence when cultured in 5-mM glucose and 10-mM l-lactate media. However, approximately 70% of those showed DCF fluorescence when cultured in the no glucose and 10-mM d-lactate media. The downregulation of AQP9 significantly increased the DCF fluorescence rate to 50.44%±6.13% in the 10-mM l-lactate medium, whereas, it did not increase the rate in the other medium conditions. These results demonstrate that AQP9 expression is required for l-lactate to maintain retinal neuronal survival.

Glutamate reduces glucose utilization while concomitantly enhancing AQP9 and MCT2 expression in cultured rat hippocampal neurons

The excitatory neurotransmitter glutamate has been reported to have a major impact on brain energy metabolism. Using primary cultures of rat hippocampal neurons, we observed that glutamate reduces glucose utilization in this cell type, suggesting alteration in mitochondrial oxidative metabolism. The aquaglyceroporin AQP9 and the monocarboxylate transporter MCT2, two transporters for oxidative energy substrates, appear to be present in mitochondria of these neurons. Moreover, they not only co-localize but they interact with each other as they were found to co-immunoprecipitate from hippocampal neuron homogenates. Exposure of cultured hippocampal neurons to glutamate 100 μM for 1 h led to enhanced expression of both AQP9 and MCT2 at the protein level without any significant change at the mRNA level. In parallel, a similar increase in the protein expression of LDHA was evidenced without an effect on the mRNA level. These data suggest that glutamate exerts an influence on neuronal energy metabolism likely through a regulation of the expression of some key mitochondrial proteins.

Aquaporin and brain diseases

BACKGROUND

The presence of water channel proteins, aquaporins (AQPs), in the brain led to intense research in understanding the underlying roles of each of them under normal conditions and pathological conditions.

SCOPE OF REVIEW

In this review, we summarize some of the recent knowledge on the 3 main AQPs (AQP1, AQP4 and AQP9), with a special focus on AQP4, the most abundant AQP in the central nervous system.

MAJOR CONCLUSIONS

AQP4 was most studied in several brain pathological conditions ranging from acute brain injuries (stroke, traumatic brain injury) to the chronic brain disease with autoimmune neurodegenerative diseases. To date, no specific therapeutic agents have been developed to either inhibit or enhance water flux through these channels. However, experimental results strongly underline the importance of this topic for future investigation. Early inhibition of water channels may have positive effects in prevention of edema formation in brain injuries but at later time points during the course of a disease, AQP is critical for clearance of water from the brain into blood vessels.

GENERAL SIGNIFICANCE

Thus, AQPs, and in particular AQP4, have important roles both in the formation and resolution of edema after brain injury. The dual, complex function of these water channel proteins makes them an excellent therapeutic target. This article is part of a Special Issue entitled Aquaporins.

The grape aquaporin VvSIP1 transports water across the ER membrane

Water diffusion through biological membranes is facilitated by aquaporins, members of the widespread major intrinsic proteins (MIPs). In the present study, the localization, expression, and functional characterization of a small basic intrinsic protein (SIP) from the grapevine were assessed. VvSIP1 was expressed in leaves and berries from field-grown vines, and in leaves and stems from in vitro plantlets, but not in roots. When expressed in tobacco mesophyll cells and in Saccharomyces cerevisiae, fluorescent-tagged VvSIP1 was localized at the endoplasmic reticulum (ER). Stopped-flow spectroscopy showed that VvSIP1-enriched ER membrane vesicles from yeast exhibited higher water permeability and lower activation energy for water transport than control vesicles, indicating the involvement of protein-mediated water diffusion. This aquaporin was able to transport water but not glycerol, urea, sorbitol, glucose, or inositol. VvSIP1 expression in Xenopus oocytes failed to increase the water permeability of the plasma membrane. VvSIP1-His-tag was solubilized and purified to homogeneity from yeast ER membranes and the reconstitution of the purified protein in phosphatidylethanolamine liposomes confirmed its water channel activity. To provide further insights into gene function, the expression of VvSIP1 in mature grapes was studied when vines were cultivated in different field conditions, but its transcript levels did not increase significantly in water-stressed plants and western-exposed berries. However, the expression of the aquaporin genes VvSIP1, VvPIP2;2, and VvTIP1;1 was up-regulated by heat in cultured cells.

Expression analysis and molecular characterization of aquaporins in Rhodnius prolixus

Aquaporins (AQPs) are water channels responsible for transport of water and, in some cases, transport of small solutes such as urea and glycerol across lipid bilayer membranes. Hematophagous insects, such as Rhodnius prolixus, ingest large volumes of fluid and must rapidly eliminate the excess of water and salts from the blood meal within the gut. In order to deal with this increase in body fluid volume, a hormone-controlled diuresis is activated, during which a high rate of water and salt absorption occurs across the anterior midgut, followed by secretion of water and salts by the Malpighian tubules (MTs). Previously, one member of the MIP family (major intrinsic protein that includes the AQP family) was identified in the MTs of R. prolixus, and named RpMIP. We have described here that the RpMIP gene has different variants, and is present in tissues other than MTs. In addition, we have characterized a new AQP (RhoprAQP1) found in different tissues of R. prolixus. The expression of these transcripts in unfed insects as well as blood fed insects was evaluated using real-time quantitative PCR. Molecular models of the predicted proteins were constructed and the characteristics of their pores evaluated. A yeast complementation assay was used to validate that the products of these transcripts were bona fide AQPs. Both RhoprAQP1 and RhoprMIP-A were capable of transporting water whereas RhoprMIP-A was also capable of transporting H2O2. Taken together, these analyses suggest that RhoprMIP is probably an aquaglyceroporin, while RhoprAQP1 appears to be a strict aquaporin that transports only water.

Viruses as modulators of mitochondrial functions

Mitochondria are multifunctional organelles with diverse roles including energy production and distribution, apoptosis, eliciting host immune response, and causing diseases and aging. Mitochondria-mediated immune responses might be an evolutionary adaptation by which mitochondria might have prevented the entry of invading microorganisms thus establishing them as an integral part of the cell. This makes them a target for all the invading pathogens including viruses. Viruses either induce or inhibit various mitochondrial processes in a highly specific manner so that they can replicate and produce progeny. Some viruses encode the Bcl2 homologues to counter the proapoptotic functions of the cellular and mitochondrial proteins. Others modulate the permeability transition pore and either prevent or induce the release of the apoptotic proteins from the mitochondria. Viruses like Herpes simplex virus 1 deplete the host mitochondrial DNA and some, like human immunodeficiency virus, hijack the host mitochondrial proteins to function fully inside the host cell. All these processes involve the participation of cellular proteins, mitochondrial proteins, and virus specific proteins. This review will summarize the strategies employed by viruses to utilize cellular mitochondria for successful multiplication and production of progeny virus.

AQP5 is differentially regulated in astrocytes during metabolic and traumatic injuries

Water movement plays vital roles in both physiological and pathological conditions in the brain. Astrocytes are responsible for regulating this water movement and are the major contributors to brain edema in pathological conditions. Aquaporins (AQPs) in astrocytes play critical roles in the regulation of water movement in the brain. AQP1, 3, 4, 5, 8, and 9 have been reported in the brain. Compared with AQP1, 4, and 9, AQP3, 5, and 8 are less studied. Among the lesser known AQPs, AQP5, which has multiple functions identified outside the central nervous system, is also indicated to be involved in hypoxia injury in astrocytes. In our study, AQP5 expression could be detected both in primary cultures of astrocytes and neurons, and AQP5 expression in astrocytes was confirmed in 1- to 4-week old primary cultures of astrocytes. AQP5 was localized on the cytoplasmic membrane and in the cytoplasm of astrocytes. AQP5 expression was downregulated during ischemia treatment and upregulated after scratch-wound injury, which was also confirmed in a middle cerebral artery occlusion model and a stab-wound injury model in vivo. The AQP5 increased after scratch injury was polarized to the migrating processes and cytoplasmic membrane of astrocytes in the leading edge of the scratch-wound, and AQP5 over-expression facilitated astrocyte process elongation after scratch injury. Taken together, these results indicate that AQP5 might be an important water channel in astrocytes that is differentially expressed during various brain injuries.

Aquaporin water channels in the nervous system

The aquaporins (AQPs) are plasma membrane water-transporting proteins. AQP4 is the principal member of this protein family in the CNS, where it is expressed in astrocytes and is involved in water movement, cell migration and neuroexcitation. AQP1 is expressed in the choroid plexus, where it facilitates cerebrospinal fluid secretion, and in dorsal root ganglion neurons, where it tunes pain perception. The AQPs are potential drug targets for several neurological conditions. Astrocytoma cells strongly express AQP4, which may facilitate their infiltration into the brain, and the neuroinflammatory disease neuromyelitis optica is caused by AQP4-specific autoantibodies that produce complement-mediated astrocytic damage.

Loss of aquaporin 9 expression adversely affects the survival of retinal ganglion cells

Aquaporin 9 (AQP9), an aquaglyceroporin belonging to the AQP water channel family, is permeable not only to water but also to noncharged solutes such as lactate. In neurons, lactate presumably acts as an energy substrate and as a source of NADH (the reduced form of nicotinamide adenine dinucleotide), a scavenger of reactive oxygen species (ROS). We previously reported that retinal ganglion cells (RGCs) express AQP9 and that elevated intraocular pressure reduces AQP9 expression and increases death of neurons in the retinal ganglion cell layer of rodents. In the present study, we investigated the association of AQP9 expression with serum deprivation-induced death of RGC-5 cells and with death of neurons in the rat retinal ganglion cell layer after optic nerve transection (ONT). The effect of AQP9 RNA interference on serum deprivation-induced apoptosis, ROS accumulation, and the NAD(+)/NADH ratio in RGC-5 cells was examined. Both serum deprivation and ONT significantly reduced AQP9 protein expression in RGCs and increased the rate of RGC death. Retinal AQP9 gene expression also declined after ONT. Down-regulation of AQP9 significantly increased apoptosis, ROS accumulation, and the NAD(+)/NADH ratio in the RGC-5 cells. These findings suggest that AQP9 loss adversely affects survival of RGCs, at least partly because of decreased transport of lactate as a substrate for energy and/or ROS scavenger.

Relative CO(2)/NH(3) selectivities of mammalian aquaporins 0-9

Previous work showed that aquaporin 1 (AQP1), AQP4-M23, and AQP5 each has a characteristic CO(2)/NH(3) and CO(2)/H(2)O permeability ratio. The goal of the present study is to characterize AQPs 0-9, which traffic to the plasma membrane when heterologously expressed in Xenopus oocytes. We use video microscopy to compute osmotic water permeability (P(f)) and microelectrodes to record transient changes in surface pH (ΔpH(S)) caused by CO(2) or NH(3) influx. Subtracting respective values for day-matched, H(2)O-injected control oocytes yields the channel-specific values P(f)* and ΔpH(S)*. We find that P(f)* is significantly >0 for all AQPs tested except AQP6. (ΔpH(S)*)(CO(2)) is significantly >0 for AQP0, AQP1, AQP4-M23, AQP5, AQP6, and AQP9. (ΔpH(S)*)(NH(3)) is >0 for AQP1, AQP3, AQP6, AQP7, AQP8, and AQP9. The ratio (ΔpH(S)*)(CO(2))/P(f)* falls in the sequence AQP6 (∞) > AQP5 > AQP4-M23 > AQP0 ≅ AQP1 ≅ AQP9 > others (0). The ratio (ΔpH(S)*)(NH(3))/P(f)* falls in the sequence AQP6 (∞) > AQP3 ≅ AQP7 ≅ AQP8 ≅ AQP9 > AQP1 > others (0). Finally, the ratio (ΔpH(S)*)(CO(2))/(-ΔpH(S)*)(NH(3)) falls in the sequence AQP0 (∞) ≅ AQP4-M23 ≅ AQP5 > AQP6 > AQP1 > AQP9 > AQP3 (0) ≅ AQP7 ≅ AQP8. The ratio (ΔpH(S)*)(CO(2))/(-ΔpH(S)*)(NH(3)) is indeterminate for both AQP2 and AQP4-M1. In summary, we find that mammalian AQPs exhibit a diverse range of selectivities for CO(2) vs. NH(3) vs. H(2)O. As a consequence, by expressing specific combinations of AQPs, cells could exert considerable control over the movements of each of these three substances.

Physiology of potassium channels in the inner membrane of mitochondria

The inner membrane of the ATP-producing organelles of endosymbiotic origin, mitochondria, has long been considered to be poorly permeable to cations and anions, since the strict control of inner mitochondrial membrane permeability is crucial for efficient ATP synthesis. Over the past 30 years, however, it has become clear that various ion channels--along with antiporters and uniporters--are present in the mitochondrial inner membrane, although at rather low abundance. These channels are important for energy supply, and some are a decisive factor in determining whether a cell lives or dies. Their electrophysiological and pharmacological characterisations have contributed importantly to the ongoing elucidation of their pathophysiological roles. This review gives an overview of recent advances in our understanding of the functions of the mitochondrial potassium channels identified so far. Open issues concerning the possible molecular entities giving rise to the observed activities and channel protein targeting to mitochondria are also discussed.

Characterization of Leishmania donovani aquaporins shows presence of subcellular aquaporins similar to tonoplast intrinsic proteins of plants

Leishmania donovani, a protozoan parasite, resides in the macrophages of the mammalian host. The aquaporin family of proteins form important components of the parasite-host interface. The parasite-host interface could be a potential target for chemotherapy. Analysis of L. major and L. infantum genomes showed the presence of five aquaporins (AQPs) annotated as AQP9 (230aa), AQP putative (294aa), AQP-like protein (279aa), AQP1 (314aa) and AQP-like protein (596aa). We report here the structural modeling, localization and functional characterization of the AQPs from L. donovani. LdAQP1, LdAQP9, LdAQP2860 and LdAQP2870 have the canonical NPA-NPA motifs, whereas LdAQP putative has a non-canonical NPM-NPA motif. In the carboxyl terminal to the second NPA box of all AQPs except AQP1, a valine/alanine residue was found instead of the arginine. In that respect these four AQPs are similar to tonoplast intrinsic proteins in plants, which are localized to intracellular organelles. Confocal microscopy of L. donovani expressing GFP-tagged AQPs showed an intracellular localization of LdAQP9 and LdAQP2870. Real-time PCR assays showed expression of all aquaporins except LdAQP2860, whose level was undetectable. Three-dimensional homology modeling of the AQPs showed that LdAQP1 structure bears greater topological similarity to the aquaglyceroporin than to aquaporin of E. coli. The pore of LdAQP1 was very different from the rest in shape and size. The cavity of LdAQP2860 was highly irregular and undefined in geometry. For functional characterization, four AQP proteins were heterologously expressed in yeast. In the fps1Δ yeast cells, which lacked the key aquaglyceroporin, LdAQP1 alone displayed an osmosensitive phenotype indicating glycerol transport activity. However, expression of LdAQP1 and LdAQP putative in a yeast gpd1Δ strain, deleted for glycerol production, conferred osmosensitive phenotype indicating water transport activity or aquaporin function. Our analysis for the first time shows the presence of subcellular aquaporins and provides structural and functional characterization of aquaporins in Leishmania donovani.

Neuromyelitis optica (NMO)--an autoimmune disease of the central nervous system (CNS)

In the past 10 years, neuromyelitis optica (NMO) has evolved from Devic's categorical clinical description into a broader disease spectrum. Serum IgG antibodies have been identified in NMO patients with the water channel aquaporin-4 (AQP4) as their main target antigen. AQP4 antibodies/NMO-IgG have been shown to be a highly specific and moderately sensitive serum biomarker for NMO. The immunopathology of NMO lesions supports that anti-AQP4 antibodies/NMO-IgG are involved in the pathogenesis of NMO. In vitro studies have demonstrated that human NMO-IgG induce necrosis and impair glutamate transport in astrocytes. Certain ethnic groups, notably of Asian and African origin, seem to be more susceptible to NMO than others. The genetic background for these putative differences is not known, a weak human leucocyte antigen association has been identified. AQP4 gene variants could represent a genetic susceptibility factor for different clinical phenotypes within the NMO spectrum. Experimental models have been described including a double-transgenic myelin-specific B- and T-cell mouse. NMO-like disease has been induced with passive transfer of human anti-AQP4 antibodies to the plasma of mice with pre-established experimental autoimmune encephalomyelitis or by intrathecal administration to naive mice. NMO may be characterized as a channelopathy of the central nervous system with autoimmune characteristics.

Reactive oxygen species in the regulation of synaptic plasticity and memory

The brain is a metabolically active organ exhibiting high oxygen consumption and robust production of reactive oxygen species (ROS). The large amounts of ROS are kept in check by an elaborate network of antioxidants, which sometimes fail and lead to neuronal oxidative stress. Thus, ROS are typically categorized as neurotoxic molecules and typically exert their detrimental effects via oxidation of essential macromolecules such as enzymes and cytoskeletal proteins. Most importantly, excessive ROS are associated with decreased performance in cognitive function. However, at physiological concentrations, ROS are involved in functional changes necessary for synaptic plasticity and hence, for normal cognitive function. The fine line of role reversal of ROS from good molecules to bad molecules is far from being fully understood. This review focuses on identifying the multiple sources of ROS in the mammalian nervous system and on presenting evidence for the critical and essential role of ROS in synaptic plasticity and memory. The review also shows that the inability to restrain either age- or pathology-related increases in ROS levels leads to opposite, detrimental effects that are involved in impairments in synaptic plasticity and memory function.

Aquaporins in cerebrovascular disease: a target for treatment of brain edema?

In cerebrovascular disease, edema formation is frequently observed within the first 7 days and is characterized by molecular and cellular changes in the neurovascular unit. The presence of water channels, aquaporins (AQPs), within the neurovascular unit has led to intensive research in understanding the underlying roles of each of the AQPs under normal conditions and in different diseases. In this review, we summarize some of the recent knowledge on AQPs, focusing on AQP4, the most abundant AQP in the central nervous system. Several experimental models illustrate that AQPs have dual, complex regulatory roles in edema formation and resolution. To date, no specific therapeutic agents have been developed to inhibit water flux through these channels. However, experimental results strongly suggest that this is an important area for future investigation. In fact, early inhibition of water channels may have positive effects in the prevention of edema formation. At later time points during the course of disease, AQP is important for the clearance of water from the brain into blood vessels. Thus, AQPs, and in particular AQP4, have important roles in the resolution of edema after brain injury. The function of these water channel proteins makes them an excellent therapeutic target.

TIP5;1 is an aquaporin specifically targeted to pollen mitochondria and is probably involved in nitrogen remobilization in Arabidopsis thaliana

In plant sexual reproduction, water and solute movement are tightly regulated, suggesting the involvement of aquaporins. We previously identified TIP5;1 and TIP1;3 as the only Arabidopsis aquaporin genes that are selectively and highly expressed in mature pollen, and showed that they can transport both water and urea when expressed in Xenopus oocytes. Here, we show that TIP5;1 has unusual characteristics, as its water transport activity is regulated by pH. Analysis of the water transport activity of a mutant version of TIP5;1 (TIP5;1-H131A) and amino acid alignment with other plant aquaporins regulated by pH suggested that a conserved motif is involved in pH sensing. GFP-TIP5;1 is located in the mitochondria of pollen tubes. The single mutants tip1;3 and tip5;1, as well as the tip1;3 tip5;1 double mutant, are fertile, but all mutants had shorter than normal pollen tubes when germinated in vitro in the absence of exogenous nitrogen. Thus, we propose that TIP5;1 and TIP1;3 are involved in nitrogen recycling in pollen tubes of Arabidopsis thaliana.

Aquaporin biology and nervous system

Our understanding of the movement of water through cell membranes has been greatly advanced by the discovery of a family of water-specific, membrane-channel proteins: the Aquaporins (AQPs). These proteins are present in organisms at all levels of life, and their unique permeability characteristics and distribution in numerous tissues indicate diverse roles in the regulation of water homeostasis.Phenotype analysis of AQP knock-out mice has confirmed the predicted role of AQPs in osmotically driven transepithelial fluid transport, as occurs in the urinary concentrating mechanism and glandular fluid secretion. Regarding their expression in nervous system, there are evidences suggesting that AQPs are differentially expressed in the peripheral versus central nervous system and that channel-mediated water transport mechanisms may be involved in cerebrospinal fluid formation, neuronal signal transduction and information processing.Moreover, a number of recent studies have revealed the importance of mammalian AQPs in both physiological and pathophysiological mechanisms and have suggested that pharmacological modulation of AQP expression and activity may provide new tools for the treatment of variety of human disorders in which water and small solute transport may be involved. For all the AQPs, new contributions to physiological functions are likely to be discovered with ongoing work in this rapidly expanding field of research.

Aquaporins and neurodegenerative diseases

Aquaporins (AQPs) are a family of widely distributed membrane-inserted water channel proteins providing a pathway for osmotically-driven water, glycerol, urea or ions transport through cell membranes and mechanisms to control particular aspects of homeostasis. Beside their physiological expression patterns in Central Nervous System (CNS), it is conceivable that AQPs are also abnormally expressed in some pathological conditions interesting CNS (e.g. neurodegenerative diseases) in which preservation of brain homeostasis is at risk.The purpose of this review was to take in consideration those neurodegenerative diseases in whose pathogenetic processes it was possible to hypothesize some alterations in CNS AQPs expression or modulation leading to damages of brain water homeostasis.

Cell locations for AQP1, AQP4 and 9 in the non-human primate brain

The presence of three water channels (aquaporins, AQP), AQP1, AQP4 and AQP9 were observed in normal brain and several rodent models of brain pathologies. Little is known about AQP distribution in the primate brain and its knowledge will be useful for future testing of drugs aimed at preventing brain edema formation. We studied the expression and cellular distribution of AQP1, 4 and 9 in the non-human primate brain. The distribution of AQP4 in the non-human primate brain was observed in perivascular astrocytes, comparable to the observation made in the rodent brain. In contrast with rodent, primate AQP1 is expressed in the processes and perivascular endfeet of a subtype of astrocytes mainly located in the white matter and the glia limitans, possibly involved in water homeostasis. AQP1 was also observed in neurons innervating the pial blood vessels, suggesting a possible role in cerebral blood flow regulation. As described in rodent, AQP9 mRNA and protein were detected in astrocytes and in catecholaminergic neurons. However additional locations were observed for AQP9 in populations of neurons located in several cortical areas of primate brains. This report describes a detailed study of AQP1, 4 and 9 distributions in the non-human primate brain, which adds to the data already published in rodent brains. This relevant species differences have to be considered carefully to assess potential drugs acting on AQPs non-human primate models before entering human clinical trials.

Higher order structure of aquaporin-4

Unlike other mammalian AQPs, multiple tetramers of AQP4 associate in the plasma membrane to form peculiar structures called Orthogonal Arrays of Particles (OAPs), that are observable by freeze-fracture electron microscopy (FFEM). However, FFEM cannot give information about the composition of OAPs of different sizes, and due to its technical complexity is not easily applicable as a routine technique. Recently, we employed the 2D gel electrophoresis BN-SDS/PAGE that for the first time enabled the biochemical isolation of AQP4-OAPs from several tissues. We found that AQP4 protein is present in several higher-order complexes (membrane pools of supra-structures) which contain different ratios of M1/M23 isoforms corresponding to AQP4-OAPs of different size. In this paper, we illustrate in detail the potentiality of 2D BN/SDS-PAGE for analyzing AQP4 supra-structures, their relationship with the dystrophin glycoprotein complex and other membrane proteins, and their role as a specific target of Neuromyelitis Optica autoantibodies.

Water transport between CNS compartments: functional and molecular interactions between aquaporins and ion channels

The physiological ability of the mammalian CNS to integrate peripheral stimuli and to convey information to the body is tightly regulated by its capacity to preserve the ion composition and volume of the perineuronal milieu. It is well known that astroglial syncytium plays a crucial role in such process by controlling the homeostasis of ions and water through the selective transmembrane movement of inorganic and organic molecules and the equilibration of osmotic gradients. Astrocytes, in fact, by contacting neurons and cells lining the fluid-filled compartments, are in a strategic position to fulfill this role. They are endowed with ion and water channel proteins that are localized in specific plasma membrane domains facing diverse liquid spaces. Recent data in rodents have demonstrated that the precise dynamics of the astroglia-mediated homeostatic regulation of the CNS is dependent on the interactions between water channels and ion channels, and their anchoring with proteins that allow the formation of macromolecular complexes in specific cellular domains. Interplay can occur with or without direct molecular interactions suggesting the existence of different regulatory mechanisms. The importance of molecular and functional interactions is pinpointed by the numerous observations that as consequence of pathological insults leading to the derangement of ion and volume homeostasis the cell surface expression and/or polarized localization of these proteins is perturbed. Here, we critically discuss the experimental evidence concerning: (1) molecular and functional interplay of aquaporin 4, the major aquaporin protein in astroglial cells, with potassium and gap-junctional channels that are involved in extracellular potassium buffering. (2) the interactions of aquaporin 4 with chloride and calcium channels regulating cell volume homeostasis. The relevance of the crosstalk between water channels and ion channels in the pathogenesis of astroglia-related acute and chronic diseases of the CNS is also briefly discussed.

A look inside: localization patterns and functions of intracellular plant aquaporins

Aquaporins form a superfamily of intrinsic channel proteins in the plasma and intracellular membranes of plant cells. While a lot of research effort has substantiated the importance of plasma membrane aquaporins for the regulation of plant water homeostasis, comparably little is known about the function of intracellular aquaporins. Yet, various low-molecular-weight compounds, in addition to water, were recently shown to permeate some of these aquaporins. In this review, we examine the diversity of transport properties and localization patterns of intracellular aquaporins. The discussed profiles include, for example, water and ammonia transport across the tonoplast or CO2 transport through the chloroplast envelope. Furthermore, we try to assess to what extent the diverse aquaporin distribution patterns, in relation to the high degree of compartmentation of plant cells, can be linked to a wide range of cellular functions.

Aquaporins: translating bench research to human disease

There is considerable potential for translating knowledge of aquaporin structure, function and physiology to the clinic. One area is in aquaporin-based diagnostics. The discovery of AQP4 autoantibodies as a marker of the neuromyelitis optica form of multiple sclerosis has allowed precise diagnosis of this disease. Other aquaporin-based diagnostics are possible. Another area is in aquaporin-based genetics. Genetic diseases caused by loss-of-function mutations in aquaporins include nephrogenic diabetes insipidus and cataracts, and functionally significant aquaporin polymorphisms are beginning to be explored. Perhaps of greatest translational potential is aquaporin-based therapeutics. Information largely from aquaporin knockout mice has implicated key roles of aquaporin-facilitated water transport in transepithelial fluid transport (urinary concentrating, gland fluid secretion), water movement into and out of the brain, cell migration (angiogenesis, tumor metastasis, wound healing) and neural function (sensory signaling, seizures). A subset of aquaporins that transport both water and glycerol, the 'aquaglyceroporins', regulate glycerol content in epidermal, fat and other tissues, and are involved in skin hydration, cell proliferation, carcinogenesis and fat metabolism. Aquaporin-based modulator drugs are predicted to be of broad potential utility in the treatment of edematous states, cancer, obesity, wound healing, epilepsy and glaucoma. These exciting possibilities and their associated challenges are reviewed.

Analysis of mice with targeted deletion of AQP9 gene provides conclusive evidence for expression of AQP9 in neurons

AQP9 is an aquaglyceroporin that serves important functions in peripheral organs, including the liver. Reflecting the lack of AQP9 knockout mice, uncertainties still prevail regarding the localization and roles of AQP9 in the central nervous system. Here we present a comprehensive analysis of AQP9 gene expression in brain, based on a quantitative and multipronged approach that includes the use of animals with targeted deletion of the AQP9 gene. We show by real-time PCR that AQP9 mRNA concentration in rat and mouse brain is approximately 3% and approximately 0.5%, respectively, of that in rat and mouse liver, the organ with the highest level of AQP9. By blue native gel analysis it could be demonstrated that the brain contains tetrameric AQP9, corresponding to the functional form of AQP9. The band corresponding to the AQP9 tetramer was absent in AQP9 knockout brain and liver. Immunocytochemistry and in situ hybridization analyses with AQP9 knockout controls show that subpopulations of nigral neurons express AQP9 both at the mRNA and at the protein levels and that populations of cortical cells (including hilar neurons in the hippocampus) contain AQP9 mRNA but no detectable AQP9 immunosignal. The present data provide conclusive evidence for the presence of tetrameric AQP9 in brain and for the expression of AQP9 in neurons.

Enhanced expression of aquaporin-9 in rat brain edema induced by bacterial lipopolysaccharides

To investigate the role of AQP9 in brain edema, the expression of AQP9 in an infectious rat brain edema model induced by the injection of lipopolysaccharide (LPS) was examined. Immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR) analysis demonstrated that the expressions of AQP9 mRNA and protein at all observed intervals were significantly increased in LPS-treated animals in comparison with the control animals. Time-course analysis showed that the first signs of blood-brain barrier disruption and the increase of brain water content in LPS-treated animals were evident 6 h after LPS injection, with maximum value appearing at 12 h, which coincided with the expression profiles of AQP9 mRNA and protein in LPS-treated animals. The further correlation analysis revealed strong positive correlations among the brain water content, the disruption of the blood-brain barrier and the enhanced expressions of AQP9 mRNA and protein in LPS-treated animals. These results suggested that the regulation of AQP9 expression may play important roles in water movement and in brain metabolic homeostasis associated with the pathophysiology of brain edema induced by LPS injection.

Aquaporin water channels in mammals

Water channels, aquaporins (AQPs), are a family of small integral plasma membrane proteins that primarily transport water across the plasma membrane. There are 13 members (AQP0-12) in humans. This number is final as the human genome project has been completed. They are divided into three subgroups based on the primary sequences: water selective AQPs (AQP0, 1, 2, 4, 5, 6, 8), aquaglyceroporins (AQP3, 7, 9, 10), and superaquaporins (AQP11, 12). Since no specific inhibitors are yet available, functional roles of AQPs are suggested by AQP null mice and humans. Abnormal water metabolism was shown with AQP1, 2, 3, 4, 5 null mice, especially with AQP2 null mice: fatal at neonate due to diabetes insipidus. Abnormal glycerol transport was shown with AQP3, 7, 9 null mice, although they appeared normal. AQP0 null mice suffer from cataracts, although the pathogenesis is not clear. Unexpectedly, AQP11 null mice die from uremia as a result of polycystic kidneys. Interestingly, AQP6, 8, 10, 12 null mice are almost normal. AQP null humans have been reported with AQP0, 1, 2, 3, 7: only AQP2 null humans show an outstanding phenotype, diabetes insipidus. This review summarizes the current knowledge on all mammalian AQPs and hopefully will stimulate future research in both clinical and basic fields.

Recombinant hirudin treatment modulates aquaporin-4 and aquaporin-9 expression after intracerebral hemorrhage in vivo

Edema formation has been linked to thrombin toxicity induced by blood clot at the acute stage of intracerebral hemorrhage. Thrombin induces cell toxicity in neuron, microglia and astrocyte. Aquaporin (AQP) 4 and 9 are proteins expressed on astrocyte in rat brain and involved in the brain water accumulation in brain edema. Recombinant hirudin (r-Hirudin) is a direct inhibitor of thrombin that can block the toxicitic effect of thrombin. In this study, we demonstrated that autologous whole blood infusion in caudate nucleus up-regulates the expression of AQP4 and AQP9 mRNAs and proteins. AQP4 and AQP9 mRNAs expression peaked at about 6 h after blood infusion. The AQP4 protein peaked at about 48 h while AQP9 at about 24 h after blood infusion. Thrombin induced up-regulation of AQP4 and AQP9 were inhibited by r-Hirudin administration and significantly decreased the expression of both AQPs. We further investigated the relationship between edema formation and expression of AQP4 and AQP9. The data presented here may be helpful in optimizing r-Hirudin as an anti-thrombin drug in the treatment of edema at the acute stage of ICH.

Carrier-mediated urea transport across the mitochondrial membrane of an elasmobranch (Raja erinacea) and a teleost (Oncorhynchus mykiss) fish

In osmoregulating teleost fish, urea is a minor nitrogen excretory product, whereas in osmoconforming marine elasmobranchs it serves as the major tissue organic solute and is retained at relatively high concentrations (∼400 mmol/l). We tested the hypothesis that urea transport across liver mitochondria is carrier mediated in both teleost and elasmobranch fishes. Intact liver mitochondria in rainbow trout ( Oncorhynchus mykiss) demonstrated two components of urea uptake, a linear component at high concentrations and a phloretin-sensitive saturable component [Michaelis constant ( Km) = 0.58 mmol/l; maximal velocity ( Vmax) = 0.12 μmol·h−1·mg protein−1] at lower urea concentrations (<5 mmol/l). Similarly, analysis of urea uptake in mitochondria from the little skate ( Raja erinacea) revealed a phloretin-sensitive saturable transport ( Km= 0.34 mmol/l; Vmax= 0.054 μmol·h−1·mg protein−1) at low urea concentrations (<5 mmol/l). Surprisingly, urea transport in skate, but not trout, was sensitive to a variety of classic ionophores and respiration inhibitors, suggesting cation sensitivity. Hence, urea transport was measured in the reverse direction using submitochondrial particles in skate. Transport kinetics, inhibitor response, and pH sensitivity were very similar in skate submitochondrial particle submitochondrial particles ( Km= 0.65 mmol/l, Vmax= 0.058 μmol·h−1·mg protein−1) relative to intact mitochondria. We conclude that urea influx and efflux in skate mitochondria is dependent, in part, on a bidirectional proton-sensitive mechanism similar to bacterial urea transporters and reminiscent of their ancestral origins. Rapid equilibration of urea across the mitochondrial membrane may be vital for cell osmoregulation (elasmobranch) or nitrogen waste excretion (teleost).

Water and ion channels: crucial in the initiation and progression of apoptosis in central nervous system?

Programmed cell death (PCD), is a highly regulated and sophisticated cellular mechanism that commits cell to isolated death fate. PCD has been implicated in the pathogenesis of numerous neurodegenerative disorders. Countless molecular events underlie this phenomenon, with each playing a crucial role in death commitment. A precedent event, apoptotic volume decrease (AVD), is ubiquitously observed in various forms of PCD induced by different cellular insults. Under physiological conditions, cells when subjected to osmotic fluctuations will undergo regulatory volume increase/decrease (RVI/RVD) to achieve homeostatic balance with neurons in the brain being additionally protected by the blood-brain-barrier. However, during AVD following apoptotic trigger, cell undergoes anistonic shrinkage that involves the loss of water and ions, particularly monovalent ions e.g. K(+), Na(+) and Cl(-). It is worthwhile to concentrate on the molecular implications underlying the loss of these cellular components which posed to be significant and crucial in the successful propagation of the apoptotic signals. Microarray and real-time PCR analyses demonstrated several ion and water channel genes are regulated upon the onset of lactacystin (a proteosomal inhibitor)-mediated apoptosis. A time course study revealed that gene expressions of water and ion channels are being modulated just prior to apoptosis, some of which are aquaporin 4 and 9, potassium channels and chloride channels. In this review, we shall looked into the molecular protein machineries involved in the execution of AVD in the central nervous system (CNS), and focus on the significance of movements of each cellular component in affecting PCD commitment, thus provide some pharmacological advantages in the global apoptotic cell death.

Aquaporin 1 - a novel player in spinal cord injury

The role of water channel aquaporin 1 (AQP-1) in uninjured or injured spinal cords is unknown. AQP-1 is weakly expressed in neurons and gray matter astrocytes, and more so in white matter astrocytes in uninjured spinal cords, a novel finding. As reported before, AQP-1 is also present in ependymal cells, but most abundantly in small diameter sensory fibers of the dorsal horn. Rat contusion spinal cord injury (SCI) induced persistent and significant four- to eightfold increases in AQP-1 levels at the site of injury (T10) persisting up to 11 months post-contusion, a novel finding. Delayed AQP-1 increases were also found in cervical and lumbar segments, suggesting the spreading of AQP-1 changes over time after SCI. Given that the antioxidant melatonin significantly decreased SCI-induced AQP-1 increases and that hypoxia inducible factor-1alpha was increased in acutely and chronically injured spinal cords, we propose that chronic hypoxia contributes to persistent AQP-1 increases after SCI. Interestingly; AQP-1 levels were not affected by long-lasting hypertonicity that significantly increased astrocytic AQP-4, suggesting that the primary role of AQP-1 is not regulating isotonicity in spinal cords. Based on our results we propose possible novel roles for AQP-1 in the injured spinal cords: (i) in neuronal and astrocytic swelling, as AQP-1 was increased in all surviving neurons and reactive astrocytes after SCI and (ii) in the development of the neuropathic pain after SCI. We have shown that decreased AQP-1 in melatonin-treated SCI rats correlated with decreased AQP-1 immunolabeling in the dorsal horns sensory afferents, and with significantly decreased mechanical allodynia, suggesting a possible link between AQP-1 and chronic neuropathic pain after SCI.

Water movements in the brain: role of aquaporins

About 80% of the brain is water. This review discusses the importance of the three brain water-channel proteins (AQP1, AQP4, AQP9) in brain physiology. AQP1 is expressed in the choroid plexus and participates in forming cerebrospinal fluid. AQP4, found in astrocyte foot processes, glia limitans and ependyma, facilitates water movement into and out of the brain, accelerates astrocyte migration and alters neuronal activity. Recently, AQP4 autoantibodies were discovered in patients with neuromyelitis optica, a demyelinating disease, and are now being used to diagnose this condition. AQP9 is present in some glia and neurons, but its function is unclear. Finally, we discuss how the discovery of AQP activators and inhibitors will be the next major step in this field.

Aquaporins in the brain: from aqueduct to "multi-duct"

The aquaporin channel family was first considered as a family of water channels, however it is now clear that some of these channels are also permeable to small solutes such glycerol, urea and monocarboxylates. In this review, we will consider AQP4 and AQP9 expressed in the rodent brain. AQP4 is present on astrocytic end-feet in contact with brain vessels and could be involved in ionic homeostasis. However, AQP4 may also be involved in cell adhesion. AQP4 expression is highly modified in several brain disorders and it can play a key role in the cerebral edema formation. However, the exact role of AQP4 in edema formation is still debated. Recently, AQP4 has been shown to be also involved in astrocyte migration during glial scar formation. AQP9 is expressed in astrocytes and in catecholaminergic neurons. Two isoforms of AQP9 are expressed in brain cells, the shortest isoform is localized in the inner membrane of mitochondria and the longest in the cell membrane. The level of expression of AQP9 is negatively regulated by high concentrations of insulin. Taken together, these results suggest that AQP9 could be involved in brain energy metabolism. The induction of AQP9 in astrocytes is observed with time after stroke onset suggesting participation in the clearance of excess lactate in the extracellular space. These recent exciting results suggest that AQPs may not only be involved in water homeostasis in the brain but could also participate in other important physiological functions.

Cytotoxic edema: mechanisms of pathological cell swelling

Cerebral edema is caused by a variety of pathological conditions that affect the brain. It is associated with two separate pathophysiological processes with distinct molecular and physiological antecedents: those related to cytotoxic (cellular) edema of neurons and astrocytes, and those related to transcapillary flux of Na+ and other ions, water, and serum macromolecules. In this review, the authors focus exclusively on the first of these two processes. Cytotoxic edema results from unchecked or uncompensated influx of cations, mainly Na+, through cation channels. The authors review the different cation channels that have been implicated in the formation of cytotoxic edema of astrocytes and neurons in different pathological states. A better understanding of these molecular mechanisms holds the promise of improved treatments of cerebral edema and of the secondary injury produced by this pathological process.

The role of mtDNA mutations in the pathogenesis of age-related hearing loss in mice carrying a mutator DNA polymerase gamma

Mitochondrial DNA (mtDNA) mutations may contribute to aging and age-related diseases. Previously, we reported that accumulation of mtDNA mutations is associated with age-related hearing loss in mice carrying a mutator allele of the mitochondrial Polg DNA polymerase. To elucidate the role of mtDNA mutations in the pathogenesis of age-related hearing loss or presbycusis, we performed large scale gene expression analysis to identify mtDNA mutation-responsive genes and biological process categories associated with mtDNA mutations by comparing the gene expression patterns of cochlear tissues from 9-month-old mitochondrial mutator and control mice. mtDNA mutations were associated with transcriptional alterations consistent with impairment of energy metabolism, induction of apoptosis, cytoskeletal dysfunction, and hearing dysfunction in the cochlea of aged mitochondrial mutator mice. TUNEL staining and caspase-3 immunostaining analysis demonstrated that the levels of apoptotic markers were significantly increased in the cochleae of mitochondrial mutator mice compared to age-matched controls. These observations support a new model of how mtDNA mutations impact cochlear function whereby accumulation of mtDNA mutations lead to mitochondrial dysfunction, an associated impairment of energy metabolism, and the induction of an apoptotic program. The data presented here provide the first global assessment at the molecular level of the pathogenesis of age-related disease in mitochondrial mutator mice and reveal previously unrecognized biological pathways associated with mtDNA mutations.

Aquaporins: a promising target for drug development

Aquaporins (AQPs) are a family of small hydrophobic, integral membrane proteins that are expressed in all living organisms and play critical roles in controlling the water flow into and out of cells. So far, 13 different AQPs have been identified in mammals (AQP 0-12). AQPs have recently been implicated in various diseases such as cancer, cataract, brain oedema, gallstone disease and nephrogenic diabetes insipidus, as well as in the development of obesity and polycystic kidney disease. Interfering with the expression of AQPs will undoubtedly have therapeutic applications. Hence, in this review, the authors look at each AQP and its association with various pathological conditions in humans and demonstrate that they form potential targets for the treatment of such diseases.

Projection map of aquaporin-9 at 7 A resolution

Aquaporin-9, an aquaglyceroporin present in diverse tissues, is unique among aquaporins because it is not only permeable to water, urea and glycerol, but also allows passage of larger uncharged solutes. Single particle analysis of negatively stained recombinant rat aquaporin-9 revealed a particle size characteristic of the tetrameric organization of all members of the aquaporin family. Reconstitution of aquaporin-9 into two-dimensional crystals enabled us to calculate a projection map at 7 A resolution. The projection structure indicates a tetrameric structure, similar to GlpF, with each square-like monomer forming a pore. A comparison of the pore-lining residues between the crystal structure of GlpF and a homology model of aquaporin-9 locates substitutions in these residues predominantly to the hydrophobic edge of the tripathic pore of GlpF, providing first insights into the structural basis for the broader substrate specificity of aquaporin-9.