Involvement of aquaglyceroporins in energy metabolism in health and disease.

Aquaglyceroporins are a group of the aquaporin (AQP) family of transmembrane water channels. While AQPs facilitate the passage of water, small solutes, and gases across biological membranes, aquaglyceroporins allow passage of water, glycerol, urea and some other solutes. Thanks to their glycerol permeability, aquaglyceroporins are involved in energy homeostasis. This review provides an overview of what is currently known concerning the functional implication and control of aquaglyceroporins in tissues involved in energy metabolism, i.e. liver, adipose tissue and endocrine pancreas. The expression, role and (dys)regulation of aquaglyceroporins in disorders affecting energy metabolism, and the potential relevance of aquaglyceroporins as drug targets to treat the alterations of the energy balance is also addressed.

Role of aquaglyceroporins and caveolins in energy and metabolic homeostasis.

Aquaglyceroporins and caveolins are submicroscopic integral membrane proteins that are particularly abundant in many mammalian cells. Aquaglyceroporins (AQP3, AQP7, AQP9 and AQP10) encompass a subfamily of aquaporins that allow the movement of water, but also of small solutes, such as glycerol, across cell membranes. Glycerol constitutes an important metabolite as a substrate for de novo synthesis of triacylglycerols and glucose as well as an energy substrate to produce ATP via the mitochondrial oxidative phosphorylation. In this sense, the control of glycerol influx/efflux in metabolic organs by aquaglyceroporins plays a crucial role with the dysregulation of these glycerol channels being associated with metabolic diseases, such as obesity, insulin resistance, non-alcoholic fatty liver disease and cardiac hypertrophy. On the other hand, caveolae have emerged as relevant plasma membrane sensors implicated in a wide range of cellular functions, including endocytosis, apoptosis, cholesterol homeostasis, proliferation and signal transduction. Caveolae-coating proteins, namely caveolins and cavins, can act as scaffolding proteins within caveolae by concentrating signaling molecules involved in free fatty acid and cholesterol uptake, proliferation, insulin signaling or vasorelaxation, among others. The importance of caveolae in whole-body homeostasis is highlighted by the link between homozygous mutations in genes encoding caveolins and cavins with metabolic diseases, such as lipodystrophy, dyslipidemia, muscular dystrophy and insulin resistance in rodents and humans. The present review focuses on the role of aquaglyceroporins and caveolins on lipid and glucose metabolism, insulin secretion and signaling, energy production and cardiovascular homeostasis, outlining their potential relevance in the development and treatment of metabolic diseases.

Yeast reveals unexpected roles and regulatory features of aquaporins and aquaglyceroporins.

BACKGROUND: The yeast Saccharomyces cerevisiae provides unique opportunities to study roles and regulation of aqua/glyceroporins using frontline tools of genetics and genomics as well as molecular cell and systems biology. SCOPE OF REVIEW: S. cerevisiae has two similar orthodox aquaporins. Based on phenotypes mediated by gene deletion or overexpression as well as on their expression pattern, the yeast aquaporins play important roles in key aspects of yeast biology: establishment of freeze tolerance, during spore formation as well as determination of cell surface properties for substrate adhesion and colony formation. Exactly how the aquaporins perform those roles and the mechanisms that regulate their function under such conditions remain to be elucidated. S. cerevisiae also has two different aquaglyceroporins. While the role of one of them, Yfl054c, remains to be determined, Fps1 plays critical roles in osmoregulation by controlling the accumulation of the osmolyte glycerol. Fps1 communicates with two osmo-sensing MAPK signalling pathways to perform its functions but the details of Fps1 regulation remain to be determined. MAJOR CONCLUSIONS: Several phenotypes associated with aqua/glyceroporin function in yeasts have been established. However, how water and glycerol transport contribute to the observed effects is not understood in detail. Also many of the basic principles of regulation of yeast aqua/glyceroporins remain to be elucidated. GENERAL SIGNIFICANCE: Studying the yeast aquaporins and aquaglyceroporins offers rich insight into the life style, evolution and adaptive responses of yeast and rewards us with discoveries of unexpected roles and regulatory mechanisms of members of this ancient protein family. This article is part of a Special Issue entitled Aquaporins.

Functional characterization of three aquaglyceroporins from Trypanosoma brucei in osmoregulation and glycerol transport.

Previous studies using bloodstream form Trypanosoma brucei have shown that glycerol transport in this parasite occurs via specific membrane proteins, namely a glycerol transporter and glycerol channels [1]. Later, we cloned, expressed and characterized the transport properties of all three aquaglyceroporins (AQP1-3) [2], which were found permeable for water, glycerol and other small uncharged solutes like dihydroxyacetone [3]. Here, we report on the cellular localization of TbAQP1 and TbAQP3 in bloodstream form trypanosomes. Indirect immunofluorescence analysis showed that TbAQP1 is exclusively localized in the flagellar membrane, whereas TbAQP3 was found in the plasma membrane.In addition, we analyzed the functions of all 3 AQPs, using an inducible inheritable double-stranded RNA interference methodology. All AQP knockdown cell lines were still able to survive hypo-osmotic stress conditions, except AQP2 knockdown parasites. Depleted TbAQP2 negatively impacted cell growth and the regulatory volume recovery, whereas AQP1 und 3 knockdown trypanosomes displayed phenotypes consistent with their localization in external membranes. A simultaneous knockdown of all 3 AQPs showed that the cells were able to substitute the missing glycerol uptake capability through a putative glycerol transporter.

The sense of water in the blowfly Protophormia terraenovae.

The gustatory system of the blowfly, Protophormia terraenovae, is a relatively simple biological model for studies on chemosensory input and behavioral output. It appears to have renewed interest as a model for studies on the role of water channels, namely aquaporins or aquaglyceroporins, in water detection. To this end, we investigated the presence of water channels, their role in "water" and "salt" cell responsiveness and the transduction mechanism involved. For the first time our electrophysiological results point to the presence of an aquaglyceroporin in the chemoreceptor membrane of the "water" cell in the blowfly taste chemosensilla whose transduction mechanism ultimately involves an intracellular calcium increase and consequently cell depolarization. This hypothesis is also supported by calcium imaging data following proper stimulation. This mechanism is triggered by "water" cell stimulation with hypotonic solutions and/or solutes such as glycerol which crosses the membrane by way of aquaglyceroporins. Behavioral output indicates that the "sense" of water in blowflies is definitely not dependent on the "water" cell only, but also on the "salt" cell sensitivity. These findings also hypothesize a new role for aquaglyceroporin in spiking cell excitability.

Aquaglyceroporins are involved in uptake of arsenite into murine gastrointestinal tissues.

Aquaglyceroporins (AQGPs) are members of aquaporin (AQP) family and belong to a subgroup of this water channel family; they are transmembrane proteins that transport water as well as glycerol and other solutes of small molecules. Recent studies have also identified that AQGPs are important transporters of trivalent metalloid in some mammalian cells. However, the uptake routes of arsenite in mammals are still less defined. In this study, to understand the routes of arsenite intake in mammals, mice were treated with Hg(II), glycerol, and As(III) and uptake of As(III) into the gastrointestinal tissues was measured. The level of inorganic arsenic (iAs) in gastrointestinal tissues after As(III) stimulation was much higher than Hg(II) +As(III) or glycerol+As(III) group. RT-PCR results showed that AQGPs were extensively expressed in gastrointestinal tissues of mice. We also treated Caco-2 cells with Hg(II) and As(III); the level of iAs in a group treated with Hg(II)+As(III) decreased compared with As(III)-treated group. Our results suggested that AQGPs could be important transporters in arsenite uptake into gastrointestinal tissues of mice, but more data are need to prove if AQGPs is the only pathway involved in As transport in mammals or just one of them.

Aquaglyceroporins: ancient channels for metalloids.

The identification of aquaglyceroporins as uptake channels for arsenic and antimony shows how these toxic elements can enter the food chain, and suggests that food plants could be genetically modified to exclude arsenic while still accumulating boron and silicon.

Role of aquaglyceroporin (AQP1) gene and drug uptake in antimony-resistant clinical isolates of Leishmania donovani.

Antimonial-containing drugs are the first line of treatment against Leishmaniasis. Resistance to antimonials in Leishmania is proposed to be due to reduced uptake of trivalent antimony (SbIII) through the aquaglyceroporin (AQP1). We investigated the uptake of SbIII and involvement of aquaglyceroporin in developing antimony resistance phenotype in Leishmania donovani clinical isolates. SbIII accumulation, copy number of AQP1 gene, and transcript levels were compared in antimony-sensitive versus -resistant isolates. Antimony-resistant field isolates showed reduced uptake of SbIII. The copy number of AQP1 gene showed higher copy number in the antimony-resistant isolates when compared with the sensitive isolates and did not correlate to the reduced uptake of SbIII. Downregulation of AQP1 RNA levels was not consistently found in the antimony-resistant isolates. Our studies indicate that while downregulation of AQP1 may be one of the mechanisms of antimony resistance, it is however not an invariable feature.

Plant aquaporins: membrane channels with multiple integrated functions.

Aquaporins are channel proteins present in the plasma and intracellular membranes of plant cells, where they facilitate the transport of water and/or small neutral solutes (urea, boric acid, silicic acid) or gases (ammonia, carbon dioxide). Recent progress was made in understanding the molecular bases of aquaporin transport selectivity and gating. The present review examines how a wide range of selectivity profiles and regulation properties allows aquaporins to be integrated in numerous functions, throughout plant development, and during adaptations to variable living conditions. Although they play a central role in water relations of roots, leaves, seeds, and flowers, aquaporins have also been linked to plant mineral nutrition and carbon and nitrogen fixation.

A current view of the mammalian aquaglyceroporins.

The discovery of aquaporin water channels by Agre and coworkers answered a long-standing biophysical question of how the majority of water crosses biological membranes. The identification and study of aquaporins have provided insight, at the molecular level, into the fundamental physiology of water balance regulation and the pathophysiology of water balance disorders. In addition to the originally identified classical aquaporins, a second class of aquaporins has been identified. Aquaporins in this latter class, the so-called aquaglyceroporins, transport small uncharged molecules such as glycerol and urea as well as water. Aquaglyceroporins have a wide tissue distribution, and emerging data suggest that several of them may play previously unappreciated physiological or pathophysiological roles. Analyses of transgenic mice have revealed potential roles of aquaglyceroporins in skin elasticity, gastrointestinal function and metabolism, and metabolic diseases such as diabetes mellitus. This review comprehensively discusses the recent discoveries in the field of aquaglyceroporins, alongside a brief overview of the so-called unorthodox aquaporins.

Metalloids: essential, beneficial or toxic? Major intrinsic proteins sort it out.

Major intrinsic proteins (MIPs) are a family of selective membrane channels comprising water-channelling aquaporins and glycerol-channelling aquaglyceroporins. Recently, several MIPs within all domains of life were shown to facilitate the diffusion of reduced and non-charged species of the metalloids silicon, boron, arsenic and antimony. Metalloids encompass a group of biologically important elements ranging from the essential to the highly toxic. Consequently, all organisms require efficient membrane transport systems to control the exchange of metalloids with the environment. Recent genetic evidence has demonstrated a crucial role for specific MIPs in metalloid homeostasis. We propose that specific MIPs represent an ancient and indispensable transport mechanism for metalloids, which suggests that they could be potential pharmacological targets.

Yeast osmoregulation.

Osmoregulation is the active control of the cellular water balance and encompasses homeostatic mechanisms crucial for life. The osmoregulatory system in the yeast Saccharomyces cerevisiae is particularly well understood. Key to yeast osmoregulation is the production and accumulation of the compatible solute glycerol, which is partly controlled by the high osmolarity glycerol (HOG) signaling system. Genetic analyses combined with studies on protein-protein interactions have revealed the wiring scheme of the HOG signaling network, a branched mitogen-activated protein (MAP) kinase (MAPK) pathway that eventually converges on the MAPK Hog1. Hog1 is activated following cell shrinking and controls posttranscriptional processes in the cytosol as well as gene expression in the nucleus. HOG pathway activity can easily and rapidly be controlled experimentally by extracellular stimuli, and signaling and adaptation can be separated by a system of forced adaptation. This makes yeast osmoregulation suitable for studies on system properties of signaling and cellular adaptation via mathematical modeling. Computational simulations and parallel quantitative time course experimentation on different levels of the regulatory system have provided a stepping stone toward a holistic understanding, revealing how the HOG pathway can combine rigorous feedback control with maintenance of signaling competence. The abundant tools make yeast a suitable model for an integrated analysis of cellular osmoregulation. Maintenance of the cellular water balance is fundamental for life. All cells, even those in multicellular organisms with an organism-wide osmoregulation, have the ability to actively control their water balance. Osmoregulation encompasses homeostatic processes that maintain an appropriate intracellular environment for biochemical processes as well as turgor of cells and organism. In the laboratory, the osmoregulatory system is studied most conveniently as a response to osmotic shock, causing rapid and dramatic changes in the extracellular water activity. Those rapid changes mediate either water efflux (hyperosmotic shock), and hence cell shrinkage, or influx (hypoosmotic shock), causing cell swelling. The yeast S. cerevisiae, as a free-living organism experiencing both slow and rapid changes in extracellular water activity, has proven a suitable and genetically tractable experimental system in studying the underlying signaling pathways and regulatory processes governing osmoregulation. Although far from complete, the present picture of yeast osmoregulation is both extensive and detailed (de Nadal et al., 2002; Hohmann, 2002; Klipp et al., 2005). Simulations using mathematical models combined with time course measurements of different molecular processes in signaling and adaptation have allowed elucidation of the first system properties on the yeast osmoregulatory network.

Aquaglyceroporin PbAQP during intraerythrocytic development of the malaria parasite Plasmodium berghei.

The malaria parasite can use host plasma glycerol for lipid biosynthesis and membrane biogenesis during the asexual intraerythrocytic development. The molecular basis for glycerol uptake into the parasite is undefined. We hypothesize that the Plasmodium aquaglyceroporin provides the pathway for glycerol uptake into the malaria parasite. To test this hypothesis, we identified the orthologue of Plasmodium falciparum aquaglyceroporin (PfAQP) in the rodent malaria parasite, Plasmodium berghei (PbAQP), and examined the biological role of PbAQP by performing a targeted deletion of the PbAQP gene. PbAQP and PfAQP are 62% identical in sequence. In contrast to the canonical NPA (Asn-Pro-Ala) motifs in most aquaporins, the PbAQP has NLA (Asn-Leu-Ala) and NPS (Asn-Leu-Ser) in those positions. PbAQP expressed in Xenopus oocytes was permeable to water and glycerol, suggesting that PbAQP is an aquaglyceroporin. In P. berghei, PbAQP was localized to the parasite plasma membrane. The PbAQP-null parasites were viable; however, they were highly deficient in glycerol transport. In addition, they proliferated more slowly compared with the WT parasites, and mice infected with PbAQP-null parasites survived longer. Taken together, these findings suggest that PbAQP provides the pathway for the entry of glycerol into P. berghei and contributes to the growth of the parasite during the asexual intraerythrocytic stages of infection. In conclusion, we demonstrate here that PbAQP plays an important role in the blood-stage development of the rodent malaria parasite during infection in mice and could be added to the list of targets for the design of antimalarial drugs.

Ammonia permeability of the aquaglyceroporins from Plasmodium falciparum, Toxoplasma gondii and Trypansoma brucei.

Plasmodium falciparum uses amino acids from haemoglobin degradation mainly for protein biosynthesis. Glutamine, however, is mostly oxidized to 2-oxoglutarate to restore NAD(P)H + H+. In this process two molecules of ammonia are released. We determined an ammonia production of 9 mmol h(-1) per litre of infected red blood cells in the early trophozoite stage. External application of ammonia yielded a cytotoxic IC50 concentration of 2.8 mM. As plasmodia cannot metabolize ammonia it must be exported. Yet, no biochemical or genomic evidences exist that plasmodia possess classical ammonium transporters. We expressed the P. falciparum aquaglyceroporin (PfAQP) in Xenopus laevis oocytes and examined whether it may serve as an exit pathway for ammonia. We show that injected oocytes: (i) acidify the medium due to ammonia uptake, (ii) take up [14C]methylamine and [14C]formamide, (iii) swell in solution with formamide and acetamide and (iv) display an ammonia-induced NH4+-dependent clamp current. Further, a yeast strain lacking the endogenous aquaglyceroporin (Fps1) is rescued by expression of PfAQP which provides for the efflux of toxic methylamine. Ammonia permeability was similarly established for the aquaglyceroporins from Toxoplasma gondii and Trypanosoma brucei. Apparently, these aquaglyceroporins are important for the release of ammonia derived from amino acid breakdown.

The structure, function and regulation of the nodulin 26-like intrinsic protein family of plant aquaglyceroporins.

The nodulin 26-like intrinsic protein family is a group of highly conserved multifunctional major intrinsic proteins that are unique to plants, and which transport a variety of uncharged solutes ranging from water to ammonia to glycerol. Based on structure-function studies, the NIP family can be subdivided into two subgroups (I and II) based on the identity of the amino acids in the selectivity-determining filter (ar/R region) of the transport pore. Both subgroups appear to contain multifunctional transporters with low to no water permeability and the ability to flux multiple uncharged solutes of varying sizes depending upon the composition of the residues of the ar/R filter. NIPs are subject to posttranslational phosphorylation by calcium-dependent protein kinases. In the case of the family archetype, soybean nodulin 26, phosphorylation has been shown to stimulate its transport activity and to be regulated in response to developmental as well as environmental cues, including osmotic stresses. NIPs tend to be expressed at low levels in the plant compared to other MIPs, and several exhibit cell or tissue specific expression that is subject to spatial and temporal regulation during development.

Distinct transport selectivity of two structural subclasses of the nodulin-like intrinsic protein family of plant aquaglyceroporin channels.

Major intrinsic proteins (MIPs) are a diverse class of integral membrane proteins that facilitate the transport of water and some small solutes across cellular membranes. X-ray structures of MIPs indicate that a tetrad of residues (the ar/R region) form a narrow pore constriction that constitutes the selectivity filter. In comparison with mammalian and microbial species, plants have a greater number and diversity of MIPs with greater than 30 genes encoding four phylogenetic subfamilies with eight different classes of ar/R sequences. The nodulin 26-like intrinsic protein (NIP) subfamily in Arabidopsis can be subdivided into two ar/R subgroups: the NIP subgroup I, which resembles the archetype of the family, soybean nodulin 26, and the NIP subgroup II, which is represented by the Arabidopsis protein AtNIP6;1. These two NIPs differ principally by the substitution of a conserved alanine (NIP subgroup II) for a conserved tryptophan (NIP subgroup I) in the helix 2 position (H2) of the ar/R filter. A comparison of the water and solute tranport properties of the two proteins was performed by expression in Xenopus laevis oocytes. Nodulin 26 is an aquaglyceroporin with a modest osmotic water permeability (P(f)) and the ability to transport uncharged solutes such as glycerol and formamide. In constrast, AtNIP6;1 showed no measurable water permeability but transported glycerol, formamide, as well as larger solutes that were impermeable to nodulin 26. By site-directed mutagenesis, we show that the H2 position is the crucial determinant that confers these transport behaviors. A comparison of the NIPs and tonoplast-intrinsic proteins (TIP) shows that the H2 residue can predict the transport profile for water and glycerol with histidine found in TIP-like aquaporins, tryptophan found in aquaglyceroporins (NIP I), and alanine found in water-impermeable glyceroporins (AtNIP6;1).