Reference: Biol. Bull. 229: 3–5. (August 2015) © 2015 Marine Biological Laboratory
New Insights into Aquaporin Evolution and Physiology in Eukaryotic Organisms: Introduction to a Virtual Symposium in The Biological Bulletin `* JOAN CERDA Institut de Recerca i Tecnologia Agroalimenta`ries (IRTA)-Institut de Cie`ncies del Mar, Consejo Superior de Investigaciones Cientı´ficas (CSIC), 08003 Barcelona, Spain
Water is the solvent of life, and consequently the control of water homeostasis is vital for the survival of any organism. The mechanisms involved in water transport across biological barriers have long been the subject of physiological investigations. However, although membrane water channels were predicted to exist as early as the 1970s, it was not until 1992 that the first aquaporin water channel was serendipitously discovered by Peter Agre and collaborators (Preston et al., 1992. Science 256: 385–387), for which Agre was later awarded the 2003 Nobel Prize in Chemistry. Aquaporins are pore-forming membrane channels that primarily allow the passage of water and other non-charged solutes across biological membranes following an osmotic gradient, while excluding the transit of ions. Aquaporins are present in all kingdoms of life, from bacteria to humans, where they form a superfamily with up to 17 subfamilies in vertebrates and up to 71 paralogs in plants. It is now well established that aquaporins are permeable to water, glycerol, and urea, but also to other unconventional solutes such as ammonia, metalloids, and hydrogen peroxide, as well as nitrite, chloride, and other anions. Aquaporins play important roles in fluid secretion and absorption in different tissues, and in other unexpected processes including cell migration and neural signaling. Genetic alterations in aquaporins, leading to channel malfunction or defects in their intracellular trafficking, have been related to several pathophysiological conditions, including nephrogenic diabetes insipidus, brain edema, obesity, wound healing, and congenital cataracts. Crystallographic studies and methods for molecular dynamics simulation are providing the necessary tools for the future design of specific aquaporin inhibitory
molecules, thus setting the basis for the development of novel therapies. Despite the exceptionally fast growth of the aquaporin field during the past several years, numerous questions remain. Indeed, the physiological functions of many aquaporins, as well as the physiological relevance of their selectivity to unconventional permeants, still remain poorly understood. One of the underlying obstacles is the failure of gene knockout approaches to uncover the biological role and complex regulatory networks of certain aquaporins in a given tissue or organ. In some cases, however, the lack of information can also be attributed to the almost exclusive use of established model organisms, which may not necessarily be best suited for studying the physiological role of a specific aquaporin. In recent years, the use of technological advances in genomics and transcriptomics across species has revealed the diversity of aquaporins in many non-model organisms, including those with astonishingly adaptive mechanisms for controlling fluid homeostasis under extreme environmental conditions of temperature, water availability, or osmolality. These new resources may thus facilitate the application of August Krogh’s longstanding principle, “For many problems there is an animal on which it can be most conveniently studied” (Krogh, 1929. Am. J. Physiol. 90: 243–251), to modern aquaporin biology research, which may help to uncover novel roles for aquaporins, shed light on other poorly understood mechanisms in humans, and open new avenues for biotechnological applications. Bringing together experts who are investigating the physiology and biology of aquaporins in alternative model organisms, including protozoans, sponges, insects, fish, amphibians, and rodents, is the objective of this virtual symposium issue of The Biological Bulletin.
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` J. CERDA
The symposium opens with an introductory review by Finn and Cerda` (pp. 6 –23), summarizing the great diversity of aquaporin water channels in all domains of life, from Archaea and Bacteria to Eukaryota. This study gives a short overview of the structure and function of aquaporins, and provides a more in-depth view of the phylogenetic interrelationships and evolutionary mechanisms, such as horizontal gene transfer and genomic and tandem duplications, that may have given rise to the four major grades of eukaryotic aquaporins: the classical water-selective aquaporins, the water- and glycerol-transporting aquaporins, known as aquaglyceroporins (Glps), the aquaporin-8 (Aqp8)-type aquaammoniaporins, and the unorthodox aquaporins. The spicules constituting the skeleton of sponges (Porifera) produce a biosilica matrix, which is formed enzymatically via silicatein from amorphous hydrated silica. During this process, but after the polycondensation reaction, biosilica undergoes a process of syneresis, during which water is extruded, spicule diameter shrinks, and the biosilica is hardened to function as a solid rod. Wang and Mu¨ller (pp. 24 –37) review the current evidence that an Aqp8-related channel is involved in the removal of reaction water at the sites where the siliceous spicules are formed. This process is also driven by a mucin/nidogen-like protein, which induces contraction of the biosilica product. Interestingly, the commonly used aquaporin inhibitor, mercury chloride, as well as manganese ions, which are potent blockers of the gating of some plant aquaporins, prevent the hardening of the biosilica product and, ultimately, formation of the spicules. This original research illustrates the role of an aquaporin in a rather unexpected process, and may lead to new applications for biosilica-based materials in biomedicine and nanooptics. von Bu¨low and Beitz (pp. 38 – 46) review the diversity of aquaporins in protozoa that show different lifestyles. Although some protozoa live freely, in direct contact with the environment, many of these organisms are well-known human parasites that develop inside cells or swim in the bloodstream and cause debilitating diseases such as malaria, toxoplasmosis, Chagas’ disease, leishmaniasis, or trypanosomiasis (sleeping sickness). In this contribution, the authors present an interesting comparative study of the number, differential subcellular localization, and function of aquaporins in these different protozoa. They conclude that free-living protozoa show a higher number of aquaporins, more highly specialized, stage-dependent expression and localization during development, and even more complex mechanisms for channel regulation, than endogenous protozoa with a protected life inside a host. Thus, aquaporin diversity in these organisms likely reflects adaptations to changing environments. Since protozoan aquaporins, such as the Glp of the human malaria parasite Plasmodium falciparum, have been suggested as potential drug targets for
disease treatments, this study summarizes information of interest for the future design of therapeutic strategies. Insects are the most successful group of terrestrial organisms, accounting for more than half of the world’s eukaryotic biodiversity. Such a remarkable explosion of life forms underscores the ability of insects to colonize extremely diverse niches, from those with exceptionally arid conditions to those in sub-zero temperatures. The success of these organisms has been associated with the emergence of a number of adaptations for survival in various harsh environments. Therefore, insects appear to be excellent models for investigating the role of aquaporin-mediated fluid transport in osmoregulation and freeze tolerance. The next review and research paper cover these aspects specifically. Goto et al. (pp. 47–57) focus on the aquaporins of the Antarctic midge Belgica antarctica, which extensively dehydrates to survive the low temperatures and desiccation stress that occur in its habitat. Although these studies are still in their initial phase, the data suggest a wide tissue distribution of aquaporins in this insect, which change abundance in response to dehydration, rehydration, and freezing. The research paper by Maruyama et al. (pp. 58 – 69) investigates the distribution of two insect-specific aquaporins, the Pyrocoelia rufa integral protein (PRIP)- and Drosophila intrinsic protein (DRIP)-like channels, in ovarian follicles of the silk moth Bombyx mori. Interestingly, DRIP was localized in peripheral yolk granules of diapause-destined oocytes, whereas PRIP was found only in the oocyte plasma membrane. There, during oogenesis, PRIP might mediate oocyte water uptake facilitated by transepithelial ion movements, thus producing a water reservoir for further embryonic development. Remarkably, this mechanism resembles that which occurs in marine teleost oocytes undergoing meiotic maturation, where water influx, driven by an increased osmolyte pool of free amino acids resulting from yolk proteolysis, is mediated by a teleost-specific Aqp1-like channel (Fabra et al., 2005. Science 307: 545). Fish are constantly exposed to the aquatic environment and have evolved adaptations for the maintenance of body fluid homeostasis in both saline and freshwater environments. Teleost fish show the largest repertoire of aquaporins among vertebrates as a result of whole genome- and lineage-specific duplications; interestingly, many duplicated genes are redundantly expressed in some tissues, which may reflect the existence of teleost aquaporin paralogs with specialized functions. However, the physiological roles of most fish aquaporins are still unknown. The next two papers review existing information on aquaporin physiology during fish osmoregulation and male germ cell development, including activation of spermatozoa, in which the control of fluid transport is critical for successful fertilization. The review by Madsen et al. (pp. 70 –92) summarizes the distribution of aquaporins in the major osmoregulatory organs of teleosts, elasmobranchs, and agnathans, such as the
AQUAPORIN EVOLUTION AND PHYSIOLOGY
gills, intestine, and kidney. Current data on the potential role of aquaporins in transepithelial water transport are also discussed. This study includes an interesting discussion on the most likely concerted action of transcellular (aquaporin-mediated) and paracellular (non-aquaporinmediated) mechanisms for water absorption across the intestinal epithelium. Boj et al. (pp. 93–108) provide a comparative view of aquaporin biology in the male reproductive tissues of mammals and teleosts. Although detailed studies in teleosts have only been carried out in one marine species so far, data suggest that both mammalian and teleost aquaporins may play similar roles in regulating osmosis-induced volume changes and the passage of metabolites such as glycerol during germ cell development and the maturation and activation of spermatozoa. Recent findings suggesting the possibility that aquaporins may function beyond water transport, such as in signaling pathways during spermatogenesis, or the sensing of cell swelling and mitochondrial peroxide transport in motile sperm, are also reviewed. Suzuki et al. (pp. 109 –119) provide an overview of the diversity of aquaporins in amphibians, which represent the first vertebrates to adapt to terrestrial environments. Unlike marine teleosts, most amphibians do not drink water to counteract evaporative water loss through the skin; instead, they absorb water from shallow water sources or moist substrates across the ventral skin, and they also reabsorb water from the renal tubular fluid and from urine stored in the urinary bladder. The authors summarize a series of studies showing that several different aquaporins play key roles in transepithelial water absorption and reabsorption in these organs. As observed in mammals, the function of some of these channels appears to be tightly regulated by the neurohypophyseal hormone, arginine vasotocin, the non-mammalian counterpart of vasopressin. Some of these studies are particularly noteworthy, as they provide the best characterization to date of the molecular regulatory mech-
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anisms of aquaporin intracellular trafficking in a non-mammalian vertebrate. The final contribution in the symposium, by Thomas Pannabecker (pp. 120 –128), deals with the role of aquaporins in desert rodents. These mammals have the ability to produce unusually concentrated urine and very dry feces to minimize evaporative water losses through the lungs, nose, and skin, all adaptive mechanisms for maintaining salt and water homeostasis in an arid environment. Pannabecker reviews physiological and anatomical studies of aquaporins in the organ systems of desert rodents involved in desiccation resistance, principally in the kidney, but also in the gastrointestinal tract, ear, and nose. The author also presents evidence of the role that different aquaporins may play in facilitating effective water preservation. A comparison of the findings with laboratory strains of rats and mice indicates that the pattern of aquaporin expression in the organs of desert rodents does not always follow those in established laboratory models. Deeper investigation into the aquaporin pathways involved in transmembrane and/or transepithelial water flows in the organs of desert rodents may offer new insights into the cellular roles and regulation of aquaporins, and particularly in the urine concentrating mechanism. The contributions included in this virtual symposium issue of The Biological Bulletin represent a limited set of non-model eukaryotic organisms in which aquaporin physiology is currently being investigated. Nevertheless, I believe they illustrate how the organisms studied may be advantageous for investigating basic aspects of aquaporins, showing unexpected scenarios where aquaporins are involved, and highlighting how comparative physiology can contribute to our understanding of the function and evolution of these important channels. As editor, I thank the authors for their insights and hope that this symposium will stimulate further studies in non-model organisms to uncover the many questions and facets that still remain to be addressed and answered in the field of aquaporin biology.
