What are aquaporins? Aquaporins (often called aquaporin water channels) room a household of small, integral membrane proteins that are expressed extensively throughout the animal and plant kingdoms. They have a comparable basic structure, v aquaporin monomers consisting of six transmembrane helical segments and also two short helical segments the surround cytoplasmic and also extrathedesigningfairy.comular vestibules connected by a small aqueous spicy (Figure 1A). Lock contain several conserved motifs, including NPA sequences in their brief helical segments. Aquaporin monomers assemble together tetramers in membranes, with each monomer to work independently. Some aquaporins, such as mammalian AQP4, have the right to further aggregate in thedesigningfairy.com membrane to type supramolecular crystalline assemblies dubbed orthogonal arrays the particles.

You are watching: Which of the structure represents an aquaporin?


*

(A) Membrane topography that the aquaporin monomer (left) and crystal framework (AQP1, PDB 1j4n) (side view, left; optimal view, right) with four water molcules (red balls) displayed in aqueous sharp region. Helices room labeled H1–H8. (B) Epithelial fluid secretion. High transepithelial water permeability facilitates active, near-isosmolar liquid secretion in exocrine glands. (C) thedesigningfairy.com migration. AQP-facilitated thedesigningfairy.com migration involves water entry right into protruding lamellipodia in migrating thedesigningfairy.coms. (D) Skin hydration. AQP3 maintains high level of glycerol in the stratum corneum; glycerol acts as a humectant to maintain water. (E) Adipocyte metabolism. AQP7 facilitates glycerol departure from adipocytes, avoiding intrathedesigningfairy.comular glycerol and also triglyceride accumulation. AQP, aquaporin; TG, triglyceride; FFA, cost-free fatty acid. (Panels B–E adapted from Verkman, A.S. (2011). Aquaporins in ~ a glance. J. Thedesigningfairy.com Sci. 124, 2107–2112.)
What execute aquaporins do at the molecule level? The primary role of many aquaporins is to transfer water across thedesigningfairy.com membranes in solution to osmotic gradients produced by energetic solute transport. Because the water deliver capacity that aquaporin monomers is low, membranes frequently contain a high density of aquaporins, as much as 10,000 per square micron, to boost water permeability substantially over that in the lack of aquaporins. Molecule dynamics simulations indicate that steric factors and also electrostatic interaction in the aqueous pore are responsible because that the selectivity that aquaporins for water. A subset that aquaporins, called aquaglyceroporins additionally transport glycerol. The sharp diameter of the aquaglyceroporins is slightly better than the of the water-selective aquaporins, and the pore is lined by reasonably hydrophobic residues compared with the pore of a water-selective aquaporin. In addition to water and glycerol, over there is evidence, several of which is controversial, that some aquaporins happen gases (CO2, NH3, NO, O2), various tiny solutes such as H2O2 and arsenite, and also even ion (K+, Cl-). Non-transporting features for part aquaporins have additionally been suggested, such as thedesigningfairy.com–thedesigningfairy.com adhesion, membrane polarization and also regulation of connecting proteins, such together ion channels.
Why do some membranes require high water or glycerol permeability? many thedesigningfairy.coms carry out not refer aquaporins. Practically all biological membranes are fairly water permeable as a an effect of water diffusion despite membrane lipids, such that thedesigningfairy.com volume equilibrates in minutes or much less in an answer to one osmotic gradient. So then why should high membrane water permeability be necessary in some thedesigningfairy.coms? One instance is fluid secretion and absorption throughout epithelial thedesigningfairy.com layers, such together in kidney tubules and also exocrine glands, where normal kidney role and secretion of bodily fluids, such as saliva, require high, aquaporin-facilitated transepithelial water permeability. High water permeability likewise facilitates water movement throughout fluid–tissue barriers, such as in between blood and brain parenchyma throughout the blood–brain barrier. Other thedesigningfairy.comular and tissue functions of aquaporin water transport, and of aquaglyceroporin-facilitated glycerol transport, room not obvious a priori.
Where are aquaporins expressed? The mammalian aquaporins, i beg your pardon number around a dozen, room expressed in many thedesigningfairy.com species involved in fluid transport, including epithelia and also endothelia in kidney, lung, exocrine glands, eye and also gastrointestinal organs. However, aquaporins are likewise expressed in thedesigningfairy.coms that carry out not have actually an obvious function in fluid transport, such together erythrocytes and also some leukocytes, adipocytes and also skeletal muscle. Aquaporins are likewise expressed in astrocytes throughout the central nervous system, and also in supportive thedesigningfairy.coms (but not electrically excitable thedesigningfairy.coms) in sensory organs such as retinal Müller thedesigningfairy.coms. In the eye, aquaporins room expressed in cornea, lens and also ciliary epithelium. The tissue distribution of mammalian aquaporins provided initial clues around their thedesigningfairy.com and also organ functions, though plenty of of the early guesses have not been evidenced in phenotype researches on aquaporin knockout mice.
What around aquaporins in plants and lower organisms? Aquaporins space expressed in miscellaneous plants and microbes, consisting of bacteria and also yeast. A huge number of plant aquaporins have actually been identified, dubbed PIPs (plasma membrane intrinsic proteins) and also TIPs (tonoplast intrinsic proteins) for your thedesigningfairy.comular expression patterns. Deletion studies have actually elucidated a range of interesting roles for aquaporins in plants, consisting of transpiration, metabolism and also reproduction, i m sorry are necessary in plant adaptation to various ecological stresses. Though aquaporins are additionally expressed in part microbes, deletion studies have not revealed any type of clear phenotype, though there is suggestive evidence that aquaporins might be safety in freeze–thaw stress, perhaps to preserve the water permeability that thedesigningfairy.com membrane at short temperature. The high surface-to-volume ratio of microbes predicts rapid osmotic equilibration, making aquaporins unnecessary for most functions, such together osmoregulation.
What anticipated functions of aquaporins have been shown from knockout mice? Phenotypic analysis of aquaporin knockout mice has shown the anticipated duty of aquaporins in transepithelial fluid transport (Figure 1B). Deletion of assorted aquaporins in kidney tubules reasons excessive urine output and also impaired urinary concentrating ability. High, aquaporin-dependent water permeability is required in the kidney proximal tubule, because that near-isosmolar liquid absorption, in the slim descending body of Henlé, because that countercurrent multiplication, and in the collecting duct, because that water absorption. Aquaporins additionally facilitate epithelial liquid secretion in salivary and airway submucosal glands, and in the choroid plexus (from i m sorry cerebrospinal fluid is secreted) and ciliary epithelium (from i m sorry ocular aqueous liquid is secreted). Over there is an important caveat, however: aquaporins are forced for transepithelial fluid transport when fluid transport rate (per epithelial surface area) is really high, as in proximal tubule and salivary gland. Though aquaporins room expressed and functional in lung, intestinal and also sweat gland epithelia, they do not show up to have actually a far-ranging physiological role.
What are some less noticeable roles that aquaporin-facilitated water transport? an unanticipated role of aquaporins is in thedesigningfairy.com migration. AQP1 deletion in mice impairs angiogenesis as a consequence of a palliation in the migration rate of microvascular endothelial thedesigningfairy.coms. Aquaporin-dependent migration has been uncovered in a wide selection of thedesigningfairy.com types. Biophysical studies indicate that aquaporin polarization come the top edge of moving thedesigningfairy.coms facilitates water influx throughout lamellipodial expansion (Figure 1C). In comparison to transepithelial liquid transport, where aquaporins facilitate water movement across thedesigningfairy.com layers, aquaporins in moving thedesigningfairy.coms facilitate local, transient water transport. One more unanticipated function of an aquaporin is in neuroexcitation: deletion of AQP4 impairs neurosensory signaling and also alters seizure and also cortical dispersing depression dynamics. Return the mechanism has actually not been totally resolved, it appears that AQP4-facilitated water carry is essential for rapid transforms in extrathedesigningfairy.comular room volume and also K+ concentration, and that lose of AQP4 leads to delayed K+ reuptake following neuroexcitation. Other unexpected roles of water-selective aquaporins incorporate their involvement in corneal and also lens transparency, mind and spinal cord swelling, neuroinflammation and also pain.
What about glycerol transfer by aquaglyceroporins? The mammalian aquaglyceroporins encompass AQP3, AQP7 and AQP9. AQP3 is express in the basal layer of proliferating keratinocytes in the epidermis (Figure 1D). Mice doing not have AQP3 have reduced skin hydration and elasticity, which is due to impaired glycerol transfer from the blood come the epidermis and also consequent diminished glycerol content in the epidermis and stratum corneum. Reduced glycerol impairs skin hydration and also elasticity, together glycerol is a significant humectant (water-retaining osmolyte). Reduced glycerol additionally impairs epidermal proliferation, together glycerol is an essential epidermal thedesigningfairy.com metabolite affiliated in ATP generation and also membrane lipid biosynthesis. There has been considerable interest in AQP3 in the cosmetics industry, through marketed assets claiming to rise AQP3 expression. Current studies have suggested an essential role of AQP3 in macrophage and also T-lymphocyte function, perhaps entailing AQP3-facilitated H2O2 transport. Another intriguing observation, made in mice lacking AQP7, is progressive obesity v adipocyte hypertrophy, which appears to be a an effect of impaired glycerol efflux indigenous adipocytes. Reduced plasma membrane glycerol permeability result from AQP7 deficiency leader to intrathedesigningfairy.comular buildup of glycerol and triglycerides (Figure 1E), arguing the opportunity of AQP7 upregulation to reduce fat fixed in obesity. Finally, AQP9 expression in hepatocytes has been proposed to facilitate hepatic glycerol uptake, though the significance of hepatic glycerol absorb in human metabolism is unclear.
What around aquaporins in cancer? Provocative data implicate aquaporins in some cancers. A large descriptive literary works reports solid expression that aquaporins in brain, skin, gastrointestinal, lung and other cancers, regularly with correlations in between aquaporin expression and tumor grade. Tumors often overexpress aquaporin(s) uncovered in the thedesigningfairy.com kind of origin, such together AQP4 in astrocyte-derived glioblastomas, although sometimes tumor thedesigningfairy.coms refer aquaporins that are not checked out in the thedesigningfairy.coms the origin. Numerous mechanisms have been suggest that attach aquaporin expression with cancer. AQP1 is strongly expressed in tumor microvessels, and also its deletion in mice reduces tumor angiogenesis. Expression of assorted aquaporins in tumor thedesigningfairy.coms rises their migration and metastatic potential in thedesigningfairy.com culture and pet models. AQP3 rises the proliferation of miscellaneous tumor thedesigningfairy.coms, and also mice doing not have AQP3 are highly resistant to skin cancer. Though many questions stay to be addressed, aquaporin inhibition because that cancer therapy is an attractive idea.
Are any human conditions caused by aquaporin abnormalities? One rare hereditary an illness is led to by loss-of-function mutations in AQP2, a water channel expressed in the kidney collecting duct. AQP2 facilitates transepithelial water absorb in the kidney collecting duct in response to antidiuretic hormone, and this process involves the exocytic insertion that AQP2-containing vesicles into the apical membrane that the thedesigningfairy.com. Mutations in AQP2 the interfere v its thedesigningfairy.comular trafficking or water move function result in the disease nephrogenic diabetes insipidus, in which big volumes that dilute urine, periodically exceeding 30 liters every day, room excreted. Mutations in AQP0 (originally dubbed ‘major intrinsic protein of lens fiber’) are linked with congenital cataracts; however, the system of cataractogenesis is not well-known as the is unclear even if it is the main function of AQP0 is in lens structure or the water permeability of lens fiber thedesigningfairy.coms. Loss-of-function mutations in several other aquaporins have been established in a handful of individuals, although none appears to be linked with far-reaching disease. An interesting autoimmune neurological disease, neuromyelitis optica, is brought about by autoantibodies directed against extrathedesigningfairy.comular epitopes top top AQP4. Autoantibody binding reasons complement- and also thedesigningfairy.com-mediated astrocyte cytotoxicity, which leads to inflammation, v cytokine release, leukocyte infiltration and also blood–brain barrier disruption. The inflammation demyelinating lesions in the spinal cord and optic nerve result in paralysis and also blindness. So far other aquaporin-based autoimmune conditions have no been discovered.
What about aquaporin-based treatment for person diseases? there is considerable interest though little progress in aquaporin-based therapeutics. Data indigenous thedesigningfairy.com and animal models carry out a compelling rationale because that the advance of aquaporin modulators as therapeutics: AQP1 inhibitors space predicted to have actually utility in diuretic-refractory edema, tumor angiogenesis and also glaucoma; AQP4 inhibitors in some develops of mind edema and also injury; and AQP3 inhibitors in some cancers. Upregulators of the expression of part aquaporins have potential therapeutic utility for obesity, wound healing and spinal cord injury. Details heavy steel (mercury, gold) containing link that react non-specifically v protein sulfhydryls inhibit the water permeability of part aquaporins. However, the identification of non-toxic, small-molecule aquaporin inhibitors has not been possible, possibly for a mix of reasons, including obstacles in function-based screening assays and also the intrinsic refractoriness of aquaporins to drug exploration (termed non-druggability). One area the recent progression is in AQP4-based therapeutics for neuromyelitis optica. The idea is to block the binding the pathogenic autoantibodies to astrocyte AQP4, i m sorry may provide a targeted, non-immunosuppressive method to block the disease-initiating occasion in neuromyelitis optica. One method has been the design of a non-pathogenic, tight-binding anti-AQP4 antibody (‘aquaporumab’) that competes v pathogenic autoantibodies; one more approach has actually been the identification, through high-throughput screening, of small-molecule blockers of the autoantibody–AQP4 interaction.

See more: What Is A G? Acceleration Of Gravity In Cm/S2 Standard Gravity


What’s next? over there remain significant gaps in our knowledge about the thedesigningfairy.comular instrument of certain aquaporin functions, such together aquaporin-facilitated thedesigningfairy.com migration, thedesigningfairy.com proliferation, and neuroexcitatory phenomena. Though much of the low-hanging fruit in the aquaporin ar has been picked and digested, together in locations of aquaporin protein structure and phenotypic analysis of knockout mice, brand-new discoveries and also ideas proceed to emerge. Compelling recent data warrant additional investigation the aquaporins in cancer, immune thedesigningfairy.com role and obesity. Lastly, nevertheless the difficulties and limited progress in the identification of aquaporin inhibitors, broad opportunities stay in aquaporin-based therapeutics.