Involvement of water channels in synaptic vesicle swelling

Vesicle swelling is critical for secretion; however, the underlying mechanism of synaptic vesicle (SV) swelling is unknown. A G alphai3-phospholipase A2 (PLA2)-mediated involvement of the water channel aquaporin-1 (AQP1) in the regulation of secretory vesicle swelling in the exocrine pancreas has been previously reported. In the present study, the association and involvement of water channels in SV swelling was explored. Results from the study demonstrate that water channels AQP1 and AQP6, and the heterotrimeric Go protein are associated with SVs and participate in their swelling.     

Roles of aquaporins in the brain

It is now over 10 years ago that aquaporin 1 (AQP1) was discovered and cloned from the red blood cells, and in 2003 the Nobel price in Chemistry was awarded to Pr. Peter Agre for his work on AQPs, highlighting the importance of these proteins in life sciences. AQPs are water channels. To date this protein family is composed of 11 sub-types in mammalians. Three main AQPs described in the mammalian brain are AQP1, AQP4 and AQP9. Several recent studies have shown that these channels are implicated in numerous physiological functions. AQP1 has a role in cerebrospinal fluid formation, whereas AQP4 is involved in water homeostasis and extracellular osmotic pressure in brain parenchyma. AQP4 seems also to have an important function in oedema formation after brain trauma or brain ischemia. AQP9 is implicated in brain energy metabolism. The level of expression of each AQP is highly regulated. After a trauma or an ischemia perturbation of the central nervous system, the level of expression of each AQP is differentially modified, resulting in facilitating oedema formation. At present, the exact role of each AQP is not yet determined. A better understanding of the mechanisms of AQP regulation should permit the development of new pharmacological strategies to prevent oedema formation. AQP9 has been recently specifically detected in the catecholaminergic neurons of the brain. This new result strengthens the hypothesis that the AQPs are not only water channels, but that some AQPs may play a role in energy metabolism as metabolite channels.     

The structural basis of water permeation and proton exclusion in aquaporins (Review)

Aquaporins (AQPs) represent a ubiquitous class of integral membrane proteins that play critical roles in cellular osmoregulations in microbes, plants and mammals. AQPs primarily function as water-conducting channels, whereas members of a sub-class of AQPs, termed aquaglyceroporins, are permeable to small neutral solutes such as glycerol. While AQPs facilitate transmembrane permeation of water and/or small neutral solutes, they preclude the conduction of protons. Consequently, openings of AQP channels allow rapid water diffusion down an osmotic gradient without dissipating electrochemical potentials. Molecular structures of AQPs portray unique features that define the two central functions of AQP channels: effective water permeation and strict proton exclusion. This review describes AQP structures known to date and discusses the mechanisms underlying water permeation, proton exclusion and water permeability regulation.     

The structural basis of water permeation and proton exclusion in aquaporins

Aquaporins (AQPs) represent a ubiquitous class of integral membrane proteins that play critical roles in cellular osmoregulations in microbes, plants and mammals. AQPs primarily function as water-conducting channels, whereas members of a sub-class of AQPs, termed aquaglyceroporins, are permeable to small neutral solutes such as glycerol. While AQPs facilitate transmembrane permeation of water and/or small neutral solutes, they preclude the conduction of protons. Consequently, openings of AQP channels allow rapid water diffusion down an osmotic gradient without dissipating electrochemical potentials. Molecular structures of AQPs portray unique features that define the two central functions of AQP channels: effective water permeation and strict proton exclusion. This review describes AQP structures known to date and discusses the mechanisms underlying water permeation, proton exclusion and water permeability regulation.     

Rapid aquaporin translocation regulates cellular water flow: mechanism of hypotonicity-induced subcellular localization of aquaporin 1 water channel

The control of cellular water flow is mediated by the aquaporin (AQP) family of membrane proteins. The structural features of the family and the mechanism of selective water passage through the AQP pore are established, but there remains a gap in our knowledge of how water transport is regulated. Two broad possibilities exist. One is controlling the passage of water through the AQP pore, but this only has been observed as a phenomenon in some plant and microbial AQPs. An alternative is controlling the number of AQPs in the cell membrane. Here, we describe a novel pathway in mammalian cells whereby a hypotonic stimulus directly induces intracellular calcium elevations through transient receptor potential channels, which trigger AQP1 translocation. This translocation, which has a direct role in cell volume regulation, occurs within 30 s and is dependent on calmodulin activation and phosphorylation of AQP1 at two threonine residues by protein kinase C. This direct mechanism provides a rationale for the changes in water transport that are required in response to constantly changing local cellular water availability. Moreover, because calcium is a pluripotent and ubiquitous second messenger in biological systems, the discovery of its role in the regulation of AQP translocation has ramifications for diverse physiological and pathophysiological processes, as well as providing an explanation for the rapid regulation of water flow that is necessary for cell homeostasis.     

Volume regulation in cortical collecting duct cells: role of AQP2

BACKGROUND INFORMATION: The renal CCD (cortical collecting duct) plays a role in final volume and concentration of urine by a process that is regulated by the antidiuretic hormone, [arginine]vasopressin. This hormone induces an increase in water permeability due to the translocation of AQP2 (aquaporin 2) from the intracellular vesicles to the apical membrane of principal cells. During the transition from antidiuresis to diuresis, CCD cells are exposed to changes in environmental osmolality, and cell-volume regulation may be especially important for the maintenance of intracellular homoeostasis. Despite its importance, cell-volume regulation in CCD cells has not been widely investigated. Moreover, no studies have been carried out till date to evaluate the putative role of AQPs during this process in renal cells. RESULTS: In the present study, we have studied the regulatory cell-volume responses to hypo-osmotic or hyperosmotic challenges in two CCD cell lines: one not expressing AQPs and the other stably transfected with AQP2. We have used a fluorescent probe technique in which the acquisition of single-cell kinetic data can be simultaneously recorded with the intracellular pH. Experiments with hyperosmotic mannitol media demonstrated that, independent of AQP2 expression, CCD cells shrink but fail to show regulatory volume increase, at least under the studied conditions. In contrast, under hypo-osmotic shocks, regulatory volume decrease occurs and the activation of these mechanisms is more rapid in AQP2 transfected cells. This regulatory response takes place in parallel with intracellular acidification, which is faster in cells expressing AQP2. The acidification and the initial regulatory volume decrease response were inhibited by glibenclamide and BaCl2 only in AQP2 cells. CONCLUSIONS: These results suggest that increases in the osmotic water permeability due to the expression of AQP2 are critical for a rapid activation of regulatory volume decrease mechanisms, which would be linked to cystic fibrosis transmembrane conductance regulator and to barium-sensitive potassium channels.     

Aquaporin proteins in murine trophectoderm mediate transepithelial water movements during cavitation

Mammalian blastocyst formation is dependent on establishment of trophectoderm (TE) ion and fluid transport mechanisms. We have examined the expression and function of aquaporin (AQP) water channels during murine preimplantation development. AQP 3, 8, and 9 proteins demonstrated cell margin-associated staining starting at the 8-cell (AQP 9) or compacted morula (AQP 3 and 8) stages. In blastocysts, AQP 3 and 8 were detected in the basolateral membrane domains of the trophectoderm, while AQP3 was also observed in cell margins of all inner cell mass (ICM) cells. In contrast, AQP 9 was predominantly observed within the apical membrane domains of the TE. Murine blastocysts exposed to hyperosmotic culture media (1800 mOsm; 10% glycerol) demonstrated a rapid volume decrease followed by recovery to approximately 80% of initial volume over 5 min. Treatment of blastocysts with p-chloromercuriphenylsulfonic acid (pCMPS, > or =100 microM) for 5 min significantly impaired (P < 0.05) volume recovery, indicating the involvement of AQPs in fluid transport across the TE. Blastocysts exposure to an 1800-mOsm sucrose/KSOMaa solution did not demonstrate volume recovery as observed following treatment with glycerol containing medium, indicating glycerol permeability via AQPs 3 and 9. These findings support the hypothesis that aquaporins mediate trans-trophectodermal water movements during cavitation.     

Membrane water permeability and aquaporin expression increase during growth of maize suspension cultured cells

Aquaporins (AQPs) are water channels that allow cells to rapidly alter their membrane water permeability. A convenient model for studying AQP expression and activity regulation is Black Mexican Sweet (BMS) maize cultured cells. In an attempt to correlate membrane osmotic water permeability coefficient (P_f) with AQP gene expression, we first examined the expression pattern of 33 AQP genes using macro-array hybridization. We detected the expression of 18 different isoforms representing the four AQP subfamilies, i.e. eight plasma membrane (PIP), five tonoplast (TIP), three small basic (SIP) and two NOD26-like (NIP) AQPs. While the expression of most of these genes was constant throughout all growth phases, mRNA levels of ZmPIP1;3 , ZmPIP2;1 , ZmPIP2;2, ZmPIP2;4 and ZmPIP2;6 increased significantly during the logarithmic growth phase and the beginning of the stationary phase. The use of specific anti-ZmPIP antisera showed that the protein expression pattern correlated well with mRNA levels. Cell pressure probe and protoplast swelling measurements were then performed to determine the P_f. Interestingly, we found that the P_f were significantly increased at the end of the logarithmic growth phase and during the steady-state phase compared to the lag phase, demonstrating a positive correlation between AQP abundance in the plasma membrane and the cell P_f.     

Human aquaporins: Regulators of transcellular water flow

Background

Emerging evidence supports the view that (AQP) aquaporin water channels are regulators of transcellular water flow. Consistent with their expression in most tissues, AQPs are associated with diverse physiological and pathophysiological processes.

Scope of review

AQP knockout studies suggest that the regulatory role of AQPs, rather than their action as passive channels, is their critical function. Transport through all AQPs occurs by a common passive mechanism, but their regulation and cellular distribution varies significantly depending on cell and tissue type; the role of AQPs in cell volume regulation (CVR) is particularly notable. This review examines the regulatory role of AQPs in transcellular water flow, especially in CVR. We focus on key systems of the human body, encompassing processes as diverse as urine concentration in the kidney to clearance of brain oedema.

Major conclusions

AQPs are crucial for the regulation of water homeostasis, providing selective pores for the rapid movement of water across diverse cell membranes and playing regulatory roles in CVR. Gating mechanisms have been proposed for human AQPs, but have only been reported for plant and microbial AQPs. Consequently, it is likely that the distribution and abundance of AQPs in a particular membrane is the determinant of membrane water permeability and a regulator of transcellular water flow.

General significance

Elucidating the mechanisms that regulate transcellular water flow will improve our understanding of the human body in health and disease. The central role of specific AQPs in regulating water homeostasis will provide routes to a range of novel therapies. This article is part of a Special Issue entitled Aquaporins.     

Water channel proteins in the inner ear and their link to hearing impairment and deafness

The inner ear is a fluid-filled sensory organ that transforms mechanical stimuli into the senses of hearing and balance. These neurosensory functions depend on the strict regulation of the volume of the two major extracellular fluid domains of the inner ear, the perilymph and the endolymph. Water channel proteins, or aquaporins (AQPs), are molecular candidates for the precise regulation of perilymph and endolymph volume. Eight AQP subtypes have been identified in the membranous labyrinth of the inner ear. Similar AQP subtypes are also expressed in the kidney, where they function in whole-body water regulation. In the inner ear, AQP subtypes are ubiquitously expressed in distinct cell types, suggesting that AQPs have an important physiological role in the volume regulation of perilymph and endolymph. Furthermore, disturbed AQP function may have pathophysiological relevance and may turn AQPs into therapeutic targets for the treatment of inner ear diseases. In this review, we present the currently available knowledge regarding the expression and function of AQPs in the inner ear. We give special consideration to AQP subtypes AQP2, AQP4 and AQP5, which have been studied most extensively. The potential functions of AQP2 and AQP5 in the resorption and secretion of endolymph and of AQP4 in the equilibration of cell volume are described. The pathophysiological implications of these AQP subtypes for inner ear diseases, that appear to involve impaired fluid regulation, such as Menière's disease and Sj?gren's syndrome, are discussed.     

Expression and function of aquaporins in human skin: Is aquaporin-3 just a glycerol transporter?

The aquaporins (AQPs) are a family of transmembrane proteins forming water channels. In mammals, water transport through AQPs is important in kidney and other tissues involved in water transport. Some AQPs (aquaglyceroporins) also exhibit glycerol and urea permeability. Skin is the limiting tissue of the body and within skin, the stratum corneum (SC) of the epidermis is the limiting barrier to water loss by evaporation. The aquaglyceroporin AQP3 is abundantly expressed in keratinocytes of mammalian skin epidermis. Mice lacking AQP3 have dry skin and reduced SC hydration. Interestingly, however, results suggested that impaired glycerol, rather than water transport was responsible for this phenotype. In the present work, we examined the overall expression of AQPs in cells from human skin and we reviewed data on the functional role of AQPs in skin, particularly in the epidermis. By RT-PCR on primary cell cultures, we found that up to 6 different AQPs (AQP1, 3, 5, 7, 9 and 10) may be selectively expressed in various cells from human skin. AQP1, 5 are strictly water channels. But in keratinocytes, the major cell type of the epidermis, only the aquaglyceroporins AQP3, 10 were found. To understand the role of aquaglyceroporins in skin, we examined the relevance to human skin of the conclusion, from studies on mice, that skin AQP3 is only important for glycerol transport. In particular, we find a correlation between the absence of AQP3 and intercellular edema in the epidermis in two different experimental models: eczema and hyperplastic epidermis. In conclusion, we suggest that in addition to glycerol, AQP3 may be important for water transport and hydration in human skin epidermis.     

Expression and function of aquaporins in human skin: Is aquaporin-3 just a glycerol transporter?

The aquaporins (AQPs) are a family of transmembrane proteins forming water channels. In mammals, water transport through AQPs is important in kidney and other tissues involved in water transport. Some AQPs (aquaglyceroporins) also exhibit glycerol and urea permeability. Skin is the limiting tissue of the body and within skin, the stratum corneum (SC) of the epidermis is the limiting barrier to water loss by evaporation. The aquaglyceroporin AQP3 is abundantly expressed in keratinocytes of mammalian skin epidermis. Mice lacking AQP3 have dry skin and reduced SC hydration. Interestingly, however, results suggested that impaired glycerol, rather than water transport was responsible for this phenotype. In the present work, we examined the overall expression of AQPs in cells from human skin and we reviewed data on the functional role of AQPs in skin, particularly in the epidermis. By RT-PCR on primary cell cultures, we found that up to 6 different AQPs (AQP1, 3, 5, 7, 9 and 10) may be selectively expressed in various cells from human skin. AQP1, 5 are strictly water channels. But in keratinocytes, the major cell type of the epidermis, only the aquaglyceroporins AQP3, 10 were found. To understand the role of aquaglyceroporins in skin, we examined the relevance to human skin of the conclusion, from studies on mice, that skin AQP3 is only important for glycerol transport. In particular, we find a correlation between the absence of AQP3 and intercellular edema in the epidermis in two different experimental models: eczema and hyperplastic epidermis. In conclusion, we suggest that in addition to glycerol, AQP3 may be important for water transport and hydration in human skin epidermis.     

Reciprocity in the Developmental Regulation of Aquaporins 1, 3 and 5 during Pregnancy and Lactation in the Rat

Milk secretion involves significant flux of water, driven largely by synthesis of lactose within the Golgi apparatus. It has not been determined whether this flux is simply a passive consequence of the osmotic potential between cytosol and Golgi, or whether it involves regulated flow. Aquaporins (AQPs) are membrane water channels that regulate water flux. AQP1, AQP3 and AQP5 have previously been detected in mammary tissue, but evidence of developmental regulation (altered expression according to the developmental and physiological state of the mammary gland) is lacking and their cellular/subcellular location is not well understood. In this paper we present evidence of developmental regulation of all three of these AQPs. Further, there was evidence of reciprocity since expression of the rather abundant AQP3 and less abundant AQP1 increased significantly from pregnancy into lactation, whereas expression of the least abundant AQP5 decreased. It would be tempting to suggest that AQP3 and AQP1 are involved in the secretion of water into milk. Paradoxically, however, it was AQP5 that demonstrated most evidence of expression located at the apical (secretory) membrane. The possibility is discussed that AQP5 is synthesized during pregnancy as a stable protein that functions to regulate water secretion during lactation. AQP3 was identified primarily at the basal and lateral membranes of the secretory cells, suggesting a possible involvement in regulated uptake of water and glycerol. AQP1 was identified primarily at the capillary and secretory cell cytoplasmic level and may again be more concerned with uptake and hence milk synthesis, rather than secretion. The fact that expression was developmentally regulated supports, but does not prove, a regulatory involvement of AQPs in water flux through the milk secretory cell.