Effects of lens major intrinsic protein on glycerol permeability and metabolism

Loss of fiber cell communication may contribute to the development of cataracts of many different etiologies

The lens is an avascular organ that is supported by an internal circulation of water and solutes. This circulation is driven by ion pumps, channels and transporters in epithelial cells and by ion channels in fiber cells and is maintained by fiber-fiber and fiber-epithelial cell communication. Gap junctional intercellular channels formed of connexin46 and connexin50 are critical components of this circulation as demonstrated by studies of connexin null mice and connexin mutant mice. Moreover, connexin mutants are one of the most common causes of autosomal dominant congenital cataracts. However, alterations of the lens circulation and coupling between lens fiber cells are much more prevalent, beyond the connexin mutant lenses. Intercellular coupling and levels of connexins are decreased with aging. Gap junction-mediated intercellular communication decreases in mice expressing mutant forms of several different lens proteins and in some mouse models of lens protein damage. These observations suggest that disruption of ionic homeostasis due to reduction of the lens circulation is a common component of the development of many different types of cataracts. The decrease in the lens circulation often reflects low levels of lens fiber cell connexins and/or functional gap junction channels.

Aquaporin Protein-Protein Interactions

Aquaporins are tetrameric membrane-bound channels that facilitate transport of water and other small solutes across cell membranes. In eukaryotes, they are frequently regulated by gating or trafficking, allowing for the cell to control membrane permeability in a specific manner. Protein–protein interactions play crucial roles in both regulatory processes and also mediate alternative functions such as cell adhesion. In this review, we summarize recent knowledge about aquaporin protein–protein interactions; dividing the interactions into three types: (1) interactions between aquaporin tetramers; (2) interactions between aquaporin monomers within a tetramer (hetero-tetramerization); and (3) transient interactions with regulatory proteins. We particularly focus on the structural aspects of the interactions, discussing the small differences within a conserved overall fold that allow for aquaporins to be differentially regulated in an organism-, tissue- and trigger-specific manner. A deep knowledge about these differences is needed to fully understand aquaporin function and regulation in many physiological processes, and may enable design of compounds targeting specific aquaporins for treatment of human disease.

Functional expression of aquaporins in embryonic, postnatal, and adult mouse lenses

Aquaporin 0 (AQP0) and AQP1 are expressed in the lens, each in a different cell type, and their functional roles are not thoroughly understood. Our previous study showed that these two AQPs function as water transporters. In order to further understand the functional significance of these two different aquaporins in the lens, we investigated their initiation and continued expression. AQP0 transcript and protein were first detected at embryonic stage (E) 11.25 in the differentiating primary fiber cells of the developing lens; its synthesis continued through the adult stage in the secondary fiber cells. Low levels of AQP1 expression were first seen in lens anterior epithelial cells at E17.5; following postnatal day (P) 6.5, the expression gradually progressed towards the equatorial epithelial cells. In the postnatal lens, the increase in membrane water permeability of epithelial cells and lens transparency coincides with the increase in AQP1 expression. AQP1 expression reaches its peak at P30 and continues through the adult stage both in the anterior and equatorial epithelial cells. The enhancement in AQP1 expression concomitant with the increase in the size of the lens suggests the progression in the establishment of the lens microcirculatory system. In vitro and in vivo studies show that both aquaporins share at least one important function, which is water transport in the lens microcirculatory system. However, the temporal expression of these two AQPs suggests an apparently unique role/s in lens development and transparency. To our knowledge, this is the first report on the expression patterns of AQP0 and AQP1 during lens development and differentiation and their relation to lens transparency.

The channel architecture of aquaporin 0 at a 2.2-A resolution

We determined the x-ray structure of bovine aquaporin 0 (AQP0) to a resolution of 2.2 A. The structure of this eukaryotic, integral membrane protein suggests that the selectivity of AQP0 for water transport is based on the identity and location of signature amino acid residues that are hallmarks of the water-selective arm of the AQP family of proteins. Furthermore, the channel lumen is narrowed only by two, quasi-2-fold related tyrosine side chains that might account for reduced water conductance relative to other AQPs. The channel is functionally open to the passage of water because there are eight discreet water molecules within the channel. Comparison of this structure with the recent electron-diffraction structure of the junctional form of sheep AQP0 at pH 6.0 that was interpreted as closed shows no global change in the structure of AQP0 and only small changes in side-chain positions. We observed no structural change to the channel or the molecule as a whole at pH 10, which could be interpreted as the postulated pH-gating mechanism of AQP0-mediated water transport at pH >6.5. Contrary to the electron-diffraction structure, the comparison shows no evidence of channel gating induced by association of the extracellular domains of AQP0 at pH 6.0. Our structure aids the analysis of the interaction of the extracellular domains and the possibility of a cell-cell adhesion role for AQP0. In addition, our structure illustrates the basis for formation of certain types of cataracts that are the result of mutations.

Interaction of major intrinsic protein (aquaporin-0) with fiber connexins in lens development

We observed that chick lens-fiber gap-junction-forming proteins, connexin (Cx) 45.6 and Cx56, were associated with an unknown protein, which was then identified as major intrinsic protein (MIP), also known as aquaporin-0 (AQP0), the most abundant membrane protein in lens fibers. A 1063 bp cDNA of chick MIP(AQP0) was identified that encodes a 262 amino acid protein with a predicted molecular weight of 28.1 kDa. Dual immunofluorescence and confocal microscopy of sagittal and coronal sections of the lens tissues showed that MIP(AQP0) consistently localized with gap junction plaques formed by Cx45.6 and Cx56 during the early stages of embryonic chick lens development. Immunoprecipitation combined with immunoblotting analyses revealed that MIP(AQP0) was associated with Cx45.6 and Cx56 at these developmental stages. The specificity of this interaction was further confirmed with the silver staining of the protein components of immunoprecipitates. The pull-down analysis of lens lysates revealed that C-terminus of MIP(AQP0) probably interacted with these two fiber connexins. In late embryonic and adult lenses, however, uniform co-distribution of MIP(AQP0) and fiber connexins was largely disrupted, except for the area surrounding the actively differentiating bow regions, as was revealed by immunofluorescence and immunoprecipitation experiments. The interaction of MIP(AQP0) with lens fiber connexins in differentiating lens cells but not in mature lens fibers suggests a potential role for MIP(AQP0) in the facilitation of fiber connexins for the formation of gap junctions during lens development.

Cloning and functional characterization of a novel aquaporin from Xenopus laevis oocytes

We have cloned a novel aquaporin (AQP) from Xenopus laevis oocytes, which we have provisionally named AQPxlo. The predicted protein showed highest homology (39-50%) to aquaglyceroporins. Northern blot analysis showed strong hybridization to an approximately 1.4-kb transcript in X. laevis fat body and oocytes, whereas a weaker signal was obtained in kidney. We injected in vitro transcribed cRNA encoding AQPxlo into Xenopus oocytes for functional characterization. AQPxlo expression increased osmotic water permeability (P(f)), as well as the uptake of glycerol and urea. However, AQPxlo excluded larger polyols and thiourea. An alkaline extracellular pH (pH(o)) increased P(f) and to a lesser extent urea uptake but not glycerol uptake. Remarkably, low HgCl(2) concentrations (0.3-10 microm) reduced P(f) and urea uptake, whereas high concentrations (300-1000 microm) reversed the inhibition. We propose that AQPxlo is a new AQP paralogue unknown in mammals.

Aquaglyceroporins: channel proteins with a conserved core, multiple functions, and variable surfaces

Membrane channels for water and small nonionic solutes are required for osmoregulation in bacteria, plants, and animals. Aquaporin-1, the water channel of human erythrocytes, is the first channel demonstrated to conduct water, by expression in Xenopus oocytes. Phylogenetic analyses reveal the existence of two clusters of subfamilies, the aquaporins (AQPs) and glycerol facilitators (GLPs). Sequence-based structure prediction provided a model comprising six membrane-spanning helices, while sequence analyses suggested strategic residues that are important for structure and function. The surface topography of several AQPs has been mapped by atomic force microscopy, revealing different features that correlate with differences in the loops connecting transmembrane helices. The 3D structures of AQP1 and GlpF have been determined by electron cryomicroscopy. The 3.8-A density map allowed the first atomic model of AQP1 to be built, taking into account data from sequence analyses. This model provides some insight into the permeation of water through a channel that blocks the passage of protons. GIpF has been resolved to 6.9 A, revealing helices that are similar to those of AQP1. Homology modeling shows the channel region of these distant aquaglyceroporins to be similar, as confirmed by the 2.2-A structure of GlpF from X-ray crystallography.

pH-Dependent channel activity of heterologously-expressed main intrinsic protein (MIP) from rat lens

Wild-type rat lens main intrinsic protein (MIP) was heterologously expressed in the membrane of Spodoptera frugiperda (Sf21) cells using the baculovirus expression system and in mouse erythroid leukaemia cells (MEL C88). Both MEL and Sf21 cell lines expressing wild-type MIP were investigated for the conductance of ions using a whole cell patch clamp technique. An increase in conductance was seen in both expression systems, particularly on lowering the pH to 6.3. In Sf21 cells, addition of antibodies to the NPA1 box resulted in a reduction of current flow. These results suggest that MIP has pH-dependent ion channel activity, which involves the NPA1 box domain.

Heterologous expression and topography of the main intrinsic protein (MIP) from rat lens

Wild type rat lens main intrinsic protein (MIP) and MIP mutated (F73I, F75L) to resemble the glycerol facilitator of Escherichia coli in the region of the NPA1 box were used to investigate the topology of MIP in the membrane of Spodoptera frugiperda (Sf21) cells using the baculovirus expression system and expression in mouse erythroid leukaemia cells (MEL C88). Differential fixation for staining was used, with paraformaldehyde for externally exposed antigenic sites, and acetone for both externally and internally exposed protein antigenic sites. Immunofluorescence using antibodies to synthetic MIP peptides showed that wild type MIP had a six transmembrane topography. The N- and C-termini were intracellular in both expression systems, and both NPA boxes were found to be extracellular. These results show that residues around the NPA1 box can influence the folding of the MIP in the membrane, and provide structural evidence for the poor water transport properties of MIP, as the NPA boxes lie outside the plane of the membrane.

Cloning and functional expression of an MIP (AQP0) homolog from killifish (Fundulus heteroclitus) lens

The major intrinsic protein (MIP) of lens fiber cells is a member of the aquaporin (AQP) water channel family. The protein is expressed at very high levels in lens fiber cells, but its physiological function is unclear. By homology to known AQPs, we have cloned a full-length cDNA encoding an MIP from the lens of killifish (Fundulus heteroclitus). The predicted protein (263 amino acids; GenBank accession no. AF191906) shows 77% identity to amphibian MIPs, 70% identity to mammalian MIPs, and 46% identity to mammalian AQP1. Expression of MIPfun in Xenopus laevis oocytes causes an approximately 40-fold increase in oocyte water permeability. This stimulation is comparable to that seen with AQP1 and substantially larger than that seen with other MIPs. The mercurials HgCl(2) and p-chloromercuribenzenesulfonate inhibit the water permeability of MIPfun by approximately 25%. MIPfun is not permeable to glycerol, urea, or formic acid but is weakly permeable to CO(2).

Molecular solutions to mammalian lens transparency

The mammalian lens generates an internal microcirculation that maintains transparency in the avascular lens. Significant progress has been made in characterizing the membrane transport proteins associated with this circulation. By combining physiological and molecular evidence, a more comprehensive understanding of normal lens function and cataractogenesis is emerging.

Glycation decreases calmodulin binding to lens transmembrane protein, MIP

Channels of the major intrinsic protein (MIP) of the lens transport water, thus playing an important role in lens fiber cell homeostasis. Calmodulin (CAM) interacts with MIP and possibly regulates MIP channel permeability. Protein glycation has been implicated in lens opacification. We previously identified sites of glycation of MIP, which are in close proximity to the putative CAM binding site. This study is aimed to show the effect of in vitro and in vivo glycation on CAM binding to MIP. Our results show that MIP and MP20 are the major CAM binding proteins of the lens membrane. In vitro incubation of lens membranes with 1 M glucose decreased CAM binding by 38% (P<0.001). Similarly, there was a progressive decrease in CAM binding to diabetic lens membranes compared to age-matched controls (up to 30% decrease, P<0.01). Mutation of K228 and K238 as well as a triple K mutation (K228N, K238N, K259N) of MIP resulted in a decrease in CAM binding. Thus, post-translational protein modifications of MIP influence CAM binding. Since CAM is the ubiquitous Ca(2+) receptor, decreases in CAM binding to the target protein will affect the Ca(2+)-mediated cellular processes leading to lens opacification in diabetic and aging lenses.

Structural clues in the sequences of the aquaporins

The large number of sequences available for the aquaporin family represents a valuable source of information to incorporate into three-dimensional structure determination. Phylogenetic analysis was used to define type sequences to avoid extreme over-representation of some subfamilies, and as a measure of the quality of multiple sequence alignment. Inspection of the sequence alignment suggested eight conserved segments that define the core architecture of six transmembrane helices and two functional loops, B and E, projecting into the plane of the membrane. The sum of the core segments and the minimum lengths of the interlinking loops constitute the 208 residues necessary to satisfy the aquaporin architecture. Analysis of hydrophobic and conservation periodicity and of correlated mutations across the alignment indicated the likely assignment and orientation of the helices in the bilayer. This assignment is examined with respect to the structure of the erythrocyte aquaporin 1 determined by electron crystallography. The aquaporin 1 tetramer is described as three rings of helices, each ring with a different exposure to the lipid environment. The sequence analysis clearly suggests that two helices are exposed along their whole lengths, two helices are exposed only at their N termini, and two helices are not exposed to lipid. It is further proposed that, besides loops B and E, the highly conserved motifs on helices 1 and 4, ExxxTxxF/L, could line the water channel.

Aquaporins: Phylogeny, Structure, and Physiology of Water Channels

How water permeates cellular membranes and what this means for cell functioning and several diseases are now emerging from the study of the aquaporins (AQPs), the water channel family. A combination of sequence analysis, three-dimensional structure determination, and physiology of the AQP family proteins provides a glimpse into the workings of water channels.

Epithelial aquaporins

The past year has brought significant advances in our understanding of the involvement of aquaporins in the regulation of water balance. Besides the identification of new mammalian aquaporins, highlights include the progress in our understanding of their cell-biological regulation and their roles in physiology and pathophysiology as deduced from natural and engineered knockout models.