1.A.8 The Major Intrinsic Protein (MIP) Family
The Major Intrinsic Protein (MIP) of the human lens of the eye (Aqp0), after which the MIP family was named, represents about 60% of the protein in the lens cell. In the native form, it is an aquaporin, but during lens development, it becomes proteolytically truncated. The channel, which normally houses 6-9 water molecules, becomes constricted so only three remain, and these are trapped in a closed conformation (Gonen et al., 2004a,b). These truncated tetramers form intercellular adhesive junctions (head to head), yielding a crystalline array that mediates lens formation with cells tightly packed as required to form a clear lens (Gonen and Walz, 2006). Lipids crystallize with the protein (Gonen et al., 2005). Ion channel activity has been shown for Aquaporins 0, 1, and 6, Drosophila Big Brain and plant Nodulin-26 (Yool and Campbell, 2012). Roles of aquaporins in human cancer have been reviewed (Pareek et al. 2013) as have their folding pathways (Klein et al. 2015). AQPs may act as transmembrane osmosensors in red cells, secretory granules and microorganisms (Hill and Shachar-Hill 2015). MIP superfamly proteins and variations of their selectivity filters have been reviewed (Verma et al. 2015). Their evolution has been discussed (Ishibashi et al. 2017). AQPs have a variety of functions and are related to inner ear diseases such as Meniere's disease, sensorineural hearing loss, and presbycusis (Dong et al. 2019). AQPs are also important for male reproductive health (Carrageta et al. 2019). The evolution of the aquaporin superfamily has been discussed (Ishibashi et al. 2020). The cellular functions of aquaporins are regulated mainly by posttranslational modifications, e.g., phosphorylation, ubiquitination, glycosylation, subcellular distribution, degradation, and protein interactions (Li et al. 2020). Aquaporins play roles in inflammation (Mariajoseph-Antony et al. 2020) and in various aspects of health and disease (Magouliotis et al. 2020). They play major roles in secretion of saliva by salivary glands, and their disruption can cause a variety of diseases (D'Agostino et al. 2020). Aquaporins in mamalian lungs have been reviewed (Yadav et al. 2020). Lineage-level divergence of copepod glycerol transporters and the emergence of isoform-specific trafficking regulation has been documented (Catalán-García et al. 2021).
The MIP family is large and diverse, possessing thousands of members that form transmembrane channels. These channel proteins function in water, small carbohydrate (e.g., glycerol), urea, NH3, CO2, H2O2 and ion transport by energy-independent mechanisms. For example, the glycerol channel, FPS1p of Saccharomyces cerevisiae mediates uptake of arsenite and antimonite (Wysocki et al., 2001). Ion permeability appears to occur through a pathway different than that used for water/glycerol transport and may involve a channel at the 4 subunit interface rather than the channels through the subunits (Saparov et al., 2001). MIP family members are found ubiquitously in bacteria, archaea and eukaryotes. Phylogenetic clustering of the proteins is largely according to phylum of the organisms of origin, but one or more clusters are observed for each phylogenetic kingdom (plants, animals, yeast, bacteria and archaea) (Park and Saier, 1996). MIPs are classified into five subfamilies in higher plants, including plasma membrane (PIPs), tonoplast (TIPs), NOD26-like (NIPs), small basic (SIPs) and unclassified X (XIPs) intrinsic proteins. One of the plant clusters includes only tonoplast (TIP) proteins, while another includes plasma membrane (PIP) proteins (de Paula Santos Martins et al. 2015). Aquaporins in Nicotiana tabacum have been tabulated, and their relationships to other Solanaceae species have been described (De Rosa et al. 2020). A genome analysis of Betula pendula (silver birch) identified 33 putative genes encoding full-length AQP sequences (BpeAQPs). They are grouped into five subfamilies, representing ten plasma membrane intrinsic proteins (PIPs), eight tonoplast intrinsic proteins (TIPs), eight NOD26-like intrinsic proteins (NIPs), four X intrinsic proteins (XIPs), and three small basic intrinsic proteins (SIPs) (Venisse et al. 2021). Fungal X-intrinsic protein aquaporins from Trichoderma atroviride have been studied (Amira et al. 2021). In the parasidic helminthes, AQPs play roles in promoting the transport of water, osmoregulation, uptake of nutrients, release of toxic metabolic products and transport of antiparasitic drugs (Wang and Ye 2020). Their involvement in diseases pathogenesis has been reviewed (Ala et al. 2021).
The known aquaporins cluster loosely together as do the known glycerol facilitators. MIP family proteins are believed to form aqueous pores that selectively allow passive transport of their solute(s) across the membrane with minimal apparent recognition. Aquaporins selectively transport water (but not glycerol) while glycerol facilitators selectively transport glycerol but not water. Some aquaporins can transport NH3 and CO2. Glycerol facilitators function as solute nonspecific channels, and may transport glycerol, dihydroxyacetone, propanediol, urea and other small neutral molecules in physiologically important processes. Some members of the family, including the yeast Fps1 protein (TC #1.A.8.5.1) and tobacco NtTIPa (TC #1.A.8.10.2) may transport both water and small solutes. A constriction within the pore, the aromatic/arginine (ar/R) selectivity filter, is thought to control solute permeability: narrow channels conduct water, whilst wider channels permit passage of solutes. Substrate discrimination depends on a complex interplay between the solute, pore size, and polarity (Kitchen et al. 2019). Brain fluid can be secreted against an osmotic gradient, suggesting that conventional osmotic water flow via aquaporins may not fully describe transmembrane fluid transport in the brain (MacAulay 2021). Aquaporins are essential to maintain motility and membrane lipid architecture during mammalian sperm capacitation (Delgado-Bermúdez et al. 2021).
Calamita et al. 2018 review the expression, regulation and physiological roles of AQPs in adipose tissue, liver and endocrine pancreas that are involved in energy metabolism. The review also summarizes the involvement of AQPs in metabolic disorders, such as obesity, diabetes and liver diseases. Challenges and recent advances related to pharmacological modulation of AQPs expression and function to control and treat metabolic diseases are discussed (Calamita et al. 2018). Jain et al. 2018 have shown that an intra-helical salt-bridge in the Loop E half-helix can influence the transport properties of AQP1 and GlpF channels. AQPs are homotetramers with two conserved asparagine-proline-alanine (NPA) motifs embedding in the plasma membrane. The cellular functions of aquaporins are regulated mainly by posttranslational modifications, e.g., phosphorylation, ubiquitination, glycosylation, subcellular distribution, degradation, and protein interactions. Aquaporins, in particular, AQP2, play important roles in some disease conditions such as water loss and gain (Li et al. 2020). The expression of AQP-1, -3, -4, -5, -8 and -9 were documented in the digestive system, where these six AQP isoforms serve essential roles including mediating the transmembrane water transport and regulating the secretion of gastrointestinal (GI) fluids, consequently facilitating the digestion and absorption of GI contents (Liao et al. 2021). The expression levels of AQPs are controlled by various factors, and AQPs can stimulate various signaling pathways; however, aberrant expression of AQPs in the GI tracts are associated with the initiation and development of numerous diseases (Liao et al. 2021). Altered iris aquaporin expression and aqueous humor osmolality in glaucoma have been compared (Huang et al. 2021).
Zardoya and Villalba (2001) have conducted phylogenetic analyses of the MIP family, analyzing 153 homologues. They divided the proteins into six major 'paralogous' groups: (1) GLPs, or glycerol-transporting channel proteins, which include mammalian AQP3, AQP7, and AQP9, several nematode paralogues, a yeast paralogue, and Escherichia coli GLP; (2) AQPs, or aquaporins, which include metazoan AQP0, AQP1, AQP2, AQP4, AQP5, and AQP6; (3) PIPs, or plasma membrane intrinsic proteins of plants, which include PIP1 and PIP2; (4) TIPs, or tonoplast intrinsic proteins of plants, which include αTIP, γTIP, and δTIP; (5) NODs, or nodulins of plants; and (6) AQP8s, or metazoan aquaporin 8 proteins. Of these groups, AQPs, PIPs, and TIPs cluster together as noted above. Wild and domesticated olive species have 52 and 79 genes encoding full-length AQP sequences, respectively (Faize et al. 2020). They fall into five established subfamilies: PIP, TIP, NIP, SIP, and XIP and their substrate specificities and cellular localizations were predicted (Faize et al. 2020).
In agreement with their divergent sequences, human AQP1-9 have very different physiological functions. They are involved in (1) nephrogenic diabetes insipidus, (2) brain water balance and hearing and (3) salivary secretion (Li and Verkman, 2001). Bacterial homologues also have diverse functions. Two proteins in E. coli function as water and glycerol transporters, respectively. Lactobacillus plantarum has 6 homologues, some of which transport water, glycerol and dihydroxyacetone, and some which transporter these compounds as well as D,L-lactic acid (Bienert et al. 2013). The pH sensitivities of Aqp0 channels in lenses of tetraploid and diploid teleosts have been reported (Chauvigné et al. 2015). In the heart, AQPs are implicated in proper cardiac water homeostasis and energy balance as well as heart failure and arsenic cardiotoxicity (Verkerk et al. 2019). Because of their glycerol permeability, aquaglyceroporins are involved in energy homeostasis. Calamita and Delporte 2021 provided an overview of the functional implication and control of aquaglyceroporins in tissues involved in energy metabolism, i.e. liver, adipose tissue and the endocrine pancreas. The expression, role and (dys)regulation of aquaglyceroporins in disorders affecting energy metabolism is also addressed.
Several reports of MIP family proteins transporting ions may or may not be physiologically significant. For example, the influx of arsenite and antimonite via the Fps1 protein into yeast cells is well documented (Wysocki et al., 2001). Similarly, these compounds are taken up via aquaporins in Leishmania (Gourbal et al., 2004). Moreover, AQP6 of renal epithelia have been reported to transport anions at low pH (Yasui et al., 1999). Demonstration of the involvement of the cyanobacterial channel protein (TC #1.A.8.4.1) in copper homeostasis suggests that it may transport Cu2+. Finally, Yang et al. (2005) showed that arsenite exists the Mesorhizobium meliloti cell by downhill movement through AqpS (1.A.8.15.1). The physiological functions of many MIP family proteins are unknown.
MIP family channels consist of homotetramers (e.g., GlpF of E. coli; TC #1.A.8.1.1, AqpZ of E. coli; TC #1.A.8.3.1, and MIP or Aqp0 of Bos taurus; TC #1.A.8.8.1). Each subunit spans the membrane six times as putative α-helices and arose from a 3-spanner-encoding genetic element by a tandem, intragenic duplication event. The two halves of the proteins are therefore of opposite orientation in the membrane. However, a well-conserved region between TMSs 2 and 3 and TMSs 5 and 6 dip into the membrane, each loop forming a half TMS. A voltage-related gating mechanism involving the conserved arginine of the channel's main constriction was captured for human aquaporins through molecular dynamics studies. Mom et al. 2020 showed that this voltage-gating is probably conserved among members of this family and that the underlying mechanism may explain part of plant AQPs diversity when contextualized to high ionic concentrations provoked by drought.
Several MIPs within all domains of life have been shown to facilitate the diffusion of reduced and non-charged species of the metalloids silicon, boron, arsenic and antimony (Bienert et al., 2008). 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 (Bienert et al., 2008).
The crystal structure of the glycerol facilitator of E. coli was solved at 2.2 Å resolution (Fu et al., 2000). Glycerol molecules line up in single file within the amphipathic channel. In the narrow selectivity filter of the channel, the glycerol alkyl backbone is wedged against a hydrophobic corner, and successive hydroxyl groups form hydrogen bonds with a pair of acceptor and donor atoms. The two conserved D-P-A motifs in the loops between TMSs 2 and 3 and TMSs 5 and 6 form the interface between the two duplicated halves of each subunit. Thus each half of the protein forms 3.5 TMSs surrounding the channel. The structure explains why GlpF is selectively permeable to straight chain carbohydrates, and why water and ions are excluded. Aquaporin-1 (AQP1) and the bacterial glycerol facilitator, GlpF can transport O2, CO2, NH3, glycerol, urea, and water to varying degrees. For small solutes permeating through AQP1, a remarkable anticorrelation between permeability and solute hydrophobicity was observed whereas the opposite trend was observed for permeation through the membrane (Hub and Groot, 2008). AQP1 is thus a selective filter for small polar solutes, whereas GlpF is highly permeable to small solutes and less permeable to larger solutes.
Aquaporin-1 (Aqp1) from the human red blood cell has been solved by x-ray crystallography to 3.8 Å resolution (Murata et al., 2000). The aqueous pathway is lined with conserved hydrophobic residues that permit rapid water transport. Water selectivity is due to a constriction of the inner pore diameter to about 3 Å over the span of a single residue, superficially similar to that in the glycerol facilitator of E. coli.
AqpZ, a homotetramer (tAqpZ) of four water-conducting channels that facilitate rapid water movements across the plasma membrane of E. coli, has been solved to 3.2 Å resolution. All channel-lining residues in the four monomeric channels are found orientated in nearly identical positions with one marked exception at the narrowest channel constriction, where the side chain of a conserved Arg-189 adopts two distinct conformational orientations. In one of the four monomers, the guanidino group of Arg-189 points toward the periplasmic vestibule, opening up the constriction to accommodate the binding of a water molecule through a tridentate H-bond. In the other three monomers, the Arg-189 guanidino group bends over to form an H-bond with carbonyl oxygen of Thr-183 occluding the channel. Therefore, the tAqpZ structure reveals two distinct Arg-189 conformations associated with water permeation through the channel constrictions. Alternating between the two Arg-189 conformations disrupts continuous flow of water, thus regulating the open probability of the water pore. Further, the difference in Arg-189 displacements is correlated with a strong electron density found between the first transmembrane helices of two open channels, suggesting that the observed Arg-189 conformations are stabilized by asymmetrical subunit interactions in tAqpZ (Jiang et al., 2006).
Plants exhibits high diversity in aquaporin isoforms and broadly classified into five different subfamilies on the basis of phylogenetic distribution and subcellular occurrence: plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin 26-like proteins (NIPs), small basic intrinsic proteins (SIPs) and uncharacterized intrinsic proteins (XIPs) (Singh et al. 2020). The gating mechanism of aquaporin channels is regulated by post-translational modifications such as phosphorylation, methylation, acetylation, glycosylation, and deamination. Aquaporin expression and transport functions are also modulated by the various phytohormone-mediated signalling in plants. Combined physiology and transcriptome analyses revealed the role of aquaporins in regulating hydraulic conductance in roots and leaves. Aquaporin activities during solute transport, plant development, abiotic stress response, and plant-microbe symbiosis have been reviewed (Singh et al. 2020).
The 3-D structures of the open and closed forms of plant aquaporins, PIP1 and PIP2, have been solved (Törnroth-Horsefield et al., 2006). In the closed conformation, loop D caps the channel from the cytoplasm and thereby occludes the pore. In the open conformation, loop D is displaced up to 16 Å, and this movement opens a hydrophobic gate blocking the channel entrance from the cytoplasm. These results reveal a molecular gating mechanism which appears conserved throughout all plant plasma membrane aquaporins. In plants it regulates water intake/export in response to water availability and cytoplasmic pH during anoxia (Törnroth-Horsefield et al., 2006).
The MIP superfamily includes three subfamilies: aquaporins, aquaglyceroporins and S-aquaporins. (1) The aquaporins (AQPs) are water selective. (2) The aquaglyceroporins are permeable to water, but also to other small uncharged molecules. (3) The third subfamily, with little conserved amino acid sequences around the NPA boxes, include 'superaquaporins' (S-aquaporins).The phylogeny of insect MIP family channels has been published (Finn et al. 2015). The arylsulfonamide AqB011 which selectively blocks the central ion pore of mammalian AQP1 has been shown to impair migration of HT29 colon cancer cells. Traditional herbal medicines are sources of selective AQP1 inhibitors that also slow cancer cell migration (Kourghi et al. 2018).
13 isoforms of mammalian aquaporins (AQP0 - AQP12),are known, nine of which localize to different parts of the renal tubular epithelium. Additional transport functions of renal AQPs (AQP3, AQP6, AQP7 and AQP8) are known. Aquaglyceroporins are most probably key elements in the renal regulation of nitrogen balance and maintenance of the correct pH of body fluids (Michalek 2016). In human sperm, AQP3 and AQP11 are expressed mainly in the tail, AQP7 in the head and AQP8 in the midpiece (Pellavio and Laforenza 2021). AQPs are important for the normal functioning of sperm to ensure normal fertility. AQP3, AQP7 and AQP11 are involved in sperm volume regulation, a key role for fertility because osmoadaptation protects the sperm against swelling and tail bending that could affect sperm motility. AQP8 has a fundamental role in regulating the elimination of hydrogen peroxide, the most abundant reactive oxygen species (ROS), and therefore plays a role in the response to oxidative stress (Pellavio and Laforenza 2021).
Otitis media (OM) refers to inflammatory diseases of the middle ear (ME), regardless of cause or pathological mechanism. The expression of aquaporins (AQPs) in the ME and Eustachian tube (ET) is relevant. Eleven types of AQPs, AQP1 to AQP11, have been found to be expressed in mammalian ME and ET (Jung et al. 2017). The distribution and levels of expression of AQPs depend on the presence or absence of inflammation. Fluid accumulation in the ME and ET is a common mechanism for all types of OM, causing edema in the tissue and inducing inflammation involving various AQPs. The expression patterns of several AQPs, especially AQP1, 4 and 5, may have immunological functions in OM.
Some classes of AQPs conduct ions, glycerol, urea, CO2 , nitric oxide, and other small solutes. Ion channel activity has been demonstrated for mammalian AQPs 0, 1, 6, Drosophila big brain (BIB), soybean nodulin 26, and rockcress AtPIP2;1 (Kourghi et al. 2017). Classification of AQPs into three categories (orthodox AQPs, aquaglyceroporins and superaquaporins) is based on their sequence similarities and substrate selectivities. In the male reproductive tract of mammals, most AQPs (except AQP6 and AQP12) are found in different organs (including testis, efferent ducts and epididymis). AQP1 and AQP9 are the most abundant AQPs in the efferent ducts and epididymis and play a crucial role for the secretion/reabsorption dynamics of luminal fluid during sperm transport and maturation. AQP3, AQP7, AQP8 and AQP11 are the most abundant AQPs in sperm and are involved in the regulation of their volumes, which is required for the differentiation of spermatids into spermatozoa during spermatogenesis, as well as in sperm transit along environments of different osmolality (male and female reproductive tracts). Mounting evidence indicates that AQP3, AQP7 and AQP11 are involved in cryotolerance as well as the sperm response to variations of osmolality and to freeze-thawing procedures (Yeste et al. 2017).
In mammals, aquaporins are subdivided into classical aquaporins that are permeable to water; aquaglyceroporins that are permeable to water, glycerol and urea; peroxiporins that facilitate the diffusion of H2O2 through cell membranes; and so called unorthodox aquaporins. Aquaporins ensure important physiological functions in both exocrine and endocrine pancreas and are involved in pancreatic fluid and insulin secretion. Modification of aquaporin expression and/or subcellular localization may be involved in the pathogenesis of pancreatic insufficiencies, diabetes and pancreatic cancer (Arsenijevic et al. 2019).
Mechanisms that drive the development of multiple inflammatory diseases that occur in the nose and contribute to the process of olfactory recognition of compounds entering the nasal cavity involve the action of water channels such as AQPs. Jung et al. 2020 reviewed the relationship between AQPs and rhinologic conditions, focusing on the current state of knowledge and mechanisms that link AQPs and rhinologic conditions. Key conclusions include the following: (1) Various AQPs are expressed in both nasal mucosa and olfactory mucosa; (2) the expression of AQPs in these tissues is different in inflammatory diseases such as AR or CRS, as compared with that in normal tissues; (3) the expression of AQPs in CRS differs depending on the presence or absence of nasal polyps; and (4) the expression of AQPs in tissues associated with olfaction is different from that in the respiratory epithelium.
Water homeostasis plays a crucial role in different reproductive processes, e.g., oocyte transport, hormonal secretion, completion of successful fertilization, blastocyst formation, pregnancy, and birth (Kordowitzki et al. 2020). Further, aquaporins are involved in the process of spermatogenesis, and they have been reported to be involved in the storage of spermatozoa. Aquaporins are relevant for seveeral physiological functions in the female reproductive system, and they are relevant to different pathologies in the female reproductive system. Four Impatiens walleriana aquaporins: IwPIP1;4, IwPIP2;2, IwPIP2;7 and IwTIP4;1, have been characteerized (Đurić et al. 2021). Drought stress affected the aquaporin expression in I. walleriana leaves, which was up- or downregulated depending on stress intensity. Expression of IwPIP2;7 was the most affected of these four aquaporins. At 15% and 5% soil moisture and recovery from 15% and 5% soil moisture, IwPIP2;7 expression significantly decreased and increased, respectively. Aquaporins IwPIP1;4 and IwTIP4;1 had lower expression than IwPIP2;7, with moderate expression changes in response to drought and recovery, while IwPIP2;2 expression was of significance only in recovered plants (Đurić et al. 2021).
Humans contain 13 AQPs (AQP0-AQP12) which are divided into three sub-classes namely orthodox aquaporin (AQP0, 1, 2, 4, 5, 6, and 8), aquaglyceroporin (AQP3, 7, 9, and 10) and super or unorthodox aquaporin (AQP11 and 12) based on their pore selectivity. They are involved in a wide variety of non-infectious diseases including cancer, renal dysfunction, neurological disorders, epilepsy, skin diseases, metabolic syndrome, and even cardiac diseases. AQPs can be regulated by microbial and parasitic infections that suggest their involvement in microbial pathogenesis, inflammation-associated responses and AQP-mediated cell water homeostasis. In a review, Azad et al. 2021 examine the involvement of AQPs in infectious and non-infectious diseases and potential AQPs-target modulators. AQP structures, tissue-specific distributions and physiological relevance, functional diversity and regulation were considered.
The generalized transport reaction for channel proteins of the MIP family is:
H2O (out) → H2O (in) (e.g., aquaporins)
solute (out) → solute (in) (e.g., glycerol facilitators).