The water molecules that transport in and out of the cells are a fundamental component of any physiological process to maintain life. This movement of water helps in the regulation of cell activity, functioning of organ systems and maintaining the fluid homeostasis. This concept of transport of water across the cell membranes was suspected to be facilitated by channels or pores, which eventually led to the discovery of the water channels while working on membrane proteins of red blood cells in 1992 by Peter Agre and his colleagues. These water channels formerly named as CHIP28 are now called as aquaporin 1 (
Brown, 2017). The discovery of aquaporins (AQPs) explained its primary role in water transport across biological membrane in response to the osmotic gradients formed as a result of active transport of the solute. This transport function exerted by aquaporins would be required for cell homeostasis to play a vital role in maintaining endothelial function
(Uttara et al., 2009). There are thirteen distinct AQPs in mammals that are sub-divided into (
Ishibashi, 2009).
Conducts water only (AQP0, AQP1, AQP2, AQP4, AQP5, AQP6 and AQP8).
Aquaglyceroporins
Transport water and small neutral solutes, particularly glycerol (AQP3, AQP7, AQP9 and AQP10).
Unorthodox aquaporins
Indicated as unorthodox aquaporins due to their distinct evolutionary pathway and primary sequence distinctions (AQP11 or AQP12).
The above isoforms of AQPs are expressed in cell types which are involved in transport of fluids like-epithelia and endothelia in kidney, lung, exocrine glands, eye (cornea, lens, ciliary epithelium), CNS (Astrocytes) and gastro-intestinal organs. They are also expressed in erythrocytes, certain leukocytes, adipocytes and skeletal muscle.
Stress arises as a reflexive response to unfavourable conditions in a particular environment in which the animal lives. Animals are exposed to various stressors that disrupt the homeostasis with oxidative stress being one among them. In addition to the excessive creation of reactive oxygen species (ROS) that antioxidants are unable to control, oxidative stress is also characterised as a disruption of the redox balance of cells (
Pisoschi and Pop, 2015). Over the years, it has been widely established that the ROS produced in the biological system enters the cells by diffusing freely through the lipid bilayer of the cell membrane and not through specific transporters or channels. This concept has changed considerably over time by the discovery of AQPs.
Cells implicated in inflammation, can suffer osmotic microenvironmental changes, resulting in increased cell hydraulic permeability and size, as well as variations in cytoskeletal structure (
Maidhof et al., 2014). The participation of AQPs in multiple inflammatory processes, as evidenced by dysregulation in animal illnesses, reveals their novel significance in protection and response to various noxious stimuli, including bacterial infection. This discovery may provide a new key to resolving the understanding of host-pathogen interactions, as well as new situations for investigating the regulation of individual AQPs as targets for future pharmaceutical therapeutics
(Meli et al., 2018). This review focuses on summarizing the evidences regarding the control that AQPs have over oxidative stress in physiological and pathological conditions in animals. The knowledge gaps have been identified to stimulate further research.
Importance of assessing stress in animals
Stress is a multi-dimensional phenomenon that challenges the homeostasis of the animal by affecting the body systems comprehensively. Exposure of animals to any acute or chronic stressors invariably leads to unfavourable consequences ranging from discomfort to death
(Alberto et al., 2022). Ninan et al., (2019) in their studies on haematology in Gir cattle concluded that alterations due to metabolic, nutrient deficiency, physiological and health status can be compared for diagnostic and therapeutic purpose in different age and physiological states of Gir cattle breed, which are unique cattle breed adapted to existing climatic conditions. Stress evaluation has been crucial in production systems, research and in knowing the well-being of the animals and is needed in animals for developing therapeutic, nutritive and management strategies for better animal welfare.
Oxidative stress, significantly affects a wide range of living organisms. It develops when ROS production surpasses a live organism’s antioxidant capability, which can result in numerous diseases
(Alberto et al., 2022). Environmental agitations like nutritional deficiency, salinity, solar heat, radiation, drought, exposure to pesticides, heavy metals and herbicides stimulate the production of ROS, leading to oxidative distress that subsequently leads to various disease conditions
(Desikan et al., 2005).
A growing body of literature clearly indicates that overproduction of ROS and the resulting oxidative stress is a major detrimental consequence of most common commercial stressors
(Surai et al., 2019). Transportation of animals drives the affected animals to oxidative stress, thereby provoking morbidity, mortality, carcass trim loss and undesirable meat characteristics (
Rosas-Valencia et al., 2019).
In dogs, sustained stress results in oxidative damage causing chronic disease conditions like; cardiovascular diseases, gastrointestinal diseases, immune dysfunction and development of abnormal behaviours
(Fan et al., 2023). Animals subjected to chronic environmental stress exhibit alterations in endocrine function, basal metabolism, water and electrolyte metabolism, acid-base balance (
Padodara and Ninan, 2013). In this regard, AQPs have a regulatory role in fluid homeostasis. Thus, the significance of assessing stress in animals plays an important role in identifying its impact on production as well as the welfare of the animal.
Different techniques used in quantifying Aquaporins
Quantification methods used to determine the aquaporins are Immunohistochemical analysis, Transmission electron microscopy visualization, Mass spectrometry, Gel electrophoresis, Immunoblot quantification, Liquid chromatography, Knockout animal models, Quantitative phenotypic assay, Inductively Coupled Plasma - Mass Spectrometry, Radio immuno assay, Western blotting and Sandwich ELISA.
Functions of Aquaporins in animals
The fundamental role of most aquaporins is to transport water molecules across the biological membranes in response to osmotic changes. A subset of aquaporins termed as aquaglyceroporins transports glycerol and evidence suggests that some aquaporins allow gases (CO2, NH3, NO, O2) to pass (
Verkman, 2013). These features have attracted researchers due to their physiological significance of transporting gases of biological relevance. In addition, many small solutes such as H2O2, arsenite and ions (K+, Cl-) are also being transported by some aquaporins. The AQPs that are able to conduct H2O2 and / or ammonia are called as peroxiporins and ammoniaporins / aqua ammoniaporins respectively
(Tamma et al., 2018). Non-transporting functions for certain AQPs have been studied, such as cell to cell interactions, polarization of the cell membranes and regulation of the interacting proteins, such as the ion channels (
Verkman, 2013).
Aquaporins (AQPs) feature ubiquitously in different species, organs, tissues and cells in the biological system.
i) AQPs are expressed particularly in cell types that involve in transport of fluid, such as epithelial cells in various organs and in some cell types that do not have vital role in fluid transport, such as adipocytes
(Verkman et al., 2014).
ii) AQPs are associated with the functioning of gametes. In sperm, they are involved in osmo-adaptation, after the sperm enters reproductive tract of the female, which is decisive for activation of sperm motility, capacitation and for the fertilizing ability. AQPs play principal role in the maturation of oocytes (
Delgado-Bermúdez et al., 2022).
iii) In gastro-intestinal tissues, AQP1 in the epithelium of intra-hepatic bile duct is involved in bile formation, in lacteals of intestine involved in fat absorption and in salivary gland microvascular endothelium taking part in secretion of saliva; AQP4 in parietal cells of stomach involved in acid/fluid secretion; AQP3 and AQP4 in colon surface epithelium involved in faecal dehydration; AQP5 in acinar cells in salivary glands take part in saliva secretion; and AQP8 in jejunal villi. AQP8 and AQP9 transcripts were localised in hepatocytes, indicating their role in regulation of hepatocyte volume and bile secretion (
Ma and Verkman, 1999). AQPs regulation is carried out by the osmotic forces and AQPs have a major transcellular pathway for the bidirectional transport of water molecules by the digestive tract epithelium
(Zhu et al., 2016).
iv) AQPs have a significant function in maintaining the fluid homeostasis in lungs (
Wittekindt and Dietl, 2019). The expression levels of AQP1, AQP4 and AQP5 decreased in the respiratory tract in conditions like oxidative stress associated with heat-stress in buffaloes
(Rebez et al., 2023).
v) AQP4 is relatively the most plenteous water channel in the central nervous system and is expressed in astrocytes and is involved in water movement, cell migration and neuro-excitation. AQP1 is localised in choroid plexus facilitating cerebrospinal fluid secretion and in neurons of dorsal root ganglion tuning pain perception (
Papadopoulos and Verkman, 2013).
vi) In cardiovascular function. AQP1, AQP4, AQP7 and AQP9 express in the endothelial cells, vascular smooth muscle cells and heart. AQPs are expressed differentially in various cardiovascular tissues and take part in transmembrane transport of water, metabolism, cell migration and inflammatory response
(Shangzu et al., 2022).
vii) The transport of solutes and water molecules through gut epithelia is regulated by different isoforms of AQPs and are characterised by their particular gastrointestinal tract distribution pattern
(Zhu et al., 2016).
viii) AQP1 expression was found in renal epithelium particularly in the proximal tubule, descending limb of the loop of Henle and in descending vasa recta endothelium. Several evidences demonstrate that AQP2 is under the control of arginine vasopressin playing a critical role in reabsorption of water in kidney
(Su et al., 2020). It has been found that the mice with deficient functional AQP2, AQP3, or AQP4 exhibit nephrogenic diabetes insipidus of various degrees due to reduced water permeability in the collecting duct. Mice that lack AQP7 and AQP8 have the ability to concentrate urine, although AQP7 null mice exhibit a unique defect in glycerol reabsorption (
Verkman, 2006). Thus, AQPs are important in varied aspects of the concentrating mechanism of urine.
ix) In goats
Shukla et al., (2023) identified AQP3 was higher in renal medulla as compared to renal cortex during summer season and opined AQP3 plays huge role in movement of water in kidney, particularly in renal medulla where anti-diuretic hormone acts in water reabsorption. Further the study suggested a seasonal variation in the expression of AQPs in goat’s renal system.
Thus, numerous isoforms of AQPs have been identified and are differentially expressed and are involved in multiple physiological processes in animals, facilitating tissue-specific osmoregulation.
Regulatory role of Aquaporins in stressed animals
Aquaporins are membrane channels that are extensively distributed in various body systems. The 13 different AQPs, encoded by the genes AQP0–AQP12, identified in mammals help in regulation of important physiological functions in heart, kidney, brain, lung, digestive system and various other systems and have a functional role in stress associated with various pathological conditions. The underlying mechanisms of aquaporin involvement in various organ systems are discussed.
Cardiovascular system
Aquaporins in mammals expressed in heart have specific distribution pattern of each subtype in cardiomyocytes. Any dysfunction leads to wide range of disorders. In cardiac tissues of various animal species (mouse, rat, sheep, goat), AQP subtypes have been discovered and the first AQP (AQP1) was identified in a rat’s cardiac tissue by Agre and co-workers
(Verkerk et al., 2019). Recently, it has been found that aquaporins (AQP1, AQP4, AQP7 and AQP9) are localised in the endothelial cells, smooth muscle cells of vascular system and heart. The cardiac AQP localisation and expression depends on different factors that include; species, sex, development and aging
(Verkerk et al., 2019). AQP are also involved in the pathology of related disease conditions like; cerebral ischemia, congestive heart failure and angiogenesis
(Tie et al., 2017).
AQP1
In sheep and rabbits AQP1 is concentrated in microvascular endothelial cells and cardiomyocytes
(Jonker et al., 2003, Ding et al., 2013). Myocardial ischemia and the severity of myocardial edema are consistent with AQP1 expression
(Song et al., 2018). The dysfunction of mitochondria is considered to be the prominent feature of myocardial ischemia as this organelle is the prime contributor and major target for ROS inflicted damage
(Kurian et al., 2016) leading to oxidative stress.
AQP4
Aquaporin4 maintains the water balance of cardiomyocytes. However, the role of AQP4 in cardiovascular disease is not studied and is poorly understood. AQP4 is distributed in intercalated discs, endothelial cells, sarcolemma and serosa of heart. AQP4 expression was down-regulated following myocardial injury, thus having a protective effect
(Rutkovskiy et al., 2012). Butler et al., (2006) reported that AQP4 protein play significant role in myocardial oedema. It has been established that, when myocardial infarction with increased water content in cardiomyocytes induced myocardial oedema, AQP4 expression upregulated in cardiomyocytes
(Song et al., 2018). Thus, the decrease in level of AQP4 in endoplasmic reticulum stress causes cardiac injury.
AQP7
For energy production, cardiomyocytes require fatty acids and glucose. AQPs of the sub-family, aquaglyceroporin allow the passage of glycerol playing significant role in energy regulation
(Gladka et al., 2009). The most predominant aquaglyceroporin in heart is AQP7 which is upregulated in many conditions when energy and/or substrate are altered
(Verkerk et al., 2019). AQP7 expression is upregulated in exercising rats
(Verkerk et al., 2019). In cardiomyocytes, AQP7 may act as a facilitator of glycerol and alterations in AQP7 may elevate its susceptibility to hypertrophy and mortality rate. AQP7 can act as an original pathway for delivery of nutrients into heart and mediate toxicity of different poisons
(Rutkovskiy et al., 2013).
AQP9
AQP9 is predominantly present in liver and not in heart, suggesting that AQP9 affects cardiac activity by decreased arsenic clearance by liver
(Carbrey et al., 2009). AQP9 is localised in valvular tissue of subjects with infective endocarditis and their expression is related to the development of acute heart failure
(Verkerk et al., 2019). Oxidative stress occurrence in the heart proves there is enhanced myocardial generation of H2O2 induced by active infective endocarditis
(Ostrowski et al., 2013). In this context, it is found that expression of AQP9 can be a reliable prognostic marker in infective endocarditis. In polymicrobial sepsis, AQP9 may be used as a pharmacological target
(Mohammad et al., 2022).
Digestive system
Fluid transfer such as secretion and absorption play crucial function in digestive system ensuring normal gut activities. Water transfer across the digestive epithelium is suggested to occur through AQPs and also through other channel mechanisms
(Matsuzaki et al., 2004). Aquaporins play a vital role in maintaining intestinal stability by controlling the uptake and release of water molecules and small solutes. These aquaporins not only regulate cellular growth and movement but also contribute to processes like intestinal inflammation and tumour formation, underscoring their significant role in promoting intestinal well-being
(Lv et al., 2022). Until now, at least 11 AQPs (AQP 1 to 11) are found to be expressed in stomach, small intestine and large intestine of the digestive system. Oxidative stress has a role in the pathogenesis of number of gastrointestinal disorders, like peptic ulcers, gastrointestinal malignancies and inflammatory bowel disease
(Bhattacharyya et al., 2014).
AQP1
AQP1 is widely distributed in the endothelial cells of GI tract and plays crucial role in the transport of water between the GI tract mucosa and blood in addition to being involved in salivary secretion. AQP1 is found to be localised in the endothelial cells of the small intestine’s central lacteals, which facilitates in formation of chylomicron, impairing adipose absorption. Several areas of the enteric nervous system, including the rat ileum and the ovine duodenum, have been reported to contain AQP1
(Volkart et al., 2023). AQP1 deletion is linked to a noticeably different bacterial stool microbiome composition and may affect mice’s capacity to concentrate their stools
(Volkart et al., 2023).
AQP1 was found in the small intestine and colon of buffalo and co-localization with enteric neurons in the jejunum, the ileum, the cecum and the colon were shown, suggesting involvement in osmoregulation in gastrointestinal physiology and also maturation of intestinal structures within the first week following birth (
De Luca et al., 2015).
AQP3 and AQP4
The basolateral membrane of epithelial cells in the stomach and intestines is where AQP3 and AQP4 are primarily found. In the
Citrobacterium rodent infection model, the results suggested that, AQP3-dependent H2O2 transport help in a ROS host defence mechanism
(Thiagarajah et al., 2017). The preservation of intestinal epithelial barrier function depends heavily on AQP3. It modulates the import of extracellular H2O2, activating cyto-protective pathways such as epidermal growth factor receptor (EGFR). In intestinal damage, mice lacking AQP3 show aberrant H2O2 signalling, defective lamellipodia and focal adhesion formation, reduced wound healing and increased inflammation
(Yde et al., 2021). AQP3 influences the formation of actin-driven lamellipodial projections in the injured cell model by influencing the membrane’s permeability to hydrogen peroxide, aiding in endothelial cell migration and epithelial healing
(Lv et al., 2022). Changing the location of AQPs may change the water homoeostasis of intestinal cells, which results in diarrhoea
(Lv et al., 2022). AQPs are implicated in the pathophysiology of bile acid-induced diarrhoea
(Yde et al., 2016).
AQP7, AQP8, AQP10 and AQP11
In small intestine and large intestine, AQP7, AQP8, AQP10 and AQP11 are dispersed in the enterocytes
(Zhu et al., 2016). It has been found that, in intestinal tract, AQPs also participate in cell proliferation, related cell functions and participation in intestinal mucosal damage; regulation of cell signalling and innate mucosal immune responses
(Lv et al., 2022).
AQP8 are involved in absorption of water in the intestine, the secretion of bile in liver and pancreatic juice in pancreas
(Calamita et al., 2005). AQP8 is engaged in the metabolism of H2O2 and downregulating AQP8 may be a defence mechanism against extremely high levels of oxidative stress
(Te et al., 2008). These facts suggest that H2O2 is a common mediator of the inflammatory process in the colon. Additionally, it was discovered that the colon of rats with IBD had downregulated levels of AQP1, AQP3 and AQP8 (
Chao and Zhang, 2018).
The GI tract’s aquaporin function should be further studied to develop innovative treatments and biomarkers to control fluid transport and to aid in the detection of GI illnesses.
Urinary system
It is well known that the pathogenesis of renal disorders may be influenced by the dysregulation of redox homeostasis and the excessive generation of free radicals.
Oxidative stress is found to develop in animals with renal dysfunctions and is caused by an overall rise in ROS accompanied by a decline in antioxidant capacity (Tamma and Valenti, 2016). Ureteral obstruction induces renal injury and leads to oxidative stress
(Kaeidi et al., 2020). Accordingly, many studies in animal models have explored the effect of unilateral ureteral obstruction-induced renal oxidative stress, inflammation and apoptosis. Unilateral Urethral Obstruction is linked to the severe downregulation of AQP2, AQP3, AQP4 and AQP1, which play significant roles in the impaired urinary concentrating capacity in the obstructed kidney. The downregulation of AQPs contribute to the impaired water reabsorption and urinary concentrating capacity in obstructive kidney disease, a serious and frequent clinical complication associated with the impaired renal tubular function in modulating fluid and electrolyte homeostasis. In oxidative stress associated with heat stressed chickens AQP2 levels increased in renal tissues
(Rebez et al., 2023).
AQP1
Specifically caused by mitochondrial reactive oxygen species (mtROS), high glucose-induced cellular hypoxia was inhibited in the glomeruli of diabetic mice by mitochondrial blockades or manganese superoxide dismutase (MnSOD) overexpression, which boost the expression of AQP1. Endothelial cell overexpression of AQP1 inhibited the effects of hyperglycemia on cellular hypoxia and death
(Sada et al., 2016).
AQP2
The extracellular cation, sodium and intracellular cation, potassium have significant physiological functions and any imbalance could lead to stress
(Ninan et al., 2018). A study using rats found that water restriction lowered AQP2 while increasing plasma sodium and diuresis. This study offers novel evidence of the regulation of AQP-2 expression by renal endogenous Angiotensin II-oxidative stress in hypernatremic rats exposed to acute sodium overload (Della
Penna et al., 2014). Bioinformatic data integration tools were used to identify the mechanisms causing AQP2 depletion and the research identified autophagy/apoptosis, oxidative stress and inflammatory signalling as crucial components of the system
(Mak et al., 2023).
AQP11
Functional role of AQP11 includes the regulation of intracellular H2O2 homeostasis to avoid endoplasmic reticulum stress as a result of the functional identification of AQP11 as a peroxiporin. In the kidney’s proximal tubular epithelial cells, AQP11 is abundantly expressed and associates with the endoplasmic reticulum. A more recent study reveals expression of loss-of-function mutant of water-specific AQP11, due to sudden juvenile death syndrome in mice model. It has been demonstrated that in AQP11 mutant mice the renal level of superoxide was elevated and that an antioxidant ameliorated ROS-related kidney injury. These findings suggest that the mechanism of progression of kidney injury involving an alteration of oxygen homeostasis leads to apoptosis
(Hoshino et al., 2019).
Respiratory system
AQPs are channel proteins that facilitate transport of fluid in alveolar space, humidification of the air passage, fluid absorption in pleura and secretion of sub-mucosal glands
(Song et al., 2017). The four AQPs, AQP1, AQP2, AQP4 and AQP5, expressed in the lungs under diverse physiological and pathological circumstances are associated with various lung disorders. In the peripheral lung and airways, water is transported via epithelia and endothelia during airway hydration, alveolar fluid transfer and submucosal gland secretion. The expression of AQP1 in microvascular endothelia, AQP3 and AQP4 in the airway epithelia and AQP5 in type I alveolar epithelial cells, submucosal gland acini and a minority of airway epithelial cells (
Verkman, 2007).
AQP1
It has been established that AQP-1a is involved in transendothelial/transepithelial permeability of water and is the major pathway involved in water flux across pulmonary microvascular endothelium (
Wittekindt and Dietl, 2019). Because it reduces pulmonary oedema and inflammation, AQP1 may protect against Ischemia-Reperfusion Injury (IRI)-induced lung injury. There may be a use for AQP1 upregulation in the treatment of ischemia-reperfusion injury to the lung
(Wang et al., 2023a).
AQP5
In normal conditions, AQP-5 is localized in the respiratory epithelium mediating transcellular water permeability
(Ma et al., 2015). Reactive oxygen or nitrogen species (ROS or RNS) and oxidative stress in the respiratory system increase the production of mediators of pulmonary inflammation and initiate or promote mechanisms of carcinogenesis
(Valavanidis et al., 2013). It is possible to use AQP5 as a prognostic biomarker by understanding its involvement in the development of lung adenocarcinoma
(Jaskiewicz et al., 2023). However, there is still paucity of information about the role of animal AQPs in respiratory system.
The regulatory AQPs in various body systems of stressed animals are presented in Table 1.
Regulatory role of Aquaporins in animal’s resistance to drought conditions
Extreme weather condition like drought as a result of rising temperatures, are likely to decrease water quality for animal consumption, through increased concentration of pathogens, sediments, salts, nutrients or pollutants in water
(Godde et al., 2021) impacting health and productivity of animals. Animals have developed varied adaptations to drought circumstances. These adaptations aid in their survival and growth in arid settings. Water homeostasis is one such mechanism that is regulated by aquaporins, even if their function in animal drought tolerance is less well understood than that of plants. Aquaporins help maintain cellular hydration and prevent dehydration in harsh environmental circumstances in animals by controlling water transport.
In summary, animals require aquaporins for water balance and stress tolerance during unfavourable environmental conditions. The attention of researchers is drawn favourably in understanding the complex regulatory role of aquaporins in animals affected by drought conditions, particularly in relation to defence mechanisms and drought adaption strategies.
