Sepsis represents a prevalent complication in Intensive Care Units across Germany and the United States (1), with persistently high mortality due to its complex immunological nature. In the United States septic conditions accounts for more than $22 billion (11.2%) of total US hospital costs in 2017 (2). Incidence and mortality of sepsis differ among the regions worldwide. Incidence reaches from 158/100 000 population in 2015 in Germany (3) to 780/100000 population in Sweden (4) For patients with clearly documented sepsis (including severe sepsis), the mortality rates from 2010 to 2015 fell from 26.6% to 23.5%, while for those with severe sepsis alone, the rates decreased from 47.8% to 41.7%. in Germany (5). These figures are comparable to the rates in England (6) but significantly higher than those in the USA (15%) and Australia (18.4%) (7, 8).The absence of predictive biomarkers specific to this syndrome prevents tailored individual therapies based on patients’ immune status. Aquaporins (AQPs) are potential biomarkers due to their significant roles in inflammation, particularly in sepsis, as evidenced by experimental and association studies (9–11). AQPs are emerging as promising candidates in sepsis research due to their significant roles in inflammation and immune responses (12). Experimental and association studies indicate that AQPs are not merely transport proteins; their dysregulation is observed in immune and epithelial cells when exposed to infectious and inflammatory stimuli (13). Recent findings have firmly established the involvement of AQPs in inflammatory processes, particularly since several AQP isoforms are expressed in both innate and adaptive immune cells (14). They play crucial roles in phagocytic functions and specific immune processes such as cell activation and migration (15, 16).

The recognition of AQPs in inflammation enhances our understanding of the complex mechanisms governing host-pathogen interactions. As such, AQPs represent potential therapeutic targets for modulating edema, cell migration, and the release of inflammatory cytokines and mediators (17, 18).

Aquaporins are a family of membrane proteins that facilitate the transport of water across biological membranes. They are integral to maintaining water balance in cells and tissues. As described, Aquaporins are essential for water homeostasis in all organisms (19). These proteins are known for their remarkable ability to transport water selectively and efficiently (20). Aquaporins play critical roles in various physiological processes, including kidney water conservation, brain water balance, and secretion of cerebrospinal fluid (19).

AQPs comprise a group of 13 membrane proteins crucial for regulating cellular water, salt fluxes, and the transport of small solutes like glycerol, urea, and carbon dioxide (17). Water-selective AQPs play roles in transepithelial fluid transport, cell migration, brain edema, and neuroexcitation (17), while aquaglyceroporins are involved in cell proliferation, adipocyte metabolism, and epidermal water retention. Table 1 illustrates the various families of aquaporins. The objective of this study is to provide an update on the potential contributions of aquaporins (AQPs) to the pathomechanisms of sepsis, based on the current literature findings (21).

In conclusion, aquaporins exert a significant influence on the pathophysiology of sepsis, affecting fluid balance, organ function and the inflammatory response. Further research is required to fully explore the potential of targeting aquaporins as a therapeutic strategy. The following paragraph will delineate the role of aquaporins in various organ systems.

In recent years, single-nucleotide polymorphisms (SNPs) in aquaporin genes have been linked to various pathological conditions, highlighting their significant clinical value (91–93). Notably, the -1364 A/C (rs3759129) polymorphism in the promoter region of the AQP5 gene has been extensively studied by our group in the context of sepsis (10). The initial investigation sought to ascertain the correlation between the AQP5 promoter -1364A/C polymorphism and 30-day survival in patients with severe sepsis. The findings revealed a notable increase in survival rates among patients with combined AC/CC genotypes, in comparison to those with AA genotypes. This observation remained consistent even after adjusting for various clinical covariates. The findings emphasize the potential prognostic significance of AQP5 expression variations in severe sepsis, underscoring the role of AQP5 channels in influencing patient outcomes (9). Furthermore, we demonstrated that the C-allele of the AQP5-1364A/C polymorphism, which is associated with decreased AQP5 expression and improved outcomes in sepsis, is linked to higher promoter methylation of AQP5 in neutrophils, monocytes, and lymphocytes in both septic patients and healthy controls. Furthermore, decreased AQP5 promoter methylation was correlated with increased AQP5 expression in cell-line models, indicating that AQP5 promoter methylation may serve as a crucial mechanism in genotype-dependent AQP5 expression regulation. This suggests that AQP5 promoter methylation may represent a potential target for interventions in sepsis (94). In patients with sepsis, elevated methylation levels at the cytosine site nt-937 within the AQP5 promoter are linked to augmented AQP5 mRNA expression and are predictive of an elevated risk of mortality within 30 days. This indicates that epigenetic regulation of AQP5 via NF-κB binding at nt-937 is of pivotal importance in influencing the outcome of sepsis, thereby underscoring the potential prognostic significance of AQP5 promoter methylation in septic patients (95). Aqp5 knockout (KO) mice exhibited significantly higher survival rates post-LPS injection compared to wild-type (WT) mice, indicating that Aqp5 deficiency exerts a protective effect in sepsis. Furthermore, AQP5 expression and the AQP5 -1364A/C polymorphism were observed to regulate immune cell migration, with neutrophils from individuals with the AA genotype demonstrating earlier and more precise migration compared to those with AC/CC genotypes. This suggests that AQP5 plays a role in modulating immune responses and survival outcomes in sepsis (96).

We also examined the association between complications in septic patients and the AQP5 -1364A/C polymorphism. Here we investigated the association between the promoter polymorphism and major adverse kidney events in septic patients, as well as its impact on 90-day survival. The results demonstrated that individuals with AC/CC genotypes exhibited a reduced incidence of major adverse kidney events in comparison to those with AA genotypes. Furthermore, C-allele carriers demonstrated enhanced 90-day survival rates. Subsequent multiple proportional hazard analysis substantiated the association between AC/CC genotypes and a diminished risk of mortality within 90 days, thereby corroborating the AQP5 -1364A/C polymorphism as an independent prognostic factor in sepsis, with implications for precision medicine (97). In acute respiratory distress syndrome (ARDS) caused by bacterial pneumonia, the AQP5 -1364A/C promoter polymorphism’s C-allele was linked to reduced pulmonary inflammation and improved 30-day survival rates, offering potential insights for characterizing and treating ARDS on an individualized basis. This finding highlights the impact of AQP5 genotype on inflammation and prognosis in ARDS, suggesting a significant advancement in understanding and managing this condition (98). The association between the AQP5 promoter -1364A/C polymorphism and AKI in patients with pneumonia-induced acute respiratory distress syndrome (ARDS) were examined. Results show that while the incidence of AKI upon admission did not differ between genotypes, by day 30, the AA genotype had a significantly higher prevalence of AKI compared to AC/CC genotypes. Moreover, the AA genotype was identified as an independent risk factor for AKI persistence, indicating that AQP5 genotype may influence AKI development and resolution beyond fluid balance considerations in ARDS (99).

Furthermore, a polymorphism in the AQP3 gene was examined. There was an association between AQP3 polymorphism (rs17553719) and expression with survival outcomes in sepsis patients. Results showed that the CC genotype was linked to decreased 30-day survival, higher AQP3 mRNA expression, and elevated IL-33 concentration, suggesting a potential role of AQP3 in sepsis prognosis (100). Polymorphisms in AQP genes could therefore influence the disease progression in sepsis.

The role of AQPs in inflammasome activation has been described intensively in other reviews (21, 29). The inflammasome, crucial in the immune response, is found in macrophages and neutrophil granulocytes and recognizes various pathogen antigens. The NLRP3 inflammasome, upregulated in sepsis, triggers the release of IL-1β, dependent on cell pH and facilitated by aquaporin-mediated water influx in macrophages (101). The movement of water by AQPs appears to be a pivotal factor in unifying the activators of the NLRP3 inflammasome. The absence of AQP1 in a mouse model of acute lung injury resulted in a reduction in IL-1β release and neutrophilic inflammation, which serves to underscore the role of AQPs as a danger signal for NLRP3 activation. AQP3, which is highly expressed in THP-1 cells, plays a role in the rapid changes in cell volume and the activation of the inflammasome in response to stimuli such as reswelling, nigericin, and ATP. The increased expression of AQP3 serves to amplify these responses, while its peroxiporin activity has been observed to enhance intracellular ROS and inflammasome activation. Furthermore, AQP4 resulted in a reduction of NLRP3, caspase-1, and IL-1β proteins in the treatment group, indicating the inactivation of the inflammasome (102). Furthermore, the absence of AQP5 was observed to facilitate NLRP3 inflammasome activation via the generation of reactive oxygen species (ROS). The inhibition of ROS or the blockade of the NLRP3 inflammasome was observed to markedly diminish the extent of damage and pyroptosis in AQP5-deficient lacrimal gland epithelial cells (103).

The term “immunometabolism in sepsis” denotes the intricate interplay between the immune system and the body’s metabolic processes during the course of sepsis. Immune cells undergo metabolic alterations during sepsis in order to rapidly provide energy and materials for defense responses. These involve increased glycolysis and changes in fatty acid and amino acid utilization, which occur during the proinflammatory phase of sepsis (104). Similar to tubular epithelial cells (TECs), immune cells in sepsis may undergo a profound metabolic transformation, transitioning from oxidative phosphorylation (OXPHOS) to a predominance of aerobic glycolysis. Within this metabolic realignment, the majority of pyruvate generated through glycolysis avoids mitochondrial entry and is instead converted into lactate—a process catalyzed by lactate dehydrogenase (LDH). This strategic metabolic adaptation is crucial, as it supports increased ATP production through glycolysis to meet the elevated energy demands imposed by the septic challenge (105). Aquaporins (AQPs), particularly those involved in glycerol transport, play a crucial role in enhancing glycolysis during sepsis by ensuring the availability of key substrates and supporting the increased metabolic demands of cells. Specifically, aquaglyceroporins such as AQP3, AQP7, AQP9, and AQP10 facilitate the transport of glycerol across cell membranes ( Table 1 ). In sepsis, the body’s need for energy surges, leading to the upregulation of these AQPs. Especially AQP3 and AQP1 seem to be upregulated in immune cells after inflammatory stimulus (106). It is known that AQP3 and AQP9 can influence gluconeogenesis by transporting glycerol into the cell (107). The glycerol transported into cells is converted into glycerol-3-phosphate (G3P) by glycerol kinase, and then into dihydroxyacetone phosphate (DHAP), an intermediate in the glycolytic pathway (108) ( Figure 4 ). This process directly feeds glycerol into glycolysis, enhancing its flux and thereby increasing ATP production.

Furthermore, during sepsis, hypoxia-inducible factor 1-alpha (HIF-1α) triggers the upregulation of glucose transporter 1 (GLUT1), enhancing glucose uptake into cells (104). AQP3 supports this process by facilitating the transport of glycerol, which complements glucose metabolism ( Figure 4 ). Additionally, elevated levels of O-GlcNAcylation, a post-translational modification of proteins that occurs during sepsis, further increase glucose uptake via GLUT1 and regulate glycerol transport through AQP3. This dual availability of glucose and glycerol ensures rapid glycolysis, meeting the high energy demands of immune cells (106, 109).

AQP7 also plays a significant role in metabolic regulation by influencing lipid metabolism and glycerol availability. In sepsis, increased intracellular glycerol levels or active AQP7 expression could enhance p38 signaling, which is associated with the upregulation of glycolysis and nitric oxide production. This metabolic flexibility might allow immune cells to adapt to the energy demands of the septic environment (110).

The interaction between glycolysis and glycerol metabolism is crucial in sepsis. For instance, glycerol processed through glycolysis can enter the pentose phosphate pathway (PPP), which is important for producing NADPH and ribose-5-phosphate, essential for biosynthesis and redox balance in immune cells. AQP1, although primarily known for water transport, is upregulated by glycolysis ( Figure 4 ), may also indirectly influence glycolysis by regulating glucose availability and interacting with other metabolic pathways, including those involving lactate and hydrogen ion (H+) transport, which are byproducts of glycolysis (111).

In summary, aquaporins contribute to the increased glycolysis observed in sepsis by facilitating glycerol transport, supporting glucose uptake, and interacting with various metabolic signaling pathways. This enhancement of glycolysis ensures that immune cells have sufficient energy to respond to infection and maintain cellular function under the stress of sepsis.

Aquaporins may be useful drug targets in sepsis. For example, we recently showed that methazolamide and furosemide reduced AQP5 expression in REH cells, with methazolamide also reducing immune cell migration. However, only methazolamide pre-treatment showed potential to reduce LPS-induced AQP5 expression, suggesting it may be useful in sepsis prophylaxis (112). AQP9, found in hepatocytes and leukocytes, is being investigated as a potential target to reduce mortality in septic shock. Aqp9 knockout (KO) mice showed prolonged survival and reduced inflammation compared to wild-type (WT) mice after LPS-induced endotoxic shock. KO mice exhibited reduced production of pro-inflammatory nitric oxide (NO) and superoxide anion (O2-), as well as reduced levels of iNOS and COX-2, which was attributed to impaired NF-κB p65 activation in various tissues. Blocking AQP9 with HTS13286 in FaO cells also prevented LPS-induced inflammation, suggesting a role for AQP9 in early phases of endotoxic shock via modulation of NF-κB signalling. These findings highlight AQP9 as a promising target for the development of new therapies against endotoxemia (113). In a study, the novel AQP9 inhibitor RG100204 was shown to normalise oxidative stress and improve survival in mouse models of sepsis. RG100204 reduced cardiac and renal dysfunction, decreased activation of the NLRP3 inflammasome pathway and reduced myeloperoxidase activity in lung tissue, suggesting that AQP9 is a potential drug target for polymicrobial sepsis (55). Another study investigated the role of FGD5-AS1 in sepsis and LPS-induced inflammation and showed that FGD5-AS1 overexpression increased AQP1 and decreased miR-133a-3p expression, subsequently reducing inflammatory cytokines such as TNF-α, IL-6 and IL-1β. Dual-luciferase reporter and miRNA pull-down assays confirmed that FGD5-AS1 acts as a competitive endogenous RNA for miR-133a-3p on AQP1, suggesting that overexpression of FGD5-AS1 may inhibit the inflammatory response in sepsis (114). Another study investigated the relationship between AQP1, miRNA-874 and lncRNA H19 in LPS-induced sepsis. It was found that H19 and AQP1 expressions decreased while miR-874 expression increased in sepsis samples, mouse models and cardiomyocytes. H19 acted as a competitive endogenous RNA (ceRNA) for AQP1 by regulating miR-874, reversing LPS-induced inflammatory responses and myocardial dysfunction, suggesting H19 as a potential therapeutic target for sepsis-associated myocardial dysfunction (54). The development of aquaporin (AQP) inhibitors faces significant challenges, particularly due to potential off-target and side effects. Many putative AQP modulators reported in literature have failed to show consistent activity upon retesting (115). This inconsistency is often attributed to the limitations of assays used, such as oocyte swelling or calcein fluorescence, which are prone to artifacts (116). Apparent inhibition of osmotic swelling may result from factors unrelated to AQPs, such as changes in cell volume regulation or the activity of non-AQP ion or solute transporters. Common inhibitors of ion transport processes, like bumetanide or acetazolamide, may alter resting cell volume, further complicating the assessment of true AQP inhibition (117). The complexity of identifying specific AQP inhibitors is compounded by the structural characteristics of AQPs and their high abundance in cell membranes. In some cases, reported inhibitors, such as loop diuretics or antiepileptics, have confused the literature due to their lack of specificity and inability to be confirmed in subsequent studies (115). For example, AER-270, a claimed selective AQP4 inhibitor, showed only partial inhibition in mouse and human models, and its effects may be due to its role as an NF-κB inhibitor rather than directly targeting AQP4 (118, 119). This highlights the need to thoroughly evaluate off-target effects, as inhibitors may influence pathways beyond AQPs. This underscores the critical need to better understand these unintended interactions to develop more selective and effective AQP inhibitors in the future (120). Further development of AQP-targeted therapeutics requires well-designed, large-scale functional screens to identify true small-molecule inhibitors. Additionally, targeting AQP signaling pathways or intracellular trafficking, as seen with vasopressin receptor antagonists like vaptans, may offer alternative therapeutic approaches. Nonetheless, off-target effects remain a significant concern, underscoring the need for careful validation in future studies (17, 116). It is possible that another potential drug under investigation, phloretin, may be able to interfere with AQPs. In vitro studies have shown that inhibition of AQP9 with phloretin can reduce mortality, inflammatory responses and organ damage in sepsis models (38).

AQPs are emerging as crucial players in sepsis, influencing various organ systems. Their roles in immune cell activation, fluid regulation, and inflammatory processes make them attractive therapeutic targets. In sepsis, AQP1, AQP4, AQP5, and AQP9 have been shown to significantly affect organs like the brain, heart, lungs, kidneys, and liver. For example, AQP4 plays a key role in SAE by contributing to cerebral edema, while AQP1 and AQP9 are implicated in myocardial injury and ALI. Modulating these AQPs shows potential to alleviate organ damage and improve patient outcomes.

Therapeutic strategies have been developed to target these proteins. Dexamethasone and traditional Chinese medicines have shown potential in reducing cardiac and pulmonary damage. Specific inhibitors such as HTS13286 and RG100204 targeting AQP9 have demonstrated promise in reducing inflammation, improving survival, and mitigating organ dysfunction in sepsis models. These approaches highlight the therapeutic potential of modulating AQPs in sepsis-related complications like ALI, SAE, and AKI, which affects around 50% of sepsis patients.

However, the development of AQP inhibitors faces significant challenges due to potential off-target effects. Many inhibitors reported in the literature have shown inconsistent activity, often complicated by nonspecific interactions with other ion transporters or signaling pathways. Future research must focus on refining these inhibitors, exploring alternative pathways such as intracellular trafficking, and conducting large-scale screening to discover more selective and effective therapeutic options.

In summary, AQPs represent promising biomarkers and therapeutic targets in sepsis, especially in modulating inflammation and organ injury in critical systems such as the brain, heart, lungs, kidneys, and liver. However, the complexity of their inhibition and the risk of off-target effects necessitate further investigation into selective therapeutic approaches.