Wnt signaling - using the bloodstream to send a message

Abstract

Wnt signaling is one of the cell’s most complex and important signal transduction pathways. This pathway, which is divided into additional sub-pathways, regulates cell growth, motility, polarity, and differentiation during embryonic development as well as stem cell regeneration. In addition, the Wnt cascades are involved in maintaining different aspects of adult homeostasis. The Wnt ligands, which normally initiate this cascade, are secreted glycoproteins that bind specific receptors and co-receptors to execute their intracellular signaling activity. The Wnt pathways have been extensively studied in anchored cells and in tissues. However, recent evidence now shows that the Wnt cascades are functional in the circulation and that these important signals can affect different circulating blood cells by traveling through the bloodstream. Wnt signaling can function in both paracrine and autocrine manner; however, in the current review, we will discuss the exocrine properties of the pathway and address the following topics: the source of Wnt ligands and their ability to travel in the bloodstream; which cell populations express Wnt signaling components; and finally, what are the physiological roles of the Wnt cascade in the different circulating blood cells.

Introduction

The Wnt signaling pathway is an evolutionarily conserved signaling cascade that regulates crucial aspects of cell growth, motility, and differentiation throughout the body [1]. The secreted Wnt ligands interact with other secreted and membrane-bound proteins to initiate cellular signaling cascades [2–4]. Transmembrane receptors include members of the frizzled (Fz) family, as well as several co-receptors (such as LRP-5/6) that interact with the different Wnt ligands to initiate signaling. The Wnt pathways include the canonical-β-catenin dependent pathway that involves nuclear translocation of β-catenin and activation of target genes via TCF/LEF transcription factors, and the non-canonical Wnt/Ca²⁺ and Planar cell polarity pathways. These conserved signaling cascades control key biological processes such as cell fate determination, proliferation, differentiation, and migration, in both embryonic development and adults [5, 6]. The Wnt proteins are considered mostly paracrine molecules, and thus, Wnt signaling is largely defined in tissues and their resident cells, where the pathway, as well as pathological changes, are relatively well characterized. However, data has been accumulating demonstrating active Wnt signaling in fluid compartments, such as the bloodstream, where the circulating cells are mostly dispersed.

Numerous findings show that Wnt signaling is crucial for hematopoietic stem cell (HSC) self-renewal and homeostasis [7] and for the maturation of hematopoietic progenitors [8, 9]. Indeed, multiple members of the Wnt proteins are expressed in hematopoietic cells [8], and the Wnt cascade was shown to control the proliferation of immune cell progenitors and affect cell-fate decisions [10, 11]. The transmission and function of the Wnt signal depends on the cell type and tissue context [12] and was shown to have different functions in the regulation of immune cells, depending on these parameters [13].

Dysregulation of Wnt signaling is the hallmark of many solid tumors as well as various hematological malignancies. For example, aberrant activation of the Wnt/β-catenin pathway is associated with all leukemia types [14, 15], and different Wnt-related genes, such as FAT-1, have been shown to be associated with the disease [16]. Increasing evidence indicates that abnormal Wnt/β-catenin signaling promotes cancer progression by regulating the tumor-immune cycle, as it directly alters critical regulators of T cells’ anti-tumor activities [17]. Aberrant canonical Wnt signaling is also implicated in developing and progressing inflammation processes, including immunological responses in the lung, intestine, and other systemic diseases, possibly through recently discovered cross-talk between the Wnt and NF-κB pathways [18]. In addition, Wnt signaling has also been shown to affect the regulation of immune cells [13].

Although signaling pathways are not generally known to function in the circulation, it is possible that these important signals can be used to affect different cell types by traveling through the bloodstream. Also, the Wnt ligands’ hydrophobicity enables their movement in the blood circulation system as the plasma substance contains serum and other proteins that may serve as chaperons (for example: afamin [19]). Indeed, of the 19 human Wnt ligands, the expression of 12 ligands was reliably detected in human blood [20–22]. Despite their lipid-rich moieties, Wnt ligands have long-range effects [23] and can be packaged into exosomes. These extracellular secreted vesicles are trafficked between cells and help maintain Wnts’ biological activity [24, 25]. Exosomes can be found in the bloodstream and are important mediators of cell-cell communication by delivering their cargoes across the circulation system [26].

There are three main categories of blood cells: red blood cells (RBCs), white blood cells (WBCs), and platelets. WBCs in the bloodstream consist largely of polymorphonuclear cells, or granulocytes (neutrophils, eosinophils, and basophils) and peripheral blood mononuclear cells (PBMCs, i.e., lymphocytes and monocytes). Figure 1 illustrates the hierarchy of blood cells [27–32].

Fig. 1.

The hematopoietic hierarchy of Blood cells differentiation. This schematic reflects the current understanding of the hematopoietic lineage stem cells differentiating hierarchy. Created with BioRender.com

In the current review, we will focus on Wnt signaling in mature circulating blood cells and address the following questions:

• What is the source of Wnt ligands in the bloodstream? Which circulating cell populations express Wnt receptors, co-receptors, and intracellular Wnt signaling components, thus capable of transducing a Wnt signal? And finally, what are the physiological functions of the Wnt cascade in the different circulating blood cells?

What is the source of Wnt ligands in the bloodstream?

The Wnt ligands have been broadly divided into two groups depending on their downstream effects: canonical Wnt ligands, which induce a β-catenin dependent pathway (Wnt1, 2, 3, 8a, 8b, 10a, and 10b), and non-canonical Wnt ligands (Wnt4, 5a, 5b, 6, 7a, 7b, and 11) [33]. Crosstalk between the different Wnt ligands has also been documented [34, 35]. These ligands belong to a large family of secreted glycoproteins, of approximately 40 kDa, which are highly hydrophobic. Extracellular movement is achieved by specific carriers such as Secreted Wnt interacting molecules (SWIM) [36], lipoprotein particles (LPP) [37–39], and Heparan sulfate proteoglycans (HSPGs) [40, 41]. Exosomes and cytonemes are also used to facilitate the extracellular movement of the hydrophobic Wnt proteins [24, 42–47], and exosome-mediated Wnt signaling was shown to modulate tumor progression [48]. In addition, Menck et al. have demonstrated that under certain conditions, macrophages can shuttle Wnt5a via plasma membrane-derived microvesicles (MV) [49]. The ability of Wnt ligands to function on distant cells has been studied to some extent in Drosophila melanogaster [50]. However, the aptitude of Wnt proteins to travel in the bloodstream is yet to be determined, although these ligands were detected in physiological concentrations in the plasma of healthy and septic patients [20, 22]. Wnt regulators (such as secreted Frizzled-related protein (sFRP), Dkk-1 and WIF), antagonize signaling by preventing ligand-receptor interactions or Wnt receptor maturation [51]. It was shown that some of these proteins can be secreted by blood cells. Platelets are a major source of Dkk-1 in pathological type 2 inflammation [52], and platelet-derived Dkk-1 was implicated in ICAM-1/VCAM-1–mediated neutrophilic acute lung inflammation [53]. Also, Multiple Myeloma cells from patients with advanced bone lesions constitutively produce sFRP-2 [54].

In general, endothelial cells (ECs), as well as several immune cells, have been implicated as the source of Wnt ligands in the blood circulation:

Endothelial Cells (EC)

Vascular endothelial cells forming the inner layer of blood vessels harbor key roles in developing and maintaining a functional circulatory system, providing paracrine support to surrounding non-vascular cells [55]. ECs were shown to express various Wnt proteins. Mouse primary brain microvascular ECs express mRNAs encoding Wnt7a and Wnt10b [56], and mRNAs of Wnt5a, Wnt7a, Wnt10b, and Wnt13 were detected in human umbilical vein ECs [57]. Bovine aortic ECs were shown to exclusively express the Wnt2b ligand, while distinct human ECs exhibit a differential pattern of Wnt mRNA expression [58]. Also, transcripts of Wnt1, Wnt2b, Wnt4, Wnt5a, and Wnt8b were detected in human ECs, though, due to the lack of specific reagents, the results were not confirmed at the protein level [59]. These EC-derived Wnt ligands exhibited the capacity to induce β-catenin-dependent signaling in effector T cells. A later study revealed that in mice, both lung and retinal ECs express the non-canonical Wnt5a and Wnt5b as well as the canonical Wnt2 ligands [60]. An additional study showed that human pulmonary microvascular ECs express Wnt5a, which is released via exosomes, allowing it to exert long-range effects [61].

While ECs express Wnt ligands, their production appears to primarily support local signaling [62, 63]. Further studies are needed to determine whether they contribute significantly to systemic Wnt ligand levels.

Immune cells

Dendritic cells (DCs) are derived from the bone marrow’s hematopoietic stem cells (HSCs) forming a widely distributed cellular system throughout the body, while a small number enter the bloodstream. Different DC subtypes were shown to express the Wnt5a ligand at both the mRNA and protein levels [64, 65]. It was also demonstrated that follicular DCs secrete Wnt5a, which protects isolated germinal center B cells from apoptosis through activation of the Wnt/Ca + 2 pathway [65]. Both autocrine and paracrine activity of this secreted Wnt was determined.

Monocytes are immune cells originating in the bone marrow and differentiating into macrophages or dendritic cells following organ infiltration. Wnt5a was detected in circulating monocytes of healthy human individuals [21], and human subjects with obesity or type 2 diabetes [66]. Arderiu et al. found that circulating monocytes secrete Wnt5a, which interacts with Fz5 in microvascular ECs and induces angiogenesis [67].

Lymphocytes are immune cells that populate the bloodstream and are largely divided into B- and T-lymphocytes. Flow-cytometry assays revealed that both B- and T-lymphocytes express the Wnt5a ligand [21]. Ghosh et al. have shown an interesting pattern of Wnt expression in CXCL12-treated T cells: Wnt5a, Wnt10a, and Wnt7a were significantly up-regulated, whereas the canonical Wnt3a was down-regulated [68]. A previous study on the other hand, found that T cell activation with anti-CD3 and anti-CD28 resulted in a 3-fold increase in Wnt3a mRNA expression [69]. In addition, Koch et al. found that side population cells originate from B-cell lymphomas secrete Wnt3a-containing exosomes [48]. Numerous studies show that Wnt expression is up-regulated during inflammatory response [70, 71] and in cases of septic shock [22, 71, 72]. Also, Wnt5a up-regulation was detected in PBMCs upon treatment with interleukin (IL)−13 and IL-4 [73]. In summation, although specific Wnt proteins have been shown to have pro- or anti-inflammatory functions, their exact contribution to inflammation in vivo is yet to be determined.

Which cell populations express Wnt signaling components and can thus transduce the Wnt cascade?

Cells must express the different signaling components to transduce a functional Wnt cascade. Several studies describe the expression of specific Wnt-signaling-related components in various blood cells. For example, granulocytes and monocytes express the Wnt signaling receptors Fz1, Fz5, and Fz7 and, to a lower degree, Fz4 and Fz8 [74]. Human Mast cells were found to express the Fz receptors (mostly 1 and 7), Disheveled- (Dvl) 1–3, and the co-receptors LRP5-6 [75]. Valencia et al. showed that human CD14⁺ monocytes express low mRNA levels of most Wnt receptors (Fz1-5, Fz7, Ryk, ROR1 and ROR2), with Fz1 being the most highly expressed [76]. Interestingly, peripheral blood monocytes from patients with chronic kidney disease exhibit a robust up-regulation of Wnt signaling genes [77]. LRP5 is expressed in human ECs, monocytes, and macrophages [78]. Mature T cells were shown to express the Fz receptors, LRP5/6 and Dvl1-3 [54], and activated T cells express transcripts for Fz5 receptor and the wnt5a ligand [79]. Human neutrophils also express Fz2, Fz5 and Fz8 receptors [80]. Human platelet proteomic datasets revealed several Wnt signaling pathway components, including Dvl-2 and LRP5. The following proteins were detected in both resting and activated platelet lysates: Fz isoforms 1–9, LRP5/6, Dvl-2, Axin-1, APC, FRAT-1, CK1α, GSK3β, and β-catenin in [81]. Another study found that all known Fz receptors except Fz1 and 10 were detected in rat platelets. Fz3, 5, 6, and 7 displayed higher expression than Fz2, 4, 8, and 9. All Fz receptors were present in human platelets, though the expression of Fz8, 9, and 10 was relatively low. The expression of Axin was observed in rat and human platelets and Fz3 was the most abundant receptor detected [82].

To provide a crude comparison between various Wnt signaling components, we used human single-cell RNA‐sequencing (scRNA‐seq) data obtained from publicly available sources (Fig. 2). Healthy PBMC dataset was processed as described in Stephenson et al. [83], with cell annotations provided by Levinger et al. [84]. The data was further processed using Scanpy (version 1.10.4). Cells with fewer than 200 detected genes or over 5% of total counts mapping to mitochondrial genes were excluded. A curated set of Wnt‐related genes comprising Wnt ligands (Fig. 2A), Fz receptors and co‐receptors (Fig. 2B), Wnt pathway components (Fig. 2C) and Wnt targets (Fig. 2D) was defined, and average expression values of these genes were calculated for each cell type using the normalized expression matrix. Genes with a mean expression of zero in each cell type were considered not expressed for that subset.

Fig. 2.

Plot of mean Wnt signaling factors expression: The dot plot displays the expression of Wnt-related genes in a PBMC dataset. The x-axis represents the curated Wnt-related genes (e.g., DVL1, APC, CTNNB1, etc.), while the y-axis corresponds to the cell-type annotations derived from the PBMCs (e.g., “T Naïve (CD4)”, “Monocyte (CD14)”, etc.). Each dot represents the mean normalized expression level of a specific Wnt-related gene in that cell type subset—calculated as the average value from the normalized expression matrix. Larger dots indicate higher average expression, whereas the absence of a dot means the gene’s average expression in that cell type is zero. The dot plot was generated using Matplotlib (version 3.9.2) and Seaborn (version 0.13.2). Note that the Wnt ligands mean expression is 10 fold smaller than the other Wnt factors. (A) Wnt ligands, (B), Fz receptors and co-receptors, (C) Wnt pathway components and (D) Wnt targets

It is worth mentioning that many of these cells express both the Wnt ligands and receptors, as well as other components of the Wnt cascade, and are known to use Wnt signaling for multiple cellular functions (as detailed below). Further studies are needed to determine their contribution to Wnt signaling in distant tissues.

What are the physiological roles of the Wnt cascade in the different circulating blood cells?

Anucleated blood cells

Though the different Wnt signaling pathways are usually studied in nucleated, dividing cells, platelets and RBCs, which are anucleated, express various Wnt pathway components [81, 85].

Platelets are circulating blood cells derived from bone marrow megakaryocytes that form clots to prevent blood loss following vascular injury. Platelets express nearly all the elements of the Wnt/β-catenin canonical pathway, though β-catenin stabilization is probably due to non-transcriptional mechanisms, such as coupling adhesion receptor activation with cytoskeletal changes transduced by actin [81]. Steele et al. showed a functional role for Wnt signaling in regulating platelet function by demonstrating that Wnt3a inhibits activation, alters shape changes and hinders the small GTPase RhoA activity [81]. Later work showed that Wnt3a differentially modulates four key GTPase proteins and affects the Dvl/Daam-1 signaling complex [86]. Other studies have shown that Wnt5a can potentiate induced platelet aggregation via the PI3K/Akt pathway [82], and induced β-catenin degradation following sustained platelets aggregation in the presence of calcium [87]. The non-canonical Wnt signaling is part of a negative feedback loop that restricts platelet activation and possibly limits thrombus growth in sync with other platelet regulators [88].

Erythrocytes (RBCs), which function as oxygen carriers, are the most abundant cells in the circulation system. Like platelets, erythrocytes are anucleated cells and thus are unusual candidates for Wnt signaling. Still, Siman-Tov et al. have found that both Wnt3a and Wnt5a activate the non-canonical Wnt cascade in RBCs. This study has demonstrated that Wnts improve the erythrocyte cytoskeleton flexibility and strength, leading to prolonged survival both ex-vivo, and in post-transfusion recipient mice [21].

Peripheral Blood Mononuclear Cells (PBMCs)

In PBMCs derived from asthma patients, many genes known to be involved in the Wnt/β-catenin pathway (such as Wnt5a, Fz7, DVL3, and cyclin D1), display a > 2-fold change in expression level compared to normal subjects [89]. In general, positive Wnt regulators were up-regulated, whereas Wnt activity inhibitors showed no change or down-regulated expression patterns, suggesting that Wnt signaling is intracellularly activated in asthma conditions [89].

B-lymphocytes. Early studies implied that β-catenin expression was dispensable for normal B cell development and function [90]. Indeed, the canonical Wnt cascade was shown to be silenced in mature B cells as reflected by the low expression of TCF and LEF [91]. The low Wnt activity in mature B cells has also been demonstrated using different in vivo Wnt reporter mice [92]. Interestingly, mature B-1 cells are able to respond to Wnt ligands in vitro, despite lack of activated Wnt signaling [93]. Wnt signaling is mainly indicated in mature B-cell-derived hematological malignancies such as chronic lymphocytic leukemia (CLL). In these cells, β-catenin-independent signaling, predominantly the Wnt/planar cell polarity (PCP) pathway, has a prominent role (reviewed in [94]). Wnt/β-catenin pathway inhibitor genes were shown to be hyper-methylated in both CLL cell lines and patient samples [95]. Even though both Wnt cascades were suggested to be involved in the pathogenesis of CLL [96–100], no consensus has been reached about the importance of the individual Wnt branches in different physiological processes and cell–cell interactions controlling CLL progression. The roles of distinct components of the canonical and non-canonical Wnt cascades are methodically presented by Janovská and Bryja [94].

T-lymphocytes. The Role of Wnt signaling in mature T cells and the involvement of the Wnt-responsive TFs, such as Tcf1, and Lef1 in CD8 + T cells memory formation was previously discussed (reviewed in [101, 102]). However, many of the data originated from murine spleen or thymus residing T lymphocytes. Despite their important contribution to the understanding of Wnt signaling in mature lymphocytes, as the current review focuses on circulating cells, studies conducted in spleenocytes/thymocytes were omitted. Additional studies, conducted in peripheral blood cells, will be discussed. The maturation of precursor cells to circulating lymphocytes has been found to be Wnt-dependent [103], and Wnt/β-catenin signaling in DCs regulates CD8 + T-cell response to A. fumigatus [104]. Willinger et al. found that LEF1 and TCF7 (TCF-1) are expressed in mature T cells, and that resting T cells preferentially express inhibitory LEF1 and TCF7 isoforms. Importantly, they show that T cell activation changes the isoform balance in favor of the stimulatory TCF7, suggesting that Wnt signaling regulates peripheral T cell differentiation [105]. In CD4 + T cells LEF1 is dominantly expressed in Th1 but not in Th2 cells. A high affinity LEF1-binding site was identified in the proximal promoter region of the Th2-specific cytokine IL-4 suggesting that LEF1 negatively controls the IL-4 gene expression [106]. Th2 differentiation was also shown to be regulated in Wnt-dependent manner. The special AT-rich binding protein 1 (SATB1), which is the T lineage-enriched chromatin organizer and global regulator, interacts with β-catenin and recruits it to SATB1’s genomic binding sites in order to orchestrate Th2 lineage commitment [107]. In activated T cells Wnt signaling was suggested to affect T cells immunotherapy resistance via NFAT nuclear translocation [79]. Wu et al. found that Wnt activity directly up-regulates matrix metallo-proteinase (MMP) proteases, which are crucial for T cell migration, suggesting a role for Wnt signaling in T cell extravasation [59]. In T-cell acute lymphoblastic leukemia adult cells (MOLT4), the chemokine CCL25, which induces chemotaxis in mature CD4 + or CD8 + thymocytes [108], promotes Wnt5a expression via Protein kinase C (PKC) activation. Wnt5a induces cell migration and invasion through RhoA activation and cooperates with CCL25 to promote MOLT4 cell metastasis [109]. Lee et al. reported a significant correlation between the Wnt inhibitor - sFRP1 and IL-17 levels in the synovial fluid of rheumatoid arthritis patients, suggesting a role in the differentiation and/or function of human Th17 cells. They also showed that sFRP1 enhances the production of Th17 cytokines from human T cells by potentiating TGF-β-induced phosphorylation of Smad2/3 in human CD4 + T cells. Together, these data suggest that TCF1 functions as a direct transcriptional repressor of IL-17 in the presence of β-catenin [110].

Regulatory T (Treg) cells are a specific CD4 + CD25 + Foxp3 + T cell lineage that are crucial for the induction of self-tolerance [111]. TCF1, which is a Wnt-associated transcription factor was found to interact with Foxp3, and Wnt activation impaired the transcriptional activity of the latter. Furthermore, Wnt signaling (induced by Wnt3a and other factors) abolished the suppressive capacity of Treg Cells. The authors have shown Wnt-mediated activation of Fz receptors expressed on Treg cells and proposed that Wnt produced by mononuclear cells could modulate the strength of the immune response through the inhibition of Foxp3 transcriptional output [69]. Shen et al. demonstrated that Wnt16b derived from fibroblasts could regulate Tregs differentiation through DCs, and that Tregs differentiation was inhibited by Wnt16B depletion [112]. Also, fungal activation of the Wnt/β-catenin pathway in DCs was shown to promote PD-L1-mediated polarization of CD4 + T cells into Treg cells [104].

Natural Killer (NK) cells play an important role in the immune response to cancer as they induce T-cell independent apoptosis of neoplastic cells. One study showed that the CD244 stimulation of NK cells, resulted in GSK-3β inhibition and the stabilization of β-catenin, leading to increased IFN-γ and granzyme B expression, associated with enhanced cytotoxicity [113]. Aoyama et al. demonstrated that Wnt signaling can play a significant role in early T versus NK cell differentiation in the presence of Notch signaling [114]. It was also shown that intestinal mucus-derived nanoparticle–mediated activation of Wnt/β-catenin signaling plays a role in the induction of liver Natural killer T (NKT) cell anergy in mice [115].

Monocytes. Early studies demonstrated adherence-dependent accumulation of β-catenin in human monocytes and identified TCF-4 as the LEF/TCF transcription factor in these cells [116]. Tickenbrock et al. showed that β-catenin protein can be stabilized in monocytes by inhibiting GSK-3β. Wnt3a stimulation markedly decreased the migratory potential of monocytes, though their adhesion to the endothelial layer was increased following Wnt activation. These events correlate with an enhanced β-catenin stability but are most likely not mediated by activation of Wnt-target genes such as c-myc or Connexin43 [74]. A more recent work demonstrated that circulating human primary monocytes are susceptible to Wnt3a stimulation. Wnt activation was shown to induce the secretion of cytokines and chemokines, enhancing CCL2-mediated monocyte migration. Interestingly, in patients with rheumatic joint diseases, in which monocytes are known to be dysfunctional, Wnt3a treatment generated a unique cytokine expression profile, significantly distinct from healthy donors [20]. Abdulkareem et al. found that Wnt/β-catenin pathway components are significantly up-regulated in chronic kidney disease monocytes and signaling activity is associated with the inflammatory response [77]. In human monocytic THP-1 cells, Wnt5a lead to enhanced adhesion to ECs and induced the expression of various pro-inflammatory cytokines and inflammatory mediators, particularly IL-8 and CXCL2. Wnt5a also induced JNK phosphorylation and NF-κB activation via β-catenin-independent signaling [117]. Monocytes were shown to respond to Wnt5a by secreting cytokines such as IL-6 or IL-10 that can interfere with DC differentiation [118]. The addition of Wnt5a to monocyte cultures interfered with β-catenin accumulation and reduced its levels, thus suggesting that Wnt5a effects could be independent of β-catenin transcriptional activities. In support of this notion, stabilization of β-catenin by LiCl treatment of GM-CSF/IL-4 monocyte cultures did not mimic the Wnt5a effects [119].

Polymorphonuclear cells (granulocytes)

In granulocytes derived from the CRC liver metastasis microenvironment, approximately 82% of the genes in the Wnt signaling pathway were up-regulated, and migration was significantly reduced following Wnt inhibition [120].

Neutrophils are recruited to an infected area or injured site by a chemoattractant gradient and harbor a crucial role in regulating the immune response [121]. Wnt5a was shown to induce chemotaxis in human neutrophils, a response that was almost completely inhibited by sFRP1 [80]. The Wnt5a-stimulated neutrophil chemotaxis was partially mediated by phosphoinositide 3-kinase activity. The Wnt5a ligand is most likely secreted by macrophages as neutrophil migration was strongly induced by LPS-stimulated macrophage supernatant and significantly inhibited by anti-Wnt5a antibody [80]. Interestingly, Wnt3a markedly reduced the recruitment of neutrophils into the alveolar space [53]. Furthermore, in injured lungs, the mRNA expression of both ICAM-1 and VCAM-1 (which mediate neutrophils activation [122]) was significantly inhibited by Wnt3a. Similar outcome was observed by the neutralization of platelet-derived Dkk-1, a major Wnt antagonist [53]. These results reveal a potentially novel mechanism that Wnt/β-catenin signaling utilizes to control neutrophil infiltration during acute inflammation. Dkk-1 also suppresses PTGS2-induced neutrophil recruitment in lung metastases by antagonizing cancer cell non-canonical Wnt/PCP–RAC1–JNK signaling [123].

Eosinophils are white blood cells that are involved in various cellular processes but are best known for their role in combating parasitic infection [124]. Eosinophils from asthmatic subjects enhance Wnt5a expression in airway smooth muscle cells and promote their proliferation by increased extracellular matrix proteins production [125, 126]. Survival of eosinophils has been reported to require the nuclear presence of β-catenin, which can be triggered via IL-5 in a Wnt-independent manner [127]. In a recent study, Shi et al. suggested that sFRP5 modulate the production of cytokines relevant to eosinophil infiltration and through the inhibition of Wnt5a signaling [128].

Mast cells are important for surveillance of and responses to pathogens and cell injury but are also implicated in the contexts of allergies, anaphylaxis, asthma, and other hypersensitivity reactions [129]. Human Mast cells express several Fz receptors, the co-receptors LRP5-6 and Dvl- 1–3. The addition of Wnt3a activated mature Mast cells and induced IL-8 release [75].

Concluding remarks

The role of the Wnt pathway and other signaling cascades in circulating blood cells is a rapidly evolving field of study that includes signal transduction, immunology, and hematology. Different Wnt ligands are present in the bloodstream in both normal and pathological conditions and can modulate immune cell function, suggesting novel roles for this cascade - beyond its classical activity in development and oncogenesis. One open question that remains is whether circulating immune cells act as mobile delivery vehicles that home on to specific tissues to release Wnt ligands locally, or do they secrete Wnt ligands into the bloodstream for systemic distribution (either freely or in bound forms). Our review suggests that both mechanisms are feasible; the Wnt ligands have been detected in human plasma and can travel in the bloodstream when tethered to a carrier, which supports the idea that the Wnt ligands can act in an endocrine manner, beyond the site of secretion. On the other hand, several types of immune cells have been shown to express and secrete Wnt ligands and are known to infiltrate tissues. Thus, it is plausible that in some contexts, tissue-resident or infiltrating immune cells provide localized and specific Wnt signals, especially in pathological conditions such as inflammation or cancer. While extensive studies have characterized the implications of Wnt signaling in hematopoietic stem cells and immune cell differentiation, their role in mature circulating immune cells is only beginning to unravel. This review highlights the diverse aspects of Wnt signaling in various circulating blood cell populations. However, significant gaps remain in our understanding of how these events are regulated and their physiological consequences in the circulatory system. Future research should focus on elucidating the precise mechanisms by which Wnt ligands travel through the bloodstream, their sources, and their interactions with different immune cells.

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