The pathological role of Wnt5a in psoriasis and psoriatic arthritis

Abstract Psoriasis (PsO) is a chronic inflammatory skin disease with both local and systemic components. PsO‐associated arthritis, known as psoriatic arthritis (PsA), develops in approximately 13%‐25% of PsO patients. Various factors associated with both PsO and PsA indicate that these conditions are part of a single disease. Identification of novel targets for the development of drugs to treat both PsO and PsA is desirable to provide more patient‐friendly treatment regimens. Such targets will likely represent ‘common checkpoints’ of inflammation, for example key components or transduction cascades of the signalling pathways involved. Emerging evidence supports involvement of the non‐canonical Wnt signalling pathways in the development of both PsO and PsA, especially the Wnt5a‐activated signalling cascades. These, together with interlinked factors, are crucial in the interactions among keratinocytes, immune cells and inflammatory factors in PsO, as well as among chondrocytes, osteoblasts and osteoclasts that trigger both subchondral bone remodelling and cartilage catabolism in PsA. This review focuses on the pathological role of Wnt5a signalling and its interaction with other interlinked pathways in both PsO and PsA, and also on the main challenges for future research, particularly with respect to molecules targeting Wnt signalling pathways for the treatment of PsO and PsA.


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TIAN eT Al regimens. These targets will most likely represent 'common checkpoints' of inflammation, including key components or transduction cascades of the signalling pathways involved, rather than 'common denominators' such as cytokines. 10,11 For example, small molecules that inhibit enzymes such as Janus kinases or phosphodiesterase 4 have proved effective in treating PsO and PsA. 12,13 In this context, the Wnt5a signalling pathway is an attractive target for the treatment of PsO as well as PsA.

| Wnt5a signalling pathway
Wnt signalling, which plays important roles in regulating cell proliferation, differentiation, polarity, migration and inflammation, [14][15][16][17][18] is classified into β-catenin-dependent canonical and β-catenin-independent non-canonical pathways. In the canonical pathway, Wnt signalling is activated by Wnt proteins binding to their respective dimeric cell surface receptors composed of the seven-transmembrane Frizzled proteins and the low-density lipoprotein receptor-related proteins (LRP5/6). Upon Wnt-Fz/LRP signalling, Dvl is activated and dissociates from a multiprotein complex leading to inactivation of GSK3β. This inhibits the phosphorylation and degradation of βcatenin, which accumulates in the cytoplasm and then translocates to the nucleus and interacts with lymphoid enhancer-binding factors (LEF) and T cell factors (TCF), causing transcriptional activation of target genes 14 (Figure 1).
The key molecules and cascades in the non-canonical pathway have been previously summarized. 19,20 Briefly, non-canonical Wnt signal transduction, predominantly of Wnt5a, mainly involves planar cell polarity (PCP) and Wnt/Ca 2+ pathways. Non-canonical Wnt signalling pathways, which are independent of β-catenin, rely on Wnt signal transduction through Fzd and its coreceptors, such as receptor tyrosine kinase-like orphan receptor 2 (ROR2) or receptor-like tyrosine kinase (RYK). Through the activation of calcium signalling (phospholipase C/protein kinase C (PKC)/Ca 2+ ) and calmodulin-sensitive protein kinase II (CamkII), the Wnt/Ca 2+ /CamkII pathway activates nuclear factor as-

| Wnt5a is differentially expressed in psoriatic skin
Reischl et al 23 found that Wnt5a expression was fourfold higher than normal in skin lesions from patients with plaque-type psoriasis.
Another study with more subjects showed that Wnt5a transcripts were up-regulated fivefold in skin lesions and that FZD2 and FZD5 F I G U R E 1 Model of the role and proposed mechanism of Wnt5a in psoriasis. Activation of Wnt5a signaling and its downstream effectors by local or systemic pathogens stimulate keratinocyte proliferation and secretion of inflammatory cytokines, which further regulate Wnt5a expression and promote keratinocyte proliferation and activation through Wnt5a-mediated signalling pathways. This cross-talk forms a signalling loop that promotes the persistence of PsO inflammation and disease progression expression was also increased, while mRNA levels of WIF1 (a Wnt antagonist) were down-regulated >10-fold. 24 We previously demonstrated overexpression of Wnt5a in PsO lesions, and in vitro analysis of Wnt5a knockdown in HaCaT and NHK cells suppressed cell proliferation and induced apoptosis. 25 A recent analysis of gene pathogenicity using samples from psoriatic and healthy skin revealed that Wnt5a is one of five key genes in psoriasis. 26 However, Wnt5a expression remains high in resolving psoriatic lesions 27 ; therefore, the role of enhanced Wnt5a expression in PsO is more complicated than proposed. Mice with overexpression of Wnt5a in the epidermis do not exhibit a psoriasis phenotype. 28 The correlation between epigenetic modifications and psoriasis was investigated by Verma et al 29

| Wnt5a and keratinocytes
Interplay between the immune system and the epithelium is the pathological trigger of PsO. The activated adaptive and innate im- and IL-23. IL-1α, TNF-α, transforming growth factor-α and interferon-γ stimulated keratinocytes to produce higher levels of Wnt5a, which, in turn, repressed both Notch1 and HES1. 32 Knockdown of Wnt5a suppressed keratinocyte proliferation and induced apoptosis by repressing the Wnt5a/Ca 2+ or Wnt/β-catenin pathways. 25 The calcium-sensing receptor (CaSR) is essential in calcium-induced differentiation of normal human epidermal keratinocytes (NHEKs) because it increases the level of free intracellular calcium, which up-regulates the expression of Wnt5a. Subsequently, autocrine Wnt5a promotes the differentiation of NHEKs. 33 In contrast, Wnt5a treatment can suppress HaCaT keratinocyte proliferation and differentiation, although the expression of IL-8, IL-17A and interferon-γ was up-regulated. 34 It is difficult to interpret these somewhat contradictory results, but they indicate a complex and as yet unclear

| Interaction with inflammatory cytokines
Immune cell infiltration is one of the main characteristics of psoriatic lesions. As a potent signalling molecule, Wnt5a is strongly implicated in a number of inflammatory diseases, including PsO, rheumatoid arthritis and sepsis. 19 24 However, stimulation of fibroblast-like synoviocytes with TNF-α and IL-17A led to increased expression of Wnt5a. 39 In vitro costimulation of mouse fibroblasts with purified IL-17A and Wnt5a resulted in transforming growth factor-β1 secretion and collagen transcription. 40 These results indicate interplay between Wnt5a signalling and inflammatory responses, which may be dependent on Wnt5a-mediated interaction with different leucocytes and keratinocytes.
Accordingly, emerging evidence supports the regulatory roles of Wnt signalling pathways in leucocyte function. In contrast, DCs isolated from the colon of Wnt5a-and receptor tyrosine kinase-like orphan receptor 2 (Ror2)-deficient mice impair the differentiation of naïve CD4 + T cells into interferon-γ-producing CD4 + Th1 cells. Furthermore, the Wnt5a-Ror2 signalling axis augments the priming effect of DCs on interferon-γ production, which subsequently enhances lipopolysaccharide (LPS)-induced IL-12 expression. 45 The dual role of Wnt5a in DCs as a pro-inflammatory and tolerogenic molecule indicates a complicated mechanism by which Wnt5a modulates DC differentiation and function. Wnt5a may modulate DC responses to limit inflammation, and its regulation of the immune response is a primordial mechanism for achieving immune homoeostasis.

| Macrophages and neutrophils
Macrophage recruitment is another characteristic of inflammation, including in PsO. An in vitro study focusing on Wnt5a interaction with macrophages in castration-resistant prostate cancer (CRPC) indicated that Wnt5a may be a crucial regulator that induces CRPC in the bone niche by recruiting and regulating macrophages. 46 Another in vitro study confirmed that Wnt5a induces macrophage chemotaxis and activation. 47 Recombinant Wnt5a-induced cytokine secretion by macrophages from C57BL/6 mice was dependent on TLR4 and was repressed by polymyxin B. 48 Moreover, Wnt5a is upregulated in macrophages stimulated with endotoxin (LPS), which induces the expression of IL-1β, IL-6, IL-8 and macrophage inflammatory protein-1β. 22 In fact, macrophage-derived Wnt5a is an important regulator of macrophage immune function, pro-inflammatory cytokine release, angiogenesis and lymphangiogenesis. 49 Human neutrophils express a number of Wnt5a receptors, including FZD2, 5 and 8. Wnt5a stimulation of human neutrophils leads to chemotactic migration and the secretion of CXCL8 and CCL2.
Neutrophil chemotaxis induced by supernatant collected from LPSstimulated macrophages was markedly inhibited by an antagonist of Wnt5a, which indicates that Wnt5a may contribute to neutrophil recruitment, thereby regulating the inflammation response. 50  silencing Wnt5a significantly reduced the enhanced permeability and improved barrier function in IL-4-treated HCAEC monolayers. 56 In this context, Wnt5a may not only participate in angiogenesis by stimulating endothelial cell proliferation, but may also enhance the permeability of vascular endothelial cells, which is supposed to contribute to leucocyte effusion and infiltration in psoriatic lesions.

| Wnt5a and angiogenesis
Taking these observations together, we present a model in which Wnt5a activation is involved in keratinocyte proliferation and secretion of inflammatory cytokines, which further regulate Wnt5a expression and promote keratinocyte proliferation and activation through Wnt5a-mediated signalling pathways (Figure 1). This crosstalk forms a signalling loop that promotes the persistence of PsO inflammation and disease progression.

| WNT5A AND PSA
PsA targets the spine, peripheral joints and the entheses. 55 The aetiology of PsA is unclear, but is thought to be an interplay of genetic, immunological and environmental factors that promote pathological bone remodelling and joint damage. Sixty-seven per cent of PsA patients exhibit erosive bone disease, 56 in which increased osteoclast activity causes destructive bone loss in both a localized and a systemic manner. [57][58][59] The simultaneous presence of bone erosions and bony spurs in PsA joints indicates that PsA leads to activated bone remodelling with both enhanced bone resorption and bone formation. Abnormal bone remodelling therefore plays a crucial role in PsA. 60 In contrast to PsO, in which Wnt5a is overexpressed, there has been no report of Wnt5a expression in PsA tissues, although These data support the hypothesis that Wnt5a plays a dual role, modulating bone remodelling as well as interacting with the immune system involved in psoriasis, and may thereby participate in the development of PsA. The Wnt5a-Ror2 signalling pathway regulates the activity of chondrocytes, osteoblasts and osteoclasts and is overexpressed in arthritis tissues. We therefore hypothesize that Wnt5a-Ror2-mediated interaction between the above-mentioned cells triggers both subchondral bone remodelling and cartilage catabolism ( Figure 2).

| CON CLUS IONS
Based on the findings presented in this review, we propose that Wnt5a-activated signalling pathways and other potentially interlinked factors mediate interactions among keratinocytes, immune cells and inflammatory factors, and that Wnt5a plays an important role in the development of PsO and PsA. However, the degree to which these responses in keratinocytes and leucocytes require Wnt5a remains uncertain. More research, particularly in vivo studies using exogenous Wnt5a inhibitors or conditional Wnt5a knockout in keratinocytes or other interacting cells, is needed to clarify the precise role and mechanism of the Wnt5a-mediated immune response and inflammation in PsO and PsA. This will reveal whether Wnt5a is a 'common checkpoint' for PsO and PsA and, if so, would confirm Wnt5a as a potential target for the treatment of both PsO and PsA.

ACK N OWLED G EM ENTS
The study was supported by the National Natural Science Foundation of China (NSFC 81773327; 81874029), the Project of Nature Science Foundation of Hebei Province (H2013209255), Funding for Young Talent of Hebei Province, and the Project of Nature Science Foundation of Shaanxi Province (2019JM-303). We thank Jeremy Allen, PhD, from Liwen Bianji, Edanz Group China (www.liwen bianji. cn/ac), for editing the English text of a draft of this manuscript.

CO N FLI C T O F I NTE R E S T
The authors confirm that there are no conflicts of interest.

AUTH O R CO NTR I B UTI O N S
FMT wrote much of the first draft, MTM provided suggestions and assistance in the writing, and ZXL guided the project and manuscript to its final form. All authors have read and approved the final manuscript.

DATA ACCE SS I B I LIT Y
The datasets in the current study are available from the corresponding author on reasonable request.