The roles of Autotaxin/Lysophosphatidic acid in immune regulation and Asthma

Abstract

Lysophosphatidic acid (LPA) species are present in almost all organ systems and play diverse roles through its receptors. Asthma is an airway disease characterized by chronic allergic inflammation where various innate and adaptive immune cells participate in establishing Th2 immune response. Here, we will review the contribution of LPA and its receptors to the functions of immune cells that play a key role in establishing allergic airway inflammation and aggravation of allergic asthma.

1. Overview of autotaxin/LPA pathway

Lysophosphatidic acid (LPA), a membrane derived from glycerophospholipid, is present in most biological fluids like plasma, urine, bronchoalveolar fluid (BALF), saliva, cerebrospinal fluid, and tissue extracts [1–4]. Depending on their difference in length and saturation degree, LPA species can be subdivided into saturated (C16:0 and C18:0), monounsaturated (C18:1), and polyunsaturated (C18:2, C20:4, C22:5, and C22:6) forms [4–6]. As a smallest water-soluble bioactive lipid, LPA elicits a wide variety of biological effects through their binding to at least six canonical G protein-coupled LPA receptors (LPA_10–6) and several non-canonical receptors such as the receptors for advanced glycation end products (RAGE) [7], transient receptor potential cation channel V1 (TRPV1)[8], and peroxisome proliferator-activated receptor γ (PPARγ) nuclear receptor [9]. LPA influences diverse cellular and tissue activities including proliferation, survival, chemotaxis, and tissue development [1]. Diversity of LPA biological activities seems to come from various cell-type specific expressions of LPA receptors, which may preferentially bind to specific LPA species [10, 11]. Local LPA concentration is determined by the balance between the activities of LPA producing and degrading enzymes, which are enriched in a certain pathophysiological conditions such as asthma, resulting in the selective increase or decrease of specific LPA species [1, 4].

LPA species are produced extracellularly as well as intracellularly through multiple mechanisms involved by different enzymes and substrates [1, 3, 4]. As intermediate products in the biosynthesis of triacylglycerol and phopholipids, LPA species are de novo synthesized from glycerol-3-phosphate and acyl-CoA by glycerophosphate acyltransferase activity in the mitochondria and endoplasmic reticulum [12]. LPA can be also generated at the cell membrane as a byproduct of N-acylphosphatidylethanolamine (NAPE) degradation by Ca²⁺⁻dependent phosphohydrolase [13] during the biosynthesis of N-acylethanolamines, the endocannabinoid related bioactive amines [14]. However, the biologic significance of de novo LPA biosynthesis has not been well established. In in vivo biologic conditions, two major pathways are responsible for generating most of the LPA species. The first route is through utilizing membrane-derived phosphatidic acid (PA) as a substrate. Phospholipase activity of phospholipase A1 (PLA1) or A2 (PLA2) on cell membrane converts PA into LPA through hydrolyzing fatty acid of the PA. The second pathway is one where LPA is generated from lysophosphatidylcholine (LPC), which is an intermediate product of membrane phospholipids produced by PLA activity. Converting from LPC to LPA requires lysophospholipase D (LysoPLD) activity, which cleaves choline group from LPC [1, 3]. LysoPLD in this pathway was found to be identical to Autotaxin (ATX), previously identified as an autocrine factor for cancer cell motility [15–17]. Local LPA concentration is also regulated by several classes of degradation enzymes, including lipid phosphate phosphatases (LPPs), LPA acyltransferase, and phospholipases [18–20]. Because LPPs’ expression and their enzymatic activities vary in organ systems [21], tissue-specific LPA concentrations are also determined by its degradation rate.

ATX, a secretory enzyme responsible for most of the extracellular LPA production, has a broad tissue expression and has been found in most of the normal and diseased biological fluids [22]. High expression of ATX has been demonstrated in adipose tissue [23], bronchial epithelium of the lung [2, 24], choroid plexus epithelium of the brain [25], and high endothelial venules (HEVs) of lymphoid tissue [26]. Based on its cDNA homology with ectonucleotide pyrophosphatase-phosphodiesterase (ENPP) family, ATX is classified as an ENPP2 protein [27]. Although ATX has the in vitro enzyme activities for hydrolyzing nucleotides and sphingosylphosphorylcholine to produce sphingosine 1-phosphate (S1P) [28, 29], in vivo enzymatic studies using ATX-deficient mice and the selective chemical inhibitors indicate that the major function of ATX is related with LysoPLD, but not with nucleotide metabolism or S1P production [28, 30]. Of interest, several studies showing unique ATX functions in airway epithelial cell activation and cancer cell migration [31, 32] also showed the non-enzymatic function of ATX through its binding with integrin. ATX has been shown to play a role in brain development though an unknown mechanism that is independent from its catalytic activity [33]. ATX gene (ENPP2) expression is induced by pro-inflammatory mediators, including TLR ligands, type I interferons, TNF, IL1β, and IL-6 in many cells [34–38], suggesting an important role of ATX/LPA pathway in amplifying inflammatory signals. ATX gene expression is negatively regulated by its enzymatic products, LPA and S1P, in a feedback mechanism in the cancer cell lines [35].

A critical role of ATX/LPA pathway in organ development is well-implicated in embryonic neural and vascular systems by the studies using the knock-out mice of ATX (Enpp2 −/−) and LPA receptors (reviewed in[39]). Unlike global deletion of ATX gene which is embryonically lethal, a Tamoxifen-induced conditional deletion of ATX gene in adult mouse (Rosa26Cre-^ERT2/Enpp2^fl/fl) did not affect mouse survival and major systemic physiology, though it resulted in an 80 % reduction of plasma LPA [40]. Although LPA contribution to local milieu at steady state remains unknown, this finding suggests minimal systemic requirement of ATX/LPA function. It will be interesting to see how LPA controls the fine-tuning of local micro-environmental milieu in order to maintain tissue homeostasis at steady state. However, in the diseased status, such as wound repair [41], chronic viral infections [42], autoimmune diseases [34, 43], allergic inflammation [2, 44], obesity [45], and certain types of malignancy [46], the local activity of ATX and LPA concentrations are significantly altered, implying the pathological roles of this pathway. In this review, we discuss the role of ATX/LPA pathway in the pathogenesis of asthma and regulating the immune functions involved in allergic Th2 immune response.

2. ATX/LPA as immunological modifiers

In the normal process of wound repair and tissue remodeling, ATX/LPA mediates platelet aggregation, cytokine production, and cellular migration and proliferation of immune cells, fibroblasts, endothelial cells, and keratinocytes [41]. However, in the unresolved chronic pathological conditions like rheumatoid arthritis, asthma, lung fibrosis, liver cirrhosis, chronic colitis, and the malignancies related with inflammation, aberrantly activated ATX/LPA signals aggravate chronic inflammation through producing cytokines and recruiting inflammatory cells [47]. The expression of canonical (LPA_1–6) and non-canonical LPA receptors (PPARγ and RAGE) varies depending on the types of immune cells [44]. The studies targeting the ATX/LPA pathway using pharmacologic selective agonists and antagonists as well as the genetically-engineered mice have elucidated the functions of each LPA receptors in immune-mediated chronic diseases [1].

2.1. ATX/LPA in regulating immune cell migration.

To perform effective immune surveillance, naïve or central memory T lymphocytes in circulating blood constantly enter lymph nodes via high endothelial venules (HEVs) to encounter activating, tolerogenic or homeostatic antigens presented by dendritic cells (DCs) with the help of supporting fibroblastic reticular cells (FRC) network [48, 49]. ATX is constitutively expressed and released by the endothelial cells of HEVs and the FRC in lymph nodes, and facilitates transendothelial migration and intranodal motility of the lymphocytes [26, 50, 51]. The endothelial cell-secreted ATX binds to the cell surface receptors of lymphocytes, such as β−1 integrin [26] and Mn²⁺ activatable receptors [52], and produces LPA species locally using the cell membrane-derived substrates. LPA receptors mediating T lymphocyte migration is not yet identified [26], but LPA₄ and LPA₆ of endothelial cells have been proposed to be involved [53]. LPA₂ signal pathway was also demonstrated to mediate interstitial T lymphocyte motility through the lymph node FRC network [51].

HEVs are normally located in lymph nodes at steady state, but detected in non-lymphoid organs in the conditions of chronic inflammation and solid tumors [49]. Not only do naïve and central memory T lymphocytes migrate through HEVs, but other immune cells like B lymphocytes, pre-DCs, natural killer (NK) cells and plasmacytoid DCs (pDCs) also migrate through HEVS, though with notably less frequency [49]. Furthermore, many in vitro and in vivo studies have reported that ATX/LPA pathway induces direct chemotactic migration of various immune cells through the activation of the LPA receptors [54]: naïve CD4⁺ T cell through LPA₂ [55], immature DCs through LPA₃[56, 57], NK cells through LPA_1–3[58], and monocytes through LPA₁ and LPA₃[59]. Together, these data suggest that the LPA-mediated immune cells migration occurs not only in normal lymph nodes, but also in inflammatory and malignant microenvironments where the highly-expressed ATX plays a role in keeping normal and pathological immune homeostasis.

Although plasma LPA level is high enough (μM ranges) to transduce cellular signals, the functions of extracellular plasma LPAs for circulating immune cells are not yet elucidated. It has been suggested that the activities of plasma LPA species are blocked by binding to plasma proteins or sequestered by relocation into microparticles [54]. Besides LPA production, ATX binds with integrin through its SMB domain at T cell membrane [60], which may be required for efficient cell migration.

2.2. Regulation of various immune cell functions through LPA receptors.

2.2.1. Lymphocytes

In the in vitro experiments [61–63], LPA has induced different responses of human CD4⁺ T lymphocytes, depending on LPA receptor expression which is modified by T cell activation status. In naïve CD4⁺ T cells usually expressing high LPA₂, LPA increases chemotaxis but decreases IL-2 production [61–63]. However, in activated T cells which express low LPA₂ but high LPA₁, LPA signals result in the opposite direction – an increase in IL-2 production with the attenuated chemotactic responses. These data suggest that LPA functions differently depending on the expression of LPA receptors during the course of T cells activation [61–63]. Furthermore, LPA induces the production of Th2 cytokines by T lymphocytes, such as IL-13 in the synergy with other T cell activation signals [64] providing evidences for the association between ATX/LPA pathway and Th2 immune diseases such as asthma. On the contrary, the recent studies demonstrated that LPA is a suppressive signal to CD8⁺ cytotoxic T lymphocytes. The LPA₅ expressed by CD8⁺ T lymphocytes suppresses T cell receptor (TCR) mediated activation in in vivo and in vitro experiments [65, 66]. This finding could provide a clue for understanding the impaired tumor cytotoxicity seen in the ATX/LPA- enriched tumor microenvironments. The roles of LPA/ATX pathway in B lymphocytes remain unclear because of the conflicting reports. One group reported that LPA inhibits B cell receptor (BCR) signaling and antibody production through the binding with LPA₅ [67] but others demonstrated that ATX expressed by intestinal B lymphocytes promotes immune responses, resulting in severe colitis [68].

2.2.2. Dendritic cells

Dendritic cells (DCs) are the most potent antigen presenting cells and function as a bridge between innate and adaptive immune systems. DCs express various LPA receptors to mediate chemotaxis, cytokine productions, and cell surface receptor expression [56, 57, 69, 70]. In human monocyte derived-DCs (moDCs), the mRNAs for LPA₁-₃ were detected [57, 69], although only LPA₂ protein expression was confirmed [70]. On the other hand, mRNAs for LPA_1–5 were detected in mouse DCs [56]. It has been known that LPA induces chemotaxis of the immature DCs, but not mature DCs in both human and mouse [57]. Of interest, only unsaturated LPA species (C18:1 and C20:4), not saturated LPAs (C16:0 and C18:0), induce DC chemotaxis through mainly LPA₃[56]. In moDCs, ATX/LPA signals differentially modulate the production of cytokines such as IL-12, IL-6, IL-8, TNFα and IL-10 and affect the subsequent T cell polarization [57, 69, 70]. Plasma LPA inhibits the cell surface expression of CD1, a lipid antigen-presenting molecule of DCs, in a PPAR nuclear receptor-dependent way, which results in the reduced DCs’ capacity to activate T cells [71]. In addition, the signals through LPA₂ expressed on DCs play as negative regulators in the adoptive DC transfer model where the transfer of donor LPA₂ deficient DCs aggravates allergic lung inflammation [72]. These findings suggest that ATX/LPA pathway acts in the ways of either stimulating or inhibiting DC functions depending on the immunologic contexts and expressing receptors.

The limitation of the current understanding of ATX/LPA pathway in DCs is that most of the studies have been carried out with in vitro cultured DCs, such as moDCs and bone marrow derived DCs (bmDCs). It is important to have in vivo DC study models to investigate ATX/LPA pathway because of the cellular complexity and plasticity of DC subtypes including conventional DC1 (cDC1), conventional DC2 (cDC2), and plasmacytoid DC (pDC), which can be better assessed in an in vivo system. Of interest, we found that ATX is exclusively expressed by cDC2, but not by other DC subtypes (unpublished data). Because cDC2 plays a critical role in sensitizing allergen and initiating Th2 immune responses [73, 74], it will be interesting to study the LPA/ATX dependent cDC2 roles in allergic lung inflammation. Future studies addressing the ATX/LPA role in the various DC subtypes in the context of immune modulation would be essential for dissecting complex inflammatory diseases and cancers where DCs play a critical role in inducing immune potentiation or tolerance.

2.2.3. Monocytes/Macrophages

Tissue macrophages existing in the forms of tissue-resident or monocyte-derived recruited macrophages play an important role in local homeostasis, innate immune response, normal tissue development, and the repair of damaged tissue. Tissue resident macrophages are colonized in various tissues during embryonic development and maintain their population by self-renewal throughout adult life [75]. Bone marrow-derived blood monocytes can be recruited to local tissues and differentiated into tissue macrophage to replenish tissue resident macrophage pool at the steady state or function as potent pro-inflammatory immune cells during active inflammation and later as anti-inflammatory cells during the resolution [76].

LPA induces monocyte migration [77–80] and stimulates the conversion from monocytes to macrophages in mouse and human [81]. Intracellular PLA2-LPA pathway plays an important role at the leading edge of monocyte migration in response to the potent chemoattractant MCP1 [77, 78]. Indirectly, LPA also modulates monocytes migration through the endothelial cell production of chemokines MCP1 and IL-8, which is mediated by LPA₁ and LPA₃ [79]. During the resolution phase of inflammation, monocyte migration in response to a pro-resolving chemoattractant, protein annexin A1 (ANXA1), is dependent on the binding of LPA to LPA₂ [80]. LPA facilitates the differentiation of human blood CD14⁺ and mouse CD11b⁺ monocytes to macrophages through the activation of PPARγ, a non-canonical LPA receptor [81].

A critical role of ATX/LPA pathway in modulating macrophage functions was implicated in chronic inflammatory diseases, such as atherosclerosis [82], rheumatoid arthritis, [83] and asthma [2]. In in vitro cultured macrophages, LPA directly stimulates the production of pro-inflammatory mediators including IL-1β and reactive oxygen species (ROS) [84]. LPA induces the transformation of recruited macrophages into foam cells, which are an early feature of atherosclerosis development [82]. The proposed mechanism for LPA-mediated foam cell transformation is in part due to an enhanced oxidized phospholipid uptake [85] through LPA₁ and LPA₃. Once foam cells are formed, LPA inhibits the migration of foam cells, which contributes foam cell accumulation in atherosclerotic plaque [86], suggesting a critical role of ATX/LPA pathway in the development of atherosclerosis. The role of ATX/LPA has been well studied in the formation of osteoclasts which are the specialized monocyte/macrophage linage cells in bone. LPA enhances the differentiation, survival, and bone resorption activity of osteoclasts [87–89] indicating its pathological role in excessive bone loss condition such as chronic joint inflammation, malignant bony metastasis, and osteoporosis.

The role of ATX/LPA in embryonic fetal-originated resident macrophages has not been well studied other than in microglial cells, which are yolk-sac originated resident macrophages in the central nervous system. LPA controls various biological functions of microglial cells including migration, metabolism, proliferation, and growth factor expression [90, 91]. It also induces pro-inflammatory phenotype of microglia cells through LPA₁ and LPA₅ [92–94]. Because the growing evidences support the idea that embryonic fetal originated tissue-resident macrophages are functionally different from macrophages originated from hematopoietic stem cells [95], it would be interesting to see how different the role of ATX/LPA is in the ontologically diverse tissue-resident macrophages.

2.2.4. Granulocytes and Mast cells

ATX/LPA affects granulocyte functions. LPA activates neutrophil through triggering calcium mobilization [96], induces neutrophil migration in response to chemoattractants through the binding with LPA₁ in the a mouse model of vasculitis [97, 98], and increases neutrophil degranulation [99]. LPA significantly increased the chemotaxis of neutrophils isolated from patients with pneumonia [100]. Eosinophilic responses to LPA are similar to those of neutrophils. LPA induced chemotaxis, ROS production, and calcium mobilization in human eosinophils [101]. Inhalation with LPA directly enhanced the infiltration of not only neutrophils, but also eosinophils in BALF in an in vivo animal experiment [102].

Mast cells in mucosal and epithelial tissues regulate vascular homeostasis at a steady state and play an important pathophysiological roles in allergy, asthma, and anaphylaxis [103]. LPA induces mast cell differentiation [104], the release of the intracellular granules such as histamine and tryptase, and the production of MIP-1β, IL-8, and MCP-1 [105–107].

3. The role of ATX/LPA in asthma

In the normal healthy state, a constitutive ATX expression is found in airway epithelial cells and alveolar macrophages, and the secreted ATX and its byproducts, LPA species, are also detected in BAL fluids [2, 24, 108] However, the normal phenotypes with no pathologic lung defect seen in multiple ATX transgenic mice, including whole body ATX deficient (Enpp2+/− , Rosa26Cre-^ERT2/Enpp2^fl/fl [1, 40], airway epithelial cell-specific ATX deleted (CC10Enpp2−/−), and ATX overexpressed transgenic mice (TgCC10hEnpp2+/+) [109] suggests that ATX/LPA pathway seems to be minimally required for overall lung physiology at the steady state.

We and others [2, 44] reported that allergen challenge in patients with mild asthma results in a remarkable increase in BAL polyunsaturated LPA species along with elevated ATX protein level. In in vivo and in vitro conditions, ATX/LPA exerts multiple effects on airway and lung inflammation, suggesting its potential role in the pathobiology of asthma [2, 4]. As we discussed above, LPS/ATX pathway modulates the functions of immune cells involved in asthma pathogenesis. Because many of the studies were not performed in allergic lung inflammation, the ATX/LPA effects on immune cells should be carefully interpreted in asthma. However, the collective data still supports the pro-asthmatic roles of ATX/LPA pathway. For example, LPA enhances the chemotaxis of various immune cells involved in asthma pathogenesis, including DCs [56, 57], T lymphocytes [55], eosinophils [101], and monocytes [77–80]. In addition, ATX/LPA is important for expressing IL-13 in Th2 cells [64], inducing mast cell maturation and granulation [104], and augmenting smooth muscle cell growth and contractility [110–112].

Furthermore, the pro-asthmatic role of ATX/LPA has been demonstrated in the in vivo allergic lung inflammation studies utilizing the selective agonists/antagonists for ATX and LPA receptors and the genetically modified animals. Overexpression of ATX in the transgenic mice resulted in an accentuation of allergic lung inflammation with increased eosinophils and elevated Th2 cytokines (IL-4 and IL-5), along with the greater level of polyunsaturated LPA species (LPA22:5 and 22:6) in BAL fluids. On the other hand, blocking ATX/LPA pathway in the transgenic mouse models (LPA₂ knock-out and ATX deficient mice) or using a selective pharmacological ATX inhibitor resulted in a marked attenuation of asthmatic lung inflammation [2].

On the contrary, the deletion of LPA₂ showed negative regulatory function on allergic inflammation in the ovalbumin-induced asthma model, where LPA₂ knock-out mice had greater allergen-driven lung inflammation [72]. In addition, the selective pharmacologic agonist for LPA₂ alleviated house dust mite (HDM)-induced allergic lung inflammation [113]. Of interest, the experiments with bone marrow-derived DCs isolated from the LPA₂ knock-out mice, the LPA₂ depleted DCs were hyperactive to activate T cells in vitro and became more pathologic in the in vivo model of adaptive transfer, suggesting that the inhibitory signals of LPA₂ in allergic asthma may originate from its suppressive role on DC functions. The conflicting results of LPA₂ signals in asthma may stem from the differences in the mouse models of asthma, the severity of allergic inflammation, or the recruited cell types. Since LPA₂ expression is shared by many immune cells involved in asthma pathogenesis, a cell-type specific approach for LPA₂ receptor is needed to figure out the exact role of LPA₂ in the immune cell activation and allergic lung inflammation.

In response to allergens, airway epithelial cells (AECs) secrete cytokines (TSLP, IL-1α, GMCSF and IL-33), chemokines (CCL2 and CCL20), and danger signals (ATP and uric acid) to facilitate immune cell recruitment and induce allergic airway inflammation [114, 115]. AECs also modulate the migration of cDC2 to regional lymph node in order to trigger allergen reactive-Th2 immune responses through the production of Colony Stimulating Factor 1(CSF1) [73]. AECs also express LPA receptors and respond to LPA species at the physiologic concentration. The direct response of AECs to LPA in an in vivo mouse model was to secrete innate Th2 cytokines such as TSLP and CCL20 [116]. In contrast, the in vitro experiments performed in human bronchial epithelial cells were contradictory. In these experiments, LPA treatment induced the epithelial release of anti-Th2 inflammatory mediators such as decoy receptors for IL-13 and IL-33 (IL-13Rα2 and soluble ST2 respectively) and PGE2 [117–120]. However, these experiments have the limitation of using in vitro culture of single type cells stimulated with single LPA species, LPA 18:1, which may not reflect an actual asthmatic microenvironment where molecular interaction among various immune cells, cytokines, and chemokines exist with the characteristic elevation of polyunsaturated LPA22:5 and 22:6 [2, 44].

Like other groups have reported [44], we also found that the polyunsaturated LPA species (22:5 and 22:6) are predominantly increased in BAL fluids of the patients enrolled in the protocol for subsegmental bronchoprovocation with allergen (SBP-AG), whereas the saturated or monounsaturated LPAs (16:0, 18:0, and 18:1) remained unchanged. Since there was no increase in plasma LPA levels, it is unlikely that increase in polyunsaturated LPA species was due to a plasma leak [2]. This characteristic increase of polyunsaturated LPA22:5 was also found in a mouse model of asthma [2, 121]. The mechanism for preferential biosynthesis of polyunsaturated LPA species in asthma is still unclear, although the increased substrate availability of polyunsaturated LPC species by the activation of PLA1 has been proposed [4]. Though it still remains unknown, the biologic functions of polyunsaturated LPA species may be different from those of the saturated forms. It has been suggested that polyunsaturated LPA species have differential specificity and/or affinity to the specific LPA receptors as exemplified by the studies demonstrating the unique effects of unsaturated LPA on migration of immature mouse DCs through the binding with LPA₃ [56] and the activation of PPARγ in the early stage of atherosclerosis [11].

PPARγ has been known as an important immune modulator of lung through the development and differentiation of alveolar macrophages. PPARγ also serves as a non-canonical LPA receptor [122] and a key regulator in the progression of asthma. The studies revealed that PPARγ regulates DCs’ migration to regional lymph nodes and modulates Th2 response in allergic inflammation [123, 124]. Furthermore, the recent evidence implies the PPARγ involvement in resident memory T cells (T_RM) formation in chronic asthma [125]. However, it remains to be seen whether LPA is a ligand for these PPARγ activities in the process of immune modulation of allergic lung inflammation.

A pivotal role of ATX/LPA pathway in asthma pathogenesis has also been implicated by the recent findings of another non-canonical LPA receptor, RAGE [7]. RAGE, a multi-ligand receptor, is expressed abundantly in the lung and promotes allergic airway disease, although it is unclear whether LPA is a ligand for the receptor in this process [126]. RAGE and its well-known ligand, HMGB1, play a role in allergic airway sensitization and inflammatory response through modulating DC functions [127]. Furthermore, the genetic or pharmacological inhibition of RAGE blocked the Th2 cytokines responses in mice and human bronchial epithelial cells [128]. It would be interesting to study whether LPA is involved as a ligand for RAGE-mediated allergic lung inflammation and there is any synergy between its ligands.

Bronchoconstriction by the reflex arc, from vagal afferent to parasympathetic efferent pathway, is a part of neural regulation for airway hyper-responsiveness in asthma. The activation of petrosal neurons innervating carotid body glomus cells by the afferent arm elicits bronchoconstriction [129]. LPA may mediate this reflex bronchoconstriction through its non-canonical receptor, transient receptor potential cation channel V1 (TRPV1), on the axons of petrosal neurons. Direct LPA activation of the carotid body increases efferent vagal activity resulting in acute bronchoconstriction. Systemic inhibition on TRPV1 or LPA receptor prevents reflex bronchoconstriction induced by bradykinin [130]. Cough-variant asthma is a unique form of dysfunctional airway disease manifesting as chronic cough. Although our understanding for its etiology is limited, it has been regarded as a neuropathic disease where TRPV1 channels may be involved (PMID 27483288). Of interest, therapeutic targeting TRPV1 did reduce the severity of chronic cough (PMID:24139494)

Along with other publications that suggest an important role of LPA in normal neuronal development and signaling neuropathic pain [131, 132], this study indicates that LPA plays a role in synaptic regulation involved in neuronal regulation of asthmatic bronchoconstriction.

4. Future direction

Asthma is a complex immunologic airway disease where various types of immune cells are intertwined to establish a chronic Th2 allergic immune response. Intercellular communication between immune cells in the milieu of airways and lungs is an integral part of asthma pathogenesis. Locally produced LPA plays a key role in cellular communication through its binding to six different receptors that immune cells express variably during the course of the disease process. The local concentration of LPA is also regulated by its production and degradation in Th2 cytokine enriched asthmatic milieu which induces the enzymes involved in LPA biosynthesis, such as ATX. Especially, a certain LPA species are preferentially produced in the asthmatic milieu and may have a preferential binding to a specific receptor(s). To solve this puzzle, it is necessary to have in vivo animal models to address a cell-specific analysis of LPA production and LPA receptors. Immune cell type-specific knock-out and knock-in models of LPA receptors and LPA biosynthesis enzymes are imperative to decode this entanglement.

Figure 1.

Summary of the production and potential biologic actions of polyunsaturated LPA in asthma