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Background: Autotaxin is essential for vascular development in mice, but the underlying mechanism remains unknown. Results: Autotaxin had similar vascular functions in zebrafish. Furthermore, suppression of lysophosphatidic acid receptors (LPA 1 and LPA 4 ) led to similar vascular defects. Conclusion: Autotaxin exerts its vascular functions by activating LPA 1 and/or LPA 4 in zebrafish. Significance: LPA is a critical factor for regulating angiogenesis in vertebrates.

The lipid mediator lysophosphatidic acid (LPA) signals via six distinct G protein-coupled receptors to mediate both unique and overlapping biological effects, including cell migration, proliferation and survival. LPA is produced extracellularly by autotaxin (ATX), a secreted lysophospholipase D, from lysophosphatidylcholine. ATX-LPA receptor signaling is essential for normal development and implicated in various (patho)physiological processes, but underlying mechanisms remain incompletely understood. Through gene targeting approaches in zebrafish and mice, we show here that loss of ATX-LPA1 signaling leads to disorganization of chondrocytes, causing severe defects in cartilage formation. Mechanistically, ATX-LPA1 signaling acts by promoting S-phase entry and cell proliferation of chondrocytes both in vitro and in vivo, at least in part through β1-integrin translocation leading to fibronectin assembly and further extracellular matrix deposition; this in turn promotes chondrocyte-matrix adhesion and cell proliferation. Thus, the ATX-LPA1 axis is a key regulator of cartilage formation.

Lysophosphatidic acid (LPA) is a blood-derived bioactive lipid with numerous biological activities exerted mainly through six defined G protein-coupled receptors (LPA 1 -LPA 6 ). LPA was first identified as a vasoactive compound because it induced transient hypertension when injected intravenously in rodents. Here, we examined the molecular mechanism underlying the LPA-induced hypertensive response. The LPA-induced hypertensive response was significantly attenuated by pretreatment with a Rho kinase inhibitor, which blocks Gα 12/13 signaling. Consistent with this, the response was weakened in KO mice of LPA 4 , a Gα 12/13 -coupling LPA receptor. KO mice of another Gα 12/13 -coupling LPA receptor, LPA 6 , also showed an attenuated LPA-induced hypertensive response. However, LPA 6 KO mice also displayed attenuated pressor responses to an adrenergic agent and abnormal blood vessel formation. Using several LPA analogs with varied affinity for each LPA receptor, we found a good correlation between the hypertensive and LPA 4 agonistic activities. Incubated mouse plasma, which contained abundant LPA, also induced a hypertensive response. Interestingly the response was completely abolished when the plasma was incubated in the presence of an ATX inhibitor. Together, these results indicate that circulating LPA produced by ATX contributes to the elevation of blood pressure through multiple LPA receptors, mainly LPA 4 .

Lysophosphatidic acid (LPA) is emerging as an angiogenic factor, because knockdown of the enzyme that produces it (autotaxin, also known as ENPP2) and its receptors cause severe developmental vascular defects in both mice and fish. In addition, overexpression of autotaxin in mice causes similar vascular defects, indicating that the extracellular amount of LPA must be tightly regulated. Here, we focused on an LPA-degrading enzyme, lipid phosphate phosphatase 3 (LPP3, also known as PPAP2B), and showed that LPP3 was localized in specific cell–cell contact sites of endothelial cells and suppresses LPA signalling through the LPA6 receptor (also known as LPAR6). In HEK293 cells, overexpression of LPP3 dramatically suppressed activation of LPA6. In human umbilical vein endothelial cells (HUVECs), LPA induced actin stress fibre formation through LPA6, which was substantially upregulated by LPP3 knockdown. LPP3 was localized to cell–cell contact sites and was missing in non-contact sites to which LPA-induced actin stress fibre formation mediated by LPA6 was restricted. Interestingly, the expression of LPP3 in HUVECs was dramatically increased after forskolin treatment in a process involving Notch signalling. These results indicate that LPP3 regulates and localizes LPA signalling in endothelial cells, thereby stabilizing vessels through Notch signalling for proper vasculature.