Non-EDG family LPA receptors

Although description of the EDG LPA receptors clarified a large amount of the extracellular actions of LPA, there are several responses, including platelet aggregation and certain aspects of vascular development that could not be fully explained by the three EDG family LPA GPCRs (278, 279). In 2003, Noguchi and colleagues, using ligand screening by Ca²⁺-mobilization assay, identified a previously known GPCR, P2Y₉ of the purinergic cluster, as an LPA receptor (280). This observation gave new momentum to the lysophospholipid field, and till 2009 two other GPCRs were found to be specific to LPA (281-284). These receptors (LPA4, LPA5 and LPA6 in order of confirmation) all belong to the P2Y purinergic family, however none of them responds to nucleotide ligands. A recent study in 2017 deciphered the crystal structure and ligand

binding properties of LPA6 (181). During their research, Taniguchi and colleagues found that LPA6 possesses a gap between TM4 and TM5, which forms vertical cleft open towards the plasma membrane. Besides, the presence of several hydrophobic amino acids in the cleft indicates its role as a lipid-binding site. They speculate that LPA6, in contrast with LPA1, can be accessible for its ligands from the extracellular environment as well as laterally from the plasma membrane. Considering the fact, that the ligand-binding pocket of LPA6, formed by TM3, TM4 and TM5 is highly conserved in the P2Y family, binding properties of LPA6 may provide insight into these functions of the other two non-EDG LPA receptors as well (181).

LPA₄ (formerly P2Y₉ and GPR23) was the first LPA receptor identified, that does not belong to the EDG family (280). LPAR4 encodes a 370 amino acid GPCR and is located on chromosome X (178). LPA₄ shows high abundance in human ovary, lower abundance in thymus, pancreas, brain, heart, small intestine, testis, prostate, colon, spleen, and platelets (66, 285). Among murine tissues, LPA₄ is present in heart, ovary, skin, thymus, and bone marrow (66). During development, LPA₄ is found in brain, maxillary process, branchial arches, limb buds, liver, and somites (204).

LPA4 KO mice do not exhibit any obvious phenotypical alteration (286). However, approximately 30% of the embryos do not survive gestation, which can be attributed to hemorrhage and abnormal, dilated blood vessels with abrupt VSM and perycyte recruitment (279). Besides, LPA4 is also assumed to have a role in lymphatic vessel development, as null mice show dilated lymphatic vessels and lymph sacs (279).

Furthermore, LPA4 KO mice display increased trabecular bone volume, number, and thickness, which are in contrast with what has been seen in LPA1 KO, suggesting counteracting roles for LPA1 and LPA4 in bone formation (287).

LPA4 couples with Gα12/13, Gαq/11, Gαi/o, and uniquely among LPA receptors Gαs as well (288). Through these G proteins, LPA4 can trigger Rho/ROCK activation, intracellular cyclic adenosine monophosphate (cAMP) accumulation, and ERK and PI3K activation (288-290). LPA₄ facilitates cell adhesion via N-cadherin and, in contrast with other LPA GPCRs, e.g. LPA1, inhibits cell migration (286, 289).

LPA₅ (formerly GPR92) was first identified as an LPA receptor in 2006 (281, 282).

LPAR5 encodes a 372 amino acid GPCR, which is highly expressed in human spleen, and mast cells while in a lesser extent in heart, small intestine, placenta, colon, liver,

and platelets (281, 285, 291). In mice LPA5 mRNA shoes high abundance in small intestine, whilst moderately high in lung, heart, stomach, colon, spleen, thymus, skin, liver, platelets, mast cells, gastrointestinal lymphocytes, and dorsal root ganglia (281, 282). LPA5 expression is high in the early embryonic forebrain, rostral midbrain, and hindbrain; however later on (from embryonic days 9.5-12.5) the expressional pattern becomes more diffuse throughout the whole brain (204).

LPA5 KO mice appear lean; however, they seem to be protected against neuropathic pain, caused by partial sciatic nerve ligation (PSNL) (292).

LPA₅ couples with Gα_12/13, and Gα_q/11, through which elicits neurite retraction, stress fiber formation, receptor internalization and intracellular Ca²⁺ mobilization respectively (282). LPA₅ has been reported to increase intracellular cAMP levels in a presumably Gα_s-independent manner (281, 282). It is of note, that LPA₅ shows a marked preference for alkyl-LPA analogues (25, 26). Besides LPA₅ can interact with NHERF2, through which it enhances the recruitment of the Na⁺/H⁺ exchanger 3 to the microvilli of the colon, and facilitates Na⁺-dependent water resorption (293).

LPA₆ (P2Y₅) is the most recently identified LPA receptor described by Pasternack et colleagues in 2008 (283). Expression of LPA6 was reported in human intestinal mucosa cells, scalp hair follicles and skin (283, 294). Currently LPA6 is the only non-EDG LPA receptor with an available crystal structure (181), which was briefly covered in the introduction of this section.

LPA6 KO mice have been recently reported. Although these mice exhibited normal blood pressure and heart rate, they showed decreased vascular responses to adrenergic stimuli by phenylephrine or noradrenalin. Furthermore, LPA6 KO mice showed abruption in postnatal retinal vessel formation, indicating that LPA6 signaling is essential for the development of the normal vasculature (295).

LPA6 signaling is still obscure, however evidences suggest, that LPA6 couples with Gα12/13 and Gαi/o and can activate Rho, ROCK, PLC, Ras, and PI3K on the other side inhibits AC (284, 294, 296). LPA₆ has been shown to have marked preference for sn2 regioisomers of LPA (294).

At the time of its characterization, LPA₆ was reported to be a genetic risk factor for an autosomal recessive form for hypotrichosis simplex and woolly hair (283, 297).

Besides, mutations of the lipase member H, the human orthologue of PA-PLA₁ enzyme,

that produces sn-2 LPA also proved to cause the same condition (298, 299). These results hint a possible future role for LPA6 signaling in the therapy of human hair loss.

Homozygous inactivation of LPA6 has been reported in bladder cancer (300).