Characterization of lpa₂ (Edg4) and lpa₁/lpa₂ (Edg2/Edg4) Lysophosphatidic Acid Receptor Knockout Mice: Signaling Deficits without Obvious Phenotypic Abnormality Attributable to lpa₂

Abstract

Lysophosphatidic acid (LPA), a bioactive lipid produced by several cell types including postmitotic neurons and activated platelets, is thought to be involved in various biological processes, including brain development. Three cognate G protein-coupled receptors encoded by lpa₁ /lp _A1/Edg-2/Gpcr26, lpa₂ /lp _A2/Edg-4, and lpa₃ /lp _A3/Edg-7 mediate the cellular effects of LPA. We have previously shown that deletion of lpa₁ in mice results in craniofacial dysmorphism, semilethality due to defective suckling behavior, and generation of a small fraction of pups with frontal hematoma. To further investigate the role of these receptors and LPA signaling in the organism, we deleted lpa₂ in mice. Homozygous knockout (lpa₂ ^(−/−)) mice were born at the expected frequency and displayed no obvious phenotypic abnormalities. Intercrosses allowed generation of lpa₁ ^(−/−) lpa₂ ^(−/−) double knockout mice, which displayed no additional phenotypic abnormalities relative to lpa₁ ^(−/−) mice except for an increased incidence of perinatal frontal hematoma. Histological analyses of lpa₁ ^(−/−) lpa₂ ^(−/−) embryonic cerebral cortices did not reveal obvious differences in the proliferating cell population. However, many LPA-induced responses, including phospholipase C activation, Ca²⁺ mobilization, adenylyl cyclase activation, proliferation, JNK activation, Akt activation, and stress fiber formation, were absent or severely reduced in embryonic fibroblasts derived from lpa₁ ^(−/−) lpa₂ ^(−/−) mice. Except for adenylyl cyclase activation [which was nearly abolished in lpa₁ ^(−/−) fibroblasts], these responses were only partially affected in lpa₁ ^(−/−) and lpa₂ ^(−/−) fibroblasts. Thus, although LPA₂ is not essential for normal mouse development, it does act redundantly with LPA₁ to mediate most LPA responses in fibroblasts.

Isao Ishii and James Contos contributed equally to this work.

We thank Forrest Liu at the UCSD Core facility for ES cell work and blastocyst injection, Carol Akita for the maintenance of mice, and Matthew McCreight for help in counting cells in the proliferation assay.

This work was supported by the National Institute of Mental Health (grant no. K02MH01723 to J.C.), a research grant from Allelix Biopharmaceuticals (to J.C.), a grant from the Mitsubishi Pharma Research Foundation (to I.I.), and an unrestricted gift from Merck Research Laboratories (to J.C.).