▪ Abstract Lysophospholipids (LPs), such as lysophosphatidic acid and sphingosine 1-phosphate, are membrane-derived bioactive lipid mediators. LPs can affect fundamental cellular functions, which include proliferation, differentiation, survival, migration, adhesion, invasion, and morphogenesis. These functions influence many biological processes that include neurogenesis, angiogenesis, wound healing, immunity, and carcinogenesis. In recent years, identification of multiple cognate G protein-coupled receptors has provided a mechanistic framework for understanding how LPs play such diverse roles. Generation of LP receptor-null animals has allowed rigorous examination of receptor-mediated physiological functions in vivo and has identified new functions for LP receptor signaling. Efforts to develop LP receptor subtype-specific agonists/antagonists are in progress and raise expectations for a growing collection of chemical tools and potential therapeutic compounds. The rapidly expanding literature on the LP receptors is herein reviewed.

There are two isoforms of sphingosine kinase (SphK) that catalyze the formation of sphingosine 1-phosphate, a potent sphingolipid mediator. Whereas SphK1 stimulates growth and survival, here we show that SphK2 enhanced apoptosis in diverse cell types and also suppressed cellular proliferation. Apoptosis was preceded by cytochrome c release and activation of caspase-3. SphK2-induced apoptosis was independent of activation of sphingosine 1-phosphate receptors. Sequence analysis revealed that SphK2 contains a 9-amino acid motif similar to that present in BH3-only proteins, a pro-apoptotic subgroup of the Bcl-2 family. As with other BH3-only proteins, co-immunoprecipitation demonstrated that SphK2 interacted with Bcl-x_L. Moreover, site-directed mutation of Leu-219, the conserved leucine residue present in all BH3 domains, markedly suppressed SphK2-induced apoptosis. Hence, the apoptotic effect of SphK2 might be because of its putative BH3 domain.

Lysophosphatidic acid (LPA) is a simple bioactive phospholipid with diverse physiological actions on many cell types. LPA induces proliferative and/or morphological effects and has been proposed to be involved in biologically important processes including neurogenesis, myelination, angiogenesis, wound healing, and cancer progression. LPA acts through specific G protein-coupled, seven-transmembrane domain receptors. To date, three mammalian cognate receptor genes,lp_A1/vzg-1/Edg2,lp_A2/Edg4, andlp_A3/Edg7, have been identified that encode high-affinity LPA receptors. Here, we review current knowledge on these LPA receptors, including their isolation, function, expression pattern, gene structure, chromosomal location, and possible physiological or pathological roles.

Lysophospholipids (LPs), including lysophosphatidic acid and sphingosine 1-phosphate, produce many cellular effects. However, the prolonged absence of any cloned and identified LP receptor has left open the question of how these lipids actually bring about these effects. The cloning and functional identification of the first LP receptor, lp_A1/vzg-1, has led rapidly to the identification and classification of multiple orphan receptors/expression sequence tags known by many names (e.g. edg, mrec1.3, gpcr26, H218, AGR16, nrg-1) as members of a common cognate G protein-coupled receptor family. We review features of the LP receptor family, including molecular characteristics, genomics, signaling properties, and gene expression. A major question for which only partial answers are available concerns the biological significance of receptor-mediated LP signaling. Recent studies that demonstrate the role of receptor-mediated LP signaling in the nervous system, cardiovascular system, and other organ systems indicate the importance of this signaling in development, function, and pathophysiology and portend an exciting time ahead for this growing field.

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.

Enhancement of cerebral blood flow by hypoxia is critical for brain function, but signaling systems underlying its regulation have been unclear. We report a pathway mediating hypoxia-induced cerebral vasodilation in studies monitoring vascular disposition in cerebellar slices and in intact mouse brains using two-photon intravital laser scanning microscopy. In this cascade, hypoxia elicits cerebral vasodilation via the coordinate actions of H₂S formed by cystathionine β-synthase (CBS) and CO generated by heme oxygenase (HO)-2. Hypoxia diminishes CO generation by HO-2, an oxygen sensor. The constitutive CO physiologically inhibits CBS, and hypoxia leads to increased levels of H₂S that mediate the vasodilation of precapillary arterioles. Mice with targeted deletion of HO-2 or CBS display impaired vascular responses to hypoxia. Thus, in intact adult brain cerebral cortex of HO-2–null mice, imaging mass spectrometry reveals an impaired ability to maintain ATP levels on hypoxia.