Ji Woong Choi

LPA Receptors: Subtypes and Biological Actions

Lysophosphatidic acid (LPA) is a small, ubiquitous phospholipid that acts as an extracellular signaling molecule by binding to and activating at least five known G protein-coupled receptors (GPCRs): LPA(1)-LPA(5). They are encoded by distinct genes named LPAR1-LPAR5 in humans and Lpar1-Lpar5 in mice. The biological roles of LPA are diverse and include developmental, physiological, and pathophysiological effects. This diversity is mediated by broad and overlapping expression patterns and multiple downstream signaling pathways activated by cognate LPA receptors. Studies using cloned receptors and genetic knockout mice have been instrumental in uncovering the significance of this signaling system, notably involving basic cellular processes as well as multiple organ systems such as the nervous system. This has further provided valuable proof-of-concept data to support LPA receptors and LPA metabolic enzymes as targets for the treatment of medically important diseases that include neuropsychiatric disorders, neuropathic pain, infertility, cardiovascular disease, inflammation, fibrosis, and cancer.

FTY720 (fingolimod) efficacy in an animal model of multiple sclerosis requires astrocyte sphingosine 1-phosphate receptor 1 (S1P1) modulation

Sphingosine 1-phosphate (S1P), a lysophospholipid, has gained relevance to multiple sclerosis through the discovery of FTY720 (fingolimod), recently approved as an oral treatment for relapsing forms of multiple sclerosis. Its mechanism of action is thought to be immunological through an active phosphorylated metabolite, FTY720-P, that resembles S1P and alters lymphocyte trafficking through receptor subtype S1P1. However, previously reported expression and in vitro studies of S1P receptors suggested that direct CNS effects of FTY720 might theoretically occur through receptor modulation on neurons and glia. To identify CNS cells functionally contributing to FTY720 activity, genetic approaches were combined with cellular and molecular analyses. These studies relied on the functional assessment, based on clinical score, of conditional null mouse mutants lacking S1P1 in CNS cell lineages and challenged by experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. All conditional null mutants displayed WT lymphocyte trafficking that responded normally to FTY720. In marked contrast, EAE was attenuated and FTY720 efficacy was lost in CNS mutants lacking S1P1 on GFAP-expressing astrocytes but not on neurons. In situ hybridization studies confirmed that astrocyte loss of S1P1 was the key alteration in functionally affected mutants. Reductions in EAE clinical scores were paralleled by reductions in demyelination, axonal loss, and astrogliosis. Receptor rescue and pharmacological experiments supported the loss of S1P1 on astrocytes through functional antagonism by FTY720-P as a primary FTY720 mechanism. These data identify nonimmunological CNS mechanisms of FTY720 efficacy and implicate S1P signaling pathways within the CNS as targets for multiple sclerosis therapies.

Biological roles of lysophospholipid receptors revealed by genetic null mice : an update

Two lysophospholipids (LPs), lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), are known to affect various cellular events. Their actions are mediated by binding to at least ten bona fide high-affinity G protein-coupled receptors referred to as LPA1-5 and S1P1-5. These LPs are expressed throughout the body and are involved in a range of biological activities including normal development, as well as functioning in most organ systems. A growing number of biological functions have been uncovered in vivo using single- or multiple-null mice for each LP receptor. This review will focus on findings from in vivo as well as in vitro studies using genetic null mice for the LP receptors, LPA1,2,3 and S1P1,2,3,5, and for the LP producing enzymes, autotaxin and sphingosine kinase 1/2.

Lysophospholipids and their receptors in the central nervous system

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), two of the best-studied lysophospholipids, are known to influence diverse biological events, including organismal development as well as function and pathogenesis within multiple organ systems. These functional roles are due to a family of at least 11 G protein-coupled receptors (GPCRs), named LPA(1-6) and S1P(1-5), which are widely distributed throughout the body and that activate multiple effector pathways initiated by a range of heterotrimeric G proteins including G(i/o), G(12/13), G(q) and G(s), with actual activation dependent on receptor subtypes. In the central nervous system (CNS), a major locus for these signaling pathways, LPA and S1P have been shown to influence myriad responses in neurons and glial cell types through their cognate receptors. These receptor-mediated activities can contribute to disease pathogenesis and have therapeutic relevance to human CNS disorders as demonstrated for multiple sclerosis (MS) and possibly others that include congenital hydrocephalus, ischemic stroke, neurotrauma, neuropsychiatric disorders, developmental disorders, seizures, hearing loss, and Sandhoff disease, based upon the experimental literature. In particular, FTY720 (fingolimod, Gilenya, Novartis Pharma, AG) that becomes an analog of S1P upon phosphorylation, was approved by the FDA in 2010 as a first oral treatment for MS, validating this class of receptors as medicinal targets. This review will provide an overview and update on the biological functions of LPA and S1P signaling in the CNS, with a focus on results from studies using genetic null mutants for LPA and S1P receptors. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Lysophosphatidic Acid Signaling May Initiate Fetal Hydrocephalus

Blockade of lysophosphatidic acid signaling provides a new strategy for treating fetal hydrocephalus. Is the Cause of Hydrocephalus Blood Simple? Hydrocephalus or “water on the brain” is caused by accumulation of cerebrospinal fluid (CSF) in the cerebral ventricles during fetal development and is one of the most common neurological disorders of newborns, occurring in 1 in 1500 live births. One apparent cause of hydrocephalus is bleeding into the cerebral ventricles or brain tissue of the fetus, suggesting that factors or components in blood may trigger development of this severe neurological disorder. The most common treatment is surgical insertion of an intraventricular shunt that drains excess CSF from the cerebral ventricles, but this approach only relieves intracranial pressure and does not solve the root cause of the disorder. Yung et al. set out to investigate which factors in blood trigger hydrocephalus using an in vivo fetal mouse model that they developed. They identify a blood-borne lipid called lysophosphatidic acid (LPA) as a potential cause of hydrocephalus and show that when LPA is prevented from binding to its receptor LPA1 by a receptor antagonist, that hydrocephalus does not develop in fetal mice. The authors injected serum, plasma, or red blood cells into the cerebral ventricles of the brains of fetal mice in utero at 13.5 days of gestation. The animals were then assessed prenatally 1 or 5 days later or postnatally at several different time points. Injection of serum or plasma but not red blood cells induced CSF accumulation and hydrocephalus, with animals displaying enlarged heads, dilated ventricles, and thinning of the cortex. The investigators reasoned that LPA, a blood-borne lipid that is known to be important for the developing cerebral cortex, might be involved in the development of hydrocephalus. When they injected a solution containing LPA into the cerebral ventricles of fetal mice in utero, the mice did indeed develop severe hydrocephalus. The authors wondered how an increase in LPA might affect cortical development and lead to hydrocephalus. They show that injection of LPA resulted in altered adhesion and mislocalization of neural progenitor cells along the surface of the ventricles and that this mislocalization depended on expression of the LPA1 receptor by these cells. When the researchers repeated their experiments with fetal mice lacking the LPA1 receptor, they were unable to induce hydrocephalus. The key finding came with their demonstration that an LPA1 receptor antagonist blocked the ability of LPA to induce hydrocephalus in the fetal mice. These results suggest that LPA and its LPA1 receptor may be new therapeutic targets for developing drugs that could be used in conjunction with surgery to treat this debilitating neurological disease. Fetal hydrocephalus (FH), characterized by the accumulation of cerebrospinal fluid, an enlarged head, and neurological dysfunction, is one of the most common neurological disorders of newborns. Although the etiology of FH remains unclear, it is associated with intracranial hemorrhage. Here, we report that lysophosphatidic acid (LPA), a blood-borne lipid that activates signaling through heterotrimeric guanosine 5′-triphosphate–binding protein (G protein)–coupled receptors, provides a molecular explanation for FH associated with hemorrhage. A mouse model of intracranial hemorrhage in which the brains of mouse embryos were exposed to blood or LPA resulted in development of FH. FH development was dependent on the expression of the LPA1 receptor by neural progenitor cells. Administration of an LPA1 receptor antagonist blocked development of FH. These findings implicate the LPA signaling pathway in the etiology of FH and suggest new potential targets for developing new treatments for FH.

Lysophosphatidic Acid Receptor-dependent Secondary Effects via Astrocytes Promote Neuronal Differentiation *□

Lysophosphatidic acid (LPA) is a simple phospholipid derived from cell membranes that has extracellular signaling properties mediated by at least five G protein-coupled receptors referred to as LPA1–LPA5. In the nervous system, receptor-mediated LPA signaling has been demonstrated to influence a range of cellular processes; however, an unaddressed aspect of LPA signaling is its potential to produce specific secondary effects, whereby LPA receptor-expressing cells exposed to, or “primed,” by LPA may then act on other cells via distinct, yet LPA-initiated, mechanisms. In the present study, we examined cerebral cortical astrocytes as possible indirect mediators of the effects of LPA on developing cortical neurons. Cultured astrocytes express at least four LPA receptor subtypes, known as LPA1–LPA4. Cerebral cortical astrocytes primed by LPA exposure were found to increase neuronal differentiation of cortical progenitor cells. Treatment of unprimed astrocyte-progenitor cocultures with conditioned medium derived from LPA-primed astrocytes yielded similar results, suggesting the involvement of an astrocyte-derived soluble factor induced by LPA. At least two LPA receptor subtypes are involved in LPA priming, since the priming effect was lost in astrocytes derived from LPA receptor double-null mice (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{LPA}_{1}^{(-{/}-)}{/}\mathrm{LPA}_{2}^{(-{/}-)}\) \end{document}). Moreover, the loss of LPA-dependent differentiation in receptor double-null astrocytes could be rescued by retrovirally transduced expression of a single deleted receptor. These data demonstrate that receptor-mediated LPA signaling in astrocytes can induce LPA-dependent, indirect effects on neuronal differentiation.

Neurological S1P signaling as an emerging mechanism of action of oral FTY720 (Fingolimod) in multiple sclerosis

FTY720 (fingolimod, Novartis) is a promising investigational drug for relapsing forms of multiple sclerosis (MS), an autoimmune and neurodegenerative disorder of the central nervous system. It is currently under FDA review in the United States, and could represent the first approved oral treatment for MS. Extensive, ongoing clinical trials in Phase II/III have supported both the efficacy and safety of FTY720. FTY720 itself is not bioactive, but when phosphorylated (FTY720-P) by sphingosine kinase 2, it becomes active through modulation of 4 of the 5 known G protein-coupled sphingosine 1-phosphate (S1P) receptors. The mechanism of action (MOA) is thought to be immunological, where FTY720 alters lymphocyte trafficking via S1P1. However, MOA for FTY720 in MS may also involve a direct, neurological action within the central nervous system in view of documented S1P receptor-mediated signaling influences in the brain, and this review considers observations that support an emerging neurological MOA.

The complex morphology of reactive astrocytes controlled by fibroblast growth factor signaling.

Astrocytes are the most abundant cell‐type of the human brain and play a variety of roles in brain homeostasis and synaptic maturation, under normal conditions. However, astrocytes undergo dramatic pathological changes in response to brain injury, such as reactive gliosis and glial scar formation. Although abnormal hypertrophy and massive proliferation of astrocytes are obvious, the molecular identity and cues that dictate the structural changes in reactive astrocytes remain unclear. This study proposes that fibroblast growth factor (FGF) signaling is responsible for making astrocyte morphology more complex and hypertrophic in response to an inflammatory stimulus such as lipopolysaccharide. Primary astrocytes isolated from perinatal brains developed more branches in the presence of FGF8 or lesser branches in the presence of FGF2. Introduction of the constitutively active form of the FGF receptor 3 (caFGFR3) into the brain increases the structural complexity, with greater glial fibrillary acidic protein level in astrocytes, while overexpression of a dominant‐negative form of FGFR3 (dnFGFR3) reduces it. Treatment of FGF8 facilitated the wound‐healing process of primary astrocytes in vitro by changing their morphology, indicating that the FGF signal may control the responsiveness of astrocytes in injury conditions. Finally, the blockade of FGF signaling by introducing dnFGFR3 at the site of reactive gliosis reduces astrocyte branch formation and minimizes hypertrophic responses during reactive gliosis. Taken together, these results indicate that FGF8–FGFR3 signaling controls structural changes in astrocytes during reactive gliosis, under pathogenic conditions. GLIA 2014;62:1328–1344

Lysophosphatidic acid (LPA) and its receptor, LPA1, influence embryonic schwann cell migration, myelination, and cell‐to‐axon segregation

Schwann cell (SC) migration is an important step preceding myelination and remyelination in the peripheral nervous system, and can be promoted by peptide factors like neuregulins. Here we present evidence that a lipid factor, lysophosphatidic acid (LPA), influences both SC migration and peripheral myelination through its cognate G protein‐coupled receptor (GPCR) known as LPA1. Ultrastructural analyses of peripheral nerves in mouse null‐mutants for LPA1 showed delayed SC‐to‐axon segregation, polyaxonal myelination by single SCs, and thinner myelin sheaths. In primary cultures, LPA promoted SC migration through LPA1, while analysis of conditioned media from purified dorsal root ganglia neurons using HPLC/MS supported the production of LPA by these neurons. The heterotrimeric G‐alpha protein, Gαi, and the small GTPase, Rac1, were identified as important downstream signaling components of LPA1. These results identify receptor mediated LPA signaling between neurons and SCs that promote SC migration and contribute to the normal development of peripheral nerves through effects on SC‐axon segregation and myelination. GLIA 2013;61:2009–2022

Early immature neuronal death initiates cerebral ischemia-induced neurogenesis in the dentate gyrus

Throughout adulthood, neurons are continuously replaced by new cells in the dentate gyrus (DG) of the hippocampus, and this neurogenesis is increased by various neuronal injuries including ischemic stroke and seizure. While several mechanisms of this injury-induced neurogenesis have been elucidated, the initiation factor remains unclear. Here, we investigated which signal(s) trigger(s) ischemia-induced cell proliferation and neurogenesis in the hippocampal DG region. We found that early apoptotic cell death of the immature neurons occurred in the DG region following transient forebrain ischemia/reperfusion in mice. Moreover, early immature neuronal death in the DG initiated transient forebrain ischemia/reperfusion-induced neurogenesis through glycogen synthase kinase-3β/β-catenin signaling, which was mediated by microglia-derived insulin-like growth factor-1 (IGF-1). Additionally, we observed that the blockade of immature neuronal cell death, early microglial activation, or IGF-1 signaling attenuated ischemia-induced neurogenesis. These results suggest that early immature neuronal cell death initiates ischemia-induced neurogenesis through microglial IGF-1 in mice.