Non-cell autonomous and non-catalytic activities of ATX in the developing brain

The Role of Autotaxin and LPA Signaling in Embryonic Development, Pathophysiology and Cancer

Autotaxin (ATX) or Ectonucleotide Pyrophosphatase/Phosphodiesterase 2 (ENPP2) is a secreted enzyme with lysophospholipase D activity, with its primary function being the extracellular hydrolysis of lysophosphatidylcholine (LPC) to lysophosphatidic acid (LPA), a bioactive lipid [...].

Tissue-Wide Effects Override Cell-Intrinsic Gene Function in Radial Neuron Migration

The mammalian neocortex is composed of diverse neuronal and glial cell classes that broadly arrange in six distinct laminae. Cortical layers emerge during development and defects in the developmental programs that orchestrate cortical lamination are associated with neurodevelopmental diseases. The developmental principle of cortical layer formation depends on concerted radial projection neuron migration, from their birthplace to their final target position. Radial migration occurs in defined sequential steps, regulated by a large array of signaling pathways. However, based on genetic loss-of-function experiments, most studies have thus far focused on the role of cell-autonomous gene function. Yet, cortical neuron migration in situ is a complex process and migrating neurons traverse along diverse cellular compartments and environments. The role of tissue-wide properties and genetic state in radial neuron migration is however not clear. Here we utilized mosaic analysis with double markers (MADM) technology to either sparsely or globally delete gene function, followed by quantitative single-cell phenotyping. The MADM-based gene ablation paradigms in combination with computational modeling demonstrated that global tissue-wide effects predominate cell-autonomous gene function albeit in a gene-specific manner. Our results thus suggest that the genetic landscape in a tissue critically affects the overall migration phenotype of individual cortical projection neurons. In a broader context, our findings imply that global tissue-wide effects represent an essential component of the underlying etiology associated with focal malformations of cortical development in particular, and neurological diseases in general.

Histological, functional and transcriptomic alterations in the juvenile hippocampus in a mouse model of thyroid hormone resistance

BACKGROUND

Proper thyroid hormone signaling via the TRα1 nuclear receptor is required for normal neurodevelopmental processes. The specific downstream mechanisms mediated by TRα1 that impact brain development remain to be investigated.

METHODS

In this study, the structure, function and transcriptome of hippocampal tissue in a mouse model expressing the first RTHα mutation discovered in a patient, THRA E403X, were analyzed. RNAscope was used to visualize the spatial and temporal expression of Thra1 mRNA in the hippocampus of WT mice, which is corresponding to THRA1 mRNA in humans. The morphological structure was analyzed by Nissl staining, and the synaptic transmission was analyzed on the basis of long-term potentiation. The Morris water maze test and the zero maze test were used to evaluate the behavior. RNA-seq and quantitative real-time PCR were used to analyze the differentially expressed genes (DEGs) of the hippocampal tissues in the mouse model expressing the Thra E403X mutation.

RESULTS

The juvenile mutant Thra E403X mice presented with delayed neuronal migration, disordered neuronal distribution, and decreased synaptic plasticity. A total of 754 DEGs, including 361 upregulated genes and 393 downregulated genes, were identified by RNA-seq. DEG-enriched Gene Ontology (GO) and KEGG pathways were associated with PI3K-Akt signaling, ECM-receptor interaction, neuroactive ligand-receptor interaction, and a range of immune-related pathways. 25 DEGs were validated by qPCR.

CONCLUSIONS

The ThraE403X mutation results in histological and functional abnormalities, as well as transcriptomic alterations in the juvenile mouse hippocampus. This study of the ThraE403X mutant offers new insights into the biological cause of RTHα-associated neurological diseases.

Role of lysophosphatidic acid and its receptors in health and disease: novel therapeutic strategies

Lysophosphatidic acid (LPA) is an abundant bioactive phospholipid, with multiple functions both in development and in pathological conditions. Here, we review the literature about the differential signaling of LPA through its specific receptors, which makes this lipid a versatile signaling molecule. This differential signaling is important for understanding how this molecule can have such diverse effects during central nervous system development and angiogenesis; and also, how it can act as a powerful mediator of pathological conditions, such as neuropathic pain, neurodegenerative diseases, and cancer progression. Ultimately, we review the preclinical and clinical uses of Autotaxin, LPA, and its receptors as therapeutic targets, approaching the most recent data of promising molecules modulating both LPA production and signaling. This review aims to summarize the most update knowledge about the mechanisms of LPA production and signaling in order to understand its biological functions in the central nervous system both in health and disease.

NSun2 promotes cell migration through methylating autotaxin mRNA

NSun2 is an RNA methyltransferase introducing 5-methylcytosine into tRNAs, mRNAs, and noncoding RNAs, thereby influencing the levels or function of these RNAs. Autotaxin (ATX) is a secreted glycoprotein and is recognized as a key factor in converting lysophosphatidylcholine into lysophosphatidic acid (LPA). The ATX-LPA axis exerts multiple biological effects in cell survival, migration, proliferation, and differentiation. Here, we show that NSun2 is involved in the regulation of cell migration through methylating ATX mRNA. In the human glioma cell line U87, knockdown of NSun2 decreased ATX protein levels, whereas overexpression of NSun2 elevated ATX protein levels. However, neither overexpression nor knockdown of NSun2 altered ATX mRNA levels. Further studies revealed that NSun2 methylated the 3'-UTR of ATX mRNA at cytosine 2756 in vitro and in vivo Methylation by NSun2 enhanced ATX mRNA translation. In addition, NSun2-mediated 5-methylcytosine methylation promoted the export of ATX mRNA from nucleus to cytoplasm in an ALYREF-dependent manner. Knockdown of NSun2 suppressed the migration of U87 cells, which was rescued by the addition of LPA. In summary, we identify NSun2-mediated methylation of ATX mRNA as a novel mechanism in the regulation of ATX.

Non-Cell-Autonomous Mechanisms in Radial Projection Neuron Migration in the Developing Cerebral Cortex

Concerted radial migration of newly born cortical projection neurons, from their birthplace to their final target lamina, is a key step in the assembly of the cerebral cortex. The cellular and molecular mechanisms regulating the specific sequential steps of radial neuronal migration in vivo are however still unclear, let alone the effects and interactions with the extracellular environment. In any in vivo context, cells will always be exposed to a complex extracellular environment consisting of (1) secreted factors acting as potential signaling cues, (2) the extracellular matrix, and (3) other cells providing cell–cell interaction through receptors and/or direct physical stimuli. Most studies so far have described and focused mainly on intrinsic cell-autonomous gene functions in neuronal migration but there is accumulating evidence that non-cell-autonomous-, local-, systemic-, and/or whole tissue-wide effects substantially contribute to the regulation of radial neuronal migration. These non-cell-autonomous effects may differentially affect cortical neuron migration in distinct cellular environments. However, the cellular and molecular natures of such non-cell-autonomous mechanisms are mostly unknown. Furthermore, physical forces due to collective migration and/or community effects (i.e., interactions with surrounding cells) may play important roles in neocortical projection neuron migration. In this concise review, we first outline distinct models of non-cell-autonomous interactions of cortical projection neurons along their radial migration trajectory during development. We then summarize experimental assays and platforms that can be utilized to visualize and potentially probe non-cell-autonomous mechanisms. Lastly, we define key questions to address in the future.

Genetic deletion of Autotaxin from CD11b+ cells decreases the severity of experimental autoimmune encephalomyelitis

Autotaxin (ATX) is a secreted lysophospholipase D catalyzing the extracellular production of lysophosphatidic acid (LPA), a growth factor-like signaling lysophospholipid. ATX and LPA signaling have been incriminated in the pathogenesis of different chronic inflammatory diseases and various types of cancer. In this report, deregulated ATX and LPA levels were detected in the spinal cord and plasma of mice during the development of experimental autoimmune encephalomyelitis (EAE). Among the different sources of ATX expression in the inflamed spinal cord, F4/80⁺ CD11b⁺ cells, mostly activated macrophages and microglia, were found to express ATX, further suggesting an autocrine role for ATX/LPA in their activation, an EAE hallmark. Accordingly, ATX genetic deletion from CD11b⁺ cells attenuated the severity of EAE, thus proposing a pathogenic role for the ATX/LPA axis in neuroinflammatory disorders.

The roles of autotaxin/lysophosphatidic acid in immune regulation and asthma

Lysophosphatidic acid (LPA) species are present in almost all organ systems and play diverse roles through its receptors. Asthma is an airway disease characterized by chronic allergic inflammation where various innate and adaptive immune cells participate in establishing Th2 immune response. Here, we will review the contribution of LPA and its receptors to the functions of immune cells that play a key role in establishing allergic airway inflammation and aggravation of allergic asthma.

Autotaxin and chronic inflammatory diseases

Autotaxin (ATX) is a secreted glycoprotein, widely present in biological fluids including blood. ATX catalyzes the hydrolysis of lysophosphatidylcholine (LPC) to lysophosphatidic acid (LPA), a growth factor-like, signaling phospholipid. LPA exerts pleiotropic effects mediated by its G-protein-coupled receptors that are widely expressed and exhibit overlapping specificities. Although ATX also possesses matricellular properties, the majority of ATX reported functions in adulthood are thought to be mediated through the extracellular production of LPA. ATX-mediated LPA synthesis is likely localized at the cell surface through the possible interaction of ATX with integrins or other molecules, while LPA levels are further controlled by a group of membrane-associated lipid-phosphate phosphatases. ATX expression was shown to be necessary for embryonic development, and ATX deficient embryos exhibit defective vascular homeostasis and aberrant neuronal system development. In adult life, ATX is highly expressed in the adipose tissue and has been implicated in diet-induced obesity and glucose homeostasis with multiple implications in metabolic disorders. Additionally, LPA has been shown to affect multiple cell types, including stromal and immune cells in various ways. Therefore, LPA participates in many processes that are intricately involved in the pathogenesis of different chronic inflammatory diseases such as vascular homeostasis, skeletal and stromal remodeling, lymphocyte trafficking and immune regulation. Accordingly, increased ATX and LPA levels have been detected, locally and/or systemically, in patients with chronic inflammatory diseases, most notably idiopathic pulmonary fibrosis (IPF), chronic liver diseases, and rheumatoid arthritis. Genetic and pharmacological studies in mice have confirmed a pathogenetic role for ATX expression and LPA signaling in chronic inflammatory diseases, and provided the proof of principle for therapeutic interventions, as exemplified by the ongoing clinical trials for IPF.

Pleotropic Roles of Autotaxin in the Nervous System Present Opportunities for the Development of Novel Therapeutics for Neurological Diseases

Autotaxin (ATX) is a soluble extracellular enzyme that is abundant in mammalian plasma and cerebrospinal fluid (CSF). It has two known enzymatic activities, acting as both a phosphodiesterase and a phospholipase. The majority of its biological effects have been associated with its ability to liberate lysophosphatidic acid (LPA) from its substrate, lysophosphatidylcholine (LPC). LPA has diverse pleiotropic effects in the central nervous system (CNS) and other tissues via the activation of a family of six cognate G protein-coupled receptors. These LPA receptors (LPARs) are expressed in some combination in all known cell types in the CNS where they mediate such fundamental cellular processes as proliferation, differentiation, migration, chronic inflammation, and cytoskeletal organization. As a result, dysregulation of LPA content may contribute to many CNS and PNS disorders such as chronic inflammatory or neuropathic pain, glioblastoma multiforme (GBM), hemorrhagic hydrocephalus, schizophrenia, multiple sclerosis, Alzheimer's disease, metabolic syndrome-induced brain damage, traumatic brain injury, hepatic encephalopathy-induced cerebral edema, macular edema, major depressive disorder, stress-induced psychiatric disorder, alcohol-induced brain damage, HIV-induced brain injury, pruritus, and peripheral nerve injury. ATX activity is now known to be the primary biological source of this bioactive signaling lipid, and as such, represents a potentially high-value drug target. There is currently one ATX inhibitor entering phase III clinical trials, with several additional preclinical compounds under investigation. This review discusses the physiological and pathological significance of the ATX-LPA-LPA receptor signaling axis and summarizes the evidence for targeting this pathway for the treatment of CNS diseases.

The LPA-LPA4 axis is required for establishment of bipolar morphology and radial migration of newborn cortical neurons

Newborn neurons in the developing neocortex undergo radial migration, a process that is coupled with their precise passage from multipolar to bipolar shape. The cell-extrinsic signals that govern this transition are, however, poorly understood. Here, we find that lysophosphatidic acid (LPA) signaling contributes to the establishment of a bipolar shape in mouse migratory neurons through LPA receptor 4 (LPA4). LPA4 is robustly expressed in migratory neurons. LPA4-depleted neurons show impaired multipolar-to-bipolar transition and become arrested in their migration. Further, LPA4-mediated LPA signaling promotes formation of the pia-directed process in primary neurons overlaid on neocortical slices. In addition, LPA4 depletion is coupled with altered actin organization as well as with destabilization of the F-actin-binding protein filamin A (FlnA). Finally, overexpression of FlnA rescues the morphology and migration defects of LPA4-depleted neurons. Thus, the LPA-LPA4 axis regulates bipolar morphogenesis and radial migration of newborn cortical neurons via remodeling of the actin cytoskeleton.

Autotaxin in Pathophysiology and Pulmonary Fibrosis

Lysophospholipid signaling is emerging as a druggable regulator of pathophysiological responses, and especially fibrosis, exemplified by the relative ongoing clinical trials in idiopathic pulmonary fibrosis (IPF) patients. In this review, we focus on ectonucleotide pyrophosphatase-phosphodiesterase 2 (ENPP2), or as more widely known Autotaxin (ATX), a secreted lysophospholipase D (lysoPLD) largely responsible for extracellular lysophosphatidic acid (LPA) production. In turn, LPA is a bioactive phospholipid autacoid, forming locally upon increased ATX levels and acting also locally through its receptors, likely guided by ATX's structural conformation and cell surface associations. Increased ATX activity levels have been detected in many inflammatory and fibroproliferative conditions, while genetic and pharmacologic studies have confirmed a pleiotropic participation of ATX/LPA in different processes and disorders. In pulmonary fibrosis, ATX levels rise in the broncheoalveolar fluid (BALF) and stimulate LPA production. LPA engagement of its receptors activate multiple G-protein mediated signal transduction pathways leading to different responses from pulmonary cells including the production of pro-inflammatory signals from stressed epithelial cells, the modulation of endothelial physiology, the activation of TGF signaling and the stimulation of fibroblast accumulation. Genetic or pharmacologic targeting of the ATX/LPA axis attenuated disease development in animal models, thus providing the proof of principle for therapeutic interventions.

2-O-Carba-oleoyl cyclic phosphatidic acid induces glial proliferation through the activation of lysophosphatidic acid receptor

Lysophosphatidic acid (LPA) and cyclic phosphatidic acid (cPA) are one of the lipid mediators regulating cell proliferation and differentiation through the activation of LPA receptors. An LPA receptor-mediated signal is important for the development of the central nervous system, while it has been demonstrated that LPA caused microglial activation and astroglial dysfunction. Previously, we have reported that cPA and carba analog of cPA, 2-O-carba-cPA (2ccPA), protected neural damage caused by transient ischemia. However, little is known about the target cell of cPA/2ccPA in the central nervous systems. Here, we examined the effect of 2ccPA on glial proliferation and differentiation using the primary astrocytes and oligodendrocyte precursor cells (OPCs) cultures. 2ccPA increased the DNA synthesis of astrocytes and OPCs, but it did not reduce the formazan production in the mitochondria. Further, 2ccPA increased the cell number and cell survival against oxidative stress. The inhibition of LPA receptors by ki16425 abolished 2ccPA-induced DNA synthesis. Extracellular signal-regulated kinase (ERK) was activated by 2ccPA, which contributed to the astroglial DNA synthesis. These results suggest that 2ccPA is a beneficial regulator of glial population through the activation of LPA receptor without reduction of mitochondrial activity.

A SLM2 Feedback Pathway Controls Cortical Network Activity and Mouse Behavior

The brain is made up of trillions of synaptic connections that together form neural networks needed for normal brain function and behavior. SLM2 is a member of a conserved family of RNA binding proteins, including Sam68 and SLM1, that control splicing of Neurexin1-3 pre-mRNAs. Whether SLM2 affects neural network activity is unknown. Here, we find that SLM2 levels are maintained by a homeostatic feedback control pathway that predates the divergence of SLM2 and Sam68. SLM2 also controls the splicing of Tomosyn2, LysoPLD/ATX, Dgkb, Kif21a, and Cask, each of which are important for synapse function. Cortical neural network activity dependent on synaptic connections between SLM2-expressing-pyramidal neurons and interneurons is decreased in Slm2-null mice. Additionally, these mice are anxious and have a decreased ability to recognize novel objects. Our data reveal a pathway of SLM2 homeostatic auto-regulation controlling brain network activity and behavior.

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SLM2 splicing targets are spatially controlled within the hippocampus

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RNA-seq reveals SLM2 feedback control and synaptic protein splicing targets

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Loss of SLM2 dampens patterns of hippocampal γ oscillations

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Loss of SLM2 changes mouse behavior that depends on these neural networks

Mosaic Analysis with Double Markers Reveals Distinct Sequential Functions of Lgl1 in Neural Stem Cells

The concerted production of neurons and glia by neural stem cells (NSCs) is essential for neural circuit assembly. In the developing cerebral cortex, radial glia progenitors (RGPs) generate nearly all neocortical neurons and certain glia lineages. RGP proliferation behavior shows a high degree of non-stochasticity, thus a deterministic characteristic of neuron and glia production. However, the cellular and molecular mechanisms controlling RGP behavior and proliferation dynamics in neurogenesis and glia generation remain unknown. By using mosaic analysis with double markers (MADM)-based genetic paradigms enabling the sparse and global knockout with unprecedented single-cell resolution, we identified Lgl1 as a critical regulatory component. We uncover Lgl1-dependent tissue-wide community effects required for embryonic cortical neurogenesis and novel cell-autonomous Lgl1 functions controlling RGP-mediated glia genesis and postnatal NSC behavior. These results suggest that NSC-mediated neuron and glia production is tightly regulated through the concerted interplay of sequential Lgl1-dependent global and cell intrinsic mechanisms.

Serping1/C1 Inhibitor Affects Cortical Development in a Cell Autonomous and Non-cell Autonomous Manner

Current knowledge regarding regulation of radial neuronal migration is mainly focused on intracellular molecules. Our unbiased screen aimed at identification of non-cell autonomous mechanisms involved in this process detected differential expression of Serping1 or C1 inhibitor, which is known to inhibit the initiation of the complement cascade. The complement cascade is composed of three pathways; the classical, lectin, and the alternative pathway; the first two are inhibited by C1 inhibitor, and all three converge at the level of C3. Knockdown or knockout of Serping1 affected neuronal stem cell proliferation and impaired neuronal migration in mice. Knockdown of Serping1 by in utero electroporation resulted in a migration delay of the electroporated cells as well as their neighboring cells demonstrating a non-cell autonomous effect. Cellular polarity was also affected. Most importantly, expression of protein components mimicking cleaved C3 rescued the knockdown of Serping1, indicating complement pathway functionality. Furthermore, we propose that this activity is mediated mainly via the complement peptide C5a receptors. Whereas addition of a selective C3a receptor agonist was minimally effective, the addition of a dual C3aR/C5a receptor agonist significantly rescued Serping1 knockdown-mediated neuronal migration defects. Our findings suggest that modulating Serping1 levels in the developing brain may affect the complement pathway in a complex way. Collectively, our findings demonstrate an unorthodox activity for the complement pathway during brain development.