Genetics and cell biology of lysophosphatidic acid receptor-mediated signaling during cortical neurogenesis

Maternal choline supplementation in neurodevelopmental disorders: mechanistic insights from animal models and future directions

OBJECTIVES

To synthesize evidence from animal models of neurodevelopmental disorders (NDD) using maternal choline supplementation, to characterize current knowledge on the mechanisms of choline's protective effects against NDD, and to identify gaps in knowledge for future study.

METHODS

A literature review was conducted in PubMed to identify studies using prenatal choline supplementation interventions in rodent models of neurodevelopmental disorders. 24 studies were identified, and behavioral and biological findings were extracted from each. Studies examining both genetic and environmental risk factors were included.

RESULTS

Maternal choline supplementation during gestation is protective against both genetic and environmental NDD risk factors. Maternal choline supplementation improves both cognitive and affective outcomes throughout the lifespan in NDD models. Prenatal choline improved these outcomes through its participation in processes like neurogenesis, epigenetic regulation, and anti-inflammatory signaling.

DISCUSSION

Maternal choline supplementation improves behavioral and neurobiological outcomes in animal models of NDD, paralleling findings in humans. Animal models provide a unique opportunity to study the mechanisms by which gestational choline improves neurodevelopmental outcomes. This is especially important since nearly 90% of pregnant people in the United States are deficient in choline intake. However, much is still unknown about the mechanisms through which choline and its derivatives act. Further research into this topic, especially mechanistic studies in animal models, is critical to modernize maternal choline intake guidelines and to develop interventions to increase maternal choline intake in vulnerable populations.

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.

Altered cleavage plane orientation with increased genomic aneuploidy produced by receptor-mediated lysophosphatidic acid (LPA) signaling in mouse cerebral cortical neural progenitor cells

The brain is composed of cells having distinct genomic DNA sequences that arise post-zygotically, known as somatic genomic mosaicism (SGM). One form of SGM is aneuploidy-the gain and/or loss of chromosomes-which is associated with mitotic spindle defects. The mitotic spindle orientation determines cleavage plane positioning and, therefore, neural progenitor cell (NPC) fate during cerebral cortical development. Here we report receptor-mediated signaling by lysophosphatidic acid (LPA) as a novel extracellular signal that influences cleavage plane orientation and produces alterations in SGM by inducing aneuploidy during murine cortical neurogenesis. LPA is a bioactive lipid whose actions are mediated by six G protein-coupled receptors, LPA₁-LPA₆. RNAscope and qPCR assessment of all six LPA receptor genes, and exogenous LPA exposure in LPA receptor (Lpar)-null mice, revealed involvement of Lpar1 and Lpar2 in the orientation of the mitotic spindle. Lpar1 signaling increased non-vertical cleavage in vivo by disrupting cell-cell adhesion, leading to breakdown of the ependymal cell layer. In addition, genomic alterations were significantly increased after LPA exposure, through production of chromosomal aneuploidy in NPCs. These results identify LPA as a receptor-mediated signal that alters both NPC fate and genomes during cortical neurogenesis, thus representing an extracellular signaling mechanism that can produce stable genomic changes in NPCs and their progeny. Normal LPA signaling in early life could therefore influence both the developing and adult brain, whereas its pathological disruption could contribute to a range of neurological and psychiatric diseases, via long-lasting somatic genomic alterations.

The Role of Lysophosphatidic Acid in Adult Stem Cells

Stem cells are undifferentiated multipotent precursor cells that are capable both of perpetuating themselves as stem cells (self-renewal) and of undergoing differentiation into one or more specialized types of cells. And these stem cells have been reported to reside within distinct anatomic locations termed “niches”. The long-term goals of stem cell biology range from an understanding of cell-lineage determination and tissue organization to cellular therapeutics for degenerative diseases. Stem cells maintain tissue function throughout an organism’s lifespan by replacing differentiated cells. To perform this function, stem cells provide a unique combination of multilineage developmental potential and the capacity to undergo self-renewing divisions. The loss of self-renewal capacity in stem cells underlies certain degenerative diseases and the aging process. This self-renewal regulation must balance the regenerative needs of tissues that persist throughout life. Recent evidence suggests lysophosphatidic acid (LPA) signaling pathway plays an important role in the regulation of a variety of stem cells. In this review, we summarize the evidence linking between LPA and stem cell regulation. The LPA-induced signaling pathway regulates the proliferation and survival of stem cells and progenitors, and thus are likely to play a role in the maintenance of stem cell population in the body. This lipid mediator regulatory system can be a novel potential therapeutics for stem cell maintenance.

Lysophosphatidic acid induces neuronal cell death via activation of asparagine endopeptidase in cerebral ischemia-reperfusion injury

Lysophosphatidic acid (LPA), an extracellular signaling molecule, influences diverse biological events, including the pathophysiological process induced after ischemic brain injury. However, the molecular mechanisms mediating the pathological change after ischemic stroke remain elusive. Here we report that asparagine endopeptidase (AEP), a lysosomal cysteine proteinase, is regulated by LPA during stroke. AEP proteolytically cleaves tau and generates tauN368 fragments, triggering neuronal death. Inhibiting the generation of LPA reduces the expression of AEP and tauN368, and alleviates neuronal cell death. Together, this evidence indicates that the LPA-AEP pathway plays a key role in the pathophysiological process induced after ischemic stroke. Inhibition of LPA could be a useful therapeutic for treating neuronal injury after stroke.

Increased gyrification and aberrant adult neurogenesis of the dentate gyrus in adult rats

A remarkable example of maladaptive plasticity is the development of epilepsy after a brain insult or injury to a normal animal or human. A structure that is considered central to the development of this type of epilepsy is the dentate gyrus (DG), because it is normally a relatively inhibited structure and its quiescence is thought to reduce hippocampal seizure activity. This characteristic of the DG is also considered to be important for normal hippocampal-dependent cognitive functions. It has been suggested that the brain insults which cause epilepsy do so because they cause the DG to be more easily activated. One type of brain insult that is commonly used is induction of severe seizures (status epilepticus; SE) by systemic injection of a convulsant drug. Here we describe an alteration in the DG after this type of experimental SE that may contribute to chronic seizures that has not been described before: large folds or gyri that develop in the DG by 1 month after SE. Large gyri appeared to increase network excitability because epileptiform discharges recorded in hippocampal slices after SE were longer in duration when recorded inside gyri relative to locations outside gyri. Large gyri may also increase excitability because immature adult-born neurons accumulated at the base of gyri with time after SE, and previous studies have suggested that abnormalities in adult-born DG neurons promote seizures after SE. In summary, large gyri after SE are a common finding in adult rats, show increased excitability, and are associated with the development of an abnormal spatial distribution of adult-born neurons. Together these alterations may contribute to chronic seizures and associated cognitive comorbidities after SE.

The G protein-coupled receptor GPRC5B contributes to neurogenesis in the developing mouse neocortex

Neural progenitor cells in the developing brain give rise to neurons and glia. Multiple extrinsic signalling molecules and their cognate membrane receptors have been identified to control neural progenitor fate. However, a role for G protein-coupled receptors in cell fate decisions in the brain remains largely putative. Here we show that GPRC5B, which encodes an orphan G protein-coupled receptor, is present in the ventricular surface of cortical progenitors in the mouse developing neocortex and is required for their neuronal differentiation. GPRC5B-depleted progenitors fail to adopt a neuronal fate and ultimately become astrocytes. Furthermore, GPRC5B-mediated signalling is associated with the proper regulation of β-catenin signalling, a pathway crucial for progenitor fate decision. Our study uncovers G protein-coupled receptor signalling in the neuronal fate determination of cortical progenitors.

Autotaxin/ENPP2 regulates oligodendrocyte differentiation in vivo in the developing zebrafish hindbrain

During development, progenitors that are committed to differentiate into oligodendrocytes, the myelinating cells of the central nervous system (CNS), are generated within discrete regions of the neuroepithelium. More specifically, within the developing spinal cord and hindbrain ventrally located progenitor cells that are characterized by the expression of the transcription factor olig2 give temporally rise to first motor neurons and then oligodendrocyte progenitors. The regulation of this temporal neuron-glial switch has been found complex and little is known about the extrinsic factors regulating it. Our studies described here identified a zebrafish ortholog to mammalian atx, which displays evolutionarily conserved expression pattern characteristics. Most interestingly, atx was found to be expressed by cells of the cephalic floor plate during a time period when ventrally-derived oligodendrocyte progenitors arise in the developing hindbrain of the zebrafish. Knock-down of atx expression resulted in a delay and/or inhibition of the timely appearance of oligodendrocyte progenitors and subsequent developmental stages of the oligodendrocyte lineage. This effect of atx knock-down was not accompanied by changes in the number of olig2-positive progenitor cells, the overall morphology of the axonal network or the number of somatic abducens motor neurons. Thus, our studies identified Atx as an extrinsic factor that is likely secreted by cells from the floor plate and that is involved in regulating specifically the progression of olig2-positive progenitor cells into lineage committed oligodendrocyte progenitors.

Ligand-based autotaxin pharmacophore models reflect structure-based docking results

The autotaxin (ATX) enzyme exhibits lysophospholipase D activity responsible for the conversion of lysophosphatidyl choline to lysophosphatidic acid (LPA). ATX and LPA have been linked to the initiation of atherosclerosis, cancer invasiveness, and neuropathic pain. ATX inhibition therefore offers currently unexploited therapeutic potential, and substantial interest in the development of ATX inhibitors is evident in the recent literature. Here we report the performance-based comparison of ligand-based pharmacophores developed on the basis of different combinations of ATX inhibitors in the training sets against an extensive database of compounds tested for ATX inhibitory activity, as well as with docking results of the actives against a recently reported ATX crystal structure. In general, pharmacophore models show better ability to select active ATX inhibitors binding in a common location when the ligand-based superposition shows a good match to the superposition of actives based on docking results. Two pharmacophore models developed on the basis of competitive inhibitors in combination with the single inhibitor crystallized to date in the active site of ATX were able to identify actives at rates over 40%, a substantial improvement over the <10% representation of active site-directed actives in the test set database.

Sphingosine-1-phosphate regulation of mammalian development

Sphingosine-1-phosphate (S1P) was first identified as a lysophospholipid metabolite whose formation is required for the irreversible degradation of sphingolipids. Years later, it was discovered that S1P is a bioactive lipid that provokes varied cell responses by acting through cell-surface receptors to drive cell signaling. More recent findings in model organisms have now established that S1P metabolism and signaling are integrated into many physiological systems. We describe here the surprising breadth of function of S1P in mammalian development and the underlying biologic processes that S1P regulates.

Galpha12/Galpha13 deficiency causes localized overmigration of neurons in the developing cerebral and cerebellar cortices

The heterotrimeric G proteins G(12) and G(13) link G-protein-coupled receptors to the regulation of the actin cytoskeleton and the induction of actomyosin-based cellular contractility. Here we show that conditional ablation of the genes encoding the alpha-subunits of G(12) and G(13) in the nervous system results in neuronal ectopia of the cerebral and cerebellar cortices due to overmigration of cortical plate neurons and cerebellar Purkinje cells, respectively. The organization of the radial glia and the basal lamina was not disturbed, and the Cajal-Retzius cell layer had formed normally in mutant mice. Embryonic cortical neurons lacking G(12)/G(13) were unable to retract their neurites in response to lysophosphatidic acid and sphingosine-1-phosphate, indicating that they had lost the ability to respond to repulsive mediators acting via G-protein-coupled receptors. Our data indicate that G(12)/G(13)-coupled receptors mediate stop signals and are required for the proper positioning of migrating cortical plate neurons and Purkinje cells during development.

Stem cell regulation by lysophospholipids

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) regulate a diverse range of mammalian cell processes, largely through engaging multiple G protein-coupled receptors specific for these lysophospholipids. LPA and S1P have been clearly identified to have widespread physiological and pathophysiological actions, controlling events within the reproductive, gastrointestinal, vascular, nervous and immune systems, and also having a prominent role in cancer. Here we review the recent literature showing the additional emerging role for LPA and S1P in the regulation of stem cells and their progenitors. We discuss the role of these lysophospholipids in regulating the proliferation, survival, differentiation and migration of a range of adult and embryonic stem cells and progenitors, and thus are likely to play a substantial role in the maintenance, generation, mobilisation and homing of stem cell and progenitor populations in the body.

Absence of LPA1 signaling results in defective cortical development

Lysophosphatidic acid (LPA) is a simple phospholipid with extracellular signaling properties mediated by specific G protein-coupled receptors. At least 2 LPA receptors, LPA(1) and LPA(2), are expressed in the developing brain, the former enriched in the neurogenic ventricular zone (VZ), suggesting a normal role in neurogenesis. Despite numerous studies reporting the effects of exogenous LPA using in vitro neural models, the first LPA(1) loss-of-function mutants reported did not show gross cerebral cortical defects in the 50% that survived perinatal demise. Here, we report a role for LPA(1) in cortical neural precursors resulting from analysis of a variant of a previously characterized LPA(1)-null mutant that arose spontaneously during colony expansion. These LPA(1)-null mice, termed maLPA(1), exhibit almost complete perinatal viability and show a reduced VZ, altered neuronal markers, and increased cortical cell death that results in a loss of cortical layer cellularity in adults. These data support LPA(1) function in normal cortical development and suggest that the presence of genetic modifiers of LPA(1) influences cerebral cortical development.

A new method of embryonic culture for assessing global changes in brain organization

While dissociated, reaggregated cells and organotypic slice cultures are useful models for understanding brain development, they only partially mimic the processes and organization that exist in vivo. Towards bridging the gap between in vitro and in vivo paradigms, a method for culturing intact brain tissue was developed using whole cerebral cortical hemispheres in which the anatomical and cellular organization of nervous system tissue is preserved. Single, free-floating telencephalic hemispheres were dissected from embryonic mice and placed into defined culture medium on an orbital shaker. Orbital shaking was necessary for optimal growth, and cortices grown under these conditions closely approximated in vivo parameters of cell division, differentiation, migration and cell death for up to 24 h. In addition to wild-type cultures, the method was compatible with genetically altered tissues. One particular advantage of this method is its ability to reveal global anatomical alterations in the embryonic brain following exposure to soluble growth factors. This method should thus be helpful for assessing a wide range of soluble molecules for their systemic effects on the embryonic brain.

EDG receptors as a potential therapeutic target in retinal ischemia-reperfusion injury

LPA (lysophosphatidic acid) specific endothelial differentiation gene (EDG) receptors have been implicated in various anti-apoptotic pathways. Ischemia of the brain and retina causes neuronal apoptosis, which raises the possibility that EDG receptors participate in anti-apoptotic signaling in ischemic injury. We examined the expression of EDG receptors in a model of retinal ischemia-reperfusion injury and also tested LXR-1035, a novel analogue of LPA, in the rat following global retinal ischemic injury. Rats were subjected to 45 or 60 min of raised intraocular pressure. Animals were sacrificed at 24 h post-ischemia and retinal tissue was stained for EDG receptors. In separate experiments, animals were randomized to receive LXR or saline vehicle by intravitreal injection 24 h prior to ischemia. The degree of retinal damage was assessed morphologically by measuring the thickness of the inner retinal layers as well as functionally by electroretinography (ERG). We found that the normal retina has a baseline expression of the LPA receptors, EDG-2 and EDG-4, which are significantly upregulated in the inner layers in response to ischemia. Animals pretreated with LXR-1035 had dose-dependent, significant reductions in histopathologic damage and significant improvement in functional deficits compared with corresponding vehicle-controls, after 45 and 60 min of ischemia. These results suggest that LPA receptor signaling may play an important role in neuroprotection in retinal ischemia-reperfusion injury.

The role of LPA1 in formation of synapses among cultured hippocampal neurons

We studied the lysophosphatidic acid receptor-1 (LPA1) gene, which we found to be expressed endogenously in cultured hippocampal neurons, and in vivo in young (1-week-old) rat brain slices. Overexpressed green fluorescent protein (GFP)-tagged, membrane-associated LPA1 accumulated in a punctate manner over the entire dendritic tree and caused an increase in dendritic spine density. About half of the dendritic spines in the LPA1-transfected neurons displayed distinct fluorescent puncta, and this subset of spines was also substantially larger than puncta-free, LPA1-transfected or control GFP spines. This phenotype could also be seen in cells transfected with a ligand-binding, defective mutant and is therefore not dependent on interaction with an ambient ligand. While spontaneous miniature excitatory synaptic currents were of the same amplitudes, they decayed slower in LPA1-transfected neurons compared with GFP controls. We propose that LPA1 may play a role in the formation and modulation of the dendritic spine synapse.

Phosphodiesterase-Ialpha/autotaxin (PD-Ialpha/ATX): a multifunctional protein involved in central nervous system development and disease

Phosphodiesterase-Ialpha/autotaxin (PD-Ialpha/ATX) was originally identified as a cell-motility-stimulating factor secreted by a variety of tumor cells. Thus, studies related to its potential functional roles have traditionally focused on tumorigenesis. PD-Ialpha/ATX's catalytic activity, initially defined as nucleotide pyrophosphatase/phosphodiesterase, was soon recognized as being necessary for its tumor cell-motility-stimulating activity. However, only the discovery of PD-Ialpha/ATX's identity with lysophospholipase D, an extracellular enzyme that converts lysophosphatidylcholine into lysophosphatidic acid (LPA) and potentially sphingosylphosphoryl choline into sphingosine 1-phosphate (S1P), revealed the actual effectors responsible for PD-Ialpha/ATX's ascribed motogenic functions, i.e., its catalytic products. PD-Ialpha/ATX has also been detected during normal development in a number of tissues, in particular, the central nervous system (CNS), where expression levels are high. Similar to tumor cells, PD-Ialpha/ATX-expressing CNS cells secrete catalytically active PD-Ialpha/ATX into the extracellular environment. Thus, it appears reasonable to assume that PD-Ialpha/ATX's CNS-related functions are mediated via lysophospholipid, LPA and potentially S1P, signaling. However, recent studies identified PD-Ialpha/ATX as a matricellular protein involved in the modulation of oligodendrocyte-extracellular matrix interactions and oligodendrocyte remodeling. This property of PD-Ialpha/ATX was found to be independent of its catalytic activity and to be mediated by a novel functionally active domain. These findings, therefore, uncover PD-Ialpha/ATX, at least in the CNS, as a multifunctional protein able to induce complex signaling cascades via distinct structure-function domains. This Mini-Review describes PD-Ialpha/ATX's multifunctional roles in the CNS and discusses their potential contributions to CNS development and pathology.

Structural characteristics of lysophosphatidic acid biological targets

Lysophosphatidic acid (LPA; 1-acyl-3-phosphoglycerol) exerts its biological activity through both extracellular and intracellular targets. Receptor targets include the cell-surface G-protein-coupled receptors LPA(1-4) and the nuclear PPAR-gamma (peroxisome-proliferator-activated receptor gamma). Enzyme targets include the secreted cancer cell motility factor, autotaxin, and the transmembrane phosphatases, LPP1-3 (where LPP stands for lipid phosphate phosphatase). Ion channel targets include the two pore domain ion channels in the TREK family, TREK-1, TREK-2 and TRAAK. Structural features of these targets and their interactions with LPA are reviewed.