Expression of the chemokine receptor Cxcr4 mRNA during mouse brain development

Chemokines in neuroectodermal development and their potential implication in cancer stem cell-driven metastasis

Chemokines regulate proliferation and migration of various types of normal stem and progenitor cells, including precursor cells of neuroectodermal origin. Based on this it is conceivable that the established role of chemokines in cancer cell proliferation and organ-specific metastasis might also be associated with stem cell-like cells present in the tumor. Such cancer stem cells (CSCs) represent a small subpopulation of tumor cells that are thought to initiate and sustain tumor formation. More recently, characteristics of stem cells have also been observed in metastatic cancer cells, and it has been suggested that CSCs might play a crucial role in the metastatic process as such. Intriguingly, first evidence has been provided that the metastatic spread of specific CSCs is driven by chemokine signaling. Thus it is possible that chemokine-mediated CSC regulation might be a general feature of metastasis formation.

Regional and cellular localization of the CXCl12/SDF-1 chemokine receptor CXCR7 in the developing and adult rat brain

The chemokine stromal cell-derived factor-1 (SDF-1) regulates neuronal development via the chemokine receptor CXCR4. In the adult brain the SDF-1/CXCR4 system was implicated in neurogenesis, neuromodulation, brain inflammation, tumor growth, and HIV encephalopathy. Until the recent identification of RDC1/CXCR7 as the second SDF-1 receptor, CXCR4 was considered to be the only receptor for SDF-1. Here we provide the first map of CXCR7 mRNA expression in the embryonic and adult rat brain. At embryonic stages, CXCR7 and CXCR4 were codistributed in the germinative zone of the ganglionic eminences, caudate putamen, and along the routes of GABAergic precursors migrating toward the cortex. In the cortex, CXCR7 was identified in GABAergic precursors and in some reelin-expressing Cajal-Retzius cells. Unlike CXCR4, CXCR7 was abundant in neurons forming the cortical plate and sparse in the developing dentate gyrus and cerebellar external germinal layer. In the adult brain, CXCR7 was expressed by blood vessels, pyramidal cells in CA3, and mature dentate gyrus granule cells, which is reminiscent of the SDF-1 pattern. CXCR7 and CXCR4 overlapped in the wall of the four ventricles. Further neuronal structures expressing CXCR7 comprised the olfactory bulb, accumbens shell, supraoptic and ventromedial hypothalamic nuclei, medial thalamus, and brain stem motor nuclei. Also, GLAST-expressing astrocytes showed signals for CXCR7. Thus, CXCR4 and CXCR7 may cooperate or act independently in SDF-1-dependent neuronal development. In mature neurons and blood vessels CXCR7 appears to be the preponderant SDF-1-receptor.

SLITs suppress tumor growth in vivo by silencing Sdf1/Cxcr4 within breast epithelium

The genes encoding Slits and their Robo receptors are silenced in many types of cancer, including breast, suggesting a role for this signaling pathway in suppressing tumorigenesis. The molecular mechanism underlying these tumor-suppressive effects has not been delineated. Here, we show that loss of Slits, or their Robo1 receptor, in murine mammary gland or human breast carcinoma cells results in coordinate up-regulation of the Sdf1 and Cxcr4 signaling axis, specifically within mammary epithelium. This is accompanied by hyperplastic changes in cells and desmoplastic alterations in the surrounding stroma. A similar inverse correlation between Slit and Cxcr4 expression is identified in human breast tumor tissues. Furthermore, we show in a xenograft model that Slit overexpression down-regulates CXCR4 and dominantly suppresses tumor growth. These studies classify Slits as negative regulators of Sdf1 and Cxcr4 and identify a molecular signature in hyperplastic breast lesions that signifies inappropriate up-regulation of key prometastatic genes.

Hypoxia- and vascular endothelial growth factor-induced stromal cell-derived factor-1alpha/CXCR4 expression in glioblastomas: one plausible explanation of Scherer's structures

The morphological patterns of glioma cell invasion are known as the secondary structures of Scherer. In this report, we propose a biologically based mechanism for the nonrandom formation of Scherer's secondary structures based on the differential expression of stromal cell-derived factor (SDF)-1alpha and CXCR4 at the invading edge of glioblastomas. The chemokine SDF-1alpha was highly expressed in neurons, blood vessels, subpial regions, and white matter tracts that form the basis of Scherer's secondary structures. In contrast, the SDF-1alpha receptor, CXCR4, was highly expressed in invading glioma cells organized around neurons and blood vessels, in subpial regions, and along white matter tracts. Neuronal and endothelial cells exposed to vascular endothelial growth factor up-regulated the expression of SDF-1alpha. CXCR4-positive tumor cells migrated toward a SDF-1alpha gradient in vitro, whereas inhibition of CXCR4 expression decreased their migration. Similarly, inhibition of CXCR4 decreased levels of SDF-1alpha-induced phosphorylation of FAK, AKT, and ERK1/2, suggesting CXCR4 involvement in glioma invasion signaling. These studies offer one plausible molecular basis and explanation of the formation of Scherer's structures in glioma patients.

CXCR4 signaling in the regulation of stem cell migration and development

The regulated migration of stem cells is a feature of the development of all tissues and also of a number of pathologies. In the former situation the migration of stem cells over large distances is required for the correct formation of the embryo. In addition, stem cells are deposited in niche like regions in adult tissues where they can be called upon for tissue regeneration and repair. The migration of cancer stem cells is a feature of the metastatic nature of this disease. In this article we discuss observations that have demonstrated the important role of chemokine signaling in the regulation of stem cell migration in both normal and pathological situations. It has been demonstrated that the chemokine receptor CXCR4 is expressed in numerous types of embryonic and adult stem cells and the chemokine SDF-1/CXCL12 has chemoattractant effects on these cells. Animals in which SDF-1/CXCR4 signaling has been interrupted exhibit numerous phenotypes that can be explained as resulting from inhibition of SDF-1 mediated chemoattraction of stem cells. Hence, CXCR4 signaling is a key element in understanding the functions of stem cells in normal development and in diverse pathological situations.

CXCR4/SDF-1 system modulates development of GnRH-1 neurons and the olfactory system

Stromal cell-derived factor 1 (SDF-1) and its receptor CXCR4 influence neuronal migration and have been identified in nasal regions. Gonadotropin releasing hormone-1 (GnRH-1) neurons migrate from nasal regions into the developing forebrain, where postnatally they control reproduction. This study examined the role of SDF-1/CXCR4 in development of the GnRH-1/olfactory systems. Migrating GnRH-1 neurons were CXCR4 immunopositive as were the fibers along which they migrate. SDF-1 transcripts were detected in olfactory epithelium and vomeronasal organ, while SDF-1 immunoreactivity highlighted the GnRH-1 migratory pathway. CXCR4-deficient mice showed a decrease in GnRH-1 cells at the nasal forebrain junction and in brain, but the overall migratory pathway remained intact, no ectopic GnRH-1 cells were detected and olfactory axons reached the olfactory bulb. To further characterize the influence of SDF-1/CXCR4 in the GnRH-1 system, nasal explants were used. CXCR4 expression in vitro was similar to that in vivo. SDF-1 was detected in a dorsal midline cell cluster as well as in migrating GnRH-1 cells. Treatment of explants with bicyclam AMD3100, a CXCR4 antagonist, attenuated GnRH-1 neuronal migration and sensory axon outgrowth. Moreover, the number of GnRH-1 neurons in the explant periphery was reduced. The effects were blocked by coincubation with SDF-1. Removal of midline SDF-1 cells did not alter directional outgrowth of olfactory axons. These results indicate that SDF-1/CXCR4 signaling in not necessary for olfactory axon guidance but rather influences sensory axon extension and GnRH-1 neuronal migration, and maintains GnRH-1 neuronal expression as the cells move away from nasal pit regions.

Tonic activation of CXC chemokine receptor 4 in immature granule cells supports neurogenesis in the adult dentate gyrus

Stromal-cell-derived factor-1 (SDF-1) and its receptor CXC chemokine receptor 4 (CXCR4) play a well-established role during embryonic development of dentate gyrus granule cells. However, little is known about the regulation and function of CXCR4 in the postnatal dentate gyrus. Here, we identify a striking mismatch between intense CXCR4 mRNA and limited CXCR4 protein expression in adult rat subgranular layer (SGL) neurons. We demonstrate that CXCR4 protein expression in SGL neurons is progressively lost during postnatal day 15 (P15) to P21. This loss of CXCR4 protein expression was paralleled by a reduction in the number of SDF-1-responsive SGL neurons and a massive upregulation of SDF-1 mRNA in granule cells. Intraventricular infusion of the CXCR4-antagonist AMD3100 dramatically increased CXCR4 protein expression in SGL neurons, suggesting that CXCR4 is tonically activated and downregulated by endogenous SDF-1. Infusion of AMD3100 also facilitated detection of CXCR4 protein in bromodeoxyuridine-, nestin-, and doublecortin-labeled cells and showed that the vast majority of adult-born granule cells transiently expressed CXCR4. Chronic AMD3100 administration impaired formation of new granule cells as well as neurogenesis-dependent long-term recognition of novel objects. Therefore, our findings suggest that tonic activation of CXCR4 in newly formed granule cells by endogenous SDF-1 is essential for neurogenesis-dependent long-term memory in the adult hippocampus.

Chemokine receptors and neurotrophic factors: potential therapy against aids dementia?

Chemokine receptors, in particular, CXCR4 and CCR5, mediate human immunodeficiency virus type 1 (HIV-1) infection of immunocompetent cells and the apoptosis of these cells. However, the virus does not infect neurons. Yet through a variety of mechanisms, HIV promotes glial cell activation, synaptodendritic alterations, and neuronal loss that ultimately lead to motor and cognitive impairment. Chemokines and chemokine receptors are abundant in the adult central nervous system and play a role in neuronal apoptosis evoked by HIV proteins. Thus, reducing the availability of chemokine receptors may prevent the neuronal degeneration seen in HIV-positive patients. In this article, we present and discuss a recent experimental approach aimed at testing effective neuroprotective therapies against HIV-mediated neuronal degeneration.

Regional distribution of cortical interneurons and development of inhibitory tone are regulated by Cxcl12/Cxcr4 signaling

Interneurons are born in subcortical germinative zones and tangentially migrate in multiple streams above and below the developing cortex, and then, at the appropriate developmental stage, migrate radially into the cortex. The factors that control the formation of and the timing of exit from the streams remain obscure; moreover, the rationale for this complicated developmental plan is unclear. We show that a chemokine, Cxcl12, is an attractant for interneurons during the stage of stream formation and tangential migration. Furthermore, the timing of exit from the migratory streams accompanies loss of responsiveness to Cxcl12 as an attractant. Mice with mutations in Cxcr4 have disorganized migratory streams and deletion of Cxcr4 after the streams have formed precipitates premature entry into the cortical plate. In addition, constitutive deletion of Cxcr4 specifically in interneurons alters the regional distribution of interneurons within the cortex and leads to interneuron laminar positioning defects in the postnatal cortex. To examine the role of interneuron distribution on the development of cortical circuitry, we generated mice with focal defects in interneuron distribution and studied the density of postnatal inhibitory innervation in areas with too many and too few interneurons. Interestingly, alterations in IPSC frequency and amplitude in areas with excess interneurons tend toward normalization of inhibitory tone, but in areas with reduced interneuron density this system fails. Thus, the processes controlling interneuron sorting, migration, regional distribution, and laminar positioning can have significant consequences for the development of cortical circuitry and may have important implications for a range of neurodevelopmental disorders.

Multiple roles of chemokine CXCL12 in the central nervous system: a migration from immunology to neurobiology

Chemotactic cytokines (chemokines) have been traditionally defined as small (10-14kDa) secreted leukocyte chemoattractants. However, chemokines and their cognate receptors are constitutively expressed in the central nervous system (CNS) where immune activities are under stringent control. Why and how the CNS uses the chemokine system to carry out its complex physiological functions has intrigued neurobiologists. Here, we focus on chemokine CXCL12 and its receptor CXCR4 that have been widely characterized in peripheral tissues and delineate their main functions in the CNS. Extensive evidence supports CXCL12 as a key regulator for early development of the CNS. CXCR4 signaling is required for the migration of neuronal precursors, axon guidance/pathfinding and maintenance of neural progenitor cells (NPCs). In the mature CNS, CXCL12 modulates neurotransmission, neurotoxicity and neuroglial interactions. Thus, chemokines represent an inherent system that helps establish and maintain CNS homeostasis. In addition, growing evidence implicates altered expression of CXCL12 and CXCR4 in the pathogenesis of CNS disorders such as HIV-associated encephalopathy, brain tumor, stroke and multiple sclerosis (MS), making them the plausible targets for future pharmacological intervention.

Chemokines: a new class of neuromodulator?

Chemokines are not only found in the immune system or expressed in inflammatory conditions: they are constitutively present in the brain in both glial cells and neurons. Recently, the possibility has been raised that they might act as neurotransmitters or neuromodulators. Although the evidence is incomplete, emerging data show that chemokines have several of the characteristics that define neurotransmitters. Moreover, their physiological actions resemble those of neuromodulators in the sense that chemokines usually have few effects by themselves in basal conditions, but modify the induced release of neurotransmitters or neuropeptides. These findings, together with the pharmacological development of agonists and antagonists that are selective for chemokine receptors and can cross the blood-brain barrier, open a new era of research in neuroscience.

GTPgammaS incorporation in the rat brain: a study on mu-opioid receptors and CXCR4

Chemokine and opioid receptors are G-protein-coupled receptors that play important roles in both the central nervous system and the immune system. The long-term goal of our research is to establish whether opioids regulate the activity of the chemokine receptor CXCR4 (one of the major HIV co-receptors) in the brain. In this research, we studied the anatomical distribution of functional receptors in young and adult animals by using the [(35)S]GTPgammaS "binding" assay as an indication of G-protein activation by CXCL12 (the natural CXCR4 ligand) or by mu-opioid agonists. Brain slices or homogenates from Holtzmann rats of different ages (from 2 to 21 days old and adult animals) were treated with CXCL12 (0.001-100 nM), D: -ala2,MePhe4,gly-ol5]enkephalin (DAMGO; 0.0003-10 microM) or morphine (0.0003-10 microM) and then processed for the assay. Our results show stimulation of both mu-OR and CXCR4 in several brain areas, including cortex and hippocampus (p < 0.001); this effect is dose and age dependent, and the magnitude of response varies among different brain regions. Furthermore, AMD3100 (100 ng/ml), a specific CXCR4 antagonist, abolished CXCL12 stimulation in all the brain regions analyzed (p < 0.001). Our findings suggest a similar pattern of expression for mu-OR and CXCR4 in the brain, supporting the possibility of an interaction between the two G-protein-coupled receptors in vivo. This might be relevant to the role of opiates in HIV neuropathogenesis.

Brain-derived neurotrophic factor prevents the nigrostriatal degeneration induced by human immunodeficiency virus-1 glycoprotein 120 in vivo

Glycoprotein 120 (gp120) from the T-tropic strain of the human immunodeficiency virus type 1 has been shown to cause neuronal apoptosis through activation of the chemokine receptor CXCR4. Therefore, reducing CXCR4 expression may prevent gp120-mediated apoptosis. Brain-derived neurotrophic factor (BDNF) is known to reduce both gp120 neurotoxicity and CXCR4 expression in vitro. The scope of this work is to establish whether BDNF is neuroprotective against gp120 in vivo and, if so, whether this effect correlates with its ability to down-regulate CXCR4. Serotype 2 adeno-associated viral vector encoding for BDNF (rAAV-BDNF) or control vector was microinjected into the striata of adult rats. Two weeks later gp120 was injected into the same striatum, and apoptosis determined. Pretreatment with rAAV-BDNF prior to gp120 microinjection prevented caspase-3 activation as well as in situ terminal deoxynucleotidyl transferase biotin-dUTP nick end labelling in the striatum and substantia nigra. In addition, rAAV-BDNF reversed the loss of tyrosine hydroxylase immunoreactivity induced by gp120 in both areas. CXCR4 expression was then determined by immunohistochemistry and RT-PCR, and found to be decreased in striata of rAAV-BDNF-treated rats. Conversely, BDNF heterozygous mice exhibited an increase in CXCR4 mRNA levels compared to wild-type littermates. Our data suggest that down-regulation of CXCR4 expression may contribute to the neuroprotective activity of BDNF against gp120 toxicity in the basal ganglia.

Patterns of SDF-1alpha and SDF-1gamma mRNAs, migration pathways, and phenotypes of CXCR4-expressing neurons in the developing rat telencephalon

Cortical GABAergic neurons originate in the ventral telencephalon, invade the cortex via tangential migration, and integrate into the cortical plate by surface-directed and ventricle-directed migration. In mice lacking CXCR4 or SDF-1, GABAergic neurons fail to complete their migration. It is presently unknown which parts of the migration of CXCR4-expressing GABAergic neurons are driven by SDF-1. Here we compared patterns of SDF-1 isoforms and CXCR4 in the developing rat telencephalon. In the ventral telencephalon, radial glia, striatal, and migratory GABAergic neurons expressed CXCR4. Tangentially migrating CXCR4-expressing neurons populated the marginal zone and started to invade the lateral intermediate zone at embryonic day (E)14. Until E17 the spread of CXCR4-expressing neurons in the dorsomedial direction was accompanied by progressive upregulation of SDF-1alpha in the dorsomedial intermediate/subventricular zone. In the meninges, SDF-1alpha and SDF-1gamma were expressed persistently. During invasion of the cortical plate the orientation of CXCR4-immunoreactive neurons changed gradually from tangential (E17/E18) to radial (postnatal day [P] 0), which was paralleled by downregulation of SDF-1alpha in the intermediate/subventricular zone. At E17, CXCR4-immunoreactive cells were colabeled with markers for ventral forebrain-derived neurons (Dlx) but not markers for glutamatergic (Tbr) or subplate (calretinin) neurons. Postnatally, calretinin- and somatostatin-expressing but not parvalbumin-expressing GABAergic neurons or pyramidal cells contained CXCR4. Pyramidal cells and few large blood vessels expressed SDF-1alpha, while microvessels contained SDF-1gamma transcripts. In summary, SDF-1alpha is expressed along cortical but not subcortical migration routes of GABAergic neurons. We propose that regulated expression of SDF-1 in the intermediate/subventricular zone influences lateromedial tangential migration of CXCR4-expressing GABAergic neurons.

Cxcl12/Cxcr4 chemokine signaling is required for placode assembly and sensory axon pathfinding in the zebrafish olfactory system

Positioning neurons in the right places and wiring axons to the appropriate targets are essential events for establishment of neural circuits. In the zebrafish olfactory system, precursors of olfactory sensory neurons (OSNs) assemble into a compact cluster to form the olfactory placode. Subsequently, OSNs differentiate and extend their axons to the presumptive olfactory bulb with high precision. In this study, we aim to elucidate the molecular mechanism underlying these two developmental processes. cxcr4b, encoding a chemokine receptor, is expressed in the migrating olfactory placodal precursors, and cxcl12a (SDF-1a), encoding a ligand for Cxcr4b, is expressed in the abutting anterior neural plate. The expression of cxcr4b persists in the olfactory placode at the initial phase of OSN axon pathfinding. At this time, cxcl12a is expressed along the placode-telencephalon border and at the anterior tip of the telencephalon, prefiguring the route and target of OSN axons, respectively. Interfering with Cxcl12a/Cxcr4b signaling perturbs the assembly of the olfactory placode, resulting in the appearance of ventrally displaced olfactory neurons. Moreover, OSN axons frequently fail to exit the olfactory placode and accumulate near the placode-telencephalon border in the absence of Cxcr4b-mediated signaling. These data indicate that chemokine signaling contributes to both the olfactory placode assembly and the OSN axon pathfinding in zebrafish.

Stromal derived factor-1 exerts differential regulation on distinct cortical cell populations in vitro

BACKGROUND

Stromal derived factor (SDF-1), an alpha chemokine, is a widely known chemoattractant in the immune system. A growing body of evidence now suggests multiple regulatory roles for SDF-1 in the developing nervous system.

RESULTS

To investigate the role of SDF-1 signaling in the growth and differentiation of cortical cells, we performed numerous in vitro experiments, including gene chip and quantitative RT-PCR analysis. Using SDF-1 medium and AMD3100, a receptor antagonist, we demonstrate that the chemokine signaling regulates key events during early cortical development. First, SDF-1 signaling maintains cortical progenitors in proliferation, possibly through a mechanism involving connexin 43 mediated intercellular coupling. Second, SDF-1 signaling upregulates the differentiation of cortical GABAergic neurons, independent of sonic signaling pathway. Third, SDF-1 enables the elongation and branching of axons of cortical glutamatergic neurons. Finally, cortical cultures derived from CXCR4-/- mutants show a close parallel to AMD3100 treatment with reduced cell proliferation and differentiation of GABAergic neurons.

CONCLUSION

Results from this study show that SDF-1 regulates distinct cortical cell populations in vitro.

Chemokine receptor expression by neural progenitor cells in neurogenic regions of mouse brain

We previously demonstrated that chemokine receptors are expressed by neural progenitors grown as cultured neurospheres. To examine the significance of these findings for neural progenitor function in vivo, we investigated whether chemokine receptors were expressed by cells having the characteristics of neural progenitors in neurogenic regions of the postnatal brain. Using in situ hybridization we demonstrated the expression of CCR1, CCR2, CCR5, CXCR3, and CXCR4 chemokine receptors by cells in the dentate gyrus (DG), subventricular zone of the lateral ventricle, and olfactory bulb. The pattern of expression for all of these receptors was similar, including regions where neural progenitors normally reside. In addition, we attempted to colocalize chemokine receptors with markers for neural progenitors. In order to do this we used nestin-EGFP and TLX-LacZ transgenic mice, as well as labeling for Ki67, a marker for dividing cells. In all three areas of the brain we demonstrated colocalization of chemokine receptors with these three markers in populations of cells. Expression of chemokine receptors by neural progenitors was further confirmed using CXCR4-EGFP BAC transgenic mice. Expression of CXCR4 in the DG included cells that expressed nestin and GFAP as well as cells that appeared to be immature granule neurons expressing PSA-NCAM, calretinin, and Prox-1. CXCR4-expressing cells in the DG were found in close proximity to immature granule neurons that expressed the chemokine SDF-1/CXCL12. Cells expressing CXCR4 frequently coexpressed CCR2 receptors. These data support the hypothesis that chemokine receptors are important in regulating the migration of progenitor cells in postnatal brain.

Expression of SDF-1 and CXCR4 during reorganization of the postnatal dentate gyrus

Previous studies have demonstrated that stromal cell-derived factor 1 (SDF-1) is crucial for early dentate development; however, the mouse mutants for this chemokine and its only receptor, CXCR4, are neonatally lethal, making conclusions about the role of these molecules in postnatal development difficult to sustain. Previous expression analyses have used single labeling, but the distribution of CXCR4 is complex and to determine the cell types expressing CXCR4 requires multiple marker labeling. In this study, we examined the distribution of SDF-1 and CXCR4 mRNAs during the first postnatal weeks, combining these markers with several other cell-type-specific markers. We found that SDF-1 has three sites of expression: (1) continuation of prenatal expression in the meninges; (2) expression in Cajal-Retzius cells occupying the molecular layer of the upper and lower blades of the dentate, and (3) the maturing dentate granule neurons themselves. The timing of expression in these three sites corresponds to alterations in the distribution of the primary cell types expressing CXCR4 during the same periods, notably the expression of CXCR4 in radial-glial-like GFAP-expressing dentate precursors and immature dentate granule neurons. Taken together, our data suggest potential ongoing roles for SDF-1/CXCR4 signaling in the dentate gyrus during the early postnatal period that will be tested in the future with more precise genetic approaches.

Temporal mRNA profiles of inflammatory mediators in the murine 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrimidine model of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). With the exception of a few rare familial forms of the disease, the precise molecular mechanisms underlying PD are unknown. Inflammation is a common finding in the PD brain, but due to the limitation of postmortem analysis its relationship to disease progression cannot be established. However, studies using the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD have also identified inflammatory responses in the nigrostriatal pathway that precede neuronal degeneration in the SNpc. To assess the pathological relevance of these inflammatory responses and to identify candidate genes that might contribute to neuronal vulnerability, we used quantitative reverse-transcription polymerase chain reaction (qRT-PCR) to measure mRNA levels of 11 cytokine and chemokine encoding genes in the striatum of MPTP-sensitive (C57BL/6J) and MPTP-insensitive (Swiss Webster, SWR) mice following administration of MPTP. The mRNA levels of all 11 genes changed following MPTP treatment, indicating the presence of inflammatory responses in both strains. Furthermore, of the 11 genes examined only 3, interleukin 6 (Il-6), macrophage inflammatory protein 1 alpha/CC chemokine ligand 3 (Mip-1alpha/Ccl3) and macrophage inflammatory protein 1 beta/CC chemokine ligand 4 (Mip-1beta/Ccl4), were differentially regulated between C57BL/6J and SWR mice. In both mouse strains, the level of monocyte chemoattractant protein 1/CC chemokine ligand 2 (Mcp-1/Ccl2) mRNA was the first to increase following MPTP administration, and might represent a key initiating component of the inflammatory response. Using Mcp-1/Ccl2 knockout mice backcrossed onto a C57BL/6J background we found that MPTP-stimulated Mip-1alpha/Ccl3 and Mip-1beta/Ccl4 mRNA expression was significantly lower in the knockout mice; suggesting that Mcp-1/Ccl2 contributes to MPTP-enhanced expression of Mip-1alpha/Ccl3 and Mip-1beta/Ccl4. However, stereological analysis of SNpc neuronal loss in Mcp-1/Ccl2 knockout and wild-type mice showed no differences. These findings suggest that it is the ability of dopaminergic SNpc neurons to survive an inflammatory insult, rather than genetically determined differences in the inflammatory response itself, that underlie the molecular basis of MPTP resistance.

Molecular interaction between projection neuron precursors and invading interneurons via stromal-derived factor 1 (CXCL12)/CXCR4 signaling in the cortical subventricular zone/intermediate zone

Most cortical interneurons are generated in the subpallial ganglionic eminences and migrate tangentially to their final destinations in the neocortex. Within the cortex, interneurons follow mainly stereotype routes in the subventricular zone/intermediate zone (SVZ/IZ) and in the marginal zone. It has been suggested that interactions between invading interneurons and locally generated projection neurons are implicated in the temporal and spatial regulation of the invasion process. However, so far experimental evidence for such interactions is lacking. We show here that the chemokine stromal-derived factor 1 (SDF-1; CXCL12) is expressed in the main invasion route for cortical interneurons in the SVZ/IZ. Most SDF-1-positive cells are proliferating and express the homeodomain transcription factors Cux1 and Cux2. Using MASH-1 mutant mice in concert with the interneuron marker DLX, we exclude that interneurons themselves produce the chemokine in an autocrine manner. We conclude that the SDF-1-expressing cell population represents the precursors of projection neurons during their transition and amplification in the SVZ/IZ. Using mice lacking the SDF-1 receptor CXCR4 or Pax6, we demonstrate that SDF-1 expression in the cortical SVZ/IZ is essential for recognition of this pathway by interneurons. These results represent the first evidence for a molecular interaction between precursors of projection neurons and invading interneurons during corticogenesis.

Meninges control tangential migration of hem-derived Cajal-Retzius cells via CXCL12/CXCR4 signaling

Cajal-Retzius cells are critical in the development of the cerebral cortex, but little is known about the mechanisms controlling their development. Three focal sources of Cajal-Retzius cells have been identified in mice-the cortical hem, the ventral pallium and the septum-from where they migrate tangentially to populate the cortical surface. Using a variety of tissue culture assays and in vivo manipulations, we demonstrate that the tangential migration of cortical hem-derived Cajal-Retzius cells is controlled by the meninges. We show that the meningeal membranes are a necessary and sufficient substrate for the tangential migration of Cajal-Retzius cells. We also show that the chemokine CXCL12 secreted by the meninges enhances the dispersion of Cajal-Retzius cells along the cortical surface, while retaining them within the marginal zone in a CXCR4-dependent manner. Thus, the meningeal membranes are fundamental in the development of Cajal-Retzius cells and, hence, in the normal development of the cerebral cortex.

Stromal cell-derived factor-1 (chemokine C-X-C motif ligand 12) and chemokine C-X-C motif receptor 4 are required for migration of gonadotropin-releasing hormone neurons to the forebrain

Gonadotropin-releasing hormone (GnRH) neurons migrate from the vomeronasal organ (VNO) in the nasal compartment to the basal forebrain in mice, beginning on embryonic day 11 (E11). These neurons use vomeronasal axons as guides to migrate through the nasal mesenchyme. Most GnRH neurons then migrate along the caudal branch of the vomeronasal nerve to reach the hypothalamus. We show here that stromal cell-derived factor-1 [SDF-1, also known as chemokine C-X-C motif ligand 12 (CXCL12)] is expressed in the embryonic nasal mesenchyme from as early as E10 in an increasing rostral to caudal gradient that is most intense at the border of the nasal mesenchyme and the telencephalon. Chemokine C-X-C motif receptor 4 (CXCR4), the receptor for SDF-1, is expressed by neurons in the olfactory epithelium and VNO. Cells derived from these sensory epithelia, including migrating GnRH neurons and ensheathing glial precursors of the migrating mass (MM), also express CXCR4, suggesting that they may use SDF-1 as a chemokine. In support of this, most GnRH neurons of CXCR4-/- mice fail to exit the VNO at E13, and comparatively few GnRH neurons reach the forebrain. There is also a significant decrease in the total number of GnRH neurons in CXCR4-/- mice and an increase in cell death within the VNO relative to controls. The MM is smaller in CXCR4-/- mice, suggesting that some MM cells also require SDF-1/CXCR4 function for migration and survival.

Laminating the hippocampus

Lamination of neurons and fibre projections is a fundamental organizational principle of the mammalian cerebral cortex. A laminated organization is likely to be essential for cortical function, as studies in mutant mice have revealed causal relationships between lamination defects and functional deficits. Unveiling the determinants of the laminated cortical architecture will contribute to our understanding of how cortical functions have evolved in phylogenetic and ontogenetic development. Recently, the hippocampus, with its clearly segregated cell and fibre layers, has become a major subject of studies on cortical lamination.

Neurons produce type I interferon during viral encephalitis

Type I interferons, also referred to as IFN-alpha/beta, form the first line of defense against viral infections. Major IFN-alpha/beta producers in the periphery are the plasmacytoid dendritic cells (pDCs). Constitutive expression of the IFN regulatory factor (IRF)-7 enables pDCs to rapidly synthesize large amounts of IFN-alpha/beta after viral infection. In the central nervous system (CNS), pDCs are considered to be absent from the parenchyma, and little is known about the cells producing IFN-alpha/beta. The study presented here aimed to identify the cells producing IFN-alpha/beta in the CNS in vivo after infection by neurotropic viruses such as Theiler's virus and La Crosse virus. No cells with high constitutive expression of IRF-7 were detected in the CNS of uninfected mice, suggesting the absence of cells equivalent to pDCs. Upon viral infection, IFN-beta and some subtypes of IFN-alpha, but not IFN-epsilon or IFN-kappa, were transcriptionally up-regulated. IFN-alpha/beta was predominantly produced by scattered parenchymal cells and much less by cells of inflammatory foci. Interestingly, in addition to some macrophages and ependymal cells, neurons turned out to be important producers of both IFN-alpha and IFN-beta. However, only 3% of the infected neurons produced IFN-alpha/beta, suggesting that some restriction to IFN-alpha/beta production existed in these cells. All CNS cell types analyzed, including neurons, were able to respond to type I IFN by producing Mx or IRF-7. Our data show that, in vivo, neurons take an active part to the antiviral defense by being both IFN-alpha/beta producers and responders.

Massive loss of Cajal-Retzius cells does not disrupt neocortical layer order

Cajal-Retzius (CR) cells, the predominant source of reelin in developing neocortex, are thought to be essential for the inside out formation of neocortical layers. Fate mapping revealed that a large population of neocortical CR cells arises from the cortical hem. To investigate the function of CR cells, we therefore genetically ablated the hem. Neocortical CR cells were distributed beneath the pial surface in control mice, but were virtually absent in hem-ablated mice from embryonic day (E) 10.5 until birth. CR cells derived from other sources did not invade the neocortical primordium to compensate for hem loss. We predicted that neocortical layers would be inverted in hem-ablated animals, as in reeler mice, deficient in reelin signaling. Against expectation, layers showed the standard order. Low levels of reelin in the cortical primordium, or diffusion of reelin from other sites, may have allowed lamination to proceed. Our findings indicate, however, that the sheet of reelin-rich CR cells that covers the neocortical primordium is not required to direct layer order.

The mouse forkhead gene Foxc1 is required for primordial germ cell migration and antral follicle development

Foxc1 encodes a forkhead/winged helix transcription factor expressed in many embryonic tissues. Previous studies have investigated defects in the urogenital system of Foxc1 null mutants, but the mechanisms underlying the abnormal development of the gonad have not been explored. From earliest stages, the mutant ovaries are smaller than normal, with fewer germ cells and disorganized somatic issue. No bursa membrane is formed, and the oviduct remains uncoiled. Although germ cells are specified correctly, many of them do not migrate to the gonadal ridge, remaining trapped in the hindgut. Consequently, the number initially reaching the gonad is less than 25% of normal. Once in the ovary, germ cells proliferate normally, but the supporting somatic cells are not organized correctly. Since mutant embryos die at birth, further development was followed in ovaries grafted underneath the kidney capsule of ovariectomized females. Transplanted ovaries display normal folliculogenesis up to preantral stages. However, no follicles develop beyond early antral stages. Mutant follicles are often polyovulatory and have disrupted theca and granulosa cell layers. We conclude that alongside its previously known roles in kidney, cardiovascular and eye development, Foxc1 has essential functions during at least two stages of gonad development-germ cell migration and folliculogenesis.

Membrane properties of retinal stem cells/progenitors

The membrane properties of cells help integrate extrinsic information relayed through growth factors, chemokines, extracellular matrix, gap junctions and neurotransmitters towards modulating cell-intrinsic properties, which in turn determine whether cells remain quiescent, proliferate, differentiate, establish contact with other cells or remove themselves by activating programmed cell death. This review highlights some of the membrane properties of early and late retinal stem cells/progenitors, which are likely to be helpful in the identification and enrichment of these cells and in understanding mechanisms underlying their maintenance and differentiation. Understanding of membrane properties of retinal stem cells/progenitors is essential for the successful formulation of approaches to treat retinal degeneration and diseases by cell therapy.

Chemokines: integrators of pain and inflammation

Chronic (neuropathic) pain is one of the most widespread and intractable of human complaints, as well as being one of the most difficult syndromes to treat successfully with drugs or surgery. The development of new therapeutic approaches to the treatment of painful neuropathies requires a better understanding of the mechanisms that underlie the development of these chronic pain syndromes. It is clear that inflammatory responses often accompany the development of neuropathic pain, and here we discuss the idea that chemokines might be key to integrating the development of pain and inflammation and could furnish new leads in the search for effective analgesic agents for the treatment of painful neuropathies.

Perinatal subplate neuron injury: implications for cortical development and plasticity

Perinatal brain injury may result in widespread deficits in visual, motor and cognitive systems suggesting disrupted brain development. Neurosensory and cognitive impairment are observed at increasing frequency with decreasing gestational ages, suggesting a unique vulnerability of the developing brain. The peak of human subplate neuron development coincides with the gestational ages of highest vulnerability to perinatal brain injury in the premature infant. At the same time, human thalamocortical connections are forming and being refined by activity-dependent mechanisms during critical periods. Subplate neurons are the first cortical neurons to mature and are selectively vulnerable to early hypoxic-ischemic brain injury in animal models. Timing of subplate neuron death determines the resulting defect in thalamocortical development: very early excitotoxic subplate neuron death results in failure of thalamocortical innervation, while later subplate neuron death interferes with the refinement of thalamocortical connections into mature circuits. We suggest that subplate neuron injury may be a central component of perinatal brain injury resulting in specific neurodevelopmental consequences.

The SDF-1/CXCR4 pathway and the development of the cerebellar system

Mice deficient for the chemokine receptor CXCR4 show premature translocation of granule cell neuroblasts from their germinal zone into the nascent cerebellum [Y.-R. Zuo et al. (1998) Nature, 393, 595-599]. Here, we used CXCR4-null mice to analyse the early development of cerebellar cortical inhibitory interneurons and pontine neurons which, in the adult, are synaptically integrated with granule cells. Cortical inhibitory interneuronal precursors normally invade the cerebellar anlage of CXCR4-deficient mice, but their dispersal is impeded by dislocated foci of proliferating granule cells, from which they are excluded. This is reminiscent of the strict exclusion of inhibitory interneuronal precursors from the superficial external granule cell layer. As inhibitory interneuronal precursors readily mingle with post-mitotic granule cells both in wild-type and CXCR4-null mice, these findings indicate that the developmentally regulated interactions between granule and inhibitory interneuronal precursors are independent of SDF-1/CXCR4 signalling. In contrast, the transit of pontine neurons from the rhombic lip through the anterior extramural stream to the basilar pons is disrupted in CXCR4-deficient animals. Migrating pontine neurons express CXCR4, and in CXCR4-null animals these cells are found displaced deep into the brainstem. Consequently, nascent pontine nuclei in CXCR4-deficient animals are hypoplastic. Moreover, they fail to express plexin D1, suggesting that SDF-1/CXCR4 signalling may also impinge on axon guidance critical to the orderly formation of granule cell mossy fibre afferents.

A role for CXCR4 signaling in survival and migration of neural and oligodendrocyte precursors

Oligodendrocyte development is controlled by a number of survival and migratory factors. The present study shows that signaling of CXCR4 receptor by the chemokine CXCL12 regulates survival and migration of neural precursors (NP) as well as oligodendrocyte progenitors (OP). CXCR4 is expressed by E14 striatal NP and OP generated by neurospheres. In CXCR4-defective mice, the number of NP in neurosphere outgrowth was twofold less than in wild-type (WT) mice; NP radial cell migration was also decreased. In contrast, the addition of CXCL12 to WT NP increased radial migration from the sphere in a dose-dependent manner with a maximal response at 200 nM. When oligodendrocytes differentiated in neurosphere outgrowth, CXCR4 was downregulated. OP isolated from newborn brain coexpressed CXCR4 with platelet-derived growth factor receptor-alpha (PDGFR alpha) or chondroitin sulfate proteoglycan; receptor expression also decreased during differentiation in vitro. Neonatal OP showed a peak migratory response to 20 nM of CXCL12 in chemotactic chambers, a migration inhibited by a CXCR4 antagonist and anti-CXCL12 antibody. In the embryonic spinal cord, the number of OP-expressing PDGFR alpha was reduced more than twofold in CXCR4-defective mice compared with WT and the ratio of ventral to dorsal OP was significantly increased. This indicates a defect in OP survival and their dorsal migration from the ventral cord region, probably because CXCR4(-/-) OP are unable to respond to CXCL12 made by vascular endothelia and the pia mater. We propose that CXCR4 signaling regulate survival and outward chemotactic migration of OP during embryonic and postnatal CNS development.

Protocadherin Celsr3 is crucial in axonal tract development

In the embryonic CNS, the development of axonal tracts is required for the formation of connections and is regulated by multiple genetic and microenvironmental factors. Here we show that mice with inactivation of Celsr3, an ortholog of Drosophila melanogaster flamingo (fmi; also known as starry night, stan) that encodes a seven-pass protocadherin, have marked, selective anomalies of several major axonal fascicles, implicating protocadherins in axonal development in the mammalian CNS for the first time. In flies, fmi controls planar cell polarity (PCP) in a frizzled-dependent but wingless-independent manner. The neural phenotype in Celsr3 mutant mice is similar to that caused by inactivation of Fzd3, a member of the frizzled family. Celsr3 and Fzd3 are expressed together during brain development and may act in synergy. Thus, a genetic pathway analogous to the one that controls PCP is key in the development of the axonal blueprint.

The chemokine SDF-1 differentially regulates axonal elongation and branching in hippocampal neurons

Recent data have shown that the chemokine SDF-1 plays a critical role in several aspects of brain development such as cell migration and axon pathfinding. However, its potential function in the generation of axons and dendrites is poorly characterized. In order to better understand the role of SDF-1 in the development of central neurons, we studied the cellular distribution of the SDF-1 receptor CXCR4 by immunocytochemistry of developing hippocampal neurons and tested the effect of SDF-1 in process patterning at the early stages of neuronal development. We found that CXCR4 immunoreactivity undergoes a striking redistribution during development. At the early stages, from day 2 to day 4 in culture, CXCR4 is particularly concentrated at the leading edge of growing neurites. As the cells mature, staining declines at the tip of the processes and becomes more broadly distributed along axons and, to a lesser extent, dendrites. SDF-1 stimulation of neurons at day 1-2 in culture triggers several effects on neuronal morphogenesis. SDF-1 reduces growth cone number and axonal outgrowth but stimulates axonal branching. These latter two effects are not observed in other neurites. This study unravels a new role for SDF-1/CXCR4 in specifying hippocampal neuron morphology by regulating axonal patterning at an early stage of neuronal development.

Stromal cell-derived factor-1 (SDF-1) expression in embryonic mouse cerebral cortex starts in the intermediate zone close to the pallial–subpallial boundary and extends progressively towards the cortical hem

We describe the onset and the expansion of stromal cell-derived factor 1 (SDF-1) expression in the intermediate zone of embryonic mouse cerebral cortex between embryonic days (E)11.5 and 18.5, and on postnatal day 1. Using in situ hybridisation with a digoxigenin-labeled probe, SDF-1 mRNA was detectable by E 12.5 in a small area of the intermediate zone just dorsal to the pallial-subpallial boundary. During the following days, SDF-1 expression extended towards the dorso-lateral pallium, and then the hippocampus and cortical hem. The position of the SDF-1 positive cells within the intermediate zone was closely correlated with the stream of tangentially migrating cells carrying the polysialylated form of neural cell adhesion molecule (PSA-NCAM). However, whereas these cells form a ventro-dorsal stream passing from the subpallium into the pallium, SDF-1 was not detectable on the ventral side of the pallial-subpallial boundary at any of the developmental stages studied. By E 16.5, the intensity of SDF-1 hybridisation signal in the intermediate zone decreased, to become undetectable by E 18.5.

Developmental roles of p73 in Cajal-Retzius cells and cortical patterning

To examine the role of the p53 homolog p73 in brain development, we studied p73-/-, p73+/-, E2F1-/-, and reeler mutant mice. p73 in developing brain is expressed in Cajal-Retzius (CR) cells, the cortical hem, and the choroid plexus. p73-expressing CR cells are lost in p73-/- embryos, although Reelin is faintly expressed in the marginal zone. Ectopic neurons in the p73-/- preplate and cortical hem at embryonic day 12 implicate p73 in the early developmental program of the cortex; however, preplate partition and early cortical plate formation are not disturbed. Postnatal p73-/- mice show a mild hypoplasia of the rostral cortex and a severely disrupted architecture of the posterior telencephalon. In the developing p73-/- hippocampus, the most striking abnormality is the absence of the hippocampal fissure, suggesting a role of p73 in cortical folding. p73+/- mice have a less severe cortical phenotype; they display a dorsal shift of the entorhinal cortex and a reduced size of occipital and posterior temporal areas, which acquire entorhinal-like features such as Reelin-positive cells in layer II. CR cells appear unaffected by heterozygosity. We relate the malformations of the posterior pole in p73 mutant mice to alterations of p73 expression in the cortical hem and suggest that p73 forms part of an early signaling network that controls neocortical and archicortical regionalization. In mice deficient for the transcription factor E2F1, a main activator of the TAp73 (transactivating p73) isoform, we find a defect of the caudal cortical architecture resembling the p73+/- phenotype along with reduced TAp73 protein levels and propose that an E2F1-TAp73 dependent pathway is involved in cortical patterning.