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

Severe cerebellar malformations in mutant mice demonstrate a role for PDGF-C/PDGFRα signalling in cerebellar development

Formation of the mouse cerebellum is initiated in the embryo and continues for a few weeks after birth. Double mutant mice lacking platelet-derived growth factor-C and that are heterozygous for platelet-derived growth factor receptor alpha (Pdgfc-/-; PdgfraGFP/+) develop cerebellar hypoplasia and malformation with loss of cerebellar lobes in the posterior vermis. This phenotype is similar to those observed in Foxc1 mutant mice and in a human neuroimaging pattern called Dandy Walker malformation. Pdgfc-Pdgfra mutant mice also display ependymal denudation in the 4th ventricle and gene expression changes in cerebellar meninges, which coincide with the first visible signs of cerebellar malformation. Here we show that PDGF-C/PDGFRα signalling is a critical component in the network of molecular and cellular interactions that take place between the developing meninges and neural tissues, and which are required to build a fully functioning cerebellum.

Vascular Regulation of Developmental Neurogenesis

Evolutionary studies indicate that the nervous system evolved prior to the vascular system, but the increasing complexity of organisms prompted the vascular system to emerge in order to meet the growing demand for oxygen and nutrient supply. In recent years, it has become apparent that the symbiotic communication between the nervous and the vascular systems goes beyond the exclusive covering of the demands on nutrients and oxygen carried by blood vessels. Indeed, this active interplay between both systems is crucial during the development of the central nervous system (CNS). Several neural-derived signals that initiate and regulate the vascularization of the CNS have been described, however less is known about the vascular signals that orchestrate the development of the CNS cytoarchitecture. Here, we focus on reviewing the effects of blood vessels in the process of neurogenesis during CNS development in vertebrates. In mammals, we describe the spatiotemporal features of vascular-driven neurogenesis in two brain regions that exhibit different neurogenic complexity in their germinal zone, the hindbrain and the forebrain.

Origins, Development, and Compartmentation of the Granule Cells of the Cerebellum

Granule cells (GCs) are the most numerous cell type in the cerebellum and indeed, in the brain: at least 99% of all cerebellar neurons are granule cells. In this review article, we first consider the formation of the upper rhombic lip, from which all granule cell precursors arise, and the way by which the upper rhombic lip generates the external granular layer, a secondary germinal epithelium that serves to amplify the upper rhombic lip precursors. Next, we review the mechanisms by which postmitotic granule cells are generated in the external granular layer and migrate radially to settle in the granular layer. In addition, we review the evidence that far from being a homogeneous population, granule cells come in multiple phenotypes with distinct topographical distributions and consider ways in which the heterogeneity of granule cells might arise during development.

TrkB Signaling Influences Gene Expression in Cortistatin-Expressing Interneurons

Brain-derived neurotrophic factor (BDNF) signals through its cognate receptor tropomyosin receptor kinase B (TrkB) to promote the function of several classes of inhibitory interneurons. We previously reported that loss of BDNF-TrkB signaling in cortistatin (Cort)-expressing interneurons leads to behavioral hyperactivity and spontaneous seizures in mice. We performed bulk RNA sequencing (RNA-seq) from the cortex of mice with disruption of BDNF-TrkB signaling in cortistatin interneurons, and identified differential expression of genes important for excitatory neuron function. Using translating ribosome affinity purification and RNA-seq, we define a molecular profile for Cort-expressing inhibitory neurons and subsequently compare the translatome of normal and TrkB-depleted Cort neurons, revealing alterations in calcium signaling and axon development. Several of the genes enriched in Cort neurons and differentially expressed in TrkB-depleted neurons are also implicated in autism and epilepsy. Our findings highlight TrkB-dependent molecular pathways as critical for the maturation of inhibitory interneurons and support the hypothesis that loss of BDNF signaling in Cort interneurons leads to altered excitatory/inhibitory balance.

CXCL12/CXCR4 signal transduction in diseases and its molecular approaches in targeted-therapy

Chemokines are small molecules called "chemotactic cytokines" and regulate many processes like leukocyte trafficking, homing of immune cells, maturation, cytoskeletal rearrangement, physiology, migration during development, and host immune responses. These proteins bind to their corresponding 7-membrane G-protein-coupled receptors. Chemokines and their receptors are anti-inflammatory factors in autoimmune conditions, so consider as potential targets for neutralization in such diseases. They also express by cancer cells and function as angiogenic factors, and/or survival/growth factors that enhance tumor angiogenesis and development. Among chemokines, the CXCL12/CXCR4 axis has significantly been studied in numerous cancers and autoimmune diseases. CXCL12 is a homeostatic chemokine, which is acts as an anti-inflammatory chemokine during autoimmune inflammatory responses. In cancer cells, CXCL12 acts as an angiogenic, proliferative agent and regulates tumor cell apoptosis as well. CXCR4 has a role in leukocyte chemotaxis in inflammatory situations in numerous autoimmune diseases, as well as the high levels of CXCR4, observed in different types of human cancers. These findings suggest CXCL12/CXCR4 as a potential therapeutic target for therapy of autoimmune diseases and open a new approach to targeted-therapy of cancers by neutralizing CXCL12 and CXCR4. In this paper, we reviewed the current understanding of the role of the CXCL12/CXCR4 axis in disease pathology and cancer biology, and discuss its therapeutic implications in cancer and diseases.

Cerebellar involvement in warts Hypogammaglobulinemia immunodeficiency myelokathexis patients: neuroimaging and clinical findings

Background

Warts Hypogammaglobulinemia Immunodeficiency Myelokathexis (WHIM) syndrome is a primary immunodeficiency characterized by recurrent bacterial infections, severe chronic neutropenia, with lymphopenia, monocytopenia and myelokathexis which is caused by heterozygous gain of functions mutations of the CXC chemokine receptor 4 (CXCR4). WHIM patients display an increased incidence of non-hematopoietic conditions, such as congenital heart disease suggesting that abnormal CXCR4 may put these patients at increased risk of congenital anomalies.

Studies conducted on CXCR4 and SDF-1-deficient mice have demonstrated the role of CXCR4 signaling in neuronal cell migration and brain development. In particular, CXCR4 conditional knockout mice display abnormal cerebellar morphology and poor coordination and balance on motor testing.

Results

In order to evaluate a possible neurological involvement in WHIM syndrome subjects, we performed neurological examination, including International Cooperative Ataxia Rating Scale, cognitive and psychopathological assessment and brain Magnetic Resonance Imaging (MRI) in 6 WHIM patients (age range 8–51 years) with typical gain of functions mutations of CXCR4 (R334X or G336X). In three cases (P3, P5, P6) neurological evaluation revealed fine and global motor coordination disorders, balance disturbances, mild limb ataxia and excessive talkativeness. Brain MRI showed an abnormal orientation of the cerebellar folia involving bilaterally the gracilis and biventer lobules together with the tonsils in four subjects (P3, P4, P5, P6). The neuropsychiatric evaluation showed increased risk of internalizing and/or externalizing problems in four patients (P2, P3, P4, P6).

Conclusions

Taken together, these observations suggest CXCR4 gain of function mutations can be associated with cerebellar malformation, mild neuromotor and psychopathological dysfunction in WHIM patients.

Non-cell autonomous control of precerebellar neuron migration by Slit and Robo proteins

During development, precerebellar neurons migrate tangentially from the dorsal hindbrain to the floor plate. Their axons cross it but their cell bodies stop their ventral migration upon reaching the midline. It has previously been shown that Slit chemorepellents and their receptors, Robo1 and Robo2, might control the migration of precerebellar neurons in a repulsive manner. Here, we have used a conditional knockout strategy in mice to test this hypothesis. We show that the targeted inactivation of the expression of Robo1 and Robo2 receptors in precerebellar neurons does not perturb their migration and that they still stop at the midline. The selective ablation of the expression of all three Slit proteins in floor-plate cells has no effect on pontine neurons and only induces the migration of a small subset of inferior olivary neurons across the floor plate. Likewise, we show that the expression of Slit proteins in the facial nucleus is dispensable for pontine neuron migration. Together, these results show that Robo1 and Robo2 receptors act non-cell autonomously in migrating precerebellar neurons and that floor-plate signals, other than Slit proteins, must exist to prevent midline crossing.

Embryology

With the growing recognition of the extent and prevalence of human cerebellar disorders, an understanding of developmental programs that build the mature cerebellum is necessary. In this chapter we present an overview of the basic epochs and key molecular regulators of the developmental programs of cerebellar development. These include early patterning of the cerebellar territory, the genesis of cerebellar cells from multiple spatially distinct germinal zones, and the extensive migration and coordinated cellular rearrangements that result in the formation of the exquisitely foliated and laminated mature cerebellum. This knowledge base is founded on extensive analysis of animal models, particularly mice, due in large part to the ease of genetic manipulation of this important model organism. Since cerebellar structure and function are largely conserved across species, mouse cerebellar development is highly relevant to humans and has led to important insights into the developmental pathogenesis of human cerebellar disorders. Human fetal cerebellar development remains largely undescribed; however, several human-specific developmental features are known which are relevant to human disease and underline the importance of ongoing human fetal research.

Phenotypic outcomes in Mouse and Human Foxc1 dependent Dandy-Walker cerebellar malformation suggest shared mechanisms

FOXC1 loss contributes to Dandy-Walker malformation (DWM), a common human cerebellar malformation. Previously, we found that complete Foxc1 loss leads to aberrations in proliferation, neuronal differentiation and migration in the embryonic mouse cerebellum (Haldipur et al., 2014). We now demonstrate that hypomorphic Foxc1 mutant mice have granule and Purkinje cell abnormalities causing subsequent disruptions in postnatal cerebellar foliation and lamination. Particularly striking is the presence of a partially formed posterior lobule which echoes the posterior vermis DW 'tail sign' observed in human imaging studies. Lineage tracing experiments in Foxc1 mutant mouse cerebella indicate that aberrant migration of granule cell progenitors destined to form the posterior-most lobule causes this unique phenotype. Analyses of rare human del chr 6p25 fetal cerebella demonstrate extensive phenotypic overlap with our Foxc1 mutant mouse models, validating our DWM models and demonstrating that many key mechanisms controlling cerebellar development are likely conserved between mouse and human.

DOI:http://dx.doi.org/10.7554/eLife.20898.001

Insights into the mechanism of CXCL12-mediated signaling in trophoblast functions and placental angiogenesis

The chemokine CXCL12 and its receptor CXCR4 are important signaling components required for human blastocyst implantation and the progression of pregnancy. Growing evidence indicates that the CXCL12/CXCR4 axis can regulate trophoblast function and uterine spiral artery remodeling, which plays a fundamental role in placentation and fetal outcome. The orphan receptor CXCR7 is also believed to partly regulate the function of the CXCL12/CXCR4 axis. Additionally, the CXCL12/CXCR4/CXCR7 axis can enhance the cross-talk between trophoblasts and decidual cells such as uterine natural killer cells and decidual stromal cells which are involved in regulation of trophoblast differentiation and invasion and placental angiogenesis. In addition, recent studies proved that CXCL12 expression is elevated in the placenta and mid-trimester amniotic fluid of pregnant women with preeclampsia, implying that dysregulation of CXCL12 plays a role in the pathogenesis of preeclampsia. Further understanding of the regulatory mechanisms of CXCL12-mediated signaling in trophoblast functions and placental angiogenesis may help to design novel therapeutic approaches for pregnancy-associated diseases.

Are neural crest stem cells the missing link between hematopoietic and neurogenic niches?

Hematopoietic niches are defined as cellular and molecular microenvironments that regulate hematopoietic stem cell (HSC) function together with stem cell autonomous mechanisms. Many different cell types have been characterized as contributors to the formation of HSC niches, such as osteoblasts, endothelial cells, Schwann cells, and mesenchymal progenitors. These mesenchymal progenitors have themselves been classified as CXC chemokine ligand (CXCL) 12-abundant reticular (CAR) cells, stem cell factor expressing cells, or nestin-positive mesenchymal stem cells (MSCs), which have been recently identified as neural crest-derived cells (NCSCs). Together, these cells are spatially associated with HSCs and believed to provide appropriate microenvironments for HSC self-renewal, differentiation, mobilization and hibernation both by cell-cell contact and soluble factors. Interestingly, it appears that regulatory pathways governing the hematopoietic niche homeostasis are operating in the neurogenic niche as well. Therefore, this review paper aims to compare both the regulation of hematopoietic and neurogenic niches, in order to highlight the role of NCSCs and nervous system components in the development and the regulation of the hematopoietic system.

Development of the cerebellum: simple steps to make a 'little brain'

The cerebellum is a pre-eminent model for the study of neurogenesis and circuit assembly. Increasing interest in the cerebellum as a participant in higher cognitive processes and as a locus for a range of disorders and diseases make this simple yet elusive structure an important model in a number of fields. In recent years, our understanding of some of the more familiar aspects of cerebellar growth, such as its territorial allocation and the origin of its various cell types, has undergone major recalibration. Furthermore, owing to its stereotyped circuitry across a range of species, insights from a variety of species have contributed to an increasingly rich picture of how this system develops. Here, we review these recent advances and explore three distinct aspects of cerebellar development - allocation of the cerebellar anlage, the significance of transit amplification and the generation of neuronal diversity - each defined by distinct regulatory mechanisms and each with special significance for health and disease.

Foxc1 dependent mesenchymal signalling drives embryonic cerebellar growth

Loss of Foxc1 is associated with Dandy-Walker malformation, the most common human cerebellar malformation characterized by cerebellar hypoplasia and an enlarged posterior fossa and fourth ventricle. Although expressed in the mouse posterior fossa mesenchyme, loss of Foxc1 non-autonomously induces a rapid and devastating decrease in embryonic cerebellar ventricular zone radial glial proliferation and concurrent increase in cerebellar neuronal differentiation. Subsequent migration of cerebellar neurons is disrupted, associated with disordered radial glial morphology. In vitro, SDF1α, a direct Foxc1 target also expressed in the head mesenchyme, acts as a cerebellar radial glial mitogen and a chemoattractant for nascent Purkinje cells. Its receptor, Cxcr4, is expressed in cerebellar radial glial cells and conditional Cxcr4 ablation with Nes-Cre mimics the Foxc1-/- cerebellar phenotype. SDF1α also rescues the Foxc1-/- phenotype. Our data emphasizes that the head mesenchyme exerts a considerable influence on early embryonic brain development and its disruption contributes to neurodevelopmental disorders in humans.

Sema3E/PlexinD1 regulates the migration of hem-derived Cajal-Retzius cells in developing cerebral cortex

During the development of the cerebral cortex, Cajal-Retzius (CR) cells settle in the preplate and coordinate the precise growth of the neocortex. Indeed, CR cells migrate tangentially from specific proliferative regions of the telencephalon (for example, the cortical hem (CH)) to populate the entire cortical surface. This is a very finely tuned process regulated by an emerging number of factors that has been sequentially revealed in recent years. However, the putative participation of one of the major families of axon guidance molecules in this process, the Semaphorins, was not explored. Here we show that Semaphorin-3E (Sema3E) is a natural negative regulator of the migration of PlexinD1-positive CR cells originating in the CH. Our results also indicate that Sema3E/PlexinD1 signalling controls the motogenic potential of CR cells in vitro and in vivo. Indeed, absence of Sema3E/PlexinD1 signalling increased the migratory properties of CR cells. This modulation implies negative effects on CXCL12/CXCR4 signalling and increased ADF/Cofilin activity.

Characterization of a novel CC chemokine CCL4 in immune response induced by nitrite and its expression differences among three populations of Megalobrama amblycephala

A novel CC chemokine gene, chemokine CC motif ligand 4 (CCL4), was isolated from Megalobrama amblycephala. The full-length cDNA was 913 bp, encoding 94 amino acid residues. The deduced amino acid sequence possessed the typical arrangement of four cysteines as found in other known CC chemokines. The expression of M. amblycephala CCL4 during the early development showed the mRNA levels before hatching and at 62 h post fertilized (hpf) were significantly higher than other post-hatching stages (P < 0.05). Besides, it was widely expressed in all detected tissues with the highest transcription in liver, followed by intestine, spleen and gill, where a larger number of immune cells including lymphocytes and macrophages are present. Our findings had fully confirmed that CCL4 expression was strongly induced in vitro and quickly up-regulated after nitrite stress, then substantially altered in all tested tissues, supporting a potential pro-inflammatory function. We also indicated that inflammation effect might firstly happen in blood after nitrite stress. Furthermore, the tissue expression differences of CCL4 among three natural populations revealed that CCL4 mRNA in Yuni Lake population was obviously higher than the other two populations, Liangzi Lake population and Poyang Lake population, which will provide valuable insights into breeding strategies for selecting population with better immune property of M. amblycephala.

Emerging Targets in Pituitary Adenomas: Role of the CXCL12/CXCR4-R7 System

Chemokines are chemotactic regulators of immune surveillance in physiological and pathological conditions such as inflammation, infection, and cancer. Several chemokines and cognate receptors are constitutively expressed in the central nervous system, not only in glial and endothelial cells but also in neurons, controlling neurogenesis, neurite outgrowth, and axonal guidance during development. In particular, the chemokine CXCL12 and its receptors, CXCR4 and CXCR7, form a functional network that controls plasticity in different brain areas, influencing neurotransmission, neuromodulation, and cell migration, and the dysregulation of this chemokinergic axis is involved in several neurodegenerative, neuroinflammatory, and malignant diseases. CXCR4 primarily mediates the transduction of proliferative signals, while CXCR7 seems to be mainly responsible for scavenging CXCL12. Importantly, the multiple intracellular signalling generated by CXCL12 interaction with its receptors influences hypothalamic modulation of neuroendocrine functions, although a direct modulation of pituitary functioning via autocrine/paracrine mechanisms was also reported. Both CXCL12 and CXCR4 are constitutively overexpressed in pituitary adenomas and their signalling induces cell survival and proliferation, as well as hormonal hypersecretion. In this review we focus on the physiological and pathological functions of immune-related cyto- and chemokines, mainly focusing on the CXCL12/CXCR4-7 axis, and their role in pituitary tumorigenesis. Accordingly, we discuss the potential targeting of CXCR4 as novel pharmacological approach for pituitary adenomas.

Sequence analysis and expression differentiation of chemokine receptor CXCR4b among three populations of Megalobrama amblycephala

Chemokine (C-X-C motif), receptor 4 (CXCR4), a member of the family of seven transmembrane G-protein-coupled receptors, plays important roles in immunomodulation, organogenesis, hematopoiesis, and derailed cerebellar neuron migration. We characterized the sequences and expression profiles of CXCR4b in Megalobrama amblycephala. The full-length cDNA was 1638bp, encoding 353 amino acid residues. Multiple alignment and phylogenetic analysis indicated that M. amblycephala CXCR4b contained the similar conservative sequences and motifs with other organisms. The CXCR4b expression in different development stages of M. amblycephala showed the mRNA levels before hatching and at 62h post fertilization (hpf) were significantly higher than at other post hatching stages (P<0.05). Besides, CXCR4b was constitutively expressed in a wide range of tissues, at higher levels in headkidney, liver, intestine spleen, blood and gill, where a larger number of immune cells including lymphocytes and macrophages reside, suggesting its specific roles in inflammatory responses. The CXCR4b expression after high nitrite concentration (ρNO(2-)-N: 20.29mg/L) exposure supported a potential pro-inflammatory function for CXCR4b. In order to identify the better population with immune property for breeding, we compared the tissue expression of CXCR4b among Liangzi Lake population (L), Yuni Lake population (Y) and Poyang Lake population (P), it was indicated that the expression levels in the population Y were obviously higher than that of the other two populations.

Chemokines induce axon outgrowth downstream of Hepatocyte Growth Factor and TCF/β-catenin signaling

Axon morphogenesis is a complex process regulated by a variety of secreted molecules, including morphogens and growth factors, resulting in the establishment of the neuronal circuitry. Our previous work demonstrated that growth factors [Neurotrophins (NT) and Hepatocyte Growth Factor (HGF)] signal through β-catenin during axon morphogenesis. HGF signaling promotes axon outgrowth and branching by inducing β-catenin phosphorylation at Y142 and transcriptional regulation of T-Cell Factor (TCF) target genes. Here, we asked which genes are regulated by HGF signaling during axon morphogenesis. An array screening indicated that HGF signaling elevates the expression of chemokines of the CC and CXC families. In line with this, CCL7, CCL20, and CXCL2 significantly increase axon outgrowth in hippocampal neurons. Experiments using blocking antibodies and chemokine receptor antagonists demonstrate that chemokines act downstream of HGF signaling during axon morphogenesis. In addition, qPCR data demonstrates that CXCL2 and CCL5 expression is stimulated by HGF through Met/b-catenin/TCF pathway. These results identify CC family members and CXCL2 chemokines as novel regulators of axon morphogenesis downstream of HGF signaling.

Growth factor receptor-Src-mediated suppression of GRK6 dysregulates CXCR4 signaling and promotes medulloblastoma migration

BACKGROUND

Metastasis in medulloblastoma (MB) is associated with poor survival. Recent genetic studies revealed MB to comprise distinct molecular subgroups, including the sonic hedgehog (SHH) subgroup that exhibits a relatively high rate of progression. To identify targeted therapeutics against metastasis, a better understanding of the regulation of MB cell migration is needed. G protein-coupled receptor kinases (GRKs) have been implicated in cancer metastasis through their regulation of G-protein coupled receptors (GPCRs) involved in growth factor (GF)-mediated cell migration. However, the specific roles and regulation of GRKs in MB have not been investigated.

METHODS

Microarray mRNA analysis was performed for GRKs, GPCRs, and GFs in 29 human MB, and real time RT-PCR was used to detect GRK6 expression in MB cells. Lenti- or retro-virus infection, and siRNA or shRNA transfection, of MB cells was used to overexpress and knockdown target genes, respectively. Western blot was used to confirm altered expression of proteins. The effect of altered target protein on cell migration was determined by Boyden chamber assay and xCELLigence migration assays.

RESULTS

We observed co-overexpression of PDGFRA, CXCR4, and CXCL12 in the SHH MB subtype compared to non-SHH MB (5, 7, and 5-fold higher, respectively). GRK6, which typically acts as a negative regulator of CXCR4 signaling, is downregulated in MB, relative to other GRKs, while the percentage of GRK6 expression is lower in MB tumors with metastasis (22%), compared to those without metastasis (43%). In SHH-responsive MB cells, functional blockade of PDGFR abolished CXCR4-mediated signaling. shPDGFR transfected MB cells demonstrated increased GRK6 expression, while PDGF or 10% FBS treatment of native MB cells reduced the stability of GRK6 by inducing its proteosomal degradation. Overexpression or downregulation of Src, a key mediator of GF receptor/PDGFR signaling, similarly inhibited or induced GRK6 expression, respectively. siRNA downregulation of GRK6 enhanced CXCR4 signaling and promoted MB migration, while lentiviral-GRK6 overexpression suppressed CXCR4 signaling, potentiated the effect of AMD3100, a CXCR4 antagonist, and impaired migration.

CONCLUSIONS

Our findings demonstrate a novel mechanism of GF receptor/PDGFR-Src-mediated dysregulation of CXCR4 signaling that promotes MB cell migration, which could potentially be exploited for therapeutic targeting in SHH MB.

Regulation of a vascular plexus by gata4 is mediated in zebrafish through the chemokine sdf1a

Using the zebrafish model we describe a previously unrecognized requirement for the transcription factor gata4 controlling embryonic angiogenesis. The development of a vascular plexus in the embryonic tail, the caudal hematopoietic tissue (CHT), fails in embryos depleted of gata4. Rather than forming a normal vascular plexus, the CHT of gata4 morphants remains fused, and cells in the CHT express high levels of osteogenic markers ssp1 and runx1. Definitive progenitors emerge from the hemogenic aortic endothelium, but fail to colonize the poorly vascularized CHT. We also found abnormal patterns and levels for the chemokine sdf1a in gata4 morphants, which was found to be functionally relevant, since the embryos also show defects in development of the lateral line, a mechano-sensory organ system highly dependent on a gradient of sdf1a levels. Reduction of sdf1a levels was sufficient to rescue lateral line development, circulation, and CHT morphology. The result was surprising since neither gata4 nor sdf1a is obviously expressed in the CHT. Therefore, we generated transgenic fish that conditionally express a dominant-negative gata4 isoform, and determined that gata4 function is required during gastrulation, when it is co-expressed with sdf1a in lateral mesoderm. Our study shows that the gata4 gene regulates sdf1a levels during early embryogenesis, which impacts embryonic patterning and subsequently the development of the caudal vascular plexus.

CXCL12 signaling in the development of the nervous system

Chemokines are small, secreted proteins that have been shown to be important regulators of leukocyte trafficking and inflammation. All the known effects of chemokines are transduced by action at a family of G protein coupled receptors. Two of these receptors, CCR5 and CXCR4, are also known to be the major cellular receptors for HIV-1. Consideration of the evolution of the chemokine family has demonstrated that the chemokine Stromal cell Derived Factor-1 or SDF1 (CXCL12) and its receptor CXCR4 are the most ancient members of the family and existed in animals prior to the development of a sophisticated immune system. Thus, it appears that the original function of chemokine signaling was in the regulation of stem cell trafficking and development. CXCR4 signaling is important in the development of many tissues including the nervous system. Here we discuss the manner in which CXCR4 signaling can regulate the development of different structures in the central and peripheral nervous systems and the different strategies employed to achieve these effects.

Should I stay or should I go? Becoming a granule cell

Cerebellar granule cells undergo profound and rapid morphological modifications during development while they migrate from their birthplace at the surface of the cerebellar cortex to its deepest layer. Post-mitotic granule cells extend bipolar axons and sequentially use the two main modes of migration, tangential and radial, to reach their final destinations. Recent studies show that protein degradation involving key cell-cycle regulators controls granule cell axon extension. The use of knockout mice deficient in different axon-guidance molecules combined with cutting-edge imaging methods has started to shed light on the molecular mechanisms that trigger granule cell migration. These studies suggest that a major reorganization of the cytoskeleton occurs as granule cells switch from tangential to radial migration.

Chemokine-like factor 1, a novel cytokine, induces nerve cell migration through the non-extracellular Ca2+-dependent tyrosine kinases pathway

Chemokine-like factor 1 (CKLF1) is a newly cloned chemotactic cytokine. The roles of CKLF1 in the immune system and the respiratory system have been reported, but its function in the nervous system is still remaining unclear. We aimed to investigate the role of CKLF1 in the nerve cell migration and its regulatory mechanisms. By chemotaxis assays and wound-healing assays, CKLF1 stimulated the migration of SH-SY5Y cells dose-dependently. By immunofluorescence staining, CKLF1 induced actin polymerization. By western blotting, proline-rich tyrosine kinase 2 (PYK2) was phosphorylated at Tyr-402 in response to CKLF1 and this phosphorylation was apparently suppressed by phospholipase C-gamma inhibitor U73122, but not extracellular Ca(2+) chelator EGTA. Furthermore, after transfection of dominant-negative mutant PYK2 plasmid, the chemotaxis upon CKLF1 was significantly attenuated in SH-SY5Y cells. Concluding, CKLF1 stimulates the migration of SH-SY5Y cells dose-dependently by activating non-extracellular Ca(2+)-dependent tyrosine kinases pathway and inducing actin polymerization.

CXCR4 and CXCR7 cooperate during tangential migration of facial motoneurons

Migration of facial motoneurons in the zebrafish hindbrain depends on SDF1/CXCL12 signaling. Recent studies demonstrated that SDF1 can bind two chemokine receptors, CXCR4 and CXCR7. Here we explore the expression and function of the cxcr7b gene in zebrafish hindbrain development. By the time cxcr4b-expressing motoneurons migrate from rhombomere (r) r4 to r6, expression of cxcr7b is rapidly restricted to the ventral part of r5. Inactivation of either cxcr7b or cxcr4b impairs motoneuron migration, with however different phenotypes. Facial motoneurons preferentially accumulate in r5 in cxcr7b morphant embryos, while they are distributed between r4, r5 and r6 in cxcr4b morphants. Simultaneous inactivation of both receptors leads to yet a third phenotype, with motoneurons mostly distributed between r4 and r5. The latter phenotype resembles that of sdf1a morphant embryos. Double inactivation of sdf1a and cxcr7b indeed did not lead to a complete arrest of migration but rather to a partial rescue of r5 arrest of motoneuron migration. This result is in accordance with the functional hypothesis that SDF1 might interact with CXCR7 and that they have an antagonistic effect within r5. The ectopic expression of a truncated CXCR7 receptor leads to a motoneuron migration defect. Altogether, we show that CXCR7 is required, for proper tangential migration of facial motoneurons, by determining a permissive migration pathway through r5.

SDF1/CXCR4 signalling regulates two distinct processes of precerebellar neuronal migration and its depletion leads to abnormal pontine nuclei formation

The development of mossy-fibre projecting precerebellar neurons (PCN)presents a classical example of tangential neuronal migration. PCN migrate tangentially along marginal streams beneath the pial surface from the lower rhombic lip to specific locations in the hindbrain, where they form precerebellar nuclei. Among them, the pontine neurons follow a stereotypic anteroventral-directed pathway to form the pontine nuclei in the pons. The guidance mechanisms that determine the marginal migration of PCN and the anterior migration of pontine neurons are poorly understood. Here, we report that a chemokine SDF1 (also known as CXCL12) derived from the meningeal tissue regulates the migratory pathways of PCN. PCN are chemoattracted by the meningeal tissue, an effect that is mimicked by an SDF1 source. Analysis of knockout mice for the Sdf1 receptor Cxcr4 shows that both the marginal migration of PCN and the anterior migration of pontine neurons are disrupted. We provide further evidence that SDF1/CXCR4 signalling regulates these two processes cell-autonomously. As a result of disrupted neuronal migration, pontine nuclei formation was highly abnormal, with the presence of multiple ectopic pontine clusters posteriorly. The ectopic pontine clusters led to ectopic collateral branch formation from the corticospinal tract. Our results together demonstrate crucial roles for SDF1/CXCR4 in multiple aspects of PCN migration and highlight the deleterious consequence of derailed migration on proper nuclei formation. Furthermore, we provide the first in vivo evidence that pontine neurons themselves induce collateral branching from the corticospinal axons.

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.

Implantation of olfactory ensheathing cells promotes neuroplasticity in murine models of stroke

Murine olfactory ensheathing cells (OECs) promote central nervous system axonal regeneration in models of spinal cord injury. We investigated whether OECs could induce a neuroplastic effect to improve the neurological dysfunction caused by hypoxic/ischemic stress. In this study, human OECs/olfactory nerve fibroblasts (hOECs/ONFs) specifically secreted trophic factors including stromal cell-derived factor-1alpha (SDF-1alpha). Rats with intracerebral hOEC/ONF implantation showed more improvement on behavioral measures of neurological deficit following stroke than control rats. [18F]fluoro-2-deoxyglucose PET (FDG-PET) showed increased glucose metabolic activity in the hOEC/ONF-treated group compared with controls. In mice, transplanted hOECs/ONFs and endogenous homing stem cells including intrinsic neural progenitor cells and bone marrow stem cells colocalized with specific neural and vascular markers, indicating stem cell fusion. Both hOECs/ONFs and endogenous homing stem cells enhanced neuroplasticity in the rat and mouse ischemic brain. Upregulation of SDF-1alpha and CXCR4 in hOECs/ONFs promoted neurite outgrowth of cocultured primary cortical neurons under oxygen glucose deprivation conditions and in stroke animals through upregulation of cellular prion protein (PrP C) expression. Therefore, the upregulation of SDF-1alpha and the enhancement of CXCR4 and PrP C interaction induced by hOEC/ONF implantation mediated neuroplastic signals in response to hypoxia and ischemia.

Regulation of axonal elongation and pathfinding from the entorhinal cortex to the dentate gyrus in the hippocampus by the chemokine stromal cell-derived factor 1 alpha

During the early developmental stage, a neural circuit is established between the entorhinal cortex (EC) and the hippocampal dentate gyrus (DG) via the perforant pathway. However, the manner in which the perforant fibers are navigated has mostly remained a mystery. Here, we analyzed the functional role of a chemokine, namely, stromal cell-derived factor 1alpha (SDF-1alpha), in the navigation of the perforant fibers. SDF-1alpha was observed to promote neurite growth, which is dependent on mDia1, in cultured entorhinal cortical neurons obtained from rats at postnatal day 0. We then used entorhino-hippocampal cocultures comprising green fluorescence-labeled EC and DG slices to assess the projection of the perforant fibers from the EC. Although the specific laminar termination of the entorhinal axons was observed with this system, the number of appropriately terminating entorhinal axons decreased significantly when the SDF-1alpha signaling pathway was blocked by a neutralizing antibody against SDF-1alpha or by the specific SDF-1alpha receptor antagonist AMD3100 (1,1'-[1,4-phenylenebis(methylene)]bis-1,4,8,11-tetra-azacyclotetradecane octahydrochloride). Furthermore, inhibition of the SDF-1alpha signaling pathway resulted in a decrease in the immunoreactivity for PSD-95 (postsynaptic density protein-95) in the DG, possibly because of a reduction in the number of projecting perforant fibers. These results demonstrate that SDF-1alpha plays a critical role in promoting the growth of perforant fibers from the EC to the DG.

Besides Purkinje cells and granule neurons: an appraisal of the cell biology of the interneurons of the cerebellar cortex

Ever since the groundbreaking work of Ramon y Cajal, the cerebellar cortex has been recognized as one of the most regularly structured and wired parts of the brain formed by a rather limited set of distinct cells. Its rather protracted course of development, which persists well into postnatal life, the availability of multiple natural mutants, and, more recently, the availability of distinct molecular genetic tools to identify and manipulate discrete cell types have suggested the cerebellar cortex as an excellent model to understand the formation and working of the central nervous system. However, the formulation of a unifying model of cerebellar function has so far proven to be a most cantankerous problem, not least because our understanding of the internal cerebellar cortical circuitry is clearly spotty. Recent research has highlighted the fact that cerebellar cortical interneurons are a quite more diverse and heterogeneous class of cells than generally appreciated, and have provided novel insights into the mechanisms that underpin the development and histogenetic integration of these cells. Here, we provide a short overview of cerebellar cortical interneuron diversity, and we summarize some recent results that are hoped to provide a primer on current understanding of cerebellar biology.

Functional response to SDF1 alpha through over-expression of CXCR4 on adult subventricular zone progenitor cells

The chemokine receptor CXCR4 and its ligand, stromal cell derived factor-1 alpha (SDF1 alpha) regulate neuroblast migration towards the ischemic boundary after stroke. Using loss- and gain-function, we investigated the biological effect of CXCR4/SDF1 alpha on neural progenitor cells. Neural progenitor cells, from the subventricular zone (SVZ) of the adult rat, were transfected with rat CXCR4-pLEGFP-C1 and pSIREN-RetroQ-CXCR4-siRNA retroviral vectors. Migration assay analysis showed that inhibition of CXCR4 by siRNA significantly reduced cell migration compared to the empty vector, indicating that CXCR4 mediated neural progenitor cell motility. When neural progenitor cells were cultured in growth medium containing bFGF (20 ng/ml), over-expression of CXCR4 significantly reduced the cell proliferation as measured by the number of bromodeoxyuridine+ (BrdU+) cells (26.4%) compared with the number in the control group (54.0%). Addition of a high concentration of SDF1 alpha (500 ng/ml) into the progenitor cells with over-expression of CXCR4 reversed the cell proliferation back to the control levels (57.6%). Immunostaining analysis showed that neither over-expression nor inhibition of CXCR4 altered the population of neurons and astrocytes, when neural progenitor cells were cultured in differentiation medium. These in vitro results suggest that CXCR4/SDF1 alpha primarily regulates adult neural progenitor cell motility but not differentiation, while over-expression of CXCR4 in the absence of SDF1 alpha decreases neural progenitor cell proliferation.

Hox paralog group 2 genes control the migration of mouse pontine neurons through slit-robo signaling

The pontine neurons (PN) represent a major source of mossy fiber projections to the cerebellum. During mouse hindbrain development, PN migrate tangentially and sequentially along both the anteroposterior (AP) and dorsoventral (DV) axes. Unlike DV migration, which is controlled by the Netrin-1/Dcc attractive pathway, little is known about the molecular mechanisms guiding PN migration along the AP axis. Here, we show that Hoxa2 and Hoxb2 are required both intrinsically and extrinsically to maintain normal AP migration of subsets of PN, by preventing their premature ventral attraction towards the midline. Moreover, the migration defects observed in Hoxa2 and Hoxb2 mutant mice were phenocopied in compound Robo1;Robo2, Slit1;Slit2, and Robo2;Slit2 knockout animals, indicating that these guidance molecules act downstream of Hox genes to control PN migration. Indeed, using chromatin immunoprecipitation assays, we further demonstrated that Robo2 is a direct target of Hoxa2 in vivo and that maintenance of high Robo and Slit expression levels was impaired in Hoxa2 mutant mice. Lastly, the analysis of Phox2b-deficient mice indicated that the facial motor nucleus is a major Slit signaling source required to prevent premature ventral migration of PN. These findings provide novel insights into the molecular control of neuronal migration from transcription factor to regulation of guidance receptor and ligand expression. Specifically, they address the question of how exposure to multiple guidance cues along the AP and DV axes is regulated at the transcriptional level and in turn translated into stereotyped migratory responses during tangential migration of neurons in the developing mammalian brain.

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.

Gating of Sema3E/PlexinD1 signaling by neuropilin-1 switches axonal repulsion to attraction during brain development

The establishment of functional neural circuits requires the guidance of axons in response to the actions of secreted and cell-surface molecules such as the semaphorins. Semaphorin 3E and its receptor PlexinD1 are expressed in the brain, but their functions are unknown. Here, we show that Sema3E/PlexinD1 signaling plays an important role in initial development of descending axon tracts in the forebrain. Early errors in axonal projections are reflected in behavioral deficits in Sema3E null mutant mice. Two distinct signaling mechanisms can be distinguished downstream of Sema3E. On corticofugal and striatonigral neurons expressing PlexinD1 but not Neuropilin-1, Sema3E acts as a repellent. In contrast, on subiculo-mammillary neurons coexpressing PlexinD1 and Neuropilin-1, Sema3E acts as an attractant. The extracellular domain of Neuropilin-1 is sufficient to convert repulsive signaling by PlexinD1 to attraction. Our data therefore reveal a "gating" function of neuropilins in semaphorin-plexin signaling during the assembly of forebrain neuronal circuits.

Signals on the Move: Chemokine Receptors and Organogenesis in Zebrafish

The chemokine SDF1 (stromal cell-derived factor 1) directs cell migration in many different contexts, ranging from embryogenesis to inflammation. SDF1a is the guidance cue for the zebrafish lateral line primordium, a tissue that moves along the flank of the embryo and deposits cells that form mechanosensory organs. The SDF1a receptor CXCR4b acts in cells at the leading edge of the primordium to direct its migration. Two new studies show that a second SDF1 receptor, CXCR7, is required only in the trailing cells of the primordium, and they explore how these two receptors orchestrate migration of the primordium. CXCR4b and CXCR7 are expressed in complementary domains, possibly through mutual repression in which each receptor inhibits expression of the other. These studies illustrate how the entire primordium can respond to a single signal, yet generate cell type-specific responses by using different receptors.

NSCL-1 and -2 control the formation of precerebellar nuclei by orchestrating the migration of neuronal precursor cells

During embryonic development post-mitotic neurons of the precerebellar neuroepithelium, migrate from the rhombic lip to the ventral part of the neural tube to form the precerebellar nuclei of the pons and medulla oblongata. In this study, we show that neural basic helix-loop-helix transcription factors NSCL-1 and -2 are expressed in all cells of the anterior extramural migration stream (aes), which forms the precerebellar nuclei. The combined inactivation of NSCL-1 and -2 led to a complete absence of the pontine nuclei and a strong reduction in the reticulotegmental nuclei. We demonstrate that NSCL-1/2 were required for a sustained expression of the netrin receptor and cell guidance molecule Dcc in the aes. Unc5H3, a second netrin receptor, which serves as a stop signal for migratory cells was up-regulated in NSCL-1/2 deficient cells, which ceased migration and accumulated ectopically. Furthermore, we observed a massive increase of apoptosis in cells of the aes in the absence of NSCL-1/2, which together with the arrest of migration might explain the virtually complete loss of aes-derived structures in NSCL-1/2 mutant mice. We conclude that NSCL-1/2 maintain migration and survival of cells in the aes.

Plexin-B2 controls the development of cerebellar granule cells

Cerebellar granule cell progenitors proliferate postnatally in the upper part of the external granule cell layer (EGL) of the cerebellum. Postmitotic granule cells differentiate and migrate, tangentially in the EGL and then radially through the molecular and Purkinje cell layers. The molecular control of the transition between proliferation and differentiation in cerebellar granule cells is poorly understood. We show here that the transmembrane receptor Plexin-B2 is expressed by proliferating granule cell progenitors. To study Plexin-B2 function, we generated a targeted mutation of mouse Plexin-B2. Most Plexin-B2(-/-) mutants die at birth as a result of neural tube closure defects. Some mutants survive but their cerebellum cytoarchitecture is profoundly altered. This is correlated with a disorganization of the timing of granule cell proliferation and differentiation in the EGL. Many differentiated granule cells migrate inside the cerebellum and keep proliferating. These results reveal that Plexin-B2 controls the balance between proliferation and differentiation in granule cells.

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.

Expression of classical cadherins in the cerebellar anlage: quantitative and functional aspects

During central nervous system (CNS) development, cell migration precedes and is key to the integration of diverse sets of cells. Mechanistically, CNS histogenesis is realized through a balanced interplay of cell-cell and cell-matrix adhesion molecules. Here, we summarize experiments that probe the developmental expression and potential significance of a set of cadherins, including M-, N- and R-cadherin, for patterning of the cerebellar cortex. We established a transgenic marker that allows cerebellar granule cells to be followed from the neuroblast stage to their final, postmitotic settlement. In conjunction with flow cytometry, this allowed us to derive a quantitative view of cadherin expression in differentiating granule cells and relate it to the expression of the same cadherins in cerebellar inhibitory interneuronal precursors. In vitro reaggregation analysis supports a role for cadherins in cell sorting and migration within the nascent cerebellar cortex that may be rationalized within the context of the differential adhesion hypothesis (Foty, R.A. and Steinberg, M.S., 2005. The differential adhesion hypothesis: a direct evaluation. Dev. Biol. 278, 255-263.).

Absence of the basilar pons in mice lacking a functional Large glycosyltransferase gene suggests a defect in pontine neuron migration

Several forms of congenital muscular dystrophy result from mutations in glycosyltransferases that modify alpha-dystroglycan. As pontine hypoplasia has been reported in some clinical cases of congenital muscular dystrophy, we have begun to examine whether these glycosyltransferases are required for the normal development of the basilar pons, one of several precerebellar nuclei of the hindbrain. In veils (Large(vls)) mice, which carry a loss-of-function mutation in the Large glycosyltransferase gene, the basilar pons is absent. Instead, ectopic clusters of pontine neurons are found lateral to their normal site, suggesting that these neurons are unable to migrate to their appropriate site. Two other precerebellar nuclei, the lateral reticular nucleus and the inferior olive, are present in Large(vls) mice. In addition, the basilar pons forms normally in dystrophin-deficient mice. These results demonstrate that the Large glycosyltransferase but not dystrophin is required for normal basilar pontine development.

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.