Development and reorganization of corticospinal projections in EphA4 deficient mice

Ephedrine and cocaine cause developmental neurotoxicity and abnormal behavior in zebrafish

Ephedrine (EPH) and cocaine (COC) are illegal stimulant drugs, and have been frequently detected in aquatic environments. EPH and COC have negative effects on the nervous system and cause abnormal behaviors in mammals and fish at high concentrations, but their mechanisms of neurotoxicity remain unclear in larvae fish at low concentrations. To address this issue, zebrafish embryos were exposed to EPH and COC for 14 days post-fertilization (dpf) at 10, 100, and 1000 ng L-1. The bioaccumulation, development, behavior, cell neurotransmitter levels and apoptosis were detected to investigate the developmental neurotoxicity (DNT) of EPH and COC. The results showed that EPH decreased heart rate, while COC increased heart rate. EPH caused cell apoptosis in the brain by AO staining. In addition, behavior analysis indicated that EPH and COC affected spontaneous movement, touch-response, swimming activity and anxiety-like behaviors. EPH and COC altered the levels of the neurotransmitters dopamine (DA) and γ-aminobutyric acid (GABA) with changes of the transcription of genes related to the DA and GABA pathways. These findings indicated that EPH and COC had noticeable DNT in the early stage of zebrafish at environmentally relevant concentrations.

Activation of multiple Eph receptors on neuronal membranes correlates with the onset of optic neuropathy

BACKGROUND

Optic neuropathy is a major cause of irreversible blindness, yet the molecular determinants that contribute to neuronal demise have not been fully elucidated. Several studies have identified 'ephrin signaling' as one of the most dysregulated pathways in the early pathophysiology of optic neuropathy with varied etiologies. Developmentally, gradients in ephrin signaling coordinate retinotopic mapping via repulsive modulation of cytoskeletal dynamics in neuronal membranes. Little is known about the role ephrin signaling plays in the post-natal visual system and its correlation with the onset of optic neuropathy.

METHODS

Postnatal mouse retinas were collected for mass spectrometry analysis for erythropoietin-producing human hepatocellular (Eph) receptors. Optic nerve crush (ONC) model was employed to induce optic neuropathy, and proteomic changes during the acute phase of neuropathic onset were analyzed. Confocal and super-resolution microscopy determined the cellular localization of activated Eph receptors after ONC injury. Eph receptor inhibitors assessed the neuroprotective effect of ephrin signaling modulation.

RESULTS

Mass spectrometry revealed expression of seven Eph receptors (EphA2, A4, A5, B1, B2, B3, and B6) in postnatal mouse retinal tissue. Immunoblotting analysis indicated a significant increase in phosphorylation of these Eph receptors 48 h after ONC. Confocal microscopy demonstrated the presence of both subclasses of Eph receptors within the retina. Stochastic optical reconstruction microscopy (STORM) super-resolution imaging combined with optimal transport colocalization analysis revealed a significant co-localization of activated Eph receptors with injured neuronal cells, compared to uninjured neuronal and/or injured glial cells, 48 h post-ONC. Eph receptor inhibitors displayed notable neuroprotective effects for retinal ganglion cells (RGCs) after six days of ONC injury.

CONCLUSIONS

Our findings demonstrate the functional presence of diverse Eph receptors in the postnatal mammalian retina, capable of modulating multiple biological processes. Pan-Eph receptor activation contributes to the onset of neuropathy in optic neuropathies, with preferential activation of Eph receptors on neuronal processes in the inner retina following optic nerve injury. Notably, Eph receptor activation precedes neuronal loss. We observed a neuroprotective effect on RGCs upon inhibiting Eph receptors. Our study highlights the importance of investigating this repulsive pathway in early optic neuropathies and provides a comprehensive characterization of the receptors present in the developed retina of mice, relevant to both homeostasis and disease processes.

Activation of Multiple Eph Receptors on Neuronal Membranes Correlates with The Onset of Traumatic Optic Neuropathy

Background

Optic neuropathy (ON) is a major cause of irreversible blindness, yet the molecular determinants that contribute to neuronal demise have not been fully elucidated. Several studies have identified 'ephrin signaling' as one of the most dysregulated pathways in the early pathophysiology of ON with varied etiologies. Developmentally, gradients in ephrin signaling coordinate retinotopic mapping via repulsive modulation of cytoskeletal dynamics in neuronal membranes. Little is known about the role ephrin signaling played in the post-natal visual system and its correlation with the onset of optic neuropathy.

Methods

Postnatal mouse retinas were collected for mass spectrometry analysis for Eph receptors. Optic nerve crush (ONC) model was employed to induce optic neuropathy, and proteomic changes during the acute phase of neuropathic onset were analyzed. Confocal and super-resolution microscopy determined the cellular localization of activated Eph receptors after ONC injury. Eph receptor inhibitors assessed the neuroprotective effect of ephrin signaling modulation.

Results

Mass spectrometry revealed expression of seven Eph receptors (EphA2, A4, A5, B1, B2, B3, and B6) in postnatal mouse retinal tissue. Immunoblotting analysis indicated a significant increase in phosphorylation of these Eph receptors 48 hours after ONC. Confocal microscopy demonstrated the presence of both subclasses of Eph receptors in the inner retinal layers. STORM super-resolution imaging combined with optimal transport colocalization analysis revealed a significant co-localization of activated Eph receptors with injured neuronal processes, compared to uninjured neuronal and/or injured glial cells, 48 hours post-ONC. Eph receptor inhibitors displayed notable neuroprotective effects after 6 days of ONC injury.

Conclusions

Our findings demonstrate the functional presence of diverse Eph receptors in the postnatal mammalian retina, capable of modulating multiple biological processes. Pan-Eph receptor activation contributes to the onset of neuropathy in ONs, with preferential activation of Eph receptors on neuronal processes in the inner retina following optic nerve injury. Notably, Eph receptor activation precedes neuronal loss. We observed neuroprotective effects upon inhibiting Eph receptors. Our study highlights the importance of investigating this repulsive pathway in early optic neuropathies and provides a comprehensive characterization of the receptors present in the developed retina of mice, relevant to both homeostasis and disease processes.

Mechanistic and therapeutic implications of EphA-4 receptor tyrosine kinase in the pathogenesis of Alzheimer's disease

Erythropoietin-producing hepatoma (Eph) receptors belong to a family of tyrosine kinase receptors that plays a pivotal role in the development of the brain. Eph can be divided broadly into two groups, namely, EphA and EphB, comprising nine and five members, respectively. In recent years, the role of EphA-4 has become increasingly apparent in the onset of Alzheimer's disease (AD). Emerging evidence suggests that EphA-4 results in synaptic dysfunction, which in turn promotes the progression of AD. Moreover, pharmacological or genetic ablation of EphA-4 in the murine model of AD can alleviate the symptoms. The current review summarizes different pathways by which EphA-4 can influence pathogenesis. Since, majority of the studies had reported the protective effect of EphA-4 inhibition during AD, designing therapeutics based on decreasing its enzymatic activity might be necessary for introducing the novel interventions. Therefore, the review described peptide and nanobodies inhibitors of EphA-4 that exhibit the potential to modulate EphA-4 and could be used as lead molecules for the targeted therapy of AD.

Imidacloprid and thiamethoxam affect synaptic transmission in zebrafish

Imidacloprid (IMI) and thiamethoxam (THM) are two commonly applied neonicotinoid insecticides. IMI and THM could cause negative impacts on non-target organisms like bees. However, the information about neurotoxicity of IMI and THM in fish is still scarce. Here we investigated the effects of IMI and THM on locomotor behavior, AChE activity, and transcription of genes related to synaptic transmission in zebrafish exposed to IMI and THM with concentrations of 50 ng L^-1 to 50,000 ng L^-1 at 14 day post fertilization (dpf), 21 dpf, 28 dpf and 35 dpf. Our results showed that IMI and THM significantly influenced the locomotor activity in larvae at 28 dpf and 35 dpf. THM elevated AChE activity at 28 dpf. The qPCR data revealed that IMI and THM affected the transcription of marker genes belonging to the synapse from 14 dpf to 35 dpf. Furthermore, IMI and THM mainly affected transcription of key genes in γ-aminobutyric acid, dopamine and serotonin pathways in larvae at 28 dpf and 35 dpf. These results demonstrated the neurotoxicity of IMI and THM in zebrafish. The findings from this study suggested that IMI and THM in the aquatic environment may pose potential risks to fish fitness and survival.

Remarkable complexity and variability of corticospinal tract defects in adult Semaphorin 6A knockout mice

The corticospinal tract (CST) has a complex and long trajectory that originates in the cerebral cortex and ends in the spinal cord. Semaphorin 6A (Sema6A), a member of the semaphorin family, is an important regulator of CST axon guidance. Previous studies have shown that postnatal Sema6A mutant mice have CST defects at the midbrain-hindbrain boundary and medulla. However, the routes the aberrant fibers take throughout the Sema6A mutant brain remain unknown. In this study, we performed 3D reconstruction of immunostained CST fibers to reevaluate the details of the abnormal CST trajectories in the brains of adult Sema6A mutant mice. Our results showed that the axon guidance defects reported in early postnatal mutants were consistently observed in adulthood. Those abnormal trajectories revealed by 3D analysis of brain sections were, however, more complex and variable than previously thought. In addition, 3D analysis allowed us to identify a few new patterns of aberrant projections. First, a subset of fibers that separated from and descended in parallel to the main bundle projected laterally at the caudal pons, subsequently changed direction by turning caudally, and extended to the medulla. Second, some abnormal fibers returned to the correct trajectory after deviating substantially from the original tract. Third, some fibers reached the pyramidal decussation normally but did not enter the dorsal funiculus. Section immunostaining combined with 3D reconstruction is a powerful method to track long projection fibers and to examine the entire nerve tracts of both normal and abnormal animals.

The corticospinal tract: Evolution, development, and human disorders

The corticospinal tract (CST) plays a major role in cortical control of spinal cord activity. In particular, it is the principal motor pathway for voluntary movements. Here, we discuss: (i) the anatomic evolution and development of the CST across mammalian species, focusing on its role in motor functions; (ii) the molecular mechanisms regulating corticospinal tract formation and guidance during mouse development; and (iii) human disorders associated with abnormal CST development. A comparison of CST anatomy and development across mammalian species first highlights important similarities. In particular, most CST axons cross the anatomical midline at the junction between the brainstem and spinal cord, forming the pyramidal decussation. Reorganization of the pattern of CST projections to the spinal cord during evolution led to improved motor skills. Studies of the molecular mechanisms involved in CST formation and guidance in mice have identified several factors that act synergistically to ensure proper formation of the CST at each step of development. Human CST developmental disorders can result in a reduction of the CST, or in guidance defects associated with abnormal CST anatomy. These latter disorders result in altered midline crossing at the pyramidal decussation or in the spinal cord, but spare the rest of the CST. Careful appraisal of clinical manifestations associated with CST malformations highlights the critical role of the CST in the lateralization of motor control. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 810-829, 2017.

Targeted Sequencing Reveals Low-Frequency Variants in EPHA Genes as Markers of Paclitaxel-Induced Peripheral Neuropathy

Purpose: Neuropathy is the dose-limiting toxicity of paclitaxel and a major cause for decreased quality of life. Genetic factors have been shown to contribute to paclitaxel neuropathy susceptibility; however, the major causes for interindividual differences remain unexplained. In this study, we identified genetic markers associated with paclitaxel-induced neuropathy through massive sequencing of candidate genes.Experimental Design: We sequenced the coding region of 4 EPHA genes, 5 genes involved in paclitaxel pharmacokinetics, and 30 Charcot-Marie-Tooth genes, in 228 cancer patients with no/low neuropathy or high-grade neuropathy during paclitaxel treatment. An independent validation series included 202 paclitaxel-treated patients. Variation-/gene-based analyses were used to compare variant frequencies among neuropathy groups, and Cox regression models were used to analyze neuropathy along treatment.Results: Gene-based analysis identified EPHA6 as the gene most significantly associated with paclitaxel-induced neuropathy. Low-frequency nonsynonymous variants in EPHA6 were present exclusively in patients with high neuropathy, and all affected the ligand-binding domain of the protein. Accumulated dose analysis in the discovery series showed a significantly higher neuropathy risk for EPHA5/6/8 low-frequency nonsynonymous variant carriers [HR, 14.60; 95% confidence interval (CI), 2.33-91.62; P = 0.0042], and an independent cohort confirmed an increased neuropathy risk (HR, 2.07; 95% CI, 1.14-3.77; P = 0.017). Combining the series gave an estimated 2.5-fold higher risk of neuropathy (95% CI, 1.46-4.31; P = 9.1 × 10^-4).Conclusions: This first study sequencing EPHA genes revealed that low-frequency variants in EPHA6, EPHA5, and EPHA8 contribute to the susceptibility to paclitaxel-induced neuropathy. Furthermore, EPHA's neuronal injury repair function suggests that these genes might constitute important neuropathy markers for many neurotoxic drugs. Clin Cancer Res; 23(5); 1227-35. ©2016 AACR.

Identification of the genomic mutation in Epha4(rb-2J/rb-2J) mice

The EphA4 receptor tyrosine kinase is involved in numerous cell-signalling activities during embryonic development. EphA4 has the ability to bind to both types of ephrin ligands, the ephrinAs and ephrinBs. The C57BL/6J-Epha4rb-2J/GrsrJ strain, denoted Epha4(rb-2J/rb-2J), is a spontaneous mouse mutant that arose at The Jackson Laboratory. These mutants exhibited a synchronous hind limb locomotion defect or "hopping gait" phenotype, which is also characteristic of EphA4 null mice. Genetic complementation experiments suggested that Epha4(rb-2J) corresponds to an allele of EphA4, but details of the genomic defect in this mouse mutant are currently unavailable. We found a single base-pair deletion in exon 9 resulting in a frame shift mutation that subsequently resulted in a premature stop codon. Analysis of the predicted structure of the truncated protein suggests that both the kinase and sterile α motif (SAM) domains are absent. Definitive determination of genotype is needed for experimental studies of mice carrying the Epha4(rb-2J) allele, and we have also developed a method to ease detection of the mutation through RFLP. Eph-ephrin family members are reportedly expressed as numerous isoforms. Hence, delineation of the specific mutation in EphA4 in this strain is important for further functional studies, such as protein-protein interactions, immunostaining and gene compensatory studies, investigating the mechanism underlying the effects of altered function of Eph family of receptor tyrosine kinases on phenotype.

The Corticospinal System: From Development to Motor Control

The corticospinal system is the principal motor system for controlling movements that require the greatest skill and flexibility. It is the last motor system to develop. The pattern of termination of corticospinal axons, as they grow into the spinal gray matter, bears little resemblance to the pattern later in development and in maturity. Refinement of corticospinal terminations occurs during a protracted postnatal period and includes both elimination of transient terminations and growth to new targets. This refinement is driven by neural activity in the motor cortical areas and by limb motor experience. Developing corticospinal terminals compete with each other for synaptic space on spinal neurons. More active terminals are more competitive and are able to secure more synaptic space than their less active counterparts. Corticospinal terminals can activate spinal neurons from very early in development. The importance of this early synaptic activity appears to be more for refining corticospinal connections than for transmitting signals to spinal motor circuits for movement control. The motor control functions of the corticospinal system are not expressed until development of connectional specificity with spinal cord neurons, a strong capacity for corticospinal synapses to facilitate spinal motor circuits, and the formation of the cortical motor map.

Developmental expression of EphA4-tyrosine kinase receptor in the mouse brain and spinal cord

Eph receptor tyrosine kinases and their ephrin ligands are involved in some of the most important steps during the development of the central nervous system, including cell migration, axon guidance, topographic mapping and synapse formation. Moreover, in the adult, they have been implicated in plasticity and regulation of neural stem cell function. One member of the Eph family, EphA4, can bind to both classes of ephrins and may have multiple functions in nervous system development. In order to look for potential sites of EphA4 action during central nervous system development, we conducted a spatio-temporal analysis of EphA4 protein expression. We used immunohistochemistry in preference to in situ hybridization because of the high likelihood that EphA4 protein is expressed on axon tracts, long distances from EphA4 mRNA. In the telencephalon, EphA4 was expressed in the developing cortex from embryonic day 11 (E11) and, later, on major cortical tracts including the corpus callosum and cortico-spinal tract. Robust EphA4 expression was also found in the hippocampus and fornix, and cells and tracts in the striatum. In the diencephalon, the thalamus, the hypothalamus and thalamo-cortical projection were strongly positive. In the mesencephalon, a number of different nuclei were weakly positive, most prominently the red nucleus. In the rhombencephalon, many nuclei were strongly positive including the cerebellum and one of its afferent paths, the inferior cerebellar peduncle, as well as the olivary region. In the spinal cord, there was a dynamic pattern of expression through development, with persistent expression in the dorsal funiculus and ventral grey matter.

Cartilage-specific deletion of ephrin-B2 in mice results in early developmental defects and an osteoarthritis-like phenotype during aging in vivo

BACKGROUND

Ephrins and their related receptors have been implicated in some developmental events. We have demonstrated that ephrin-B2 (EFNB2) could play a role in knee joint pathology associated with osteoarthritis (OA). Here, we delineate the in vivo role of EFNB2 in musculoskeletal growth, development, and in OA using a cartilage-specific EFNB2 knockout (EFNB2(Col2)KO) mouse model.

METHODS

EFNB2(Col2)KO was generated with Col2a1-Cre transgenic mice. The skeletal development was evaluated using macroscopy, immunohistochemistry, histomorphometry, radiology, densitometry, and micro-computed tomography. Analyses were performed at P0 (birth) and on postnatal days P15, P21, and on 8-week- and 1-year-old mice.

RESULTS

EFNB2(Col2)KO mice exhibited significant reduction in size, weight, length, and in long bones. At P0, the growth plates of EFNB2(Col2)KO mice displayed increased type X collagen, disorganized hyphertrophic zone, and decreased mineralization. At P15, mutant mice demonstrated a significant reduction in VEGF and TRAP at the chondro-osseous junction and a delay in the secondary ossification, including a decrease in bone volume and trabecular thickness. At P21 and 8 weeks old, EFNB2(Col2)KO mice exhibited reduced bone mineral density in the total skeleton, femur and spine. One-year-old EFNB2(Col2)KO mice demonstrated OA phenotypic features in both the knee and hip. By P15, 27 % of the EFNB2(Col2)KO mice developed a hip locomotor phenotype, which further experiments demonstrated reflected the neurological midline abnormality involving the corticospinal tract.

CONCLUSION

This in vivo study demonstrated, for the first time, that EFNB2 is essential for normal long bone growth and development and its absence leads to a knee and hip OA phenotype in aged mice.

Variable laterality of corticospinal tract axons that regenerate after spinal cord injury as a result of PTEN deletion or knock-down

Corticospinal tract (CST) axons from one hemisphere normally extend and terminate predominantly in the contralateral spinal cord. We previously showed that deleting the gene phosphatase and tensin homolog (PTEN) in the sensorimotor cortex enables CST axons to regenerate after spinal cord injury and that some regenerating axons extend along the "wrong" side. Here, we characterize the degree of specificity of regrowth in terms of laterality. PTEN was selectively deleted via cortical adeno-associated virus (AAV)-Cre injections in neonatal PTEN-floxed mice. As adults, mice received dorsal hemisection injuries at T12 or complete crush injuries at T9. CST axons from one hemisphere were traced by unilateral biotinylated dextran amine (BDA) injections in PTEN-deleted mice with spinal cord injury and in noninjured PTEN-floxed mice that had not received AAV-Cre. In noninjured mice, 97.9 ± 0.7% of BDA-labeled axons in white matter and 88.5 ± 1.0% of BDA-labeled axons in gray matter were contralateral to the cortex of origin. In contrast, laterality of CST axons that extended past a lesion due to PTEN deletion varied across animals. In some cases, regenerated axons extended predominantly on the ipsilateral side; in other cases, axons extended predominantly contralaterally, and in others, axons were similar in numbers on both sides. Similar results were seen in analyses of cases from previous studies using short hairpin (sh)RNA-mediated PTEN knock-down. These results indicate that CST axons that extend past a lesion due to PTEN deletion or knock-down do not maintain the contralateral rule of the noninjured CST, highlighting one aspect of how the resultant circuitry from regenerating axons may differ from that of the uninjured CST. J. Comp. Neurol. 524:2654-2676, 2016. © 2016 Wiley Periodicals, Inc.

Nonspecific labeling limits the utility of Cre-Lox bred CST-YFP mice for studies of corticospinal tract regeneration

Studies of axon regeneration in the spinal cord often assess regeneration of the corticospinal tract (CST). Emx1-Cre x Thy1-STOP-YFP mice have been reported to have yellow fluorescent protein (YFP) selectively expressed in forebrain neurons leading to genetic labeling of CST axons in the spinal cord, and it was suggested that these CST-YFP mice would be useful for studies of CST regeneration. Because regeneration past a lesion may involve only a few axons, the presence of labeled non-CST axons compromises interpretation. We show here that in CST-YFP mice, some YFP-labeled axons are not from the CST. Specifically, YFP-labeled axons are present in regions beyond those with anterogradely labeled CST axons, most YFP-labeled axons beyond established CST locations do not undergo Wallerian degeneration following a large lesion of the sensorimotor cortex, some rubrospinal and reticulospinal neurons are labeled with YFP, and some YFP-labeled cells in the spinal gray matter have YFP-labeled projections into the spinal cord white matter. We further demonstrate that the density of YFP-labeled axon arbors hinders tracing of single axons to their point of origin in the main descending tracts. In light of recent advances in 3D imaging for visualizing axons in unsectioned blocks of spinal cord, we also assessed CST-YFP mice for 3D imaging and found that YFP fluorescence in CST-YFP mice is faint for clearing-based 3D imaging in comparison with fluorescence in Thy1-YFP-H mice and fluorescence of mini-ruby biotinylated dextran amine (BDA). Overall, the nonspecific and faint YFP labeling in CST-YFP mice limits their utility for assessments of CST axon regeneration.

What are the Best Animal Models for Testing Early Intervention in Cerebral Palsy?

Interventions to treat cerebral palsy should be initiated as soon as possible in order to restore the nervous system to the correct developmental trajectory. One drawback to this approach is that interventions have to undergo exceptionally rigorous assessment for both safety and efficacy prior to use in infants. Part of this process should involve research using animals but how good are our animal models? Part of the problem is that cerebral palsy is an umbrella term that covers a number of conditions. There are also many causal pathways to cerebral palsy, such as periventricular white matter injury in premature babies, perinatal infarcts of the middle cerebral artery, or generalized anoxia at the time of birth, indeed multiple causes, including intra-uterine infection or a genetic predisposition to infarction, may need to interact to produce a clinically significant injury. In this review, we consider which animal models best reproduce certain aspects of the condition, and the extent to which the multifactorial nature of cerebral palsy has been modeled. The degree to which the corticospinal system of various animal models human corticospinal system function and development is also explored. Where attempts have already been made to test early intervention in animal models, the outcomes are evaluated in light of the suitability of the model.

EphA4 receptor is a novel negative regulator of osteoclast activity

Of the ephrin (Eph) receptors, mature osteoclasts express predominantly EphA4. This study sought to determine if EphA4 has a regulatory role in osteoclasts. Treatment of RAW/C4 cells with Epha4 small interfering RNAs (siRNAs) increased average size, Ctsk mRNA expression level, and bone resorption activity of the derived osteoclast-like cells. Activation of the EphA4 signaling in osteoclast precursors with EfnA4-fc chimeric protein reduced cell size and resorption activity of the derived osteoclasts. Homozygous Epha4 null mice had substantially less trabecular bone in femur and vertebra compared to wild-type controls. The bone loss was due to a decrease in trabecular number and an increase in trabecular spacing, but not to an increase in osteoclast-lined bone surface or an increase in the number of osteoclasts on bone surface. Dynamic histomorphometry and serum biomarker analyses indicate that bone formation in Epha4 null mice was reduced slightly but not significantly. Osteoclasts of Epha4 null mice were also larger, expressed higher levels of Mmp3 and Mmp9 mRNAs, and exhibited greater bone resorption activity than wild-type osteoclasts in vitro. Deficient Epha4 expression had no effects on the total number of osteoclast formed in response to receptor activator of NF-κB ligand nor on apoptosis of osteoclasts in vitro. It also did not affect the protein-tyrosine phosphorylation status of its ligands, EfnB2, EfnA2, and EfnA4, in osteoclasts. Deficient Epha4 expression in Epha4 null osteoclasts activated the β3 -integrin signaling through reduced phosphorylation of the tyr-747 residue, which led to increased binding of the stimulatory talin and reduced binding of the inhibitory Dok1 to β3 -integrin. This in turn activated Vav3 and the bone resorption activity of osteoclasts. In conclusion, we demonstrate for the first time that EphA4 is a potent negative regulator of osteoclastic activity, mediated in part through increased Dok1 binding to β3 -integrin via an increase in EphA4-dependent tyr-747 phosphorylation.

EphrinB3/EphA4-mediated guidance of ascending and descending spinal tracts

The spinal cord contains many descending and ascending longitudinal tracts whose development appears to be controlled by distinct guidance systems. We identified a population of dorsal spinal neurons marked by coexpression of the transcription factor Zic2 and the guidance receptor EphA4. Zic2+;EphA4+ neurons are surrounded by mechanosensory terminals, suggesting innervation by mechanoreceptor afferents. Their axons form an ipsilateral ascending pathway that develops during embryogenesis and projects within the ventral aspect of the dorsal funiculus, the same location as the descending corticospinal tract (CST), which develops postnatally. Interestingly, the same guidance mechanism, namely, ephrinB3-induced EphA4 forward signaling, is required for the guidance of both ascending and descending axon tracts. Our analysis of conditional EphA4 mutant mice also revealed that the development of the dorsal funiculus occurs independently of EphA4 expression in descending CST axons and is linked to the distribution of Zic2+;EphA4+ spinal neurons and the formation of the ascending pathway.

In and out from the cortex: development of major forebrain connections

In this review we discuss recent advances in the understanding of the development of forebrain projections attending to their origin, fate determination, and axon guidance. Major forebrain connections include callosal, corticospinal, corticothalamic and thalamocortical projections. Although distinct transcriptional programs specify these subpopulations of projecting neurons, the mechanisms involved in their axonal development are similar. Guidance by short- and long-range molecular cues, interaction with intermediate target populations and activity-dependent mechanisms contribute to their development. Moreover, some of these connections interact with each other showing that the development of these axonal tracts is a well-orchestrated event. Finally, we will recapitulate recent discoveries that challenge the field of neural wiring that show that these forebrain connections can be changed once formed. The field of reprogramming has arrived to postmitotic cortical neurons and has showed us that forebrain connectivity is not immutable and might be changed by manipulations in the transcriptional program of matured cells.

EphA4 receptor tyrosine kinase is a modulator of onset and disease severity of experimental autoimmune encephalomyelitis (EAE)

The EphA4 receptor tyrosine kinase is a major regulator of axonal growth and astrocyte reactivity and is a possible inflammatory mediator. Given that multiple sclerosis (MS) is primarily an inflammatory demyelinating disease and in mouse models of MS, such as experimental autoimmune encephalomyelitis (EAE), axonal degeneration and reactive gliosis are prominent clinical features, we hypothesised that endogenous EphA4 could play a role in modulating EAE. EAE was induced in EphA4 knockout and wildtype mice using MOG peptide immunisation and clinical severity and histological features of the disease were then compared in lumbar spinal cord sections. EphA4 knockout mice exhibited a markedly less severe clinical course than wildtype mice, with a lower maximum disease grade and a slightly later onset of clinical symptoms. Numbers of infiltrating T cells and macrophages, the number and size of the lesions, and the extent of astrocytic gliosis were similar in both genotypes; however, EphA4 knockout mice appeared to have decreased axonal pathology. Blocking of EphA4 in wildtype mice by administration of soluble EphA4 (EphA4-Fc) as a decoy receptor following induction of EAE produced a delay in onset of clinical symptoms; however, most mice had clinical symptoms of similar severity by 22 days, indicating that EphA4 blocking treatment slowed early EAE disease evolution. Again there were no apparent differences in histopathology. To determine whether the role of EphA4 in modulating EAE was CNS mediated or due to an altered immune response, MOG primed T cells from wildtype and EphA4 knockout mice were passively transferred into naive recipient mice and both were shown to induce disease of equivalent severity. These results are consistent with a non-inflammatory, CNS specific, deleterious effect of EphA4 during neuroinflammation that results in axonal pathology.

Lateralization of motor control in the human nervous system: genetics of mirror movements

Mirror movements (MM) are a peculiar motor defect in humans where the intended unilateral movement of a body part results in involuntary movement of the same body part on the opposite side. This loss in the lateralization of motor control can be caused by genetic mutations that result in an aberrant projection of the corticospinal tract. However, recent evidence suggests that the same genes controlling corticospinal tract development also play roles in the development of other circuits involved in motor control, including local spinal circuits and the corpus callosum. These recent studies in humans and mouse models of MM will be discussed to provide an overview of the basis of MM and the molecular mechanisms underlying the lateralization of motor control.

Inhibition and Termination of Physiological Responses by GTPase Activating Proteins

Physiological processes are strictly organized in space and time. However, in cell physiology research, more attention is given to the question of space rather than to time. To function as a signal, environmental changes must be restricted in time; they need not only be initiated but also terminated. In this review, we concentrate on the role of one specific protein family involved in biological signal termination. GTPase activating proteins (GAPs) accelerate the endogenously low GTP hydrolysis rate of monomeric guanine nucleotide-binding proteins (GNBPs), limiting thereby their prevalence in the active, GTP-bound form. We discuss cases where defective or excessive GAP activity of specific proteins causes significant alteration in the function of the nervous, endocrine, and hemopoietic systems, or contributes to development of infections and tumors. Biochemical and genetic data as well as observations from human pathology support the notion that GAPs represent vital elements in the spatiotemporal fine tuning of physiological processes.

Development and maturation of the spinal cord: implications of molecular and genetic defects

The human central nervous system (CNS) may be the most complex structure in the universe. Its development and appropriate specification into phenotypically and spatially distinct neural subpopulations involves a precisely orchestrated response, with thousands of transcriptional regulators combining with epigenetic controls and specific temporal cues in perfect synchrony. Understandably, our insight into the sophisticated molecular mechanisms which underlie spinal cord development are as yet limited. Even less is known about abnormalities of this process - putative genetic and molecular causes of well-described defects have only begun to emerge in recent years. Nonetheless, modern scientific techniques are beginning to demonstrate common patterns and principles amid the tremendous complexity of spinal cord development and maldevelopment. These advances are important, given that developmental anomalies of the spinal cord are an important cause of mortality and morbidity (Sadler, 2000); it is hoped that research advances will lead to better methods to detect, treat, and prevent these lesions.

Change in the balance of excitatory and inhibitory midline fiber crossing as an explanation for the hopping phenotype in EphA4 knockout mice

Neuronal networks in the spinal cord termed central pattern generators (CPGs) are responsible for the generation of rhythmic movements, such as walking. The axon guidance molecule EphA4 has been suggested to play a role in the configuration of spinal CPG networks in mammals. In EphA4 knockout (EphA4-KO) mice, the normal alternating walking pattern is replaced by a rabbit-like hopping gait, which can be reproduced when locomotor-like activity is induced in the isolated spinal cord. This hopping phenotype has been explained by an abnormal midline crossing of ipsilateral axons. Here, we investigated the nature of this overcrossing in heterozygous EphA4 (EphA4(lacZ/+) ) mice that showed normal alternating gait and homozygous EphA4 (EphA4(lacZ/lacZ) ) mice with hopping gait. Localized lesions showed that the hopping phenotype is maintained by fibers crossing in the ventral commissure. Using transgenic mouse lines in which glutamatergic, GABAergic and glycinergic neurons are intrinsically labeled, we showed a significant increase in the number of crossing excitatory β-galactosidase-positive neurons and a decrease in the number of inhibitory neurons crossing the midline in EphA4(lacZ/lacZ) mice compared with EphA4(lacZ/+) mice. These results show that the hopping phenotype is the result of a change in the balance between excitatory and inhibitory signals across the midline and that EphA4-positive neurons play an essential role in the mammalian CPG.

Corticospinal tract fibers cross the ephrin-B3-negative part of the midline of the spinal cord after brain injury

The fibers of corticospinal tract (CST), which control fine motor function, predominantly project to the contralateral spinal cord, not recross to the ipsilateral side. Ephrin-B3, which is expressed in the midline of the spinal cord, and its receptor, EphA4, are crucial for preventing CST fibers from recrossing the midline in the developing spinal cord. However, these fibers can cross the midline to the denervated side after a unilateral CST or cortical injury. We determined the reason CST fibers can cross the midline after a cortical injury and the changes in ephrin-B3-EphA4 signaling associated with such a crossing. We first examined axonal sprouting from CST fibers after unilateral ablation of the motor cortex in postnatal and adult mice. CST fibers crossed the midline of the spinal cord after cortical ablation, especially when conducted during the early postnatal period. These fibers were well associated with functional recovery after the injury. We next assessed the mRNA expression of ephrin-B3 and EphA4 before and after the ablation. Surprisingly, no changes were detected in the expression patterns. We found, however, that ephrin-B3 expression in the ventral part of the midline disappeared after postnatal day 9 (P9), but was pronounced along the entire midline before P6. Most of the CST fibers crossed the midline through the ventral region, where ephrin-B3 expression was absent. Our results suggest that ephrin-B3 is not expressed along the entire midline of the spinal cord, and sprouting axons can cross the midline at ephrin-B3-negative areas.

Cortical control of adaptive locomotion in wild-type mice and mutant mice lacking the ephrin-Eph effector protein alpha2-chimaerin

In voluntary control, supraspinal motor systems select the appropriate response and plan movement mechanics to match task constraints. Spinal circuits translate supraspinal drive into action. We studied the interplay between motor cortex (M1) and spinal circuits during voluntary movements in wild-type (WT) mice and mice lacking the α2-chimaerin gene (Chn1(-/-)), necessary for ephrinB3-EphA4 signaling. Chn1(-/-) mice have aberrant bilateral corticospinal systems, aberrant bilateral-projecting spinal interneurons, and disordered voluntary control because they express a hopping gait, which may be akin to mirror movements. We addressed three issues. First, we determined the role of the corticospinal system in adaptive control. We trained mice to step over obstacles during treadmill locomotion. We compared performance before and after bilateral M1 ablation. WT mice adaptively modified their trajectory to step over obstacles, and M1 ablation increased substantially the incidence of errant steps over the obstacle. Chn1(-/-) mice randomly stepped or hopped during unobstructed locomotion but hopped over the obstacle. Bilateral M1 ablation eliminated this obstacle-dependent hop selection and increased forelimb obstacle contact errors. Second, we characterized the laterality of corticospinal action in Chn1(-/-) mice using pseudorabies virus retrograde transneuronal transport and intracortical microstimulation. We showed bilateral connections between M1 and forelimb muscles in Chn1(-/-) and unilateral connections in WT mice. Third, in Chn1(-/-) mice, we studied adaptive responses before and after unilateral M1 ablation. We identified a more important role for contralateral than ipsilateral M1 in hopping over the obstacle. Our findings suggest an important role for M1 in the mouse in moment-to-moment adaptive control, and further, using Chn1(-/-) mice, a role in mediating task-dependent selection of mirror-like hopping movements over the obstacle. Our findings also stress the importance of subcortical control during adaptive locomotion because key features of the trajectory remained largely intact after M1 ablation.

Activation of ephrin A proteins influences hematopoietic stem cell adhesion and trafficking patterns

OBJECTIVE

To determine if Eph receptors and ephrins can modulate the homing of hematopoietic cells in a murine bone marrow transplantation model.

MATERIALS AND METHODS

EphA and ephrin A gene expression by mouse hematopoietic stem cells and the progenitor cell line FDCP-1 was determined by real-time reverse transcription polymerase chain reaction and flow cytometry. The effect of ephrin A activation on adhesion of hematopoietic progenitors was determined by in vitro adhesion assays in which cells were exposed to fibronectin or vascular cell adhesion molecule-1 (VCAM-1) and an increasing gradient of immobilized EphA3-Fc. Adhesion to fibronectin and VCAM-1 was further investigated using soluble preclustered EphA3-Fc. We used soluble unclustered EphA3-Fc as an antagonist to block endogenous EphA-ephrin A interactions in vivo. The effect of injecting soluble EphA3-Fc on the mobilization of hematopoietic progenitor cells was examined. We determined the effect on short-term homing by pretreating bone marrow cells with EphA3-Fc or the control IgG before infusion into lethally irradiated mice.

RESULTS

Preclustered and immobilized EphA3-Fc increased adhesion of progenitor cells and FDCP-1 to fibronectin and VCAM-1 (1.6- to 2-fold higher adhesion; p < 0.05) relative to control (0 μ/cm(2) EphA3-Fc extracellular molecule alone). Injection of the antagonist soluble EphA3-Fc increased progenitor cell and colony-forming unit-spleen cells in the peripheral blood (42% greater colony-forming unit in culture; p < 0.05, 3.8-fold higher colony-forming unit-spleen) relative to control.

CONCLUSION

Treating bone marrow cells with EphA3-Fc resulted in a reduction by 31% in donor stem cells homing to the bone marrow and accumulation of donor cells in recipient spleens (50% greater than control) and greater recovery of donor stem cells from the peripheral blood.

Variations in ventral root axon morphology and locomotor behavior components across different inbred strains of mice

Locomotion is a complex behavior affected by many different brain- and spinal cord systems, as well as by variations in the peripheral nervous system. Recently, we found increased gene expression for EphA4, a gene intricately involved in motor neuron development, between high-active parental strain C57BL/6J and the low-active chromosome substitution strain 1 (CSS1). CSS1 mice carry chromosome 1 from A/J mice in a C57BL/6J genetic background, allowing localization of quantitative trait loci (QTL) on chromosome 1. To find out whether differences in motor neuron anatomy, possibly related to the changes in EphA4 expression, are involved in the motor activity differences observed in these strains, motor performance in various behavioral paradigms and anatomical differences in the ventral roots were investigated. To correlate the behavioral profiles to the spinal motor neuron morphology, not only CSS1 and its parental strains C57BL/6J (host) and A/J (donor) were examined, but also a set of other mouse inbred strains (AKR/J, 129x1/SvJ and DBA/2J). Significant differences were found between inbred strains on home cage motor activity levels, the beam balance test, grip test performance, and on alternating versus synchronous hind limb movement (hind limb hopping). Also, considerable differences were found in spinal motor neuron morphology, with A/J and CSS1 showing smaller, possibly less developed, motor neuron axons compared to all other inbred strains. For CSS1 and C57BL/6J, only genetically different for chromosome 1, a correlation was found between motor activity levels, synchronous hind limb movement and neuro-anatomical differences in spinal motor neurons. Inclusion of the other inbred strains, however, did not show this direct correlation. These data verifies the complex nature of the mammalian motor system that may be further dissected using genetic mapping populations derived from these inbred strains.

Developmental course of EphA4 cellular and subcellular localization in the postnatal rat hippocampus

From embryonic development to adulthood, the EphA4 receptor and several of its ephrin-A or -B ligands are expressed in the hippocampus, where they presumably play distinct roles at different developmental stages. To help clarify these diverse roles in the assembly and function of the hippocampus, we examined the cellular and subcellular localization of EphA4 in postnatal rat hippocampus by light and electron microscopic immunocytochemistry. On postnatal day (P) 1, the EphA4 immunostaining was robust in most layers of CA1, CA3, and dentate gyrus and then decreased gradually, until P21, especially in the cell body layers. At the ultrastructural level, focal spots of EphA4 immunoreactivity were detected all over the plasma membrane of pyramidal and granule cells, between P1 and P14, from the perikarya to the dendritic and axonal extremities, including growth cones and filopodia. This cell surface immunoreactivity then became restricted to the synapse-associated dendritic spines and axon terminals by P21. In astrocytes, the EphA4 immunolabeling showed a similar cell surface redistribution, from the perikarya and large processes at P1-P7, to small perisynaptic processes at P14-P21. In both cell types, spots of EphA4 immunoreactivity were also detected, with an incidence decreasing with maturation, on the endoplasmic reticulum, Golgi apparatus, and vesicles, organelles involved in protein synthesis, posttranslational modifications, and transport. The cell surface evolution of EphA4 localization in neuronal and glial cells is consistent with successive involvements in the developmental movements of cell bodies first, followed by process outgrowth and guidance, synaptogenesis, and finally synaptic maintenance and plasticity.

Excitatory components of the mammalian locomotor CPG

Locomotion in mammals is to a large degree controlled directly by intrinsic spinal networks, called central pattern generators (CPGs). The overall function of these networks is governed by interaction between inhibitory and excitatory neurons. In the present review, we will discuss recent findings addressing the role of excitatory synaptic transmission for network function including the role of specific excitatory neuronal populations in coordinating muscle activity and in generating rhythmic activity.

There's more than one way to skin a chimaerin

In two manuscripts published in Neuron (Beg et al. and Wegmeyer et al.) and one published in Cell (Iwasato et al.), investigators have found that a particular GAP, alpha-chimaerin, is required in vivo for ephrinB3/EphA4-dependent motor circuit formation.

Combinatorial treatments for promoting axon regeneration in the CNS: strategies for overcoming inhibitory signals and activating neurons' intrinsic growth state

In general, neurons in the mature mammalian central nervous system (CNS) are unable to regenerate injured axons, and neurons that remain uninjured are unable to form novel connections that might compensate for ones that have been lost. As a result of this, victims of CNS injury, stroke, or certain neurodegenerative diseases are unable to fully recover sensory, motor, cognitive, or autonomic functions. Regenerative failure is related to a host of inhibitory signals associated with the extracellular environment and with the generally low intrinsic potential of mature CNS neurons to regenerate. Most research to date has focused on extrinsic factors, particularly the identification of inhibitory proteins associated with myelin, the perineuronal net, glial cells, and the scar that forms at an injury site. However, attempts to overcome these inhibitors have resulted in relatively limited amounts of CNS regeneration. Using the optic nerve as a model system, we show that with appropriate stimulation, mature neurons can revert to an active growth state and that when this occurs, the effects of overcoming inhibitory signals are enhanced dramatically. Similar conclusions are emerging from studies in other systems, pointing to a need to consider combinatorial treatments in the clinical setting.

Deletion of EphA4 enhances deafferentation-induced ipsilateral sprouting in auditory brainstem projections

Axonal selection of ipsilateral and/or contralateral targets is essential for integrating bilateral sensory information and for coordinated movement. The molecular processes that determine ipsilateral and contralateral target choice are not fully understood. We examined this target selection in the developing auditory brainstem. Ventral cochlear nucleus (VCN) axons normally project to the medial nucleus of the trapezoid body (MNTB) only on the contralateral side. However, after unilateral removal of cochlear input in neonates, we found that axons from the unoperated VCN sprout and project to MNTB bilaterally. We found that EphA4 is expressed in the mouse auditory brainstem during development and during a sensitive period for ipsilateral sprouting, so we hypothesized that deletion of the Eph receptor EphA4 would impair target selection in these auditory pathways. Lipophilic dyes were used to evaluate quantitatively the brainstem projections in wild-type and EphA4-null mice. VCN-MNTB projections in EphA4-null mice were strictly contralateral, as in wild-type mice. However, after deafferentation, EphA4-null mice had a significant, threefold increase in the proportion of axons from the intact VCN that sprouted into ipsilateral MNTB compared with wild-type mice. Heterozygous mice had a twofold increase in these projections. These results demonstrate that EphA4 influences auditory brainstem circuitry selectively in response to deafferentation. Although this axon guidance molecule is not by itself necessary for appropriate target choice during normal development, it is a strong determinant of ipsilateral vs. contralateral target choice during deafferentation-induced plasticity.

EphA4 regulates central nervous system vascular formation

Molecules involved in axon guidance have recently also been shown to play a role in blood vessel guidance. To examine whether axon guidance molecules, such as the EphA4 receptor tyrosine kinase, might also play a role in development of the central nervous system (CNS) vasculature and repair following CNS injury, we examined wild-type and EphA4 null mutant (-/-) mice. EphA4-/- mice exhibited an abnormal CNS vascular structure in both the cerebral cortex and the spinal cord, with disorganized branching and a 30% smaller diameter. During development, EphA4 was expressed on endothelial cells. This pattern of expression was not maintained in the adult. After spinal cord injury in wild-type mice, expression of EphA4 was markedly up-regulated on activated astrocytes, many of which were tightly associated with blood vessels. In EphA4-/- spinal cord following injury, astrocytes were not as tightly associated with blood vessels as the wild-type astrocytes. In uninjured EphA4-/- mice, the blood-brain barrier (BBB) appeared normal, but it showed prolonged leakage following spinal cord injury. These results support a role for EphA4 in CNS vascular formation and guidance during development and an additional role in BBB repair.

Eph tyrosine kinase receptor EphA4 is required for the topographic mapping of the corticospinal tract

Fine movement in the body is controlled by the motor cortex, which signals in a topographically specific manner to neurons in the spinal cord by means of the corticospinal tract (CST). How the correct topography of the CST is established is unknown. To investigate the possibility that the Eph tyrosine kinase receptor EphA4 is involved in this process, we have traced CST axons in mice in which the EphA4 gene has been deleted. The forelimb subpopulation of CST axons is unaffected in the EphA4-/- mice, but the hindlimb subpopulation branches too early within the cord, both temporally and spatially. EphA4 shows a dynamic expression pattern in the environment of the developing CST in the spinal cord: high at the time of forelimb branching and down-regulated before hindlimb branching. To examine whether the fore- and hindlimb subpopulations of CST axons respond differently to EphA4 in their environment, neurons from fore- and hindlimb motor cortex were cultured on a substrate containing EphA4. Neurons from the hindlimb cortex showed reduced branching on the EphA4 substrate compared with their forelimb counterparts. Neurons from the hindlimb cortex express ephrinA5, a high-affinity ligand for EphA4, at higher levels compared with forelimb cortex neurons, and this expression is down-regulated before hindlimb branching. Together, these findings suggest that EphA4 regulates topographic mapping of the CST by controlling the branching of CST axons in the spinal cord.

Distribution of EphB receptors and ephrin-B1 in the developing vertebrate spinal cord

Contact-dependent interactions between EphB receptors and ephrin-B ligands mediate a variety of cell-cell communication events in the developing and mature central nervous system (CNS). These predominantly repulsive interactions occur at the interface between what are considered to be mutually exclusive EphB and ephrin-B expression domains. We previously used receptor and ligand affinity probes to show that ephrin-B ligands are expressed in the floor plate and within a dorsal region of the embryonic mouse spinal cord, while EphB receptors are present on decussated segments of commissural axons that navigate between these ephrin-B domains. Here we present the generation and characterization of two new monoclonal antibodies, mAb EfB1-3, which recognizes EphB1, EphB2, and EphB3, and mAb efrnB1, which is specific for ephrin-B1. We use these reagents and polyclonal antibodies specific for EphB1, EphB2, EphB3, or ephrin-B1 to describe the spatiotemporal expression patterns of EphB receptors and ephrin-B1 in the vertebrate spinal cord. Consistent with affinity probe binding, we show that EphB1, EphB2, and EphB3 are each preferentially expressed on decussated segments of commissural axons in vivo and in vitro, and that ephrin-B1 is expressed in a dorsal domain of the spinal cord that includes the roof plate. In contrast to affinity probe binding profiles, we show here that EphB1, EphB2, and EphB3 are present on the ventral commissure, and that EphB1 and EphB3 are expressed on axons that compose the dorsal funiculus. In addition, we unexpectedly find that mesenchymal cells, which surround the spinal cord and dorsal root ganglion, express ephrin-B1.

Roles of Eph receptors and ephrins in the normal and damaged adult CNS

Injury to the central nervous system (CNS) usually results in very limited regeneration of lesioned axons, which are inhibited by the environment of the injury site. Factors that have been implicated in inhibition of axonal regeneration include myelin proteins, astrocytic gliosis and cell surface molecules that are involved in axon guidance during development. This review examines the contribution of one such family of developmental guidance molecules, the Eph receptor tyrosine kinases and their ligands, the ephrins in normal adult CNS and following injury or disease. Eph/ephrin signaling regulates axon guidance through contact repulsion during development of the CNS, inducing collapse of neuronal growth cones. Eph receptors and ephrins continue to be expressed in the adult CNS, although usually at lower levels, but are upregulated following neural injury on different cell types, including reactive astrocytes, neurons and oligodendrocytes. This upregulated expression may directly inhibit regrowth of regenerating axons; however, in addition, Eph expression also regulates astrocytic gliosis and formation of the glial scar. Therefore, Eph/ephrin signaling may inhibit regeneration by more than one mechanism and modulation of Eph receptor expression or signaling could prove pivotal in determining the outcome of injury in the adult CNS.

LOCOMOTOR CIRCUITS IN THE MAMMALIAN SPINAL CORD

Intrinsic spinal networks, known as central pattern generators (CPGs), control the timing and pattern of the muscle activity underlying locomotion in mammals. This review discusses new advances in understanding the mammalian CPGs with a focus on experiments that address the overall network structure as well as the identification of CPG neurons. I address the identification of excitatory CPG neurons and their role in rhythm generation, the organization of flexor-extensor networks, and the diverse role of commissural interneurons in coordinating left-right movements. Molecular and genetic approaches that have the potential to elucidate the function of populations of CPG interneurons are also discussed.

Chemorepellent axon guidance molecules in spinal cord injury

Regenerating axons stop growing when they reach the border of the glial-fibrotic scar, presumably because they encounter a potent molecular barrier inhibiting growth cone advance. Chemorepulsive axon guidance molecules provide a non-permissive environment restricting and channeling axon growth in the developing nervous system. These molecules could also act as growth-inhibitory molecules in the regenerating nervous system. The receptors for repulsive guidance cues are expressed in the mature nervous system, suggesting that adult neurons are sensitive to the activity of developmentally active repulsive proteins. In this review, we summarize recent observations on semaphorins, ephrins, and slits in the injured brain and spinal cord, providing evidence that these proteins are major players in inhibiting axonal regeneration and establishing the glial-fibrotic scar.

EphB2 regulates axonal growth at the midline in the developing auditory brainstem

Eph receptors play important roles in axon guidance at the midline. In the auditory system, growth of axons across the midline is an important determinant of auditory function. The avian cochlear nucleus, n. magnocellularis (NM), makes bilateral projections to its target, n. laminaris (NL). We examined the time course of NM axon growth toward the midline, the expression of Eph proteins at the midline during this growth, and the effects of Eph receptor misexpression on axonal growth across the midline. We found that NM axons reach the midline at E4. At this age, EphB receptors are expressed at the ventral floor plate. Expression extends dorsally to the ventricular zone beginning at E5. NM axons thus grow across the midline at a time when EphB receptor expression levels are low. Overexpression of EphB2 at E2 resulted in misrouted axons that deflected away from transfected midline cells. This effect was observed when midline cells were transfected but not when NM cells alone were transfected, suggesting that EphB2 acts non-cell autonomously and through reverse signaling. These data suggest an inhibitory role for midline Eph receptors, in which low levels permit axon growth and subsequently high levels prohibit growth after axons have crossed the midline.

Identification of candidate genes at the corticoseptal boundary during development

Cortical midline glia are critical to the formation of the corpus callosum during development. The glial wedge is a population of midline glia that is located at the corticoseptal boundary and expresses repulsive/growth-inhibitory molecules that guide callosal axons as they cross the midline. The glial wedge are the first cells within the cortex to express GFAP and thus may express molecules specific for glial maturation. The corticoseptal boundary is a genetically defined boundary between the cingulate cortex (dorsal telencephalon) and the septum (ventral telencephalon). The correct dorso-ventral position of this boundary is vital to the formation of both the glial wedge and the corpus callosum. Our aim was to identify genes expressed specifically within the glial wedge that might be involved in either glial differentiation, formation of the corticoseptal boundary or development of the corpus callosum. To identify such genes we have performed a differential display PCR screen comparing RNA isolated from the glial wedge with RNA isolated from control tissues such as the neocortex and septum, of embryonic day 17 mouse brains. Using 200 different combinations of primers, we identified and cloned 67 distinct gene fragments. In situ hybridization analysis confirmed the differential expression of many of the genes, and showed that clones G24F3, G39F8 and transcription factor LZIP have specific expression patterns in the telencephalon of embryonic and postnatal brains. An RNase Protection Assay (RPA) revealed that the expression of G39F8, G24F3 and LZIP increase markedly in the telencephalon at E16 and continue to be expressed until at least P0, during the period when the corpus callosum is forming.

An immunohistochemical study of the development of sensorimotor components of the early fetal human spinal cord

Sections from all spinal cord levels from 20 human fetuses, age range 7.5-17 gestational weeks (GW) were immunostained for non-phosphorylated neurofilaments (to reveal motoneurones, spinocerebellar neurones and other large neurones), the calcium-binding protein parvalbumin (large proprioreceptive afferents), growth-associated protein 43 kDa (growing axons), glial fibrillary acidic protein (radial glia), synaptophysin (synaptic terminals) the cell-cell recognition molecule ephrin A4 (EphA4) and the ETS transcription factor Er81 (subclasses of motoneurone and proprioreceptive neurone). Muscle afferents crossed the dorsal horn by 7.5 GW and innervated motoneurones by 9 GW. An alignment of glial fibres guided them from dorsal columns to ventral horn, at right angles to the radial glia. They continued to provide a dense innervation of motoneurone pools up to 17 GW. By 13 GW motoneurones were segregated into distinct columns, all of which expressed EphA4 although only certain lateral groups expressed Er81. However, Er81 expression was more widespread amongst dorsal root ganglion neurones. From 9 GW Clarke's column neurones were identified and by 14 GW were heavily innervated by parvalbumin-positive afferents whilst their efferent axons could be traced to the lateral funiculus. This investigation contributes towards a timetable for the functional development of human motor control and makes comparisons with well-studied rodent models.

Methylmercury Alters Eph and Ephrin Expression During Neuronal Differentiation of P19 Embryonal Carcinoma Cells

Developmental exposure to methylmercury (MeHg) induces a spectrum of neurological impairment characterized by cognitive disturbance, sensory/motor deficit, and diffuse structural abnormalities of the brain. These alterations may arise from neural path-finding errors during brain development, resulting from disturbances in the function of morphoregulatory guidance molecules. The Eph family of tyrosine kinase receptors and their ligands, the ephrins, guide neuronal migration and neurite pathfinding mainly via repulsive intercellular interactions. The present study examined the effects of MeHg on mRNA and protein expression profiles of Ephs and ephrins in the P19 embryonal carcinoma (EC) cell line and its neuronal derivatives. Undifferentiated control P19 cells displayed low- to undetectable levels of mRNA for ephrins or Ephs, with the sole exception of EphA2 which was highly expressed. Upon differentiation into neurons, the ephrin expression increased progressively through day 10. Similarly, expression of the Ephs, including EphsA3, -A4, -A8, -B2, -B3, -B4, and -B6, increased significantly. In contrast, EphA2 expression decreased in day 2, 6 and 10 control neurons. Treatment with MeHg did not affect the expression of mRNA for ephrins or Ephs in undifferentiated P19 cells. However, treatment of differentiating neurons with MeHg for 24 h caused consistent increases in ligand mRNA expression, particularly ephrin-A5, -A6, -B1, and -B2. Similarly, MeHg induced variable increases in mRNA expression of receptors EphA2, -A3, -B3, and -B6. A trend toward a concentration-response relationship was observed for the alterations in Eph receptor mRNA expression although increases at the low and mid concentrations did not reach statistical significance. Immunoblots for ligand and receptor proteins mirrored the increases in the mRNA levels at the 0.5 and 1.5 microM MeHg concentrations but showed decreased protein levels compared to controls at the 3.0 microM concentration. Alterations in the Eph/ephrin family of repulsion molecules may represent an important mechanism in developmental MeHg neurotoxicity.