Slit-mediated repulsion is a key regulator of motor axon pathfinding in the hindbrain

Robo2-Slit1 dependent cell-cell interactions mediate assembly of the trigeminal ganglion

Vertebrate cranial sensory ganglia, responsible for sensation of touch, taste and pain in the face and viscera, are composed of both ectodermal placode and neural crest cells. The cellular and molecular interactions allowing generation of complex ganglia remain unknown. Here, we show that proper formation of the trigeminal ganglion, the largest of the cranial ganglia, relies on reciprocal interactions between placode and neural crest cells in chick, as removal of either population resulted in severe defects. We demonstrate that ingressing placode cells express the Robo2 receptor and early migrating cranial neural crest cells express its cognate ligand Slit1. Perturbation of this receptor-ligand interaction by blocking Robo2 function or depleting either Robo2 or Slit1 using RNA interference disrupted proper ganglion formation. The resultant disorganization mimics the effects of neural crest ablation. Thus, our data reveal a novel and essential role for Robo2-Slit1 signaling in mediating neural crest-placode interactions during trigeminal gangliogenesis.

Slit2 involvement in glioma cell migration is mediated by Robo1 receptor

Slit and Robo proteins are evolutionarily conserved molecules whose interaction underlies axon guidance and neuronal precursor cell migration. During development secreted Slit proteins mediate chemorepulsive signals on cells expressing Robo receptors. Because similar molecular mechanisms may be utilized in glioma cell invasion and neuroblast migration, we studied the expression of Slit2 and its transmembrane receptor Robo1 as well as their functional role in migration in glioma cells. qRT-PCR and immunohistochemistry of human specimens revealed that Slit2 was distinctly expressed by non-neoplastic neurons, but at only very low levels in fibrillary astrocytoma and glioblastoma. Robo1 also was mainly restricted to neurons in the normal brain, whereas astrocytic tumor cells in situ as well as glioblastoma cell lines overexpressed Robo1 at mRNA and protein levels. Recombinant human Slit2 in a concentration of 0.45 nM was repulsive for glioma cell lines in a modified Boyden chamber assay. RNAi-mediated knockdown of Robo1 in glioma cell lines neutralized the repulsive effect of Slit2, demonstrating that Robo1 served as the major Slit2 receptor. Our findings suggest that a chemorepulsive effect mediated by interaction of Slit2 and Robo1 participates in glioma cell guidance in the brain.

Beyond Parkinson disease: amyotrophic lateral sclerosis and the axon guidance pathway

Background

We recently described a genomic pathway approach to study complex diseases. We demonstrated that models constructed using single nucleotide polymorphisms (SNPs) within axon guidance pathway genes were highly predictive of Parkinson disease (PD) susceptibility, survival free of PD, and age at onset of PD within two independent whole-genome association datasets. We also demonstrated that several axon guidance pathway genes represented by SNPs within our final models were differentially expressed in PD.

Methodology/Principal Findings

Here we employed our genomic pathway approach to analyze data from a whole-genome association dataset of amyotrophic lateral sclerosis (ALS); and demonstrated that models constructed using SNPs within axon guidance pathway genes were highly predictive of ALS susceptibility (odds ratio = 1739.73, p = 2.92×10⁻⁶⁰), survival free of ALS (hazards ratio = 149.80, p = 1.25×10⁻⁷⁴), and age at onset of ALS (R² = 0.86, p = 5.96×10⁻⁶⁶). We also extended our analyses of a whole-genome association dataset of PD, which shared 320,202 genomic SNPs in common with the whole-genome association dataset of ALS. We compared for ALS and PD the genes represented by SNPs in the final models for susceptibility, survival free of disease, and age at onset of disease and noted that 52.2%, 37.8%, and 34.9% of the genes were shared respectively.

Conclusions/Significance

Our findings for the axon guidance pathway and ALS have prior biological plausibility, overlap partially with PD, and may provide important insight into the causes of these and related neurodegenerative disorders.

Molecular guidance cues necessary for axon pathfinding from the ventral cochlear nucleus

During development, multiple guidance cues direct the formation of appropriate synaptic connections. Factors that guide developing axons are known for various pathways throughout the mammalian brain; however, signals necessary to establish auditory connections are largely unknown. In the auditory brainstem the neurons whose axons traverse the midline in the ventral acoustic stria (VAS) are primarily located in the ventral cochlear nucleus (VCN) and project bilaterally to the superior olivary complex (SOC). The circumferential trajectory taken by developing VCN axons is similar to that of growing axons of spinal commissural neurons. Therefore, we reasoned that netrin-DCC and slit-robo signaling systems function in the guidance of VCN axons. VCN neurons express the transcription factor, mafB, as early as embryonic day (E) 13.5, thereby identifying the embryonic VCN for these studies. VCN axons extend toward the midline as early as E13, with many axons crossing by E14.5. During this time, netrin-1 and slit-1 RNAs are expressed at the brainstem midline. Additionally, neurons within the VCN express RNA for DCC, robo-1, and robo-2, and axons in the VAS are immunoreactive for DCC. VCN axons do not reach the midline of the brainstem in mice mutant for either the netrin-1 or DCC gene. VCN axons extend in pups lacking netrin-1, but most DCC-mutant samples lack VCN axonal outgrowth. Stereological cell estimates indicate only a modest reduction of VCN neurons in DCC-mutant mice. Taken together, these data show that a functional netrin-DCC signaling system is required for establishing proper VCN axonal projections in the auditory brainstem.

The Slit receptor Robo1 is predominantly expressed via the Dutt1 alternative promoter in pioneer neurons in the embryonic mouse brain and spinal cord

Robo1 is a member of the Roundabout (Robo) family of receptors for the Slit axon guidance cues. In mice (and humans), the Robo1 locus has alternative promoters producing two transcript isoforms, Robo1 and Dutt1. These isoforms have unique 5' termini, predicted to encode distinct N-terminal amino acids, but share the rest of their 3' exons. To determine the spatial expression of the Robo1 and Dutt1 isoforms, we generated isoform-specific RNA probes, and carried out in situ hybridization on E10.5 mouse embryos, the stage in early neuron differentiation when many major axon pathways are established. The two isoforms had distinct expression patterns that partially overlapped. Dutt1 was the predominant isoform, with widespread expression in regions of post-mitotic neurons and neuroepithelial cells. The Robo1 isoform had a distinct expression pattern restricted to subsets of neurons, many of which were Dutt1-negative. Dutt1 was the main isoform expressed in spinal cord commissural neurons. For both probes, the main hybridization signal was limited to two spots in the nuclei of individual cells. This study shows distinct expression patterns for the Dutt1 and Robo1 alternative promoters in the embryonic nervous system.

Electroporation of cDNA/Morpholinos to targeted areas of embryonic CNS in Xenopus

Background

Blastomere injection of mRNA or antisense oligonucleotides has proven effective in analyzing early gene function in Xenopus. However, functional analysis of genes involved in neuronal differentiation and axon pathfinding by this method is often hampered by earlier function of these genes during development. Therefore, fine spatio-temporal control of over-expression or knock-down approaches is required to specifically address the role of a given gene in these processes.

Results

We describe here an electroporation procedure that can be used with high efficiency and low toxicity for targeting DNA and antisense morpholino oligonucleotides (MOs) into spatially restricted regions of the Xenopus CNS at a critical time-window of development (22–50 hour post-fertilization) when axonal tracts are first forming. The approach relies on the design of "electroporation chambers" that enable reproducible positioning of fixed-spaced electrodes coupled with accurate DNA/MO injection. Simple adjustments can be made to the electroporation chamber to suit the shape of different aged embryos and to alter the size and location of the targeted region. This procedure can be used to electroporate separate regions of the CNS in the same embryo allowing separate manipulation of growing axons and their intermediate and final targets in the brain.

Conclusion

Our study demonstrates that electroporation can be used as a versatile tool to investigate molecular pathways involved in axon extension during Xenopus embryogenesis. Electroporation enables gain or loss of function studies to be performed with easy monitoring of electroporated cells. Double-targeted transfection provides a unique opportunity to monitor axon-target interaction in vivo. Finally, electroporated embryos represent a valuable source of MO-loaded or DNA transfected cells for in vitro analysis. The technique has broad applications as it can be tailored easily to other developing organ systems and to other organisms by making simple adjustments to the electroporation chamber.

Slit-Robo interactions during cortical development

Interneurons are an integral part of cortical neuronal circuits. During the past decade, numerous studies have shown that these cells, unlike their pyramidal counterparts that are derived from the neuroepithelium along the lumen of the lateral ventricles, are generated in the ganglionic eminences in the subpallium. They use tangential migratory paths to reach the cortex, guided by intrinsic and extrinsic cues. Evidence is now emerging which suggests that the family of Slit proteins, acting through Robo receptors, play a role not only in axon guidance in the developing forebrain, but also as guiding signals in the migration of cortical interneurons. Here we describe the patterns of expression of Slit and Robo at different stages of forebrain development and review the evidence in support of their role in cortical interneuron migration. Slit-Robo signal transduction mechanisms are also important during normal development in a number of systems in the body and in disease states, making them potential therapeutic targets for the treatment of neurological disorders and certain types of cancer.

UNC5C is required for spinal accessory motor neuron development

In both invertebrates and vertebrates, UNC5 receptors facilitate chemorepulsion away from a Netrin source. Unlike most motor neurons in the embryonic vertebrate spinal cord, spinal accessory motor neuron (SACMN) cell bodies and their axons translocate along a dorsally directed trajectory away from the floor plate/ventral midline and toward the lateral exit point (LEP). We have recently shown that Netrin-1 and DCC are required for the migration of SACMN cell bodies, in vivo. These observations raised the possibility that vertebrate UNC5 proteins mediate the presumed repulsion of SACMN away from the Netrin-rich ventral midline. Here, we show that SACMN are likely to express UNC5A and UNC5C. Whereas SACMN development proceeds normally in UNC5A null mice, many SACMN cell bodies fail to migrate away from the ventral midline and inappropriately cluster in the ventrolateral spinal cord of mouse embryos lacking UNC5C. These results support an important role for UNC5C in SACMN development.

Recognition molecules and neural repair

Neural recognition molecules were discovered and characterized initially for their functional roles in cell adhesion as regulators of affinity between cells and the extracellular matrix in vitro. They were then recognized as mediators or co-receptors which trigger signal transduction mechanisms affecting cell adhesion and de-adhesion. Their involvement in contact attraction and repulsion relies on cell-intrinsic properties that are modulated by the spatial contexts of their expression at particular stages of ontogenetic development, in synaptic plasticity and during regeneration after injury. The functional roles of recognition molecules in cell proliferation and migration, determination of developmental fate, growth cone guidance, and synapse formation, stabilization and modulation have been well documented not only by in vitro, but also by in vivo studies that have been greatly aided by generation of genetically altered mice. More recently, the functions of recognition molecules have been investigated under conditions of neural repair and manipulated using a broad range of genetic and pharmacological approaches to achieve a beneficial outcome. The principal aim of most therapeutically oriented approaches has been to neutralize inhibitory factors. However, less attention has been paid to enhancing repair by stimulating the stimulatory factors. When considering potential therapeutic strategies, it is worth considering that a single recognition molecule can possess domains that are conducive or repellent and that the spatial distribution of recognition molecules can determine the overall function: Recognition molecules may be repellent for neurite outgrowth when presented as barriers or steep-concentration gradients and conducive when presented as uniform substrates. The focus of this review will be on the more recent attempts to study the conducive mechanisms with the expectation that they may be able to tip the balance from a regeneration inhospitable to a hospitable environment. It is likely that a combination of the two principles, as multifactorial as each principle may be in itself, will be of therapeutic value in humans.

Zebrafish Gastrulation: Cell Movements, Signals, and Mechanisms

Gastrulation is a morphogenetic process that results in the formation of the embryonic germ layers. Here we detail the major cell movements that occur during zebrafish gastrulation: epiboly, internalization, and convergent extension. Although gastrulation is known to be regulated by signaling pathways such as the Wnt/planar cell polarity pathway, many questions remain about the underlying molecular and cellular mechanisms. Key factors that may play a role in gastrulation cell movements are cell adhesion and cytoskeletal rearrangement. In addition, some of the driving force for gastrulation may derive from tissue interactions such as those described between the enveloping layer and the yolk syncytial layer. Future exploration of gastrulation mechanisms relies on the development of sensitive and quantitative techniques to characterize embryonic germ-layer properties.

Forced expression of Phox2 homeodomain transcription factors induces a branchio-visceromotor axonal phenotype

What causes motor neurons to project into the periphery is not well understood. We here show that forced expression of the homeodomain protein Phox2b, shown previously to be necessary and sufficient for branchio-visceromotor neuron development, and of its paralogue Phox2a imposes a branchiomotor-like axonal phenotype in the spinal cord. Many Phox2-transfected neurons, whose axons would normally stay within the confines of the neural tube, now project into the periphery. Once outside the neural tube, a fraction of the ectopic axons join the spinal accessory nerve, a branchiomotor nerve which, as shown here, does not develop in the absence of Phox2b. Explant studies show that the axons of Phox2-transfected neurons need attractive cues to leave the neural tube and that their outgrowth is promoted by tissues, to which branchio-visceromotor fibers normally grow. Hence, Phox2 expression is a key step in determining the peripheral axonal phenotype and thus the decision to stay within the neural tube or to project out of it.

Axonal growth regulation of fetal and embryonic stem cell-derived dopaminergic neurons by Netrin-1 and Slits

The physical restoration of dopamine circuits damaged or lost in Parkinson disease by implanting embryonic stem (ES)-derived cells may become a treatment. It is critical to understand responses of ES-derived dopamine (DA) neurons to guidance signals that determine axonal path and targeting. Using a collagen gel culture system, we examined effects of secreted molecules Netrin-1 and Slits on neurite outgrowth of fetal DA neurons and murine ES-differentiated DA neurons. We have previously shown that fetal DA neurons express DCC and Robo1/2 receptors and that Netrin-1 and Slit2 function as an attractant and a repellent for DA neurite outgrowth. In the present study, we observe that both Slit1 and Slit3 repel and inhibit neurite growth of fetal DA neurons. Here, we also demonstrate that ES-differentiated neurons including DA neurons express the Netrin receptor DCC and Slit receptor Robo proteins. In the gel culture system of ES cells, Netrin-1 promoted neurite outgrowth mediated by DCC receptor, and Slit1 and Slit3 were inhibitory for neurite outgrowth through Robo receptors. Slit2 appeared to exert inhibitory as well as repulsive effects in the coculture assay. However, unlike fetal DA neurites, no directed neurite outgrowth was observed in the cocultures of ES-derived DA neurons with Netrin-1-, Slit1-, and Slit3-producing cells. The findings suggest that ES-derived DA neurons generated by current protocols can respond to guidance cues in vitro in a similar manner to fetal cells but also exhibit distinct responses. This may result from developmental differences generated by present in vitro methods of cell patterning or conditioning during ES cell differentiation.