Dorsal Interneurons Research Articles

Insights into the organization of dorsal spinal cord pathways from an evolutionarily conserved raldh2 intronic enhancer

Comparative studies of the tetrapod raldh2 (aldh1a2) gene, which encodes a retinoic acid (RA) synthesis enzyme, have led to the identification of a dorsal spinal cord enhancer. Enhancer activity is directed dorsally to the roof plate and dorsal-most (dI1) interneurons through predicted Tcf- and Cdx-homeodomain binding sites and is repressed ventrally via predicted Tgif homeobox and ventral Lim-homeodomain binding sites. Raldh2 and Math1/Cath1 expression in mouse and chicken highlights a novel, transient, endogenous Raldh2 expression domain in dI1 interneurons, which give rise to ascending circuits and intraspinal commissural interneurons, suggesting roles for RA in the ontogeny of spinocerebellar and intraspinal proprioceptive circuits. Consistent with expression of raldh2 in the dorsal interneurons of tetrapods, we also found that raldh2 is expressed in dorsal interneurons throughout the agnathan spinal cord, suggesting ancestral roles for RA signaling in the ontogenesis of intraspinal proprioception.

SOX6 controls dorsal progenitor identity and interneuron diversity during neocortical development

SummaryThe extraordinary neuronal diversity of the central nervous system emerges largely from controlled spatial and temporal segregation of cell type-specific molecular regulators. Here, we report that the transcription factor SOX6 controls the molecular segregation of dorsal (pallial) from ventral (subpallial) telencephalic progenitors, and the differentiation of cortical interneurons, regulating forebrain progenitor and interneuron heterogeneity. During corticogenesis in mice, SOX6 and highly related SOX5 expression is largely mutually exclusive in pallial and subpallial progenitors, respectively, and remains mutually exclusive in a reverse pattern in postmitotic neuronal progeny. Loss of SOX6 from pallial progenitors causes their inappropriate expression of normally subpallium-restricted developmental controls, conferring mixed dorsal-ventral identity. In postmitotic cortical interneurons, loss of SOX6 dramatically disrupts the differentiation and diversity of cortical interneuron subtypes, analogous to SOX5 control over cortical projection neuron development. These data reveal SOX6 as a novel transcription factor regulator of both progenitor and cortical interneuron diversity during neocortical development.

S1.3 Phox2b and the autonomic control of breathing

In all vertebrates, the Phox2b transcription factor is specifically expressed and required in a limited set of neuronal types, which match for the most part the neurons of the visceral reflex circuits that regulate cardio-vascular, respiratory and digestive functions. Until recently, the role of Phox2b in the circuits that control breathing in mammals seemed patchier than in those that control blood pressure and digestion. This changed with the discovery that dominant mutations in Phox2b cause congenital central hypoventilation syndrome (CCHS). CCHS is a rare, but severe congenital disease characterized by central hypoventilation in sleep and an absent response to hypercapnia, which have been attributed to a defect in the autonomic regulation of breathing. We have modeled the most frequent human mutation in the mouse. The heterozygous Phox2b27Ala/+ mice breathe irregularly, do not respond to hypercapnia and die in the first hours after birth from central apnoea. The sole anatomical defect we found was the lack of CO2-sensitive, Phox2b-expressing glutamatergic cells in the retrotrapezoid nucleus (RTN). The location and molecular phenotype of RTN neurons characterized by their chemosensitivity matches that of the parafacial respiratory group (pFRG) identified as intrinsically rhythmic pre-inspiratory neurons. We found that the respiratory-like spontaneous rhythmic activity of the pFRG was lost in Phox2b27Ala/+ embryos, while the pre-Bötzinger complex, the best studied respiratory rhythm generator, functioned normally when tested in isolation. We thus propose that, in addition to the unresponsiveness to CO2, the lack of the pFRG oscillator contributes to the severe phenotype of the Phox2b27Ala/+ pups.pFRG/RTN neurons not only succumb to the dominant Phox2b27Ala mutation, but are also depleted in the absence of Phox2b. Using genetic tracing to study the ontogeny of pFRG/RTN neurons, we found that they originate from a population of Lbx1+ dorsal interneurons in rhombomeres 3 or 5. This gave us the opportunity to generate conditional Phox2b mutants using cre recombinase expression in either rhombomere 3/5 or in Lbx1+ interneurons. Like the Phox2b27Ala/+ mutation, the two conditional Phox2b mutations resulted in fatal central apnoea and abolished the activity of the embryonic pFRG.

Transcriptional control of axonal guidance and sorting in dorsal interneurons by the Lim-HD proteins Lhx9 and Lhx1

BackgroundLim-HD proteins control crucial aspects of neuronal differentiation, including subtype identity and axonal guidance. The Lim-HD proteins Lhx2/9 and Lhx1/5 are expressed in the dorsal spinal interneuron populations dI1 and dI2, respectively. While they are not required for cell fate acquisition, their role in patterning the axonal trajectory of dI1 and dI2 neurons remains incompletely understood.ResultsUsing newly identified dI1- and dI2-specific enhancers to trace axonal trajectories originating from these interneurons, we found that each population is subdivided into several distinct groups according to their axonal pathways. dI1 neurons project axons rostrally, either ipsi- or contra-laterally, while dI2 are mostly commissural neurons that project their axons rostrally and caudally. The longitudinal axonal tracks of each neuronal population self-fasciculate to form dI1- and dI2-specific bundles. The dI1 bundles are spatially located ventral relative to dI2 bundles. To examine the functional contribution of Lim-HD proteins to establishment of dI axonal projections, the Lim-HD code of dI neurons was altered by cell-specific ectopic expression. Expression of Lhx1 in dI1 neurons caused a repression of Lhx2/9 and imposed caudal projection to the caudal commissural dI1 neurons. Complementarily, when expressed in dI2 neurons, Lhx9 repressed Lhx1/5 and triggered a bias toward rostral projection in otherwise caudally projecting dI2 neurons, and ventral shift of the longitudinal axonal fascicule.ConclusionThe Lim-HD proteins Lhx9 and Lhx1 serve as a binary switch in controlling the rostral versus caudal longitudinal turning of the caudal commissural axons. Lhx1 determines caudal turning and Lhx9 triggers rostral turning.

Cux2functions downstream of Notch signaling to regulate dorsal interneuron formation in the spinal cord

Obtaining the diversity of interneuron subtypes in their appropriate numbers requires the orchestrated integration of progenitor proliferation with the regulation of differentiation. Here we demonstrate through loss-of-function studies in mice that the Cut homeodomain transcription factor Cux2 (Cutl2) plays an important role in regulating the formation of dorsal spinal cord interneurons. Furthermore, we show that Notch regulates Cux2 expression. Although Notch signaling can be inhibitory to the expression of proneural genes, it is also required for interneuron formation during spinal cord development. Our findings suggest that Cux2 might mediate some of the effects of Notch signaling on interneuron formation. Together with the requirement for Cux2 in cell cycle progression, our work highlights the mechanistic complexity in balancing neural progenitor maintenance and differentiation during spinal cord neurogenesis.

In vivo analysis of morphogenesis of choroid plexus in transgenic zebrafish

Materials and methods We generated a range of random transgenics by means of enhancer trapping using the Tol2 transposon that carries the EGFP gene controlled by a partial keratin8 promoter. It inserts randomly and reveals activity of enhancers in tissue-specific manner in stable transgenic lines. (Parinov et al., 2004; Choo et al.2006). One of these lines, ET33 expressed GFP in the roof plate, dorsal interneurons and meninx. The injection of the transposase mRNA into ET33 embryos resulted in transposition of Tol2 in germ cells and many novel transgenic lines including ET33E20 (Gateways) were generated. Gateways embryos demonstrate the GFP expression pattern mainly restricted to the brain and spinal cord, similar to that of several genes located close to the insertion site that we cloned in an attempt to define which genes expression is recapitulated by GFP expression in Gateways. Interestingly, the early GFP expression is localized to sites, where several cranial blood vessels develop later on. GFP also appears in the roof plate and in groups of cells at the midline in contact with brain ventricles, including the choroid plexus. Using the enhancer trap lines we studied the morphogenesis of the choroid plexus of IV ventricle using a combination of histology, immunohistochemistry and in vivo analysis during normal development.

State-, Timing-, and Pattern-Dependent Neuromodulation of Synaptic Strength by a Serotonergic Interneuron

Here we report that a serotonergic neuron evokes two distinct neuromodulatory actions with different state, timing, and firing pattern dependencies. These neuromodulatory actions may have important behavioral functions. In the mollusc, Tritonia diomedea, EPSCs evoked by ventral swim interneuron B (VSI) exhibited intrinsic plasticity; after a spike train, EPSC amplitude increased from a basal state to a potentiated state, which usually lasted >10 min. While the synapse was in a potentiated state, stimulation of a serotonergic dorsal swim interneuron (DSI) decreased VSI synaptic strength, returning it to a basal state. The extent of the DSI-evoked decrement was strongly correlated with the magnitude of the homosynaptic potentiation. This synaptic reset, or depotentiation, by DSI was blocked by the serotonin receptor antagonist methysergide and mimicked by a serotonin puff. In contrast to this state-dependent neuromodulatory action, we found that a previously described DSI-evoked transient enhancement of VSI synaptic strength was state-independent, producing the same multiplicative increase in EPSC amplitude regardless of whether the synapse was in a potentiated or basal state. These two actions also differed in their dependencies on the firing pattern of DSI and VSI action potentials. Results suggest that state-independent synaptic enhancement by DSI may play a short-term role during a swim motor pattern, whereas state-dependent actions may have longer-lasting consequences, resetting VSI synaptic strength after a swim bout. Thus, differences in two neuromodulatory actions at one synapse may allow a serotonergic neuron to play distinct roles at different stages of a motor pattern.

Axon Initial Segment Ca 2+ Channels Influence Action Potential Generation and Timing

Although action potentials are typically generated in the axon initial segment (AIS), the timing and pattern of action potentials are thought to depend on inward current originating in somatodendritic compartments. Using two-photon imaging, we show that T- and R-type voltage-gated Ca(2+) channels are colocalized with Na(+) channels in the AIS of dorsal cochlear nucleus interneurons and that activation of these Ca(2+) channels is essential to the generation and timing of action potential bursts known as complex spikes. During complex spikes, where Na(+)-mediated spikelets fire atop slower depolarizing conductances, selective block of AIS Ca(2+) channels delays spike timing and raises spike threshold. Furthermore, AIS Ca(2+) channel block can decrease the number of spikelets within a complex spike and can even block single, simple spikes. Similar results were found in cortex and cerebellum. Thus, voltage-gated Ca(2+) channels at the site of spike initiation play a key role in generating and shaping spike bursts.

Control of Precerebellar Neuron Development byOlig3bHLH Transcription Factor

The rhombic lip (RL) is the neuroepithelium immediately adjacent to the roof plate of the fourth ventricle, and it gives rise to various brainstem and cerebellar cell types. Our study shows that the bHLH (basic helix-loop-helix) transcription factor Olig3 is expressed in the progenitors of RL, and ablation of Olig3 significantly affects the development of RL. In Olig3-/- caudal RL, the expression level of Math1 in the dorsal interneuron 1 (dI1) domain is reduced, and the formation of four mossy-fiber nuclei is compromised; dI2-dI3 neurons are misspecified to dI4 interneurons, and the climbing-fiber neurons (inferior olive nucleus) are completely lost. In addition, the formation of brainstem (nor)adrenergic centers and first-order relay visceral sensory neurons is also dependent on Olig3. Therefore, Olig3 plays an important role in the fate specification and differentiation of caudal RL-derived neurons.

Transient Enhancement of Spike-Evoked Calcium Signaling by a Serotonergic Interneuron

Enhancement of presynaptic Ca(2+) signals is widely recognized as a potential mechanism for heterosynaptic potentiation of neurotransmitter release. Here we show that stimulation of a serotonergic interneuron increased spike-evoked Ca(2+) in a manner consistent with its neuromodulatory effect on synaptic transmission. In the gastropod mollusk, Tritonia diomedea, stimulation of a serotonergic dorsal swim interneuron (DSI) at physiological rates heterosynaptically enhances the strength of output synapses made by another swim interneuron, C2, onto neurons in the pedal ganglion. Using intracellular electrophysiological recording combined with real-time confocal imaging of C2 (loaded with Oregon Green Bapta 1), it was determined that DSI stimulation increases the amplitude of spike-evoked Ca(2+) signals in C2 without altering basal Ca(2+) signals. This neuromodulatory action was restricted to distal neurites of C2 where synapses with pedal neurons are located. The effect of DSI stimulation on C2 spike-evoked Ca(2+) signals resembled DSI heterosynaptic enhancement of C2 synapses in several measures: both decayed within 15 s, both were abolished by the serotonin receptor antagonist, methysergide, and both were independent of DSI's depolarizing actions on C2. A brief puff of serotonin could mimic the enhancement of spike-evoked Ca(2+) signals in the distal neurites of C2, but larger puffs or bath-applied serotonin elicited nonphysiological effects. These results suggest that DSI heterosynaptic enhancement of C2 synaptic strength may be mediated by a local enhancement of spike-evoked Ca(2+) signals in the distal neurites of C2.

Different functions for homologous serotonergic interneurons and serotonin in species-specific rhythmic behaviours

Closely related species can exhibit different behaviours despite homologous neural substrates. The nudibranch molluscs Tritonia diomedea and Melibe leonina swim differently, yet their nervous systems contain homologous serotonergic neurons. In Tritonia, the dorsal swim interneurons (DSIs) are members of the swim central pattern generator (CPG) and their neurotransmitter serotonin is both necessary and sufficient to elicit a swim motor pattern. Here it is shown that the DSI homologues in Melibe, the cerebral serotonergic posterior-A neurons (CeSP-As), are extrinsic to the swim CPG, and that neither the CeSP-As nor their neurotransmitter serotonin is necessary for swim motor pattern initiation, which occurred when the CeSP-As were inactive. Furthermore, the serotonin antagonist methysergide blocked the effects of both the serotonin and CeSP-As but did not prevent the production of a swim motor pattern. However, the CeSP-As and serotonin could influence the Melibe swim circuit; depolarization of a cerebral serotonergic posterior-A was sufficient to initiate a swim motor pattern and hyperpolarization of a CeSP-A temporarily halted an ongoing swim motor pattern. Serotonin itself was sufficient to initiate a swim motor pattern or make an ongoing swim motor pattern more regular. Thus, evolution of species-specific behaviour involved alterations in the functions of identified homologous neurons and their neurotransmitter.

A Molecular Program for Contralateral Trajectory: Rig-1 Control by LIM Homeodomain Transcription Factors

Despite increasing evidence for transcriptional control of neural connectivity, how transcription factors regulate discrete steps in axon guidance remains obscure. Projection neurons in the dorsal spinal cord relay sensory signals to higher brain centers. Some projection neurons send their axons ipsilaterally, whereas others, commissural neurons, send axons contralaterally. We show that two closely related LIM homeodomain proteins, Lhx2 and Lhx9, are expressed by a set of commissural relay neurons (dI1c neurons) and are required for the dI1c axon projection. Midline crossing by dI1c axons is lost in Lhx2/9 double mutants, a defect that results from loss of expression of Rig-1 from dI1c axons. Lhx2 binds to a conserved motif in the Rig-1 gene, suggesting that Lhx2/9 regulate directly the expression of Rig-1. Our findings reveal a link between the transcriptional programs that define neuronal subtype identity and the expression of receptors that guide distinctive aspects of their trajectory.

Neuroendocrine secretory protein 55 (NESP55) in the spinal cord of rat: An immunocytochemical study

The immunohistochemical expression of a novel chromogranin-like protein, neuroendocrine secretory protein 55 (NESP55), in the rat spinal cord was investigated. NESP55-immunoreactive cells were detected in the ventral horn, intermediate laminae, and deep dorsal horn, comprising motoneurons, autonomic neurons, and interneurons throughout all spinal segments. Within laminae I-II of the dorsal horn, one or two NESP55-positive cells were often seen. Nerve fibers also contained NESP55 immunoreactivity (IR) and were particularly prominent in the ventral horn. No nerve terminals/varicosities appeared to contain NESP55 in any spinal lamina. Double-staining experiments revealed that a high proportion of the NESP55-positive neurons were cholinergic. Moreover, NESP55-IR in the motoneurons was evenly distributed in the whole cytoplasm with a finely granular appearance. In contrast, the fluorescent material in the preganglionic neurons was concentrated in the perinuclear region and largely overlapped with the trans-Golgi network marker TGN38. Our data provide detailed morphological information on the distribution of NESP55-IR in the rat spinal cord. Also, the differential intracellular expression of NESP55-IR in the spinal motoneurons and autonomic neurons suggests that NESP55 may be processed into different secretory granules and may be involved in both constitutive and regulated pathways in these neurons.

Potentiation Phase of Spike Timing-Dependent Neuromodulation by a Serotonergic Interneuron Involves an Increase in the Fraction of Transmitter Release

In the mollusk, Tritonia diomedea, the serotonergic dorsal swim interneuron (DSI) produces spike timing-dependent neuromodulation (STDN) of the synaptic output of ventral swim interneuron B (VSI) resulting in a biphasic, bidirectional change of synaptic strength characterized by a rapid heterosynaptic potentiation followed by a more prolonged heterosynaptic depression. This study examined the mechanism underlying the potentiation phase of STDN. In the presence of 4-aminopyridine, which blocks the depression phase and enhances transmitter release from VSI, rapidly stimulating VSI led to a steady-state level of transmitter depletion during which potentiation by DSI or serotonin (5-HT) was eliminated. Cumulative plots of excitatory postsynaptic currents were used to estimate changes in the size and replenishment rate of the readily releasable pool (RRP) and the fraction of release. 5-HT application increased transmitter release without altering replenishment rate. The magnitude of 5-HT-evoked potentiation correlated with the increase in the fraction of release. A phenomenological model of the synapse further supported the hypothesis that 5-HT-induced potentiation was caused by an increase in the fraction of release and correctly predicted no change in frequency facilitation. A dynamic version of the model correctly predicted the effect of DSI stimulation under a variety of conditions. Finally, depletion of internal Ca(2+) stores with cyclopiazonic acid showed that Ca(2+) from internal stores is necessary for the 5-HT-induced potentiation. The data indicate that 5-HT released from DSI increases the fraction of the RRP discharged during VSI action potentials using a mechanism that involves Ca(2+) extrusion from internal stores, resulting in time- and state-dependent neuromodulation.

Parameter Space Analysis Suggests Multi-Site Plasticity Contributes to Motor Pattern Initiation inTritonia

This research examines the mechanisms that initiate rhythmic activity in the episodic central pattern generator (CPG) underlying escape swimming in the gastropod mollusk Tritonia diomedea. Activation of the network is triggered by extrinsic excitatory input but also accompanied by intrinsic neuromodulation and the recruitment of additional excitation into the circuit. To examine how these factors influence circuit activation, a detailed simulation of the unmodulated CPG network was constructed from an extensive set of physiological measurements. In this model, extrinsic input alone is insufficient to initiate rhythmic activity, confirming that additional processes are involved in circuit activation. However, incorporating known neuromodulatory and polysynaptic effects into the model still failed to enable rhythmic activity, suggesting that additional circuit features are also required. To delineate the additional activation requirements, a large-scale parameter-space analysis was conducted (~2 x 10(6) configurations). The results suggest that initiation of the swim motor pattern requires substantial reconfiguration at multiple sites within the network, especially to recruit ventral swim interneuron-B (VSI) activity and increase coupling between the dorsal swim interneurons (DSIs) and cerebral neuron 2 (C2) coupling. Within the parameter space examined, we observed a tendency for rhythmic activity to be spontaneous and self-sustaining. This suggests that initiation of episodic rhythmic activity may involve temporarily restructuring a nonrhythmic network into a persistent oscillator. In particular, the time course of neuromodulatory effects may control both activation and termination of rhythmic bursting.

Cdk5 selectively affects the migration of different populations of neurons in the developing spinal cord

It has been shown that cyclin-dependent kinase 5 (Cdk5) is crucial for neuronal migration and survival in the brain. However, the role of Cdk5 in neuronal migration in the spinal cord has never been investigated. The present study is the first to show that Cdk5 affects the migration of different populations of neurons in the developing spinal cord. In the absence of Cdk5, at least four neuronal populations failed to migrate to their final destinations: sympathetic and parasympathetic preganglionic neurons, as well as dorsally originating and ventrally originating (U-shaped group) diaphorase-positive dorsal horn interneurons. In contrast, the migration of somatic motor neurons and various types of ventral and dorsal interneurons was unaffected by the absence of Cdk5. Moreover, our results suggest that Cdk5-dependent migration in the developing spinal cord is axon- or glial fiber-mediated. Finally, our results show that sympathetic preganglionic neurons and somatic motor neurons in Cdk5-deficient mice continue to extend processes and project toward their normal target areas, suggesting that Cdk5 has no obvious effects on axonal outgrowth and guidance mechanisms of these two neuronal populations in spinal cord development.

Selective expression of Bhlhb5 in subsets of early‐born interneurons and late‐born association neurons in the spinal cord

Accumulating evidence has suggested that the basic helix-loop-helix transcription factors play important roles in controlling neuronal fate specification and differentiation in the developing central nervous system. In this study, we report a detailed immunological study on the expression of Bhlhb5 in embryonic mouse spinal cord with a newly developed antibody. At the early stage of neural development, Bhlhb5 is specifically expressed in dI6 dorsal interneurons and in V1 and V2 ventral interneurons. At late stages of development, Bhlhb5 expression is detected in a subset of late-born dorsal association interneurons that migrate into the uppermost layer of the dorsal horn.

Expression of D-type cyclins in differentiating cells of the mouse spinal cord

The D-type cyclins form complexes with the cyclin dependent (CD) kinases CDK4 and CDK6 and promote the G1-S phase transition of the cell cycle by antagonizing the retinoblastoma suppresser protein pRB. In the developing nervous system D-type cyclins show spatially and temporally dynamic patterns of expression. We demonstrated that cyclin D1 was transiently expressed in differentiating spinal cord ventral interneurons while cyclin D3 protein was expressed in differentiating motor neurons and dorsal interneurons. This expression of cyclin D3 in neurons of the mantle zone was extended to all regions of the spinal cord at E15.5. The results suggest that cyclin D1 and D3 have specific functions in differentiating neurons. Similarly, in the developing midbrain-hindbrain region the D-type cyclins were expressed in different subsets of cells. Our results argue in favor of different functions for D-type cyclins during proliferation and differentiation of neural progenitors.

Homologues of serotonergic central pattern generator neurons in related nudibranch molluscs with divergent behaviors

Homologues of a neuron that contributes to a species-specific behavior were identified and characterized in species lacking that behavior. The nudibranch Tritonia diomedea swims by flexing its body dorsally and ventrally. The dorsal swim interneurons (DSIs) are components of the central pattern generator (CPG) underlying this rhythmic motor pattern and also activate crawling. Homologues of the DSIs were identified in six nudibranchs that do not exhibit dorsal-ventral swimming: Tochuina tetraquetra, Melibe leonina, Dendronotus iris, D. frondosus, Armina californica, and Triopha catalinae. Homology was based upon shared features that distinguish the DSIs from all other neurons: (1) serotonin immunoreactivity, (2) location in the Cerebral serotonergic posterior (CeSP) cluster, and (3) axon projection to the contralateral pedal ganglion. The DSI homologues, named CeSP-A neurons, share additional features with the DSIs: irregular basal firing, synchronous inputs, electrical coupling, and reciprocal inhibition. Unlike the DSIs, the CeSP-A neurons were not rhythmically active in response to nerve stimulation. The CeSP-A neurons in Tochuina and Triopha also excited homologues of the Tritonia Pd5 neuron, a crawling efferent. Thus, the CeSP-A neurons and the DSIs may be part of a conserved network related to crawling that may have been co-opted into a rhythmic swim CPG in Tritonia.

Bmp and Wnt/β-catenin signals control expression of the transcription factor Olig3 and the specification of spinal cord neurons

In the developing spinal cord, signals of the roof plate pattern the dorsal progenitor domain and control the specification of three neuron types, dorsal interneurons dI1, dI2, and dI3. Bmp and Wnt/β-catenin signals as well as transcription factors like Olig3 or Ngn1/2 are essential in this process. We have studied the epistatic relationship between Bmp and Wnt/β-catenin signals and the transcription factor Olig3 in dorsal spinal cord patterning. Using β-catenin gain-of-function and compound β-catenin gain-of-function/ Olig3 loss-of-function mutations in mice, we could show that Wnt/β-catenin signals act upstream of Olig3 in the specification of dI2 and dI3 neurons. The analysis of such compound mutant mice allowed us to distinguish between the two functions of Wnt/β-catenin signaling in proliferation and patterning of dorsal progenitors. Using electroporation of chick spinal cords, we further demonstrate that Bmp signals act upstream of Wnt/β-catenin in the regulation of Olig3 and that Wnt/β-catenin signals play an instructive role in controlling Olig3 expression. We conclude that Wnt/β-catenin and BMP signals coordinately control the specification of dorsal neurons in the spinal cord.