BMPs as mediators of roof plate repulsion of commissural neurons

Assaying the ability of diffusible signaling molecules to reorient embryonic spinal commissural axons

Dorsal commissural axons in the vertebrate spinal cord(1) have been an invaluable model system in which to identify axon guidance signals. Here, we describe an in vitro assay, "the reorientation assay", that has been used extensively to study the effect of extrinsic and intrinsic signals on the orientation of commissural axons(2). This assay was developed by numerous people in the laboratories of Jane Dodd, Thomas Jessell and Andrew Lumsden (see acknowledgements for more details) and versions of this assay were used to demonstrate the reorientation activities of key axon guidance molecules, including the BMP chemorepellent in the roof plate(3,4) and the chemoattractive activities of Netrin1(5) and Sonic Hedgehog (Shh)(6) in the floor plate in the spinal cord. Explants comprising 2-3 segments of the dorsal two-thirds of spinal cord are dissected from embryonic day (E) 11 rats and cultured in three dimensional collagen gels(7). E11 dorsal spinal explants contain newly born commissural neurons, which can be identified by their axonal expression of the glycoprotein, Tag1(8). Over the course of 30-40 hours in culture, the commissural axon trajectory is recapitulated in these dorsal explants with a time course similar to that seen in vivo. This axonal trajectory can be challenged by placing either test tissues or a COS cell aggregate expressing a candidate signaling molecule in contact with one of the lateral edges of the dorsal explant. Commissural axons extending in the vicinity of the appended tissue will grow under the influence of both the endogenous roof plate and signals from the ectopic lateral tissue. The degree to which commissural axons are reoriented under these circumstances can be quantified. Using this assay, it is possible both to examine the sufficiency of a particular signal to reorient commissural axons(3,4) as well the necessity for this signal to direct the commissural trajectory(9).

A midline switch of receptor processing regulates commissural axon guidance in vertebrates

Commissural axon guidance requires complex modulations of growth cone sensitivity to midline-derived cues, but underlying mechanisms in vertebrates remain largely unknown. By using combinations of ex vivo and in vivo approaches, we uncovered a molecular pathway controlling the gain of response to a midline repellent, Semaphorin3B (Sema3B). First, we provide evidence that Semaphorin3B/Plexin-A1 signaling participates in the guidance of commissural projections at the vertebrate ventral midline. Second, we show that, at the precrossing stage, commissural neurons synthesize the Neuropilin-2 and Plexin-A1 Semaphorin3B receptor subunits, but Plexin-A1 expression is prevented by a calpain1-mediated processing, resulting in silencing commissural responsiveness. Third, we report that, during floor plate (FP) in-growth, calpain1 activity is suppressed by local signals, allowing Plexin-A1 accumulation in the growth cone and sensitization to Sema3B. Finally, we show that the FP cue NrCAM mediates the switch of Plexin-A1 processing underlying growth cone sensitization to Sema3B. This reveals pathway-dependent modulation of guidance receptor processing as a novel mechanism for regulating guidance decisions at intermediate targets.

Axon guidance at the midline: of mice and flies

In bilaterally symmetric organisms, the midline is a critical organizing center for the developing central nervous system. There is a striking conservation of the molecules and mechanisms that control axon path finding at the midline in vertebrate and invertebrate nervous systems. The majority of axons in the CNS cross the midline before projecting to their contralateral synaptic targets and this crossing decision is under exquisite spatial and temporal regulation. Growing commissural axons initially respond to attractive signals, while inhibiting responses to repulsive signals. Once across, repulsion dominates, allowing axons to leave and preventing them from re-entering the midline. Here we review recent advances in flies and mice that illuminate the molecular mechanisms underlying the establishment of precise connectivity at the midline.

ActRIIA and BMPRII Type II BMP receptor subunits selectively required for Smad4-independent BMP7-evoked chemotaxis

Bone morphogenetic protein (BMP)-evoked reorientation and chemotaxis of cells occurs with rapid onset and involves events local to the cell membrane. The signaling pathways underlying these rapid processes likely diverge from those mediating classical transcriptional responses to BMPs but it remains unclear how BMP receptors are utilized to generate distinct intracellular mechanisms. We show that BMP7-evoked chemotaxis of monocytic cells depends on the activity of canonical type II BMP receptors. Although the three canonical type II BMP receptors are expressed in monocytic cells, inhibition of receptor subunit expression by RNAi reveals that ActRIIA and BMPRII, but not ActRIIB, are each essential for BMP7-evoked chemotaxis but not required individually for BMP-mediated induction. Furthermore, the chemotactic response to BMP7 does not involve canonical Smad4-dependent signaling but acts through PI3K-dependent signaling, illustrating selective activation of distinct intracellular events through differential engagement of receptors. We suggest a model of a BMP receptor complex in which the coordinated activity of ActRIIA and BMPRII receptor subunits selectively mediates the chemotactic response to BMP7.

Emerging mechanisms in morphogen-mediated axon guidance

Early in animal development, gradients of secreted morphogenic molecules, such as Sonic hedgehog (Shh), Wnt and TGFbeta/Bmp family members, regulate cell proliferation and determine the fate and phenotype of the target cells by activating well-characterized signalling pathways, which ultimately control gene transcription. Shh, Wnt and TGFbeta/Bmp signalling also play an important and evolutionary conserved role in neural circuit assembly. They regulate neuronal polarization, axon and dendrite development and synaptogenesis, processes that require rapid and local changes in cytoskeletal organization and plasma membrane components. A key question then is whether morphogen signalling at the growth cone uses similar mechanisms and intracellular pathway components to those described for morphogen-mediated cell specification. This review discusses recent advances towards the understanding of this problem, showing how Shh, Wnt and TGFbeta/Bmp have adapted their 'classical' signalling pathways or adopted alternative and novel molecular mechanisms to influence different aspects of neuronal circuit formation.

Characterization of wise protein and its molecular mechanism to interact with both Wnt and BMP signals

Cross-talk of BMP and Wnt signaling pathways has been implicated in many aspects of biological events during embryogenesis and in adulthood. A secreted protein Wise and its orthologs (Sostdc1, USAG-1, and Ectodin) have been shown to modulate Wnt signaling and also inhibit BMP signals. Modulation of Wnt signaling activity by Wise is brought about by an interaction with the Wnt co-receptor LRP6, whereas BMP inhibition is by binding to BMP ligands. Here we have investigated the mode of action of Wise on Wnt and BMP signals. It was found that Wise binds LRP6 through one of three loops formed by the cystine knot. The Wise deletion construct lacking the LRP6-interacting loop domain nevertheless binds BMP4 and inhibits BMP signals. Moreover, BMP4 does not interfere with Wise-LRP6 binding, suggesting separate domains for the physical interaction. Functional assays also show that the ability of Wise to block Wnt1 activity through LRP6 is not impeded by BMP4. In contrast, the ability of Wise to inhibit BMP4 is prevented by additional LRP6, implying a preference of Wise in binding LRP6 over BMP4. In addition to the interaction of Wise with BMP4 and LRP6, the molecular characteristics of Wise, such as glycosylation and association with heparan sulfate proteoglycans on the cell surface, are suggested. This study helps to understand the multiple functions of Wise at the molecular level and suggests a possible role for Wise in balancing Wnt and BMP signals.

Traction on immobilized netrin-1 is sufficient to reorient axons

During embryonic development, axons are guided to their target by patterning proteins encountered along their trajectory. These cues can be linked to the cells that produce them or secreted into the extracellular matrix. Whether secreted cues, like netrin-1, provide traction for the growth cone when they become attached to the extracellular matrix is unclear. Advancing spinal commissural neuron growth cones were shown to generate local forces of 4 to 15 piconewtons but, when confronted with immobilized netrin-1, generated traction forces in excess of 63 piconewtons on netrin-1 that can redirect the trajectory of the axon.

Sonic hedgehog guides axons through a noncanonical, Src-family-kinase-dependent signaling pathway

Sonic hedgehog (Shh) plays essential roles in developmental events such as cell fate specification and axon guidance. Shh induces cell fate specification through canonical Shh signaling, mediated by transcription. However, the mechanism by which Shh guides axons is unknown. To study this, we developed an in vitro assay for axon guidance, in which neurons can be imaged while responding to a defined gradient of a chemical cue. Axons of dissociated commissural neurons placed in a Shh gradient turned rapidly toward increasing concentrations of Shh. Consistent with this rapid response, we showed that attraction by Shh does not require transcription. Instead, Shh stimulates the activity of Src family kinase (SFK) members in a Smoothened-dependent manner. Moreover, SFK activity is required for Shh-mediated guidance of commissural axons, but not for induction of Gli transcriptional reporter activity. Together, these results indicate that Shh acts via a rapidly acting, noncanonical signaling pathway to guide axons.

FGF8 signaling regulates growth of midbrain dopaminergic axons by inducing semaphorin 3F

Accumulating evidence indicates that signaling centers controlling the dorsoventral (DV) polarization of the neural tube, the roof plate and the floor plate, play crucial roles in axon guidance along the DV axis. However, the role of signaling centers regulating the rostrocaudal (RC) polarization of the neural tube in axon guidance along the RC axis remains unknown. Here, we show that a signaling center located at the midbrain-hindbrain boundary (MHB) regulates the rostrally directed growth of axons from midbrain dopaminergic neurons (mDANs). We found that beads soaked with fibroblast growth factor 8 (FGF8), a signaling molecule that mediates patterning activities of the MHB, repelled mDAN axons that extended through the diencephalon. This repulsion may be mediated by semaphorin 3F (sema3F) because (1) FGF8-soaked beads induced an increase in expression of sema3F, (2) sema3F expression in the midbrain was essentially abolished by the application of an FGF receptor tyrosine kinase inhibitor, and (3) mDAN axonal growth was also inhibited by sema3F. Furthermore, mDAN axons expressed a sema3F receptor, neuropilin-2 (nrp2), and the removal of nrp-2 by gene targeting caused caudal growth of mDAN axons. These results indicate that the MHB signaling center regulates the growth polarity of mDAN axons along the RC axis by inducing sema3F.

Motor neurons with axial muscle projections specified by Wnt4/5 signaling

Axial muscles are innervated by motor neurons of the median motor column (MMC). In contrast to the segmentally restricted motor columns that innervate limb, body wall, and neuronal targets, MMC neurons are generated along the entire length of the spinal cord. We show that the specification of MMC fate involves a dorsoventral signaling program mediated by three Wnt proteins (Wnt4, Wnt5a, and Wnt5b) expressed in and around the floor plate. These Wnts appear to establish a ventral^high to dorsal^low signaling gradient and promote MMC identity and connectivity by maintaining expression of the LIM homeodomain proteins Lhx3/4 in spinal motor neurons. Elevation of Wnt4/5 activity generates additional MMC neurons at the expense of other motor neuron columnar subtypes, whereas depletion of Wnt4/5 activity inhibits the production of MMC neurons. Thus, two dorsoventral signaling pathways, mediated by Shh and Wnt4/5, are required to establish an early binary divergence in motor neuron columnar identity.

Draxin, a repulsive guidance protein for spinal cord and forebrain commissures

Axon guidance proteins are critical for the correct wiring of the nervous system during development. Several axon guidance cues and their family members have been well characterized. More unidentified axon guidance cues are assumed to participate in the formation of the extremely complex nervous system. We identified a secreted protein, draxin, that shares no homology with known guidance cues. Draxin inhibited or repelled neurite outgrowth from dorsal spinal cord and cortical explants in vitro. Ectopically expressed draxin inhibited growth or caused misrouting of chick spinal cord commissural axons in vivo. draxin knockout mice showed defasciculation of spinal cord commissural axons and absence of all forebrain commissures. Thus, draxin is a previously unknown chemorepulsive axon guidance molecule required for the development of spinal cord and forebrain commissures.

UNC-129 regulates the balance between UNC-40 dependent and independent UNC-5 signaling pathways

The UNC-5 receptor mediates axon repulsion from UNC-6/netrin through UNC-40 dependent (‘UNC-5+UNC-40’) and independent (‘UNC-5-alone’) signaling pathways. A requirement for UNC-40 dependent signaling has been shown in long-range repulsion from UNC-6/netrin, however, the mechanisms used to regulate distinct UNC-5 signaling pathways are poorly understood. Here we demonstrate that the C. elegans TGF-β family ligand UNC-129, graded opposite to UNC-6/netrin, functions independent of the canonical TGF-β receptors to regulate UNC-5 cellular responses. We provide evidence that UNC-129 facilitates long-range repulsive guidance of UNC-6 by enhancing ‘UNC-5+UNC-40’ signaling at the expense of ‘UNC-5-alone’ signaling through interaction with the UNC-5 receptor. This increases the set point sensitivity of growth cones to UNC-6/netrin as they simultaneously migrate up the UNC-129 gradient and down the UNC-6 gradient. Similar regulatory interactions between oppositely graded extracellular cues may be a common theme in guided cell and axon migrations.

The growth cone as seen through Cajal's original histological preparations and publications

During the development of the nervous system, each neuron must contact its appropriate target cell in order to establish its specific connections. More than a century ago, Ramon y Cajal discovered an amoeboid-like structure at the end of the axon of developing nerve cells. He called this structure the growth cone [cono de crecimiento] and he proposed that this structure was guided towards its target tissue by chemical substances secreted by the different cells that line its course. We have reviewed the discovery of the growth cone by Cajal using his original publications, his original scientific drawings, and by studying his histological preparations conserved at the "Instituto Cajal" (Madrid, Spain).(1) We found a very good correlation between the structure of the growth cone in the Golgi-impregnated and reduced silver-nitrate-stained material used by Cajal, and that which is revealed with present-day methods. Finally, Cajal's view of the function of the growth cone and his chemotactic hypothesis will also be considered in the light of present-day knowledge.

Identification and subclassification of new Atoh1 derived cell populations during mouse spinal cord development

At spinal levels, sensory information pertaining to body positioning (proprioception) is relayed to the cerebellum by the spinocerebellar tracts (SCTs). In the past we revealed the basic helix-loop-helix transcription factor Atoh1 (Math1) to be important for establishing Dorsal Progenitor 1 (DP1) commissural interneurons, which comprise a subset of proprioceptive interneurons. Given there exists multiple subdivisions of the SCT we asked whether Atoh1 may also play a role in specifying other cell types in the spinal cord. Here, we reveal the generation of at least three DP1 derived interneuron populations that reside at spatially restricted positions along the rostral-caudal axis. Each of these cell populations expresses distinct markers and anatomically coincides with the cell bodies of the various subdivisions of the SCT. In addition, we found that as development proceeds (e.g. by E13.5) Atoh1 expression becomes apparent in the dorsal midline in the region of the roof plate (RP). Interestingly, we find that cells derived from Atoh1 expressing RP progenitors express SSEA-1, and in the absence of Atoh1 these progenitors become SOX9 positive. Altogether we reveal the existence of multiple Atoh1 dependent cell types in the spinal cord, and uncover a novel progenitor domain that arises late in development.

TGF-beta signals regulate axonal development through distinct Smad-independent mechanisms

Proper nerve connections form when growing axons terminate at the correct postsynaptic target. Here I show that Transforming growth factor beta (TGFbeta) signals regulate axon growth. In most contexts, TGFbeta signals are tightly linked to Smad transcriptional activity. Although known to exist, how Smad-independent pathways mediate TGFbeta responses in vivo is unclear. In Drosophila mushroom body (MB) neurons, loss of the TGFbeta receptor Baboon (Babo) results in axon overextension. Conversely, misexpression of constitutively active Babo results in premature axon termination. Smad activity is not required for these phenotypes. This study shows that Babo signals require the Rho GTPases Rho1 and Rac, and LIM kinase1 (LIMK1), which regulate the actin cytoskeleton. Contrary to the well-established receptor activation model, in which type 1 receptors act downstream of type 2 receptors, this study shows that the type 2 receptors Wishful thinking (Wit) and Punt act downstream of the Babo type 1 receptor. Wit and Punt regulate axon growth independently, and interchangeably, through LIMK1-dependent and -independent mechanisms. Thus, novel TGFbeta receptor interactions control non-Smad signals and regulate multiple aspects of axonal development in vivo.

Neuroregenerative strategies in the brain: emerging significance of bone morphogenetic protein 7 (BMP7)

Every year thousands of people suffer from brain injuries and stroke, and develop motor, sensory, and cognitive problems as a result of neuronal loss in the brain. Unfortunately, the damaged brain has a limited ability to enact repair and current modes of treatment are not sufficient to offset the damage. An extensive list of growth factors, neurotrophic factors, cytokines, and drugs has been explored as potential therapies. However, only a limited number of them may actually have the potential to effectively offset the brain injury or stroke-related problems. One of the treatments considered for future brain repair is bone morphogenetic protein 7 (BMP7), a factor currently used in patients to treat non-neurological diseases. The clinical application of BMP7 is based on its neuroprotective role in stroke animal models. This paper reviews the current approaches considered for brain repair and discusses the novel convergent strategies by which BMP7 potentially can induce neuroregeneration.

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.

Dissection and culture of embryonic spinal commissural neurons

Studies of spinal commissural neurons have provided substantial insight into the mechanisms that regulate axon guidance. Explants of embryonic spinal cords and isolated spinal commissural neurons have been important experimental tools for the identification and characterization of several guidance cues, including netrins, semaphorins, slits, sonic hedgehog, BMPs, and wnts. In this unit, protocols are provided for establishing these explant assays to assess the outgrowth and turning capacity of commissural axons. In addition, methods are included for preparing cultures highly enriched with embryonic commissural neurons, which have been used to probe the biochemical signaling mechanisms regulating axon guidance.

Phosphatidylinositol-3-kinase-atypical protein kinase C signaling is required for Wnt attraction and anterior-posterior axon guidance

Wnt proteins are conserved axon guidance cues that control growth cone navigation. However, the intracellular signaling mechanisms that mediate growth cone turning in response to Wnts are unknown. We previously showed that Wnt-Frizzled signaling directs spinal cord commissural axons to turn anteriorly after midline crossing through an attractive mechanism. Here we show that atypical protein kinase C (aPKC), is required for Wnt-mediated attraction of commissural axons and proper anterior-posterior (A-P) pathfinding. A PKCzeta pseudosubstrate, a specific blocker of aPKC activity, and expression of a kinase-defective PKCzeta mutant in commissural neurons resulted in A-P randomization in "open-book" explants. Upstream of PKCzeta, heterotrimeric G-proteins and phosphatidylinositol-3-kinases (PI3Ks), are also required for A-P guidance, because pertussis toxin, wortmannin, and expression of a p110gamma kinase-defective construct all resulted in A-P randomization. Overexpression of p110gamma, the catalytic subunit of PI3Kgamma, caused precocious anterior turning of commissural axons before midline crossing in open-book explants and caused dissociated precrossing commissural axons, which are normally insensitive to Wnt attraction, to turn toward Wnt4-expressing cells. Therefore, we propose that atypical PKC signaling is required for Wnt-mediated A-P axon guidance and that PI3K can act as a switch to activate Wnt responsiveness during midline crossing.

BMP type I receptor complexes have distinct activities mediating cell fate and axon guidance decisions

The finding that morphogens, signalling molecules that specify cell identity, also act as axon guidance molecules has raised the possibility that the mechanisms that establish neural cell fate are also used to assemble neuronal circuits. It remains unresolved, however, how cells differentially transduce the cell fate specification and guidance activities of morphogens. To address this question, we have examined the mechanism by which the Bone morphogenetic proteins (BMPs) guide commissural axons in the developing spinal cord. In contrast to studies that have suggested that morphogens direct axon guidance decisions using non-canonical signal transduction factors, our results indicate that canonical components of the BMP signalling pathway, the type I BMP receptors (BMPRs), are both necessary and sufficient to specify the fate of commissural neurons and guide their axonal projections. However,whereas the induction of cell fate is a shared property of both type I BMPRs,axon guidance is chiefly mediated by only one of the type I BMPRs, BMPRIB. Taken together, these results indicate that the diverse activities of BMP morphogens can be accounted for by the differential use of distinct components of the canonical BMPR complex.

Analysis of gene expression during neurite outgrowth and regeneration

Background

The ability of a neuron to regenerate functional connections after injury is influenced by both its intrinsic state and also by extrinsic cues in its surroundings. Investigations of the transcriptional changes undergone by neurons during in vivo models of injury and regeneration have revealed many transcripts associated with these processes. Because of the complex milieu of interactions in vivo, these results include not only expression changes directly related to regenerative outgrowth and but also unrelated responses to surrounding cells and signals. In vitro models of neurite outgrowth provide a means to study the intrinsic transcriptional patterns of neurite outgrowth in the absence of extensive extrinsic cues from nearby cells and tissues.

Results

We have undertaken a genome-wide study of transcriptional activity in embryonic superior cervical ganglia (SCG) and dorsal root ganglia (DRG) during a time course of neurite outgrowth in vitro. Gene expression observed in these models likely includes both developmental gene expression patterns and regenerative responses to axotomy, which occurs as the result of tissue dissection. Comparison across both models revealed many genes with similar gene expression patterns during neurite outgrowth. These patterns were minimally affected by exposure to the potent inhibitory cue Semaphorin3A, indicating that this extrinsic cue does not exert major effects at the level of nuclear transcription. We also compared our data to several published studies of DRG and SCG gene expression in animal models of regeneration, and found the expression of a large number of genes in common between neurite outgrowth in vitro and regeneration in vivo.

Conclusion

Many gene expression changes undergone by SCG and DRG during in vitro outgrowth are shared between these two tissue types and in common with in vivo regeneration models. This suggests that the genes identified in this in vitro study may represent new candidates worthy of further study for potential roles in the therapeutic regrowth of neuronal connections.

Navigating their way to the clinic: emerging roles for axon guidance molecules in neurological disorders and injury

The mechanisms underlying formation of the basic network of the nervous system are of fundamental interest in developmental neurobiology. During the wiring of the nervous system, newborn neurons send axons that travel long distances to their targets. These axons are directed by environmental cues, known as guidance cues, to their correct destinations. Through extensive studies in vertebrates and invertebrates many of the guidance cues and their receptors have been identified. Recently, guidance molecules have been suggested to have important roles in pathological conditions of the nervous system. Mutations in guidance receptors have been associated with hereditary neurological disorders, and deregulation of guidance cues might be associated with predisposition to epilepsy. In addition, it was suggested that guidance molecules play roles in the ability of the adult nervous system to recover and repair after injury. Thus, molecules that were first discovered as "developmental cues" are now emerging as important factors in neurological disease and injury in the adult.

Proteolysis and membrane capture of F-spondin generates combinatorial guidance cues from a single molecule

The formation of neuronal networks is governed by a limited number of guidance molecules, yet it is immensely complex. The complexity of guidance cues is augmented by posttranslational modification of guidance molecules and their receptors. We report here that cleavage of the floor plate guidance molecule F-spondin generates two functionally opposing fragments: a short-range repellent protein deposited in the membrane of floor plate cells and an adhesive protein that accumulates at the basement membrane. Their coordinated activity, acting respectively as a short-range repellant and a permissive short-range attractant, constricts commissural axons to the basement membrane beneath the floor plate cells. We further demonstrate that the repulsive activity of the inhibitory fragment of F-spondin requires its presentation by the lipoprotein receptor–related protein (LRP) receptors apolipoprotein E receptor 2, LRP2/megalin, and LRP4, which are expressed in the floor plate. Thus, proteolysis and membrane interaction coordinate combinatorial guidance signaling originating from a single guidance cue.

BMP gradients steer nerve growth cones by a balancing act of LIM kinase and Slingshot phosphatase on ADF/cofilin

Bone morphogenic proteins (BMPs) are involved in axon pathfinding, but how they guide growth cones remains elusive. In this study, we report that a BMP7 gradient elicits bidirectional turning responses from nerve growth cones by acting through LIM kinase (LIMK) and Slingshot (SSH) phosphatase to regulate actin-depolymerizing factor (ADF)/cofilin-mediated actin dynamics. Xenopus laevis growth cones from 4–8-h cultured neurons are attracted to BMP7 gradients but become repelled by BMP7 after overnight culture. The attraction and repulsion are mediated by LIMK and SSH, respectively, which oppositely regulate the phosphorylation-dependent asymmetric activity of ADF/cofilin to control the actin dynamics and growth cone steering. The attraction to repulsion switching requires the expression of a transient receptor potential (TRP) channel TRPC1 and involves Ca²⁺ signaling through calcineurin phosphatase for SSH activation and growth cone repulsion. Together, we show that spatial regulation of ADF/cofilin activity controls the directional responses of the growth cone to BMP7, and Ca²⁺ influx through TRPC tilts the LIMK-SSH balance toward SSH-mediated repulsion.

Neogenin-RGMa Signaling at the Growth Cone Is Bone Morphogenetic Protein-independent and Involves RhoA, ROCK, and PKC

The repulsive guidance molecule RGMa has been shown to induce outgrowth inhibition of neurites by interacting with the transmembrane receptor neogenin. Here we show that RGMa-induced growth cone collapse is mediated by activation of the small GTPase RhoA, its downstream effector Rho kinase and PKC. In contrast to DRG cultures from neogenin-/- mice, in which no RGMa-mediated growth cone collapse and activation of RhoA occurred, treatment of wild type DRG neurites with soluble RGMa led to a marked activation of RhoA within 3 min followed by collapse, but left Rac1 and Cdc42 unaffected. Furthermore, preincubation of DRG axons with the bone morphogenetic protein (BMP) antagonist noggin had no effect on RGMa-mediated growth cone collapse, implying that the role of RGM in axonal guidance is neogenin- and not BMP receptor-dependent. Pretreatment with 1) C3-transferase, a specific inhibitor of the Rho GTPase; 2) Y-27632, a specific inhibitor of Rho kinase; and 3) Gö6976, the general PKC inhibitor, strongly inhibited the collapse rate of PC12 neurites. Growth cone collapse induced by RGMa was abolished by the expression of dominant negative RhoA, but not by dominant negative Rac1. In contrast to RGMa, netrin-1 induced no growth cone retraction but instead reduced RGMa-mediated growth cone collapse. These results suggest that activation of RhoA, Rho kinase, and PKC are physiologically relevant and important elements of the RGMa-mediated neogenin signal transduction pathway involved in axonal guidance.

The retinal ganglion cell axon's journey: insights into molecular mechanisms of axon guidance

The developing visual system has proven to be one of the most informative models for studying axon guidance decisions. The pathway is composed of the axons of a single neuronal cell type, the retinal ganglion cell (RGC), that navigate through a series of intermediate targets on route to their final destination. The molecular basis of optic pathway development is beginning to be elucidated with cues such as netrins, Slits and ephrins playing a key role. Other factors best characterised for their role as morphogens in patterning developing tissues, such as sonic hedgehog (Shh) and Wnts, also act directly on RGC axons to influence guidance decisions. The transcriptional basis of the spatial-temporal expression of guidance cues and their cognate receptors within the developing optic pathway as well as mechanisms underlying the plasticity of guidance responses also are starting to be understood. This review will focus on our current understanding of the molecular mechanisms directing the early development of functional connections in the developing visual system and the insights these studies have provided into general mechanisms of axon guidance.

The LIM-only factor LMO4 regulates expression of the BMP7 gene through an HDAC2-dependent mechanism, and controls cell proliferation and apoptosis of mammary epithelial cells

The nuclear LIM-only protein 4 (LMO4) is upregulated in breast cancer, especially estrogen receptor-negative tumors, and its overexpression in mice leads to hyperplasia and tumor formation. Here, we show that deletion of LMO4 in the mammary glands of mice leads to impaired lobuloalveolar development due to decreased epithelial cell proliferation. With the goal of discovering potential LMO4-target genes, we also developed a conditional expression system in MCF-7 cells for both LMO4 and a dominant negative (DN) form of its co-regulator, cofactor of LIM domains (Clim/Ldb/Nli). We then used DNA microarrays to identify genes responsive to LMO4 and DN-Clim upregulation. One of the genes common to both data sets was bone morphogenic protein 7 (BMP7), whose expression is also significantly correlated with LMO4 transcript levels in a large dataset of human breast cancers, suggesting that BMP7 is a bona fide target gene of LMO4 in breast cancer. Inhibition of BMP7 partially blocks the effects of LMO4 on apoptosis, indicating that BMP7 mediates at least some functions of LMO4. Gene transfer studies show that LMO4 regulates the BMP7 promoter, and chromatin immunoprecipitation studies show that LMO4 and its cofactor Clim2 are recruited to the BMP7 promoter. Furthermore, we demonstrate that HDAC2 recruitment to the BMP7 promoter is inhibited by upregulation of LMO4 and that HDAC2 knockdown upregulates the promoter. These studies suggest a novel mechanism of action for LMO4: LMO4, Clim2 and HDAC2 are part of a transcriptional complex, and increased LMO4 levels can disrupt the complex, leading to decreased HDAC2 recruitment and increased promoter activity.

p130CAS is required for netrin signaling and commissural axon guidance

Netrins are an important family of axon guidance cues. Here, we report that netrin-1 induces tyrosine phosphorylation of p130(CAS) (Crk-associated substrate). Our biochemical studies indicate that p130(CAS) is downstream of the Src family kinases and upstream of the small GTPase Rac1 and Cdc42. Inhibition of p130(CAS) signaling blocks both the neurite outgrowth-promoting activity and the axon attraction activity of netrin-1. p130(CAS) RNA interference inhibits the attraction of commissural axons in the spinal cord by netrin-1 and causes defects in commissural axon projection in the embryo. These results demonstrate that p130(CAS) is a key component in the netrin signal transduction pathway and plays an important role in guiding commissural axons in vivo.

Morphogens as conserved axon guidance cues

Morphogen family proteins are now widely appreciated as axon guidance cues. Because their roles as morphogens are highly conserved across phylogeny, their functional conservation in axon guidance is now being rigorously examined. Recent studies suggest that morphogens are important in shaping topographic projections in chick and Drosophila visual systems, a process that occurs even later in development.

Bone morphogenetic protein signalling and vertebrate nervous system development

Transforming growth factor-beta (TGFbeta) signalling, particularly signalling from the bone morphogenetic protein (BMP) members of this protein family, is crucial for the development of both the central and peripheral nervous systems in vertebrates. Experimental embryology and genetics performed in a range of organisms are providing insights into how BMPs establish the neural tissue and control the types and numbers of neurons formed. These studies also highlight the interactions between different developmental signals that are necessary to form a functional nervous system. The challenges ahead will be to uncover functions of TGFbeta signalling in later stages of CNS development, as well as to determine possible associations with neurological diseases.

Bone morphogenetic protein down-regulation of neuronal pituitary adenylate cyclase-activating polypeptide and reciprocal effects on vasoactive intestinal peptide expression

Among bone morphogenetic proteins (BMPs), the decapentaplegic (Dpp; BMP2, BMP4) and glass bottom boat (Gbb/60A; BMP5, BMP6, BMP7) subgroups have well-described functions guiding autonomic and sensory neuronal development, fiber formation and neurophenotypic identities. Evaluation of rat superior cervical ganglia (SCG) post-ganglionic sympathetic neuron developmental regulators identified that selected BMPs of the transforming growth factor beta superfamily have reciprocal effects on neuronal pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) expression. Dpp and Gbb/60A BMPs rapidly down-regulated PACAP expression, while up-regulating other sympathetic neuropeptides, including PACAP-related VIP. The suppressive effects of BMP on PACAP mRNA and peptide expression were potent, efficacious and phosphorylated mothers against decapentaplegic homolog (Smad) signaling-dependent. Axotomy of SCG dramatically increases PACAP expression, and the possibility that abrogation of inhibitory retrograde target tissue BMP signaling may contribute to this up-regulation of sympathetic neuron PACAP was investigated. Replacement of BMP6 to SCG explant preparations significantly blunted the injury-induced elevated PACAP expression, with a concomitant decrease in sympathetic PACAP-immunoreactive neuron numbers. These studies suggested that BMPs modulate neuropeptide identity and diversity by stimulating or restricting the expression of specific peptidergic systems. Furthermore, the liberation of SCG neurons from target-derived BMP inhibition following axotomy may be one participating mechanism associated with injury-induced neuropeptidergic plasticity.

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.

Concise Review: Bone Morphogenetic Protein Pleiotropism in Neural Stem Cells and Their Derivatives-Alternative Pathways, Convergent Signals

Bone morphogenetic proteins (BMPs) are a class of morphogens that are critical regulators of the central nervous system (CNS), peripheral nervous system, and craniofacial development. Modulation of BMP signaling also appears to be an important component of the postnatal stem cell niche. However, describing a comprehensive model of BMP actions is complicated by their paradoxical effects in precursor cells, which include dorsal specification, promoting proliferation or mitotic arrest, cell survival or death, and neuronal or glial fate. In addition, in postmitotic neurons BMPs can promote dendritic growth, act as axonal chemorepellants, and stabilize synapses. Although many of these responses depend on interactions with other incoming signals, some reflect the recruitment of distinct BMP signal transduction pathways. In this review, we classify the diverse effects of BMPs on neural cells, focus on the known mechanisms that specify distinct responses, and discuss the remaining challenges in identifying the cellular basis of BMP pleiotropism. Addressing these issues may have importance for stem cell mobilization, differentiation, and cell integration/survival in reparative therapies.

Axon guidance cues in auditory development

The innervation of the cochlear sensory epithelium is intricately organized, allowing the tonotopy established by the auditory hair cells to be maintained along the ascending auditory pathways. These auditory projections are patterned by several gene families that regulate neurite attraction and repulsion, known as axon guidance cues. In this review, the roles of various axon guidance molecules, including fibroblast growth factor, ephs, semaphorins, netrins and slits, are examined in light of their known contribution to auditory development. Additionally, morphogens are discussed in the context of their recently described influence on axonal pathfinding in other sensory systems. The elucidation of these various mechanisms may guide the development of therapies aimed at maximizing the connectivity of auditory neurons in the context of congenital or acquired sensorineural hearing loss, especially as pertains to cochlear implants. Further afield, improved understanding of the molecular processes which regulate innervation of the organ of Corti during normal development may prove useful in connecting regenerated hair cells to the central nervous system.

FGF as a Target-Derived Chemoattractant for Developing Motor Axons Genetically Programmed by the LIM Code

LIM transcription factors confer developing axons with specific navigational properties, but the downstream guidance receptors and ligands are not well defined. The dermomyotome, a transient structure from which axial muscles arise, is the source of a secreted long-range chemoattractant specific for medial-class spinal motor neuron axons (MMCm axons). We show that fibroblast growth factors (FGFs) produced by the dermomyotome selectively attract MMCm axons in vitro. FGF receptor 1 (FGFR1) expression is restricted to MMCm neurons, and conditional deletion of FGFR1 causes motor axon guidance defects. Furthermore, reprogramming the identity of limb-innervating motor neurons to that of dermomyotome-innervating MMCm cells using the LIM factor Lhx3 induces FGFR1 expression and shifts an increased number of motor axons to an FGF-responsive state. These results point to a role for FGF signaling in axon guidance and further unravel how downstream effectors of LIM codes direct wiring of the developing nervous system.

Protein kinase A regulates the sensitivity of spinal commissural axon turning to netrin-1 but does not switch between chemoattraction and chemorepulsion

Bifunctional axon guidance cues have been grouped into two classes depending on whether changes in intracellular cAMP or cGMP switch the response of the growth cone between attraction and repulsion. According to this model, axons respond to netrin-1, a group I guidance cue, as a chemoattractant when cAMP levels are high in the growth cone but switch and are repelled when the intraneuronal concentration of cAMP is low. The model is complicated by the proposal that cAMP-dependent kinase, protein kinase A (PKA), functions as a downstream effector for several guidance cues, including netrin-1, suggesting a close inter-relationship between guidance cue signal transduction and mechanisms regulating the switch between attraction and repulsion. Here, we examine possible interactions between netrin-1-mediated axon guidance and cAMP signaling in embryonic rat spinal commissural neurons. We report that netrin-1 does not alter the concentration of cAMP or PKA activity in these neurons across a wide range of netrin-1 concentrations and time points after application, leading us to conclude that netrin-1 does not regulate PKA in these cells. In contrast to the cyclic nucleotide switch model, we report that, despite inhibiting PKA, embryonic spinal commissural axons were always attracted to netrin-1 and never repelled. Instead, manipulating PKA regulated the sensitivity of chemoattraction to netrin-1: PKA inhibition reduced, and PKA activation increased, the distance over which axons turn toward a source of netrin-1. These findings indicate that the mechanisms underlying cyclic nucleotide-regulated switching are separable from the signal transduction mechanisms required for chemoattraction to netrin-1.

Sonic hedgehog has a dual effect on the growth of retinal ganglion axons depending on its concentration

The stereotypical projection of retinal ganglion cell (RGC) axons to the optic disc has served as a good model system for studying axon guidance. By both in vitro and in vivo experiments, we show that a secreted molecule, Sonic hedgehog (Shh), may play a critical role in the process. It is expressed in a dynamic pattern in the ganglion cell layer with a relatively higher expression in the center of the retina. Through gel culture and stripe assays, we show that Shh has a dual effect on RGC axonal growth, acting as a positive factor at low concentrations and a negative factor at high concentrations. Results from time-lapse video microscopic and stripe assay experiments further suggest that the effects of Shh on axons are not likely attributable to indirect transcriptional regulation by Shh. Overexpression of Shh protein or inhibition of Shh function inside the retina resulted in a complete loss of centrally directed projection of RGC axons, suggesting that precise regulation of Shh level inside the retina is critical for the projection of RGC axons to the optic disc.

Molecular mechanisms of axon guidance

In order to form a functional nervous system, neurones extend axons, often over long distances, to reach their targets. This process is controlled by extracellular receptors and their ligands, several families of which have been identified. These proteins may act to either repel or attract growth cones and a given receptor may transduce either type of signal, depending on the cellular context. In addition to these archetypal axon guidance molecules, it is becoming apparent that molecules previously known for their role in patterning can also direct axonal outgrowth. The growth cone receptors do not act in isolation and combine with members of the same or other families to produce a graded response or even a complete reversal in its polarity. These signals can be further combined and/or modulated by processing of the molecule both directly at the cell surface and by the network of intracellular signalling pathways which are activated. The result is a sophisticated and dynamic set of cues that enable a growth cone to successfully navigate to its destination, modulating its response to changing environmental cues along its pathway.

Longitudinal axon guidance

Our knowledge about molecular mechanisms underlying axon guidance along the antero-posterior axis in contrast to the dorso-ventral axis of the developing nervous system is very limited. During the past two years in vitro and in vivo studies have indicated that morphogens have a role in longitudinal axon guidance. Morphogens are secreted proteins that act in a concentration-dependent manner on susceptible groups of precursor cells and induce their differentiation to a specific cell fate. Thus, gradients of morphogens are responsible for the appropriate patterning of the nervous system during early phases of neural development. Therefore, it was surprising to find that gradients of two of these morphogens, Wnt4 and Shh, can be re-used for longitudinal axon guidance during later stages of nervous system development.

Baboon/dSmad2 TGF-beta signaling is required during late larval stage for development of adult-specific neurons

The intermingling of larval functional neurons with adult-specific neurons during metamorphosis contributes to the development of the adult Drosophila brain. To better understand this process, we characterized the development of a dorsal cluster (DC) of Atonal-positive neurons that are born at early larval stages but do not undergo extensive morphogenesis until pupal formation. We found that Baboon(Babo)/dSmad2-mediated TGF-beta signaling, known to be essential for remodeling of larval functional neurons, is also indispensable for proper morphogenesis of these adult-specific neurons. Mosaic analysis reveals slowed development of mutant DC neurons, as evidenced by delays in both neuronal morphogenesis and atonal expression. We observe similar phenomena in other adult-specific neurons. We further demonstrate that Babo/dSmad2 operates autonomously in individual neurons and specifically during the late larval stage. Our results suggest that Babo/dSmad2 signaling prior to metamorphosis may be widely required to prepare neurons for the dynamic environment present during metamorphosis.

Netrins guide Drosophila commissural axons at short range

Netrins are secreted axon guidance molecules required for commissure formation in a wide range of animal species, including Caenorhabditis elegans, Drosophila melanogaster and mice. They are generally thought to function as chemoattractants, acting at a distance to direct commissural axon growth toward the midline of the central nervous system. We show here, however, that D. melanogaster commissural axons still orient normally and reach the midline even in the complete absence of netrins, though some of them fail to cross the midline. Tethering endogenous netrin to the membrane selectively disrupts its long-range but not short-range activity, yet still allows normal commissure formation. We therefore propose that netrins act in commissural axon guidance as short-range cues that promote midline crossing, not as long-range chemoattractants.

Neural crest delamination and migration: integrating regulations of cell interactions, locomotion, survival and fate

During the entire process of neural crest development from specification till final differentiation, delamination and migration are critical steps where nascent crest cells face multiple challenges: within a relatively short period of time that does not exceed several hours, they have to change drastically their cell- and substrate-adhesion properties, lose cell polarity and activate the locomotory machinery, while keeping proliferating, surviving and maintaining a pool of precursors in the neural epithelium. Then, as soon as they are released from the neural tube, neural crest cells have to adapt to a new, rapidly-changing environment and become able to interpret multiple cues which guide them to appropriate target sites and prevent them from distributing in aberrant locations. It appears from recent studies that, behind an apparent linearity and unity, neural crest development is subdivided into several independent steps, each being governed by a multiplicity of rules and referees. Here resides probably one of the main reasons of the success of neural crest cells to accomplish their task.

Mechanisms of morphogen movement

Morphogens are defined as signaling molecules that are produced locally, yet act directly at a distance to pattern the surrounding field of cells in a concentration-dependent manner. In recent years many laboratories have devoted their attention to how morphogens actually reach distant cells. Several models have been proposed, including diffusion in the extracellular space and planar transcytosis. A combination of genetic, developmental, and cell-biological approaches have been taken to tackle this issue. I will present the models and discuss the types of experiments that have been designed to test them. It stands out that most of the work has been carried out in Drosophila. Morphogens contribute to patterning of the vertebrate nervous system, and the same signaling molecules have recently been shown to play important, possibly instructive, roles in axon guidance. Little, if anything, is known about the movement of morphogens in the context of nervous system development. The long-standing tradition of biophysical studies on diffusion in the brain extracellular space, along with the sophisticated in vitro culture systems developed in neurobiology laboratories, may provide new tools and ideas to test these models in a new context.

Morphogens as growth cone signalling molecules

Morphogen signalling among cells is one of the most important mechanisms underlying the progressive patterning of embryos. Members of the hedgehog (Hh), wingless (Wnt), transforming growth factor-beta (TGFbeta), and fibroblast growth factor (Fgf) families of extracellular signalling molecules act as morphogens. Recent studies have demonstrated that members of these four families of proteins, secreted by well-characterised organiser centres in the central nervous system (CNS) as floor plate or midbrain-hindbrain boundary, are reused at later developmental stages to control axon growth. Here, we have summarised the evidence for this novel idea with a particular emphasis on those related to Shh and Wnt signalling-the object of some works in our laboratory.

The contributions of BMP4, positive guidance cues, and repulsive molecules to cutaneous nerve formation in the chick hindlimb

Our previous surgical manipulations have shown that the target ectoderm is necessary for the initial formation of one of the major cutaneous nerves in the embryonic chick limb (Honig, M.G., Camilli, S.J., Xue, Q.S., 2004. Ectoderm removal prevents cutaneous nerve formation and perturbs sensory axon growth in the chick hindlimb. Dev. Biol. 266, 27-42.). Moreover, the target ectoderm is required during a critical time period, at approximately St. 24, when those axons are about to diverge from the hindlimb plexus. To elucidate the underlying mechanisms, here we examined the effects of removing the ectoderm at St. 24 on a variety of molecules expressed within the limb. We find that, while ectoderm removal is accompanied by changes in the expression of Lmx1, fibronectin, EphA7, cDermo-1, and in the complement of muscle cells, these changes do not account for the cutaneous nerve deficit. In contrast, an upregulation of PNA-binding sites and a downregulation of Bmp4 appear to be associated with this nerve deficit. Exogenous BMP4 reversed the effect of ectoderm removal on cutaneous nerve formation, but did not act as a chemoattractant. Our results suggest that BMP4, together with permissive and repulsive molecules that growing cutaneous axons encounter in the local environment and with signaling molecules, originating from and/or dependent on the ectoderm, work in concert to ensure proper cutaneous nerve formation.