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Boundary cap cells constrain spinal motor neuron somal migration at motor exit points by a semaphorin-plexin mechanism.

Bron R, Vermeren M, Kokot N, Andrews W, Little GE, Mitchell KJ, Cohen J - Neural Dev (2007)

Bottom Line: We conclude that semaphorin-mediated repellent interactions between boundary cap cells and immature spinal motor neurons regulates somal positioning by countering the drag exerted on motor neuron cell bodies by their axons as they emerge from the CNS at motor exit points.Our data support a model in which BC cell semaphorins signal through Npn-2 and/or Plexin-A2 receptors on motor neurons via a cytoplasmic effector, MICAL3, to trigger cytoskeletal reorganisation.This leads to the disengagement of somal migration from axon extension and the confinement of motor neuron cell bodies to the spinal cord.

View Article: PubMed Central - HTML - PubMed

Affiliation: MRC Centre for Developmental Neurobiology, King's College London, Guy's Campus, London Bridge, London, SE1 1UL, UK. rbron@unimelb.edu.au

ABSTRACT

Background: In developing neurons, somal migration and initiation of axon outgrowth often occur simultaneously and are regulated in part by similar classes of molecules. When neurons reach their final destinations, however, somal translocation and axon extension are uncoupled. Insights into the mechanisms underlying this process of disengagement came from our study of the behaviour of embryonic spinal motor neurons following ablation of boundary cap cells. These are neural crest derivatives that transiently reside at motor exit points, central nervous system (CNS):peripheral nervous system (PNS) interfaces where motor axons leave the CNS. In the absence of boundary cap cells, motor neuron cell bodies migrate along their axons into the periphery, suggesting that repellent signals from boundary cap cells regulate the selective gating of somal migration and axon outgrowth at the motor exit point. Here we used RNA interference in the chick embryo together with analysis of mutant mice to identify possible boundary cap cell ligands, their receptors on motor neurons and cytoplasmic signalling molecules that control this process.

Results: We demonstrate that targeted knock down in motor neurons of Neuropilin-2 (Npn-2), a high affinity receptor for class 3 semaphorins, causes their somata to migrate to ectopic positions in ventral nerve roots. This finding was corroborated in Npn-2 mice, in which we identified motor neuron cell bodies in ectopic positions in the PNS. Our RNA interference studies further revealed a role for Plexin-A2, but not Plexin-A1 or Plexin-A4. We show that chick and mouse boundary cap cells express Sema3B and 3G, secreted semaphorins, and Sema6A, a transmembrane semaphorin. However, no increased numbers of ectopic motor neurons are found in Sema3B mouse embryos. In contrast, Sema6A mice display an ectopic motor neuron phenotype. Finally, knockdown of MICAL3, a downstream semaphorin/Plexin-A signalling molecule, in chick motor neurons led to their ectopic positioning in the PNS.

Conclusion: We conclude that semaphorin-mediated repellent interactions between boundary cap cells and immature spinal motor neurons regulates somal positioning by countering the drag exerted on motor neuron cell bodies by their axons as they emerge from the CNS at motor exit points. Our data support a model in which BC cell semaphorins signal through Npn-2 and/or Plexin-A2 receptors on motor neurons via a cytoplasmic effector, MICAL3, to trigger cytoskeletal reorganisation. This leads to the disengagement of somal migration from axon extension and the confinement of motor neuron cell bodies to the spinal cord.

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Genetic ablation of Sema3B in the mouse does not lead to ectopic positioning of spinal motor neurons. (a) As seen in the chick, in the E12.5 mouse trunk, Sema3B is expressed by BC cells located at the spinal cord DREZ (red arrows) and MEP (black arrows). Bar = 100 μm. (b) A quantitative analysis of the incidence of ectopic motor neurons at hindlimb level reveals that there is no significant difference between E12.5 Sema3B  mice and heterozygous and wild-type littermates (n = 3 each). This is in sharp contrast to E12.5 Npn-2  mice, which is included for comparison (n = 4). **P ≤ 0.01; two-tailed t-test.
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Figure 5: Genetic ablation of Sema3B in the mouse does not lead to ectopic positioning of spinal motor neurons. (a) As seen in the chick, in the E12.5 mouse trunk, Sema3B is expressed by BC cells located at the spinal cord DREZ (red arrows) and MEP (black arrows). Bar = 100 μm. (b) A quantitative analysis of the incidence of ectopic motor neurons at hindlimb level reveals that there is no significant difference between E12.5 Sema3B mice and heterozygous and wild-type littermates (n = 3 each). This is in sharp contrast to E12.5 Npn-2 mice, which is included for comparison (n = 4). **P ≤ 0.01; two-tailed t-test.

Mentions: Therefore, we next analysed Sema3B mutant mice [27]. We first confirmed that Sema3B is also strongly expressed in mouse BC cells (Figure 5a). Analysis of the incidence of ectopic motor neurons throughout the rostro-caudal axis, however, did not reveal a difference between Sema3B mutant mice and heterozygous and wild-type littermates (Figure 5b). Therefore, loss of Sema3B expression despite its distinct expression by BC cells has no effect on positioning of motor neuron cell bodies.


Boundary cap cells constrain spinal motor neuron somal migration at motor exit points by a semaphorin-plexin mechanism.

Bron R, Vermeren M, Kokot N, Andrews W, Little GE, Mitchell KJ, Cohen J - Neural Dev (2007)

Genetic ablation of Sema3B in the mouse does not lead to ectopic positioning of spinal motor neurons. (a) As seen in the chick, in the E12.5 mouse trunk, Sema3B is expressed by BC cells located at the spinal cord DREZ (red arrows) and MEP (black arrows). Bar = 100 μm. (b) A quantitative analysis of the incidence of ectopic motor neurons at hindlimb level reveals that there is no significant difference between E12.5 Sema3B  mice and heterozygous and wild-type littermates (n = 3 each). This is in sharp contrast to E12.5 Npn-2  mice, which is included for comparison (n = 4). **P ≤ 0.01; two-tailed t-test.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2131750&req=5

Figure 5: Genetic ablation of Sema3B in the mouse does not lead to ectopic positioning of spinal motor neurons. (a) As seen in the chick, in the E12.5 mouse trunk, Sema3B is expressed by BC cells located at the spinal cord DREZ (red arrows) and MEP (black arrows). Bar = 100 μm. (b) A quantitative analysis of the incidence of ectopic motor neurons at hindlimb level reveals that there is no significant difference between E12.5 Sema3B mice and heterozygous and wild-type littermates (n = 3 each). This is in sharp contrast to E12.5 Npn-2 mice, which is included for comparison (n = 4). **P ≤ 0.01; two-tailed t-test.
Mentions: Therefore, we next analysed Sema3B mutant mice [27]. We first confirmed that Sema3B is also strongly expressed in mouse BC cells (Figure 5a). Analysis of the incidence of ectopic motor neurons throughout the rostro-caudal axis, however, did not reveal a difference between Sema3B mutant mice and heterozygous and wild-type littermates (Figure 5b). Therefore, loss of Sema3B expression despite its distinct expression by BC cells has no effect on positioning of motor neuron cell bodies.

Bottom Line: We conclude that semaphorin-mediated repellent interactions between boundary cap cells and immature spinal motor neurons regulates somal positioning by countering the drag exerted on motor neuron cell bodies by their axons as they emerge from the CNS at motor exit points.Our data support a model in which BC cell semaphorins signal through Npn-2 and/or Plexin-A2 receptors on motor neurons via a cytoplasmic effector, MICAL3, to trigger cytoskeletal reorganisation.This leads to the disengagement of somal migration from axon extension and the confinement of motor neuron cell bodies to the spinal cord.

View Article: PubMed Central - HTML - PubMed

Affiliation: MRC Centre for Developmental Neurobiology, King's College London, Guy's Campus, London Bridge, London, SE1 1UL, UK. rbron@unimelb.edu.au

ABSTRACT

Background: In developing neurons, somal migration and initiation of axon outgrowth often occur simultaneously and are regulated in part by similar classes of molecules. When neurons reach their final destinations, however, somal translocation and axon extension are uncoupled. Insights into the mechanisms underlying this process of disengagement came from our study of the behaviour of embryonic spinal motor neurons following ablation of boundary cap cells. These are neural crest derivatives that transiently reside at motor exit points, central nervous system (CNS):peripheral nervous system (PNS) interfaces where motor axons leave the CNS. In the absence of boundary cap cells, motor neuron cell bodies migrate along their axons into the periphery, suggesting that repellent signals from boundary cap cells regulate the selective gating of somal migration and axon outgrowth at the motor exit point. Here we used RNA interference in the chick embryo together with analysis of mutant mice to identify possible boundary cap cell ligands, their receptors on motor neurons and cytoplasmic signalling molecules that control this process.

Results: We demonstrate that targeted knock down in motor neurons of Neuropilin-2 (Npn-2), a high affinity receptor for class 3 semaphorins, causes their somata to migrate to ectopic positions in ventral nerve roots. This finding was corroborated in Npn-2 mice, in which we identified motor neuron cell bodies in ectopic positions in the PNS. Our RNA interference studies further revealed a role for Plexin-A2, but not Plexin-A1 or Plexin-A4. We show that chick and mouse boundary cap cells express Sema3B and 3G, secreted semaphorins, and Sema6A, a transmembrane semaphorin. However, no increased numbers of ectopic motor neurons are found in Sema3B mouse embryos. In contrast, Sema6A mice display an ectopic motor neuron phenotype. Finally, knockdown of MICAL3, a downstream semaphorin/Plexin-A signalling molecule, in chick motor neurons led to their ectopic positioning in the PNS.

Conclusion: We conclude that semaphorin-mediated repellent interactions between boundary cap cells and immature spinal motor neurons regulates somal positioning by countering the drag exerted on motor neuron cell bodies by their axons as they emerge from the CNS at motor exit points. Our data support a model in which BC cell semaphorins signal through Npn-2 and/or Plexin-A2 receptors on motor neurons via a cytoplasmic effector, MICAL3, to trigger cytoskeletal reorganisation. This leads to the disengagement of somal migration from axon extension and the confinement of motor neuron cell bodies to the spinal cord.

Show MeSH
Related in: MedlinePlus