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IGF-I specifically enhances axon outgrowth of corticospinal motor neurons

Nature Neuroscience volume 9, pages 1371–1381 (2006)Cite this article

Abstract

Corticospinal motor neurons (CSMN) are among the most complex CNS neurons; they control voluntary motor function and are prototypical projection neurons. In amyotrophic lateral sclerosis (ALS), both spinal motor neurons and CSMN degenerate; their damage contributes centrally to the loss of motor function in spinal cord injury. Direct investigation of CSMN is severely limited by inaccessibility in the heterogeneous cortex. Here, using new CSMN purification and culture approaches, and in vivo analyses, we report that insulin-like growth factor-1 (IGF-I) specifically enhances the extent and rate of murine CSMN axon outgrowth, mediated via the IGF-I receptor and downstream signaling pathways; this is distinct from IGF-I support of neuronal survival. In contrast, brain-derived neurotrophic factor (BDNF) enhances branching and arborization, but not axon outgrowth. These experiments define specific controls over directed differentiation of CSMN, indicate a distinct role of IGF-I in CSMN axon outgrowth during development, and might enable control over CSMN derived from neural precursors.

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Figure 1: Retrograde labeling and FACS purification yields pure populations of CSMN.
Figure 2: FACS-purified CSMN retain the cellular and molecular characteristics of mature CSMN in culture.
Figure 3: FACS-purified CSMN express a distinct set of growth factor and neurotrophin receptors, both in vitro and in vivo.
Figure 4: IGF-I and BDNF have distinct effects on CSMN morphology.
Figure 5: IGF-I induces a marked and specific increase in CSMN axon outgrowth.
Figure 6: Local application of IGF-I results in immediate and marked increase in the rate of CSMN axon outgrowth.
Figure 7: Blockade of IGF-I signaling via the IGF-I receptor and the PI3K and ERK/MAPK pathways results in axon outgrowth defects in vitro.
Figure 8: Localized in vivo blockade of IGF-I signaling markedly reduces CSMN axon growth.

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Acknowledgements

We thank B. Molyneaux for help and advice on neonatal surgery; L. Catapano and J. Chen for help with neuron isolation; D. Dombkowski for expert advice and optimization of FACS methods; D. Scadden for generous access to tissue culture facilities near the FACS facility; N. Kishi for help with Sholl analysis; J. Emsley for advice on statistical analysis; L. Reichardt, D. Kaplan, T. Jessell and M. Leid for gifts of antibodies; F. Briggs, A. Eswar and A. Palmer for technical assistance; A. Chandawarkar and J. Nagurney for help with blinded data analysis and immunocytochemistry; T. Jakobs for help with live imaging; and J. Emsley, P. Arlotta, S. Sohur, J. Menezes and other members of the Macklis lab for critical reading of the manuscript. This work was supported by grants from the US National Institutes of Health (NS49553, NS45523 and NS41590) and the ALS Association (to J.D.M.). P.H.O. was supported by a Harvard Center for Neurodegeneration and Repair Postdoctoral Fellowship.

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Authors and Affiliations

  1. Departments of Neurosurgery and Neurology, MGH-HMS Center for Nervous System Repair, and Program in Neuroscience, Harvard Medical School, Massachusetts General Hospital, Boston, 02114, Massachusetts, USA

    P Hande Özdinler & Jeffrey D Macklis

  2. Harvard Stem Cell Institute, Harvard University, Boston, 02114, Massachusetts, USA

    P Hande Özdinler & Jeffrey D Macklis

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Contributions

This study was jointly designed by P.H.Ö. and J.D.M.; experiments were performed by P.H.Ö.; P.H.Ö. and J.D.M. jointly analyzed and interpreted data and jointly wrote the paper.

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Correspondence to Jeffrey D Macklis.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Fig. 1

The effects of IGF-I are specific to axon extension. (PDF 961 kb)

Supplementary Fig. 2

Other growth factors do not substantially affect CSMN morphology. (PDF 1221 kb)

Supplementary Fig. 3

IGF-I receptor is expressed in CSMN axons, anti-IGF-IRα, and anti-TrkB antibodies binds to CSMN axons in vivo, and CSMN death does not occur following antibody application. (PDF 1362 kb)

Supplementary Video 1

Local application of IGF-I via placement of a single IGF-I-coated bead near CSMN cell body/axon hillock results in immediate and dramatic effects on the rate of CSMN axon outgrowth. (MOV 18131 kb)

Supplementary Video 2

In the presence of control beads, coated with BSA or PBS, the rate of axon outgrowth is quite slow. (MOV 24296 kb)

Supplementary Video 3

BDNF-coated beads induce immediate and local branching at the site of placement. (MOV 12508 kb)

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Özdinler, P., Macklis, J. IGF-I specifically enhances axon outgrowth of corticospinal motor neurons. Nat Neurosci 9, 1371–1381 (2006). https://doi.org/10.1038/nn1789

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