Kathryn W. Tosney


    Contact Information
    Kathryn Tosney
    1301 Memorial Drive
    Cox Science Center, 229D
    The University of Miami
    Coral Gables, FL 33146
    Office (305) 284-2673
    FAX (305) 284-3039
    ktosney@miami.edu



Growth Cone Motility and Guidance


Once we identify tissues that guide axons, we then analyze at the cellular level to ask questions that are inaccessible at either the tissue or the molecular levels. For instance, even if we had an inhibitory molecule in hand, we would still need to know how the molecule causes the growth cone, the leading tip of the axon, to turn away on contact. Contact of a single filopodial tip suffices to turn a growth cone. The signal detected by the filopodial tip adhesion must induce a signal cascade that modulates cytoskeletal dynamics.

To learn how cues signal and modulate the growth cones’ motile machinery, we optically record growth cones in culture as they contact relevant cells or substrates, and assess growth cone and cytoskeletal dynamics.



Our analysis reveals that filopodial contact with cellular guidance cues can invariably induce responses that are impressively discrete. (figure below; see Oakley & Tosney 1993, Steketee & Tosney 1999, Polinsky & Tosney 1999). Filopodial contact with Schwann cells locally induces veil extension. Filopodial contact with anterior sclerotome cells stimulates veil and filopodial extension throughout the growth cone and locally induces contacting processes to consolidate. Filopodial contact with posterior sclerotome cells locally prohibits veil extension.

We will call these responses “the discrete responses.”


  • top left Activity unbiased by contact is shown as symetrical, with veils extending on both sides.
  • top right Contact with a Schwann cell induces a large veil locally, without altering extension on the non-contacting side.
  • bottom right Contact with an anterior cell increases filoodial extension
  • bottom left Contact with a posterior cell prohibits veil extension locally.


These discrete responses are interesting for three reasons:

1) They are physiologically relevant. The cells that elicit these responses actually guide axons in the embryo.

2) They are the invariant consequence of contact. Their invariance and simplicity is consistent with the idea that each discrete response is THE direct physiological consequence of filopodial contact with the guidance cue.

3) These responses have another property, discreteness, which makes them particularly valuable as guidance mechanisms. Each of these responses is impressively stereotyped. It does not affect just extension, or engorgement or consolidation: it affects a very precise subset of one activity. For instance, a response can alter only the advance of veils down a filopodium, without changing the initiation of veils on the same filopodium (Steketee and Tosney, 1999). These cues thus have a constant and highly discrete effect on motility, that suffices to steer the growth cone.

The invariance and discreteness of each response also make them highly useful as bio-assays. We are using these assays to dissect out the cell surface, signaling and cytoskeletal mechanisms of guidance.

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Figure
shows dual label of phosphotyrosine (red) and actin (green) in a growth cone that had been optically recorded through the moment of fixation.The phosphorotyrosine is preferentially localized at the leading edge of actively extending veils (white arrow); it is diminished at regions of veil retraction (lower left). Figure by M. Steketee.


Publications

  • Polinsky, M., K. Balazovich and K.W. Tosney (2000). Identification of an invariant response: Contact with Schwann cells induces veil extension in growth cones. J. Neurosci. 20: 1044-1055.
  • Steketee M and Tosney KW (1999). Contact with isolated sclerotome cells steers sensory growth cones by altering distinct elements of extension. J. Neurosci. 19: 3495-3506.
  • Oakley RA and Tosney KW (1993). Contact-mediated mechanisms of motor axon segmentation. J Neurosci 13:3773-3792.
  • Tosney KW and Wessells NK (1983). Neuronal motility: the ultrastructure of veils and microspikes correlates with their motile activities. J.Cell Sci. 6:389-411.

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