Different signaling pathways mediate regenerative versus developmental sensory axon growth.
Recent advances in defining neurotrophin signaling mediators have provided insights into the signal transduction mechanisms that underlie axon growth. Evidence is accumulating that major Trk effectors regulate the morphological development of embryonic peripheral neurons. Less is known about signaling related to the robust axon extension that follows peripheral axotomy of adult neurons. Regenerative axon growth can be mimicked in vitro by a "conditioning" lesion performed 2 weeks before culture (Smith and Skene, 1997). Previous work has implicated both neurotrophins and cytokines in this response. Because signal transduction mediators of both of these families of growth factors are well characterized, we have compared the role of neurotrophin and cytokine signaling in developmental versus regenerative sensory axon growth. Chemical inhibitors were administrated to embryonic and axotomized sensory neurons in vitro to block the activation of Erk kinase (MEK)-extracellular signal-regulated kinase (ERK), phosphatidylinositol-3 kinase (PI3-K), and janus kinase (JAK) signaling. As expected, both MEK and PI3-K inhibition blocked axon growth from both naive and NGF-stimulated embryonic day 13 sensory neurons, whereas inhibition of JAK phosphorylation had no effect. In contrast, neither MEK nor PI3-K inhibitors blocked elongation of adult sensory neurons after a conditioning lesion. However, the addition of a JAK2 inhibitor prevented the regenerative axon response. Consistent with these pharmacological results, the percentage of neurons showing intense nuclear signal transducers and activators of transcription 3 phosphorylation after a conditioning lesion was markedly increased compared with controls. These observations demonstrate that the signaling mediators that underlie regenerative axon growth are distinct from those used during development and suggest that cytokine signaling may be critical to peripheral nervous system regeneration.
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Neuroscience Center, University of North Carolina, Chapel Hill, North Carolina 27599, USA.
Snider
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D000818:Animals; D001369:Axons / Q000187:drug effects / Q000378:metabolism*; D019771:Axotomy; D002455:Cell Division / Q000187:drug effects / Q000502:physiology; D002470:Cell Survival / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D016207:Cytokines / Q000378:metabolism / Q000494:pharmacology; D004268:DNA-Binding Proteins / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005727:Ganglia, Spinal / Q000166:cytology / Q000196:embryology / Q000378:metabolism*; D053614:Janus Kinase 2; D051379:Mice; D018345:Mice, Knockout; D008817:Mice, Mutant Strains; D020929:Mitogen-Activated Protein Kinase Kinases / Q000037:antagonists & inhibitors; D009414:Nerve Growth Factors / Q000378:metabolism / Q000494:pharmacology; D009416:Nerve Regeneration / Q000187:drug effects / Q000502:physiology*; D009475:Neurons, Afferent / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010766:Phosphorylation / Q000187:drug effects; D011505:Protein-Tyrosine Kinases / Q000037:antagonists & inhibitors; D011518:Proto-Oncogene Proteins / Q000172:deficiency / Q000235:genetics; D019253:Proto-Oncogene Proteins c-bcl-2; D050796:STAT3 Transcription Factor; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D015534:Trans-Activators / Q000378:metabolism; D051028:bcl-2-Associated X Protein
D016207:Cytokines; D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors; D009414:Nerve Growth Factors; D000081082:Phosphoinositide-3 Kinase Inhibitors; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D050796:STAT3 Transcription Factor; C494087:Stat3 protein, mouse; D015534:Trans-Activators; D051028:bcl-2-Associated X Protein; D011505:Protein-Tyrosine Kinases; C507926:Jak2 protein, mouse; D053614:Janus Kinase 2; D020929:Mitogen-Activated Protein Kinase Kinases