Limits...
Neuronal activity biases axon selection for myelination in vivo.

Hines JH, Ravanelli AM, Schwindt R, Scott EK, Appel B - Nat. Neurosci. (2015)

Bottom Line: Using zebrafish, we found that activity-dependent secretion stabilized myelin sheath formation on select axons.Instead, oligodendrocyte processes wrapping silenced axons retracted more frequently.We propose that axon selection for myelination results from excessive and indiscriminate initiation of wrapping followed by refinement that is biased by activity-dependent secretion from axons.

View Article: PubMed Central - PubMed

Affiliation: Department of Pediatrics, University of Colorado School of Medicine, Aurora, Colorado, USA.

ABSTRACT
An essential feature of vertebrate neural development is ensheathment of axons with myelin, an insulating membrane formed by oligodendrocytes. Not all axons are myelinated, but mechanisms directing myelination of specific axons are unknown. Using zebrafish, we found that activity-dependent secretion stabilized myelin sheath formation on select axons. When VAMP2-dependent exocytosis was silenced in single axons, oligodendrocytes preferentially ensheathed neighboring axons. Nascent sheaths formed on silenced axons were shorter in length, but when activity of neighboring axons was also suppressed, inhibition of sheath growth was relieved. Using in vivo time-lapse microscopy, we found that only 25% of oligodendrocyte processes that initiated axon wrapping were stabilized during normal development and that initiation did not require activity. Instead, oligodendrocyte processes wrapping silenced axons retracted more frequently. We propose that axon selection for myelination results from excessive and indiscriminate initiation of wrapping followed by refinement that is biased by activity-dependent secretion from axons.

Show MeSH
Veratridine reduces nascent sheath length but not sheath number or axon selection(a) Quantification of touch response assays on control and veratridine-treated larvae. Data shown represent the proportion of larvae exhibiting either wild-type or prolonged touch-response phenotypes (see Methods). Scoring was performed three hours after treatment and again immediately prior to confocal imaging (24 hr post-treatment). For both control and treated groups, n = 80 (3 h post-treatment) and 76 (24 h post-treatment). (b) Representative confocal images show reporter expression in control and veratridine-treated Tg(phox2B:GFP); Tg(sox10:mRFP) larvae. M indicates the position of the Mauthner axon and arrowheads point to sites of phox2B+ axon wrapping. Scale bar, 5 µm. (c–e) Summary of axon selection, sheath number, and sheath length measurements from (b). Error bars show s.e.m.; t-test; *P < 0.05, **P < 0.01; n.s., not significant. For (c–e) n = 30 control and 28 veratridine-treated larvae. P = 0.2147 (c), P = 0.2173 (d), P = 0.0011 (e, left), and P = 0.0342 (e, right); t-test.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC4414883&req=5

Figure 2: Veratridine reduces nascent sheath length but not sheath number or axon selection(a) Quantification of touch response assays on control and veratridine-treated larvae. Data shown represent the proportion of larvae exhibiting either wild-type or prolonged touch-response phenotypes (see Methods). Scoring was performed three hours after treatment and again immediately prior to confocal imaging (24 hr post-treatment). For both control and treated groups, n = 80 (3 h post-treatment) and 76 (24 h post-treatment). (b) Representative confocal images show reporter expression in control and veratridine-treated Tg(phox2B:GFP); Tg(sox10:mRFP) larvae. M indicates the position of the Mauthner axon and arrowheads point to sites of phox2B+ axon wrapping. Scale bar, 5 µm. (c–e) Summary of axon selection, sheath number, and sheath length measurements from (b). Error bars show s.e.m.; t-test; *P < 0.05, **P < 0.01; n.s., not significant. For (c–e) n = 30 control and 28 veratridine-treated larvae. P = 0.2147 (c), P = 0.2173 (d), P = 0.0011 (e, left), and P = 0.0342 (e, right); t-test.

Mentions: If activity is necessary for biased axon choice, can heightened activity enhance wrapping? To test this we treated embryos with the Na+ channel modulator veratridine at 72 hpf, immediately prior to the onset of myelination. Veratridine prolongs Na+ channel opening and, as expected, injected embryos showed striking and sustained behavioral phenotypes. Upon touch stimulation, control embryos initiated short bursts of swim movements (Supplementary Video 1). By contrast, veratridine-treated embryos initiated a prolonged swim response that eventually terminated in seizure-like behavior and brief paralysis (Supplementary Video 2). Although veratridine treatment had a pronounced effect on neural behavior (Fig. 2a), we observed no change within the spinal cord in the selection of phox2B+ axons or the overall level of wrapping (Fig. 2b–d). However, the length of nascent sheaths on both phox2B+ and phox2B− axons was slightly reduced in veratridine-treated embryos relative to controls (Fig. 2e). We conclude that TTX-sensitive activity is necessary for selection of axons at high fidelity but that widespread elevated neuronal activity is not sufficient to increase wrapping of spinal cord axons.


Neuronal activity biases axon selection for myelination in vivo.

Hines JH, Ravanelli AM, Schwindt R, Scott EK, Appel B - Nat. Neurosci. (2015)

Veratridine reduces nascent sheath length but not sheath number or axon selection(a) Quantification of touch response assays on control and veratridine-treated larvae. Data shown represent the proportion of larvae exhibiting either wild-type or prolonged touch-response phenotypes (see Methods). Scoring was performed three hours after treatment and again immediately prior to confocal imaging (24 hr post-treatment). For both control and treated groups, n = 80 (3 h post-treatment) and 76 (24 h post-treatment). (b) Representative confocal images show reporter expression in control and veratridine-treated Tg(phox2B:GFP); Tg(sox10:mRFP) larvae. M indicates the position of the Mauthner axon and arrowheads point to sites of phox2B+ axon wrapping. Scale bar, 5 µm. (c–e) Summary of axon selection, sheath number, and sheath length measurements from (b). Error bars show s.e.m.; t-test; *P < 0.05, **P < 0.01; n.s., not significant. For (c–e) n = 30 control and 28 veratridine-treated larvae. P = 0.2147 (c), P = 0.2173 (d), P = 0.0011 (e, left), and P = 0.0342 (e, right); t-test.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC4414883&req=5

Figure 2: Veratridine reduces nascent sheath length but not sheath number or axon selection(a) Quantification of touch response assays on control and veratridine-treated larvae. Data shown represent the proportion of larvae exhibiting either wild-type or prolonged touch-response phenotypes (see Methods). Scoring was performed three hours after treatment and again immediately prior to confocal imaging (24 hr post-treatment). For both control and treated groups, n = 80 (3 h post-treatment) and 76 (24 h post-treatment). (b) Representative confocal images show reporter expression in control and veratridine-treated Tg(phox2B:GFP); Tg(sox10:mRFP) larvae. M indicates the position of the Mauthner axon and arrowheads point to sites of phox2B+ axon wrapping. Scale bar, 5 µm. (c–e) Summary of axon selection, sheath number, and sheath length measurements from (b). Error bars show s.e.m.; t-test; *P < 0.05, **P < 0.01; n.s., not significant. For (c–e) n = 30 control and 28 veratridine-treated larvae. P = 0.2147 (c), P = 0.2173 (d), P = 0.0011 (e, left), and P = 0.0342 (e, right); t-test.
Mentions: If activity is necessary for biased axon choice, can heightened activity enhance wrapping? To test this we treated embryos with the Na+ channel modulator veratridine at 72 hpf, immediately prior to the onset of myelination. Veratridine prolongs Na+ channel opening and, as expected, injected embryos showed striking and sustained behavioral phenotypes. Upon touch stimulation, control embryos initiated short bursts of swim movements (Supplementary Video 1). By contrast, veratridine-treated embryos initiated a prolonged swim response that eventually terminated in seizure-like behavior and brief paralysis (Supplementary Video 2). Although veratridine treatment had a pronounced effect on neural behavior (Fig. 2a), we observed no change within the spinal cord in the selection of phox2B+ axons or the overall level of wrapping (Fig. 2b–d). However, the length of nascent sheaths on both phox2B+ and phox2B− axons was slightly reduced in veratridine-treated embryos relative to controls (Fig. 2e). We conclude that TTX-sensitive activity is necessary for selection of axons at high fidelity but that widespread elevated neuronal activity is not sufficient to increase wrapping of spinal cord axons.

Bottom Line: Using zebrafish, we found that activity-dependent secretion stabilized myelin sheath formation on select axons.Instead, oligodendrocyte processes wrapping silenced axons retracted more frequently.We propose that axon selection for myelination results from excessive and indiscriminate initiation of wrapping followed by refinement that is biased by activity-dependent secretion from axons.

View Article: PubMed Central - PubMed

Affiliation: Department of Pediatrics, University of Colorado School of Medicine, Aurora, Colorado, USA.

ABSTRACT
An essential feature of vertebrate neural development is ensheathment of axons with myelin, an insulating membrane formed by oligodendrocytes. Not all axons are myelinated, but mechanisms directing myelination of specific axons are unknown. Using zebrafish, we found that activity-dependent secretion stabilized myelin sheath formation on select axons. When VAMP2-dependent exocytosis was silenced in single axons, oligodendrocytes preferentially ensheathed neighboring axons. Nascent sheaths formed on silenced axons were shorter in length, but when activity of neighboring axons was also suppressed, inhibition of sheath growth was relieved. Using in vivo time-lapse microscopy, we found that only 25% of oligodendrocyte processes that initiated axon wrapping were stabilized during normal development and that initiation did not require activity. Instead, oligodendrocyte processes wrapping silenced axons retracted more frequently. We propose that axon selection for myelination results from excessive and indiscriminate initiation of wrapping followed by refinement that is biased by activity-dependent secretion from axons.

Show MeSH