Synaptic vesicle release regulates myelin sheath number of individual oligodendrocytes in vivo

Abstract

The myelination of axons by oligodendrocytes markedly affects CNS function, but how this is regulated by neuronal activity in vivo is not known. We found that blocking synaptic vesicle release impaired CNS myelination by reducing the number of myelin sheaths made by individual oligodendrocytes during their short period of formation. We also found that stimulating neuronal activity increased myelin sheath formation by individual oligodendrocytes. These data indicate that neuronal activity regulates the myelinating capacity of single oligodendrocytes.

Figure 1. Synaptic vesicle release regulates myelinated axon number

(a) Lateral view of control (left) and TeNT expressing (right) zebrafish at 3 dpf. Boxes indicate areas in b. (Scale bar 250 μm) (b) Lateral view of Tg(mbp:mCherry-CAAX) spinal cords in controls (left) and TeNT expressing animals (right) (Scale bar 25 μm). White lines indicate the areas of ventral spinal cord illustrated in c. (c) Transmission electron micrographs of control (left) and TeNT expressing (right) animals at 4 dpf (Scale bar 2 μm). Unmyelinated axons > 0.3 μm diameter indicated in turquoise. (d) Number of myelinated and unmyelinated axons > 0.3 μm diameter in control and TeNT expressing hemi-spinal cords (Student’s two-tailed t-tests, myelinated axon number p = 0.0017, unmyelinated axon number, p = 0.0005, total axon > 0.3 μm number p = 0.4, control n = 5, TeNT n = 5). (e) Axon diameter distribution of axons between 0.3 μm and 1.9 μm (control n = 5, TeNT n = 5).

Figure 2. Synaptic vesicle release regulates myelin sheath formation by individual oligodendrocytes

(a) Individual oligodendrocytes labelled by mbp:mCherry-CAAX in control (top), and TeNT (bottom) animals at 4 dpf. Scale bar 10 μm. (b) Myelin sheath number per cell (oligodendrocytes in dorsal plus ventral spinal cord) (Student’s two-tailed t-test, p = 1.23872E-05, n = 45 cells in 24 animals; TeNT, n = 47 cells in 26 animals). (c) Time-lapse images of oligodendrocytes in a control (left) and TeNT expressing animal (right). Scale bar 10 μm. The colours in d correspond to relevant traces in D and E. (d + e) Total number of myelin sheaths for 13 different control (d) and 11 TeNT (e) oligodendrocytes over time. Time point zero is the time at which each individual oligodendrocyte initiates formation of its first myelin sheath. (f) Number of myelin sheaths per oligodendrocyte 6 hours after initiation of first myelin sheath, and number of sheath retractions within that 6 hour period of sheath generation (Student’s two-tailed t-test, p = 0.007 for sheath number and p = 0.55 for sheath retraction, n = 13 cells in 13 control and n = 11 cells in 11 in TeNT expressing animals).

Figure 3. Pentylenetetrazole increases myelin sheath number per oligodendrocyte in a synaptic vesicle dependent manner

(a) Individual oligodendrocytes in the ventral spinal cord labelled by mbp:mCherry-CAAX in control (top), PTZ (middle) and PTZ + TeNT (bottom) treated animals at 4 dpf. Scale bar 10 μm. (b) Myelin sheath number per cell (oligodendrocytes in ventral spinal cord) (One way ANOVA, p = 1.32E-05: Student’s two-tailed t-tests, Control vs PTZ, p = 0.0001, PTZ vs PTZ+TeNT, p = 3.40762E-06, Control n = 31, cells in 25 animals; PTZ, n = 18 cells in 14 animals, PTZ + TeNT n = 46 cells in 29 animals).