Motor skill learning requires active central myelination

Abstract

Myelin-forming oligodendrocytes (OLs) are formed continuously in the healthy adult brain. In this work, we study the function of these late-forming cells and the myelin they produce. Learning a new motor skill (such as juggling) alters the structure of the brain's white matter, which contains many OLs, suggesting that late-born OLs might contribute to motor learning. Consistent with this idea, we show that production of newly formed OLs is briefly accelerated in mice that learn a new skill (running on a "complex wheel" with irregularly spaced rungs). By genetically manipulating the transcription factor myelin regulatory factor in OL precursors, we blocked production of new OLs during adulthood without affecting preexisting OLs or myelin. This prevented the mice from mastering the complex wheel. Thus, generation of new OLs and myelin is important for learning motor skills.

Fig. 1. Deleting Myrf in OPs prevents new myelination.

(A) Many YFP⁺ (newly formed) cells accumulated 1 month after tamoxifen treatment in the P-Myrf^(+/−) corpus callosum, including Pdgfra⁺,CC1⁻ OPs (arrowheads) and CC1⁺, Pdgfra⁻ OLs (arrows). In contrast, the P-Myrf^(−/−) corpus callosum contained few YFP⁺ cells, mainly Pdgfra⁺ OPs. Some YFP⁺,CC1⁺ cells appeared fragmented, presumably because they are apoptotic (yellow arrow). (B) Numbers of YFP⁺,CC1⁺ OLs in the P-Myrf^(−/−) versus P-Myrf^(+/−) corpus callosum (****P < 0.0001). Error bars indicate SEM. (C) Very few GFP⁺ (newly formed) myelin sheaths are present in the P-Myrf^(−/−):Tau-mGFP corpus callosum 1 month post-tamoxifen relative to P-Myrf^(+/−) siblings. Asterisk indicates third ventricle. (D) The number densities of Pdgfra⁺ OPs or CC1⁺,YFP⁻ (preexisting) OLs did not change between P60 and P150. Error bars indicate SEM. Scale bars: 50 μm, (A) and (C).

Fig. 2. Deleting Myrf in OPs does not trigger demyelination.

Tamoxifen was administered to P60 mice and, 5 weeks later, their brains were examined by Eri-C histochemistry (A to F) or electron microscopy (G to M). There was no visible loss of myelin in P-Myrf^(−/−) [(A), (B), (G), and (H)] or P-Myrf^(+/−) [(C), (D), (I), and (J)] brains, but there was marked demyelination in S10-Myrf^(−/−) [(E), (F), (K), and (L)]. In S10-Myrf^(−/−) white matter, phagocytic cells containing membrane debris were present (M). Performance on an accelerating rotarod was not impaired in P-Myrf^(−/−) mice for at least 8 weeks post-tamoxifen compared with their P-Myrf^(+/−) littermates (N),whereas S10-Myrf^(−/−) mice were seriously impaired after 4 to5 weeks (O). Error bars indicate SEM. *P < 0.05; ***P < 0.001; ****P < 0.0001. Scale bars: 2 mm, (A), (C), and (E); 1 mm, (B), (D), and (F); 5 μm, (G), (I), and (K); 1 μm, (H), (J), and (L); and 2 μm (M).

Fig. 3. Mice learn general strategies for coping with uneven rung spacing.

(A) On the regular running wheel, WT mice place fore- and hindpaws on consecutive rungs while reaching forward with the contralateral forepaw. (B to E) On the complex wheel, they grasp the same rung with fore- and hindpaws (red dots), selecting rungs preceded by a one- or two-rung gap [e.g., (B) and (D)]. These strategies are transferable to other rung patterns. (B) and (C) and (D) and (E) are consecutive video frames (240 frames/s).

Fig. 4. Active CNS myelination is required for motor skill learning.

(A) The complex wheel pattern. (B) Experimental design. P-Myrf^(−/-) and P-Myrf^(+/−) mice were housed singly, and tamoxifen was administered from P60 or P90. Three weeks later, they were introduced to the complex wheel (CW). (C and D) On the wheel, P-Myrf^(−/−) mice were impaired relative to P-Myrf^(+/−) [tamoxifen on P90; means ± SEM (error bars), n = 7 and 5, respectively]. (E) Time on wheel at >1 m/min was not different between cohorts. Error bars denote SEM. (F) Individual performances, distance versus time. (G to J) Five pooled experiments confirm divergence between P-Myrf^(+/−) and P-Myrf^(−/−) mice [P = 0.0063 for accumulated distances, P = 0.0003 for average speeds; K-S test, n = 36 (20 males) and 32 (17 males), respectively]. Error bars in (G) denote SEM. (K and L) P-Myrf^(+/flox) and P-Myrf^(flox/flox) mice were introduced to the complex wheel before tamoxifen exposure and reintroduced 3 weeks after treatment. Both before and after, there was no difference between cohorts (n = 7 and 8, respectively). Error bars in (L) denote SEM. *P < 0.05; **P < 0.01; ***P < 10⁻³; ****P < 10⁻⁴. Also see fig. S6.

Fig. 5. Running stimulates OP proliferation and OL production.

Running on the complex wheel (CW) caused (A) a transient increase in the EdU labeling index of Pdgfra⁺ OPs in the corpus callosum after 2 days, (B) an increase in the number density of OPs at 6 days, and (C) increased production of OLs (EdU⁺,Pdgfra⁻) by 11 days. The latter were a mixture of mature CC1⁺ and immature CC1⁻ OLs. The numbers of both cell types were greater in runners than nonrunners at 11 days, although individually the increases did not reach significance (P = 0.15 and 0.06, respectively). (D) After 3 weeks running, there were ~50% more EdU⁺,CC1⁺ OLs in runners than nonrunners. (E) Experimental design. EdU was administered in the drinking water for 4 days during running, as indicated (arrows 1 and 2). (F) The EdU labeling index was increased by the first but not the second encounter with the wheel. Each data point represents multiple fields from at least three sections from three or more mice. Error bars in (A) to (D) and (F) represent SEM. Also see fig. S7.