Astrocytes promote myelination in response to electrical impulses

Abstract

Myelin, the insulating layers of membrane wrapped around axons by oligodendrocytes, is essential for normal impulse conduction. It forms during late stages of fetal development but continues into early adult life. Myelination correlates with cognitive development and can be regulated by impulse activity through unknown molecular mechanisms. Astrocytes do not form myelin, but these nonneuronal cells can promote myelination in ways that are not understood. Here, we identify a link between myelination, astrocytes, and electrical impulse activity in axons that is mediated by the cytokine leukemia inhibitory factor (LIF). These findings show that LIF is released by astrocytes in response to ATP liberated from axons firing action potentials, and LIF promotes myelination by mature oligodendrocytes. This activity-dependent mechanism promoting myelination could regulate myelination according to functional activity or environmental experience and may offer new approaches to treating demyelinating diseases.

Figure 1.

Action Potentials Stimulate Myeli-nation of DRG Axons by Mature Oligodendrocytes(A) Mouse DRG neurons were cultured 3-4 weeks in multicompartment culture dishes equipped with electrodes for stimulating action potentials in axons. Axons from DRG neurons plated into the two-side compartments grow under the barrier with the central compartment, providing a high resistance connection for electrical stimulation through platinum electrodes (Fields et al., 1992). Oligodendrocyte progenitor cells (OPCs) were added to the side compartments and cultured with DRG neurons for 7 days. Electrical stimulation in a phasic pattern (see Experimental Procedures) was applied to axons for 2-3 weeks, and the number of myelinated profiles stained with Sudan black was compared with unstimulated controls. (B) Electrical stimulation increased myelination through a mechanism requiring activation of sodium-dependent action potentials, as shown by blocking the effects of stimulation in the presence of 1 μM of the sodium channel blocker tetrodotoxin (TTX). (C) Treating cultures of mature oligodendrocytes and DRG neurons with adenosine (300μM) or the adenosine receptor agonist NECA over a wide range of concentrations failed to increase the number of Sudan-black-stained myelin profiles, but treatment with the ATP receptor agonist 2MeSATP (300 μM) mimicked the effect of electrical stimulation in increasing myelination by oligodendrocytes (n = 156 cultures, eight independent experiments). ATP is known to be released by axons firing action potentials (Stevens and Fields, 2000), suggesting that ATP may be an activity-dependent intercellular messenger regulating myelination. (D) A similar increase in myelination was seen by using immunocytochemical staining with antibodies against myelin oligodendrocyte glycoprotein (MOG) to quantify the number of myelin segments/microscope field in unstimulated (1Ca) and stimulated cultures (1Cb) (24 cultures, three independent experiments). p < 0.005 with respect to control; scale bar = 20 μm. Error bars represent SEM.

Figure 2.

Electrical Activity Stimulates Myelination through a Mechanism Involving ATP Receptor Activation and the Cytokine Leukemia Inhibitory Factor (A) Both electrical stimulation (10 Hz, phasic; 0.5 s pulse every 2 s) and ATP receptor activation with 300 μM 2MeSATP for 2 weeks increased myelination, with nonadditive effects when used in combination, suggesting action through a similar mechanism. (B) Activity-blocking antibodies against the cytokine leukemia inhibitory factor (LIF) blocked the increase in myelination produced by 2MeSATP treatment. No effects on myelination were seen after antibody treatment in the absence of 2MeSATP stimulation, suggesting the effects of LIF are activity dependent and require ATP receptor stimulation (93 experiments; three replicates). None of the treatments affected the number of oligodendrocytes (data not shown). Asterisk, p < 0.05; double asterisks, p < 0.005. Error bars represent SEM.

Figure 3.

Developmental Profile of Oligodendrocytes in Cocultures and Effects of P2-Receptor Activation The state of differentiation of oligodendrocytes in cocultures was determined by staining with antibodies against markers of OPC differentiation after 7, 9, 11, and 14 days in coculture with DRG neurons in the presence and absence of 100 μM 2MeSATP, and cell proliferation was measured by BrdU incorporation (magenta). Hoechst nuclear stain was used to identify all cells in the culture (blue) and to calculate the percentage of cells identified with each marker. Mean percentage and SEM of cells stained positively with each marker are indicated in the lower right corner of each figure (n = 40 microscope fields in each). Immunocytochemical staining for a marker of early oligodendrocytes O4 is shown in green; a marker for intermediate stage oligodendrocytes, myelin basic protein (MBP), is shown in red; and the myelin oligodendrocyte glycoprotein (MOG) expressed by mature oligodendrocytes is shown in yellow. The results show that OPCs have progressed into a postmitotic phase and begun to express the O4 antigen in large numbers by 7 days in culture, the time when treatment with 2MeSATP was begun. ATP receptor activation with 2MeSATP had no effects on maturation or proliferation of oligodendrocytes when applied after 7 days in coculture, but it did increase the number of cells expressing the mature oligodendrocyte marker MOG after 11 and 14 days (4-7 days 2MeSATP treatment; p < 0.001). The results show that ATP receptor activation increases myelination by increasing development of oligodendrocytes at the mature MOG positive stage, rather than acting on cells at the progenitor stage.

Figure 4.

Action Potential Firing Causes LIF Release from Cocultures of DRG Neurons and Oligodendrocytes, and LIF Treatment Affects Myelination in a Concentration-Dependent Manner(A) 10 Hz phasic stimulation of DRG axons (open triangles) increased the concentration of LIF in medium sampled from cocultures compared with unstimulated cultures (filled circles). The stimulus-induced release of LIF decreased with time in culture, but remained elevated for approximately 1 week (asterisk, p < 0.03 at day 7). Antibody against LIF did not interfere with the LIF release (data not shown). Error bars represent SEM. (B) LIF was applied to 7 day old cocultures over a wide range of concentrations, and myelination was assessed by Sudan black and MOG staining. Low concentrations of exogenous LIF (<5 ng/ml) promoted myelination (p < 0.003, linear regression). Error bars represent SEM. (C) LIF concentrations above 5 ng/ml inhibited myelination (p < 0.0001, linear regression). Error bars represent SEM. (D) Representative images of cocultures stained with MOG are shown; the optimal concentration of LIF was near 0.1 ng/ml. This suggests that endogenously released LIF could influence the timing and extent of myelination. An average of 11 myelinated segments/microscope field were seen in response to 300 μM 2MeSATP. There were no significant differences in the numbers of oligodendrocytes or DRGs in these experiments at the time of myelin quantification. Bar = 10 μm. Error bars represent SEM.

Figure 5.

Action Potentials Increase Myelination via an LIF-Dependent Mechanism with Astrocytes Providing the Major Source of LIF in Response to 2MeSATP Treatment(A) LIF neutralizing antibody blocked both the 2MeSATP and action potential-mediated increase in myelination (ANOVA, p = 0.43, n = 68), suggesting that action potentials and 2MeSATP increase myelination via the same LIF-dependent mechanism. Error bars represent SEM. (B) The release of LIF after 24 hr electrical stimulation of DRG axons in coculture with oligodendrocytes was blocked by stimulation in the presence of 30 U/ml apyrase, an enzyme that rapidly degrades extracellular ATP (p < 0.0001, ANOVA among all four groups; p < 0.05 elect. Stim. versus elect. Stim. + apyrase). This is consistent with the hypothesis that ATP released from axons firing action potentials in turn induces release of LIF by a mechanism involving activation of P2 purinergic receptors. Treatment with apyrase alone had no effect on LIF release. Error bars represent SEM. (C) Astrocytes release LIF in response to 2MeSATP treatment in a concentration-dependent manner, with a maximal response between 100 and 300 μM. Error bars represent SEM. (D) To identify the cellular source of LIF in co-cultures, astrocytes, DRG neurons, and oligodendrocytes were cultured separately and treated with 100 μM 2MeSATP for 1-3 days. In contrast to the large amount of LIF released by 2MeSATP treatment of astrocytes, monocultures of DRG neurons or oligodendrocytes released little LIF. The concentration of LIF in treated versus untreated cultures of astrocytes was significantly different (0.247 6 0.055 versus 7.21 6 0.049 ng/ml LIF in control versus 2MeSATP; p < 0.0001). The small amount of LIF released in oligodendrocyte cultures could derive from contaminating GFAP-positive cells, which increase in oligodendrocyte cultures from 3% to 8% by 10 days in culture. Double asterisks, p < 0.001. Error bars represent SEM.

Figure 6.

LIF Is Synthesized by Astrocytes and Regulated by Action Potential Firing(A) Astrocytes from E19 rat cortex stained strongly with anti-LIF antibody. DRG neurons and oligodendrocytes showed only weak staining with anti-LIF antibody, and astrocytes from LIF^-/- mice were not stained (data not shown). (B) Western blot analysis confirmed the immunostaining results and indicated astrocytes as the predominant source of LIF in cocultures. (No microglia were detectable by immunocytochemistry.) Equal concentrations of protein (35 mg) from purified cultures of DRG neurons, astrocytes, and oligodendrocytes were compared. Antibody against ankyrin G was used as a control for equal loading in each lane. Purified mouse LIF confirmed the specificity of the LIF antibody and indicated that the amount of LIF in the astrocyte sample exceeds 20 ng. (C) The level of LIF mRNA in astrocytes grown on DRG axons was increased after 24 hr electrical stimulation of axons (top). This increase was prevented by stimulation in the presence of apyrase (30 U/ml), which degrades extracellular ATP (bottom). This indicates that the activity-dependent regulation of LIF mRNA levels in astrocytes occurs through a mechanism involving P2 purinergic receptor activation by ATP released from axons firing action potentials. (D) The experiments on effects of action potential firing on LIF mRNA in astrocytes were performed in multicompartment cell-culture chambers that separate the DRG cell bodies from axons. Astrocytes were derived from rats and plated onto mouse axons in the central compartment. Species-specific primers for LIF were used to ensure that the measurements did not derive from LIF mRNA possibly synthesized by neurons. As a control, PCR for 28s ribosomal RNA showed no change after any of the treatments in (C) (data not shown). (E) LIF is present in glia in vivo during the period of myelination in mouse optic nerve, as indicated by immunocytochemical staining of optic nerve from 10 day old mice. (F) Specificity of the antibody is shown by the lack of staining in LIF^-/-mice.

Figure 7.

Astrocytes Promote Myelination in Response to ATP Receptor Stimulation by Releasing LIF (A) A positive effect of astrocytes on myelination was show by supplementing cocultures of DRG neurons and oligodendrocytes with astrocytes. Myelination after 2 weeks in coculture increased in proportion to the percentage of additional astrocytes added 3 hr after plating OPCs on DRG neurons (solid dots). Similar effects were seen in cultures supplemented with astrocytes from mice heterozygous for LIF (open circle). However, supplementing cultures with astrocytes derived from homozygous LIF^-/- mice failed to increase myelination (solid triangle). The inability of astrocytes from LIF^-/- mice to increase myelination could be restored by adding 0.1 ng/ ml LIF to the cultures (open triangle) (n = 1200 microscope fields). Error bars represent SEM. (B) Eliminating astrocytes from cultures also eliminates the positive effect of 2MeSATP on myelination. This was shown by making co-cultures from O1+ oligodendrocytes purified by immunopanning. Cultures made from mature O1+ oligodendrocytes were still responsive to LIF, as shown by the increase in myelination produced by adding 0.1 ng/ml LIF to these cultures. The positive effect of 100 μM 2MeSATP on myelination could be restored by supplementing these cocultures of O1-positive oligodendrocytes with 8% astrocytes, strongly implicating astrocytes as the primary source of LIF in response to electrical stimulation causing the release of ATP from axons. Double asterisks, p < 0.001; n = 400 microscope fields. Error bars represent SEM.