The Quest for Remyelination: A New Role for Neurotrophins and Their Receptors

Abstract

The formation of myelin is dependent on a reciprocal and intimate relationship between neurons and the myelin‐forming glia. Recently, the neurotrophin family of growth factors has been shown to regulate the complex cell–cell interactions that control myelination. Neurotrophins and their receptors influence myelin formation via two distinct mechanisms, either by acting on the neurons, changing the axonal signals that control myelination, or by acting directly on the myelin‐forming glia. In this review, we will discuss research highlighting the ability of neurotrophins to both promote and inhibit the myelination process. As reflected in the work presented here, these effects are dependent on a delicate balance of which neurotrophins are expressed, and what receptors are activated. Additionally, we examine an emerging model in which the growth factors that promote the early survival and differentiation of particular sets of neurons later play important roles as key regulators in glial development. Characterizing the temporal expression and the cellular targets of neurotrophins, both during development and after injury, represents a pivotal step in developing a greater understanding of the myelination process, contributing to the development of effective treatments for demyelinating conditions. We conclude this review by discussing the potential for neurotrophins as therapeutics in the quest for remyelination.

INTRODUCTION

In the nervous system, neurons and glia share a mutual dependence in establishing a functional relationship. Nowhere is this interdependence more evident than in the process by which glia form myelin around axons. The formation of myelin is an exquisite and dynamic example of cell–cell interaction, which consists of the unidirectional wrapping of multiple layers of membrane concentrically around an axon, initiated at the site of the axon–glial junction. The myelin sheath is a product of evolution, necessary for saltatory conduction, allowing action potentials to propagate more efficiently and rapidly along an axon. Demyelination, caused by disease or nerve injury, severely disrupts the efficient transmission of signals, ultimately resulting in loss of function. In order to more effectively treat these devastating conditions, it is essential to expand our knowledge of how myelin formation occurs. Although there is still much to be learned about the factors necessary for myelination, both during development and after injury, a growing body of work suggests that neurotrophins may play a key role in this process. This family of growth factors, which includes nerve growth factor (NGF), brain‐derived neurotrophic factor (BDNF), neurotrophin‐3 (NT‐3) and NT‐4/5, was initially shown to be important in the development of specific neuronal populations (2, 23, 26, 28). These trophic factors stimulate growth and promote survival of neurons through their interaction with the tropomyosin‐related kinase (Trk) receptors. Neurotrophins exhibit selective affinity for the Trk receptors: NGF binds to TrkA, BDNF and NT‐4/5 bind to TrkB, and NT‐3 binds to TrkC. All of the neurotrophins, both in their mature form and in their uncleaved state as proneurotrophins, also bind to the pan‐neurotrophin receptor (NTR) p75. Interaction with p75^NTR can result in a variety of downstream events, including neuronal cell survival, cell death and myelination, depending on both the state of the neurotrophin and whether p75^NTR acts alone or in a receptor complex (27)

In addition to their vital role in the growth and survival of neurons, neurotrophins serve a variety of other functions, including effects on non‐neuronal tissues as seen in the cardiovascular, endocrine, reproductive and immune systems (39). Within the nervous system, neurotrophins also modulate cell‐fate decisions, dendritic pruning, synaptic strength and plasticity, and neurotransmitter release (12, 23, 39). Neurotrophins exercise control over various stages of glial development (Figure 1), effects that are particularly exciting because of their relevance to remyelination. For example, NT‐3 impacts the survival, proliferation and differentiation of oligodendrocytes, the myelin‐forming cell of the central nervous system (3, 4). BDNF can induce oligodendrocyte proliferation and differentiation (17, 29). In the periphery, NT‐3 is involved in the survival and migration of Schwann cells, the myelin‐forming cell of the peripheral nervous system (1, 6, 30, 42), and NGF acting through p75^NTR has also been shown to play a role in Schwann cell migration (1, 6). Understanding the effects of neurotrophins on both neuronal and glial development is necessary to determine the appropriate environment in which remyelination can occur. Equally important is determining how neurotrophins are involved in the actual process of myelin formation. A clear understanding of the precise mechanisms by which neurotrophins regulate myelination requires the use of both in vivo and in vitro systems. Recent advances in in vitro myelination techniques allow for the careful reduction and manipulation of experimental conditions in order to characterize the dynamic relationship between neurons and glia. In this review, we will discuss research obtained from both cell culture and animal models, highlighting the ability of neurotrophins both to promote and to inhibit the myelination process. As reflected in the work presented here, these effects are dependent on a delicate balance of which neurotrophins are expressed, and what receptors are activated. In order to develop appropriate and effective therapeutics, it is also important to understand where and when the neurotrophins and their receptors are expressed. For this reason, we choose to focus this review on the dual role neurotrophins play in the myelination process. Neurotrophins regulate myelination either by mediating axonal signals or by acting directly on the myelinating glia. Understanding this difference will hopefully enhance the development of therapeutics that will specifically target the site of dysfunction in demyelinating conditions. We will conclude by examining the use of neurotrophins in transplantation studies performed after injury. Promising results from these efforts highlight the exciting potential of neurotrophins in the quest for remyelination.

Figure 1.

Stages of Schwann cell and oligodendrocyte development. A. Illustration of Schwann cell morphology during proliferation/migration; premyelination, which includes elongation and ensheathment; and myelination. B. Illustration of oligodendrocyte progenitor cells during the proliferation/migration stage of development, the differentiation of the progenitor cell into an oligodendrocyte, and finally active myelination.

NEUROTROPHINS REGULATE THE AXONAL CONTROL OF MYELINATION

The quest for remyelination depends in part on understanding why some axons become myelinated during development and others do not. Cues from axons have been shown to play a role in determining which axons undergo myelination (15). There is still much to learn about both the identity of these signals and how these signals are regulated. A key study demonstrates an increase in both axonal diameter and myelination following an increase in the size of the axonal target (40). This work suggests that augmentation of axon diameter may represent an important step in inducing myelination. The results of this study also imply that extrinsic factors are instrumental in regulating axonal signals to the myelinating glia. Recent work shows that neurotrophins represent one type of extrinsic factor that can influence these axonal cues. NGF has been shown to regulate myelination of TrkA‐expressing dorsal root ganglion (DRG) neurons by both Schwann cells and oligodendrocytes in vitro (11). This effect is mediated through a direct interaction between NGF and its receptor on the neuron, and was demonstrated by the ability of NGF to regulate myelination in the absence of p75^NTR. Instead, NGF modulates myelination through an interaction with TrkA, which was shown by using a TrkA‐activating antibody to successfully recapitulate the effects of NGF on myelin protein expression (11). The failure of NGF to regulate myelination of TrkB‐expressing neurons confirms that the role of NGF in myelination occurs through a specific interaction with neuronal TrkA, and not through a receptor on the glial cells.

Surprisingly, the interaction between NGF and TrkA promotes myelination by Schwann cells, but inhibits myelination by oligodendrocytes (11). This intriguing observation provides new insight into the role of axonal signals in myelination. Myelin formation tends to occur along the entire length of an axon, without any noticeable disruption caused by the crossing of the axon from the central nervous system (CNS) into the peripheral nervous system (PNS). It was therefore assumed that a common axonal signal was likely to control myelination in both the CNS and the periphery. However, the differential effects of NGF on myelination suggest the probable existence of distinct axonal signals, specific to either Schwann cells or oligodendrocytes. Recent work identifies neuregulin‐1 (NRG1) type III as an axonal signal that controls various aspects of myelin formation by Schwann cells (32, 38). It is possible that the effect of NGF on Schwann cell myelination occurs through regulation of NRG1 type III. The finding that NGF promotes the release of soluble forms of neuregulin from DRG sensory neurons provides support for this scenario (19). However, it is important to note that NRG1 type III was recently shown to be necessary, but not sufficient, for the induction of Schwann cell myelination (38). It is therefore possible that the interaction of NGF with TrkA regulates PNS myelination through the control of additional axonal signals that have not yet been identified (Figure 2A).

Figure 2.

Neurotrophins and their receptors regulate myelination of the central (CNS) and peripheral nervous systems (PNS). A. The effects of neurotrophins and their receptors on myelination of the PNS. Neurotrophins impact multiple stages of the myelination process through direct interaction with receptors on the Schwann cells. For instance, neuron‐derived BDNF (34), signals through p75^NTR expressed on the Schwann cells to promote myelin formation, but also to inhibit the migration and proliferation of Schwann cells (16). In contrast, NT‐3 activates TrkC on the Schwann cells to inhibit myelination (10, 42, 43) by activating JNK to promote migration by Schwann cells (10, 42, 43). On the contrary, Krox‐20, expressed by the Schwann cell, promotes myelination and inhibits proliferation by suppressing JNK (33). Finally, the truncated TrkB‐T1 acts as a scavenger for excess BDNF (16) once myelination has completed. Peripheral myelin formation is also regulated through the effects of neurotrophins on axonal signals, that is, NGF activates axonal TrkA to promote myelin formation by Schwann cells (11). Possible downstream signaling targets of TrkA include the neuregulins (32, 38), LINGO‐1 (31), PSA‐NCAM (13) and the adhesion protein L1 (41). Additionally, the promotion of Schwann cell myelination by GDNF (22) may occur through activation of the axonal Ret receptor, which may subsequently signal through similar downstream candidates as TrkA. Electrical activity may also play a role in PNS myelination. ATP, released by axons following transmission of action potentials, acts on the glial receptor P2 to inhibit Schwann cell myelination, migration and proliferation (36). B. Effects of neurotrophins and their receptors on CNS myelination. Direct effects of neurotrophins on oligodendrocytes include the promotion of migration and survival through the combined signaling of NT‐3 and PDGF to activate the glial receptors, TrkC and PDGFR‐α respectively (3, 4, 25). BDNF has also been reported to promote oligodendrocyte differentiation via the full‐length TrkB (18). Furthermore, adenosine, released by axons, acts through the purinergic receptor A2 to inhibit migration and proliferation and promote myelin formation by oligodendrocytes (37). Additionally, ATP release by axons stimulates the release of leukemia inhibitory factor (LIF) by astrocytes, which promote myelination by oligodendrocytes (24). The ability of neurotrophins to regulate axonal myelination cues is demonstrated by the inhibition of myelination through the activation of axonal TrkA by NGF (11). Possible downstream signaling targets of TrkA include the neuregulins (32, 38), LINGO‐1 (31), PSA‐NCAM (13) and L1 (41). In addition, the ability of BDNF to promote myelination in the CNS has been hypothesized to occur through effects on the axons, though the identity of the receptor that BDNF activates, and whether the receptor is truly located on the neuron has not yet been determined (9). Abbreviations: ATP = adenosine triphosphate; BDNF = brain‐derived neurotrophic factor; GDNF = glial cell‐line derived neurotrophic factor; JNK = c‐Jun N‐terminal kinase; NGF = nerve growth factor; p75^NTR = pan‐neurotrophin receptor p75; PDGF = platelet‐derived growth factor; PDGFR‐α = platelet‐derived growth factor receptor alpha; PSA‐NCAM = polysialic acid neural cell adhesion molecule; PDGF‐AA = PDGF A‐chain homodimer; Trk = tropomyosin‐related kinase.

Interestingly, this work shows that neuronal expression of a threshold level of NRG1 type III determines whether axons will become myelinated (38). Increases in the expression of NRG1 type III above this minimum level dictate a corresponding increase in the number of times the myelin membrane will wrap around the axon. Importantly, these findings imply that the ability of NRG1 type III to regulate the thickness of the myelin sheath appears likely to be controlled by changes in axon diameter (32, 38). Myelinated axons with larger diameters express higher amounts of NRG1 type III per unit membrane area and show a corresponding increase in the number of myelin layers around the axon. In contrast, the induction of myelination by NRG1 type III appears to be independent of axon diameter (38). This conclusion is supported by the finding that axons of similar diameter can differ greatly in their level of expression of NRG1 type III, at or below the threshold required to induce myelination (38). These findings are important because they demonstrate that changes in axon diameter alone are not sufficient to induce myelination in the periphery.

Regulation of myelin formation, independent of axon diameter, can also be seen in the effects of NGF in the CNS. The presence of NGF promotes an increase in axon diameter, but also inhibits myelin formation by oligodendrocytes (11). This result suggests that NGF acts through TrkA to regulate axonal signals other than axon diameter to prevent the formation of myelin by oligodendrocytes. It is important to note that the effects of NGF on oligodendrocytes include inhibition of both the induction of myelination, and the differentiation and maturation of oligodendrocytes before myelin formation (Figure 2B) (11). The specific effect of NGF on the development of oligodendrocytes was shown by staining DRG–oligodendrocyte precursor cell (OPC) cocultures for 2′,3′‐cyclic nucleotide 3′‐phosphodiesterase (CNPase), myelin‐associated glycoprotein (MAG) and myelin basic protein (MBP), markers of progressive stages of oligodendrocyte maturation. In the presence of NGF, the expression of these markers was significantly decreased compared with cells cultured in the absence of NGF. These results suggest that NGF greatly inhibits the differentiation and maturation of oligodendrocytes, an effect that contributes to the overall reduction in myelin formation in the presence of NGF. However, NGF also directly inhibits the induction of myelination, independent of its effects on oligodendrocyte development. This was demonstrated by the ability of NGF to inhibit myelin formation in cultures containing purified mature oligodendrocytes seeded onto DRG axons (11). Thus, the interaction between NGF and TrkA may activate separate axonal signaling cascades to control both oligodendrocyte development and the induction of myelination. Although none of the axonal signals downstream of TrkA have currently been identified, potential candidates include the neuregulins (32, 38), LINGO‐1 (31), and various cell adhesion molecules such as polysialic acid neural cell adhesion molecule (PSA‐NCAM) (13), and L1 (41). Additional candidates include the purinergic receptors and compounds such as adenosine (37). Adenosine, generated through the hydrolysis of adenosine triphosphate (ATP) released by electrically active axons, has been shown to act through the purinergic receptor A2 to inhibit migration and proliferation and promote myelin formation by oligodendrocytes (37) (Figure 2B). Additionally, ATP release by axons stimulates the release of leukemia inhibitory factor by astrocytes, which promote myelination by oligodendrocytes (24). Some of these candidates may also represent additional factors involved in peripheral myelination (Figure 2A). In particular, ATP has been shown to act on the glial receptor P2 to inhibit Schwann cell myelination, migration and proliferation (36).

The novel capability of NGF to regulate axonal signals involved in myelination suggests that other neurotrophins may also direct myelin formation through neuronal signaling cascades. A reduction in the size of retinal ganglion cell (RGC) axons was shown to be accompanied by hypomyelination of the optic nerve in BDNF knockout mice (9). These findings may indicate that direct effects of BDNF on axons are necessary for proper myelination by oligodendrocytes (9). It is possible that BDNF controls myelination of the optic nerve through direct modulation of RGC axon diameter. Another interesting study demonstrates the ability of glial cell‐line derived neurotrophic factor (GDNF) to promote myelination of nociceptive neurons by Schwann cells (22). These results may represent yet another example of an extrinsic growth factor regulating myelination through effects on the axons, particularly because GDNF has also been shown to promote the release of neuregulin from sensory neurons (19). The enhancement of Schwann cell myelination by GDNF (22) may occur through activation of the axonal Ret receptor, which could initiate downstream signaling cascades similar to those hypothesized to be activated by TrkA (Figure 2A). Hopefully, future studies will confirm whether these examples are indeed cases in which extrinsic growth factors regulate axonal cues for myelination. Elucidating the exact signaling pathways involved in myelin formation is absolutely necessary for the development of therapeutics to effectively treat demyelinating conditions.

NEUROTROPHIN EFFECTS ON MYELIN‐FORMING GLIA

In recent years, neurotrophins have been shown to elicit diverse and sometimes opposing effects on the myelin‐forming glia of the PNS and CNS, both during development and after injury. Understanding the ability of neurotrophins both to promote and to inhibit the myelination process is essential for discovering the appropriate means by which to treat demyelinating conditions.

Neurotrophins directly influence Schwann cell development.  It has long been thought that cell proliferation, migration and differentiation are distinct and separate cellular processes, and that a cell must complete these initial events before making cell‐fate decisions and discriminating its ultimate function. However, recent work suggests that the determination of Schwann cell fate, as either a myelinating or a nonmyelinating cell, depends heavily on the modulation of these earlier developmental events. Thus, the formation of myelin by Schwann cells can be regarded as a complex process encompassing multiple interdependent stages of Schwann cell development. This process includes a period of proliferation in which Schwann cells receive signals to rapidly proliferate and migrate along axons, a premyelination phase involving elongation and ensheathment of the axon by the glia, and finally the active formation of myelin (Figure 1A). Accumulating evidence suggests that neurotrophins and their receptors may have a variety of effects on the developmental stages involved in peripheral myelin formation. The effects of NT‐3 and BDNF on Schwann cell myelination highlight the intricate relationship between the various stages of the myelination process. BDNF greatly enhances the expression of myelin proteins and the formation of Schwann cell myelin internodes in vitro (10, 16, 34). Unexpectedly, NT‐3 has the opposite effect, inhibiting the expression of the myelin proteins and the formation of the internodes (10, 34). In order to identify the neurotrophin receptors responsible for the BDNF enhancement and the NT‐3 inhibition of myelin formation, the expression of neurotrophin receptors was initially monitored throughout the myelination process (16). p75^NTR, the full‐length TrkC, and the truncated TrkB‐T1 receptors were the only neurotrophin receptors detected in the Schwann cells. Through a combination of blocking antibodies, inhibitors and the p75^NTR knockout mouse, the effect of BDNF was shown to be mediated by p75^NTR, whereas the effect of NT‐3 was mediated by the full‐length TrkC receptor in vitro (16, 34). These effects were recapitulated by in vivo studies whereby exogenous neurotrophins, soluble neurotrophin scavengers (TrkB‐Fc, TrkC‐Fc) or function‐blocking antibodies were injected along the developing sciatic nerve (10, 16). Additionally, it was shown that TrkB‐T1 was induced at the initiation of active myelin formation and followed a similar expression profile as the myelin proteins both in vitro and in vivo (16). As blocking antibodies to TrkB‐T1 enhanced myelination, it appears that this truncated receptor may act as a sink or scavenger to remove excess BDNF once myelination is well underway. This is an intriguing alternative to the down‐regulation of BDNF gene expression to achieve a similar purpose (16). These studies confirm that BDNF and NT‐3 directly affect the active formation of myelin by Schwann cells. However, it is also important to evaluate the effects of neurotrophins on other stages of the myelination process.

Interestingly, the p75^NTR and TrkC receptor are expressed both in Schwann cell/DRG neuronal cocultures and during the development of the sciatic nerve, with their levels decreasing gradually as myelination progresses (16). These expression profiles suggest that BDNF and NT‐3, acting through p75^NTR and TrkC, respectively, are impacting the early stages of the myelination process, in addition to controlling the induction of myelin formation. Recently, growth factors have been shown to inhibit Schwann cell myelination by possibly inducing or maintaining Schwann cells in a nonmyelinating proliferative state (44). In a similar fashion, NT‐3 acting through TrkC was shown to greatly enhance Schwann cell migration while inhibiting myelination, whereas BDNF acting through p75^NTR inhibits migration but promotes myelination (42, 43). Specifically, the NT‐3‐induced cell migration is dependent on the activation of the Rho GTPases, Rac1 and Cdc42, and downstream c‐Jun N‐terminal kinase (JNK). Conversely, the BDNF inhibition is dependent on the activation of RhoA and Rho kinase (16, 42, 43). These opposing signals from BDNF and NT‐3 are dependent on the expression of the neurotrophin, the activation of the specific receptor and the identity of the downstream effector molecules. Is it possible that cell migration and myelination are intimately related such that inhibiting the pathway activated by NT‐3/TrkC promotes Schwann cell myelination? Consistent with this emerging theme, the transcription factor Krox‐20, which is known to control Schwann cell differentiation and myelination, abolishes Schwann cell proliferation by suppressing the JNK (33). The work discussed here strongly suggests that the various stages of Schwann cell development are closely interrelated such that the regulation by neurotrophins of early developmental stages directly impacts the ultimate decision by the Schwann cell of whether or not to form myelin (Figure 2A).

Finally, it is also important to consider the effects of neurotrophins and their receptors on myelination by Schwann cells after nerve injury. Endogenous BDNF was found to be essential for regeneration and remyelination of axons after injury (45). Similarly, the role of p75^NTR in remyelination was also examined after injury and was found to be consistent with the previous reports. Functional p75^NTR was necessary for remyelination; however, it must be noted that axonal effects by BDNF and p75^NTR cannot be excluded from these particular studies (35). Defining the delicate balance of neurotrophins during development and the fundamental signaling pathways necessary to promote myelination will hopefully relate specifically to cell transplantation studies and remyelination paradigms.

Neurotrophins directly influence oligodendrocyte development.  During development, the oligodendrocyte progenitor cell, the myelin‐forming cell of the central nervous system, receives signals to proliferate, migrate along axons, differentiate into mature oligodendrocytes and then extend processes to multiple axons and initiate the formation of myelin (Figure 1B). Neurotrophins released from either paracrine or autocrine sources, play a key role in regulating proliferation, cell survival and differentiation of both the progenitor cell and the mature oligodendrocyte. Specifically, studies have documented the effects of NT‐3 on oligodendrocyte survival and proliferation, in addition to promoting the clonal expansion of oligodendrocyte progenitor cells in vitro when combined with the mitogen platelet‐derived growth factor (Figure 2B) (3, 4, 25). In these studies, full‐length TrkC, expressed by both the progenitors and the mature oligodendrocytes, mediated the effect of NT‐3 on oligodendrocyte proliferation and survival (3, 4, 14, 25). Additional studies showed that NGF significantly enhanced oligodendrocyte survival, but did not promote the proliferation of progenitor cells unless coadministered with the mitogen fibroblast growth factor (14). Recently, the role of neurotrophins in oligodendrocyte differentiation was reported (17, 18). BDNF, NGF and NT‐3 were found to promote differentiation of basal forebrain oligodendrocytes by modulating the expression of MBP (17). However, when working with cortical oligodendrocytes, only NGF and NT‐3 promoted differentiation, without any effect from BDNF (17). The expression of specific neurotrophin receptors, including all of the Trk receptors and p75^NTR, was found to be responsible for the regional effects of the neurotrophins (18). These findings suggest that oligodendrocytes may exist as a heterogeneous population of cells, expressing different neurotrophin receptors in various combinations or separately. Contrary to the neurotrophin effects mediated by p75^NTR on the differentiation of oligodendrocytes, p75^NTR has previously been shown to induce oligodendrocyte cell death. Specifically, NGF induced the p75‐mediated cell death of cortical oligodendrocytes cultured from neonatal rats without any effect from BDNF or NT‐3 (8). Recent findings illustrate that p75^NTR expression is induced in spinal cord oligodendrocytes after injury and that the pro‐neurotrophin (unprocessed precursor) pro‐NGF induces p75‐mediated death of oligodendrocytes (5). It was later demonstrated that the pro‐form of NGF was indeed secreted in the CNS after injury, and that this secreted pro‐NGF was capable of inducing cell death in vitro (21). The direct effects of neurotrophins on oligodendrocyte proliferation, survival and differentiation are complex, eliciting diverse and sometimes opposing events in oligodendrocyte development. Taken together, the potential role for neurotrophins to promote remyelination will depend largely on a careful balance between the timing and expression profiles of which neurotrophins are expressed, and what receptors may be activated.

THE ROLE OF NEUROTROPHINS IN REMYELINATION

Demyelination by injury or disease results in loss of function leading to severe debilitation. Determining the appropriate environment in which remyelination can occur is essential for proper treatment of this devastating condition. As demonstrated by the work reviewed here, neurotrophins exert effects on both neurons and glia that contribute to the formation of myelin during development. It is therefore plausible that these same factors may prove to be important in promoting remyelination after injury and disease. Indeed, recent work involving transplantation studies demonstrates the remarkable potential of neurotrophins to establish functional recovery following nerve injury and demyelination. Transplantation of fibroblasts expressing either BDNF or NT‐3 enhanced axonal growth, OPC proliferation and myelination in adult rat spinal cords after injury (29). The effectiveness of these two neurotrophins in promoting remyelination is also shown through transplantation of BDNF or NT‐3‐expressing Schwann cells into demyelinated mouse spinal cords (20). These Schwann cell grafts not only increased OPC proliferation and differentiation, but also encouraged remyelination and recovery of locomotor function (20). An additional example of functional recovery was seen following transplantation of glial‐restricted precursor cells (GRPs) expressing D15A, a multi‐neurotrophin with the signaling capabilities of BDNF and NT‐3 (7). In this study, performed on rats subjected to spinal cord injury, signaling by neurotrophins promoted GRPs both to differentiate and to actively form myelin, resulting in improved locomotor function (7). Clearly, these studies provide exciting evidence that neurotrophins are effective at promoting remyelination and functional recovery after spinal cord injury. However, it remains to be seen whether these effects result from the actions of neurotrophins on the neurons or on the glia. In order for neurotrophins to be successfully used in the treatment of demyelinating conditions, it is crucial to determine their exact site of action in promoting remyelination. By understanding where neurotrophins and their receptors are expressed, therapeutics can be targeted to specific sites of dysfunction, hopefully resulting in more effective treatment and minimizing unwanted side effects. In addition, the method of neurotrophin administration requires careful consideration, that is, timing‐dependent mechanisms associated with glia, axonal regeneration and survival, and the cellular targets of the neurotrophin action. This is made extremely clear in transplantation studies where the locomotor activity of injured rats did not improve until 4 weeks after transplantation occurred (7). It is quite possible that therapeutic induction of remyelination will require a combination of neurotrophins and neurotrophin receptors expressed at various time points after injury. The use of combinatorial treatment is supported by the work discussed previously demonstrating the effects of neurotrophins at various stages of the myelination process. Naturally, it must be noted that the formation of myelin results from complex cell–cell interactions, and the precise mechanisms promoting myelination are still not entirely clear. The studies discussed here provide insight into the effects of neurotrophins on myelination, both during development and after injury. By characterizing the temporal expression and the cellular targets of neurotrophins, future studies will provide a greater understanding of the myelination process, contributing to the development of effective therapeutics in the quest for remyelination.