Jonah R. Chan

NGF Controls Axonal Receptivity to Myelination by Schwann Cells or Oligodendrocytes

Axons dictate whether or not they will become myelinated in both the central and peripheral nervous systems by providing signals that direct the development of myelinating glia. Here we identify the neurotrophin nerve growth factor (NGF) as a potent regulator of the axonal signals that control myelination of TrkA-expressing dorsal root ganglion neurons (DRGs). Unexpectedly, these NGF-regulated axonal signals have opposite effects on peripheral and central myelination, promoting myelination by Schwann cells but reducing myelination by oligodendrocytes. These findings indicate a novel role for growth factors in regulating the receptivity of axons to myelination and reveal that different axonal signals control central and peripheral myelination.

Neurotrophins are key mediators of the myelination program in the peripheral nervous system

Although knowledge of the functions of neurotrophins has advanced rapidly in recent years, studies concerning the involvement of neurotrophins in glial–neuronal interactions rarely extend further than their roles in supporting the survival and differentiation of neuronal cells. In this study endogenous brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3) were identified in Schwann cell/dorsal root ganglia neuronal cocultures and shown to modulate the myelination program of the peripheral nervous system. The differential expression of BDNF and NT3 were examined and compared with the expression profiles of myelin proteins in the cocultures throughout the myelination process. BDNF levels correlated with active myelin formation, whereas NT3 expression was initially high and then down regulated throughout the proliferation and premyelination periods. Addition of exogenous BDNF enhanced myelination, whereas the removal of the endogenous BDNF by using the BDNF receptor TrkB-Fc fusion protein inhibited the formation of mature myelin internodes. Interestingly, exogenous NT3 significantly inhibited myelination, whereas the removal of the endogenous NT3 by using the NT3 receptor TrkC-Fc fusion protein resulted in an enhancement similar to that obtained with the addition of BDNF. In addition, in vivo studies were performed during the development of the mouse sciatic nerve. Subcutaneous injections of BDNF resulted in an enhancement of myelin formation in the sciatic nerve, whereas the removal of the endogenous BDNF dramatically inhibited myelination. Injections of NT3 inhibited myelin formation, and the removal of the endogenous NT3 enhanced myelination. These results demonstrate that BDNF and NT3 possess different modulatory roles in the myelination program of the peripheral nervous system and that their mechanisms of action are specific and highly regulated.

Myelin Regeneration: A Recapitulation of Development?

The developmental process of myelination and the adult regenerative process of remyelination share the common objective of investing nerve axons with myelin sheaths. A central question in myelin biology is the extent to which the mechanisms of these two processes are conserved, a concept encapsulated in the recapitulation hypothesis of remyelination. This question also has relevance for translating myelin biology into a better understanding of and eventual treatments for human myelin disorders. Here we review the current evidence for the recapitulation hypothesis and discuss recent findings in the development and regeneration of myelin in the context of human neurological disease.

A culture system to study oligodendrocyte myelination processes using engineered nanofibers

Current methods for studying central nervous system myelination necessitate permissive axonal substrates conducive to myelin wrapping by oligodendrocytes. We have developed a neuron-free culture system in which electron-spun nanofibers of varying sizes substitute for axons as a substrate for oligodendrocyte myelination, thereby allowing manipulation of the biophysical elements of axonal-oligodendroglial interactions. To investigate axonal regulation of myelination, this system effectively uncouples the role of molecular (inductive) cues from that of biophysical properties of the axon. We use this method to uncover the causation and sufficiency of fiber diameter in the initiation of concentric wrapping by rat oligodendrocytes. We also show that oligodendrocyte precursor cells display sensitivity to the biophysical properties of fiber diameter and initiate membrane ensheathment before differentiation. The use of nanofiber scaffolds will enable screening for potential therapeutic agents that promote oligodendrocyte differentiation and myelination and will also provide valuable insight into the processes involved in remyelination.

Olig2 Targets Chromatin Remodelers to Enhancers to Initiate Oligodendrocyte Differentiation

Establishment of oligodendrocyte identity is crucial for subsequent events of myelination in the CNS. Here, we demonstrate that activation of ATP-dependent SWI/SNF chromatin-remodeling enzyme Smarca4/Brg1 at the differentiation onset is necessary and sufficient to initiate and promote oligodendrocyte lineage progression and maturation. Genome-wide multistage studies by ChIP-seq reveal that oligodendrocyte-lineage determination factor Olig2 functions as a prepatterning factor to direct Smarca4/Brg1 to oligodendrocyte-specific enhancers. Recruitment of Smarca4/Brg1 to distinct subsets of myelination regulatory genes is developmentally regulated. Functional analyses of Smarca4/Brg1 and Olig2 co-occupancy relative to chromatin epigenetic marking uncover stage-specific cis-regulatory elements that predict sets of transcriptional regulators controlling oligodendrocyte differentiation. Together, our results demonstrate that regulation of the functional specificity and activity of a Smarca4/Brg1-dependent chromatin-remodeling complex by Olig2, coupled with transcriptionally linked chromatin modifications, is critical to precisely initiate and establish the transcriptional program that promotes oligodendrocyte differentiation and subsequent myelination of the CNS.

NGF Regulates the Expression of Axonal LINGO-1 to Inhibit Oligodendrocyte Differentiation and Myelination

Neurons and glia share a mutual dependence in establishing a functional relationship, and none is more evident than the process by which axons control myelination. Here, we identify LRR and Ig domain-containing, Nogo receptor-interacting protein (LINGO-1) as a potent axonal inhibitor of oligodendrocyte differentiation and myelination that is regulated by nerve growth factor and its cognate receptor TrkA in a dose-dependent manner. Whereas LINGO-1 expressed by oligodendrocyte progenitor cells was previously identified as an inhibitor of differentiation, we demonstrate that axonal expression of LINGO-1 inhibits differentiation with equal potency. Disruption of LINGO-1 on either cell type is sufficient to overcome the inhibitory action and promote differentiation and myelination, independent of axon diameter. Furthermore, these results were recapitulated in transgenic mice overexpressing the full length LINGO-1 under the neuronal promoter synapsin. Myelination was greatly inhibited in the presence of enforced axonal LINGO-1. The implications of these results relate specifically to the development of potential therapeutics targeting extrinsic growth factors that may regulate the axonal expression of modulators of oligodendrocyte development.

Multiple sclerosis: Prospects and promise

We have entered a golden era in multiple sclerosis (MS) research. Two decades ago, our understanding of the disease was largely descriptive and there were no approved therapies to modify the natural history of MS. Today, delineation of immune pathways relevant to MS have been clarified; a comprehensive map of genes that influence risk compiled; clues to environmental triggers identified; noninvasive in vivo monitoring of the MS disease process has been revolutionized by high‐field MRI; and many effective therapies for the early, relapsing, component of MS now exist. However, major challenges remain. We still have no useful treatment for progressive MS (the holy grail of MS research), no means to repair injured axons or protect neurons, and extremely limited evidence to guide treatment decisions. Recent advances have set in place a foundation for development of increasingly selective immunotherapy for patients; application of genetic and genomic discoveries to improve therapeutic options; development of remyelination or neuroprotection therapies for progressive MS; and integrating clinical, imaging and genomic data for personalized medicine. MS has now advanced from the backwaters of autoimmune disease research to the front‐line, and definitive answers, including cures, are now realistic goals for the next decade. Many of the breakthrough discoveries in MS have also resulted from meaningful interactions across disciplines, and especially from translational and basic scientists working closely with clinicians, highlighting that the clinical value of discoveries are most often revealed when ideas developed in the laboratory are tested at the bedside. Ann Neurol 2013;74:317–327

The geometric and spatial constraints of the microenvironment induce oligodendrocyte differentiation.

The oligodendrocyte precursor cell (OPC) arises from the subventricular zone (SVZ) during early vertebrate development to migrate and proliferate along axon tracts before differentiating into the myelin-forming oligodendrocyte. We demonstrate that the spatial and temporal regulation of oligodendrocyte differentiation depends intimately on the axonal microenvironment and the density of precursor cells along a specified axonal area. Differentiation does not require dynamic axonal signaling, but instead is induced by packing constraints resulting from intercellular interactions. Schwann cells and even artificial beads bound to the axonal surface can mimic these constraints and promote differentiation. Together, these results describe the coordinately controlled biophysical interaction of oligodendrocyte precursors within an axonal niche leading to self-renewal and differentiation.

Schwann cells and astrocytes induce synapse formation by spinal motor neurons in culture

Glia constitute 90% of cells in the human nervous system, but relatively little is known about their functions. We have been focusing on the potential synaptic roles of glia in the CNS. We recently found that astrocytes increase the number of mature, functional synapses on retinal ganglion cells (RGCs) by sevenfold and are required for synaptic maintenance in vitro. These observations raised the question of whether glia similarly enhance synapse formation by other neuron types. Here we have investigated whether highly purified motor neurons isolated from developing rat spinal cords are able to form synapses in the absence of glia or whether glia similarly enhance synapse number. We show that spinal motor neurons (SMNs) form few synapses unless Schwann cells or astrocytes are present. Schwann cells increase the number of functional synapses by ninefold as measured by immunostaining, and increase spontaneous synaptic activity by several hundredfold. Surprisingly, the synapses formed between spinal motor neurons were primarily glutamatergic, as they could be blocked by CNQX. This synapse-promoting activity is not mediated by direct glial-neuronal cell contact but rather is mediated by secreted molecule(s) from the Schwann cells, as we previously found for astrocytes. Interestingly, the synapse-promoting activity from astrocytes and Schwann cells was functionally similar: Schwann cells also promoted synapse formation between retinal ganglion cells, and astrocytes promoted synapse formation between spinal motor neurons. These studies show that both astrocytes and Schwann cells strongly promote synapse formation between spinal motor neurons and demonstrate that glial regulation of synaptogenesis extends to other neuron types.

Neurotrophin 3 activation of TrkC induces Schwann cell migration through the c-Jun N-terminal kinase pathway

During development and nerve injury, complex interactions between glial cells and neurons are essential for establishing proper nerve function. Neurotrophins play multiple roles in the developing nervous system, including cell survival, growth, and differentiation. Here we show that migration of Schwann cells, isolated from sciatic nerves, is significantly enhanced by neurotrophin 3, but not by nerve growth factor or brain-derived neurotrophic factor. The neurotrophin-3-induced cell migration was also observed in Schwann cells isolated from sciatic nerves of p75NTR-/- mice, indicating that neurotrophin 3 enhances cell migration through TrkC. This effect was blocked by K252a, an inhibitor of the Trk receptor family. Additionally, the neurotrophin-3-induced cell migration depended on Rho GTPases (Rac1 and Cdc42) and c-Jun N-terminal kinase. We obtained the same results with Cos-7 cells expressing TrkC. Taken together, these results suggest that neurotrophin 3 activation of TrkC induces Schwann cell migration through the c-Jun N-terminal kinase signaling pathway.