The extracellular factors that are responsible for inducing myelination in the central nervous system (CNS) remain elusive. We investigated whether brain-derived neurotrophic factor (BDNF) is implicated, by first confirming that BDNF heterozygous mice exhibit delayed CNS myelination during early postnatal development. We next established that the influence of BDNF upon myelination was direct, by acting on oligodendrocytes, using co-cultures of dorsal root ganglia neurons and oligodendrocyte precursor cells. Importantly, we found that BDNF retains its capacity to enhance myelination of neurons or by oligodendrocytes derived from p75NTR knockout mice, indicating the expression of p75NTR is not necessary for BDNF-induced myelination. Conversely, we observed that phosphorylation of TrkB correlated with myelination, and that inhibiting TrkB signalling also inhibited the promyelinating effect of BDNF, suggesting that BDNF enhances CNS myelination via activating oligodendroglial TrkB-FL receptors. Together, our data reveal a previously unknown role for BDNF in potentiating the normal development of CNS myelination, via signalling within oligodendrocytes.

The generation of biologically active proteins by regulated intramembrane proteolysis is a highly conserved mechanism in cell signaling. Presenilin-dependent ␥-secretase activity is responsible for the intramembrane proteolysis of selected type I membrane proteins, including ␤-amyloid precursor protein (APP) and Notch. A small fraction of intracellular domains derived from both APP and Notch translocates to and appears to function in the nucleus, suggesting a generic role for ␥-secretase cleavage in nuclear signaling. Here we show that the p75 neurotrophin receptor (p75 NTR ) undergoes presenilin-dependent intramembrane proteolysis to yield the soluble p75-intracellular domain. The p75 NTR is a multifunctional type I membrane protein that promotes neurotrophininduced neuronal survival and differentiation by forming a heteromeric co-receptor complex with the Trk receptors. Mass spectrometric analysis revealed that ␥-secretase-mediated cleavage of p75 NTR occurs at a position located in the middle of the transmembrane (TM) domain, which is reminiscent of the amyloid ␤-peptide 40 (A␤40) cleavage of APP and is topologically distinct from the major TM cleavage site of Notch 1. Size exclusion chromatography and co-immunoprecipitation analyses revealed that TrkA forms a molecular complex together with either full-length p75 or membrane-tethered C-terminal fragments. The p75-ICD was not recruited into the TrkAcontaining high molecular weight complex, indicating that ␥-secretase-mediated removal of the p75 TM domain may perturb the interaction with TrkA. Independent of the possible nuclear function, our studies suggest that ␥-secretase-mediated p75 NTR proteolysis plays a role in the formation/disassembly of the p75-TrkA receptor complex by regulating the availability of the p75 TM domain that is required for this interaction.The p75 neurotrophin receptor (p75 NTR

The neurotrophin brain-derived neurotrophic factor (BDNF) has been implicated in regulating CNS myelination. BDNF mutant mice exhibit a hypomyelinating phenotype, and BDNF exerts distinct effects upon oligodendroglial proliferation, differentiation, and myelination in vitro. To investigate the precise influence that BDNF exerts in regulating CNS myelination in vivo, we have generated conditional knock-out mice in which TrkB has been deleted specifically in oligodendrocytes. Deletion of TrkB disrupted normal oligodendrocyte myelination, resulting in a significant reduction in myelin protein expression and myelination of CNS white matter tracts during development. Importantly, conditional knock-out mice exhibited normal numbers of mature oligodendrocytes and normal numbers of myelinated axons; however, myelin thickness was significantly reduced during development. These data indicate that while TrkB expression in oligodendrocytes plays no role in the initial contact with axons, it exerts an important influence in subsequent stages to promote myelin ensheathment. The conditional knock-out mice also exhibited an increased density of oligodendrocyte progenitor cells (OPCs) in CNS white matter tracts. Concordant with these results, in vitro analyses using OPCs subjected to TrkB knockdown also revealed increased OPC proliferation. Our data suggested this effect was dependent upon TrkC and p75 expression. Thus, our data demonstrate that TrkB expression in oligodendroglia exerts a direct effect on oligodendrocytes to promote myelination and an indirect effect upon the OPC population, modifying their proliferative potential.