Brain-derived neurotrophic factor (BDNF) plays vitally important roles in neural development and plasticity in both health and disease. Recent studies using mutant mice to selectively manipulate BDNF signalling in desired cell types, in combination with animal models of demyelinating disease, have demonstrated that BDNF not only potentiates normal central nervous system myelination in development but enhances recovery after myelin injury. However, the precise mechanisms by which BDNF enhances myelination in development and repair are unclear. Here, we review some of the recent progress made in understanding the influence BDNF exerts upon the myelinating process during development and after injury, and discuss the cellular and molecular mechanisms underlying its effects. In doing so, we raise new questions for future research.

Methods to promote myelin regeneration in response to central myelin loss are essential to prevent the progression of clinical disability in demyelinating diseases. The neurotrophin brain-derived neurotrophic factor (BDNF) is known to promote myelination during development via oligodendrocyte expressed TrkB receptors. Here, we use a structural mimetic of BDNF to promote myelin regeneration in a preclinical mouse model of central demyelination. In female mice, we show that selective targeting of TrkB with the BDNF-mimetic enhances remyelination, increasing oligodendrocyte differentiation, the frequency of myelinated axonsnd myelin sheath thickness after a demyelinating insult. Treatment with exogenous BDNF exerted an attenuated effect, increasing myelin sheath thickness only. Further, following conditional deletion of TrkB from pre-myelinating oligodendrocytes, we show the effects of the BDNF-mimetic on oligodendrocyte differentiation and remyelination are lost, indicating these are dependent on oligodendrocyte expression of TrkB. Overall, these studies demonstrate that targeting oligodendrocyte TrkB promotes in vivo remyelination in the brain. SIGNIFICANCE STATEMENT Novel strategies to promote myelin regeneration are required to prevent progressive neurodegeneration and clinical disability in patients with central demyelinating disease. Here, we test if selectively targeting the TrkB receptor on the myelin-producing oligodendrocytes, can promote remyelination in the brain. Using a structural mimetic of its native ligand, BDNF, we show that stimulation of TrkB enhances remyelination, increasing oligodendrocyte differentiation, the frequency of myelinated axons and thickness of the myelin sheath following a demyelinating insult. Further, we show that these effects are dependent on the phosphorylation of oligodendrocyte expressed TrkB receptors in vivo. Overall, we demonstrate that selective targeting of TrkB has therapeutic potential to promote remyelination in the brain.

Developing a high-throughput approach to quantify the extent of myelin integrity in preclinical models of demyelinating diseases will enhance our capacity to identify novel therapies for myelin repair. In light of the technical limitations of electron microscopy and immunohistochemical analyses of myelination, we have utilized a novel imaging technique, spectral confocal reflectance (SCoRe) microscopy. SCoRe takes advantage of the optically reflective properties of compact myelin, allowing the integrity of compact myelin to be quantified over the course of the cuprizone-induced model of central demyelination. We applied SCoRe imaging on fixed frozen brain sections. SCoRe analysis of control mice identified an increase in corpus callosum myelination during the period of cuprizone administration and recovery, suggesting that the normal developmental processes of myelination are ongoing at this time. Importantly, analysis of mice subjected to cuprizone identified a significant reduction in compact myelin in both rostral and caudal corpus callosum compared to age-matched control mice. SCoRe microscopy also allowed the visualization and quantification of the amount of myelin debris in demyelinating lesions. Combining SCoRe imaging with immunohistochemistry, we quantified the amount of myelin debris within IBA-1+ microglia and found that 11% of myelin debris colocalized in microglia irrespective of the callosal regions, with the vast majority of debris outside of microglia. In summary, we have demonstrated that SCoRe microscopy is an effective and powerful tool to perform both quantitative and qualitative analyses of compact myelin integrity in health or after injury in vivo, demonstrating its future application in high-throughput assessments and screening of the therapeutic efficacy of myelin repair therapies in preclinical animal models of demyelinating diseases.

The sphingolipids galactosylceramide (GalCer), sulfatide (ST) and sphingomyelin (SM) are essential for myelin stability and function. GalCer and ST are synthesized mostly from C22‐C24 ceramides, generated by Ceramide Synthase 2 (CerS2). To clarify the requirement for C22‐C24 sphingolipid synthesis in myelin biosynthesis and stability, we generated mice lacking CerS2 specifically in myelinating cells (CerS2^ΔO/ΔO). At 6 weeks of age, normal‐appearing myelin had formed in CerS2^ΔO/ΔO mice, however there was a reduction in myelin thickness and the percentage of myelinated axons. Pronounced loss of C22‐C24 sphingolipids in myelin of CerS2^ΔO/ΔO mice was compensated by greatly increased levels of C18 sphingolipids. A distinct microglial population expressing high levels of activation and phagocytic markers such as CD64, CD11c, MHC class II, and CD68 was apparent at 6 weeks of age in CerS2^ΔO/ΔO mice, and had increased by 10 weeks. Increased staining for denatured myelin basic protein was also apparent in 6‐week‐old CerS2^ΔO/ΔO mice. By 16 weeks, CerS2^ΔO/ΔO mice showed pronounced myelin atrophy, motor deficits, and axon beading, a hallmark of axon stress. 90% of CerS2^ΔO/ΔO mice died between 16 and 26 weeks of age. This study highlights the importance of sphingolipid acyl chain length for the structural integrity of myelin, demonstrating how a modest reduction in lipid chain length causes exposure of a denatured myelin protein epitope and expansion of phagocytic microglia, followed by axon pathology, myelin degeneration, and motor deficits. Understanding the molecular trigger for microglial activation should aid the development of therapeutics for demyelinating and neurodegenerative diseases.

The neurotrophin, brain-derived neurotrophic factor (BDNF) promotes central nervous system (CNS) myelination during development and after injury. This is achieved via activation of oligodendrocyte-expressed tropomyosin-related kinase (Trk) B receptors. However, while administration of BDNF has shown beneficial effects, BDNF itself has a poor pharmacokinetic profile. Here, we compare two TrkB-targeted BDNF-mimetics, the structural-mimetic, tricyclic dimeric peptide-6 (TDP6) and the non-peptide small molecule TrkB agonist LM22A-4 in a cuprizone model of central demyelination in female mice. Both mimetics promoted remyelination, increasing myelin sheath thickness and oligodendrocyte densities after 1-week recovery. Importantly, LM22A-4 exerts these effects in an oligodendroglial TrkB-dependent manner. However, analysis of TrkB signaling by LM22A-4 suggests rather than direct activation of TrkB, LM22A-4 exerts its effects via indirect transactivation of Trk receptors. Overall, these studies support the therapeutic strategy to selectively targeting TrkB activation to promote remyelination in the brain.

Fucosidosis results from lack of α-L-fucosidase activity, with accumulation of fucose-linked substrates in the nervous system and viscera leading to progressive motor and mental deterioration, and death. The naturally occurring dog model of fucosidosis was used to evaluate the neuropathological responses to partial enzyme replacement, and substrate reduction in early disease following treatment with recombinant canine α-L-fucosidase delivered through cerebrospinal fluid. Neuropathology in both treated (n = 3) and untreated fucosidosis-affected (n = 3) animals was evaluated with immunohistochemistry, image analysis, manual quantification and gene expression analysis and compared with unaffected age-matched controls (n = 3) in an extension of our previous biochemical report on the same cohort. Data were analyzed by ANOVA. Quantification demonstrated a consistent trend to reduction in vacuolation, pyramidal neuron loss, astrocytosis, microgliosis, perivascular storage, apoptosis, oligodendrocyte loss, and hypomyelination throughout the central nervous system of enzyme treated animals compared to placebo-treated, age-matched affected controls. Key lesions including lysosomal expansion in neurons of deep cortex, astrocytosis in cerebral cortex and medulla, and increased lysosomal membrane associated protein-1 (LAMP-1) gene expression were ameliorated in treated animals. There was no change in spheroid formation and loss of Purkinje cells, but Purkinje cell vulnerability to apoptosis was reduced with treatment. Despite reduced severity of fucosidosis neuropathology with partial enzyme replacement, more complete and sustained biochemical correction is required to halt neuropathological processes in this large animal model of lysosomal storage disease.