Development of a human pluripotent stem cell-derived in vitro model of myelination

Abstract

Myelination is essential for central nervous system (CNS) formation, health, and function. Its development is an adaptive and regulated process that, when perturbed, leads to disease. However, our understanding of myelin formation in health and disease is limited by the paucity of human models of myelination. I sought to develop a human stem cell-derived in vitro model of myelination that would enable studies of myelin development in the context of pharmacological, physiological, or genetic perturbations. Three-dimensional human induced pluripotent stem cell-derived, spinal cord organoids were generated containing NF-H+ neurons, GFAP+ astrocytes, PDGFRα+ oligodendrocyte progenitor cells, and MBP+ oligodendrocytes. With prolonged culture, myelin formation was evident, demonstrated by (i) thick MBP+ tubular structures co-localising with NF-H+ axons; (ii) organisation of myelinated axon domains, as determined by appropriate clustering of paranodal and nodal proteins; and (iii) compact myelin lamellae visualised by transmission electron microscopy. iPSC ‘myelinoids’ demonstrate temporal development of myelination, with both myelin sheath length and compaction increasing over time. The morphology of individual myelinating oligodendrocytes could also be analysed, and exposure to a pharmacological cytoskeletal modulator potentiated myelin sheath number per cell, as expected. Furthermore, a novel automated pipeline to quantify myelin volume across entire myelinoids was developed, which showed that myelin volume—specific to myelinated axons—increased over time. Automated analysis of oligodendrogenesis and myelin formation was demonstrated as an alternative method for investigating the effects of small molecules or trophic factor signalling on oligodendrogenesis and myelin development. Neuronal activity can regulate myelination by oligodendrocytes in model organisms. However, whether myelination by human oligodendrocytes is responsive to neuronal activity has not previously been investigated. I found that blocking synaptic vesicle release via tetanus toxin (TeNT) impaired myelin sheath generation by individual oligodendrocytes and led to a reduction in total myelin volume across myelinoids.These results demonstrate that human myelinating oligodendrocytes respond to changes in neuronal activity. An advantage of iPSC-based models is the generation of patient-derived cultures that enable aspects of human disease to be modelled in vitro. In order to demonstrate that myelinoids could be used to model disorders of myelin, myelinoids were generated from a patient with a homozygous recessive mutation in the gene for Neurofascin (NFASC). This nonsense mutation is predicted to specifically affect the glial-expressed form of Neurofascin, Nfasc155 (a component of the paranodal axoglial junction assembly expressed by glia), the deficiency of which results in a neurodevelopmental disorder characterised by hypotonia, amimia, and areflexia. Patient-derived myelinoids demonstrated impaired formation of paranodal axoglial junctions, while nodal Neurofascin remained intact, recapitulating the major pathology of this disease. Finally, oligodendrocyte pathology is prevalent in amyotrophic lateral sclerosis (ALS), a rapidly progressing neurodegenerative disorder of both upper and lower motor neurons. However, the mechanism by which oligodendrocytes contribute to disease is not yet understood. TDP-43 is a critical DNA/RNA binding protein involved in RNA metabolism whose cytoplasmic misaccumulation and aggregation in neurons and glia is a pathological hallmark of ALS. Disease causing mutations in the encoding gene, TARDBP, account for 5-10% of familial forms of ALS. To determine whether mutant TDP-43 affects myelin development, myelinoids were generated from both patient-derived and CRISPR/Cas9 gene-corrected iPSCs. I found that myelinoids derived from mutant and gene-corrected iPSCs showed no difference in oligodendrogenesis or myelin formation. Collectively, this work shows that iPSC myelinoids provide a robust platform for investigating human myelin development in both health and disease.