A human cellular model to study peripheral myelination and demyelinating neuropathies

Peripheral myelination is an essential process in the development of the peripheral nervous system. Peripheral myelin permits saltatory action potential propagation, and also supports axons via an intricate system of interactions between Schwann cells and the axolemma. Both hereditary and acquired inflammatory processes can damage peripheral myelin and this is the pathological basis of demyelinating peripheral neuropathies such as Charcot-Marie-Tooth disease type 1, Guillain-Barre syndrome and chronic inflammatory demyelinating polyradiculoneuropathy (CIDP). The study of peripheral myelination and demyelination has relied heavily on animal models as systems that allow for dissecting the molecular mechanisms by which Schwann cells regulate myelin production, and for exploring the interaction between these cells and peripheral axons. Another classical approach to investigate peripheral myelination is based on in vitro coculture systems of rodent dorsal root ganglia neurons and Schwann cells; however, the lack of an analogous human cellular system has limited the study of human peripheral myelination. In this issue of Brain, Clark and co-workers address this obstacle by using cellular reprogramming to create a system to study peripheral myelination of human axons in vitro (Clark et al., 2017). Induced pluripotent stem cells (iPSCs) are generated by forcing expression of pluripotency factors in somatic cells such as fibroblasts, a technique termed cellular reprogramming. Human iPSCs can be differentiated into cells from all three germinal layers, including several neuronal types, and are an invaluable tool for modelling human neurological disorders at the cellular level. IPSCs have been used in the past to generate human motor neurons and to model inherited (Saporta et al., 2015) and acquired (Harschnitz et al., 2016) neuropathies. However, neurons behave quite differently in the absence of myelin. Juxtacrine signalling from Schwann cells helps organize the entire length of myelinated axons into a series of polarized domains centred around nodes of Ranvier, which are necessary for normal saltatory nerve conduction (Salzer, 2003). Axonal calibre, neurofilament phosphorylation and packing density, as well as axonal transport, are all disrupted when axons are ensheathed by abnormal myelin. This has been shown in elegant studies in rodent models (de Waegh et al., 1992; Kirkpatrick and Brady, 1994), such as Trembler (Tr) mice, a naturally occurring dysmyelinating model caused by missense mutations in the peripheral myelin protein 22 (PMP22) gene (Suter et al., 1992). Accordingly, many human disorders cannot be accurately modelled using iPSC-derived neurons owing to the absence of myelin and axonSchwann cell interactions. Human neuronal Schwann cell co-cultures have not been possible to date because of the inability to differentiate myelinating Schwann cells from iPSCs, as will be discussed below. However, in this issue of Brain, Clark et al. (2017) establish the first myelinating co-culture systems using human iPSC-derived sensory neurons and rat Schwann cells, and use them to demonstrate the important role of the neuregulin-ErbB signalling pathway in the myelination of human sensory axons. In addition, they use their system to demonstrate the effects of anti-disialosyl antibodies in myelinated axons, and thereby establish an in vitro system to model acquired demyelinating neuropathies. By successfully establishing co-cultures of human iPSC-derived sensory neurons and rat Schwann cells, Clark et al. showed that two different species have enough molecular signalling homology to allow for efficient Schwann cell-axonal interactions and to initiate alignment, basal lamina formation and myelination. The authors also introduced adaptations to the classical protocol for co-culture assays, including the use of a dedicated myelination medium consisting of a neuronal medium (N2) supplemented with Matrigel and ascorbic acid (Fig. 1). This simple modification yielded cultures with enhanced neuronal health and alignment of Schwann BRAIN 2017: 140; 856–867 | 856