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Investigating the mechanisms controlling the asymmetric division of neocortical progenitors that generate neurones in the mammalian brain is crucial for understanding the abnormalities of cortical development. Partitioning of fate determinants is a key instructive step and components of the apical junctional complex (adherens junctions), including the polarity proteins PAR3 and aPKC as well as adhesion molecules such as N‐cadherin, have been proposed to be candidate determinants. In this study, however, we found no correlation between the partitioning of N‐cadherin and fate determination. Rather, we show that adherens junctions comprise three membrane domains, and that during asymmetrical division these are split such that both daughters retain the adhesive proteins that control cell position, but only one daughter inherits the polarity proteins along with the apical membrane. This provides a molecular explanation as to how both daughters remain anchored to the ventricular surface after mitosis, while adopting different fates.

Peripheral nervous system (PNS) neurons support axon regeneration into adulthood, whereas central nervous system (CNS) neurons lose regenerative ability after development. To better understand this decline whilst aiming to improve regeneration, we focused on phosphoinositide 3‐kinase (PI3K) and its product phosphatidylinositol (3,4,5)‐trisphosphate (PIP 3 ). We demonstrate that adult PNS neurons utilise two catalytic subunits of PI3K for axon regeneration: p110α and p110δ. However, in the CNS, axonal PIP 3 decreases with development at the time when axon transport declines and regenerative competence is lost. Overexpressing p110α in CNS neurons had no effect; however, expression of p110δ restored axonal PIP 3 and increased regenerative axon transport. p110δ expression enhanced CNS regeneration in both rat and human neurons and in transgenic mice, functioning in the same way as the hyperactivating H1047R mutation of p110α. Furthermore, viral delivery of p110δ promoted robust regeneration after optic nerve injury. These findings establish a deficit of axonal PIP 3 as a key reason for intrinsic regeneration failure and demonstrate that native p110δ facilitates axon regeneration by functioning in a hyperactive fashion. Synopsis CNS axons lose the ability to regenerate with maturity, whilst PNS axons do not. This study shows that PIP 3 levels decline in CNS neurons at the time when regenerative ability is lost. CNS overexpression of one isoform of PI3K, p110δ, enhances axonal PIP 3, axon transport, and regenerative ability. p110α and p110δ were found to be required for axon regeneration in adult PNS neurons, however PI3K and PIP 3 declined in CNS neurons as they developed to maturity. p110α or p110δ were overexpressed in mature CNS neurons, but only p110δ restored PIP 3 and regeneration, whilst the activating H1047R mutation was required in p110α to promote regeneration similarly. p110δ mediated regeneration through multiple downstream pathways, including mTOR, pS6, CRMP2, ARF6, and increased axonal transport of integrins and Rab11‐positive endosomes. Transgenic expression of p110δ or hyperactive p110α H1047R stimulated axon regeneration after optic nerve injury and increased RGC survival, whilst viral delivery of p110δ led to further enhanced axon regeneration. Graphical Abstract CNS axons lose the ability to regenerate with maturity, whilst PNS axons do not. This study shows that PIP 3 levels decline in CNS neurons at the time when regenerative ability is lost. CNS overexpression of one isoform of PI3K, p110δ, enhances axonal PIP 3, axon transport, and regenerative ability.

Multiple sclerosis (MS) is characterized by an immune system attack targeting myelin, which is produced by oligodendrocytes (OLs). We performed single-cell transcriptomic analysis of OL lineage cells from the spinal cord of mice induced with experimental autoimmune encephalomyelitis (EAE), which mimics several aspects of MS. We found unique OLs and OL precursor cells (OPCs) in EAE and uncovered several genes specifically alternatively spliced in these cells. Surprisingly, EAE-specific OL lineage populations expressed genes involved in antigen processing and presentation via major histocompatibility complex class I and II (MHC-I and -II), and in immunoprotection, suggesting alternative functions of these cells in a disease context. Importantly, we found that disease-specific oligodendroglia are also present in human MS brains and that a substantial number of genes known to be susceptibility genes for MS, so far mainly associated with immune cells, are expressed in the OL lineage cells. Finally, we demonstrate that OPCs can phagocytose and that MHC-II-expressing OPCs can activate memory and effector CD4-positive T cells. Our results suggest that OLs and OPCs are not passive targets but instead active immunomodulators in MS. The disease-specific OL lineage cells, for which we identify several biomarkers, may represent novel direct targets for immunomodulatory therapeutic approaches in MS. Single-cell RNA-seq of a mouse model of multiple sclerosis uncovers new oligodendrocyte populations and putative disease markers and suggests new mechanisms underlying the pathogenesis of the disease.

Two independent systems impairing hepatocyte proliferation during liver injury cause physiologically significant levels of functional hepatocyte regeneration from biliary cells. Bile duct sends on substitutes for injured liver Liver injury triggers regeneration through proliferation of hepatocytes to compensate for the loss of cells. However, in humans, this process is impaired in chronic liver diseases and it has been difficult to reproduce this impairment in mouse models of liver injury. Stuart Forbes and colleagues have used two systems to impair hepatocyte proliferation, via impairment of β-integrin or overexpression of the cell cycle inhibitor p21. They observe that, following injury under these conditions, epithelial bile duct cells are able to adopt hepatic fate and regenerate up to 25% of functional hepatocytes. After liver injury, regeneration occurs through self-replication of hepatocytes. In severe liver injury, hepatocyte proliferation is impaired—a feature of human chronic liver disease 1 , 2 . It is unclear whether other liver cell types can regenerate hepatocytes 3 , 4 , 5 . Here we use two independent systems to impair hepatocyte proliferation during liver injury to evaluate the contribution of non-hepatocytes to parenchymal regeneration. First, loss of β1-integrin in hepatocytes with liver injury triggered a ductular reaction of cholangiocyte origin, with approximately 25% of hepatocytes being derived from a non-hepatocyte origin. Second, cholangiocytes were lineage traced with concurrent inhibition of hepatocyte proliferation by β1-integrin knockdown or p21 overexpression, resulting in the significant emergence of cholangiocyte-derived hepatocytes. We describe a model of combined liver injury and inhibition of hepatocyte proliferation that causes physiologically significant levels of regeneration of functional hepatocytes from biliary cells.

Key Points CNS remyelination is the regenerative process by which myelin sheaths are restored to demyelinated axons. Unlike the poor regeneration that occurs following neuronal injury, remyelination can occur as a spontaneous and efficient process in experimental models and many clinical conditions. Although remyelination can occur in multiple sclerosis (MS; a widely occurring demyelinating disease), it often fails, leaving axons demyelinated and vulnerable to degeneration. Recent studies have revealed the importance of the myelin sheath for maintaining axonal integrity and hence the importance of promoting remyelination in diseases such as MS as an effective means of neuroprotection. Remyelination is mediated by a population of adult neural stem cells that are widely distributed throughout the CNS and that are commonly referred to as oligodendrocyte precursor cells (OPCs). These cells respond to demyelination by activation, proliferation, migration and finally differentiation into remyelinating oligodendrocytes; it is the last of these processes that is most likely to fail in MS and leave areas of demyelination containing oligodendrocyte-lineage cells that are unable to fully differentiate. Remyelination is governed by a complex interaction of environmental signals and cell-intrinsic mechanisms that are triggered by the inflammatory response to injury. This response therefore has a key role in initiating remyelination. The network of signals that is involved in remyelination shows high levels of redundancy. In theory, remyelination can be enhanced either by promoting endogenous remyelination or by transplanting myelinating cells. The first approach, which may have a pharmacological basis, is especially attractive for diseases such as MS in which remyelination occurs and will involve either the antagonism of negative regulatory pathways and/or the enhancement of positive regulatory pathways. Cell therapy (transplantation) approaches to remyelination are expected to be of particular benefit for genetic demyelinating diseases in which there is an inherent defect in the oligodendrocyte lineage. Recent studies have provided proof-of-principle that human cells can be used to remyelinate the entire CNS in laboratory animal models. In the CNS, remyelination of denuded axons occurs to reinstate neuronal function. Franklin and ffrench-Constant consider the cells and molecular signals that are required for remyelination and how this knowledge can be channelled towards more effective therapies for demyelinating diseases. Remyelination involves reinvesting demyelinated axons with new myelin sheaths. In stark contrast to the situation that follows loss of neurons or axonal damage, remyelination in the CNS can be a highly effective regenerative process. It is mediated by a population of precursor cells called oligodendrocyte precursor cells (OPCs), which are widely distributed throughout the adult CNS. However, despite its efficiency in experimental models and in some clinical diseases, remyelination is often inadequate in demyelinating diseases such as multiple sclerosis (MS), the most common demyelinating disease and a cause of neurological disability in young adults. The failure of remyelination has profound consequences for the health of axons, the progressive and irreversible loss of which accounts for the progressive nature of these diseases. The mechanisms of remyelination therefore provide critical clues for regeneration biologists that help them to determine why remyelination fails in MS and in other demyelinating diseases and how it might be enhanced therapeutically.

Experience and changes in neuronal activity can alter CNS myelination, but the signalling pathways responsible remain poorly understood. Here we define a pathway in which endothelin, signalling through the G protein-coupled receptor endothelin receptor B and PKC epsilon, regulates the number of myelin sheaths formed by individual oligodendrocytes in mouse and zebrafish. We show that this phenotype is also observed in the prefrontal cortex of mice following social isolation, and is associated with reduced expression of vascular endothelin. Additionally, we show that increasing endothelin signalling rescues this myelination defect caused by social isolation. Together, these results indicate that the vasculature responds to changes in neuronal activity associated with experience by regulating endothelin levels, which in turn affect the myelinating capacity of oligodendrocytes. This pathway may be employed to couple the metabolic support function of myelin to activity-dependent demand and also represents a novel mechanism for adaptive myelination.

Retinitis pigmentosa, which affects one in 3000 people, causes blindness and has no treatment. Mutations in the retinitis pigmentosa GTPase regulator (RPGR) gene cause 20% of all cases. Recent work suggests that RPGR, localised to the photoreceptor connecting cilium, regulates rhodopsin transport to the outer segment through its effect on the turnover of actin. We set out to establish a novel model for RPGR disease to test the hypothesis that RPGR mutations lead to retinal degeneration due to a dysregulation of the actin cytoskeleton. Patients with RPGR mutations and their unaffected relatives were recruited and skin biopsy samples taken. Fibroblast lines were established and reprogrammed to generate induced pluripotent stem cell (iPSC) lines. A three-dimensional organogenesis protocol was optimised whereby embryoid bodies were formed and patterned towards an eye field fate in a 100-day retinal differentiation protocol, allowing three-dimensional optic cups to form. RPGR-mutated cultures were compared with their healthy controls. Mutant and wild-type iPSC lines were generated and characterised. Differentiation of all lines resulted in the generation of optic cups in a self-organising manner after 100 days in culture. These cultures contained mature photoreceptors, as evidenced by morphology and both RNA and protein expression. Photoreceptor cultures from RGPR-mutated iPSCs had increased actin polymerisation compared with controls (mean confocal pixel intensity count 59·02 [SD 16·24] vs 23·70 [8·20], p=0·0081). This finding was confirmed by assessment of F-actin with western blot. Pathways regulating actin turnover were explored; western blot analysis showed a reduction in both Src and ERK phosphorylation in RGPR-mutated photoreceptor cultures. An unbiased protein array confirmed this reduction in ERK and Src activation. Several other pathways were also shown to be dysregulated in the RGPR-mutated photoreceptor cultures. This study supports the hypothesis that RPGR mutations lead to actin dysregulation. We have identified several pathways that are interrupted in RPGR-mutant photoreceptor cultures and could be contributing to disease. This study is the first use, to our knowledge, of human iPSCs with retinitis pigmentosa-causing mutations to look at pathophysiology of disease. Wellcome Trust.

The authors report a transcriptional profile of the discrete stages of spontaneous remyelination following toxin-induced focal demyelination in rats. They find an enrichment of retinoid X receptor (RXR) signaling pathways over the course of remyelination and show that RXR activation stimulates oligodendrocyte differentiation to enhance remyelination. The molecular basis of CNS myelin regeneration (remyelination) is poorly understood. We generated a comprehensive transcriptional profile of the separate stages of spontaneous remyelination that follow focal demyelination in the rat CNS and found that transcripts that encode the retinoid acid receptor RXR-γ were differentially expressed during remyelination. Cells of the oligodendrocyte lineage expressed RXR-γ in rat tissues that were undergoing remyelination and in active and remyelinated multiple sclerosis lesions. Knockdown of RXR-γ by RNA interference or RXR-specific antagonists severely inhibited oligodendrocyte differentiation in culture. In mice that lacked RXR-γ, adult oligodendrocyte precursor cells efficiently repopulated lesions after demyelination, but showed delayed differentiation into mature oligodendrocytes. Administration of the RXR agonist 9- cis -retinoic acid to demyelinated cerebellar slice cultures and to aged rats after demyelination caused an increase in remyelinated axons. Our results indicate that RXR-γ is a positive regulator of endogenous oligodendrocyte precursor cell differentiation and remyelination and might be a pharmacological target for regenerative therapy in the CNS.

Myelination is essential for rapid impulse conduction in the CNS, but what determines whether an individual axon becomes myelinated remains unknown. Here we show, using a myelinating coculture system, that there are two distinct modes of myelination, one that is independent of neuronal activity and glutamate release and another that depends on neuronal action potentials releasing glutamate to activate NMDA receptors on oligodendrocyte lineage cells. Neuregulin switches oligodendrocytes from the activity-independent to the activity-dependent mode of myelination by increasing NMDA receptor currents in oligodendrocyte lineage cells 6-fold. With neuregulin present myelination is accelerated and increased, and NMDA receptor block reduces myelination to far below its level without neuregulin. Thus, a neuregulin-controlled switch enhances the myelination of active axons. In vivo, we demonstrate that remyelination after white matter damage is NMDA receptor-dependent. These data resolve controversies over the signalling regulating myelination and suggest novel roles for neuregulin in schizophrenia and in remyelination after white matter damage.

In recent years, the central roles of neuronal and axonal damage, as well as axon–glial and axon–myelin interactions, in the pathogenesis and progression of multiple sclerosis (MS) have become increasingly apparent. Franklin et al . review advances in our understanding of the molecular mechanisms underlying these MS-related events, and discuss approaches towards axonal neuroprotection and repair, particularly through the regenerative process remyelination. Multiple sclerosis (MS) is an inflammatory demyelinating disease that is considered by many people to have an autoimmune aetiology. In recent years, new data emerging from histopathology, imaging and other studies have expanded our understanding of the disease and may change the way in which it is treated. Conceptual shifts have included: first, an appreciation of the extent to which the neuron and its axon are affected in MS, and second, elucidation of how the neurobiology of axon–glial and, particularly, axon–myelin interaction may influence disease progression. In this article, we review advances in both areas, focusing on the molecular mechanisms underlying axonal loss in acute inflammation and in chronic demyelination, and discussing how the restoration of myelin sheaths via the regenerative process of remyelination might prevent axon degeneration. An understanding of these processes could lead to better strategies for the prevention and treatment of axonal loss, which will ultimately benefit patients with MS. Key Points Axonal damage—now a recognized pathological feature of multiple sclerosis—is most severe in new inflammatory demyelinating lesions, and occurs at a slower rate during progressive disease Multiple mechanisms contribute to acute axonal injury in new inflammatory lesions, including oxidative damage, energy deprivation and sodium accumulation, all of which are amenable to therapy In chronic lesions, loss of the trophic support that is normally provided by intact myelinating oligodendrocytes may contribute to axonal damage Remyelination represents a powerful means of preventing axonal damage attributable to loss of myelin trophic support In recent years we have witnessed the emergence of an increasing number of therapeutic targets to enhance remyelination by endogenous progenitor cells, thereby reducing chronic axonal loss