MicroRNA mediated repression does work in a binary fashion, which explains why GFAP+ glial cells in homeostatic conditions do in fact express a low basal level of c-Jun. extracellular matrix remodeling and axon guidance. This work identifies a unique role for miR-200a in inhibiting reactive gliosis in axolotl glial cells during spinal cord regeneration. Introduction Salamanders have retained the remarkable ability to functionally regenerate after spinal cord injury (SCI)1C9. In response ZL0420 to SCI, glial fibrillary acidic protein (GFAP)+ glial cells proliferate and migrate through the lesion to create a permissive environment for axon regeneration9C12. This is in stark contrast to the mammalian response to SCI where ZL0420 damaged astrocytes undergo reactive gliosis and contribute to the glial scar by secreting axon growth inhibitory proteins like chondroitin sulfate proteoglycans (CSPGs) and collagens13C16. The glial scar is usually a complex subject, it has been shown to be beneficial by preventing more damage to the spinal cord but it also expresses proteins that are inhibitory to axon regeneration16. Many different vertebrate animals, in addition to salamanders; have the ability to regenerate a functional spinal cord after injury, including lamprey, xenopus and zebrafish. Common to all these animals is usually that regeneration occurs in the absence of reactive gliosis and glial scar formation10C12,17. The molecular pathways that promote functional spinal cord regeneration without glial scar formation are poorly understood. Recent advances in molecular genetics and transcriptional profiling techniques are beginning to elucidate the molecular and cellular responses necessary for functional spinal cord regeneration. Lampreys, which represent the most basal vertebrate ancestor that diverged from a shared common ancestor to humans more than 560 million years ago, can regenerate locomotive function within 12 weeks of a full spinal cord transection. After SCI in lamprey resident GFAP+ astrocytes elongate and form a glial bridge that facilitates axons to regenerate through the lesion18C26. This is reminiscent of the injury-induced glial bridge formed by GFAP+ glial cells in zebrafish spinal cord, which is usually similarly necessary for axon regeneration27,28. Xenopus display robust functional spinal cord regeneration in the larval stages by activating the GFAP+/Sox2+ glial cells to divide, migrate, and repair the lesion which allows axons to regenerate. However the tadpoles ability to regenerate is usually lost after metamorphoses into an adult frog29C41. Similar events occur in axolotl, GFAP?+?/Sox2?+?cells adjacent to the injury site are activated in response to injury and will migrate to repair the lesion, however axolotls can regenerate throughout life4,7C10,42. In axolotls an injury to the spinal cord is usually fully repaired, rostral and caudal sides of the spinal cord reconnect but there is no glial bridge structure formed as is seen in zebrafish43. A common theme in these species is the absence of reactive gliosis and ZL0420 the lack of a glial scar. To facilitate functional recovery these remarkable animals activate glial cells to regenerate the ependymal tube or form a glial bridge both of which act as a highway to guide axon regeneration through the lesion site. In contrast mammalian glial cells; often referred to as astrocytes; undergo a process of reactive gliosis in response to injury. Historically, reactive astrocytes were characterized as highly proliferative, hypertrophic cells that express high levels of GFAP. Advances in lineage tracing and transcriptomic profiling approaches have revealed a much higher degree of heterogeneity among reactive astrocytes44,45. Recent publications suggest that reactive astrocytes and components of glial scar are beneficial for mitigating the inflammatory Ras-GRF2 response, resulting in less neuronal death early after injury46C48. However, the chronic persistence of the glial scar remains a major barrier to axon regeneration. Despite the high degree of heterogeneity across reactive astrocytes, several injury models have identified a critical role for the transcriptional complex AP-1 in promoting reactive gliosis by activating the GFAP promoter and other downstream pathways leading to glial scar formation49C54. AP-1 is commonly formed as a heterodimeric complex of FOS and JUN proteins capable of regulating the expression.

MicroRNA mediated repression does work in a binary fashion, which explains why GFAP+ glial cells in homeostatic conditions do in fact express a low basal level of c-Jun