Summary Transected axons fail to regrow in the mature central nervous system (CNS). Astrocyte scars are widely regarded as causal in this failure. Here, using three genetically targeted loss-of-function manipulations in adult mice, we show that preventing astrocyte scar formation, attenuating scar-forming astrocytes, or deleting chronic astrocyte scars all failed to result in spontaneous regrowth of transected corticospinal, sensory or serotonergic axons through severe spinal cord injury (SCI) lesions. In striking contrast, sustained local delivery via hydrogel depots of required axon-specific growth factors not present in SCI lesions, plus growth-activating priming injuries, stimulated robust, laminin-dependent sensory axon regrowth past scar-forming astrocytes and inhibitory molecules in SCI lesions. Preventing astrocyte scar formation significantly reduced this stimulated axon regrowth. RNA sequencing revealed that astrocytes and non-astrocyte cells in SCI lesions express multiple axon-growth supporting molecules. Our findings show that contrary to prevailing dogma, astrocyte scar formation aids rather than prevents CNS axon regeneration.

Signaling mechanisms that regulate astrocyte reactivity and scar formation after spinal cord injury (SCI) are not well defined. We used the Cre recombinase (Cre)-loxP system under regulation of the mouse glial fibrillary acidic protein (GFAP) promoter to conditionally delete the cytokine and growth factor signal transducer, signal transducer and activator of transcription 3 (STAT3), from astrocytes. After SCI in GFAP-Cre reporter mice, Ͼ99% of spinal cord cells that exhibited Cre activity as detected by reporter protein expression were GFAP-expressing astrocytes. Conditional deletion (or knock-out) of STAT3 (STAT3-CKO) from astrocytes in GFAP-Cre-loxP mice was confirmed in vivo and in vitro. In uninjured adult STAT3-CKO mice, astrocytes appeared morphologically similar to those in STAT3ϩ/ϩ mice except for a partially reduced expression of GFAP. In STAT3ϩ/ϩ mice, phosphorylated STAT3 (pSTAT3) was not detectable in astrocytes in uninjured spinal cord, and pSTAT3 was markedly upregulated after SCI in astrocytes and other cell types near the injury. Mice with STAT3-CKO from astrocytes exhibited attenuated upregulation of GFAP, failure of astrocyte hypertrophy, and pronounced disruption of astroglial scar formation after SCI. These changes were associated with increased spread of inflammation, increased lesion volume and partially attenuated motor recovery over the first 28 d after SCI. These findings indicate that STAT3 signaling is a critical regulator of certain aspects of reactive astrogliosis and provide additional evidence that scar-forming astrocytes restrict the spread of inflammatory cells after SCI.

Astroglial scars surround damaged tissue after trauma, stroke, infection, or autoimmune inflammation in the CNS. They are essential for wound repair, but also interfere with axonal regrowth. A better understanding of the cellular mechanisms, regulation, and functions of astroglial scar formation is fundamental to developing safe interventions for many CNS disorders. We used wild-type and transgenic mice to quantify and dissect these parameters. Adjacent to crush spinal cord injury (SCI), reactive astrocytes exhibited heterogeneous phenotypes as regards proliferation, morphology, and chemistry, which all varied with distance from lesions. Mature scar borders at 14 d after SCI consisted primarily of newly proliferated astroglia with elongated cell processes that surrounded large and small clusters of inflammatory, fibrotic, and other cells. During scar formation from 5 to 14 d after SCI, cell processes deriving from different astroglia associated into overlapping bundles that quantifiably reoriented and organized into dense mesh-like arrangements. Selective deletion of STAT3 from astroglia quantifiably disrupted the organization of elongated astroglia into scar borders, and caused a failure of astroglia to surround inflammatory cells, resulting in increased spread of these cells and neuronal loss. In cocultures, wild-type astroglia spontaneously corralled inflammatory or fibromeningeal cells into segregated clusters, whereas STAT3-deficient astroglia failed to do so. These findings demonstrate heterogeneity of reactive astroglia and show that scar borders are formed by newly proliferated, elongated astroglia, which organize via STAT3-dependent mechanisms to corral inflammatory and fibrotic cells into discrete areas separated from adjacent tissue that contains viable neurons.

After complete spinal cord transections that removed all supraspinal inputs in adult rats, combinations of serotonergic agonists and epidural electrical stimulation were able to acutely transform spinal networks from nonfunctional to highly functional and adaptive states as early as 1 week after injury. Using kinematics, physiological and anatomical analyses, we found that these interventions could recruit specific populations of spinal circuits, refine their control via sensory input and functionally remodel these locomotor pathways when combined with training. The emergence of these new functional states enabled full weight-bearing treadmill locomotion in paralyzed rats that was almost indistinguishable from voluntary stepping. We propose that, in the absence of supraspinal input, spinal locomotion can emerge from a combination of central pattern-generating capability and the ability of these spinal circuits to use sensory afferent input to control stepping. These findings provide a strategy by which individuals with spinal cord injuries could regain substantial levels of motor control.Severe spinal cord injuries that remove all supraspinal input to lumbosacral spinal circuits lead to permanent paralysis of the legs in adult rodents 1-3 and humans. Nevertheless, networks of neurons in the lumbosacral spinal cord retain an intrinsic capability to oscillate and generate coordinated rhythmic motor outputs. Circuits underlying such rhythmic and oscillatory outputs are commonly referred to as central pattern generators (CPGs) and are found in all invertebrate and vertebrate animals 4,5 . Although the anatomical architecture of locomotor CPGs remains poorly understood, especially in mammals 5 , the functional phenomenon, central pattern generation, has been documented extensively. Indirect evidence suggests that CPGs are present in human spinal cord 6,7 . These observations offer the possibility of directly accessing and activating spinal cord CPGs to facilitate locomotor recovery after a severe spinal cord injury (SCI).Correspondence should be addressed to G.C. (gregoire.courtine@bli.uzh.ch). Note: Supplementary information is available on the Nature Neuroscience website.Author Contributions: G.C., Y.P.G., I.L., M.V.S. and V.R.E. designed the study. G.C., R.R.R., H.Z. and P.M. performed the surgeries. G.C., Y.P.G., R.v.d.B., P.M. and A.Y. carried out the experiments. G.C., A.Y., B.S., Y.A., R.I. and M.V.S. conducted the anatomical assessments. G.C. analyzed the data. G.C., R.R.R., M.V.S. and V.R.E. wrote the manuscript. G.C. supervised the study. 16,20,21 that lead to specific improvements of stepping patterns. These interventions, however, have shown limited potential for promoting weight-bearing capacities and there have been few attempts to correlate the specific functional states induced pharmacologically 9,10 , electrically [11][12][13]19,22 or by locomotor training 2,20 with distinct characteristics of stepping motor patterns. When studied in sufficient statistical detail, analyses of kinematics and...

Spinal cord injuries (SCIs) in humans and experimental animals are often associated with varying degrees of spontaneous functional recovery during the first months after injury. Such recovery is widely attributed to axons spared from injury that descend from the brain and bypass incomplete lesions, but its mechanisms are uncertain. To investigate the neural basis of spontaneous recovery, we used kinematic, physiological and anatomical analyses to evaluate mice with various combinations of spatially and temporally separated lateral hemisections with or without the excitotoxic ablation of intrinsic spinal cord neurons. We show that propriospinal relay connections that bypass one or more injury sites are able to mediate spontaneous functional recovery and supraspinal control of stepping, even when there has been essentially total and irreversible interruption of long descending supraspinal pathways in mice. Our findings show that pronounced functional recovery can occur after severe SCI without the maintenance or regeneration of direct projections from the brain past the lesion and can be mediated by the reorganization of descending and propriospinal connections. Targeting interventions toward augmenting the remodeling of relay connections may provide new therapeutic strategies to bypass lesions and restore function after SCI and in other conditions such as stroke and multiple sclerosis.