Chondroitinase ABC combined with Schwann cell transplantation enhances restoration of neural connection and functional recovery following acute and chronic spinal cord injury

Schwann cell transplantation is considered one of the most promising cell-based therapy to repair injured spinal cord due to its unique growth-promoting and myelin-forming properties.A the Food and Drug Administration-approved Phase I clinical trial has been conducted to evaluate the safety of transplanted human autologous Schwann cells to treat patients with spinal cord injury.A major challenge for Schwann cell transplantation is that grafted Schwann cells are confined within the lesion cavity,and they do not migrate into the host environment due to the inhibitory barrier formed by injury-induced glial scar,thus limiting axonal reentry into the host spinal cord.Here we introduce a combinatorial strategy by suppressing the inhibitory extracellular environment with injection of lentivirus-mediated transfection of chondroitinase ABC gene at the rostral and caudal borders of the lesion site and simultaneously leveraging the repair capacity of transplanted Schwann cells in adult rats following a mid-thoracic contusive spinal cord injury.We report that when the glial scar was degraded by chondroitinase ABC at the rostral and caudal lesion borders,Schwann cells migrated for considerable distances in both rostral and caudal directions.Such Schwann cell migration led to enhanced axonal regrowth,including the serotonergic and dopaminergic axons originating from supraspinal regions,and promoted recovery of locomotor and urinary bladder functions.Importantly,the Schwann cell survival and axonal regrowth persisted up to 6 months after the injury,even when treatment was delayed for 3 months to mimic chronic spinal cord injury.These findings collectively show promising evidence for a combinatorial strategy with chondroitinase ABC and Schwann cells in promoting remodeling and recovery of function following spinal cord injury.

Lin28 as a therapeutic target for central nervous system regeneration and repair

Axon disconnection in the central nervous system(CNS) usually causes signal transduction failure and severe functional deficits in patients with neurological disorders. Currently, there is no cure for patients with CNS axon injury and they usually suffer from life-long neurological defects(e.g., paralysis, loss of sensory function, and autonomic dysfunction) and life-threatening complications(e.g., autonomic dysreflexia).

PTEN inhibition and axon regeneration and neural repair

The intrinsic growth ability of all the neurons declines during development although some may grow better than others. Numerous intracellular signaling proteins and transcription factors have been shown to regulate the intrinsic growth capacity in mature neurons. Among them, PI3 kinase/Akt pathway is important for controlling axon elongation. As a negative regulator of this pathway, the tumor suppressor phosphatase and tensin homolog （PTEN） appears critical to con- trol the regenerative ability of young and adult neurons. This review will focus on recent research progress in axon regeneration and neural repair by PTEN inhibition and therapeutic potential of blocking this phosphatase for neurological disorders. Inhibition of PTEN by deletion in con- ditional knockout mice, knockdown by short-hairpin RNA, or blockade by pharmacological approaches, including administration of selective PTEN antagonist peptides, stimulates various degrees of axon regrowth in juvenile or adult rodents with central nervous system injuries. Im- portantly, post-injury PTEN suppression could enhance axonal growth and functional recovery in adult central nervous system after injury.

Environmental cues determine the fate of astrocytes after spinal cord injury

Reactive astrogliosis occurs after central nervous system（CNS） injuries whereby resident astrocytes form rapid responses along a graded continuum. Following CNS lesions, na？ve astrocytes are converted into reactive astrocytes and eventually into scar-forming astrocytes that block axon regeneration and neural repair. It has been known for decades that scarring development and its related extracellular matrix molecules interfere with regeneration of injured axons after CNS injury, but the cellular and molecular mechanisms for controlling astrocytic scar formation and maintenance are not well known. Recent use of various genetic tools has made tremendous progress in better understanding genesis of reactive astrogliosis. Especially, the latest experiments demonstrate environment-dependent plasticity of reactive astrogliosis because reactive astrocytes isolated from injured spinal cord form scarring astrocytes when transplanted into injured spinal cord, but revert in retrograde to naive astrocytes when transplanted into naive spinal cord. The interactions between upregulated type I collagen and its receptor integrin β1 and the N-cadherin-mediated cell adhesion appear to play major roles for local astrogliosis around the lesion. This review centers on the environment-dependent plasticity of reactive astrogliosis after spinal cord injury and its potential as a therapeutic target.

Role of axon resealing in retrograde neuronal death and regeneration after spinal cord injury

Spinal cord injury leads to persistent behavioral deficits because mammalian central nervous system axons fail to regenerate. A neuron's response to axon injury results from a complex interplay of neuron-intrinsic and environmental factors. The contribution of axotomy to the death of neurons in spinal cord injury is controversial because very remote axotomy is unlikely to result in neuronal death, whereas death of neurons near an injury may reflect environmental factors such as ischemia and inflammation. In lampreys, axotomy due to spinal cord injury results in delayed apoptosis of spinal-projecting neurons in the brain, beyond the extent of these environmental factors. This retrograde apoptosis correlates with delayed resealing of the axon, and can be reversed by inducing rapid membrane resealing with polyethylene glycol. Studies in mammals also suggest that polyethylene glycol may be neuroprotective, although the mechanism(s) remain unclear. This review examines the early, mechanical, responses to axon injury in both mammals and lampreys, and the potential of polyethylene glycol to reduce injury-induced pathology. Identifying the mechanisms underlying a neuron's response to axotomy will potentially reveal new therapeutic targets to enhance regeneration and functional recovery in humans with spinal cord injury.

Oligodendrocyte pathology in fetal alcohol spectrum disorders

The pathology of fetal alcohol syndrome and the less severe fetal alcohol spectrum disorders includes brain dysmyelination.Recent studies have shed light on the molecular mechanisms underlying these white matter abnormalities.Rodent models of fetal alcohol syndrome and human studies have shown suppressed oligodendrocyte differentiation and apoptosis of oligodendrocyte precursor cells.Ethanol exposure led to reduced expression of myelin basic protein and delayed myelin basic protein expression in rat and mouse models of fetal alcohol syndrome and in human histopathological specimens.Several studies have reported increased expression of many chemokines in dysmyelinating disorders in central nervous system,including multiple sclerosis and fetal alcohol syndrome.Acute ethanol exposure reduced levels of the neuroprotective insulin-like growth factor-1 in fetal and maternal sheep and in human fetal brain tissues,while ethanol increased the expression of tumor necrosis factor α in mouse and human neurons.White matter lesions have been induced in the developing sheep brain by alcohol exposure in early gestation.Rat fetal alcohol syndrome models have shown reduced axon diameters,with thinner myelin sheaths,as well as reduced numbers of oligodendrocytes,which were also morphologically aberrant oligodendrocytes.Expressions of markers for mature myelination,including myelin basic protein,also were reduced.The accumulating knowledge concerning the mechanisms of ethanol-induced dysmyelination could lead to the development of strategies to prevent dysmyelination in children exposed to ethanol during fetal development.Future studies using fetal oligodendrocyte-and oligodendrocyte precursor cell-derived exosomes isolated from the mother's blood may identify biomarkers for fetal alcohol syndrome and even implicate epigenetic changes in early development that affect oligodendrocyte precursor cell and oligodendrocyte function in adulthood.By combining various imaging modalities with molecular studies,it may be possible to determine which fetuses are at risk and to intervene therapeutically early in the pregnancy.

Synapsing with NG2 cells(polydendrocytes),unappreciated barrier to axon regeneration?

Have you heard of NG2 cells or NG2 glia or polydendro- cytes~. These are new names for the precursor cells that used to be referred to as oligodendrocyte precursor cells （OPCs）, which become the oligodendrocytes that myelinate central nervous system （CNS） axons. Evidence suggests, however, that they have other functions, besides differentiating into oligodendrocytes. Most notably, the OPCs/NG2 cells are uni- formly distributed in grey matter as well as in white matter, which matches poorly with the distribution of myelinating oligodendrocytes. Furthermore, not every NG2 cell is fated to become an oligodendrocyte. Hence the term OPC can be fairly applied only when discussing the role of these cells in the oligodendrocyte lineage.

Heterogeneity in the regenerative abilities of central nervous system axons within species: why do some neurons regenerate better than others?

Some neurons,especially in mammalian peripheral nervous system or in lower vertebrate or in vertebrate central nervous system(CNS)regenerate after axotomy,while most mammalian CNS neurons fail to regenerate.There is an emerging consensus that neurons have different intrinsic regenerative capabilities,which theoretically could be manipulated therapeutically to improve regeneration.Population-based comparisons between"good regenerating"and"bad regenerating"neurons in the CNS and peripheral nervous system of most vertebrates yield results that are inconclusive or difficult to interpret.At least in part,this reflects the great diversity of cells in the mammalian CNS.Using mammalian nervous system imposes several methodical limitations.First,the small sizes and large numbers of neurons in the CNS make it very difficult to distinguish regenerating neurons from non-regenerating ones.Second,the lack of identifiable neurons makes it impossible to correlate biochemical changes in a neuron with axonal damage of the same neuron,and therefore,to dissect the molecular mechanisms of regeneration on the level of single neurons.This review will survey the reported responses to axon injury and the determinants of axon regeneration,emphasizing non-mammalian model organisms,which are often under-utilized,but in which the data are especially easy to interpret.

Axon injury induced endoplasmic reticulum stress and neurodegeneration

Injury to central nervous system axons is a common early characteristic of neurodegenerative diseases. Depending on its location and the type of neuron, axon injury often leads to axon degeneration, retrograde neuronal cell death and progressive permanent loss of vital neuronal functions. Although these sequential events are clearly connected, ample evidence indicates that neuronal soma and axon degenerations are active autonomous processes with distinct molecular mechanisms. By exploiting the anatomical and techni- cal advantages of the retinal ganglion cell （RGC）/optic nerve （ON） system, we demonstrated that inhibition of the PERK-eIF2a-CHOP pathway and activation of the X-box binding protein 1 pathway synergistically protect RGC soma and axon, and preserve visual function, in both acute ON traumatic injury and chronic glaucomatous neuropathy. The autonomous endoplasmic reticulum （ER） stress pathway in neurons has been implicated in several other neurodegenerative diseases. In addition to the emerging role of ER mor- phology in axon maintenance, we propose that ER stress is a common upstream signal for disturbances in axon integrity, and that it leads to a retrograde signal that can subsequently induce neuronal soma death. Therefore manipulation of the ER stress pathway may be a key step toward developing the effective neuro- protectants that are greatly needed in the clinic.

The necessary role of mTORC1 in central nervous system axon regeneration

Permanent loss of vital functions after central nervous system （CNS） injury, e.g., blindness in traumatic optic nerve （ON） injury or paralysis in spinal cord injury, occurs in part because axons in the adult mammalian CNS do not regenerate after injury. Growth failure is due to the diminished intrinsic regenerative capacity of mature neurons and the inhibitory environment of the adult CNS. Neutralizing extracellular inhibitory molecules genetically or pharmacologically yields only limited regeneration and functional recovery, highlighting the critical importance of neuron-intrinsic factors.

Inhibition of central axon regeneration: perspective from chondroitin sulfate proteoglycans in lamprey spinal cord injury

Background:Failure of axon regeneration after spinal cord injury(SCI)underlies the paralysis that so profoundly affects patients’quality of life.Many factors are involved in the regeneration failure.Chondroitin sulfate proteoglycans(CSPGs),normal constituents of the perineuronal nets in central nervous system(CNS),are secreted at the injury site and initially were thought to act as a purely physical barrier.In the past decade,the receptor-like protein tyrosine phosphatases,protein tyrosine phosphatase sigma(PTPσ),and leukocyte common antigen-related phosphatase(LAR),have been identified as transmembrane receptors for CSPGs.The two receptors for myelin-associated growth inhibitors,Nogo receptors 1 and 3(NgR1 and NgR3)also have been found to bind with CSPGs(Sharma et al.,2012).These findings suggest that CSPGs inhibit regeneration by interacting with these receptors,initiating downstream inhibitory signaling(Figure 1).

Reactive astrocyte scar and axon regeneration:suppressor or facilitator?

Several major factors are known to contribute to CNS axon regenerative failure after injury, including reduced intrinsic growth capacity of developed neurons and extrinsic factors mediating axon outgrowth. For the latter, a non-permissive environment around the lesion and the lack of sufficient neurotrophic support within the adult CNS play important roles （Silver et al., 2015）. In addition to generation of various inhibitory substrates by oligodendrocytes, fibrotic tissues, inflammatory cells and other cell types, reactive astrocytes surrounding lesions are thought to highly suppress regeneration of injured CNS axons （Silver and Miller, 2004; Ohtake and Li, 2014）. A great number of studies suggest that reactive astrocytic scars form one of the major barriers preventing axon regeneration after CNS iniuries, including spinal cord injury （SCI）. However, reactive astrocytes were reported to provide a beneficial role by reducing infiltrating immunoreactive cells into adjacent domains, protecting bordering neural tissue from damage and generating numerous supportive extracellular matrix （ECM） components to promote cell survival and growth （Bush et al., 1999）. Previ- ous data showed that ablation of reactive astrocytes increased inflammation and secondary tissue damage, prevented blood- brain barrier formation and increased local neurite growth. Interestingly, a recent study by Anderson et al （2016） provides evidence that reactive astrocytes around the lesioned spinal cord support axon regeneration after SCI, rather than block regrowth （Anderson et al., 2016）.

Metabolic reprogramming of glial cells as a new target for central nervous system axon regeneration

After central nervous system(CNS)injury,severed axons fail to regenerate and their disconnections to the original targets result in permanent functional deficits in patients(Mahar and Cavalli,2018).Both the diminished intrinsic regenerative capacity of mature neurons and the inhibitory CNS milieu contribute to the regenerative failure following CNS injury.

The expression of histone deacetylases and the regenerative abilities of spinal-projecting neurons after injury

Epigenetic control of regeneration after spinal cord injury： Com- plete spinal cord injury （SCI） in humans and other mammals leads to irreversible paralysis below the level of injury, due to failure of axonal regeneration in the central nervous system （CNS）. Previous work has shown that successful axon regeneration is dependent upon transcription of a large number of regeneration-associated genes （RAGs） and transcription factors （TFs） （Van Kesteren et al., 2011）. A prominent theory in the field of axon regeneration is that the large differences in regenerative potential between peripheral nervous system （PNS） neurons, which regenerate well, and CNS neurons, which do not, reflect differences in intrinsic transcriptional net- works, rather than individual genes （Van Kesteren et al., 2011）.

Acetylation as a mechanism that regulates axonal regeneration

The capacity for adult axons to regenerate after injury is diminished compared with developing axons.In the case of central nervous system（CNS）axons,injury causes a total failure to regenerate.This failure is due to both the intrinsic developmental decrease in growth capacity and the extrinsic inhibitory environment formed because of the injury.

Coordination of the axonal cytoskeleton during the emergence of axon collateral branches

The formation of branches during development allows a single axon to make synaptic contacts with numerous target neurons,often in different parts of the nervous system,thereby allowing for the establishment of complex patterns of neuronal connectivity.

Localized regulation of the axon shaft during the emergence of collateral branches

The ability of the axon to form de novo collateral branches along its length is fundamental to the establishment of complex patterns of connectivity during development and is also a major response of many axonal populations following injury.The emergence of branches is under both positive and negative control by extracellular signals.