Highlights • Regeneration and sprouting are two forms of injury-induced axonal growth. • Myelin inhibitors modulate axonal sprouting after CNS injury. • Regeneration elicited intrinsically may be ...further modulated by myelin inhibitors. • Promoting sprouting to restore function may be a more attainable near-term goal.
Highlights • Neuron-extrinsic and -intrinsic factors regulate axon plasticity. • Circuit rewiring involves formation of detour circuits or activation of latent tracts. • Rehabilitation may strengthen ...adaptive connections after injury. • Transneuronal tracing and 3D imaging may reveal post-injury network alterations.
Neural stem cells (NSCs) expressing GFP were embedded into fibrin matrices containing growth factor cocktails and grafted to sites of severe spinal cord injury. Grafted cells differentiated into ...multiple cellular phenotypes, including neurons, which extended large numbers of axons over remarkable distances. Extending axons formed abundant synapses with host cells. Axonal growth was partially dependent on mammalian target of rapamycin (mTOR), but not Nogo signaling. Grafted neurons supported formation of electrophysiological relays across sites of complete spinal transection, resulting in functional recovery. Two human stem cell lines (566RSC and HUES7) embedded in growth-factor-containing fibrin exhibited similar growth, and 566RSC cells supported functional recovery. Thus, properties intrinsic to early-stage neurons can overcome the inhibitory milieu of the injured adult spinal cord to mount remarkable axonal growth, resulting in formation of new relay circuits that significantly improve function. These therapeutic properties extend across stem cell sources and species.
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► Neural stem cells grow axons over very long distances after severe spinal cord injury ► New synaptic relays are formed, improving electrophysiological and functional outcome ► Rodent and human neural stem cells exhibit similar growth properties ► Mechanisms intrinsic to early-stage neurons overcome adult nervous system inhibition
Following severe spinal cord injury in rats, functional outcomes can be improved by the engraftment of neural stem cells embedded in a matrix that contains a growth factor cocktail. The resulting neurons extend axons over long distances and form reciprocal synaptic connections with neurons from the host.
Myelin-associated inhibitors of axon growth, including Nogo, MAG and OMgp, have been the subject of intense research. A myriad of experimental approaches have been applied to investigate the ...potential of targeting these molecules to promote axonal repair after spinal cord injury. However, there are still conflicting results on their role in axon regeneration and therefore a lack of a cohesive mechanism on how these molecules can be targeted to promote axon repair. One major reason may be the lack of a clear definition of axon regeneration in the first place. Nevertheless, recent data from genetic studies in mice indicate that the roles of these molecules in CNS axon repair may be more intricate than previously envisioned.
How aging impacts axon regeneration after CNS injury is not known. We assessed the impact of age on axon regeneration induced by Pten deletion in corticospinal and rubrospinal neurons, two neuronal ...populations with distinct innate regenerative abilities. As in young mice, Pten deletion in older mice remains effective in preventing axotomy-induced decline in neuron-intrinsic growth state, as assessed by mTOR activity, neuronal soma size, and axonal growth proximal to a spinal cord injury. However, axonal regeneration distal to injury is greatly diminished, accompanied by increased expression of astroglial and inflammatory markers at the injury site. Thus, the mammalian CNS undergoes an age-dependent decline in axon regeneration, as revealed when neuron-intrinsic growth state is elevated. These results have important implications for developing strategies to promote axonal repair after CNS injuries or diseases, which increasingly affect middle-aged to aging populations.
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•PTEN inhibits neuronal growth state independently of age in the mammalian CNS•Aging diminishes the regeneration-promoting effect of targeting Pten in the CNS•Pten deletion unmasks an age-dependent decline in mammalian CNS axon regeneration•Aging is associated with increased negative extrinsic influences at the injury site
Working on two different axonal pathways, Geoffroy et al. show an age-dependent decline in axon regeneration after injury in the adult mammalian CNS. Their data implicate changes in neuron-extrinsic influences at the injury site in this age-dependent decline.
In the adult mammalian CNS, chondroitin sulfate proteoglycans (CSPGs) and myelin-associated inhibitors (MAIs) stabilize neuronal structure and restrict compensatory sprouting following injury. The ...Nogo receptor family members NgR1 and NgR2 bind to MAIs and have been implicated in neuronal inhibition. We found that NgR1 and NgR3 bind with high affinity to the glycosaminoglycan moiety of proteoglycans and participate in CSPG inhibition in cultured neurons. Nogo receptor triple mutants (Ngr1(-/-); Ngr2(-/-); Ngr3(-/-); which are also known as Rtn4r, Rtn4rl2 and Rtn4rl1, respectively), but not single mutants, showed enhanced axonal regeneration following retro-orbital optic nerve crush injury. The combined loss of Ngr1 and Ngr3 (Ngr1(-/-); Ngr3(-/-)), but not Ngr1 and Ngr2 (Ngr1(-/-); Ngr2(-/-)), was sufficient to mimic the triple mutant regeneration phenotype. Regeneration in Ngr1(-/-); Ngr3(-/-) mice was further enhanced by simultaneous ablation of Rptpσ (also known as Ptprs), a known CSPG receptor. Collectively, our results identify NgR1 and NgR3 as CSPG receptors, suggest that there is functional redundancy among CSPG receptors, and provide evidence for shared mechanisms of MAI and CSPG inhibition.
Multiple lines of evidence have indicated that the inability of adult mammalian central nervous system (CNS) axons to regenerate after injury is partly due to the growth inhibitory property of ...central myelin. Three prototypical myelin-associated inhibitors of neurite outgrowth have been identified, including Nogo, myelin-associated glycoprotein (MAG) and oligodendrocyte-myelin glycoprotein (OMgp). These inhibitory ligands, their receptors and signaling pathways are being intensively investigated for their roles in CNS axon regeneration failure. In addition, several members of the axon guidance molecules have been implicated in restricting CNS axon regeneration, some of which are expressed by mature oligodendrocytes. Here we review in vitro and in vivo studies of these molecules in neurite growth and in axon regeneration failure and discuss the implications of these studies. While the increasing number of potential axon regeneration inhibitors highlights the complexity of the restrictive CNS environment, it provides new windows of opportunity as well as new challenges for therapeutic development for spinal cord injury and related neurological conditions.
A central hypothesis for the limited capacity for adult central nervous system (CNS) axons to regenerate is the presence of myelin-derived axon growth inhibitors, the role of which, however, remains ...poorly understood. We have conducted a comprehensive genetic analysis of the three major myelin inhibitors, Nogo, MAG, and OMgp, in injury-induced axonal growth, including compensatory sprouting of uninjured axons and regeneration of injured axons. While deleting any one inhibitor in mice enhanced sprouting of corticospinal or raphespinal serotonergic axons, there was neither associated behavioral improvement nor a synergistic effect of deleting all three inhibitors. Furthermore, triple-mutant mice failed to exhibit enhanced regeneration of either axonal tract after spinal cord injury. Our data indicate that while Nogo, MAG, and OMgp may modulate axon sprouting, they do not play a central role in CNS axon regeneration failure.
► Deleting Nogo, MAG, and OMgp in mice does not enhance spinal axon regeneration ► Nogo, MAG, and OMgp may modulate axon sprouting after CNS injury ► Deleting all three inhibitors has no synergistic effect on sprouting or regeneration ► Functional recovery is not improved in Nogo/MAG/OMgp mutants after spinal cord injury
Axons in the adult CNS have poor ability to grow after injury, impeding functional recovery in patients of spinal cord injury. This has been attributed to both a developmental decline in ...neuron-intrinsic growth ability and the presence of extrinsic growth inhibitors. We previously showed that genetic deletion of Nogo, an extrinsic inhibitor, promoted axonal sprouting from uninjured corticospinal tract (CST) neurons but not regeneration from injured CST neurons, whereas genetic deletion of PTEN, an intrinsic inhibitor, promoted both CST sprouting and regeneration. Here we test the hypothesis that combining an elevation of neuron-intrinsic growth ability and a reduction of extrinsic growth inhibition by genetic codeletion of PTEN and Nogo may further improve injury-induced axonal growth. In an apparent paradox, additionally deleting Nogo further enhanced CST regeneration but not sprouting in PTEN-deleted mice. Enhanced CST regeneration and sprouting in PTEN and PTEN/Nogo-deleted mice were associated with no or only temporary improvement in functional recovery. Our data illustrate that neuron-intrinsic and -extrinsic factors regulate axon regeneration and sprouting in complex ways and provide proof-of-principle evidence that targeting both can further improve regeneration. Neuron-intrinsic growth ability is an important determinant of neuronal responsiveness to changes in extrinsic growth inhibition, such that an elevated intrinsic growth state is a prerequisite for reducing extrinsic inhibition to take effect on CST regeneration. Meanwhile, additional strategies are required to unleash the full potential for functional recovery with enhanced axon regeneration and/or sprouting.
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