The failure of axons to regenerate is a major obstacle for functional recovery after central nervous system (CNS) injury. Removing extracellular inhibitory molecules results in limited axon regeneration in vivo. To test for the role of intrinsic impediments to axon regrowth, we analyzed cell growth control genes using a virus-assisted in vivo conditional knockout approach. Deletion of PTEN (phosphatase and tensin homolog), a negative regulator of the mammalian target of rapamycin (mTOR) pathway, in adult retinal ganglion cells (RGCs) promotes robust axon regeneration after optic nerve injury. In wild-type adult mice, the mTOR activity was suppressed and new protein synthesis was impaired in axotomized RGCs, which may contribute to the regeneration failure. Reactivating this pathway by conditional knockout of tuberous sclerosis complex 1, another negative regulator of the mTOR pathway, also leads to axon regeneration. Thus, our results suggest the manipulation of intrinsic growth control pathways as a therapeutic approach to promote axon regeneration after CNS injury.Axons do not regenerate after injury in the adult mammalian central nervous system (CNS), a phenomenon attributed to two properties of the adult CNS, the inhibitory extrinsic environment and a diminished intrinsic regenerative capacity of mature CNS neurons (1-4). Neutralization of the extracellular molecules identified as axon regrowth inhibitors allows only a limited degree of axon regeneration in vivo (5-7). Therefore, intrinsic mechanisms are likely to be important in controlling the process of axon regeneration. A hint about possible mechanisms of neuronal regenerative ability comes from the evolutionarily conserved molecular pathways that control cellular growth and size. For most cell types, specific mechanisms are necessary to prevent cellular overgrowth upon the completion of development (8). Because many of these molecules are often expressed in postmitotic mature neurons, we hypothesized that they may contribute to the diminished regenerative ability in adult CNS neurons.To circumvent the problem that germline knockout of individual cell growth control genes often results in compromised viability in mice, we designed a strategy based on intravitreal injection of adeno-associated viruses expressing Cre (AAV-Cre) in adult mice. This procedure resulted in the expression of Cre in more than 90% of retinal ganglion cells (RGCs) and few other non-RGC cells, as indicated in two reporter lines ( fig. S1, A and B). We thus injected AAV-Cre into the vitreous body of different adult floxed mice, including Rb f/f (9), P53 f/f (9),

Despite the essential role of the corticospinal tract (CST) in controlling voluntary movements, successful regeneration of large numbers of injured CST axons beyond a spinal cord lesion has never been achieved. Here we demonstrate a critical involvement of PTEN/mTOR in controlling the regenerative capacity of mouse corticospinal neurons. Upon the completion of development, the regrowth potential of CST axons lost and this is accompanied by a down-regulation of mTOR activity in corticospinal neurons. Axonal injury further diminishes neuronal mTOR activity in these neurons. Forced up-regulation of mTOR activity in corticospinal neurons by conditional deletion of PTEN, a negative regulator of mTOR, enhances compensatory sprouting of uninjured CST axons and even more strikingly, enables successful regeneration of a cohort of injured CST axons past a spinal cord lesion. Furthermore, these regenerating CST axons possess the ability to reform synapses in spinal segments distal to the injury. Thus, modulating neuronal intrinsic PTEN/mTOR activity represents a potential therapeutic strategy for promoting axon regeneration and functional repair after adult spinal cord injury.

SUMMARY Axon regeneration failure accounts for permanent functional deficits following CNS injury in adult mammals. However, the underlying mechanisms remain elusive. In analyzing axon regeneration in different mutant mouse lines, we discovered that deletion of suppressor of cytokine signaling 3 (SOCS3), in adult retinal ganglion cells (RGCs), promotes robust regeneration of injured optic nerve axons. This regeneration-promoting effect is efficiently blocked in SOCS3-gp130 double knockout mice, suggesting that SOCS3 deletion promotes axon regeneration via a gp130-dependent pathway. Consistently, a transient up-regulation of ciliary neurotrophic factor (CNTF) was observed within the retina following optic nerve injury. Intravitreal application of CNTF further enhances axon regeneration from SOCS3-deleted RGCs. Together, our results suggest that compromised responsiveness to injury-induced growth factors in mature neurons contributes significantly to regeneration failure. Thus, developing strategies to modulate negative signaling regulators may be an efficient strategy of promoting axon regeneration after CNS injury.