Most axons in the adult mammalian central nervous system (CNS) are unable to regenerate after injury, potentially leading to devastating and permanent functional deficits in individuals who have suffered neurologic damage. It has been proposed that the failure of axon regeneration could be due to a reduced intrinsic regenerative capacity of mature neurons and/or a hostile environment of the adult CNS, including both the presence of inhibitory factors from both the glial scar and CNS myelin and a lack of neurotrophic support. Despite recent progress in identifying some molecular players in each of these processes, how these extrinsic and intrinsic factors determine the regenerative decision of lesioned axons in vivo remains largely unknown. In addition, it remains to be determined whether various types of CNS axons differ in their requirements for successful axon regeneration. The ultimate goal of this study is to utilize the molecular tools obtained from our previous studies to address the molecular logic behind regeneration failure in the adult CNS in vivo. Recent studies from our laboratory, as well as others, have demonstrated that three proteins, Nogo, myelin-associated glycoprotein (MAG) and oligodendrocyte-myelin glycoprotein (OMgp), collectively account for the majority of the inhibitory activity associated with CNS myelin. Interestingly, all of these factors exert their inhibitory actions through a common receptor complex consisting of the ligand-binding Nogo receptor (NgR) and two signal-transducers p75/TROY and Lingo-1. These observations offer a unique opportunity for designing strategies to neutralize the inhibitory influences of CNS myelin by blocking this common signaling pathway. However, due to the likely presence of potential functional homologues of NgR, p75/TROY, and Lingo-1, a single knockout strategy may not result in the complete removal of these inhibitory activities. In fact, studies using both NgR and p75 knockout mice have failed to show a complete loss of response to myelin inhibition in vitro. In our previous studies, we have designed a truncated NgR which retains the ability to bind all three inhibitory ligands but does not interact with NgR’s signaling co-receptors p75/TROY and Lingo-1. Once over-expressed in neurons both in vitro and in vivo, truncated NgR efficiently blocks neuronal responses to myelin inhibitors in a dominant-negative manner. Thus, we have generated transgenic mice in which this dominant-negative NgR (DN-NgR) is stably expressed in the nervous system and we have demonstrated that neuronal responses to myelin inhibitors are blocked in these transgenic lines. These animal models will be employed in this study to define the role of myelin inhibitors and other extrinsic and intrinsic factors in limiting the regeneration of different axonal tracts in vivo.
