SUMMARY 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.

The myelin-derived neurite growth inhibitor Nogo has been proposed to play a major role in blocking axon regeneration in the CNS after injuries. However, past studies have produced mixed results regarding the regenerative phenotype of various Nogo-deficient mouse lines after experimental spinal cord injury. Two lines did not display enhanced corticospinal tract (CST) regeneration, and one displayed modest regeneration. A fourth line, a Nogo-A,B gene-trap mutant, was instead reported to exhibit extensive CST regeneration, but the results were later found to be inadvertently confounded with an axon labeling artifact. Of the four Nogo mutant lines studied so far, three continue to express some isoform(s) of Nogo, leaving open the question whether any remaining Nogo protein contributes to the modest regenerative phenotype reported in some. The remaining Nogo mutant line studied was confounded by the unexplained rescue of embryonic lethality associated with this mutation. To gain a better understanding of the contribution of Nogo as an inhibitor of regeneration of CNS axons, and particularly CST axons, we reanalyzed the Nogo-A,B gene-trap mutant line and analyzed a novel, fully viable Nogo deletion mutant line that is null for all known isoforms of Nogo. Our analyses failed to reveal any enhanced CST regeneration after experimental spinal cord injury in either line. These results indicate that Nogo alone does not account for lack of CST regeneration and have implications for current therapeutic development for spinal cord injury in humans by targeting Nogo.

One important aspect of recovery and repair after spinal cord injury (SCI) lies in the complex cellular interactions at the injury site that leads to the formation of a lesion scar. EphA4, a promiscuous member of the EphA family of repulsive axon guidance receptors, is expressed by multiple cell types in the injured spinal cord, including astrocytes and neurons. We hypothesized that EphA4 contributes to aspects of cell-cell interactions at the injury site after SCI, thus modulating the formation of the astroglial-fibrotic scar. To test this hypothesis, we studied tissue responses to a thoracic dorsal hemisection SCI in an EphA4 mutant mouse line. We found that EphA4 expression, as assessed by β-galactosidase reporter gene activity, is associated primarily with astrocytes in the spinal cord, neurons in the cerebral cortex and, to a lesser extent, spinal neurons, before and after SCI. However, we did not observe any overt reduction of glial fibrillary acidic protein (GFAP) expression in the injured area of EphA4 mutants in comparison with controls following SCI. Furthermore, there was no evident disruption of the fibrotic scar, and the boundary between reactive astrocytes and meningeal fibroblasts appeared unaltered in the mutants, as were lesion size, neuronal survival and inflammation marker expression. Thus, genetic deletion of EphA4 does not significantly alter the astroglial response or the formation of the astroglial-fibrotic scar following a dorsal hemisection SCI in mice. In contrast to what has been proposed, these data do not support a major role for EphA4 in reactive astrogliosis following SCI.

The cognitive impairment in Alzheimer's disease (AD) is associated with synaptic loss, neuritic sprouting and altered neuroplasticity. Compensatory neuritic sprouting might be beneficial, while aberrant sprouting could contribute to the neurodegenerative process. Nogo (or Rtn4) is a major myelin-derived inhibitor of axonal sprouting in adult CNS. Recent evidence has implicated both the Reticulon family of proteins and a receptor for Nogo, NgR, in reducing amyloid-β production, a key step in AD pathogenesis. To test the hypothesis that Nogo, as an inhibitor of axonal sprouting, modulates disease progression in a mouse model of AD, we introduced an APP transgene (a human APP minigene carrying the Swedish and Indiana mutations under the PDGFB promoter) into a Nogo null background and characterized the behavioral and neuropathological consequences. We found that deleting Nogo ameliorates learning and memory deficits of APP transgenic mice in the Morris water maze at an early/intermediate stage of the disease. Furthermore, deleting Nogo restored the expression levels of markers for synapto-dendritic complexity and axonal sprouting including synaptophysin, MAP2, GAP43 and neurofilament that are otherwise reduced in APP transgenic mice. Other aspects of disease progression including neuronal loss, astrogliosis, microgliosis and, importantly, Aβ levels and amyloid deposits were not significantly altered by Nogo deletion. These data support the hypothesis that Nogo-mediated inhibition of neuritic sprouting contributes to the disease progression in an APP transgenic model of AD in a way that is mechanistically distinct from what has been proposed for Rtn3 or NgR.

SummaryThe very limited ability to regenerate axons after injury in the mature mammalian central nervous system (CNS) has been partly attributed to the growth restrictive nature of CNS myelin. Oligodendrocyte myelin glycoprotein (OMgp) was identified as a major myelin-derived inhibitor of axon growth. However, its role in axon regeneration in vivo is poorly understood. Here we describe the generation and molecular characterization of an OMgp allelic series. With a single gene targeting event and Cre/FLP mediated recombination, we generated an OMgp null allele with a LacZ reporter, one without a reporter gene, and an OMgp conditional allele. This allelic series will aid in the study of OMgp in adult CNS axon regeneration using mouse models of spinal cord injury. The conditional allele will overcome developmental compensation when employed with an inducible Cre, and allows for the study of temporal and tissue/cell type-specific roles of OMgp in CNS injury-induced axonal plasticity. KeywordsOMgp; conditional allele; axon regeneration; CNS repair; myelin inhibition Following development, axons in the mammalian central nervous system (CNS, including the brain and the spinal cord) have largely lost the ability to regenerate in response to injuries (Ramón y Cajal, 1928). Spinal cord injury patients suffer permanent functional deficits due to this inability for axon regeneration in adult CNS, which is in contrast with the robust regenerative ability of axons in the developing CNS or in the adult peripheral nervous system (PNS). The inhibitory action of myelin on axon growth has been hypothesized as one primary reason for this loss of regenerative ability (Berry, 1982;Schwab and Bartholdi, 1996). Several myelin-derived inhibitory proteins have been identified (Filbin, 2003;Yiu and He, 2006). Oligodendrocyte Myelin glycoprotein (OMgp) was identified as a major myelin-derived inhibitor that possesses potent inhibitory activity on neurite outgrowth in vitro (Kottis et al., 2002;Wang et al., 2002). Together with two other myelin inhibitors, Nogo and myelin-associated glycoprotein (MAG), it signals through at least two receptors, NgR1 and PirB, to effect the inhibitory action of myelin (Atwal et al., 2008;Yiu and He, 2006). However, the role of OMgp in axon regeneration in vivo is not well understood. In a published report, OMgp deficient mice were found to exhibit slightly enhanced axon regeneration in the ascending dorsal column sensory tract and the descending raphespinal serotonergic tract but not in the corticospinal tract (Ji et al., 2008 Zheng et al., 2005). A MAG mutant exhibits upregulated NgR1 expression (Pernet et al., 2008). Thus, mutations in either an inhibitory ligand or receptor in the myelin inhibition pathway can lead to changes in the expression of other genes in the pathway, which may mask the effect of gene deletion (in the case of the NgR1 or MAG mutants) or complicates the interpretation of the results (in the case of the OMgp mutant). This is particularly a challenge for genetic studies of axon regener...

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