Regeneration of central nervous system axons after injury is severely restricted because several myelin-associated inhibitory proteins are present at the injury site, including the membrane protein Nogo-A. On p. 4591, Adrian Walmsley and co-workers report that the function of the receptor for Nogo-66, the C-terminal 66-residue inhibitory domain of Nogo-A, may be modulated by `ectodomain shedding'. Nogo-66 binds the neuronal glycosyl-phosphatidyl-anchored Nogo-66 receptor (NgR), which transduces the neurite outgrowth inhibitory signal via its transmembrane co-receptor p75NTR. The authors show that, in human neuroblastoma cells, human NgR is cleaved by a zinc metalloproteinase to yield a lipid-raft-associated C-terminal fragment and a soluble N-terminal fragment containing the NgR ligand-binding domain. This fragment binds Nogo-66 and blocks Nogo-66 binding to cell-surface NgR but does not associate with p75NTR and so could antagonise Nogo-66 signalling. Finally, the authors report that the same fragments of NgR are present in human brain cortex and cerebrospinal fluid, indicating that ectodomain shedding is a potential endogenous mechanism for NgR regulation.
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