ABSTRACT
Axons and their synapses distal to an injury undergo rapid Wallerian degeneration, but axons in the C57BL/WldS mouse are protected. The degenerative and protective mechanisms are unknown. We identified the protective gene, which encodes an N-terminal fragment of ubiquitination factor E4B (Ube4b) fused to nicotinamide mononucleotide adenylyltransferase (Nmnat), and showed that it confers a dose-dependent block of Wallerian degeneration. Transected distal axons survived for two weeks, and neuromuscular junctions were also protected. Surprisingly, the Wld protein was located predominantly in the nucleus, indicating an indirect protective mechanism. Nmnat enzyme activity, but not NAD+ content, was increased fourfold in WldS tissues. Thus, axon protection is likely to be mediated by altered ubiquitination or pyridine nucleotide metabolism.
Subject(s)
Axons/metabolism , Fungal Proteins/genetics , Neuromuscular Junction/metabolism , Nicotinamide-Nucleotide Adenylyltransferase/genetics , Recombinant Fusion Proteins/genetics , Saccharomyces cerevisiae Proteins , Trauma, Nervous System , Wallerian Degeneration/genetics , Wallerian Degeneration/metabolism , Action Potentials/genetics , Animals , Axons/ultrastructure , Base Sequence/physiology , Cell Nucleus/metabolism , Cell Nucleus/ultrastructure , Cell Survival/genetics , Fungal Proteins/metabolism , Immunohistochemistry , Mice , Mice, Mutant Strains , Mice, Transgenic , Microscopy, Electron , Molecular Sequence Data , Motor Neurons/cytology , Motor Neurons/metabolism , Muscle, Skeletal/growth & development , Muscle, Skeletal/innervation , Muscle, Skeletal/metabolism , Mutation/physiology , Nerve Tissue Proteins/genetics , Nerve Tissue Proteins/metabolism , Nervous System/metabolism , Nervous System/pathology , Neuromuscular Junction/ultrastructure , Nicotinamide-Nucleotide Adenylyltransferase/metabolism , Recombinant Fusion Proteins/metabolism , Sciatic Nerve/injuries , Sciatic Nerve/metabolism , Sciatic Nerve/ultrastructure , Synaptic Transmission/genetics , Synaptic Vesicles/metabolism , Ubiquitin-Conjugating Enzymes , Wallerian Degeneration/physiopathologyABSTRACT
The mechanism of attachment of acetylcholinesterase (AChE) to neuronal membranes in interneuronal synapses is poorly understood. We have isolated, sequenced, and cloned a hydrophobic protein that copurifies with AChE from human caudate nucleus and that we propose forms a part of a complex of membrane proteins attached to this enzyme. It is a short protein of 136 amino acids and has a molecular mass of 18 kDa. The sequence contains stretches of both hydrophobic and hydrophilic amino acids and two cysteine residues. Analysis of the genomic sequence reveals that the coding region is divided among five short exons. Fluorescence in situ hybridization localizes the gene to chromosome 6p21.32-p21.2. Northern blot analysis shows that this gene is widely expressed in the brain with an expression pattern that parallels that of AChE.
