ABSTRACT
Charcot-Marie-Tooth 1A (CMT1A) neuropathy, the most common inherited peripheral neuropathy, is primarily caused by a gene duplication for the peripheral myelin protein-22 (PMP22). In an accordant mouse model, we investigated the role of monocyte chemoattractant protein-1 (MCP-1/CCL2) as a regulator of nerve macrophages and neural damage including axonopathy and demyelination. By generating PMP22tg mice with reduced levels or lack of MCP-1/CCL2, we found that MCP-1/CCL2 is involved in the increase of macrophages in mutant nerves. PMP22tg mice with wild-type levels of MCP-1/CCL2 showed strong macrophage increase in the diseased nerves, whereas either 50% reduction or total absence of MCP-1/CCL2 led to a moderate or a strong reduction of nerve macrophages, respectively. Interestingly, MCP-1/CCL2 expression level and macrophage numbers were correlated with features indicative of axon damage, such as maldistribution of K+ channels, reduced compound muscle action potentials, and muscle weakness. Demyelinating features, however, were most highly reduced when MCP-1/CCL2 was diminished by 50%, whereas complete lack of MCP-1/CCL2 showed an intermediate demyelinating phenotype. We also identified the MEK1/2-ERK1/2-pathway as being involved in MCP-1/CCL2 expression in the Schwann cells of the CMT1A model. Our data show that, in a CMT1A model, MCP-1/CCL2 activates nerve macrophages, mediates both axon damage and demyelination, and may thus be a promising target for therapeutic approaches.
Subject(s)
Axons/pathology , Charcot-Marie-Tooth Disease/metabolism , Charcot-Marie-Tooth Disease/pathology , Chemokine CCL2/metabolism , Myelin Proteins/metabolism , Animals , Axons/metabolism , Cell Count , Charcot-Marie-Tooth Disease/physiopathology , Chemokine CCL2/genetics , Disease Models, Animal , Extracellular Signal-Regulated MAP Kinases/antagonists & inhibitors , Extracellular Signal-Regulated MAP Kinases/metabolism , Femoral Nerve/enzymology , Femoral Nerve/pathology , Femoral Nerve/physiopathology , Hand Strength/physiology , Humans , MAP Kinase Signaling System , Macrophages/metabolism , Mice , Mice, Transgenic , Myelin Sheath/pathology , Potassium Channels/metabolism , RNA, Messenger/genetics , RNA, Messenger/metabolism , Up-Regulation/geneticsABSTRACT
The presence and cellular localization of five membrane peptidases has been investigated in peripheral nerves, including those of the autonomic nervous system, in the pig. Endopeptidase-24.11 ("enkephalinase") peptidyl dipeptidase A, aminopeptidase N, aminopeptidase W and dipeptidyl peptidase IV were studied by both enzymic assays of membranes prepared from samples of nerve and by immunoperoxidase histochemistry at light and in two cases, endopeptidase-24.11 and aminopeptidase W, at electron microscopic levels. All five peptidases could be quantified by enzymic assay, though the activities were about 1% of those in renal microvilli and less than those of choroid plexus membranes. Endopeptidase-24.11 was associated with Schwann cell membranes in all types of nerve examined, including major nerves containing predominantly myelinated fibres as well as autonomic nerves, such as the vagus and splenic nerves and the sympathetic chain, staining being observed in membranes associated with myelinated and unmyelinated fibres. The Schwann cell location of endopeptidase-24.11 was confirmed by correlation with immunostaining for glial fibrillary acidic protein and by electron microscopy. This peptidase is known to have a wide repertoire of susceptible substrates among neuropeptides which was here shown to include vasoactive intestinal polypeptide (Km 268 microM, kcat 568 min-1), one of a number of neuropeptides present in peripheral nerve fibres. Three of the peptidases, peptidyl dipeptidase A, aminopeptidase N and dipeptidyl peptidase IV, were associated with microvessels of peripheral nerves. Aminopeptidase N was also observed in connective tissue elements, including the perineurium. Aminopeptidase W was unique among the five peptidases in having a neuronal localization. This was observed in unmyelinated and myelinated nerves and was supported by comparison with the pattern of staining observed for neurofilament protein and by electron microscopic immunoperoxidase staining. This observation was unexpected since aminopeptidase W has not been detected as a neuronal marker in the brain. Some possible roles for the membrane peptidases in peripheral nerves are discussed.
Subject(s)
Membrane Proteins/analysis , Nerve Tissue Proteins/analysis , Peptide Hydrolases/analysis , Peripheral Nerves/enzymology , Swine/metabolism , Animals , Brachial Plexus/enzymology , Exopeptidases , Femoral Nerve/enzymology , Fluorescent Antibody Technique , Immunoenzyme Techniques , Microscopy, Electron , Microscopy, Immunoelectron , Neuropeptides/metabolism , Peripheral Nerves/ultrastructureABSTRACT
Observed in this study were morphologic changes in inflamed nerves, along with biochemical changes, which appear to act concurrently to deactivate or prevent activation of the local anesthetic solution. Morphologic changes were observed along the nerve fiber distant from the inflammatory site. These neuro-degenerative changes were seen at the axon and myelin sheaths level. The biochemical data support the presence in the inflamed nerve of amino acids which may be the product of lysosomal rupture and proteolytic enzyme release. These inflammatory mediators may affect either the local anesthetic or the environment of the nerve fiber. The precise mechanism of action of these catalytic products cannot be determined from this study.
