Complement potentiates the degradation of myelin proteins by plasmin: implications for a mechanism of inflammatory demyelination

Sub-Chronic Neuropathological and Biochemical Changes in Mouse Visual System after Repetitive Mild Traumatic Brain Injury

Repetitive mild traumatic brain injury (r-mTBI) results in neuropathological and biochemical consequences in the human visual system. Using a recently developed mouse model of r-mTBI, with control mice receiving repetitive anesthesia alone (r-sham) we assessed the effects on the retina and optic nerve using histology, immunohistochemistry, proteomic and lipidomic analyses at 3 weeks post injury. Retina tissue was used to determine retinal ganglion cell (RGC) number, while optic nerve tissue was examined for cellularity, myelin content, protein and lipid changes. Increased cellularity and areas of demyelination were clearly detectable in optic nerves in r-mTBI, but not in r-sham. These changes were accompanied by a ~25% decrease in the total number of Brn3a-positive RGCs. Proteomic analysis of the optic nerves demonstrated various changes consistent with a negative effect of r-mTBI on major cellular processes like depolymerization of microtubules, disassembly of filaments and loss of neurons, manifested by decrease of several proteins, including neurofilaments (NEFH, NEFM, NEFL), tubulin (TUBB2A, TUBA4A), microtubule-associated proteins (MAP1A, MAP1B), collagen (COL6A1, COL6A3) and increased expression of other proteins, including heat shock proteins (HSP90B1, HSPB1), APOE and cathepsin D. Lipidomic analysis showed quantitative changes in a number of phospholipid species, including a significant increase in the total amount of lysophosphatidylcholine (LPC), including the molecular species 16:0, a known demyelinating agent. The overall amount of some ether phospholipids, like ether LPC, ether phosphatidylcholine and ether lysophosphatidylethanolamine were also increased, while the majority of individual molecular species of ester phospholipids, like phosphatidylcholine and phosphatidylethanolamine, were decreased. Results from the biochemical analysis correlate well with changes detected by histological and immunohistochemical methods and indicate the involvement of several important molecular pathways. This will allow future identification of therapeutic targets for improving the visual consequences of r-mTBI.

Role of Campylobacter jejuni infection in the pathogenesis of Guillain-Barré syndrome: an update

Our current knowledge on Campylobacter jejuni infections in humans has progressively increased over the past few decades. Infection with C. jejuni is the most common cause of bacterial gastroenteritis, sometimes surpassing other infections due to Salmonella, Shigella, and Escherichia coli. Most infections are acquired due to consumption of raw or undercooked poultry, unpasteurized milk, and contaminated water. After developing the diagnostic methods to detect C. jejuni, the possibility to identify the association of its infection with new diseases has been increased. After the successful isolation of C. jejuni, reports have been published citing the occurrence of GBS following C. jejuni infection. Thus, C. jejuni is now considered as a major triggering agent of GBS. Molecular mimicry between sialylated lipooligosaccharide structures on the cell envelope of these bacteria and ganglioside epitopes on the human nerves that generates cross-reactive immune response results in autoimmune-driven nerve damage. Though C. jejuni is associated with several pathologic forms of GBS, axonal subtypes following C. jejuni infection may be more severe. Ample amount of existing data covers a large spectrum of GBS; however, the studies on C. jejuni-associated GBS are still inconclusive. Therefore, this review provides an update on the C. jejuni infections engaged in the pathogenesis of GBS.

Kallikrein 6 regulates early CNS demyelination in a viral model of multiple sclerosis

Kallikrein 6 (Klk6) is a secreted serine protease that is elevated in active multiple sclerosis lesions and patient sera. To further evaluate the involvement of Klk6 in chronic progressive demyelinating disease, we determined its expression in the brain and spinal cord of SJL mice infected with Theiler's murine encephalomyelitis virus (TMEV) and assessed the effects of Klk6-neutralizing antibodies on disease progression. Klk6 RNA expression was elevated in the brain and spinal cord by 7 days postinfection (dpi). Thereafter, Klk6 expression persisted primarily in the spinal cord reaching a peak of fivefold over controls at mid-chronic stages (60 dpi-120 dpi). Significant elevations in Klk6 RNA were also induced in splenocytes stimulated with viral capsid proteins in vitro and in activated human acute monocytic leukemia cells. Klk6-neutralizing antibodies reduced TMEV-driven brain and spinal cord pathology and delayed-type hypersensitivity (DTH) responses when examined at early chronic time points (40 dpi). Reductions in spinal cord pathology included a decrease in activated monocytes/microglia and reductions in the loss of myelin basic protein (MBP). By 180 dpi, pathology scores no longer differed between groups. These findings point to regulatory activities for Klk6 in the development and progression of central nervous system (CNS) inflammation and demyelination that can be effectively targeted through the early chronic stages with neutralizing antibody.

Does thrombin play a role in the pathogenesis of brain damage after periventricular hemorrhage?

Neonatal periventricular hemorrhage (PVH) is a devastating complication of prematurity in the human infant. Based upon observations made primarily in adult rodents and the fact that the immature brain uses proteolytic systems for cell migration and growth, we hypothesized that thrombin and plasmin enzyme activities contribute to the brain damage after PVH. The viability of mixed brain cells derived from newborn rat periventricular region was suppressed by whole blood and thrombin, but not plasmin. Following injection of autologous blood into the periventricular region of newborn rat brain, proteolytic activity was detected in a halo around the hematoma using membrane overlays impregnated with thrombin and plasmin fluorogenic substrates. Two-day old rats received periventricular injection of blood, thrombin, and plasminogen. After 2 days, thrombin and blood were associated with significantly greater damage than saline or plasminogen. Two-day old mice received intracerebral injections of blood in combination with saline or the proteolytic inhibitors hirudin, alpha2macroglobulin, or plasminogen activator inhibitor-1. After 2 days, hirudin significantly reduced brain cell death and inflammation. Two-day-old mice then received low and high doses of hirudin mixed with blood after which behavioral testing was conducted repeatedly. At 10 weeks there was no statistically significant evidence for behavioral or structural brain protection. These results indicate that thrombin likely plays a role in neonatal periventricular brain damage following PVH. However, additional factors are likely important in the recovery from this result.

Expression of Myelencephalon-Specific Protease after Cryogenic Lesioning of the Rat Parietal Cortex

The gene for myelencephalon-specific protease (MSP) is a member of the kallikrein gene family and in rats is expressed mainly in the central nervous system. Its function and alteration in brain injury have not yet been clarified. We examined the expression of MSP after cryogenic injury (CI) using in situ hybridization, immunohistochemistry, and Western blotting. Analysis of MSP mRNA by in situ hybridization revealed a higher level of expression around the cryogenic area than on the contralateral side at 2-7 days after CI, with peak expression occurring 7 days after CI. Immunohistochemical analysis demonstrated expression of MSP protein at 1 day after CI, in the same region in which MSP mRNA was observed, with peak expression again at 7 days after CI, in the area around the lesion. Double immunohistochemical labeling revealed that MSP was expressed mainly in oligodendrocytes. These results suggest that expression of MSP may be related to the turnover of myelin-associated proteins and extracellular matrix proteins after CI. The regulation of active MSP may be important in the physiological or pathological changes involved in remyelination or demyelination.

Activity of a newly identified serine protease in CNS demyelination

We have identified a novel serine protease, myelencephalon-specific protease (MSP), which is preferentially expressed in the adult CNS, and therein, is abundant in both neurones and oligodendroglia. To determine the potential activity of MSP in CNS demyelination, we examined its expression in multiple sclerosis lesions and in two animal models of multiple sclerosis: Theiler's murine encephalomyelitis virus (TMEV) and myelin oligodendrocyte glycoprotein (MOG)-induced experimental allergic encephalomyelitis (EAE) in marmosets. High levels of MSP were present within infiltrating mononuclear cells, including macrophages and T cells, which characteristically fill sites of demyelination, both in multiple sclerosis lesions and in animal models of this disease. The functional consequence of excess MSP on oligodendroglia was determined in vitro by evaluating the effects of recombinant MSP (r-MSP) on oligodendrocyte survival and process number. Application of excess r-MSP resulted in a dramatic loss of processes from differentiated oligodendrocytes, and a parallel decrease in process outgrowth from immature cells. Transfection of oligodendrocyte progenitors with an MSP-green fluorescent protein construct produced similar changes in oligodendrocyte process number. Importantly, r-MSP did not affect oligodendrocyte survival or differentiation towards the sulphatide-positive lineage. We further demonstrate that myelin basic protein, and to a lesser extent myelin oligodendrocyte glycoprotein, can serve as MSP substrates. These studies support the hypothesis that excess MSP, as is present in inflammatory CNS lesions, promotes demyelination.

Immunopathology of secondary-progressive multiple sclerosis

Twenty-three plaques obtained at early autopsy from 2 patients with secondary-progressive multiple sclerosis were examined immunohistochemically for microglia/macrophages, and for immunoglobulins and components of activated complement. Most of the lesions examined in both cases exhibited evidence of low-grade active demyelination of an unusual type (frustrated phagocytosis) in periplaque white matter. This included linear groups of microglia engaging short segments of disrupted myelin that were associated with deposits of C3d, an opsonin formed during complement activation. Similar microglia/C3d/myelin profiles were not observed in newly forming lesions in cases of acute multiple sclerosis or other central white matter diseases. As C3d coupling is known to increase the immunogenicity of potential antigens enormously, present findings point to disrupted myelin close to plaques as a possible source of the putative multiple sclerosis antigen. Ongoing myelin destruction found in a high proportion of old, established plaques was surprising. It suggests that slowly expanding lesions (progressive plaques), in which ongoing myelin breakdown occurs in the absence of florid perivascular cell cuffing or other histological signs of acute inflammation, contribute to disease progression in cases of secondary-progressive multiple sclerosis.

Expression of neuropsin in oligodendrocytes after injury to the CNS

Proteases are involved in a variety of processes including demyelination after injury to the central nervous system. Neuropsin is a serine protease, which is constitutively expressed in the neurons of the limbic system. In the present study, intrahippocampal kainate injection and enucleation were performed on adult mice. Neuropsin mRNA and protein expression was detected by in situ hybridization and immunohistochemistry. Double in situ hybridization confirmed that the mRNA expression was induced in oligodendrocytes. One day after kainate injection to the hippocampus, neuropsin mRNA was expressed, peaking 4-8 days postoperatively and disappearing at 14 days. Immunohistochemistry and immunoelectron microscopy revealed that neuropsin was expressed in the cell body of oligodendrocytes and myelin. To see if neuropsin degrades myelin protein, purified myelin was incubated with recombinant neuropsin. A decrease in the intensity of the bands of myelin basic protein was observed. These results indicate that neuropsin is involved in demyelination.

Preferential expression of myelencephalon-specific protease by oligodendrocytes of the adult rat spinal cord white matter

Myelencephalon-specific protease (MSP) is a novel serine protease that is expressed predominantly in the nervous system. In the adult rat spinal cord, MSP mRNA expression was dramatically upregulated, in both the white and gray matter, after systemic exposure to the glutamate receptor agonist, kainic acid (KA) (Scarisbrick et al. J Neurosci 17: 8156-8168, 1997b). To determine the cell-specific expression patterns of MSP, we generated MSP-specific monoclonal antibodies. These have been used in immunohistochemical and in situ hybridization colocalization studies, to demonstrate that MSP mRNA and protein are produced predominantly by CNP-immunoreactive oligodendroglia, but not by GFAP-immunoreactive astrocytes, in the white matter of the normal adult cord. In vitro, the soma of oligodendrocytes were also densely MSP immunoreactive, as were their growth tips, while astrocytes were associated with lower levels. These findings suggest that the enzymatic activity of MSP is likely to be important in the biology of oligodendrocytes and/or in the maintenance of the nerve fiber tracts of the adult spinal cord.

Biochemistry of demyelination

The myelin sheath, a lipid-rich multilamellar membrane of relative stability, both insulates and enhances conduction in nerve axons. A notable feature of myelin-specific proteins, in particular myelin basic protein, is their susceptibility to proteolytic activity and their encephalitogenicity, which induces inflammatory demyelination in the CNS. The final common pathway of myelin breakdown in vivo is well documented and there is evidence that myelin disruption can be mediated directly by soluble (circulating) factors and for following receptor-driven phagocytosis by macrophages. However the exact mechanism(s) of demyelination in multiple sclerosis is still unresolved, both antigen-specific and--non-specific events having the potential to generate the myelinolytic process.

Myelin basic protein does not have a mitogenic effect on adult oligodendrocytes

Increased numbers of oligodendrocytes and remyelination are frequently observed in multiple sclerosis plaques. It is presumed the increased numbers of oligodendrocytes are due to cell division, but this has not been proven. The mitogens within the lesion which might be responsible for this are unknown. Since oligodendrocyte proliferation occurs in areas in which there is myelin breakdown, we undertook the present study to determine if myelin basic protein (MBP) or its breakdown products could induce oligodendrocyte proliferation. MBP, or MBP digested by the neutral proteinase plasmin, was added in three concentrations to the media of adult bovine oligodendrocytes in culture. Oligodendrocytes were identified by staining for galactocerebroside. Bromodeoxyuridine incorporation was used as a measure of cell division. Oligodendrocytes were found to divide only rarely in regular culture media, in the presence of MBP, plasmin, or MBP digested by plasmin. The results indicate that MBP is not a significant mitogen for the mature oligodendrocyte.

Immunopathogenesis and treatment of the Guillain-Barré syndrome--Part I

The etiology of the Guillain-Barré syndrome (GBS) still remains elusive. Recent years have witnessed important advances in the delineation of the mechanisms that may operate to produce nerve damage. Evidence gathered from cell biology, immunology, and immunopathology studies in patients with GBS and animals with experimental autoimmune neuritis (EAN) indicate that GBS results from aberrant immune responses against components of peripheral nerve. Autoreactive T lymphocytes specific for the myelin antigens P0 and P2 and circulating antibodies to these antigens and various glycoproteins and glycolipids have been identified but their pathogenic role remains unclear. The multiplicity of these factors and the involvement of several antigen nonspecific proinflammatory mechanisms suggest that a complex interaction of immune pathways results in nerve damage. Data on disturbed humoral immunity with particular emphasis on glycolipid antibodies and on activation of autoreactive T lymphocytes and macrophages will be reviewed. Possible mechanisms underlying initiation of peripheral nerve-directed immune responses will be discussed with particular emphasis on the recently highlighted association with Campylobacter jejuni infection.

An electrophysiological and histological study of trypsin induced demyelination

Ten-microliters quantities of trypsin or saline were injected into rat tibial nerve and the physiological and histological changes evaluated and compared to the focal demyelinating lesions induced by intraneural injection of rabbit EAN serum and proteinase K. The injection of trypsin produced progressive conduction block that was maximal on day 4, and a slowing of motor nerve conduction. Early retraction of myelin at paranodes, vesicular change, and macrophage stripping of myelin from nerve axons were seen on histological examination. At day 4, the first groups of completely demyelinated axons were seen, typically in a perivascular distribution. These changes were similar to those seen in the positive controls and thus support the postulate that proteolytic enzymes from macrophages--the dominant cellular species within the demyelinating lesion, play a central role in degradation of the myelin sheath in demyelinating diseases.

Selective elimination of macrophages by dichlormethylene diphosphonate-containing liposomes suppresses experimental autoimmune neuritis

The injection of liposome-encapsulated dichlormethylene diphosphonate (Cl2MDP) constitutes an effective method to selectively eliminate phagocytic cells from spleen, liver and the circulation. We evaluated the effect of Cl2MDP-liposomes on the course of actively induced and adoptively transferred experimental autoimmune neuritis (EAN), both animal models of the human Guillain-Barré syndrome. Injection of Cl2MDP-liposomes 11 and 13 days postimmunization (p.i.) of Lewis rats with bovine peripheral nerve myelin efficiently prevented clinical signs of EAN up to day 15 p.i., when all control animals were affected. Thereafter, EAN gradually also developed in Cl2MDP-liposome-treated rats, but until day 19 disease was significantly milder than in control rats injected with buffer-filled liposomes. Adoptive transfer EAN (AT-EAN) induced by injection of activated P2-specific T cells could be suppressed even more markedly by application of Cl2MDP-liposomes 1, 3, and 6 days after cell transfer. Efficient suppression of AT-EAN by Cl2MDP-liposomes rules out the possibility that EAN is prevented due to interference with the induction phase of this experimental disease and confirms that macrophages are important effector cells during EAN. Selective suppression of phagocytic cell function by drug-containing liposomes may hold promise as a novel treatment of demyelinating autoimmune diseases of the nervous system.

Phagocytosis of myelin by microglia in vitro

Previous experiments from this laboratory have shown that peritoneal macrophages in culture will phagocytize myelin. Myelin preopsonized with myelin antibodies is phagocytized to a much greater extent than untreated myelin, indicating that macrophages ingest myelin by an Fc receptor. The present work was undertaken to determine the characteristics of myelin phagocytosis by microglia, the resident macrophages of the central nervous system. Microglia isolated from 4-5 day primary cultures of newborn rat brains were shown to bind and phagocytize myelin labeled in the lipids by 14C-acetate. Both binding and phagocytosis as shown by the appearance of 14C-cholesterol ester were greatly increased if labeled myelin was preopsonized with antiserum to myelin basic protein or galactocerebroside. Both preopsonized and untreated myelin were phagocytized more actively by microglia than by peritoneal macrophages under the same culture conditions. Microglia cultured in the presence of GM-CSF showed slightly increased cholesterol ester production from opsonized myelin, but the effect of GM-CSF was significantly greater than myelin pretreated with control serum (34% increase) or untreated myelin (154% increase). There was no significant effect of GM-CSF on myelin phagocytosis by peritoneal macrophages. Cerebrospinal fluid containing immunoglobulin drawn from rabbits with acute EAE also opsonized myelin to increase phagocytosis by microglia, as has been previously shown with peritoneal macrophages. These results indicate that microglia may actively participate in myelin destruction in demyelinating diseases where myelin antibodies or a source of GM-CSF may be present.

Antibody-induced generation of reactive oxygen radicals by brain macrophages in canine distemper encephalitis: a mechanism for bystander demyelination

The mechanism of inflammatory demyelination in canine distemper encephalitis (CDE) is uncertain but macrophages are thought to play an important effector role in this lesion. Serum and cerebrospinal fluid (CSF), containing anti-canine distemper virus and anti-myelin antibodies from dogs with CDE were tested for their ability to generate reactive oxygen species (ROS) in macrophages in primary dog brain cell cultures using a chemiluminescence (CL) assay. The majority of serum samples and several CSF samples from animals with inflammatory demyelination elicited a CL signal in infected dog brain cell cultures. In contrast, none of these samples induced a positive response in uninfected cultures which contained large numbers of myelin antigen-presenting cells, although defined anti-myelin antibodies lead to a marked secretion of ROS in this system. It was concluded that antiviral antibody-induced secretion of ROS, known to be highly toxic for brain tissue, may play an important role in white matter damage in inflammatory lesions supporting a previous hypothesis of bystander demyelination in CDE. No evidence was found for a similar antibody-dependent cellular cytotoxicity-like mechanism mediated by anti-myelin antibodies in CDE, which does not support the concept of autoimmunity in this disease.

The role of mast cells in demyelination. 1. Myelin proteins are degraded by mast cell proteases and myelin basic protein and P2 can stimulate mast cell degranulation

Incubation of bovine peripheral myelin with supernatants from degranulated rat serosal mast cells led to extensive loss of P0. Similarly, when myelinated axons prepared from guinea pig CNS were incubated with degranulation supernatants, a significant loss of basic protein (MBP) was observed. As cationic peptides can stimulate mast cell degranulation, rat serosal mast cells were incubated with MBP, and with P2. Degranulation was assayed by measurement of release of the granule enzyme beta-hexosaminidase and it was found that both MBP and P2 stimulated 40-50% degranulation at a concentration of 50 micrograms/ml. The results of this study suggest that release of mast cell proteases could contribute to myelin damage in both the PNS and CNS, and that subsequent release of P2 or MBP or their breakdown products could potentiate further mast cell degranulation.

Complement depletion suppresses Lewis rat experimental allergic neuritis

Lewis rats immunized with myelin and complete Freund's adjuvant were treated with cobra venom factor (CVF) which depletes the C3 component of complement. CVF given at day 9 delayed the onset of experimental allergic neuritis (EAN) by 2-3 days and when given at days 9 and 12 delayed the onset of EAN by 4-5 days. Lumbar nerve roots of CVF-treated rats had significantly less demyelination than those from control EAN rats.