Wolfgang Brück

Neuronal injury in bacterial meningitis: mechanisms and implications for therapy

In bacterial meningitis, long-term neurological sequelae and death are caused jointly by several factors: (1) the systemic inflammatory response of the host, leading to leukocyte extravasation into the subarachnoid space, vasculitis, brain edema and secondary ischemia; (2) stimulation of resident microglia within the CNS by bacterial compounds; and (3) possible direct toxicity of bacterial compounds on neurons. Neuronal injury is mediated by the release of reactive oxygen intermediates, proteases, cytokines and excitatory amino acids, and is executed by the activation of transcription factors, caspases and other proteases. In experimental meningitis, dexamethasone as an adjunct to antibiotic treatment leads to an aggravation of neuronal damage in the hippocampal formation, suggesting that corticosteroids might not be the ideal adjunctive therapy. Several approaches that interfere selectively with the mechanisms of neuronal injury are effective in animal models, including the use of nonbacteriolytic protein synthesis-inhibiting antibiotics, antioxidants and inhibitors of transcription factors, matrix metalloproteinases, and caspases.

Multiple sclerosis: Recent developments in neuropathology, pathogenesis, magnetic resonance imaging studies and treatment

The cause of multiple sclerosis is generally considered to be entirely T cell mediated. However, recent reports of studies in a variety of animal models of inflammatory demyelinating disease, coupled with detailed pathological analysis and neuroimaging studies of multiple sclerosis patients, indicate that the events involved in the formation of the multiple sclerosis lesion may be more complicated. This complex pathogenesis is reflected in the variable response of multiple sclerosis patients to immunomodulatory therapy.

Early expression of glutamate transporter proteins in ramified microglia after controlled cortical impact injury in the rat

Traumatic brain injury is followed by increased extracellular glutamate concentration. Uptake of glutamate is mainly mediated by the glial glutamate transporters GLAST and GLT‐1. Extent and distribution of GLAST and GLT‐1 were studied in a rat model of controlled cortical impact injury (CCII). Western Blot analysis revealed lowest levels of GLAST and GLT‐1 with a decrease by 40%–54% and 42%–49% between 24 and 72 h posttrauma. By 8 h after CCII, CSF glutamate levels were increased (10.5 μM vs. 2.56 μM in controls; P < 0.001), reaching maximum values by 48 h. A significant increase in de novo GLAST and GLT‐1 expressing ramified microglia was observed within 4 h, reached a stable level by 48 h, and remained high up to 72 h after CCII. Furthermore, ramified microglia de novo expressed the neuronal glutamate transporter EAAC1 after CCII. Following CCII, GLAST/GLT‐1 and GFAP coexpressing astrocytes were immediately reduced, reaching minimum levels within 8 h. This reduction of expression could be either due to protein downregulation or loss of astrocytes. At 72 h, a marked population of GLAST‐ and GLT‐1–positive reactive astrocytes appeared. These results support the hypothesis that reduced astrocytic GLAST and GLT‐1 protein levels following CCII contribute to evolving secondary injury. Microglia are capable of de novo expressing glutamate transporter proteins, indicating that the expression of glial and neuronal glutamate transporters is not restricted to a specific glial or neuronal lineage. Ramified microglia may play an important compensatory role in the early regulation of extracellular glutamate after CCII. GLIA 35:167–179, 2001.

Selective neuronal vulnerability following mild focal brain ischemia in the mouse.

The evolution of cellular damage over time and the selective vulnerability of different neuronal subtypes was characterized in the striatum following 30‐minute middle cerebral artery occlusion and reperfusion in the mouse. Using autoradiography we found an increase in the density of [3H]PK11195 binding sites—likely reflecting microglial activation—in the lesion border at 3 days and in the whole striatum from 10 days to 6 weeks. This was accompanied by a distinct loss of [3H]flumazenil and [3H]CGP39653 binding sites from 10 days up to 6 weeks reflecting neuronal loss. Brain ischemia resulted in a substantial loss of medium spiny projection neurons as seen at three days by Nissl staining, TUNEL and immunocytochemistry using antibodies against microtubule‐associated protein (MAP2), NeuN, (μ‐opioid receptors, substance P, Lenkephalin, neurokinin B, choline acetyltransferase, parvalbumin, calretinin and somatostatin. Both patch and matrix compartments were involved in ischemic damage. In contrast, the numbers of cholinergic, GABAergic, and somatostatin‐containing interneurons in the ischemic striatum were not different from those in the contralateral hemisphere at 3 and 14 days. A low density of glutamate receptors, the ability to sequester calcium by calcium‐binding proteins and other hitherto unidentified factors may explain this relative resistance of interneurons to acute ischemia.

Neuronal Apoptosis in the Dentate Gyrus in Humans with Subarachnoid Hemorrhage and Cerebral Hypoxia

Apoptosis of dentate granule cells is a typical feature of several animal models of disease. In 20 autopsy cases of subarachnoid hemorrhage (SAH) and global cerebral hypoxia caused by protracted shock or respiratory failure, we evaluated by light microscopy and in situ tailing whether this pattern of neuronal damage also occurs in humans. In subarachnoid hemorrhage, 4.0/mm2 (0–13.0/mm2) a poptotic neurons were observed in the dentate gyrus, in cerebral hypoxia 3.6/mm2 (0–19.9/mm2) (p>0.05), and in 10 aged‐matched control cases dying rapidly from non‐neurological diseases 0/mm2 (0–0/mm2) (median [range]) (p<0.001 versus SAH and hypoxia). Neuronal apoptosis in the dentate gyrus was most frequent, when death occurred later than 24 hours and less than 11 days after disease onset. Neuronal damage in the hippocampus was always necrotic. It was more severe in hypoxia than in SAH (median neuronal damage score 3 [range: 0–3] versus 0 [0–3], p<0.001).

Adult-onset Leukoencephalopathy with vanishing white matter presenting with dementia.

We report on a case of dementia and extensive cerebral white matter abnormalities seen on magnetic resonance images which meet the criteria for leukoencephalopathy with vanishing white matter. This is an inherited condition that was first thought to occur only in children. Our patient shows that vanishing white matter should be considered in adult patients with early‐onset dementia and extensive white matter changes seen on magnetic resonance images.

Differential regulation of myelin phagocytosis by macrophages/microglia, involvement of target myelin, Fc receptors and activation by intravenous immunoglobulins

Macrophages/microglia are the key effector cells in myelin removal. Differences exist in the amount and time course of myelin uptake in the central (CNS) and peripheral nervous system (PNS), the basis of this difference, however, is not yet clarified. In the present experiments we studied the phagocytosis rate of CNS or PNS myelin by macrophages and microglia in vitro. Additionally, the effects of intravenous immunoglobulins (IVIg) on this process were investigated. In the PNS experiments, sciatic nerves were cocultured with peritoneal macrophages. Optic nerve fragments were used to characterize the myelin‐removing properties of microglia. Cocultures with peritoneal macrophages aimed at investigating the differences in phagocytosis between resident microglia and added macrophages. The myelin phagocytosis in sciatic nerve fragments was higher than in optic nerves, indicating differences in the myelin uptake rate between peripheral macrophages and microglia. IVIg increased the phagocytosis of PNS myelin by macrophages, but not by microglia in optic nerves. The addition of peritoneal macrophages to optic nerve fragments did not lead to an increase in the phagocytosis of CNS myelin either. The IVIg induced phagocytosis of PNS myelin by peripheral macrophages was associated with an increased expression of macrophage Fc receptors measured by FACS. Blocking of Fc receptors by anti‐Fc receptor antibody reduced the IVIg induced PNS myelin phagocytosis to basic levels, indicating that the induced but not the basic myelin uptake by macrophages is Fc receptor dependent. In contrast to peripheral macrophages, IVIg did not increase Fc receptor density on microglia. These data indicate that phagocytosis of PNS and CNS myelin by macrophages or microglia is differentially regulated. Local factors within the CNS or PNS may affect this process by modulating the surface receptor profile and activation state of the phagocytic cell or the structure of the myelin sheath.

Myelinopathia centralis diffusa (vanishing white matter disease): Evidence of apoptotic oligodendrocyte degeneration in early lesion development

We describe histopathological changes in a 2‐year‐old boy who died from myelinopathia centralis diffusa. Despite extensive white matter destruction, surprisingly high numbers of oligodendrocytes expressing proteolipid protein mRNA were detected. In an active demyelinating lesion in the brainstem, oligodendrocytes showed typical signs of apoptosis. We suggest that death of mature oligodendrocytes is the critical event in the disease.

Expression of death-related proteins in dentate granule cells in human bacterial meningitis.

Neuronal apoptosis in the dentate gyrus has been observed in animal models of bacterial meningitis and in humans dying in the course of the disease. To evaluate the mechanisms of neuronal cell death, hippocampal sections of 20 patients dying from bacterial meningitis were investigated by immunohistochemistry using antibodies against the proform of caspase‐3 and the active enzyme, bcl‐2, bax and p53. In the dentate granule cell layer, the median density of neurons with an apoptotic morphology was 7.6/mm2 (0–15.6/mm2). The median density of immunoreactive neurons was 2.3/mm2 (procaspase‐3), 0.9/mm2 (activated caspase‐3), 1.8/mm2 (bcl‐2), 1.1/mm2 (bax) and 0.4/mm2 (p53). 80% of neurons immunoreactive for active caspase‐3 had an apoptotic morphology, whereas only 10% of all procaspase‐3 stained neurons showed signs of apoptosis. Apoptotic cell death is present in humans dying in the course of bacterial meningitis in the dentate gyrus of the Formatio hippocampi. Neuronal expression of caspase‐3, bcl‐2 and bax suggests an involvement of these proteins in neuronal death.

Immunoglobulins induce increased myelin debris clearance by mouse macrophages.

The present study investigated the effect of human immunoglobulins on migration and myelin phagocytosis by macrophages. Mouse sciatic nerves and macrophages were cocultured and treated with 1, 10 and 20 mg/ml immunoglobulins for 10 days in vitro. Numbers of invading macrophages, myelin density within the nerves and macrophage myelin load were determined in semithin sections. Human immunoglobulins lead to an increased myelin removal by macrophages as proven by a statistically significant higher myelin load of the macrophage cytoplasm when compared with untreated control macrophages. The results suggest that one possible action of immunoglobulins in demyelinating diseases is an improved clearance of lesional debris with the removal of myelin-associated inhibitory molecules.