News and Publications

Role of Inflammatory Processes in Alzheimer's Disease

B.M. Bradt, S. O'Barr, J.X. Yu, N.R. Cooper

Alzheimer's disease is the most common cause of senile dementia, a symptom complex that involves the age-related progressive loss of cognitive functions. Pathologically, the disease is characterized by the presence of large numbers of neuritic (senile) plaques in the hippocampus and neocortex, regions of the brain concerned with cognition and memory. The plaques are largely composed of a highly insoluble fibrillar form of a 40- to 43-residue ß-amyloid peptide (Aß). Damaged (dystrophic) neuronal processes, loss of neurons and synapses, increased numbers of activated (reactive) astrocytes and microglia, components of the proinflammatory complement system, and proinflammatory cytokines are also associated with neuritic plaques.

These findings suggested that inflammatory processes involving the complement system and proinflammatory cytokines could mediate neuronal damage and activation and influx of glial cells into the area surrounding neuritic plaques. For complement to be biologically relevant to the progression of Alzheimer's disease, the Aß peptide must be able to activate complement. We used novel experimental approaches to show that the fibrillar Aß peptide efficiently activates both pathways of the complement system, leading to the formation of covalent complexes composed of Aß and activation fragments of C3, the third component of complement. Complement activation by Aß triggered the generation of C5a, a cytokine-like activation product of C5, and led to formation of the C5b-9 membrane attack complex. C5b-9 generated by Aß-mediated complement activation was functionally competent; it could insert into neuronal precursor cell membranes and make the cells permeable to small molecules. These findings provide potential inflammation-based mechanisms to account for the presence of complement in neuritic plaques, for damage to neurons in the vicinity of plaques, and for the influx and activation of glial cells.

We have begun studies to evaluate interactions between complement activation products and proinflammatory cytokines. To this end, we examined the effects of C5a and Aß on human THP-1 cells, as a surrogate for microglia. C5a and Aß together stimulated a marked 5- to 8-fold increase in IL-1 and IL-6 secretion, whereas neither C5a nor Aß alone mediated the release of either cytokine. These findings provide potential mechanisms for the presence of proinflammatory cytokines in neuritic plaques.

Several effects of human C5a on human neuroblastoma cells in culture have been determined. First, and unexpectedly, we found that the human neuronal cell lines have receptors for C5a. Second, human C5a protected the undifferentiated neuroblastoma cells from Aß-mediated cytotoxic effects in a dose-dependent manner, a finding that could be relevant to the acute neurotoxicity of fibrillar Aß in vitro but not in vivo. Third, human C5a is mitogenic for undifferentiated human neuroblastoma cells in vitro.

Studies in a murine transgenic model of Alzheimer's disease have been started in collaboration with K. Hsiao, University of Minnesota, Minneapolis. To this end, we developed potent antibodies to murine C3 by immunizing rabbits with purified recombinant C3 generated in Escherichia coli. Expression and distribution of C3 in the brains of older transgenic mice were evaluated immunohistochemically. Murine C3 was associated with neuritic plaques in the hippocampus and neocortex of the transgenic mice, findings consistent with complement activation by fibrillar Aß in neuritic plaques in the brains of the transgenic mice.

B-Cell Apoptosis Triggered by Antigen Receptors

W. Chen, N.R. Cooper

Lymphocyte development, maturation, and differentiation depend on intracellular signaling pathways triggered by ligation of antigen receptors on the cell surface. The responses that ensue depend on the maturation state of the cell, the intensity of the stimulus, and the simultaneous engagement of various additional cell-surface receptors. Our recent studies focus on the constituents of the intracellular signaling pathway that mediate apoptosis of B cells after ligation of the B-cell antigen receptor.

Antigen-specific apoptosis of B cells is important in negative selection during B-cell development and maturation. For these studies, we have used an Epstein-Barr virus--negative human B-cell lymphoma cell line with a mature phenotype (B104). We found that apoptosis of B104 cells after ligation of the B-cell antigen receptor depends on activation of caspase-3, but that upstream events do not involve activation of caspase-8 or caspase-1. The pathway thus differs from the well-studied Fas and TNF apoptotic pathways. Caspase-3 activation and apoptosis of B104 cells after ligation of the B-cell antigen receptor depend on release of cytochrome c from mitochondria into the cytosol and on activation of an unidentified caspase.

PUBLICATIONS

Bradt, B., Kolb, W.P., Cooper, N.R. Complement dependent pro-inflammatory properties of the Alzheimer's disease ß-peptide. J. Exp. Med., in press.

Cooper, N.R. Biology of the complement system. In: Inflammation: Basic Principles and Clinical Correlates, 3rd ed. Gallin, J., Snyderman, R. (Eds.). Lippincott-Raven, Philadelphia, in press.

Cooper, N.R. Complement and viruses. In: The Human Complement System in Health and Disease. Volanakis, J., Frank, M. (Eds.). Marcel Dekker, New York, 1998, p. 393.

Cooper, N.R. Complement dependent virus neutralization. In: The Complement System. Rother, K., Till, G.O., Hänsch, G.M. (Eds.). Springer-Verlag, New York, 1997, p. 302.

Cooper, N.R. Evasion of complement mediated damage by microorganisms. In: The Complement System. Rother, K., Till, G.O., Hänsch, G.M. (Eds.). Springer-Verlag, New York, 1997, p. 309.

Roberts, L., Cooper, N.R. Activation of a Ras-MAPK dependent pathway by Epstein-Barr virus latent membrane protein 1 is essential for cellular transformation. Virology 240:93, 1998.

Sugano, N., Roberts, M.L., Cooper, N.R. EBV binding to CD21 activates the initial viral promoter via NF-B induction. J. Exp. Med. 186:731, 1997.

Webster, S., Lue, L.-F., Brachova, L., Tenner, A., McGeer, P., Walker, D., Bradt, B., Cooper, N.R., Rogers, J. Molecular and cellular characterization of the membrane attack complex, C5b-9, in Alzheimer's disease. Neurobiol. Aging 18:415, 1997.