The future use of complement inhibitors for the treatment of neurological diseases

Involvement of Complement in Alzheimer's Disease: From Genetics Through Pathology to Therapeutic Strategies

Complement is a critical component of innate immunity, evolved to defend against pathogens and clear toxic debris ranging from dead and dying cells to immune complexes. These roles make complement a key player in homeostasis; however, complement has a dark side. When the rigid control mechanisms fail, complement becomes dysregulated, acting as a driver of inflammation and resultant pathology in numerous diseases. Roles of complement in Alzheimer's disease (AD) and other dementias have emerged in recent years, supported by genetic, biomarker and pathological evidence and animal model studies. Numerous questions remain regarding the precise roles of complement in the brain in health and disease, including where and when complement is expressed, how it contributes to immune defence and garbage disposal in the healthy brain, and exactly how complement contributes to pathology in dementias. In this brief review, we will summarise current knowledge on complement roles in brain, present the evidence implicating complement in AD and explore whether complement represents an attractive therapeutic target for AD.

Spontaneous complement activation on human B cells results in localized membrane depolarization and the clustering of complement receptor type 2 and C3 fragments

While our previous studies have demonstrated that complement activation induced by complement receptors type 2 (CR2/CD21) and 1 (CR1/CD35) results in C3-fragment deposition and membrane attack complex (MAC) formation in human B cells, the consequences of these events for B-cell functions remain unknown. In the present study, we show that CR2-induced complement activation results in membrane depolarization, as indicated by annexin V binding, with kinetics similar to those of C3-fragment deposition and different from those of MAC formation. On the other hand, like MAC formation, depolarization requires activation of complement via the alternative pathway, as indicated by total inhibition upon neutralization of factor D, and is abrogated by combined blockade of CR1 and CR2, but not of either receptor alone. The membrane depolarization is not associated with the apoptosis of B cells, as examined by co-staining with APO-2.7 or by the TdT-mediated biotin-dUTP nick-end labelling (TUNEL) assay. Confocal microscopy revealed that depolarization and C3 deposition, unlike MAC deposition, are limited to restricted areas on the B-cell surface. Double staining revealed a close association between the C3-fragment patches and membrane depolarization, as well as redistribution of lipid rafts to these areas. We propose that these events may play a role in the regulation of B-cell signalling and cross-talk with T cells.

[The relevance of the inflammatory response in the injured brain]

Research efforts in recent years have defined traumatic brain injury (TBI) as a predominantly immunological and inflammatory disorder. This perception is based on the fact that the overwhelming neuroinflammatory response in the injured brain contributes to the development of posttraumatic edema and to neuropathological sequelae which are, in large part, responsible for the adverse outcome. While the "key" mediators of neuroinflammation, such as the cytokine cascade and the complement system, have been clearly defined by studies in experimental TBI models, their exact pathways of interaction and pathophysiological implications remain to be further elucidated. This lack of knowledge is partially due to the concept of a "dual role" of the neuroinflammatory response after TBI. This notion implies that specific inflammatory molecules may mediate diverse functions depending on their local concentration and kinetics of expression in the injured brain. The inflammation-induced effects range from beneficial aspects of neuroprotection to detrimental neurotoxicity. The lack of success in pushing anti-inflammatory therapeutic concepts from"bench to bedside" for patients with severe TBI strengthens the further need for advances in basic research on the molecular aspects of the neuroinflammatory network in the injured brain. The present review summarizes the current knowledge from experimental studies in this field of research and discusses potential future targets of investigation.

CD4+CD25- effector T-cells inhibit hippocampal long-term potentiation in vitro

During neuroinflammation T-cells invade the CNS, and may lead to the development and progression of several pathologies, of which multiple sclerosis is the most common. In these pathologies neuroinflammation is often associated with cognitive dysfunction. Using mouse hippocampal slices, we show here that CD4(+)CD25(-) T-cells inhibit long-term potentiation (LTP) induced by high-frequency stimulation. The T-cell-mediated inhibition of LTP can be prevented by blockade of gamma-aminobutyric acid (GABA)(A) receptors. These findings provide additional insight into the multiple functions of T-cells in CNS pathologies.

Closed head injury--an inflammatory disease?

Closed head injury (CHI) remains the leading cause of death and persisting neurological impairment in young individuals in industrialized nations. Research efforts in the past years have brought evidence that the intracranial inflammatory response in the injured brain contributes to the neuropathological sequelae which are, in large part, responsible for the adverse outcome after head injury. The presence of hypoxia and hypotension in the early resuscitative period of brain-injured patients further aggravates the inflammatory response in the brain due to ischemia/reperfusion-mediated injuries. The profound endogenous neuroinflammatory response after CHI, which is phylogenetically aimed at defending the intrathecal compartment from invading pathogens and repairing lesioned brain tissue, contributes to the development of cerebral edema, breakdown of the blood-brain barrier, and ultimately to delayed neuronal cell death. However, aside from these deleterious effects, neuroinflammation has been recently shown to mediate neuroreparative mechanisms after brain injury as well. This "dual effect" of neuroinflammation was the focus of extensive experimental and clinical research in the past years and has lead to an expanded basic knowledge on the cellular and molecular mechanisms which regulate the intracranial inflammatory response after CHI. Thus, head injury has recently evolved as an inflammatory and immunological disease much more than a pure traumatological, neurological, or neurosurgical entity. The present review will summarize the so far known mechanisms of posttraumatic neuroinflammation after CHI, based on data from clinical and experimental studies, with a special focus on the role of pro-inflammatory cytokines, chemokines, and the complement system.

Early complement activation increases in the brain in some aged normal subjects

Complement activation is increased in Alzheimer's disease (AD) and may contribute to the development and progression of this disorder. To compare early complement activation between normal and AD brain specimens, C4d and iC3b concentrations were measured in hippocampus, entorhinal cortex, temporal cortex, parietal cortex, and cerebellum from aged normal and AD subjects n=10-14 for both), and in hippocampus and entorhinal cortex from younger normal subjects (n=5-6). C4d and iC3b levels increased 2.3- to 4.6-fold in AD versus aged normal specimens (all P <0.05), with lowest concentrations of these activation proteins generally in cerebellum. No significant differences were present between aged and younger normal C4d and iC3b levels in hippocampus or entorhinal cortex. However, the concentrations of these proteins were markedly increased in several aged normal specimens. Normal subject age was moderately associated with both C4d (r=0.49) and iC3b (r=0.53) concentrations in the hippocampus. Increased brain complement activation in some elderly individuals may promote the subsequent development of AD.

Synthesis and complement inhibitory activity of B/C/D-ring analogues of the fungal metabolite 6,7-diformyl-3',4',4a',5',6',7',8',8a'-octahydro-4,6',7'-trihydroxy- 2',5',5',8a'-tetramethylspiro[1'(2'H)-naphthalene-2(3H)-benzofuran]

This paper reports the synthesis and the bioassay of 4-methoxy- and 4-hydroxyspiro[benzofuran-2(3H)-cyclohexane] partial analogues (5) of the complement inhibitory sesquiterpene fungal metabolite 6,7-diformyl-3',4',4a',5',6',7',8',8a'-octahydro-4,6',7'-trihydroxy-2',5',5',8a'-tetramethylspiro[1'(2'H)-naphthalene-2(3H)-benzofuran] (1a, K-76) and its silver oxide oxidized product (1b, K-76COOH). The described target compounds represent spirobenzofuran B/C/D-ring analogues lacking the A-ring component of the prototype structure. The target compounds were evaluated by the inhibition of total hemolytic complement activity in human serum. It was observed that the structurally simplified analogue 4-methoxyspiro[benzofuran-2(3H)-cyclohexane]-6-carboxylic acid (5a) exhibited an IC(50) = 0.53 mM similar to the IC(50) = 0.57 mM that was observed for the natural product derivative 1b. Exhibiting an IC(50) = 0.16 mM, the three-ringed partial structure 6-carboxy-7-formyl-4-methoxyspiro[benzofuran-2(3H)-cyclohexane] (5k)was found to be the most potent target compound. Like the natural product, 5k appears to inhibit primarily at the C5 activation step and inhibits both the classical and alternative human complement pathways. Several other analogues inhibited complement activation in vitro at concentrations similar to those required for inhibition by the natural product 1b.

In vivo pharmacokinetics of calreticulin S-domain, an inhibitor of the classical complement pathway

Inhibition of the complement system is potentially therapeutic in diseases where uncontrolled or overshooting complement activation plays a significant role in the pathogenesis of the disorder. Calreticulin (CRT) is a multifunctional protein whose cell-surface form (ectocalreticulin) is reported to be a C1q receptor. A 124-residue domain within CRT, the S-domain, binds to C1q, prevents the formation of C1 and so inhibits activation of the classical pathway. To assess the usefulness of CRT S-domain as a complement inhibitor, recombinant S-domain was expressed, radiolabeled, and the fate of the radiolabeled peptide followed in vivo. In rats, CRT-S-domain shows a half-life of 1.21 +/- 0.34 and 40.5 +/- 2.7 min in the distribution and elimination phases from plasma, respectively. The peptide remains largely intact, and is cleared from the circulation by the kidneys, where it accumulates in the proximal convoluted tubules, but is not excreted. Much smaller amounts of the peptide accumulate in other tissues, and essentially none crosses the blood-brain barrier.

Microglial activation, emergence of ED1-expressing cells and clusterin upregulation in the aging rat CNS, with special reference to the spinal cord

With advancing age, the incidence of neuronal atrophy and dystrophy increases and, in parallel, behavioural sensorimotor impairment becomes overt. Activated microglia has been implicated in cytotoxic and inflammatory processes in neurodegenerative diseases as well as during aging. Here we have used immunohistochemistry and in situ hybridization to examine the expression of OX42, ED1, ED2, GFAP and clusterin in CNS of young adult and behaviourally tested aged rats (30-month-old), to study the occurrence of activated microglia/ED1 positive macrophages in senescence and to what extent this correlates with astrogliosis and signs of sensorimotor impairment among the individuals. The results show a massive region-specific increase in activated microglia and ED1 expressing cell profiles in aged rats. The infiltration was most prominent in the spinal cord dorsal columns, including their sensory relay nuclei, and the outer portions of the lateral and ventral columns. At such sites the occurrence of macrophages coincided with increased levels of GFAP and positive correlations were evident between the labeling for, on the one hand, OX42 and, on the other, GFAP and ED1. Also, the ventral and dorsal roots were heavily infiltrated by ED1 positive cells. The signs of gliosis were most pronounced among aged rats with advanced sensorimotor impairment. In contrast, the grey matter of aged rats showed very few activated microglia/ED1 labeled cells despite signs of focal astrogliosis. ED2 expression was confined to perivascular cells and leptominges with a similar labeling pattern in young and aged rats. In aged rats increased expression of clusterin was observed in GFAP-immunoreactive profiles of the white matter only. It is suggested that this increase may reflect a response to degenerative/inflammatory processes.

Alzheimer disease and neuroinflammation

It is now generally accepted that the lesions of Alzheimer disease (AD) are associated with a host of inflammatory molecules, including complement proteins, as well as with many activated microglia. Most inflammatory components are synthesized by brain cells. In order to estimate the intensity of the inflammatory reaction, we have measured the levels of the mRNAs for complement proteins, two complement regulators (CD59 and C1 inhibitors), an acute phase reactant (C-reactive protein, CRP) and two microglial markers, (HLA-DR and CD11b), in normal and AD brain. The mRNAs for inflammatory mediators are markedly upregulated in AD tissue while those of the complement inhibitors are almost unchanged. The upregulations for CRP and CD11b in AD hippocampus are comparable to those in osteoarthritic joints. This lends further support to the hypothesis that chronic inflammation may be causing neuronal death in AD.

Efficient transduction of neural cells in vitro and in vivo by a baculovirus-derived vector

Gene delivery to the central nervous system is central to the development of gene therapy for neurological diseases. We developed a baculovirus-derived vector, the Bac-CMV-GFP vector, containing a reporter gene encoding for the green fluorescent protein (GFP) under the control of the cytomegalovirus (CMV) promoter. Two neuroblastomal cell lines and three human primary neural cultures could be efficiently transduced. In all cases, addition of butyrate, an inhibitor of histone deacetylase, increased the level of expression in terms of the number of GFP-expressing cells and the intensity of fluorescence. The level of expression in a human telencephalic culture was over 50% of transduced cells with a multiplicity of infection of 25. GFP expression was demonstrated to be genuine expression and not pseudotransduction of the reporter protein. Most interestingly, Bac-CMV-GFP could transduce neural cells in vivo when directly injected into the brain of rodents and was not inactivated by the complement system. Thus, baculovirus is a promising tool for gene transfer into the central nervous system both for studies of the function of foreign genes and the development of gene therapy strategies.

Intracerebral complement C5a receptor (CD88) expression is regulated by TNF and lymphotoxin-alpha following closed head injury in mice

The anaphylatoxin C5a is a potent mediator of inflammation in the CNS. We analyzed the intracerebral expression of the C5a receptor (C5aR) in a model of closed head injury (CHI) in mice. Up-regulation of C5aR mRNA and protein expression was observed mainly on neurons in sham-operated and head-injured wild-type mice at 24 h. In contrast, in TNF/lymphotoxin-alpha knockout mice, the intracerebral C5aR expression remained at low constitutive levels after sham operation, whereas it strongly increased in response to trauma between 24 and 72 h. Interestingly, by 7 days after CHI, the intrathecal C5aR expression was clearly attenuated in the knockout animals. These data show that the posttraumatic neuronal expression of the C5aR is, at least in part, regulated by TNF and lymphotoxin-alpha at 7 days after trauma.

Complement regulators C1 inhibitor and CD59 do not significantly inhibit complement activation in Alzheimer disease

Proteins characteristic of activated complement are associated with Alzheimer disease (AD) lesions. The classical complement pathway can be activated only when the influence of such endogenous regulators as C1-inhibitor (C1-inh) and CD59 are overcome. We used the techniques of reverse transcriptase-polymerase chain reaction and Western blotting to assess the mRNA and protein levels of C1-inh and CD59 in AD and control brains in comparison with levels of the complement components with which they interact. The inhibitors were only slightly upregulated and then only in heavily affected areas of AD brain such as the entorhinal cortex, hippocampus, midtemporal gyrus and midfrontal gyrus. The ratio of AD to control mRNAs in these four areas was 1.17 for C1-inh and 1.12 for CD59, compared to 3.06 for C1r, 2.67 for C1s, 2.35 for C5, 2.56 for C6, 2.42 for C7, 5. 08 for C8 and 16.3 for C9. Peripheral organ expression of C1-inh and CD59 mRNAs was no different in AD than controls but was slightly upregulated in infarcted heart tissue. Again, the increase was small compared with that of the competitive complement components. These data indicate that the forces which upregulate and activate complement in AD and myocardial infarction are not effectively suppressed by the endogenous regulators, C1-inh and CD59.

The importance of inflammatory mechanisms in Alzheimer disease

Lesions in such chronic neurodegenerative disorders as Alzheimer disease (AD), Parkinson disease, the parkinsonism dementia complex of Guam, and amyotrophic lateral sclerosis have associated with them a variety of proteins known to be involved in inflammatory processes. This is particularly true of AD, where inflammatory reactions are thought to be important contributors to the neuronal loss. Proteins present include complement proteins, complement inhibitors, acute phase reactants, inflammatory cytokines, proteases, and protease inhibitors. Studies of cultured human astrocytes and microglia, obtained from postmortem brain, have established that nearly all of these proteins are produced by one or another of these cell types. Human neurons also produce many inflammatory proteins and their inhibitors, creating complex interactions. Accumulations of amyloid and extracellular tangles apparently act as irritants, causing the activation of complement, the initiation of reactive changes in microglia, and the release of potentially neurotoxic products. Such products include the membrane attack complex, oxygen free radicals, and excess glutamate. Twenty epidemiological studies that have been published to date indicate that populations taking antiinflammatory drugs have a significantly reduced prevalence of AD or a slower mental decline. One small clinical trial with indomethacin showed arrest of the disease over a six-month period. Therapeutic intervention in key inflammatory processes holds great promise for the amelioration of AD and possibly other neurodegenerative disorders.