Versatility of the complement system in neuroinflammation, neurodegeneration and brain homeostasis

Abstract

The immune response after brain injury is highly complex and involves both local and systemic events at the cellular and molecular level. It is associated to a dramatic over-activation of enzyme systems, the expression of proinflammatory genes and the activation/recruitment of immune cells. The complement system represents a powerful component of the innate immunity and is highly involved in the inflammatory response. Complement components are synthesized predominantly by the liver and circulate in the bloodstream primed for activation. Moreover, brain cells can produce complement proteins and receptors. After acute brain injury, the rapid and uncontrolled activation of the complement leads to massive release of inflammatory anaphylatoxins, recruitment of cells to the injury site, phagocytosis and induction of blood brain barrier (BBB) damage. Brain endothelial cells are particularly susceptible to complement-mediated effects, since they are exposed to both circulating and locally synthesized complement proteins. Conversely, during neurodegenerative disorders, complement factors play distinct roles depending on the stage and degree of neuropathology. In addition to the deleterious role of the complement, increasing evidence suggest that it may also play a role in normal nervous system development (wiring the brain) and adulthood (either maintaining brain homeostasis or supporting regeneration after brain injury). This article represents a compendium of the current knowledge on the complement role in the brain, prompting a novel view that complement activation can result in either protective or detrimental effects in brain conditions that depend exquisitely on the nature, the timing and the degree of the stimuli that induce its activation. A deeper understanding of the acute, subacute and chronic consequences of complement activation is needed and may lead to new therapeutic strategies, including the ability of targeting selective step in the complement cascade.

Keywords: Alzheimer’s disease; brain homeostasis; complement system; endothelium; stroke; therapeutic targets; traumatic brain injury.

Figure 1

The complement system: an overview. Classical pathway (CP): C1q, the CP initiator, recognizes and binds antigen-antibody complexes or specific molecules, including β-amyloid, C reactive protein (CRP), DNA and apoptotic bodies. After binding, the C1r and C1s proteases subsequently cleave C4 and C2 to generate C4a, C4b, C2a, C2b, permitting the formation of C4b2a (CP C3 convertase). This complex cleaves C3 into C3a, which, in turn, acts as potent anaphylatoxin, and C3b that binds to the complex forming the C4b2a3b protein block (CP C5 convertase). Lectin pathway (LP): MBL, ficolin-1, ficolin-2, ficolin-3 and collectin-11, the LP initiators, recognize and bind high-density arrays of mannose, fucose and N-acetylated sugars exposed by pathogens or by self-altered cells. After binding, the MBL-associated serine proteases (MASPs), MASP-1 and -2, associated in complex with the above recognition molecules (MBL/ficolins), cleave C4 and C2, thereby forming C3 convertase and C5 convertase in a similar manner to that of the CP (Ehrnthaller et al., 2011). The role of the third serine protease, called MASP-3, remains unclear (Kjaer et al., 2013). Alternative pathway (AP): the activation of the AP is driven by a spontaneous hydrolysis of circulating C3 (called tick-over process) to form C3(H₂O). This molecule then associates factor B and factor D to form C3bBb (AP C3 convertase). Similar to the C3 convertase generated in the CP and LP, this complex splits C3 into C3a and C3b, with the latter creating a new C3 convertase (AP amplification loop, dotted line), and/or binding C3 convertase already present to create the C3bBb3b complex (AP C5 convertase). Extrinsic pathway: the recent characterization of this activation pathway suggests that it is driven by activated proteolytic enzymes, including thrombin, plasmin, kallikrein, factor XIIa. Thrombin possesses its own C5 convertase activity and, under undefined conditions, has been shown to have the capacity to directly cleave C5 to generate the correspondent active fragments (Huber-Lang et al., 2006). Recently, it has been shown that the coagulation serine proteases are likewise able to cleave C3 (Markiewski et al., ; Amara et al., 2010). Terminal pathway: CP, AP, LP and the extrinsic pathway all converge at C5 convertase formation, activating a common cascade through the cleavage of C5 into the anaphylatoxin C5a and the active C5b. Finally, (1) C3b fragment binds the targeted cell allowing the assembly of C6, C7, C8 and C9 in a pore called membrane attack complex (C5b-9 or MAC), that causes the direct lysis of the cell; (2) many fragments, such as C3b and C4b as well as C5b, work as opsonins triggering an overactivation of the phagocytic response; (3) altogether complement components are able to orchestrate an adaptative immune reaction by communicating with multiple immune cells (Ricklin and Lambris, 2007) through different receptors, leading to a robust local and systemic inflammatory response.

Figure 2

Complement-mediated endothelial damage: working hypothesis. Healthy brain endothelial cells physically isolate the brain parenchyma from the blood compartment. Under physiological conditions, complement proteins, such as C1q, LP initiators, C3 and C5, monitor the blood and the cell surfaces for potential threats (A). The additional complement factors that normally circulate in the bloodstream, have been omitted in this figure for purposes of clarity and simplification. When a cerebrovascular injury occurs (B), endothelial cells may change their glycosylation profile, leading to the exposure of high density arrays of sugar (DAMPs). LP initiators, through their carbohydrate recognition domains, can recognize and bind altered endothelial cells, leading to LP complement activation (B). Complement activated fragments trigger the expression of adhesion molecules and the release of chemokines and cytokines (not shown) on endothelial cells, resulting in the worsening of the BBB leakage (B→C). The injured brain parenchyma is then rapidly invaded by the full immune arsenal, including the complement proteins, cytokines and immune cells (e.g., monocytes) belonging to the blood compartment, leading to amplification of local damage (C). The overactivation of the complement system in brain tissue leads to: (1) a potent inflammatory response and the recruitment of peripheral immune cells mediated by C3a and C5a anaphylatoxins, (2) direct lysis of neurons and other brain cells, including those that are potentially savable, by C5b-9, (3) opsonization with subsequent microglia/macrophage phagocytosis of the target cells by C3b and C5b. This schema proposes a key role for endothelial cells in triggering local LP complement activation and suggests that targeting the peripheral compartment may represent effective strategy for brain protection from injury and different acute CNS diseases.