Be on Target: Strategies of Targeting Alternative and Lectin Pathway Components in Complement-Mediated Diseases

Abstract

The complement system has moved into the focus of drug development efforts in the last decade, since its inappropriate or uncontrolled activation has been recognized in many diseases. Some of them are primarily complement-mediated rare diseases, such as paroxysmal nocturnal hemoglobinuria, C3 glomerulonephritis, and atypical hemolytic uremic syndrome. Complement also plays a role in various multifactorial diseases that affect millions of people worldwide, such as ischemia reperfusion injury (myocardial infarction, stroke), age-related macular degeneration, and several neurodegenerative disorders. In this review, we summarize the potential advantages of targeting various complement proteins with special emphasis on the components of the lectin (LP) and the alternative pathways (AP). The serine proteases (MASP-1/2/3, factor D, factor B), which are responsible for the activation of the cascade, are straightforward targets of inhibition, but the pattern recognition molecules (mannose-binding lectin, other collectins, and ficolins), the regulatory components (factor H, factor I, properdin), and C3 are also subjects of drug development. Recent discoveries about cross-talks between the LP and AP offer new approaches for clinical intervention. Mannan-binding lectin-associated serine proteases (MASPs) are not just responsible for LP activation, but they are also indispensable for efficient AP activation. Activated MASP-3 has recently been shown to be the enzyme that continuously supplies factor D (FD) for the AP by cleaving pro-factor D (pro-FD). In this aspect, MASP-3 emerges as a novel feasible target for the regulation of AP activity. MASP-1 was shown to be required for AP activity on various surfaces, first of all on LPS of Gram-negative bacteria.

Keywords: alternative pathway; complement inhibitors; complement system; complement-related diseases; lectin pathway.

Figure 1

Overview of the complement system. The components of all activation phases are listed in gray boxes, while their inhibitors in green boxes nearby. Serine proteases are indicated by red letters. Black arrows indicate the direction of the cascade. Certain enzymatic cleavages are emphasized by red arrows. The three activation routes merge at the cleavage of C3 highlighted by dark yellow background.

Figure 2

Domain structures of the complement components discussed in the article. Each domain type is represented by a different symbol listed at the bottom. Domain abbreviations are as follows: SP, serine protease; CCP, complement control protein; CUB, complement C1r/C1s, sea urchin Uegf and bone morphogenetic protein-1; EGF, epidermal growth factor-like; vWFA, von Willebrand factor type A; FIMAC, factor I/membrane attack complex; CD5, scavenger receptor cysteine-rich domain; LDLr, low-density lipoprotein receptor; MG, α-macroglobulin; TED, thioester domain; ANA, anaphylatoxin; LNK, linker; TSR, thrombospondin-type 1 repeat; TMD, transmembrane domain; CPD, cytoplasmic domain; CRD, carbohydrate recognition domain.

Figure 3

MASP-1 and MASP-3 play roles in the activation of the alternative pathway (AP). The activation process of the AP is divided into several phases, which are indicated by differently colored backgrounds. MASP-3 is proved to be the professional activator of pro-factor D in blood, therefore, plays an important role in the pre-initiation phase. The physiological activator of zymogen MASP-3 (zym MASP-3) is hitherto undiscovered (shown by the question mark). MASP-1 is indispensable for efficient initiation and amplification of the AP on certain surfaces, although the mechanism is yet unknown (shown by dashed arrows). Black arrows represent conversion processes, while red arrows stand for enzymatic reactions pointing from the enzyme toward its substrate. The circle-shaped red arrow symbolizes the autoactivation of MASP-1.

Figure 4

Specific inhibition of proteases requires multiple favorable contacts in a large contact area with an inhibitor. (A) The structure of MASP-1 in complex with a specific small-protein inhibitor, SGMI-1 (PDB entry 4DJZ). SGMI-1 was developed by phage-display (122). Amino acid residues in the randomized positions are colored magenta (P4, P2, P1′, P2′, P4′) and blue (P1). All amino acid residues in the randomized positions have contacts with the protease body; moreover, SGMI-1 has other contact areas with MASP-1 in the non-randomized positions as well. (B) The structure of trypsin in complex with a non-specific small-molecule inhibitor (123) (PDB entry 3LJJ). The inhibitor is based on benzamidine. A two-headed arrow indicates the movement of the terminal cyclopentane moiety, which has two equivalent binding sites.

Figure 5

Structure of factor D (FD) in complex with a selective small-molecule inhibitor. The figure is based on the structure of FD in complex with “inhibitor 6” described by Maibaum et al. (120) (PDB entry 5FCK). Inhibitor 6 has multiple polar and hydrophobic interactions with the protein body. It is notable that inhibitor 6 interacts with the self-inhibited conformation of FD, probably stabilizing FD in this form. Asp¹⁷⁷ (blue) of the S1 pocket forms a salt bridge with Arg²⁰² of the self-inhibitory loop (red). The catalytic triad is colored magenta. Numbers indicate amino acid positions in mature FD, while numbers in parenthesis reflect the traditional chymotrypsinogen numbering. Hydrogen bonds are indicated by yellow dashed lines. (A) FD shown by ribbon representation. (B) FD shown by surface representation.

Figure 6

C3c in complex with a compstatin analog. Compstatin, a cyclic peptide, was developed by phage-display. Since its discovery, several modified compstatin analogs have been developed. Compstatin and its analogs bind to C3, C3b, or C3c between the MG4 and MG5 domains. Compstatin sterically prevents the C3-convertase (C3bBb) to access its substrate C3. The depicted structure was determined using the Ac-V4W/H9A-NH2 variant of the original peptide. The figure was prepared based on the structure by Janssen et al. (170) (PDB entry 2QKI). On the left, the whole structure is shown with C3c (brown) in surface representation and compstatin (magenta) with spheres. On the right, a close-up of the binding site is shown with compstatin represented by sticks. Hydrogen bonds are indicated by yellow dashed lines.

Figure 7

Structure of the complex of C3b, mini factor H (FH), and factor I (FI). Mini FH is a potential drug candidate. In vitro, it is more effective than full-length FH in accelerating the decay of C3b by FI. The structure shows extensive contacts between the three proteins. The figure was made based on the structure of ternary complex of C3b-mini FH-FI (S525A) (184) (PDB entry 5O32). The colors of the legends match the depicted protein chains. The P1 residue (Arg¹³⁰³) of the primary cleavage site in C3b by FI is indicated by sphere representation. Mini FH and the light chain of FI are shown by surface representation, whereas C3b and the heavy chain of FI are shown by ribbon representation.