The human complement fragment receptor, C5L2, is a recycling decoy receptor

Area of Science:

• Immunology
• Cell Biology
• Biochemistry

Background:

• The complement system fragment C5a is a potent anaphylatoxin.
• C5a exerts its effects through two receptors: C5aR and C5L2.
• While C5aR is a known signaling receptor, C5L2's function remains less understood, with some evidence suggesting a signaling role.

Purpose of the Study:

• To investigate the signaling capacity of human C5L2.
• To determine if human C5L2 can couple to G proteins.
• To elucidate the primary function of human C5L2 in the context of C5a binding.

Main Methods:

• Mutagenesis of human C5L2 to incorporate G protein activation motifs from C5aR.
• Measurement of intracellular calcium (Ca2+) flux and beta-arrestin redistribution in response to mutated C5L2.
• Analysis of C5L2 internalization, C5a binding, and ligand degradation using live-cell imaging and biochemical assays.
• Comparison of C5L2 and C5aR internalization kinetics and ligand fate.

Main Results:

• Mutated C5L2 did not exhibit intracellular Ca2+ response or beta-arrestin redistribution, indicating a complete lack of G protein coupling.
• Human C5L2 demonstrated constitutive, ligand-independent internalization via a clathrin-dependent pathway.
• Internalization led to the accumulation and degradation of C5a and C5a des Arg within cells.
• C5aR showed slower internalization with significant ligand release, unlike C5L2.

Conclusions:

• Human C5L2 lacks the capacity for G protein-mediated signaling.
• A primary function of human C5L2 is the efficient clearance of active complement fragments from the extracellular milieu.
• C5L2 acts as a scavenger receptor, removing C5a and its metabolites through internalization and degradation.