The beta-arrestins (ARRB1 and ARRB2) regulate G-protein coupled receptor (GPCR) dependent- and independent-signaling pathways and are ubiquitously expressed. Here we show that ARRB2 mRNA and protein expression is enriched in macrophages, and that it regulates complement C1q expression and cell survival. Basal and Toll-like receptor (TLR) inducible expression of mRNAs encoding the complement subcomponents C1qa, C1qb and C1qc was greatly reduced in bone marrow-derived macrophages (BMM) from ARRB2-deficient, but not ARRB1-deficient mice, while factor-independent survival of ARRB2(-/-) BMM was enhanced compared to wildtype BMM. TatARRB2(23), a cell-permeable peptide that contains the MAPK JNK-binding motif from within the ARRB2 C-domain, impaired ARRB2 interaction with JNK3, down-regulated C1q expression and permitted factor-independent survival in BMM, thus suggesting that this peptide antagonises ARRB2 function in macrophages. In addition, TatARRB2(23) transiently activated the phosphorylation of JNK and ERK, but not p38 in BMM. These data imply that ARRB2 acts to limit JNK/ERK activation and survival in macrophages, but is required for basal and TLR-inducible complement C1q expression. Given that loss of C1q function is strongly associated with the development of systemic lupus erythematosus, ARRB2 may act to limit the development of autoimmune disease.

C5L2 is a 7 transmembrane domain receptor for complement fragment C5a that, unlike the classical C5a receptor, C5aR, does not couple to G proteins. However, in mice where C5L2 has been deleted, the response to C5a is altered, suggesting that C5L2 may have a signaling function. In order to investigate whether human C5L2 also has some capacity to transduce signals, we have attempted to produce a signaling competent form of human C5L2 by inserting C5aR sequences at three key G protein activation motifs. However, we detected neither an intracellular Ca(2+) response nor beta-arrestin redistribution in mutated C5L2, suggesting that the potential for G protein coupling is completely absent in this receptor and that, in humans, C5L2 may have functions that are unrelated to signaling. In confirmation of this, we detected constitutive ligand-independent internalization of C5L2 that resulted in the rapid accumulation of C5a and its stable metabolite, C5a des Arg, within the cell with only a small net change in cell surface receptor levels. Internalization was found to be through a clathrin-dependent mechanism that led to the retention and, in cells natively expressing C5L2, the degradation of the ligand within an intracellular compartment. In contrast, the classical C5a receptor, C5aR, internalized ligand much more slowly and a majority of this ligand was released back into the extracellular environment in an apparently undegraded form. These data suggest that a major function of human C5L2 is to remove active complement fragments from the extracellular environment.