Macrophage activation and differentiation signals regulate schlafen-4 gene expression: evidence for Schlafen-4 as a modulator of myelopoiesis.

BACKGROUND: The ten mouse and six human members of the Schlafen (Slfn) gene family all contain an AAA domain. Little is known of their function, but previous studies suggest roles in immune cell development. In this report, we assessed Slfn regulation and function in macrophages, which are key cellular regulators of innate immunity. METHODOLOGY/PRINCIPAL FINDINGS: Multiple members of the Slfn family were up-regulated in mouse bone marrow-derived macrophages (BMM) by the Toll-like Receptor (TLR)4 agonist lipopolysaccharide (LPS), the TLR3 agonist Poly(I∶C), and in disease-affected joints in the collagen-induced model of rheumatoid arthritis. Of these, the most inducible was Slfn4. TLR agonists that signal exclusively through the MyD88 adaptor protein had more modest effects on Slfn4 mRNA levels, thus implicating MyD88-independent signalling and autocrine interferon (IFN)-ß in inducible expression. This was supported by the substantial reduction in basal and LPS-induced Slfn4 mRNA expression in IFNAR-1⁻/⁻ BMM. LPS causes growth arrest in macrophages, and other Slfn family genes have been implicated in growth control. Slfn4 mRNA levels were repressed during macrophage colony-stimulating factor (CSF-1)-mediated differentiation of bone marrow progenitors into BMM. To determine the role of Slfn4 in vivo, we over-expressed the gene specifically in macrophages in mice using a csf1r promoter-driven binary expression system. Transgenic over-expression of Slfn4 in myeloid cells did not alter macrophage colony formation or proliferation in vitro. Monocyte numbers, as well as inflammatory macrophages recruited to the peritoneal cavity, were reduced in transgenic mice that specifically over-expressed Slfn4, while macrophage numbers and hematopoietic activity were increased in the livers and spleens. CONCLUSIONS: Slfn4 mRNA levels were up-regulated during macrophage activation but down-regulated during differentiation. Constitutive Slfn4 expression in the myeloid lineage in vivo perturbs myelopoiesis. We hypothesise that the down-regulation of Slfn4 gene expression during macrophage differentiation is a necessary step in development of this lineage.

Beta-arrestin 2 is required for complement C1q expression in macrophages and constrains factor-independent survival.

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.

Expression analysis of G Protein-Coupled Receptors in mouse macrophages.

BACKGROUND: Monocytes and macrophages express an extensive repertoire of G Protein-Coupled Receptors (GPCRs) that regulate inflammation and immunity. In this study we performed a systematic micro-array analysis of GPCR expression in primary mouse macrophages to identify family members that are either enriched in macrophages compared to a panel of other cell types, or are regulated by an inflammatory stimulus, the bacterial product lipopolysaccharide (LPS). RESULTS: Several members of the P2RY family had striking expression patterns in macrophages; P2ry6 mRNA was essentially expressed in a macrophage-specific fashion, whilst P2ry1 and P2ry5 mRNA levels were strongly down-regulated by LPS. Expression of several other GPCRs was either restricted to macrophages (e.g. Gpr84) or to both macrophages and neural tissues (e.g. P2ry12, Gpr85). The GPCR repertoire expressed by bone marrow-derived macrophages and thioglycollate-elicited peritoneal macrophages had some commonality, but there were also several GPCRs preferentially expressed by either cell population. CONCLUSION: The constitutive or regulated expression in macrophages of several GPCRs identified in this study has not previously been described. Future studies on such GPCRs and their agonists are likely to provide important insights into macrophage biology, as well as novel inflammatory pathways that could be future targets for drug discovery.