Sushi domain‐containing protein 4 (SUSD4) inhibits complement by disrupting the formation of the classical C3 convertase

Abstract

Recently discovered Sushi domain-containing protein 4 (SUSD4) contains several Sushi or complement control protein domains; therefore, we hypothesized that it may act as complement inhibitor. Two isoforms of human SUSD4, fused to the Fc part of human IgG, were recombinantly expressed in Chinese hamster ovary (CHO) cells. The secreted soluble isoform of SUSD4 (SUSD4b) inhibited the classical and lectin complement pathways by 50% at a concentration of 0.5 μM. This effect was due to the fact that 1 μM SUSD4b inhibited the formation of the classical C3 convertase by 90%. The membrane-bound isoform (SUSD4a) inhibited the classical and alternative complement pathways when expressed on the surface of CHO cells but not when expressed as a soluble, truncated protein. In all functional studies, we used known complement inhibitors as positive controls, while Coxsackie adenovirus receptor, which has no effect on complement, expressed with Fc tag, was a negative control. We also studied the mRNA expression of both isoforms of SUSD4 in a panel of human tissues using quantitative PCR and primarily found SUSD4a in esophagus and brain, while SUSD4b was highly expressed in esophagus, ovary, and heart. Overall, our results show that SUSD4 is a novel complement inhibitor with restricted expression.

RNA-Seq reveals common and unique PXR- and CAR-target gene signatures in the mouse liver transcriptome

“…In addition, TCPOBOP markedly up-regulated cyclin B1 (Ccnb1) (99-fold), which is important for cell proliferation (Porter and Donoghue, 2003); fibronectin type III domain containing 5 (Fndc5) (94-fold), which is a precursor of the anti-obesity hormone irisin (Bostrom et al, 2012), as well as polo-like kinase 1 (Plk1) (95-fold), which controls several key steps during mitosis (Schoffski, 2009; Strebhardt, 2010). The top most down-regulated gene by TCPOBOP in liver is deleted in malignant brain tumors 1 (Dmbt1) (99.8%), which is important in tissue repair following liver injury (Bisgaard et al, 2002), followed by the hormone-metabolizing enzyme hydroxy-delta-5-steroid-dehydrogenase, 3 beta- and steroid delta-isomerase 5 (Hsd3b5) (99.3%); serine (or cysteine) peptidase inhibitor clade A member 4 pseudogene 1 (Serpina4-ps1) (98.7%), which is a pseudogene that transcribes but does not appear to form a protein; odorant binding protein 2A (Obp2a) (97.1%), which has a high affinity towards binding aldehydes and fatty acids (Tcatchoff et al, 2006); the stress-response metal-binding gene metallothionein Mt2 (97%) (Klaassen et al, 1999); dopachrome tautomerase (Dct) (95%), which is important for melanogenesis (Aroca et al, 1990); as well as sushi domain containing 4 (Susd4) (94.8%), which is a complement inhibitor (Holmquist et al, 2013). In addition, several DPGs such as the uptake transporter Slco1a1 (96.3%), phase-I Ces4a (96.3%), Cyp2c40 (94%), and Cyp4a10 (94%), also rank among the top 10 most down-regulated genes by TCPOBOP in liver.…”

The role of complement inhibitors beyond controlling inflammation

“…The gene coding for SUSD4 is located at chromosome 1q41, which is not far from the cluster of well‐characterized complement inhibitors such as CD46 (1q32.2) or factor H (1q31.3). In humans, expression of the membrane‐bound form of SUSD4 at the mRNA level was detected to the largest degree in brain, oesophagus, kidneys, breast and prostate . In mice, protein expression was detected in brain, eyes, spinal cord and testes .…”

“…In humans, expression of the membrane-bound form of SUSD4 at the mRNA level was detected to the largest degree in brain, oesophagus, kidneys, breast and prostate [41]. In mice, protein expression was detected in brain, eyes, spinal cord and testes [42].…”

“…This is indeed the case but the molecular mechanism of action of SUSD4 is not what one would initially expect. Whereas CD46 binds C3b and acts at the level of C3 convertases, SUSD4 interacts with C1q within the C1 complex and therefore prevents C2 cleavage and formation of the classical C3 convertase, which is composed of C4b and C2a, activated by the C1 complex [41]. Similarly, SUSD4 also inhibited lectin pathway activation triggered by MBL.…”

What does complement do in Alzheimer’s disease? Old molecules with new insights

“…Recently, it was discovered that these two compounds inhibit the C3 convertase of the classical complement pathway [89,90]. Recently Holmquist et al discovered Sushi domain-containing protein 4 (SUSD4) is a novel complement inhibitor and it inhibited the formation of the classical C3 convertase by 90% [91]. …”