A proteolytic modification of AIM promotes its renal excretion.

Tomoko Yamazaki,Zhenghua Li,Katsuhiko Nakashima,Peter S Nelson,Jared M Lucas,Satoko Arai,Ken-Ichi Yamamura,Ryoichi Sugisawa,Ayaka Matsumoto,Yoshie Senda,Emiri Hiramoto,Ryosuke Takai,Toru Miyazaki,Andrew Morgan

doi:10.1038/srep38762

Abstract

Apoptosis inhibitor of macrophage (AIM, encoded by cd5l) is a multi-functional circulating protein that has a beneficial role in the regulation of a broad range of diseases, some of which are ameliorated by AIM administration in mice. In blood, AIM is stabilized by association with IgM pentamers and maintains its high circulating levels. The mechanism regulating the excessive accumulation of blood AIM remains unknown, although it is important, since a constitutive increase in AIM levels promotes chronic inflammation. Here we found a physiological AIM-cleavage process that induces destabilization of AIM and its excretion in urine. In blood, IgM-free AIM appeared to be cleaved and reduced in size approximately 10 kDa. Cleaved AIM was unable to bind to IgM and was selectively filtered by the glomerulus, thereby excreted in urine. Amino acid substitution at the cleavage site resulted in no renal excretion of AIM. Interestingly, cleaved AIM retained a comparable potency with full-length AIM in facilitating the clearance of dead cell debris in injured kidney, which is a key response in the recovery of acute kidney injury. Identification of AIM-cleavage and resulting functional modification could be the basis for designing safe and efficient AIM therapy for various diseases.

Highlights

Apoptosis inhibitor of macrophage (AIM; encoded by cd5l)[1] is a circulating protein belonging to the scavenger receptor cysteine-rich (SRCR) super family, the members of which share different numbers of well conserved cysteine-rich domains (AIM possesses three domains; SRCR1, 2 and 3)[2,3]
We previously reported that when mouse recombinant AIM was injected intravenously into mice, rAIM that did not bind to IgMpentamers was excreted in the urine[8]
We found that the rAIM excreted in urine was reduced in size by approximately 10 kDa compared with the original rAIM as assessed by immunoblotting (Fig. 1a)

Summary

Introduction

Apoptosis inhibitor of macrophage (AIM; encoded by cd5l)[1] is a circulating protein belonging to the scavenger receptor cysteine-rich (SRCR) super family, the members of which share different numbers of well conserved cysteine-rich domains (AIM possesses three domains; SRCR1, 2 and 3)[2,3]. We showed that AIM is incorporated into obese adipocytes and hepatocytes via CD36-mediated endocytosis where it inactivates cytoplasmic fatty acid synthase through direct binding[13] This response reduces the production of lipid droplet-coating proteins such as fat-specific protein 27 and perilipin, thereby decreasing triacylglycerol deposition within adipocytes and hepatocytes[14,15]. Under a cholesterol-rich Western diet, AIM supports the survival of inflammatory macrophages at atherosclerotic regions, resulting in disease acceleration[7] Such detrimental outcomes of high levels of AIM, which were observed in specific disease models with exaggerated diets, have led us to assess whether certain mechanisms preventing the excess accumulation of blood AIM are present. We demonstrate a newly discovered proteolytic modification of AIM which may regulate the physiological blood level of AIM, IgM-free active AIM, to avoid undesired disease occurrence

Methods

Results

Conclusion