The molecular basis of inter-α-inhibitor heavy chain transfer on to hyaluronan

The inflammation-associated protein TSG-6 (the product of tumour necrosis factor-stimulated gene-6) can form covalent complexes with the heavy chains (HC1 and HC2) of IαI (inter-α-inhibitor); namely, TSG-6·HC1 and TSG-6·HC2, which act as intermediates in the covalent transfer of HCs on to the GAG (glycosaminoglycan) HA (hyaluronan). HC·HA, which is formed for example in the synovial fluids of arthritis patients, is more aggregated than unmodified HA and has alteredmechanical and cell-binding properties. The expansion of the HA-rich cumulus ECM (extracellular matrix) during ovulation is critically dependent on the catalysis of HC·HA generation by TSG-6, with TSG-6−/− mice being female infertile because of failure of HA cross-linking. It has been shown recently that TSG-6-mediated HC·HA formation is essential for the formation of HA-rich pericellular matrix and for cell migration in a model of wound healing. In contrast, in this model, the formation of cell-associated HA cable-like structures, although requiring the transfer of HCs on to HA, might not involve TSG-6. TSG-6-mediated HC transfer involves two sequential transesterification processes, where HCs are transferred from the CS (chondroitin sulfate) of IαI first on to TSG-6 and then on to HA. TSG-6 is an essential co-factor and catalyst in this chain of events, with both TSG-6·HC formation and HC transfer being dependent on the presence of Mg2+ or Mn2+ ions. Introduction TSG-6 (tumour necrosis factor-stimulated gene 6) is produced by many cell and tissue types in response to various stimuli; in particular, it has been detected in physiological and pathological contexts that are associated with inflammation and tissue remodelling (reviewed in [1]). TSG-6 was originally described as a HA (hyaluronan)-binding protein, but its ability to interact with a diversity of ligands, including both proteins and GAGs (glycosaminoglycans), results in a wide range of biological activities [1]. Furthermore, a growing body of evidence suggests that TSG-6 is subject to functional partitioning in different tissue micro-environments, where it is regulated by factors such as pH and its association with individual ligands [1,2]. One important role of TSG6 is the catalysis of HC (heavy chain) transfer from IαI (inter-α-inhibitor) to HA [3,4], where the resulting HC·HA complexes have been observed in synovial fluids from arthritis patients [5], in the cumulus ECM (extracellular matrix) during ovulation [3] and in the bronchoalveolar fluids of asthmatics and the airway tissues and secretions of smokers [6]. In such contexts, HC/HC interactions have been implicated in the cross-linking of HA [5,7], although this is