Marilyn S. Rugg

Overexpression of Hyaluronan Synthase 2 Alters Hyaluronan Distribution and Function in Proximal Tubular Epithelial Cells

The functional consequences of increased renal cortical hyaluronan that is associated with both acute injury and progressive scarring are unclear. The aim of this study was to characterize hyaluronan synthase-2 (HAS2)-driven HA synthesis and determine its effect on renal proximal tubular epithelial cell (PTC) function, because this is known to be the inducible form of HA synthase in this cell type. Overexpression of HAS2 mRNA increased HA generation, which in the supernatant predominantly was HA of large molecular weight, whereas there was an increase in low molecular weight HA in cell-associated fractions. This was associated with increased expression of hyaluronidases, inhibition of HA cable formation concurrent with reduction in HA-dependent monocyte binding, and increased pericellular HA matrix. Overexpression of HAS2 led to enhanced cell migration. HA can be modified by the covalent attachment of heavy chains that are derived from the serum protein inter-alpha-inhibitor (IalphaI), a process that is known to be catalyzed by TNF-alpha-stimulated gene 6 (TSG-6; an inflammation-associated protein). Enhanced migration was abrogated by blocking antibodies to either IalphaI or TSG-6. Addition of recombinant full-length TSG-6 (TSG-6Q) or TSG-6Q_Y94F, a mutant variant with impaired HA binding, increased cell migration. Both of these proteins were able to mediate the covalent transfer of heavy chains, from IalphaI and pre-alpha-inhibitor, onto HA. Addition of the isolated TSG-6-Link module (Link_TSG-6), which binds HA but is unable to form covalent complexes with IalphaI/pre-alpha-inhibitor, had no effect on migration, suggesting that TSG-6-mediated formation of heavy chain-HA complexes is critical in the formation of a pericellular HA matrix.

The molecular basis of inter-alpha-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 IalphaI (inter-alpha-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 altered mechanical 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 IalphaI 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 Mg(2+) or Mn(2+) ions.

Metal Ion-dependent Heavy Chain Transfer Activity of TSG-6 Mediates Assembly of the Cumulus-Oocyte Matrix

Background: TSG-6 (TNF-stimulated gene-6)-dependent transfer of heavy chains from inter-α-inhibitor onto hyaluronan is critical for ovulation. Results: A calcium ion and chelating glutamate within TSG-6 mediate formation of the covalent heavy chain-TSG-6 intermediate. Conclusion: TSG-6 transferase activity rather than hyaluronan binding drives cumulus expansion. Significance: The role of metal ions in hyaluronan-heavy chain formation has been determined. The matrix polysaccharide hyaluronan (HA) has a critical role in the expansion of the cumulus cell-oocyte complex (COC), a process that is necessary for ovulation and fertilization in most mammals. Hyaluronan is organized into a cross-linked network by the cooperative action of three proteins, inter-α-inhibitor (IαI), pentraxin-3, and TNF-stimulated gene-6 (TSG-6), driving the expansion of the COC and providing the cumulus matrix with its required viscoelastic properties. Although it is known that matrix stabilization involves the TSG-6-mediated transfer of IαI heavy chains (HCs) onto hyaluronan (to form covalent HC·HA complexes that are cross-linked by pentraxin-3) and that this occurs via the formation of covalent HC·TSG-6 intermediates, the underlying molecular mechanisms are not well understood. Here, we have determined the tertiary structure of the CUB module from human TSG-6, identifying a calcium ion-binding site and chelating glutamic acid residue that mediate the formation of HC·TSG-6. This occurs via an initial metal ion-dependent, non-covalent, interaction between TSG-6 and HCs that also requires the presence of an HC-associated magnesium ion. In addition, we have found that the well characterized hyaluronan-binding site in the TSG-6 Link module is not used for recognition during transfer of HCs onto HA. Analysis of TSG-6 mutants (with impaired transferase and/or hyaluronan-binding functions) revealed that although the TSG-6-mediated formation of HC·HA complexes is essential for the expansion of mouse COCs in vitro, the hyaluronan-binding function of TSG-6 does not play a major role in the stabilization of the murine cumulus matrix.

Chapter 16:Hyaluronan-Binding Proteins in Inflammation

Tony Day joined the Immunochemistry Unit in September 1984 to do a 10-month research project on factor H, under the supervision of Bob Sim, for the final year of his chemistry degree at the University of Oxford. He continued working on factor H for his DPhil (1985–1987)– Tony was Bobs second PhD st...