Hyaluronan matrices in pathobiological processes

Abstract

Hyaluronan matrices are ubiquitous in normal and pathological biological processes. This remarkable diversity is related to their unique mechanism of synthesis by hyaluronan synthases. These enzymes are normally activated in the plasma membrane and utilize cytosolic substrates directly to form these large polyanionic glycosaminoglycans, which are extruded directly into the extracellular space. The extracellular matrices that are formed interact with cell surface receptors, notably CD44, that often dictate the biological processes, as described in the accompanying minireviews of this series. This article focuses on the discovery in recent studies that many cell stress responses initiate the synthesis of a monocyte-adhesive hyaluronan extracellular matrix, which forms a central focus for subsequent inflammatory processes that are modulated by the dialogue between the matrix and the inflammatory cells. The mechanisms involve active hyaluronan synthases at the cell membrane when cell stresses occur at physiological levels of glucose. However, dividing cells at hyperglycemic levels of glucose initiate the synthesis of hyaluronan in intracellular compartments, which induces endoplasmic reticulum stress and autophagy, processes that probably contribute greatly to diabetic pathologies.

Fig. 1

Model for the normal transport of hyaluronan synthase (HAS) from the endoplasmic reticulum (ER) to the plasma membrane, where it is activated to synthesize and extrude hyaluronan. The confocal micrographs show live cells that were transfected with GFP-Has3 (green) and stained for hyaluronan (red). They demonstrate ER / Golgi localization (left), transport vesicles (right), active HAS in plasma membranes (yellow) and extracellular hyaluronan (red). Micrographs provided by Kirsi Rilla (see the article by Tammi et al. [3] in this series).

Fig. 2

Model for the biosynthesis of proteoglycans (see text for details). ER, endoplasmic reticulum; PAP, phosphoadenosinephosphosulfate.

Fig. 3

U937 monocytic cells, using the receptor CD44 (red), bind to hyaluronan cable structures (green) on the surface of poly(I:C)-stimulated cultures of intestinal smooth muscle cells at 4 °C (left panel) [6]. When the cultures are warmed (37 °C for 30 min), the monocytic cells relocate, or ‘cap’, CD44 to one pole and internalize hyaluronan as shown in the enlarged inset. The left panel is reprinted from ref. [6] with permission from the American Society for Investigative Pathology.

Fig. 4

A section from a lung biopsy taken from a patient with an asthmatic flare stained for hyaluronan (green), CD44 (red) and nuclei (blue).

Fig. 5

Model for the intracellular activation of hyaluronan synthases in cells that divide in hyperglycemic medium (25 mM glucose). The images on the left are mesangial cells stimulated to divide in hyperglycemic medium, permeabilized at 16 h and stained for hyaluronan (green). Intracellular hyaluronan is observed in endoplasmic reticulum (ER) / Golgi regions and in transport vesicles [21]. The images on the right show permeabilized cells (left) and nonpermeabilized cells (right) stained for hyaluronan (green), cyclin D3 (red) and nuclei (blue) 36 h after stimulation to divide in hyperglycemic medium [21]. PKC, protein kinase C.

Fig. 6

Adhesion of U937 monocytes to kidney sections from a control and a streptozotocin-induced diabetic rat, 1 week after the induction of hyperglycemia. An enlargement of the diabetic kidney section (bottom left) shows clusters of monocytes over glomeruli. The adhesion was performed at 4 °C. When a section from the diabetic kidney was warmed to 37 °C, most of the monocytes detached. They were then spread on a slide and stained for hyaluronan (green), CD44 (red) and nuclei (blue) (bottom right). Examples of capped CD44 are apparent (arrowheads). The insets in this panel show macrophages in glomeruli in sections that co-stain for CD44 and hyaluronan (yellow), providing evidence for monocyte /macrophage activity in the glomeruli.

Fig. 7

3T3-L1 cells dividing in hyperglycemic medium undergo autophagy and synthesize an extensive monoctye-adhesive matrix. 3T3-L1 cells were stimulated to divide in hyperglycemic medium (25 mM glucose), routinely used to promote adipogenesis in this model. At 48 h, a permeabilized culture (top panel) was stained for hyaluronan (green), cyclin D3 (red) and nuclei (blue). The presence of hyaluronan cables (green) and cyclin D3-stained aggresomes (red) indicates that the cells underwent autophagy and cyclin D3-mediated formation of a hyaluronan matrix. The bottom left panel shows extensive U937 monocyte adhesion to an identically treated culture, which was lost when the culture was treated with Streptomyces hyaluronidase (selective for hyaluronan) (bottom right panel).

Fig. 8

The treatment of mesangial cells with xyloside prevents the intracellular synthesis of hyaluronan, autophagy and the formation of a monocyte-adhesive hyaluronan matrix. Mesangial cells stimulated to divide in hyperglycemic medium produce an extensive monocyte-adhesive hyaluronan matrix and stain for cyclin D3 at 48 h (middle panels). The control (left panels) and the hyperglycemic culture treated with 0.25 mM 4-methylumbelliferone-β-xyloside do not undergo autophagy, do not synthesize intracellular hyaluronan and do not produce a monocyte-adhesive hyaluronan matrix.