PERSPECTIVE ARTICLE
Hyaluronic Acid in wound Healing : Rediscovering a major player.
Kessiena L. Aya, MS; Robert Stern, MD
Department of Basic Biomedical Sciences, Touro College of Osteopathic Medicine, New York, New York
HA is a straight-chain glycosaminoglycan (GAG) carbohydrate polymer that is a major component of the extracellular matrix (ECM).1–3 It is particularly prominent during wound
repair, embryogenesis, and whenever rapid tissue turnover and repair occur.
HA can exist in a number of forms. It may be free, bound to HA-binding proteins known as hyaladherins,13 or intercalated into complex structures such as in the ECM. The presence
of HAsurrounded by multiple aggrecans in a bottlebrush configuration in the structure of cartilage is an example of such a complex structure.
HA has important hygroscopic, rheological, and viscoelastic properties that fluctuate with changes in temperature, pH, ionic environment, and binding partners. However, these properties are also highly dependent on chain length. HA can reach over 107 Da in molecular mass (HMW-HA), but also exists in multiple smaller forms, referred to as low molecular weight HA (LMW-HA).
HA can exist in a number of forms. It may be free, bound to HA-binding proteins known as hyaladherins,13 or intercalated into complex structures such as in the ECM. The presence of HA surrounded by multiple aggrecans in a bottlebrush configuration in the structure of cartilage is an example of such a complex structure.
High molecular weight hyaluronan molecules
HA has immunosuppressive effects in its HMW form. Large HA molecules protect against lymphocyte-mediated cytolysis, suppress septic responses to lipopolysaccharides, 16 maintain immune tolerance, induce production of immunosuppressive macrophages, and, in general, modulate the immune response. They also reduce expression of inflammatory cytokines. The HMW-HA molecules are thus not only immunosuppressive but also antiangiogenic and anti-inflammatory. The HMW form of HA additionally has intrinsic antiaging and anticancer effects.
Low molecular weight hyaluronan fragments
By contrast, short or LMW HA fragments are highly angiogenic, immunostimulatory and inflammatory. The presence of the small HA forms is a reflection of tissues under stress. Even the smallest fragment, the tetrasaccharide, has specific functions, with an ability to induce heat shock proteins and suppress apoptosis. The very smallest fragments apparently have the ability to ameliorate the intensity of the reactions induced by the small to intermediate-size fragments of HA. These small tetrameric to hexameric polysaccharides identify tissue injury through Toll-like receptors (TLRs). They also have the ability to inhibit the growth of tumor cells.
STAGES OF WOUND HEALING
• Formation of the initial clot and platelet plug.
• Increased vascular permeability and edema in early wound healing.
• Initial inflammatory cell response
• Mononuclear infiltrate
• Angiogenesis
• Development of myo fibroblasts and deposition of granulation tissue
• Fibroblast invasion and proliferation
• Deposition of type III and type I collagens
• Re-epithelialization
• Final repair and remodelling.
Table 1. Tabulation of hyaluronan participation in wound healing Day by Day
Day 0
Clot formation. HA binds to fibrinogen, loosening the clot while also stabilizing it. HA pours into the area from platelets as well as from wounded endothelial cells.
Day 0.1
HMW-HA comprises much of the edema fluid, the “tumour” component of the inflammatory reaction, constituting the swelling of early wound healing. Anoxia at the wound stimulates lactate production that enhances local HA synthesis. The lowered pH resulting from lactate production releases oxygen more easily from haemoglobin, known as the Bohr effect. This is particularly important at wound margins and for survival of tissue.
Day 0.2
Platelet hyaluronidase (HYAL2) is released from inhibition by an unknown inhibitor. Platelet hyaluronidase generates intermediate-sized HA fragments that attract polymorphonuclear leukocytes. The spaces created by edema facilitate entry of polymorphonuclear leukocytes and their removal of dead tissue, debris, and bacteria.
HMW-HA limits intensity of the myeloperoxidase reaction and free-radical injury of tissues. This dampens the intensity of the inflammatory response.
Day 1
The attraction of mononuclear cells into the wound may be induced by further HA fragmentation. TLR2 and TLR4 respond to small HA fragments, inducing macrophage inflammatory gene expression, including that of TNF-α and other chemokines.
Day 3–5
Signalling by HA fragments induces mitosis, sprouting, and tropism of endothelial cells, providing angiogenesis and neovascularization. Stimulation of MMPs by HA helps to regulate angiogenesis. Expression of vascular endothelial growth factor is controlled in part by HA. Organization of a fibroblast HA-rich pericellular coat precedes differentiation of fibroblasts into myofibroblasts. Granulation tissue is formed with a matrix rich in HA. This provides cushioning of the wound and structural organization of the evolving matrix.
Day 6
Migration and proliferation of fibroblasts are stimulated by HA. The HA also stimulates preferential synthesis of collagen type III by fibroblasts, as well as other extracellular matrix
components. HA, together with fibronectin, provides a guiding runway for fibroblast migration and an ECM for wound closure.
Day 8
Preferential degradation of collagen III occurs with sparing of type I collagen.
Day 9–14
Continued deposition of collagen type I occurs with growing tensile strength of the wound, perhaps initiated by decreasing levels of HA. This constitutes much of the fibrous connective tissue of the wound. In the repairing wound, HA faces the wound margin, together with CD44. This complex regulates keratinocyte proliferation and migration during the process of re-epithelization