Abstract: Collagen is a key component of a healing wound. In this review, a general description of the wound healing process is provided focusing on collagen’s unique role. The mode of action (MoA) of collagen-based dressings is also addressed. Due to a number of potential stimuli (local tissue ischemia, bioburden, necrotic tissue, repeated trauma, etc.), wounds can stall in the inflammatory phase contributing to the chronicity of the wound. One key component of chronic wounds is an elevated level of matrix metalloproteinases (MMPs). At elevated levels, MMPs not only degrade nonviable collagen but also viable collagen. In addition, fibroblasts in a chronic wound may not secrete tissue inhibitors of MMPs (TIMPs) at an adequate level to control the activity of MMPs. These events prevent the formation of the scaffold needed for cell migration and ultimately prevent the formation of the extracellular matrix (ECM) and granulation tissue. Collagen based wound dressings are uniquely suited to address the issue of elevated levels of MMPs by acting as a ‘sacrificial substrate’ in the wound. It has also been demonstrated that collagen breakdown products are chemotactic for a variety of cell types required for the formation of granulation tissue. In addition, collagen based dressings have the ability to absorb wound exudates and maintain a moist wound environment.
Address correspondence to: David Brett, BS, BS, MS Wound Management Division Smith & Nephew 920 Lake Carillon Dr., Suite 110 St. Petersburg, FL 33716 Phone: 727-399-3496 E-mail: firstname.lastname@example.org
Proteins are natural polymers and make up almost 15% of the human body. The building blocks of all proteins are amino acids. Collagen is the major protein of the extracellular matrix (ECM) and is the most abundant protein found in mammals, comprising 25% of the total protein and 70% to 80% of skin (dry weight). Collagen acts as a structural scaffold in tissues. The central feature of all collagen molecules is their stiff, triple-stranded helical structure.1 Types I, II, and III are the main types of collagen found in connective tissue and constitute 90% of all collagen in the body. Function of collagen in wound healing. Previously, collagens were thought to function only as a structural support; however, it is now evident that collagen and collagen-derived fragments control many cellular functions, including cell shape and differentiation, 2,3 migration, 4 and synthesis of a number of proteins. 5 Findings suggest that cell contact with precise extracellular matrix molecules influence cell behavior by regulating the quantity and quality of matrix deposition. Type I collagen is the most abundant structural component of the dermal matrix; migrating keratinocytes likely interact with this protein. Collagenase (via formation of gelatin) may aid in dissociating keratinocytes from collagen-rich matrix and thereby promote efficient migration over the dermal and provisional matrices. Cellular functions are regulated by the ECM. The information provided by ECM macromolecules is processed and transduced into the cells by specialized cell surface receptors. 5 Evidence demonstrates that the receptors play a major function in contraction of wounds, 6,7 migration of epithelial cells, 8 collagen deposition, 9 and induction of matrix-degrading collagenase. Although keratinocytes will adhere to denatured collagen (gelatin), collagenase production is not turned on in response to this substrate. 10 Keratinocytes have been known to recognize and migrate on Type I collagen substratum, resulting in enhanced collagenase production. 11 Collagen plays a key role in each phase of wound healing. Hemostasis (duration = minutes). Platelets aggregate around exposed collagen. Platelets then secrete factors, which interact with and stimulate the intrinsic clotting cascade, which strengthens the platelet aggregate into a stable hemostatic “plug.” Blood platelets also release αa-granules, which release a variety of growth factors (GFs) and cytokines, such as platelet derived GF (PDGF), insulin-like GF (IGF-1), epidermal GF (EGF), and transforming GF-beta (TGF-b), 12 which “call” a variety of inflammatory cells (neutrophils, eosinophils, and monocytes) to the wound site and initiate the inflammatory phase. Inflammation (duration = days). Proteolytic enzymes are secreted by inflammatory cells that migrate to wound sites, notably neutrophils, eosinophils, and macrophages. The action of proteolytic enzymes on the macromolecular constituents of the ECM (such as collagen) gives rise to many peptides (protein fragments) during wound healing. These degradation products have a chemotactic effect in the recruitment of other cells, such as mononuclear cells, additional neutrophils, and macrophages. Activated macrophages secrete TNF-a, which among other things, induces macrophages to produce IL-1b. IL-1bβ is mitogenic for fibroblast and up-regulates matrix metalloproteinase (MMP) expression. TNF-α and IL-1bβ are key pro-inflammatory cytokines, which directly influence deposition of collagen in the wound by inducing synthesis of collagen via fibroblasts and down regulation of tissue inhibitors of matrix metalloproteinases (TIMPs). 12 Inflammatory cells also secrete growth factors including TGF-b, TGF-b, bHB-EGF, and bFGF. 12 These GFs continue to stimulate migration of fibroblasts, epithelial cells and vascular endothelial cells into the wound. As a result, the cellularity of the wound increases. This begins the proliferative phase. Proliferation (duration = weeks). Cleavage products resulting from collagen degradation stimulate fibroblast proliferation. Fibroblasts secrete a variety of GFs (IGF-1, bFGF, TGF-b, PDGF, and KGF), 12 which guide the formation of the ECM. The collagen cleavage products also stimulate vascular endothelial cell proliferation. These cells secrete a variety of GFs (VEGF, βFGF, PDGF), 12 which promote angiogenesis. With a vascularized ECM, granulation is achieved. Collagen cleavage products also stimulate keratinocyte migration and proliferation. Keratinocytes secrete a variety of GFs and cytokines, such as TGF-b, TGF-b, and IL-1. 12 As keratinocytes migrate from the edge of the wound across the newly formed granulation tissue, re-epithelization is achieved. Remodeling (duration = 1 year +). A balance is reached between the synthesis of new components of the scar matrix and their degradation by MMPs, such as collagenase, gelatinase, and stromelysin. Fibroblasts are the major cell type that synthesizes collagen, elastin, and proteoglycans. They are also the major source of MMPs and TIMPs. In addition, they secrete lysyl oxidase, which cross-links components of the ECM. Angiogenesis ceases and the density of capillaries in the wound site decreases as the scar matures. The result is the creation of a stronger scar, though the skin only regains almost 75% of its original tensile strength. The phases of acute wound healing are further described in Figures 1–5.
The Role of MMPs in Wound Chronicity
Wound bed preparation (WBP) can be described as the management of the wound to accelerate endogenous healing or to facilitate the effectiveness of other therapeutic measures. 13,14 The 4 basic aspects of WBP can be represented by the acronym: TIME. T = tissue (nonviable or deficient); I = infection or inflammation; M = moisture control; E = epidermal margin. 15 Focusing on the “E” in TIME, collagen dressings possess properties, which lend themselves to creating a wound environment favorable to the migration of cells from the epidermal margin across granulation tissue, encouraging wound closure. Due to a number of potential stimuli (local tissue ischemia, bioburden, necrotic tissue, repeated trauma, etc.), the wound has stalled in the inflammatory phase contributing to the chronicity of the wound. As a result of the aforementioned pro-inflammatory stimuli, the wound is overstimulated and inflammatory cells, such as macrophages, are present in higher numbers and are more active than they typically would be in an acute wound. In addition, the cells, such as fibroblasts and endothelial cells, are senescent and unable to function properly as they would in an acute wound. With the overabundance of macrophages, there is an overabundance of key pro-inflammatory cytokines, such as TNF-b and IL-1b, secreted by the macrophages. These pro-inflammatory cytokines signal the fibroblasts to secrete MMPs, but due to the overabundance of pro-inflammatory cytokines the fibroblasts secrete elevated levels of MMPs. At this level, MMPs not only degrade nonviable collagen, but also viable collagen laid down by the fibroblasts themselves. Additionally, the fibroblasts are unable to secrete tissue inhibitors of MMPs (TIMPs) at an adequate level to control the activity of the MMPs. These events prevent the formation of the scaffold needed for cell migration and ultimately prevent the formation of the ECM. In addition, cells in a chronic wound tend to be senescent, thus unable to communicate with other cells and unable to function properly. One result of this is a lack of endothelial cell activity slowing the formation of blood vessels. Without an adequate blood supply, tissue can die and as a result, there is an increase in wound size. All of the aforementioned phenomena impede the formation of viable granulation tissue and thus inhibit re-epithelialization (ie, wound closure). 12 One of the key contributors to wound chronicity is an overabundance (and/or activity) of MMPs in the wound; the ability to inhibit or deactivate a number of excess MMPs may help create an environment more conducive to the formation of granulation tissue, and eventual wound closure.
Collagen-based Wound Dressings
There are a number of different collagen dressings available, which employ a variety of carriers/combining agents such as gels, pastes, polymers, oxidized regenerated cellulose (ORC), and ethylene diamine tetraacetic acid (EDTA). The collagen within these products tends to be derived from bovine, porcine, equine, or avian sources, which is purified in order to render it nonantigenic. The collagen in a given collagen dressing can vary in concentration and type. Certain collagen dressings are comprised of Type I (native) collagen; whereas, other collagen dressings contain denatured collagen as well. A given collagen dressing may contain ingredients, such as alginates and cellulose derivatives that can enhance absorbency, flexibility, and comfort, and help maintain a moist wound environment. Collagen dressings have a variety of pore sizes and surface areas, as well. All of these attributes are meant to enhance the wound management aspects of the dressings. Many collagen dressings contain an antimicrobial agent to control pathogens within the wound. Collagen dressings typically require a secondary dressing (see Appendix I for a summary of currently available collagen-based wound dressings). Mode of action (MoA). Research has shown that some collagen-based dressings produce a significant increase in the fibroblast production; have a hydrophilic property that may be important in encouraging fibroblast permeation; enhance the deposition of oriented, organized collagen fibers by attracting fibroblasts and causing a directed migration of cells; aid in the uptake and bioavailability of fibronectin; help preserve leukocytes, macrophages, fibroblasts, and epithelial cells; and assist in the maintenance of the chemical and thermostatic microenvironment of the wound. 16–20 The MoA of several collagen dressings includes the inhibition or deactivation excess MMPs. As mentioned, excess MMPs are a key contributor to wound chronicity. The MoA of collagen dressings is described in Figures 6–13.
Collagen: Native versus Denatured
In addition to the various sources of collagen (bovine, porcine, etc.), collagen dressings can also contain different types of collagen. These types of collagens may result in unique activity in the wound bed as they have different substrate specificity. For example, Type I (native) collagen attracts MMP-1. 1 Denatured collagen (gelatin) attracts MMP-2 and MMP-9. 1,21 Gelatin also attracts stromelysin and matrilysin.21 These MMPs (among others) are found in excess in chronic wounds and contribute to a wound’s chronicity (see Appendix II for a breakdown of collagen source/type per collagen dressing). Biochemistry of collagen types. When a migrating cell (such as a keratinocyte) encounters Type I collagen, the cell secretes MMPs in order to denature the Type I collagen to gelatin. A critical reason for this is that once Type I collagen is converted into gelatin, many active sites (RGD sequences) are made accessible to the cells. RGD (Arg-Gly-Asp) sequences are attachment sites and are chemotactic for a variety of cells responsible for creating granulation tissue. Thus, a collagen dressing containing gelatin could provide enhanced signaling to the cells responsible for creating granulation tissue. A collagen-dressing containing only Type I collagen requires MMP-1 to initially convert collagen to gelatin, so cells in the wound must first release MMP-1 to change the Type I collagen into gelatin to get this benefit. Pore size and surface area. Pore size of collagen dressings is important to allow cells to enter the dressing and concentrate therein. In addition, surface area plays a role in managing exudate. Typically the larger the surface area, the more exudate is absorbed.
Previously, collagens were thought to function only as structural support; however, collagen and collagen-derived fragments control many cellular functions, including cell shape and differentiation, migration, and synthesis of a number of proteins. Collagen also plays a critical role in all phases of wound healing (hemostasis, inflammation, proliferation, and remodelling). It is also clear that while much of the MoA of the various collagen dressings is similar, there are key differences as well. Disclosure: This review was written on behalf of and paid for by Smith & Nephew, Inc., (St. Petersburg, FL). The author is an employee of Smith & Nephew.