Chemokines and Their Receptors Are Key Players in the Orchestra That Regulates Wound Healing

Abstract

Significance: Normal wound healing progresses through a series of overlapping phases, all of which are coordinated and regulated by a variety of molecules, including chemokines. Because these regulatory molecules play roles during the various stages of healing, alterations in their presence or function can lead to dysregulation of the wound-healing process, potentially leading to the development of chronic, nonhealing wounds.

Recent advances: A discovery that chemokines participate in a variety of disease conditions has propelled the study of these proteins to a level that potentially could lead to new avenues to treat disease. Their small size, exposed termini, and the fact that their only modifications are two disulfide bonds make them excellent targets for manipulation. In addition, because they bind to G-protein-coupled receptors (GPCRs), they are highly amenable to pharmacological modulation.

Critical issues: Chemokines are multifunctional, and in many situations, their functions are highly dependent on the microenvironment. Moreover, each specific chemokine can bind to several GPCRs to stimulate the function, and both can function as monomers, homodimers, heterodimers, and even oligomers. Activation of one receptor by any single chemokine can lead to desensitization of other chemokine receptors, or even other GPCRs in the same cell, with implications for how these proteins or their receptors could be used to manipulate function.

Future directions: Investment in better understanding of the functions of chemokines and their receptors in a local context can reveal new ways for therapeutic intervention. Understanding how different chemokines can activate the same receptor and vice versa could identify new possibilities for drug development based on their heterotypic interactions.

Manuela Martins-Green, PhD

Figure 1.

Schematic representation of the structural components of the chemokine molecule. Chemokines are structurally composed of a flexible N-terminus, followed by three antiparallel β-pleated sheets separated by flexible loops and terminate with a long α-helix in the C-terminus. The molecule assumes a globular shape, because cysteine #1, near the beginning of the N-terminus, makes a disulfide bond with cysteine #3 present in the 30s loop, and cysteine #2, also in the N-terminus, establishes a disulfide bond with cysteine #4, which is located close to the C-terminal α-helix.

Figure 2.

Chemokine superfamily. It is composed of four families; the CXC family (A), in which the first two cysteines are separated by any single amino acid, the CC family (B), in which the first two cysteines are adjacent, the CX3C family (C), in which the first two cysteines are separated by three amino acids, and the C family (D), in which one of the first two cysteines is missing.

Figure 3.

Chemokine receptor structure and families. (A) These molecules are seven-transmembrane, GPCRs. They are composed of a short acidic N-terminus facing the outside of the cell, which is important for ligand binding, and three extracellular and three intracellular loops, the second of which contains the DRYY motif that is characteristic of this family. The C-terminus is intracellular and is rich in serine and threonine amino acids which, when phosphorylated, inactivate the receptor. (B) They form four families that are named after the ligand families, and they function in monomers, homodimers, and heterodimers. GPCRs, G-protein-coupled receptors.

Figure 4.

Phases of normal cutaneous wounds. Shortly after wounding and clot formation, the inflammatory phase of healing begins with neutrophils coming in first, followed by macrophages. This phase is followed by re-epithelialization and granulation tissue formation in which the keratinocytes migrate to cover the wound, and the wound tissue begins its repair by cell proliferation, ECM production, and blood vessel development. Finally, during the remodeling phase, much of the extracellular elements are removed by apoptosis, and the ECM is remodeled to produce the scar. ECM, extracellular matrix.

Figure 5.

Immediate response to wounding. After tissue damage, bleeding occurs, and prothrombin is activated to thrombin, which cleaves fibrinogen in the tissue to make fibrin. Fibrin, in turn, with the fibronectin and activated platelets, forms the clot. The degranulation of the platelets releases cytokines and growth factors that, in conjunction with thrombin, activate fibroblasts and resident macrophages present in the tissue.

Figure 6.

Chemoattraction of leukocytes into the sites of injury. Activated fibroblasts and macrophages produce more growth factors and cytokines along with prostaglandins, leukotrienes, interleukins, and TNF-α. These mediators stimulate endothelial cells to produce and/or activate cell surface adhesion molecules that capture the leukocytes. Simultaneously, chemokines produced locally chemoattract these leukocytes, generating the inflammatory phase of healing. TNF-α, tumor necrosis factor-α.

Figure 7.

Early inflammatory phase of healing. Neutrophils are the first leukocytes to arrive at the sites of injury, where they eliminate infection and produce chemokines such as CCL2, which attracts monocytes from the blood into the tissue. These monocytes differentiate into macrophages that, in turn, secrete a plethora of cytokines and growth factors that are important in the development of the granulations tissue and, eventually, in attracting lymphocytes. Shortly after that, the final phase of healing, remodeling, ensues.

Figure 8.

Chemokines and their roles in the various phases of wound healing. During clot formation, a series of events that lead to the inflammatory phase of wound healing occurs. Platelet granules release a number of molecular mediators, among them the chemokine CXCL4, which is antiangiogenic and ensures that blood vessels do not develop prematurely. CXCL1 and 8 are also produced shortly after wounding and chemoattract neutrophils, which rid the wound of bacteria and foreign materials. Neutrophils, in turn, produce CCL2, a potent chemoattractant of monocyte/macrophages. These leukocytes clean up the debris and dead neutrophils, and, along with other cells in the wound (e.g., fibroblasts), produce a plethora of growth factors and cytokines, among which are the chemokines CXCL1, 5, 6, 7, and 8, and CCL2, 3, 4, and 5. CCL2 will continue to attract more macrophages until fibrinogen in the wound disappears, at which time macrophage arrival stops. At this time in the progression of the healing process, CXCL8, along with vascular endothelial growth factor, stimulates angiogenesis. CCL3, 4, and 5 are instrumental in bringing in lymphocytes that are the last major type of leukocytes present in the wound tissue. CXCL9, 10, and 11 are produced somewhat later during healing and are known to be anti-inflammatory and antiangiogenic or angiostatic. These chemokines are present in the right place at the right time to ensure that angiogenesis is terminated and inflammation is resolved.