COMPREHENSIVE INVITED REVIEW Chemokines and Their Receptors Are Key Players in the Orchestra That Regulates Wound Healing Manuela Martins-Green,* Melissa Petreaca, and Lei Wang Department of Cell Biology and Neuroscience, University of California, Riverside, California. Significance: Normal wound healing progresses through a series of over- lapping 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, ex- posed termini, and the fact that their only modifications are two disulfide Manuela Martins-Green, PhD bonds make them excellent targets for manipulation. In addition, because they bind to G-protein-coupled receptors (GPCRs), they are highly amenable to Submitted for publication January 9, 2013. *Correspondence: Department of Cell Biology pharmacological modulation. and Neuroscience, University of California, Riv- Critical Issues: Chemokines are multifunctional, and in many situations, their erside, Biological Sciences Building, 900 Uni- functions are highly dependent on the microenvironment. Moreover, each versity Ave., Riverside, CA 92521 (email: [email protected]). specific chemokine can bind to several GPCRs to stimulate the function, and both can function as monomers, homodimers, heterodimers, and even oligo- mers. Activation of one receptor by any single chemokine can lead to desen- Abbreviations sitization of other chemokine receptors, or even other GPCRs in the same cell, and Acronyms with implications for how these proteins or their receptors could be used to Ang-2 = angiopoietin-2 manipulate function. AREs = adenine–uridine-rich Future Directions: Investment in better understanding of the functions of elements chemokines and their receptors in a local context can reveal new ways for DARC = Duffy antigen receptor therapeutic intervention. Understanding how different chemokines can acti- for chemokines vate the same receptor and vice versa could identify new possibilities for drug ECM extracellular matrix development based on their heterotypic interactions. = EGF = epidermal growth factor EST = expressed sequence tag SCOPE AND SIGNIFICANCE the 21st century that we fully real- FGF = fibroblast growth factor Chemokines are a family of small ized the wealth of proteins that this GPCRs = G-protein–coupled chemotactic cytokines that were dis- family provides, not only as regula- receptors covered in the late 1970s and early tors of immune function, but also as HDAC-1 = histone deacetylase 1 1980s,1–3 and were originally de- having functions that go well beyond. HIF-1a = hypoxic-inducible scribed as factors that chemoattract We know today that chemokines play factor 1-alpha and activate cells of the immune critical roles in many basic biological HTS = hypertrophic scars system during inflammation. Dis- processes such as angiogenesis and IFN-c = interferon gamma covery of new proteins of this family also are critically involved in chronic IL = interleukin continued at a slow pace through the inflammation, autoimmune diseases, KSRP = KH-type splicing regu- 1990s, especially those related to cancer, and viral infections. latory protein homeostasis of the immune system, In this review, we will first discuss (continued) but it was not until the beginning of the classification of these proteins and ADVANCES IN WOUND CARE, VOLUME 2, NUMBER 7 327 Copyright ª 2013 by Mary Ann Liebert, Inc. DOI: 10.1089/wound.2012.0380 j 328 MARTINS-GREEN ET AL. their receptors, then describe how one could envision that manipulation Abbreviations they are regulated at multiple levels, of this network of cytokines could and Acronyms (continued) and address some of the broad func- modulate either of these processes MDNCF = macrophage-derived tions they perform in a variety of bio- and lead to improvement of these neutrophil chemoattractant logical processes. In the latter portion conditions. In the case of wound factor of the review, we will focus in more healing, because chemokines from MMPs = matrix metallo- detail on the role of chemokines in the CXC and CC families are ex- proteinases normal and abnormal wound healing. pressed throughout the wound-healing NFjB = nuclear factor kappa-B process in specific temporal and TCR = T-cell receptor spatial patterns, this network of TGF-b = transforming growth TRANSLATIONAL RELEVANCE proteins lends itself to providing factor beta The chemokine network is a good regulated control for proper healing. TNF-a = tumor necrosis factor candidate for controlling both a vari- alpha ety of processes involved in inflam- VEGF = vascular endothelial mation, angiogenesis, and disease. A DISCUSSION OF FINDINGS growth factor lack of regulation of this complex AND RELEVANT LITERATURE network of cytokines can result in Classification of chemokines chronic inflammation, dysregulation and chemokine receptors of blood vessel development, and es- The chemokine superfamily is a tablishment of a chronic environment group of small (8–10 kDa), positively that leads to impaired healing, gen- charged, secreted proteins with a eralized fibrotic disease, and cancer. 20%–50% sequence homology, which Because these chemokines are small is reflected in shared structural proteins that do not have modifica- characteristics. They usually have tions other than the two disulfide four cysteines, the first two located bonds, are stable, and are amenable near the N-terminus of the molecule, to large-scale production, it is possi- the third in the center of the se- ble to use these proteins or peptides quence, and the fourth close to the corresponding to functional regions beginning of the C-terminal helix. as adjuvants for wound therapy. These cysteines form disulfide Furthermore, the fact that they bind bridges between the first and third G-protein-coupled receptors (GPCRs) cysteine residues and also between increases the likelihood that their the second and fourth cysteine resi- biological pathways can be controlla- dues, folding the molecule into a ble by small chemical agonists or globular shape with the N-terminus antagonists. shaped as a loop, the C-terminus as an a-helix, and the center of the molecule containing three b-pleated CLINICAL RELEVANCE sheets. The termini are both exposed Chemokines are major players in to the outside of the molecule and are inflammation and angiogenesis. important in receptor binding (Fig. Therefore, changes in their levels or 1). However, there are a small num- function can lead to chronic inflam- ber of chemokines that contain only mation and dysregulated angiogene- two cysteines and others that contain sis. These alterations can lead to six cysteines.4–6 either absent or excessive function, During the early years of their leading to impaired healing, chronic discovery, the chemokines were wounds, generalized fibrotic disease called by a variety of names, most of in response to injury, excess healing, them acronyms representing their and development of keloids and can- functions. In 1999, it was decided cer. Because chemokines are so clo- during a Gordon Conference on these sely involved in the regulation of chemotactic proteins that a system- both inflammation and angiogenesis, atic nomenclature was needed, and CHEMOKINE FUNCTION IN WOUND HEALING 329 ines are separated by three amino acids. This latter family of chemokines contains only one member, fractalkine, which is tethered to a mucin-like stalk that is linked to a transmembrane domain. The che- mokine proper can be released from the stalk by enzymatic digestion and serve as a chemoattractant, or can remain attached as a transmembrane protein and serve as an adhesion molecule. CXCL4, CXCL10, and CXCL1 were among the first members of the chemokine superfamily to be identified and sequenced.1,2,8 The first member of the chemokine superfamily known to possess a immune cell-chemoattractant activity was named macrophage-derived neutrophil chemoattractant factor (MDNCF), a chemokine isolated from a medium of lipopolysaccharide-treated monocyte/ macrophages that showed the ability to chemoat- tract neutrophils. Subsequent cloning and se- quencing of MDNCF showed that this factor was interleukin (IL)-8 (now referred to as CXCL8) and uncovered its sequence similarity to CXCL4, CXCL10, and CXCL1, and the chicken chemokine chCXCLi2.9 The identification of these chemokines was quickly followed by the isolation of CCL2, Figure 1. Schematic representation of the structural components of the which was purified from a medium conditioned chemokine molecule. Chemokines are structurally composed of a flexible N- terminus, followed by three antiparallel b-pleated sheets separated by by glioma cells and phytohemagglutinin-treated flexible loops and terminate with a long a-helix in the C-terminus. The peripheral blood mononuclear cell and chemoat- molecule assumes a globular shape, because cysteine #1, near the begin- tracted monocytes.10 Most of the remaining che- ning of the N-terminus, makes a disulfide bond with cysteine #3 present in mokines were identified by searching the mouse the 30s loop, and cysteine #2, also in the N-terminus, establishes a disulfide bond with cysteine #4,