Figure 1. Chemokine signaling pathway
Chemokines are a family of chemoattractant cytokines released by tissues in the earliest phases of infection. They are usually produced by a wide variety of cell types in response to bacterial products, viruses, and agents that trigger physical damage. These small proteins directly induce chemotaxis in nearby responsive cells, leading to the movement of the cells toward the source of the chemokines. In addition to being known for mediating chemotaxis, chemokines have the exclusive structural characteristics. All identified chemokines are approximately 8-10 kilodaltons in mass and have four cysteine residues in conserved locations that are critical for forming their 3-dimensional shape.
Chemokines are divided into 4 groups. The two major families, the CC chemokines and the CXC chemokine, differ in their genetic locations and structural features. CC chemokines, whose genes are mainly found in clusters on chromosome 4 of human, have two adjacent cysteine residues near the amino terminus. CXC chemokines, in which the equivalent cysteine residues are separated by a single amino acid, are mostly clustered in one region of chromosome 17 in human genes. There are also two minor families that have been identified and described. C chemokines are the one type which has lost the first or third cysteines. The other one is CX3C family that contains three amino acids between first two cysteines.
Chemokine receptors comprise a family of approximately 20 different G - protein - coupled, and seven transmembrane segment polypeptides on the surface of target cells. All of these receptors contain approximately 350 amino acids with a total molecular weight around 40 kDa. The extracellular domain consists of the N-terminus and three extracellular hydrophilic loops which interact with the chemokine ligand. The middle part is comprised of 7 helical transmembrane domains. The intracellular region is composed of three loops and the C-terminus which G-proteins couple to. G-proteins exist as a heterotrimer and they are composed of three distinct subunits. The C-terminal end collaborates to transduce the chemokine signal following ligand binding. There is a highly conserved amino acid sequence called “DRYLAIV” in the second intracellular loop making the chemokine receptor different from other seven-transmembrane receptors which share similar structural features.
As in the case of the chemokines, the receptors can be classified into 4 different groups, CC chemokine receptors, CXC chemokine receptors, C chemokine receptors and CX3C chemokine receptors that correspond to the 4 distinct subfamilies of chemokines they bind. They differ in the spacing of cysteine residues near N-terminus of the receptor.
Figure 2. The typical structure of chemokine receptor
Chemokines signaling pathway
The released chemokine attracts target cell and binds to its surface where the chemokine receptor is located. Intracellular signaling by chemokine receptors is significantly dependent on neighboring G-proteins which are at the C-terminus. The G-protein subunit stays inactive by being bound with guanosine diphosphate (GDP). After the binding of the chemokine ligand, chemokine receptors associate with G-proteins, allowing the exchange of GDP for another molecule called guanosine triphosphate (GTP), and the dissociation of the different G protein subunits, which is the activation of the G-protein. The released activated G-protein causes the subsequent activation of an enzyme called phospholipase C (PLC) that is associated with the cell membrane. PLC then cleaves phosphatidylinositol (4,5)-bisphosphate (PIP2) into two second messenger molecules named Inositol triphosphate (IP3) and diacylglycerol (DAG). IP3 induces calcium influx and DAG activates another enzyme called protein kinase C (PKC). These events promote many intracellular signaling cascades such as the MAP kinase pathway that initiates many cellular responses like chemotaxis, degranulation, release of superoxide anions and changes in the avidity of integrins.
Chemokine-stimulated GPCRs can activate several downstream functions that ultimately lead to actin polarization, shape change, and directed cell movement and other cellular responses. The activation of G-protein causes the dissociation of GTP-bound Gα subunit from the heterodimers which activate the downstream proteins such as GPCR kinases (GRKs), arrestin proteins and regulator of G-protein signaling (RGS) proteins. This results in the production of second messengers that further promote downstream signal transduction pathways and then initiate a cellular response. For example, the generation of 3-phosphorylated lipids act as the second messenger for downstream regulation such as PKC, AKT, and Ras pathways.
The activity of chemokines and their receptors play a significant role in the chemokines signaling pathway regulation. The chemokine receptor can be controlled to modulate the cellular levels of receptor molecules, or the presentation of functionally active receptors at the cell surface. Regulation of protein expression includes the genetic polymorphisms, gene expression, mRNA modification, mRNA splice variation, protein modification and oligomerization. These regulations effectively change the number of chemokines and receptors and vastly affect the number of chemokine-receptor combinations. However, these processes cannot be solely responsible for the changes required by individual cells to fine-tune their response according to the specific composition of the local environment. Therefore, tight control of the presence of functional chemokine receptors at the cell surface is essential, and can be achieved by affecting the activation state, signaling ability and/or cellular localization of the receptor. This rapid control can be mediated in response to ligand binding but can also be the consequence of cross-talk from other receptors. The chemokines-receptor-targeted processes include selective and competitive binding interactions, degradation and localization, down-regulation by atypical (decoy) receptors, interactions with cell-surface glycosaminoglycans, alternative signaling responses, and binding to natural or pharmacological inhibitors.
Chemokine-receptor system plays critical roles in an array of physiological processes including immune homeostasis, inflammatory responses, and cancer progress. Some chemokines are considered pro-inflammatory and can be induced during an immune response to recruit cells of the immune system to the site of infection, while other chemokines are considered homeostatic and are involved in controlling the migration of cells during normal processes of tissue maintenance or development. For example, stem cells and progenitor cells require homeostatic chemokines to home into distinct niches that regulate their stemness, quiescence, proliferation, and differentiation. The effects of homeostatic chemokines on development and generation that fuel epithelial hyperplasia, progressive fibrosis, and tissue sclerosis need to be further evaluated for therapeutic interventions. Chemokines and GPCRs are not only appreciated as important mediators for innate immune cell trafficking in response to acute inflammatory insults, but are also considered to be central cellular fate determinants of adaptive immune responses. Chemokines signaling pathway contributes to a wide range of diseases: autoimmune disorders (for example, psoriasis, rheumatoid arthritis and multiple sclerosis), pulmonary diseases (like asthma and chronic obstructive pulmonary disease), transplant rejection, metabolic and vascular diseases (such as obesity, diabetes and atherosclerosis), as well as infectious diseases including HIV, sepsis and inflammatory bowel disease. In addition, it also takes a great part in cancer metastasis as it is involved in extravasation, migration, micro-metastasis, and angiogenesis.
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