Looking at CC Chemokine Receptor 2 Function Specifically

Now that I have looked at the basics of what chemokines do in general, I am going to look specifically at the function of CC chemokine receptor 2 and its ligands. Once we know the mechanism of action of this receptor and it’s chemokines and how it functions under normal circumstances we can then go on to look at what its aberrant expression causes and how this can be treated.

Here is a little summary table to describe the properties of CCL2/MCP-1:

Produced By	Monocytes Macrophages Fibroblasts Keratinocytes Vascular Smooth Muscle Cells Endotheleial Cells T cells
Receptor	CC chemokine Receptor 2 (CCR2)
Attracts	Monocytes Natural Killer Cells Basophils Eosinophils Dendritic Cells
Major Effects	Activates macrophages Basophil histamine release Promotes T-helper 2 (TH2) cell immunity

Adapted from Janeway's Immunobiology by Murphy and Travers (2008).

MCP-1 Structure

There are 4 known MCP chemokines involved in human immunity, MCP1-4. MCP-1 is secreted in response to inflammatory signals in two forms that have molecular weights of 9kDa and 13 kDa. The differences in molecular weights are to do with O-glycosylation but this doesn’t affect their ability to attract monocytes and other immune cells. The residues in the amino terminal are crucial in the functioning of the MCP-1 protein. The mains ones include:

• Residues 1-6: contribute to the chemoattractant ability of MCP-1 (Asp-3 in particular has been noted as a key player)
• Residues 7-10: involved in receptor desensitisation
• Amino Acid at Position 1: important in the secondary structure formation of the protein and therefore the binding of MCP-1 to recpetors

It has been most commonly noted that MCP-1 binds to CCR2 in a dimeric fashion, although monomers of MCP-1 have also been known to cause activation of receptors too (17).

Functions of MCP-1

The ability of MCP-1 to cause chemotaxis and the movement of cells is thought to be due to the fact that through the signally pathway, CCR2 activation activates phospholipase C and therefore, eventually increases the concentration of intracellular calcium through IP3 signalling. A schematic diagram is shown to summarise this process along with the other theories of how chemotaxis is enabled: that of NFκB activation and Rho activation.

Adapted from Melgarejo, E. et al. 2009. Monocyte Chemoattractant Protein-1: A Key Mediator in Inflammatory Processes. The International Journal of Biochemistry and Cell Biology 41: 996-1001

MCP-1 Signalling. 

Cell motility is enabled through a variety of ways. These include the activation of phospholipase C, causing the cleavage of phosphatidylinositol 4,5-bisphosphate (PIP2). This results in 2 products: diacylglycerol (DAG) and inositol triphosphate (IP₃). IP₃ can bind to IP₃ receptors on the endoplasmic reticulum (ER) to cause calcium increase which is needed for cell movement. DAG causes the activation of protein kinase C (PKC) which can phosphorylate transcription factors like NFκB which is the basis for directional movement of the cell. Another mechanism by which cell movement is enabled is by the activation of Rho family of proteins. These proteins facilitate actin-dependent processes which result in pseudopod formation and membrane-ruffling (17).

Where to Start...

I believe it’s always best to start with the basics in order to get a good foundation of knowledge on which to build upon. For this reason, I have gone back to the beginning with regards to chemokines and looked at them as a whole group. The following is what I consider to be the keys points that are relevant to my research.

What are Chemokines?

Chemokines are a type of intracellular signalling molecule that come under the umbrella term of cytokines. They are chemoattractant in nature which means they attract and recruit certain cells and bacteria towards distinct areas in order to carry out a specific function: this is what distinguishes them from other cytokines. They are produced in response to several stimuli including:

·         Bacterial products

·         Viruses

·         Agents that cause physical damage to cells E.g. silica, alum, urate crystals in gout

Due to this fact, it is evident that they are heavily involved in immunity and responding to pathogens. This includes both the innate and adaptive immune systems as they act to induce chemotaxis in the early phases of infection as well as the in the later stages that require leukocyte recruitment (5).

How do they function?

Chemokines act on G-protein coupled receptors (GPCRs) to transmit their signal. These GPCRs have 7 transmembrane helices and are linked to trimeric G-proteins that have an attached GDP protein in the inactive state. The signalling process is by way of the classical GPCR pathway in which activation of the receptor and G-protein causes a dissociation of the three subunits into the α-subunit and the βγ-subunit. These subunits can then go on to activate other intracellular molecules independently and trigger various intracellular pathways including the phospholipase C pathway which can result in an increased intracellular calcium concentration.

For more information visit… http://www.annualreviews.org/doi/pdf/10.1146/annurev.immunol.19.1.397 
 

How are chemokine receptors classified?

There are 4 types of chemokine and correspondingly 4 types of chemokine receptor that are classified based on their amino acid sequence near the terminus of the molecule. These are: 

1.       CC chemokine receptors

These have two adjacent cysteine residues in this region. They are termed CCR1-9 and the genes for these receptors are clustered mostly on chromosome 4. β-chemokines act here.   

2.       CXC chemokines receptors

The cysteine residues on these termini are separated by a single amino acid. CXCR1-6 is used to denote these receptors with α-chemokines acting on them. The genes that encode these receptors are clustered on chromosome 17.

3.       CX3C chemokine receptor

Only one had been discovered: fractalkine or CX3CL1 which binds the sole receptor of this class CX3CR1. It has 3 amino acids between the two cysteine terminal residues.

4.       XC chemokine receptors

The chemokines that bind these receptors are unique in that they only contain two cysteine residues. They attract T cell precursors to the thymus.

 

For this reason, the chemokines that act on these two distinct sets of receptors result in varying physiological effects. The CC chemokine receptors are what I will be elaborating on as I start to look more specifically at the CC chemokine receptor 2 (CCR2) and the diseases it is implicated in.

That’s enough background information! It’s not the most exciting and innovative bit of text but I believe it’s necessary in order to give context to my research. So I hope you haven’t become disheartened as I will endeavour to bring you the latest in chemokine therapeutics and those novel agents that will hopefully make a real difference to sufferers worldwide.

A Brief Introduction

Some scientific blogs are intellectually thrilling, posting about never-before-seen research that has real value in the world of medical research. Others, are by professors trying to stake a claim as someone who is important enough to be heard and recognised by others. I, on the other hand, as a BSc Medical Pharmacology student have had the task set on me to research into cytokine therapeutics, in particular CC chemokine receptor 2 blockade. Although I am unlikely to produce any novel therapy for the treatment of some incurable disease, I hope to collate my findings here and, if not stimulate the minds of scientists worldwide, at least expand my own knowledge in what is such a current development in the scientific sphere. Without further ado, I present to you my blog of CC chemokine receptor 2 blockade research - may it be of some use to you in gaining an understanding of what chemokines and their blockade in therapeutics entails. Happy reading!