The MERS-CoV receptor story to date..

The Middle East respiratory syndrome (MERS) coronavirus (CoV) uses the dipeptidyl peptidase 4 (DPP4; also called CD26) molecule as its cellular contact for entry into the host.

The receptor was described in Nature by Raj and colleagues (led by Bart Haagman) in March 2013, part of a Dutch, German, Swiss and Saudi Arabian team. Some of this group, led by Christian Drosten, had previously found that MERS-CoV strain EMC did not use the angiotensin converting enzyme 2 (ACE2) as a receptor, which the SARS-CoV does. 

Haagman's group went on to use a modified form of the S1 domain of the Spike (S) protein to probe cell types in search of those that bound the protein, and allowed MERS-CoV replication. This included African green monkey (Vero) and human liver (Huh-7) cell lines and to a lesser extent, kidney cells from the Pipistrellus pipistrellus bat. Binding or infection failed in COS-7 African green monkey kidney cells. 

Drosten's earlier study identified that the baby hamster kidney (BHK) or African green monkey kidney (MA104) cell line could not support MERS-CoV/EMC while a rhesus monkey kidney (LLC-MK2 cell line) and bat cells or cell lines from Rousettus aegyptiacus kidney (RoNi/7),  Rhinolophus landeri lung (RhiLu), Pipistrellus pipistrellus kidney (PipNi/1 and PipNi/3), Carollia perspicillata kidney (CarNi/1), Myotis daubentonii kidney (MyDauNi/2), porcine (PS) and human kidney cancer (769-P) cell line could. The diversity of this cellular tropism is unique among the known CoVs.

Haagman's group next purified proteins from permissive cells by capturing them using the modified S1 domain, resulting in the discovery of a 100 kilodalton (kDa) protein which was characterised using mass spectrometry. DPP4 shows high amino acid and nucleotide sequence similarity across multiple species. 

DPP4 was sought and found on Vero, Huh-7 and human non-ciliated bronchial epithelial cells (?rather than ciliated cells found more widely throughout the upper and lower respiratory tract) or COS-7 cells. DPP4 is also found on human epithelial cells in the kidney, small intestine, liver and prostate as well as stimulated lymphocytes. That creates a number of spots of linkage between MERS-CoV replication and signs and symptoms of MERS.

When COS-7 cells were made to express DPP4, MERS-CoV/EMC could now bind. Blocking access to DPP4 using specific antisera resulted in blockage of cellular infection by MERS-CoV/EMC. Blockage did not result when the enzymatic function of DPP4 was inhibited using specific drugs.

But what is DPP? It's a few things. It's a protein that spans the cellular membrane, existing as a dimer (2 of itself somehow associated); it cuts dipeptides from hormones and chemokines to make them bioactive (do stuff); it has a role in glucose metabolism; it is abundant; it is involved in T-cell activation, movement of cells and organism around the body, cell adhesion, cell death (apoptosis), regulation of tumour growth. Reducing the levels of DPP4 expression or inhibiting virus binding may be useful therapeutic research areas for the future.

If the virus is limited in its tropism to cells of the deeper airways, it's transmission may also be limited because it is a little more difficult to cough and sneeze material from deep in the lungs compared to a virus happy to replicate itself in the nasal cavity and surrounds.

So to the latest advance.  Cui and colleagues, in the Virology Journal this month, cleverly looked at the receptor molecule in bats (from which CoVs often spring) as a way of saying something about the relationship between host and virus; namely MERS-CoV. Because the viruses and their receptors are predicted co-evolve, signatures can be identified in the genes for the receptor and for the bit of virus it interacts with.

In particular they found that 6 amino acids were very conserved among a batch of bat DPP4 sequences. 3 of these are predicted to occur where the MERS-CoV receptor binding domain (RBD) interacts with DPP4 protein, suggesting "co-evolutionary history" between bats and MERS-CoV. 

The authors concurred with others in concluding that MERS-CoV has an origin in bats because it has evolved to maintain its interaction with the bat DPP4.

Also check out Mike Coston's comments and Crawford Kilian's thoughts on this recent paper.