Problems with CFTR Protein in Platelets Linked to Lung Inflammation, Injury

Problems with the CFTR protein in platelets, blood cells essential to clotting, activated these cells, both driving lung inflammation and injury, and hindering the lungs’ ability to clear bacteria in a mouse models of cystic fibrosis (CF), a study found.

Blocking a calcium channel known as TRPC6 returned platelets to a non-activated state, stopping the inflammation and easing injury, suggesting this channel may be a treatment target for CF.

The study, “Cystic fibrosis transmembrane conductance regulator dysfunction in platelets drives lung hyperinflammation,” was published in the Journal of Clinical Investigation.

Most CF studies focus on how an impaired or absent CFTR protein (caused by mutations in the CFTR gene) in surface airways lead to thick mucous secretions, leaving patients unable to effectively clear their lungs. The result is chronic infections and inflammation. (CFTR stands for cystic fibrosis transmembrane conductance regulator.)

Recent studies suggest that a dysfunctional CFTR protein can produce a chronic hyperinflammatory state prior to bacterial infection.

However, the mechanisms driving hyperinflammation in CF remain unknown.

Platelets are gaining interest as a mediator of inflammation in a variety of diseases, including CF. These blood cells are thought to be key participants in immune responses.

People with CF are known to have higher numbers of platelets circulating in their blood than healthy people, and treatment with ibuprofen — a common pain reliever and anti-inflammatory medicine that inhibits platelet aggregation — has been shown to slow the progression of CF lung disease. A study in a mouse model with a CFTR deletion mutation also reported that antiplatelet therapy reduced lung injury in the animals.

These varied findings suggest that the CFTR protein in platelets may act to regulate lung inflammation in CF patients. 

Researchers at the University of California San Francisco investigated if a poorly working CFTR protein in blood platelets contributed to hyperinflammation. 

They first investigated the effects on lung inflammation of a complete (global) deletion of CFTR, infecting mice bred without the protein with either the bacterium Pseudomonas aeruginosa or its surface molecule called lipopolysaccharide (LPS). 

At 48 hours after infection, mice without CFTR had greater lung injury, a higher white blood cell (WBC) count, and higher levels of immune cells known as neutrophils that did mice with a normal CFTR protein. The mutant animals also showed  greater platelet activation, which triggered an increase in neutrophil extracellular traps (NETs) — networks of extracellular fibers whose main role is to trap and kill bacteria.

The team then tested mice were bred in such a way that the entire CFTR protein was selectively deleted from either their platelets (CF-PF4), neutrophils (CF-MRP8), or myeloid cells (CF-LysM) — encompassing a variety of blood cells. 

When these mice were infected with LPS, those with the platelet-specific CFTR deletion (CF-PF4) showed greater lung inflammation and increased lung injury, and higher white blood cell and neutrophil counts than did other mice. 

Likewise, CF-PF4 mice infected with P. aeruginosa bacteria had greater lung injury, and higher WBC levels and neutrophil counts. The animals were unable to clear the bacterial infection. 

Platelets lacking CFTR were seen to be hyperactivated. Interestingly, blocking a calcium channel called transient receptor potential channel 6 (TRPC6) — known to be functionally connected to CFTR — reversed platelet activation and lessened lung injury in the animals.

Mice lacking both CFTR and TRPC6 were then bred, and infected with LPS or P. aeruginosa bacteria. Compared to control mice, hyperinflammation and lung injury was reversed in these mutant mice. Platelet activation and the formation of NETs  were reduced in the LPS-infected mice, and bacterial clearance restored.

Researchers also evaluated TRPC6 inhibition in platelets isolated from CF patients and healthy people serving as controls. Compared to controls, calcium entry was higher in CF cells, and blocking TRPC6 normalized calcium entry and platelet activation. 

“We conclude that CFTR dysfunction in platelets produces aberrant TRPC6-dependent platelet activation, which is a major driver of CF lung inflammation and impaired bacterial clearance,” the researchers wrote.

“Platelets, and TRPC6, are what we believe to be novel therapeutic targets in the treatment of CF lung disease,” they added.