Researchers describe for the first time the role of a unique, pressure-sensing protein in the development of lung edema

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Researchers at the University of Illinois at Chicago describe for the first time the role of a unique, pressure-sensing protein in the development of lung edema – a condition in which chronic high vascular pressure in the lungs causes fluid from the bloodstream to enter the air spaces of the lungs.

The results, which are published in the Proceedings of the National Academy of Sciences, suggest that suppressing the activity of the protein could be a new approach to treating lung edema.

Lung edema can have a variety of causes, including heart failure.

Certain types of heart failure – where the heart is chronically unable to pump blood efficiently – can cause increased pressure in the blood vessels in the lungs.

The high pressure can result in capillary stress failure, where connections between the individual cells that make up the walls of the capillary blood vessels become looser, allowing fluid from the bloodstream to enter the air spaces in the lungs.

Yulia Komarova, UIC associate professor of pharmacology, and Asrar Malik, Schweppe Family Distinguished Professor and head of pharmacology in the UIC College of Medicine, have been studying adherens junctions – the structures that bind together the cells that make up blood vessels.

Adherens junctions act like adjustable nuts and bolts that can be tightened or loosened to modulate the flow of fluids and blood cells into and out of the bloodstream, such as immune cells that travel in the blood to get to areas where they are needed.

Komarova and colleagues wanted to see if a protein called piezo 1 – which is found in many cell types, including in the endothelial cells that line the blood vessels, and that can sense mechanical pressure – was involved in triggering adherens junctions to loosen up under conditions of high fluid pressure in the lungs.


Piezo Proteins

Piezo1 and Piezo2 proteins, encoded by the Piezo1/FAM38A and Piezo2/FAM38B genes, respectively, were recently identified as mechanically activated (MA) ion channels in the murine neuroblastoma cell line N2A in an siRNA screen (34).

They have subsequently been shown to induce MA cationic currents in numerous eukaryotic cell types, connecting mechanical forces to biological signals. Piezo proteins are predicted to be large integral membrane proteins with 24–40 transmembrane domains, making them the proteins with the largest number of transmembrane domains (Fig. 1) (4).

Initially described as an essential component of an MA ion channel (4), Piezo1 was later shown to possess an ion-conducting pore (3).

Piezo1 forms homotetramers, but whether the complex contains one or four pores is unknown (3).

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FIGURE 1.
Models of human PIEZO1 and PIEZO2. A and B, predicted membrane topology models of PIEZO1 (panel A, UniProt accession number Q92508) and PIEZO2 (panel B, UniProt accession number Q9H5I5) were created using Swiss-Prot prediction tools and the methodology of Eisenberg et al. (74). The locations of PIEZO1 mutations identified in hereditary xerocytosis (A) and PIEZO2 mutations identified in DA5, GS, and MWS (B) are marked. aa, amino acids.

Piezo1 and Piezo2, ∼2500 and 2800 amino acids long, respectively, share ∼50% identity. Piezo homologues are found in organisms as diverse as plants, protozoa, and invertebrates.

Phylogenetic analyses suggest that in vertebrates, Piezo1 diverged from Piezo2, as most lower organisms carry a single Piezo protein, whereas vertebrates have two (Fig. 2 and supplemental Fig. 1).

The exception is pathogenic protozoa, where homologues of Piezo are present in two groups genetically distinct from mammalian Piezo1 and Piezo2 (5).

Multispecies sequence alignments of evolutionarily distant protozoa, amoeba, plant, insect, and vertebrate Piezos reveal a remarkably conserved motif, the PFEW domain, hypothesized to be involved in channel conductance or gating (Fig. 3).

Most mutations associated with human disease occur in this domain (see below).

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FIGURE 2.
Evolutionary relationships of Piezo proteins. A phylogenetic tree based on comparison of Piezo protein sequences illustrating the relationships of Piezo proteins between species is shown. A single Piezo protein is present in lower species, including plants, worms, and flies. Near the base of the vertebrate tree, Piezo proteins were duplicated. The presence of more than one protein infers that the Piezo proteins have acquired additional functions. C. elegansCaenorhabditis elegans.
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FIGURE 3.
A highly conserved region in Piezo proteins. Multiple sequence alignment of Piezo proteins reveals a highly conserved region across species in the COOH terminus of Piezo proteins from plants to humans. This absolutely conserved region, the PFEW motif, is shown with amino acids comprising this motif shaded.

In mice and humans, many mRNA isoforms from different cell types have been identified for both Piezo loci. Whether these isoforms are translated and/or are of functional significance is unknown.

Piezo1 is broadly expressed with high levels in skin, bladder, kidney, lung, endothelial cells, erythrocytes, and periodontal ligament cells (6). Piezo2 is most prominently expressed in sensory trigeminal ganglia (TG) and dorsal root ganglia (DRG), Merkel cells, lung, and bladder (47,9).


The researchers engineered mice where piezo 1 was deleted in the adult animal in the endothelial cells.

They then elevated the pressure in blood vessels in the lungs in order to mimic the effects of heart failure.

In mice where piezo 1 was deleted in the endothelial cells, minimal fluid was seen entering the lungs, while in mice that had the piezo 1 protein, the lungs filled with fluid as blood pressure increased.

In a separate experiment, Komarova and colleagues used a mouse model where the adherens junctions were artificially reinforced in endothelial cells to keep the connections between individual cells lining the blood vessel cells tight.

In these mice, no fluid was seen to enter the lungs when high pressures were induced in the animals even when they had the piezo 1 protein.

“Our experiments suggest that by either blocking the activity of piezo 1 or by bolstering the adherens junctions we can prevent fluid from entering the lungs,” Komarova said. “Small drug molecules that achieve these goals could represent novel therapeutic approaches to treat lung edema associated with heart failure.”

More information: Endothelial cell Piezo1 mediates pressure-induced lung vascular hyperpermeability via disruption of adherens junctions, PNAS first published June 11, 2019 DOI: 10.1073/pnas.1902165116 , https://www.pnas.org/content/early/2019/06/10/1902165116

Journal information: Proceedings of the National Academy of Sciences
Provided by University of Illinois at Chicago

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