Cellular glue protein found to help clear away dead cells

A protein which sticks cells and tissues together has been found to have a surprising second job: helping cells which are packed into a continuous sealed layer, also known as epithelial cells, engulf nearby dead cells.

The debris of dying cells is one of the main causes of inflammation, so the finding by researchers at the Centre for Genomic Regulation (CRG) in Barcelona could lead to new clues for understanding chronic inflammatory conditions.

The study, published today in Nature Communications, centres around the E-cadherin complex, which consists of E-cadherin and three other proteins. They work together to link the cells which line our skin, guts and airways, providing structural support that stops tissues from falling apart. Every cell in these epithelial tissues clasp the E-cadherin on the surface of the cell directly next to it.

A group led by Dr. Verena Ruprecht studied epithelia in live zebrafish and mouse embryos and discovered the machinery also assembles at the precise spot when a dying cell drifts into contact.

The researchers asked whether the protein and its partners were gripping the dying cell as they normally grip their neighbors. They ran two tests. They first offered the tissue dying cells that had been stripped of E-cadherin, which the tissue cleared away as efficiently as normal ones. They then offered the cells droplets of fat not containing any protein at all, but harboring a signal that dying cells display on their surface. Those were swallowed too.

We were intrigued to find out that epithelial cells repurpose their molecular adhesion machinery - the 'glue' that normally holds them together - to engulf dying cells."

Verena Ruprecht, ICREA Research Professor, senior author of the study

The cells doing the swallowing are locked into a barrier that has to stay watertight, so wrapping themselves around something the size of another cell presents a real difficulty.

The imaging showed cells manage this because the top and bottom surfaces of a single cell behave independently of one another. The bottom of a cell stretches and wraps around a dead cell while the opposite side, which faces the outside world or an open space like the inside of a lumen, stayed still and continued to form a sealed barrier. When the researchers measured the top surface of a cell before, during and after it had swallowed something, the area barely changed while the bottom surface deformed significantly during the 'eating' process.

Ruprecht compares it to a line of dancers with their arms linked, whose upper bodies stay steady while their feet do the work, moving from a simple step to something far more energetic once a dying cell arrives. "It's the same dancer with a different choreography," she says.

The study also looked into the finer details of the mechanical work of cells involved. One of the proteins that form part of the E-cadherin complex worked as a rope, tethering the assembly to the cell's internal skeleton so that force can be transmitted across the surface of the target to swallow it. Cells lacking the tethering protein, or lacking the specific part of it that grips the skeleton, could no longer engulf dead cells.

Another worked as a brake for the cell's contractile motor. Unexpectedly, if this brake is removed the cell can be too stiff to do its job and fails to clear dying cells.

The team then tested whether the mechanism also exists in mammals. In early mouse embryos, blocking E-cadherin left dying cells sitting uncleared, matching what had been seen in zebrafish and indicating that the mechanism is shared across vertebrates.

The study follows Ruprecht's previous research which showed that embryos use epithelia to remove dying cells cooperatively, a type of early innate immune response.

Embryos were used back then and now for a practical reason. They are transparent and provide a window into live cell and tissue dynamics at a resolution that cannot be achieved in the human body.

While the present study takes the next step in explaining how embryos clear dying cells, there is an open question for future research about whether the same mechanism is at work in adult zebrafish or mice, or any type of human tissue.

Notably, epithelia in the adult body are already known to clear dying cells in the retina, the colon, the airways and the mammary gland. E-cadherin is present in epithelia throughout the body and its structure has changed remarkably little across species, which makes it a likely candidate for a mechanism used more widely in epithelia.

If confirmed, the clinical interest lies in what happens when clearance fails. Dying cells left in place break apart and leak, causing chronic inflammation. What this study adds is that clearance is both a question of whether a cell receives the right chemical signals to swallow apoptotic debris and how it can physically deform during this motion while keeping itself and the surrounding tissue intact.

"Studying the mechanisms of how dying cells can be removed efficiently from tissues is of very high relevance to human health," concludes Ruprecht.

The work was led by joint first authors Hanna-Maria Häkkinen, Marta Batet Palau and Laura F. Bianchi and supervised by Verena Ruprecht. It was funded by the Spanish Ministry of Science and Innovation, the Human Frontier Science Program, the European Union's Horizon Europe programme, and the "la Caixa" Foundation, with additional support from the European Social Fund. It made use of the CRG Core Facilities for Advanced Light Microscopy, Tissue Engineering and Protein Technologies.