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When a wound on the skin creates a gap, the epithelial cells of the skin, surrounding the wound, move in a concerted fashion to close this gap. The boundaries of these gaps can have different curvatures; they could either be convex or concave. Interestingly, the cells situated on the convex-shaped surfaces form large membranous outgrowths and crawl towards the empty space; while on a concave surface, the layer of cells contracts together, tugging at the margins of the wound and gradually closing the gap.

While these specific modes of cell movement had been well-documented, it was unclear how the cells near the gaps mounted such distinctly different responses. How does something so seemingly inconsequential as a curvature of a gap, at the microscopic scale, end up dictating how cells move to heal a wound?

Four years ago, Simran Rawal, a graduate student in Tamal Das's lab at the Tata Institute of Fundamental Research, Hyderabad, India, decided to take a closer look at what happens inside epithelial cells when they respond to differently shaped gaps created in the tissue.

The findings from , now published in Nature Cell Biology, reveal that the largest intracellular organelle, the endoplasmic reticulum (ER), senses the curvature of the wound gap, and in response, drastically changes its structure: becoming tubular in shape when the wound surface is convex, and flattened into sheet-like structures when the wound surface is concave. As it turns out, this distinct difference in the ER morphology ends up playing a crucial role in deciding how the cell will move to seal a wound.

The morphology of ER influences how the cells move while sealing a gap

The epithelial barrier is not only adept at mending centimeter-scale large gaps in the tissue, but also shows equal discipline while sealing off small micron-scale gaps caused by a cell or two extruding out of a cell layer.

Das and Rawal mapped out the structural changes in specific organelles—lysosomes, Golgi, endoplasmic reticulum (ER), and mitochondria—to gain a better understanding of what happens inside the cell when it mounts a response to gaps of different geometries. Rawal observed that among these organelles, the ER showed the most drastic change in morphology.

Since the ER structure of the cells situated at the concave edges is flattened and sheet-like, Rawal observed what happens when this morphological restructuring is disrupted and forcibly changed into tube-like structures. Cells that had previously been contracting, resulting in purse-string-like closure of the gap, had now switched their mode of migration and started to crawl towards the empty gap instead.

A closer look at other associated intracellular components during this remodeling revealed that the structural changes in the ER were dependent on the changing dynamics of both actin and microtubules, the two major cytoskeletal frameworks of the cell. However, at convex-shaped gap edges, the microtubules are more crucial for the ER to change into tube-like structures.

Meanwhile, it was becoming important to quantify these morphological changes and characterize the mechanical cues the cell may be experiencing. Pradeep Keshavanarayana from Fabian Spill's lab at the University of Birmingham, UK, developed a mathematical model that helped quantify the strain on the cell when it begins to migrate towards a gap at different curvatures.

Ideally, a cell tries to achieve a lower strain energy. This study revealed how the changed ER structures at both convex and concave surfaces helped lower the strain energy experienced by the cell during protrusion and contraction.

ER: A possible link between mechanical cues and cell signaling?

An intriguing bit about the ER is that it spans the entire cell dynamically, starting from the nuclear envelope to the cell periphery as a single entity. Thus, any drastic changes in its structure have the potential to exert mechanical forces or set off signaling cascades spanning the breadth of the entire cell.

This study reports that the ER is a potential mechanotransducer, acting as a link between a mechanical cue (in this case, it is the wound gap geometry) and the biochemical changes inside the cell, thus regulating the overall response of the cell to an external mechanical stimulus.

Rawal explains, "Cytoskeleton has long been recognized as a primary sensor of mechanical cues in the cells; it was fascinating to discover that multiple intracellular membrane-bound organelles, such as the ER, primarily known for their conventional roles in calcium signaling and protein synthesis, also respond to mechanical signals in their environment and reorganize themselves."

While most studies on wound healing focus on biochemical signals and protein interactions, this study reveals a surprising new player—the shapes of the wounds themselves. The findings show that the physical geometry of a wound can influence how a cell rearranges its internal structures, and its decision on how to move while sealing the gap.

The fundamental observations in this study open up multiple new avenues for investigation. As Das says, "This work is part of our larger effort to uncover unexpected roles of cell organelles in shaping how tissues behave. Simran's discovery that the endoplasmic reticulum, a structure usually known for protein synthesis, can sense wound geometry and influence how cells move, opens up many exciting questions.

"Could the ER—or for that matter, other cellular organelles—help guide how tissues form in a developing embryo? Might similar mechanisms be involved in repairing organs after injury? Could other organelles be doing similar jobs in ways we haven't yet imagined? These are some of the big questions we are now eager to explore."