Selectins are a family of proteins that mediate cellular interactions. They reside on cell surfaces, facilitating temporary attachment between cells. This adhesive capability is important for processes involving cell movement through the bloodstream and into tissues. Their role in these movements supports health and immune responses.
Understanding Selectins
Selectins are cell adhesion molecules, specifically C-type lectins, binding to carbohydrate structures in a calcium-dependent manner. There are three main types, named for the cell type where they were identified or found: L-selectin, P-selectin, and E-selectin.
L-selectin is found on leukocytes (white blood cells). P-selectin is present on platelets and endothelial cells lining blood vessels. E-selectin is expressed on endothelial cells, especially when activated by inflammatory signals. Though structurally similar, their distinct locations and regulation enable specialized functions.
The Role of Selectins in Cell Adhesion
Selectins primarily mediate “rolling adhesion,” a unique form of cell attachment. This mechanism differs from the stronger, more permanent adhesion facilitated by other cell adhesion molecules. Selectins act as initial tethering molecules, allowing circulating cells, like white blood cells, to slow down and “roll” along blood vessel walls.
Rolling occurs because selectin binding to carbohydrate ligands on opposing cells is transient and low-affinity. Rapid formation and dissociation of these bonds allow a cell to tumble along the vessel lining without sticking firmly. This low-affinity interaction enables white blood cells to survey the endothelial surface for signals without stopping, maintaining blood flow. As the cell rolls, new bonds form at its leading edge while existing bonds at the trailing edge break, propelling it forward.
Selectins and the Body’s Defenses
Selectin-mediated rolling adhesion is a first step in the body’s defense mechanisms, especially during inflammation. When tissues are infected or injured, endothelial cells lining nearby blood vessels activate, expressing E-selectin and P-selectin. These endothelial selectins then interact with L-selectin and other ligands on circulating leukocytes, initiating rolling.
This slowing allows leukocytes to sense chemical signals (chemokines) released at the inflammation site. These signals activate other adhesion molecules on leukocytes, leading to stronger, more stable attachment to the vessel wall. Subsequently, leukocytes exit the bloodstream and migrate into the inflamed tissue to combat infection or aid repair. Without initial selectin-mediated rolling, white blood cells would be carried past the inflammation site by rapid blood flow, preventing an effective immune response.