Active modulation of erythrocyte mechanics.

The classic view of the red blood cell (RBC) presents a biologically inert cell, that upon maturation, has limited capacity to alter its physical properties. This view developed largely due to the absence of translational machinery and inability to synthesise or repair proteins in circulating RBC. Recent developments have challenged this perspective, supporting the importance of post-translational modifications, and greater understanding of ion movement in these cells, that each regulate a myriad of cellular properties. There is thus now sufficient evidence to induce a step-change in understanding of RBC; rather than passively responding to the surrounding environment, these cells have the capacity to actively regulate their physical properties and thus alter blood flow. Specific evidence supports that the physical and rheological properties of RBC are subject to active modulation, primarily by the second-messenger molecules nitric oxide (NO) and calcium-ions (Ca2+). Further, an isoform of nitric oxide synthase is expressed in RBC (RBC-NOS), which has been recently demonstrated to actively modulate the physical properties of RBC. Mechanical stimulation of the cell membrane activates RBC-NOS leading to NO-generation, which has several intracellular effects, including the S-nitrosylation of membrane components. Intracellular concentration of Ca2+ is increased upon mechanical stimulation via the recently identified mechano-sensitive channel, piezo1. Increased intracellular Ca2+ modifies the physical properties of RBC by regulating cell volume and potentially altering important intracellular proteins. A synthesis of recent advances in understanding of molecular processes thus challenges the classic view of RBC, and rather indicates a highly active cell with self-regulated mechanical properties.