Cation channels, cell volume and the death of an erythrocyte

Abstract

Similar to a variety of nucleated cells, human erythrocytes activate a non-selective cation channel upon osmotic cell shrinkage. Further stimuli of channel activation include oxidative stress, energy depletion and extracellular removal of Cl-. The channel is permeable to Ca2+ and opening of the channel increases cytosolic [Ca2+]. Intriguing evidence points to a role of this channel in the elimination of erythrocytes by apoptosis. Ca2+ entering through the cation channel stimulates a scramblase, leading to breakdown of cell membrane phosphatidylserine asymmetry, and stimulates Ca(2+)-sensitive K+ channels, thus leading to KCl loss and (further) cell shrinkage. The breakdown of phosphatidylserine asymmetry is evidenced by annexin binding, a typical feature of apoptotic cells. The effects of osmotic shock, oxidative stress and energy depletion on annexin binding are mimicked by the Ca2+ ionophore ionomycin (1 microM) and blunted in the nominal absence of extracellular Ca2+. Nevertheless, the residual annexin binding points to additional mechanisms involved in the triggering of the scramblase. The exposure of phosphatidylserine at the extracellular face of the cell membrane stimulates phagocytes to engulf the apoptotic erythrocytes. Thus, sustained activation of the cation channels eventually leads to clearance of affected erythrocytes from peripheral blood. Susceptibility to annexin binding is enhanced in several genetic disorders affecting erythrocyte function, such as thalassaemia, sickle-cell disease and glucose-6-phosphate dehydrogenase deficiency. The enhanced vulnerability presumably contributes to the shortened life span of the affected erythrocytes. Beyond their role in the limitation of erythrocyte survival, cation channels may contribute to the triggering of apoptosis in nucleated cells exposed to osmotic shock and/or oxidative stress.

Cell shrinkage and monovalent cation fluxes: Role in apoptosis

“…Furthermore, several reports indicate that the mechanisms of volume loss or AVD during apoptosis can occur independent of the cell nucleus and other organelles [33,34]. For example, in erythrocytes, the Ca 2+ ionophore ionomycin was shown to induce a cell death process analogous to apoptosis with cell shrinkage, cell membrane blebbing, and externalization of phosphatidylserine [34]. These studies suggest that AVD can be caspase-independent and independent of other intracellular organelles.…”

“…Berg et al [32] suggested that upon apoptotic stimulation, AVD in mature erythrocytes devoid of caspase-9, Apaf-1, and cytochrome c resulted from the activation of calpain, not caspases. Furthermore, several reports indicate that the mechanisms of volume loss or AVD during apoptosis can occur independent of the cell nucleus and other organelles [33,34]. For example, in erythrocytes, the Ca 2+ ionophore ionomycin was shown to induce a cell death process analogous to apoptosis with cell shrinkage, cell membrane blebbing, and externalization of phosphatidylserine [34].…”

Physiology of Cell Volume Regulation in Vertebrates

“…Taurine release is also triggered during PCD (516), and erythrocytes from the taurine transporter knockout mouse are more resistant to apoptosis (513). This might be related to the fact that TauCl, which is the chlorinated product of taurine, causes PCD through direct damage to the mitochondria (454), i.e., net loss of taurine could result in a reduction in TauCl and thus a reduced risk for PCD.…”

Caspase-independent cell death?

“…In nucleated erythrocytes, experimental induction of apoptosis does not necessarily activate caspases and is not necessarily prevented by zVAD.fmk (Daugas et al, 2001). Mature, senescing erythrocytes undergo at least those aspects of an apoptotic program that mark them for clearance by phagocytes: increase in intracellular Ca 2 þ , leading to exposure of phosphatidylserine, loss of K þ i , and cell shrinkage (Lang et al, 2003a(Lang et al, , b, 2004; see also, Macho et al, 1997). This process is at least partially mimicked by hyperosmotic shock, which likewise leads to phosphatidylserine exposure.…”

“…This process is at least partially mimicked by hyperosmotic shock, which likewise leads to phosphatidylserine exposure. In this case, both increase in intracellular ceramide and influx of Ca 2 þ participate (Lang et al, 2003a(Lang et al, , b, 2004. Bratosin et al have argued that at least a partial explanation for the conflicting data concerning mature erythrocytes resides in the activity of cysteine proteases other than caspases.…”