CrossTalk proposal: Cell volume changes are an essential step in the cell death machinery
2013-12-15

Apoptosis or programmed cell death is a physiological mechanism to maintain the balance of cell number (Lang & Hoffmann, 2012). The destruction of DNA renders apoptosis an irreversible process precluding replication and protein synthesis of the affected cell. The cell membrane undergoes phospholipid scrambling with phosphatidylserine exposure at the cell surface and subsequent cell membane blebbing leads to formation of small apoptotic bodies with scrambled cell membrane (Lang & Hoffmann, 2012). Cell membrane scrambling and membrane blebbing are similarly hallmarks of suicidal death or eryptosis of erythrocytes, i.e. cells lacking nuclei (Lang et al. 2012). The exposed phosphatidylserine binds to receptors of phagocytosing cells leading to engulfment and degradation of the cellular corpses (Lang & Hoffmann, 2012). Accordingly, apoptotic cells are eliminated without release of intracellular proteins which would otherwise cause inflammation (Lang & Hoffmann, 2012). Apoptosis is further characterized by nuclear condensation, DNA fragmentation and mitochondrial depolarization (Lang & Hoffmann, 2012). The degradation of cellular proteins is accomplished by caspases (Lang & Hoffmann, 2012). Hallmarks of apoptosis include cell shrinkage (Wyllie, 1980; Lang & Hoffmann, 2012), which fosters the engulfment of the apoptotic cells by phagocytes (Lang & Hoffmann, 2012). Cell shrinkage or cytoplasmic condensation is considered to be a prerequisite for apoptosis (Maeno et al. 2006a; Ernest et al. 2008). Apoptotic volume decrease is observed in all cell types (Bortner & Cidlowski, 1998) and is an active, essential part of the machinery leading to apoptosis (Okada & Maeno, 2001; Maeno et al. 2006a). Apoptotic volume decrease is accomplished in part by activation of K+ and/or Cl− channels (Poulsen et al. 2010; Lang & Hoffmann, 2012; Fig. 1). Notably, in a wide variety of cell types apoptotic cell shrinkage precedes cytochrome c release, caspase-3 activation and DNA fragmentation, is abrogated by inhibition of K+ and Cl− channels but apparently does not require caspase activity (Okada & Maeno, 2001; Maeno et al. 2006a). Most importantly, inhibition of K+ or Cl− channels interferes with apoptosis, indicating that the activation of K+ and Cl− channels with subsequent apoptotic volume decrease is required for the induction of apoptosis (Okada & Maeno, 2001; Maeno et al. 2006a; Poulsen et al. 2010). In multidrug-resistant Ehrlich cells, Cl− channels appear to be downregulated preventing apoptotic cell shrinkage and thus protecting the cells against apoptosis (Poulsen et al 2010). In some cells under specific challenges, however, apoptosis has been observed without preceding apoptotic volume decrease (Hortelano et al. 2002; Bortner & Cidlowski, 2003; Franco et al. 2006). Major cell volume regulatory mechanisms in apoptotic cell shrinkage Osm = organic osmolytes. Dashed arrows indicate inhibition, continuous arrows indicate stimulation of transport system. Apoptotic cell shrinkage is in part due to inhibition of cell volume regulatory ion uptake by the Na+–K+–2Cl− cotransporter NKCC1 (Lang & Hoffmann, 2012). Disruption of NKCC1 mediated cellular Cl− uptake by removal of extracellular Cl− resulted in caspase-3 activation and cell death (Maeno et al. 2006a). Thus, regulatory cell volume increase by the Na+–K+–2Cl− cotransporter protects against apoptosis (Maeno et al. 2006b; Nukui et al. 2006). Apoptotic cell shrinkage is further paralleled by inhibition of Na+–H+ exchanger activity (Lang et al. 2000). Na+–H+ exchanger NHE1 is degraded by caspase-3 (Lupescu et al. 2009) and pharmacological inhibition of caspases interferes with the cytosolic acidification following expression of the proapoptotic viral protein NS1 (Lupescu et al. 2009). Apoptotic volume decrease is composed of an early and a late component (Lang & Hoffmann, 2012). The early component of apoptotic volume decrease precedes caspase activation (Maeno et al. 2006a,b; Poulsen et al. 2010), DNA fragmentation (Bortner et al. 1997), and cell membrane scrambling with exposure of phophatidylserine at the cell surface (Elliott & Higgins, 2003). The time required for cells to undergo apoptotic cell volume decrease varies, however (Lang & Hoffmann, 2012). Early apoptotic volume decrease amounts to approximately 20–40% decrease of cell volume loss in parallel to a K+–Na+ gradient reversal and is followed by late apoptotic volume decrease with cytoskeleton-sensitive massive cell shrinkage (Bortner et al. 2008). The second phase of cell shrinkage is paralleled by and required for membrane blebbing and apoptotic body formation (Nunez et al. 2010). As shown in a wide variety of cells, apoptosis is triggered by hyperosmotic (e.g. (Bortner & Cidlowski, 1996; Friis et al. 2005; Lang et al. 2012; Lang & Hoffmann, 2012) and isotonic (Maeno et al. 2006b; Nukui et al. 2006) cell shrinkage. The ability of cells to counteract hyperosmotic cell shrinkage by regulatory cell volume increase appears to determine their ability to resist hyperosmotic shock (Bortner & Cidlowski, 1996). As apoptotic cell shrinkage is accomplished by activation of K+ and/or Cl− channels (Lang & Hoffmann, 2012), it is paralleled by cellular loss of K+ and by decrease of cytosolic K+ concentration (Lang & Hoffmann, 2012). The decline of cytosolic K+ concentration is initially paralleled by an increase of cytosolic Na+ concentration, followed by a second phase of cellular loss of both K+ and Na+ (Panayiotidis et al. 2006; Bortner & Cidlowski, 2007; Bortner et al. 2008; Poulsen et al. 2010; Lang & Hoffmann, 2012). The cellular K+ loss is fostered by decreased Na+–K+-ATPase activity (Panayiotidis et al. 2006). Accordingly, the presence of Na+–K+-ATPase inhibitor ouabain enhanced the apoptosis of Jurkat lymphocytes following treatment with CD95 ligand or TRAIL (tumour necrosis factor-related apoptosis-inducing ligand; Panayiotidis et al. 2010). However, the effect of ouabain may not necessarily have resulted from Na+–K+-ATPase inhibition (Orlov & Hamet, 2004). A decrease of cytosolic K+ concentration has been shown to be essential for the full triggering of apoptosis (Bortner et al. 1997; Lang & Hoffmann, 2012). Cellular K+ loss at least contributes to cell shrinkage, caspase activation, nuclease activity and cell membrane scrambling (Lang & Hoffmann, 2012). K+ inhibits Ca2+ -induced DNA degradation and histone release and the decline of intracellular K+ concentration during apoptosis presumably contributes to the stimulation of apoptotic nuclear degradation (Ajiro et al. 2008). However, prevention of cellular K+ loss in oocytes abrogated nuclear condensation, budding, and fragmentation but not DNA degradation, indicating that in those cells cellular K+ loss was not required for DNA degradation (Perez et al. 2000). Cell volume regulation is in part accomplished by cellular accumulation or release of organic osmolytes (Lang & Hoffmann, 2012), such as sorbitol, glycerophosphorylcholine, myo-inositol, betaine and taurine (Lang & Hoffmann, 2012). The organic osmolytes stabilize intracellular proteins (Burg et al. 2007; Lang & Hoffmann, 2012). The stabilisation of the permeability transition pore by organic osmolytes counteracts mitochondrial depolarization (Zhang et al. 2010). The protein stabilisation contributes to the inhibitory effect of organic osmolytes on apoptotic cell death (Burg et al. 2007; Zhang et al. 2010). Along those lines, cellular loss of organic osmolytes could trigger apoptosis (Kitamura et al. 1998). Apoptotic cell volume decrease is typically paralleled by cellular release of organic osmolytes, which presumably exit the cells through anion channels (Lang & Hoffmann, 2012). In conclusion, even though individual hallmarks of apotosis or suicidal cell death could be triggered without cell shrinkage, decrease of cell volume is an integral part of apoptosis which stimulates further key elements of suicidal cell death including DNA fragmentation, cell membrane scrambling and blebbing. Accordingly, cell shrinkage participates in the orchestration of apoptosis and is thus an essential element in meaningful and efficient suicidal cell death. Readers are invited to give their views on this and the accompanying CrossTalk articles in this issue by submitting a brief comment. Comments may be posted up to 6 weeks after publication of the article, at which point the discussion will close and authors will be invited to submit a 'final word'. 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