Ion channels in regulated cell death

Abstract

Activation of ion channels and pores are essential steps during regulated cell death. Channels and pores participate in execution of apoptosis, necroptosis and other forms of caspase-independent cell death. Within the program of regulated cell death, these channels are strategically located. Ion channels can shrink cells and drive them towards apoptosis, resulting in silent, i.e. immunologically unrecognized cell death. Alternatively, activation of channels can induce cell swelling, disintegration of the cell membrane, and highly immunogenic necrotic cell death. The underlying cell death pathways are not strictly separated as identical stimuli may induce cell shrinkage and apoptosis when applied at low strength, but may also cause cell swelling at pronounced stimulation, resulting in regulated necrosis. Nevertheless, the precise role of ion channels during regulated cell death is far from being understood, as identical channels may support regulated death in some cell types, but may cause cell proliferation, cancer development, and metastasis in others. Along this line, the phospholipid scramblase and Cl(-)/nonselective channel anoctamin 6 (ANO6) shows interesting features, as it participates in apoptotic cell death during lower levels of activation, thereby inducing cell shrinkage. At strong activation, e.g. by stimulation of purinergic P2Y7 receptors, it participates in pore formation, causes massive membrane blebbing, cell swelling, and membrane disintegration. The LRRC8 proteins deserve much attention as they were found to have a major role in volume regulation, apoptotic cell shrinkage and resistance towards anticancer drugs.

Keywords: Anoctamin 6; Apoptosis; LRRC8A; Necroptosis; TMEM16F.

Fig. 1

Role of anoctamins in cell death. Model for activation of anoctamin 6 (ANO6) by stimulation of purinergic P2X₇ receptors with ATP in macrophages. Stimulation of P2X₇ with ATP leads to sudden cell shrinkage (at moderate Ca²⁺ increase), which quickly changes into massive blebbing and cell swelling with a subsequent disintegration of the plasma membrane at increasing intracellular Ca²⁺ concentrations [43]. At low Ca²⁺ concentration, ANO6 is relatively selective for Cl⁻ but changes into a nonselective channel/pore at larger cytosolic Ca²⁺ concentrations [43, 118]

Fig. 2

Activation of volume regulated anion currents. Model for activation of volume activated anion channels (VRAC). Intracellular Ca²⁺ is regarded as a modulatory factor for VRAC activity and volume regulation (RVD), but evidence was also provided that RVD and VRAC necessarily require Ca²⁺ [79]. A hypotonic bath solution will induce cell swelling by water influx through aquaporin (AQP) water channels, which leads to a drop in intracellular ionic strength and activation of Ca²⁺-insensitive phospholipase A2 (PLA₂). Ca²⁺ moves into the cells through transient receptor potential (TRP) channels, most likely TRPC1 and TRPM7, and is released from endoplasmic reticulum (ER) Ca²⁺ stores, which may activate additional Ca²⁺-sensitive PLA₂. PLA₂ cleaves phospholipids (PL) in the plasma membrane and in the ER membrane to lysophospholipids (LPL), thereby increasing plasma membrane tension, which activates VRAC leading to regulatory volume decrease (RVD)

Fig. 3

Cell appearances during necroptotic cell death of NIH3T3 cells. a Holographic images (HoloMonitor, I&L Biosystems) of NIH3T3 cells exposed to the necroptotic cocktail TSZ (10 ng/ml TNF, 5 µM of the SMAC mimetic birinapant, and 25 µM of the pan-caspase inhibitor Z-VAD). Note the decay of the cells after 6 h. The vertical scale bar indicates cell height ranging form 0 to 15,71 µm. b FACS analysis of TSZ-treated NIH3T3 cells indicating PS scrambling (annexin V binding) and membrane permeabilization (PI)