SummaryAfferent output in type II taste cells is mediated by ATP liberated through ion channels. It is widely accepted that pannexin 1 (Panx1) channels are responsible for ATP release in diverse cell types, including taste cells. While biophysical evidence implicates slow deactivation of ion channels following ATP release in taste cells, recombinant Panx1 activates and deactivates rapidly. This inconsistency could indicate that the cellular context specifies Panx1 functioning. We cloned Panx1 from murine taste tissue, and heterologously expressed it in three different cell lines: HEK-293, CHO and neuroblastoma SK-N-SH cells. In all three cell lines, Panx1 transfection yielded outwardly rectifying anion channels that exhibited fast gating and negligible permeability to anions exceeding 250 Da. Despite expression of Panx1, the host cells did not liberate ATP upon stimulation, making it unclear whether Panx1 is involved in taste-related ATP secretion. This issue was addressed using mice with genetic ablation of the Panx1 gene. The ATP-biosensor assay revealed that, in taste cells devoid of Panx1, ATP secretion was robust and apparently unchanged compared with the control. Our data suggest that Panx1 alone forms a channel that has insufficient permeability to ATP. Perhaps, a distinct subunit and/or a regulatory circuit that is absent in taste cells is required to enable a high ATP-permeability mode of a native Panx1-based channel.

Programmed cell death, or apoptosis, is a physiological cell suicide mechanism, which is triggered in the cells by different stimuli. It has been shown that proteases play a significant role both in the target cell killing by cytotoxic lymphocytes and in the TNF-or anti-Fas-induced cell death. The proteases involved in the early (induction) and late (cell self-destruction) stages of apoptosis are reviewed. It is suggested that the late stages are connected with the activation of a cascade of intracellular proteases, which leads to massive protein destruction. It is likely that the protein destruction is mainly designed for preventing autoimmune response to proteins released from dying cells.

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