Mitochondria are the governors of both cell life (e.g. energy generation) and cell death. Some regulation of both of these functions occurs at the level of the outer membrane in that it controls the flow of metabolites and the release of intermembrane space proteins into the cytosol. These two functions are connected in that a drastic reduction in metabolite flow through the outer membrane, associated with VDAC closure*
, can lead to protein release and apoptosis [1
]. However, Ca2+
induced mitochondrial swelling and subsequent cell death may require an increase in Ca2+
flux and thus an increase in outer membrane permeability. Perhaps these apparently conflicting changes in outer membrane permeability may be understood if VDAC closure is irrelevant to a small ion such as Ca2+
and the Ca2+
-induced mitochondrial swelling is strictly an inner-membrane phenomenon.
It is generally believed that mitochondrial swelling is caused by the opening of PTP (permeability transition pores). The pore has a molecular mass cut-off of 1500 Da. Many have proposed that PTP involves both the inner and outer membrane through the participation of VDAC. However, knockout of VDAC1 does not seem to affect the formation of PTP [3
], making the participation of VDAC in PTP questionable. Nevertheless, it is generally believed that Ca2+
should flow easily through VDAC channels because VDAC shows only a weak selectivity for small mono-valent ions [4
]. Indeed, in a recent study [6
], the authors concluded that VDAC opening promotes calcium flux into mitochondria followed by PTP and mitochondrial swelling, i.e., VDAC opening induces cell death.
This conclusion is consistent with the report that anti-apoptotic Bcl-2 family proteins close VDAC channels [7
]. It is also in agreement with studies on gelosin, hexokinase, RuR, DIDS etc. [8
] that closure of VDAC is an antiapoptotic signal but in sharp contrast to other findings that VDAC closure is a pro-apoptotic signal. For example, the anti-apoptotic protein, Bcl-xL
, favors the opening of VDAC channels [12
], while the pro-apoptotic oligonucleotide, G3139 closes VDAC [2
]. In addition, removal of required growth factors from cell lines leads to decreased MOM permeability to metabolites, such as phosphocreatine, and subsequent apoptosis. This is also consistent with VDAC closure. It is possible that apoptosis induced by VDAC closure is through a different pathway from calcium induced PTP.
In addition to the role of Ca+2 in apoptosis, Ca2+
is an important intracellular second messenger. At sub-micromolar concentration, it can also stimulate mitochondrial respiration and phosphorylation by the activation of dehydrogenases [16
]. The flux of Ca2+
into mitochondria seems come from the SR/ER-mitochondria contacts [18
], which propagate calcium signaling into the mitochondria. The permeability of the mitochondrial outer membrane would thus have implications in other Ca2+
signaling besides mitochondrial swelling and cell apoptosis.
To clarify the role of VDAC gating in Ca2+ flux through the outer membrane, we examined the permeability of VDAC to Ca2+ in the open and closed states. The states with higher Ca2+ permeability are different from the state traditionally considered to be the open state.