Differential effects of endo- and exopolyphosphatase expression on the induction of the mitochondrial permeability transition pore

Inorganic polyphosphate (polyP) is a polymer that consists of a series of orthophosphates connected by high-energy phosphoanhydride bonds, like those found in ATP. In mammalian mitochondria, polyP has been linked to the activation of the mitochondrial permeability transition pore (mPTP). However, the details of this process are not completely understood. The activation of mPTP by polyP may involve the regulation of bioenergetics, Ca²⁺ buffering, or direct involvement in mPTP channel formation. In this study, using refractive index imaging techniques, we examined mPTP induction in both wild-type (WT) SH-SY5Y cells, and mutant SH-SY5Y cells expressing either mitochondrially targeted exopolyphosphatase (MitoPPX), which depletes polyP by breaking free terminal phosphoanhydride bonds; or endopolyphosphatase (MitoPPN), which cleaves internal phosphoanhydride bonds and thus can target polyP pool with protected terminal groups. Upon treating the cells with the calcium ionophore ferutinin, the influx of Ca²⁺ triggered mitochondrial membrane depolarization and permeabilization in both WT and MitoPPX cells indicating activation of mPTP. However, in MitoPPN cells, mitochondrial depolarization occurred without mPTP activation. Based on these findings we propose the possibility that activation of mPTP is not linked to the pool of free polyP. This supports the hypothesis that polyP is either an important structural component of the mPTP channel or associated with other macromolecular complexes involved in mPTP induction.