Accumulation of lipophilic dications by mitochondria and cells

Abstract

Lipophilic monocations can pass through phospholipid bilayers and accumulate in negatively-charged compartments such as the mitochondrial matrix, driven by the membrane potential. This property is used to visualize mitochondria, to deliver therapeutic molecules to mitochondria and to measure the membrane potential. In theory, lipophilic dications have a number of advantages over monocations for these tasks, as the double charge should lead to a far greater and more selective uptake by mitochondria, increasing their therapeutic potential. However, the double charge might also limit the movement of lipophilic dications through phospholipid bilayers and little is known about their interaction with mitochondria. To see whether lipophilic dications could be taken up by mitochondria and cells, we made a series of bistriphenylphosphonium cations comprising two triphenylphosphonium moieties linked by a 2-, 4-, 5-, 6- or 10-carbon methylene bridge. The 5-, 6- and 10-carbon dications were taken up by energized mitochondria, whereas the 2- and 4-carbon dications were not. The accumulation of the dication was greater than that of the monocation methyltriphenylphosphonium. However, the uptake of dications was only described by the Nernst equation at low levels of accumulation, and beyond a threshold membrane potential of 90-100 mV there was negligible increase in dication uptake. Interestingly, the 5- and 6-carbon dications were not accumulated by cells, due to lack of permeation through the plasma membrane. These findings indicate that conjugating compounds to dications offers only a minor increase over monocations in delivery to mitochondria. Instead, this suggests that it may be possible to form dications within mitochondria that then remain within the cell.

Derivatives of Rhodamine 19 as Mild Mitochondria-targeted Cationic Uncouplers

“…The scheme suggests a transmembrane movement for two forms of our H ϩ carriers: a deprotonated neutral form R and a monoprotonated cationic form RH ϩ . We also assume that a doubly protonated form, RH 2 2ϩ , has considerably lower membrane permeability because dimers of alkyltriphenylphosphonium are known to be substantially less permeable than tetraphenylphosphonium itself (47,48). Apparently, the neutral form of the carrier R should have much higher permeability than the charged form RH ϩ .…”

Photodynamic therapy with fullerenes

“…The single cationic charge possessed by BF4 is also likely to play an important role in determining its relative phototoxicity. Many lipophilic monocations have been shown to localize fairly specifically in mitochondria [86][87][88] and this property has been proposed as a strategy to target drugs to mitochondria. 89 We have also performed preliminary experiments comparing effectiveness of BF4 to Photofrin ® , one of the clinically approved photosensitizers for cancer therapy.…”

Pharmacological Targeting of the Mitochondrial Phosphatase PTPMT1

“…Having determined that alexidine dihydrochloride was an effective chemical inhibitor of PTPMT1 in vitro, we were keen to explore whether the compound would inhibit PTPMT1 in cells. We were hopeful of this in part due to PTPMT1 being resident on the matrix-facing side of the inner mitochondrial membrane, and lipophilic dications (a structural class of which alexidine dihydrochloride is a member) have previously been shown to target the mitochondria, their hydrophobicity allowing them to cross the membranes while their positive charge encourages accumulation in the matrix (Ross et al, 2006;Murphy and Smith, 2007). As the role of PTPMT1 had previously been studied in the pancreatic ␤-cell, and genetic perturbation had been shown to affect insulin secretion, we used this system as our model.…”