Living animal cells have the plasma membrane potential (V_m), which results from a difference in the electrical charge on the two extreme sides of the plasma membrane, that is, a slight excess of positive and negative ions outside and inside a cell, respectively. Such a difference is controlled by various ion channels. In neuronal cells, the V_m is negative in the resting state and is drastically changed for neurotransmission. In response to some input, the ion channels of neuronal cells get quickly and transiently activated such that the abovementioned difference is cancelled (∆V_m > 0, depolarization). Coordinated activation and deactivation of the ion channels occurs along the neuronal axons, which generates electrical pulses. This is the mechanism of neurotransmission. Therefore, the control of the V_m using a light source is an attractive strategy that can allow targeted, fast control of neuronal activities. In this symposium, we have demonstrated a novel general strategy for the control of the V_m by using donor-acceptor-linked molecules (D-A molecules) capable of intramolecular long-lived charge separation, nanocarriers for membrane targeting of D-A molecules, and light.

The D-A molecules used in this study were triadic complexes of ferrocene (Fc), porphyrin (H₂P), and fullerene (C₆₀) (Fc-H₂P-C₆₀). We had previously reported that, under light irradiation, Fc-H₂P-C₆₀ yields the ferrocenium cation (Fc⁺)-H₂P-C₆₀ radical anion (C₆₀^•-) with a long lifetime of ~0.01 ms [1]. Fc-H₂P-C₆₀ was solubilized in the physiological media with a genetically engineered lipoprotein nanoparticle (cpHDL). cpHDL was capable of high plasma membrane binding without significant cytotoxicity [2]. The Fc-H₂P-C₆₀/cpHDL complex was added to a cell culture medium of neuronal cells (rat pheochromocytoma PC12), and its plasma membrane binding was confirmed by confocal analysis and freeze-fracture electron microscopy. Subsequent light irradiation of PC12 cells decreased the membrane potential (∆V_m > 0, depolarization). Such depolarization was not observed for the reference molecule (H₂P alone) that was incapable of charge separation, clearly demonstrating that the charge separation was responsible for this depolarization [3]. Our following study with other D-A molecules revealed that the degree of the depolarization was positively correlated with the charge separation yield [4].

To gain insight into the mechanism of this photo-induced depolarization, cytotoxicity, channel activities, and membrane capacitance were evaluated in D-A molecule-treated PC12 cells under or after irradiation. Pore formation in the plasma membrane, resulting from severe membrane damage, were not detected. Indeed, the photo-generation of reactive oxygen species (ROS) leading to membrane disruption was less efficient in the D-A molecules than in H₂P incapable of photo-induced charge separation. In contrast, it was found that a specific subtype of cation channels was inhibited, and that the membrane capacitance was decreased by irradiated D-A molecules [5]. In this symposium, I will discuss more about the potential of applying our D-A molecules/carriers system in the biomedical fields.

REFERENCES

[1] Imahori, H. et al., J. Am. Chem. Soc. 2001, 123, 2607-2617.

[2] Nakatsuji, H. et al., Angew. Chem. Int. Ed. 2015, 54, 11725-11729.

[3] Numata, T. et al., J. Am. Chem. Soc. 2012, 134, 6092-6095.

[4] Takano, Y. et al., Chem. Sci. 2016, 7, 3331-3337.

[5] Numata, T. et al., Cell. Physiol. Biochem. 2020, 54, 899-916.