Molecular biology and genomics have hugely improved our understanding of the genes and molecular components that regulate the cell cycle – most importantly the molecular checkpoints mechanisms involving cyclin and kinase cascades. Despite a detailed and comprehensive description, the biochemical cell cycle foundation still leaves room for endogenous bioelectric signalling acting as a non-specific control of cell division[1]. Bioelectricity is ubiquitous in biology and currently we observe a return to an (electro)physiological approach, whereby bioelectric cues feedback-loop with canonical cell cycle checkpoints. It is increasingly appreciated that bioelectric signals, arising from the constant flow of charged molecules in and out of the cell, may constitute a fundamental control mechanism of the cell cycle. In fission yeast S. pombe, bioelectricity may orchestrate the cell cycle through membrane potential-dependent recruitment of proteins (e.g. minD) that play key roles in the cell cycle. The potassium ion (K⁺) is crucial in regulating yeast’s bioelectricity, such as the trans-membrane potential (TMP), by creating an asymmetric distribution of ions across the plasma membrane. The importance of potassium in the cell cycle regulation is well recognized[2]. However, the exact mechanism of action is, to date, not fully understood. Early experiments have suggested a link between the TMP and mitotic activity^1,2. For instance, rapidly dividing cells have a depolarized, low membrane potential, whereas non-dividing cells (such as end-differentiated muscles and neurons) have a hyperpolarised membrane potential. Noteworthy is the similarity between two states of high mitotic activity: a tissue explant adaptation to cell culture and malignant transformation in vivo, both presenting extremely depolarized plasma membranes. Pioneering experiments and theories that linked mitotic control to bioelectric signalling succeeded at the basics several decades ago[3]. It was shown that TMP slowly oscillates as a cell passes through its cycle, hyperpolarising at G1/S and depolarizing at G2/M transitions. It was postulated that G1 blockage of mitosis may be fundamentally controlled by the intracellular ionic balances setting up the TMP, which may in turn influence the concentration of different metabolites necessary for mitotic preparation. DNA synthesis and mitosis were blocked when TMP was maintained at the level similar to the resting potential of mitotically quiescent cells. However, the exact mechanism of bioelectric mitotic control remains unclear to this day. This is especially true in the budding yeasts, since its size and steady-state TMP largely differs from somatic cells studied in the abovementioned experiments. This fascinating area of study remained in the shade until recently, perhaps because of a lack of appropriate tools, as well as the shift of attention that came with the genomic revolution.

Yeasts are excellent models to uncover the role of K⁺-dependent regulation in cell-cycle, as they possess effective strategies to distinguish among the alkali-metal cations (K⁺, Na⁺, Li⁺). In budding yeast cells, K⁺ is actively transported into the cell by the K⁺-specific TRK1,2 uniporter, using the energy generated by the proton gradient. Since a high concentration of Na⁺ is toxic, there is no active uptake of Na⁺, but its surplus is actively pumped out of the cell. This allows yeast to keep K⁺ at a fairly high intracellular concentration (200-300 mM), and to actively maintain a high cytosolic K⁺/Na⁺ ratio. The later being important for DNA synthesis associated enzymes activity.

Using genetically-encoded light-gated K⁺ channel (BLINK⁴) it is possible to manipulate the TMP to control the expression of cyclins – and thereby proliferation. Cultured cells partition into different cell cycle stages and no means exist to keep them in synchrony over many generations, making it impossible to administer a fitting perturbation for all cells simultaneously. Optogenetics offers a non-invasive way to send cell-fit activation signals considering the cell cycle stage. It permits study the effect of TMP on the yeast cell cycle, perturbing individual cells at different cell cycle stages with single-cell optogenetic control of the K⁺ uptake. Modeling and implementing bioelectric mitotic control at the single cell level allows to create crate a cell-computer interface (cybergenetics) to steer the cell cycle progress at will.

[1] Urrego D. et al., Potassium channels in cell cycle and cell proliferation. Philos Trans R Soc Lond B Biol Sci. 2014 Feb 3;369(1638)

[2] Blackiston, D. J., McLaughlin, K. A. & Levin, M. Bioelectric controls of cell proliferation: Ion channels, membrane voltage and the cell cycle. Cell Cycle 8, 3527–3536 (2009)

[3] Cone, C. D. Unified theory on the basic mechanism of normal mitotic control and oncogenesis. J. Theor. Biol. 30, 151–181 (1971)

[4] Cosentino, C. et al. (2015. Engineering of a light-gated potassium channel. Science 348, 707–710