Mechanisms of cell volume regulation
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We are working on the mechanisms of cell volume regulation in human, mouse and rat hepatocytes using, both, primary cells as well as various cell lines. In the liver, regulation of cell volume is employed in
• the maintenance of cell homoeostasis, i.e. the normal functional status of a cell,
• the modulation of cell metabolism,
• the triggering of cell proliferation, on the one hand, and of apoptosis (the “programmed cell death”), on the other.
HICCS as the main players in the regulatory volume increase (rvi)
In the last two decades, our group could identify hypertonicity-induced cation channels (HICCs) as the main players in the regulatory volume increase (RVI) of cells undergoing hypertonic stress [1-6]. On the molecular level, the α-subunit of the (epithelial Na+ channel) ENaC and the ΔC splice variant of (the transient receptor potential channel) TRPM2 could recently be defined as HICC components, in HepG2 and in HeLa cells [3, 5].
Furthermore, under isotonic conditions, HICCs are essential mediators of the cell growth proceeding proliferation and they are opposing the process of apoptosis [7, 8] (Figure 1).
As the main mediators of RVI, not unexpectedly, the activation of HICCs was of pivotal significance for the regain of cell water as it is occurring after cryo-preservation. Decoding this adaptive response of cell hydration to low temperatures was of fundamental significance for the “cryo project” in which multiple and reversible arrests of living cells for high-resolution imaging could be established as a novel approach (Masip et al. 2016).
Slow cooling leads to a passive dehydration of cells whereas rehydration during warming reflects the active regain of functionality. The ability to modulate such an energy demanding process could be instrumental in optimizing the cryo-arrest of living systems. We used various levels of hypertonic stress to disturb the water content of cells and to define the energy profiles of aquaporins and (Na+ conducting) cation channels during rehydration. Na+ import was found to be the rate-limiting step in water restoration whereas aquaporins played merely a permissive role (Christmann et al 2016). Indeed, regulated Na+ import was increased 2-fold following cryo-arrests (Figure 3A) hereby facilitating the osmotic rehydration of cells. Freezing temperatures increased cell viscosity with a remarkable hysteresis and viscosity was a trigger of cation channels. The peptide hormone vasopressin was a further activator of channels increasing the viability of post-cryo cells considerably (Figure 3B). Hence, the hormone opens the path to a novel class of cryo-protectants with an intrinsic biological activity.
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2. Wehner F, Shimizu T, Sabirov R, Okada Y (2003) Hypertonic activation of a non-selective cation conductance in HeLa cells and its contribution to cell volume regulation. FEBS Letters 551:20-24
3. Bondarava M, Li T, Endl E, Wehner F (2009) alpha-ENaC is a functional element of the hypertonicity-induced cation channel in HepG2 cells and it mediates proliferation. Pflugers Archiv - European Journal of Physiology 458:675-687
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5. Numata T, Sato K, Christmann C, Marx R, Mori Y, Okada Y, Wehner F (2012) The DC splice-variant of TRPM2 is the hypertonicity-induced cation channel (HICC) in HeLa cells and the ecto-enzyme CD38 mediates its activation. Journal of Physiology 590.5:1121-1138
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