Frank Wehner

Mechanisms of cell volume regulation



Contact

Phone: +49 (0231) 133 - 2225
Fax: +49 (0231) 133 - 2299

Research concept


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.


Current research

<strong>Figure 1: Cell volume regulation in the interplay of proliferation vs. apoptosis.</strong> Zoom Image
Figure 1: Cell volume regulation in the interplay of proliferation vs. apoptosis.

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).


<strong>Figure 3:</strong><span> </span><strong>A</strong><span> HICC currents are 2-fold increased post-cryo. </span><strong>B</strong><span> </span><span>Vasopressin in­creases the survival of αENaC expressing cells (HepG2 and HeLa).</span> Zoom Image
Figure 3: A HICC currents are 2-fold increased post-cryo. B Vasopressin in­creases the survival of αENaC expressing cells (HepG2 and HeLa). [less]

Cryo-Project

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.




Literature

1. Wehner F, Sauer H, Kinne RKH (1995) Hypertonic stress increases the Na+ conductance of rat hepatocytes in primary culture. Journal of General Physiology 105:507-535

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

4. Li T, ter Veld F, Nürnberger HR, Wehner F (2005) A novel hypertonicity-induced cation channel in primary cultures of human hepatocytes. FEBS Letters 579:2087-2091

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

6. Wehner F, Tinel H (2000) Osmolyte and Na+ transport balances of rat hepatocytes as a function of hypertonic stress. Pflugers Archiv - European Journal of Physiology 441:12-24

7. Shimizu T, Wehner F, Okada Y (2006) Inhibition of hypertonicity-induced cation channels sensitizes HeLa cells to shrinkage-induced apoptosis. Cellular Physiology and Biochemistry 18:295-302

8. Numata T, Sato K, Okada Y, Wehner F (2008) Hypertonicity-induced cation channels rescue cells from staurosporine-elicited apoptosis. Apoptosis 13:895-903

9. Furchtgott LA, Chow CC, Periwal V (2009) A model of liver regeneration. Biophys J 96:3926-35 DOI 10.1016/j.bpj. 2009.01.061

10. Miyaoka Y, Miyajima A (2013) To divide or not to divide: revisiting liver regeneration. Cell Div 8:8

11. Lang F, Busch GL, Völkl H, Häussinger D (1995) Cell volume: A second message in regulation of cellular function. News in Physiological Sciences 10:18-22

 
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