From M. A. Matzke and A. J. M. Matzke

Sir, We read with interest the recent ‘Problems and Paradigms’ feature ‘Electric fields at the plasma membrane level: a neglected element in the mechanism of cell signalling’ by Olivotto et a/.(’). We suggest, however, that Olivotto et al, did not go far enough in their analysis: most of what they proposed for electric field effects on plasma membrane pro- teins could apply equally well to nuclear membrane proteins and DNA present at the nuclear periphery. The significant ‘electrical dimension’(’) in cell biology could thus be extended to include influences on the structures and inter- actions of macromolecules present in and adjacent to the nuclear envelope. In this view, the electrical properties of the nuclear envelope could play a significant role in regulat- ing gene expression. The nuclear envelope could also mediate electrical signalling between the cell surface and peripherally localized chromatin.

Olivotto et a/. discuss membranes as ‘privileged sites for electric fields’, and explain how membrane proteins are ‘electrical devices’that respond to electric fields. They then propose that voltage-sensitive proteins are placed at the source of signal transduction mechanisms at the plasma membrane and speculate about an electrostatic code of cell signalling. These concepts can be applied to the nuclear envelope and DNA as follows.

The nuclear envelope as a ‘site for electric fields’ Olivotto et al. discuss three types of potential that contribute to the plasma membrane electrostatic profile. Does a similar profile exist for the nuclear envelope? A surface potential - consisting of the potential arising from the attraction of dif- fusible positive ions for fixed negative charges in the lipid bilayer (Gouy-Chapman cloud), together with the potential across the ‘compact layer’, generated at the interface between the monolayer of water molecules at the mem- brane surface and water in the bulk phase - should be com- mon to all membranes. But what about the third component of total transmembrane potential, Vrest, which is the poten- tial difference between extracellular and intracellular bulk phases or, in the case of the nucleus, between the nucleo- plasm and the cytoplasm?

The nuclear envelope consists of an inner and outer nuclear membrane, which are fused at the nuclear pore ~omplexes(~3~). The traditional textbook view is that the nuclear envelope is freely permeable to small inorganic ions because of the presence of large pores and therefore is

unable to maintain a gradient of diffusible ions. In the last five years, however, this view has been challenged by elec- trophysiological studies that demonstrate high nuclear envelope resistances and the presence of ion channels, some of which are possibly the same as the nuclear pore c ~ m p l e x e s ( ~ - ~ ) . These recent results support electrophysio- logical studies of nuclei performed by Loewenstein and coworkers in the early 1 960d6). Because the electrical prop- erties of the nuclear envelope appear to resemble those of the semipermeable plasma membrane(3), electric fields of the magnitude described by Olivotto eta/. (up to 100 kV/cm within a 1 nm distance from the membrane surface) can be expected at the nuclear periphery.

DNA as an ‘electrical device’ The sensitivity of proteins to electric fields derives from a high density of fixed ionic groups and large dipoles(’), which are also features of DNA molecules(7). Electric-field effects on DNA include strand separation, conformational changes, dissociation of ligands and interactions between double helices@). Several authors have suggested that DNA close to membranes might be influenced by membrane electric f i e l d ~ ( ~ 8 ~ ~ ) . The packaging of DNA with proteins into chro- matin in eukaryotic cells increases the complexity of the system, but does not detract from the basic idea that mem- brane electric fields will affect genetic material that is asso- ciated with membranes. Some peripheral chromatin appears to lie (at most) within 1-2 nm of the inner nuclear membrane(”), with no intervening lamin layer(’*). DNA in this fraction would inevitably be influenced by the electric field generated at the membrane.

With respect to gene expression, there are a number of ways in which electric fields at the nuclear periphery could influence the conformations and interactions of peripheral and transmembrane nuclear proteins and DNA(2). Chromatin condensation and gene silencing in particular have been associated with the nuclear periphery. In yeast, the loss of the peripheral positioning of telomeres abolishes telomeric silencing effects, suggesting that silencing requires an asso- ciation of telomeres with the nuclear envelope(I3).

Voltage-sensitive proteins at the source of signal transduction mechanisms of the plasma membrane Olivotto et al. propose that the conformational and func- tional states of voltage-sensitive membrane proteins are coupled to the electric fields generated at the plasma mem-

brane, a relationship that can apply to nuclear membrane proteins, membrane-associated DNA and fields generated at the nuclear envelope. Several growth factors have been localized to the nuclear membrane(14). Electrical continuity could exist between the plasma membrane and the nuclear envelope via endomembranes and/or cytoskeletal elements. Electrical signalling at the cell surface could be transmitted rapidly to the nucleus, resulting in electrically induced changes in the conformation of nuclear membrane proteins and peripherally located DNA/chromatin(2).

Concluding remarks We agree with Olivotto et a/. that deciphering the electric language of cells would be a 'crucial breakthrough in biolog- ical sciencefl). Even more than with field effects at the plasma membrane level, however, electric fields generated at nuclear membranes and their influences on the structures and activities of nuclear proteins and nucleic acids are virtu- ally uncharted territories. Perhaps molecular biologists will come to accept the electrical dimension to cell biology once the impact of membrane electric fields on DNA and nuclear proteins becomes more widely acknowledged and investi- gated.

2 Matzke, A.J.M. and Matzke, M.A. (1991). The electrical properties of the nuclear envelope, and their possible role in the regulation of eukaryotic gene expression. Bioelecfrochem. Bioenerg. 25, 357-370. 3 DeFelice, L.J. and Mazzanti, M. (1995). Biophysics of the nuclear envelope. In Cell Physiology Source Book, pp. 351-366. Academic Press: London. 4 Bustamante, J.O. (1994). Nuclear electrophysiology. J. Membr. Bid 138, 105- 112. 5 Csermely, P., Schnaider, T. and Szanto, I. (1995). Signalling and transport through the nuclear membrane. Biochim. Biophys. Acta 1241, 425-452. 6 Loewenstein, W., Kanno, Y. and Ito, S. (1966). Permeability of nuclear membranes. Ann. NYAcad. Sci. 137,708-716. 7 Porschke, D. (1994). DNA double helices with positive electric dichroism and permanent dipole moments: Non-symmetric charge distributions and 'frozen' configurations. Biophys. Chem. 49, 127-1 39. 8 Porschke, D. (1985). Effects of electric fields on biopolymers. Ann. Rev. Phys. Chem. 36, 159-178. 9 Porschke, D. (1985). Short electric-field pulses convert DNA from 'condensed' to 'free' conformation. Biopolymers24, 1981-1993. 10 Kinoshita, H., Christian, S.D., Kim, M.H., Baker, J.G. and Dryhurst, G. (1977). Interfacial behavior of biologically important purines at the mercury solution interface. In Necfrochemical Studies of Biological Systems. ACS Symposium Series38, 11 3-142. 11 Franke, W.W., Scheer,U., Krohne, G. and Jarasch, E.D. (1981). The nuclear envelope and the architecture of the nuclear periphery. J. CellBiol. 91, 39s-50s. 12 Paddy, M.R., Belmont, A.S., Saumweber, H., Agard, D. and Sedat, J.W. (1 990). lnterphase nuclear envelope lamins form a discontinuous network that interacts with only a fraction of the chromatin in the nuclear periphery. Ce//62, 89- 106. 13 Palladino, F., Laroche, T., Gilson, E., Pillus, L. and Gasser, S. (1993). The positioning of yeast telomeres depends on SIR3, SIR4 and the integrity of the nuclear membrane. Coldspring HarborSymp. Quanf. Biol. LVIII, 733-746. 14 Prochiantz, A. and Theodore, L. (1 995). Nuclear/growth factors. BioEssays 17,39-44.

References 1 Olivotto, M., Arcangeli, A., Carla, M. and Wanke, E. (1996). Electric fields at the plasma membrane level: a neglected element in the mechanisms of cell signalling. BioEssays 18,495-504.

M.A. Matzke and A.J.M. Matrke, Institute of Molecular Biology, Austrian Academy of Science, Billrothstrasse 11, A-5020 Salzburg, Austria. E-mail: mmatzke @oeaw.ac.at