Cellular physics

Abstract

Physical factors shape every aspect of cellular life-temperature, gravity, electrical fields, but also, more subtle factors such as sound or magnetic fields can be perceived by living beings to adjust cellular functions with the challenges of environment. Despite their impact on biology, the mechanisms of perception and transduction of physical fields are often barely understood, in some cases, not even investigated. One reason for this "blind spot" of cell biology might be that the otherwise so efficient repertory of molecular biology is of limited help in this case-perception of physical stimuli requires a transformation process translating physics into biochemistry (so-called susception), and knowledge about the molecular nature of the players involved in this translation process, although important, is not sufficient. When we know that the amyloplasts triggering plant gravitropism are composed of starch, this is interesting, but it does not reveal very much about the nature of gravity perception. We need cellular physics when we want to understand cellular biology. Two contributions in the present issue have adopted a physical approach to cell biology:Nanosecond pulsed electrical fields are used to electroporate and extract plant tissues very efficiently for industrial applications, for instance, during the processing of sugar beets. In a time of rising energy costs, this strategy is becoming progressively attractive. However, the energy input is by orders of magnitude lower than that required for membrane breakage indicating that it is a biological process rather than mere electrophysics that underlies this phenomenon. In fact, such nanosecond pulsed electrical fields have been shown to induce apoptosis in cancer cells and elicit, in the plant model tobacco BY-2, a rapid bundling of actin filaments followed by an actin-dependent permeabilization of the membrane (Berghöfer et al. 2009). This indicates that charging of the membrane, even much below the voltages required for irreversible membrane breakage, can cause a biological effect. Several theoretical, but competing, models have been proposed to explain this phenomenon, but experimental evidence was lacking, at least for plant cells. In the current issue, Flickinger et al. (2010) have used a newly developed Pulsed Laser Fluorescence Microscopy setup in combination with the voltage-sensitive fluorescent dye ANNINE-6 to follow membrane charging with an unprecedented time resolution of 5 ns in tobacco BY-2 protoplasts. They can show maximal voltage changes at the cell poles and can depict the geometrical distribution of charge revealing a distinct azimuthal pattern. This pattern is consistent with only one of the concurrent models proposing that nanosecond pulsed electrical fields induce small but reversible pores at the cell poles that reseal within microseconds after the pulse. In addition, voltage-gated ion channels at the hyperpolarized poles are detected from a reversion of membrane polarity at the hyperpolarized pole. Although these charges occur rapid...