Current concepts in nuclear pore electrophysiology.

Over 4 decades ago, microelectrode studies of in situ nuclei showed that, under certain conditions, the nuclear envelope (NE) behaves as a barrier opposing the nucleocytoplasmic flow of physiological ions. As the nuclear pore complexes (NPCs) of the NE are the only pathways for direct nucleocytoplasmic flow, those experiments implied that the NPCs are capable of restricting ion flow. These early studies validated electrophysiology as a useful approach to quantify some of the mechanisms by which NPCs mediate gene activity and expression. Since electron microscopy (EM) and other non-electrophysiological investigations, showed that the NPC lumen is a nanochannel, the opinion prevailed that the NPC could not oppose the flow of ions and, therefore, that electrophysiological observations resulted from technical artifacts. Consequently, the initial enthusiasm with nuclear electrophysiology faded out in less than a decade. In 1990, nuclear electrophysiology was revisited with patch-clamp, the most powerful electrophysiological technique to date. Patch-clamp has consistently demonstrated that the NE has intrinsic ion channel activity. Direct demonstrations of the NPC on-off ion channel gating behavior were published for artificial conditions in 1995 and for intact living nuclei in 2002. This on-off switching/gating behavior can be interpreted in terms of a metastable energy barrier. In the hope of advancing nuclear electrophysiology, and to complement the other papers contained in this special issue of the journal, here I review some of the main technical, experimental, and theoretical issues of the field, with special focus on NPCs.

Nuclear pore ion channel activity in live syncytial nuclei.

Nuclear pore complexes (NPCs) are important nanochannels for the control of gene activity and expression. Most of our knowledge of NPC function has been derived from isolated nuclei and permeabilized cells in cell lysates/extracts. Since recent patch-clamp work has challenged the dogma that NPCs are freely permeable to small particles, a preparation of isolated living nuclei in their native liquid environment was sought and found: the syncytial nuclei in the water of the coconut Cocos nucifera. These nuclei have all properties of NPC-mediated macromolecular transport (MMT) and express foreign green fluorescent protein (GFP) plasmids. They display chromatin movement, are created by particle aggregation or by division, can grow by throwing filaments to catch material, etc. This study shows, for the first time, that living NPCs engaged in MMT do not transport physiological ions - a phenomenon that explains observations of nucleocytoplasmic ion gradients. Since coconuts are inexpensive (less than US$1/nut per litre), this robust preparation may contribute to our understanding of NPCs and cell nucleus and to the development of biotechnologies for the production of DNA, RNA and proteins.