Calcium influx constitutes the ionic basis for the maintenance of glutamate-induced extended neuronal depolarization associated with hippocampal neuronal death

Excessive activation of neuronal glutamate receptors has been implicated in the pathophysiology of stroke, epilepsy, and traumatic brain injury. Previously, it has been demonstrated that excitotoxic glutamate exposure results in the induction of an extended neuronal depolarization (END), as well as protracted elevations in free intracellular calcium ([Ca(2+)](i)). Both END and the prolonged [Ca(2+)](i) elevations were shown to correlate with subsequent neuronal death. In the current study, we used whole-cell current-clamp electrophysiology and fura-ff Ca(2+) imaging to determine the electrophysiological basis of END. We found that removal of extracellular Ca(2+) but not Na(+) in the post-glutamate period resulted in complete reversal of END, allowing neurons to rapidly repolarize to their initial resting membrane potential (RMP). In addition, removal of extracellular Ca(2+) was sufficient to eliminate the protracted [Ca(2+)](i) elevations induced by excitotoxic glutamate exposure. To investigate the mechanism through which extracellular Ca(2+) was effecting these changes, pharmacological antagonists of well-characterized routes of Ca(2+) entry were tested for their effects on END. Antagonists of glutamate receptors and voltage-gated Ca(2+) channels (VGCCs) had no significant effect on the membrane potential of neurons in END. Likewise, inhibitors of the Na(+)/Ca(2+) exchange (NCX) were ineffective. In contrast, addition of 500 microM ZnCl(2) or 100 microM GdCl(3) to control extracellular medium (containing normal levels of extracellular Ca(2+)) in the post-glutamate period resulted in rapid and complete reversal of END. Addition of 1mM CdCl(2) to control medium had only modest effects on END. These data provide the first direct evidence that END induced by excitotoxic glutamate exposure is caused by an influx of extracellular Ca(2+) and demonstrate that the previously irreversible condition of END can be reversed by removing extracellular Ca(2+). In addition, understanding the electrophysiological basis of this novel Ca(2+)-induced extended depolarization may provide an insight into the pathophysiology of stroke, traumatic brain injury, and other forms of neuronal injury.     

Lateral gene transfer between prokaryotes and multicellular eukaryotes: ongoing and significant?

The expansion of genome sequencing projects has produced accumulating evidence for lateral transfer of genes between prokaryotic and eukaryotic genomes. However, it remains controversial whether these genes are of functional importance in their recipient host. Nikoh and Nakabachi, in a recent paper in BMC Biology, take a first step and show that two genes of bacterial origin are highly expressed in the pea aphid Acyrthosiphon pisum. Active gene expression of transferred genes is supported by three other recent studies. Future studies should reveal whether functional proteins are produced and whether and how these are targeted to the appropriate compartment. We argue that the transfer of genes between host and symbiont may occasionally be of great evolutionary importance, particularly in the evolution of the symbiotic interaction itself.     

Noncontact dipole effects on channel permeation. III. Anomalous proton conductance effects in gramicidin

Proton transport on water wires, of interest for many problems in membrane biology, is analyzed in side-chain analogs of gramicidin A channels. In symmetrical 0.1 N HCl solutions, fluorination of channel Trp(11), Trp-(13), or Trp(15) side chains is found to inhibit proton transport, and replacement of one or more Trps with Phe enhances proton transport, the opposite of the effects on K(+) transport in lecithin bilayers. The current-voltage relations are superlinear, indicating that some membrane field-dependent process is rate limiting. The interfacial dipole effects are usually assumed to affect the rate of cation translocation across the channel. For proton conductance, however, water reorientation after proton translocation is anticipated to be rate limiting. We propose that the findings reported here are most readily interpreted as the result of dipole-dipole interactions between channel waters and polar side chains or lipid headgroups. In particular, if reorientation of the water column begins with the water nearest the channel exit, this hypothesis explains the negative impact of fluorination and the positive impact of headgroup dipole on proton conductance.     

The molecular organization of the beta-globin complex of the deer mouse, Peromyscus maniculatus

Recombinant DNA clones have been isolated that contain 80 kb of the beta-globin complex from the deer mouse, Peromyscus maniculatus. Comparisons of this complex with that from the laboratory mouse, Mus domesticus (with an order 5'-Hbby, Hbb-bhO, Hbb-bhl, Hbb-bh2, Hbb-bh3, Hbb-bl, Hbb-b2 3') highlight organizational trends in the beta-globin complex since the two species diverged. Unlike other mammals studied thus far, the deer mouse possesses three adult genes. Partial sequence analysis indicates that each of the three adult genes is intact and hence may be functional. Hybridization of one of the two Mus pseudogenes, Hbb-bh3, to genomic blots from Peromyscus reveals that it has a homologous counterpart in Peromyscus. Homologous genes to the two gamma-like Mus genes, Hbb-bhO and Hbb-bhl, are also found in Peromyscus. The strong hybridization between the Hbb-bhl genes and significant nucleotide similarity between the Hbb-bhO genes suggest that both pairs are important for the ontogeny of these mice although no known product has been identified for the Hbb-bhO genes. The presence of Hbb-bhO and Hbb-bhl in Peromyscus suggests that the duplication that created this related gene set occurred before the two lineages diverged. A single gene for Hbb-y has been isolated from Peromyscus. The adult region in Peromyscus has undergone significant divergence from the same region in Mus, having three rather than two adult genes, the acquisition of at least 15 kb of extra DNA relative to Mus, and possibly the loss of the Hbb-bh2 pseudogene. The nonadult region of the complex, in contrast, contains the same set of genes apparently distributed over the same amount of DNA as in the Mus beta- globin complex. This observation suggests that the embryonic region of the complex is more evolutionarily stable than the adult region.