Allosteric regulation and spatial distribution of kainate receptors bound to ancillary proteins.

A diverse range of accessory proteins regulates the behaviour of most ligand- and voltage-gated ion channels. For glutamate receptor 6 (GluR6) kainate receptors, two unrelated proteins, concanavalin-A (Con-A) and postsynaptic density protein 95 (PSD-95), bind to extra- and intracellular domains, respectively, but are reported to exert similar effects on GluR6 desensitization behaviour. We have tested the hypothesis that distinct allosteric binding sites control GluR6 receptors via a common transduction pathway. Rapid agonist application to excised patches revealed that neither Con-A nor PSD-95 affect the onset of desensitization. The rate of desensitization elicited by 10 mM L-glutamate was similar in control (taufast = 5.5 +/- 0.4 ms), Con-A-treated patches (taufast = 6.1 +/- 0.5 ms) and patches containing PSD-95 and GluR6 receptors (taufast = 4.7 +/- 0.6 ms). Likewise, the time course of recovery from GluR6 desensitization was similar in both control and Con-A conditions, whereas PSD-95 accelerated recovery almost twofold. Peak and steady-state (SS) dose-response relationships to glutamate were unchanged by lectin treatment (e.g. control, EC50(SS) = 31 +/- 28 microM vs Con-A, EC50(SS) = 45 +/- 9 microM, n = 6), suggesting that Con-A does not convert non-conducting channels with high agonist affinity into an open conformation. Instead, we demonstrate that the effects of Con-A on macroscopic responses reflect a shift in the relative contribution of different open states of the channel. In contrast, the effect of PSD-95 on recovery behaviour suggests that the association between kainate receptors and cytoskeletal proteins regulates signalling at glutamatergic synapses. Our results show that Con-A and PSD-95 regulate kainate receptors via distinct allosteric mechanisms targeting selective molecular steps in the transduction pathway.

Transplanted bone marrow generates new neurons in human brains.

Adult bone marrow stem cells seem to differentiate into muscle, skin, liver, lung, and neuronal cells in rodents and have been shown to regenerate myocardium, hepatocytes, and skin and gastrointestinal epithelium in humans. Because we have demonstrated previously that transplanted bone marrow cells can enter the brain of mice and differentiate into neurons there, we decided to examine postmortem brain samples from females who had received bone marrow transplants from male donors. The underlying diseases of the patients were lymphocytic leukemia and genetic deficiency of the immune system, and they survived between 1 and 9 months after transplant. We used a combination of immunocytochemistry (utilizing neuron-specific antibodies) and fluorescent in situ hybridization histochemistry to search for Y chromosome-positive cells. In all four patients studied we found cells containing Y chromosomes in several brain regions. Most of them were nonneuronal (endothelial cells and cells in the white matter), but neurons were certainly labeled, especially in the hippocampus and cerebral cortex. The youngest patient (2 years old), who also lived the longest time after transplantation, had the greatest number of donor-derived neurons (7 in 10,000). The distribution of the labeled cells was not homogeneous. There were clusters of Y-positive cells, suggesting that single progenitor cells underwent clonal expansion and differentiation. We conclude that adult human bone marrow cells can enter the brain and generate neurons just as rodent cells do. Perhaps this phenomenon could be exploited to prevent the development or progression of neurodegenerative diseases or to repair tissue damaged by infarction or trauma.

Functional stoichiometry of glutamate receptor desensitization.

Potassium (K+) channels and ionotropic glutamate receptors (iGluRs) fulfill divergent roles in vertebrate nervous systems. Despite this, however, recent work suggests that these ion channels are structurally homologous, sharing an ancestral protein, architectural design, and tetrameric subunit stoichiometry. Their gating mechanisms also are speculated to have overlapping features. Here we show that the mechanism of iGluR desensitization is unique. Unlike K+ channels, AMPA- and kainate-type iGluR subunits desensitize in several ordered conformational steps. AMPA receptors operate as dimers, whereas the functional stoichiometry of kainate receptor desensitization is dependent on external ions. Contrary to conventional understanding, kinetic models suggest that partially desensitized AMPA and kainate receptors conduct ions and are likely participants in synaptic signaling. Although sharing many structural correlates with K+ channels, iGluRs have evolved unique subunit-subunit interactions, tailoring their gating behavior to fulfill distinct roles in neuronal signaling.