No abstract

NMDA receptor-mediated calcium transients play a critical role in synaptogenesis, synaptic plasticity, and excitotoxicity. NMDA receptors are heteromeric complexes of NR1A combined with NR2A, NR2B, NR2C, and/or NR2D subunits. The NR2 subunits determine a variety of electrophysiological and pharmacological properties of the NMDA receptor complex. In this report, we provide evidence for the first time that there is also a significant difference in peak channel open probability (P(o)) between NMDA receptors composed of NR1A/NR2A and those of NR1A/NR2B subunits. First, whole-cell patch-clamp recordings from human embryonic kidney (HEK) 293 cells expressing NMDA receptors revealed that NR1A/NR2A-mediated peak current densities are approximately four times larger than those of NR1A/NR2B. We show that this fourfold difference is unlikely caused by differences in receptor surface expression, since these levels were similar for the two subtypes by Western blot analysis. To determine whether P(o) contributed to the difference in peak current densities, we used two different open channel antagonists, MK-801 and 9-aminoacridine, in a variety of experimental paradigms. Our results indicate that peak P(o) is significantly higher (twofold to fivefold) for NR1A/NR2A than NR1A/NR2B, with estimated values of approximately 0.35 and 0.07, respectively. These results suggest that a change in the relative expression levels of NR2A and NR2B can regulate peak amplitude of NMDA receptor-mediated excitatory postsynaptic potentials and therefore may play a role in mechanisms underlying synaptic plasticity.

A variety of processes limit NMDA (N-methyl-D-aspartate) receptor (NMDAR) activity in response to agonist exposure, including rundown--the decline of peak current with repeated, sustained agonist application. Here we report that calcium and tyrosine phosphorylation differentially regulate rundown of synaptic versus extrasynaptic NMDAR-mediated current in rat hippocampal pyramidal neurons.

SUMMARY Primary hyperparathyroidism (PHPT) is a common cause of bone loss that is modeled by continuous PTH (cPTH) infusion. Here we show that the inflammatory cytokine IL-17A is upregulated by PHPT in humans and cPTH in mice. In humans IL-17A is normalized by parathyroidectomy. In mice treatment with anti-IL-17A antibody and silencing of IL-17A receptor IL-17RA prevent cPTH induced osteocytic and osteoblastic RANKL production and bone loss. Mechanistically, cPTH stimulates conventional T cell production of TNFα (TNF), which increases the differentiation of IL-17A producing Th17 cells via TNF receptor 1 (TNFR1) signaling in CD4+ cells. Moreover, cPTH enhances the sensitivity of naïve CD4+ cells to TNF via GαS/cAMP/Ca++ signaling. Accordingly, conditional deletion of GαS in CD4+ cells and treatment with the calcium channel blocker diltiazem prevents Th17 cell expansion and blocks cPTH induced bone loss. Neutralization of IL-17A and calcium channel blockers may thus represent novel therapeutic strategies for hyperparathyroidism.

Evidence suggests that NMDA receptor-mediated neurotoxicity plays a role in the selective neurodegeneration underlying Huntington's disease (HD). The gene mutation that causes HD encodes an expanded polyglutamine tract of >35 in huntingtin, a protein of unknown function. Both huntingtin and NMDA receptors interact with cytoskeletal proteins, and, for NMDA receptors, such interactions regulate surface expression and channel activity. To determine whether mutant huntingtin alters NMDA receptor expression or function, we coexpressed mutant or normal huntingtin, containing 138 or 15 glutamine repeats, respectively, with NMDA receptors in a cell line and then assessed receptor channel function by patch-clamp recording and surface expression by western blot analysis. It is interesting that receptors composed of NR1 and NR2B subunits exhibited significantly larger currents when coexpressed with mutant compared with normal huntingtin. Moreover, this effect was selective for NRl/NR2B, as NRl/NR2A showed similar currents when coexpressed with mutant versus normal huntingtin. However, ion channel properties and total surface expression of the NR1 subunit were unchanged in cells cotransfected with NRl/NR2B and mutant huntingtin. Our results suggest that mutant huntingtin may increase numbers of functional NRl/NRPB-type receptors at the cell surface. Because NRl/NR2B is the predominant NMDA receptor subtype expressed in medium spiny neostriatal neurons, our findings may help explain the selective vulnerability of these neurons in HD.