Distinct subunits in heteromeric kainate receptors mediate ionotropic and metabotropic function at hippocampal mossy fiber synapses

Heteromeric kainate receptors (KARs) containing both glutamate receptor 6 (GluR6) and KA2 subunits are involved in KAR-mediated EPSCs at mossy fiber synapses in CA3 pyramidal cells. We report that endogenous glutamate, by activating KARs, reversibly inhibits the slow Ca2+-activated K+ current I(sAHP) and increases neuronal excitability through a G-protein-coupled mechanism. Using KAR knockout mice, we show that KA2 is essential for the inhibition of I(sAHP) in CA3 pyramidal cells by low nanomolar concentrations of kainate, in addition to GluR6. In GluR6(-/-) mice, both ionotropic synaptic transmission and inhibition of I(sAHP) by endogenous glutamate released from mossy fibers was lost. In contrast, inhibition of I(sAHP) was absent in KA2(-/-) mice despite the preservation of KAR-mediated EPSCs. These data indicate that the metabotropic action of KARs did not rely on the activation of a KAR-mediated inward current. Biochemical analysis of knock-out mice revealed that KA2 was required for the interaction of KARs with Galpha(q/11)-proteins known to be involved in I(sAHP) modulation. Finally, the ionotropic and metabotropic actions of KARs at mossy fiber synapses were differentially sensitive to the competitive glutamate receptor ligands kainate (5 nM) and kynurenate (1 mM). We propose a model in which KARs could operate in two modes at mossy fiber synapses: through a direct ionotropic action of GluR6, and through an indirect G-protein-coupled mechanism requiring the binding of glutamate to KA2

Silencing SERCA1b in a few fibers stimulates growth in the entire regenerating soleus muscle

The neonatal isoform of the sarcoplasmic/endoplasmic reticulum Ca2+ ATPase 1 (SERCA1b) is a dominant Ca2+ pump in the young Wbers of regenerating muscle. In vivo transfection of about 1% of the Wbers with SERCA1b RNAi plasmid resulted in no apparent change in the transfected Wbers, but enhanced the increase of fresh weight and Wber size in the whole regenerating rat soleus muscle, until the normal size was reached. Co-transfection of calcineurin inhibitor cain/cabin-1 with SERCA1b RNAi was suYcient to cut down the widespread growth stimulation, but the subsequent transfection of cain into the SERCA1b RNAi transfected muscle did not inhibit muscle growth. The SERCA1b RNAi preferably upregulated the expression of the NFAT reporter lacZ compared to controls when co-transfected into the Wbers. Notably, perimuscular injection of interleukin-4 (IL-4) antibody but not that of an unrelevant antibody completely abolished the growth-promoting eVect of SERCA1b RNAi. This indicates that silencing SERCA1b in a few Wbers stimulates the calcineurin- NFAT-IL-4 pathway and Wber growth in the whole regenerating soleus. These results suggest the presence of an autocrine–paracrine coordination of growing muscle Wbers, and put forward a new method to stimulate skeletal muscle regeneration