Target cell-specific involvement of presynaptic mitochondria in post-tetanic potentiation at hippocampal mossy fiber synapses.
ABSTRACT Previous studies indicate that boutons from the same axon exhibit distinct Ca2+ dynamics depending on the postsynaptic targets. Mossy fibers of hippocampal granule cells innervate synaptic targets via morphologically distinct boutons. We investigated mitochondrial involvement in the generation of post-tetanic residual Ca2+ (Ca(res)) at large and small en passant mossy fiber boutons (MFBs). Mitochondria limited the [Ca2+]i build-up during high-frequency stimulation (HFS) at large MFBs, but not at small MFBs. The amount of Ca(res), quantified as a time integral of residual [Ca2+]i, was significantly larger at large MFBs than at small MFBs, and that at large MFBs was substantially attenuated by inhibitors of mitochondrial Ca2+ uniporter and mitochondrial Na+/Ca2+ exchanger (mitoNCX). In contrast, blockers of mitoNCX had no effect on the Ca(res) at small MFBs. Post-tetanic Ca(res) has been proposed as a mechanism for post-tetanic potentiation (PTP). We examined mitochondrial involvement in PTP at mossy fiber synapses on hilar mossy cells (MF-->MC synapse) and on hilar interneurons (MF-->HI synapse), which are presumably innervated via large and small MFBs, respectively. Consistent with the differential contribution of mitochondria to Ca(res) at large and small MFBs, mitoNCX blockers significantly reduced the PTP at the MF-->MC synapse, but not at the MF-->HI synapse. In contrast, protein kinase C (PKC) inhibitors significantly reduced the PTP at MF-->HI synapse, but not at the MF-->MC synapse. These results indicate that mitochondria- and PKC-dependent PTP are expressed at distinct hilar mossy fiber synapses depending on postsynaptic targets.
Article: Adaptive Regulation Maintains Posttetanic Potentiation at Cerebellar Granule Cell Synapses in the Absence of Calcium-Dependent PKC.[show abstract] [hide abstract]
ABSTRACT: Posttetanic potentiation (PTP) is a transient, calcium-dependent increase in the efficacy of synaptic transmission following elevated presynaptic activity. The calcium-dependent protein kinase C (PKC(Ca)) isoforms PKCα and PKCβ mediate PTP at the calyx of Held synapse, with PKCβ contributing significantly more than PKCα. It is not known whether PKC(Ca) isoforms play a conserved role in PTP at other synapses. We examined this question at the parallel fiber → Purkinje cell (PF→PC) synapse, where PKC inhibitors suppress PTP. We found that PTP is preserved when single PKC(Ca) isoforms are knocked out and in PKCα/β double knock-out (dko) mice, even though in the latter all PKC(Ca) isoforms are eliminated from granule cells. However, in contrast to wild-type and single knock-out animals, PTP in PKCα/β dko animals is not suppressed by PKC inhibitors. These results indicate that PKC(Ca) isoforms mediate PTP at the PF→PC synapse in wild-type and single knock-out animals. However, unlike the calyx of Held, at the PF→PC synapse either PKCα or PKCβ alone is sufficient to mediate PTP, and if both isoforms are eliminated a compensatory PKC-independent mechanism preserves the plasticity. These results suggest that a feedback mechanism allows granule cells to maintain the normal properties of short-term synaptic plasticity even when the mechanism that mediates PTP in wild-type mice is eliminated.Journal of Neuroscience 09/2012; 32(38):13004-9. · 7.11 Impact Factor
Article: Endocytosis of somatodendritic NCKX2 is regulated by Src family kinase-dependent tyrosine phosphorylation.[show abstract] [hide abstract]
ABSTRACT: We have previously reported that the surface expression of K(+)-dependent Na(+)/Ca(2+) exchanger 2 (NCKX2) in the somatodendritic compartment is kept low by constitutive endocytosis, which results in the polarization of surface NCKX2 to the axon. Clathrin-mediated endocytosis is initiated by interaction of the μ subunit of adaptor protein complex 2 (AP-2) with the canonical tyrosine motif (YxxΦ) of a target molecule. We examined whether endocytosis of NCKX2 involves two putative tyrosine motifs ((365)YGKL and (371)YDTM) in the cytoplasmic loop of NCKX2. Coimmunoprecipitation assay revealed that the (365)YGKL motif is essential for the interaction with the μ subunit of AP-2 (AP2M1). Consistently, either overexpression of NCKX2-Y365A mutant or knockdown of AP2M1 in cultured hippocampal neurons significantly reduced the internalization of NCKX2 from the somatodendritic surface and thus abolished the axonal polarization of surface NCKX2. Next, we tested whether the interaction between the tyrosine motif and AP2M1 is regulated by phosphorylation of the 365th tyrosine residue (Tyr-365). Tyrosine phosphorylation of heterologously expressed NCKX2-WT, but not NCKX2-Y365A, was increased by carbachol (CCh) in PC-12 cells. The effect of CCh was inhibited by PP2, a Src family kinase (SFK) inhibitor. Moreover, PP2 facilitated the endocytosis of NCKX2 in both the somatodendritic and axonal compartments, suggesting that tyrosine phosphorylation of NCKX2 by SFK negatively regulates its endocytosis. Supporting this idea, activation of SFK enhanced the NCKX activity in the proximal dendrites of dentate granule cells (GCs). These results suggest that endocytosis of somatodendritic NCKX2 is regulated by SFK-dependent phosphorylation of Tyr-365.Frontiers in Cellular Neuroscience 01/2013; 7:14. · 4.17 Impact Factor
Article: Calcium-dependent isoforms of protein kinase C mediate posttetanic potentiation at the calyx of Held.[show abstract] [hide abstract]
ABSTRACT: High-frequency stimulation leads to a transient increase in the amplitude of evoked synaptic transmission that is known as posttetanic potentiation (PTP). Here we examine the roles of the calcium-dependent protein kinase C isoforms PKCα and PKCβ in PTP at the calyx of Held synapse. In PKCα/β double knockouts, 80% of PTP is eliminated, whereas basal synaptic properties are unaffected. PKCα and PKCβ produce PTP by increasing the size of the readily releasable pool of vesicles evoked by high-frequency stimulation and by increasing the fraction of this pool released by the first stimulus. PKCα and PKCβ do not facilitate presynaptic calcium currents. The small PTP remaining in double knockouts is mediated partly by an increase in miniature excitatory postsynaptic current amplitude and partly by a mechanism involving myosin light chain kinase. These experiments establish that PKCα and PKCβ are crucial for PTP and suggest that long-lasting presynaptic calcium increases produced by tetanic stimulation may activate these isoforms to produce PTP.Neuron 06/2011; 70(5):1005-19. · 14.74 Impact Factor