Retrograde adenosine/A2A receptor signaling facilitates excitatory synaptic transmission and seizures.

Retrograde signaling at the synapse is a fundamental way by which neurons communicate and neuronal circuit function is fine-tuned upon activity. While long-term changes in neurotransmitter release commonly rely on retrograde signaling, the mechanisms remain poorly understood. Here, we identified adenosine/A2A receptor (A2AR) as a retrograde signaling pathway underlying presynaptic long-term potentiation (LTP) at a hippocampal excitatory circuit critically involved in memory and epilepsy. Transient burst activity of a single dentate granule cell induced LTP of mossy cell synaptic inputs, a BDNF/TrkB-dependent form of plasticity that facilitates seizures. Postsynaptic TrkB activation released adenosine from granule cells, uncovering a non-conventional BDNF/TrkB signaling mechanism. Moreover, presynaptic A2ARs were necessary and sufficient for LTP. Lastly, seizure induction released adenosine in a TrkB-dependent manner, while removing A2ARs or TrkB from the dentate gyrus had anti-convulsant effects. By mediating presynaptic LTP, adenosine/A2AR retrograde signaling may modulate dentate gyrus-dependent learning and promote epileptic activity.

Atypical presentation and percutaneous repair of ascending aortic pseudoaneurysm: a case report.

Background: Ascending aortic pseudoaneurysms (AAPs) are an unusual complication of cardiac or aortic surgery and are associated with a high risk of complications and mortality. Guidelines recommend surgical repair. There is few data concerning percutaneous occlusion of AAP. We present a case of syncope due to vascular and heart chamber compression by a large post-surgical AAP that was filled through a focal leak. Ascending aortic pseudoaneurysm was successfully occluded percutaneously. Case summary: A 66-year-old man with a mechanical aortic prosthesis and a Dacron tube in the ascending aorta presented with syncope due to compression of the right atrium and superior vena cava by a large peritube collection. A computed tomography angiography (CTA) showed a large AAP that was filled through a small focal dehiscence of the tube proximal suture. Patient was dismissed for surgery due to high surgical risk. Then, AAP was successfully occluded percutaneously via a 6-French radial access and local anaesthesia. Discussion: In patients with syncope and previous cardiac surgery, aortic complications should be ruled out. Although transthoracic echocardiography may be useful, CTA is the recommended diagnostic test for ruling out post-surgical AAP and allows the characterization of the number, localization, and size of the leaks. In selected patients with high surgical risk and favourable anatomic characteristics, a percutaneous closure could be indicated.

Donor-derived vasculature is required to support neocortical cell grafts after stroke.

Neural precursor cells (NPCs) transplanted into the adult neocortex generate neurons that synaptically integrate with host neurons, supporting the possibility of achieving functional tissue repair. However, poor survival and functional neuronal recovery of transplanted NPCs greatly limits engraftment. Here, we test the hypothesis that combining blood vessel-forming vascular cells with neuronal precursors improves engraftment. By transplanting mixed embryonic neocortical cells into adult mice with neocortical strokes, we show that transplant-derived neurons synapse with appropriate targets while donor vascular cells form vessels that fuse with the host vasculature to perfuse blood within the graft. Although all grafts became vascularized, larger grafts had greater contributions of donor-derived vessels that increased as a function of their distance from the host-graft border. Moreover, excluding vascular cells from the donor cell population strictly limited graft size. Thus, inclusion of vessel-forming vascular cells with NPCs is required for more efficient engraftment and ultimately for tissue repair.

Targeted deletion of AKAP7 in dentate granule cells impairs spatial discrimination.

Protein Kinase A (PKA) mediates synaptic plasticity and is widely implicated in learning and memory. The hippocampal dentate gyrus (DG) is thought to be responsible for processing and encoding distinct contextual associations in response to highly similar inputs. The mossy fiber (MF) axons of the dentate granule cells convey strong excitatory drive to CA3 pyramidal neurons and express presynaptic, PKA-dependent forms of plasticity. Here, we demonstrate an essential role for the PKA anchoring protein, AKAP7, in mouse MF axons and terminals. Genetic ablation of AKAP7 specifically from dentate granule cells results in disruption of MF-CA3 LTP directly initiated by cAMP, and the AKAP7 mutant mice are selectively deficient in pattern separation behaviors. Our results suggest that the AKAP7/PKA complex in the MF projections plays an essential role in synaptic plasticity and contextual memory formation.

Synaptotagmin-12 phosphorylation by cAMP-dependent protein kinase is essential for hippocampal mossy fiber LTP.

Synaptotagmin-12 (Syt12) is an abundant synaptic vesicle protein that--different from other synaptic vesicle-associated synaptotagmins--does not bind Ca(2+). Syt12 is phosphorylated by cAMP-dependent protein kinase-A at serine-97 in an activity-dependent manner, suggesting a function for Syt12 in cAMP-dependent synaptic plasticity. To test this hypothesis, we here generated (1) Syt12 knock-out mice and (2) Syt12 knockin mice carrying a single amino-acid substitution [the serine-97-to-alanine- (S97A)-substitution]. Both Syt12 knock-out mice and Syt12 S97A-knockin mice were viable and fertile, and exhibited no measurable change in basal synaptic strength or short-term plasticity as analyzed in cultured cortical neurons or in acute hippocampal slices. However, both Syt12 knock-out and Syt12 S97A-knockin mice displayed a major impairment in cAMP-dependent mossy-fiber long-term potentiation (LTP) in the CA3 region of the hippocampus. This impairment was observed using different experimental configurations for inducing and monitoring mossy-fiber LTP. Moreover, although the Syt12 knock-out had no effect on the short-term potentiation of synaptic transmission induced by the adenylate-cyclase activator forskolin in cultured cortical neurons and in the CA1 region of the hippocampus, both the Syt12 knock-out and the Syt12 S97A-knockin impaired the long-term increase in mossy-fiber synaptic transmission induced by forskolin. Thus, Syt12 is essential for cAMP-dependent presynaptic LTP at mossy-fiber synapses, and a single amino-acid substitution that blocks the cAMP-dependent phosphorylation of Syt12 is sufficient to impair the function of Syt12 in mossy-fiber LTP, suggesting that cAMP-dependent phosphorylation of Syt12 on serine-97 contributes to the induction of mossy-fiber LTP.

Estradiol attenuates ischemia-induced death of hippocampal neurons and enhances synaptic transmission in aged, long-term hormone-deprived female rats.

BACKGROUND: Transient global forebrain ischemia causes selective, delayed death of hippocampal CA1 pyramidal neurons, and the ovarian hormone 17ß-estradiol (E2) reduces neuronal loss in young and middle-aged females. The neuroprotective efficacy of E2 after a prolonged period of hormone deprivation is controversial, and few studies examine this issue in aged animals given E2 treatment after induction of ischemia. METHODOLOGY/PRINCIPAL FINDINGS: The present study investigated the neuroprotective effects of E2 administered immediately after global ischemia in aged female rats (15-18 months) after 6 months of hormone deprivation. We also used electrophysiological methods to assess whether CA1 synapses in the aging hippocampus remain responsive to E2 after prolonged hormone withdrawal. Animals were ovariohysterectomized and underwent 10 min global ischemia 6 months later. A single dose of E2 (2.25 µg) infused intraventricularly after reperfusion significantly increased cell survival, with 45% of CA1 neurons surviving vs 15% in controls. Ischemia also induced moderate loss of CA3/CA4 pyramidal cells. Bath application of 1 nM E2 onto brain slices derived from non-ischemic aged females after 6 months of hormone withdrawal significantly enhanced excitatory transmission at CA1 synapses evoked by Schaffer collateral stimulation, and normal long-term potentiation (LTP) was induced. The magnitude of LTP and of E2 enhancement of field excitatory postsynaptic potentials was indistinguishable from that recorded in slices from young rats. CONCLUSIONS/SIGNIFICANCE: The data demonstrate that 1) acute post-ischemic infusion of E2 into the brain ventricles is neuroprotective in aged rats after 6 months of hormone deprivation; and 2) E2 enhances synaptic transmission in CA1 pyramidal neurons of aged long-term hormone deprived females. These findings provide evidence that the aging hippocampus remains responsive to E2 administered either in vivo or in vitro even after prolonged periods of hormone withdrawal.

RIM1alpha phosphorylation at serine-413 by protein kinase A is not required for presynaptic long-term plasticity or learning.

Activation of presynaptic cAMP-dependent protein kinase A (PKA) triggers presynaptic long-term plasticity in synapses such as cerebellar parallel fiber and hippocampal mossy fiber synapses. RIM1alpha, a large multidomain protein that forms a scaffold at the presynaptic active zone, is essential for presynaptic long-term plasticity in these synapses and is phosphorylated by PKA at serine-413. Previous studies suggested that phosphorylation of RIM1alpha at serine-413 is required for presynaptic long-term potentiation in parallel fiber synapses formed in vitro by cultured cerebellar neurons and that this type of presynaptic long-term potentiation is mediated by binding of 14-3-3 proteins to phosphorylated serine-413. To test the role of serine-413 phosphorylation in vivo, we have now produced knockin mice in which serine-413 is mutated to alanine. Surprisingly, we find that in these mutant mice, three different forms of presynaptic PKA-dependent long-term plasticity are normal. Furthermore, we observed that in contrast to RIM1alpha KO mice, RIM1 knockin mice containing the serine-413 substitution exhibit normal learning capabilities. The lack of an effect of the serine-413 mutation of RIM1alpha is not due to compensation by RIM2alpha because mice carrying both the serine-413 substitution and a RIM2alpha deletion still exhibited normal long-term presynaptic plasticity. Thus, phosphorylation of serine-413 of RIM1alpha is not essential for PKA-dependent long-term presynaptic plasticity in vivo, suggesting that PKA operates by a different mechanism despite the dependence of long-term presynaptic plasticity on RIM1alpha.

Endocannabinoid-mediated long-term plasticity requires cAMP/PKA signaling and RIM1alpha.

Endocannabinoids (eCBs) have emerged as key activity-dependent signals that, by activating presynaptic cannabinoid receptors (i.e., CB1) coupled to G(i/o) protein, can mediate short-term and long-term synaptic depression (LTD). While the presynaptic mechanisms underlying eCB-dependent short-term depression have been identified, the molecular events linking CB1 receptors to LTD are unknown. Here we show in the hippocampus that long-term, but not short-term, eCB-dependent depression of inhibitory transmission requires presynaptic cAMP/PKA signaling. We further identify the active zone protein RIM1alpha as a key mediator of both CB1 receptor effects on the release machinery and eCB-dependent LTD in the hippocampus. Moreover, we show that eCB-dependent LTD in the amygdala and hippocampus shares major mechanistic features. These findings reveal the signaling pathway by which CB1 receptors mediate long-term effects of eCBs in two crucial brain structures. Furthermore, our results highlight a conserved mechanism of presynaptic plasticity in the brain.

Protein kinase A regulates calcium permeability of NMDA receptors.

Calcium (Ca2+) influx through NMDA receptors (NMDARs) is essential for synaptogenesis, experience-dependent synaptic remodeling and plasticity. The NMDAR-mediated rise in postsynaptic Ca2+ activates a network of kinases and phosphatases that promote persistent changes in synaptic strength, such as long-term potentiation (LTP). Here we show that the Ca2+ permeability of neuronal NMDARs is under the control of the cyclic AMP-protein kinase A (cAMP-PKA) signaling cascade. PKA blockers reduced the relative fractional Ca2+ influx through NMDARs as determined by reversal potential shift analysis and by a combination of electrical recording and Ca2+ influx measurements in rat hippocampal neurons in culture and hippocampal slices from mice. In slices, PKA blockers markedly inhibited NMDAR-mediated Ca2+ rises in activated dendritic spines, with no significant effect on synaptic current. Consistent with this, PKA blockers depressed the early phase of NMDAR-dependent LTP at hippocampal Schaffer collateral-CA1 (Sch-CA1) synapses. Our data link PKA-dependent synaptic plasticity to Ca2+ signaling in spines and thus provide a new mechanism whereby PKA regulates the induction of LTP.

Assessing the role of Ih channels in synaptic transmission and mossy fiber LTP.

Hyperpolarization-activated nonselective cation channels (Ih channels) play an important role in the control of membrane excitability and rhythmic neuronal activity. The functional relevance of presynaptic Ih channels in regulating synaptic function, however, is not well established. Recently, it has been proposed [Mellor, J., Nicoll, R. A. & Schmitz, D. (2002) Science 295, 143-147] that presynaptic Ih channels are necessary for hippocampal mossy fiber long-term potentiation (LTP). This observation challenges an alternative model that suggests presynaptic forms of LTP are caused by a direct modification of the transmitter release machinery. Here, we assess the role of Ih in hippocampal mossy fiber LTP as well as cerebellar parallel fiber LTP, forms of potentiation that share common mechanisms. Our results show that after Ih blockade neither mossy fiber LTP nor parallel fiber LTP are affected. Furthermore, Ih does not significantly modify basal excitatory synaptic transmission in the hippocampus, whereas the organic Ih blockers ZD7288 and DK-AH 269 induce a large Ih-independent depression of synaptic transmission. In summary, our results indicate that Ih-mediated persistent changes in presynaptic excitability do not underlie presynaptic forms of LTP.