Multifunctional role of protein kinase C in regulating the formation and maturation of specific synapses

Neuroprotective Effects of Guanosine in Ischemic Stroke-Small Steps towards Effective Therapy

Guanosine (Guo) is a nucleotide metabolite that acts as a potent neuromodulator with neurotrophic and regenerative properties in neurological disorders. Under brain ischemia or trauma, Guo is released to the extracellular milieu and its concentration substantially raises. In vitro studies on brain tissue slices or cell lines subjected to ischemic conditions demonstrated that Guo counteracts destructive events that occur during ischemic conditions, e.g., glutaminergic excitotoxicity, reactive oxygen and nitrogen species production. Moreover, Guo mitigates neuroinflammation and regulates post-translational processing. Guo asserts its neuroprotective effects via interplay with adenosine receptors, potassium channels, and excitatory amino acid transporters. Subsequently, guanosine activates several prosurvival molecular pathways including PI3K/Akt (PI3K) and MEK/ERK. Due to systemic degradation, the half-life of exogenous Guo is relatively low, thus creating difficulty regarding adequate exogenous Guo distribution. Nevertheless, in vivo studies performed on ischemic stroke rodent models provide promising results presenting a sustained decrease in infarct volume, improved neurological outcome, decrease in proinflammatory events, and stimulation of neuroregeneration through the release of neurotrophic factors. In this comprehensive review, we discuss molecular signaling related to Guo protection against brain ischemia. We present recent advances, limitations, and prospects in exogenous guanosine therapy in the context of ischemic stroke.

A decrease in eukaryotic elongation factor 2 phosphorylation is required for local translation of sensorin and long-term facilitation in Aplysia

Mechanistic target of rapamycin complex 1 (mTORC1)-dependent protein synthesis is required for many forms of synaptic plasticity and memory, but the downstream pathways important for synaptic plasticity are poorly understood. Long-term facilitation (LTF) in Aplysia is a form of synaptic plasticity that is closely linked to behavioral memory and an attractive model system for examining the important downstream targets for mTORC1 in regulating synaptic plasticity. Although mTORC1-regulated protein synthesis has been strongly linked to translation initiation, translation elongation is also regulated by mTORC1 and LTF leads to an mTORC1-dependent decrease in eukaryotic elongation factor 2 (eEF2) phosphorylation. The purpose of this study is to test the hypothesis that the decrease in eEF2 phosphorylation is required for mTORC1-dependent translation and plasticity. We show that the LTF-induced decrease in eEF2 phosphorylation is blocked by expression of an eEF2 kinase (eEF2K) modified to be resistant to mTORC1 regulation. We found that expression of this modified kinase blocked LTF. LTF requires local protein synthesis of the neuropeptide sensorin and importantly, local sensorin synthesis can be measured using a dendra fluorescent protein containing the 5' and 3' untranslated regions (UTRs) of sensorin. Using this construct, we show that blocking eEF2 dephosphorylation also blocks the increase in local sensorin synthesis. These results identify decreases in eEF2 phosphorylation as a critical downstream effector of mTOR required for long-term plasticity and identify an important translational target regulated by decreases in eEF2 phosphorylation.

Aplysia cell adhesion molecule and a novel protein kinase C activity in the postsynaptic neuron are required for presynaptic growth and initial formation of specific synapses

To explore the role of both Aplysia cell adhesion molecule (ApCAM) and activity of specific protein kinase C (PKC) isoforms in the initial formation of sensory neuron synapses with specific postsynaptic targets (L7 but not L11), we examined presynaptic growth, initial synapse formation, and the expression of the presynaptic neuropeptide sensorin following cell-specific reduction of ApCAM or of a novel PKC activity. Synapse formation between sensory neurons and L7 begins by 3 h after plating and is accompanied by a rapid accumulation of a novel PKC to sites of synaptic interaction. Reducing ApCAM expression specifically from the surface of L7 blocks presynaptic growth and initial synapse formation, target-induced increase of sensorin in sensory neuron cell bodies and the rapid accumulation of the novel PKC to sites of interaction. Selective blockade of the novel PKC activity in L7, but not in sensory neurons, with injection of a dominant negative construct that interferes with the novel PKC activity, produces the same actions as downregulating ApCAM; blockade of presynaptic growth and initial synapse formation, and the target-induced increase of sensorin in sensory neuron cell bodies. The results indicate that signals initiated by postsynaptic cell adhesion molecule ApCAM coupled with the activation of a novel PKC in the appropriate postsynaptic neuron produce the retrograde signals required for presynaptic growth associated with initial synapse formation, and the target-induced expression of a presynaptic neuropeptide critical for synapse maturation.

Synapse- and stimulus-specific local translation during long-term neuronal plasticity

Long-term memory and synaptic plasticity require changes in gene expression and yet can occur in a synapse-specific manner. Messenger RNA localization and regulated translation at synapses are thus critical for establishing synapse specificity. Using live-cell microscopy of photoconvertible fluorescent protein translational reporters, we directly visualized local translation at synapses during long-term facilitation of Aplysia sensory-motor synapses. Translation of the reporter required multiple applications of serotonin, was spatially restricted to stimulated synapses, was transcript- and stimulus-specific, and occurred during long-term facilitation but not during long-term depression of sensory-motor synapses. Translational regulation only occurred in the presence of a chemical synapse and required calcium signaling in the postsynaptic motor neuron. Thus, highly regulated local translation occurs at synapses during long-term plasticity and requires trans-synaptic signals.

Effects of axotomy on cultured sensory neurons of Aplysia: long-term injury-induced changes in excitability and morphology are mediated by different signaling pathways

To facilitate an understanding of injury-induced changes within the nervous system, we used a single-cell, in vitro model of axonal injury. Sensory neurons were individually dissociated from the CNS of Aplysia and placed into cell culture. The major neurite of some neurons was then transected (axotomized neurons). Axotomy in hemolymph-containing culture medium produced long-term hyperexcitability (LTH-E) and enhanced neuritic sprouting (long-term hypermorphogenesis [LTH-M]). Axotomy in the absence of hemolymph induced LTH-E, but not LTH-M. Hemolymph-derived growth factors may activate tyrosine receptor kinase (Trk) receptors in sensory neurons. To examine this possibility, we treated uninjured (control) and axotomized sensory neurons with K252a, an inhibitor of Trk receptor activity. K252a depressed the excitability of both axotomized and control neurons. K252a also produced a distinct pattern of arborizing outgrowth of neurites in both axotomized and control neurons. Protein kinase C (PKC) is an intracellular signal downstream of Trk; accordingly, we tested the effects of bisindolylmaleimide I (Bis-I), a specific inhibitor of PKC, on the axotomy-induced cellular changes. Bis-I blocked LTH-E, but did not disrupt LTH-M. Finally, because Trk activates the extracellular signal regulated kinase pathway in Aplysia sensory neurons, we examined whether this pathway mediates the injury-induced changes. Sensory neurons were axotomized in the presence of U0126, an inhibitor of mitogen-activated/extracellular receptor-regulated kinase. U0126 blocked the LTH-M due to axotomy, but did not impair LTH-E. Therefore distinct cellular signaling pathways mediate the induction of LTH-E and LTH-M in the sensory neurons.