Protein kinases and Hebbian function

UBC-9 Acts in GABA Neurons to Control Neuromuscular Signaling in C. elegans

Regulation of excitatory to inhibitory signaling balance is essential to nervous system health and is maintained by numerous enzyme systems that modulate the activity, localization, and abundance of synaptic proteins. SUMOylation is a key post-translational regulator of protein function in diverse cells, including neurons. There, its role in regulating synaptic transmission through pre- and postsynaptic effects has been shown primarily at glutamatergic central nervous system synapses, where the sole SUMO-conjugating enzyme Ubc9 is a critical player. However, whether Ubc9 functions globally at other synapses, including inhibitory synapses, has not been explored. Here, we investigated the role of UBC-9 and the SUMOylation pathway in controlling the balance of excitatory cholinergic and inhibitory GABAergic signaling required for muscle contraction in Caenorhabditis elegans. We found inhibition or overexpression of UBC-9 in neurons modestly increased muscle excitation. Similar and even stronger phenotypes were seen with UBC-9 overexpression specifically in GABAergic neurons, but not in cholinergic neurons. These effects correlated with accumulation of synaptic vesicle-associated proteins at GABAergic presynapses, where UBC-9 and the C. elegans SUMO ortholog SMO-1 localized, and with defects in GABA-dependent behaviors. Experiments involving expression of catalytically inactive UBC-9 [UBC-9(C93S)], as well as co-expression of UBC-9 and SMO-1, suggested wild type UBC-9 overexpressed alone may act via substrate sequestration in the absence of sufficient free SUMO, underscoring the importance of tightly regulated SUMO enzyme function. Similar effects on muscle excitation, GABAergic signaling, and synaptic vesicle localization occurred with overexpression of the SUMO activating enzyme subunit AOS-1. Together, these data support a model in which UBC-9 and the SUMOylation system act at presynaptic sites in inhibitory motor neurons to control synaptic signaling balance in C. elegans. Future studies will be important to define UBC-9 targets at this synapse, as well as mechanisms by which UBC-9 and the SUMO pathway are regulated.

Lysosomal activity associated with developmental axon pruning

Clearance of cellular debris is a critical feature of the developing nervous system, as evidenced by the severe neurological consequences of lysosomal storage diseases in children. An important developmental process, which generates considerable cellular debris, is synapse elimination, in which many axonal branches are pruned. The fate of these pruned branches is not known. Here, we investigate the role of lysosomal activity in neurons and glia in the removal of axon branches during early postnatal life. Using a probe for lysosomal activity, we observed robust staining associated with retreating motor axons. Lysosomal function was involved in axon removal because retreating axons were cleared more slowly in a mouse model of a lysosomal storage disease. In addition, we found lysosomal activity in the cerebellum at the time of, and at sites where, climbing fibers are eliminated. We propose that lysosomal activity is a central feature of synapse elimination. Moreover, staining for lysosomal activity may serve as a marker for regions of the developing nervous system undergoing axon pruning.

Emotion perception deficits following traumatic brain injury: a review of the evidence and rationale for intervention

While the cognitive disturbances that frequently follow severe traumatic brain injury (TBI) are relatively well understood, the ways in which these affect the psychosocial functioning of people with TBI are yet to be determined and have thus received little attention in treatment research. Growing evidence indicates that a significant proportion of individuals with TBI demonstrate an inability to recognize affective information from the face, voice, bodily movement, and posture. Because accurate interpretation of emotion in others is critical for the successful negotiation of social interactions, effective treatments are necessary. Until recently, however, there have been no rehabilitation efforts in this area. The present review examines the literature on emotion perception deficits in TBI and presents a theoretical rationale for targeted intervention. Several lines of research relevant to the remediation of emotion perception in people with TBI are considered. These include work on emotion perception remediation with other cognitively impaired populations, current neuropsychological models of emotion perception and underlying neural systems, and recent conceptualizations of remediation processes. The article concludes with a discussion of the importance of carrying out efforts to improve emotion perception within a contextualized framework in which the day-to-day relevance of training is clear to all recipients.

Extracellular ATP in activity-dependent remodeling of the neuromuscular junction

Electrical activity during early development affects the development and maintenance of synapses (Spitzer [2006]: Nature 4447:707-712), but the intercellular signals regulating maintenance of synapses are not well identified. At the neuromuscular junction, adenosine 5-triphosphate (ATP) is coreleased with acetylcholine at activated nerve terminals to modulate synaptic function. Here we use cocultured mouse motor neurons and muscle cells in a three-compartment cell culture chamber to test whether endogenously released ATP plays a role in activity-dependent maintenance of neuromuscular synapses. The results suggest that ATP release at the synapse counters the negative effect of electrical activity, thus stabilizing activated synapses. Confirming our previous work (Li et al. [2001]: Nat Neurosci 4:871-872), we found that in doubly innervated muscles, electrical stimulation induced heterosynaptic downregulation of the nonstimulated convergent input to the muscle fiber with no or little change of the stimulated inputs. However, in preparations that were stimulated in the presence of apyrase, an enzyme that degrades extracellular ATP, synapse downregulation of stimulated inputs was substantial and significant, and end plate potentials were reduced. Apyrase treatment for 20 h in the absence of stimulation did result in moderate diminution, but this was prevented by blocking spontaneous neural activity with tetrodotoxin. The P2 receptor blocker, suramin, also induced activity-dependent synapse diminution. The decrease in synaptic efficacy produced by prolonged stimulation in the presence of apyrase persisted for greater than 20 h, consistent with a developmental time-course and distinct from the rapid neuromodulatory actions of ATP that have been demonstrated by others. We conclude that extracellular ATP promotes stabilization of the neuromuscular junction and may play a role in activity-dependent synaptic modification during development.

The role of the theta isoform of protein kinase C (PKC) in activity-dependent synapse elimination: evidence from the PKC theta knock-out mouse in vivo and in vitro

PKC plays a critical role in competitive activity-dependent synapse modification at the neuromuscular synapse in vitro and in vivo. This action involves a reduction of the strength of inactive inputs to muscle cells that are activated by other inputs. A decrease of postsynaptic responsiveness and a loss of postsynaptic acetyl choline receptors account for the heterosynaptic loss in vitro. The loss is not seen in preparations in which PKC has been blocked pharmacologically. Here, we show that the loss does not occur in in vitro preparations made from animals genetically modified to lack the theta isoform of PKC. Synapse elimination in the newborn period in vivo is delayed but is eventually expressed in knock-out animals. PKC-dependent synapse reduction is suppressed in heterologous cultures combining normal nerve and PKC theta-deficient muscle, as might be expected from the postsynaptic locus of the changes that underlie the activity-dependent plasticity. Preparations in which PKC theta-deficient neurons innervated normal muscle also exhibited a marked deficit in PKC-deficient synapse reduction. The presynaptic action of PKC theta implied by this observation is blocked by TTX, and we propose that activity-related synapse strengthening is decreased by presynaptic PKC theta. Thus, PKC theta in both presynaptic and postsynaptic elements plays a critical role in activity-dependent synapse modulation and loss. We provide a model for activity-dependent synapse loss incorporating these findings.

Substrates for coincidence detection and calcium signaling for induction of synaptic potentiation in the neonatal visual cortex

Regulation of the efficacy of synaptic transmission by activity-dependent processes has been implicated in learning and memory as well as in developmental processes. We previously described transient potentiation of excitatory synapses onto layer 2/3 pyramidal neurons in the visual cortex that is induced by coincident presynaptic stimulation and postsynaptic depolarization. In the adult visual cortex, activation of N-methyl-d-aspartate (NMDA) glutamate receptors is necessary to induce this plasticity. These receptors act as coincidence detectors, sensing presynaptic glutamate release and postsynaptic depolarization, and cause an influx of Ca(2+) that is necessary for the potentiation. In the neurons of the neonatal visual cortex, on the other hand, coincident presynaptic stimulation and postsynaptic depolarization induce stable long-term potentiation (LTP). In addition, reduced but significant LTP can be induced in many neurons in the presence of the NMDA receptor (NMDAR) antagonist, 2-amino-5-phosphonovaleric acid despite the Ca(2+) requirement. Therefore there must be an alternative postsynaptic Ca(2+) source and coincidence detection mechanism linked to the LTP induction mechanism in the neonatal cortex operating in addition to NMDARs. In this study, we find that in layer 2/3 pyramidal neurons, release of Ca(2+) from inositol trisphosphate (InsP(3)) receptor-mediated intracellular stores and influx through voltage-gated Ca(2+) channels (VGCCs) provide alternative postsynaptic Ca(2+) sources. We hypothesize that InsP(3)Rs are coincidence detectors, sensing presynaptic glutamate release through linkage with group I metabotropic glutamate receptors (mGluRs), and depolarization, through VGCCs. We also find that the downstream protein kinases, PKA and PKC, have a role in potentiation in layer 2/3 pyramidal neurons of the neonatal visual cortex.