SUMMARY Proper establishment of synapses is critical for constructing functional circuits. Interactions between presynaptic neurexins and postsynaptic neuroligins coordinate the formation of synaptic adhesions. An isoform code determines the direct interactions of neurexins and neuroligins across the synapse. However, whether extracellular linker proteins can expand such a code is unknown. Using a combination of in vitro and in vivo approaches, we found that hevin, an astrocyte-secreted synaptogenic protein, assembles glutamatergic synapses by bridging neurexin-1α and neuroligin-1B, two isoforms that do not interact with each other. Bridging of neurexin-1α and neuroligin-1B via hevin is critical for the formation and plasticity of thalamocortical connections in the developing visual cortex. These results show that astrocytes promote the formation of synapses by modulating neurexin/neuroligin adhesions through hevin secretion. Our findings also provide an important mechanistic insight into how mutations in these genes may lead to circuit dysfunction in diseases such as autism.

Spinal cord injury (SCI) results not only in motor deficits, but produces, in many patients, excruciating chronic pain (SCI-Pain). We have previously shown, in a rodent model, that SCI causes suppression of activity in the GABAergic nucleus, zona incerta (ZI), and concomitant increased activity in one of its main targets, the posterior nucleus of the thalamus (PO); the increased PO activity is correlated with the maintenance and expression of hyperalgesia after SCI. Here, we test the hypothesis that SCI causes a similar pathological increase in other thalamic nuclei regulated by ZI, specifically the mediodorsal thalamus (MD), involved in the emotional-affective aspects of pain. We recorded single and multi-unit activity from MD of either anesthetized or awake rats, and compared data from rats with SCI with data from sham-operated controls (anesthetized experiments) or with data from the same animals pre-lesion (awake experiments). Consistent with our hypothesis, MD neurons from rats with SCI show significant increases in spontaneous firing rates, and in the magnitude and duration of responses to noxious stimuli. In a subset of anesthetized animals, similar changes in activity of MD neurons were produced by pharmacologically inactivating ZI in naïve rats, suggesting that the changes in MD after SCI are related to suppressed inhibition from ZI. These data support our hypothesis that SCI-Pain results, at least in part, from a loss of inhibition to thalamic nuclei associated with both the sensory-discriminative and emotional-affective components of pain.

The transcription factors CREB (cAMP response element binding factor), SRF (serum response factor), and MEF2 (myocyte enhancer factor 2) play critical roles in the mechanisms underlying neuronal plasticity. However, the role of the activation of these transcription factors in the different components of plasticity in vivo is not well known. In this study, we tested the role of CREB, SRF, and MEF2 in ocular dominance plasticity (ODP), a paradigm of activity-dependent neuronal plasticity in the visual cortex. These three proteins bind to the synaptic activity response element (SARE), an enhancer sequence found upstream of many plasticity-related genes (

GluA2-lacking, calcium-permeable α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPARs) have unique properties, but their presence at excitatory synapses in pyramidal cells is controversial. We have tested certain predictions of the model that such receptors are present in CA1 cells and show here that the polyamine spermine, but not philanthotoxin, causes use-dependent inhibition of synaptically evoked excitatory responses in stratum radiatum, but not s. oriens, in cultured and acute hippocampal slices. Stimulation of single dendritic spines by photolytic release of caged glutamate induced an N-methyl-d-aspartate receptor-independent, use- and spermine-sensitive calcium influx only at apical spines in cultured slices. Bath application of glutamate also triggered a spermine-sensitive influx of cobalt into CA1 cell dendrites in s. radiatum. Responses of single apical, but not basal, spines to photostimulation displayed prominent paired-pulse facilitation (PPF) consistent with use-dependent relief of cytoplasmic polyamine block. Responses at apical dendrites were diminished, and PPF was increased, by spermine. Intracellular application of pep2m, which inhibits recycling of GluA2-containing AMPARs, reduced apical spine responses and increased PPF. We conclude that some calcium-permeable, polyamine-sensitive AMPARs, perhaps lacking GluA2 subunits, are present at synapses on apical dendrites of CA1 pyramidal cells, which may allow distinct forms of synaptic plasticity and computation at different sets of excitatory inputs.

The transcription factor CREB is involved in a myriad of cellular functions in the central nervous system. For instance, the role of CREB via phosphorylation at the amino-acid residue Serine (Ser) 133 in expressing plasticity-related genes and activity-dependent neuronal plasticity processes has been extensively demonstrated. However, much less is known about the role of CREB phosphorylation at Ser 142 and 143. Here, we employed a viral vector containing a dominant negative form of CREB, with serine-to-alanine mutations at residue 142 and 143 to specifically block phosphorylation at both sites. We then transfected this vector into primary neurons in vitro or intra-cortically injected it into mice in vivo, to test if these phosphorylation events were important for activity-dependent plasticity. We demonstrated by immunohistochemistry of cortical neuronal cultures that the expression of Arc, a known plasticity-related gene, requires triple phosphorylation of CREB at Ser 133, 142, and 143. Moreover, we recorded visually-evoked field potentials in awake mice before and after a 7-day period of monocular deprivation to show that, in addition to CREB phosphorylation at Ser 133, ocular dominance plasticity in the visual cortex also requires CREB phosphorylation at Ser 142/143. Our findings suggest that Ser 142/143 phosphorylation is an additional posttranslational modification of CREB that triggers the expression of specific target genes and activity-dependent neuronal plasticity processes. SIGNIFICANCE STATEMENTThe transcription factor CREB triggers the expression of numerous different gene clusters in response to different cellular stimuli. Previous studies have shown that CREB can be activated by phosphorylation at several of its serine residues. We discovered that ocular dominance plasticity, a type of activity-dependent plasticity in the visual cortex, requires the phosphorylation of three different serine residues on CREB (Ser133, Ser142, and Ser143). The expression of the critical early gene Arc also requires this triple phosphorylation pattern. Elucidating such phosphorylation patterns of CREB required for activity-dependent gene expression could help us better understand the mechanisms of neuronal plasticity.