UBE3A expression during early postnatal brain development is required for proper dorsomedial striatal maturation

Angelman syndrome (AS) is a severe neurodevelopmental disorder (NDD) caused by loss of functional ubiquitin protein ligase E3A (UBE3A). Previous studies showed that UBE3A plays an important role in the first postnatal weeks of mouse brain development, but its precise role is unknown. Since impaired striatal maturation has been implicated in several mouse models for NDDs, we studied the importance of UBE3A in striatal maturation. We used inducible Ube3a mouse models to investigate the maturation of medium spiny neurons (MSNs) from dorsomedial striatum. MSNs of mutant mice matured properly till postnatal day 15 (P15) but remained hyperexcitable with fewer excitatory synaptic events at later ages, indicative of stalled striatal maturation in Ube3a mice. Reinstatement of UBE3A expression at P21 fully restored MSN excitability but only partially restored synaptic transmission and the operant conditioning behavioral phenotype. Gene reinstatement at P70 failed to rescue both electrophysiological and behavioral phenotypes. In contrast, deletion of Ube3a after normal brain development did not result in these electrophysiological and behavioral phenotypes. This study emphasizes the role of UBE3A in striatal maturation and the importance of early postnatal reinstatement of UBE3A expression to obtain a full rescue of behavioral phenotypes associated with striatal function in AS.

Identifying the temporal electrophysiological and molecular changes that contribute to TSC-associated epileptogenesis

Tuberous sclerosis complex (TSC), caused by heterozygous mutations in TSC1 or TSC2, frequently results in intractable epilepsy. Here, we made use of an inducible Tsc1-knockout mouse model, allowing us to study electrophysiological and molecular changes of Tsc1-induced epileptogenesis over time. We recorded from pyramidal neurons in the hippocampus and somatosensory cortex (L2/L3) and combined this with an analysis of transcriptome changes during epileptogenesis. Deletion of Tsc1 resulted in hippocampus-specific changes in excitability and adaptation, which emerged before seizure onset and progressed over time. All phenotypes were rescued after early treatment with rapamycin, an mTOR inhibitor. Later in epileptogenesis, we observed a hippocampal increase of excitation-to-inhibition ratio. These cellular changes were accompanied by dramatic transcriptional changes, especially after seizure onset. Most of these changes were rescued upon rapamycin treatment. Of the genes encoding ion channels or belonging to the Gene Ontology term action potential, 27 were differentially expressed just before seizure onset, suggesting a potential driving role in epileptogenesis. Our data highlight the complex changes driving epileptogenesis in TSC, including the changed expression of multiple ion channels. Our study emphasizes inhibition of the TSC/mTOR signaling pathway as a promising therapeutic approach to target epilepsy in patients with TSC.

Angelman Syndrome: From Mouse Models to Therapy

The UBE3A gene is part of the chromosome 15q11-q13 region that is frequently deleted or duplicated, leading to several neurodevelopmental disorders (NDD). Angelman syndrome (AS) is caused by the absence of functional maternally derived UBE3A protein, while the paternal UBE3A gene is present but silenced specifically in neurons. Patients with AS present with severe neurodevelopmental delay, with pronounced motor deficits, absence of speech, intellectual disability, epilepsy, and sleep problems. The pathophysiology of AS is still unclear and a treatment is lacking. Animal models of AS recapitulate the genotypic and phenotypic features observed in AS patients, and have been invaluable for understanding the disease process as well as identifying apropriate drug targets. Using these AS mouse models we have learned that loss of UBE3A probably affects many areas of the brain, leading to increased neuronal excitability and a loss of synaptic spines, along with changes in a number of distinct behaviours. Inducible AS mouse models have helped to identify the critical treatment windows for the behavioral and physiological phenotypes. Additionally, AS mouse models indicate an important role for the predominantly nuclear UBE3A isoform in generating the characteristic AS pathology. Last, but not least, the AS mice have been crucial in guiding Ube3a gene reactivation treatments, which present a very promising therapy to treat AS.

Loss of nuclear UBE3A causes electrophysiological and behavioral deficits in mice and is associated with Angelman syndrome

Mutations affecting the gene encoding the ubiquitin ligase UBE3A cause Angelman syndrome. Although most studies focus on the synaptic function of UBE3A, we show that UBE3A is highly enriched in the nucleus of mouse and human neurons. We found that the two major isoforms of UBE3A exhibit highly distinct nuclear versus cytoplasmic subcellular localization. Both isoforms undergo nuclear import through direct binding to PSMD4 (also known as S5A or RPN10), but the amino terminus of the cytoplasmic isoform prevents nuclear retention. Mice lacking the nuclear UBE3A isoform recapitulate the behavioral and electrophysiological phenotypes of Ube3a^m-/p+ mice, whereas mice harboring a targeted deletion of the cytosolic isoform are unaffected. Finally, we identified Angelman syndrome-associated UBE3A missense mutations that interfere with either nuclear targeting or nuclear retention of UBE3A. Taken together, our findings elucidate the mechanisms underlying the subcellular localization of UBE3A, and indicate that the nuclear UBE3A isoform is the most critical for the pathophysiology of Angelman syndrome.

Strain Differences in Presynaptic Function: PROTEOMICS, ULTRASTRUCTURE, AND PHYSIOLOGY OF HIPPOCAMPAL SYNAPSES IN DBA/2J AND C57Bl/6J MICE

The inbred strains C57BL/6J and DBA/2J (DBA) display striking differences in a number of behavioral tasks depending on hippocampal function, such as contextual memory. Historically, this has been explained through differences in postsynaptic protein expression underlying synaptic transmission and plasticity. We measured the synaptic hippocampal protein content (iTRAQ (Isobaric Tags for Relative and Absolute Quantitation) and mass spectrometry), CA1 synapse ultrastructural morphology, and synaptic functioning in adult C57BL/6J and DBA mice. DBA mice showed a prominent decrease in the Ras-GAP calcium-sensing protein RASAL1. Furthermore, expression of several presynaptic markers involved in exocytosis, such as syntaxin (Stx1b), Ras-related proteins (Rab3a/c), and rabphilin (Rph3a), was reduced. Ultrastructural analysis of CA1 hippocampal synapses showed a significantly lower number of synaptic vesicles and presynaptic cluster size in DBA mice, without changes in postsynaptic density or active zone. In line with this compromised presynaptic morphological and molecular phenotype in DBA mice, we found significantly lower paired-pulse facilitation and enhanced short term depression of glutamatergic synapses, indicating a difference in transmitter release and/or refilling mechanisms. Taken together, our data suggest that in addition to strain-specific postsynaptic differences, the change in dynamic properties of presynaptic transmitter release may underlie compromised synaptic processing related to cognitive functioning in DBA mice.

Functional properties of GABA synaptic inputs onto GABA neurons in monkey prefrontal cortex

In rodent cortex GABAA receptor (GABAAR)-mediated synapses are a significant source of input onto GABA neurons, and the properties of these inputs vary among GABA neuron subtypes that differ in molecular markers and firing patterns. Some features of cortical interneurons are different between rodents and primates, but it is not known whether inhibition of GABA neurons is prominent in the primate cortex and, if so, whether these inputs show heterogeneity across GABA neuron subtypes. We thus studied GABAAR-mediated miniature synaptic events in GABAergic interneurons in layer 3 of monkey dorsolateral prefrontal cortex (DLPFC). Interneurons were identified on the basis of their firing pattern as fast spiking (FS), regular spiking (RS), burst spiking (BS), or irregular spiking (IS). Miniature synaptic events were common in all of the recorded interneurons, and the frequency of these events was highest in FS neurons. The amplitude and kinetics of miniature inhibitory postsynaptic potentials (mIPSPs) also differed between DLPFC interneuron subtypes in a manner correlated with their input resistance and membrane time constant. FS neurons had the fastest mIPSP decay times and the strongest effects of the GABAAR modulator zolpidem, suggesting that the distinctive properties of inhibitory synaptic inputs onto FS cells are in part conferred by GABAARs containing α1 subunits. Moreover, mIPSCs differed between FS and RS interneurons in a manner consistent with the mIPSP findings. These results show that in the monkey DLPFC GABAAR-mediated synaptic inputs are prominent in layer 3 interneurons and may differentially regulate the activity of different interneuron subtypes.

Functional Maturation of GABA Synapses During Postnatal Development of the Monkey Dorsolateral Prefrontal Cortex

Development of inhibition onto pyramidal cells may be crucial for the emergence of cortical network activity, including gamma oscillations. In primate dorsolateral prefrontal cortex (DLPFC), inhibitory synaptogenesis starts in utero and inhibitory synapse density reaches adult levels before birth. However, in DLPFC, the expression levels of γ-aminobutyric acid (GABA) synapse-related gene products changes markedly during development until young adult age, suggesting a highly protracted maturation of GABA synapse function. Therefore, we examined the development of GABA synapses by recording GABAAR-mediated inhibitory postsynaptic currents (GABAAR-IPSCs) from pyramidal cells in the DLPFC of neonatal, prepubertal, peripubertal, and adult macaque monkeys. We found that the decay of GABAAR-IPSCs, possibly including those from parvalbumin-positive GABA neurons, shortened by prepubertal age, while their amplitude increased until the peripubertal period. Interestingly, both GABAAR-mediated quantal response size, estimated by miniature GABAAR-IPSCs, and the density of GABAAR synaptic appositions, measured with immunofluorescence microscopy, were stable with age. Simulations in a computational model network with constant GABA synapse density showed that the developmental changes in GABAAR-IPSC properties had a significant impact on oscillatory activity and predicted that, whereas DLPFC circuits can generate gamma frequency oscillations by prepubertal age, mature levels of gamma band power are attained at late stages of development.

The role of glutamatergic inputs onto parvalbumin-positive interneurons: relevance for schizophrenia

Cognitive impairment, a core feature of schizophrenia, has been suggested to arise from a disturbance of gamma oscillations that is due to decreased neurotransmission from the parvalbumin (PV) subtype of interneurons. Indeed, PV interneurons have uniquely fast membrane and synaptic properties that are crucially important for network functions such as feedforward inhibition or gamma oscillations. The causes leading to impairment of PV neurotransmission in schizophrenia are still under investigation. Interestingly, NMDA receptors (NMDARs) antagonism results in schizophrenia-like symptoms in healthy adults. Additionally, systemic NMDAR antagonist administration increases prefrontal cortex pyramidal cell firing, apparently by producing disinhibition, and repeated exposure to NMDA antagonists leads to changes in the GABAergic markers that mimic the impairments found in schizophrenia. Based on these findings, PV neuron deficits in schizophrenia have been proposed to be secondary to (NMDAR) hypofunction at glutamatergic synapses onto these cells. However, NMDARs generate long-lasting postsynaptic currents that result in prolonged depolarization of the postsynaptic cells, a property inconsistent with the role of PV cells in network dynamics. Here, we review evidence leading to the conclusion that cortical disinhibition and GABAergic impairment produced by NMDAR antagonists are unlikely to be mediated via NMDARs at glutamatergic synapses onto mature cortical PV neurons.

Proteomics, ultrastructure, and physiology of hippocampal synapses in a fragile X syndrome mouse model reveal presynaptic phenotype

Fragile X syndrome (FXS), the most common form of hereditary mental retardation, is caused by a loss-of-function mutation of the Fmr1 gene, which encodes fragile X mental retardation protein (FMRP). FMRP affects dendritic protein synthesis, thereby causing synaptic abnormalities. Here, we used a quantitative proteomics approach in an FXS mouse model to reveal changes in levels of hippocampal synapse proteins. Sixteen independent pools of Fmr1 knock-out mice and wild type mice were analyzed using two sets of 8-plex iTRAQ experiments. Of 205 proteins quantified with at least three distinct peptides in both iTRAQ series, the abundance of 23 proteins differed between Fmr1 knock-out and wild type synapses with a false discovery rate (q-value) <5%. Significant differences were confirmed by quantitative immunoblotting. A group of proteins that are known to be involved in cell differentiation and neurite outgrowth was regulated; they included Basp1 and Gap43, known PKC substrates, and Cend1. Basp1 and Gap43 are predominantly expressed in growth cones and presynaptic terminals. In line with this, ultrastructural analysis in developing hippocampal FXS synapses revealed smaller active zones with corresponding postsynaptic densities and smaller pools of clustered vesicles, indicative of immature presynaptic maturation. A second group of proteins involved in synaptic vesicle release was up-regulated in the FXS mouse model. In accordance, paired-pulse and short-term facilitation were significantly affected in these hippocampal synapses. Together, the altered regulation of presynaptically expressed proteins, immature synaptic ultrastructure, and compromised short-term plasticity points to presynaptic changes underlying glutamatergic transmission in FXS at this stage of development.

GABA transporter GAT1 prevents spillover at proximal and distal GABA synapses onto primate prefrontal cortex neurons

The plasma membrane GABA transporter GAT1 is thought to mediate uptake of synaptically released GABA. In the primate dorsolateral prefrontal cortex (DLPFC), GAT1 expression changes significantly during development and in schizophrenia. The consequences of such changes, however, are not well understood because GAT1's role has not been investigated in primate neocortical circuits. We thus studied the effects of the GAT1 blocker 1,2,5,6-tetrahydro-1-[2-[[(diphenylmethylene)amino]oxy]ethyl]-3-pyridinecarboxylic acid hydrochloride (NO711) on GABA transmission onto pyramidal neurons of monkey DLPFC. As in rat cortex, in monkey DLPFC NO711 did not substantially alter miniature GABA transmission, suggesting that GAT1 does not regulate single-synapse transmission. In rat cortical circuits, between-synapse GABA spillover produced by NO711 clearly prolongs the inhibitory postsynaptic currents, but whether NO711 also prolongs the inhibitory postsynaptic potentials (IPSPs) is unclear. Moreover, whether spillover differentially affects perisomatic versus dendritic inputs has not been examined. Here we found that NO711 prolonged the GABAA receptor-mediated IPSPs (GABAAR-IPSPs) evoked by stimulating perisomatic synapses. Dendritic, but not perisomatic, synapse stimulation often elicited a postsynaptic GABAB receptor-mediated IPSP that was enhanced by NO711. Blocking GABAB receptors revealed that NO711 prolonged the GABAAR-IPSPs evoked by stimulation of dendrite-targeting inputs. We conclude that a major functional role for GAT1 in primate cortical circuits is to prevent the effects of GABA spillover when multiple synapses are simultaneously active. Furthermore, we report that, at least in monkey DLPFC, GAT1 similarly restricts GABA spillover onto perisomatic or dendritic inputs, critically controlling the spatiotemporal specificity of inhibitory inputs onto proximal or distal compartments of the pyramidal cell membrane.

Parvalbumin-positive basket interneurons in monkey and rat prefrontal cortex

Differences in the developmental origin and relative proportions of biochemically distinct classes of cortical neurons have been found between rodents and primates. In addition, species differences in the properties of certain cell types, such as neurogliaform cells, have also been reported. Consequently, in this study we compared the anatomical and physiological properties of parvalbumin (PV)-positive basket interneurons in the prefrontal cortex of macaque monkeys and rats. The somal size, total dendritic length, and horizontal and vertical spans of the axonal arbor were similar in monkeys and rats. Physiologically, PV basket cells could be identified as fast-spiking interneurons in both species, based on their short spike and high-frequency firing without adaptation. However, important interspecies differences in the intrinsic physiological properties were found. In monkeys, basket cells had a higher input resistance and a lower firing threshold, and they generated more spikes at near-threshold current intensities than those in rats. Thus monkey basket cells appeared to be more excitable. In addition, rat basket cells consistently fired the first spike with a substantial delay and generated spike trains interrupted by quiescent periods more often than monkey basket cells. The frequency of miniature excitatory postsynaptic potentials in basket cells was considerably higher in rats than that in monkeys. These differences between rats and monkeys in the electrophysiological properties of PV-positive basket cells may contribute to the differential patterns of neuronal activation observed in rats and monkeys performing working-memory tasks.

Dopamine D1 receptor activation regulates sodium channel-dependent EPSP amplification in rat prefrontal cortex pyramidal neurons

Dopamine (DA) effects on prefrontal cortex (PFC) neurons are essential for the cognitive functions mediated by this cortical area. However, the cellular mechanisms of DA neuromodulation in neocortex are not well understood. We characterized the effects of D1-type DA receptor (D1R) activation on the amplification (increase in duration and area) of excitatory postsynaptic potentials (EPSPs) at depolarized potentials, in layer 5 pyramidal neurons from rat PFC. Simulated EPSPs (sEPSPs) were elicited by current injection, to determine the effects of D1R activation independent of modulation of transmitter release or glutamate receptor currents. Application of the D1R agonist SKF81297 attenuated sEPSP amplification at depolarized potentials in a concentration-dependent manner. The SKF81297 effects were inhibited by the D1R antagonist SCH23390. The voltage-gated Na+ channel blocker tetrodotoxin (TTX) abolished the effects of SKF81297 on sEPSP amplification, suggesting that Na+ currents are necessary for the D1R effect. Furthermore, blockade of 4-AP- and TEA-sensitive K+ channels in the presence of TTX significantly increased EPSP amplification, arguing against the possibility that SKF81297 up-regulates currents that attenuate sEPSP amplification. SKF81297 application attenuated the subthreshold response to injection of depolarizing current ramps, in a manner consistent with a decrease in the persistent Na+ current. In addition, D1R activation decreased the effectiveness of temporal EPSP summation during 20 Hz sEPSP trains, selectively at depolarized membrane potentials. Therefore, the effects of D1R activation on Na+ channel-dependent EPSP amplification may regulate the impact of coincidence detection versus temporal integration mechanisms in PFC pyramidal neurons.

Electrophysiological differences between neurogliaform cells from monkey and rat prefrontal cortex

Current dogma holds that a canonical cortical circuit is formed by cellular elements that are basically identical across species. However, detailed and direct comparisons between species of specific elements of this circuit are limited in number. In this study, we compared the morphological and physiological properties of neurogliaform (NGF) inhibitory neurons in the prefrontal cortex (PFC) of macaque monkeys and rats. In both species, NGF cells were readily identified based on their distinctive morphological features. Indeed, monkey NGF cells had only a few morphological features that differed from rat, including a larger soma, a greater number of dendrites, and a more compact axonal field. In contrast, whole cell recordings of the responses to injected current steps revealed important differences between monkey and rat NGF cells. Monkey NGF cells consistently generated a short-latency first spike riding on an initial depolarizing hump, whereas in rat NGF cells, the first spike appeared after a substantial delay riding on a depolarizing ramp not seen in monkey NGF cells. Thus although rat NGF cells are traditionally classified as late-spiking cells, monkey NGF cells did not meet this physiological criterion. In addition, NGF cells in monkey appeared to be more excitable than those in rat because they displayed a higher input resistance, a lower spike threshold, and a higher firing frequency. Finally, NGF cells in monkey showed a more prominent spike-frequency adaptation as compared with rat. Our findings indicate that the canonical cortical circuit differs in at least some aspects of its constituent elements across species.

Mediodorsal thalamic afferents to layer III of the rat prefrontal cortex: synaptic relationships to subclasses of interneurons

The mediodorsal nucleus of the thalamus (MD) represents the main subcortical structure that projects to the prefrontal cortex (PFC) and it regulates key aspects of the cognitive functions of this region. Within the PFC, GABA local circuit neurons shape the activity patterns and hence the "memory fields" of pyramidal cells. Although the connections between the MD and PFC are well established, the ultrastructural relationships between projecting fibers from the MD and different subclasses of GABA cells in the PFC are not known. In order to address this issue in the rat, we examined MD axons labeled by tract-tracing in combination with immunogold-silver to identify different calcium-binding proteins localized within separate populations of interneurons. Electron micrographic examination of PFC sections from these animals revealed that MD terminals made primarily asymmetric synapses onto dendritic spines and less commonly onto dendritic shafts. Most of the dendrites receiving MD synaptic input were immunoreactive for parvalbumin (ParV), whereas MD synapses onto dendrites labeled for calretinin or calbindin were less frequent. We also observed that some MD terminals were themselves immunoreactive for calcium-binding proteins, again more commonly for ParV. These results suggest that the MD exerts a dual influence on PFC pyramidal cells: direct inputs onto spines and an indirect influence mediated via synapses onto each subclass of interneurons. The apparent preferential input to ParV cells endows MD afferents with a strong indirect inhibitory influence on pyramidal neuron activity by virtue of ParV cell synapses onto soma, proximal dendrites, and axon initial segments.