Li Lian Yuan

Tau mislocalization to dendritic spines mediates synaptic dysfunction independently of neurodegeneration.

The microtubule-associated protein tau accumulates in Alzheimers and other fatal dementias, which manifest when forebrain neurons die. Recent advances in understanding these disorders indicate that brain dysfunction precedes neurodegeneration, but the role of tau is unclear. Here, we show that early tau-related deficits develop not from the loss of synapses or neurons, but rather as a result of synaptic abnormalities caused by the accumulation of hyperphosphorylated tau within intact dendritic spines, where it disrupts synaptic function by impairing glutamate receptor trafficking or synaptic anchoring. Mutagenesis of 14 disease-associated serine and threonine amino acid residues to create pseudohyperphosphorylated tau caused tau mislocalization while creation of phosphorylation-deficient tau blocked the mistargeting of tau to dendritic spines. Thus, tau phosphorylation plays a critical role in mediating tau mislocalization and subsequent synaptic impairment. These data establish that the locus of early synaptic malfunction caused by tau resides in dendritic spines.

Deletion of Kv4.2 Gene Eliminates Dendritic A-Type K+ Current and Enhances Induction of Long-Term Potentiation in Hippocampal CA1 Pyramidal Neurons

Dendritic, backpropagating action potentials (bAPs) facilitate the induction of Hebbian long-term potentiation (LTP). Although bAPs in distal dendrites of hippocampal CA1 pyramidal neurons are attenuated when propagating from the soma, their amplitude can be increased greatly via downregulation of dendritic A-type K+ currents. The channels that underlie these currents thus may represent a key regulatory component of the signaling pathways that lead to synaptic plasticity. We directly tested this hypothesis by using Kv4.2 knock-out mice. Deletion of the Kv4.2 gene and a loss of Kv4.2 protein resulted in a specific and near-complete elimination of A-type K+ currents from the apical dendrites of CA1 pyramidal neurons. The absence of dendritic Kv4.2-encoded A-type K+ currents led to an increase of bAP amplitude and an increase of concurrent Ca2+ influx. Furthermore, CA1 pyramidal neurons lacking dendritic A-type K+ currents from Kv4.2 knock-out mice exhibited a lower threshold than those of wild-type littermates for LTP induction with the use of a theta burst pairing protocol. LTP triggered with the use of a saturating protocol, on the other hand, remained indistinguishable between Kv4.2 knock-out and wild-type neurons. Our results support the hypothesis that dendritic A-type K+ channels, composed of Kv4.2 subunits, regulate action potential backpropagation and the induction of specific forms of synaptic plasticity.

Calcium–Calmodulin-Dependent Kinase II Modulates Kv4.2 Channel Expression and Upregulates Neuronal A-Type Potassium Currents

Calcium–calmodulin-dependent kinase II (CaMKII) has a long history of involvement in synaptic plasticity, yet little focus has been given to potassium channels as CaMKII targets despite their importance in repolarizing EPSPs and action potentials and regulating neuronal membrane excitability. We now show that Kv4.2 acts as a substrate for CaMKII in vitro and have identified CaMKII phosphorylation sites as Ser438 and Ser459. To test whether CaMKII phosphorylation of Kv4.2 affects channel biophysics, we expressed wild-type or mutant Kv4.2 and the K+ channel interacting protein, KChIP3, with or without a constitutively active form of CaMKII in Xenopus oocytes and measured the voltage dependence of activation and inactivation in each of these conditions. CaMKII phosphorylation had no effect on channel biophysical properties. However, we found that levels of Kv4.2 protein are increased with CaMKII phosphorylation in transfected COS cells, an effect attributable to direct channel phosphorylation based on site-directed mutagenesis studies. We also obtained corroborating physiological data showing increased surface A-type channel expression as revealed by increases in peak K+ current amplitudes with CaMKII phosphorylation. Furthermore, endogenous A-currents in hippocampal pyramidal neurons were increased in amplitude after introduction of constitutively active CaMKII, which results in a decrease in neuronal excitability in response to current injections. Thus CaMKII can directly modulate neuronal excitability by increasing cell-surface expression of A-type K+ channels.

Identification of the Hippocampal Input to Medial Prefrontal Cortex In Vitro

To delineate the cellular mechanisms underlying the function of medial prefrontal cortex (mPFC) networks, it is critical to understand how synaptic inputs from various afferents are integrated and drive neuronal activity in this region. Using a newly developed slice preparation, we were able to identify a bundle of axons that contain extraneocortical fibers projecting to neurons in the prelimbic cortex. The anatomical origin and functional connectivity of the identified fiber bundle were probed by in vivo track tracing in combination with optic and whole-cell recordings of neurons in layers 2/3 and 5/6. We demonstrate that the identified bundle contains afferent fibers primarily from the ventral hippocampus but does not include contributions from the mediodorsal nucleus of the thalamus, amygdala, or lateral hypothalamus/medial forebrain bundle. Further, we provide evidence that activation of this fiber bundle results in patterned activity of neurons in the mPFC, which is distinct from that of laminar stimulation of either the deep layers 5/6 or the superficial layer 1. Evoked excitatory postsynaptic potentials are monosynaptic and glutamatergic and exhibit bidirectional changes in synaptic efficacy in response to physiologically relevant induction protocols. These data provide the necessary groundwork for the characterization of the hippocampal pathway projecting to the mPFC.

Levetiracetam has an activity-dependent effect on inhibitory transmission

Purpose:  Previous work has shown that levetiracetam (LEV) binds the vesicular protein SV2A and reduces excitatory neurotransmitter release during trains of high‐frequency activity, most likely by accessing its binding site through vesicular endocytosis into excitatory synaptic terminals. Because there are differences in excitatory and inhibitory transmitter release mechanisms, and there are suggestions that neurons differ in their SV2A expression, we were curious whether LEV also reduces inhibitory transmission.

dbCRID: a database of chromosomal rearrangements in human diseases

Chromosomal rearrangement (CR) events result from abnormal breaking and rejoining of the DNA molecules, or from crossing-over between repetitive DNA sequences, and they are involved in many tumor and non-tumor diseases. Investigations of disease-associated CR events can not only lead to important discoveries about DNA breakage and repair mechanisms, but also offer important clues about the pathologic causes and the diagnostic/therapeutic targets of these diseases. We have developed a database of Chromosomal Rearrangements In Diseases (dbCRID, http://dbCRID.biolead.org), a comprehensive database of human CR events and their associated diseases. For each reported CR event, dbCRID documents the type of the event, the disease or symptoms associated, and—when possible—detailed information about the CR event including precise breakpoint positions, junction sequences, genes and gene regions disrupted and experimental techniques applied to discover/analyze the CR event. With 2643 records of disease-associated CR events curated from 1172 original studies, dbCRID is a comprehensive and dynamic resource useful for studying DNA breakage and repair mechanisms, and for analyzing the genetic basis of human tumor and non-tumor diseases.

Iron deficiency with or without anemia impairs prepulse inhibition of the startle reflex.

Iron deficiency (ID) during early life causes long‐lasting detrimental cognitive sequelae, many of which are linked to alterations in hippocampus function, dopamine synthesis, and the modulation of dopaminergic circuitry by the hippocampus. These same features have been implicated in the origins of schizophrenia, a neuropsychiatric disorder with significant cognitive impairments. Deficits in sensorimotor gating represent a reliable endophenotype of schizophrenia that can be measured by prepulse inhibition (PPI) of the acoustic startle reflex. Using two rodent model systems, we investigated the influence of early‐life ID on PPI in adulthood. To isolate the role of hippocampal iron in PPI, our mouse model utilized a timed (embryonic day 18.5), hippocampus‐specific knockout of Slc11a2, a gene coding an important regulator of cellular iron uptake, the divalent metal transport type 1 protein (DMT‐1). Our second model used a classic rat dietary‐based global ID during gestation, a condition that closely mimics human gestational ID anemia (IDA). Both models exhibited impaired PPI in adulthood. Furthermore, our DMT‐1 knockout model displayed reduced long‐term potentiation (LTP) and elevated paired‐pulse facilitation (PPF), electrophysiological results consistent with previous findings in the IDA rat model. These results, in combination with previous findings demonstrating impaired hippocampus functioning and altered dopaminergic and glutamatergic neurotransmission, suggest that iron availability within the hippocampus is critical for the neurodevelopmental processes underlying sensorimotor gating. Ultimately, evidence of reduced PPI in both of our models may offer insights into the roles of fetal ID and the hippocampus in the pathophysiology of schizophrenia.

Dendritic Mechanisms Controlling the Threshold and Timing Requirement of Synaptic Plasticity

Active conductances located and operating on neuronal dendrites are expected to regulate synaptic integration and plasticity. We investigate how Kv4.2‐mediated A‐type K+ channels and Ca2+‐activated K+ channels are involved in the induction process of Hebbian‐type plasticity that requires correlated pre‐ and postsynaptic activities. In CA1 pyramidal neurons, robust long‐term potentiation (LTP) induced by a theta burst pairing protocol usually occurred within a narrow window during which incoming synaptic potentials coincided with postsynaptic depolarization. Elimination of dendritic A‐type K+ currents in Kv4.2−/− mice, however, resulted in an expanded time window, making the induction of synaptic potentiation less dependent on the temporal relation of pre‐ and postsynaptic activity. For the other type of synaptic plasticity, long‐term depression, the threshold was significantly increased in Kv4.2−/− mice. This shift in depression threshold was restored to normal when the appropriate amount of internal free calcium was chelated during induction. In concert with A‐type channels, Ca2+‐activated K+ channels also exerted a sliding effect on synaptic plasticity. Blocking these channels in Kv4.2−/− mice resulted in an even larger potentiation while by contrast, the depression threshold was shifted further. In conclusion, dendritic A‐type and Ca2+‐activated K+ channels dually regulate the timing‐dependence and thresholds of synaptic plasticity in an additive way.

Activation of M2 muscarinic receptors leads to sustained suppression of hippocampal transmission in the medial prefrontal cortex

Cholinergic innervation of the prefrontal cortex is critically involved in arousal, learning and memory. Dysfunction of muscarinic acetylcholine receptors and their downstream signalling pathways has been identified in mental retardation. To assess the role played by the muscarinic receptors at the hippocampal–frontal cortex synapses, an important relay in information storage, we used a newly developed frontal slice preparation in which hippocampal afferent fibres are preserved. Transient activation of muscarinic receptors by carbachol results in a long‐lasting depression of synaptic efficacy at the hippocampal but not cortical pathways or local circuitry. On the basis of a combination of electrophysiological, pharmacological and anatomical results, this input‐specific muscarinic modulation can be partially attributed to the M2 subtype of muscarinic receptors, possibly through a combination of pre‐ and postsynaptic mechanisms.

Spatial learning deficits in mice lacking A-type K + channel subunits

Kv4.2‐mediated A‐type K+ channels in dendrites act to dampen back‐propagating action potentials, constrain coincidence detection, and modify synaptic properties. Because of naturally high concentrations in the hippocampus, genetic deletion of this protein results in enhanced CA1 dendritic excitability and a broader signal integration time window with potential implications for spatial learning. In this investigation, we tested Kv4.2 knockout mice in the Morris water maze to assess their spatial reference acquisition and recall abilities. These mice demonstrated prolonged latencies and pathlength to reach a hidden platform during learning trials that was correlated to a decreased use of spatial search strategies in favor of repetitive looping. Knockout mice also showed no preference for target areas in recall‐based probe trials but were less impaired by a switch in the platform location at the start of reversal learning. We discuss the possibility that these behavior discrepancies may be attributable to an enhancement in synaptic plasticity and loss of selectivity among synaptic pathways bearing different information into the CA1 region.