Hyun Taek Kim

Dynamics of Population Code for Working Memory in the Prefrontal Cortex

Some neurons (delay cells) in the prefrontal cortex elevate their activities throughout the time period during which the animal is required to remember past events and prepare future behavior, suggesting that working memory is mediated by continuous neural activity. It is unknown, however, how working memory is represented within a population of prefrontal cortical neurons. We recorded from neuronal ensembles in the prefrontal cortex as rats learned a new delayed alternation task. Ensemble activities changed in parallel with behavioral learning so that they increasingly allowed correct decoding of previous and future goal choices. In well-trained rats, considerable decoding was possible based on only a few neurons and after removing continuously active delay cells. These results show that neural activity in the prefrontal cortex changes dynamically during new task learning so that working memory is robustly represented and that working memory can be mediated by sequential activation of different neural populations.

Highly efficient gene knockout in mice and zebrafish with RNA-guided endonucleases

RNA-guided endonucleases (RGENs), derived from the prokaryotic Type II CRISPR-Cas system, enable targeted genome modification in cells and organisms. Here we describe the establishment of gene-knockout mice and zebrafish by the injection of RGENs as Cas9 protein:guide RNA complexes or Cas9 mRNA plus guide RNA into one-cell-stage embryos of both species. RGENs efficiently generated germline transmittable mutations in up to 93% of newborn mice with minimal toxicity. RGEN-induced mutations in the mouse Prkdc gene that encodes an enzyme critical for DNA double-strand break repair resulted in immunodeficiency both in F₀ and F₁ mice. We propose that RGEN-mediated mutagenesis in animals will greatly expedite the creation of genetically engineered model organisms, accelerating functional genomic research.

Maternal Stress Produces Learning Deficits Associated with Impairment of NMDA Receptor-Mediated Synaptic Plasticity

Stress in adulthood can have a profound effect on physiology and behavior, but the extent to which prolonged maternal stress affects the brain function of offspring when they are adult remains primarily unknown. In the present work, chronic immobilization stress to pregnant mice affected fetal growth and development. When pups born from stressed mice were reared to adulthood in an environment identical to that of nonstressed controls, several physiological parameters were essentially unaltered. However, spatial learning and memory was significantly impaired in the maternally stressed offspring in adulthood. Furthermore, electrophysiological examination revealed a significant reduction in NMDA receptor-mediated long-term potentiation in the CA1 area of hippocampal slices. Subsequent biochemical analysis demonstrated a substantial decrease in NR1 and NR2B subunits of the NMDA receptor in synapses of the hippocampus, and the interaction between these two subunits appeared to be reduced. These results suggest that prolonged maternal stress leads to long-lasting malfunction of the hippocampus, which extends to and is manifested in adulthood.

WDR11, a WD protein that interacts with transcription factor EMX1, is mutated in idiopathic hypogonadotropic hypogonadism and Kallmann syndrome.

By defining the chromosomal breakpoint of a balanced t(10;12) translocation from a subject with Kallmann syndrome and scanning genes in its vicinity in unrelated hypogonadal subjects, we have identified WDR11 as a gene involved in human puberty. We found six patients with a total of five different heterozygous WDR11 missense mutations, including three alterations (A435T, R448Q, and H690Q) in WD domains important for β propeller formation and protein-protein interaction. In addition, we discovered that WDR11 interacts with EMX1, a homeodomain transcription factor involved in the development of olfactory neurons, and that missense alterations reduce or abolish this interaction. Our findings suggest that impaired pubertal development in these patients results from a deficiency of productive WDR11 protein interaction.

Is Alpha Wave Neurofeedback Effective with Randomized Clinical Trials in Depression? A Pilot Study

Frontal asymmetric activation has been proposed to be the underlying mechanism for depression. Some case studies have reported that the enhancement of a relative right frontal alpha activity by an asymmetry neurofeedback training leads to improvement in depressive symptoms. In the present study, we examined whether a neurofeedback training designed to increase the relative activity of the right frontal alpha band would have an impact on symptoms of depressive subjects suffering from emotional, behavioral, and cognitive problems. Our results indicated that the asymmetry neurofeedback training increased the relative right frontal alpha power, and it remained effective even after the end of the total training sessions. In contrast to the training group, the placebo control group did not show a difference. The neurofeedback training had profound effects on emotion and cognition. First, we replicated earlier findings that enhancing the left frontal activity led to alleviation of depressive symptoms. Moreover, cognitive tests revealed that the asymmetry training improved performance of executive function tests, whereas the placebo treatment did not show improvement. We preliminarily concluded that the asymmetry training is important for controlling and regulating emotion, and it may facilitate the left frontal lobe function.

The Medial Prefrontal Cortex Is Involved in Spatial Memory Retrieval under Partial-Cue Conditions

Brain circuits involved in pattern completion, or retrieval of memory from fragmented cues, were investigated. Using different versions of the Morris water maze, we explored the roles of the CA3 subregion of the hippocampus and the medial prefrontal cortex (mPFC) in spatial memory retrieval under various conditions. In a hidden platform task, both CA3 and mPFC lesions disrupted memory retrieval under partial-cue, but not under full-cue, conditions. For a delayed matching-to-place task, CA3 lesions produced a deficit in both forming and recalling spatial working memory regardless of extramaze cue conditions. In contrast, damage to mPFC impaired memory retrieval only when a fraction of cues was available. To corroborate the lesion study, we examined the expression of the immediate early gene c-fos in mPFC and the hippocampus. After training of spatial reference memory in full-cue conditions for 6 d, the same training procedure in the absence of all cues except one increased the number of Fos-immunoreactive cells in mPFC and CA3. Furthermore, mPFC inactivation with muscimol, a GABA agonist, blocked memory retrieval in the degraded-cue environment. However, mPFC-lesioned animals initially trained in a single-cue environment had no difficulty in retrieving spatial memory when the number of cues was increased, demonstrating that contextual change per se did not impair the behavioral performance of the mPFC-lesioned animals. Together, these findings strongly suggest that pattern completion requires interactions between mPFC and the hippocampus, in which mPFC plays significant roles in retrieving spatial information maintained in the hippocampus for efficient navigation.

Visualization of Myelination in GFP-Transgenic Zebrafish

The insulation of axons in the vertebrate nervous system by myelin is essential for efficient axonal conduction. Myelination disruption and remyelination failure can cause human diseases, such as multiple sclerosis and hereditary myelin diseases. However, despite progress in understanding myelination regulation, many important questions remain unanswered. To investigate the mechanisms underlying myelination in vivo, we generated transgenic zebrafish expressing enhanced green fluorescent protein (EGFP) under the control of the mbp promoter. This transgenic fish displayed faithful EGFP expression in oligodendrocytes and Schwann cells in embryonic and adult zebrafish. Interestingly, although myelination progressed continuously in the postembryonic central nervous system, some of the spinal cord regions were filled with unmyelinated axons even in the adult spinal cord, suggesting functional differences between myelinated and unmyelinated axons. Our results suggest that this transgenic zebrafish could be a valuable animal model to study oligodendrocyte differentiation and myelination in vivo. Developmental Dynamics 239:592–597, 2010.

Neuralized-2 Regulates a Notch Ligand in Cooperation with Mind Bomb-1

Mutations in Drosophila neuralized (Dneur) result in a variety of developmental defects that closely resemble those of Notch mutants and other Notch pathway mutants. However, mice with disrupted neur1 do not show any aberrant cell fate specifications in neurogenesis and somitogenesis. Thus, we speculated that other vertebrate neur homolog(s) might compensate for loss of the neur gene. Here, we report the paralog of mouse Neur1, named Neuralized-2 (Neur2), which is a ubiquitin-protein isopeptide ligase (E3) that interacts with and ubiquitinates Delta. Both murine Neur1 and Neur2 have similar degrees of homology to DNeur, and neur2 is expressed in patterns similar to those of neur1 in embryos, suggesting potential functional redundancy. Interestingly, two distinct classes of E3 ligases, Mind bomb-1 (Mib1) and Neur2, have cooperative but distinct roles in Delta endocytosis to Hrs-positive vesicles, i.e. Mib1 functions in the initial step of Delta endocytosis, and Neur2 is required for targeting endocytosed Delta to Hrs-positive vesicles. Thus, our study provides a new insight into how distinct E3 ligases work together in the endocytic pathways for Notch signaling.

The maintenance of specific aspects of neuronal function and behavior is dependent on programmed cell death of adult‐generated neurons in the dentate gyrus

A considerable number of new neurons are generated daily in the dentate gyrus (DG) of the adult hippocampus, but only a subset of these survive, as many adult‐generated neurons undergo programmed cell death (PCD). However, the significance of PCD in the adult brain for the functionality of DG circuits is not known. Here, we examined the electrophysiological and behavioral characteristics of Bax‐knockout (Bax‐KO) mice in which PCD of post‐mitotic neurons is prevented. The continuous increase in DG cell numbers in Bax‐KO mice resulted in the readjustment of afferent and efferent synaptic connections, represented by age‐dependent reductions in the dendritic arborization of DG neurons and in the synaptic contact ratio of mossy fibers with CA3 dendritic spines. These neuroanatomical changes were associated with reductions in synaptic transmission and reduced performance in a contextual fear memory task in 6‐month‐old Bax‐KO mice. These results suggest that the elimination of excess DG neurons via Bax‐dependent PCD in the adult brain is required for the normal organization and function of the hippocampus.

Inositol 1,4,5-Trisphosphate 3-Kinase A Functions As a Scaffold for Synaptic Rac Signaling

Activity-dependent alterations of synaptic contacts are crucial for synaptic plasticity. The formation of new dendritic spines and synapses is known to require actin cytoskeletal reorganization specifically during neural activation phases. Yet the site-specific and time-dependent mechanisms modulating actin dynamics in mature neurons are not well understood. In this study, we show that actin dynamics in spines is regulated by a Rac anchoring and targeting function of inositol 1,4,5-trisphosphate 3-kinase A (IP3K-A), independent of its kinase activity. On neural activation, IP3K-A bound directly to activated Rac1 and recruited it to the actin cytoskeleton in the postsynaptic area. This focal targeting of activated Rac1 induced spine formation through actin dynamics downstream of Rac signaling. Consistent with the scaffolding role of IP3K-A, IP3K-A knock-out mice exhibited defects in accumulation of PAK1 by long-term potentiation-inducing stimulation. This deficiency resulted in a reduction in the reorganization of actin cytoskeletal structures in the synaptic area of dentate gyrus. Moreover, IP3K-A knock-out mice showed deficits of synaptic plasticity in perforant path and in hippocampal-dependent memory performances. These data support a novel model in which IP3K-A is critical for the spatial and temporal regulation of spine actin remodeling, synaptic plasticity, and learning and memory via an activity-dependent Rac scaffolding mechanism.