Hyun-Seob Lee

Foxa2 and Nurr1 synergistically yield A9 nigral dopamine neurons exhibiting improved differentiation, function, and cell survival.

Effective dopamine (DA) neuron differentiation from neural precursor cells (NPCs) is prerequisite for precursor/stem cell‐based therapy of Parkinsons disease (PD). Nurr1, an orphan nuclear receptor, has been reported as a transcription factor that can drive DA neuron differentiation from non‐dopaminergic NPCs in vitro. However, Nurr1 alone neither induces full neuronal maturation nor expression of proteins found specifically in midbrain DA neurons. In addition, Nurr1 expression is inefficient in inducing DA phenotype expression in NPCs derived from certain species such as mouse and human. We show here that Foxa2, a forkhead transcription factor whose role in midbrain DA neuron development was recently revealed, synergistically cooperates with Nurr1 to induce DA phenotype acquisition, midbrain‐specific gene expression, and neuronal maturation. Thus, the combinatorial expression of Nurr1 and Foxa2 in NPCs efficiently yielded fully differentiated nigral (A9)‐type midbrain neurons with clearly detectable DA neuronal activities. The effects of Foxa2 in DA neuron generation were observed regardless of the brain regions or species from which NPCs were derived. Furthermore, DA neurons generated by ectopic Foxa2 expression were more resistant to toxins. Importantly, Foxa2 expression resulted in a rapid cell cycle exit and reduced cell proliferation. Consistently, transplantation of NPCs transduced with Nurr1 and Foxa2 generated grafts enriched with midbrain‐type DA neurons but reduced number of proliferating cells, and significantly reversed motor deficits in a rat PD model. Our findings can be applied to ongoing attempts to develop an efficient and safe precursor/stem cell‐based therapy for PD. STEM CELLS 2010;28:501–512

Generation of functional dopamine neurons from neural precursor cells isolated from the subventricular zone and white matter of the adult rat brain using nurr1 overexpression

Neural precursor (NP) cells from adult mammalian brains can be isolated, expanded in vitro, and potentially used as cell replacement source material for treatment of intractable brain disorders. Reduced ethical concerns, lack of teratoma formation, and possible ex vivo autologous transplantation are critical advantages to using adult NP donor cells over cells from fetal brain tissue or embryonic stem cells. However, the usage of adult NP cells is limited by the ability to induce specific neurochemical phenotypes in these cells. Here, we demonstrate induction of a dopaminergic phenotype in NP cells isolated from the subventricular zone (SVZ) and white matter of rodent adult brains using overexpression of the nuclear receptor Nurr1 in vitro. Forced expression of Nurr1, a transcriptional factor specific to midbrain dopamine (DA) neuron development, caused in the adult cells an acquisition of the DA neurotransmitter phenotype and sufficient differentiation toward morphologically, phenotypically, and ultrastructurally mature DA neurons. Co‐expression of neurogenic factor Mash1 and treatment with neurogenic cytokines brain‐derived neurotrophic factor and neurotrophin‐3 greatly enhanced Nurr1‐induced DA neuron yield. The Nurr1‐induced DA neurons demonstrated in vitro presynaptic DA neuronal functionality, releasing DA neurotransmitter in response to depolarization stimuli and specific DA reuptake. Furthermore, Nurr1‐engineered adult SVZ NP cells survived, integrated, and differentiated into DA neurons in vivo that can reverse the behavioral deficit in the host striatum of parkinsonian rats. These findings open the possibility for the use of precursor cells from adult brains as a cell source for neuronal replacement treatment of Parkinson disease.

HFTL: hybrid flash translation layer based on hot data identification for flash memory

For the last years, a number of flash translation layers (FTL) have been proposed for hiding erase-before-write architecture of NAND flash memory. However, although many conventional FTLs efficiently provide the logical to physical address remapping algorithms, they could not escape from the performance degradation when handling the hot data which tends to generate so many overwrites on the same logical address. In this paper, we propose a novel FTL algorithm called Hybrid Flash Translation Layer (HFTL) that adaptively exploits the sector mapping and log block based mapping schemes. To do so, HFTL first separates the hot data from the cold data by using the hot data identifier. And then it dynamically manages the former by using the sector mapping scheme showing an optimal performance for intensive overwrites at the same location, and the latter by using the log block based mapping scheme. By using this adaptive hybrid method, HFTL is always guaranteed to yield good performance for the pattern with the hot data as well as the pattern without it. Through a series of experiments, we show that HFTL yields better performance than conventional FTLs.

Conditions for tumor-free and dopamine neuron-enriched grafts after transplanting human ES cell-derived neural precursor cells.

We have previously demonstrated derivation of neural precursor (NP) cells of a midbrain-type from human embryonic stem (hES) cells to yield an enriched population of dopamine (DA) neurons. These hES-derived NPs can be expanded in vitro through multiple passages without altering their DA neurogenic potential. Here, we studied two aspects of these hES-NP cells that are critical issues in cell therapeutic approaches for Parkinsons disease (PD): cell survival and tumorigenic potential. Neuroepithelial rosettes, a potentially tumorigenic structure, disappeared during hES-NP cell expansion in vitro. Although a minor population of cells positive for Oct3/4, a marker specific for undifferentiated hES cells, persisted in culture during hES-NP cell expansion, they could be completely eliminated by subculturing hES-NPs under differentiation-inducing conditions. Consistently, no tumors/teratomas are formed in rats grafted with multipassaged hES-NPs. However, extensively expanded hES-NP cells easily underwent cell death during differentiation in vitro and after transplantation in vivo. Transgenic expression of Bcl-XL and sonic hedgehog (SHH) completely overcame the cell survival problems without increasing tumor formation. These findings indicate that hES-NP cell expansion in conjunction with Bcl-XL+SHH transgene expression may provide a renewable and safe source of DA neurons for transplantation in PD.

Combined Nurr1 and Foxa2 roles in the therapy of Parkinson's disease.

Use of the physiological mechanisms promoting midbrain DA (mDA) neuron survival seems an appropriate option for developing treatments for Parkinsons disease (PD). mDA neurons are specifically marked by expression of the transcription factors Nurr1 and Foxa2. We show herein that Nurr1 and Foxa2 interact to protect mDA neurons against various toxic insults, but their expression is lost during aging and degenerative processes. In addition to their proposed cell‐autonomous actions in mDA neurons, forced expression of these factors in neighboring glia synergistically protects degenerating mDA neurons in a paracrine mode. As a consequence of these bimodal actions, adeno‐associated virus (AAV)‐mediated gene delivery of Nurr1 and Foxa2 in a PD mouse model markedly protected mDA neurons and motor behaviors associated with nigrostriatal DA neurotransmission. The effects of the combined gene delivery were dramatic, highly reproducible, and sustained for at least 1 year, suggesting that expression of these factors is a promising approach in PD therapy.

Generation of Dopamine Neurons with Improved Cell Survival and Phenotype Maintenance Using a Degradation-Resistant Nurr1 Mutant†‡

Nurr1 is a transcription factor specific for the development and maintenance of the midbrain dopamine (DA) neurons. Exogenous Nurr1 in neural precursor (NP) cells induces the differentiation of DA neurons in vitro that are capable of reversing motor dysfunctions in a rodent model for Parkinson disease. The promise of this therapeutic approach, however, is unclear due to poor cell survival and phenotype loss of DA cells after transplantation. We herein demonstrate that Nurr1 proteins undergo ubiquitin‐proteasome‐system‐mediated degradation in differentiating NP cells. The degradation process is activated by a direct Akt‐mediated phosphorylation of Nurr1 proteins and can be prevented by abolishing the Akt‐target sequence in Nurr1 (Nurr1Akt). Overexpression of Nurr1Akt in NP cells yielded DA neurons in which Nurr1 protein levels were maintained for prolonged periods. The sustained Nurr1 expression endowed the Nurr1Akt‐induced DA neurons with resistance to toxic stimuli, enhanced survival, and sustained DA phenotypes in vitro and in vivo after transplantation. STEM CELLS 2009;27:2238–2246

Editorial: An efficient index buffer management scheme for implementing a B-tree on NAND flash memory

Recently, NAND flash memory has been one of the best storage mediums for various embedded systems such as MP3 players, mobile phones and laptops because of its shock-resistant, low-power consumption, and none-volatile properties. However, since it has very distinct characteristics including erase-before-write and asymmetric read/write speed, the performance of disk based systems and applications may degrade dramatically when directly adopting them on the flash memory storage systems. Especially when a B-tree is constructed on NAND flash memory, intensive overwrite operations may be caused by record inserting, deleting, and reorganizing. These may result in severe performance degradation when building the B-tree. In this paper, we propose an efficient index buffer management scheme, called IBSF, which eliminates redundant index units in the index buffer and then delays the time that the index buffer requires to become full. Consequently, IBSF significantly reduces the number of write operations to a flash memory when constructing a B-tree. We also show that IBSF yields a better performance on a flash memory by comparing it to the related technique through various experiments.

Orphan nuclear receptor Nurr1 induces neuron differentiation from embryonic cortical precursor cells via an extrinsic paracrine mechanism

Nurr1 is an orphan nuclear receptor‐type transcription factor (TF) that plays critical roles in midbrain dopamine neuron development. This study demonstrated a novel role for Nurr1 in neuronal/astrocytic differentiation of neural precursor (NP) cells isolated from rat embryonic cortices: overexpression of this TF promoted NP cell differentiation towards neurons at the expense of astrocytic differentiation. Single cell‐based lineage analyses and experiments using co‐cultures revealed that Nurr1 elicited its neurogenic role in an extrinsic paracrine manner. We defined diffusible factors and downstream neurogenic TFs responsible for the Nurr1‐mediated neuronal differentiation.

An Efficient Buffer Management Scheme for Implementing a B-Tree on NAND Flash Memory

Recently, NAND flash memory has been used for a storage device in various mobile computing devices such as MP3 players, mobile phones and laptops because of its shock-resistant, low-power consumption, and none-volatile properties, However, due to the very distinct characteristics of flash memory, disk based systems and applications may result in severe performance degradation when directly adopting them on flash memory storage systems. Especially, when a B-tree is constructed, intensive overwrite operations may be caused by record inserting, deleting, and its reorganizing, This could result in severe performance degradation on NAND flash memory because of its distinct characteristics. In this paper, we propose an efficient buffer management scheme, called IBSF, which eliminates redundant index units in the index buffer and then delays the time that the index buffer is filled up. Consequently, IBSF significantly reduces the number of write operations to a flash memory when constructing a B-tree. We also show that IBSF yields a better performance on a flash memory by comparing it to the related technique called BFTL through various experiments.

Nanometer-scale lithography of the Langmuir–Blodgett films with atomic force microscope

Abstract The Langmuir–Blodgett (LB) films as a resist for scanning probe microscope (SPM) lithography have been studied extensively. The poly(methylphenyl methacrylate) (PMPMA) LB films were prepared and fabricated by atomic force microscopy (AFM) lithography. The induced bias voltage, scanning speed and developing conditions are very important for the high resolution AFM lithography. When the exposure was carried out at the bias voltage of −25 V, the protruding lines appeared in the exposed regions. This result provided evidence of a negative resist phenomenon due to the bond formation to build longer-chain polymer molecules which decrease the dissolution rate in the appropriate developer compared to that of the unexposed molecules. Preoptimized LB films at the various conditions exhibit 120 nm line resolution.