Eun Mi Hwang

TREK-1 and Best1 Channels Mediate Fast and Slow Glutamate Release in Astrocytes upon GPCR Activation

Astrocytes release glutamate upon activation of various GPCRs to exert important roles in synaptic functions. However, the molecular mechanism of release has been controversial. Here, we report two kinetically distinct modes of nonvesicular, channel-mediated glutamate release. The fast mode requires activation of G(αi), dissociation of G(βγ), and subsequent opening of glutamate-permeable, two-pore domain potassium channel TREK-1 through direct interaction between G(βγ) and N terminus of TREK-1. The slow mode is Ca(2+) dependent and requires G(αq) activation and opening of glutamate-permeable, Ca(2+)-activated anion channel Best1. Ultrastructural analyses demonstrate that TREK-1 is preferentially localized at cell body and processes, whereas Best1 is mostly found in microdomains of astrocytes near synapses. Diffusion modeling predicts that the fast mode can target neuronal mGluR with peak glutamate concentration of 100 μM, whereas slow mode targets neuronal NMDA receptors at around 1 μM. Our results reveal two distinct sources of astrocytic glutamate that can differentially influence neighboring neurons.

Caffeine-Mediated Inhibition of Calcium Release Channel Inositol 1,4,5-Trisphosphate Receptor Subtype 3 Blocks Glioblastoma Invasion and Extends Survival

Calcium signaling is important in many signaling processes in cancer cell proliferation and motility including in deadly glioblastomas of the brain that aggressively invade neighboring tissue. We hypothesized that disturbing Ca(2+) signaling pathways might decrease the invasive behavior of giloblastoma, extending survival. Evaluating a panel of small-molecule modulators of Ca(2+) signaling, we identified caffeine as an inhibitor of glioblastoma cell motility. Caffeine, which is known to activate ryanodine receptors, paradoxically inhibits Ca(2+) increase by inositol 1,4,5-trisphospate receptor subtype 3 (IP(3)R3), the expression of which is increased in glioblastoma cells. Consequently, by inhibiting IP(3)R3-mediated Ca(2+) release, caffeine inhibited migration of glioblastoma cells in various in vitro assays. Consistent with these effects, caffeine greatly increased mean survival in a mouse xenograft model of glioblastoma. These findings suggest IP(3)R3 as a novel therapeutic target and identify caffeine as a possible adjunct therapy to slow invasive growth of glioblastoma.

CHI3L1 (YKL-40) is expressed in human gliomas and regulates the invasion, growth and survival of glioma cells

Chitinase 3‐like 1 (CHI3L1) is a secreted glycoprotein that has pleiotropic activity in aggressive cancers. In our study, we examined the expression and function of CHI3L1 in glioma cells. CHI3L1 was highly expressed in human glioma tissue, whereas its expression in normal brain tissue was very low. CHI3L1 suppression by shRNA reduced glioma cell invasion, anchorage‐independent growth and increased cell death triggered by several anticancer drugs, including cisplatin, etoposide and doxorubicin, whereas CHI3L1 overexpression had the opposite effect in glioma cells. Because the invasive nature of glioma cells plays a critical role in the high morbidity of glioma, we have further defined the role of CHI3L1 in the process of glioma invasion. Downregulation of CHI3L1 results in decreased cell–matrix adhesion and causes a marked increase in stress fiber formation and cell size with fewer cellular processes. Furthermore, the expression and activity of matrix metalloproteinase‐2 was also decreased in glioma cells in which CHI3L1 was knocked down. Taken together, these results suggest that CHI3L1 plays an important role in the regulation of malignant transformation and local invasiveness in gliomas. Thus, targeting the CHI3L1 molecule may be a potential therapeutic molecular target for gliomas.

RhoGDI2 Expression Is Associated with Tumor Growth and Malignant Progression of Gastric Cancer

Purpose: Rho GDP dissociation inhibitor 2 (RhoGDI2) has been identified as a regulator of Rho family GTPase. However, there is currently no direct evidence suggesting whether RhoGDI2 activates or inhibits Rho family GTPase in vivo (and which type), and the role of RhoGDI2 in tumor remains controversial. Here, we assessed the effects of RhoGDI2 expression on gastric tumor growth and metastasis progression. Experimental Design: Proteomic analysis was done to investigate the tumor-specific protein expression in gastric cancer and RhoGDI2 was selected for further study. Immunohistochemistry was used to detect RhoGDI2 expression in clinical samples of primary gastric tumor tissues which have different pathologic stages. Gain-of-function and loss-of-function approaches were done to examine the malignant phenotypes of the RhoGDI2-expressing or RhoGDI2-depleting cells. Results: RhoGDI2 expression was correlated positively with tumor progression and metastasis potential in human gastric tumor tissues, as well as cell lines. The forced expression of RhoGDI2 caused a significant increase in gastric cancer cell invasion in vitro, and tumor growth, angiogenesis, and metastasis in vivo, whereas RhoGDI2 depletion evidenced opposite effects. Conclusion: Our findings indicate that RhoGDI2 is involved in gastric tumor growth and metastasis, and that RhoGDI2 may be a useful marker for tumor progression of human gastric cancer.

A disulphide-linked heterodimer of TWIK-1 and TREK-1 mediates passive conductance in astrocytes

TWIK-1 is a member of the two-pore domain K(+) (K2P) channel family that plays an essential part in the regulation of resting membrane potential and cellular excitability. The physiological role of TWIK-1 has remained enigmatic because functional expression of TWIK-1 channels is elusive. Here we report that native TWIK-1 forms a functional channel at the plasma membrane of astrocytes. A search for TWIK-1-binding proteins led to the identification of TREK-1, another member of the K2P family. The TWIK-1/TREK-1 heterodimeric channel is formed via a disulphide bridge between residue C69 in TWIK-1 and C93 in TREK-1. Gene silencing demonstrates that surface expression of TWIK-1 and TREK-1 are interdependent. TWIK-1/TREK-1 heterodimers mediate astrocytic passive conductance and cannabinoid-induced glutamate release from astrocytes. Our study sheds new light on the diversity of K2P channels.

Light-inducible receptor tyrosine kinases that regulate neurotrophin signalling

Receptor tyrosine kinases (RTKs) are a family of cell-surface receptors that have a key role in regulating critical cellular processes. Here, to understand and precisely control RTK signalling, we report the development of a genetically encoded, photoactivatable Trk (tropomyosin-related kinase) family of RTKs using a light-responsive module based on Arabidopsis thaliana cryptochrome 2. Blue-light stimulation (488 nm) of mammalian cells harbouring these receptors robustly upregulates canonical Trk signalling. A single light stimulus triggers transient signalling activation, which is reversibly tuned by repetitive delivery of blue-light pulses. In addition, the light-provoked process is induced in a spatially restricted and cell-specific manner. A prolonged patterned illumination causes sustained activation of extracellular signal-regulated kinase and promotes neurite outgrowth in a neuronal cell line, and induces filopodia formation in rat hippocampal neurons. These light-controllable receptors are expected to create experimental opportunities to spatiotemporally manipulate many biological processes both in vitro and in vivo.

BACE1 inhibitory effects of lavandulyl flavanones from Sophora flavescens.

In order to access beta-secretase (BACE1), and enzyme strongly implicated in the cause of Alzheimers disease, inhibitors must possess sufficient lipophilicity to traverse two lipid bilayers. Current drug candidates, which are almost totally peptide-derived, are thus inefficient because cell permeability presents a serious limiting factor. In this study, lipophilic alkylated (C(10)-C(5)) flavanones from Sophora flavescens were examined for their inhibitory effects against beta-secretase. Lavandulyl flavanones (1, 2, 5, 6, and 8) showed potent beta-secretase inhibitory activities with IC(50)s of 5.2, 3.3, 8.4, 2.6, and 6.7microM, respectively, while no significant activity was observed in the corresponding hydrated lavandulyl flavanones (4 and 7) and prenylated flavanone (3). As we expected, lavandulyl flavanones reduced Abeta secretion dose-dependently in transfected human embryonic kidney (HEK-293) cells. In kinetic studies, all compounds screened were shown to be noncompetitive inhibitor.

CDK5-dependent inhibitory phosphorylation of Drp1 during neuronal maturation

Mitochondrial functions are essential for the survival and function of neurons. Recently, it has been demonstrated that mitochondrial functions are highly associated with mitochondrial morphology, which is dynamically changed by the balance between fusion and fission. Mitochondrial morphology is primarily controlled by the activation of dynamin-related proteins including dynamin-related protein 1 (Drp1), which promotes mitochondrial fission. Drp1 activity is regulated by several post-translational modifications, thereby modifying mitochondrial morphology. Here, we found that phosphorylation of Drp1 at serine 616 (S616) is mediated by cyclin-dependent kinase 5 (CDK5) in post-mitotic rat neurons. Perturbation of CDK5 activity modified the level of Drp1S616 phosphorylation and mitochondrial morphology in neurons. In addition, phosphorylated Drp1S616 preferentially localized as a cytosolic monomer compared with total Drp1. Furthermore, roscovitine, a chemical inhibitor of CDKs, increased oligomerization and mitochondrial translocation of Drp1, suggesting that CDK5-dependent phosphorylation of Drp1 serves to reduce Drp1’s fission-promoting activity. Taken together, we propose that CDK5 has a significant role in the regulation of mitochondrial morphology via inhibitory phosphorylation of Drp1S616 in post-mitotic neurons.

Gadd45b Mediates Fas-induced Apoptosis by Enhancing the Interaction between p38 and Retinoblastoma Tumor Suppressor

Gadd45b has been known as a positive mediator of apoptosis induced by certain cytokines and oncogenes. Here, we identified Gadd45b as an effector of Fas-induced apoptosis and found that p38-mediated Rb hyperphosphorylation is one of the mechanisms of Fas-induced apoptosis in murine hepatocyte AML12 cells. Gadd45b has been shown to activate p38 through its physical interaction with MTK1 and induce apoptosis. However, in this study, we have showed that the function of Gadd45b during Fas-induced apoptosis in AML12 cells is different from that reported in previous studies. Depletion of Gadd45b expression did not inhibit the phosphorylation of p38, but it suppressed p38-mediated Rb phosphorylation and apoptosis in response to Fas stimulation by reducing the interaction between p38 and Rb. Ectopic expression of Gadd45b was sufficient to enhance this interaction. These findings suggest that Gadd45b mediates p38-induced Rb phosphorylation by enhancing the interaction between p38 and Rb during Fas-induced apoptosis in murine hepatocytes.

Human nuclear clusterin mediates apoptosis by interacting with Bcl-XL through C-terminal coiled coil domain.

Clusterin (CLU), a glycoprotein, is involved in apoptosis, producing two alternatively spliced isoforms in various cell types. The pro‐apoptotic CLU appears to be a nuclear isoform (nuclear clusterin; nCLU), and the secretory CLU (sCLU) is thought to be anti‐apoptotic. The detailed molecular mechanism of nCLU as a pro‐apoptotic molecule has not yet been clear. In the current study, overexpressed nCLU induced apoptosis in human kidney cells. Biochemical studies revealed that nCLU sequestered Bcl‐XL via a putative BH3 motif in the C‐terminal coiled coil (CC2) domain, releasing Bax, and promoted apoptosis accompanied by activation of caspase‐3 and cytochrome c release. These results suggest a novel mechanism of apoptosis mediated by nCLU as a pro‐apoptotic molecule. J. Cell. Physiol. 227: 1157–1167, 2012.