A tubulin binding molecule drives differentiation of acute myeloid leukemia cells.

Despite much progress in developing better drugs, many patients with acute myeloid leukemia (AML) still die within a year of diagnosis. This is partly because it is difficult to identify therapeutic targets that are effective across multiple AML subtypes. One common factor across AML subtypes is the presence of a block in differentiation. Overcoming this block should allow for the identification of therapies that are not dependent on a specific mutation for their efficacy. Here, we used a phenotypic screen to identify compounds that stimulate differentiation in genetically diverse AML cell lines. Lead compounds were shown to decrease tumor burden and to increase survival in vivo. Using multiple complementary target deconvolution approaches, these compounds were revealed to be anti-mitotic tubulin disruptors that cause differentiation by inducing a G2-M mitotic arrest. Together, these results reveal a function for tubulin disruptors in causing differentiation of AML cells.

Development of Human in vitro Brain-blood Barrier Model from Induced Pluripotent Stem Cell-derived Endothelial Cells to Predict the in vivo Permeability of Drugs.

An in vitro blood-brain barrier (BBB) model is critical for enabling rapid screening of the BBB permeability of the drugs targeting on the central nervous system. Though many models have been developed, their reproducibility and renewability remain a challenge. Furthermore, drug transport data from many of the models do not correlate well with the data for in vivo BBB drug transport. Induced-pluripotent stem cell (iPSC) technology provides reproducible cell resources for in vitro BBB modeling. Here, we generated a human in vitro BBB model by differentiating the human iPSC (hiPSC) line GM25256 into brain endothelial-type cells. The model displayed BBB characteristics including tight junction proteins (ZO-1, claudin-5, and occludin) and endothelial markers (von Willebrand factor and Ulex), as well as high trans-endothelial electrical resistance (TEER) (1560 Ω.cm2 ± 230 Ω.cm2) and γ-GTPase activity. Co-culture with primary rat astrocytes significantly increased the TEER of the model (2970 Ω.cm2 to 4185 Ω.cm2). RNAseq analysis confirmed the expression of key BBB-related genes in the hiPSC-derived endothelial cells in comparison with primary human brain microvascular endothelial cells, including P-glycoprotein (Pgp) and breast cancer resistant protein (BCRP). Drug transport assays for nine CNS compounds showed that the permeability of non-Pgp/BCRP and Pgp/BCRP substrates across the model was strongly correlated with rodent in situ brain perfusion data for these compounds (R2 = 0.982 and R2 = 0.9973, respectively), demonstrating the functionality of the drug transporters in the model. Thus, this model may be used to rapidly screen CNS compounds, to predict the in vivo BBB permeability of these compounds and to study the biology of the BBB.

Drug Distribution into Peripheral Nerve.

Little is known about the impact of the blood-nerve barrier (BNB) on drug distribution into peripheral nerves. In this study, we examined the peripheral nerve penetration in rats of 11 small-molecule drugs possessing diverse physicochemical and transport properties and ProTx-II, a tarantula venom peptide with molecular mass of 3826 Daltons. Each drug was administered as constant rate intravenous infusion for 6 hours (small molecules) or 24 hours (ProTx-II). Blood and tissues including brain, spinal cord, sciatic nerve, and dorsal root ganglion (DRG) were collected for drug concentration measurements. Unbound fractions of a set of compounds were determined by equilibrium dialysis method in rat blood, brains, spinal cords, sciatic nerves, and DRG. We also investigated the influence of N-[4-[2-(6,7-dimethoxy-3,4-dihydro-1H-isoquinolin-2-yl)ethyl]phenyl]-5-methoxy-9-oxo-10H-acridine-4-carboxamide (GF120918), a P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) inhibitor, on the peripheral nerve and central nervous system (CNS) tissue penetration of imatinib. We found that: 1) the unbound fraction in brain tissue homogenate highly correlates with that in the spinal cord, sciatic nerve, and DRG for a set of compounds and thus provides a good surrogate for spinal cord and peripheral nerve tissues, 2) small-molecule drugs investigated can penetrate the DRG and sciatic nerve, 3) P-gp and BCRP have a limited impact on the distribution of small-molecule drugs into peripheral nerves, and 4) DRG is permeable to ProTx-II, but its distribution into sciatic nerve and CNS tissues is restricted. These results demonstrate that small-molecule drugs investigated can penetrate peripheral nerve tissues, and P-gp/BCRP may not be a limiting factor at the BNB. Biologics as large as ProTx-II can access the DRG but not sciatic nerve and CNS tissues.

Endothelial cell adhesion to the extracellular matrix induces c-Src-dependent VEGFR-3 phosphorylation without the activation of the receptor intrinsic kinase activity.

RATIONALE: Integrins cooperate with growth factor receptors to promote downstream signaling for cell proliferation and migration. However, the mechanism of receptor activation is still unknown. OBJECTIVE: To analyze the mechanism of phosphorylation of the vascular endothelial growth factor receptor (VEGFR)-3 by cell adhesion. METHODS AND RESULTS: We show that VEGFR-3 phosphorylation, induced by cell attachment to the extracellular matrix, is independent from the intrinsic kinase activity of the receptor, as evidenced from phosphorylation cell adhesion experiments with a mutant kinase dead receptor or in the presence of the specific kinase inhibitor MAZ 51. Cell adhesion experiments in the presence of the c-Src inhibitor PP2 or in fibroblast triple knockout for c-Src, Yes, and Fyn (SYF) demonstrate that VEGFR-3 phosphorylation, induced by extracellular matrix, is mediated by c-Src. Kinase assays in vitro with recombinant c-Src show that VEGFR-3 is a direct c-Src target and mass spectrometry analysis identified the sites phosphorylated by c-Src as tyrosine 830, 833, 853, 1063, 1333, and 1337, demonstrating that integrin-mediated receptor phosphorylation induces a phosphorylation pattern that is distinct from that induced by growth factors. Furthermore, pull-down assays show that integrin-mediated VEGFR-3 phosphorylation activates the recruitment to the receptor of the adaptor proteins CRKI/II and SHC inducing activation of JNK. CONCLUSIONS: These data suggest that cell adhesion to extracellular matrix induces a downstream signaling using the tyrosine kinase receptor VEGFR-3 as scaffold.

In vitro screening strategies for nicotinic receptor ligands.

A common historical strategy to the discovery of nicotinic receptor ligands has involved the use of radioligand-binding assays for ligand identification in combination with two-electrode voltage clamp in Xenopus oocytes for electrophysiological characterization. More recently, higher-throughput methodologies have replaced these approaches to accommodate screening of large compound libraries and to provide increased capacity for electrophysiological profiling in mammalian cell lines. We, and others, have implemented cell-based screening assays using the fluorometric imaging plate reader (FLIPR) for primary and lead optimization screening of nicotinic receptor agonists and positive allosteric modulators (PAMs). Using GH4C1 cells expressing the rat alpha7 nicotinic receptor, both acetylcholine and nicotine produced concentration-dependent elevations of intracellular calcium with EC(50) values of 5.5 and 1.6 microM, respectively. PAM activity was robustly detected using the FLIPR assay; for example, the known alpha7 receptor PAM 5-hydroxyindole failed to directly activate the receptor but produced a leftward shift of the nicotine concentration-response curve in combination with a potentiation of the maximum evoked response to nicotine. Electrophysiological confirmation of agonist activity was achieved using the Dynaflow rapid perfusion system and patch clamp in the same GH4C1 cell expression system. Estimated EC(50) values for acetylcholine-evoked currents in GH4C1/alpha7 cells were 55 and 576 microM for area-under-the-curve (AUC) and maximum peak height calculations, respectively. Similarly, PAM activity was confirmed using electrophysiological recordings while also allowing for the mechanistic discrimination of compounds, not possible using the FLIPR assay. Specifically, PAMs capable of slowing the rapid desensitization of alpha7 receptors to different extents were discernable in these studies. Further improvements in the capacity to screen compounds using electrophysiology has been achieved by implementation of high-throughput gigaohm quality recording systems such as the QPatch and PatchXpress where agonist EC(50) values are highly comparable to those obtained using conventional manual patch clamp.

Inhibition of Wnt signaling, modulation of Tau phosphorylation and induction of neuronal cell death by DKK1.

Expression of the Wnt antagonist Dickkopf-1 (DKK1) is induced during neurodegenerative processes associated with Alzheimer's Disease and brain ischemia. However, little is known about DKK1-mediated effects on neurons. We now describe that, in cultured neurons, DKK1 is able to inhibit canonical Wnt signaling, as assessed by TCF reporter assay and analysis of beta-catenin levels, and to elicit cell death associated with loss of BCL-2 expression, induction of BAX, and TAU hyperphosphorylation. Local infusion of DKK1 in rats caused neuronal cell death and astrocytosis in the CA1 region of the hippocampus and death of cholinergic neurons in the nucleus basalis magnocellularis. Both effects were reversed by systemic administration of lithium ions, which rescue the Wnt pathway by inhibiting glycogen synthase kinase-3beta. The demonstration that DKK1 inhibits Wnt signaling in neurons and causes neuronal death supports the hypothesis that inhibition of the canonical Wnt pathway contributes to the pathophysiology of neurodegenerative disorders.

Inhibition of the canonical Wnt signaling pathway by apolipoprotein E4 in PC12 cells.

We examined the effect of the three human isoforms of apolipoprotein E (ApoE2, ApoE3, and ApoE4) on the canonical Wnt signaling pathway in undifferentiated PC12 cells. Addition of recombinant ApoE4 reduced Wingless-Int7a-stimulated gene expression at concentrations of 80 and 500 nm. Recombinant ApoE2 and ApoE3 were virtually inactive. Recombinant ApoE4 also inhibited Wnt signaling when combined with very low density lipoproteins (VLDLs) or in cells over-expressing the low density lipoprotein receptor-related protein, LRP6. In contrast, the enforced expression of LRP5 unmasked an inhibition by ApoE2 and ApoE3, which, however, were less effective than ApoE4 in inhibiting Wnt signaling. We also transfected PC12 cells with constructs encoding for the three human ApoE isoforms to examine whether endogenously expressed ApoE isoforms could modulate the Wnt pathway. Under these conditions, all three ApoE isoforms were able to inhibit Wnt signaling, although ApoE4 showed the greatest efficacy. Only the conditioned medium collected from cultures transfected with ApoE4 induced a significant inhibition of Wnt7a-stimulated gene expression, confirming that ApoE4 has an extracellular action that is not shared by the other ApoE isoforms. We conclude that ApoE4 behaves as an inhibitor of the canonical Wnt pathway in a context-independent manner.

Induction of Dickkopf-1, a negative modulator of the Wnt pathway, is associated with neuronal degeneration in Alzheimer's brain.

We used primary cultures of cortical neurons to examine the relationship between beta-amyloid toxicity and hyperphosphorylation of the tau protein, the biochemical substrate for neurofibrillary tangles of Alzheimer's brain. Exposure of the cultures to beta-amyloid peptide (betaAP) induced the expression of the secreted glycoprotein Dickkopf-1 (DKK1). DKK1 negatively modulates the canonical Wnt signaling pathway, thus activating the tau-phosphorylating enzyme glycogen synthase kinase-3beta. DKK1 was induced at late times after betaAP exposure, and its expression was dependent on the tumor suppressing protein p53. The antisense induced knock-down of DKK1 attenuated neuronal apoptosis but nearly abolished the increase in tau phosphorylation in betaAP-treated neurons. DKK1 was also expressed by degenerating neurons in the brain from Alzheimer's patients, where it colocalized with neurofibrillary tangles and distrophic neurites. We conclude that induction of DKK1 contributes to the pathological cascade triggered by beta-amyloid and is critically involved in the process of tau phosphorylation.

Functional characterization of WNT7A signaling in PC12 cells: interaction with A FZD5 x LRP6 receptor complex and modulation by Dickkopf proteins.

WNT factors represent key mediators of many processes in animal development and homeostasis and act through a receptor complex comprised of members of the Frizzled and low density lipoprotein-related receptors (LRP). In mammals, 19 genes encoding Wingless and Int-related factor (WNTs), 10 encoding Frizzled, and 2 encoding LRP proteins have been identified, but little is known of the identities of individual Frizzled-LRP combinations mediating the effects of specific WNT factors. Additionally, several secreted modulators of WNT signaling have been identified, including at least three members of the Dickkopf family. WNT7A is a WNT family member expressed in the vertebrate central nervous system capable of modulating aspects of neuronal plasticity. Gene knock-out models in the mouse have revealed that WNT7A plays a role in cerebellar maturation, although its function in the development of distal limb structures and of the reproductive tract have been more intensely studied. To identify a receptor complex for this WNT family member, we have analyzed the response of the rat pheochromocytoma cell line PC12 to WNT7A. We find that PC12 cells are capable of responding to WNT7A as measured by increased beta-catenin stability and activation of a T-cell factor-based luciferase reporter construct and that these cells express three members of the Frizzled family (Frizzled-2, -5, and -7) and LRP6. Our functional analysis indicates that WNT7A can specifically act via a Frizzled-5.LRP6 receptor complex in PC12 cells and that this activity can be antagonized by Dickkopf-1 and Dickkopf-3.

The Wnt pathway, cell-cycle activation and beta-amyloid: novel therapeutic strategies in Alzheimer's disease?

Beta-amyloid protein (betaAP) is thought to cause neuronal loss in Alzheimer's disease (AD). Applied to neurons in culture, betaAP induces neuronal death and hyperphosphorylation of tau protein, which forms neurofibrillary tangles (NFTs) in AD brains. Neurons also undergo rapid apoptotic death following reactivation of a mitotic cycle. However, the molecular events that determine the fate of neurons challenged with betaAP (apoptotic death, formation of NFTs and survival) are unclear. We discuss a scenario for the pathogenesis of AD. This links betaAP-induced changes to the Wnt signaling pathway that promotes proliferation of progenitor cells and directs cells into a neuronal phenotype during brain development. We propose that betaAP-mediated facilitation of mitogenic Wnt signaling activates unscheduled mitosis in differentiated neurons. Furthermore, late downregulation of Wnt signaling by betaAP might lead to NFT formation. We propose that drugs that both inhibit the cell cycle and rescue Wnt activity could provide novel AD therapeutics.

Pharmacological properties of rat alpha 7 nicotinic receptors expressed in native and recombinant cell systems.

The pharmacological properties of the rat alpha7 nicotinic acetylcholine receptor endogenously expressed in PC12 cells and recombinantly expressed in GH4C1 cells (alpha7-GH4C1 cells) were characterized and compared. Patch-clamp recordings demonstrated that activation by choline and block by methyllycaconitine and dihydro-beta-erythroidine were similar, but block by mecamylamine was different. Whereas in alpha7-GH4C1 cells the inhibition curve for mecamylamine was monophasic (IC(50) of 1.6 microM), it was biphasic in PC12 cells (IC(50) values of 341 nM and 9.6 microM). The same rank order of potency was obtained for various nicotinic agonists, while acetylcholine was 3.7-fold less potent and 1.5-fold more effective in PC12 cells. Dihydro-beta-erythroidine differentially blocked acetylcholine-evoked currents in both systems. Since reverse transcriptase polymerase chain reaction (RT-PCR) experiments revealed expression of alpha3, alpha4, alpha5, alpha7 and beta4 subunits in PC12 cells, whereas GH4C1 cells express only the beta4 subunit, our results suggest that more than one form of alpha7 containing heteromeric nicotinic receptors might be functionally expressed in PC12 cells.

Pharmacological and molecular characterisation of SK3 channels in the TE671 human medulloblastoma cell line.

The expression of the small conductance calcium-activated potassium channels SK1, SK2 and SK3 was investigated in the TE671 human medulloblastoma cell line using RT-PCR and transcripts were detected only for SK3. Immunodetection experiments confirmed this result, demonstrating the presence of the SK3 protein. This potassium channel was characterised in TE671 cells using whole-cell patch-clamp recordings. Voltage steps to -100 mV from a holding potential of 0 mV in equimolar 140 mM intra- and extracellular K(+) (K(+)(in/out)) elicited an inward current. The reversal potential of this current shifted 56.6 mV per 10-fold increase in K(+)(out) thus suggesting K(+) selectivity. This current was dependent on the concentration of Ca(2+)(in) with an EC(50) of 104.2 nM. A pharmacological characterisation of this current revealed that it was not blocked by 1 microM charybdotoxin (ChTX), 0.3 microM iberiotoxin (IbTX) or 10 microM clotrimazole (CLT) and only modestly inhibited (<50%) by 30 nM scyllatoxin (ScTX), 200 microM dequalinium chloride (Deq) or 300 microM d-tubocurarine (d-TC). The non-selective SK blocker d-TC blocked the current with an IC(50) of 43.2 microM while apamin blocked the current to a much greater extent (87.8% at 1 microM) with an IC(50) of 4.3 nM. Furthermore, the current was significantly increased (132.6+/-5.2%, n=7) by 500 microM 1-ethyl-2-benzimidazolinone (EBIO). Collectively, these data demonstrate the presence of an endogenous SK3 channel in human TE671 cells.

Molecular cloning and initial characterization of the MG61/PORC gene, the human homologue of the Drosophila segment polarity gene Porcupine.

Insect and vertebrate Porcupine genes encode multi-pass endoplasmic reticulum proteins involved in the processing of Wnt (wingless and int homologue) proteins, a class of secreted glycoprotein factors homologous to the Drosophila melanogaster segment polarity gene Wingless (Wg). Here we report the cloning of cDNAs encoding the human homologue of the Drosophila gene Porcupine (Porc), the characterization of its genomic structure and the quantitative analysis of its expression in a comprehensive panel of human tissues. The human Porcupine locus (MG61/PORC) spans 15 exons over approximately 12 kb of genomic sequence on Xp11.23. Real-time quantitative expression analysis reveals that MG61/PORC transcripts are expressed in multiple tissues, but are particularly abundant in the brain. Like its mouse and Xenopus homologues, MG61/PORC encodes four protein isoforms (A-D) generated through alternative splicing and expressed in a tissue-specific fashion. Finally, we present evidence indicating that MG61/PORC can influence the activity of a human Wnt7A expression construct in a T-cell factor-responsive reporter assay.

Lithium induces gene expression through lymphoid enhancer-binding factor/T-cell factor responsive element in rat PC12 cells.

Lithium inhibits glycogen synthase kinase-3 (GSK-3), which leads to an increase of cytoplasmic beta-catenin levels. In some cell types, but not in others, activated beta-catenin interacts with members of the lymphoid enhancer-binding factor (LEF)/T-cell factor (TCF) family of transcription factors and induces gene expression. Lithium effect on LEF/TCF-mediated gene expression has never been evaluated in cells with a neuronal phenotype. We have constructed a LEF/TCF-dependent luciferase reporter gene to investigate lithium effects on transcription in PC12 cells. In transiently transfected PC12 cells, lithium induced a time-dependent increase in LEF/TCF-mediated luciferase activity. These results are consistent with the known inhibitory effects of lithium on GSK-3 and represent the first demonstration that a LEF/TCF responsive element also mediates lithium-induced gene expression in PC12 cells.