Short Splice Form Research Articles

A revised model of TRAIL‐R2 DISC assembly explains how FLIP(L) can inhibit or promote apoptosis

The long FLIP splice form FLIP(L) can act as both an inhibitor and promoter of caspase‐8 at death‐inducing signalling complexes (DISCs) formed by death receptors such as TRAIL‐R2 and related intracellular complexes such as the ripoptosome. Herein, we describe a revised DISC assembly model that explains how FLIP(L) can have these opposite effects by defining the stoichiometry (with respect to caspase‐8) at which it converts from being anti‐ to pro‐apoptotic at the DISC. We also show that in the complete absence of FLIP(L), procaspase‐8 activation at the TRAIL‐R2 DISC has significantly slower kinetics, although ultimately the extent of apoptosis is significantly greater. This revised model of DISC assembly also explains why FLIP's recruitment to the TRAIL‐R2 DISC is impaired in the absence of caspase‐8 despite showing that it can interact with the DISC adaptor protein FADD and why the short FLIP splice form FLIP(S) is the more potent inhibitor of DISC‐mediated apoptosis.

Cytoplasmic FLIP(S) and nuclear FLIP(L) mediate resistance of castrate-resistant prostate cancer to apoptosis induced by IAP antagonists

Expression of tumor necrosis factor-α (TNFα) in the serum of prostate cancer patients is associated with poorer outcome and progression to castrate-resistant (CRPC) disease. TNFα promotes the activity of NFκB, which regulates a number of anti-apoptotic and proinflammatory genes, including those encoding the inhibitor of apoptosis proteins (IAPs); however, in the presence of IAP antagonists, TNFα can induce cell death. In the presence of recombinant or macrophage-derived TNFα, we found that IAP antagonists triggered degradation of cIAP1 and induced formation of Complex-IIb, consisting of caspase-8, FADD and RIPK1 in CRPC models; however, no, or modest levels of apoptosis were induced. This resistance was found to be mediated by both the long (L) and short (S) splice forms of the caspase-8 inhibitor, FLIP, another NFκB-regulated protein frequently overexpressed in CRPC. By decreasing FLIP expression at the post-transcriptional level in PC3 and DU145 cells (but not VCaP), the Class-I histone deacetylase (HDAC) inhibitor Entinostat promoted IAP antagonist-induced cell death in these models in a manner dependent on RIPK1, FADD and Caspase-8. Of note, Entinostat primarily targeted the nuclear rather than cytoplasmic pool of FLIP(L). While the cytoplasmic pool of FLIP(L) was highly stable, the nuclear pool was more labile and regulated by the Class-I HDAC target Ku70, which we have previously shown regulates FLIP stability. The efficacy of IAP antagonist (TL32711) and Entinostat combination and their effects on cIAP1 and FLIP respectively were confirmed in vivo, highlighting the therapeutic potential for targeting IAPs and FLIP in proinflammatory CRPC.

XIRP2, an actin-binding protein essential for inner ear hair-cell stereocilia.

Hair cells of the inner ear are mechanoreceptors for hearing and balance, and proteins highly enriched in hair cells may have specific roles in the development and maintenance of the mechanotransduction apparatus. We identified XIRP2/mXinβ as an enriched protein likely to be essential for hair cells. We found that different isoforms of this protein are expressed and differentially located: short splice forms (also called XEPLIN) are targeted more to stereocilia, whereas two long isoforms containing a XIN-repeat domain are in both stereocilia and cuticular plates. Mice lacking the Xirp2 gene developed normal stereocilia bundles, but these degenerated with time: stereocilia were lost and long membranous protrusions emanated from the nearby apical surfaces. At an ultrastructural level, the paracrystalline actin filaments became disorganized. XIRP2 is apparently involved in the maintenance of actin structures in stereocilia and cuticular plates of hair cells, and perhaps in other organs where it is expressed.

A short splice form of Xin-actin binding repeat containing 2 (XIRP2) lacking the Xin repeats is required for maintenance of stereocilia morphology and hearing function.

Approximately one-third of known deafness genes encode proteins located in the hair bundle, the sensory hair cell's mechanoreceptive organelle. In previous studies, we used mass spectrometry to characterize the hair bundle's proteome, resulting in the discovery of novel bundle proteins. One such protein is Xin-actin binding repeat containing 2 (XIRP2), an actin-cross-linking protein previously reported to be specifically expressed in striated muscle. Because mutations in other actin-cross-linkers result in hearing loss, we investigated the role of XIRP2 in hearing function. In the inner ear, XIRP2 is specifically expressed in hair cells, colocalizing with actin-rich structures in bundles, the underlying cuticular plate, and the circumferential actin belt. Analysis using peptide mass spectrometry revealed that the bundle harbors a previously uncharacterized XIRP2 splice variant, suggesting XIRP2's role in the hair cell differs significantly from that reported in myocytes. To determine the role of XIRP2 in hearing, we applied clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9-mediated genome-editing technology to induce targeted mutations into the mouse Xirp2 gene, resulting in the elimination of XIRP2 protein expression in the inner ear. Functional analysis of hearing in the resulting Xirp2-null mice revealed high-frequency hearing loss, and ultrastructural scanning electron microscopy analyses of hair cells demonstrated stereocilia degeneration in these mice. We thus conclude that XIRP2 is required for long-term maintenance of hair cell stereocilia, and that its dysfunction causes hearing loss in the mouse.

Inhibition of Splicing Factor SF3B1: Evaluating Effects on Iron Metabolism in K562 and CD34+ Cells

Somatic mutations in the RNA splicing factor SF3B1 have been found in hematopoietic cells from patients with myelodysplastic syndrome. The association is particularly strong for patients with anemia with ringed sideroblasts, where 65 - 75 % show alterations in SF3B1. Ringed sideroblasts are erythroid precursors with insoluble iron aggregates in mitochondria and deficient hemoglobin synthesis. Thus these cells demonstrate a defect in iron metabolism. SF3B1 is a core component of the U2 snRNP spliceosome, responsible for the majority of mRNA maturation, raising the question of how alteration of SF3B1 might perturb cellular iron metabolism. Meayamycin B, a synthetic analog of the natural product FR901464, is an inhibitor of SF3B1, and here we describe the effects of this compound on cellular iron metabolism in the K562 leukemic cell line and in CD34+ cells isolated from cord blood.Inhibition of SF3B1 by meayamycin B was confirmed by analysis of spliced forms of Mcl-1 mRNA, a known target of SF3B1. In untreated cells, a long form (Mcl-lL) arises from inclusion of exon 2 in the final mRNA. In treated cells, inhibition of SF3B1 splicing generates a short form (Mcl-lS) that lacks exon 2. In the presence of 3 to 10 nM meayamycin B the short splice-form (Mcl-1S) became the dominant species. To examine the effect of inhibition of SF3B1 on iron metabolism, K562 cells were exposed to meayamycin B for 24 hours, with controls of 50 µM hemin and 50 µM deferoxamine (DFO). Steady state mRNA levels of transferrin receptor (TFR-1), ABCB7 (mutations in which are associated with inherited sideroblastic anemia and ataxia), and NDRG1 (n-myc downstream regulated gene, previously shown to be induced by DFO) were significantly down regulated by meayamycin B, as determined by qRT-PCR. NDUFS8 mRNA that encodes a subunit of mitochondrial complex 1 and has previously been shown to be iron-responsive, did not change in response to meayamycin B, hemin, or DFO. However, MRLP19 that encodes a mitochondrial ribosomal component, was strongly down-regulated by meayamycin B. In contrast, cytosolic ribosomal protein RPLP0 was not affected by meayamycin B, hemin or DFO and was used as the endogenous control.Analysis of protein levels by western blotting indicated that meayamycin B increased levels of ferritin (heavy chain) and decreased TFR-1. In concordance with the decreased TFR-1 protein levels, meayamycin B treated K562 cells exhibited significant downregulation of iron uptake from 55Fe-transferrin, similar to that observed for hemin treated cells. DFO treatment significantly increased transferrin iron uptake, compared to untreated controls.A similar set of experiments was performed with CD34+ cells, before or after differentiation with erythropoietin. Meayamycin was efficacious in these cells, as shown by disappearance of the long splice-form Mcl-lL and appearance of the short splice form Mcl-1S in cells exposed to the compound for 16 h. TFR-1 mRNA exhibited a dose dependent decrease in response to meayamycin (both pre and post epo treatment). Neither hemin nor DFO altered TFR-1 mRNA levels, or protein levels. In non-epo treated CD34+ cells, exposure to meayamycin caused a small decrease in ferritin protein levels, but ferritin protein increased in epo-treated cells exposed to meayamycin. Hemin increased ferritin protein in both non-epo and epo treated cells. Mitochondrial ribosomal protein MRPL19 RNA was significantly decreased by meayamycin in all cases, but cytoplasmic ribosomal protein RPLP0 was unaffected by meayamycin. Ribosomal protein levels were not responsive to iron manipulations with hemin or DFO.To summarize, treatment of cells with meayamycin B inhibits SF3B1 function in K562 cells or CD34+ cells. The effects are quicker and cleaner than siRNA, making possible closer functional correlations. Meayamycin treated K562 cells exhibited downregulation of TFR-1 and TFR-1 iron uptake, as if they were sensing high cellular iron levels. Note that this is the opposite of the expected result, because iron loading in the setting of sideroblastic anemia would be expected to be associated with increased cellular iron uptake. In CD34+ cells, a similar downregulation of TFR-1 levels was observed, although iron uptake has not yet been directly measured. The strong decrease in mitochondrial ribosomal protein expression indicates a specific mitochondrial effect of SF3B1 inhibition, which may be an important mediator of iron metabolism. DisclosuresNo relevant conflicts of interest to declare.

Homer1 Alternative Splicing Is Regulated by Gonadotropin-Releasing Hormone and Modulates Gonadotropin Gene Expression

Hypothalamic gonadotropin-releasing hormone (GnRH) plays a critical role in reproductive physiology by regulating follicle-stimulating hormone (FSH) and luteinizing hormone (LH) gene expression in the pituitary. Analysis of gonadotrope deep-sequencing data identified a global regulation of pre-mRNA splicing by GnRH. Homer1, a gene encoding a postsynaptic density scaffolding protein, was selected for further study. Homer1 expresses a short splice form, Homer1a, and more-abundant long transcripts Homer1b/c. GnRH induced a modest increase in Homer1b/c expression and a dramatic increase in the Homer1a splice form. G protein knockdown studies suggested that the Homer1 induction, but not the regulated splicing, was Gαq/11 dependent. Phosphorylation of the splicing regulator SRp20 was found to be induced by GnRH. SRp20 depletion attenuated the GnRH-induced increase in the Homer1a-to-Homer1b/c ratio and modulated the effects of GnRH on FSHβ and LHβ expression. Homer1 gene knockdown resulted in increased GnRH-induced FSHβ and LHβ transcript levels. Furthermore, splice-form-specific reduction of Homer1b/c increased both FSHβ and LHβ mRNA induction, whereas reduction of Homer1a had the opposite effect on FSHβ induction. These results indicate that the regulation of Homer1 splicing by GnRH contributes to gonadotropin gene control.

Prognostic and therapeutic relevance of c‐FLIP in acute myeloid leukaemia

Chemoresistance is a major contributor to the aggressiveness of AML and is often due to insufficient apoptosis. The CFLAR gene is expressed as long and short splice forms encoding the anti-apoptotic proteins c-FLIP(L) and c-FLIP(S) (CFLAR(L) and CFLAR(S) , respectively) that play important roles in drug resistance. In univariate analyses of CFLAR mRNA expression in adult AML patients, those individuals with higher than median mRNA expression of the long splice form CFLAR(L) (but not the short splice form) had significantly lower 3 year overall survival (P = 0·04) compared to those with low expression. In cell line studies, simultaneous down-regulation of c-FLIP(L) and c-FLIP(S) proteins using siRNA induced apoptosis in U937 and NB-4 AML cells, but not K562 or OCI-AML3 cells. However, dual c-FLIP(L/S) downregulation sensitized all four cell lines to apoptosis induced by recombinant tumour necrosis factor-related apoptosis-inducing ligand (rTRAIL). Moreover, specific downregulation of c-FLIP(L) was found to recapitulate the phenotypic effects of dual c-FLIP(L/S) downregulation. The histone deacetylase (HDAC)1/2/3/6 inhibitor Vorinostat was found to potently down-regulate c-FLIP(L) expression by transcriptional and post-transcriptional mechanisms and to sensitize AML cells to rTRAIL. Further analyses using more selective HDAC inhibitors revealed that HDAC6 inhibition was not required for c-FLIP(L) down-regulation. These results suggest that c-FLIP(L) may have clinical relevance both as a prognostic biomarker and potential therapeutic target for HDAC inhibitors in AML although this requires further study.

93 Mapping the Interaction Between FLIP and FADD

Introduction: cFLIP is a major anti-apoptotic protein that blocks the apoptotic pathway mediated by death receptors. It is overexpressed in many cancers resulting in chemoresistance and limiting the effectiveness of commonly used anticancer therapies. Both the long (FLIPL) and the short (FLIPS) splice forms compete with procaspase 8 for binding with the adaptor protein FADD at the death inducing signaling complex (DISC), which is formed after stimulation of death receptors by their ligands. Materials and Methods:Molecular modeling of the FLIP–FADD interaction was performed using published structures viral FLIP (MC159) and human FADD. Site directed mutagenesis of was performed to study the importance of specific residues of FLIP. Protein–protein interactions were studied by GST-pulldown assay. DISC recruitment of FLIP mutants was investigated in a DISC IP assay. Western blotting was performed to assess protein levels. Results: Modelling identified several key residues which appeared to be important for the cFLIP–FADD interaction. Site directed mutagenesis was performed against these key residues, and a panel of cFLIP and FADD mutants was generated. These mutants were then screened for their ability to interact with either FADD or cFLIP using GST-pulldown assays. A functional assay for DISC recruitment was also carried out to confirm which of these mutations resulted in impaired recruitment. This analysis established the critical amino acids which mediate DED interactions between cFLIP and FADD. Conclusions: Iterative rounds of computer modelling and mutagenesis have characterised the molecular features of the interaction between the pro-apoptotic adaptor molecule FADD and the anti-apoptotic protein FLIP.

94 Induction of Autophagy by Imatinib Sequesters BCR-ABL in Autophagosomes and Reduces BCR-ABL Expression

Introduction: cFLIP is a major anti-apoptotic protein that blocks the apoptotic pathway mediated by death receptors. It is overexpressed in many cancers resulting in chemoresistance and limiting the effectiveness of commonly used anticancer therapies. Both the long (FLIPL) and the short (FLIPS) splice forms compete with procaspase 8 for binding with the adaptor protein FADD at the death inducing signaling complex (DISC), which is formed after stimulation of death receptors by their ligands. Materials and Methods:Molecular modeling of the FLIP–FADD interaction was performed using published structures viral FLIP (MC159) and human FADD. Site directed mutagenesis of was performed to study the importance of specific residues of FLIP. Protein–protein interactions were studied by GST-pulldown assay. DISC recruitment of FLIP mutants was investigated in a DISC IP assay. Western blotting was performed to assess protein levels. Results: Modelling identified several key residues which appeared to be important for the cFLIP–FADD interaction. Site directed mutagenesis was performed against these key residues, and a panel of cFLIP and FADD mutants was generated. These mutants were then screened for their ability to interact with either FADD or cFLIP using GST-pulldown assays. A functional assay for DISC recruitment was also carried out to confirm which of these mutations resulted in impaired recruitment. This analysis established the critical amino acids which mediate DED interactions between cFLIP and FADD. Conclusions: Iterative rounds of computer modelling and mutagenesis have characterised the molecular features of the interaction between the pro-apoptotic adaptor molecule FADD and the anti-apoptotic protein FLIP.

The expression of splice forms for the rheumatoid arthritis risk associated gene PTPN22 is significantly different for patients compared to controls

BackgroundSeveral genetic risk factors have been revealed for rheumatoid arthritis (RA) whereof HLA-DRB1 shared epitope alleles and PTPN22 remains the two most undisputed for the disease. Hypotheses for how these...

Identification of alternatively spliced Dab1 and Fyn isoforms in pig

BackgroundDisabled-1 (Dab1) is an adaptor protein that is essential for the intracellular transduction of Reelin signaling, which regulates the migration and differentiation of postmitotic neurons during brain development in vertebrates. Dab1 function depends on its tyrosine phosphorylation by Src family kinases, especially Fyn.ResultsWe have isolated alternatively spliced forms of porcine Dab1 from brain (sDab1) and liver (sDab1-Li) and Fyn from brain (sFyn-B) and spleen (sFyn-T). Radiation hybrid mapping localized porcine Dab1 (sDab1) and Fyn (sFyn) to chromosomes 6q31-35 and 1p13, respectively. Real-time quantitative RT-PCR (qRT-PCR) demonstrated that different isoforms of Dab1 and Fyn have tissue-specific expression patterns, and sDab1 and sFyn-B display similar temporal expression characteristics in the developing porcine cerebral cortex and cerebellum. Both sDab1 isoforms function as nucleocytoplasmic shuttling proteins. It was further shown that sFyn phosphorylates sDab1 at tyrosyl residues (Tyr) 185, 198/200 and 232, whereas sDab1-Li was phosphorylated at Tyr 185 and Tyr 197 (corresponding to Y232 in sDab1) in vitro.ConclusionsAlternative splicing generates natural sDab1-Li that only carries Y185 and Y197 (corresponding to Y232 in sDab1) sites, which can be phosphorylated by Fyn in vitro. sDab1-Li is an isoform that is highly expressed in peripheral organs. Both isoforms are suggested to be nucleocytoplasmic shuttling proteins. Our results imply that the short splice form sDab1-Li might regulate cellular responses to different cell signals by acting as a dominant negative form against the full length sDab1 variant and that both isoforms might serve different signaling functions in different tissues.

C-terminal Alternative Splicing Modulates Single-Channel Gating of Cav1.3 L-Type Calcium Channel

We have recently discovered a novel mechanism of channel modulation in CaV1.3 channels enabling cells to tightly control gating by C-terminal alternative splicing. The absence of a C-terminal modulatory motif (CTM) within a short splice form facilitates CaV1.3 channel activation at lower voltages and induces pronounced Ca2+ dependent-inactivation (CDI) (Singh et al., JBC 2008). Intriguingly, whole-cell measurements revealed a significant difference in current density between long and short channels for both Ba2+ and Ca2+ currents (IBa and ICa) while membrane expression of the pore-forming subunit was unchanged suggesting additional differences of unitary current properties.

Chemical Cross-linking Provides a Model of the γ-Secretase Complex Subunit Architecture and Evidence for Close Proximity of the C-terminal Fragment of Presenilin with APH-1

Gamma-secretase is an intramembrane cleaving aspartyl protease complex intimately implicated in Alzheimer disease pathogenesis. The protease is composed of the catalytic subunit presenilin (PS1 or PS2), the substrate receptor nicastrin (NCT), and two additional subunits, APH-1 (APH-1a, as long and short splice forms (APH-1aL, APH-1aS), or APH-1b) and PEN-2. Apart from the Alzheimer disease-associated beta-amyloid precursor protein, gamma-secretase has been shown to cleave a large number of other type I membrane proteins. Despite the progress in elucidating gamma-secretase function, basic questions concerning the precise organization of its subunits, their molecular interactions, and their exact stoichiometry in the complex are largely unresolved. Here we isolated endogenous human gamma-secretase from human embryonic kidney 293 cells and investigated the subunit architecture of the gamma-secretase complex formed by PS1, NCT, APH-1aL, and PEN-2 by chemical cross-linking. Using this approach, we provide evidence for the close neighborhood of the PS1 N- and C-terminal fragments (NTF and CTF, respectively), the PS1 NTF and PEN-2, the PS1 CTF and APH-1aL, and NCT and APH-1aL. We thus identify a previously unrecognized PS1 CTF/APH-1aL interaction, verify subunit interactions deduced previously from indirect approaches, and provide a model of the gamma-secretase complex subunit architecture. Finally, we further show that, like the PS1 CTF, the PS2 CTF also interacts with APH-1aL, and we provide evidence that these interactions also occur with the other APH-1 variants, suggesting similar subunit architectures of all gamma-secretase complexes.

C-FLIP knockdown induces ligand-independent DR5-, FADD-, caspase-8-, and caspase-9-dependent apoptosis in breast cancer cells

Cellular-FLICE inhibitory protein (c-FLIP) is an inhibitor of apoptosis downstream of the death receptors Fas, DR4, and DR5, and is expressed as long (c-FLIP L) and short (c-FLIP S) splice forms. We found that the knockdown of c-FLIP using small interfering RNA (siRNA) triggered ligand-independent caspase-8- and -9-dependent spontaneous apoptosis and decreased the proliferation of MCF-7 breast cancer cells. Further analysis revealed that an apoptotic inhibitory complex (AIC) comprised of DR5, FADD, caspase-8, and c-FLIP L exists in MCF-7 cells, and the absence of c-FLIP L from this complex induces DR5- and FADD-mediated caspase-8 activation in the death inducing signaling complex (DISC). c-FLIP S was not detected in the AIC, and using splice form-specific siRNAs we showed that c-FLIP L but not c-FLIP S is required to prevent spontaneous death signaling in MCF-7 cells. These results clearly show that c-FLIP L prevents ligand-independent death signaling and provides direct support for studying c-FLIP as a relevant therapeutic target for breast cancers.

Modulation of Voltage- and Ca2+-dependent Gating of CaV1.3 L-type Calcium Channels by Alternative Splicing of a C-terminal Regulatory Domain

Low voltage activation of CaV1.3 L-type Ca2+ channels controls excitability in sensory cells and central neurons as well as sinoatrial node pacemaking. CaV1.3-mediated pacemaking determines neuronal vulnerability of dopaminergic striatal neurons affected in Parkinson disease. We have previously found that in CaV1.4 L-type Ca2+ channels, activation, voltage, and calcium-dependent inactivation are controlled by an intrinsic distal C-terminal modulator. Because alternative splicing in the CaV1.3 α1 subunit C terminus gives rise to a long (CaV1.342) and a short form (CaV1.342A), we investigated if a C-terminal modulatory mechanism also controls CaV1.3 gating. The biophysical properties of both splice variants were compared after heterologous expression together with β3 and α2δ1 subunits in HEK-293 cells. Activation of calcium current through CaV1.342A channels was more pronounced at negative voltages, and inactivation was faster because of enhanced calcium-dependent inactivation. By investigating several CaV1.3 channel truncations, we restricted the modulator activity to the last 116 amino acids of the C terminus. The resulting CaV1.3ΔC116 channels showed gating properties similar to CaV1.342A that were reverted by co-expression of the corresponding C-terminal peptide C116. Fluorescence resonance energy transfer experiments confirmed an intramolecular protein interaction in the C terminus of CaV1.3 channels that also modulates calmodulin binding. These experiments revealed a novel mechanism of channel modulation enabling cells to tightly control CaV1.3 channel activity by alternative splicing. The absence of the C-terminal modulator in short splice forms facilitates CaV1.3 channel activation at lower voltages expected to favor CaV1.3 activity at threshold voltages as required for modulation of neuronal firing behavior and sinoatrial node pacemaking.

C-terminal splicing reveals intramolecular gating modulation in CaV1.3 L-type Ca2+ channels

Neuronal excitability and pace-making in the sinoatrial node are controlled by low-voltage activated CaV1.3 Ltype Ca2+ channels. We recently found that in related CaV1.4 channels a highly-structured distal C-terminal motif (CTM) modulates voltageand Ca2+-dependent gating (CDI). In CaV1.3, C-terminal splicing leads to a fulllength (CaV1.3L) and at least 1 short (CaV1.3S) splice form. If a CTM would also modulate CaV1.3 gating it would be present in CaV1.3L but not CaV1.3S variants. We therefore compared the biophysical properties of CaV1.3L or CaV1.3S coexpressed with β3 + α2δ-1 in tsA-201 cells using the whole-cell patch-clamp technique. CaV1.3S channels activated at more negative potentials compared to CaV1.3L (~ -10 mV, p < 0.0001), inactivated faster (p < 0.01) and showed more CDI (p < 0.01). These changes resulted in a decreased window current shifted to more hyperpolarized potentials likely to cause a reduction in the channels' dynamic range. Removal of the C-terminal 158 (CaV1.3Δ1158) or 76 amino acids was sufficient to induce gating properties similar to CaV1.3S. FRET experiments revealed interaction of the last 158 amino acids (C158) to a proximal C-terminal domain in CaV1.3L. Coexpression of C158 with CaV1.3Δ1158 completely restored CaV1.3L gating properties confirming this protein interaction. Thus CaV1.3 channel gating is under control of the distal C-terminus allowing alternative splicing to finetune channel activity and adapt channel function to physiological needs. from 13th Scientific Symposium of the Austrian Pharmacological Society (APHAR). Joint Meeting with the Austrian Society of Toxicology (ASTOX) and the Hungarian Society for Experimental and Clinical Pharmacology (MFT) Vienna, Austria. 22–24 November 2007

Genetic Analysis of the Short Splice Variant of the Inhibitor of Caspase-activated DNase (ICAD-S) in Chicken DT40 Cells

We have studied the regulation of the caspase-Activated DNase (CAD) by its inhibitor, ICAD. To study the role of ICAD short and long splice forms ICAD-S and ICAD-L, respectively, in vivo, we constructed chicken DT40 cell lines in which the entire coding regions of ICAD alone or ICAD plus CAD were deleted. ICAD and ICAD/CAD double knock-outs lacked both DNA fragmentation and nuclear fragmentation after the induction of apoptosis. We constructed a model humanized system in which human ICAD-L and CAD proteins expressed in DT40 ICAD/CAD double knock-out cells could rescue both DNA fragmentation and stage II chromatin condensation. ICAD-S could not replace ICAD-L as a chaperone for folding active CAD in these cells. However, a modified version of ICAD-S, in which the two caspase-3 cleavage sites were replaced with two tobacco etch virus (TEV) protease cleavage sites (ICAD-S(2TEV)) and which was therefore resistant to caspase cleavage, did inhibit CAD activation upon induction of apoptosis in vivo. Moreover, ICAD-L(2TEV) was functional as a chaperone for the production of active CAD in DT40 cells. In extracts prepared from these cells, we were able to activate CAD by cleavage of ICAD-L(2TEV) with TEV protease under non-apoptotic conditions. Thus, ICAD appears to be the only functional inhibitor of CAD activation in these cell-free extracts. Taken together, these observations indicate that ICAD-S may function together with ICAD-L as a buffer to prevent inappropriate CAD activation, particularly in cells where ICAD-S is the dominant form of ICAD protein.

C-FLIP: A Key Regulator of Colorectal Cancer Cell Death

c-FLIP is an inhibitor of apoptosis mediated by the death receptors Fas, DR4, and DR5 and is expressed as long (c-FLIP(L)) and short (c-FLIP(S)) splice forms. We found that small interfering RNA (siRNA)-mediated silencing of c-FLIP induced spontaneous apoptosis in a panel of p53 wild-type, mutant, and null colorectal cancer cell lines and that this apoptosis was mediated by caspase-8 and Fas-associated death domain. Further analyses indicated the involvement of DR5 and/or Fas (but not DR4) in regulating apoptosis induced by c-FLIP siRNA. Interestingly, these effects were not dependent on activation of DR5 or Fas by their ligands tumor necrosis factor-related apoptosis-inducing ligand and FasL. Overexpression of c-FLIP(L), but not c-FLIP(S), significantly decreased spontaneous and chemotherapy-induced apoptosis in HCT116 cells. Further analyses with splice form-specific siRNAs indicated that c-FLIP(L) was the more important splice form in regulating apoptosis in HCT116, H630, and LoVo cells, although specific knockdown of c-FLIP(S) induced more apoptosis in the HT29 cell line. Importantly, intratumoral delivery of c-FLIP-targeted siRNA duplexes induced apoptosis and inhibited the growth of HCT116 xenografts in BALB/c severe combined immunodeficient mice. In addition, the growth of c-FLIP(L)-overexpressing colorectal cancer xenografts was more rapid than control xenografts, an effect that was significantly enhanced in the presence of chemotherapy. These results indicate that c-FLIP inhibits spontaneous death ligand-independent, death receptor-mediated apoptosis in colorectal cancer cells and that targeting c-FLIP may have therapeutic potential for the treatment of colorectal cancer.

Embryonic expression of zebrafish AMPA receptor genes: Zygotic gria2α expression initiates at the midblastula transition

The AMPA-preferring receptors (AMPARs) mediate rapid excitatory synaptic transmission in the central nervous system of vertebrates. Expression profiles of 8 AMPAR genes were studied by RT-PCR analyses to elucidate the properties of AMPARs during early zebrafish development. Transcripts of all AMPAR genes are detected at the time of fertilization, suggesting maternal transcriptions of zebrafish AMPAR genes. The amounts of gria1 and gria2 transcripts are several-fold higher than that of gria3 and gria4 between 10 and 72 hpf (hour postfertilization). The edited gria2α transcript decreases during gastrulation period, suggesting that zygotic expression of gria2α begins around the time of midblastula transition. Relative to the amount of β-actin, the amounts of AMPAR transcripts increase significantly after the completion of neurulation. The amounts of gria2 transcripts exceed the total amounts of the remaining AMPAR transcripts after 36 hpf, suggesting increases in the representation of low Ca 2+ permeable AMPARs during neuronal maturation. Many but not all of the known mammalian protein–protein interaction motifs are preserved in the C-terminal domains (CTD) of zebrafish AMPARs. Before 16 hpf, the embryos express predominantly the alternative splice forms encoding longer CTD. Representations of the short CTD splice forms of gria2 and gria4α increase after 24 hpf, when neurulation is nearly completed.

Single nucleotide polymorphisms in the chicken Lmbr1 gene are associated with chicken polydactyly

Polydactyly is a common malformation of vertebrate limbs. Preaxial polydactyly (PPD) has been mapped in human, mouse and chicken to the syntenic region of human 7q36. Lmbr1 was thought as the critical candidate gene for human and mouse PPD. To understand the molecular mechanism underlying chicken polydactyly, we have cloned the open reading frame (ORF) of chicken Lmbr1, which contains 1467 nucleotides. Within this ORF, we found one short and one long splice forms. The short splice form has a complete deletion of exon 4. Six cSNPs were found in the chicken ORF, and two of these cSNPs, G797A and G1255A, lead to amino acid substitutions. However, G797A substitution had no significant association with polydactyly and the G1255A substitution had very low frequency in the population. The T1254C polymorphism in exon 13 was found to be strongly associated with polydactyly. Radiation hybrid mapping of a DNA fragment containing intron 13 of the chicken Lmbr1 assigned the gene to chromosome 2 between MCW071 (a marker within the EN2 gene) and ADL0270, a syntenic region to human 7q36.