A cytoprotective role for optineurin during mycobacterial infection of macrophages

Autophagy has emerged as a critical innate immune mechanism for host elimination of intracellular pathogens, however, the role of the autophagy receptor Optineurin during mycobacterial infection is not fully understood. To address this lacuna, we infected bone marrow-derived macrophages (BMDMs) derived from Optn+/+ and Optn-/- mice with Mycobacterium smegmatis, and observed the infection outcome at sequential time points. While low multiplicity of infection (MOI) did not show any significant difference between BMDMs from the two groups, at high MOI Optn-/- mice-derived BMDMs showed significantly lower colony forming unit counts, as well as lower cell counts at 12 h and 24 h post-infection. Quantification of cell numbers and nuclear morphologies at various time points post-infection indicated a markedly higher cell death in the Optineurin-deficient macrophages. Optineurin-deficient BMDMs showed significantly lower levels of the autophagosomal protein LC3-II upon infection, indicating a potential role for Optineurin in regulating autophagy during mycobacterial infection. Moreover, when stimulated by bacterial LPS, Optineurin deficient macrophages, showed altered levels of the inflammatory cytokine pro-IL-1β. These observations taken together suggest a novel regulatory role for Optineurin during mycobacterial infection. Its deficiency leads to an impairment in macrophage responses, directly impacting the pathophysiology of infection.

A glaucoma-associated OPTN polymorphism, M98K sensitizes retinal cells to endoplasmic reticulum stress and tumour necrosis factor α

Optineurin/OPTN polymorphism, M98K is associated with normal tension glaucoma in certain populations, and genetic evidence shows its interaction with tumour necrosis factor-alpha (TNFα) polymorphism in causing glaucoma. Endoplasmic reticulum (ER) stress is also associated with glaucoma. We hypothesized that M98K-OPTN may sensitize retinal ganglion cells to various types of stress. To test this hypothesis, stable clones of a retinal cell line, 661W, expressing either wild-type (WT)-OPTN or M98K-OPTN were generated and examined for their survival under various stress conditions. Compared with WT-OPTN expressing cells, M98K-OPTN expressing cells showed significantly lower cell survival and higher activation of caspase-3 and caspase-8 upon treatment with tunicamycin (an inducer of ER stress) or TNFα. Levels of ER stress sensors IRE1α, PERK and ATF6 were significantly higher in M98K-OPTN expressing cells. Tunicamycin treatment resulted in significantly higher induction of ER stress marker CHOP and several other ER stress response genes regulated by IRE1α-XBP1, PERK-ATF4 and ATF6 pathways, in M98K-OPTN expressing cells. Splicing of XBP1 and ATF6 activation was higher in tunicamycin-treated M98K-OPTN expressing cells. Increased levels of PERK and IRE1α proteins in M98K-OPTN expressing cells were dependent on autophagy. Overall, our results show that M98K-OPTN sensitizes retinal cells to TNFα and ER stress-induced cell death. We also show that M98K-OPTN alters ER stress response signalling, which possibly enhances the sensitivity of retinal cells to ER stress. Our results provide support to the hypothesis that M98K-OPTN may cooperate with other genetic or environmental factors to cause retinal ganglion cell death associated with glaucoma.

Cold-induced loss of interaction with HSC70 triggers inflammasome activity of familial cold autoinflammatory syndrome-causing mutants of NLRP3

NLRP3 is a cytoplasmic receptor protein, which initiates caspase-1 mediated inflammatory immune response upon detection of invading pathogen or a wide array of internal distress signals. Several gain-of function mutations of NLRP3 cause hereditary disorder of cold-induced hyper-inflammation known as familial cold autoinflammatory syndrome-1 (FCAS1). Although, caspase-1 activation and downstream interleukin-1β/interleukin-18 maturation are common effectors in pathophysiology of this disorder, molecular mechanisms of how exposure to subnormal temperature triggers mutant NLRP3-inflammsome activity is not understood. Here, we show that endogenous NLRP3 is in complex with HSC70 (HSPA8), and this interaction is reduced upon exposure to cold. FCAS-causing NLRP3-L353P and NLRP3-R260W mutants show enhanced interaction with HSC70. Upon exposure to subnormal temperature, NLRP3-L353P and NLRP3-R260W show enhanced inflammasome formation, increased caspase-1 activation and reduced interaction with HSC70. Knockdown of HSC70 results in increased inflammasome formation by L353P and R260W mutants of NLRP3. Our results suggest that interaction with HSC70 suppresses inflammasome formation by FCAS-causing NLRP3 mutants at physiological temperature, and loss of this inhibitory association at subnormal temperature causes aggravated inflammasome formation and caspase-1 activation leading to interleukin-1β maturation. These results provide evidence for HSC70 being a cold-sensor and a temperature-dependent regulator of inflammatory signaling by FCAS-causing NLRP3 mutants.

Regulation of transferrin receptor trafficking by optineurin and its disease-associated mutants

Transferrin receptor (TFRC) is a transmembrane protein that plays a crucial role in mediating homeostasis of iron in the cell. The binding of transferrin (that is bound to iron) to TFRC at the cell membrane generally starts endocytosis of TFRC-transferrin complex, which leads to formation of vesicles that are positive for TFRC. These vesicles travel to the early endosomes and later to the endocytic recycling compartment. Release of iron occurs in the early endosomes because of acidic pH. Major fraction of the transferrin and TFRC is transported back to the cell membrane; however, a minor fraction of it is transported to lysosomes through the process of autophagy. Optineurin (OPTN) is a multi-functional adaptor protein that plays a pivotal role in the control of TFRC trafficking, recycling and autophagy dependent degradation. Optineurin also plays a role in cargo-selective and non-selective autophagy. Here, we review our understanding of the function of OPTN in regulating TFRC trafficking, recycling and autophagy dependent degradation. We also discuss the mechanisms by which certain disease-associated mutations of OPTN alter these processes.

HSC70 as a sensor of low temperature: role in cold-triggered autoinflammatory disorders

Familial cold autoinflammatory syndrome (FCAS) is a subset of heritable autoinflammatory disorders wherein inflammatory symptoms aggravate upon exposure of the individual to subnormal temperature. In the past two decades, several mutations in various genes such as NLRP3, NLRP12, PLCG2 and NLRC4 have been identified that cause cold-triggered inflammation. However, our understanding of the mechanisms by which cells perceive subnormal temperature, and what keeps the inflammation under check until exposure to low temperature, is very limited. We hypothesise that recognition of FCAS-associated mutants as misfolded polypeptides by temperature-sensitive HSC70 (HSPA8) chaperone determines the FCAS phenotype. At 37 °C, HSC70 would interact with the mutant proteins, keeping them almost inactive, and loss of interaction at low temperature due to a conformational change in HSC70 would lead to their activation. The proposed mechanism of low temperature sensing in the context of FCAS may have wider implications for HSC70 as a cold temperature sensor in various pathological conditions where symptoms get aggravated upon exposure to low temperature.

Autophagy-independent cytoprotection by optineurin from toxicity of aggregates formed by mutant huntingtin and mutant ataxin-3

An important feature of several neurodegenerative diseases is the formation of pathological structures containing aggregated proteins. The autophagy receptor optineurin/OPTN is frequently observed in these structures. The role played by optineurin in these aggregates is not clear. In this study, we explored whether optineurin has a cytoprotective role in the cells having mutant protein aggregates. We overexpressed mutant huntingtin having 97 glutamine repeats (mHtt), and mutant ataxin-3 having 130 glutamine repeats (mAtax-3) in wild-type and optineurin-deficient neuronal (N2A) and non-neuronal cells (Optn-/- mouse embryonic fibroblasts), and determined the percentage of dead cells with mutant protein aggregates. Optineurin-deficient cells having mHtt or mAtax-3 aggregates showed higher cell death as compared to wild-type cells having mutant protein aggregates. Confocal microscopy revealed that optineurin formed a shell around mHtt and mAtax-3 aggregates through its C-terminal domain. The C-terminal domain of optineurin, which lacks LC3-interacting region required for autophagy, was necessary and sufficient to reduce cytotoxicity of mHtt and mAtax-3 aggregates. Our results show that in the absence of optineurin, mutant protein aggregates are highly toxic, revealing an autophagy-independent cytoprotective function of optineurin, which is mediated by its C-terminal domain.

Human primary retinal cells as an in-vitro model for investigating defective signalling caused by OPTN mutants associated with glaucoma

Studies carried out on the pathogenesis of glaucoma using murine cell lines and animal models require to be validated in human cells. Therefore, we explored the possibility of using human primary retinal cells (hPRCs) in culture as a model for molecular studies and testing of potential therapeutic drugs. For this purpose, central retinal tissue, obtained from the enucleated eyes of patients with anterior staphyloma, was digested with trypsin and grown in a medium containing supplements (basic fibroblast growth factor and fetal bovine serum). hPRCs at passage 1 and 2, show expression of either GFAP, a glial cell marker, or β-III tubulin, a retinal ganglion cell (RGC)-specific marker. But at passages 3-5 nearly all of hPRCs express several RGC-specific markers (Brn3 proteins, Thy-1, β-III tubulin, RBPMS and NeuN) but not GFAP. Expression of these markers indicated that these cells may have functional properties of RGCs. As RGCs are sensitive to glaucoma-associated mutants of OPTN, we analysed the survival of hPRCs upon overexpression of OPTN mutants. Glaucoma-associated mutants, E50K-OPTN and M98K-OPTN, induced significantly higher cell death in hPRCs compared to WT-OPTN, whereas an amyotrophic lateral sclerosis-associated mutant, E478G-OPTN, did not. TBK1 inhibitor Amlexanox protected hPRCs from E50K-OPTN and M98K-OPTN induced cell death. M98K-OPTN induced cell death was suppressed by inhibitors of CaMKKβ and AMPK in hPRCs as well as in 661W, a mouse cell line that expresses several markers of RGCs and RGC precursor cells. Our results suggest that hPRCs under appropriate culture condition show RGC-like properties. These cells can be used to explore the molecular mechanisms of cell death relevant for glaucoma pathogenesis and for testing of cytoprotective compounds.

A glaucoma- and ALS-associated mutant of OPTN induces neuronal cell death dependent on Tbk1 activity, autophagy and ER stress

Mutations in OPTN are associated with glaucoma, an eye disease, and also with amyotrophic lateral sclerosis (ALS), a motor neuron disease. A 2-bp insertion in OPTN (691_692insAG or 2bpIns-OPTN) is associated with both glaucoma and ALS. This mutation results in frame shift after 127 amino acids, giving rise to a protein with C-terminal aberrant sequence. We have explored the mechanism of induction of cell death by this mutant in a motor neuron cell line, NSC-34, and also in a retinal cell line, 661W. Compared to wild-type OPTN, this mutant induced more cell death in NSC-34 and 661W cells. This mutant localizes predominantly in the nucleus whereas normal OPTN localizes in the cytoplasm. Deletion analysis of 2bpIns-OPTN showed that the aberrant sequence was not essential for cell death induction. This mutant interacts with TANK-binding kinase 1 (Tbk1) but not with OPTN and activates Tbk1. This mutant induced ER stress in NSC-34 cells as seen by induction of C/EBP homologous protein (CHOP) and some other genes. Induction of CHOP, autophagosomal protein LC3-II and cell death by this mutant were abrogated by Tbk1 knockdown and also by 4-phenylbutyric acid, that inhibits ER stress. Induction of CHOP and cell death by 2bpIns-OPTN was autophagy dependent as shown by the effect of Atg5 knockdown. This mutant caused increased formation of LC3-positive aggregates. Treatment of cells with autophagy inducer rapamycin reduced LC3-positive aggregates, CHOP and cell death induced by 2bpIns-OPTN. These results suggest that constitutive activation of Tbk1 by 2bpIns-OPTN leads to impaired autophagy that results in ER stress and cell death.

Optineurin modulates ER stress-induced signaling pathways and cell death

We have investigated the physiological role of the autophagy receptor Optineurin/Optn in endoplasmic reticulum (ER) stress response using cellular and animal models. In comparison to their normal counterparts, Optn-deficient mouse embryonic fibroblasts showed significantly higher cell death and caspase-3 activation upon treatment with tunicamycin and thapsigargin, inducers of ER stress. The transcript levels of some of the genes regulated by the IRE1-XBP1 and PERK-ATF4 pathways were upregulated in Optn-deficient cells, in comparison with normal cells, upon treatment with tunicamycin, and also in the brain cortex and liver of tunicamycin treated Optn-deficient mice. Also, the basal levels of IRE1α and PERK were higher in Optn-deficient cells. These results suggest that Optn modulates ER stress-induced signaling pathways and provides protection from ER stress-induced cell death.

Altered Functions and Interactions of Glaucoma-Associated Mutants of Optineurin

Optineurin (OPTN) is an adaptor protein that is involved in mediating a variety of cellular processes such as signaling, vesicle trafficking, and autophagy. Certain mutations in OPTN (gene OPTN) are associated with primary open angle glaucoma, a leading cause of irreversible blindness, and amyotrophic lateral sclerosis, a fatal motor neuron disease. Glaucoma-associated mutations of OPTN are mostly missense mutations. OPTN mediates its functions by interacting with various proteins and altered interactions of OPTN mutants with various proteins primarily contribute to functional defects. It interacts with Rab8, myosin VI, Huntigtin, TBC1D17, and transferrin receptor to mediate various membrane vesicle trafficking pathways. It is an autophagy receptor that mediates cargo-selective as well as non-selective autophagy. Glaucoma-associated mutants of OPTN, E50K, and M98K, cause defective vesicle trafficking, autophagy, and signaling that contribute to death of retinal ganglion cells (RGCs). Transgenic mice expressing E50K-OPTN show loss of RGCs and persistent reactive gliosis. TBK1 protein kinase, which mediates E50K-OPTN and M98K-OPTN induced cell death, is emerging as a potential drug target. Autoimmunity has been implicated in glaucoma but involvement of OPTN or its mutants in autoimmnity has not been explored. In this review, we highlight the main functions of OPTN and how glaucoma-associated mutants alter these functions. We also discuss some of the controversies, such as the role of OPTN in signaling to transcription factor NF-κB, interferon signaling, and use of RGC-5 cell line as a cell culture model.

Autophagy receptor optineurin promotes autophagosome formation by potentiating LC3-II production and phagophore maturation

Autophagy is an essential physiological process that maintains cellular homeostasis by eliminating harmful protein aggregates, damaged organelles and certain pathogens through lysosomal degradation. During autophagy specialized structures, known as autophagosomes are formed that recruit the cargo through autophagy receptors, and deliver it to lysosomes. Optineurin (Optn) is an autophagy receptor that mediates cargo selective autophagy. Recently, we have identified a novel function of Optn that promotes autophagosome formation during non-selective autophagy. Optn-deficient cells show reduced formation of autophagosomal protein LC3-II and lower number of autophagosomes as well as autolysosomes. Interestingly, formation of phagophores is increased in Optn-deficient cells. This suggests that Optn promotes autophagosome formation by potentiating LC3-II production and phagophore maturation. Phosphorylation of Optn at Ser-177 is required for promoting autophagosome formation. Here, we discuss various aspects of the role of Optn in the formation of autophagosomes and Atg16L1-positive vesicles. We also discuss the potential role of Rab1a-Optn interaction.

661W is a retinal ganglion precursor-like cell line in which glaucoma-associated optineurin mutants induce cell death selectively

A photoreceptor cell line, 661W, derived from a mouse retinal tumor that expresses several markers of cone photoreceptor cells has been described earlier. However, these cells can be differentiated into neuronal cells. Here, we report that this cell line expressed certain markers specific to retinal ganglion cells such as Rbpms, Brn3b (Pou4f2), Brn3c (Pou4f3), Thy1 and γ-synuclein (Sncg), and some other markers of neuronal cells (beta-III tubulin, NeuN and MAP2). These cells also expressed Opn1mw, a cone-specific marker and nestin, a marker for neural precursor cells. Two glaucoma-associated mutants of OPTN, E50K and M98K, but not an amyotrophic lateral sclerosis-associated mutant, E478G, induced cell death selectively in 661W cells. However, in a motor neuron cell line, NSC34, E478G mutant of OPTN but not E50K and M98K induced cell death. We conclude that 661W is a retinal ganglion precursor-like cell line, which shows properties of both retinal ganglion and photoreceptor cells. We suggest that these cells could be utilized for exploring the mechanisms of cell death induction and cytoprotection relevant for glaucoma pathogenesis. RGC-5 cell line which probably arose from 661W cells showed expression of essentially the same markers of retinal ganglion cells and neuronal cells as seen in 661W cells.

Optineurin promotes autophagosome formation by recruiting the autophagy-related Atg12-5-16L1 complex to phagophores containing the Wipi2 protein

Autophagy is a quality-control mechanism that helps to maintain cellular homeostasis by removing damaged proteins and organelles through lysosomal degradation. During autophagy, signaling events lead to the formation of a cup-shaped structure called the phagophore that matures into the autophagosome. Recruitment of the autophagy-associated Atg12-5-16L1 complex to Wipi2-positive phagophores is crucial for producing microtubule-associated protein 1 light chain 3-II (LC3-II), which is required for autophagosome formation. Here, we explored the role of the autophagy receptor optineurin (Optn) in autophagosome formation. Fibroblasts from Optn knock-out mouse showed reduced LC3-II formation and a lower number of autophagosomes and autolysosomes during both basal and starvation-induced autophagy. However, the number of Wipi2-positive phagophores was not decreased in Optn-deficient cells. We also found that the number of Atg12/16L1-positive puncta and recruitment of the Atg12-5-16L1 complex to Wipi2-positive puncta are reduced in Optn-deficient cells. Of note, Optn was recruited to Atg12-5-16L1-positive puncta, and interacted with Atg5 and also with Atg12-5 conjugate. A disease-associated Optn mutant, E478G, defective in ubiquitin binding, was also defective in autophagosome formation and recruitment to the Atg12-5-16L1-positive puncta. Moreover, we noted that Optn phosphorylation at Ser-177 was required for autophagosome formation but not for Optn recruitment to the phagophore. These results suggest that Optn potentiates LC3-II production and maturation of the phagophore into the autophagosome, by facilitating the recruitment of the Atg12-5-16L1 complex to Wipi2-positive phagophores.

A Disease-associated Mutant of NLRC4 Shows Enhanced Interaction with SUG1 Leading to Constitutive FADD-dependent Caspase-8 Activation and Cell Death

Nod-like receptor family card containing 4 (NLRC4)/Ipaf is involved in recognition of pathogen-associated molecular patterns leading to caspase-1 activation and cytokine release, which mediate protective innate immune response. Point mutations in NLRC4 cause autoinflammatory syndromes. Although all the mutations result in constitutive caspase-1 activation, their phenotypic presentations are different, implying that these mutations cause different alterations in properties of NLRC4. NLRC4 interacts with SUG1 and induces caspase-8-mediated cell death. Here, we show that one of the autoinflammatory syndrome-causing mutants of NLRC4, H443P, but not T337A and V341A, constitutively activates caspase-8 and induces apoptotic cell death in human lung epithelial cells. Compared with wild type NLRC4, the H443P mutant shows stronger interaction with SUG1 and with ubiquitinated cellular proteins. Phosphorylation of NLRC4 at Ser⁵³³ plays a crucial role in caspase-8 activation and cell death. However, H443P mutant does not require Ser⁵³³ phosphorylation for caspase-8 activation and cell death. Caspase-8 activation by NLRC4 and its H443P mutant are dependent on the adaptor protein FADD. A phosphomimicking mutant of NLRC4, S533D does not require SUG1 activity for inducing cell death. Ubiquitin-tagged NLRC4 could induce cell death and activate caspase-8 independent of Ser⁵³³ phosphorylation. Our work suggests that SUG1-mediated signaling results in enhanced ubiquitination and regulates FADD-dependent caspase-8 activation by NLRC4. We show that the autoinflammation-associated H443P mutant is altered in interaction with SUG1 and ubiquitinated proteins, triggering constitutive caspase-8-mediated cell death dependent on FADD but independent of Ser⁵³³ phosphorylation.

M98K-OPTN induces transferrin receptor degradation and RAB12-mediated autophagic death in retinal ganglion cells

Mutations in the autophagy receptor OPTN/optineurin are associated with the pathogenesis of glaucoma and amyotrophic lateral sclerosis, but the underlying molecular basis is poorly understood. The OPTN variant, M98K has been described as a risk factor for normal tension glaucoma in some ethnic groups. Here, we examined the consequence of the M98K mutation in affecting cellular functions of OPTN. Overexpression of M98K-OPTN induced death of retinal ganglion cells (RGC-5 cell line), but not of other neuronal and non-neuronal cells. Enhanced levels of the autophagy marker, LC3-II, a post-translationally modified form of LC3, in M98K-OPTN-expressing cells and the inability of an LC3-binding-defective M98K variant of OPTN to induce cell death, suggested that autophagy contributes to cell death. Knockdown of Atg5 reduced M98K-induced death of RGC-5 cells, further supporting the involvement of autophagy. Overexpression of M98K-OPTN enhanced autophagosome formation and potentiated the delivery of transferrin receptor to autophagosomes for degradation resulting in reduced cellular transferrin receptor levels. Coexpression of transferrin receptor or supplementation of media with an iron donor reduced M98K-induced cell death. OPTN complexes with RAB12, a GTPase involved in vesicle trafficking, and M98K variant shows enhanced colocalization with RAB12. Knockdown of Rab12 increased transferrin receptor level and reduced M98K-induced cell death. RAB12 is present in autophagosomes and knockdown of Rab12 resulted in reduced formation of autolysosomes during starvation-induced autophagy, implicating a role for RAB12 in autophagy. These results also show that transferrin receptor degradation and autophagy play a crucial role in RGC-5 cell death induced by M98K variant of OPTN.

Emerging role of tyrosine phosphatase, TCPTP, in the organelles of the early secretory pathway

T-cell protein tyrosine phosphatase, TCPTP, is a ubiquitously expressed non-receptor type tyrosine phosphatase. There are two splice variants of TCPTP, TC48 and TC45, which differ in their sub-cellular localizations and functions. TC45 is a nuclear protein, which has both nuclear and cytoplasmic substrates, and is involved in many signaling events including endocytic recycling of platelet-derived growth factor β-receptor. TC48 is a predominantly endoplasmic reticulum (ER)-localizing protein, which dephosphorylates some of the substrates of TC45 at the ER. However, recently few specific substrates for TC48 have been identified. These include C3G (RapGEF1), syntaxin 17 and BCR-Abl. TC48 moves from the ER to post-ER compartments, the ER-Golgi intermediate compartment (ERGIC) and Golgi, and it is retrieved back to the ER. The retrieval of ER proteins from post-ER compartments is generally believed as a mechanism of targeting these proteins to the ER. However, it is possible that this shuttling of TC48 serves to regulate signaling in the early secretory pathway. For example, TC48 dephosphorylates phosphorylated C3G at the Golgi and inhibits neurite outgrowth. TC48 interacts with and dephosphorylates syntaxin 17, which is an ER and ERGIC-localizing protein involved in vesicle transport. A yeast two-hybrid screen identified several unique interacting partners of TC48 belonging to two groups - proteins involved in vesicle trafficking and proteins involved in cell adhesion. These interacting proteins could be substrates or regulators of TC48 function and localization. Thus, the role of TC48 seems to be more diverse, which is still to be explored.

Tyrosine phosphorylation of a SNARE protein, syntaxin 17: implications for membrane trafficking in the early secretory pathway

The T-cell protein tyrosine phosphatase is expressed as two splice variants - TC45, a nuclear protein, and TC48, which is localized predominantly in the ER (endoplasmic reticulum). Yeast two-hybrid screening revealed direct interaction of TC48 with Syntaxin17, a SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein localized predominantly in the ER and to some extent in the ER-Golgi intermediate compartment. Syntaxin 17 did not interact with TC45. C-terminal 40 amino acids of TC48 were sufficient for interaction with syntaxin 17. Overexpressed syntaxin 17 was phosphorylated at tyrosine upon pervanadate treatment (a tyrosine phosphatase inhibitor/tyrosine kinase activator) of COS-1 cells. Mutational analysis identified Tyr156 in the cytoplasmic domain as the major site of phosphorylation. Endogenous syntaxin 17 was phosphorylated by pervanadate treatment in CHO and MIN6 cells but was not phosphorylated in a variety of other cell lines tested. c-Abl was identified as one of the kinases, which phosphorylates syntaxin 17 in MIN6 cells. Phosphorylation of endogenous and overexpressed syntaxin 17 was reduced in the presence of IGF receptor and EGF receptor kinase inhibitors. Serum depletion reduced pervanadate-induced phosphorylation of endogenous syntaxin 17. TC48 coexpression reduced phosphorylation of syntaxin 17 by pervanadate and purified TC48 directly dephosphorylated syntaxin 17. β-COP dispersal by overexpressed syntaxin 17 was reduced after pervanadate-induced phosphorylation. A phospho-mimicking mutant (Y156E) of syntaxin 17 showed reduced interaction with COPI vesicles. These results suggest that tyrosine phosphorylation of syntaxin 17 is likely to have a role in regulating syntaxin 17 dependent membrane trafficking in the early secretory pathway.

Optineurin mediates a negative regulation of Rab8 by the GTPase-activating protein TBC1D17

Rab GTPases regulate various membrane trafficking pathways but the mechanisms by which GTPase-activating proteins recognise specific Rabs are not clear. Rab8 is involved in controlling several trafficking processes, including the trafficking of transferrin receptor from the early endosome to the recycling endosome. Here, we provide evidence to show that TBC1D17, a Rab GTPase-activating protein, through its catalytic activity, regulates Rab8-mediated endocytic trafficking of transferrin receptor. Optineurin, a Rab8-binding effector protein, mediates the interaction and colocalisation of TBC1D17 with Rab8. A non-catalytic region of TBC1D17 is required for direct interaction with optineurin. Co-expression of Rab8, but not other Rabs tested, rescues the inhibition of transferrin receptor trafficking by TBC1D17. The activated GTP-bound form of Rab8 is localised to the tubules emanating from the endocytic recycling compartment. Through its catalytic activity, TBC1D17 inhibits recruitment of Rab8 to the tubules and reduces colocalisation of transferrin receptor and Rab8. Knockdown of optineurin or TBC1D17 results in enhanced recruitment of Rab8 to the tubules. A glaucoma-associated mutant of optineurin, E50K, causes enhanced inhibition of Rab8 by TBC1D17, resulting in defective endocytic recycling of transferrin receptor. Our results show that TBC1D17, through its interaction with optineurin, regulates Rab8-mediated endocytic recycling of transferrin receptor and recruitment of Rab8 to the endocytic recycling tubules. We describe a mechanism of regulating a Rab GTPase by an effector protein (optineurin) that acts as an adaptor to bring together a Rab (Rab8) and its GTPase-activating protein (TBC1D17).

Optineurin is required for CYLD-dependent inhibition of TNFα-induced NF-κB activation

The nuclear factor kappa B (NF-κB) regulates genes that function in diverse cellular processes like inflammation, immunity and cell survival. The activation of NF-κB is tightly controlled and the deubiquitinase CYLD has emerged as a key negative regulator of NF-κB signalling. Optineurin, mutated in certain glaucomas and amyotrophic lateral sclerosis, is also a negative regulator of NF-κB activation. It competes with NEMO (NF-κB essential modulator) for binding to ubiquitinated RIP (receptor interacting protein) to prevent NF-κB activation. Recently we identified CYLD as optineurin-interacting protein. Here we have analysed the functional significance of interaction of optineurin with CYLD. Our results show that a glaucoma-associated mutant of optineurin, H486R, is altered in its interaction with CYLD. Unlike wild-type optineurin, the H486R mutant did not inhibit tumour necrosis factor α (TNFα)-induced NF-κB activation. CYLD mediated inhibition of TNFα-induced NF-κB activation was abrogated by expression of the H486R mutant. Upon knockdown of optineurin, CYLD was unable to inhibit TNFα-induced NF-κB activation and showed drastically reduced interaction with ubiquitinated RIP. The level of ubiquitinated RIP was increased in optineurin knockdown cells. Deubiquitination of RIP by over-expressed CYLD was abrogated in optineurin knockdown cells. These results suggest that optineurin regulates NF-κB activation by mediating interaction of CYLD with ubiquitinated RIP thus facilitating deubiquitination of RIP.

Regulation of endocytic trafficking of transferrin receptor by optineurin and its impairment by a glaucoma-associated mutant

Background

Optineurin is a multifunctional protein involved in several functions such as vesicular trafficking from the Golgi to the plasma membrane, NF-κB regulation, signal transduction and gene expression. Mutations in optineurin are associated with glaucoma, a neurodegenerative eye disease that causes blindness. Genetic evidence suggests that the E50K (Glu50Lys) is a dominant disease-causing mutation of optineurin. However, functional alterations caused by mutations in optineurin are not known. Here, we have analyzed the role of optineurin in endocytic recycling and the effect of E50K mutant on this process.

Results

We show that the knockdown of optineurin impairs trafficking of transferrin receptor to the juxtanuclear region. A point mutation (D474N) in the ubiquitin-binding domain abrogates localization of optineurin to the recycling endosomes and interaction with transferrin receptor. The function of ubiquitin-binding domain of optineurin is also needed for trafficking of transferrin to the juxtanuclear region. A disease causing mutation, E50K, impairs endocytic recycling of transferrin receptor as shown by enlarged recycling endosomes, slower dynamics of E50K vesicles and decreased transferrin uptake by the E50K-expressing cells. This impaired trafficking by the E50K mutant requires the function of its ubiquitin-binding domain. Compared to wild type optineurin, the E50K optineurin shows enhanced interaction and colocalization with transferrin receptor and Rab8. The velocity of Rab8 vesicles is reduced by co-expression of the E50K mutant. These results suggest that the E50K mutant affects Rab8-mediated transferrin receptor trafficking.

Conclusions

Our results suggest that optineurin regulates endocytic trafficking of transferrin receptor to the juxtanuclear region. The E50K mutant impairs trafficking at the recycling endosomes due to altered interactions with Rab8 and transferrin receptor. These results also have implications for the pathogenesis of glaucoma caused by the E50K mutation because endocytic recycling is vital for maintaining homeostasis.

Optineurin and its mutants: molecules associated with some forms of glaucoma

AIMS

Optineurin is a gene (OPTN) associated with normal-tension glaucoma and primary open-angle glaucoma. The aim of our study is to understand the functions of the protein optineurin in retinal ganglion cells, and the modifications that come about in its functions when mutations occur in its sequence.

METHODS

We have worked with the rat retinal ganglion cell line RGC5, as well as with some others such as HeLa (human cancer cell line), COS-1 (monkey kidney cell line), IMR-32 (human neuroblastoma cell line), and D407 (human retinal pigment epithelial cell line). The cDNAs of wild-type OPTN, and of its mutants H26D, E50K, R454Q and H486R, were transfected into each of these cell lines and the effects studied. Localization of the proteins in cells was monitored by confocal microscopy and cell imaging, and their interaction with other cellular proteins by yeast two-hybrid assay and co-immunoprecipitation.

RESULTS

We found optineurin to interact with several proteins, which are involved in NF-kappaB regulation, vesicular trafficking at recycling endosomes, immune response and transcription regulation. Some of these proteins showed altered/reduced interactions with E50K and H486R mutants. E50K, a severe phenotype-related mutant of optineurin, was found to selectively kill RGC5 cells through oxytosis and apoptosis, while none of the other mutants did so.

CONCLUSIONS

Our results indicate that several independent mechanisms are likely to be involved in the pathogenesis of glaucoma caused by mutations in optineurin.

NF-kappaB mediates tumor necrosis factor alpha-induced expression of optineurin, a negative regulator of NF-kappaB

Optineurin is a ubiquitously expressed multifunctional cytoplasmic protein encoded by OPTN gene. The expression of optineurin is induced by various cytokines. Here we have investigated the molecular mechanisms which regulate optineurin gene expression and the relationship between optineurin and nuclear factor κB (NF-κB). We cloned and characterized human optineurin promoter. Optineurin promoter was activated upon treatment of HeLa and A549 cells with tumor necrosis factor α (TNFα). Mutation of a putative NF-κB-binding site present in the core promoter resulted in loss of basal as well as TNFα-induced activity. Overexpression of p65 subunit of NF-κB activated this promoter through NF-κB site. Oligonucleotides corresponding to this putative NF-κB-binding site showed binding to NF-κB. TNFα-induced optineurin promoter activity was inhibited by expression of inhibitor of NF-κB (IκBα) super-repressor. Blocking of NF-κB activation resulted in inhibition of TNFα-induced optineurin gene expression. Overexpressed optineurin partly inhibited TNFα-induced NF-κB activation in Hela cells. Downregulation of optineurin by shRNA resulted in an increase in TNFα-induced as well as basal NF-κB activity. These results show that optineurin promoter activity and gene expression are regulated by NF-κB pathway in response to TNFα. In addition these results suggest that there is a negative feedback loop in which TNFα-induced NF-κB activity mediates expression of optineurin, which itself functions as a negative regulator of NF-κB.

Tumor necrosis factor-alpha-induced caspase-1 gene expression. Role of p73

Tumour necrosis factor-alpha (TNF-alpha) is a cytokine that is involved in many functions, including the inflammatory response, immunity and apoptosis. Some of the responses of TNF-alpha are mediated by caspase-1, which is involved in the production of the pro-inflammatory cytokines interleukin-1beta, interleukin-18 and interleukin-33. The molecular mechanisms involved in TNF-alpha-induced caspase-1 gene expression remain poorly defined, despite the fact that signaling by TNF-alpha has been well studied. The present study was undertaken to investigate the mechanisms involved in the induction of caspase-1 gene expression by TNF-alpha. Treatment of A549 cells with TNF-alpha resulted in an increase in caspase-1 mRNA and protein expression, which was preceded by an increase in interferon regulatory factor-1 and p73 protein levels. Caspase-1 promoter reporter was activated by the treatment of cells with TNF-alpha. Mutation of the interferon regulatory factor-1 binding site resulted in the almost complete loss of basal as well as of TNF-alpha-induced caspase-1 promoter activity. Mutation of the p53/p73 responsive site resulted in reduced TNF-alpha-induced promoter activity. Blocking of p73 function by a dominant negative mutant or by a p73-directed small hairpin RNA reduced basal as well as TNF-alpha-induced caspase-1 promoter activity. TNF-alpha-induced caspase-1 mRNA and protein levels were reduced when p73 mRNA was down-regulated by small hairpin RNA. Caspase-5 gene expression was induced by TNF-alpha, which was inhibited by the small hairpin RNA-mediated down-regulation of p73. Our results show that TNF-alpha induces p73 gene expression, which, together with interferon regulatory factor-1, plays an important role in mediating caspase-1 promoter activation by TNF-alpha.

A Glaucoma-Associated Mutant of Optineurin Selectively Induces Death of Retinal Ganglion Cells Which Is Inhibited by Antioxidants

PURPOSE

Mutations in the coding region of the OPTN gene are associated with certain glaucomas. Although the function of the optineurin protein is yet to be elucidated, the most common mutation, E50K, is associated with a severe phenotype. This study explores some functional features of optineurin and its mutants.

METHODS

Plasmids expressing normal or wild-type (WT) and E50K, R545Q, H26D, and H486R mutant optineurin were transfected into HeLa, Cos-1, IMR32, and the rat retinal ganglion cell (RGC) line RGC-5, and their effects on cell survival monitored by morphologic observation of cells were studied. Expression of optineurin and its mutants was monitored by immunofluorescence staining of cells and by Western blotting.

RESULTS

The E50K mutant of optineurin selectively induced the death of retinal ganglion cells but not of the other cell lines tested. Although the expression of optineurin and E50K mutant suppressed cell death induced by tumor necrosis factor-alpha in HeLa cells, they potentiated this cell death in retinal ganglion cells. Cell death induced by the optineurin mutant in retinal ganglion cells was inhibited by the antioxidants N-acetylcysteine and Trolox. Reactive oxygen species (ROS) were produced upon expression of E50K, which were reduced by antioxidants. Coexpression of manganese superoxide dismutase with the E50K mutant abolished ROS production and inhibited cell death.

CONCLUSIONS

The E50K mutation of optineurin acquired the ability to induce cell death selectively in retinal ganglion cells. This cell death was mediated by oxidative stress. The present findings raise the possibility of antioxidant use for delaying or controlling some forms of glaucoma.

C3G is required for c-Abl-induced filopodia and its overexpression promotes filopodia formation

The Rap1 guanine nucleotide exchange factor, C3G (also known as Rap1GEF-1) is involved in signaling from growth factors, cytokines and integrins and plays a role in cell adhesion and migration, but the mechanism by which C3G regulates various cellular functions is poorly understood. We, therefore, investigated the ability of C3G to affect actin cytoskeleton-dependent morphological changes in cells. Using RNA interference, we provide evidence that C3G is required for c-Abl-induced filopodia during cell spreading on fibronectin. C3G expression induces actin cytoskeletal reorganization and promotes filopodia formation independent of its catalytic activity. It showed enrichment at filopodia tips characteristic of molecules involved in filopodia dynamics. C3G-induced filopodia were not inhibited by dominant negative mutants of Rho, Rac and Cdc42, but required Abl catalytic activity. Coexpression of N-Wasp-Crib inhibited C3G induced as well as c-Abl-induced filopodia and wiskostatin, a pharmacological inhibitor of N-Wasp attenuates C3G-induced filopodia. Cellular C3G interacts with c-Abl and C3G expression results in enhanced localization of endogenous c-Abl in the cytoplasm. We suggest that C3G and c-Abl function in an interdependent manner, in linking external signals to remodeling the cytoskeleton to induce filopodia.

Involvement of caspase 1 and its activator Ipaf upstream of mitochondrial events in apoptosis

PTP-S2/TC45 is a nuclear protein tyrosine phosphatase that activates p53 and induces caspase 1-dependent apoptosis. We analyzed the role of ICE protease-activating factor (Ipaf), an activator of caspase 1 in p53-dependent apoptosis. We also determined the sequence of events that lead to apoptosis upon caspase 1 activation by Ipaf. PTP-S2 expression induced Ipaf mRNA in MCF-7 cells which was dependent on p53. PTP-S2-induced apoptosis was inhibited by a dominant-negative mutant of Ipaf and also by an Ipaf-directed short-hairpin RNA. Doxorubicin-induced apoptosis was potentiated by the expression of caspase 1 (but not by a catalytic mutant of caspase 1) and required endogenous Ipaf. Doxorubicin treatment of MCF-7 cells resulted in activation of exogenous caspase 1, which was partly dependent on endogenous Ipaf. An activated form of Ipaf induced caspase 1-dependent apoptosis that was inhibited by Bcl2 and also by a dominant inhibitor of caspase 9 (caspase 9s). Caspase 1-dependent apoptosis induced by doxorubicin was also inhibited by Bcl2 and caspase 9s, but caspase 1 activation by activated Ipaf was not inhibited by Bcl2. Mitochondrial membrane permeabilization was induced by caspase 1 and activated Ipaf, which was inhibited by Bcl2, but not by caspase 9s. Expression of caspase 1 with activated Ipaf resulted in the activation of Bax at mitochondria. Our results suggest that Ipaf is involved in PTP-S2-induced apoptosis and that caspase 1, when activated by Ipaf, causes release of mitochondrial proteins (cytochrome c and Omi) through Bax activation, thereby functioning as an initiator caspase.

Evidence for a role of transmembrane protein p25 in localization of protein tyrosine phosphatase TC48 to the ER

T-cell protein tyrosine phosphatase gives rise to two splice isoforms: TC48, which is localized to the endoplasmic reticulum (ER) and TC45, a nuclear protein. The present study was undertaken to identify proteins that are involved in targeting TC48 to the ER. We identified two TC48-interacting proteins, p25 and p23, from a yeast two-hybrid screen. p23 and p25 are members of a family of putative cargo receptors that are important for vesicular trafficking between Golgi complex and ER. Both p23 and p25 associate with overexpressed TC48 in Cos-1 cells as determined by coimmunoprecipitation. A significant amount of TC48 colocalized initially with ERGIC and Golgi complex markers (in addition to ER and nuclear membrane localization) and was then retrieved to the ER. Coexpression with p25 enhanced ER localization of TC48, whereas coexpression with p23 resulted in its trapping in membranous structures. Coexpression of a p25 mutant lacking the ER-localization signal KKxx resulted in enhanced Golgi localization of TC48. Forty C-terminal amino acid residues of TC48 (position 376-415) were sufficient for interaction with p23 (but not with p25) and targeted green fluorescence protein (GFP) to the Golgi complex. Targeting of GFP to the ER required 66 C-terminal amino acid residues of TC48 (position 350-415), which showed interaction with p25 and p23. We suggest that TC48 translocates to the Golgi complex along the secretory pathway, whereas its ER localization is maintained by selective retrieval enabled by interactions with p25 and p23.

Role of p73 in Regulating Human Caspase-1 Gene Transcription Induced by Interferon-γ and Cisplatin

Caspase-1, a cysteine protease is primarily involved in proteolytic activation of proinflammatory cytokines such as interleukin-1beta. It is also involved in some forms of apoptosis. Here we have analyzed the role of p73, a homolog of tumor suppressor p53, in regulating human caspase-1 gene transcription. The caspase-1 promoter was strongly activated by p73alpha and p73beta primarily through a p53/p73 responsive site. Overexpression of p73 by transient transfection increased the caspase-1 mRNA level. Treatment of cells with cisplatin (which increases p73 protein level) resulted in increased caspase-1 promoter activity and its mRNA level. Blocking of p73 function by a dominant negative mutant reduced basal as well as cisplatin-induced caspase-1 promoter activity. Mutation of the p73 responsive site abolished cisplatin-induced activation of the promoter. Interferon-gamma induced caspase-1 promoter activity and this was reduced by p73-directed small hairpin RNA and also by a dominant negative mutant of p73. Abrogation of the p73 responsive site partially inhibited interferon-gamma-induced activation of the caspase-1 promoter. Treatment of HeLa cells with interferon-gamma resulted in an increase in p73 protein as well as its activity. Mutation of the IRF-1 binding site abolished interferon-gamma-induced caspase-1 promoter activity but p73-induced activation was only marginally reduced. IRF-1 cooperated with p73 and cisplatin cooperated with interferon-gamma in the activation of the caspase-1 promoter. Our results show that p73 is a regulator of caspase-1 gene transcription, and is required for optimal activation of the caspase-1 promoter by interferon-gamma.

Caspase-1 activator Ipaf is a p53-inducible gene involved in apoptosis

The tumor suppressor protein p53 regulates transcription of many genes that mediate cell cycle arrest, apoptosis, DNA repair and other cellular responses. Here we show that Ipaf, a human CED-4 homologue and an activator of caspase-1, is induced by p53. Overexpression of p53 by transfection in U2OS and A549 cells increased Ipaf mRNA levels. Treatment of p53-positive cell lines U2OS and MCF-7 with the DNA damaging drug, doxorubicin, which increases p53 protein level, induced expression of Ipaf mRNA but similar treatment of MCF-7-mp53 (a clone of MCF-7 cells expressing mutant p53) and p53-negative K562 cells showed much less induction of Ipaf gene expression. Expression analysis for Ipaf mRNA in doxorubicin-treated human tumor cell lines suggests that p53-dependent as well as p53-independent mechanisms are involved in the regulation of Ipaf gene expression in a cell-type-specific manner. The Ipaf promoter was activated by normal p53 but not by His(273) mutant of p53. A functional p53-binding site was identified in the Ipaf promoter. A dominant-negative mutant of Ipaf inhibited p53-induced and doxorubicin-induced apoptosis by about 50%. Ipaf-directed small hairpin RNA downregulated p53-induced Ipaf gene expression and also reduced p53-induced apoptosis. Doxorubicin-induced apoptosis was also inhibited by Ipaf-directed small hairpin RNA. Our results show that p53 can directly induce Ipaf gene transcription, which contributes to p53-dependent apoptosis in at least some human cells.

Phosphorylated guanine nucleotide exchange factor C3G, induced by pervanadate and Src family kinases localizes to the Golgi and subcortical actin cytoskeleton

BACKGROUND

The guanine nucleotide exchange factor C3G (RapGEF1) along with its effector proteins participates in signaling pathways that regulate eukaryotic cell proliferation, adhesion, apoptosis and embryonic development. It activates Rap1, Rap2 and R-Ras members of the Ras family of GTPases. C3G is activated upon phosphorylation at tyrosine 504 and therefore, determining the localization of phosphorylated C3G would provide an insight into its site of action in the cellular context.

RESULTS

C3G is phosphorylated in vivo on Y504 upon coexpression with Src or Hck, two members of the Src family tyrosine kinases. Here we have determined the subcellular localization of this protein using antibodies specific to C3G and Tyr 504 phosphorylated C3G (pY504 C3G). While exogenously expressed C3G was present mostly in the cytosol, pY504 C3G formed upon Hck or Src coexpression localized predominantly at the cell membrane and the Golgi complex. Tyrosine 504-phosphorylated C3G showed colocalization with Hck and Src. Treatment of Hck and C3G transfected cells with pervanadate showed an increase in the cytosolic staining of pY504 C3G suggesting that tyrosine phosphatases may be involved in dephosphorylating cytosolic phospho-C3G. Expression of Src family kinases or treatment of cells with pervanadate resulted in an increase in endogenous pY504 C3G, which was localized predominantly at the Golgi and the cell periphery. Endogenous pY504 C3G at the cell periphery colocalized with F-actin suggesting its presence at the subcortical actin cytoskeleton. Disruption of actin cytoskeleton by cytochalasin D abolished phospho-C3G staining at the periphery of the cell without affecting its Golgi localization.

CONCLUSIONS

These findings show that tyrosine kinases involved in phosphorylation of C3G are responsible for regulation of its localization in a cellular context. We have demonstrated the localization of endogenous C3G modified by tyrosine phosphorylation to defined subcellular domains where it may be responsible for restricted activation of signaling pathways.

Physical and functional interaction between Hck tyrosine kinase and guanine nucleotide exchange factor C3G results in apoptosis, which is independent of C3G catalytic domain

The hematopoietic cell kinase Hck is a Src family tyrosine kinase expressed in cells of myelomonocytic lineage, B lymphocytes, and embryonic stem cells. To study its role in signaling pathways we used the Hck-SH3 domain in protein interaction cloning and identified C3G, the guanine nucleotide exchange factor for Rap1 and R-Ras, as a protein that associated with Hck. This interaction was direct and was mediated partly through the proline-rich region of C3G. C3G could be co-immunoprecipitated with Hck from Cos-1 cells transfected with Hck and C3G. C3G was phosphorylated on tyrosine 504 in cells when coexpressed with Hck but not with a catalytically inactive mutant of Hck. Phosphorylation of endogenous C3G at Tyr-504 was increased by treatment of human myelomonocytic THP-1 cells with mercuric chloride, which is known to activate Hck tyrosine kinase specifically. Coexpression of Hck with C3G induced a high level of apoptosis in many cell lines by 30-42 h of transfection. Induction of apoptosis was not dependent on Tyr-504 phosphorylation or the catalytic domain of C3G but required the catalytic activity of Hck. Using dominant negative constructs of caspases we found that caspase-1, -8, and -9 are involved in this apoptotic pathway. These results suggest that C3G and Hck interact physically and functionally in vivo to activate kinase-dependent and caspase-mediated apoptosis, which is independent of catalytic domain of C3G.

A nuclear protein tyrosine phosphatase activates p53 and induces caspase-1-dependent apoptosis

PTP-S2/TC45 is a nuclear protein tyrosine phosphatase, which induces p53-dependent apoptosis. Here we show that the p53 protein level increased in MCF-7 cells in response to PTP-S2 overexpression. PTP-S2-induced p53 protein was transcriptionally active and it could activate caspase-1 gene expression from endogenous as well as ectopic promoter. Coexpression of an active site mutant of procaspase-1 strongly inhibited PTP-S2-induced apoptosis. Mutant procaspase-1 also inhibited apoptosis induced by p53 overexpression or doxorubicin treatment, which induce caspase-1 gene expression. In contrast, apoptosis induced by staurosporine or cycloheximide, which do not increase caspase-1 gene expression, was not affected by mutant procaspase-1. These results suggest that caspase-1 may be one of the mediators of p53-dependent apoptosis in human cells.

Induction of cytochrome c release and apoptosis by Hck-SH3 domain-mediated signalling requires caspase-3

The function of key components of signal transduction, the Src family tyrosine kinases is dependent on catalytic activity as well as on intermolecular interaction achieved by their SH2 and SH3 modular domains. We have analyzed the effect of overexpression of the hematopoietic cell kinase (Hck) and its N-terminal unique and SH3 domains on cell survival. Overexpression of the N-terminal unique and SH3 domains (Hck-USH3) induced about 25% of expressing Cos-1 cells to undergo apoptosis 30 hrs after transfection. The full length p59 and p56 forms and the unique domain alone induced low levels of cell death. The unique and SH3 domain of a closely related kinase, Lyn did not induce apoptosis. Overexpression of a mutant USH3 domain (Gly --> Ala), that disrupts membrane localization, did not induce high level of apoptosis. Cells overexpressing Hck-USH3 showed activation of caspase-3 and release of cytochrome c from mitochondria into cytosol. Caspase-3 defective MCF-7 cells were resistant to apoptosis and cytochrome c release induced by Hck-USH3, which were restored by introducing the caspase-3 gene. These results suggest that Hck SH3 domain mediated signalling at the plasma membrane triggers a pathway leading to caspase-3 dependent cyto- chrome c release and apoptosis.

Inhibition of anchorage-independent cell growth, adhesion, and cyclin D1 gene expression by a dominant negative mutant of a tyrosine phosphatase

PTP-S4/TC48 protein tyrosine phosphatase is localized in the nuclear and cytoplasmic membranes. To investigate the role of PTP-S4 in cell growth, adhesion, and transformation, normal and a catalytically inactive mutant form of this phosphatase were expressed in polyoma virus-transformed F111 fibroblast cell line, PyF. Expression of mutant PTP-S4 in PyF cells resulted in strong inhibition of anchorage-independent growth in soft agar but had no significant effect on growth in liquid culture. Tumor formation in nude mice was also reduced by mutant PTP-S4. Expression of normal PTP-S4 in PyF cells significantly increased anchorage-independent cell growth and tumor formation in nude mice. Overexpression of catalytically inactive mutant of PTP-S2/TC45 (a splice variant of PTP-S4 that is nuclear) did not inhibit anchorage-independent growth of PyF cells. Mutant PTP-S4-expressing cells were inhibited in adhesion and spreading on tissue culture plates compared to control cells. Expression of mutant PTP-S4 in PyF cells reduced the levels of cyclin D1 and cyclin A mRNA, whereas cyclin D2 mRNA level was not affected significantly. Expression of antisense cyclin D1 strongly inhibited anchorage-independent growth. Inhibition of anchorage-independent growth by mutant PTP-S4 was overcome to a large extent by coexpression of cyclin D1. These results suggest that mutant PTP-S4 inhibits anchorage-independent growth and adhesion of polyoma virus-transformed cells by interfering with the normal function of PTP-S4 upstream of cyclin D1 gene expression.

A nuclear protein tyrosine phosphatase induces shortening of G1 phase and increase in c-Myc protein level

PTP-S2 is a ubiquitously expressed nuclear protein tyrosine phosphatase which shows increased expression upon mitogenic stimulation in a variety of cells in vitro and in vivo. In order to understand the role of this enzyme in cell cycle progression, tetracycline-regulated HeLa clones expressing PTP-S2 were isolated and characterized. Tetracycline-controlled expression of PTP-S2 increased the rate of cell proliferation. An analysis of the distribution of cells in various phases of the cell cycle in an exponentially growing cell population showed that there was a large decrease in the percentage of cells in G1 phase in a PTP-S2-expressing population of cells compared to nonexpressing cells. This decrease in the percentage of cells in G1 was dependent on the level of PTP-S2 expression. There was a corresponding increase in the percentage of cells in G2/M but no significant increase in the percentage of cells in S phase. An analysis of the time course of cell cycle progression after release from double thymidine block showed that the duration of G1 phase was significantly shortened in cells induced to express exogenous PTP-S2. However, the duration of S phase was not significantly altered and the duration of G2 phase was increased to some extent. Induction of PTP-S2 expression was associated with an increase in c-Myc protein levels, although the c-Myc mRNA level was not changed. Our results suggest that overexpression of PTP-S2 promotes progression of cells through G1 to S phase and is associated with increased level of c-Myc protein through a posttranscriptional mechanism.

Direct transcriptional activation of human caspase-1 by tumor suppressor p53

The tumor suppressor protein p53 is a sequence-specific DNA-binding protein, and its biological responses are very often mediated by transcriptional activation of various target genes. Here we show that caspase-1 (interleukin-1beta converting enzyme), which plays a role in the production of proinflammatory cytokines and in apoptosis, is a transcriptional target of p53. Caspase-1 mRNA levels increased upon overexpression of p53 by transfection in MCF-7 cells. Human caspase-1 promoter showed a sequence homologous to the consensus p53-binding site. This sequence bound to p53 in gel shift assays. A caspase-1 promoter-reporter construct was activated 6-8-fold by cotransfection with normal p53 but not by mutant p53 (His(273)) in HeLa, as well as MCF-7, cells. Mutation of the p53-binding site in caspase-1 promoter abolished transactivation by p53. Treatment of p53-positive MCF-7 cells with the DNA-damaging drug, doxorubicin, which increases p53 levels, enhanced caspase-1 promoter activity 4-5-fold, but similar treatment of MCF-7-mp53 (a clone of MCF-7 cells expressing mutant p53) and p53-negative HeLa cells with doxorubicin did not increase caspase-1 promoter activity. Doxorubicin treatment increased caspase-1 mRNA levels in MCF-7 cells but not in MCF-7-mp53 or HeLa cells. These results show that endogenous p53 can regulate caspase-1 gene expression.

PTP-S2, a nuclear tyrosine phosphatase, is phosphorylated and excluded from condensed chromosomes during mitosis

PTP-S2 is a tyrosine specific protein phosphatase that binds to DNA and is localized to the nucleus in association with chromatin. It plays a role in the regulation of cell proliferation. Here we show that the subcellular distribution of this protein changes during cell division. While PTP-S2 was localized exclusively to the nucleus in interphase cells, during metaphase and anaphase it was distributed throughout the cytoplasm and excluded from condensed chromosomes. At telophase PTP-S2 began to associate with chromosomes and at cytokinesis it was associated with chromatin in the newly formed nucleus. It was hyperphosphorylated and showed retarded mobility in cells arrested in metaphase. In vitro experiments showed that it was phosphorylated by CK2 resulting in mobility shift. Using a deletion mutant we found that CK2 phosphorylated PTP-S2 in the C-terminal non-catalytic domain. A heparin sensitive kinase from mitotic cell extracts phosphorylated PTP-S2 resulting in mobility shift. These results are consistent with the suggestion that during metaphase PTP-S2 is phosphorylated (possibly by CK2 or a CK2-like enzyme), resulting in its dissociation from chromatin.

Identification of a novel splice variant of C3G which shows tissue-specific expression

C3G is a guanine nucleotide-releasing protein that binds to the Src homology 3 (SH3) domain of the adapter protein Crk. In this study, we isolated cDNAs coding for rat C3G. Northern blot analysis of RNA from various rat tissues and cell lines showed a major transcript of about 7 kb which was present at the highest level in testis. A comparison of the amino acid sequence (derived from the cDNA sequence) of rat C3G with the human form showed 87.3% sequence identity. The principal difference was the presence of an additional 51 amino acids in the rat C3G sequence after the fifth PXXP motif. This difference may be attributable to alternative splicing of the primary transcript. This interpretation was supported by reverse transcription-polymerase chain reaction (RT-PCR) assays, which resulted in two products differing by 153 bp. The RT-PCR analysis of RNA from various rat tissues showed that the relative expression levels of the two splice forms were variable. The form of C3G with the insertion of 51 amino acids (named C3G-2) was present in rat testis at a high level and, to a lesser extent, in brain, but it was not seen (or was present at a very low level) in other rat tissues and certain rat and mouse cell lines. This expression pattern of the C3G-2 form was confirmed by Northern blotting using the insert region as a probe. The C3G-1 form, without the insertion of 51 amino acids, was present in almost all rat tissues except testis and in cell lines of rat, mouse, or human origin. Thus, in rat cells, we have identified a novel splice variant of C3G. The expression pattern indicates that the form of C3G described here is likely to serve a tissue- or cell-specific physiological function.

Induction of p53 dependent apoptosis upon overexpression of a nuclear protein tyrosine phosphatase

Two ubiquitously expressed protein tyrosine phosphatases, PTP-S2 and PTP-S4 (also known as TC45 and TC48, respectively), are alternately spliced products of the same gene. Overexpression of PTP-S2 by transient transfection induced chromatin condensation and nuclear fragmentation, typical of apoptosis. Expression of PTP-S4 resulted in a much lower number of cells with apoptotic phenotype. PTP-S2 induced apoptosis in MCF7 and A549 human tumor cell lines which are p53 positive but not in HeLa and SW620 cells which are p53 negative. Apoptosis induced by PTP-S2 in MCF7 cells was inhibited by cotransfection with mutant p53 (Arg-273 --> His) but not by wild type p53. PTP-S2 induced apoptosis was inhibited by antiapoptotic protein Bcl2 and certain inhibitors of caspases. These results suggest that the nuclear tyrosine phosphatase PTP-S2 induces p53 dependent, serum starvation independent and caspase mediated apoptosis.

Protein phosphatase 2A inhibitors, phoslactomycins. Effects on the cytoskeleton in NIH/3T3 cells

Protein phosphorylation is a key regulatory mechanism of the organization and dynamics of the actin cytoskeleton during cell motility, differentiation, and cytokinesis. The level of protein phosphorylation is dependent on the relative activities of both protein kinases and protein phosphatases. In this paper, we examined the effect of phoslactomycins (PLMs) on the regulation of the cytoskeleton of NIH/3T3 fibroblasts. Treatment of cells with PLM-F (10 microM) induced actin filament depolymerization after 4 h. This effect was reversible and actin filaments were reformed 1 h after removal of the inhibitors. As PLM-F had no effect at all on polymerization of purified actin in vitro, it is thought that PLMs induce actin depolymerization through an indirect mechanism. An in vitro assay showed PLMs inhibited protein phosphatase 2A at lower concentrations (IC50 4.7 microM) than protein phosphatase 1. An in situ phosphorylation assay also revealed that PLM-F treatment stimulated the phosphorylation of intracellular vimentin. These results suggest that phoslactomycins are protein phosphatase 2A-specific inhibitors and that protein phosphatase 2A is involved in regulation of the organization of the actin cytoskeleton.

Overexpression of a nuclear protein tyrosine phosphatase increases cell proliferation

PTP-S2 is a widely expressed nuclear protein tyrosine phosphatase which shows increased expression upon mitogenic stimulation of a variety of cells. In order to elucidate the role of this enzyme in cell division, stable clones of HeLa cells expressing rat PTP-S2 were isolated and their growth properties analysed. Overexpressed PTP-S2 was located in the cell nucleus and there was no significant change in the total tyrosine phosphatase activity of PTP-S2 overexpressing cells. PTP-S2 overexpressing clones, D3 and B5, showed increased rate of cell division and lower serum requirement as compared with control cells. D3 and B5 cells formed larger colonies in soft agar, were not contact inhibited upon confluency, grew in multilayers, and showed altered morphology. These results are consistent with the suggestion that PTP-S2 may be a positive regulator of cell proliferation.

Evaluation of aperture efficiency of a radiofrequency electric field strength meter antenna

With present-day rapid increases in the applications of radiofrequency microwave radiation in research, medicine, defense, communications, and industry, it has become necessary to have simple inexpensive portable radiofrequency microwave field strength meters for health and safety surveys around operating equipment. This paper presents the results of a study conducted at Trombay calibrating grid to evaluate the receiving aperture efficiency of the 1-m long antenna of a simple narrow band electric field strength meter, with frequency response centered around 27.12 MHz. An aperture efficiency of 0.47 with a coefficient of variation of +/-3.46% is considered adequate for field use.

Interaction of SH3 domain of Hck tyrosine kinase with cellular proteins containing proline-rich regions: evidence for modulation by unique domain

The Hck tyrosine kinase, a member of Src family, is predominantly expressed in myeloid cells. In this report we have analyzed interaction of cellular proteins with Src homology 3 (SH3) domain of Hck. For this purpose we used various GST-Hck fusion proteins comprising a part of unique region, complete unique region and/or complete SH3 domain of Hck, and glutathione S-transferase (GST). When these fusion proteins (or GST), immobilized on glutathione-agarose beads were incubated with [35S] methionine labelled cell extracts, multiple proteins which interact specifically with SH3 domain of Hck were detected by SDS-PAGE followed by autoradiography. The Hck interacting proteins could also be detected by a tandem blot binding assay in which the blot was incubated with purified fusion protein (or GST) and then the interacting proteins were identified by using antibody against GST. When a part of or complete unique domain was present along with SH3 domain, the interaction of some specific proteins was reduced several fold. These results raise the possibility of unique domain altering the properties of SH3 domain, thus modulating or restricting the interaction of SH3 domain with specific cellular proteins. This modulatory effect of unique domain was localized to 28 amino acids upstream of SH3 domain. SH3 interacting proteins were associated with serine/threonine and tyrosine kinase activities towards exogenous substrates. Most of the SH3 binding proteins were soluble in Triton X-100. Differentiation of promyelocytic leukemia cell line HL-60 into macrophage like cells resulted in appearance of novel SH3 binding proteins. Hck was detected in the eluate of WGA-Sepharose column, suggesting that it interacts with WGA binding glycoprotein (s). A rat spleen cDNA library was screened for the SH3 binding proteins by protein interaction cloning. Sequence analysis of the clones showed the presence of proline rich regions containing PPXP motifs.

Two splice variants of a tyrosine phosphatase differ in substrate specificity, DNA binding, and subcellular location

Four different forms of a non-receptor type protein-tyrosine phosphatase are generated by alternative splicing; two of these forms (PTP-S2 and PTP-S4) are major forms, which are expressed in rat as well as human cells. Here we report that PTP-S2 binds to nonspecific DNA in vitro and localizes in the nucleus upon transfection in HeLa cells. PTP-S4 does not bind to nonspecific DNA and shows perinuclear and cytoplasmic localization. Removal of the C-terminal 34 amino acids of PTP-S4 gives rise to a truncated protein, which binds to nonspecific DNA and localizes to the nucleus. PTP-S4, but not PTP-S2, interacts strongly with the isolated nuclear matrix. The two forms of this tyrosine phosphatase show different substrate specificity in vitro, a feature novel to splice variants of tyrosine phosphatases. Mitogenic stimulation induces mRNAs for PTP-S2 as well as for PTP-S4 in the G1 phase during liver regeneration. These results suggest that alternative splicing gives rise to two protein-tyrosine phosphatases with distinct substrate specificities and subcellular locations. The 34 amino acids at the C terminus of PTP-S4 play a critical role in determining substrate specificity, subcellular location, and interaction with nuclear matrix and DNA.

Association of Lyn tyrosine kinase with the nuclear matrix and cell-cycle-dependent changes in matrix-associated tyrosine kinase activity

The nuclear matrix isolated from HeLa cells and Rat2 fibroblasts harbors tyrosine kinase and tyrosine phosphatase activities. Polypeptides of 53, 56 and 60 kDa, associated with this subnuclear structure, were phosphorylated at tyrosine in vivo. By immunoblot and immunolabelling experiments, we identified one of the nuclear-matrix-associated tyrosine kinases as Lyn, a Src family member. Lyn was distributed as foci throughout the matrix. The p56 and p53 isoforms of Lyn remained firmly associated with the nuclear matrix after a variety of matrix preparation procedures, and were not detectable in the chromatin fraction of the nucleus. The tyrosine kinase activity associated with the nuclear matrix showed cell-cycle-dependent changes, maximum activity being observed at the G1/S transition phase. Polyoma-virus-transformed rat fibroblast cells showed sixfold higher tyrosine kinase activity in the nuclear matrix preparations compared to that in untransformed cells. These observations are consistent with the suggestion that tyrosine kinase activity associated with the nuclear matrix may be an important determinant of cellular proliferation.