Targetable leukaemia dependency on noncanonical PI3Kγ signalling

Phosphoinositide-3-kinase-γ (PI3Kγ) is implicated as a target to repolarize tumour-associated macrophages and promote antitumour immune responses in solid cancers1-4. However, cancer cell-intrinsic roles of PI3Kγ are unclear. Here, by integrating unbiased genome-wide CRISPR interference screening with functional analyses across acute leukaemias, we define a selective dependency on the PI3Kγ complex in a high-risk subset that includes myeloid, lymphoid and dendritic lineages. This dependency is characterized by innate inflammatory signalling and activation of phosphoinositide 3-kinase regulatory subunit 5 (PIK3R5), which encodes a regulatory subunit of PI3Kγ5 and stabilizes the active enzymatic complex. We identify p21 (RAC1)-activated kinase 1 (PAK1) as a noncanonical substrate of PI3Kγ that mediates this cell-intrinsic dependency and find that dephosphorylation of PAK1 by PI3Kγ inhibition impairs mitochondrial oxidative phosphorylation. Treatment with the selective PI3Kγ inhibitor eganelisib is effective in leukaemias with activated PIK3R5. In addition, the combination of eganelisib and cytarabine prolongs survival over either agent alone, even in patient-derived leukaemia xenografts with low baseline PIK3R5 expression, as residual leukaemia cells after cytarabine treatment have elevated G protein-coupled purinergic receptor activity and PAK1 phosphorylation. Together, our study reveals a targetable dependency on PI3Kγ-PAK1 signalling that is amenable to near-term evaluation in patients with acute leukaemia.

Biallelic USP14 variants cause a syndromic neurodevelopmental disorder

PURPOSE

Imbalances in protein homeostasis affect human brain development, with the ubiquitin-proteasome system (UPS) and autophagy playing crucial roles in neurodevelopmental disorders (NDD). This study explores the impact of biallelic USP14 variants on neurodevelopment, focusing on its role as a key hub connecting UPS and autophagy.

METHODS

Here, we identified biallelic USP14 variants in 4 individuals from 3 unrelated families: 1 fetus, a newborn with a syndromic NDD and 2 siblings affected by a progressive neurological disease. Specifically, the 2 siblings from the latter family carried 2 compound heterozygous variants c.8T>C p.(Leu3Pro) and c.988C>T p.(Arg330∗), whereas the fetus had a homozygous frameshift c.899_902del p.(Lys300Serfs∗24) variant, and the newborn patient harbored a homozygous frameshift c.233_236del p.(Leu78Glnfs∗11) variant. Functional studies were conducted using sodium dodecyl-sulfate polyacrylamide gel electrophoresis, western blotting, and mass spectrometry analyses in both patient-derived and CRISPR-Cas9-generated cells.

RESULTS

Our investigations indicated that the USP14 variants correlated with reduced N-terminal methionine excision, along with profound alterations in proteasome, autophagy, and mitophagy activities.

CONCLUSION

Biallelic USP14 variants in NDD patients perturbed protein degradation pathways, potentially contributing to disorder etiology. Altered UPS, autophagy, and mitophagy activities underscore the intricate interplay, elucidating their significance in maintaining proper protein homeostasis during brain development.

Targetable leukemia dependency on noncanonical PI3Kγ signaling

Phosphoinositide 3-kinase gamma (PI3Kγ) is implicated as a target to repolarize tumor-associated macrophages and promote anti-tumor immune responses in solid cancers. However, cancer cell-intrinsic roles of PI3Kγ are unclear. Here, by integrating unbiased genome-wide CRISPR interference screening with functional analyses across acute leukemias, we define a selective dependency on the PI3Kγ complex in a high-risk subset that includes myeloid, lymphoid, and dendritic lineages. This dependency is characterized by innate inflammatory signaling and activation of phosphoinositide 3-kinase regulatory subunit 5 ( PIK3R5 ), which encodes a regulatory subunit of PI3Kγ and stabilizes the active enzymatic complex. Mechanistically, we identify p21 (RAC1) activated kinase 1 (PAK1) as a noncanonical substrate of PI3Kγ that mediates this cell-intrinsic dependency independently of Akt kinase. PI3Kγ inhibition dephosphorylates PAK1, activates a transcriptional network of NFκB-related tumor suppressor genes, and impairs mitochondrial oxidative phosphorylation. We find that treatment with the selective PI3Kγ inhibitor eganelisib is effective in leukemias with activated PIK3R5 , either at baseline or by exogenous inflammatory stimulation. Notably, the combination of eganelisib and cytarabine prolongs survival over either agent alone, even in patient-derived leukemia xenografts with low baseline PIK3R5 expression, as residual leukemia cells after cytarabine treatment have elevated G protein-coupled purinergic receptor activity and PAK1 phosphorylation. Taken together, our study reveals a targetable dependency on PI3Kγ/PAK1 signaling that is amenable to near-term evaluation in patients with acute leukemia.

Proteostasis and lysosomal quality control deficits in Alzheimer's disease neurons

The role of proteostasis and organelle homeostasis dysfunction in human aging and Alzheimer's disease (AD) remains unclear. Analyzing proteome-wide changes in human donor fibroblasts and their corresponding transdifferentiated neurons (tNeurons), we find aging and AD synergistically impair multiple proteostasis pathways, most notably lysosomal quality control (LQC). In particular, we show that ESCRT-mediated lysosomal repair defects are associated with both sporadic and PSEN1 familial AD. Aging- and AD-linked defects are detected in fibroblasts but highly exacerbated in tNeurons, leading to enhanced neuronal vulnerability, unrepaired lysosomal damage, inflammatory factor secretion and cytotoxicity. Surprisingly, tNeurons from aged and AD donors spontaneously develop amyloid-β inclusions co-localizing with LQC markers, LAMP1/2-positive lysosomes and proteostasis factors; we observe similar inclusions in brain tissue from AD patients and APP-transgenic mice. Importantly, compounds enhancing lysosomal function broadly ameliorate these AD-associated pathologies. Our findings establish cell-autonomous LQC dysfunction in neurons as a central vulnerability in aging and AD pathogenesis.

Targeting pleckstrin-2/Akt signaling reduces proliferation in myeloproliferative neoplasm models

Myeloproliferative neoplasms (MPNs) are characterized by the activated JAK2/STAT pathway. Pleckstrin-2 (Plek2) is a downstream target of the JAK2/STAT5 pathway and is overexpressed in patients with MPNs. We previously revealed that Plek2 plays critical roles in the pathogenesis of JAK2-mutated MPNs. The nonessential roles of Plek2 under physiologic conditions make it an ideal target for MPN therapy. Here, we identified first-in-class Plek2 inhibitors through an in silico high-throughput screening approach and cell-based assays, followed by the synthesis of analogs. Plek2-specific small-molecule inhibitors showed potent inhibitory effects on cell proliferation. Mechanistically, Plek2 interacts with and enhances the activity of Akt through the recruitment of downstream effector proteins. The Plek2-signaling complex also includes Hsp72, which protects Akt from degradation. These functions were blocked by Plek2 inhibitors via their direct binding to the Plek2 dishevelled, Egl-10 and pleckstrin (DEP) domain. The role of Plek2 in activating Akt signaling was further confirmed in vivo using a hematopoietic-specific Pten-knockout mouse model. We next tested Plek2 inhibitors alone or in combination with an Akt inhibitor in various MPN mouse models, which showed significant therapeutic efficacies similar to that seen with the genetic depletion of Plek2. The Plek2 inhibitor was also effective in reducing proliferation of CD34-positive cells from MPN patients. Our studies reveal a Plek2/Akt complex that drives cell proliferation and can be targeted by a class of antiproliferative compounds for MPN therapy.

Frontotemporal Dementia Patient Neurons With Progranulin Deficiency Display Protein Dyshomeostasis

Haploinsufficiency of progranulin (PGRN) causes frontotemporal dementia (FTD), a devastating neurodegenerative disease with no effective treatment. PGRN is required for efficient proteostasis, as loss of neuronal PGRN results in dysfunctional lysosomes and impaired clearance and cytoplasmic aggregation of TDP-43, a protein involved in neurodegeneration in FTD. These and other events lead to neurodegeneration and neuroinflammation. However, the detailed mechanisms leading to protein dyshomeostasis in PGRN-deficient cells remain unclear. We report here the development of human cell models of FTD with PGRN-deficiency to explore the molecular mechanisms underlying proteostasis breakdown and TDP-43 aggregation in FTD. Neurons differentiated from FTD patient induced pluripotent stem cells (iPSCs) have reduced PGRN levels, and the neurons recapitulate key disease features, including impaired lysosomal function, defective TDP-43 turnover and accumulation, neurodegeneration, and death. Proteomic analysis revealed altered levels of proteins linked to the autophagy-lysosome pathway (ALP) and the ubiquitin-proteasome system (UPS) in FTD patient neurons, providing new mechanistic insights into the link between PGRN-deficiency and disease pathobiology.

Liver RBFOX2 regulates cholesterol homeostasis via Scarb1 alternative splicing in mice

RNA alternative splicing (AS) expands the regulatory potential of eukaryotic genomes. The mechanisms regulating liver-specific AS profiles and their contribution to liver function are poorly understood. Here, we identify a key role for the splicing factor RNA-binding Fox protein 2 (RBFOX2) in maintaining cholesterol homeostasis in a lipogenic environment in the liver. Using enhanced individual-nucleotide-resolution ultra-violet cross-linking and immunoprecipitation, we identify physiologically relevant targets of RBFOX2 in mouse liver, including the scavenger receptor class B type I (Scarb1). RBFOX2 function is decreased in the liver in diet-induced obesity, causing a Scarb1 isoform switch and alteration of hepatocyte lipid homeostasis. Our findings demonstrate that specific AS programmes actively maintain liver physiology, and underlie the lipotoxic effects of obesogenic diets when dysregulated. Splice-switching oligonucleotides targeting this network alleviate obesity-induced inflammation in the liver and promote an anti-atherogenic lipoprotein profile in the blood, underscoring the potential of isoform-specific RNA therapeutics for treating metabolism-associated diseases.

ALS/FTD mutations in UBQLN2 are linked to mitochondrial dysfunction through loss-of-function in mitochondrial protein import

UBQLN2 mutations cause amyotrophic lateral sclerosis (ALS) with frontotemporal dementia (FTD), but the pathogenic mechanisms by which they cause disease remain unclear. Proteomic profiling identified 'mitochondrial proteins' as comprising the largest category of protein changes in the spinal cord (SC) of the P497S UBQLN2 mouse model of ALS/FTD. Immunoblots confirmed P497S animals have global changes in proteins predictive of a severe decline in mitochondrial health, including oxidative phosphorylation (OXPHOS), mitochondrial protein import and network dynamics. Functional studies confirmed mitochondria purified from the SC of P497S animals have age-dependent decline in nearly all steps of OXPHOS. Mitochondria cristae deformities were evident in spinal motor neurons of aged P497S animals. Knockout (KO) of UBQLN2 in HeLa cells resulted in changes in mitochondrial proteins and OXPHOS activity similar to those seen in the SC. KO of UBQLN2 also compromised targeting and processing of the mitochondrial import factor, TIMM44, resulting in accumulation in abnormal foci. The functional OXPHOS deficits and TIMM44-targeting defects were rescued by reexpression of WT UBQLN2 but not by ALS/FTD mutant UBQLN2 proteins. In vitro binding assays revealed ALS/FTD mutant UBQLN2 proteins bind weaker with TIMM44 than WT UBQLN2 protein, suggesting that the loss of UBQLN2 binding may underlie the import and/or delivery defect of TIMM44 to mitochondria. Our studies indicate a potential key pathogenic disturbance in mitochondrial health caused by UBQLN2 mutations.

Structures of chaperone-associated assembly intermediates reveal coordinated mechanisms of proteasome biogenesis

The proteasome mediates most selective protein degradation. Proteolysis occurs within the 20S core particle (CP), a barrel-shaped chamber with an α₇β₇β₇α₇ configuration. CP biogenesis proceeds through an ordered multistep pathway requiring five chaperones, Pba1-4 and Ump1. Using Saccharomyces cerevisiae, we report high-resolution structures of CP assembly intermediates by cryogenic-electron microscopy. The first structure corresponds to the 13S particle, which consists of a complete α-ring, partial β-ring (β2-4), Ump1 and Pba1/2. The second structure contains two additional subunits (β5-6) and represents a later pre-15S intermediate. These structures reveal the architecture and positions of Ump1 and β2/β5 propeptides, with important implications for their functions. Unexpectedly, Pba1's N terminus extends through an open CP pore, accessing the CP interior to contact Ump1 and the β5 propeptide. These results reveal how the coordinated activity of Ump1, Pba1 and the active site propeptides orchestrate key aspects of CP assembly.

Maternal Iron Deficiency Modulates Placental Transcriptome and Proteome in Mid-Gestation of Mouse Pregnancy

BACKGROUND

Maternal iron deficiency (ID) is associated with poor pregnancy and fetal outcomes. The effect is thought to be mediated by the placenta but there is no comprehensive assessment of placental responses to maternal ID. Additionally, whether the influence of maternal ID on the placenta differs by fetal sex is unknown.

OBJECTIVES

To identify gene and protein signatures of ID mouse placentas at mid-gestation. A secondary objective was to profile the expression of iron genes in mouse placentas across gestation.

METHODS

We used a real-time PCR-based array to determine the mRNA expression of all known iron genes in mouse placentas at embryonic day (E) 12.5, E14.5, E16.5, and E19.5 (n = 3 placentas/time point). To determine the effect of maternal ID, we performed RNA sequencing and proteomics in male and female placentas from ID and iron-adequate mice at E12.5 (n = 8 dams/diet).

RESULTS

In female placentas, 6 genes, including transferrin receptor (Tfrc) and solute carrier family 11 member 2, were significantly changed by maternal ID. An additional 154 genes were altered in male ID placentas. A proteomic analysis quantified 7662 proteins in the placenta. Proteins translated from iron-responsive element (IRE)-containing mRNA were altered in abundance; ferritin and ferroportin 1 decreased, while TFRC increased in ID placentas. Less than 4% of the significantly altered genes in ID placentas occurred both at the transcriptional and translational levels.

CONCLUSIONS

Our data demonstrate that the impact of maternal ID on placental gene expression in mice is limited in scope and magnitude at mid-gestation. We provide strong evidence for IRE-based transcriptional and translational coordination of iron gene expression in the mouse placenta. Finally, we discover sexually dimorphic effects of maternal ID on placental gene expression, with more genes and pathways altered in male compared with female mouse placentas.

Membrane skeleton modulates erythroid proteome remodeling and organelle clearance

The final stages of mammalian erythropoiesis involve enucleation, membrane and proteome remodeling, and organelle clearance. Concomitantly, the erythroid membrane skeleton establishes a unique pseudohexagonal spectrin meshwork that is connected to the membrane through junctional complexes. The mechanism and signaling pathways involved in the coordination of these processes are unclear. The results of our study revealed an unexpected role of the membrane skeleton in the modulation of proteome remodeling and organelle clearance during the final stages of erythropoiesis. We found that diaphanous-related formin mDia2 is a master regulator of the integrity of the membrane skeleton through polymerization of actin protofilament in the junctional complex. The mDia2-deficient terminal erythroid cell contained a disorganized and rigid membrane skeleton that was ineffective in detaching the extruded nucleus. In addition, the disrupted skeleton failed to activate the endosomal sorting complex required for transport-III (ESCRT-III) complex, which led to a global defect in proteome remodeling, endolysosomal trafficking, and autophagic organelle clearance. Chmp5, a component of the ESCRT-III complex, is regulated by mDia2-dependent activation of the serum response factor and is essential for membrane remodeling and autophagosome-lysosome fusion. Mice with loss of Chmp5 in hematopoietic cells in vivo resembled the phenotypes in mDia2-knockout mice. Furthermore, overexpression of Chmp5 in mDia2-deficient hematopoietic stem and progenitor cells significantly restored terminal erythropoiesis in vivo. These findings reveal a formin-regulated signaling pathway that connects the membrane skeleton to proteome remodeling, enucleation, and organelle clearance during terminal erythropoiesis.

Global proteomics of Ubqln2-based murine models of ALS

Familial neurodegenerative diseases commonly involve mutations that result in either aberrant proteins or dysfunctional components of the proteolytic machinery that act on aberrant proteins. UBQLN2 is a ubiquitin receptor of the UBL/UBA family that binds the proteasome through its ubiquitin-like domain and is thought to deliver ubiquitinated proteins to proteasomes for degradation. UBQLN2 mutations result in familial amyotrophic lateral sclerosis (ALS)/frontotemporal dementia in humans through an unknown mechanism. Quantitative multiplexed proteomics was used to provide for the first time an unbiased and global analysis of the role of Ubqln2 in controlling the composition of the proteome. We studied several murine models of Ubqln2-linked ALS and also generated Ubqln2 null mutant mice. We identified impacts of Ubqln2 on diverse physiological pathways, most notably serotonergic signaling. Interestingly, we observed an upregulation of proteasome subunits, suggesting a compensatory response to diminished proteasome output. Among the specific proteins whose abundance is linked to UBQLN2 function, the strongest hits were the ubiquitin ligase TRIM32 and two retroelement-derived proteins, PEG10 and CXX1B. Cycloheximide chase studies using induced human neurons and HEK293 cells suggested that PEG10 and TRIM32 are direct clients. Although UBQLN2 directs the degradation of multiple proteins via the proteasome, it surprisingly conferred strong protection from degradation on the Gag-like protein CXX1B, which is expressed from the same family of retroelement genes as PEG10. In summary, this study charts the proteomic landscape of ALS-related Ubqln2 mutants and identifies candidate client proteins that are altered in vivo in disease models and whose degradation is promoted by UBQLN2.

ALS/FTD mutations in UBQLN2 impede autophagy by reducing autophagosome acidification through loss of function

Mutations in UBQLN2 cause amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and other neurodegenerations. However, the mechanism by which the UBQLN2 mutations cause disease remains unclear. Alterations in proteins involved in autophagy are prominent in neuronal tissue of human ALS UBQLN2 patients and in a transgenic P497S UBQLN2 mouse model of ALS/FTD, suggesting a pathogenic link. Here, we show UBQLN2 functions in autophagy and that ALS/FTD mutant proteins compromise this function. Inactivation of UBQLN2 expression in HeLa cells reduced autophagic flux and autophagosome acidification. The defect in acidification was rescued by reexpression of wild type (WT) UBQLN2 but not by any of the five different UBQLN2 ALS/FTD mutants tested. Proteomic analysis and immunoblot studies revealed P497S mutant mice and UBQLN2 knockout HeLa and NSC34 cells have reduced expression of ATP6v1g1, a critical subunit of the vacuolar ATPase (V-ATPase) pump. Knockout of UBQLN2 expression in HeLa cells decreased turnover of ATP6v1g1, while overexpression of WT UBQLN2 increased biogenesis of ATP6v1g1 compared with P497S mutant UBQLN2 protein. In vitro interaction studies showed that ATP6v1g1 binds more strongly to WT UBQLN2 than to ALS/FTD mutant UBQLN2 proteins. Intriguingly, overexpression of ATP6v1g1 in UBQLN2 knockout HeLa cells increased autophagosome acidification, suggesting a therapeutic approach to overcome the acidification defect. Taken together, our findings suggest that UBQLN2 mutations drive pathogenesis through a dominant-negative loss-of-function mechanism in autophagy and that UBQLN2 functions as an important regulator of the expression and stability of ATP6v1g1. These findings may have important implications for devising therapies to treat UBQLN2-linked ALS/FTD.

Chromatin accessibility promotes hematopoietic and leukemia stem cell activity

Chromatin organization is a highly orchestrated process that influences gene expression, in part by modulating access of regulatory factors to DNA and nucleosomes. Here, we report that the chromatin accessibility regulator HMGN1, a target of recurrent DNA copy gains in leukemia, controls myeloid differentiation. HMGN1 amplification is associated with increased accessibility, expression, and histone H3K27 acetylation of loci important for hematopoietic stem cells (HSCs) and leukemia, such as HoxA cluster genes. In vivo, HMGN1 overexpression is linked to decreased quiescence and increased HSC activity in bone marrow transplantation. HMGN1 overexpression also cooperates with the AML-ETO9a fusion oncoprotein to impair myeloid differentiation and enhance leukemia stem cell (LSC) activity. Inhibition of histone acetyltransferases CBP/p300 relieves the HMGN1-associated differentiation block. These data nominate factors that modulate chromatin accessibility as regulators of HSCs and LSCs, and suggest that targeting HMGN1 or its downstream effects on histone acetylation could be therapeutically active in AML.

The deubiquitinating enzyme Usp14 controls ciliogenesis and Hedgehog signaling

Primary cilia are hair-like organelles that play crucial roles in vertebrate development, organogenesis and when dysfunctional result in pleiotropic human genetic disorders called ciliopathies, characterized by overlapping phenotypes, such as renal and hepatic cysts, skeletal defects, retinal degeneration and central nervous system malformations. Primary cilia act as communication hubs to transfer extracellular signals into intracellular responses and are essential for Hedgehog (Hh) signal transduction in mammals. Despite the renewed interest in this ancient organelle of growing biomedical importance, the molecular mechanisms that trigger cilia formation, extension and ciliary signal transduction are still not fully understood. Here we provide, for the first time, evidence that the deubiquitinase ubiquitin-specific protease-14 (Usp14), a major regulator of the ubiquitin proteasome system (UPS), controls ciliogenesis, cilia elongation and Hh signal transduction. Moreover, we show that pharmacological inhibition of Usp14 positively affects Hh signal transduction in a model of autosomal dominant polycystic kidney disease. These findings provide new insight into the spectrum of action of UPS in cilia biology and may provide novel opportunities for therapeutic intervention in human conditions associated with ciliary dysfunction.

The Proteasome and Its Network: Engineering for Adaptability

The proteasome, the most complex protease known, degrades proteins that have been conjugated to ubiquitin. It faces the unique challenge of acting enzymatically on hundreds and perhaps thousands of structurally diverse substrates, mechanically unfolding them from their native state and translocating them vectorially from one specialized compartment of the enzyme to another. Moreover, substrates are modified by ubiquitin in myriad configurations of chains. The many unusual design features of the proteasome may have evolved in part to endow this enzyme with a robust ability to process substrates regardless of their identity. The proteasome plays a major role in preserving protein homeostasis in the cell, which requires adaptation to a wide variety of stress conditions. Modulation of proteasome function is achieved through a large network of proteins that interact with it dynamically, modify it enzymatically, or fine-tune its levels. The resulting adaptability of the proteasome, which is unique among proteases, enables cells to control the output of the ubiquitin-proteasome pathway on a global scale.

Thiol-based direct threat sensing by the stress-activated protein kinase Hog1

The yeast stress-activated protein kinase Hog1 is best known for its role in mediating the response to osmotic stress, but it is also activated by various mechanistically distinct environmental stressors, including heat shock, endoplasmic reticulum stress, and arsenic. In the osmotic stress response, the signal is sensed upstream and relayed to Hog1 through a kinase cascade. Here, we identified a mode of Hog1 function whereby Hog1 senses arsenic through a direct physical interaction that requires three conserved cysteine residues located adjacent to the catalytic loop. These residues were essential for Hog1-mediated protection against arsenic, were dispensable for the response to osmotic stress, and promoted the nuclear localization of Hog1 upon exposure of cells to arsenic. Hog1 promoted arsenic detoxification by stimulating phosphorylation of the transcription factor Yap8, promoting Yap8 nuclear localization, and stimulating the transcription of the only known Yap8 targets, ARR2 and ARR3, both of which encode proteins that promote arsenic efflux. The related human kinases ERK1 and ERK2 also bound to arsenic in vitro, suggesting that this may be a conserved feature of some members of the mitogen-activated protein kinase (MAPK) family. These data provide a mechanistic basis for understanding how stress-activated kinases can sense distinct threats and perform highly specific adaptive responses.

Loss of the deubiquitinase USP36 destabilizes the RNA helicase DHX33 and causes preimplantation lethality in mice

Deubiquitinases are proteases with a wide functional diversity that profoundly impact multiple biological processes. Among them, the ubiquitin-specific protease 36 (USP36) has been implicated in the regulation of nucleolar activity. However, its functional relevance in vivo has not yet been fully described. Here, we report the generation of an Usp36-deficient mouse model to examine the function of this enzyme. We show that Usp36 depletion is lethal in preimplantation mouse embryos, where it blocks the transition from morula to blastocyst during embryonic development. USP36 reduces the ubiquitination levels and increases the stability of the DEAH-box RNA helicase DHX33, which is critically involved in ribosomal RNA synthesis and mRNA translation. In agreement with this finding, O-propargyl-puromycin incorporation experiments, Northern blot, and electron microscopy analyses demonstrated the role of USP36 in ribosomal RNA and protein synthesis. Finally, we show that USP36 down-regulation alters cell proliferation in human cancer cells by inducing both apoptosis and cell cycle arrest, and that reducing DHX33 levels through short hairpin RNA interference has the same effect. Collectively, these results support that Usp36 is essential for cell and organism viability because of its role in ribosomal RNA processing and protein synthesis, which is mediated, at least in part, by regulating DHX33 stability.

An inhibitor of the proteasomal deubiquitinating enzyme USP14 induces tau elimination in cultured neurons

The ubiquitin-proteasome system (UPS) is responsible for most selective protein degradation in eukaryotes and regulates numerous cellular processes, including cell cycle control and protein quality control. A component of this system, the deubiquitinating enzyme USP14, associates with the proteasome where it can rescue substrates from degradation by removal of the ubiquitin tag. We previously found that a small-molecule inhibitor of USP14, known as IU1, can increase the rate of degradation of a subset of proteasome substrates. We report here the synthesis and characterization of 87 variants of IU1, which resulted in the identification of a 10-fold more potent USP14 inhibitor that retains specificity for USP14. The capacity of this compound, IU1-47, to enhance protein degradation in cells was tested using as a reporter the microtubule-associated protein tau, which has been implicated in many neurodegenerative diseases. Using primary neuronal cultures, IU1-47 was found to accelerate the rate of degradation of wild-type tau, the pathological tau mutants P301L and P301S, and the A152T tau variant. We also report that a specific residue in tau, lysine 174, is critical for the IU1-47-mediated tau degradation by the proteasome. Finally, we show that IU1-47 stimulates autophagic flux in primary neurons. In summary, these findings provide a powerful research tool for investigating the complex biology of USP14.

Ubiquilin1 promotes antigen-receptor mediated proliferation by eliminating mislocalized mitochondrial proteins

Ubiquilins (Ubqlns) are a family of ubiquitin receptors that promote the delivery of hydrophobic and aggregated ubiquitinated proteins to the proteasome for degradation. We carried out a proteomic analysis of a B cell lymphoma-derived cell line, BJAB, that requires UBQLN1 for survival to identify UBQLN1 client proteins. When UBQLN1 expression was acutely inhibited, 120 mitochondrial proteins were enriched in the cytoplasm, suggesting that the accumulation of mitochondrial client proteins in the absence of UBQLN1 is cytostatic. Using a Ubqln1^−/− mouse strain, we found that B cell receptor (BCR) ligation of Ubqln1^−/− B cells led to a defect in cell cycle entry. As in BJAB cells, mitochondrial proteins accumulated in BCR-stimulated cells, leading to protein synthesis inhibition and cell cycle block. Thus, UBQLN1 plays an important role in clearing mislocalized mitochondrial proteins upon cell stimulation, and its absence leads to suppression of protein synthesis and cell cycle arrest.

UBE2O remodels the proteome during terminal erythroid differentiation

During terminal differentiation, the global protein complement is remodeled, as epitomized by erythrocytes, whose cytosol is ~98% globin. The erythroid proteome undergoes a rapid transition at the reticulocyte stage; however, the mechanisms driving programmed elimination of preexisting cytosolic proteins are unclear. We found that a mutation in the murine Ube2o gene, which encodes a ubiquitin-conjugating enzyme induced during erythropoiesis, results in anemia. Proteomic analysis suggested that UBE2O is a broad-spectrum ubiquitinating enzyme that remodels the erythroid proteome. In particular, ribosome elimination, a hallmark of reticulocyte differentiation, was defective in Ube2o-/- mutants. UBE2O recognized ribosomal proteins and other substrates directly, targeting them to proteasomes for degradation. Thus, in reticulocytes, the induction of ubiquitinating factors may drive the transition from a complex to a simple proteome.

Multi-omics analysis identifies ATF4 as a key regulator of the mitochondrial stress response in mammals

Mitochondrial stress activates a mitonuclear response to safeguard and repair mitochondrial function and to adapt cellular metabolism to stress. Using a multiomics approach in mammalian cells treated with four types of mitochondrial stressors, we identify activating transcription factor 4 (ATF4) as the main regulator of the stress response. Surprisingly, canonical mitochondrial unfolded protein response genes mediated by ATF5 are not activated. Instead, ATF4 activates the expression of cytoprotective genes, which reprogram cellular metabolism through activation of the integrated stress response (ISR). Mitochondrial stress promotes a local proteostatic response by reducing mitochondrial ribosomal proteins, inhibiting mitochondrial translation, and coupling the activation of the ISR with the attenuation of mitochondrial function. Through a trans-expression quantitative trait locus analysis, we provide genetic evidence supporting a role for Fh1 in the control of Atf4 expression in mammals. Using gene expression data from mice and humans with mitochondrial diseases, we show that the ATF4 pathway is activated in vivo upon mitochondrial stress. Our data illustrate the value of a multiomics approach to characterize complex cellular networks and provide a versatile resource to identify new regulators of mitochondrial-related diseases.

Highly Multiplexed Quantitative Mass Spectrometry Analysis of Ubiquitylomes

System-wide quantitative analysis of ubiquitylomes has proven to be a valuable tool for elucidating targets and mechanisms of the ubiquitin-driven signaling systems, as well as gaining insights into neurodegenerative diseases and cancer. Current mass spectrometry methods for ubiquitylome detection require large amounts of starting material and rely on stochastic data collection to increase replicate analyses. We describe a method compatible with cell line and tissue samples for large-scale quantification of 5,000-9,000 ubiquitylation forms across ten samples simultaneously. Using this method, we reveal site-specific ubiquitylation in mammalian brain and liver tissues, as well as in cancer cells undergoing proteasome inhibition. To demonstrate the power of the approach for signal-dependent ubiquitylation, we examined protein and ubiquitylation dynamics for mitochondria undergoing PARKIN- and PINK1-dependent mitophagy. This analysis revealed the largest collection of PARKIN- and PINK1-dependent ubiquitylation targets to date in a single experiment, and it also revealed a subset of proteins recruited to the mitochondria during mitophagy.

A TRAF2 binding independent region of TNFR2 is responsible for TRAF2 depletion and enhancement of cytotoxicity driven by TNFR1

Tumor Necrosis Factor (TNF) interacts with two receptors known as TNFR1 and TNFR2. TNFR1 activation may result in either cell proliferation or cell death. TNFR2 activates Nuclear Factor-kappaB (NF-kB) and c-Jun N-terminal kinase (JNK) which lead to transcriptional activation of genes related to cell proliferation and survival. This depends on the binding of TNF Receptor Associated Factor 2 (TRAF2) to the receptor. TNFR2 also induces TRAF2 degradation. In this work we have investigated the structural features of TNFR2 responsible for inducing TRAF2 degradation and have studied the biological consequences of this activity. We show that when TNFR1 and TNFR2 are co-expressed, TRAF2 depletion leads to an enhanced TNFR1 cytotoxicity which correlates with the inhibition of NF-kB. NF-kB activation and TRAF2 degradation depend of different regions of the receptor since TNFR2 mutants at amino acids 343-349 fail to induce TRAF2 degradation and have lost their ability to enhance TNFR1-mediated cell death but are still able to activate NF-kB. Moreover, whereas NF-kB activation requires TRAF2 binding to the receptor, TRAF2 degradation appears independent of TRAF2 binding. Thus, TNFR2 mutants unable to bind TRAF2 are still able to induce its degradation and to enhance TNFR1-mediated cytotoxicity. To test further this receptor crosstalk we have developed a system stably expressing in cells carrying only endogenous TNFR1 the chimeric receptor RANK-TNFR2, formed by the extracellular region of RANK (Receptor activator of NF-kB) and the intracellular region of TNFR2.This has made possible to study independently the signals triggered by TNFR1 and TNFR2. In these cells TNFR1 is selectively activated by soluble TNF (sTNF) while RANK-TNFR2 is selectively activated by RANKL. Treatment of these cells with sTNF and RANKL leads to an enhanced cytotoxicity.

Proteomic analysis of annexin A2 phosphorylation induced by microtubule interfering agents and kinesin spindle protein inhibitors

Microtubule interfering agents (MIAs) are antitumor drugs that inhibit microtubule dynamics, while kinesin spindle protein (KSP) inhibitors are substances that block the formation of the bipolar spindle during mitosis. All these compounds cause the accumulation of mitotic cells and subsequently cell death. We used two-dimensional gel electrophoresis (2DE) followed by MALDI-MS analysis to demonstrate that the MIAs vinblastine (Velban) and paclitaxel (Taxol), as well as the KSP inhibitor S-tritil-L-cysteine, induce the phosphorylation of annexin A2 in human lung carcinoma A549 cells. Further tandem mass spectrometry analysis using a combination of peptide fragmentation methods (CID and ETD) and multiple reaction monitoring (MRM) analysis determined that this modification occurs mainly at threonine 19. We show that MIAs and KSP inhibitors only induce this phosphorylation in cells capable of reaching the M phase. Furthermore, we demonstrate that CDK activity is required for the phosphorylation of annexin A2 induced by MIAs and KSP inhibitors. Finally, we have used double thymidine block synchronization to demonstrate that annexin A2 is not phosphorylated during a normal mitosis, indicating that this phosphorylation of annexin A2 is a specific response to these drugs.

Microtubule interfering agents and KSP inhibitors induce the phosphorylation of the nuclear protein p54(nrb), an event linked to G2/M arrest

Microtubule interfering agents (MIAs) are anti-tumor drugs that inhibit microtubule dynamics, while kinesin spindle protein (KSP) inhibitors are substances that block the formation of the bipolar spindle during mitosis. All these compounds cause G2/M arrest and cell death. Using 2D-PAGE followed by Nano-LC-ESI-Q-ToF analysis, we found that MIAs such as vincristine (Oncovin) or paclitaxel (Taxol) and KSP inhibitors such as S-tritil-l-cysteine induce the phosphorylation of the nuclear protein p54(nrb) in HeLa cells. Furthermore, we demonstrate that cisplatin (Platinol), an anti-tumor drug that does not cause M arrest, does not induce this modification. We show that the G2/M arrest induced by MIAs is required for p54(nrb) phosphorylation. Finally, we demonstrate that CDK activity is required for MIA-induced phosphorylation of p54(nrb).

Phosphorylation of human eukaryotic elongation factor 1Bgamma is regulated by paclitaxel

Paclitaxel (Ptx) is an antitumoural drug that inhibits microtubule dynamics, causes G2/M arrest and induces cell death. 2-D PAGE and MALDI-TOF-MS analysis of HeLa cells extracts revealed that Ptx up-regulates a form of the eukaryotic elongation factor 1Bgamma (eEF1Bgamma) and down-regulates another one. This event, linked to the lack of Ptx effect over eEF1Bgamma mRNA or protein levels suggested a PTM of this elongation factor. Further 2-D PAGE analysis followed by a phosphospecific staining with PRO-Q Diamond showed the staining of the Ptx up-regulated form only. Moreover, this Ptx up-regulated form of eEF1Bgamma disappears upon treatment with protein phosphatase. Thus, we demonstrate that human eEF1Bgamma phosphorylation is regulated by Ptx.

Characterization of HSP27 phosphorylation induced by microtubule interfering agents: Implication of p38 signalling pathway

Vincristine and paclitaxel are widely used antitumoral drugs that interfere with microtubule dynamics. We have previously demonstrated that vincristine induces phosphorylation of HSP27 at serine 82 in MCF-7 cells. In this report, we show that vincristine also causes phosphorylation of serines 78 and 15. Moreover, we demonstrate that phosphorylation of this chaperone is induced by the p38 signalling pathway while the JNK pathway is not implicated. Differences between vincristine and paclitaxel treatments are also appreciated. Thus, while vincristine induces a strong phosphorylation of the three serines, paclitaxel induces a weak phosphorylation of serine 78 and has no effect over serines 82 and 15 phosphorylation. Interestingly, pre-treatment of cells with a ten-fold excess of paclitaxel abolishes vincristine-induced phosphorylation of HSP27.

Cockroaches as carriers of fungi of medical importance

The fungal pathogenic flora of the external surface of 103 cockroaches (Periplaneta americana) collected from the intensive care unit of a hospital were investigated. In this study, a high percentage of test cockroaches (93.2%) were found to carry fungi of medical importance. The main fungi isolated were species of Candida, Aspergillus and Penicillium. Information about the carriage of pathogenic fungi by cockroaches in hospital environment is scanty. The results suggest that cockroaches can play a role in dissemination of fungi, which they can carry on their external surface.

Receptor component protein (RCP): a member of a multi-protein complex required for G-protein-coupled signal transduction

The calcitonin-gene-related peptide (CGRP) receptor component protein (RCP) is a 148-amino-acid intracellular protein that is required for G-protein-coupled signal transduction at receptors for the neuropeptide CGRP. RCP works in conjunction with two other proteins to constitute a functional CGRP receptor: calcitonin-receptor-like receptor (CRLR) and receptor-activity-modifying protein 1 (RAMP1). CRLR has the stereotypical seven-transmembrane topology of a G-protein-coupled receptor; it requires RAMP1 for trafficking to the cell surface and for ligand specificity, and requires RCP for coupling to the cellular signal transduction pathway. We have made cell lines that expressed an antisense construct of RCP and determined that CGRP-mediated signal transduction was reduced, while CGRP binding was unaffected. Furthermore, signalling at two other endogenous G-protein-coupled receptors was unaffected, suggesting that RCP was specific for a limited subset of receptors.

Halothane-induced intracellular calcium release in cholinergic cells

Low concentrations of halothane and isoflurane can release acetylcholine in an extracellular Ca(2+)-independent manner. In the present study, a cholinergic cell line (SN56) was used to examine whether release of calcium from intracellular stores occurs in the presence of halothane. Changes in intracellular calcium concentration ([Ca(2+)](i)) were measured using fluo-3, a fluorescent calcium-sensitive dye and laser scanning confocal microscopy. Halothane, at sub-anesthetic concentrations (14, 28, 40 and 56 microM), increased [Ca(2+)](i) in SN56 cells. This effect remained even when the cells were perfused with medium lacking extracellular calcium, suggesting the involvement of intracellular Ca(2+) sources. SN56 cells responded to ryanodine by increasing [Ca(2+)](i) and this effect was blocked by dantrolene, an inhibitor of Ca(2+)-release from ryanodine-sensitive stores. The effect of halothane was attenuated after the increase in [Ca(2+)](i) induced by ryanodine and it was suppressed by dantrolene, suggesting the participation of ryanodine-sensitive stores. Using cyclopiazonic acid, a Ca(2+)-ATPase inhibitor, we investigated whether the depletion of intracellular Ca(2+) stores interfered with the effect of halothane. Cyclopiazonic acid significantly decreased the increase in [Ca(2+)](i) induced by the volatile anesthetic. It is suggested that sub-anesthetic concentrations of halothane may increase [Ca(2+)](i) by releasing Ca(2+) from intracellular stores in cholinergic cells.

The role of the CGRP-receptor component protein (RCP) in adrenomedullin receptor signal transduction

G protein-coupled receptors are usually thought to act as monomer receptors that bind ligand and then interact with G proteins to initiate signal transduction. In this study we report an intracellular peripheral membrane protein named the calcitonin gene-related peptide (CGRP)-receptor component protein (RCP) required for signal transduction at the G protein-coupled receptor for adrenomedullin. Cell lines were made that expressed an antisense construct of the RCP cDNA, and in these cells diminished RCP expression correlated with loss of adrenomedullin signal transduction. In contrast, loss of RCP did not diminish receptor density or affinity, therefore RCP does not appear to act as a chaperone protein. Instead, RCP represents a novel class of protein required to couple the adrenomedullin receptor to the cellular signal transduction pathway. A candidate adrenomedullin receptor named the calcitonin receptor-like receptor (CRLR) has been described, which forms high affinity adrenomedullin receptors when co-expressed with the accessory protein receptor-activity modifying protein 2 (RAMP2). RCP co-immunoprecipitated with CRLR and RAMP2, indicating that a functional adrenomedullin receptor is composed of at least three proteins: the ligand binding protein (CRLR), an accessory protein (RAMP2), and a coupling protein for signal transduction (RCP).

Changes in Ca(2+) channel expression upon differentiation of SN56 cholinergic cells

The SN56 cell line, a fusion of septal neurons and neuroblastoma cells, has been used as a model for central cholinergic neurons. These cells show increased expression of cholinergic neurochemical features upon differentiation, but little is known about how differentiation affects their electrophysiological properties. We examined the changes in Ca(2+) channel expression that occur as these cells undergo morphological differentiation in response to serum withdrawal and exposure to dibutyryl-cAMP. Undifferentiated cells expressed a T-type current with biophysical and pharmacological properties similar, although not identical, to those reported for the current generated by the alpha(1H) (CaV3.2) Ca(2+) channel subunit. Differentiated cells expressed, in addition to this T-type current, high voltage activated currents which were inhibited 38% by the L-type channel antagonist nifedipine (5 microM), 37% by the N-type channel antagonist omega-conotoxin-GVIA (1 microM), and 15% by the P/Q-type channel antagonist omega-agatoxin-IVA (200 nM). Current resistant to these inhibitors accounted for 15% of the high voltage activated current in differentiated SN56 cells. Our data demonstrate that differentiation increases the expression of neuronal type voltage gated Ca(2+) channels in this cell line, and that the channels expressed are comparable to those reported for native basal forebrain cholinergic neurons. This cell line should thus provide a useful model system to study the relationship between calcium currents and cholinergic function and dysfunction.

Internalization of mammalian fluorescent cellular prion protein and N-terminal deletion mutants in living cells

The cellular prion protein (PrP(c)) is a glycosylphosphatidylinositol (GPI)-anchored plasma membrane protein whose conformational altered forms (PrP(sc)) are known to cause neurodegenerative diseases in mammals. In order to investigate the intracellular traffic of mammalian PrP(c) in living cells, we have generated a green fluorescent protein (GFP) tagged version of PrP(c). The recombinant protein was properly anchored at the cell surface and its distribution pattern was similar to that of the endogenous PrP(c), with labeling at the plasma membrane and in an intracellular perinuclear compartment. Comparison of the steady-state distribution of GFP-PrP(c) and two N-terminal deletion mutants (Delta32-121 and Delta32-134), that cause neurological symptoms when expressed in PrP knockout mice, was carried out. The mutant proteins accumulated in the plasma membrane at the expense of decreased labeling in the perinuclear region when compared with GFP-PrP(c). In addition, GFP-PrP(c), but not the two mutants, internalized from the plasma membrane in response to Cu2+ treatment and accumulated at a perinuclear region in SN56 cells. Our data suggest that GFP-PrP(c) can be used to follow constitutive and induced PrP(c) traffic in living cells.

Beta-scorpion toxin induces the release of gamma-[3 H]aminobutyric acid in rat brain slices

The effect of the beta-scorpion toxin, TiTX gamma on the release of [3H]GABA from rat brain cortical slices is described. The stimulatory effect of TiTX gamma on the release of [3H]GABA was dependent on incubation time and TiTX gamma concentration with an EC50 of 0.19 microM. The scorpion toxin effect was calcium dependent and was completely inhibited by tetrodotoxin. beta-Alanine also induced the release of [3H]GABA and this effect was not inhibited by tetrodotoxin but was additive in the presence of TiTX gamma. The data suggest a neuronal origin for the release of [3H]GABA by TiTX gamma.

Trafficking of green fluorescent protein tagged-vesicular acetylcholine transporter to varicosities in a cholinergic cell line

Synaptic vesicle proteins are suggested to travel from the trans-Golgi network to active zones via tubulovesicular organelles, but the participation of different populations of endosomes in trafficking remains a matter of debate. Therefore, we generated a green fluorescent protein (GFP)-tagged version of the vesicular acetylcholine transporter (VAChT) and studied the localization of VAChT in organelles in the cell body and varicosities of living cholinergic cells. GFP-VAChT is distributed to both early and recycling endosomes in the cell body and is also observed to accumulate in endocytic organelles within varicosities of SN56 cells. GFP-VAChT positive organelles in varicosities are localized close to plasma membrane and are labeled with FM4-64 and GFP-Rab5, markers of endocytic vesicles and early endosomes, respectively. A GFP-VAChT mutant lacking a dileucine endocytosis motif (leucine residues 485 and 486 changed to alanine residues) accumulated at the plasma membrane in SN56 cells. This endocytosis-defective GFP-VAChT mutant is localized primarily at the somal plasma membrane and exhibits reduced neuritic targeting. Furthermore, the VAChT mutant did not accumulate in varicosities, as did VAChT. Our data suggest that clathrin-mediated internalization of VAChT to endosomes at the cell body might be involved in proper sorting and trafficking of VAChT to varicosities. We conclude that genesis of competent cholinergic secretory vesicles depends on multiple interactions of VAChT with endocytic proteins.

[Building the meaning of disease recurrence: experience of women with breast cancer]

Regarding recurrence as a possibility in the lives of women with breast cancer, this work aimed at identifying how they construct the meaning of such possibility from their own existence. The study was based on the conceptions of symbolic interactionism and the content analysis of interviews with 12 mastectomized women was used. The possibility of recurrence was present in the lives of the women under study. It was configured by the uncertainty that they experienced and represented in the thematic units identified in their accounts when they stated to be "living with a stigmatizing disease", "experiencing a negative feeling" and "facing prejudice".

Effects of a Lasiodora spider venom on Ca2+ and Na+ channels

The venom of a Brazilian spider, Lasiodora sp (Mygalomorphae, Theraphosidae), was screened for activity against ion channels using Ca2+ imaging and whole-cell patch clamp in GH3 cells. When tetrodotoxin (TTX) was present to block Na+ channels, the venom abolished the Ca2+ oscillations that are normally present in these cells and reduced the basal level of intracellular Ca2+. Under patch clamp, the venom reduced the L-type Ca2+ channel conductance and caused a positive shift in its voltage dependence of activation. In addition to these effects, when applied without TTX, the venom also caused a slow and noisy increase in intracellular Ca2+. The sensitivity of this second effect to TTX suggested an effect on Na+ channels, which was tested using patch clamp. Control Na+ currents inactivated completely as a single exponential. Treatment with the venom did not affect the amplitude of I(Na), but caused it to divide in two slower exponential components plus a sustained component, all of which were suppressed by TTX. The venom also caused a negative shift in the voltage dependence of activation and steady-state inactivation of I(Na). The observed effects of this venom on whole-cell currents explain the changes it causes in intracellular Ca2+ in GH3 cells and demonstrate that the venom of this spider is a source of toxins active against ion channels.

Spider neurotoxins block the beta scorpion toxin-induced calcium uptake in rat brain cortical synaptosomes

In this paper we describe the effects of the beta scorpion toxin Tityus gamma (TiTX gamma) and spider neurotoxins Tx3-3 and Tx3-4 in the (45)Ca(2+) uptake in synaptosomes. The TiTX gamma-stimulatory effect on (45)Ca(2+) uptake in synaptosomes was inhibited omega-Conotoxin MVIIC (omega-CgTX MVIIC) (0.1 microM) and omega-Agatoxin IVA (0.1 microM) by 70% and 41%, respectively. omega-CgTX MVIIC (1.0 microM) almost completely blocked the TiTX gamma-induced (45)Ca(2+) uptake in synaptosomes. Verapamil (1.0 microM) and omega-Conotoxin GVIA (0.1 microM) had no effect in the scorpion toxin-induced (45)Ca(2+) influx. The spider neurotoxins Tx3-3 and Tx3-4 inhibited the TiTX gamma-induced calcium uptake with an IC(50) of 10.0 and 30.0 nM, respectively. It is suggested that spider neurotoxins Tx3-3 and Tx3-4 blocking effect in the TiTX gamma-induced calcium uptake involves P/Q-type calcium channels.

Calcium channels coupled to depolarization-evoked glutamate release in the myenteric plexus of guinea-pig ileum

Glutamate is the major excitatory neurotransmitter in the CNS. The recent characterization of glutamate as a neurotransmitter in the enteric nervous system opened a new line of investigation concerning the role of glutamate in that system. The present study aimed to further characterize the enteric glutamate release and the calcium channels coupled to it. For this study the myenteric plexus-longitudinal muscle of guinea-pig ileum was stimulated with potassium chloride or with electrical pulses. The released glutamate was detected by spectrofluorimetry. Laser scanning confocal microscopy was used for analysis of immunolabeled enteric tissue for co-localization studies of calcium channels (N- and P/Q-type) and glutamate transporters (EAAC1). Here we report the effects of known Ca(2+)-channel blockers on glutamate release evoked by KCl-depolarization or electrical stimulation in the myenteric plexus. We find that N-type Ca(2+) channels control a major portion of evoked glutamate release from this system, with a very small contribution from L-type Ca(2+) channels. Moreover, alpha(1A)-like (P-type Ca(2+) channel) and alpha(1B)-like (N-type Ca(2+ )channel) immunoreactivity co-localized with glutamate transporters in the myenteric plexus. In addition, KCl-evoked or electrically stimulated glutamate release was sensitive to omega-agatoxin IVA, in a frequency-dependent manner, suggesting that P-type channels are also coupled to the release of glutamate. We, thus, conclude that both N-type and P-type Ca(2+) channels control most of the evoked glutamate release from the enteric nervous system, as also occurs in some parts of the CNS.

Molecular cloning of cDNAs encoding insecticidal neurotoxic peptides from the spider Phoneutria nigriventer

From a Phoneutria nigriventer venom gland cDNA library several clones coding for the insect specific neurotoxin Tx4(6-1) were isolated. cDNA analysis showed that the encoded protein contained three distinct segments, comprising a signal sequence of 16 amino acids, followed by a glutamate-rich sequence of 18 amino acids and, finally, the coding region for the mature toxin. The deduced amino acid sequence for the mature polypeptide was identical to the protein sequence determined chemically. In addition, two new putative toxins called Pn4A and Pn4B were characterized and their predicted complete amino acid sequence revealed approximately 78% similarity to Tx4(6-1).

Inhibition of glutamate uptake by Tx3-4 is dependent on the redox state of cysteine residues

Glutamate transporters are essential for the homeostasis of glutamate and normal function of glutamatergic synapses. Their function was shown to be regulated by redox agents and dimerizations that involves redox changes of cysteine residues. Peptide neurotoxins are also known to be rich in cysteine residues that contribute to their activity and stability. Among them is the toxin Tx3-4, from the spider Phoneutria nigriventer, which is able to inhibit glutamate uptake in rat hippocampal synaptosomes. Based on results obtained with manipulation of the redox state of cysteine residues in synaptosomes and in Tx3-4, we suggest that the effect of this toxin on glutamate uptake is due to interactions that involve cysteines both in the toxin and in the transporters.

The effect of isoflurane on the release of [(3)H]-acetylcholine from rat brain cortical slices

Volatile general anaesthetics are believed to affect synaptic transmission, but their actions in the central nervous system (CNS) remains unclear. Acetylcholine (ACh) is one of the most important neurotransmitter in the CNS and thus, it is possible that its release could be one of the targets for volatile anaesthetic action. However, the effects of these agents on the release of ACh are not yet fully understood. Rat brain cortical slices were loaded with [(3)H]-choline in order to study the effect of isoflurane on the release of [(3)H]-ACh from this preparation. Isoflurane (28, 43, 54, 95 and 182 nM) significantly increased the basal release of [(3)H]-ACh. This effect was independent of the extracellular sodium and calcium concentration but was decreased by tetracaine and dantrolene, inhibitors of Ca(2+-)release from intracellular stores. These findings indicate that isoflurane may cause a Ca(2+-)release from internal stores that increases [(3)H]-ACh release in rat brain cortical slices.

Visualization and trafficking of the vesicular acetylcholine transporter in living cholinergic cells

The present experiments investigated the trafficking of the vesicular acetylcholine transporter (VAChT) tagged with the enhanced green fluorescent protein (EGFP) in living cholinergic cells (SN56). The EGFP-VAChT chimera was located in endosomal-like compartments in the soma of SN56 cells, and it was also targeted to varicosities of neurites. In contrast, EGFP alone in cells was soluble in the cytoplasm. The C-terminal cytoplasmic tail of VAChT has been implicated in targeting of VAChT to synaptic vesicles; thus, we have examined the role of the C-terminal region in the trafficking to varicosities. A C-terminal fragment tagged with EGFP appeared to be selectively accumulated in varicosities when expressed in SN56 cells. Interestingly, the protein was not freely soluble in the cytosol, and it presented a punctate pattern of expression. However, EGFP-C terminus did not present this peculiar pattern of expression in a nonneuronal cell line (HEK 293). Moreover, the C-terminal region of VAChT did not seem to be essential for VAChT trafficking, as a construct that lacks the C-terminal tail was, similar to EGFP-VAChT, partially targeted to endocytic organelles in the soma and sorted to varicosities. These experiments visualize VAChT for the first time in living cells and suggest that there might be multiple signals that participate in trafficking of VAChT to sites of synaptic vesicle accumulation.

Muscarinic regulation of Ca2+ oscillation frequency in GH3 cells

The GH3 anterior pituitary cell line has been used as a model to investigate diverse aspects of pituitary cell physiology including Ca2+ homeostasis and secretion. These cells possess muscarinic receptors which, by activating K+ channels and inhibiting Ca2+ channels, should decrease electrical excitability. We measured the effect of carbachol (10 microM) on the frequency of Ca2+ oscillations caused by Ca2+ action potentials in the plasma membrane. Carbachol reduced oscillation frequency by approximately 85% (p < 0.001). This inhibition was reversed by atropine (1 microM), and was prevented by pre-incubation with pertussis toxin (200 ng/ml, 24 h). Since many anterior pituitary cell types secrete acetylcholine, the presence of muscarinic receptors coupled to cell excitability in these cells suggest that ACh could exert a paracrine- or autocrine-like action in GH3 cell cultures. In experiments designed to test this idea, perfusion with 1 microM atropine caused a small but significant increase (p < 0.05) in oscillation frequency when the cells had previously been incubated for 30 min without perfusion. However, this effect was not blocked by either pre-treatment with pertussis toxin or by including atropine during the entire experiment (including the 30-min incubation without perfusion). We conclude that these cells respond to muscarinic agonists by decreasing oscillation frequency but find no evidence for feedback control by endogenous ACh under these conditions.

Expression of the vesicular acetylcholine transporter, proteins involved in exocytosis, and functional calcium signaling in varicosities and soma of a murine septal cell line

The expression and localization of the vesicular acetylcholine transporter in a septal cell line, SN56, were investigated. Immunoprecipitation and immunoblot analysis of postnuclear supernatants indicated that this cell line expresses reasonable amounts of the transporter. Immunofluorescence and confocal microscopy experiments showed that the vesicular transporter is present in varicosities and also in the cell body of differentiated cells. Varicosities have the potential to be functional sites of transmitter release because they responded to depolarization with calcium influx through voltage-gated calcium channels and expressed the synaptic proteins synaptotagmin, SV2, synaptophysin, and a subunit of P/Q calcium channels. In the soma of SN56 cells, the transporter immunoreactivity was similar to that for synaptotagmin, and it colocalized with synaptophysin, but it did not colocalize with SV2. Labeling for SV2 appeared prominently in a defined perinuclear structure, whereas the two former proteins were widely distributed in the soma, where several endocytic compartments could be identified with the vital dye FM4-64. These data suggest that distinct synaptic vesicle proteins exist in different subcellular compartments, and consequently they may follow distinct pathways in neurites before reaching sites of transmitter storage and release in SN56 cells.

Inhibition of glutamate uptake by a polypeptide toxin (phoneutriatoxin 3-4) from the spider Phoneutria nigriventer

Glutamate concentration increases significantly in the extracellular compartment during brain ischaemia and anoxia. This increase has an important Ca(2+)-independent component, which is due in part to the reversal of glutamate transporters of the plasma membrane of neurons and glia. The toxin phoneutriatoxin 3-4 (Tx3-4) from the spider Phoneutria nigriventer has been reported to decrease the evoked glutamate release from synaptosomes by inhibiting Ca(2+) entry via voltage-dependent Ca(2+) channels. However, we report here that Tx3-4 is also able to inhibit the uptake of glutamate by synaptosomes in a time-dependent manner and that this inhibition in turn leads to a decrease in the Ca(2+)-independent release of glutamate. No other polypeptide toxin so far described has this effect. Our results suggest that Tx3-4 can be a valuable tool in the investigation of function and dysfunction of glutamatergic neurotransmission in diseases such as ischaemia.