LRRK2 is required for CD38-mediated NAADP-Ca2+ signaling and the downstream activation of TFEB (transcription factor EB) in immune cells.

CD38 is a cell surface receptor capable of generating calcium-mobilizing second messengers. It has been implicated in host defense and cancer biology, but signaling mechanisms downstream of CD38 remain unclear. Mutations in LRRK2 (leucine-rich repeat kinase 2) are the most common genetic cause of Parkinson disease; it is also a risk factor for Crohn disease, leprosy, and certain types of cancers. The pathogenesis of these diseases involves inflammation and macroautophagy/autophagy, processes both CD38 and LRRK2 are implicated in. Here, we mechanistically and functionally link CD38 and LRRK2 as upstream activators of TFEB (transcription factor EB), a host defense transcription factor and the master transcriptional regulator of the autophagy/lysosome machinery. In B-lymphocytes and macrophages, we show that CD38 and LRRK2 exist in a complex on the plasma membrane. Ligation of CD38 with the monoclonal antibody clone 90 results in internalization of the CD38-LRRK2 complex and its targeting to the endolysosomal system. This generates an NAADP-dependent calcium signal, which requires LRRK2 kinase activity, and results in the downstream activation of TFEB. lrrk2 KO macrophages accordingly have TFEB activation defects following CD38 or LPS stimulation and fail to switch to glycolytic metabolism after LPS treatment. In overexpression models, the pathogenic LRRK2G2019S mutant promotes hyperactivation of TFEB even in the absence of CD38, both by stabilizing TFEB and promoting its nuclear translocation via aberrant calcium signaling. In sum, we have identified a physiological CD38-LRRK2-TFEB signaling axis in immune cells. The common pathogenic mutant, LRRK2G2019S, appears to hijack this pathway.Abbreviations:ADPR: ADP-ribose; AMPK: AMP-activated protein kinase; BMDM: bone marrow-derived macrophage; cADPR: cyclic-ADP-ribose; COR: C-terminal of ROC; CTSD: cathepsin D; ECAR: extracellular acidification rate; EDTA: ethylenediaminetetraacetic acid; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GFP: green fluorescent protein; GPN: Gly-Phe ß-naphthylamide; GSK3B/GSK3ß: glycogen synthase kinase 3 beta; GTP: guanosine triphosphate; KD: knockdown; LAMP1: lysosomal-associated membrane protein 1; LRR: leucine rich repeat; LRRK2: leucine rich repeat kinase 2; mAb: monoclonal antibody; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; MAPK/ERK: mitogen-activated protein kinase; MCOLN1: mucolipin 1; MFI: mean fluorescence intensity; mRNA: messenger RNA; MTOR: mechanistic target of rapamycin kinase; NAADP: nicotinic acid adenine dinucleotide phosphate; NAD: nicotinamide adenine dinucleotide; NADP: nicotinamide adenine dinucleotide phosphate; PD: Parkinson disease; PPP3CB: protein phosphatase 3, catalytic subunit, beta isoform; q-RT-PCR: quantitative reverse transcription polymerase chain reaction; ROC: Ras of complex; siRNA: small interfering RNA; SQSTM1/p62: sequestome 1; TFEB: transcription factor EB; TPCN: two pore channel; TRPM2: transient receptor potential cation channel, subfamily M, member 2; ZKSCAN3: zinc finger with KRAB and SCAN domains 3.

AKT Regulates NLRP3 Inflammasome Activation by Phosphorylating NLRP3 Serine 5.

The cytosolic pattern recognition receptor NLRP3 senses host-derived danger signals and certain microbe-derived products in both humans and rodents. NLRP3 activation assembles an inflammasome complex that contains the adapter proteins ASC and caspase-1, whose activation triggers the maturation and release of the proinflammatory cytokines IL-1ß and IL-18. S5 phosphorylation of NLRP3 prevents its oligomerization and activation, whereas dephosphorylation of this residue by the phosphatase PP2A allows NLRP3 activation. However, the protein kinase that mediates NLRP3 S5 phosphorylation is unknown. In this study, we show that AKT associates with NLRP3 and phosphorylates it on S5, limiting NLRP3 oligomerization. This phosphorylation event also stabilizes NLRP3 by reducing its ubiquitination on lysine 496, which inhibits its proteasome-mediated degradation by the E3 ligase Trim31. Pharmacologic manipulation of AKT kinase activity reciprocally modulates NLRP3 inflammasome-mediated IL-1ß production. Inhibition of AKT reduced IL-1ß production following the i.p. injection of LPS into mice. We propose that AKT, Trim31, and PP2A together modulate NLRP3 protein levels and the tendency to oligomerize, thereby setting a tightly regulated threshold for NLRP3 activation.

Bcl-2 regulates pyroptosis and necroptosis by targeting BH3-like domains in GSDMD and MLKL.

Apoptosis is a form of programmed cell death in multicellular organisms. Bcl-2 prevents apoptosis and promotes cellular survival by neutralizing BH3 domain-containing proteins, which directly activate the pore-forming proteins BAX and BAK. However, Bcl-2 is not known to regulate other cell death effectors such as gasdermin D (GSDMD) or mixed lineage kinase domain-like (MLKL), whose activation causes pyroptosis and necroptosis, respectively. Here, we identify a BH3-like domain in both GSDMD and MLKL that mediates an interaction with B-cell lymphoma 2 (Bcl-2). The presence of Bcl-2 reduced GSDMD cleavage at D275 by caspase-1, 4 or 5, and enhanced the GSDMD cleavage at D87. The GSDMD D87 cleavage inactivates the pyroptotic execution program. The presence of Bcl-2 also limited RIP3 mediated phosphorylation of MLKL, which reduced MLKL oligomerization and tempered the induction of necroptosis. Our observations suggest that the presence of Bcl-2 limits the induction of three forms of cell death apoptosis, pyroptosis, and necroptosis.

SARS-Coronavirus Open Reading Frame-8b triggers intracellular stress pathways and activates NLRP3 inflammasomes.

The SARS (severe acute respiratory syndrome) outbreak was caused by a coronavirus (CoV) named the SARS-CoV. SARS pathology is propagated both by direct cytotoxic effects of the virus and aberrant activation of the innate immune response. Here, we identify several mechanisms by which a SARS-CoV open reading frame (ORF) activates intracellular stress pathways and targets the innate immune response. We show that ORF8b forms insoluble intracellular aggregates dependent on a valine at residue 77. Aggregated ORF8b induces endoplasmic reticulum (ER) stress, lysosomal damage, and subsequent activation of the master regulator of the autophagy and lysosome machinery, Transcription factor EB (TFEB). ORF8b causes cell death in epithelial cells, which is partially rescued by reducing its ability to aggregate. In macrophages, ORF8b robustly activates the NLRP3 inflammasome by providing a potent signal 2 required for activation. Mechanistically, ORF8b interacts directly with the Leucine Rich Repeat domain of NLRP3 and localizes with NLRP3 and ASC in cytosolic dot-like structures. ORF8b triggers cell death consistent with pyroptotic cell death in macrophages. While in those cells lacking NLRP3 accumulating ORF8b cytosolic aggregates cause ER stress, mitochondrial dysfunction, and caspase-independent cell death.

SARS-Coronavirus Open Reading Frame-3a drives multimodal necrotic cell death.

The molecular mechanisms underlying the severe lung pathology that occurs during SARS-CoV infections remain incompletely understood. The largest of the SARS-CoV accessory protein open reading frames (SARS 3a) oligomerizes, dynamically inserting into late endosomal, lysosomal, and trans-Golgi-network membranes. While previously implicated in a non-inflammatory apoptotic cell death pathway, here we extend the range of SARS 3a pathophysiologic targets by examining its effects on necrotic cell death pathways. We show that SARS 3a interacts with Receptor Interacting Protein 3 (Rip3), which augments the oligomerization of SARS 3a helping drive necrotic cell death. In addition, by inserting into lysosomal membranes SARS 3a triggers lysosomal damage and dysfunction. Consequently, Transcription Factor EB (TFEB) translocates to the nucleus increasing the transcription of autophagy- and lysosome-related genes. Finally, SARS 3a activates caspase-1 either directly or via an enhanced potassium efflux, which triggers NLRP3 inflammasome assembly. In summary, Rip3-mediated oligomerization of SARS 3a causes necrotic cell death, lysosomal damage, and caspase-1 activation-all likely contributing to the clinical manifestations of SARS-CoV infection.

Cytochrome c Negatively Regulates NLRP3 Inflammasomes.

The release of cytochrome c from the inner mitochondrial membrane, where it is anchored by caridolipin, triggers the formation of the Apaf-1 apoptosome. Cardiolipin also interacts with NLRP3 recruiting NLRP3 to mitochondria and facilitating inflammasome assembly. In this study we investigated whether cytosolic cytochrome c impacts NLRP3 inflammasome activation in macrophages. We report that cytochrome c binds to the LRR domain of NLRP3 and that cytochrome c reduces the interactions between NLRP3 and cardiolipin and between NLRP3 and NEK7, a recently recognized component of the NLRP3 inflammasome needed for NLRP3 oligomerization. Protein transduction of cytochrome c impairs NLRP3 inflammasome activation, while partially silencing cytochrome c expression enhances it. The addition of cytochrome c to an in vitro inflammasome assay severely limited caspase-1 activation. We propose that there is a crosstalk between the NLRP3 inflammasome and apoptosome pathways mediated by cytochrome c, whose release during apoptosis acts to limit NLRP3 inflammasome activation.

Activator of G-Protein Signaling 3-Induced Lysosomal Biogenesis Limits Macrophage Intracellular Bacterial Infection.

Many intracellular pathogens cause disease by subverting macrophage innate immune defense mechanisms. Intracellular pathogens actively avoid delivery to or directly target lysosomes, the major intracellular degradative organelle. In this article, we demonstrate that activator of G-protein signaling 3 (AGS3), an LPS-inducible protein in macrophages, affects both lysosomal biogenesis and activity. AGS3 binds the Gi family of G proteins via its G-protein regulatory (GoLoco) motif, stabilizing the Gα subunit in its GDP-bound conformation. Elevated AGS3 levels in macrophages limited the activity of the mammalian target of rapamycin pathway, a sensor of cellular nutritional status. This triggered the nuclear translocation of transcription factor EB, a known activator of lysosomal gene transcription. In contrast, AGS3-deficient macrophages had increased mammalian target of rapamycin activity, reduced transcription factor EB activity, and a lower lysosomal mass. High levels of AGS3 in macrophages enhanced their resistance to infection by Burkholderia cenocepacia J2315, Mycobacterium tuberculosis, and methicillin-resistant Staphylococcus aureus, whereas AGS3-deficient macrophages were more susceptible. We conclude that LPS priming increases AGS3 levels, which enhances lysosomal function and increases the capacity of macrophages to eliminate intracellular pathogens.

Canonical and noncanonical g-protein signaling helps coordinate actin dynamics to promote macrophage phagocytosis of zymosan.

Both chemotaxis and phagocytosis depend upon actin-driven cell protrusions and cell membrane remodeling. While chemoattractant receptors rely upon canonical G-protein signaling to activate downstream effectors, whether such signaling pathways affect phagocytosis is contentious. Here, we report that Gαi nucleotide exchange and signaling helps macrophages coordinate the recognition, capture, and engulfment of zymosan bioparticles. We show that zymosan exposure recruits F-actin, Gαi proteins, and Elmo1 to phagocytic cups and early phagosomes. Zymosan triggered an increase in intracellular Ca(2+) that was partially sensitive to Gαi nucleotide exchange inhibition and expression of GTP-bound Gαi recruited Elmo1 to the plasma membrane. Reducing GDP-Gαi nucleotide exchange, decreasing Gαi expression, pharmacologically interrupting Gßγ signaling, or reducing Elmo1 expression all impaired phagocytosis, while favoring the duration that Gαi remained GTP bound promoted it. Our studies demonstrate that targeting heterotrimeric G-protein signaling offers opportunities to enhance or retard macrophage engulfment of phagocytic targets such as zymosan.

SARS-coronavirus open reading frame-9b suppresses innate immunity by targeting mitochondria and the MAVS/TRAF3/TRAF6 signalosome.

Coronaviruses (CoV) have recently emerged as potentially serious pathogens that can cause significant human morbidity and death. The severe acute respiratory syndrome (SARS)-CoV was identified as the etiologic agent of the 2002-2003 international SARS outbreak. Yet, how SARS evades innate immune responses to cause human disease remains poorly understood. In this study, we show that a protein encoded by SARS-CoV designated as open reading frame-9b (ORF-9b) localizes to mitochondria and causes mitochondrial elongation by triggering ubiquitination and proteasomal degradation of dynamin-like protein 1, a host protein involved in mitochondrial fission. Also, acting on mitochondria, ORF-9b targets the mitochondrial-associated adaptor molecule MAVS signalosome by usurping PCBP2 and the HECT domain E3 ligase AIP4 to trigger the degradation of MAVS, TRAF3, and TRAF 6. This severely limits host cell IFN responses. Reducing either PCBP2 or AIP4 expression substantially reversed the ORF-9b-mediated reduction of MAVS and the suppression of antiviral transcriptional responses. Finally, transient ORF-9b expression led to a strong induction of autophagy in cells. The induction of autophagy depended upon ATG5, a critical autophagy regulator, but the inhibition of MAVS signaling did not. These results indicate that SARS-CoV ORF-9b manipulates host cell mitochondria and mitochondrial function to help evade host innate immunity. This study has uncovered an important clue to the pathogenesis of SARS-CoV infection and illustrates the havoc that a small ORF can cause in cells.

Normal autophagic activity in macrophages from mice lacking Gαi3, AGS3, or RGS19.

In macrophages autophagy assists antigen presentation, affects cytokine release, and promotes intracellular pathogen elimination. In some cells autophagy is modulated by a signaling pathway that employs Gαi3, Activator of G-protein Signaling-3 (AGS3/GPSM1), and Regulator of G-protein Signaling 19 (RGS19). As macrophages express each of these proteins, we tested their importance in regulating macrophage autophagy. We assessed LC3 processing and the formation of LC3 puncta in bone marrow derived macrophages prepared from wild type, Gnai3(-/-), Gpsm1(-/-), or Rgs19(-/-) mice following amino acid starvation or Nigericin treatment. In addition, we evaluated rapamycin-induced autophagic proteolysis rates by long-lived protein degradation assays and anti-autophagic action after rapamycin induction in wild type, Gnai3(-/-), and Gpsm1(-/-) macrophages. In similar assays we compared macrophages treated or not with pertussis toxin, an inhibitor of GPCR (G-protein couple receptor) triggered Gαi nucleotide exchange. Despite previous findings, the level of basal autophagy, autophagic induction, autophagic flux, autophagic degradation and the anti-autophagic action in macrophages that lacked Gαi3, AGS3, or RGS19; or had been treated with pertussis toxin, were similar to controls. These results indicate that while Gαi signaling may impact autophagy in some cell types it does not in macrophages.

Regulator of G-protein signaling 3 isoform 1 (PDZ-RGS3) enhances canonical Wnt signaling and promotes epithelial mesenchymal transition.

The Wnt ß-catenin pathway controls numerous cellular processes including cell differentiation and cell-fate decisions. Wnt ligands engage Frizzled receptors and the low-density-lipoprotein-related protein 5/6 (LRP5/6) receptor complex leading to the recruitment of Dishevelled (Dvl) and Axin1 to the plasma membrane. Axin1 has a regulator of G-protein signaling (RGS) domain that binds adenomatous polyposis coli and Gα subunits, thereby providing a mechanism by which Gα subunits can affect ß-catenin levels. Here we show that Wnt signaling enhances the expression of another RGS domain-containing protein, PDZ-RGS3. Reducing PDZ-RGS3 levels impaired Wnt3a-induced activation of the canonical pathway. PDZ-RGS3 bound GSK3ß and decreased its catalytic activity toward ß-catenin. PDZ-RGS3 overexpression enhanced Snail1 and led to morphological and biochemical changes reminiscent of epithelial mesenchymal transition (EMT). These results indicate that PDZ-RGS3 can enhance signals generated by the Wnt canonical pathway and that plays a pivotal role in EMT.

Activation of autophagy by inflammatory signals limits IL-1ß production by targeting ubiquitinated inflammasomes for destruction.

Autophagosomes delivers cytoplasmic constituents to lysosomes for degradation, whereas inflammasomes are molecular platforms activated by infection or stress that regulate the activity of caspase-1 and the maturation of interleukin 1ß (IL-1ß) and IL-18. Here we show that the induction of AIM2 or NLRP3 inflammasomes in macrophages triggered activation of the G protein RalB and autophagosome formation. The induction of autophagy did not depend on the adaptor ASC or capase-1 but was dependent on the presence of the inflammasome sensor. Blocking autophagy potentiated inflammasome activity, whereas stimulating autophagy limited it. Assembled inflammasomes underwent ubiquitination and recruited the autophagic adaptor p62, which assisted their delivery to autophagosomes. Our data indicate that autophagy accompanies inflammasome activation to temper inflammation by eliminating active inflammasomes.

Traf6 and A20 differentially regulate TLR4-induced autophagy by affecting the ubiquitination of Beclin 1.

Toll-like receptor 4 (TLR4) signaling triggers autophagy, which has been linked to both adaptive and innate immunity. Engagement of TLR4 recruits to the receptor complex Beclin 1, a key component of a class III phosphatidylinositol 3-kinase complex (PI3KC3) that initiates autophagosome formation. Recently, we found that tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6)-mediates Lys(63) (K63)- linked ubiquitination of Beclin 1 is crucial for TLR4-triggered autophagy in macrophages. We identified two TRAF6-binding motifs in Beclin 1 that facilitate the binding of TRAF6 and the ubiquitination of Beclin 1. A lysine located in the Bcl-2 homology 3 (BH3) domain of Beclin 1 serves as a major site for K63-linked ubiquitination. Opposing TRAF6, the deubiquitinating enzyme A20 reduces the extent of K63-linked ubiquitination of Beclin 1 and limits the induction of autophagy in response to TLR4 signaling. Furthermore, treatment of macrophages with either interferonγ or interleukin-1 triggers the K63-linked ubiquitination of Beclin 1 and the formation of autophagosomes. These results indicate that the status of K63-linked ubiquitination of Beclin 1 plays a key role in regulating autophagy during inflammatory responses.

TRAF6 and A20 regulate lysine 63-linked ubiquitination of Beclin-1 to control TLR4-induced autophagy.

Autophagy delivers cytoplasmic constituents to autophagolysosomes and is linked to both innate and adaptive immunity. Toll-like receptor 4 (TLR4) signaling induces autophagy and recruits Beclin-1, the mammalian homolog of yeast Atg6, to the receptor complex. We found that tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6)-mediated, Lys(63) (K63)-linked ubiquitination of Beclin-1 is critical for TLR4-triggered autophagy in macrophages. Two TRAF6-binding motifs in Beclin-1 facilitated the binding of TRAF6 and the ubiquitination of Beclin-1. Lys(117), which is strategically located in the Bcl-2 homology 3 (BH3) domain of Beclin-1, was a major site for K63-linked ubiquitination. The deubiquitinating enzyme A20 reduced the extent of K63-linked ubiquitination of Beclin-1 and limited the induction of autophagy in response to TLR signaling. Treatment of macrophages with either interferon-gamma or interleukin-1 also triggered the K63-linked ubiquitination of Beclin-1 and the formation of autophagosomes. These results indicate that the status of K63-linked ubiquitination of Beclin-1 plays a key role in regulating autophagy during inflammatory responses.

MyD88 and Trif target Beclin 1 to trigger autophagy in macrophages.

The Toll-like receptors (TLR) play an instructive role in innate and adaptive immunity by recognizing specific molecular patterns from pathogens. Autophagy removes intracellular pathogens and participates in antigen presentation. Here, we demonstrate that not only TLR4, but also other TLR family members induce autophagy in macrophages, which is inhibited by MyD88, Trif, or Beclin 1 shRNA expression. MyD88 and Trif co-immunoprecipitate with Beclin 1, a key factor in autophagosome formation. TLR signaling enhances the interaction of MyD88 and Trif with Beclin 1, and reduces the binding of Beclin 1 to Bcl-2. These findings indicate TLR signaling via its adaptor proteins reduces the binding of Beclin 1 to Bcl-2 by recruiting Beclin 1 into the TLR-signaling complex leading to autophagy.

The mitogen-activated protein kinase kinase kinase kinase GCKR positively regulates canonical and noncanonical Wnt signaling in B lymphocytes.

Wnt ligands bind receptors of the Frizzled (Fz) family to control cell fate, proliferation, and polarity. Canonical Wnt/Fz signaling stabilizes beta-catenin by inactivating GSK3beta, leading to the translocation of beta-catenin to the nucleus and the activation of Wnt target genes. Noncanonical Wnt/Fz signaling activates RhoA and Rac, and the latter triggers the activation of c-Jun N-terminal kinase (JNK). Here, we show that exposure of B-lymphocytes to Wnt3a-conditioned media activates JNK and raises cytosolic beta-catenin levels. Both the Rac guanine nucleotide exchange factor Asef and the mitogen-activated protein kinase kinase kinase kinase germinal center kinase-related enzyme (GCKR) are required for Wnt-mediated JNK activation in B cells. In addition, we show that GCKR positively affects the beta-catenin pathway in B cells. Reduction of GCKR expression inhibits Wnt3a-induced phosphorylation of GSK3beta at serine 9 and decreases the accumulation of cytosolic beta-catenin. Furthermore, Wnt signaling induces an interaction between GCKR and GSK3beta. Our findings demonstrate that GCKR facilitates both canonical and noncanonical Wnt signaling in B lymphocytes.

Rgs1 and Gnai2 regulate the entrance of B lymphocytes into lymph nodes and B cell motility within lymph node follicles.

Signaling by G protein-coupled receptors coupled to Galpha(i) assists in triggering lymphocyte movement into and out of lymph nodes. Here, we show that modulating the signaling output from these receptors dramatically alters B cell trafficking. Intravital microscopy of adoptively transferred B cells from wild-type and Rgs1-/- mice revealed that Rgs1-/- B cells stick better to lymph node high endothelial venules, home better to lymph nodes, and move more rapidly within lymph node follicles than do wild-type B cells. In contrast, B cells from Gnai2-/- mice enter lymph nodes poorly and move more slowly than do wild-type B cells. The Gnai2-/- mice often lack multiple peripheral lymph nodes, and their B cells respond poorly to chemokines, indicating that Galpha(i1) and Galpha(i3) poorly compensate for the loss of Galpha(i2). These results demonstrate opposing roles for Rgs1 and Gnai2 in B cell trafficking into and within lymph nodes.

Pyk2 amplifies epidermal growth factor and c-Src-induced Stat3 activation.

Signal transducers and activators of transcription factors (STATs) mediate many of the cellular responses that occur following cytokine, growth factor, and hormone signaling. STATs are activated by tyrosine and serine phosphorylation, which normally occurs as a tightly regulated process. Dysregulated STAT activity may facilitate oncogenesis, as constitutively activated STATs have been found in many human tumors as well as in v-abl- and v-src-transformed cell lines. Pyk2 is a member of the focal adhesion kinase family and can be activated by c-Src, epidermal growth factor receptor (EGFR), Janus kinase 1, tyrosine kinases, and G-protein-coupled receptor signaling. Although Pyk2 has been implicated in Janus kinase-dependent activation of MAPK and Stat1, no role for Pyk2 in the activation of other STAT proteins has been ascribed. Here, we provide evidence that Pyk2, along with c-Src, facilitates EGFR-mediated Stat3 activation. Pyk2 expression in HeLa cells induces Stat3 reporter gene activation and Stat3 phosphorylation on amino acid residues Tyr-705 and Ser-727. Together Pyk2 and c-Src potently activate Stat3, and Pyk2 enhances Stat3-induced cell proliferation. Moreover, the expression of a dominant negative version of Pyk2 impairs c-Src-induced Stat3 activation and cell proliferation. The treatment of A431 cells with EGF results in the recruitment of c-Src, Pyk2, and Stat3 to the EGFR and the phosphorylation of c-Src, Pyk2, and Stat3. Expression of constructs for dominant negative forms of either Pyk2 or c-Src impair EGF-induced Stat3 phosphorylation. These results indicate that Pyk2 facilitates EGFR- and c-Src-mediated Stat3 activation, thereby implicating Pyk2 activation as a potential co-mediator in triggering Stat3-induced oncogenesis.

Tumor necrosis factor (TNF)-induced germinal center kinase-related (GCKR) and stress-activated protein kinase (SAPK) activation depends upon the E2/E3 complex Ubc13-Uev1A/TNF receptor-associated factor 2 (TRAF2).

Tumor necrosis factor (TNF)-induced activation of apoptosis signal-regulating kinase 1 (ASK1) and germinal center kinases (GCKs) and the subsequent activation of stress-activated protein kinases (SAPKs and c-Jun NH(2)-terminal kinases) requires TNF receptor-associated factor 2 (TRAF2). Although the TRAF2 TRAF domain binds ASK1, GCK, and the highly related kinase GCKR, the RING finger domain is needed for their activation. Here, we report that TNF activates GCKR and the SAPK pathway in a manner that depends upon TRAF2 and Ubc13, a member along with Uev1A of a dimeric ubiquitin-conjugating enzyme complex. Interference with Ubc13 function or expression inhibits both TNF- and TRAF2-mediated GCKR and SAPK activation, but has a minimal effect on ASK1 activation. TNF signaling leads to TRAF2 polyubiquitination and oligomerization and to the oligomerization, ubiquitination, and activation of GCKR, all of which are sensitive to the disruption of Ubc13 function. These results indicate that the assembly of a TRAF2 lysine 63-linked polyubiquitin chain by Ubc13/Uev1A is required for TNF-mediated GCKR and SAPK activation, but may not be required for ASK1 activation.

Additional 5' exons in the RGS3 locus generate multiple mRNA transcripts, one of which accounts for the origin of human PDZ-RGS3.

Regulators of G-protein signaling (RGS) proteins can be broadly divided into those that consist predominantly of an RGS domain and those that possess an RGS domain along with additional domains. RGS3 fits into both categories, as both short and longer forms exist. Recently, a novel form of mouse RGS3 that possesses a PDZ domain was identified. Here we show that the human PDZ-RGS3 isoform arises from 10 upstream exons along with 6 exons from the previously characterized RGS3. We found that 47,000 nucleotides span the last of the 10 upstream exons and the first exon used from the original cluster of RGS3 exons. These 10 upstream exons encode 398 amino acids, which show strong conservation with those from mouse PDZ-RGS3. In addition, another isoform exists that uses 17 upstream exons, 9 of which overlap with those in PDZ-RGS3, along with the same 6 downstream exons used in PDZ-RGS3. Finally, a short form of human RGS3 arises from an unrecognized RGS3 exon that encodes an amino-terminal 140 amino acids. For each RGS3 isoform, RT-PCR detected specific mRNA transcripts and immunoblot analysis identified specific bands for RGS3 and PDZ-RGS3. RGS3 provides an example of the complex origins of the coding regions of mammalian proteins.