H-Ras does not need COP I- or COP II-dependent vesicular transport to reach the plasma membrane

Biology, pathology, and therapeutic targeting of RAS

RAS was identified as a human oncogene in the early 1980s and subsequently found to be mutated in nearly 30% of all human cancers. More importantly, RAS plays a central role in driving tumor development and maintenance. Despite decades of effort, there remain no FDA approved drugs that directly inhibit RAS. The prevalence of RAS mutations in cancer and the lack of effective anti-RAS therapies stem from RAS' core role in growth factor signaling, unique structural features, and biochemistry. However, recent advances have brought promising new drugs to clinical trials and shone a ray of hope in the field. Here, we will exposit the details of RAS biology that illustrate its key role in cell signaling and shed light on the difficulties in therapeutically targeting RAS. Furthermore, past and current efforts to develop RAS inhibitors will be discussed in depth.

Small GTPases SAR1A and SAR1B regulate the trafficking of the cardiac sodium channel Nav1.5

BACKGROUND

The cardiac sodium channel Nav1.5 is essential for the physiological function of the heart and causes cardiac arrhythmias and sudden death when mutated. Many disease-causing mutations in Nav1.5 cause defects in protein trafficking, a cellular process critical to the targeting of Nav1.5 to cell surface. However, the molecular mechanisms underlying the trafficking of Nav1.5, in particular, the exit from the endoplasmic reticulum (ER) for cell surface trafficking, remain poorly understood.

METHODS AND RESULTS

Here we investigated the role of the SAR1 GTPases in trafficking of Nav1.5. Overexpression of dominant-negative mutant SAR1A (T39N or H79G) or SAR1B (T39N or H79G) significantly reduces the expression level of Nav1.5 on cell surface, and decreases the peak sodium current density (INa) in HEK/Nav1.5 cells and neonatal rat cardiomyocytes. Simultaneous knockdown of SAR1A and SAR1B expression by siRNAs significantly reduces the INa density, whereas single knockdown of either SAR1A or SAR1B has minimal effect. Computer modeling showed that the three-dimensional structure of SAR1 is similar to RAN. RAN was reported to interact with MOG1, a small protein involved in regulation of the ER exit of Nav1.5. Co-immunoprecipitation showed that SAR1A or SAR1B interacted with MOG1. Interestingly, knockdown of SAR1A and SAR1B expression abolished the MOG1-mediated increases in both cell surface trafficking of Nav1.5 and the density of INa.

CONCLUSIONS

These data suggest that SAR1A and SAR1B are the critical regulators of trafficking of Nav1.5. Moreover, SAR1A and SAR1B interact with MOG1, and are required for MOG1-mediated cell surface expression and function of Nav1.5.

Activated G Protein Gαs Samples Multiple Endomembrane Compartments

Heterotrimeric G proteins are localized to the plasma membrane where they transduce extracellular signals to intracellular effectors. G proteins also act at intracellular locations, and can translocate between cellular compartments. For example, Gαs can leave the plasma membrane and move to the cell interior after activation. However, the mechanism of Gαs translocation and its intracellular destination are not known. Here we use bioluminescence resonance energy transfer (BRET) to show that after activation, Gαs rapidly associates with the endoplasmic reticulum, mitochondria, and endosomes, consistent with indiscriminate sampling of intracellular membranes from the cytosol rather than transport via a specific vesicular pathway. The primary source of Gαs for endosomal compartments is constitutive endocytosis rather than activity-dependent internalization. Recycling of Gαs to the plasma membrane is complete 25 min after stimulation is discontinued. We also show that an acylation-deacylation cycle is important for the steady-state localization of Gαs at the plasma membrane, but our results do not support a role for deacylation in activity-dependent Gαs internalization.

Ras chaperones: new targets for cancer and immunotherapy

The Ras inhibitor S-trans,trans-farnesylthiosalicylic acid (FTS, Salirasib®) interferes with Ras membrane interactions that are crucial for Ras-dependent signaling and cellular transformation. FTS had been successfully evaluated in clinical trials of cancer patients. Interestingly, its effect is mediated by targeting Ras chaperones that serve as key coordinators for Ras proper folding and delivery, thus offering a novel target for cancer therapy. The development of new FTS analogs has revealed that the specific modifications to the FTS carboxyl group by esterification and amidation yielded compounds with improved growth inhibitory activity. When FTS was combined with additional therapeutic agents its activity toward Ras was significantly augmented. FTS should be tested not only in cancer but also for genetic diseases associated with abnormal Ras signaling, as well as for various inflammatory and autoimmune disturbances, where Ras plays a major role. We conclude that FTS has a great potential both as a safe anticancer drug and as a promising immune modulator agent.

Rasosomes originate from the Golgi to dispense Ras signals

Ras proteins undergo an incompletely understood trafficking process in the cell. Rasosomes are protein nanoparticles of 80–100 nm diameter that carry lipidated Ras isoforms (H-Ras and N-Ras) as well as their effectors through the cytoplasm and near the plasma membrane (PM). In this study, we identified the subcellular origin of rasosomes and how they spread Ras proteins through the cell. We found no dependency of rasosome formation on galectins, or on the GDP-/GTP-bound state of Ras. We found that significantly more rasosomes are associated with forms of Ras that are localized to the Golgi, namely N-Ras or the singly palmitoylated H-Ras mutant (C181S). To explore the possibility that rasosome originate from the Golgi, we used photoactivatable (PA)-GFP-H-Ras mutants and showed that rasosomes bud from the Golgi in a two-step mechanism. Newly released rasosomes first move in an energy-dependent directed fashion and then convert to randomly diffusing rasosomes. Dual fluorescence time-lapse imaging revealed the appearance of dually labeled rasosomes, indicating a dynamic exchange of cytoplasmic and PM-associated Ras with rasosome-associated Ras. Finally, higher levels of rasosomes correlate with higher levels of ERK phosphorylation, a key marker of Ras downstream signaling. We suggest that H-Ras and N-Ras proteins exchange with rasosomes that can function as carriers of palmitoylated Ras and its signals.

Trafficking of an endogenous potassium channel in adult ventricular myocytes

The roles of several small GTPases in the expression of an endogenous potassium current, I(to,f), in adult rat ventricular myocytes have been investigated. The results indicate that forward trafficking of newly synthesized Kv4.2, which underlies I(to,f) in these cells, requires both Rab1 and Sar1 function. Expression of a Rab1 dominant negative (DN) reduced I(to,f) current density by roughly one-half relative to control, mCherry-transfected myocytes. Similarly, expression of a Sar1DN nearly halved I(to,f) current density. Rab11 is not essential to trafficking of Kv4.2, as expression of a Rab11DN had no effect on I(to,f) over the time frames investigated here. In a process dependent on intact endoplasmic reticulum (ER)-to-Golgi transport, however, overexpression of wild-type Rab11 resulted in a doubling of I(to,f) density; block of ER-to-Golgi traffic by Brefeldin A completely abrogated the effect. Also implicated in the trafficking of Kv4.2 are Rab5 and Rab4. Rab5DN expression increased endogenous I(to,f) by two- to threefold, nonadditively with inhibition of dynamin-dependent endocytosis. And, in a phenomenon similar to that previously reported for myoblast-expressed Kv1.5, Rab4DN expression roughly doubled endogenous peak transient currents. Colocalization experiments confirmed the involvement of Rab4 in postinternalization trafficking of Kv4.2. There was little role evident for the lysosome in the degradation of internalized Kv4.2, as overexpression of neither wild-type nor DN isoforms of Rab7 had any effect on I(to,f). Instead, degradation may depend largely on the proteasome; the proteasome inhibitor MG132 significantly increased I(to,f) density.

One-step split GFP staining for sensitive protein detection and localization in mammalian cells

Although epitope tags are useful to detect intracellular proteins and follow their localization with antibodies, background and nonspecific staining often remain problematic. We describe a simple assay based on the split GFP complementation system. Proteins tagged with the 15-amino acid GFP 11 fragment are detected with a solution of the recombinant nonfluorescent complementary GFP 1-10 fragment to reconstitute a fluorescent GFP. In contrast to antibody-based staining methods, this one-step assay presents high specificity and very low background of fluorescence, thus conferring higher signal-to-noise ratios. We demonstrate that this new application of the split GFP tagging system facilitates detection of proteins displaying various subcellular localizations using flow cytometry and microscopy analysis.

Multiple cellular proteins modulate the dynamics of K-ras association with the plasma membrane

Although specific proteins have been identified that regulate the membrane association and facilitate intracellular transport of prenylated Rho- and Rab-family proteins, it is not known whether cellular proteins fulfill similar roles for other prenylated species, such as Ras-family proteins. We used a previously described method to evaluate how several cellular proteins, previously identified as potential binding partners (but not effectors) of K-ras4B, influence the dynamics of K-ras association with the plasma membrane. Overexpression of either PDEδ or PRA1 enhances, whereas knockdown of either protein reduces, the rate of dissociation of K-ras from the plasma membrane. Inhibition of calmodulin likewise reduces the rate of K-ras dissociation from the plasma membrane, in this case in a manner specific for the activated form of K-ras. By contrast, galectin-3 specifically reduces the rate of plasma membrane dissociation of activated K-ras, an effect that is blocked by the K-ras antagonist farnesylthiosalicylic acid (salirasib). Multiple cellular proteins thus control the dynamics of membrane association and intercompartmental movement of K-ras to an important degree even under basal cellular conditions.

H-ras resides on clathrin-independent ARF6 vesicles that harbor little RAF-1, but not on clathrin-dependent endosomes

Internalization of H-Ras from the cell surface onto endomembranes through vesicular endocytic pathways may play a significant role(s) in regulating the outcome of Ras signaling. However, the identity of Ras-associated subcellular vesicles and the means by which Ras localize to these internal sites remain elusive. In this study, we show that H-Ras is absent from endosomes initially derived from a clathrin-dependent endocytic pathway. Instead, both oncogenic H-Ras-61L and wild type H-Ras (basal or EGF-stimulated) bind Arf6-associated clathrin-independent endosomes and vesicles of the endosomal-recycling center (ERC). K-Ras4B-12V can also be internalized via Arf6 endosomes, and the C-terminal tails of both H-Ras and K-Ras4B are sufficient to mediate localization of GFP chimeras to Arf6-associated vesicles. Interestingly, little Raf-1 was found on these Arf6-associated endosomes even when active H-Ras was present. Instead, endogenous Raf-1 distributed primarily on EEA1-containing vesicles, suggesting that this H-Ras effector, although accessible for H-Ras interaction on the plasma membrane, appears to separate from its regulator during early stages of endocytosis. The discrete and dynamic distribution of Ras pathway components with spatio-temporal complexity may contribute to the specificity of Ras:effector interaction.

Anterograde and retrograde transport of neutral sphingomyelinase-2 between the Golgi and the plasma membrane

The activation of neutral sphingomyelinase-2 (nSMase2) and consequent ceramide production are implicated in many stress-induced signaling pathways. Trafficking of nSMase2 from the Golgi compartment to the plasma membrane (PM) in response to signaling stimuli has been described. However, the precise mechanisms of transport remain unknown. This study aimed to investigate the trafficking of nSMase2 between the Golgi and the PM. We show here that V5-nSMase2 localizes at the PM and Golgi in MCF-7 cells and confirm relocalization of nSMase2 to the PM at confluence. Although cycloheximide (CHX) treatment partially inhibited the Golgi localization of GFP-nSMase2, recovery of GFP-nSMase2 to an intracellular compartment was still observed after photobleaching. Moreover, in the presence of CHX, GFP- and V5-nSMase2 co-localized with endosomal/recycling markers. In HEK293 cells, activation of either protein kinase C-alpha or betaII, with the phorbol ester PMA led to relocalization of both wild-type and inactive nSMase2 to the pericentrion, a PKC-dependent subset of recycling endosomes. Finally, inhibition of nSMase2 endocytosis by K+depletion reduced the intracellular pool of nSMase2 and increased nSMase2 activity resulting in elevated ceramide levels. Altogether, these results suggest that nSMase2 traffics from the Golgi to the PM as a membrane protein en route to the cell surface and recycles back to the Golgi through the endosomal/recycling compartment. Moreover, the recycling of nSMase2 from the PM is important for its catalytic regulation.

Sar1-dependent trafficking of the human calcium receptor to the cell surface

The molecular mechanisms underlying the exit from the endoplasmic reticulum (ER) for cell surface trafficking of the human calcium receptor (hCaR) remain poorly understood. We investigated the role of the Sar1 small GTP-binding protein in cell surface transport of the hCaR. Disruptions of endogenous Sar1 function with the constitutively active Sar1H79G mutant or depletion using small interfering RNA, attenuates cell surface expression of the hCaR. Mutation of several putative di-acidic ER export motifs in the carboxyl-tail of the receptor revealed no apparent defect in cell surface expression. Truncated mutants lacking most of the carboxyl-terminal sequences or all intracellular domains also showed no impairment in cell surface expression at steady state. A truncated receptor containing only the large amino-terminal extracellular ligand-binding domain (ECD) is secreted into the culture medium and Sar1H79G inhibits this secretion. ECD receptor variants with the cysteines essential for intermolecular disulfide-linked dimerization mutated to serine or four of the asparagine sites for N-glycosylation mutated to alanine also disrupt secretion, indicating proper ECD conformation is critical for forward transport of this receptor.

The deubiquitinating enzyme USP17 blocks N-Ras membrane trafficking and activation but leaves K-Ras unaffected

The proto-oncogenic Ras isoforms (H, N, and K) have a C-terminal CAAX motif and undergo the same post-translational processing steps, although they traffic to the plasma membrane through different routes. Previously, we have shown that overexpression of the deubiquitinating enzyme USP17 inhibits H-Ras localization to the plasma membrane. Now we report that whereas H-Ras and N-Ras were unable to localize to the plasma membrane in the presence of USP17, K-Ras4b localization was unaffected. EGF stimulation was unable to induce N-Ras membrane localization in USP17-expressing cells. In addition, N-Ras activity and downstream signaling through the MAPK MEK/ERK and PI3K/JNK pathways were blunted. However, we still detected abundant N-Ras localization at the ER and Golgi in USP17-expressing cells. Collectively, our data showed that the deubiquitinating enzyme USP17 blocks EGF-induced N-Ras membrane trafficking and activation, but left K-Ras unaffected.

Rasosomes spread Ras signals from plasma membrane 'hotspots'

Ras proteins regulate cell growth, differentiation, and apoptosis from various cellular platforms. We have recently identified a novel potential signaling platform, the rasosome, which moves rapidly near the plasma membrane (PM) and in the cytosol, carrying multiple copies of palmitoylated Ras proteins. In the present study we demonstrate that rasosomes are unique entities distinct from PM nanoclusters or from endocytotic compartments. In addition, we examine whether rasosomes can act as regulated Ras signaling platforms. We show that a single rasosome simultaneously carries different types of Ras molecules in their active and inactive state, suggesting that rasosomes can upload and download Ras signals. Total internal reflection fluorescence (TIRF) microscopy combined with fast time-lapse and a new spatial analysis algorithm demonstrate that rasosome movement near the PM is restricted to distinctive areas, rasosomal 'hotspots', localized between actin filament cages. In addition, Ras-binding domain of Raf-1 (RBD) is recruited to Ras in rasosomal hotspots as revealed by bimolecular fluorescence complementation experiments. Interestingly, epidermal growth factor stimulates H/NRas activation on rasosomes and the subsequent recruitment of RBD to rasosomes. Moreover, we show that rasosomes are loaded with Ras downstream effectors and modulators. These findings establish that physiological stimulation originating from PM hotspots is transduced to rasosomes, which appear to serve as robust Ras signaling platforms that spread signals across the cell.

Developmentally regulated and evolutionarily conserved expression of SLITRK1 in brain circuits implicated in Tourette syndrome

Tourette syndrome (TS) is an inherited developmental neuropsychiatric disorder characterized by vocal and motor tics. Multiple lines of neurophysiological evidence implicate dysfunction in the corticostriatal-thalamocortical circuits in the etiology of TS. We recently identified rare sequence variants in the Slit and Trk-like family member 1 (SLITRK1) gene associated with TS. SLITRK1, a single-pass transmembrane protein, displays similarities to the SLIT family of secreted ligands, which have roles in axonal repulsion and dendritic patterning, but its function and developmental expression remain largely unknown. Here we provide evidence that SLITRK1 has a developmentally regulated expression pattern in projection neurons of the corticostriatal-thalamocortical circuits. SLITRK1 is further enriched in the somatodendritic compartment and cytoplasmic vesicles of cortical pyramidal neurons in mouse, monkey, and human brain, observations suggestive of an evolutionarily conserved function in mammals. SLITRK1 is transiently expressed in the striosomal/patch compartment of the mammalian striatum and moreover is associated with the direct output pathway; adult striatal expression is confined to cholinergic interneurons. These analyses demonstrate that the expression of SLITRK1 is dynamic and specifically associated with the circuits most commonly implicated in TS and related disorders, suggesting that SLITRK1 contributes to the development of corticostriatal-thalamocortical circuits.

Murine CENPF interacts with syntaxin 4 in the regulation of vesicular transport

Syntaxin 4 is a component of the SNARE complex that regulates membrane docking and fusion. Using a yeast two-hybrid screen, we identify a novel interaction between syntaxin 4 and cytoplasmic murine CENPF, a protein previously demonstrated to associate with the microtubule network and SNAP-25. The binding domain for syntaxin 4 in CENPF was defined by yeast two-hybrid assay and co-immunoprecipitation. Confocal analyses in cell culture reveal a high degree of colocalization between endogenously expressed proteins in interphase cells. Additionally, the endogenous SNARE proteins can be isolated as a complex with CENPF in immunoprecipitation experiments. Further analyses demonstrate that murine CENPF and syntaxin 4 colocalize with components of plasma membrane recycling: SNAP-25 and VAMP2. Depletion of endogenous CENPF disrupts GLUT4 trafficking whereas expression of a dominant-negative form of CENPF inhibits cell coupling. Taken together, these studies demonstrate that CENPF provides a direct link between proteins of the SNARE system and the microtubule network and indicate a diverse role for murine CENPF in vesicular transport.

Spatial control of protein binding on lipid bimembrane using photoeliminative linker

Protein adsorption and dissociation on cell membrane surfaces is a topic of important study to reveal biological processes including signal transduction and protein trafficking. We demonstrated here the establishment of a mimic model system for the spatial control of protein adsorption/elimination on a lipid bimembrane using a photochemical technique. The novel photoeliminative linker that we synthesized here consists of three distinct components: a substrate (biotin), a photoeliminative group (4-(4-(1-hydroxyethyl)-2-methoxy-5-nitrophenoxy)butanoic acid), and a lipid bimembrane-adsorbent group (farnesyl). The photoeliminative linker was inserted on the entire surface of the lipid bimembrane and two-dimensionally eliminated by spatial UV irradiation onto the membrane to create a biotin pattern. A target protein, streptavidin was selectively immobilized on the patterned biotin, although it was almost not attached on the nonirradiated region. The streptavidin array was selectively dissociated by UV irradiation onto the entire membrane.

Altered localization of H-Ras in caveolin-1-null cells is palmitoylation-independent

Caveolin-1 is a palmitoylated protein involved in the formation of plasma membrane subdomains termed caveolae, intracellular cholesterol transport, and assembly and regulation of signaling molecules in caveolae. Caveolin-1 interacts via a consensus binding motif with several signaling proteins, including H-Ras. Ras oncogene products function as molecular switches in several signal transduction pathways regulating cell growth and differentiation. Post-translational modifications, including palmitoylation, are critical for the membrane targeting and function of H-Ras. Subcellular localization regulates the signaling pathways engaged by H-Ras activation. We show here that H-Ras is localized at the plasma membrane in caveolin-1-expressing cells but not in caveolin-1-deficient cells. Since palmitoylation is required for trafficking of H-Ras from the endomembrane system to the plasma membrane, we tested whether the altered localization of H-Ras in caveolin-1-null cells is due to decreased H-Ras palmitoylation. Although the palmitoylation profiles of cultured embryo fibroblasts isolated from wild type and caveolin-1 gene-disrupted mice differed, suggesting that caveolin-1, or caveolae, play a role in the palmitate incorporation of a subset of palmitoylated proteins, the palmitoylation of H-Ras was not decreased in caveolin-1-null cells. We conclude that the altered localization of H-Ras in caveolin-1-deficient cells is palmitoylation-independent. This article shows two important new mechanisms by which loss of caveolin-1 expression may perturb intracellular signaling, namely the mislocalization of signaling proteins and alterations in protein palmitoylation.

Topology of molecular machines of the endoplasmic reticulum: a compilation of proteomics and cytological data

The endoplasmic reticulum (ER) is a key organelle of the secretion pathway involved in the synthesis of both proteins and lipids destined for multiple sites within and without the cell. The ER functions to both co- and post-translationally modify newly synthesized proteins and lipids and sort them for housekeeping within the ER and for transport to their sites of function away from the ER. In addition, the ER is involved in the metabolism and degradation of specific xenobiotics and endogenous biosynthetic products. A variety of proteomics studies have been reported on different subcompartments of the ER providing an ER protein dictionary with new data being made available on many protein complexes of relevance to the biology of the ER including the ribosome, the translocon, coatomer proteins, cytoskeletal proteins, folding proteins, the antigen-processing machinery, signaling proteins and proteins involved in membrane traffic. This review examines proteomics and cytological data in support of the presence of specific molecular machines at specific sites or subcompartments of the ER.