GDI2 deletion alleviates neurodegeneration and memory loss in the 5xFAD mice model of Alzheimer's disease

Accumulation of insoluble deposits of amyloid β-peptide (Aβ), derived from amyloid precursor protein (APP) processing, represents one of the major pathological hallmarks of Alzheimer's disease (AD). Perturbations in APP transport and hydrolysis could lead to increased Aβ production. However, the precise mechanisms underlying APP transport remain elusive. The GDP dissociation inhibitor2 (GDI2), a crucial regulator of Rab GTPase activity and intracellular vesicle and membrane trafficking, was investigated for its impact on AD pathogenesis through neuron-specific knockout of GDI2 in 5xFAD mice. Notably, deficiency of GDI2 significantly ameliorated cognitive impairment, prevented neuronal loss in the subiculum and cortical layer V, reduced senile plaques as well as astrocyte activation in 5xFAD mice. Conversely, increased activated microglia and phagocytosis were observed in GDI2 ko mice. Further investigation revealed that GDI2 knockout led to more APP co-localized with the ER rather than the Golgi apparatus and endosomes in SH-SY5Y cells, resulting in decreased Aβ production. Collectively, these findings suggest that GDI2 may regulate Aβ production by modulating APP intracellular transport and localization dynamics. In summary, our study identifies GDI2 as a pivotal regulator governing APP transport and process implicated in AD pathology; thus highlighting its potential as an attractive pharmacological target for future drug development against AD.

Exploring the impact of CYP2D6 and UGT2B7 gene-drug interactions, and CYP-mediated DDI on oxycodone and oxymorphone pharmacokinetics using physiologically-based pharmacokinetic modeling and simulation

Oxycodone is one of the most commonly used opioids to treat moderate to severe pain. It is metabolized mainly by CYP3A4 and CYP2D6, while only a small fraction of the dose is excreted unchanged into the urine. Oxymorphone, the metabolite primarily formed by CYP2D6, has a 40- to 60-fold higher mu-opioid receptor affinity than the parent compound. While CYP2D6-mediated gene-drug-interactions (GDIs) and drug-drug interactions (DDIs) are well-studied, they only account for a portion of the variability in oxycodone and oxymorphone exposure. The combined impact of CYP2D6-mediated GDIs and DDIs, CYP3A4-mediated DDIs, and UGT2B7 GDIs is not fully understood yet and hard to study in head-to-head clinical trials given the relatively large number of scenarios. Instead, we propose the use of a physiologically-based pharmacokinetic model that integrates available information on oxycodone's metabolism to characterize and predict the impact of DDIs and GDIs on the exposure of oxycodone and its major, pharmacologically-active metabolite oxymorphone. To this end, we first developed and verified a PBPK model for oxycodone and its metabolites using published clinical data. The verified model was then applied to determine the dose-exposure relationship of oxycodone and oxymorphone stratified by CYP2D6 and UGT2B7 phenotypes respectively, and administered perpetrators of CYP-based drug interactions. Our simulations demonstrate that the combination of CYP2D6 UM and a UGT2B7Y (268) mutation may lead to a 2.3-fold increase in oxymorphone exposure compared to individuals who are phenotyped as CYP2D6 NM / UGT2B7 NM. The extent of oxymorphone exposure increases up to 3.2-fold in individuals concurrently taking CYP3A4 inhibitors, such as ketoconazole. Inhibition of the CYP3A4 pathway results in a relative increase in the partial metabolic clearance of oxycodone to oxymorphone. Oxymorphone is impacted to a higher extent by GDIs and DDIs than oxycodone. We predict oxymorphone exposure to be highest in CYP2D6 UMs/UGT2B7 PMs in the presence of ketoconazole (strong CYP3A4 index inhibitor) and lowest in CYP2D6 PMs/UGT2B7 NMs in the presence of rifampicin (strong CYP3A4 index inducer) covering a 55-fold exposure range.

Effect of different ceramic systems on antagonist dental structure by microtomographic analysis

OBJECTIVES

This study aims to identify the two-dimensional and three-dimensional analyses and evaluate the loss of tooth structure in the tooth's different constituent elements and the ceramic antagonist's surface.

METHODS

In this study, three groups (n = 10) represented by different ceramic systems (lithium disilicate [GDis], lithium silicate reinforced with zirconia [GSil], and monolithic zirconia [GZir]) were evaluated. Each group obtained ten ceramic blocks and submitted them to the sintering/crystallization process. To carry out the wear test, healthy mandibular premolars were used as the specimens and the ceramic blocks as antagonists. The premolars were submitted to two-dimensional and three-dimensional analyses using a computerized microtomography (µTC) before and after the wear test. The wear test was performed with 30 N and 300,000 cycles load, with occlusion, laterality, and disocclusion movements.

RESULTS

The statistical analysis comparing the loss of two-dimensional tooth structure showed a statistically significant difference among all groups (p < 0.05). Statistical analysis comparing the percentage of loss of three-dimensional tooth structure showed a statistically significant difference between groups GDis and GSil and between groups GDis and GZir. However, when comparing GSil with GZir, no statistically significant difference was found. The qualitative analysis of the teeth showed that GDis showed considerable enamel loss and dentin exposure, GSil showed enamel wear with flattening the cusp without dentin exposure, and GZir showed minimal enamel wear without dentin exposure. In the qualitative analysis of ceramic antagonists, more significant wear of the ceramic material for GDis was observed, followed by the GSil and GZir groups, respectively.

SIGNIFICANCE

The use of the lithium disilicate should be cautious, restricting it to areas with lower masticatory forces. Areas of higher masticatory forces showed a large amount of antagonist wear, with dentin involvement. This can lead to dentinal hypersensitivity, risk of compromising patients' occlusion, by harming group disocclusion guides, causing pain and temporomandibular disorders.

ECM1-associated miR-1260b promotes osteogenic differentiation by targeting GDI1

Osteoporosis (OP) is a common disease among older adults. The promotion of osteoblast differentiation plays a crucial role in alleviating OP symptoms. Extracellular matrix protein 1 (ECM1) has been reported to be closely associated with osteogenic differentiation. In this study, we constructed U2OS cell lines with ECM1 knockdown and ECM1a overexpression based on knockdown, and identified the target miRNA (miR-1260b) by sequencing. Overexpression of miR-1260b promoted the osteogenic differentiation of U2OS and MG63 cells, as demonstrated by increased alkaline phosphatase (ALP) activity, matrix mineralization, and Runt-Related Transcription Factor 2 (RUNX2), Osteopontin (OPN), Collagen I (COL1A1), and Osteocalcin (OCN) protein expressions, whereas low expression of miR-1260b had the opposite effect. In addition, miR-1260b expression was decreased in OP patients than in non-OP patients. Next, we predicted the target gene of miRNA through TargetScan and miRDB and found that miR-1260b negatively regulated GDP dissociation inhibitor 1 (GDI1) by directly binding to its 3'-untranslated region. Subsequent experiments revealed that GDI1 overexpression decreased ALP activity and calcium deposit, reduced RUNX2, OPN, COL1A1, and OCN expression levels, and reversed the effects of miR-1260b on osteogenic differentiation. In conclusion, ECM1-related miR-1260b promotes osteogenic differentiation by targeting GDI1 in U2OS and MG63 cells. Thus, this study has significant implication for osteoporosis treatment.

GDP dissociation inhibitor 1 (GDI1) attenuates β-amyloid-induced neurotoxicity in Alzheimer's diseases

OBJECTIVE

β-Amyloid (Aβ) induced neurotoxicity is an implicated mechanism in Alzheimer's disease (AD). This study focused on the role of GDP dissociation inhibitor 1 (GDI1) in Aβ-induced neurotoxicity.

METHODS

Data from the GEO database for AD-related datasets GSE140829, GSE63061, GSE36980, and GSE60360 were downloaded and identified common differentially expressed genes (coDEGs). The mRNA levels of GDI1 in the serum of AD patients were analyzed by RT-qPCR. ROC curve evaluated the diagnostic value. Aβ25-35 induced SH-SY5Y cells to construct an AD cell model. CCK-8, flow cytometry, and ELISA assay were used to analyze cell viability, apoptosis, and concentrations of inflammatory factors. KEGG enrichment was employed to analyze the signal pathway of targets from GDI1 in the AD.

RESULTS

The GEO database identifies coDEGs including GDI1. GDI1 is generally elevated in serum from AD patients as well as in Aβ-induced SH-SY5Y cells. GDI1 has 77.97% sensitivity and 84.44% specificity to identify AD patients from controls. Aβ induced decreased cell viability, increased apoptosis, and promoted over-secretion of inflammatory cytokines, but they were all partially weakened by reduced GDI1. What's more, the GDI1 interacting gene and AD target gene were co-enriched on Endocytosis and MAPK signaling pathway.

CONCLUSION

Elevated GDI1 is a potential diagnostic biomarker for AD and that inhibition of GDI1 attenuates Aβ-induced neurotoxicity in AD. Our study offers new horizons for AD treatment.

The effects of the first versus second glaucoma drainage implant surgery in patients with primary open-angle glaucoma

BACKGROUND

To investigate the outcome of non-valved glaucoma drainage implant surgery (GDIS) in primary open-angle glaucoma (POAG) patients divided in the first GDI group (patients who underwent the first GDIS) and the second GDI group (patients who underwent the second GDIS because of the failed first GDIS).

METHODS

Intraocular pressure (IOP), visual acuity (VA), visual field defect (VFD), medication score (MS), survival rate of GDIS, complications, and patient background was retrospectively analyzed. Two success criteria were set: Criteria (1) IOP reduction ≥ 20% and 5 < IOP ≤ 21, Criteria (2) IOP reduction ≥ 20% and 5 < IOP ≤ 14.

RESULTS

There were 136 eyes of 109 patients in the first GDI group and 32 eyes of 27 patients in the second GDI group. In the first GDI group and II, mean preoperative IOP was 26.7 ± 6.7 mmHg and 23.7 ± 3.5 mmHg, respectively (P = 0.09). No statistically significant difference in postoperative IOP reduction was found between the two groups (P = 0.39). At 5-years postoperative, the Criteria 1 (Criteria 2) survival rate in the first GDI group and the second GDI group was 60.4% (31.7%) and 61.2% (25.6%), respectively (Criteria 1: hazard ratio [HR]: 0.64, 95% confidence interval [CI]: 0.30-1.35 [P = 0.24]; Criteria 2: HR: 0.81, 95% CI: 0.46-1.44, P = 0.48). No significant difference in VA, VFD change, MS, or complications was observed. Young patient age was the only significant factor for failure in the first GDI group (odds ratio: 0.95, 95% confidence interval: 0.91-1.00, P = 0.03).

CONCLUSION

The second GDIS may be as effective as the first GDIS for IOP reduction in POAG patients, however, there is a high risk of failure in young-age patients and the surgery may be ineffective in eyes requiring Criteria 2.

Implementation of pharmacogenetic testing in medication reviews in a hospital setting

AIM

To investigate whether it is feasible to perform pharmacogenetic testing and implement the test results as part of medication reviews during hospitalization of multimorbid patients.

METHODS

Patients with ≥2 chronic conditions and ≥5 regular drugs with at least one potential gene-drug interaction (GDI) were included from one geriatric and one cardiology ward for pharmacogenetic testing. After inclusion by the study pharmacist, blood samples were collected and shipped to the laboratory for analysis. For patients still hospitalized at the time when the pharmacogenetic test results were available, the information was used in medication reviews. Recommendations from the pharmacist on actionable GDIs were communicated to the hospital physicians, who subsequently decided on potential immediate changes or forwarded suggestions in referrals to general practitioners.

RESULTS

The pharmacogenetic test results were available for medication review in 18 of the 46 patients (39.1%), where median length of hospital stay was 4.7 days (1.6-18.3). The pharmacist recommended medication changes for 21 of 49 detected GDIs (42.9%). The hospital physicians accepted 19 (90.5%) of the recommendations. The most commonly detected GDIs involved metoprolol (CYP2D6 genotype), clopidogrel (CYP2C19 genotype) and atorvastatin (CYP3A4/5 and SLCOB1B1 genotype).

CONCLUSIONS

The study shows that implementation of pharmacogenetic testing for medication review of hospitalized patients has the potential to improve drug treatment before being transferred to primary care. However, the logistics workflow needs to be further optimized, as test results were available during hospitalization for less than half of the patients included in the study.

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.

Activators of G-protein signaling 3: a drug addiction molecular gateway

Drug addiction is marked by continued drug-seeking behavior despite deleterious consequences and a heightened propensity to relapse not withstanding long, drug-free periods. The enduring nature of addiction has been hypothesized to arise from perturbations in intracellular signaling, gene expression, and brain circuitry induced by substance abuse. Ameliorating some of these aberrations should abate behavioral and neurochemical markers associated with an 'addiction phenotype'. This review summarizes data showing that protein expression and signaling through the nonreceptor activator of G-protein signaling 3 (AGS3) are altered by commonly abused substances in rat and in in-vitro addiction models. AGS3 structure and function are unrelated to the more broadly studied regulator of G-protein signaling family. Thus, the unique role of AGS3 is the focus of this review. Intriguingly, AGS3 protein changes persist into drug abstinence. Accordingly, studies probing the role of AGS3 in the neurochemistry of drug-seeking behavior and relapse are studied in detail. To illuminate this study, AGS3 structure, cellular localization, and function are covered so that an idealized AGS3-targeted pharmacotherapy can be proposed.

Distribution of activator of G-protein signaling 3 within the aggresomal pathway: role of specific residues in the tetratricopeptide repeat domain and differential regulation by the AGS3 binding partners Gi(alpha) and mammalian inscuteable

AGS3, a receptor-independent activator of G-protein signaling, is involved in unexpected functional diversity for G-protein signaling systems. AGS3 has seven tetratricopeptide (TPR) motifs upstream of four G-protein regulatory (GPR) motifs that serve as docking sites for Gialpha-GDP. The positioning of AGS3 within the cell and the intramolecular dynamics between different domains of the proteins are likely key determinants of their ability to influence G-protein signaling. We report that AGS3 enters into the aggresome pathway and that distribution of the protein is regulated by the AGS3 binding partners Gialpha and mammalian Inscuteable (mInsc). Gialpha rescues AGS3 from the aggresome, whereas mInsc augments the aggresome-like distribution of AGS3. The distribution of AGS3 to the aggresome is dependent upon the TPR domain, and it is accelerated by disruption of the TPR organizational structure or introduction of a nonsynonymous single-nucleotide polymorphism. These data present AGS3, G-proteins, and mInsc as candidate proteins involved in regulating cellular stress associated with protein-processing pathologies.

Activator of G protein signaling 3 null mice: I. Unexpected alterations in metabolic and cardiovascular function

Activator of G protein signaling (AGS)-3 plays functional roles in cell division, synaptic plasticity, addictive behavior, and neuronal development. As part of a broad effort to define the extent of functional diversity of AGS3-regulated-events in vivo, we generated AGS3 null mice. Surprisingly, AGS3 null adult mice exhibited unexpected alterations in cardiovascular and metabolic functions without any obvious changes in motor skills, basic behavioral traits, and brain morphology. AGS3 null mice exhibited a lean phenotype, reduced fat mass, and increased nocturnal energy expenditure. AGS3 null mice also exhibited altered blood pressure control mechanisms. These studies expand the functional repertoire for AGS3 and other G protein regulatory proteins providing unexpected mechanisms by which G protein systems may be targeted to influence obesity and cardiovascular function.

Overexpression of Kruppel-like factor 5 in esophageal epithelia in vivo leads to increased proliferation in basal but not suprabasal cells

Krüppel-like factor 5 (Klf5; also called IKLF or BTEB2), a zinc-finger transcription factor with proproliferative and transforming properties in vitro, is expressed in proliferating cells of gastrointestinal tract epithelia, including in basal cells of the esophagus. Thus, Klf5 is an excellent candidate to regulate esophageal epithelial proliferation in vivo. Nonetheless, the function of Klf5 in esophageal epithelial homeostasis and tumorigenesis in vivo has not previously been determined. Here, we used the ED-L2 promoter of the Epstein-Barr virus to express Klf5 throughout esophageal epithelia. ED-L2/Klf5 transgenic mice were born at the appropriate Mendelian ratio, survived to at least 1 yr of age, and showed no evidence of esophageal dysplasia or cancer. Staining for bromodeoxyuridine (BrdU) demonstrated increased proliferation in the basal layer of ED-L2/Klf5 mice, but no proliferation was seen in suprabasal cells, despite ectopic expression of Klf5 in these cells. Notably, expression of the KLF family member Klf4, which binds the same DNA sequences as Klf5 and which inhibits proliferation and promotes differentiation, was not altered in ED-L2/Klf5 transgenic mice. In primary esophageal keratinocytes that overexpressed Klf5, expression of Klf4 still inhibited proliferation and promoted differentiation, providing a possible mechanism for the persistence of keratinocyte differentiation in ED-L2/Klf5 mice. To identify additional targets for Klf5 in esophageal epithelia, we performed functional genomic analyses and identified a total of 15 differentially expressed genes. In summary, while Klf5 positively regulates proliferation in basal cells, it is not sufficient to maintain proliferation in the esophageal epithelium.

Activation of Rac1 by Rho-guanine nucleotide dissociation inhibitor-beta with defective isoprenyl-binding pocket

Rho-guanine nucleotide dissociation inhibitor-beta (RhoGDIbeta), a regulator for Rho GTPases, is implicated in cancer cell progression. We reported that C-terminal truncated RhoGDIbeta (DeltaC(166-201)-RhoGDIbeta) promoted metastasis through activating Rac1 signaling pathway in ras-transformed fibroblast cells. To better understand the mechanism of Rac1 activation by DeltaC(166-201)-RhoGDIbeta during metastasis, the amount of GTP-bound Rac1 was measured as the activation level of Rac1 in cells expressing various mutant RhoGDIbeta with sequential C-terminal deletions. Three C-terminal hydrophobic amino acid residues (Trp191, Leu193, and Ile195) supposed to interact with isoprenyl groups of Rac1, was indispensable for a proper regulation of Rac1 activation/inhibition. Deletion of this region led RhoGDIbeta to continuously associate with GTP-bound Rac1, provoking constitutive activation of Rac1. Thus, impaired interaction of RhoGDIbeta with Rac1 isoprenyl groups possibly makes RhoGDIbeta function as a positive regulator for Rac1 during metastasis.

The "ups and downs" of signaling cascades in addiction

Drug addiction is a chronic disease characterized by compulsive drug use despite the severe negative consequences associated with it. Repeated exposure to drugs of abuse results in molecular adaptations in neuronal signaling pathways, which eventually manifest in the complex behavioral alterations that characterize addiction. These include tolerance, sensitization, dependence, drug craving, and relapse. In this Review, we focus on recent studies highlighting signaling cascades initiated by cocaine, as a representative of a drug of abuse with a defined site of action, and alcohol, as a drug with an undefined primary site of action. Specifically, we describe recent studies that emphasize the role of protein-protein interactions, phosphorylation, and compartmentalization in the molecular mechanisms that result in the cellular and behavioral adaptations that underlie addiction. Signaling cascades that contribute to addiction, as well as those that protect or delay the development of addiction, are presented.

Interaction of transducin-alpha with LGN, a G-protein modulator expressed in photoreceptor cells

LGN and activator of G-protein signaling 3 (AGS3) belong to the class of G-protein modulators containing G-protein regulatory motifs (GPR proteins). Evidence for the functions of these molecules has only started to emerge. Immunostaining of mouse retina cross-sections and serial tangential sectioning of the retina combined with immunoblot analysis revealed that LGN is expressed in the inner segments of photoreceptor cells. Double immunolabeling demonstrated that, following light-dependent translocation from the outer segments, the alpha-subunit of the visual G-protein transducin (Gtalpha) colocalizes with LGN in the basal part of the inner segments. LGN and Gtalpha coprecipitate from the retinal extracts, supporting the notion of the interaction between the proteins. Furthermore, the GPR domain of LGN potently inhibits receptor-mediated guanine nucleotide exchange and steady-state GTPase activity of transducin. The localization and interaction with Gtalpha suggest LGN roles in modulation of transducin translocation and other photoreceptor cell functions.

The presence of a Leu-Gly-Asn repeat-enriched protein (LGN), a putative binding partner of transducin, in ROD photoreceptors

PURPOSE

Heterotrimeric G proteins are regulated by receptors that act as guanine nucleotide exchange factors (GEFs) and by RGS proteins, which act as guanosine triphosphatase (GTPase) activating proteins (GAPs). Guanosine diphosphate (GDP) dissociation inhibitors (GDIs), such as activators of G protein signaling (AGS)-1 and -3 and Leu-Gly-Asn repeat-enriched (LGN) proteins regulate the Gi family of G proteins. AGS3 and LGN contain four characteristic G protein regulator (GPR) domains that are responsible for its GDI function. This study investigates the presence of a GDI for transducin in photoreceptor cells.

METHODS

Western blot analysis of bovine and mouse retina was performed using specific antibodies to AGS and LGN proteins. The subcellular localization of LGN in retina was studied by immunofluorescence microscopy of mouse retinal sections and fractionation of retinal lysates, using sucrose density gradients. The interaction of LGN with transducin was studied using pull-down assays with GST-fused LGN constructs, co-immunoprecipitation and assays for GTPgammaS binding.

RESULTS

LGN, but not AGS3 and AGS1, was present in the retina, where it was localized mostly in the inner segments and outer plexiform layer of photoreceptor cells in both light and dark conditions. LGN was present in the cytosol, membrane, and the detergent-resistant cytoskeletal fraction. The amount of LGN relative to transducin was at least 1:15. The alpha subunit of transducin in its GDP-bound state interacted with endogenous and recombinant LGN, and the recombinant GPR domain of LGN reduced the rate of GTP exchange.

CONCLUSIONS

Photoreceptor inner segments contain LGN, which can bind to the alpha subunit of transducin and potentially regulate its function.

Nuclear Translocation of Cleaved LyGDI Dissociated from Rho and Rac during Trp53-Dependent Ionizing Radiation-Induced Apoptosis of Thymus CellsIn Vitro

LyGDI inhibits the dissociation of GDP from Rho family GTPases and is found in abundance in hematopoietic cells. Here we report truncation of LyGDI after irradiation in mouse 3SB thymus cells. A 21-kDa fragment of LyGDI, resulting from activated caspase 3-induced cleavage at an N-terminal consensus site following the Asp(18) residue, accumulated at peak quantities between 5 and 12 h after irradiation. Cleavage of LyGDI was inhibited by the caspase inhibitor benzoyloxycarbonyl-Val-Asp-fluoromethylketone. Subcellular fractionation and immunofluorescence revealed the truncated 21-kDa fragment of LyGDI within the nuclear fraction of irradiated 3SB cells, whereas full-length LyGDI was found only in the cytoplasmic fraction. Truncated LyGDI within the nucleus had no association with the Rho family proteins RhoA and Rac1, since these proteins were observed only in the cytoplasmic fractions. These data demonstrate that regulation of Rho family GTPases by LyGDI is disrupted during apoptosis, suggesting that fragmentation of LyGDI implicates the transmission of a signal from the cytoplasm to the nucleus during Trp53-dependent apoptosis of thymus cells after irradiation.

LyGDI functions in cancer metastasis by anchoring Rho proteins to the cell membrane

Rho family GTPases play an important role in a number of processes related to metastasis, and RhoGDP dissociation, inhibitors (RhoGDIs) regulate Rho family proteins. We cloned genomic DNA from colon carcinoma SW480 cells capable of transforming nonmetastatic ras-transformed 1-1ras1000 cells into metastatic cells. This DNA contained a truncated human ras homolog gene family GDP dissociation inhibitor beta (ARHGDIB) gene, resulting in a C-terminal truncated form of LyGDI (Delta C-LyGDI, 166-201 deletion), a member of the RhoGDIs. The stable expression of Delta C-LyGDI induced pulmonary metastasis in 1-1ras1000 cells, whereas expression of full-length LyGDI did not induce metastasis. Delta C-LyGDI was preferentially localized in the membrane, detected in a NP-40-insoluble fraction, and co-purified with radixin, moesin, Rac1, Cdc42, and RhoA. In Delta C-LyGDI transfectant, an activation state of Rac1 was elevated and Delta C-LyGDI was associated with Rac1-GTP. In keeping with the observed localization of Rac1 to the cell membrane and the elevated level of Rac1-GTP, Delta C-LyGDI transfectants were found to be more invasive than mock transfectant. These results suggest that LyGDI functions in the cell membrane to afford spatial regulation of Rho family GTPase signaling through ezrin radixin moesin (ERM) proteins during metastasis.

Glutamate Transmission and Addiction to Cocaine

A variety of data point to the possibility that neuroadaptations in glutamate transmission are produced by repeated exposure to cocaine that result in the expression of behaviors characteristic of addiction, such as craving and relapse. Using the reinstatement model of relapse in rats, glutamate release in the projection from the prefrontal cortex to the nucleus accumbens has been shown to underlie cocaine- and stress-primed reinstatement. In this report, four adaptations produced by withdrawal from repeated cocaine are described that may regulate the release of glutamate underlying reinstatement of drug-seeking resulted. (1) Neurons in the prefrontal cortex have increased levels of activator of G protein signaling 3 (AGS3) that causes reduced signaling through Gi coupled receptors, and normalization of AGS3 blocked cocaine-primed reinstatement. (2) The activity of the cystine-glutamate exchanger is reduced resulting in decreased extracellular glutamate in the nucleus accumbens, and normalization of exchanger activity prevented cocaine-primed reinstatement. (3) Metobotropic glutamate receptor function is diminished after repeated cocaine administration that results in reduced regulation of glutamate release. (4) Homer1 protein is reduced in the nucleus accumbens, and Homer2 knockout mice show enhanced responsiveness to cocaine. Taken together, there appears to be both pre- and postsynaptic changes in glutamate transmission that dysregulates the glutamatergic projection from the prefrontal cortex to the nucleus accumbens. These adaptations are hypothesized to facilitate glutamate release in response to a cocaine injection or acute stress and lead to the reinstatement of drug-seeking behavior.

Vav1 and Ly-GDI two regulators of Rho GTPases, function cooperatively as signal transducers in T cell antigen receptor-induced pathways

The Rho family GTPases are pivotal for T cell signaling; however, the regulation of these proteins is not fully known. One well studied regulator of Rho GTPases is Vav1; a hematopoietic cell-specific guanine nucleotide exchange factor critical for signaling in T cells, including stimulation of the nuclear factor of activated T cells (NFAT). Surprisingly, Vav1 associates with Ly-GDI, a hematopoietic cell-specific guanine nucleotide dissociation inhibitor of Rac. Here, we studied the functional significance of the interaction between Vav1 and Ly-GDI in T cells. Upon organization of the immunological synapse, both Ly-GDI and Vav1 relocalize to T cell extensions in contact with the antigen-presenting cell. Ly-GDI is phosphorylated on tyrosine residues following T cell receptor stimulation, and it associates with the Src homology 2 region of an adapter protein, Shc. In addition, the interaction between Ly-GDI and Vav1 requires tyrosine phosphorylation. Overexpression of Ly-GDI alone is inhibitory to NFAT stimulation and calcium mobilization. However, when co-expressed with Vav1, Ly-GDI enhances Vav1 induction of NFAT activation, phospholipase Cgamma phosphorylation, and calcium mobilization. Moreover, Ly-GDI does not alter the regulation of these phenomena when coexpressed with oncogenic Vav1. Since oncogenic Vav1 does not bind Ly-GDI, this suggests that the functional cooperativity of Ly-GDI and Vav1 is dependent upon their association. Thus, our data suggest that the interaction of Vav1 and Ly-GDI creates a fine tuning mechanism for the regulation of intracellular signaling pathways leading to NFAT stimulation.

TNF-alpha-mediated neutrophil apoptosis involves Ly-GDI, a Rho GTPase regulator

We investigated intracellular signaling events involved in fibronectin-accelerated TNF-alpha-mediated PMN apoptosis by means of 2-D gel electrophoresis and western blotting. Proteins were sequenced with electrospray ionization mass spectrometry. Apoptosis was quantitated by flow cytometry. We detected a cluster of acidic, high molecular-weight proteins that were only tyrosine phosphorylated when TNF-alpha-treated PMN interacted with fibronectin. Sequence analysis revealed that one of these proteins was Ly-GDI, a regulator of Rho GTPases. Fibronectin increased the TNF-alpha-induced Ly-GDI cleavage, yielding a 23-kD fragment. At 8 h, intact Ly-GDI was decreased to 33% on fibronectin, compared with 69% on PolyHema (P<0.05). Inhibition of tyrosine phosphorylation prevented phosphorylation of Ly-GDI, fibronectin-accelerated Ly-GDI cleavage, and fibronectin-accelerated apoptosis in TNF-alpha-treated PMN. We found that Ly-GDI cleavage was dependent on caspase-3 activation and that caspase-3 inhibition decreased apoptosis. We conclude that tyrosine phosphorylation of Ly-GDI, followed by increased caspase-3-mediated Ly-GDI cleavage, is a signaling event associated with accelerated TNF-alpha-mediated apoptosis on fibronectin.

Rho family GTPase Cdc42 is essential for the actin-based motility of Shigella in mammalian cells

Shigella, the causative agent of bacillary dysentery, is capable of directing its movement within host cells by exploiting actin dynamics. The VirG protein expressed at one pole of the bacterium can recruit neural Wiskott-Aldrich syndrome protein (N-WASP), a downstream effector of Cdc42. Here, we show that Cdc42 is required for the actin-based motility of Shigella. Microinjection of a dominant active mutant Cdc42, but not Rac1 or RhoA, into Swiss 3T3 cells accelerated Shigella motility. In add-back experiments in Xenopus egg extracts, addition of a guanine nucleotide dissociation inhibitor for the Rho family, RhoGDI, greatly diminished the bacterial motility or actin assembly, which was restored by adding activated Cdc42. In N-WASP-depleted extracts, the bacterial movement almost arrested was restored by adding exogenous N-WASP but not H208D, an N-WASP mutant defective in binding to Cdc42. In pyrene actin assay, Cdc42 enhanced VirG-stimulating actin polymerization by N-WASP-actin-related protein (Arp)2/3 complex. Actually, Cdc42 stimulated actin cloud formation on the surface of bacteria expressing VirG in a solution containing N-WASP, Arp2/3 complex, and G-actin. Immunohistological study of Shigella-infected cells expressing green fluorescent protein-tagged Cdc42 revealed that Cdc42 accumulated by being colocalized with actin cloud at one pole of intracellular bacterium. Furthermore, overexpression of H208D mutant in cells interfered with the actin assembly of infected Shigella and diminished the intra- and intercellular spreading. These results suggest that Cdc42 activity is involved in initiating actin nucleation mediated by VirG-N-WASP-Arp2/3 complex formed on intracellular Shigella.

General role of GDP dissociation inhibitor 2 in membrane release of Rab proteins: modulations of its functional interactions by in vitro and in vivo structural modifications

The GDP dissociation inhibitors (GDIs) represent an important class of regulatory proteins in the functional cycle and recycling of Rab GTPases. Previous studies have demonstrated that GDI-1 can operate with multiple Rab proteins. In this study we have addressed a plausible general activity of GDI-2 in supporting Rab membrane release and have analyzed the requirements of sequence-conserved vs variable regions of GDI-2 in these functional interactions. The in vitro function of expressed recombinant GDI-2 wild-type-, point-, or deletion-mutant proteins was investigated toward several Rab family members, divergent in structure, localized and operating on different membranes, including Rab2, Rab4, Rab5, Rab8, Rab9, and Rab11. We demonstrate here a general and nearly invariant ability of GDI-2(WT) to release from membranes this subset of diverse Rabs. Deletion of an 18-residue segment from the C-terminal variable region yielded a fully functional or only slightly defective GDI-2. Conversely, substitution of Met at position 250 of the conserved region markedly abrogated the activity toward all Rabs. Surprisingly, a replacement of an adjacent conserved residue (Y249V) resulted in a selective Rab-dependent response and a profound gain of function toward specific Rabs. To further test whether the endogenous GDI-2 can adopt a gain-of-function conformation, we pharmacologically stimulated intact 3T3-L1 adipocytes to induce GDI-2 tyrosine phosphorylation. We found a pronounced increase of the Rab4 soluble form and its soluble complexes with the tyrosine-phosphorylated GDI-2. Together, these results indicate that (a) GDI-2 displays a general activity to release Rabs from membranes, (b) GDI-2-conserved residues, but not the C-terminal variable region, are essential for this activity, and (c) structural modifications in GDI-2 can enhance its functional activity, directing selective interactions with individual Rabs.

How RhoGDI binds Rho

Like all Rho (Ras homology) GTPases, RhoA functions as a molecular switch in cell signaling, alternating between GTP- and GDP-bound states, with its biologically inactive GDP-bound form maintained as a cytosolic complex with RhoGDI (guanine nucleotide-exchange inhibitor). The crystal structures of RhoA-GDP and of the C-terminal immunoglobulin-like domain of RhoGDI (residues 67-203) are known, but the mechanism by which the two proteins interact is not known. The functional human RhoA-RhoGDI complex has been expressed in yeast and crystallized (P6(5)22, unit-cell parameters a = b = 139, c = 253 A, two complexes in the asymmetric unit). Although diffraction from these crystals extends to 3.5 A and is highly anisotropic, the experimentally phased (MAD plus MIR) electron-density map was adequate to reveal the mutual disposition of the two molecules. The result was validated by molecular-replacement calculations when data were corrected for anisotropy. Furthermore, the N-terminus of RhoGDI (the region involved in inhibition of nucleotide exchange) can be identified in the electron-density map: it is bound to the switch I and switch II regions of RhoA, occluding an epitope which binds Dbl-like nucleotide-exchange factors. The entrance of the hydrophobic pocket of RhoGDI is 25 A from the last residue in the RhoA model, with its C-terminus oriented to accommodate the geranylgeranyl group without conformational change in RhoA.

Neurite extension occurs in the absence of regulated exocytosis in PC12 subclones

We have investigated the process leading to differentiation of PC12 cells. This process is known to include extension of neurites and changes in the expression of subsets of proteins involved in cytoskeletal rearrangements or in neurosecretion. To this aim, we have studied a PC12 clone (trk-PC12) stably transfected with the nerve growth factor receptor TrkA. These cells are able to undergo both spontaneous and neurotrophin-induced morphological differentiation. However, both undifferentiated and nerve growth factor-differentiated trk-PC12 cells appear to be completely defective in the expression of proteins of the secretory apparatus, including proteins of synaptic vesicles and large dense-core granules, neurotransmitter transporters, and neurotransmitter-synthesizing enzymes. These results indicate that neurite extension can occur independently of the presence of the neurosecretory machinery, including the proteins that constitute the fusion machine, suggesting the existence of differential activation pathways for the two processes during neuronal differentiation. These findings have been confirmed in independent clones obtained from PC12-27, a previously characterized PC12 variant clone globally incompetent for regulated secretion. In contrast, the integrity of the Rab cycle appears to be necessary for neurite extension, because antisense oligonucleotides against the neurospecific isoform of Rab-guanosine diphosphate-dissociation inhibitor significantly interfere with process formation.

The amphioxus rab GDP-dissociation inhibitor (GDI) gene is neural-specific: implications for the evolution of chordate rab GDI genes

The rab GDP-dissociation inhibitor (rab GDI) proteins are involved in the regulation of vesicle-mediated cellular transport. We isolated the amphioxus rab GDI gene, analyzed its expression during amphioxus development, and performed a phylogenetic analysis of the rab GDI family. In contrast to the two major rab GDI forms in mammals, the alpha and beta forms, there is only one rab GDI isoform in amphioxus. Our analysis indicates that the occurrence of the alpha and beta forms of rab GDI preceded the divergence of lineages leading to birds and mammals, and that the amphioxus rab GDI may have evolved directly from the common ancestor of both forms. While the mammalian rab GDI beta-genes are ubiquitously expressed, the rab GDI alpha genes are predominantly expressed in neural tissues. The expression analysis of the amphioxus rab GDI gene shows predominantly neural expression similar to that of the mammalian rab GDI alpha form, suggesting that the ancestral expression pattern of chordate rab GDI was neural. In addition, the chicken rab GDI beta-like gene also shows neural-specific expression, which indicates that the neural expression was retained in both early postduplication alpha and beta isoforms and that a novel function associated with ubiquitous expression may have evolved uniquely in mammals. These results reveal a likely scenario of functional divergence of the rab GDI genes after duplication of the ancestral gene. A similar pattern of evolution, in which one of the duplicated genes retained a role similar to that of the ancestral one while other genes were recruited into novel roles, was also observed in the analysis of chordate Otx and hedgehog genes. In the rab GDI, hedgehog, and Otx gene families, the gene retaining the ancestral role shows a lower rate of sequence evolution than its counterpart, which was recruited for a novel function.

Expression level, subcellular distribution and rho-GDI binding affinity of merlin in comparison with Ezrin/Radixin/Moesin proteins

Merlin, a neurofibromatosis type-2 tumor suppressor, shows significant sequence similarity to ERM (Ezrin/Radixin/Moesin) proteins, general actin filament/plasma membrane cross-linkers, which are regulated in a Rho-dependent manner. To understand its physiological functions, we compared merlin with ERM proteins in vivo and in vitro. Quantitative immunoblotting revealed that the molar ratio of merlin/ERM in cultured epithelial or non-epithelial cells was approximately 0.14 or approximately 0.05, respectively. After centrifugation of cell homogenate, merlin was mostly recovered in the insoluble fraction, whereas almost half of ERM proteins were found in the soluble fraction. Merlin and ERM proteins were concentrated at microvilli when introduced into fibroblasts. In contrast, in epithelial cells, introduced merlin was co-distributed with E-cadherin in lateral membranes, whereas ERM proteins were concentrated in apical microvilli. Finally, we examined the binding affinity of merlin to Rho GDP dissociation inhibitor (Rho-GDI), to which N-terminal halves of ERM proteins but not the full-length molecules specifically bind. In vitro binding assays revealed that the N-terminal halves of merlin isoform-I and -II as well as full-length merlin isoform-II bound to Rho-GDI with similar binding affinity to ERM proteins. Immunoprecipitation confirmed these findings in vivo. These findings do not favor the notion that merlin functions simply in a redundant or competitive manner to ERM proteins.

Rho guanine dissociation inhibitors: pivotal molecules in cellular signalling

The small G proteins of the Ras family act as bimodal relays in the transfer of intracellular signals. This is a dynamic phenomenon involving a cascade of protein-protein interactions modulated by chemical modifications, structural rearrangements and intracellular relocalisations. Most of the small G proteins could be operationally defined as proteins having two conformational states, each of which interacts with different cellular partners. These two states are determined by the nature of the bound nucleotide, GDP or GTP. This capacity to cycle between a GDP-bound conformation and a GTP-bound conformation enables them to filter, to amplify or to temporise the upstream signals that they receive. Thus the control of this cycle is crucial. Membrane anchoring of the proteins in the Ras family is a prerequisite for their activity. Most of the proteins in the Rho/Rac and Rab subfamilies of Ras proteins cycle between cytosol and membranes. Then the control of membrane association/dissociation is an other important regulation level. This review will describe one family of crucial regulators acting on proteins in the Rho/Rac family-the Rho guanine nucleotide dissociation inhibitors, or RhoGDIs. As yet, only three RhoGDIs have been described: RhoGDI-1, RhoGDI-2 (or D4/Ly-GDI) and RhoGDI-3. RhoGDI 1 and 2 are cytosolic and participate in the regulation of both the GDP/GTP cycle and the membrane association/dissociation cycle of Rho/Rac proteins. The non-cytosolic RhoGDI-3 seems to act in a slightly different way.

Regulation of Rho protein binding to membranes by rhoGDI: inhibition of releasing activity by physiological ionic conditions

The Rho GDP dissociation inhibitor (GDI) is an ubiquitously expressed regulatory protein involved in the cycling of Rho proteins between membrane-bound and soluble forms. Here, we characterized the Rho solubilization activity of a glutathione S-transferase (GST) - GDI fusion protein in a cell-free system derived from rat kidney. Addition of GST-GDI to kidney brush border membranes resulted in the specific release of Cdc42 and RhoA from the membranes, while RhoB and Ras were not extracted. The release of Cdc42 and RhoA by GST-GDI was dose dependent and saturable with about 50% of both RhoA and Cdc42 extracted. The unextracted Rho proteins were tightly bound to membranes and could not be solubilized by repeated GST-GDI treatment. These results demonstrated that kidney brush border membranes contained two populations of RhoA and Cdc42. Furthermore, the GST-GDI solubilizing activity on membrane-bound Cdc42 and RhoA was abolished at physiological conditions of salt and temperature in all tissues examined. When using bead-immobilized GST-GDI, KCl did not reduced the binding of Rho proteins. However, washing brush border membranes with KCl prior treatment by GST-GDI inhibited the extraction of Rho proteins. Taken together, these results suggest that the binding of GDI to membrane-bound Cdc42 and RhoA occurs easily under physiological ionic strength conditions, but a complementary factor is required to extract these proteins from membranes. These observations suggest that the shuttling activity of GDI upon Rho proteins could be normally downregulated under physiological conditions.

Role of Rho small GTP binding protein in the regulation of actin cytoskeleton in hepatic stellate cells

BACKGROUND/AIMS

In the fibrotic response to liver injury, hepatic stellate cells are activated, leading to the myofibroblastic cell shape, with actin cytoskeletal reorganization and increased extracellular matrix production. The reorganization of actin cytoskeleton suggests that the small GTP binding protein Rho might modulate the process of this myofibroblastic change. The aim of this study was to investigate the role of Rho in the phenotypic changes of hepatic stellate cells.

METHODS

The phenotypic changes were investigated by the overexpression of Rho regulator, Rho GDI or dominant negative mutant of Rho in mouse hepatic stellate cell line, GRX cells. In activated rat hepatic stellate cells, the effects of microinjection of Botulinus toxin C3, which is the specific inhibitor for Rho, were analyzed. Furthermore, the effect of C3 on the type I collagen accumulation in hepatic stellate cells was investigated.

RESULTS

Overexpression of Rho GDI or the dominant negative mutant of Rho caused the shrinkage cell shape and suppressed stress fiber formation. Microinjection of toxin C3 caused a markedly distorted cell shape and the disappearance of stress fibers in rat stellate cells. In addition, C3 strongly suppressed collagen accumulation in activated stellate cells.

CONCLUSIONS

These results suggest that Rho regulates the actin cytoskeletal reorganization, and may be implicated in the collagen accumulation in activated stellate cells. These findings provide evidence for the role of Rho in the myofibroblastic phenotype in hepatic stellate cells.

Characterization of the ORF YBR264c in Saccharomyces cerevisiae, which encodes a new yeast Ypt that is degraded by a proteasome-dependent mechanism

We identified the ORF YBR264c during the systematic sequencing of the Saccharomyces cerevisiae genome. It encodes a putative protein of 218 amino acids. We demonstrate here that the gene is indeed expressed and encodes a new Ypt in yeast. This protein specifically binds guanine nucleotides and interacts via its C-terminal end with the unique Rab GDP Dissociation Inhibitor (RabGDI). In accordance with a recent proposal, the gene is now designated YPT10. No mutant phenotype could be associated with inactivation of the gene. However, overexpression of YPT10 resulted in defects in growth; microscopic examination of such cells revealed an overabundance of vesicular and tubular structures, suggesting some alteration in the function of the Golgi apparatus. In addition, degradation of the Ypt10 protein, which possesses a PEST sequence, is shown to be dependent on proteasome activity.

Phosphoinositide-dependent activation of Rho A involves partial opening of the RhoA/Rho-GDI complex

Rho GTPases have two interconvertible forms and two cellular localizations. In their GTP-bound conformation, they bind to the cell membrane and are activated. In the inactive GDP-bound conformation, they associate with a cytosolic protein called GDP dissociation inhibitor (GDI). We previously reported that the RhoA component of the RhoA/Rho-GDI complex was not accessible to the Clostridium botulinum C3 ADP-ribosyl transferase, unless the complex had been incubated with phosphoinositides. We show here that PtdIns, PtdIns4P, PtdIns3,4P2, PtdIns4,5P2 and PtdInsP3 enhance not only the C3-dependent ADP-ribosylation, but also the GDP/GTP exchange in the RhoA component of the prenylated RhoA/Rho-GDI complex. In contrast, in the nonprenylated RhoA/Rho-GDI complex, the levels of ADP-ribosylation and GDP/GTP exchange are of the same order as those measured on free RhoA and are not modified by phosphoinositides. In both cases, phosphoinositides partially opened, but did not fully dissociate the complex. Upon treatment of the prenylated RhoA/Rho-GDI complex with phosphoinositides, a GTP-dependent transfer to neutrophil membranes was evidenced. Using an overlay assay with the prenylated RhoA/Rho-GDI complex pretreated with PtdIns4P and labeled with [alpha32P]GTP, three membrane proteins with molecular masses between 26 and 32 kDa were radiolabeled. We conclude that in the presence of phosphoinositides, the prenylated RhoA/Rho-GDI complex partially opens, which allows RhoA to exchange GDP for GTP. The opened GTP-RhoA/Rho-GDI complex acquires the capacity to target specific membrane proteins.

Rho GTPases are over-expressed in human tumors

Small GTPases of the Rho family are involved in the regulation of a variety of cellular processes, such as the organization of the microfilamental network, cell-cell contact and malignant transformation. To address the question of whether Rho proteins are involved in carcinogenesis in man, we compared their expression in tumors from colon, breast and lung with that of the corresponding normal tissue originating from the same patient. As shown by Rho-specific 32P-ADP-ribosylation, as well as Western-blot analysis, the amount of RhoA protein was largely increased in all 3 types of tumors tested. The most dramatic differences in the expression of Rho GTPases were observed in breast tissue. All breast tumors analyzed showed high levels of RhoA, Rac and Cdc42 proteins, whereas in the corresponding normal tissue these Rho proteins were hardly or not detectable. Progression of breast tumors from WHO grade I to grade III was accompanied by a significant average increase in RhoA protein. Overall, increase in the amount of Rho GTPases, in particular RhoA, appears to be a frequent event in different types of human tumors. This supports the view that Rho GTPases are involved in human carcinogenesis.

The interaction between rac1 and its guanine nucleotide dissociation inhibitor (GDI), monitored by a single fluorescent coumarin attached to GDI

The interaction of rac with guanine nucleotide dissociation inhibitor protein (rhoGDI) is described, using GDI fluorescently labeled on its single cysteine with N-[2-(1-maleimidyl)ethyl]-7-diethylaminocoumarin-3-carboxamide (MDCC). The labeled GDI shows a 70% decrease in fluorescence emission on binding geranylgeranylated rac1.GDP and has an affinity for rac1 within a factor of 2 of the unlabeled GDI. The labeled GDI was used to determine the kinetic mechanism of the interaction by measuring the association and dissociation in real time. The kinetics are interpreted in terms of a two-step mechanism: binding of rac to GDI and then a conformational change of the complex with an overall dissociation constant of 0.4 nM. The conformational change has a rate constant of 7.3 s-1 (pH 7.5, 30 degrees C), and the reverse has a rate constant of 1.4 x 10(-)3 s-1. To overcome difficulties inherent in using and manipulating lipid-modified rac, we also used a combination of unmodified rac1, expressed in Escherichia coli and produced with C-terminal truncation (thus lacking the cysteine that is the site of lipid attachment), and farnesylated C-terminal peptide. This combination can mimic geranylgeranylated rac1, producing a complex with the coumarin-labeled GDI, and was used to examine the relative importance of different regions of rac1 in interaction with GDI.

Molecular dissection of guanine nucleotide dissociation inhibitor function in vivo. Rab-independent binding to membranes and role of Rab recycling factors

Guanine nucleotide dissociation inhibitor (GDI) is an essential protein required for the recycling of Rab GTPases mediating the targeting and fusion of vesicles in the exocytic and endocytic pathways. Using site-directed mutagenesis of yeast GDI1, we demonstrate that amino acid residues required for Rab recognition in vitro are critical for function in vivo in Saccharomyces cerevisiae. Analysis of the effects of Rab-binding mutants on function in vivo reveals that only a small pool of recycling Rab protein is essential for growth, and that the rates of recycling of distinct Rabs are differentially sensitive to GDI. Furthermore, we find that membrane association of Gdi1p is Rab-independent. Mutant Gdi1 proteins unable to bind Rabs were able to associate with cellular membranes as efficiently as wild-type Gdi1p, yet caused a striking loss of the endogenous cytosolic Gdi1p-Rab pools leading to dominant inhibition of growth when expressed at levels of the normal, endogenous pool. These results demonstrate a potential role for a new recycling factor in the retrieval of Rab-GDP from membranes, and illustrate the importance of multiple effectors in regulating GDI function in Rab delivery and retrieval from membranes.

Cleavage and nuclear translocation of the caspase 3 substrate Rho GDP-dissociation inhibitor, D4-GDI, during apoptosis

While investigating endonucleases potentially involved in apoptosis, an antisera was raised to bovine deoxyribonuclease II, but it recognized a smaller protein of 26 kDa protein in a variety of cell lines. The 26 kDa protein underwent proteolytic cleavage to 22 kDa concomitantly with DNA digestion in cells induced to undergo apoptosis. Sequencing of the 26 kDa protein identified it as the Rho GDP-dissociation inhibitor D4-GDI. Zinc, okadaic acid, calyculin A, cantharidin, and the caspase inhibitor z-VAD-fmk, all prevented the cleavage of D4-GDI, DNA digestion, and apoptosis. The 26 kDa protein resided in the cytoplasm of undamaged cells, whereas following cleavage, the 22 kDa form translocated to the nucleus. Human D4-GDI, and D4-GDI mutated at the caspase 1 or caspase 3 sites, were expressed in Chinese hamster ovary cells which show no detectable endogenous D4-GDI. Mutation at the caspase 3 site prevented D4-GDI cleavage but did not inhibit apoptosis induced by staurosporine. The cleavage of D4-GDI could lead to activation of Jun N-terminal kinase which has been implicated as an upstream regulator of apoptosis in some systems. However, the results show that the cleavage of D4-GDI and translocation to the nucleus do not impact on the demise of the cell.

Arrest of endosome acidification by bafilomycin A1 mimics insulin action on GLUT4 translocation in 3T3-L1 adipocytes

In insulin-sensitive fat and muscle cells, the major glucose transporter GLUT4 is constitutively sequestered in endosomal tubulovesicular membranes, and moves to the cell surface in response to insulin. While sequence information within GLUT4 appears to be responsible for its constitutive intracellular sequestration, the regulatory elements and mechanisms that enable this protein to achieve its unique sorting pattern under basal and insulin-stimulated conditions are poorly understood. We show here that arrest of endosome acidification in insulin-sensitive 3T3-L1 adipocytes by bafilomycin A1, a specific inhibitor of the vacuolar proton pump, results in the rapid and dose-dependent translocation of GLUT4 from the cell interior to the membrane surface; the effects of maximally stimulatory concentrations of bafilomycin A1 (400-800 nM) were equivalent to 50-65% of the effects of acute insulin treatment. Like insulin, bafilomycin A1 induced the redistribution of GLUT1 and Rab4, but not that of other proteins whose membrane localization has been shown to be insulin-insensitive. Studies to address the mechanism of this effect demonstrated that neither autophosphorylation nor internalization of the insulin receptor was altered by bafilomycin A1 treatment. Bafilomycin-induced GLUT4 translocation was not blocked by cell pretreatment with wortmannin. Taken together, these data indicate that arrest of endosome acidification mimics insulin action on GLUT4 and GLUT1 translocation by a mechanism distal to insulin receptor and phosphatidylinositol 3-kinase activation, and suggest an important role for endosomal pH in the membrane dynamics of the glucose transporters.

Cytosolic RAB3D is associated with RAB escort protein (REP), not RAB-GDP dissociation inhibitor (GDI), in dispersed chief cells from guinea pig stomach

Rab3D, a low-molecular-weight GTP-binding protein believed to be involved with regulated exocytosis, is associated with secretory granules in gastric chief cells. Although Rab3D is predominantly membrane associated, a significant fraction is cytosolic. Rab proteins are geranylgeranylated on their C-terminal cysteine motifs by geranylgeranyltransferase (GGTase). Rab escort protein (REP) is required to present Rab proteins to GGTase and may accompany newly modified Rab proteins to their target membrane. In most tissues, cytosolic Rab proteins are complexed with rab-GDP dissociation inhibitor (rab-GDI). In the present study, we examined the interactions of Rab3D with cytosolic proteins in dispersed chief cells. Two REP isoforms and at least two GDI isoforms are present in chief cell and brain cytosol. When chief cell cytosol was fractionated by gel filtration chromatography, Rab3D eluted with REP at >150 kDa, whereas rab-GDI eluted as a separate 65-kDa peak, suggesting that Rab3D exists as a complex with REP, but not with rab-GDI. In addition, a small fraction of Rab3D eluted as a monomer at 29 kDa. As has been demonstrated previously, in brain cytosol, Rab3 proteins co-elute with rab-GDI at approx. 90 kDa, suggesting that Rab3 proteins undergo active cycling between membrane and cytosolic compartments in this tissue. In vitro experiments revealed that Rab3D remains associated with REP after geranylgeranylation. Our findings suggest that, in gastric chief cells, Rab3D remains associated with REP after geranylgeranylation until it is presented to its target membrane.

Rho family small G proteins play critical roles in mechanical stress-induced hypertrophic responses in cardiac myocytes

-Mechanical stress induces a variety of hypertrophic responses, such as activation of protein kinases, reprogramming of gene expression, and an increase in protein synthesis. In the present study, to elucidate how mechanical stress induces such events, we examined the role of Rho family small GTP-binding proteins (G proteins) in mechanical stress-induced cardiac hypertrophy. Treatment of neonatal rat cardiomyocytes with the C3 exoenzyme, which abrogates Rho functions, suppressed stretch-induced activation of extracellular signal-regulated protein kinases (ERKs). Overexpression of the Rho GDP dissociation inhibitor (Rho-GDI), dominant-negative mutants of RhoA (DNRhoA), or DNRac1 significantly inhibited stretch-induced activation of transfected ERK2. Overexpression of constitutively active mutants of RhoA slightly activated ERK2 in cardiac myocytes. Overexpression of C-terminal Src kinase, which inhibits functions of the Src family of tyrosine kinases, or overexpression of DNRas had no effect on stretch-induced activation of transfected ERK2. The promoter activity of skeletal alpha-actin and c-fos genes was increased by stretch, and these increases were completely inhibited by either cotransfection of Rho-GDI or pretreatment with C3 exoenzyme. Mechanical stretch increased phenylalanine incorporation into cardiac myocytes by approximately 1.5-fold compared with control, and this increase was also significantly suppressed by pretreatment with C3 exoenzyme. Overexpression of Rho-GDI or DNRhoA did not affect angiotensin II-induced activation of ERK. ERKs were activated by culture media conditioned by stretch of cardiomyocytes without any treatment, but not of cardiomyocytes with pretreatment by C3 exoenzyme. These results suggest that the Rho family of small G proteins plays critical roles in mechanical stress-induced hypertrophic responses.

Acquisition of aluminum tolerance in Saccharomyces cerevisiae by expression of the BCB or NtGDI1 gene derived from plants

Eleven aluminum stress-induced genes derived from plants (wheat, Arabidopsis and tobacco) were introduced into Saccharomyces cerevisiae to test if expression of these genes confers Al tolerance. Al sensitivity tests showed that expression of two genes, either an Arabidopsis gene for blue copper binding protein (BCB), or a tobacco gene for the GDP dissociation inhibitor (NtGDI1), conferred Al tolerance. Determinations of total content and localization of Al ions in these transformants suggested that the BCB gene product functions in restricting Al uptake, while expression of the NtGDI1 gene promotes release of Al ions after uptake.

Vac1p coordinates Rab and phosphatidylinositol 3-kinase signaling in Vps45p-dependent vesicle docking/fusion at the endosome

The vacuolar protein sorting (VPS) pathway of Saccharomyces cerevisiae mediates transport of vacuolar protein precursors from the late Golgi to the lysosome-like vacuole. Sorting of some vacuolar proteins occurs via a prevacuolar endosomal compartment and mutations in a subset of VPS genes (the class D VPS genes) interfere with the Golgi-to-endosome transport step. Several of the encoded proteins, including Pep12p/Vps6p (an endosomal target (t) SNARE) and Vps45p (a Sec1p homologue), bind each other directly [1]. Another of these proteins, Vac1p/Pep7p/Vps19p, associates with Pep12p and binds phosphatidylinositol 3-phosphate (PI(3)P), the product of the Vps34 phosphatidylinositol 3-kinase (PI 3-kinase) [1] [2]. Here, we demonstrate that Vac1p genetically and physically interacts with the activated, GTP-bound form of Vps21p, a Rab GTPase that functions in Golgi-to-endosome transport, and with Vps45p. These results implicate Vac1p as an effector of Vps21p and as a novel Sec1p-family-binding protein. We suggest that Vac1p functions as a multivalent adaptor protein that ensures the high fidelity of vesicle docking and fusion by integrating both phosphoinositide (Vps34p) and GTPase (Vps21p) signals, which are essential for Pep12p- and Vps45p-dependent targeting of Golgi-derived vesicles to the prevacuolar endosome.

Kinetics of Cdc42 membrane extraction by Rho-GDI monitored by real-time fluorescence resonance energy transfer

The mechanisms underlying the ability of the Rho-GDP dissociation inhibitor (RhoGDI) to elicit the release of Rho-related GTP-binding proteins from membranes is currently unknown. In this report, we have set out to address this issue by using fluorescence resonance energy transfer approaches to examine the functional interactions of the RhoGDI with membrane-associated Cdc42. Two fluorescence assays were developed to monitor the interactions between these proteins in real time. The first involved measurements of resonance energy transfer between N-methylanthraniloyl GDP (MantGDP) bound to Cdc42 and fluorescein maleimide covalently attached to cysteine 79 of RhoGDI (RhoGDI-FM). This assay allowed us to directly monitor the binding of RhoGDI to membrane-associated Cdc42. The second fluorescence assay involved measurements of resonance energy transfer between membrane-associated Cdc42-MantGDP and hexadecyl(amino) fluorescein that was randomly inserted into the membrane bilayer. This assay enabled us to directly monitor the (GDI-induced) release of Cdc42 from membranes. Analyses of the rates of change in the fluorescence of Cdc42-MantGDP, which serves as a resonance energy transfer donor in both of these assays, as a function of RhoGDI concentration suggests a two-step mechanism to explain the ability of RhoGDI to stimulate the release of Cdc42 from membranes. Specifically, we propose that the GDI first binds rapidly to membrane-associated Cdc42 and then a slower isomerization occurs which represents the rate-limiting step for the dissociation of the Cdc42-RhoGDI complex from membranes. We propose that this slow step in the observed kinetics reflects the time-course of translocation of the geranyl-geranyl lipid tail of Cdc42 from the outer leaflet of the membrane to the isoprenyl binding site observed in the previously reported NMR structure of the Cdc42-RhoGDI complex [Gosser et al. (1997) Nature 387, 814].

Overcoming expression and purification problems of RhoGDI using a family of "parallel" expression vectors

We describe the construction of expression vectors based on three of the most frequently used gene fusion affinity tags [glutathione S-transferase (GST), maltose binding protein (MBP), and the His6 peptide]. The polylinkers of pGEX4T1, pMal-c2, and a pET vector were replaced with the polylinker isolated from the baculovirus expression plasmid pFastBac. Once appropriate restriction sites have been introduced into a gene, it can be fused to all three affinity tags with little effort, allowing expression-screening experiments to be performed efficiently. We discuss the development and use of these vectors with respect to overcoming purification problems encountered for the RhoA GDP/GTP nucleotide dissociation inhibitor (RhoGDI) and their advantages over commercially available expression vectors.

Mechanism of activation of Pak1 kinase by membrane localization

Pak kinases are a family of serine/threonine protein kinases homologous to Ste20p of yeast. Paks can be activated in vivo and in vitro by binding to GTP-bound Cdc42 and Rac1, members of the Rho family of small GTPases implicated in regulating the organization of the actin cytoskeleton. We have previously reported that the SH2/SH3-containing adaptor protein Nck binds Pak kinase through its second SH3 domain. Pak1 can be targeted to the membrane by Nck in response to tyrosine phosphorylation, and membrane association of Pak1 is sufficient to increase its specific activity. The mechanism whereby Pak is activated by membrane localization, however, is unknown. We show here that expression of three proteins that inhibit Rho-family GTPases by different mechanisms (RhoGDI, Bcr and D57Y Cdc42) all block the activation of Pak by a membrane-targeted Nck SH3 domain, demonstrating that the in vivo activation of Pak1 induced by membrane localization is dependent on Rho-family GTPases. This implies that Pak activity can be regulated in cells both by the level of GTP loading of various Rho-family GTPases and the local concentration of Pak relative to these GTPases. Our data also suggest the existence of Rho-family GTPases in addition to Cdc42 and Rac1 that can activate Pak on membranes.

Defining the functions of trans-SNARE pairs

The homotypic fusion of yeast vacuoles includes a 'docking' step, which we show here to consist of two sequential reactions: a reversible 'tethering' mediated by the GTPase Ypt7, and 'SNARE pairing', in which SNARE proteins from opposite membranes form a complex in trans. The function of this trans-SNARE complex must be transient, as the complex can be disassembled by excess Sec18 in the presence of Sec17 and ATP without influencing the fusion rate. These data indicate that SNARE pairing may transiently signal to downstream factors, leading to fusion.

Nuclear transport factor p10/NTF2 functions as a Ran-GDP dissociation inhibitor (Ran-GDI)

The cytosolic nuclear transport factor p10/NTF2 is required for the translocation of karyophilic molecules through nuclear pores [1] [2] [3], and the small GTPase Ran is a key regulator of protein transport between the nucleus and cytoplasm [4] [5]. It has been reported that p10/NTF2 interacts directly and specifically with Ran-GDP but not with Ran-GTP [6]. The precise role(s) of p10/NTF2 in the Ran GTP/GDP cycle are thus far unclear, however. In this study, we show that mammalian p10/NTF2 dramatically inhibits the dissociation of [3H]GDP from Ran and the binding of [35S]GTPgammaS to Ran following the dissociation of non-radioactive GDP by RCC1, the only known mammalian guanine nucleotide exchange factor for Ran (Ran-GEF) [7]. In contrast, the dissociation of [35S]GTP gamma S from Ran, which was also catalyzed by RCC1, was not affected by p10/NTF2. Furthermore, the activities of wild-type p10/NTF2 and the mutant forms M84T and D92G in an assay of nuclear protein import in a digitonin-permeabilized cell-free system correlated with their level of inhibition of the dissociation of nucleotide from Ran-GDP. These results suggest that p10/NTF2 acts as a GDP dissociation inhibitor for Ran (Ran-GDI), thereby coordinating the Ran-dependent reactions that underlie nuclear protein import.

Rab11 is required for trans-golgi network-to-plasma membrane transport and a preferential target for GDP dissociation inhibitor

The rab11 GTPase has been localized to both the Golgi and recycling endosomes; however, its Golgi-associated function has remained obscure. In this study, rab11 function in exocytic transport was analyzed by using two independent means to perturb its activity. First, expression of the dominant interfering rab11S25N mutant protein led to a significant inhibition of the cell surface transport of vesicular stomatitis virus (VSV) G protein and caused VSV G protein to accumulate in the Golgi. On the other hand, the expression of wild-type rab11 or the activating rab11Q70L mutant had no adverse effect on VSV G transport. Next, the membrane association of rab11, which is crucial for its function, was perturbed by modest increases in GDP dissociation inhibitor (GDI) levels. This led to selective inhibition of the trans-Golgi network to cell surface delivery, whereas endoplasmic reticulum-to-Golgi and intra-Golgi transport were largely unaffected. The transport inhibition was reversed specifically by coexpression of wild-type rab11 with GDI. Under the same conditions two other exocytic rab proteins, rab2 and rab8, remained membrane bound, and the transport steps regulated by these rab proteins were unaffected. Neither mutant rab11S25N nor GDI overexpression had any impact on the cell surface delivery of influenza hemagglutinin. These data show that functional rab11 is critical for the export of a basolateral marker but not an apical marker from the trans-Golgi network and pinpoint rab11 as a sensitive target for inhibition by excess GDI.