Characterization and biological actions of N-terminal truncated forms of glucose-dependent insulinotropic polypeptide

The N-terminal domain of glucose-dependent insulinotropic polypeptide (GIP) plays an important role in regulating biological activity. This study examined biological properties of several N-terminal truncated forms of GIP and two novel forms with substitutions at Phe position-6 with Arg or Val. GIP(6-42), GIP(R6-42), GIP(V6-42), GIP(7-42) and GIP(9-42) stimulated cAMP production in BRIN-BD11 cells similar to native GIP, whereas responses to GIP(3-42), GIP(4-42), GIP(5-42) and GIP(8-42) were reduced (P < 0.01 to P < 0.001). GIP-induced cyclic AMP production was significantly inhibited by GIP(3-42), GIP(4-42), GIP(5-42), GIP(6-42), GIP(R6-42), GIP(7-42) and GIP(8-42) (P < 0.001). Compared with native GIP, in vitro insulinotropic activity of GIP(3-42), GIP(4-42), GIP(5-42), GIP(7-42) and GIP(8-42) was reduced (P < 0.05 to P < 0.001), with GIP(4-42), GIP(5-42), GIP(7-42) and GIP(8-42) also potently inhibiting GIP-stimulated insulin secretion (P < 0.001). In ob/ob mice, GIP(4-42) and GIP(8-42) increased (P < 0.05 to P < 0.01) plasma glucose concentrations compared to the glucose-lowering action of native GIP. When GIP(8-42) was co-administered with native GIP it countered the ability of the native peptide to lower plasma glucose and increase circulating insulin concentrations. These data confirm the importance of the N-terminal region of GIP in regulating bioactivity and reveal that sequential truncation of the peptide yields novel GIP receptor antagonists which may have functional significance.     

In vivo priming of mouse CTL precursors directed to product of a newly defined minor H-42 locus is under a novel control of class II MHC gene

In a previous study, we discovered a new mouse minor histocompatibility antigen encoded by a locus at 8.5 cM apart from the H-2 complex, and we have since named the locus H-42. One allele of H-42, which is named H-42a, had been elucidated, but the other alleles, which we tentatively named H-42b, have not been elucidated. In the present study, we explored MHC control on the anti-H-42a cytotoxic T lymphocyte (CTL) responsiveness in H-42b mice. In vivo immunization (i.v. injection) of H-42b mice with 5 to 30 X 10(6) spleen cells (SC) bearing allogeneic H-42a antigen but carrying H-2 complex (mouse MHC) matched with the H-42b mice failed to prime anti-H-42a CTL but induced stable and specific anti-H-42a CTL unresponsiveness, i.e., tolerance, in the H-42b recipient mice. In contrast, H-2 heterozygous H-42b F1 mice injected with SC bearing H-42a alloantigen on either of the parental H-2 haplotypes were effectively primed to generate anti-H-42a CTL. Exploration of the region or subregion in the H-2 complex of H-42a donor SC that should be compatible with H-42b recipient mice for the induction of their anti-H-42a CTL tolerance demonstrated that the compatibility at I region, most probably I-A subregion, but not at K, S, or D region, determined the induction of the tolerance. MHC class II compatible H-42a skin graft (SG) to H-42b mice, however, consistently primed the anti-H-42a CTL in the H-42b recipients. These results were discussed in several aspects, including uniqueness of MHC class II control on the CTL response to minor H-42a antigen, possibility of inactivation of responding anti-H-42a precursor CTL or helper T cells in H-42b mice by encountering the veto cells present in MHC class II-matched H-42a SC population, and significance of the present observations as a mechanism of CTL tolerance to self-components.     

Role of histidine 42 in ascorbate peroxidase. Kinetic analysis of the H42A and H42E variants.

To examine the role of the distal His42 residue in the catalytic mechanism of pea cytosolic ascorbate peroxidase, two site-directed variants were prepared in which His42 was replaced with alanine (H42A) or glutamic acid (H42E). Electronic spectra of the ferric derivatives of H42A and H42E (pH 7.0, mu = 0.10 m, 25.0 degrees C) revealed wavelength maxima [lambda(max) (nm): 397, 509, approximately equal to 540(sh), 644 (H42A); 404, 516, approximately equal to 538(sh), 639 (H42E)] consistent with a predominantly five-co-ordinate high-spin iron. The specific activity of H42E for oxidation of L-ascorbate (8.2 +/- 0.3 U.mg(-1)) was approximately equal to 30-fold lower than that of the recombinant wild-type enzyme (rAPX); the H42A variant was essentially inactive but activity could be partially recovered by addition of exogenous imidazoles. The spectra of the Compound I intermediates of H42A [lambda(max) (nm) = 403, 534, 575(sh), 645] and H42E [lambda(max) (nm) = 404, 530, 573(sh), 654] were similar to those of rAPX. Pre-steady-state data for formation of Compound I for H42A and H42E were consistent with a mechanism involving accumulation of a transient enzyme intermediate (K(d)) followed by conversion of this intermediate into Compound I (k'(1)). Values for k'(1) and K(d) were, respectively, 4.3 +/- 0.2 s(-1) and 30 +/- 2.0 mM (H42A) and 28 +/- 1.0 s(-1) and 0.09 +/- 0.01 mM (H42E). Photodiode array experiments for H42A revealed wavelength maxima for this intermediate at 401 nm, 522 nm and 643 nm, consistent with the formation of a transient [H42A-H(2)O(2)] species. Rate constants for Compound I formation for H42A were independent of pH, but for rAPX and H42E were pH-dependent [pKa = 4.9 +/- 0.1 (rAPX) and pK(a) = 6.7 +/- 0.2 (H42E)]. The results provide: (a) evidence that His42 is critical for Compound I formation in APX; (b) confirmation that titration of His42 controls Compound I formation and an assignment of the pK(a) for this group; (c) mechanistic and spectroscopic evidence for an intermediate before Compound I formation; (d) evidence that a glutamic acid residue at position 42 can act as the acid-base catalyst in ascorbate peroxidase.     

Transport of molybdate by Clostridium pasteurianum.

The transport of 99MoO42- into dinitrogen-fixing cells of Clostridium pasteurianum was investigated. Transport of molybdate in this organism is energy dependent; sucrose is required in the minimal media, and the system is inhibited by the glycolysis inhibitors, NaF, iodoacetic acid, and arsenate. The cells accumulate molybdate against a concentration gradient, and the uptake shows a marked dependence on temperature (optimum 37 C) and pH (optimum 6.0). The rate of molybdate uptake with increasing molybdate concentrations shows saturation kinetics with an apparent Km and Vmax of 4.8 X 10(-5) M and 55 nmol/g of dry cells per min, respectively. Inhibition studies with the anions SO42-, S2O32-, WO42-, and VO32- show that SO42- and WO42- competitively inhibit MoO42- uptake (apparent Ki [SO42-] is 3.0 X 10(-5) M; apparent Ki [WO42-] is 2.4 X 10(-5), whereas S2O32- and VO32- have no inhibitory effect. Exchange experiments with MoO42- show that only a small percentage of the 99MoO42- taken up by the cells is exchangeable. Exchange experiments with WO42- and SO42- indicate that once inside the cells WO42- and SO42- cannot substitute for MoO42-.     

Cytotoxic T lymphocyte response to minor H-42a alloantigen in H-42b mice: clonal inactivation of the precursor cytotoxic T lymphocytes by veto-like spleen cells that express the H-42a antigen

When (B10.BR X CWB)F1 (BWF1; H-2k/b) mice carrying the H-42b allele at the minor H-42 locus were injected with H-42a C3H.SW (CSW; H-2b) or C3H (H-2k) spleen cells (SC), self-H-2Kb restricted anti-H-42a pCTL in the BWF1 recipients were primed and differentiated to anti-H-42a CTL after in vitro stimulation with (B10.BR X CSW)F1 (BSF1; H-2k/b, H-42b/a) SC. In contrast, anti-H-42a pCTL in H-42b mice were inactivated by injection with H-42-congenic H-42a SC, and stable anti-H-42a CTL tolerance was induced. Preference of H-2Kb restriction of anti-H-42a CTL was strict, and self-H-2Kb-restricted anti-H-42a CTL did not lyse target cells carrying H-42a antigen in the context of H-2Kbm1. Involvement of suppressor cells in the anti-H-42a CTL tolerance was ruled out by the present cell transfer study and the previous cell-mixing in vitro study. Notably, treatment with anti-Thy-1.2 antibody (Ab) plus complement (C) wiped out the ability of CSW SC in the priming of anti-H-42a pCTL of BWF1 mice but left that of C3H SC unaffected, and injection of the anti-Thy-1.2 Ab plus C-treated CSW SC induced anti-H-42a CTL tolerance in the BWF1 recipients. Furthermore, H-42a/b, I-Ab/bm12 [CSW X B6.CH-2bm12 (bm12)]F1 SC could not prime anti-H-42a pCTL in H-42b, I-Ab (CWB X B6)F1 recipients, whereas H-42a/b, I-Ab (CSW X B6)F1 SC primed anti-H-42a pCTL in H-42b, I-Ab/bm12 (CWB X bm12)F1 recipients. The unresponsiveness of anti-H-42a pCTL in H-42b mice to H-42-congenic H-42a SC was sometimes corrected by immunization of H-42b female mice with H-42-congenic H-42a male SC. Taking all of the results together, we propose the following. Unresponsiveness of anti-H-42a pCTL in H-42b mice to H-42-congenic H-42a SC is caused by "veto cells" contained in the antigenic H-42a SC. Anti-H-42a pCTL in the H-42b recipients directly interacting with H-42-congenic H-42a SC, which bear H-42a antigen and H-2Kb restriction element, are inactivated or vetoed.(ABSTRACT TRUNCATED AT 400 WORDS)     

Antiapoptotic Cdc42 mutants are potent activators of cellular transformation

Cdc42 is a small GTP-binding protein which has been implicated in a number of cellular activities, including cell morphology, motility, cell-cycle progression, and malignant transformation. While GTPase-defective forms of Cdc42 inhibit cell growth, a mutation [Cdc42(F28L)] that allows the constitutive exchange of GDP for GTP and is GTPase-competent induces cellular transformation. These results suggest that Cdc42 must cycle between its GTP- and GDP-bound states to stimulate cell growth. In attempting to design Cdc42 molecules with more potent transforming activity, we set out to generate other types of Cdc42 mutants capable of constitutive GDP-GTP exchange. Here, we describe one such mutant, generated by changing a conserved aspartic acid residue at position 118 to an asparagine. The Cdc42(D118N) protein exchanges GDP for GTP more rapidly than wild-type Cdc42, but significantly more slowly than the Cdc42(F28L) mutant. Despite its slower rate of activation, the Cdc42(D118N) mutant is more potent at inducing cellular transformation than the Cdc42(F28L) protein, and causes a significant loss in actin stress fibers, reminiscent of what is observed with fibroblasts transformed by oncogenic Ras mutants. Effector-loop mutations made within the D118N background inhibit Cdc42-induced transformation and Cdc42-mediated antiapoptotic (survival) activity to similar extents. In addition, mutating aspartic acid 121 (to asparagine), which forms part of a caspase cleavage site (DLRD, residues 118-121 of Cdc42), in combination with the F28L mutation generates a Cdc42 molecule [Cdc42(F28L/D121N)] with transforming activity significantly stronger than that of Cdc42(F28L). Thus, mutations that combine some capacity for cycling between the GTP- and GDP-bound states with increased survival against apoptotic signals yield Cdc42 molecules with the maximum capability for inducing cellular transformation.     

Positive effects of Forskolin (stimulator of lipolysis) treatment on cryosurvival of in vitro matured porcine oocytes

In order to examine its effect on oocyte lipid content and cryosurvival, Forskolin was added to the medium for in vitro maturation of porcine oocytes. Treatments were control (IVM without Forskolin during the 42 h incubation period), addition of 10 μM Forskolin for the entire 42 h (0-42) and addition of 10 μM Forskolin between 24 and 42 h only (24-42). In Experiment 1, treatments did not differ significantly in cleavage rate, but the blastocyst formation rate was lower in the 0-42 group than for control and 24-42 group oocytes (17, 32 and 40%, respectively; P < 0.05). It was shown in Experiment 2 that Forskolin treatment from 0-42 h and from 24-42 h significantly reduced lipid content of oocytes compared to that of control cells (65 and 99 vs. 140 μm² intensity of fluorescence, respectively; P < 0.05). In Experiment 3, the percentage of oocyte survival after cryopreservation and thawing was significantly higher in both Forskolin treatment groups than in control oocytes (72% for 0-42, 65% for 24-42 and 52% for control; P < 0.05). However, Forskolin treatment did not increase cleavage rates of vitrified in vitro matured porcine oocytes (Control group 28%, 0-42 h group 0%, 24-42 h group 26.67%). Addition of Forskolin affected the nuclear maturation of porcine oocytes. The percentage of PBE (polar body extrusion) were significantly reduced in the 0-42 h group (0-42 h group 42.00 ± 2.08 vs. Control group 79.70 ± 2.82 and 24-42 h group 70.60 ± 2.83; P < 0.05). The 24-42 h group showed similar nuclear status to that of the Control group. We propose that delipation engendered by incubation with 10 μM Forskolin during 24-42 hours of maturation increased cryosurvival of in vitro-maturated porcine oocytes and that attendant chemical lipolysis did not impair their further development as it may have done in oocytes incubated with Forskolin for the full 42 h.     

Identification of the human platelet GTPase activating protein for the CDC42Hs protein

The CDC42Hs protein appears to be an isoform of the ras-related GTP-binding protein G25K and is an apparent human homolog of the Saccharomyces cerevisiae cell-division-cycle protein, CDC42Sc. In this study, we report the identification of a GTPase-activating protein (GAP) for CDC42Hs from human platelets (designated from here on as CDC42Hs-GAP). The CDC42Hs-GAP activity was solubilized from platelet membranes, recovered through successive chromatography steps (the final step being Mono-Q chromatography), and purified approximately 3500-fold. The CDC42Hs-GAP activity appeared to correspond to a polypeptide with an apparent Mr of approximately 25,000. The GTPase activities of the purified human platelet CDC42Hs, the Escherichia coli-recombinant CDC42Hs, and the Spodoptera frugiperda-recombinant GTP-binding proteins are all stimulated by the CDC42Hs-GAP to identical extents, which indicates that the recombinant CDC42Hs proteins are as effective as the native human platelet protein in coupling to the GAP. However, a mutant form of the E. coli-recombinant CDC42Hs which contains a valine residue at position 12 (CDC42HsVal-12) has a significantly reduced intrinsic GTPase activity (relative to the wild type CDC42HsGly-12) which is not stimulated by the CDC42Hs-GAP. The CDC42Hs-GAP also does not stimulate the GTPase activities of the ras or rap GTP-binding proteins; however, it is capable of a weak stimulation of the GTPase activity of mammalian rho. Based on the apparent similarities in the molecular size of the CDC42Hs- and rho-GAPs (i.e. 25-30 kDa), and the cross-reactivity of rho with the CDC42Hs-GAP, it seems likely that the CDC42Hs- and rho-GAPs will constitute a specific subclass of the ras-related GAP superfamily.     

Identification of a 42-kilodalton phosphotyrosyl protein as a serine(threonine) protein kinase by renaturation.

We have surveyed fibroblast lysates for protein kinases that might be involved in mitogenesis. The assay we have used exploits the ability of blotted, sodium dodecyl sulfate-denatured proteins to regain enzymatic activity after guanidine treatment. About 20 electrophoretically distinct protein kinases could be detected by this method in lysates from NIH 3T3 cells. One of the kinases, a 42-kilodalton serine(threonine) kinase (PK42), was found to possess two- to fourfold-higher in vitro activity when isolated from serum-stimulated cells than when isolated from serum-starved cells. This kinase comigrated on sodium dodecyl sulfate-gels with a protein (p42) whose phosphotyrosine content increased in response to serum stimulation. The time courses of p42 tyrosine phosphorylation and PK42 activation were similar, reaching maximal levels within 10 min and returning to basal levels within 5 h. Both p42 tyrosine phosphorylation and PK42 activation were stimulated by low concentrations of phorbol esters, and the responses of p42 and PK42 to TPA were abolished by chronic 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment. Chronic TPA treatment had less effect on serum-induced p42 tyrosine phosphorylation and PK42 activation. PK42 and p42 bound to DEAE-cellulose, and both eluted at a salt concentration of 250 mM. Thus, PK42 and p42 comigrate and cochromatograph, and the kinase activity of PK42 correlates with the tyrosine phosphorylation of p42. These findings suggest that PK42 and p42 are related or identical, that PK42 is activated by tyrosine phosphorylation, and that this tyrosine phosphorylation can be regulated by protein kinase C.     

Autographa californica multiple nucleopolyhedrovirus nucleocapsid protein BV/ODV-C42 mediates the nuclear entry of P78/83

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) BV/ODV-c42 (orf101; c42), which encodes a 41.5-kDa viral nucleocapsid protein with a putative nuclear localization signal (NLS) motif at the C terminus, is a highly conserved gene among members of the Baculoviridae family. C42 is demonstrated to be essential for AcMNPV propagation and can bind to nucleocapsid protein P78/83, a viral activator for the actin-related protein 2/3 (ARP2/3) complex to initiate nuclear actin polymerization, which is essential for viral nucleocapsid morphogenesis during AcMNPV infection. Here, we report the identification of a novel pathway through which c42 functions in nucleocapsid assembly. Cotransfection of Sf9 cells with c42 and p78/83 plasmids demonstrated that C42 was capable of recruiting P78/83 to the nuclei of uninfected cells and that the NLS motif of C42 was essential for this process. To validate this nuclear relocation mode in bacmid-transfected cells, a c42-disrupted bacmid (vAc(c42ko-gfp)) and rescued bacmids with wild-type c42 (vAc(c42res-gfp)) or with NLS coding sequence-mutated c42 (vAc(c42nls-gfp)) were prepared. By immuno-staining, P78/83 was found to be localized in the cytoplasm of either vAc(c42ko-gfp)- or vAc(c42nls-gfp)-transfected cells, whereas P78/83 was relocated to the nuclei of vAc(c42res-gfp)-transfected cells. Furthermore, F-actin-specific staining confirmed that there was no actin polymerization activity in the nuclei of either vAc(c42ko-gfp)- or vAc(c42nls-gfp)-transfected cells, which might be attributed to the absence of nuclear P78/83, an activator of the ARP2/3 complex to initiate nuclear actin polymerization. We therefore hypothesize a mode of action where C42 binds to P78/83 in the cytoplasm to form a protein complex and cotransports to the nucleus under the direction of the NLS motif in C42 during AcMNPV infection.     

Actions of thrombin and thrombin receptor peptide analogues in gastric and aortic smooth muscle: development of bioassays for structure-activity studies.

We have examined the biological activities of thrombin and the thrombin-receptor-related polypeptides, S42FLLRNPNDKYEPF55(TRP42-55), S42FLLRNPND50(TRP42-50), and A42FLLRNPND50(A42-TRP42-50) as well as an arginine-containing basic peptide beginning with the SF motif (SFRGHITR), in rat aortic (RA) rings and in a gastric guinea pig longitudinal (LM) smooth muscle preparation. In the RA preparation, thrombin, as well as the three receptor-related peptides caused a relaxation in tissue that was precontracted with noradrenaline; the basic peptide, SFRGHITR, was inactive either as an agonist or as an antagonist to TRP42-55. In the LM bioassay, which unlike the RA preparation did not persistently desensitize in response to thrombin, all three receptor-related peptides, like thrombin, caused a prompt phasic reproducible contraction. The basic peptide, SFRGHITR, was inactive. In the LM assay, TRP42-55, TRP42-50 and A42-TRP42-55 all caused comparable contractile responses. We conclude that the gastric LM smooth muscle possesses a thrombin receptor and provides a convenient and reliable assay for the activities of thrombin receptor-related peptides. Our data also demonstrated that neither the C-terminal hirudin-related pentapeptide nor the N-terminal serine hydroxyl group are required for the biological activity of the thrombin receptor-derived peptide previously described (TRP42-55). Based on our findings we suggest that only a small portion of the N-terminal sequence of TRP42-55 may be required for thrombin-like biological activity.     

Sequence determinants of enhanced amyloidogenicity of Alzheimer A{beta}42 peptide relative to A{beta}40

Aggregation of proteins into insoluble deposits is associated with a variety of human diseases. In Alzheimer disease, the aggregation of amyloid beta (Abeta) peptides is believed to play a key role in pathogenesis. Although the 40-mer (Abeta40) is produced in vivo at higher levels than the 42-mer (Abeta42), senile plaque in diseased brains is composed primarily of Abeta42. Likewise, in vitro, Abeta42 forms fibrils more rapidly than Abeta40. The enhanced amyloidogenicity of Abeta42 could be due simply to its greater length. Alternatively, specific properties of residues Ile(41) and Ala(42) might favor aggregation. To distinguish between these two possibilities, we constructed a library of sequences in which residues 41 and 42 were randomized. The aggregation behavior of the resulting sequences was assessed using a high throughput screen, based on the finding that fusions of Abeta42 to green fluorescence protein (GFP) prevent the folding and fluorescence of GFP, whereas mutations in Abeta42 that disrupt aggregation produce green fluorescent fusions. Correlations between the sequences of Abeta42 mutants and the fluorescence of Abeta42-GFP fusions in vivo were confirmed in vitro through biophysical studies of synthetic 42-residue peptides. The data reveal a strong correlation between aggregation propensity and the hydrophobicity and beta-sheet propensities of residues at positions 41 and 42. Moreover, several mutants containing hydrophilic residues and/or beta-sheet breakers at positions 41 and/or 42 were less prone to aggregate than Abeta40 wherein these two residues are deleted entirely. Thus, properties of the side chains at positions 41 and 42, rather than length per se, cause Abeta42 to aggregate more readily than Abeta40.     

Production of recombinant amyloid-beta peptide 42 as an ubiquitin extension

Amyloid-beta peptide 42 (Abeta42) mediates neuronal degeneration in Alzheimer's disease (AD). We sought to produce recombinant Abeta42 as an ubiquitin extension. A synthetic oligonucleotide encoding Abeta42 was constructed and cloned as an extended polypeptide of hexahistidine-tagged ubiquitin (H(6)Ub) using the pET vector. Isopropyl-beta-D-thiogalactopyranoside induction of transformed Escherichia coli resulted in the production of large amounts of insoluble H(6)Ub-Abeta42 fusion protein. H(6)Ub-Abeta42 was solubilized in 8 M urea and applied to a nickel-nitrilotriacetic acid affinity column for purification. Column washing removed the urea and soluble H(6)Ub-Abeta42 was eluted, indicating that covalently attached ubiquitin prevented Abeta42 from aggregating. Abeta42 was cleaved from H(6)Ub using recombinant yeast ubiquitin hydrolase-1 (YUH-1) and purified using reverse-phase chromatography. The recombinant Abeta42 prepared in this study has the same toxic effect on human neuroblastoma SH-SY5Y cells comparing with chemically synthesized, commercial one. The peptide yield was more than 4 mg/L culture, indicating this ubiquitin fusion technique is an attractive method for production of aggregation-prone peptides such as Abeta42.     

GIP-(3-42) does not antagonize insulinotropic effects of GIP at physiological concentrations

Glucose-dependent insulinotropic polypeptide [GIP-(1-42)] is degraded by dipeptidyl peptidase IV (DPP IV), forming GIP-(3-42). In mice, high concentrations of synthetic GIP-(3-42) may function as a GIP receptor antagonist, but it is unclear whether this occurs at physiological concentrations. In COS-7 cells transiently transfected with the human GIP receptor, GIP-(1-42) and -(3-42) bind with affinities (IC(50)) of 5.2 and 22 nM, respectively. GIP-(1-42) was a potent agonist, stimulating cAMP accumulation (EC(50), 13.5 pM); GIP-(3-42) alone had no effect. When incubated together with native GIP, GIP-(3-42) behaved as a weak antagonist (IC(50), 92 and 731 nM for inhibition of cAMP accumulation elicited by 10 pM and 1 nM native GIP, respectively). In the isolated perfused rat pancreas, GIP-(3-42) alone had no effect on insulin output and only reduced the response to GIP (1 nM) when coinfused in >50-fold molar excess (IC(50), 138 nM). The ability of GIP-(3-42) to affect the antihyperglycemic or insulinotropic actions of GIP-(1-42) was examined in chloralose-anesthetized pigs given intravenous glucose. Endogenous DPP IV activity was inhibited to reduce degradation of the infused GIP-(1-42), which was infused alone and together with GIP-(3-42), at rates sufficient to mimic postprandial concentrations of each peptide. Glucose, insulin, and glucagon responses were identical irrespective of whether GIP-(1-42) was infused alone or together with GIP-(3-42). We conclude that, although GIP-(3-42) can weakly antagonize cAMP accumulation and insulin output in vitro, it does not behave as a physiological antagonist in vivo.     

Changes in phosphorylation of an F-actin capping protein of Physarum polycephalum during the cell cycle

The Cap 42(b), a Ca2+-dependent F-actin capping phosphoprotein of 42,000 daltons, was shown to be localized in the cytosol of Physarum polycephalum by measurements of phosphorylatability in the absence of Ca2+. The phosphorylation of Cap 42(b) in the cytosol changed during the cell cycle: it was high in the S and G2 phase, and low in the M phase and boundary phase between S and G2 phase. When the isolated Cap 42(b) was added to M phase cytosol, the phosphorylation of Cap 42(b) was significantly increased by at least 6-fold. Compared with this result, about 2-fold increase in the phosphorylation of Cap 42(b) was observed when the Cap 42(b) kinase was added to M phase cytosol. Therefore, it is likely that the low level of Cap 42(b) phosphorylation in M phase cytosol is mostly due to the decreased amount of phosphorylatable Cap 42(b) and to a lesser extent due to a low level of the Cap 42(b) kinase activity.     

Role of Cdc42-Cla4 interaction in the pheromone response of Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the highly conserved Rho-type GTPase Cdc42 is essential for cell division and controls cellular development during mating and invasive growth. The role of Cdc42 in mating has been controversial, but a number of previous studies suggest that the GTPase controls the mitogen-activated protein (MAP) kinase cascade by activating the p21-activated protein kinase (PAK) Ste20. To further explore the role of Cdc42 in pheromone-stimulated signaling, we isolated novel alleles of CDC42 that confer resistance to pheromone. We find that in CDC42(V36A) and CDC42(V36A, I182T) mutant strains, the inability to undergo pheromone-induced cell cycle arrest correlates with reduced phosphorylation of the mating MAP kinases Fus3 and Kss1 and with a decrease in mating efficiency. Furthermore, Cdc42(V36A) and Cdc42(V36A, I182T) proteins show reduced interaction with the PAK Cla4 but not with Ste20. We also show that deletion of CLA4 in a CDC42(V36A, I182T) mutant strain suppresses pheromone resistance and that overexpression of CLA4 interferes with pheromone-induced cell cycle arrest and MAP kinase phosphorylation in CDC42 wild-type strains. Our data indicate that Cla4 has the potential to act as a negative regulator of the mating pathway and that this function of the PAK might be under control of Cdc42. In conclusion, our study suggests that control of pheromone signaling by Cdc42 not only depends on Ste20 but also involves interaction of the GTPase with Cla4.     

Epidermal growth factor-dependent regulation of Cdc42 is mediated by the Src tyrosine kinase

Treatment of cells with epidermal growth factor (EGF) promotes the activation of the small GTP-binding protein Cdc42, as well as its phosphorylation in cells. The EGF-dependent phosphorylation of Cdc42 occurs at tyrosine 64 in the Switch II domain and appears to be mediated through the Src tyrosine kinase, because both the expression of a dominant-negative Src mutant (mouse Src(K297R)) and treatment of cells with the Src kinase inhibitor PP2 blocks the EGF-stimulated phosphorylation of Cdc42, whereas expression of an activated Src mutant (Src(Y529F)) promotes phosphorylation in the absence of EGF treatment. The EGF-stimulated phosphorylation of Cdc42 is not required for its activation, nor does it directly affect the interactions of activated Cdc42 with target/effector proteins including PAK, ACK, WASP, or IQGAP. However, the EGF-stimulated phosphorylation of Cdc42 is accompanied by an enhancement in the interaction of Cdc42 with the Rho-GDP dissociation inhibitor (RhoGDI). The EGF-stimulated activation of Cdc42 does require activated Src, as well as the Vav2 protein, a member of the Dbl family of guanine nucleotide exchange factors. Src catalyzes the tyrosine phosphorylation of Vav2, and overexpression of Vav2 together with activated Src (Src(Y529F)) can completely bypass the need for EGF to promote the activation of Cdc42. Thus, EGF signaling through Src appears to have dual regulatory effects on Cdc42: 1). it leads to the activation of Cdc42 as mediated by the Vav2 guanine nucleotide exchange factor, and 2). it results in the phosphorylation of Cdc42, which stimulates the binding of RhoGDI, perhaps to direct the movement of Cdc42 to a specific cellular site to trigger a signaling response, because Cdc42-RhoGDI interactions are essential for Cdc42-induced cellular transformation.     

Matrix rigidity regulates spatiotemporal dynamics of Cdc42 activity and vacuole formation kinetics of endothelial colony forming cells

Recent evidence has shown that endothelial colony forming cells (ECFCs) may serve as a cell therapy for improving blood vessel formation in subjects with vascular injury, largely due to their robust vasculogenic potential. The Rho family GTPase Cdc42 is known to play a primary role in this vasculogenesis process, but little is known about how extracellular matrix (ECM) rigidity affects Cdc42 activity during the process. In this study, we addressed two questions: Does matrix rigidity affect Cdc42 activity in ECFC undergoing early vacuole formation? How is the spatiotemporal activation of Cdc42 related to ECFC vacuole formation? A fluorescence resonance energy transfer (FRET)-based Cdc42 biosensor was used to examine the effects of the rigidity of three-dimensional (3D) collagen matrices on spatiotemporal activity of Cdc42 in ECFCs. Collagen matrix stiffness was modulated by varying the collagen concentration and therefore fibril density. The results showed that soft (150 Pa) matrices induced an increased level of Cdc42 activity compared to stiff (1 kPa) matrices. Time-course imaging and colocalization analysis of Cdc42 activity and vacuole formation revealed that Cdc42 activity was colocalized to the periphery of cytoplasmic vacuoles. Moreover, soft matrices generated faster and larger vacuoles than stiff matrices. The matrix-driven vacuole formation was enhanced by a constitutively active Cdc42 mutant, but significantly inhibited by a dominant-negative Cdc42 mutant. Collectively, the results suggest that matrix rigidity is a strong regulator of Cdc42 activity and vacuole formation kinetics, and that enhanced activity of Cdc42 is an important step in early vacuole formation in ECFCs.     

Chitinase gene expression during mycoparasitic interaction of Trichoderma harzianum with its host.

For monitoring chitinase expression during mycoparasitism of Trichoderma harzianum in situ, we constructed strains containing fusions of green fluorescent protein (GFP) to the 5'-regulatory sequences of the T. harzianum nag1 (N-acetyl-beta-d-glucosaminidase-encoding) and ech42 (42-kDa endochitinase-encoding) genes. Confronting these strains with Rhizoctonia solani led to induction of gene expression before (ech42) or after (nag1) physical contact. A 12-kDa cut-off membrane separating the two fungi abolished ech42 expression, indicating that macromolecules are involved in its precontact activation. No ech42 expression was triggered by culture filtrates of R. solani or by placing T. harzianum onto plates previously colonized by R. solani. Instead, high expression occurred upon incubation of T. harzianum with the supernatant of R. solani cell walls digested with culture filtrates or purified endochitinase 42 (CHIT42, encoded by ech42) from T. harzianum. The chitinase inhibitor allosamidin blocked ech42 expression and reduced inhibition of R. solani growth during confrontation. The results indicate that ech42 is expressed before contact of T. harzianum with R. solani and its induction is triggered by soluble chitooligosaccharides produced by constitutive activity of CHIT42 and/or other chitinolytic enzymes.     

Binding of Cdc42 to phospholipase D1 is important in neurite outgrowth of neural stem cells

We previously demonstrated that phospholipase D (PLD) expression and PLD activity are upregulated during neuronal differentiation. In the present study, employing neural stem cells from the brain cortex of E14 rat embryos, we investigated the role of Rho family GTPases in PLD activation and in neurite outgrowth of neural stem cells during differentiation. As neuronal differentiation progressed, the expression levels of Cdc42 and RhoA increased. Furthermore, Cdc42 and PLD1 were mainly localized in neurite, whereas RhoA was localized in cytosol. Co-immunoprecipitation revealed that Cdc42 was bound to PLD1 during differentiation, whereas RhoA was associated with PLD1 during both proliferation and differentiation. These results indicate that the association between Cdc42 and PLD1 is related to neuronal differentiation. To examine the effect of Cdc42 on PLD activation and neurite outgrowth, we transfected dominant negative Cdc42 (Cdc42N17) and constitutively active Cdc42 (Cdc42V12) into neural stem cells, respectively. Overexpression of Cdc42N17 decreased both PLD activity and neurite outgrowth, whereas co-transfection with Cdc42N17 and PLD1 restored them. On the other hand, Cdc42V12 increased both PLD activity and neurite outgrowth, suggesting that active state of Cdc42 is important in upregulation of PLD activity which is responsible for the increase of neurite outgrowth.