Liraglutide Protects Against Diastolic Dysfunction and Improves Ventricular Protein Translation

PURPOSE

Diastolic dysfunction is an increasingly common cardiac pathology linked to heart failure with preserved ejection fraction. Previous studies have implicated glucagon-like peptide 1 (GLP-1) receptor agonists as potential therapies for improving diastolic dysfunction. In this study, we investigate the physiologic and metabolic changes in a mouse model of angiotensin II (AngII)-mediated diastolic dysfunction with and without the GLP-1 receptor agonist liraglutide (Lira).

METHODS

Mice were divided into sham, AngII, or AngII+Lira therapy for 4 weeks. Mice were monitored for cardiac function, weight change, and blood pressure at baseline and after 4 weeks of treatment. After 4 weeks of treatment, tissue was collected for histology, protein analysis, targeted metabolomics, and protein synthesis assays.

RESULTS

AngII treatment causes diastolic dysfunction when compared to sham mice. Lira partially prevents this dysfunction. The improvement in function in Lira mice is associated with dramatic changes in amino acid accumulation in the heart. Lira mice also have improved markers of protein translation by Western blot and increased protein synthesis by puromycin assay, suggesting that increased protein turnover protects against fibrotic remodeling and diastolic dysfunction seen in the AngII cohort. Lira mice also lost lean muscle mass compared to the AngII cohort, raising concerns about peripheral muscle scavenging as a source of the increased amino acids in the heart.

CONCLUSIONS

Lira therapy protects against AngII-mediated diastolic dysfunction, at least in part by promoting amino acid uptake and protein turnover in the heart. Liraglutide therapy is associated with loss of mean muscle mass, and long-term studies are warranted to investigate sarcopenia and frailty with liraglutide therapy in the setting of diastolic disease.

Metformin preconditioning protects against myocardial stunning and preserves protein translation in a mouse model of cardiac arrest

Cardiac arrest (CA) causes high mortality due to multi-system organ damage attributable to ischemia-reperfusion injury. Recent work in our group found that among diabetic patients who experienced cardiac arrest, those taking metformin had less evidence of cardiac and renal damage after cardiac arrest when compared to those not taking metformin. Based on these observations, we hypothesized that metformin's protective effects in the heart were mediated by AMPK signaling, and that AMPK signaling could be targeted as a therapeutic strategy following resuscitation from CA. The current study investigates metformin interventions on cardiac and renal outcomes in a non-diabetic CA mouse model. We found that two weeks of metformin pretreatment protects against reduced ejection fraction and reduces kidney ischemia-reperfusion injury at 24 h post-arrest. This cardiac and renal protection depends on AMPK signaling, as demonstrated by outcomes in mice pretreated with the AMPK activator AICAR or metformin plus the AMPK inhibitor compound C. At this 24-h time point, heart gene expression analysis showed that metformin pretreatment caused changes supporting autophagy, antioxidant response, and protein translation. Further investigation found associated improvements in mitochondrial structure and markers of autophagy. Notably, Western analysis indicated that protein synthesis was preserved in arrest hearts of animals pretreated with metformin. The AMPK activation-mediated preservation of protein synthesis was also observed in a hypoxia/reoxygenation cell culture model. Despite the positive impacts of pretreatment in vivo and in vitro, metformin did not preserve ejection fraction when deployed at resuscitation. Taken together, we propose that metformin's in vivo cardiac preservation occurs through AMPK activation, requires adaptation before arrest, and is associated with preserved protein translation.

Abstract P1039: Overexpressing Mitochondrial Transcription Factor A Protects Against Myocardial Stunning And Improves Mortality In A Mouse Model Of Cardiac Arrest

Background: Cardiac arrest (CA) affects over 600,000 Americans every year and is associated with overwhelming mortality. Patients who survive CA frequently develop multi-system injury, including cardiac dysfunction and reduced cardiac ejection fraction (EF). Nearly 25% of these patients die from cardiogenic shock. This cardiac injury is primarily attributed to ischemia-reperfusion injury. Mitochondria make up 30% of the heart by volume and are a major site of ischemia-reperfusion damage. Such damage affects mitochondrial DNA (mtDNA), but these changes may be limited by manipulating mitochondrial transcription factor A (TFAM). We investigated the role of cardiac-specific TFAM overexpression on cardiac function after CA in a mouse model.

Methods: Wild-type (WT) and transgenic mice featuring cardiac-specific TFAM overexpression (TFAM OE) were subjected to 8-min of asystole by direct injection with potassium chloride, followed by cardiopulmonary resuscitation. Surviving mice were assessed by echocardiography at 1-day, 1-week, and 4-weeks and monitored for survival over the time frame. Hearts were collected and sent for bulk RNA sequencing as well as assessment of mtDNA copy number, mitochondrial morphology, and protein expression.

Results: TFAM OE mice express 40% more TFAM than WT littermates. TFAM OE mice had protected cardiac EF (51.11±2.95%) compared to WT mice (38.49±3.76%) at one day and improved survival over 4 weeks. Bulk-RNA sequencing implicated changes to ribosomal expression and protein synthesis in the arrest TFAM OE mice. Western blot analysis of 1-day post-arrest hearts demonstrated increased expression of multiple heat shock proteins and increased phosphorylation of eukaryotic translation initiation factor 4E-binding protein (4E-BP1), suggesting that TFAM OE mice support protein translation to aid in cardiac recovery after arrest.

Conclusions: Cardiac TFAM overexpression protects cardiac function and survival after cardiac arrest. Early evidence suggests that this protection may be mediated by supporting protein translation, a novel potential pathway for improving outcomes after arrest.

A novel ultrasound-guided mouse model of sudden cardiac arrest

AIM

Mouse models of sudden cardiac arrest are limited by challenges with surgical technique and obtaining reliable venous access. To overcome this limitation, we sought to develop a simplified method in the mouse that uses ultrasound-guided injection of potassium chloride directly into the heart.

METHODS

Potassium chloride was delivered directly into the left ventricular cavity under ultrasound guidance in intubated mice, resulting in immediate asystole. Mice were resuscitated with injection of epinephrine and manual chest compressions and evaluated for survival, body temperature, cardiac function, kidney damage, and diffuse tissue injury.

RESULTS

The direct injection sudden cardiac arrest model causes rapid asystole with high surgical survival rates and short surgical duration. Sudden cardiac arrest mice with 8-min of asystole have significant cardiac dysfunction at 24 hours and high lethality within the first seven days, where after cardiac function begins to improve. Sudden cardiac arrest mice have secondary organ damage, including significant kidney injury but no significant change to neurologic function.

CONCLUSIONS

Ultrasound-guided direct injection of potassium chloride allows for rapid and reliable cardiac arrest in the mouse that mirrors human pathology without the need for intravenous access. This technique will improve investigators' ability to study the mechanisms underlying post-arrest changes in a mouse model.

Abstract 476: Mitochondrial DNA Preservation Preserves Cardiac Function Following Sudden Cardiac Arrest

Background: Sudden cardiac arrest (SCA) affects over 350,000 Americans yearly with greater than 70% mortality. Survivors frequently develop cardiomyopathy after SCA. Cardiac reperfusion causes mitochondrial ROS production, which is known to damage mitochondrial DNA (mtDNA), but the physiologic consequences of mtDNA damage are unclear. We investigated the role of mtDNA damage by altering the expression of TFAM, a nuclear-encoded transcription factor that protects mtDNA from ROS, in a mouse model of SCA.

Methods: WT and transgenic mice featuring cardiac-specific TFAM overexpression (TFAM-OE) and under-expression (TFAM Flox) underwent either 8 min of SCA or sham surgery followed by cardiopulmonary resuscitation. Survivors were assessed by echocardiography at 1-day, 1-week, and 4-weeks. Tissues were collected for assessment of mtDNA copy number and damage and assessment of mitochondrial morphology, protein expression, and function.

Results: WT, TFAM-OE, and TFAM Flox mice had no significant changes to baseline body weight or ejection fraction (EF). There were no changes in time to return of spontaneous circulation or body temperature between groups. 1 day after SCA, WT mice have reduced EF (38.49±3.76%) compared to sham WT mice (59.73±1.42). EF is protected in TFAM-OE mice (51.11±2.95%) and exacerbated in TFAM-UE mice (29.36±5.40%). TFAM-OE have significantly higher survival at 4 weeks (80%, 8 of 10) when compared to WT mice (38%, 5 of 13), but there is no change in TFAM-Flox mice (43%, 3 of 7). TFAM OE mice have higher mtDNA copy number and lower mtDNA damage when compared to WT mice.

Conclusions: TFAM OE protects cardiac function 1-day after SCA and improves 4-week survival. This is likely driven by TFAM-mediated protection of mtDNA. TFAM-Flox mice have lower EF at one day but no change to survival. This work suggests a role for mtDNA damage as a mechanism and potential therapeutic target of cardiomyopathy after SCA.

Gingival solitary chemosensory cells are immune sentinels for periodontitis

Solitary chemosensory cells (SCCs) are epithelial sentinels that utilize bitter Tas2r receptors and coupled taste transduction elements to detect pathogenic bacterial metabolites, triggering host defenses to control the infection. Here we report that SCCs are present in mouse gingival junctional epithelium, where they express several Tas2rs and the taste signaling components α-gustducin (Gnat3), TrpM5, and Plcβ2. Gnat3-/- mice have altered commensal oral microbiota and accelerated naturally occurring alveolar bone loss. In ligature-induced periodontitis, knockout of taste signaling molecules or genetic absence of gingival SCCs (gSCCs) increases the bacterial load, reduces bacterial diversity, and renders the microbiota more pathogenic, leading to greater alveolar bone loss. Topical treatment with bitter denatonium to activate gSCCs upregulates the expression of antimicrobial peptides and ameliorates ligature-induced periodontitis in wild-type but not in Gnat3-/- mice. We conclude that gSCCs may provide a promising target for treating periodontitis by harnessing innate immunity to regulate the oral microbiome.

Aggravated gut inflammation in mice lacking the taste signaling protein α-gustducin

Inflammatory bowel disease (IBD) is a debilitating immune-related condition that affects over 1.4 million Americans. Recent studies indicate that taste receptor signaling is involved in much more than sensing food flavor, and taste receptors have been localized in a variety of extra-oral tissues. One of the newly revealed functions of taste receptors and downstream signaling proteins is modulation of immune responses to microbes and parasites. We previously found that components of the taste receptor signaling pathway are expressed in subsets of the intestinal epithelial cells. α-Gustducin, a key G-protein α subunit involved in sweet, umami, and bitter taste receptor signaling, is expressed in the intestinal mucosa. In this study, we investigated the role of α-gustducin in regulation of gut mucosal immunity and inflammation using α-gustducin knockout mice in the dextran sulfate sodium (DSS)-induced IBD model. DSS is a chemical colitogen that can cause intestinal epithelial damage and inflammation. We analyzed DSS-induced colitis in α-gustducin knockout versus wild-type control mice after administration of DSS in drinking water. Our results show that the knockout mice had aggravated weight loss, diarrhea, intestinal bleeding, and inflammation over the experimental period compared to wild-type mice, concurrent with augmented immune cell infiltration and increased expression of TNF and IFN-γ but decreased expression of IL-13 and IL-5 in the colon. These results suggest that the taste receptor signaling pathway may play critical roles in regulating gut immune balance and inflammation.

Aggravated gut inflammation in mice lacking the taste signaling protein α-gustducin

Inflammatory bowel disease (IBD) is a debilitating immune-related condition that affects over 1.4 million Americans. Recent studies indicate that the taste system is involved in much more than sensing food flavor, and taste receptors have been localized in a variety of extra-oral tissues. One of the newly revealed functions of taste receptors and signaling proteins is modulation of immune responses to microbes and parasites. We previously found that components of the taste receptor signaling pathway are expressed in subsets of the intestinal epithelial cells. α-Gustducin, a key G-protein α subunit involved in sweet, umami, and bitter taste receptor signaling, is expressed in the intestinal mucosa. In this study, we investigated the role of α-gustducin in regulation of gut mucosal immunity and inflammation using gustducin knockout mice in the dextran sulfate sodium (DSS)-induced IBD model. DSS is a chemical colitogen that can cause intestinal epithelial damage and inflammation. We analyzed DSS-induced colitis in gustducin knockout versus wild-type mice after administration of 3% DSS in drinking water to mice. Our results show that the knockout mice had increased weight loss, diarrhea, intestinal bleeding, and inflammation over the experimental period compared to wild-type mice, concurrent with increased expression of TNF and IFN-γ but decreased expression of IL-13 and IL-5 in the colon. These results suggest that the taste receptor signaling pathway may play critical roles in regulating gut immune balance and inflammation.

Gli3 is a negative regulator of Tas1r3-expressing taste cells

Mouse taste receptor cells survive from 3-24 days, necessitating their regeneration throughout adulthood. In anterior tongue, sonic hedgehog (SHH), released by a subpopulation of basal taste cells, regulates transcription factors Gli2 and Gli3 in stem cells to control taste cell regeneration. Using single-cell RNA-Seq we found that Gli3 is highly expressed in Tas1r3-expressing taste receptor cells and Lgr5+ taste stem cells in posterior tongue. By PCR and immunohistochemistry we found that Gli3 was expressed in taste buds in all taste fields. Conditional knockout mice lacking Gli3 in the posterior tongue (Gli3CKO) had larger taste buds containing more taste cells than did control wild-type (Gli3WT) mice. In comparison to wild-type mice, Gli3CKO mice had more Lgr5+ and Tas1r3+ cells, but fewer type III cells. Similar changes were observed ex vivo in Gli3CKO taste organoids cultured from Lgr5+ taste stem cells. Further, the expression of several taste marker and Gli3 target genes was altered in Gli3CKO mice and/or organoids. Mirroring these changes, Gli3CKO mice had increased lick responses to sweet and umami stimuli, decreased lick responses to bitter and sour taste stimuli, and increased glossopharyngeal taste nerve responses to sweet and bitter compounds. Our results indicate that Gli3 is a suppressor of stem cell proliferation that affects the number and function of mature taste cells, especially Tas1r3+ cells, in adult posterior tongue. Our findings shed light on the role of the Shh pathway in adult taste cell regeneration and may help devise strategies for treating taste distortions from chemotherapy and aging.

Honoring Jean-David Rochaix

We honor Jean-David Rochaix, an outstanding scholar of chloroplast biogenesis and photosynthesis, who received the prestigious Lifetime Achievement Award of the International Society of Photosynthesis Research at its 17th International Photosynthesis Congress held in Maastricht, The Netherlands (August 5-12, 2016). With this award he joins other major discoverers in the field of photosynthesis: Pierre Joliot (of France, 2013); Ulrich W. Heber* (of Germany, 2010) and Kenneth Sauer (of USA, 2010); Jan M. Anderson* (of Australia, 2007); and Horst T. Witt* (of Germany, 2004). See "Appendix 1" for the list of those who have received the ISPR Communication, Innovation, Calvin-Benson, and Hill awards.

Tuft cells, taste-chemosensory cells, orchestrate parasite type 2 immunity in the gut

The intestinal epithelium forms an essential barrier between a host and its microbiota. Protozoa and helminths are members of the gut microbiota of mammals, including humans, yet the many ways that gut epithelial cells orchestrate responses to these eukaryotes remain unclear. Here we show that tuft cells, which are taste-chemosensory epithelial cells, accumulate during parasite colonization and infection. Disruption of chemosensory signaling through the loss of TRMP5 abrogates the expansion of tuft cells, goblet cells, eosinophils, and type 2 innate lymphoid cells during parasite colonization. Tuft cells are the primary source of the parasite-induced cytokine interleukin-25, which indirectly induces tuft cell expansion by promoting interleukin-13 production by innate lymphoid cells. Our results identify intestinal tuft cells as critical sentinels in the gut epithelium that promote type 2 immunity in response to intestinal parasites.

The Effect of Bacteriochlorophyll g Oxidation on Energy and Electron Transfer in Reaction Centers from Heliobacterium modesticaldum

The heliobacteria are a family of strictly anaerobic, Gram-positive, photoheterotrophs in the Firmicutes. They make use of a homodimeric type I reaction center (RC) that contains ∼20 antenna bacteriochlorophyll (BChl) g molecules, a special pair of BChl g' molecules (P800), two 8(1)-OH-Chl aF molecules (A0), a [4Fe-4S] iron-sulfur cluster (FX), and a carotenoid (4,4'-diaponeurosporene). It is known that in the presence of light and oxygen BChl g is converted to a species with an absorption spectrum identical to that of Chl a. Here, we show that main product of the conversion is 8(1)-OH-Chl aF. Smaller amounts of two other oxidized Chl aF species are also produced. In the presence of light and oxygen, the kinetics of the conversion are monophasic and temperature dependent, with an activation energy of 66 ± 2 kJ mol(-1). In the presence of oxygen in the dark, the conversion occurs in two temperature-dependent kinetic phases: a slow phase followed by a fast phase with an activation energy of 53 ± 1 kJ mol(-1). The loss of BChl g' occurs at the same rate as the loss of Bchl g; hence, the special pair converts at the same rate as the antenna Chl's. However, the loss of P800 photooxidiation and flavodoxin reduction is not linear with the loss of BChl g. In anaerobic RCs, the charge recombination between P800(+) and FX(-) at 80 K is monophasic with a lifetime of 4.2 ms, but after exposure to oxygen, an additional phase with a lifetime of 0.3 ms is observed. Transient EPR data show that the line width of P800(+) increases as BChl g is converted to Chl aF and the rate of electron transfer from A0 to FX, as estimated from the net polarization generated by singlet-triplet mixing during the lifetime of P800(+)A0(-), is unchanged. The transient EPR data also show that conversion of the BChl g results in increased formation of triplet states of both BChl g and Chl aF. The nonlinear loss of P800 photooxidiation and flavodoxin reduction, the biphasic backreaction kinetics, and the increased EPR line width of P800(+) are all consistent with a model in which the BChl g'/BChl g' and BChl g'/Chl aF' special pairs are functional but the Chl aF'/Chl aF' special pair is not.

Non-specific immunostaining by a rabbit antibody against gustducin α subunit in mouse brain

Gustducin is a guanosine nucleotide-binding protein functionally coupled with taste receptors and thus originally identified in taste cells of the tongue. Recently, bitter taste receptors and gustducin have been detected in the airways, digestive tracts and brain. The existing studies showing taste receptors and gustducin in the brain were carried out exclusively on frozen sections. In order to avoid the technical shortcomings associated with frozen sectioning, we performed immunofluorescence staining using vibratome-cut sections from mouse brains. Using a rabbit gustducin antibody, we could not detect neurons or astrocytes as reported previously. Rather, we found dense fibers in the nucleus accumbens and periventricular areas. We assumed these staining patterns to be specific after confirmation with conventional negative control staining. For the verification of this finding, we stained gustducin knockout mouse brain and tongue sections with the same rabbit gustducin antibody. Whereas negative staining was confirmed in the tongue, intensive fibers were constantly stained in the brain. Moreover, immunostaining with a goat gustducin antibody could not demonstrate the fibers in the brain tissue. The present study implies a cross immunoreaction that occurs with the rabbit gustducin antibody in mouse brain samples, suggesting that the conventional negative controls may not be sufficient when an immunostaining pattern is to be verified.

Functional modeling identifies paralogous solanesyl-diphosphate synthases that assemble the side chain of plastoquinone-9 in plastids

It is a little known fact that plastoquinone-9, a vital redox cofactor of photosynthesis, doubles as a precursor for the biosynthesis of a vitamin E analog called plastochromanol-8, the physiological significance of which has remained elusive. Gene network reconstruction, GFP fusion experiments, and targeted metabolite profiling of insertion mutants indicated that Arabidopsis possesses two paralogous solanesyl-diphosphate synthases, AtSPS1 (At1g78510) and AtSPS2 (At1g17050), that assemble the side chain of plastoquinone-9 in plastids. Similar paralogous pairs were detected throughout terrestrial plant lineages but were not distinguished in the literature and genomic databases from mitochondrial homologs involved in the biosynthesis of ubiquinone. The leaves of the atsps2 knock-out were devoid of plastochromanol-8 and displayed severe losses of both non-photoactive and photoactive plastoquinone-9, resulting in near complete photoinhibition at high light intensity. Such a photoinhibition was paralleled by significant damage to photosystem II but not to photosystem I. In contrast, in the atsps1 knock-out, a small loss of plastoquinone-9, restricted to the non-photoactive pool, was sufficient to eliminate half of the plastochromanol-8 content of the leaves. Taken together, these results demonstrate that plastochromanol-8 originates from a subfraction of the non-photoactive pool of plastoquinone-9. In contrast to other plastochromanol-8 biosynthetic mutants, neither the single atsps knock-outs nor the atsps1 atsps2 double knock-out displayed any defects in tocopherols accumulation or germination.

Lumen Thiol Oxidoreductase1, a disulfide bond-forming catalyst, is required for the assembly of photosystem II in Arabidopsis

Here, we identify Arabidopsis thaliana Lumen Thiol Oxidoreductase1 (LTO1) as a disulfide bond-forming enzyme in the thylakoid lumen. Using topological reporters in bacteria, we deduced a lumenal location for the redox active domains of the protein. LTO1 can partially substitute for the proteins catalyzing disulfide bond formation in the bacterial periplasm, which is topologically equivalent to the plastid lumen. An insertional mutation within the LTO1 promoter is associated with a severe photoautotrophic growth defect. Measurements of the photosynthetic activity indicate that the lto1 mutant displays a limitation in the electron flow from photosystem II (PSII). In accordance with these measurements, we noted a severe depletion of the structural subunits of PSII but no change in the accumulation of the cytochrome b(6)f complex or photosystem I. In a yeast two-hybrid assay, the thioredoxin-like domain of LTO1 interacts with PsbO, a lumenal PSII subunit known to be disulfide bonded, and a recombinant form of the molecule can introduce a disulfide bond in PsbO in vitro. The documentation of a sulfhydryl-oxidizing activity in the thylakoid lumen further underscores the importance of catalyzed thiol-disulfide chemistry for the biogenesis of the thylakoid compartment.

Additive effect of mutations affecting the rate of phylloquinone reoxidation and directionality of electron transfer within photosystem I

Optical pump-probe spectroscopy in the nanosecond-microsecond timescale has been used to study the electron transfer reactions taking place within the Photosystem I reaction center of intact Chlamydomonas reinhardtii cells. The biphasic kinetics of phylloquinone (PhQ) reoxidation were investigated in double mutants that combine a mutation (PsaA-Y696F) near the primary acceptor chlorophyll, ec3A, with those near PhQA (PsaA-S692A, PsaA-W697F). The PsaA-S692A and PsaA-W697F mutations selectively lengthened the 200 ns lifetime component observed in the wild-type (WT). The reverse similar 20 ns component was unaltered in the single mutant, both in terms of lifetime and relative amplitude. However, both double mutants possessed a reverse similar 20 ns component (PhQB(-) reoxidation) with increased amplitude compared with the WT and the individual PhQA mutants. The component assigned to PhQA(-) reoxidation was slowed, like the individual PhQA mutants, and of lower amplitude, as observed in the single ec3A mutant. Hence, the effects of these mutations are almost entirely additive, providing strong support for the previously proposed bidirectional electron transfer model, which attributes the reverse similar 20 and reverse similar 200 ns phases to reoxidation of PhQB or PhQA, respectively. Moreover, in all the mutants investigated, it was also possible to observe an intermediate (approximately 180 ns) component, as previously reported for mutants of the PhQ(A) binding pocket (Biochim. Biophys. Acta [2006] 1757, 1529-1538), which we have tentatively attributed to forward electron transfer between the iron-sulfur clusters FX and FA/B.

Two equilibration pools of chlorophylls in the Photosystem I core antenna of Chlamydomonas reinhardtii

Femtosecond transient absorption spectroscopy was applied for a comparative study of excitation decay in several different Photosystem I (PSI) core preparations from the green alga Chlamydomonas reinhardtii. For PSI cores with a fully interconnected network of chlorophylls, the excitation energy was equilibrated over a pool of chlorophylls absorbing at approximately 683 nm, independent of excitation wavelength [Gibasiewicz et al. J Phys Chem B 105:11498-11506, 2001; J Phys Chem B 106:6322-6330, 2002]. In preparations with impaired connectivity between chlorophylls, we have found that the spectrum of chlorophylls connected to the reaction center (i.e., with approximately 20 ps decay time) over which the excitation is equilibrated becomes excitation-wavelength-dependent. Excitation at 670 nm is finally equilibrated over chlorophylls absorbing at approximately 675 nm, whereas excitation at 695 nm or 700 nm is equilibrated over chlorophylls absorbing at approximately 683 nm. This indicates that in the vicinity of the reaction center there are two spectrally different and spatially separated pools of chlorophylls that are equally capable of effective excitation energy transfer to the reaction center. We propose that they are related to the two groups of central PSI core chlorophylls lying on the opposite sides of reaction center.

Signaling through Galphai2 protein is required for recruitment of neutrophils for antibody-mediated elimination of larval Strongyloides stercoralis in mice

The heterotrimeric guanine nucleotide-binding protein Galphai2 is involved in regulation of immune responses against microbial and nonmicrobial stimuli. Galphai2-/- mice have a selectively impaired IgM response consistent with a disorder in B cell development yet have augmented T cell effector function associated with increased production of IFN-gamma and IL-4. The goal of the present study was to determine if a deficiency in the Galphai2 protein in mice would affect the protective immune response against Strongyloides stercoralis, which is IL-4-, IL-5-, and IgM-dependent. Galphai2-/- and wild-type mice were immunized and challenged with S. stercoralis larvae and analyzed for protective immune responses against infection. Galphai2-/- mice failed to kill the larvae in the challenge infection as compared with wild-type mice despite developing an antigen-specific Th2 response characterized by increased IL-4, IL-5, IgM, and IgG. Transfer of serum collected from immunized Galphai2-/- mice to naïve wild-type mice conferred passive protective immunity against S. stercoralis infection thus confirming the development of a protective antibody response in Galphai2-/- mice. Differential cell analyses and myeloperoxidase assays for quantification of neutrophils showed a significantly reduced recruitment of neutrophils into the microenvironment of the parasites in immunized Galphai2-/- mice. However, cell transfer studies demonstrated that neutrophils from Galphai2-/- mice are competent in killing larvae. These data demonstrate that Galphai2 signaling events are not required for the development of the protective immune responses against S. stercoralis; however, Galphai2 is essential for the recruitment of neutrophils required for host-dependent killing of larvae.

Assignment of a kinetic component to electron transfer between iron–sulfur clusters FX and FA/B of Photosystem I

We studied the kinetics of reoxidation of the phylloquinones in Chlamydomonas reinhardtii Photosystem I using site-directed mutations in the PhQ(A)-binding site and of the residues serving as the axial ligand to ec3(A) and ec3(B) chlorophylls. In wild type PS I, these kinetics are biphasic, and mutations in the binding region of PhQ(A) induced a specific slowing down of the slow component. This slowing allowed detection of a previously unobserved 180-ns phase having spectral characteristics that differ from electron transfer between phylloquinones and F(X). The new kinetic phase thus reflects a different reaction that we ascribe to oxidation of F(X)(-) by the F(A/B) FeS clusters. These absorption changes partly account for the differences between the spectra associated with the two kinetic components assigned to phylloquinone reoxidation. In the mutant in which the axial ligand to ec3(A) (PsaA-Met688) was targeted, about 25% of charge separations ended in P(700)(+)A(0)(-) charge recombination; no such recombination was detected in the B-side symmetric mutant. Despite significant changes in the amplitude of the components ascribed to phylloquinone reoxidation in the two mutants, the overall nanosecond absorption changes were similar to the wild type. This suggests that these absorption changes are similar for the two different phylloquinones and that part of the differences between the decay-associated spectra of the two components reflect a contribution from different electron acceptors, i.e. from an inter-FeS cluster electron transfer.

Directing electron transfer within Photosystem I by breaking H-bonds in the cofactor branches

Photosystem I has two branches of cofactors down which light-driven electron transfer (ET) could potentially proceed, each consisting of a pair of chlorophylls (Chls) and a phylloquinone (PhQ). Forward ET from PhQ to the next ET cofactor (FX) is described by two kinetic components with decay times of approximately 20 and approximately 200 ns, which have been proposed to represent ET from PhQB and PhQA, respectively. Immediately preceding each quinone is a Chl (ec3), which receives a H-bond from a nearby tyrosine. To decrease the reduction potential of each of these Chls, and thus modify the relative yield of ET within the targeted branch, this H-bond was removed by conversion of each Tyr to Phe in the green alga Chlamydomonas reinhardtii. Together, transient optical absorption spectroscopy performed in vivo and transient electron paramagnetic resonance data from thylakoid membranes showed that the mutations affect the relative amplitudes, but not the lifetimes, of the two kinetic components representing ET from PhQ to F(X). The mutation near ec3A increases the fraction of the faster component at the expense of the slower component, with the opposite effect seen in the ec3B mutant. We interpret this result as a decrease in the relative use of the targeted branch. This finding suggests that in Photosystem I, unlike type II reaction centers, the relative efficiency of the two branches is extremely sensitive to the energetics of the embedded redox cofactors.

Mutation of the putative hydrogen-bond donor to P700 of photosystem I

The primary electron donor of photosystem I (PS1), called P(700), is a heterodimer of chlorophyll (Chl) a and a'. The crystal structure of photosystem I reveals that the chlorophyll a' (P(A)) could be hydrogen-bonded to the protein via a threonine residue, while the chlorophyll a (P(B)) does not have such a hydrogen bond. To investigate the influence of this hydrogen bond on P(700), PsaA-Thr739 was converted to alanine to remove the H-bond to the 13(1)-keto group of the chlorophyll a' in Chlamydomonas reinhardtii. The PsaA-T739A mutant was capable of assembling active PS1. Furthermore the mutant PS1 contained approximately one chlorophyll a' molecule per reaction center, indicating that P(700) was still a Chl a/a' heterodimer in the mutant. However, the mutation induced several band shifts in the visible P(700)(+) - P(700) absorbance difference spectrum. Redox titration of P(700) revealed a 60 mV decrease in the P(700)/P(700)(+) midpoint potential of the mutant, consistent with loss of a H-bond. Fourier transform infrared (FTIR) spectroscopy indicates that the ground state of P(700) is somewhat modified by mutation of ThrA739 to alanine. Comparison of FTIR difference band shifts upon P(700)(+) formation in WT and mutant PS1 suggests that the mutation modifies the charge distribution over the pigments in the P(700)(+) state, with approximately 14-18% of the positive charge on P(B) in WT being relocated onto P(A) in the mutant. (1)H-electron-nuclear double resonance (ENDOR) analysis of the P(700)(+) cation radical was also consistent with a slight redistribution of spin from the P(B) chlorophyll to P(A), as well as some redistribution of spin within the P(B) chlorophyll. High-field electron paramagnetic resonance (EPR) spectroscopy at 330-GHz was used to resolve the g-tensor of P(700)(+), but no significant differences from wild-type were observed, except for a slight decrease of anisotropy. The mutation did, however, provoke changes in the zero-field splitting parameters of the triplet state of P(700) ((3)P(700)), as determined by EPR. Interestingly, the mutation-induced change in asymmetry of P(700) did not cause an observable change in the directionality of electron transfer within PS1.

A high-field EPR study of P700+ in wild-type and mutant photosystem I from Chlamydomonas reinhardtii

High-frequency, high-field EPR at 330 GHz was used to study the photo-oxidized primary donor of photosystem I (P(700)(+)(*)) in wild-type and mutant forms of photosystem I in the green alga Chlamydomonas reinhardtii. The main focus was the substitution of the axial ligand of the chlorophyll a and chlorophyll a' molecules that form the P(700) heterodimer. Specifically, we examined PsaA-H676Q, in which the histidine axial ligand of the A-side chlorophyll a' (P(A)) is replaced with glutamine, and PsaB-H656Q, with a similar replacement of the axial ligand of the B-side chlorophyll a (P(B)), as well as the double mutant (PsaA-H676Q/PsaB-H656Q), in which both axial ligands were replaced. We also examined the PsaA-T739A mutant, which replaces a threonine residue hydrogen-bonded to the 13(1)-keto group of P(A) with an alanine residue. The principal g-tensor components of the P(700)(+)(*) radical determined in these mutants and in wild-type photosystem I were compared with each other, with the monomeric chlorophyll cation radical (Chl(z)(+)(*)) in photosystem II, and with recent theoretical calculations for different model structures of the chlorophyll a(+) cation radical. In mutants with a modified P(B) axial ligand, the g(zz) component of P(700)(+)(*) was shifted down by up to 2 x 10(-4), while mutations near P(A) had no significant effect. We discuss the shift of the g(zz) component in terms of a model with a highly asymmetric distribution of unpaired electron spin in the P(700)(+)(*) radical cation, mostly localized on P(B), and a deviation of the P(B) chlorophyll structure from planarity due to the axial ligand.

Excitonic interactions in wild-type and mutant PSI reaction centers

Femtosecond excitation of the red edge of the chlorophyll a Q(Y) transition band in photosystem I (PSI), with light of wavelength > or = 700 nm, leads to wide transient (subpicosecond) absorbance changes: positive DeltaA between 635 and 665 nm, and four negative DeltaA bands at 667, 675, 683, and 695 nm. Here we compare the transient absorbance changes after excitation at 700, 705, and 710 nm at 20 K in several PSI preparations of Chlamydomonas reinhardtii where amino acid ligands of the primary donor, primary acceptor, or connecting chlorophylls have been mutated. Most of these mutations influence the spectrum of the absorbance changes. This supports the view that the chlorophylls of the electron transfer chain as well as the connecting chlorophylls are engaged in the observed absorbance changes. The wide absorption spectrum of the electron transfer chain revealed by the transient measurements may contribute to the high efficiency of energy trapping in photosystem 1. Exciton calculations, based on the recent PSI structure, allow an assignment of the DeltaA bands to particular chlorophylls: the bands at 675 and 695 nm to the dimers of primary acceptor and accessory chlorophyll and the band at 683 nm to the connecting chlorophylls. The subpicosecond transient absorption bands decay may reflect rapid charge separation in the PSI reaction center.

Disassembly and degradation of photosystem I in an in vitro system are multievent, metal-dependent processes

An in vitro system was created to study the process of membrane protein degradation by using photosystem I (PS1) as a model membrane protein. Purified chloroplast membranes were incubated at 30 degrees C in a defined buffer along with various extracts or reagents to reconstitute the disassembly and degradation of PS1, which was monitored by a variety of techniques that probe the integrity of the PS1 complex: photo-biochemical assays, semi-native gel electrophoresis, low temperature fluorescence spectroscopy, and immunoblots using antibodies against different PS1 subunits. During a typical time course, degradation of PS1 appeared to be a multievent process, with disassembly of the complex preceding proteolysis of the subunits. The first change seen was a rapid (<5 min) decrease in PS1 photochemical activity. This was followed by a diminution of far-red fluorescence emission from the core antenna of PS1 and a slower disassembly of the PS1 chlorophyll-protein core complex, as visualized by semi-native gel electrophoresis. Surprisingly, the latter was not accompanied by a similar rate of proteolysis of the PsaA core subunit. In contrast, addition of soluble proteases caused rapid loss of immuno-detectable PS1 polypeptides and cleavage of the major PS1 polypeptides in interhelical loops. The in vitro degradation process was time- and temperature-dependent but did not require ATP, GTP, or soluble chloroplast proteins. Chelation of divalent cations by EDTA inhibited the later steps of disassembly and proteolysis, and this effect could be reversed by addition of micromolar Zn2+, with Co2+ and Ca2+ providing somewhat lower activity.

Mutation induced modulation of hydrogen bonding to P700 studied using FTIR difference spectroscopy

Site-directed mutagenesis in combination with Fourier transform infrared difference spectroscopy has been used to study how hydrogen bonding modulates the electronic and physical organization of P700, the primary electron donor in photosystem I. Wild-type PS I particles from Chlamydomonas reinhardtii and a mutant in which ThrA739 is changed to alanine [TA(A739) mutant] were studied. ThrA739 is thought to provide a hydrogen bond to the chlorophyll-a' molecule of P700 (the two chlorophylls of P700 (P700(+)) will be called P(A) and P(B) (P(A)(+) and P(B)(+))). The mutation considerably alters the (P700(+)-P700) FTIR difference spectra. However, we were able to describe all of the mutation induced changes in the difference spectra in terms of difference band assignments that were proposed recently (Hastings, G., Ramesh, V. M., Wang, R., Sivakumar, V. and Webber, A. (2001) Biochemistry 40, 12943-12949). Upon comparison of mutant and wild type (P700(+)-P700) FTIR difference spectra, it is shown that (1) the 13(3) ester carbonyl modes of P(A) and P(B) are unaltered upon mutation of ThrA739 to alanine. (2) The 13(3) ester carbonyl modes of P(A)(+)/P(B)(+) upshift/downshift upon mutation. These oppositely directed shifts indicate that the mutation modifies the charge distribution over the pigments in the P700(+) state, with charge on P(B) being relocated onto P(A). We also show that the 13(1) keto carbonyl mode of P(B)/P(B)(+) is unaltered/downshifted upon mutation, as is expected for the above-described mutation induced charge redistribution in P700(+). Although the 13(3) ester carbonyl modes of the chlorophylls of P700 in the ground state are unaltered upon mutation, the 13(1) keto carbonyl mode of P(A) upshifts upon mutation, as does the 13(1) keto carbonyl mode of P(A)(+). For P700 in the ground state, bands that we associate with HisA676/HisB656 upshift/downshift upon mutation. For the P700(+) state, bands that we associate with HisA676/HisB656 also upshift/downshift upon mutation. These observations are also consistent with the notion that the mutation leads to the charge on P(B)(+) being relocated onto P(A)(+). In addition, we suggest that a hydrogen bond to the 13(1) keto carbonyl of P(A) is still present in the TA(A739) mutant, probably mediated through an introduced water molecule.

Use of aminoglycoside adenyltransferase translational fusions to determine topology of thylakoid membrane proteins

We have developed a system to examine the topology of thylakoid membrane proteins using the bacterial aadA gene as a reporter. Translational fusions that place the aminoglycoside adenyltransferase domain in the stroma should provide high antibiotic resistance, while those that place it in the thylakoid lumen should give rise to low resistance. Genes encoding chimeric polypeptides consisting of AadA fused to varying lengths of the PsaA polypeptide, whose topology is known, were introduced into the chloroplast genome of Chlamydomonas reinhardtii. As expected, chimeras with an even number of alpha-helices in general resulted in higher resistance. This effect was not due to differences in expression or in catalytic activity. This system should prove useful in analysis of novel proteins predicted to be localized to the thylakoid membrane.

In vivo interactions between photosynthesis, mitorespiration, and chlororespiration in Chlamydomonas reinhardtii

Interactions between photosynthesis, mitochondrial respiration (mitorespiration), and chlororespiration have been investigated in the green alga Chlamydomonas reinhardtii using flash illumination and a bare platinum electrode. Depending on the physiological status of algae, flash illumination was found to induce either a fast (t(1/2) approximately 300 ms) or slow (t(1/2) approximately 3 s) transient inhibition of oxygen uptake. Based on the effects of the mitorespiratory inhibitors myxothiazol and salicyl hydroxamic acid (SHAM), and of propyl gallate, an inhibitor of the chlororespiratory oxidase, we conclude that the fast transient is due to the flash-induced inhibition of chlororespiration and that the slow transient is due to the flash-induced inhibition of mitorespiration. By measuring blue-green fluorescence changes, related to the redox status of the pyridine nucleotide pool, and chlorophyll fluorescence, related to the redox status of plastoquinones (PQs) in C. reinhardtii wild type and in a photosystem I-deficient mutant, we show that interactions between photosynthesis and chlororespiration are favored when PQ and pyridine nucleotide pools are reduced, whereas interactions between photosynthesis and mitorespiration are favored at more oxidized states. We conclude that the plastid oxidase, similar to the mitochondrial alternative oxidase, becomes significantly engaged when the PQ pool becomes highly reduced, and thereby prevents its over-reduction.

Mutations in both sides of the photosystem I reaction center identify the phylloquinone observed by electron paramagnetic resonance spectroscopy

The core of photosystem I (PS1) is composed of the two related integral membrane polypeptides, PsaA and PsaB, which bind two symmetrical branches of cofactors, each consisting of two chlorophylls and a phylloquinone, that potentially link the primary electron donor and the tertiary acceptor. In an effort to identify amino acid residues near the phylloquinone binding sites, all tryptophans and histidines that are conserved between PsaA and PsaB in the region of the 10th and 11th transmembrane alpha-helices were mutated in Chlamydomonas reinhardtii. The mutant PS1 reaction centers appear to assemble normally and possess photochemical activity. An electron paramagnetic resonance (EPR) signal attributed to the phylloquinone anion radical (A(1)(-)) can be observed either transiently or after illumination of reaction centers with pre-reduced iron-sulfur clusters. Mutation of PsaA-Trp(693) to Phe resulted in an inability to photo-accumulate A(1)(-), whereas mutation of the analogous tryptophan in PsaB (PsaB-Trp(673)) did not produce this effect. The PsaA-W693F mutation also produced spectral changes in the time-resolved EPR spectrum of the P(700)(+) A(1)(-) radical pair, whereas the analogous mutation in PsaB had no observable effect. These observations indicate that the A(1)(-) phylloquinone radical observed by EPR occupies the phylloquinone-binding site containing PsaA-Trp(693). However, mutation of either tryptophan accelerated charge recombination from the terminal Fe-S clusters.

Evidence for two active branches for electron transfer in photosystem I

All photosynthetic reaction centers share a common structural theme. Two related, integral membrane polypeptides sequester electron transfer cofactors into two quasi-symmetrical branches, each of which incorporates a quinone. In type II reaction centers [photosystem (PS) II and proteobacterial reaction centers], electron transfer proceeds down only one of the branches, and the mobile quinone on the other branch is used as a terminal acceptor. PS I uses iron-sulfur clusters as terminal acceptors, and the quinone serves only as an intermediary in electron transfer. Much effort has been devoted to understanding the unidirectionality of electron transport in type II reaction centers, and it was widely thought that PS I would share this feature. We have tested this idea by examining in vivo kinetics of electron transfer from the quinone in mutant PS I reaction centers. This transfer is associated with two kinetic components, and we show that mutation of a residue near the quinone in one branch specifically affects the faster component, while the corresponding mutation in the other branch specifically affects the slower component. We conclude that both electron transfer branches in PS I are active.

Flexibility in photosynthetic electron transport: a newly identified chloroplast oxidase involved in chlororespiration

Besides electron transfer reactions involved in the 'Z' scheme of photosynthesis, alternative electron transfer pathways have been characterized in chloroplasts. These include cyclic electron flow around photosystem I (PS I) or a respiratory chain called chlororespiration. Recent work has supplied new information concerning the molecular nature of the electron carriers involved in the non-photochemical reduction of the plastoquinone (PQ) pool. However, until now little is known concerning the nature of the electron carriers involved in PQ oxidation. By using mass spectrometric measurement of oxygen exchange performed in the presence of 18O-enriched O2 and Chlamydomonas mutants deficient in PS I, we show that electrons can be directed to a quinol oxidase sensitive to propyl gallate but insensitive to salicyl hydroxamic acid. This oxidase has immunological and pharmacological similarities with a plastid protein involved in carotenoid biosynthesis.

Electron flow between photosystem II and oxygen in chloroplasts of photosystem I-deficient algae is mediated by a quinol oxidase involved in chlororespiration

In Chlamydomonas reinhardtii mutants deficient in photosystem I because of inactivation of the chloroplast genes psaA or psaB, oxygen evolution from photosystem II occurs at significant rates and is coupled to a stimulation of oxygen uptake. Both activities can be simultaneously monitored by continuous mass spectrometry in the presence of (18)O(2). The light-driven O(2) exchange was shown to involve the plastoquinone pool as an electron carrier, but not cytochrome b(6)f. Photosystem II-dependent O(2) production and O(2) uptake were observed in isolated chloroplast fractions. Photosystem II-dependent oxygen exchange was insensitive to a variety of inhibitors (azide, carbon monoxide, cyanide, antimycin A, and salicylhydroxamic acid) and radical scavengers. It was, however, sensitive to propyl gallate. From inhibitors effects and electronic requirements of the O(2) uptake process, we conclude that an oxidase catalyzing oxidation of plastoquinol and reduction of oxygen to water is present in thylakoid membranes. From the sensitivity of flash-induced O(2) exchange to propyl gallate, we conclude that this oxidase is involved in chlororespiration. Clues to the identity of the protein implied in this process are given by pharmacological and immunological similarities with a protein (IMMUTANS) identified in Arabidopsis chloroplasts.

Photosystem I is indispensable for photoautotrophic growth, CO2 fixation, and H2 photoproduction in Chlamydomonas reinhardtii

Certain Chlamydomonas reinhardtii mutants deficient in photosystem I due to defects in psaA mRNA maturation have been reported to be capable of CO2 fixation, H2 photoevolution, and photoautotrophic growth (Greenbaum, E., Lee, J. W., Tevault, C. V., Blankinship, S. L. , and Mets, L. J. (1995) Nature 376, 438-441 and Lee, J. W., Tevault, C. V., Owens, T. G.; Greenbaum, E. (1996) Science 273, 364-367). We have generated deletions of photosystem I core subunits in both wild type and these mutant strains and have analyzed their abilities to grow photoautotrophically, to fix CO2, and to photoevolve O2 or H2 (using mass spectrometry) as well as their photosystem I content (using immunological and spectroscopic analyses). We find no instance of a strain that can perform photosynthesis in the absence of photosystem I. The F8 strain harbored a small amount of photosystem I, and it could fix CO2 and grow slowly, but it lost these abilities after deletion of either psaA or psaC; these activities could be restored to the F8-psaADelta mutant by reintroduction of psaA. We observed limited O2 photoevolution in mutants lacking photosystem I; use of 18O2 indicated that this O2 evolution is coupled to O2 uptake (i.e. respiration) rather than CO2 fixation or H2 evolution. We conclude that the reported instances of CO2 fixation, H2 photoevolution, and photoautotrophic growth of photosystem I-deficient mutants result from the presence of unrecognized photosystem I.

A systematic survey of conserved histidines in the core subunits of Photosystem I by site-directed mutagenesis reveals the likely axial ligands of P700

The Photosystem I complex catalyses the transfer of an electron from lumenal plastocyanin to stromal ferredoxin, using the energy of an absorbed photon. The initial photochemical event is the transfer of an electron from the excited state of P700, a pair of chlorophylls, to a monomer chlorophyll serving as the primary electron acceptor. We have performed a systematic survey of conserved histidines in the last six transmembrane segments of the related polytopic membrane proteins PsaA and PsaB in the green alga Chlamydomonas reinhardtii. These histidines, which are present in analogous positions in both proteins, were changed to glutamine or leucine by site-directed mutagenesis. Double mutants in which both histidines had been changed to glutamine were screened for changes in the characteristics of P700 using electron paramagnetic resonance, Fourier transform infrared and visible spectroscopy. Only mutations in the histidines of helix 10 (PsaA-His676 and PsaB-His656) resulted in changes in spectroscopic properties of P700, leading us to conclude that these histidines are most likely the axial ligands to the P700 chlorophylls.

Limited photosynthetic electron flow but no CO2 fixation in Chlamydomonas mutants lacking photosystem I

By measuring O2 and CO2 exchange in mutants of the green alga Chlamydomonas reinhardtii in which genes encoding the reaction center of photosystem I (psaA or psaB) have been deleted, we found that a photosystem II-dependent electron flow using O2 as the final acceptor can be sustained in the light. However, in contrast with recent reports using other Chlamydomonas mutants (B4 and F8), we show here that CO2 fixation does not occur in the absence of photosystem I. By deleting the psaA gene in both B4 and F8 strains, we conclude that the ability of these mutants to fix CO2 in the light is due to the presence of residual amounts of photosystem I.

Allele-specific suppression of a defective trans-Golgi network (TGN) localization signal in Kex2p identifies three genes involved in localization of TGN transmembrane proteins

Kex2 protease (Kex2p) and Ste13 dipeptidyl aminopeptidase (Ste13p) are required in Saccharomyces cerevisiae for maturation of the alpha-mating factor in a late Golgi compartment, most likely the yeast trans-Golgi network (TGN). Previous studies identified a TGN localization signal (TLS) in the C-terminal cytosolic tail of Kex2p consisting of Tyr-713 and contextual sequences. Further analysis of the Kex2p TLS revealed similarity to the Ste13p TLS. Mutation of the Kex2p TLS results in transport of Kex2p to the vacuole by default. When expression of a GAL1 promoter-driven KEX2 gene is shut off in MAT(alpha) cells, the TGN becomes depleted of Kex2p, resulting in a gradual decline in mating competence which is greatly accelerated by TLS mutations. To identify the genes involved in localization of Kex2p, we isolated second-site suppressors of the rapid loss of mating competence observed upon shutting off expression of a TLS mutant form of Kex2p (Y713A). Seven of 58 suppressors were allele specific, suppressing point mutations at Tyr-713 but not deletions of the TLS or entire C-terminal cytosolic tail. By linkage analysis, the allele-specific suppressors defined three genetic loci, SOI1, S0I2, and S0I3. Pulse-chase analysis demonstrated that these suppressors increased net TGN retention of both Y713A Kex2p and a Ste13p-Pho8p fusion protein containing a point mutation in the Ste13p TLS. SOI1 suppressor alleles reduced the efficiency of localization of wild-type Kex2p to the TGN, implying an impaired ability to discriminate between the normal TLS and a mutant TLS. soi1 mutants also exhibited a recessive defect in vacuolar protein sorting. Suppressor alleles of S0I2 were dominant. These results suggest that the SOI1 and S0I2 genes encode regulators or components of the TLS recognition machinery.

The effects of clathrin inactivation on localization of Kex2 protease are independent of the TGN localization signal in the cytosolic tail of Kex2p

Localization of Kex2 protease (Kex2p) to the yeast trans-Golgi network (TGN) requires a TGN localization signal (TLS) in the Kex2p C-terminal cytosolic tail. Mutation of the TLS accelerates transport of Kex2p to the vacuole by an intracellular (SEC1-independent) pathway. In contrast, inactivation of the clathrin heavy-chain gene CHC1 results in transport of Kex2p and other Golgi membrane proteins to the cell surface. Here, the relationship of the two localization defects was assessed by examining the effects of a temperature-sensitive CHC1 allele on trafficking of wild-type (WT) and TLS mutant forms of Kex2p. Inactivation of clathrin by shifting chc1-ts cells to 37 degrees C caused WT and TLS mutant forms of Kex2p to behave identically. All forms of Kex2p appeared at the plasma membrane within 30-60 min of the temperature shift. TLS mutant forms of Kex2p were stabilized, their half-lives increasing to that of wild-type Kex2p. After inactivation of clathrin heavy chain, vacuolar protease-dependent degradation of all forms of Kex2p was blocked by a sec1 mutation, which is required for secretory vesicle fusion to the plasma membrane, indicating that transport to the cell surface was required for degradation by vacuolar proteolysis. Finally, after clathrin inactivation, all forms of Kex2p were degraded in part by a vacuolar protease-independent pathway. After inactivation of both chc1-ts and sec1-ts, Kex2 was degraded exclusively by this pathway. We conclude that the effects of clathrin inactivation on Kex2p localization are independent of the Kex2p C-terminal cytosolic tail. Although these results neither prove nor rule out a direct interaction between the Kex2 TLS and a clathrin-dependent structure, they do imply that clathrin is required for the intracellular transport of Kex2p TLS mutants to the vacuole.

Selectable marker recycling in the chloroplast

The bacterial gene aadA is an important and widely used selectable marker for manipulation of the chloroplast genome through biolistic transformation. Because no other such marker is available, two strategies for recycling of the aadA cassette have been developed. One utilizes homologous recombination between two direct repeats flanking the aadA cassette to allow its loss under non-selective growth conditions. A second strategy is to perform co-transformation with a plasmid containing a modified, non-essential chloroplast gene and another plasmid in which the aadA cassette disrupts a chloroplast gene known to be essential for survival. Under selective growth conditions the first mutation can be transferred to all chloroplast DNA copies whereas the aadA insertion remains heteroplasmic. Loss of the selectable marker can be achieved subsequently by growing the cells on non-selective media. In both cases it is possible to reuse the aadA cassette for the stepwise disruption or mutagenesis of any gene in the same strain.

Mutation of a tyrosine localization signal in the cytosolic tail of yeast Kex2 protease disrupts Golgi retention and results in default transport to the vacuole

Kex2 protease processes pro-alpha-factor in a late Golgi compartment in Saccharomyces cerevisiae. The first approximately 30 residues of the 115 amino acid CO2H-terminal cytosolic tail (C-tail) of the Kex2 protein (Kex2p) contain a Golgi retention signal that resembles coated-pit localization signals in mammalian cell surface receptors. Mutation of one (Tyr713) of two tyrosine residues in the C-tail or deletion of sequences adjacent to Tyr713 results in loss of normal Golgi localization. Surprisingly, loss of the Golgi retention signal resulted in transport of C-tail mutant Kex2p to the vacuole (yeast lysosome), as judged by kinetics of degradation and by indirect immunofluorescence. Analysis of the loss of Kex2 function in vivo after shutting off expression of wild-type or mutant forms proved that mutations that cause rapid vacuolar turnover do so by increasing the rate of exit of the enzyme from the pro-alpha-factor processing compartment. The most likely explanation for these results is that mutation of the Golgi retention signal in the C-tail results in transport of Kex2p to the vacuole by default. Wild-type Kex2p also was transported to the vacuole at an increased rate when overproduced, although apparently not due to saturation of a Golgi-retention mechanism. Instead, the wild-type and C-tail mutant forms of Kex2p may follow distinct paths to the vacuole.

Immunolocalization of Kex2 protease identifies a putative late Golgi compartment in the yeast Saccharomyces cerevisiae

The Kex2 protein of the yeast Saccharomyces cerevisiae is a membrane-bound, Ca2(+)-dependent serine protease that cleaves the precursors of the mating pheromone alpha-factor and the M1 killer toxin at pairs of basic residues during their transport through the secretory pathway. To begin to characterize the intracellular locus of Kex2-dependent proteolytic processing, we have examined the subcellular distribution of Kex2 protein in yeast by indirect immunofluorescence. Kex2 protein is located at multiple, discrete sites within wild-type yeast cells (average, 3.0 +/- 1.7/mother cell). Qualitatively similar fluorescence patterns are observed at elevated levels of expression, but no signal is found in cells lacking the KEX2 gene. Structures containing Kex2 protein are not concentrated at a perinuclear location, but are distributed throughout the cytoplasm at all phases of the cell cycle. Kex2-containing structures appear in the bud at an early, premitotic stage. Analysis of conditional secretory (sec) mutants demonstrates that Kex2 protein ordinarily progresses from the ER to the Golgi but is not incorporated into secretory vesicles, consistent with the proposed localization of Kex2 protein to the yeast Golgi complex.

Localization of components involved in protein transport and processing through the yeast Golgi apparatus

Saccharomyces cerevisiae sec7 mutants exhibit pleiotropic deficiencies in the transit of proteins through the Golgi apparatus, and elaborate an array of Golgi apparatus-like cisternae at a restrictive growth temperature (37 degrees C). The SEC7 gene encodes an essential high-molecular weight protein (227 kD) that is phosphorylated in vivo. In cell lysates, Sec7 protein (Sec7p) is recovered in both sedimentable and soluble fractions. A punctate immunofluorescent pattern of Sec7p-associated structures seen in SEC cells coalesces in sec14 mutant yeast that accumulate exaggerated Golgi cisternae at 37 degrees C. Sec7p may function as a peripheral membrane protein that cycles between a soluble, cytosolic pool and a sedimentable, membrane-associated complex for its essential role in vesicular traffic through the Golgi apparatus. The transmembrane Kex2 protease, which processes precursors of secreted peptides within the yeast secretory pathway, is also localized by indirect immunofluorescence to multiple structures in the yeast cell (Redding, K., and R. Fuller, manuscript submitted for publication). In double-immunofluorescence labeling experiments, significant colocalization of Sec7 and Kex2 proteins was found. Colocalization of the two antigens, one implicated in protein transport through the Golgi apparatus and the other in processing within a late Golgi compartment, supports the conclusion that we have visualized the yeast Golgi apparatus.

Atrotoxin: a specific agonist for calcium currents in heart

A specific label for voltage-dependent calcium channels is essential for the isolation and purification of the membrane protein that constitutes the calcium channel and for a better understanding of its function. A fraction of Crotalus atrox that increases voltage-dependent calcium currents in single, dispersed guinea pig ventricular cells was isolated. In the doses used, neither sodium nor potassium currents were changed. The fraction was active in the absence of detectable phospholipase or protease activity, and the active component, designated atrotoxin, produced its effect rapidly and reversibly. The effect was produced by extracellular but not intracellular application of the agent. The increase in Ca2+ current was blocked by the Ca2+ channel blockers cobalt and nitrendipine. The active fraction completely blocked specific [3H]nitrendipine binding to guinea pig ventricular membrane preparations. The inhibition of nitrendipine binding by atrotoxin was apparently via an allosteric mechanism. Thus atrotoxin was shown to bind to the Ca2+ channel and to act as a specific Ca2+ channel agonist.