PIEZO1 regulates leader cell formation and cellular coordination during collective keratinocyte migration

The collective migration of keratinocytes during wound healing requires both the generation and transmission of mechanical forces for individual cellular locomotion and the coordination of movement across cells. Leader cells along the wound edge transmit mechanical and biochemical cues to ensuing follower cells, ensuring their coordinated direction of migration across multiple cells. Despite the observed importance of mechanical cues in leader cell formation and in controlling coordinated directionality of cell migration, the underlying biophysical mechanisms remain elusive. The mechanically-activated ion channel PIEZO1 was recently identified to play an inhibitory role during the reepithelialization of wounds. Here, through an integrative experimental and mathematical modeling approach, we elucidate PIEZO1's contributions to collective migration. Time-lapse microscopy reveals that PIEZO1 activity inhibits leader cell formation at the wound edge. To probe the relationship between PIEZO1 activity, leader cell formation and inhibition of reepithelialization, we developed an integrative 2D continuum model of wound closure that links observations at the single cell and collective cell migration scales. Through numerical simulations and subsequent experimental validation, we found that coordinated directionality plays a key role during wound closure and is inhibited by upregulated PIEZO1 activity. We propose that PIEZO1-mediated retraction suppresses leader cell formation which inhibits coordinated directionality between cells during collective migration.

PIEZO1-HaloTag hiPSCs: Bridging Molecular, Cellular and Tissue Imaging

PIEZO1 channels play a critical role in numerous physiological processes by transducing diverse mechanical stimuli into electrical and chemical signals. Recent studies underscore the importance of endogenous PIEZO1 activity and localization in regulating mechanotransduction. To enable physiologically and clinically relevant human-based studies, we genetically engineered human induced pluripotent stem cells (hiPSCs) to express a HaloTag fused to endogenous PIEZO1. Combined with super-resolution imaging, our chemogenetic approach allows precise visualization of PIEZO1 in various cell types. Further, the PIEZO1-HaloTag hiPSC technology allows non-invasive monitoring of channel activity via Ca2+-sensitive HaloTag ligands, with temporal resolution approaching that of patch clamp electrophysiology. Using lightsheet imaging of hiPSC-derived neural organoids, we also achieve molecular scale PIEZO1 imaging in three-dimensional tissue samples. Our advances offer a novel platform for studying PIEZO1 mechanotransduction in human cells and tissues, with potential for elucidating disease mechanisms and development of targeted therapeutics.

Piezo1 regulates cholesterol biosynthesis to influence neural stem cell fate during brain development

Mechanical forces and tissue mechanics influence the morphology of the developing brain, but the underlying molecular mechanisms have been elusive. Here, we examine the role of mechanotransduction in brain development by focusing on Piezo1, a mechanically activated ion channel. We find that Piezo1 deletion results in a thinner neuroepithelial layer, disrupts pseudostratification, and reduces neurogenesis in E10.5 mouse embryos. Proliferation and differentiation of Piezo1 knockout (KO) mouse neural stem cells (NSCs) isolated from E10.5 embryos are reduced in vitro compared to littermate WT NSCs. Transcriptome analysis of E10.5 Piezo1 KO brains reveals downregulation of the cholesterol biosynthesis superpathway, in which 16 genes, including Hmgcr, the gene encoding the rate-limiting enzyme of the cholesterol biosynthesis pathway, are downregulated by 1.5-fold or more. Consistent with this finding, membrane lipid composition is altered, and the cholesterol levels are reduced in Piezo1 KO NSCs. Cholesterol supplementation of Piezo1 KO NSCs partially rescues the phenotype in vitro. These findings demonstrate a role for Piezo1 in the neurodevelopmental process that modulates the quantity, quality, and organization of cells by influencing cellular cholesterol metabolism. Our study establishes a direct link in NSCs between PIEZO1, intracellular cholesterol levels, and neural development.

Global PIEZO1 Gain-of-Function Mutation Causes Cardiac Hypertrophy and Fibrosis in Mice

PIEZO1 is a subunit of mechanically-activated, nonselective cation channels. Gain-of-function PIEZO1 mutations are associated with dehydrated hereditary stomatocytosis (DHS), a type of anaemia, due to abnormal red blood cell function. Here, we hypothesised additional effects on the heart. Consistent with this hypothesis, mice engineered to contain the M2241R mutation in PIEZO1 to mimic a DHS mutation had increased cardiac mass and interventricular septum thickness at 8–12 weeks of age, without altered cardiac contractility. Myocyte size was greater and there was increased expression of genes associated with cardiac hypertrophy (Anp, Acta1 and β-MHC). There was also cardiac fibrosis, increased expression of Col3a1 (a gene associated with fibrosis) and increased responses of isolated cardiac fibroblasts to PIEZO1 agonism. The data suggest detrimental effects of excess PIEZO1 activity on the heart, mediated in part by amplified PIEZO1 function in cardiac fibroblasts.

Endothelial Piezo1 sustains muscle capillary density and contributes to physical activity

Piezo1 forms mechanically activated nonselective cation channels that contribute to endothelial response to fluid flow. Here we reveal an important role in the control of capillary density. Conditional endothelial cell-specific deletion of Piezo1 in adult mice depressed physical performance. Muscle microvascular endothelial cell apoptosis and capillary rarefaction were evident and sufficient to account for the effect on performance. There was selective upregulation of thrombospondin-2 (TSP2), an inducer of endothelial cell apoptosis, with no effect on TSP1, a related important player in muscle physiology. TSP2 was poorly expressed in muscle endothelial cells but robustly expressed in muscle pericytes, in which nitric oxide (NO) repressed the Tsp2 gene without an effect on Tsp1. In endothelial cells, Piezo1 was required for normal expression of endothelial NO synthase. The data suggest an endothelial cell-pericyte partnership of muscle in which endothelial Piezo1 senses blood flow to sustain capillary density and thereby maintain physical capability.

Spatiotemporal dynamics of PIEZO1 localization controls keratinocyte migration during wound healing

Keratinocytes, the predominant cell type of the epidermis, migrate to reinstate the epithelial barrier during wound healing. Mechanical cues are known to regulate keratinocyte re-epithelialization and wound healing; however, the underlying molecular transducers and biophysical mechanisms remain elusive. Here, we show through molecular, cellular, and organismal studies that the mechanically activated ion channel PIEZO1 regulates keratinocyte migration and wound healing. Epidermal-specific Piezo1 knockout mice exhibited faster wound closure while gain-of-function mice displayed slower wound closure compared to littermate controls. By imaging the spatiotemporal localization dynamics of endogenous PIEZO1 channels, we find that channel enrichment at some regions of the wound edge induces a localized cellular retraction that slows keratinocyte collective migration. In migrating single keratinocytes, PIEZO1 is enriched at the rear of the cell, where maximal retraction occurs, and we find that chemical activation of PIEZO1 enhances retraction during single as well as collective migration. Our findings uncover novel molecular mechanisms underlying single and collective keratinocyte migration that may suggest a potential pharmacological target for wound treatment. More broadly, we show that nanoscale spatiotemporal dynamics of Piezo1 channels can control tissue-scale events, a finding with implications beyond wound healing to processes as diverse as development, homeostasis, disease, and repair.

Mechanically activated Piezo1 channels of cardiac fibroblasts stimulate p38 mitogen-activated protein kinase activity and interleukin-6 secretion

Piezo1 is a mechanosensitive cation channel with widespread physiological importance; however, its role in the heart is poorly understood. Cardiac fibroblasts help preserve myocardial integrity and play a key role in regulating its repair and remodeling following stress or injury. Here we investigated Piezo1 expression and function in cultured human and mouse cardiac fibroblasts. RT-PCR experiments confirmed that Piezo1 mRNA in cardiac fibroblasts is expressed at levels similar to those in endothelial cells. The results of a Fura-2 intracellular Ca²⁺ assay validated Piezo1 as a functional ion channel that is activated by its agonist, Yoda1. Yoda1-induced Ca²⁺ entry was inhibited by Piezo1 blockers (gadolinium and ruthenium red) and was reduced proportionally by siRNA-mediated Piezo1 knockdown or in murine Piezo1^+/− cells. Results from cell-attached patch clamp recordings on human cardiac fibroblasts established that they contain mechanically activated ion channels and that their pressure responses are reduced by Piezo1 knockdown. Investigation of Yoda1 effects on selected remodeling genes indicated that Piezo1 activation increases both mRNA levels and protein secretion of IL-6, a pro-hypertrophic and profibrotic cytokine, in a Piezo1-dependent manner. Moreover, Piezo1 knockdown reduced basal IL-6 expression from cells cultured on softer collagen-coated substrates. Multiplex kinase activity profiling combined with kinase inhibitor experiments and phosphospecific immunoblotting established that Piezo1 activation stimulates IL-6 secretion via the p38 mitogen-activated protein kinase downstream of Ca²⁺ entry. In summary, cardiac fibroblasts express mechanically activated Piezo1 channels coupled to secretion of the paracrine signaling molecule IL-6. Piezo1 may therefore be important in regulating cardiac remodeling.

Yoda1 analogue (Dooku1) which antagonizes Yoda1-evoked activation of Piezo1 and aortic relaxation

Background and Purpose

The mechanosensitive Piezo1 channel has important roles in vascular physiology and disease. Yoda1 is a small‐molecule agonist, but the pharmacology of these channels is otherwise limited.

Experimental Approach

Yoda1 analogues were generated by synthetic chemistry. Intracellular Ca²⁺ and Tl⁺ measurements were made in HEK 293 or CHO cell lines overexpressing channel subunits and in HUVECs, which natively express Piezo1. Isometric tension recordings were made from rings of mouse thoracic aorta.

Key Results

Modification of the pyrazine ring of Yoda1 yielded an analogue, which lacked agonist activity but reversibly antagonized Yoda1. The analogue is referred to as Dooku1. Dooku1 inhibited 2 μM Yoda1‐induced Ca²⁺‐entry with IC₅₀s of 1.3 μM (HEK 293 cells) and 1.5 μM (HUVECs) yet failed to inhibit constitutive Piezo1 channel activity. It had no effect on endogenous ATP‐evoked Ca²⁺ elevation or store‐operated Ca²⁺ entry in HEK 293 cells or Ca²⁺ entry through TRPV4 or TRPC4 channels overexpressed in CHO and HEK 293 cells. Yoda1 caused dose‐dependent relaxation of aortic rings, which was mediated by an endothelium‐ and NO‐dependent mechanism and which was antagonized by Dooku1 and analogues of Dooku1.

Conclusion and Implications

Chemical antagonism of Yoda1‐evoked Piezo1 channel activity is possible, and the existence of a specific chemical interaction site is suggested with distinct binding and efficacy domains.

Meta-analysis: the effect of antibiotic resistance status on the efficacy of triple and quadruple first-line therapies for Helicobacter pylori

BACKGROUND

Information regarding the effects of drug resistance on therapies for Helicobacter pylori is limited.

AIMS

To determine the effect of drug resistance on the efficacy of first-line treatment regimens for H. pylori and identify the most efficacious treatments in the presence of drug resistance.

METHODS

We searched for studies using the keywords: 'Helicobacter pylori','resistance' and 'treatment' or 'therapy'. Multilevel meta-regression models were used to determine the effect of drug resistance on treatment efficacy.

RESULTS

We analysed data from 93 studies with 10,178 participants. For triple therapies, clarithromycin resistance had a greater effect on treatment efficacy than nitroimidazole resistance. Metronidazole resistance reduced efficacy by 26% in triple therapies containing a nitroimidazole, tetracycline and bismuth, while efficacy was reduced by only 14% when a gastric acid inhibitor was added to the regimen. Quadruple therapies containing both clarithromycin and metronidazole were the most efficacious; >80% of H. pylori infections were consistently eradicated with these regimens.

CONCLUSIONS

Drug resistance was a strong predictor of efficacy across triple therapies for the eradication of H. pylori in adults. Resistance to either clarithromycin or metronidazole, but not both simultaneously, may be overcome by using quadruple therapies, especially those containing both clarithromycin and metronidazole.

Comparison of IgA-alpha1-antitrypsin levels in rheumatoid arthritis and seronegative oligoarthritis: complex formation is not associated with inflammation per se

Serum complexes between IgA and alpha1-antitrypsin (IgA alpha1AT) have been found at raised levels in early rheumatoid arthritis (RA), where they appear to be associated with more erosive disease. We have now measured the levels of these complexes in the sera and synovial fluid of patients with RA and seronegative oligoarthritis to determine whether there is a relationship between complex levels and joint inflammation, and if bacterial stimulation of the mucosa is associated with complex formation in seronegative oligoarthritis. IgA-alpha1AT complexes were measured in patients with RA (n = 75) and seronegative oligoarthritis, with or without definite reactive arthritis (n = 28), using a newly developed sandwich ELISA. The results were compared with serum levels from healthy volunteers (n = 30). IgA and alpha1AT were also measured using ELISA and radial immunodiffusion (RID) techniques, respectively. IgA-alpha1AT complex levels in the sera of RA patients [mean = 25.6 arbitrary units (au)] were significantly higher (P = 0.0034) than those in patients with seronegative oligoarthritis (mean = 12.36 au) and healthy controls (mean = 8.08 au). There was no evidence for the inflamed joint being the source of IgA-alpha1AT complexes since synovial fluid levels were lower than corresponding serum levels, although higher amounts were found in RA than seronegative oligoarthritis (12.84 au vs 4.11 au, P = 0.01). Serum levels of IgA and alpha1AT were similar in RA and seronegative oligoarthritis patients, and were higher than in normals. There was a significant correlation between complex and serum IgA levels in RA (r = 0.49, P < 0.001) and seronegative oligoarthritis (r = 0.53, P < 0.001) patients, although no relationship was found with alpha1AT levels. There was no correlation with other markers of the acute-phase response (C-reactive protein, erythrocyte sedimentation rate), nor with any clinical markers. In RA patients, serum complex levels were significantly higher in seropositive than seronegative patients (30.75 vs 16.48 au, P = 0.03), and we have demonstrated that a small amount of alpha1AT may be complexed with IgA rheumatoid factor. Our data suggest that the formation of IgA-alpha1AT complexes is not associated with inflammation per se, and does not appear to be related to bacterial stimulation of the mucosal immune system in patients with seronegative oligoarthritis.

Effects of rumpshaker mutation on CNS myelin composition and structure

Myelinated CNS tissues from homozygous/hemizygous and heterozygous jimpy rumpshaker jprsh mutant mice were examined to determine the consequences on myelin structure of this mutation in the proteolipid protein (PLP) gene. Polyacrylamide gel electrophoresis and immunoblotting of brain homogenates confirmed that there was a decrease in PLP levels on the B6C3 genetic background onto which this gene was bred. We also observed an increase in level of a protein band that could correspond to the uncharacterized 10-kDa PLP previously reported in jprsh mice on an Rb(1.3) 1Bnr background. High-performance TLC and densitometry of lipids from brain homogenate and isolated myelin revealed a decrease in content of cerebrosides and sulfatides. Electron microscopy on optic nerves revealed that normal radial component is retained in jprsh myelin, further substantiating that PLP is not a component of this junctional complex. X-ray diffraction measurements on unfixed optic nerves showed that the jprsh period is 5-10 A larger than normal. Moreover, jprsh optic nerve myelin was unstable, as evidenced by a continual increase in the period postdissection. jprsh myelin that was equilibrated at varying pH and ionic strength typically had a larger than normal period under all conditions (both swelling and compacting). Our findings thus demonstrate that the biochemical abnormalities in the jprsh mutant correlate with a wider periodicity and less stable packing of the myelin.

Therapist efficiency and clinic accessibility with the Mental Health Research Institute brief therapy model

A health maintenance organization (HMO) mental health clinic used the Mental Research Institute (MRI) brief therapy model to achieve striking therapist efficiency and clinic accessibility. In the two-year period from January 1985 through December 1986, the clinic averaged 834 separate patients per therapist, compared with a regional average of 456 patients. The rate of hospitalization from the clinic catchment area was two thirds that of the region. This article describes the MRI approach as practiced at the clinic and discusses its applicability to community mental health centers and other mental health clinics.

Lymphocyte surface antigen L25 is a member of the integrin receptor superfamily

Monoclonal antibody (mAb) anti-L25 identifies an antigen on the surface of human lymphocytes. This mAb immunoprecipitated three distinct polypeptides of Mr 150,000, 85,000, and 75,000 from Nonidet P-40 lysates of surface radioiodinated lymphocytes. The three polypeptides were found under both nonreducing and reducing conditions. An additional polypeptide of Mr 130,000 was detected in mAb anti-L25 immuno-precipitates when cells were lysed with CHAPS (3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate). Epitope localization experiments indicated that both the Mr 150,000 and 85,000 polypeptides contained antibody reactive sites. Peptide mapping studies demonstrated structural similarities in the Mr 150,000 and 85,000 components. The analysis of L25 subunits from lysates of antigen-stimulated T lymphocytes revealed a loss of the Mr 150,000 polypeptides in mAb anti-L25 immunoprecipitates. Solid phase double determinant binding assays demonstrated that L25 is similar and probably identical to lymphocyte surface antigen very late activation-4. This relationship placed L25 as a member of the integrin receptor superfamily.

Monoclonal antibodies that distinguish between class II antigens (HLA-DP, DQ, and DR) in 14 haplotypes

The specificity of three commonly used monoclonal antibodies (MoAbs) reacting with human class II histocompatibility antigens, was analyzed to determine whether these MoAbs would distinguish between HLA-DP, DQ, and DR in a large number of haplotypes. The reactivity of these MoAbs (L243, Anti-Leu 10, and B7/21) was compared by serial immunoprecipitation of class II antigens from 11 B-cell lines. The cell lines examined expressed a total of five DP, three DQ, and nine DR types, which together represent most of the well-defined class II specificities. This is the first demonstration that one of these antibodies, B7/21. binds to at least five DP specificities, and does not bind to DR or DQ molecules as defined by reactivity with the two other MoAbs. Within the scope of these experiments, the B7/21 antibody was shown to react with a monomorphic DP determinant. A variant clone of the B7/21 hybridoma was isolated that secretes IgG1 antibody with the same specificity as the original IgG3 antibody. The two other antibodies studied have been previously shown to react with DR molecules (L243) or DQ molecules (Anti-Leu 10). Here, their lack of cross-reaction with DP molecules is demonstrated. Thus, each of the three MoAbs reacts exclusively with a distinct class II molecule in all haplotypes studied, and therefore should be useful for comparing the independent expression and function of DP, DQ, and DR molecules.

Expression of Leu-19 (NKH-1) antigen on IL 2-dependent cytotoxic and non-cytotoxic T cell lines

The Leu-19 (NKH-1) antigen is expressed on human peripheral blood NK cells and a subset of peripheral blood cytotoxic T lymphocytes that kill "NK-sensitive" tumor cell targets without major histocompatibility complex restriction. In the present study, we demonstrate that the Leu-19 (NKH-1) antigen is also expressed on most interleukin 2 (IL 2) dependent T cell lines and clones that have been maintained in long term culture. The Leu-19 (NKH-1) antigen expressed on an antigen-specific, class I directed cytotoxic T lymphocyte cell line was an approximately 200,000 to 220,000 dalton protein, similar to Leu-19 (NKH-1) protein expressed on natural killer cells and KG1a, an immature stem cell leukemia cell line. Furthermore, Leu-19 (NKH-1) was expressed on both CD4+ and CD8+ IL 2 dependent T cell clones, and was present on both cytotoxic and non-cytotoxic T cell clones. Thus expression of Leu-19 (NKH-1) antigen on cultured cell lines does not directly correlate with cytotoxic function, antigenic specificity, or cell lineage.

Expression of Leu-19 (NKH-1) antigen on IL 2-dependent cytotoxic and non-cytotoxic T cell lines

The Leu-19 (NKH-1) antigen is expressed on human peripheral blood NK cells and a subset of peripheral blood cytotoxic T lymphocytes that kill "NK-sensitive" tumor cell targets without major histocompatibility complex restriction. In the present study, we demonstrate that the Leu-19 (NKH-1) antigen is also expressed on most interleukin 2 (IL 2) dependent T cell lines and clones that have been maintained in long term culture. The Leu-19 (NKH-1) antigen expressed on an antigen-specific, class I directed cytotoxic T lymphocyte cell line was an approximately 200,000 to 220,000 dalton protein, similar to Leu-19 (NKH-1) protein expressed on natural killer cells and KG1a, an immature stem cell leukemia cell line. Furthermore, Leu-19 (NKH-1) was expressed on both CD4+ and CD8+ IL 2 dependent T cell clones, and was present on both cytotoxic and non-cytotoxic T cell clones. Thus expression of Leu-19 (NKH-1) antigen on cultured cell lines does not directly correlate with cytotoxic function, antigenic specificity, or cell lineage.

Identification and characterization of two novel lymphocyte function-associated antigens, L24 and L25

We describe the function and cell distribution of two novel cell surface antigens, L24 and L25. These antigens are broadly distributed on human lymphocytes. Monoclonal antibodies specific for these molecules block lysis by Class I- and II-specific cytotoxic T lymphocytes, but do not affect any other T cell functions tested. Anti-L24 antibody immunoprecipitates a molecule composed of two disulfide-linked monomers of 140 kd each. Anti-L25 antibody immunoprecipitates three proteins of 150, 85, and 75 kd. The study of these and other function associated molecules may provide insight into mechanisms of cytotoxic T lymphocyte recognition and/or function.

Identification and characterization of two novel lymphocyte function-associated antigens, L24 and L25

We describe the function and cell distribution of two novel cell surface antigens, L24 and L25. These antigens are broadly distributed on human lymphocytes. Monoclonal antibodies specific for these molecules block lysis by Class I- and II-specific cytotoxic T lymphocytes, but do not affect any other T cell functions tested. Anti-L24 antibody immunoprecipitates a molecule composed of two disulfide-linked monomers of 140 kd each. Anti-L25 antibody immunoprecipitates three proteins of 150, 85, and 75 kd. The study of these and other function associated molecules may provide insight into mechanisms of cytotoxic T lymphocyte recognition and/or function.

S-phase detection with an antibody to bromodeoxyuridine. Role of DNase pretreatment

We have previously described a monoclonal antibody (BU-1) to 5-bromo-2-deoxyuridine (BrdUrd) that is useful for measurement of cell cycle S-phase. BU-1 hybridoma supernatant reacted with incorporated BrdUrd after the cells had been ethanol fixed; without a requirement for acid or base denaturation. We have found that this reactivity is lost if purified antibody is used, if the culture supernatants are heated, or if a mycoplasma-free hybridoma line is isolated. The supernatant contained endogenous DNase activity that was a result of mycoplasma infection of the cell line. This DNase activity was required for staining the cells with BU-1 in the absence of other denaturation steps. The endogenous DNase could be substituted for by the addition of bovine pancreatic DNase I. The disruption of the double stranded DNA structure with an enzyme rather than with harsh chemical or heat treatments does not affect protein structure or cellular morphology and allows the detection of incorporated BrdUrd of morphologic or antigenic cell subsets. DNase pre-treatment may also be useful for detection of other 'hidden' DNA antigens.

Evaluation of diallate and triallate herbicides for genotoxic effects in a battery of in vitro and short-term in vivo tests

Commercial-grade preparations of two thiocarbamate herbicides, diallate and triallate, were evaluated for their mutagenic potential in a battery of short-term bioassays. All in vitro bioassays were performed with and without mammalian metabolic activation, and all such tests were repeated after an interval of at least 1 week. Diallate and triallate were tested in the Salmonella/microsome assay over dose ranges of 0.59 to 118.0 micrograms/plate and 6.37 to 1273 micrograms/plate, respectively. Both diallate and triallate gave positive results in S. typhimurium strains TA1535, TA98, and TA100 only in the presence of a rat-liver metabolic activation system. In Saccharomyces cerevisiae strain D7, diallate was tested at concentrations from 1.18 to 29.50 micrograms/ml, and triallate was tested at 0.955 to 9.548 micrograms/ml. Both diallate and triallate gave negative results for mitotic gene conversion, mitotic crossing-over, and reverse mutation. In the mouse lymphoma L5178Y TK+/- assay, diallate was tested at concentrations ranging from 1 to 72 micrograms/ml, and triallate was tested at 0.5 to 60 micrograms/ml. Both herbicides produced mutagenic responses in the mouse lymphoma assay in the presence of metabolic activation. In the Drosophila sex-linked recessive lethal test, flies were exposed to 0.0004% diallate and 0.001% triallate. In this assay, diallate was considered mutagenic, whereas triallate did not produce a detectable mutagenic response.

Properties of a mutant Escherichia coli phosphoenolpyruvate carboxylase deficient in coregulation by intermediary metabolites

Phosphoenolpyruvate carboxylase of Escherichia coli is activated by three different mechanisms: contiguous by acetyl coenzyme A, precursor by fructose 1,6-bisphosphate, and compensatory feedback by cytidine 5'-diphosphate (CDP). Even though each activator can interact independently with the enzyme, synergistic effects are observed with some combinations, namely, fructose 1,6-bisphosphate or CDP (coregulators), with acetyl coenzyme A. A mutant was isolated that has a phosphoenolpyruvate carboxylase which is refractory to activation by fructose, 1,6-bisphosphate and CDP. The mutant enzyme was shown to be active primarily as the dimer and to lack cooperativity in substrate binding. The binding of acetyl coenzyme A and substrate, however, was essentially the same as that of the wild-type enzyme. The mutant cells grew extremely slowly on glucose alone as the sole carbon source. The only defect in the mutant appeared to be the inability of this enzyme to be activated by the coregulators. These data are consistent with the thesis that coregulation by fructose 1,6-bisphosphate or CDP is an essential requirement for the activation in vivo of this enzyme.

Phycobilisomes in Anacystis nidulans

Phycobilisomes were demonstrated in Anacystis nidulans by chemical and morphological studies on cells grown in red light. These cells showed a marked reduction in the chlorophyll-phycocyanin ratio owing to a decreased chlorophyll content. Granular structures of approximately 35 nm were observed throughout red light-grown cells, but were most distinct in the peripheral region. The presence of phycobilisomes in cells grown in red light as well as in cells grown in white light is supported by experiments in which glutaraldehyde was used to stabilize the attachment between the phycobiliprotein and the thylakoids, allowing the isolation of both in the same fraction by sucrose density gradient centrifugation.

Kinetics of Single-Layer Graphite Oxidation: Evaluation by Electron Microscopy

Etch-decoration reveals that the rate of removal of carbon atoms exposed at monolayer steps on graphite surfaces is very different from the rate of removal, under identical conditions, at multilayer steps. At 1113 degrees K and a pressure of 1.33 newtons per square meter of oxygen, the rate of oxidation (along the layer planes) is less by a factor of nearly 100 than that at multilayer steps.

Osteochondritis of the Lower End of the Left Femur in a Child aged 5 years

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Amniotic Band

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Myositis Ossificans Traumatica

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Two Cases of Congenital Kyphosis

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Case of Severe Comminution of the Femur

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Three Cases of Congenital Œdema (Milroy's Disease) in Two Generations of the Same Family

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Case of Kienböch's Disease

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Fracture of the Transverse Process of the Third Lumbar Vertebra

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Congenital Absence of Tibia

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Case of Left Unilateral complete Absence of the Tibia

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