Changes in Pneumonia Incidence and Infant Mortality 5 Years Following Introduction of the 13-valent Pneumococcal Conjugate Vaccine in a “3+0” Schedule in Nicaragua

Background

Streptococcus pneumoniae causes an estimated 826,000 deaths of children in the world each year and many health facility visits. To reduce the burden of pneumococcal disease, many nations have added pneumococcal conjugate vaccines to their national immunization schedules. Nicaragua was the first country eligible for funding from the GAVI Alliance to introduce the 13-valent pneumococcal conjugate vaccine (PCV13), provided to infants at 2, 4, and 6 months of age. The goal of this study was to evaluate the population impact of the first five years of the program.

Methods

Numbers of visits for pneumonia, pneumonia-related deaths, bacterial meningitis, and infant deaths between 2008 and 2015 were collected from all 107 public health facilities in León Department. Vital statistics data provided additional counts of pneumonia-related deaths that occurred outside health facilities. Adjusted incidence rates and incidence rate ratios (IRRa) in the vaccine (2011–2015) and pre-vaccine periods (2008–2010) were estimated using official population estimates as exposure time.

Results

The IRRa for pneumonia hospitalizations was 0.70 (95% confidence interval [CI]: 0.66, 0.75) for infants, and 0.92 (95% CI: 0.85, 0.99) for one year olds. The IRRa for post-neonatal infant mortality was 0.56 (95% CI: 0.41, 0.77). In the population as a whole, ambulatory visits and hospitalizations for pneumonia, as well as pneumonia-related mortality and rates of bacterial meningitis were lower in the vaccine period.

Conclusion

Five years following program introduction, reductions were observed in health facility visits for pneumonia in immunized age groups and infant mortality, which would be hard to achieve with any other single public health intervention. Future study is warranted to understand whether the lack of a booster dose (e.g.,, at 12 months) may be responsible for the small reductions in pneumonia hospitalizations observed in one year-olds as compared with infants.

Disclosures

S. Becker-Dreps, Pfizer: Consultant and Grant Investigator, Consulting fee and Research grant; D. J. Weber, Pfizer: Consultant and Speaker’s Bureau, Consulting fee and Speaker honorarium

Changes in the incidence of pneumonia, bacterial meningitis, and infant mortality 5 years following introduction of the 13-valent pneumococcal conjugate vaccine in a "3+0" schedule

Background

Streptococcus pneumoniae causes about 826,000 deaths of children in the world each year and many health facility visits. To reduce the burden of pneumococcal disease, many nations have added pneumococcal conjugate vaccines to their national immunization schedules. Nicaragua was the first country eligible for GAVI Alliance funding to introduce the 13-valent pneumococcal conjugate vaccine (PCV13) in 2010, provided to infants at 2, 4, and 6 months of age. The goal of this study was to evaluate the population impact of the first five years of the program.

Methods

Numbers of visits for pneumonia, pneumonia-related deaths, and bacterial meningitis in both children and adults, and infant deaths between 2008 and 2015 were collected from all 107 public health facilities in León Department. Vital statistics data provided additional counts of pneumonia-related deaths that occurred outside health facilities. Adjusted incidence rates and incidence rate ratios (IRRa) in the vaccine (2011–2015) and pre-vaccine periods (2008–2010) were estimated retrospectively using official population estimates as exposure time.

Results

The IRRa for pneumonia hospitalizations was 0.70 (95% confidence interval [CI]: 0.66, 0.75) for infants, and 0.92 (95% CI: 0.85, 0.99) for one year-olds. The IRRa for post-neonatal infant mortality was 0.56 (95% CI: 0.41, 0.77). In the population as a whole, ambulatory visits and hospitalizations for pneumonia, as well as pneumonia-related mortality and rates of bacterial meningitis were lower in the vaccine period.

Conclusions

During the first five years of program implementation, reductions were observed in health facility visits for pneumonia in immunized age groups and infant mortality, which would be hard to achieve with any other single public health intervention. Future study is warranted to understand whether the lack of a booster dose (e.g., at 12 months) may be responsible for the small reductions in pneumonia hospitalizations observed in one year-olds as compared to infants.

Levels and predictors of persistent organic pollutants in an adult population from four Spanish regions

This research aimed to assess serum concentrations of a group of persistent organic pollutants (POPs) in a sample of adults recruited in four different regions from Spain and to assess socio-demographic, dietary, and lifestyle predictors of the exposure. The study population comprised 312 healthy adults selected from among controls recruited in the MCC-Spain multicase-control study. Study variables were collected using standardized questionnaires, and pollutants were analyzed by means of gas chromatography with electron capture detection. Multivariable analyses were performed to identify predictors of log-transformed pollutant concentrations, using combined backward and forward stepwise multiple linear regression models. Detection rates ranged from 89.1% (hexachlorobenzene, HCB) to 93.6% (Polychlorinated biphenyl-153 [PCB-153]); p,p'-dichlorodiphenyldichloroethylene (p,p'-DDE) showed the highest median concentrations (1.04ng/ml), while HCB showed the lowest (0.24ng/ml). In the multivariable models, age was positively associated with HCB, p,p'-DDE, and PCB-180. BMI was associated positively with p,p'-DDE but negatively with PCB-138. Total accumulated time residing in an urban area was positively associated with PCB-153 concentrations. The women showed higher HCB and lower p,p'-DDE concentrations versus the men. Notably, POP exposure in our study population was inversely associated with the breastfeeding received by participants and with the number of pregnancies of their mothers but was not related to the participants' history of breastfeeding their children or parity. Smoking was negatively associated with HCB and PCB-153 concentrations. Consumption of fatty foods, including blue fish, was in general positively associated with POP levels. Although POP environmental levels are declining worldwide, there is a need for the continuous monitoring of human exposure in the general population. The results of the present study confirm previous findings and point to novel predictors of long-term exposure to persistent organic pollutants.

Impact of a combined pediatric and adult pneumococcal immunization program on adult pneumonia incidence and mortality in Nicaragua

BACKGROUND

In 2010, Nicaragua implemented an adult immunization program with the 23-valent pneumococcal polysaccharide vaccine (PPSV-23) and a pediatric immunization program with the 13-valent pneumococcal conjugate vaccine (PCV-13). We assessed incidence rates of ambulatory visits and hospitalizations for pneumonia and pneumonia-related mortality in adults over the age of 50 years before and after the program's implementation in the Department of León, Nicaragua.

METHODS

We collected visit diagnoses from all 107 public health facilities between 2008 and 2012 in León. We compared incidence rates of ambulatory visits for pneumonia, pneumonia hospitalizations, and pneumonia-related mortality in the pre-vaccine (2008-2009) and vaccine (2011-2012) periods among older adults using Poisson regression with generalized estimating equations (GEE), controlling for age group, municipality, and proportions of adults who were immunized against influenza. Exposure time was estimated by official municipality population estimates.

RESULTS

We did not observe lower incidence rates of ambulatory visits or hospitalizations for pneumonia among adults during the vaccine period versus the pre-vaccine period. However, pneumonia-related mortality was lower in the vaccine period versus the pre-vaccine period, with an adjusted incidence rate ratio (IRRa) of 0.73 (0.56, 0.94) among adults aged 50-64 years, and 0.55 (0.43, 0.70) among adults aged ≥65 years.

CONCLUSIONS

These early results following introduction of a combined pediatric and adult pneumococcal immunization program in Nicaragua show a probable impact of the program on the reduction of pneumonia-related deaths in older adults, but a less clear impact on the reduction of health facility visits for pneumonia.

Characterization of enterotoxigenic Escherichia coli strains isolated from Nicaraguan children in hospital, primary care and community settings

Enterotoxigenic Escherichia coli (ETEC) is one of the most common causes of diarrhoea among young children in developing countries. ETEC vaccines offer promise in reducing the burden of ETEC disease, but the development of these vaccines relies on the characterization of ETEC isolates from a variety of settings. To best reflect the full spectrum of ETEC disease in León, Nicaragua, the aim of this study was to characterize ETEC strains isolated from children with diarrhoea attending different settings (hospital, primary care clinics and in the community) and children from different age groups. We characterized ETEC isolates in terms of their colonization factors (CFs) and enterotoxins, and determined whether these factors varied with setting and age group. Diarrhoeal stool samples were obtained from children under the age of 60 months from: (1) the regional public hospital, (2) four public primary care clinics, and (3) a population-based cohort. In total, 58 ETEC-positive isolates were analysed by multiplex-PCR assays for the identification of CFs (CS1, CS2, CS3, CS4, CS5, CS6, CS7, CS8, CS12, CS13, CS14, CS15, CS17, CS18, CS19, CS20, CS21, CS22 and CFA/I), and enterotoxins [heat-labile toxin (LT) and heat-stable variants STh and STp]. The frequency of CFs and enterotoxins was compared among the three settings and for different age groups, using Fisher's exact test or a χ(2) test. At least one CF was detected among one-half of samples; CS19 was detected among all strains in which a CF was identified, either alone or in combination with another CF. Among all CFs detected, 91.7 % were identified as members of the class 5 fimbrial family. CFs were detected more commonly among samples from infants captured in the health facility setting compared with the community setting. Overall, LT was detected among 67.2 % of samples, STh was detected among 20.7 % and both enterotoxins were detected among 12.1 %. The enterotoxin STh was detected more commonly among cases in the community, whilst a combination of STh and LT was detected more commonly among cases treated in health facilities. Our results suggest that, to protect against diarrhoeal cases associated with this E. coli pathotype in León, Nicaragua, an ETEC vaccine that effectively targets the archeotype CFA/I of the class 5 fimbrial family would be the most effective in this setting.

Placental concentrations of heavy metals in a mother-child cohort

Heavy metals are environmental contaminants with properties known to be toxic for wildlife and humans. Despite strong concerns about their harmful effects, little information is available on intrauterine exposure in humans. The aim of this study was to evaluate prenatal exposure to As, Cd, Cr, Hg, Mn, and Pb and its association with maternal factors in a population-based mother-child cohort in Southern Spain. Between 2000 and 2002, 700 pregnant women were recruited and 137 placentas from the cohort were randomly selected and analyzed for the selected metals by atomic absorption. Maternal sociodemographic and lifestyle factors were obtained by questionnaire after delivery. Bivariate analysis and multivariate linear regression were performed. Cd and Mn concentrations were detected in all placentas, while Cr, Pb, and Hg were found in 98.5%, 35.0%, and 30.7% of samples, respectively. The highest concentrations were observed for Pb (mean: 94.80 ng/g wet weight of placenta), followed by Mn (63.80 ng/g), Cr (63.70 ng/g), Cd (3.45 ng/g), and Hg (0.024 ng/g). Arsenic was not detected in any sample. Gestational age and smoking during pregnancy were associated with placental Cd concentrations, while no factor appeared to influence concentrations of Cr, Hg, Mn, or Pb. In comparison to results of European studies, these concentrations are in a low-intermediate position. Studies are required to investigate the factors contributing to early exposure to heavy metals and to determine how placental transfer of these toxic compounds may affect children's health.

Low-dose radiation hyper-radiosensitivity in multicellular tumour spheroids

OBJECTIVE

We propose and study a new model aimed at describing the low-dose hyper-radiosensitivity phenomenon appearing in the survival curves of different cell lines.

METHODS

The model uses the induced repair assumption, considering that the critical dose at which this mechanism begins to act varies from cell to cell in a given population. The model proposed is compared with the linear-quadratic model and the modified linear-quadratic model, which is commonly used in literature and in which the induced repair is taken into account in a heuristic way. The survival curve for the MCF-7 line of human breast cancer is measured at low absorbed doses and the uncertainties in these doses are estimated using thermoluminiscent dosemeters.

RESULTS

It is shown that these multicellular spheroids present low-dose hyper-radiosensitivity. The new model permits an accurate description of the data of two human cell lines (previously published) and of the multicellular spheroids of the MCF-7 line here measured.

CONCLUSION

The model shows enough flexibility to account for data with very different characteristics and considers in a faithful way the hypothesis of the repair induction.

Antibiotic resistance patterns of Escherichia coli isolates from different aquatic environmental sources in León, Nicaragua

Antibiotic-resistant bacteria have emerged due to the selective pressure of antimicrobial use in humans and animals. Water plays an important role in dissemination of these organisms among humans, animals and the environment. We studied the antibiotic resistance patterns among 493 Escherichia coli isolates from different aquatic environmental sources collected from October 2008 to May 2009 in León, Nicaragua. High levels of antibiotic resistance were found in E. coli isolates in hospital sewage water and in eight of 87 well-water samples. Among the resistant isolates from the hospital sewage, ampicillin, chloramphenicol, ciprofloxacin, nalidixic acid, trimethoprim-sulphamethoxazole was the most common multi-resistance profile. Among the resistant isolates from the wells, 19% were resistant to ampicillin, ceftazidime, ceftriaxone, cefotaxime, chloramphenicol, ciprofloxacin, gentamicin, nalidixic acid and trimethoprim-sulphamethoxazole. E. coli producing ESBL and harbouring bla(CTX-M) genes were detected in one of the hospital sewage samples and in 26% of the resistant isolates from the well-water samples. The bla(CTX-M-9) group was more prevalent in E. coli isolates from the hospital sewage samples and the bla(CTX-M-1) group was more prevalent in the well-water samples.

Antibiotic resistance patterns of intestinal Escherichia coli isolates from Nicaraguan children

In developing countries, diarrhoeal diseases are one of the major causes of death in children under 5 years of age. It is known that diarrhoeagenic Escherichia coli (DEC) is an important aetiological agent of infantile diarrhoea in Nicaragua. However, there are no recent studies on antimicrobial resistance among intestinal E. coli isolates in Nicaraguan children. The aim of the present study was to determine the antimicrobial resistance pattern in a collection of 727 intestinal E. coli isolates from the faeces of children in León, Nicaragua, between March 2005 and September 2006. All samples had been screened previously for the presence of DEC by multiplex PCR. Three hundred and ninety-five non-DEC isolates (270 from children with diarrhoea and 125 from children without diarrhoea) and 332 DEC isolates (241 from children with diarrhoea and 91 from children without diarrhoea) were analysed in this study. In general, antimicrobial resistance among the 727 intestinal E. coli isolates was high for ampicillin (60 %), trimethoprim–sulfamethoxazole (64 %) and chloramphenicol (11 %). Among individual E. coli categories, enteroaggregative E. coli isolates from children with and without diarrhoea exhibited significantly higher levels of resistance (P<0.05) to ampicillin and trimethoprim–sulfamethoxazole compared to the other E. coli categories. Resistance to ceftazidime and/or ceftriaxone and a pattern of multi-resistance was related to CTX-M-5- or CTX-M-15-producing E. coli isolates. The results suggest that E. coli isolates from Nicaraguan children have not reached the high levels of resistance to the most common antibiotics used for diarrhoea treatment as in other countries.

Antibiotic resistance patterns in gram-negative and gram-positive bacteria causing septicemia in newborns in León, Nicaragua: correlation with environmental samples

The aim of this study was to identify the bacteria causing neonatal septicemia in a neonatal intensive care unit (NICU) in León, Nicaragua and its relation with bacteria isolated from the environment at the NICU. Our data showed that 74% (34/46) of the bacteria related to newborns with septicemia were Gram-negative and highly resistant to beta-lactams (>85%) and aminoglycosides (80%), leading to treatment failure in 10 neonates with fatal outcome. Although, the prevalence of Gram-positive bacteria (26%) was lower than Gram-negative bacteria, Staphylococcus epidermidis was related to the death of three newborns. No clonal similarity was found among Enterobacter cloacae , Escherichia coli and Serratia liquefaciens isolated from the neonates with septicemia and the NICU environment. However, in order to improve the outcome for neonates with septicemia, infection control practices and appropriate empirical therapy should be considered to reduce the high prevalence of extended-spectrum beta-lactamase (ESBL)-producing Gram-negative bacteria isolated from neonates with septicemia (80%) and from the NICU environment (34%).

Transgenic Xenopus embryos reveal that anterior neural development requires continued suppression of BMP signaling after gastrulation

In vertebrates, BMP signaling before gastrulation suppresses neural development. Later in development, BMP signaling specifies a dorsal and ventral fate in the forebrain and dorsal fate in the spinal cord. It is therefore possible that a change in the competence of the ectoderm to respond to BMP signaling occurs at some point in development. We report that exposure of the anterior neural plate to BMP4 before gastrulation causes suppression of all neural markers tested. To determine the effects of BMP4 after gastrulation, we misexpressed BMP4 using a Pax-6 promoter fragment in transgenic frog embryos and implanted beads soaked in BMP4 in the anterior neural plate. Suppression of most anterior neural markers was observed. We conclude that most neural genes continue to require suppression of BMP signaling into the neurula stages. Additionally, we report that BMP4 and BMP7 are abundantly expressed in the prechordal mesoderm of the neurula stage embryo. This poses the paradox of how the expression of most neural genes is maintained if they can be inhibited by BMP signaling. We show that at least one gene in the anterior neural plate suppresses the response of the ectoderm to BMP signaling. We propose that the suppressive effect of BMP signaling on the expression of neural genes coupled with localized suppressors of BMP signaling result in the fine-tuning of gene expression in the anterior neural plate.

Analysis of promoter recognition in vivo directed by sigma(F) of Bacillus subtilis by using random-sequence oligonucleotides

Formation of spores from vegetative bacteria by Bacillus subtilis is a primitive system of cell differentiation. Critical to spore formation is the action of a series of sporulation-specific RNA polymerase sigma factors. Of these, sigma(F) is the first to become active. Few genes have been identified that are transcribed by RNA polymerase containing sigma(F) (E-sigma(F)), and only two genes of known function are exclusively under the control of E-sigma(F), spoIIR and spoIIQ. In order to investigate the features of promoters that are recognized by E-sigma(F), we studied the effects of randomizing sequences for the -10 and -35 regions of the promoter for spoIIQ. The randomized promoter regions were cloned in front of a promoterless copy of lacZ in a vector designed for insertion by double crossover of single copies of the promoter-lacZ fusions into the amyE region of the B. subtilis chromosome. This system made it possible to test for transcription of lacZ by E-sigma(F) in vivo. The results indicate a weak sigma(F)-specific -10 consensus, GG/tNNANNNT, of which the ANNNT portion is common to all sporulation-associated sigma factors, as well as to sigma(A). There was a rather stronger -35 consensus, GTATA/T, of which GNATA is also recognized by other sporulation-associated sigma factors. The looseness of the sigma(F) promoter requirement contrasts with the strict requirement for sigma(A)-directed promoters of B. subtilis. It suggests that additional, unknown, parameters may help determine the specificity of promoter recognition by E-sigma(F) in vivo.

Comparison of morpholino based translational inhibition during the development of Xenopus laevis and Xenopus tropicalis

Morpholino (MO) based inhibition of translational initiation represents an attractive methodology to eliminate gene function during Xenopus development (Heasman et al., 2000). However, the degree to which a given target protein can be eliminated and the longevity of this effect during embryogenesis has not been documented. To examine the efficacy of MOs, we have used transgenic Xenopus lines that harbour known numbers of integrations of a GFP reporter under the control of the ubiquitous and highly expressed CMV promoter (Fig. 1a). In addition we have investigated the longevity of the inhibitory effect by using transgenic lines expressing GFP specifically in the lens of tadpoles. These transgenic lines represent the ideal control for the technique as the promoters are highly expressed and GFP can be easily detected by fluorescence and immunoblotting. Moreover, as GFP has no function in development, the levels of inhibition can be tested in an otherwise normal individual. Here we report that MOs are able to efficiently and specifically inhibit the translation of GFP in transgenic lines from Xenopus laevis and Xenopus tropicalis and the inhibitory effect is long-lived, lasting into the tadpole stages. genesis 30:110--113, 2001.

Xenopus Sprouty2 inhibits FGF-mediated gastrulation movements but does not affect mesoderm induction and patterning

Signal transduction through the FGF receptor is essential for the specification of the vertebrate body plan. Blocking the FGF pathway in early Xenopus embryos inhibits mesoderm induction and results in truncation of the anterior-posterior axis. The Drosophila gene sprouty encodes an antagonist of FGF signaling, which is transcriptionally induced by the pathway, but whose molecular functions are poorly characterized. We have cloned Xenopus sprouty2 and show that it is expressed in a similar pattern to known FGFs and is dependent on the FGF/Ras/MAPK pathway for its expression. Overexpression of Xsprouty2 in both embryos and explant assays results in the inhibition of the cell movements of convergent extension. Although blocking FGF/Ras/MAPK signaling leads to an inhibition of mesodermal gene expression, these markers are unaffected by Xsprouty2, indicating that mesoderm induction and patterning occurs normally in these embryos. Finally, using Xenopus oocytes we show that Xsprouty2 is an intracellular antagonist of FGF-dependent calcium signaling. These results provide evidence for at least two distinct FGF-dependent signal transduction pathways: a Sprouty-insensitive Ras/MAPK pathway required for the transcription of most mesodermal genes, and a Sprouty-sensitive pathway required for coordination of cellular morphogenesis.

A role for BMP signalling in heart looping morphogenesis in Xenopus

The heart develops from a linear tubular precursor, which loops to the right and undergoes terminal differentiation to form the multichambered heart. Heart looping is the earliest manifestation of left-right asymmetry and determines the eventual heart situs. The signalling processes that impart laterality to the unlooped heart tube and thus allow the developing organ to interpret the left-right axis of the embryo are poorly understood. Recent experiments in zebrafish led to the suggestion that bone morphogenetic protein 4 (BMP4) may impart laterality to the developing heart tube. Here we show that in Xenopus, as in zebrafish, BMP4 is expressed predominantly on the left of the linear heart tube. Furthermore we demonstrate that ectopic expression of Xenopus nodal-related protein 1 (Xnr1) RNA affects BMP4 expression in the heart, linking asymmetric BMP4 expression to the left-right axis. We show that transgenic embryos overexpressing BMP4 bilaterally in the heart tube tend towards a randomisation of heart situs in an otherwise intact left-right axis. Additionally, inhibition of BMP signalling by expressing noggin or a truncated, dominant negative BMP receptor prevents heart looping but allows the initial events of chamber specification and anteroposterior morphogenesis to occur. Thus in Xenopus asymmetric BMP4 expression links heart development to the left-right axis, by being both controlled by Xnr1 expression and necessary for heart looping morphogenesis.

Analysis of vertebrate SCL loci identifies conserved enhancers

The SCL gene encodes a highly conserved bHLH transcription factor with a pivotal role in hemopoiesis and vasculogenesis. We have sequenced and analyzed 320 kb of genomic DNA composing the SCL loci from human, mouse, and chicken. Long-range sequence comparisons demonstrated multiple peaks of human/mouse homology, a subset of which corresponded precisely with known SCL enhancers. Comparisons between mammalian and chicken sequences identified some, but not all, SCL enhancers. Moreover, one peak of human/mouse homology (+23 region), which did not correspond to a known enhancer, showed significant homology to an analogous region of the chicken SCL locus. A transgenic Xenopus reporter assay was established and demonstrated that the +23 region contained a new neural enhancer. This combination of long-range comparative sequence analysis with a high-throughput transgenic bioassay provides a powerful strategy for identifying and characterizing developmentally important enhancers.

A gene trap approach in Xenopus

The frog transgenesis technique ultimately promises to make mutagenesis possible through random insertion of plasmid DNA into the genome. This study was undertaken to evaluate whether a gene trap approach combined with transgenesis would be appropriate for performing insertional mutagenesis in Xenopus embryos. Firstly, we confirmed that the transgenic technique results in stable integration into the genome and that transmission through the germline occurs in the expected Mendelian fashion. Secondly, we developed several gene trap vectors, using the green fluorescent protein (GFP) as a marker. Using these vectors, we trapped several genes in Xenopus laevis that are expressed in a spatially restricted manner, including expression in the epiphysis, the olfactory bulb and placodes, the eyes, ear, brain, muscles, tail and intestine. Finally, we cloned one of the trapped genes using 5' rapid amplification of cDNA ends polymerase chain reaction (RACE PCR). These results suggest that the transgenic technique combined with a gene trap approach might provide a powerful method for generating mutations in endogenous genes in Xenopus.

Transgenic Xenopus embryos from sperm nuclear transplantations reveal FGF signaling requirements during gastrulation

We have developed a simple approach for large-scale transgenesis in Xenopus laevis embryos and have used this method to identify in vivo requirements for FGF signaling during gastrulation. Plasmids are introduced into decondensed sperm nuclei in vitro using restriction enzyme-mediated integration (REMI). Transplantation of these nuclei into unfertilized eggs yields hundreds of normal, diploid embryos per day which develop to advanced stages and express integrated plasmids nonmosaically. Transgenic expression of a dominant negative mutant of the FGF receptor (XFD) after the mid-blastula stage uncouples mesoderm induction, which is normal, from maintenance of mesodermal markers, which is lost during gastrulation. By contrast, embryos expressing XFD contain well-patterned nervous systems despite a putative role for FGF in neural induction.

Inhibition of FGF receptor activity in retinal ganglion cell axons causes errors in target recognition

Native fibroblast growth factor receptor (FGFR) function was inhibited in developing Xenopus retinal ganglion cells (RGCs) by in vivo transfection of a dominant negative FGFR. Axons expressing the dominant negative protein advanced at 60% of the normal speed, but nevertheless navigated appropriately in the embryonic optic pathway. When they neared the optic tectum, however, many axons made erroneous turns, causing them to bypass rather than enter their target. By contrast, RGC axons expressing nonfunctional FGFR mutants entered the tectum correctly. These findings demonstrate a role for FGFR signaling in the extension and targeting of RGC axons and suggest that receptor tyrosine kinase/growth factor interactions play a critical function in establishing initial connectivity in the vertebrate visual system.

Temporal regulation of the Xenopus FGF receptor in development: a translation inhibitory element in the 3′ untranslated region

Early frog embryogenesis depends on a maternal pool of mRNA to execute critical intercellular signalling events. FGF receptor-1, which is required for normal development, is stored as a stable, untranslated maternal mRNA transcript in the fully grown immature oocyte, but is translationally activated at meiotic maturation. We have identified a short cis-acting element in the FGF receptor 3′ untranslated region that inhibits translation of synthetic mRNA. This inhibitory element is sufficient to inhibit translation of heterologous reporter mRNA in the immature oocyte without changing RNA stability. Deletion of the poly(A) tract or polyadenylation signal sequences does not affect translational inhibition by this element. At meiotic maturation, we observe the reversal of translational repression mediated by the inhibitory element, mimicking that seen with endogenous maternal FGF receptor mRNA at meiosis. In addition, the activation of synthetic transcripts at maturation does not appear to require poly(A) lengthening. We also show that an oocyte cytoplasmic protein specifically binds the 3′ inhibitory element, suggesting that translational repression of Xenopus FGF receptor-1 maternal mRNA in the oocytes is mediated by RNA-protein interactions. These data describe a mechanism of translational control that appears to be independent of poly(A) changes.

FGF signalling in the early specification of mesoderm in Xenopus

We have examined the role of FGF signalling in the development of muscle and notochord and in the expression of early mesodermal markers in Xenopus embryos. Disruption of the FGF signalling pathway by expression of a dominant negative construct of the FGF receptor (XFD) generally results in gastrulation defects that are later evident in the formation of the trunk and tail, though head structures are formed nearly normally. These defects are reflected in the loss of notochord and muscle. Even in embryos that show mild defects and gastrulate properly, muscle formation is impaired, suggesting that morphogenesis and tissue differentiation each depend on FGF. The XFD protein inhibits the expression of the immediate early gene brachyury throughout the marginal zone, including the dorsal side; it does not, however, inhibit the dorsal lip marker goosecoid, which is expressed in the first involuting mesoderm at the dorsal side that will underlie the head. The XFD protein also inhibits Xpo expression, an immediate early marker of ventral and lateral mesoderm. These results suggest that FGF is involved in the earliest events of most mesoderm induction that occur before gastrulation and that the early dorsal mesoderm is already composed of two cell populations that differ in their requirements for FGF.

Fibroblast growth factor receptors contain a conserved HAV region common to cadherins and influenza strain A hemagglutinins: a role in protein-protein interactions?

The first extracellular domain of the cadherins has been shown to exhibit extensive sequence homology with the amino termini of the HA1 chains of influenza strain A hemagglutinins. These regions of homology are known to be functionally important in both the cadherins and the hemagglutinins. The homologous regions harbor the tripeptide HAV, which has been identified as being the cadherin cell adhesion recognition sequence. Here we report that members of the rapidly expanding family of fibroblast growth factor receptors also possess HAV-containing regions. These regions are homologous to the HAV-containing regions present within both the hemagglutinins and the cadherins and appear to be involved in regulating the function of the fibroblast growth factor receptors. We speculate that the HAV motif may represent an evolutionarily conserved amino acid sequence that will prove to be functionally important in a wide variety of proteins.

Expression of a dominant negative mutant of the FGF receptor disrupts mesoderm formation in Xenopus embryos

Peptide growth factors may play a role in patterning of the early embryo, particularly in the induction of mesoderm. We have explored the role of fibroblast growth factor (FGF) in early Xenopus development by expressing a dominant negative mutant form of the FGF receptor. Using a functional assay in frog oocytes, we found that a truncated form of the receptor effectively abolished wild-type receptor function. Explants from embryos expressing this dominant negative mutant failed to induce mesoderm in response to FGF. In whole embryos the mutant receptor caused specific defects in gastrulation and in posterior development, and overexpression of a wild-type receptor could rescue these developmental defects. These results demonstrate that the FGF signaling pathway plays an important role in early embryogenesis, particularly in the formation of the posterior and lateral mesoderm.

Regulation of the fibroblast growth factor receptor in early Xenopus embryos

Recent evidence suggests that fibroblast growth factor (FGF) is a primary mesoderm inducer in Xenopus development. We have isolated a full-length cDNA clone for the Xenopus FGF receptor. Like other FGF receptors, the Xenopus homolog is a membrane-spanning protein with a split intracellular tyrosine kinase domain. The Xenopus FGF receptor mRNA is present as a maternal message whose levels are constant through early development. There is no specific regional localization of the transcript by analysis of FGF receptor mRNA levels in microdissected embryonic tissue. In isolated animal-pole blastomeres, FGF receptor mRNA declines over 16 hr in culture and this loss can be prevented by incubation with FGF or activin. Despite the presence of the FGF receptor mRNA in the oocyte, oocytes in culture do not respond to added FGF. However, injection of exogenous Xenopus FGF receptor transcripts into oocytes does generate a functional response to FGF. Our data suggest that posttranscriptional response to FGF. Our data suggest that posttranscriptional mechanisms regulate the FGF receptor in the oocyte and early embryo and further suggest that mesoderm-inducing factors influence receptor mRNA levels during the time of early tissue formation.

Selective excision of the centromere chromatin complex from Saccharomyces cerevisiae

We have taken advantage of the known structural parameters associated with centromere DNA in vivo to construct a CEN fragment that can be selectively excised from the chromatin DNA with restriction endonucleases. CEN3 DNA is organized in chromatin such that a 220-250-bp region encompassing the elements of centromere homology is resistant to nuclease digestion. Restriction enzyme linkers encoding the Bam HI-recognition site were ligated to a 289 base pair DNA segment that spans the 220-250-bp protected core (Bloom et al., 1984). Replacement of this CEN3-Bam HI linker cassette into a chromosome or plasmid results in formation of a complete structural and functional centromeric unit. A centromere core complex that retains its protected chromatin conformation can be selectively excised from intact nuclei by restriction with the enzyme Bam HI. The centromeric protein-DNA complex is therefore not dependent upon the intact torsional constrains on linear chromosomes for its structural integrity. Isolation of this complex provides a novel approach to characterizing authentic centromeric proteins bound to DNA in their native state.

UV-induced damage and repair in centromere DNA of yeast

The centromere is the region within a chromosome that is required for proper segregation during mitosis and meiosis. Lesions in this sequence represent a unique type of damage, as loss of function could result in catastrophic loss of the genetic material of an entire chromosome. We have measured the induction by ultraviolet (UV) light of pyrimidine dimers in a 2550-bp restriction fragment that includes the centromere region of chromosome III in Saccharomyces cerevisiae. Yeast cells were exposed to ultraviolet light, cellular DNA was gently extracted, and subsequently treated with a UV-specific endonuclease to cleave all pyrimidine dimers. The sites of UV-specific nuclease scission within the centromere were determined by separating the DNA according to molecular weight, transferring the fragments to nitrocellulose, and hybridizing to a radiolabeled 624-bp fragment homologous to the centromere DNA from chromosome III. Several hotspots were identified in chromatin DNA from cells, as well as in irradiated deproteinized DNA. Double strand damage due to closely opposed pyrimidine dimers was also observed. At biological doses (35% survival) there are approximately 0.1 to 0.2 pyrimidine dimers per centromere. These dimers are efficiently repaired in the centromere and surrounding region.

Chromatin conformation of yeast centromeres

The centromere region of Saccharomyces cerevisiae chromosome III has been replaced by various DNA fragments from the centromere regions of yeast chromosomes III and XI. A 289-base pair centromere (CEN3) sequence can stabilize yeast chromosome III through mitosis and meiosis. The orientation of the centromeric fragments within chromosome III has no effect on the normal mitotic or meiotic behavior of the chromosome. The structural integrity of the centromere region in these genomic substitution strains was examined by mapping nucleolytic cleavage sites within the chromatin DNA. A nuclease-protected centromere core of 220-250 base pairs was evident in all of the genomic substitution strains. The position of the protected region is determined strictly by the centromere DNA sequence. These results indicate that the functional centromere core is contained within 220-250 base pairs of the chromatin DNA that is structurally distinct from the flanking nucleosomal chromatin.