Titan is unique in the solar system as it hosts a dense atmosphere mainly made of molecular nitrogen N2 and methane CH4. The Cassini-Huygens Mission revealed the presence of an intense atmospheric ...photochemistry initiated by the photo-dissociation and ionization of N2 and CH4. In the upper atmosphere, Cassini detected signatures compatible with the presence of heavily charged molecules which are precursors for the solid core of the aerosols. These observations have indicated that ion chemistry has an important role for organic growth. However, the processes coupling ion chemistry and aerosol formation and growth are still mostly unknown. In this study, we investigated the cation chemistry responsible for an efficient organic growth that we observe in Titan's upper atmosphere, simulated using the PAMPRE plasma reactor. Positive ion precursors were measured by in situ ion mass spectrometry in a cold plasma and compared with INMS observations taken during the T40 flyby. A series of positive ion measurements were performed in three CH4 mixing ratios (1%, 5% and 10%) showing a variability in ion population. Low methane concentrations result in an abundance of amine cations such as NH4+ whereas aliphatic compounds dominate at higher methane concentrations. In conditions of favored tholin production, the presence of C2 compounds such as HCNH+ and C2H5+ is found to be consistent with copolymeric growth structures seen in tholin material. The observed abundance of these two ions particularly in conditions with lower CH4 amounts is consistent with modeling work simulating aerosol growth in Titan's ionosphere, which includes mass exchange primarily between HCNH+ and C2H5+ and negatively charged particles. These results also confirm the prevalent role of C2 cations as precursors to molecular growth and subsequent mass transfer to the charged aerosol particles as the CH4 abundance decreases towards lower altitudes.
•Cation chemistry is investigated in an N2-CH4 plasma discharge.•Laboratory spectra are compared with INMS observations.•C2 cations are important precursors in efficient gas-to-solid conversion conditions.•Results are consistent with HCNH+ and C2H5+ mass exchange for efficient tholin growth.•The presence of these C2 compounds such as HCNH+ and C2H5+ is consistent with tholin copolymeric growth patterns.
•In situ cryogenic trap in a plasma discharge enables quantification of gas-phase precursors in Titan-like conditions•C2H4 is a major hydrocarbon precursor to tholin formation•HCN and NH3 are also ...important volatile products•NH3 is the most abundant product formed with a 1% CH4 mixing ratio
The Cassini Mission has greatly improved our understanding of the dynamics and chemical processes occurring in Titan's atmosphere. It has also provided us with more insight into the formation of the aerosols in the upper atmospheric layers.
However, the chemical composition and mechanisms leading to their formation were out of reach for the instruments onboard Cassini. In this context, it is deemed necessary to apply and exploit laboratory simulations to better understand the chemical reactivity occurring in the gas phase of Titan-like conditions. In the present work, we report gas phase results obtained from a plasma discharge simulating the chemical processes in Titan's ionosphere. We use the PAMPRE cold dusty plasma experiment with an N2CH4 gaseous mixture under controlled pressure and gas influx. An internal cryogenic trap has been developed to accumulate the gas products during their production and facilitate their detection. The cryogenic trap condenses the gas-phase precursors while they are forming, so that aerosols are no longer observed during the 2 h plasma discharge. We focus mainly on neutral products NH3, HCN, C2H2 and C2H4. The latter are identified and quantified by in situ mass spectrometry and infrared spectroscopy. We present here results from this experiment with mixing ratios of 90–10% and 99–1% N2CH4, covering the range of methane concentrations encountered in Titan's ionosphere. We also detect in situ heavy molecules (C7). In particular, we show the role of ethylene and other volatiles as key solid-phase precursors.
Keller's systemic model of youth mentoring posits there are multiple pathways through which all stakeholders in the youth mentoring process, including the program staff who support the match (or case ...managers), influence youth outcomes. This study examines case managers' direct and indirect contributions to match outcomes and tests how transitive interactions facilitate a theorized sequence of mentoring interactions to effect greater closeness and length, specifically in nontargeted mentoring programs. A structural equations model of case manager contributions to match outcomes was tested using data from 758 mentor-mentee matches, supported by 73 case managers across seven mentoring agencies. Results reveal direct effects of mentor-reported match support quality on match length and indirect influences on match length through increasing youth-centeredness, goal-focused orientation, and closeness. The findings confirm the presence of multiple pathways of influence, including indirect effects on outcomes via transitive interactions in match support that scaffold youth-centeredness and goal-focused interactions in the match. Findings also suggest supervisors' evaluations of case managers may provide little information about how match support influences the nature of mentor-mentee interactions.
Members of the Apicomplexa phylum possess specialized secretory organelles that discharge, apically and in a timely regulated manner, key factors implicated in parasite motility, host cell invasion, ...egress and subversion of host cellular functions. The mechanisms regulating trafficking and apical docking of these secretory organelles are only partially elucidated. Here, we characterized two conserved endosomal trafficking regulators known to promote vesicle transport and/or fusion, HOOK and Fused Toes (FTS), in the context of organelle discharge in Toxoplasma gondii. TgHOOK and TgFTS form a complex with a coccidian-specific partner, named HOOK interacting partner (HIP). TgHOOK displays an apically enriched vesicular pattern and concentrates at the parasite apical tip where it colocalizes with TgFTS and TgHIP. Functional investigations revealed that TgHOOK is dispensable but fitness conferring. The protein regulates the apical positioning and secretion of micronemes and contributes to egress, motility, host cell attachment, and invasion. Conditional depletion of TgFTS or TgHIP impacted on the same processes but led to more severe phenotypes. This study provides evidence of endosomal trafficking regulators involved in the apical exocytosis of micronemes and possibly as a consequence or directly on the discharge of the rhoptries.
Toxoplasma gondii affects between 30 and 80% of the human population, poses a life-threatening risk to immunocompromised individuals, and is a cause of abortion and birth defects following congenital transmission. T. gondii belongs to the phylum of Apicomplexa characterized by a set of unique apical secretory organelles called the micronemes and rhoptries. Upon host cell recognition, this obligatory intracellular parasite secretes specific effectors contained in micronemes and rhoptries to promote parasite invasion of host cells and subsequent persistence. Here, we identified novel T. gondii endosomal trafficking regulators and demonstrated that they regulate microneme organelle apical positioning and exocytosis, thereby strongly contributing to host cell invasion and parasite virulence.
Plasmodium falciparum and Toxoplasma gondii are obligate intracellular parasites that belong to the phylum of Apicomplexa and cause major human diseases. Their access to an intracellular lifestyle is ...reliant on the coordinated release of proteins from the specialized apical organelles called micronemes and rhoptries. A specific phosphatidic acid effector, the acylated pleckstrin homology domain-containing protein (APH) plays a central role in microneme exocytosis and thus is essential for motility, cell entry, and egress. TgAPH is acylated on the surface of the micronemes and recruited to phosphatidic acid (PA)-enriched membranes. Here, we dissect the atomic details of APH PA-sensing hub and its functional interaction with phospholipid membranes. We unravel the key determinant of PA recognition for the first time and show that APH inserts into and clusters multiple phosphate head-groups at the bilayer binding surface.
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•Solution structures of APH from Toxoplasma gondii and Plasmodium falciparum•APH represents a new class of PH domain•APH phosphatidic acid binding site encompasses canonical and atypical sites•APH inserts into the bilayer and clusters multiple phosphate head-groups
High-resolution structures of a phosphatidic acid effector protein (APH) from both Plasmodium falciparum and Toxoplasma gondii parasites and its interaction with the lipid mediator unravel a new class of PH domain that initiates microneme-plasma membrane fusion by inserting and clustering phosphatidic acid within membranes.
We describe a rare case of Soemmering’s ring-induced uveitis-glaucoma-hyphema (UGH) syndrome, caused by an undisplaced, in-the-bag intraocular lens (IOL) 16 years after implantation. The presenting ...symptoms were recurrent episodes of transient monocular vision loss, which prompted extensive unremarkable investigations. Ultrasound biomicroscopy (UBM) eventually revealed an enlarging Soemmering’s ring contacting the posterior iris and causing UGH syndrome. This is a unique case because the IOL haptics remained in the bag. Anterior vitrectomy, Soemmering’s ring extraction, IOL exchange to a 3-piece IOL, and implantation of a trabecular meshwork bypass microstent were performed. IOP control and both functional and structural stability were achieved long term. Soemmering’s ring-induced UGH syndrome should be on the differential of a patient with previous cataract surgery and elevated IOP complaining of visual disturbances. Early diagnosis with UBM and surgical intervention may provide optimal outcomes.
This thoroughly updated Second Edition of the Handbook of Youth Mentoring presents the only comprehensive synthesis of current theory, research, and practice in the field of youth mentoring. Editors ...David L. DuBois and Michael J. Karcher gather leading experts in the field to offer critical and informative analyses of the full spectrum of topics that are essential to advancing our understanding of the principles for effective mentoring of young people. This volume includes twenty new chapter topics and eighteen completely revised chapters based on the latest research on these topics. Each chapter has been reviewed by leading practitioners, making this handbook the strongest bridge between research and practice available in the field of youth mentoring.
We used meta‐analysis to review 55 evaluations of the effects of mentoring programs on youth. Overall, findings provide evidence of only a modest or small benefit of program participation for the ...average youth. Program effects are enhanced significantly, however, when greater numbers of both theory‐based and empirically based “best practices” are utilized and when strong relationships are formed between mentors and youth. Youth from backgrounds of environmental risk and disadvantage appear most likely to benefit from participation in mentoring programs. Outcomes for youth at‐risk due to personal vulnerabilities have varied substantially in relation to program characteristics, with a noteworthy potential evident for poorly implemented programs to actually have an adverse effect on such youth. Recommendations include greater adherence to guidelines for the design and implementation of effective mentoring programs as well as more in‐depth assessment of relationship and contextual factors in the evaluation of programs.
Objectives: The aim of this repeated-measures study was to examine the roles of cultural mistrust and perceived mentor support for ethnic-racial identity in a sample of girls of color. It was ...hypothesized that mentors' support for ethnic-racial identity measured at baseline would influence relationship quality, as well as the girls' ethnic identity and cultural mistrust, at the end of the intervention, adjusting for baseline measures. It was also hypothesized that girls' cultural mistrust toward Whites at baseline would be negatively associated with mentoring relationship quality at the end of the intervention. Method: Participants were 40 adolescent girls of color who were matched with racially-ethnically diverse women mentors in a community-based mentoring program. Results: Mentor support for ethnic-racial identity as reported by youth significantly predicted relative increases in youth reports of relational but not instrumental satisfaction. Higher mentor support for ethnic-racial identity also significantly predicted increases in ethnic identity exploration, but only among girls with White mentors. Further, youth's reported greater cultural mistrust toward Whites was a significant predictor of decreased instrumental relationship satisfaction among girls with White mentors. Conclusions: Findings support the importance of further efforts to understand the roles of culturally relevant relationship processes and youth attitudes in mentoring interventions for girls of color.
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