The dynamics of coupled populations subject to control

The dynamics of coupled populations have mostly been studied in the context of metapopulation viability with application to, for example, species at risk. However, when considering pests and pathogens, eradication, not persistence, is often the end goal. Humans may intervene to control nuisance populations, resulting in reciprocal interactions between the human and natural systems that can lead to unexpected dynamics. The incidence of these human-natural couplings has been increasing, hastening the need to better understand the emergent properties of such systems in order to predict and manage outbreaks of pests and pathogens. For example, the success of the growing aquaculture industry depends on our ability to manage pathogens and maintain a healthy environment for farmed and wild fish. We developed a model for the dynamics of connected populations subject to control, motivated by sea louse parasites that can disperse among salmon farms. The model includes exponential population growth with a forced decline when populations reach a threshold, representing control interventions. Coupling two populations with equal growth rates resulted in phase locking or synchrony in their dynamics. Populations with different growth rates had different periods of oscillation, leading to quasiperiodic dynamics when coupled. Adding small amounts of stochasticity destabilized quasiperiodic cycles to chaos, while stochasticity was damped for periodic or stable dynamics. Our analysis suggests that strict treatment thresholds, although well intended, can complicate parasite dynamics and hinder control efforts. Synchronizing populations via coordinated management among farms leads to more effective control that is required less frequently. Our model is simple and generally applicable to other systems where dispersal affects the management of pests and pathogens.     

Lysosomal Targeting of the CLN7 Membrane Glycoprotein and Transport Via the Plasma Membrane Require a Dileucine Motif

CLN7 is a polytopic lysosomal membrane protein deficient in variant late infantile neuronal ceroid lipofuscinosis, a neurodegenerative lysosomal storage disorder. In this study fluorescence protease protection assays and mutational analyses revealed the N‐ and C‐terminal tails of CLN7 in the cytosol and two N‐glycosylation sites at N371 and N376. Both partially and non‐glycosylated CLN7 were correctly transported to lysosomes. To identify lysosomal targeting motifs, we generated CD4‐chimera fused to the N‐ and C‐terminal domains of CLN7. Lysosomal localization of the chimeric proteins requires a consensus acidic dileucine‐based motif in the N‐terminus and two tandem tyrosine‐based signals in the C‐terminus. Mutation of these sorting motifs resulted in cell surface redistribution of CD4 chimeras. However, the dileucine‐based motif is of critical importance for lysosomal localization of the full‐length CLN7 in different cell lines. Cell surface biotinylation revealed that at equilibrium 22% of total CLN7 is localized at the plasma membrane. Mutation of the dileucine motif or the co‐expression of dominant‐negative mutant dynamin K44A led to a further increase of CLN7 at the plasma membrane. Our data demonstrate that CLN7 contains several cytoplasmic lysosomal targeting signals of which the N‐terminal dileucine‐based motif is required for the predominant lysosomal targeting along the indirect pathway and clathrin‐mediated endocytosis of CLN7.     

The Novel Membrane Protein TMEM59 Modulates Complex Glycosylation,Cell Surface Expression,and Secretion of the Amyloid Precursor Protein

Ectodomain shedding of the amyloid precursor protein (APP) by the two proteases α- and β-secretase is a key regulatory event in the generation of the Alzheimer disease amyloid β peptide (Aβ). At present, little is known about the cellular mechanisms that control APP shedding and Aβ generation. Here, we identified a novel protein, transmembrane protein 59 (TMEM59), as a new modulator of APP shedding. TMEM59 was found to be a ubiquitously expressed, Golgi-localized protein. TMEM59 transfection inhibited complex N- and O-glycosylation of APP in cultured cells. Additionally, TMEM59 induced APP retention in the Golgi and inhibited Aβ generation as well as APP cleavage by α- and β-secretase cleavage, which occur at the plasma membrane and in the endosomes, respectively. Moreover, TMEM59 inhibited the complex N-glycosylation of the prion protein, suggesting a more general modulation of Golgi glycosylation reactions. Importantly, TMEM59 did not affect the secretion of soluble proteins or the α-secretase like shedding of tumor necrosis factor α, demonstrating that TMEM59 did not disturb the general Golgi function. The phenotype of TMEM59 transfection on APP glycosylation and shedding was similar to the one observed in cells lacking conserved oligomeric Golgi (COG) proteins COG1 and COG2. Both proteins are required for normal localization and activity of Golgi glycosylation enzymes. In summary, this study shows that TMEM59 expression modulates complex N- and O-glycosylation and suggests that TMEM59 affects APP shedding by reducing access of APP to the cellular compartments, where it is normally cleaved by α- and β-secretase.     

Augmented Insulinotropic Action of Arachidonic Acid through the Lipoxygenase Pathway in the Obese Zucker Rat

Objective: The metabolism of arachidonic acid (AA) has been shown to be altered in severe insulin resistance that is present in obese (fa/fa) Zucker rats. We examined the effects and mechanism of action of AA on basal and glucose‐stimulated insulin secretion in pancreatic islets isolated from obese (fa/fa) Zucker rats and their homozygous lean (Fa/Fa) littermates. Research Methods and Procedures: Islets were isolated from 10‐ to 12‐week‐old rats and incubated for 45 minutes in glucose concentrations ranging from 3.3 to 16.7 mM with or without inhibitors of the cyclooxygenase or lipoxygenase pathways. Medium insulin concentrations were measured by radioimmunoassay, and islet production of the 12‐lipoxygenase metabolite, 12‐hydroxyeicosatetraenoic acid (12‐HETE), was measured by enzyme immunoassay. Results: In islets from lean animals, AA stimulated insulin secretion at submaximally stimulatory glucose levels (< 11.1 mM) but not at 16.7 mM glucose. In contrast, in islets derived from obese rats, AA potentiated insulin secretion at all glucose concentrations. AA‐induced insulin secretion was augmented in islets from obese compared with lean rats at high concentrations of AA in the presence of 3.3 mM glucose. Furthermore, the inhibitor of 12‐lipoxygenase, esculetin (0.5 μM), inhibited AA‐stimulated insulin secretion in islets from obese but not lean rats. Finally, the islet production of the 12‐HETE was markedly enhanced in islets from obese rats, both in response to 16.7 mM glucose and to AA. Discussion: The insulin secretory response to AA is augmented in islets from obese Zucker rats by a mechanism related to enhanced activity of the 12‐lipoxygenase pathway. Therefore, augmented action of AA may be a mechanism underlying the adaptation of insulin secretion to the increased demand caused by insulin resistance in these animals.     

Protein kinase A-mediated phosphorylation of connexin36 in mouse retina results in decreased gap junctional communication between AII amacrine cells

Gap junctions in AII amacrine cells of mammalian retina participate in the coordination of the rod and cone signaling pathway involved in visual adaptation. Upon stimulation by light, released dopamine binds to D(1) receptors on AII amacrine cells leading to increased intracellular cAMP (cyclic adenosine monophosphate) levels. AII amacrine cells express the gap junctional protein connexin36 (Cx36). Phosphorylation of Cx36 has been hypothesized to regulate gap junctional activity of AII amacrine cells. However, until now in vivo phosphorylation of Cx36 has not been reported. Indeed, it had been concluded that Cx36 in bovine retina is not phosphorylated, but in vitro phosphorylation for Cx35, the bass ortholog of Cx36, had been shown. To clarify this experimental discrepancy, we examined protein kinase A (PKA)-induced phosphorylation of Cx36 in mouse retina as a possible mechanism to modulate the extent of gap junctional coupling. The cytoplasmic domains of Cx36 and the total Cx36 protein were phosphorylated in vitro by PKA. Mass spectroscopy revealed that all four possible PKA consensus motifs were phosphorylated; however, domains point mutated at the sites in question showed a prevalent usage of Ser-110 and Ser-293. Additionally, we demonstrated that Cx36 was phosphorylated in cultured mouse retina. Furthermore, activation of PKA increased the level of phosphorylation of Cx36. cAMP-stimulated, PKA-mediated phosphorylation of Cx36 protein was accompanied by a decrease of tracer coupling between AII amacrine cells. Our results link increased phosphorylation of Cx36 to down-regulation of permeability through gap junction channels mediating light adaptation in the retina.     

N‐terminal residues of the yeast pheromone receptor,Ste2p,mediate mating events independently of G1‐arrest signaling

In Saccharomyces cerevisiae, mechanisms modulating the mating steps following cell cycle arrest are not well characterized. However, the N‐terminal domain of Ste2p, a G protein‐coupled pheromone receptor, was recently proposed to mediate events at this level. Toward deciphering receptor mechanisms associated with this mating functionality, scanning mutagenesis of targeted regions of the N‐terminal domain has been completed. Characterization of ste2 yeast overexpressing Ste2p variants indicated that residues Ile 24 and Ile 29 as well as Pro 15 are critical in mediating mating efficiency. This activity was shown to be independent of Ste2p mediated G1 arrest signaling. Further analysis of Ile 24 and Ile 29 highlight the residues' solvent accessibility, as well as the importance of the hydrophobic nature of the sites, and in the case of Ile 24 the specific size and shape of the side chain. Mutation of these Ile's led to arrest of mating after cell contact, but before completion of cell wall degradation. We speculate that these extracellular residues mediate novel receptor interactions with ligand or proteins, leading to stimulation of alternate signaling effector pathways. J. Cell. Biochem. 107: 630–638, 2009. © 2009 Crown in the right of Canada.     

Developing the sterile insect technique for Cryptophlebia leucotreta (Lepidoptera: Tortricidae): influence of radiation dose and release ratio on fruit damage and population growth in field cages

The effect of radiation dose and different release ratios of treated (T) to untreated (U) Cryptophlebia leucotreta (Meyrick) (Lepidoptera: Tortricidae), on the incidence of fruit damage, the competitiveness of the treated males, and population growth was examined inside field cages. Navel orange trees were individually enclosed in large nylon mesh cages. Newly emerged adult moths treated with either 150 or 200 Gy of gamma radiation were released into the cages at ratios of 5T:1U or 10T:1U. The fruit was collected after 4 wk, and the number of damaged fruit and larval entries per cage were recorded for each treatment. Infested fruit was maintained in the laboratory until all emerging F1 progeny were collected and outcrossed to untreated moths of the opposite sex. Treatment had a significant effect on the mean number of larval entries and on the number of undamaged fruit per cage. The number of larval entries as well as the number of F1 progeny per cage decreased as the overflooding ratio increased. A significant reduction in egg hatch was observed in the progeny of crosses between F1 females or F1 males originating from the treatment cages compared with crosses of F1 moths originating from the control cages. The lowest mean number of fertile F1 adult females and males was obtained from the 150 Gy and 10T:1U ratio treatment. This treatment also showed the lowest per generation rate of increase (< 1 from the parental [P1] to the F1 generation), suggesting that growth in the fertile population would have been prevented if releases of treated moths at this dose and ratio were maintained in the field.     

Cost-effectiveness of pooled nucleic acid amplification testing for acute HIV infection after third-generation HIV antibody screening and rapid testing in the United States: a comparison of three public health settings

Background

Detection of acute HIV infection (AHI) with pooled nucleic acid amplification testing (NAAT) following HIV testing is feasible. However, cost-effectiveness analyses to guide policy around AHI screening are lacking; particularly after more sensitive third-generation antibody screening and rapid testing.

Methods and Findings

We conducted a cost-effectiveness analysis of pooled NAAT screening that assessed the prevention benefits of identification and notification of persons with AHI and cases averted compared with repeat antibody testing at different intervals. Effectiveness data were derived from a Centers for Disease Control and Prevention AHI study conducted in three settings: municipal sexually transmitted disease (STD) clinics, a community clinic serving a population of men who have sex with men, and HIV counseling and testing sites. Our analysis included a micro-costing study of NAAT and a mathematical model of HIV transmission. Cost-effectiveness ratios are reported as costs per quality-adjusted life year (QALY) gained in US dollars from the societal perspective. Sensitivity analyses were conducted on key variables, including AHI positivity rates, antibody testing frequency, symptomatic detection of AHI, and costs. Pooled NAAT for AHI screening following annual antibody testing had cost-effectiveness ratios exceeding US$200,000 per QALY gained for the municipal STD clinics and HIV counseling and testing sites and was cost saving for the community clinic. Cost-effectiveness ratios increased substantially if the antibody testing interval decreased to every 6 months and decreased to cost-saving if the testing interval increased to every 5 years. NAAT was cost saving in the community clinic in all situations. Results were particularly sensitive to AHI screening yield.

Conclusions

Pooled NAAT screening for AHI following negative third-generation antibody or rapid tests is not cost-effective at recommended antibody testing intervals for high-risk persons except in very high-incidence settings. Please see later in the article for the Editors'' Summary