Individual-based modeling of the spread of pine wilt disease: vector beetle dispersal and the Allee effect

Pine wilt disease is caused by the pinewood nematode Bursaphelenchus xylophilus, which is vectored by the Japanese pine sawyer beetle Monochamus alternatus. Due to their mutualistic relationship, according to which the nematode weakens and makes trees available for beetle reproduction and the beetle in turn carries and transmits the nematode to healthy pine trees, this disease has resulted in severe damage to pine trees in Japan in recent decades. Previous studies have worked on modeling of population dynamics of the vector beetle and the pine tree to explore spatial expansion of the disease using an integro-difference equation with a dispersal kernel that describes beetle mobility over space. In this paper, I revisit these previous models but retaining individuality: by considering mechanistic interactions at the individual level it is shown that the Allee effect, an increasing per-capita growth rate as population abundance increases, can arise in the beetle dynamics because of the necessity for beetles to contact pine trees at least twice to reproduce successfully. The incubation period after which a tree contacted by a first beetle becomes ready for beetle oviposition by later beetles is crucial for the emergence of this Allee effect. It is also shown, however, that the strength of this Allee effect depends strongly on biological mechanistic properties, especially on beetle mobility. Realistic individual-based modeling highlights the importance of how spatial scales are dealt with in mathematical models. The link between mechanistic individual-based modeling and conventional analytical approaches is also discussed.     

Evaluation of the protein interfaces that form an intermolecular four-helix bundle as studied by computer simulation

Rational design of protein surface is important for creating higher order protein structures, but it is still challenging. In this study, we designed in silico the several binding interfaces on protein surfaces that allow a de novo protein–protein interaction to be formed. We used a computer simulation technique to find appropriate amino acid arrangements for the binding interface. The protein–protein interaction can be made by forming an intermolecular four-helix bundle structure, which is often found in naturally occurring protein subunit interfaces. As a model protein, we used a helical protein, YciF. Molecular dynamics simulation showed that a new protein–protein interaction is formed depending on the number of hydrophobic and charged amino acid residues present in the binding surfaces. However, too many hydrophobic amino acid residues present in the interface negatively affected on the binding. Finally, we found an appropriate arrangement of hydrophobic and charged amino acid residues that induces a protein–protein interaction through an intermolecular four-helix bundle formation.     

Modeling the Expansion of an Introduced Tree Disease

Pine wilt disease is caused by the introduced pinewood nematode, Bursaphelenchus xylophilus, for which the vector is the pine sawyer beetle, Monochamus alternatus. Native Japanese pines, black pine (Pinus thunbergii) and red pine (P. densiflora), are extremely sensitive to the nematode's infection, and the parasite has been expanding nationwide in the last few decades, despite intensive control efforts. To understand the parasite's range expansion in Japan, we modeled the dynamics of the pines and the beetle that disperses the nematode, using an integro-difference equation in a one-dimensional space. Based on field data collected in Japan, we investigated the dependence of the parasite's rate of range expansion on the eradication rate of the beetle, the initial pine density, and the beetle dispersal ability. Our model predicts several results. (1) The Allee Effect operates on beetle reproduction, and consequently the parasite cannot invade a pine stand, once the beetle density decreases below a threshold. (2) The distribution of the dispersal distance of the beetles critically affects the expansion rate of the disease. As the fraction of the beetles that travel over long distance increases from zero, the range expansion accelerates sharply. (3) However, too frequent long-range dispersal results in a failure of the parasite invasion due to the Allee Effect, suggesting the importance of correctly assessing the beetle's mobility to predict the speed of range expansion of the parasite. (4) As the eradication rate is increased, the range expansion speed decreases gradually at first and suddenly drops to zero at a specific value of the eradication rate. This revised version was published online in July 2006 with corrections to the Cover Date.     

TSH-stimulated electrical excitation in thyroid cells

This report demonstrates TSH-stimulated electrical excitation in cultured porcine thyroid cells. TSH depolarizes the thyroid cell membrane potentials and causes the appearance of action potentials, which occur in a burst. The burst is preceded by depolarization and after the burst, during which usually 2 spikes are seen, a repolarization occurs. This TSH-induced electrical excitation is associated with iodide discharge.     

Interrelationship between insulin-like growth factor I-induced activation of the Na+/H(+)-antiporter and intracellular Ca(2+)-mobilization in thyroid cells.

Insulin-like growth factor I (IGF-I) increased cytoplamic pH (pHi) and cytoplasmic Ca2+ [( Ca2+]i) in cultured porcine thyroid cells. Inhibition of the Na+/H(+)-antiporter by dimethylamiloride or a reduction of external Na(+)-concentrations attenuates the increases in pHi and [Ca2+]i. The [Ca2+]i response to IGF-I is a pHi-dependent process. IGF-I activates Na+/H(+)-antiporter and alkalinizes thyroid cells. The resulting increase in pHi facilitates the [Ca2+]i response by adjusting the pHi closer to the pHi-optimum of the intracellular Ca(2+)-mobilizing system. One of the biological functions of IGF-I-induced activation of the Na+/H(+)-antiporter is to shift the pHi to an optimal value for the [Ca2+]i response.     

Cytoplasmic pH in cultured porcine thyroid cells: alkalinization by epidermal growth factor

We studied the effects of epidermal growth factor (EGF), thyroid-stimulating hormone (TSH) and amiloride on cytoplasmic pH (pHi) in cultured porcine thyroid cells. We used 2',7'-bis(2-carboxyethyl)-5- (and 6-)carboxyfluorescein (BCECF), an internalized fluorescent pH indicator, to measure pHi. EGF stimulated thyroid cell alkalinization and proliferation, which were blocked by amiloride. EGF-stimulated thyroid cell alkalinization depended on extracellular Na+ concentrations. EGF stimulation resulted in an activation of Na+/H+ exchange, which alkalinized the cells. The results indicated that Na+/H+ exchange or cell alkalinization might function as a transmembrane signal transducer in the action of EGF. In the present system, TSH did not stimulate alkalinization or proliferation.     

Epidermal growth factor (EGF) and tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA) stimulate PG synthesis and thymidine incorporation in cultured porcine thyroid cells

This is the first report to show that epidermal growth factor (EGF) and 12-O-tetradecanoylphorbol 13-acetate (TPA) stimulate the production of PGE2 and 6-keto PGF1 alpha, an end metabolite of PGI2, in the thyroid gland. In cultured porcine thyroid cells, EGF and TPA stimulate PGE2 and 6-keto PGF1 alpha production; the maximum PG levels were obtained after 3-4 h incubation with EGF or TPA; the addition of as little as 10(-11) M EGF or 5 X 10(-11) M TPA resulted in increases in PGE2 and 6-keto PGF1 alpha, and the maximum levels were obtained with 10(-8)-10(-7) M EGF or TPA. This report also shows that EGF and TPA stimulate [3H] thymidine incorporation.     

Insulin-like growth factor I stimulates inositol phosphate accumulation, a rise in cytoplasmic free calcium, and proliferation in cultured porcine thyroid cells

Insulin-like growth factor (IGF-I) stimulates thyroid cell proliferation. Using primary cultured porcine thyroid cells, we studied the intracellular pathways that mediate the action of IGF-I on thyroid cell proliferation. IGF-I stimulates inositol phosphate accumulation, a rise in cytoplasmic free calcium [( Ca2+]i), and cell proliferation. Exposure to IGF-I results in a time- and dose-dependent accumulation of inositol monophosphate, inositol bisphosphate, and inositol trisphosphate. IGF-I also increases [Ca2+]i, measured using fura-2, a fluorescent Ca2+ indicator; the IGF-I-induced [Ca2+]i response occurs immediately, reaches a maximum within 1 min, and then slowly declines. IGF-I stimulates thyroid cell proliferation, stimulates thymidine incorporation, and increases cell numbers. The IGF-I-induced inositol phosphate accumulation and [Ca2+]i response parallel thyroid cell proliferation in a dose-dependent manner; the maximal response is observed at a concentration of 100 ng/ml IGF-I, with half-maximal stimulation at approximately 10 ng/ml. Inositol phosphate accumulation and [Ca2+]i response after IGF-I stimulation may function as intracellular messengers for thyroid cell proliferation. This report may constitute the first demonstration of IGF-I-stimulated inositol phosphate accumulation and [Ca2+]i response in the cells.     

Characterization of swine leukocyte antigen alleles and haplotypes on a novel miniature pig line,Microminipig

Microminipigs are extremely small‐sized, novel miniature pigs that were recently developed for medical research. The inbred Microminipigs with defined swine leukocyte antigen (SLA) haplotypes are expected to be useful for allo‐ and xenotransplantation studies and also for association analyses between SLA haplotypes and immunological traits. To establish SLA‐defined Microminipig lines, we characterized the polymorphic SLA alleles for three class I (SLA‐1, SLA‐2 and SLA‐3) and two class II (SLA‐DRB1 and SLA‐DQB1) genes of 14 parental Microminipigs using a high‐resolution nucleotide sequence‐based typing method. Eleven class I and II haplotypes, including three recombinant haplotypes, were found in the offspring of the parental Microminipigs. Two class I and class II haplotypes, Hp‐31.0 (SLA‐1*1502–SLA‐3*070102–SLA‐2*1601) and Hp‐0.37 (SLA‐DRB1*0701–SLA‐DQB1*0502), are novel and have not so far been reported in other pig breeds. Crossover regions were defined by the analysis of 22 microsatellite markers within the SLA class III region of three recombinant haplotypes. The SLA allele and haplotype information of Microminipigs in this study will be useful to establish SLA homozygous lines including three recombinants for transplantation and immunological studies.