Mode of action and growth toxicity of arsenic to tilapia Oreochromis mossambicus can be determined bioenergetically.

We present a bioenergetics-based approach to analyze the chronic effects and growth toxicity mode of action in tilapia Oreochromis mossambicus exposed to waterborne As and to predict fish growth under different exposure scenarios. 7-day exposure bioassays showed that tilapia accumulate As when exposed to waterborne As. We conducted growth bioassays to assess chronic As toxicity to tilapia. We incorporated a universal ontogenetic growth model with the DEB(tox )theory to explore the mode of action of As toxicity. Our results show that the specific growth rates of exposed tilapia are inversely proportional to As concentrations and are calculated as 0.76% d(-1) in 0 microg mL(-1), 0.57% d(-1) in 1 microg mL(-1), 0.2 % d(-1) in 2 microg mL(-1), and 0.04% d(-1) in 4 microg mL(-1) As, respectively. We showed that the internal threshold concentration did not change significantly with time, demonstrating that the critical body residue approach is applicable for As toxicity assessment. We distinguished between three modes of action of As, including direct effects on growth and indirect effects by way of maintenance and food consumption. Our results support that decreased feeding accounts for the growth decrease in the case of feeding ad libitum. The feeding decrease model also illustrates the growth trajectories of tilapia during the entire whole life span, suggesting that the maximum biomass of tilapia are 1038.75 g in uncontaminated water and 872.97 g in 1 microg mL(-1), 403.06 g in 2 microg mL(-1), and 336.65 g in 4 microg mL(-1) As, respectively. We suggest that considering modes of action in ecotoxicology not only improves our understanding of the toxicities of chemicals, it is also useful in setting up models and avoiding pitfalls in species- and site-specific environmental risk assessment. This proposed framework for tilapia gives preliminary information relevant to aquacultural and ecologic management.

Organ-specific toxicokinetics and dose-response of arsenic in tilapia Oreochromis mossambicus.

We appraised organ-specific toxicokinetics and dose responses of arsenic burdens in tilapia Oreochromis mossambicus. We kinetically linked an Area-under-the-curve (AUC)-based acute toxicity model and a pharmacodynamic model to derive dose-response relationships between equilibrium organ-specific arsenic concentrations and mortality effects. The AUC-based acute toxicity model was also used to derive organ-specific internal effect concentration (IEC)-time-response relationships, which can also be applied to predict a time-mortality profile. We conducted a 7-day exposure experiment to obtain toxicokinetic parameters, whereas the AUC-based acute toxicity model was verified with LC50(t) data obtained from a 7-day acute toxicity bioassay. Our results demonstrated that 96-hour LC50 and incipient LC50 for tilapia exposed to arsenic are 28.68 (95% confidence interval to 24.92 to 32.44) and 25.55 mg L(-1), respectively. Dose-response relationships followed the Hill equation, which could be expressed as organ-specific bioconcentration factors and incipient LC50. Organ-specific dose-response relationships showed that muscle, gill, and liver have a relatively steep sigmoid dose-response profile in that IEC50 were 26.6, 62.5, and 78.5 microg g(-1) dry wt (dw), respectively. Organ-specific arsenic internal lethal burdens were the highest in the gill and the lowest in the muscle in waterborne-exposed tilapia. The IEC and target-organ concentrations derived in this study can be used in site-specific risk assessment.

A simple modeling approach towards hydroperiod effects on fish dynamics in a northern Taiwan wetland ecosystem.

In this paper we develop a simple dynamic model to describe the seasonality of fish population subjected to the pattern of water level fluctuations (i.e., hydroperiod) in a northern Taiwan wetland ecosystem, Kuandu Natural Reservation. We use a harmonic oscillation to characterize hydroperiod quantitatively by obtaining the amplitude and timing of the dominant periodic component in a time series of water levels. The model illustrates the temporal pattern of fish dynamics through the year that can result in very high density of fish at the end of a hydroperiod as well as the importance of ponds and other depressions that both are refuges and sinks during dry periods. We suggest that a 9-mon effective threshold in the length of the hydroperiod must be exceeded to produce high fish population densities. Our results also indicate that the large, piscivorous fish appear to have a major impact on smaller fish in the marsh habitat. Simulation results reveal that the recovery of small fish populations in the marsh following a major drought requires up to two years. The collective results imply that human activities such as drainage or other alterations of the hydrology can exacerbate natural cycles and results in detrimental stresses on fish production and the higher trophic levels dependent on this production.

Proteolysis of the Caulobacter McpA chemoreceptor is cell cycle regulated by a ClpX-dependent pathway.

Proteolysis is involved in cell differentiation and the progression through the cell cycle in Caulobacter crescentus. We have constitutively expressed the transmembrane chemoreceptor McpA from a multicopy plasmid to demonstrate that McpA degradation is modulated during the cell cycle. The level of McpA protein starts to decrease only when the swarmer cells differentiate into stalked cells. The reduction in McpA protein levels is maintained until the stalked cells develop into predivisional cells, at which point the level returns to that observed in swarmer cells. The cell-cycle-regulated degradation of McpA does not require the last 12 C-terminal amino acids, but it does require three amino acids (AAL) located 15 residues away from the C terminus. The ClpXP protease is essential in C. crescentus for viability, and thus, we tested McpA degradation in xylose conditional mutants. The effect on McpA degradation occurred within two generations from the start of ClpX depletion. The conditional mutants' growth rate was only slightly affected, suggesting that ClpX is directly involved in McpA proteolysis.

Proteolysis of the McpA chemoreceptor does not require the Caulobacter major chemotaxis operon.

The degradation of the McpA chemoreceptor in Caulobacter crescentus accompanies the swarmer cell to the stalked-cell differentiation event. To further analyze the requirements for its degradation, we have constructed a series of strains that have deletions in the mcpA gene and in the mcpA chemotaxis operon. Internal deletions of the mcpA gene demonstrate that the highly conserved domain (signalling unit) and the methylation domains are not required for cell cycle-regulated proteolysis. The deletion of the chemotaxis operon, which is absolutely required for chemotaxis and McpA chemoreceptor methylation, has no effect on McpA proteolysis.

Targeting of aminopeptidase N to bile canaliculi correlates with secretory activities of the developing canalicular domain.

We have used human hepatoma cell lines as an in vitro model to study the development of hepatic bile canaliculi (BC). Well-differentiated hepatoma cells cultured for 72 hours could develop characteristic spheroid structures at sites of cell-cell contact that contained tight junctions and various membrane protein markers, resembling BC found in vivo. Intact cytoskeleton was essential for this differentiation process. In the coculture experiments in which cells of different origins were populated together, BC only formed between hepatic cells and preferentially among well-differentiated cells. Poorly differentiated hepatoma cells never formed BC among themselves, but could be induced to undergo canalicular differentiation by interacting with well-differentiated cells. During BC morphogenesis, integral canalicular membrane proteins were gradually delivered and accumulated at the developing BC. Among them, targeting of aminopeptidase N (APN) seemed to correlate with activation of certain secretory functions. Specifically, only APN-positive BC supported excretion of fluorescein diacetate (FDA) and 70-kd dextran, but had no relationship with secretion of horseradish peroxidase (HRP). Targeting of another BC protein, dipeptidyl peptidase IV (DPPIV), on the other hand, bore no association with any secretory activity examined. In addition, inhibition of enzymatic activity of APN could perturb canalicular differentiation without affecting cell proliferation. Our results suggest that targeting of APN proteins may reflect or even play an important role in the development and functional maturation of the canalicular structures.

Assessment of heart wall motion: modified spatial modulation of magnetization for MR imaging.

This paper reports a new noninvasive method for detecting a regional myocardial infarction. The method consists of superimposing a magnetization grid onto different image planes of the human heart. In this study the spatial modulation of magnetization (SPAMM) in magnetic resonance imaging was modified to accommodate the use of a conventional clinical magnet. The percentages of circumferential shortening (PCS) and the percentages of wall thickening (PWT) during a cardiac cycle were estimated by measuring the amount of deformation of each grid from the end of the diastole to the end of the systole. A small data base was established on 10 subjects (6 normal volunteers and 4 patients). Patients with regional myocardial infarction, when monitored by the modified SPAMM investigation, showed smaller values of PCS and PWT in the areas where myocardial infarction was diagnosed by catheterization. The amount in decrement was directly proportional to the severity of the regional lesions.

Dilaceration of a central incisor with iatrogenic root perforation--a case report.

Dilacerated tooth is caused by disruption of the follicle of the developing tooth secondary to traumatic injury of its deciduous predecessor. This case presented a dilacerated permanent maxillary incisor which was perforated in root canal at buccal side by general practitioner and treated successfully by conservative endodontic therapy.