Uptake Mechanisms (uptake + mechanism)

Distribution by Scientific Domains

Selected Abstracts

Uptake Mechanism of Oppositely Charged Fluorescent Nanoparticles in HeLa Cells

Julia Dausend
Abstract The endocytotic mechanisms involved in the uptake of charged polystyrene nanoparticles into HeLa cells were investigated. Uptake experiments were done in the presence or absence of drugs known to inhibit various factors in endocytosis. Independent of the particle charge, endocytosis is highly dependent on dynamin, F-actin, and tyrosine-specific protein kinases, which suggests a dynamin-dependent and lipid raft-dependent mechanism. However, cholesterol depletion did not hinder particle uptake. Regarding positively charged particles, macropinocytosis, the microtubule network, and cyclooxygenases are also involved. The clathrin-dependent pathway plays a minor role. [source]

Medium-sized peptides as built in carriers for biologically active compounds

Ferenc Hudecz
Abstract A growing number of oligopeptides of natural and/or synthetic origin have been described and considered as targeting structures for delivery bioactive compounds into various cell types. This review will outline the discovery of peptide sequences and the corresponding mid-sized oligopeptides with membrane translocating properties and also summarize de novo designed structures possessing similar features. Conjugates and chimera constructs derived from these sequences with covalently attached bioactive peptide, epitope, oligonucleotide, PNA, drug, reporter molecule will be reviewed. A brief note will refer to the present understanding on the uptake mechanism at the end of each section. 2005 Wiley Periodicals, Inc. Med Res Rev [source]

When to say when: can excessive drinking explain silicon uptake in diatoms?

BIOESSAYS, Issue 3 2009
Kimberlee Thamatrakoln
Abstract Diatoms are the single most important drivers of the oceanic silicon biogeochemical cycle. Due to their considerable promise in nanotechnology, there is tremendous interest in understanding the mechanism by which they produce their intricately and ornately decorated silica-based cell wall. Although specific proteins have been implicated in some of the key steps of silicification, the exact mechanisms are poorly understood. Silicon transporters, identified in both diatoms and silicoflagellates, are hypothesized to mediate silicon uptake. Recently, macropinocytosis, the non-specific engulfment of extracellular fluid, was proposed as a more energetically favorable uptake mechanism, which can also explain the long-observed effect of salinity on frustule morphology. We explore the bioenergetic, membrane recycling, and vacuolar volume requirements that must be satisfied for pinocytosis-mediated silicon uptake. These calculated requirements contrast starkly with existing data on diatom physiology, uptake kinetics, growth, and ultrastructure, leading us to conclude that pinocytosis cannot be the primary mechanism of silicon uptake. [source]

Site-Specific Investigation of the Steady-State Kinetics and Dynamics of the Multistep Binding of Bile Acid Molecules to a Lipid Carrier Protein

Dr. Clelia Cogliati
Abstract The investigation of multi-site ligand,protein binding and multi-step mechanisms is highly demanding. In this work, advanced NMR methodologies such as 2D 1H,15N line-shape analysis, which allows a reliable investigation of ligand binding occurring on micro- to millisecond timescales, have been extended to model a two-step binding mechanism. The molecular recognition and complex uptake mechanism of two bile salt molecules by lipid carriers is an interesting example that shows that protein dynamics has the potential to modulate the macromolecule,ligand encounter. Kinetic analysis supports a conformational selection model as the initial recognition process in which the dynamics observed in the apo form is essential for ligand uptake, leading to conformations with improved access to the binding cavity. Subsequent multi-step events could be modelled, for several residues, with a two-step binding mechanism. The protein in the ligand-bound state still exhibits a conformational rearrangement that occurs on a very slow timescale, as observed for other proteins of the family. A global mechanism suggesting how bile acids access the macromolecular cavity is thus proposed. [source]

Dynamics of genome evolution in facultative symbionts of aphids

Patrick H. Degnan
Summary Aphids are sap-feeding insects that host a range of bacterial endosymbionts including the obligate, nutritional mutualist Buchnera plus several bacteria that are not required for host survival. Among the latter, ,Candidatus Regiella insecticola' and ,Candidatus Hamiltonella defensa' are found in pea aphids and other hosts and have been shown to protect aphids from natural enemies. We have sequenced almost the entire genome of R. insecticola (2.07 Mbp) and compared it with the recently published genome of H. defensa (2.11 Mbp). Despite being sister species the two genomes are highly rearranged and the genomes only have ,55% of genes in common. The functions encoded by the shared genes imply that the bacteria have similar metabolic capabilities, including only two essential amino acid biosynthetic pathways and active uptake mechanisms for the remaining eight, and similar capacities for host cell toxicity and invasion (type 3 secretion systems and RTX toxins). These observations, combined with high sequence divergence of orthologues, strongly suggest an ancient divergence after establishment of a symbiotic lifestyle. The divergence in gene sets and in genome architecture implies a history of rampant recombination and gene inactivation and the ongoing integration of mobile DNA (insertion sequence elements, prophage and plasmids). [source]

Biomagnification and polychlorinated biphenyl congener distribution in an aquatic predator-prey, host-parasite system

Maria E. Persson
Abstract Biomagnification and polychlorinated biphenyl (PCB) congener distribution was examined in a predator-prey, hostparasite system, in which Atlantic salmon (Salmo salar) preyed upon sprat (Sprattus sprattus). Eubothrium crassum was an intestinal parasite in salmon that also "preyed upon" sprat, because the parasites gained access to foodstuffs via the host (salmon) gut. Salmon contained significantly higher concentrations of total PCBs compared to both parasites and prey (sprat), but no difference in PCB concentration was found between sprat and E. crassum. Salmon biomagnified several PCB congeners from their diet (sprat), whereas parasites did not, despite the fact that both salmon and their parasites ingested the same prey. Differences in nutrient uptake mechanisms between the host and their parasites, in addition to the lack of a gastrointestinal tract in the cestode, may explain the lack of biomagnification in E. crassum. No difference was found in PCB congener distribution between parasites, salmon, and sprat, and none of the animal types showed a preference for accumulating more or less lipophilic congeners (congeners with a high or low octanol/water partition coefficient [KOW]). Biomagnification factors for individual congeners in salmon did not increase with KOW; rather, they were constant, as shown by a linear relationship for congener concentration in prey and predator. [source]

Expression of Pit2 sodium-phosphate cotransporter during murine odontogenesis is developmentally regulated

Dawei Zhao
Different sodium-dependent inorganic phosphate (Pi) uptake mechanisms play a major role in cellular Pi homeostasis. The function and detailed distribution patterns of the type III Na+ -phosphate cotransporter, PiT-2, in different organs during development are still largely unknown. We therefore examined the temporospatial expression patterns of Pit2 during murine odontogenesis. Odontoblasts were always devoid of Pit2 expression, whereas a transient, but strong, expression was detected in young secretory ameloblasts. However, the stratum intermedium and, later on, the papillary layer and cells of the subodontoblastic layer, exhibited high levels of Pit2 mRNA, which increased gradually as the tooth matured. Hormonal treatment or Pi starvation of tooth germs in vitro did not alter Pit2 levels or patterns of expression, indicating mechanisms of regulation different from those of PiT-1 or other cell types. PiT-2 also functions as a retroviral receptor, and functional membrane-localized protein was confirmed throughout the dental papilla/pulp by demonstrating cellular permissiveness to infection by a gammaretrovirus that uses PiT-2 as a receptor. The distinct pattern of Pit2 expression during odontogenesis suggests that its Pi -transporter function may be important for homeostasis of dental cells and not specifically for mineralization of the dental extracellular matrices. The expression of viral receptors in enamel-forming cells and the dental pulp may be of pathological significance. [source]

The uptake by cells of 2-arachidonoylglycerol, an endogenous agonist of cannabinoid receptors

FEBS JOURNAL, Issue 7 2001
Tiziana Bisogno
It is not yet clear if the endocannabinoid 2-arachidonoylglycerol (2-AG) is transported into cells through the same membrane transporter mediating the uptake of the other endogenous cannabinoid, anandamide (N -arachidonoylethanolamine, AEA), and whether this process (a) is regulated by cells and (b) limits 2-AG pharmacological actions. We have studied simultaneously the facilitated transport of [14C]AEA and [3H]2-AG into rat C6 glioma cells and found uptake mechanisms with different efficacies but similar affinities for the two compounds (Km 11.0 2.0 and 15.3 3.1 m, Bmax 1.70 0.30 and 0.24 0.04 nmolmin,1mg protein,1, respectively). Despite these similar Km values, 2-AG inhibits [14C]AEA uptake by cells at concentrations (Ki = 30.1 3.9 m) significantly higher than those required to either 2-AG or AEA to inhibit [3H]2-AG uptake (Ki = 18.9 1.8 and 20.5 3.2 m, respectively). Furthermore: (a) if C6 cells are incubated simultaneously with identical concentrations of [14C]AEA and [3H]2-AG, only the uptake of the latter compound is significantly decreased as compared to that observed with [3H]2-AG alone; (b) the uptake of [14C]AEA and [3H]2-AG by cells is inhibited with the same potency by AM404 (Ki = 7.5 0.7 and 10.2 1.7 m, respectively) and linvanil (Ki = 9.5 0.7 and 6.4 1.2 m, respectively), two inhibitors of the AEA membrane transporter; (c) nitric oxide (NO) donors enhance the uptake of both [14C]AEA and [3H]2-AG, thus suggesting that 2-AG action can be regulated through NO release; (d) AEA and 2-AG induce a weak release of NO that can be blocked by a CB1 cannabinoid receptor antagonist, and significantly enhanced in the presence of AM404 and linvanil, thus suggesting that transport into C6 cells limits the action of both endocannabinoids. [source]


Erica B. Young
When NH4+ or NO3, was supplied to NO3, -stressed cells of the microalga Dunaliella tertiolecta Butcher, immediate transient changes in chl a fluorescence were observed over several minutes that were not seen in N-replete cells. These changes were predominantly due to nonphotochemical fluorescence quenching. Fluorescence changes were accompanied by changes in photosynthetic oxygen evolution, indicating interactions between photosynthesis and N assimilation. The magnitude of the fluorescence change showed a Michaelis-Menten relationship with half-saturation concentration of 0.5 ,M for NO3, and 10 ,M for NH4+. Changes in fluorescence responses were characterized in D. tertiolecta both over 5 days of N starvation and in cells cultured at a range of NO3, -limited growth rates. Variation in responses was more marked in starved than in limited cells. During N starvation, the timing and onset of the fluorescence responses were different for NO3, versus NH4+ and were correlated with changes in maximum N uptake rate during N starvation. In severely N-starved cells, the major fluorescence response to NO3, disappeared, whereas the response to NH4+ persisted. N-starved cells previously grown with NH4+ alone showed fluorescence responses with NH4+ but not NO3, additions. The distinct responses to NO3, and NH4+ may be due to the differences between regulation of the uptake mechanisms for the two N sources during N starvation. This method offers potential for assessing the importance of NO3, or NH4+ as an N source to phytoplankton populations and as a diagnostic tool for N limitation. [source]

Intracellular calcium regulation among subpopulations of rat dorsal root ganglion neurons

Shao-Gang Lu
Primary afferent neurons are functionally heterogeneous. To determine whether this functional heterogeneity reflects, in part, heterogeneity in the regulation of the concentration of intracellular Ca2+ ([Ca2+]i), the magnitude and decay of evoked Ca2+ transients were assessed in subpopulations of dorsal root ganglion (DRG) neurons with voltage clamp and fura-2 ratiometric imaging. To determine whether differences in evoked Ca2+ transients among subpopulations of DRG neurons reflected differences in the contribution of Ca2+ regulatory mechanisms, pharmacological techniques were employed to assess the contribution of influx, efflux, release and uptake pathways. Subpopulations of DRG neurons were defined by cell body size, binding of the plant lectin IB4 and responsiveness to the algogenic compound capsaicin (CAP). Ca2+ transients were evoked with 30 mm K+ or voltage steps to 0 mV. There were marked differences between subpopulations of neurons with respect to both the magnitude and decay of the Ca2+ transient, with the largest and most slowly decaying Ca2+ transients in small-diameter, IB4 -positive, CAP-responsive neurons. The smallest and most rapidly decaying transients were in large-diameter, IB4 -negative and CAP-unresponsive DRG neurons. These differences were not due to a differential distribution of voltage-gated Ca2+ currents. However, these differences did appear to reflect a differential contribution of other influx, efflux, release and uptake mechanisms between subpopulations of neurons. These results suggest that electrical activity in subpopulations of DRG neurons will have a differential influence on Ca2+ -regulated phenomena such as spike adaptation, transmitter release and gene transcription. Significantly more activity should be required in large-diameter non-nociceptive afferents than in small-diameter nociceptive afferents to have a comparable influence on these processes. [source]

Sources and bioavailability of phosphorus fractions in freshwaters: a British perspective

ABSTRACT This paper seeks a perspective on the forms of phosphorus which promote aquatic eutrophication, with the particular quest of establishing their sources. A short background traces the development of understanding of nutrient enrichment and the suppositions about the relative contributions of agriculture, sewage and detergent residues. Most aquatic systems, and their primary producers, are naturally deficient in biologically-available phosphorus. Aquatic plants have evolved very efficient phosphorus uptake mechanisms. The biomass responses to an increase in the supply of phosphorus are stoichiometrically predictable. The most bioavailable forms of phosphorus are in solution, as orthophosphate ions, or are readily soluble or elutable from loose combinations. Ready bioavailability coincides well with what is measurable as molybdate-reactive (MRP) or soluble-reactive phosphorus (SRP). Most other forms, including phosphates of the alkaline earth metals, aluminium and iron are scarcely available at all. Orthophosphate ions sorbed to metal oxides and hydroxides are normally not biologically available either, except through weak dissociation (,desorption'). The production of alkaline phosphatase provides organisms with an additional mechanism for accelerating the sequestration of phosphate from organic compounds. Bioavailable phosphate is liberated when redox- or alkali-sensitive metal hydroxides dissolve but these processes are minor contributors to the biological responses to nutrient enrichment. Most of the familiar eutrophication is attributable to the widespread application of secondary sewage treatment methods to the wastes emanating from a burgeoning and increasingly urbanised human population. The use of polyphosphate-based detergents, now in decline, has contributed to the problem. In aquatic systems, the additional phosphorus raises the biological supportive capacity, sometimes to the capacity of the next limiting factor (carbon, light, hydraulic retention or of another nutrient). At high orthophosphate loadings, the straight stoichiometric yield relationship between biomass yield and phosphorus availability is lost. Movements of phosphorus and its recycling within aquatic systems do not prevent the slow gravitation of phosphorus to the bottom substrata. The phosphorus retentivity of sediments depends upon their chemical composition. While oxide-hydroxide binding capacity in the surface sediments persists, they act as a sink for phosphorus and a control on further cycling. Iron-rich and clay-rich sediments perform best in these conditions; calcareous sediments least so. Eutrophication may lead to the exhaustion of sediment P-binding capacity. Non-sorbed phosphate is readily recyclable if primary producers have access to it. Recycling is most rapid in shallow waters (where sediment disturbance, by flow, by wind action and through bioturbation, is frequent) and least in deep ventilated sediments. The contributions of phosphorus from catchments are assessed. The slow rate of weathering of (mostly apatitic) minerals, the role of chemical binding in soils and the incorporation and retentivity by forested terrestrial ecosystems each contribute to the minimisation of phosphorus leakage to drainage waters. Palaeolimnological and experimental evidence confirms that clearance of land and ploughing its surface weakens the phosphorus retentivity of catchments. The phosphorus transferred from arable land to drainage remains dominated by sorbed fractions which are scarcely bioavailable. Some forms of intensive market gardening or concentrated stock rearing may mobilise phosphates to drainage but it is deduced that drainage from agricultural land is not commonly a major source of readily bioavailable phosphorus in water. Careful budgeting of the phosphates in run-off from over-fertilised soils may nevertheless show that a proportionately small loss of bioavailable phosphorus can still be highly significant in promoting aquatic plant production. The bioavailable-phosphorus (BAP) load achieving the OECD threshold of lake eutrophy (35 mg P m,3) is calculated to be equivalent to a terrestrial loss rate of approximately 17.5 kg BAP km,2 year,1), or only 1,2% of a typical fertiliser application. The output is shown to be comparable with the P yield from secondary treatment of the sewage produced by a resident population of 30,44 persons km,2. With tertiary treatment, the equivalence is with approximately 200 persons km,2. [source]