On the microstructurally driven heterogeneous response of brain white matter to drug infusion pressure

Delivering therapeutic agents into the brain via convection-enhanced delivery (CED), a mechanically controlled infusion method, provides an efficient approach to bypass the blood–brain barrier and deliver drugs directly to the targeted focus in the brain. Mathematical methods based on Darcy’s law have been widely adopted to predict drug distribution in the brain to improve the accuracy and reduce the side effects of this technique. However, most of the current studies assume that the hydraulic permeability and porosity of brain tissue are homogeneous and constant during the infusion process, which is less accurate due to the deformability of the axonal structures and the extracellular matrix in brain white matter. To solve this problem, a multiscale model was established in this study, which takes into account the pressure-driven deformation of brain microstructure to quantify the change of local permeability and porosity. The simulation results were corroborated using experiments measuring hydraulic permeability in ovine brain samples. Results show that both hydraulic pressure and drug concentration in the brain would be significantly underestimated by classical Darcy’s law, thus highlighting the great importance of the present multiscale model in providing a better understanding of how drugs transport inside the brain and how brain tissue responds to the infusion pressure. This new method can assist the development of both new drugs for brain diseases and preoperative evaluation techniques for CED surgery, thus helping to improve the efficiency and precision of treatments for brain diseases.

     

Invertebrate models of drug abuse

Susceptibility to drug addiction depends on genetic and environmental factors and their complex interactions. Studies with mammalian models have identified molecular targets, neurochemical systems, and brain regions that mediate some of the addictive properties of abused drugs. Yet, our understanding of how the primary effects of drugs lead to addiction remains incomplete. Recently, researchers have turned to the invertebrate model systems Drosophila melanogaster and Caenorhabditis elegans to dissect the mechanisms by which abused drugs modulate behavior. Due to their sophisticated genetics, relatively simple anatomy, and their remarkable molecular similarity to mammals, these invertebrate models should provide useful insights into the mechanisms of drug action. Here we review recent behavioral and genetic studies in flies and worms on the effects of ethanol, cocaine, and nicotine, three of the most widely abused drugs in the world.     

The blood-brain barrier choline transporter as a brain drug delivery vector

Choline is a ubiquitous molecule, found throughout almost every tissue in the body. Given it is a charged cation, nearly every cellular membrane has a transport mechanism to meet the intracellular and membrane need for choline. The blood-brain barrier is no exception in that a carrier-mediated transport mechanism is present to deliver choline from plasma to brain. The carrier consists of an anionic binding area that attracts positively charged quaternary ammonium groups or simple cations. Recent reports have shown this vector to be efficacious in delivering quaternary ammonium analogs of nicotine to brain. Future work is being completed to determine if other cationic or positively charged therapeutics can be effectively delivered to brain via this carrier.     

Transport processes through the blood-brain barrier

The existence of the blood-brain barrier is due to tight junctions between endothelial cells preventing the passage of liquid and solute material at the capillary level. Substances can thus pass across the blood-brain barrier if they are lipophilic or if they have transport systems in the membranes of endothelial cells. The luminal membrane brings metabolites needed for the brain function, the abluminal one plays an important part in removing substances from brain, this can happen against a concentration gradient and thus needs energy. Ions are transported differently by the 2 membranes. Sodium and chloride have carriers and potassium is transported very actively by the sodium-potassium ATPase of the abluminal membrane. Blood-brain glucose influx is very important and happens by carrier transport at the 2 membranes. Efflux seems to use the same transport system as the influx. Transport of ketone bodies seems to happen only from blood to brain, the carriers being reversibly used for brain-blood transport of pyruvic and lactic acid. Amino-acid transport is very different on the luminal and abluminal membranes. On the luminal membrane there are 2 transport systems, one for basic amino acids, the other one, the L system, for neutral amino-acids. All neutral amino-acids are transported through the abluminal membrane by the L, A and ASC systems. There exists a system of transport for basic amino-acids, and a very active one for acid amino-acids. Some systems for the transport of hormones, vitamins and for some peptides exist also at the blood-brain barrier which thus plays a very important role in the regulation of brain metabolism.     

Active export proteins mediating drug resistance in staphylococci

Drug resistance mediated by integral membrane transporters is an important mode of cellular resistance to cytotoxic agents across all classes of living organisms. Gram-positive bacteria, such as staphylococcal species, are not encapsulated by a selective outer membrane permeability barrier. Therefore, these organisms often employ integral membrane drug transport systems to maintain cellular concentrations of antimicrobials at subtoxic levels. Staphylococcal species, including the opportunistic human pathogen Staphylococcus aureus, encode a multitude of drug exporters, encompassing transporters from each of the five currently recognized families of bacterial drug resistance transporters. A number of these transporters are chromosomally encoded and allow the host cell to realize clinically significant levels of drug resistance after minor mutations to regulatory regions. Others are plasmid-encoded and can be easily passed between staphylococcal strains and species, or acquired from other Gram-positive genera. In combination, staphylococcal drug transporters potentiate resistance to a vast array of antimicrobial compounds, including macrolide, quinolone, tetracycline and streptogramin antibiotics, as well as a broad range of biocides, such as quaternary ammonium compounds, biguanidines and diamidines. An understanding of the genetic and molecular properties of drug transporters will lead to effective treatments of staphylococcal infections. Here we provide a detailed review of the active drug transporters of the staphylococci.     

CFTR in cystic fibrosis and cholera: from membrane transport to clinical practice

We have used a brief analysis of transport via cystic fibrosis (CF) transmembrane conductance regulators (CFTRs) in various organ systems to highlight the importance of basic membrane transport processes across epithelial cells for first-year medical students in physiology. Because CFTRs are involved in transport both physiologically and pathologically in various systems, we have used this clinical correlation to analyze how a defective gene leading to defective transport proteins can be directly involved in the symptoms of cholera and CF. This article is a "Staying Current" approach to transport via CFTRs including numerous helpful references with further information for a teaching faculty member. The article follows our normal presentation which begins with a discussion of the involvement of CFTR transport in the intestine and how cholera affects intestinal transport, extends to CFTR transport in various organ systems in CF, and concludes with the logic behind many of the treatments that improve CF. Student learning objectives are included to assist in assessment of student understanding of the basic concepts.     

TRANSPORT OF TYROSINE, PHENYLALANINE, TRYPTOPHAN AND GLYCINE IN NEUROBLASTOMA CLONES

Amino acid transport was studied in three neuroblastoma clones, N-TD6, which synthesizes norepinephrine, N-T16, which synthesizes small amounts of serotonin, and N-S20Y, which synthesizes acetylcholine. All three clones exhibited high-affinity saturable transport systems for tyrosine, phenylalanine, tryptophan and glycine as well as systems unsaturated at amino acid concentrations of 1 mM in the external medium. Tyrosine, phenylalanine and tryptophan enter all three clones by rapidly exchanging transport systems which appear to be relatively insensitive to lowered external [Na⁺] or to the presence of 2,4-dinitrophenol (DNP). Glycine uptake was slower and was much more sensitive to lowered external [Na⁺] and to the presence of DNP in the medium. Glycine transport in N-T16 cells was decreased more markedly at low temperature than was transport of the three aromatic amino acids. K_m and V_max values found for saturable transport of tyrosine, phenylalanine and tryptophan were sufficiently low to suggest that, if similar amino acid transport systems exist in neuronal membranes, and if amino acid levels in brain extracellular fluid are similar to levels in plasma, such systems may serve, in conjunction with transport systems in cerebral capillaries, to limit the entry of amino acids into brain cells when blood amino levels are near the normal physiological range.     

Biochemical mechanisms of neurotoxic lead activity

This article presents the latest study results on lead (Pb2+) neurotoxicity, in order to draw attention of the Polish public to the issue and initiate a nation-wide programme eliminating lead contamination effects, especially in children. We discuss the after-effect of exposure to lead in concentrations lower than presently accepted as 'safe'. The pathway of lead transport to the brain, and the effects of lead accumulation in neurons, oligodendroglia and astroglia, are examined. We also present the impairing influence of lead on the cognitive brain functions and learning abilities as a result of affecting three main neurotransmission systems: dopaminergic, cholinergic and glutaminergic. The present knowledge on the influence of lead on receptors, neutransmitter release and synaptic proteins.     

Neural diffusivity and pre-emptive epileptic seizure intervention

The propagation of epileptic seizure activity in the brain is a widespread pathophysiology that, in principle, should yield to intervention techniques guided by mathematical models of neuronal ensemble dynamics. During a seizure, neural activity will deviate from its current dynamical regime to one in which there are significant signal fluctuations. In silico treatments of neural activity are an important tool for the understanding of how the healthy brain can maintain stability, as well as of how pathology can lead to seizures. The hope is that, contained within the mathematical foundations of such treatments, there lie potential strategies for mitigating instabilities, e.g. via external stimulation. Here, we demonstrate that the dynamic causal modelling neuronal state equation generalises to a Fokker-Planck formalism if one extends the framework to model the ways in which activity propagates along the structural connections of neural systems. Using the Jacobian of this generalised state equation, we show that an initially unstable system can be rendered stable via a reduction in diffusivity–i.e., by lowering the rate at which neuronal fluctuations disperse to neighbouring regions. We show, for neural systems prone to epileptic seizures, that such a reduction in diffusivity can be achieved via external stimulation. Specifically, we show that this stimulation should be applied in such a way as to temporarily mirror the activity profile of a pathological region in its functionally connected areas. This counter-intuitive method is intended to be used pre-emptively–i.e., in order to mitigate the effects of the seizure, or ideally even prevent it from occurring in the first place. We offer proof of principle using simulations based on functional neuroimaging data collected from patients with idiopathic generalised epilepsy, in which we successfully suppress pathological activity in a distinct sub-network prior to seizure onset. Our hope is that this technique can form the basis for future real-time monitoring and intervention devices that are capable of treating epilepsy in a non-invasive manner.     

Interactions and regulation of molecular motors in Xenopus melanophores

Many cellular components are transported using a combination of the actin- and microtubule-based transport systems. However, how these two systems work together to allow well-regulated transport is not clearly understood. We investigate this question in the Xenopus melanophore model system, where three motors, kinesin II, cytoplasmic dynein, and myosin V, drive aggregation or dispersion of pigment organelles called melanosomes. During dispersion, myosin V functions as a "molecular ratchet" to increase outward transport by selectively terminating dynein-driven minus end runs. We show that there is a continual tug-of-war between the actin and microtubule transport systems, but the microtubule motors kinesin II and dynein are likely coordinated. Finally, we find that the transition from dispersion to aggregation increases dynein-mediated motion, decreases myosin V--mediated motion, and does not change kinesin II--dependent motion. Down-regulation of myosin V contributes to aggregation by impairing its ability to effectively compete with movement along microtubules.     

Confidence limits and sample size for determining nonhost status of fruits and vegetables to tephritid fruit flies as a quarantine measure

Quarantine measures including treatments are applied to exported fruit and vegetable commodities to control regulatory fruit fly pests and to reduce the likelihood of their introduction into new areas. Nonhost status can be an effective measure used to achieve quarantine security. As with quarantine treatments, nonhost status can stand alone as a measure if there is high efficacy and statistical confidence. The numbers of insects or fruit tested during investigation of nonhost status will determine the level of statistical confidence. If the level of confidence of nonhost status is not high, then additional measures may be required to achieve quarantine security as part of a systems approach. Certain countries require that either 99.99 or 99.9968% mortality, as a measure of efficacy, at the 95% confidence level, be achieved by a quarantine treatment to meet quarantine security. This article outlines how the level of confidence in nonhost status can be quantified so that its equivalency to traditional quarantine treatments may be demonstrated. Incorporating sample size and confidence levels into host status testing protocols along with efficacy will lead to greater consistency by regulatory decision-makers in interpreting results and, therefore, to more technically sound decisions on host status.     

The blood–brain barrier transport and cerebral distribution of guanidinoacetate in rats: involvement of creatine and taurine transporters

Although the cerebral accumulation of guanidinoacetate (GAA) contributes to neurological complications in S -adenosylmethionine:guanidinoacetate N -methyltransferase (GAMT) deficiency, how GAA is abnormally distributed in the brain remains unknown. The purpose of this study was to investigate the transport of GAA across the blood–brain barrier (BBB) and in brain parenchymal cells in rats. [¹⁴C]GAA microinjected into the rat cerebrum was not eliminated from the brain, implying the negligible contribution of GAA efflux transport across the BBB. In contrast, in vivo analysis and an uptake study by TR-BBB cells, a rat in vitro BBB model, revealed that GAA was transported from the circulating blood across the BBB most likely via a creatine transporter (CRT). Although CRT at the BBB is almost saturated by endogenous creatine under physiological conditions, the creatine level in the blood significantly decreases in GAMT deficiency. This might lead to the increase of CRT-mediated blood-to-brain transport of GAA at the BBB. Furthermore, [¹⁴C]GAA was taken up by brain parenchymal cells in a concentrative manner most likely via taurine transporter and CRT. These characteristics of GAA transport across the BBB and in the brain parenchymal cells could be the key factors that facilitate GAA accumulation in the brains of patients with GAMT deficiency.     

Intraspecific variation influences natural settlement of eastern oysters

As populations decline, their intraspecific diversity also diminishes. Population decline may be exacerbated if a decrease in intraspecific diversity also reduces important ecological functions that maintain population numbers. Oyster reefs are severely overharvested, declining by ~85 % worldwide. We tested how increasing within-species diversity of eastern oysters (Crassostrea virginica) using transplants would affect recruitment of oyster larvae, a key function necessary to maintain future populations. If harvesting reduces population numbers, within-species diversity, and connectivity, then oysters may lose the ability to adapt to changing environmental conditions as well as incur lower levels of recruitment that may hasten their decline. Results from laboratory and field studies indicated that oyster larvae use chemical cues from adult oysters and not from associated fouling communities to select settlement sites. To test how increasing within-species diversity of oysters affected recruitment, we collected oysters from three distinct bay systems in Texas, USA, and compared natural settlement in treatments where all oysters were from a single bay to a mixture of all three bays. Significantly greater recruitment occurred in mixed treatments in 2010, 2011, and 2012 even though oyster recruitment varied by order of magnitude during this time. The net biodiversity effect was positive in all 3 years, indicating that increased recruitment in mixed treatments can be greater than the additive effect of the single bay treatments. Losing intraspecific diversity may reduce recruitment and lead to further declines in oyster populations, illustrating the need for understanding how intraspecific diversity influences ecological functions.     

Transport of threonine and tryptophan by rat brain slices: relation to other amino acids at concentrations found in plasma

Threonine content of brain decreases in young rats fed a threonine-limiting, low protein diet containing a supplement of small neutral amino acids (serine, glycine and alanine), which are competitors of threonine transport in other systems (Tews et al., 1977). Threonine transport by brain slices was inhibited more by a complex amino acid mixture resembling plasma from rats fed the small neutral amino acid supplement than by mixtures resembling plasma from control rats or from rats fed a supplement of large neutral amino acids. Greater inhibition was seen with mixtures containing only the small neutral amino acids than with mixtures containing only large neutral amino acids. On an equimolar basis, serine and alanine were the most inhibitory; large neutrals were moderately so; and glycine and lysine were without effect. Threonine transport was also strongly inhibited by α-amino-n-butyric acid and homoserine, less so by α-aminoisobutyric acid, and not at all by GABA. The complex amino acid mixtures strongly inhibited α-aminoisobutyric acid transport by brain or liver slices but, in contrast to effects in brain, the extent of the inhibition in liver was not much affected by altering the composition of the mixture. Tryptophan accumulation by brain slices was effectively inhibited by other large neutral amino acids in physiologically occurring concentrations. Threonine, or a mixture of serine, glycine and alanine only slightly inhibited tryptophan uptake; basic amino acids were without effect and histidine stimulated tryptophan transport slightly. These results support the conclusion that a diet-induced decrease in the concentration in brain of a specific amino acid may be related to increased inhibition of its transport into brain by increases in the concentrations of transport-related, plasma amino acids.     

Molecular modeling of blood-brain barrier nutrient transporters: in silico basis for evaluation of potential drug delivery to the central nervous system

For drugs that act in the brain, the blood-brain barrier (BBB) is a considerable physical barrier which influences the distribution of drugs to the brain. The BBB is essentially impermeable for hydrophilic and/or charged compounds. Nutrient membrane transporters have an important physiological role in the transport of essential substances across the BBB required for normal brain function. We and others have shown that these transporters may have utility as drug delivery vectors, thereby increasing brain distribution of these compounds via these systems. In this review, we evaluate molecular (in silico) models of BBB transport proteins. Few BBB membrane transporters have been crystallized, but their crystal structures have a possibility for use in homology modeling. Other techniques commonly used are 2D quantitative structure-activity relationships (QSAR), as well as 3D-QSAR techniques including comparative molecular field analysis (CoMFA) and comparative similarity index analysis (CoMSIA). Each of these models provides valuable information for ascertaining their potential basis for BBB transport and brain drug delivery.     

Does bioturbation by a benthic fish modify the effects of sediment contamination on saltmarsh benthic microalgae and meiofauna?

A microcosm experiment was conducted using a replicated factorial design to determine if a benthic fish modifies the effects of sediment-bound contaminants (diesel fuel and two levels of a Cu, Cr, Pb, Hg and Cd mixture) on saltmarsh benthic primary producers and consumers. The naked goby, Gobiosoma bosc, a burrowing fish that preys on small macrofauna, was added to experimental microcosms. Goby burrowing and foraging significantly increased turbidity and disrupted the sediment surface. Results were typified by complex and varied responses with many statistically significant effects and interactions among treatments. Although G. bosc modified the responses to both diesel and metal pollution in invertebrates (but not microalgae), bioturbation did not increase or decrease the toxic effects of metals or diesel, and diesel-metal interactions did not vary in response to G. bosc. Specifically, G. bosc inhibited a trend toward diesel-induced increases in nematode abundance, and diesel toxicity inhibited increases in ostracod abundance stimulated by G. bosc. Diatoms, nematodes and the copepod Pseudostenhelia wellsi decreased in treatments with G. bosc. However, G. bosc lead to increases in cyanobacteria and ostracods and a trend toward increases in the copepod Pseudobradya sp. Our findings suggest that microcosm experiments are potentially poor mimics of natural systems without bioturbation. Conclusions about the direct and indirect effects of contaminants may differ with and without bioturbation. Finally, our work suggests that the direct effects of toxicants may inhibit or mask bioturbation effects that stimulate population growth of some meiofauna.     

Ribonucleoprotein complexes in neurologic diseases

Ribonucleoprotein (RNP) complexes regulate the tissue-specific RNA processing and transport that increases the coding capacity of our genome and the ability to respond quickly and precisely to the diverse set of signals. This review focuses on three proteins that are part of RNP complexes in most cells of our body: TAR DNA-binding protein (TDP-43), the survival motor neuron protein (SMN), and fragile-X mental retardation protein (FMRP). In particular, the review asks the question why these ubiquitous proteins are primarily associated with defects in specific regions of the central nervous system? To understand this question, it is important to understand the role of genetic and cellular environment in causing the defect in the protein, as well as how the defective protein leads to misregulation of specific target RNAs. Two approaches for comprehensive analysis of defective RNA-protein interactions are presented. The first approach defines the RNA code or the collection of proteins that bind to a certain cis-acting RNA site in order to lead to a predictable outcome. The second approach defines the RNA map or the summary of positions on target RNAs where binding of a particular RNA-binding protein leads to a predictable outcome. As we learn more about the RNA codes and maps that guide the action of the dynamic RNP world in our brain, possibilities for new treatments of neurologic diseases are bound to emerge.     

Expression of substrate specificity in facilitated transport systems

Summary In facilitated transport systems the carrier reorientation step is shown to be largely independent of the forces of interaction between the substrate and the carrier site, whereas in coupled systems (obligatory exchange or cotransport) reorientation proceeds at the expense of the binding force developed in the transition state. In consequence, the expression of substrate specificity is expected to differ in the two systems. In the facilitated transport of analogs no larger than the normal substrate, the affinity but not the maximum rate of transport can vary widely; with larger analogs, both the affinity and rale can vary if steric constraints are more severe in the translocation step than in binding. In coupled transport, by contrast, the translocation step can be highly sensitive to the structure of the substrate, and binding much less sensitive. The theory agrees with published observations on facilitated systems for choline and glucose in erythrocytes, as well as on Na⁺-coupled systems for the same substrates in other cells. The following mechanism, which could account for the behavior, is proposed. In facilitated systems, the transport site fits the substrate closely and retains its shape as the carrier undergoes reorientation. In coupled systems, the site is initially looser, but during carrier reorientation it contracts around the substrate. In both systems, the carrier encloses the substrate during the translocation step, though for a different reason: in coupled but not in facilitated systems the binding force enormously increases in the enclosed state, through a chelation effect. In both systems, steric interference with enclosure retards the translocation of bulky substrate analogs.