River macrophyte indices: not the Holy Grail!

1. Recent studies have demonstrated that there is generally no unambiguous relationship between plant species composition and specific environmental conditions in rivers. Nevertheless, indices of environmental pressures based on macrophytes are flourishing, because of the requirements of the Water Framework Directive (WFD). 2. We first reviewed nine such indices against 13 criteria for bioindicators. Then, using data from France and England, we tested whether the IBMR (Macrophyte Biological Index for Rivers) and LEAFPACS (predictions and classification system for macrophytes) methods could reliably indicate nutrient and hydromorphological pressures. Finally, we used an improved bootstrapping method to estimate accuracy. 3. Currently, most indices lack ecological meaning for a variety of reasons, including partial sampling (backwaters are excluded); reliance on list of taxa (there are identification difficulties) rather than structure and functions; correlation rather than causation; application within a limited biogeographical area; reliance on ‘expert’ judgement; high precision but poor accuracy; poorly defined reference conditions; lack of independent tests; and an inability to discriminate reliably between the target pressures of interest from confounding background variables. 4. IBMR was a far better indicator of pH (or HCO₃‐pCO₂) than it was of soluble reactive phosphorus, SRP (or SRP‐NH₄). While there was a highly significant correlation between IBMR and SRP after removing the effect of pH, the relationship was weak (r² = 0.08, n = 215, P < 0.001). 5. LEAFPACS is a multi‐metric method summing up five individual indices, each compliant with the WFD. Its individual metrics were not better correlated with nutrient and hydromorphological pressures (with r² < 0.1, n = 62, P < 0.05) than was the IBMR. The meaning of the overall metric is questionable. 6. There are problems in determining the precision of the indices, owing to uncertainties in recording, but they are less than the uncertainties in determining accuracy (because species optima and tolerances are sometimes poorly known). 7. Reliable information is needed to improve the state of our rivers. Macrophyte indices are able to detect statistically significant pressures from a large population of sites but cannot be applied at specific sites, as required by the WFD, owing to large uncertainties and low explanatory power. Typically, more than 90% of the variability in macrophyte indices is attributed to factors other than human pressure. The WFD would be better served by a simpler, holistic approach based on our current mechanistic understanding of river processes. These findings are likely to apply also to other taxonomic groups (macroinvertebrates, diatoms, fish) used in the assessment of purported ecological quality and to palaeolimnological measures of reference status.     

Neural circuitry: seeing the parts that make the picture

What are the neural correlates of vision? A recent study on Drosophila has described the incredible neuronal diversity in the fly visual system, and traced the circuits that underlie color vision.     

Reactive oxygen species act remotely to cause synapse loss in a Drosophila model of developmental mitochondrial encephalopathy

Mitochondrial dysfunction is a hallmark of many neurodegenerative diseases, yet its precise role in disease pathology remains unclear. To examine this link directly, we subtly perturbed electron transport chain function in the Drosophila retina, creating a model of Leigh Syndrome, an early-onset neurodegenerative disorder. Using mutations that affect mitochondrial complex II, we demonstrate that mild disruptions of mitochondrial function have no effect on the initial stages of photoreceptor development, but cause degeneration of their synapses and cell bodies in late pupal and adult animals. In this model, synapse loss is caused by reactive oxygen species (ROS) production, not energy depletion, as ATP levels are normal in mutant photoreceptors, and both pharmacological and targeted genetic manipulations that reduce ROS levels prevent synapse degeneration. Intriguingly, these manipulations of ROS uncouple synaptic effects from degenerative changes in the cell body, suggesting that mitochondrial dysfunction activates two genetically separable processes, one that induces morphological changes in the cell body, and another that causes synapse loss. Finally, by blocking mitochondrial trafficking into the axon using a mutation affecting a mitochondrial transport complex, we find that ROS action restricted to the cell body is sufficient to cause synaptic degeneration, demonstrating that ROS need not act locally at the synapse. Thus, alterations in electron transport chain function explain many of the neurodegenerative changes seen in both early- and late-onset disorders.     

Diet fat influences liver plasma-membrane lipid composition and glucagon-stimulated adenylate cyclase activity.

Rats were fed diets that differed in fatty acid composition or in the proportion of energy derived from fat to determine if alteration of dietary fat intake influences the structural lipid composition of liver plasma membrane and the expression of an associated hormone-receptor-mediated function. Weanling rats were fed 9% (w/w) or 20% (w/w) low-erucic acid rape-seed oil or 9% (w/w) soya-bean oil for 24 days. Plasma membranes were isolated and the effect of diet fat on the fatty acid composition of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol and sphingomyelin was determined. Diet fat significantly altered total saturated and (omega-9) and (omega-6)-unsaturated fatty acid composition in addition to the (omega-6)- to (omega-3)-unsaturated fatty acid ratio in these polar lipids. Feeding the high-fat diet increased the (omega-6)- to (omega-3)-unsaturated fatty acid ratio and the (omega-9)-unsaturated fatty acid content in all lipids except sphingomyelin. Assay of glucagon-stimulated adenylate cyclase activity at both high and low glucagon concentrations indicated that high-fat intake also decreased cyclic AMP formation. In a second experiment the fat intake was held constant (40% of energy) and oleic acid was substituted for linoleic acid by blending high- and low-linoleic acid-type safflower oils. This experiment established that a dose-response relationship exists between dietary intake of fatty acid and the fatty acid composition of plasma-membrane phospholipids. Specific diet-induced transitions in membrane phospholipid fatty acid composition were paralleled by changes in glucagon-stimulated adenylate cyclase activity. This study suggests that transitions in dietary fat intake can alter a hormone-receptor-mediated enzyme function in vivo by changing the surrounding lipid environment.     

Fish oil-enriched diet is mucosal protective against acetic acid-induced colitis in rats.

Products of arachidonic acid metabolism are elevated in patients with inflammatory bowel disease and this elevation is correlated with disease activity. Eicosapentaenoic acid competes with arachidonic acid and alters eicosanoid biosynthesis. In this experiment, the possibility that eicosapentaenoic acid could be used in the treatment of inflammatory bowel disease was investigated by determining the effect of 6 weeks of a fish oil-supplemented diet, enriched in eicosapentaenoic acid, on colonic and ileal morphology, histology, and in vivo fluid absorption in rats with 4% acetic acid-induced colitis. The results of an eicosapentaenoic acid-enriched diet were compared with results of saturated and polyunsaturated fatty acid-enriched diets. In rats with misoprostol pretreated acetic acid-induced colitis, an eicosapentaenoic acid-enriched diet reversed net colonic fluid secretion to absorption and prevented macroscopic and histologic injury, compared with saturated and poly-unsaturated fatty acid-enriched diets, which did not. The fish oil mucosal protective effect occurred in the presence of a 30-fold enhancement of PGE2 synthesis. In rats with non-misoprostol pretreated acetic acid-induced colitis, an eicosapentaenoic acid-enriched diet returned ileal fluid absorption to control levels, as compared with saturated and polyunsaturated fatty acid-enriched diets, which did not. In conclusion, a fish oil (eicosapentaenoic acid)-enriched diet, but not a saturated- or a polyunsaturated-enriched diet, protected colonic and ileal net fluid absorption in an experimental model of inflammatory bowel disease.     

Influence of dietary fat on the lipid composition of rat brain synaptosomal and microsomal membranes.

The modulation of rat brain microsomal and synaptosomal membrane lipid by diet fat was examined. Brain synaptosomal and microsomal membrane composition was compared for rats fed on diets containing either soya-bean oil (SBO), SBO plus choline, SBO lecithin, sunflower oil (SFO), chow or low-erucic acid rape-seed oil (LER) for 24 days. Cholesterol and phosphatidylcholine levels in both membranes were altered by diet. Diet fat also affected the microsomal content of sphingomyelin. Change in membrane phosphatidylcholine level was related to the relative balance of omega-6, omega-3 and monounsaturated fatty acids within the diets fed. The highest phosphatidylcholine levels appeared in membranes of animals fed on SBO lecithin and the lowest in those fed on LER. Microsomal membrane cholesterol and sphingomyelin content increased by feeding on SBO lecithin. In both synaptosomal and microsomal membranes a highly significant correlation was observed between membrane phosphatidylcholine and cholesterol content. The fatty acyl composition of phospholipids from both membranes also altered with diet and age. Alteration in fatty acid composition was observed in response to dietary levels of omega-6, omega-3 and monounsaturated fatty acids, but the unsaturation index of each phospholipid remained constant for all diet treatments. These changes in lipid composition suggest that dietary fat may be a significant modulator in vivo of the physicobiochemical properties of brain synaptosomal and microsomal membranes.     

Synthesis of chain elongation-desaturation products of linoleic acid by liver and brain microsomes during development of the pig.

Microsomes isolated from liver and brain tissue were assayed to examine transitions in metabolic capability to synthesize chain elongation-desaturation products of C18:2(9,12) during the perinatal development of the pig. Microsomal synthesis of trienes, tetraenes and pentaenes was compared for fetal, neonatal and postnatal piglets. Rates of synthesis of tetraenes and pentaenes by chain elongation-desaturation of C18:2(9,12) were greatest in liver. During the later half of gestation, the capability to synthesize tetraenes increased 2-3-fold on a per mg of microsomal protein basis. Increase in the capacity to synthesize tetraenes suggests a significant transition in the activity of delta 5 desaturase during the last half of gestation. For brain, synthesis of C22:5(4,7,10,13,16) from C18:2(9,12) was greatest at term. These observations indicate that in liver and brain the capability to chain elongate-desaturate C18:2(9,12) to longer chain homologues increases significantly during early development. It is suggested that during gestation the activity of the delta 5 desaturase limits synthesis of C20 and C22 homologues of C18:2(9,12). The metabolic conversion of C20 and C22 fatty acids by chain elongation of C20:4(5,8,11,14) does not appear to limit the synthesis of very long chain homologues of linoleic acid in fetal liver or brain.     

Delta 5-desaturase activity in liver and brain microsomes during development of the pig.

Microsomes isolated from liver and brain tissue were assayed to examine transitions in metabolic capability to synthesize tetraenes and pentaenes by chain elongation-desaturation of C20:3(8,11,14) during the perinatal development of the pig. Rates of synthesis of tetraenes and pentaenes by chain elongation-desaturation of C20:3(8,11,14) were greatest in liver. During the latter half of gestation, the capability to synthesize tetraenes increased 7- or 23-fold on a per mg of microsomal protein basis for brain and liver respectively. Increase in the capacity to synthesize tetraenes from C20:3(8,11,14) suggests a significant transition in the activity of the delta 5-desaturase during the last half of gestation. These observations indicate that in liver and brain the capability to chain elongate-desaturate C18:2(9,12) to longer chain homologues increases significantly during early development as a function of transitions in the activity of the delta 5-desaturase.     

Phosphatidylethanolamine methyltransferase: evidence for influence of diet fat on selectivity of substrate for methylation in rat brain synaptic plasma membranes

Male weanling rats were fed diets containing 20% (w/w) fat differing in fatty acid composition for 24 days. Synaptic plasma membranes were isolated from the brain and the fatty acid composition of phosphatidylethanolamine and phosphatidylcholine was determined. In vitro assays of phosphatidylethanolamine methyl-transferase activity were performed on fresh membrane samples to assess effect of dietary fat on the rate of phosphatidylethanolamine methylation for phosphatidylcholine synthesis via the phosphatidylethanolamine methyltransferase pathway. Dietary level of n-6 and ratio of n-6 to n-3 fatty acids influenced membrane phospholipid fatty acid composition and activity of the lipid-dependent phosphatidylethanolamine methyltransferase pathway. Rats fed a diet rich in n-6 fatty acids produced a high ratio of n-6/n-3 fatty acids in synaptosomal membrane phosphatidylethanolamine, and elevated rates of methylation of phosphatidylethanolamine to phosphatidylcholine by phosphatidylethanolamine methyltransferases, suggesting that the pathway exhibits substrate selectivity for individual species of phosphatidylethanolamine containing long-chain homologues of dietary n-6 and n-3 fatty acids (20:4(n-6), 22:4(n-6), 22:5(n-6) and 22:6(n-3). It may be concluded that diet alters the membrane content of n-6, n-3 and monounsaturated fatty acids, and that change in phosphatidylethanolamine species available for methylation to phosphatidylcholine alters the rate of product synthesis in vivo by the phosphatidylethanolamine methyltransferase pathway.     

Dietary ganglioside decreases cholesterol content, caveolin expression and inflammatory mediators in rat intestinal microdomains

Membrane microdomains rich in cholesterol and sphingolipids, including gangliosides (GGs), are known to be important regions for cell signaling and binding sites for various pathogens. Cholesterol depletion inhibits the cellular entry of pathogens and also reduces inflammatory signals by disrupting microdomain structure. Our previous study showed that dietary gangliosides increased total ganglioside incorporation while decreasing cholesterol in the intestinal mucosa. We hypothesized that diet-induced reduction in cholesterol content in the intestinal mucosa disrupts microdomain structure resulting in reduced pro-inflammatory signals. Male weanling Sprague-Dawley rats were fed semipurified diets for 2 weeks. Experimental diets were formulated to include either ganglioside-enriched lipid (GG diet, 0.02% gangliosides [w/w of diet] ) or polyunsaturated fatty acid (PUFA diet, 1% arachidonic acid and 0.5% docosahexaenoic acid, w/w of total fat), in a control diet containing 20% fat. Levels of cholesterol, GG, caveolin, platelet activating factor (PAF), and diglyceride (DG) were measured in the microdomain isolated from the intestinal brush border. The GG diet increased total gangliosides by 50% with a relative increase in GD3 and a relative decrease in GM3. Cholesterol content was also reduced by 23% in the intestinal microdomain. These changes resulted in a significant decrease in the ratio of cholesterol to ganglioside. The GG diet and the PUFA diet were both associated with reduction in caveolin, PAF, and DG content in microdomains, whereas no change occurred in the ganglioside profile of animals fed the PUFA diet. Dietary gangliosides decrease the cholesterol/ganglioside ratio, caveolin, PAF and DG content in microdomains thus exerting a potential anti-inflammatory effect during gut development.