Neuronal assemblies: single cortical neurons are obedient members of a huge orchestra

Spontaneous cortical activity of single neurons is often either dismissed as noise, or is regarded as carrying no functional significance and hence is ignored. Our findings suggest that such concepts should be revised. We explored the coherent population activity of neuronal assemblies in primary sensory area in the absence of a sensory input. Recent advances in real-time optical imaging based on voltage-sensitive dyes (VSDI) have facilitated exploration of population activity and its intimate relationship to the activity of individual cortical neurons. It has been shown by in vivo intracellular recordings that the dye signal measures the sum of the membrane potential changes in all the neuronal elements in the imaged area, emphasizing subthreshold synaptic potentials and dendritic action potentials in neuronal arborizations originating from neurons in all cortical layers whose dendrites reach the superficial cortical layers. Thus, the VSDI has allowed us to image the rather illusive activity in neuronal dendrites that cannot be readily explored by single unit recordings. Surprisingly, we found that the amplitude of this type of ongoing subthreshold activity is of the same order of magnitude as evoked activity. We also found that this ongoing activity exhibited high synchronization over many millimeters of cortex. We then investigated the influence of ongoing activity on the evoked response, and showed that the two interact strongly. Furthermore, we found that cortical states that were previously associated only with evoked activity can actually be observed also in the absence of stimulation, for example, the cortical representation of a given orientation may appear without any visual input. This demonstration suggests that ongoing activity may also play a major role in other cortical function by providing a neuronal substrate for the dependence of sensory information processing on context, behavior, memory and other aspects of cognitive function.     

Coordinate induction of glutathione biosynthesis and glutathione-metabolizing enzymes is correlated with salt tolerance in tomato

The acclimation of reduced glutathione (GSH) biosynthesis and GSH-utilizing enzymes to salt stress was studied in two tomato species that differ in stress tolerance. Salt increased GSH content and GSH:GSSG (oxidized glutathione) ratio in oxidative stress-tolerant Lycopersicon pennellii (Lpa) but not in Lycopersicon esculentum (Lem). These changes were associated with salt-induced upregulation of gamma-glutamylcysteine synthetase protein, an effect which was prevented by preincubation with buthionine sulfoximine. Salt treatment induced glutathione peroxidase and glutathione-S-transferase but not glutathione reductase activities in Lpa. These results suggest a mechanism of coordinate upregulation of synthesis and metabolism of GSH in Lpa, that is absent from Lem.     

Metabolic complications of HIV-1 antiretroviral therapy: the lipodystrophy syndrome

The lipodystrophy syndrome is one of the complications reported with increased frequency in patients with HIV-1 infection receiving antiretroviral therapy. The wide range of prevalence estimates may be due to differing definitions, methods and patient populations. We described the various pathogenic theories and the morphological and metabolic alterations associated with this syndrome. Even if no effective treatment exists, a correct lifestyle, adequate diet and physical exercise seem to be very important. Moreover drug therapies should be used with care to avoid potentially harmful interactions with antiretroviral agents. Ideally, the future effort to define the mechanism of lipodystrophy would be multidisciplinary and would involve not only experts in AIDS research but also nutritionists, endocrinologists and cardiologists.     

Rat liver acyl-CoA synthetase 4 is a peripheral-membrane protein located in two distinct subcellular organelles,peroxisomes, and mitochondrial-associated membrane

Obesity and non-insulin-dependent diabetes favor storage of fatty acids in triacylglycerol over oxidation. Recently, individual acyl-CoA synthetase (ACS) isoforms have been implicated in the channeling of fatty acids either toward lipid synthesis or toward oxidation. Although ACS1 had been localized to three different subcellular regions in rat liver, endoplasmic reticulum, mitochondria, and peroxisomes, the study had used an antibody raised against the full-length ACS1 protein which cross-reacts with other isoforms, probably because all ACS family members contain highly conserved amino acid sequences. Therefore, we examined the subcellular location of ACS1, ACS4, and ACS5 in rat liver to determine which isoform was present in peroxisomes, whether the ACSs were intrinsic membrane proteins, and which ACS isoforms were up-regulated by PPAR alpha ligands. Non-cross-reacting ACS1, ACS4, and ACS5 peptide antibodies showed that ACS4 was the only ACS isoform present in peroxisomes isolated from livers of gemfibrozil-treated rats. ACS4 was also present in fractions identified as mitochondria-associated membrane (MAM). ACS1 was present in endoplasmic reticulum fractions and ACS5 was present in mitochondrial fractions. Incubation with troglitazone, a specific inhibitor of ACS4, decreased ACS activity in the MAM fractions 30-45% and in the peroxisomal fractions about 30%. Because the signal for ACS4 protein in peroxisomes was so strong compared to the MAM fraction, we examined ACS4 mRNA abundance in livers of rats treated with the PPAR alpha agonist GW9578. Treatment with GW9578 increased ACS4 mRNA abundance 40% and ACS1 mRNA 25%. Although we had originally proposed that ACS4 is linked to triacylglycerol synthesis, it now appears that ACS4 may also be important in activating fatty acids destined for peroxisomal oxidation. We also determined that, unlike ACS1 and 5, ACS4 is not an intrinsic membrane protein. This suggests that ACS4 is probably targeted and linked to MAM and peroxisomes by interactions with other proteins.     

A covarion-based method for detecting molecular adaptation: application to the evolution of primate mitochondrial genomes

A new method for detecting site-specific variation of evolutionary rate (the so-called covarion process) from protein sequence data is proposed. It involves comparing the maximum-likelihood estimates of the replacement rate of an amino acid site in distinct subtrees of a large tree. This approach allows detection of covarion at the gene or the amino acid levels. The method is applied to mammalian-mitochondrial-protein sequences. Significant covarion-like evolution is found in the (simian) primate lineage: some amino acid positions are fast-evolving (i.e. unconstrained) in non-primate mammals but slow-evolving (i.e. highly constrained) in primates, and some show the opposite pattern. Our results indicate that the mitochondrial genome of primates reached a new peak of the adaptive landscape through positive selection.     

Salt stress induces up-regulation of an efficient chloroplast antioxidant system in the salt-tolerant wild tomato species Lycopersicon pennellii but not in the cultivated species

The response of the chloroplastic antioxidant system of the cultivated tomato Lycopersicon esculentum (Lem) and its wild salt-tolerant related species L. pennellii (Lpa) to NaCl stress was studied. An increase in H₂O₂ level and membrane lipid peroxidation was observed in chloroplasts of salt-stressed Lem. In contrast, a decrease in these indicators of oxidative stress characterized chloroplasts of salt-stressed Lpa plants. This differential response of Lem and Lpa to salinity, correlates with the activities of the antioxidative enzymes in their chloroplasts. Increased activities of total superoxide dismutase (SOD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), glutathione-S-transferase (GST), phospholipid hydroperoxide glutathione peroxidase (PHGPX) and several isoforms of non-specific peroxidases (POD) were found in chloroplasts of salt-treated Lpa plants. In these chloroplasts, in contrast, activity of lipoxygenase (LOX) decreased while in those of salt-stressed Lem it increased. Although total SOD activity slightly increased in chloroplasts of salt-treated Lem plants, differentiation between SOD types revealed that only stromal Cu/ZnSOD activity increased. In contrast, in chloroplasts of salt-treated Lpa plants FeSOD activity increased while Cu/ZnSOD activity remained unchanged. These data indicate that salt-dependent oxidative stress and damage, suffered by Lem chloroplasts, was effectively alleviated in Lpa chloroplasts by the selective up-regulation of a set of antioxidative enzymes. Further support for the above idea was supplied by leaf discs experiments in which pre-exposure of Lpa plants to salt-treatment conferred cross-tolerance to paraquat-induced oxidative stress while increased oxidative damage by paraquat-treatment was found in salt-stressed Lem plants.     

The N-terminal region of Arabidopsis cystathionine gamma-synthase plays an important regulatory role in methionine metabolism

Cystathionine gamma-synthase (CGS) is a key enzyme of Met biosynthesis in bacteria and plants. Aligning the amino acid sequences revealed that the plant enzyme has an extended N-terminal region that is not found in the bacterial enzyme. However, this region is not essential for the catalytic activity of this enzyme, as deduced from the complementation test of an Escherichia coli CGS mutant. To determine the function of this N-terminal region, we overexpressed full-length Arabidopsis CGS and its truncated version that lacks the N-terminal region in transgenic tobacco (Nicotiana tabacum) plants. Transgenic plants expressing both types of CGS had a significant higher level of Met, S-methyl-Met, and Met content in their proteins. However, although plants expressing full-length CGS showed the same phenotype and developmental pattern as wild-type plants, those expressing the truncated CGS showed a severely abnormal phenotype. These abnormal plants also emitted high levels of Met catabolic products, dimethyl sulfide and carbon disulfide. The level of ethylene, the Met-derived hormone, was 40 times higher than in wild-type plants. Since the alien CGS was expressed at comparable levels in both types of transgenic plants, we further suggest that post-translational modification(s) occurs in this N-terminal region, which regulate CGS and/or Met metabolism. More specifically, since the absence of the N-terminal region leads to an impaired Met metabolism, the results further suggest that this region plays a role in protecting plants from a high level of Met catabolic products such as ethylene.     

Characterization of SP1, a stress-responsive,boiling-soluble,homo-oligomeric protein from aspen

In flowering plants, the vegetative nucleus and the two sperm cells are proposed to form a functional assemblage, the male germ unit (MGU). Here, we describe the developmental pathway of MGU assembly in Arabidopsis and report two classes of mutations that affect the integrity and/or the positioning of the MGU in the mature pollen grain. In germ unit malformed (gum) mutants, the vegetative nucleus is positioned adjacent to the pollen grain wall, separate from the two sperm cells, whereas in MGU displaced (mud) mutants, the intact MGU is displaced to the pollen grain wall. mud and gum mutants correspond to male-specific gametophytic mutations that also reduce pollen fitness. Genetic mapping showed that the gum1 and gum2 mutations are genetically linked, possibly allelic, whereas the mud1 and mud2 mutations correspond to two unlinked loci mapping on different chromosomes. The hierarchical relationship between mud and gum mutations was investigated by phenotypic analysis of double mutants. gum1 appeared to act earlier than mud1 and mud2, affecting initial MGU assembly and its stability during pollen maturation. In contrast, mud1 and mud2 mutations appear to act only on MGU positioning during final maturation. From in planta analyses of pollen germination in mud and gum mutants, we conclude that the initial proximity and positioning of MGU components is not required for their entrance into the pollen tube, but the efficiency of MGU translocation is reduced.     

Coupling of dopamine receptors to G proteins: studies with chimeric D2/D3 dopamine receptors

D₂ and D₃ dopamine receptors belong to the superfamily of G protein-coupled receptors; they share a high degree of homology and are structurally similar. However, they differ from each other in their second messenger coupling properties. Previously, we have studied the differential coupling of these receptors to G proteins and found that while D₂ receptor couples only to inhibitory G proteins, D₃ receptor couples also to a stimulatory G protein, Gs. We aimed to investigate the molecular basis of these differences and to determine which domains in the receptor control its coupling to G proteins. For this purpose four chimeras were constructed, each composed of different segments of the original D₂ and D₃ receptors. We have demonstrated that chimeras with a third cytoplasmic loop of D₂ receptor couple to Gi protein in a pattern characteristic of D₂ receptor. On the other hand chimeras containing a third cytoplasmic loop of D₃ receptor have coupling characteristics like those of D₃ receptor, and they couple also to Gs protein. These findings demonstrate that the third cytoplasmic loop determines and accounts for the coupling of dopamine receptors D₂ and D₃ to G proteins.