Dietary Carbohydrates and Insulin Resistance in Adolescents from Marginalized Areas of Chiapas, México.

Evidence of the role that dietary carbohydrates (total carbohydrates, dietary fiber, total sugars, dietary glycemic index (GI) and glycemic load (GL)) exerts on insulin levels in adolescents is controversial. Thus, the aim of this study was to assess the association between dietary carbohydrates and insulin resistance in adolescents from Chiapas, México. A cross-sectional study was conducted in 217 adolescents. Sociodemographic, anthropometric, dietary and biochemical data were obtained. Total carbohydrates, dietary fiber, total sugars, dietary GI and GL were calculated from 24 h recalls. Two validated cut-off points for the homeostasis model assessment of insulin resistance (HOMA-IR) were used as surrogates of insulin resistance. Fasting insulin levels ≥ 14.38 µU/mL were considered as abnormal. Multivariate logistic regression models were fitted to assess the association between tertiles of dietary carbohydrates and insulin resistance or hyperinsulinemia. In our study, adolescents with the highest dietary fiber intake had lower odds of HOMA-IR > 2.97 (OR = 0.34; 95% CI: 0.13-0.93) when adjusted for sex, age, body fat percentage and saturated fatty acids intake. No significant associations were found for the rest of the carbohydrate variables. In summary, high-fiber diets reduce the probability of insulin resistance in adolescents from marginalized areas of Chiapas, México.

Characterization of the cystine/glutamate antiporter in cultured Bergmann glia cells.

Glutamate, the major excitatory transmitter in the vertebrate brain is a potent neurotoxin through the over-stimulation of its specific membrane receptors. In accordance, a tight regulation of its extracellular levels by plasma membrane transporters is present. A family of excitatory amino acid transporters is expressed in neurons and glia cells and is responsible of the removal of the neurotransmitter from the synaptic cleft. Glial transporters account for more than 80% of the brain uptake activity. The cystine/glutamate antiporter is another plasma membrane-bound protein critically involved in glutamatergic transmission. Upon oxidative stress, it begins to pump out glutamate in exchange for cystine, mostly needed for glutathione production. Taking into consideration that all of these glutamate transporter proteins are present in glia cells that surround glutamatergic synapses, we reasoned that a functional coupling of them should exist to prevent an excitotoxic insult to the neighboring neuronal cells. To this end, we used the established model of chick cerebellar Bergmann glia cultures. Once we could establish the expression of the cystine/glutamate antiporter in our system, we characterized its kinetic properties and started to gain insight into its regulation and plausible coupling to other transporters. Exposure to glutamate reduces the uptake activity and favors a physical interaction with the excitatory amino acid transporter 1 and the Na+-dependent neutral amino acids transporter 3. In contrast, treatment of the cultured cells with a nitric oxide donor such as sodium nitroprussiate augments the exchanger activity. Longer sodium nitroprussiate exposure periods down-regulates the cystine/glutamate protein levels. These results suggest that a coordinated interplay between glutamate transporters and exchangers takes place in glia cells to prevent excitotoxic insults.

Coupling of glutamate and glucose uptake in cultured Bergmann glial cells.

Glutamate, the main excitatory neurotransmitter in the vertebrate brain, exerts its actions through specific membrane receptors present in neurons and glial cells. Over-stimulation of glutamate receptors results in neuronal death, phenomena known as excitotoxicity. A family of sodium-dependent, glutamate uptake transporters mainly expressed in glial cells, removes the amino acid from the synaptic cleft preventing neuronal death. The sustained sodium influx associated to glutamate removal in glial cells, activates the sodium/potassium ATPase restoring the ionic balance, additionally, glutamate entrance activates glutamine synthetase, both events are energy demanding, therefore glia cells increase their ATP expenditure favouring glucose uptake, and triggering several signal transduction pathways linked to proper neuronal glutamate availability, via the glutamate/glutamine shuttle. To further characterize these complex transporters interactions, we used the well-established model system of cultured chick cerebellum Bergmann glia cells. A time and dose-dependent increase in the activity, plasma membrane localization and protein levels of glucose transporters was detected upon d-aspartate exposure. Interestingly, this increase is the result of a protein kinase C-dependent signaling cascade. Furthermore, a glutamate-dependent glucose and glutamate transporters co-immunoprecipitation was detected. These results favour the notion that glial cells are involved in glutamatergic neuronal physiology.

Glia plasma membrane transporters: Key players in glutamatergic neurotransmission.

Glutamate, the main excitatory amino acid in the central nervous system, elicits its functions through the activation of specific membrane receptors that are expressed in neurons and glial cells. The re-cycling of this amino acid is carried out mostly through a continuous interplay between neurons and glia cells, given the fact that the removal of glutamate from the synaptic cleft depends mainly on glial glutamate transporters. Therefore, a functional and physical interaction between membrane transporters links glutamate uptake, transformation to glutamine and its release to the extra-synaptic space and its uptake to the pre-synaptic terminal. This sequence of events, best known as the glutamate/glutamine shuttle is central to glutamatergic transmission. In this sense, the uptake process triggers a complex series of biochemical cascades that modify the physiology of glial cells in the immediate, short and long term so as to be capable to take up, transform and release these amino acids in a regulated amount and in an appropriate time frame to sustain glutamatergic neurotransmission. Among the signaling cascades activated in glial cells by glutamate transporters, a sustained Na(+) and Ca(2+) influx, protein posttranslational modifications and gene expression regulation at the transcriptional and translational levels are present. Therefore, it is clear that the pivotal role of glial cells in the context of excitatory transmission has been constantly underestimated.

Glutamate Receptor Stimulation Up-Regulates Glutamate Uptake in Human Müller Glia Cells.

Glutamate, the main excitatory amino acid in the vertebrate retina, is a well know activator of numerous signal transduction pathways, and has been critically involved in long-term synaptic changes acting through ionotropic and metabotropic glutamate receptors. However, recent findings underlining the importance of intensity and duration of glutamate stimuli for specific neuronal responses, including excitotoxicity, suggest a crucial role for Na(+)-dependent glutamate transporters, responsible for the removal of this neurotransmitter from the synaptic cleft, in the regulation of glutamate-induced signaling. Transporter proteins are expressed in neurons and glia cells, albeit most of glutamate uptake occurs in the glial compartment. Within the retina, Müller glia cells are in close proximity to glutamatergic synapses and participate in the recycling of glutamate through the glutamate/glutamine shuttle. In this context, we decided to investigate a plausible role of glutamate as a regulatory signal for its own transport in human retinal glia cells. To this end, we determined [(3)H]-D-aspartate uptake in cultures of spontaneously immortalized human Müller cells (MIO-M1) exposed to distinct glutamatergic ligands. A time and dose-dependent increase in the transporter activity was detected. This effect was dependent on the activation of the N-methyl D-aspartate subtype of glutamate receptors, due to a dual effect: an increase in affinity and an augmented expression of the transporter at the plasma membrane, as established via biotinylation experiments. Furthermore, a NMDA-dependent association of glutamate transporters with the cystoskeletal proteins ezrin and glial fibrillary acidic protein was also found. These results add a novel mediator of the glutamate transporter modulation and further strengthen the notion of the critical involvement of glia cells in synaptic function.

Functional plasticity of GAT-3 in avian Müller cells is regulated by neurons via a glutamatergic input.

GABA (γ-amino butyric acid) is the major inhibitory transmitter in the central nervous system and its action is terminated by specific transporters (GAT), found in neurons and glial cells. We have previously described that GAT-3 is responsible for GABA uptake activity in cultured avian Müller cells and that it operates in a Na(+) and Cl(-) dependent manner. Here we show that glutamate decreases [(3)H] GABA uptake in purified cultured glial cells up to 50%, without causing cell death. This effect is mediated by ionotropic glutamatergic receptors. Glutamate inhibition on GABA uptake is not reverted by inhibitors of protein kinase C or modified by agents that modulate cyclic AMP/PKA. Biotinylation experiments demonstrate that this reduction in GABA uptake correlates with a decrease in GAT-3 plasma membrane levels. Interestingly, both GAT-1 and GAT-3 mRNA levels are also decreased by glutamate. Conditioned media (CM) prepared from retinal neurons could also decrease GABA influx, and glutamate receptor antagonists (MK-801 + CNQX) were able to prevent this effect. However, glutamate levels in CM were not different from those found in fresh media, indicating that a glutamatergic co-agonist or modulator could be regulating GABA uptake by Müller cells in this scenario. In the whole avian retina, GAT-3 is present from embryonic day 5 (E5) increasing up to the end of embryonic development and post-hatch period exclusively in neuronal layers. However, this pattern may change in pathological conditions, which drive GAT-3 expression in Müller cells. Our data suggest that in purified cultures and upon extensive neuronal lesion in vivo, shown as a Brn3a reduced neuronal cells and an GFAP increased gliosis, Müller glia may change its capacity to take up GABA due to GAT-3 up regulation and suggests a regulatory interplay mediated by glutamate between neurons and glial cells in this process.