An intelligent tutoring system for supporting active learning: A case study on predictive parsing learning.

The way in which people learn and institutions teach is changing due to the ever-increasing impact of technology. People access the Internet anywhere, anytime and request online training. This has brought about the creation of numerous online learning platforms which offer comprehensive and effective educational solutions which are 100% online. These platforms benefit from intelligent tutoring systems that help and guide students through the learning process, emulating the behavior of a human tutor. However, these systems give the student little freedom to experiment with the knowledge of the subject, that is, they do not allow him/her to propose and carry out tasks on his/her own initiative. They are very restricted systems in term of what the student can do, as the tasks are defined in advance. An intelligent tutoring system is proposed in this paper to encourage students to learn through experimentation, proposing tasks on their own initiative, which involves putting into use all the skills, abilities tools and knowledge needed to successfully solve them. This system has been designed developed and applied for learning predictive parsing techniques and has been used by Computer Science students during four academic courses to evaluate its suitability for improving the student's learning process.

Insulin promotes the hydrolysis of a glycosyl phosphatidylinositol in cultured rat astroglial cells.

Glycosyl phosphatidylinositols have been implicated in insulin signaling through their action as precursors of second messenger molecules in peripheral tissues. In the present study, cultured rat astrocytes were used to investigate whether glycosyl phosphatidylinositol might be involved in the mechanism of insulin signal transduction in neural cells. A glycosyl phosphatidylinositol sensitive to hydrolysis by both phosphatidylinositol-specific phospholipase C and glycosyl phosphatidylinositol-specific phospholipase D and to nitrous acid deamination was purified. When astrocytes were exposed to 10 nM insulin, a rapid and significant reduction in the content of glycosyl phosphatidylinositol was observed within 1-2 min. In addition, an inverse concentration-dependent relationship between glycosyl phosphatidylinositol and diacylglycerol levels was found, suggesting a phospholipase C-mediated hydrolysis of glycosyl phosphatidylinositol in response to insulin. The effects of insulin were mediated through its own receptors and not through insulin-like growth factor (IGF)-I and/or IGF-II receptors, as demonstrated by affinity cross-linking studies. Also, the effects of 5 nM IGF-1 or 5 nM IGF-II on glycosyl phosphatidylinositol and diacylglycerol levels were different from those caused by insulin and were not essentially modified by pretreatment of the cells with either platelet-derived growth factor (PDGF) or epidermal growth factor (EGF). When cells were sequentially incubated with PDGF and EGF, a reduction in both glycosyl phosphatidylinositol and diacylglycerol contents was observed; the diacylglycerol but not the glycosyl phosphatidyl content was reversed after incubation with IGF-I, and especially with IGF-II, for 10 min. Despite the remarkable homology among insulin, IGF-I, and IGF-II, our results indicate that in astrocytes these compounds probably use different signal transduction pathways.

Isolation of a glycosyl-phosphatidylinositol (GPI) from rat brain.

Brain lipids were labelled with [1-14C]-isethionyl acetimidate and purified by sequential thin layer chromatography. Four labelled peaks were obtained, the first ones migrating with the same Rf as glycosyl-phosphatidylinositol (GPI). Further proof of the isolation of GPI was obtained by the observations that 44.8% of the radioactivity associated with the lipid in peak I was converted to the water phase by the effect of a PI-specific phospholipase C, and that the soluble material so obtained produced a dose-dependent inhibition of cAMP-dependent protein kinase activity. These findings indicate a biological equivalence between GPI and its polar head group from rat brain and those described in other cell types, and are consistent with the proposed role of these molecules in cellular signalling.

Insulin does not induce the hydrolysis of a glycosyl phosphatidylinositol in rat fetal hepatocytes.

An inositol phosphoglycan that is the polar head group of a glycosyl phosphatidylinositol has been considered as a putative mediator of insulin action. To gain insight into the functions of this hormone during development, the relationships between insulin, insulin receptors, glycosyl phosphatidylinositol, and inositol phosphoglycan were studied. Glycosyl phosphatidylinositol was isolated and characterized in fetal liver as early as day 15 of intrauterine life. In isolated hepatocytes from fetal and adult rats labeled with [3H]glucosamine, [3H]galactose, or [3H]myo-inositol, these molecules were incorporated into glycosyl phosphatidylinositol. In hepatocytes labeled with [3H]glucosamine and then allowed to react with [1-14C]IAI, the [3H]glycosyl phosphatidylinositol was purified as the 14C-labeled amidinated lipid. Glycosyl phosphatidylinositol molecules from fetal and adult cells were sensitive to hydrolysis by a phosphatidylinositol-specific phospholipase C from B. cereus. The product of this hydrolysis inhibits the activity of a cAMP-dependent protein kinase, whereas this effect was abolished by nitrous acid deamination. In isolated hepatocytes from adult animals, an inverse correlation between extracellular insulin and the number of insulin receptors and the cellular content of glycosyl phosphatidylinositol was observed. However, in fetal hepatocytes insulin failed to reduce the glycosyl-phosphatidylinositol content when labeled either with [1-14C]isethionyl acetimidate or [3H]glucosamine, whereas insulin-like growth factor I produced a significant hydrolysis of glycosyl phosphatidylinositol. Fetal and adult hepatocytes were incubated with insulin or inositol phosphoglycan after which glycogen phosphorylase activities were determined. Inositol phosphoglycan mimicked the action of insulin on both forms of the enzyme from adult hepatocytes, whereas in fetal cells insulin did not change, and purified inositol phosphoglycan reduced the activities of glycogen phosphorylase. These findings suggest a dissociation between insulin receptor occupancy and the expected hormonal effects in fetal hepatocytes. This could be related to alterations at a postreceptor level.

Evidence that circadian variations of circulating melatonin levels in fetal and suckling rats are dependent on maternal melatonin transfer.

Although the circadian variation of melatonin content in the pineal gland appears during the 3rd week of extrauterine life, recent studies suggest that the fetus perceives the day length through maternal melatonin transfer. Accordingly, we determined serum melatonin concentrations in pregnant and lactating rats and in their offsprings during the day (D) and at night (N). As compared with nonpregnant adult female rats (D: 6.0 +/- 0.5 and N: 112.0 +/- 5.0 pg/ml), significant increases of serum melatonin concentrations, both during the day and at night, were observed in 21-day pregnant (D: 21.0 +/- 2.6 and N: 222.0 +/- 7.2 pg/ml) and in 10-day (D: 20.0 +/- 2.6 and N: 145.0 +/- 12.3 pg/ml) and 20-day (D: 19.0 +/- 1.2 and N: 140.0 +/- 8.0 pg/ml) lactating animals, while a profound decrease was found in 5-day (D: 13.0 +/- 1.0 and N: 50.0 +/- 5.6 pg/ml) lactating rats. In addition, melatonin levels were significantly higher during the night than during the day in all the experimental groups. High levels and circadian variation of serum melatonin were detected also in 21-day-old fetuses (D: 13.0 +/- 0.8 and N: 108.0 +/- 4.8 pg/ml) and in 5- and 10-day-old suckling (in 5-day-old, D: 13.0 +/- 0.6 and N: 71.0 +/- 3.1 pg/ml; in 10-day-old, D: 17.0 +/- 1.4 and N: 63.0 +/- 3.8 pg/ml) rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholesterol exchange between human serum isolated lipoproteins. Effect of protein concentration.

The in vitro transfer of radiolabelled unesterified cholesterol from human low- and very low- to high-density lipoproteins in the presence of either lipoprotein deficient serum or bovine serum albumin has been studied. The rate of transfer was faster from LDL (t1/2 = 44 min) than from VLDL (55 min). The presence of 2% protein had no effect on the transfer. However, 10% albumin or lipoprotein deficient serum reduced t1/2 by 35%, which indicates no specific effect of a plasma protein on the rate of transfer of unesterified cholesterol.

Free cholesterol transfer from human lower-density lipoproteins (d less than 1.063) to lipoprotein-deficient serum and high-density lipoproteins.

The in vitro transfer of free cholesterol (FC) between human serum lipoproteins in the absence of lecithin:cholesterol acyltransferase (LCAT) activity has been examined. The results show that the amount of FC that the high-density lipoprotein (HDL) and lipoprotein-deficient serum (LDS) fractions were able to capture from low-density lipoproteins (LDL) and very-low-density lipoproteins (VLDL) was proportional to the amount of FC present in d less than 1.063 lipoproteins. The presence of HDL increased this transfer markedly. These results indicate that, in the absence of LCAT activity, FC can transfer from lower-density lipoproteins to higher-density serum fractions, and this transfer might increase under hypercholesterolemic conditions. The possible importance of this phenomena in regard to the exchange of FC between serum lipoproteins and tissue cells is discussed.

Asymmetric distribution of the phosphatidylinositol-linked phospho-oligosaccharide that mimics insulin action in the plasma membrane.

We have investigated the topography of a glycosyl-phosphatidylinositol implicated in insulin action by a combination of two complementary methods: (a) chemical labelling with a non-permeable (isethionyl acetimidate) and a permeable (ethyl acetimidate) probe; and (b) enzymatic modifications with beta-galactosidase (EC 3.2.1.23) or phosphatidylinositol-specific phospholipase C (EC 3.1.4.3). Using the first approach the majority of the glycosyl-phosphatidylinositol is found in the outer surface of intact hepatocytes, adipocytes, fibroblasts and lymphocytes, but not in erythrocytes which presented only a 20% of the total labelled glycosyl-phosphatidylinositol to the exterior. Upon insulin addition (10 nM), about 60% of the total glycosyl-phosphatidylinositol was hydrolysed in both hepatocytes and adipocytes but not in erythrocytes. In agreement with the extracellular localization in hepatocytes and with the proposed role of this glycolipid in insulin action, treatment of rat hepatocytes with beta-galactosidase from Escherichia coli, an enzyme that hydrolyses the oligosaccharide moiety of the glycosyl-phosphatidylinositol, cleaved 65% of the total glycophospholipid and blocked the effect of insulin (but not of glucagon) on pyruvate kinase (EC 2.7.1.40). Similar treatment with phosphatidylinositol-specific phospholipase C from Bacillus cereus hydrolysed 62% of the total glycosyl-phosphatidylinositol. From the various approaches used it is concluded that the majority of this glycophospholipid is at the outer surface in a variety of insulin-sensitive cells.

Insulin induces a similar reduction in the concentrations of its own receptor and of an insulin-sensitive glycosyl-phosphatidylinositol in isolated rat hepatocytes.

We have used isolated rat hepatocytes to study whether the insulin-induced reduction of its own receptors may modify the transduction of hormone signals by changes in the content of a glycosyl-phosphatidylinositol. Both subsequent insulin binding and glycosyl-phosphatidylinositol concentrations markedly decreased as a function of time and insulin concentration during preincubation of hepatocytes with insulin. The modifications observed in insulin binding were due to changes in receptor concentration. These results show that insulin regulates both the number of its own receptors and glycosyl-phosphatidylinositol concentrations in target cells, which may be of interest in many pathophysiological situations.

Different phosphorylated forms of an insulin-sensitive glycosylphosphatidylinositol from rat hepatocytes.

Labeling with [3H]galactose was employed to isolate a glycosylphosphatidylinositol from rat hepatocytes which might be involved in the action of insulin. The polar head group of this glycosylphosphatidylinositol was generated by phosphodiesterase hydrolysis with a phosphatidylinositol-specific phospholipase C from Bacillus cereus. By Dowex AG1 x 8 chromatography the polar head group could be separated into three radioactive peaks eluting at 100 mM (peak I), 200 mM (peak II) and 500 mM (peak III) ammonium formate, respectively. Peak III was the most active as an inhibitor of the cAMP-dependent protein kinase. Treatment of peak III with alkaline phosphatase markedly reduced its activity on cAMP-dependent protein kinase. When peaks I, II or III were treated with alkaline phosphatase and analyzed again by Dowex AG1 x 8 chromatography, the radioactivity eluted with the aqueous fraction. The above results indicate that the polar head group of the insulin-sensitive glycosylphosphatidylinositol from rat hepatocytes exists in three different phosphorylated forms and that the biological activity of this molecule depends on its phosphorylation state.

Localisation of the insulin-sensitive phosphatidylinositol glycan at the outer surface of the cell membrane.

A phosphatidylinositol-glycan has been implicated in the mechanism of action of insulin. Some of the actions of insulin may be mediated by the generation of the polar head group of this phosphatidylinositol-glycan. Localisation of the insulin-sensitive phosphatidylinositol-glycan was investigated by reacting the glycophospholipid with the imidoester [1-14C]-isethionyl acetimidate. The present results indicate that most of the insulin-sensitive phosphatidylinositol-glycan is localized at the plasma membrane of rat liver, being 85% of the glycophospholipid present at the outer surface of the cell. These results suggest a paracrine action of insulin.

Differential precipitation of isolated human plasma lipoproteins with heparin and manganese chloride.

We studied the precipitation of isolated lipoproteins with heparin and MnCl2. Lipoproteins were isolated from human plasma by preparative ultracentrifugation and their free cholesterol was labeled. Each lipoprotein fraction was then precipitated at various pHs, with or without bovine serum albumin (60 g/L) present. Under no set of conditions was one class of lipoproteins completely separated from the other two. Specifically, under standard conditions for precipitation of serum lipoproteins (pH 7.4 and protein 60 g/L), 12% of the very-low-density lipoprotein (VLDL) and 8% of the low-density lipoprotein (LDL) remained in the supernatant liquid, and 30% of the high-density lipoprotein (HDL) was precipitated. These results indicate that, under these conditions, so-called HDL cholesterol may be a mixture of VLDL, LDL, and HDL, although the sum of the amount of these three fractions remaining in the supernate is fortuitously very close to the value for HDL cholesterol isolated by ultracentrifugation.

Estrogen binding in rat liver during pregnancy.

Estrogen binding in the liver of pregnant rats has been studied. When the results are expressed in pmol/mg of protein there is a marked decrease in relation to control rats on 12th or 21st days of gestation. In spite of the liver weight increase on day 21, however, the binding capacity for estrogens in the whole liver is still lower than in controls. It is suggested that the changes in rat liver estrogen receptors in late gestation cannot explain the action of estrogens on triglyceride rich lipoprotein synthesis and the consequent hypertriglyceridemia of late gestation.

Apparent inhibition of cholesteryl esters exchange by high density lipoproteins.

The kinetics of the exchange of Cholesteryl esters between low density lipoproteins (LDL) and high density lipoproteins (HDL) stimulated by lipoprotein depleted plasma has been studied in vitro. The results indicate that the exchange is inhibited with the increase of HDL present in the assay, although the limiting factor is not the absolute concentration of HDL, since in a simultaneous LDL increase, the exchange augments proportionally to the total cholesteryl esters pool. Implications regarding overall metabolism of body cholesterol are discussed.