Stressed cybrids model demyelinated axons in multiple sclerosis

Multiple sclerosis is likely caused by a complex interaction of multiple genes and environmental factors. The contribution of mitochondrial DNA genetic backgrounds has been frequently reported. To evaluate the effect of mitochondrial DNA haplogroups in the same genetic and environmental circumstances, we have built human transmitochondrial cell lines and simulated the effect of axon demyelination, one of the hallmarks of multiple sclerosis pathology, by altering the ionic gradients through the plasmalemma and increasing ATP consumption. In this model, mitochondrial biogenesis is observed. This process is larger in Uk cybrids, which mirrors their lower oxidative phosphorylation capacity in basal conditions. This model replicates a process occurring in both patients suffering from multiple sclerosis and several animal models of axon demyelination. Therefore, it can be used to analyze the contribution of various mitochondrial DNA genotypes to multiple sclerosis. In this sense, a longer or stronger energy stress, such as that associated with demyelinated axons in multiple sclerosis, could make Uk individuals more susceptible to this pathology. Finally, pharmacologic compounds targeted to mitochondrial biogenesis could be a potential therapy for multiple sclerosis.

Survival and mitochondrial function in septic patients according to mitochondrial DNA haplogroup

Introduction

We recently found that platelet cytochrome c oxidase (COX) activities and quantities in 6-month-survival septic patients are significantly higher than those of patients who died before 6 months. Other studies suggested that the mitochondrial DNA (mtDNA) genotype could play a major role in sepsis survival. Given that COX catalytic subunits are encoded by mtDNA, the objective of the present study was to explore whether mtDNA population genetic variation could affect COX activity and quantity and favors sepsis survival.

Methods

A prospective, multicenter, observational study was carried out in six Spanish ICUs. We included 96 patients with severe sepsis. We determined the mtDNA haplogroup, the COX specific activity/citrate synthase specific activity (COXa/CSa) ratio and the COX quantity/citrate synthase specific activity (COXq/CSa) ratio in circulating platelets at the time of diagnosis, day 4 and day 8. We used survival at 1 and 6 months as endpoints.

Results

Patients with the JT mtDNA haplogroup (n = 15) showed higher COXq/CSa ratio at day 4 (P = 0.04) and day 8 (P = 0.02) than those with other haplogroups (n = 81). Logistic regression analysis showed that the JT mtDNA haplogroup (odds ratio = 0.18; 95% confidence interval = 0.04 to 0.94; P = 0.04) and COXq/CSa ratio (odds ratio = 0.53; 95% confidence interval = 0.30 to 0.93; P = 0.03) were associated with 1-month survival after controlling for age and lactic acid levels.

Conclusions

The novel findings of our study are that 1-month surviving septic patients showed higher COXq/CSa ratio than nonsurviving individuals, that patients from the JT mtDNA haplogroup showed a higher COXq/CSa ratio and that JT patients had a higher 1-month survival than patients from other mtDNA haplogroups.

Melatonin inhibits serotonin transporter activity in intestinal epithelial cells

Gastrointestinal serotonin (5-HT) and melatonin are two closely related neuromodulators which are synthesised in the enterochromaffin cells of the intestinal epithelium and which have been shown to be involved in the physiopathology of the gastrointestinal tract. The effects of 5-HT depend on 5-HT availability which is, in part, modulated by the serotonin transporter (SERT). This transporter provides an efficient 5-HT uptake after release and is expressed in the membrane of the enterocytes. Although the origin and effects of 5-HT and melatonin are similar, the interrelationship between them in the gastrointestinal tract is unknown. The main aim of this study was to determine whether melatonin affects SERT activity and expression, and, if so, to elucidate the mechanisms involved. Caco-2 cell line was used to carry out the study as these cells have been shown to endogenously express SERT. The results showed that melatonin inhibits SERT activity by affecting both V(max) and kt kinetic constants although SERT synthesis or intracellular trafficking did not appear to be affected. The melatonin effect seemed to be independent of melatonin receptors MT(1) and MT(2) and protein kinase C and cAMP intracellular pathways. Our results suggest that the inhibition of SERT might be due to a catalytic effect of melatonin on the allosteric citalopram-sensitive site in SERT. This study shows, for the first time, that melatonin modulates SERT activity, thus demonstrating the feedback system between melatonin and the serotoninergic system in the gastrointestinal tract.

Lipopolysaccharide induces alteration of serotonin transporter in human intestinal epithelial cells

Intestinal serotoninergic activity and serotonin transporter (SERT) function have been shown to be altered in intestinal inflammatory diseases. Serotonin (5-HT) plays a critical role in the regulation of gastrointestinal physiology. Activity of 5-HT depends on its extracellular availability, partly modulated by SERT that transports 5-HT into the cell. Lipopolysaccharide (LPS) is a component of Gram-negative bacteria outer membrane, which acts as a potent activator of the inflammatory system in the intestine. The aim of this work was to determine, in the enterocyte-like cell line Caco-2, whether LPS treatment affects serotoninergic activity by acting on SERT. The results demonstrate that LPS treatment diminishes SERT activity in a dose- and period-dependent way. The kinetic study shows that V(max) was significantly reduced after treatment with LPS. The LPS effect on 5-HT uptake was, in part, mediated by protein kinase C (PKC) activation. The molecular expression of SERT revealed that LPS treatment did not affect the mRNA level or the SERT protein content in cell homogenate. The level of SERT protein, however, was reduced on brush border membrane. The LPS effect might be due to an alteration of the intracellular traffic of SERT which may, in part, be mediated by PKC activity.

Regulation of serotonin transporter activity by adenosine in intestinal epithelial cells

Serotonin plays a critical role in the regulation of intestinal physiology. The serotonin transporter (SERT) expressed in the intestinal epithelium determines 5-HT availability and activity. The serotoninergic system and SERT activity have been described as being altered in chronic intestinal pathologies such as inflammatory diseases. Adenosine has also been shown to be involved in a variety of intestinal functions and to play a central role in the regulation of inflammatory responses of injured tissue. Since the modulation of SERT by adenosine in the intestine remains unknown, the aim of the present work was to study the effect of adenosine on SERT activity and expression and to determine the molecular mechanism involved. The study has been carried out using human enterocyte-like Caco-2 cells which endogenously express SERT. The results show that adenosine diminishes SERT activity in both the apical and basal membranes by acting in the intrinsic molecule with no alteration of either SERT mRNA or protein levels. The effect of adenosine appears to be mediated by A(2) receptors and activation of the cAMP/PKA signalling pathway. Moreover, the adenosine effect did not seem to involve the activation of AMP activated protein kinase. Adenosine effects are reached at high concentrations, which suggests that adenosine modulation of SERT may be significant under conditions of inflammation and tissue injury.

Expression of 5-HT1A and 5-HT7 receptors in Caco-2 cells and their role in the regulation of serotonin transporter activity

Serotonin (5-HT) receptors are expressed in the gastrointestinal tract and play an important role in gastrointestinal activity regulation. 5-HT binding to receptors depends on 5-HT availability, which is, in part, modulated by the 5-HT transporter (SERT) expressed in enterocytes. This work concerns the expression of 5-HT(1A) and 5-HT(7) receptors (5-HTR(1A) and 5-HTR(7)) in the human enterocyte-like Caco-2 cell line and their role in SERT activity modulation. The results demonstrate the mRNA and protein expression of 5-HTR(1A) and 5-HTR(7) in these cells. In addition, both receptors are shown to modulate SERT activity; 5-HTR(1A) activation increased 5-HT uptake and 5-HTR(7) activation inhibited it. In both cases, SERT modulation might involve a cAMP/PKA pathway. Effects on SERT disappeared after long-term activation of 5-HTR(1A) and 5-HTR(7), indicating their desensitization. However, in both cases, the desensitization did not show itself to be mediated by a reduction of the amount of receptors in the membrane.

Regulation of the human serotonin transporter mediated by long-term action of serotonin in Caco-2 cells

AIM

The aim of this study was to determine the effect of long-term serotonin (5-hydroxytryptamine, 5-HT) treatment on the human serotonin transporter (hSERT) function and its expression.

METHODS

This study was carried out in the enterocyte-like cell line Caco-2. These cells constitutively express the hSERT and have been shown to be an excellent model for the study of this protein. We measured serotonin transport, levels of mRNA expression and of the SERT protein after treating the cells with serotonin.

RESULTS

Serotonin treatment diminished hSERT activity in a concentration and period-dependent way by increasing the K(t) value and reducing V(max). This inhibition was reversible and was not mediated by either the action of 5-HT(2), 5-HT(3) or 5-HT(4) receptors, or by the intracellular second messengers, protein kinase C and cAMP. 5-HT did not seem to affect either the mRNA level of the SERT or the protein transporter measured in either the membrane or the cell lysate. The 5-HT treatment effect was additive to the inhibitory effect of treatment with a low concentration of citalopram and fluoxetine. Nevertheless, 5-HT did not increase the inhibition yielded by treatment with high concentration citalopram.

CONCLUSION

The chronic increase in serotonin in the extracellular medium diminishes the function of the SERT. This effect seems to be due to an effect on the transporter molecule itself in the membrane, without altering protein synthesis, intracellular traffic, or its availability.

Effect of long-term fluoxetine treatment on the human serotonin transporter in Caco-2 cells

Fluoxetine is a selective serotonin reuptake inhibitor (SSRI) broadly used in the treatment of human mood disorders and gastrointestinal diseases involving the serotoninergic system. The effectiveness of this therapy depends on repeated long-term treatment. Most of the long-term studies in vivo of SSRI effects on serotoninergic activity have focused on their effects on autoreceptors or postsynaptic receptors. The chronic effect of SSRIs on the activity of the serotonin transporter (SERT) has been less studied and the results have been contradictory. The aim of this study was to determine the specific effect of long-term fluoxetine treatment on human serotonin transporter (hSERT) in vitro, by using the human enterocyte-like cell line Caco-2. Results show that fluoxetine diminished the 5-HT uptake in a concentration-dependent way and that this effect was reversible. Fluoxetine affected mainly the hSERT transport rate by reducing the availability of the transporter in the membrane with no significant alteration of either the total hSERT protein content or the hSERT mRNA level. These results suggest that the effect of fluoxetine on the expression of hSERT is post-translational and has shown itself to be independent of PKC and PKA activity. This study may be useful to clarify the effect of the long-term fluoxetine therapy in both gastrointestinal and central nervous system disorders.

Molecular characterization and intracellular regulation of the human serotonin transporter in Caco-2 cells

The serotonin transporter (SERT) has shown itself to be an effective pharmacological target in the treatment of mood disorders and some kinds of gastrointestinal syndromes. Most of the molecular studies of SERT in humans have been carried out using heterologous models. In this work, we have investigated the human enterocyte-like Caco-2 cell line as a potential "in vitro" model to study the human SERT. The results show that these cells express a SERT mRNA identical to the human brain SERT, and a 70 kDa protein immunodetected using a specific antibody. The SERT activity levels in Caco-2 cells increased in correlation with the onset and maintenance of the morphological and functional differentiation of the cells. Caco-2 SERT was also shown to be a high affinity (Kt=0.216 microM) saturable, Na(+) -dependent transporter that was inhibited by fluoxetine (IC(50)=17.6 nM). In addition, SERT activity was inhibited by the intracellular modulators protein kinase C and cAMP, either after short or long-term treatment. In short, the expression and molecular characteristics of the human SERT in Caco-2 cells indicate that this cell line may be an ideal tool to study in vitro the physiology and pharmacology of human SERT.