Regulation of Na+/myo-inositol cotransporter gene expression in hyperglycemic rat hippocampus

A review of the role of inositols in conditions of insulin dysregulation and in uncomplicated and pathological pregnancy

Inositols, a group of 6-carbon polyols, are highly bioactive molecules derived from diet and endogenous synthesis. Inositols and their derivatives are involved in glucose and lipid metabolism and participate in insulin-signaling, with perturbations in inositol processing being associated with conditions involving insulin resistance, dysglycemia and dyslipidemia such as polycystic ovary syndrome and diabetes. Pregnancy is similarly characterized by substantial and complex changes in glycemic and lipidomic regulation as part of maternal adaptation and is also associated with physiological alterations in inositol processing. Disruptions in maternal adaptation are postulated to have a critical pathophysiological role in pregnancy complications such as gestational diabetes and pre-eclampsia. Inositol supplementation has shown promise as an intervention for the alleviation of symptoms in conditions of insulin resistance and for gestational diabetes prevention. However, the mechanisms behind these affects are not fully understood. In this review, we explore the role of inositols in conditions of insulin dysregulation and in pregnancy, and identify priority areas for research. We particularly examine the role and function of inositols within the maternal-placental-fetal axis in both uncomplicated and pathological pregnancies. We also discuss how inositols may mediate maternal-placental-fetal cross-talk, and regulate fetal growth and development, and suggest that inositols play a vital role in promoting healthy pregnancy.

Identification and Functional Implications of Sodium/Myo-Inositol Cotransporter 1 in Pancreatic β-Cells and Type 2 Diabetes

Myo-inositol (MI), the precursor of the second messenger phosphoinositide (PI), mediates multiple cellular events. Rat islets exhibit active transport of MI, although the mechanism involved remains elusive. Here, we report, for the first time, the expression of sodium/myo-inositol cotransporter 1 (SMIT1) in rat islets and, specifically, β-cells. Genetic or pharmacological inhibition of SMIT1 impaired glucose-stimulated insulin secretion by INS-1E cells, probably via downregulation of PI signaling. In addition, SMIT1 expression in INS-1E cells and isolated islets was augmented by acute high-glucose exposure and reduced in chronic hyperglycemia conditions. In corroboration, chronic MI treatment improved the disease phenotypes of diabetic rats and islets. On the basis of our results, we postulate that the MI transporter SMIT1 is required to maintain a stable PI pool in β-cells in order that PI remains available despite its rapid turnover.

Osmoregulatory inositol transporter SMIT1 modulates electrical activity by adjusting PI(4,5)P2 levels

Myo-inositol is an important cellular osmolyte in autoregulation of cell volume and fluid balance, particularly for mammalian brain and kidney cells. We find it also regulates excitability. Myo-inositol is the precursor of phosphoinositides, key signaling lipids including phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. However, whether myo-inositol accumulation during osmoregulation affects signaling and excitability has not been fully explored. We found that overexpression of the Na(+)/myo-inositol cotransporter (SMIT1) and myo-inositol supplementation enlarged intracellular PI(4,5)P2 pools, modulated several PI(4,5)P2-dependent ion channels including KCNQ2/3 channels, and attenuated the action potential firing of superior cervical ganglion neurons. Further experiments using the rapamycin-recruitable phosphatase Sac1 to hydrolyze PI(4)P and the P4M probe to visualize PI(4)P suggested that PI(4)P levels increased after myo-inositol supplementation with SMIT1 expression. Elevated relative levels of PIP and PIP2 were directly confirmed using mass spectrometry. Inositol trisphosphate production and release of calcium from intracellular stores also were augmented after myo-inositol supplementation. Finally, we found that treatment with a hypertonic solution mimicked the effect we observed with SMIT1 overexpression, whereas silencing tonicity-responsive enhancer binding protein prevented these effects. These results show that ion channel function and cellular excitability are under regulation by several "physiological" manipulations that alter the PI(4,5)P2 setpoint. We demonstrate a previously unrecognized linkage between extracellular osmotic changes and the electrical properties of excitable cells.

Expression Profiling of Solute Carrier Gene Families at the Blood-CSF Barrier

The choroid plexus (CP) is a highly vascularized tissue in the brain ventricles and acts as the blood-cerebrospinal fluid (CSF) barrier (BCSFB). A main function of the CP is to secrete CSF, which is accomplished by active transport of small ions and water from the blood side to the CSF side. The CP also supplies the brain with certain nutrients, hormones, and metal ions, while removing metabolites and xenobiotics from the CSF. Numerous membrane transporters are expressed in the CP in order to facilitate the solute exchange between the blood and the CSF. The solute carrier (SLC) superfamily represents a major class of transporters in the CP that constitutes the molecular mechanisms for CP function. Recently, we systematically and quantitatively examined Slc gene expression in 20 anatomically comprehensive brain areas in the adult mouse brain using high-quality in situ hybridization data generated by the Allen Brain Atlas. Here we focus our analysis on Slc gene expression at the BCSFB using previously obtained data. Of the 252 Slc genes present in the mouse brain, 202 Slc genes were found at detectable levels in the CP. Unsupervised hierarchical cluster analysis showed that the CP Slc gene expression pattern is substantially different from the other 19 analyzed brain regions. The majority of the Slc genes in the CP are expressed at low to moderate levels, whereas 28 Slc genes are present in the CP at the highest levels. These highly expressed Slc genes encode transporters involved in CSF secretion, energy production, and transport of nutrients, hormones, neurotransmitters, sulfate, and metal ions. In this review, the functional characteristics and potential importance of these Slc transporters in the CP are discussed, with particular emphasis on their localization and physiological functions at the BCSFB.

Sodium-dependent Transport of [3H](1d)chiro-Inositol by Tetrahymena

The transport characteristics of (1D)chiro-inositol by the ciliate Tetrahymena were examined in competition studies employing [3H](1D)chiro-inositol. (1D)chiro-Inositol transport was competed by unlabeled (1D)chiro-inositol, myo-inositol, scyllo-inositol, and D-glucose in a concentration-dependent manner. Conversely, (1D)chiro-inositol competed for [3H]myo- and [3H]scyllo-inositol transport. Lineweaver-Burke analysis of the competition data indicated a Km of 10.3 mM and a Bmax of 4.7 nmol/min/mg for (1D)chiro-inositol. Transport of (1D)chiro-inositol was inhibited by cytochalasin B, an inhibitor of facilitated glucose transporters, and phlorizin, an inhibitor of sodium-dependent transporters. Removal of sodium from the radiolabeling buffer also inhibited uptake. The presence of 0.64 mM calcium or magnesium ions exerted negligible effects on transport, although potassium was inhibitory. [3H](1D)chiro-Inositol was shown to be incorporated into Tetrahymena phosphoinositides.

Subarachnoid hemorrhage induces Na+/myo-inositol cotransporter in the rat brain

Neurons and glial cells respond to extracellular hyperosmolarity by accumulating small organic solutes, called "osmolytes." Na+/myo-inositol is one of the major organic osmolytes in the brain and Na+/myo-inositol cotransporter (SMIT) regulates extracellular Na+/myo-inositol content. Subarachnoid hemorrhage (SAH) is an osmotic stress-inducing event of the brain. The expression of SMIT messenger ribonucleic acid (mRNA) and protein was investigated with in situ hybridization and immunohistochemistry in rat brains with SAH induced by endovascular perforation. SMIT riboprobe was raised from a 490-bp rat SMIT complementary deoxyribonucleic acid. Anti-SMIT antibody was raised in rabbits. SMIT mRNA was expressed strongly in the cortex, hippocampus, and hypothalamus of the perforated side at 6 to 24 hours after SAH. Mild upregulation was noted in the contralateral cortex, hippocampus, and hypothalamus. The ventral aspect of the pons showed mild upregulation. Microautoradiography and immunostaining showed SMIT expression mainly in the neurons, but also in some non-neural cells in the hippocampus. The present results indicate that diffuse osmotic stress occurs in the host brain after SAH.

Diffuse brain injury induces local expression of Na+/myo-inositol cotransporter in the rat brain

We studied expression of an osmoprotective gene, sodium/myo-inositol cotransporter (SMIT) in Marmarou's animal model for human diffuse brain injury by in situ hybridization and immunohistochemistry. In rats with diffuse brain injury, transient upregulation of SMIT mRNA was exclusively observed in the lateral area of pyramidal tract in lower brainstem. The expression was induced at 1 h after injury, peaked at 24 h, and returned to almost control levels at 48 h. Upregulated expression was found mainly in small glia-like cells. By immunohistochemistry using antibodies to phosphorylated mitogen-activated protein (MAP) kinases, inductions of phosphorylated p44/42 MAP kinase were also observed after diffuse brain injury. Interestingly, the distribution patterns of induced phosphorylated p44/42 MAP kinase were completely coincident with those of upregulated SMIT mRNA after diffuse brain injury. These results suggest that diffuse brain injury induces local expression of SMIT by activation of p44/42 MAP kinase cascade. The confined SMIT induction may reflect regional differences of damage and/or cellular differences in sensitivity to neuropathological stresses caused by this injury.