Differential expression, activity and regulation of the sodium/myo-inositol cotransporter in astrocyte cultures from different regions of the rat brain

Abstract

The high-affinity sodium/myo-inositol cotransporter (SMIT) is involved in osmoregulation in several cells and tissues. In the CNS the activity of SMIT also determines the individual susceptibility of neural cells to the inositol depleting effect of lithium, which is considered to be important in lithium's therapeutic effects in manic-depressive illness. Among neural cells SMIT is particularly active in astrocytes. In the present work we have cloned the cDNA of SMIT of the rat and assessed its activity, expression and regulation in primary astroglia cultures derived from five different rat brain regions: cerebellum, cortex, diencephalon, hippocampus and tegmentum. After an incubation period of 24 h in medium containing 3[H]labeled myo-inositol different steady-state concentrations were detected which were dependent on the brain region from which the astrocytes were cultured. In addition, myo-inositol uptake in astrocytes from different areas was characterized by two different Km values (27 microM for cerebellum and diencephalon, 50 microM for cortex, hippocampus and tegmentum) and by three different v(max) values (approx. 200 pmol/mg protein/min for astrocytes from cerebellum and tegmentum, 298 for hippocampus and 465 for cortex), indicating that the active myo-inositol uptake into astroglial cells is distinct in the various brain regions. The efficacy of uptake as determined by v(max) values of 3[H]myo-inositol uptake correlated with the level of mRNA of SMIT in the astrocyte cultures from the various brain regions as determined by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR). Both 3[H]myo-inositol uptake and SMIT mRNA content was upregulated by incubation of astrocytes in medium of increased osmolarity. In astrocytes from cerebellum, cortex, hippocampus and tegmentum 3[H]myo-inositol uptake was downregulated by chronic incubation with 400 microM inositol. This effect was not observed in astrocytes from diencephalon. Furthermore, in astrocytes from cortex and hippocampus but not from cerebellum, diencephalon and tegmentum incubation with corticosterone for three days upregulated 3[H]myo-inositol uptake. It is concluded that SMIT is differentially expressed and regulated in astrocytes from distinct brain regions. These regional differences suggest particular consideration of localized effects in investigations of the role of myo-inositol in the mechanism of action of antibipolar drugs.