Transcription factor tonicity-responsive enhancer-binding protein (TonEBP) which transactivates osmoprotective genes...

Tonicity-responsive enhancer-binding protein (TonEBP) was initially identified as a transcription factor involved in adaptation of renal cells to hypertonicity by activation of osmoprotective genes encoding proteins for accumulation of compatible osmolytes. Since brain osmoadaptation is observed in relationship to neurological disorders resulting from pathological osmotic disbalances of blood plasma we have investigated through immunocytochemistry TonEBP expression in cerebral cortex and hippocampus of normal rat and rats submitted to an acute systemic hypertonicity or to a prolonged systemic hypotonicity. TonEBP-expressing cells were identified using double immunofluorescence and appropriate cell type markers. Their relative proportion was determined by quantitative image analysis. In normal rats TonEBP expressed primarily in neurons where it was strictly located in the cell nucleus but heterogeneously distributed into a nucleoplasmic pool and a granular pool. In animals made acutely hypertonic TonEBP labeling increased dramatically exclusively in the nuclei of neurons and reached a maximum within 1 h. In hypertonic animals TonEBP labeling covered the whole cell nucleus of virtually all neurons, appeared finely punctuated but was no more granular. Optical density of the labeling as determined by image analysis correlated linearly with the increased plasma osmolality. In animals made hypotonic for several days no conspicuous decrease of TonEBP labeling was observed. In normal animals a very minor proportion of non-neuronal cells showed a faint TonEBP nuclear labeling. This proportion increased slightly in hypertonic animals. Nevertheless these non-neuronal TonEBP-positive nuclei which belonged to oligodendrocytes and to a small subpopulation of astrocytes remained always very weakly labeled when compared with neuron nuclei. Brain capillary endothelial cells as well as microglial cells showed no TonEBP-labeling even in hypertonic animals. Our data demonstrate that in brain TonEBP is significantly expressed and tonicity-overexpressed in neurons and accordingly suggest that neurons only among brain cells accumulate compatible osmolytes through TonEBP-mediated activation of osmoprotective genes to adapt to acute systemic hypertonicity.