Transportin 1 in the mouse brain: Appearance in regions of neurogenesis, cerebrospinal fluid production/sensing, and circadian clock

Transportin1 (Tnpo1) is a carrier protein belonging to the importin-β family, which transports substrates between the cytoplasm and the nucleus. To gain insight into the role of Tnpo1 gene in the brain, we investigated the localization of Tnpo1-, Tnpo2-, and Tnpo3-expressing cells by in situ hybridization histochemistry. Tnpo1 mRNA-positive cells were distributed throughout the brain from the olfactory bulb to the medulla oblongata. The cells in the subventricular zone of the lateral ventricle, where neurogenesis occurs even in the adult, and its progeny neurons in the granular cells of the olfactory bulb and the islands of Calleja were strongly labeled. It is also noteworthy that cerebrospinal fluid (CSF)-generating epithelial cells in the choroid plexus and CSF-contacting and -sensing circumventricular organs, including organum vasculosum lamina terminalis, subfornical organ, and subcommissural organ, expressed high amounts of Tnpo1. The strongest signals were found in the suprachiasmatic nucleus (SCN), where the biological clock resides, which prompted us to examine the circadian characteristics of Tnpo1. Under constant-dark conditions, the circadian expression profiles of Tnpo1 mRNA in the SCN showed a peak in the subjective night and a trough in the subjective day. Tnpo2 and Tnpo3 showed similar patterns of expression, except in the choroids plexus, the subventricular zone, and the SCN, where the expression was notably weaker. These findings suggest that Tnpo1 is involved in a variety of functions in the adult brain, including neurogenesis, CSF production and sensing, and circadian rhythms.