Thyroid hormone, but not photoperiod, is a potent modulator of the chroroid plexus transciptome in the ewe
Three groups of adult ewes were initally submitted to short days. One group remained in short days (Short), the other two were transferred to long days - one was intact (Long), the other had been subject to thyroidectomy before the photoperiodic transfer (THX). At the end of the treatment (see Lomet et al 2018 CelMolLifeSci 75:905-918) ewes were killed and extracts from the choroid plexus (from lateral ventricles) were taken and analyzed. Abstract: The photoperiodic control of seasonal functions requires the action of melatonin at the pars tuberalis of the pituitary and subsequent control of local thyroid hormone (TH) signaling by tanycytes lining the basolateral part of the third ventricle. Therefore, TH supply of tancytes through the cerebrospinal fluid (CSF) is central to photoperiodism and the choroid plexuses (CP), which produce the CSF, play a pivotal role. Here, the transcriptome of the CP of the lateral ventricles was established by RNAseq in ewes maintained under three experimental conditions: ewes exposed to a short photoperiod (SP), intact ewes submitted to an acute 3-week exposure to a long photoperiod (LP-Sham) and ewes thyroidectomized prior to the LP exposure (LP-THX). Statistical analysis of the data revealed 1169 genes impacted by photoperiod (SP vs LP comparison) and 575 genes sensitive to the TH status (LP vs LP-THX comparison). Even though twice as numerous as the genes impacted by TH, genes responsive to photoperiod comparatively displayed rather weak transcriptional changes associated with a lower statistical significance. In line with this, validation of a few candidate DEG by RT-qPCR confirmed the impact of the TH status, but not that of photoperiod. We further showed that the expression level of the melatonin MT1 receptor is at best very low in the ewe CP, which provides a functional rationale for the lack of impact of photoperiod. In conclusion, while the CP is responsive to TH, which it provides to distant target tissues through the CSF, it is unlikely to be a major target of melatonin. Overall design: The study was performed in 24 ewes - 8 per group (except THX - 7 ewes). RNA was individually extracted from each sample of choroid plexus. Then, 6 equimolar pools were prepared by mixing RNA from 3-4 individuals for each condition (i.e. duplicate for each condition).