Defining diurnal fluctuations in mouse choroid plexus and CSF at high molecular, spatial, and temporal resolution

Abstract

Transmission and secretion of signals via the choroid plexus (ChP) brain barrier can modulate brain states via regulation of cerebrospinal fluid (CSF) composition. Here, we developed a platform to analyze diurnal variations in male mouse ChP and CSF. Ribosome profiling of ChP epithelial cells revealed diurnal translatome differences in metabolic machinery, secreted proteins, and barrier components. Using ChP and CSF metabolomics and blood-CSF barrier analyses, we observed diurnal changes in metabolites and cellular junctions. We then focused on transthyretin (TTR), a diurnally regulated thyroid hormone chaperone secreted by the ChP. Diurnal variation in ChP TTR depended on Bmal1 clock gene expression. We achieved real-time tracking of CSF-TTR in awake TtrmNeonGreen mice via multi-day intracerebroventricular fiber photometry. Diurnal changes in ChP and CSF TTR levels correlated with CSF thyroid hormone levels. These datasets highlight an integrated platform for investigating diurnal control of brain states by the ChP and CSF.

Circadian Mechanisms in Brain Fluid Biology

“…Choroid plexus epithelial cells display cell-autonomous circadian rhythmicity in the gene expression of core components of the molecular clock including BMAL1, CLOCK, CRY, and PER2. [98][99][100][101][102][103] The circadian rhythmicity in choroid plexus explants exhibit multiday autonomous oscillations of Per2::luciferase, 98,101,102 which can be resynchronized with dexamethasone. 102 Developmentally, choroid plexus rhythmicity is evident by birth, but the 24-hour period is not solidified in mice until 11 days after birth, around the time of eye opening.…”

“…[98][99][100][101][102][103] The circadian rhythmicity in choroid plexus explants exhibit multiday autonomous oscillations of Per2::luciferase, 98,101,102 which can be resynchronized with dexamethasone. 102 Developmentally, choroid plexus rhythmicity is evident by birth, but the 24-hour period is not solidified in mice until 11 days after birth, around the time of eye opening. 98 While circadian rhythmicity in choroid plexus has been reported in both male 102 and female rodents, 100 studies from ovariectomized rats demonstrate that rhythmicity in females is dependent on estrogen and mediated through the estrogen receptor.…”

“…104 Recently, ribosomal pulldown analyses showed choroid plexus translation and protein synthesis are regulated diurnally, with higher levels of translation taking place during the dark phase when rodents are more active. 102 These oscillations in translation may drive molecular cues to long-distance brain targets through choroid plexus output into the CSF. For example, choroid plexus TTR (transthyretin)-the main transporter of thyroid hormone from the blood through choroid plexus epithelial cells into CSF (Figure 2, right inset)-is higher during the active phase.…”

“…This corresponds to higher levels of thyroid hormone T3 in the CSF. 102 Why these day/night differences in TTR and CSF thyroid hormone exist and how they may maintain the circadian period remains to be elucidated.…”

“…[106][107][108] In addition to the metabolic profile of the choroid plexus, gene expression of membrane transporters and tight junction components, together with transmission electron micrographs of the apicolateral tight junctions suggest that the blood-CSF barrier may be more permeable during the rest phase. 102 This could have large implications not only for drug transport across the barrier 105 but also for immune cell trafficking. 109,110 For example, intravital 2-photon microscopy of the choroid plexus showed real-time macrophage infiltration into the choroid plexus after immune activation in adult mice 94 and during development after maternal inflammation.…”