Proceedings of The Physiological Society
King's College London (2011) Proc Physiol Soc 22, PC22
Effect of transthyretin on thyroxine and +
R. Chen1, W. Selway1, S. Nadarajah1, H. Farhat1, J. E. Preston1
1. Institute of Pharmaceutical Science, King's College London, London, United Kingdom.
Transthyretin (TTR) is a serum and cerebrospinal fluid (CSF) chaperone protein for the thyroid hormones, retinol and β-amyloid peptide. TTR is synthesized systemically in the liver, and centrally by the choroid plexus (CP) and ependymal cells lining the cerebral ventricles. The secretion of TTR by the CP into CSF has been suggested to play a key role in the transfer of T4 from the blood to the CSF, as a unidirectional secretion of TTR to the CSF may drive T4 across the blood CSF barrier. On the other hand, T4 transport from CSF into blood is significantly reduced by TTR in a dose-dependent manner. TTR also binds β-amyloid peptide to attenuate amyloid plaque development, a hallmark of Alzheimer’s disease. It is however, unclear whether TTR affects the clearance of β-amyloid from the brain. The aim of this study was to investigate the role of TTR in β-amyloid and T4 efflux from the brain. Eight week old male 129sv mice were anaesthetized i.p. with 0.1 ml ketamine (100mg.ml-1) and 0.2 ml medetomidine (1mg.ml-1). The mice were placed in a stereotaxic frame and a cannula inserted into one lateral cerebral ventricle. A total 1μl artificial CSF was infused containing 125I-Aβ40 and 3H-inulin, or 125I-T4 and 3H-mannitol and the brain removed 2, 4, 8, 16 or 32 minutes after infusion. Brain samples were dissolved in Solvable and scintillation fluid added before counting. All procedures were within the guidelines of the Animals (Scientific procedures) Act, UK, 1986. At baseline, levels of both 125I-T4 and 125I-Aβ40 in the brain were significantly higher than the extracellular markers and the net uptake of 125I-T4 into the brain was significantly greater than that for 125I-Aβ40. With increased time post infusion, levels of 125I-T4, 125I-Aβ40 and extracellular makers declined in brain. These generated linear relation curves, by which the half time for efflux was calculated. For 125I-T4, the half time for efflux was the fastest, 5.14 min, and faster than the marker mannitol (7.6 min), suggesting removal was via a transport system and not just bulk drainage of CSF and interstitial fluid. The half time for 125I-Aβ40 efflux was slower than T4 (17.5 min) but still faster than its extracellular marker, 3H-inulin (23.7 min). The introduction of TTR resulted in a significant increase in whole brain uptake and retention of 125I-T4 doubling the half time for efflux whilst extracelllar markers remained similar to control. TTR increased 125I-Aβ40 accumulation sigmificantly in the CPs which may act as a route for delayed Aβ40 removal. This study indicates that TTR acts differentially on T4 and Aβ40 efflux from brain. TTR prevents loss thyroxine from the brain but increases the accumulation of Aβ40 only in the CP which may enhance later CNS removal.
Where applicable, experiments conform with Society ethical requirements