Paracrine signaling by glial cell–derived triiodothyronine activates neuronal gene expression in the rodent brain and human cells
Beatriz C.G. Freitas, … , Ronald M. Lechan, Antonio C. Bianco
Beatriz C.G. Freitas, … , Ronald M. Lechan, Antonio C. Bianco
Published May 10, 2010
Citation Information: J Clin Invest. 2010;120(6):2206-2217. https://doi.org/10.1172/JCI41977.
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Research Article Endocrinology

Paracrine signaling by glial cell–derived triiodothyronine activates neuronal gene expression in the rodent brain and human cells

Abstract

Hypothyroidism in humans is characterized by severe neurological consequences that are often irreversible, highlighting the critical role of thyroid hormone (TH) in the brain. Despite this, not much is known about the signaling pathways that control TH action in the brain. What is known is that the prohormone thyroxine (T4) is converted to the active hormone triiodothyronine (T3) by type 2 deiodinase (D2) and that this occurs in astrocytes, while TH receptors and type 3 deiodinase (D3), which inactivates T3, are found in adjacent neurons. Here, we modeled TH action in the brain using an in vitro coculture system of D2-expressing H4 human glioma cells and D3-expressing SK-N-AS human neuroblastoma cells. We found that glial cell D2 activity resulted in increased T3 production, which acted in a paracrine fashion to induce T3-responsive genes, including ectonucleotide pyrophosphatase/phosphodiesterase 2 (ENPP2), in the cocultured neurons. D3 activity in the neurons modulated these effects. Furthermore, this paracrine pathway was regulated by signals such as hypoxia, hedgehog signaling, and LPS-induced inflammation, as evidenced both in the in vitro coculture system and in in vivo rat models of brain ischemia and mouse models of inflammation. This study therefore presents what we believe to be the first direct evidence for a paracrine loop linking glial D2 activity to TH receptors in neurons, thereby identifying deiodinases as potential control points for the regulation of TH signaling in the brain during health and disease.

Authors

Beatriz C.G. Freitas, Balázs Gereben, Melany Castillo, Imre Kalló, Anikó Zeöld, Péter Egri, Zsolt Liposits, Ann Marie Zavacki, Rui M.B. Maciel, Sungro Jo, Praful Singru, Edith Sanchez, Ronald M. Lechan, Antonio C. Bianco

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Figure 2

H4 astrocyte cells activate T4 into T3 and establish a mRNA footprint in neuronal SK-N-AS cells.

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H4 astrocyte cells activate T4 into T3 and establish a mRNA footprint in...
(A) Effects of T4 addition on ENPP2 or UCP2 gene expression in cultures of SK-N-AS or cocultures of SK-N-AS and H4 cells as indicated. Cells were cultured as in legend to Figure 1 and treated with 20 pM T4 (free fraction) for 48 hours. (B) Fractional conversion of T4 to T3 as measured after addition of 125I-T4 and determination of outer ring deiodination via measurement of free 125I. Media samples were collected at the indicated times. (C) Chromatograms of H4 cell medium at the indicated times after addition of 125I-T4. Typical peaks of 125-T3 and 125I are shown after 24 hours. (D) Same as in C, except that 125I-T3 was added to cultures of SK-N-AS cells and 125I-T2 and 125I-T1 peaks are visualized. (E) Same as in C, except that 125I-T4 was added to H4 and SK-N-AS cocultures and the indicated peaks are visualized. (F) Same as in A, except that different concentrations of free T4 were used and 20 nM rT3 was added at the beginning of incubation as indicated. T3 indicates that 100 nM T3 was added at the beginning of incubation. (G) Same as in F, except that α-T3 antiserum was added at time zero as indicated. In all experiments, values are mean ± SEM of 5–9 independent wells; *P < 0.01 versus control.