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 6

LPS signaling amplifies the astrocyte-mediated mRNA footprint in neuronal cells.

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LPS signaling amplifies the astrocyte-mediated mRNA footprint in neurona...
Effect of LPS on TRH mRNA in the PVN of WT and D2-KO mice. Note apparent reduction in TRH mRNA in WT mice following the systemic administration of LPS (A–B), but absence of a response in D2-KO animals (C–D). Scale bar: 100 μm. III, third ventricle. (E) Quantification of the TRH mRNA hybridization signal from each of the 4 animal groups (n = 4 per group). (F) Luciferase activity in HEK-TLR4-MD2-ELAM cells, which have a functional LPS–NF-κB signaling via stable expression of the TLR4 receptor and an NF-κB–responsive Luciferase reporter after the addition of 1 μg/ml LPS (bacterial lipopolysaccharide). (G) The coculture system described in Figure 2 was exposed to LPS as in F for the indicated times in the presence of 20 pM T4 (free fraction). Cells were harvested at the indicated times and processed for deiodinase activity or quantitative RT-PCR. Shown are the time courses of D2 and D3 mRNA levels or activities (H) after addition of LPS. (I) ENPP2 mRNA levels in SK-N-AS cells at the indicated time points. (J) UCP2 mRNA levels in SK-N-AS cells at the indicated time points. Values are mean ± SEM of 6–9 independent wells; *P < 0.01 versus control.