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 1

TH signaling in neuronal (SK-N-AS) and glial (H4) cell lines.

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TH signaling in neuronal (SK-N-AS) and glial (H4) cell lines. (A) Immuno...
(A) Immunocytochemistry of SK-N-AS and H4 probed with α-tubulin and stained with DAPI. Original magnification, ×20. (B) Endogenous deiodinase activity in H4 and SK-N-AS (SK) cell lines. nd, not detectable. (C) RNA expression pattern of genes involved in TH metabolism and action as determined by quantitative RT-PCR in H4 and SK-N-AS cell lines. (D) Responsiveness of putative T3-target genes (UCP2, RC3, and ENPP2) in H4 and SK-N-AS cell lines treated with media containing charcoal-stripped serum or 100 nM T3 for 48 hours as indicated. 5× indicates 5-fold induction of expression. (E) Schematic representation of the Transwell System in which an insert is placed on a 6-well plate and cells (H4) are seeded inside the insert; SK-N-AS cells are seeded at the bottom of the 6-well plate. After cells are seeded, both cell types are kept separated overnight and then placed together in the same multiwell plate as indicated. (F) Dose responsiveness of ENPP2 gene to T3 in H4 and SK-N-AS cell lines incubated alone as in D or coincubated in the Transwell system for 48 hours as indicated. (G) Same as in F, except that this is a time-responsiveness to T3. In all experiments, values are mean ± SEM of 5–9 independent wells; *P < 0.01 versus control.