Thyroid Hormones Reorganize the Cytoskeleton of Glial Cells Through Gfap Phosphorylation and Rhoa-Dependent Mechanisms

Abstract

Thyroid hormones (3,5,3'-triiodo-L: -thyronine, T3; 3,5,3',5'-L: -tetraiodothyronine, T4; TH) play crucial roles in the growth and differentiation of the central nervous system. In this study, we investigated the actions of TH on proliferation, viability, cell morphology, in vitro phosphorylation of glial fibrillary acidic protein (GFAP) and actin reorganization in C6 glioma cells. We first observe that long-term exposure to TH stimulates cell proliferation without induce cell death. We also demonstrate that after 3, 6, 12, 18, and 24 h treatment with TH, C6 cells and cortical astrocytes show a process-bearing shape. Furthermore, immunocytochemistry with anti-actin and anti-GFAP antibodies reveals that TH induces reorganization of actin and GFAP cytoskeleton. We also observe an increased in vitro 32P incorporation into GFAP recovered into the high-salt Triton insoluble cytoskeletal fraction after 3 and 24 h exposure to 5 x 10(-8) and 10(-6) M T3, and only after 24 h exposure to 10(-9) M T4. These results show a T3 action on the phosphorylating system associated to GFAP and suggest a T3-independent effect of T4 on this cytoskeletal protein. In addition, C6 cells and astrocytes treated with lysophosphatidic acid, an upstream activator of the RhoA GTPase pathway, totally prevented the morphological alterations induced by TH, indicating that this effect could be mediated by the RhoA signaling pathway. Considering that IF network can be regulated by phosphorylation leading to reorganization of IF filamentous structure and that alterations of the microfilament organization may have important implications in glial functions, the effects of TH on glial cell cytoskeleton could be implicated in essential neural events such as brain development.

Maternal Hypothyroxinemia-Induced Neurodevelopmental Impairments in the Progeny

“…We postulate that an aberrant location would delay neurons in reaching their normal destination and thus affect normal brain function. The results suggested the independent role of T 4 , and T 4 is more than likely to regulate Factin content and the organization of the action cytoskeleton and exerted a profound effect on neuronal migration [94,95]. These studies raised a novel neurophysiological aspect of T 4 , t h e t r a n s c r i p t i o n a l i n a c t i v e h o r m o n e , i m p a c t neurodevelopment by mechanisms other than regulate gene expression.…”

“…Cytoskeleton dynamics is controlled by a series of actin-binding proteins, which may be the molecular targets for T 4 . It is reported that T 4 regulated cytoskeleton organization via Rho-GTPase signaling pathway, which is involved with actin-binding proteins [95]. In another aspect, cytoskeleton is the major internal structure determining the neurite outgrowth, which is related to dendritic and axonal morphogenesis [97].…”

Thyroid hormones and the central nervous system of mammals (Review)

“…As astrocytes contain both α and ß isotypes MOLECULAR MEDICINE REPORTS 1: 279-295, 2008 of nTRs, they are probably direct targets of THs. Notably, it was also noticed that THs had an effect on GFAP phosphorylation and cytoskeletal organization, which seemed to be mediated by a pathway involving the RhoA small GTPase and to depend directly on T4 (217).…”

Intermediate Filaments as a Target of Signaling Mechanisms in Neurotoxicity

“…GFAP filaments that normally spread in the cytoplasm of astrocytes became organized around the cell nucleus. In addition, Zamoner and coworkers [51] showed that both T 3 and T 4 induced GFAP phosphorylation and reorganization in glioma C6 cells through the inhibition of RhoA GTPase. The modulation of GFAP was accompanied by increased proliferation of glioma cells.…”

“…In brain, these hormones are essential for myelination [43,44], neuritogenesis [45], synaptic plasticity [46][47][48], IF phosphorylation [49][50][51][52][53][54], cell differentiation and maturation [55]. Considering the role of these hormones on brain development, thyroid diseases might account for brain injury as well as alteration in mood and cognition [56].…”