Short-Term Effects of Thyroid Hormones on Cytoskeletal Proteins Are Mediated by GABAergic Mechanisms in Slices of Cerebral Cortex from Young Rats

Abstract

: Thyroid hormones play important roles in brain function. However, few information is available about the effect of 3,5,3'-triiodo-L-thyronine (T(3)) or thyroxine (T(4)) on the in vitro phosphorylation of intermediate filament (IF) proteins from cerebral cortex of rats. In this study we investigated the involvement of GABAergic mechanisms mediating the effects of T(3) and T(4) on the in vitro incorporation of (32)P into IF proteins from cerebral cortex of 10-day-old male rats. Tissue slices were incubated with or without T(3), T(4), gamma-aminobutiric acid (GABA), kinase inhibitors or specific GABA antagonists and (32)P-orthophosphate for 30 min. The IF-enriched cytoskeletal fraction was extracted in a high salt Triton-containing buffer and the in vitro (32)P incorporation into IF proteins was measured. We first observed that 1 microM T(3) and 0.1 microM T(4) significantly increased the in vitro incorporation of (32)P into the IF proteins studied through the PKA and PKCaMII activities. A similar effect on IF phosphorylation was achieved by incubating cortical slices with GABA. Furthermore, by using specific GABA antagonists, we verified that T(3) induced a stimulatory effect on IF phosphorylation through noncompetitive mechanisms involving GABA(A), beyond GABA(B) receptors. In contrast, T(4) effects were mediated mainly by GABA(B) mechanisms. In conclusion, our results demonstrate a rapid nongenomic action of T(3) and T(4) on the phosphorylating system associated to the IF proteins in slices of cerebral cortex of 10 day-old male rats and point to GABAergic mechanisms mediating such effects.

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

“…Although Friedman et al (1977) have described the inhibition of thyroid adenylate cyclase by TH as a possible negative feedback mechanism, several evidences in the literature point to the ability of TH to increase cyclicAMP (cAMP) levels in other cell types (Leitman et al, 1996;Sarkar et al, 1999;Zamoner et al, 2006). In this context, our group demonstrated that TH modulate PKA-mediated IF phosphorylation in cerebral cortex (Zamoner et al, 2006), supporting the hypothesis that the increased GFAP phosphorylation we have observed could be through PKAdependent mechanisms.…”

“…PKA and PKCaMII activities from cerebral cortex of young rats (Zamoner et al, 2006). Because aberrant cytoskeletal phosphorylation/dephosphorylation may have serious consequences for cellular function and structure and Rho proteins are well known for their effects on the actin cytoskeleton (Ridley, 2001), in the present study we investigated the involvement of GFAP phosphorylation and RhoA mechanisms Fig.…”

“…Moreover, we have recently reported that TH alter phosphorylation of glial and neuronal IF proteins in cerebral cortex of rats (Zamoner et al, 2006). Therefore, taking into account that phosphorylation is an important mechanism regulating cytoskeletal dynamics and that cytoskeleton is essential for normal cell morphology, in the present study we investigated the effects of exposing cultured C6 glioma cells or cortical astrocytes to different concentrations of TH on cell morphology, viability, proliferation, and GFAP phosphorylation.…”

Intermediate Filaments as a Target of Signaling Mechanisms in Neurotoxicity

“…In this context, it has been previously demonstrated that both T 3 and T 4 may modulate the GABAergic system and induce PKA-and PKCaMII-mediated hyperphosphorylation of vimentin, GFAP, NF-M and NF-L in cerebral cortex from very young rats (up to 10 days of age) [50]. However, only T 4 caused hyperphosphorylation of the same proteins later in development (15 days of age) through GABA-independent mechanisms [49].…”

“…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].…”