l-Triiodothyronine differentially and nongenomically regulates synaptosomal protein phosphorylation in adult rat brain cerebral cortex: Role of calcium and calmodulin

Adult-onset thyroid disorders in humans impair several important central nervous system functions, causing various neuropsychiatric diseases. However, the mechanisms of thyroid hormone (TH) action in the mature mammalian brain remain unclear. Recent nongenomic actions of TH in adult brains are spotlighted. Many nongenomic mechanisms are modulated by phosphorylation–dephosphorylation of substrate proteins. In the present study, l-triiodothyronine (l-T3) demonstrated differential regulation of phosphorylation status of five different synaptosomal proteins (63, 53, 38, 23, and 16 kD) in both a Ca2+/calmodulin (CaM)-dependent and -independent manner. l-T3 increased the level of phosphorylation of all these five proteins. Ca2+/CaM further stimulated phosphorylation of 63- and 53-kD proteins by l-T3, which were inhibited both by EGTA (Ca2+-chelator) or KN62 (Ca2+/CaM kinase-II [CaMK-II] inhibitor), suggesting the role of CaMK-II. l-T3 increased the phosphorylation of 23- and 38-kD proteins; the effect was independent of EGTA or KN62. The presence of Ca2+ decreased l-T3-induced phosphorylation of 63-, 53- and 38-kD proteins. Surprisingly, l-T3-induced phosphorylation of 16-kD protein was not augmented further with Ca2+ or Ca2+/CaM; instead, the presence of CaM abolished the l-T3-induced phosphorylation. EGTA or KN62 could not restore the effect of CaM-induced dephosphorylation of this protein. This study identified the role of Ca2+/CaM in the regulation of l-T3-induced protein phosphorylation and supported a unique nongenomic mechanism of second messenger-mediated regulation of protein phosphorylation by TH in mature rat brain. This has profound implications for higher mental functions and strategies for novel therapeutics.