In vitro thyroid hormone rapidly modulates protein phosphorylation in cerebrocortical synaptosomes from adult rat brain

Thyroid hormones induced rapid changes in phosphorylation in a membrane-containing lysate of synaptosomes purified from adult rat cerebral cortex. The in vitro addition of 3,5,3′-l-triiodothyronine or l-thyroxine strongly influenced incorporation of label from [γ-32P]-ATP into proteins in a cerebrocortical synaptosomal lysate. Incubation with 3,5,3′-l-triiodothyronine or l-thyroxine had strong biphasic dose-dependent effects on the phosphorylation of 38±1, 53±1, 62±1, and 113±1 kDa proteins (which we termed α, β, γ, and δ, respectively) and several others. Although we observed differing levels of phosphorylation among the four proteins, doses of 3,5,3′-l-triiodothyronine or l-thyroxine ranging from 1 to 30nM caused significant dose-dependent stimulation of the phosphorylation of all of them, an effect which occurred within three minutes. In each case, the enhancement of phosphorylation diminished with higher concentrations (100nM-1μM) of 3,5,3′-l-triiodothyronine. In contrast, incubations with similar doses of 3,3′,5′-l-triiodothyronine (reverse l-triiodothyronine) were without significant effect, indicating a specificity for 3,5,3′-l-triiodothyronine and l-thyroxine. Western blots of synaptosomal lysates incubated with 3,5,3′-l-triiodothyronine (1nM-1μM) demonstrated phosphorylation at the serine residues of a 112 kDa protein (matching δ) and phosphorylation at tyrosyl residues of a distinct 95 kDa protein. These data support the contention that thyroid hormones have a variety of rapid nongenomic pathways for regulation of protein phosphorylation in mature mammalian brain.