Affiliations: [a] Department of Research, Center for Stem Cell Biology, Department of Research, Roger Williams Medical Center, Chalkstone, RI, USA | [b] Department of Pathology, Rhode Island Hospital, Brown Medical School, Providence, RI 02903, USA
Correspondence:
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Corresponding author: Dr. LuGuang Luo, Department of Research, Center for Stem Cell Biology, Roger Williams Medical Center, 825 Chalkstone, RI 02908, USA. Tel.: +1 401 456 5344; Fax: +1 401 456 5759; E-mail: Lluo@RWMC.org.
Abstract: Depletion of thyrotropin releasing hormone (TRH) gene expression resulted in augmented tau and glycosynthetase kinase-3β (GSK-3β), in contrast, TRH administration resulted in decreases of 75% in GSK-3β and 90% in Tau phosphorylation in cultured rat hippocampal neurons. To further study TRH regulation of tau phosphorylation, immunoblotting was used to explore G-protein coupled TRH receptor activation of the phosphokinase C (PKC) and phosphokinase A (PKA) signaling pathways. TRH was found to rapidly activate PKA (2.5 fold in 10 min) while it suppressed PKC (levels decreased by 85% vs. control) in hippocampal neurons. This process was also discovered to be a cell type-specific response, as TRH activated PKC in only hypothalamic neurons. Further investigation revealed that the Src inhibitor Protein Phosphatase 2 (PP2, 50 uM) could block TRH inhibition of PKC, GSK-3β, and tau phosphorylation with no effects on PKA. In addition, the PKC inhibitor GF109203 Bis (10 uM) was also able to suppress TRH inhibition of GSK-3β, leading to increased GSK-3 β activity. Independent of these effects, inhibition of PKA by H89 (10 uM) significantly blocked TRH inhibition of GSK-3 β. These data suggests that both PKA and PKC are independently crucial to TRH's effects on GSK-3 β, and support the roles of two distinct pathways involving suppression of PKC via the Src kinase and activation of PKA in mediating TRH effects on GSK-3 β and tau. These dual signaling pathways between TRH and tau may provide mechanisms for the precise regulation of tau phosphorylation and dephosphorylation in neurons.
Keywords: TRH, Tau, PKA, PKC, Src, signal transduction, hippocampal neurons
