Transthyretin Induces Insulin-like Growth Factor I Nuclear Translocation Regulating Its Levels in the Hippocampus

Transthyretin (TTR) is the carrier protein of thyroxine (T-4) and binds to retinol-binding protein (RBP)-retinol complex. It is mainly synthesized by both liver and choroid plexuses of the brain. Besides these properties, it has a neuroprotective role in several contexts such as Alzheimer's disease (AD) and cerebral ischemia. Activation of insulin-like growth factor receptor I (IGF-IR) pathways and increased levels of TTR are associated with absence of neurodegeneration in an AD mouse model. In the present study, we verified that young/adult TTR null mice had decreased levels of IGF-IR in the hippocampus, but not in choroid plexus when compared with wild-type age-matched controls. Moreover, we could also demonstrate that conditional silencing of peripheral TTR did not have any influence in hippocampal IGF-IR levels, indicating that TTR effect on IGF-IR levels is due to TTR mainly synthesized in the choroid plexus. In vitro cellular studies, using NIH3T3 cell line and primary cultured hippocampal neurons, we showed that TTR upregulates IGF-IR at the transcription and translation levels and that is dependent on receptor internalization. Using a GFP-IGF-IR fusion protein, we also found that TTR triggers IGF-IR nuclear translocation in cultured neurons. We could also see an enrichment of IGF-IR in the nuclear fraction, after TTR stimulation in NIH3T3 cells, indicating that IGF-IR regulation, triggered by TTR is induced by nuclear translocation. In summary, the results provide evidence of a new role of TTR as a transcription inducer of IGF-IR in central nervous system (CNS), unveiling a new role in neuroprotection.