S-equol is a major bacterial metabolite of the soy isoflavone daidzein. It is known to be a phytoestrogen that acts by binding to the nuclear estrogen receptors (ERs) that are expressed in various brain regions, including the cerebellum. However, the effects of S-equol on cerebellar development and function have not yet been extensively studied. In this study, the effects of S-equol were evaluated using a mouse primary cerebellar culture, Neuro-2A clonal cells, and an astrocyte-enriched culture. S-equol augmented the dendrite arborization of Purkinje cells induced by triiodothyronine (T₃) and the neurite growth of Neuro-2A cell differentiation. Such augmentation was suppressed by G15, a selective G-protein coupled ER (GPR30) antagonist, and ICI 182,780, an antagonist for ERs in both cultures. On the other hand, in astrocytes, S-equol induced cell proliferation and cell migration with an increase in the phosphorylated extracellular-signal-regulated kinase 1/2 and F-actin rearrangements. Such effects were suppressed by G15, but not by ICI. These findings indicated that S-equol may enhanced cerebellar development by affecting both neurons and astrocytes through several signaling pathways, including GPR30 and ERs. We here report a novel mechanism of S-equol in cerebellar development that may provide a novel possibility to use S-equol supplementation during development.

Perfluorooctane sulfonate (PFOS) has been used in a wide variety of industrial and commercial products. The adverse effects of PFOS on the developing brain are becoming of a great concern. However, the molecular mechanisms of PFOS on brain development have not yet been clarified. We investigated the effect of early-life exposure to PFOS on brain development and the mechanism involved. We investigated the change in thyroid hormone (TH)-induced dendrite arborization of Purkinje cells in the primary culture of newborn rat cerebellum. We further examined the mechanism of PFOS on TH signaling by reporter gene assay, quantitative RT-PCR, and type 2 iodothyronine deiodinase (D2) assay. As low as 10⁻⁷ M PFOS suppressed thyroxine (T₄)-, but not triiodothyronine (T₃)-induced dendrite arborization of Purkinje cells. Reporter gene assay showed that PFOS did not affect TRα1- and TRβ1-mediated transcription in CV-1 cells. RT-PCR showed that PFOS suppressed D2 mRNA expression in the absence of T₄ in primary cerebellar cells. D2 activity was also suppressed by PFOS in C6 glioma-derived cells. These results indicate that early-life exposure of PFOS disrupts TH-mediated cerebellar development possibly through the disruption of D2 activity and/or mRNA expression, which may cause cerebellar dysfunction.

Thyroid hormones (THs) and estrogen (E2) play essential roles in neuronal differentiation and plasticity during brain development. S-equol, a plant-derived isoflavone metabolite, is a selective E2 receptor (ER) ligand that exhibits neurotrophic effects; however, its interaction with TH receptor (TR) signaling remains unclear. In this study, we investigated the effects of S-equol on TRβ-associated transcriptional activity and neuronal morphogenesis in mouse neuroblastoma-derived Neuro-2a cells or rat C6 glioma cells. Luciferase reporter assays demonstrated that S-equol significantly enhanced T3-induced TRβ transcriptional activity in a concentration- and time-dependent manner. Additionally, exposure to S-equol or T3 alone promoted neurite outgrowth and wound closure, whereas co-exposure to both compounds resulted in a more significant enhancement of these processes. Furthermore, mRNA expression levels of synapse-related genes (Dlg4, Syn1, Syp, Camk2b, and Bdnf) were significantly increased by S-equol co-exposure in the presence of T3. In silico docking analysis revealed that S-equol exhibited moderate to high binding affinity for TRβ (−8.7 kcal/mol), ERα, and ERβ, suggesting a structural basis for TR–ER crosstalk. Collectively, these findings indicate that S-equol functions as a dual-acting modulator that may modulate T3 signaling involving TR–ER interaction. Although S-equol may exert beneficial effects on neurodevelopment, it may also act as an endogenous endocrine modulator that alters the fine regulation of TH action during development, warranting careful evaluation from physiological and toxicological perspectives.