Cerebellar astrocytes treated by thyroid hormone modulate neuronal proliferation

Thyroid hormones are important for neurogenesis and gliogenesis during brain development. We have previously demonstrated that triiodothyronine (T3) treatment induced proliferation in primary culture astrocytes derived from the cerebellum of neonatal rats. Conditioned medium obtained from those T3-treated astrocytes (T3CM) mimicked the effect of hormonal treatment on these cells. Because neuron-glia interaction plays an important role in brain development, we tested the ability of such T3-glial CM to influence neuronal physiology. With that aim, neurons from 19-day embryonic cerebella were cultivated for 24 h in the presence of CM obtained from T3-treated cerebellar astrocytes. Interestingly, the cerebellar neuronal population increased by 60-80% in T3CM. Addition of 5 microM forskolin enhanced the responsiveness of cerebellar neurons to astrocytes T3CM, but it did not interfere with neuronal survival in control medium. Conversely, inhibition of adenylate cyclase by its specific inhibitor, SQ22536, reversed the T3CM effect on neurons. These data strongly suggest that cAMP signal transduction pathways might be implicated in such an event. Analysis of bromodeoxyuridil incorporation revealed that the increase in neuron number in T3CM was partially due to neuron proliferation, because the proliferation index was three times higher in T3CM than in control medium. Neutralizing antibody assays demonstrated that T3CM effects on neurons are due, at least in part, to the presence of tumor necrosis factor-beta and epidermal growth factor. Thus, we report here a novel molecular mechanism of action of thyroid hormone on cerebellar neuronal cells: Thyroid hormone induces astrocytes to secrete growth factors that can interfere with neuronal proliferation via a paracrine pathway.