Distribution of Smoothened at hippocampal mossy fiber synapses.

The seven-transmembrane receptor Smoothened is essential for hedgehog signal transduction. In adulthood, the highest density of Smoothened mRNA is found in the granule cell layer of the dentate gyrus. There, Smoothened expression is regulated by the synaptic activity involving the glutamatergic transmission. The precise localization of Smoothened proteins, however, has not yet been reported. Here, we describe Smoothened protein distribution in the hippocampal mossy fibers using specific Smoothened antibodies. We provide evidences for their presynaptic localization, and using electron microscopy, show that Smoothened is located in close association with synaptic vesicles and rarely with the plasma membrane. These findings demonstrate that Smoothened is localized presynaptically and suggest that Smoothened signal transduction may be implicated in the complex aspects of mossy fiber function.