A Novel Striated Membrane Contact Site in Vestibular Hair Cells

The mammalian vestibular system has two types of sensory receptor hair cells (HCs), each with different neurotransmission mechanisms. Type II HCs use ribbon synapses to transmit neurotransmitters like glutamate to bouton terminals of afferent neurons. Type I HCs are nearly engulfed by a calyceal afferent ending. They regulate afferent activity through both quantal and non-quantal transmission (NQT), the latter faster than classic neurotransmitter release and potentially key to stabilizing vision and balance during rapid head movements. Here, we describe a novel striated contact, present only in the mouse type I HC and its calyceal afferent ending. Electron tomography reveals periodic striations on both the HC and calyx plasma membranes. These contact sites are intimately associated with a plasma membrane mitochondria contact site on the type I HC side of the synapse. Given that mitochondrion-plasma membrane contact sites in other systems regulate local ion homeostasis, this distinctive arrangement has the potential to serve or modulate NQT at the type I HC-calyx synapse.