A novel membrane contact site in vestibular hair cells

Abstract

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 afferent neurons. On the other hand, type I HCs are nearly engulfed by a calyceal afferent ending and also form ribbon synapses. These HCs regulate afferent activity through non-quantal transmission (NQT), which is faster than classic neurotransmitter release and may play a key role in stabilizing vision and balance during rapid head movements. Here, we describe a novel striated contact, present between the mouse type I HC and its calyceal afferent ending, and intimately associated with atypical plasma membrane-apposed (PMA) mitochondria. This distinctive arrangement has the potential to serve or modulate NQT.

Figure 1:

A novel striated junction between the type I hair cell and its calyceal afferent ending in the mouse vestibular sensory organs A: Schematic drawing of the utricular epithelium organisation in an adult mouse (t.I, type I cell; t.II, type II cell; calyx; end, endolymph; ep., sensory epithelium; aff. fib., afferent fibres). B: Electron micrograph of mouse utricular epithelium showing t.I, t.II and calyx (C). C: Adult t.I transverse section, right below the nucleus; yellow *, PMA mitochondria; red *, other mitochondria. D: Segmentation from Serial Blockface (SBF) imaging on t.I showing all mitochondria. Blue ovoid: t.I nucleus. The proportion of contact with the plasma membrane for each mitochondrion is represented in colour ranging from no contact (Red) to 50% of surface contact (white-yellow): mitochondria; (see Suppl. Video 1) E: Quantification of the size of the mitochondria depending on their proximity to the plasma membrane. We bin the mitochondria in 2 groups, either < 20% or > 20% surface contact; n=325 on 3 segmented SBF cells, unpaired t-test **** p>0,0001. F-G: High magnification electron micrographs of striations. Depending on the angle of the section, striations can be visible or not, highlighting the need for electron tomography (imm: inner mitochondrial membrane; omm: outer mitochondrial membrane; m: mitochondria; pm: plasma membrane; t.I: type I HC; c.: calyx.). H-K: ssTEM tomogram H-I: transversal image (H) and corresponding in-face view (I) of a tomogram highlighting the periodic striations on the calyx membrane J: 3D rendering of serial tomograms segmentation of the calyx membrane (green) and the PMA mitochondria (blue) (see Suppl. Video 2). K: pseudo-coloured plasma membranes from segmentations in two serial tomograms (grey: type I HC plasma membrane, green: calyx plasma membrane, orange contour: the surface of contact between the mitochondria and the type I cell plasma membrane).