Innervation regulates synaptic ribbons in lateral line mechanosensory hair cells

Abstract

Failure to form proper synapses in mechanosensory hair cells, the sensory cells responsible for hearing and balance, leads to deafness and balance disorders. Ribbons are electron-dense structures that tether synaptic vesicles to the presynaptic zone of mechanosensory hair cells where they are juxtaposed with the post-synaptic endings of afferent fibers. They are initially formed throughout the cytoplasm, and, as cells mature, ribbons translocate to the basolateral membrane of hair cells to form functional synapses. We have examined the effect of post-synaptic elements on ribbon formation and maintenance in the zebrafish lateral line system by observing mutants that lack hair cell innervation, wild-type larvae whose nerves have been transected and ribbons in regenerating hair cells. Our results demonstrate that innervation is not required for initial ribbon formation but suggest that it is crucial for regulating the number, size and localization of ribbons in maturing hair cells, and for ribbon maintenance at the mature synapse.

Excessive activation of ionotropic glutamate receptors induces apoptotic hair-cell death independent of afferent and efferent innervation

“…2G), supporting that ngn1 morphant zebrafish hair cells are morphologically and, at least in part, functionally mature despite lacking afferent and efferent innervation. It should be noted that hair-cell innervation is important for synaptic-ribbon maintenance in zebrafish36 and afferent innervation plays a significant and variable role in long-term hair-cell maintenance and viability in mice37. Nevertheless, previous studies have shown that short term hair-cell survival is possible in the absence of innervation38, indicating ngn1 morphants as a useful tool in determining whether iGluR agonist exposure damages hair cells in the absence of innervation.…”

Cnr2 is important for Ribbon Synapse Maturation and Function in Hair Cells and Photoreceptors

“…The first mature and functional HCs can be found in the LL and inner ear as early as 3dpf, even if the maturation process is ongoing in both structures into adulthood [13,85]. HC innervation also happens early and is extensively documented in the LL [86][87][88], and it was demonstrated that innervation regulates ribbon synapses development and maturation [32]. Interestingly, we did not find overt defects in the sensory or motor innervation of NMs, thus suggesting that the defects observed in the sensory synapses were HC autonomous, but this remains to be tested.…”

“…Furthermore, both the size and number of ribbons per sensory cell vary depending on the species [25], the cell type [25], the position in the sensory epithelium [26], the developmental stage [27,28], and the sensory activity [29]. In the LL of zebrafish larva, mature HCs arise as early as 3dpf, in which three to four spherical ribbons (diameter ~200-300nm) neatly arranged in the basolateral portion can be visualized [24,[30][31][32]. Tethered spherical (diameter ~20-30nm) glutamate-filled vesicles surround the ribbon body.…”

Transmission Disrupted: Modeling Auditory Synaptopathy in Zebrafish

“…In contrast to electron micrographs, quantitative analysis of immunolabeled epithelial whole mounts provide the advantage of being able to examine synaptic features of large numbers of hair cells. This advantage has been used by a number of groups to characterize relative variances in size and morphologies of pre- and postsynaptic components in mammalian and zebrafish hair-cell organs ( Wong et al, 2014 ; Paquette et al, 2016 ; Suli et al, 2016 ; Becker et al, 2018 ; Jean et al, 2018 ). Further benefits of using the larval zebrafish lateral-line system for quantitative imaging of immunolabeled structures are twofold.…”

“…Further benefits of using the larval zebrafish lateral-line system for quantitative imaging of immunolabeled structures are twofold. First, the relative simplicity; each neuromast contains 10–16 hair cells with ∼3 synapses per cell of similar morphology, making comparative analyses of synapses in numerous lateral-line organs straightforward ( Sheets et al, 2012 ; Suli et al, 2016 ). This in contrast to mammalian auditory hair cells where the number and size of synapses can vary among individual hair cells depending on location within the Organ of Corti—the sensory organ for hearing ( Meyer et al, 2009 ).…”