synaptojanin1 Is Required for Temporal Fidelity of Synaptic Transmission in Hair Cells

Abstract

To faithfully encode mechanosensory information, auditory/vestibular hair cells utilize graded synaptic vesicle (SV) release at specialized ribbon synapses. The molecular basis of SV release and consequent recycling of membrane in hair cells has not been fully explored. Here, we report that comet, a gene identified in an ENU mutagenesis screen for zebrafish larvae with vestibular defects, encodes the lipid phosphatase Synaptojanin 1 (Synj1). Examination of mutant synj1 hair cells revealed basal blebbing near ribbons that was dependent on Cav1.3 calcium channel activity but not mechanotransduction. Synaptojanin has been previously implicated in SV recycling; therefore, we tested synaptic transmission at hair-cell synapses. Recordings of post-synaptic activity in synj1 mutants showed relatively normal spike rates when hair cells were mechanically stimulated for a short period of time at 20 Hz. In contrast, a sharp decline in the rate of firing occurred during prolonged stimulation at 20 Hz or stimulation at a higher frequency of 60 Hz. The decline in spike rate suggested that fewer vesicles were available for release. Consistent with this result, we observed that stimulated mutant hair cells had decreased numbers of tethered and reserve-pool vesicles in comparison to wild-type hair cells. Furthermore, stimulation at 60 Hz impaired phase locking of the postsynaptic activity to the mechanical stimulus. Following prolonged stimulation at 60 Hz, we also found that mutant synj1 hair cells displayed a striking delay in the recovery of spontaneous activity. Collectively, the data suggest that Synj1 is critical for retrieval of membrane in order to maintain the quantity, timing of fusion, and spontaneous release properties of SVs at hair-cell ribbon synapses.

Ribeye is required for presynaptic CaV1.3a channel localization and afferent innervation of sensory hair cells

“…Previously, we have shown that in the lateralline organ, synaptic transmission from hair cells is required for postsynaptic spiking (Obholzer et al, 2008). Thus, spiking recorded in response to direct mechanical stimulation of innervated hair cells provides an indirect measure of hair-cell transmission (Trapani et al, 2009). We found that compared with control larvae, a morphant injected with 0.75 mM ribeye b MO showed a slight reduction in the number of spikes recorded in response to a 20 Hz stimulus (Fig.…”

Input timing for spatial processing is precisely tuned via constant synaptic delays and myelination patterns in the auditory brainstem

“…Here it is well established that the unusual arrangement of vesicles in the ribbon and specialized proteins are required for fast docking and efficient exocytosis (40,41). Moreover, specific short-latency calcium channels have been suggested to be involved in allowing rapid vesicle release also in the ongoing response (42) as well as calcium-induced calcium release from specific internal stores (43).…”

The Sac domain-containing phosphoinositide phosphatases: structure, function, and disease

“…In D. rerio , synaptojanin 1 mutant displays a defect in cone photoreceptor ribbon anchoring, abnormal synaptic transmission at synapses (Holzhausen et al, 2009) as well as a role in maintaining the quantity, fusion and release of synaptic vesicles at hair-cell ribbon synapses (Trapani et al, 2009). In D. melanogaster , synaptojanin is recruited by endophilin to promote synaptic vesicle uncoating (Verstreken et al, 2003).…”