title: Disruption of the presynaptic cytomatrix protein bassoon degrades ribbon anchorage, multiquantal release, and sound encoding at the hair cell afferent synapse.
abstract: Inner hair cells (IHCs) of the cochlea use ribbon synapses to transmit auditory information faithfully to spiral ganglion neurons (SGNs). In the present study, we used genetic disruption of the presynaptic scaffold protein bassoon in mice to manipulate the morphology and function of the IHC synapse. Although partial-deletion mutants lacking functional bassoon (Bsn(ΔEx4/5)) had a near-complete loss of ribbons from the synapses (up to 88% ribbonless synapses), gene-trap mutants (Bsn(gt)) showed weak residual expression of bassoon and 56% ribbonless synapses, whereas the remaining 44% had a loosely anchored ribbon. Patch-clamp recordings and synaptic CaV1.3 immunolabeling indicated a larger number of Ca(2+) channels for Bsn(gt) IHCs compared with Bsn(ΔEx4/5) IHCs and for Bsn(gt) ribbon-occupied versus Bsn(gt) ribbonless synapses. An intermediate phenotype of Bsn(gt) IHCs was also found by membrane capacitance measurements for sustained exocytosis, but not for the size of the readily releasable vesicle pool. The frequency and amplitude of EPSCs were reduced in Bsn(ΔEx4/5) mouse SGNs, whereas their postsynaptic AMPA receptor clusters were largely unaltered. Sound coding in SGN, assessed by recordings of single auditory nerve fibers and their population responses in vivo, was similarly affected in Bsn(gt) and Bsn(ΔEx4/5) mice. Both genotypes showed impaired sound onset coding and reduced evoked and spontaneous spike rates. In summary, reduced bassoon expression or complete lack of full-length bassoon impaired sound encoding to a similar extent, which is consistent with the comparable reduction of the readily releasable vesicle pool. This suggests that the remaining loosely anchored ribbons in Bsn(gt) IHCs were functionally inadequate or that ribbon independent mechanisms dominated the coding deficit.
authors: Auditory Systems Physiology Group, InnerEarLab, University Medical Center, 37075 Göttingen, Germany.
authors: Zhizi
authors: Z
authors: Jing
authors: Mark A
authors: MA
authors: Rutherford
authors: Hideki
authors: H
authors: Takago
authors: Thomas
authors: T
authors: Frank
authors: Anna
authors: A
authors: Fejtova
authors: Darina
authors: D
authors: Khimich
authors: Tobias
authors: T
authors: Moser
authors: Nicola
authors: N
authors: Strenzke
mesh_terms: D000161:Acoustic Stimulation; D000162:Acoustics; D000200:Action Potentials / Q000187:drug effects / Q000235:genetics; D000429:Alcohol Oxidoreductases; D000818:Animals; D001309:Auditory Threshold / Q000502:physiology; D001703:Biophysics; D002118:Calcium / Q000378:metabolism; D020746:Calcium Channels, L-Type / Q000235:genetics / Q000378:metabolism; D056970:Co-Repressor Proteins; D003051:Cochlea / Q000166:cytology*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D004533:Egtazic Acid / Q000494:pharmacology; D004558:Electric Stimulation; D016057:Evoked Potentials, Auditory, Brain Stem / Q000235:genetics; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects / Q000235:genetics; D005260:Female; D005786:Gene Expression Regulation / Q000235:genetics; D006199:Hair Cells, Auditory, Inner / Q000378:metabolism*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D009154:Mutation / Q000235:genetics*; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D017084:Otoacoustic Emissions, Spontaneous / Q000187:drug effects / Q000235:genetics; D018408:Patch-Clamp Techniques; D010750:Phosphoproteins / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D018091:Receptors, AMPA / Q000235:genetics / Q000378:metabolism; D013569:Synapses / Q000502:physiology*
chemical_list: C498441:Bsn protein, mouse; C485995:Cacna1d protein, mouse; D020746:Calcium Channels, L-Type; D056970:Co-Repressor Proteins; D004268:DNA-Binding Proteins; D009419:Nerve Tissue Proteins; D010750:Phosphoproteins; D012333:RNA, Messenger; D018091:Receptors, AMPA; D004533:Egtazic Acid; D000429:Alcohol Oxidoreductases; C111341:Ctbp2 protein, mouse; C104722:glutamate receptor ionotropic, AMPA 2; D002118:Calcium