Edward C. and Amy H. Sewall Professor of Otolaryngology — Head & Neck Surgery


  • Navigating Hereditary Hearing Loss: Pathology of the Inner Ear. Frontiers in cellular neuroscience Nicolson, T. 2021; 15: 660812


    Inherited forms of deafness account for a sizable portion of hearing loss among children and adult populations. Many patients with sensorineural deficits have pathological manifestations in the peripheral auditory system, the inner ear. Within the hearing organ, the cochlea, most of the genetic forms of hearing loss involve defects in sensory detection and to some extent, signaling to the brain via the auditory cranial nerve. This review focuses on peripheral forms of hereditary hearing loss and how these impairments can be studied in diverse animal models or patient-derived cells with the ultimate goal of using the knowledge gained to understand the underlying biology and treat hearing loss.

    View details for DOI 10.3389/fncel.2021.660812

    View details for PubMedID 34093131

    View details for PubMedCentralID PMC8172992

  • Disruption of tmc1/2a/2b genes in zebrafish reveals subunit requirements in subtypes of inner ear hair cells. The Journal of neuroscience : the official journal of the Society for Neuroscience Smith, E. T., Pacentine, I., Shipman, A., Hill, M., Nicolson, T. 2020


    Detection of sound and head movement requires mechanoelectrical transduction (MET) channels at tips of hair-cell stereocilia. In vertebrates, the transmembrane channel-like (TMC) proteins TMC1 and TMC2 fulfill critical roles in MET and substantial evidence implicates these TMCs as subunits of the MET channel. To identify developmental and functional roles of this Tmc subfamily in the zebrafish inner ear, we tested the effects of truncating mutations in tmc1, tmc2a, and tmc2b on in vivo mechanosensation at the onset of hearing and balance, before gender differentiation. We find that tmc1/2a/2b triple-mutant larvae cannot detect sound or orient with respect to gravity. They lack acoustic-evoked behavioral responses (AEBR), vestibular-induced eye movements (VIEM), and hair-cell activity as assessed with FM dye labeling and microphonic potentials. Despite complete loss of hair-cell function, tmc triple-mutant larvae retain normal gross morphology of hair bundles and proper trafficking of known MET components Protocadherin 15a (Pcdh15a), Lipoma HMGIC fusion partner-like 5 (Lhfpl5), and Transmembrane inner ear protein (Tmie). Transgenic, hair cell-specific expression of Tmc2b-mEGFP rescues the behavioral and physiological deficits in tmc triple mutants. Results from tmc single- and double- mutants evince a principle role for Tmc2a and Tmc2b in hearing and balance, respectively, whereas Tmc1 has lower overall impact. Our experiments reveal that in developing cristae, hair cells stratify into an upper, Tmc2a-dependent layer of teardrop shaped cells and a lower, Tmc1/2b-dependent tier of gourd shaped cells. Collectively our genetic evidence indicates that auditory/vestibular end organs and subsets of hair cells therein rely on distinct combinations of Tmc1/2a/2b.Significance StatementWe assessed the effects of tmc1/2a/2b truncation mutations on mechanoelectrical transduction (MET) in the inner-ear hair cells of larval zebrafish. tmc triple mutants lacked behavioral responses to sound and head movements, while further assays demonstrated no observable mechanosensitivity in the tmc1/2a/2b triple mutant inner ear. Examination of tmc double mutants revealed major contributions from Tmc2a and Tmc2b to macular function; however, Tmc1 had less overall impact. FM labeling of lateral cristae in tmc double mutants revealed the presence of two distinct cell types, an upper layer of teardrop shaped cells that rely on Tmc2a, and a lower layer of gourd shaped cells that rely on Tmc1/2b.

    View details for DOI 10.1523/JNEUROSCI.0163-20.2020

    View details for PubMedID 32371604

  • The lhfpl5 Ohnologs lhfpl5a and lhfpl5b Are Required for Mechanotransduction in Distinct Populations of Sensory Hair Cells in Zebrafish FRONTIERS IN MOLECULAR NEUROSCIENCE Erickson, T., Pacentine, I. V., Venuto, A., Clemens, R., Nicolson, T. 2020; 12: 320


    Hair cells sense and transmit auditory, vestibular, and hydrodynamic information by converting mechanical stimuli into electrical signals. This process of mechano-electrical transduction (MET) requires a mechanically gated channel localized in the apical stereocilia of hair cells. In mice, lipoma HMGIC fusion partner-like 5 (LHFPL5) acts as an auxiliary subunit of the MET channel whose primary role is to correctly localize PCDH15 and TMC1 to the mechanotransduction complex. Zebrafish have two lhfpl5 genes (lhfpl5a and lhfpl5b), but their individual contributions to MET channel assembly and function have not been analyzed. Here we show that the zebrafish lhfpl5 genes are expressed in discrete populations of hair cells: lhfpl5a expression is restricted to auditory and vestibular hair cells in the inner ear, while lhfpl5b expression is specific to hair cells of the lateral line organ. Consequently, lhfpl5a mutants exhibit defects in auditory and vestibular function, while disruption of lhfpl5b affects hair cells only in the lateral line neuromasts. In contrast to previous reports in mice, localization of Tmc1 does not depend upon Lhfpl5 function in either the inner ear or lateral line organ. In both lhfpl5a and lhfpl5b mutants, GFP-tagged Tmc1 and Tmc2b proteins still localize to the stereocilia of hair cells. Using a stably integrated GFP-Lhfpl5a transgene, we show that the tip link cadherins Pcdh15a and Cdh23, along with the Myo7aa motor protein, are required for correct Lhfpl5a localization at the tips of stereocilia. Our work corroborates the evolutionarily conserved co-dependence between Lhfpl5 and Pcdh15, but also reveals novel requirements for Cdh23 and Myo7aa to correctly localize Lhfpl5a. In addition, our data suggest that targeting of Tmc1 and Tmc2b proteins to stereocilia in zebrafish hair cells occurs independently of Lhfpl5 proteins.

    View details for DOI 10.3389/fnmol.2019.00320

    View details for Web of Science ID 000509937300001

    View details for PubMedID 32009898

    View details for PubMedCentralID PMC6974483

  • Temporal Vestibular Deficits in synaptojanin 1 (synj1) Mutants. Frontiers in molecular neuroscience Gao, Y., Nicolson, T. 2020; 13: 604189


    The lipid phosphatase synaptojanin 1 (synj1) is required for the disassembly of clathrin coats on endocytic compartments. In neurons such activity is necessary for the recycling of endocytosed membrane into synaptic vesicles. Mutations in zebrafish synj1 have been shown to disrupt the activity of ribbon synapses in sensory hair cells. After prolonged mechanical stimulation of hair cells, both phase locking of afferent nerve activity and the recovery of spontaneous release of synaptic vesicles are diminished in synj1 mutants. Presumably as a behavioral consequence of these synaptic deficits, synj1 mutants are unable to maintain an upright posture. To probe vestibular function with respect to postural control in synj1 mutants, we developed a method for assessing the vestibulospinal reflex (VSR) in larvae. We elicited the VSR by rotating the head and recorded tail movements. As expected, the VSR is completely absent in pcdh15a and lhfpl5a mutants that lack inner ear function. Conversely, lhfpl5b mutants, which have a selective loss of function of the lateral line organ, have normal VSRs, suggesting that the hair cells of this organ do not contribute to this reflex. In contrast to mechanotransduction mutants, the synj1 mutant produces normal tail movements during the initial cycles of rotation of the head. Both the amplitude and temporal aspects of the response are unchanged. However, after several rotations, the VSR in synj1 mutants was strongly diminished or absent. Mutant synj1 larvae are able to recover, but the time required for the reappearance of the VSR after prolonged stimulation is dramatically increased in synj1 mutants. Collectively, the data demonstrate a behavioral correlate of the synaptic defects caused by the loss of synj1 function. Our results suggest that defects in synaptic vesicle recycling give rise to fatigue of ribbons synapses and possibly other synapses of the VS circuit, leading to the loss of postural control.

    View details for DOI 10.3389/fnmol.2020.604189

    View details for PubMedID 33584199

  • Subunits of the mechano-electrical transduction channel, Tmc1/2b, require Tmie to localize in zebrafish sensory hair cells PLOS GENETICS Pacentine, I. V., Nicolson, T. 2019; 15 (2): e1007635


    Mutations in transmembrane inner ear (TMIE) cause deafness in humans; previous studies suggest involvement in the mechano-electrical transduction (MET) complex in sensory hair cells, but TMIE's precise role is unclear. In tmie zebrafish mutants, we observed that GFP-tagged Tmc1 and Tmc2b, which are subunits of the MET channel, fail to target to the hair bundle. In contrast, overexpression of Tmie strongly enhances the targeting of Tmc1-GFP and Tmc2b-GFP to stereocilia. To identify the motifs of Tmie underlying the regulation of the Tmcs, we systematically deleted or replaced peptide segments. We then assessed localization and functional rescue of each mutated/chimeric form of Tmie in tmie mutants. We determined that the first putative helix was dispensable and identified a novel critical region of Tmie, the extracellular region and transmembrane domain, which is required for both mechanosensitivity and Tmc2b-GFP expression in bundles. Collectively, our results suggest that Tmie's role in sensory hair cells is to target and stabilize Tmc channel subunits to the site of MET.

    View details for PubMedID 30726219

  • Subunits of the mechano-electrical transduction channel, Tmc1/2b, require Tmie to localize in zebrafish sensory hair cells. PlosGenetics Pacentine, I. V., Nicolson, T. 2019
  • Zebrafish: from genes and neurons to circuits, behavior and disease. Journal of neurogenetics Chandrasekhar, A., Guo, S., Masai, I., Nicolson, T., Wu, C. F. 2017; 31 (3): 59-60

    View details for DOI 10.1080/01677063.2017.1359589

    View details for PubMedID 28868983

  • The genetics of hair-cell function in zebrafish. Journal of neurogenetics Nicolson, T. 2017; 31 (3): 102-112


    Our ears are remarkable sensory organs, providing the important senses of balance and hearing. The complex structure of the inner ear, or 'labyrinth', along with the assorted neuroepithelia, have evolved to detect head movements and sounds with impressive sensitivity. The rub is that the inner ear is highly vulnerable to genetic lesions and environmental insults. According to National Institute of Health estimates, hearing loss is one of the most commonly inherited or acquired sensorineural diseases. To understand the causes of deafness and balance disorders, it is imperative to understand the underlying biology of the inner ear, especially the inner workings of the sensory receptors. These receptors, which are termed hair cells, are particularly susceptible to genetic mutations - more than two dozen genes are associated with defects in this cell type in humans. Over the past decade, a substantial amount of progress has been made in working out the molecular basis of hair-cell function using vertebrate animal models. Given the transparency of the inner ear and the genetic tools that are available, zebrafish have become an increasingly popular animal model for the study of deafness and vestibular dysfunction. Mutagenesis screens for larval defects in hearing and balance have been fruitful in finding key components, many of which have been implicated in human deafness. This review will focus on the genes that are required for hair-cell function in zebrafish, with a particular emphasis on mechanotransduction. In addition, the generation of new tools available for the characterization of zebrafish hair-cell mutants will be discussed.

    View details for DOI 10.1080/01677063.2017.1342246

    View details for PubMedID 28705044

    View details for PubMedCentralID PMC6080859

  • Enlargement of Ribbons in Zebrafish Hair Cells Increases Calcium Currents But Disrupts Afferent Spontaneous Activity and Timing of Stimulus Onset. The Journal of neuroscience : the official journal of the Society for Neuroscience Sheets, L., He, X. J., Olt, J., Schreck, M., Petralia, R. S., Wang, Y. X., Zhang, Q., Beirl, A., Nicolson, T., Marcotti, W., Trapani, J. G., Kindt, K. S. 2017; 37 (26): 6299-6313


    In sensory hair cells of auditory and vestibular organs, the ribbon synapse is required for the precise encoding of a wide range of complex stimuli. Hair cells have a unique presynaptic structure, the synaptic ribbon, which organizes both synaptic vesicles and calcium channels at the active zone. Previous work has shown that hair-cell ribbon size is correlated with differences in postsynaptic activity. However, additional variability in postsynapse size presents a challenge to determining the specific role of ribbon size in sensory encoding. To selectively assess the impact of ribbon size on synapse function, we examined hair cells in transgenic zebrafish that have enlarged ribbons, without postsynaptic alterations. Morphologically, we found that enlarged ribbons had more associated vesicles and reduced presynaptic calcium-channel clustering. Functionally, hair cells with enlarged ribbons had larger global and ribbon-localized calcium currents. Afferent neuron recordings revealed that hair cells with enlarged ribbons resulted in reduced spontaneous spike rates. Additionally, despite larger presynaptic calcium signals, we observed fewer evoked spikes with longer latencies from stimulus onset. Together, our work indicates that hair-cell ribbon size influences the spontaneous spiking and the precise encoding of stimulus onset in afferent neurons.SIGNIFICANCE STATEMENT Numerous studies support that hair-cell ribbon size corresponds with functional sensitivity differences in afferent neurons and, in the case of inner hair cells of the cochlea, vulnerability to damage from noise trauma. Yet it is unclear whether ribbon size directly influences sensory encoding. Our study reveals that ribbon enlargement results in increased ribbon-localized calcium signals, yet reduces afferent spontaneous activity and disrupts the timing of stimulus onset, a distinct aspect of auditory and vestibular encoding. These observations suggest that varying ribbon size alone can influence sensory encoding, and give further insight into how hair cells transduce signals that cover a wide dynamic range of stimuli.

    View details for DOI 10.1523/JNEUROSCI.2878-16.2017

    View details for PubMedID 28546313

    View details for PubMedCentralID PMC5490065

  • Integration of Tmc1/2 into the mechanotransduction complex in zebrafish hair cells is regulated by Transmembrane O-methyltransferase (Tomt). eLife Erickson, T., Morgan, C. P., Olt, J., Hardy, K., Busch-Nentwich, E., Maeda, R., Clemens, R., Krey, J. F., Nechiporuk, A., Barr-Gillespie, P. G., Marcotti, W., Nicolson, T. 2017; 6


    Transmembrane O-methyltransferase (TOMT/LRTOMT) is responsible for non-syndromic deafness DFNB63. However, the specific defects that lead to hearing loss have not been described. Using a zebrafish model of DFNB63, we show that the auditory and vestibular phenotypes are due to a lack of mechanotransduction (MET) in Tomt-deficient hair cells. GFP-tagged Tomt is enriched in the Golgi of hair cells, suggesting that Tomt might regulate the trafficking of other MET components to the hair bundle. We found that Tmc1/2 proteins are specifically excluded from the hair bundle in tomt mutants, whereas other MET complex proteins can still localize to the bundle. Furthermore, mouse TOMT and TMC1 can directly interact in HEK 293 cells, and this interaction is modulated by His183 in TOMT. Thus, we propose a model of MET complex assembly where Tomt and the Tmcs interact within the secretory pathway to traffic Tmc proteins to the hair bundle.

    View details for DOI 10.7554/eLife.28474

    View details for PubMedID 28534737

    View details for PubMedCentralID PMC5462536