Thecochleaisinnervatedbytwomainclassesofneurons:thespiralganglionneuron(SGN)afferents,which transmitinformationfromtheeartothebrain,andtheolivocochlearneuron(OCN)efferents,whichprovide feedbackfromthebraintotheear.Housedintheauditorybrainstem,OCNscomprisetwosmallpopulations of cholinergic neurons that send axons along the eighth nerve and into the cochlea. One subset, the medial olivocochlear(MOC)efferents,extendmyelinatedaxonsthatfasciculatewithSGNafferentsinradialbundles and terminate on outer hair cells in the organ of Corti. The other subset, the lateral olivocochlear (LOC) efferents,developthinner,unmyelinatedaxonsthatalsofollowalongtheradialbundles,butterminateinstead ontheendingsofTypeISGNafferentscontactingtheinnerhaircells.Together,theLOCandMOCneurons modulatetheoutputofthecochlea,therebyimprovingbinauralhearingandprotectingthecochleafromthe effects of excess noise and aging. By investigating how LOC and MOC neurons develop and establish connections, we can gain valuable insights into how the cochlea is wired and maintained for a lifetime of hearing. This knowledge will improve cochlear implant technology and identify new molecular entry points forrewiringthedamagedcochlea. OCNaxonsdevelopintightassociationwiththeSGNafferents,whichappeartoprovideascaffoldforgrowth within the cochlea. In turn, OCN efferents influence SGN activity both indirectly, by forming transient synapses with the IHCs during development, and directly, by forming synapses on Type I peripheral processes that can regulate mature SGN firing properties. Based on the intimate relationship between these two populations, we hypothesize that reciprocal interactions between efferents and afferents sculpt the final wiring pattern of the cochlea. To investigate this idea, we propose to launch a new research project aimed at defininghowandwhenOCNaxonsinteractwithSGNafferents,bothatthecellularlevelandatthemolecular level. We will start by using genetic approaches to document afferent?efferent interactions with high spatial and temporal resolution. In parallel, we will use newly available molecular biology techniques to identify genes that are differentially expressed in LOC and MOC neurons, including those that might direct each populationtowardsdistincttargetsinthecochlea.Thesestudieswillbecomplementedwithafocusedanalysis of the transcription factor Gata3, which we found is required in OCNs for proper innervation of the cochlea, with secondary effects on SGN afferent growth and targeting. Results from the proposed experiments will establishaframeworkforstudyingthedevelopmentandfunctionofOCNsandprovidenewinsightsintothe molecularpathwaysthatguidethedualinnervationofthecochleabyafferentsandefferents.

Public Health Relevance

Thesenseofhearingdependsontheactivityofneuronsintheinnerear,whichallowanimalstohear,locate andinterpretawidevarietyofsoundsintheworld.Wewillstudythedevelopmentofacriticalpopulationof neurons that are housed in the brain and provide feedback to the neurons in the ear, enhancing cochlear functionoverallwhilealsoprovidingcrucialprotectionfromthedamagingeffectsofexcessnoiseandaging. Our work will improve the design of cochlear implants, inform efforts to repair the damaged cochlea, and provideinsightsintotheendogenoussystemsthatmaintaintheearforalifetimeofhearing.

Agency
National Institute of Health (NIH)
Institute
National Institute on Deafness and Other Communication Disorders (NIDCD)
Type
Research Project (R01)
Project #
5R01DC015974-02
Application #
9382877
Study Section
Special Emphasis Panel (ZRG1)
Program Officer
Freeman, Nancy
Project Start
2016-12-01
Project End
2021-11-30
Budget Start
2017-12-01
Budget End
2018-11-30
Support Year
2
Fiscal Year
2018
Total Cost
Indirect Cost
Name
Harvard Medical School
Department
Biology
Type
Schools of Medicine
DUNS #
047006379
City
Boston
State
MA
Country
United States
Zip Code
Shrestha, Brikha R; Chia, Chester; Wu, Lorna et al. (2018) Sensory Neuron Diversity in the Inner Ear Is Shaped by Activity. Cell 174:1229-1246.e17
Frank, Michelle M; Goodrich, Lisa V (2018) Talking back: Development of the olivocochlear efferent system. Wiley Interdiscip Rev Dev Biol 7:e324