SYNACTAUD · Requirement for hair cell electrical activity in the auditory sensory map formation: Assessment by genetically controlled inhibition of synaptic activity in mice

Requirement for hair cell electrical activity in the auditory sensory map formation: Assessment by genetically controlled inhibition of synaptic activity in mice

Project context Precise wiring of neurons in sensory systems is necessary for the precise detection of stimuli. The precision of these neural circuits is achieved by two processes: developmentally programmed pattern formation and experience dependent refinement. The inherited pattern formation of neural connectivity results from spatially and temporally coordinated expression of axon guidance molecules, and is completed in embryo. After the map is established, it is refined, adapting itself better to the environment by determining whether certain connections are maintained or not. It has been proposed that this fine-tuning is accomplished by experience-dependent neural activity, meaning that neuronal survival depends on the presence of afferent inputs. Humans are able to discriminate sounds with frequencies that are separated by no more than 0.1 %. This level of performance arises from the exceptional sensory capacities of our cochlea, which detects stimuli through an array of sensory hair cells, spatially organised to follow a gradient of preferred frequency (a tonotopic map). This map is conserved downstream of the cochlea, implying that cochlear neurons must connect hair cells to the cochlear nuclei in the brain with high precision so as to retain the performance of the auditory system. Project objectives The objective of this project is to address the question as to how this precision is achieved, in particular to assess a possibility that hair cell activity - both prehearing spontaneous, and stimulus dependent - might play a fundamental role in the fine-tuning of this wiring by silencing the hair cell activity and measuring the effect of this on their afferent innervation. To do this, an exogenous hyperpolarising channel is expressed in hair cells so that activity-dependent plasticity might be temporally delimited in development. For the technique to enable the exogenous hyperpolarising channel expression, we considered several methods like constructing transgenic mice or in vitro culturing and infection system. However, direct injection of adenovirus into mouse cochlea was finally chosen. The technique has only been established in a few laboratories in the world(see references), and the application of the system is infinite because it allows us to investigate in vivo a transgenic mouse system while saving years in the time required to prepare the transgenics, and can be applied to the wide range of injection timing, from neonatal, to early postnatal, or even adult mouse. Project work We have set up the systems to prepare adenoviral vectors and to perform the surgery to inject adenovirus in our laboratory, and are now optimising the efficiency of the surgery and infection. We will then test if the silencing of a given hair cell affects the afferent innervation of itself and/or its neighbours. This project will provide a new framework for studying the wiring of the cochlea, through which new questions may be addressed at the level of the afferent neurons and downstream. The medical implications of this are twofold: it will help us understand pathologies related to the improper innervation of the cochlea, and to develop strategies to improve the wiring of cochlear implants in the brain. References Iizuka, T. et al., Human Gene Therapy 19: 384-390 (2008); Kilpatrick, LA. et al., Gene Therapy 6: 569-578 (2011).

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