Rhombomere 4-derived auditory and vestibular systems in Hoxb1 mutants

Rhombomere r4 (r4) and Hox associated genes Hoxb1 and Hoxb2 contribute to the formation of specific auditory and vestibular subcircuits. In particular, sensory and motor components of the sound transmission pathway, sensorimotor reflex circuits, as well as the hindlimb vestibulospinal reflex, derive from r4 and are strongly affected in Hoxb1 mutants.1,2 Inner ear efferent (IEE) neurons also originate from r4 and form vestibular (VEN) and cochlear (CEN) efferent neuron subpopulations. The CEN is further subdivided into medial (MOC) and lateral (LOC) olivocochlear motoneurons. MOC neurons inhibit the motility of outer hair cells (OHCs), which amplify low intensity sounds, while LOCs innervate the afferent terminations on the inner hair cells (IHC) modulating the excitability of the cochlear nerve, thus protecting the cochlea from acoustic damage. Hoxb1null mutants lack MOC and LOC efferent neurons leading to defects in OHCs and in cochlear amplification, and mice have increasing auditory thresholds.1 A hypothesis is that MOC neuron endings play a trophic function on OHCs and that the physical interaction between MOC efferents and OHCs is essential for proper maturation and functioning of OHCs. Further genetic intersectional studies impinging either on motor or sensory components of r4-derived auditory subcircuits are needed to understand the involvement of efferent innervations for proper functioning of the cochlea. Regarding the vestibular system, Hoxb1null mutant mice also fail to form the VEN at early developmental stages.2 However, transmission electron microscopy (TEM) in adult mice reveals the presence of both afferent and efferent neuronal endings on receptor cells. To understand whether projections are missing at birth and new connections gradually appear during the first month by compensatory plasticity mechanisms, we are in the process of testing newborn mutant pups for the presence of efferent endings on hair cells by TEM. We also aim to use retrograde labelling in 3-month old mutant mice to assess their eventual presence and identify their origin.