A mouse model for benign paroxysmal positional vertigo with genetic predisposition for displaced otoconia

Abstract

Abnormal formation of otoconia, the biominerals of the inner ear, results in balance disorders. The inertial mass of otoconia activates the underlying mechanosensory hair cells in response to change in head position primarily during linear and rotational acceleration. Otoconia associate exclusively with the two gravity receptors, the utricle and saccule. The cristae sensory epithelium is associated with an extracellular gelatinous matrix known as cupula, equivalent to otoconia. During head rotation, the inertia of endolymphatic fluids within the semicircular canals deflects the cupula of the corresponding crista and activates the underlying mechanosensory hair cells. It is believed that detached free-floating otoconia particles travel ectopically to the semicircular canal and cristae and are the culprit for benign paroxysmal positional vertigo (BPPV). The Slc26a4 mouse mutant harbors a missense mutation in pendrin. This mutation leads to impaired transport activity of pendrin and to defects in otoconia composition and distribution. All Slc26a4 ^loop/loop homozygous mutant mice are profoundly deaf but show inconsistent vestibular deficiency. A panel of behavioral tests was utilized in order to generate a scoring method for vestibular function. A pathological finding of displaced otoconia was identified consistently in the inner ears of mutant mice with severe vestibular dysfunction. In this work, we present a mouse model with a genetic predisposition for ectopic otoconia with a clinical correlation to BPPV. This unique mouse model can serve as a platform for further investigation of BPPV pathophysiology, and for developing novel treatment approaches in a live animal model.

Keywords: SLC26A4; balance; deafness; hearing; vertigo.

FIGURE 1

The Slc26a4^loop/loop mutant mice are profoundly deaf but show variable vestibular dysfunction. Tone-burst ABR tests demonstrate that Slc26a4^loop/loop are all deaf with no exception and regardless of any vestibular abnormality.

FIGURE 2

A battery of behavioral tests distinguish between two subgroups of Slc26a4^loop/loop mutant mice, based on variable vestibular dysfunction. Selected behavioral tests were used in the behavioral pipeline: (a) the forced swimming test; (b) the trunk curl test; (c) presence of circling behavior and/or head tilt; and (d) observation for gait abnormality. (e) The scoring system with a description for each individual test is depicted. A cumulative score was given to each mouse following the completion of all tests. (f) A significant difference between two subgroups of mutant mice with and without vestibular deficiency is shown. The average cumulative score of each group is shown for each behavioral test separately (column bar graph) and collectively (box plot), summarizing all behavioral tests score together. For mutants with vestibular dysfunction, N=32; without vestibular dysfunction, N=47.

FIGURE 3

Displaced otoconia strongly correlate with severe vestibular dysfunction of Slc26a4^loop/loop mutant mice. (a, b) In wild-type mice, thousands of 5um otoconia are associated with the sensory macula of the utricle and saccule. (c) The vestibular membranous labyrinth of Slc26a4^loop/loop mutant mice with severe vestibular dysfunction is shown. An ectopic giant otoconia is present at the posterior crista ampullaris (white arrow). (d, e) In all Slc26a4^loop/loop mutant mice, giant otoconia were detected within the utricle and saccule. (f) Among the subgroup with severe vestibular dysfunction, an ectopic giant otoconia was found in at least one ear in 30 out of 32 mutant mice (93.8%). In the subgroup of mutant mice with normal vestibular function, ectopic giant otoconia was found in only 3 out of 47 mice (6.4%). For mutant with vestibular dysfunction, N=32; and without vestibular dysfunction, N=47.

FIGURE 4

Ectopic otoconia underlies severe vestibular dysfunction of Slc26a4^loop/loop mutant mice. (a) The inner ear is divided into two functional anatomical units, the cochlear system for hearing and the vestibular system for balance and motion perception. The vestibule is comprised of five sensory organ, two maculae, utricle and saccule, for perception of gravity, linear and vertical acceleration (green). (b) The three cristae ampullaris and associated semicircular canals are responsible for perception of angular and rotational head movement in the three-dimensional (3D) axis. (c) In wild type mice, thousands of otoconia are associated with the sensory maculae of the utricle and saccule (green). (d) In Slc26a4^loop/loop mutant mice, otoconia are replaced with a giant mineralized body. (e) The abnormal otoconia particle is occasionally displaced from maculae, either floating in endolymph (canalithiasis) or attached to the gelatinous cupula of the cristae (cupulithiasis) and leads to pathological vestibular stimulation. (f) Under normal conditions, head rotation in one direction leads to inertia of endolymphatic fluids within the semicircular canal to the opposite direction. Depending on the direction of fluid inertia, deflection of the cupula either activates or suppresses the underlying sensory hair cells. An excitatory stimulus triggered by a semicircular canal in one ear is simultaneously coupled with inhibitory suppression of the contralateral semicircular canal corresponding to the same plan of rotational axis. The balance between primary afferent firing rates of both sides is processed towards a central vestibular sensation.