Calcium Oxalate Stone Formation in the Inner Ear as a Result of an Slc26a4 Mutation

Abstract

Calcium oxalate stone formation occurs under pathological conditions and accounts for more than 80% of all types of kidney stones. In the current study, we show for the first time that calcium oxalate stones are formed in the mouse inner ear of a genetic model for hearing loss and vestibular dysfunction in humans. The vestibular system within the inner ear is dependent on extracellular tiny calcium carbonate minerals for proper function. Thousands of these biominerals, known as otoconia, are associated with the utricle and saccule sensory maculae and are vital for mechanical stimulation of the sensory hair cells. We show that a missense mutation within the Slc26a4 gene abolishes the transport activity of its encoded protein, pendrin. As a consequence, dramatic changes in mineral composition, size, and shape occur within the utricle and saccule in a differential manner. Although abnormal giant carbonate minerals reside in the utricle at all ages, in the saccule, a gradual change in mineral composition leads to a formation of calcium oxalate in adult mice. By combining imaging and spectroscopy tools, we determined the profile of mineral composition and morphology at different time points. We propose a novel mechanism for the accumulation and aggregation of oxalate crystals in the inner ear.Biomineralization processes in the human body normally occur in a variety of different tissues, including bones, teeth, and otoconia within the vestibular system of the inner ear. The vestibular system is comprised of five sensory organs. Three cristae connected to semicircular canals are sensitive for angular movement, and the saccule and utricle are sensitive for linear acceleration and gravity. Otoconia are small highly dense calcitic minerals that associate exclusively with the saccule and utricle. Thousands of otoconia, partially embedded in a gelatinous matrix, are supported on the sensory epithelium and serve as an inertial mass that is critical for mechanical stimulation (1, 2) Movement of the otoconial layer through action of gravitational or inertial forces activate the underlying mechanosensory hair cells to generate action potentials that are transmitted to the brain.The biomineralization process, such as in otoconia formation, involves organic and inorganic components and results in biominerals that differ significantly in morphology and mechanical properties from similar synthetic or geological minerals (3, 4). Otoconia formation occurs outside the cells and therefore depends on secretion of the required assembly components into the endolymphatic spaces (5). Otoconia seeding in mice begins as early as embryonic day (E)14.5 4 and initiates extensive mineral growth, with the highest rate of calcification at E15-16 (6). By postnatal day (P)7, otoconia achieve their final size and are maintained at progressive ages with a low rate of calcium turnover (7). The main inorganic fraction of otoconia in birds and mammals is calcite (CaCO 3 ), a polymorph of calcium carbonate (2,8). The organic fraction of otoconia contains s...

Reduction of Cellular Expression Levels Is a Common Feature of Functionally Affected Pendrin (SLC26A4) Protein Variants

“…The functional test used in the pres ent work evaluates the chloride/iodide exchange activity of pendrin (8,31,35,36). Despite that iodide transport may be of no physiological relevance in the inner ear, the chloride/iodide exchange activity generally reflects the chloride/ bicarbonate exchange activity (10) and can therefore be used as a tool to discrim inate between variants with and without pathogenic potential.…”

Genetics of Nonsyndromic Recessively Inherited Moderate to Severe and Progressive Deafness in Humans

“…SLC26A4 plays a significant role in the maintenance of the ionic balance within the inner ear and is involved in bicarbonate secretion (Wangemann et al, 2007). There are three different mouse mutants of Slc26a4 which lack pendrin but none of them model the less severe hearing loss observed in humans (Dror et al, 2010;Everett et al, 2001;Lu et al, 2011). However, recently, transgenic mice with Slc26a4 expression inducible by doxycycline on a background of mice lacking endogenous pendrin expression were generated.…”

Proteins Involved in Otoconia Formation and Maintenance

“…Recently, Dror et al have also demonstrated that a recessive missense mutation within the highly conserved region of slc26a4 results in a mutant pendrin protein with impaired transport activity. This mutant mouse has severely abnormal mineral composition, size and shape of otoconia, i.e., giant CaCO 3 crystals in the utricle at all ages, giant CaOx crystals in the saccule of older adults, and ectopic giant stones in the crista (Dror et al 2010). Therefore, pendrin participates in otoconia formation through providing HCO 3 -, which is essential for forming CaCO 3 crystals and for buffering the endolymphatic pH.…”