osteoprotegerin and Hearing-Loss

Prolactin is a versatile hormone with over 300 known functions and predominantly expressed in the pituitary. However, its expression has additionally been found in a number of extrapituitary organs. Recently, we described the expression of prolactin in the inner ear of mice, where it was correlated to age. Previous research has shown prolactin to be linked to abnormal bone metabolism and hearing loss due to changes in morphology of the bony otic capsule. Here we further investigated the relationship between prolactin, hearing loss and cochlea bone metabolism. BALB/c mice were tested for hearing using ABR at 6 and 12 months of age. Bone mineral density of the cochlea was evaluated using microCT scanning. Prolactin expression was calculated using quantitative real time PCR. Expression of the key regulators of bone metabolism, osteoprotegerin and receptor activator of nuclear factor-kappaB ligand were also determined. We found that prolactin expression was exclusive to the female mice. This also correlated to a greater threshold shift in hearing for the females between 6 and 12 months of age. Analyses of the cochlea also show that the bone mineral density was lower in females compared to males. However, no gender differences in expression of osteoprotegerin or receptor activator of nuclear factor-kappaB ligand could be found. Further analysis of cochlea histological sections revealed larger ostocyte lacunae in the females. These results provide a possible mechanism for an age related hearing loss sub-type that is associated with gender and provides clues as to how this gender bias in hearing loss develops. In addition, it has the potential to lead to treatment for this specific type of hearing loss.

Topics: Acid Phosphatase; Aging; Animals; Auditory Threshold; Bone and Bones; Bone Density; Cochlea; Evoked Potentials, Auditory, Brain Stem; Female; Gene Expression Regulation; Hearing Loss; Isoenzymes; Male; Mice; Mice, Inbred BALB C; Osteoprotegerin; Prolactin; RANK Ligand; Sex Characteristics; Tartrate-Resistant Acid Phosphatase

Three auditory ossicles including the malleus, incus, and stapes conduct sound in the middle ear from the tympanic membrane to the inner ear. Auditory ossicles are massively resorbed by osteoclasts in Opg(-/-) mice, which lack osteoprotegerin (OPG), a soluble decoy receptor for the osteoclastogenic cytokine RANKL. Opg(-/-) mice exhibit progressive hearing loss and are a model for juvenile Paget's disease. However, effects of antiresorptive treatment on auditory ossicles and on hearing function in Opg(-/-) mice are unknown. We intraperitoneally injected Opg(-/-) mice with bisphosphonate risedronate 5 d/wk for 9 wk. Morphology of auditory ossicles was examined microscopically, radiographically, and histologically. Hearing function was monitored by measuring the auditory brain stem response (ABR). Control Opg(-/-) mice exhibited thinning of all three ossicles and tibia. In contrast, risedronate treatment significantly inhibited bone loss in auditory ossicles as well as in long bones of Opg(-/-) mice. Bony fusion of the junction between the stapes and the otic capsule was reduced after treatment. Moreover, ABR measurement showed that hearing in Opg(-/-) mice was significantly improved by risedronate treatment. These data suggest that hearing loss in pathologies characterized by excessive resorption of the auditory ossicles may be prevented by bisphosphonates.

Topics: Animals; Bone Density Conservation Agents; Cochlea; Diphosphonates; Etidronic Acid; Evoked Potentials, Auditory, Brain Stem; Female; Hearing; Hearing Loss; Mice; Mice, Inbred C57BL; Mice, Transgenic; Osteoprotegerin; Risedronic Acid; Time Factors

The otic capsule, when compared with other bones in the body, is unique in that it undergoes no significant remodeling of bone after development. We previously demonstrated that osteoprotegerin (OPG), which inhibits formation and function of osteoclasts, is produced at high levels in the inner ear of normal mice and secreted into the perilymph from where it diffuses into the surrounding otic capsule bone through a lacunocanalicular system. To test our hypothesis that the high level of OPG may be important in the inhibition of otic capsule remodeling, we studied the light microscopic histology of the otic capsule in OPG knockout mice for evidence of abnormal remodeling of bone. We also tested the hearing in OPG knockout mice to determine whether OPG and its influence on surrounding bone is important for auditory function.. Temporal bone histopathology and pathophysiology were compared in homozygous OPG knockout mice and C57BL/6 (B6) mice, the background strain for the knockouts. Auditory function in age-matched animals from each group was evaluated at approximately 4-week intervals from 8 to 21 weeks using frequency-specific auditory brainstem responses (ABR) and distortion product otoacoustic emissions (DPOAE). After each of the last three evaluations, the cochleae from one mouse of each group were harvested, processed, and examined by light microscopy.. Osteoprotegerin knockout mice demonstrated abnormal remodeling of bone within the otic capsule with multiple foci showing osteoclastic bone resorption and formation of new bone. Such changes were not seen in the age-matched B6 controls. The active bone remodeling process in the knockout animals showed many similarities to otosclerosis seen in human temporal bones. Over the time period that we monitored, auditory function was significantly and progressively compromised in the knockout animals relative to B6 controls. At the earliest age of test (8 wk), the loss was apparent as a mild, high-frequency reduction in sensitivity by ABR. In contrast, DPOAE losses in the knockouts were substantial even at 8 weeks, and by 21 weeks, these losses exceeded our equipment limits. Results of ABR testing showed hearing sensitivity changes in the animals of the background strain were confined largely to the high frequencies, whereas OPG knockouts demonstrated substantial low-frequency shifts in addition to those at high frequencies.. The histopathological and pathophysiological findings in OPG knockout mice support the hypothesis that OPG is important in the inhibition of bone remodeling within the otic capsule and the maintenance of normal auditory function. This mouse may provide a valuable animal model of human otosclerosis.

Topics: Acoustic Stimulation; Animals; Bone Remodeling; Disease Models, Animal; Disease Progression; Ear, Inner; Evoked Potentials, Auditory, Brain Stem; Glycoproteins; Hearing Loss; Mice; Mice, Inbred C57BL; Mice, Knockout; Osteoprotegerin; Receptors, Cytoplasmic and Nuclear; Receptors, Tumor Necrosis Factor; Temporal Bone

Bones conduct sound in the middle ear. The three ossicles-the malleus, incus, and stapes-form a chain that transmits vibrations from the tympanic membrane to the oval window of the inner ear. Little is known about bone remodeling events in these ossicles and about potential effects of osteoporosis on hearing loss. Osteoclastic bone resorption is enhanced in Opg(-/-) mice lacking osteoprotegerin, which is a soluble decoy receptor for the osteoclastogenic cytokine RANKL. We asked whether auditory ossicles are resorbed in Opg(-/-) mice, and whether these mice suffer from impaired auditory function. All three ossicles in Opg(-/-) mice showed thinning, especially at the malleal manubrium and incus body. Most notably, unlike in the case in wild-type mice, the junction between the stapes and the otic capsule was fixed in Opg(-/-) mice, and the stapedial footplate was thinner and broader. Radiological analyses revealed that malleal cortical thickness was positively correlated with tibial bone mineral density in Opg(-/-) and control littermate mice. Furthermore, progressive hearing loss was detected in Opg(-/-) mice starting at 6 to 15 weeks of age. These data suggest that osteoprotegerin plays a crucial role in hearing by protecting the auditory ossicles and otic capsule from osteoclastic bone resorption.

Topics: Acoustic Stimulation; Animals; Bone Resorption; Ear Ossicles; Evoked Potentials, Auditory, Brain Stem; Female; Glycoproteins; Hearing Loss; Heterozygote; Mice; Mice, Inbred C57BL; Mice, Knockout; Osteoprotegerin; Radiography; Receptors, Cytoplasmic and Nuclear; Receptors, Tumor Necrosis Factor; Time Factors

Familial expansile osteolysis (FEO) is a rare autosomal dominant disorder characterized by striking focal expansile osteolytic bone lesions and generalized osteopenia, often accompanied by characteristic early hearing loss and dental disease. The TNFRSF11A gene encodes the receptor activator of nuclear factor-kappaB (RANK), which has been demonstrated to be essential in bone remodeling and osteoclast differentiation. Identical insertional mutations in the first exon of RANK have been identified in all published FEO kindreds. The mutation is an 18 base pair tandem duplication in the sequence coding for the signal peptide of RANK, which causes an increase in NF-kappaB signaling. We report the identification and mutational analysis of two unrelated FEO patients. One had no family history of FEO, but presented with bilateral hearing loss at an early age, deterioration of teeth, and severe pain and swelling in the distal tibia before the age of 20. The second patient had a family history of FEO and exhibited an extensive expansile tibial lesion and lesions in one humerus and a phalanx. She also had early hearing loss and dental disease. Mutational analysis of the TNFRSF11A gene in our patients demonstrated an 18 base pair tandem duplication, one base proximal to the duplications previously reported. This novel mutation results in addition of the same six amino acids to the RANK signal peptide that has been observed previously. Further analysis of the exon 1 sequence demonstrated that it has the ability to form a stable secondary structure that may facilitate the generation of tandem duplications.

Topics: Aged; Amino Acid Sequence; Amino Acids; Base Sequence; Bone and Bones; DNA Mutational Analysis; Exons; Female; Gene Duplication; Glycoproteins; Hearing Loss; Humans; Molecular Sequence Data; Mutation; NF-kappa B; Osteolysis; Osteoprotegerin; Protein Sorting Signals; Protein Structure, Secondary; Radiography; Receptors, Cytoplasmic and Nuclear; Receptors, Tumor Necrosis Factor; Signal Transduction