Macrophage contribution to the response of the rat organ of Corti to amikacin

Supporting cell characteristics in long-deafened aged mouse ears

Significant sensory hair cell loss leads to irreversible hearing and balance deficits in humans and other mammals. Future therapeutic strategies to repair damaged mammalian auditory epithelium may involve inserting stem cells into the damaged epithelium, inducing non-sensory cells remaining in the epithelium to transdifferentiate into replacement hair cells via gene therapy, or applying growth factors. Little is currently known regarding the status and characteristics of the non-sensory cells that remain in the deafened auditory epithelium, yet this information is integral to the development of therapeutic treatments. A single high-dose injection of the aminoglycoside kanamycin coupled with a single injection of the loop diuretic furosemide was used to kill hair cells in adult mice, and the mice were examined 1 year after the drug insult. Outer hair cells are lost throughout the entire length of the cochlea and less than a third of the inner hair cells remain in the apical turn. Over 20% and 55% of apical organ of Corti support cells and spiral ganglion cells are lost, respectively. We examined the expression of several known support cell markers to investigate for possible support cell dedifferentiation in the damaged ears. The support cell markers investigated included the microtubule protein acetylated tubulin, the transcription factor Sox2, and the Notch signaling ligand Jagged1. Non-sensory epithelial cells remaining in the organ of Corti retain acetylated tubulin, Sox2 and Jagged1 expression, even when the epithelium has a monolayer-like appearance. These results suggest a lack of marked SC dedifferentiation in these aged and badly damaged ears.

Gene transfer using bovine adeno-associated virus in the guinea pig cochlea

Gene transfer into the cells of the cochlea is useful for both research and therapy. Bovine adeno-associated virus (BAAV) is a new viral vector with potential for long-term gene expression with little or no side effects. In this study, we assessed transgene expression using BAAV with beta-actin-GFP as a reporter gene, in the cochleae of normal and deafened guinea pigs. We used two different routes to inoculate the cochlea: scala media (SM) or scala tympani (ST). Auditory brainstem response assessments were carried out before inoculation, 7 days after inoculation and immediately before killing, to assess the functional consequences of the treatment. We observed threshold shifts because of the surgical invasion, but no apparent pathology associated with the virus. Fourteen days after the injection, animals were killed and cochleae assessed histologically. Epi-fluorescence showed that BAAV transduced the supporting cells of both normal and deafened animals through SM and ST inoculations. Transgene expression in cells of the membranous labyrinth after ST inoculation is an important outcome because of the greater feasibility of this route for future clinical application. BAAV facilitates efficient transduction of the membranous labyrinth epithelium with minimum pathogenicity and may become clinically applicable for inner ear gene therapy.

Bone marrow-derived cells expressing Iba1 are constitutively present as resident tissue macrophages in the mouse cochlea

Immune-mediated inner ear disorder has been well established as a clinical entity; however, the innate immune system of the inner ear is a poorly understood area of research with high clinical and immunological importance. Although the presence of resident tissue macrophages in the inner ear has been suggested, there has been some controversy. In this study, we analyzed the origin of cochlear resident macrophages and the contribution of hematopoietic bone marrow (BM) to the recruitment of macrophages in the cochlea. To visualize the localization of BM-derived cells, BM chimeric mice were made by transplantation of hematopoietic stem cells, which were genetically labeled with enhanced green fluorescent protein, into lethally irradiated C57BL/6 mice. The distribution and characteristics of BM-derived cells in the mouse cochlea were studied immunohistochemically. We successfully identified the constitutive presence of tissue resident macrophages in the spiral ligament and spiral ganglion that are derived from BM in larger numbers than previously reported. Moreover, cochlear resident macrophages gradually turn over for several months during steady-state replacement by BM-derived cells, and the number of cochlear macrophages immediately increased in response to local surgical stress. The present findings demonstrate the hematopoietic origin of cochlear resident and infiltrating macrophages. Our study provides a novel anatomical and immunological basis for the inner ear and indicates that the cochlear resident macrophages would be a therapeutic target in inner ear disorders.

Rapid hair cell loss: a mouse model for cochlear lesions

In comparison to other mammals, mice have proved extremely resistant to aminoglycoside-induced hair cell ablation in vivo. In this paper we examine the pattern and extent of cochlear lesions rapidly induced with a combination of a single dose of aminoglycoside (kanamycin) followed by a loop diuretic (bumetanide). With this protocol, the vestibular system was unaffected, but in the cochlea, there was extensive loss of outer hair cells (OHC) that commenced in the basal coil and progressed apically so that, by 48 h, OHC loss was almost complete. TUNEL-positive nuclei and activated caspase-3 labeling demonstrated that most OHC died via a classical apoptotic pathway. However, scattered debris within the OHC region suggested that many apoptotic cells ruptured prior to completion of apoptosis. Following lesion repair, supporting cells retained characteristics of differentiated cells but positional shift occurred. In comparison to OHC loss, inner hair cell (IHC) death was delayed and only observed in 50% of all cochleae examined even after extensive reorganization of the tissue. The coadmininstration of diuretic with FM1-43, used as a tracer for aminoglycoside uptake, indicated entry into IHC as readily as OHC, suggesting that the differential response to aminoglycoside was not due to differential uptake. Where IHC death was ongoing, there were indications of different modes of cell death: cells with morphological features of autophagy, necrosis, and apoptosis were apparent. In addition to damage to the organ of Corti, there was a significant and progressive decrease in strial thickness beginning as early as 7 days posttreatment. This was due predominantly to degeneration of marginal cells. The strial pathology resembled that reported after noise damage and with aging. This in vivo protocol provides a robust model in which to obtain extensive OHC loss in the mature cochleae of mice and is a means with which to examine different aspects of cochlear pathology in transgenic or mutant strains.