Histochemistry of glycoconjugates of the auditory receptor-functional implications

Gangliosides and hearing

Severe auditory impairment observed in GM3 synthase-deficient mice and humans indicates that glycosphingolipids, especially sialic-acid containing gangliosides, are indispensable for hearing. Gangliosides associate with glycoproteins to form membrane microdomains, the composition of which plays a special role in maintaining the structural and functional integrity of hair cells. These microdomains, also called lipid rafts, connect with intracellular signaling and cytoskeletal systems to link cellular responses to environmental cues. During development, ganglioside species are expressed in distinctive spatial and temporal patterns throughout the cochlea. In both mice and humans, blocking particular steps of ganglioside metabolism produces distinctive neurological and auditory phenotypes. Thus each ganglioside species may have specific, non-overlapping functions within the cochlea, central auditory network, and brain.

Physiopathological function of hematoside (GM3 ganglioside)

Since I was involved in the molecular cloning of GM3 synthase (SAT-I), which is the primary enzyme for the biosynthesis of gangliosides in 1998, my research group has been concentrating on our efforts to explore the physiological and pathological implications of gangliosides especially for GM3. During the course of study, we demonstrated the molecular pathogenesis of type 2 diabetes and insulin resistance focusing on the interaction between insulin receptor and gangliosides in membrane microdomains and propose a new concept: Life style-related diseases, such as type 2 diabetes, are a membrane microdomain disorder caused by aberrant expression of gangliosides. We also encountered an another interesting aspect indicating the indispensable role of gangliosides in auditory system. After careful behavioral examinations of SAT-I knockout mice, their hearing ability was seriously impaired with selective degeneration of the stereocilia of hair cells in the organ of Corti. This is the first observation demonstrating a direct link between gangliosides and hearing functions.

Mice lacking ganglioside GM3 synthase exhibit complete hearing loss due to selective degeneration of the organ of Corti

The ganglioside GM3 synthase (SAT-I), encoded by a single-copy gene, is a primary glycosyltransferase for the synthesis of complex gangliosides. In SAT-I null mice, hearing ability, assessed by brainstem auditory-evoked potentials (BAEP), was impaired at the onset of hearing and had been completely lost by 17 days after birth (P17), showing a deformity in hair cells in the organ of Corti. By 2 months of age, the organ of Corti had selectively and completely disappeared without effect on balance or motor function or in the histology of vestibule. Interestingly, spatiotemporal changes in localization of individual gangliosides, including GM3 and GT1b, were observed during the postnatal development and maturation of the normal inner ear. GM3 expressed in almost all regions of cochlea at P3, but at the onset of hearing it distinctly localized in stria vascularis, spiral ganglion, and the organ of Corti. In addition, SAT-I null mice maintain the function of stria vascularis, because normal potassium concentration and endocochlear potential of endolymph were observed even when they lost the BAEP completely. Thus, the defect of hearing ability of SAT-I null mice could be attributed to the functional disorganization of the organ of Corti, and the expression of gangliosides, especially GM3, during the early part of the functional maturation of the cochlea could be essential for the acquisition and maintenance of hearing function.

Distribution of glycoconjugates during cochlea development in mice: light microscopic lectin study

During development, different epithelial cells in the mouse cochlea express different cell surface glycoconjugates, which may reflect membrane specialization. Some of the lectins tested in this study (SBA, succ-WGA, and PSA) labeled the sensory cells of the cochlea around birth. Other lectins (WGA, Con A, RCA-II, and PHA-E) labeled surfaces of the sensory cells, particularly the stereocilia, from early stages of development (gestation day (GD) 16) through 21 days after birth. These may be adhesion molecules needed to attach the newly forming tectorial membrane (TM) to the stereocilia. Lectin staining of the developing TM revealed that the substructures of the TM are biochemically distinct. Lectin staining also showed the temporal sequence of the expression of cytoplasmic glycoconjugates of the cochlear epithelium during development. Biochemical changes during development are probably the result of different cells being involved in the production of glycoconjugates, and may have functional significance, specifically with regard to the expression of adhesion and/or signaling molecules.

Sequential changes in anti-GAL-1 staining of the rat organ of Corti following amikacin exposure

Hair cell loss and a non-functional epithelial reorganization appeared in the organ of Corti after acoustic or toxic damage. Moreover, in the drug damaged organ of Corti, transient atypical cells were recently described with characteristics of both immature hair cells and/or non-sensory epithelial cells. The phenotype of these atypical cells has been now investigated by using the galectine 1 (GAL-1) antibody. In the normal organ of Corti, this antibody recognizes all the epithelial cells except the sensory hair cells and their supporting cells. At PD 21, transient atypical cells were not stained by GAL-1 antibody, suggesting that they were originated from hair cells or their supporting cells. Later, the organ of Corti was substituted by an epithelial scare, GAL-1 stained. This study also emphasizes the particular resistance of the cochlear apex to degeneration after antibiotic intoxication.