Microtubule-associated proteins in adult human sensory organs

Neuronal development in the cochlea of a nonhuman primate model, the common marmoset

Precise cochlear neuronal development is vital to hearing ability. Understanding the developmental process of the spiral ganglion is useful for studying hearing loss aimed at aging or regenerative therapy. Although interspecies differences have been reported between rodents and humans, to date, most of our knowledge about the development of cochlear neuronal development has been obtained from rodent models because of the difficulty in using human fetal samples in this field. In this study, we investigated cochlear neuronal development in a small New World monkey species, the common marmoset (Callithrix jacchus). We examined more than 25 genes involved in the neuronal development of the cochlea and described the critical developmental steps of these neurons. We also revealed similarities and differences between previously reported rodent models and this primate animal model. Our results clarified that this animal model of cochlear neuronal development is more similar to humans than rodents and is suitable as an alternative for the analysis of human cochlear development. The time course established in this report will be a useful tool for studying primate‐specific neuronal biology of the inner ear, which could eventually lead to new treatment strategies for human hearing loss.

Structure and innervation of the cochlea

The role of the cochlea is to transduce complex sound waves into electrical neural activity in the auditory nerve. Hair cells of the organ of Corti are the sensory cells of hearing. The inner hair cells perform the transduction and initiate the depolarization of the spiral ganglion neurons. The outer hair cells are accessory sensory cells that enhance the sensitivity and selectivity of the cochlea. Neural feedback loops that bring efferent signals to the outer hair cells assist in sharpening and amplifying the signals. The stria vascularis generates the endocochlear potential and maintains the ionic composition of the endolymph, the fluid in which the apical surface of the hair cells is bathed. The mechanical characteristics of the basilar membrane and its related structures further enhance the frequency selectivity of the auditory transduction mechanism. The tectorial membrane is an extracellular matrix, which provides mass loading on top of the organ of Corti, facilitating deflection of the stereocilia. This review deals with the structure of the normal mature mammalian cochlea and includes recent data on the molecular organization of the main cell types within the cochlea.

Microtubule associated protein (MAP1A) mRNA was up-regulated by hypergravity in the rat inner ear

Differential display analysis of differential mRNA expression in the rat inner ear under hypergravity identified two down- and four up-regulated genes. The up-regulation of microtubule associated protein 1A (MAP1A) in one of these was confirmed by real-time polymerase chain reaction. Since MAP1A is believed to work as a cell stabilizer connecting the actin with microtubule, this is possibly a response to strengthen this stabilizer under hypergravity. The MAP1A gene is the first found to be affected by gravity change in the inner ear.

Changes in MAP2 and tyrosinated alpha-tubulin expression in cochlear inner hair cells after amikacin treatment in the rat

The expression of MAP2 (microtubule-associated protein 2) and of tyrosinated alpha-tubulin was investigated immunocytochemically in the cochleas of normal and amikacin-treated rats. For MAP2, two different antibodies were used: anti-MAP2ab, against the high molecular weight forms, and anti-MAP2abc, additionally against the embryonic form c. In the cochlea of the normal rat, the outer (OHCs) and inner (IHCs) hair cells were labeled for MAP2abc. The labeling was weaker in IHCs than in OHCs. The hair cells were rarely labeled for MAPab. Both OHCs and IHCs were labeled for tyrosinated alpha-tubulin. In the cochlea of the amikacin-treated rat, aggregates of anti-MAP2abc and anti-tyrosinated alpha-tubulin antibodies were seen in the apical region of the IHCs as early as the end of the antibiotic treatment. In rats investigated during the following week, the cell body of most of the surviving IHCs were not labeled for MAP2abc and tyrosinated alpha-tubulin. Then, labeling for these two antibodies reappeared in the surviving IHCs, including their giant stereocilia. Fewer surviving IHCs were labeled for tyrosinated alpha-tubulin than for MAP2abc. The amikacin-poisoned IHCs were rarely labeled for MAP2ab. These results suggest that cochlear hair cells essentially express form c of MAP2. In the amikacin-damaged cochlea, the apical aggregation of MAP2c and tyrosinated alpha-tubulin within the poisoned IHCs could be implicated in a cell degenerative process. By contrast, the extinction and recovery of MAP2c and tyrosinated alpha-tubulin labeling in the remaining IHCs suggest the occurrence of a limited repair process. A possible role of MAP2 and tubulin in hair cell survival is discussed.