Lance Zirpel

Dynamic Studies of Ototoxicity in Mature Avian Auditory Epithelium

ABSTRACT: Hearing loss induced by ototoxicity is a worldwide problem despite the development of newer antibiotics and chemotherapy agents. The cellular mechanisms responsible for aminoglycoside‐induced hearing loss are still poorly understood. We have developed two different methods of studying the dynamic cellular and subcellular changes in the chick auditory sensory epithelium that occur during hair cell death. The first study was performed in mature chicks after a single, high dose injection of gentamicin, which results in the rapid loss of all hair cells in the basal third of the cochlea. Chicks were sacrificed at discrete time points after drug treatment, and transmission electron microscopy was performed to study the ultrastructural changes in basal hair cells during the course of cell death. We noted various changes in the cell morphology including accumulation of cytoplasmic inclusion bodies, dispersion of the cytoplasmic polyribosomes, mitochondrial swelling, and cellular extrusion by 24 h after injection. The next two studies were performed using tissue cultures from mature avian auditory sensory epithelium. Cultured cells were labeled using vital fluorescent markers, and levels of intracellular calcium and reactive oxygen species within hair cells were studied following aminoglycoside exposure. We identified a dose‐dependent increase in the levels of intracellular calcium, which was blocked by an inhibitor of voltage‐gated calcium channels. We also found that levels of reactive oxygen species in hair cells greatly increased after exposure to gentamicin, and this response was blocked by two different antioxidants. These studies serve to identify key cellular and molecular changes in hair cells in response to ototoxic drugs. Further study of these processes may lead to a better understanding of how ototoxicity is induced and to potential preventative interventions.

Deafferentation-Induced Activation of NFAT (Nuclear Factor of Activated T-Cells) in Cochlear Nucleus Neurons during a Developmental Critical Period: A Role for NFATc4-Dependent Apoptosis in the CNS

During the development and maturation of sensory neurons, afferent activity is required for normal maintenance. There exists a developmental window of time when auditory neurons, including neurons of the anteroventral cochlear nucleus (AVCN), depend on afferent input for survival. This period of time is often referred to as a critical period. The cellular and molecular mechanisms that underlie AVCN neuron susceptibility to deafferentation-induced death remain unknown. Here, we show that only during this critical period deafferentation of mouse AVCN neurons by in vivo cochlea removal results in rapid nuclear translocation and activation of the transcription factor NFATc4 (nuclear factor of activated T-cells isoform 4). NFAT activation is abolished by in vivo treatment with the calcineurin inhibitor FK506 and the specific NFAT-inhibitor 11R-VIVIT. Inhibition of NFAT significantly attenuates deafferentation-induced apoptosis of AVCN neurons and abolishes NFAT-mediated expression of FasL, an initiator of apoptotic pathways, in the cochlear nucleus. These data suggest that NFAT-mediated gene expression plays a role in deafferentation-induced apoptosis of cochlear nucleus neurons during a developmental critical period.

Developmental changes in metabotropic glutamate receptor–mediated calcium homeostasis

Neurons of the chick cochlear nucleus, nucleus magnocellularis (NM), require eighth nerve activation of metabotropic glutamate receptors (mGluRs) for maintenance of intracellular calcium homeostasis. Interrupting this activation results in an increase in intracellular calcium concentration ([Ca2+]i) followed by cell atrophy, degeneration, and death of many neurons. Although these phenomena are well characterized in late embryonic and posthatch chicks, little is known about the role of mGluRs and calcium homeostasis during the development of synaptic activity in NM. Using Fura‐2 imaging, fluorescent immunohistochemistry, and Western immunoblotting, we investigated (1) the expression and function of group I mGluRs and their role in calcium regulation during development of NM, and (2) the expression of two other key molecules involved in regulating neuronal [Ca2+]i : inositol trisphosphate receptors (IP3Rs) and sarcoplasmic/endoplasmic reticulum calcium ATPases (SERCAs). Confocal imaging of Fluo‐3‐labeled NM was used to investigate the kinetics of global NM neuron calcium signals. Measurements were made at four ages that extend from before synaptic function begins in NM, through functional onset, to mature patterns of spontaneous activity, namely, embryonic days (E) 10, 13, 15, and 18. mGluR5, mGluR1, and SERCA expression peaked at E13 and then decreased with age. IP3R expression increased to peak at E18. [Ca2+]i response to mGluR activation increased with age. The rise time of [Ca2+]i signals in NM neurons did not change with development, but E13 neurons were slower to reestablish baseline [Ca2+]i. These results suggest that the mGluR‐mediated calcium homeostasis of NM neurons develops in parallel with synaptic activity and appears to be refined with increasing synaptic activity. J. Comp. Neurol. 421:95–106, 2000.

Zinc inhibition of group I mGluR-mediated calcium homeostasis in auditory neurons.

Zinc is widely distributed in the central nervous system (CNS), it functions normally as a synaptic modulator, and it contributes to neuronal death under pathologic conditions. Zinc colocalizes with glutamate in excitatory synapses, and the presence of zinc is well characterized in the synapses of the auditory system. Since chick cochlear nucleus neurons depend upon synaptic activation of metabotropic glutamate receptors (mGluRs) for maintenance and survival, the goal of this study was to determine (1) if zinc is released from the eighth nerve calyces onto nucleus magnocellularis (NM) neurons in the chick cochlear nucleus, and, if so, (2) what effect it has on group I mGluR-mediated calcium homeostasis of these neurons. Using in vitro slices and a fluorescent dye relatively specific to vesicularized zinc, we show that zinc is indeed localized to the presynaptic calyces and is released upon nerve stimulation or KCl depolarization. Experiments employing fura-2 calcium imaging show that zinc inhibits group I mGluR release of calcium from internal stores of NM neurons and disrupts activity-dependent calcium homeostasis in a manner identical to the mGluR5-specific antagonist 2-methyl-6-(phenylethynyl)pyridine. The mGluR1-specific antagonist 7-hydroxyiminocyclopropan-[b]chromen-la-carboxylic acid ethyl ester did not affect release of calcium from stores by the nonspecific mGluR agonist aminocyclopentane dicarboxylic acid, nor did it affect activity-dependent calcium homeostasis. We conclude that zinc is present in and released from the glutamatergic eighth nerve calcyes. The presence of zinc inhibits mGluR5, a major component of calcium homeostasis of NM neurons, and plays a modulatory role in the activity-dependent, mGluR-mediated calcium homeostasis of auditory neurons.

Assembling, Connecting, and Maintaining the Cochlear Nucleus

The cochlear nucleus (CN) is an essential synaptic intermediary in the ascending auditory pathway and the site of remarkable neuronal specializations that allow this pathway to represent most of the behaviorally relevant information available in sounds (Cant 1992; Rhode and Greenberg 1992; Romand and Avan 1997; Ryugo and Parks 2003). Because of the powerful influence that the developing ear exerts on the developing auditory central nervous system (CNS) (Rubel 1978; Parks 1997; Friauf and Lohmann 1999; Rubel and Fritzsch 2002), considerable research has been directed at understanding the basic events of normal development and the central effects of early deafness. The large literature on normal structural and functional development of the CN has been reviewed in a previous volume of this series (Cant 1998; Sanes and Walsh 1998), and various aspects of abnormal development are discussed in other chapters of the book (Friauf, Chapter 3 and Moore and King, Chapter 4).