Analysis of nicotinic acetylcholine receptor subunits in the cochlea of the mouse

The α1 subunit of nicotinic acetylcholine receptors in the inner ear: transcriptional regulation by ATOH1 and co-expression with the γ subunit in hair cells

Acetylcholine is a key neurotransmitter of the inner ear efferent system. In this study, we identify two novel nAChR subunits in the inner ear: α1 and γ, encoded by Chrna1 and Chrng, respectively. In situ hybridization shows that the messages of these two subunits are present in vestibular and cochlear hair cells during early development. Chrna1 and Chrng expression begin at embryonic stage E13.5 in the vestibular system and E17.5 in the organ of Corti. Chrna1 message continues through P7, whereas Chrng is undetectable at post-natal stage P6. The α1 and γ subunits are known as muscle-type nAChR subunits and are surprisingly expressed in hair cells which are sensory-neural cells. We also show that ATOH1/MATH1, a transcription factor essential for hair cell development, directly activates CHRNA1 transcription. Electrophoretic mobility-shift assays and supershift assays showed that ATOH1/E47 heterodimers selectively bind on two E boxes located in the proximal promoter of CHRNA1. Thus, Chrna1 could be the first transcriptional target of ATOH1 in the inner ear. Co-expression in Xenopus oocytes of the α1 subunit does not change the electrophysiological properties of the α9α10 receptor. We suggest that hair cells transiently express α1γ-containing nAChRs in addition to α9α10, and that these may have a role during development of the inner ear innervation.

Old mice lacking high-affinity nicotine receptors resist acoustic trauma

There is presently no clearly effective preventative medication against noise-induced hearing loss (NIHL). However, negative feedback systems that presumably evolved to modulate the sensitivity of the organ of Corti may incidentally confer protection. One feedback system implicated in protection from NIHL involves synaptic connections between the lateral olivocochlear efferent terminals and the afferent fibers of spiral ganglion neurons (SGNs). These connections operate via high-affinity nicotinic acetylcholine receptors containing the β2 subunit. We unexpectedly observed protection from NIHL in 9-month old knockout mice lacking the β2 subunit (β2(-/-)); however, the same protection was not observed in 2-month old β2(-/-) mice. This enigmatic observation led to the discovery that protection from acoustic trauma in older β2(-/-) mice is mainly mediated by an age-related increase of corticosterone, not disruption of efferent cholinergic transmission. Significant protection of inner hair cells after acoustic trauma in β2(-/-) mice was linked to the activation of glucocorticoid signaling pathways. However, significant loss of SGNs was observed in animals with chronically high systemic levels of corticosterone. These results suggested a "double-edge sword" nature of glucocorticoid signaling in neuronal protection, and a need for caution regarding when to apply synthetic glucocorticoid drugs to treat neural injury such as accompanies acoustic trauma.

Retinoic acid and nerve growth factor induce differential regulation of nicotinic acetylcholine receptor subunit expression in SN56 cells

Retinoic acid (RA) and nerve growth factor (NGF) have multiple functions in the regulation of neuronal development. In the present study, we characterized the expression of different nicotinic acetylcholine receptor (nAChR) subtypes in the cholinergic SN56 cell line and investigated the roles of RA and NGF in the expression of choline acetyltransferase (ChAT) and different nAChR subtypes. The nAChR agonist [(3)H]epibatidine was bound to two sites, with apparent affinities of 13 and 380 pM. RT-PCR analysis revealed expression of alpha3, alpha4, alpha5, alpha7, beta2, and beta4 nAChR subunits. RA treatment induced morphological changes, and the mRNA level of ChAT was maximally elevated after 4 days of exposure. The density of [(3)H]epibatidine binding sites and the mRNA and protein level of the alpha3 and beta2 nAChR subunits were also increased by RA-induced differentiation. RA down-regulated the mRNA and protein level of the alpha4 nAChR subunit, whereas no significant change was observed in the mRNA and protein level of the alpha7 nAChR subunit. NGF treatment increased the mRNA and protein level of the alpha3 and beta2 nAChR subunits. No morphological effects of NGF were observed, and the mRNA level of ChAT and mRNA and protein level of the alpha4 and alpha7 nAChR subunits were not significantly altered. Validation was performed with real-time RT-PCR. The present results show that RA and NGF have different effects on the expression of ChAT and the morphology and the expression pattern of different nAChR subunits in cholinergic SN56 cells.

Requirement of nicotinic acetylcholine receptor subunit beta2 in the maintenance of spiral ganglion neurons during aging

Age-related hearing loss (presbycusis) is a major health concern for the elderly. Loss of spiral ganglion neurons (SGNs), the primary sensory relay of the auditory system, is associated consistently with presbycusis. The causative molecular events responsible for age-related loss of SGNs are unknown. Recent reports directly link age-related neuronal loss in cerebral cortex with the loss of high-affinity nicotine acetylcholine receptors (nAChRs). In cochlea, cholinergic synapses are made by olivocochlear efferent fibers on the outer hair cells that express alpha9 nAChR subunits and on the peripheral projections of SGNs that express alpha2, alpha4-7, and beta2-3 nAChR subunits. A significantly decreased expression of the beta2 nAChR subunit in SGNs was found specifically in mice susceptible to presbycusis. Furthermore, mice lacking the beta2 nAChR subunit (beta2-/-), but not mice lacking the alpha5 nAChR subunit (alpha5-/-), have dramatic hearing loss and significant reduction in the number of SGNs. Our findings clearly established a requirement for beta2 nAChR subunit in the maintenance of SGNs during aging.

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.

Involvement of nicotinic acetylcholine receptors in controlling Chlamydia pneumoniae growth in epithelial HEp-2 cells

Nicotinic acetylcholine receptors (nAChRs) play an essential role in neurotransmission. Recent studies have indicated that nAChRs may be involved in the regulation of some bacterial infections through immunological mechanisms in macrophages. However, the regulation of infection with Chlamydia pneumoniae, which is a ubiquitous pneumonia-causing bacterium, by an nAChR-mediated mechanism is still unclear. In the present study, it was found that stimulation of nAChRs with ligands such as nicotine and acetylcholine altered the growth of C. pneumoniae in epithelial HEp-2 cells. Thus, the results revealed a possible pathophysiological role of nAChRs in the regulation of intracellular bacterial infection.

Colocalization but differential regulation of neuronal NO synthase and nicotinic acetylcholine receptor in C2C12 myotubes

In mammalian skeletal muscle, neuronal-type nitric oxide synthase (nNOS) is found to be enriched at neuromuscular endplates. Here we demonstrate the colocalization of the nicotinic acetylcholine receptor (nAChR, stained with alpha-bungarotoxin) and nNOS (stained with a specific antibody) in murine C(2)C(12) myotubes. However, coimmunoprecipitation experiments demonstrated no evidence for a direct protein-protein association between the nAChR and nNOS in C(2)C(12) myotubes. An antibody to the alpha(1)-subunit of the nAChR did not coprecipitate nNOS, and an nNOS-specific antibody did not precipitate the alpha(1)-subunit of the nAChR. Treatment of mice with bacterial LPS downregulated the expression of nNOS in skeletal muscle, and treatment of C(2)C(12) cells with bacterial LPS and interferon-gamma markedly decreased nNOS mRNA and protein expression. In contrast, mRNA and protein of the nAChR (alpha-, gamma-, and epsilon-subunits) remained unchanged at the mRNA and protein levels. These data demonstrate that nNOS and the nAChR are colocalized in murine skeletal muscle and C(2)C(12) cells but differ in their expressional regulation.

Effect of dimethoate on the function and expression of nicotinic acetylcholine receptor in primary skeletal muscle cell culture

To investigate the molecular mechanism of intermediate myasthenia syndrome (IMS), we analyzed the toxic effects of the representative organophosphate dimethoate on the function and expression of the nicotinic acetylcholine receptor (nAChR) in primary skeletal muscle cell culture. The results showed that the expression of nAChR on the muscle cell membrane was significantly increased after cells were exposed to dimethoate (130 microM). AChR function measured by carbachol-induced (22)Na+ influx demonstrated that dimethoate may inhibit the nAChR function either by binding to a noncompetitive site and changing the conformational state of nAChR or by blocking the nAChR channel directly. This study also demonstrated that dimethoate could rapidly induce the expression of c-fos, with a maximal effect at about 40 min, and c-fos might act as a transcriptional factor in regulating the expression of nAChR in the primary skeletal muscle cell culture following organophosphate exposure.

Expression of the nicotinic acetylcholine receptor subunit, alpha9, in the guinea pig cochlea

Acetylcholine is a major neurotransmitter of the cochlear efferent system. Based on its high level of expression in hair cells, the recently cloned nicotinic receptor subunit, alpha9 [Elgoyhen et al., Cell 79 (1994) 705-715], is likely to be the postsynaptic receptor for acetylcholine in hair cells either as a homomeric complex or with other subunits yet to be identified. To further study this receptor, we cloned and sequenced alpha9 cDNA from the guinea pig organ of Corti library [Wilcox and Fex, Hear. Res. 62 (1992) 124-126]. The sequence of the guinea pig alpha9 cDNA is similar to that of the rat, with identities of 85% and 89% at the nucleotide and amino acid levels, respectively. Most differences are in the cytoplasmic loop domain between the transmembrane segments 3 and 4. We also observed minor differences in the putative ligand binding regions. Pharmacological differences between acetylcholine receptors on outer hair cells of rat and guinea pig have been reported, and the minor structural changes we observe could account for these differences. Reverse transcription-polymerase chain reaction analysis showed a high expression of alpha9 in the organ of Corti while expression was low or not detected in the spiral ganglion. In situ hybridization histochemistry showed expression of alpha9 mRNA in both inner and outer hair cells, with much higher expression in outer hair cells than in inner hair cells. In the inner hair cell, silver grains were more abundant over the basal part of the cell than over the apical part. Immunocytochemistry showed a pattern of distribution of the alpha9 protein similar to that seen for mRNA with in situ hybridization. Immunolabeling was most intense at the bases of both inner and outer hair cells. To determine the effect of hair cell loss on alpha9 expression, hair cells were destroyed by either systemic or local application of kanamycin. This treatment led to a down regulation of alpha9 in hair cells; this down regulation appeared to precede hair cell degeneration. In the spiral ganglion, a transient up regulation of alpha9, as determined by RT-PCR, was seen 4-6 weeks after kanamycin treatment.