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ARTICLES Table of Contents   
Year : 2001  |  Volume : 3  |  Issue : 11  |  Page : 19-35
Differential gene expression following noise trauma in birds and mammals

1 Kresge Hearing Research Institute, Department of Otolaryngology/Head-Neck Surgery; Department of Cell and Developmental Biology, University of Michigan Medical School, Ann Arbor, MI 48109 USA
2 Kresge Hearing Research Institute, Department of Otolaryngology/Head-Neck Surgery
3 Kresge Hearing Research Institute, Department of Otolaryngology/Head-Neck Surgery; Department of Otolaryngology-Head and Neck Surgery, Seoul National University College of Medicine, 28 Yongon- Dong, Chongno-Ku, 110-744 Seoul, Korea
4 Kresge Hearing Research Institute, Department of Otolaryngology/Head-Neck Surgery; NIDCD/NIH, Bethesda, MD 20892, U.S.A.

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  Abstract 

Acoustic overstimulation has very different outcomes in birds and mammals. When noise exposure kills hair cells in birds, these cells can regenerate and hearing will recover. In mammals, however, the hair cell loss, and resulting hearing loss, is permanent. Changes in gene expression form the basis for important biological processes, including repair, regeneration, and plasticity. We are therefore using a battery of molecular approaches to identify and compare changes in gene expression following noise trauma in birds and mammals. Both differential display and subtractive hybridisation were used to identify genes whose expression increased in the chick basilar papilla immediately following exposure to an octave band noise (118 dB, centre frequency 1.5 kHz) for 4-6 hr. Among those upregulated genes were two involved in actin signalling: the CDC42 gene encoding a Rho GTPase, and WDR1, which encodes a protein involved in actin dynamics. A third gene, UBE3B, encodes an E3 ubiquitin ligase involved in protein turnover. A fourth gene encodes a cystein-rich secreted protein that may interact with calcium channels. To examine the mammalian response, gene microarrays on nylon membranes (Clontech Atlas Gene Arrays) were used to examine global changes in gene expression 30 minutes after TTS (110 dB broadband noise 50% duty cycle) or PTS (125 dB, 100% duty cycle) noise overstimulation (each for 90 minutes) in the rat cochlea. Several genes, including classic immediate early response genes such as c-fos, EGR1/NGFI-A, and NGFI-B, were upregulated at this early time point following the PTS exposure, but were not upregulated following the TTS exposure.

Keywords: regeneration, repair, TTS, PTS, gene arrays, differential display

How to cite this article:
Lomax MI, Gong TWL, Cho Y, Huang L, Oh SH, Adler HJ, Raphael Y, Altschuler RA. Differential gene expression following noise trauma in birds and mammals. Noise Health 2001;3:19-35

How to cite this URL:
Lomax MI, Gong TWL, Cho Y, Huang L, Oh SH, Adler HJ, Raphael Y, Altschuler RA. Differential gene expression following noise trauma in birds and mammals. Noise Health [serial online] 2001 [cited 2020 Dec 5];3:19-35. Available from: https://www.noiseandhealth.org/text.asp?2001/3/11/19/31767

  Introduction Top


The function of sensory organs such as the ear depends on the expression of subsets of genes in those cells and tissues that comprise both the peripheral sensory organ and the central auditory pathway. Environmental stimuli such as excessive noise may induce changes in gene expression that provide clues to the biochemical pathways activated by a specific stimulus. The morphological and physiological consequences of acoustic overstimulation in birds and mammals have been studied extensively, but little is known about the molecular consequences of acoustic trauma. Following a noise exposure that damages the cochlea and leads to hair cell death, the avian cochlea undergoes regenerative repair (Cotanche, 1987; Cotanche, et al., 1987; Ryals and Rubel; 1988, Corwin and Warchol, 1991; Cotanche and Corwin, 1991; Cotanche and Lee, 1994; Cotanche et al., 1994; Cotanche, 1997; Cotanche 1999), but the mammalian cochlea does not. Depending on the level and duration of the noise exposure, mammals undergo either a temporary threshold shift (TTS) with complete functional recovery, or undergo a permanent threshold shift (PTS) with loss of hair cells and incomplete recovery of auditory function. A noise exposure that produces a TTS response in the rat is known to invoke protective mechanisms such as the classic heat shock protein stress response (Lim et al., 1993). Induction of HSP70 in the cochlea may protect outer hair cells from permanent damage (Yoshida et al., 1999; Mitchell et al., In press).

Analysis of differential gene expression in the cochlea after acoustic trauma may identify biochemical pathways involved in repair or regeneration in the chick, or in protective or degenerative mechanisms in mammals. We have used several different molecular biological approaches to identify global changes in gene expression following noise damage in the avian cochlea, with the long-term goal of understanding which specific biochemical pathways are activated preceding hair cell death and during regeneration. We have also initiated molecular analysis of the response of the mammalian cochlea to acoustic overstimulation. For these mammalian studies we turned to gene arrays on nylon membranes to identify differentially expressed genes. Analysis of gene expression in the mammalian cochlea following TTS can identify genes for proteins required to re-establish homeostasis and restore normal auditory function. Analysis of gene expression following a PTS noise exposure can identify the signals leading to apoptosis and thus provide possible therapeutic strategies for blocking this response.

This paper reviews our current knowledge of regenerative repair in the avian cochlea and summarizes the results of our molecular studies on differential gene expression following acoustic trauma in the chick inner ear. These studies have identified several proteins that interact with cytoskeletal elements or that are involved in actin dynamics. The remaining challenge is to define the biochemical signalling pathways in which these proteins function. We also summarize our data using gene arrays to identify changes in gene expression in the rat cochlea following a noise overstimulation that produces either a TTS or a PTS. Our data show that PTS, but not TTS, noise exposure induces several immediate early and early growth response genes. This finding is the first molecular correlate of PTS noise damage in the mammalian cochlea.

Regenerative repair in the avian cochlea

The discovery of regenerative repair following noise (Cotanche, 1987; Corwin and Cotanche, 1988; Ryals and Rubel, 1988) or ototoxic damage (Cruz et al., 1987) to the avian cochlea surprised the auditory research community. The biology of avian hair cell regeneration (Corwin and Warchol, 1991; Cotanche et al., 1994; Stone et al., 1998; Cotanche, 1999) and the restoration of auditory function (Smolders, 1999) have been reviewed extensively since this phenomenon was first observed. Staecker and Van De Water (1998) proposed that hair cells in the avian cochlea can respond to noise damage by activating several different intracellular biological pathways. There is evidence that severely damaged mammalian hair cells undergo apoptosis (Pirvola et al., 2000), and the assumption is that apoptotic mechanisms occur in the avian cochlea. Nevertheless, the molecular signals that trigger the apoptotic pathway are currently unknown. At least two different mechanisms for production of new hair cells are invoked. A supporting cell may re-enter the cell cycle, initiate de novo DNA synthesis, then divide to regenerate a supporting cell and produce a new hair cell (Raphael, 1992; Stone and Cotanche, 1994). There is also evidence for transdifferentiation as a mechanism of converting supporting cells to hair cells in the absence of mitosis (Adler and Raphael, 1996; Roberson et al., 1996; Adler et al., 1997). Finally, repair processes may be invoked in hair cells that sustain only minor damage.

Few molecular studies have examined changes in gene expression following acoustic trauma (reviewed in Riedl et al., 1997). Such studies would help to identify the signalling mechanisms underlying these different cellular pathways. Lee and Cotanche (1995, 1996) examined the levels of mRNA for two important structural proteins found in stereocilia, β-actin, the major structural protein of stereocilia, and fimbrin, an actin bundling protein that cross-links the parallel actin filaments. They used RT-PCR assays specific for each gene and detected higher levels of (3-actin mRNA, but not fimbrin mRNA in the chick basilar papilla after noise trauma. Other studies have attempted to define the kinetics of supporting cell mitosis. Stone and Cotanche (1994) established by BrdU labelling that supporting cells enter S phase 18 hours after the initiation of either a 4 hour or 24 hour noise exposure. Warchol and Corwin (1996) showed that supporting cells within 200 mm of laser ablated individual hair cells re-enter the cell cycle about 12 hours after the hair cells die.

Several groups have attempted to identify the signals that trigger hair cell regeneration in the avian cochlea by examining the effects of specific mitogens, signalling agents, and growth factors on hair cell regeneration in culture systems (Oesterle and Rubel, 1993; Lee and Cotanche, 1996; Oesterle and Rubel, 1996). Oberholtzer and colleagues (Navaratnam, et al., 1996) showed that regeneration of inner ear hair cells in birds involves activation of the cyclic AMP/protein kinase A intracellular signal transduction pathway.

Raphael and colleagues (Raphael, 1992, 1993; Adler et al., 1995) have shown morphologically that a limited noise exposure produces a lesion in the chick basilar papilla in which hair cells are eliminated ([Figure - 1], centre). New stereocilia bundles, presumably from newly produced hair cells, appear by four days following the noise exposure ([Figure - 1], right). Although these hair cell bundles initially appear disorganized, they mature and eventually assume the normal stair step morphology (Tilney et al., 1992). We have chosen to use this acoustic overstimulation paradigm for our molecular studies.

Differential gene expression in the regenerating chick auditory epithelium

We have taken a fairly global approach to the problem of identifying genes that are differentially expressed in the regenerating chick auditory epithelium after acoustic trauma. Rather than examining changes in expression of individual genes after acoustic overstimulation, we have used molecular approaches such as differential display of mRNA (Liang and Pardee, 1992; Welsh et al., 1992; Liang et al., 1993; McClelland et al., 1995) and suppressive subtractive hybridisation (Diatchenko et al., 1999), which are more global and do not depend on the sequence of a particular gene. In principle, these methods can identify all genes that are differentially expressed following noise overstimulation; in practice, they identify only a subset of these genes. By cloning and characterizing genes that are differentially expressed following noise trauma, we aim to identify molecular processes and intracellular biochemical pathways that are required for hair cell degeneration, with subsequent regenerative repair of auditory hair cells. Results from the chick might then be applied to the mammalian auditory system to enhance survival of hair cells and neurons after noise damage, or even eventually to promote hair cell regeneration in mammals.

Our studies on differential gene expression following acoustic trauma in the chick [Table - 1] have identified many novel genes and implicated several interesting signalling pathways in the response to acoustic overstimulation. These may play a role in regeneration, but on the other hand, may be part of a more general stress response that is also present in the mammalian cochlea. These genes could even be relevant to hereditary deafness in humans. The following sections discuss selected results from our molecular studies in terms of emerging themes.

Cytoskeletal proteins

Regeneration of the chick basilar papilla requires the production of new hair cells with their profusion of actin-filled stereocilia on the apical surface. One might expect, therefore, to see increased expression of genes for cytoskeletal proteins, which play an important role in maintaining the shape and rigidity of stereocilia, following noise trauma and during hair cell regeneration. We detected increased levels of mRNA for (3-actin [Table - 1] after acoustic overstimulation (Adler et al., 1999). These results are consistent with and confirmed the findings of Lee and Cotanche (1995), who demonstrated increased expression of the (3-actin gene after noise trauma by RT-PCR.

Proteins that signal to the actin cytoskeleton

An important theme that has emerged from our molecular studies is that pathways that activate or signal through the actin cytoskeleton may play a critical role in normal auditory function. Genes for proteins that signal to the actin cytoskeleton are upregulated after noise trauma. We demonstrated increased expression of CDC42 (Gong et al., 1996, 1997) following acoustic trauma. CDC42 is a member of the Rho family of small GTPases, which comprises Rho, Rac, and CDC42. These proteins are required for formation of stress fibbers, lamellipodia (membrane ruffles) and filopodia (membrane projections or spikes), respectively (Hall, 1998). CDC42 signals to the actin cytoskeleton see [Figure - 2] through an adapter protein called N­WASP (Carlier et al., 1999; Rohatgi, et al., 1999) and additional downstream effectors such as Diaphanous (Watanabe et al., 1997). N-WASP interacts with both CDC42 and the Arp2/3 complex, which is required for nucleation of actin polymerisation (Carlier et al., 1999; Higgs and Pollard, 1999; Machesky and Gould, 1999; Blanchoin et al., 2000; Mullins, 2000). The importance of this signalling pathway is supported by studies of human hereditary deafness. Analysis of affected members of DFNA1 families identified mutations in the human gene for diaphanous (DIAPH). Diaphanous is known to be a downstream effector of CDC42 and a ligand for the protein profilin, which is required for actin polymerisation. Thus, results from our molecular studies, combined with studies of human deafness genes, suggest that this signalling pathway is critically important for normal auditory function.

A second gene involved in this pathway is upregulated after acoustic trauma. WDR1 encodes a 68 kDa protein, WDR1, with nine WD40 repeats (Adler et al., 1999). WDR1 is the vertebrate homologue of actin-interacting protein 1 (AIP1), which was first identified in a two-hybrid screen for actin binding proteins in yeast. Yeast AIP1 interacts with cofilin to disassemble actin filaments (Rodal et al., 1999). Xenopus AIP1 was identified as a protein that interacts with actin-depolymerizing factor (ADF) (Okada et al., 1999). ADF and cofilin (often referred to as ADF/cofilin) are two different proteins with high sequence similarity that perform essentially the same function, namely, the calcium-independent cleavage of actin filaments to produce actin monomers [Figure - 2]. DAip1, a Dictyostelium homologue of yeast AIP1, is involved in endocytosis, cytokinesis, and motility (Konzok et al., 1999). Thus, these studies on AIP1 (WDR1) in lower eukaryotes point to an important role for this protein in actin dynamics [Figure - 2].

Northern blot analysis (Adler et al. 1999) revealed increased expression of WDR1 in the lesioned area of the chick basilar papilla immediately after acoustic overstimulation. In situ hybridisation (ISH) experiments in the normal basilar papilla detected high levels of WDR1 mRNA in homogene cells, cuboidal cells [Figure - 3]A, and hair cells [Figure - 3]C, but not in supporting cells (Oh et al., submitted). Acoustic overstimulation, however, resulted in ectopic expression of WDR1 in supporting cells [Figure - 3]B. WDR1 protein was detected in homogene cells by immunohistochemistry with antibodies that we developed to the N-terminus of the chick WDR1 protein (Oh et al., 2000). Homogene cells are elongated cells filled with actin filaments that presumably anchor the tectorial membrane (Shiel and Cotanche, 1990; Goodyear et al., 1996; Fekete et al., 1998). Heller et al. (1998) showed that both F-actin and homogenin are present at high levels in homogene cells. Homogenin is related to gelsolin, a protein that cleaves actin filaments in a calcium-dependent manner. The high level of F-actin, WDR1, and homogenin in homogene cells may indicate that actin dynamics plays an important role in the function of these cells.

Motor proteins

Motor proteins are ATP-hydrolysing proteins that contain highly conserved motor domains. These proteins, and their associated cargo, move along cytoskeletal elements. The two major classes of motor proteins are myosins and kinesins. Myosins move along actin filaments and are active in cell movement, membrane traffic, and signal transduction (Mermall, et al., 1998). Kinesins move along microtubules and transport organelles and vesicles along these structures (Hisanaga, et al., 1989; Hirokawa, 1998). Mutations in the genes for at least three unconventional myosins, myosin VI, myosin VIIa and Myosin XV, cause deafness in either mice, humans, or both (Friedman, et al., 1999; Raphael, et al., 2000). These unconventional myosins clearly play important roles in hair cells and are known to be important for normal auditory function in mammals (Gibson et al., 1995; Weil et al., 1995; Liu et al., 1997; Probst et al., 1998; Liang et al., 1999). Their exact role, though, remains to be determined (Wu et al., 2000). We showed that the KIF1B gene (KH129, [Table - 1]), which encodes a novel kinesin, is up­regulated after noise trauma (Gong et al., 1999). The function of KIF1B in the cochlea and its involvement in human hereditary deafness remain to be determined.

Proteins with known roles in signalling pathways

Our initial studies on the response of the basilar papilla to noise overstimulation (Gong et al., 1996) demonstrated increased expression of genes for several important and well­characterized signalling pathways. CaM Kinase II, the neuronal calcium-calmodulin regulated kinase, signals through protein phosphorylation. Parathyroid hormone related protein, PTHrP, a small secreted protein, is believed to have both autocrine and paracrine functions. However, the specific intracellular pathways that are activated by these signalling molecules remain to be determined.

Proteins with putative roles in signalling pathways

In addition to the genes for these well-studied signalling molecules, we identified genes for two novel proteins that may also function in intracellular signalling pathways. For example, UBE3B (KH125; [Table - 1]) is up-regulated only in the lesioned area of the chick basilar papilla after noise overstimulation (see Lomax et al., 2000) and encodes a novel protein involved in ubiquitin-mediated protein degradation. This process requires the sequential activity of three enzymes to activate and transfer ubiquitin to target proteins: E1, a ubiquitin-activating enzyme; E2, ubiquitin-conjugating enzyme; and E3, a ubiquitin ligase. UBE3B is related to the oncoprotein E6-AP through the conserved active site resides in the C-terminal HECT (Homology to E6-AP C-T erminus) domain (Scheffner, 1998). E6-AP targets the tumour suppressor p53 for rapid degradation via the proteosome pathway. Although the HECT domain identifies the UBE3B protein as a novel E3 ubiquitin ligase, there are no well-defined sequences that identify the substrate protein that is targeted for degradation. Thus, in spite of knowing the enzymatic role of UBE3B, we do not understand its intracellular signalling function.

Some functional inferences can be drawn from data on related proteins in lower eukaryotes, however. The UBE3B protein has 75% sequence identity to the Oxi-1 protein of C. elegans (worm). The Oxi-1 gene was isolated in an experiment designed to identify oxidative stress­responsive genes. This homology suggests that UBE3B may play a protective role in either the classic stress response or in the stress response invoked by oxidative damage. The remaining challenge is to identify the protein or proteins that UBE3B targets for rapid turnover, and to determine the roles of both UBE3B and its target in the vertebrate cochlea following noise trauma.

The second novel gene that may be involved in intracellular signalling pathways (KH603/KH604; [Table - 1]) encodes a cysteine (Cys)-rich protein related to a family of mammalian proteins, the cysteine-rich secreted proteins (CRISPs). Northern blot analysis indicated that the chick CRISP-related gene is expressed only in the eye and ear [Figure - 4]. Like the UBE3B gene, this novel gene is expressed at high levels in the lesioned area of the chick basilar papilla following noise trauma ([Figure - 4], right). The complete molecular cloning of this gene will be necessary to determine its identity and to deduce its function in the vertebrate ear. Again, some functional inferences can be drawn from related proteins in lower organisms. The chick CRISP gene is highly similar (59% identity; 72% similarity) to two small, secreted Cys-rich venom proteins (CRVP), metalloproteins with 12 Cys-Cys disulfide bridges that act as toxins. Helothermine (HLTx), from the Mexican beaded lizard is a 25.5-kDa peptide toxin that blocks ryanodine receptors (Morrissette et al., 1995) and also inhibits calcium channels in cerebellar granule cells of newborn rats (Nobile et al., 1996). The second CRVP is a venom allergen protein from an insect. These similarities suggest a potential role for the chick CRISP protein in regulating calcium signalling in the ear in response to noise overstimulation.

The COCH gene is up-regulated after noise trauma and is mutated in DFNA9 families. One of the most interesting results from these studies on differential gene expression after acoustic trauma is our finding that the COCH gene is differentially expressed after noise. Two partial cDNAs derived from the 3'UTR of COCH (KH627 and KH654, [Table - 1]) were isolated in a subtractive hybridisation experiment, suggesting that this gene is upregulated following acoustic trauma. Additional experiments, particularly Northern blot analysis of the COCH gene, are required to confirm this result. The COCH gene encodes a novel extracellular matrix protein and mutations in the COCH gene in affected members of DFNA9 families lead to sensorineural deafness and balance problems (Robertson et al., 1998). Based on our results, the chick system may provide an important experimental model in which to study the function of the COCH protein. Furthermore, as the chick homologues of other deafness genes are cloned, it will be of interest to determine whether or not the genes for other proteins shown to be required for normal auditory function are upregulated following acoustic overstimulation.

Response to noise trauma in the mammalian cochlea

The mammalian cochlea can only recover from hearing loss if the trauma does not result in hair cell death. The recovery from a TTS in the mammalian cochlea can involve intracellular pathways for protection and repair, including the classic stress response pathway controlled by the heat shock transcription factors (Morimoto, 1993; 1998). A noise exposure that induces TTS also upregulates heat shock proteins (HSPs), including HSP72, in the rat cochlea 6 to 8 hours after TTS noise (Lim et al., 1993). These increased levels of HSP72 provide protection from a second noise exposure given at a time at which HSP72 levels are high (Mitchell et al., in press). Noise induced HSP72 expression is seen in outer hair cells and stria vascularis and may help protect hair cells from permanent damage. In general agreement with these studies, Liberman and colleagues (Yoshida et al., 1999) demonstrated that whole body heat stress in mice also induces HSP70 and protects the ear from acoustic injury during the period of high HSP70 expression. Other pathways, in addition to the classic stress response, may also be involved. For example, the TTS level noise also results in induction of neurotrophic factors such as glial cell-line derived neurotrophic factor (GDNF) (Altschuler et al., 1999; Nam et al., 2000). Thus, induction of the stress response may serve to protect the ear from acoustic trauma.

A PTS level noise exposure results in hair cell loss, and hearing will never completely return to normal. However, there is often a TTS component, with some improvement in hearing over time. A PTS exposure might therefore induce pathways for protection and repair, as well as pathways for cell death. Following acoustic overstimulation that damages the organ of Corti, the FGFR3 gene is rapidly upregulated in supporting cells surrounding the outer hair cells (Pirvola et al., 1995). It has been proposed that acidic FGF and FGFR-3 may be involved in repair processes within the damaged cochlea. PTS noise overstimulation in mammals also has been shown to induce apoptosis, or programmed cell death, through activation of Jun-N-terminal kinase (JNK), also known as stress-activated protein kinase (SAPK) (Derijard et al., 1994). JNKs phosphorylate and activate c-Jun, which then interacts with c-Fos to form the transcription factor AP-1. The Jun-N-terminal kinase pathway is involved in many different cellular processes, such as cellular proliferation, apoptosis, and tissue morphogenesis (reviewed in Ip and Davis, 1998). Blocking this enzyme with a specific inhibitor protects hair cells from apoptosis and has been shown to reduce noise­induced hearing loss (Pirvola et al., 2000; Ylikoski and Pirvola, in press).

Changes in gene expression after TTS or PTS in the rat cochlea

We are examining differential gene expression in the rat cochlea following TTS and PTS level noise exposures. We used a noise exposure paradigm that produces a TTS of 30 to 50 dB, with complete recovery after 3 hr, as confirmed by ABR analysis. This TTS noise exposure has been shown to upregulate HSP70 (Lim et al., 1993) and GDNF (Nam et al., 2000) in the rat cochlea 6 hr or 8 hr following noise, respectively. We also used a noise exposure condition that produces PTS of >70 dB and hair cell loss, as assessed by cytocochleograms. Both TTS and PTS noise exposures were for 90 minutes, and animals were sacrificed following ABR analysis, or approximately 15 minutes after cessation of the noise. Changes in immediate early genes following these noise exposures might therefore be expected.

For these mammalian studies, we isolated RNA from the whole cochlea of 6-8 animals sacrificed within 15 min following cessation of the noise (105 min following its start) and examined changes in global gene expression using the new technology of gene arrays (see Lomax et al, 2000, for a review). Duplicate Clontech gene array membranes that contained 1176 known rat genes were hybridised with cDNA from either the TTS or PTS groups. Resulting phosphorimager signals from the membranes were then analysed to detect changes in gene expression (see scheme in [Figure - 5]). Criteria for differential expression were based on our previous experience using these membranes to profile normal gene expression in the rat auditory pathway (Cho et al., in press; Cho et al., unpublished). A similar approach has been employed by Salvi and colleagues (Taggert et al., in press) to assess changes in gene expression following a TTS noise exposure in the chinchilla.

Several well characterized immediate early genes (reviewed in Akins et al, 1996) and early growth response (EGR) genes (O'Donovan et al., 1999) showed increased expression after the PTS noise exposure, but not the TTS exposure [Table - 2], and this increased expression has been confirmed by RT-PCR analysis (data not shown). These genes included c-fos, the classical immediate-early gene (Akins et al., 1996; Walton et al., 1999), and three genes (NGFI­A/EGR1, NGFI-B, and PC3) that were activated by NGF treatment of PC12 cells and named NFG-inducible (NGFI) genes (Milbrandt 1986; 1987; 1988). In addition to these genes, most of which are important transcription factors, several genes for small secreted signalling molecules were also upregulated following PTS but not TTS noise overstimulation [Table - 2]. IP-10 and LIF are known to be transiently expressed after neuronal injury. Thus, the PTS noise exposure used in this study activates genes that can be characterized as immediate early genes, based on their rapid and transient expression following various stimuli.

Several of these genes were identified in different experimental systems and assigned different names. For example, NGFI-A is also known as EGR1 (early growth response gene 1), and NGFI-B has been called Nur77, which encodes a nuclear receptor. Most of these immediate early genes encode important transcription factors that initiate genetic cascades of subsequent gene activation in response to both pathological and physiological signals. Thus, the increased expression of c-fos after PTS was not an isolated response of one immediate early gene, but part of a concerted response of several immediate early and early growth response genes. O'Donovan et al. (1999) reviewed the history of the immediate early and early growth response genes and noted that they are at the interface between molecular biology and systems physiology. Our studies in the rat provide a molecular correlate for the PTS response in the mammalian cochlea.

Just as we could not readily assign the novel genes found from differential display in the avian cochlea following noise to either the regeneration or stress pathways, we also cannot always assign the genes that are differentially expressed following a PTS noise exposure to a specific pathway. These genes may be part of pathways leading to protection, recovery or repair, but could alternatively be part of cell death pathways. Indeed many genes, including several we find differentially expressed by noise, have multiple roles and can be involved in different pathways. As noted earlier, the c-Fos and c-Jun proteins are components of the AP-1 transcription complex, which is required to activate many downstream genes and pathways. JNK is known to affect varied and often diametrically opposite intracellular signalling pathways, ranging from activation of cell proliferation to cell death (Minden, et al., 1995; Leppa and Rohmann, 1999). Additional experiments, with later times of assessment, will be required to identify genes further downstream in the different pathways activated by these acoustic overstimulation conditions.


  Conclusions Top


The studies reviewed in this paper summarize changes in gene expression we have observed following acoustic overstimulation in both the avian and mammalian cochlea. The different classes of genes observed following noise in these two families may be due to the different duration of noise exposures and times of assessment, as well as potential differences in response(s) between species. A further analysis of gene expression in response to noise in both species is therefore necessary, examining different exposure conditions and different times following noise. We can then examine and compare the expression of specific genes between species and among noise exposure conditions. Such analysis should lead to new insights on the molecular pathways involved in the stress response, apoptosis and regeneration following noise. Determining the targets of the important transcription factors we have already identified should provide clues to the type of signal transduction pathways that are invoked. Furthermore, localization of mRNA transcripts and the corresponding proteins will help identify cells that participate in the response of the auditory epithelium to acoustic trauma.


  Acknowledgements Top


Supported by NIH grants DC02492 (M.I.L.), DC02982 (M.I.L. and R.A.A.) and R01 DC01634 (Y.R.). H.J.A. was supported by a supplement to DC02492. S.H.O. is a scholar of the Korean Science and Engineering Foundation. We thank James Beals and Kathy Chen for assistance with figures.[89]

 
  References Top

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Correspondence Address:
Margaret I Lomax
Department of Otolaryngology/ Head - Neck Surgery, University of Michigan Medical School, Ann Arbor, MI 48109 USA

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