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First published online 27 June 2006
doi: 10.1242/jcs.03013
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Research Article |
1 and TRß differentially regulate gene expression of Kcnq4 and prestin during final differentiation of outer hair cells
1 University of Tübingen, Department of Otolaryngology, Tübingen Hearing Research Centre (THRC), Laboratory of Molecular Neurobiology, Elfriede-Aulhorn-Str. 5, 72076 Tübingen, Germany
2 University of Tübingen, Institute of Physiology II and Department of Otolaryngology, THRC, Gmelinstr. 5, 72076 Tübingen, Germany
3 Max-Planck-Institute for Experimental Endocrinology, Feodor-Lynen-Str. 7, 30625 Hannover, Germany
4 Developmental Auditory Physiology Laboratory, Boys Town National Research Hospital, 555 North 30th Street, Omaha, NE 68131, USA
* Author for correspondence (e-mail: marlies.knipper{at}uni-tuebingen.de)
Accepted 6 April 2006
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Summary |
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1 on neither hearing function nor cochlear T3 target genes has been described to date. It is also uncertain whether TR1 and TRß can act simultaneously on different target genes within a single cell. We focused on two concomitantly expressed outer hair cell genes, the potassium channel Kcnq4 and the motor protein prestin Slc26a5. In outer hair cells, TH enhanced the expression of the prestin gene through TRß. Simultaneously Kcnq4 expression was activated in the same cells by derepression of TR1 aporeceptors mediated by an identified THresponse element, which modulates KCNQ4 promoter activity. We show that T3 target genes can differ in their sensitivity to TH receptors having the ligand either bound (holoreceptors) or not bound (aporeceptors) within single cells, and suggest a role for TR1 in final cell differentiation.
Key words: KCNQ4, Prestin, Final differentiation, TR aporeceptors
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Introduction |
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and TRß (Wu and Koenig, 2000
) that can either positively or negatively regulate target genes in response to T3 (Lazar, 2003). To date it is unknown whether T3 target genes can differ in their sensitivity to distinct thyroid hormone receptor (TR) isoforms or to TRs having the ligand bound (holoreceptors) or not bound (aporeceptors). This is a crucial question considering the pathologies associated with hypothyroidism or TR mutations (O'Shea and Williams, 2002). The presumptive redundant effects of coexpressed TRs could definitively mask the individual TR responsiveness of genes and dramatically define the pathological phenotype. To unravel the question of TR specificity at the single-cell level, we focused on two cochlear outer hair cell (OHC) genes that are concomitantly expressed during a crucial time period during which hypothyroidism leads to irreversible hearing deficits in adults (Deol, 1973; Uziel et al., 1985). Neurosensory deafness in humans and rodents is presumed to be related to mutations in the TRß, leading to resistance to thyroid hormone (RTH), caused by a transdominant negative transcriptional effect (Refetoff et al., 1993; Weiss and Refetoff, 2000). A role of TR1 for auditory function was suggested in a study of TR2-deficient mice exhibiting increased expression of TR1, which was proposed to compensate for missing TRß activity (Ng et al., 2001). However, no T3 target genes have been identified in the cochlea at the level of TR-isoform-specific transcriptional regulation.
During a developmental period before the onset of hearing, the gene that encodes the outer hair cell motor protein prestin, Slc26a5 (Zheng et al., 2000), is expressed from approximately postnatal day (P) 3 in euthyroid rats, whereas its expression is delayed under conditions of hypothyroidism (Weber et al., 2002). The voltage-dependent K+ channel KCNQ4, which is responsible for the predominant K+ conductance, IK,n, of mature OHCs (Marcotti and Kros, 1999) is expressed in rodents from approximately P6 onwards (present study) (Kharkovets et al., 2000). In order to study the specific effect of TH on the expression of these genes, a variety of animal models of hypothyroidism and several TR mutant mice were used to examine their TH dependency. These included (1) animals with goitrogen-induced hypothyroidism; (2) animals that acquired hypothyroidism as a consequence of a naturally occurring point mutation in the thyrotropin receptor (Tshrhyt mutants) (for a review, see Walsh and McGee, 2001); (3) athyroid, Pax8–/– mutants (Christ et al., 2004; Macchia, 2000; Mansouri et al., 1998; Tell et al., 1999); (4) TR1–/– (Rusch et al., 1998; Wikstrom et al., 1998), TRß–/– (Forrest et al., 1996a; Forrest et al., 1996b; Gauthier et al., 1999), TR1–/–/ß–/– (Gothe et al., 1999; Rusch et al., 2001) mutants; and (5) euthyroid TR1m/+ mice carrying a dominant-negative TR1 R384C point mutation (Tinnikov et al., 2002). The different mutant strains used in this study are listed and summarized in Table 1.
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Both the prestin gene (Weber et al., 2002) and Kcnq4 (present study) were identified as simultaneously expressed T3-dependent genes that are, however, differentially regulated by either TRß or TR1. A role for TR1 in inner-ear development and specifically final differentiation of OHCs was demonstrated. Furthermore, the data show for the first time that apo-TR1 can repress a T3 target gene independently but in parallel to TRß acting on a different T3 target gene within the same cell.
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Results |
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KCNQ4 and prestin protein expression are differentially affected by hypothyroidism
In the absence of thyroid hormone, the expression of prestin was reduced and the distribution pattern remained immature (Fig. 2A, hypo, red) (Weber et al., 2002). By contrast, KCNQ4 expression was completely lacking in the OHCs of hypothyroid rats (Fig. 2A, hypo, green). Using northern blot analysis both the 
3.9 kb and the 3.8 kb Kcnq4 transcripts were detected (Fig. 2B, con as) (Kubisch et al., 1999), which were significantly reduced in the cochleae of hypothyroid animals (Fig. 2B, hypo, as). The corresponding sense probes showed no signal (Fig. 2B, con, sense). Reduced Kcnq4 mRNA levels were also noted in the cochleae of P12 hypothyroid rats (Fig. 2C, hypo) compared with control animals (Fig. 2C, con) using semi-quantitative RT-PCR. The housekeeping gene cyclophilin was used as an internal control. In addition, Kcnq4 mRNA was detected by in situ hybridization in both outer and inner hair cells of control animals (Fig. 2D, control) (see also Oliver et al., 2003) and was absent in age-matched hypothyroid rats (Fig. 2D, hypo).
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KCNQ4 but not prestin protein expression differs depending on the absence of thyroid hormone or its receptors
KCNQ4 and prestin protein expression profiles in the absence of TH were compared with those in TR1–/–/ß–/– mutant mice, which carry deletions of both TR and TRß. Immunohistochemical analyses were carried out with antibodies against KCNQ4 or prestin, along with the efferentspecific marker synaptophysin (Gil-Loyzaga and Pujol, 1988; Knipper et al., 1995), shown for P10 animals in Fig. 3.
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Similar to prestin expression profiles in hypothyroid animals (Fig. 3A, red) prestin was expressed in TR1–/–/ß–/– mutants (Fig. 3C, red), however its distribution remained immature. Surprisingly, in contrast to absent KCNQ4 expression in hypothyroid animals (Fig. 3B, red) KCNQ4 was expressed when both TR and TRß were deleted (Fig. 3D, red), although its subcellular distribution was still immature. The results suggest that Kcnq4 gene expression is regulated by a TH aporeceptor that may repress transcription (Perissi et al., 1999), and when deleted, might cause the less-severe KCNQ4 phenotype observed in the TR1–/–/ß–/– mutants.
KCNQ4 and prestin protein expression is differentially regulated by either TR1 or TRß
To test this inference, the effect of hypothyroidism in animals with single TR deletions was considered. The TR subtype responsible for repression of Kcnq4 should be identifiable, as its deletion should render the Kcnq4 gene independent of the influence of TH.
KCNQ4 and prestin protein expression was analyzed in both TR1–/– and TRß–/– mutants in which hypothyroidism had been induced by treating neonates with the goitrogen MMI (methylmercapto-imidazol). A subset of goitrogen-treated mutant mice were rescued from hypothyroidism by daily injections of T4 from birth onwards to serve as a control condition. When hypothyroidism was induced in neonatal TRß–/– mutants, KCNQ4 protein was not expressed in OHCs (Fig. 4A, red), whereas prestin was expressed but the immature subcellular distribution persisted (Fig. 4C, red). By contrast, KCNQ4 expression was normal in TR1–/– mutants even under conditions of induced hypothyroidism (Fig. 4B, red), confirming that the TR1 deletion promoted the expression of KCNQ4. In the case of the prestin gene, the deletion of TR1 did not alter the typical expression profile observed in hypothyroidism; i.e. an immature subcellular distribution pattern persisted (Fig. 4D, red). These findings collectively validate the view that apo-TR1 represses KCNQ4 expression, but does not affect the expression of prestin.
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Treatment with T4 rescued the KCNQ4 phenotype from neonatally induced hypothyroidism in TRß–/– mutants (Fig. 4E, red), while not affecting the normal phenotype in TR1–/– mutants (Fig. 4F, red), a finding that is consistent with the idea that TH mediates KCNQ4 expression independently of TRß activity by releasing the Kcnq4 gene from the repressive action of TR1. Consistent with the idea that TRß rather than TR1 controls prestin expression and distribution, prestin expression is insensitive to TH when TRß is deleted (Fig. 4G, red) whereas its expression remains dependent on TH in TR1–/– mutants (Fig. 4H, red). Thus, OHCs express two genes that are simultaneously but differentially regulated by either TR1 or TRß.
A TRE in the KCNQ4 gene mediates repressive apoTR1 activity on the Kcnq4 promoter in the absence of TH in vitro
The data so far clearly indicate a difference in the regulation of prestin and KCNQ4 expression by TRß or TR1, respectively. Although TREs in the prestin gene have been described and one, TREPrest, has been functionally characterized (Weber et al., 2002), information about TREs associated with the Kcnq4 gene is currently unavailable. Using MatInspector professional software (release 2.0; http://www.genomatix.de) (Quandt et al., 1995), putative response elements, TREKCNQ4, were identified 3-10 kb upstream of the transcription start site, as deduced by Kubisch et al. (Kubisch et al., 1999) for the human KCNQ4 gene (Fig. 5A). We focused on three identified putative TREKCNQ4 because they contained two hexamers that are almost identical to the consensus sequence AGGTCA. The hexamers are organized as either a direct repeat (TRE-1KCNQ4), a palindrome (TRE-2KCNQ4), or an inverted palindrome (TRE-3KCNQ4) with a 4, 8 and 8 bp spacer, respectively (Fig. 5A).
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(cTR) shifted [32P]TRE-3KCNQ4 (Fig. 5B, lane 4) to similar positions as a 32P-labeled control TRE (DR4; Fig. 5B, lane 1), which correspond to a homodimer complex with a lower electromobility and a monomer complex with a higher electromobility. Neither [32P]TRE-1KCNQ4 (Fig. 5B, lane 2) nor [32P]TRE-2KCNQ4 (Fig. 5B, lane 3) was able to bind cTR. The shift of [32P]TRE-3KCNQ4 (Fig. 5B, lane 4) was equally diminished whether unlabeled TRE-3KCNQ4 (Fig. 5B, lane 5) or DR4 (Fig. 5B, lane 6) oligomers were added as a competitor. Unlabeled oligomers specific for the transcription factor TFIID had no effect on this interaction (Fig. 5B, lane 7). Nucleotide exchanges introduced in the 5' half-site of the unlabeled TRE-3KCNQ4 (Fig. 5C, TRE-3mut1) used as a competitor had no influence on the capability of cTR to shift the [32P]TRE-3KCNQ4 (Fig. 5B, lane 8), indicating that competitor function is lost upon mutation of the 5' half-site. Nucleotide exchanges introduced in the 3' half-site of TRE3KCNQ4 (Fig. 5C, TRE-3mut2) still abolished the interaction of cTR and [32P]TRE-3KCNQ4 (Fig. 5B, lane 9), indicating that this oligomer still functions as a competitor. The results indicate that a TRE upstream of the KCNQ4 promoter is capable of binding TR.
Using PromoterInspector software (release 2.0; http://www.genomatix.de) (Scherf et al., 2000), a minimal promoter sequence in the 5' upstream region starting at position –495 bp to +45 bp relative to the start point of transcription of the KCNQ4 gene was identified. The introduction of this sequence into a promoterless reporter gene vector led to a significant 4.3-fold (±0.3 s.d.; n=4) increase in reporter gene activation (Fig. 5D, white column) when compared with the promoterless vector, specifying the minimal promoter of KCNQ4.
A series of transfections were performed to examine the TH responsiveness of the reporter gene expression upon the subcloning of the TRE-3KCNQ4 upstream of the KCNQ4 promoter sequence. The introduction of the TRE-3KCNQ4 upstream of the identified promoter sequence led to a significant decline of the reporter gene activity to 2.1±0.5 s.d. (P<0.05; n=4) with co-transfected TR in the absence of ligands (Fig. 5D, black column). The addition of T3 had only a subtle effect on the reduction of the reporter gene activity (data not shown). Only recently, Lee and Privalsky (Lee and Privalsky, 2005) showed that TR-RAR (retinoic acid receptor) heterodimers can also form on different TREs, including inverted palindromes. In contrast to TR-RXR (retinoid X receptor) heterodimers which were shown to recruit only co-activators, TR-RAR dimers can also recruit co-repressors, which are, however, only efficiently released from TR-RAR heterodimers upon the addition of both ligands, T3 and all-trans retinoic acid (ATRA) (Lee and Privalsky, 2005). Indeed, in contrast to adding only T3, the addition of T3 and ATRA led to a significant increase of reporter gene activity (4.1±1.1 s.d.; P<0.05; n=3) to similar levels seen with the KCNQ4 promoter alone (Fig. 5D, compare gray column with white column), indicating an involvement of endogenous RARs.
In line with previous experiments (Yen et al., 1992), we noted that in EMSAs T3 decreased TR homodimer binding to TRE-3KCNQ4 whereas monomer binding remained unaffected (Fig. 5E).
Both, TR and TRß bind to TRE-3KCNQ4 and to TREPrest
To gain further insight into the mechanism of how differential TR activity on two genes is achieved in a single cell, we verified in a first approach the parallel expression of TR1 and TRß in OHCs at the time of final hair cell differentiation. To date, evidence of TR expression in OHCs has come from separate studies done at the protein level, for TR1 (Lautermann and ten Cate, 1997) and TRß (Knipper et al., 2001). Using a novel technique (Michna et al., 2003) to specifically dissect OHCs from rodents, both TR1 and TRß transcripts were detected in isolated OHCs around the time of onset of hearing by RT-PCR (data not shown). In a next step we questioned a preference of TRß or TR for either TREPrest or TRE-3KCNQ4. Using EMSA, in vitro translated rat TR (rTR) and TRß (rTRß) shifted the [32P]TRE-3KCNQ4 (Fig. 6, lane 1 and 3) and [32P]TREPrest (Fig. 6, lane 5 and 7) to positions comparable to those of cTR. Unlabeled TRE-3KCNQ4 and TREPrest competitor oligonucleotides specifically blocked the binding of rTR and rTRß to [32P]TRE-3KCNQ4 (Fig. 6, lane 2 and 4) and [32P]TREPrest (Fig. 6, lane 6 and 8), respectively, indicating that both receptors, TR and TRß, bind to both TRE-3KCNQ4 and TREPrest.
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A dominant-negative mutation in the TR1 receptor causes repression of Kcnq4 gene expression but has no effect on prestin
The repressive activity of TR1 on Kcnq4 but not on the prestin gene was confirmed by analyzing a mouse model in which the TR1 gene expressed a dominant-negative point mutation (R384C) that renders the receptor insensitive to TH (Tinnikov et al., 2002). Genes that are repressed by TR1 can be identified by a persistence of gene repression. The expression profiles of KCNQ4 and prestin were analyzed in wild-type (TR1+/+) and heterozygous TR1m/+ mutants at P13 (Fig. 7). Unlike wild-type animals, KCNQ4 was absent in OHCs of all cochlear turns of TR1m/+ mutants (Fig. 7A,C, KCNQ4, red), whereas prestin expression and distribution was normal in OHCs of both wild-type and mutant mice (Fig. 7B,D, prestin, red). Interestingly, in the same sections where KCNQ4 was absent in OHCs, KCNQ4 expression in vestibular hair cells (VHC) (Kharkovets et al., 2000) of wild-type and TR1m/+ mutants was normal (Fig. 7E,F, KCNQ4, red). This clearly demonstrates the selective nature of the apo-TR1 repressive influence on KCNQ4 expression in OHCs but not in VHCs. Furthermore, the normal expression of prestin in OHCs of TR1m/+ mutants provides additional support for the view that TR aporeceptors act independently on the two simultaneously expressed T3 target genes in OHCs.
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1 and TRß, respectively. A theoretical mechanism for this TR-specific regulation of KCNQ4 and prestin expression is shown in Fig. 8.
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In this model, prestin expression is reduced under the condition of hypothyroidism, while it is enhanced through the actions of TRß in the presence of TH (Weber et al., 2002). Because prestin is normally expressed in TR1m/+ mutant mice (Fig. 7), but its immature distribution is observed in both TH-deficient and TRß–/– mice (Fig. 3), it is reasonable to characterize the apo-TRß as a `silent' TH-receptor (Chassande, 2003) in the case of prestin (Fig. 8A). The fact that prestin is expressed in the absence of TH or TRß (Fig. 3) indicates that a baseline level of transcription is independent of TH-TR complexes (Fig. 8A). In this regard, it has been suggested that recently identified cis-active elements in the upstream region of exon 1 of the prestin gene (Knipper et al., 2005) may recruit a pre-initiation complex mediating THindependent basal levels of prestin transcription. Furthermore, prestin expression is transcriptionally enhanced when TH is bound to TRß (Fig. 8B), a finding that confirms the role played by TRß.
Likewise, findings from this investigation support a model in which apo-TR1 represses KCNQ4 expression (Fig. 8C). This conclusion is based on several independent findings. First, it is notable that KCNQ4 is not expressed under hypothyroid conditions (Fig. 2), nor is the protein expressed in TR1m/+ mutant mice (Fig. 7). When these observations are taken together with the observation that the deletion of TR1, but not TRß, rescues KCNQ4 expression from the consequences of hypothyroidism (Fig. 4), as does T4 treatment of hypothyroid TRß–/– mice (Fig. 4), one is drawn to the conclusion that the repressive action of apo-TR1 is overcome in the presence of TH, leading to activation of Kcnq4 transcription by the dynamics of derepression (Fig. 8D).
Taking into account that the identified regulatory elements TREPrest (Weber et al., 2002) and TRE-3KCNQ4 (Fig. 5) did not display TR isoform-specific binding properties (Fig. 6), the presence of both receptors in isolated OHCs suggests that distinct sequence characteristics in the regulatory region of both individual genes are required to define the differential recruitment of proteins (co-factors), influencing the assembly and activity of the pre-initiation complex.
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Discussion |
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1 or TRß, respectively. The findings reported here reveal for the first time a role of TR1 in inner-ear development, and suggest a molecular mechanism for the occurrence of gene repression parallel to the gene enhancement mediated by specific TR-isoforms in a single cell. Moreover, a crucial time period is defined during which a switch from apo-TR1 to holo-TR1 occurs independently of the presence of TRß activity, an event that is presumably defined by a local peak of TH level.
TR1 activity during final differentiation of the inner ear
The abnormal regulation of genes controlled by TRß has long been presumed to be the cause of profound neurosensory deafness associated with congenital hypothyroidism (Forrest et al., 1996a; Forrest et al., 1996b). Resistance to TH owing to a point mutation within the TRß gene (Adams et al., 1994; Refetoff et al., 1967; Weiss and Refetoff, 2000) has suggested a crucial role of apo-TRß in normal developmental processes, including development of hearing. However, genes that are affected by TRß have not been identified so far. Until now no evidence for a role of TR in hearing existed (Ng et al., 2001). This may be due to the fact that to date most terminal differentiation processes in the organ of Corti have not been regarded in the context of hypothyroidism.
Apo-TR1 has recently been shown to exert a repressive influence on gene regulation (Flamant et al., 2002). The unlocking from TR1 aporeceptor-mediated gene repression appears to become necessary towards the second to third postnatal week in rodents. T3-based compensatory treatments indicate that the brain (Eayrs, 1971; Morte et al., 2002), bone, spleen, intestine (Flamant et al., 2002), heart (Mai et al., 2004) and cochlea (Christ et al., 2004; Knipper et al., 2001) depend on T3 during the second postnatal week. If, however, T3 replacement occurs beyond the second postnatal week, the capacity to rescue hypothyroidism-induced pathology is significantly reduced (Christ et al., 2004; Flamant et al., 2002; O'Shea and Williams, 2002; Tinnikov et al., 2002).
A life-threatening situation resulting from a failure of timely activation of distinct genes repressed by the apo-TR1 was demonstrated by the early postnatal death of Pax8–/– mice, which could be prevented by the deletion of TR (Flamant et al., 2002). Although recent findings suggest that in addition to TR1, TR2 has to be deleted to rescue Pax8–/– mice (Mittag et al., 2005), data in the present study (Figs 4, 7) point to a restricted role of apo-TR1 on Kcnq4. Considering the detrimental influence of a persistent gene repression mediated by TH aporeceptors (Flamant and Samarut, 2003), it will be challenging to investigate whether postnatal death of OHCs in Pax8–/– mice (Christ et al., 2004) is due to persistent repressive activity of apo-TR1 on Kcnq4. This is important in light of the previously reported death of OHCs caused by either a decreased KCNQ4 expression (Rüttiger et al., 2004), a pharmacological blockade of KCNQ4 channels (Nouvian et al., 2003) or deletion of Kcnq4 (Kharkovets et al., 2006).
Specificity of TR-mediated transcriptional control of two concomitantly expressed T3 target genes in a single cell
Based on recent efforts to classify TRs (Chassande, 2003), we suggest that TRß acts on the prestin gene as a `silent' receptor in the absence of TH (Fig. 8A), whereas TR1 acts as aporeceptor on Kcnq4. Previously acquired data (Knipper et al., 1999; Lautermann and ten Cate, 1997) and data resulting from the present study showing that both TR and TRß are expressed in OHCs at the time of KCNQ4 and prestin expression, exclude the possibility that tissue-specific differences in TR levels define TR specificity, as previously suggested (Dillmann, 2002). We cannot, however, rule out the fact that differences in the level of either TR or TRß in OHCs may play a role in TR specificity. As both TRs were able to bind to both TREPrest and TREKCNQ4 (Fig. 6), we can be certain that the TR specificity seen in the case of Kcnq4 and prestin is not determined by differences in sequence-specific binding properties of the TREs, as previously described (Olson et al., 1998).
Local changes in T3 level should instead be considered to play a crucial role in influencing TR specificity. Indeed, in humans and rodents the critical developmental time period of the inner ear occurs in parallel to the natural rise of TH blood plasma levels (Deol, 1973; Knipper et al., 2000; Uziel et al., 1985).
The importance of T3 availability as a factor controlling TRregulated transcription was clearly demonstrated in a recent study of TR1m/m mutant mice. In this strain, the affinity of TR1 for TH is reduced as a consequence of a point mutation (Tinnikov et al., 2002) leading to mortality within the first 3 postnatal weeks, as in Pax8–/– mice. The pathological consequences of the condition were reversible upon compensatory T3 levels (Tinnikov et al., 2002). The importance of circulating T3 for the conversion of apo-TR1 to the holoreceptor configuration was also emphasized in studies analyzing TR1 function in heart tissue, intestine, bone and brain (Flamant et al., 2002; Mai et al., 2004).
The role of local T3 levels, which are determined by deiodinase activity, has been shown in target tissues during amphibian metamorphosis (Becker et al., 1997; Huang et al., 1999) and rat brain development (Kaplan and Yaskoski, 1981; Obregon et al., 1991). Hearing loss in 5'-deiodinase type 2 (D2)-deficient mice (Ng et al., 2004) was only recently discussed in the context of a striking peak of D2 activity in the cochlea between P5 and P10 (Campos-Barros et al., 2000). Although the role of deiodinase in the switch from apo-TR to holo-TR has been hypothesized (Chassande, 2003), supporting data have been unavailable until now.
The data in the present study show that the time period over which the two T3 target genes in OHCs are activated and/or modulated is brief and occurs between P6 and P10, when there is a peak in 5'-deiodinase activity (Campos-Barros et al., 2000) and a steep rise in the TH blood plasma level (Knipper et al., 2000). Local T3 levels thus may split the genes into TR1affected or -unaffected categories. T3 may induce a conformational change in the receptor that destabilizes its bond with co-repressors and facilitates the binding of co-activators (Lazar, 2003).
Retinoic acid, shown to be required as an additional ligand to overcome the repressive effect on Kcnq4 promoter activity in vitro (Fig. 6) should be considered as a further ligand needed for derepression. Lee and Privalsky (Lee and Privalsky, 2005) reported that TR1 forms heterodimers with RAR, which in contrast to TR-RXR heterodimer recruit both co-activators and co-repressors. TR1-RAR heterodimers bound to the inverted palindrome TRE-3KCNQ4 in OHCs may thus recruit corepressors until TH levels rise during early postnatal development. Although several studies underscore the importance of RAR for inner-ear development (Raz and Kelley, 1999; Romand et al., 2002), changes in retinoic acid levels during postnatal development have not been reported (Romand, 2003).
TRß activity in the maturation of OHCs
As higher T3 levels seem to be required to release apo-TR1 from TRE-3KCNQ4, lower T3 levels may be sufficient for the recruitment of the TRß complex to TREPrest (Fig. 8). Prestin expression and distribution are known to be diminished and immature in hypothyroidism (Weber et al., 2002), as well as in TRß–/– and TR1–/–/ß–/– mutants as shown in the present study. Although we cannot rule out the possibility that the influence of TH on the redistribution of prestin is indirect, this condition may be causally linked to the reduced nonlinear capacitance of OHCs observed in TRß–/– and TR1–/–/ß–/– mutants (Rusch et al., 2001).
In conclusion, the characterization of differentially regulated T3 target genes in the same cell may provide the basis for novel insight into the mechanism of subcellular TR specificity. Given the fact that TR1 and TRß show no preference in binding to TREPrest or TRE-3KCNQ4, the specificity of TR binding in postnatal OHCs must be achieved by additional gene-specific differences in the upstream regions of the prestin and Kcnq4 genes. These differences influence the level of TR-associated DNA-bound transcription factors, as previously proposed (Glass and Rosenfeld, 2000). The composition of regulatory elements may then act in concert with specific local T3 levels to alter dissociation or recruitment rates of co-repressor or coactivator complexes (Fig. 8) (Hermanson et al., 2002; Kamei et al., 1996; Onate et al., 1995).
Until now human patients with mutant TR1 have not been identified; as either the effects on a human phenotype are too mild, spontaneous miscarriages occur at embryonic or fetal stages of development, and/or the symptoms have not been attributed to defects in TR1 (Tinnikov et al., 2002). Based on the findings reported here, the latter explanation may apply most appropriately in the case of the auditory system.
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Materials and Methods |
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), Tshrhyt (hyt/hyt) (Walsh and McGee, 2001), TR1–/– (Wikstrom et al., 1998), TRß–/– (Forrest et al., 1996a), TR1–/–/ß–/– (Gothe et al., 1999) and TR1m/+ (Tinnikov et al., 2002) mice were used (Table 1). MMI treatment of mice, thyroxin (T4) administration, and T4 and T3 plasma titer determinations were performed as described (Knipper et al., 2000; Weber et al., 2002). Animal experiments were approved and complied with all protocol requirements at the University of Tübingen.
Tissue preparation
Cochleae of untreated (control), MMI-treated (hypothyroid) and T4-treated (hypothyroid+T4) animals were prepared and cryosectioned as described (Knipper et al., 1998).
Riboprobe synthesis and in situ hybridization
Using the KCNQ4-specific oligonucleotide primers rKCNQ4-USP3 and rKCNQ4-DSP9 (supplementary material, Table S1) a PCR fragment was amplified from rat cochlear cDNA, cloned into pCR®II TOPO Vector (Invitrogen) and sequenced. Kcnq4-specific riboprobes were synthesized and in situ hybridization was performed as described (Knipper et al., 1999; Knipper et al., 2000; Weber et al., 2002).
Northern blot
Northern blots were performed as described (Knipper et al., 1998; Knipper et al., 1999). The effect of TH on mRNA levels was semi-quantitatively evaluated using mRNAs isolated from a similar number of cochleae as described (Knipper et al., 1998). To ensure the loading of similar amounts of mRNA per lane blots were probed with the housekeeping gene cyclophilin (Thellin et al., 1999).
Semi-quantitative RT-PCR
Semi-quantitative RT-PCR was performed as described (Rüttiger et al., 2006). In brief, cochlear mRNA from P12 hypothyroid and control rats was isolated using the Dynabeads mRNA Direct Kit (Dynal) and reverse-transcribed with Superscript II (Invitrogen). Using the oligonucleotides rKCNQ4-USP3 and rKCNQ4-DSP3 and cyclophilin-f and cyclophilin-r (supplementary material Table S1) as internal controls semi-quantitative RT-PCR was performed on equivalent amounts of mRNA isolated from four cochleae of control and hypothyroid rats. PCR was repeated three times using different animals.
Generation of KCNQ4 antibody
An antibody against a conserved part of the C-terminus of KCNQ4 (CQTLSISRSVSTNMD-COOH) was raised in rabbits, purified by affinity chromatography and tested by immunoblotting and immunohistochemistry by preincubation with the synthetic peptide (data not shown).
Fluorescence immunohistochemistry
Cochlear sections of rats and mice were stained and imaged as described (Knipper et al., 1998; Knipper et al., 2000). The rabbit and goat (Santa Cruz) polyclonal antibody against KCNQ4, rabbit polyclonal antibody against prestin (Weber et al., 2002), sheep polyclonal antibody against synaptophysin (The Binding Site) were visualized with Cy3(Jackson ImmunoResearch Laboratories) or Alexa Fluor 488conjugated secondary antibodies (Molecular Probes) and counter-stained with DAPI (Vector Laboratories).
Oligonucleotides and labeling
In EMSA, 3.85 pmol of the double-stranded synthetic oligonucleotides TRE1KCNQ4, TRE-2KCNQ4, TRE-3KCNQ4, TRE-3mut1, TRE-3mut2, DR4, TREPrest and TFIID (supplementary material, Table S1) were labeled with [
-32P]ATP (5000 Ci/mmol; Amersham Biosciences) by phosphorylation with T4 polynucleotide kinase (Promega) and purified with the QIAquick Nucleotide Removal Kit (Qiagen).
Cloning of the human KCNQ4 promoter
Genomic DNA was isolated from human blood with the QIAmp Blood Kit (Qiagen). PCR was performed with the oligonucleotide primers PromK4USP and PromK4DSP and nested primers sPromK4USP(BglII) and sPromK4DSP(HindIII) (supplementary material Table S1). A 739 bp PCR fragment was amplified, cloned into the pGL3basic vector (Promega) and sequenced.
Cloning of the reporter gene constructs
The synthetic double-stranded oligonucleotide TRE-3KCNQ4 (supplementary material Table S1) was cloned into the SacI and XhoI sites of the pGL3 vector containing the putative human KCNQ4 promoter. The previously described rat TR expression vector was used (Weber et al., 2002).
Transfection and reporter gene assays
HEK293 cells were cultured and transfected with 500 ng of each reporter construct, 500 ng of the control plasmid RSV-LacZ, and 500 ng of the TR expression vector for 12 hours as described (Weber et al., 2002). Transfected cells were incubated for 48 hours in fresh medium containing serum either depleted of T3 and retinoic acid (Weber et al., 2002) or containing 150 nM T3 (Sigma) or 1.5 µM all-trans-retinoic acid (ATRA; Sigma). The luciferase assay system (Promega) and the ßgalactosidase enzyme system (Promega) were used to determine luciferase activity and transfection efficiency as described (Weber et al., 2002). Four independent experiments were performed, each containing three to six independently prepared transfection mixtures.
In vitro translation
Rat TR and TRß were translated in vitro from plasmids (Weber et al., 2002) using the TNT® T7 coupled reticulocyte lysate system (Promega).
Electromobility-shift assay (EMSA)
EMSAs were performed as described (Weber et al., 2002). Unlabeled competitor oligonucleotides (200-fold excess) or T3 (150 nM) were preincubated with recombinant chicken TR (Santa Cruz) for 30 minutes before adding the radiolabeled oligonucleotides.
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Acknowledgments |
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1m/+ mice, R. Panford-Walsh for reading and correcting this manuscript and D. Schmollinger for technical assistance. This work was supported by the Federal Ministry of Education and Research (01KS9602), the Interdisciplinary Center of Clinical Research Tübingen, the Deutsche Forschungsgemeinschaft DFG KN316/4-1 and the NIH-NIDCD-5R01DC04566. |
Footnotes |
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