Prevention of auditory dysfunction in hypothyroid Tshr mutant mice by thyroxin treatment during development

Thyroid hormone deprival and TSH/TSHR signaling deficiency lead to central hypothyroidism-associated intestinal dysplasia

BACKGROUND

Central hypothyroidism (CH) is characterized by low T4 levels and reduced levels or bioactivity of circulating TSH. However, there is a lack of studies on CH-related intestinal maldevelopment. In particular, the roles of TH and TSH/TSHR signaling in CH-related intestinal maldevelopment are poorly understood. Herein, we utilized Tshr-/- mice as a congenital hypothyroidism model with TH deprival and absence of TSHR signaling.

METHODS

The morphological characteristics of intestines were determined by HE staining, periodic acid-shiff staining, and immunohistochemical staining. T4 was administrated into the offspring of homozygous mice from the fourth postnatal day through weaning or administrated after weaning. RT-PCR was used to evaluate the expression of markers of goblet cells and intestinal digestive enzymes. Single-cell RNA-sequencing analysis was used to explore the cell types and gene profiles of metabolic alternations in early-T4-injected Tshr-/- mice.

KEY FINDINGS

Tshr deletion caused significant growth retardation and intestinal maldevelopment, manifested as smaller and more slender small intestines due to reduced numbers of stem cells and differentiated epithelial cells. Thyroxin supplementation from the fourth postnatal day, but not from weaning, significantly rescued the abnormal intestinal structure and restored the decreased number of proliferating intestinal cells in crypts of Tshr-/- mice. Tshr-/- mice with early-life T4 injections had more early goblet cells and impaired metabolism compared to Tshr+/+ mice.

SIGNIFICANCE

TH deprival leads to major defects of CH-associated intestinal dysplasia while TSH/TSHR signaling deficiency promotes the differentiation of goblet cells and impairs nutrition metabolism.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

Genetic background of Prop1(df) mutants provides remarkable protection against hypothyroidism-induced hearing impairment

Hypothyroidism is a cause of genetic and environmentally induced deafness. The sensitivity of cochlear development and function to thyroid hormone (TH) mandates understanding TH action in this sensory organ. Prop1(df) and Pou1f1(dw) mutant mice carry mutations in different pituitary transcription factors, each resulting in pituitary thyrotropin deficiency. Despite the same lack of detectable serum TH, these mutants have very different hearing abilities: Prop1(df) mutants are mildly affected, while Pou1f1(dw) mutants are completely deaf. Genetic studies show that this difference is attributable to the genetic backgrounds. Using embryo transfer, we discovered that factors intrinsic to the fetus are the major contributor to this difference, not maternal effects. We analyzed Prop1(df) mutants to identify processes in cochlear development that are disrupted in other hypothyroid animal models but protected in Prop1(df) mutants by the genetic background. The development of outer hair cell (OHC) function is delayed, but Prestin and KCNQ4 immunostaining appear normal in mature Prop1(df) mutants. The endocochlear potential and KCNJ10 immunostaining in the stria vascularis are indistinguishable from wild type, and no differences in neurofilament or synaptophysin staining are evident in Prop1(df) mutants. The synaptic vesicle protein otoferlin normally shifts expression from OHC to IHC as temporary afferent fibers beneath the OHC regress postnatally. Prop1(df) mutants exhibit persistent, abnormal expression of otoferlin in apical OHC, suggesting delayed maturation of synaptic function. Thus, the genetic background of Prop1(df) mutants is remarkably protective for most functions affected in other hypothyroid mice. The Prop1(df) mutant is an attractive model for identifying the genes that protect against deafness.

Atypical fetal voice processing in preeclamptic pregnancy

OBJECTIVE

To compare fetal heart rate (HR) response to the mother's voice in pregnancies complicated by preeclampsia with those of fetuses in uneventful, normotensive pregnancies.

METHOD

Fifty fetuses (n = 22, preeclampsia; n = 28, uneventful, normotensive pregnancies) between 32 and 40 weeks gestational age were recruited. Each fetus was presented with a 2-min no-sound baseline period followed by a 2-min voice period during which an audio recording of his/her mother reading a story was played through a loud speaker over the maternal abdomen at an average of 95 dBA followed by a 2-min no-voice offset period. HR was recorded continuously.

RESULTS

Fetuses in the preeclamptic group showed no response to the mother's voice when it was played. In comparison, fetuses in the uneventful, normotensive group responded to the mother's voice with a HR increase.

CONCLUSION

Fetuses in pregnancies complicated by preeclampsia show atypical auditory processing of the mother's voice. Such atypical responding may reflect a delay in auditory system maturation, functional elevation of sensorineural threshold, or decreased thyroid hormone.

Thyroxine treatments do not correct inner ear defects in tmprss1 mutant mice

Complete deficiency of a member of the type II transmembrane serine protease family, tmprss1 (also known as hepsin), is associated with severe to profound hearing loss in mice and a gross enlargement of the tectorial membrane in the cochlea. Levels of thyroxine in these mice have been shown to be significantly lower when compared with wild-type controls. As thyroxine is critical for inner ear development, we delivered thyroxine to these mice during the prenatal or postnatal stage of development. Both the treatments could not ameliorate hearing loss or correct deformities in the tectorial membrane of these mutant mice, suggesting that a deficiency in tmprss1 affects thyroxine responsiveness in the inner ear in vivo.

A protective role for type 3 deiodinase, a thyroid hormone-inactivating enzyme, in cochlear development and auditory function

Thyroid hormone is necessary for cochlear development and auditory function, but the factors that control these processes are poorly understood. Previous evidence indicated that in mice, the serum supply of thyroid hormone is augmented within the cochlea itself by type 2 deiodinase, which amplifies the level of T(3), the active form of thyroid hormone, before the onset of hearing. We now report that type 3 deiodinase, a thyroid hormone-inactivating enzyme encoded by Dio3, is expressed in the immature cochlea before type 2 deiodinase. Dio3-/- mice display auditory deficits and accelerated cochlear differentiation, contrasting with the retardation caused by deletion of type 2 deiodinase. The Dio3 mRNA expression pattern in the greater epithelial ridge, stria vascularis, and spiral ganglion partly overlaps with that of thyroid hormone receptor beta (TRbeta), the T(3) receptor that is primarily responsible for auditory development. The proposal that type 3 deiodinase prevents premature stimulation of TRbeta was supported by deleting TRbeta, which converted the Dio3-/- cochlear phenotype from one of accelerated to one of delayed differentiation. The results indicate a protective role for type 3 deiodinase in hearing. The auditory system illustrates the considerable extent to which tissues can autoregulate their developmental response to thyroid hormone through both type 2 and 3 deiodinases.

The influence of thyroid hormone deficiency on the development of cochlear nonlinearities

It is well known that failure to treat severe congenital hypothyroidism leads to profound auditory disability, and it has been suggested that an intracochlear defect, or defects, associated with the condition diminishes the efficacy of an active, physiologically vulnerable nonlinear transduction process commonly referred to as cochlear amplification. We address this question directly by tracking the development of threshold-frequency (tuning) curves and two-tone suppression in hypothyroid, Tshr mutant mice born to hypothyroid dams and comparing those findings with findings observed in euthyroid mice. Like sharp tuning, two-tone suppression is a product of transduction nonlinearity and is a useful indicator of the functional status of cochlear amplification. In contrast to euthyroid mice that acquire sharp tuning, normal two-tone suppression, and adultlike sensitivity by the end of the third postnatal week, as shown in earlier studies, hypothyroid mice remained grossly insensitive to sound throughout life. In addition, tuning was generally broad in hypothyroid mice, tuning curve "tips" were frequently missing, and two-tone suppression was rarely observed. However, unlike tip thresholds, tuning curve "tail" thresholds, a feature that reflects the functional status of passive elements of transduction, acquired normal values over a roughly 2-month postnatal time period. These observations collectively suggest that active transduction micromechanics, at least in the frequency region studied here, are profoundly affected by thyroid hormone and support speculation that abnormal outer hair cell function may be the cause of the primary, enduring peripheral auditory defect associated with profound, congenital hypothyroidism in the Tshr mutant mouse.

Dietary thyroid hormone replacement ameliorates hearing deficits in hypothyroid mice

Thyroid hormone (TH) insufficiency causes variable hearing impairment and mental deficiency in humans. Rodents lacking TH have congenital hearing deficiency that has been attributed to physiologic, morphologic, and developmental abnormalities of the auditory system. We examined four genetically defined strains of hypothyroid mice for development of hearing and response to TH replacement initiated during late gestation and continued through six weeks of age. Auditory brain stem response studies showed variable hearing impairment in homozygous mutants of each strain at three weeks of age relative to normal littermates. Mutants from three of the strains still had hearing deficiencies at six weeks of age. TH-enriched diet significantly improved hearing in three-week-old mutants of each strain relative to untreated mutants. Differences in the level of hearing impairment between the Prop1df and Pit1dw mutants, which have defects in the same developmental pathway, were determined to be due to genetic background modifier genes. Further physiologic and morphologic studies in the Cgatm1Sac strain indicated that poor hearing was due to cochlear defects. We conclude that TH supplement administered during the critical period of hearing development in mice can prevent deafness associated with congenital hypothyroidism of heterogeneous genetic etiology.

Thyroid hormone receptors TRalpha1 and TRbeta differentially regulate gene expression of Kcnq4 and prestin during final differentiation of outer hair cells

Thyroid hormone (TH or T3) and TH-receptor beta (TRbeta) have been reported to be relevant for cochlear development and hearing function. Mutations in the TRbeta gene result in deafness associated with resistance to TH syndrome. The effect of TRalpha1 on neither hearing function nor cochlear T3 target genes has been described to date. It is also uncertain whether TRalpha1 and TRbeta 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 TRbeta. Simultaneously Kcnq4 expression was activated in the same cells by derepression of TRalpha1 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 TRalpha1 in final cell differentiation.

Consequences of combined maternal, fetal and persistent postnatal hypothyroidism on the development of auditory function in Tshrhyt mutant mice

Tshrhyt/hyt mutant mice express a point mutation in the gene encoding the thyrotropin receptor, and affected animals are congenitally hypothyroid and profoundly deaf as a consequence when the condition is untreated. In this investigation, a previously unrecognized developmental stage was identified in the hypothyroid, mutant progeny of hypothyroid dams by tracking developmental changes in the auditory brainstem response (ABR). ABR thresholds develop rapidly in normal, euthyroid animals, decreasing as much as 80 dB between P12 (postnatal day 12) and P15, with mature sensitivity being gradually acquired by P18. In contrast, Tshrhyt/hyt mutant mice remained profoundly deaf on P24 and although thresholds improved by approximately 30 dB by P60, residual frequency-dependent deficits of 20-70 dB were observed in animals exhibiting end-stage disease. The rate of threshold improvement in mutant mice was approximately ten times slower than in normal mice. While ABR wave latencies and interpeak intervals decreased early in postnatal life, values decreased over a delayed and protracted time period, reaching adult values well after those of controls attained maturity. As with normal mice, slopes of wave I latency-intensity curves were significantly steeper in immature animals than those observed in adults and decreased during development, but failed to achieve normal adult values and remained significantly steeper than those for controls. Findings reported here suggest that passive aspects of electromechanical transduction achieve maturity in Tshrhyt/hyt progeny of Tshrhyt/hyt mice and that development, limited as it may be, occurs most prominently in the basal half of the cochlea.

Developmental disruption of thyroid hormone: correlations with hearing dysfunction in rats

A wide variety of environmental contaminants adversely affect thyroid hormone (TH) homeostasis. Hypothyroidism and/or hypothyroxinemia during the early postnatal period in the rat leads to permanent structural damage and loss of function in the cochlea. A major uncertainty in assessing the risks of developmental exposure to thyroid-disrupting chemicals (TDCs) is the lack of a clear characterization of the dose-response relationship, especially in the lower region, between disruption of hormones and adverse consequences. The current work correlated early postnatal hypothyroxinemia with hearing loss in the adult rat. Linear regression was performed on the log transform for total serum thyroxine (T4) concentrations on postnatal day 14 or 21 versus dB(SPL) of hearing loss in adult animals developmentally exposed to TDCs. Regression analyses revealed a highly significant correlation between T4 concentration and hearing loss. In the rat, a 50-60% decrease in circulating T4 was needed to significantly impact hearing function. This correlation suggests that T4 serum concentrations at 14 or 21 days of postnatal age may be a good predictive biomarker in rodents of the adverse consequence of developmental exposure to TDCs.