Thyroid hormone (TH) is essential for cochlear development and normal auditory function. Considering the importance of TH in mediating cochlear development, understanding the degree to which developing tissues can adapt to perturbations in thyroid hormone signaling is extremely important. The deiodinases (D2 and D3) are enzymes that tightly control TH availability at the tissue level and have been proposed to function as adaptive mechanisms that maintain tissue TH homeostasis. D2 converts thyroxine (T4) to a biologically active ligand triiodothyronine (T3); locally amplifying a T3 signal. Conversely, Dio3 inactivates T3 and T4 by converting these iodothyronines to the inactive metabolites diiodothyronine (T2) and reverse T3 (rT3), respectively. During cochlear development, Dio3 expression is high prenatally while Dio2 expression is low. During the first postnatal week, the expression levels of these enzymes invert, resulting in high expression of Dio2 and low expression of Dio3. Together, the deiodinases control the timing of postnatal cochlear remodeling; suggesting a genetic developmental clock controls TH-mediated cochlear development. Considering the role deiodinases have as an adaptive mechanism, it is important to understand whether such a developmental clock can be negated during times of developmental thyroid hormone insufficiency. We hypothesize that the perinatal change in deiodinase expression is controlled by a developmental clock rather than environmental clues and therefore have limited capacity to function as compensatory mechanisms in response to low TH during development. To test this, timed-pregnant mice were treated with thyroid gland inhibitors to induced hypothyroidism from gestational day 12.5 until pup sacrifice. A parallel set of untreated timed-pregnant mice served as controls. Cochlea were harvest from control and hypothyroid mice at postnatal ages P1, P5, P10 and P15 for qRT-PCR. We observed D2 and D3 mRNA levels were similar or reduced in hypothyroid animals compared to controls at P1, P5, and P10. This finding indicates that at these ages, D2 at did not respond in a manner consistent with the idea of compensation. However, at P15, D2 mRNA levels were increased in hypothyroid animals compared to controls; a finding that is consistent with a compensatory mechanism. Interestingly, decreased D3 mRNA levels observed in the hypothyroid cochlea is indicative of an adaptive response that could be attempting to compensate for the goitrogen-induced reductions in T4. Taken together, our results suggest that developmental programs in tissues may be dominant over potential compensatory mechanisms and that developing tissues may be more susceptible to perturbations in tissue TH levels due to a reduced capacity to compensate.
Date of Degree
Master of Science (MS)
Science, Engineering and Technology
Uscategui Calderon, M. A. (2020). Characterization of deiodinase expression in the hypothyroid developing cochlea [Master’s thesis, Minnesota State University, Mankato]. Cornerstone: A Collection of Scholarly and Creative Works for Minnesota State University, Mankato. https://cornerstone.lib.mnsu.edu/etds/1064/
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