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Morphological and functional colonic defects caused by a mutated thyroid hormone receptor alpha.

BACKGROUND
Mutations of thyroid hormone receptor alpha 1 (TRalpha1) result in resistance to thyroid hormone (RTHalpha) exhibiting symptoms of retarded growth, delayed bone maturation, anemia, and severe constipation. Using a mouse model of RTHalpha (Thra1PV/+ mouse), we aimed to understand the molecular basis underlying the severe constipation observed in patients.


METHODS
The Thra1PV/+ mouse expresses a strong dominant negative mutant, PV, which has lost T3 binding and transcription activity (Thra1PV/+ mouse) faithfully reproduces growth abnormalities and anemia as shown in RTHalpha patients and therefore is a valid model to examine causes of severe constipation in patients. We used histopathological analysis, confocal fluorescence imaging, transmission electron microscopy (TEM), and gene expression profiles to comprehensively analyze the colonic abnormalities of Thra1PV/+ mouse.


RESULTS
We found a significant increase in colonic transit time and decrease stool water content in Thra1PV/+ mouse, mimicking constipation as found in patients. Histopathological analysis showed expanded lamina propria filled with interstitium fluid between crypt columns, enlarged muscularis mucosa, and increased content of collagen in expanded submucosa. TEM analysis revealed shorter muscle fibers with wider gap junctions between muscle cells, fewer caveolae, and hypoplastic interstitial cells of Cajal (ICC) in the rectal smooth muscles of Thra1PV/+ mice. These abnormal histological manifestations suggested defective intercellular transfer of small molecules, electrolytes, and signals for communication among muscles cells, validated by Lucifer Yellow transferring assays. Expression of key smooth muscle contractility regulators, such as calmodulin, myosin light-chain kinase, and phosphorylated myosin light chain, was markedly lower, and c-KIT signaling in ICC was attenuated, resulting in decreased contractility of the rectal smooth muscles of Thra1PV/+ mice. Collectively, these abnormal histopathological alterations and diminished contractility regulators led to the constipation exhibited in patients.


CONCLUSIONS
This is the first demonstration that TRalpha1 mutants could act to cause abnormal rectum smooth muscle organization, defects in intercellular exchange of small molecules, and decreased expression of contractility regulators to weaken the contractility of rectal smooth muscles. These findings provide new insights into the molecular basis underlying constipation found in RTHalpha patients.