ABSTRACT
PURPOSE: Opaque chick corneas become thin and transparent from embryonic day (E)9 to E20 of incubation. Thyroxine (T4) injected in ovo on E9 induces precocious transparency by E12. The present study was conducted to determine whether corneal cells differentially express genes for T4 regulation, keratan sulfate proteoglycan (KSPG) synthesis, crystallins, and endothelial cell ion transporters during transparency development and whether these expressions are altered when E9 embryos are treated with T4. METHODS: E9 eggs received T4 or buffer; corneas were dissected on E12. Corneal transparency was measured digitally and thickness was determined from cryostat cross sections. mRNA expressions were determined by real-time PCR using cDNA synthesized from whole-cell RNA, cells expressing T4 receptor mRNAs assessed by in situ hybridization, and KS disaccharide sulfation measured by electrospray ionization tandem mass spectrometry (ESI-MS/MS). RESULTS: All corneal layers expressed T4 receptor alpha (THRA) mRNA; keratocytes and endothelial cells expressed T4 receptor beta (THRB) mRNA. During normal development, THRB expression increased 20-fold from E12 to E20; THRA expression remained constant. Expressions of most genes involved in KS synthesis increased from E9 to E16, and then decreased from E16 to E20. From E9 to E20, expressions of crystallin genes increased; T4/3-deiodinase DIII (DIO3) increased 10-fold; and sodium-potassium ATPase transporter (ATP1A1), sodium-bicarbonate transporter (NBC), and carbonic anhydrase II (CA2) increased 5- to 10-fold. E9 T4 administration decreased corneal thickness by E12; increased DIO3, THRB, and CA2 expressions 5- to 20-fold; decreased KSPG core protein genes and galactose sulfotransferase CHST1 expressions 2-fold; and reduced KS disulfated/monosulfated disaccharide (DSD/MSD) ratios. CONCLUSIONS: Thyroxine modifies expressions of KSPG synthesis and carbonic anhydrase genes.
