micro-Crystallin as an intracellular 3,5,3'-triiodothyronine holder in vivo

Area of Science:

• Biochemistry
• Molecular Biology
• Genetics

Background:

• Cytosolic T(3)-binding protein, identified as mu-crystallin (CRYM), is involved in thyroid hormone (T3) transport.
• CRYM mutations are linked to hereditary deafness, highlighting its clinical significance.
• In vitro studies suggest CRYM's role in T3 reservation and nuclear transport, but in vivo functions remain unclear.

Purpose of the Study:

• To investigate the in vivo functions of CRYM gene products.
• To elucidate the precise role of CRYM in thyroid hormone regulation and its impact on physiological processes.

Main Methods:

• Generation of CRYM knockout mice through targeted gene disruption.
• Assessment of T3 binding activity in various tissues of knockout mice.
• Measurement of serum T3 and T4 concentrations, growth, heart rate, and hearing ability in euthyroid knockout mice.
• Analysis of TSHbeta, glutathione-S-transferase alpha2, and deiodinase 1 mRNA expression.
• Intravenous injection of radiolabeled T3 to track its tissue distribution and turnover.

Main Results:

• CRYM knockout mice exhibited a loss of NADPH-dependent T3 binding activity in multiple tissues.
• Serum concentrations of T3 and T4 were significantly suppressed in knockout mice at the euthyroid state.
• Despite suppressed thyroid hormone levels, knockout mice showed normal growth, heart rate, and hearing ability.
• Gene disruption did not affect the expression of key regulatory mRNAs (TSHbeta, GSTalpha2, DIO1).
• Radiolabeled T3 showed rapid entry and escape from tissues in CRYM-knockout mice, indicating accelerated turnover.

Conclusions:

• CRYM plays a crucial role in vivo in regulating thyroid hormone (T3) concentrations.
• Disruption of the CRYM gene leads to decreased T3 turnover, resulting in lower tissue and serum T3 levels.
• The study suggests that CRYM's primary in vivo function relates to T3 turnover rather than direct action or regulation of peripheral T3 effects.