Characterization of a major development-regulated serum thyroxine-binding globulin in the euthyroid mouse

Area of Science:

• Endocrinology
• Developmental Biology
• Biochemistry

Background:

• Thyroid hormones (T4 and T3) are crucial for development and metabolism.
• Thyroxine-binding globulin (TBG) is a primary carrier protein for thyroid hormones in circulation.
• The role and regulation of TBG during development are not fully understood in all species.

Purpose of the Study:

• To characterize the developmental regulation, binding properties, structure, and regulatory mechanisms of mouse thyroxine-binding globulin (TBG).
• To investigate the ontogenic pattern of thyroid hormone binding activity in mouse serum during development.
• To explore the influence of non-esterified fatty acids (NEFA) and thyroid status on TBG function.

Main Methods:

• Serum protein electrophoresis (PAGE) to identify TBG.
• Scatchard analysis to determine binding affinities and capacities.
• Isoelectric focusing (i.e.f.) to analyze TBG isoforms.
• Experimental hypothyroidism to assess thyroid hormone control.

Main Results:

• Mouse serum exhibits a striking developmental pattern of T4 and T3 binding, peaking 3-5 days postnatally (7-8 times adult levels).
• PAGE identified an alpha-1 globulin as the primary perinatal binding protein, consistent with TBG.
• Scatchard analysis revealed high affinity (10(9) M-1) and low capacity binding sites for T4 on TBG.
• Isoelectric focusing showed mouse TBG is microheterogeneous with 10-12 isoforms, with developmental shifts in their abundance.
• Non-esterified fatty acids (NEFAs) inhibited TBG binding, while hypothyroidism significantly increased serum TBG levels.
• TBG binding patterns were distinct from other thyroid hormone carriers like transthyretin (TTR) and albumin.

Conclusions:

• Mouse TBG is a major, development-regulated protein crucial for thyroid hormone transport during maturation.
• TBG exhibits distinct binding kinetics and structural heterogeneity (isoforms) that change with development.
• TBG synthesis and/or secretion are under thyroid hormone control, with hypothyroidism increasing TBG levels.
• The presence of major TBGs in mice and rats suggests a broader role for this protein in mammalian development than previously recognized.