Induction of type 3 iodothyronine deiodinase by nerve injury in the rat peripheral nervous system

Thyroid hormones are essential for the development and repair of the periph eral nervous system. The type 2 deiodinase, which is responsible for the ac tivation of T-4 into T-3, is induced in injured sciatic nerve. To obtain in formation on the type 3 deiodinase (D3) responsible for the degradation of thyroid hormones, we looked for its expression (mRNA and activity) in the s ciatic nerve after injury. D3 was undetectable in the intact sciatic nerve of adult rats, but was rapidly and highly increased in the distal and proxi mal segments after nerve lesion. After cryolesion, D3 up-regulation disappe ared after 3 d in the proximal segment, whereas it was sustained for 10 d i n the distal segment, then declined to reach basal levels after 28 d, when functional recovery was completed. After a trans-section preventing the ner ve regeneration, up-regulation of D3 persisted up to 28 d at high levels in the distal segment. D3 was expressed in peripheral connective sheaths and in the internal endoneural compartment. D3 mRNA was inducible by 12-O-tetra decanoylphorbol-13-acetate in cultured fibroblasts or Schwann cells. In con clusion, induction of D3 in the peripheral nervous system after injury may play an important role during the regeneration process by adjusting intrace llular T-3 levels.