Isolation and characterization of a cDNA for the beta subunit of chicken Thyroid-Stimulating Hormone

Abstract

The beta subunit of Thyroid Stimulating Hormone (TSHP) has been isolated and sequenced in many species including several mammals and the frog. However, TSHO has not been isolated from any avian species. The objective of this study was to isolate a CDNA for chicken TSHP and use this CDNA to characterize TSHP gene expression around the time of hatch. Comparison of the CDNA sequences for TSHP from five species (four mammals and the frog) identified three highly conserved regions. Polymerase chain reaction (PCR) was implemented and a homologous CDNA sequence was produced. Tissue specificity of the CDNA probe was analysed by Northern blot. The "P-labeled probe hybridized to an RNA species of approximately 500 bases in pituitary RNA samples alone; no signal was detected in the other tissues. This length corresponds with the length of TSHP MRNA in other species (that range from 450 to 575 bases). Once a probe was developed, TSHP MRNA levels were measured along with thyroxine (T,), triiodothyronine (T 3), and growth hormone (GH) during late embyonic and early post-hatch ages of chickens. In these experiments it was shown that TSHO MRNA peaked on embryonic day 19 and T4 levels followed that pattern, with a sharp increase on post-hatch day 1. T3 levels increased significantly on post-hatch day I after the detection of GH in circulation on day 19 of embryonic development. The regulation of TSHP gene expression by T3 and thyrotropin releasing hormone (TRH) is well documented in mammals. The objective of the next study was to test the effect of TRH or T3 on chicken TSHP gene expression on embryonic day 19 and post-hatch day 1. In these studies we found that on embryonic day 19 TRH stimulated TSHP MRNA levels, while T3 had no significant effects. However, on day I T 3 decreased levels of TSHP MRNA, while TRH had no effect.