Type-I insulin-like growth factor receptor gene expression in the chick. Developmental changes and the effect of selection for increased growth on the amount of receptor mRNA

An RNase protection assay is described that allowed the quantitative analysis of chicken type-I IGF receptor mRNA transcripts. The transcripts were measured in extracts of total nucleic acid (TNA) and, under the hybridization conditions described, protected probes of the expected size were obtained. The RNA-RNA hybrids could be quantified in the presence of at least a 1000-fold molar excess of DNA containing sequences which were complimentary to the RNA probe. The amount of protected probe was linearly related to the amount of TNA in the hybridization reaction medium, and this allowed the results to be expressed in the form of mRNA molecules/cell. Type-I IGF receptor mRNA transcripts were detected in all the tissues examined from a 20-day-old chick embryo. Their amount ranged from 5 to 24 molecules/cell, in the order liver < breast muscle < leg muscle < heart < brain. The amount of receptor mRNA was 65- to 300-fold less than that of beta-actin mRNA. The quantity of type-I IGF receptor mRNA varied significantly throughout embryonic and post-hatch development. Maximum amounts were measured in 21-day-old embryos (a two- to fourfold increase relative to 16-day-old embryos). Thereafter the amount of receptor mRNA decreased, during the 4-week period after hatching, to levels which were significantly lower than that observed in 16-day-old embryos. Throughout the period of embryonic and post-hatch development described here the amount of beta-actin mRNA remained constant, indicating that the changes in the quantity of receptor mRNA were due to specific mechanisms acting directly on the steady-state levels of type-I IGF receptor mRNA. Selection for increased growth had no effect on the amount of type-I IGF receptor mRNA. The result was the same when expressed either as molecules/cell or as a percentage of beta-actin mRNA.